Catalysis of Organic..

352Heteropoly AcidsSiO 2 is the best catalyst support for this reaction. As Al 2 O 3 is incorporated intothe SiO 2 the catalyst activity and selectivity decreases significantly. In these mixedoxide support materials Brønsted acid sites favour acylation, whilst the presence ofLewis acid sites results in hydrolysis of the anhydride.Supported catalysts could be reused once with little loss of activity; furtherreuse led to a significant drop in activity, as a result of strong absorption of productsand by-products on the catalyst surface (indicated by colour change of the catalyst).More rapid catalyst deactivation was observed in a trickle bed reactor than in a batchslurry reactor.Experimental SectionCatalysts were prepared by impregnation of the appropriate support material withaqueous solutions of silicotungstic acid using JM proprietary methods. Theconcentration of STA in the impregnation solution was such as to achieve the desiredloading on the catalyst support. Thereafter the impregnated material was dried in air(LOD