The relationship between adsorption and solid acidity of heteropolyacids

The adsorption energy of a base to a solid acid catalyst is often assumed to correlate with the acid strength of the catalyst. In this study, the influences of adsorbate type, binding configuration and solid acid composition on the adsorption energy are explored using quantum chemical methods. In particular, density functional theory is used to calculate the adsorption energies of functionalized hydrocarbons containing O, N, or S heteroatoms or CC to phosphotungstic (H3PW12O40) and phosphomolybdic (H3PMo12O40) acids. The adsorption energies of the different molecules bound to the same solid acid are not easily predicted by the proton affinity of the adsorbate because the stabilization of the protonated adsorbate also varies with composition. Bond order conservation helps to explain the relatively small variance in adsorption energies among reactants of widely varying base strength. The activation barriers to form carbenium-ion transition states from adsorbed olefins are also calculated over the two heteropolyacids. The stronger adsorption of propylene to phosphotungstic acid compared to phosphomolybdic acid results in a higher activation barrier to form the carbenium-ion transition state. These heteropolyacids are predicted to have higher activation barriers than zeolites for carbenium-ion formation, which is typically thought to be the rate controlling step in many hydrocarbon conversion processes. This contrasts with the ranking of acid strength based solely on the magnitude of the adsorption energy.