Selective conversion of acetone to isobutene and acetic acid on aluminosilicates: Kinetic coupling between acid-catalyzed and radical-mediated pathways

Abstract

Solid Brønsted acids catalyze aldol condensations that form CC bonds and remove O-atoms from oxygenate reactants, but sequential β-scission reactions also cleave CC bonds, leading to isobutene and acetic acid products for acetone reactants. The elementary steps and site requirements that cause these selectivities to depend sensitively on Al content and framework type in aluminosilicate solid acids remain speculative. Acetone reactions on microporous and mesoporous aluminosilicates (FER, TON, MFI, BEA, MCM-41) showed highest β-scission selectivities on MFI and BEA; they increased as the Al content and the intracrystalline density of active protons decreased. The effects of acetone and H2O pressure on turnover rates and selectivities indicate that an equilibrated pool of reactive C6 ketols and alkenones are present at pseudo-steady-state concentrations during catalysis and that they act as intermediates in β-scission routes. Two distinct C6 β-scission pathways contribute to the formation of isobutene and acetic acid: (i) a minor H2O-mediated route involving β-scission of C6 ketols on protons and (ii) the predominant anhydrous path, in which H-transfer forms unsaturated C6 enols at protons and these enols propagate radical chains mediated by transition states stabilized by van der Waals contacts within vicinal microporous voids. This latter route is consistent with coupled-cluster free energy estimates for these unsaturated C6 enols and their respective free radicals. It accounts for β-scission selectivities that increase with decreasing proton density, a finding that precludes the sole involvement of acid sites and requires instead the kinetic coupling between reactions at protons and propagation steps mediated by transition states confined within proximate voids, even when such voids lack a specific binding site. These mechanistic interpretations also account for the observed effects of residence time, of the loss of active protons by deactivation, of acetone and H2O pressures, and of aluminosilicate framework structure on selectivity. These mechanistic insights also demonstrate the ability of voids to stabilize transition states that mediate homogeneous reactions by mere confinement, even in the absence of chemical binding onto specific sites, as well as the essential requirement of intimate proximity for the effective kinetic coupling between reactions on protons and in proton-free voids, a process mediated by the diffusion of very reactive and unstable intermediates present at very low local concentrations within microporous frameworks.