A temporal analysis of products (TAP) study of C2-C4 alkene reactions with a well-defined pool of methylating species on ZSM-22 zeolite

[Display omitted] •Alkene methylation on ZSM-22 zeolite was studied by TAP pulse-response experiments.•Alkene adsorption and diffusion within the zeolite nanopores does not conform to simple models.•Low-pressure flow of DME at 400 °C followed by evacuation produces a stable population of methoxy groups with 5% coverage.•The kinetics of alkene reactions with the methoxy groups agree well with DFT-derived kinetics.•TAP provides a unique bridge between ab initio and experimental kinetics of MTH-relevant chemistry. Reactions between alkenes and methanol or dimethyl ether (DME) on zeolite catalysts are involved in industrial processes that are highly relevant for the transition to renewable carbon sources, such as the Methanol-To-Hydrocarbons (MTH) process. In MTH chemistry, alkene methylation increases the length of product carbon chains, and its relative rate with respect to other reactions largely controls the overall selectivity. Experimental studies of alkene methylation present a considerable challenge because they are typically accompanied by cracking, hydrogen transfer, and aromatization reactions. Herein, the pulse-response Temporal Analysis of Products (TAP) methodology and complementary FTIR measurements were employed to isolate a well-defined population of surface species, consistent with Surface Methoxy Species (SMS) on Bronsted acid sites that are reactive in alkene methylation on a ZSM-22 (TON) zeolite. Their coverage was determined by TAP titration to be ca. 5% of the total amount of Brønsted acid sties, which was also indirectly suggested by FTIR data. C2-C4 alkenes were quantitatively reacted with SMS to precisely measure the intrinsic kinetic parameters of isolated alkene methylation steps. The rate constants increased and the activation energies decreased as functions of the carbon number (EC2H4 = 52 kJ/mol > EC3H6 = 32 kJ/mol > Ec-C4H8 = 16 kJ/mol). However, the rate constant for iso-butene was comparable to propene, despite its activation energy (Ei-C4H8 = 19 kJ/mol) being much lower than propene’s. This effect is in agreement with the increased steric hindrance predicted by DFT for isobutene adsorption and methylation in TON zeolites. Our results considerably extend previously available TAP data on alkene methylation reactions and furthermore validate ab initio models of these crucial steps in the complex MTH chemistry on acidic zeolites.