Conversion of methanol to hydrocarbons: II. Reaction paths for olefin formation over HZSM-5 zeolite catalyst

Abstract

Methanol was converted to hydrocarbons, primarily olefins, and water over a low-activity HZSM-5 zeolite catalyst with a SiO 2 Al 2O 3 ratio of 1600/1. Mixtures of methanol and individual C 2-C 4 olefins were also used to simulate conditions in the catalyst bed at low conversions. Facile methylation of olefins with methanol to produce the next higher olefinic homolog was observed. Ethylene was the major primary hydrocarbon produced from methanol at low conversion. The olefin mixtures isolated under various conditions of reaction were fitted to the chain-growth kinetics as described by the Flory equation. Good to excellent correlation coefficients were found at low and medium conversions of methanol. This is consistent with a stepwise growth in molecular weight of the olefins, starting with propylene, by alkylation of the olefins with methanol. However, since ethylene is produced by a different mechanism, it does not fit the Flory equation. At higher temperatures and conversions, the olefins undergo scrambling or thermodynamic equilibration reactions also producing olefin mixtures with good correlation coefficients. Under the latter conditions, a distinction between the stepwise growth and thermodynamic equilibration reactions cannot be made since both contribute to the product mixture.