Homogeneous catalyst immobilization studies

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). ... 

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. 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

Many efforts have been undertaken to graft transition metal complexes onto various supports in order to retain the performance of the soluble catalyst precursors and to allow easy separation of the catalysts from the reaction products. Most studies have been concerned with polymers, particularly with functionalized styrene-divinylbenzene resins. This approach to immobilize homogeneous catalysts has been reviewed, with all the strategies to anchor metal complexes on organic or inorganic supports examined (57-59). 

In addition to the immobilization of boron-derived catalysts, other commonly used homogeneous catalysts have been supported on polymers. Sharpless and others [82-87] prepared various quinine-based catalysts to achieve asymmetric dihydroxylations of alkenes. Initial studies were performed with catalyst 109 (Fig. 3), obtained by co-polymerization of 9-(4-chlo-robenzoyl)quinidine with acrylonitrile [82]. 

Polymeric membranes also show potential for application in the area of chiral catalysis. Here metallocomplexes find use as homogeneous catalysts, since they show high activity and enantioselectivity. They are expensive, however, and their presence in the final product is undesirable they must be, therefore, separated after the reaction ends. Attempts have been made to immobilize these catalysts on various supports. Immobilization is a laborious process, however, and often the catalyst activity decreases upon immobilization. An alternative would be a hybrid process, which combines the homogeneous catalytic reactor with a nanofiltration membrane system. Smet et al. [2.98] have presented an example of such an application. They studied the hydrogenation of dimethyl itaconate with Ru-BINAP as a homogeneous chiral catalyst. The nanofiltration membrane helps separate the reaction products from the catalyst. Two different configurations can be utilized, one in which the membrane is inserted in the reactor itself, and another in which the membrane is extraneous to the reactor. Ru-BINAP is known to be an excellent hydrogenation catalyst... 

Yet another approach which can be used to immobilize ILs involves treatment of a solid with a substantial amount of IL (5-50 wt.%). In contrast to the earlier studies, the I Ls used here were non-acidic and did not undergo reactions with the support This approach resulted in the formation of multiple layers of free IL on the carrier which could then act as an inert reaction phase to dissolve various homogeneous catalysts [14]. Although the resulting material was a solid, the active species was dissolved in the IL phase and acted like a homogeneous catalyst (see Figure 2). 

Numerous studies have appeared in the literature in which transition metal complexes. Immobilized on functionalized-polymers, or in which sulfonated resins have been used as catalysts. Polystyrene is the most common polymer support and has been used in the gel-form and macroretlcular forms. Review articles on "immobilized catalysts" have presented the advantages and disadvantages of Immobilizing the catalytic site O- ). The primary motivation is to "hetrogenize" a homogenous catalyst and thereby avoid costly separations of the catalyst from the reaction mixture and, in the case of acids, to minimize contact of the corrosive catalyst with the reactor vessel. 

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

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