Photocatalytic Cascade Nitrogen Fixation for Selective Purification of Methane-Rich Coal-Bed Gas Over a Bimetallic MOF.

CH4-rich coal-bed methane (CBM) is often contaminated with N2, but their similar properties make conventional separation energy-intensive and inefficient. Herein, we report a photocatalytic strategy for selective N2 conversion over CH4, achieving simultaneous CBM purification and nitrogen valorization. A Co-Ni metal-organic framework (CoNi-PYZ) features isolated bimetallic sites, where ligand/Co units donate electrons to Ni, establishing spatially separated reduction and oxidation centers. This structural motif drives a photocatalytic cascade nitrogen fixation that N2 is first reduced to NH3 at the Ni sites; subsequently, the generated NH3 outcompetes CH4 for the Co sites, leading to its oxidation to NO3 -. This remarkable selectivity is governed by the high polarity and lone-pair electron donation of NH3, favoring its adsorption over nonpolar CH4. Notably, the optimal catalyst achieves high activity under mild conditions without sacrificial agents, delivering NH4 + and NO3 - production rates of 599.8 and 199.8 µmol g- 1 h- 1, respectively. Mechanism analysis reveals that ligand engineering optimizes the d-band center to balance adsorption and desorption, thereby minimizing the rate-determining step barriers for both half-reactions, consistent with the Sabatier principle. This work provides a reaction-driven alternative to traditional phase-separation, establishing a novel paradigm for CBM upgrading coupled with selective N2 transformation.