Direct Hydroxylation of Benzene to Phenol Using Palladium–Titanium

Article pubs.acs.org/IECR

Direct Hydroxylation of Benzene to Phenol Using Palladium− Titanium Silicalite Zeolite Bifunctional Membrane Reactors Xiaobin Wang,*,† Bo Meng,† Xiaoyao Tan,†,∥ Xiongfu Zhang,‡ Shujuan Zhuang,† and Lihong Liu§ †

School of Chemical Engineering, Shandong University of Technology, Zibo 255049, People’s Republic of China School of Chemical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China § Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia ∥ Department of Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, People’s Republic of China ‡

ABSTRACT: A series of titanium silicalite zeolite catalysts were successfully incorporated inside a Pd membrane reactor aiming to improve the direct hydroxylation of benzene to phenol. The correlation between the membrane structure and the reaction efficiency was investigated. The influences of reactor configuration, feed mode, and catalysts on benzene conversion, product yield, hydrogen conversion, and water production rate were examined in detail. The reaction was very sensitive to the porosity of Ti-containing zeolite film and the bonding state of the titanium atom in the titanosilicates (i.e., framework and extraframework titanium). The framework titanium could adsorb active oxygen species to form Ti peroxo species which would suppress the decomposition, while the extraframework titanium promoted the decomposition of active oxygen species leading to more water production. Large inter- and intracrystalline pores as well as mesopores provided the reactive species greater opportunity to contact directly with framework titanium resulting in high reaction activity and hydrogen selectivity (based on the phenol production). Furthermore, these intraparticle pores helped the reactants more favorably to reach the active site than these intercrystalline pores. In contrast, the compact titanium silicalite film with smaller pore size was disadvantageous to the reaction due to the slower diffusion of the reactants and products through the zeolite layer. A possible reaction pathway of palladium− titanium silicalite zeolite (Pd−TS) composite membrane for the direct hydroxylation of benzene to phenol was also proposed based on the reaction results.

1. INTRODUCTION Phenol is a valuable intermediate product for the chemical synthesis of drugs, petrochemicals, agrochemicals, and synthetic resin. In practice, phenol production in the chemical industry is mainly carried out through a multistep cumene process featuring disadvantages such as high energy consumption, low atom utilization, safety issues related to the explosive intermediate (cumene hydroperoxide), and the expensive downstream separation of the large amount of acetone as the byproduct. Therefore, one-step hydroxylation of benzene to phenol has been identified as a potentially attractive alternative over the state of the art due to the economic incentive and environmental concerns. Different oxygen sources such as N2O, H2O2, and O2 have been explored to improve the reaction efficiency. The results of direct oxidation by nitrous oxide or hydrogen peroxide are quite encouraging,1−3 but the high cost of these two oxidizing reagents limits their practical application. It is desirable to directly produce phenol from benzene by using oxygen as an oxidant due to its considerable economic advantage. There are a number of reports on phenol production using gaseous hydrogen and oxygen as the reactants. It is believed that active oxygen species capable of directly converting benzene to phenol was produced by reacting between oxygen and hydrogen on the catalyst. Although high phenol selectivity was obtained using molecular oxygen as oxidant, the phenol yield was still low (1 × 10−6 mol·m−2·s−1·Pa−1) than Pd membranes at 473 K. Therefore, the H 2 permeability of Pd−TS composite membranes was mainly controlled by the Pd membrane layer. The similar hydrogen flux and H2/N2 ideal selectivity may be due to the comparable Pd membrane thickness and microstructure. The hydrogen flux and H2/N2 ideal selectivity

Figure 8. UV−vis spectra of TS-1, TS-1e, TS-1p, and Ti-MCM-41. 5640

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present.32,33 In order to investigate the effect of titanium species on the benzene hydroxylation, the TS-1′ catalyst with extraframework titanium was also used.25 The TS-1′ catalyst with a Ti/Si molar ratio of 0.04 in the mother liquid shows absorbance at 330 nm for extraframework anatase phase together with the absorbance at 210 nm featuring the framework titanium.25 3.2. Direct Hydroxylation of Benzene to Phenol over Pd−TS Composite Membranes. In order to improve the Pd catalytic membrane reactor and clarify the joint influence from Pd membrane and Ti-containing zeolite catalysts on the reaction, various Pd−zeolite composite membranes have been probed in benzene hydroxylation. It has been estimated that it is proper to choose 473 K as the reaction temperature for the direct hydroxylation of benzene to phenol.17 Therefore, the reaction temperature was controlled around 473 K in this work. Figure 9 illustrates the benzene conversion and phenol selectivity in Pd−TS (palladium−titanium silicalite) composite membranes under different flow configurations (Figure 2). In reactor mode-1, the benzene/oxygen mixture was fed to the Pd membrane side, while hydrogen was fed to the Ti-containing zeolite layer on the support. This reactor configuration displays comparable benzene conversion of ca. 5% and phenol yield of 3.6−4%. Cyclohexane (CYH) and cyclohexanone (CYC) were the main byproducts of the reaction, and a comparable selectivity (Figure 9c) was obtained for this configuration. Such a result reveals that the titanium silicalite catalysts did not participate in the reaction and the zeolite layer only served as a modifier for the macroporous support. This is reasonable as the oxygen/benzene mixture just flowed along the Pd surface and the direct hydroxylation of benzene to phenol only occurred on the surface of the Pd membrane in this reactor configuration (mode-1). The similar hydrogen conversions and water generation rates of different Pd composite membranes shown in Figure 10 further verify this assumption. Under this operation condition, the hydrogen must diffuse across the Ticontaining zeolite catalysts to the opposite of the Pd membrane with mass transferring limitation through these titanium silicalite films. Therefore, the hydrogen permeation rate may affect the contact between oxygen and active hydrogen atoms, directly influencing the final reaction performance for this reaction mode. However, different Pd−TS composite membranes displayed comparable hydrogen fluxes (Figure 6) at the same operation condition. Furthermore, the H2 permeation fluxes were kept consistent in mode-1 and mode-2 for all Pd− TS membranes during the reaction in order to eliminate the effect of H2 permeation on the reaction. Therefore, the effect of hydrogen permeation on the benzene hydroxylation reaction could be ignored for these Pd−TS membranes. In contrast to the results from mode-1 reactor, the reaction activity had a distinct difference in mode-2 configuration which fed the benzene/oxygen mixture to the support side (i.e., titanium silicalite zeolite side) and hydrogen to the Pd membrane side. The titanium silicalite catalysts were expected to participate in the reaction for this configuration, and the nature of the zeolite catalysts may play an important role in the reaction. Indeed, this could be reflected by the much higher phenol and lower byproduct selectivities as well as water/ phenol ratios on mode-2 than mode-1. A higher phenol selectivity up to 90% and a lower cyclohexane and cyclohexanone selectivity below 9% were achieved in mode-2 for these Pd−TS membranes as shown in Figure 9. Especially, 98.5 and 97.4% phenol selectivities with a small amount of

Figure 9. (a) Benzene conversion, (b) phenol selectivity, and (c) cyclohexane and cyclohexanone selectivity on the different Pd−TS membrane reactors under two feed modes.

byproducts were obtained for Pd−TS-1p and Pd−Ti-MCM membranes, respectively. Furthermore, the water/phenol ratios had a more than 50% decrease in mode-2 than in mode-1 for Pd−TS-1p and Pd−Ti-MCM (Figure 10c). The dramatic difference of reaction results for Pd−TS membranes on mode-2 also proved the above assumption that the Ti-containing zeolite catalyst really participated in the reaction on this configuration. A low benzene conversion of 1.35% and phenol yield of 1.26% were obtained for the Pd−TS-1 membrane, probably resulting from the diffusion resistance of benzene/oxygen on the TS-1 layer owing to the compact structure of the film and small microporous channels of TS-1. Replacement of the TS-1 film by TS-1e or Ti-MCM-41 in Pd−TS-1e and Pd−Ti-MCM 5641

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achieved by the Pd−TS-1p membrane reactor because of its large intraparticle voids in the TS-1p layer. Figure 11 further illustrates the most possible diffusion and reaction pathway on the mode-2 configuration. The benzene/

Figure 11. Schematic of diffusion and reaction in Pd−TS membrane reactor (mode-2).

oxygen mixture must go through the titanium silicalite layer via inter- or intraparticle pores to react with the active hydrogen atom dissociated from the Pd membrane surface. By the analysis of the nitrogen flux (Figure 5) and surface morphologies of four Ti-containing zeolite films (Figures 3 and 4), it is recognized that the small pore size and compact structure of the TS-1 film would impede the reactants from diffusing into the channels, resulting in low benzene conversion and phenol yield. Faster diffusion in the TS-1e and Ti-MCM-41 film with the presence of large interparticle and mesoporous pores led to the increase of benzene conversion and phenol yield. The best benzene conversion and phenol yield correlated well with the fastest diffusion rate of reactants on TS-1p film. The different catalytic performances gave strong evidence that porous structure in the titanium silicalite catalysts significantly affects the reaction efficiency. The hydrogen conversion and water production results shown in Figure 10 further support the hypothesis that the titanium silicalite zeolite catalyzed the reaction compared with mode-1. The water/phenol ratio was 420−430 in mode-1 reactors with little differences for five Pd− TS composite membranes. However, in the mode-2 reactor, the water/phenol ratio was dramatically reduced; in particular Pd− TS-1p displayed the lowest water/phenol ratio of 188. This observation, together with higher benzene conversion and phenol yield, proved that the titanium silicalite catalysts could suppress the water generation and improve the hydrogen efficiency in mode-2 configuration. However, the influence of intraparticle pores and interparticle pores on the reactions could not be ignored. Figure 9 displays that the Pd−Ti-MCM membrane has slightly higher benzene conversion and phenol yield than Pd−TS-1e, which exhibited higher hydrogen conversion and water generation rate than Ti-MCM-41 modified Pd membrane (Figure 10). This reveals that more hydrogen was converted into water for the Pd−TS-1e membrane. Indeed, the water/phenol ratios in the Pd−TS-1e and Pd−Ti-MCM membranes were 289 and 206 (Figure 10c), respectively. The different diffusion pathways of reactants in the titanium silicalite layer were probably the main reason for this phenomenon. As illustrated in Figure 11, the diffusion of the oxygen/benzene mixture was fulfilled via interparticle pores between the neighboring crystals and intraparticle zeolite pores in TS-1e and Ti-MCM-41 layers, respectively. For Pd catalytic membrane, the hydrogen dissociated in the bulk metal and permeated to the opposite side followed by the reaction with oxygen to form active oxygen species (i.e., H2O2, O*, HO*, HOO*, etc.), which further reacted with benzene to convert phenol. The unreacted hydrogen atom could readily be reassociated to form molecular

Figure 10. (a) Hydrogen conversion, (b) water generation rate, and (c) water/phenol ratio on the different Pd−TS membrane reactors under two feed modes.

composite membranes resulted in immediate increases in benzene conversion and phenol yield. Higher benzene conversions of 4.32 and 4.87% and phenol yields of 3.96 and 4.74% were obtained by Pd−TS-1e and Pd−Ti-MCM composite membranes, respectively. The reduction of diffusion resistance due to the large interparticle pores of TS-1e and mesopores of Ti-MCM-41 could explain the increase of catalytic activity compared to Pd−TS-1. Especially, the benzene conversion and phenol yield in the Pd−TS-1p membrane are about 5 times higher than those in Pd−TS-1. The highest benzene conversion of 7.23% and phenol yield of 7.12% were 5642

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Figure 12. Proposed possible reaction pathways in Pd−TS composite membrane reactor (a, mode-1; b, mode-2).

from 322 to 509 compared to Pd−TS-1 membrane. Furthermore, the Pd−TS-1′ in the mode-2 reactor exhibited a much higher water/phenol ratio than five Pd−TS membranes under mode-1 configuration. These facts verified that the isolated framework titanium inside the zeolite channels played an important role in the benzene hydroxylation. The higher phenol selectivity of Pd−TS-1 also explained the function of titanium species in the reaction. These results manifest that it is possible to improve the direct hydroxylation of benzene to phenol by incorporating proper Ti-containing zeolite catalysts into the Pd membrane reactor. Figure 12 summarizes the proposed possible reaction pathways in the Pd−TS composite membranes for mode-1 and mode-2 based on the results and previous studies.6,35−37 It is likely that this membrane reactor could produce hydrogen peroxide and lead to the formation of hydroxyl radicals according to the previous publications.6,35,36 During the first step, hydrogen is dissociated while permeating through the Pd membrane to the opposite side in the form of H*. Then the active hydrogen could immediately react with oxygen to give H2O2, HOO*, and other active oxygen species. For the mode-1 configuration, the benzene/oxygen flow along the surface of the Pd membrane and the Ti-containing zeolite did not participate in the reaction; the benzene must timely react with these unstable active oxygen species in order to obtain a high material utilization efficiency. Otherwise, these unreacted unstable active oxygen species could easily convert into water. Actually, the water generation rate was high in mode-1 as shown in Figure 10. However, in the mode-2 reactor, the benzene/oxygen must diffuse across the Ti-containing zeolite catalysts and the titanium silicalite zeolite participated in the reaction. The framework titanium would absorb these unstable active oxygen species to form Ti−peroxo complex, avoiding the quick decomposition of active oxygen species.22−24 In that circumstance, more benzene could react with active oxygen species. This would help to explain why Pd composite membranes modified with Ti-containing zeolite catalysts can suppress the water production, and improve the hydrogen utilization efficiency without compromising the benzene conversion in mode-2.

hydrogen and desorbed from the Pd surface. It is noteworthy that these active oxygen species are unstable and could easily convert to water before reacting with benzene in time. Thus, it is clearly seen that the hydrogen efficiency is closely associated with the effective decomposition of active oxygen species. On the other hand, it is well-known that framework titanium could adsorb hydrogen peroxide to form a Ti−peroxo complex that could further react with a hydroxyl to form a stable complex of a five-membered cyclic structure to suppress the decomposition rate of hydrogen peroxide.22−24 The diffusion across the intraparticle pores facilitated the contact opportunity between reactants and the framework titanium which is identified as an active site for the Ti-containing zeolite catalysts. For these reasons, only the effective contact inside the zeolite pores was expected to suppress the water generation and enhance the raw materials utilization efficiency by the framework titanium atom. Therefore, Pd−Ti-MCM membrane exhibited higher phenol production (i.e., water/phenol ratio = 206 versus 289 for Pd− TS-1e). However, due to the slow mass transfer limited by the zeolite channels in Pd−TS-1e, the framework titanium was not being fully used during the reaction. This derivation is also supported by the experimental results of other Pd membrane reactors. A lower reactivity was observed in the Pd−TS-1 membrane because the contact opportunities between reactants and framework titanium were dramatically reduced due to the slow diffusion of reactants in the TS-1 layer resulting from the compact structure and small pore size of TS-1 zeolite. On the contrary, the TS-1p layer improved the diffusion rate owing to the presence of large intraparticle voids, leading to the increase of contact opportunity of reactants with framework titanium, which further produced the higher hydrogen utilization efficiency in the Pd−TS-1p membrane. Overall, the water/ phenol ratio of the Pd−TS-1p membrane was the lowest followed by Pd−Ti-MCM, Pd−TS-1e, and Pd−TS-1 (Figure 10c). The different reactivities of Pd−TS composite membranes are attributed to the combined effects of the porosity of titanium silicalite films and the contact opportunity between reactants and the framework titanium. To better understand the catalytic efficiency of titanium, Pd− TS-1′ membrane by incorporating extraframework titanium (Figure 8) was also prepared. It is very clear that the extraframework titanium species could decompose hydrogen peroxide and lead to low H2O2 efficiency.26,27,30,34 A higher hydrogen conversion and lower benzene conversion were obtained in the Pd−TS-1′ membrane compared to Pd−TS-1. Moreover, it is noteworthy that Pd−TS-1′ membrane displayed higher water production than Pd−TS-1. It can be seen from Figure 10c that the water/phenol ratio of Pd−TS-1′ increased

4. CONCLUSION The application of a Pd membrane reactor with Ti-containing zeolite catalysts in the direct hydroxylation of benzene to phenol has been investigated; in particular, the emphasis was focused on the clarification of the feed modes and catalysis. Ticontaining zeolite film has two benefits by not only providing an effective protective diffusion barrier for the deposition of a 5643

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(5) Ehrich, H.; Berndt, H.; Pohl, M. M.; Jähnisch, K.; Baerns, M. Oxidation of benzene to phenol on supported Pt-VOx and Pd-VOx catalysts. Appl. Catal., A 2002, 230, 271. (6) Niwa, S.; Eswaramoorthy, M.; Nair, J.; Raj, A.; Itoh, N.; Shoji, H.; Namba, T.; Mizukami, F. A one-step conversion of benzene to phenol with a palladium membrane. Science 2002, 295, 105. (7) Itoh, N.; Niwa, S.; Mizukami, F.; Inoue, T.; Igarashi, A.; Namba, T. Catalytic palladium membrane for reductive oxidation of benzene to phenol. Catal. Commun. 2003, 4, 243. (8) Sato, K.; Niwa, S.; Hanaoka, T.; Komura, K.; Namba, T.; Mizukami, F. Direct hydroxylation of methyl benzoate to methyl salicylate by using new Pd membrane reactor. Catal. Lett. 2004, 96, 107. (9) Sato, K.; Hanaoka, T.; Niwa, S.; Stefan, C.; Namba, T.; Mizukami, F. Direct hydroxylation of aromatic compounds by a palladium membrane reactor. Catal. Today 2005, 104, 260. (10) Sato, K.; Hanaoka, T.; Hamakawa, S.; Nishioka, M.; Kobayashi, K.; Inoue, T.; Namba, T.; Mizukami, F. Structural changes of a Pdbased membrane during direct hydroxylation of benzene to phenol. Catal. Today 2006, 118, 57. (11) Vulpescu, G. D.; Ruitenbeek, M.; van Lieshout, L. L.; Correia, L. A.; Meyer, D.; Pex, P. A. C. One-step selective oxidation over a Pdbased catalytic membrane; evaluation of the oxidation of benzene to phenol as a model reaction. Catal. Commun. 2004, 5, 347. (12) Sayyar, M. H.; Wakeman, R. J. Comparing two new routes for benzene hydroxylation. Chem. Eng. Res. Des. 2008, 86, 517. (13) Shu, S. L.; Huang, Y.; Hu, X. J.; Fan, Y. Q.; Xu, N. P. On the membrane reactor concept for one-step hydroxylation of benzene to phenol with oxygen and hydrogen. J. Phys. Chem. C 2009, 113, 19618. (14) Bortolotto, L.; Dittmeyer, R. Direct hydroxylation of benzene to phenol in a novel microstructured membrane reactor with distributed dosing of hydrogen and oxygen. Sep. Purif. Technol. 2010, 73, 51. (15) Dittmeyer, R.; Bortolotto, L. Modification of the catalytic properties of a Pd membrane catalyst for direct hydroxylation of benzene to phenol in a double-membrane reactor by sputtering of different catalyst systems. Appl. Catal., A 2011, 391, 311. (16) Wang, X. B.; Zhang, X. F.; Liu, H. O.; Qiu, J. S.; Han, W.; Yeung, K. L. Investigation of Pd membrane reactors for one-step hydroxylation of benzene to phenol. Catal. Today 2012, 193, 151. (17) Wang, X. B.; Tan, X. Y.; Meng, B.; Zhang, X. F.; Liang, Q.; Pan, H.; Liu, S. M. One-step hydroxylation of benzene to phenol via a Pd capillary membrane microreactor. Catal. Sci. Technol. 2013, 3, 2380. (18) Wang, X. B.; Guo, Y.; Zhang, X. F.; Wang, Y.; Liu, H. O.; Wang, J. Q.; Qiu, J. S.; Yeung, K. L. Catalytic properties of benzene hydroxylation by TS-1 film reactor and Pd-TS-1 composite membrane reactor. Catal. Today 2010, 156, 288. (19) Wang, X. B.; Tan, X. Y.; Meng, B.; Zhang, X. F.; Liang, Q.; Pan, H.; Liu, S. M. TS-1 zeolite as an effective diffusion barrier for highly stable Pd membrane supported on macroporous α-Al2O3 tube. RSC Adv. 2013, 3, 4821. (20) Taramasso, M.; Perego, G.; Notari, B. Preparation of porous crystalline synthetic material comprised of silicon and titanium oxides. U.S. Patent 4,410,501, 1983. (21) Wang, X. B.; Zhang, X. F.; Wang, Y.; Liu, H. O.; Qiu, J. S.; Wang, J. Q.; Han, W.; Yeung, K. L. Investigating the role of zeolite nanocrystal seeds in the synthesis of mesoporous catalysts with zeolite wall structure. Chem. Mater. 2011, 23, 4469. (22) Bellussi, G.; Carati, A.; Clerici, M. G.; Maddinelli, G.; Millini, R. Reactions of titanium silicalite with protic molecules and hydrogen peroxide. J. Catal. 1992, 133, 220. (23) Clerici, M. G.; Ingallina, P. Epoxidation of lower olefins with hydrogen peroxide and titanium silicalite. J. Catal. 1993, 140, 71. (24) Wu, P.; Komatsu, T.; Yashima, T. Hydroxylation of aromatics with hydrogen peroxide over titanosilicates with MOR and MFI structures: effect of Ti peroxo species on the diffusion and hydroxylation activity. J. Phys. Chem. B 1998, 102, 9297. (25) Wang, X. B.; Zhang, X. F.; Liu, H. O.; Yeung, K. L.; Wang, J. Q. Preparation of titanium silicalite-1 catalytic films and application as catalytic membrane reactors. Chem. Eng. J. 2010, 156, 562.

dense Pd membrane on macroporous alumina ceramic tube, but also being the important catalyst to improve the benzene hydroxylation reaction. Membrane configuration and porosity of the titanium silicalite layer are important factors affecting the reaction efficiency. Larger inter- or intraparticle pores promoted the effective contact between reactants and framework titanium, leading to the higher reactivity and lower water production rate. The framework titanium could adsorb active oxygen species to form Ti−peroxo species and suppress the decomposition, and the extraframework titanium promoted the decomposition of active oxygen species resulting in more water production. Intraparticle pores were more favorable for reactants to reach the isolated framework titanium active site than intercrystalline pores. Indeed, the Pd−Ti-MCM membrane possessed intraparticle mesopores and exhibited the decrease of the water/ phenol ratio up to 28% compared to Pd−TS-1e possessing large interparticle pores in the titanium silicalite layer. Especially the Pd−TS-1p membrane with intracrystalline voids enabled the highest benzene conversion (7.2%) and phenol yield (7.1%) as well as the lowest water/phenol ratio (188) compared to the other Pd membranes because the framework titanium in the zeolite pores could significantly suppress the water production and increase the hydrogen utilization efficiency. It is possible to improve the direct hydroxylation of benzene in a Pd membrane reactor by incorporating the proper Ti-based zeolite catalyst. Our ongoing efforts are devoted to depositing a loose nanosized titanium silicalite zeolite with high catalytic activity on the surface of a Pd capillary membrane to further match the synergistic effects between the Pd membrane and Ti-based zeolite catalysts.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 533 2786292; Fax: +86 533 2786292; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant 21276147), the Natural Science Foundation of Shandong Province (Grant ZR2012BQ003), and the Young Teacher Supporting Scheme of Shandong University of Technology.

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