Metal catalysis with nanostructured metals supported inside strongly acidic cross-linked polymer frameworks: Influence of reduction conditions of AuIII-containing resins on metal nanoclusters formation in macroreticular and gel-type materials
Graphical abstract
TEM pictures of Au0 nanoparticles in the beads of ion exchangers Amberlyst BD20 after reduction of the partially dry materials with gaseous hydrogen.
Highlights
► Synthesis of gold nanoclusters inside strongly acidic ion exchangers. ► Metallated resin beads are reduced with H2 in the absence of added water. ► Metallated resin beads are also reduced with NaBH4 in water. ► Metal distribution inside the resin beads is homogeneous when the reductant is H2. ► Metal distribution inside the resin beads is peripheral when the reductant is NaBH4.
Introduction
Over the years, a variety of transition metals have been successfully immobilized on functional resins. Such resin-supported metal catalysts are used in the production of many industrially important materials including, solvents such as MIBK (methylisobutyl ketone), oxygenate petrol additives such as TAME (t-amylmethylether), 1,2-propanediol and certain fine organic chemicals [1]. Nanostructured Au0 supported on typically inorganic, non-reducible metal oxides has become a catalytic center witnessing an interest in the catalysis Community comparable to that historically devoted to nano-structured Pd0 and Pt0 in the second half of the past century [2]. Given our established attention to catalytically relevant materials based on nanostructured metal(0) supported on cross-linked functional polymers (CFPs) [3], [4] we decided to focus our attention on Au0/functional resin nanocomposites.
Considerable attention to materials conceptually related to these ones has been recently paid by Japanese researchers like Ishida and Haruta [5] who synthesized Au0 nanoclusters stabilized by “surface functional groups on an anion-exchange resin” and Kobayashi and co-workers [6] who synthesized polystyrene-based “copolymer-microencapsulated gold(0) nanocatalysts”. Apart from the content of these contributions, it should be stressed that the catalytic chemistry of polymer-supported Au0 materials is today practically terra incognita and we believe that the availability of further easily accessible Au0/functional resin materials will be a useful circumstance for boosting the interest in Au0 catalytic chemistry occurring inside these non-conventional supports.
In this paper, we report on a systematic investigation on nanostructured Au0 stemming from the dispersion, followed by reduction, of a very convenient hydrolytically stable cationic AuIII precursor [7], [Au(en)2]Cl3, inside four commercially available sulfonic resins, with the aim of obtaining potentially useful oxidation and reduction catalysts [2b].
We report herein on the synthesis, micro- and nanometer scale structure, XRMA and TEM characterization of Au0/CFP nanocomposites obtained upon acting with molecular hydrogen on the metallated “partially dry resins” (see later) in the absence of any added liquid phase and with aqueous NaBH4, i.e. a reducing system that we have been using for fifteen years in our endeavor through the “world” of M0/CFP nanocomposites [4].
Section snippets
Materials and Instruments
For this study we obtained commercially available strongly acidic ion exchangers manufactured by Dow Chemicals, USA (Amberlyst) and Lanxess, Germany (Lewatit) of following types: two gel-types (Amberlyst BD20 and Lewatit K1221) and two macroreticular types (Amberlyst EBD400, Lewatit K2621). The polymers were supplied in acidic form and before use they were thoroughly washed, with 2 l of deionized water per 100 g of material, filtered on a Gooch filter and finally dried at 55 °C in a ventilated
Morphology of the employed ion exchanger supports
Swollen-state morphology of the supports selected for this study was characterized using ISEC [10] and the results are shown in Table 1. ISEC provides a thorough description of the overall morphology of a macromolecular material in terms of pores diameter, pore volumes, polymer chain density and polymer gel fraction volume.
The gel-type resins exhibit only porosity generated by swelling of their polymer matrix which is described as set of discrete fractions, each characterized by single average
Conclusions
Functional sulfonic acid resins -SO3H hosting AuIII ions can be easily transformed into Au0/-SO3H nanocomposites by action of molecular hydrogen in the “partially dry state” of the polymer support. In fact, action of gaseous dihydrogen on AuIII/-SO3H materials permits the formation of Au0/-SO3H only if partially hydrated -SO3H matrices are employed. Metal distribution inside the polymer beads appears to be homogeneous (Fig. 2), under these conditions, and metal nanoclusters size turns out to be
Acknowledgements
This work was partially supported by the Academy of Sciences of the Czech Republic (Grant No. M200720902) and by CNR, Italy. Donation of Amberlyst and Lewatit resins by Dow Chemicals, USA and Lanxess, Germany, respectively, is gratefully acknowledged.
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