Ind. Eng. Chem. Res. 2008, 47, 973-975
973
Functionalized Hexagonal Mesoporous Silica as an Oxidizing Agent for the Oxidative Desulfurization of Organosulfur Compounds Paolo De Filippis* and Marco Scarsella Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, UniVersita` degli Studi di Roma “La Sapienza”, Via Eudossiana, 18, 00184 Roma, Italy
The use of peroxycarboxylic-acid-functionalized hexagonal mesoporous silica (HMS) as an oxidizing solid agent was investigated. The experimentation was performed on organic sulfur compounds that have been selected as representative of those contained in crude distillates. The results clearly show the potentiality of this oxidizing agent for the development of oxidative desulfurization (ODS) processes for the purpose of producing ultralow-sulfur fuels and sulfoxides and sulfones as secondary products. Introduction Oxidative desulfurization (ODS) seems to be particularly promising for the development of processes capable of higher desulfurization efficiency on oil refinery streams.1-12 Actually, one of the most investigated oxidative routes to produce ultralow-sulfur fuels is based on the well-known procedure where the oxidizing agent, a peroxycarboxylic acid, is generated in situ by hydrogen peroxide and the corresponding carboxylic acid.3,13-19 However, its industrial applicability is still difficult, because of the liquid phase of the reactant mixture that makes necessary a liquid-liquid separation step to remove both the spent reactant and the oxidized products from the treated stream. The use of solid oxidizing agents in ODS processes is highly desirable,20-24 with evident advantages both for the separation of oxidized products and the recovery of the spent reactant in an environmental and safe perspective. In the present work, we have specifically studied the possibility to develop an oxidation step using a solid oxidizing agent. Particularly, the oxidizing properties toward organosulfur compounds of a peroxycarboxylic-acid-functionalized hexagonal mesoporous silica (HMS) have been successfully investigated. The study has been conducted on simplified model systems, constituted by sulfur compounds selected between the most representative of those contained in fuels, dissolved in an aromatic solvent. Experimental Section Materials. Tetraethyl orthosilicate (TEOS), 2-cyanoethyltriethoxysilane (CETS), n-dodecylamine, methanesulfonic acid, and hydrogen peroxide (50%) were used in the peroxyacidfunctionalized HMS preparation. The sulfur compounds were selected as being representative of the organosulfur families found more frequently in the light and medium distillates from which commercial gasoline and diesel oil pools are produced. Specifically, the chosen sulfur compounds were 1-benzothiophene (BT), dibenzothiophene (DBT), and diphenyl sulfide (DPS). Toluene was used as an organic solvent. All the products were commercial reagent grade, supplied by Aldrich and Fluka, and used as received. Experimental Procedure. The preparation of the silicasupported peroxycarboxylic acid was made by acid hydrolysis, * To whom correspondence should be addressed. E-mail address:
[email protected].
followed by oxidation (with hydrogen peroxide) of the cyanoethyl-HMS, which was synthesized via the sol-gel alkaline route, using dodecylamine as templating agent, in accordance with the work reported by Elings et al.25 The amount of carboxylic groups on the acid-functionalized HMS was determined using acid-base titration, whereas the peroxycarboxylic groups content was determined using iodometric titration. The reaction mixtures were prepared by adding a weighted amount of the selected sulfur compounds in a known volume of toluene. The amount of added sulfur compounds was calculated to give, for all of the solutions, an elemental sulfur concentration of ∼480 ppm (BT:DBT:DPS molar ratios equal to 1:1:1). The oxidation experimental procedure was as follows. A quantity (20 mL) of the selected reaction mixture was put in a 100-mL two-necked flask that was equipped with a magnetic stirrer and a reflux condenser. The mixture was heated under stirring (at ∼750 rpm) at the selected reaction temperature and an appropriate amount of solid peroxycarboxylic acid (PC-HMS) was added and the reaction was started. The molar ratio between peroxycarboxylic acid groups and sulfur compounds in the mixture was ∼3. To determine the initial and residual concentration of the selected sulfur compounds in the organic phase, ∼0.5 mL aliquots of the liquid samples were withdrawn from the reactor at fixed time intervals and then each sample was analyzed by gas chromatography. Results and Discussion The loading in the synthesized functionalized silica of both acid and peroxyacid groups, determined by titration, were ∼2 and ∼1.6 mmol/g, respectively. The nonquantitative yield obtained in the oxidation of the carboxylic groups with hydrogen peroxide probably indicates that a fraction of the carboxylic groups is embedded in the silica structure and is not accessible to the peroxide oxidation. The obtained silica-supported peroxycarboxylic acid was tested as an oxidant for organosulfur compounds, as described in the Experimental Procedure. The tests were conducted using model molecules that are representative of the classes of sulfur compounds normally present in the crude oil medium distillates. These particularly involve aromatic compounds with high steric hindrance, which are often refractive to hydrodesulfurization.
10.1021/ie071057y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/11/2008
974
Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008
Figure 1. Plot of 1-benzothiophene (BT), dibenzothiophene (DBT), and diphenyl sulfide (DPS) conversion (T ) 40 °C). Table 1. Conversion after 4 min at Different Temperatures Conversion (%) sulfur compound
30 °C
40 °C
50 °C
60 °C
70 °C
diphenyl sulfide, DPS benzothiophene, BT dibenzothiophene, DBT
99.9 17.1 68.3
99.9 29.7 78.6
99.9 35.6 84.0
99.9 42.2 95.2
99.9 47.3 96.7
Figure 1 shows a typical plot of the advancement of the oxidation reaction performed at 40 °C, which has been evaluated considering the conversion of the selected sulfur compound, calculated as
conversion )
C0 - C C0
where C0 is its initial concentration and C its concentration after t minutes of reaction. The tested solid peroxycarboxylic acid seems to be highly reactive toward the heteroaromatic sulfur compounds (which generally are refractive to hydrodesulfurization), yielding a total conversion of >90% within 2 h. In the selected operative conditions, sulfur compounds are characterized by values of oxidation rate that are comparable to those calculated for the conventional “pseudo-homogeneous”
oxidizing systems: 4 min are sufficient to convert 100% of DPS, whereas the overall conversion for thiophenic compounds is ∼60%. The oxidation kinetics seems to be higher for DBT, for which the conversion is quantitative within
