ZSM-5分子筛酸性修饰及探针分子表征

MODIFICATION OF ACIDITY OF ZSM-5 ZEOLITE AND ITS ISOMERIZATION PROPERTIES

采用柠檬酸、硫醚、苯胺、二苯胺、磷酸二氢铵及硅油作为改性试剂对二甲苯异构化专用HZSM-5分子筛进行液相浸渍改性,结合甲苯、邻二甲苯、间二甲苯、乙苯及间二异丙苯的脉冲反应表征,对各种改性手段的效果进行评估。各种改性方法对分子筛微孔体积和比表面积的影响很小,均减弱了酸性,对L酸的影响大于B酸。5种探针反应结果表明:柠檬酸改性强化了分子筛酸性,提高了歧化/异构化活性;磷酸二氢铵改性提高了分子筛歧化和脱乙基反应活性;苯胺、二苯胺、硫醚改性手段部分钝化了分子筛酸性,降低了歧化/异构化活性;随着改性剂分子尺寸的增大,钝化外表面酸性越明显,从而改变分子筛孔内外的酸位分布,产生不同程度的择形效果;硅油改性充分地钝化了外表面酸性,形成最明显的择形效应。

The modifications of acidity of HZSM-5 were conducted by liquid impregnation method using citric acid, thioether, aniline, diphenylamine, ammonium dihydrogen phosphate and silicone oil as modifier, respectively and were characterized by XRD, X ray fluorescence, FTIR, and BET techniques. The isomerization properties of the modified HZSM-5 were evaluated by pulse reactions of toluene, ortho-xylene, meta-xylene, ethylbenzene, and meta- biisopropylbenzene probes. It is found that the impacts of various modifiers on pore volume and surface area are weak, except the acidities which are all weakened. The impact of modification on L acid is greater than B acid. The results of the isomerization reactions for probe compounds indicate that the citric acid enhances the acidity, resulting in increased isomerization and disproportionation activities; the ammonium dihydrogen phosphate modification can enhance the disproportionation and de-ethylation performance; while the modifications of aniline, diphenylamine, thioether passivate partly the acidity and reduce the disproportionation activity. The larger the molecular size of the modifier, the more obvious passivation of surface acidity, resulting in changes of acid site distribution in the zeolite and different shape-selective properties, the silicone modification greatly passivate the acidity on outer surface of the zeolite, leading the most obvious shape-selective effect.