Preparation of Large-Size, Superparamagnetic, and Highly Magnetic Fe₃O₄@PDA Core–Shell Submicrosphere-Supported Nano-Palladium Catalyst and Its Application to Aldehyde Preparation through Oxidative Dehydrogenation of Benzyl Alcohols

Abstract

Large-size, superparamagnetic, and highly magnetic Fe₃O₄@PDA core–shell submicrosphere-supported nano-palladium catalysts were prepared in this study. Dopamine was encapsulated on the surface of Fe₃O₄ particles via self-polymerization and then protonated to positively charge the microspheres. PdCl₄²⁻ was dispersed on the surface of the microspheres by positive and negative charge attraction and then reduced to nano-palladium. With air as oxidant, the catalyst can successfully catalyze the dehydrogenation of benzyl alcohols to produce the corresponding aldehydes at 120 °C.

1. Introduction

Fe₃O₄ nanoparticles have been used extensively in many fields due to their superior magnetic properties [1,2,3,4,5] and microwave absorption function [6,7,8,9]. Compared with other metal nanoparticles, the preparation of Fe₃O₄ nanoparticles is simple, and the materials used are inexpensive and easily available. Thus, these materials are suitable for industrial production. However, single Fe₃O₄ nanospheres will usually experience an agglomeration phenomenon, which hinders the catalytic reaction. Hence, substances such as SiO₂ [10,11], dopamine [12,13,14,15], and surfactant [16,17,18] are usually used to wrap Fe₃O₄ so that it becomes uniformly dispersed, achieving favorable hydrophilic interactions. This type of Fe₃O₄ with a core–shell structure has enormous application potential in various fields, including bioseparation [19,20], diagnostic analysis [21,22], molecular imprinting [23], and battery energy storage [24]. Moreover, other metal nanoparticles can be introduced on the shell surface, which not only maintains the original high efficiency of other metal catalysts but also has the magnetism of Fe₃O₄. These properties facilitate the separation of catalysts from the reaction mixture.

Benzaldehyde derivative, an important organic chemical raw material, has been widely applied in pesticides [25], medicines [26], spices [27], dyestuff [28], and cosmetics [29]. The preparation of benzaldehyde derivative through catalytic oxidative dehydrogenation is a green synthetic method [30,31]. Magnetic catalysts play a very important role, such as Pd/Fe₃O₄ catalysts, which have been successfully studied. In their catalytic reactions, there is a medium yield of benzaldehyde derivatization under oxygen or air [32], which can be greatly improved by adding equivalent molar alkali as co-catalyst [33,34,35]. Moreover, a high yield was achieved when peroxide was used as oxidant [36,37,38]. Therefore, we focused on the development of a new and efficient Fe₃O₄@PDA@Pd microparticles catalyst for oxidative dehydrogenation of benzyl alcohols without base under air.

To form the Fe₃O₄@PDA core–shell structure, a polydopamine (PDA) layer was wrapped on the surface of large Fe₃O₄ particles through dopamine autoagglutination. The amino group of the microspheres was combined with protons through protonation with positive electricity. PdCl₄²⁻ was dispersed on the surface through positive and negative charge attraction. The large-size, superparamagnetic, and highly magnetic Fe₃O₄@PDA core–shell-submicrosphere-loaded nano-palladium catalyst was prepared through further reduction (Scheme 1). This catalyst was applied in the preparation of aldehyde through oxidative dehydrogenation of benzyl alcohol compounds under air.

2. Results and Discussion

We characterized and analyzed Fe₃O₄@PDA@Pd. As shown in the x-ray diffraction patterns (see Supplementary File), the peaks of Fe₃O₄ and Fe₃O₄@PDA@Pd microparticles were basically identical, that is, they were both characteristic peaks of Fe₃O₄. Fe₃O₄@PDA@Pd had no characteristic palladium peak, because the palladium particles were very fine under uniform dispersion.

According to the scanning electron microscope (SEM) graph (Figure 1), the particle size of Fe₃O₄@PDA@Pd submicrospheres was within 600–800 nm. Thus, their particle size was large, and nano-palladium particles could be seen in the 100,000× SEM graph. During energy-dispersive x-ray spectroscopy (EDS) analysis (Figure 2 and

Table 1. EDS spectra of Fe₃O₄@PDA@Pd.

Element	wt%
C K	13.43
O K	33.67
Fe K	48.66
Pd L	4.24
Total	100.00

), the content of palladium weight was 4.24%. In the mapping graph, palladium was uniformly distributed on the surface. According to the mixed weight in the reaction, the theoretical palladium content in Fe₃O₄@PDA@Pd was approximately 2.5%, and the palladium content was measured as 2.3% through inductively coupled plasma atomic emission spectroscopy (ICP-AES). Through EDS analysis, the palladium content was 4.24%, as this analysis is mainly aimed at element composition on the material surface, and palladium particles were distributed on the surface of submicrospheres. Moreover, the measured palladium content was partially high.

The transmission electron microscopy (TEM) graph (Figure 3) shows that the prepared Fe₃O₄@PDA@Pd catalyst presented a core–shell structure that centered on Fe₃O₄ submicrospheres. The dopamine layer was wrapped on the surface of Fe₃O₄ submicrospheres, and the thickness of the shell-layer dopamine was uniformly distributed within the interval of 80–90 nm. Nano-palladium particles were dispersed on dopamine. The palladium (0) particle size was within 7–12 nm, and the average particle size was 9.2 nm.

Figure 4 shows the magnetization curves of Fe₃O₄ and Fe₃O₄@PDA@Pd submicrospheres under room temperature (300 K). As shown in the figure, the maximum saturated magnetic intensity of the two submicrospheres was 75 and 45 emu/g, respectively, and the coercive force for both was 0. The existence of the PDA layer decreased the maximum saturated magnetic intensity of Fe₃O₄@PDA@Pd submicrospheres, but they had superparamagnetic and highly magnetic features. Thus, they could easily disperse and separate from the reaction system.

We used Fe₃O₄@PDA@Pd for aldehyde preparation through oxidative dehydrogenation of benzyl alcohols. The reaction temperature, reaction time, and oxidizing agent for aldehyde preparation through the Fe₃O₄@PDA@Pd-catalyzed oxidative dehydrogenation of benzyl alcohols were optimized using benzyl alcohol as the substrate (

Table 2. Optimization of reaction conditions.

Entry	Temperature (°C)	Time (h)	Oxidant	Conversion a (%)	Selectivity a (%)
1	80	24	Air	0	–
2	100	24	Air	<5	>99
3	120	12	Air	62	95
4	120	24	Air	80	92
5	120	48	Air	87	79
6	140	24	Air	100	42 b
7	120	24	O2	86	70
8	120	48	None c	31	>99

^a Determined by gas chromatography (GC) using area normalization method; ^b another product (benzoic acid) was 58%; ^c argon protection.

). The results show that the dehydrogenation reaction did not occur when the reaction temperature was low. The increase in reaction temperature promoted a dehydrogenation reaction. However, when the temperature reached 140 °C, benzyl alcohol products were totally converted, and their selectivity rapidly declined. The product was mainly benzoic acid; hence, the appropriate reaction temperature was 120 °C. Similarly, when the reaction time was too long, some products were converted into benzoic acids; thus, the appropriate reaction time was 24 h. When oxygen was used as an oxidizing agent, the conversion rate of benzyl alcohols slightly increased, but selectivity declined, indicating that benzaldehyde was easily oxidized under oxygen conditions. Partial benzyl alcohol was converted into benzaldehyde when no oxidizing agent was used, indicating that this catalyst could catalyze benzyl alcohol dehydrogenation even under oxygen-free conditions so as to prepare benzaldehyde. However, the conversion rate was low.

With optimal reaction conditions (Table 2, Entry 4), we checked the reaction scope, and the results are shown in

Table 3. Catalytic dehydrogenation of substituted benzylic alcohols to aldehydes.

Entry	Substituted Benzylic Alcohol	Aldehyde	Yield a (%)
1			57
2			76
3			77
4			84
5			73
6			71
7			60
8			86
9			69
10			53
11			65 b
12			72 b
13			72
14			74

^a Isolated yield; ^b 12 h.

. There was a medium yield of synthetic aldehyde prepared through Fe₃O₄@PDA@Pd-catalyzed substituent benzyl alcohol dehydrogenation. The dehydrogenation of substituent benzyl alcohols was easier under high-electron cloud density (Entries 2, 3, 4, 8) than under low-electron cloud density (Entries 5, 6, 7). When a substituent group (Entries 1, 9) was present at the adjacent position, it could exert an inhibitory effect on the dehydrogenation reaction of benzyl alcohols, possibly because the existence of adjacent groups affected C–OH bond adsorption by the catalyst. In addition, heterocyclic ring aromatic methanol (Entries 11, 12, 13, 14) could also experience oxidative dehydrogenation to generate the corresponding aldehyde, and a large quantity of acids was generated after 24 h reaction of 2-furancarbinol (Entry 11) and 2-thienylmethanol. All products were characterized by ¹H NMR and ¹³C NMR (see Supplementary File).

Fe₃O₄@PDA@Pd permits easy recovery of the catalyst from the reaction mixture by a magnet. The reusability of the recovered catalyst was evaluated for the oxidative dehydrogenation of benzyl alcohol as a model reaction, and nearly 95% of its original activity was retained even after five cycles (Figure 5). After six cycles, its activity decreased but its catalytic selectivity remained almost invariant. Compared with the fresh catalysts by SEM and EDS (see Supplementary File), we found that the catalysts were partially fractured and the palladium content was reduced by nearly half.

3. Materials and Methods

Commercially available reagents were used without further purification. The x-ray diffraction (XRD) analysis used a Bruker AXS D8 Advance diffractometer. Scanning electron microscope (SEM) and energy-dispersive spectrometry (EDS) images were recorded on a Hitachi S-4800 field emission scanning electron microscope equipped with an energy-dispersive spectrometer. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F field transmission electron microscope. Magnetic information was obtained on a Quantum Design DynaCool-9 vibrating sample magnetometer. The ¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer using tetramethylsilane (TMS) as internal standard.

Fe₃O₄ particles were prepared according to the method specified in Reference [39].

The general procedure was for the preparation of Fe₃O₄@PDA. Approximately 0.15 g of strong ammonia water was dissolved in 50 mL of deionized water. Then, 0.1 g of Fe₃O₄ and 0.16 g of dopamine hydrochloride were added, and the mixture was placed under uniform ultrasonic dispersion and mechanical agitatation under 40 °C for 24 h. After the reaction ended, solid and liquor were separated using a magnet. The product was placed under ultrasonic washing using 25 mL × 3 deionized water and 25 mL × 3 ethanol, and the solid was directly used in the next step.

The general procedure for the preparation of Fe₃O₄@PDA@Pd was as follows. To prepare palladium sodium chloride solution, 0.115 g (approximately 0.65 mmol) of palladium chloride and 0.076 g (approximately 1.3 mmol) of sodium chloride were taken, and deionized water was added dropwise until the weight reached 8 g. The mixture was heated to 50 °C, and a palladium sodium chloride solution was prepared and placed in a brown bottle for later use. To prepare Fe₃O₄@PDA-loaded nano-palladium, Fe₃O₄@PDA solid was placed under ultrasonic washing using 25 mL of deionized water, 0.1 N of 25 mL hydrochloric acid, 25 mL of deionized water, and 25 mL of ethanol. Approximately 40 mL of ethanol and 4 mL of deionized water were added, and the mixture was mechanically agitated under 10 °C. Subsequently, 0.29 g of palladium sodium chloride solution was slowly added dropwise, and then the mixture was continuously agitated for 3 h. A solution comprising 60 mg of ascorbic acid and 6 mL of deionized water was added slowly using a needle cylinder for 20 min. Afterward, the reaction lasted for 2 h. The solid and liquor were separated using a magnet; the reaction product was placed under ultrasonic washing using 25 mL of ethanol, 25 mL × 3 deionized water, and 25 mL × 3 ethanol. The solid was preserved in 25 mL of ethanol and sealed using nitrogen (solid was approximately 0.1 g after drying).

The general procedure for the oxidative dehydrogenation of benzyl alcohol was as follows. Approximately 0.1 g of Fe₃O₄@PDA@Pd catalyst (2 mol% of Pd) was used. The aforementioned solid and liquor were separated using a magnet and placed under ultrasonic washing using 10 mL × 3 O-xylene. Then, 1 mmol of benzyl alcohol derivative and 5 mL of O-xylene were successively added and blended under ultrasonic conditions. The mixture was then magnetically stirred for 24 h under 120 °C. After the reaction ended, the solid and liquor were separated using a magnet. The liquor was concentrated, and the dehydrogenation product could be obtained after purification through column chromatography (petroleum ether/ethyl acetate).

4. Conclusions

We developed a method for preparing large-size, superparamagnetic, and highly magnetic Fe₃O₄@PDA core–shell submicrosphere-supported nano-palladium catalysts. The catalysts could catalyze the dehydrogenation of benzyl alcohols to produce the corresponding aldehydes with medium to high yields under air as oxidant without base. Additionally, they could be easily removed from the reaction media by an external magnet and reused five times without a considerable reduction in reactivity.