含β沸石结构单元的介孔硅铝分子筛合成

Synthesis of Mesoporous Aluminosilicate Molecular Sieves by Assembling β Zeolite Subunits

以β沸石在Na2SiO3水溶液中选择性脱硅降解所得沸石次级结构单元、脱除的硅物种及Na2SiO3共同作为硅铝源,十六烷基三甲基溴化铵为模板剂,水热法合成出介孔硅铝分子筛BM。采用XRD、N2吸附-脱附、TEM、FT-IR、27 Al MAS NMR、Py-IR、XRF及NH3-TPD手段对BM进行了表征,并以异丙苯裂化为探针反应,对比考察了Hβ、HBM和HAl-MCM-41分子筛的催化性能。结果表明,BM为孔壁含有β沸石结构单元的介孔分子筛。与常规方法合成的Al-MCM-41分子筛相比,其表面硅羟基数量明显减少,孔壁增厚,且四配位铝比例明显增加,酸中心密度、强酸中心比例、B/L酸比值及水热稳定性都明显提高。采用HBM分子筛催化异丙苯裂化反应的苯收率约为采用HAl-MCM-41分子筛时的3倍,且前者的催化稳定性高于Hβ沸石。

β zeolite particles were controllably desilicated in Na2SiO3 aqueous solution. The resulting zeolitic structural units and the dissolved silica together with Na2SiO3 were used as silicon and aluminum sources in the hydrothermal synthesis of mesoporous aluminosilicate molecular sieves BM with the aid of cetyltrimethylammonium bromide as the template. The obtained BM was characterized by means of XRD, N2 adsorption-desorption, TEM, FT-IR, 27Al MAS NMR, Py-IR, XRF and NH3-TPD. Cumene cracking was tested over HBM, Hβ and HAl-MCM-41 to evaluate the catalytic performance of BM. Compared with conventional mesoporous sieves Al-MCM-41, BM exhibited considerably fewer isolated silanols, larger pore wall thickness, and higher percentage of tetracoordinated Al, leading to improved hydrotermal stability. The acidity site density, the fraction of strong acid sites and B/L ratio in HBM were significantly higher than those in HAl-MCM-41 Benzene yield of cumene cracking over HBM was roughly 3 times as high as that over HAl-MCM-41, whereas HBM showed much higher stability than Hβ.