C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 459–463© 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS. Tous droits reservesS1387-1609(00)00128-6/SCO

Chimie des surfaces et calalyse / Surface chemistry and catalysis

How could organic synthesis help theunderstanding of the problems of deephydrodesulfurization of gasoils?Alexandra Milenkovica,b, Mathieu Macauda,b, Emmanuelle Schulza, Tamas Koltaia,b,David Loffredab, Michel Vrinatb, Marc Lemairea,*

aLaboratoire de catalyse et synthese organique, UMR 5622, UCBL, CPE Lyon, 43, bd du 11-Novembre-1918,69622 Villeurbanne cedex, FrancebInstitut de recherches sur la catalyse, 2, av. Albert-Einstein, 69626 Villeurbanne cedex, France

Received 2 February 2000, accepted 3 February 2000

Communicated by Francois Mathey

Abstract – Various alkyldibenzothiophenes bearing bulky groups in positions 4 and 6 were synthesized and their sensitivityto HDS compared over a NiMo/Al2O3 industrial catalyst in a batch reactor at 573 K and under 5 MPa H2 pressure. It wasdemonstrated that their reactivity is correlated to the steric hindrance near the sulfur atom. A new method for removing theserefractory compounds is described, involving the selective formation of insoluble charge transfer complexes betweendibenzothiophene derivatives and tetranitrofluorenone in synthetic solutions. Following the same procedure, the global sulfurlevel was lowered in gasoils. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

hydrodesulfurization / hydrogenation / alkyldibenzothiophenes / kinetics / charge-transfer complex / p-accep-tors

Version francaise abregee — Quid de la chimie organique au service de l’hydrodesulfuration profonde ? Plusieursalkyldibenzothiophenes substitues par des groupements alkyles en position 4 et 6 ont ete synthetises. Nous avons compareleur reactivite en hydrodesulfuration sur un catalyseur industriel NiMo/Al2O3 dans un reacteur ferme a 573 K et sous unepression d’hydrogene de 5 MPa. Il a ete demontre que leur reactivite etait directement liee a l’encombrement autour del’atome de soufre, l’etape limitante de leur transformation etant la rupture de la liaison C–S. Nous decrivons ensuite unenouvelle methode d’elimination de ces composes refractaires a l’hydrotraitement. Cette methode est basee sur l’analyse deproprietes physico-chimiques des composes refractaires et utilise la formation selective de complexes de transfert de charge(CTC) entre le 4,6-dimethyldibenzothiophene et la tetranitrofluorenone en solution synthetique. La procedure a ete etendueaux coupes gazoles et conduit a une baisse significative de leur taux de soufre. © 2000 Academie des sciences / Editionsscientifiques et medicales Elsevier SAS

hydrodesulfuration / hydrogenation / alkyldibenzothiophenes / cinetique / complexes de transfert de charge /accepteurs p

1. Introduction

In order to eliminate sulfur from distillates, con-ventional catalytic hydrotreating processes have

been used for many years. However, the ‘hard sulfur’compounds which remain after conventional hy-drodesulfurization (HDS) are still present in a signifi-cant amount (0.2–0.3 wt-% of sulfur) of the diesel

* Correspondence and reprints.E-mail address: marc.lemaire@univ-lyon1.fr (M. Lemaire).

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A. Milenkovic et al. / C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 459–463

fraction [1, 2]. Such refractory compounds have beenrecognized to be b-substituted alkyldibenzothio-phenes (alkylDBTs) [3–5] such as 4,6-dimethyldiben-zothiophene (4,6-DMDBT). Kinetic investigationsincluding competitive experiments [6] indicated that,after adsorption via an aromatic ring, thesemolecules were firstly hydrogenated into dihy-drodibenzothiophene derivatives. These unstable in-termediates were then transformed following twopathways on similar catalytic sites, by hydrogenationinto tetrahydro- and hexahydrodibenzothiophenecompounds, or by desulfurization according to anelimination mechanism, to obtain biphenyl deriva-tives. However, it was established that other sulfur-containing molecules, with higher molecularweights, were particularly resistant to classical HDS[7]. The identification of these refractory compoundsin hydrotreated gasoils is here reported as well asthe comparison of their reactivities in a batch reactor,on an industrial NiMo/Al2O3 hydrotreating catalyst.Then, a new procedure for removing these refractorycompounds from oil fractions is proposed: consider-ing the electron rich structure of alkylDBTs, theirability of forming charge-transfer complexes (CTC)with p-acceptors was studied [8]. The formation ofinsoluble CTCs, which are easily removable by filtra-tion, led to an efficient method for specific removalof alkylDBTs from gasoils [9].

2. Results and discussion

2.1. Synthesis of 4,6-dialkyldibenzothiophenes

4,6-DMDBT and two higher homologues, 4,6-di-ethyldibenzothiophene (4,6-DEDBT) and 4,6-di-isobutyldibenzothiophene (4,6-DiBuDBT) have beenprepared by a metallation and a subsequent alkyla-tion [10]. We have also obtained a more hinderedcompound (4,6-diisopropyldibenzothiophene, 4,6-DiPrDBT), with a four-step synthesis [11].

Sulfur derivatives issued from these syntheses,given in figure 1, have been used as authentic sam-ples for the identification, with the help of gas-chro-matography, of the structure of some refractorycompounds contained in desulfurized gasoils (figure2). Then, their transformation in a batch reactor onan industrial hydrotreating catalyst was studied.

2.2. Hydrodesulfurization of4,6-dialkyldibenzothiophenes. Kinetic results

Studies were performed in a batch reactor with anindustrial NiMo/Al2O3 catalyst. Classical kinetic tech-niques were used to determine the rate of transfor-mation and the selectivity observed for each product.This study suggested that each alkylDBT was trans-formed according to two parallel routes in agree-

Figure 1. Structure of the DBT derivatives synthesized.

Figure 2. Gas chromatography (sulfur-specific detection) of a desulfurized gasoil: identification of some refractory compounds.

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Figure 3. Reaction scheme for the transformation of alkyl-DBT.

the size of the alkyl groups (table). In the case ofdibenzothiophene (DBT) the major product ob-served was BP. For 4,6-DMDBT, 4,6-DEDBT and4,6-DiBuDBT, the products were mainly resultingfrom the hydrogenation pathway, leading to CHBand hydrogenated undesulfurized molecules. For4,6-DiPrDBT, such a tendency of the reactant to betransformed according to the hydrogenation path-way was even more pronounced: only hydrogenatedproducts were observed and no desulfurization wasnoticeable under the experimental conditions. Itmust be noted that 4,6-DiPrDBT was more than sixtimes less reactive than 4,6-DMDBT, which was al-ready considered as a model of an unreactivecompound.

The use of competitive experiments [13] allowedthe determination of the adsorption equilibrium con-stants ratio between two considered molecules.These ratios proved to be similar, suggesting that allthe studied compounds were identically adsorbed onthe catalytic surface and probably via the p electronsof the aromatic ring. Moreover, adsorption strengthwas not responsible for the difference of reactivityobserved between the various alkylDBT. Indeed,lower reactivity is related to lower reaction rate forthe C–S bond cleavage (elimination reaction) due tosteric hindrance, in the basic attack, generated by thealkyl group near the sulfur atom. This was proved bythe results obtained with 4,6-DiPrDBT where thesteric hindrance near the sulfur atom is more impor-tant, the tertiary carbon being closer to the sulfurthan in the case of 4,6-DiBuDBT, for example.

The challenge for the conventional catalytic hy-drotreating processes is now to eliminate these veryrefractory compounds. Research is being carried outto develop new catalysts and for the improvingcatalytic processes. But, this research does not seemto be really fully efficient for removing ‘hard sulfur’species from distillates.

We thus propose a process, based on a newconcept, to eliminate alkyldibenzothiophenes fromoil fractions: the selective formation of insolublecharge-transfer complexes between dibenzothio-phene derivatives and p-acceptors.

ment with previous proposals of the literature [6, 12](figure 3). The first route, called the direct desulfur-ization pathway (DDS), gives biphenyl derivatives(BP). The second pathway, called the hydrogenationroute (HYD) consists of a preliminary hydrogenationof an aromatic ring, giving tetrahydro- and hexahy-drodibenzothiophene or analogues (HN). These in-termediates can then be desulfurized, givingcyclohexylbenzenes derivatives (CHB).

Selectivities in each product (HN, CHB and BP)appeared to be deeply related to the presence and to

Table Table. Catalytic activity results (selectivities calculated at 10 % conversion rate).

Transformation rate (relative to DBT=100) S(BP) (%) S(CHB) (%) S(HN) (%)

100DBT 90 10 030915.94,6-DMDBT 61

11.94,6-DEDBT 5829136.3 104,6-DiBuDBT 40 502.34,6-DiPrDBT 0 0 100

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Figure 4. Structure of the p-acceptors tested and their respective LUMO.

2.3. Desulfurization by formation of CTC

Dibenzothiophene has already been reported tobehave as an electron donor in the formation of CTC[14]. But gasoils contain a large variety of aromaticcompounds, with or without heteroatoms. Most ofthem are able to form CTCs and can compete there-fore with DBT derivatives. The experimental deter-mination of their oxidation potential and thecalculation of their HOMO (performed with the PM3semi-empirical method within the HyperChem 4.0software) may allow a rough classification of theircomplexing ability: the dibenzothiophene derivativespossess high donor ability [15]. The selective com-plexation between these compounds and commonp-acceptors was tested. Various p-acceptors (figure4) with quinone, fluorenone and anthraquinonestructure have been tested in order to obtain thehighest selective removal of 4,6-DMDBT againstother non-heteroatom-containing aromatics. Theoret-ical calculations have been performed to determinethe LUMO level of the chosen acceptors. Thestrength of the interaction and the rate of the com-plexation have been correlated to the energy differ-ence between the LUMO of acceptor (A) and theHOMO of donor (B): the lower the LUMO level of A,the stronger will be the interaction with 4,6-DMDBT.

Two parameters have been checked in order tochoose the best acceptor: the selectivity of acceptorstowards the specific removal of 4,6-DMDBT amongnumerous polyaromatic compounds and their capac-ity which can be defined as the trapped amount of4,6-DMDBT. Among all tested combinations be-tween donor/acceptor molecules, tetranitro-fluorenone and 4,6-DMDBT show an optimaloverlap of the frontier molecular orbitals, as ex-

pected from the theoretical calculations. In this casethe observed selectivity towards 4,6-DMDBT ismaximal.

Considering this preliminary study on modelmolecules in synthetic solutions, the complexationprocedure was tested with a gasoil containing 11 300ppm of sulfur. The general experiment is describedin figure 5, where TNF has been chosen as insolublep-acceptor. The concentration of TNF introduced inthe gasoil was chosen to be equal to the initial sulfurlevel. The resulting suspension was stirred at roomtemperature leading to the formation of black insolu-ble CTC, then filtered. The initial gasoil, the filtrateand the residue were studied by GC (figure 5)equipped with a sulfur-specific detector (FPD). Aftertreatment with tetranitrofluorenone, the filtrate con-tained only benzothiophene compounds (figure 5B),whereas the BT/DBT ratio was initially 60:40 (figure5A). While dibenzothiophene derivatives were ableto form CTC with TNF, no BT derivative wastrapped.

This treatment led thus to an efficient and selectiveelimination of the sulfur- containing compounds thatare refractory to the classical HDS process. Studiesare now being performed to finalize a new desulfur-ization process according to CTC formation andbased on bed technology.

3. Conclusion

Hydrodesulfurization of polyaromatic sulfur com-pounds is still a serious challenge for refiners. Kinet-ics studies have proved that the significant decreasesin the global reaction rate, compared to DBT itself,

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Figure 5. General procedure for complexation and chromatogram obtained at the different stages: A) gasoil containing 11 300 ppm sulfur,B) gasoil obtained after filtration of the complexed aromatic compound, C) trapped compounds from the gasoil.

were not due to adsorption problems, but to sterichindrance in the elimination reaction involved in theC–S bond scission. In the case of 4,6-DiPrDBT, sucha steric effect led to a complete disappearance of thedesulfurization pathway under the chosen condi-tions. This compound could be considered as ahighly unreactive polyaromatic sulfur, since its trans-formation rate is about two orders of magnitudelower than that of dibenzothiophene.

Considering these refractory properties ofalkyldibenzothiophenes, a new procedure for deepdesulfurization, i.e. the formation of charge-transfercomplexes, was developed. Dibenzothiophenederivatives proved to form selectively insoluble CTCwith p-acceptors in gasoils, leading, after filtration, toa significant decrease of the global sulfur content.The original properties of alkyldibenzothiophenederivatives described in this article could be of greatimportance for developing new processes for thedeep desulfurization of gasoil.

Acknowledgements

Financial support for this work by Total-Fina, Elf, IFPand CNRS-Ecodev is gratefully acknowledged.

References

[1] Kabe T., Ishihara A., Tajima H., Ind. Eng. Chem. Res. 31(1992) 1577.

[2] Landau M.V., Catal. Today 36 (1997) 393.[3] Ma X., Sakanishi K., Mochida I., Fuel 73 (1994) 1667.[4] Houalla M., Broderick D., Sapre A.V., Nag N.K., de Beer

V.J.H., Gates B.C., Kwart H., J. Catal. 61 (1980) 523.[5] Ishihara A., Tajima A., Kabe T., Chem. Lett. 4 (1992) 669.[6] Meille V., Schulz E., Lemaire M., Vrinat M., J. Catal. 170 (1997)

29.[7] Depauw G.A., Froment G.F., J. Chromatogr. A 761 (1997)

231.[8] Foster R., Organic charge-transfer complexes, Academic

Press, New York, 1969.[9] Meille V., Schulz E., Vrinat M., Lemaire M., Chem. Commun.

(1998) 305–306; Lemaire M., Breysse M., Vrinat M., Meille V.,Schulz E., French Patent 97 07538, 1997.

[10] Caubere C., Adach-Becker S., Caubere P., Tetrahedron 52(1996) 9087.

[11] Macaud M., Milenkovic A., Schulz E., Lemaire M., Vrinat M.,J. Catal. (accepted).

[12] Singhal G.H., Espino R.L., Sobel J.E., Huff G.A., J. Catal. 67(1981) 457.

[13] Vrinat M., Germain J.E., J. Chim. Phys. 74 (1977) 524.[14] Mukherjee T.K.J., Phys. Chem. 73 (1969) 3442–3445;

Mukherjee T.K.J., Phys. Chem. 74 (1970) 3006–3014.[15] Milenkovic A., Schulz E., Meille V., Loffreda D., Forissier M.,

Vrinat M., Sautet P., Lemaire M., Energ. Fuels 13 (1999)881–887.

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