Methanol, reaction homologation

During this same time period, ethylene glycol was reported as a product in cobalt-catalyzed methanol homologation reactions methanol, ethanol, and glycol ethers were also found in similar reactions carried out to homologate w-propanol and w-butanol (34). Reaction conditions were 800-1000 atm of H2/CO at 225°C. 

Although several studies have examined the effects of various promoters and ligands on the methanol homologation reaction, none has identified a system with substantially improved selectivity. However, there are many claims that iodide accelerates the rate of the reaction 62-64). While the possible sources of this enhancement have been discussed in Section IV,B, it should be noted that the systems from which these interpretations were extracted are by no means simple. Qualitative comparisons among the various studies of promoted and unpromoted systems are difficult for the reasons given above, but, in addition, because the variety of forms by which iodine is introduced (e.g., I2, CH3I, or iodide salts) apparently produce different effects (57, 63, 64). Also, many of the systems involve two promoter components (e.g., triphenylphosphine + methyl iodide or tri-p-tolylphosphine + I2X which further complicates the interpretations as to the role(s) of the halide. 

Besides cobalt, other catalyst metals haw also been examined for the methanol homologation reaction [26], More detailed studies have been reported for iron [85]. ruthenium [86], and rhodium [87]. whereas little is known about nickel [88]. osmium, iridium, and palladium [5]. 

To achieve, then, high acetic acid selectivity directly from synthesis gas (eq. 1) it is necessary to balance the rates of the two consecutive steps of this preparation - ruthenium-carbonyl catalyzed methanol formation (10) (Figures 2 and 5) and cobalt-carbonyl catalyzed carbonylation to acetic acid (Figure 6) - such that the instantaneous concentration of methanol does not build to the level where competing secondary reactions, particularly methanol homologation (7, H), ester homologation (12, 13), and acid esterification (1 ), become important. 

The reductive carbonylation (Equation 1) and homologation (Equation 2) of methanol are reactions of considerable interest to the chemical industry (1). These reactions provide a route to... 

The carbonylation-homologation reaction may also be carried out on a mixture of alcohols and their formates. For instance, at a very high conversion of the reagents, methanol-methyl formate and i-butaiol -i-butyl formate produce a mixture of oxygenates particularly rich in acetates that are useful as octane improvers for gasoline (Fig. 3). 

Several mixed-metal clusters containing platinum and cobalt are known and some of them have been employed as methanol homologation catalysts.1 Among them, the title compound2 was first prepared unambiguously from the reaction of dichloro[l,2-ethanediylbis(diphenylphosphine)]platinum with sodium tetracarbonylcobaltate, Na[CO(CO)4]. The compound also may be prepared by the reaction of [l,2-ethanediylbis(diphenyl-phosphine)]bis(phenylethynyl)pIatinum with Co COJg.1... 

Hydrogenation of the acyl complex [Co(COMe)(CO)3PPh2Me] gave a product distribution very similar to that obtained when [Co(CO)3PPh2Me] was used as a catalyst for methanol homologation.402 This was regarded as further evidence for the presence of an acyl intermediate in the catalytic reaction. 

The transformation was called an homologation reaction because essentially it consisted in going from one alcohol to an alcohol containing one carbon atom more than the starting material (Wender, Levine, and Orchin, 14). Tertiary alcohols reacted most rapidly, secondary alcohols less rapidly and primary alcohols only very slowly. It was of considerable importance to ascertain whether the olefin intermediate was essential and for this purpose, methanol and benzyl alcohol, neither of which can dehydrate to an olefin, were used in the reaction. Both compounds, contrary to other primary alcohols, reacted quite rapidly and gave the homologous alcohol of the methanol converted, about 40 mole per cent went to ethanol and with benzyl alcohol, a 30% yield of 2-phenylethanol was secured. In both examples, however, reduction products were also present of the methanol converted, 8 mole per cent went to methane and from benzyl alcohol, a 50 to 60% yield of toluene was secured. The conversion of methanol to methane appears to be the only case in which an appreciable quantity of hydrocarbon is secured from a purely aliphatic alcohol. The behavior of benzyl alcohol and its derivatives will be discussed later. 

This is unlikely, however, since in methanol homologation studies, methane is generated preferentially, even when there are comparable amounts of methanol and ethanol present in the reaction media (55). Acetic acid decarboxylation has also been suggested as a pathway for methane formation ... 

The same difficulties encountered in determining the nature of the species or cycles involved in the carbonylation reaction exist for the homologation reaction, and so little is actually known about the specifics of the latter reaction. However, most workers in the area of methanol homologation seem to believe that the resulting ethanol is not derived from hydrogenating free acetaldehyde. Rather, the carbonylated species is reduced before leaving the metal center, for example,... 

Interest in coal-derived syntheses of base chemicals has led to a fast growing number of publications in open and patent literature concerning homologation reaction. -Most of that work is devoted to the hydrocarbonylation of methanol and aims at the optimi7ation of catalysis, product separation, and catalyst recycling. 

At prolonged reaction times, increasing amounts of high molecular weight condensates are funned. In the gas phase, products like methane, dimethylether and CO2 are found in addition to the syngas components. All reaclion products have been identified by GC/MS measurements and by comparison of CjCproduct composition of a typical methanol homologation run obtained by a cobalt/iudine catalyst is given in Table H. 

Another unsolved problem is catalyst recycling, especially when bimetallic cobalt systems are used. In summary, it must be Slated that the known processes for methanol homologation still lack sufficient conversion rates and ethanol/ acetaldehyde selectivittes. 0 far. no ethanol yields exceeding 40% have been reported under acceptable catalyst concentrations and reaction conditions. 

AJthaugh various propiisals for the ni chani m of methanol homologation exist, the course of the reaction is still not fully understood. This is especially true for the activation of methanol with a concomitant C-0 bond scission. Also, the folc of the iodine promoter and of ligands remains unclear. This situation is controversial to the closely-related carbonylation of methanol to acetic acid with rhodium catalysts, where the oxidative addition of the intermediate methyl iodide to a rhodium (1) is a generally-accepted reaction path [SR]. 

The role of iodine promoters in methanol homologation still remains unclear. Controversial reports can be found in the literature describing methyl iodide 3 an intermediate that can be formed at reaction conditions (of. Equation (20)). 

Tlic product distribution observed in methanol homologation can be deduced from the reaction steps catalyzed by metal cmnplexes or acids 5]. Side products such as alkanes can be explained by reductive elimination steps, as diown in Equation (31). 

Methanol homologation catalyzed by ruthenium has been studied by Braca etal. [86, 89, 90]. Catalyst systems such as Ru(acac)3/Nal and Ru(C0)4lj/NaI have been shown to be active. In contrast to cobalt catalysts, no reaction occurs in the absence of 1" and a proton supplier is needed. As can be taken from Table XI, the reaction is higidy selective to C -products and no higlter products are formed. Due to the high hydrogenation activity of ruthenium, however, methane and ethane arc formed as side products in considerable amounts as well as dimethyl ether. Thus, the overall yield of ethanol is limited. The same catalyst systems have also been shown to be active in the homologation/carbonylation of ethers and esters. 

Since the reactions in Equations (11) and (12) cover the field of methanol homologation, they will be not discussed here, although they are related to the class of reactions discussed here, from a mechanistic point of view,... 

Thus the homologation reaction can be used, for example, for the synthesis of acetaldehyde from methanol [48], propionic acid from acetic acid [47], or ethyl acetate from methyl acetate [50]. Styrene may be produced from toluene by oxidation to benzyl alcohol [51] and homologation to 2-phenylethanol, which in turn can be dehydrated to styrene. From the chemical point of view, the applications of homologation reactions are broad and useful. But, as mentioned before, low selec-... 

Scheme 2. Insertion and Sn2 mechanism of the homologation reaction with methanol.

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