晶粒大小对ZSM-5分子筛甲醇制低碳烯烃催化性能的影响

对三种HZSM-5分子筛进行Ca改性,获得两组酸性接近的催化剂,考察了晶粒大小对甲醇制低碳烯烃(MTO)反应的影响。通过进一步与Na改性的比较,探讨了Ca在催化反应中的作用。采用扫描电镜(SEM)测定晶粒大小及形貌,氨气程序升温脱附(NH3-TPD)及吡啶红外吸附(Py-IR)表征催化剂的酸性。MTO催化性能测定结果表明,HZSM-5的低碳烯烃选择性较低且下降较快,催化活性降低也快;Ca改性降低酸性,提高了低碳烯烃选择性和催化稳定性;晶粒大小主要影响催化稳定性,小晶粒分子筛催化剂稳定性更好。高Ca含量改性效果更好;钠改性也提高了低碳烯烃选择性,但其稳定性较差。对于HZSM-5和Ca/HZSM-5,小晶粒的催化剂具有较好的催化稳定性。提出Ca参与了催化反应,MTO是一个酸碱协同作用的催化过程。

Effect of zeolite crystal size on the catalytic performance of HZSM-5 in the reaction of methanol to light olefins

To investigate the effect of zeolite crystal size on the reaction of methanol to light olefins (MTO), two groups of catalysts with nearly the same acidity were obtained by modification of three kinds of HZSM-5 zeolites with Ca. Na was also used as a reference modifier to investigate the role of Ca in MTO. The crystal size and morphology of the catalysts were characterized by SEM and the acidity was measured by NH₃-TPD and Py-IR. The results showed that for MTO over HZSM-5, the selectivity to light olefins is low; moreover, the conversion of methanol as well as the selectivity to light olefins decrease quickly with time on stream. The selectivity to light olefin and the catalytic stability of HZSM-5 is increased significantly by the modification with Ca, especially at high Ca loading. Na modification may enhance the selectivity to light olefins, but the conversion of methanol over Na modified HZSM-5 still decreases quickly with the time on stream. The catalysts (either HZSM-5 or Ca/HZSM-5) with smaller crystal size also exhibit better stability. The MTO performance of HZSM-5 catalyst is affected both by the acidity and by the crystal size. Ca-modified HZSM-5 with lower acidity exhibits higher catalytic stability and higher selectivity to light olefins, which may be ascribed to that Ca is involved in the MTO cycle via both acidic and basic sites in the Ca-modified HZSM-5. Small crystal size is favorable to enhance the catalytic stability.