The Effect of Ru Precursor and Support on the Hydrogenation of Aromatic Aldehydes/Ketones to Alcohols

Supported Ru catalysts were prepared by using different Ru precursors and examined for the hydrogenation of benzaldehyde to CHM. The catalyst prepared from Ru3(CO)12 precursor and HT support exhibited high yield of CHM. Moderate acidic and basic nature of HT was favourable to control the selectivity for CHM. The physicochemical properties analysis revealed that highly dispersed Ru nanoparticles were effective for the hydrogenation of benzaldehyde. RuCO/HT catalyst was tolerant to different functional groups and was stable until 7th cycle of recycle study.


Introduction
Cyclohexanemethanol (CHM) is an important intermediate for the synthesis of pharmaceutical drugs and biodegradable detergents. [1] Traditionally, CHM is produced by catalytic hydrogenation of cyclohexanecarboxaldehyde or benzyl alcohol, [2][3][4][5][6][7][8][9][10][11][12] hydroformylation of cyclohexene, [13] hydroboration of cyclohexanecarboxaldehyde. [14] Among these methods, catalytic hydrogenation is a simple and atom-efficient route for the synthesis of CHM. However, cyclohexanecarboxaldehyde and benzyl alcohol are generally produced via hydrogenation benzaldehyde. Therefore, one-step CHM synthesis from benzaldehyde is highly desirable. Nevertheless, this transformation is still a challenging task due to the instability of CÀ O bond which undergoes hydrogenolysis reaction and generates toluene or methylcyclohexane as byproduct. [15,16] In 2012, supported Ru catalyst have been reported for the hydrogenation of benzaldehyde by using high pressure of H 2 (6 MPa) and compressed (CO 2 + H 2 O) system. [17] High yield of CHM was achieved using mild acidic additive (CO 2 ). Later, colloidal Ru and Ir nanoparticles catalysts have been developed. [18,19] Interestingly, in these catalytic methods, the selectivity for benzyl alcohol and CHM was modified using BMIMBF 4 and H 2 O as reaction solvent respectively. Not only solvent but the crystallographic phase of Ru was also a crucial factor hydrogenation of benzaldehyde. [20] Ru catalyst with hcp phase demonstrated higher activity than fcc phase for the hydrogenation of aromatic ring. Furthermore, hcp phase was useful to suppress hydrogenolysis reaction in the hydrogenation of benzaldehyde, but aromatic hydrogenation of benzyl alcohol was also affected. Recently, succinyl-βcyclodextrin modified Ru catalyst showed high efficiency under mild reaction conditions (30°C and 0.1 MPa H 2 ). This catalytic system suffers from limited scope and long reaction time (17 h) and limited scope. [21] Even though the green solvent (water) is used in all catalytic methods, solvent-free conditions are more favorable for the industrial applications to reduce cost and energy consumption ( Scheme 1).
Generally, supported Ru catalysts are prepared using RuCl 3 or Ru(NO)(NO 3 ) 3 precursors. [22][23][24][25][26][27][28] Nevertheless, the counter ion (Cl À ) could not remove completely from the surface of catalyst and consequently, it affects the catalytic activity of Ru. Furthermore, these two precursors are unable to produce highly dispersed Ru nanoparticles on the support. On the other hand, Ru 3 (CO) 12 precursor provides highly dispersed Ru nanoparticles without any counter ion effect. [27][28][29] Recently, our group demonstrated that the choice of Ru precursor can modify not only dispersion but Ru morphology also. [30] For instance, Ru 3 (CO) 12 precursor was dispersed on Pr 2 O 3 support in the form of low-crystalline Ru nanolayers. Whereas, Ru nanoparticles were obtained from Ru(acac) 3 precursor and Pr 2 O 3 support. Interestingly, this Ru nanolayers catalyst showed higher activity than Ru nanoparticles catalyst for the synthesis of 2-substituted quinolines. [31] For the hydrogenation of benzaldehyde, Ru was the most studied metal, but Ru precursor effect has never been explored systematically. [17,18,[20][21][22] Hydrotalcites (HT) or layered double hydroxides are a class of two-dimension (2D) layered nanostructured inorganic materials. which is often used as catalyst or support. [32][33][34] HT exhibits several properties such as high specific surface area, high adsorption capacity, tunable acidic-basic properties and good thermal stability. With such desirable properties of HT and Ru metal, a new catalyst can be developed to control the selectivity of the desired product (CHM). Herein, we report HT-supported Ru catalyzed hydrogenation of benzaldehyde to CHM under solvent-free conditions.

Results and Discussion
HT-supported Ru catalysts were prepared using different Ru precursor by impregnation method. The designation "CO", "AC", "NO" and "CL" indicates that the catalyst was prepared by using Ru 3 (CO) 12 , Ru(acac) 3 , Ru(NO)(NO 3 ) 3 and RuCl 3 precursor respectively. Initially, we examined these catalysts for hydrogenation of benzaldehyde as a model reaction under solvent-free conditions. As shown in Table 1, Ru CO /HT was found to be the most active catalyst for selective hydrogenation of benzaldehyde to CHM (entry 1). The catalyst prepared from Ru(acac) 3 precursor showed moderate (82 %) yield of CHM with methylcyclohexane (5 %). Surprisingly, Ru NO /HT catalyst was unable for aromatic hydrogenation of benzyl alcohol and benzyl alcohol was obtained (entry 2). Ru CL /HT catalyst was ineffective for the hydrogenation of carbonyl group as well as aromatic ring. Next, we examined the effect of support on the activity of Ru catalysts. MgO and Al 2 O 3 gave 82 % and 48 % yield of CHM respectively. The difference in selectivity of HT, MgO and Al 2 O 3 support can be explained by the nature of the support. In previous reports, it is suggested that the adsorption of substrate can exist in coplanner or non-planar form according to the nature of support and affects the selectivity of desired product. [35,36] For instance, acidic support (Al 2 O 3 ) showed co-planner and basic nature of support (MgO) showed non-planner adsorption form. Al 2 O 3 supported Ru catalyst showed high yield (37 %) of methylcyclohexane which indicated that co-planner adsorption might be responsible for the formation of methylcyclohexane (Table 1, entry 6). Basic support (MgO) gave low yield (2 %) of methylcyclohexane (entry 5). However, Ru/ MgO catalyst was unable to produce CHM selectively. Probably, acidic-basic sites of hydrotalcite in the vicinity of Ru were useful to produce CHM. Moreover, neutral silica gave also methylcyclohexane as major product. Ru 3 CO 12 and HT were unable hydrogenate benzaldehyde (entries 8 and 9).
To understand high activity of Ru CO /HT catalyst, we characterized all catalysts by BET analysis, H 2 chemisorption and TEM analysis ( Table 2). The specific surface area of all catalyst was not changed significantly. Furthermore, the Ru loading amount and particle size were nearly same. Nevertheless, the aggregated Ru nanoparticles were also observed in TEM images of Ru NO /HT and Ru CL /HT catalysts. Ru particle size of Ru NO /HT and Ru CL /HT was calculated by omitting the aggregated particles. Ru dispersion was remarkably changed in all catalysts. Ru CO /HT catalyst showed higher Ru dispersion than that of other catalysts. Probably, high activity of Ru CO /HT was attributed to high Ru dispersion on HT. In previous reports, it is suggested that hydroxyl groups on the supports plays a crucial role for highly dispersed Ru nanoparticles from Ru 3 (CO) 12 precursor. [29,37] The reaction between Ru 3 (CO) 12 precursor and hydroxyl groups on the support produces ruthenium-hydride species which reacts with some fractions of CO in Ru surface species and finally, it gives highly dispersed Ru nanoparticles. Furthermore, Li et al. described high metal dispersion using HT support. During reduction process, metal redispersed on the surface of HT via interacting with Al 3 + or the surface defects related to Al 3 + which leads to formation of abundant low-coordinated Pd sites (terrace, edge, and defect). [38] The catalysts prepared from other Ru precursors showed low dispersion and gave low yield of CHM. With physicochemical properties and Table 1, we concluded that high dispersion of Ru metal over support was a crucial factor for the high selectivity of Ru CO /HT catalyst. Next, we investigated the general applicability of Ru CO /HT catalyst for the hydrogenation of different benzaldehyde ( Table 3). The reactivity of ortho, meta and para methylsubstituted benzaldehyde was investigated (entries 2-4). p-Methyl-benzaldehyde showed the higher yield than o-methyl-benzaldehyde due to the steric effect of substituent. Furthermore, high yield (94 %) of 4-tert-cyclohehanemethanol was achieved when 4-tert-benzaldehyde was used as a reactant (entry 5). Hydrogenation of 4-chlorobenzaldehyde underwent dehalogenation reaction and CHM was observed as product (entry 6). In addition, heterocyclic aldehydes were successfully transformed to the corresponding products (entries 7 and 8).
We further expanded the general applicability of Ru CO /HT for the hydrogenation of aromatic ketones (Table 4). Acetophenone bearing with electron-donating or electron-withdrawing group afforded high yield of corresponding product (entries 1-3). 3-acetopyridine was smoothly converted to 1-(piperidin-3-yl)ethanol.
The reusability of Ru CO /HT was examined for the hydrogenation of benzaldehyde under optimized reaction conditions (1 mol% Ru CO /HT, 3.5 MPa H 2 , 100°C, and 9 h). After each cycle, the catalyst separated from reaction mixture by centrifugation and washed with acetone three times. Then, the catalyst was dried at 40°C for 12 h, reduced and used for the next cycle. Ru CO /HT showed good stability and was recycled for 6 cycles (Figure 1). In 7 th cycle, the reaction rate for aromatic hydrogenation of benzyl alcohol was decreased and 75 % yield of CHM was observed with 25 % yield of benzyl alcohol. To study the loss in catalytic activity, we examined the fresh and recovered catalysts after 7 th cycle by scanning transmission electron.   microscopy (STEM). The recovered catalyst showed partially aggregated Ru nanoparticles ( Figure S3). To understand Ru aggregation in every cycle, we carried out the reaction under modified reaction conditions for two cycles (1 mol% Ru CO /HT, 3.5 MPa H 2 , 100°C, and 2 h). STEM analysis showed the catalyst was slightly aggregated after 2 nd cycle ( Figure S4). However, the degree of Ru aggregation was small, and it did not affect the CHM yield until 7 th cycle. Moreover, we analyzed the electronic state of fresh and recovered catalyst after 7 th cycle by X-ray photoelectron microscopy (XPS). In previous reports, different electronic states of Ru were reported. [39][40][41][42][43] The metallic Ru 3P 3/2 state was observed at 461.1 eV � 0.3 eV and Ru valence between + 1 to + 3 The metallic Ru 3P 3/2 was observed at 461.1 eV � 0.3 eV and Ru valence between + 1 to + 3 appeared at 462.3 eV � 0.3 eV. Higher oxidation states (+ 3) and (+ 4) of Ru was observed at 463.5 eV � 0.3 and 466.7 eV � 0.5, respectively. The XPS spectrum of fresh catalyst showed two bands on 461.5 eV and 483.6 eV which were assigned to metallic Ru (Figure 2). On the hand, the recovered catalyst showed higher binding energies than fresh catalyst and consistent with Ru valence between + 1 to + 3. Therefore, we concluded that Ru metal nanoparticles was aggregated after 7th cycle which led to the loss in catalytic activity of Ru/HT. Although Ru metal was aggregated, the formation of toluene and methylcyclohexane were not observed in 7 th cycle. High yield of CHM in 7 th cycle can be achieved by a long reaction time (> 9 h) and/or high H 2 pressure (> 3.5 MPa).

Conclusion
In summary, we have developed an efficient catalytic method for aromatic hydrogenation of aldehydes and ketones under solvent-free conditions. HT-supported Ru catalyst prepared from Ru 3 CO 12 precursor showed higher activity than that of Ru(acac) 3 , Ru(NO)(NO 3 ) and RuCl 3 precursors. High dispersion and small Ru particle size in Ru CO /HT catalyst were attributed to high catalytic activity of Ru CO /HT catalyst. This catalyst was tolerant of different functional groups. N-or O-containing heterocyclic aldehydes/ketones were transformed to saturated cyclic alcohols. Ru CO /HT catalyst was stable until 7th cycle of recycle study. HAADF-STEM and XPS analysis showed that Ru metal was partially aggregated after 7th cycle and resultant in the loss of activity was observed.