Characterization Of Aromatic Structures In Coal Tar Pitch And Preparation Of Benzoic Acids By Oxidation Degradation

As a valuable heavy carbon resource,coal tar pitch（CTP）is overcapacity and its utilization rate is low in China.Aromatic structures（ASs）are the main component of CTP,which have important influence on the properties and uses of CTP.Therefore,study of the composition and distribution of ASs in CTP from the molecular level is the basis for the efficient utilization of CTP.In this study,DCTP and JCTP were used as samples,respectively.First,they were directly characterized through modern analytical instruments.The results show the carbon skeleton of CTP mainly consisted of aromatic carbon,and the proportion of aromatic carbon is about 95%,the average aromatic ring number of ASs unit is close to 5.The aromatic layer of JCTP is more than that of DCTP in the microcrystalline structure indicating that the microcrystalline structure of JCTP is more significant.The existing forms of nitrogen in CTP are pyridine nitrogen,amino nitrogen,pyrrole nitrogen,quaternary nitrogen and pyridine oxide,of which pyridine and pyrrole nitrogen are the highest.Sulfur forms are aliphatic sulfur,aromatic sulfur and sulfoxide,among which aromatic sulfur has the highest content.The result of ruthenium ion catalytic oxidation（RICO）shows the ASs in CTP are main in the form of dense compacting condensation,and there are a small amount of biphenyl aromatics,but the amount of aromatic ring with alkyl side chain and methylene bridge bond is less.Then the method of separation and selective destruction was adopted,and they were divided into the soluble components（extracts,E₁-E₃）and insoluble component(the residue,R_CTP)by sequential and extraction with methanol,cyclohexane,and toluene,respectively.The composition and distribution characteristics of Ass in E₁-E₃ were comprehensively analyzed by gas chromatography-mass spectrometry（GC/MS）and other analytical instruments.The results show the compounds in E₁-E₃ were detected by GC/MS mainly including aromatics and a small amount of heteroatomic compounds.The aromatics are mainly condensed aromatics,and the ring number of aromatics is 2-7.There are a few cyclic aromatic ring structures and a small number of aromatic ring structures with alphatic side chains with bridge bonds,and most of the side chains are methyl groups.Heteroatomic compounds are mainly composed of benzofuran,benzo quinoline,benzo pyridine and benzo thiophene.The total contents of 16 US EPA priorty polycyclic aromatic hydrocarbons（PAHs）in DCTP and JCTP soluble components are 110.79 mg/g and135.08 mg/g,respectively.16 kinds of PAHs in DCTP cyclohexane soluble substance（DE₂）are as high as 72.07 mg/g.Therefore,16 kinds of PAHs can be selected from DE₂ by certain means.R_CTPTP was carried out by RICO,which was selectively degraded into small molecules that could be analyzed,and the structure characteristics of ASs in R_CTPTP were deduced by detecting these small molecules.The result of R_CTP RICO shows that the hard soluble residue(DR_CTP)of DCTP and(JR_CTP)of JCTP are composed of large molecular aromatic rings composed of Coroneneand Benzene and[ghi]perylene as structural units.CTP is rich in macromolecular ASs and the soluble acids obtained from oxidative degradation are main benzoic carboxylic acids（BCAs）.In this paper,air preoxidation or solvent extraction pretreatment was applied to CTP.The aim was to increase the yield of BCAs in the alkali/O₂oxidation process of CTP,and the feasibility of obtaining BCAs from CTP was explored.After the pretreatment of DCTP,the weight loss rate of alkali/O₂ oxidation increased by 20.06%,the total yield of BCAs increased by 5.84%,the selectivity of benzene six formic acid and benzene pentacarbonic acid increased by 2.73%and 1.99%respectively,indicating that the high added value BCAs could be obtained by oxidation of the residue of DCTP first methanol extraction and air preoxidation treatment as oxidation substrate.