High-level ab initio enthalpies of formation of 2,5-dimethylfuran, 2-methylfuran, and furan

Computational insight into excited states of the ring-opening radicals from the pyrolysis of furan biofuels

The low-lying valence excited states and Rydberg states of the radical species from the ring-opening reactions in pyrolysis of furan biofuels have been determined by extensive density functional theory and sophisticated wave function theory calculations. The radicals 1-C₄ H₅ O-2, 2-furylCH₂ , and 4-C₆ H₇ O with the delocalized π-type single electron are predicted to be most stable among the reactive species here for furan, 2-methyfuran, and 2,5-dimethylfuran, respectively. Predicted vertical transition energies by TD-CAM-B3LYP show good agreement with those by CASPT2. Some among the electronic excitations to low-lying states can take place in the visible light region, and they may be involved in the combustion process. Further surface hopping dynamics simulations on the excited states of the most stable ring-opening radical 1-C₄ H₅ O-2 of furan as an example reveal that 89.9% sampling trajectories at the initial excited state of 2² A"(π¹ π*² ) decay to the 1² A'(n¹ π*² ) state within an average of 384 fs, and then 81.2% trajectories at the 1² A' state go to the ground state within an average of 114 fs. At the end of the simulation for 1000 fs, 18.8% trajectories still stay on the excited states of 2² A" and 1² A', suggesting that the reactive radicals in the ground state are mainly responsible for the combustion chemistry of furan biofuels. © 2018 Wiley Periodicals, Inc.

Infrared and Electronic Spectroscopy of the Jet-Cooled 5-Methyl-2-furanylmethyl Radical Derived from the Biofuel 2,5-Dimethylfuran

The electronic and infrared spectra of the 5-methyl-2-furanylmethyl (MFM) radical have been characterized under jet-cooled conditions in the gas phase. This resonance-stabilized radical is formed by H atom loss from one of the methyl groups of 2,5-dimethylfuran (DMF), a promising second-generation biofuel. As a resonance-stabilized radical, it plays an important role in the flame chemistry of DMF. The D0-D1 transition was studied using two-color resonant two-photon ionization (2C-R2PI) spectroscopy. The electronic origin is in the middle of the visible spectrum (21934 cm(-1) = 455.9 nm) and is accompanied by Franck-Condon activity involving the hindered methyl rotor. The frequencies and intensities are fit to a one-dimensional methyl rotor potential, using the calculated form of the ground state potential. The methyl rotor reports sensitively on the local electronic environment and how it changes with electronic excitation, shifting from a preferred ground state orientation with one CH in-plane and anti to the furan oxygen, to an orientation in the excited state in which one CH group is axial to the plane of the furan ring. Ground and excited state alkyl CH stretch infrared spectra are recorded using resonant ion-dip infrared (RIDIR) spectroscopy, offering a complementary view of the methyl group and its response to electronic excitation. Dramatic changes in the CH stretch transitions with electronic state reflect the changing preference for the methyl group orientation.

The pyrolysis of 2-methylfuran: a quantum chemical, statistical rate theory and kinetic modelling study

Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice-Ramsperger-Kassel-Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1-C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

Pyrolysis of furan in a microreactor

A silicon carbide microtubular reactor has been used to measure branching ratios in the thermal decomposition of furan, C4H4O. The pyrolysis experiments are carried out by passing a dilute mixture of furan (approximately 0.01%) entrained in a stream of helium through the heated reactor. The SiC reactor (0.66 mm i.d., 2 mm o.d., 2.5 cm long) operates with continuous flow. Experiments were performed with a reactor inlet pressure of 100-300 Torr and a wall temperature between 1200 and 1600 K; characteristic residence times in the reactor are 60-150 μs. The unimolecular decomposition pathway of furan is confirmed to be: furan (+ M) ⇌ α-carbene or β-carbene. The α-carbene fragments to CH2=C=O + HC≡CH while the β-carbene isomerizes to CH2=C=CHCHO. The formyl allene can isomerize to CO + CH3C≡CH or it can fragment to H + CO + HCCCH2. Tunable synchrotron radiation photoionization mass spectrometry is used to monitor the products and to measure the branching ratio of the two carbenes as well as the ratio of [HCCCH2]/[CH3C≡CH]. The results of these pyrolysis experiments demonstrate a preference for 80%-90% of furan decomposition to occur via the β-carbene. For reactor temperatures of 1200-1400 K, no propargyl radicals are formed. As the temperature rises to 1500-1600 K, at most 10% of the decomposition of CH2=C=CHCHO produces H + CO + HCCCH2 radicals. Thermodynamic conditions in the reactor have been modeled by computational fluid dynamics and the experimental results are compared to the predictions of three furan pyrolysis mechanisms. Uncertainty in the pressure-dependency of the initiation reaction rates is a possible a source of discrepancy between experimental results and theoretical predictions.

A comprehensive experimental and detailed chemical kinetic modelling study of 2,5-dimethylfuran pyrolysis and oxidation

The pyrolytic and oxidative behaviour of the biofuel 2,5-dimethylfuran (25DMF) has been studied in a range of experimental facilities in order to investigate the relatively unexplored combustion chemistry of the title species and to provide combustor relevant experimental data. The pyrolysis of 25DMF has been re-investigated in a shock tube using the single-pulse method for mixtures of 3% 25DMF in argon, at temperatures from 1200-1350 K, pressures from 2-2.5 atm and residence times of approximately 2 ms. Ignition delay times for mixtures of 0.75% 25DMF in argon have been measured at atmospheric pressure, temperatures of 1350-1800 K at equivalence ratios (ϕ) of 0.5, 1.0 and 2.0 along with auto-ignition measurements for stoichiometric fuel in air mixtures of 25DMF at 20 and 80 bar, from 820-1210 K. This is supplemented with an oxidative speciation study of 25DMF in a jet-stirred reactor (JSR) from 770-1220 K, at 10.0 atm, residence times of 0.7 s and at ϕ = 0.5, 1.0 and 2.0. Laminar burning velocities for 25DMF-air mixtures have been measured using the heat-flux method at unburnt gas temperatures of 298 and 358 K, at atmospheric pressure from ϕ = 0.6-1.6. These laminar burning velocity measurements highlight inconsistencies in the current literature data and provide a validation target for kinetic mechanisms. A detailed chemical kinetic mechanism containing 2768 reactions and 545 species has been simultaneously developed to describe the combustion of 25DMF under the experimental conditions described above. Numerical modelling results based on the mechanism can accurately reproduce the majority of experimental data. At high temperatures, a hydrogen atom transfer reaction is found to be the dominant unimolecular decomposition pathway of 25DMF. The reactions of hydrogen atom with the fuel are also found to be important in predicting pyrolysis and ignition delay time experiments. Numerous proposals are made on the mechanism and kinetics of the previously unexplored intermediate temperature combustion pathways of 25DMF. Hydroxyl radical addition to the furan ring is highlighted as an important fuel consuming reaction, leading to the formation of methyl vinyl ketone and acetyl radical. The chemically activated recombination of HȮ2 or CH3Ȯ2 with the 5-methyl-2-furanylmethyl radical, forming a 5-methyl-2-furylmethanoxy radical and ȮH or CH3Ȯ radical is also found to exhibit significant control over ignition delay times, as well as being important reactions in the prediction of species profiles in a JSR. Kinetics for the abstraction of a hydrogen atom from the alkyl side-chain of the fuel by molecular oxygen and HȮ2 radical are found to be sensitive in the estimation of ignition delay times for fuel-air mixtures from temperatures of 820-1200 K. At intermediate temperatures, the resonantly stabilised 5-methyl-2-furanylmethyl radical is found to predominantly undergo bimolecular reactions, and as a result sub-mechanisms for 5-methyl-2-formylfuran and 5-methyl-2-ethylfuran, and their derivatives, have also been developed with consumption pathways proposed. This study is the first to attempt to simulate the combustion of these species in any detail, although future refinements are likely necessary. The current study illustrates both quantitatively and qualitatively the complex chemical behavior of what is a high potential biofuel. Whilst the current work is the most comprehensive study on the oxidation of 25DMF in the literature to date, the mechanism cannot accurately reproduce laminar burning velocity measurements over a suitable range of unburnt gas temperatures, pressures and equivalence ratios, although discrepancies in the experimental literature data are highlighted. Resolving this issue should remain a focus of future work.

Harmonising production, properties and environmental consequences of liquid transport fuels from biomass--2,5-dimethylfuran as a case study

The rapid development in methods for transforming non-edible biomass into platform chemicals and fuels has accelerated over recent years. However, the determination of whether these 'next-generation' biofuels perform in a satisfactory manner in engines, turbines and burners has lagged behind. The evaluation of the ecological and toxicological aspects has also been unable to keep up. We show, by using 2,5-dimethylfuran (DMF) as a concrete example, how a range of studies is needed to establish the benefits and risks of using a particular biofuel. In this regard, the variable with the largest impact about which little is known is probably the behaviour of DMF when it is accidentally introduced into groundwater. A primary consideration is to avoid a repetition of the methyl tert-butyl ether (MTBE) fiasco.