A hierarchical Fe/ZSM-5 zeolite with superior catalytic performance for benzene hydroxylation to phenolw

Phenol is an important industrial intermediate for the production of various chemicals such as bisphenol A, phenolic resins, caprolactam, alkylphenols and adipic acid. The cumene hydroperoxide process is the currently used industrial method to produce phenol. Its environmentally stressing aspects have spurred research to replace this process with more benign approaches such as the direct oxidation of benzene to phenol with nitrous oxide. In Solutia’s AlphOx technology, the nitrous oxide by-product of the manufacture of adipic acid has been considered as a cheap source, because otherwise it has to be treated as waste. The most active and selective catalyst is Fe/ZSM-5 zeolite, but the strong catalyst deactivation due to formation of carbonaceous side-products remains an obstacle in its commercialization. As these deposits can be easily removed by calcination, the process can be operated in a cyclic manner. Yet, the economic viability is hampered by the relatively strong deactivation of the catalyst. Thus, increasing the catalytic activity or decreasing the rate of deactivation remains a research goal. Steamcalcination of Fe/ZSM-5 substantially improves the catalytic activity and the lifetime before regeneration is required. The effect of steaming is understood in terms of an increased number of active Fe sites. Hierarchical zeolites are of considerable current interest. The creation of mesoporosity in ordinary zeolite crystals strongly improves the catalytic performance in reactions that tend to suffer from diffusion limitations. A relevant example is the creation of mesoporosity in HZSM-5 with trace amounts of iron by desilication. Such a catalyst has a lower activity than Fe/ZSM-5 materials. Herein, we report that a hierarchical Fe/ZSM-5 catalyst outperforms an optimized steam-calcined Fe/ZSM-5 zeolite catalyst: the turnover number to phenol of the hierarchical zeolite is almost four times higher after 24 h on stream. The high accessibility of the small microporous domains renders the zeolite more resistant to deactivation by coke deposits. In our approach, we added [3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride ([(CH3O)3SiC3H6N(CH3)2C18H37, TPOAC) to the gel to prepare Fe/ZSM-5. The use of amphiphilic organosilanes has recently been explored as a straightforward method to prepare HZSM-5 zeolite with a high degree of mesoporosity. This strategy is based on the covalent interaction of a long-chain alkylammonium surfactant molecule with the growing zeolite crystal surface. The linkage is brought about by a hydrolysable methoxysilyl moiety in the amphiphilic surfactant. In their acidic form, these mesoporous zeolites exhibit improved performance in a number of demanding reactions. The active sites for benzene oxidation with nitrous oxide, however, are highly dispersed Fe centers. These ions are formed upon autoreduction of isolated or at least very dispersed extra-framework ferric ions in the zeolite micropores. The desired initial dispersion is preferably obtained by isolating the Fe ions in the zeolite framework during hydrothermal synthesis. Thus Fe was added as the nitrate to a gel to prepare hierarchical ZSM-5 with the final molar composition Al2O3/Fe2O3/Na2O/SiO2/ TPABr/H2SO4/H2O/TPOAC = 1.2/0.26/40/95/10/26/9000/5. The gel was autoclaved at 150 1C for 4 days (mesoFe/ZSM-5-as). We compared direct calcination in air at 550 1C (mesoFe/ZSM-5) to the alternative procedure which involved the removal of part of the mesoporogen in mesoFe/ZSM-5 by extraction with methanol in a Soxhlet apparatus prior to calcination (mesoFe/ZSM-5-E). The final activation step involved steam calcination at 700 1C following established methods (mesoFe/ ZSM-5-S and mesoFe/ZSM-5-ES). A reference Fe/ZSM-5 catalyst was prepared according to a standard recipe (Fe/ZSM-5-S). The crystallinity of calcined and steamed mesoFe/ZSM-5 zeolites is evident from the vibrational band at 550 cm 1 in their infrared spectra, which is due to the double five-ring vibration of ZSM-5, and the XRD patterns (ESIw). The crystallinity of the hierarchical zeolites estimated by IR spectroscopy is considerably higher (>90%) than that estimated by XRD (50–60%). Fig. 1 shows the phenol reaction rates of the steamed catalysts. The initial reaction rate of mesoFe/ZSM-5-ES is substantially higher than that of conventional steam-calcined Fe/ZSM-5. Clearly, deactivation of mesoFe/ZSM-5-ES is much lower compared to Fe/ZSM-5-S. As a result, the reaction rate of the hierarchical zeolite is almost four times a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, China. E-mail: [email protected]; Fax: +86-411-84694447; Tel: +86-411-84379070 b Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: [email protected]; Fax: +31-40-2455054; Tel: +31-40-2475178 w Electronic supplementary information (ESI) available: Experimental details, UV-Vis, XRD, IR, SEM, active site titration, spent catalyst analysis, reaction data. See 10.1039/b917038c