Effects of NaOH Modification on the Macromolecular Structural Evolution and Carbon Dioxide Adsorption Behavior of Bituminous Coal

Alkali treatment can enhance the CO2 adsorption capacity of coal; however, existing studies have primarily focused on characterizing individual structures and properties, with a lack of a systematic understanding of microstructural changes. In this study, Fourier transform infrared spectroscopy was used to characterize changes in functional groups in modified bituminous coal before and after CO2 adsorption at different temperatures. Furthermore, molecular models of raw and NaOH-modified coal were established based on integrated characterization results, and molecular simulations were employed to elucidate the microscopic mechanisms underlying the enhanced CO2 adsorption. The results indicate that (i) NaOH modification decreases the contents of C═O and −COOH groups while promoting the formation of stable sodium carboxylate structures. The reduction of aromatic carbon and enrichment of long-chain aliphatic structures alter the coal surface chemistry and improve CO2 adsorption stability; (ii) NaOH selectively removes less stable components and enriches highly condensed structures, increasing the aromatic condensation degree of the coal by 25.29%. It promotes the development of the coal’s pore structure, resulting in a 82.72% increase in the specific surface area; (iii) NaOH modification enhances the CO2 adsorption capacity of coal by 15.79% and significantly improves the interaction between CO2 and the coal matrix under high-temperature, low-pressure conditions. Pore structure evolution is the primary factor governing adsorption enhancement, while the contribution of the molecular composition effect rises from 17.39% to 43.38% as temperature rises, becoming a key controlling factor at elevated temperatures. This study provides molecular-scale theoretical support for CO2 geological storage in coal seams and related technologies.