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

Abstract

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.