Effects of CaO-modified zeolite on one-step synthesis of dimethyl ether

Synthesis of dimethyl ether (DME) was carried out in a fixed bed reactor over a bifunctional catalyst CaO/HZSM-5+JC207, which was composed of a methanol-synthesis catalyst JC207 (Jingjing Catalyst Plant, China) and a dehydrator HZSM-5 (Si/Al = 38) modified with CaO. The effects of zeolite modification with CaO on the performance of the bifunctional catalyst in one-step synthesis of DME were investigated. XRD analysis of the modified zeolite CaO/HZSM-5 indicates that CaO is highly dispersed on the zeolite HZSM-5 without any new species formed. Pyridine-IR analysis shows that the modification of HZSM-5 with CaO not only brings on significant changes in acid type and acid amount but also turns some of Bronstead acid sites into Lewis acid sites. NH3-temperature programming desorption analysis indicates that the number of acid sites on the zeolite surface, especially, that of the strong acid sites decreases with the increase of CaO loading. Modification of zeolite HZSM-5 with appropriate dose of CaO turns the strong acid sites into weak or less strong acid sites; such a redistribution of acid sites may be beneficial for the increase of DME selectivity.