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Ind. Eng. Chem. Res. 1997, 36, 5110-5117
Hydrodesulfurization of Catalytic Cracked Gasoline. 2. The Difference between HDS Active Site and Olefin Hydrogenation Active Site S. Hatanaka*,† and M. Yamada Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980-77, Japan
O. Sadakane Petroleum Research Laboratory, Mitsubishi Oil Company Ltd., 4-1 Ohgimachi, Kawasaki-ku, Kawasaki 210, Japan
The inhibiting effect of olefin on CCG HDS (found in our previous report) was further investigated from a mechanistic point of view. A small amount of thiophene (corresponding to ca. 100 wt ppm of sulfur content) and varying amounts of olefins (diisobutylene and 1-octene) dissolved in toluene were hydrotreated on Co-Mo/Al2O3 at 150 °C at 1.3 MPa. Both the total HDS (thiophene HDS minus thiol and sulfide formation) and thiophene HDS were strongly inhibited by a small amount of olefin (1 mol %). However, when the olefin content was increased over 20 mol %, thiophene HDS rather recovered while the total HDS monotonously decreased. Both isoolefin and n-olefin can interact with the thiophene HDS active site, resulting in the inhibition on thiophene HDS. The interaction between the HDS active site and olefins was further studied by examining the effect of H2S, an well-known inhibitor for HDS, on hydrogenation of olefins. The hydrogenation of n-olefin as well as thiophene HDS was sensitively inhibited by H2S, while the hydrogenation of 2,4,4-trimethyl-2-pentene was promoted. The interaction was also studied by examining the effects of Co, an well-known promoter for HDS, on HDS and olefin reaction over different Co/(Co + Mo) ratio catalysts. HDS was promoted by Co; however, isoolefin hydrogenation was retarded slightly and n-olefin hydrogenation was largely retarded. Three types of active sites for thiophene HDS, n-olefin hydrogenation, and isoolefin hydrogenation, were proposed. Introduction Catalytic cracked gasoline (CCG) contains olefins in 20-40 vol % as well as paraffins and aromatics and is one of the major components of motor gasoline. Since CCG sometimes contains high levels of sulfur, CCG HDS (hydrodesulfurization) is a prospective process to be developed from the environmental point of view. In order to keep a high octane value of CCG during HDS treatment, higher activity for thiophene HDS and lower activity for olefin hydrogenation are expected for the catalyst at the same time. Therefore, it is important to make clear the possibility of the selective CCG HDS that gives minimum olefin loss by controlling the olefin hydrogenation active site. If the active site for olefin hydrogenation is different from that of HDS, it may be possible to control the activity of both sites separately. The relation between the active site for olefin hydrogenation and that for HDS on Co-Mo/γ-Al2O3 has been reported recently, in our previous work (Hatanaka et al., 1997). HDS reactivity of thiophenes (alkylthiophenes and alkylbenzothiophenes) in CCG has been compared with the reactivity of pure sulfur compounds in toluene on Co-Mo/γ-Al2O3 at 150-230 °C and 1.3 MPa, and it has been newly found that C6-C10 olefins contained in CCG strongly inhibited HDS reaction. These results indicated that C6-C10 olefins can interact * To whom correspondence should be addressed. Tel.: +8144-344-3128. Fax: +81-44-344-3645. † Present address: Petroleum Research Laboratory, Mitsubishi Oil Co. Ltd, 4-1 Ohgimachi, Kawasaki-ku, Kawasaki 210, Japan. S0888-5885(97)00349-7 CCC: $14.00
with the HDS active site, resulting in the inhibiting effect on HDS. The hydrogenation active site may be closely connected with that of HDS. On the other hand, concerning the relations between thiophene HDS and butene hydrogenation on Mo/γAl2O3 catalysts, the different view has been reported. For example, thiophene HDS was carried out at 400 °C, and atmospheric pressure on MoO3/Al2O3 varied the degree of sulfidation (Okamoto et al., 1980). kbutene-HG/ kthiophene-HDS was constant as long as the S/Mo atomic ratio was between 0 and 1.2; however, kbutene-HG/ kthiophene-HDS increased at a S/Mo atomic ratio over 1.2. Thiophene HDS was also examined at 300 °C and atmospheric pressure on the MoO/carbon catalyst modified by phosphate. Thiophene HDS was strongly retarded by phosphate modification; however, butene hydrogenation reaction was little affected (Bouwens et al., 1988). From the kinetic studies on thiophene HDS and successive butene hydrogenation by recirculation flow reactor on Co-Mo/γ-Al2O3 at 235-265 °C and atmospheric pressure, it was proposed that H2S affects the HDS reaction by competitive adsorption to the HDS active site; however, butene does not affect HDS and moves to the hydrogenation active site, which is different from the HDS active site (Satterfield and Roberts, 1968). The H2S and cis-2-butene (reaction products of thiophene HDS) addition study was also carried out in thiophene HDS on the Mo(100) single-crystal surface, and only H2S showed inhibition on HDS (Bussell and Somorjai, 1987). In this way, most of these works have indicated that the HDS-active site is different from butene hydrogenation active site. These indications by thiophene HDS and butene © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5111
hydrogenation studies seem to be inconsistent to our previous work that C6-C10 olefins inhibit thiophenes HDS by the interaction with HDS active site. The possible reasons for this inconsistency are the difference of olefin types, olefin carbon number, and reaction conditions. In this paper, the effects of olefin types and olefin concentration were further studied under the same reaction conditions of our previous work. The present work tries to make clear the mechanism of C6-C10 olefin inhibition on HDS by the careful study of thiophene HDS in the presence of these olefins. Diisobutylene, 2,4,4-trimethyl-2-pentene (2,4,4-TM-2-P), 1-octene, 1-hexene, and cyclohexene were selected as the olefin compounds because the olefins contained in CCG are mostly iso-C6-C10 olefins and some n- and cycloolefins. Further, in order to classify the HDS active site and olefin reaction active site, two well-known effects (i.e., the inhibiting effect of H2S and the promoting effect of Co on HDS reaction) (Hagenbach et al., 1971; Kasahara et al., 1995) were applied for thiophene HDS and olefin hydrogenation tests.
activity tests. Four mL of the catalyst crushed to 0.61.0 mm particles was packed in a microreactor. The catalyst was presulfided and aged by the same procedures of the previous work (Hatanaka et al., 1997). After these catalyst pretreatments, the feedstocks were flown into the reactors with hydrogen or H2S (0.01-1.0 vol %)/hydrogen. The reaction conditions were as follows: 150-200 °C, 1.3 MPa, catalyst/feed 0.17 or 0.34 g of cat. min/mol, H2/feed ratio 1.6 mol/mol. Definition. In this paper, total HDS (%) and thiophene HDS (%) are defined in the following equations, respectively.
Experimental Section
1. Reaction Products of Thiophene HDS in the Presence of Olefins. The product distribution of thiophene HDS (feed sulfur content: ca. 100 wt ppm) in the presence of olefins (diisobutylene and 1-octene) was studied at 150 °C and 1.3 MPa. In every case, the reduction of thiophene and the production of butane, butene, and THT were observed. The thiophene conversion to THT was 2-4% (analyzed by a GC-AED and identified by a GC-MS). THT has been observed in some thiophene HDS studies and reported as one of the reaction intermediates of thiophene HDS (Hensen et al., 1996). In addition to the reaction products of thiophene, several reaction products were also observed. That is, in the case of 2,4,4-TMP-2, 2,4,4-TMP-1 and 2,2,4trimethylpentane were found. In the case of 1-octene, 2-, 3-, and 4-octenes and n-octane were detected; however, the structural isomer of 1-octene was not found. These products were thought to be produced by isomerization and hydrogenation of fed olefins. The dimer and trimer of diisobutylene were formed, and the conversion of diisobutylene to the oligomer was less than 1%; however, the oligomer of 1-octene was not detected. A trace ( 1-octene ) 1-hexene > diisobutylene. Considering the molecular dimensions, the order of the molecular effective diameter is presumed to be cyclohexene > diisobutylene > 1-octene ) 1-hexene (Barrer, 1959), and the order of the molecular length is 1-octene > 1-hexene > diisobutylene > cyclohexene. As diisobutylene shows the most weak inhibition, and little difference is observed between 1-octene and 1-hexene, the strength of the inhibiting effects is considered not to depend on the steric effects of the olefin molecules but to depend on the interaction strength of olefin molecules to the HDS active site of the catalyst. In other words, it is suggested that diisobutylene is weakly adsorbed and 1-octene is strongly adsorbed on the HDS active site. 3. Thiol and Sulfide Formation. As mentioned above in section 1, thiols and sulfides were produced. These formation mechanisms were further studied because thiophene HDS was compensated by these productions. Thiol and sulfide production has been reported in the study of gasification and liquefaction of coal with or without Fe2O3 catalyst (Farmer, 1947; Attar, 1978;
Weisser, 1968). 2-Pentanethiol has been proposed to be produced by the reaction between 1-pentene and hydrogen sulfide on Fe2O3 (Miki, 1996). In the present work, no thiol and sulfide were formed without thiophene. And the types of thiols and sulfides, as mentioned above in 1, suggest the addition of H2S to reactant olefins. Olefins are quickly isomerized to the equilibrium composition, so 2,4,4-TM-2-P and 2-octene give the same thiol and sulfide distribution with diisobutylene and 1-octene. Therefore, they are also considered to be formed by the reaction of olefin with H2S from thiophene HDS. The selectivity of thiol and sulfide was clearly influenced by olefin content. As olefin content increased, thiol yield increased at first. By the further increase of olefin content, sulfide yield began to increase whereas thiol yield decreased. This may suggest that thiols are produced by the reaction between olefins and H2S and then sulfides are successively produced by the reaction between thiols and olefins. The above results suggest that olefin effectively scavenges H2S from the reaction system to produce thiols and sulfides. Considering the strong inhibiting effect of H2S on HDS, thiophene HDS recovery over 20 mol % of olefin content mentioned above may be brought about by the removal of H2S inhibition. The effects of H2S on HDS and olefin reactions will be discussed more precisely in 5. 4. Olefin Hydrogenation. The inhibiting effect of olefins on thiophene HDS suggests that olefin interacts with HDS active site. In this section, the interaction of olefin with active sites including HDS active site is further studied by examining olefin hydrogenation and isomerization reactions. In the thiophene HDS study with varying olefin content (2a, Figure 1), olefins are hydrogenated and isomerized. The results of olefin hydrogenation are plotted in Figure 3. Hydrogenation of diisobutylene decreases as its concentration increases. The calculated reaction order of diisobutylene hydrogenation is roughly 0.8. This suggests that isoolefin is strongly adsorbed on the hydrogenation active site. On the other hand, hydrogenation of 1-octene is only slightly affected by its concentration, and the reaction order of 1-octene hydrogenation is calculated to be 1.0, which is the same as butene hydrogenation (Satterfield and Roberts, 1968). A small amount of oligomer is formed from diisobutylene; however, oligomer is not formed from 1-octene. Oligomer may be produced by the strong adsorption of diisobutylene to the olefin reaction site.
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5113
Figure 2. Effects of olefin types on thiophene HDS. Reaction conditions: pressure 1.3 MPa, catalyst/feed 0.17g of cat. min/mol. Feed: thiophene concentration 2.83 × 10-4 mol/mol, olefin 20 mol %.
5. H2S Addition Study for HDS and Olefin Reactions. If the olefin hydrogenation active site is different from the HDS active site, it may be possible to improve the selectivity of CCG HDS by controlling these active sites independently. In other words, as mentioned in Introduction, in order to improve the HDS selectivity of CCG, it is necessary to make clear the difference between the HDS active site and the olefin hydrogenation active site. The present results about the thiophene HDS study in the presence of C6-C10 olefins indicate that olefin can interact with the HDS active site resulting in the poisoning of HDS. Therefore, if C6-C10 olefins, which can interact with the HDS active site, are hydrogenated on the same HDS active site, it is difficult to improve the HDS selectivity by selectively deactivating the hydrogenation active site. However, it is still unclear
whether C6-C10 olefins are hydrogenated on the thiophene HDS active site or not. And it should be mentioned that most of the previous works about thiophene HDS have proposed that produced butene moves to the hydrogenation active site which is different from HDS active site. In this section, the difference between them was further studied by using the inhibiting effects of H2S. As is well known, H2S is a strong inhibitor for HDS reaction (Satterfield and Roberts, 1968; Yamada et al., 1990; Kasahara et al., 1997), and its inhibiting effect is explained as follows. That is, H2S is adsorbed on the HDS active site having coordinative unsaturation and competitively inhibits the other sulfur compounds’ access to the HDS active site. Therefore, by using this well-known concept, it may be possible to make clear the difference. That is, if olefin reactions are affected
5114 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Figure 3. Effect of olefin content on olefin hydrogenation. Reaction conditions and feedstock: temperature 150 °C, pressure 1.3 MPa, H2/feed ratio 1.6 mol/mol, catalyst/feed 0.34 g of cat. min/ mol, thiophene concentration 2.83 × 10-4 mol/mol, toluene 9960 mol %, olefin 1-40 mol %. Table 1. Effects of H2S on Alkyl-(benzo)-thiophenes HDSa HDS, %, at H2S concn in H2, vol % sulfur compound
0
0.1
thiophene 2-methylthiophene 3-methylthiophene 2,5-dimethylthiophene 2-ethylthiophene benzothiophene
86.0 63.8 75.3 45.3 47.2 99.4
27.0 10.9 22.3 12.0 10.7 63.8
a Reaction conditions: temperature 160 °C, pressure 1.3 MPa, H2/feed ratio 1.60 mol/mol, catalyst/feed 0.34 g of cat. min/mol sulfur compounds concentration 2.83 × 10-4 mol/mol.
by H2S as HDS is affected, the structure of the olefin hydrogenation active site is considered to be similar to that of the HDS active site. If olefin reactions are not affected by H2S, the structure of the active site for olefin reactions may be different from that of the HDS active site. In this way, the structural difference between the HDS active site and olefin active site could be clarified by H2S addition study on HDS reaction and olefin reactions. (a) Effects of H2S on Thiophene HDS. The effects of H2S on HDS of thiophene, alkylthiophenes, and benzothiophene were examined in the H2 stream with or without H2S (Table 1). As is well known, even with a small amount of H2S (0.1 vol % in H2), all thiophenes’ HDS was strongly retarded, and the reaction rate constants obtained were 15-20% of the original. (b) Effects of H2S on Olefin Hydrogenation. The reactions of 2,4,4-TM-2-P, 1-octene and 1-hexene were examined in the H2 stream with or without H2S. The results are shown in Table 2 (2,4,4-TM-2-P), Table 3 (1octene), and Table 4 (1-hexene). The intrinsic hydrogenation reactivity of olefins without H2S is as follows: 1-octene > 1-hexene > 2,4,4-TM-2-P. With H2S, hydrogenation of 2,4,4-TM-2-P (production of 2,2,4-trimethylpentane) is promoted while isomerization of 2,4,4-TM2-P is little affected by H2S. On the other hand, both hydrogenation and isomerization reactions of 1-octene and 1-hexene are retarded by H2S. It is noted that the hydrogenation of 2,4,4-TM-2-P is promoted by H2S, because the effect of H2S on hydrogenation of higher isoolefins has never been reported and is completely different from that on the hydrogena-
tion of butene. For example, the effects of H2S addition on thiophene HDS and butene hydrogenation have been studied on Co-Mo/γ-Al2O3 by pulse technique (Owens and Amberg, 1961), and both thiophene HDS and butene hydrogenation are retarded by H2S. In contrast to olefin hydrogenation, in the study of the hydrogenation of aromatics on Co-Mo/γ-Al2O3, it has been reported that H2S retarded the hydrogenation of benzene, little affected the hydrogenation of toluene and promoted hydrogenation of o-xylene (Yamada et al., 1987). The various response of these hydrogenations to H2S addition may be due to the strength of their interactions with the active sites. The present finding, that hydrogenation of diisobutylene is promoted by H2S, means that the response of the diisobutylene hydrogenation to H2S addition is in contrast to those of thiophene HDS and n-olefin hydrogenation. Therefore, the diisobutylene hydrogenation active site is different from the HDS active site, and it may be possible to control the selectivity of CCG HDS. (c) Effects of H2S on Thiol and Sulfide Formation. Thiol and sulfide are found by the reaction of olefins and H2S at 150 °C as shown in Table 5. Since thiol and sulfide are not found without H2S, it is confirmed that thiol and sulfide are formed by the reaction between H2S and olefins. In the case of diisobutylene, conversion of H2S is low and the reaction order of H2S is roughly 0.7. On the other hand, in the case of 1-octene and 1-hexene, conversion of H2S is very high and the reaction order of H2S is roughly 1.0. As mentioned above in section 2a, the recovery of thiophene HDS was observed over 20 mol % of olefin concentration (Figure 1). As H2S strongly inhibits the HDS reaction, the removal of H2S from the reaction system is bound with higher thiophene HDS. Thiol and sulfide formation from olefin and H2S was also ascertained by the experiment in this section. By these results, it was suggested that olefins effectively scavenged H2S and removed the inhibiting effect of H2S. In the case of 1-octene, the possible explanation is that large recovery was brought by the complete removal of H2S by the high reactivity of 1-octene with H2S to sulfide. 6. The Effect of Co on HDS and Olefin Hydrogenation. As mentioned above in 5a and 5b, the response to H2S of the active site for thiophene HDS is similar to that for n-olefin hydrogenation but totally different from that for isoolefin hydrogenation. Considering that most of olefins contained in CCG are isoolefins, this difference between the active site for HDS and that for isoolefin hydrogenation is very important to improve the HDS selectivity of the catalyst. Then, in order to further clarify the difference between the HDS active site and the isoolefin active site, the response of the catalyst activity to the well-known promoting effects of Co was studied by using the catalysts with varying Co content. The results are shown in Figures 4 and 5. As is well-known, thiophene HDS drastically increases with increasing Co/(Co + Mo) ratio. On the other hand, with increasing Co/(Co + Mo) ratio, it is noted that the hydrogenation of diisobutylene decreases a little and that of 1-octene largely decreases. The similar decreasing trend has been reported about the activity for butene hydrogenation on unsupported Co-Mo sulfide catalysts, in which thiophene HDS and successive butene hydrogenation were undertaken at 320 °C and atmospheric pressure (Inamura and Prins, 1995). They have explained about the phenomena that
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5115 Table 2. Hydrogenation and Isomerization of 2,4,4-Trimethyl-2-pentene in the Presence of H2Sa composition in C8 hydrocarbons (composition in 2,4,4-trimethylpentene), mol % H2S concn, vol % in H2
2,4,4-TM-1-pentene
2,4,4-TM-2-pentene
2,2,4-TM-pentane
others
0 0.01 0.1 0.2 1.0
71.6 (73.7) 71.4 (73.0) 71.3 (74.0) 70.7 (74.0) 68.9 (73.8)
25.6 (26.3) 26.4 (27.0) 25.0 (26.0) 24.8 (26.0) 24.4 (26.2)
2.2 1.8 3.1 3.9 6.0
0.6 0.4 0.5 0.5 0.7
a Reaction conditions and feedstock: temperature 150 °C, pressure 1.3 MPa, H /feed ratio 1.60 mol/mol, catalyst/feed 0.34 g of cat. 2 min/mol, feedstock 2,4,4-trimethyl-2-pentene 20 mol % in toluene.
Table 3. Hydrogenation and Isomerization of 1-Octene in the Presence of H2Sa composition in C8 hydrocarbons (composition in olefins), mol % H2S concn, vol % in H2
1-C8d b
trans-2-C8d
cis-2-C8d
trans-3- and -4-C8d
cis-3- and -4-C8d
n-C8
0 0.01 0.1 0.2 1.0
1.5 (2.2) 2.5 (2.7) 4.1 (4.5) 13.9 (14.4) 25.5 (26.0)
17.1 (25.9) 30.8 (34.0) 45.4 (49.6) 43.7 (45.4) 37.5 (38.3)
6.8 (10.3) 11.6 (12.8) 17.9 (19.6) 21.8 (22.6) 21.4 (21.8)
31.0 (47.1) 35.5 (39.1) 17.8 (19.5) 11.8 (12.3) 9.1 (9.3)
9.4 (14.3) 10.3 (11.4) 6.3 (6.8) 5.1 (5.3) 4.5 (4.6)
34.2 9.4 8.5 3.7 2.0
a Reaction conditions and feedstock: temperature 150 °C, pressure 1.3 MPa, H /feed ratio 1.60 mol/mol, catalyst/feed 0.34 g of cat. 2 min/mol, feedstock 1-octene 20 mol % in toluene. b 1-Octene.
Table 4. Hydrogenation and Isomerization of 1-Hexene in the Presence of H2Sa composition in C6 hydrocarbons (composition in olefins), mol % d b
H2S concn, vol % in H2
1-C6
0 0.01 0.1 0.2 1.0
3.2 (4.4) 3.9 (4.3) 29.5 (30.7) 37.0 (38.3) 37.4 (38.5)
trans-2-C6d
cis-2-C6d
trans- and cis-3-C6d
n-C6
39.2 (48.6) 53.9 (60.2) 38.1 (39.7) 32.3 (33.4) 34.4 (35.4)
16.8 (20.8) 19.5 (21.8) 21.6 (22.5) 20.7 (21.4) 20.4 (21.0)
21.4 (26.6) 12.3 (13.7) 6.8 (7.1) 6.7 (6.9) 5.0 (5.1)
19.4 10.4 4.0 3.3 2.8
a Reaction conditions and feedstock: temperature 150 °C, pressure 1.3 MPa, H /feed ratio 1.60 mol/mol, catalyst/feed 0.34 g of cat. 2 min/mol, feedstock 1-hexene 20 mol % in toluene. b 1-Hexene.
Table 5. Thiols and Sulfides Formation by the Reactiona between Olefins and H2S composition in thiolc and sulfide,d mol % olefin
H2S concn, vol % in H2
diisobutylenee diisobutylene diisobutylene 1-octene 1-octene 1-octene 1-octene 1-hexene 1-hexene 1-hexene 1-hexene
0.01 0.1 0.2 0.01 0.1 0.2 1.0 0.01 0.1 0.2 1.0
thiols and sulfides yield, × 10 61 84 17 160 300 1000 16 110 220 410
10-5 mol/molb
1-thiol
2+-thiols
sulfides
66 72 67 1 14 31 44 1 19 35 44
8 12 16 6 55 52 50 5 38 45 48
26 16 17 93 31 17 6 94 43 20 8
a Reaction conditions and feedstock: temperature 150 °C, pressure 1.3 MPa, H /feed ratio 1.60 mol/mol, catalyst/feed 0.34 g of cat. 2 min/mol., feedstock toluene 80 mol %, olefin 20 mol %. b mol/mol-feed (olefin + toluene). c Octanethiol or hexanethiol. d Dioctyl sulfide or dihexyl sulfide. e Composition: 2,4,4-trimethyl-1-pentene 74.9 mol %, 2,4,4-trimethyl-2-pentene 20.6 mol %, others 4.5 mol %.
the addition of Co ions blocks the coordinatively unsaturated Mo sites on the edges of MoS2 and decreases the hydrogenation activity. If Co blocks the unsaturated Mo sites, the small effects of Co on isoolefin hydrogenation suggest that isoolefin hydrogenation proceeds on the unsaturated Mo sites not blocked by Co. In the case of the catalyst having half metal content (* marked), thiophene percent conversion largely decreases; however, percentage of hydrogenation slightly decreases. These results also indicate that HDS and olefin hydrogenation proceed at the different active sites. 7. Some Speculation on the Active Sites for HDS and for Olefin Reaction. In CCG HDS, higher activity for thiophenes HDS and lower activity for olefin hydrogenation are expected for the catalyst at the same time. The present series aims to clarify the condition to realize this expectation.
In our previous paper, it has been found that C6-C10 olefins contained in CCG show strong inhibiting effect on HDS. This is a new finding, because it has been reported that butene has no relation with the active site for thiophene HDS during thiophene HDS. Therefore, in order to improve the HDS selectivity of the catalyst for CCG HDS, the present work tried to investigate the inhibiting effect of olefins on thiophene HDS from the mechanistic point of view. From the study of thiophene HDS in the presence of iso- and n-olefins (Figure 1), it was found that both types of olefins can interact with the HDS active site, resulting in the inhibition on HDS reaction. In this study, the effects of olefin types and olefin concentration were further studied under the same reaction conditions of our previous paper. However, the reason for the inconsistency of the results between C6 and C8 olefins
5116 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Figure 4. Effects of Co on HDS and isoolefin hydrogenation. MoO3 content: 15.0 wt % (*: 7.5 wt %). Reaction conditions: temperature 175 °C, pressure 1.3 MPa, H2/feed ratio 1.6 mol/mol, catalyst/feed 0.34 g of cat. min/mol. Feed: thiophene concentration 2.83 × 10-4 mol/mol, toluene 80 mol %, diisobutylene 20 mol %.
Type 2: Active site for n-olefin hydrogenation. This site is inhibited by H2S and is also inhibited by Co. Type 3: Active site for isoolefin hydrogenation. This site is promoted by H2S but slightly inhibited by Co. By the kinetic study of benzothiophene HDS on CoMo/γ-Al2O3, it has been reported that some of the HDS sites are converted to hydrogenation active sites by the H2S addition (Kasahara et al., 1994). These results suggest that the structure of each active site is not so rigid and the HDS active site can hydrogenate olefin in some conditions. This may also indicate that the HDS active site has some similarity with hydrogenation active site like our results in which type 1 has some similarity with type 2. However, the type 3 site (isoolefin hydrogenation) is totally different from the type 1 site (thiophene HDS). These phenomena suggest the possibility of the selective CCG HDS in which HDS reaction performs with minimizing olefin hydrogenation because most of olefins contained in CCG are isoolefins. Literature Cited
Figure 5. Effects of Co on HDS and n-olefin hydrogenation. MoO3 content: 15.0 wt % (*: 7.5 wt %). Reaction conditions: temperature 190 °C, pressure 1.3 MPa, H2/feed ratio 1.6 mol/mol, catalyst/feed 0.34 g of cat. min/mol. Feed: thiophene concentration 2.83 × 10-4 mol/mol, toluene 80 mol %, 1-octene 20 mol %.
(our work) and butene (mentioned in introduction) is still unclear. The active sites for thiophene HDS and olefin hydrogenation were then classified based on their response to H2S, a well-known inhibitor to the HDS active site. The HDS reaction (Table 1) and hydrogenation of n-olefin (Tables 3 and 4) were retarded by H2S. On the other hand, by H2S, hydrogenation of isoolefin was rather promoted (Table 2). As the result, it was suggested that the active site for HDS has some similarity with that for n-olefin hydrogenation but not with the active site for isoolefin hydrogenation. The study of the effects of Co, a well-known promoter, suggested that the HDS active site is different from n-olefin and isoolefin hydrogenation active sites (Figures 4 and 5). From these results, we speculated on the following three types of active sites for HDS, n-olefin reactions, and isoolefin reactions. Type 1: HDS active site where n- and isoolefin can approach. This site is inhibited by H2S and promoted by Co.
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Received for review May 15, 1997 Revised manuscript received September 26, 1997 Accepted October 1, 1997X IE9703494
X Abstract published in Advance ACS Abstracts, November 1, 1997.
