Formation mechanism of phenols during pyrolysis of low-rank coal macerals swelled with different polar solvents based on Py-GC/MS and DFT analysis

Abstract The scope of this study is to investigate the effect of swelling with different polar solvents, including tetrahydrofuran (THF), benzene (BE) and n -heptane (Hep), on the pyrolysis properties of low-rank coal and the possible formation pathways of phenols. In this paper, swelled de-ashed vitrinite (VD-S: VD-THF, VD-BE and VD-Hep) and swelled de-ashed inertinite (ID-S: ID-THF, ID-BE and ID-Hep) were prepared and characterized by FTIR. The weight loss and kinetic analysis were investigated using thermogravimetry (TG), and the distribution of pyrolysis products were investigated by Py-GC/MS and fast pyrolysis fixed bed. Combining the selectivity changes of pyrolysis products with density functional theory (DFT) calculations, the possible formation pathways of phenols derived from VD-S and ID-S pyrolysis are proposed. TG results show that the weight loss of VD-THF and ID-THF reaches up to 32.22 wt% and 26.48 wt%. The comparison of activation energy exhibits that THF primarily affects the first stage of pyrolysis. The highest selectivity of PCXC was observed in VD-THF and ID-THF, at up to 19.99% and 17.40%. By integrating the selectivity changes of the detected compounds using Py-GC/MS with the comprehensive analysis of electrostatic potential (ESP) and frontier molecular orbital, seven possible formation pathways of cresol preparation by VD-S and ID-S are proposed. DFT calculations demonstrate that Path 5 of VD-Hep possesses the lowest reaction energy of 140.79 kJ/mol. Ultimately, the possible swelling mechanism exhibits that the C ar –C al bonds are prone to cracking when macerals are swelled by BE. THF promotes the cleavage of C–O bonds, and Hep affects the cleavages of C al –C al bonds.