nature of the SiO 2 shell made it possible for the reactants to reach the nanoparticles,
which enabled this hybrid catalyst to be used in a relay sequence where its dual
modes of reactivity could be showcased. For their demonstration, the authors used
4-nitrophenyl-β-glucopyranoside as the substrate, which was first hydrolyzed by
the glucosidase into glucose and 4-nitrophenol. Once the hydrolysis reaction was
complete, NaBH 4 was added to the reaction mixture, and then the Au NPs catalyzed
the reduction of the 4-nitrophenol into 4-hydroxyaniline.
The interfacing of Cu NPs and CalB for tandem catalysis purposes has also been
recently described by Ge and coworkers [55]. In their study, the authors demonstrated that the addition of copper sulfate to a phosphate buffer saline (PBS) solution
containing CalB led to the self-assembly of a three-dimensional copper phosphateenzyme composite. The driving force for this assembly process was the Cu(II) ions
that created a well-ordered enzyme-metal ion network. Interestingly, the authors
found that it was possible to re-assemble this copper phosphate-enzyme composite
into well-defined Cu/CalB NPs, by reducing the Cu(II) with NaBH 4 . Furthermore,
they showed that polyvinyl pyrrolidone (PVP) could be used as an additive in
this re-assembly process to control the degree of aggregation, which allowed
them to prepare smaller and more active Cu/CalB NPs. The dual catalytic function
of this Cu/CalB hybrid was demonstrated in a two-step chemoenzymatic cascade,
involving CalB-catalyzed hydrolysis of p-nitrophenyl butyrate and Cu-catalyzed
nitro-group reduction of the liberated 4-nitrophenol.
Very recently, Wang et al. reported on the design of another Pd/CalB hybrid [56],
which was obtained by co-immobilizing the two catalytic species within separate
compartments of the metal-organic framework (MOF) UiO-66-NH 2 . This particular
MOF can be conveniently synthesized by mixing ZrCl 4 , 2-aminoterephthalic
acid, and benzoic acid in DMF, and allowing the reaction mixture to incubate for
24 h at 120
C. A useful feature of this MOF is that the benzoic acid ligands are
exchangeable, which makes it a flexible platform for interfacing different catalysts
since its structure and chemical properties can be sequentially tuned to fit each
catalytic species that is being immobilized. In their synthesis of this Pd/CalB hybrid,
the authors started from the unmodified MOF UiO-66-NH 2 and generated Pd NPs
within its inner pores. Next, the authors performed a ligand exchange of the benzoic
acid for lauric acid, which dramatically changed the polarity of the MOF and made
it significantly more hydrophobic. The increase in the hydrophobic nature of the
MOF was not only of benefit for its catalytic applications in organic solvents,
but it also allowed for more efficient immobilization of the CalB. The authors
carried out this enzyme immobilization by simply stirring the Pd@UiO with CalB
at room temperature for 1 h. The CalB enzyme was too large to enter the pores of
the MOF, and instead it deposited on the outer surface of the support particles,
well-separated from the Pd NPs. The authors prepared two different Pd/CalB
hybrids that differed in the amount of lauric acid, and the most efficient one
named CalB-Pd@-UiO-LA50 exhibited good activity in a one-pot hydrogenation/
esterification cascade (Scheme 5). In this cascade, benzaldehyde could be efficiently
converted into benzyl hexanoate in >99% yield within 8 h at room temperature.
The recyclability of this hybrid also proved to be good, and it could be reused three
times with only a minor loss in activity of the CalB component.
252
O. Verho and J.-E. Bäckvall
which enabled this hybrid catalyst to be used in a relay sequence where its dual
modes of reactivity could be showcased. For their demonstration, the authors used
4-nitrophenyl-β-glucopyranoside as the substrate, which was first hydrolyzed by
the glucosidase into glucose and 4-nitrophenol. Once the hydrolysis reaction was
complete, NaBH 4 was added to the reaction mixture, and then the Au NPs catalyzed
the reduction of the 4-nitrophenol into 4-hydroxyaniline.
The interfacing of Cu NPs and CalB for tandem catalysis purposes has also been
recently described by Ge and coworkers [55]. In their study, the authors demonstrated that the addition of copper sulfate to a phosphate buffer saline (PBS) solution
containing CalB led to the self-assembly of a three-dimensional copper phosphateenzyme composite. The driving force for this assembly process was the Cu(II) ions
that created a well-ordered enzyme-metal ion network. Interestingly, the authors
found that it was possible to re-assemble this copper phosphate-enzyme composite
into well-defined Cu/CalB NPs, by reducing the Cu(II) with NaBH 4 . Furthermore,
they showed that polyvinyl pyrrolidone (PVP) could be used as an additive in
this re-assembly process to control the degree of aggregation, which allowed
them to prepare smaller and more active Cu/CalB NPs. The dual catalytic function
of this Cu/CalB hybrid was demonstrated in a two-step chemoenzymatic cascade,
involving CalB-catalyzed hydrolysis of p-nitrophenyl butyrate and Cu-catalyzed
nitro-group reduction of the liberated 4-nitrophenol.
Very recently, Wang et al. reported on the design of another Pd/CalB hybrid [56],
which was obtained by co-immobilizing the two catalytic species within separate
compartments of the metal-organic framework (MOF) UiO-66-NH 2 . This particular
MOF can be conveniently synthesized by mixing ZrCl 4 , 2-aminoterephthalic
acid, and benzoic acid in DMF, and allowing the reaction mixture to incubate for
24 h at 120
C. A useful feature of this MOF is that the benzoic acid ligands are
exchangeable, which makes it a flexible platform for interfacing different catalysts
since its structure and chemical properties can be sequentially tuned to fit each
catalytic species that is being immobilized. In their synthesis of this Pd/CalB hybrid,
the authors started from the unmodified MOF UiO-66-NH 2 and generated Pd NPs
within its inner pores. Next, the authors performed a ligand exchange of the benzoic
acid for lauric acid, which dramatically changed the polarity of the MOF and made
it significantly more hydrophobic. The increase in the hydrophobic nature of the
MOF was not only of benefit for its catalytic applications in organic solvents,
but it also allowed for more efficient immobilization of the CalB. The authors
carried out this enzyme immobilization by simply stirring the Pd@UiO with CalB
at room temperature for 1 h. The CalB enzyme was too large to enter the pores of
the MOF, and instead it deposited on the outer surface of the support particles,
well-separated from the Pd NPs. The authors prepared two different Pd/CalB
hybrids that differed in the amount of lauric acid, and the most efficient one
named CalB-Pd@-UiO-LA50 exhibited good activity in a one-pot hydrogenation/
esterification cascade (Scheme 5). In this cascade, benzaldehyde could be efficiently
converted into benzyl hexanoate in >99% yield within 8 h at room temperature.
The recyclability of this hybrid also proved to be good, and it could be reused three
times with only a minor loss in activity of the CalB component.
252
O. Verho and J.-E. Bäckvall