Synergistic Promotion of Methane Hydrate Formation by l -Tryptophan and PCN-250-Fe2Co for Efficient Hydrate-Based Separation of Low-Concentration Coalbed Methane

Abstract Hydrate-based separation is an effective method for CH4 recovery from low-concentration coalbed methane. The hydrate separation performance, such as the generation rate and the efficiency of separation and purification, is influenced by kinetic promoters. In this work, a dual-promoter system consisting of l-tryptophan (l-Trp) and PCN-250-Fe2Co was developed to enhance hydrate-based CH4 separation. l-Trp regulates the gas–liquid interfacial microenvironment through its amphiphilic structure, suppressing compact ice crystal nucleation and enhancing CH4 diffusive transport across the interface. Meanwhile, the high specific surface area and abundant microporous structure of PCN-250-Fe2Co provide numerous heterogeneous nucleation sites for CH4, shorten the CH4 diffusion distance, and significantly enhance the mass transfer within the liquid phase. As a result, the optimized system containing 0.5 wt % l-Trp and 0.1 wt % PCN-250-Fe2Co framework achieved a CH4 recovery of 86.64%. This value is 11.46% and 5.02% higher than those obtained with PCN-250-Fe2Co alone and l-Trp alone, respectively. This significant enhancement is attributable to the synergistic interplay between the carboxylate anchors of l-Trp and the metal sites of PCN-250-Fe2Co, which simultaneously facilitate CH4 capture and mitigate mass-transfer resistance for CH4. Theoretical analysis further showed that the CH4/N2 selective adsorption ratio of the l-Trp&PCN-250-Fe2Co system (44.932) was 16.6% higher than that of the PCN-250-Fe2Co system (38.523). Overall, the l-Trp&PCN-250-Fe2Co composite system exhibits promising potential for hydrate-based CH4 separation and purification from low-concentration coalbed methane, providing a new method for hydrate-based gas separation.