Homogeneous molecular catalysts

Homogeneous molecular catalysts, which have far greater control over selectivity than heterogeneous sohd catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of mass-transfer problems can be achieved (Andrew et al., 1995 Birnbaum et al., 1999). 

This book focuses on homogeneous (molecular) catalysts. There is a need to replace precious metals used in heterogeneous catalysis as well, but that topic is beyond the scope of material that can be covered in one book. 

A drawback of homogenous catalytic processes is often the complex and costly separation and recycling of the catalyst. Therefore, considerable efforts are made to combine the easy separation of heterogenous catalysts with the high potential activity and selectivity of homogenous molecular catalysts. 

To orient the reaction selectively, various heterogeneous catalysts, mostly metals, have been used but also homogeneous molecular catalysts. Concerning the latter, a distinction should be made between redox catalysts and chemical catalysts. Arcxnatic and heteroaromatic amoa radicals ofien... 

Bliimel et al. [22-32] presented a strategy where previously successful homogeneous molecular catalysts are covalently bound to oxidic supports. Subsequent analysis and investigation was performed by conventional and suspension solid-state NMR spectroscopy. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Hydrogenation of aromatic nitro compounds [8,18,29] and hydrogenation of benzene derivatives [2,9,21] have been generally accepted as model reactions to check the heterogeneous nature of catalyst, because homogeneous species are not believed to be active. But at least two well-studied examples show that molecular catalysts can hydrogenate benzene [36,37]. 

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

Kragl and Dreisbach (1996) have carried out the enantioselective addition of diethyl zinc to benzaldehyde in a continuous asymmetric membrane reactor using a homogeneous soluble catalyst, described in their paper. Here a,a-diphenyl-L-proline was used as a chiral ligand, coupled to a copolymer made from 2-hydroxy ethyl methacrylate and octadecyl methacrylate, which had a sufficiently high molecular weight to allow separation by ultra-filtration (U/F). The solvent-stable polyaramide U/F Hoechst Nadir UF PA20 retained more than 99.8% of the catalyst. The ee was 80 %, compared to 98 % for a noncoupled catalyst. 

Molecular catalysis. The term molecular catalysis is used for catalytic systems where identical molecular species are the catalytic entity, like the molybdenum complex in Figure 8.1, and also large molecules such as enzymes. Many molecular catalysts are used as homogeneous catalysts (see (5) below), but can also be used in multiphase (heterogeneous) systems, such as those involving attachment of molecular entities to polymers. 

Central to catalysis is the notion of the catalytic site. It is defined as the catalytic center involved in the reaction steps, and, in Figure 8.1, is the molybdenum atom where the reactions take place. Since all catalytic centers are the same for molecular catalysts, the elementary steps are bimolecular or unimolecular steps with the same rate laws which characterize the homogeneous reactions in Chapter 7. However, if the reaction takes place in solution, the individual rate constants may depend on the nonreactive ligands and the solution composition in addition to temperature. 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Fig. 45.2 Possible qualitative concentration profiles in the liquid boundary layer for homogeneous hydrogenations with molecular catalysts.

Catalysts other than homogeneous (molecular) compounds such as nanoparticles have been used in ionic liquids. For example, iridium nanoparticles prepared from the reduction of [IrCl(cod)2] (cod = cyclooctadiene) with H2 in [bmim][PF6] catalyses the hydrogenation of a number of alkenes under bipha-sic conditions [27], The catalytic activity of these nanoparticles is significantly more effective than many molecular transition metal catalysts operating under similar conditions. 

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