Structure and Reactivity of Zn-Modified ZSM-5 Zeolites: The Importance of Clustered Cationic Zn Complexes

Abstract

A novel route for the introduction of well-defined zinc species into ZSM-5 zeolite via chemical vapor deposition of dimethylzinc (CVD(DMZ)) is explored. The structural properties and catalytic reactivity of the synthesized material (Zn/ZSM-5-CVD(DMZ)) are investigated against a set of Zn/ZSM-5 catalysts prepared by incipient wetness impregnation (IWI), ion exchange (IE), and high-temperature reaction with zinc vapor (CVD(m)). The materials are characterized by a range of physicochemical methods including temperature programmed reduction (TPR), in situ FTIR, magic-angle spinning (MAS) NMR, and X-ray absorption spectroscopy (XAS). The catalysts are tested for their activity in the dehydrogenation of propane. Catalysts prepared by IE and IWI exhibit a high degree of heterogeneity of extraframework zinc species. These include, besides isolated Zn2+ cations, multinuclear oxygenated zinc clusters and bulk zinc oxide aggregates. The CVD(m) method results in quantitative replacement of all Brønsted acid protons by isolated Zn2+. In CVD(DMZ) the Brønsted acid sites (BAS) react stoichiometrically with dimethylzinc Zn(CH3)2 (DMZ) yielding grafted [Zn-CH3]+ species, which can further be transformed to isolated Zn2+ ions by reduction in hydrogen. The presence of zinc in ZSM-5 enhances the rate of alkane dehydrogenation. The initial activity of Zn/ZSM-5 prepared by IWI and IE correlates with the Zn content. The samples with a more heterogeneous distribution of extraframework Zn species are more active than the samples with isolated Zn2+. The activity of reduced Zn/ZSM-5-CVD(DMZ) containing predominantly isolated Zn2+ ions can be substantially increased by oxidation prior to the reaction. However, the resulting oxygenated complexes easily decompose during the reaction. Propane dehydrogenation and catalyst stability of Zn/ZSM-5-CVD(DMZ) can be improved by addition of steam to the hydrocarbon feed. This rate enhancement is ascribed to an increase of the steady-state concentration of the reactive oxygenated sites.