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

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 Bronsted acid protons b...