Homogeneous Catalyst Systems

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

In view of the fact that the propagation center in metathesis closely resembles metal carbene complexes, it is not surprising that preformed carbene complexes show catalytic activity for metathesis. Table 2 gives examples of carbene complexes that are effective as metathesis catalyst. 

Well-defined, relatively stable, Lewis acid-free catalysts, such as Mt(=CHCHMe3)(=NC6H3-/-Pr2)[OCMe(CF3)2]2 (Mt = Mo, W), can provide living polymers with very narrow molecular weight distributions. Certain carbene complexes are active catalysts for the metathesis of internal alkynes [59]. W and 

Tungsten-based catalysts Molybdenum-based catalysts  

WCl4(OC6H3-Br2-2,6)/ BuPb4 85 [30] EtAlCb MoCl3(NO)(OPPh,)2/ 20 [38] 

Uchida, T. Ishikawa, and M. Takagi, Reel. Trav. Chim. Pays-Bas, 1977, 96, Ml3. 

Mortreux, J. Nicole, and F. Petit, J. Chem. Soc., Chem. Commun., 

Anchoring of metal complexes through interaction with surface hydroxyl groups of inorganic supports continues to be of interest. Studies with catalysts prepared with allyl, carbonyl, chloride, and ethoxy ligands have been reported. Kuznetsov and co-workers conclude that the precursors of metathesis-active centres of surface metal complexes, prepared by anchoring allyl and ethoxy compounds of Mo, W, and Re to silica, are co-ordinatively unsaturated metal ions with oxidation number +4. Metathesis activity of the surface species depends on the ligand environment of the metal ion. 

Tungsten hexachloride reacts with dry silica gel to form a surface complex with average composition ( i02)WCl4. Van Roosmalen and associates reported that with tetramethyltin the complex yields a solid catalyst for the metathesis of alkenes with the same activity as the WOQ4-SnMe4 system, but is less active for the metathesis of methyl oleate. The supported complex can be used several times without additional SnMe4.  

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In the Institut Fransais du Petrc le process (62), ethylene is dimerized into polymer-grade 1-butene (99.5% purity) suitable for the manufacture of linear low density polyethylene. It uses a homogeneous catalyst system that eliminates some of the drawbacks of heterogeneous catalysts. It also inhibits the isomerization of 1-butene to 2-butene, thus eliminating the need for superfractionation of the product (63,64). The process also uses low operating temperatures, 50—60°C, and pressures (65). 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficulties of catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two rival plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases. 

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

With reference to the homogeneous catalyst systems thus far reported for the synthesis of hydrocarbons/chemicals from carbon monoxide and hydrogen, only the anionic rhodium systems of Union Carbide show any appreciable shift activity. With neutral species of the type M3(CO)12 (M = Ru or Os), only small quantities of carbon dioxide are produced under the synthesis conditions (57). 

A possible mechanistic pathway for the M3(CO)i2 (M = Ru or Os) Fischer-Tropsch catalysts is presented in Scheme 3. It should be emphasized that most of the ideas outlined above are extremely speculative. However, it is to be hoped that with the advent of homogeneous catalyst systems, detailed kinetic and mechanistic studies will lead to a clarification of the situation in the not-too-distant future. 

In the case of homogeneous catalyst systems, it is frequently possible to vary the catalyst composition and thereby some structural features of the catalyst in a systematic manner. The study of the influence of such modifications on the nature and the distribution of the products is an important method for improving the efficiency of a homogeneous catalyst and is often referred to as the tailoring of a catalyst" (2). Such inves-... 

Establishment of a free radical mechanism via H-atom transfer for hydrogenation using HMn(CO)5 (see Section II,D), and possibly also HCo(CO)4 (see Section II,C), suggests that more serious consideration for such mechanisms should be given for other hydridocarbonyl catalyst systems, and indeed for other homogeneous catalysts systems in general. The pentacyanocobaltate(II) catalyst can certainly operate by such a mechanism (see Section II,D). 

Table 41.17 Comparative hydrogenation studies using supported ionic liquid catalysts, biphasic catalyst systems and the classical homogeneous catalyst systems [116].a) ... 

The Wacker-Hoechst process has been studied in great detail and in all textbooks it occurs as the example of a homogeneous catalyst system illustrating nucleophilic addition to alkenes. Divalent palladium is the oxidising agent and water is the oxygen donor according to the equation ... 

Methane to Methanol and/or Formaldehyde Recent research indicates that a catalyst system in the presence of H2SO4 can convert methane directly into methanol. Homogeneous catalyst systems show promise. Also, heterogeneous Fe-ZSM-5 catalysts are reported to be attractive for this chemistry. Novel plasma reactors to generate hydroxyl radicals are also being investigated. 

This chapter has focused on inorganic and heterogeneous catalysts, because historically these are the major systems with which chemical engineers have been concerned. There are number of important homogeneous catalytic processes such as the Wacker process to make vinyl acetate from ethylene and acetic acid, and there are many acid and base homogeneous catalyst systems. 

Rhodium-based, homogeneous catalyst systems are reported that can... 

We will here report on homogeneous catalyst systems which allow the homologation of esters to proceed under very mild reaction conditions, i.e. 20-60 bar and 150-160 C, and according to two alternative stoichiometries ... 

The solvent is a sine qua non of a homogeneous catalyst system. Solvent properties are indeed very important in determining the activity, selectivity, and stability of a catalyst. Solvent stability is also essential, if the catalytic system as a whole is to be stable. As described above, several solvents have been employed in studies of cobalt-catalyzed CO reduction. Keim et al. (39) noted a substantial difference in activity and selectivity between catalyst solutions in toluene and W-methylpyrrolidone (Table I). Most of the information in this area again comes from the work of Feder and Rathke (36). Listed in Table IV are their results showing changes in the activity of the cobalt catalyst corresponding to changes in solvent polarity. The rates... 

Despite numerous screening studies, the literature contains little evidence that homogeneous catalyst systems based on metals other than Co, Rh, or Ru have significant activity for catalytic CO reduction. As seen for the known active catalytic systems, however, properties of solvents and additives or promoters can have enormous effects on catalytic activities. Solvents and additives can serve many roles in these catalytic systems. One important function of promoters in the Rh and Ru systems appears to be that of stabilizing metal oxidation states involved in catalytic chemistry. Other... 

Acetophenone 0-methyloxime is reduced by incremental addition of the substrate and borane to the homogeneous catalyst system to result in the formation of the corresponding methoxyamine in 67 % ee and in 860% chemical yield based on the chiral auxiliary (Scheme 14) (37). 

This reaction was one of the first examples of catalysis by a supported organometallic compound. In 1964 it was observed that Mo (CO) 6/ A1203, after activation by heating in vacuo at 120°C, catalyzed the conversion of propylene into ethylene and 2-butene (82). The nature of the active site in this catalyst system is still not fully defined (83). Since the initial discovery many heterogeneous and homogeneous catalyst systems have been reported (84, 85), the latter being more amenable to kinetic and mechanistic studies. 

Ford and co-workers have also recently developed a homogeneous catalyst system for the water-gas shift reaction (95). Their system consists of ruthenium carbonyl, Ru3(CO)12, in an ethoxyethanol solvent containing KOH and H20 under a CD atmosphere. Experiments have been conducted from 100-120°C. The identity of the H2 and CD2 products has been confirmed, and catalysis by both metal complex and base has been verified since the total amount of H2 and COz produced exceeds the initial amounts of both ruthenium carbonyl and KOH. The authors point out that catalysis by base in this system depends on the instability of KHC03 in ethoxyethanol solution under the reaction conditions (95). Normally the hydroxide is consumed stoichiometrically to produce carbonate, and this represents a major reason why a water-gas shift catalyst system has not been developed previously under basic conditions. As has been noted above, coordinated carbonyl does not have to be greatly activated in order for it to undergo attack by the strongly nucleophilic hydroxide ion. Because of the instability of KHC03... 

Several different homogeneous catalyst systems for the reduction of NO by CO have been described to date (183-185, 187, 189), and in all cases, the reduction follows (113) with nitrous oxide as the reduced N-containing product, this despite the fact that reduction to N2 is more favored thermodynamically. The reason for adherence to the stoichiometry of (113), rather than (114), for example, may relate to the fact that N20, once formed, is a very poor ligand. 

A different homogeneous catalyst system for the carbon monoxide reduc-tition of NO is based on transition metal dinitrosyl complexes and has been investigated by both Johnson and co-workers (184,186, 235), and Ibers and co-workers (185, 236). In 1973 Johnson and Bhaduri (184) reported the separate reactions (116) and (117) which appeared to presage catalysis of NO reduction via (113). Shortly thereafter, Haymore and Ibers (185) showed that Ir(NO)2L2 + and related dinitrosyl... 

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