Structural behaviour and adsorption properties of Sc-based metal-organic frameworks

Abstract

Some of the challenges faced when developing novel functional materials, cannot be resolved without the correct understanding of their structure‐property relationships. Metal‐organic frameworks (MOFs) constitute a representative example where in-depth structural knowledge can greatly help improve and optimise their application into industrially relevant settings. Fortunately, the inherent crystalline nature of MOFs allows for analysis using the wide range of crystallographic experimental techniques that are currently available. This work covers the study of the structural properties of a particular family of MOFs, which have shown significant potential as molecular sieves and for gas storage. Sc-based MOFs first attracted attention for their particularly robust and inert nature, bypassing some of the physical challenges many MOFs have when undergoing industrial implementation. After an initial review of the state of the art in the field of MOFs and the techniques utilised to analyse their properties, this work then focuses on the mechanical properties of a series of functionalised and unfunctionalised Sc‐dicarboxylate MOFs. Using nano‐indentation techniques and high‐pressure crystallography, the hardness and elasticity of these materials are correlated to their different structural features, confirming their relative robustness when compared to other MOFs in the literature. An interesting property of Sc2BDC3 is its selective uptake of CO2 over other fuel-related gases such as CH4 and CO. In this context, the in situ adsorption crystallographic analysis of Sc2BDC3 and its amino‐functionalised derivative Sc2(BDC‐NH2)3 (BDC‐NH2 = 1,4‐amino‐2‐benzenedicarboxylate) is described, as performed using the gas cell set up of beamline I19 at the Diamond Light Source synchrotron. This study is the first example of a mixed gas atmosphere experiment using single‐crystal diffraction, which in conjunction with in silico, adsorption and breakthrough experiments, provides direct insight into the interactions that drive the selective behaviour of both frameworks. Following this, the MOF Sc2BDC3 (BDC = 1,4‐benzenedicarboxylate), is selected as a case study for branched and unbranched alkane separation. Here, high‐pressure crystallography shows how these relatively oversized guest molecules, can be forced at thousands of atmospheres of pressure inside the narrow triangular channels (< 4 Å diameter) of the framework. It is also possible to resolve the structural changes the framework undergoes upon uptake of the different guests, as well as locate the adsorption sites of the hydrocarbons in the pores of Sc2BDC3, which can be then correlated to the gas adsorption behaviour of the different guests. To conclude, the high‐pressure inclusion study of both CO2 and CH4 inside Sc2BDC3 shows how combining cryoloading techniques and molecular crystallography for the first time, can provide improved models of the adsorbed gaseous guests inside Sc2BDC3. This example not only provides a novel alternative in which to study more easily the adsorption sites in MOFs via diffraction techniques, but also reveals some of the interesting structural behaviour MOFs can have in these extreme conditions.