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, 1 H NMR, and solid-state 13 C 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 E CYC possess similar basic arene structures and E CYC is rich in macromolecular clusters with lots of side chains and bridge bonds. The chemical structure of E CYC 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.