Making porous materials with supercritical carbon dioxide
Metal-organic frameworks (MOFs) are crystals full of tiny pores, with possible uses in clean energy, gas capture and water treatment. Many are made using long heating times and large volumes of organic solvents, which adds cost and waste.
Much of my research has explored another option: using carbon dioxide as a tool.
What is supercritical CO2?
When carbon dioxide is held above a certain temperature and pressure, it becomes supercritical, behaving partly like a gas and partly like a liquid. Adding it to a liquid can make that liquid expand, which changes how well it holds dissolved ingredients.
Growing crystals on demand
In one study, we used this effect to make a copper-based MOF called HKUST-1 quickly from a solution containing greatly reduced amounts of organic solvent. The solution stayed stable for months until CO2 was added, and crystals then formed under mild conditions of 40 °C and 40 to 100 bar.
Changing the pressure gave us control over crystal size and yield. The process also created larger pores alongside the natural small ones, like adding motorways to a network of side streets. These larger routes can help molecules to move through the material.
It does not suit every MOF
We also compared this route with a conventional method across several other MOFs and related materials. In those systems, CO2 did not create extra large pores or significantly improve yields, but it could affect crystallinity, crystal size and shape. That is useful to know, because it helps to show where the method is most valuable.
Why it matters
Using less solvent and gaining more control could make MOF production cleaner and more predictable. Showing that these benefits hold up at larger scale is the next challenge.
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