Catalytic properties of activated carbon surface in the process of adsorption/oxidation of methyl mercaptan

Activated carbons of different origins were used as adsorbents of methyl mercaptan (MM). Before the MM breakthrough capacity tests were carried out, the surface of carbons was characterized from the point of view of its chemistry (Boehm titration) and porosity (adsorption of nitrogen at its boiling point). The results showed that the ability of carbon to adsorb methyl mercaptan depends strongly on its surface chemistry, particularly on the presence of basic oxygen-containing groups and ash content. Catalytic effect of one metal, iron, was studied in more details. It was found that introduction of iron enhances the removal capacity significantly as a result of electron transfer reaction in which thiolate ion is oxidized to dimethyl disulfide (DMDS). This reaction likely involves the reduction of iron sites, which are regenerated after further re-exposure to oxygen. DMDS as a main reaction product is strongly adsorbed in small pores. Water is required for the formation of DMDS since it facilitates the dissociation of MM. That dissociation occurs in water film when the pH of the local system is greater than the apparent pKa of MM in the confined pore space.