Palladium metal nanoparticles system

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

Me]x-MCM-41 containing nanosized particles of platinum, palladium, rhodium, ruthenium and iridium were directly synthesised from surfactant stabilised spherical metal nanoparticles in the synthesis gel, and characterised with XRD, ICP-AES, TG/DSC, TEM, nitrogen physisorption and carbonmonoxide chemisorption, and Si MAS NMR. During the synthesis some agglomeration of the particles took place, but the metal particles were present inside the pore system of MCM-41. The matericils were active and selective catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene and norbomene. 

Platinum/palladium nanoparticulate catalysts were synthesized [421] from microemulsions in the system PEGDE/hexadecane/water. The conversion of the platinum precursor to metal nanoparticles (10-100 nm, formed by aggregation of 2-5 nm particles) was achieved through hydrazine at ambient temperature. The nanoparticles could be deposited on alumina substrates. 

Scenario 4. Nanoparticles are formed during the reaction initiated by soluble palladium precatalysts. Our experience in phosphine-free (mostly aqueous) systems led us at an early stage to hypothesize that these protocols go hand in hand with formation of palladium sols [4, 83]. Since 2000, the observation of palladium sols in Mizoroki-Heck reactions has become ubiquitous. So, if one were to decide to compile a list of references that mention the formation of sols, that list would practically coincide with a list of references which deal with the development of new phosphine-free catalytic systems. In fact, the idea that it is the dispersed palladium metal which catalyses (or, more correctly, serves as precatalyst) the Mizoroki-Heck reaction should be traced back to the seminal pubUcations by Heck in 1972 [2], who clearly attiibuted catalytic activity in a phosphine-free catalytic system to palladium metal formed by reduction of Pd(OAc)2 by the alkene or the amine. This idea fell into oblivion until its rediscovery in the course of a general interest in nanoscale systems in the late 1990s. 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

The partial oxidation of alcohols, to afford carbonyl or carboxylic compounds, is another synthetic route of high industrial interest For this, scC02 was investigated as a reaction medium for the aerobic oxidation of aliphatic, unsaturated, aromatic and benzylic acids with different catalytic systems, mainly based on the use of noble metals, both in batch [58-64] and in continuous fixed-bed reactors [65-70]. In this context, very promising results have been obtained when studying the catalytic activity of supported palladium and gold nanoparticles in the oxidation of benzyl alcohol to benzaldehyde these allowed conversions and selectivities in excess of 90% to be achieved [71-73]. 

In most cases, classical preparahon methods well known from heterogeneous catalysis have been applied for the preparation of the Pd parhcles/species supported on solids. However, the palladium parhcle size was inveshgated in detail in only a few reports in the literature. A strong influence of the preparation method, Pd particle size (dispersion) and of the chemical nature of the Pd nanoparticles (e.g. metal or oxide) has been reported in several papers. A rather complete overview of the literature on heterogeneous palladium systems applied to Heck catalysis including detailed discussion on mechanishc aspects is given in reviews by Biffis et al. [20] and Jones et al. [136] for the time before 2001 and 2006, respectively. 

Arcoleo et al [220] reported synthesis of palladium nanoparticles from the micellar system AOT/n-heptane. In one of the methods (A), two microemulsions were prepared in one, the water phase was an aqueous solution of K2PdCl4 and in the other, an aqueous solution of hydrazine monohydrate N2H4.H2O. The same volumes of the two were mixed to obtain the metal particles. In another method (B), hydrazine monohydrate was directly added to a Pd(AOT)2/NaAOT/n-heptane solution. Method B produced 3-5 nm particles under specific conditions, which were stable in size as a function of time or hydrogen concentration. Arcoleo etal [422] also used NaAOT /n-heptane reverse micelles with solubilized aqueous... 

Since then, statements that palladium nanoparticles or sols are true catalysts in a given catalytic system have become not infrequent. Indeed, in many cases it is clearly seen that, after the addition of precatalyst, a brownish-grey colour of palladium sol appears immediately and hansmission electron microscope measurements reveal the nanoparticles of metal. From the discussion of various preformed sols in Mizoroki-Heck reactions (Scenarios 1-3), it becomes evident that palladium nanoparticles display rather hmited activity with respect to scope, TONs and TOF values. On the other hand, in the discussion of various SRPCs, we see many examples of high activity and quite extended scope of substrates reaching to aryl chlorides. 

Recent developments aiming at highly active and versatile catalytic materials for Mizoroki-Heck and Suzuki reactions in various reaction media, including SCCO2, are exemplified by dendrimer-encapsulated metal (palladium) nanoparticles [121-123]. Although those systems are not yet fully developed and optimized, they have already delivered... 

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