Homogeneous catalysis metal salts

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

Hydrogen addition to multiple bonds is catalyzed by certain complex metal salts in solution. This may be described as homogeneous catalysis and, compared to heterogeneous catalysis, is a relatively new development in the area of hydrogenation reactions. Rhodium and ruthenium salts appear to be generally useful catalysts ... 

This chapter has focused on heterogeneous catalysis in supercritical media, but the relationship between supercritical fluids and catalysis is much broader. There have been numerous studies of homogeneous catalysis in SCFs. Examples include hydroformylation via cobalt carbonyl complexes in supercritical CO2, oxidation via metal salts dissolved in supercritical water, and acid-catalyzed dehydration of alcohols in supercritical water. 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

The use of naked but supported colloids and clusters is well established in heterogeneous catalysis [6]. Normally, the metal particles are directly generated on the support from metal salts. Such catalysts usually possess high activities however, compared with homogeneously working catalysts, they show considerably reduced selectivities. This is due to a broad particle size distribution as well as to the lack of ligand molecules influencing the selectivity decisively. 

Catalytic activity of rare earth elements (i.e., lanthanides, symbol Ln) in homogeneous catalysis was mentioned as early as 1922 when CeCls was tested as a true catalyst for the preparation of diethylacetal from ethanol and acetaldehyde [1]. Solutions of inorganic Ln salts were subsequently reported to catalyze the hydrolysis of carbon and phosphorous acid esters [2], the decarboxylation of acids [3], and the formation of 4-substituted 2,6-dimethylpyrimidines from acetonitrile and secondary amines [4]. In the meantime, the efficiency of rare earth metals in heterogeneous catalysis, e. g., as promoters in lanthanide (element mixtures)-... 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

Carbonylation of methanol catalyzed by soluble Group IX transition metal complexes remains the dominant method for the commercial production of acetic acid. The Monsanto process stands as one of the major success stories of homogeneous catalysis, and for three decades it was the preferred technology because of the excellent activity and selectivity of the catalyst. It has been demonstrated by workers at Celanese, however, that addition of iodide salts can significantly benefit the process by improving the catalytic reaction rate and catalyst stability at low water concentrations. Many attempts have been made to enhance the activity of... 

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