244
A. Dhakshinamoorthy and H. Garcia
The main reasons for this excellent catalytic performance are the small particle size of
the incorporated metal NPs, commonly about 3 nm or below, the favorable diffusion
of substrates and reagents to the active sites, due to the high porosity of the MOFs,
and the remarkable stability of the metal NP/MOF composite due to the structural
robustness of the host that thwarts agglomeration of embedded NPs.
In some cases, the occurrence of a synergy between the host and the metal NPs
has been proposed. It has also been determined that the catalytic activity, selectivity
and stability of the metal NP/MOF composite can be improved by the presence of
substituents on the aromatic ring. These substituents can establish additional interactions with the metal NPs in the cavities, providing additional stability, while still
leaving many atoms of the NP, mainly at places where no linkers are present, free to
interact with substrates and reagents. Additionally, the presence of substituents on
the linker can modify the polarity of the internal spaces and can tune the dimensions
of the pores. It is clear that the influence of substituents present on the organic linkers
and the influence of modification of the nodes are factors that deserve further study
in order to determine the performance gain that can be reached by applying these
strategies.
Although the number of examples of the use of metal NPs inside MOFs is likely
to continue growing in the near future, by exploring new reactions as well as other
hosts based on highly porous and robust MOFs, a vast potential of these metal
NPs/MOF composite catalysts remains to be exploited for the promotion of tandem
reactions. Tandem reactions in where two or more elementary reactions occur in a
single step typically require the action of catalytic sites of different nature. Metal
NPs encapsulated within MOFs can offer the combination of the catalytic activity
of the metal NP with that of other sites located at the metal nodes or at the organic
linker. Metal nodes having exchangeable positions are Lewis acid centers, while
substituents on the linker can be either Brönsted acids or bases. The combination of
all these sites on the encapsulated metal NP and at the MOF lattice can easily renders
a multifunctional catalyst that could be suitable to promote tandem reactions.
Thus, metal NPs included in MOFs can be suitable solid catalysts, not only for
those reactions promoted by metal NPs in other high surface area supports, but they
can also act as multifunctional catalysts and in this way promote one-pot multiple
reactions.
Acknowledgements AD thanks the University Grants Commission, New Delhi, for the award of
an Assistant Professorship under its Faculty Recharge Programme. AD also thanks the Department
of Science and Technology, India, for the financial support through Extra Mural Research Funding
(EMR/2016/006500). Financial support by the Spanish Ministry of Economy and Competitiveness
(Severo Ochoa and RTI2018-098237-B-C21) and Generalitat Valenciana (Prometeo 2017-083) is
gratefully acknowledged.
A. Dhakshinamoorthy and H. Garcia
The main reasons for this excellent catalytic performance are the small particle size of
the incorporated metal NPs, commonly about 3 nm or below, the favorable diffusion
of substrates and reagents to the active sites, due to the high porosity of the MOFs,
and the remarkable stability of the metal NP/MOF composite due to the structural
robustness of the host that thwarts agglomeration of embedded NPs.
In some cases, the occurrence of a synergy between the host and the metal NPs
has been proposed. It has also been determined that the catalytic activity, selectivity
and stability of the metal NP/MOF composite can be improved by the presence of
substituents on the aromatic ring. These substituents can establish additional interactions with the metal NPs in the cavities, providing additional stability, while still
leaving many atoms of the NP, mainly at places where no linkers are present, free to
interact with substrates and reagents. Additionally, the presence of substituents on
the linker can modify the polarity of the internal spaces and can tune the dimensions
of the pores. It is clear that the influence of substituents present on the organic linkers
and the influence of modification of the nodes are factors that deserve further study
in order to determine the performance gain that can be reached by applying these
strategies.
Although the number of examples of the use of metal NPs inside MOFs is likely
to continue growing in the near future, by exploring new reactions as well as other
hosts based on highly porous and robust MOFs, a vast potential of these metal
NPs/MOF composite catalysts remains to be exploited for the promotion of tandem
reactions. Tandem reactions in where two or more elementary reactions occur in a
single step typically require the action of catalytic sites of different nature. Metal
NPs encapsulated within MOFs can offer the combination of the catalytic activity
of the metal NP with that of other sites located at the metal nodes or at the organic
linker. Metal nodes having exchangeable positions are Lewis acid centers, while
substituents on the linker can be either Brönsted acids or bases. The combination of
all these sites on the encapsulated metal NP and at the MOF lattice can easily renders
a multifunctional catalyst that could be suitable to promote tandem reactions.
Thus, metal NPs included in MOFs can be suitable solid catalysts, not only for
those reactions promoted by metal NPs in other high surface area supports, but they
can also act as multifunctional catalysts and in this way promote one-pot multiple
reactions.
Acknowledgements AD thanks the University Grants Commission, New Delhi, for the award of
an Assistant Professorship under its Faculty Recharge Programme. AD also thanks the Department
of Science and Technology, India, for the financial support through Extra Mural Research Funding
(EMR/2016/006500). Financial support by the Spanish Ministry of Economy and Competitiveness
(Severo Ochoa and RTI2018-098237-B-C21) and Generalitat Valenciana (Prometeo 2017-083) is
gratefully acknowledged.