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.