Encapsulation of a Polyoxometalate into an Organosilica Microcapsule for Highly Active Solid Acid Catalysis

We have developed a highly active and water-tolerant solid acid catalyst by encapsulating a polyoxometalate into hydrophobic hollow polymethylsiloxane microspherical particles. 12-Tungstophosphoric acid and a cesium salt of a heteropoly acid (Cs2.5H0.5PW12O40, abbreviated as CsPW) were used for this purpose. The encapsulated catalysts were prepared by a sol–gel reaction of methyltrichlorosilane around water droplets containing heteropoly compounds in a water-in-oil emulsion. Both the catalysts were active in the hydrolysis of ethyl acetate in a large excess of water. While the 12-tungstophosphoric acid in the polymethylsiloxane microcapsules leached into the water during the repetition of the reaction, the activity of the CsPW encapsulated catalyst (0.86 mol·(acid-mol)−1·min–1) was maintained. This activity was superior to that of bare cesium salt (0.34 mol·(acid-mol)−1·min–1). To our knowledge, the CsPW encapsulated catalyst exhibits the highest activity among solid acid catalysts with respect to reaction rate per acidic proton. The polymethylsiloxane shell plays several roles including shielding of CsPW and allowing penetration of reactants (ethyl acetate and water) and products (ethanol and acetic acid) through the microporous membrane.