Impact of OH-groups on the mobility of linkers and guests in UiO-66 (Zr)

TL;DR

This study investigates how hydroxylation and dehydroxylation affect the mobility of guest molecules and separation performance in UiO-66 (Zr) using $^{2}$H NMR, revealing that hydroxylation state influences diffusion and selectivity.

Contribution

It provides experimental insights into the impact of hydroxylation on UiO-66's structural mobility and guest diffusion, highlighting subtle changes' significance for material performance.

Findings

Dehydroxylation alters guest molecule mobility in UiO-66.

Hydroxylation state affects separation selectivity.

Structural mobility correlates with diffusivity improvements.

Abstract

UiO-66 (Zr) is a metal-organic framework known for its thermal and chemical stability and wide range of possible applications. In particular, this material has high separation selectivity of various hydrocarbon mixtures. Moreover, it is was shown that the performance depends on the hydroxylation state of material (reversible phase transition $Zr_{6}O_{4}(OH)_{4} {\longrightarrow} Zr_{6}O_{6}$ at 250 - 300 ${\deg}C)$. Despite all the attention the UiO-66 received over past few years, the impact of the dehydroxylation on its properties remains poorly understood. In this contribution we apply $^{2}$H NMR experimental method in order to compare the mobility of butane isomers in hydroxylated and dehydroxylated form of UiO-66. We provide estimations of the translational diffusion coefficients and show which hydroxylation state of the material has higher separation selectivity. Moreover, the impact of the hydroxylation on the structural mobility of UiO-66 is discussed. Correlation between parameters of structural dynamics and the diffusivity of guest molecules indicates that for the microporous material with small windows even subtle changes can lead to a tangible improvement of the properties.