Liquid pores that trap carbon

The Glasgow climate talks — COP26 — recently concluded with a Glasgow Climate Pact and countries agreed to come back earlier to the table next year to report their climate actions, rather than in 2025. The world is under pressure to prevent carbon dioxide emissions. The world, therefore, is looking at technologies to capture carbon dioxide at the source and convert it into something useful or easily storable.

The challenge is to develop a material that best captures the gas. Solids can have lots of pores for the gas molecules to snuggle in, but solids soon get saturated and need replacement. Liquids, being able to flow, can be circulated to continuously capture carbon dioxide, but they absorb less than solids.

In the recent past, newer materials have come up, called ‘porous liquids’. These ‘liquids with holes’, first produced in 2015, are engineered materials that contain microscopic cavities that could be as small as a molecule. According to James Stuart, an inventor of porous liquid and a co-founder of porousliquidtechnologies.com, these liquids have 10,000 times the number of cavities found in conventional fluids, a lot of which is just empty space.

These pores, therefore, can absorb big volumes of gas and can even be tuned to choose a particular gas — like carbon dioxide.

Recently, Prof Kamendra P Sharma of IIT Bombay announced that his team had developed one such porous liquid.

Usually, porous liquids are made with large organic molecules, or molecular cages, which remain intact even if dissolved in a solvent. This, however, involves a tedious, multi-step process, says Sharma.

His method is simpler. Sharma made a porous liquid using hollow silica nanorods and a polymer (wetting agent). This liquid, according to IIT Bombay, can capture carbon dioxide at room temperature.

But the next problem is, what to do with the carbon dioxide thus captured?

Sharma’s team added to the mix an enzyme called bio-conjugated carbonic anhydrase (bCA) and calcium chloride. The enzyme reacts with the carbon dioxide in the hollows of the silica nanorods and converts it into bicarbonate ions.

Most enzymes need water to be active. However, bCA works very well in the polymer environment that the porous liquid offers. These ions then react with the calcium ions from calcium chloride to form micrometre-sized calcium carbonate crystals.

These crystals can be removed by heating the system and letting them sediment out. Calcium carbonate has several industrial uses. Once the calcium carbonate is removed, the porous liquid is ready for another round of work.

Thus, Sharma’s process is a combination of two separate streams — in the first, the silica nanorods in the liquid capture the carbon dioxide; and in the second, the enzyme converts the gas into calcium carbonate.

“The proposed porous liquid can be used for all gases since the pore size of the nanorods is larger than gas molecules. By choosing the right catalyst and a suitable reactant, different gases can be converted into useful chemicals,” says Sharma.

There are further advantages. If the porous liquid gets saturated with carbon dioxide, it turns from acidic to neutral. A dye that changes colour with acidity can be used to find out when the liquid is full of carbon dioxide.

Sharma admits that the cost of the porous liquid is still too high for commercial viability, mainly due to the enzyme. Over time, an inexpensive alternative would come up. “We have presented a proof-of-concept,” says Sharma in an article published by IIT Bombay. He notes that the rate of carbon dioxide capture works well till the temperature reaches 50 degrees Celsius.

However, on cost, Stuart says the product is “highly economical”.

Now that porous liquids are ‘real’, a new area of materials science may open up, with clear potential for long-term applications in chemical processes, he says in a 2016 scientific paper that discusses his discovery.

COMMENT NOW