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

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.


Introduction
Formic acid (FA, HCO 2 H) 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 (H 2 ) 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 H 2 . The dehydrogenative decomposition of FA using various metal particles can produce CO-free H 2 , 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 H 2 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 NH 3 and H 2 SO 4 [14]. This FA synthetic method produces vast amounts of (NH 4 ) 2 SO 4 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 (H 2 O 2 ) as an oxidant, where utilization efficiency of H 2 O 2 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 H 2 [25][26][27][28][29][30]. Watanabe et al. reported using an RuCl 2 (PPh 3 ) 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(BF 4 ) 2 ·6H 2 O catalyst

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 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].

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. Table 1. Reaction of Benzaldehyde with formic acid using carbon-supported metal catalysts.
Catalysts 2019, 9, x FOR PEER REVIEW 2 of 10 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. 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.

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 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]. Next, the effect of Pd support was investigated for the hydrogenation of benzaldehyde under same reaction conditions. Besides commercial Pd/carbon and Pd/Al 2 O 3 catalysts, we prepared supported Pd catalysts by an impregnation method using an aqueous solution of Pd(NO 3 ) 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 Al 2 O 3 , SiO 2 , CeO 2 , ZrO 2 , TiO 2 , 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).  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, 4methoxybenzaldehyde, 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 3pyridine methanol in a high yield (entry 7). However, furfural and 5-hydroxymethylfurfural (HMF) were hardly reduced under the present conditions (entries 8 and 9). 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.  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.  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.

4-hydroxybenzaldehyde
Catalysts 2019, 9, x FOR PEER REVIEW 4 of 10 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.  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.  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.  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.  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.  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.  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.  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.  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.  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.  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.  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.  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.  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.  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. 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 βhydrogen 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.

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]. 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 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). 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.
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 βhydrogen 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.

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]. 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.

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/Al 2 O 3 was the best catalyst among Pd/carbon, Pd/CaO, Pd/zeolites (MCM-41, ZSM-5), and Pd/TiO 2 , 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/Al 2 O 3 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 paminobenzonitrile, p-chlorobenzonitrile, and p-iodobenzonitrile were found inactive in this reaction system [44]. In Figure S2, the time courses of the reaction of benzonitrile using a Pd/Al 2 O 3 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/Al 2 O 3 catalyst was recyclable for three times without a significant loss of the product yield and selectivity ( Figure 2). 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 paminobenzonitrile, p-chlorobenzonitrile, and p-iodobenzonitrile were found inactive in this reaction system [44].  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]. 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 paminobenzonitrile, 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.

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/Al 2 O 3 catalyst was added and stirred to start the reaction at room temperature. After the completion of the reaction, the catalyst was separated by filtration.

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/Al 2 O 3 , 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 H 2 SO 4 (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/Al 2 O 3 catalyst were calculated using the equations shown below:

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.

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/Al 2 O 3 , 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/Al 2 O 3 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 http://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/Al 2 O 3 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/Al 2 O 3 catalyst.