Reaction rates cobalt catalysis

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.)... 

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. 

The cobalt(III) initiation and catalysis pathways are very effective in many oxidations but suffer some limitations, e.g., Co " is strongly inhibited by cobalt(II) ions, which seem to form dimers with Co ". Such dimers are only weak catalysts in arene oxidations. As a result the rate of oxidations is inversely dependent on the concentration of Co " in the reaction mixture thus the cleavage of such dimers by addition of small amounts of co-catalysts will attain the reaction rate [11c, 12]. Additionally in the case of deactivated, electron-poor systems such as toluic acid or p-nitrotoluene, cobalt(III) alone is not an efficient catalyst - synergistic co-catalysts are necessary to achieve good results. 

The most important t5q)es of homogeneous catalysis in water are performed by acids, bases and trace metals. A wide variety of mechanisms have been outlined for acid/base catalysis and are presented in kinetics texts (e.g. Moore and Pearson, 1981 Laidler, 1965). A number of bases have been observed to catalyze the hydration of carbon dioxide (Moore and Pearson, 1981 Dennard and Williams, 1966). Examples are listed in Table 9.7 for OH and the base Co(NH3)gOH2. The most dramatic effect is the catalysis of HS-oxidation by cobalt-4,4, 4",4"-tetrasulfophthalocyanine (Co-TSP ). At concentrations of 0.1 nM Co-TSP the reaction rate was catalyzed from a mean life of roughly 50 h to about 5 min. The investigators attributed the reason for historically inconsistent experimentally determined reaction rates for the H2S-O2 system by different researchers partly to contamination by metals. Clearly, catalysis by metal concentrations that are present in less than nanomolar concentrations is likely to be effective in aquatic systems. We shall see that similar arguments apply to catalysis by surfaces and enzymes. 

Three types of catalytic experiments were achieved on each Co-substituted P-zeolites. First, the catalytic activity of the as-made zeolite was evaluated. Then, the activity of the calcined one was investigated. Finally, the as-synthetised Co-substituted zeolite was treated in acetic acid at reflux and the filtrate was fed into reaction. The results are reported in Table 4. The boric (as-made or calcined) Co-substituted P-zeolites presented a catalytic activity higher than the activities of the uncatalysed reaction and the reaction with boric zeolite in the proton form. As the boric Co-substituted zeolites were not acid, they did not decrease the reaction rate of the oxidation. The filtrate of the boric Co-substituted zeolite was as active as the solids. This demonstrated that the catalysis resulted from the cobalt in solution. 

A common feature of all catalysis for F-T synthesis, whether they are cobalt or iron based, is that the catalytic activity is reduced due to the oxidation of active species. Under the typical reaction conditions, this oxidation may be caused by water, which is one of the primary products in the F-T process. On the other hand, at low partial pressure water can also help to increase the product quality by increasing the chain growth probability. Thus, in situ removing some of the water from the product and keeping the water pressure at an optimal value may improve the catalysis activity and promote the reaction rate. Zhu and coworkers [22] have evaluated the potential separation using NaA zeolite membrane to in situ removal of water Irom simulated F-T product stream. High selectivity for water removal from CO, H2 and CH4 were obtained. This result opened an opportunity for in situ water removal from F-T synthesis under the reaction conditions. 

Lim derivative was also studied using HP-NMR, and a mechanism for the formation of branched aldehyde products was proposed. Further work published by Crause et a described the effect of variation of the Lim alkyl chain. HP-NMR and HP-IR studies, combined with molecular modeling and autoclave experiments, showed that the primary factor governing performance of these ligands was the extent of phosphine-modified versus unmodified catalysis. Control of catalyst equilibria in favor of cobalt—phosphine species resulted in improved product linearities, but slower reaction rates. 

Various metals catalyze reaction (5). Homogeneous catalysis by dissolved salts, such as nickel sulfate, is useful in the treatment of plant wastes. The higher rate of reaction makes treatment in vessels of reasonable size feasible. These catalysts would not be applicable to streams, such as depleted brine, that recycle to the process. Instead, over the years, there have been attempts to produce heterogeneous catalysts for use in fixed beds. These rely on catalytically active metals such as cobalt and nickel. Section 7.5.9.3B discusses the process and apparatus now used commercially with a nickel catalyst [89]. 

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. 

A complete mechanism for the autoxidation of alkylaromatic hydrocarbons by cobalt(n) in acetic acid has not been established,25 6 although a complex rate law has been determined for tetralin. 22 The reaction most likely proceeds by a fiiee radical chain mechanism in which the purpose of the cobalt ions is to provide a hi h steady state concentration of free radicals by catalysis of the decomposition of THP. The free radical nature of the autoxidation of tettalin with the colloidal CoPy catalysts is supported by experiments which showed inhibition of the reaction by 2,6-di-rerr-butylphenol and 2,6-di-rm-butyl-4-methylphenol, and by a shortening of the induction period and increase of the reaction rate when azobis(isobut nitrile) was added to the reaction mixture as a free radical initiator. 

The properties of polyurethanes derived from the hydroformylation of fatty acid derivatives, subsequent hydrogenation, and reaction with isocyanates such as toluene diisocyanate (TDl), methylene diphenyl-4,4-diisocyanate (MDI), and 1,6-hexamethylenediisocyanate (HDI) may be strongly dependent on the metal used for the hydroformylation [12a, 62]. At high conversion rates with a rhodium catalyst, a rigid polyurethane A is formed, whereas under the conditions of cobalt catalysis and low conversion a hard rubber or rigid plastic (polyurethane B) with lower mechanical strength results (Scheme 6.100). 

Catalysis by reversed micelles was also discussed previously in connection with substitution reactions at cobalt(ra). > In a similar study rate constants for the transacts isomerization of the [Cr(ox)2(OH2)2] ion in the polar cavities of reversed alkylammonium carboxylate micelles in benzene are up to 63 times as large as values found in pure water. At constant water activity increasing the concentration of... 

The bipyridyl complexes of Co(ID showed electrocatalytic activity for reduction of allyl halides to 1,5-hexadiene in micellar media. Catalysis lowered the overpotential for reduction of allyl chloride in 0.1 M SDS and CTAB by 1.4 V compared to direct reduction. Yields of about 60% of l,5 hexadiene were obtained from electrolyses at carbon felt electrodes. Small micellar enhancements of reaction rates were found for tris 2,2 bipyridyl)cobalt(II) (Table 1). Catalytic efficiency followed the order CTAB > SDS = acetonitrile. Preliminary work with the long chain derivative bis(2,2 -bipyridyl)(4,4 -hexadecyl-2,2 -bipyridyl)cobalt(II)... 

In the case of cobalt ions, the inverse reaction of Co111 reduction with hydroperoxide occurs also rather rapidly (see Table 10.3). The efficiency of redox catalysis is especially pronounced if we compare the rates of thermal homolysis of hydroperoxide with the rates of its decomposition in the presence of ions, for example, cobalt decomposes 1,1-dimethylethyl hydroperoxide in a chlorobenzene solution with the rate constant kd = 3.6 x 1012exp(—138.0/ RT) = 9.0 x 10—13 s—1 (293 K). The catalytic decay of hydroperoxide with the concentration [Co2+] = 10 4M occurs with the effective rate constant Vff=VA[Co2+] = 2.2 x 10 6 s— thus, the specific decomposition rates differ by six orders of magnitude, and this difference can be increased by increasing the catalyst concentration. The kinetic difference between the homolysis of the O—O bond and redox decomposition of ROOH is reasoned by the... 

The chemical reactivity of crown-ether complexes with neutral molecules has received little attention. Nakabayashi et al. (1976) have reported crown-ether catalysis in the reaction of thiols with l-chloro-2,4-dinitrobenzene. The catalytic activity was attributed to deprotonation of thiols by dicyclohexyl-18-crown-6 in acetonitrile solution. Blackmer et al. (1978) found that the rate of aquation of the cobalt(III) complex [333] increases on addition of... 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

Operando methodology aims to define and characterize structure/function relationships which must be interfaced with rate and dynamics measurements of the elementary steps. Recent years have shown a marked increase in the presence of spectroscopic investigations of catalytic reactions in literature (see Catalysis Today, 113 issues 1-2). For example, operando techniques were used to determine the temperature stability range of two NOx reduction catalyst types, (NH4)[Co(H20)2]Ga(P04)3 vi. (NH4)[Mn(H20)2]Ga(P04)3. Fig. 5 shows that the catalyst with manganese changes in structural stability around 673 K. Inspection of the catalyst with cobalt shows that there is no structure modification at a temperature below 673 K. 

I think in the current paper, Dr. Wilmarth s paper worked on by Dr. Haim, the acid catalysis of the aquation of the azide system is an example of what I call an off-site reaction. The attachment of hydrogen to nitrogen, which is three atoms away from the cobalt atom bringing about a weakening of the cobalt nitrogen bond and—if I remember the figures correctly—a 3500-or 5800-fold increase in the rate of aquation. 

---