Metal-organic framework (MOF) as adsorbents for hydrogen separation from steam methane reforming: an in-depth review

Hydrogen (H₂), acknowledged as a clean and advanced fuel, has attracted research focus for its production, purification, and energy generation in accordance with the Sustainable Development Goal (UN-SDG 7). H₂ is produced by both fossil fuel (such as reforming, pyrolysis, gasification) and non-fossil fuel-based technologies (such as water electrolysis). Currently, fossil fuel-based hydrogen production predominates in meeting the current demands. However, hydrogen obtained through these methods is impure and requires purification before application. Metal-organic frameworks (MOFs) are emerging novel adsorbent materials that surpass conventional adsorbents owing to their favorable physicochemical characteristics and adaptability. This review elucidates the influences and correlations between MOF adsorbents and the performance of the pressure swing adsorption (PSA) process in the separation of H₂ from steam methane reforming (SMR) off-gas. The PSA performance is dictated by the adsorbent's properties and the operational parameters. The gas separation on MOF adsorbents occurs through equilibrium, kinetic, or size exclusion mechanisms. The H₂ separation is largely governed by the van-der Waals interaction of various components of SMR off-gas with the MOF, and the gases interact in the order CO₂ ≫ CH₄ > CO > N₂ > H₂. It is noted that the MOF-gas interaction can be tuned by functioning MOFs with polar (e.g., -OH, NO₂, SO₃H) and non-polar functional groups (e.g., ester and alkanes). The operational parameters influence PSA performance indicators, and a general trend is seen among them. This review presents the critical analysis, summary, challenges, and outlook of the MOF-based PSA hydrogen separation, providing notable examples of MOFs reported.