Pyrolysis Pathways of the Furanic Ether 2‑Methoxyfuran

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.