Synthesis, structural characterization, and catalytic properties of tungsten-exchanged H-ZSM5

W-exchanged H-ZSM5 was prepared by sublimation of WCl at 673 K followed by hydrolysis of exchanged WCl species at 523 K. D exchange with residual OH groups showed that each W initially replaced about two zeolitic protons for W/Al ratios of 0.29 and 0.44, consistent with the formation of (WO) containing W species bridging two cation exchange sites. As temperatures reached ∼973 K during D-OH exchange, these species reduced to (WO) with the concurrent formation of one OD group. CH conversion turnover rates (per W) and C-C selectivities are very similar to those observed on a Mo/H-ZSM5 sample with similar cation exchange level. As in the case of Mo/H-ZSM5, WO/H-ZSM5 precursors are initially inactive in CH reactions, but they activate during induction with the concurrent evolution of CO, HO, and an excess amount of H. The reduction and carburization processes occurring during CH reactions and the structure of the exchanged WO precursors was probed using in situ X-ray absorption spectroscopy (XAS). XAS studies confirmed the isolated initial nature of the exchanged WO precursors after hydrolysis and dehydration and the formation of WC clusters ∼0.6 nm in diameter during CH reactions at 973 K. The structural and catalytic resemblance between W- and Mo-exchanged H-ZSM5 is not unexpected, in view of chemical similarities between oxides or carbides of Mo and W. The synthesis of exchanged WO precursors and their subsequent carburization during CH reactions, however, are more difficult than the corresponding processes for the MoO counterparts. This may account for previous reports of lower CH reaction rates and aromatics selectivities on W/H-ZSM5 compared with those observed on Mo/H-ZSM5 and with those reported here for rigorously exchanged W/H-ZSM5.