Molecular representation and atomic-level coking evolution investigation of modified coal tar pitch via 13C NMR, MALDI-TOF-MS, SAXS, and ReaxFF MD

Abstract

Modified coal tar pitch (MCTP) is a high-quality feedstock for high value-added (HVA) carbon materials. The coking process of MCTP is essential for the preparation of the carbon products and determines the product performance. Here, the MCTP molecular representation (C93H59NO) created by computational modeling strategy duplicates the microstructure and composition data from XRD, 13C NMR, FT-IR, XPS, and MALDI-TOF-MS analysis. The mechanism of volatile removal and coking evolution for MCTP was revealed via the obtained model, reactive force field molecular dynamics (ReaxFF MD) simulations, TG-MS, XRD, and SAXS. The removal of volatiles during the MCTP coking process was caused by the breaking of aromatic rings at the edge of MCTP molecules and the generation of radicals. The main volatile products are H2, H2O, CO, CH4, and C2H4, and the volatiles were mainly removed in the temperature range of 430–900 K. For the coking evolution, the XRD and SAXS analysis showed the crystalline and graphitization degree of pitch coke samples were destructed slightly below 973 K and improved above 973 K. The true density (finally 2.2 g/cm3), RDF critical peak intensity, sp2 hybridized bond proportion (finally 41.4%) and six-membered ring proportion (finally 63.3%) for the pitch coke nucleus underwent the decline and then increase during the ReaxFF MD simulations. The patterns of MCTP coking evolution obtained by the detection methods (XRD & SAXS) and ReaxFF MD were demonstrated mutually. The coking evolution could be divided into two stages: the growth and order of the coking nucleus. The destruction of the aromatic moieties in MCTP molecules and the cross-link of the aliphatic chains resulted in the primary nucleation. The condensation and aromatization proceeded within the primary nucleus to finally form the crystalline nucleus of pitch coke. This work provides theoretical guidance for the quality improvement of HVA carbon products.