Influence of Extraframework Metal Species on the Kinetics and Spatial Distribution of Coke during Methanol-to-Hydrocarbons Conversion over ZSM-5 Catalysts

Abstract

The incorporation of selected extraframework cations is a common approach to tuning the performance of zeolite catalysts in industrially relevant methanol-to-hydrocarbons (MTH) conversion processes, although the mechanistic origin of their effects remains incompletely understood. In this study, we investigated the relationships between MTH and coking kinetics, the nature and distribution of coke deposits, and the positions of extraframework calcium and zinc species in the ZSM-5 catalyst, which promote alkene and aromatic formation, respectively. At comparable Brønsted acid site densities, enhanced aromatics formation over zinc-containing ZSM-5 is accompanied by a 2× higher intrinsic MTH activity and a 150× faster active site loss than over the calcium-containing catalyst. Photoelectron photoion coincidence spectroscopy shows that faster coke-forming kinetics over the zinc-containing catalyst is linked to enhanced formaldehyde formation via methanol dehydrogenation, triggering a cascade of intermediates, such as dienes and aromatics, a pathway inhibited in the calcium-containing catalyst. This also leads to a higher fraction of high-molecular-weight coke in the zinc-containing catalyst and a predominance of surface graphitic coke in the calcium-containing catalyst, as revealed by complementary thermogravimetric, microscopic, and spectroscopic analyses. Powder neutron diffraction indicates that coke preferentially occupies the channel intersections and extends along the straight channels in both catalysts. However, the average coke orientation differs as calcium-containing ZSM-5 comprises more distorted intrapore aromatic deposits, whereas zinc-containing ZSM-5 accommodates more planar aromatic coke. The confinement of coke in micropores is consistent with the preferential location of cations determined by high-resolution X-ray diffraction analysis, which shows that calcium occupies two sites in the sinusoidal channels, with one of these pointing toward the channel intersections, whereas the smaller zinc-associated density is found at sites in five- and six-membered rings between main channels, at the expense of occupancy of sinusoidal channel-based positions. Consequently, faster coke-forming kinetics linked to the dehydrogenation activity of extraframework zinc species, together with lower spatial strain at channel intersections, favor the formation of more condensed, planar-like coke in zinc-containing catalysts. Conversely, the larger size and higher channel occupancy of calcium sites destabilize aromatics and hinder coke formation in this key intersecting zone. These results show that extraframework species influence not only the reaction kinetics but also the local organization and orientation of confined coke, in part through steric constraints associated with their crystallographic location.