Abstract
The present review compiles the advances in the dendritic catalysis within the last two decades, in particular concerning heterogeneous dendrimer-based catalysts and their and application in various processes, such as hydrogenation, oxidation, cross-coupling reactions, etc. There are considered three main approaches to the synthesis of immobilized heterogeneous dendrimer-based catalysts: (1) impregnation/adsorption on silica or carbon carriers; (2) dendrimer covalent grafting to various supports (silica, polystyrene, carbon nanotubes, porous aromatic frameworks, etc.), which may be performed in a divergent (as a gradual dendron growth on the support) or convergent way (as a grafting of whole dendrimer to the support); and (3) dendrimer cross-linking, using transition metal ions (resulting in coordination polymer networks) or bifunctional organic linkers, whose size, polarity, and rigidity define the properties of the resulted material. Additionally, magnetically separable dendritic catalysts, which can be synthesized using the three above-mentioned approaches, are also considered. Dendritic catalysts, synthesized in such ways, can be stored as powders and be easily separated from the reaction medium by filtration/centrifugation as traditional heterogeneous catalysts, maintaining efficiency as for homogeneous dendritic catalysts.
Highlights
Various catalytic processes, such as hydrogenation, oxidation, polymerization, etc., are of great importance in the modern petrochemistry and pharmaceutical industry [1–6]
Hydrogen bonding provided the effective adsorption of poly(amido amine) (PAMAM) dendrimers, impregnated with Pt nanoparticles, on the surface of nitrogen-doped single-wall carbon nanotubes (SWCNT), synthesized by chemical vapor deposition (CVD) method (Scheme 9) [329]
The convergent approach, implying the covalent attachment of the already synthesized dendrimers or dendrons to the preliminary functionalized carrier, escapes the defects resulting from irregular dendrimer coatings, which are inevitable for multi-step divergent synthesis due to diffusion limitations increasing with the increase in the dendron generation [92,146,147,252,339,349,350,364,368,381]
Summary
Various catalytic processes, such as hydrogenation, oxidation, polymerization, etc., are of great importance in the modern petrochemistry and pharmaceutical industry [1–6]. The synthesis was usually carried out in the organic solvent (toluene or THF) as a metal source and reducing agent, respectively; the combinations of decomposed organic metal salt (e.g., Pd(OAc)2) or complex (e.g., [Ru(Cumene)Cl2]2 or [Ru(COD)(COT)]) and molecular hydrogen [119,120,129], or chloride complex salt or acid (e.g., K2PdCl4, HAuCl4 or H2PtCl6), often stabilized by amphiphilic ionic liquid (e.g., [N(C8H17)4]Br), and sodium borohydride [121,137,194–200], were used Another approach implied the modification of nanoparticles, already stabilized by bifunctional ligands (e.g., Au NPs, stabilized by 11-mercaptoundecanoic acid), with hydroxyl poly(aryl ether) dendrons of various generations (Scheme 4), providing the nanoparticle uniform and narrow in size distribution [201].
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