ZSM-5分子筛的P改性研究进展

P改性是调变ZSM-5分子筛催化性能的重要手段。ZSM-5分子筛的P改性方法主要由磷酸、磷酸盐水溶液及有机P化合物浸渍或通过气相沉积等方法进行。改性ZSM-5分子筛中,P与Al物种相互作用, P物种主要以四配位形式存在,调变分子筛的酸性,通过对表面的修饰阻止骨架Al的脱除,提高ZSM-5分子筛的水热稳定性。P改性后,HZSM-5分子筛的酸密度,尤其是表面强酸密度下降,降低正碳离子机理反应中的二次反应,减少氢转移及齐聚,促进轻烯烃的生成。P改性修饰HZSM-5分子筛的孔口,提高择形性能,部分进入体相的P减少裂化过程中ZSM-5交叉口处大分子正碳离子的形成,减少双分子裂化机理中间体的生成,提高轻烯烃选择性。P改性ZSM-5分子筛强酸密度降低,择形效应提高,焦炭生成得到抑制,催化剂寿命延长。

Recent progress of P modified ZSM-5 molecular sieve

Phosphorus modification is one of the most important approaches in tuning the catalytic properties of ZSM-5 molecular sieve, and have been widely applied in catalytic cracking, methanol to hydrocarbons (MTH), toluene disproportionation and ethanol to olefin reactions. The P modification of ZSM-5 is mainly achieved by impregnation method of phosphorus acid, phosphate, organophosphorus compounds solution, or by chemical vapor deposition. After P modification, the introduced P interacts with Al in the ZSM-5 molecular sieve, and recent prevailing views consider that the P species is existed as four coordinated form, adjusting the acidity of the molecular sieve. At the same time, the P species have prevented the removal of framework Al atoms through surface modification, which significantly improves the hydrothermal stability of ZSM-5 molecular sieve after P modification. Also, the density of acid sites is decreased obviously, especially the density of strong acid sites, after P modification, lowering the secondary reactions of most carbenium ion reactions, suppressing hydrogen transfer and olefin oligomerization, promoting the production of light olefins. On the other side, the pore opening of ZSM-5 is further decorated by deposition of P species, leading to the improving of shape selectivity, and during the catalytic cracking process, the P species entering the pores of ZSM-5 will hinder the formation of voluminous carbenium ions in the intersections, reducing the intermediates of bimolecular cracking, enhancing the light olefin selectivity. Due to the decreasing of strong acid site density and improving of shape selectivity, the formation of coke is also inhibited, which is benefit for improving the life of ZSM-5 catalyst.