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Ind. Eng. Chem. Res. 1996, 35, 2487-2494
2487
KINETICS, CATALYSIS, AND REACTION ENGINEERING Hydrodesulfurization Reactivities of Various Sulfur Compounds in Vacuum Gas Oil Xiaoliang Ma,*,† Kinya Sakanishi, and Isao Mochida Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan
The hydrodesulfurization (HDS) of a vacuum gas oil (VGO) was performed at 360 °C (6.9 MPa) over a commercial NiMo catalyst to examine the HDS reactivities of various sulfur compounds which exist in the VGO by means of quantitative pseudo-first-order kinetic analysis. Four representative types of aromatic-skeleton sulfur compounds were observed in the VGO: alkylbenzothiophenes (BTs), alkyldibenzothiophenes (DBTs), alkylphenanthro[4,5-b,c,d]thiophenes (PTs), and alkylbenzonaphthothiophenes (BNTs). Among these, alkyl-BTs exhibited the highest HDS reactivity, whereas alkyl-DBTs with alkyl substituents at the 4 and/or 6 positions appeared to have the least reactivity even though their aromatic-skeleton is smaller than those of both alkyl-PTs and -BNTs. Steric hindrance of alkyl groups at specific positions appears to be a major reason for the low reactivity. Quantum chemical calculations on representative sulfur compounds were carried out to compare molecular parameters with their different HDS reactivities. Introduction Environmental concerns and the requirements for severe upgrading of the petroleum and coal-derived liquids strongly demand reduction of sulfur content in heavy distillates and impact the operating configuration of many refineries. In order to develop highly active catalysts and design a more efficient hydrodesulfurization (HDS) process, it is necessary to fully understand the HDS reactivities and mechanism of various sulfur compounds, especially the refractory sulfur compounds, existing in heavy distillates under practical desulfurization conditions. A large amount of polycyclicaromatic sulfur compounds and their alkyl-substituted derivatives was found to exist in the heavy distillates (Willey et al., 1981; Kong and Lee, 1982, Kong et al., 1984; Nishioka et al., 1985, 1986, Ma et al., 1996). Unfortunately, their HDS reactivities are still not welldefined. A number of researchers have reported on the HDS of model sulfur compounds such as thiophene, benzothiophene (BT), and dibenzothiophene (DBT) (Singhal et al., 1981; Ho and Sobel, 1991; Miki et al., 1992) as well as some of their alkyl-substituted derivatives (Houalla et al., 1980; Kilanowski et al., 1978; Miki et al., 1993). To date, benzonaphthothiophenes (BNT) were the largest polycyclic organic sulfur compounds that have been investigated. HDS networks of benzo[b]naphtho[2,3-d]thiophene (B[b]N[2,3-d]T) and benzo[b]naphtho[1,2-d]thiophene (B[b]N[1,2-d]T) were reported by Sapre et al. (1980) and Vrinat (1983), respectively. However, HDSs of such model sulfur compounds reported have been performed alone in the pure solvent. In the practical HDS of heavy distillates, the reaction environment is much more complex, coexisting aromatic † Present address: Fuel Science Program, Department of Materials Science and Engineering, 209 Academic Projects Building, The Pennsylvania State University, University Park, PA 16802.
species as well as various types of sulfur compounds compete for the active sites on the HDS catalyst surface. Hence, the reactivities of various sulfur compounds in heavy distillates still need to be defined in the practical desulfurization process. Determination of the HDS reactivities of various sulfur compounds in the practical process is always limited by the analytic technique in identification and quantification of the sulfur compounds, especially in high-boiling-point distillates, since the ring size, substituent structure, and electronic properties of sulfurcontaining heterocyclic aromatic compounds are very similar to those of the polycyclic aromatic hydrocarbons of much larger concentration, which are dominant in the same boiling range distillates and interfere in the identification and quantification of sulfur compounds. In a previous paper (Ma et al., 1994), the present authors have reported the HDS reactivities of sulfur species in a diesel fuel, where a high-resolution gas chromatograph (GC) with a sulfur-selective flame photometry detector (FPD) was used to quantify the sulfur contents. It was found that dibenzothiophenes having two alkyl substituents at the 4 and 6 positions are the most resistive to the desulfurization. In another paper (Ma et al., 1996), the authors reported both identification and quantification of high-molecular-weight sulfur compounds in a vacuum gas oil (VGO). The objective of this study was to determine HDS reactivities of sulfur compounds in a VGO over a commercial NiMo/Al2O3 catalyst under conventional conditions. In addition, quantum chemical calculation of representative sulfur compounds was carried out to correlate the molecular parameters with their different HDS reactivities based on the proposed HDS mechanism as described below. Experimental Section A VGO derived from a Middle East crude was supplied from Research Institute of Petroleum, Japan
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2488 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 1. Properties and Composition of VGO density (15 °C) (g/mL) composition H (wt %) C (wt %) S (wt %) N (wt %) V (wt ppm) Ni (wt ppm) boiling range (°C) IBP 5% 10% 20% 30% 40% 50% 60% 70% 80%
0.9263 12.00 85.27 2.45 0.17 0.14 0.05 205 341 378 410 434 450 473 494 510 530
correction factor which is a function of the retention time. The value of n was determined with standard samples. The details of identification and quantification of sulfur compounds in the VGO refer to a previous paper (Ma et al., 1996). The Computer Aided Chemistry (CAChe) worksystem provided by CAChe Scientific Inc. was used to calculate the electron density of the sulfur atom and bond orders of the unsaturated bonds in the sulfur compounds using the Molecular Orbital Package (MOPAC, Version 6.10). The PM3 (Modified Neglect of Diatomic Overlap, Parametric Method 3) semiempirical Hamiltonian developed by Stewart (1989) was employed to solve the Schro¨dinger equation to calculate the optimum geometry and electronic properties of the sulfur compounds using the standard parameters. Results
Energy Co. Its boiling range was from 340 to more than 530 °C, and the sulfur content was 2.45 wt %. Some properties and composition of the VGO are summarized in Table 1. The catalyst used in the present study was commercial NiMo/Al2O3 (NiO, 3 wt %; MoO3, 15 wt %; average diameter, 1.3 × 1.1 mm; average length, 3.5 mm), which was presulfided with a 5% H2S/H2 flow under atmospheric pressure at 360 °C for 6 h at a heating rate of 120 °C/h before their use. The HDS of the VGO was performed over the NiMo catalyst at 360 °C (6.9 MPa) in a 50 mL magnetically stirred (1000 rpm) batch autoclave with a catalyst-tooil weigh ratio of 0.10. The heating rate was ca. 25 °C/ min, and the cooling rate was ca. 30 °C/min. The reaction was counted from the moment when the temperature in the reactor reached the prescribed level. The hydrogen gas-to-oil ratio was 250 v/v (normalized). The total reaction pressure was controlled at 6.9 MPa throughout the reaction by adding gaseous hydrogen into the autoclave to compensate for its consumption. This ensured the approximately constant hydrogen partial pressure throughout the reaction. The sulfur compounds in the VGO and desulfurized oils were analyzed by a Yanaco gas chromatograph (G6800) equipped with a methylsilicone capillary column (0.25 mm i.d. × 50 m length × 0.25 µm film) and FPD. The column temperature was programmed from 70 to 120 °C at 10 °C/min, 120 to 200 at 4 °C/min, and 200 to 310 °C at 2 °C/min, and then kept at 310 °C. Major sulfur compounds were identified using Shimadzu GC-mass (GC-17A, QP-5000, quadrupole-type; EI, electron voltage, 70 eV) by comparing mass spectra with those of authentic standards, while most peaks were tentatively identified by their molecular ions (m/z). Some methyl- and dimethyl-DBTs were identified by referring to the relative retention times available in the literature (Vassilaros et al., 1982; Chawla and Di Sanzo, 1982; Kong et al., 1984; Rollmann et al., 1995). The peaks of DBT, 4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and benzo[b]naphtho[2,1-d]thiophene (B[b]N[2,1-d]T) were further verified by standard samples. The sulfur concentration corresponding to each sulfur compound was quantified by GC-FPD according to the following equation:
Ci(ppm) ) fAin where Ci is the sulfur concentration of sulfur compound i, Ai is the peak area of sulfur compound i, and f is a
1. Composition and Distribution of Sulfur Compounds in the VGO. FPD gas chromatograms of the VGO are shown in Figure 1, indicating more than 100 peaks in the VGO. The identification and quantification of major sulfur compounds in the VGO are summarized in Table 2, where their sulfur concentrations (ppm) are listed. Four types of sulfur compounds with different aromatic skeletons were basically present in the VGO. The first type of species was alkyl-BTs with alkyl carbon atoms from 2 to 16. The second type was alkyl-DBTs with alkyl carbon atoms from 0 to 6. The third type was alkyl-BNTs with alkyl carbon atoms from 0 to 5. The last one was alkylphenanthro[4,5-b,c,d]thiophenes (P[4,5-b,c,d]Ts) with alkyl carbon atoms from 2 to 7. Essentially neither sulfides nor alkylthiophenes were found in the present VGO. 2. HDS Reactivities of Sulfur Compounds. Figure 1 also shows FPD gas chromatograms of oils desulfurized at different reaction times at 360 °C (6.9 MPa H2). Almost all of the alkyl-BTs were desulfurized within the first 10 min, and most of the alkyl-BNTs were desulfurized within 40 min. The most refractory sulfur species were alkyl-DBTs with one or two alkyl substituents at 4 and/or 6 positions, such as 4-MDBT (No. 6), 4,6-DMDBT (No. 11), 2,6-DMDBT (No. 12), 3,6-DMDBT (No. 13), 1,4- and/or 1,6-DMDBT (No. 15). Figure 2 shows the progress of HDS conversion with reaction time for overall sulfur and selected representative sulfur compounds. The loss of sulfur from the major sulfur-containing compounds with reaction time was found to follow pseudo-first-order kinetics, which can be expressed in integral form as follows:
Ln(C0,i/Ct,i) ) kdisa,it where C0,i and Ct,i are sulfur concentrations (ppm) of the sulfur compound i at initial time and reaction time t (min), respectively, and kdisa,i is its pseudo-first-order disappearing rate constant (min-1). This kinetic treatment relies on following two assumptions. (1) The main sulfur compounds found in the GC region suffer no interference from partially hydrogenated sulfur compounds since most of the hydrogenated species, such as 2,3-dihydrobenzothiophene, 1a,2,3,4,4a-hexahydrodibenzothiophene, 1a,2,3,4,4a-hexahydro4,6-dimethyldibenzothiophene, and 5b,6,11,11a-tetrahydrobenzo[b]naphtho[2,3-d]thiophene, display the higher reactivities by an order of magnitude than those of their
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2489
Figure 1. GC-FPD chromatograms of feed and its hydrodesulfurized oils [reaction conditions: 360 °C, 6.9 MPa, NiMo catalyst].
parents (Kilanowski et al., 1978; Houalla et al., 1978; Isoda et al., 1994; Sapre et al., 1980). (2) Most of the alkyl-BTs were removed within the first 10 min due to their high reactivities (Nag et al., 1979; Ma et al., 1994). The kdisa values of alkyl-DBTs, alkyl-PTs, and alkyl-BNTs were calculated based on the conversions within the reaction times between 10 and 40 min, where the interference in the main alkyl-DBTs, -BNTs, and -PTs from alkyl-BTs is negligible. The pseudo-first-order plots of some representative sulfur compounds are shown in Figure 3. Fairly good straight lines were obtained. The rate constants of major sulfur compounds at 360 °C were calculated by the least-squares method and are listed in Table 2. The HDS reactivities relative to DBT (the ratio of rate constants of the sulfur compounds to that of DBT) are also listed in Table 2 for comparison of their reactivities. Most alkyl-BTs, except C3-BT (No. 2) and C12-BT (No. 31), showed considerably higher reactivities than
that of DBT, their relative rate constants being threefold higher than that of the latter. Alkyl-DBTs had low and very different reactivities, and their relative rate constants varied from 1.14 to 0.09, depending on the positions of their alkyl substituents. Among alkyl-DBTs, DBT and the alkyl-DBTs without alkyl substituents at the 4 and 6 positions, such as 2- and/or 3-MDBT (No. 7), 1-MDBT (No. 8), and 2,8and/or 3,7- and/or 3,8-DMDBT (No. 14), had their relative rate constants between 1.14 and 0.74. AlkylDBTs with one of the alkyls substituting at either the 4 or 6 position, such as 4-MDBT (No. 6), 2,6-DMDBT (No. 12), 3,6-DMDBT (No. 13), and 1,4- and/or 1,6DMDBT (No. 15), showed very low HDS reactivity but were still higher than that of 4,6-DMDBT. Among sulfur compounds found in the present study, 4,6DMDBT displayed the lowest reactivity, its relative rate constant being only 0.09. Some of alkyl-DBTs with three or more alkyl carbon atoms appeared to have
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2490 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 2. Hydrodesulfurization Reactivities of Sulfur Compounds (See Figure 1 for the Peak Assignments) peak no.
molecular ion
proposed identification
S concentration (ppm)
rate constant k (min-1)
ki/kDBT
1 2 3 4 5 6
176 176 176 184 218 198 232 198 198 232 232 212 212 212 212 246 212 246 212 246 212 246 226 226 226 226 226 226 260 240 260 240 240 240 236 236 302 234 268 302 302 250 316 250 316 248 250 248 248 330 264 330 264 330 262 264 266 262 330 262 278 330 278 330 276 292 344 276 344 276 292 344 276 344 276 344 290 358
C3-benzothiophene C3-benzothiophene C3-benzothiophene dibenzothiophene C6-benzothiophene 4-methyldibenzothiophene C7-benzothiophene 2 and/or 3-dimethyldibenzothiophene 1-methyldibenzothiophene C7-benzothiophene C7-benzothiophene C2-dibenzothiophene 4,6-dimethyldibenzothiophene 2,6-dimethyldibenzothiophene 3,6-dimethyldibenzothiophene C8-benzothiophene 3,7- and/or 2,8- and/or 3,8-dimethyldibenzothiophenes C8-benzothiophene 1,4 and/or 1,6-dimethyldibenzothiophene C8-benzothiophene C2-dibenzothiophene C8-benzothiophene C3-dibenzothiophene C3-dibenzothiophene C3-dibenzothiophene C3-dibenzothiophene C3-dibenzothiophene C3-dibenzothiophene C8-benzothiophene C4-dibenzothiophene C8-benzothiophene C4-dibenzothiophene C4-dibenzothiophene C4-dibenzothiophene C2-phenanthrothiophene C2-phenanthrothiophene C12-benzothiophene benzo[b]naphtho[2,1-d]thiophene C6-dibenzothiophene C12-benzothiophene C12-benzothiophene C3-phenanthrothiophene C13-benzothiophene C3-phenanthrothiophene C13-benzothiophene C1-benzonaphthothiophene C3-phenanthrothiophene C1-benzonaphthothiophene C1-benzonaphthothiophene C14-benzothiophene C4-phenanthrothiophene C14-benzothiophene C4-phenanthrothiophene C14-benzothiophene C2-benzonaphthothiophene C4-phenanthrothiophene C4-phenylbenzothiophene C2-benzonaphthothiophene C14-benzothiophene C2-benzonaphthothiophene C5-phenanthrothiophene C14-benzothiophene C5-phenanthrothiophene C14-benzothiophene C3-benzonaphthothiophene C6-phenanthrothiophene C15-benzothiophene C3-benzonaphthothiophene C15-benzothiophene C3-benzonaphthothiophene C6-phenanthrothiophene C15-benzothiophene C3-benzonaphthothiophene C15-benzothiophene C3-benzonaphthothiophene C15-benzothiophene C4-benzonaphthothiophene C16-benzothiophene
22 27 23 53 30 65 15 69 49 16 29 47 69 55 76 19 46 15 67 18 73 48 61 84 77 78 91 71 15 66 13 76 80 85 79 57 32 50 29 20 76 69 10 66 32 81 17 94 84 15 52 36 38 62 38 35 24 77 23 66 37 58 40 40 47 37 29 78 41 58 18 20 45 59 67 36 56 53
0.1012 0.0374 0.1204 0.0296 0.0887 0.0051
3.42 1.26 4.07 1.00 3.00 0.17
0.0246 0.0219
0.83 0.74
0.1184 0.0129 0.0025 0.0116 0.0050
4.00 0.44 0.09 0.39 0.17
0.0339
1.14
0.0047
0.16
0.0088 0.0852 0.0062 0.0056 0.0089 0.0058 0.0051 0.0182
0.30 2.88 0.21 0.19 0.30 0.19 0.17 0.61
0.0052
0.17
0.0112 0.0098 0.0089 0.0174 0.0091
0.38 0.33 0.30 0.59 0.31
0.0151
0.51
0.0311 0.0179
1.05 0.60
0.0107
0.36
0.0136
0.46
0.0259 0.0196
0.88 0.66
0.0112
0.38
0.0236
0.80
0.0171
0.58
0.0148
0.50
0.0148
0.50
0.0081
0.27
0.0180
0.61
0.0178
0.60
0.0132
0.45
0.0256
0.87
0.0121
0.41
0.0303
1.02
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
}
} } } }
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2491
Figure 2. HDS conversion of representative sulfur compounds and total sulfur vs reaction time.
reactivity similar to that of 4,6-DMDBT. Two of their alkyl substituents are most likely to be located at the 4 and 6 positions. Alkyl-BNTs showed moderate HDS reactivities, with their relative rate constants changing from 1.02 to 0.41. It is worthwhile to note that such reactivities are much higher than those of the alkyl-DBTs with one of the alkyl substituents at either the 4 or 6 position. Relative rate constants of alkyl-P[4,5-b,c,d]Ts were in the range from 0.27 to 0.70, which was also much higher than that of 4-MDBT but lower than that of DBT. 3. Electron Density on the Sulfur Atom and Bond Order in the Representative Sulfur Compounds. The electron density on the sulfur atoms of representative sulfur compounds, their methyl-substituted derivatives, and their partially hydrogenated derivatives are shown in Table 3. The electron densities of the S atom in four types of aromatic-skeleton structures are included in the range from 5.739 to 5.774. Alkyl substituents on the aromatic ring hardly change the electron density of the S atom, whereas the hydrogenation of the benzene ring or olefinic bond adjacent to the S atom increases the electron density of the S atom by ca. 0.2. The bond orders of the representative sulfur compounds are also shown in Table 3. BT has the largest bond order (1.769) at the C2-C3 bond in its heterocyclic ring among the four types of aromatic sulfur compounds. Even if two methyl substituents are present at C2 and C3, respectively, the bond order of the C2-C3 bond is still as high as 1.720. The maximum bond order in DBT is found to be 1.448 at symmetrical bonds of C1-C2 and C8-C9, which is the smallest among the sulfur compounds. The bond orders in DBT and its alkylsubstituted derivatives are all similar, probably due to a well-conjugated system. B[b]N[2,1-d]T has the highest bond order of 1.625 at the C5-C6 bond in the ring adjacent to the S atom and the second highest bond order of 1.595 at C8-C9 and C10-C11 bonds in the ring apart from the S atom. P[4,5-b,c,d]T has the first highest bond order of 1.734 at the C5-C6 bond in the ring apart from the S atom. Symmetrical bonds of C3C4 and C7-C8 in the rings adjacent to the S atom have the second highest bond order of 1.511 in P[4,5-b,c,d]T. Alkyl substituents on the aromatic rings change the bond order in the rings only slightly. Discussion The sulfur species identified in VGO may be basically classified into four types of aromatic skeletons. However, the reactivity for HDS in these structures is not a
simple function of aromatic ring size or type. AlkylBTs exhibit the highest HDS reactivity, whereas alkylDBTs exhibit very different reactivities, strongly depending on the positions of the alkyl substituents. Alkyl-DBTs with alkyl substituents at the 4 and/or 6 positions appeared to have the least reactivity as observed in diesel fuel (Ma et al., 1994). Both alkylPTs and alkyl-BNTs exhibit lower reactivities than those of alkyl-BTs but much higher than those of alkylDBTs with alkyl substituents at the 4 and/or 6 positions. Two routes of HDS have been proposed for these structures: One is the hydrogenolysis route where the sulfur atom is directly eliminated without hydrogenation of the aromatic ring, while the other is the hydrogenation route where the hydrogenation of an olefinic bond or an aromatic ring takes place prior to the hydrogenolysis of the C-S bond. Hence, the HDS reaction networks of the four types of sulfur compounds can be represented as follows:
where R is an alkyl substituent. kH1 and kH2 are the rate constants of the hydrogenation at the aromatic ring adjacent to and apart from the S atom, respectively. kS1, kS2, and kS3 are the hydrogenolysis rate constants of the sulfur species and their derivatives hydrogenated at the aromatic ring adjacent to and apart from the S atom, respectively. If each step of the reaction is assumed to follow the first-order kinetics, the disappearing rate of
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2492 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996
Figure 3. Pseudo-first-order plots of representative sulfur compounds. Table 3. Electron Density (the Data Underlined) on the Sulfur Atom and the Bond Order (the Data Next to the Bond) of Representative Sulfur Compounds and Some of Their Hydrogenated Derivatives representative species
suggested alkyl species with steric hindrance
partial hydrogenated species
the sulfur compound i can be described as follows:
Ratei ) -(kS1,i + kH1,i + kH2,i)Ci ) -kdisa,iCi where kdisa,i ) kS1,i + kH1,i + kH2,i. It has been reported that hydrogenolysis reactivities are governed by the electron density on the S atom and alkyl steric hindrance, while the hydrogenation route involves the bond order of the bond which is hydrogenated prior to the S elimination (Ma et al., 1995). Thus, the hydrogenolysis rate constants (kS1) of the four types of sulfur compounds are expected to be much the same, except for sterically hindered species, because of their similar electron densities on the S atom. The difference in the total HDS reactivities (kdisa) of the four types of species can be ascribed to their different hydrogenation reactivities (kH). Under the conventional conditions, the HDS of alkylBTs has been reported to dominantly proceed through the hydrogenation at the C2-C3 bond, followed by the hydrogenolysis (Devanneaux and Maurin, 1981), where the hydrogenation step is the rate-determining step.
Larger bond order (1.769) at C2-C3 also supports this route. In the present study, HDS rate constants (kdisa) of most alkyl-BTs relative to DBT were ca. 4. If DBT is assumed to be desulfurized dominantly by hydrogenolysis under the present conditions, the higher (three-fold) reactivities of alkyl-BTs are ascribed to their hydrogenation route, suggesting that kH1/kS1 is about 3 for alkyl-BTs. A very satisfactory agreement is noted between the values predicted in this way and those in the HDS of model BT reported by Devanneaux and Maurin (1981), where kH1/kS1 for BT was 3.2 at 250 °C over a Co-Mo/Al2O3 catalyst. The influence of alkyl substituents on the hydrogenation route of alkyl-BTs can also be ascribed to their influence on the bond order of the C2-C3 bond as discussed in our previous paper (Ma et al., 1995). Two methyl substituents at the 2 and 3 positions, respectively, reduce the hydrogenation rate of BT by ca. 10 times due to the reduction of the bond order from 1.769 to 1.718. Steric hindrance of the two methyl substituents may be also responsible. Thus, such alkyl-BTs, for example, C3-BT (No. 2) and C12-BT (No. 31), exhibit HDS reactivities comparable to that of DBT. HDS of alkyl-DBTs takes place predominantly by the hydrogenolysis route under the present conditions as reported by Houalla et al. (1978) and Aubert et al. (1986). The values of kH1/kS1 are calculated on the basis of their data to be 670 and 4.5 under the conditions of 300 °C, 102 atm, and Co-Mo/Al2O3 and 340 °C, 70 atm, and Ni-Mo/Al2O3, respectively. An even distribution of the π-electrons on the conjugated system of DBT reduces their bond order ( kH1,DBTs) which promotes its HDS through the prior hydrogenation. There are three aromatic-skeletal isomers of BNT, B[b]N[2,3-d]T, B[b]N[1,2-d]T, and B[b]N[2,1-d]T. According to Sapre et al. (1980) and Vrinat (1983), the (kH1 + kH2)/kS1 values for B[b]N[2,3-d]T and B[b]N[1,2-d]T were 0.97 and 0.94, respectively, being ascribed to their
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2493
higher bond order in the aromatic rings. The contribution of the hydrogenation route to the overall HDS reaches to ca. 50%. Nag et al. (1979) reported that B[b]N[2,3-d]T showed higher HDS reactivity than DBT, with their relative reactivities being 2.6. The higher HDS reactivity of the former can be principally ascribed to the contribution of the hydrogenation route even though it has a slightly higher electron density on the S atom. Although alkyl substituents at the 2 and/or 11 positions may block the hydrogenolysis of alkyl-B[b]N[2,3-d]T, its HDS may proceed readily through the hydrogenation route. C11 in the aromatic skeleton of B[b]N[2,1-d]Ts have an effect similar to the methyl substituent at the 4 or 6 position in DBT which sterically hinder the hydrogenolysis. The higher bond order (1.625) at the C6C7 bond in B[b]N[2,1-d]T allows the hydrogenation route, where enhancement of desulfurization reactivity by reducing steric hindrance and increasing the electron density on the S atom is expected. It is worthwhile to note that hydrogenation is not always favorable for HDS. For example, the hydrogenation of the benzene ring apart from the S atom for alkyl-P[4,5-b,c,d]T and alkyl-B[b]N[2,1-d]T changes the electron density on the S atom very little (the electron density is 5.763 for both 5,6-dihydrophenanthro[4,5b,c,d]thiophene and 8,9,10,11-tetrahydrobenzo[b]naphtho[2,1-d]thiophene). Hence, this has little effect on their hydrogenolysis reactivity (kS3 ∼ kS1). The hydrogenation of the aromatic ring adjacent to the S atom is exclusively favorable for enhancing the electron density, thereby the HDS reactivities as observed by Sapre et al. (1980) and Vrinat (1983) for the HDS of B[b]N[2,3d]T and B[b]N[1,2-d]T, kS3/kS1, were 4.1 and 31, respectively. The relative HDS reactivities also depend strongly on the reaction conditions, such as temperature, pressure, catalyst, and even other non-sulfur-containing compounds of the mixture (Isoda et al., 1994b, 1995). HDS reactivities of 4-MDBT and 4,6-DMDBT relative to that of DBT (being 0.17 and 0.09, respectively) in the present study were much lower than those (being 0.35 and 0.14, respectively) as observed in the HDS of the diesel fuel (Ma et al., 1994). A higher concentration of larger polyaromatics with higher bond orders in their aromatic skeletons exists in VGO than in diesel fuel and thus inhibits the hydrogenation of 4-MDBT and 4,6-DMDBT by preferential occupation of the hydrogenation site on the catalyst. The alkyl-DBTs with alkyl substituents at the 4 and/ or 6 positions are the most refractory sulfur species in both diesel fuel and VGO. Therefore, it is necessary to pay closer attention to the reactivity of such sulfur species when the HDS process and active catalysts are to be designed. It is worthwhile to point out that the refractory sulfur species exist predominantly in the lighter fraction of the VGO; hence, it may be advantageous to separate the fraction rich in these species from the more reactive fraction to allow the desulfurization of these low reactivity species under more optimum conditions. Acknowledgment The authors express their gratitude to Dr. D. Duayne Whitehurst for his helpful comments and kind review. X.Ma expresses his special thanks to the Fukken Kaikan of Koube for providing him scholarship throughout this work.
Nomenclature BNT ) benzonaphthothiophene B[b]N[1,2-d]T ) benzo[b]naphtho[1,2-d]thiophene B[b]N[2,1-d]T ) benzo[b]naphtho[2,1-d]thiophene B[b]N[2,3-d]T ) benzo[b]naphtho[2,3-d]thiophene BT ) benzothiophene DBT ) dibenzothiophene DMDBT ) dimethyldibenzothiophene FPD ) flame photometry detector GC ) gas chromatography HDS ) hydrodesulfurization kdisa ) pseudo-first-order disappearing rate constant of the sulfur compound kH1 ) pseudo-first-order hydrogenation rate constant of the sulfur compound at the aromatic ring adjacent to the S atom kH2 ) pseudo-first-order hydrogenation rate constant of the sulfur compound at the aromatic ring apart from the S atom kS1 ) pseudo-first-order hydrogenolysis rate constant of the sulfur compound kS2 ) pseudo-first-order hydrogenolysis rate constant of the sulfur compound derivatives hydrogenated at the aromatic ring adjacent to the S atom kS3 ) pseudo-first-order hydrogenolysis rate constant of the sulfur compound derivatives hydrogenated at the aromatic ring apart from the S atom MDBT ) methyldibenzothiophene P[4,5-b,c,d]T ) phenanthro[4,5-b,c,d]thiophene VGO ) vacuum gas oil
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Received for review March 12, 1996 Accepted May 17, 1996X IE960137R
X Abstract published in Advance ACS Abstracts, July 1, 1996.
