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.
Highlights
China is relatively rich in low-rank coals (LRCs) such as lignites and subbituminous coals, whose reserves account for about 50% of total coal reserves in China [1,2]
Zhaotong lignite (ZT) and Yining subbituminous coal (YN) were subjected to thermal dissolution to afford thermal insoluble portions (i.e., RZT and RYN) with macromolecular structures
The fact indicated that compared with YN, ZT had low coalification degree and rich alphatic structures, which could account for the higher yield of RYN than that of RZT
Summary
China is relatively rich in low-rank coals (LRCs) such as lignites and subbituminous coals, whose reserves account for about 50% of total coal reserves in China [1,2]. LRCs inherently contain abundant aliphatic structures and oxygen-containing functional groups, which are helpful for their oxidation It is promising for producing carboxylic acids by oxidation of LRCs [5,6,7]. The yield of BCAs is low and a large number of alphatic acids with small molecules coexist in the watersoluble portion obtained by LRC oxidation, which are different from the resulting products from bituminous coal and anthracite oxidation. It is a huge challenge for the subsequent separation of BCAs [25]. The effect of thermal dissolution on oxidation of LRCs to prepare BCAs was studied, whilst the intrinsic relationship between the thermal pretreatment of LRCs and the yield and selectivity of BCAs were investigated by ultimate analysis, Fourier transform infrared (FTIR) spectroscopy, and nitrogen adsorption-desorption analysis
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