A novel topological indium-organic framework for reversible ammonia uptake under mild conditions and catalysis

As a new generation of porous materials, metal-organic frameworks (MOFs) are promising candidates for NH₃ uptake. However, obtaining stable MOFs with good NH₃ adsorption capacity and reliable recyclability under mild conditions remains a big challenge. Herein, we report a novel topological 3D porous indium-organic framework (InOF), [In₈(μ₂-OH)₆(μ₂-H₂O)₃L₆Cl₆]·5DMF·4H₂O (1), built from 2,2′-dimethylbiphenyl-4,4′-dicarboxylic acid (H₂L, DMF: N,N-dimethylformamide). Crystal structure analysis reveals that 1 possesses a rare tetranuclear [In₄(μ₂-OH)₃]⁹⁺ cluster as the secondary building unit (SBU). Each SBU is linked through the μ₂-H₂O bridges to three adjacent ones, leading to a 2D honeycomb layer consisting of the unusual eighteen-membered (In³⁺)₁₈ ring with a diameter of 15 Å. The 2D inorganic layers are further connected by the L²⁻ ligands to form an unprecedented trinodal 3,3,4-connected 3D framework with the Schläfli symbol (4·8·10)₃(4·8³·10²)₃(8³). Thanks to the existence of the football-shaped nanosized cages decorated with the abundant hydrogen-donating/-accepting groups like μ₂-OH⁻ and Cl⁻ ions, μ₂-H₂O and -CH₃, 1 shows an NH₃ uptake of 10.4 mmol g⁻¹ at 273 K and 1 bar. Notably, 1 can be regenerated at 60 °C, enabling the adsorption/desorption of NH₃ to be completely reversible and the adsorption capacity to be almost fully maintained after 10 cycles. Furthermore, 1 exhibits promising catalytic activity and cycling stability for the acetalization of benzaldehydes.