Computational Study of the Combustion and Atmospheric Decomposition of 2-Methylfuran

Search for transition states with external forces

It is much more difficult to find on the potential energy surface (PES) a transition state (TS) than a local minimum (LM). We propose a new methodology which makes this task much easier. Applying external forces to nuclei in a molecule we can locate on PES not just one particular TS but a number of consecutive transition states. With our approach it is possible to move over PES from one transition state to another without involving any local minima. The latter can be located in a separate step through the reaction path calculations performed for every transition state found before. Preliminary results for the 2-fluorofuran molecule illustrate the usefulness of the proposed method.

Kinetics of the Reactions of Hydroxyl Radicals with Furan and Its Alkylated Derivatives 2-Methyl Furan and 2,5-Dimethyl Furan

Furans are promising second generation biofuels with comparable energy densities to conventional fossil fuels. Combustion of furans is initiated and controlled to a large part by reactions with OH radicals, the kinetics of which are critical to understand the processes occurring under conditions relevant to low-temperature combustion. The reactions of OH radicals with furan (OH + F, R1), 2-methyl furan (OH + 2-MF, R2), and 2,5-dimethyl furan (OH + 2,5-DMF, R3) have been studied in this work over the temperature range 294-668 K at pressures between 5 mbar and 10 bar using laser flash photolysis coupled with laser-induced fluorescence (LIF) spectroscopy to generate and monitor OH radicals under pseudo-first-order conditions. Measurements at p ≤ 200 mbar were made in N₂, using H₂O₂ or (CH₃)₃COOH radical precursors, while those at p ≥ 2 bar were made in He, using HNO₃ as the radical precursor. The kinetics of reactions R1-R3 were observed to display a negative dependence on temperature over the range investigated, indicating the dominance of addition reactions under such conditions, with no significant dependence on pressure observed. Master equation calculations are in good agreement with the observed kinetics, and a combined parametrization of addition channels and abstraction channels for R1-R3 is provided on the basis of this work and previous shock tube measurements at higher temperatures. This work significantly extends the temperature range previously investigated for R1 and represents the first temperature-dependent measurements of R2 and R3 at temperatures relevant for atmospheric chemistry and low-temperature combustion.

Mechanisms and kinetics of the low-temperature oxidation of 2-methylfuran: insight from DFT calculations and kinetic simulations

The low-temperature oxidation (LTO) mechanisms of the 2-methylfuran (2-MF) biofuel and the corresponding thermodynamic and kinetic properties have been explored by density functional theory (DFT) and composite G4 methodologies as well as kinetic simulations. The O₂ addition to the main furylCH₂ radical from the methyl dehydrogenation in 2-MF forms three peroxide radicals PO1, PO2, and PO3 with the energy barriers of 15.1, 19.3, and 20.6 kcal mol^-1 and the reaction ΔG of -8.2, 5.7, and -0.1 kcal mol^-1 (298 K and 1 atm), respectively. Through hydrogen transfer followed by dehydroxylation, these nascent products evolve into stable aldehydes and cyclic ketones, which may further decompose into smaller species under the action of OH. Calculations and simulations show that the product P1 from the dehydroxylation of PO1 has a dominant population (higher than 96%) among the final products, although the temperature and pressure may influence the species profiles and rate constants to some extent. Based on the G4-calibrated thermodynamic parameters, the temperature and pressure dependence of the rate constants and the two- and three-parameter Arrhenius coefficients for all reactions considered here have been determined by using the transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) methods. The present results provide a comprehensive understanding of the mechanisms and kinetics of the LTO process of the 2-MF biofuel.

Driving proton transfer reactions in the 2-methylfuran ring with external forces

In this paper we investigate the proton transfer reactions in 2-methylfuran.

Isomerization and Decomposition of 2-Methylfuran with External Forces

The primary goal of this project was to evaluate the performance of the Standard and Enforced Geometry Optimization (SEGO) method which we have recently developed. The SEGO method has been designed for an automatic location of multiple minima on the molecular Potential Energy Surface (PES), and its usefulness has been demonstrated so far for three molecules only. In this project we applied the SEGO method to explore the 2-methylfuran (2MF) PES. Our choice was not accidental: this molecule recently gained a great deal of interest as a potential candidate for biofuel, and therefore its pyrolysis is extensively studied. To understand pyrolysis of 2MF a detailed knowledge about its PES is needed. In these studies we explored the 2MF PES and located a surprisingly large number of local minima corresponding to 2MF isomers and decomposition products. Some of the 2MF isomers and fragments have amazing structures which most likely were previously unknown. All structures presented in this paper were found in an automatic manner as a result of the enforced chemical reactions driven by the SEGO method. Thus, in these studies we have not only proven that the SEGO method is a very efficient tool for exploring molecular potential energy surfaces, but we also obtained very detailed knowledge about the 2MF PES.

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.

Peroxy self-reaction leading to the formation of furfural

Experimental and theoretical results show the importance of peroxy radical self-reaction.

High temperature pyrolysis of 2-methyl furan

Experiments and theory reveal the complex dissociation of 2-methylfuran and the surprising importance of H-atom loss.

Gas-phase ozonolysis of furans, methylfurans, and dimethylfurans in the atmosphere

Ozonolysis of methylfurans contributes significantly to their atmospheric degradation.

Study of the Synchrotron Photoionization Oxidation of 2-Methylfuran Initiated by O(3P) under Low-Temperature Conditions at 550 and 650 K

The O-(³P)-initiated oxidation of 2-methylfuran (2-MF) was investigated using vacuum-ultraviolet synchrotron radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Reaction species were studied by multiplexed photoionization mass spectrometry at 550 and 650 K. The oxygen addition pathway is favored in this reaction, forming four triplet diradicals that undergo intersystem crossing into singlet epoxide species that lead to the formation of products at m/z 30 (formaldehyde), 42 (propene), 54 (1-butyne, 1,3-butadiene, and 2-butyne), and 70 (2-butenal, methyl vinyl ketone, and 3-butenal). Mass-to-charge ratios, photoionization spectra, and adiabatic ionization energies for each primary reaction species were obtained and used to characterize their identities. In addition, by means of electronic structure calculations, potential energy surface scans of the different species produced throughout the oxidation were examined to further validate the primary chemistry occurring. Branching fractions for the formation of the primary products were calculated at the two temperatures and contribute 81.0 ± 21.4% at 550 K and 92.1 ± 25.5% at 650 K.

Absolute photoionization cross sections of furanic fuels: 2-ethylfuran, 2-acetylfuran and furfural

Absolute photoionization cross sections of the molecules 2-ethylfuran, 2-acetylfuran and furfural, including partial ionization cross sections for the dissociative ionized fragments, are measured for the first time. These measurements are important because they allow fuel quantification via photoionization mass spectrometry and the development of quantitative kinetic modeling for the complex combustion of potential fuels. The experiments are carried out using synchrotron photoionization mass spectrometry with an orthogonal time-of-flight spectrometer used for mass analysis at the Advanced Light Source of Lawrence Berkeley National Laboratory. The CBS-QB3 calculations of adiabatic ionization energies and appearance energies agree well with the experimental results. Several bond dissociation energies are also derived and presented.

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).