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A novel oxidative desulfurization process to remove refractory sulfur compounds from diesel fuel
Applied Catalysis B: Environmental 63 (2006) 85–93 www.elsevier.com/locate/apcatb
A novel oxidative desulfurization process to remove refractory sulfur compounds from diesel fuel Jeyagowry T. Sampanthar *, Huang Xiao, Jian Dou, Teo Yin Nah, Xu Rong, Wong Pui Kwan Applied Catalysis Technology Group, Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), No. 1, Pesek Road, Jurong Island, Singapore 627833, Singapore Received 17 June 2005; received in revised form 12 September 2005; accepted 12 September 2005 Available online 25 October 2005
Abstract Manganese and cobalt oxide catalysts supported on g-Al2O3 have been found to be effective in catalyzing air oxidation of the sulfur impurities in diesel to corresponding sulfones at a temperature range of 130–200 8C and atmospheric pressure. The sulfones were removed by extraction with polar solvent to reduce the sulfur level in diesel to as low as 40–60 ppm. Oxidation of model compounds showed that the most refractory sulfur compounds in hydrodesulfurization of diesel were more reactive in oxidation. The oxidative reactivity of model impurities in diesel follows the order: trialkylsubstituted dibenzothiophene > dialkyl-substituted dibenzothiophene > monoalkyl-substituted dibenzothiophene > dibenzothiophene. # 2005 Elsevier B.V. All rights reserved. Keywords: Oxidative desulfurization; Diesel; Sulfur; Catalyst; MnO2/g-Al2O3; Co3O4/g-Al2O3; Solvent extraction
1. Introduction Deep desulfurization of diesel fuel has become an important research subject due to the upcoming legislative regulations to reduce sulfur content in most western countries. The US Clean Air Act Amendments of 1990 and the new regulations by the US Environmental Protection Agency (EPA) and government regulations in many countries call for the production and use of more environment-friendly transportation fuels with lower contents of sulfur and aromatics. The demand for transportation fuels has been increasing in most countries for past two decades. For example, US Environmental Protection Agency has set up guidelines to limit the sulfur content of diesel fuel to 15 ppm by 2006 [1]. Conventional hydrodesulfurization (HDS) process has been employed by refineries to remove organic sulfur from fuels for several decades and the lowest sulfur content achieved by such process in the fuels is around 500 ppm. However, to meet the challenges of producing ultraclean fuels, especially with sulfur content lower than 15 ppm, both capital investment and operational costs would be rather high due to more severe operating conditions [2]. Consequently, * Corresponding author. Tel.: +65 67963819; fax: +65 63166182. E-mail address: [email protected] (J.T. Sampanthar). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.09.007
several alternative approaches have been used, such as biodesulfurization [3], selective adsorption [4], extraction by ionic-liquid [5] and oxidative desulfurization (ODS) [6–14]. Various studies on the ODS process have reported the use of differing oxidizing agents and catalysts, such as H2O2/acetic acid [7] and H2O2/formic acid [8], H2O2/heteropolyacids [9], H2O2/inorganic solid acids [10], NO2/heterogeneous catalysts [11], ozone/heterogeneous catalysts [12], tert-butylperoxides/ heterogeneous catalysts [13] and O2/aldehyde/cobalt catalysts [14]. The ODS process is usually carried out under mild conditions which present competitiveness over the conventional HDS process [15]. In this process, the sulfur compounds present in diesel are oxidized by the oxidizing agent to give rise to the corresponding sulfones. These sulfones are highly polarized compounds, such that they are removed from the diesel by subsequent solvent extraction using water-soluble polar solvents, such as NMP, DMF, DMSO and MeOH, etc. [15]. By combination of the processes, the sulfur content of the diesel can be reduced to 50 ppm [16]. Scheme 1 shows the oxidation of organic sulfur compounds. The resulting sulfones can be removed by either extraction and/or adsorption. Here, we report the effective use of air as an environmentally benign and low-cost oxidant to oxidize the sulfur compounds in diesel at ambient pressure and moderate temperature in the
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Scheme 1.
presence of heterogeneous based simple transition metal oxides loaded on g-Al2O3. 2. Experimental 2.1. Materials Untreated diesel with a sulfur content in the range of 430– 465 ppm was obtained from Shell Petroleum Corporation, Singapore. Co(NO3)6H2O, Mn(CH3COO)24H2O, dibenzothiophene (DBT), 4-methyl-dibenzothiophene (4-MDBT), 4,6-dimethyl-dibenzothiophene (4,6-DMDBT), 4,6-diethyldibenzothiophene (4,6-DEDBT) and n-tetradecane were purchased from Sigma–Aldrich, Singapore and used without further purification. Model diesel was prepared by adding equimolar amounts of dibenzothiophene, 4-methyl-dibenzothiophene, 4,6-dimethyl-dibenzothiophene and 4,6-diethyl-dibenzothiophene to n-tetradecane to make up a solution with a total sulfur content of 400 ppm. 2.2. Catalyst preparation A 10 g of g-Al2O3 pellet (obtained from Singapore Catalyst Technology Center, diameter 3–4 mm, length 6–10 mm with a specific surface area and a total pore volume of 370 m2 g 1 and 0.87 ml g 1, respectively) was impregnated with cobalt nitrate and/or manganese acetate aqueous solutions by an incipient wetness method. The total metal oxide loading with respect to g-Al2O3 ranged from 2 to 13 wt%. The impregnated samples were left on a roller which was set at 25 rpm for 18 h to obtain better dispersion. The water content of the samples were removed and dried at 120 8C in the oven for 18 h followed by calcination in a static furnace at 550 8C for 5 h with a ramp of 5 8C min 1. Other catalysts, such as W, Ni, Fe and Cu, were also prepared in a similar manner. 2.3. Catalyst characterization The physical and chemical properties of the prepared catalysts were characterized by various analytical techniques. The monolayer deposition of metal oxides on g-Al2O3 was confirmed using powder X-ray diffraction (XRD) technique using a Bruker AXS D8 Advance instrument with Cu Ka radiation at 40 kV and 40 mA. The N2 adsorption–desorption isotherm of the catalysts were studied using an Autosorb-1 at
77 K. Prior to the measurement, the calcined catalysts were degassed at 300 8C. The thermal behavior of uncalcined catalysts were studied by thermo gravimetric analysis (TGA) using Universal V2.5H TA, Model SDT 2690 instrument under laminar flow of air with a flow rate of 90 ml min 1. X-ray photoelectron spectroscopy (XPS) investigation was conducted on a VGESCALAB 250 spectrometer using monochromatic Al Ka, X-ray source (1486.6 eV) at a constant analyzer pass energy of 20.0 eV. All binding energies were referenced to the C 1s peak arising from adventitious carbon (BE 285.0 eV). The metal oxides composition and loading in the catalysts were also characterized and confirmed by ICP (Model Vista-MPX) and SEM-EDX (Model Jeol JSM-6700F). 2.4. Analysis of S—content in the model and real diesel Sulfur content of the model and real diesel samples were analyzed by XRF and GC-AED. The total sulfur content in the model and real diesel samples were analyzed using XRF (Bruker AXS S4 Exporer), which was calibrated with six liquid calibration standards (obtained from AccuStandard), and concentration ranging from 0 to 500 ppm sulfur by wt%. The 10 ml of samples were placed into 40 mm diameter plastic cells equipped with 2.5 mm MYLAR polyester film window. Each cell was vented to prevent the polyester film window from bulging during the analysis. The samples were then placed in the automatic sample chamber and the optical path of the XRF was flushed with helium gas prior to the measurement. A 6890 GC coupled with atomic emission detector (JASAED) was used to identify the various sulfur compounds and their concentration. The GC was equipped with a split/splitless injection port and operated in split mode. A 30 m 0.32 mm i.d. 1 mm film thickness HP-1 MS capillary column was used for separation as it has a lower specified rate of column bleed than conventional methyl silicone capillary columns. Hydrogen and oxygen gases were used as reagent gases for both carbon (179 nm) and sulfur (181 nm). In order to improve the sulfur selectivity over carbon, the AED gas flows (hydrogen, oxygen and helium makeup) were optimized to minimize interferences from hydrocarbons. The samples volume of 1 ml was injected without any solvent dilution. The GC-AED instrument was calibrated with sulfur in diesel fuel SRM 2724 obtained from National Institute of Standards and Technology (NIST), Reference Material Department, US. The SRM 2724 is a commercial no. 2-distillate fuel oil as defined by ASTM with
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the certified total sulfur content of 425 ppm. In addition to the SRM 2724, the AccuStandard (also obtained from NIST) with sulfur content in diesel fuel 0, 100, 200, 300, 400 and 500 ppm samples were also used for the calibration. 2.5. Solvent extraction on real diesel without oxidative treatment Solvent extraction studies for the removal of sulfur compounds in untreated diesel (obtained from Shell Petroleum Corporation, Singapore with a sulfur content in the range of 430– 465 ppm) were carried out with four different organic solvents of different polarities, such as acetonitrile (AcN), dimethylforamide (DMF), 1-methyl-2-pyrrolidinone (NMP) and methonal (MeOH). A 25.0 ml of untreated diesel were mixed with the known volume of polar organic solvents to determine the efficiency of solvent extraction. The diesel–solvent mixture was stirred for 30 min before separating the two layers. After extraction by the respective polar solvents, the sulfur content in the diesel was measured by XRF and GC-AED. 2.6. Catalytic oxidation followed by solvent extraction on model diesel The oxidation experiments in this study were carried out with 20.0 ml of model diesel in a refluxed round bottom flask. Approximately, 20–30 mg of g-Al2O3 supported Mn and/or Co oxides in the form of pellets were used as catalysts. The reactions were carried out at a temperature range of 90–180 8C, during which air was introduced via a gas disperser at a constant flow rate of 100 ml min 1 while the reaction mixture was stirred throughout the experiment. Awater-cooled reflux condenser was mounted on top of the reaction flask to prevent solvent loss and
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Table 1 BET surface areas and total pore volume of the prepared catalysts after calcinations at 550 8C Catalysta
Surface area (m2 g 1)
TPV (ml)
g-Al2O3 2%MnO2/g-Al2O3 5%MnO2/g-Al2O3 8%MnO2/g-Al2O3 11%MnO2/g-Al2O3 13%MnO2/g-Al2O3 2%Co3O4//g-Al2O3 5%Co3O4//g-Al2O3 8%Co3O4//g-Al2O3 5%MnO2/3%Co3O4//g-Al2O3 3%MnO2/3%Co3O4//g-Al2O3 3%MnO2/5%Co3O4//g-Al2O3
377 361 350 331 317 305 368 350 323 322 331 310
0.87 0.86 0.86 0.78 0.80 0.77 0.79 0.76 0.76 0.72 0.75 0.72
a All the above samples were calcined at 550 8C under static air and degas at 300 8C for 5 h before measurements.
serve as an air outlet. The progress of the reaction was monitored periodically withdrawing 0.5 ml aliquots of the reaction mixture for GC-AED analysis. Blank experiments were carried out with model diesel and pure support of g-Al2O3 under exactly similar experimental conditions. The oxidized products in the model diesel were extracted with NMP or methanol after the completion of the reaction. The reacted model diesel was mixed with one of these polar solvents at different volume ratio (e.g. diesel:polar solvent = 4:1 when NMP as a solvent, 1:1 when MeOH as solvent) and was magnetically stirred for 30 min. The mixture was then transferred into a separating funnel and separated into two layers of model diesel and polar solvent which were analyzed for sulfur compounds using GC-AED. When MeOH was used as solvent, it was removed from solvent–sulfone mixture using rotary
Fig. 1. X-ray photoelectron spectroscopy analysis of metals oxides loaded on gamma-alumina: (A) 11%MnO2/g-Al2O3; (B) 5%MnO2/3%Co3O4/g-Al2O3; (C) 3%MnO2/5%Co3O4/g-Al2O3; (D) 8%Co3O4/g-Al2O3.
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Fig. 2. Sulfur-specific gas chromatograms of model diesel (air oxidation of model diesel (400 ppm sulfur) catalyzed MnO2 (11%) loaded on g-alumina support; solvent extraction was carried out using NMP:oxidized diesel = 1:1, single extraction).
evaporator and the product of sulfones mixture was precipitated at the bottom of the flask. 2.7. Catalytic oxidation by Mn and/or Co oxides supported on g-Al2O3 followed by solvent extraction on real diesel A 150.0 ml real diesel underwent oxidative desulfurization reaction in the presence of about 100 mg of catalyst at temperature range of 130–200 8C in a two-necked round bottom flask. The reaction mixture was magnetically stirred to ensure a good mixing and bubbled with purified air at constant flow of 100 ml min 1. The reaction mixture was periodically
Fig. 3. Conversion % of the thiophenic compounds to the corresponding sulfone in model diesel (catalyst: 11%MnO2 loaded in g-Al2O3, temperature 150 8C).
sampled and analyzed using GC-AED and reaction was ceased after about 18 h. The oxidized diesel was then cooled to room temperature and 25.0 ml of reacted diesel was treated with varying volume of different solvents for solvent extraction. Sulfur content of the extracted oxidized real diesel was measured by XRF and GC-AED. Similar reaction and solvent extraction method were carried out with different loading of both Mn and/or Co oxide catalysts. 3. Results and discussion 3.1. Characterization of catalyst The TGA studies showed that the most of the metal salts loaded on the g-Al2O3 converted into their corresponding oxides at below 500 8C under laminar flow of air. Table 1 summarizes the specific surface area and total pore volume of the prepared catalysts. It shows that the specific surface area in the series considerably lowered from 377 m2 g 1 for pure g-Al2O3 support to the lowest value of 305 m2 g 1 for the sample loaded with the maximum amount of transition metal oxide (13%MnO2/g-Al2O3). The total pore volume of the calcined samples also decreases as the loading of the transition metal oxides increases. The decreasing behavior of both surface area and total pore volume with the increasing loading of the metal oxides are consistent due to the possible blockage of the inner pores, especially the smaller ones, and dilution of the
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Fig. 4. Sulfur-specific gas chromatograms of real diesel (air oxidation of real diesel diesel (450 ppm sulfur) catalyzed MnO2 (11%) loaded on g-alumina support; solvent extraction was carried out using NMP:oxidized diesel = 1:1, single extraction).
initial support material, g-Al2O3, by the uniformly dispersed and dense metal oxide, MnO2 and Co3O4, phase. The absence of characteristic diffraction peaks in XRD patterns confirmed that the deposition of metal oxides on the support g-Al2O3 were in the form of amorphous layer. The data obtained for the actual loadings of metal oxides from ICP analysis, XRF and SEM coupled with EDX were almost equal to the initial calculated values. This confirmed the calcinations process and its process conditions were optimum and virtually all the metal oxides coated on the support material. XPS spectra with binding energies (eV) for metal elements are shown in Fig. 1. The binding energies of Mn 2p (641.5 eV) and Co 2p (780.4 eV) for manganese oxide sample A and cobalt oxide sample D are consistent with the formation of MnO2 and Co3O4 in these samples. However, for samples B and C which contains binary Mn–Co oxides coatings, there is a pronounced shift from +0.7 to +1.2 eV for Mn 2p which could be attributed to the interaction of manganese with alumina support. The positive shift of Al 2p binding energies in sample B and C also suggests that existence of interaction between the coated metal oxides and the g-Al2O3 support when both Mn and Co oxides are present.
conversion increased with time and it reached its maximum conversion of 80–90% at 8 h. Fig. 3 also shows that the oxidative reactivity of the model thiophene compounds follows the order of 4,6-dEDBT > 4,6-dMDBT > 4-MDBT > DBT. The observed order of reactivity is opposite to that observed in the hydrodesulfurization process where the most sterically hindered thiophenes, 4,6-dEDBT and 4,6-dMDBT, are the least reactive. Apparently, the increased electron density of the sulfur atoms in disubstituted thiophenes can overcompensate for the steric hindrance of the C4 and C6 alkyl groups in the oxidative process.
3.2. Selective catalytic sulfur oxidation followed by solvent extraction on model diesel As shown by the sulfur-specific gas chromatograms in Fig. 2 and % of conversion versus time in Fig. 3, the thiophenes
Fig. 5. Conversion % thiophenic compounds to corresponding sulfone in real diesel (catalyst: 11%MnO2 loaded in g-Al2O3, temperature 150 8C).
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3.3. Selective catalytic sulfur oxidiation followed by solvent extraction on real diesel
Table 2 Sulfur content analysis results after solvent extraction of diesel with and without oxidation
Similar results were obtained with real diesel containing approximately 450 ppm sulfur as shown by the sulfur-specific GC-AED chromatograms in Fig. 4. The conversion of the substituted thiophenes (Fig. 5) to corresponding sulfones was in the range of 65–75% depending on the type of catalysts and operating temperatures in the range of 130–200 8C. The selectivity was about 90–100%. The total sulfur content of the diesel before and after was same in most of the cases and when the operating temperature increases, some of the sulfur compounds were over oxidized and converted (see Scheme 1) into SO2 (gas). The elimination of SO2 was confirmed by scrubbing the outlet gas with a AgNO3 solution to form AgSO3 precipitate. Table 2 summarizes the results of extracting real diesel before and after oxidation. Among the polar solvents tested, NMP was found to be the most efficient in extracting sulfur compounds from both diesel and oxidized diesel. While both thiophenes and sulfones can be extracted from diesel, the sulfones are significantly easier to be removed from diesel by polar solvents due to higher polarity. The results also show that the extraction efficiency for both thiophenes and sulfones with the polarity of the extraction solvent. The GC-AED carbon chromatogram shows there were no significant changes in the product distribution before and after oxidation of the real diesel samples. The trisubstitued dibenzothiophenes compounds were easier to be oxidized than the monosubstituted dibenzothiophene, such as 4-methyldibenzothiophene is difficult to oxidize compared to 4,6diethyl-dibenzothiophene (Table 3).
Catalysta
Extraction solvent (vol)
S content (ppm)b (treated diesel)
Diesel, no oxidation Diesel, no oxidation Diesel, no oxidation Diesel, no oxidation Diesel, no oxidation 2%Co3O4/g-Al2O3 2%Co3O4/g-Al2O3 2%Co3O4/g-Al2O3 2%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3
No extraction AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml) AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml) AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml) AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml)
430 310 226 219 314 237 146 129 215 236 145 134 215 198 117 108 172
a Oxidation reaction carried out at 130 8C; 25.0 ml oxidized diesel extracted with solvent. b S content was measured by XRF and GC-AED.
3.4. Properties of oxydesulfurized real diesel The treated diesel (oxidized followed with solvent extraction) was analyzed for diesel specification parameters, such as density, cetane index, pour point, kinematic viscosity, etc. and the results are given in Table 4. The studies show that the olefin content of the diesel was increased and aromatic content of the diesel was reduced substantially. Cetane index increased
Table 3 Sulfur content analysis results after solvent extraction of oxidized diesel at various temperatures and various catalysts system Catalysta
Reaction temperature (8C)
Extraction solvent (vol)
2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 5%MnO2/g-Al2O3 5%MnO2/g-Al2O3 5%MnO2//g-Al2O3 5%MnO2/g-Al2O3 5%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 11%MnO2/g-Al2O3 11%MnO2/g-Al2O3 13%MnO2/g-Al2O3 13%MnO2/g-Al2O3 5%MnO2/3%Co3O4//g-Al2O3 3%MnO2/5%Co3O4//g-Al2O3
130 130 130 130 130 130 130 130 130 130 150 150 150 150 150 180 150 180 180 180
NMP NMP NMP NMP DMF NMP NMP DMF DMF DMF NMP NMP NMP NMP NMP NMP NMP NMP NMP NMP
(10 ml 3) (10 ml) (20 ml) (30 ml) (10 ml) (10 ml 3) (10 ml) (10 ml) (20 ml) (30 ml) (10 ml) (20 ml) (30 ml) (40 ml) (10 ml) (30 ml) (10 ml) (30 ml) (10 ml) (10 ml)
Oxidation reaction temperature in 8C. a 30.0 ml oxidized diesel extracted with different amount of different solvent (single or multiple extraction). b S content was measured by XRF and GC-AED.
S (ppm)b 64 188 126 96 193 51 168 187 145 115 143 119 103 61 104 66 84 44 109 123
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Table 4 Some diesel specification analysis for the untreated and treated diesel samples Test a
Method
Real diesel
Treated diesel
S content (ppmw) (wt%) Kinematic viscosity @ 40 8C (cSt) Density @ 15 8C (kg l 1) Olefins (vol%) Aromatics (vol%) Water content (ppm) Pour point (8C) Lubricity (mm) Cetane index
ASTM D3120-96 ASTM D445-01 ASTM D4052-96 ASTM D1319-99 ASTM D1319-99 Karl Fischer ASTM D97-96a CEC F06-A-96 ASTM D976-91(00)
0.043 4.376 0.8541 2.4 46.4 120 +6 175 53
0.001 4.982 0.8286 3.6 12.5 139 +12 474 62.8
a
Analysis carried out by Intertek Testing Services (S) Pvt. Ltd., Singapore Technical Centre.
approximately by 20%. Density and other parameters were within the required limits. Lubricity of the treated diesel was increased by substantial amount. 3.5. Effect of the catalyst composition Metal oxide catalysts derived from Mn, Co, W, Ni, V, Fe and Cu, respectively, were tested for their ability to oxidize sulfur impurities in real diesel and a model diesel mixture composed of n-tetradecane and various substituted dibenzothiophene compounds in air at a temperature of 110–180 8C. Among these catalysts, only manganese and cobalt oxides were found to be effective in catalyzing the oxidation of the thiophenes to sulfones. The unmodified support, g-Al2O3, was also ineffective. The effects of metal loading and reaction temperature were investigated. Below 110 8C, the oxidation reaction was not observed. There was no significant difference in conversions for the oxidation of model diesel catalyzed by either 2 or 5% Co3O4/g-Al2O3 at 130 or 150 8C. However, in the case of MnO2/g-Al2O3 catalysts, higher metal loading led to higher conversion at all test temperatures: 130, 150, 180 and 200 8C. The best results were obtained with catalysts containing the highest MnO2 loadings (11 and 13%) at 180 8C. Similar effect was observed for the oxidation of real diesel (Table 2). For binary mixed metal (Mn and Co) oxide catalysts, higher activity was observed with higher Mn/Co ratio. Thus, 5%MnO2/3%Co3O4//g-Al2O3 showed better oxidation activity than 5%Co3O4/3%MnO2//g-Al2O3 (Table 3 and Fig. 6). 3.6. Simplified process diagram for the treatment of production S-free diesel Fig. 7 shows a possible process diagram for the integration of oxidative desulfurization process into an existing refinery process unit for desulfurization of diesel. The ODS reactor unit may be installed as downstream of a conventional hydrodesulfurization reactor unit. Oxidative desulfurization reaction can be carried out as a secondary desulfurization process for diesel that have been treated by conventional HDS process. The treated/oxidized diesel is channeled to a stirred/mixing tank containing polar solvent for removing the oxidized sulfur compounds. The diesel/solvent mixture is then channeled to a
Fig. 6. Conversion % thiophenic compounds to corresponding sulfone in real diesel (catalyst: (A) 8%MnO2 loaded in g-Al2O3, temperature 150 8C; (B) 5%MnO2/3%Co3O4 loaded in g-Al2O3, temperature 180 8C; (C) 3%MnO2/ 5%Co3O4 loaded in g-Al2O3, temperature 180 8C).
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Fig. 7. Simplified process diagram for the treatment and production of S-free diesel.
settler where the treated diesel is separated from the solvent. The solvent can be recycled by distillation and can be reused. The treated diesel is further passed through a basic adsorbentbed unit for further removal of remaining sulfur-containing compounds in the diesel. The remaining sulfones in the treated diesel could be easily removed by adsorption compare to thiophinic compounds. 4. Conclusion It has been demonstrated that Mn- and Co-containing oxide catalysts are highly effective for selective oxidation of the refractory sulfur compounds in diesel fuel using molecular oxygen in air at atmospheric pressure and the sulfur content can be easily reduced to 40–60 ppm after coupled with extraction by polar solvent. During our research studies, Murata et al. [14] have reported recently oxidation of sulfur compounds using molecular oxygen in the presence of cobalt catalysts and aldehydes in monophasic system. In our system, the catalyst (heterogeneous) can be easily reactivated and reused. The polar solvent used for extraction can be recycled by vacuum distillation. The low-sulfur (10–15 ppm sulfur) diesel was obtained by simply pass through treated diesel (oxidized and solvent extracted diesel) into the activated basic g-Al2O3 adsorbent-bed at room temperature. This oxidative desulfurization process has several advantages over other oxidative desulfurization processes which were reported. One advantage of this process that the reaction can be carried out using inexpensive oxygen found air compare to costly oxidants, such as H2O2 or ozone, which were reported in the literature for the oxidative desuflurisation processes. In addition, the use of air as oxidant also eliminates the need to carry out any oxidant recovery process that is usually required if liquid oxidants (tert-butylperoxide or H2O2) are used. Another advantage of this process is the mild operating conditions compared to hydrodesulfurization process which more severe
conditions are needed. Yet another advantage of this process is the ease of integration into any existing refinery for the production of diesel, as afforded by the mild process conditions of liquid phase contacting and the use of air. Furthermore, the use of a selective oxidation catalyst also permits the tuning of experimental parameters, such as temperature and contacting time, to achieve optimal conversion and selectivity. The simplified process flow sheet of the oxidative desulfurization process which can be adopted for the refineries without major changes in the infrastructure is shown in Fig. 7. Acknowledgements This work was financially supported by the Agency for Science, Technology and Research (Project No. ICES/04112001). J.T. Sampanthar wish to thank Prof. Hua Chun Zeng (National University of Singapore, Singapore) and Mr. Sam Mylvaganam (ICES) for their valuable comments and useful discussion. References [1] US EPA, Regulatory Announcement: Heavy-Duty Engine and Vehicle Standards and Highway Fuel Sulfur Control Requirements, December, 2000. [2] C. Song, Catal. Today 86 (2003) 211; R. Shafi, G.J. Hutchings, Catal. Today 59 (2000) 423; K.G. Knudsen, B.H. Cooper, H. Topsoe 189 (1999) 205. R.G. Tailleur, J. Ravigli, S. Quenza, N. Valencia, Appl. Catal. A Gen. 282 (2005) 227; T. Viveros-Garcia, J.A. Ochoa-Tapia, R. Lobo-Oehmichen, J.A. ReyesHeredia, E.S. Perez-Cisneros, Chem. Eng. J. 106 (2005) 119. [3] F. Li, P. Xu, C. Ma, Y. Zheng, Y. Qu, J. Chem. Eng. Jpn. 36 (2003) 1174. [4] R.T. Yang, A.J. Hernandez_Maldonado, F.H. Yang, Science 301 (2003) 73; Y. Sano, K.H. Choi, Y. Korai, I. Mochida, Appl. Catal. B Environ. 49 (2004) 219; X.L. Ma, L. Sun, C.S. Song, Catal. Today 77 (2002) 107;
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A novel oxidative desulfurization process to remove refractory sulfur compounds from diesel fuel Jeyagowry T. Sampanthar *, Huang Xiao, Jian Dou, Teo Yin Nah, Xu Rong, Wong Pui Kwan Applied Catalysis Technology Group, Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), No. 1, Pesek Road, Jurong Island, Singapore 627833, Singapore Received 17 June 2005; received in revised form 12 September 2005; accepted 12 September 2005 Available online 25 October 2005
Abstract Manganese and cobalt oxide catalysts supported on g-Al2O3 have been found to be effective in catalyzing air oxidation of the sulfur impurities in diesel to corresponding sulfones at a temperature range of 130–200 8C and atmospheric pressure. The sulfones were removed by extraction with polar solvent to reduce the sulfur level in diesel to as low as 40–60 ppm. Oxidation of model compounds showed that the most refractory sulfur compounds in hydrodesulfurization of diesel were more reactive in oxidation. The oxidative reactivity of model impurities in diesel follows the order: trialkylsubstituted dibenzothiophene > dialkyl-substituted dibenzothiophene > monoalkyl-substituted dibenzothiophene > dibenzothiophene. # 2005 Elsevier B.V. All rights reserved. Keywords: Oxidative desulfurization; Diesel; Sulfur; Catalyst; MnO2/g-Al2O3; Co3O4/g-Al2O3; Solvent extraction
1. Introduction Deep desulfurization of diesel fuel has become an important research subject due to the upcoming legislative regulations to reduce sulfur content in most western countries. The US Clean Air Act Amendments of 1990 and the new regulations by the US Environmental Protection Agency (EPA) and government regulations in many countries call for the production and use of more environment-friendly transportation fuels with lower contents of sulfur and aromatics. The demand for transportation fuels has been increasing in most countries for past two decades. For example, US Environmental Protection Agency has set up guidelines to limit the sulfur content of diesel fuel to 15 ppm by 2006 [1]. Conventional hydrodesulfurization (HDS) process has been employed by refineries to remove organic sulfur from fuels for several decades and the lowest sulfur content achieved by such process in the fuels is around 500 ppm. However, to meet the challenges of producing ultraclean fuels, especially with sulfur content lower than 15 ppm, both capital investment and operational costs would be rather high due to more severe operating conditions [2]. Consequently, * Corresponding author. Tel.: +65 67963819; fax: +65 63166182. E-mail address: [email protected] (J.T. Sampanthar). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.09.007
several alternative approaches have been used, such as biodesulfurization [3], selective adsorption [4], extraction by ionic-liquid [5] and oxidative desulfurization (ODS) [6–14]. Various studies on the ODS process have reported the use of differing oxidizing agents and catalysts, such as H2O2/acetic acid [7] and H2O2/formic acid [8], H2O2/heteropolyacids [9], H2O2/inorganic solid acids [10], NO2/heterogeneous catalysts [11], ozone/heterogeneous catalysts [12], tert-butylperoxides/ heterogeneous catalysts [13] and O2/aldehyde/cobalt catalysts [14]. The ODS process is usually carried out under mild conditions which present competitiveness over the conventional HDS process [15]. In this process, the sulfur compounds present in diesel are oxidized by the oxidizing agent to give rise to the corresponding sulfones. These sulfones are highly polarized compounds, such that they are removed from the diesel by subsequent solvent extraction using water-soluble polar solvents, such as NMP, DMF, DMSO and MeOH, etc. [15]. By combination of the processes, the sulfur content of the diesel can be reduced to 50 ppm [16]. Scheme 1 shows the oxidation of organic sulfur compounds. The resulting sulfones can be removed by either extraction and/or adsorption. Here, we report the effective use of air as an environmentally benign and low-cost oxidant to oxidize the sulfur compounds in diesel at ambient pressure and moderate temperature in the
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Scheme 1.
presence of heterogeneous based simple transition metal oxides loaded on g-Al2O3. 2. Experimental 2.1. Materials Untreated diesel with a sulfur content in the range of 430– 465 ppm was obtained from Shell Petroleum Corporation, Singapore. Co(NO3)6H2O, Mn(CH3COO)24H2O, dibenzothiophene (DBT), 4-methyl-dibenzothiophene (4-MDBT), 4,6-dimethyl-dibenzothiophene (4,6-DMDBT), 4,6-diethyldibenzothiophene (4,6-DEDBT) and n-tetradecane were purchased from Sigma–Aldrich, Singapore and used without further purification. Model diesel was prepared by adding equimolar amounts of dibenzothiophene, 4-methyl-dibenzothiophene, 4,6-dimethyl-dibenzothiophene and 4,6-diethyl-dibenzothiophene to n-tetradecane to make up a solution with a total sulfur content of 400 ppm. 2.2. Catalyst preparation A 10 g of g-Al2O3 pellet (obtained from Singapore Catalyst Technology Center, diameter 3–4 mm, length 6–10 mm with a specific surface area and a total pore volume of 370 m2 g 1 and 0.87 ml g 1, respectively) was impregnated with cobalt nitrate and/or manganese acetate aqueous solutions by an incipient wetness method. The total metal oxide loading with respect to g-Al2O3 ranged from 2 to 13 wt%. The impregnated samples were left on a roller which was set at 25 rpm for 18 h to obtain better dispersion. The water content of the samples were removed and dried at 120 8C in the oven for 18 h followed by calcination in a static furnace at 550 8C for 5 h with a ramp of 5 8C min 1. Other catalysts, such as W, Ni, Fe and Cu, were also prepared in a similar manner. 2.3. Catalyst characterization The physical and chemical properties of the prepared catalysts were characterized by various analytical techniques. The monolayer deposition of metal oxides on g-Al2O3 was confirmed using powder X-ray diffraction (XRD) technique using a Bruker AXS D8 Advance instrument with Cu Ka radiation at 40 kV and 40 mA. The N2 adsorption–desorption isotherm of the catalysts were studied using an Autosorb-1 at
77 K. Prior to the measurement, the calcined catalysts were degassed at 300 8C. The thermal behavior of uncalcined catalysts were studied by thermo gravimetric analysis (TGA) using Universal V2.5H TA, Model SDT 2690 instrument under laminar flow of air with a flow rate of 90 ml min 1. X-ray photoelectron spectroscopy (XPS) investigation was conducted on a VGESCALAB 250 spectrometer using monochromatic Al Ka, X-ray source (1486.6 eV) at a constant analyzer pass energy of 20.0 eV. All binding energies were referenced to the C 1s peak arising from adventitious carbon (BE 285.0 eV). The metal oxides composition and loading in the catalysts were also characterized and confirmed by ICP (Model Vista-MPX) and SEM-EDX (Model Jeol JSM-6700F). 2.4. Analysis of S—content in the model and real diesel Sulfur content of the model and real diesel samples were analyzed by XRF and GC-AED. The total sulfur content in the model and real diesel samples were analyzed using XRF (Bruker AXS S4 Exporer), which was calibrated with six liquid calibration standards (obtained from AccuStandard), and concentration ranging from 0 to 500 ppm sulfur by wt%. The 10 ml of samples were placed into 40 mm diameter plastic cells equipped with 2.5 mm MYLAR polyester film window. Each cell was vented to prevent the polyester film window from bulging during the analysis. The samples were then placed in the automatic sample chamber and the optical path of the XRF was flushed with helium gas prior to the measurement. A 6890 GC coupled with atomic emission detector (JASAED) was used to identify the various sulfur compounds and their concentration. The GC was equipped with a split/splitless injection port and operated in split mode. A 30 m 0.32 mm i.d. 1 mm film thickness HP-1 MS capillary column was used for separation as it has a lower specified rate of column bleed than conventional methyl silicone capillary columns. Hydrogen and oxygen gases were used as reagent gases for both carbon (179 nm) and sulfur (181 nm). In order to improve the sulfur selectivity over carbon, the AED gas flows (hydrogen, oxygen and helium makeup) were optimized to minimize interferences from hydrocarbons. The samples volume of 1 ml was injected without any solvent dilution. The GC-AED instrument was calibrated with sulfur in diesel fuel SRM 2724 obtained from National Institute of Standards and Technology (NIST), Reference Material Department, US. The SRM 2724 is a commercial no. 2-distillate fuel oil as defined by ASTM with
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the certified total sulfur content of 425 ppm. In addition to the SRM 2724, the AccuStandard (also obtained from NIST) with sulfur content in diesel fuel 0, 100, 200, 300, 400 and 500 ppm samples were also used for the calibration. 2.5. Solvent extraction on real diesel without oxidative treatment Solvent extraction studies for the removal of sulfur compounds in untreated diesel (obtained from Shell Petroleum Corporation, Singapore with a sulfur content in the range of 430– 465 ppm) were carried out with four different organic solvents of different polarities, such as acetonitrile (AcN), dimethylforamide (DMF), 1-methyl-2-pyrrolidinone (NMP) and methonal (MeOH). A 25.0 ml of untreated diesel were mixed with the known volume of polar organic solvents to determine the efficiency of solvent extraction. The diesel–solvent mixture was stirred for 30 min before separating the two layers. After extraction by the respective polar solvents, the sulfur content in the diesel was measured by XRF and GC-AED. 2.6. Catalytic oxidation followed by solvent extraction on model diesel The oxidation experiments in this study were carried out with 20.0 ml of model diesel in a refluxed round bottom flask. Approximately, 20–30 mg of g-Al2O3 supported Mn and/or Co oxides in the form of pellets were used as catalysts. The reactions were carried out at a temperature range of 90–180 8C, during which air was introduced via a gas disperser at a constant flow rate of 100 ml min 1 while the reaction mixture was stirred throughout the experiment. Awater-cooled reflux condenser was mounted on top of the reaction flask to prevent solvent loss and
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Table 1 BET surface areas and total pore volume of the prepared catalysts after calcinations at 550 8C Catalysta
Surface area (m2 g 1)
TPV (ml)
g-Al2O3 2%MnO2/g-Al2O3 5%MnO2/g-Al2O3 8%MnO2/g-Al2O3 11%MnO2/g-Al2O3 13%MnO2/g-Al2O3 2%Co3O4//g-Al2O3 5%Co3O4//g-Al2O3 8%Co3O4//g-Al2O3 5%MnO2/3%Co3O4//g-Al2O3 3%MnO2/3%Co3O4//g-Al2O3 3%MnO2/5%Co3O4//g-Al2O3
377 361 350 331 317 305 368 350 323 322 331 310
0.87 0.86 0.86 0.78 0.80 0.77 0.79 0.76 0.76 0.72 0.75 0.72
a All the above samples were calcined at 550 8C under static air and degas at 300 8C for 5 h before measurements.
serve as an air outlet. The progress of the reaction was monitored periodically withdrawing 0.5 ml aliquots of the reaction mixture for GC-AED analysis. Blank experiments were carried out with model diesel and pure support of g-Al2O3 under exactly similar experimental conditions. The oxidized products in the model diesel were extracted with NMP or methanol after the completion of the reaction. The reacted model diesel was mixed with one of these polar solvents at different volume ratio (e.g. diesel:polar solvent = 4:1 when NMP as a solvent, 1:1 when MeOH as solvent) and was magnetically stirred for 30 min. The mixture was then transferred into a separating funnel and separated into two layers of model diesel and polar solvent which were analyzed for sulfur compounds using GC-AED. When MeOH was used as solvent, it was removed from solvent–sulfone mixture using rotary
Fig. 1. X-ray photoelectron spectroscopy analysis of metals oxides loaded on gamma-alumina: (A) 11%MnO2/g-Al2O3; (B) 5%MnO2/3%Co3O4/g-Al2O3; (C) 3%MnO2/5%Co3O4/g-Al2O3; (D) 8%Co3O4/g-Al2O3.
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Fig. 2. Sulfur-specific gas chromatograms of model diesel (air oxidation of model diesel (400 ppm sulfur) catalyzed MnO2 (11%) loaded on g-alumina support; solvent extraction was carried out using NMP:oxidized diesel = 1:1, single extraction).
evaporator and the product of sulfones mixture was precipitated at the bottom of the flask. 2.7. Catalytic oxidation by Mn and/or Co oxides supported on g-Al2O3 followed by solvent extraction on real diesel A 150.0 ml real diesel underwent oxidative desulfurization reaction in the presence of about 100 mg of catalyst at temperature range of 130–200 8C in a two-necked round bottom flask. The reaction mixture was magnetically stirred to ensure a good mixing and bubbled with purified air at constant flow of 100 ml min 1. The reaction mixture was periodically
Fig. 3. Conversion % of the thiophenic compounds to the corresponding sulfone in model diesel (catalyst: 11%MnO2 loaded in g-Al2O3, temperature 150 8C).
sampled and analyzed using GC-AED and reaction was ceased after about 18 h. The oxidized diesel was then cooled to room temperature and 25.0 ml of reacted diesel was treated with varying volume of different solvents for solvent extraction. Sulfur content of the extracted oxidized real diesel was measured by XRF and GC-AED. Similar reaction and solvent extraction method were carried out with different loading of both Mn and/or Co oxide catalysts. 3. Results and discussion 3.1. Characterization of catalyst The TGA studies showed that the most of the metal salts loaded on the g-Al2O3 converted into their corresponding oxides at below 500 8C under laminar flow of air. Table 1 summarizes the specific surface area and total pore volume of the prepared catalysts. It shows that the specific surface area in the series considerably lowered from 377 m2 g 1 for pure g-Al2O3 support to the lowest value of 305 m2 g 1 for the sample loaded with the maximum amount of transition metal oxide (13%MnO2/g-Al2O3). The total pore volume of the calcined samples also decreases as the loading of the transition metal oxides increases. The decreasing behavior of both surface area and total pore volume with the increasing loading of the metal oxides are consistent due to the possible blockage of the inner pores, especially the smaller ones, and dilution of the
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Fig. 4. Sulfur-specific gas chromatograms of real diesel (air oxidation of real diesel diesel (450 ppm sulfur) catalyzed MnO2 (11%) loaded on g-alumina support; solvent extraction was carried out using NMP:oxidized diesel = 1:1, single extraction).
initial support material, g-Al2O3, by the uniformly dispersed and dense metal oxide, MnO2 and Co3O4, phase. The absence of characteristic diffraction peaks in XRD patterns confirmed that the deposition of metal oxides on the support g-Al2O3 were in the form of amorphous layer. The data obtained for the actual loadings of metal oxides from ICP analysis, XRF and SEM coupled with EDX were almost equal to the initial calculated values. This confirmed the calcinations process and its process conditions were optimum and virtually all the metal oxides coated on the support material. XPS spectra with binding energies (eV) for metal elements are shown in Fig. 1. The binding energies of Mn 2p (641.5 eV) and Co 2p (780.4 eV) for manganese oxide sample A and cobalt oxide sample D are consistent with the formation of MnO2 and Co3O4 in these samples. However, for samples B and C which contains binary Mn–Co oxides coatings, there is a pronounced shift from +0.7 to +1.2 eV for Mn 2p which could be attributed to the interaction of manganese with alumina support. The positive shift of Al 2p binding energies in sample B and C also suggests that existence of interaction between the coated metal oxides and the g-Al2O3 support when both Mn and Co oxides are present.
conversion increased with time and it reached its maximum conversion of 80–90% at 8 h. Fig. 3 also shows that the oxidative reactivity of the model thiophene compounds follows the order of 4,6-dEDBT > 4,6-dMDBT > 4-MDBT > DBT. The observed order of reactivity is opposite to that observed in the hydrodesulfurization process where the most sterically hindered thiophenes, 4,6-dEDBT and 4,6-dMDBT, are the least reactive. Apparently, the increased electron density of the sulfur atoms in disubstituted thiophenes can overcompensate for the steric hindrance of the C4 and C6 alkyl groups in the oxidative process.
3.2. Selective catalytic sulfur oxidation followed by solvent extraction on model diesel As shown by the sulfur-specific gas chromatograms in Fig. 2 and % of conversion versus time in Fig. 3, the thiophenes
Fig. 5. Conversion % thiophenic compounds to corresponding sulfone in real diesel (catalyst: 11%MnO2 loaded in g-Al2O3, temperature 150 8C).
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3.3. Selective catalytic sulfur oxidiation followed by solvent extraction on real diesel
Table 2 Sulfur content analysis results after solvent extraction of diesel with and without oxidation
Similar results were obtained with real diesel containing approximately 450 ppm sulfur as shown by the sulfur-specific GC-AED chromatograms in Fig. 4. The conversion of the substituted thiophenes (Fig. 5) to corresponding sulfones was in the range of 65–75% depending on the type of catalysts and operating temperatures in the range of 130–200 8C. The selectivity was about 90–100%. The total sulfur content of the diesel before and after was same in most of the cases and when the operating temperature increases, some of the sulfur compounds were over oxidized and converted (see Scheme 1) into SO2 (gas). The elimination of SO2 was confirmed by scrubbing the outlet gas with a AgNO3 solution to form AgSO3 precipitate. Table 2 summarizes the results of extracting real diesel before and after oxidation. Among the polar solvents tested, NMP was found to be the most efficient in extracting sulfur compounds from both diesel and oxidized diesel. While both thiophenes and sulfones can be extracted from diesel, the sulfones are significantly easier to be removed from diesel by polar solvents due to higher polarity. The results also show that the extraction efficiency for both thiophenes and sulfones with the polarity of the extraction solvent. The GC-AED carbon chromatogram shows there were no significant changes in the product distribution before and after oxidation of the real diesel samples. The trisubstitued dibenzothiophenes compounds were easier to be oxidized than the monosubstituted dibenzothiophene, such as 4-methyldibenzothiophene is difficult to oxidize compared to 4,6diethyl-dibenzothiophene (Table 3).
Catalysta
Extraction solvent (vol)
S content (ppm)b (treated diesel)
Diesel, no oxidation Diesel, no oxidation Diesel, no oxidation Diesel, no oxidation Diesel, no oxidation 2%Co3O4/g-Al2O3 2%Co3O4/g-Al2O3 2%Co3O4/g-Al2O3 2%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 5%Co3O4/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3
No extraction AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml) AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml) AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml) AcN (10 ml) DMF (10 ml) NMP (10 ml) MeOH (25 ml)
430 310 226 219 314 237 146 129 215 236 145 134 215 198 117 108 172
a Oxidation reaction carried out at 130 8C; 25.0 ml oxidized diesel extracted with solvent. b S content was measured by XRF and GC-AED.
3.4. Properties of oxydesulfurized real diesel The treated diesel (oxidized followed with solvent extraction) was analyzed for diesel specification parameters, such as density, cetane index, pour point, kinematic viscosity, etc. and the results are given in Table 4. The studies show that the olefin content of the diesel was increased and aromatic content of the diesel was reduced substantially. Cetane index increased
Table 3 Sulfur content analysis results after solvent extraction of oxidized diesel at various temperatures and various catalysts system Catalysta
Reaction temperature (8C)
Extraction solvent (vol)
2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 2%MnO2/g-Al2O3 5%MnO2/g-Al2O3 5%MnO2/g-Al2O3 5%MnO2//g-Al2O3 5%MnO2/g-Al2O3 5%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 8%MnO2/g-Al2O3 11%MnO2/g-Al2O3 11%MnO2/g-Al2O3 13%MnO2/g-Al2O3 13%MnO2/g-Al2O3 5%MnO2/3%Co3O4//g-Al2O3 3%MnO2/5%Co3O4//g-Al2O3
130 130 130 130 130 130 130 130 130 130 150 150 150 150 150 180 150 180 180 180
NMP NMP NMP NMP DMF NMP NMP DMF DMF DMF NMP NMP NMP NMP NMP NMP NMP NMP NMP NMP
(10 ml 3) (10 ml) (20 ml) (30 ml) (10 ml) (10 ml 3) (10 ml) (10 ml) (20 ml) (30 ml) (10 ml) (20 ml) (30 ml) (40 ml) (10 ml) (30 ml) (10 ml) (30 ml) (10 ml) (10 ml)
Oxidation reaction temperature in 8C. a 30.0 ml oxidized diesel extracted with different amount of different solvent (single or multiple extraction). b S content was measured by XRF and GC-AED.
S (ppm)b 64 188 126 96 193 51 168 187 145 115 143 119 103 61 104 66 84 44 109 123
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Table 4 Some diesel specification analysis for the untreated and treated diesel samples Test a
Method
Real diesel
Treated diesel
S content (ppmw) (wt%) Kinematic viscosity @ 40 8C (cSt) Density @ 15 8C (kg l 1) Olefins (vol%) Aromatics (vol%) Water content (ppm) Pour point (8C) Lubricity (mm) Cetane index
ASTM D3120-96 ASTM D445-01 ASTM D4052-96 ASTM D1319-99 ASTM D1319-99 Karl Fischer ASTM D97-96a CEC F06-A-96 ASTM D976-91(00)
0.043 4.376 0.8541 2.4 46.4 120 +6 175 53
0.001 4.982 0.8286 3.6 12.5 139 +12 474 62.8
a
Analysis carried out by Intertek Testing Services (S) Pvt. Ltd., Singapore Technical Centre.
approximately by 20%. Density and other parameters were within the required limits. Lubricity of the treated diesel was increased by substantial amount. 3.5. Effect of the catalyst composition Metal oxide catalysts derived from Mn, Co, W, Ni, V, Fe and Cu, respectively, were tested for their ability to oxidize sulfur impurities in real diesel and a model diesel mixture composed of n-tetradecane and various substituted dibenzothiophene compounds in air at a temperature of 110–180 8C. Among these catalysts, only manganese and cobalt oxides were found to be effective in catalyzing the oxidation of the thiophenes to sulfones. The unmodified support, g-Al2O3, was also ineffective. The effects of metal loading and reaction temperature were investigated. Below 110 8C, the oxidation reaction was not observed. There was no significant difference in conversions for the oxidation of model diesel catalyzed by either 2 or 5% Co3O4/g-Al2O3 at 130 or 150 8C. However, in the case of MnO2/g-Al2O3 catalysts, higher metal loading led to higher conversion at all test temperatures: 130, 150, 180 and 200 8C. The best results were obtained with catalysts containing the highest MnO2 loadings (11 and 13%) at 180 8C. Similar effect was observed for the oxidation of real diesel (Table 2). For binary mixed metal (Mn and Co) oxide catalysts, higher activity was observed with higher Mn/Co ratio. Thus, 5%MnO2/3%Co3O4//g-Al2O3 showed better oxidation activity than 5%Co3O4/3%MnO2//g-Al2O3 (Table 3 and Fig. 6). 3.6. Simplified process diagram for the treatment of production S-free diesel Fig. 7 shows a possible process diagram for the integration of oxidative desulfurization process into an existing refinery process unit for desulfurization of diesel. The ODS reactor unit may be installed as downstream of a conventional hydrodesulfurization reactor unit. Oxidative desulfurization reaction can be carried out as a secondary desulfurization process for diesel that have been treated by conventional HDS process. The treated/oxidized diesel is channeled to a stirred/mixing tank containing polar solvent for removing the oxidized sulfur compounds. The diesel/solvent mixture is then channeled to a
Fig. 6. Conversion % thiophenic compounds to corresponding sulfone in real diesel (catalyst: (A) 8%MnO2 loaded in g-Al2O3, temperature 150 8C; (B) 5%MnO2/3%Co3O4 loaded in g-Al2O3, temperature 180 8C; (C) 3%MnO2/ 5%Co3O4 loaded in g-Al2O3, temperature 180 8C).
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Fig. 7. Simplified process diagram for the treatment and production of S-free diesel.
settler where the treated diesel is separated from the solvent. The solvent can be recycled by distillation and can be reused. The treated diesel is further passed through a basic adsorbentbed unit for further removal of remaining sulfur-containing compounds in the diesel. The remaining sulfones in the treated diesel could be easily removed by adsorption compare to thiophinic compounds. 4. Conclusion It has been demonstrated that Mn- and Co-containing oxide catalysts are highly effective for selective oxidation of the refractory sulfur compounds in diesel fuel using molecular oxygen in air at atmospheric pressure and the sulfur content can be easily reduced to 40–60 ppm after coupled with extraction by polar solvent. During our research studies, Murata et al. [14] have reported recently oxidation of sulfur compounds using molecular oxygen in the presence of cobalt catalysts and aldehydes in monophasic system. In our system, the catalyst (heterogeneous) can be easily reactivated and reused. The polar solvent used for extraction can be recycled by vacuum distillation. The low-sulfur (10–15 ppm sulfur) diesel was obtained by simply pass through treated diesel (oxidized and solvent extracted diesel) into the activated basic g-Al2O3 adsorbent-bed at room temperature. This oxidative desulfurization process has several advantages over other oxidative desulfurization processes which were reported. One advantage of this process that the reaction can be carried out using inexpensive oxygen found air compare to costly oxidants, such as H2O2 or ozone, which were reported in the literature for the oxidative desuflurisation processes. In addition, the use of air as oxidant also eliminates the need to carry out any oxidant recovery process that is usually required if liquid oxidants (tert-butylperoxide or H2O2) are used. Another advantage of this process is the mild operating conditions compared to hydrodesulfurization process which more severe
conditions are needed. Yet another advantage of this process is the ease of integration into any existing refinery for the production of diesel, as afforded by the mild process conditions of liquid phase contacting and the use of air. Furthermore, the use of a selective oxidation catalyst also permits the tuning of experimental parameters, such as temperature and contacting time, to achieve optimal conversion and selectivity. The simplified process flow sheet of the oxidative desulfurization process which can be adopted for the refineries without major changes in the infrastructure is shown in Fig. 7. Acknowledgements This work was financially supported by the Agency for Science, Technology and Research (Project No. ICES/04112001). J.T. Sampanthar wish to thank Prof. Hua Chun Zeng (National University of Singapore, Singapore) and Mr. Sam Mylvaganam (ICES) for their valuable comments and useful discussion. References [1] US EPA, Regulatory Announcement: Heavy-Duty Engine and Vehicle Standards and Highway Fuel Sulfur Control Requirements, December, 2000. [2] C. Song, Catal. Today 86 (2003) 211; R. Shafi, G.J. Hutchings, Catal. Today 59 (2000) 423; K.G. Knudsen, B.H. Cooper, H. Topsoe 189 (1999) 205. R.G. Tailleur, J. Ravigli, S. Quenza, N. Valencia, Appl. Catal. A Gen. 282 (2005) 227; T. Viveros-Garcia, J.A. Ochoa-Tapia, R. Lobo-Oehmichen, J.A. ReyesHeredia, E.S. Perez-Cisneros, Chem. Eng. J. 106 (2005) 119. [3] F. Li, P. Xu, C. Ma, Y. Zheng, Y. Qu, J. Chem. Eng. Jpn. 36 (2003) 1174. [4] R.T. Yang, A.J. Hernandez_Maldonado, F.H. Yang, Science 301 (2003) 73; Y. Sano, K.H. Choi, Y. Korai, I. Mochida, Appl. Catal. B Environ. 49 (2004) 219; X.L. Ma, L. Sun, C.S. Song, Catal. Today 77 (2002) 107;
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