About the Mechanism of Methanol Conversion on Zeolites

Schulz Hans

摘要

Time- and space resolved selectivity in methanol conversion on zeolites has been investigated and exploited for insight about the reaction mechanism. Particular focus is directed to constraints on diffusion and reaction provided by the pore structure. 3 zeolites with well-ordered voids and channels of different size have been used comparatively. Novel experimental tools have been developed. Product composition is normalized, providing a deactivation pattern, which reflects the reaction mechanism, as changing with time and characterizing essential constraints. The higher product complexity, the more information is present about the mechanism. But in methanol conversion on zeolites, there are difficulties in making use this wealth of information-selectivity meaning a mixture of many compounds, ranging from gases to liquids and substances retained on the catalyst. As catalysts, zeolites with 8-, 10- and 12-ring channels and distinct void's size have been used in a specifically designed fixed bed reactor, applying ampoule product sampling and advanced chromatography. On catalyst retained substances are taken in account and deactivation is elucidated profoundly. Progress about the mechanism of hydrocarbon formation concerns autocatalysis, constraints with zeolite topologies and changes with time when "C-Retained" accumulates in the pores or on surface of the zeolite crystallites. A surprising reanimation regime with zeolite HZSM5 has been observed and disclosed as thermodynamically controlled arene alkylation with olefins. At low temperature with HZSM5, a different reaction mechanism-as compared with application at high temperature-is noticed, with regimes of incubation, acceleration, and deactivation-at short catalyst live time. The novel experimental techniques allow revealing chemistry in the migrating reaction zone-and in the zones of deactivated catalyst behind, and of fresh catalyst in front. Comparing the results with the 3 zeolite topologies provides insight about controlling constraints.

About the Mechanism of Methanol Conversion on Zeolites

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