| Lignite is the lowest rank of coal, which has a low heat value and high volatility, so it is significant to develop a non-fuel use of lignite. Some studies indicate that the coal can be oxidized into chemicals in alkali aqueous solutions with O2 as oxidant at high temperatures. The products mainly contain benzene poly carboxylic acids (BPCAs) and small-molecular fatty acids. BPCAs are valuable chemicals widely used in medicine, resin, plasticizer, synthetic fiber, food and electron industries, which are mainly produced from the oxidation of petroleum aromatics. As the exhaustion of petroleum resource, finding a new path to produce BPCAs is in accord with sustainable development strategy. The main contents of this study include the analysis of products from alkali-oxygen oxidation of lignite, the effect of reaction parameters on the yield of BPCAs, and structure information of lignite and humic acid (HA) via the distribution of oxidation products.The process of alkali-oxygen oxidation of lignite has undergone the generation of humic acids, water-soluble acids (WSA), and BPCAs and small-molecular fatty acids. The components of WSA are seldom studied in the previous research. In this study, LC/MS was employed to analyze the liquid products of alkali-oxygen oxidation. The research shows that the water-soluble acids include some naphthalene carboxylic acids, biphenyl carboxylic acids, and monocyclic products. Among the monocyclic products detected, the phenol group is easiest to be oxidized, the next is acetyl and aldehyde group, and the methyl is relatively inactive. The oxidation temperature, reaction time and alkali/coal ratio have significant effect on the yield of BPCAs. The result shows that 25.8 wt% of BPCAs and 40.7 wt% of small-molecular fatty acids including oxalic acid, acetic acid and formic acid were obtained from the alkali-oxygen oxidation of Xilinhaote lignite.In this study, we used several analysis and testing techniques to characterize lignite and HA, and we proposed organic structures of lignite and HA mainly based on the products from alkali-oxygen oxidation of lignite and humic acid. From the analysis of FT-IR and 13C CP/MAS NMR, we got the information of aromaticity, aromatic cluster size and the distribution of oxygen-containing functional groups. From the elemental analysis, we got the H/C ratio, O/C ratio and the content of heteroatom like N and S. From the GPC and fluorescence spectroscopy, we obtained the mass distribution and aromatic character of lignite and HA. The distribution of BPCAs could give some information of the aromatic clusters of lignite and HA. The proposed models of organic structures of lignite and HA can reflect the oxidation product characteristic, the aromaticity, aromatic cluster size and H/C ratio, which are in accord with the experimental data. |
PDF Full Text Request