Synthesis Of Hierarchical Zeolite And Its Application

Zeolite, a crystalline aluminosilicates with micropores, has been broadly used as catalysts in industry. But its narrow pore limts the usage in some large molecule reaction. During the past decade, significant efforts have been devoted to developing methods to introduce mesoporosity into zeolite materials by different approaches.In some studies, organosilanes have been used to synthesis of hierarchical zeolites with enhanced surface area, porosity, and catalytic activity. The method was based on perturbing the growth of zeolite crystals by silanization of the zeolitic seeds to hinder their further aggregation. The molecular sieves have been prepared earlier by addition of organosilanes to the raw gel, which caused their incorporation within the zeolite micropores.In this paper,3-(trimethoxysilyl)propyl)octadecyldimethylammoni (TPOAC) and cetyhriethylammonium bromide(CTAB) as two templates, tetraethylorthosilicate(TEOS), fumed silica, water glass and silicasol as silica sources have been used to synthesize the hierarchical zeolites in basic medium. The synthesized samples were characterized by SEM, TEM, XRD and N2 adsorption/desorption. The effects of inorganic silica sources, ratio of TPOAC/CTAB, ageing time on order degree and hydrothermal stability of the assembly mesoporous structure were studied. The results show that the hierarchical zeolite with TEOS has much higher order degree than that with other silica sources. Less ageing time causes worse order degree.Some experiments were carried out to investigate the effects of alumina sources, ratio of TPOAC/CTAB, concentration of TPAOH and mixing ways of the templates to introduce the alumina into hierarchical zeolite. A typical zeolite, TEOS and NaAlO2 as silica source and alumina source respectively with stirring for two hours, TPOAC/CTAB=1:1, used in the selective synthesis of 2,6-DMN, the selectivity of 2,6-DMN kept above 40% in ten hours, and the maximum yield got to 5.2%.Another approach to design the hierarchical zeolites is based on the pre-crystallization method. By attachment of TPOAC and CTAB into the solution of ZSM-5 seed, the mesoporous material is achieved. TEM shows the zeolites with nuclear shell structure under pre-crystallization tempreture 90℃. In the selective synthesis of 2,6-DMN, reaction time 1 Oh, the conversation of 2-MN was 25%, the selectivity of 2,6-DMN was 31% and the yield of 2,6-DMN reached 5%. In the work, carboned rice was used as hard template to synthesize the hierarchical zeolites, as spherical crystals, with crystallization temperature 150-170℃for 24-48 h. The synthesized hierarchical zeolite was used in alkylation of 2-methylnaphthalene (2-MN) with methnol to produce 2,6-dimethylnaphthalene (2,6-DMN). The results show that the mesoporous zeolite performs high activitation and stability, high 2,6-DMN selectivity and yield.