Carbon monoxide with dihydrogen

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

The reaction of carbon monoxide with steam to produce carbon dioxide and dihydrogen is called the water gas shift (WGS) reaction and is an important process in the production of dihydrogen, ammonia, and other bulk chemicals. The overall reaction is shown below ... 

The reaction of OsHCl(CO)(P Pr3)2 with HC1 gives the dichloro derivative OsCl2( n2-H2)(CO)(PIPr3)2.35 In solution, this complex is stable under argon for a matter of days. However, the dihydrogen unit is highly activated toward heterolytic cleavage, as demonstrated by deprotonation with NaH and by reactions with carbon monoxide and /ert-butyl isocyanide, which afford OsHCl(CO)L(P Pr3)2 (L = CO, r-BuNC) and HC1. 

In the case of the tantalum complexes 100, reversible hydrogen migration may occur at room temperature in the presence of carbon monoxide, or at 70°C with dihydrogen or dimethylphosphino ethane to afford complexes 106, 107, and 108, respectively.92,95 In contrast, the phosphametallacycle remains intact when 100 is treated with halogenated reagents such as CH3X (X = Cl, Br, I).92,95... 

We have already reviewed the activation of alkenes, alkynes, and carbon monoxide towards nucleophilic attack. The heterolytic splitting of dihydrogen is also an example of this activation it will be discussed in Section 2.10. The reaction of nucleophiles with silanes co-ordinated to an electrophilic metal can be regarded as an example of activation towards nucleophilic attack (Figure 2.28). Complexes of Ir(III) and Pd(II) give t.o.f. for this reaction as high as 300,000 mol.mol. fh"1. 

The mechanism of the shift reaction in this catalyst system involves the attack of hydroxide anion at coordinated carbon monoxide, forming a metallacarboxylic acid. Elimination of C02 gives a rhodium hydride species that can react with the proton stemming from water to give dihydrogen. Rhodium may be either Rh(I) or Rh(III) as the valence of the metal does not change during this process. 

The reaction is first order in rhodium catalyst concentration, first order in dihydrogen pressure and has an order of minus one in carbon monoxide pressure. In our Scheme 6.1 this would be in accord with a rate-determining step at the end of the reaction sequence, e.g. reaction 6. Since the reaction order in substrate is zero, the rhodium catalyst under the reaction conditions predominates as the alkyl or acyl species any appreciable amount of rhodium hydride occurring under fast pre-equilibria conditions would give rise to a positive depence of the rate of product formation on the alkene concentration. The minus one order in CO suggests that the acyl species rather than the alkyl species is dominant under the reaction conditions. The negative order in CO is explained [20] by equilibrium... 

In addition to catalyzing hydroformylation, the platinum SPO complexes are excellent hydrogenation catalysts for aldehydes (as already demonstrated by the side products of hydroformylation), in particular, in the absence of carbon monoxide. Further, in ibis process, the facile heterolytic splitting of dihydrogen may play a role. The hydrogenation of aldehydes requires the presence of carboxylic acids, and perhaps the release of alkoxides from platinum requires a more reactive proton donor than that available on the nearby SPO. For example, 4 hydrogenates 2-methylpropanal at 95 °C and 40 bar of H2 to give the alcohol, with a TOF of 9000 mol moN h (71). 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

The activation and functionalization of C-H bonds by the Pt" ion is particularly attractive because of the unusual regioselectivity, high oxidation level specificity, and mildness of reaction conditions. Moreover, Sen has recently reported that, in the presence of copper chloride at 120-160 °C, Shilov chemistry can be made catalytic with dioxygen as the ultimate oxidant [39]. A number of aliphatic acids were tested, and turnover numbers of up to 15/hour with respect to platinum were observed. H/D exchange studies also confirm the marked preference for the activation of primary C-H bonds in the presence of weaker secondary C-H bonds. This study constituted the first example of the direct use of dioxygen in the catalytic oxidation of unactivated primary C-H bonds under mild conditions that does not involve the use of a co-reductant (e. g., sacrificial metals, 2H + 2e", dihydrogen, or carbon monoxide see below). 

Studies indicate that the overall transformation encompasses three catalytic steps in tandem (Scheme 6) [9 a, 40]. The first is the water-gas shift reaction involving the oxidation of carbon monoxide to carbon dioxide with the simultaneous formation of dihydrogen. It is possible to by-pass this step by replacing... 

At temperatures below 70 C, any dihydrogen impurity in the carbon monoxide does not react (in the dark) with the Clj to form HCl [89,1650]. This observation may be significant in connection with the manufacture of phosgene from carbon monoxide containing unseparated And, by using a 2.5% molar excess of CO in the temperature range of... 

M(C0)5X , and H2 in the presence of a general base to provide anionic metal hydrides. This process was shown to be first-order in both metal complex and dihydrogen and was not inhibited by addition of carbon monoxide. Consistent with the rds in catalysis being formation of the metal hydride intermediate, the metal catalyzed reaction of RX/CO2/H2 to provide HCOOR is not inhibited by CO. The well-established formation of metalloformate, M(C0)s02CH", from M(C0)5H and CO2 is followed by a less facile process involving the reaction of the metalloformate with RX. This latter reaction is first-order in both metal complex and alkyl halide and is inhibited by carbon monoxide. 

Carbon monoxide has a high affinity for transition metals, forming the metal carbonyls (see 14.6.2.). Despite this, CO reacts slowly or not at all with metals. Some finely divided metals (Fe, Co) are converted slowly to the corresponding carbonyls under drastic conditions of T and P. Active Ni, as it is obtained by reducing nickel oxide with dihydrogen at 400°C, is, on the other hand, easily carbonylated to Ni(CO)4 even at temperatures as low as 30°C ... 

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