Modification of nanocrystalline HZSM-5 zeolite with tetrapropylammonium hydroxide and its catalytic performance in methanol to gasoline reaction

Abstract

Nanocrystalline ZSM-5 zeolite was modified with tetrapropylammonium hydroxide (TPAOH) solution. The effect of the TPAOH treatment time on the catalytic performance of HZSM-5 was investigated in methanol to gasoline (MTG) reaction. The modified samples were characterized using X-ray diffraction, scanning electron microscopy, 27 Al and 29 Si magic-angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, N 2 adsorption, and NH 3 temperature-programmed desorption. The HZSM-5 structure was basically unchanged after modification, but the relative crystallinity increased and the crystal morphology was more regular. The Brunauer-Emmett-Teller surface area, the micropore surface area, the surface Si/Al ratio, and the amount of strong acid sites increased; this might be caused by desilication and dealumination, and secondary crystallization of the zeolite during the treatment. The stability of the catalyst was significantly improved after modification, as reflected by the increased lifetime from 70 h to above 170 h in MTG reaction. The catalyst lifetime increased with increasing TPAOH treatment time. Hydrogen transfer reactions became faster and led to more isoparaffins and less olefins in the liquid products. 采用四丙基氢氧化铵(TPAOH)溶液对纳米ZSM-5分子筛进行改性, 运用X射线衍射/扫描电镜/ 27 Al和 29 Si固体核磁/X射线光电子能谱/N 2 物理吸附脱附法和NH 3 程序升温脱附等手段对所制样品进行了表征, 并评价了其催化甲醇制汽油反应性能. 结果表明, 改性后的HZSM-5相对结晶度增加, 晶体形貌更加规整, 表面硅铝比增加, 比表面积和微孔表面积增大, 强酸位酸量增多. 同时, TPAOH改性不仅可以使分子筛脱硅脱铝, 而且伴有二次晶化补硅补铝, 改变了分子筛的硅铝分布. 改性的HZSM-5在甲醇制汽油反应中的稳定性大幅度提高, 其寿命由70 h增至170 h以上, 随着TPAOH处理时间的增加, 催化剂寿命增加, 氢转移反应加快, 导致油相产品中异构烷烃增多, 烯烃减少. Tetrapropylammonium hydroxide treatment of the HZSM-5 zeolite improved the diffusion capacity and coke tolerance through desilication, dealumination, and secondary crystallization, reflected by the significantly increased catalyst lifetime.