Homogeneous catalysis shifts

Oxidation of carbon monoxide by metal ions and homogeneous catalysis of the water gas shift reaction and related processes. J. Halpern, Comments Inorg. Chem., 1981,1, 3-15 (42). 

Several studies have reported the catalysis of the liquid-phase water gas shift reaction (WGSR). Actually, homogeneous catalysis of the WGSR is not competitive with its heterogeneous counterpart due to the limited rate, instability of the catalysts, and high costs. Scheme 64 shows the most important steps. 

FIGURE 9.26 Fischer-Tropsch synthesis vs. Oxo synthesis on cobalt catalysts. The thermodynamically controlled shift from heterogeneous to homogeneous catalysis. 

Homogeneous Catalysis of the Water Gas Shift Reaction Using Simple Mononuclear Carbonyls... 

Ng and Tsakiri—Mo, W, and Ru carbonyl catalysts—kinetics over Mo supported formate mechanism. Ng and Tsakiri127 reported the homogeneous catalysis of water-gas shift using a number of different metal carbonyl complexes (e.g., Mo(CO)6, W(CO)6, and Ru3(CO)12) under basic conditions in toluene-water emulsions (Table 45). The conditions used were PCo = 20.7 atm T = 180 °C 70 ml solution containing 71.4 mmol KOH 2.5 hours 550 revs/min stirring rate. 

The nucleophilic attack by alkoxides, amines, and water is of great interest to homogeneous catalysis. A dominant reaction in syn-gas systems is the conversion of carbonyls with water to metal hydrides and carbon dioxide ("Shift Reaction"), see Figure 2.27. 

The second class of reactions, with CO and H20 serving as a source of two electrons, is not a catalytic reaction as such but is examined closely here since this is an integral step in the homogeneous catalysis of the water gas shift. The reaction may take two forms, with the reduction product being either a reduced metal center, (29), or a metal hydride, (30). 

This section has dealt with the oxidation of CO to C02, especially as it enters into the water-gas shift reaction (26a). A reasonable view of the homogeneous catalysis of this reaction, whether in basic or acidic media, is emerging in which CO formation proceeds from nucleophilic attack of water or OH" on an activated carbonyl followed by either reductive decarboxylation or hetero-atom -elimination yielding, respectively, a reduced metal or a metal hydride species. 

Scheme 3 forms a catalytic cycle for the water-gas shift reaction (63) employing [Rh2(/i-CO)(CO)2(dpm)2] in the presence of acid as a catalyst (62). It should be reiterated that alternative cycles might be written which do not involve formate intermediates. For example, a possible mechanism for catalysis of the water-gas shift reaction involving the binuclear metal species, [Pt2H2( -HXdpm)2]+, is outlined below (Scheme 4) (64). We have critically discussed the role of formate versus carboxylic acid intermediates in homogeneous catalysis of the water-gas shift reaction by mononuclear metal catalysts elsewhere (34).

There have been several recent studies of homogeneous catalysis of the water gas shift reaction (equation 7) by mononuclear and cluster catalysts, including mononuclear platinum complexes (15). 

Hydrogenation with homogeneous catalysis has received increasing attention over the last few decades. Hexarhodium hexadecacarbonyl (181 Scheme 35) under water gas shift conditions forms 1,2,3,4-tetrahydroquinoline (128) or N-formyl-1,2,3,4-tetrahydroisoquinoline (182) with the parent hete-roaromatic. When hydrogen is substituted for carbon monoxide, 4-methylquinoline is reduced to 4-methyl-5,6,7,8-tetrahydroquinoline (183) exclusively.Isoquinoline behaves similarly generating A/-formyl-l,2,3,4-tetrahydroisoquinoline (184). Similar reductions under water gas shift or synthesis gas conditions using transition metal carbonyls derived from Mn, Co and Fe, have been recorded.Promotion by phase transfer agents is observed in some cases. 

FIGURE 14-28 Homogeneous Catalysis of Water Gas Shift Reaction. (Adapted with permission from H. Ishida, K. Tanaka,... 

A second application to organometallic chemistry is related to NMR observation under high gas pressure. It makes it possible to strongly increase the concentration of a gas in a solution, to shift chemical equilibria and to detect intermediates under conditions similar to those used in homogenous catalysis. 

In recent years there has been much interest in the use of supercritical fluids (SCFs) as replacements for conventional liquid solvents, particularly in separation science, but also as reaction media. In addition to their environmental benefits, SCFs have further advantages over conventional liquid solvents, and these are briefly outlined in Section 2. The remainder of the chapter describes the use of SCFs as a medium for NMR spectroscopic studies. First we look briefly at motives for such NMR studies and the techniques employed. We then examine in more detail chemical shifts and nuclear spin relaxation in SCFs. The lower relaxation rates associated with SCFs and consequent sharper lines obtained for quadrupolar nuclei make SCFs excellent solvents. Section 8 describes some NMR studies of organometallic reactions in SCFs. Here the miscibility of supercritical solvents with gaseous reagents proves to be a tremendously useful feature in, for example, homogeneous catalysis. Finally we comment on future possibilities for NMR studies in SCFs. 

Altogether the different possibilities of shifting products to the desired chain length and double-bond position make the SHOP the most elegant and flexible process operating today. It is furthermore one of the larger applications of homogeneous catalysis. 

The water-gas shift reaction is normally an unwanted side reaction of homogeneous catalysis when carbon monoxide is engaged as a substrate and if water is present as the medium or as a product. Both a pH-basic medium (formation of the nucleophilic [OH] ) and metals or metal complexes that deprotonate the water favor the shift reaction. For example, in the hydrocarboxylation process to make propionic acid directly from C2H4, CO, and H2O (eq. (14)), the formation of hydrogen via the water-gas shift reaction leads to (minor) hydrogenation and hydroformylation products (cf. Section 2.1.2.2). 

R. M. Laine, R. B. Wilson, Recent developments in the homogeneous catalysis of the water-gas shift reaction , in Aspects of Homogeneous Catalysis (Ed. R. Ugo), Vol. 5, Reidel (Kluwer), Dordrecht 1984. 

Trends in the wCo chemical shifts of a wide variety of organocobalt complexes have been usefully interpreted in terms of structure-activity relationships in homogeneous catalysis (42) and in terms of the influence of various pi-ligands on structure and... 

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