Cobalt reaction rate

Linear terminal olefins are the most reactive in conventional cobalt hydroformylation. Linear internal olefins react at less than one-third that rate. A single methyl branch at the olefinic carbon of a terminal olefin reduces its reaction rate by a factor of 10 (2). For rhodium hydroformylation, linear a-olefins are again the most reactive. For example, 1-butene is about 20—40 times as reactive as the 2-butenes (3) and about 100 times as reactive as isobutylene. 

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

Catalysts other than the above cobalt salts have been considered. Several patents suggest that cobalt bromide gives improved yields and faster reaction rates (12—16). The bromide salts are, however, very corrosive and require that expensive materials of constmction, such as HastaHoy C or titanium, be used in the reaction system. Only one faciHty, located in the UK, is beHeved to uti1i2e cobalt bromide catalyst in the production of ben2oic acid. 

Fischer Tropsch synthesis is catalyzed by a variety of transition metals such as iron, nickel, and cobalt. Iron is the preferred catalyst due to its higher activity and lower cost. Nickel produces large amounts of methane, while cobalt has a lower reaction rate and lower selectivity than iron. By comparing cobalt and iron catalysts, it was found that cobalt promotes more middle-distillate products. In FTS, cobalt produces... 

Sulfite reacts readily with oxygen, particularly under hot, alkaline conditions, but the reaction rate is slow in colder, neutral waters thus complete FW deaeration cannot be guaranteed. Consequently, it is standard practice to add a small amount of catalyst to the sulfite. The catalyst is usually cobalt sulfate [more properly, cobaltous sulfate (CoS04) supplied as an anhydrous, monohydrate, or heptahydrate salt] or sometimes cobaltous nitrate. The catalyst is added to 100% sodium sulfite at a concentration level of 0.2 to 0.25%. 

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

Stereochemistry and reaction rates of anionopentaaminc complexes of cobalt(III) and chro-mium(Ill). D. A. House, Coord. Chem. Rev., 1977, 23, 223-322 (451). 

Now the second cobalt was not oxidized, and the reaction rate became... 

It is interesting to note that cobalt cobaltite, C03O4, is a good catalyst, too, for anodic chlorine evolution. In this case, too, a correlation is observed between the reaction rate and the spinel s defect concentration (amount of nonstoichiometric oxygen). 

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

Iglesia, E., Soled, S.L., and Fiato, R.A. 1992. Fischer-Tropsch synthesis on cobalt and ruthenium. Metal dispersion and support effects on reaction rate and selectivity. J. Catal. 137 212-24. 

FIGURE 9.18 Influence of pressure on reaction rate, olefin content in the C3 fraction, and methane selectivity with cobalt as the catalyst for FT synthesis. Catalyst 100Co-18Th02-100 Si02 (Kieselguhr), H2/CO = 1.8, 175°C. 

The most difficult problem to solve in the design of a Fischer-Tropsch reactor is its very high exothermicity combined with a high sensitivity of product selectivity to temperature. On an industrial scale, multitubular and bubble column reactors have been widely accepted for this highly exothermic reaction.6 In case of a fixed bed reactor, it is desirable that the catalyst particles are in the millimeter size range to avoid excessive pressure drops. During Fischer-Tropsch synthesis the catalyst pores are filled with liquid FT products (mainly waxes) that may result in a fundamental decrease of the reaction rate caused by pore diffusion processes. Post et al. showed that for catalyst particle diameters in excess of only about 1 mm, the catalyst activity is seriously limited by intraparticle diffusion in both iron and cobalt catalysts.1... 

The reaction rates of various types of olefins follow much the same pattern with both cobalt- and rhodium-catalyzed systems. Wender and co-workers (47) classified the nonfunctional substrates as straight-chain terminal, internal, branched terminal, branched internal, and cyclic olefins. The results they obtained are given in Table III. 

Another significant and positive characteristic of phosphine-modified cobalt systems is that a high proportion of linear products can be obtained from internal olefins, with only a small sacrifice in reaction rates (58), as shown in Table X. 

It was concluded that in this case an equilibrium existed which gave 100 ppm of soluble cobalt at reaction temperature. The polymer support acted as a reservoir for furnishing soluble metal at reaction temperature and reabsorbing it after completion (about 10 ppm in the product after cooling to ambient temperature). The rate approximated that obtained in a standard cobalt reaction with 100 ppm of cobalt catalyst. 

Of the three catalytic systems so far recognized as being capable of giving fast reaction rates for methanol carbonylation—namely, iodide-promoted cobalt, rhodium, and iridium—two are operated commercially on a large scale. The cobalt and rhodium processes manifest some marked differences in the reaction area (4) (see Table I). The lower reactivity of the cobalt system requires high reaction temperatures. Very high partial pressures of carbon monoxide are then required in the cobalt system to... 

Figure 9.20 Correlation between the activity of a series of Co-Mo/AI203 catalysts for the HDS reaction, expressed in the reaction rate constant /cT, and the cobalt phases observed in Mossbauer spectra (left) as well as the NO adsorption sites probed with infrared spectra of adsorbed NO (right) (left figure from Wivel et al. [70], right figure adapted from [49] and [74]).

Reppe reaction involves carbonylation of methanol to acetic acid and methyl acetate and subsequent carbonylation of the product methyl acetate to acetic anhydride. The reaction is carried out at 600 atm and 230°C in the presence of iodide-promoted cobalt catalyst to form acetic acid at over 90% yield. In the presence of rhodium catalyst the reaction occurs at milder conditions at 30 to 60 atm and 150-200°C. Carbon monoxide can combine with higher alcohols, however, at a much slower reaction rate. 

Thus mixtures containing methyl iodide, which generates the protonic acids, HI and HRu(CO)3l3, and ionic iodides (Nal, KI), which provide the Lewis acid (K , Na ), give the highest yields of ethanol and the highest reaction rates (Table II) analogously to that found with cobalt-ruthenium catalysts (Ru/Co 2 I /Co 5) (13). ... 

The coordination atmosphere of the metal ion in solution can also be expected to affect the reaction rate. Microanalytical results indicate that the active catalysts in cobalt and nickel systems could well be metal thiolic species produced in situ. However, these complexes are appreciably more soluble in the, alkaline solutions than are metal hydroxides (see, for example, the analysis results reported in Table IV), and it is not possible on the present evidence to differentiate between catalysis as a result of increased solubility (comparing metal hydroxides and metal thiolic complexes), and catalysis as a result of differences in the allowed ease of electron transfer. It is apparent, however, that most of the metals investigated (Table I) are poor catalysts because they form only the insoluble hydroxide complexes. 

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