Probing into the building and evolution of primary hydrocarbon pool species in the process of methanol to olefins over H-ZSM-5 zeolite

Abstract

It is well known that the reaction process of methanol to olefins (MTO) keeps to the hydrocarbon pool (HCP) mechanism; however, there is no clear consensus over how the primary HCP species are built and how they evolve in the transition period of MTO. In this work, the building and evolution of the primary HCP species for MTO over H-ZSM-5 zeolite were probed through combined pulse-quench, GC–MS and 13C-methanol labeling techniques. The results indicate that propene/ethene carrying the first C–C bonds formed in the induction period can quickly constitute long chain alkenes through oligomerization. Subsequently, the long chain alkenes are inclined to form methylcyclopentadienes (MCPdienes) via cyclization. After that, methylbenzenes (MBs) are produced through the ring expansion of MCPdienes. Both MCPdienes and MBs can boost the production of light olefins in MTO; MCPdienes decompose or convert to MBs easily at elevated temperature, whereas MBs accumulate gradually on the zeolite channels and contribute more greatly to the production of olefins at steady stage and high temperature. As a result, MCPdienes are identified as the primary HCP intermediates in the early transition period of MTO and their appearance promotes the build-up of aromatic HCP (dominated by MBs) and drives the MTO process from the induction period to the steady period. The insight shown in this work helps to clarify the reaction mechanism of MTO in the initial period, which should be of benefit to further researches into MTO.