Pyrolysis of Furan at Low Pressures:  Vibrational Relaxation, Unimolecular Dissociation, and Incubation Times

Abstract

Vibrational relaxation, incubation times, and unimolecular dissociation of C4H4O have been investigated over the extended temperature range 500−3000 K in 2−5% furan−krypton mixtures, 2% furan−neon mixtures, and in pure furan. The experiments were performed in shock waves using laser-schlieren (LS) densitometry and time-of-flight (TOF) mass spectrometry. At low temperatures and low pressures, only vibrational relaxation was observed using the LS technique. This relaxation is unexpectedly slow and shows a strong nonexponential time dependence. Unimolecular dissociation is observed in TOF experiments between 1300 and 1700 K in a pressure range of 175−250 Torr as well as LS experiments between 1700 and 3000 K for pressures between 100 and 600 Torr. The TOF experiments show that under the given conditions two molecular dissociation channels leading to C2H2 + CH2CO or to C3H4 + CO are dominant. The branching ratio between these channels has been determined between 1300 and 1700 K. At low temperatures, the molecular channel leading to C3H4 and CO is preferred, but a channel switching was observed around 1700 K. The domination of these molecular channels is consistent with the shape of the LS profiles, and these have been successfully modeled with just these two reactions. The overall unimolecular rate constant is in the falloff regime close to the low-pressure limit. By use of statistical reaction rate theory, the total unimolecular rate constant could be modeled over an extended temperature and pressure range using a value of 〈ΔE〉all = 50 cm-1 for the furan dissociation. In a small range of conditions at low pressures and high temperatures, both the vibrational relaxation and dissociation were resolved and incubation times estimated.

Pyrolysis of furan in a microreactor

“…At the lowest elevated temperature studied (1000 K), there are clearly products from both channels. A weak temperature dependence is observed, which is qualitatively consistent with that found by a shock-tube study fifteen years ago 16 under slightly different reaction conditions. While the absolute ratios found here by four different approaches are in quite good mutual agreement, they are about a factor of two smaller than those found in Fulle et al 16 .…”

“…A weak temperature dependence is observed, which is qualitatively consistent with that found by a shock-tube study fifteen years ago 16 under slightly different reaction conditions. While the absolute ratios found here by four different approaches are in quite good mutual agreement, they are about a factor of two smaller than those found in Fulle et al 16 . At the lowest temperatures that we study (1000 K -1100 K), roughly 85% of the reaction goes through the β-carbene.…”

“…The first two measurements are carried out at a common photon energy, which as elaborated upon in Section IV.b, is the preferred route; the other two latter estimates require detailed considerations described below. Also included for comparison is the curve fit from Fulle et al 16 who measured the ratio in a shock tube, quantifying these species using electron impact, time-of-flight mass spectrometry.…”

“…The uncertainty in the photon flux measurements, Φ(E), is likely the cause that this ratio is less than 1. The results of a shock tube study 16 (Fig. 4), the propargyl radical/methylacetylene ratio was measured at 10.4 eV () and 10.5 eV ().…”

“…In earlier studies, the thermal decomposition of furan was studied in flow tubes, 12,13 shock tubes, 14,15,16 and by IR homogeneous pyrolysis. 17 These studies were conducted over a wide range of pressure (1 mTorr -20 atm) and temperature (500 K -3000 K).…”