A review of the catalytic preparation of mesophase pitch

Abstract

Because of its high purity and excellent orientation, mesophase pitch is a superior precursor for high-performance carbon materials. However, the preparation of top-notch mesophase pitch faces challenges. Catalytic polycondensation at low temperatures is more favorable for synthesizing mesophase pitch, because it circumvents the high-temperature free radical reaction of other thermal polycondensation approaches. The reaction is gentle and can be easily controlled. It has the potential to significantly improve the yield of mesophase pitch and easily introduce naphthenic characteristics into the molecules, catalytic polycondensation is therefore a preferred method of synthesizing highly spinnable mesophase pitch. This review provides a synopsis of the selective pretreatment of the raw materials to prepare different mesophase pitches, and explains the reaction mechanism and associated research advances for different catalytic systems in recent years. Finally, how to manufacture high-quality mesophase pitch by using a catalyst-promoter system is summarized and proposed, which may provide a theoretical basis for the future design of high-quality pitch molecules.

1. Introduction

Pitch-based carbon fiber can be segregated into two categories including general-purpose fiber and high-performance fiber. General-purpose pitch-based carbon fiber, derived from isotropic spinning pitch and usually a short fiber, has a tensile strength of less than 1000 MPa and an elastic modulus lower than 100 GPa[1]. It could be employed as functional materials such as anti-corrosion, heat preservation, abrasion resistance, and adsorption. High-performance pitch-based carbon fiber refers to fiber with a strength > 1000 MPa and a modulus > 100 GPa. Despite the complex process including spinning, oxidizing, carbonizing, and graphitizing by using mesophase pitch as the raw material, the superior modulus, thermal conductivity, electronic conductivity, vibration attenuation and low thermal expansion coefficient are endowing it with wide application[2–6].

High-quality mesophase pitch as the precursor can produce not only carbon fibers with excellent performance, but also other high-end carbon products such as carbon foam, carbon microspheres, needle coke, etc. It plays an important role in military industry, aerospace, nuclear energy industry. It can also be used in other fields, such as LED heat transfer materials with superior electronic and thermal conductivity, composites with superior structures and functions, as well as high-end fabrication, energy storage, transformation, and many potential uses. Mesophase pitch is a kind of anisotropic liquid crystal with large molecular weight and thermodynamically stability formed through pyrolysis, dehydrogenation and polycondensation of polycyclic aromatic hydrocarbons when heated under an inert atmosphere. Layered nematic liquid crystal with optical anisotropy is formed by orientation, alignment and self-assembly under liquid phase interaction involving chemical and physical reactions of organic substances[7–9]. The emergence, expansion, coalescence and distortion of mesophase spheres are necessary for the transition of aromatic small molecule to a highly graphitized carbon. The size of anisotropic isochromatic region is a gauge of the liquid-phase carbonization and thus determines the further utilization of mesophase pitch[10–12].