Pt/TiO2 Catalyzed Hydrogenation of Benzoic Acid with Unprecedented High Activity


 The hydrogenation of benzoic acid (BA) to corresponding cyclohexanecarboxylic acid has important industry and academia significance, however, the electron deficient aromatic ring and the catalyst “poison” by carboxyl group make BA hydrogenation as one of the most challenging transformations. Herein, we found that Pt NPs deposited on TiO2 were very effective for BA hydrogenation with a record TOF of 4490 h− 1 under 80 oC and 50 bar H2 in hexane, one order higher than the reported results. DFT calculation showed that Pt NPs had a weaker interaction with BA than Ru and Pd NPs commonly used for BA hydrogenation, which improved the toxicity resistance of catalyst to BA. Pt/TiO2 catalysts with electron deficient and electron enriched Pt sites were successfully synthesized by modifying the electron transfer direction between Pt and TiO2. Isotopic experiments suggested the participation of dissociated H from carboxyl group in BA hydrogenation. Consequently, the electron deficient Pt sites with stronger adsorption of BA were more active than electron rich Pt sites in BA hydrogenation. In addition to BA, terephthalic acid, iso-phthalic acid, trimesic acid and other BA derivatives could also be efficiently converted to corresponding aromatic saturated products, demonstrating the wide substrate scope of Pt/TiO2.


Introduction
The selective hydrogenation of benzoic acid (BA) or its derivatives has been widely used for the production of ne chemicals, intermediates and industrial raw materials [1][2][3]. For example, BA hydrogenation to cyclohexanecarboxylic acid (CCA) is an important step in the production of nylon-6 in industry [4][5]. However, the need to overcome the high resonance energy of the electron de cient aromatic ring [6] and the catalyst "poison" by carboxyl group [7,8] make BA hydrogenation as one of the most challenging transformations. Harsh conditions (100~250 o C, 50~150 bar H 2 ) are typically required in order to obtain high BA conversion, which inevitably causes the decrease in selectivity due to the side reactions of decarboxylation and over hydrogenation [9,10]. Up to now, various supported metal catalysts (e.g. Pd, Ru, Rh, and Ni) have been used for BA hydrogenation under relatively mild conditions, but the activity is still relatively low [11,12].
Previous results demonstrated that most of the supported metal catalysts were active only with water as solvent under mild condition and showed low or no activity in organic solvents for BA hydrogenation [13].
The typical solvation effect is possibly related with the preferential adsorption of aromatic ring on metal surface induced by the interaction of carboxyl group with H 2 O molecules [13,14] and the participation of H* from the dissociated H 2 O molecules in the reaction [2,15]. Though water could modify the adsorption mode of substrates, it may also block the surface active sites [16,17]. Taken together the low H 2 solubility in water (e.g. 0.792 mmol L -1 , 298.15 K, 1 atm H 2 ) [18], water is not a good choice for e cient BA hydrogenation under mild conditions. Carboxylates tend to strongly adsorb on metal surface, which signi cantly deteriorates the catalytic activity through so-called "poison effects" [19]. In contrast, the adsorption of aromatic ring of BA on metal surface is weak considering that the electron de cient phenyl ring does not easily bind to the surface unoccupied d-metal orbitals via π-bonds [20,21]. This may be the reason that most metal NPs showed relatively low activity in BA hydrogenation. Recently, our group reported that the activity of Ru NPs was greatly enhanced in BA hydrogenation by tuning the adsorption mode of BA on metal surface with phosphine ligands [2]. Therefore, to realize the e cient BA hydrogenation, the supported metal NPs with appropriate adsorption strength towards carboxyl group and aromatic ring may be the good choice.
Herein, we reported that Pt/TiO 2 is a highly active and selective catalyst for BA hydrogenation under mild conditions in either organic solvents or water by screening a series of supported metal NPs. Pt/TiO 2 gives a record activity with apparent TOF up to 4490 h -1 under 80 o C and 50 bar H 2 in hexane. The studies indicated that electron de cient Pt sites were more active than electron rich Pt sites, possibly due to the facilitated participation of H from carboxyl group in BA hydrogenation on electron de cient Pt sites.

Results And Discussion
Screening the Supported Metal NPs for BA Hydrogenation Firstly, commercially available carbon supported metal NPs were tested in BA hydrogenation (Figure 1a). Pd/C (5 wt%) and Ru/C (5 wt%) were almost inactive in hexane at 40 °C and 10 bar H 2 , similar to previous report [13,22]. To our delight, Pt/C (5 wt%) afforded 51% conversion with > 99% selectivity to CCA under identical conditions. Inspired by this result, different types of supported Pt catalysts with Pt loading of 2 wt% were screened in BA hydrogenation (Figure 1a) considering that the support with different acid/base or redox properties may in uence the catalytic performance of supported metal NPs [23,24]. Pt/MgO, Pt/CeO 2 , Pt/CN and Pt/γ-Al 2 O 3 afforded less than 20% BA conversion. Pt/SiO 2 and Pt/ZrO 2 gave moderate BA conversion respectively of 65% and 35%. Pt/TiO 2 afforded high BA conversion of 96% with > 99% selectivity to CCA. The catalyst screen results suggest that the base and acid support respectively deteriorate [25] and promote [26]  To increase S/C ratio is very important for practical applications. Thus, the BA hydrogenation was performed at S/C as high as 1200 over Pt/TiO 2 in the presence of acetic acid to facilitate the dissolution of BA in hexane. Under such harsh conditions, Pt/TiO 2 could still afford 90% conversion with the apparent TOF of 4490 h -1 at 80 °C and 50 bar H 2 , an order of magnitude higher than the supported metal NPs ever reported (Table S1).  Pt/TiO 2 was active in hexane, H 2 O, cyclohexane, isopropyl alcohol and EtOH (Tables 1 and S2), showing the wide solvent tolerance. Even using acetic acid as the solvent, 68% conversion could still be obtained, showing the high anti-carboxyl poisoning ability of Pt NPs. The product selectivity to CCA is > 93% for all the solvents investigated with cyclohexenecarboxylic acid as the side product. BA conversion in aprotic and oxygenate free solvents (e.g. n-hexane) is much higher than that in protic and oxygenate solvents. The decreased hydrogenation rate may be related to the hydrogen bonding of protic solvent and BA, which may hinder the BA adsorption on Pt surface [27]. It is noteworthy to mention that the activity of Pt/TiO 2 was much lower in water than in hexane, which is possibly related with the active sites blocking and low solubility of H 2 in water [16,18,28].  Table 2). The Pt dispersion of Pt/TiO 2 -200 and Pt/TiO 2 -450 slightly decreased, which may be caused by the weak adsorption ability of CO at the interface of Pt and TiO 2 after H 2 treatment [29]. The Pt dispersion obtained by H 2 molecules was almost the same for Pt/TiO 2 , Pt/TiO 2 -200 and Pt/TiO 2 -450 (Table 2). It should be noted that the Pt dispersion of Pt/TiO 2 before and after H 2 treatment was comparable, showing that the H 2 treatment of Pt/TiO 2 did not induce the severe coverage of Pt surface by TiO x , which may be due to pre-nucleation reduction method for the synthesis of the parent Pt/TiO 2 [30,31].
The reaction pro les for BA hydrogenation displayed that more BA was converted to CCA with Pt/TiO 2 than with Pt/TiO 2 -450 within the same reaction time ( Figure S4). Under similar conditions, Pt/TiO 2 with > 99% conversion was more active than Pt/TiO 2 -200 and Pt/TiO 2 -450 with conversion respectively of 59% and 10% (Table 1). To make reasonable comparisons, the TOF of Pt/TiO 2 catalysts was normalized to Pt dispersion obtained with CO chemisorption. Pt/TiO 2 , Pt/TiO 2 -200 and Pt/TiO 2 -450 respectively afforded TOF of 2200 h -1 , 757 h -1 and 103 h -1 , further con rming that Pt/TiO 2 is more active than Pt/TiO 2 -200 and Pt/TiO 2 -450.
Generally, the H 2 treatment of Pt/TiO 2 at high temperature would induce the change in electronic and geometric structure of Pt due to the strong metal-support interaction (SMSI) [32]. In order to understand the different catalytic properties of Pt/TiO 2 catalysts, the electronic structure of Pt was rstly characterized by X-ray photoelectron spectroscopy (XPS) (Figure 2c, Table 2 Pt NPs induced by SMSI [33], which was further con rmed by the higher Ti 2p 3/2 BEs of Pt/TiO 2 than those of Pt/TiO 2 -200 and Pt/TiO 2 -450 ( Figure S5). It should be noted that Ti 2p 3/2 BEs of Pt/TiO 2 were lower than those of TiO 2 , implying the electron transfer from Pt to Ti cations. The Pt 0 /Pt δ+ ratio of Pt/TiO 2 increased from 68/32 to 74/26 after heat treatment in H 2 , showing that the reduction degree of Pt increased at high temperature ( Table 2). The electronic structure of Pt/TiO 2 catalysts could be facilely modi ed due to the electron-withdrawing ability of acidic TiO 2 support [34] and the SMSI effect of Pt-TiO 2 system [35].
The electronic and geometric structures of Pt NPs were further characterized with in situ diffuse re ectance infrared Fourier transform spectra (DRIFTS) of CO chemisorption (Figure 2d). DRIFTS of adsorbed CO for Pt/TiO 2 showed four distinct υ CO bands in the linear carbonyl region located at approximately 2115, 2096, 2079 and 2062 cm -1 . The band at 2115 cm -1 can be assigned to Pt δ+ [36]. The lower frequency vibrational stretch at 2062 cm -1 was assigned to CO molecules adsorbed on low-coordination Pt-edge and -corner sites. The higher frequency vibrational stretch at 2096 and 2079 cm −1 can be assigned to CO molecules that are adsorbed on the Pt (111) terrace sites (the coordination number of 9) and Pt (110) (the coordination number of 8), respectively [37][38][39][40][41][42]. The DRIFTS of adsorbed CO for Pt/TiO 2 -200 was similar to that of Pt/TiO 2 with the exception that the red shift of the vibration peaks was observed, showing the electron donation from Ti cations to Pt due to SMSI effect. The lower activity of Pt/TiO 2 -200 than Pt/TiO 2 suggested that Pt with electron de cient surface was favorable for the BA hydrogenation considering that the two catalysts had similar geometric surface structure of Pt. The Role of the Carboxyl Group Generally, the de cient phenyl ring does not easily bond on metal surface [20], which always results in low catalytic activity [50]. However, the high activity of Pt/TiO 2 in BA hydrogenation suggests that carboxyl group may affect the hydrogenation activity. To identify the role of carboxyl group in BA hydrogenation, hydrogenation of benzotri uoride and toluene were conducted (Figure 4a). Pt/TiO 2 could e ciently catalyze the hydrogenation of the above substrates to the corresponding aromatic ring hydrogenated products. The activity followed the order of BA > toluene > benzotri uoride under similar reaction conditions. The unusual high activity of BA hydrogenation was in contrast to the previous ndings that the de cient phenyl ring was di cult to be hydrogenated. This suggests that the carboxyl group may be involved in the whole reaction process although it cannot easily be hydrogenated at mild reaction conditions [10].
DFT calculation results showed that the most favorable adsorption con guration of toluene on Pt(111) was the benzene ring adsorption parallel to the metal plane, and the methyl group was far away from the Pt surface. This mainly due to better superposition of its π-orbitals with the Pt conduction band ( Figure  4b) [51,52]. Different from toluene, BA molecule adopted a con guration that an O atom of the carboxyl group was co-adsorbed on the Pt surface. The strong adsorption of BA on Pt/TiO 2 as discussed above may be derived from the co-adsorption of carboxyl group on Pt surface [53]. The dissociated hydrogen stems from the carboxyl group may act as one of the H sources. To con rm this, a control experiment was conducted by using benzoic acid-d 5 and D 2 as reactants. The MS analysis of the product showed the appearance of molecular ion peaks at m/z of 139.2 and 138.2 with the intensity ratio of 1.9, denoting the presence of 6 deuterated and 5 deuterated CCA in the product (Figure 4c). An obvious sharp peak at 1.255 was observed in the 1 H-NMR spectrum of the product assigned to the H on the m-or p-position of cyclohexane ring (Figure 4c), further con rming the results of MS analysis. The above results show that the dissociated H from carboxyl group is involved in the hydrogenation process. To this end, the active Pt-H species from homolytic dissociation of hydrogen and dissociated H from carboxyl group attack the activated BA molecule to produce CCA. Besides, the adsorption of carboxyl group on Pt/TiO 2 favored the orientation of the aromatic ring on the Pt surface, which may facilitate the hydrogen transfer from Pt surface to BA molecules [54]. On the basis of this mechanism, the Pt with electron de cient surface favored the adsorption and dissociation of carboxyl group, which could enhance the BA hydrogenation activity.
Substrate scopes of Pt/TiO 2 Pt/TiO 2 was also used for the hydrogenation of BA derivatives at mild conditions (Table 3). Firstly, the hydrogenations of methyl-substituted BA (o-, m-, and p-), p-ethyl benzoic acid and p-pentyl benzoic acid were investigated and the full conversion was obtained in 6 h with the kinetically favored cis isomer [55]. The cis/trans ratio varied in the range of 20/30 to 86/14 (Table 3, entry 1-5). For p-isopropyl benzoic acid, it needed 10 h to reach full conversion with the cis/trans ratio of 68/32 (Table 3, entry 6). Even for ptri uoromethylbenzoic acid with more electron de cient aromatic ring, full conversion was achieved in 10 h though S/C ratio was decreased to 50/1 ( Table 3, entry 7), demonstrating the high activity of Pt/TiO 2 . p-Hydroxylbenzoic acid, phenyl propionic acid and phenyl pentanoic acid could be e ciently transferred to corresponding products over Pt/TiO 2 ( Table 3, entry 8-10). The hydrogenation of methyl benzoate, monomethyl terephthalate and dioctyl phthalate resulted in the formation of the aromatic hydrogenated products using Pt/TiO 2 as catalyst ( Table 3, entry [11][12][13]. It should be mentioned that Pt/TiO 2 could also catalyze the hydrogenation of terephthalic acid, phthalic acid, iso-phthalic acid and even the challenging trimesic acid and trimethyl trimesate to corresponding aromatic ring saturated product under mild conditions, further demonstrating the high e ciency of Pt/TiO 2 for the hydrogenation of aromatic acids (Table 3, entry [14][15][16][17][18]. The hydrogenation of dioctyl phthalate/phthalate acid and trimesic acid/trimethyl trimesate respectively produced the trans and cis isomers, and the hydrogenation of the other substrates investigated in this paper resulted in the formation of cis isomer as the main product on the basis of NMR analysis (see SI), which may be caused by the steric hindrance effect [56]. Table 3. Hydrogenation of BA derivatives using Pt/TiO 2 as catalyst. a The recycle stability of Pt/TiO 2 was investigated using BA hydrogenation as a model reaction (Figure 4d).
The recovered Pt/TiO 2 can be repeatedly used in the hydrogenation reactions, for example in 5 runs of our demonstration, without showing discernible decrease in the conversion of BA or the reaction selectivity. The Pt/TiO 2 after ve cycles had similar particle size as the fresh one on the basis of TEM characterizations ( Figure S6), con rming that no aggregation of Pt NPs occurred during recycling process.

Conclusion
In summary, Pt/TiO 2 was found to be a superior catalyst for BA hydrogenation in comparison with Ru/C and Pd/C due to the weak interaction strength between Pt and BA which inhibited the toxic of BA to the catalyst. A record TOF of 4490 h -1 was achieved with Pt/TiO 2 under 80 o C and 50 bar H 2 in hexane, more than 10 times higher than the literature results under similar conditions. Isotopic studies con rmed that the dissociated hydrogen from the carboxyl group was involved in BA hydrogenation which could be facilitated by the strong adsorption of BA on Pt surface. By comparing the activity of Pt/TiO 2 catalysts with different surface electronic and geometric structures, it could be concluded that electron de cient and low coordination Pt sites had higher catalytic activity than electron rich and high coordination Pt sites in BA hydrogenation, possibly due to the combined effect of higher H 2 activation ability and the stronger adsorption of BA by electron de cient Pt sites. The wide substrate scope including e.g. very challenging terephthalic acid, phthalate acid, phthalic acid, iso-phthalic and trimesic acid demonstrates the potential practical applications of Pt/TiO 2 in hydrogenation of BA and its derivatives.

Experimental Section
Chemicals and Materials The chemicals were purchased from commercial suppliers and used as received with details listed below: 4-methylbenzoic acid (98%, Innochem (Beijing) chemicals), anisic acid (98%), benzamide (98%), cyclohex- Pt catalysts were prepared with a similar method to Pt/TiO 2 except that the corresponding oxide was used as supports. Analysis by inductively couple plasma atomic emission analysis (ICP-AES) clearly indicated that the desired amounts of metal species were successfully loaded onto each of the catalysts.

Characterization
Transmission electron microscopy (TEM) image were obtained using a HITACHI HT7700 at an acceleration voltage of 100 kV. High-resolution scanning electron microscopy (HRSEM) was undertaken by using a HITACHI S5500 apparatus operating at an acceleration voltage of 1-30 kV. X-ray photoelectron (XPS) was performed on an ESCALAB 250xi spectrometer using Al Kα radiation. All the XPS spectra were calibrated by the C1s peak (284.6 eV) from contamination to compensate the charge effects. N 2 sorption isotherms were carried out on a Micromeritics ASAP2020 volumetric adsorption analyzer. Liquid 1 H and 13 C, NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 25 °C.
In situ diffuse re ectance infrared Fourier transform spectra (DRIFTS) of CO chemisorption was measured on a Thermo Scienti c IR spectrometer with a mercury cadmium telluride (MCT) detector, recorded with a resolution of 4 cm −1 [59]. Prior to CO adsorption, the samples were treated in situ in the DRIFT cell under a H 2 ow (20 mL min −1 ) at the desired temperatures for 1 h, followed by purging with a He ow at the same temperature for 30 min. After cooling to room temperature, a background spectrum was collected. Then the He ow was switched to a pure CO ow (20 mL min −1 ) until saturated adsorption was achieved. CO adsorption experiments were carried out sequentially on a single sample. Gas-phase CO spectra were collected at the same pressure and subtracted from the corresponding sample spectra.
CO/H 2 chemisorption measurement was performed at 50 °C on a Autochem II 2920 chemisorption instrument with a thermal conductivity detector (TCD for calculation of turnover frequency (TOF) [61]. Products (HD, H 2 , and D 2 ) were analysed with an online mass spectrometer (GAM200, InProcess Instruments). The mass/charge ratio (m/z) values used are 2 for H 2 , 4 for D 2 , and 3 for HD. The background HD exchanges from the corresponding support were deducted from the results.
General Procedure for the Hydrogenation of Benzoic Acid (BA).
The hydrogenation reactions were carried out in a stainless steel autoclave (300 mL) with a thermocouple-probed detector. In a typical process for benzoic acid (BA) hydrogenation, a desired amount of the solid catalyst was placed in an ampule tube, followed by the addition of BA (0.12 mmol) and 3 mL of n-hexane (for reaction performed at S/C of 1200, 0.3 mL of acetic acid was added to assist the dissolution of BA). The ampule tube was loaded into the reactor. After the tube was purged six times with hydrogen, the nal pressure was adjusted to 10 bar and the reactor was heated to desired temperature with vigorous stirring. After the reaction, the solid catalyst was separated by centrifugation and the ltrate was collected, diluted with n-hexane, and analyzed by an Agilent 6890N GC instrument equipped with an Agilent J&W GC HP-INNOWax capillary column (30 m × 0.32 mm × 0.25 μm). The diphenyl ether was used as the internal standard to determine the conversion, selectivity and carbon balance. The carbon balance was ~ 100%.
For the recycle experiments, the liquid was decanted after centrifugation of the reaction mixture. The residual catalyst was thoroughly washed with n-hexane, and used directly for the next run.

Computational Setup
All the calculations were performed with density functional theory (DFT) by using the Vienna Ab-initio Simulation Package (VASP) [62,63]. The projector augmented-wave pseudopotential method with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional including zero-damping DFT-D3 of Grimme's correction was employed [64,65]. A plane-wave basis with cutoff energy of 400 eV was adopted. Four-atomic-layer slab models of Pt(111), Pd(111), and Ru(0001) with the bottom two layers xed consisting of 144 metal atoms were built. The vacuum spaces were set as 15 Å between the layers. A gamma k-point sampling of 1×1×1 was selected. The convergence energy and force were set to be 1 × 10 −5 eV and 0.02 eV/Å, respectively.
The adsorption energy E ad was calculated as: The E ads/sub , E mol and E sub are the total energy of the adsorbed molecule, the molecule in the gas phase, and the pure slab, respectively.