Metal-organic frameworks (MOFs) have shown great potential for a number of applications, including as carbon-capture materials, but their use commercially has been limited by production methods, which are generally expensive batch processes. With the development of a continuous process for producing a range of MOFs at industrial scale, Promethean Particles (Nottingham, U.K.; www.prometheanparticles.co.uk) aims to lower the costs of making MOFs and expand production volumes.
MOFs are a class of materials where metal ions or clusters are linked with rigid organic linker molecules to form highly porous crystalline compounds. “We’ve seen current prices for the supply of a common MOF material, known as UiO-66, quoted as high as $60,000 per kg by other MOF manufacturers,” says James Stephenson, Promethean CEO. “With further development of our continuous-flow process, we think we can reach a target of $30 per kg for our MOF products,” he says, adding, “We’re trying to change the perception of what is commercially viable when it comes to MOFs.”
Promethean recently announced funding from an investment round that will be used to build a new facility with the capability to expand their production capacity by 5–10 times, compared to their current capacity of 1,000 tons per year. Production at those costs and volumes would open the door for more widespread use of MOFs in carbon-capture applications, water-harvesting applications in water-stressed regions, and other uses.
In Promethean’s process, called continuous flow hydrothermal or solvothermal synthesis (CFHS or CFSS), a solution of metal salts and a solution of organic linkers are combined at a specified temperature in a patented counter-current reactor, which can be operated at elevated pressure. Within the reactor, the two solutions mix in turbulent flow and nucleation of MOF particles occurs (diagram).
Promethean product manager Selina Ambrose explains that by carefully controlling the temperature and pressure of the reactor, as well as the fluid flowrates, the mixing dynamics can be influenced, meaning the structural properties of the resulting MOF could be controlled. These properties include particle size, shape or crystallinity. The product from the reactor is a suspension of MOF particles in water or other carrier solvent. Once the particles form, the continuous-flow nature of the process allows an option for secondary and tertiary ingredients to be introduced in situ, to further modify the MOFs and obtain the desired properties.