Mechanistic insight into low temperature toluene production via benzene methylation over mesopore-rich fibrous silica HZSM-5 zeolite

Abstract

The fibrous silica HZSM-5 (HFZ) was synthesized by microemulsion method assisted with ZSM-5 seed crystallization under different amounts of a silica source, tetraethyl orthosilicate (TEOS, 1.0–1.5 mol) for superior benzene methylation activity. The aptness of the physicochemical properties of HFZ offered an abundance of active sites, allow benzene conversion up to 90% and toluene production at the lower temperature of 300 °C, with the following order of yields: 1.0HFZ (66.1%) > 1.5HFZ (46.2%) > commercial HZSM-5 (41.9%). Additionally, the less extraframework alumina-possessing 1.0HFZ was found to display the lowest activation energy and substrate diffusion limitation, compared to microstructured commercial HZSM-5 (HZ). The 1.0HFZ also possessed a lower Lewis acidic nature and able to decrease subsequent production of ethylbenzene (EB) while significantly improved further methylation to xylene. The in-situ FTIR study gave insights on the adsorption of both methanol and benzene on the active sites of catalyst, revealing that 1.0HFZ followed the associative mechanism in which both methanol and benzene were adsorbed onto the active sites. Meanwhile, HZ followed the consecutive mechanism due to the presence of intermediate methoxy.