The mechanisms of ethene and propene formation from methanol over high silica H-ZSM-5 and H-beta

Selectivity control is a key issue in the zeolite facilitated conversion of methanol to hydrocarbons. This work addresses: (a) How the zeolite topology (MFI versus BEA) determines the type of intermediate participating in the catalytic cycle thereby controlling the ethene/propene product selectivity in the methanol to alkenes reaction, and (b) to what extent light alkene formation occurs via aromatic and alkene reaction intermediates for H-ZSM-5. Three catalyst samples have been studied in experiments relying on isotopic labeling. For H-beta, penta- and hexamethyl benzene are involved in an aromatics based hydrocarbon pool type mechanism leading predominantly to propene and the higher alkenes (a very low yield of ethene is observed), whereas for H-ZSM-5, the lower methylbenzenes are the aromatic intermediates and represent the major route for ethene formation. For H-ZSM-5, an alkene based and an aromatics based catalytic cycle for product formation are distinguishable. By conducting co-reaction experiments (MeOH + propene or MeOH + p-xylene) over H-ZSM-5 we were able to evaluate the relative rates of ethene and propene formation from both the alkene and the aromatics based catalytic cycles. (C) 2009 Elsevier B.V. All rights reserved.