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...

Functional Resins as Hydrophilic Supports for Nanoclustered Pd(0) and Pd(0)‐Au(0) Catalysts Designed for the Direct Synthesis of Hydrogen Peroxide

“…Model for the generation of size-controlled metal nanoparticles inside metallated resins. a) Pd(II) is homogeneously dispersed inside the polymer framework; b) Pd(II) is reduced to Pd(0); c) Pd(0) atoms start to aggregate; d) a single 3 nm nanocluster is formed and "blocked" inside of the largest mesh present in that "slice" of polymer framework (reproduced from Angewandte Chemie [15] ).…”

“…We have recently reported [15] on an innovative protocol useful to produce size-controlled palladium nanoclusters (Template Controlled Synthesis, TCS, approach, see Figure 1 below) supported on designed gel-type resins. We have applied our expertise to obtain potential catalysts related both to Hutching and Fierros approaches, i.e., based on i) nanometer-sized, expectedly (see below) nanoalloyed Au(0) and Pd(0) supported on gel-type resins [16] and ii) designed gel-type sulfonic resins bearing either "Pd 2 þ " ions or Pd(0) nanoclusters to be possibly employed in water and featured by good stability and consequently reusability.…”

“…For sake of further simplification, we code occasionally MTEMA-DMAA-MBA 10 -4 also as * PSMe and SEMA-DMAA-DVB also as * P -SO 3 À H þ . MTEMA-DMAA-MBA 10 -4 is a macromolecular ligand chemically and structurally very close to MTE-MA-DMAA-MBA 4 -4 (4% mol -92% mol -4% mol), which resin was thoroughly investigated by us both as a ligand towards Pd(II) and Au(III) and as a macromolecular "mold" for the generation of size-controlled Pd(0) and Au(0) nanoclusters, [15,16] Figure 1.…”

Synthesis and catalytic activity of metal nanoclusters inside functional resins: an endeavour lasting 15 years

“…This process is based on a catalyst such as P -NR − (Corain et al 2004b), which is able to withstand working conditions that are not particularly mild. Reaction 3 is the access to a plethora of hybrid-phase metal catalysts (Corain et al 2004b Figure 2. Schematic of the micro-and nanoscale morphology of (a) gel-type and (b) macroreticular resins ).…”

Formation and growth of palladium nanoparticles inside porous poly(4-vinyl-pyridine) monitored by operando techniques: The role of different reducing agents

“…For example, polymersupported palladium-based catalysts have shown remarkable performance in coupling and hydrogenation reactions [14]. The polymer may act simultaneously as support and stabilizer, thanks to the dual effect of porosity (steric stabilization) and polar functional groups (electrostatic stabilization) [15][16][17]. In polymers without specific functional groups, such as polystyrene-based polymers, stabilization occurs only via electronic interaction between the π electrons of the benzene rings of the polymer and the empty orbitals of metal atoms [11,18,19].…”