Differences in the methanol-to-olefins reaction catalyzed by SAPO-34 with dimethyl ether as reactant

Dimethyl ether (DME) was compared to methanol as the reactant for making olefins with a SAPO-34 catalyst. Their similar selectivities showed that they have the same mechanism, but they also gave notable differences in catalyst stability and how product selectivities changed with time-on-stream. These differences were caused by DME hydrolysis equilibrium being pushed toward DME by a low water concentration, which resulted in a low methanol partial pressure and slower kinetics. The higher methanol partial pressure with the methanol feed caused the hydrocarbon pool to form faster. Conversely, the lower methanol partial pressure with the DME feed formed the hydrocarbon pool slower and showed that during the early induction period, propene selectivity was much higher than ethene selectivity. The use of DME and low water concentration were advantageous because the lower methanol concentration in the catalyst resulted in much slower deactivation, which in addition to better catalyst stability, also gave a higher selectivity to propene.