Effects of cesium ion exchange on acidity of 12-tungstophophoric acid

The acidity of heteropolytungstic acids containing cesium is of great interest because of their unique catalytic and physical properties. Preparation and characterization of CsxH3-xPW12O40 (X = 1, 2, 2.5 and 3) have been performed using FTIR for functional groups, XRD for crystal structure, P-31 MAS NMR for the environment of the phosphorous. Calorimetry and adsorption of pyridine interaction in cyclohexane slurry (Cal-ad method) was used to quantify the number of acid sites and the corresponding enthalpy of adsorption. The distribution of proton and cesium ions in the salt structures is nearly homogeneous, as confirmed by XRD, 31p MAS NMR and enthalpy measurements. The acidity, systematically obtained in this work, indicates different proton strengths, with the following order: H3PW > Cs2HPW congruent to Cs2.5H0.5PW much greater than CsH2PW much greater than CS3PW, with DeltaH(1) = -32.7, -28.3, -27.4, -20.8 and -7.8kcalmol(-1), respectively. These enthalpies are strongly influenced by an endothermic term related to interstitial expansion of the structure by entrance of the pyridine molecule, which is evidenced by XRD results. The total amount of protons titrated by pyridine in all solids is larger compared to the calculated population on surface. This supports penetration by the polar probe into all of the solids. For Cs2.5H0.5PW the strongest protons are exposed nearly exclusively on the surface of this solid, as demonstrated by Cal-ad results for site-1 calculation. The combination of FTIR and adsorption site density demonstrated the effect of substituting Cs for H. changing the Bronsted acid site density to hydrogen-bonded ones. (C) 2004 Elsevier Inc. All rights reserved.