Imprinting of the Surface of Mesoporous Silicates using Organic Structure Directing Agents: Catalysts for Large Organic Molecules

摘要

The M41S family and the related SB A-15 mesoporous materials are a class of ordered porous (alumino) silicates with pore diameters in the range of 20-500 A (see review [3]). Despite their large pore dimensions compared to that of microporous zeolites, mesoporous materials are restricted in their use as catalysts for large organic molecules by their poor thermal stability and broad distribution of acid sites. A combination of the large pore dimensions of mesoporous materials with the strong acid sites present in zeolite-like structures would be highly advantageous leading to a novel and probably useful catalytic material. Hydrocarbon reactions involving large organic molecules that cannot enter the small pores of a zeolite catalyst may be catalyzed by the new material having a mesoporous framework structure with accessible strong acid sites on the surface of these large pores. Various strategies have been reported in the literature to combine some of the features of different porous materials. Partial "recrystallization" of the surface of MCM-41 resulted in a mesoporous material with surface-tectosilicate structures giving enhanced acidity and catalytic cracking activity. However, the walls of MCM-41 have a thickness of ～ 10 A and can contain, at most, very small organized units of silica/alumina. A dual templating method has been used for the synthesis of macroporous silicates with zeolitic microporous frameworks and MCM-41/ZSM-5 composites. In addition, another technique for the synthesis of macroporous materials with zeolitic microporous frameworks by self-assembly of colloidal zeolites has been reported. Finally, to obtain a mesoporous material with acid sites on the surface a procedure has been reported for the syntheses of organically functionalized mesoporous materials with sulfonic acid groups.