Impact of acid site concentration and temperature on the operating regime, activity and selectivity in methanol-to-olefins conversion over H-ZSM-5

Abstract

The effect of Si/Al ratio on operating regime, activity and selectivity in methanol-to-olefins conversion has been assessed over H-ZSM-5 zeolites with high (Si/Al=25), moderate (Si/Al=40) and low (Si/Al=140) acid site concentration, in a wide temperature (325 – 450 °C) and space time range (0.44 – 110.0 kgcat s molMeOH-1). For Si/Al=25 no stable regime is achieved, shifting immediately from transition to deactivation, while Si/Al=140 always ensures stable operation. At lower Si/Al ratio and increasing temperature, reaction pathways shift to produce more C3-C5 olefins. Suppression of bimolecular methylation and hydride transfer in favour of monomolecular alkene cracking is at the origin of such behaviour, either by increased acid sites distance or due to reduced physisorbed concentrations and activation energy differences. The latter phenomena cause a peculiar conversion decrease with increasing temperature between 350 and 400 °C. Consistent, well-designed data acquisition allowed insights into the complex mechanism and the effect of catalyst properties and temperature thereon.