Effects of Composition and Acid Strength on Elimination and Isomerization Catalysis by Solid Bronsted Acids.

The effects of the identity of the central atom X (P, Si, Al, and Co) in SiO2-supported Keggin-type polyoxometalate clusters and the effects of acid-strength were examined for acid-catalyzed alkanol dehydration, ether cleavage, and bifunctional n-hexane isomerization reactions by theory and experiment. Alkanol dehydration and ether cleavage turnover rates reflect the rate-constant for C-O cleavage via carbenium ion type transition states and the equilibrium constant for the formation of unreactive reactant dimers. Kinetic inhibition by n-donors, such as alkanol and ether reactants or H2O products is ubiquitous for alkanol dehydration and ether cleavage reactions and reflects solvation effects that also make alkanol dimers unreactive. The late carbenium ion type elimination transition states evident in density functional theory (DFT) treatments are consistent with the small kinetic isotope effects measured for deuterated alkanols, with the strong substitutent effects for alkanol reactants, and with the high pre-exponential factors for the elimination rate constant. The rate constants for elimination elementary steps decreased in parallel with decreasing valence of the central atom X in POM clusters and decreased exponentially as deprotonation energies (DPE), a rigorous descriptor of acid strength, increased (and acid strength decreased) on all POM clusters and zeolite H-BEA. Thermochemical cycles rigorously describe reactivity and activation barriers in terms of the stability of protonated cationic transition states and intermediates, which depend, in turn, on the DPE of the acid, the proton affinity of reactants, and the ion-pair stabilization energy (DeltaE int). The higher DPE of weaker acids is compensated in part by a more negative DeltaEint. n-Hexane isomerization rate constants showed a similar but stronger exponential sensitivity to the DPE of the solid acid. This reflects a less effective DPE-DeltaEint compensation, consistent with the larger charge separation because of the more delocalized positive charge at the isomerization transition state relative to those involved in alkanol elimination reactions. The correlations of DPE and rate constants of acid-catalyzed elementary steps for catalysts with known structure for which DPE values can be calculated suggest that these rate constants, in turn, can be used as a measure of acid strength for acid catalysts with unknown structures. Apparent DPE values were estimated in this manner for SO4-ZrO2, WOx-ZrO 2 and persulfonic acid resins.