Sulfated Nanocapsular Aluminas: Controlling their Brönsted and Lewis Acidity

Abstract

Sulfated aluminas were prepared by a sol−gel method, using sulfuric acid as both hydrolysis catalyst and sulfate source. The nominal amount of sulfate was varied between ∼2 and 17% wt. Solids thus obtained present unique nanocapsular morphology with capsules’ shells made up of Al−O polynuclear species, which crystallized into boehmite when aged for 30 days. Upon calcination, small γ-alumina crystals were obtained, which present large surface areas and pore volumes. The presence of sulfate ions generated strong Brönsted acid sites, while Lewis acid sites were present in the coordinatively unsaturated sites of tetracoordinated and pentacoordinated aluminum atoms, as was demonstrated by both pyridine adsorption followed by FTIR and 27Al MAS NMR. Catalytic performance was tested using 2-propanol conversion as a model reaction, showing a marked dependence of activity and selectivity with sulfate loading.