A comparison of direct synthesis and vapour phase alumination of MCM-41

Mesoporous AlMCM-41 materials were prepared by both direct synthesis and the reaction of siliceous MCM-41 with aluminium chloride vapour. The products were characterised using XRD, N-2 sorption, EDAX and both MAS NMR and FTIR spectroscopy, and acid catalytic activity was evaluated using the conversion of 1,3,5-triisopropylbenzene (1,3,5-TIPB) as a test reaction. Acid site concentrations (Bronsted and Lewis) were estimated from sorption of pyridine, using FTIR. The concentrations of acid sites were correlated with the fraction of aluminium in the solids and comparisons were made with results published previously for similar mesoporous materials. Both the compositions of the gels used in the synthesis of Al-mesoporous materials and the procedures used to introduce aluminium into mesoporous silica strongly influenced the above correlation. In all cases, the ratio of total acid site concentration to aluminium concentration was less than unity. Consideration of the present catalytic results for conversion of 1,3,5-TIPB along with results published previously for conversion of cumene, demonstrates that acid catalytic activity is better correlated with Bronsted rather than with Lewis site concentration.