孔壁含有沸石次级结构单元的MCM-48的合成及催化研究

Synthesis of mesoporous MCM-48 containing zeolite secondary building units and it′s catalytic properties

以双表面活性剂(双极性头表面活性剂和三乙醇胺)为模板,以含有沸石次级结构单元的溶胶为前驱体,在碱性条件下合成了新型介孔分子筛MCM-48。研究了沸石前驱体和三乙醇胺对产物MCM-48形成的影响,XRD和TEM表征结果表明,样品具有较高的结晶度和规则的孔径;FTIR和N2吸附表征结果表明,产物MCM-48的孔壁中含有沸石的次级结构单元,从而使得样品具有更大的比表面积。产物的重芳烃轻质化反应催化活性表明,实验样品对重芳烃的转化率比常规的MCM-48高,转化率约高出11%,进一步证明了实验样品的孔壁中含有沸石的结构单元。

Mesoporous aluminosilicate MCM-48 containing zeolite secondary building units in the pore wall, has been synthesized in alkaline media by a two-step procedure： the zeolite structure precursor was first synthesized by carefully controlling reaction conditions and then the pre-synthesized zeolite precursor was assembled using mixed templates of gemini surfactant [ C18 H37 N （ CH3 ）2 （ CH2 ）3N （ CH3 ）2 C18H37 ]^2＋ and triethanolamine （TEA）. Small angle XRD patterns of the as-made samples display strong diffraction peaks, indicating the highly ordered mesostructured MCM-48 was formed. TEM images further show that the as-made sample has uniform cubic pore channel system with the pore diameter of about 2.5 nm. FT-IR technique was employed to characterize the pore wall of the MCM-48 containing zeolite building units. IR spectra reveal an extra band at 520 cm^-1 ～ 600 cm^-1 assigned to five-membered rings from zeolite structure, suggesting the presence of zeolite building units in the pore wall. N2 adsorption/desorption result shows the material has much higher specific surface area （ 1 200 m^2/g）, compared with the reported data （1 100 m^2/g） of MCM-48. This may be ascribed to the presence of micropores produced by zeolite secondary building units in its pore wall. The hydrogenation dealkylation reaction of heavy aromatic hydrocarbons on the as-made MCM-48 catalyst gives higher conversion than that on traditional MCM-48. Thus, it is concluded that as-made MCM-48 containing the secondary building units of zeolites is of potential application in the catalytic conversion of large molecules. The key factors in our synthesis of MCM-48 are the controlled preparation of zeolite precursor and the selection of auxiliary template.