Oxidation of Zhundong subbituminous coal in aqueous Fe2+/ glacial acetic acid solution with H2O2

The improvement of the reactivity of H 2 O 2 and Zhundong subbituminous coal (ZS) was focused on in this work. The mild oxidation of ZS was carried out under different conditions. The liquid oxidation products were analyzed using gas chromatography /mass spectrometry (GC/MS). The results suggest that Fe 2+ and glacial acetic acid (GAA) have a synergistic effect on the H 2 O 2 oxidation of ZS, which can signicantly promote the conversion of ZS and the production of liquid products. In total, 40 compounds are identied, which could be categorized as 8 group components. Most of them are valued-added chemicals. Among the detected compounds, carboxylic acids (CAs) is the most abundant, accounting for 47.22% of all the group components in TRC. Moreover, according to the compounds detected by GC/MS, condensed aromatic rings are signicantly abundant than aliphatic moieties in organic matter of ZS. -CH 2 CH 2 - are predominant bridged linkages connecting aromatic rings in ZS. The existing forms of organic nitrogen and sulfur species contain quinolins, pyrroles, pyrimidins, quinoxalines, enamides, oxazoles, isoquinolines, thiazols, sulfonamides in ZS.

Therefore, it is necessary to exploit a new conversion method for utilizing LRCs e ciently based on its structural characteristics (Liu et al. 2019b;Liu et al. 2018).
As is known to all, oxygen-containing functional groups, such as − OH, −COOH, −COOR, etc., are abundant in LRCs, which make LRCs have the potential to produce value-added oxygenated organic chemicals via the liquid oxidation (Doskočil et al. 2014;Liu et al. 2016;Mae et al. 2001;Miura et al. 1996). Hence, the oxidation of LRCs have attracted much attention from researchers. Classi ed by the kinds of oxidants, the method of oxidative depolymerization of LRCs includes RuCl 3 -NaIO 4 oxidation (Huang et al. 2008;Liu et al. 2014;Lv et al. 2016;Murata et al. 2001;Stock and Wang 1986), O 2 oxidation Wang et al. 2012), NaClO oxidation (Liu et al. 2013a;Lv et al. 2018;Wang et al. 2014;Yao et al. 2010) and H 2 O 2 oxidation (Doskočil et al. 2014;Liu et al. 2019b;Liu et al. 2018;Liu et al. 2015;Mae et al. 2001;Miura et al. 1996;Tahmasebi et al. 2015;Wang et al. 2018). The high cost of RuCl 3 -NaIO 4 makes it di cult for industrial applications. O 2 oxidation of LRCs needs high temperature, high pressure and strong acids or alkalis. NaClO oxidation of LRCs produces large amount of chloro-subsituted species, leading to the di culties in separating liquid oxidation products (Liu et al. 2015;Wang et al. 2015b). H 2 O 2 oxidation of coal can be carried out at mild condition, and cannot be involved in other elements except H and O. Moreover, H 2 O 2 is low price, easy availability, and eco-friendliness. Based on the above factors, H 2 O 2 oxidation is a suitable way to utilize LRCs. Miura et al. (Miura et al. 1996) found that the oxidation of LRCs using H 2 O 2 under mild conditions could produce small molecule fatty acids in high yield and in high selectivity in 1996. After that, H 2 O 2 oxidation of coal was used to produce carboxylic acids, including aliphatic carboxylic acids and benzenepolycarboxylic acids, and reveal molecular structure of coal (Doskočil et al. 2014;Liu et al. 2018;Liu et al. 2015;Wang et al. 2015b;Wang et al. 2018). However, to our knowledge, few reports were issued on improving the reactivity of H 2 O 2 and coal.
Oxidation of coal with H 2 O 2 was a free radical reaction, which hydroxyl radicals (·OH), derived from the decomposition of H 2 O 2 , reacted with coal to achieve oxidative depolymerization of coal (Liu et al. 2018;Pan et al. 2013). Fe 2+ is prone to catalyze H 2 O 2 decomposition to form ·OH, as shown in Eq. 1 (Jiang et al. 2013). Moreover, the yield of the liquid products abundant in value-added organic chemicals produced via oxidative depolymerization of coal with H 2 O 2 is improved with the aid of glacial acetic acid (GAA) (Liu et al. 2015). In this paper, we extend the above ideas and propose to improve the yield of liquid products produced through H 2 O 2 oxidation of coal by introducing Fe 2+ and GAA. We investigated conversion of Zhundong subbituminous coal (ZS) at different reaction conditions and the liquid products were separated and analyzed with gas chromatography/mass spectroscopy (GC/MS). The result suggests that Fe 2+ and GAA exists in a synergistic effect on the oxidation of ZS in aqueous H 2 O 2 solution, which can signi cantly promote the conversion of ZS and the production of liquid products. The conversion of coal, and the yield and selectivity of E were calculated using the following Equations.
where m 0 , m 1 and m 2 are the weights of raw coal, FC and E, respectively.
Page 5/17 2.3 Sample analyses MEE was analyzed using an Agilent 7890/5977 GC/MS equipped with a DB-EUPAH capillary column and a quadrupole mass analyzer with a split flow rate of 10:1, a flow velocity of 1.0 mL/s, and operation under electron bombardment (70 eV) mode. The mass scanning range was from 30 to 500 amu. and the electron bombardment voltage is 70 eV.
The capillary column was held at 45 °C for 3 min, and then heated at 10 °C min −1 from 45 to 310°C, and held at 310 °C for 20 min. Data were obtained and processed using GC/MS software, and the compounds were identified by comparing the mass spectra to the NIST05 spectral library data and referring to available references. The relative content (RC) of the compound was determined by normalization of the peak area, which was defined as the peak area of the compound divided by the sum of the peak areas of all identified compounds in the total ion chromatogram (Wang et al. 2015b). The total relative content (TRC) was acquired by the sum of the RC from a class of compounds.
As shown in Table 3 and Fig. 3, 19  Then, ·OH reacts with the cluster ring, producing a large amount of water-soluble organic compounds. Meanwhile, Liu et al. (Liu et al. 2018) supposed that ·OH can break condensed aromatic rings in Xianfeng lignite-derived residue followed by rupturing the condensed aromatic rings to produce aromatic carboxylic acids. Among the detected CAs, BCAs are the most abundant in detected compounds, accounting for 34.89% of all group components in TRC, suggesting that that condensed aromatic rings are significantly abundant than aliphatic moieties in organic matter of ZS. Interestingly, benzene-1,2,3,4,5,6-hexacarboxylic acid , accounts for 25.03% of all the group components in TRC. The resulting benzene-1,2,3,4,5,6hexacarboxylic acid could be derived from the oxidation of highly condensed aromatic rings (Scheme 1). Among the detected ADAs, succinic acid is predominate, accounting for 4.3% of all the group components in TRC, implying that -CH 2 CH 2 -is dominant bridged linkages connecting aromatic rings in ZS. Moreover, most of the detected CAs are valued-added chemicals. For example, palmitic acid can be used as a precipitant, chemical reagent and waterproofing agent. Succinic acid can be used to produce surfactant, detergent additive and foaming agent. Benzene-1,2,3,4,5,6-hexacarboxylic acid is a useful polydentate ligand (Liu et al. 2018).
As shown in Table 3 and Fig. 3,