Acidic cesium salts of the Keggin-type heteropolyacids, Cs2.5H0.5PW12O40 and Cs2.5H1.5SiW12O40, retained their strong acidity in water-containing organic media, and worked as insoluble, efficient acid catalysts in the hydrolysis of ethyl acetate and the estenfication of acetic acid with ethanol. Silica-bound Cs2.5H0.5PW12O40, a newly prepared composite, was an active and easily recoverable solid acid catalyst for the hydrolysis of ethyl acetate with excess water.

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