Designing small-molecule catalysts for CO 2 capture

Sergio E. Wong, Edmond Y Lau, Heather J. Kulik, Joseph H. Satcher, Carlos Valdez, Marcus Worsely, Felice C Lightstone, Roger Aines

Research output: Chapter in Book/Report/Conference proceedingConference contribution

10 Scopus citations

Abstract

One method for CO 2 capture is to dissolve CO 2 in water to form carbonic acid. This reaction (CO 2+H 2O→ H 2CO 3(aq)) is remarkably slow but is catalyzed in biological systems by an enzyme called carbonic anhydrase (CA). The catalyzed reaction is diffusion limited and occurs at near neutral pH. The enzyme catalytic center is composed of a Zn(II) ion that is coordinated by 3 histidine residues and an axial water/hydroxyl group. A nucleophilic attack by the hydroxyl group on the CO 2 molecule is the first step in the reaction mechanism. Cu(II), Hg(II), Cd(II), Ni(II), Co(II) and Mn(II) can bind the CA binding site and substitute the zinc ion; however, only Co(II) yields rates comparable to Zn(II). Unfortunately, an enzyme, such as carbonic anhydrase, is not amenable for industrial applications where a wide range of physico-chemical conditions exist. Enzymes are vulnerable to large pressures, high temperature, and high ionic strength. Efforts to isolate the key structural features responsible for catalysis led to the development of small-molecule mimetics of the CA catalytic site. These mimetics can, in turn, be used as catalyst for CO 2 sequestration. Two of the fastest catalyst are the cyclic molecules: 1, 4, 7, 10-tetraazacyclododedacane and 1, 5, 9-triazacyclododedacane (both complexed with a Zn(II) ion). Nitrogen atoms in these cyclic molecules mimic the imidazole nitrogens of the CA active site. It is possible to examine the energetics of these compounds using transition state theory for the purposes of designing more efficient catalysts. Transition state theory predicts that the reaction rate constant is proportional to exp(-E a/kT), where E a denotes the activation energy, k the Boltzmann constant, and T the temperature of the reaction. The activation energy is the energetic cost of forming the reaction transition state from the reactants. Ab initio calculations can yield the activation energy, which can, in principle, be used as a design metric for more efficient catalysts. Using this approach, the difference in kinetic rate constant between the tetra- and tri-aza dodecane catalysts can be determined. Furthermore, the rates of the corresponding Co(II) catalysts were explored. Our data suggests this is a viable method for the design of inorganic small-molecule catalysts.

Original languageEnglish (US)
Title of host publicationEnergy Procedia
Pages817-823
Number of pages7
Volume4
DOIs
StatePublished - 2011
Externally publishedYes
Event10th International Conference on Greenhouse Gas Control Technologies - Amsterdam, Netherlands
Duration: Sep 19 2010Sep 23 2010

Other

Other10th International Conference on Greenhouse Gas Control Technologies
CountryNetherlands
CityAmsterdam
Period9/19/109/23/10

Keywords

  • Catalyst
  • CO absorption
  • CO cycle

ASJC Scopus subject areas

  • Energy(all)

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