Hydrogen Peroxide as a Green Oxidant for the Selective Catalytic Oxidation of Benzylic and Heterocyclic Alcohols in Different Media: An Overview

Abstract

Among a plethora of known and established oxidant in organic chemistry, hydrogen peroxide stands in a special position. It is commercially and inexpensively available, highly effective, selective, and more importantly it is compatible with current environmental concerns, dictated by principles of green chemistry. Several chemicals or their intermediates that are important in our daily life such as pharmaceuticals, flavors, fragrances, etc. are products of oxidation of alcohols. In this review, we introduce hydrogen peroxide as an effective, selective, green and privileged oxidant for the catalyzed oxidation of primary and secondary benzylic and heterocyclic alcohols to corresponding carbonyl compounds in different media such as aqueous media, under solvent-free conditions, various organic solvent, and dual-phase system.

1. Introduction

Nowadays, energy and environmental crises have resulted in the discovery of green technologies for progressive use in the chemical industry. Exploration of green chemical reaction approaches have especially shaped our daily lives. The use of alternative non-toxic and green solvents, benign reagents and substrates, and the application of new catalytic systems with the same efficiency or even superior efficiency have also reduced the risk of chemical reactions and resulted in safety [1,2].

Oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively, are considered as a pivotal reaction in organic transformations due to the wide applications of these products as precursors or intermediates for construction of several drugs, fragrances, and vitamins. Because of the low selectivity of some reactions such as oxidation, the resulting byproducts generate waste and pollution. Therefore, selective oxidation of organic compounds as a critical chemical transformation has wide-ranging applications in the chemical, industrial, and biological processes which needs more attention. Traditionally, selective oxidation of alcohols has needed toxic, corrosive, and expensive stoichiometric inorganic oxidants such as Cr(VI), Mn(VII), heptavalent iodine, and DMSO-coupled reagents which create both environmental and economic concerns [3,4]. Due to an urgent and important demand for greener approaches, the use of toxic solvents and reagents such as organic peroxides must be avoided. Thus, green solvents such as water or solvent-free systems, clean oxidants such as O₂ or H₂O₂, and also recyclable catalysts should be considered in order to comply with the principles of green chemistry proposed by Anastas and Warner [2]. According to the atom economy principle, established by Trost in 1991, in oxidation reactions, with respect to the total mass of the oxidant, the mass amount of oxygen transferred to the substrate should be optimal [5]. Although molecular oxygen is considered to be an ideal oxidant, its use sometimes is problematic since it needs harsh reaction conditions such as high temperature or pressure and it also shows poor selectivity.

Nowadays, among the common oxidants, hydrogen peroxide (H₂O₂) with 47% oxygen content has been found to be a more suitable and practical oxidant. Moreover, with the transfer of one oxygen atom to the substrate in the oxidation reaction, one equivalent of H₂O is practically formed as an ideal and green expected byproduct [6,7,8,9,10]. In addition, due to its good solubility in water and many organic solvents, H₂O₂ is a very operative oxidant in liquid phase reactions [9,10]. Furthermore, its safe storage, operation, and transportation of aqueous hydrogen peroxide, along with its commercially availability, justify its wide applications [11,12].

Selective oxidation of aliphatic or aromatic alcohols to the corresponding carbonyl compounds using hydrogen peroxide either in the absence or presence of catalysts in different solvents under neutral, alkaline, or acid conditions have been extensively studied [13,14,15,16,17,18]. Our research group was also interested in oxidation [19,20,21,22], using hydrogen peroxide as an oxidant of choice [23,24] and especially, selective oxidation of alcohols to the corresponding carbonyl compounds [25,26,27]. We recently reviewed the applications of pyridinium chlorochromate (PCC) as an important selective oxidant for oxidation of primarily and secondary alcohols to their corresponding carbonyl compounds [28]. As a continuation of these interests, herein, we try to highlight the importance and efficacy of hydrogen peroxide as an effective and green oxidant in the catalyzed selective oxidation of primary and secondary benzylic alcohols and primary heterocyclic alcohols to their corresponding carbonyl compounds. This review is also intended to cover recent literature, focusing on new methods of selective catalytic oxidation of benzylic and heterocyclic alcohols using hydrogen peroxide in aqueous phase, a solvent-free system, in various solvents, and dual-phase systems. In addition, we present an overview on the reasonable reaction mechanism and promising catalytic effects of the selective oxidation of alcohols which actually can be considered as dehydrogenation of alcohols.

2. Selective Catalytic Oxidation of Benzylic and Heterocyclic Alcohols

2.1. Oxidation of Benzylic and Heterocyclic Alcohols in Aqueous Media

Epichlorohydrin (ECH)-modified Fe₃O₄ microspheres (1) for the selective oxidation of benzyl alcohol to benzaldehyde with H₂O₂ has been reported [29,30]. The ECH-derived hydroxyl groups have been shown to be milder and beneficial for the structural stability of Fe₃O₄ as compare with corrosive organic acid and iron-chelating additives. With ECH modification, a few surface active sites have been occupied, so the activation energy for H₂O₂ on Fe₃O₄ microspheres decomposition increased from 50.1 to 116.3 kJ mol⁻¹. Therefore, the promotional effect of short-chain saturated alcohols as additives for Fe₃O₄-catalyzed decomposition in H₂O₂ was established for the first time, and exploited to improve the catalytic performance of Fe₃O₄ microspheres for the selective oxidation of benzyl alcohol to corresponding benzaldehyde with H₂O₂ in water (Table 1) [31]. The results showed that applications of only Fe₃O₄ microspheres converted only 5.7% of PhCH₂OH (Table 1, Entry 1). Although, the catalytic activity of Fe₃O₄ microspheres were suggestively improved by the addition of small amounts of short-chain saturated alcohols (0.4 mol% with regards to H₂O₂) (Table 1, Entries 2–7), the selectivity for benzaldehyde reduced to 83 to 87%, due to over-oxidation to benzoic acid as a byproduct. The better promotional effect (same dosage, 0.4 mol%) of secondary and tertiary alcohols (41.8% to 48.9% in Table 1, Entries 2–4) on the conversion of benzyl alcohol (13.5% to 22.8% in Table 1, Entries 5–7) are observed as compared with primary alcohols. As shown in Table 1, for each alcohol, the optimal dosage could be different additives to accomplish the highest conversion of benzyl alcohol. Although the dosage of all alcohol additives was fixed (0.4 mol% with regard to hydrogen peroxide), the conversion of PhCH₂OH may not be the optimal value for each alcohol additive.

Table 1. Catalytic oxidation of benzyl alcohol to benzaldehyde with H₂O₂ in water on Fe₃O₄ microspheres 1 with 0.4 mol% of alcohol additives ^a.

Entry	Additive	Conv. (%)	Yield (%)	Sel. (%)
1	No additive	5.7	5.6	98.8
2	Glycerol	48.9	42.6	87.2
3	Tert-butanol	45.3	38.9	86
4	Iso-propanol	41.8	34.9	83.5
5	Methanol	22.8	19.5	85.5
6	Ethanol	13.5	11.5	85.4
7	MEG	17.4	14.5	83.2

^a Reaction conditions: Substrate (40 mmol and 4.1 mL), short-chain saturated alcohols (0.4 mol% with regards to H₂O₂), Fe₃O₄ microspheres 1 (0.2 g), H₂O₂ (30 wt%, 8 mL), H₂O (8 mL), reflux at 100 °C, 1.5 h.

Recyclable and reusable nanocatalyst (MgAl₂O₄@SiO₂–PTA) (2) was applied for the selective oxidation of primary and secondary alcohols (Table 2) to the corresponding aldehydes (for example, benzyl alcohol Entry 1) and ketones (for example, 1-phenyl ethanol Entry 7) with hydrogen peroxide as oxidant and water [32]. The oxidation reaction of 1-phenyl ethanol to acetophenone was more efficient than other reactions (98%). The catalyst was successfully used five times without any loss of its high catalytic activity. In benzylic alcohol oxidation, since the replacement was different from electron donating to electron withdrawing (Table 2, Entries 2–8), the yield of products decreased from 94% to 81% [33].

Table 2. Synthesis of diversified benzylic alcohol in the presence of MgAl₂O₄@ SiO₂–PTA 2 ^a.

Entry	Substrate	Product	Yield (%) b
1	C6H5CH2OH	C6H5CHO	96
2	4-ClC6H4CH2OH	4-ClC6H4CHO	91
3	4-NO2C6H4CH2OH	4-NO2C6H4CHO	85
4	3-NO2C6H4CH2OH	3-NO2C6H4CHO	81
5	4-OMeC6H4CH2OH	4-OMeC6H4CHO	93
6	4-MeC6H4CH2OH	4-MeC6H4CHO	94
7	C6H5CH(OH)CH3	C6H5COCH3	98
8	4-MeC6H4CH(OH)CH3	4-MeC6H4COCH3	97
9	4-ClC6H4CH(OH)CH3	4-ClC6H4COCH3	95
10	3-BrC6H4CH(OH)CH3	3-BrC6H4COCH3	92
11 c	C6H5CH2OH	-	-

^a Reaction condition: Substrate (2 mmol), MgAl₂O₄@ SiO₂–PTA 2 (0.05 g), H₂O₂ (30%, 1 mmol), H₂O (3 mL), 90 °C, 1.5 h. ^b Isolated yields. ^c Substrate (2 mmol), without MgAl₂O₄@ SiO2–PTA 2, H₂O₂ (1 mmol), H₂O (3 mL), 90 °C, 4 h.

A heterogeneous organocatalyst “glycoluril” (3) was applied for the oxidation of alcohols to the corresponding carbonyls with excellent conversion and selectivity (more than 90%) in an aqueous medium using hydrogen peroxide (Table 3) [34]. The catalyst was successfully recycled for >10 times. Depending on the functional groups substituted on the aromatic rings of benzylic alcohols (Table 3, Entries 1–5), good conversions and selectivities were obtained with different reaction times. In addition, heteroaromatic substrates were successfully converted into the corresponding aldehydes in a short reaction time.

Table 3. Substrate scope for the oxidation reaction ^a.

Entry	Substrate	Product	t (h)	Conv. (%)	Sel. (%)
1	C6H5CH2OH	C6H5CHO	3	99	98
2	4-OHC6H4CH2OH	4-OHC6H4CHO	5	93	97
3	4-FC6H4CH2OH	4-FC6H4CHO	4	95	98
4	4-OMeC6H4CH2OH	4-OMeC6H4CHO	4.5	92	98
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	4.5	93	96
6	3-Pyridinemethanol	3-Pyridinecarboxaldehyde	2.5	100	59
7	2-Pyridinemethanol	Picolinaldehyde	2.5	99	52
8	C6H5CH(OH)CH3	C6H5COCH3	3.5	99	98
9	4-MeC6H4CH(OH)CH3	4-MeC6H4COCH3	4	94	97
10	4-NO2C6H4CH(OH)CH3	4-NO2C6H4COCH3	6.5	72	98
11	(C6H5)2CHOH	(C6H5)2CO	3	99	99
12	C6H5CH(OH)(CH)2C6H5	C6H5CO(CH)2C6H5	3	99	91

^a Reaction conditions: Substrate (1 mmol), glycoluril 3 (5 mol%), H₂O₂ (50%, 1.2 equiv.), H₂O (2 mL), 60 °C.

Green, chemoselective, facile, and efficient oxidation of primary and secondary benzylic alcohols to corresponding aldehydes or ketones in aqueous H₂O₂ was reported by Kamal Amani and co-workers [35] in the presence of GO/Fe₃O₄/HPW (4) nanocomposite catalyst with approximately 75% to 99% conversions, without any overoxidation to acid (100% selectivity), as shown in Table 4.

Table 4. Catalytic performance of the GO/Fe₃O₄/HPW 4 nanocatalyst for the selective oxidation of alcohols with H₂O₂ under optimal reaction conditions ^a.

Entry	Substrate	Product	t/h	Yield (%) b	Sel. (%) c
1	C6H5CH2OH	C6H5CHO	3	99	100
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	1.5	100	100
3	4-ClC6H4CH2OH	4-ClC6H4CHO	2	99	100
4	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	4	94	100
5	2-NO2C6H4CH2OH	2-NO2C6H4CHO	9	70	100
6	4-NO2C6H4CH2OH	4-NO2C6H4CHO	7	90	100
7	2-OHC6H4CH2OH	2-OHC6H4CHO	6	85	100
8	4-CH2OHC6H5CH2OH	4-CHOC6H5CHO	7	70	100

^a Reaction conditions: Substrate (1 mmol), GO/Fe₃O₄/HPW 4 (20 mg), H₂O₂ (10%, 5 mmol), 70 °C, 1.5-9 h. ^b Yields are referring to isolated yields. ^c Selectivity was based on the corresponding aldehydes or ketones. ^d Byproduct is phenylacetic acid. ^e Byproduct is 3-phenylpropanoic acid.

In water, exchanging the protons of HPW with PEG-bridged di-imidazolium cations produced double catalytic sites in a single molecule of Keggin-type phosphotungstic acid (H₃PW₁₂O₄₀, HPW)-based di-imidazolium ionic liquid (IL) hybrid (5) catalyst which enhanced the reaction rate and resulted in higher selectivity of benzyl alcohol oxidation with H₂O₂ (30 wt%) in water with excellent catalytic efficiency, convenient recovery, and steady reusability. Higher conversions (71%) and selectivity (82%) were obtained by the di-cation IL-based PIPA-0 due to the di-cationic structure of imidazolium IL (Table 5, Entry 4). In addition, the built-in phase transfer capability of PIPA-n that arose from PEG modifier provided the higher activity of PIPA-n (n = 4, 8, and 13) [36].

Table 5. Results of the selective oxidation of alcohols to aldehydes with H₂O₂ in water over various PW-based catalysts ^a.

Entry	Substrate	Product	Catalyst	Conv. (%) b	Sel. (%) c
1	C6H5CH2OH	C6H5CHO	HPW	48	67
2			[BMIm]Cl	Trace	-
3			IL-PW	59	80
4			PIPA-0	71	82
5			PIPA-4	88	79
6			PIPA-8	91	84
7			PIPA-13	96	86
8	C6H5CH(OH)CH3	C6H5COCH3	HPW	48	>99
9			IL-PW	70	>99
10			PIPA-13	95	>99
11	(C6H5)2CHOH	(C6H5)2CO	HPW IL-PW	37	>99
12				56	>99
13			PIPA-13	74	>99

^a Reaction conditions: Substrates (10 mmol), PIPA-13 5 (1.3 mol% substrate, based on PW loading,), H₂O₂ (30 wt%, 15 mmol), H₂O (1.5 mL), 95 °C, 6 h. ^b Conversion as based on alcohols. ^c Selectivity was based on the corresponding aldehydes or ketones.

Incipient wetness impregnation method was applied for the preparation of ceria (CeO₂) supported tungstophosphoric acid (H₃PW₁₂O₄₀; HPW) catalysts. As illustrated in Table 6, H₃PW₁₂O₄₀/CeO₂ (6) catalyst among the various catalysts showed the best results for selective oxidation of benzyl alcohol (BzOH) with hydrogen peroxide (20 wt% H₂O₂) [37]. In addition, response surface methodology (RSM) based on the Box–Behnken design model showed 95.2% conversion of benzyl alcohol and a 94.2% yield of benzaldehyde with 98.9% selectivity which was in good agreement with the experimental results [37]. Reusability of the catalyst were further successfully tested for six consecutive runs.

Table 6. Comparison of catalytic performances over various catalyst during oxidation of benzyl alcohol ^a.

Entry	Catalyst	BzOH Conv. (%)	BzH Sel. (%)	BzH yield (%)
1	CeO2	Nil	Nil	Nil
2	15HPW/CeO2	77.9	97.6	76
3	20HPW/CeO2	94	98.2	92.3
4	25HPW/CeO2	93.2	94.7	88.4
5	20HPW/TiO2	84	96.6	81.2
6	20HPW/ZrO2	90.8	91.7	83.3
7	20HPW/CeTiO b	89.3	93.2	83.2

^a Reaction conditions: BzOH/H₂O₂ (1:2 (mol mol⁻¹)), catalyst (0.8 g), H₂O (20 mL), 110 °C, 4 h. ^b CeTiO was prepared by sol-gel method.

Unver [38] reported the synthesis of a water-soluble dinuclear Cu(II) complex, [Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂] (7) (4-bromobenzoic acid = HOOCC₆H₄Br; 2-2′-bipyridyl = C₁₀H₈N₂) which was successfully used without any additives under mild conditions in the oxidation of primary and secondary alcohols in water (Table 7). The catalyst showed high TON values (up to 100), good to moderate yields, and high selectivity with no traces of carboxylic acid producing during or after the reactions. In addition, the competition between the pure substrates as compared with the mixtures of alcohols, under the same conditions, resulted in higher product yields (Table 8) [38].

Table 7. Oxidation of various alcohols with copper (II) complex 7 in water ^a.

Entry	Substrate	Product	Yield (%)b	TON	TOF (h−1)
1	C6H5CH2OH	C6H5CHO	100	100	16
2	C6H5CH(OH)CH3	C6H5COCH3	100	100	17

^a Reaction conditions: Substrate (0.57 mmol), [Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂] 7 (5.7 × 10⁻³ mmol), H₂O₂ (19.5 mmol), H₂O (10 mL), 70 °C, 6 h. Blank experiment was conducted without catalyst for each substrate and negligible conversion was obtained (<3%). ^b Determined with GC.

Table 8. Selective oxidation of selected alcohols with copper (II) complex 7 in water ^a.

Entry	Substrate	Product(s)	Conv. (%) b
1	C6H5CH2OH + C6H5CH(OH)CH3	C6H5CHO + C6H5COCH3	16.4 + 62.4

^a Reaction conditions: Substrate (5.7 × 10⁻⁴ mmol), [Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂] 7 (5.7 × 10⁻⁶ mmol), H₂O₂ (19.5 mmol), H₂O (10 mL), 70 °C, 6 h. 1 mmol each of alcohols was tested with mmol complex 7, mmol H₂O₂ in H₂O at 70 °C. ^b Determined with GC.

Cobalt zeolitic imidazolate framework (ZIF-9@Zeolite) (8) was prepared and under a simple and clean protocol, aldehydes, as oxidative products of alcohols, were efficiently obtained in high yields (Table 9), generating water as the only byproduct and with no decrease in catalyst activity even after four catalytic cycles. Benzyl alcohols with electron-withdrawing substituent also yielded the corresponding aldehydes high percent (Table 9, Entries 4, 7, 11, and 12) [39].

Table 9. Oxidation of alcohols to aldehydes using ZIF-9@Zeolite 8 ^a.

Entry	Substrate	Product	t/min	Yield (%)	B.P/M.P (°C)
					Found	Lit b
1	C6H5CH2OH	C6H5CHO	28	98	177	178
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	27	99	246	248
3	4-ClC6H4CH2OH	4-ClC6H4CHO	28	93	47	48
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	29	84	104–106	106
5	4-MeC6H4CH2OH	4-MeC6H4CHO	26	98	206	204–205
6	4-BrC6H4CH2OH	4-BrC6H4CHO	30	85	58	55–58
7	4-FC6H4CH2OH	4-FC6H4CHO	31	76	179–180	180
8	4-PhC6H4CH2OH	4-PhC6H4CHO	28	86	58–59	57–59
9	3-ClC6H4CH2OH	3-ClC6H4CHO	33	88	213–214	213–314
10	4-CO2MeC6H4CH2OH	4-CO2MeC6H4CHO	30	83	60–62	59–63
11	3-NO2C6H4CH2OH	3-NO2C6H4CHO	31	93	58–60	58.5
12	2-NO2C6H4CH2OH	2-NO2C6H4CHO	29	95	42–44	43
13	3-MeC6H4CH2OH	3-MeC6H4CHO	33	98	199	198–200
14	3-OMeC6H4CH2OH	3-OMeC6H4CHO	27	99	142–144	143
15	3,4,5-OMeC6H2CH2OH	3,4,5-OMeC6H2CHO	26	99	72–74	73

^a Reaction conditions: Substrate (1 mmol), ZIF-9@Zeolite 8 (0.5 mmol), H₂O₂ (30%, 2 mmol), H₂O (5 mL), 80 °C, 26 to 33 min. ^b Sigma-Aldrich.

The influence of polyetheramine (Jeffamine^®) as a di-block copolymer with ethylene oxide and propylene oxide moieties along with terminal amine on phosphotungstic acid (PTA) (9) which is a polyoxometalate catalyst was determined, in detail, in H₂O₂-mediated oxidation of BzOH in water. Recyclable PTA-Jffamine^® catalyst not only enhanced the conversion of BzOH as compared with pristine PTA, but also facilitated the easy separation of catalyst and benzaldehyde (BzH) from the reaction mixture. In addition, with and without pure-PTA as the catalyst, the BzOH oxidation reaction was investigated (Table 10). Very low conversions of BzOH were obtained for reactions with and without pure-PTA (Entries 1 and 2). In pH = 3.5, the conversion was only 25% after 1.5 h at low Jeffamine^® concentration, but dramatically increased to 100% at higher Jeffamine^® contents at (pH = 4.5, 6.5, 7.5, and 8.5) which had not been reported earlier (Table 10) [40].

Table 10. Summary of GC-MS results obtained from reactions performed at various conditions ^a.

Entry	Catalyst	pH	Conv. (%)	Sel. (%)
1	No catalyst	-	2.9	5
2	PTA (0.5 g)	2.5	9	100
3	Potassium salt of PTA (0.5 g)	7.5	5	100
4	PTA-Jffamine® (0.5 g)	7.5	100	74
5	PTA-Jffamine® (0.5 g)	3.5	25	73
6	PTA-Jffamine® (0.5 g)	4.5	100	63
7	PTA-Jffamine® (0.5 g)	6.5	100	59
8	PTA-Jffamine® (0.5 g)	8.5	100	73

^a Amount of catalyst (PTA) 9 taken is mentioned in brackets, Medium-water, pH = 2.5–8.5, 90 °C, 1.5 h.

Water-soluble heteropolyacid-based ionic liquids were prepared by modifying tungstophosphoric acid (H₃PW₁₂O₄₀) and propyl sulfonic acid-functionalized ionic complex. Among various organic TPA salts, the [DMBPSH]H₂PW₁₂O₄₀ (10) catalyst, due to strong acidity and excellent surface activity played as an effective and reusable catalyst, exhibited the best oxidative activity with a desirable BzH selectivity of 97.0% and an excellent BzOH conversion of 98.5% under optimum conditions (Table 11) [41]. Additionally, after six consecutive experimental cycles, the catalyst showed no decreasing in conversion and selectivity.

Table 11. Catalytic performance of various catalysts during oxidation of benzyl alcohol with H₂O₂ ^a.

Entry	Catalyst	Conv. (%) b	Yield (%)	Sel. (%) b
1	H3PW12O40	62.3	60.4	93.5
2	DMBPS	5.4	5.3	99.7
3	[TEAPSH]H2PW12O40 c	68.1	65.1	95.6
4	[DMPPSH]H2PW12O40 d	89.5	85.3	95.3
5	[DMBPSH]H2PW12O40	94.4	91.6	96.4
6	[DMBPSH]2HPW12O40	92.5	88.4	95.6
7	[DMBPSH]3PW12O40	90.6	86.7	95.7

^a Reaction conditions: BzOH/H₂O₂ molar ratio (1:2 (mol/mol)), catalyst (5 wt%), H₂O (20 mL), 120 °C, 3 h. ^b Analyzed by GC. ^c TEAPS, triethylammonium propyl sulfobetaine. ^d DMPPS, N,N-dimethyl(phenyl)ammonium propyl sulfobetaine.

A series of diverse amino acids such as phenylalanine, alanine, and glycine functionalized tungstophosphoric acid (TPA;H₃PW₁₂O₄₀) composite and were efficiently applied as recyclable thermally stable, eco-friendly, and cost-effective heterogeneous catalysts in the selective oxidation of benzyl alcohol (Table 12) [42]. Although [GlyH]H₂PW₁₂O₄₀ (11) had more Brønsted acidity than [PheH]H₂PW₁₂O₄₀ (12), [PheH]H₂PW₁₂O₄₀ (12) exhibited the best catalytic activity including conversion 97.9%, selectivity 97.4%, and yield of 95.4% [42,43]. This phenomenon indicated that catalytic oxidation required only a modest acidity.

Table 12. Catalytic performances of various catalysts during oxidation of benzyl alcohol with hydrogen peroxide ^a.

Entry	Catalyst	Conv. (%) b	Yield (%) b	Sel. (%) b
1	Phe	Nil	Nil	Nil
2	H3PW12O40	96.7	87	90
3	[PheH]H2PW12O40	97.9	95.4	97.4
4	[PheH]2HPW12O40	97.5	89.7	92
5	[PheH]3PW12O40	96.3	88.4	91.8
6	[AlaH]H2PW12O40	97.8	92.7	94.7
7	[GlyH]H2PW12O40	97.7	90.8	92.9

^a Reaction conditions: BzOH/H₂O₂ (1:2 (mol/mol)), catalyst (6 wt%), H₂O (30 mL), 110 °C, 4 h. ^b Analyzed by GC.

Oxidation of primary alcohols to aldehydes catalyzed by H₂O₂ as an oxidant and a reusable and water-soluble iron (III) catalyst in water (Table 13) [44]. This novel and reusable FeCl₃ complex (13) in situ formed with quaternary ammonium salt-functionalized 8-aminoquinoline. The reaction showed not only unique chemoselectivity similar to the oxidation a benzylic primary alcohol even in the presence of an aliphatic one, but also exhibited broad functional-group tolerance.

Table 13. The oxidation of benzylic primary alcohols to aldehydes ^a.

Entry	Substrate	Product	t/h	Yield (%) b
1	2-OMeC6H4CH2OH	2-OMeC6H4CHO	2	87
2	4-PhC6H4CH2OH	4-PhC6H4CHO	2	91
3	3-ClC6H4CH2OH	3-ClC6H4CHO	3	89
4	3,4-OCH2OC6H3CH2OH	3,4-OCH2OC6H3CHO	3	94
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	3	82
6	2-FC6H4CH2OH	2-FC6H4CHO	3	73
7	4-N(CH3)2C6H4CH2OH	4-N(CH3)2C6H4CHO	3	85
8	4-COOCH3C6H4CH2OH	4-COOCH3C6H5CHO	3	86
9	1-Naphthylmethanol	1-Naphthaldehyde	3	83
10	Indole-3-carbinol	Indole-3-carboxaldehyde	3	80
11	4-Pyridinemethanol	Isonicotinaldehyde	2	79
12	(5-Bromothiophen-2-yl)methanol	5-Bromothiophene-2-carbaldehyde	3	77
13	[5-(4-Bromophenyl)-2-furyl]methanol	5-(4-Bromophenyl)furan-2-carbaldehyde	3	88
14	C6H5CH(OH)Et	C6H5COEt	5	Trace c 91 recovery
15	4-OMeC6H4CH2OH (a) + C6H5CH(OH)Et (b)	4-OMeC6H4CHO	2	85 d

^a Reaction conditions: Substrate (1 mmol), the in situ formed FeCl₃/I complex 13 (2 mol%), H₂O₂ (30 wt%, 2 mmol), H₂O (1 mL), room temperature, 2 to 5 h. ^b Isolated yield. ^c Apart from at room temperature, heating at 80 °C was also performed. ^d A mixture of a and b in a molar ratio of 1:1 was used.

Various alcohols were oxidized with hydrogen peroxide over ammonium tungstate promoted by GO (14) as heterogeneous acid catalyst (Table 14) [45]. The aromatic primary alcohols and secondary alcohols were converted to the corresponding aldehydes and ketones in excellent to satisfactory yields. In addition, the catalytic system could be efficiently reused in at least seven cycles.

Table 14. Substrate scope of the alcohol oxidation reaction ^a.

Entry	Substrate	Product	Yield (%) b
1	C6H5CH(OH)CH3	C6H5COCH3	96
2	3-MeC6H4CH(OH)CH3	3-MeC6H4COCH3	94
3	3-ClC6H4CH(OH)CH3	3-ClC6H4COCH3	93
4	(C6H5)2CHOH	(C6H5)2CO	98
5	C6H5CH(OH)CH2C6H5	C6H5COCH2C6H5	96
6	C6H5CH2OH	C6H5CHO	91

^a Reaction conditions: Substrate (2 mmol), GO 14 (0.01 g), (NH₄)₃H₅[H₂(WO₄)₆] (0.03 mmol, 0.05 g), H₂O₂ (30%, 8 mmol, 0.92 g), H₂O (3 mL), 70 °C, 11 h. ^b Isolated yield.

Cu(II) nanoparticles immobilized on nanocage-like mesoporous KIT-5 as a support and a 3-aminopropyltriethoxysilane (APTES) group as a coordinating agent for Cu(II) provided an active catalyst in the selective oxidation of primary and secondary alcohols in water. The computational investigation also confirmed the catalytic role of APTES-KIT-5 silica-supported copper(II) nanocatalyst (15). The results in Table 15 showed that the oxidation of various benzylic alcohols were considerably dependent on substituents (–Cl, –OCH₃, –OH, –NO₂, and –NH₂) and on their positions [46]. In addition, aldehydes obtained with excellent selectivity without overoxidation into carboxylic acids from the oxidation of primary alcohols and secondary benzylic alcohols oxidation provided the corresponding ketone in satisfactory yield (Entry 12). The mentioned heterogeneous nanocatalyst could be recovered and reused six times.

Table 15. Cu(OAc)₂ supported on AK 15 as catalyst in oxidation of alcohols to corresponding aldehydes ^a.

Entry	Substrate	Product	t/min c	Yield (%) d,e
1	C6H5CH2OH	C6H5CHO	21	90
2	2-ClC6H4CH2OH	2-ClC6H4CHO	23	75
3 b	3-OHC6H4CH2OH	3-OHC6H4CHO	50	85
4	2-OHC6H4CH2OH	2-OHC6H4CHO	5	97
5	4-OHC6H4CH2OH	4-OHC6H4CHO	5	98
6	2-NH2C6H5CH2OH	2-NH2C6H5CHO	7	97
7	3-OMeC6H5CH2OH	3-OMeC6H5CHO	22	83
8	2-NO2C6H4CH2OH	2-NO2C6H4CHO	23	48
9	4-NO2C6H4CH2OH	4-NO2C6H4CHO	35	61
10	3-NO2C6H4CH2OH	3-NO2C6H4CHO	22	56
11	Hydroquinone	Benzoquinone	5	96
12	(C6H5)2CHOH	(C6H5)2CO	60	70

^a Reaction conditions: Substrate (1 mmol), Cu(OAc)₂/APTES-KIT-5 15 (0.05 g), H₂O₂ (3 mmol), H₂O (2 mL), reflux, all reactions carried out in reflux condition, except Entries 5, 9,11, which were performed at room temperature, 5 to 60 min. ^b Acetone was used as a solvent. ^c Time of maximum conversion (determined by TLC/or GC). ^d Isolated yield. ^e All reactions give corresponding aldehyde in 100% selectivity.

Tungstate salt with imidazolium (((1,3,5-triazine-2,4,6-triyl)tris(1-octyl-1H-imidazol-3-ium))₂(WO₄⁼)₃ (16) framework provided a catalytic system, under neutral aqueous reaction conditions, for the highly selective oxidation of primary benzylic alcohols (bearing both electron-releasing and electron-withdrawing groups) using H₂O₂ as a green oxidant (Table 16) [47]. The catalyst could be reused for at least seven subsequent reaction cycles.

Table 16. Oxidation of substituted benzylic alcohols to aldehydes using imidazolium WO₄⁼ salt 16 as catalyst ^a.

Entry	Substrate	Product	t/min	Yield (%)	B.P/M.P (°C)
					Found	Lit.
1	C6H5CH2OH	C6H5CHO	5	99	177–178	178
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	7	93	246–248	248
3	4-ClC6H4CH2OH	4-ClC6H4CHO	7	95	47–48	48
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	10	94	104–105	106
5	4-MeC6H4CH2OH	4-MeC6H4CHO	11	95	204–206	205
6	4-BrC6H4CH2OH	4-BrC6H4CHO	13	93	56–58	55–58
7	4-PhC6H4CH2OH	4-PhC6H4CHO	8	97	57–59	56–58
8	4-CO2MeC6H4CH2OH	4-CO2MeC6H4CHO	3	87	59–61	61–62
9	4-FC6H4CH2OH	4-FC6H4CHO	13	86	178–180	180
10	3-ClC6H4CH2OH	3-ClC6H4CHO	16	90	213–215	211–213
11	4-(tert-Butyl)-C6H4CH2OH	4-(tert-Butyl)-C6H4CHO	11	94	128–130	130
12	4-N(CH3)2C6H4CH2OH	4-N(CH3)2C6H4CHO	5	96	73–75	74
13	3-NO2C6H4CH2OH	3-NO2C6H4CHO	14	87	58-60	57–59
14	3-OPhC6H4CH2OH	3-OPhC6H4CHO	14	92	168–169	169
15	2-NO2C6H4CH2OH	2-NO2C6H4CHO	12	92	43–45	43
16	2-ClC6H4CH2OH	2-ClC6H4CHO	14	95	210–215	212–213
17	2-OHC6H4CH2OH	2-OHC6H4CHO	15	85	196–198	197
18	3,4-ClC6H3CH2OH	3,4-ClC6H3CHO	10	96	40–42	43
19	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	15	86	71–73	69–70
20	3,4-OMeC6H3CH2OH	3,4-OMeC6H3CHO	10	88	42–44	42–43

^a Reaction conditions: Substrate (0.5 mmol), ((1,3,5-triazine-2,4,6-triyl)tris(1-octyl-1H-imidazol-3-ium))₂(WO₄⁼)₃ 16 (0.21 mmol), H₂O₂ (30%, 1 mmol), H₂O (2 mL), room temperature, 3 to 16 min.

Via simple method, Na₇PW₁₁O₃₉ (PW₁₁) was immobilized on quanternary ammonium functionalized chloromethylated polystyrene (DMA16/CMPS) (PW₁₁-DMA16/CMPS (17)) and used as high active, stable, recoverable, and recyclable catalyst in the oxidation of aliphatic and aromatic alcohol with H₂O₂ (Table 17) [48]. Although, benzhydrol oxidation is difficult because of the deficiency of interaction between the alcohol moiety and the catalyst [49], in this catalytic system, benzhydrol attained 100% conversion and 97.5% selectivity (Entry 1).

Table 17. Oxidation of various alcohols catalyzed by PW₁₁-DMA16/CMPS 17 with 30% H₂O₂ ^a.

Entry	Substrate	Product	T (°C)	XANOL (%)	SANOL (%)
1	(C6H5)2CHOH	(C6H5)2CO	80	100	97.5
2	C6H5CH(OH)CH3	C6H5COCH3	90	92	98.7
3	C6H5CH2OH	C6H5CHO	80	91.7	84

^a Reaction conditions: PW₁₁-DMA16/CMPS 17 (0.3 g), H₂O₂:alcohols (2:1 mol/mol), H₂O (10 mL), 80 to 90 °C, 6 h.

As reported in Table 18, Ramazani et al. [50] introduced nanomagnetic MgFe₂O₄ (18) as an active, and reusable (seven runs) catalyst for the oxidation of various primary and secondary alcohols in good yields in water as a solvent, and either oxone (at room temperature) or H₂O₂ (at 60 °C) as an oxidant. Overoxidation of aldehydes to the corresponding carboxylic acids was not observed which emphasized that the aldehyde selectivity, in most cases, was quite high (>99%).

Table 18. Oxidation of various alcohols using MgFe₂O₄ MNPs 18 as catalyst in the present of H₂O₂ in water ^a.

Entry	Substrate	Product	t/min	Yield (%) b
1	C6H5CH2OH	C6H5CHO	55	88
2	4-ClC6H4CH2OH	4-ClC6H4CHO	60	86
3	3-ClC6H4CH2OH	3-ClC6H4CHO	65	87
4	2-ClC6H4CH2OH	2-ClC6H4CHO	60	85
5	2,5-ClC6H3CH2OH	2,5-ClC6H3CHO	80	85
6	2-BrC6H4CH2OH	2-BrC6H4CHO	65	87
7	3-BrC6H4CH2OH	3-BrC6H4CHO	65	86
8	4-BrC6H4CH2OH	4-BrC6H4CHO	60	84
9	4-FC6H4CH2OH	4-FC6H4CHO	70	88
10	3-FC6H4CH2OH	3-FC6H4CHO	70	88
11	4-NO2C6H4CH2OH	4-NO2C6H4CHO	120	81
12	3-NO2C6H4CH2OH	3-NO2C6H4CHO	120	85
13	2-NO2C6H4CH2OH	2-NO2C6H4CHO	120	86
14	4-OMeC6H4CH2OH	4-OMeC6H4CHO	50	92
15	(C6H5)2CHOH	(C6H5)2CO	70	87
16	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	100	72

^a Reaction conditions: Substrate (1 mmol), MgFe₂O₄ MNPs 18 (5 mol%, 10 mg), H₂O₂ (1.3 mmol), H₂O (2 mL), 60 °C, 50 to 120 h. ^b Isolated.

A solution based chemical reduction method was applied for the production of gold nanoparticles supported on magnesium oxide nanorods (Au-MgO) (19) which were found to be an efficient heterogeneous catalyst with hydrogen peroxide for the base free oxidation of alcohols in aqueous medium at room temperature, Table 19 [51]. It is worthwhile to mention that the catalyst was reused for five cycles.

Table 19. Oxidation of alcohols using Au-MgO nanorodes 19 ^a.

Entry	Substrate	Product	t/h	Yield (%) b	Sel. (%) d
1	C6H5CH(OH)CH3	C6H5COCH3	5	68	100
2	4-ClC6H4CH(OH)CH3	4-ClC6H4COCH3	24	21	100
3	4-BrC6H4CH(OH)CH3	4-BrC6H4COCH3	24	4	100
4	1-(naphthalene-2-yl)ethanol	2-Acetonaphthanone	5	98	100
5	(C6H5)2CHOH	(C6H5)2CO	5	41 c	100
6	1-Indanol	1-Indanone	6	90	100
7	C6H5CH2OH	C6H5CHO	4	50	99
8	4-OMeC6H4CH2OH	4-OMeC6H4CHO	24	15c	100
9	4-NO2C6H4CH2OH	4-NO2C6H4CHO	3	99	99
10	4-CH2ClC6H4CH2OH	4-CH2ClC6H4CHO	4	50	67

^a Reaction conditions: Substrate (1 mM), Au-MgO nanorodes 19 (30 mg), H₂O₂ (15 equiv.), H₂O (20 mL), 27 °C, 3 to 24 h. ^b Determined from GC. ^c Determined from GC-MS. ^d Selectivity = [(peak area of substrate + peak area of product/total peak area)] × 100.

Perez [52] reported the application of copper(II) complexes, copper, and copper oxide nanoparticles supported on SBA-15 applied in the benzyl alcohol oxidation in aqueous phase as catalyst and in the presence of H₂O₂ (Table 20). Immobilization of ionic liquid containing copper followed by chemical reduction method provided the catalyst which showed the highest benzyl alcohol oxidation activity with a 73% conversion and 54% selectivity with 30 min reaction time. Different copper species of the catalyst explored different conversion and selectivity. The highest activity, with 73% conversion and 54% selectivity for benzaldehyde, was obtained in metallic copper nanoparticle catalyst such as Cu/IMILeSBA-15-G1 (20) (Entry 9), in 30 min. The 20 had also the higher activity as compare with similar previously reported catalyst such as copper nanoparticle-polyacrylamide/SBA-15 and polymer-supported copper(II)-L-valine complexes [53,54].

Table 20. Benzyl alcohol oxidation catalyzed by the different copper supported catalysts ^a.

Entry	Catalyst	Conv. (%)	Sel. (%) b	TOF (h−1) c
1	SBA-15	0.9	100	-
2	Cu(II)-PADO-HMDS-SBA-15	16	78	23
3	Cu(II)-IMIL-SBA-15-G1	32	69	39
4	Cu(II)-IMIL-SBA-15-G2	65	51	32
5	Cu(II)-IMIL-SBA-15-G2-R d	14	71	7
6	CuO/SBA-15-C1	19	83	63
7	3.7%CuO/SBA-15-C2	11	82	11
8	5%CuO/SBA-15-C2	20	73	15
9	Cu/IMIL-SBA-15-G1	73	54	66
10	Cu/IMIL-SBA-15-G2	66	33	27
11	Cu/IMIL-SB-15-G2-R d	16	66	7
12	2%Cu/PEG/SBA-15	27	68	52
13	2%Cu/PEG/SBA-15-R d	25	67	48

^a Reaction conditions: Benzyl alcohol (3 mmol), catalyst (0.1 g), H₂O₂ (9 mmol), H₂O (5 mL), 80 °C, 30 min. ^b Selectivity towards benzaldehyde determined by GC. ^c TOF = moles of benzyl alcohol converted per mol of copper per hour. ^d Reused material.

The efficient and selective oxidation of primary and secondary alcohols were reported by Shi [55] who immobilized ionic liquids/peroxotungstates/SiO₂ (21) catalyzed reactions in the presence of H₂O₂ as an oxidant in neat water (Table 21). Hydrophilic imidazoliums and hydrophobic hydrocarbon chains in the ILs caused to diffusion of both hydrophobic alcohols and hydrophilic H₂O₂ oxidant into the micro reactor and provided carbonyls catalyzed by the peroxotungstates. Substituted-benzylic alcohols were oxidized to selective carbonyl products in satisfied yields but electronic effect of substituted groups on the activity was significantly observed. In addition, aromatic secondary alcohols were also selectively oxidized to ketones in good yields. Moreover, the catalyst was easily recovered by filtration and reused at least for six times.

Table 21. The selective oxidation of different alcohols catalyzed by SiO₂-BisILs[W₂O₃(O₂)₄] 21 with H₂O₂ ^a.

Entry	Substrate	Product	t/h	Yield (%) b	Sel. (%) c
1	C6H5CH2OH	C6H5CHO	18	96	98
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	18	94	95
3	2-MeC6H4CH2OH	2-MeC6H4CHO	18	88	96
4	4-ClC6H4CH2OH	4-ClC6H4CHO	22	81	87
5	4-BrC6H4CH2OH	4-BrC6H4CHO	22	80	85
6	2-BrC6H4CH2OH	2-BrC6H4CHO	22	73	88
7	4-NO2C6H4CH2OH	4-NO2C6H4CHO	30	75	82
8	3,4-OMeC6H3CH2OH	3,4-OMe C6H3CHO	26	67	97
9	(C6H5)2CHOH	(C6H5)2CO	24	58	100
10	C6H5CH(OH)CH3	C6H5COCH3	18	88	99
11	C6H5CH2OH	C6H5CHO	18	89	99

^a Reaction conditions: Substrate (1 mmol), SiO₂-BisILs[W₂O₃(O₂)₄] 21 (1 mol%) based on content of tungstate, H₂O₂ (30%, 1.4 mmol), H₂O (2 mL), 90 °C, 18 to 30 h. ^b GC yields. ^c The selectivity of product was calculated based on GC.

Bis-imidazolium tungstate ionic liquid produced magnetically recoverable catalyst, at least 5 times, which were extremely dispersible in water and could selectively oxidized a wide variety of alcohols and sulfides using H₂O₂ oxidant. The results in Table 22 demonstrated the key role of MNP@IL/W (22) as both oxidant and phase transfer catalyst in various alcohol oxidations [56]. Good to excellent conversion and selectivity was also obtained in oxidation of secondary alcohols.

Table 22. Oxidation of alcohols to aldehydes catalyzed by MNP@IL/W 22 ^a.

Entry	Substrate	Product	t/h	Conv. (%) b	Yield (%) c
1	C6H5CH2OH	C6H5CHO	3	99	95
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	4	89	85
3	4-MeC6H4CH2OH	4-MeC6H4CHO	5	96	95
4	4-ClC6H4CH2OH	4-ClC6H4CHO	3	99	94
5	4-BrC6H4CH2OH	4-BrC6H4CHO	3	99	97
6	2-ClC6H4CH2OH	2-ClC6H4CHO	8	85	85 d
7	2-NO2C6H4CH2OH	2-NO2C6H4CHO	10	68	65 d
8	C6H5CH(OH)Et	C6H5COEt	4	99	99 d

^a Reaction conditions: Substrate (1 mmol), MNP@IL/W 22 (3 mol%), H₂O₂ (30%, 3 mmol), H₂O (2 mL), 90 °C, 3 to 10 h. ^b Conversion calculated based on initial mmol of substrates. ^c Yields of aldehydes were determined by GC. ^d Solvent, 1CH₃CN:2H₂O (2 mL).

Using the anion-exchange method produced some hybrid materials ([C4mim]_3+xPMo_12−xV_xO₄₀, x = 0, 1, 2) based on V-substituted phosphomolybdic acid H_3+xPMo_12−xV_xO₄₀ (x = 0, 1, 2) and ionic liquid 1-butyl-3-methyl imidazolium bromide ([C4mim]Br) and applied in the benzyl alcohol oxidation reaction Table 23 [57]. The hybrids [C4mim]_3+xPMo_12−xV_xO₄₀ demonstrated much higher catalytic activities than both corresponding moieties. Particularly, [C4mim]₄PMo₁₁VO₄₀ (23) provided 34% benzyl alcohol conversion and 99% selectivity for benzaldehyde under the optimized conditions (Entry 7) and was reused for five runs without much decrease in selectivity and conversion. In this case, the reaction mixture turned out to be L-L-S triphase system because of the insolubility of the solid hybrid and immiscibility between water and benzyl alcohol [57]. Although high selectivities for benzaldehyde were obtained, in all cases, conversions of the benzyl alcohol were below 50% which justified more efforts to improve the catalytic performances by modifiying morphologies and hydrophilic and hydrophobic properties of the hybrid catalysts in upcoming work.

Table 23. Catalytic performances of various POM catalyst for oxidation of benzyl alcohol with H₂O₂ ^a.

Entry	Catalyst	Reaction System b	Conv. (%)	Sel. (%) c	H2O2-Efficiency (%)
1	None	L-L	3	99	10
2	[C4mim] Br	L-L	3	99	10
3	H3PMo12O40	L-L	22	99	48
4	H4PMo11VO40	L-L	20	99	45
5	H5PMo10V2O40	L-L	12	99	33
6	[C4mim]3PMo12O40	L-L	47	88	66
7	[C4mim]4PMo11VO40	L-L-S	34	99	59
8	[C4mim]5PMo10V2O40	L-L	16	99	40

^a Reaction conditions: Benzyl alcohol (30 mmol), catalyst (100 mg), H₂O₂ (30 wt%, 36 mmol), H₂O (6 mL), 80 °C, 6 h. ^b L (liquid, water phase), L (liquid, organic phase), and S (solid, catalyst). ^c Selectivity for benzaldehyde.

Catalytic activities of Fe₃O₄@C materials in neat water were examined in the selective oxidation of alcohols using H₂O₂ under base-free conditions. Both aryl and alkyl alcohols as a comprehensive substrate scope were oxidized with high activity and selectivity over the B-600 materials (24) (Table 24) [58]. In addition, the magnetic catalyst could be easily removed using an external magnetic field and reused at least four times. Different primary and secondary benzylic alcohols with electron-donating or -withdrawing functional groups converted their corresponding substituted aldehyde or ketones into good to excellent yields (Entries 1–14) but difference in activities were seen which were attributed to their compositions and structures. Secondary benzylic alcohols such as 1-phenylethanol and derivatives were converted to the corresponding ketones in relatively lower yields as compared with primary benzylic alcohols due to the steric effect of the α-CH position in secondary alcohols (Entries 11–15).

Table 24. Oxidation of various alcohols by B-600 24 in water ^a.

Entry	Substrate	Product	Conv. (%) b	Sel. (%) b
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	86	99
2	2-MeC6H4CH2OH	2-MeC6H4CHO	93	99
3	4-MeC6H4CH2OH	4-MeC6H4CHO	91	99
4	2-NH2C6H4CH2OH	2-NH2C6H4CHO	>99	99
5	4-COOMeC6H4CH2OH	4-COOMeC6H4CHO	88	99
6	4-NO2C6H4CH2OH	4-NO2C6H4CHO	84	99
7	4-FC6H4CH2OH	4-FC6H4CHO	96	99
8	4-ClC6H4CH2OH	4-ClC6H4CHO	94	99
9	3-ClC6H4CH2OH	3-ClC6H4CHO	83	99
10	4-BrC6H4CH2OH	4-BrC6H4CHO	>99	99
11	C6H5CH(OH)CH3	C6H5COCH3	81	99
12	4-MeC6H4CH(OH)CH3	4-MeC6H4COCH3	82	99
13	4-ClC6H4CH(OH)CH3	4-ClC6H4COCH3	89	99
14	4-BrC6H4CH(OH)CH3	4-BrC6H4COCH3	91	99
15	(C6H5)2CHOH	(C6H5)2CO	81	99
16	4,4-(CH2OH)2C6H4	4,4-(CHO)2C6H4	91	90
17 c	2-Pyridinemethanol	Picolinaldehyde	98	96
18 c	Furfuryl alcohol	Furfural	90	99

^a Reaction conditions: Substrate (0.5 mmol), B-600 24 (10 mol% Fe), H₂O₂ (30 wt%, 1.5 mmol), H₂O (3 mL), 110 °C, 48 h. ^b Conversion and selectivity were determined by GC-MS with an external standard. ^c 60 h.

Recyclable heterogeneous catalyst β-CD grafted on lignin cross linked by epichlorohydrin (EPI) as crosslinking agent (l-β-CD) (25) were prepared and under mild reaction were used in the oxidation of BzOH to BzH conditions. The catalyst provided the solution selective oxidation of BzOH in high selectivity (>99%) and catalytic activity was not significantly decreased after five cycles. As illustrated in Table 25, (25) was applied for different benzyl alcohol oxidations to the corresponding BzHs. The results not only emphasized the catalytic power of the 25/H₂O₂/NaHCO₃ system for the oxidation of various substituted benzyl alcohols in good yields (79% to 99%) and high selectivity (>99%), but also obviously showed the electronic and steric hindrance effect of substituent groups on the catalytic oxidation of substrates [59].

Table 25. Scope oxidation of alcohols catalyzed by l-β-CD 25 in water ^a.

Entry	Substrate	Product	Yield (%) b
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	99
2	3-OMeC6H4CH2OH	3-OMeC6H4CHO	98
3	2-OMeC6H4CH2OH	2-OMeC6H4CHO	97
4	2-MeC6H4CH2OH	2-MeC6H4CHO	99
5	3-MeC6H4CH2OH	3-MeC6H4CHO	97
6	4-MeC6H4CH2OH	4-MeC6H4CHO	96
7	4-NO2C6H4CH2OH	4-NO2C6H4CHO	83
8	3-NO2C6H4CH2OH	3-NO2C6H4CHO	81
9	2-NO2C6H4CH2OH	2-NO2C6H4CHO	79
10	(C6H5)2CHOH	(C6H5)2CO	Trace

^a Reaction condition: Substrate (1 mmol), l-β-CD 25 (0.8 g), NaHCO₃ (1.5 mmol), H₂O₂ (30%, 2.5 mL), H₂O (25 mL), 60 °C, 240 min. ^b GC yield.

Yadollahi [60] reported the application of sodium and potassium salts of a sandwich-type tetracobalt tungstophosphate, [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ catalysts for the selective oxidation of alcohols with H₂O₂ in water. In general, the Na₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂]·27H₂O complex (26) presented better activity and was recycled for five times. Results in Table 26 showed that alcohols with electron-donating substituent converted to their corresponding aldehydes even faster than benzyl alcohol [60].

Table 26. Selective oxidation of different alcohols to the corresponding aldehyde using Na-Co-POM 26 ^a.

Entry	Substrate	Product	t/h	Yield (%) b
1	C6H5CH2OH	C6H5CHO	3	97
2	4-NO2C6H4CH2OH	4-NO2C6H4CHO	4	95
3	3-NO2C6H4CH2OH	3-NO2C6H4CHO	4.15	95
4	2-NO2C6H4CH2OH	2-NO2C6H4CHO	4.30	95
5	4-FC6H4CH2OH	4-FC6H4CHO	6	98
6	4-ClC6H4CH2OH	4-ClC6H4CHO	6.15	97
7	4-BrC6H4CH2OH	4-BrC6H4CHO	6.45	98
8	2-ClC6H4CH2OH	2-ClC6H4CHO	6.15	92
9	4-MeC6H4CH2OH	4-MeC6H4CHO	2	97
10	2-MeC6H4CH2OH	2-MeC6H4CHO	2.10	97
11	4-OMeC6H4CH2OH	4-OMeC6H4CHO	2	99
12	2-OMeC6H4CH2OH	2-OMeC6H4CHO	2.05	99
13	3-OMeC6H4CH2OH	3-OMeC6H4CHO	2.30	99
14	C6H5CH(OH)Et	C6H5COEt	3	96

^a Reaction conditions: Substrate (1 mmol), Na-Co-POM 26 (1 µmol), H₂O₂ (30%, 9.8 mmol), H₂O (3 mL), reflux, 2 to 6.15 h. ^b Yield refers to GC yield.

Direct solvothermal synthesis or post-synthetic modification were applied for the preparation of ECH-modified Fe₃O₄ microspheres in ethylene glycol (MEG). As summarized in Table 27, the catalyst was successfully performed for the selective oxidation of BzOH to BzH in water with H₂O₂ [29]. The reaction was sensitive to both reaction temperature and the molar ratio of H₂O₂ to BzOH. Magnetic Fe₃O₄ microspheres (27) were recoverable and reusable at least five times without loss of catalytic selectivity and activity after NaBH₄ reduction and ECH modification. In light of the above, surface modification with organic groups increased the catalytic performance of Fe₃O₄ and exhibited suitable interactions with H₂O₂.

Table 27. Catalytic oxidation of benzyl alcohol to benzaldehyde with H₂O₂ ^a.

Entry	Catalyst	BzOH Conv. (%)	BzH yield (%)	Sel. (%) b
1	None	0	0	0
2	ECH (1 g)	2	2	100
3	Fe3O4-blank	8.3	7.9	94.8
4	Fe3O4-ECH-P-1g	25.5	25.2	98.9
5	Fe3O4-ECH-P-3g	39.2	33.2	84.8
6	Fe3O4-ECH-P-4g	34.7	32.6	93.9
7	Fe3O4-ECH-P-5g	37.7	32.5	86.3
8	Fe3O4-ECH-D	36.6	34.2	93.5
9 c	Fe3O4-ECH-D	23.1	22.4	96.9
10 d	Fe3O4-ECH-D	1.7	1.7	100
11 e	Fe3O4-ECH-D	23.4	21.7	92.9

^a Reaction conditions: Benzyl alcohol (40 mmol), catalyst (0.2 g), H₂O₂ (80 mmol), H₂O (8 mL), 100 °C, 1.5 h. ^b Selectivity based on the conversion of BzOH. ^c Reaction temperature, 90 °C. ^d Reaction temperature, 80 °C. ^e H₂O₂, 40 mmol added.

Benzyl alcohol oxidation catalyzed by gold nanoparticles supported on gamma alumina (γ-Al₂O₃) were prepared using a deposition-precipitation method. The Nano Au/c-Al₂O₃ (28) selectivity provided benzaldehyde (over 98%) under environment-friendly conditions and could be recovered after three times. More investigation showed that changes in support material, even the same type of alumina (γ-Al₂O₃), had a significant effect on both catalytic activity and physical–chemical of the catalysts. Gold particles were responsible for catalytic efficiency since in the tests conducted in like conditions using no catalyst or γ-alumina catalyst caused a very low conversion of benzyl alcohol and yield of benzaldehyde (<1%) ()(Table 28) [61]. For all the supports, both the conversion of benzyl alcohol and the selectivity of benzaldehyde also increased. As a result, for the 2% Au/γ-Al₂O₃ catalysts, higher yields of benzaldehyde were achieved8)(Table 29) [61]. While the benzaldehyde selectivities were high in the S723-supported catalysts, the conversion of the benzyl alcohol was comparatively low.

Table 28. Effect of Au catalyst concentration on the oxidation of benzyl alcohol ^a.

Entry	Catalyst	Conv. (%)	Yield (%)	Sel. (%)
1	No catalyst	1.8	-	-
2	S823	3.4	0.8	25
3	1%Au/S823	24.7	12.4	50.2
4	2%Au/S823	29.6	18.4	61.9
5	4%Au/S823	33.3	21.3	63.9

^a Reaction conditions: Benzyl alcohol (5.3 g), catalyst (0.3 g), molar ratio BzOH/H₂O₂ (1:1.3), 80 °C, 2 h.

Table 29. Effect of nanogold on the oxidation of benzyl alcohol over various support materials ^a.

Entry	Type of Al2O3 Support	Conv. (%)		Yield (%)		Sel. (%)
		1%Au	2%Au	1%Au	2%Au	1%Au	2%Au
1	M	37.1	33..8	11.5	16.7	31	49.4
2	S823	24.7	29.6	12.4	18.4	50.2	61.9
3	S723	20.4	18.1	10.9	15.6	53.7	86.3

^a Reaction conditions: Benzyl alcohol (5.3 g), catalyst (0.3 g), molar ratio BzOH/H₂O₂ (1:1.3), 80 °C, 2 h.

Platinum nanoparticles supported on Ca(Mg)-ZSM-5 (29) were a highly stable and selective catalyst for the room-temperature oxidation of alcohols in water. In situ EPR measurement and the radical trapping technique demonstrated that •OH radicals generated by O–O cleavage bond of H₂O₂ intermediate as the rate determining step, contributed to the H abstraction of the α-C–H bond of alcohols to provide aldehydes/ketones. PtNPs on different supports applied for the oxidation reactions of alcohols in aqueous solution at room temperature and the results are summarized in Table 30 [62]. From the results, the basic zeolites, such as Mg-ZSM-5 and Ca-ZSM-5, exhibited the optimal performance during benzyl alcohol oxidation, with 95% conversion and 99% of selectivity at a carbon balance of 98%, (Entries 1 and 2). The PtNPs catalytic power extremely depended on the size of Pt which was sized similar to the PtNPs/Ca-ZSM-5 catalyst with a Pt mean size of ~5.8 nm, and when prepared by the impregnation method, could only convert 89% of the benzyl alcohol in 20 h of reaction (Entry 2). In contrast, high basicity supports, such as CaO and MgO, significantly suppressed the oxidation process (Entries 6 and 7). The PtNPs on inert supports, such as silica gel, exhibited some activity but did not exceed the conversion upper of 70%, even with a prolonged reaction time (Entry 5). Moreover, the application of AuNPs or PdNPs, instead of PtNPs, yielded no benzaldehyde product under identical experimental conditions which could be attributed to the unique catalytic property of PtNPs (Entries 8 and 9). In addition, by replacing water with other solvents, only a slight conversion of benzyl alcohol was observed (Entries 13–15) which indicated the important role of water in the PtNP catalyzed alcohol oxidation procedure. The catalyst could be steady and reused even after four reaction cycles with no Pt leaching and less than 1% Ca leaching.

Table 30. The catalytic property of PtNPs on different supports ^a.

Entry	Catalyst	Substrate b	Solvent	Conv. (%)	Sel. (%)
1	Pt/Ca-ZSM-5	BA	H2O	98.8	99
2	Pt/Mg-ZSM-5	BA	H2O	95.5	98
3	Pt/Ca-ZSM-5 c	BA	H2O	89	96
4	Pt/HZSM-5	BA	H2O	65	87
5	Pt/SiO2	BA	H2O	64	87
6	Pt/CaO	BA	H2O	0	-
7	Pt/MgO	BA	H2O	0	-
8	Au/Ca-ZSM-5	BA	H2O	0	-
9	Pd/Ca-ZSM-5	BA	H2O	0	-
10	Pt/Mg-ZSM-5	CHA	H2O	20.7	85.5
11	Pt/Ca-ZSM-5	CHA	H2O	21.8	82
12	Pt/SiO2	CHA	H2O	5.1	90
13	Pt/Ca-ZSM-5	BA	CH3CN	<1	-
14	Pt/Ca-ZSM-5	BA	EtOH	<1	-
15	Pt/Ca-ZSM-5	BA	CCl4	<1	-

^a Reaction conditions: A mixture of 50 mg catalyst with a unified 1 wt% Pt and 20 mM alcohol in various solvents was stirred at room temperature (25 °C) in open air for 20 h. ^b BA, benzyl alcohol; CHA, cyclohexanol. ^c Impregnation method was used to prepare the catalyst.

An acid-base reaction using a Keggin-type phosphotungstic acid and TEA was applied for the synthesis of several triethylamine (TEA) salts of phosphotungstic acid ((TEAH)nH_3-nPW₁₂O₄₀ (n = 1, 2, 3)). These catalysts used for the alcohol oxidation reactions, and their catalytic activity, selectivity, and recovery rate are listed in Table 31 [63]. The (TEAH)H₂PW₁₂O₄₀ (30) catalyst showed the best results, with 99.6% conversion of benzyl alcohol and 100% of selectivity to benzaldehyde under optimized reaction conditions. The activity and selectivity were also essentially unchanged even in the third and the fifth cycles.

Table 31. Activity and selectivity in the oxidation of benzyl alcohol with the catalysts ^a.

Entry	Catalyst	Conv. of BA (%) c	Sel. for BzH (%) c	Yield of Benzoic Acid	Recovery Rate of Catalyst (%)
1	Blank	17.9	100	0	-
2	H3PW12O40	100	92.4	7.6	0
3	[TEAH]3PW12O40	95	100	0	77
4	[TEAH]3P W12O40+H2SO4 b	84.4	100	0	62
5	[TEAH]2HPW12O40	97.5	100	0	58
6	[TEAH]H2PW12O40	99.6	100	0	53
7	[TEAH]H2PW12O40 + H2SO4 b	86.7	100	0	42
8	[TEAH]H2PW12O40 (cycle 3)	99.8	100	0	-
9	[TEAH]H2PW12O40 (cycle 5)	99.5	100	0	-

^a Reaction conditions: Benzyl alcohol (1 mL, 9.6 mmol), catalyst (0.04 mmol), H₂O₂ (30%, 1.2 mL, 11.8 mmol), H₂O (10 mL), 100 °C, 3 h. ^b H₂SO₄ (1 mol/L) was added to adjust the acidity from pH ≈ 5 to pH = 1 before heating the system. ^c BA, benzyl alcohol; BzH, benzlaldehyde.

An iron(III) complex formulated as L(14,28-[1,3-diiminoisoindolinato]phthalocyaninato)Fe(III) (31) in which L was a labile axial ligand was synthesized. The efficiency of the prepared catalyst was investigated by the oxidations of primary and secondary benzylic alcohols with both hydrogen peroxide and tert-butyl hydroperoxide (TBHP). Benzyl alcohol, 4-chlorobenzyl alcohol, 1-phenylethanol, and diphenylmethanol (benzhydrol), as illustrated in Table 32, were effectively oxidized without needing highly problematic oxidants or adding organic solvent. In primary alcohol oxidation (benzyl alcohol and 4-chlorobenzyl alcohol) no significant carboxylic acid as overoxidation products were observed. Moreover, secondary alcohols produced ketones with excellent selectivity [64].

Table 32. Summary of results from catalysis and control experiments ^a.

Entry	Substrate	Product	t/min	Yield (%) e	TON f	TOF (h−1) g
1	C6H5CH2OH	C6H5CHO	15 15 b	1.6 0.03	84 -	340 -
2	4-ClC6H4CH2OH	4-ClC6H4CHO	15 c 15 b	40 0.41	940 -	3800 -
3	C6H5CH(OH)CH3	C6H5COCH3	6 6 b	4.9 0.085	230 -	2300 -
4	(C6H5)2CHOH	(C6H5)2CO	10 d 10 b	3.7 0.77	210 -	1200 -

^a All reactions were run in magnetically stirred round-bottomed flasks open to the air at room temperature. Catalyst 31 (1.0 to 2.8 mg, 1.2 to 3.5 μmol) was dissolved in the alcohol substrate before adding the oxidant as an aqueous solution. Caution: solutions containing significant concentrations of hydrogen peroxide and metal-containing compounds are potentially dangerous, and reactions should be carried out behind adequate protection. Optimal reaction times for hydrogen peroxide oxidations were determined by quenching the reaction at various points with sodium thiosulfate. ^b No catalyst. ^c 72 °C. ^d 70 °C. ^e Oxidized product yields on the basis of total oxidant added. ^f TON, moles product/moles catalyst. Products were identified and quantified by HPLC vs. external standard. The catalyst 31 used in this study is prepared in crystalline form, where the composition of the crystals can vary slightly. In the crystallographically determined structure, ligand L is observed to be a mixture of methanol and water, with two co-crystallized methanol molecules, also present for every complex bearing a methanol ligand, whereas three co-crystallized methanol molecules are present for every complex bearing a water ligand. In a desire to report catalyst performance conservatively, in light of this issue, we calculated TON values based on an assumption that the catalyst had 100% methanol as the L ligand, which along with the two co-crystallized methanol molecules gave a formula weight of 806.65, less than that for the case where L = water and three co-crystallized methanol molecules were present, in which case the formula weight was 824.66. This results in slightly under-reported turnover numbers (TON), assuming a mixture of the two species is actually present, because the number of moles of catalyst actually present in the reaction mixtures is slightly less than the number used in TON calculations. ^g TOF, turnovers per hour, average over entire reaction period.

A straightforward method was suggested for the oxidation of alcohols to their corresponding carbonyl compounds using nanomagnetic Fe₃O₄ (32) catalyst in water by H₂O₂ and the results are summarized in Table 33 [65]. High yields and the best selectivity (>99%) were obtained by producing aldehyde without observing overoxidation of aldehydes to parallel carboxylic acids [66].

Table 33. Oxidation of various alcohols by using H₂O₂ in the presence of nanomagnetic Fe₃O₄ 32 catalyst in water at 50 °C ^a.

Entry	Substrate	Product	t/min	Yield (%) b
1	C6H5CH2OH	C6H5CHO	20	91
2	4-ClC6H4CH2OH	4-ClC6H4CHO	30	89
3	3-ClC6H4CH2OH	3-ClC6H4CHO	35	87
4	2-ClC6H4CH2OH	2-ClC6H4CHO	40	90
5	2,5-ClC6H3CH2OH	2,5-ClC6H3CHO	45	87
6	2-BrC6H4CH2OH	2-BrC6H4CHO	30	85
7	3-BrC6H4CH2OH	3-BrC6H4CHO	35	88
8	4-BrC6H4CH2OH	4-BrC6H4CHO	35	85
9	4-FC6H4CH2OH	4-FC6H4CHO	40	88
10	3-FC6H4CH2OH	3-FC6H4CHO	40	85
11	4-NO2C6H4CH2OH	4-NO2C6H4CHO	120	85
12	3-NO2C6H4CH2OH	3-NO2C6H4CHO	120	81
13	2-NO2C6H4CH2OH	2-NO2C6H4CHO	120	82
14	4-OMeC6H4CH2OH	4-OMeC6H4CHO	15	90
15	(C6H5)2CHOH	(C6H5)2CO	30	88
16	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	120	85

^a Reaction conditions: Substrate (1 mmol), nanomagnetic Fe₃O₄ 32 (10 mol%, 23 mg), H₂O₂ (1.2 mmol), H₂O (1 mL), 50 °C, 15 to 120 min. ^b Yield refers to isolated products. The products were characterized from their spectra data (IR) and compared with authentic samples.

Grafting the Cu(II) Schiff base complex onto the channels of mesoporous silica material SBA-15 (Cu(II)-Shiff base-SBA-15 (33)) was conducted by Ma et al. [67], and provided an effective catalyst for the selective oxidation of alcohols in water phase with hydrogen peroxide. Benzyl alcohol converted 98.5% with 100% of the selectivity to benzyl aldehyde. Other substituted benzyl alcohol was also successfully oxidized under the optimal conditions in which the substituted groups impacted the catalytic activity (Table 34).

Table 34. Selective oxidation of other primary alcohols ^a.

Entry	Substrate	Product	t/h	Conv. (%)	Sel. (%)
1	4-MeC6H4CH2OH	4-MeC6H4CHO	3	43.9	>99
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	3	30.5	>99
3	4-ClC6H4CH2OH	4-ClC6H4CHO	2	72.7	>99
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	2	48.6	>99

^a Reaction conditions: Substrate (2 mmol, 0.2 mL), Cu(II)-Schiff base-SBA-15 33 (0.02 g), H₂O₂ (30%, 2 mL), H₂O (7.5 to 15 mL), reflux, 2 to 3 h.

Malakooti et al. [68] reported the catalytic activity of the Fe₂O₃/SBA-15 (34) catalyst in oxidation of alcohols. As compared with different heterogeneously catalyzed procedures [7,69] the catalyst showed good activity, and therefore the catalyst was successfully examined for the oxidation of a variety of primary and secondary alcohols using H₂O₂ (Table 35). Benzyl alcohols were converted to their corresponding aldehydes with excellent yields and also alcohols containing heteroatom such as furfuryl alcohol, hindered substituted alcohols, and secondary benzylic alcohols were oxidized. No further oxidation of aldehyde products to carboxylic acid were seen and the selectivity of the catalyst was also confirmed. The recovered catalyst was reused for at least six successive runs with only a few decreasing in conversion.

Table 35. Oxidation of alcohols to aldehyde using Fe₂O₃/SBA-15 34 catalyst in water in the presence of H₂O₂ ^a.

Entry	Substrate	Product	t/h	Conv. (%)
1	C6H5CH2OH	C6H5CHO	3.5	95
2	C6H5CH(OH)CH3	C6H5COCH3	4	80
3	Furfuryl alcohol	Furfural	2	100
4	2-OHC6H4CH2OH	2-OHC6H4CHO	2.5	100
5	4-OMeC6H4CH2OH	4-OMeC6H4CHO	2	100
6	4-NO2C6H4CH2OH	4-NO2C6H4CHO	3	100
7	4-ClC6H4CH2OH	4-ClC6H4CHO	4.5	90
8	2-ClC6H4CH2OH	2-ClC6H4CHO	4.5	80
9	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	2	95
10	C6H5CH(OH)Et	C6H5COEt	4	40
11	(C6H5)2CHOH	(C6H5)2CO	6	45
12	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	6	85

^a Reaction conditions: Substrate (1 mmol), Fe₂O₃/SBA-15 34 (0.02 g, 1.64 mol% Fe), H₂O₂ (2 mmol, 0.2 mL), H₂O (2 mL), 80 °C, 2 to 6 h.

Simple and inexpensive iron(III)-benzenetricarboxylate (Fe-BTC) (35) metal-organic gel catalyst, which was previously synthesized [70], was applied in a catalytic system for the oxidation of alcohols with H₂O₂ oxidant in water with no proof of other side products (Table 36) [71].

Table 36. Fe-BTC gel 35 catalyzed oxidation of various alcohols ^a.

Entry	Substrate	Product	t/min	Conv. (%)	Sel. (%)	TON b
1	C6H5CH2OH	C6H5CHO	180	98	>99	504
2	C6H5CH(OH)CH3	C6H5COCH3	30 180	56 100	>99 >99	288 514

^a Reaction conditions: Substrate (9.26 mmol), dried Fe-BTC gel 35 (0.01 g, 2 mg Fe, 36 µmol Fe, 18 µmol [Fe₂(C₉H₃O₆)(NO₃)₃(H₂O)₃ catalyst), molar ratio alcohol/Fe = 9.26/0.036 = 257, H₂O₂ (20 mL of 10% H₂O₂ (c = 3.07 mmol/mL) at an addition rate of 0.12 mL min⁻¹, H₂O (5 mL), molar ratio H₂O₂/alcohol (6.6), 30 to 180 min. ^b TON = [mol product/mol catalyst], the turnover frequency TOF [mol product/(mol catalyst x time)] is obtained by dividing TON through time.

Two cobalt (II) and cobalt (III) complexes of a terpyridine based ligand, (40-(2-thienyl)-2,2′,6′,2′′-terpyridine (L)), were prepared in which each complex had two units of the tridentate ligand. The cobaltous complex and the cobaltic complex showed the formula [Co(L)₂](NO₃)₂.2CH₃OHH₂O (36) and [Co(L)₂](NO₃)₃.2CH₃OH (37), respectively. The aromatic alcohol oxidation reactions in the presence of catalysts was performed and the results are summarized in Table 37 [72]. As it was clear, in oxidation by hydrogen peroxide, the alcohols with more solubility in an aqueous media provided more reactivity towards and needed less reaction times. It is worthwhile mentioning that the cobaltous species was more effective than the cobalt (III) catalyst for alcohol oxidation reactions.

Table 37. Oxidation of a wide range of alcohols using the Co(II) and Co(III) complexes (36, 37) as catalyst in water at room temperature ^a,b.

Entry	Substrate	Product	Co(II) catalyst		Co(III) catalyst
			Conv. (%)	Carbonyl Compound: Carboxylic Acid/Hydroperoxide	Conv. (%)	Carbonyl Compound: Carboxylic Acid/Hydroperoxide
1	C6H5CH2OH	C6H5CHO	94	13:1	34	6:1
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	95	15:1	37	12:1
3	4-ClC6H4CH2OH	4-ClC6H4CHO	73	9:1	24	8:1
4	1-Indanol	1-Indanone	91	31:1	30	22:1
5	Furfuryl alcohol	Furfural	89 c	5:1	42	4:1
6	2-Thiophenemethanol	2-Thiophenecarboxaldehyde	87 c	8:1	49	5:1
7	(C6H5)2CHOH	(C6H5)2CO	93 d	19:1	36	17:1
8	4,4-(OMe)2(C6H4)2CHOH	4,4-(OMe)2(C6H4)2CO	96 d	22:1	39	19:1
9	2-FC6H4CH(OH)C6H5	2-FC6H4COC6H5	94 e	19:1	37	18:1

^a Reaction conditions: Substrate/catalyst/oxidant (20:1:60), H₂O (2 mL), room temperature, 8 h. ^b The conversion was calculated based on the starting substrate, by using GC and with a comparison of authentic samples. ^c The reaction time was 2 h. ^d The reaction time was 12 h. ^e The reaction was 6 h.

Efficient usage of Lacunary Keggin-tungstoborate of K₈[BW₁₁O₃₉H]·13H₂O (38) catalyst was reported for the first time in an aqueous/oil system for the oxidation of alcohols [73]. Benzyl alcohol oxidation provided benzaldehyde in high conversion and selectivity and secondary alcohols delivered high yields of ketones. The molar ratio of H₂O₂/benzyl alcohol was optimized (1:1) because a greater content of oxidant decreased the activity and selectivity of the oxidation product which benzoic acid produced (Table 38 and Table 39). The catalyst could be reused after four consecutive cycles of the reaction.

Table 38. Oxidation of benzyl alcohol catalyzed by K₈[BW₁₁O₃₉H].13H₂O 38 with H₂O₂ ^a.

Entry	H2O2/BzOH (mol/mol)	Sel. of BzH (mol%)	BzH Yield (mol%)
1 b	1/2	98	98
2	1/1	90	86
3	2/1	83	81
4	3/1	80	79
5	4/1	78	77
6	5/1	61	61

^a Reaction conditions: Benzyl alcohol (1 mmol), K₈[BW₁₁O₃₉H]·13H₂O 38 (0.015 mmol), H₂O₂ (30%), H₂O (3 mL), 90 °C, 6 h, the conversion was based on benzyl alcohol. ^b H₂O₂ (1 mmol), benzyl alcohol (2 mmol), the conversion was based on H₂O₂.

Table 39. Oxidation of various alcohols catalyzed by K₈[BW₁₁O₃₉H].13H₂O 38 with H₂O₂ ^a.

Entry	Substrate	Product	t/h	Conv. (mol%)	Sel. (mol%)	H2O2 Efficiency (%)
1	C6H5CH(OH)CH3	C6H5COCH3	3	99	99	67
2	C6H5CH2OH	C6H5CHO C6H5COOH	6 6	98 98	83 16	67

^a Reaction conditions: Substrate (1 mmol), K₈[BW₁₁O₃₉H].13H₂O 38 (0.015 mmol), H₂O₂ (30% aq), H₂O (3 mL), 90 °C, 3 to 6 h, the conversion was based on alcohols, the selectivity was based on ketone, aldehyde, or acid. H₂O₂ efficiency (%) = products (mol)/consumed H₂O₂ (mol) × 100.

Using easily prepared water-soluble POMs, K₈[γ-SiW₁₀O₃₆]·13H₂O (39) precatalyst for the selective oxidation of alcohols were reported for the first time. Benzyl alcohol, 1-phenylethanol, and benzhydrol, etc. [74] as activated benzylic alcohols with 30% H₂O₂ at 90 °C were selectively oxidized to the corresponding ketone in high yields (Table 40) and the catalyst was recycled five times. With the molar ratio of H₂O₂ to benzyl alcohol 5:1, benzyl alcohol was absolutely converted to benzoic acid after 7 h at 90 °C (Entry 1). By decreasing the temperature of the reaction from 90 to 70 °C and the molar ratio of H₂O₂ to benzyl alcohol 1:1, benzaldehyde was obtained as the only oxidation product (Entry 2).

Table 40. Selective oxidation of alcohol catalyzed by K₈[γ-SiW₁₀O₃₆]·13H₂O 39 with H₂O₂ at 90 °C ^a.

Entry	Substrate	Product	t/h	Conv. (mol%)	Yield (mol%)
1	C6H5CH2OH	C6H5CO2H	8	100	100
2 b	C6H5CH2OH	C6H5CHO	7	84	84
3	C6H5CH(OH)CH3	C6H5COCH3	7	100	100
4	(C6H5)2CHOH	(C6H5)2CO	7	100	100

^a Reaction conditions: Substrate (1 mmol), K₈[γ-SiW₁₀O₃₆].13H₂O 39 (6.7 µmol), H₂O₂ (30%, 5 mmol), H₂O (2 mL), 90 °C (oil bath temperature), 7 to 8 h. Conversion% = consumed alcohol (mol)/alcohol added (mol) × 100. ^b Substrate (1 mmol), K₈[γ-SiW₁₀O₃₆].13H₂O (6.7 µmol), H₂O₂ (1 mmol), 70 °C (oil bath temperature).

To gain more understanding of the catalytic system, the reactions were performed at 20 °C and the results are shown in Table 41. Dropping the reaction temperature from 90 to 20 °C was needed to achieve good yield, longer time (even some days), and more amount of catalyst (Entries 1 and 2). These data indicated that the high reaction temperature and the good water-solubility of alcohols provided high yields of products.

Table 41. Selective oxidation of alcohol catalyzed by K₈[γ-SiW₁₀O₃₆].13H₂O 39 with H₂O₂ at 20 °C ^a.

Entry	Substrate	Product	Conv. (mol%)	Yield (mol%)
1	C6H5CH2OH	C6H5CHO	64	64
2	C6H5CH(OH)CH3	C6H5COCH3	78	78

^a Reaction conditions: Substrate (1 mmol), K₈[γ-SiW₁₀O₃₆]·13H₂O 39 (67 µmol), H₂O₂ (30%, 5 mmol), H₂O (2 mL), 20 °C, 72 h. Conversion (%) = consumed alcohol (mol)/alcohol added (mol) × 100.

Keggin-type heteropolyacids (40) and hydrogen peroxide as a multiphase system were applied for the selective oxidation of alcohols to ketones or aldehydes (Table 42) with no appreciable detection of higher oxidation state by-product in most cases (Table 42). In the case of benzyl alcohols and 4-chloro benzyl alcohol, the formation of small amounts of benzoic and 4-chlorobenzoic acid due to the addition of hydrogen peroxide twice was decomposed rapidly under homogeneous conditions (Table 42, Entries 6 to 10) [75].

Table 42. Oxidation of alcohols with hydrogen peroxide catalyzed by different heteropolyacids at 70 °C in multiphase conditions ^a.

Entry	Substrate	Product	Catalyst	t/h	C6H5COCH3 (%)	Other Products (%)
1 2 3	C6H5CH(OH)CH3 C6H5CH(OH)CH3 C6H5CH(OH)CH3	C6H5COCH3 C6H5COCH3 C6H5COCH3	H3PMo12O40 Py3PMo12O40 H6PMo11AlO40	9 5 7	95 (5) b 100 (6) b 99	None None None
4 5	(C6H5)2CHOH (C6H5)2CHOH	(C6H5)2CO (C6H5)2CO	H3PMo12O40 Py3PMo12O40	5 4	99 (8) b 99	None None
6 7 8	C6H5CH2OH C6H5CH2OH C6H5CH2OH	C6H5CHO C6H5CHO C6H5CHO	H3PMo12O40 Py3PMo12O40 H6PMo11AlO40	5 3 4	84 (12) b 89 (5) b 85	(Benzoic acid) (4) (Benzoic acid) (2) (Benzoic acid) (2)
9 10	4-ClC6H4CH2OH 4-ClC6H4CH2OH	4-ClC6H4CHO 4-ClC6H4CHO	H3PMo12O40 H6PMo11AlO40	53	93 (10) b 95	(4-Chlorobenzoic acid) (1) (4-Chlorobenzoic acid) (2)
11 12	4-MeC6H4CH2OH 4-MeC6H4CH2OH	4-MeC6H4CHO 4-MeC6H4CHO	H3PMo12O40 Py3PMo12O40	43	96 (7) b 96	(4-Methylbenzoic acid) (3) (4-Methylbenzoic acid) (3)
13 14	4-OMeC6H4CH2OH 4-OMeC6H4CH2OH	4-OMeC6H4CHO 4-OMeC6H4CHO	H3PMo12O40 Py3PMo12O40	3 2	90 91	(4-Methoxybenzoic acid) (6) (4-Methoxybenzoic acid) (5)

^a Reaction conditions: Substrate (0.7 mmol), HPA 40 (3% in mmol), H₂O₂ (35 w/v%, 1 mL), Aliquat 336 (tricaprylmethylammonium chloride, 0.232 g, 0.59 mmol), H₂O (4 mL), n-decane as internal standard (0.056 g, 0.39 mmol), 70 °C, 2 to 9 h, 700 rpm. ^b In homogeneous system corresponding to the numbers in the parentheses, substrate (0.7 mmol), HPA (3% in mmol), H₂O₂ (35 w/v%, 1 mL), CH3CN (5 mL), n-decane as internal standard (0.056 g, 0.39 mmol), 700 rpm.

Easy product isolation, good turnovers, and high selectivities were achieved using the Zn substituted polyoxoanion (NH₄)₇Zn_0.5[α-ZnO₄W₁₁O₃₀ZnO₅(OH₂)]·nH₂O (41) as the catalyst for the oxidation of organic functionalities. As illustrated in Table 43, only minor (≤5%) amounts of benzoic acids as overoxidation byproduct was provided and the catalyst could be reused three times with little loss in its efficacy [76]. In addition, in contrast to a previously reported system [77,78] using Na₂WO₄, a phase transfer catalyst and H₂O₂ in water, in this system, no phase transfer co-catalyst was required.

Table 43. Oxidation of benzyl alcohol by (NH₄)₇Zn_0⋅5[α-ZnO₄W₁₁O₃₀ZnO₅(OH₂)]⋅nH₂O 41 in water with aqueous hydrogen peroxide as an oxidant ^a.

Entry	Substrate	Product	Conv. (%) b
1	C6H5CH2OH	C6H5CHO	80
2	4-MeC6H4CH2OH	4-MeC6H4CHO	82

^a Reaction conditions: Substrate (1 mmol), (NH₄)₇Zn_0⋅5[α-ZnO₄W₁₁O₃₀ZnO₅(OH₂)]·nH₂O 41 (n ≈ 18, 4 µmol), H₂O₂ (30 w/v%, 4 mmol), H₂O (5 mL), 80 °C, 10 h, with vigorous stirring. ^b Conversion was determined by GC or proton NMR.

Alkali-treated ZSM-5 zeolite (42) was obtained by the alkali-treatment modification of the commercially available ZSM-5 zeolite with high concentration NaOH solution at low temperature. The oxidation reaction in the presence of the catalyst (Table 44) showed 53% conversion of BzOH and 86% selectivity to benzaldehyde (BzH). Additionally, the catalyst could be reused for more than six times and was very stable [79]. The results also showed that with an increase of the SiO₂/Al₂O₃ ratio in as-received zeolites, the conversion of BzOH decreased seriously which could be attributed to decreasing the concentration of Lewis acids on the surface of treated zeolite.

Table 44. Selective oxidation of benzyl alcohol to benzaldehyde by hydrogen peroxide over different catalyst ^a.

Entry	Catalyst	Amount of Catalyst (g)	Conv. (%)	Sel. (%)
				BzH	BzA	Benzyl Benzoate (%)
1	None	-	2	73	27	0
2	25ZSM(AT-0)	1	44	80	19	1
3	25HZSM(AT-0.5) b	1	52	85	14	1
4	25ZSM(AT-0.5)	0.1	37	61	31	8
5	25ZSM(AT-0.5)	0.2	45	68	27	5
6	25ZSM(AT-0.5)	0.5	52	80	18	2
7	25ZSM(AT-0.5)	1	53	86	13	1
8	25ZSM(AT-0.5)	1.5	49	87	13	0
9	25ZSM(AT-0.5)	2	47	88	11	1
10	25ZSM(AT-1.0)	1	50	85	14	1
11	25ZSM(AT-1.5)	1	48	86	13	1
12	25ZSM(AT-3.0)	1	37	89	10	1
13	38ZSM(AT-0.5)	1	41	86	14	0
14	50ZSM(AT-0.5)	1	33	84	15	1

^a Reaction condition: Catalyst (0.1 to 2 g), H₂O₂/BzOH (1:3), reaction at reflux temperature, 4 h. ^b Acid exchange with HCl after alkali-treatment for 0.5 h.

Among different W- and Mo-based heteropolyoxometalate catalysts which were used in the oxidation of aromatic alcohols in the presence of hydrogen peroxide in water, dodecatungstophosphoric acid, H₃PW₁₂O₄₀ (43), had the most efficiency. This catalytic system provided a highly selective, efficient, fast, environmentally friendly, and inexpensive approach for the conversion of alcoholic functions to carbonyl groups with H₂O₂ in water, and even after fifteen runs, the efficiency of the oxygenation system was ~10% decreased (Table 45) [80].

Table 45. Oxidation of some aromatic alcohols with 34% H₂O₂ in water catalyzed by H₂PW₁₂O₄₀ 43 ^a.

Entry	Substrate	Product	t/min	Conv. (%)	Sel. (%)
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	120	73	100
2	C6H5CH2OH	C6H5CHO	150	78	100
3	C6H5CH(OH)CH3	C6H5COCH3	150	85	100
4	3-NO2C6H4CH2OH	3-NO2C6H4CHO	150	90	100

^a Reaction conditions: Substrate (0.94 mmol), H₂PW₁₂O₄₀ 43 (0.018 mmol), H₂O₂ (34%, 5 mmol), H₂O (5 mL), room temperature, 120 to 150 min. Progress of the reactions was followed by aliquots withdrawn directly and periodically from the reaction mixture and analyzed by gas chromatography. Products were isolated as described in the experimental section.

Table 46 [81] summarizes the application of cheap, effective, organic-solvent-free and phase-transfer WO₄²⁻ catalyst (Na₂WO₄·2H₂O (44)) and aqueous hydrogen peroxide for six primary or secondary alcohol oxidations which are liquids or melt below 90 °C [77,82]. Primary alcohols, such as benzyl alcohol, were easily oxidized to aldehydes with no overoxidation.

Table 46. Phase-transfer catalyzed oxidation of alcohols ^a.

Entry	Substrate	Product	Work-Up	Yield (%)	Purity (%) b
1	C6H5CH(OH)CH3	C6H5COCH3	Extractive	88 91 94	>98 96 c 94
2	C6H5CH(OH)Et	C6H5COEt	Extractive	85 94	>98 >98
3	(C6H5)2CHOH	(C6H5)2CO	Extractive	93 98	>98 >98
4	4-MeC6H4CH(OH)C6H5	4-MeC6H4COC6H5	Vacuum filtration	99 65 d	>98 >98
5	C6H5CH2OH	C6H5CHO	Extractive	91 81 88 e	98 94 97

^a Reaction conditions: Substrate (1 mol equiv.), Na₂WO₄·2H₂O 44 (1 mol%), [CH₃(C₈H₁₇)₃N]HSO₄ (1 mol%), H₂O₂ (30%, 1.1 mol equiv.), H₂O, 90 °C, 1 to 3 h. ^b Determined by GC analysis. The remainder was the starting alcohol. ^c Determined by ¹H NMR spectroscopy by integration of product and substrate methyl resonances. ^d Yield after recrystallization from methanol. ^e [CH₃(C₈H₁₇)₃N]HSO₄ prepared in situ from [CH₃(C₈H₁₇)₃N]Cl and NaHSO₄·H₂O.

2.2. Oxidation of Benzylic and Heterocyclic Alcohols in a Solvent-Free System

Green oxidation of primary and secondary alcohols in the presence of nano-MoO₃/copper Schiff base complex (45), using H₂O₂, was investigated under solvent-free conditions with high conversion and excellent selectivity. The benefits of the reaction also included the ease of isolating products from green media, and the reusability of the catalyst for six times without loss of activity and selectivity (Table 47) [83]. Further oxidation such as the preparation of acid or ester were not investigating (Table 47, Entries 1–4 and 6–12) and, additionally, benzyl phenyl sulfide oxidation provided no sulfoxide or sulfone byproducts (Entry 5).

Table 47. The oxidation of different alcohols catalyzed by NMCS bio-nanocomposite 45 using H₂O₂ within 3 h at 80 °C under ultrasonic irradiation ^a.

Entry	Substrate	Product	Yield (%)
1	4-(tert-Butyl)C6H4CH(OH)CH3	4-(tert-Butyl)C6H4COCH3	100
2	C6H5CH(OH)Et	C6H5COEt	90
3	(C6H5)2CHOH	(C6H5)2CO	60
4	C6H5CH(OH)CH3	C6H5COCH3	100
5	2-SHC6H4CH2OH	2-SHC6H4CHO	100
6	4-MeC6H4CH2OH	4-MeC6H4CHO	100
7	Furfuryl alcohol	Furfural	100
8	2-OHC6H4CH2OH	2-OHC6H4CHO	90
9	C6H5CH2OH	C6H5CHO	100
10	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	90
11	4-NO2C6H4CH2OH	4-NO2C6H4CHO	85
12	4-ClC6H4CH2OH	4-ClC6H4CHO	80

^a Reaction conditions: Substrate (0.1 mmol), NMCS bio-nanocomposite 45 (0.007 g), H₂O₂ (30%, 0.2 mmol, 20 µL), 80 °C, solvent free, air, 3 h.

Dioxo-molybdenum(VI) complex supported functionalized Merrifield resin (MR-SB-Mo) (46). The catalyst efficiently and selectively oxidized a wide variety of alcohols to aldehydes or ketones using H₂O₂ as an oxidant with reasonably good TOF (660 h⁻¹ in case of benzyl alcohol) under solvent-free reaction conditions and did not lead to overoxidized products under optimized conditions (Table 48) [84]. The catalyst afforded regeneration and could be reused for at least five reaction cycles without loss of efficiency and product selectivity. In the case of substituted benzyl alcohols, different types of substituents such as –F, –Cl, –Br, –OMe, –OH, and –NO₂ were well-tolerated during the oxidation process (Table 48, Entries 2–7), some of which could be utilized for further derivation. One of the notable aspects of the developed catalytic system was its ability to oxidize benzyl alcohol to benzaldehyde at a relatively higher scale (10 g scale) without losing the catalytic efficiency and product selectivity (Table 48, Entry 1^d), which provides its potential application in commercial processes.

Table 48. Oxidation of alcohols to aldehydes or ketones catalyzed by MR-SB-Mo 46 using 30% aqueous H₂O₂ as oxidant ^a.

Entry	Substrate	Product	t (min)	Yield (%) b	TOF (h−1) c
1	C6H5CH2OH	C6H5CHO	90 90 90	99 98 d 97 e	660 653 646
2 f	4-FC6H4CH2OH	4-FC6H4CHO	100	97	582
3 f	4-ClC6H4CH2OH	4-ClC6H4CHO	100	96	576
4 f	4-BrC6H4CH2OH	4-BrC6H4CHO	100	97	582
5 f	4-NO2C6H4CH2OH	4-NO2C6H4CHO	105	97	554
6 f	4-OMeC6H4CH2OH	4-OMeC6H4CHO	100	99	594
7	4-OHC6H4CH2OH	4-OHC6H4CHO	100	98	588
8	(C6H5)2CHOH	(C6H5)2CO	120	98	490
9	C6H5CH2OH	C6H5CHO	105	97	554

^a Reaction conditions: Unless otherwise stated, all reactions were performed, substrate (2.5 mmol), MR-SB-Mo 46 (5.6 mg, contain 0.0025 mmol of Mo), H₂O₂ (30%, 2.75 mmol), solvent free, 65 °C, 90 to 120 min. ^b Isolated yield. ^c TOF = (mmol of product)/[(mmol of catalyst) × (time)]. ^d Yield at 5th reaction cycle. ^e Yield at 10 g scale reaction. ^f Reaction conducted with 2 mL acetonitrile.

Active Au-Pd catalysts on carbon and titanium oxide (Au-Pd/C (47) and Au-Pd/TiO₂ (48)) were applied for the benzyl alcohol oxidation without adding solvents. The results showed that carbon-supported bimetallic catalysts provided a higher conversion as compared with Au-Pd/TiO₂ 48 catalyst, due to the vast superficial area of carbon supported catalyst as compared with the titanium supported catalyst (Figure 1) [85,86]. These results demonstrated the critical importance of correct selection of support for the metal nanoparticles immobilization in the catalyst preparation approaches. Additionally, some dissimilarities in the selectivity and catalytic activities of catalysts could be the result of both the interaction of catalyst support and the influence of the shape of metal particles.

Figure 1. Curves conversion versus time for carbon-supported catalysts and titanium supported catalysts.

Fe₃O₄/GrOSi(CH₂)₃–NH₂/HPMo nanocomposite (49) as a magnetically recyclable (at least four times) heterogeneous separable oxidation catalyst was applied for selective oxidation of alcohols to corresponding aldehydes (Table 49, Entries 1–10) and ketones (Table 49, Entries 11–13) with very high selectivity (≥99%) in moderate to excellent yields (60% to 96%) with H₂O₂ under solvent-free conditions (Table 49) [87].

Table 49. Results of various alcohol oxidations with H₂O₂ catalyzed by the Fe₃O₄/GrOSi(CH₂)₃-NH₂/HPMo 49 catalyst ^a.

Entry	Substrate	Product	Yield (%)b
1	C6H5CH2OH	C6H5CHO	90
2	4-iPrC6H4CH2OH	4-iPrC6H4CHO	96
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	94
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	86
5	2-NO2C6H4CH2OH	2-NO2C6H4CHO	60
6	3-NO2C6H4CH2OH	3-NO2C6H4CHO	82
7	4-BrC6H4CH2OH	4-BrC6H4CHO	92
8	4-ClC6H4CH2OH	4-ClC6H4CHO	94
9	2-ClC6H4CH2OH	2-ClC6H4CHO	82
10	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	90
11	(C6H5)2CHOH	(C6H5)2CO	90
12	C6H5CH(OH)CH3	C6H5COCH3	90
13	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	90

^a Reaction conditions: Substrate (10 mmol), Fe₃O₄/GrOSi(CH₂)₃-NH₂/HPMo 49 (0.2 g), H₂O₂ (30 wt%, 15 mmol), solvent free, 100 °C, 4 h. ^b Yields were determined by GC analysis with n-decane as internal standard.

Homotrinuclear copper catalyst [Cu₃(slmh)(µ-Cl)₂(CH₃OH)₃]·0.5CH₃OH (50), where H₄slmh stands for disalicylaldehyde malonoyldihydrazone was prepared by Lal et al., in 2017 [88]. This heterogeneous catalyst oxidized several aromatic ring substituted benzylic alcohols by aqueous hydrogen peroxide and the results are summarized in Table 50 [88]. No benzoic acid production was observed in benzylic alcohol oxidations. Moreover, recyclability of the catalyst showed the five consecutive runs ability with no loss of activity.

Table 50. Oxidation of alcohols catalyzed by [Cu₃(slmh)(µ-Cl)₂(CH₃OH)₃]·0.5CH₃OH 50 under solvent-free conditions in the absence of base and co-catalyst ^a.

Entry	Substrate	Product	Yield (%) b
1	C6H5CH2OH	C6H5CHO	78
2	2-MeC6H4CH2OH	2-MeC6H4CHO	80
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	88
4	3-ClC6H4CH2OH	3-ClC6H4CHO	94
5	4-MeC6H4CH2OH	4-MeC6H4CHO	81
6	2-pyridinemethanol	picolinaldehyde	93
7	3,4,5-OMeC6H2CH2OH	3,4,5-OMeC6H2CHO	88
8	Furfuryl alcohol	Furfural	92
9	2-Thiophenemethanol	2-Thiophenecarboxaldehyde	90
10	4-NO2C6H4CH2OH	4-NO2C6H4CHO	97
11	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	85
12	C6H5CH(OH)CH3	C6H5COCH3	1
13	4-OMeC6H4CH2OH:C6H5CH(OH)CH3	4-OMeC6H4CHO:C6H5COCH3	77:1

^a Reaction conditions: Substrate (4.62 mmol), [Cu₃(slmh)(µ-Cl)₂(CH₃OH)₃].0.5CH₃OH 50 (0.04 mmol), H₂O₂ (15%, 8.46 mmol), solvent free, 70 °C, 15 h. ^b Isolated yield.

The selective oxidation of alcohols using green oxidants, H₂O₂, in the presence of Mo₃O₁₀(C₅H₆N)₂.H₂O (51) showed the efficiency of the catalyst for the oxidations of alcohols to the corresponding ketones or aldehydes under solvent-free conditions over reaction-controlled phase-transfer catalysis (Table 51) [89]. The method was so selective with no overoxidation product preparation even with longer reaction time. In addition, MoO_x-pyridine could efficiently oxidize secondary benzylic alcohols to their corresponding ketones with excellent conversions and high selectivity (Table 51, Entries 8–11) [89]. It is worthwhile to mention that the catalyst could be reused in H₂O₂ systems for at least four runs.

Table 51. Oxidation of alcohols to corresponding aldehydes or ketones using Mo₃O₁₀(C₅H₆N)₂·H₂O wires (MoOx-pyridine) 51 catalyst ^a.

Entry	Substrate	Product	t/min	Conv. (%) b
1	C6H5CH2OH	C6H5CHO	45	>95
2	4-ClC6H4CH2OH	4-ClC6H4CHO	90	95
3	2-ClC6H4CH2OH	2-ClC6H4CHO	60	>95
4	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	120	90
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	10	50
6	2-OHC6H4CH2OH	2-OHC6H4CHO	70	95
7	4-OMeC6H4CH2OH	4-OMeC6H4CHO	30	90
8	C6H5CH(OH)Et	C6H5COEt	75	95
9	C6H5CH(OH)CH3	C6H5COCH3	90	95
10	(C6H5)2CHOH	(C6H5)2CO	60	95
11	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	50	95
12	Furfuryl alcohol	Furfural	40	80

^a Reaction conditions: Substrate (1 mmol), Mo₃O₁₀(C₅H₆N)₂·H₂O wires 51 (1.6 mol%), H₂O₂ (1 mmol), solvent free, 80 °C, 10 to 120 min. ^b Conversions were obtained by TLC and GC.

Water-soluble heterobimetallic complex [CuZn(bz)₃(bpy)₂]BF₄ (52) was applied for selective oxidations of primary alcohols to the corresponding aldehydes with no base or co-catalyst. In addition, the catalyst could be recyclable and keep its activity intact over five cycles (Table 52) [90].

Table 52. Oxidation of alcohols catalyzed by [CuZn(bz)₃(bpy)₂]BF₄ 52 under solvent-free condition in the absent of base ^a.

Entry	Substrate	Product	t/h	Yield (%) b Isolated Product (GC) c
1	C6H5CH2OH	C6H5CHO	7	84 (86)c
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	3	98 (100)c
3	2-MeC6H4CH2OH	2-MeC6H4CHO	9	97
4	3-MeC6H4CH2OH	3-MeC6H4CHO	10	93
5	4-MeC6H4CH2OH	4-MeC6H4CHO	4	98
6	3-ClC6H4CH2OH	3-ClC6H4CHO	9	97
7	3-NO2C6H4CH2OH	3-NO2C6H4CHO	3	85 (97) c
8	Furfuryl alcohol	Furfural	3	97 (100) c
9	2-Pyridinemethanol	Picolinaldehyde	3	96
10	2-Thiophenemethanol	2-Thiophenecarboxaldehyde	3	98
11	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	2	98 (100) c
12	3,4,5-OMeC6H2CH2OH	3,4,5-OMeC6H2CHO	6	98
13	C6H5CH(OH)CH3	C6H5COCH3	16	1 c

^a Reaction conditions: Substrate (4.84 mmol), [CuZn(bz)₃(bpy)₂]BF₄52 (0.023 mmol), H₂O₂ (30%, 8.83 mmol), solvent free, 70 °C, 2 to 16 h. ^b Isolated yield via column chromatography. ^c GC conversion.

Table 53 presents the application of Cu(II) Schiff base complex [Cu(HL)(H₂O)NO₃] (53) using 2-[(2-hydroxy-1,1-dimethyl-ethylimino)methylphenol (H₂L) in the oxidation of different alcohols [91]. Primary benzylic alcohols either with electron donating or electron withdrawing groups underwent selective and efficient oxidation to the corresponding aldehydes with no overoxidation (Table 53, Entries 1–8). The oxidation of secondary alcohols provided ketones in good to high yield (Table 53, Entries 10–12). The catalyst was also reused five times without losing of its activity.

Table 53. Oxidation of alcohols using H₂O₂ catalyzed by [Cu(HL)(H₂O)NO₃] 53 ^a.

Entry	Substrate	Product b	Yield (%) c
1	C6H5CH2OH	C6H5CHO	94
2	2-OHC6H4CH2OH	2-OHC6H4CHO	62
3	4-OHC6H4CH2OH	4-OHC6H4CHO	69
4	2-ClC6H4CH2OH	2-ClC6H4CHO	65
5	4-ClC6H4CH2OH	4-ClC6H4CHO	84 d
6	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	90
7	4-MeC6H4CH2OH	4-MeC6H4CHO	81 e
8	4-OMeC6H4CH2OH	4-OMeC6H4CHO	82
9	Furfuryl alcohol	Furfural	68
10	4-OMeC6H5CH(OH)CH3	4-OMeC6H5COCH3	86
11	C6H5CH(OH)CH3	C6H5COCH3	87
12	1-Tetralol	1-Tetralone	70

^a Reaction conditions: Substrate (1 mmol), [Cu(HL)(H₂O)NO₃] 53 (0.02 mmol), H₂O₂ (30%, 3 mmol), solvent free, 70 °C, 3 h. ^b All products are identified by comparison of their physical data with those of authentic samples. ^c Isolated yield. Yields are determined by TLC and GC based on the starting alcohol. ^{d 1}H NMR (500 MHz, CDCl₃) δ: 7.26 to 7.82 (m, 4H, ArH) and 9.96 (s, 1H, aldehyde H). ^{e 1}H NMR (500 MHz, CDCl₃) δ: 2.31 (s, 3H, CH₃), 7.20 (d, J = 15 Hz, 2H, ArH), 7.66 (d, J = 15 Hz, 2H, ArH), and 9.95 (s, 1H, aldehyde H).

The treatment of mordenite acid using hydrochloric acid (MOR-HC) (54), nitric acid (MOR-HN) (55), and oxalic acid (MOR-O_x) improved its textural properties such as increasing the BET surface area, total pore volume, mesopore volume, and external surface area (Table 54) [92]. These improvements influenced the catalytic activity of the mordenite in oxidation of benzyl alcohol not only in benzyl alcohol conversion both also in benzaldehyde selectivities. In terms of catalytic properties, the highest improvement was observed in the case of the nitric acid treated sample as follows: benzyl alcohol conversion >99% and benzaldehyde selectivities >99%. In addition, the fittingness of the MOR-HN for various acid catalyzed bulky molecular transformations was observed. Decreasing acid site density of mordenite by dealumination and the increasing porosity, resulted in controlling overoxidation of benzaldehyde to undesired benzoic acid in the MOR-HN catalyst [92].

Table 54. Performance of various catalysts in benzyl alcohol oxidation ^a.

Entry	Catalyst	Conv. of BzOH (wt%)	Product Yield (wt%) BzH	Product Yield (wt%) Benzoic Acid
1	MOR	42.95	41.15	1.80
2	MOR-HN	99.94	99.84	0.1
3	MOR-HC	54.49	54.35	0.14
4	MOR-OX	60.37	60.25	0.12

^a Reaction conditions: Benzyl alcohol (14 mL), catalyst (0.5 g), H₂O₂ (30 wt%, 13 mL), deionized H₂O (26 mL), pressure (atmospheric), 90 °C, 4 h.

Tin-containing 1,3-din-butylimidazolium bromide ([BBIM]Br)-SnCl₂ (56) was prepared by Xing et al. [93] and successfully applied for the simple and efficient oxidation of benzyl alcohol using hydrogen peroxide as the oxidant (Table 55). In this catalyst, coordination of Sn species with the imidazole ring resulted in the easy recovery and reusability of the catalyst for six reaction runs without loss of catalytic activity.

Table 55. Catalytic properties of [BBIM]Br-SnCl₂ 56 in the oxidation of various alcohols with H₂O₂ ^a.

Entry	Substrate	Product	t/min	Conv. (%) b	Sel. (%) b
1	C6H5CH2OH	C6H5CHO	15	100	95.1
2	4-NO2C6H4CH2OH	4-NO2C6H4CHO	60	71	99
3	4-CH2OHC6H4CH2OH	4-CH2OHC6H4CHO	15 c	99	90
4	4-OMeC6H4CH2OH	4-OMeC6H4CHO	160	87	91
5	4-iPrC6H4CH2OH	4-iPrC6H4CHO	15	99	50
6	4-OHC6H4CH2OH	4-OHC6H4CHO	15	99	98
7	(C6H5)2CHOH	(C6H5)2CO	15	99	>99

^a Reaction conditions: Substrate (1.5 mmol), [BBIM]Br–SnCl₂ 56 (0.11 g), H₂O₂ (30%, 2 mmol), solvent free, 65 °C, 15 to 160 min. ^b GC analysis. ^c Reaction was carried out using H₂O₂ (30%, 3.5 mmol).

The CoFe₂O₄ nanoparticles (NPs) (57) catalyzed the solvent-free oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) with hydrogen peroxide (Table 56) [94]. Excellent results, >99% conversation of BzOH and 100% selectivity, were obtained with no obvious loss of catalyst activity after three consecutive runs.

Table 56. Selective oxidation of benzyl alcohol to benzaldehyde catalyzed by CoFe₂O₄ NPs 57 ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)
1	C6H5CH2OH	C6H5CHO	>99	100

^a Reaction conditions: Substrate (3 mL, 29 mmol), CoFe₂O₄ NPs 57 (40 mg), H₂O₂ (35%, 145 mmol, 12.5 mL), solvent free, 110 °C, 5 h.

The Mn- and Co-substituted polyoxotungstates [MPW11O39]⁵⁻ (M = Mn or Co) was immobilized on MCM-41 or into the interlayer of Mg₃Al-layered double hydroxide. The catalysts showed catalytic activity in the presence of H₂O₂ for the solvent-free oxidation of benzyl alcohol for at least four times with no loss of activity and selectivity. As illustrated in Table 57 [95], POM loaded-LDH (58) and POM loaded-LDH-adipate (59) catalysts were better catalysts than POM-free catalysts due to easier reduction of metal ion via interaction of POM anion with the clay surface [96]. In addition, the basic sites of LDH-supported catalysts resulted in higher selectivity for benzaldehyde as compared with the unsupported catalysts [97].

Table 57. Oxidation of benzyl alcohol with various catalyst ^a.

Entry	Catalyst	t/h	Conv. (%)	Sel. for BzH (%)
1	-	5	2	70
2	MnPOM/MCM	5 8	67 76	75 69
3	CoPOM/MCM	5 8	70 78	71 67
4	TBA-MnPOM b	5	49	75
5	TBA-CoPOM b	5	52	70
6	TBA-MnPOM c	2.5	35	Not determined
7	TBA-CoPOM c	2.5	13	Not determined
8	LDH	5	3	91
9	MnPOM/LDH	5	45	97
10	CoPOM/LDH	5	37	96
11	LDH-adi	5	4	90
12	MnPOM/LDH-adi	5	66	100
13	CoPOM/LDH-adi	5	69	99

^a Reaction conditions: Benzyl alcohol (10 mL), catalyst (5 wt%), H₂O₂/alcohol molar ratio (3:1), 90 °C, 2.5 to 8 h. Oxidation reaction: Substrate (benzyl alcohol), catalyst, and aqueous H₂O₂ (30%) were inserted into a 60 mL parr reactor. The reactor was heated to a desired temperature and the mixture was stirred. The catalyst was separated, and the products were extracted with diethyl ether, dried, and analyzed by GC (chromatograph, Varian CP-3800 GC; column, CP-Sil 30 m × 0.25 mm). ^b H₂O₂/alcohol molar ratio of 2:1. ^c H₂O₂/alcohol molar ratio of 5:1, in H₂O, 100 °C [98].

Two long chain multi-SO₃H functionalized heteropolyanion-based ionic liquids S4SiIL (60) and S3PIL (61) as homogeneous catalysts provided aldehydes and ketones with 63% to 100% yields through the oxidation of alcohols with no phase transfer catalyst and could be reused five times. The corresponding benzoic acids of benzyl alcohols were obtained with 64% to 94% yields (Table 58) [99]. For 60, benzaldehyde was obtained in 94% yield, whereas for 61, the benzoic acid provided 83% yield.

Table 58. Results of selective oxidation of alcohols catalyzed by S4SiIL 60 and S3PIL 61 ^a.

Entry	Substrate	Product	Conv. (%)	Yield (%)
1	C6H5CH2OH	C6H5CHO C6H5CO2H	100 b 99 c	94 b 83 c
2	4-MeC6H4CH2OH	4-MeC6H4CHO 4-MeC6H4CO2H	100 b 99 c	100 b 87 b
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO 4-OMeC6H4CO2H	100 b 99 c	100 c 94 c
4	4-ClC6H4CH2OH	4-ClC6H4CHO 4-ClC6H4CO2H	99 b 97 c	91 c 77 c
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO 4-NO2C6H4CO2H	96 b 95 c	81 c 64 c
6 d	C6H5CH(OH)CH3	C6H5COCH3	98 b/95c	94 b/80 c
7 d	(C6H5)2CHOH	(C6H5)2CO	95 b/89c	83 b/76 c

^a Reaction conditions: Substrate (30 mmol), S4SiIL 60 and S3PIL 61 (0.05 mmol), H₂O₂ (35%, 45 mmol), solvent free, 70 °C, 4 h. ^b Results of S4SiIL. ^c Results of S3PIL. ^d Substrate (30 mmol), S4SiIL and S3PIL (0.05 mmol), H₂O₂ (60 mmol), 70 °C, 4 h.

A polyethylene oxide-supported long-chain imidazolium polyoxometalate hybrid catalyst ([PEO-didodecylimidazolium]₃[PW₁₂O₄₀]₂) (62) was employed as the recyclable catalyst (could be reused six times) for the selective oxidation reactions of alcohols using H₂O₂ at room temperature, without significant loss of its activity, as shown in Table 59 [100]. Ketones or aldehydes were the only produced products. Primary benzylic alcohols had the best activities with no significant effects on the electronic properties, electron-withdrawing, or electron-donating substituents, or steric hindrance of the groups on the benzene ring [100].

Table 59. Oxidation of various alcohols ^a.

Entry	Substrate	Product	t/h	Conv. (%)	Yield (%)
1	C6H5CH2OH	C6H5CHO	6	100	98
2	2-OMeC6H4CH2OH	2-OMeC6H4CHO	6	99	96
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	5	>99	98
4	3-OMeC6H4CH2OH	3-OMeC6H4CHO	8	98	94
5	4-MeC6H4CH2OH	4-MeC6H4CHO	5	>99	99
6	4-ClC6H4CH2OH	4-ClC6H4CHO	8	96	95
7	4-NO2C6H4CH2OH	4-NO2C6H4CHO	8	95	93
8	(C6H5)2CHOH	(C6H5)2CO	16	81	80
9	C6H5CH(OH)CH3	C6H5COCH3	14	83	81
10	2-Pyridinemethanol	Picolinaldehyde	16	86	84
11	Furfuryl alcohol	Furfural	12	90	82

^a Reaction conditions: Substrate (10 mmol), [PEO-didodecylimidazolium]₃[PW₁₂O₄₀]₂ 62 (0.05 mol % of alcohol), H₂O₂ (30%, 2 equiv.), solvent free, room temperature, 5 to 16 h.

Schiff base complexes of transition metal ions supported on silica coated magnetic metallic cobalt nanoparticles (Co@SiO₂@[M(II)SBC], M = Co, Cu, Ni, Zn) (63, 64, 65, 66) as heterogeneous catalysts were prepared and explored for the oxidations of the alcohols to corresponding aldehydes in high yields. The catalysts could be recyclable five times. The obtained results, as shown in Table 60 [101], indicated that alcohol conversions occurred during approximately half an hour with 100% selectivity. Although all of these catalysts selectively converted different alcohols to corresponding aldehyde, the Ni(II) catalysts were more active than three others and converted alcohols to aldehydes in shorter time.

Table 60. Investigation of catalytic activity of Co@SiO₂@[Co(II)SBC] 63 catalysts on the oxidation reaction ^a.

Entry	Substrate	Product	t/min	Yield (%) b	Sel. (%)
1	4-ClC6H4CH2OH	4-ClC6H4CHO	27–28	91	100
2	4-MeC6H4CH2OH	4-MeC6H4CHO	24–26	93	100
3	4-FC6H4CH2OH	4-FC6H4CHO	32–33	94	100
4	4-OMeC6H4CH2OH	4-OMeC6H4CHO	22–25	90	100
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	25–27	91	100
6	C6H5CH2OH	C6H5CHO	24–26	92	100

^a Reaction conditions: Substrate (0.5 mmol), Co@SiO₂@[Co(II)SBC] 63 (0.04 g), H₂O₂ (30%, 60 μL), solvent free, 50 °C, 24 to 33 min. ^b Isolated yield.

Bismuth tribromide (BiBr₃) (67) catalyzed oxidations of both primary and secondary benzylic alcohols with aqueous H₂O₂ to corresponding carbonyl compounds in high yields, as summarized in Table 61 [102]. Without overoxidation, primary benzylic alcohols resulted in exclusive formation of aldehydes. Generally, the oxidation of primary benzylic alcohols proceeded slower than primary secondary alcohols.

Table 61. Oxidation of alcohols with H₂O₂ and BiBr₃ 67 ^a.

Entry	Substrate	Product	t/min	Yield (%) b
1	C6H5CH2OH	C6H5CHO	30	90
2	2-MeC6H4CH2OH	2-MeC6H4CHO	30	93
3	4-MeC6H4CH2OH	4-MeC6H4CHO	30	92
4	2-ClC6H4CH2OH	2-ClC6H4CHO	30	85
5	4-ClC6H4CH2OH	4-ClC6H4CHO	30	89
6	4-BrC6H4CH2OH	4-BrC6H4CHO	30	85
7	4-OMeC6H4CH2OH	4-OMeC6H4CHO	30	72
8	4-NO2C6H4CH2OH	4-NO2C6H4CHO	30	80
9	1-Naphthalenmethanol	1-Naphthaldehyde	10	75
10	C6H5CH(OH)CH3	C6H5COCH3	10	80
11	C6H5CH(OH)C2H5	C6H5COC2H5	10	85
12	4-MeC6H5CH(OH)CH3	4-MeC6H5COCH3	20	90
13	4-ClC6H5CH(OH)CH3	4-ClC6H5COCH3	10	95
14	4-BrC6H5CH(OH)CH3	4-BrC6H5COCH3	10	95
15	4-FC6H5CH(OH)CH3	4-FC6H5COCH3	10	96
16	1-(Naphthalen-2-yl)ethanol	2-Acetonaphthone	10	80
17	1-Tetralol	1-Tetralone	10	85
18	(C6H5)2CHOH	(C6H5)2CO	40	85
19	C6H5CH(OH)CO2Et	C6H5COCO2Et	10	88

^a Reaction conditions: Substrate (1 mmol), BiBr₃ 67 (10 mol%), H₂O₂ (30%, 5 mmol), solvent free, 70 °C, 10 to 40 min. ^b Isolated yield.

Three amphiphilic sulphonato-salen-chromium(III) complexes immobilized on MCM-41 (Cr(SO₃-salphen)-MCM-41) (68) were applied for the selective oxidation of BzOH to BzH with H₂O₂, and the results are listed in Table 62 [103]. The introduction of hydrophilic sulfonic acid groups to the catalyst structure enhanced accessibility of the catalyst with oxidant which increased the catalytic act in oxidation reactions as compare with their corresponding lipophilic complexes. The best results were also obtained, i.e., 60.3% benzyl alcohol conversion with 100% benzaldehyde selectivity. In addition, Cr(SO₃-salphen)-MCM-41 complex could be reused for five runs.

Table 62. Catalytic performance of samples in benzyl alcohol oxidation ^a.

Entry	Catalyst	Conv. of BzOH (%)	Sel. (%)			H2O2 Efficiency (mol %)
			BzH	Benzoic Acid	Benzyl Benzoate
1	Blank	3	100	0	0	5.1
2	Cr(salen)	12.2	65.4	23.5	11.1	15.3
3	Cr(salten)	15	70.2	19.8	10	17.8
4	Cr(salphen)	20.5	75.6	18.2	6.2	23.6
5	Cr(SO3-salen)	25	76.5	17.5	6	28
6	Cr(SO3-salten)	31.4	82	15.3	2.7	35.5
7	Cr(SO3-salphen)	34.7	80.5	15.4	4.1	37.9
8	Cr(salen)-MCM-41	42.5	98.2	1.3	0.4	45.6
9	Cr(salten)-MCM-41	43.2	100	0	0	48.1
10	Cr(salphen)-MCM-41	45.2	100	0	0	52
11	Cr(SO3-salen)-MCM-41	53.1	100	0	0	62.5
12	Cr(SO3-salten)-MCM-41	57	100	0	0	68.3
13	Cr(SO3-salphen)-MCM-41	60.3	100	0	0	72.5

^a Reaction conditions: Benzyl alcohol (50 mmol), H₂O₂ (30%, 125 mmol), heterogenized catalysts (0.25 g) or homogeneous catalysts (1.25 mmol), 50 °C, 4 h.

Cesium salt of transition metal (M = Co, Mn, Ni) substituted phosphomolybdates PMo₁₁M as catalysts for the oxidation of alcohol were applied and the catalyst showed the activity in the order of activity PMo₁₁Co (69) ≥ PMo₁₁Ni (70) > PMo₁₁Mn (71) as illustrated in Table 63 [104]. In addition, the catalysts could be reused up to 2 cycles.

Table 63. Oxidation of various alcohols catalyzed by PMo₁₁M (M = Co, Mn, Ni), under optimized conditions ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)
1	C6H5CH2OH	C6H5CHO	56.5 b 37.9 c 56.2 d	90.9 84.9 81.3

^a Reaction conditions: PMo₁₁M (M = Co, Ni, Mn) 69, 70, 71 (20 mg), mole ratio of alcohol to H₂O₂ (1:3), 90 °C, 24 h. ^b [M] = Co. ^c [M] = Mn. ^d [M] = Ni.

Using cationic surfactants with different carbon-chain lengths for the functionalization of the Vcontaining Keggin POM H₄PMo₁₁VO₄₀ provided a series of polyoxometalate (POM)-based amphiphilic catalysts. The catalysts were for selective benzyl alcohol oxidation by H₂O₂ (Table 64) [105] and the best catalytic efficiency was obtained by (ODA)₄PMo₁₁VO₄₀ (72) (ODA: octadecylmethylammonium) due to its amphiphilic property (60.6% of benzyl alcohol conversion with 99% of selectivity for benzaldehyde). Furthermore, the catalyst could be recycled over four run reactions without losing activity.

Table 64. Selective oxidation of benzyl alcohol to benzaldehyde with H₂O₂ in the presence of the catalysts investigated ^a.

Entry	Catalyst	Conv. (%) b	Sel. (%) c
1	H3PMo12O40	19.1 ± 1.9 d	88.6 ± 1.6 d
2	H4PMo11VO40	28.2 ± 1.8	90 ± 1.4
3	(ODA)4PMo11VO40	60.6 ± 1.5	99 ± 0.8
4	ODACl	0	0
5	(ODA)3PMo12VO40	36.5 ± 1.7	94.5 ± 1.1
6	(ODA)4PMo11VO40-P	58.9 ± 1.6	99 ± 0.8
7	(ODACl + H4PMo11VO40) e	57.1 ± 1.7	98.8 ± 0.9
8	(DODA)4PMo11VO40	53.3 ± 1.8	99 ± 0.9
9	(DDA)4PMo11VO40	51.6 ± 1.8	98 ± 1
10	(HDA)PMo11VO40	56.3 ± 1.8	99 ± 0.9
11	(TBA)4PMo11VO40	28.9 ± 1.2	91.1 ± 1.4

^a Reaction conditions: Benzyl alcohol (60 mmol), catalyst (0.03 mmol), H₂O₂ (50 mmol), solvent free, 90 °C, 6 h. ^b Conversion of benzyl alcohol. ^c Selectivity for benzaldehyde. ^d Standard deviation. ^e ODACl (0.12 mmol) and H₄PMo₁₁VO₄₀ (0.03 mmol) were simultaneously added into the reaction system.

A simple adsorption method provided heterogeneous copper(II)-cysteine/SiO₂-Al₂O₃ (73) catalyst which represented good activity and selectivity in aromatic alcohol oxidation reactions, as shown in Table 65 [106]. All reactants were completely converted to selective carbonyl compounds. In addition, shifting the substituent from electron withdrawing (Entries 2–5) to electron donating (Entries 6–9) remarkably increased the yield of corresponding aldehydes from 45% to 98%. The catalyst could also be recycled over five times.

Table 65. Solvent-free oxidation of various aromatic alcohols over Cu(II)-cysteine/SiO₂-Al₂O₃ 73 ^a.

Entry	Substrate	Product	t/min	Yield (%) b
1	C6H5CH2OH	C6H5CHO	60	95
2	4-BrC6H4CH2OH	4-BrC6H4CHO	45	80
3	4-ClC6H4CH2OH	4-ClC6H4CHO	45	75
4	3-NO2C6H4CH2OH	3-NO2C6H4CHO	90	45
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	85	45
6	2-MeC6H4CH2OH	2-MeC6H4CHO	50	90
7	3-OHC6H4CH2OH	3-OHC6H4CHO	45	98
8	3-OMeC6H4CH2OH	3-OMeC6H4CHO	45	98
9	4-ClC6H4CH(OH)C6H5	4-ClC6H4COC6H5	80	80
10	(C6H5)2CHOH	(C6H5)2CO	70	85

^a Reaction conditions: Substrate (1 mmol), Cu(II)-cysteine/SiO₂-Al₂O₃ 73 (0.15 g, 17.5% Cu(II)-cysteine/SiO₂-Al₂O₃), H₂O₂ (4 mmol), solvent free, room temperature, 45 to 90 min. ^b Isolated yield.

Lal et al. [107] reported the solvent-free oxidation of primary (Entries 2–5, 9) and secondary (Entries 6–8) benzylic alcohols using a heterotrinuclear complex containing a dicopper(II)–monozinc(II) centre, [ZnCu₂(slsch)(NO₃)₂(H₂O)₈]·2H₂O (74), with hydrogen peroxide, as summarized in Table 66, with good yield and excellent selectivity and four consecutive run reusability.

Table 66. Hydrogen peroxide mediated oxidation of alcohols to aldehydes and ketones ^a.

Entry	Substrate	Product	t/h	Yield (%) b
1	C6H5CH2OH	C6H5CHO	10	85
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	11	87
3	4-NMe2C6H4CH2OH	4-NMe2C6H4CHO	11	88
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	9	82
5	2-ClC6H4CH2OH	2-ClC6H4CHO	10	78
6	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	12	80
7	C6H5CH(OH)CH3	C6H5COCH3	12	80
8	4-ClC6H4CH(OH)CH3	4-ClC6H4COCH3	14	82
9	4-MeC6H4CH(OH)CH3	4-MeC6H4COCH3	12	78

^a Reaction condition: Substrate (5 mmol), [ZnCu₂(slsch)(NO₃)₂(H₂O)₈]·2H₂O 74 (8 mol%), H₂O₂ (30%, 10 mmol), solvent free, 100 °C, 9 to 14 h. ^b Isolated yields.

Under mild conditions, primary and secondary alcohols were oxidized by applying rutheniumbis(benzimidazole)pyridinedicarboxylate complex [Ru(bbp)(pydic)] (75) as catalyst and H₂O₂ as oxidant. Aldehydes and ketones were prepared with good yield and excellent selectivity (Table 67) [108].

Table 67. Oxidation of various alcohols with H₂O₂ catalyzed by Ru(bbp)(pydic) 75 ^a.

Entry	Substrate	Product	t/h	Conv. (%) b	Yield (%) b
1	C6H5CH2OH	C6H5CHO	1	97	96
2	4-MeC6H4CH2OH	4-MeC6H4CHO	1	98	96
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	1	96	93
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	2	82	81
5 c	4-Pyridinemethanol	Isonicolinaldehyde	3	76	74
6	C6H5CH(OH)CH3	C6H5COCH3	1	92	92
7	(C6H5)2CHOH	(C6H5)2CO	3	82	82

^a Reaction condition: Substrate (2 mmol), Ru(bbp)(pydic) 75 (2 × 10⁻³ mmol), H₂O₂ (30%, 10 mmol), solvent free, 60 °C, 1 to 3 h. ^b Determined by GC. ^c Ru(bbp)(pydic) (4 × 10⁻³ mmol).

Keggintype polyoxometallates, such as M₃(PW₁₂O₄₀)₂ [M = Ni, Zn, Co, Mn, Cu; denoted as MPW₁₂], FePW₁₂O₄₀ [FePW₁₂] (76), Ni₃(PMo₁₂O₄₀)₂ [NiPMo₁₂] (77), and Ni₂SiW₁₂O₄₀ [NiSiW₁₂] (78) were synthesized as catalysts, under solvent-free condition, for the selective oxidation of BzOH into BzH with hydrogen peroxide (Table 68) [109]. Among them, Ni₃(PW₁₂O₄₀)₂·26H₂O (77) provided high catalytic activity, TON of 550.6 mol/(mol cat.), and 87.3% selectivity. The above catalyst was also recovered and reused for three times.

Table 68. Oxidation of alcohols ^a.

Entry	Substrate	Product	Conv. (%)	Yield (%)	Sel. (%)
1	C6H5CH2OH	C6H5CHO	90.9	79.4	87.3
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO 4-OMeC6H4CHO b	93.9 82	27.1 78.7	28.9 96

^a Reaction Condition: Substrate (47.5 mmol, 5 mL), Ni₃(PW₁₂O₄₀)₂·26H₂O 77 (0.5 g), H₂O₂ (30%, 93 mmol, 10 mL), solvent free, 90 °C (in an water bath), 3.5 h. ^b Conditions: substrate (47.5 mmol, 5 mL), Ni₃(PW₁₂O₄₀)₂·26H₂O (0.5 g), H₂O₂ (30%, 93 mmol, 10 mL), solvent free, 70 °C, 3.5 h.

Appling L-aspartic acid coupled with imidazolium based ionic liquid [L-AAIL] (79), in the presence of hydrogen peroxide as an ideal oxidant, provided a green protocol for the selective oxidation of alcohols which could be recycled and reused for seven runs. The substituted primary benzylic alcohols were selectively oxidized to aldehydes with 88% to 96% yield (Table 69, Entries 1–6, 9) and the secondary alcohols also produced ketones with 76% to 84% yield (Table 69, Entries 7, 8, 10) [110].

Table 69. L-AAIL 79 catalyzed oxidation of alcohols by hydrogen peroxide at 25 °C ^a.

Entry	Substrate	Product	t/min	Yield (%) b,c
1	C6H5CH2OH	C6H5CHO	30	96
2	4-ClC6H4CH2OH	4-ClC6H4CHO	38	93
3	4-BrC6H4CH2OH	4-BrC6H4CHO	40	92
4	4-NH2C6H4CH2OH	4-NH2C6H4CHO	38	94
5	4-OMeC6H4CH2OH	4-OMeC6H4CHO	35	96
6	4-MeC6H4CH2OH	4-MeC6H4CHO	35	88
7	C6H5CH(OH)CH3	C6H5COCH3	65	84
8	(C6H5)2CHOH	(C6H5)2CO	85	82
9	Furfuryl alcohol	Furfural	210	88
10	2-Pyridinemethanol	Picolinaldehyde	190	90

^a Reaction condition: Substrate (2 mmol), L-AAIL 79 (0.0025 mmol), H₂O₂ (5 mmol), solvent free, 25 °C, 30 to 210 min. ^b Quantified by HPLC of the crude product. ^c Isolated yield by flash chromatography.

Under solvent-free conditions, hydrogen peroxide as oxidant and metal dodecanesulfonate salts, M(DS)_x [M = Fe³⁺ (80), Cu²⁺ (81), Ni²⁺ (82) and Sn²⁺ (83), DS = dodecanesulfonate, x = 3 or 2] as catalysts were conducted by biphasic catalysis for benzyl alcohol oxidation to benzaldehyde. The results are summarized in Table 70 [111] and indicate that Fe(DS)₃ (80) catalyst represented a surprisingly high activity, exhibiting nearly 100% conversion of BzOH and TON of 194.4 mol/(mol cat) as compare with other catalytic systems, due to its difference in Lewis acidity (Entry 1) [112]. In addition, this catalyst could be recovered and reused three times. Overoxidation of BzH into benzylic acid was minimized in biphasic operation systems due to reducing the contact opportunity between BzH in bulk organic phase and the catalyst on the interphase or H₂O₂ that was soluble in the aqueous phase.

Table 70. Catalytic activity of various dodecanesulfonate salts ^a.

Entry	Catalyst (0.25 mmol)	Conv. (%)	TON (mol/molcat)
1	Fe(DS)3	100	194.4
2	Cu(DS)2	47.3	92
3	Ni(DS)2	7.8	15.2
4	Sn(DS)2	4	7.8

^a Reaction conditions: Ratio of H₂O₂ to alcohol (0.22:0.0486, 4.5:1), catalyst (0.2 g), solvent free, 90 °C, 6 h, stirring speed 250 rpm.

A series of conventional chromium(III) Schiff base complexes were immobilized on MCM-41. Without any organic solvent, phase transfer catalyst, or additive, the mentioned complexes showed much higher catalytic performance in benzyl alcohol conversion to benzaldehyde, due to their corresponding homogeneous analogs. The catalyst represented difference activity by varying ligand in the following order: 84-L4 > 84-L2 > 84-L1 > 84-L5 > 84-L3 (Figure 2) [113]. The difference could be attributed to ligand structures which 84-L4 with π-extended coordination structure exhibiting the best catalytic performance.

Figure 2. Homogeneous chromiun (III) Schiff base complexes and their immobilized analouge were prepared as described in “experimental” section.

Different alcohol oxidations were also applied over the representative catalyst 84-L4 (Table 71) [113]. All benzylic alcohols were oxidized with good yields, and the major products were the corresponding ketones or aldehydes. Overoxidation of aldehydes into their corresponding acids were also observed. The catalyst could be reusedfor four catalytic runs, with only a little change in the catalytic performance.

Table 71. Catalytic performance of the representative immobilized complex in the oxidation of alcohols ^a.

Entry	Substrate	Product	Conv. (mol%) b	Yield (mol%) c
1	C6H5CH2OH	C6H5CHO	45 d 44 e 43 f 41 g	45 44 42 38

^a Reaction conditions: Substrate (0.05 mol), 84-L4 (0.25 g), H₂O₂ (0.125 mol), solvent free, 50 °C, 4 h. ^b Conversion = (moles of substrate reacted/moles of substrate in the feed) × 100. ^c Yield = (moles of substrate converted to the products/moles of substrate in the feed) × 100. ^{d, e, f, g} Recycle runs: 1, 2, 3, 4 respectively.

As illustrated in Table 72 [114], green oxidant hydrogen peroxide and cyclopentadienyl molybdenum acetylide catalyst, CpMo(CO)₃(C≡CPh) (85), were used to explore selective oxidations of different aromatic alcohols to aldehydes with very high conversion (90%) and aldehyde selectivity (90%). The catalytic system could be reused even after five recycles with no decrease in alcohol conversion and aldehyde selectivity.

Table 72. Oxidation of different alcohols ^a.

Entry	Substrate	Product	Conv. (%)	Yield (%)	Sel. (%)		TON
					Aldehyde	Acid
1	C6H5CH2OH	C6H5CHO	86	79	92	8	396
2	4-MeC6H4CH2OH	4-MeC6H4CHO	90	78	87	13	391
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	90	90	90	10	392
4	3,5-OMeC6H3CH2OH	3,5-OMeC6H3CHO	83	71	85	15	352
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	60	53	88	12	264
6	4-ClC6H4CH2OH	4-ClC6H4CHO	65	62	91	9	296
7	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	78	70	90	10	343

^a Reaction conditions: Substrate (0.05 mol), CpMo(CO)₃(C≡CPh) 85 (1 to 0.01 mmol), H₂O₂ (30%, 0.1 mol), solvent free, 80 °C, 8 h.

Nano-γ-Fe₂O₃ 86 as a selective and active catalyst for alcohol and olefin oxidation yielded the corresponding aldehyde, with good to excellent selectivity, without losing catalyst activity even after reusing 5 times. As illustrated in Table 73 [115], significant impact on the catalyst activity was provided by substitutions on the aromatic ring of the benzyl alcohol. Moreover, as compared with benzyl alcohol, 1-phenylethanol, diphenylmethanol, and 1-phenyl-1-propanol gave good conversions with lower selectivity (Table 73, Entries 4–6).

Table 73. Selective oxidation of alcohols to aldehydes and ketones with nano-γ-Fe₂O₃ 86 ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)	TON
1	4-(tert-Butyl)C6H4CH2OH	4-(tert-Butyl) C6H4CHO	10	>99	10
2	4-FC6H4CH2OH	4-FC6H4CHO	87	51	44
3	4-ClC6H4CH2OH	4-ClC6H4CHO	98	61	60
4	C6H5CH(OH)CH3	C6H5COCH3	42	60	25
5	C6H5CH(OH)Et	C6H5COEt	35	42	44
6	(C6H5)2CHOH	(C6H5)2CO	49	52	25
7	2-Biphenylmethanol	2-Biphenylcarbaldehyde	6	>99	6

^a Reaction conditions: Substrate (10 mmol, 1.08 g), nano-γ-Fe₂O₃ 86 (1 mol%, 16 mg), H₂O₂ (30 wt%, 1 mL, 10 mmol), solvent free, 75 °C, 12 h.

Some methyl-containing Cr(salen) complexes which immobilized on MCM-41 were prepared as catalysts and exhibited for the selective oxidation of benzyl alcohol (BzOH) with H₂O₂. All organic solvent-free systems, with no phase transfer catalyst or additive, showed much higher catalytic activity than their homogeneous analogue. Benzaldehyde (BzH), benzoic acid, and benzyl benzoate were only detected as products (Table 74) [116]. By increasing methyl content of catalysts, from Cr(salen)-MCM-41(CH₃)₁ (87) to Cr(salen)-MCM-41(CH₃)₃ (89), BzOH conversion and H₂O₂ efficiency decreased slowly, whereas BzH selectivity increased, as can be seen in the presence of Cr(salen)-MCM-41(CH₃)₂ (88) and 89, BzOH was completely oxidized to BzH with no the formation of byproducts. The best BzOH conversion of 65.0% with 100% selectivity to benzaldehyde (BzH) was reached using catalytic system 88 [116].

Table 74. Catalytic performance of the obtained complexes in the selective oxidation of benzyl alcohol ^a.

Entry	Catalyst	Conv. of BzOH (mol %) b	Sel. (mol%) c			H2O2 Efficiency (mol %) d
			BzH	Benzoic Acid	Benzyl Benzoate
1	Blank	3	100	0	0	5.1
2	Cr(salen)	12.2	65.4	23.5	11.1	15.3
3	Cr(salen)-MCM-41	40.5	80.6	12.4	7	46.5
4	Cr(salen)-MCM-41(CH3)1	66.1	92.3	5.6	2.1	77.3
5	Cr(salen)-MCM-41(CH3)2	65	100	0	0	76.6
6	Cr(salen)-MCM-41(CH3)3	61.3	100	0	0	70.9

^a Reaction conditions: Benzyl alcohol (0.05 mol), heterogenized catalyst (0.25 g) or homogeneous complex (1 mol%), H₂O₂ (30%, 0.125 mol), solvent free, 50 °C, 4 h. ^b Conversion = (moles of BzOH reacted/moles of BzOH in the feed) × 100. ^c Selectivity = (moles of BzOH converted to the products/moles of BzOH reacted) × 100. ^d Efficiency of H₂O₂ for BzOH oxidation = (moles of H₂O₂ converted to the products/moles of H₂O₂ consumed) × 100.

Wang et al. reported the preparation of Mo^VI oxo-diperoxo complex [MoO(O₂)₂(TEDA)₂] (90) (TEDA = 1,4-diazabicyclo [2.2.2]octane) [117], which successfully catalyzed the high yield oxidations of alcohols to their corresponding carbonyl groups by H₂O_2. The catalyst was active even for three recycling experiments with no overoxidation (Table 75).

Table 75. [MoO(O₂)₂(TEDA)₂]-catalyzed oxidation of selected alcohols using H₂O₂ as oxidant ^a.

Entry	Substrate	Product	Yield (%) b	Sel. (%)
1	C6H5CH2OH	C6H5CHO	96	98
2	4-MeC6H4CH2OH	4-MeC6H4CHO	96	96

^a Reaction conditions: Substrate (1 mmol), [MoO(O₂)₂(TEDA)₂] 90 (0.01 mol, 1 mol %), H₂O₂ (30%, 2 mmol), solvent free, 80 °C, 6 h, control experiments without complex 90 showed no oxidation under the same reaction conditions. ^b Determined by GC using an internal standard technique.

Some tetra-alkylpyridinium octamolybdate (91–94) were used as catalysts for selective oxidations of benzyl alcohol. Among them, as illustrated in Table 76, [118], tetraalkylpyridinium octamolybdate exhibited high activity and selectivity due to oxygen transfer ability from hydrogen peroxide to the substrate. This matter is clear by 82.3% to 94.8% benzyl alcohol conversion and 87.9% to 96.7% benzaldehyde selectivity. The simple and easy catalyst preparations and utilization, high recovery, and short reaction time made the catalytic system an ideal choice for future investigations.

Table 76. Oxidation of benzyl alcohol to benzaldehyde by hydrogen peroxide over tetra-alkylpyridinium octamolybdate catalysts ^a.

Entry	Catalyst	Amount of Catalyst/g	BzOH/H2O2 Molar Ratio	t/h	Conv. (%)	Sel. (%)
1	PyC4	0.5	1:1.2	1.7	93.6	90.5
2		0.8	1:1.1	1	86.4	91.8
3		0.8	1:1	0.5	82.3	94.6
4		Recycle b	1:1	0.6	83.9	93.3
5	PyC8	0.5	1:1.1	0.6	90.5	89.4
6	PyCl4	0.5	1:1	0.4	85.5	96.7
7	PyCl6	0.5	1:1.1	0.6	92.7	89.5
8	PyCl6	Recycle c	1:1.1	1	94.8	87.9
9	PyC4	0.8	1:2	6.5	99.5	76.5
10	PyC4	0.8	1 d:1.1	0.4	22.6	99.6

^a Reaction conditions: Benzyl alcohol (10.8 g, 0.1 mol), catalyst (0.5 to 0.8 g), H₂O₂ (15%, 25 g, 0.12 mol), reflux, 0.4 to 1.7 h. ^b Catalyst of Entry 4 was recycled. ^c Catalyst of Entry 8 was recycled. ^d Here, the alcohol is cyclohexanol. The catalysts of PyC4, PyC6, PyCl4, and PyCl6 are [n-C₄H₉(π-C₅H₅N)]₄Mo₈O₂₆, [n-C₈H₁₇(π-C₅H₅N)]₄Mo₈O₂₆, [n-C₁₄H₂₉(π-C₅H₅N)]₄Mo₈O₂₆, and [n-C₁₆H₃₃(π-C₅H₅N)]₄Mo₈O₂₆, respectively.

Na₄H₃[SiW₉Al₃(H₂O)₃O₃₇]·12H₂O(SiW₉Al₃) (95) was synthesized and applied as the catalyst for organic-solvent-free selective oxidations of alcohol to ketones using H₂O₂ without any phase-transfer catalyst under mild, safe, and simple reaction conditions (Table 77) [119]. Benzaldehydes was prepared with selective benzylic alcohol oxidations in moderate to good yields without overoxidation. Additionally, N-atom of 2-pyridinemethanol and S-atom of 2-thiophenemethanol showed no oxidation.

Table 77. The selective oxidation of alcohols with H₂O₂ catalyzed by SiW₉Al₃ 95 without solvent ^a.

Entry	Substrate	Product	t/h	Conv. (%)	Sel. (%)
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	6	100	99
2	4-MeC6H4CH2OH	4-MeC6H4CHO	7	100	99
3	C6H5CH2OH	C6H5CHO	5	100	99
4	4-ClC6H4CH2OH	4-ClC6H4CHO	6	100	99
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	12	74	99
6	2-Pyridinemethanol	Picolinaldehyde	5	100	99
7	Furfuryl alcohol	Furfural	2	100	99
8	2-Thiophenemethanol	2-Thiophenecarboxaldehyde	4	100	99

^a Reaction condition: Substrate (20 mmol), SiW₉Al₃ 95 (1.7 µmol), H₂O₂ (30%, 30 to 50 mmol), solvent free, 60 to 95 °C, 2 to 12 h.

2.3. Oxidation of Benzylic and Heterocyclic Alcohols in the Presence of Various Solvents

2.3.1. Acetonitrile Solvent

Keggin-type polyoxometalate [n-C₄H₉)₄N]_x[PW₁₁ZnO₃₉]·nH₂O was successfully immobilized on imidazole functionalized ionic liquid-modified mesoporous MCM-41 by physical adsorption (PW₁₁Zn@MCM-41-Im) (96) [120]. The supported ionic liquid catalyst was easily recovered by simple filtration and reused in four reaction runs. Different alcohols were efficiently oxidized in reflux condition, as shown in Table 78, with high yields and good selectivity. A slight influence on the reaction of benzylic alcohols and electronic nature (electron-withdrawing and electron-donating groups) of the substituent were observed (Table 78, Entries 2–6).

Table 78. Oxidation of alcohols with H₂O₂ catalyzed by PW₁₁Zn@MCM-41-Im 96 under reflux conditions ^a.

Entry	Substrate	Product	t (h)	Yield (%) b	TOF (h−1)
1	C6H5CH2OH	C6H5CHO	10	94	15.83
2	2-ClC6H4CH2OH	2-ClC6H4CHO	14	88	10.47
3	4-ClC6H4CH2OH	4-ClC6H4CHO	12	94	13.09
4	2-BrC6H4CH2OH	2-BrC6H4CHO	15	90	10
5	4-BrC6H4CH2OH	4-BrC6H4CHO	12	92	12.77
6	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	13	60	7.69
7	2-NO2C6H4CH2OH	2-NO2C6H4CHO	12	75	10.41
8	4-NO2C6H4CH2OH	4-NO2C6H4CHO	12	70	9.72
9	4-OHC6H4CH2OH	4-OHC6H4CHO	12	82	11.08
10	2-OMeC6H4CH2OH	2-OMeC6H4CHO	12	92	12.77
11	4-OMeC6H4CH2OH	4-OMeC6H4CHO	14	94	9.64
12	2-MeC6H4CH2OH	2-MeC6H4CHO	12	94	11.05
13	4-Tert-butylC6H4CH2OH	4-Tert-butylC6H4CHO	11	65	9.84

^a Reaction conditions: Substrates (0.5 mmol), PW₁₁Zn@MCM-41-Im 96 (50 mg, 0.003 mmol), H₂O₂ (30%, 5 mmol), CH₃CN (3 mL), reflux, 10 to 15 h. ^b GC yield based on the starting alcohol.

Three paramagnetic metal complexes of 3-hydroxy-3,3′-biindoline-2,2′-dione (dihydroindolone, H₄ID) (MH₂ID) with Ni²⁺, Cu²⁺, and VO²⁺ ions with were synthesized and applied in oxidation reactions using aqueous H₂O₂ in acetonitrile. Although all the NiH₂ID (97), CuH₂ID (98), and VOH₂ID (99) catalysts showed good catalytic activity, chemo-, and regioselectivity, VOH₂ID had the highest potential due to more Lewis acid character and the high oxidation number of the central V⁴⁺ ion in 99 as compared with both Ni²⁺- and Cu²⁺-species in the same homogenous aerobic atmosphere [121]. As illustrated in Table 79, 99 afforded the best amount of the selective product such as benzaldehyde (85%) [122].

Table 79. Oxidation process catalyzed by VOH₂ID 99 using an aqueous H₂O₂ in acetonitrile ^a.

Entry	Substrate	Product	t/h	Conv. (%)			Conv. (%)	Sel. (%)	TON d	TOF e
				R b (Residual)	P c	Side Products
1	C6H5CH2OH	C6H5CHO	1	50	47	3	50	94	23.5	23.5
2			2	28	64	8	72	88	32	16
3			3	3	85	12	97	87	42.5	14.1
4			4	0	71	29	100	71	35.5	8.87

^a Reaction conditions: Benzyl alcohol (R) (1 mmol), VOH₂ID 99 (0.02 mmol), H₂O₂ (3 mmol), CH₃CN (10 mL), 85 °C, 4 h. ^b Residual amount (%) of the reactant after finishing the catalytic process (R). ^c Selectivity percentage of benzaldehyde (P), and the other side products. ^d TON (turn over number) is the ratio of moles of product obtained to the moles of catalyst. ^e Corresponding TOF (turn over frequency) (TON/h) is shown as mol (mol catalyst)⁻¹ h⁻¹.

Iron catalysts supported on porous furfuryl alcohol derived resins were synthesized and applied for the selective comparison of benzyl alcohol to benzaldehyde. As shown in Table 80, FeCl₂ (0.5 mol%) as a reference reaction was tested for the oxidation of 0.77 M benzyl alcohol with 2.3 eq. H₂O₂ in acetonitrile [123]. After microwave irradiation (5 min), 53% conversion with 78% selectivity to benzaldehyde was detected. Loading iron catalysts supported on P420 (Fe/P420 (100)) or P500resin (Fe/P500 (101)) with 10 times lower Fe content (0.05 mol%), similar selectivity with lower conversion in the product mixture was obtained. (Table 80 and Table 81) [123]. In addition, the selectivity in the final reaction mixture by preventing further oxidation to benzoic acid was optimized by continuous addition of H₂O₂ between 1.5 and 2.3 eq.

Table 80. Benzyl alcohol conversion and selectivity using Fe nanoparticles supported on P420 resin 100 as compared with free dissolved FeCl₂, using 1.5 and 2.3 equiv. H₂O₂ ^a.

Entry	Substrate	Product	Catalyst (mol%)		t/min	H2O2 (equiv.)	Conv. (%)	Sel. (%)
			Fe/P420	FeCl2				BzH
1	C6H5CH2OH	C6H5CHO	-	0.5	5	2.3	53	78
2			0.03	-	5	2.3
3			0.05	-	5	2.3	22	78
4			0.23	-	1.2	2.3	20
5			0.05	-	5	1.5
6			0.12	-	2.3	1.5

^a Reaction conditions: Substrate (0.77 M, 0.2 mL, 1.92 mmol), FeCl₂ (0.5 mol%), Fe/P420 100 (0.3 to 0.0.23 mol%), H₂O₂ (1.5 and 2.3 equiv., 0.3 mL of a 33 or 50 w/v% aqueous H₂O₂ solution), CH₃CN (2 mL), microwave irradiation (300 W), autogenous pressure (5 to 17 bar with an average of 16 bar), 90 to 132 °C, 1.2 to 5 min.

Table 81. Benzyl alcohol conversion and selectivity using Fe nanoparticles supported on P500 101 as compared with dissolved FeCl₂, using 1.5 and 2.3 equiv. H₂O₂ ^a.

Entry	Substrate	Product	Catalyst (mol%)		t/min	H2O2 (equiv.)	Conv. (%)
			Fe/P500	FeCl2
1	C6H5CH2OH	C6H5CHO	-	0.5	5	2.3
2			0.03	-	5	2.3	6–8
3			0.05	-	5	2.3
4			0.05	-	5	1.5
5			0.12	-	2.3	1.5
6			0.23	-	1.5	1.5

^a Reaction conditions: Substrate (0.77 M, 0.2 mL, 1.92 mmol), FeCl₂ (0.5 mol%), Fe/P500 101 (0.3 to 0.0.12 mol%), H₂O₂ (1.5 and 2.3 equiv., 0.3 mL of a 33 or 50 w/v% aqueous H₂O₂ solution), CH₃CN (2 mL), microwave irradiation (300 W), autogenous pressure (5 to 17 bar with an average of 16 bar), 90 to 132 °C, 2.3 to 5 min.

Metal molybdates were coupled with Co, Ni, and Cu with Mo, respectively, for the preparation of bimetallic complexes which was used successfully in alcohol oxidation reactions by Xinhua et al., in 2018 [124]. The 100% selectivity in conversion of the alcohols to corresponding carbonyl compounds showed noticeable performance of theses catalysts which could be reused three times. The results represented that CoMoO₄ (102) catalyst provided lower conversion and much higher selectivity (approximately 100%). In addition, non-terminal alcohols could be oxidized to ketones sufficiently (Table 82, Entries 2 and 3). Although NiMoO₄ (103) had near 100% selectivity, Cu₄Mo₃O₁₂ (104) provided no product (Table 82, Entry 1) due to the existence of a bimetallic combination effect in the oxidation reactions.

Table 82. Selective oxidation of different alcohols ^a.

Entry	Substrate	Product	Conv. (%) b	Yield (%) b
1	C6H5CH2OH	C6H5CHO	43 37 c,d - d	43 36 c,d - d
2	C6H5CH(OH)CH3	C6H5COCH3	37	37
3 c	(C6H5)2CHOH	(C6H5)2CO	30	30
4	4-BrC6H4CH2OH	4-BrC6H4CHO	46	46
5 e	1-Naphthylmethanol	1-Naphthaldehyde	13	13
6 e	4-NO2C6H4CH2OH	4-NO2C6H4CHO	25	25

^a Reaction conditions: Substrate (1 mmol), CoMoO₄ 102 (0.05 g, 23 mol%), H₂O₂ (30%, 0.23 g, 2 mmol), CH₃CN (5 mL), 70 °C, 18 h. ^b Calculated by GC with nonane as the internal standard. ^c With NiMoO₄ as the catalyst (0.05 g, 23 mol%). ^d With Cu₄Mo₃O₁₂ as the catalyst (0.04 g, 6 mol%). ^e Isolated by column chromatography.

Baghbanian et al. [125] immobilized 12-tungstophosphoric acid onto poly(N-vinylimidazole) which was modified by magnetic nanozeolite (MNZ@PVImW) (105). The results of successful catalytic oxidations of benzylic alcohols in the presence of synthesized catalyst under optimized reaction conditions are summarized in Table 83. In addition, without loss of catalytic activity, the catalyst could be reused eight times.

Table 83. Oxidation of various alcohols catalyzed by MNZ@PVImW 105 ^a.

Entry	Substrate	Product		t/min	Conv. (%) b	Yield (%)
		A	B
1	C6H5CH2OH	C6H5CHO	C6H5CO2H	30	96	95
2	4-MeC6H4CH2OH	4-MeC6H4CHO	4-MeC6H4CO2H	25	92	92
3	2-MeC6H4CH2OH	2-MeC6H4CHO	2-MeC6H4CO2H	25	91	90
4	4-OMeC6H4CH2OH	4-OMeC6H4CHO	4-OMeC6H4CO2H	25	96	95
5	3,4-OMeC6H3CH2OH	3,4-OMeC6H3CHO	3,4-OMeC6H3CO2H	20	91	90
6	4-NO2C6H4CH2OH	4-NO2C6H4CHO	4-NO2C6H4CO2H	60	82	80
7	3-NO2C6H4CH2OH	3-NO2C6H4CHO	3-NO2C6H4CO2H	90	79	75
8	4-ClC6H4CH2OH	4-ClC6H4CHO	4-ClC6H4CO2H	45	92	91
9	3-ClC6H4CH2OH	3-ClC6H4CHO	3-ClC6H4CO2H	45	92	90
10	4-BrC6H4CH2OH	4-BrC6H4CHO	4-BrC6H4CO2H	40	92	91
11	3-BrC6H4CH2OH	3-BrC6H4CHO	3-BrC6H4CO2H	40	90	89
12	C6H5CH(OH)CH3	C6H5COCH3	-	30	90	88
13	(C6H5)2CHOH	(C6H5)2CO	-	45	92	90
14	4-OMeC6H4CH(OH)C6H5	4-OMeC6H4COC6H5	-	40	91	90
15	4-ClC6H4CH(OH)C6H5	4-ClC6H4COC6H5	-	40	88	85

^a Reaction conditions: Substrate (1 mmol), MNZ@PVImW 105 (2 mg), H₂O₂ (4 mmol), CH₃CN (3 mL), 40 °C, 20 to 90 min. ^b Conversions were calculated based on initial mmol of benzyl alcohol.

A simple and efficient catalytic oxidation of benzylic and hetero-aryl alcohols to their corresponding carbonyl compound using recoverable heterobimetallic sodium-dioxidovanadium (V) complexes (106) was reported (Table 84) [126].

Table 84. Catalytic oxidation of alcohols by heterobimetallic vanadium(V) complexes 106 ^a.

Entry	Substrate	Product	t/h	Yield (%) b	TON	TOF (h−1)	Yield (%) c
1	C6H5CH2OH	C6H5CHO	3.4	89	445	130.88	93
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	4	91	455	113.75	94
3	4-NO2C6H4CH2OH	4-NO2C6H4CHO	4.3	80	400	93.02	87
4	C6H5CH(OH)CH3	C6H5COCH3	6	No reaction	-	-	-
5	C6H5CH(OH)Et	C6H5COEt	7.2	No reaction	-	-	-
6	3-ClC6H4CH2OH	3-ClC6H4CHO	4.5	74	370	82.22	79
7	4-MeC6H4CH2OH	4-MeC6H4CHO	4.5	88	400	97.78	90
8	3-MeC6H4CH2OH	3-MeC6H4CHO	5	86	430	86	88
9	2-MeC6H4CH2OH	2-MeC6H4CHO	5	84	420	84	86
10	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	4	86	430	107.5	88
11	Furfuryl alcohol	Furfural	3	85	425	141.67	89
12	2-Pyridinemethanol	Picolinaldehyde	4	89	445	111.25	92
13	2-Thiophenemethanol	2-Thiophenecarboxaldehyde	2.3	78	390	169.57	82
14	2,3,4-OMeC6H2CH2OH	2,3,4-OMeC6H2CHO	3.5	94	470	134.29	96

^a Reaction condition: Substrate (4.84 mmol), heterobimetallic dioxido-vanadium(V) complexes 106 (0.01 mmol), H₂O₂ (15%, 4.85 mmol), 70 °C, 2.3 to 7.2 h. ^b Isolated yield. ^c GC Yield.

The hydrothermal method was applied for the preparation of MCM-41 nanostructure-modified with vanadium, iron, and cobalt. Under optimized conditions, a molar ratio substrate/oxidant of 4/1 and 7 h of reaction, as summarized in Table 85, BzH was the main product with 7% and 12% yield for V-M(60) (107) and Fe-M(60) (108), respectively [127]. High TON (1100 mol/mol V), 95% selectivity to BzH, and 31.7% yield made V-M(60) performance better than 108 and Co-M(60) (109) due to effective dispersion of vanadium species in the framework that could be considered as the active sites for the oxidation reaction. Moreover, the catalyst could be effectively reused and recovered for at least three cycles without loss of its activity and selectivity.

Table 85. Benzyl alcohol oxidation with H₂O₂ on M-M(x) catalysts under standards conditions ^a.

Entry	Sample	Conv. of BzOH	Sel. (%)			BzH Yield (%)	TON
			BzH	BzA	BzB
1	V-M(60)	7.1 b (28.3) c	100	-	-	7.1	235
2	Fe-M(60)	13.3 b (53.2) c	90.5	6.0	3.5	12	50.5
3	Co-M(60)	0.9 b (3.5) c	100	-	-	0.9	2.1

^a Reaction conditions: BzOH/H₂O₂ molar ratio (4:1), M-M(x) (100 mg), CH₃CN (91.15 mmol, Sintorgan, 99.5%), 70 °C, 7 h. ^b BzOH conversion (%). ^c BzOH conversion (mol% of max).

Using a synthetic manganese catalyst for catalytic oxidation and oxidative kinetic resolution (OKR) of secondary alcohols in the presence of an environmentally benign oxidant hydrogen peroxide and a small amount of additive sulfuric acid provided the high yields of products (up to 93%) with excellent enantioselectivity (>90% ee in the OKR of secondary alcohols). Moreover, 111 provided a higher enantiomeric excess (90% ee) than 110 (65%) in 1-phenylethanol oxidation (Table 86, Entry 1 and footnote c) [128].

Table 86. Substrate scope of oxidative kinetic resolution of secondary alcohols using the 111/H₂O₂/H₂SO₄ catalytic system. ^a,b

Entry	Substrate	Product	H2O2 (equiv.)	Conv. (%)	ee (%)
1 c	C6H5CH(OH)CH3	C6H5COCH3	0.8	69	90
2	4-ClC6H4CH(OH)CH3	4-ClC6H4COCH3	0.8	74	92
3	3-ClC6H4CH(OH)CH3	3-ClC6H4COCH3	0.9	69	92
4	2-ClC6H4CH(OH)CH3	2-ClC6H4COCH3	0.9	68	96
5 d	4-NO2C6H4CH(OH)CH3	4-NO2C6H4COCH3	0.9	60	96
6 d	4-PhC6H4CH(OH)CH3	4-PhC6H4COCH3	0.9	65	93
7	C6H5CH(OH)Et	C6H5COEt	0.8	71	92
8	4-BrC6H4CH(OH)Et	4-BrC6H4COEt	0.8	69	96
9	4-ClC6H4CH(OH)Et	4-ClC6H4COEt	0.7	62	90
10	4-FC6H4CH(OH)Et	4-FC6H4COEt	0.8	68	96
11	3-FC6H4CH(OH)Et	3-FC6H4COEt	0.9	69	94
12	2-FC6H4CH(OH)Et	2-FC6H4COEt	0.8	61	90
13	C6H5CH(OH)iPr	C6H5COiPr	0.8	66	92
14	C6H5CH(OH)c-Hex	C6H5COc-Hex	0.8	65	92
15	C6H5CH(OH)(CH2)5Me	C6H5CO(CH2)5Me	0.8	64	93

^a Reaction conditions: An CH₃CN (0.5 mL) solution containing H₂O₂ (30%, 0.70 to 0.90 equiv.) was added dropwise into an CH₃CN (1 mL) solution containing secondary alcohol (0.5 mmol), 111 (0.2 mol%), and H₂SO₄ (1 mol%) by using a syringe pump at 0 °C for 1 h. ^b Conversion yields and ee values were determined by GC with a CP-Chirasil-Dex CB column. ^c When 110 was used as a catalyst under the identical reaction conditions, the conversion yields and ee values were 66% and 65%, respectively. ^d Conversion yields were calculated from the isolated products and the ee values were determined by HPLC with IA column.

A variety of picolinic/quinaldinic acids as ligands mixed in situ with Fe(OAc)₂ (112) catalyzed the H₂O₂ oxidation of 1-phenylethanol especially with 6-methylpicolinic acid (6-MepicH) and 4-chloropicolinic acid (4-ClpicH) (Table 87) [129]. The oxidation possesses proceeded using 35% aq. H₂O₂ in CH₃CN solution with isolated Fe complexes (113) as the catalyst (Table 88) [129]. In addition, the results showed that the redox potential of Fe III and the lability of picolinate or quinaldinate ligand were important factors for the catalytic reaction.

Table 87. Iron-catalyzed oxidation of 1-phenylethanol by various combinations of picolinic/quinaldinic acids ^a.

Entry	Picolinic Quinaldinic Acids		Conv. (%) b	Yield (%) b	Sel. (%) c
1	6-MepicH	picH	78	71	91
2	6-MepicH	4-OMepicH	74	69	93
3	6-MepicH	4-MepicH	70	63	93
4	6-MepicH	4-ClpicH	85 94 d	81 93 d 91 e	95 99 d
5	qnH	picH	46	39	85
6	qnH	4-OMepicH	35	25	71
7	qnH	4-MepicH	39	31	79
8	qnH	4-ClpicH	73	65	89
9 f	picH	-	57	33	59
10 f	4-OMepicH	-	13	8	59
11 f	4-MepicH	-	44	40	91
12 f	4-ClpicH	-	39	36	93
13 f	6-MepicH	-	25	20	77
14 f	qnH	-	9	6	60

^a Reaction conditions: 1-Phenylethanol (12 g, 0.1 mol), Fe(OAc)₂ 112 (0.5 equiv.), picolinic/quinaldinic acids (0.5 equiv. + 0.5 equiv.), H₂O₂ (35%, 2 equiv.), CH₃CN (2 mL), 25 °C, 30 min, and further stirring for 5 min, unless otherwise stated. ^b Determined by GC by using biphenyl as an internal standard. Average of two runs. ^c Yield/conversion × 100. ^d H₂O₂ (35%) in CH₃CN was used. ^e Yield of isolated product, 10 g scale. ^f 0.10 equiv. of acid were used.

Table 88. Catalytic oxidation of 1-phenylethanol by isolated Fe complexes 113 ^a.

Entry	Fe complex	Conv. (%)b	Yield (%) b	Sel. (%) c
1	[Fe(6-Mepic)2(pic)]	69	59	85
2	[Fe(6-Mepic)2(4-Mepic)]	66	56	85
3	[Fe(6-Mepic)2(4-Clpic)]	76 (67) d	65 (63) d	86 (95) d
4	[Fe(qn)2(4-Mepic)]	39	33	85
5	[Fe(qn)2(4-Clpic)]	76	64	84
6	[Fe(4-OMepic)3]	24	4	17
7	[Fe(4-Mepic)3]	8	6	75
8	[Fe(4-Clpic)3]	14	10	71
9	[Fe(pic)3]	24	13	54
10	[Fe(6-Mepic)3]	22	17	74

^a Reaction conditions: 1- Phenylethanol (12 g, 0.1 mol), Fe complex 113 (0.05 equiv.), H₂O₂ (35%, 2 equiv.), CH₃CN (2 mL), 25 °C, 30 min, and further stirring for 5 min, unless otherwise stated. ^b Determined by GC using biphenyl as an internal standard. Average of two runs. ^c Yield/conversion × 100. ^d 0.1 equiv. of CH₃COOH was added.

The coprecipitation approach was applied for hybrid chromium (VI)-based magnetic nanocomposite catalyst (Fe₃O₄@SiO₂@PPh₃@Cr₂O₇²⁻ (114)) preparation by Maleki et al., in 2016 [130]. This catalyst was used for the first report of using magnetic nanocomposites with ultrasonic irradiation for the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide at room temperature (Table 89). Total conversion of benzyl alcohol to benzaldehyde and the reusability of the catalyst at least five times were the benefit of this system.

Table 89. The oxidations of benzyl alcohol to benzaldehyde in the presence of Fe₃O₄@SiO₂@PPh₃@Cr₂O₇²⁻ nanocomposite 114 ^a.

Entry	Substrate	Product	Conv. (%) b	TON	TOF (min−1)
1	C6H5CH2OH	C6H5CHO	100	416	21.83

^a Reaction conditions: Substrate (5 mmol), Fe₃O₄@SiO₂@PPh₃@Cr₂O₇²⁻ 114 (0.05 g), H₂O₂ (30%, 0.6 mL), CH₃CN (2 mL), room temperature, 20 min, ultrasonic irradiation. ^b Obtained by gas chromatography mass spectrometry analysis.

Yadollahi et al. [131] first reported using transition metal-substituted polyoxometalate [PW₁₁ZnO₃₉]⁵⁻ (PW₁₁Zn) (POMs) supported on activated carbon (AC) for oxidation of alcohols in the presence of H₂O₂ and CH₃CN as solvent (Table 90). In addition, with high selectivity, the oxidation of p-hydroxybenzyl alcohol without oxidation of the hydroxyl group was carried out (Table 90, Entry 13) and showed no change in the catalytic activity/selectivity of PW₁₁Zn@AC (115) for at least five catalytic cycles of sequence loading.

Table 90. Oxidation of various alcohols with hydrogen peroxide catalyzed by PW₁₁Zn@AC 115 ^a.

Entry	Substrate	Product	t/min b	TOF (h−1) c
1	C6H5CH2OH	C6H5CHO	45	267
2	2-MeC6H4CH2OH	2-MeC6H4CHO	35	343
3	4-MeC6H4CH2OH	4-MeC6H4CHO	25	480
4	2-OMeC6H4CH2OH	2-OMeC6H4CHO	35	343
5	4-OMeC6H4CH2OH	4-OMeC6H4CHO	25	480
6	3-OMeC6H4CH2OH	3-OMeC6H4CHO	70	171
7	2-NO2C6H4CH2OH	2-NO2C6H4CHO	180	67
8	4-NO2C6H4CH2OH	4-NO2C6H4CHO	240	50
9	4-BrC6H4CH2OH	4-BrC6H4CHO	50	240
10	4-FC6H4CH2OH	4-FC6H4CHO	25	480
11	4-ClC6H4CH2OH	4-ClC6H4CHO	35	343
12	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	90	133
13	4-OHC6H4CH2OH	4-OHC6H4CHO	50	240
14	3-Br-2-HOC6H3CH2OH	3-Br-2-HOC6H3CH2OH	25	480

^a Reaction conditions: Substrate (1 mmol), PW₁₁Zn@AC 115 (5 μmol), H₂O₂ (30%, 1 mL), CH₃CN (3 mL), reflux, 25 to 240 min. ^b Yields are quantitative and refer to GC yields. ^c Turnover frequency.

Two thiosemicarbazide Schiff bases, 1-(4-dimethylaminobenzylidene) thiosemicarbazide (ABTSC) and 1-(2-pyridincarboxyl-idene) thiosemicarbazide (TCTS), were applied for the coordination with Co(II), Ni(II), Zn(II), Cd(II), and Ag(I) transition metal salts, chloride, and acetate, and then under optimized conditions proceeded successfully for the oxidation of benzylic alcohols, Table 91 [132]. The environmentally friendly oxidation catalytic systems exhibited great activities in successive runs without loss in activity. The best results under the optimum conditions were obtained with Co(ABTSC)₂(OAc)₂ (116) catalyst as in 4-methoxybenzyl alcohol oxidation to aldehyde with 95% conversion and 100% benzaldehyde selectivity.

Table 91. Oxidation benzyl alcohol derivatives for complex [Co(ABTSC)₂(OAc)₂] 116 ^a.

Entry	Substrate	Product	Yield (%)	Sel. (%)
1	C6H5CH2OH	C6H5CHO	85	100
2	2-NO2C6H4CH2OH	2-NO2C6H4CHO	65	97
3	3-NO2C6H4CH2OH	3-NO2C6H4CHO	65	97
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	75	100
5	2-OMeC6H4CH2OH	2-OMeC6H4CHO	85	100
6	3-OMeC6H4CH2OH	3-OMeC6H4CHO	90	100
7	4-OMeC6H4CH2OH	4-OMeC6H4CHO	95	100

^a Reaction conditions: Substrate (1 mmol), [Co(ABTSC)₂(OAc)₂] 116 (0.03 mmol), H₂O₂ (30%, 3 mmol), CH₃CN (3 mL), 82 °C, 80 min.

Nanocrystalline cobalt aluminate (CoAl₂O₄), using both conventional (sample A: CoAl₂O₄-CCM) (117) and microwave combustion method (sample B: CoAl₂O₄-MCM) (118), was synthesized for comparative investigation in various alcohol oxidations to corresponding carbonyl compounds, as shown in Table 92 and Table 93 [133]. 117 showed weaker ferromagnetic in nature as compared with CoAl₂O₄-MCM and both also showed a lower conversion rate in the oxidation of benzyl alcohol [134]. This result was confirmed more powerful for 118 and similar results were achieved within a shorter time. In addition, both catalytic systems catalytic were reusable activity/selectivity during at least five catalytic cycles.

Table 92. Oxidation of substituted alcohols to aldehydes using cobalt aluminate (CoAl₂O₄) (sample A) 117 under the optimum conditions ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	74.11	81.34
2	4-NO2C6H4CH2OH	4-NO2C6H4CHO	63.09	79.12
3	C6H5CH(OH)CH3	C6H5COCH3	63.45	83.25
4	C6H5CH(OH)Et	C6H5COEt	58.34	80.94
5	C6H5CH2OH	C6H5CHO	80.91	98.68

^a Reaction conditions: Substrate (5 mmol), CoAl₂O₄ (sample A) 117 (0.5 g), H₂O₂ (5 mmol), CH₃CN (5 mmol), 80 °C, 8 h.

Table 93. Oxidation of substituted alcohols to aldehydes using cobalt aluminate (CoAl₂O₄) (sample B) 118 under optimum conditions ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	73.71	78.84
2	4-NO2C6H4CH2OH	4-NO2C6H4CHO	58.07	74.89
3	C6H5CH(OH)CH3	C6H5COCH3	78.54	85.43
4	C6H5CH(OH)Et	C6H5COEt	71.23	81.87
5	C6H5CH2OH	C6H5CHO	95.98	98.90

^a Reaction conditions: Substrate (5 mmol), CoAl₂O₄ (sample B) 118 (0.5 g), H₂O₂ (5 mmol), CH₃CN (5 mmol), 80 °C, 8 h.

The sol-gel technique via hydrolysis of tetraethyl orthosilicate (TEOS) was applied for the preparation of mono substituted Keggin type POMs, [n-C₄H₉)₄N]_x[PW₁₁MO₃₉]·nH₂O (PWM) (M = Cr, Mn, Fe, Co, Ni, and Cu) in silica matrix (PWM/SiO₂) and were examined in the oxidation reactions of different aromatic alcohols with aqueous 30% H₂O₂. Because for the preparation of benzaldehyde, PWFe/SiO₂ (119) composite showed the highest activity (80% yields) among all of catalytic systems, further oxidation was performed by this catalyst (Table 94) [33]. Benzyl alcohol its derivatives with electron donating and withdrawing groups in para-, orto- and meta-positions, provided yields as excellent as primarily aromatic alcohols to the corresponding aldehydes. The 119 catalyst could also be recovered for five oxidation runs.

Table 94. Oxidation of alcohols with hydrogen peroxide catalyzed by PWFe/SiO₂ 119 ^a.

Entry	Substrate	Product	t/h	Yield (%) b
1	C6H5CH2OH	C6H5CHO	4	92
2	2-MeC6H4CH2OH	2-MeC6H4CHO	3	98
3	3-MeC6H4CH2OH	3-MeC6H4CHO	3	100
4	4-MeC6H4CH2OH	4-MeC6H4CHO	2.5	100
5	4-(tert-Butyl)C6H4CH2OH	4-(tert-Butyl)MeC6H4CHO	4	65
6	2-OMeC6H4CH2OH	2-OMeC6H4CHO	4.5	92
7	4-OMeC6H4CH2OH	4-OMeC6H4CHO	2.5	100
8	3-BrC6H4CH2OH	3-BrC6H4CHO	3.15	98
9	4-BrC6H4CH2OH	4-BrC6H4CHO	3	99
10	2-BrC6H4CH2OH	2-BrC6H4CHO	4.5	96
11	2-NO2C6H4CH2OH	2-NO2C6H4CHO	3	60
12	4-NO2C6H4CH2OH	4-NO2C6H4CHO	3	55
13	4-OHC6H4CH2OH	4-OHC6H4CHO	4	85
14	2-OHC6H4CH2OH	2-OHC6H4CHO	5	75
15	4-ClC6H4CH2OH	4-ClC6H4CHO	3.15	99
16	2,4-ClC6H3CH2OH	2,4-ClC6H3CHO	50	96
17	3-IC6H4CH2OH	3-IC6H4CHO	4	90
18	C6H5CH(OH)Et	C6H5COEt	2.5	99

^a Reaction condition: Substrate (1 mmol), PWFe/SiO₂ 119 (0.03 mmol), H₂O₂ (30%, 1 mL), CH₃CN (3 mL), reflux, 2.5 to 50 h. ^b Yields refer to GC yields.

Oxidation of para-substituted phenyl methyl sulfides and benzyl alcohols with H₂O₂ in acetonitrile solution were examined by a series of diiron (III) complexes of 1,3-bis(2′-arylimino)isoindoline, [(Fe(L)Cl)₂O], and 1,4-di-(2′-aryl)aminophthalazine, [Fe₂(µ-OMe)₂(H₂L)Cl₄], 1,4-di-(4′-methyl-2′-thiazolyl)aminophthalazine, and 1,4-di-(2′-benzthiazolyl)-aminophthalazine (Table 95) [135]. The results of the [Fe₂(µ-O)(L^3,5,7)₂Cl₂] (120–122) catalysts confirmed that complex 120 provided the highest conversion because of catalyzing both oxygen-atom transfer and hydrogen-atom abstraction. In addition, among phthalazine-based complexes, the best oxidative activity was found with 124, since benzyl alcohol gave the benzaldehyde with 100% selectivity by catalysts in all these reactions, and no other products were provided.

Table 95. Results of the oxidation of benzyl alcohols catalyzed by isoindoline- ([Fe₂(µ-O)(L^3,5,7)₂Cl₂]) (120–122) and phthalazine-based ([Fe₂(µ-OMe)₂(H₂^4,6,8)Cl₄]) (123–125) diiron complexes ^a.

Entry	Catalyst	Substrate 4′R-PhCH2OH	t/h	Conv. (%)	Product distribution (%)
					4′R-PhCHO	4′R-PhCO2H
1	[Fe2(µ-O)(L3)2Cl2]	H H	1 2	2.4 12.5	100 100	0 0
2	[Fe2(µ-O)(L5)2Cl2]	H H	1 2	0.97 0.97	100 100	0 0
3	[Fe2(µ-O)(L7)2Cl2]	H H	1 2	0.15 1.2	100 100	0 0
4	[Fe2(µ-OMe)2(H2L4)Cl4]	H H	1 2	7 11	100 100	0 0
5	[Fe2(µ-OMe)2(H2L6)Cl4]	H H OMe OMe Me Me Cl Cl NO2 NO2	1 2 1 2 1 2 1 2 1 2	9.5 12 10 13 9 11 9 10 4 5	100 100 100 100 100 100 100 100 100 100	0 0 0 0 0 0 0 0 0 0
6	[Fe2(µ-OMe)2(H2L8)Cl4]		0.17 b	3	100	0

^a Reaction conditions: Substrate (1 mmol), ([Fe₂(µ-O)(L^3,5,7)₂Cl₂]) (120–122) and ([Fe₂(µ-OMe)₂(H₂^4,6,8)Cl₄]) (123–125) (10 µmol), H₂O₂ (2.5 mmol), CH₃CN (5 mL), 20 °C, 1 to 2 h. ^b Complex precipitation after 10 min.

The oxidation of alcohols employing H₂O₂ as oxidant by manganese-containing catalytic system [Mn^IV,IV₂O₃(tmtacn)₂]²⁺ (126)/carboxylic acid (where tmtacn = N,N’,N’’-trimethyl-1,4,7-triazacyclononane was reported (Table 96) [136]. Co-catalyst trichloroacetic acid provided the most active catalyst system and side reactions due to side products were not observed [137,138].

Table 96. Oxidation of secondary alcohols to ketones using H₂O₂ catalyzed by [Mn^IV,IV₂O₃(tmtacn)](PF₆)₂ (126) ^a.

Entry	Substrate	Product	Conv. (%) b	Yield (%) c
1	C6H5CH(OH)CH3	C6H5COCH3	100	77
2	C6H5CH(OH)Et	C6H5COEt	100	63
3	(C6H5)2CHOH	(C6H5)2CO	100	93

^a Reaction conditions: Substrate (3 mmol), [Mn^IV,IV₂O₃(tmtacn)](PF₆)₂ 126 (0.1 mol%), Cl₃CCO₂H (1 mol%), H₂O₂ (50%, 1.45 equiv.), CH₃CN ([substrate] = 1M), room temperature, 16 h. ^b Conversion was determined by ¹H NMR spectroscopy. ^c Isolated yields.

As illustrated in Table 97, an in situ prepared catalyst based on manganese(II) salts (Mn(ClO₄)₂·6H₂O (127)), pyridine-2-carboxylic acid, and butanedione with H₂O₂ as oxidant at ambient temperatures as catalyst system provided good-to-excellent yields and conversions with high TONs (up to 10,000) [139]. In addition, secondary alcohols were converted to ketones selectively in substrates bearing multiple alcohol groups which reduced the need to introduction protecting groups prior to the oxidation and subsequent removal. In general, the results confirmed that benzyl CH oxidation proceeded in preference to aliphatic CH oxidation.

Table 97. Oxidation of secondary alcohols ^a.

Entry	Substrate	Product	Conv. (%) b	Yield (%) c
1	C6H5CH(OH)CH3	C6H5COCH3	97	90
2	C6H5CH(OH)Et	C6H5COEt	92	77
3	(C6H5)2CHOH	(C6H5)2CO	90	80
4	1-Tetralol	1-Tetralone	88	75
5 d	4-OMeC6H4CH(OH)CH3	4-OMeC6H4COCH3	70	64
6	4-BrC6H4CH(OH)CH3	4-BrC6H4COCH3	78	76

^a Reaction conditions: Substrate (0.5 M, 1 mmol), Mn(ClO₄)₂·6H₂O 127 (50 µM, 0.01 mol%), pyridine-2-carboxylic acid (2.5 mM, 0.5 mol%), NaOAc (5 mM, 1 mol%), butanedione (0.25 M, 0.5 equiv.), H₂O₂ (50%, 1.5 M, 3 equiv.) in CH₃CN, room temperature, 12 to 16 h. ^b Conversions and yields based on the substrate, determined by ¹H NMR spectroscopy. ^c Isolated yields, unless stated otherwise. ^g Substrate (0.25 M, 0.5 mmol).

Chemoselective oxidation of alcohols to their corresponding carbonyl compounds in short reaction times and high yields were reported by an efficient protocol including H₂O₂ as oxidant at 80 °C in acetonitrile in the presence of nanoparticles of Fe₂O₃-SiO₂ (128) (Table 98) [140]. The catalyst was successfully recycled at least five times with no substantial loss in the activity of the reused catalyst.

Table 98. Oxidation of various alcohols catalyzed by (9.6 wt% Fe) Fe₂O₃-SiO₂/Imidazole/H₂O₂ ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%) b	TON
1 d	C6H5CH2OH	C6H5CHO	94	100	55
2 d	4-ClC6H4CH2OH	4-ClC6H4CHO	100	100	58

^a Reaction conditions: Substrate (1 mmol), Fe₂O₃-SiO₂ 128 (9.6 wt% Fe, 10 mg, 17.2 µmol), imidazole (40 µmol), H₂O₂ (30%, 14 mmol), CH₃CN (3 mL), 80 ± 1 °C, 5 h. Conversions and yields are based on the starting substrate. ^b All products were identified by comparison of their physical and spectral data with those of authentic samples or GC-MS. ^c Reaction time = 4 h.

In the presence of H₂O₂, a reusable (three times) and high stable heteropolyoxometalates [C₇H₇N(CH₃)₃]₃PMo₄O₁₆ (BTPM) (140) catalyst was catalyzed for selective oxidation of BzH with selectivity more than 99% and 92.8% of BzOH conversion (Scheme 1) [141].

Scheme 1. Process of the catalytic oxidation of BzOH.

Vanadium phosphorus oxide (VPO) (129) was introduced as an operational catalyst in the presence of hydrogen peroxide for the preparation of aldehydes and ketones from alcohol oxidations without employing any sacrificial oxidants or base at 65 to 70 °C in acetonitrile solvent (Table 99) [142]. Although both activated and non-activated alcohols were selectively and efficiently converted to the corresponding carbonyls with no producing byproducts, activated alcohols such as benzhydrol, benzyl alcohol, and 1-phenylethanol (Entries 1, 2, and 3, respectively) afforded a much higher conversion. The catalyst could also be easily reused for five successive cycle runs with a slight decrease in its efficiency.

Table 99. Oxidation of various alcohols over VPO 129 catalyst using H₂O₂ and acetonitrile ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)	TON
1	(C6H5)2CHOH	(C6H5)2CO	52	100	80
2	C6H5CH2OH	C6H5CHO	66	78 b	102
3	C6H5CH(OH)CH3	C6H5COCH3	77	100	118
4	2-FC6H4CH2OH	2-FC6H4CHO	42	62 c	65

^a Reaction conditions: Substrate (10 mmol), VPO 129 (10 mg), H₂O₂ (30%, 40 mmol), CH₃CN (10 mL), 65 °C, 4 h, stir, N₂. ^b Remaining benzoic acid. ^c Remaining fluoro benzoic acid.

An environmentally friendly protocol including hydrogen peroxide, liquid phase at atmospheric pressure over Fe³⁺/montmorillonite-K10 (130) catalyst synthesized by the ion-exchange method at pH = 4 in acetonitrile solvent was proposed for the oxidation of various primary and secondary aromatic, as illustrated in Table 100 [143]. Oxidation of the -OH group adjacent to the benzene ring (activated alcohols) was easier such as in 1-phenylethanol (Entry 1) and benzyl alcohols (Entries 2–3) and also it showed higher conversions (86% to 95%). The catalyst was also reused for five times.

Table 100. Oxidation of various alcohols over Fe³⁺/K10 130 catalyst using H₂O₂ and acetonitrile ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)	TON
1	C6H5CH(OH)CH3	C6H5COCH3	86	95 b	48
2	C6H5CH2OH	C6H5CHO	>95	32 c	54
3	3-ClC6H4CH2OH	3-ClC6H4CHO	>95	5 c	54
4	4-FC6H4CH2OH	4-FC6H4CHO	>95	41 d	54

^a Reaction conditions: Substrate (12.5 mmol), Fe³⁺/K10 130 (0.25 g), H₂O₂ (30%, 50 mmol), CH₃CN (10 mL), 65 °C, 8 h, stir, air. ^b 4 h, remaining benzaldehyde. ^c 4 h, remaining respective benzoic acids. ^d 0.5 h, remaining fluoro benzoic acid.

2.3.2. Toluene Solvent

Oxo-vanadium (V=O) was immobilized on the surface of Fe₃O₄@PDA [VO(PDA)@Fe₃O₄] as a magnetic adsorbent and stabilizing agent. VO(PDA)@ Fe₃O₄ (131) nanoparticles demonstrated high catalytic activity as a recyclable catalyst in the chemoselective oxidation of benzylic alcohols to aldehydes using hydrogen peroxide in high yield (Table 101) [144]. The synthesized catalyst could be reused seven times and could be easily separated.

Table 101. Oxidation of benzylic alcohols catalyzed by VO(PDA)@Fe₃O₄ 131 ^a.

Entry	Substrate	Product	t (h)	Yield (%)
1	C6H5CH2OH	C6H5CHO	6	96
2	4-ClC6H4CH2OH	4-ClC6H4CHO	8	90
3	4-MeC6H4CH2OH	4-MeC6H4CHO	8	88
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	12	90
5	(C6H5)2CHOH	(C6H5)2CO	8	92
6	C6H5CH(OH)CH3	C6H5COCH3	10	90

^a Reaction conditions: Substrate (1 mmol), VO(PDA)@Fe₃O₄ 131 (60 mg, 1.5 mol%), H₂O₂ (30%, 2.5 mmol), toluene (5 mL), reflux, 6 to 12 h.

The deposition-precipitation method was applied for the preparation of a series of bimetallic copper-nickel (CuNix, x = 0.1, 0.2, 0.5, and 1) nanoparticles supported on activated carbon (AC) for assessing the effect of different ratios of Ni added into the Cu catalyst. A comparison between bimetallic Cu-Ni nanoparticles and monometallic Cu and Ni nanoparticles showed that the catalytic activity increased in the presence of the CuNi₁/AC (132) catalyst for the oxidation of benzyl alcohols to the corresponding aldehyde in a short reaction time at 80 °C (Table 102) [145].

Table 102. Catalytic performance of the catalysts for the oxidation of benzyl alcohol to benzaldehyde ^a.

Entry	Catalyst	Conv. (%)
1	Cu/AC	13.8
2	Ni/AC	33.6
3	CuNi0.1/AC	35.1
4	CuNi0.2/AC	36.5
5	CuNi0.5/AC	37.1
6	CuNi1/AC	46.8

^a Reaction conditions: Benzyl alcohol (2.6 mL), catalyst (0.1 g), H₂O₂ (30%, 3.3 mL), toluene (20 mL), 80 °C, 2 h.

Redox reaction between in situ generated zero-valent cobalt nanomaterial and KMnO₄ produced gram-scale synthesis of recoverable heterobimetallic Co-Mn oxide (Co₂Mn₃O₈) (133) in water was successfully applied for oxidation of benzyl, aliphatic, cinamyl, pyridine, and thiophene moiety alcohols to aldehydes/ketones, both in the presence of hydrogen peroxide, with excellent selectivity (>99%) and yield (Table 103) [146].

Table 103. Substrate scope for Co₂Mn₃O₈-promoted oxidation of a variety of alcohols in the presence of hydrogen peroxide ^a.

Entry	Substrate	Product	t/h	Conv. (%)	TOF (h−1)
1	4-OMeC6H4CH2OH	4-OMeC6H4CHO	10	85	170 b
2	Indole-3-carbinol	Indole-3-carboxaldehyde	12	78	130 b
3	2-Thiophenemethanol	2-Thiophenecarboxaldehyde	12	72	120 b
4	2-Pyridinemethanol	Picolinaldehyde	10	80	160 b
5	C6H5CH(OH)CH3	C6H5COCH3	10	95	190 c
6	(C6H5)2CHOH	(C6H5)2CO	10	85	170 c

^a Reaction conditions: Substrate (10 mmol), Co₂Mn₃O₈ 133 (0.05 mol%), H₂O₂ (0.7 mL), toluene (30 mL), 130 °C, 10 to 12 h. ^b Yields were calculated from GC with o-xylene as an internal standard. ^c NMR yield.

The selective oxidation of alcohols catalyzed successfully by Fe₃O₄@mSiO₂/NH-PV₂W (134) as a novel magnetically recyclable catalyst was prepared by immobilization of divanadium-substituted Keggin phosphotungstic acid H₅PV₂W₁₀O₄₀ on mesoporous silica-coated Fe₃O₄ core-shell nanoparticles (Table 104) [147]. With an external magnetic field, the catalyst was easily separated after the completion of the reactions and without any loss of its catalytic activity, the recovered catalyst could be reused at least five times. Although the catalyst showed high catalytic activities with 98% and 99% selectivity for conversion of benzyl alcohol to benzaldehyde, the secondary benzyl alcohols were reluctantly oxidated to corresponding ketones such as 40% and 36% conversion oxidation for 1-phenylethanol, as shown in Entry 4.

Table 104. Selective oxidation of different alcohols catalyzed by Fe₃O₄@SiO₂/NH-V₂PW (20%) 134 ^a.

Entry	Substrate	Product	Conv. (%)	Sel. (%)
1	C6H5CH2OH	C6H5CHO	98	99
2	3-MeC6H4CH2OH	3-MeC6H4CHO	97	99
3	4-MeC6H4CH2OH	4-MeC6H4CHO	98	99
4	C6H5CH(OH)CH3	C6H5COCH3	40	99

^a Reaction condition: Substrate (1 mM), Fe₃O₄@SiO₂/NH-V₂PW 134 (0.02 g), H₂O₂ (2 mL), toluene (2 mL), 80 °C, 8 h.

2.3.3. Acetic Acid Solvent

A simple combination included NaBr (135), H₂O₂, and AcOH was overlooked among many reported catalytic systems for catalytic oxidations of alcohols [148,149,150]. Oxidation of secondary benzylic alcohols bearing substituents such as Cl, Br, NO₂, and diarylmethanol derivatives in aqueous hydrogen peroxide as terminal oxidant were explored by this system with excellent yields (Table 105) [151].

Table 105. Substrate scope under optimized conditions ^a.

Entry	Substrate	Product	Yield (%)
1	C6H5CH(OH)n-Oct	C6H5COn-Oct	98
2	4-ClC6H4CH(OH)n-Pr	4-ClC6H4COn-Pr	93
3	4-BrC6H4CH(OH)CH3	4-BrC6H4COCH3	>99
4	4-NO2C6H4CH(OH)CH3	4-NO2C6H4COCH3	95
5	3-NO2C6H4CH(OH)CH3	3-NO2C6H4COCH3	95
6	1-(Naphthalen-2-yl)propan-1-ol	1-(Naphthalen-2-yl)propan-1-one	91
7	4-OMeC6H4CH(OH)CH3	4-OMeC6H4COCH3	7
8	C6H5CH(OH)(CH2)2Cl	C6H5CO(CH2)2Cl	93
9	(C6H5)2CHOH	(C6H5)2CO	98
10	1-Naphthyl(phenyl)methanol	1-Benzoylnaphthalene	90
11	9-Fluorenol	9-Fluorenone	97

^a Reaction conditions: Substrate (0.5 mmol), NaBr 135 (10 mol%), H₂O₂ (2 equiv.), CH₃COOH (1 mL), 60 °C, 2 h.

Under optimized conditions, substrate having p-methoxypenyl group provided the oxidation product c′ with poor yield due to formation of major product 4-methoxy-α-methylbenzyl acetate (d) and 4-methoxy-α-methylbenzyl hydroperoxide (e). Without NaBr catalyst, d and e products were largely prepared which indicated that the formation of these products occurred independently from the oxidation reaction pathway. With more experience, the best optimized condition was performed in AcOH-EtOAc (3:7, 0.25 M), giving the desired ketone c′ in 86% isolated yield (Table 106, Entries 1–5).

Table 106. Optimization of conditions for substrate c ^a.

Entry	Solvent	Yield of c′ (%) b	Yield of d (%) b	Yield of e (%) b
1	AcOH	7	27	15
2	AcOH-EtOAc (7:3)	31	31	10
3	AcOH-EtOAc (5:5)	47	13	7
4	AcOH-EtOAc (3:7)	60	5	4
5 c	AcOH-EtOAc (3:7)	86 d	Trace	Trace

^a Reaction conditions: p-Methoxybenzyl alcohol (0.5 mmol), NaBr 135 (10 mol%), solvent (1 mL), 60 °C for 2 h. ^b Determined by ¹H NMR analysis of crude material. ^c Volume of solvent used was 2 mL. ^d Isolated yield.

As illustrated in Table 107, this protocol was successfully applied for the preparation of corresponding oxidative products f′–j′ a variety of substrates such as 1-o-methoxyphenyl-1-ethanol (f), cyclopropylphenylmethanol (g), and various cyclic substrates h–j with good yields (Table 107) [150,151].

Table 107. The preparation of corresponding oxidative products f′–j′ ^a.

Entry	Substrate	Product	Yield (%)
1	4-OMeC6H4CH(OH)CH3	4-OMeC6H4COCH3 (c′)	86
2	2-OMeC6H4CH(OH)CH3	2-OMeC6H4COCH3 (f′)	73
3 b	C6H5CH(OH)c-Pr	C6H5CH(OH)c-Pr (g′)	61
4	3,3-Dimethyl-1-indanol	3,3-Dimethyl-1-indanone (h′)	70
5	4,4-Dimethyl-1-tetralol	4,4-Dimethyl-1-tetralone (i′)	73
6	2,3,4,5-Tetrahydrobenzo[b]oxepin-5-ol	3,4-Dihydrobenzo[b]oxepin-5(2H)-one (j′)	66

^a Reaction conditions: Substrate (0.5 mmol), NaBr 135 (10 mol%), CH₃COOH (2 mL), 60 °C, 2 h. ^b NaBr 135 (20 mol%).

Heterogeneous catalytic oxidation of preyssler type heteropolyacid supported onto silica gel, H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂ (136) was reported for the converting of substituted benzyl alcohols converted to the corresponding benzaldehyde derivatives, as shown in Table 108 [26]. Using 0.03 g H₁₄-P₅ supported onto silica gel in refluxing acetic acid represented the best results with excellent yields. After extended reaction times, overoxidation products such as carboxylic acids were not observed and the catalytic activity remained after three runs.

Table 108. Oxidation of alcohols using H₁₄-P₅ supported onto silica gel 136 ^a.

Entry	Substrate	Product	t/min	Yield (%) a
1	C6H5CH2OH	C6H5CHO	13	99
2	2-OHC6H4CH2OH	2-OHC6H4CHO	10	98
3	2-MeC6H4CH2OH	2-MeC6H4CHO	12	99
4	4-BrC6H4CH2OH	4-BrC6H4CHO	14	99
5	4-NO2C6H4CH2OH	4-NO2C6H4CHO	15	98
6	Furfuryl alcohol	Furfural	30	97

^a Reaction conditions: Substrate (1 mmol), H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂ 136 (0.03 g), CH₃COOH (5 mL), reflux, 10 to 30 min. ^b Yields analyzed by GC.

2.3.4. Dimethylacetamide Solvent

As shown in Table 109, a series of heteropolytungstates catalysts, Q₃PW₁₂O₄₀ (137), Q₃PMo₁₂O₄₀ (138), and Q₃PMo₄O₂₄ (139) (Q⁺ represents [C₇H₇N(CH₃)₃]⁺), were applied for the BzOH oxidation reactions by Jian [152]. Using 137 and 138 as catalyst needed no feature of the reaction-controlled phasetransfer catalysis, within the determined reaction time, the catalysts were still soluble in the solvent, and the reaction conversions were poor (38.5% and 24.6%, respectively). Applying [C₇H₇N(CH₃)₃]₉PW₉O₃₄ (140) as catalyst under optimal conditions, DMAc, H₂O₂/BzOH:1, 95.0% conversion of benzyl alcohol (based on H₂O₂) and over 99% selectivity of benzaldehyde was achieved. In addition, the catalyst could be reused and recycled three times.

Table 109. Catalytic oxidation of BzOH by various heteropolyoxometalates with H₂O₂ ^a.

Entry	Catalyst	Solvent	t/h	Solubility of Catalyst		Conv. (%)	TON (mmol)/Catalyst (mmol)	TOF (h−1)
				During Reaction	After Reaction
1	Q3PW12O40	DMAc	3.5	soluble	soluble	38.5	87	25
2	Q9PW9O34	DMAc	0.5	soluble	insoluble	95	214	428
3	Q3PMo12O40	CH3CN	3.5	soluble	soluble	24.6	55	16
4	Q3PMo4O24	CH3CN	3.5	soluble	insoluble	92.8	209	60

^a Reaction conditions: Catalyst/H₂O₂/alcohol molar ratio (1:225:250), DMAc (10 mL), 80 °C, 0.5 to 3.5 h.

2.3.5. Methanol Solvent

As presented in the Table 110, Dabiri et al. [153] reported oxidation of alcohols to the related carbonyl compounds using a vanadium Schiff base complex on nano silica catalyst (VSBC@NS) (141). Nano silica NH₂-functionalized, 2,4-dihydroxy benzaldehyde, and VO(acac)₂ were reacted and provided heterogeneous 141 catalysts which showed good efficiency, chemical stability, and at least five times reusability in selective oxidation of alcohols to aldehydes.

Table 110. Oxidation of alcohols ^a.

Entry	Substrate	Product	Conv. (%)	Yield (%) b	TON	TOF (h−1)
1	C6H5CH2OH	C6H5CHO	98	95	4.75	1.19
2	4-MeC6H4CH2OH	4-MeC6H4CHO	93	89	4.45	1.11
3	4-ClC6H4CH2OH	4-ClC6H4CHO	90	88	4.4	1.1
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	81	76	3.8	0.95
5	3-NO2C6H4CH2OH	3-NO2C6H4CHO	84	81	4.05	1.01
6	1-Indanol	1-Indanone	94	91	4.55	1.14
7	C6H5CH(OH)CH3	C6H5COCH3	89	86	4.3	1.08

^a Reaction conditions: Substrate (0.5 mmol), VSBC@NS 141 (20 mol%), H₂O₂ (1.2 equiv.), MeOH (2 mL), reflux, 4 h. ^b GC yield, n-dodecane was used as an internal standard.

2.3.6. tert-Buthanol Solvent

Tandon and co-workers [154] reported a quasi-homogenous reaction system included using 1:1.5 t-butanol-H₂O_2, not expensive PTC tetrabutyl ammonium hydrogen sulfate (TBAHSO₄) as the phase transfer and heteropolyanion Na₂WO₄ (142) as the catalyst for simple secondary alcohol oxidations to ketones at 90 °C. As shown in Table 111, 100% yield of products in 30 min confirmed the power of this catalytic system.

Table 111. Oxidation of secondary alcohols with hydrogen peroxide in tert-butanol ^a.

Entry	Substrate	Product	Yield (%)
1	1-Naphthaleneethanol	1-Acetonaphthone	98
2	(C6H5)2CHOH	(C6H5)2CO	100
3	C6H5CH(OH)CH3	C6H5COCH3	100

^a Reaction conditions: Substrate (1.84 g, 10 mmol), Na₂WO₄ 142 (164 mg, 0.5 mmol), TBAHSO₄ (169 mg, 0.5 mmol), H₂O₂ (30%, 1.72 mL, 15 mmol), t-BuOH (2 mL), 90 °C, 30 min.

Oxidation of benzylic alcohols also provided corresponding aldehydes or acids with good yields (Table 112) [154]. Substitution of the aromatic ring influenced the state of oxidation like p-methoxybenzaldehyde was produced with 87% yield from p-methoxybenzyl alcohol in four hours and further oxidation did not occur; whereas p-nitrobenzaldehyde (62%) and p-nitrobenzoic acid (30%) were produced from p-nitrobenzyl alcohol oxidation. In addition, non-benzylic primary alcohols were directly converted to their corresponding carboxylic acids (Table 112, Entry 3). The results showed that, generally, secondary alcohols were more reactive towards oxidation than primary alcohols.

Table 112. Oxidation of primary alcohols with hydrogen peroxide in tert-butanol.

Entry	Substrate	Product	Yield (%)
1	C6H5CH2OH	C6H5CHO a	75 g
2	C6H5CH2OH	C6H5CO2H b	100
3	C6H5CH2OH	C6H5CH2CO2H b	84
4	4-OMeC6H4CH2OH	4-OMeC6H4CHO c	87 h
5	4-OMeC6H4CH2OH	4-OMeC6H4CH2CO2H d	87
6	4-NO2C6H4CH2OH	4-NO2C6H4CHO b,e	62
7	4-NO2C6H4CH2OH	4-NO2C6H4CHO e,f	52

Reaction conditions: Alcohol/H₂O₂/Na₂WO₄/PTC = ^a 20:30:1:1 (8 h), ^b 20:40:1:1, ^c 20:30:1:1, ^d 20:50:1:1, ^e 18 h, ^f 100:200:1:2 (18 h). ^g Remaining 25%: benzoic acid. ^h Rest is unreacted alcohol.

2.3.7. Xylene Solvent

Hayashi, in 2012 [155], reported using 30% H₂O₂ in the presence of activated carbon (143) for the oxidation of secondary benzylic alcohols to ketones (Table 113). At first, 9-fluorenol was selected as a substrate (Table 114) and 9-fluorenone was obtained with 77% yield (100 weight% of activated carbon, m-xylene, 95 °C, 18 h). When ten equivalents of 30% H₂O₂ were used, less (4 equiv.) and more amounts (15 equiv.) of 30% H₂O₂ caused a decrease of yield (Table 114, Entries 1 (68%) and 3 (35%), respectively). After drying, the activated carbon was recovered and reused without deactivation.

Table 113. Oxidation of various alcohols to the corresponding carbonyl compounds ^a.

Entry	Substrate	Product	Yield (%)
1	Phenyl(2-quinolinyl)methanol	Phenyl(2-quinolinyl)methanone	74
2	α-Quinolyl-ethanol	1-(2-Quinolinyl)ethanone	67
3	Phenyl(2-pyridyl)methanol	Phenyl 2-pyridyl ketone	64
4	9-Fluorenol	9-Fluorenone	77
5	C6H5CH(OH)COC6H5	(C6H5)2(CO)2	91

^a Reaction conditions: Substrate (1 mmol), activated carbon 143 (100 wt%), H₂O₂ (30%, 10 mmol), anhydrous xylene (10 mL), 95 °C, 18 h.

Table 114. Oxidation of 9-fluorenol ^a.

Entry	X (equiv.)	Yield (%) b
1	4	68
2	10	77
3	15	35

^a Reaction conditions: 9-Fluorenol (1 mmol), activated carbon 143 (100 wt%), H₂O₂ (30%, 4, 10, 15 mmol), anhydrous xylene (10 mL), 95 °C, 18 h. ^b Isolated yield after silica gel column chromatography.

2.3.8. Chloroform Solvent

A solvent-controlled selective oxidation of benzyl alcohol (BnOH) by 30 wt% H₂O₂ with iron(III) tosylate (Fe(OTs)₃·6H₂O) (144) was reported (Table 115) [156]. The results confirmed that using different solvents provided dissimilar products such as in chloroform, quantitative conversion to benzaldehyde (BzH) was shown, while in acetonitrile, benzoic acid was detected with high yield.

Table 115. Oxidation of benzyl alcohol catalyzed by Fe(OTs)₃.6H₂O 144 with H₂O₂ ^a.

Entry	Substrate	Product	Conv. (%)	Yield (%)	Sel. (%)	TON (mol/molcat)
1	C6H5CH2OH	C6H5CHO	89.3	85.6	94.1	85.5

^a Reaction conditions: Benzyl alcohol (1.08 g, 10 mmol), Fe(OTs)₃.6H₂O 144 (0.1 mmol), H₂O₂ (30%, molar ratio n(H₂O₂)/n(BnOH) = 6/1), CHCl₃ (10 g), 60 °C, 16 h.

2.4. Oxidation of Benzylic and Heterocyclic Alcohols in Dual-Phase System

2.4.1. Acetonitrile-Water

Tungstate ions immobilized on the homemade periodic mesoporous organosilica with imidazolium ionic liquid framework which was denoted as WO₄⁼@PMO-IL (145) were prepared and applied with hydrogen peroxide in the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds (Table 116) [157]. Various primary even pretty hindered substrate 2-nitro-benzyl alcohol were also converted to the corresponding aldehyde with excellent selectivities and good yields without any overoxidation (Table 116, Entry 5). With acceptable to excellent yield, under the same reaction conditions, secondary aromatic alcohols were also converted to the corresponding ketones (Table 116, Entries 7–9). In addition, the catalyst could be also efficiently recovered and reused in seven subsequent reaction cycles without deactivation [157].

Table 116. Oxidation of various alcohols with WO⁼₄@PMO-IL 145 catalyst ^a.

Entry	Substrate	Product	t/h	Conv. (%) b	Sel. (%)
1	C6H5CH2OH	C6H5CHO	12	75	100
2	4-OMeC6H4CH2OH	4-OMeC6H4CHO	7.5	76	100
3	4-ClC6H4CH2OH	4-ClC6H4CHO	11	82	100
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	24	65	93
5	2-NO2C6H4CH2OH	2-NO2C6H4CHO	33	77	97
6	3-ClC6H4CH2OH	3-ClC6H4CHO	21	80	100
7	C6H5CH(OH)CH3	C6H5COCH3	15	94	100
8	C6H5CH(OH)Et	C6H5COEt	15	73	100
9	C6H5CH(OH)CH2C6H5	C6H5COCH2C6H5	16	42	100

^a Reaction condition: Substrate (1 mmol), CH₃CN:H₂O (1:1, 0.5 mL), WO₄⁼@PMO-IL 145 (1.5 mol %), H₂O₂ (30%, 5 equiv.), 90 °C, 7.5 to 33 h. ^b GC yield using standard addition method.

The mild alcohol oxidations with hydrogen peroxide were catalyzed by heterogeneous catalyst iron(III) tetrakis(p-sulfonatophenylporphyrinato)acetate supported on polyvinylpyridine and Amberlite IRA-400 (146). As illustrated in Table 117, benzyl alcohol and substituted benzyl alcohols were efficiently converted to their corresponding aldehydes with no significant effect of substituents on the oxidation process [158]. In addition, secondary alcohols were converted to their corresponding ketones with good yields. The catalysts could also be applied in four regeneration cycles without significant loss of their activity.

Table 117. Performance of catalysts in the oxidation of alcohols with H₂O₂ ^a.

Entry	Substrate	Product	Fe(TPPS)-Ad IRA-400		Fe(TPPS)-PVP
			t/h	Yield (%) b	t/h	Yield (%) b
1	C6H5CH2OH	C6H5CHO	4	91	4	90
2	4-ClC6H4CH2OH	4-ClC6H4CHO	4	90	3	90
3	4-BrC6H4CH2OH	4-BrC6H4CHO	3	89	3	90
4	4-OMeC6H4CH2OH	4-OMeC6H4CHO	4	92	4	92
5	2-ClC6H4CH2OH	2-ClC6H4CHO	4	90	4	93
6	3-OMeC6H4CH2OH	3-OMeC6H4CHO	5	91	5	90
7	1-Indanol	1-Indanone	8	76	7	74
8	C6H5CH(OH)CH3	C6H5COCH3	5	90	4	86

^a Reaction conditions: Substrate (1 mmol), supported Fe(III) porphyrins 146 (20 µmol), imidazole (0.4 mmol), H₂O₂ (6 mmol), CH₃CN:H₂O (1:1, 10 mL), room temperature, 3 to 8 h. ^b GC yield based on starting alcohol.

Tetrabutylammonium decatungstate (VI) (TBADT) (147) as catalyst was applied in the oxidation of selected alcohols with hydrogen peroxide as a green oxidant using acetonitrile/water 1,2- or dichloroethane/water as a solvent system. Microwave irradiation combined with elevated pressure in two phase system acetonitrile/water resulted in the highest conversions of substrates in the range of 80% to 100% (Table 118) [159]. TBADT also conserved its catalytic activity for two regeneration cycles.

Table 118. Oxidation of selected alcohols in the acetonitrile/water system ^a.

Entry	Substrate	Product	Conv. (mol%)		Products Proportion (mol%)
			MW b	MW c	MW b	MWP c
1	C6H5CH2OH	C6H5CHO C6H5CO2H	64	100	955	964
2	C6H5CH(OH)CH3	C6H5COCH3	81	96	100	100

^a Reaction conditions: Substrate (6 mmol), TBADT 147 (1 mol%), H₂O₂ (30%, 18 mmol), CH₃CN:H₂O (3 mL), 80 °C, 10 to 15 min. ^b MW, microwave heating for 30 min. ^c MWP, microwave heating under higher pressure for 15 min.

2.4.2. Water-Toluene

H₂O₂ in the presence of catalytic amounts of titanium oxide loaded SiO₂-based phase-boundary (148) was used to investigate oxidation of various hydrophobic alcohols, especially relatively bulky alcohols with their oxidation limited by well-known titanium-containing microporous materials due to their limitations of pore size (Table 119) [160,161]. Primary benzylic alcohols were efficiently converted into corresponding aldehydes. The rate of their oxidation depended not only on substituents (–Cl, –OCH₃, and –CH₃) but also on their positions (p-, m-, and o-) (Entries 1–6). Correspondingly, in relatively high selectivity, secondary benzylic alcohols were oxidized to corresponding ketones (Entries 7 and 8).

Table 119. Oxidation of various alcohols catalyzed by w/o-Ti-SiO₂ 148 using H₂O₂ as an oxidant ^a.

Entry	Substrate	Product	Yield (mmol) b	Sel. (%) c
1	C6H5CH2OH	C6H5CHO	1.29	97
2	4-ClC6H4CH2OH	4-ClC6H4CHO	1.33	92
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	1.20	74
4	4-MeC6H4CH2OH	4-MeC6H4CHO	1.81	80
5	3-MeC6H4CH2OH	3-MeC6H4CHO	1.48	92
6	2-MeC6H4CH2OH	2-MeC6H4CHO	1.15	99
7	C6H5CH(OH)CH3	C6H5COCH3	0.85	100
8	(C6H5)2CHOH	(C6H5)2CO	0.91	100

^a Reaction conditions: Substrate (8 mmol), w/o-Ti-SiO₂ 148 (67 mg), H₂O₂ (30%, 1.8 mL), toluene (3 mL), 60 °C, 16 h. ^b Yield of the produced aldehyde determined by GC using internal standard technique. ^c Selectivity for aldehyde production is defined as the percentage of the aldehyde yield to the total amount of products.

2.4.3. Dichloromethane-Water

A tetrameric DABCO–bromine complex (149) as a novel active bromine complex was introduced for the fast oxidation of allylic, benzylic, primary, and secondary alcohols to carbonyl compounds with excellent yields and without overoxidation (Table 120) [27].

Table 120. Oxidation of alcohols to aldehydes and ketones with the DABCO-bromine complex 149 ^a.

Entry	Substrate	Product	t/h	Yield (%)
1	C6H5CH2OH	C6H5CHO	4	97
2	4-ClC6H4CH2OH	4-ClC6H4CHO	5	98
3	4-OMeC6H4CH2OH	4-OMeC6H4CHO	4	97
4	4-NO2C6H4CH2OH	4-NO2C6H4CHO	5	87
5	2-MeC6H4CH2OH	2-MeC6H4CHO	3	95
6	4-MeC6H4CH2OH	4-MeC6H4CHO	3	96

^a Reaction conditions: Substrate (1 mmol), DABCO–Br₂ 149 (0.262 g, 0.166 mol), CH₂Cl₂:H₂O (6 mL:2 mL), room temperature, 3 to 5 h.

3. Conclusions

In this review, an attempt has been made to summarize some efficient catalytic systems for the selective oxidation of primary and secondary alcoholic OH groups to carbonyl or carboxyl functional groups, using H₂O₂ as the green oxidant. Different chemical environments such as an aqueous media, a solvent-free system, and various organic solvent and dual-phase systems in the present of hydrogen peroxide oxidant indicated good selectivity and in most cases no overoxidation to acid were observed. Moreover, in many articles, electron-donating were compared with electron-withdrawing functional groups in benzyl alcohol derivative oxidations. In most literature, it was found that there were different yields and conversions of primary and secondary benzylic alcohols with electron-donating and -withdrawing functional groups. It was also shown that electron-donating or -withdrawing functional groups had the same effect on the oxidation results of many researches. In light of the above, it is hoped that this review can assist as a valuable critical overview of the area and the contributions help in further research in this field.