X分子筛表面Silicalite-1膜的制备及形成过程

Preparation of Silicalite-1 Films on X Zeolite and Its Formation Mechanism

为了提高对二甲苯吸附选择性,采用四丙基氢氧化铵溶液对X分子筛预处理,制备出X/Silicalite-1核壳材料,采用X射线衍射、扫描电镜、透射电镜、氮气物理吸附等手段表征其物化性质,并进行静态吸附评价实验。结果表明,X分子筛表面形成了完整、均一的Silicalite-1膜层,膜层厚度约87 nm,而X分子筛没有被明显破坏。水热处理过程中,正硅酸四乙酯水解形成无定形硅物种并附着在X分子筛表面,在四丙基氢氧化铵和氢氧根离子的作用下,无定形硅物种逐渐晶化为Silicalite-1膜层,同时形成独立Silicalite-1分子筛;随着晶化时间的延长,独立Silicalite-1分子筛转化为膜层,使Silicalite-1膜层增厚。与X分子筛相比,X/Silicalite-1的对二甲苯分离系数αPX/EB、αPX/MX、αPX/OX均提高,三者从大到小依次为αPX/MX、αPX/OX、αPX/EB。

In order to achieve high para-xylene adsorptive selectivity,X/Silicalite-1 core-shell composite was prepared successfully through the pre-treatment of X zeolite.X-ray diffraction,scanning electron microscope,transmission electron microscope and nitrogen physical adsorption were employed to characterize its texture properties,and static adsorption test was used to evaluate its adsorptive selectivity.Experimental results showed that complete and continuous Silicalite-1 films of 87 nm had been formed on the surface of X zeolite,while X zeolite crystals were well maintained.During the hydrothermal treatment,amorphous silica species,which resulted from the hydrolysis of tetraethyl orthosilicate,were attached to the surface of X zeolite crystals,and then transformed into Silicalite-1 films.Meanwhile,independent Silicalite-1 crystals were also formed.With the prolonging of the treatment,Silicalite-1 crystals gradually disappeared and the thickness of the films correspondingly increased.Compared with X zeolite,the prepared X/Silicalite-1 core-shell composite exhibited higher para-xylene separation factors in the order ofαPX/MX>αPX/OX>αPX/EB.