The fractional pyrolysis of lignin model compound para-coumaryl alcohol (p-CMA) containing a propanoid side chain and a phenolic OH group was studied using the System for Thermal Diagnostic Studies at temperatures from 200 to 900 °C, in order to gain mechanistic insight into the role of large substituents in high-lignin feedstocks pyrolysis. Phenol and its simple derivatives p-cresol, ethyl-, propenyl-, and propyl-phenols were found to be the major products predominantly formed at low pyrolysis temperatures (<500 °C). A cryogenic trapping technique was employed combined with EPR spectroscopy to identify the open-shell intermediates registered at pyrolysis temperatures above 500 °C. These were characterized as radical mixtures primarily consisting of oxygen-linked conjugated radicals. A comprehensive potential energy surface analysis of p-CMA and p-CMA + H atom systems was performed using various DFT protocols to examine the possible role of concerted molecular eliminations and free-radical mechanisms in the formation of major products. Other significant unimolecular concerted reactions along with formation and decomposition of primary radicals are also described and evaluated. The calculations suggest that a set of the chemically activated secondary radical channels is relevant to the low temperature product formation under fractional pyrolysis conditions.
The experimental results on detection and identification of intermediate radicals and molecular products from gas-phase pyrolysis of cinnamyl alcohol (CnA), the simplest non-phenolic lignin model compound, over the temperature range of 400–800 °C are reported. The low temperature matrix isolation – electron paramagnetic resonance (LTMI-EPR) experiments along with the theoretical calculations, provided evidences on the generation of the intermediate carbon and oxygen centered as well as oxygen-linked, conjugated radicals. A mechanistic analysis is performed based on density functional theory to explain formation of the major products from CnA pyrolysis; cinnamaldehyde, indene, styrene, benzaldehyde, 1-propynyl benzene, and 2-propenyl benzene. The evaluated bond dissociation patterns and unimolecular decomposition pathways involve dehydrogenation, dehydration, 1,3-sigmatropic H-migration, 1,2-hydrogen shift, C—O and C—C bond cleavage processes.
Acrolein, a beta-unsaturated (acrylic) aldehyde, is one of the simplest multifunctional molecules, containing both alkene and aldehyde groups. Acrolein is an atmospheric pollutant formed in the photochemical oxidation of the anthropogenic VOC 1,3-butadiene, and serves as a model compound for methacrolein (MACR) and methyl vinyl ketone (MVK), the major oxidation products of the biogenic VOC isoprene. In addition, acrolein is involved in combustion and biological oxidation processes. This study presents a comprehensive theoretical analysis of the acrolein + OH + O(2) addition reactions, which is a key photochemical oxidation sequence, using the G3SX and CBS-QB3 theoretical methods. Both ab initio protocols provide relatively similar results, although the CBS-QB3 method systematically under-predicts literature heats of formation using atomization enthalpies, and also provides lower transition state barrier heights. Several new low-energy pathways for unimolecular reaction of the acrolein-OH-O(2) radicals are identified, with energy at around or below that of the acrolein-OH isomers + O(2). In each case these novel reactions have the potential to reform the hydroxyl radical (OH) and form coproducts that include glyoxal, glycolaldehyde (HOCH(2)CHO), formaldehyde (HCHO), CO, and substituted epoxides. Analogous reaction schemes are developed for the photochemical oxidation of MACR and MVK, producing a number of observed oxidation products. The reaction MACR + OH + O(2) --> hydroxyacetone + OH + CO is expected to be of particular importance. This study also proposes that O(2) addition to chemically activated acrolein-OH adducts can provide prompt regeneration of OH in the atmospheric oxidation of acrolein, via a double activation mechanism. This mechanism can also be extended to isoprene, MVK, and MACR. The importance of the novel chemistry revealed here in the atmospheric oxidation of acrolein and other structurally related OVOCs and VOCs requires further investigation. Additionally, a critical evaluation of the acrolein heat of formation is presented, and a new value of -16.7 +/- 1.0 kcal mol(-1) is recommended along with other thermochemical properties, from a W1 level calculation.
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