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

• Published in: The journal of physical chemistry. B Vol. 105; no. 18; pp. 3928 - 3936
• Main Authors: Ding, Weiping, Meitzner, George D, Marler, David O, Iglesia, Enrique
• Format: Journal Article
• Language: English
• Published: American Chemical Society 10.05.2001
• DOI: 10.1021/jp003413v
• ISSN: 1520-6106

W-exchanged H−ZSM5 was prepared by sublimation of WCl6 at 673 K followed by hydrolysis of exchanged WCl x species at 523 K. D2 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 (WO2)2+ containing W6+ species bridging two cation exchange sites. As temperatures reached ∼973 K during D2−OH exchange, these species reduced to (WO2)+ with the concurrent formation of one OD group. CH4 conversion turnover rates (per W) and C2−C12 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 CH4 reactions, but they activate during induction with the concurrent evolution of CO, H2O, and an excess amount of H2. The reduction and carburization processes occurring during CH4 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 CH4 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 CH4 reactions, however, are more difficult than the corresponding processes for the MoO x counterparts. This may account for previous reports of lower CH4 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.