Marc
Sévignon
a,
Mathieu
Macaud
a,
Alain
Favre-Réguillon
*ab,
Jürgen
Schulz
a,
Muriel
Rocault
a,
René
Faure
c,
Michel
Vrinat
d and
Marc
Lemaire
*a
aLaboratoire de Catalyse et Synthèse Organique, Université Claude Bernard Lyon1, UMR 5181, CPE, 43 bld du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. E-mail: marc.lemaire@univ-lyon1.fr; Fax: +(33) 472431408; Tel: +(33) 472431407
bLaboratoire de Chimie Organique, UMR 7084, Conservatoire National des Arts et Métiers, 2 rue Conté, 75003, Paris. E-mail: favrereg@cnam.fr
cLaboratoire des Sciences Analytiques, UMR 5180, Bât. Raulin, 43 bld du 11 Novembre 1918, 69622 Villeurbanne Cedex, France
dInstitut de Recherches sur la Catalyse, UPR 5401, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
First published on 26th April 2005
Very stringent regulations on the maximum sulfur content of gas oil have led to intense research into all aspects of desulfurization. Deep desulfurization of diesel fuels is particularly challenging due to the difficulty of removing refractory sulfur compounds, particularly 4,6-dialkyldibenzothiophenes, using conventional hydrodesulfurization processes (HDS). A novel approach to the potential desulfurization of fuels such as diesel is proposed. It relies on the ability of 4,6-dialkyldibenzothiophene to form charge-transfer complexes (CTC) with π-acceptor molecules. We present the synthesis of a new π-acceptor molecule, 4,5-dicyano-2,7-dinitrofluorenone, solution redox behaviour and the crystal structure of the charge-transfer complex with the refractory 4,6-dimethyldibenzothiophene. This π-acceptor compound was then immobilized on a hydrophobic support (poly(styrene-co-divinylbenzene)). The selectivity of the CTC process was confirmed by the analysis of the sulfur compounds trapped from straight run Arabian Light (SR) containing 13600 ppm S. The functionalized polymer can be used in multiple cycles for the removal of refractory S-containing compounds from hydrotreated SR. It can also be regenerated with toluene. The high selectivity of this material permits diesel fuel to be desulfurized to a level that meets future regulatory requirements, i.e. less than 10 ppm S, at ambient temperature and without hydrogen consumption.
There are three major types of transportation fuels: gasoline, jet fuels and diesel, which differ in composition and properties. The common types of sulfur compounds in the diesel fuel range are alkylated benzothiophenes (BTs), dibenzothiophene, and its alkylated derivatives (DBTs) (Fig. 1).
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Fig. 1 Types of sulfur compounds present in the diesel fuel range. |
The state of the art in desulfurization technology is hydrodesulfurization (HDS). In this process, the sulfur is removed from sulfur containing compounds by reaction with hydrogen, forming H2S on a sulfur type active phase catalyst. HDS is effective for a range of sulfur containing compounds that exhibit varying reactivities towards desulfurization. Reactivity is dependent upon the local environment of the sulfur atom in the molecule, and the overall shape of the molecule. The process is widely employed throughout the world and has been used for over 60 years. HDS is highly efficient at removing thiols, sulfides and disulfides, but is less effective for aromatic thiophenes and thiophene derivatives, especially those containing functional groups that hinder the sulfur atoms. Due to the current 350–500 ppm specifications mandated in several countries, the diesel fuel range available today has low total sulfur content but a disproportionately high concentration of refractory sulfur species.
Improvement of today's technology to enable production of ultralow sulfur content fuel requires that the difficulty of desulfurizing refractory sulfur compounds be overcome. Investigations have demonstrated that the sulfur compounds remaining in diesel fuels with a sulfur level lower than 500 ppm are the dibenzothiophenes with alkyl substituents at the 4- and/or 6- position, (Fig. 1). Both steric hindrance and electronic factors are responsible for the observed low reactivity. To meet the new regulations, the refining industry has demonstrated its capability to adapt hydrotreatment to the stringent regulations by improving catalytic activity and increasing the process severity, especially by increasing the hydrogen pressure or designing new reactor configurations.4 However, the selective elimination of this family of compounds from fuel is difficult due to their relatively low concentration in the feed and their low reactivity towards catalytic hydrodesulfurization.
The elimination of refractory sulfur compounds by non-catalytic processes is becoming a new challenge, because it could be accomplished at ambient temperature and pressure and without leading to an increase in the hydrogen consumption. New process concepts3 include design approaches for ultra-deep desulfurization focusing on adsorption,5–7 selective extraction using ionic liquids,8–10 oxidation-extraction,11,12 precipitation of S-alkylsulfonium salts13 and biodesulfurization.14
The planarity and electron-rich structure of 4,6-dimethyldibenzothiophene 1 (Fig. 1), lead us to expect that it would be capable of forming charge-transfer complexes (CTCs) with suitable π-acceptors. Charge-transfer complexes (CTCs) arising from the interaction of aromatic π-donor and π-acceptor molecules have been studied extensively15 and recently charge-transfer complex interactions have been evidenced by chemical force microscopy.16 CTCs have been used in supramolecular chemistry,17 for chiral recognition,18 and for selective organic transformation.19
An adsorption process is being explored in our laboratory for ultra-deep desulfurization of distillate fuels based on selective removal of electron rich refractory sulfur compounds like 4,6-dimethyl dibenzothiophene 1 (Fig. 1) by charge-transfer complex formation.20 However, diesel fuel contains a large variety of aromatic compounds, with and without heteroatoms, that compete with 1 in the formation of CTCs. Theoretical calculations21 and experimental studies on synthetic and real feed22–25 have shown that symmetrical π-acceptors based on polynitrosubstituted 9-fluorenones (e.g. 2,4,5,7-tetranitro-9-fluorenone 2, Fig. 2) efficiently and selectively complex dibenzothiophenes in preference to benzothiophenes and aromatic compounds.
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Fig. 2 Structure of π-acceptor molecules. |
Polynitrosubstituted 9-fluorenones (Fig. 2) have been widely employed as acceptors for charge-transfer complex formation,16 as photosensitizers and in electron transport materials.26 The structure of acceptor 2 has been determined by single X-ray analysis and reveals significant steric strain between the nitro substituents in the 4- and 5- positions.27 Thus their acoplanarity with the fluorene nucleus probably leads to a decrease in the electron-acceptor efficiency of these groups on account of the reduction in their contribution to the conjugation effect,27 and could interfere with the formation of a CTC.21
With the objective of improving the selectivity and the efficiency of the complexation of 4,6-dialkyldibenzothiophene, we carried out the synthesis of planar symmetrical π-acceptors bearing less bulky electron-withdrawing groups in the 4- and 5- position. It was expected that the substitution of nitro groups in the 4- and 5- positions by linear cyano (compound 4, Fig. 2) groups would decrease the steric strain and thus the dihedral angle between the two aromatic rings. Then, we determined their redox properties and studied the crystal structures of the new fluorenone derivatives and the CTC complex formed by π-acceptor 4 and refractory compound 1. Compound 4 was then immobilized on an insoluble resin for the ultra-deep desulfurization of distillate fuels by solid–liquid extraction.
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Fig. 3 Synthesis of the acceptor 4. |
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Fig. 4 Cyclic voltammogram of compounds 2, 3 and 4 under the conditions stated in Table 1. |
Compd | E 1red 1/2/V | E 1red 1/2/V | E 1red 1/2/V | EA (eV)a |
---|---|---|---|---|
Solvent acetonitrile; electrolyte 0.2 M Bu4N+BF4−; scan rate 100 mV s−1. All potentials given in the table were recalculated to Ag/AgCl scale.a Calculated using equation in ref. 32. | ||||
2 | −0.47 | −0.71 | −2.69 | 3.01 |
3 | −0.20 | −0.47 | −1.35 | 2.74 |
5 32 | −0.27 | −0.64 | −1.33 | 2.27 |
4 | −0.32 | −0.59 | −0.94 | 2.86 |
By analogy with compounds 2, 3 and 5, π-acceptor 4 shows clear amphoteric multiredox behaviour (Fig. 4, Table 1), consisting of 3 reversible single electron reduction waves.
Replacement of the nitro groups by cyano groups at the 4- and 5- positions slightly decreases the electron acceptor properties: the first reduction of compound 4 is shifted to a more positive value by 0.15 V (Table 1), but electron affinity (EA) calculations indicate that 4 is a rather strong acceptor, comparable to the strongest known acceptor of the fluorene series, i.e. 2,4,5,7-tetranitro-9-dicyanomethylene-fluorene32 or 3. The comparison of the reduction potentials for compounds 532 and 4 (Fig. 2) shows that the location of the cyano substituents (2,7 versus 4,5) does not exert a notable effect upon the electrochemical behavior of these compounds although a more planar structure, which is more favorable for CTC formation, was expected for compound 4.
The X-ray structure analysis of compound 2 has already been determined.33,34 The asymmetric unit consists of two molecules which differ slightly in their planarities and torsional angles. The two nitro groups in the 4- and 5- positions cannot remain in the fluorenyl plane and tend to move apart from one another. This repulsion causes rotation of these groups, inclination of C–N bonds and deformation of the fluorenone moiety. There are significant deviations from planarity of the fluorenone moiety of 2 in contrast to the planar molecule of 2,9-dinitrofluorenone 6.35 The dihedral angle values between the outer rings [C(4)–C(4a)–C(4b)–C(5)] (Fig. 5) of 2 are 13.5° and 7.7°33 (Table 2) although it is equal to 1.5° for 6.35 This deviation from planarity of the fluorenone moiety of 2 is expected to minimize the CT interactions between planar electron rich compounds 136 and 2.21 The same trend is amplified in compound 7, bearing bulky bromides in the 4- and 5- positions (Table 2), which has a dihedral angle value of 25.6°. In compound 4, introduction of the linear cyano groups in the 4- and 5- positions, strongly reduced the steric strain and the fluorenone moiety is almost planar. The dihedral angle between the outer rings [C(4)–C(4a)–C(4b)–C(5)] of compound 4 is 5.8° (Table 2).
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Fig. 5 Structure of the π-acceptor 7 (top) and 4 (bottom). |
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Fig. 6 Structure of the charge-transfer complex between 4,6-dimethyldibenzothiophene 1 and π-acceptor 4. |
The donor 1 and π-acceptor molecule 4 lie parallel with interplanar separation of 3.4 Å, which corresponds to the value expected of a charge-transfer interaction.21 Surprisingly the interactions between 1 with π-acceptor 4 involve a distortion of the structure of 4. An increase in the dihedral angles values between the outer rings [C(4)–C(4a)–C(4b)–C(5)] from 5.8° to 14.1° was observed for 4 (Table 2) although almost no modification was noticed for compound 1. Fig. 7 shows the crystal packing of the CTC formed between compound 1 and π-acceptor 4. The CTC crystal is composed of alternate layers of donors and acceptors. These layers are not coplanar for the same entity.
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Fig. 7 Crystal packing of the CTC between compound 1 and π-acceptor 4. H atoms are omitted. |
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Fig. 8 Immobilization of π-acceptor 4 on poly(styrene) resin 7. |
Poly(styrene) resins were chloromethylated (1.37 mmol Cl g−1) and then treated with an excess of hydrazine (35% wt) in EtOH at room temperature to give methyl hydrazine polymer-bound resin 10. π-Acceptor 4 was then added to resin 10 as a suspension in a toluene–acetic acid mixture at 75 °C. The excess of reagent was removed by soxhlet to yield resin 7. The loading of resin 7 was determined by elemental analysis to be 0.4 mmol of π-acceptor 4 per g.
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Fig. 9 Carbon and sulfur chromatograms of SR oil and sulfur chromatogram of trapped compounds. * and # correspond to benzothiophene and 4,6-dimethyldibenzothiophene respectively. General conditions: ambient temp., contact time 24 h, ratio fuel/polymer = 2. |
SR 13600 | SR 390 | SR 60 | |
---|---|---|---|
Density (288 K)/g l−1 | 853 | 838 | 835 |
Sulfur (ppm) | 13600 | 390 | 60 |
Total nitrogen (ppm) | 104 | 29 | 6 |
Aromatics (wt%) | 32.1 | 26.2 | 23.4 |
In order to reach very low sulfur levels we used a multistage extraction process with hydrotreated SR. The desulfurized oil from the first extraction step was re-treated with regenerated resin 7. Resin 7 could be regenerated by back extraction with 5 resin volumes of toluene and could be reused up to 10 times without loss of activity.
This process was repeated up to four times with two hydrotreated SRs obtained after classical HDS and deep HDS (390 and 60 ppm S respectively). The results are summarized in Table 4.
As shown in Table 4, the results of experiments with hydrotreated SR are promising. In three steps, a fuel containing less than 50 ppm S was obtained starting from diesel with 390 ppm S. Resin 7 is effective even with low S content SR (60 ppm S) obtained after severe HDS. The sulfur compounds remaining in the feed are refractory DBTs and in four steps the future specification was obtained (<10 ppm S).
Qualitative analysis of the sulfur-containing molecules was performed using a gas chromatograph associated with a sulfur specific detector (Sievers Model 355 B SCD). For qualification of the components, the BT and the DBT were commercially available and various alkyldibenzothiophenes have been synthesized as previously reported.37
The relevant properties of straight run oil are summarized in Table 3.
Parameter | 7 | 4 | CTC 4/1 |
---|---|---|---|
Formula | C13H4Br2N2O5 | C15H4N4O5 | C29H16N4O5S |
M | 428.00 | 320.22 | 532.52 |
System | Monoclinic | Monoclinic | Monoclinic |
Space group | P21/c | P21/n | P21/n |
a/Å | 10.3660(2) | 14.1690(8) | 7.7310(4) |
b/Å | 16.0720(5) | 6.4510(4) | 15.1430(9) |
c/Å | 8.3120(2) | 15.2380(11) | 21.1520(14) |
β/° | 103.372(2) | 110.614(2) | 98.189(2) |
V/Å3 | 1347.26(6) | 1303.64(14) | 2451.0(3) |
Z | 4 | 4 | 4 |
ρ/g cm−3 | 2.110 | 1.632 | 1.443 |
µ/mm−1 | 6.043 | 0.128 | 0.182 |
θ range/° | 2.38–31.00 | 2.44–27.69 | 1.66–24.39 |
Number reflections measured | 14094 | 5902 | 13094 |
Number of independent reflections | 4181 | 2562 | 3983 |
R int | 0.078 | 0.049 | 0.135 |
Final R value (I > 2 σ(I)) | 0.063 | 0.054 | 0.068 |
Final R (all data) | 0.115 | 0.090 | 0.147 |
CCDC reference numbers 253625 (for compound 4), 253627 (for compound 7) and 253626 (for CTC 4/1). See http://www.rsc.org/suppdata/gc/b5/b502672e/ for crystallographic data in CIF or other electronic format.
The loading capacity was calculated based on nitrogen elemental analysis and was found to be 0.4 mmol π-acceptor 4 per g. FTIR (cm−1): 3020, 2925, 1700, 1606, 1540, 1347.
This journal is © The Royal Society of Chemistry 2005 |