Influence Of Cocatalyst On The Formation And Transformation Of Mesophase Pitch

Mesophase pitch is an optical anisotropic liquid crystal formed by polycondensation of aromatic hydrocarbons such as pitch in inert gas atmosphere.It is an intermediate transition state of pitch from liquid to transform into coke and other solid carbon materials.It is a kind of excellent carbon precursors,mainly containing planar polycyclic aromatic hydrocarbons.The pitch quality directly determines the performance of mesophase pitch-based carbon fibers.At present,the best method for catalytic preparation of mesophase pitch is using aluminum trichloride（AlCl₃）as catalyst,but AlCl₃ is easy to react with co-catalyst water to form aluminum oxide insoluble substance in the reaction,which is difficult to remove,which directly affects the performance of subsequent pitch-based carbon fibers.How to completely remove aluminum compounds is the key to prepare high-performance mesophase pitches.The objective of this work is to use medium coal tar pitch as raw material,anhydrous AlCl₃ as catalyst,and explore the influence of different co-catalysts on the formation and transformation of mesophase pitch.Using polarization microscope,thermogravimetric analysis（TG）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）and other characterization methods to measure and analyze the textures and structures of different pitch products.Finally suitable mesophase preparation conditions were obtained,which will provide a technical approach for the preparation of high performance pitch-based carbon fibers.The basic reaction conditions for AlCl₃ catalyzed coal tar pitch polymerization were studied,and the effects of reaction temperature,catalyst addition amount and holding time on the formation and transformation of mesophase were investigated.The results show that increasing the temperature,catalyst content and prolongating reaction time are beneficial to the formation of mesophase pitch,and the three factors have equivalent effects in a certain range.Under the process condition of 370℃,8%anhydrous AlCl₃ addition and constant temperature for 8 h,the product mesophase shows spherical anisotropic texture.The effects of acetic acid（HAc）and iso-propane chloride（iso-PrCl）on the formation of mesophase pitch were investigated by using HAC and iso-PrCl as co-catalysts,respectively.The results show that with the increase of the amount of cocatalyst,the mesophase content increases first and then decreases,and the mesophase texture changes from mosaic type to large domain,and then to coarse mosaic type.When the amount of acetic acid was 0.06m L and 0.07 mL,and the amount of iso-propane chloride was 0.6 mL,respectively,more than 50%mesophase pitch was formed with the coalescence large domain texture,indicating that both the catalytic activity of AlCl₃(aluminum trichloride and the mesophase content could be effectively improved by the cocatalyst at suitable addition dosage.Naphthalene-based isotropic pitch was prepared by using naphthalene as model carbon source,with 8%anhydrous AlCl₃ and a certain proportion of cocatalyst at 230℃for 6 h.Then the naphthalene pitch was dissolved with toluene/pyridine as solvent to get the soluble component of naphthalene-based pitch for the purpose of removing aluminum trichloride.Then the naphthalene-based mesophase pitch was obtained by direct thermal condensation of purified naphthalene pitch.The results show that when the naphthalene mesophase pitch is prepared by the two-step process,the catalytic activity of AlCl₃ can be improved by adding 0.6 mL acetic acid as the cocatalyst during the preliminary preparation of naphthalene pitch.Under the condition of 8%AlCl₃ addition,naphthalene-based pitch with H/C value of 0.85 and softening point of 210℃can be obtained.The mesophase pitch with a softening point of 270℃and a nearly 100%anisotropic content can be obtained by direct thermal condensation of purified naphthalene pitch at 380℃for 8h.