Confinement-Controlled Rearrangements in Dioxolane Upgrading on H-ZSM-5 Revealed by Periodic DFT

Abstract

Zeolitic Brønsted acid sites catalyze carbocation rearrangements central to upgrading biomass-derived oxygenates. Here we elucidate the mechanism of dioxolane conversion to methyl ethyl ketone and isobutanal on H-ZSM-5 using periodic density functional theory on the MFI model, complemented by ab initio molecular dynamics to probe confinement effects. Dioxolane adsorption at the Brønsted site is followed by protonation-assisted ring opening to form an oxocarbenium intermediate stabilized by the deprotonated framework. From this common intermediate, selectivity is governed by two competing rearrangements, namely, the 1,2-hydride shift with a free-energy barrier of 18.05 kcal mol–1 at 498 K leading toward MEK, and the 1,2-methyl shift with a higher barrier of 25.40 kcal mol–1 leading toward isobutanal. The hydride-shift channel is kinetically preferred over the methyl-shift channel, lowering the isobutanal/MEK ratio below the 3:1 limit expected for equal branching. Adsorption thermodynamics further indicate stronger stabilization of MEK than isobutanal within ZSM-5 channels, suggesting that confinement-controlled binding can bias product distributions in addition to intrinsic rearrangement barriers. These results highlight how Brønsted acidity and pore confinement jointly shape the rearrangement landscape in MFI zeolites.