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

Abstract

A new, air-stable, permanently porous uranium(IV) metal–organic framework U(bdc)₂ (1, bdc²⁻ = 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.