On the Way to Biofuels from Furan: Discriminating Diels–Alder and Ring-Opening Mechanisms

We
performed kinetics experiments and quantum calculations to investigate
the reaction of furan to benzofuran catalyzed by the acidic zeolite
HZSM-5, which is a key step in the conversion of biomass to biofuels
through catalytic fast pyrolysis. The reaction was studied experimentally
by placing the zeolite in contact with solution-phase furan and detecting
the benzofuran product over the temperature range 270–300 °C,
yielding an apparent activation energy of 72 ± 3 kJ/mol. The
reaction was modeled in gas and zeolite phases to determine the energetics
of the following two competing pathways: a Diels–Alder mechanism
often assumed in interpretations of experimental data and a ring-opening
pathway predicted by the chemoinformatic software RING. Quantum calculations
on the zeolite/guest system were performed using the ONIOM embedded
cluster approach. We computed the energetics of reactants, products,
and all intermediate steps. Locating relevant transition states fell
beyond our computational resources because of system size and the
ruggedness of the energy landscape. The Diels–Alder mechanism
in the gas phase was found to pass through a high-energy intermediate
roughly 380 kJ/mol above the reactant energy, which 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 energy
of the ring-opening mechanism over HZSM-5 fits into the experimentally
determined energy “budget” of 72 ± 3 kJ/mol. These
experimental and computational results highlight the importance of
the ring-opening mechanism for this key step in making biofuels. 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 the reactants.