Increase in pore size and gas uptake capacity in indium-organic framework materials

Abstract

We herein report the synthesis and characterization of a new charged metal-organic framework, [Et₂NH₂][In(BPTC)] (InOF-2) (InOF = indium-organic framework; BPTC = biphenyl-3,3′,5,5′-tetracarboxylate), featuring an unc-type topology. Interestingly, InOF-2 undergoes a distinct porosity transition on going from a typical reversible type-I N₂ isotherm to a type-IV N₂ isotherm with a hysteresis loop when employing thermal treatment. A Li⁺-exchanged material [Li_0.9(H₃O)_0.1][In(BPTC)] (InOF-2-Li⁺) is obtained using an ion-exchange method with BET surface areas estimated to be 1494 m² g⁻¹, and 867 m² g⁻¹ for InOF-2, indicating that the N₂ adsorption capacity of InOF-2-Li⁺ significantly increases by ca. 72.3%. Meanwhile, H₂ adsorption measurements show a corresponding improvement in H₂ storage capacity (ca. 70.6%) on going from InOF-2 (1.14 wt%) to desolvated InOF-2-Li⁺ (1.95 wt%) at 1.0 bar and 77 K, which is comparable to the calculated increase in BET surface area. A similar improvement is also confirmed in other single gas adsorption measurements, including CH₄ and CO₂. Overall, the performance characteristics of InOF-2 and InOF-2-Li⁺ indicate these two materials to be unprecedented examples for the charged metal-organic frameworks to improve their gas storage capacity with both the thermal treatment and cation-exchange method, showing promise for more applications in producing more porous metal-organic frameworks.