Professor Andrew Livingston talks microporous polymer membranes for chemical seperation

Materials World magazine
,
1 Jun 2016

Professor Andrew Livingston is Head of the Department of Chemical Engineering at Imperial College London, specialising in membrane separations. He speaks to Natalie Daniels about his team’s recent work developing microporous polymer membranes for chemical separation.

Tell us about what you have been working on.

For the past 10–15 years at Imperial College London, UK, I have been working with my research group on developing membranes to separate molecules in organic liquids. If you look at the world of membranes, there is ongoing work on aqueous systems – desalination, kidney dialysis and on gas separations. There are, however, no membranes that separate organic liquids, such as solvents and crude oil. We have been working on how we can get the membranes into separations in organic systems. It has been interesting to us because, in organic systems, the way the separations are normally carried out is through a process called distillation, which involves boiling liquids, and then evaporating them, forcing the liquid to turn into a gas. However, this process is very energy consuming. In order to get around this, we looked at whether we could use a membrane instead and do those separations within the liquid phase – so there wouldn't be a need to boil the liquid – saving a lot of energy. The difficulty was finding a membrane that can discriminate between molecules and liquid with high enough fluxes that a good amount of liquid is able to pass through them in a given time, while also remaining stable in the organic liquid. 

What was the process behind making these membranes?

We initially started making membranes from one material known as integrally skinned asymmetric membranes – these are the first generation that have just been commercialised. We were then interested in membranes that have two layers – the first is the separating layer and the second is a support layer to hold the separating layer in place, also known as a thin-film composite membrane. These are widely used in desalination systems or processes but have not yet been used commercially with organic solvents. After we developed these membranes, we worked on developing membranes with even-thinner top layers. Our work has been focused on how we make the separating layer that goes on top of the membrane. 

What inspired the work?

Two things – one was that we wanted to make membranes thinner. We have previously reported one of the thinnest membranes at, 8nm thick, in Science last year, through a process called interfacial polymerisation. 

There have also been many reports of membranes being developed with linear polymers that are unable to pack closely together – called polymers of intrinsic microporosity – made by combining monomers to form a linear polymer and making a membrane from these. We tried to take the interfacial polymerisation approach to make a network polymer and combine that with the intrinsic microporosity approach to make a new type of membrane. The idea is that we could manipulate the monomers that have been combined in a polymer network, allowing us to design a high free-volume in the filtration nano-films.

Chemical separation processes can often be costly. How did you overcome this?

The distillation process requires boiling the liquid and, to do this, a lot of energy is needed because the molecules and the liquid are packed very closely. To turn this into a vapour, the molecules need to be forced far apart by applying energy. When you conduct the process with a membrane, you can perform the separation in the liquid without the high energy costs – amounting to less than 10% of the energy used in the distillation process.  

What further research is needed?

Starting to look at interfacial polymerisation, we have used one chemical system – a polyester-based chemistry. We are now looking at other polymerisation chemistries and pulling other monomers to make a variety of membranes. An issue with these membranes is that we would like them to be immortal – these polymers tend to age and, as they are packed closer together, it restricts permeation through them. We would like to explore whether, by using these highly networked systems, we can get membranes that will age more slowly and last longer. Another aspect we are working on is whether these membranes can be even more chemically resistant to a wider range of solvents. The last direction is to scale the membranes up to a larger area. I hope these goals will be achievable over the next two-to–five years. 

Were you happy with the outcome of your research?

We are very excited and pleased that we have been able to design these membranes from the bottom up by looking at the monomers and designing the micropores in them, by predicting that these would achieve a much higher free-volume – so far it has worked successfully. It is always exciting to see your experiment and work come together in the way this has.

To read the paper Polymer nanofilms with enhanced microporosity by interfacial polymerization, visit bit.ly/23rPZJG