Synthesis and characterization of new zirconium 4-sulfophenylphosphonates

Abstract

New way for the preparation of 4-sulfophenylphosphonic acid is proposed based on the hydrolysis of sulfamoylphenylphosphonic acid under hydrothermal conditions. 4-Sulfophenylphosphonic acid was successfully used for the preparation of zirconium sulfophenylphosphonate and zirconium phosphate sulfophenylphosphonate with formulae Zr(HO 3SC 6H 4PO 3) 2·2H 2O and Zr(HPO 4) 0.7(HO 3SC 6H 4PO 3) 1.3·2H 2O. Both compounds are layered with the interlayer distances of around 19.9 Å, which agrees well with the presumed structure derived from the structure of the alpha modification of zirconium phosphate monohydrate (α-ZrP). The inorganic part of the title compounds is identical with that of α-ZrP, with the sulfophenyl groups jutting out of the layers into the interlayer space. Both compounds were further characterized by FTIR and solid-state NMR, which confirmed the presence of strongly acidic hydrogen on the sulfonic group. Based on the NMR findings, the structure of hydrogen bonding in the interlayer space was proposed. The 2D 1H– 1H spin-exchange and double-quantum NMR experiments further revealed various mutually interacting hydrogen atoms differing in their mobility. In contrast to α-ZrP, protons in the newly prepared systems exhibit significantly higher dynamics. The proton conductivity ( σ) of both compounds was determined at 100 °C as a function of relative humidity (RH) in the range 50–90% and as a function of temperature between 70 and 150 °C, at RH = 75%. The phosphate phosphonate compound is more conductive than the pure phosphonate compound, the highest σ values being 0.063 S cm − 1 (at 100 °C and 90% RH) and 0.073 S cm − 1 (at 150 °C and 75% RH). Due to their insolubility in water, good thermal stability and the considerably high protonic conductivity the described layered compounds are promising candidates for application as components of composite proton exchange membranes in fuel cells.