Micromechanism of the effect of coal functional groups on the catalytic/esterification reaction of acetic acid

Acidification modification has proven to be a successful approach for enhancing the permeability of low-permeability coal seams and increasing coalbed methane extraction. To explore the influence of acetic acid on the organic structure of coal, a combined approach of experimental research and quantum chemical calculation is employed to examine the reaction mechanism of coal organic structure in response to acetic acid. Fourier transform infrared spectroscopy reveals a reduction in the content of hydroxyl and C–O bonds in the treated samples, along with the appearance of an absorption peak of C=C bonds. This demonstrates that the main site of acetic acid acidification modification on coal molecules is the hydroxyl functional group. Therefore, glycerol can be used as a simplified model of coal molecules for further investigation. In addition, three typical functional groups (phenyl, hydroxyl, amino) are selected as the research objects. The electronic density topological properties, molecular surface electrostatic potential, Mayer bond orders and CM5 charge of coal molecules were calculated based on quantum chemical theory. This reveals the intrinsic strength of each chemical bond between single atoms in a molecule, thereby predicting potential reaction pathways between acetic acid and coal molecules. The micro-mechanism of acetic acid catalyzing/esterifying the organic structure of coal is studied using transition state methods. The computational results show that there are two main modes of reaction between acetic acid and coal molecules: (1) acetic acid catalyzes the dehydration reaction of coal molecules; (2) acetic acid reacts with coal molecules to form ester intermediates, followed by decomposition through various reactions. Among the 14 reaction pathways, acetic acid provides protons or groups to trigger the dehydration of coal molecules. Interestingly, it is found that the overall hydroxyl group participates in the reaction is easier than the individual hydrogen atom on the hydroxyl group, and the overall carboxyl group participates in the reaction is easier than the individual hydroxyl group. This indicates that the higher the overall atomic reactivity of acetic acid, the lower the reaction barrier. Compared with catalytic reactions, acetic acid tends to undergo esterification reactions with hydroxyl groups, with the phenyl ring functional group significantly affecting the reaction barrier. The introduction of various oxygen-containing functional groups like ketone, aldehyde, and ester in the reaction products will hinder the adsorption of non-polar methane, reduce the adsorption capacity of coal seams for gas, and play an essential role in promoting gas desorption. The research findings can provide theoretical guidance for the organic acidification modification of low-permeability coal seams to enhance gas permeability and coalbed methane extraction.