MCM-48介孔材料相转化合成的形成机制

Formation Mechanism of Highly Ordered MCM-48 via Phase Transformation

利用水热合成的方法,使用新型的表面活性剂十六烷基三甲基对苯磺酸盐作为模板剂合成了高质量的MCM-48介孔分子筛,并用X-射线衍射(XRD)、扫描电镜(SEM)、高分辨透射电镜(HRTEM)以及N2吸附-脱附进行了表征。合成过程的研究表明该合成体系经历了三相,起始相为具有六方对称性的MCM-41,随着加热时间的延长,生成了具有立方对称性的MCM-48,进一步延长加热时间则生成了层状相MCM-50。三相转变发生的核心驱动力来自于表面活性剂有效堆积参数g因子的改变,随着反应时间的延长,由于对甲基苯磺酸根离子(Tos-)的流失,表面活性剂极性头所占的有效面积(a0)明显减小,g值变大。另外,XRD、傅立叶变换的红外光谱(FT-IR)以及固体魔角自旋核磁共振(^(29)Si MAS NMR)的表征结果证明:随着晶化时间的延长,相转变的同时伴随着介孔材料的孔壁逐渐由原子无序的非晶态向原子有序的晶态结构转变。最终形成的原子有序层状介孔分子筛可以作为扩孔型微孔分子筛合成的有效前驱体。

Highly ordered mesoporous molecular sieve MCM-48 with Ia3 d cubic structure was hydrothermally synthesized using new surfactant cetyltrimethylammonium tosylate（CTATos） as template via phase transformation（PT）. Structural study by small angle X-ray diffraction（XRD）, scanning electron microscopy（SEM）, highresolution transmission electron microscopy（HR-TEM） and nitrogen adsorption-desorption shows the timedependent structural change in M41 S mesoporous silica from MCM-41 to MCM-48 and MCM-50. The driving force for this transformation seems to be an increase of the local effective surfactant packing parameter（ g）, which resulted from the leaching of surfactant counter anions（Tos-） in the pore with the prolongation of the reaction time. One-dimensional solid state ^（29）Si NMR spectra, X-ray diffraction patterns, and infrared spectra show the progression of molecular organization in the self-assembled mesophases from structures with initially amorphous silica networks into sheets with very high degrees of atomic order when the phase transformation is happening.The finally obtained self-assembled lamellar silica-surfactant mesophase composites with crystal-like ordering in the silica frameworks can be an ideal precursor to fabricate the three-dimensional microporous zeolites with the expanded pore size.