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Article

Formic Acid as a Hydrogen Source for the Additive-Free Reduction of Aromatic Carbonyl and Nitrile Compounds at Reusable Supported Pd Catalysts

by
Pooja Tomar
1,2,
Yuou Nozoe
1,
Naoto Ozawa
1,
Shun Nishimura
1,3 and
Kohki Ebitani
1,3,*
1
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
2
M. Tech (CSPT), Department of Chemistry, University of Delhi, Delhi 110007, India
3
Graduate School of Advanced Science and Technology, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(8), 875; https://doi.org/10.3390/catal10080875
Submission received: 30 June 2020 / Revised: 24 July 2020 / Accepted: 27 July 2020 / Published: 4 August 2020
(This article belongs to the Section Catalysis in Organic and Polymer Chemistry)
Browse Figures
Versions Notes

Abstract

:
Formic acid can be used as a hydrogen source for the hydrogenations of various aromatic carbonyl and nitrile compounds into their corresponding alcohols and amines using reusable heterogeneous Pd/carbon and Pd/Al2O3 catalysts, respectively, under additive-free and mild reaction conditions.

Graphical Abstract

1. Introduction

Formic acid (FA, HCO2H) has attracted much attention recently since it may constitute an optional process to store hydrogen in a dense and safe form (hydrogen density: 53.4 g L−1, 4.4 wt%; b.p.: 100.75 °C) [1] instead of as molecular hydrogen (H2) gas with a low hydrogen density (0.089 g L−1 at 0 °C, 1 atm); FA has a higher hydrogen density than 690 atm H2. The dehydrogenative decomposition of FA using various metal particles can produce CO-free H2, which is suitable for low-temperature fuel cells [2,3,4,5,6,7,8,9,10,11,12,13]. For example, Mori et al. reported that PdAg nanoparticles, having a heteroatomic Pd–Ag bonding combined with vicinal weakly basic functional groups of the active metal centers, were responsible for the production of high-quality H2 from FA [13].
Industrially, FA has been produced via methyl formate, formed by the reaction between high-pressured CO (395 atm) and methanol in the presence of strong base, such as sodium methoxide, followed by a reaction with NH3 and H2SO4 [14]. This FA synthetic method produces vast amounts of (NH4)2SO4 as a non-renewable waste. Recently, it has been found that FA can be obtained from woody biomass [15,16,17,18,19,20,21,22,23,24]. We also reported selective FA synthesis from sugars, using reusable precious metal-free heterogeneous catalyst and aqueous hydrogen peroxide (H2O2) as an oxidant, where utilization efficiency of H2O2 was found extremely high, at ~100% [24].
The FA has been explored as a hydrogen source in the transfer-hydrogenation reaction instead of high-pressured H2 [25,26,27,28,29,30]. Watanabe et al. reported using an RuCl2(PPh3)3 catalyst for the reduction of nitroarenes into aminoarenes with FA in the presence of triethylamine at 125 °C [25]. Hydrogenation under mild reaction conditions abbreviates the protection procedure in organic synthesis. Therefore, Beller et al. developed a base-free transfer-hydrogenation of nitroarenes using an Fe(BF4)2·6H2O catalyst in the presence of PPh3 at 40 °C [26]. The above two complex catalysts, however, have drawbacks in the catalyst reusability and use of additives.
We also have been developing several transfer-hydrogenations using FA as a hydrogen source with reusable supported metal catalysts under additive-free conditions [31,32,33,34]. For example, the Au/ZrO2 catalyst showed a high specificity for a base-free hydrogenation of biomass-derived levulinic acid into γ-valerolactone using FA in water [31]. As an ongoing study of this research, here we are reporting an additive-free reduction of aromatic carbonyl and nitrile compounds, which are known as important reactions in the organic synthesis [26,35,36,37], using FA as a hydrogen source and reusable supported Pd catalysts under mild reaction conditions (Scheme 1). This study demonstrates that organic transformation with formic acid agent is a potential way to proceed an additive-free hydrogenation reaction with commercially-available and simple catalyst usage such as commercial Pd/carbon and Pd/Al2O3.

2. Results and Discussion

2.1. Reduction of Aromatic Carbonyl Compounds

Preliminarily, we have examined the effect of carbon-supported metal catalysts (5wt%, commercial) for the reduction of benzaldehyde into benzyl alcohol using 1.5 eq. FA at 100 °C in THF. As shown in Table 1, the commercial Pd/carbon catalyst showed an exceptionally high activity (100% conv. and 93% yield) among Rh/carbon, Pt/carbon, and Ru/carbon under additive-free conditions.
Next, the effect of Pd support was investigated for the hydrogenation of benzaldehyde under same reaction conditions. Besides commercial Pd/carbon and Pd/Al2O3 catalysts, we prepared supported Pd catalysts by an impregnation method using an aqueous solution of Pd(NO3)2 at room temperature, followed by drying at 110 °C (see Section 3.1., Materials section, in detail). Table 2 shows that the carbon support is the best among Al2O3, SiO2, CeO2, ZrO2, TiO2, and MgO (vide infra). We also screened several solvents and found that THF was the best among 1,4-dioxane, toluene, and ethanol [38].
Time courses of the reaction of benzaldehyde using the Pd/carbon catalyst are shown in Figure 1. The highest yield of 93% for benzyl alcohol was obtained with >99% conversion of benzaldehyde at 6 h. As an another experiment, the solid catalyst was removed by filtration at 1 h, and the filtrate was continuously heated at 100 °C, where the conversion and yield unchanged significantly (open marks in Figure 1). Therefore, leaching of the catalytically active sites into the reaction mixture scarcely proceed for this reaction. Accordingly, it is suggested that the reduction proceeds heterogeneously with the Pd/carbon catalyst. The used Pd/carbon catalyst was recyclable at least twice without a significant loss of the conversion and yield (Figure S1).
Using the highly active Pd/carbon catalyst and FA, the substrate scope had been explored under additive-free conditions (Table 3). Aromatic aldehydes such as 4-hydroxybenzaldehyde, 4-methoxybenzaldehyde, and 4-methylbenzaldehyde could be smoothly reduced into the corresponding alcohols with fair yields (entries 1–4). This catalytic system was successfully applied for the reduction of aromatic ketones such as acetophenone and benzophenone (entries 5 and 6). Interestingly, a heteroaromatic aldehyde such as 3-pyridine carbaldehyde was reduced into 3-pyridine methanol in a high yield (entry 7). However, furfural and 5-hydroxymethylfurfural (HMF) were hardly reduced under the present conditions (entries 8 and 9).
It is notable that the present catalytic system could not reduce nonanal, cyclohexane carbaldehyde, thiophene-2-carbaldehyde, and 4-chrolobenzaldehyde at all (Scheme 2). The inactivity of the aliphatic and heteroatomic carbonyl compounds could be ascribed to the low electron density at the carbonyl carbons and the strong coordination of heteroatoms on active Pd sites, respectively.
The intermolecular competitive reduction between benzaldehyde (1) and acetophenone (2) was examined with FA and the Pd/carbon catalyst at 100 °C for 6 h in THF (Scheme 3). It was observed that the present catalytic system preferentially reduced aldehyde over ketone moiety; the conversion of (1) and yield of (3) greatly exceeded the conversion of (2) and yield of (4), whereas the reduction of acetophenone (2) alone occurred with almost same reduction rate as benzaldehyde (1) (Table 3, entry 1 vs. 5).
The same phenomenon was observed in the intermolecular competitive oxidation between benzyl alcohol (3) and 1-phenylethanol (4) using molecular oxygen catalyzed by surface Ru monomeric cation species on hydrotalcite, where the Ru-alkoxide intermediate species undergoes β–hydrogen elimination to produce the carbonyl compound and a metal hydride species [39]. Since the formation of the metal-alkoxide intermediates of primary alcohols is favored over secondary alcohols in the ligand exchange step [40], it is suggested that Pd-alkoxide intermediate species are involved in the present catalytic reduction system using FA as a hydrogen source and the Pd/carbon catalyst.

2.2. Reduction of Aromatic Nitriles

In the reduction of aromatic nitriles with FA under additive-free conditions, benzonitrile was chosen as a sample substrate to determine the optimum catalyst and reaction conditions. The screening of the supported Pd catalyst in EtOH at room temperature revealed that the Pd/Al2O3 was the best catalyst among Pd/carbon, Pd/CaO, Pd/zeolites (MCM-41, ZSM-5), and Pd/TiO2, affording benzylamine in a high yield (76%) and selectivity (87%) [41]. Protonic solvents such as MeOH, EtOH, 1-PrOH, and 2-PrOH showed good reactivity for the reduction of benzonitrile, using three equivalents of FA and Pd/Al2O3 catalyst, in comparition with aprotic solvents (DMF and ethyl acetate) [42]. Especially, use of 2-PrOH as a solvent gave benzylamine with an 88% yield and 97% selectivity at 90% conversion of the benzonitrile [43]. Using 3 mmol of FA in the reaction mixture showed the highest selectivity, and FA utilization efficiency was determined to be 84% (Scheme 4), which was quite larger than those for the reductions of nitro and carbonyl compounds using FA as a hydrogen source (20-30%), along with the Fe and Ir complexes [26,29].
In Figure S2, the time courses of the reaction of benzonitrile using a Pd/Al2O3 catalyst are shown. Similar to the reduction of benzaldehyde, product yield of the benzylamine was found to be constant when the catalyst was removed at 1 h, indicating the heterogeneous nature of this reaction. The recovered Pd/Al2O3 catalyst was recyclable for three times without a significant loss of the product yield and selectivity (Figure 2).
This catalytic system was successfully applied for the reduction of para-substituted benzonitriles with electron donating groups such as p-methylbenzonitrile and p-methoxybenzonitrile, as shown in Table 4. Other para-substituted benzonitriles with electron withdrawing groups such as p-aminobenzonitrile, p-chlorobenzonitrile, and p-iodobenzonitrile were found inactive in this reaction system [44].
A Hammett plot using 6 mmol of substrate had a negative slope (Figure S3), indicating the cationic nature of the intermediate for the reduction of benzonitriles. Accordingly, the following reaction steps are proposed where the dissociative cleavage of FA molecule occurs on the surface of the Pd particles and the cationic intermediates are formed in the vicinity of the Pd particles.
R-C≡N + H+(ads) → R-C+=NH
R-C+=NH + CO2H−(ads) → R-CH=NH + CO2(g)
R-CH=NH + H+(ads) → R-CH+-NH2
R-CH+-NH2 + CO2H−(ads) → R-CH2-NH2 + CO2(g)

3. Materials and Methods

3.1. Materials

First, 5wt% Pd/carbon (BET surface area: 1117 m2 g−1; average pore diameter: 2.5 nm) and Pd/Al2O3 (BET surface area: 96 m2 g−1; average pore diameter: 8.6 nm) catalysts, Pd(NO3)2, FA, tertahydrofurane (THF), 2-propanol (2-PrOH), 1-butanol, benzaldehydes, benzylalcohols, benzonitriles, and benzylamines were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and used without further purification. The 5wt% loadings of Rh/carbon, Pt/carbon, and Ru/C were also obtained from Wako Pure Chemical Industries, Ltd., whereas the 5wt% Pd loadings of SiO2, MgO, CeO2, TiO2 and ZrO2 were prepared by an impregnation method (adsorption) with Pd(NO3)2 as a metal resource under the pH = 9 adjusted with Na2CO3. Briefly, the support (300 mg) and Pd source were dispersed in an aqueous solution at pH = 9 for 3 h at RT, and then the precipitate was filtered, washed, and dried at 110 °C overnight.

3.2. Reaction Procedure

Typically, 50 mg of Pd/carbon, 3 mL of THF, 1 mmol of benzaldehyde, and 1.5 mmol of FA were added to a pressure glass tube (20 mL) and heated at 100 °C for 6 h under a stirring of 500 rpm. After the completion of the reaction, the reaction mixture was cooled to room temperature and the catalyst was separated by filtration.
For the reduction of benzonitrile using FA, benzonitrile (1 mmol), FA (3 mmol) and 2,-PrOH solvent (5 mL) were added to the pressure-proof glass tube, and the air was purged by a nitrogen flow for several minutes. After that, the Pd/Al2O3 catalyst was added and stirred to start the reaction at room temperature. After the completion of the reaction, the catalyst was separated by filtration.

3.3. Product and FA Analysis

The products in the filtrate were analyzed by an FID-GC (Shimadzu, GC-2014) equipped with a capillary column (Agilent J&W GC DB-1, 50 m length, 0.32 mm diameter) using 1-buthanol as an internal standard for a calibration curve.
For the determination of the FA amount after the reaction (50 mg Pd/Al2O3, 1 mmol benzonitrile, 3 mmol FA, 3 mL 2-PrOH, 3 h, room temperature), the catalyst was filtered and the filtrate was diluted with 47 mL of pure water, and then analyzed with HPLC (WATERS 600) using an Aminex HPX-87H column (Bio-Rad Lab. Inc., Hercules, CA, USA) attached to a refractive index detector. Aq H2SO4 (10 mM) at a flow rate of 0.5 mL min−1 was run through the column, which was maintained at 50 °C [24].
The conversion, yield, selectivity, and FA utilization efficiency for the benzonitrile reduction using the Pd/Al2O3 catalyst were calculated using the equations shown below:
Conversion (%) = [Amount of substrate used (mmol) − Amount of substrate after reaction (mmol)]/[Amount of substrate used (mmol)] × 100
Yield (%) = [Amount of product (mmol)]/[Amount of substrate used (mmol)] × 100,
Selectivity (%) = [Yield/Conversion] × 100
FA utilization efficiency (%) = [Amount of Amine (mmol) × 2]/[Amount of consumed FA (mmol)] × 100

3.4. Catalyst Recycling Experiment

Recycling experiments were carried out to establish the stability of the catalysts in the reductions using FA. After a catalytic run, the reaction mixture was transferred to a centrifugation tube and was washed using the reaction solvent, followed by centrifugation to decant the supernatant liquid. The process was repeated three times, and the collected catalyst was dried under a vacuum overnight before the next reaction.

4. Conclusions

Here, we demonstrated the highly efficient reduction of aromatic carbonyl and nitrile compounds using FA as a hydrogen source with reusable and heterogeneous supported Pd catalysts, such as Pd/carbon and Pd/Al2O3, under additive-free and mild reaction conditions.
The Pd/carbon catalyst showed hydrogenation of aromatic carbonyl compounds, such as primary and secondary substituted benzaldehydes including 3-pyridine carbaldehyde, using FA in THF with significant yields (>74%). Meanwhile, Pd/Al2O3 preferred to catalyze the hydrogenation of para-substituted benzonitriles towards the corresponding amines at room temperature in 2-propanol with >96% yields. An FA utilization efficiency of 84% was reached in the latter case. The cationic reaction intermediates were proposed based on the results of the intermolecular competitive reaction between primary and secondary alcohols and the Hammett plot for the reduction of p-substituted benzonitriles. The substrate scope and the limitation of the present catalytic systems were explored. This study might open up a new avenue for the utilization of formic acid, derived from biomass resources, as a hydrogen source for versatile chemical applications.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/8/875/s1: Figure S1: Reusability of Pd/carbon catalyst. Figure S2: Time course of the reduction of benzonitrile with FA with Pd/Al2O3 catalyst. After 1 h, the catalyst was removed from the reaction mixture. Red symbols of conversion, yield, and selectivity are results after the catalyst removal, whereas the blue symbols are the reaction progress in the presence of a catalyst. Figure S3: Hammett plot for the reduction of p-substituted benzonitriles (4 mmol). Table S1: Effect of the FA amount on the reduction of benzonitrile using Pd/Al2O3 catalyst.

Author Contributions

Conceptualization, K.E.; methodology, K.E.; investigation, P.T., Y.N., N.O. and S.N.; data curation, P.T., Y.N., N.O. and S.N.; writing—original draft preparation, P.T. and K.E.; writing—review and editing, K.E. and S.N.; visualization, P.T., Y.N. and N.O.; project administration, K.E.; funding acquisition, K.E. and S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Grant-in-Aids for Scientific Research (B) in the JSPS KAKENHI (No. 17H03455) and Young Scientists (A) in the JSPS KAKENHI (No. 17H04966), Japan.

Conflicts of Interest

The authors declare that they have no competing financial interest.

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  42. Effect of FA amount on the reduction of benzonitrile was investigated using EtOH solvent. See, Table S1. It is notable that use of excess amount of FA decreased the product yield.
  43. Therefore, 2-PrOH could not act as a hydrogen source for the reduction of benzonitrile under the present conditions.
  44. Additionally, ortho-phthalonitrile and hexane nitrile were not reduced into the corresponding amines using the present catalytic system.
Catalysts 10 00875 sch001 550
Scheme 1. Selective reduction of aromatic carbonyls and nitriles using supported Pd catalysts and formic acid (FA) as a hydrogen source under additive-free conditions.
Scheme 1. Selective reduction of aromatic carbonyls and nitriles using supported Pd catalysts and formic acid (FA) as a hydrogen source under additive-free conditions.
Catalysts 10 00875 sch001
Catalysts 10 00875 g001 550
Figure 1. Time courses of the reduction of benzaldehyde in the presence of formic acid and Pd/carbon catalyst. Reaction conditions: Pd/carbon (50 mg), benzaldehyde (1 mmol), formic acid (1.5 mmol), THF (3 mL), temp. (100 °C). * Time course of the reaction when the solid catalyst was removed from the reaction mixture after 1 h by filter (open marks).
Figure 1. Time courses of the reduction of benzaldehyde in the presence of formic acid and Pd/carbon catalyst. Reaction conditions: Pd/carbon (50 mg), benzaldehyde (1 mmol), formic acid (1.5 mmol), THF (3 mL), temp. (100 °C). * Time course of the reaction when the solid catalyst was removed from the reaction mixture after 1 h by filter (open marks).
Catalysts 10 00875 g001
Catalysts 10 00875 sch002 550
Scheme 2. Inactive aldehydes in the present catalytic system.
Scheme 2. Inactive aldehydes in the present catalytic system.
Catalysts 10 00875 sch002
Catalysts 10 00875 sch003 550
Scheme 3. Intermolecular competitive reduction of benzaldehyde (1) and acetophenone (2) into benzyl alcohol (3) and 1-phenylethanol (4) using formic acid and Pd/carbon catalyst. Reaction conditions: Pd/carbon (50 mg), 1 (1 mmol), 2 (1 mmol), formic acid (1.5 mmol), 100 °C, 6 h, 500 pm.
Scheme 3. Intermolecular competitive reduction of benzaldehyde (1) and acetophenone (2) into benzyl alcohol (3) and 1-phenylethanol (4) using formic acid and Pd/carbon catalyst. Reaction conditions: Pd/carbon (50 mg), 1 (1 mmol), 2 (1 mmol), formic acid (1.5 mmol), 100 °C, 6 h, 500 pm.
Catalysts 10 00875 sch003
Catalysts 10 00875 sch004 550
Scheme 4. Selective reduction of benzonitrile using 5wt% Pd/Al2O3 catalyst and formic acid as a hydrogen source under additive-free conditions.
Scheme 4. Selective reduction of benzonitrile using 5wt% Pd/Al2O3 catalyst and formic acid as a hydrogen source under additive-free conditions.
Catalysts 10 00875 sch004
Catalysts 10 00875 g002 550
Figure 2. Reusability of the Pd/Al2O3 catalyst for the reduction of benzonitrile with FA. Reaction conditions: catalyst (50 mg), benzonitrile (1 mmol), FA (3 mmol), 2-PrOH (3 mL), RT, 400 rpm.
Figure 2. Reusability of the Pd/Al2O3 catalyst for the reduction of benzonitrile with FA. Reaction conditions: catalyst (50 mg), benzonitrile (1 mmol), FA (3 mmol), 2-PrOH (3 mL), RT, 400 rpm.
Catalysts 10 00875 g002
Table
Table 1. Reaction of Benzaldehyde with formic acid using carbon-supported metal catalysts.
Table 1. Reaction of Benzaldehyde with formic acid using carbon-supported metal catalysts.
Catalysts 10 00875 i001
EntryCatalystConversion/%Yield/%
1Pd/carbon10093
2Rh/carbon73
3Pt/carbon52
4Ru/carbon42
Reaction conditions: catalyst (50 mg), benzaldehyde (1 mmol), formic acid (1.5 mmol), THF (3 mL), 100 °C, 6 h, 500 rpm.
Table
Table 2. Effect of Pd support on the reduction of benzaldehyde with formic acid.
Table 2. Effect of Pd support on the reduction of benzaldehyde with formic acid.
EntryCatalystConversion/%Yield/%
1Pd/carbon10093
2Pd/Al2O36230
3Pd/SiO23915
4Pd/MgO162
5Pd/CeO276
6Pd/TiO261
7Pd/ZrO221
Reaction conditions: catalyst (50 mg), benzaldehyde (1 mmol), formic acid (1.5 mmol), THF (3 mL), 100 °C, 6 h, 500 rpm.
Table
Table 3. Reduction of various alcohols with formic acid using a Pd/carbon catalyst.
Table 3. Reduction of various alcohols with formic acid using a Pd/carbon catalyst.
EntrySubstrateProductConversion/%Yield/%
1 Catalysts 10 00875 i002
benzaldehyde		 Catalysts 10 00875 i003
benzylalcohol		>9993
2 Catalysts 10 00875 i004
4-hydroxybenzaldehyde		 Catalysts 10 00875 i005
4-hydroxybenzylalcohol		9088
3 Catalysts 10 00875 i006
4-methoxybenzaldehyde		 Catalysts 10 00875 i007
4-methoxybenzylalcohol		7974
4 Catalysts 10 00875 i008
4-methylbenzaldehyde		 Catalysts 10 00875 i009
4-methylbenzylalcohol		8879
5 Catalysts 10 00875 i010
acetophenone		 Catalysts 10 00875 i011
1-phenyl ethanol		9586
6 Catalysts 10 00875 i012
benzophenone		 Catalysts 10 00875 i013
benzhydrol		9378
7 a Catalysts 10 00875 i014
3-pyridine carbaldehyde		 Catalysts 10 00875 i015
3-pyridine methanol		>9993
8 b Catalysts 10 00875 i016
furfural		 Catalysts 10 00875 i017
furfurylalcohol		5630
9 a Catalysts 10 00875 i018
HMF		 Catalysts 10 00875 i019
2,5-dimethanol furan		5430
Reaction conditions: Pd/carbon catalyst (50 mg), substrate (1 mmol), formic acid (1.5 mmol), THF (3 mL), 100 °C, 6 h, 500 rpm. a 120 °C, 6 h, b 100 °C, 9 h.
Table
Table 4. Reduction of p-substituted benzonitriles with formic acid using Pd/Al2O3 catalysts.
Table 4. Reduction of p-substituted benzonitriles with formic acid using Pd/Al2O3 catalysts.
Catalysts 10 00875 i020
RConversion/%Yield/%Selectivity/%
CH3>99>99>99
OCH3>999697
Reaction conditions: catalyst (50 mg), substrate (1 mmol), formic acid (6 mmol), EtOH (3 mL), RT, 8 h, 500 rpm.

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MDPI and ACS Style

Tomar, P.; Nozoe, Y.; Ozawa, N.; Nishimura, S.; Ebitani, K. Formic Acid as a Hydrogen Source for the Additive-Free Reduction of Aromatic Carbonyl and Nitrile Compounds at Reusable Supported Pd Catalysts. Catalysts 2020, 10, 875. https://doi.org/10.3390/catal10080875

AMA Style

Tomar P, Nozoe Y, Ozawa N, Nishimura S, Ebitani K. Formic Acid as a Hydrogen Source for the Additive-Free Reduction of Aromatic Carbonyl and Nitrile Compounds at Reusable Supported Pd Catalysts. Catalysts. 2020; 10(8):875. https://doi.org/10.3390/catal10080875

Chicago/Turabian Style

Tomar, Pooja, Yuou Nozoe, Naoto Ozawa, Shun Nishimura, and Kohki Ebitani. 2020. "Formic Acid as a Hydrogen Source for the Additive-Free Reduction of Aromatic Carbonyl and Nitrile Compounds at Reusable Supported Pd Catalysts" Catalysts 10, no. 8: 875. https://doi.org/10.3390/catal10080875

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