Carbon homogeneous catalysts

Synthesis of carbonates (Homogeneous catalyst) alkyl phenyl carbonate from dialkyl carbonate and phenol 2-hydroxyethyl methyl carbonate from dimethyl carbonate... 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Transition metal-mediated phosphorus-carbon bond cleavage and its relevance to homogeneous catalyst deactivation. P. E. Gorrou, Chem. Rev., 1985,85,171 (109). 

Although a catalyst does not appear in the balanced equation for a reaction, the concentration of a homogeneous catalyst does appear in the rate law. For example, the reaction between the triiodide ion and the azide ion is very slow unless a catalyst such as carbon disulfide is present ... 

The potential for the use of catalysis in support of sustainability is enormous [102, 103]. New heterogeneous and homogeneous catalysts for improved reaction selectivity, and catalyst activity and stabihty, are needed, for example, new catalytic materials with new carbon modifications for nanotubes, new polymers. 

Although most industrial catalysts are heterogeneous, a growing number of industrial reactions use homogeneous catalysts. One example is the production of acetic acid. Most of the 2.1 billion kilograms of acetic acid produced annually is used in the polymer industry. The reaction of methanol and carbon monoxide to form acetic acid is catalyzed by a rhodium compound that dissolves in methanol ... 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

Our interest in silicon chemistry quite naturally led to a study of the hydrosilation reaction, the addition of the Si-H group across an olefin or an acetylene. This reaction is one of the most useful methods of making silicon-carbon bonds and is an important industrial process. Typically, homogeneous catalysts based on platinum, rhodium or ruthenium are used, and while very efficient, they are not recoverable(46). 

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 search for alternative energy supplies has triggered efforts to develop efficient homogeneous catalysts for Fischer-Tropsch-type syntheses via hydrogenation of carbon monoxide, a likely future key material available, for example, through oxidation of coal (33, 327, 328, 417, 418). Metal cluster systems have been used in attempts to emulate the presently used heterogeneous catalysts. The important reactions are methanation,... 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

The potential importance of homogeneous catalytic reactions in synthesis gas transformations (i.e., hydrogenation of carbon monoxide) has been widely recognized in recent years. In the first place, such systems could provide structural and mechanistic models for the currently more important, but more difficult to study, heterogeneous catalysts. Secondly, product selectivity is generally more readily achievable with homogeneous catalysts, and this would be an obviously desirable feature in an efficient process converting synthesis gas to useful chemicals and fuels. 

The immobilization of Pd(acac)2 as hydrogenation catalyst in the ionic liquids [BMIM][BF4] and [BMIM][PF6] was reported by Dupont et al. in 2000 [70]. These authors compared the biphasic hydrogenation of butadiene with the homogeneous system with all reactants being dissolved in CH2C12, with the reaction in neat butadiene and with a heterogeneous system using Pd on carbon as catalyst. 

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