Thermal Decomposition Mechanisms of the Methoxyphenols: Formation of Phenol, Cyclopentadienone, Vinylacetylene, and Acetylene

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 115; no. 46; pp. 13381 - 13389
• Main Authors: Scheer, Adam M, Mukarakate, Calvin, Robichaud, David J, Nimlos, Mark R, Ellison, G. Barney
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24.11.2011
• Subjects: 09 BIOMASS FUELS; A: Kinetics, Spectroscopy; Acetylene - analogs & derivatives; Acetylene - chemistry; Cobalt; combustion; Cyclopentanes - chemistry; Decomposition; gas phase; gasification; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Molecular Structure; Phenol; Phenols - chemical synthesis; Phenols - chemistry; Powder injection molding; Radicals; Reactors; Silicon carbide; Spectroscopy; Temperature; thermochemical decomposition
• ISSN: 1089-5639; 1520-5215; 1520-5215
• DOI: 10.1021/jp2068073

The pyrolyses of the guaiacols or methoxyphenols (o-, m-, and p-HOC6H4OCH3) have been studied using a heated SiC microtubular (μ-tubular) reactor. The decomposition products are detected by both photoionization time-of-flight mass spectroscopy (PIMS) and matrix isolation infrared spectroscopy (IR). Gas exiting the heated SiC μ-tubular reactor is subject to a free expansion after a residence time of approximately 50–100 μs. The PIMS reveals that, for all three guaiacols, the initial decomposition step is loss of methyl radical: HOC6H4OCH3 → HOC6H4O + CH3. Decarbonylation of the HOC6H4O radical produces the hydroxycyclopentadienyl radical, C5H4OH. As the temperature of the μ-tubular reactor is raised to 1275 K, the C5H4OH radical loses a H atom to produce cyclopentadienone, C5H4O. Loss of CO from cyclopentadienone leads to the final products, acetylene and vinylacetylene: C5H4O → [CO + 2 HCCH] or [CO + HCC–CHCH2]. The formation of C5H4O, HCCH, and CH2CHCCH is confirmed with IR spectroscopy. In separate studies of the (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectra, we observe the presence of C6H5OH in the molecular beam: C6H5OH + λ275.1 nm → [C6H5OH Ã] + λ275.1nm → C6H5OH+. From the REMPI and PIMS signals and previous work on methoxybenzene, we suggest that phenol results from a radical/radical reaction: CH3 + C5H4OH → [CH3–C5H4OH]* → C6H5OH + 2H.