Crack-proofing MOF membranes

Chinese chemists have developed a way to reinforce metal-organic framework-based membranes to toughen them against cracking. The process, which involves growing the porous polycrystalline films onto a wire mesh, could see the membranes used to collect hydrogen from waste gas streams. 

MOF(metal organic framework)-based membranes have the potential to   be used for a variety of gas separation and sensing purposes. But creating these films on a large scale has been difficult as the crystalline structure is prone to splitting apart. 

Now, the team lead by Guangshan Zhu, at Jilin University in Changchun,  have solved this problem by oxidising the surface of a copper wire mesh to create active copper (II) sites. As the MOF membrane grows on the mesh support, the fixed-copper ions become integrated into the copper-based MOF structure. 

’Since the MOF membrane is tightly integrated with the copper net, it provides support in the same way that steel bars are used to make reinforced concrete,’ explains Zhu.  

Using the net serves another purpose, Zhu adds, as it seeds  the growth of the MOF - helping to template an ordered crystal structure across a large area. This ordered structure is crucial for the formation of regular sized pores that allow MOFs to accommodate specific gases. 

’Our net-supported membrane shows a high permeability and selectivity for hydrogen gas,’ Zhu says - indicating that it could have potential to extract hydrogen from waste gas streams, although further optimisation would be required before this could be commercialised. To demonstrate the material’s resilience, the team re-used the membranes over a six month period, showing that they were stable and recyclable. 

Zhu is also confident that different metals and other types of framework films - such as zeolitic imidazolate frameworks (ZIFs), which have a high affinity for CO₂ - could benefit from being grown in this way. 

’This is very interesting work, as one of the biggest challenges in using microporous solids for gas separations is fashioning membranes that don’t have cracks,’ says Jeffrey Long, an expert on MOFs at the  University of California, Berkeley, US. ’Using a copper wire mesh apparently leads to intimate contacts between the mesh and the dense, polycrystalline framework layer - something that is worth investigating further.’

Lewis Brindley