Crystallographic studies of gas sorption in metal–organic frameworks

Abstract

Metal-organic frameworks (MOFs) are a class of porous crystalline materials of modular design. One of the primary applications of these materials is in the adsorption and separation of gases, with potential benefits to the energy, transport and medical sectors. In situ crystallography of MOFs under gas atmospheres has enabled the behaviour of the frameworks under gas loading to be investigated and has established the precise location of adsorbed gas molecules in a significant number of MOFs. This article reviews progress in such crystallographic studies, which has taken place over the past decade, but has its origins in earlier studies of zeolites, clathrates etc. The review considers studies by single-crystal or powder diffraction using either X-rays or neutrons. Features of MOFs that strongly affect gas sorption behaviour are discussed in the context of in situ crystallographic studies, specifically framework flexibility, and the presence of (organic) functional groups and unsaturated (open) metal sites within pores that can form specific interactions with gas molecules.

Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity

“…[1][2][3][4][5][6][7] Among MOFs that permit reversible guest removal and uptake, most have a rigid pore structure reminiscent of harder inorganic porous materials such as zeolites. A recent estimate indicates that only around 100 out of some 20,000 reported MOFs exhibit breathing or flexible behaviour in response to external stimuli.…”

Exchange of Coordinated Solvent During Crystallization of a Metal–Organic Framework Observed by In Situ High‐Energy X‐ray Diffraction

“…[5] These changes are well within the range that can be clearly resolved using high-resolution powder diffraction, and indeed this method has been used extensively in structural studies of the effect of adsorbed molecules on MOFs under gas atmospheres. [6] In many cases of MOF synthesis using solvothermal methods,i ti su nclear whether the framework is initially formed with coordinated solvent that is then exchanged with another ligand to reach the final product, or if the final product is formed from the start as the only species.T his knowledge would be valuable to the large scale deployment of MOFs,a llowing the optimization of syntheses to reduce or eliminate the need for certain types of post-synthetic processing,s uch as the high-temperature dehydroxylation of UiO-66.…”

Chemical Reactions followed by in situ Neutron Powder Diffraction

“…For more details the interested reader is referred to review articles. [125] The gases in question for adsorption in MOFs usually are comprised of light elements, e.g., H 2 , H 2 O, CO 2 , N 2 , NO, NO 2 , and the MOFs themselves contain large amounts of light elements as well. Therefore, neutron diffraction is very well suited for structural studies.…”