Aluminum impregnation into mesoporous silica molecular sieves for catalytic application to Friedel-Crafts alkylation

Abstract

The catalytic activity of mesoporous molecular sieves for Friedel-Crafts alkylation of benzene, toluene, and m-xylene with benzyl alcohol was measured with three different types of materials, KIT-1 (disordered three-dimensional channel network), MCM-41 (hexagonal packing of one-dimensional channels), and MCM-48 (cubic Ia3d structure composed of two independent three-dimensional channel systems). Aluminum was incorporated into the mesoporous materials by various routes such as direct syntheses, grafting of the Al species with anhydrous AlCl3 and impregnation with an aqueous solution of AlCl3. The catalytic activity was analyzed with respect to the effects of the preparation methods, channel structures, and Al content. It turned out that postsynthesis impregnation was the most effective means for increasing the catalytic activity. The three types of mesoporous materials exhibited approximately the same surface character with regard to the catalytic reaction, independent of the different structures and synthesis conditions. Magic angle spinning Al-27 NMR spectroscopy of the impregnated samples indicated that the coordination of the Al sites thus generated lost the tetrahedral symmetry upon complete dehydration, supporting the possibility of three-coordinated catalytic sites. (C) 2000 Academic Press.