Generation of Size‐Controlled Pd⁰ Nanoclusters inside Nanoporous Domains of Gel‐Type Resins: Diverse and Convergent Evidence That Supports a Strategy of Template‐Controlled Synthesis

Abstract

In the realm of supported metal catalysis, the metal component is usually present as nanoparticles dispersed on the surface of suitable metal oxides [1] or on active carbon. [2] Synthetic procedures are normally directed to the generation of size-controlled metal nanoclusters, whose circumstances become mandatory when reactions to be catalyzed are "structure sensitive". [3] A paramount example of the necessity of size control is given in the area of Au 0 catalytic chemistry, specifically, the low-temperature oxidation of carbon monoxide in the presence of excess dihydrogen and the case of water gas shift reaction. [4] In general, size control has been achieved by: directly manufacturing metal-oxide-supported catalysts; [5] by the generation of kinetically stabilized metal nanoclusters in the liquid phase [6] and subsequent transfer of the protected nanoclusters on to suitable supports; [7] by generating metal nanoclusters inside isoporous inorganic materials such as mesoporous silicas. [8] In addition to the still-common practice of employing inorganic supports (and active carbon), a few examples of metal catalysts supported on functional resins are cited in the realm of chemical processing (such as industrial synthesis of methylisobutylketone, chemoselective hydrogenation of diolefins, acetylenes, carbonyl compounds in the presence of isobutene) and very efficient removal of dioxygen (down to the ppb levels) from industrial waters upon hydrogenation. [9] A considerable advantage of these catalysts is that they are multifunctional, [9] and may allow size control in the generation of metal nanoclusters inside polymer frameworks after the metallation-reduction steps (Figure 1).[10]Herein, we report on three independent, convergent pieces of structural evidence of this template-controlled synthesis strategy to obtain size-controlled metal nanoclusters suitable for synthesizing resin-supported metal catalysts. In fact, in the course of our long-standing interest in the generation and catalytic exploitation of resin-supported Pd 0 nanoclusters [9] we discovered in 1998 [11] and subsequently confirmed in 2001 [12] that gel-type, lightly cross-linked resins (2-8 % mol, in the absence of any porogenic agent [9] ) are suitable templates for the generation of 2-4 nm Pd 0 nanoclusters. The strategy for this achievement is the dispersion of individual "Pd 2+ " centers in the interior of the organic functional frameworks followed by their chemical reduction to Pd 0 atoms that rapidly evolve into metal nanoclusters, the size of which was tentatively expected to be controlled by the matrix nanoporosity (Figure 1). Details of the concepts outlined in Figure 1 are given in reference [12]. In this context, our previous results were encouraging, [11,12] but a quantitative confirmation of both findings and relevant rationale was lacking. We report herein on a detailed unambiguous quantitative support of the tentative conclusions of our previous papers. II is homogeneously dispersed inside the polymer framework; b) Pd II is reduced t...

Palladium(0) Nanoparticles on Glass‐Polymer Composite Materials as Recyclable Catalysts: A Comparison Study on their Use in Batch and Continuous Flow Processes

“…Also the nature of the polymer composition has a pronounced influence on the Pd particle size. [41] Thus, we prepared a set of polymers which differed in the degree of cross-linking (5.3, 10, 20 and 30%wt DVB). These were functional-ized with Pd particles under flow conditions.…”

Gel‐Type Polyacrylic Resins Cross‐Linked with Trimethylolpropanetrimethacrylate: The Issue of Their Nanostructure and Molecular Accessibility Unveiled with a Combination of Inverse Steric Exclusion Chromatography (ISEC), and ESR and CP‐MAS 13C NMR Spectroscopy

“…These techniques provide a convergent picture of a homogeneous macromolecular three-dimensional structure that is featured by "nanopores" potentially able to play a decisive role in controlling the growth of metal nanoclusters [22] ranging from 2 to 4 nm in diameter. In its turn, the preliminary analysis of the polymer gels by CP-MAS 13 C NMR spectroscopy permitted us to describe qualitatively the solvating properties of the polymer chain at variance of solvents exhibiting different polarity.…”

“…However, it turns out to be in fact very useful if ISEC analysis is aimed at predicting the physical ability of the polymer framework to control and limit the size of metal nanoclusters generated inside of it upon spontaneous aggregation of individual metal atoms. [22] Thus, although nanopores are not really present in the polymer framework, the framework itself behaves towards the mobility of the steric probes along the column as if cylindrical pores were existing entities.…”

“…The relevance of these conceptual and physical tools to metal catalysis technology has been recently highlighted by us. [21][22][23][24] Inter alia, we have in fact discovered that functional resins, thanks to their controllable nanoporosity, can behave as templates in the production of size-controlled metal nanoclusters (template controlled synthesis approach); [22] this fact is relevant for applications in chemical processing.…”

Electrochemical, Pulsed‐Field‐Gradient Spin‐Echo NMR Spectroscopic, and ESR Spectroscopic Study of the Diffusivity of Molecular Probes inside Gel‐Type Cross‐Linked Polymers

“…During the last decade we focused on the synthesis and physicochemical characterization of ℗/M 0 catalysts and of promising CFPs. Significant achievements range from the production of resin‐supported size‐controlled Pd 0 and Au 0 nanoclusters (Figure 1)10a,b to a detailed description of typical CFP frameworks, taking advantage of substantial information obtained from various physicochemical tools such as inverse steric exclusion chromatography (ISEC),12 electron spin resonance (ESR) spectroscopy,12b pulsed‐field‐gradient spin‐echo NMR (PGSE‐NMR) spectroscopy,13 and cross‐polarization magic angle spinning (CP MAS) 13 C NMR spectroscopy 14. These techniques are particularly useful for investigating what the interior of swollen resin particles could look like 12.…”