Homogeneous catalysts, immobilization

Table 11.6 adapted from Doraiswamy (2001) provides a partial list of these categories. The main features and applications of these two categories of catalysts are discussed in more detail by Doraiswamy (2001). Of these zeolites, TS-1 catalysts and homogeneous catalysts immobilized on solid supports deserve special mention. 

Nowadays MAS measurements are the standard experiments which are routinely used in the majority of Solid State NMR. CP-MAS, the combination of MAS and CP experiments has been routinely utilized to obtain high-resolution NMR spectra of dilute spins such as N, and P Nuclei in solid materials.CP-MAS plays a key role in the characterization of homogeneous catalysts immobilized on solid supports, providing direct information about the structure of support and the metal complex. Accordingly a number of NMR studies were performed in order to investigate the morphology and chemical properties of several types of pure and functionalized silica materials (ref. 52 and references therein). ... 

In addition to the standard solid state MAS-NMR methods, Bliimel and co-workers employed high-resolution magic angle spinning NMR of suspensions, which they abbreviated as HRMAS experiments, for the investigation of the structure of phosphine linkers and homogenous catalyst immobilized on the silica surface. The resulting HRMAS NMR... 

Employing solid state NMR techniques it is nowadays easily feasible to study homogeneous catalysts immobilized on a solid support surface. From these studies it is possible to reveal the structure of the catalyst on the surface and its modifications, due to binding. Combined with the recent improvement in NMR methodology (hyperpolarization, indirect detection and FAST-MAS), the detailed study of surface processes of technical catalysts on a molecular level by solid-state NMR is now within reach. 

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. 

Fig. 1. shows the P MAS NMR chemical shifts for the immobilized and homogeneous catalyst. The chemical shifts at the -15.2 and -13.7 ppm correspond to PTA while the chemical shifts in the range from 20 and 40 ppm correspond to phosphine oxide. The chemical shifts at the 66 and 118 ppm seems to be those of BINAP ligand, which is confirmed by the spectrum of Ru-BINAP catalyst. This spectrum shows that PTA exist in large amount on the surface of immobilized catalyst and that BINAP ligand is intact after immobilization. 

Stack and coworkers immobilized phenantrohne derivative 16 on micelle-templated silica SBA-15 (Scheme 8) [55,56]. The system showed more selective and efficient catalytic activity for olefin epoxidations with peracetic acid than the analogous homogeneous catalyst. 

Another way of immobilizing catalyst complexes might be to trap them in the pores of solid particles, for instance by synthesizing the complex inside the pores of a zeolite ( ship in a bottle ). Another method could be to trap catalyst complexes in porous materials and deposit a membrane at the outer. surface. These methods of immobilizing a homogeneous catalyst do not involve chemical linkage between the catalyst and the carrier. The fixation is the result of steric hindrance. 

Cationic palladium(II) complexes are homogeneous catalysts for both intramolecular and inter-molecular hydroamination reactions.267 Palladium species immobilized on silica can be prepared by the simple addition of alkyl- or hydroxopalladium(II) complexes to partially dehydroxylated silica. The silica-bound species are more stable than their molecular precursors and are efficient catalysts for the cyclization of aminoalkynes.268... 

With tethering technology, immobilized Rh(DiPFc) catalyst, a veiy selective hydrogenation catalyst was developed by Engelhard (26) and Chirotech (27). Rh(DiPFc)(COD)BF4 is one of most interesting homogeneous catalysts due to its chemo-selectivity and sulfur tolerant behavior. The anchored complex y-... 

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