WGSR

Write a balanced chemical equation for (a) the hydrogenation of ethyne (acetylene, C2H2) to ethene (C2H4) by hydrogen (give the oxidation number of the carbon atoms in the reactant and product) (b) the shift reaction (sometimes called the water gas shift reaction, WGSR) (c) the reaction of barium hydride with water. 

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. 

Reduction of nitro compounds is a classic application of WGSR systems (Equation (17)). 

The past several years have seen renewed interest in the catalyst chemistry of the water gas shift reaction (WGSR, Eq. 1). 

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

A report in 1977 (3) of an active system prepared from [Rh(CO)2CI]2, CH3CO2H, cone. HC1 and Nal in water demonstrated that a basic medium is not a necessary condition for WGSR catalysis. This result stimulated us to examine the potential activity of several simple metal carbonyls in acidic solution as well. Attempts with Fe(CO)5 and Iri (CO)12 (17), both active in alkaline and amine solutions, proved unfruitful. However Ru3(CO)12 in acidic (0.5 N H2SOtf) aqueous ethoxyethanol gave WGSR activity substantilly larger than found in basic solutions under otherwise analogous conditions (Pco=0.9 atm, T=100°C, [Ru]Total=0 036 mol/L) (15). This solution proved unstable and... 

The Scheme depicts our most comprehensive understanding of the processes operative during the WGSR catalyzed by group 6b metal carbonyls at temperature < 100°C. As noted in the Scheme when the reaction is carried out in the presence of 13C0 both Cr(C0)3 and Cr(C0)sH are enriched in 13C-carbon monoxide, the latter species to a greater extent (see Figure 4). 

Figure 5 displays a typical time dependent trace of the hydrogen production during catalysis of the WGSR by Cr(CO)e. The decrease in activity of mature catalyst solutions is due to the consumption of KOH by C02, i.e., the formation of bicarbonate (C02 + 0H" HC03"). Reaction solutions prepared from Cr(CO)e with KHC03 as the added alkaline were much less active than their KOH counterparts. Experiments are planned at higher reaction temperatures in an effort to minimize this behavior. However, at 100° the Cr(C0) catalyst is quite active for the decomposition of formate ion to H2 plus C02 (vide infra). 

Catalysts and Catalyst Concentration. The most active catalyst for benzaldehyde reduction appears to be rhodium [Rh6(C0)i6 precursor], but iron [as Fe3(C0)i2] and ruthenium [as Ru3(C0)12] were also examined. The results of these experiments are shown in Table 1. Consistent with earlier results (12), the rhodium catalyst is by far the most active of the metals investigated and the ruthenium catalyst has almost zero activity. The latter is consistent with the fact that ruthenium produces only aldehydes during hydroformylation. Note that a synergistic effect of mixed metals does not appear to be present in aldehyde reduction as contrasted with the noticeable effects observed for the water-gas shift reaction (WGSR) and related reactions (13). 

Studies analyzing the effects of the remaining reactants, H20 and C6HsCH0 indicate that the reaction appears to be zero order with respect to both reactants. It is interesting that in previous work we also found similar behavior for H20 in ruthenium catalyzed hydroformylation (12), as did Ungermann et al. with the WGSR (14). 

Another noncatalytic step proposed by King et al. (18) in iron carbonyl/base catalysis of the WGSR involves the formation of formate ion however, we recently observed that formate formation appears to have little importance in the related rhodium catalysis of hydrohydroxymethylation. We plan to perform studies of the CO + KOH and C02 + KOH reactions independent of catalysis to more fully appreciate the relationship of these reactions to solution pH and thus the catalytic activity. 

The role of formate in the WGSR will be discussed below. Generally more H2 than CO2 is observed at the end of the reaction. Experiments (26) suggest that this is due to the solubility of CO2 in the solvent system. 

Effect of Concentration and CO Pressures on the Ruthenium Carbonyl-Trimethylamine WGSR System. As shown in Figure 1, the RU3(CO) 2/NMe3 WGSR system demonstrates a nearly first-order rate dependence on CO pressure at 0.5 mM Ru3(CO) 2 concentration. (Throughout this discussion, the total ruthenium carbonyl concentration is expressed as moles Ru3(00) 2 added per liter of solution this should not be construed to be the actual solution concentration of the trimer under operating conditions.) Here the initial rates of H2 production are 14.6 mmol /hr at 415 psi CO and 46.0 mmol /hr at 1200 psi. Thus, within experimental uncertainty, a threefold increase in CO pressure leads to a threefold increase in rate. 

Ford and co-workers (7) have reported a first-order rate dependence on CO pressure in the Ru3(CO)-l2/KOH system and ascribed this effect to CO participation in a rate-limiting elimination of hydrogen from a cluster species. This explanation does not fit our observations, because if loss of H2 were rate-limiting, the use of KOH and NMe3 as bases would be expected to lead to comparable rates for the WGSR. A comparison of activities (Laine (9) 2.3 mol H2 per mol Ru3(CO) 2 Per using KOH/MeOH... 

As shown in Table I, at 0.1 mM Ru (C0) 2 concentration, CO pressure has little if any effect on activity. On the other hand, at fixed pressure, the concentration of ruthenium carbonyl has a dramatic effect on activity (see Figure 2). At 0.1 mM Ru CCO), ruthenium carbonyl is very active for the WGSR, small decreases in catalyst concentration lead to substantial increases in activity, and no activity dependenee on CO pressure is observed. At concentrations of 0.5 mM or more, less activity is observed, changes in concentration cause smaller effects in activity and rate dependence on pressure is manifested. Diffusion effects have been shown to be unimportant (26). 

Thus, increases in CO pressure favor cluster dissociation and the formation of larger quantities of 2, High dilution should also favor the formation of 2, resulting in greater Ru(C0)3 to cluster ratios and greater WGSR activity. 

Assuming the WGSR has a first-order dependence on Ru(C0)3 concentration and that only trimeric and monomeric species are present, it can be shown that rate P q (26), in accord with... 

Small amounts of hydrocarbons added to the normal tetrahydrofuran or diglyme solvent system result in improved WGSR activity, but larger quantities inhibit the reaction (Table II). When 1-butene or 1-hexene is used, hydroformylation competes with the WGSR (4 ), but the rate of this process is small compared with the rate of H2 production. With pentane, no olefin or aldehyde products could be detected. Calderazzo (29) has reported that Ru(C0) is the principal product when the acetylacetonate of ruthenium is treated with synthesis gas in heptane,... 

The base has a very important effect on the efficiency of ruthenium carbonyl for the WGSR (see Table III). Amines were found to provide much better activity than Bronsted bases, and trimethylamine appears to be the base of choice, affording rates more than two orders of magnitude greater than those of Bronsted bases. 

Ammonia, primary and secondary amines are known to undergo side reactions under WGSR conditions ... 

Angelici and Brink (40) have found that in the reactions of amine with trans-M(CO),(PPhMe2)2+ (M = Mn or Re), the rate of carbamoyl formation follows the order, n-butylamine > cyclohexyl-amine >, isopropylamine > sec-butylamine >> tert-butylamine, implying a strong steric effect in carbamoyl formation. A similar order has been observed in the rate of reaction of organic esters with amines to form amides (41). The data in Table III indicate that a steric effect may be operative in the Ru (CO) /NR3-catalyzed WGSR, since with tertiary amines the rate follows the order, NMeQ > MeNC.H > NEt > NBu0, which does not reflect the basicity of these amines. 

The use of amines allows much higher nucleophile concentrations than those achievable with Bronsted bases. We have used solutions as concentrated as 6 M Me N. This vast difference in available nucleophile concentration partially explains the huge increase in rate afforded by NMe3 over the rate with Bronsted bases. Very large concentrations of hydroxide may promote the base attack step but can decrease the rate of the WGSR due to inhibition of the protonation of the metal hydride species. 

Additional Comments Regarding the Ruthenium Carbonyl-Tri-methylamine WGSR System. A potential mechanistic pathway for a WGSR system involves the production of formate, followed by its catalytic decomposition ... 

Ruthenium carbonyl decomposes the formate ion in basic media, but at a rate slower than the rate of the WGSR. At 100° and 0.10 mM Rug(C0)] 2 under 3 atm N2> the rate of decomposition of trimethyl ammonium formate to H2 and CO2 is 0.6 mmol/hr. Under 5 atm CO the rate is slower (<0.1 mmol/hr), but the overall rate of H2 production is >0.4 mnol/hr. At this low CO pressure, the rate of H2 production directly from CO and H2O is more than three times that from formate decomposition. Furthermore, since increases in CO pressure result in improved H2 production rates (10 mnol/hr at 50 atm CO), while apparently inhibiting the rate of formate decomposition, it may be concluded that formate decomposition has little mechanistic significance in the WGSR activity of Ru (CO)... 

On the basis of this discussion, we propose that the Ru3( CO) i2/NMe3-catalyzed WGSR follows the mechanism shown in Figure 3. A similar mechanism, involving nucleophilic attack by hydroxide instead of amine, has been proposed by Pettit and coworkers (4) for the Fe(C0)5/base system. 

By judicious adjustment of conditions, the rate of the Ru (CO) 2/NMe3 catalyze[Pg.330]

As mentioned earlier, those factors which favor formation of Ru(CO) — decreases in concentration and increases in CO pressure — favor higher turnover numbers in the WGSR. Increases in amine concentration and in temperature also improve the rates of H2 production. Thus, at 155°, 0.0082 mM Ru CO)- and 1080 psi... 

The Group VI metal carbonyls demonstrate good activity in the WGSR, but differ significantly from ruthenium carbonyl in several ways. Tables IV and V summarize some WGSR experiments with chromium and tungsten carbonyls in a tetrahydrofuran-water solvent system. 

With Cr(C0) , base clearly promotes the WGSR. However, unlike ruthenium carbonyl, chromium and tungsten carbonyls demonstrate less activity with trimethylamine than with carbonate as base. 

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