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

Abstract

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 CC 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 decrease in aluminosilicates containing acid sites of similar acid strength and approach 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 cause 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.

Stability of bound species during alkene reactions on solid acids

“…(49)]. These data suggest that more stable anions (in stronger acids) do not lead to more stable alkoxides, an experimental demonstration of the covalent nature of the alkoxide structures, which is consistent with DFT-derived formation energies and partial charges for s-propoxides that are independent of acid strength for a series of Keggin POM clusters with different central atoms and very different acid strength (HPW, HSiW, HAlW) (12). These data also show that stabilization by confinement is similar among different alkoxides and not very consequential for their stability within 12-MR MOR channels, but becomes detectable within smaller 10-MR TON channels.…”

“…from NH 4 + decomposition (6)] are proportional to the alkene pressure (10-500 kPa; 503 K) (Fig. 1A), consistent with the kinetic relevance of the alkene-alkoxide reactions that form the C-C bond (12). At lower propene pressures (1-5 kPa), dimerization rates deviate from first order (Fig.…”

“…Infrared spectra also show that O-H stretching bands [3,600 cm , the frequency range expected from theoretical treatments for π-bonded alkenes at protons (7,12). Thus, bound species predominantly consist of covalently bound alkoxides.…”

“…This uncertainty is particularly prevalent in isobutene-derived species, for which steric effects may disfavor the formation of covalently bound butoxides (7)(8)(9). Infrared spectra that lack intact or perturbed O-H stretches (10,11) during oligomerization of propene or isobutene on TON and MFI (12) indicate the proton has been transferred to the stable species, which is consistent for either the covalently bound alkoxide or carbenium ion. However, density functional theory (DFT) shows that alkoxides acquire carbenium ion character only at the transition states that mediate their transformations during catalysis on solid acids, but not at their lowest energy stable bound states (13)(14)(15)(16)(17)(18)(19), which suggests the prevalence of bound alkoxides at near saturation coverages during catalysis.…”

“…We combine here these methods to report the thermodynamics of alkoxides derived from C 2 -C 4 alkenes during their co-oligomerization on acids of diverse strength and confining environments (H-TON [10-membered ring (MR)] zeolite; H-MOR (12-MR) zeolite; Keggin polyoxometalates dispersed on mesoporous SiO 2 (H 3 PW 12 O 40 ; HPW)). Theory (DFT) and experiment show that C-C bond formation rates are limited by the addition of an alkene to bound alkoxides, present at saturation coverages at all alkene pressures (10-500 kPa) (12). At lower pressures, absolute alkoxide formation equilibrium constants for ethoxide and propoxides are measured from homodimerization rates and from the integration of infrared bands that are dependent on alkene pressure; their relative values agree with DFT-derived adsorption free energies and values measured from heterodimerization where competition for surface sites exists.…”