(349z) Spatiotemporal Coke Coupling Enhances Para-Xylene Selectivity in Highly Stable MCM-22 Catalysts

Para-xylene is one of the most important aromatic compounds used in the synthesis of various fine chemicals. Toluene alkylation with methanol (TAM) catalyzed by zeolites is an emerging and commercially attractive route to produce p-xylene; however, this process often suffers from low catalyst stability and requires the use of diluents (hydrogen and/or water), low space velocity, and high toluene-to-methanol ratios, which collectively results in low p-xylene yield. Here, we present a study where we have compared the performance of medium pore zeolites for TAM under high pressure conditions (4.2 MPa).

Our findings reveal that ZSM-23 and TNU-10 deactivate rapidly due to high diffusion limitations and a propensity to form coke within large pores (i.e. supercages, Figure 1a). Alternative structures, ZSM-5, IM-5 and TNU-9, show moderately higher catalyst stability, but experience significant coke build-up in channel intersections. Among the zeolites tested, MCM-22 shows exceptional catalyst lifetime with the highest p-xylene yield reported to date.

In this presentation, we will discuss the role of topological features on TAM reactions in these zeolites, based on a combination of catalyst testing, density functional theory (DFT) calculations, and molecular dynamic simulations. Our findings reveal that active sites in external surface pockets of MCM-22 are unselective and their deactivation is necessary to achieve high p-xylene selectivity. We also show that the nature of coke species in supercages greatly influences catalyst performance through a unique pathway that is referred to as spatiotemporal coke coupling (Figure 1b). Overall, this study identifies effective zeolite catalysts for p-xylene production as well as the new insights into the role of coking.