Effect of different functional groups on CH4 adsorption heat and surface free energy of vitrinite during coalification

Abstract

Here vitrinite of various coal ranks was collected to study the characteristics of initial isosteric heat of adsorption (Qst0) and surface free energy (Δσ) by the isothermal methane-adsorbing at various temperatures on dry basis. The surface area of demineralized dry samples was examined for the calculation of Qst0 and Δσ, and the macromolecular structures of dry vitrinite samples were determined using Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance experiments. The Qst0 shows no obvious correlations with oxygen-containing groups, however, there is a positive correlation between the alcoholic hydroxyl group (R–OH) and maximal surface free energy (Δσmax), indicating that the decreasing in R–OH content with the increasing coal rank can reduce adsorption capacity. Qst0 decrease as the amount of nonprotonated aromatic carbon attached to phenolic (including phenoxy or phenol groups) increases, indicating that phenoxy or phenol group loss with maturation is beneficial to increasing Qst0. As the degree of coal metamorphism increases, the aliphatic side chains gradually decline, inducing Qst0 initially decreases and then increases. For aromatic groups, with increases of aromatic carbons, aromatic bridgehead carbons, and ratio of aromatic CC to aromatic CH, Qst0 initially decreases and then increases.