Methanol-to-hydrocarbons reaction over SAPO-34 and ZSM-5 catalysts: A thermodynamic equilibrium simulation

The equilibrium conversion and selectivity of methanol to oxygenates and hydrocarbons reaction over SAPO-34 and ZSM-5 catalysts was studied. The equilibrium compositions were calculated using constrained Gibbs free energy minimization. The effect of catalyst type was considered by setting upper limits to products carbon number due to shape selectivity according to catalyst cage size; that is, n5 for SAPO-34 and n6 for aliphatic and n10 for aromatic compounds over ZSM-5. The equilibrium selectivity of paraffins is negligible over SAPO-34 system while that of olefins is very small over ZSM-5 both consistent with experimental results. Within temperature range of 250-450 °C, the equilibrium selectivity to ethylene and propylene increases with temperature over SAPO-34 whereas that of butenes and C5 hydrocarbons decreases. However, the main product selectivities are little affected over ZSM-5. The equilibrium selectivities of individual components showed fair agreement with experimental results for SAPO-34. For ZSM-5, the lumped product composition showed reasonable agreements. However, the products are mostly methane and benzene. The elimination of methane, and ethane from calculations by additional considerations, e.g., mechanism and kinetic evidences, significantly improved the results.