Mild Oxidative Depolymerization Of Coals And Char Powders From Coal Pyrolysis

Mild oxidative depolymerization is not only an important method for getting insight into the organic structures of coals and their derivatives, but also a potential approach for obtaining carboxylic acids from coals and their derivatives. Herein, sodium hypochlorite and hydrogen peroxide-hydrogen peroxide(HPO-AAH) were adopted as oxidants to oxidize coals and char powders. The original sample, the resulting oxidation residues and organic compounds were comprehensively characterized by multiple analytical methods to understand the organic structures of these heavy carbon resources at the molecular level and facilitate the possibility of obtaining carboxylic acids from oxidation of these heavy carbon resources.The reserve of Jincheng No. 15 anthracite(J15A) is significantly abundant. However, because of the high ash yield, high sulfur content, and high fusion temperature, J15 A cannot be used for combustion and gasification. As a high rank coal, J15 A has a weak react activity. Whereas, its abundant condensed aromatic structures would be greatly beneficial to the production of aromatic chemicals. In order to improve the react activity, J15 A was first pretreated with H2O2 to introduce hydroxyl groups into the organic structures, which would improve the hydrophilicity and the react activity of the organic matters. Subsequently, the pretreated J15 A was subjected to sequential oxidation for 6 times with Na OCl to fully degrade the organic matters. Benzenecarboxylic acids(BCAs) dominate in the resulting products, and their relative content increases with increasing oxidation times. Benzenehexacarboxylic acid can be concentrated over 34% by subsequent solvent extraction, suggesting that there exists a huge possibility to get BCAs from the mild oxidation of the high rank coals. Based on the yields of the oxidation products and the FTIR analysis of the oxidation residues, the organic matters could be divided into external portion(EP) and internal portion(IP). According to the BCAs distributions, cata-condensed aromatics in the EP are signi?cantly more than those in the IP, while peri-condensed aromatics in the IP are remarkably more abundant than those in the EP. The investigation presents a useful method to investigate aromatic structures of organic matters in high rank coals.Oxidants play a key role in the coal oxidation. As an oxidant, HPO-AAH not only completely avoids the introduction of nitrogen and sulfur into the products, but also strongly interacts with organic portion due to the presence of organic medium. These advantages will contribute to the oxidation of coals or their derivatives and separation of the resulting reaction mixtures. According to the oxidation of some coal related model compounds(CRMCs) using HPO-AAH, the oxidizing species OH., released from HPO-AAH, could preferentially attack the active part of the sample, inducing the sample oxidation by addition reaction and elimination reaction. Comparing the oxidation effects of three coals with different coalification degrees, Xiaolongtan lignite(XL) is the easiest to be oxidized by HPO-AAH. Reaction temperature and reaction time have an obvious effect on the XL oxidation, and the optimal ones are 50 oC and 9 h, respectively. The resulting oxidation products are mainly characterized by malonic and succinic acids, suggesting that dominant linkages connecting aromatic rings are-CH2- and-CH2CH2-. Meantime, lignites are proved to be feedstocks to produce some aliphatic acids, providing a new route for the non-fuel use of the lignites.The char powder yield is huge, but the char powder is usually discarded as a waste. In the study, a char powder from Shenmu County, Shaanxi province, which is called SCP for convenience, was selected as a sample to be mildly oxidized. The optimized conditions for the SCP oxidation with HPO-AAH are 50 oC, 60/1 of AAH/SCP(m L/g), 60 h, and 2/1 of HPO/AAH(m L/m L). Under the optimal condition, the yield of the oxidation products is 16.77%, while the yield of the remained oxidation residue is still up to 71%, implying SCP is difficult to be oxidized by HPO-AAH for one time. After sequential oxidation of SCP with HPO-AAH for 6 times, the most of organic matters are degraded. Alkanedioic acids are the dominant products in the first oxidation of SCP, while a lot of BCAs are generated in the next oxidations. Based on the product distributions, the organic matters of SCP could be divided into EP, middle portion(MP), and IP. EP is abundant in diarylmethane, 9,10-dihydrophenanthrene and acenaphthene. These species may be produced from depolymerization of macromolecules during coal pyrolysis, and they build EP of SCP because they are not distilled from the system in time. Lots of condensed aromatic species exist in MP, and the condensation degree gradually increases from the external to internal part of MP. These aromatic species come from the thermal condensation of the original species in the coal. IP contains many diarylmethanes and aromatic species with low condensation degrees, which may originally exist in the coal and be embedded in the IP when the coal was pyrolyzed to form SCP.A quick and comprehensive identification of the molecular components of the oxidation products is significantly important for the coal oxidation. As a new ion source, atmospheric pressure solids analysis probe(ASAP) could directly ionize sample without redundant treatments, and ASAP could analyze some macromolecular compounds over 500 u. A series of CRMCs were selected to be analyzed by ASAP/time of ?ight-mass spectrometer(ASAP/TOF-MS) to investigate the ionization mechanism of ASAP. Condensed arenes are ionized to produce stable M+? and [M+H]+, respectively, by electron transfer and proton transfer. M+? is the predominant ion when an arene is ionized by ASAP, while the content of [M+H]+ is positively related to the superdelocalizability of the arene, and no fragmental ions are produced. The bridged arens would be fragmented at different degrees when they are ionized in ASAP. Fragmental ions are formed via cleavage and rearrangement of [M+H]+ or M+? from the bridged arens, whose formation closely depends on their stability. Based on the analysis of CRMCs, ASAP/TOF-MS could quickly and accurately analyze the resulting products from Tongchuan bituminite oxidation and J15 A oxidation. Furthermore, a series of compounds over 500 u, which may be tetraphenylenepolycarboxylic acids, are detected in the products from the oxidation of insoluble macromolecular network in a lignite. The fact suggests ASAP/TOF-MS could comprehensively analyze the resulting oxidation products, facilitating the understanding of the macromolecular structures in coals.