Ind. Eng. Chem. Res. 2008, 47, 9617–9622
9617
Deep Desulfurization of Diesel Feedstock by Selective Adsorption of Refractory Sulfur Compounds Alain Favre-Re´guillon,*,†,‡ Marc Se´vignon,† Muriel Rocault,† Emmanuelle Schulz,§ and Marc Lemaire*,† UniVersite´ de Lyon, F-69622 Lyon, France, CNRS, UMR 5246, Institut de Chimie et Biochimie Mole´culaires et Supramole´culaires, 43 bouleVard du 11 noVembre 1918, 69100 Villeurbanne, France, ConserVatoire National des Arts et Me´tiers, CNRS, UMR 7084, Laboratoire des Transformations Chimiques et Pharmaceutiques, 2 rue Conte´, 75003 Paris, France, and Equipe de Catalyse Mole´culaire, ICMMO, UMR 8182, UniVersite´ Paris-Sud 11, Baˆt 420, 91405 Orsay Cedex, France
Adsorptive desulfurization was studied using straight-run gas oil (SRGO) with various sulfur contents (390 and 60 ppm S). Immobilization of electron-poor molecules (π-acceptors) on macroporous resins was achieved in three steps starting from commercially available poly(styrene-co-divinylbenzene) beads. Influence of the temperature on adsorption was studied, and the process was examined on the basis of breakthrough curves. The regeneration of the support was studied using fixed-bed technology and was efficiently performed with toluene as the solvent. The extracted compounds were easily recovered after evaporation of the toluene and the affinity of the resins toward alkyldibenzothiophenes was confirmed by GC-FPD analysis. On the basis of these preliminary experimental results, a new concept is proposed for ultradeep desulfurization of SRGO. 1. Introduction Because of government mandates worldwide, refiners must produce increasingly cleaner fuels. Sulfur represents a major petroleum pollutant that contributes to air pollution and catalyst poisoning. The production of ultralow sulfur diesel (down to 10 ppm) is motivated by the need for using new emission-control technologies that are sensitive to sulfur. To date, the major desulfurization process has been conventional hydrodesulfurization (HDS), through which sulfur compounds are converted by reaction with hydrogen, forming H2S on Ni/Co promoted Mo/W sulfides supported on γ-Al2O3.1,2 Deep desulfurization of diesel fuel is particularly challenging because the major S containing compounds (S-compounds) present in current commercial Diesel are alkyldibenzothiophenes (DBTs), which have been considered to be refractory S-compounds.3 Thus, ultradeep HDS requires important capital investment and consequently high costs because of the need to employ higher H2 pressures and/or large catalyst volumes despite the development of new catalysts.4 The selective removal of this family of S-compounds in diesel fuel is a challenge and new processes have been proposed.5-7 Among them, adsorption is playing an increasingly important role.8,9 However, due to the relatively low concentration of DBTs in the feed, conventional sorbents, which are restricted to van der Waals or electrostatic interactions between sorbents and sorbates, have shown limited selectivity. Therefore, a tremendous opportunity exists for developing new sorbents based on noncovalent but specific chemical bonds such as π-complexation10-13 and charge transfer interaction.14-21 * To whom correspondence should be addressed. E-mail:
[email protected] (A.F.-R.) and marc.lemaire@ univ-lyon1.fr (M.L.). † Universite´ de Lyon and Institut de Chimie et Biochimie Mole´culaires et Supramole´culaires. ‡ Conservatoire National des Arts et Me´tiers and Laboratoire des Transformations Chimiques et Pharmaceutiques. § Universite´ Paris-Sud 11.
An adsorption process based on the selective extraction of DBTs using charge transfer complexation has been developed in our laboratory.14-19 DBTs are planar and electron-rich structures which can form charge-transfer complexes (CTCs) with planar electron-poor molecules (π-acceptors). We have shown that the selectivity of the complexation phenomenon is controlled by the orbital interactions.22 The immobilization of π-acceptor molecules on polystyrene based polymer can be successfully performed, and thus the polymer could be applied to the desulfurization of straight-run gas oils (SRGO) in batch.18,19 Recently 2,4,5,7-tetranitro-9-fluorenone immobilized on inorganic support has been used for the extraction of 4,6dimethyldibenzothiophene from n-heptane.21 In the present study, adsorptive desulfurization of Arabian Light SRGO, which contained a variable amount of DBTs over immobilized π-acceptor molecules (i.e., 2,4,5,7-tetranitro-9fluorenone) on polystyrene support, was conducted in a fixedbed adsorption system. The process was further evaluated on the basis of the breakthrough curves and regeneration of the support was performed. 2. Experimental Section Instrumentation and Chemicals. Infrared spectra were recorded on a Bruker Vektor 22 spectrometer. NMR spectra were recorded on a Bruker AC 200 (1H, 200.13 MHz; 13C, 50.32 MHz) or Brucker DRX 300 (1H,: 300.131 MHz; 13C, 75.46 MHz). Elemental analyses were carried out by the Service Central d’Analyze du CNRS (F-69360 Solaize). Quantitative analysis of the total sulfur concentration was determined by UV fluorescence on an Antek 9000 Series sulfur analyzer equipped with a robotic liquid autosampler. Qualitative analysis of the S-compounds was performed using HP 5890 series II gas chromatograph associated with a sulfur specific detector (Sievers Model 355 D SCD) equipped with a 30 m capillary column (Pona HP). All chemicals used were of analytical grade from Aldrich and Acros. Commercially available poly(styrene-co-dinvinyl-
10.1021/ie801065z CCC: $40.75 2008 American Chemical Society Published on Web 11/01/2008
9618 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 1. Composition and Physical Properties of the Studied Straight-Run Oil density (g/cm3) kinematic viscosity at 40 °C (cSt) bp (°C) sulfur (ppm) as DBTs aromatics (wt %) monoaromatics diaromatics total
0.8376 3.91 164-348 390
0.8353 3.84 153-348 60
20.7 5.4 26.1
19.3 4.0 23.3
Table 2. Structural Properties of Initial Poly(styrene-co-dinvinylbenzene) Beads and Polymer Supported π-Acceptor (PSA)
particle size surface area pore volume
PS
PSA
300-800 µm 1208 m2/g 0.84 cm3/g
300-800 µm 623 m2/g 0.70 cm3/g
benzene) beads (300-800 µm) were supplied by Aldrich (ref no. 42,698-9) and were sieved after the immobilization procedure using sieved shaker RETSCH WS 1. The Arabian Light SRGO examined in the present study was supplied by the Institut Franc¸ais du Pe´trole (Solaize). The two SRGO studied, containing 390 and 60 ppm of sulfur, were derived from a SRGO initially containing 13 600 ppm S which has been differently treated by HDS. The S-compounds present in these feeds are thus DBTs. The properties of these SRGO are presented in Table 1. Synthesis of the Polymer Supported π-Acceptor. 2,4,5,7Tetranitro-9-fluorenone was synthesized according to the previously described procedure.19 Synthesis of chloromethyl-methyl ether was prepared according to Stadlweiser.23 Caution: chloromethyl methyl ether (CAS No. 107-30-2) is a carcinogen and a hazardous air pollutant. Furthermore, the synthesis of chloromethyl-methyl ether (Figure 1) implies the concomitant production of HCl and carbon monoxide and should be realized by working with a properly functioning laboratory fume hood equipped with a water-washing tower and a carbon monoxide detector. A total of 495.5 g (2.8 mol) of thionyl chloride was added slowly on 251.96 g (2.8 mol) of methoxyacetic acid. After the addition of 1 mL of anhydrous N,N-dimethylformamide (DMF), a strong release of HCl was observed. The mixture was then heated at 70 °C during 3 h. NMR spectrum of the crude mixture indicated a complete conversion of methoxyacetic acid to methoxyacetyl chloride: NMR 1H (CDCl3): 4.4 (s, 2H, -CH2-COCl); 3.5 (s, 3H, CH3-O-). Then, 4 g (30 mmol) of aluminum chloride were added to the mixture at ambient temperature and then heated at 70 °C for 8 h. The chloromethylmethyl ether obtained was purified by distillation (bp ) 70 °C) to obtain 157.93 g of a colorless liquid. Yield: 65%. NMR 1H (CDCl3): 5.4 (s, 2H, -O-CH2-Cl); 3.5 (s, 3H, CH3-O-). Synthesis of the Chloromethyl Resins (Merrifield Resin). A 250 mL reactor was charged with 60 g of sieved poly(styrene-co-dinvinylbenzene) beads and a solution of 185 mL of chloroform and 12.5 g (155 mmol) of chloromethylmethyl ether at 20 °C. A solution prepared by the cautious addition of 8 mL (68.5 mmol) of tin(IV) chloride to 12.5 g (155 mmol) of chloromethyl-methyl ether and 25 mL of chloroform was added dropwise with gentle stirring to the cooled mixture over a 30 min period. After 24 h at ambient temperature, the resin was collected by filtration and washed successively with dioxane/water (1/1), dioxane/water/HCl (5/5/1), water, dioxane, tetrahydrofuran (THF), and finally pentane. Anal. Found: C, 72.09; H, 6.22; Cl, 8.85. Determination of the accessible chlorine on the resin by Volhard titration:24 1.37 mmol Cl/g. FTIR (cm-1): 3017, 2925, 1600, 1510, 1450, 1280, 1120, 610.
Modification of the Chloromethyl Resins. A total of 74 mL (820 mmol) of hydrazine in 35% weight in water were slowly added to 40 g of chloromethyl resin (1.37 mmol Cl/g) in 60 mL of ethanol at ambient temperature. At the end of the addition, the mixture was mechanically stirred for 24 h at 60 °C. The resin was then filtered and washed with water and then with ethanol. Anal. Found: C, 69.59; H, 6.19; N, 1.88; Cl, 1.00. Determination of the accessible chlorine on the resin by Volhard titration:24 0.15 mmol Cl/g. FTIR (cm-1): 3018, 2927, 1608, 1452. Synthesis of Polymer Supported π-Acceptor (PSA). A total 5.01 g (14 mmol) of 2,4,5,7-tetranitro-9-fluorenone were dissolved in 650 mL of toluene and 10 mL of acetic acid, and then 10 g of the resin previously obtained was added. The mixture was mechanically stirred for 3 days at 75 °C. The resin was then washed for 48 h with toluene with a Soxhlet apparatus and then with pentane and was dried at 60 °C yielding brown resins. Anal. Found: C, 72.61; H, 5.85; N, 7.03; Cl, 0.83. Determination of the accessible chlorine on the resin by Volhard titration:24 < 0.05 mmol Cl/g. FTIR (cm-1): 3020, 2925, 1700, 1606, 1540, 1347. Determination of the loading was achieved by nitrogen elemental analysis: 0.65 mmol of 2,4,5,7-tetranitro9-fluorenone/g. Determination of the Capacity of the Resins. The amount of immobilized π-acceptor molecules was determined by elemental analysis and by FTIR using the Lambert-Beer law. A calibration curve was made by mechanically mixing various amounts of 2,4,5,7-tetranitro-9-fluorenone and chloromethyl resins. Known amounts of KBr were then carefully added. Nitro groups and characteristic polystyrene absorptions at 1345 and 2925 cm-1 respectively were measured, and a linear relationship between the concentration of 2,4,5,7-tetranitro-9-fluorenone and absorbance ratio (Abs at 1345 cm-1/Abs at 2925 cm-1) was found. The loading determined for PSA by FTIR was 0.6 mmol of π-acceptor/g. Batch Adsorption. In a jacketed glass reactor with a volume of 250 mL volume, equipped with mechanical stirring by Teflon paddles, 100 g of hydrotreated straight-run feed containing 390 ppm of S-compounds were loaded. The temperature of the organic phase was adjusted to 30, 50, or 70 °C using a water/ glycol thermocirculator HAAK K20. When the temperature was stable, 50 g of the resin was added and the concentration of the S was analyzed as a function of time. Extraction efficiency E (%) and distribution coefficients D were determined according to eqs 1 and 2 where Ci and Cf are the initial and final concentrations of the studied molecule and msol and mr are the mass of solution and resin, respectively. E(%) ) D)
Ci - Cf × 100 Ci
( )
msol Ci -1 × Cf mr
(1)
(2)
Column Breakthrough Experiments. A jacketed glass column (3 × 25 cm) was used, and the resin was wet packed with toluene to avoid entrapped air bubbles within the column. The temperature of the organic phase inside the column was regulated at 50 °C using a water/glycol thermocirculator HAAK K20. Feeds containing 390 or 60 ppm of S-compounds were then added. The SRGO was injected into the adsorbent at a liquid hourly space velocity (LHSV) of 0.24 h-1. Two different samples were collected over the time and analyzed periodically to determine S concentration. The first one was collected at the outlet of the column. The breakthrough data were plotted as
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9619
Figure 1. Immobilization of 2,4,5,7-tetranitro-9-fluorenone on poly(styrene-co-divinylbenzene) beads.
Figure 2. Sulfur concentration as a function of the time for different temperatures (Initial S content: 390 ppm, msol/mr ) 2).
the ratio of the S-compounds’ concentration in the sample to S-compounds’ concentration in the feed (C/C0) as a function of the number of bed volumes (BVs) of the feed that passed through the column. The effluent was collected in a 1 L flask, and the second sample was collected from the flask over time. The S-compounds’ concentration in the flask was also plotted against the number of BVs of the feed that passed through the column. 3. Results and Discussion 3.1. Immobilization of 2,4,5,7-Tetranitro-9-fluorenone on an Organic Support. The synthesis of π-acceptor 4,5dicyano-2,7-dinitro-9-fluorenone, previously immobilized on Merrifield resin, is not straightforward,19 whereas 2,4,5,7tetranitro-9-fluorenone could be prepared on a 20 g scale in the laboratory in a one-step procedure starting from commercially available 9-fluorenone. Thus, we further evaluated 2,4,5,7tetranitro-9-fluorenone. π-Acceptor was immobilized on a
hydrophobic support according to Figure 1. Poly(styrene) beads were first chloromethylated in chloroform using chloromethylmethyl ether and a catalytic amount of tin(IV)chloride. After 24 h at ambient temperature, the beads were collected by filtration and carefully washed. The chloromethylated polystyrene beads were then treated with an excess of hydrazine (35 wt %) in EtOH at room temperature to produce hydrazine beads. 2,4,5,7-Tetranitro-9-fluorenone was immobilized on the beads using a toluene/acetic acid mixture at 75 °C. The beads obtained were carefully washed and noncovalently adsorbed 2,4,5,7tetranitro-9-fluorenone was removed by Soxhlet extraction using toluene for 48 h. The beads were then sieved to 300-800 µm, and the loading of the resin was determined by elemental analysis and by quantitative FTIR to contain 0.65 and 0.6 mmol of π-acceptor per g, respectively. Despite significant work on polystyrene resins and on inorganic support18 we were not able to increase the amount of 2,4,5,7-tetranitro-9-fluorenone immobilized on the support. The
9620 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 3. Distribution Coefficients of S Compounds as a Function of the Temperature (Hydrotreated SRGO Containing 390 ppm of S)a
a
temperature (°C)
polymer
D
30 50 70 70
PSA PSA PSA PS
5.3 4.8 3.5 0.6
msol/mr )2, mechanical stirring 3 h.
Figure 3. Column breakthrough experiments of SRGO containing 390 ppm S over PSA at 50 °C and LVSH of 0.24 h-1.
Figure 4. Column regeneration with toluene at 50 °C and at a LVSH of 3 h-1. Table 4. S Compounds’ Concentration of the Feed and the Eluted Solution and Weight and S Content of the Extracted Compounds feed
eluted solution
S (ppm)
S (ppm)
weight (g)
S content (%)
220
8
1.5
390
extracted compounds
characteristics of this resin are showed in Table 2. The decrease in the surface as well as the pores’ volume could be explained by intramolecular side-reactions during chloromethylation which introduce additional cross-linking. 3.2. Batch Extraction of S-Compounds as a Function of the Temperature. Desulfurization of the SRGO with 390 ppm S by adsorption over polymer supported π-acceptor (PSA) was studied as a function of the temperature. Figure 2 shows the time-dependent profiles of sulfur concentration at different temperatures using PSA and PS. As expected, the adsorption rate and the distribution coefficient for S-compounds were
dependent on the temperature. The time required to reach a plateau was 15, 30, and 60 min for 70, 50, and 30 °C, respectively. As expected, the distribution coefficient of Scompounds was also dependent on the temperature (Figure 2). The close level of desulfurization obtained at 30 and 50 °C may suggest a limit of desulfurization due to saturation of the surface. However, because of the relatively low concentration of the S-compounds in the feed (down to 200 ppm), less than 0.02 mM/g of S-compounds were adsorbed under those conditions. This value could be compared to the loading of the PSA which is closed to 0.65 mM/g. Furthermore, it should be emphasized that the distribution coefficients obtained at 30 and 50 °C are 5.3 and 4.8, respectively (Table 3). Thus saturation of the PSA could not be suspected. The presence of polyaromatics in the feed that could compete with the with the adsorption of S-compounds16 could explain the low capacity of the PSA using real feed. When increasing the temperature to 70 °C, lower distribution coefficients for S-compounds were obtained as expected from temperature-dependent adsorption equilibrium models.25 We also evaluated nonfunctionalized polystyrene beads (PS) under the same conditions. A lower absorption was observed and the distribution coefficient determined for PS at 70 °C was below 0.6 (Table 3). Several criteria are important in the design of adsorbent polymers. In our case, specific and fast binding of the Scompounds is essential. Therefore it can be concluded that a temperature of 50 °C could be an optimum value for this selective extraction process and this temperature was chosen for the column breakthrough experiments. With the SRGO containing 60 ppm of S-compounds, at 50 °C with a ratio feed/ PSA of 10, a distribution coefficient of S-compounds of 10 was obtained. 3.3. Column Breakthrough Experiments with SRGO Containing 390 ppm S. The resin was further evaluated in column flow-through experiments using the SRGO initially containing 390 ppm of sulfur. The breakthrough curve of the S-compounds over the supported π-acceptor at 50 °C and at a LVSH of 0.24 h-1 is shown in Figure 3. At 50 °C, S-compounds’ breakthrough occurred almost immediately after the feed solution was eluted, and the sulfur concentration in the breakthrough curve increased gradually showing that S-compounds present in the feed are adsorbed by PSA. This early breakthrough could be explained by the relatively low amount of π-acceptors immobilized on the resins (i.e.,
