SAPO-5分子筛中硅的取代及其性能研究

本文采用三乙胺为模板剂,用水热法合成不同含硅量的SAPO-5分子筛,并用X射线衍射、扫描电镜、电子探针分析、真空重量吸附等方法对其进行了表征,结果表明硅已进入了骨架,硅同晶非等比取代磷和铝,并且分子筛的孔道是畅通的。 用微型反应器测定了AlPO_4-5及不同量硅取代的SAPO-5分子筛上的异丙苯裂解和邻二甲苯异构化催化反应,结果表明Si引入了催化活性很低的磷酸铝骨架后使其催化活性明显增加,进一步证实了硅是同晶非等比取代磷和铝,因而使分子筛骨架带负电性。 硅进入磷酸铝分子筛骨架的SAPO-5分子筛经红外光谱实验发现不仅具有表面羟基,而且都是酸性羟基。吸附吡啶后的红外光谱结果表明SAPO-5上同时存在B酸和L酸中心,B酸中心数目随着硅含量的增加而增加。

PROPERTIES OF SAPO-5 MOLECULAR SIEVES AND SILICON SUBSTITUTION IN ITS FRAMEWORKS

The SAPO-5 molecular sieves with different silicon contents have been synthesized hydrothermally by using Et3N as a templete. The products were characterized by the methods of XRD, SEM, scanning electron probe analysis and adsorption. The results show that the silicon substitutes unequal-proportionally for phosphorus and aluminum in the hypothetical aluminophosphate frameworks. The catalytic properties of the cumene cracking and o-xylene isomerization on the samples have been investigated. The results demonstrate that after the Si is substituted into the aluminophosphate frameworks the resulting samples of SAPO-5 become more catalytic active than A1PO4 - 5. It further gives the evidence that silicon has substituted for phosphorus and aluminum unequal-proportionally in the frameworks, leading to SAPO-5 compositions having anionic frameworks with negative charge with concomitant cation exchange properties and potential for Bronstcd acid sites. The hydroxyl groups and their acidic properties of SAPO-5 have been examined by means of IR spectroscopy. Four hydroxyl groups are clearly observed and they are acidic. From the IR experiments of pyridine adsorption, it is evident that both Bronsted acid sites and Lewis acid sites exist. The experiments also show that the amount of Bronsted acid sites increases as the silicon content in the SAPO-5 increases.