Synthesis, Structural Characterization, and Catalytic Properties of Tungsten-Exchanged H-ZSM5

W-exchanged H−ZSM5 was prepared by sublimation of WCl 6 at 673 K followed by hydrolysis of exchanged WCl x species at 523 K. D 2 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 2 ) 2+ containing W 6+ species bridging two cation exchange sites. As temperatures reached ∼973 K during D 2 −OH exchange, these species reduced to (WO 2 ) + with the concurrent formation of one OD group. CH 4 conversion turnover rates (per W) and C 2 −C 12 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 x /H-ZSM5 precursors are initially inactive in CH 4 reactions, but they activate during induction with the concurrent evolution of CO, H 2 O, and an excess amount of H 2 . The reduction and carburization processes occurring during CH 4 reactions and the structure of the exchanged WO x precursors was probed using in situ X-ray absorption spectroscopy (XAS). XAS studies confirmed the isolated initial nature of the exchanged WO x precursors after hydrolysis and dehydration and the formation of WC x clusters ∼0.6 nm in diameter during CH 4 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 x precursors and their subsequent carburization during CH 4 reactions, however, are more difficult than the corresponding processes for the MoO x counterparts. This may account for previous reports of lower CH 4 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.