ZSM-5分子筛的合成及其在丁烯催化裂解反应中的应用

Synthesis of ZSM-5 and its application in butylene catalytic cracking

采用水热合成法制备了无模板剂ZSM-5分子筛并用正硅酸甲酯(TMOS)对其进行外表面修饰改性,利用XRD、SEM、^(29)Si MAS NMR、^(27)Al MAS NMR、NH_3-TPD、BET和UV-vis DRS对合成分子筛的物相、形貌和酸性等进行了表征,并将其应用于催化丁烯裂解反应。研究表明,经水热合成的无模板剂ZSM-5结晶度较好,与添加模板剂合成的ZSM-5拥有相似的孔道结构和晶体结构以及相近的酸量,但在酸中心分布上有明显差异:孔道内酸中心数量增加且分布更加均匀,孔道交叉处酸中心数量减少;经过外表面修饰改性后,ZSM-5分子筛外表面部分不具备择形性的酸中心被钝化,使其择形选择能力增强。在催化丁烯裂解反应中,用TMOS进行外表面修饰改性的无模板剂ZSM-5分子筛作为催化剂能够有效抑制副反应的发生,丙烯和乙烯的总收率高达58%。

The template-free ZSM-5 was prepared by hydrothermal synthesis and then modified by tetramethoxysilane（ TMOS）. The structure,morphology,and acidity of all samples were studied by various techniques,such as XRD,SEM,29 Si MAS NMR,27 Al MAS NMR,NH3-TPD,BET and UV-vis DRS.Comparing with the zeolites synthesized by traditional method,the template-free ZSM-5 exhibited the analogical acidity,morphology and structure,but obvious difference in acid distribution. Due to the absence of the structure directing agent,the template-free ZSM-5 possessed more acid sites situated at pore channels of catalyst and less acid sites stayed at the intersection of straight and sinusoidal channels. Consequently,the shape selectivity enhanced significantly. After TMOS modification,the non-shape acid sites located at the external surface were covered by a single SiO2 layer. The template-free ZSM-5 achieved the highest total yield of ethylene and propylene,longest working life-span and low est level of coke deposition among the studied catalysts,ascribing to the suppression of the side reactions.