Nanoreactors molecular catalysts

Lebedeva MA, Chamberlain TW, Schroder M, Khlobystov AN. New pathway for heter-ogenization of molecular catalysts by noncovalent interactions with carbon nanoreactors. Chem Mater 2014 26 6461-6. 

In this chapter, we summarize the recent advances in the development of nanaoreactors based on porous solid materials for chemical reactions, including the general methods for the fabrication of typical porous materials, (mesoporous silicas (MSs), carbon nanotubes (CNTs), and the MOFs), the assembly of the molecular catalysts in the cavities and pores of the porous materials, the chemical reactions in the porous-material-based nanoreactors, and some important issues concerning the porous-material-based nanoreactor, such as the pore confinement effect, the isolation effect, and the cooperative activation effect We close this chapter with an outlook of the future development of the nanoreactors. 

The traditional methods, such as hydrothermal synthesis, impregnation, and chemical vapor deposition (CVD), can be employed to incorporate heteroatom and metal/metal oxide nanoparticles as catalysts into the nanopores of MSs. The advances in this area have been well summarized in recent reviews [35 - 38]. Herein, we will mainly focus on the assembly of molecular catalysts in the nanopore of MSs and MOFs. Using the molecular chiral catalyst as a model, we will address the general strategies for incorporating molecular catalysts in the nanoreactor, including the covalent and noncovalent bonding methods. 

Incorporating Chiral Molecular Catalysts In Nanoreactors through Covalent-Bonding Methods... 

Immobilizing Chiral Molecular Catalysts in Nanoreactors through Noncovalent... 

Encapsulating Molecular Catalyst in Nanoreactors by Reducing the Pore Entrance Size... 

Scheme 10.9 General process for encapsulating molecular catalysts into the nanocages of MS and the chemical reactions in the nanoreactor catalyzed by...

The encapsulation method uses preformed metal complexes and can thus avoid the formation of undesired species in the solid matrix. This strategy could be basically applied for the encapsulation of various kinds of molecular catalysts in the nanoreactor. 

Through covalent and noncovalent bonding methods, different kinds of molecular catalysts could be incorporated into MSs and MOFs. These porous materials with the incorporated molecular catalyst could catalyze various kinds of chemical reactions. A review of all the related works is impossible and not necessary in this chapter. We only review some representative examples for demonstrating the unique properties of the nanoreactor for catalytic reactions, including the pore confinement effect, the enhanced cooperative activation effect, and the isolation effect, as well as the microenvironment and the porous structure engineering of the nanoreactor and the catalytic nanoreactor engineering. 

In addition to the enhanced cooperative activation effect of the nanoreactor, the isolation effect could also be expected in the confined nanospace if the diameter of nanopore is similar to the size of the molecular catalysts, because the limited nanospace could restrict the free movement of the molecular catalysts. Two issues relevant to the isolation effect of the nanoreactor, namely selectivity control in organic reactions and inhibition dimerization of the molecular catalysts, will be discussed. 

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. 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

Later, more sophisticated supramolecular complexes capable of improved molecular recognition started to be studied. New supramolecular approaches to constmct synthetic biohybrid catalysts were developed [190]. An example is the giant amphi-philes, formed by a (hydrophilic) enzyme headgroup and a synthetic apolar tail. These biohybrid amphiphilic compounds self-assemble in water to yield enzyme fibers and enzyme reaction vessels (nanoreactors [202]). 

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