Engineering of Nanoreactors

The microenvironment can greatly influence the performance of the nanoreactor. First, it can affect the diffusion rate of the reactants and products in the nanoreactor. Second, the nanoreactor with a specific microenvironment may enrich a specific reagent, and thus the concentration of substrates and products inside the 

As we mentioned in Section 10.4.1, Mn(Salen) immobilized in a hydrophobic nanopore affords much higher activity and even enantioselectivity than that in a hydrophilic nanopore in the asymmetric epoxidation of olefins [80]. This is due to the increased diffusion rate of the hydrophobic substrate into the nanopore with hydrophobic surface properties. Thus, the surface modification of the nanoreactor, depending on the polarity of the reactants and products, may be an efficient method for increasing the activity of the immobilized molecular catalysts. 

We reported the incorporation of (lJ ,2J )-diaminocyclohexane in hydrophobic and hydrophilic nanopores by the co-condensation of N-[(triethoxysilyl)propyl]-(-)-(ll ,2R)-diaminocyclohexane, respectively, with (MeO)3SiCH2CH2Si(OMe)3, (BTME) and TEOS in basic media [120]. After complexing with [Rh(cod)Cl]2, the chiral catalyst in the hydrophobic nanopore affords 96% conversion with 23% ee in the ATH of acetophenone using i-PrOH as the hydrogen source, while the chiral catalyst in hydrophilic nanopore only shows 48% conversion with 14% ee under identical conditions. This is probably due to the specific adsorption and physical properties of the mesoporous network bridged with ethane groups, particularly the hydrophobic properties. 

The diffusion of reactants and products in porous-material-based nanoreactor could be greatly affected by the surface properties, which in turn could influence the catalytic activity and even selectivity of a chemical reaction taking place in the confined nanospace. Generally, organic molecules are hydrophobic and the silica-based mesoporous nanopores are hydrophihc. The difficulty in the diffusion of reactants and products in hydrophobic nanopores may reduce the reaction conversions [123]. Thus, the surface hydrophobic modification of the nanopore may benefit fast diffusion of the substrate, which may, in turn, contribute to the improved activity. When a reaction involves incompatible substrates, such as oil and water, the amphiphilic modification of the nanopore microenvironment is a smart strategy because the amphiphilic nanopore should provide a microenvironment 

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