Alkaline Modification of MCM-22 to a 3D Interconnected Pore System and its Application in Toluene Disproportionation and Alkylation
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Modification of HMCM-22 zeolite by alkaline treatment was investigated by various characterization techniques and in toluene disproportionation and alkylation with isopropyl alcohol. This ‘desilication’ process led for mild alkaline concentrations (~0.10–0.20 M NaOH at 323 K for 45 min) to the partial destruction of the zeolite framework, but also to the formation of additional mesoporosity. Furthermore, the accessibility/availability of Lewis acid sites, investigated by d 3-acetonitrile and pyridine adsorption using FTIR spectroscopy, increased for these mild alkaline treatments, while the Brønsted acidity decreased. Higher alkaline concentrations (up to 0.50 M NaOH) led to a too severe framework and pore destruction and a decrease of both the Lewis and Brønsted acid site concentration. Decomposition and deconvolution of 29Si MAS-NMR spectra confirmed the Si extraction and partial framework destruction, since more Q3 SiOH groups were formed at the expense of the Q4 T-atoms in the framework. Furthermore, the T6 and T7 Si-atoms were preferentially extracted, which would indicate that an interconnection between the intralayer and the interlayer and/or outer surface is formed. The toluene conversion in its disproportionation reaction increased for the mildly treated sample, while the selectivity to xylene isomers (and cymene and n-propyltoluene isomers in the alkylation reaction with isopropyl alcohol) was similar to the thermodynamic equilibrium, suggesting that the reaction primarily occurs at outer surface cups of the HMCM-22 zeolite.
KeywordsDesilication Mesopores MWW 29Si MAS-NMR Toluene disproportionation and alkylation
The authors would like to thank the Marie Curie Research Training Networks (EU FP6 INDENS project no.: MRTN-CT-2004-005503) for financial support. The work of J. Čejka was also supported by the Grant Agency of the Czech Republic (203/08/0604). Linn Sommer from the University of Oslo is acknowledged for the measurement of the N2 sorption isotherms.
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