The threshold dilution ratio of acetaldehyde is much larger than those of other odor compounds generated during the spontaneous combustion process and so it is the most important odorant. Studying the mechanism by which acetaldehyde is generated can provide the necessary theoretical support for acetaldehyde-based odor analysis. In the present work, the release of acetaldehyde was monitored while heating lignite, long-flame coal, and coking coal specimens under either air or nitrogen. The data show that acetaldehyde was primarily produced by the oxidation of active sites in the coal rather than by the pyrolysis of oxygen-containing functional groups. Based on quantum chemistry and coal−oxygen reaction theory, the transition state approach was used to further study the formation of acetaldehyde during the low-temperature oxidation of coal. Using density functional theory, three different coal molecule structures were modeled and optimized structures for acetaldehyde formation and the energies, bond lengths, and virtual frequencies of each reaction stagnation point were obtained at the B3LYP-D3/6-311G** and M062X-D3/Def2-TZVP levels. The results indicate that the low-temperature oxidation of coal to generate acetaldehyde involves the capture of H atoms from aliphatic side chains to generate peroxy radicals. These radicals then attack unsaturated C atoms through complex inversions to generate peroxides. In the third step of this process, the O−O single bonds in the peroxides break in response to thermal energy to form carbonyl groups. Finally, specific C−C or C−O bonds on the aliphatic side chains are thermally cleaved to generate acetaldehyde.
A FeMnCe-activated semi-coke catalyst (FeMnCe/ ASC) was prepared by the co-precipitation method using semi-coke as the raw material. The structure and morphology were characterized by X-ray diffraction, Brunauer−Emmett−Teller, scanning electron microscopy, and transmission electron microscopy analyses. The catalytic activity and stability of the FeMnCe/ASC catalyst were investigated with piperazine as the target degradation pollutant and ammonia nitrogen and chemical oxygen demand (COD) as the evaluation indexes. The results showed that the average pore size of FeMnCe/ASC mesopores was 6.68 nm, and the active components were uniformly dispersed on the carrier surface. Under the optimum conditions of piperazine solution including a mass concentration of 100 mg/L, a catalyst mass concentration of 4.0 g/L, a reaction temperature of 240 °C, an oxygen partial pressure of 1.2 MPa, a stirring speed of 500 rpm, and a reaction time of 120 min, the degradation rates of both ammonia nitrogen and COD reached 100%. After the catalyst was recycled five times, the degradation rates of ammonia nitrogen and COD still reached more than 90%. The elemental valence changes before and after the reaction were analyzed by X-ray photoelectron spectroscopy, and the intermediate products generated from piperazine degradation were analyzed by gas chromatography−mass spectroscopy to evaluate the mechanism of piperazine degradation and speculate about the degradation pathway of piperazine.
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