## Sunday, 17 April 2016

### Sifting for carbon dioxide

Some colleagues and I have just had a paper accepted for publication and with a title like "Enhancement of Chain Rigidity and Gas Transport Performance for Polymers of Intrinsic Microporosity via Intramolecular Locking of the Spiro-carbon", I thought I would explain it in more simple language.

#### In short

• If we could filter out $CO_{2}$ from the emissions of powerplants we would be able to collect and economically pump it into underground storage without having to compress all of the other gases emitted from the powerplant, such as nitrogen (which can be up to 60%).
• New plastics have been recently made called 'polymers of instrinic microporosity' (PIM). They can let through $CO_{2}$ selectively, but more importantly they are very permeable which means they can separate out $CO_{2}$ on an industrial scale.
• Jianyong Jin (University of Auckland) and his team found a way to make the best PIM so far by introducing a locking mechanism between the molecules.
• To show how this locking mechanism improves the polymer, I used computer simulations to show that the lock increased the rigidity of the polymer and also produced the optimal geometry for the polymer. This made the pores just the right size for the $CO_{2}$ to pass through, and the large rigidity made it selective and very permeable.
• Here is a video of a similar material and the sort of separations that this polymer enables.

#### What is the most interesting detail if you are a scientist?

The most interesting detail for scientists is the idea of locking polymer molecules using an intermolecular locking mechanism. Many different polymer systems rely on the interplay between entropy and enthalpy. By locking the polymer backbone you can play with how the polymer packs and distributes its vibrational energy. One really interesting application we comment on in the paper is locking protein molecules using intramolecular locks. We are very interested in exploring this idea with other groups.

From a materials point of view, the idea of engineering the size of the pore by changing the geometry of the linkage is an interesting concept. You can go straight from a chemical structure to a material property.

#### Overview of the paper with some further details

The polymer monomer that was locked is called a spirobisindane, SBI for short, formed by adding bromine onto the 6-membered rings and then performing a silver oxidation which forms an ester bridge. This forms an 8-membered ring which bridges the weakly bonded spiro centre. Here is what it looks like with a 2D drawing.

It looks quite ridiculous in a two dimensional drawing. How can those two atoms be linked all the way across by an oxygen atom? Well, looking at the 3D drawing it makes a lot more sense, the 6-membered rings are actually a lot closer together than you might think and the bridge is only slightly strained. I have coloured the different rings so the 5-membered rings are coloured blue, the 6-membered rings are coloured pink and the 8-membered ring is coloured yellow (this also helps when comparing with the 2D representation shown earlier).

The image above is actually a 3D model you can rotate in the web page so click and hold on the picture and you can rotate you can also zoom in with the mouse wheel. Thanks to molview.org for the plugin.

The polymer, made up of thousands of repeats of the monomer, forms a very rigid structure which packs very poorly leaving lots of space (pores) for the gases to permeate through. With PIMs you want to optimise the pore size for the molecule you want to sort and also it is important this pore doesn't change size by much. Thermal energy from the polymer being at room temperature causes the pores to change size as the polymers move around. Changing the pore size reduces the selectivity for $CO_{2}$ over other gases so one method that has been used in the past is to rigidify the polymer. By replacing the spirobisindane centre with something more rigid you can make it more selective. Many of these attempts, however, changed the pore geometry so it was no longer big enough for $CO_{2}$. So what we have done was to rigidify a particular PIM, PIM-1, which we know has a good geometry for $CO_{2}$ separation, while keeping the large pore size.

To show how this intramolecular lock improves the rigidity of the polymer I simulated the movement of the polymer in the computer. This heats the polymer, causing it to vibrate and the rigidity is tracked by looking at the distance between the atoms at the end of the chain (end-to-end distance). This fluctuates over time performing a repetitive motion, almost like a snake. The polymer that is more rigid will have the less movement over time. There is a plot of the end-to-end distance over time (a) and also the bar graph showing the frequency of the end-to-end distances being at a certain distance.

What we found was that the unlocked polymer, labelled SBI, was much more mobile and the end-to-end distance varied a lot more than for the locked polymer.

Using a more advanced technique (taking into account the quantum mechanical description of all of the electrons in the molecule) we compared how much more rigid this locked PIM was, compared with other methods used to rigidify the polymer. The plots you see below show how the potential energy increases, from the baseline value, as I forcefully twist the polymer in the computer. Think of it like a spring - as it is twisted past its natural equilibrium, it will want to spring back. I actually used the equation for a spring to describe how rigid each polymer is. When describing a spring there is something called the spring constant - the larger this value, the more the energy increases with the deflection. The equation for the spring's potential energy is a parabola:

$$E_{spring}=\frac{1}{2}kx^{2}$$

Fitting the spring model to the potential energy plots I calculated for the different polymers allowed us to compare with other linkages. We showed that there was a 230% increase in the rigidity for the locked SBI compared with the unlocked SBI. It also showed there are other linkages that are actually more rigid. However the locked SBI has both the correct geometry to get the right pores and has reasonably large rigidity.

Excellent gas separation experiments were performed by Tim Merkel and Sylvie Thomas at the Membrane Technology and Research, Inc. in the US. These  are called Robeson plots and have selectivity on the y axis and permeability on the x axis. It was good to have a selective polymer but for industrial scale separations the real winner is having a permeable polymer. A permeable polymer allows for industrial scale separations.

All of the black open circles are common polymers. They have a trend that Robeson saw - as you increase their permeabilty you decrease their selectivity. He set an upper bound, the black line, that was not surpassed until these rigid polymers of instrinic microporosity were developed. You can see that for these two gas pairs $O_{2}/N_{2}$ and $CO_{2}/CH_{4}$ PIM-C1 is the most permeable polymer above the line. The first gas pair $O_{2}/N_{2}$ is useful for separating oxygen from the air which is mainly used to generate oxygen in medical applications. The second gas pair $CO_{2}/CH_{4}$ is important for removing carbon dioxide from natural gas which means you only need to transport the methane not the carbon dioxide as well; this is called natural gas sweetening. However, the important application is removing $CO_{2}$ from factory emissions. This is the $CO_{2}/N_{2}$ pair, which we didn't show in the paper, so I plotted it below.
I plotted the data with a linear scale and a log-log scale (which is most often used) as it might be more intuitive to see the data on a linear scale. You can see you get some good selectivity and a huge permeability for $CO_{2}$ which is exactly what we wanted for greenhouse gas capture. So the take home message is that careful design of the molecules leads to a huge change in the material properties and from a chemist's point of view, this was very exciting.