Proton conductivity of mesoporous aluminum organophosphonate enhanced by the affinity of integral organic linker to water molecule

Abstract

The precise design of aluminophosphate (AlPO) based frameworks is quite promising for improving smooth transport of protons at the surfaces having abundant sites such as free phosphoric acid (P-OH) groups and water (H2O) molecules attached to tetrahedral AlO4 units. Our latest study has demonstrated that the surface hydrophobicity enhanced by embedding bulky aromatic groups such as phenylene (-C6H4-, -Ph-) linker is essential for improving the stability of aluminum organophosphonate (AOP) based frameworks. However, such a strong hydrophobicity did not seem suitable for amplifying the proton conduction because the length of proton hopping was increased by the presence of bulky organic linkers. In this study, we suggest a rational guideline to design the molecular structure for enhancing the proton conductivity inside surfactant-assisted mesopores, especially that at the surfaces of AOP based frameworks. Two AOP type materials containing hydrophobicity-increased biphenyl (-C6H4-C6H4-, -BP-) and pathway-added hydroquinonyl (-C6H2(OH)2-, -HQ-) groups were prepared as AOP-BP and AOP-HQ using Pluronic P123 (EO20PO70EO20). Consequently, the AOP-HQ type mesoporous material showed a superprotonic conductivity of 1.31 × 10-2 S cm-1 under 95% RH at 90 ℃. The HQ linker, e.g., abundant and additional -OH groups over the hydrophobic -Ph- linker, are quite helpful for promoting the formation of a continuous networking of H2O molecules whole the entire surfaces, leading to an efficient proton conduction by the Grotthuss mechanism.