Effects of Ca loadings on catalytic behaviors of Ca/HZSM-5 for methanol-to-olefin process

HZSM-5 zeolite was modified with different loadings of Ca by incipient impregnation and the samples were characterized by FTIR spectra of adsorbed pyridine and NH_3-TPD. Methanol to olefin (MTO) over Ca modified HZSM-5 catalysts was carried in a continuous flow fixed bed reactor. The results showed that Ca modification enhanced both catalytic stability and selectivity to light olefins (C_2~= - C_4~=), whereas excessive Ca loadings resulted in declining catalytic behaviors. The optimum Ca loading was 6%. Correlating the reaction results with the acidity characterization ones, it was found that the catalyst whose Brcnsted acid sites combined almost completely with Ca species exhibited the best catalytic behaviors. Furthermore, by comparing catalytic behaviors of Ca/HZSM-5 with those of Na/HZSM-5 and 2Ca/1Na/HZSM-5, function of Ca in the catalytic reaction was discussed. It was proposed that Ca combined with Brφnsted acid sites to form catalytic centers and thus participated in conversion of methanol.