Study On Orderly Polycondensation Of Naphthenic Base Aromatic-rich Components And Formation Mechanism Of Mesophase Structure

Mesophase pitch is recognized as excellent precursor for making high quality carbon material.Mesophase pitch based carbon material has great exploitation potentiality and broad application prospects in the field of new carbon materials due to its low price,high strength,good electric performance,and environmental protection etc.The naphthenic base aromatic-rich components is the favorable material used for the preparation of high quality mesophase pitch from the point of the average molecular structure,moreover,the vacuum residue is abundant in source and cheap in price.Therefore,the preparation and formation mechanism research of petroleum base mesophase pitch is the hotspot and focus in the field of carbon materials at home and abroad.The raw materials naphthenic base aromatic-rich components(HCTO)and its furfural extraction oil(HCTORF)were used for preparing mesophase pitch by direct thermal polycondensation method.The characterization methods of polarised optical microscopy,fourier transform infrared spectrometer(FT-IR),~1H nuclear magnetic resonance(~1H-NMR),thermogravimetric analysis(TGA),X-ray diffraction(XRD)analysis and Raman spectroscopy were used to analyze the composition and structure of mesophase pitch.The results showed that the raw material HCTORF with higher aromaticity,more naphthenic structure and less alkyl chains was easy to form mesophase pitch with lower softening point,higher carbon residue,larger domains optical texture,higher content of soluble mesophase,lower condensation degree and more aliphatic hydrocarbons.Moreover,the mesophase pitch prepared by HCTORF possesses more ordered crystal structure,less crystalline imperfection and higher degree of graphitization than the mesophase pitch from HCTO.The kinetics of thermal conversion research demonstrated that the thermal transition processes of the two kinds of raw materails follow the first order reaction kinetics model.And the apparent rate constant of HCTORF is smaller than that of HCTO.Three distillate oils(HCTORF,WHCTORF and NHCTORF)with different boiling ranges abtained from HCTORF were used for preparing mesophase pitch by orderly thermal polycondensation.The influence of the molecular weight distribution and structural composition of distillates on the properties of mesophase pitch were studied.The results showed that the composition of the raw material play a critical role on orderly thermal polycondensation.The mesophase pitch with better optical structure was produced due to the narrower molecular weight distribution and more naphthenic groups and shorter alkyl side chains in the raw materials with similar aromaticity.Moreover,the anisotropic fraction,the soluble mesophase contents and the carbon residue increased,while the softening points decreased gradually.Meanwhile,the maximum yields of the two intermediate components(n-heptane-insoluble/toluene-soluble fraction:HI-TS;toluene-insoluble/pyridine-soluble fraction:TI-PS)in mesophase pitches decreased.Additionally,with the increase of naphthenic groups and shortening of alkyl chain length in raw materials,the condensation degree of mesophase pitches decreased respectively and the alkyl structures increased.The TGA analysis indicated that the the initial decomposition temperature were higher,and the final temperature of weightlessness were lower with the increase of naphthenic groups and shortening of alkyl side chains,so the thermal polycondensation reaction was restrained.The X-ray diffraction and Raman spectroscopy analyses demestrated that the mesophase pitch became highly crystalized simultaneously:the stack height increased,the interlayer spacing decrease and the alignment degree was improved with the increase of naphthenic groups and shortening of alkyl side chains.In addition,the half-peak breadth of mesophase pitches became narrow,the crystal size increased and the crystalline imperfection decreased.The important factor in improving the optical and crystal structures of mesophase pitch is the increase of alkyl structure in mesophase molecules.The raw material HCTORF undergone heat treatment and the pretreated material(HCTORF)were gained.The pretreated material is extracted by n-heptane and methylbenzene and the fractions(HS,HITS and TI)are abtained.Next,the fractions are used for preparing mesophase pitch by direct thermal polycondensation method.The polycondensation mechanism of the three fractions during the carbonization reaction was discussed.The results show that mesophase pitch from HITS has good optical structures,high content of soluble mesophase and ratio of H/C,low softening point and high carbon residue.Moreover,the mesophase pitch became highly crystalized,the crystal size increased,the alignment degree was improved and degree of graphitization improved.The parametersδandηwere defined to describe the synergistic effect of anisotropic content and the carbonized degree.The interaction of the three fractions was discussed.The light fraction HS plays a role in reducing the system viscosity and reducing the system viscosity as solvent.The HITS fraction with abundant naphthenic structures plays a part in moderating the violence of thereaction.While the heavy fraction TI which acted as the nucleolus of mesophase pitch can expedite the generation of mesophase pitch.However,when the molecular structures of two fractions have significant difference,the molecular weight distribution is wide and the fractions do not match,so the role of synergistic effect between the fractions was weakened greatly.Additionally,the presence of alkyl chains in raw materails is benefit to the improvement of mesophase pitch,but too less or too more alkyl structures would affect the orderly arrangement of mesophase macromolecular lamella,and then it would affect the macroscopic properties of mesophase pitch.The main reason of the improvement of mesophase pitch is the increase of the naphthenic structure and secondary cause is the raise of alkyl chains.The raw materials was pretreated by polyacrylic acid,oleic acid,tetrahydronaphthalene and polyethylene glycol to obtaine the modified materials.Then the modified materials were used for preparing mesophase pitch by orderly thermal polycondensation method.The action mechanism of alkyl side chains and naphthenic structures in raw materials during the process of carbonization were studied.The results showed that the increased alkyl structure in modified materials from polyacrylic acid,oleic acid,tetrahydronaphthalene are mainly methyl,ethyl and naphthene groups respectively,while the increased alkyl structure in modified materials from polyethylene glycol contain methyl and naphthene groups.Moreover,the the alkyl structure in modified materials increased with raising amount of additives.When the additive amounts of polyacrylic acid,oleic acid,tetrahydronaphthalene and polyethylene glycol are 20wt%,5wt%,30wt%and 20wt%,the mesophase pitch have good optical structures,low softening point,high carbon residue and contain many alkyl structures.Moreover,the mesophase pitch became highly crystalized,the stack height increased,the interlayer spacing decrease,the alignment degree was improved,the crystal size increased and degree of graphitization improved.The crystal texture of mesophase pitch prepared from tetrahydronaphthalene is the best,Second is polyacrylic acid,next is polyethylene glycol and the last is oleic acid.The mesophase pitch prepared by modified materails retains some alkyl stuctures(methyl,ethyl and naphthene groups).Furthermore,the groups which contribute most to the improvement of the propoties of mesophase pitch is naphthene groups,next is methyl and the weakest factor is ethyl.The aromatic-rich components with more naphthenic structures,an appropriate amount of methyl or less number of methyl can generate superior quality mesophase pitch with excellent optical texture,ordered molecular structure and high graphitization degree.