凝胶中硅含量对SAPO-5分子筛合成及其性能的影响

Effect of Silicon Content in Gel Mixture on the Structure and Acid Catalytic Performance of SAPO 5 Molecular Sieve

通过改变反应物凝胶中的硅含量，用水热法合成了系列ＳＡＰＯ－５分子筛，并用ＸＲＤ、ＳＥＭ和ＭＡＳＮＭＲ分别研究了不同硅含量的分子筛结构和形貌．以正己烷的催化裂解为模型反应，用微反色谱研究了不同硅含量的ＳＡＰＯ－５分子筛的催化性能．结果表明，分子筛骨架中的硅含量随反应物中硅含量的增加而增加；硅开始进入ＳＡＰＯ－５分子筛骨架时，是以取代单个磷原子而进行的；当反应物中硅含量较高时，硅进入分子筛骨架是通过两个硅原子取代一对铝和磷原子，从而在ＳＡＰＯ－５中形成富硅区．硅进入分子筛骨架，虽然未改变分子筛的晶体结构，但改变了分子筛的晶形晶貌．ＳＡＰＯ－５分子筛中存在３种酸中心，即弱酸、中强酸和强酸中心．在正己烷催化裂解中，较低温度时，催化活性中心主要是中强酸中心；当反应温度较高时，弱酸中心也可充当活性中心．在强酸中心上易发生结炭现象．

A series of SAPO 5 molecular sieves has been synthesized by changing Si content in reaction mixture. XRD, SEM and MAS NMR were employed to characterize the structure and morphology of SAPO 5 with different Si contents. The acid catalysis of SAPO 5 catalysts was evaluated by micro reactor chromatography. The results show that the Si content in the framework of SAPO 5 increases with the Si content in gel mixture. Si atoms start to enter the framework of SAPO 5 by replacing single P atom in the framework of AlPO 4 5. When the Si content is high in gel mixture Si atoms start to replace a pair of P and Al in the framework of AlPO 4 5, and thus the silicon rich zone is formed in SAPO 5. Although the structure of SAPO 5 can not be changed by the entrance of Si atoms into the framework, the morphology of SAPO 5 is changed. In SAPO 5 catalyst, there are three types of acid sites, i.e weak, medium strong and strong acid sites. In the catalytic cracking reaction of n hexane , at lower reaction temperature the medium and strong acid active sites play the main role; while at higher temperature, the weak acid sites can also play some role. During cracking reaction, coke is easily to be formed on the strong acid sites.