Faster ways to make porous materials: microwaves and ball milling
A promising material is only useful if it can be made practically. Metal-organic frameworks (MOFs), crystals full of tiny pores, are often made by heating ingredients in organic solvents. In one of our own studies, this step took 20 hours at 110 °C.
Long heating uses energy and limits how much material can be made. So we have been testing quicker alternatives.
Microwaves
Microwave heating can transform how quickly crystals form. We used it with water-based chemistry to make a family of water-stable MOFs for capturing carbon dioxide and hydrogen sulfide, avoiding hazardous organic solvents.
We found the best results after 30 minutes at 100 °C. The top-performing material had an internal surface area of 1,877 square metres per gram, or about seven tennis courts.
Ball milling
Ball milling takes a different approach. Instead of heating ingredients for hours, it grinds them together in a mill, and the mechanical energy drives the reaction.
Using a statistical approach to find the best conditions, we made a copper-based MOF in minutes. This used less than 2% of the energy of the conventional method. The material stored 3.2% of its weight in hydrogen at 20 bar and 77 K, close to conventionally made material.
Why it matters
Speed and energy use affect both the cost and the environmental footprint of a material. Faster methods could help to make these materials more practical, although scaling them up reliably still needs to be demonstrated.
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