Pyrolysis Pathways of the Furanic Ether 2-Methoxyfuran

Abstract

Substituted furans, including furanic ethers, derived from nonedible biomass have been proposed as second-generation biofuels. In order to use these molecules as fuels, it is important to understand how they break apart thermally. In this work, a series of experiments were conducted to study the unimolecular and low-pressure bimolecular decomposition mechanisms of the smallest furanic ether, 2-methoxyfuran. Electronic structure (CBS-QB3) calculations indicate this substituted furan has an unusually weak O-CH3 bond, approximately 190 kJ mol(-1) (45 kcal mol(-1)); thus, the primary decomposition pathway is through bond scission resulting in CH3 and 2-furanyloxy (O-C4H3O) radicals. Final products from the ring opening of the furanyloxy radical include 2 CO, HC≡CH, and H. The decomposition of methoxyfuran is studied over a range of concentrations (0.0025-0.1%) in helium or argon in a heated silicon carbide (SiC) microtubular flow reactor (0.66-1 mm i.d., 2.5-3.5 cm long) with reactor wall temperatures from 300 to 1300 K. Inlet pressures to the reactor are 150-1500 Torr, and the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. Products formed at early pyrolysis times (100 μs) are detected by 118.2 nm (10.487 eV) photoionization mass spectrometry (PIMS), tunable synchrotron VUV PIMS, and matrix infrared absorption spectroscopy. Secondary products resulting from H or CH3 addition to the parent and reaction with 2-furanyloxy were also observed and include CH2═CH-CHO, CH3-CH═CH-CHO, CH3-CO-CH═CH2, and furanones; under the conditions in the reactor, we estimate these reactions contribute to at most 1-3% of total methoxyfuran decomposition. This work also includes observation and characterization of an allylic lactone radical, 2-furanyloxy (O-C4H3O), with the assignment of several intense vibrational bands in an Ar matrix, an estimate of the ionization threshold, and photoionization efficiency. A pressure-dependent kinetic mechanism is also developed to model the decomposition behavior of methoxyfuran and provide pathways for the minor bimolecular reaction channels that are observed experimentally.

Broadband Microwave Spectroscopy of 2-Furanyloxy Radical: Primary Pyrolysis Product of the Second-Generation Biofuel 2-Methoxyfuran

“…Even though the pyrolysis experiments were carried out under very dilute conditions, we noticed the presence of weak transitions due to products of radical recombination reactions in the spectrum at higher temperatures. Based on the work of Ellison and co-workers, 13 these reactions involved H atom and methyl radicals recombining with the 2-furanyloxy radical to produce acrolein and crotonaldehyde, respectively. In this work, these products were used to determine the best heating conditions, as their presence acts as an indicator of secondary reactions with 2-furanyloxy radical, which we seek to minimize.…”

“…Table 2 compares select bond lengths and bond angles for the equilibrium structures of 2-methoxyfuran and the 2furanyloxy radical, calculated at the ANO0/CCSD(T) level of theory. With a signal-to-noise ratio of no more than 30 on the microwave transitions of the radical, it was not possible to observe the 13 C isotopes of the radical in natural abundance, limiting determination of experimental bond lengths and angles. However, the good comparison between experiment and theory on both the 2-furanyloxy radical (Table 2) and 2methoxyfuran molecular parameters (Table S2) lends confidence in an analysis of the differences between the calculated r e structures for 2-methoxyfuran and the 2-furanyloxy radical.…”

Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol

“…As observed earlier, ,,, phenoxy radicals decarbonylate rapidly to release cyclopentadienyl radicals that fragment to substituted cyclopentadienones and CH 3 radicals. A more interesting fate of the phenoxy radicals is that they may suffer a Mulcahy rearrangement ,− to produce salicylaldehyde derivatives and H atoms. The salicylaldehydes follow the pathway of Scheme to substituted fulveneketenes and more H atoms.…”

Boundary‐Layer Model to Predict Chemically Reacting Flow within Heated, High‐Speed, Microtubular Reactors

“…These reactive species are produced at high temperatures under very short contact times in miniature silicon carbide (SiC) microtubular reactors. In recent years, this Chen nozzle radical source has been used to interrogate complex high‐temperature dissociation processes in biomass surrogates , and polycyclic aromatic hydrocabrons (PAH)/soot precursor formation . There has been an emphasis on using these microreactors to make high‐temperature kinetics measurements.…”