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Y. Min et al.
a consequence, the catalyst displayed some catalytic activity when off. The authors
point out that the possible reason is the displacement of some surface ligands by
coordination of reagents or products, which leads to the loss of the photoresponsive
azobenzene-thiol ligand.
The photoisomerization of the azobenzene unit from the trans to the cis form under
UV-light irradiation was also exploited to generate another photoswitchable catalyst
[212]. In this case, the host–guest interaction of Au NP containing azobenzene units
with a β-cyclodextrin dimer was controlled by light. Under UV-light irradiation, the
cis form of the azobenzene weakened the binding with the β-cyclodextrin, allowing
the host to encapsulate the substrates and catalyze the reaction. Under visible light,
trans azobenzene forms the inclusion complex with the β-cyclodextrin, turning off
the catalytic reaction. Several catalytic cycles of the catalyzed ester hydrolysis could
be achieved with this system, and in addition, the reaction rate of this system was
higher than that of other Au-based catalysts.
5.4 Conclusion
A considerable effort of the catalysis community is focused on creating more active,
selective, and robust catalysts. In particular, in heterogeneous catalysis, both experimental and theoretical detailed studies have led to a rational catalyst development.
The knowledge obtained from these works make that some (but still few) heterogeneous catalysts are almost comparable to well-defined homogeneous systems, in
terms of understanding of their catalytic activity, and selectivity, thereby opening the
door for improvement. However, for most heterogeneous systems, important parameters such as NP size, inter-particle distance, and surface interactions with the support
at the atomic level are not well-controlled yet.
On the other hand, metal NP are nowadays well-accepted catalysts, which to a
certain extent are in the borderline of heterogeneous and homogeneous systems. In
line with the heterogeneous catalysts, in recent years the fine-tuning of their properties (size, shape, exposed crystallographic phase, surface ligands, among others)
permitted us to better understand and develop solutions for the catalysis of tomorrow.
These systems, however, suffer from a difficult separation (if not impossible for
large-scale processes). In that context, the formation of covalent NP assemblies
from a bottom-up approach constitutes an interesting way to produce heterogeneous
catalysts with controlled NP size, inter-particle distances, and surface coordination
chemistry.
In this chapter, the covalent assembly of the metal NP has been reviewed, focusing
on the methodologies of synthesis and their applications in catalysis. As far as the
synthesis of covalent assemblies of metal NP is concerned, while the first assemblies
were produced with simple methods, efficient and more complex systems have been
engineered and applied recently. In particular, the formation of reversible covalent
networks of metal NP is now possible by application of diverse stimuli. This concept,
which has been rarely employed in catalysis, definitively deserves further studies. If
Y. Min et al.
a consequence, the catalyst displayed some catalytic activity when off. The authors
point out that the possible reason is the displacement of some surface ligands by
coordination of reagents or products, which leads to the loss of the photoresponsive
azobenzene-thiol ligand.
The photoisomerization of the azobenzene unit from the trans to the cis form under
UV-light irradiation was also exploited to generate another photoswitchable catalyst
[212]. In this case, the host–guest interaction of Au NP containing azobenzene units
with a β-cyclodextrin dimer was controlled by light. Under UV-light irradiation, the
cis form of the azobenzene weakened the binding with the β-cyclodextrin, allowing
the host to encapsulate the substrates and catalyze the reaction. Under visible light,
trans azobenzene forms the inclusion complex with the β-cyclodextrin, turning off
the catalytic reaction. Several catalytic cycles of the catalyzed ester hydrolysis could
be achieved with this system, and in addition, the reaction rate of this system was
higher than that of other Au-based catalysts.
5.4 Conclusion
A considerable effort of the catalysis community is focused on creating more active,
selective, and robust catalysts. In particular, in heterogeneous catalysis, both experimental and theoretical detailed studies have led to a rational catalyst development.
The knowledge obtained from these works make that some (but still few) heterogeneous catalysts are almost comparable to well-defined homogeneous systems, in
terms of understanding of their catalytic activity, and selectivity, thereby opening the
door for improvement. However, for most heterogeneous systems, important parameters such as NP size, inter-particle distance, and surface interactions with the support
at the atomic level are not well-controlled yet.
On the other hand, metal NP are nowadays well-accepted catalysts, which to a
certain extent are in the borderline of heterogeneous and homogeneous systems. In
line with the heterogeneous catalysts, in recent years the fine-tuning of their properties (size, shape, exposed crystallographic phase, surface ligands, among others)
permitted us to better understand and develop solutions for the catalysis of tomorrow.
These systems, however, suffer from a difficult separation (if not impossible for
large-scale processes). In that context, the formation of covalent NP assemblies
from a bottom-up approach constitutes an interesting way to produce heterogeneous
catalysts with controlled NP size, inter-particle distances, and surface coordination
chemistry.
In this chapter, the covalent assembly of the metal NP has been reviewed, focusing
on the methodologies of synthesis and their applications in catalysis. As far as the
synthesis of covalent assemblies of metal NP is concerned, while the first assemblies
were produced with simple methods, efficient and more complex systems have been
engineered and applied recently. In particular, the formation of reversible covalent
networks of metal NP is now possible by application of diverse stimuli. This concept,
which has been rarely employed in catalysis, definitively deserves further studies. If