Structural characterization, proton conductivity and furfural catalysis of novel polyfunctional zirconium...

Zirconium phosphonates combine the thermal robustness of Zr-O frameworks with the rich functionality of organophosphonic linkers, making them attractive for both energy-related and catalytic applications. Here, we report three new crystalline Zr(IV) phosphonates built from 5-(dihydroxyphosphoryl)-isophthalic acid (PiPhtA), 5-(dihydroxyphosphoryl)-nicotinic acid (PNA) and benzene-1,2,3-triyltris(methylenephosphonic acid) (BTTMPA). Phase-pure Zr[(HO₃P-C₆H₃-(COOH)₂)₂(X)₂]·6H₂O (X = F^-/OH^-) (Zr-PiPhtA), Zr(O₃P-NH⁺-C₅H₃-COOH)₂F₂ (Zr-PNA) and Zr[(H₂O₃PCH₂)(O₃PCH₂)₂-C₆H₃]·H₂O (Zr-BTTMP) were obtained under mild solvothermal conditions and characterized by synchrotron powder X-ray diffraction, pair distribution function (PDF) analysis, solid-state NMR and thermogravimetry. The crystal structures of 1D Zr-PNA (P2₁/c) and 3D Zr-BTTMP (P2₁/a) were solved ab initio from powder diffraction data, while combined structural X-ray diffraction and PDF analyses together with ammonia adsorption suggest that nanocrystalline Zr-PiPhtA exhibits features resembling those of the analogous Ca-PiPhtA derivative and Zr-BTTMP. Given that all of them exhibit characteristics adequate for facilitating proton transfer pathways, a study of proton conductivity was undertaken. Under 95% relative humidity, bulk proton conductivities reach 1.2 × 10^-3 S cm^-1 (Zr-BTTMP) at 80 °C. On exposure to NH₃ vapour, the conductivity of Zr-PiPhtA and Zr-PNA increased by almost one order of magnitude, up to 3.2 × 10^-3 S cm^-1 at 80 °C for Zr-PiPhtA, highlighting the decisive role of ammonium-assisted proton hopping. The same acid sites that promote proton mobility also endow the materials with bifunctional catalytic behavior. In the one-pot cascade upgrading of furfural, Zr-PiPhtA afforded the highest overall conversions, benefiting from the nanocrystalline morphology and a higher density of strong Brønsted acid sites.