晶种导向干凝胶法制备成型ZSM-5分子筛

摘要

在于凝胶法制备ZSM-5分子筛的体系中添加晶种导向剂,控制分子筛的生长,制备了纳米沸石组装的无粘结剂成型多级孔ZSM-5分子筛,一步完成纳米分子筛的制备及组装成型,即克服了传统纳米粒子难以过滤分离的问题,同时组装所形成的多级孔有助于改善分子在催化剂内的扩散,从而提高催化反应效率,以硅胶、薄水铝石为原料,四丙基氢氧化铵(TPAOH)和ZSM-5晶种导向胶作为粘结剂,通过混捏、挤条得到直径2 mm的条状前驱物,随后通过干凝胶转换法制备成型分子筛.所用晶种导向剂组成为0.35TPAOH:1SiO2:20H2O:4C2H5OH.通过X射线衍射(XRD),热重(TG)分析和傅里叶变换红外(FTIR)光谱等方法对分子筛晶化过程进行了表征,结果表明晶种导向剂加入量对分子筛生长速度及多级孔结构均有影响.当所加晶种导向剂中TPAOH与SiO2的摩尔比为0.025时,经过3h晶化,分子筛相对结晶度达到100％.扫描电镜(SEM)结果表明,合成的分子筛尺寸约为200 nm,组装形成的多级孔分子筛的介孔体积为0.28 cm3·g-1.通过NH3程序升温脱附(NH3-TPD)考察了所得成型分子筛的酸性,发现该分子筛酸性与市售的粉末H-ZSM-5分子筛类似.%Shaped binderless ZSM-5 zeolites were prepared via a rapid dry-gel conversion technique from aluminosilicate extrudate.Silica gel and boehmite were well mixed and extruded into cylindrical shaped extrudates with diameter of 2 mm with the aid of silica sol,ZSM-5 zeolite seed gel,and/or tetrapropylammonium hydroxide (TPAOH) solution.The total molar ratio of TPAOH to SiO2 was fixed at 0.025.With same amounts of structure directing agent (TPAOH),the addition of seed gels accelerated the crystallization of the zeolite.The seed gel directing agent composition was 0.35TPAOH:1SiO2:20H2O:4C2H5OH.The seed gel not only provided crystal nuclei for rapid crystallization but also controlled the size of the ZSM-5 crystals.Especially,the morphology of these extrudates was well maintained in the crystallization process.The crystallization processes of zeolite were characterized by X-ray diffraction (XRD),thermogravimetric (TG) analysis,and Fourier transform infrared (FTIR) spectroscopy.The results showed that the growth of the zeolite was accompanied by the occupation of the zeolite channels by TPAOH.When the TPAOH/SiO2 molar ratio was 0.025,the sample achieved 100％ relative crystallinity after 3 h crystallization.The morphology and textural properties were characterized by scanning electron microscopy (SEM) and nitrogen sorption isotherms.The obtained shaped zeolites comprised nanosized crystals (200 nm) and exhibited a hierarchical structure with high mesopore volume (0.28 cm3· g-1).The temperature-programmed desorption of ammonia (NH3-TPD) was used to assess the acidity of these shaped zeolites.The results showed that this shaped zeolite possessed an acidity similar to that of a commercial H-ZSM-5 catalyst.