Preparation of a high-performance synthetic pitch from aromatic hydrocarbons containing N/Cl

Abstract

The preparation of a synthetic pitch from aromatic monomers could easily regulate structure orientation at the molecular level, which would be useful in fabrication. An isotropic synthetic pitch was prepared by a chlorine- and/or nitrogen-induced substitution polymerization reaction method using aromatic hydrocarbon precursors containing Cl and N, which for this study were chloromethyl naphthalene and quinoline. This method was verified by investigating the structural changes under different synthesis conditions, and the synthesis mechanism induced by aromatics containing Cl was also probed. The result shows that the pyridinic N in quinoline contains a lone pair of electrons, and is an effective active site to induce the polymerization reaction by coupling with aromatic hydrocarbons containing Cl. The reaction between such free radicals causes strong homopolymerization and oligomerization. A higher reaction temperature and longer reaction time significantly increased the degree of polymerization and thus increased the softening point of the pitch. A linear molecular structure was formed by the Cl substitution reaction, which produced a highly spinnable pitch with a softening point of 258.6 °C, and carbon fibers with a tensile strength of 1 163.82 MPa were obtained. This study provides a relatively simple and safe method for the preparation of high-quality spinnable pitch.

1. Introduction

At present, preparation of carbon fibers with excellent mechanical properties and facile production is still a hot topic[1–4]. Specifically, the isotropic pitch-based carbon fiber (IP-CF) is a favorable candidate offering high performance and low cost[5–7]. The IP-CF is mainly produced from coal tar pitch, petroleum asphalt or ethylene base oil[8,9]. However, such carbonaceous materials contain large quantity of organic compounds requiring extremely complicated regulation issues[10]. Besides, the impurity and molecular polymerization degree in the raw material will also produce strong impact on the performance of pitch-based carbon fiber[11–13].

The aromatic monomers can reasonably reconstruct the pitch structure at the molecular level to avoid the influence of impurities and molecular polymerization degree on the properties of IP-CFs[12,14]. Generally, aromatic compounds, such as naphthalene, methylnaphthalene, anthracene, and other condensed aromatic compounds can be used as raw materials to synthesize pitch[15,16]. However, most of the aromatic compounds have relatively low reactivity, making it hard to obtain well assembled molecular structures with specific requirement[17,18]. Proton acid catalyst HF/BF3 was used to catalyze the polymerization reaction of naphthalene/ methylnaphthalene by decreasing the electron cloud density of the aromatic ring[19,20]. Nevertheless, the large scale preparation of synthetic pitch using super acidic catalyst usually causes very strong corrosion, limiting the production efficiency[15]. Alternatively, halogen elements, such as Br[21,22], I and Cl[23,24], with relatively mild reaction conditions, have been employed to enhance the polymerization reaction of aromatic hydrocarbons[25]. High performance synthetic pitch was successfully prepared through bromination of methylnaphthalene[23]. Based on this idea, development of novel synthesis routes by combing halogenation reaction is expected to be a promising approach for obtaining excellent pitch precursors in an easier way[26].

On the other hand, quinoline is a good precursor for preparation of synthetic pitch. The quinoline-based pitch as a nitrogen-rich carbon material has shown excellent performance in energy storage, catalysis, and functional materials[27–29]. The preparation of quinoline pitch usually needs the AlCl3 catalyst[30,31]. Unfortunately, such a catalyst leaves significant residue in the pitch, leading to the high impurity content. It would seriously affect the subsequent processing and application of quinoline pitch[32,33]. The synthesis of quinoline pitch by combining the halogenated reaction using aromatic side chain chloride can realize the regulation of synthesis process at molecular level. Moreover, it would avoid the influence of ash residue. The halogenation-induced synthesis of quinoline is of great significance for the development of new carbon materials.