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Research at two institutions – the RIKEN Advanced Science Institute, Japan, and the A*STAR Institute of Chemical and Engineering Sciences (ICES), Singapore – has led to the development of processes to capture waste carbon dioxide from power plants before it is emitted into the atmosphere. These methods could be important in reducing carbon dioxide emissions in the future, besides providing useful products with economic potential. Over recent years there has been increasing concern about the levels of anthropogenic carbon dioxide emissions into the atmosphere primarily because of their potential to cause global climate change. It is expected that the main energy source over the next century or so will be dependent on fossil fuels, aggravating the greenhouse gas effect. Research has therefore focused on ways to reduce carbon dioxide emissions for sustainable economic development. Up to now carbon sequestration or the capture of carbon dioxide and its storage underground has been a main solution to the problem. The newly developed processes, however, capture the gas and convert it into a useable product. In Singapore carbon dioxide emissions from power plants are estimated at 14-28 million tonnes per year for the next decade. The island state has no geological formations and land availability for carbon storage is limited. So the novel, fast and energy efficient processes of carbon dioxide sequestration developed by ICES are of particular relevance. Silicate minerals that contain magnesium are used to capture carbon dioxide waste from power plants in two different processes. In the first process, magnetite, hydromagnesite, iron hydroxides and silica all of high purity are produced; these high-value products can be sold giving the carbon capture process economic as well as environmental benefits. In the second process, nesquehonite, iron hydroxides and coarse silica are by-products, all of which are valuable materials in the construction industry and can be used in landfill sites given Singapore’s extensive land reclamation projects. An estimated 18-35 million cubic metres of carbonate sand product could be produced per year which would help to meet the expected 1km2 expansion in landfill per year over the next decade. These new processes provide a promising approach to carbon sequestration because they consume little energy because the actions take place at ambient temperatures and there is no secondary contamination. The required infrastructure investment, moreover, is low. A demonstration unit is presently being designed and set up, and carbon dioxide and energy balance will be analysed. CATALYST FOR CARBON CAPTURE: Many of the carbon dioxide conversion techniques presently in use require expensive metal catalysts and involve protracted procedures. But recent work at the RIKEN Institute has led to the development of a catalyst system that uses waste carbon dioxide as a free source of carbon to produce useful organic materials. For the past two decades N-heterocyclic carbenes (NHCs), which are molecules, have been used as replacements for metal catalysts or attached to metals to influence their catalytic
UWE HERMANN
Capturing carbon in novel ways
CHEMISTRY
behaviour. NHCs added to the naturally abundant metal copper created a complex which catalysed the addition of carbon dioxide to boron ester molecules. It was hoped that the NHC-copper catalyst could do the same with aromatic compounds – hydrocarbon molecules that contain a benzene ring-like structure – made up of six carbon and six hydrogen atoms. The most efficient way of introducing the carbon dioxide molecule into the aromatic molecule is to replace a carbon to hydrogen (C-H) bond, but this bond is generally unreactive. However, the aromatic compound benzoxazole has a C-H bond situated between a nitrogen atom and an oxygen atom, which makes it easier to activate. The crystal structures of intermediate compounds in this process were analysed, and they showed that a carbon-copper bond was formed on the benzoxazole at the C-H bond site. The carbon dioxide molecule was inserted into this bond, releasing the copper atom and the catalyst was then regenerated. The binding of the NHC to copper in the catalyst was essential in providing electrons to activate the C-H bond of the benzoxazole and make the insertion of carbon dioxide into the aromatic molecule easier. Solid carboxylic acid derivatives and esters were produced in high yields which can be utilised in pharmaceuticals, agrichemicals and dyes. Further experiments will adjust the catalyst complex and reaction conditions, so that they can be used for other aromatic carbon molecules with less reactive C-H bonds.
For further information contact: Carbon sequestration processes using magnesium silicate minerals: Dr Bu Jie Institute of Chemical and Engineering Sciences Agency for Science, Technology and Research (A*STAR), Singapore Email: Bu_Jie@ices.a-star.edu.sg New catalyst for carbon capture: Dr Zhaomin Hou Organometallic Chemistry Laboratory RIKEN Advanced Science Institute, Japan Email: houz@riken.jp