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

Area of Science:

• Catalysis
• Materials Science
• Chemical Engineering

Background:

• Zeolitic Brønsted acid sites are crucial for converting biomass-derived oxygenates.
• Understanding reaction mechanisms on zeolites like H-ZSM-5 is key for optimizing fuel and chemical production.

Purpose of the Study:

• To elucidate the mechanism of dioxolane conversion to methyl ethyl ketone (MEK) and isobutanal on H-ZSM-5.
• To investigate the roles of Brønsted acidity and pore confinement in product selectivity.

Main Methods:

• Periodic density functional theory (DFT) calculations on the MFI model of H-ZSM-5.
• Ab initio molecular dynamics (AIMD) to study confinement effects.
• Analysis of reaction pathways, free-energy barriers, and adsorption thermodynamics.

Main Results:

• Dioxolane conversion proceeds via protonation-assisted ring opening to an oxocarbenium intermediate.
• A 1,2-hydride shift (18.05 kcal mol⁻¹ at 498 K) favors MEK formation, while a 1,2-methyl shift (25.40 kcal mol⁻¹) favors isobutanal.
• The hydride shift is kinetically preferred, leading to a lower isobutanal/MEK ratio than statistically expected.
• MEK is more strongly stabilized than isobutanal within ZSM-5 channels, indicating confinement-influenced selectivity.

Conclusions:

• Brønsted acidity and pore confinement in MFI zeolites jointly control the rearrangement landscape during dioxolane conversion.
• Kinetic and thermodynamic factors, including confinement effects, dictate product distribution.
• This study provides fundamental insights into zeolite-catalyzed biomass upgrading reactions.