Abstract:

A series of ZSM-22 molecular sieves were synthesized by adjusting the time of adding aluminum source by two-step crystallization method, and the distribution of acid sites was regulated. XRD, SEM, N₂^{adsorption/desorption, NH₃-TPD, FTIR, Py-IR,ICP, TEM-EDS and other characterization methods were used to characterize the phase, morphology, texture properties, acidity and element. The results show that ZSM-22 molecular sieve is synthesized in situ from amorphous silicate which is not crystallized on the surface of high silicon ZSM-22 after the first step of crystallization with the addition of aluminum source under the induction of template agent, and a layer of ZSM-22 molecular sieve with low Si/Al ratio is grown on the surface of high silicon ZSM-22, thus changing the acid site of the molecular sieve. The Pt-supported bifunctional catalyst was prepared and its hydroisomerization performance of n-dodecane was studied. The results show that the two-step crystallization of ZSM-22 can inhibit the cracking reaction in micropores and obtain higher isomerization selectivity. The conversion of S6-C at 330℃ is 89.3%, the selectivity is 89.4%, and the yield is 79.8%. ZSM-22 molecular sieve synthesized by two-step crystallization had more active sites located on the outer surface and near the pore mouth of the molecular sieve, thus changing the distribution of its isomers, reducing the proportion of isomers with branch chains at the end of the carbon chain, and preferring to generate isomers with branch chains at the middle of the carbon chain. The distribution ratio of 2-methylundecane in S12-C is about 10% lower than that of conventional ZSM-22.}

Key words: molecular sieve, catalyst, two-step crystallization, hydroisomerization, n-dodecane

摘要：

采用两步晶化法，通过调节加入铝源的时间合成了一系列ZSM-22分子筛，并对其酸性位点的分布进行调控。采用XRD、SEM、N₂吸附/脱附、NH₃-TPD、FTIR、Py-IR、ICP、TEM-EDS等表征方法对其物相、形貌、织构性质、酸性和元素进行表征分析。结果表明，第二步晶化时，加入铝源，在模板剂诱导作用下，促使第一步晶化后高硅ZSM-22表面未晶化的无定形硅酸盐原位合成了ZSM-22分子筛，使高硅ZSM-22表面生长一层低硅铝比ZSM-22分子筛，从而使分子筛的酸性位点发生改变。制备了Pt负载双功能催化剂，研究其正十二烷的临氢异构化反应性能。结果表明，两步晶化ZSM-22分子筛可以抑制微孔内的裂化反应，获得更高异构产物选择性，其中S6-C在330℃时，转化率为89.3%，选择性89.4%，产率79.8%。通过两步晶化方式合成的ZSM-22分子筛，活性位点更多地位于分子筛的外表面和孔口附近，进而改变其异构体产物的分布，降低支链位于碳链端位异构体的比例，更倾向于生成支链位于碳链中间位置的异构体，S12-C的2-甲基十一烷分布比例比常规ZSM-22降低10%左右。

关键词: 分子筛, 催化剂, 两步晶化, 临氢异构化, 正十二烷