Effects of methylating agent and Brønsted acidity on methylation activity of olefins in CHA-structured zeolites: A periodic DFT study

Abstract

Chemical conversion of methane into light olefins involving intermediate CH3X (X = OH, SH, Cl, and Br) provides an alternative to syngas-based technologies. In this contribution, periodic density functional theory calculations with van der Waals correlation have been performed to achieve a full picture of how the CH3X methylating agents would affect the catalytic activity of olefin methylation in H-SAPO-34 and other metal-substituted CHA-structured zeotypes. The methylation enthalpy barriers depend strongly on the protonation ability of the methylating agent and how readily the CX bond can be broken. Gibbs free energy calculations indicate that the reactivity of different methylating agents decreases in the trend CH3OH > CH3Br ∼ CH3Cl > CH3SH. The linear transition state enthalpy scaling relations expressed in terms of adsorption enthalpy of ammonia hold very well for the coupling of propene and tetramethylethene to all the methylating agents. The olefin methylation by CH3Cl and CH3Br is much less sensitive to acid strength than that by CH3OH and CH3SH. The charge transfer between the organic species and the zeolite framework, and hence the nature of ion-pair interaction, play a key role in determining the sensitivity of the olefin methylation to acid strength.