Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon

Abstract

Desulfurization of diesel fuels with hydrogen peroxide was studied using activated carbons as the catalysts. Adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT) were investigated. The higher the adsorption capacity of the carbons is, the better the catalytic performance in the oxidation of DBT is. The effect of aqueous pH on the catalytic activities of the activated carbons was also investigated. Oxidation of DBT is enhanced when the aqueous pH is less than 2, and addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT shows a strong dependence on carboxylic group content. The oxidative removal of DBT increases as the surface carbonyl group content increases. Oxidative desulfurization of a commercial diesel fuel (sulfur content, 800 wt. ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. Much lower residual sulfur content (142 wt. ppm) was found in the oxidized oil after the oxidation by using the hydrogen peroxide–activated carbon–formic acid system, compared with a hydrogen peroxide–formic acid system. The resulting oil contained 16 wt. ppm of sulfur after activated carbon adsorption without any negative effects in the fuel quality, and 98% of sulfur could be removed from the diesel oil with 96.5% of oil recovery. Activated carbon has high catalytic activity and can be repeatedly used following simple water washing, with little change in catalytic performance after three regeneration cycles.

Introduction

Due to increasing environmental concern, special interest has been paid to reduction of organosulfur compounds in transportation fuels, because these compounds are converted into sulfur oxide, i.e., SO_x, during their combustion. In addition to this, SO_x in flue gas of automobiles poisons the catalysts for nitrogen oxide (NO_x) reduction [1]. As an alternative to hydrodesulfurization (HDS), oxidative desulfurization (ODS) has received much attention as a new method for deep desulfurization of diesel fuels [2], [3]. Various studies on the ODS process using different oxidizing agents such as NO₂[4], tert-butyl-hydroperoxide [5] and H₂O₂ have been reported. H₂O₂ is the most common oxidant because it is environmentally friendlier. Typically, H₂O₂ is used in the presence of a catalyst such as acetic acid [6], [7], formic acid [8], polyoxomethalate [9], [10], CF₃COOH [11], titano silicates [12], and solid bases [13]. However, reaction safety and cost are the important concerns for the selection of oxidant, catalyst and operating conditions for ODS processing. High concentration of peroxyacid [2], [7] and hydrogen peroxide [6], [8], [9] are necessary to oxidize sulfur compounds in light oil. However, the use of these oxidants at high concentrations should be avoided in terms of safety and the loss of oil quality. Although ODS is an efficient and promising method, there are still some issues that need to be considered, such as safety and economy of the process. The object of this study is to find a way to perform the oxidation reaction economically and safely under mild conditions.

Activated carbon (AC) is widely used as an adsorbent of organic contaminants due to its porous nature and large specific surface area. Nowadays AC is being used in an increasing number of catalytic reactions. It is used as either the support for the active phases, or as the catalyst itself because of its physical performance and surface functional groups containing primarily oxygen [14]. Both texture and surface chemistry determine their performance, and the final application of the carbon material will depend on its characteristics. Carbons were known to catalyze a number of oxidation reactions due to their capability of accepting electrons, as expected from the existence of unpaired electrons at crystallite edges [15]. Catalytic wet oxidation is a well-established method to remove hazardous substances in relatively mild conditions by using active oxidation catalysts [16], [17], [18], [19]. The hydroxyl radicals produced during the activation of hydrogen peroxide by AC provide such a strong oxidizing agent that it can oxidize organic compounds under ambient conditions [20]. The hydroxyl radicals generated from hydrogen peroxide can be resonance-stabilized on the carbon surfaces [21], [22], which results in the oxidation of heterocyclic organosulfur compounds to form ${\mathrm{SO}}_4^{2-}$[23], [24]. AC has a strong affinity for the oil phase; it is wetted by the oil and interacts with hydrogen peroxide which is present in an aqueous phase. We found that hydrogen peroxide catalyzed by AC could effectively oxidize organic sulfur compounds in light oil while the aqueous phase pH was adjusted by the addition of formic acid, this deriving from the combination of the hydroxyl radicals and the performic acid generated in the reaction medium. Organic sulfur compounds in the light oil were adsorbed and reacted on the active carbon simultaneously in the reactor. Therefore, the reaction and the adsorption in the desulfurization process were coupled, which led to low residual sulfur content in the oxidized oil.

The aim of the present work was threefold. On the one hand, oxidative removal of dibenzothiopene (DBT) in n-octane with hydrogen peroxide catalyzed by the activated carbons was investigated with respect to: (a) the structural characteristics of the activated carbons and adsorption performance of the carbons for DBT; (b) the catalytic performance of the activated carbons in the oxidative removal of DBT, the correlation between the adsorption and catalysis by the carbons; and (c) the effect of the aqueous phase pH on catalytic activities of the carbons and the cocatalysis by formic acid. On the other hand, the nature of the surface functional groups of AC was modified by chemical and thermal treatments in an attempt to clarify the role of the surface chemistry of AC in the removal of DBT. The effect of surface chemistry of AC on DBT adsorption and catalytic activity was investigated. Finally, the authors examined the applicability of AC catalyst to ultra-deep desulfurization of diesel fuels and the reusability of AC catalyst.