Study On The Depolymerization Of Lignite In Ionic Liquids

An understanding of the basic structure of lignite and its thermal depolymerization behaviors at the molecular level is the basis for coal efficiency and cleaning uses.It is also the key scientific problem needed to be resolved for the development of the theory of coal chemistry and the conversion technology of lignite. In order to overcome the problem of the low extraction yield of lignite with many conventional organic solvents at mild extraction conditions, depolymerization performance of lignite in new ionic liquids 1-butyl-3-methylidazolium tetrafluoromethanesulfonate([Bmim]OTf), 1-ethyl-3-meth-ylidazolium acetate([Emim]Ac) and the cosolvents consist of 1-butyl-3-meth ylidazolium chloride([Bmim]Cl) with different solvents were investigated in this paper. Also the thermal extracts and residues obtained from thermal depolymerization of lignite at different extraction conditions were analyzed to explore the structure of thermal products.It was found that ionic liquid [Bmim]OTf had a good performance in the depolymerization of Xianfeng lignite(XL). The extraction yield of XL in [Bmim]OTf, which is defined as tetrahydrofuran solubles(THFS), reaches to 20.1%. The depolymerization of XL was promoted markedly in ionic liquids [Bmim]OTf with the addition of tetralin(THN), H2 and catalyst ZnCl2 to depolymerization system and the yield of THFS rose to 30.4%, 36.9% and 46.8% respectively. The amount of hydrogen bonds in thermal extracts increased significantly with the addition of THN, the content of fatty compounds in thermal extracts increased greatly under the existence of H2 and the content of aliphatic hydrocarbon and aryl ethers increased obviously as the addition of catalyst ZnCl2.It was found that the ionic liquid [Emim]Ac also had a good performance on the thermal depolymerization of XL compared with that of the ionic liquid [Bmim] OTf. With increasing the thermal temperature the depolymerization performance improved significantly and the extraction yield of thermal extracts obtained from the depolymerization of XL was 92.3% in [Emim]Ac at 200 oC. Under the same extraction conditions, the extraction yields of different type of lignite in ionic liquid [Emim]Ac were significantly different. The extraction yields of different lignite follows the order: XF lignite > SL lignite > XLGL lignite. In addition, the type of the ionic liquids markedly affects the depolymerization performance of XL in ionic liquids. The extraction yields of lignite in different ionic liquids follow the order as follows: [Emim]Ac>[Bmim]Cl>[Bmim]OTf. The thermal extracts obtained from the depolymerization of XL in ionic liquid [Emim]Ac mainly contains aromatic compounds and short chain fatty compounds. It was found that there were a large number of hydrogen bonds presenting in the thermal extracts obtained from the depolymerization of XL in ionic liquid [Bmim]Cl. The thermal extracts obtained from depolymerization of XL in ionic liquid [Bmim]OTf mainly contain ether and aromatic compounds.Based on the good performance of cleavages hydrogen bonds in XL dissolution with ionic liquid [Bmim]Cl, the depolymerization of XL in the cosolvents consist of 1-butyl-3-methylidazolium chloride([Bmim]Cl) with different solvents were investigated. The results showed that the addition of 1–methylnaphthalene(1–MN), crude methyl naphthalene oil(CMNO) and THN in ionic liquid [Bmim]Cl promoted the dissolution performance of XL.