The influence of aluminium sources on the acidic behaviour as well as on the catalytic activity of mesoporous H-AlMCM-41 molecular sieves

Abstract

A series of mesoporous molecular sieves (AlMCM-41) were synthesized with varying silicon-to-aluminium ratios and using three different aluminium sources, viz., sodium aluminate, aluminium isopropoxide and aluminium sulphate. The samples were characterized systematically using XRD, TG-DTA, BET surface area, and ICP-AES. In addition, the extent of framework substitution as well as the nature of acid sites was deduced employing 27Al MAS-NMR, pyridine FT-IR, and NH3-TPD methods. These studies indicate that in case of samples prepared with sodium aluminate, most of the aluminium existed in tetrahedral positions even after calcination. Also, such samples posses moderate-to-strong (Br€onsted) acid sites along with small amounts of structural as well as non-framework Lewis acid sites. The density of non-framework acid sites depended upon the source of aluminium following a trend in the order of sodium aluminate < aluminium sulphate < aluminium isopropoxide. Among the various catalysts investigated, those having a silicon-toaluminium ratio of around 60 irrespective of aluminum sources exhibited highest conversion and good selectivity for vapour phase tertiary-butylation of phenol. This characteristic is attributed to the presence of larger number of moderate-to-strong acid sites and a smaller number of Lewis acid sites.