Unimolecular thermal decomposition of phenol and d(5)-phenol: Direct observation of cyclopentadiene formation via cyclohexadienone
National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, Colorado 80401-3393, USA.The Journal of Chemical Physics (Impact Factor: 2.95). 01/2012; 136(4):044309. DOI: 10.1063/1.3675902
The pyrolyses of phenol and d(5)-phenol (C(6)H(5)OH and C(6)D(5)OH) have been studied using a high temperature, microtubular (μtubular) SiC reactor. Product detection is via both photon ionization (10.487 eV) time-of-flight mass spectrometry and matrix isolation infrared spectroscopy. Gas exiting the heated reactor (375 K-1575 K) is subject to a free expansion after a residence time in the μtubular reactor of approximately 50-100 μs. The expansion from the reactor into vacuum rapidly cools the gas mixture and allows the detection of radicals and other highly reactive intermediates. We find that the initial decomposition steps at the onset of phenol pyrolysis are enol/keto tautomerization to form cyclohexadienone followed by decarbonylation to produce cyclopentadiene; C(6)H(5)OH → c-C(6)H(6) = O → c-C(5)H(6) + CO. The cyclopentadiene loses a H atom to generate the cyclopentadienyl radical which further decomposes to acetylene and propargyl radical; c-C(5)H(6) → c-C(5)H(5) + H → HC≡CH + HCCCH(2). At higher temperatures, hydrogen loss from the PhO-H group to form phenoxy radical followed by CO ejection to generate the cyclopentadienyl radical likely contributes to the product distribution; C(6)H(5)O-H → C(6)H(5)O + H → c-C(5)H(5) + CO. The direct decarbonylation reaction remains an important channel in the thermal decomposition mechanisms of the dihydroxybenzenes. Both catechol (o-HO-C(6)H(4)-OH) and hydroquinone (p-HO-C(6)H(4)-OH) are shown to undergo decarbonylation at the onset of pyrolysis to form hydroxycyclopentadiene. In the case of catechol, we observe that water loss is also an important decomposition channel at the onset of pyrolysis.
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ABSTRACT: A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH(3)CDO, CD(3)CHO, and CD(3)CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1%-1%) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 μs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 μs) are identified: H, H(2), CH(3), CO, CH(2)=CHOH, HC≡CH, H(2)O, and CH(2)=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH(3)CHO (+M) → CH(3) + H + CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH(2)CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.
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ABSTRACT: time-dependent oxidation of carbon fibers in boiling nitric acid was used to investigate the influence of a modification of the fiber surface properties on the adhesion strength with an acrylate resin cured by electron beam (EB). For each time of treatment, a characterization of the surface topography and the surface chemistry was done (topography at a micrometric and nanometric scale, specific surface area, temperature programmed desorption, X-ray photoelectron spectroscopy analysis). The oxidation of the fiber surface in boiling nitric acid created a rough surface, which significantly increased the specific surface area, and also generated a high density of hydroxyl groups, carboxylic acids and lactones in comparison to untreated fibers. The adhesion strength with the acrylate resin cured by EB was measured by a pull-out test. For comparison, an isothermal ultraviolet curing of the matrix was also investigated. The value of the interfacial shear strength, determined by the Greszczuk's model, was increased by the oxidation of the carbon fiber surface for both curing processes, but lower values were systemically obtained with EB curing
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ABSTRACT: A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C5H4=O) in rare gas matrices has been carried out. Ab initio coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of o-phenylene sulfite (C6H4O2SO) was used to provide a molecular beam of C5H4=O entrained in a rare gas carrier. The beam was interrogated with time-of-flight photoionization mass spectrometry (PIMS), confirming the clean, intense production of C5H4=O. Matrix isolation infrared spectroscopy coupled with 355 nm polarized UV for photo-orientation and linear dichroism experiments was used to determine the symmetries of the vibrations. Cyclopentadienone has 24 fundamental vibrational modes, Γvib = 9a1 ⊕ 3a2 ⊕ 4b1 ⊕ 8b2. Using vibrational perturbation theory and a deperturbation-diagonalization method we report assignments of the following fundamental modes in a 4 K neon matrix; the a1 modes (in cm-1) of X ̃ 1A1 C5H4=O are found to be: ν1 = 3107, ν2 = (3100, 3099), ν3 = 1735, ν5 = 1333, ν7 = 952, ν8 = 843 and ν9 = 651. The inferred a2 modes are: ν10 = 933, and ν11 = 722. The b1 modes are: ν13 = 932, ν14 = 822, and ν15 = 629. The b2 fundamentals are: ν17 = 3143, ν18 = (3078, 3076) ν19 = (1601 or 1595), ν20 = 1283, ν21 = 1138, ν22 = 1066, ν23 = 738, and ν24 = 458. The modes ν4 and ν6 were too weak to be detected, ν12 is dipole-forbidden and its position cannot be inferred from combination and overtone bands, and ν16 is below our detection range (<400 cm-1). Additional features were observed and compared to anharmonic calculations, assigned as two quantum transitions, and used to assign some of the weak and infrared inactive fundamental vibrations.
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