Catalysis on solid acids: Mechanism and catalyst descriptors in oligomerization reactions of light alkenes

This study addresses fundamental descriptions of confinement and acid strength effects on stability for transition states and intermediates involved in alkene oligomerization on solid acids. Kinetic and infrared data and theoretical treatments that account for dispersive interactions show that turnover rates (per H⁺) on aluminosilicates and heterosilicates with microporous voids (TON, MFI, BEA, FAU) and on mesoporous acids (amorphous silica-alumina, dispersed polyoxometalates) reflect the free energy of C[sbnd]C bond formation transition states referenced to gaseous alkenes and bound alkene-derived precursors present at saturation coverages. These free energy barriers decrease as the size of confining voids decreases in aluminosilicates containing acid sites of similar acid strength and approaches bimolecular transition state (TS) sizes derived from density functional theory (DFT) for propene and isobutene reactants. Such TS structures are preferentially stabilized over smaller bound precursors via contacts with the confining framework. These effects of size, typically based on heuristic geometric analogies, are described here instead by the dispersive component of DFT-derived energies for TS and intermediates, which bring together the effects of size and the shape, for different framework voids and TS and precursor structures derived from alkenes of different size; these organic moieties differ in “fit” within voids but also in their proton affinity, as a result of the ion-pair character of TS structures. The larger charge in TS structures relative to their alkene-derived precursors causes free energy barriers to decrease as conjugate anions become more stable in stronger acids. Consequently, oligomerization rate constants decrease exponentially with increasing deprotonation energy on unconfined acid sites in polyoxometalates and silica-alumina and on confined sites within MFI frameworks with Al, Ga, Fe, or B heteroatoms. Reactivity descriptions based on geometry or acid strength are replaced by their more relevant energetic descriptors–van der Waals confinement energies, proton affinities of organic molecules, and deprotonation energies–to account for reactivity, here for different reactants on diverse solid acids, but in general for acid catalysis.