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

Abstract

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.

Coke formation of model compounds relevant to pyrolysis bio-oil over ZSM-5

“…follows Diels Alder and ring-opening pathways [12,25,66]. Here we observe that these reactions are also responsible for carbon formation.…”

“…This pathway is specific to the pyrolysis of furan and reflects the contribution of oxygen in the formation of coke. In this scheme, furan reacts with itself, forming water and benzofuran via ring opening [66]. Subsequently, benzofuran decomposes to benzene, coke and CO [25].…”

Effects of water and alcohols on the polymerization of furan during its acid-catalyzed conversion into benzofuran

“…20 Benzofuran continued to react with the intermediate derived from furan, forming dibenzofuran. In methanol, the furan ring could also be opened to form 1,1,4,4-tetramethoxybutane.…”

Study on furfural conversion into aromatics over Zn/HZSM‐5 and Fe/HZSM‐5 catalysts

“…[5] Furanic compounds are converted into aromatic hydrocarbons and other hydrocarbons by process of pyrolysis on ZSM-5 zeolite. The reaction medium can be inert gas (N 2 , He), hydrogen, methane, propylene, etc., the reaction temperature usually ranges from 400 to 700 o C. [7][8][9] ZSM-5 zeolite catalyst is a reasonable choice because its pore size and structure that is suitable for a higher selectivity of aromatics. The metals such as Zn, Ga, Ag, Pt, Pd, Ir, etc.…”

“…are chosen to be doped into zeolite because these metals are reported to be beneficial for the formation of aromatic compounds such as deoxygenation and hydrogenation reactions. [7][8][9][10][11][12][13][14] In this paper, furfural, a specific compound of the furan family, will be conducted pyrolysis on HZSM-5 catalyst and HZSM-5 catalyst is modified by oxides of zinc (Zn) and iron (Fe). Results are valuated according to conversion into aromatics and aromatic hydrocarbons selectivity such as benzene, toluene,…”