Designing small-molecule catalysts for CO2 capture


One method for CO2 capture is to dissolve CO2 in water to form carbonic acid. This reaction (CO2+H20→H2CO3(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 CO2 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 CO2 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(−Ea/kT), where Ea 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.

Energy Procedia, 4, 817-823 (2011)
Heather J. Kulik
Heather J. Kulik
Professor of Chemical Engineering and Chemistry