olid-state framework materials represent a unique class of solids in which chemists are able to exercise fine control of the crystalline structure through a carefully tuned combination of strong, directional bonding interactions and rigid organic struts. This level of control has enabled the design of highly effective materials for uses as varied as ion transport and gas capture and storage. However, despite these achievements, normal liquids remain the media of choice for many industrial adsorption applications as their fluidity greatly simplifies process design. As a means of bridging the liquid and solid adsorption paradigm we took inspiration from the clathrate hydrates. These are crystalline networks of hydrogen-bonded water cages encapsulating gas molecules or other non-polar guests which can form from liquid water or ice under low temperature and high gas pressures. Remarkably these structures are only stable in the presence of their guests and so removal of these molecules results in incongruent melting and recovery of the liquid phase. Recognizing that many hydrogen-bonded organic frameworks (HOFs) are effectively clathrates without an accessible liquid phase we hypothesized that successfully melting a HOF would give rise to clathrate behavior.<br/><br/>I will discuss our work on a HOF based on the methylguanidinium cation and an aryl disulfonate anion which shows an accessible liquid phase and clathrate behavior. In the absence of guest molecules, this salt behaves as a typical ionic liquid and does not crystallize, instead vitrifying at 45 °C. However, when exposed to non-polar solutes or high gas pressures the liquid crystallizes forming a layered HOF and incorporating the solute molecules into the structure. We map the ionic liquid-guest binary phase diagram of this system and demonstrate that it has phase behavior strongly reminiscent of the clathrate hydrates, namely that the decomposition temperature of the HOF is guest dependent. Finally, we show that strategies for clathrate promotion from the clathrate hydrate literature can be readily extended to these ionic liquid systems, stabilizing the gas clathrates at lower pressures. This work is, to the best of our knowledge, the first demonstration of conversion of a HOF to an ionic liquid and the clathrate behavior which results.

Clathrate behavior from a molten hydrogen bonded organic framework

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.