Spatiotemporal Coke Coupling Enhances para-Xylene Selectivity in Highly Stable MCM-22 Catalysts.

ABSTRACT

Identifying zeolite catalysts that can simultaneously optimize p-xylene selectivity and feed utilization is critical to toluene alkylation with methanol (TAM). Here, we show that zeolite MCM-22 (MWW) has an exceptional catalyst lifetime in the TAM reaction at high operating pressure, conversion, and selectivity. We systematically probe the catalytic behavior of active sites in distinct topological features of MCM-22, revealing that high p-xylene yield and catalyst stability are predominantly attributed to sinusoidal channels and supercages, respectively. Using a combination of catalyst design and testing, density functional theory, and molecular dynamics simulations, we propose a spatiotemporal coke coupling phenomenon to explain a multistage p-xylene selectivity profile wherein the formation of light coke in supercages initiates the deactivation of unselective external surface sites. Our findings indicate that the specific nature of coke is critical to catalyst performance. Moreover, they provide unprecedented insight into the synchronous roles of distinct topological features giving rise to the exceptional stability and selectivity of MCM-22 in the TAM reaction.