Abstract
Hydrogenation of benzoic acid (BA) to cyclohexanecarboxylic acid (CCA) has important industrial and academic significance, however, the electron deficient aromatic ring and catalyst poisoning by carboxyl groups make BA hydrogenation a challenging transformation. Herein, we report that Pt/TiO2 is very effective for BA hydrogenation with, to our knowledge, a record TOF of 4490 h−1 at 80 °C and 50 bar H2, one order higher than previously reported results. Pt/TiO2 catalysts with electron-deficient and electron-enriched Pt sites are obtained by modifying the electron transfer direction between Pt and TiO2. Electron-deficient Pt sites interact with BA more strongly than electron-rich Pt sites, helping the dissociated H of the carboxyl group to participate in BA hydrogenation, thus enhancing its activity. The wide substrate scope, including bi- and tri-benzoic acids, further demonstrates the high efficiency of Pt/TiO2 for hydrogenation of BA derivatives.
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Introduction
The selective hydrogenation of benzoic acid (BA) or its derivatives has been widely used for the production of fine chemicals, intermediates, and industrial raw materials1,2,3. For example, BA hydrogenation to cyclohexanecarboxylic acid (CCA) is an important step in the production of nylon-6 in industry4,5. However, the need to overcome the high resonance energy of the electron-deficient aromatic ring6 and the catalyst “poisoning” by the carboxyl group7,8 make BA hydrogenation one of the most challenging transformations. Harsh conditions (100–250 °C, 50–150 bar H2) are typically required in order to obtain high BA conversion, which inevitably causes a decrease in selectivity due to the side reactions of decarboxylation and over-hydrogenation9,10. 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 low11,12.
Previous results demonstrated that most of the supported metal catalysts are active only with water as solvent under mild conditions and show low or no activity in organic solvents for BA hydrogenation13. The typical solvation effect is possibly related with preferential adsorption of aromatic ring on metal surface induced by the interaction of carboxyl groups with H2O molecules13,14 and the participation of H* from the dissociated H2O molecules in the reaction2,15. Though water could modify the adsorption mode of substrates, it may also block the metal surface-active sites16,17. Taken together, the low H2 solubility in water (e.g., 0.792 mmol L−1, 298.15 K, 1 atm H2)18, water is not a good choice for efficient BA hydrogenation under mild conditions.
Carboxylates tend to strongly adsorb on metal surface, which significantly deteriorates the catalytic activity through so-called “poisoning effects”19. In contrast, the adsorption of aromatic ring of BA on metal surface is weak considering that the electron-deficient phenyl ring does not easily bind to the surface unoccupied d-metal orbitals via π-bonds20,21. This may be the reason that most metal nanoparticles (NPs) show relatively low activity in BA hydrogenation. Recently, our group reported that the activity of Ru NPs is greatly enhanced in BA hydrogenation by tuning the adsorption mode of BA on Ru surface with phosphine ligands2. Therefore, to realize the efficient BA hydrogenation, the supported metal NPs with appropriate adsorption strength toward carboxyl groups and aromatic rings may be a good choice.
Herein, we report that Pt/TiO2 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. To our knowledge, Pt/TiO2 gives a record activity with an apparent TOF up to 4490 h−1 at 80 °C and 50 bar H2 in hexane. It was found that electron-deficient Pt site is more active than electron-rich Pt site, which is contributed to the participation of dissociated H from carboxyl groups in BA hydrogenation.
Results and discussion
Catalyst screening
First, commercially available carbon-supported metal NPs were tested in BA hydrogenation (Fig. 1a). Pd/C (5 wt%) and Ru/C (5 wt%) are almost inactive in hexane at 40 °C and 10 bar H2, similar with previous reports13,22. To our delight, Pt/C (5 wt%) affords 51% conversion with >99% selectivity to CCA under identical conditions. Inspired by this result, different types of supported Pt catalysts with 2 wt% Pt loading were screened (Fig. 1a) considering that the support with different acid/base or redox properties may influence the catalytic performance of supported metal NPs23,24. Pt/MgO, Pt/CeO2, Pt/CN, and Pt/γ-Al2O3 afford <20% BA conversion. Pt/SiO2 and Pt/ZrO2 give moderate BA conversion, 65% and 35%, respectively. Pt/TiO2 affords high BA conversion, 96%, with >99% selectivity to CCA. The catalyst screen results suggest that the base and acid support, respectively, deteriorate25 and promote26 the activity of Pt in BA hydrogenation. Pd/TiO2 and Ru/TiO2 were prepared using the same method with Pt/TiO2. TEM images show that the particle size of Pd and Ru NPs is ca. 4 and 2 nm, respectively (Supplementary Fig. 1). However, Pd/TiO2 and Ru/TiO2 are inactive using hexane as the solvent (Supplementary Table 1), further confirming the advantage of Pt NPs in BA hydrogenation. Even at 25 °C and 1 bar H2, Pt/TiO2 could still afford >99% BA conversion and >99% selectivity to CCA (Table 1). The apparent TOF of Pt/TiO2 was calculated to be 115 h−1 at 25 °C, 1 bar H2, and 638 h−1 at 40 °C, 10 bar H2. 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/TiO2 in the presence of acetic acid to facilitate the dissolution of BA in hexane. Under such harsh conditions, Pt/TiO2 could still afford 90% conversion with an apparent TOF of 4490 h−1 at 80 °C and 50 bar H2, an order of magnitude higher than the supported metal NPs ever reported (Supplementary Table 2).
a The catalytic results of supported metal NPs for BA hydrogenation in hexane (40 °C, 10 bar H2, S/C of 700, 2 h). b Adsorption energies of BA and acetic acid on Pt (111), Pd (111), and Ru (0001) obtained by DFT calculations. BA benzoic acid.
The catalyst screening results suggest that Pt NPs are active for BA hydrogenation in hexane irrespective of the supports, different from Pd and Ru NPs. Density functional theory (DFT) calculation shows that the adsorption energies of BA on Pt (111), Pd (111), and Ru (1000) are, respectively, −1.53, −2.87, and −2.95 eV, showing the stronger adsorption of BA on Ru and Pd than on Pt. The adsorption energies of acetic acid follow the order of Pt (111) < Pd (111) < Ru (1000) in a similar tendency to BA (Fig. 1b). This suggests that the relatively weak adsorption strength of BA on Pt may contribute to the high activity of Pt NPs.
Pt/TiO2 is active in hexane, H2O, cyclohexane, isopropyl alcohol, and EtOH (Table 1 and Supplementary Table 3), showing its wide solvent tolerance. Even using acetic acid as solvent, 68% conversion could still be obtained, showing the high anti-carboxyl poisoning ability of Pt NPs. The product selectivity to CCA is >93% in all the solvents investigated with cyclohexenecarboxylic acid as the only 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 with BA, which may hinder the BA adsorption on Pt surface27. It is noteworthy to mention that the activity of Pt/TiO2 is much lower in water than in hexane, which is very different from previous results. This can be explained by the two reasons. First, the water may block the active sites of metal NPs due to strong coordination of H2O (or dissociated OH species) on metal surface16. Second, the low solubility of H2 in water may deteriorate the hydrogenation activity18,28. In the case of metal NPs (e.g., Pd, Ru) which have strong adsorption strength for BA, the presence of water could weaken the carboxyl adsorption on metal surface by forming H-bonds with BA, which may induce the preferential adsorption of aromatic ring of BA on metal surface to accelerate the activity13,14. On the basis of above results, the water could promote the activity of metal NPs with strong adsorption strength for BA but deteriorate the activity of metal NPs with weak adsorption strength for BA as in the case of Pt/TiO2.
Electronic and geometric structures
Pt/TiO2-200 and Pt/TiO2-450 were prepared by treatment of Pt/TiO2 under H2 atmosphere at 200 and 450 °C, respectively. The TEM, HRSEM, and HRTEM images of Pt/TiO2, Pt/TiO2-200, and Pt/TiO2-450 showed the uniform distribution of Pt with particle size of 2.9 nm, showing no aggregation of Pt NPs during H2-treatment process (Fig. 2a, b, Supplementary Figs. 2–4). The plane spacing of ca 0.23 nm could be clearly observed in the HRTEM image of Pt/TiO2, which is in agreement with the (111) crystal plane of Pt NPs. The XRD patterns Pt/TiO2 and Pt/TiO2-450 clearly show the diffraction peaks assigned to rutile TiO2. No diffraction peaks from Pt NPs could be observed due to the small particle size of Pt (Supplementary Fig. 5). The CO chemisorption results show that Pt dispersion for Pt/TiO2, Pt/TiO2-200, and Pt/TiO2-450 is, respectively, 29.0%, 22.6%, and 24.2% (Table 2). The slight decrease in Pt dispersion of Pt/TiO2-200 and Pt/TiO2-450 may be caused by the weak adsorption of CO at the interface of Pt and TiO2 after H2 treatment29. The Pt dispersion was also measured by a HOT method (H2-O2 titration), which affords the similar Pt dispersion tendency to that obtained by CO chemisorption method (Table 2). The higher Pt dispersion obtained from HOT method than from CO chemisorption is possibly due to the H2 spillover effect of Pt/TiO230. It should be noted that the Pt dispersion of Pt/TiO2 before and after H2 treatment is comparable, showing that the H2 treatment of Pt/TiO2 did not induce the severe coverage of Pt surface by TiOx, which may be due to pre-nucleation reduction method for the synthesis of the parent Pt/TiO231,32.
a HRSEM image and b HRTEM image of Pt/TiO2. c Pt 4f XPS core level spectra and d CO DRIFTS results for (i) Pt/TiO2, (ii) Pt/TiO2-200, and (iii) Pt/TiO2-450.
The reaction profiles for BA hydrogenation display that reaction rate is faster with Pt/TiO2 than with Pt/TiO2-450 (Supplementary Fig. 6). Under similar conditions, Pt/TiO2 with >99% conversion is more active than Pt/TiO2-200 (59% conversion) and Pt/TiO2-450 (10% conversion) (Table 1). To make reasonable comparisons, the TOFs of Pt/TiO2 catalysts were normalized to Pt dispersion obtained with CO chemisorption. Pt/TiO2, Pt/TiO2-200 and Pt/TiO2-450, respectively, afford TOFs of 2200, 757, and 103 h−1, further confirming that Pt/TiO2 is more active than Pt/TiO2-200 and Pt/TiO2-450.
Generally, Pt/TiO2 treated at high temperature under H2 atmosphere would induce the change in electronic and geometric structure of Pt due to the strong metal-support interaction (SMSI) effect. In order to understand the different catalytic properties of Pt/TiO2 catalysts, the electronic structure of Pt was first characterized by X-ray photoelectron spectroscopy (XPS) (Fig. 2c, Table 2). In comparison with Pt/TiO2, Pt 4f binding energies (BEs) of Pt/TiO2-200 and Pt/TiO2-450 show an obvious downward shift, respectively, by 0.3 and 0.4 eV, suggesting that Pt/TiO2 has more electron-deficient Pt site. The decrease in Pt 4f BEs indicates the charge transfer from Ti cations to Pt NPs induced by SMSI33, which was further confirmed by the higher Ti 2p3/2 BEs of Pt/TiO2 than those of Pt/TiO2-200 and Pt/TiO2-450 (Supplementary Fig. 7). It should be noted that Ti 2p3/2 BEs of Pt/TiO2 are lower than those of TiO2, implying the electron transfer from Pt to Ti cations. The Pt0/Ptδ+ ratio of Pt/TiO2 was increased from 68/32 to 74/26 after heat treatment in H2, showing that the reduction degree of Pt increases at high temperature (Table 2). The electronic structure of Pt/TiO2 catalysts could be facilely modified due to the electron-withdrawing ability of acidic TiO2 support34 and the SMSI effect of Pt–TiO2 system35.
The electronic and geometric structures of Pt NPs were further characterized with in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) of CO chemisorption (Fig. 2d). DRIFTS of adsorbed CO for Pt/TiO2 shows four distinct υCO bands in linear carbonyl region located at approximately 2115, 2096, 2079, and 2062 cm−1. The band at 2115 cm−1 can be assigned to Ptδ+ 36. The lower frequency vibrational stretch at 2062 cm−1 is assigned to CO molecules adsorbed at 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 at the Pt (111) terrace sites (the coordination number of 9) and Pt (110) (the coordination number of 8), respectively37,38,39,40,41,42. The DRIFTS of adsorbed CO for Pt/TiO2-200 is similar with that of Pt/TiO2 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/TiO2-200 than Pt/TiO2 suggests that Pt with electron-deficient surface is favorable for the BA hydrogenation considering that the two catalysts have similar geometric surface structure of Pt.
The DRIFTS of adsorbed CO for Pt/TiO2-450 is quite different from those of Pt/TiO2 and Pt/TiO2-200. The obvious change in peak intensities of Pt/TiO2-450 suggests the reconstruction of surface Pt atoms under H2 treatment at high temperature39. The relatively high peak intensity at 2083 cm−1 indicates that Pt surface has more well-ordered Pt (111). The peak assigned to Pt (111) is gradually red-shifted with H2 treatment temperature increasing, implying the TiO2 donates more electrons to Pt at higher temperature43. In comparison with Pt/TiO2 and Pt/TiO2-200, the much lower activity of Pt/TiO2-450 indicates that the electron-deficient and low-coordination Pt sites may be active for BA hydrogenation.
The reaction orders of BA and H2 were investigated with Pt/TiO2 and Pt/TiO2-450 as representative catalysts considering that the reaction kinetics are particularly sensitive to the Pt structure (Fig. 3a, b)44,45. To ensure the collection of reliable kinetic data, the system was verified to be free of mass transfer resistances (see Supporting Information 1). The order of BA hydrogenation with respect to BA is, respectively, −0.34 and +0.45 for Pt/TiO2 and Pt/TiO2-450, implying the stronger adsorption of BA on Pt/TiO2 than on Pt/TiO2-450. The reaction rate of Pt/TiO2 is increased along with H2 pressure and no plateau was observed with H2 pressure from 4 to 20 bar, which is possibly related with the strong adsorption of BA. The reaction order with respect to H2 for Pt/TiO2 and Pt/TiO2-450 is +0.54 and ~0, respectively. The positive order in hydrogen for the BA hydrogenation is a logical consequence of hydrogenation being involved in rate-determining step (RDS). The kinetic results show that the overall reaction order of BA hydrogenation on Pt/TiO2-450 is much larger than that on Pt/TiO2 (+0.47 vs. +0.20, Supplementary Table 4), indicating different reaction mechanisms for the two catalysts. Temperature-dependent reactivity measurements were taken to obtain apparent activation barriers with Pt/TiO2 and Pt/TiO2-450 as representatives (Fig. 3c). The activation energies for Pt/TiO2 and Pt/TiO2-450 are, respectively, ~38 and ~56 kJ mol−1, showing the two catalysts have different active sites for BA hydrogenation46,47,48. The higher energy barriers of Pt/TiO2-450 explains its low activity in BA hydrogenation.
Reaction orders with respect to a BA and b H2, c Arrhenius plots showing apparent activation barriers, and d H2-D2 exchange results of (i) Pt/TiO2 and (ii) Pt/TiO2-450. Reaction conditions for (a) and (b): 60 °C, 3 mL n-hexane, H2 pressure: 1–30 bar, BA concentration: ~3–10 mg mL−1. Reaction conditions for (c): T = 30–70 °C, S/C = 250, 3 mL hexane. The BA conversion was maintained ~10–20% by adjusting reaction time. TOF turnover frequency, BA benzoic acid.
The characterization data show that the electron density of Pt NPs follows the order of Pt/TiO2 < Pt/TiO2-200 < Pt/TiO2-450. In combination with the catalytic activity, we can infer that the electronic deficient Pt may favor the high BA hydrogenation. From kinetic data, BA is strongly adsorbed on Pt/TiO2 with electron-rich Pt sites. According to the Sabatier rule, too strong or too weak adsorption of reactants on catalysts both disfavor high activity. The minus reaction order of BA over Pt/TiO2 means that BA (or CCA) in fact is “a poison” for the catalyst, which blocked the active sites49. The higher reaction order of H2 over Pt/TiO2 than over Pt/TiO2-450 indicated that hydrogen adsorption is relative more difficult on the former sample. But Pt/TiO2 still shows much higher activity than Pt/TiO2-450, suggesting that the hydrogen activation plays an important role in BA hydrogenation. The H2 activation ability of the catalysts was measured using H2-D2 exchange experiments (Fig. 3d, Supplementary Table 5). The normalized HD formation rate of Pt/TiO2 is more than 10-fold that of Pt/TiO2-450, showing that Pt/TiO2 with electronic deficient Pt surface is more active for H2 activation than Pt/TiO2-450 with electronic rich Pt surface. This result suggests that the high activity of Pt/TiO2 is partly attributed to the high H2 dissociation capacity.
Pt/TiO2 treated at high temperature under reductive atmosphere would induce a significant change in surface structure of TiO230, which may influence the H2 spillover50. H2-TPD measurement was performed to reveal the hydrogen species formed on Pt/TiO2 and Pt/TiO2-450 (Supplementary Fig. 8). For Pt/TiO2, the low-temperature peak can be assigned to the desorption of H2 on metallic Pt and the peaks above 200 °C are related to hydrogen species on TiO2 derived from hydrogen spillover30,51. However, only two broad and weak peaks were observed for Pt/TiO2-450, showing the difficulty in H2 spillover possibly due to the dehydroxylation of TiO2 during heat treatment30. The facile H2 spillover on Pt/TiO2 may also contribute to the high activity.
The role of the carboxyl group
Generally, the deficient phenyl ring does not easily bind on metal surface20, which always results in low catalytic activity52. However, the high activity of Pt/TiO2 in BA hydrogenation suggests that the carboxyl group may affect the hydrogenation activity. To identify the role of carboxyl group in BA hydrogenation, hydrogenation of benzotrifluoride and toluene were conducted (Fig. 4a). Pt/TiO2 could efficiently catalyze the hydrogenation of the above substrates to the corresponding aromatic ring hydrogenated products. The activity followed the order of BA > toluene > benzotrifluoride under similar reaction conditions. The unusual high activity of BA hydrogenation is in contrast to the previous findings that the deficient phenyl ring was difficult 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 conditions10.
a Comparison of the catalytic activity of Pt/TiO2 in hydrogenation of benzotrifluoride, toluene, and BA. b The adsorption mode of (i) BA and (ii) toluene on Pt (111). c 1H-NMR (i) and MS analysis (ii) of the product for benzoic-d5 acid hydrogenation with D2. d Recycling stability of Pt/TiO2 in the hydrogenation of BA. BA benzoic acid, MS mass spectrum.
DFT calculation results show that the benzene ring adsorption in parallel to the metal plane is the most favorable adsorption configuration of toluene on Pt (111), and the methyl group is far away from the Pt surface. This is mainly due to better superposition of its π-orbitals with the Pt conduction band (Fig. 4b)53. Different from toluene, BA molecule adopted a configuration that an O atom of the carboxyl group is coadsorbed on the Pt surface. The strong adsorption of BA on Pt/TiO2 as discussed above may be derived from the coadsorption of carboxyl group on Pt surface54. The dissociated H from the carboxyl group may act as one of the H sources. To confirm this, a control experiment was conducted by using benzoic acid-d5 and D2 as reactants. The mass spectrum (MS) analysis of the product shows 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 (Fig. 4c). An obvious sharp peak at 1.255 was observed in the 1H-NMR spectrum of the product assigned to the H on the m- or p-position of cyclohexane ring (Fig. 4c), further confirming 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/TiO2 favors the orientation of the aromatic ring on the Pt surface, which may facilitate the hydrogen transfer from Pt surface to BA molecules55. On the basis of this mechanism, the Pt with electron-deficient surface favors the adsorption and dissociation of carboxyl group, which could enhance BA hydrogenation activity.
The stability of Pt/TiO2 was tested in BA hydrogenation. During 5 cycles, no obvious decrease in conversion and selectivity could be observed (Fig. 4d). To identify the structure and composition of the used catalysts, Pt/TiO2 after 5 cycles was characterized by TEM and XPS techniques. The results show that used Pt/TiO2 has similar particle size and electronic structure as the fresh one (Supplementary Fig. 9), implying no obvious change in structure and composition during recycling process. The hot filtration reaction was performed by removing Pt/TiO2 from the reaction suspension after BA conversion reaching 50%. Then the filtrate was recharged with 10 bar H2. After 48 min, no further increase in conversion was observed (Supplementary Fig. 6). Meanwhile, the concentrations of Pt in the reaction solution were below the detection limit of ICP-AES, confirming the heterogeneous nature of Pt/TiO2. The above results confirm the high stability of Pt/TiO2 during recycling process.
Substrate scopes
Pt/TiO2 was also used for hydrogenation of BA derivatives at mild conditions (Table 3). First, 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-isomer56. The cis/trans-ratio varied in the range of 20/30 to 86/14 (Table 3, entries 1–5, Supplementary Figs. 10–14). For p-isopropyl benzoic acid, it needs 10 h to reach full conversion with the cis/trans-ratio of 68/32 (Table 3, entry 6, Supplementary Fig. 15). Even for p-trifluoromethylbenzoic acid with more electron-deficient aromatic ring, full conversion was achieved in 10 h though S/C ratio was decreased to 50/1 (Table 3, entry 7, Supplementary Fig. 16), demonstrating the high activity of Pt/TiO2. p-Hydroxylbenzoic acid, phenyl propionic acid, and phenyl pentanoic acid could be efficiently transferred to corresponding products over Pt/TiO2 (Table 3, entries 8–10, Supplementary Figs. 17–19). The hydrogenation of methyl benzoate, mono-methyl terephthalate, and dioctyl phthalate resulted in the formation of the aromatic hydrogenated products using Pt/TiO2 as catalyst (Table 3, entries 11–13, Supplementary Figs. 20–22). It should be mentioned that Pt/TiO2 could also catalyze the hydrogenation of terephthalic acid, phthalic acid, isophthalic acid, and even the challenging trimesic acid and trimethyl trimesate to corresponding aromatic ring saturated product under mild conditions, further demonstrating the high efficiency of Pt/TiO2 for the hydrogenation of aromatic acids (Table 3, entries 14–18, Supplementary Figs. 23–27). The hydrogenation of dioctyl phthalate/phthalate acid and trimesic acid/trimethyl trimesate, respectively, produces the trans- and cis-isomers, and the hydrogenation of the other substrates investigated in this paper results in the formation of cis-isomer as the main product on the basis of NMR analysis, which may be caused by the steric hindrance effect4.
In conclusion, Pt/TiO2 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 inhibits the toxic of BA to the catalyst. A record TOF of 4490 h−1 was achieved with Pt/TiO2 under 80 °C and 50 bar H2 in hexane, more than 10 times higher than the literature results under similar conditions. Isotopic studies confirm that the dissociated H from the carboxyl group is involved in BA hydrogenation which could be facilitated by the strong adsorption of BA on Pt surface. By comparing the activity of Pt/TiO2 catalysts with different surface electronic and geometric structures, it could be concluded that electron-deficient and low-coordination Pt sites show higher activity than electron-rich and high coordination Pt sites in BA hydrogenation, possibly due to the combined effect of higher H2 activation ability and the stronger adsorption of BA at electron-deficient Pt sites. The wide substrate scope including very challenging terephthalic acid, phthalate acid, phthalic acid, isophthalic, and trimesic acid demonstrates the potential practical applications of Pt/TiO2 in hydrogenation of BA and its derivatives.
Methods
Preparation of the Pt/TiO2
Pt catalysts with Pt loading of 2 wt% were prepared by the deposition precipitation method using NaBH4 as the reductant34,57,58. Typically, 200 mg of TiO2 and the desired amount of H2PtCl6 (4 mg Pt) was initially dispersed into 50 mL of aqueous solution. After stirring for 1 h at room temperature, a freshly prepared NaBH4 aqueous solution (2.3 mg, 0.2 mg mL−1) was added slowly. After stirring for another 1 h, the solid was collected by filtration and washed with water and ethanol three times. Finally, the obtained powder was dried at room temperature overnight. The catalyst was denoted as Pt/TiO2.
Preparation of the Pt/TiO2-200 and Pt/TiO2-450
Pt/TiO2 was treated in H2 atmosphere with a flow rate of 20 mL min−1 at the desired temperature for 2 h with a heating rate of 1 °C min−1. The sample after treatment was denoted as Pt/TiO2-T, where T (200 and 450) refers to the treatment temperature.
Synthesis of other oxide support loaded Pt catalyst
Pt/SiO2 with Pt loading of 2 wt% was prepared by the wet impregnation method by dispersing SiO2 (200 mg) in 2 mL of aqueous solution of H2PtCl6 (4 mg Pt) for 5 h. Then the solid product after drying by an evaporator and reduced under H2 atmosphere at 300 °C for 2 h to afford Pt/SiO2. Other oxide-supported Pt catalysts were prepared with a similar method to Pt/TiO2 except that the corresponding oxide was used as supports. Analysis by inductively coupled plasma atomic emission analysis (ICP-AES) clearly indicated that the desired amounts of metal species were successfully loaded onto each of the catalysts.
Catalyst 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. N2 sorption isotherms were carried out on a Micromeritics ASAP2020 volumetric adsorption analyzer. Liquid 1H and 13C, NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 25 °C.
In situ DRIFTS
In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) of CO chemisorption was measured on a Thermo Scientific 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 H2 flow (20 mL min−1) at the desired temperatures for 1 h, followed by purging with a He flow at the same temperature for 30 min. After cooling to room temperature, a background spectrum was collected. Then the He flow was switched to a pure CO flow (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 and H2 chemisorption experiments
CO/H2 chemisorption measurement was performed at 50 °C on Autochem II 2920 chemisorption instrument with a thermal conductivity detector (TCD). For CO chemisorption, the sample (~100 mg) was pretreated with hydrogen at desired temperatures for 1 h, followed by purging with high-purity He for 30 min. After the sample was cooled down to 50 °C, a 5% CO/He mixture was injected into the reactor repeatedly until CO adsorption was saturated. The dispersion of Pt was calculated from the amount of CO adsorbed by assuming the CO/Pt adsorption stoichiometry to be 1/1. Pt dispersion obtained from H2 chemisorption was measured by a HOT method (H2-O2 titration)60. Typically, 100 mg of Pt catalyst was reduced in 5 vol% H2/Ar at 120, 200, and 450 °C for 2 h, respectively, for Pt/TiO2, Pt/TiO2-200, and Pt/TiO2-450. The sample was then cooled to the 140 °C under a flow rate of 60 mL/min of Ar. Then plus O2 was introduced into the carrier gas until O2 peak reached saturation to completely oxide the surface Pt and followed by H2 reduction. Assuming that one hydrogen molecule reduced one surface PtO to Pt and 0.5 hydrogen adsorbed on one Pt atom, the Pt dispersion was calculated as follows:
H2-TPD experiments
H2-TPD experiments were conducted in a U-type quartz tube connected to a mass spectrometer (Autochem 2910). Hundred milligrams of catalyst sample was placed in a U-type quartz tube, heated to 120 and 450 °C, respectively, for Pt/TiO2 and Pt/TiO2-450 and kept for 60 min in Ar flow (30 ml min–1) to remove adsorbed species from catalyst surface. When the sample was cooled down to 25 °C, the flow was switched to H2 (30 ml min–1) for 60 min, followed by purging with Ar (30 ml min–1) for 40 min. The sample was then heated to 800 °C with a ramp rate of 10 °C min–1 in Ar flow and the TPD profiles were recorded simultaneously.
H2−D2 exchange
H2−D2 exchange reactions were carried out in a flow quartz reactor at 22 °C61. The formation rate of HD was measured by mass signal intensity (ion current). Before the test, the catalysts were heated in H2 (10 mL min−1) at 200 °C for 20 min. After the sample was cooled down to room temperature, D2 (10 mL min−1) mixed with H2 was passed through the sample. The gas hourly space velocity (GHSV) is 2.9 × 107 mL h−1 gmetal−1. Under these conditions, the H2-D2 exchange conversions were always kept below 10% for calculation of TOF. Products (HD, H2, and D2) were analyzed with an online mass spectrometer (GAM200, InProcess Instruments). The mass/charge ratio (m/z) values used are 2 for H2, 4 for D2, and 3 for HD. The background HD exchanges from the corresponding support were deducted from the results.
Hydrogenation test
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 final pressure was adjusted to 10 bar and the reactor was heated to the desired temperature with vigorous stirring. After the reaction, the solid catalyst was separated by centrifugation and the filtrate 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 employed64,65. 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 fixed 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 optimized fractional coordinates for different adsorbates on Pt(111)/Pd(111)/Ru(1000) surface see Supplementary Data 1.
The adsorption energy Ead was calculated as:
The Eads/sub, Emol, and Esub are the total energy of the adsorbed molecule, the molecule in the gas phase, and the pure slab, respectively.
Data availability
Any relevant data are available from the authors upon reasonable request.
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Acknowledgements
We acknowledge financial support from the National Key R&D Program of China (2017YFB0702800), the National Natural Science Foundation of China (21733009), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200).
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Q.H.Y. conceived the idea. Q.H.Y., M.G., X.T.K., and C.Z.L. co-wrote the paper. M.G. synthesized the nanomaterials and carried out the catalysis experiments. X.T.K. carried out the model construction and DFT calculations. C.Z.L. analyzed part of NMR results. All the authors contributed to the overall scientific interpretation and edited the manuscript.
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Guo, M., Kong, X., Li, C. et al. Hydrogenation of benzoic acid derivatives over Pt/TiO2 under mild conditions. Commun Chem 4, 54 (2021). https://doi.org/10.1038/s42004-021-00489-z
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DOI: https://doi.org/10.1038/s42004-021-00489-z
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