Organic frames made of metal to act as liquids

Innovative materials called organic frameworks (MOFs) could become much of a researcher after research showing that they can be manipulated as liquids.

MOFs are highly porous crystalline solids with metal ions or metal clusters linked by organic (carbon-based) linker groups. Different these parts can form a large variety of solids with internal pores that can 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 of the KAUST Catalysis Center research team.

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

“This is revolutionary,” Bavykina says. Propylene is an important feedstock for the chemical industry; it is used to make the polymropropylene that is used in many products. It can also be converted to other polymers and industrial useful chemicals, but it must first be separated from the propane it generally comes with.

“If current energy-intensive propane-propylene separation technologies, based on distillation, could be replaced by our MOF membrane technology, this could save about 0.1 percent of global energy consumption,” points out co-author Shuvo Datta.

One challenge for the team was to wear a crystalline MOF as a porous liquid. The team discovered how the surface of relatively large MOF nanoparticles with appropriate chemical groups could be adapted. This “surface functionalization” allowed the nanoparticles to form stable dispersions in a liquid solvent.

Another challenge was to ensure that the internal pores of the MOFs remained empty and able to absorb permeability of desired gas molecules. The porous spaces and the solvent molecules must be carefully controlled to prevent the solvent from filling the holes.

“It’s also not easy to actually demonstrate that a liquid is porous,” Bavykina adds. The researchers had to develop a new experimental setup to achieve this.

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

The team now wants to scale up its procedure to demonstrate its commercial potential. They will also try to apply it to other important processes for individual gas separation.

The study is published in Natural materials.