Acidity of Keggin-Type Heteropolycompounds Evaluated by Catalytic Probe Reactions, Sorption Microcalorimetry, and Density Functional Quantum Chemical Calculations

Abstract

The acidity and its effects on reactivity of Keggin-type heteropolycompounds were examined by catalytic probe reactions, microcalorimetry of ammonia sorption, and density functional quantum chemical calculations. Phosphotungstic, phosphomolybdic, silicotungstic, and silicomolybdic acids were used as model compounds. The specific rates of double-bond isomerization of both 1-butene and cis-2-butene were orders of magnitude greater on the tungsten heteropolyacids than on molybdenum heteropolyacids, which suggests the tungsten-containing solids are stronger acids. The rate of double-bond isomerization over silicotungstic acid was similar to that over phosphotungstic acid, indicating the minor role of the heteroatom. Results from ammonia sorption microcalorimetry showed ΔHsorp on tungsten-based heteropolyacids was approximately 40 kJ mol-1 higher than the corresponding enthalpy obtained on molybdenum-based heteropolyacids. Residual waters of hydration significantly affected both reaction rates and sorption ent...