Metal–organic frameworks for biogas upgrading: Recent advancements, challenges, and future recommendations

Biogas with a methane content of ≥97% (called biomethane) has the potential to overcome the dependency on fossil fuel-based natural gas. Biomethane is obtained by the efficient upgrade (mainly the removal of CO₂) of biogas. For this purpose, metal–organic frameworks (MOFs) are considered promising materials because of the advantages of inherent porosity, structural diversity, functionality, tailorability, and versatility. In this review, several aspects of MOFs with respect to biogas upgrading have been highlighted. In addition, significant factors have been discussed, such as the key progress in the removal of CO₂ via the selective passage of CH₄, challenges and issues related to CO₂ removal, commercialization, and future prospects of MOFs in terms of materials science and process system engineering (PSE) to provide an in-depth understanding of MOFs for biogas upgrading. This novel review provides comprehensive overview of biogas upgrading via adsorptive and membrane-based separation techniques using MOFs in a single study. In addition, all of the important and influential parameters involved in enhancing CO₂ capture, possible limitations and improvement strategies associated with these technological directions (adsorptive separation and membrane separation) have been explained for biogas upgrading. Most importantly, this study suggests that moisture-stable MOFs such as zeolitic imidazolate framework-8 (ZIF-8), the combination of ZIF-8 and ZIF-67 (ZIF-8/ZIF67), Universitetet i Oslo-66 (UiO-66), hybrid MOFs structures with appropriate functional groups and 2D MOF nanosheets with polymers of high intrinsic permeabilities, all have the potential to be ideal candidates for economical and efficient biogas upgrading. For better economical results at the industrial scale, the performance of MOFs should be evaluated in terms of both the swing adsorption process and the integrated membrane separation process under optimum conditions (pressure/temperature) to achieve fuel-grade biomethane.