Pressure of carbon monoxide, partial

Figure 3.11 illustrates the mass transfer coefficient for batch-grown R. rubrum and was computed with various acetate concentrations at 200 rpm agitation speed, 500 lux light intensity, and 30 °C. As the experiment progressed, there was an increase in the rate of carbon monoxide uptake in the gas phase and a gradual decrease in die partial pressure of carbon monoxide. Also, a decrease in the partial pressure of carbon monoxide was affected by acetate concentration in the culture media. The value of the slope of the straight line increased with the decrease in acetate concentrations, i.e. 2.5 to 1 g-l. The maximum mass transfer coefficient was obtained for 1 g-l 1 acetate concentration (KLa = 4.3-h 1). The decrease in mass transfer coefficient was observed with the increase in acetate concentration. This was due to acetate inhibition on the microbial cell population as acetate concentration increased in the culture media. The minimum KLa was 1.2h 1 at 3g l 1 acetate concentration. 

In the case of vanadium, the suboxide, vanadium monoxide, would be more volatile than carbon monoxide except at very high carbon concentrations in the metal. The removal of the residual oxygen from this metal by carbon deoxidation is, therefore, difficult. In the case of niobium and tantalum, the partial pressure of carbon monoxide is higher than that of niobium monoxide or tantalum monoxide, even when the residual carbon concentration in the metal is as low as 200 ppm. It may therefore be expected that practically all the oxygen would be removed by evaporation of carbon monoxide without any metal loss from niobium and tantalum metals containing both oxygen and carbon. 

For cobalt as catalyst, variations in reaction parameters have been studied as a means of controlling the product composition (or isomer ratio). Thus, variations in isomer ratio from 1 1 to about 4 1 were observed under widely differing conditions of temperature, catalyst concentration, partial pressure of hydrogen, and partial pressure of carbon monoxide. 

In an early investigation (28, 59, 60), critical combinations of several reaction parameters were discovered to produce unusually high yields of the linear isomer. The parameters included low partial pressure of carbon monoxide, high concentration of phosphite or aryl phosphine ligands, and low total gas pressure. The catalyst was a soluble complex of rhodium, formed in situ from rhodium metal in many cases. Isomer ratios of 10 1 to 30 1 were obtained by appropriate selection of these reaction parameters. Losses to alkane were minimal, even with Pm as low as 10 psi. Tables XI-XIV illustrate the effects of these various reaction parameters on the product composition. 

High ligand concentrations and/or low partial pressures of carbon monoxide cause a predominance of species substituted by more than one phosphorus ligand. These species containing multiple ligands present a greater sterically hindered environment for the olefin substrate and favor the linear product (24). Trialkylphosphines, the more basic ligands of the... 

In these studies it was also reported that butyraldehyde isomer ratios were increased by lowering the partial pressure of carbon monoxide, but that this decrease in Pco caused a dramatic and parallel increase in propane formation. It was concluded that propane was formed in lieu of isobutyraldehyde at low Pco. This effect is illustrated in Fig. 8. 

Fig. 8. Effect of the partial pressure of carbon monoxide on normal to isoaldehyde ratio and conversion to propane. Reprinted with permission from Hydrocarbon Process, p. 112 (1970). Copyright by Gulf Publishing Company.

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... 

The only dependencies noted in the kinetic studies were first-order dependencies on iodide promoter and rhodium concentrations. Thus there was no observed effect of varying methanol concentration, and the partial pressure of carbon monoxide had no effect on the reaction rate. Similarly, the concentration of the products, methyl acetate and acetic acid, has no effect on the reaction rate. Thus we have the unusual situation of a reaction, CH3OH + CO — CH3COzH, in which the concentrations of the reactants and product have no kinetic influence. 

By contrast, in 2000 Shibata reported the Ir-catalyzed enantioselective Pauson-Khand-type reaction of enynes [30aj. The chiral Ir catalyst was readily prepared in situ from [lrCl(cod)]2 and tolBINAP (2,2 -bis(di-p-tolylphosphino)-l,T-binaphthyl), both of which are commercially available and air-stable, and the reaction proceeded under an atmospheric pressure of carbon monoxide. The Ir-catalyzed carbonylative coupling had a wide generality in enynes with various tethers (Z), substituents on the alkyne terminus (R ) and the olefinic moiety (R ). In the case of less-reactive enynes, a lower partial pressure of carbon monoxide achieved a higher yield and ee-value (Table 11.1) [30b]. 

Figure 4. Effect of partial pressure of carbon monoxide for amorphous Fe,0NitoPti> catalyst at 230°C, and PH, — 0.5 atm. Key to PCo is the same as in Figure 3.

At the equilibrium point of this reaction the partial pressure of carbon monoxide is many times that of the carbon dioxide thus it would be much above 1 atmosphere, and carbon monoxide would escape from the crucible. With excess powdered charcoal in the crucible, therefore, both reactions would continue to run until all the barium carbonate had changed to barium oxide. Carbon monoxide does not react with barium oxide. 

The workers at the Bureau of Mines performed a series of three experiments at 185° using reduced cobalt metal and butyraldehyde as a substrate (Wender, Orchin, and Storch, 10). The catalyst was prepared fresh in each of the three experiments by treating cobaltous formate in cyclohexane with hydrogen at 185° for 2 hours. An initial partial pressure of 2000 p.s.i. of hydrogen was used in each experiment and the partial pressure of carbon monoxide was varied. 

These experiments strongly indicate that hydrogenations that proceed in the presence of cobalt and a high partial pressure of carbon monoxide very likely proceed by homogeneous catalysis in which either [Co(CO)4]2 or HCo(CO)4 or both function as the catalyst. 

The anion Co(CO)4 is isoelectronic with nickel carbonyl. Nickel does not form a hydrocarbonyl, which may account for its inability to function as a hydrogenation catalyst in the presence of a large partial pressure of carbon monoxide. 

In fact, decreasing the partial pressure of carbon monoxide also increases the selectivity of the reaction, and as far as the nature of (he tertiary phosphine ligand is concerned, of those tested, triphenylphosphine appears to be optimal in terms of reaction rate, selectivity and cost ... 

This equation thus gives Ceo s as a function of the partial pressure of carbon monoxide, and is an equation for the adsorption isotherm. This particular type of isotherm equation is called a Langmuir isotherm. Figure 10-8... 

The n/i selectivity of modified oxo catalysts increases with lower partial pressure of carbon monoxide and with high concentration of ligand. The effect of temperature is less pronounced. Under such conditions the predominant catalyst species is coordinated by more than one phosphine ligand. The metal center presents a more sterically hindered environment to the olefin and the formation of linear alkyl and acyl species is favored. Table 1 summarizes experimental evidence for these effects [8]. 

According to Natta s law (eq. (7)) the overall reaction rate is independent of the total pressure as long as the ratio of p(CO) to jp(H2) is 1 1 and a minimum carbon monoxide pressure is maintained to stabilize the metal carbonyl species. The influence of the partial pressure of carbon monoxide is depicted in Figure 5 (cf. p. 58). Low p(CO) initially increases the reaction rate whereas at higher partial pressures the rate drops (cf. Section 2.1.1.3.2) [96e, 123]. Raising the hydrogen partial pressure increases the reaction velocity [124] and to some extent the n/i ratio [125]. The latter effect is much less pronounced than for p(CO). Above a p(H2) of 60-80 bar almost no improvement in the n/i ratio is observed. 

The reaction is reversible, such that when the partial pressure of carbon monoxide is reduced, the acyl derivative reverts to the starting material. 

The IR spectra in the carbonyl stretching region, measured under hydroformylation conditions for 1-octene and other olefins, are consistent with the presence of acyl species, whose concentration increases by increasing the partial pressure of carbon monoxide, as expected on the basis of a step similar to reaction (b"). 

The reaction takes place in a rotating autoclave over eight hours, and under a partial pressure of carbon monoxide of 12 MPa. After bleeding off the carbonyl difluoride co-product and the excess of carbon monoxide, the COFI was removed by distillation at about 26.7 kPa, foliowed by a further distillation in vacuo over antimony powder to give a yield of 12% for COFI (based on IF ). The material is suitably stored in quartz vessels cooled with dry ice [1196,1751]. 

The relatively low partial pressure of carbon monoxide in the flash-tank has implications for catalyst stability. Since the rhodium catalyst exists principally as iodocarbonyl complexes (e.g., [Rh(CO)2l2] and [RtKCO LJ-), loss of CO ligands and precipitation of insoluble species (e.g., Rhl3) can be problematic. The conventional Monsanto process operates with a relatively high water concentration (10-15%, w/w) that helps to maintain catalyst stability and solubility (as discussed later). However, this operation results in a costly separation process to dry the product, typically requiring three distillation columns. The presence of water also results in the occurrence of the WGS reaction (Equation (2)), in competition with the desired carbonylation process, resulting in a lower utilization of CO. 

The experiments were carried out in small stainless steel autoclaves having an internal volume of 700 mL. The autoclaves, having been charged with a particular catalyst solution and gas mixture of interest, were mounted vertically in electrically heated ovens. The factors affecting the rate of the reaction are partial pressure of carbon monoxide, partial pressure of ethylene, catalyst concentration, temperature, base concentration/pH, and the nature of the base. Carbon monoxide has an inhibitory effect upon the reaction. The rate of reaction increases linearly with ethylene pressure in the low-pressure regime but exhibits saturation at ethylene pressures exceeding 17 atm. The reaction is second order with respect to catalyst concentration. The nature of the base used deter-... 

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