介孔结构和助剂Zn对不同晶粒大小ZSM-5催化甲醇制芳烃反应性能的影响

Effects of mesoporous structure and Zn promoter on methanol to aromatics performance over different crystal sized ZSM-5 catalysts

通过碱处理和添加助剂Zn对微米ZSM-5和纳米ZSM-5进行改性,获得具有不同孔结构和酸性质的催化剂。采用氮气吸附、X射线衍射、透射电镜、氨气程序升温脱附(NH_3-TPD)和热重(TG)技术对不同催化剂进行表征,结合催化性能评价,考察晶粒尺寸、介孔结构和Zn助剂对其催化甲醇制芳烃(MTA)反应性能的影响。结果表明,碱处理引入介孔后,孔体积均增大,总酸量都降低;微米催化剂外表面积显著增加,但纳米催化剂外表面积却有所下降;负载金属Zn后,比表面积、结晶度和总酸量都降低。在P=0.5MPa、T=430℃、WHSV=2h-1的反应条件下,负载Zn的微米催化剂由于具有较高的酸量,其芳烃与苯、甲苯和二甲苯(BTX)选择性最高,分别为85.11%和66.85%,但是稳定性较差,催化寿命仅为12h。但相较于未改性的纳米ZSM-5原粉来说,碱处理后又负载Zn的催化剂,液烃中芳烃选择性从纳米原粉的65.20%增加到80.82%,BTX选择性从纳米原粉的42.30%提高到49.56%,而在甲醇进样量增加4倍,即WHSV=8h-1时,催化剂仍显示出较好的稳定性,寿命可达84h。可见,在小晶粒ZSM-5上碱处理扩孔并引入Zn助剂可以有效提高甲醇制芳烃反应性能。

ZSM-5 catalysts with different pore structures and acidities were modified by alkali treatment and Zn introduction. The obtained catalysts were characterized by N2 adsorption, XRD, TEM, NH3-TPD and TG techniques, and the influences of crystal size, mesoporous structure and Zn metal additive on the catalytic performance of ZSM-5 for conversion of methanol to aromatics(MTA) were investigated in addition to the catalyst evaluation. The results indicated that after the formation of mesopore via alkali treatment, the pore volume of the catalysts increased, and the total acid amount decreased. The external surface area of the micro-ZSM-5 increased significantly but that of the nano-ZSM-5 decreased after alkali treatment. The surface area, crystallinity and total acid amount of ZSM-5 were decreased after the introduction of Zn species. MTA reaction was carried out in a fixed-bed reactor at 430℃ and WHSV of2 h-1 under 0.5 MPa and the Zn-loaded micron catalyst exhibited high acid amount, high aromatic selectivity of 85.11% and BTX selectivity of 66.85% in liquid hydrocarbons, but a short catalytic lifetime of only 12 hours. Compared with the unmodified nano-ZSM-5, the Zn-promoted mesoporous nano-ZSM-5 increased the aromatic selectivity from 65.20% to 80.82%, and the BTX value from 42.30% to 49.56%.The catalyst still displayed good catalytic stability when space velocity was up to 8 h-1 and the catalytic lifetime could reach 84 h. This work demonstrated that mesopore fabricating of nano-ZSM-5 catalyst by alkali treatment and then introducing Zn could effectively improve its MTA performance.