Anchoring of catalysts

The first examples of noncovalent anchoring of catalysts to soluble supports appeared in the literature in 2001. Concurrently, Mecking and ourselves were... 

Scheme 8.11 Concept of reversible anchoring of catalysts to a (dendritic) support that is functionalized with a binding motif complementaryto that ofthe catalyst, and atypical example used in our group.

Covalent Versus Noncovalent Anchoring of Catalysts to Dendrimers. .. 41... 

Fig-1 Schematic presentation of supramolecular anchoring of catalysts to dendritic... 

Table 1 Advantages and disadvantages of covalently and noncovalent anchoring of catalysts to dendrimers...

Fig. 7 The concept of supramolecular anchoring of catalysts to dendritic binding sites immobilized on silica (A) and the binding motif based on hydrogen bonding (HB) that has been used...

In many cases, the black box approach to anchoring of catalysts has been followed A support and a metal compound are simply brought together, and it is hoped that the association between metal and support is persistent even under drastic oxidation conditions. However, this approach frequently leads to failure. It is preferable first to have a clear idea of all the chemical states of the catalyst that must be retained by the support. Thus, knowledge of the full catalytic cycle and of all metal species in the system is needed. Only then can a mechanism for catalyst immobilization be proposed. Although this is a demanding approach, it may often be the only one that is rewarding. 

Schweb and Mecking have reported one of the first examples of noncovalent anchoring of catalysts to soluble polymeric supports in 2001. This noncovalently anchored catalyst featured phosphine ligands that were bound by multiple sulfonate groups to soluble polyelectrolytes using electrostatic interactions. The catalyst system was employed in the hydroformylation of 1-hexene and exhibited typical selectivity for a bis-triphenylphosphine-bound rhodium catalyst. The complex was readily recovered and recycled by ultrafiltration. Independently, Reek et al. described similar systems, but these systems made use of a soluble... 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Efforts have been made to propose a heterogeneous version of this reaction by polymerization or support-anchoring of these N-containing hgands. In most cases, however, even if success was obtained by using these heterogeneous catalysts, their recycling remained non-efficient, mainly due to the poor stabihty of the active Pd(0) species. 

Previous studies on the use of Anchored Homogeneous Catalysts (AHC s) have been concerned with studying the effect which different reaction variables had on the activity, selectivity and stability of these catalysts (1-9). These reactions were typically ran at relatively low substrate/catalyst ratios (turnover numbers-TON s), usually between 50 and 100. While these low TON reactions made it possible to obtain a great deal of information concerning the AHC s, in order to establish that these catalysts could be used in commercial applications it was necessary to apply them to reactions at much higher TON S and, also, to make direct comparisons with the corresponding homogeneous catalyst under the same reaction conditions. 

The anchoring and the reduction methods of precious metal precursors influence the particle size, the dispersion and the chemical composition of the catalyst. The results of SEM and H2 chemisorption measurements are summarised in Table 3. The XPS measurements indicate that the catalysts have only metallic Pd phase on their surface. The reduction of catalyst precursor with sodium formate resulted in a catalyst with lower dispersion than the one prepared by hydrogen reduction. The mesoporous carbon supported catalysts were prepared without anchoring agent, this explains why they have much lower dispersion than the commercial catalyst which was prepared in the presence of a spacing and anchoring agent (15). 

PdCI2 and Pd(OAc)2 anchored on a diphenylphosphinated polymer of styrene-divinylbenzene were used as the catalyst for the reaction of acetic acid. The product distribution was essentially the same for the reactions catalyzed by both homogeneous and anchored palladium catalysts (53). 

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