The KAUST team have found a way to make a crystalline MOF behave like a porous liquid. Photo: 2020 KAUST.
The KAUST team have found a way to make a crystalline MOF behave like a porous liquid. Photo: 2020 KAUST.

Innovative materials called metal organic frameworks (MOFs) could become much more versatile thanks to new research that shows they can be manipulated as liquids.

MOFs are highly porous crystalline solids with metal ions or metal clusters joined by organic (carbon-based) linker groups. By varying these components, researchers can create a huge variety of solids with internal pores able to trap selected molecules or catalyze chemical reactions.

"These crystalline materials are difficult to process, but we have developed a way to solubilize them," says Anastasiya Bavykina from the King Abdullah University of Science & Technology (KAUST) Catalysis Center in Saudi Arabia.

The KAUST researchers produced membranes composed of a liquid MOF embedded in a polymer, which they say can achieve outstanding performance in the challenging separation of propylene gas from propane.

"This is revolutionary," says Bavykina. Propylene is a key feedstock for the chemical industry. Not only can it be used to make the polymer polypropylene, but it can also be converted into other polymers and industrially useful chemicals. But first it must be separated from the propane it generally comes mixed with.

"If the current energy-intensive propane-propylene separation technologies, based on distillation, could be replaced by our MOF membrane technology, then this could save about 0.1% of global energy consumption," said co-author Shuvo Datta from the KAUST Advanced Membranes & Porous Materials Center.

As the researchers report in a paper in Nature Materials, one challenge was to how make a crystalline MOF behave like a porous liquid. The approach they came up with involves modifying the surface of relatively large MOF nanoparticles with suitable chemical groups. This ‘surface functionalization’ allowed the nanoparticles to form stable dispersions in a liquid solvent.

Another challenge was how to ensure the internal pores of the MOFs remain empty, so they can take up and allow permeation of desired gas molecules. The porous spaces and the solvent molecules must be carefully controlled to prevent the solvent from filling the gaps.

"It is also not easy to actually demonstrate that a liquid is porous," Bavykina adds. The researchers had to develop a novel experimental setup to achieve this.

The liquid phase MOF dispersions can separate gas mixtures that are bubbled through them, but the team achieved greater flexibility by incorporating this MOF into their flexible and robust polymer membranes. This allowed a continuous flow system to run for up to 30 days, producing 97% pure propylene from a 50/50 propane-propylene mixture that was effectively filtered by the membrane.

The team now want to scale up their procedure to demonstrate its commercial potential. They will also seek to apply it to other important industrial gas separation processes.

This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.