Catalyst types homogeneous

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. 

The epoxidation of olefins plays an important role in the industrial production of several commodity compounds, as well as in the synthesis of many intermediates, fine chemicals, and pharmaceuticals. The scale of production ranges from millions of tons per year to a few grams per year. The diversity of catalysts is large and encompasses all the known categories of catalyst type homogeneous, heterogeneous, and biological. 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

The basic idea of immobilising a catalytically active solution on a solid support to obtain a solid catalyst with homogeneous type selectivity and efficiency is not new. In the late seventies, Scholten [69] and Hjortkjaer [70] described supported liquid phase... 

As can be seen from Figure 2.1, cobalt was deposited on the carbon nanomaterials quite homogeneously. Hence, the cobalt particle sizes of the three catalyst types vary only little. The Co/nanofiber materials exhibit cobalt particle diameters of roughly 10 nm. In case of the nanotubes, particle sizes ranging from 5 to 7 nm were observed. 

Only a few detailed studies of the reaction mechanism of the homogeneous hydrogenation of imines have been published until now. A generalization seems to be very difficult for two reasons. First, rather different catalyst types are effective and probably act by different mechanisms. Second, the effect of certain additives (especially iodide or iodine and acid/base) is often decisive for ee and rate, but a promoter in one case can be a deactivator in another case. 

A classical example is the development of soluble chiral catalysts for homogenous asymmetric hydrogenation. The story began with the discovery of Wilkinson s catalyst [4]. In 1968, Horner [5] and Knowles [6], independently, reported the feasibility of asymmetric hydrogenations in the presence of optically active Wilkinson-type catalyst. Although the optical yields were rather low, further studies in this direction were the basis of the success of Monsanto s asymmetric synthesis of the anti-Parkinson s drug L-DOPA. The key steps of the synthesis are outlined in Scheme 11.1. 

Table 3.15. Fischer-type carbene complexes as catalysts for homogeneous-phase alkene metathesis.

The robustness of TMSP-type catalysts allows one to address many of the limitations regarding the development of optimal catalysts for homogeneous selective oxidation, including the five general limitations articulated above (1) inadequate... 

However, despite a clear understanding of the homogeneous-type problems, the development of practically suitable catalysts for homogeneous or heterogeneous catalysis face difficulties associated with resolving particular questions of targeted catalyst design. 

An analogous technology commercialized by IFP, Paris, is their continuous, chloroaluminate ionic liquid dimerization of //-butene to isooctane, promoted by a Ziegler-Natta-type homogeneous catalyst. The poorly miscible isooctane product is readily separated. 

The very soluble hexachloroplatinic acid, H2 [PtCl ] nH20, is the most useful precursor for synthetic and catalytic work. For the hydrosilation of unsaturated substrates, the catalyst of choice is chloroplatinic acid, because of its very high activity. There is currently some doubt as to whether the true catalyst is homogeneous or colloidal metal and therefore heterogeneous see Hydrosilation Catalysis). Nearly all halides andpseudohahdes (= X) form salts of the [PtXe] ion and of the [PtX4(NH3)2] type. 

The units of space velocity are the reciprocal of time. Usually, the hourly volumetric feed-gas flow rate is calculated at 60 °F (15.6 C) and 1.0 atm (1.01 bar). The volumetric liquid-feed flow rate is calculated at 60 F (15.6 °C). Space velocity depends on the design of the reactor, reactor inlet conditions, catalyst type and diameter, and fractional conversion. Walas [7] has tabulated space velocities for 102 reactions. For exanple, for the homogeneous conversion of benzene to toluene in the gas phase, the hoiuly-volumetric space velocity is 815 h . This means that 815 reactor volumes of benzene at standard conditions will be converted in one hoiu. Although space velocity has limited usefulness, it allows estimating the reaction volume rapidly at specified conditions. Other conditions require additional space velocities. A kinetic model is more useful than space velocities, allowing the calculation of the reaction volume at different operating conditions, but a model requires more time to develop, and frequently time is not available. 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

Adsorption of standard-type homogeneous complexes on supports [90], such as Vaska s complex, is possible but these catalysts clearly cannot be used with solvents that dissolve the complex. These studies, however, have to be seen in the context of supported liquid-phase catalysts. 

Catalysis are classified into two types homogeneous and heterogeneous. In homogeneous catalysis the catalyst is present in the same phase as the reactants, as when a gas-phase catalyst speeds up a gas-phase reaction, or a species dissolved in solution speeds up a reaction in solution. Chlorofluorocarbons and oxides of nitrogen are homogeneous catalysts responsible for the destruction of ozone in the stratosphere. These reactions are examined in more detail in Section 20.5. A second example is the catalysis of the oxidation-reduction reaction... 

Table 1 shows total sulfur and carbon contents of Ni catalysts after poisoning in a fixed bed. Sulfur content varied with temperature, pressure and HjS concentration. In addition, the content of sulfur was affected by the catalyst type. The homogeneous catalyst with a calcium aluminate carrier (C) adsorbed more sulfur under the same conditions. This is probably due, in part, to the calcium content of the catalyst [17] in contrast to Catalyst (A) with an AljOj carrier. 

Busico, V. Cipullo, R. Ronca, S. Budzelaar, P.H.M. Mimicking Ziegler-Natta catalysts in homogeneous phase, 1 C2-symmetric octahedral Zr(TV) complexes with tetradentate [ONNO]-type ligands. Macromol. Rapid Commun. 2001, 22, 1405. 

A close and well-researched homogeneous analogue of the epoxidation catalyst is the so-called Sharpless catalyst, a homogeneous titanium (TV) catalyst for fhe asymmefric epoxidation of allylic alcohols with f-butyl hydroperoxide (TBHP) [14]. A mechanism has been proposed for this type of epoxidation [15], which can be expressed in a generic form as shown in Fig. 13.4. 

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