A systematic study of CO methylation by surface methoxy species in different zeolites

Ketene chemistry is a topic of continuous investigation in zeolite catalysis, as it is key to carbon valorization processes towards high value hydrocarbons. Ketene formation typically occurs through CO methylation by a surface methoxy species (SMS) formed at Brønsted acid sites of zeolites. Here, we present a systematic study of SMS carbonylation reactions on all the possible active sites of six of the most commonly used zeolite frameworks: MOR, FER, CHA, MFI, Beta polymorph A (BEA), and FAU. Density Functional Theory (DFT) calculations were performed using the PBE-D3 functional with single point calculations with the ω B97M-D4 functional. The lowest Gibbs free energy barrier obtained for CO methylation by an SMS is 143 kJ mol −1 at the most favorable site of both MFI and MOR zeolites. For MOR, this is the well-known position in the side pocket, which allows for a favorable positioning of the transition state with a high dispersion contribution from the surrounding framework. On FER and BEA zeolites, comparable sites do not exist, and thus higher free energy barriers were obtained. The CHA and FAU zeolites, which present large cavities, showed the highest barriers, indicating their lower activity for this reaction, compared to other reactions with larger molecules. The results were compared to those obtained experimentally, with an overall good agreement in terms of carbonylation rates