Self-adjusting binding pockets enhance H₂ and CH₄ adsorption in a uranium-based metal-organic framework

A new, air-stable, permanently porous uranium(iv) metal-organic framework U(bdc)₂ (1, bdc^2- = 1,4-benzenedicarboxylate) was synthesized and its H₂ and CH₄ adsorption properties were investigated. Low temperature adsorption isotherms confirm strong adsorption of both gases in the framework at low pressures. In situ gas-dosed neutron diffraction experiments with different D₂ loadings revealed a rare example of cooperative framework contraction (ΔV = -7.8%), triggered by D₂ adsorption at low pressures. This deformation creates two optimized binding pockets for hydrogen (Q _st = -8.6 kJ mol^-1) per pore, in agreement with H₂ adsorption data. Analogous experiments with CD₄ (Q _st = -24.8 kJ mol^-1) and N,N-dimethylformamide as guests revealed that the binding pockets in 1 adjust by selective framework contractions that are unique for each adsorbent, augmenting individual host-guest interactions. Our results suggest that the strategic combination of binding pockets and structural flexibility in metal-organic frameworks holds great potential for the development of new adsorbents with an enhanced substrate affinity.