A First-Principles Investigation of Gas-Phase Ring-Opening Reaction of Furan over HZSM-5 and Ga-Substituted ZSM-5

The furan ring-opening reaction using three catalytic models, HZSM-5, the extra-framework sites [GaO]/ZSM-5, and [Ga(OH)₂]/ZSM-5, was investigated using periodic density functional theory as a model reaction for mechanistic understanding of catalytic vapor phase biomass upgrading. The formation of formyl allene from the furan was investigated in detail by computing the energy profiles and reaction barriers. Based on the computed free energy profiles, the HZSM-5 and [Ga(OH)₂]/ZSM-5 are active catalysts, whereas [GaO]/ZSM-5 is not active for the furan ring-opening reaction. In HZSM-5, the likely rate-controlling step is furan CH₂-O bond cleavage upon protonation (1.70 eV). The computed energy difference between the highest and lowest points in the free energy profile is 1.89 eV. The [Ga(OH)₂]⁺ is likely the catalytic center for the gallium-incorporated ZSM-5 catalyst. The rate-controlling step is a hydrogen transfer reaction, which requires a reaction barrier of 2.48 eV. The computed energy difference between the highest and lowest points in the Ga(OH)₂ catalyzed free energy profile is 2.61 eV. These computational studies provide new mechanistic understanding of the role of catalytic extra-framework sites in the vapor phase upgrading reactions.