Structural Features and Pyrolysis Behaviors of Extracts from Microwave-Assisted Extraction of a Low-Rank Coal with Different Solvents

Microwave-assisted extraction (MAE) of a low-rank Naomaohu (NMH) coal was conducted with two solvents, cyclohexanone (CYC) and tetrahydrofuran (THF), to obtain extracts (E-cyc and E-THF) and residues (R-cyc and R-THF). The parent coal, extract, and residue were characterized by ultimate analysis, TG, FTIR, GPC, NMR, and solid-state C-13 NMR. The results showed that MAE process is more efficient than Soxhlet extraction. The solvent CYC exhibits a higher total extract yield of 8.3 wt % than THF, being 4.7 wt %, during MAE and more efficiency in extracting organic components of NMH coal, especially condensed arenes and macromolecular compounds. The average molecular weight of E-cyc is higher than that of average aromatic cluster of NMH coal. Online pyrolysis-vacuum ultraviolet photoionization mass spectrometry was taken to determine the initial pyrolysis products of parent coal, R-cyc, and E-cyc. The distribution of initial pyrolysis products suggested that NMH coal and Ecyc possess similar basic arene structures and Ecyc is rich in macromolecular clusters with lots of side chains and bridge bonds. The chemical structure of Ecyc could reflect the macromolecular network structure of NMH coal to some extent. This could be an effective method to understand the organic structure of coal.