Silver‐Promoted Dehydroaromatization of Ethylene over ZSM‐5 Catalysts

Abstract

AbstractThe shape selectivity of ZSM‐5 (MFI type) catalysts is ideal for the production of C6–C8 aromatics. Developing high‐performance zeolite catalysts with improved selectivity to aromatics, particularly from diversified (non‐petroleum) feedstocks, has broad commercial appeal. Non‐oxidative coupling (NOC) of ethylene was examined over Ag‐ZSM‐5 catalysts at 400 °C and shows that Ag+ sites promote dehydroaromatization with enhanced selectivity to toluene and xylenes. Metal exchange of H‐ZSM‐5 results in Ag zoning wherein Ag+ site density is higher on the exterior of ZSM‐5 particles. Catalyst performance was characterized with varying Ag loading as well as the use of methane co‐feed. Aromatic selectivity is about 60 % on Ag‐ZSM‐5 compared to 20 % on H‐ZSM‐5, which is qualitatively consistent with density functional theory (DFT) showing that ethylene forms strong complexes with Ag+ (Lewis acid) sites. DFT calculations also reveal that ethylene activation on H+ (Brønsted acid) sites is more energetically favorable, and likely constitutes the first mechanistic step in ethylene‐to‐liquids (ETL) reactions. Ag‐ZSM‐5 is thus identified as an effective catalyst for low‐temperature ETL reactions that has the potential to outperform conventional metal‐exchanged zeolites.