珠磨-重结晶制纳米ZSM-5及催化正庚烷裂解性能

Preparation of Nano ZSM-5 Through Combining Bead-Milling and Porogen-Directed Recrystallization and Its Catalytic Performance for n-Heptane Cracking Reaction

纳米化和多级孔化是提高分子筛催化裂化性能的有效途径,本研究将珠磨“破坏”过程和重结晶“构筑”过程相结合,先将微米ZSM-5分子筛珠磨成半结晶的纳米晶粒,再加入聚二烯二甲基氯化铵作为致孔剂进行二次结晶造孔,形成了珠磨-致孔剂诱导重结晶制备纳米ZSM-5分子筛的新方法。探讨了制备过程中珠磨转速和时间、重结晶时间、致孔剂加入量等因素对制备过程和产物结构的影响。结合X-射线衍射、扫描电子显微镜、N_(2)吸附-脱附和固体核磁对所制得纳米分子筛的形貌和孔道结构进行了表征,并通过氨气程序升温脱附和吡啶红外表征了其酸性。以正庚烷催化裂化作为探针反应,对制得的ZSM-5分子筛经P、La修饰后的催化性能进行了评价,结果表明:所制纳米ZSM-5有利于单分子机理裂解反应,抑制了双分子裂解反应和氢转移等副反应,因而具有更高的催化活性和乙烯、丙烯选择性,更低的芳烃选择性和更长的催化剂寿命。

Generating nano and hierarchical zeolites provides an effective way to improve the catalytic performance.In this work,by combining destructive mechanochemical bead-milling and constructive porogen-assisted recrystallization,a new method to generate nano ZSM-5 was proposed.The parent micron-meter sized zeolites were first bead-milled into nanocrystals of low crystallinity,followed by adding polydiallydimethylammonium chloride as a porogen and growth inhibitor to proceed recrystallization,to fabricate highly crystalline nanozeolites with preserved acidity and micro-porosity.The morphology and pore structure of the obtained ZSM-5 nanozeolites were disclosed by combined X-Ray diffraction,scanning electron microscopy,N_(2) adsorption-desorption isotherms and solid-state magic angle spinning nuclear magnetic resonance.The acidity of the ZSM-5 zeolites was probed by ammonium temperature programmed desorption and pyridine infrared spectroscopy.The catalytic performance of the prepared ZSM-5 zeolites modified with P and La was evaluated using n-heptane catalytic cracking as a probe reaction.The results showed that the prepared nanoscale ZSM-5 was beneficial for single molecule mechanism cracking reactions,inhibited side reactions such as bimolecular cracking and hydrogen transfer.Therefore,the high er catalytic activity,higher selectivity for ethylene and propylene,lower aromatics selectivity and longer catalyst life were obtained.