Designing of UiO-66 Based Super Protonic Conductor with the Highest MOF Based Proton Conductivity.

MOF based proton conductors have received immense importance recently. The present study endeavors to design two post-synthetically modified UiO-66 based MOFs and study the effects of their structural differences on their proton conductivity. UiO-66-NH2 is modified by reaction with sultones to prepare two homologous compounds i.e., PSM 1 and PSM 2, which have SO3H groups in comparable extent (Zr:S ≈ 2: 1) in both. But the pendant alkyl chain holding the -SO3H group is of different length. PSM 2 has longer alkyl chain attachment than that of PSM 1. This difference in length of side arm results in huge difference in proton conducting behavior of the two compounds. PSM 1 is observed to have highest MOF based proton conductivity (1.64 × 10-1 Scm-1) at 80 °C, which is comparable to commercially available Nafion while PSM 2 shows significantly lower conductivity ((4.6 × 10-3 Scm-1). Again, the activation energy for proton conductivity is one of the lowest among all MOF based proton conductors in case of PSM 1 while, PSM 2 requires larger activation energy (almost three times).This profound effect of variation of chain length of side arm by 1 carbon atom in case of PSM 1 and PSM 2 was rather surprising and never documented before. This effect of length of side arm can be very useful to understand proton conduction mechanism of MOF based compounds and also to design better proton conductors. Besides, PSM 1 showed proton conductivity as high as 1.64 × 10-1 Scm-1 at 80 °C temperature, which is the highest reported value till date among all MOF based systems. The lability of the ‒SO3H proton of the post synthetically modified UiO-66 MOFs has theoretically been determined by molecular electrostatic potential (MEP) analysis and theoretical pKa calculation of models of functional sites along with relevant NBO analyses.