Unravelling the structure and adsorption properties of a highly stable porous hafnium phosphonate framework

The highly stable hafnium-phosphonate framework based on 2,4,6-tris(4-phosphonophenyl)-1,3,5-triazine as the linker (Hf-ttbp) displays exceptional separation performance towards uranium and transuranium elements in acidic environment. Here, we report the crystal structure of Hf-ttbp, solved from powder X-ray diffraction data, revealing that it is not isostructural to the previously reported Zr-btbp, as originally hypothesised. Hf-ttbp features honeycomb-like layers with cavities having diameter of about 10 Å filled with water molecules. The same structural arrangement is displayed by the newly synthesised Zr-ttbp. Different from Zr-btbp, where the inorganic unit is a trinuclear Zr-phosphonate cluster, Hf-ttbp contains isolated Hf atoms. The removal of water from the pores of Hf-ttbp does not induce any structural rearrangement, generating accessible porosity. Multinuclear solid-state nuclear magnetic resonance spectroscopy was employed to shed light on the local structure upon removal of water from the pores, finding evidence of symmetry breaking in the Hf environment, not detectable by X-ray diffraction, following reallocation of acidic protons. The adsorption properties of Hf-ttbp towards Ar, CO2, N2 and H2O were also assessed, finding that it displays CO2 adsorption capacity comparable to other porous metal phosphonates and high affinity for H2O, attributed to the highly polar nature of the pores.