MOFs: turning porous materials into industrial solutions
MOFs (metal-organic frameworks) are attracting interest due to their ability to capture and separate molecules. To bring them into industrial applications, they must be produced reproducibly and given a suitable format for their intended use. Mechanochemistry and reactive extrusion offer routes to reduce solvent use during synthesis, move towards continuous manufacturing and explore their incorporation into polymeric materials.
What are MOFs and why are they of interest?
MOFs, from the English term metal-organic frameworks, are metal-organic networks formed by metal centres and organic ligands.
Many of these structures have:
- molecular-sized pores
- a high internal surface area
The selection of their components makes it possible to tune their affinity towards specific substances: a feature of interest for separation and capture applications, including those related to security and defence.
Why does manufacturing determine feasibility?
Bringing these materials into industry requires producing them reproducibly and at a viable cost.
Some synthesis routes use large quantities of solvent, which then need to be separated and managed. In addition, the resulting powder requires a suitable format for use while maintaining access to its pores. Mechanochemistry and reactive extrusion address both of these needs.
How does mechanochemistry simplify synthesis?
Mechanochemistry uses impact and friction energy to promote reactions. In a ball mill, mixing and surface renewal maintain contact between reactants, making it possible to work directly with solids. For synthesis routes compatible with this approach, it reduces the need to dissolve components in large volumes of liquid.
The practical advantage is to concentrate the process on the reactants and reduce the solvent that needs to be stored, handled and recovered. This simplifies operations associated with its use and creates opportunities to reduce consumption and waste.
Reactive extrusion for continuous production
Reactive extrusion (REX) combines feeding, mixing and reaction in a single piece of equipment. Its screws renew contact between components as they move through temperature-controlled zones. The extruder can synthesise the MOF directly from its precursors, acting as a chemical reactor.
The continuous operation makes it possible to produce larger quantities by extending operation time, with simultaneous reagent input and product output. In addition, adjusting dosage, temperature and residence time provides tools to maintain reproducible manufacturing conditions. This facilitates moving from studying a reaction to preparing material for formulations and application testing.
At AIMPLAS, we have obtained ZIF-8 and MOF-5 both by ball milling and twin-screw extrusion, with the following results:
- Production rates of 1 kg/h in continuous operation were achieved.
- The materials showed the expected three-dimensional structure and a BET specific surface area comparable to commercial references.
In these developments, solvents were limited to post-processing washing steps to remove potential residual reagents.
From powder to a processable material
Integrating a MOF into a polymer opens up the possibility of obtaining functional parts or films that are easier to handle than the powder.
Extrusion enables particle distribution within the matrix and, when a reactive process is designed, allows the same equipment to be used to synthesise the MOF or modify the interaction between components.
Bringing these operations together can reduce preparation steps and facilitate composite manufacturing.
Our experience with functionalised nanoparticles shows how this interface can be engineered. In plastics developments aimed at electronic applications, REX enabled these particles to be bonded to polymer chains and used to prepare highly loaded concentrates or masterbatches, improving their distribution and reducing agglomeration. Subsequent dilution makes it possible to adjust the particle content in the final material.
The literature also describes ZIF-8 and ZIF-67 composites with polypropylene and polystyrene obtained through reactive extrusion. These precedents, together with our experience in nanoparticle modification and integration into polymer matrices, open new possibilities for incorporating MOFs into processable materials, selecting the appropriate chemistry and processing conditions to preserve their porous functionality.
How can the manufacturing route be connected with the application?
For companies, connecting synthesis, modification and processing makes it possible to approach material development from the perspective of the final function to be achieved. In filtration applications, for example, this opens up the possibility of combining the adsorption capacity of MOFs with the ease of processing of a polymer matrix, creating materials adaptable to different formats and end-use requirements.
At AIMPLAS, we combine:
- synthesis
- chemical modification
- processing
- and pilot plants
to develop advanced materials and move from porous materials to their incorporation into formulations and products.
Are you looking to develop a MOF, explore a more efficient production route or integrate it into a polymer matrix?
We can help you define and validate the most suitable process for your application.
Frequently asked questions about MOFs
It is a porous material formed by metal centres connected to organic ligands. Many of these structures have molecular-sized pores and a high internal surface area, and the selection of their components makes it possible to tune their affinity towards specific substances.
Yes, certain mechanochemical synthesis routes can be carried out without adding solvent during the reaction. In AIMPLAS developments with ZIF-8 and MOF-5, solvent use was limited to subsequent washing steps to remove potential residual reagents. This reduces solvent requirements during the synthesis stage.
At AIMPLAS, we have achieved continuous production rates of 1 kg/h in developments with ZIF-8 and MOF-5, obtaining the expected crystalline structure and a BET specific surface area comparable to commercial references. This throughput does not represent a technological limit: once the synthesis has been validated, production can be increased by making use of the extruder’s capacity or by using larger-scale equipment, adjusting feeding, mixing and residence time to maintain material quality.
This is the main challenge when developing composites. Reactive extrusion combines mixing, which distributes the particles, with reactions that improve their interaction with the matrix. The chemistry and processing conditions must be carefully selected to preserve access to the pores.