Introduction

Mercaptan molecules are undesired compounds present in crude petroleum and also produced as a decay of animal and vegetable matter. Worldwide regulations for environmental protection are forcing the petroleum refining industry to drastically decrease the sulfur content in transportation fuels.1'2 Deep desulfurization using conventional hydrotreating technology (HDS) exhibits important limitations when dealing with larger molecules such as benzothiophene and di-benzothiophene (refractory molecules). In fact, an

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improved catalyst or increased reactor volume and/or more severe reaction conditions, especially higher temperatures and higher H2 pressures, would be required.3 Consequently, removal under mild conditions of organic sulfur compounds, both in gas and liquid streams, is becoming actually a challenging task in environmental chemistry.

One of the most promising "non-HDS" based technologies for sulfur removal is desulfurization by physical adsorption on a solid adsorbent. More specifically, recent studies have proposed activated carbon as sorbent for methyl mercaptan (MM) removal from air streams.4-7 Activated carbons are well known as versatile adsorbents for many different applications.8'9 The main advantages of activated carbon compared to conventional oxide-based adsorbents are their high surface area, the presence of a well-developed microporous structure, the high adsorption capacity and the possibility to tailor their surface chemistry and porous structure during the manufacture or by post-treatment.

Gas-phase removal experiments have shown that methyl mercaptan (MM) adsorption on activated carbon is highly dependent on both the porous structure and surface chemistry. Additionally, these studies have shown that the adsorption process is accompanied by surface assisted oxidation reactions of MM to dimethyl disulfide (DMDS) and even to sulfonic acid.4-6 Similar studies also report some carbon dioxide formation under excess oxygen at higher temperatures (473 K).5 Katoh et al. proposed an oxidation mechanism through hydroxyl and oxygen radicals created on the surface of wet activated carbon fibers, a process that was assisted by trace amounts of ferrous cations present on the carbon surface.4 Similar studies using activated carbons from different origins proposed the presence of two reaction mechanisms for the oxidation of methyl mercaptan depending on the presence or absence of moisture.6 In dry conditions, the authors proposed an oxidation process with MM and oxygen adsorbed from the gas phase on the carbon surface, with water and DMDS as final reaction products. However, an "island" mechanism was proposed for wet conditions, with MM and oxygen firstly being dissolved in water cluster followed by surface reaction between thiolate ions (CH3S) and dissociatively adsorbed oxygen in water, a surface pH dependent process.

The scenario in liquid phase adsorption becomes more complex due to the presence of the solvent and many other compounds such as aromatics, olefins, etc. which will modify the adsorption/oxidation behavior. Preliminary results obtained by our group in liquid-phase removal of propanethiol using different activated carbons have shown that the process is highly dependent on the porous structure and surface chemistry of the sorbent.10 Interestingly, even in the absence of an external oxygen supply (only the mercaptan molecule and the solvent are present together with the activated carbon), the results show the presence of parallel oxidation reactions of propanethiol to dipropyl disulfide. Unfortunately, to our knowledge no studies have been reported in the literature concerning this oxidation reaction on liquid-phase experiments using activated carbon.

With this, the aim of the present work is to give more insight into the liquid-phase removal of mercaptan molecules using activated carbons. The effect of the porous structure and surface chemistry of different activated carbons on the adsorption/oxidation of a simple mercaptan molecule such as propanethiol will be overviewed. Additionally, the oxidation of propanethiol to dipropyl disulfide will be studied in more detail by performing fixed-bed adsorption/breakthrough experiments. The effect of dissolved and chemi-sorbed oxygen in the oxidation process will be discussed.

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