Molecular sieve membranes look to greener separations


mof_fibre

The hollow fibre metal-organic framework membrane modules could cut energy costs in propylene separation substantially © Georgia Tech Photo: Rob Felt

US researchers have made molecular sieving fibres that open up new possibilities for large scale chemical separations that use much less energy than conventional distillation methods. Sankar Nair and his Georgia Institute of Technology team have shown their metal–organic framework (MOF)-lined fibres can perform similarly to distillation in separating propylene and propane. That’s the same post-cracking raw mixture that the majority of the 77 million tonnes of propylene produced in 2011 was distilled from. ‘The degree of separation by the membrane is comparable, but uses vastly less energy,’ Nair says. 

Though scientists recognise MOF membranes’ separation potential, they’ve mostly been fabricated on large tubes, whose surface area-to-volume ratio is too low for producing cheap separation modules. Micron-scale fibres would be better, but inside them conditions for successful membrane formation change significantly, Nair explains. ‘The amount of reactant present in the fibre is miniscule, and molecular transport and reaction processes are quite different,’ he says.

The Georgia Tech team’s new approach uses two solutions – one containing zinc ions and another containing methyl imidazole. With one solution inside 250µm diameter hollow poly(amide-imide) porous fibres and another outside them, they start forming a zeolitic imidazolite framework (ZIF) where the two meet. They initially explored conditions where the solution inside the fibre didn’t move, but got patchy coatings. When they realised this setup wasn’t supplying enough reactant, they pumped the inner solution through the fibre, getting better coatings but struggling to control thickness and quality. Finally, they devised an intermittent approach, allowing the membrane to grow under static conditions, then periodically replenishing the reactant supply.

This lets Nair’s team direct film growth inside the fibre or on its inner or outer surfaces through different reagent and solvent permutations. Using tiny pores in MOF films inside the bore of the fibres as selective molecular scale sieves they then separated hydrogen or propylene from propane. ‘We have a demonstrated path to “scale down” MOF membranes to hollow fibres, in a manner that we can now look to “scale up” by an in-parallel replication of the process to many fibres bundled together into a module,’ Nair explains.

Zhiping Lai, a chemical engineer from the King Abdullah University of Science and Technology in Saudi Arabia, says this result ‘beautifully demonstrates’ that ZIF-8 membranes can be produced in bulk quantities on cheap supports. ‘Replacing distillation by an efficient membrane process can cut energy use up to 80%,’ Lai says. ‘It significantly increases the economic viability of the membrane process, which may lead to a technical revolution in the chemical industry.’


Related Content

Better separations with more permeable membranes

3 September 2013 Research

news image

Embedding conducting polymer nanoparticles in traditional membrane polymers improves their separation performance

Novel nanoparticle filter

23 January 2011 News Archive

news image

Researchers create a recyclable membrane based on non-covalent bonds to filter nanoparticles

Most Read

Higher levels of some metals in e-cigarette smoke

8 September 2014 Research

news image

Scientists call for regulators to help clear smoke and mirrors surrounding vaping safety

Isotope effect produces new type of chemical bond

22 October 2014 Research

news image

Evidence emerges for vibrational bond first proposed 30 years ago

Most Commented

Higher levels of some metals in e-cigarette smoke

8 September 2014 Research

news image

Scientists call for regulators to help clear smoke and mirrors surrounding vaping safety

Not all science is created equal

16 October 2014 Comments

news image

John Ioannidis explains why researchers should be curious about the differences between disciplines