Mechanistic Insights into the Induction Period of Methanol-to-Olefin Conversion over ZSM-5 Catalysts: A Combined Temperature-Programmed Surface Reaction and Microkinetic Modeling Study

The induction period of propylene formation from methanol is compared to that of dimethyl ether (DME) over ZSM-5 catalysts of different compositions using a combination of temperature-programmed surface reaction experiments and microkinetic modeling. Transient reactor performance is simulated by solving coupled 1D nonlinear partial differential equations accounting for elementary steps based on the methoxy methyl mechanism and axial dispersion and convection in the reactor. Three binding site ensembles and three active site ensembles are observed. These sites constitute up to 3% (binding sites ∼70%, active sites ∼30%) of the total acid sites during methanol conversion and up to 1% (binding sites ∼30%, active sites ∼70%) of the total acid sites during DME conversion. Over the binding sites, during the induction period of methanol, and DME conversion, the acid site density is the key descriptor. Over the active sites, acid site density is the key descriptor with higher site densities correlating with lower barriers of propylene formation during the induction period of methanol conversion. The barrier to DME desorption is the key descriptor, with lower desorption barriers correlating with lower barriers of propylene formation during the induction period of DME conversion. Barriers to propylene formation are lower during the induction period of methanol conversion (up to 141 kJ mol^-1) compared to that of DME conversion (up to 200 kJ mol^-1) over ZSM-5 catalysts.