Energy & Fuels 1995,9, 33-37
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Quantum Chemical Calculation on the Desulfurization Reactivities of Heterocyclic Sulfur Compounds Xiaoliang Ma, Kinya Sakanishi, Takaaki Isoda, and Isao Mochida" Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received May 16, 1994. Revised Manuscript Received October 6, 1994@
The relative hydrodesulfurization reactivities of heterocyclic sulfur compounds collected from our experiments and the literature were correlated with their electronic parameters estimated using MOPAC-PM3. It is found that the direct hydrogenolytic S elimination (direct hydrogenolysis) of sulfur-containing compounds is definitely correlated with the electron density of their S atoms, while the hydrogenation of the thiophenic ring or neighboring ring prior to hydrogenolysis correlated with the bond order of the most unsaturated bond in the ring. Methyl groups at 4andor 6-positions of dibenzothiophene may sterically retard direct hydrogenolysis. Hence, the hydrogenation of the aromatic ring adjacent to the thiophenic ring followed by the hydrogenolysis is a major route for such species. The molecular orbital calculation clarified that the hydrogenation of refractory sulfur compounds with alkyl steric hindrance accelerates their hydrogenolysis by reducing the steric hindrance through molecular puckering and by increasing the electron density on S. The calculation allows us to estimate reactivities of sulfur-containing compounds in both routes and to design the reaction scheme to ensure the most efficient hydrodesulfurization.
Introduction Deep desulfurization is a target of petroleum refining. Petroleum distillates often contain a large number of unreactive sulfur-containing compounds of concern, because to achieve deep desulfurization, they must be reacted.lW4Thus, it is very important to evaluate their reactivities. There have been accumulated a variety of catalytic desulfurization reactivity data under variable conditions. Fortunately, many data include the reactivity of dibenzothiophene (DBT), hence their relative reactivities to DBT allow the direct comparison of many data In the present study, attempts are made to correlate the relative reactivities of sulfur-containing compounds with their electronic parameters estimated by molecular orbital calculations. Hydrodesulfurization (HDS) has been proposed t o proceed through two routes: (1)the direct hydrogenolysis route where the sulfur atom is eliminated through direct interaction of the sulfur atom with the desulfurization active site on the catalyst surface without hydrogenation of the aromatic ring; (2) the hydrogenation route where the hydrogenation of an olefinic bond or aromatic ring neighboring the S atom takes place, followed by the hydrogenolysis of the C-S @Abstractpublished in Advance ACSAbstracts, November 15,1994. (1)Sakanishi, K.; Ando, M.; Mochida, I. Sekiyu Gakkaishi 1992,35, 403-408. (2)Shih, S. S.;Mizrahi, S.; Green, L. A.; Sarli, M. S. Ind. Eng. Chem. Res. 1992,31,1232-1235. (3)Kabe, T.;Ishihara, A. Znd. Eng. Chem. Res. 1992,31, 1577-
imn.
(4)Ma, X.;Sakanishi, K.; Mochida, I. Ind.Eng. Chem. Res. 1994, 33,219-222. (5)Kilanowski, D.R.;Teeuwen, H.; de Beer, V. H. J.; Gates, B. C.; Schuit, G. C. A.; Kwart, H. J. Catal. 1978,55,129-137. (6)Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C. J.Catal. 1979,57,509-512. (7)Geneste, P.;Amblard, P.; Bonnet, M.; Graffin, P. J . Catal. 1980, 61,115-127. (8)Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; de Beer, V. H. J.; Gates, B. C.; Kwart, H. J. Catal. 1980,61,523-527.
bond. The first route suggests a correlation of the hydrogenolysis reactivity with the electron density on the sulfur atom, while the second route suggests a correlation of the hydrogenation with the electronic characteristics of the unsaturated bond. There have been several reports of correlations of the reactivity of limited sulfur species with their parameters such as their n-electron densityg or ionization p ~ t e n t i a l .How~ ever, the logic for the selection of the reactivity parameters, which should be consistent with the desulfurization scheme, is not clear. In this study, the relative reactivities in the reactions through the first and second routes are correlated respectively with the electron density of the sulfur atom to be directly eliminated and with the bond order of the unsaturated bond to be hydrogenated prior to the desulfurization.
Computational Details The Computer Aided Chemistry (CAChe)worksystem provided by CAChe Scientific Inc. was used to calculate the electron density on the sulfur atom and bond order of the unsaturated bonds in the sulfur-containing compounds using the Molecular Orbital Package (MOPAC, Version 6.10). The PM3 (Modified Neglect of Diatomic Overlap, Parametric Method 3) semiempirical Hamiltonian developed by Stewartlo was employed to solve the Schrodinger equation to calculate the optimum geometry and electronic properties of the sulfur compounds using the standard parameters. Results 1. Electron Density on Sulfur Atom of SulfurContaining Compounds. The electron densities on the sulfur atoms of heterocyclic sulfur compounds and their hydrogenated derivatives are shown in Table 1. (9)Nagai, M.; Urimoto, H.; Uetake, K.; Sakikawa, N.; Gonzalez, R.
D.Prep.-Am. Chem. SOC.,Diu. Pet. Chem. 1986,31(4),857-861. (10)Stewart, J. J. P. J. Comput. Chem. 1989,10, 209.
0887-0624/95/2509-0033$09.00/00 1995 American Chemical Society
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34 Energy & Fuels, Vol. 9, No. 1, 1995
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Desulfurization Reactivities of Sulfur Compounds
According t o the electron density on the S atom, heterocyclic sulfur compounds are classified into three groups: (I) The electron density ranges between 5.690 and 5.780. The lone pair electrons on sulfur atom are conjugated with two unsaturated groups (olefinic bonds and benzene rings), such as thiophene (l),benzothiophenes, dibenzothiophenes, and benzonaphthothiophenes. (11) The electron density ranges between 5.900 and 6.000. The lone pair electrons are conjugated with only one unsaturated group such as 2,3-dihydrothiophene (2), 2,3-dihydrobenzothiophene(6), and (13).(111)The l a ,1,2,3,4,4a-hexahydrodibenzothiophene electron density is larger than 6.000. The lone pair electrons are never conjugated with any unsaturated group such as tetrahydrothiophene (3) and dialkyl sulfides (4). The smallest electron density, 5.696, is found on the S atom of thiophene in the first group (I). More aromatic rings fused to thiophene slightly change the electron density on the S atoms, whereas alkyl substituents on the aromatic ring hardly change the electron density of the S atom. 2. Bond Order of Heterocyclic Sulfur Compounds. The bond orders of the sulfur compounds are also shown in Table 1. Thiophene and benzothiophene (BT) (5) appear to carry isolated double bonds in their heterocyclic rings, exhibiting the largest bond order of 1.689 and 1.769, respectively. The aromatic ring adjacent t o the thiophenic ring in DBT carries a smaller bond order than that of thiophene, the largest two bond orders of 1.448 in the ring being found a t symmetrical Cl-C2 and C8-C9 bonds. By comparison, the alkyl substituents of alkyl DBTs change the bond orders slightly. In the four-cyclic sulfur compounds, benzoblnaphtho[2,3-d]thiophene(B[b]N[2,3-dlT)(23) carries the largest two bond orders of 1.614 and 1.613 at C7-C8 and C9-C10 bonds, respectively, and the second and the third largest ones (1.563 and 1.551) are found a t the C11-Clla and C5b-C6 bonds, respectively. The largest bond order in benzo[b]naphtho[l,2-dlthiophene (B[blN[1,2-dlT, 1.627) (26)is found at C10-C11 bond, and the second and the third largest ones (1.590 and 1.588) are found at C8-C9 and C6-C7 bonds, respectively. These bond orders are larger by at least 0.10 than that in DBT and less by ca. 0.14 than that of the C2-C3 bond in BT. 3. Correlation of Direct Hydrogenolysis Reactivity with the Electron Density on the S Atom. The direct hydrogenolysis reactivities of DBT, B[blN(251, and [2,3-dlT, 7,8,9,10-tetrahydro-B[blN[2,3-dlT 5b,6,ll,lla-tetrahydro-B[b]N[2,3d]T (24)were reported by Nag et a1.6 and Sapre et al.ll Their rate constants were determined at 300 "C and 71 atm over a CoOMoOdfil203 catalyst. The HDS reactivities of thiophene, BT, DBT, and some of their hydrogenated derivatives reported by Kilanowski et al.5 at 450 "C and 1 atm of H2, where direct hydrogenolysis of C-S bond is expected to be the principal reaction since the hydrogenation of the aromatic and double bond are not thermodynamically favorable under these conditions. The HDS reactivities of methyl-substituted DBTs were reported by Houalla and co-workers* at 300 "C, 102 atm over a sulfided CoO-MoOdAl203 catalyst and by the present authors4 a t 360 "C, 30 atm over a sulfided COO-Mood (11)Sapre, A.V.;Broderick, D. H.; Fraenkel, D.; Gates,B. C.; Nag, N. It AIChE J. 1980,26,690-694.
Energy &Fuels, Vol. 9,No. 1, 1995 36
.01 5.6
5.1
5.8 5.9 6.0 6.1 Electron density of the sulfur atom
6.2
Figure 1. Correlation between relative hydrogenolysis reactivity and the electron densities on sulfur.(0)Reported by Nag et a1.6and Sapre et al.ll (12) DBT, (23) Benzo[b]naphtho[2,3dlthiophene,(24) 5b,6,1l,lla-tetrahydrobenzo[b]naphtho[2,3dlthiophene, (25) 7,8,9,10-tetrahydrobenzo[b]naphtho[2,3-d1thiophene. ( 0 )Reported by Kilanowski et aLs (1) thiophene, (3) tetrahydrothiophene, (5) benzothiophene, (6) 2,3-dihydrobenzothiophene, (12) DBT. (+) Reported by Ma et al.4(12) DBT, (14)1-methyl-DBT,(15)or (16)2- or 3-methyl-DBT,(17a) 4-methyl-DBT,(21a)4,6-dimethyl-DBT.(A) Reported by Houalla et aL8(12) DBT, (19)2,8-dimethyl-DBT,(17b)4-methyl-DBT, (20) 3,7-dimethyl-DBT,(21b) 4,6-dimethyl-DBT. Numbering of data corresponds to Table 1. A1203 catalyst. Their HDS reactions are temporarily assumed to occur dominantly via the direct hydrogenolysis route since the ratio of the direct hydrogenolysis t o hydrogenation for DBT at 300 "C and 102 atm over a CoO-M003/A1203 catalyst was reported to be 667.12 In order to compare the reactivities of various sulfur compounds obtained under different reaction conditions by different authors, their hydrogenolysis reactivity (RHR) relative to DBT (the ratio of rate constants of sulfur species to that of DBT) was employed in the present study, assuming that the pseudo-first-orderrate equation is applicable t o all the sulfur compounds examined here. The electron densities on the S atoms are compared with their RHR in Figure 1. A fair correlation is observed, except for some alkylated DBTs with methyl substituents at the 4- and/or 6-positions. Tetrahydrothiophene showed the highest reactivity, reflecting its having the highest electron density on the S atom. BT, B[blN[2,3-d]T, and DBT and its methyl substituted derivatives have similar electron densities on their sulfur atoms (between 5.739 and 5.773) and, accordingly, similar direct hydrogenolysis reactivities, except for 4-methyldibenzothiophene (CMDBT) (17a, 1%) and 4,6-dimethyldibenzothiophene(4,6-DMDBT) (21a, 21b). It should be noted that thiophene showed the least hydrogenolysis reactivity (RHR 0.5) at 450 "C under 1 atm due to its having the lowest electron density. However, it exhibited 22.6 times larger HDS reactivity than that of DBT at 300 "C under 71 atm of hydrogen pressure,6 where another mechanism may operate. 4. Correlation of Hydrogenation Reactivity w i t h the Bond Order: HDS of Benzothiophenes. Geneste et al.7reported that the HDS of both methyl substituted (12) Houalla, M.;Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C.AIChE J. 1978,46, 1015-1021.
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36 Energy & Fuels, Vol. 9,No. I , 1995
e,
."
-n 2
.I
1.7
1.8 1.9 Bond order
2.0
Figure 2. Relative hydrogenation reactivity of methylsubstituted benzothiophenes and methyl-substituted 1,l-di-
oxodibenzothiophenes versus their Cz-Cs bond order. ( 0 ) Benzothiophenes: (5) BT, (7) a-methyl-BT,(8) 3-methyl-BT, (9)2,3-dimethyl-BT.(0)1,l-Dioxobenzothiophenes(BT0z):(11) BT02, (29) a-methyl-BTOz, (30) 3-methyl-BTOz, (31) 2,3dimethyl-BTOz. Numbering of data corresponds to Table 1. benzothiophenes and methyl substituted 1,l-dioxodibenzothiophenes at 300 "C, 50 atm over a COO-MOO$ A 2 0 3 catalyst proceeds via hydrogenation of their C2C3 bonds followed by the hydrogenolysis, and rate constants of their hydrogenation step were determined. Their hydrogenation reactivities relative to that of BT are correlated with their C2-C3 bond orders as shown in Figure 2. A monotonic correlation is observed. Among them, 2,3-dimethylbenzothiophene(9) exhibits the least hydrogenation reactivity corresponding to its smallest bond order value at C2-C3.
Discussion The present study revealed that the direct hydrogenolysis reactivity of most sulfur compounds is definitely correlated with the electron density on the S atom. Some of the methyl-substituted DBTs such as 4-MDBT and 4,6-DMDBT are definitely out of the correlation as shown in Figure 1. Methyl groups a t 4andlor 6-positions of DBT may sterically retard the direct hydrogenolysis because they prevent the S atom from interacting with the catalyst surface. Hydrogenation of a neighboring benzene ring prior to the S elimination is suggested. This hydrogenation will modify the plane of the substrate by causing molecular puckering, allowing the S atom to interact with the active sides on the catalyst surface. In addition, the hydrogenation is expected to activate the S atom by increasing its electron density as suggested by the calculation. For example, hydrogenation enhances the electron density from 5.768 for 4,6-DMDBT to 5.926 for la,1,2,3,4,4ahexahydro-4,6-dimethyldibenzothiophene (22). It should be pointed out that 3,7-dimethyldibenzothiophene(20) is slightly out of correlation. The two methyl groups may hinder the interaction of the S atom with the catalyst to a certain extent. This retardation is consistent with our previous paper.13 HDS of 4-MDBT and 4,6-DMDBTdominantly proceeds via the hydrogenation route, which occurs to 78.7 and 93.2% of total HDS, respectively, a t 320 "C, 2.5 MPa over a NiMo/AnOs catalyst. The hydrogenation rate constants of 4-MDBT and 2.76 x and 4,SDMDBT were 2.55 x min-I, respectively. Their similar values are consistent (13)Isoda, T.;Ma, X.; Mochida, I. Sekiyu Gukkuishi 1994,37,368375.
with their calculated similar bond orders. It is worthwhile to point out that la,1,2,3,4,4a-hexahydro-4-methyldibenzothiophene (18)and la,1,2,3,4,4a-hexahydro4,6-dimethyldibenzothiophene(20) exhibit similar direct hydrogenolysis reactivities, indicating that the steric hindrance of methyl groups against their direct hydrogenolysis should be relieved in such cases. The molecular parameter calculation shows that the C2-C3 olefinic bond in BTs has a much higher bond order than the unsaturated bonds of the other sulfurcontaining compounds. This suggests that the HDS of methylbenzothiophenes under usual HDS conditions should take place predominantly through the hydrogenation of C2-C3 olefinic bond followed by hydrogenolysis of the C-S bond, in agreement with the results observed by Geneste et al.7 and by Devawneaux and Maurin. l4 The correlation between the hydrogenation reactivities and the bond orders demonstrates that their hydrogenation reactivities increase monotonically with the increasing bond order of C2-C3. The much higher HDS reactivities of thiophene (the disappearance rate constant relative to DBT is 22.6) and BT (13.3) than DBT (1.0) reported by Nag et a1.6 a t 300 "C and 71 atm can be ascribed to S atoms more easily activated by the hydrogenation of the neighboring double bond due to their higher bond orders. The largest bond orders in each of them are 1.689 and 1.769, respectively. In contrast, the S atom in DBT is hardly activated since the hydrogenation of the ring is difficult due to its much lower bond order (1.448). Some authors reported higher reactivities of thiophene and BTs than that of DBT,6s7J5J6 while other authors obtained the opposite result^.^ The present study clarifies that their reactivity order varies with the mechanism and thus depends on the reaction conditions (temperature, hydrogen pressure, and the kind of catalyst). Under commercial conditions, the HDS of thiophene and BTs is dominated by the hydroof methyl substitugenation r o ~ t e . ~ JThe ~ J influence ~ ents on hydrogenation reactivity is explained by their influence on the bond order of the C2-C3 bond which is hydrogenated before S removal. The bond order and electron density can also successfully explain the HDS reaction networks of B[blN[2,3dlT and B[blN[1,2-dlT reported by Sapre et al.ll and Vrinat,lg respectively (see Figure 3). The bond orders of B[blN[2,3-blT suggest that the hydrogenation occurs most readily at C7-C8 (1.614) and C9-C10 (1.613) bonds in the aromatic ring isolated from the thiophene ring, and next at C5b-C6 (1.551) and C11-Clla (1.563), and hardly occurs at the other bonds due to their lower bond order ( < 1.500). The hydrogenation rates of B[blN[2,3-blT to give 7,8,9,10-tetrahydrobenzo[blnaphtho[2,3-d]thiophene (25) (8.5 x m3/kgof catalyst-s) and to give 5b,6,ll,llb-tetrahydrobenzo[blnaphtho[2,3dlthiophene (24) (4.2 x m3kg of catalyst-s) and no other hydrogenated derivatives rather than (24) and (26) are consistent with their bond orders. The largest three bond orders in B[blN[1,2-dlT are found a t C9C10 (1.6271, C8-C9 (1.5901, and C6-C7 (1.5881,respectively. This ranking also agrees with their relative (14)Devanneaux, J.; Maurin, J. J. Cutul. 1981,69,202-205. (15)Frye, C.G.; Mosby, J. F. Chem. Eng. Prog. 1967,63,66. (16)Miki, Y.; Sugimoto, Y.; Yamadaya, S.Sekiyu Gukkaishi 1992, 35,332-338. (17)Devanneaux, J.; Maurin, J. J. Cutul. 1981,69,202-205. (18)Irandoust, S.;Gahne, 0. AIChE J. 1990,36,746-752. (19)Vrinat, M. L. Appl. Cutul. 1983,6,137-158.
Desulfurization Reactivities of Sulfur Compounds A: The numbers next to the arrow denote pesudo first-order rate constants
Energy & Fuels, Vol. 9, No. 1, 1995 37 The successful correlation of the reactivities with the electron density or the bond order allows us to predict the reactivity of other sulfur species and their reaction pathways. Using this understanding, the catalyst and reaction conditions can be designed to follow the easiest route for desulfurization.
Conclusion The present study concludes the following: 1. On the basis of molecular orbital calculations, B: The numbers next to the arrows are relative values of the pseudo heterocyclic sulfur compounds are classified into three first-order rate constants groups according to their electron density on the S atom, depending on the extent of conjugation of the electron lone pair on the sulfur atom with the neighboring unsaturated bonds. More aromatic rings fused to the thiophene ring slightly change the electron density, whereas alkyl substituents on the aromatic ring hardly change it. 2. Direct hydrogenolysis reactivity of sulfur-containing compounds without alkyl steric hindrance is defiFigure 3. Reaction networks proposed for (A) benzo[blnitely correlated with the calculated electron density of naphtho[2,3-dlthiopheneby Sapre et aLll and (B) benzo[bltheir S atom. naphtho[l,a-d]thiophene by Vrinat.lg 3. The hydrogenation reactivity of the unsaturated bond is related to its bond order. HDS reactivity of hydrogenation rates (0.96 and 0.84) described in the methyl BTs is monotonically correlated with the bond reaction networks (Figure 3). orders of their C2-C3 bonds. The methyl substituents It is worthwhile to note that hydrogenation of each in BTs influence the bond order of the C2-C3 bond benzene ring is not always favorable for HDS. For which is hydrogenated before S elimination. example, 7,8,9,10-tetrahydrobenzo[bI-naphtho[2,3-d14. Exclusive hydrogenation of the benzene ring or thiophene (25) exhibited lower hydrogenolysis reactivity olefinic bond neighboring the S atom favors the hydro(8.4 x than its parent (23) (1.3x lop4),since such genolysis of the C-S bond by increasing the electron hydrogenation decreases the electron density of the S density on the S atom. atom from 5.773 to 5.763. The exclusive hydrogenation 5. The low HDS reactivity of 4-methyl and 4,6of the benzene ring adjacent t o the thiophene ring is dimethyl DBTs is ascribed to the steric hindrance of the favorable for the C-S bond cleavage because it increases methyl substituents rather than electronic effects. The the electron density on the sulfur atom. It is emphahydrogenation of the benzene ring promotes HDS not sized that the preference between direct hydrogenolysis only by reducing the methyl steric hindrance through and hydrogenation routes depends not only on the molecular puckering but also by increasing the electron molecular structures of sulfur species themselves, but density of the S atom. also strongly on the reaction conditions and the kind of EF9400878 the catalyst.
