Prof Kamendra P Sharma of the Indian Institute of Technology, Bombay, has designed a ‘porous liquid’ that can be used for capturing and storing carbon dioxide, and subsequently converting it into calcium carbonate, a useful industrial material.
According to an article in IIT Bombay website, the novelty lies in combining porosity, and catalytic activity within a liquid to convert carbon dioxide into calcium carbonate, which can be removed from the porous liquid, for the latter to be reused.
This porous liquid with honey-like viscosity is simple to manufacture and easy to integrate with continuous flow industrial processes. It is stable at industrial working temperatures.
Historically, porous solid materials have been used to trap gases from industrial effluents. Since solids cannot flow, it becomes difficult to retrofit or integrate such filters in continuous flow industrial processes. Liquids can absorb gases too, but their capacity to store gases is much less than porous solids. Besides, unlike solids, the vacant spaces in liquids are not permanent.
Prof Sharma showed that a porous liquid made by simply combining hollow silica nanorods and a polymer (wetting agent) could capture carbon dioxide at room temperature. However, converting the captured carbon dioxide to a useful chemical without spending more energy would be more valuable.
In the current study, Sharma created a liquid composite by combining the porous liquid and an enzyme called bioconjugated carbonic anhydrase(bCA) and adding calcium chloride to it. The enzyme reacts with carbon dioxide adsorbed in hollow silica nanorods and turns 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.
When carbon dioxide is passed over the porous liquid, the hollow cavities of silica nanorods trap the carbon dioxide. After the capture, the carbon dioxide slowly escapes from the nanorods. It combines with bCA to form bicarbonate ions at room temperature. These ions react with 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. The porous liquid can be reused after removing the calcium carbonate. Calcium carbonate is used in making building materials, ceramic tiles, chalk and health supplements.
The porous liquid can also store the trapped carbon dioxide at low temperatures. When the porous liquid is frozen to -60 ℃, the polymer shell coating the silica nanorods in the porous liquid becomes glassy, and the carbon dioxide stays trapped in the nanorods. When the temperature is restored, the carbon dioxide starts releasing again, kickstarting the conversion to calcium carbonate.
The researchers experimentally confirmed that adding the enzyme does not alter the shape of the nanorods or its capacity to capture carbon dioxide.
“We have presented a proof of concept for an end-to-end solution to capture, store and convert carbon dioxide from industrial emissions to calcium carbonate. All the materials used are stable at high temperatures; the porous liquid can work without degradation at industrial temperatures. The rate of carbon dioxide capture conversion does not drop significantly till a temperature of 50 ℃. We need to evaluate the efficiency of carbon dioxide capture at higher temperatures,” says Prof Sharma.
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