(603d) A Nature-Inspired Approach to the Confinement and Control of Catalytic Active Sites
The location of the active site in a catalytic material, specifically the environment that surrounds the active site, often directly influences the activity and selectivity of a catalyst. The best examples of this environmental control of the active sites are found in enzymatic systems where the active site is located in an environment in which there is control of electrostatic interactions, precise placement of chemical functional groups, and control of hydrophobicity/hydrophilicity, as some common examples.
Using inspiration from the GroEL/GroES system in E. coli, a catalyst has been synthesized where rhodium complexes have been immobilized inside of a mesoporous support material, such as SBA-15. In catalytic tests of 1-octene hydroformylation, these nano-confined sites in an inorganic support demonstrate an increased degree of activity and stability, which provide insight into structure-function relationships in catalysis. Additional catalytic examples of using nano-confined Rh complexes will also be presented.
Another bio-inspired method of nano-confinement is demonstrated using a shell of organic ligands as the confining medium around the active site. Examples using calixarene ligands bound to small gold clusters where the accessibility to the surface, cluster stability against agglomeration, and electronic tunablity are controlled by the bound calixarene ligand are demonstrated. [2, 3] Additional examples of the synthesis, characterization, and catalytic activity of noble metal clusters confined in shells of different organic ligands including the use of enzymes as ligands will be discussed.  These approaches to nano-confinement of active sites enable a greater understanding of the site requirements for reactions as well as the development of novel approaches to control catalysis on metal cluster surfaces.
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2. N. de Silva, et al. Nature Chem. 2010, 2, 1062.
3. M. M. Nigra, et al. Dalton Trans. 2013, 42, 12762.
4. M. M. Nigra, et al. Catal. Sci. Technol. 2013, 3, 2976.