Selective Nanocatalysis of Organic Transformation by Metals: Concepts, Model Systems, and Instruments

Abstract

Monodispersed transition metal (Pt, Rh, Pd) nanoparticles (NP) in the 0.8-15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions. For NP systems, the turnover rates and product distributions depend on their size, shape, oxidation states, and their composition in case of bimetallic NP systems. Dendrimersupported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis. Transition metal nanoparticles in metal core (Pt, Co)--inorganic shell (SiO 2 ) structure exhibit exceptional thermal stability and are well-suited to perform catalytic reactions at high temperatures ([400°C). Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems (heterogeneous, homogeneous and enzyme) excellent catalysts and provides opportunities to develop hybrid heterogeneoushomogeneous, heterogeneous-enzyme and homogeneousenzyme catalyst systems.

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

“…30,31 At present, a common route to drive selective oxidation by transition metals is to modify the oxidation state of the catalytically active metal centre. [32][33][34][35] However, the diradical nature of molecular oxygen means that it can also react with organic substrates via autoxidation pathways. 36,37 As these are not driven by catalysts, autoxidation reactions are difficult to study unambiguously.…”

Nanocatalysts for Rearrangement Reactions

“…Concerning the former, they are usually synthesized via two classical methods [19][20][21][22][23] : the reduction of metal salts and the decomposition of organometallic complexes. Regardless of the route employed, a key issue in their synthesis is their stabilization to avoid the formation of catalytically inactive bulk metal.…”

“…On the other hand, the use of nanoparticles by themselves as supports for the heterogenization of catalytically active transition metal complexes has expanded considerably during the last years. [19][20][21][22][23] In this context, silica-coated magnetic Fe 3 O 4 -NPs 1 decorated on their surface with an organometallic Ru(II)-arene complex containing the watersoluble 1,3,5-triaza-7-phosphatricyclo[3.3.1.1 3,7 ]decane (PTA) ligand, usually abbreviated as RAPTA complexes, have been designed (see Scheme 8.1) and successfully employed in different isomerization reactions. 26 In some cases, ultraviolet-visible (UV-Vis) measurements have been performed in order to determine the disposition of the nanoparticles along the surface (or the internal cavities) of the supports.…”

“…[19][20][21][22][23] In particular, the use of metal nanoparticles (NPs) as catalysts (or as catalyst supports) allows the increase of the exposed surface area of the active component of the catalyst, thereby enhancing its contact with the substrates and mimicking the homogeneous systems (heterogeneous NPs provide a quasihomogeneous phase). On the other hand, like in the case of heterogeneous catalysts, the insolubility of the metal NPs allows their easy separation from the reaction mixture, thus minimizing the effort of the product isolation stage.…”

Tailored Nanoparticles for Clean Technology–Achieving Size and Shape Control

“…In classical catalysis by bulk solid phases, high catalytic activity is often associated with specific surface ensembles defined by the Miller indices [218] of exposed facets that possess favorable geometries, bond distances, and coordination numbers, or with the presence of surface defects containing coordinatively highly unsaturated metal atoms [219][220][221][222]. Similar considerations can apply to model catalysts based on nanoparticles; atomic ensembles defined by the exposed planar surfaces (facets), defects, and coordinatively unsaturated atoms located at the particle periphery (corners, edges, etc.)…”