270994 Boosting the Catalytic Performance of Complexes Confined in Mesopores by Controlling Pore Surface Curvature and Chemistry
The catalytic properties of large, homogeneous catalytic complexes, including enzymes, can be considerably enhanced by immobilizing them on the surface of a mesoporous support of controlled pore surface curvature [1, 2]. Both the geometrical and the chemical properties of the nanopore surface are important.
To illustrate this phenomenon, two distinct classes of catalysts, supported on SBA-15 mesoporous silica with well-controlled pore diameter, will be presented: (1) lysozyme and myoglobin for, respectively, a hydrolysis and an oxidation reaction; (2) a Rh-diphosphine complex based on xantphos-type ligands for the selective hydroformylation of 1-alkenes to linear aldehydes.
The catalytic activity of electrostatically adsorbed lysozyme or myoglobin on the surface of SBA-15 increases when the pore diameter is decreased, up to several times that of the free enzyme in aqueous solution. It is highest when the pore diameter is only slightly larger than the size of the enzyme, as long as there are no diffusion limitations. The immobilized enzymes are also more chemically stable to degrading environments, such as low pH and the presence of pepsin. ATR-FTIR spectroscopy shows how the secondary structure of the enzyme is protected by confinement in the mesopores of SBA-15. This is in contrast to enzymes supported on a flat silica surface or on silica nanoparticles, for which activity decreases. Propylating the surface of SBA-15 renders it hydrophobic, severely affecting the structure of lysozyme and myoglobin, and leading to a sharp decrease in activity.
Hydroformylation of linear alkenes using a Rh-diphosphine complex anchored on disordered, amorphous silica may largely preserve selectivity, but at the expense of a significant drop in activity; on the surface of ordered SBA-15, with controlled local surface curvature, both activity and selectivity can be preserved, while catalyst stability is improved by avoiding leaching and isolating the active sites .
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