Reticular chemical strategies—the linking of strongly bound, rigid building blocks to create open extended materials—has resulted in the creation of a huge number of porous framework materials over the past several decades. While this reticular approach has proved very successful in a variety of chemical contexts, it has not yet been applied to a ubiquitous family of naturally occurring framework materials: the clathrate hydrates. Under high gas pressure and low temperature conditions, liquid water freezes, not as normal ice, but as a clathrate hydrate. These structures consist of networks of cages made of hydrogen-bonded water, each encapsulating a single gas molecule. A large portion of Earth’s methane reserves are stored as clathrate hydrates under the polar ice sheets or as sediments on the ocean floor. Methane and carbon dioxide clathrate hydrates are also believed to be present on Titan, Mars, and other moons and planets. Technologically, clathrate hydrates are attractive for gas capture and storage applications due to their high gas capacities and the fact that their primary component is water. However, the sub-ambient temperatures and high pressures required to form clathrate hydrates currently makes their use in these applications economically infeasible.<br/>We show that applying reticular chemical principles to clathrate hydrates increases their stability, allowing them to form at higher temperature and/or lower pressures. A major source of clathrate hydrate instability is that a large number of waters—5-6 per gas molecule—are required to form the structure, resulting in large, negative entropies of formation. We designed organic molecules which mimic the structure and hydrogen-bonding requirements of portions of the clathrate cage enabling them to replace multiple waters at once in the clathrate structure. Solutions of these molecules in water result in clathrate formation at significantly higher temperatures and lower pressures than for pure water. We map the clathrate hydrate phase diagram as a function of temperature, pressure, and additive concentration with various gases, and determine how the additive inserts into the clathrate hydrate structure via X-ray diffraction. This work opens up a new direction in clathrate hydrate research and could eventually lead to their viability as gas capture and storage media.

Making a framework out of water: Reticular chemistry applied to clathrate hydrates

ACS Spring 2022

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