Chlorination–dechlorination of coal liquefaction residue using benzyl chloride for general-purpose carbon fiber synthesis

Coal liquefaction residue (CLR) has a high content of carbon and organic substances. However, sulfur and inorganics pose a challenge for the high-value-added utilization of CLR. In this work, general-purpose carbon fiber was successfully prepared by the new method of chlorination–dechlorination using CLR as raw material and benzyl chloride as chlorinating agent. The effect of benzyl chloride addition amount on the property of isotropic pitch precursor and the performance of general-purpose carbon fiber were investigated. The results showed that the as-prepared pitch precursors were dominated by carbon elements and contained small numbers of hydrogen, oxygen, nitrogen, and sulfur. The characteristic peaks generated from chlorine elements were observed in both FTIR and XPS spectra. The peaks at the wavenumbers of 1490 cm−1 and 696 cm−1 corresponded to the vibration of C–Cl bond, and a tiny Cl 2p peak located at 200.0 eV was observed by reducing the noise of XPS spectra. It indicates that the trace chlorine remains in the as-synthesized isotropic pitch precursor even after undergoing dechlorination. The molecular compositions of isotropic pitches were mainly polyaromatic hydrocarbons, and the chlorination–dechlorination promotes the oligomerization of polyaromatic hydrocarbons to form dimers and trimers. Therefore, the polycondensed degree and carbon aromaticity were improved. It is known from 13C NMR analysis that the effect of chlorine–dechlorination makes the molecular structure of pitches transform more linear, thus enhancing their spinnability. In addition, the higher the amount of benzyl chloride added, the better the property of isotropic pitch was. The pitch of BC-30 and BC-40 showed excellent spinnability, stabilized property, and carbonized property compared to other pitches. The morphology of obtained carbon fiber was homogeneous and smooth without defects in surface and section. The as-prepared general-purpose carbon fiber demonstrated good tensile strength up to 1220 MPa with an average diameter of 8.5 μm.Graphical abstract