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The research is focused on CO2 capture, with a focus on post-combustion capture. The capture of CO2 from exhaust gas streams is one of the foremost prospects for reducing greenhouse gas emissions. The team is actively designing new porous solid materials at the sub-nanometer scale to trap CO2 from power plants or mixed in with natural gas from unconventional gas reservoirs. These crystalline porous structures known as metal organic frameworks (MOFs) will be used to trap and release CO2 in the presence of water vapor with low cost and high efficiency.
The main goal is to generate water stable MOFs that can sorb CO2 in the presence of water vapor and nitrogen. After development of quality materials with exceptional properties, Shimizu hopes to carry the research beyond the bench top to an actual system; the ability of MOFs to interface at an actual power plant will be examined. Other goals include continuous development of other gas sorbents (in addition to CO2) for many industrial applications. For example, sorbents can be developed for oxyfuel combustion (oxygen and nitrogen separation), natural gas purification, and biogas purification.
Current capture technology typically uses the formation of a chemical bond between an amine molecule and CO2, in an aqueous solution. Regeneration of this sorbent requires heating both the amine and the solution to over 100°C. The ‘solid sponge’ developed by Shimizu’s team will directly capture CO2 from the combusted gas stream; an aqueous solution will no longer be necessary to capture CO2. Due to this modification, much less heat is necessary to release the CO2 binding component, and this will result in large savings in energy and productivity at the regeneration step. At a power plant, this could save 50% in capture expense. Computer modeling will also be used to help design these MOFs, allowing the team to observe CO2 capture at the molecular level. The MOFs which have the highest affinities for CO2 – versus water and nitrogen gas- in a given pore will be selected.
This technology is applicable to both the oil and gas industry and beyond. Currently, Shimizu’s group is targeting post combustion CO2capture materials; however, the research can easily transfer to broader problems. For example, other areas of potential gas separation application include, pre-combustion capture after coal gasification, O2 separation for oxyfuel combustion, and biogas purification.
Physical adsorption mechanisms of a selective sorbent will be translated into dollar savings for CO2 capture; a material which is capable of low pressure CO2 sorption in the presence of water will be game changing. The technological impact of the work is enormous, because cheaper CO2 capture will have a global market. A better understanding of MOF agents used for gas separations will be profound, and methods developed with this research could be extended to any targeted gas separation process.
Amines are a conventional technology used to uptake and attract CO2, also known as amine scrubbing. Shimizu’s group made nanoporous materials lined with amine groups, and could clearly observe the structure of the CO2 bound to the amine and the amount of CO2 taken up. The location of the where the CO2 sat in the amine was discovered; additionally, with computation, the role of every interaction involved in CO2 binding and stabilization was determined. To date, this is the most well understood material involved in CO2 binding. This research was published in Science, and has been widely cited.
Figure: CO2 binding within an amine-functionalized nanoporous solid determined by experiment (left) and modeling (right)
Phosphonate monoesters as carboxylate-like linkers for metal organic frameworks, Iremonger, S. S.; Liang, J.; Vaidhyanathan, R.; Martens, I.; Shimizu, G. K. H., Daff, T. D.; Aghaji, M. A.; Yeganegi, S.; Woo. T. K., J. Am. Chem. Soc. 2011, 133, 20048.
A permanently porous van der Waals solid by using phosphonate monoester linkers in a metal organic framework, Iremonger, S. S.; Liang, J.; Vaidhyanathan, R.; Shimizu, G. K. H., Chem Commun. 2011,47, 4430.