Recyclable polymer-stabilized nanocatalysts with enhanced accessibility for reactants

doi:10.1016/j.cattod.2012.02.003

Catalysis Today

Volume 193, Issue 1, 15 October 2012, Pages 200-206

https://doi.org/10.1016/j.cattod.2012.02.003 Get rights and content

Abstract

Immobilization of nanocatalysts in polymers and the use of such polymer-metal nanocomposites in catalytic reactions is interesting due to their high activity and recyclability. A nanocomposite material containing Pd-NPs has been prepared by Intermatrix Synthesis (IMS) technique by using a functional polymer (granulated ion exchange resin). The resulting nanocomposite material contains immobilized core–shell Pd@Co-NPs distributed mainly by the surface of the polymer. Such arrangement enhances the accessibility of the nanocatalysts for reagents in catalytic applications. It has been shown that the Suzuki-Miyaura coupling reaction proceeds under mild reaction conditions when using the new NC material.

Graphical abstract

Highlights

► In this study we synthetized both monometallic (Co) and bimetallic (Pd@Co) metal nanoparticles inside various functional polymers. ► The synthesis was made by using a simple and efficient Intermatrix Synthesis technique coupled with the Donnan exclusion effect. ► We examined the location of metal nanoparticles which is near the surface of nanocomposite, confirmed by SEM and TEM analysis. ► Core shell Pd@Co nanoparticles synthesis results in the formation of the easily recoverable and recyclable catalysts. ► Pd@Co nanocomposites were catalytically tested by Suzuki Cross-Coupling reaction.

Introduction

In the last decade, heterogeneous catalysts have attracted much interest because of their general advantages that have been boosted thanks to the use of nanomaterials due to their large surface area, high activity and recyclability [1], [2], [3]. Regarding the last property, magnetic catalysts present some outstanding advantages because they can be conveniently recovered by using an external magnetic field [2], [4], [5].

Up to now, several types of magnetic materials have been used including Fe oxides such as magnetite, hematite, maghemite and wüstite [6], [7]. Yet, for catalytic purposes, magnetic nanoparticles surface is often chemically functionalized binding molecular catalytic complexes because of the poor catalytic properties of the bare Fe oxides or other catalytic materials (e.g. Co) [2], [8], [9].

Magnetic aggregation and the need of functionalization do still hinder the application of magnetic nanoparticles in industry. Thus, searching for more suitable magnetic materials to overcome these restrictions is still a challenge for realizing practical catalytic applications. Quite the reverse, some nanometer-sized transition metals already display useful catalytic properties (e.g. Au, Pt, Pd) but do not possess magnetic properties [10], [11]. Hence, the use of core–shell nanoparticles (NPs) with a magnetic core and a catalytic shell is a win-win strategy which, moreover, would reduce the overall cost of the catalyst since the amount of expensive catalytic metal is lower as it is only located in the shell of the NP [12], [13], [14], [15]. Concerning the potential applications, Pd, Pt, Rh, and Au nanoparticles have proven to be very versatile as they are efficient and selective catalysts for several types of catalytic reactions, such as olefin hydrogenation and C–C coupling including Heck, Suzuki and Sonogashira reactions [16], [17], [18]. The C–C coupling reactions are considered the most important and essential reactions in organic synthesis. In fact, Suzuki reaction between aryl halides and arylboronic acids for preparation of unsymmetrical biphenyls is mainly focused on solid supported Pd systems [12].

A double security level (magnetic and steric) towards NPs leakage can be achieved by combining the inherent advantages of magnetic NPs with the use of polymeric supports that prevent NPs to escape while keep their catalytic efficiency [2], [19]. Polymer stabilized metal nanoparticles (PSMNPs) developed in our research group are a clear example of such approach [12], [13], [14], [20], [21], [22].

The development of the catalytically-active nanocomposites was focused on the following main points: (1) intermatrix synthesis (IMS) of the superparamagnetic Co-NPs coated with catalytically-active Pd shell (core–shell Pd@Co MNPs); (2) synthesis of nanocomposites with enhanced distribution of catalyst MNPs providing their maximum accessibility for reactants by using IMS coupled with Donnan exclusion effect technique; (3) evaluation the applicability of various reducing agents for IMS of MNPs; (4) evaluation of possible influence of the physical shape of the polymer (spherical granules or fibers) on the final properties of nanocomposite and some others [10], [23].

In this work we have focused our research in the development of highly active, insoluble and easily separable catalysts based on cross-linked polymers (granulated beads) and core–shell catalytic-magnetic NPs (Pd@Co). The desired distribution of NPs (mainly on the surface of the polymer) was achieved by using the Donnan exclusion driven IMS [12]. The characterization of the Pd@Co synthesized NPs and their application in Suzuki reaction catalysis is discussed and the effect of synthetic conditions in the catalysis yield is also reported. In our regards, the study of such catalytic nanocomposites bearing the aforementioned properties can enlarge de knowledge basis of catalytic supports what could be of great interest for Membrane Reactors field.

Section snippets

Materials

Metal salts, Co(NO₃)₂·6H₂O and Pd(NH₃)₄Cl₂, reducing agents, Na₂S₂O₄ and NaBH₄, phenylboronic acid 98% and 4-bromoacetophenone 98% (all from Aldrich, Germany), acids and organic solvents and K₂CO₃ (all from Panreac S.A., Spain) were used as received. Ion-exchange granulated polymers were received from Purolite Iberia S.A. (Spain). Carboxylic ion-exchangers are based on carboxylated polyacrylic acid cross-linked with divinylbenzene (macroporous type) when the sulfonic ones are based on

Synthesis and composition of catalytically active nanocomposites

The IMS of PSMNPs inside the matrix of functional polymer can be described by the combination of following consecutive steps: (1) loading of the functional groups of the polymer with MNP precursors (respective metal ions) by using ion-exchange reaction and (2) metal ion reduction inside the polymer leading to the MNPs formation. The reaction scheme, which includes the metal reduction when using NaBH₄ has been reported in our recent publication [12], [27]. The same reaction scheme can be also

Conclusions

The following conclusions can be derived from the results obtained in this study:

1.
Both monometallic (e.g., Co) and bimetallic (e.g., Pd@Co) MNPs can be easily synthesized inside various functional polymers (e.g., bearing either sulfonic or carboxylic functional groups) by using a simple and efficient IMS technique. The main experimental parameter, which influence the properties of the final polymer-metal nanocomposite are the concentration and the strength of the reducing agent.
2.
The IMS of MNPs

Acknowledgments

This work was supported by research grant MAT2006-03745, 2006–2009 from the Ministry of Science and Technology of Spain, which is also acknowledged for financial support of Dmitri N. Muraviev and Alexandr Shafir within the Program Ramon y Cajal. We also thank the Ministry of Science and Research for the research grants CTQ2008-05409-C02-01, CTQ2011-22649 and Consolider In-genio2010 CSD2007-00006. Also, DURSI-Generalitat de Catalunya is acknowledged for the grant 2009SGR-1441. Special thanks are

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Recyclable polymer-stabilized nanocatalysts with enhanced accessibility for reactants

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Synthesis and composition of catalytically active nanocomposites

Conclusions

Acknowledgments

J.Solid State Chem.

Nano Today

Prog. Cryst. Growth Charact. Mater.

Nano Today

React. Funct. Polym.

Catal. Today

Catal. Today

React. Funct. Polym.

J. Catal.

Solid State Commun.

Nanoparticles from Theory to Application

Adv. Synth. Catal.

Chem. Mater.

Anal. Chim. Acta

J. Am. Chem. Soc.

Chem. Eur. J.

J. Am. Chem. Soc.

Dalton Trans.

Pure Appl. Chem.

Nanoscale Res Lett.