Catalysis by Metallic Nanoparticles in Solution: Thermosensitive Microgels as Nanoreactors

Abstract

Metallic nanoparticles have been used as catalysts for various reactions, and the huge literature on the subject is hard to overlook. In many applications, the nanoparticles must be affixed to a colloidal carrier for easy handling during catalysis. These "passive carriers" (e.g., dendrimers) serve for a controlled synthesis of the nanoparticles and prevent coagulation during catalysis.Recently, hybrids from nanoparticles and polymers have been developed that allow us to change the catalytic activity of the nanoparticles by external triggers. In particular, single nanoparticles embedded in a thermosensitive network made from poly(N-isopropylacrylamide) (PNIPAM) have become the most-studied examples of such hybrids: Immersed in cold water, the PNIPAM network is hydrophilic and fully swollen. In this state, hydrophilic substrates can diffuse easily through the network, and react at the surface of the nanoparticles. Above the volume transition located at 32°C, the network becomes hydrophobic and shrinks. Now hydrophobic substrates will preferably diffuse through the network and react with other substrates in the reaction catalyzed by the enclosed nanoparticle. Such "active carriers", may thus be viewed as true nanoreactors that open new ways for the use of nanoparticles in catalysis. 3In this review, we give a survey on recent work done on these hybrids and their application in catalysis. The aim of this review is threefold: We first review hybrid systems composed of nanoparticles and thermosensitive networks and compare these "active carriers" to other colloidal and polymeric carriers (e.g., dendrimers). In a second step we discuss the model reactions used to obtain precise kinetic data on the catalytic activity of nanoparticles in various carriers and environments. These kinetic data allow us to present a fully quantitative comparison of different nanoreactors. In a final section we shall present the salient points of recent efforts in the theoretical modeling of these nanoreactors. By accounting for the presence of a free-energy landscape for the reactants' diffusive approach towards the catalytic nanoparticle, arising from solvent-reactant and polymeric shell-reactant interactions, these models are capable of explaining the emergence of all the important features observed so far in studies of nanoreactors. The present survey also suggests that such models may be used for the design of future carrier systems adapted to a given reaction and solvent. 4

Tuning catalysis of boronic acids in microgels byin situreversible structural variations

“…37 While the interpretation of the apparent activation parameters is complicated by the fact that these represent the combination of the diffusion and the catalysis processes, for the reaction catalyzed by the PBA@PM upon bubbling with N 2 gas, the observation of the smaller E a (theoretically indicating a lower energy required to reach transition state of reactants, thereby a lower energy required to initiate the reaction) but smaller A (theoretically indicating a smaller total number of collisions per minute) when adding glucose is consistent with a model between diffusional control of the reaction and control by the reaction at the catalytic sites. 37,40 In agreement with the observation on the inuence of the [Glu] on k (Fig. 6a) discussed above that could be associated with the counterbalance of the two effects, the much denser polymer network (to some extent) of the collapsed microgels imposes a higher diffusional resistance, [38][39][40] and the formation of more boronate anions that is prior to trigonal boronic acids for facilitating the nucleophilic activation should prefer the aza-Michael addition.…”

“…37,40 In agreement with the observation on the inuence of the [Glu] on k (Fig. 6a) discussed above that could be associated with the counterbalance of the two effects, the much denser polymer network (to some extent) of the collapsed microgels imposes a higher diffusional resistance, [38][39][40] and the formation of more boronate anions that is prior to trigonal boronic acids for facilitating the nucleophilic activation should prefer the aza-Michael addition. 3,4,11 More importantly, in comparison with the apparent activation parameters obtained for the reaction upon bubbling with N 2 gas, both the smaller E a and A values are obtained for the reaction upon bubbling with CO 2 gas; both the collapsed gel and trigonal boronic acids can be envisaged to hinder the reaction to move.…”

“…This non-linear relationship can be assigned to the formation of glucose-bis-boronate complexes(Fig. 4), consequently the counterbalance of the deswelling of microgels (rendering an inhibiting effect) and the increased amount of boronate anions (rendering a promoting effect here):3,4,[38][39][40] at low [Glu]s that the microgels are in relatively…”

Planet–Satellite Nanostructures Based on Block Copolymer‐Surfactant Nanoparticles Surface‐Decorated with Gold and Silver: A New Strategy for Interfacial Catalysis

“…In the last few years, an expressive number of reports dealing with polymer‐metal NP hybrid nanostructures have reached the literature in the field of catalysis and important contributions were found in a variety of catalyzed chemical reactions . The present metal–polymer–surfactant conjugates represent a relevant example of using nanotechnology tools by combining advantageous properties of an assembly structure to those of functional materials such as the metal NP catalysts.…”

Influence of patch size and chemistry on the catalytic activity of patchy hybrid nonwovens

“…22 In addition, metal-organic frameworks (MOFs) have been employed as versatile hosts for metal NPs. [23][24][25] Besides these examples polymeric supports, like nanocellulose, 26 micro-or nano-porous polymers, [27][28][29][30][31] microgels, 32,33 polyelectrolyte brushes, [34][35][36] dendrimers and unimolecular micelles, 37,38 as well as polymer micelles, [39][40][41][42][43][44] are highly attractive for NP stabilization, as their properties can be easily tailored to the demands of a specic catalytic application (NP stabilization, selectivity, and responsivity). In a different approach, polymer nanobers produced by electrospinning 45,46 or using polyelectrolyte brushes 47 can be used as a sacricial support for NPs, which can be removed by subsequent calcination.…”