A series of HZSM-5 zeolites modified with various Ca loadings were prepared by an incipient impregnation technique and characterized by Fourier transform infrared spectra of adsorbed pyridine and temperature-programmed desorption of NH3 and CO2. Methanol to olefin (MTO) over Ca-modified HZSM-5 zeolites was performed in a flow-type fixed bed reactor. The results indicated that Ca modification increased both the catalytic stability and the light olefin selectivity. However, an excess Ca loading resulted in an inferior catalytic performance. Hence, there was an optimal Ca loading for a specific HZSM-5, and the thus prepared Ca/HZSM-5 had little free Brönsted acid sites and presented the best catalytic performance of MTO. A possible mechanism for MTO involving acid−base centers on Ca-modified HZSM-5 zeolites has been suggested on the basis of the results obtained.

The highest propylene selectivity and best catalytic stability were gained over Ca modified ZSM-5 that had little free Brønsted acid sites, which indicated that Ca interacted with Brønsted acid sites and thus participated in the catalytic reaction as supported by the MTO data of Na modified ZSM-5. A possible mechanism that involves the formation of acid-base centers by hydrolyzing Ca species in the presence of water is suggested.