Tuning of Magnesium Distribution in ZSM-5 via Different Impregnation Methods and Its Effect on Methanol to Propene Reaction

Two modified zeolites, ZSM-5-Mg(OH)2 and ZSM-5-Mg(NO3)2, were achieved by Mg(NO3)2 impregnation methods with or without NH3·H2O as a precipitator, respectively. The Brønsted acid sites of ZSM-5 can be reduced by Mg modification, especially for the strong acid. The results from XRF and XPS showed surface Mg-rich for both modified ZSM-5 zeolites. However, Mg concentration on the surface of ZSM-5-Mg(OH)2 is significantly lower than that of ZSM-5-Mg(NO3)2, indicating the addition of NH3·H2O in the Mg(NO3)2 impregnation could suppress Mg enrichment on the surface. The results from methanol conversion showed significant effects of Mg-modification method on the catalytic performance. Severe surface Mg rich distribution favored the higher propene selectivity at the cost of reduction of catalytic lifetime. The suppressed surface Mg rich distribution prolonged catalytic lifetime and maintained the light olefin selectivity at high level. This work demonstrated the catalytic performance could be adjusted by Mg modification methods via tuning Mg distribution.