Efficient post-synthesis alumination of MCM-41 using aluminium chlorohydrate containing Al polycations

Abstract

Aluminium chlorohydrate (ACH) which contains Al polycations such as the Al₁₃⁷⁺ Keggin ion is an efficient source of Al for post-synthesis alumination of purely siliceous MCM-41 resulting in materials (herein designated AlMCM) with excellent structural integrity and enhanced acidity and catalytic activity. The amount of Al incorporated into the MCM-41 framework is dependent on the concentration of Al in the grafting solution; ²⁷Al MAS NMR confirms that a large proportion of the Al is inserted into tetrahedral positions within the framework. TEM and XPS indicate that there are no separate surface alumina phases. Alumination results in modest increases in the d₁₀₀ basal spacing. This is accompanied by much larger reductions in pore size with the result that the pore wall thickness increases with Al content. However the alumination has little or no effect on the long range structural ordering or pore size uniformity. Alumination generates Brønsted acid sites which increase in population as the Al content rises. The ACH-grafted AlMCM materials exhibit considerable catalytic activity for cumene cracking and are superior to AlCl₃-grafted MCM-41 or ACH-grafted amorphous silica.

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