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Zinc-ZSM-5 catalysts which contained zinc either in the framework ([Zn]ZSM-5) or in the extraframework (ZnZSM-5) positions were prepared by a gas-solid (ZnClz(g) + HZSM-5) reaction or by ion exchange. Characterizalion using XRD, IR, TPDA, and 27Al MAS NMR showed that isomorphous substitution of the framework aluminium for zinc occurred during the gas-solid reaction at 773 K. A loss of strong acidity and a generation of weak acidity were observed as a result of the substitution. Zinc insertion into the vacancies formed by aluminium migration out of the framework was assumed to be responsible for the substitution. In contrast to ZnZSM-5, H[ZnlZSM-5 showed no activity in the hydrogenation of benzene and in promoting aromatization of propylene. |
