Dissociation, carbon monoxide

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

Complexes of the type RMn(CO)5, where R is a primary alkyl group, undergo facile CO insertion at room temperature. Carbonylated to the corresponding acyls have been the pentacarbonyls with R = Me 50, 69), Et 51, 70), n-Pr 51), and CHjSiMe, 243). The phenyl compound, PhMn(CO)j, also inserts CO, but the benzyl analog does not 51). The claim 194) that CX3Mn(CO)5 (X = H, D, or F) converts to CX3COMn-(CO) ( < 5) upon irradiation in an Ar matrix at 17°K has been disputed 209). Carbon monoxide dissociation and recombination have been proposed instead for MeMn(CO)5. 

Most Group VIII metals adsorb carbon monoxide dissociatively, and, consequently, they are good Fischer-Tropsch catalysts.240 In contrast, Pd, Pt, Ir, and Cu do not dissociate carbon monoxide. Of these metals, copper and more recently palladium were found to be excellent methanol-forming catalysts. 

Alcohols and jlkenes are also primary products and are not shown in the simplified Eq. 15.182. The overall reaction is complicated and, as a result, its mechanism has been the subject of considerable debate.188 The reaction may be viewed as the reductive polymerization of carbon monoxide, with molecular hydrogen as the reducing agent. A variety of heterogeneous catalysts, such as metallic iron and cobalt on alumina, have been used. It is believed that carbon monoxide dissociates on the catalytic surface to give carbides and that these are in turn hydrogenated to give surface carbenes 1 " n ... 

This variance in intimate mechanisms is likely seen in the reactions of HCr(CO)5 and HW(CO)j with C02 to provide HC02M(C0)j derivatives (45). For example, carbon monoxide dissociation in the chromium anion, as evinced by l3CO exchange studies, occurs at a rate indistinguishable by conventional techniques from that of C02 insertion. Consistent with this observation, the rate of decarboxylation of HC02Cr(CO)jis retarded in an atmosphere of carbon monoxide. Similar behavior was noted in decarboxy-... 

M. Torrent, M. Duran, and M. Sola, Density Functional Study on the Preactivation Scenario of the Dotz Reaction Carbon Monoxide Dissociation versus Alkyne Addition as the First Reaction Step, Organometallics 17, 1492-1501 (1998). 

As obtained in this way iron pentacarbonyl is a somewhat viscous liquid, pale yellow in colour, which solidifies at about — 21° C. to a mass of yellow needle-shaped crystals. On heating the vapour it dissociates into metallic iron and carbon monoxide, dissociation being practically complete at 216° C.2... 

The photochemistry of these complexes has been studied extensively (50-63a), and the nature of the primary photoprocess has been a controversial subject. Both carbon monoxide dissociation [Eq. (64)] (50, 51a,b, 52-56, 59, 60, 61-63a) and metal-alkyl bond homolysis [Eq. (65)] (51, 51c, 57, 58, 60a) have been proposed on the basis of different experimental results. However, more recent work has shown that CO... 

Buatier F, de Mongeot A, Toma AM, Lizzit S, Petaccia L, Baraldi A (2006) Carbon monoxide dissociation on Rh nanopyramids. Phys Rev Lett 97 056103... 

Dissociative coking occurs when carbon monoxide dissociates on a catalytic site, a follows ... 

Possible mechanisms of carbon monoxide dissociation in electrical discharges have recently been discussed in Ref.39. ... 

While the hydrogenation of the active surface carbon that forms from CO dissociation appears to be the predominant mechanism of CH4 formation, it is not the only mechanism that produces methane. Poutsma et al. [85] have detected the formation of CH4 over paliadium surfaces that do not readily dissociate carbon monoxide. They also observed methane formation over nickel surfaces at 300 K under conditions in which only molecular carbon m.onoxide appears to be present on the catalyst surfaces [81]. Vannice [86] also reported the formation of methane over platinurh, palladium, and iridium surfaces, and independent experiments indicate the absence of carbon monoxide dissociation over these transition-metal catalysts in most cases. It appears that the direct hydrogenation of molecular carbon monoxide can also occur but that this reaction has a much lower rate than methane formation via the hydrogenation of the active carbon that is produced from the dissociation of carbon monoxide in the appropriate temperature range. 

The mechanism of this reaction was investigated to find out if carbon monoxide dissociation is the rate-determining step (16). The rate of the reaction of acetylcobalt tetracarbonyl with iodine is too fast to measure under conditions which allow dissociation rate to be measured easily. Thus, dissociation is not rate-determining, and the acylcobalt tetracarbonyl and the iodine must be reacting directly. Further studies with the less reactive acyl(triphenylphosphine)cobalt tricarbonyls showed that the first step in the reaction with iodine is a rapid cleavage of the cobalt-carbon bond to form acyl iodide and iodo(triphenylphosphine)cobalt tricarbonyl. 

Metallic iron itself has very low FTS activity. Although, under operational conditions the activity of metallic iron gradually increases over time. To improve the FTS activity and tune the product selectivity of iron catalysts, promoters such as alkali metals, transition metals and other additives are incorporated into the catalyst structure. Typical promoters and additives include copper, potassium and silica. Copper acts to enhance the rate of catalyst activation, silica improves the dispersion of catalytically active iron species, while alkali metals aid carbon-monoxide dissociation from surface iron. ... 

Coordinatively Unsaturated Cobalt Carbonyls Relevant to Hydro-formylation. The negative effect of carbon monoxide partial pressure on the rate of hydroformylation was the first indication of the participation of coordinatively unsaturated cobalt carbonyls in the catalysis of aldehyde formation and of the accompanying olefin isomerization. The retarding effect of carbon monoxide has also been observed in cobalt-catalyzed olefin and aldehyde hydrogenation and in various other reactions of cobalt carbonyls as well. It was assumed that in these reactions in fast reversible carbon monoxide dissociation highly reactive coordinatively unsaturated complexes are formed in very low concentrations, undetectable by conventional analytical methods. By using sophisticated new methods, in some cases the detection and characterization of these elusive species has become possible. 

Fig. 30. Carbon monoxide dissociation on a Rh(l 11) surface as a function of potassium coverage as determined by thermal desorption isotope scrambling experiments with C 0 and C ...

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