The Effect of Pressure, Guest Uptake and Structural Flexibility on Porous Framework Materials

In recent years the development of new methods of storing, trapping or separating light gases, such as CO2, CH4 and CO has become of outmost importance from an environmental and energetic viewpoint. Porous materials such as zeolites and porous organic polymers have long been considered good candidates for this purpose. More recently, metal organic frameworks (MOFs) have attracted further interest with many aspects of their functional and mechanical properties investigated. The porous channels found in MOFs are ideal for the uptake of guests of different shapes and sizes, and with careful design they can show high selectivity for particular species from a mixture.[1] Adsorption properties of MOFs have been thoroughly studied,[2]  however obtaining in depth ‘structural’ insight into the adsorption/desorption mechanism is not so common place. For example, out of ca. forty thousand published framework structures there are less than 10 entries in which CO2 molecules have been unambiguously located experimentally within the pores.[3]

Over the last 6 years, we have been using high-pressure crystallographic techniques to explore the uptake of guest species in the pores of MOFs.  We do this, by taking advantage of the fact that the small molecules that encompass the pressure transmitting fluids used frequently in high-pressure crystallographic studies, can penetrate the pores on increasing pressure.  This has revealed unexpected flexibility,[4] explain unusual adsorption phenomena under milder gas pressures, and increase reactivity in MOFs.[5]  The potential for using high-pressure to explain structure-property relationships has also been revealed in porous magnetic materials.  Here, we will give an overview of the effect of high-pressure on both micro and nanoporous materials, and in-particular, highlight some recent work on gas-loaded framework materials.