(583db) Elucidation of the Mechanism for the Formation of Benzofuran From Furan: Investigation of a Key Step in Catalytic Fast Pyrolysis

The formation of benzofuran from furan, catalyzed by the acidic zeolite HZSM-5, is a key step in the conversion of biomass to biofuels by catalytic fast pyrolysis.  This presentation examines the energetics of two competing pathways for this reaction: a Diels-Alder mechanism, often assumed in interpretations of experimental data, and a ring-opening pathway predicted by the Rule Input Network Generator program (RING).  The reaction was modeled both in the gas phase and in zeolite clusters, using the ONIOM embedded cluster approach.  In the gas phase, the Diels-Alder mechanism was found to pass through a high-energy intermediate roughly 380 kJ/mol above the reactant energy.  This energy barrier reduces to approximately 200 kJ/mol in HZSM-5.  In contrast, the ring-opening mechanism passes through a gas-phase intermediate roughly 500 kJ/mol above the reactant energy, which falls to approximately 50 kJ/mol in HZSM-5.  The theoretical results were compared to an experimentally determined activation energy collected from the liquid-phase reaction of furan with H-ZSM5 over the temperature range of 270-300°C.  It was determined that the energy of the ring-opening mechanism, 50 kJ/mole, fits into the experimentally measured activation energy of 72±3 kJ/mole.  Our results strongly indicate that, in the cavities of HZSM-5, the condensation of two furan molecules to form benzofuran and water does not proceed by a Diels-Alder reaction between furan monomers. These experimental and computational results highlight the importance of the ring-opening mechanism for this key step in making biofuels.