Insight into Relationship between Thermal Dissolution of Low-Rank Coals and Their Subsequent Oxidative Depolymerization

Abstract

Oxidative depolymerization of low-rank coals is promising for obtaining benzene carboxylic acids (BCAs). However, it is hindered by the low yield of BCAs along with a large number of alphatic acids. Thermal dissolution could modify the physico-chemical structural features of low-rank coals, which is expected to improve the oxidation of LRCs. In this paper, lignite and subbituminous coal were firstly subjected to thermal dissolution with cyclohexane at 250 °C for 2 h. Then, the raw coal and the corresponding thermal insoluble portion (TIP) were oxidized by NaOCl under the same conditions. The residual yields of TIPs oxidation were both lower than those of raw coals oxidation, indicating that TIPs were more easily oxidized than the raw coals. The yield of BCAs obtained by TIPs oxidation was above 19% higher than that from the oxidation of raw coals. Meanwhile, the selectivity of BCAs was improved in the resulting oxidation products from TIPs compared with that from the raw coals. The relationship between BCAs generation and thermal dissolution of low rank coals was investigated by ultimate analysis, Fourier transform infrared spectroscopy, and nitrogen adsorption-desorption analysis. The results suggested that thermal dissolution could enrich aromatic portion in the remaining TIPs, resulting in an increasing of the yield and selectivity of BCAs. Simultaneously, thermal dissolution raised the specific surface area and expanded the looser space structure of TIPS, which were beneficial for the sufficient collision between aromatic structures and oxidant, facilitating the oxidative depolymerization of TIPs. This investigation would provide a novel route for promoting BCAs production by mild oxidative depolymerization of low-rank coals.