The kinetics and mechanism of the autoxidation of 2-mercaptoethanol, 2-aminoethanethiol and ethanethiol catalyzed by Co(II)- 4,4',4'',4'''-tetrasulfophthalocyanine, abbreviated as Co(II)TSP, has been examined. Kinetic data showed that the catalytic autoxidation of all three mercaptans proceeded via the same mechanism. The ultimate products of the autoxidation were found to be the corresponding disulfide (RSSR) and hydroxide ion. The kinetic data indicated that 50% to 80% of the disappearance of mercaptans was controlled by the catalytic cycle as opposed to oxidation by the intermediate, H2O2. The overall stoichiometry of the catalytic autoxidation is the sum of the following reactions:
[Chemical equation; see abstract in scanned thesis for details]
Stoichiometric ratio of mercaptan to oxygen of 1:4 was found for each mercaptan.
When the mercaptan was added, a dimeric Co(II)TSP was formed by the bridging of the two Co(II)TSP monomers by mercaptan anion. This dimer is proposed as the catalytic center for the cycle. The electron transfer from Co(II) to bound O2 forming a superoxide-like species is assumed to be the rate-determining step.
The experimental rate law for the autoxidation of each mercaptan was found to be
[Equation; see abstract in scanned thesis for details]
and an expression for kobsd of each mercaptans was derived from the postulated mechanism.
The kinetics reveal that the catalytic cycle is sensitive to variations in pH.The pH dependence at pH < 11 can be easily explained by the acid-base equilibria of the mercaptan (RSH ⇌ RS- + H+.However, the pH dependence at pH > 11 suggests that the deprotonation of Co(II)TSP dimer must occur and that pK'1 of the dimer must be between 11 and 13.
The rate of autoxidation of mercaptans follows the relative order of ethanethiol > 2-aminoethanethiol > 2-mercaptoethanol. Linear free energy relationship (LFER) was established 1) between Taft σ* value of the substituent group and the rate constant; 2) between Taft σ* value and the pK'2 where K'2 is the apparent acid dissociation constant of the catalytic dimer.
The hydrogen peroxide produced from the catalytic cycle oxidizes the mercaptan anions giving the disulfide as the product.The stoichiometry is found to be
[Chemical equation; see abstract in scanned thesis for details]
The kinetics and mechanism for the oxidation of 2-mercaptoethanol by hydrogen peroxide have been examined. The rate expression for the oxidation is as follows:
[Equation; see abstract in scanned thesis for details]
k'obsd depends on the apparent acid dissociation constants of HOC2H4SH, HOC2H4S- and H2O2.The kinetics and mechanism of reduction of Co(II)TSP by 2-mercaptoethanol under anoxic conditions were studied. Results from this study provide estimates of the rate of complexation of Co(II)TSP by 2-mercaptoethanol and the acid dissociation constant, Ka1, of thebound water on the Co(II)TSP-RS=Co(II)TSP. Co(II)TSP was reduced to Co(I)TSP by 2-mercaptoethanol. The rate expression is given by
[Equation; see abstract in scanned thesis for details]
where k2 is the rate constant for the electron transfer step, and K1 is the equilibrium constant for the complexation of CoTSP dimer with thioethanol.Ka1 and Ka2 are the apparent acid dissociation constantsof HOC2H4SH and HOC2H4S-, respectively; and α is K1/(1 + aH+/Ka1 + Ka2/aH+). Results of this system further enhance the argument thatthe electron transfer step rather than the preceding mercaptan complexation step is the rate-determining step in the oxygenated system.
This study indicates that Co(II)TSP is a very effective catalyst for the autoxidation of mercaptans. pH is an important physiochemical parameter for the autoxidation, whereas the effect of dissolved O2 concentration is rather insignificant. From a practical standpoint, use of the homogeneous catalyst is not very economical because of the relatively expensive recovery process. Solid-supported Co(II)TSP such as (SiO2)-Co(II)TSP, TiO2-Co(II)TSP, and polystyrene-divinyl benzene resin-Co(II)TSP contained in fixed bed reactors may be an economic alternative for mercaptan waste treatment. Nevertheless, the results of this study should provide useful insight into the design of a suitable hybrid system.
