Modified MOFs improve propane-propylene separation

Modified metal organic frameworks that can behave as porous liquids offer new possibilities for propane-propylene and other gas separation technologies, claim researchers in Saudi Arabia.

Cross-sectional SEM images show differences in physical characteristics of the membranes. Reproduced with permission from reference 1 (© 2020 Springer Nature)

MOFs are highly porous crystalline solids with metal ions or metal clusters joined by groups of organic linkers. Varying these parts can create solids with internal pores that trap selected molecules or catalyse chemical reactions. The team’s findings have been published in Nature Materials

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“These crystalline materials are difficult to process, but we have developed a way to solubilise them,” said Anastasiya Bavykina from the research team at the KAUST Catalysis Center.

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

Propylene is used to make the polypropylene, which is widely used in applications including packaging. It can also be converted into other polymers and industrial chemicals after it is separated from propane.

“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 per cent of global energy consumption,” said co-author Shuvo Datta.

propane-propylene
One challenge for the team was to make a crystalline MOF behave as a porous liquid (Credit: KAUST 2020)

According to KAUST, one challenge for the team was to make a crystalline MOF behave as a porous liquid. The team discovered how to modify the surface of relatively large MOF nanoparticles with suitable chemical groups. This surface functionalisation allowed the nanoparticles to form stable dispersions in a liquid solvent.

Similarly, the internal pores of the MOFs had to remain empty and able to 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 said in a statement. 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 a MOF into their polymer membranes. This allowed a continuous flow system to run for up to 30 days, producing 97 per cent pure propylene from an equal propane-propylene mixture that was filtered by the membrane.

The team now want to scale up their procedure to demonstrate its commercial potential.