Investigation of Thermochemistry Associated with the Carbon–Carbon Coupling Reactions of Furan and Furfural Using ab Initio Methods

Abstract

Upgrading furan and small oxygenates obtained from the decomposition of cellulosic materials via formation of carbon-carbon bonds is critical to effective conversion of biomass to liquid transportation fuels. Simulation-driven molecular level understanding of carbon-carbon bond formation is required to design efficient catalysts and processes. Accurate quantum chemical methods are utilized here to predict the reaction energetics for conversion of furan (C4H4O) to C5-C8 ethers and the transformation of furfural (C5H6O2) to C13-C26 alkanes. Furan can be coupled with various C1 to C4 low molecular weight carbohydrates obtained from the pyrolysis via Diels-Alder type reactions in the gas phase to produce C5-C8 cyclic ethers. The computed reaction barriers for these reactions (∼25 kcal/mol) are lower than the cellulose activation or decomposition reactions (∼50 kcal/mol). Cycloaddition of C5-C8 cyclo ethers with furans can also occur in the gas phase, and the computed activation energy is similar to that of the first Diels-Alder reaction. Furfural, obtained from biomass, can be coupled with aldehydes or ketones with α-hydrogen atoms to form longer chain aldol products, and these aldol products can undergo vapor phase hydrocycloaddition (activation barrier of ∼20 kcal/mol) to form the precursors of C26 cyclic hydrocarbons. These thermochemical studies provide the basis for further vapor phase catalytic studies required for upgrading of furans/furfurals to longer chain hydrocarbons.

Catalytic Upgrading of Biomass-Derived Compounds via C–C Coupling Reactions: Computational and Experimental Studies of Acetaldehyde and Furan Reactions in HZSM-5

“…In the Diels−Alder pathway ((reactants → intermediates (FF-1 → FF-9) → products), shown in Figure 5a, the cycloaddition reaction between two furans (FF-1 → FF-2) requires a barrier of 0.78 eV (FF-1-TS) to form the Diels− Alder adduct (4,7-dihydro-4,7-epoxybenzofuran, FF-2). From our previous studies, the barrier required for the Diels−Alder reaction of two furan molecules is 1.24 eV, 28 indicating a reduction of the barrier by 0.46 eV in the presence of HZSM-5 for the furan coupling reaction. Upon the formation of Diels− Alder product (FF-2), a proton of HZSM-5 is transferred to the adduct (see FF-2 → FF-3), followed by a C−OH bond cleavage and proton transfer (see FF-4 → FF-5) back to the zeolite to form benzofuran-4-ol (FF-5).…”

“…In addition to methanol related reactions, investigations have been also carried out on other oxygenated species. Recently, furans derived from biomass have received increasing attention in catalytic biomass conversion to fuels. ,,,, Huber and co-workers have reported that furan forms aromatics and olefins in HZSM-5, in which benzofuran, as the major furan coupling product (see Figure ), is an important intermediate for the formation of benzene and alkenes. , In addition, Williams et al have reported a “green” route to convert 2,5-dimethylfuran (DMF) and ethylene to p -xylene in HY zeolite with a 75% selectivity at 300 °C. The reaction occurs via Diels–Alder reaction followed by dehydration reaction .…”

“…Although optimizing the reaction condition of certain model compounds is relatively straightforward, it is challenging to control the catalytic reactions of a mixture of various oxygenated compounds. The above studies − ,− have suggested that certain functional groups (e.g., aldehydes) are more likely to deactivate zeolites via coke formation than others. Then a few interesting questions can be raised: Are there any effective reactions that could convert aldehydes to other, more manageable oxygenates?…”

“…Our previous study using highly accurate computational approaches (e.g., G4 method) has mapped the thermochemistry associated with the gas phase coupling reactions of furan with various C 1 –C 4 oxygenated compounds (e.g., aldehydes, alkenes, and furans) to produce C 5 –C 8 cyclic ethers . The calculated apparent barriers of these reactions (∼1 eV) are lower than those of the cellulose activation or decomposition reactions (>1.5 eV) .…”

Eliminating Systematic Errors in DFT via Connectivity-Based Hierarchy: Accurate Bond Dissociation Energies of Biodiesel Methyl Esters

“…The chemical composition of biodiesel is an inherent mixture of methyl esters of long, unsaturated fatty acids, such as oleic acid or linolenic acid, typically contain 16–18 carbon atoms with one to three CC double bonds in the hydrocarbon chain. A complete study of the intrinsic thermodynamic parameters controlling the different bond strengths is a crucial factor in the search of more efficient biofuels. − …”