The Acidity and Catalytic Activity of Supported Acidic Cesium Dodecatungstophosphates Studied by MAS NMR, FTIR, and Catalytic Test Reactions

Abstract

Acidic cesium salts of dodecatungstophosphoric acid (H3PW) supported on silica and MCM-41 were characterized by multinuclear MAS NMR, FTIR, and catalytic test reactions. 31P-NMR data indicate that the supported samples contain salts of various stoichiometries (Cs3PW and Cs2HPW) and even H3PO4. According to 29Si-NMR data, deposition of the Cs salt does not change the relative amount of various SiOH groups in the case of the silica support. In contrast, when the support is MCM-41, the relative amount of the SiOH groups increases significantly, indicating the breaking of the siloxane bonds resulting from their interaction with H3PW. The 133Cs MAS-NMR spectra also suggest that the CsPW salt interacts more strongly with the MCM-41 support than with SiO2. The results of catalytic studies show that cesium salts of dodecatungstophosphoric acid with low Cs content supported on silica or MCM-41 (Cs1.7H1.3[PW12O40]-on-SiO2 and Cs1.8H1.2[PW12O40]-on-MCM41) are active, selective, and recyclable catalysts in Friedel–Crafts alkylations (isopropylation, transalkylation), the aromatic ortho-Claisen rearrangement, and the dehydration of pinacol. The specific activity of these catalyst preparations are comparable with that of neat Cs2.6H0.4[PW12O40]. This indicates that the active species are in a well-dispersed form.