Studies of the oxidation of methyl phenyl sulfide (MFS) by hydrogen peroxide (H2O2) and urea peroxysolvate (CO(NH2)2·H2O2), catalyzed by organomodified montmorillonite (Claytone HY – montmorillonite modified with a tall alkonium cation) were performed. The kinetics of MFS oxidation were studied in the following decontamination environments: 1 – water; 2 –iso-propyl alcohol (i-PrOH)/water mixture; 3 – ethanol (EtOH); 4 – micellar solution of cetyltrimethylammonium bromide (CTABr); "oil-in-water" type microemulsion, containing either cationic CTABr (5), neutral Triton X-100 (6), or SDS - anionic sodium dodecyl sulfate (7) as a detergent. Claytone HY nanoclay was introduced into the decontamination systems in the amount of 1%. The choice of specific components and composition of reaction media is not random and was based on the results of previous research [1,2].
The selectivity of oxidation under the conditions used in measuring the speed of the processes ([MFS]о=2,0·10-3, [H2O2]о=[CO(NH2)2·H2O2]о = 0,1–1,0 M-1) was confirmed using gas chromatography. It was established that oxidation mainly results in the formation of methylphenylsulfoxide:
$$MeSPh + H_{2}O_{2} \rightarrow MeS(O)Ph + H_{2}O \tag{1}$$Further oxidation of sulfoxide to toxic sulfone does not occur within at least 24 hours from the starting point of MFS oxidation. It should be emphasized that uncatalyzed processes of oxidation by hydrogen peroxide are characterized by extremely low reaction rates and are not of practical interest. The present study is aimed at demonstrating the catalytic effect of layered silicate in reaction (1). Table below contains the values of the rate constants (k2, M-1s-1) for second-order reactions of MFS consumption in reaction medias 1–7, in the presence and absence of Claytone HY nanoclay. Numerical values of the catalytic effect (CE), which is calculated as the ratio of the corresponding k2 values for catalytic and non-catalytic oxidation, are also provided in the Table.
| Decontamination system | k2, M-1s-1104 | |||||
|---|---|---|---|---|---|---|
| H2O2 | CO(NH2)2·H2O2 | |||||
| – | Claytone HY | CE | – | Claytone HY | CE | |
| 1 | 15,1 | 34,5 | 2,3 | 20,6 | 38,4 | 1,9 |
| 2 | 6,12 | 21,3 | 3,5 | 8,04 | 22,4 | 2,8 |
| 3 | 2,45 | 12,4 | 5,1 | 3,43 | 18,9 | 5,5 |
| 4 | 0,42 | 1,89 | 4,5 | 0,89 | 3,23 | 3,6 |
| 5 | 1,01 | 3,21 | 3,2 | 2,12 | 8,32 | 3,9 |
| 6 | 2,3 | 8,94 | 3,9 | 3,67 | 15,3 | 4,2 |
Kinetic data allow us to state that a slight acceleration of the oxidation rate occurs when a solid source of hydrogen peroxide – CO(NH2)2·H2O2 urea complex, is used. An increase in the rate of MFS decomposition is also observed in systems containing nanoclay. The scope of the experiment does not make it possible to establish the exact reason for this acceleration of the reaction in the presence of montmorillonite clay. The main hypothesis is based on the assumption that the catalytic effect of clays is largely due to molecular interactions of active centers of montmorillonite (Brønsted-Lewis centers) leading to the formation of H-bonds and stabilization of transition states of intermediates (characteristical for the SN2 mechanism). Claytone HY montmorillonite clay with its large surface area is also likely to act as a strong acid due to its interlayer cations and surface protons. Either acid-base interaction, or the formation of hydrogen bonds between the silicon atom in clay and hydrogen peroxide molecules, shall increase the electrophilicity and reactivity of the intermediate hydroperoxide compound compared to inactivated H2O2. These hypotheses require further study and development. The established effect of accelerating the process of decomposition of organophosphorus substrates in a micellar medium by montmorillonite derivatives can be used in designing the fast-acting eco-friendly chemical warfare agents' neutralization systems.