Carbon capture: Solar-powered option

Artificial photosynthesis

 

Photosynthesis is the ultimate solar-powered carbon capture technology. Plants extract carbon dioxide from the atmosphere and use sunlight to convert it into chemical energy. Now, writing in Angewandte Chemie, a US team has suggested a novel reaction mechanism that can mimic photosynthesis in the binding of carbon dioxide using light energy captured by silicon nanowires. The researchers have even demonstrated proof of principle in the synthesis of two precursors of the anti-inflammatory, pain reducing drugs ibuprofen and naproxen.

Photosynthesis is not wholly light driven. It exploits two biochemical processes, a light and a dark reaction. In the light reactions, solar photons are trapped and their energy ultimately used to generate the cellular energy compounds NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate). These are then used to promote the binding of the carbon source, carbon dioxide during the synthesis of complex sugar molecules. This binding takes place in the dark reactions and pivots on forming a link between carbon dioxide and a sugar phosphate (ribulose-1,5-bisphosphate) to generate a beta-keto acid, which is then converted to the main building block for sugar synthesis.

Now, Kian Tan and Dunwei Wang at Boston College in Chestnut Hill, have focused on the dark reactions for their inspiration, although utilise p-doped silicon nanowires to as a photocathode to capture photons initially. These entities convert solar energy to electrical energy readily and are very stable under the reaction conditions used subsequently. The trapped photons then release electrons from the nanowires, which transfer to the organic molecules put in place for the chemical processing; the starting materials are aromatic ketones.

The researchers explain that transfer of electrons from the photocathode "activates" these starting materials to attack and bind carbon dioxide. The whole process requires numerous steps to produce an alpha-hydroxy acid, which is converted to the ibuprofen and naproxen precursor with high selectivity and in high yield, the team says. The researchers used proton NMR spectroscopy to monitor the crude photoelectrochemical reaction mixture along the way, which demonstrated proof of principle and vindicated their high yield.

Selective synthesis

Overall, the reaction sequence resembles natural photosynthesis more closely than approaches taken by other teams to trap and bind carbon dioxide using sunlight. Those efforts have often been hindered by very poor selectivity, which is not an issue with the current approach, the team suggests. The new strategy might ultimately deliver the selectivity required for the production of complex organic intermediates and high-value fine chemicals. The setup does, however, require an additional electrical energy input, so although it demonstrates proof of principle further work now needs to be done to make it a wholly solar-powered process. Presumably, however, the electrical potential could be produced by an adjunct photovoltaic panel.

"We are currently working on reducing the external applied potentials in the hope to perform the reported reactions using light as the sole energy supply. We are also seeking to broaden the scope of the reactions for a wider variety of molecules," Wang told SpectroscopyNOW.