Turning foe to friend
(appeared in Mar 2016)

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Finding a use for CO2 may blunt the edge of carbon emissions, , says S.Ananthanarayanan.

The earth’s atmosphere weighs about 5 million billion tonnes. With a carbon dioxide content of 400 volune parts per million (ppm), the weight of the CO2 component comes to about 8 billion tonnes. This is the residue, on balance, after annual emission of over nearly 800 billion tonnes, mostly by natural processes, but also by man-made causes, and then the absorption of nearly as much by vegetation and by the sea. But the man-made contribution, which is a fraction of the total, has been pushing up the atmospheric load, which we urgently need to reduce

While the rate of emission may not see a fall for some years, it is priority to find ways to sequester, or prevent some of the CO2 that we emit from entering the atmosphere. Methods of physical containment of emissions, in natural, underground cavities as a gas or in solution, are limited by cost and technology. But Aanindeeta Banerjee, Graham R. Dick, Tatsuhiko Yoshino and Matthew W. Kanan, from Stanford University, report in the journal, Nature, a way to do one better – to new make use of CO2 to produce raw material for products that we need.

The difficulty of using CO2 in this way is that the gas is already at the lower energy end of processes like burning, which release energy. Carbon dioxide is thus relatively inert and does not react readily, except in conditions that take energy to create. Photosynthesis, by which plants create sugars from CO2 with the help of the sun’s energy, is a way to separate the carbon atom from its low energy bond with oxygen atoms and create bonds with other carbon atoms. Ways to harness the process artificially and create energy from sunlight are hence being worked on (The Statesman, 22 April 2015). More modest conversion of CO2 into chemicals that could be used in other processes, have also been devised and used in industry, albeit sparingly, as they were generally not energy efficient. Commercially viable methods have been mostly with the help of catalysts, or special substances that provide an intermediate platform as a way around the need for high energy route to create specific chemicals. The Stanford group adds a new instance, which emulates the process in photosynthesis, of creating a useful chemical building block with CO2 as a starting point.

Nature's way

In the action of plants and photosynthesis, energy from sunlight is first converted into a chemically available form, in the form of molecules called ATP and NADPH, which store and transport energy. In the next stage, in the presence of an enzyme, RuBisCO, which catalyses, or enables and accelerates the reaction, each carbon atom, which is attached to two oxygen atoms in CO2, forms bonds with carbon atoms in 5-carbon-atom intermediary, to form chains of three carbon atoms. As a next stage, the 5-carbon intermediary is regenerated, leaving a carbon atom ready to combine, and repeated cycles lead to a 6-carbon molecule, like glucose or even other carbohydrates, like starch or cellulose.

The Stanford team notes that the basic enabler of the sequence is that RuBisCO promotes a carbon-hydrogen bond in the 5 carbon intermediate to separate and then form a bond with the carbon atom in CO2. The team then seeks to recreate these conditions and they find that caesium carbonate is a material that enables the separation of carbon-hydrogen bonds and reaction with the carbon atoms in CO2. As an application of this function, the team demonstrates that a substance called furoic acid can take up CO2 and form a derivative called FDCA, which has industrial utility.

FDCA is a material, manufactured from sugars like fructose, so far, that can be processed into the polymer, polyethylene furandicarboxylate (PEF) which could take the place of polyethylene terephthalate (PET), a widely used plastic packaging material which is manufactured with petroleum based terephthalic acid. Manufacture of PEF, which is now from fructose, would emit 50% less CO2 emissions than PET, of which the annual production is 50 million tonnes, the Stanford paper says. The problem, however, is that producing FDCA from fructose is not scalable. On the other hand, production from a cellulose base, which is abundant in non-edible bio-mass (corn husk, sawdust, etc.), and which is being processed on a large scale to a related compound, Furfural, would be economical, except that in this route as well, the last stage in conversion of bio-mass to FDCA is not efficient.

This is where the principle of catalysing the conversion of FC to FDCA, to enable the use of bio-mass for manufacture of PEF, with the consumption of CO2 along the way, could be a double winner. While the process needs to be refined and studied to demonstrate the economics, the team also finds the possibility of production of terephthalic acid, for the production of PET, using CO2, like in the case of PEF. This would again lead to a sizeable ‘sink’ for sequestering CO2 and still getting some value out of it.

“Our results demonstrate a very simple strategy for engaging CO2 in C–C bond formation……the ability to make use of the carbon–hydrogen in this way bond opens the possibility of using this approach to prepare numerous high-volume targets….. synthesis of multi-carbon alcohols and hydrocarbons using CO2 and renewable hydrogen,” the authors of the paper say.

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