Energy & Fuels 1994,8, 147-150
147
C7 Alkene Conversion and Reactivity under Desulfurization Conditions Scott A. Goddard* and Simon G. Kukes Amoco Research Center, 150 W . Warrenville Rd., Naperville, Illinois 60563 Received August 13, 1993. Revised Manuscript Received September 27, 1993"
The reactivities and reaction pathways of C7 alkenes over a CoMo/AlzOa hydrotreating catalyst were studied using both model compound feeds and heavy catalytic naphtha (HCN). The three C7 compounds studied included a normal alkene (1-heptene),an isoalkene (2-methyl-l-hexene), and a .cycloalkene(1-methylcyclohexene). All three compounds underwent double bond isomerization and hydrogenation, but only the iso- and cycloalkenes reacted by skeletal isomerization. No appreciable amounts of cyclization products were observed. The rates of hydrogenation were of the order normal alkenes (heptenes) > isoalkenes (methylhexenes) > cycloalkenes (methylcyclohexenes). The distributions of the unconverted alkenes for each type of compound generally approached predicted equilibrium values as the reaction temperature increased. The alkenes in HCN showed the same order of reactivity and similar activation energies as in the model compound studies. These HCN studies also revealed the dependence of the reactivity on the length of the carbon chain as c6 > C7
> cg.
Introduction Heavy catalytic naphtha (HCN),a high-octane gasoline component, contains most of the gasoline-rangesulfur and alkenes. Its high sulfur content could potentially limit the use of HCN in the future gasoline pool or at least require severe desulfurization prior to end-product blending. Although desulfurization is a faster reaction up to high levels of sulfur removal, some alkene saturation does occur;' this results in a severe octane penalty. An example of the large octane losses from saturation of the most prevalent normal alkenes in HCN is shown in Table 1. This potential conflict motivated the present investigation of the reactivities and reaction pathways of C7 alkenes. Both model compound and HCN feeds were employed, the former to probe detailed pathways and mechanisms and the latter to reveal effects of the real oil environment. For the model compound studies, C7 alkenes were chosen since they represent one of the largest contributors to the alkene pool in heavy catalytic naphtha, and because of the difficulty in separating and identifying all of the species present in HCN. The reactions of these compounds over metal oxide or sulfide catalysts have been studied previously, mostly in terms of metathesis,2double bond isomerization,394and dehydrocyclization.~However, they have not been studied in as much detail as have the reactions of c6 alkenes.610 Abstract published in Aduance ACS Abstracts, November 15,1993. (1)Gutberlet,L.C.;Bertolacini,R. J. Ind. Eng. Chem. Prod. Res. and Deu. 1983,22,246. (2) Kawai, T.; Okada, T.; Ishikawa, T. J.Mol. Catal. 1992,76,249. (3)Panchenkov, M.; Vagin, M. F.; Aslanyan, M. V.; Timofeeva, G. V. Zh.Fiz. Khim. 1973,47,2565. (4)Maure1,R.; Guienet,M.,Marcq,M.;Germain,J.-E.Bull. SOC. Chim. Fr. 1966,10,3082. (5)Usov, Yu. N.;Kuvshinova, N. I.; Skvortaova, Ye. V.; Nasledskova, G.G. Neftekhimiya 1966,6,872. (6)Panchenkov,G. M.; Vagin, M. F.; Podobedov, S. I.;Boiko,M. I. Zh. Fir. Khim. 1973,47,1707. (7)Maurel, R.;Guisnet,M.; Bove, L. Bull. SOC. Chim.Fr. 1969,6,1975. (8)Karmal, S.;Perot, G.; Duprez, D. J. Catal. 1991,130, 212. (9)Resofszki, G.; Gati, Gy.; Halasz, I. Appl. Catal. 1985,19, 241. (10)Kijenski,J.; Malinowski, S. Bull. Acad. Pol.Sci., Ser. Sci. Chim. 1977,25, 669.
Table 1. Octane Differences between C ~ C Normal S Alkenes and Alkanes
c6 alkenes C6 alkanes
research octane no. 90 25 76 0 62 -20
(av)
C, alkenes (av) C, alkanes Cs alkenes (av) Cs alkanes
diff in octane no. between alkenes and alkanes 65 76 a2
Table 2. Properties of Akzo Hydrotreating Catalyst KF-742 co0,wt % MOO3, W t %
activated alumina nominal size, in. shape surface area, m2/g pore volume, mL/g
4.0 15.0 balance 1/16 Quadralobe 260 0.52
The approach in this investigation was to study C7 alkenes through the reactions of their different structural isomers (i.e., the normal, iso, and cyclostructures). Thus, 1-heptane, 2-m-l-hexene, and 1-m-cyclohexene (where m = methyl) were studied at temperatures from 408 to 477 K and a pressure of 2 MPa over a commercial CoMo/ A1203 hydrotreating catalyst. In addition, the reactivity of C7 alkenes in HCN was examined under similar conditions over the same catalyst.
Q
Experimental Section Conditions. The experiments were conducted in a 1-in. fixed bed pilot plant operated a t 2 MPa and using a hydrogen flow rate of 50 sccm. The feed was delivered to the unit at a LHSV of 2. The catalyst was a commercial hydrotreating catalyst made by Akzo Chemical Co. (KF-742); its properties are listed in Table 2. The active catalyst (10 cm3) was diluted with low surface area alumina (20 cm3) to improve flow distribution and minimize temperature gradients along the length of the reactor. The catalyst was presulfided using a solution of toluene containing 1wt% sulfur in a toluene-soluble polysulfide for 16 h at 480 K, 1 h at 530 K and 1 h at 590 K.
0887-0624/94/2508-0147$04.50/00 1994 American Chemical Society
Goddard and Kukes
148 Energy & Fuels, Vol. 8, No. 1, 1994
Table 3. Skeletal Isomerization and Hydrogenation Products from Each Alkene products reactants hydrogenation skeletal isomerization 1-heptene 2-m-1-hexene 1-m-cyclohexene
heptane
2-m-hexane m-cyclohexane
none
di-m-pentenes e-cyclopentenes
Feeds. Three different C, alkenes were studied: a normal alkene (1-heptene),an isoalkene (2-m-l-hexene),and a cycloalkene (1-m-cyclohexene). These chemicals were obtained from Aldrich Chemical Co. The alkeneswere reacted both individually and as a mixture. Most of the results reported here are from the feeds containing all three alkenes. A xylene mixture was used as the solvent in each feed blend. In the experiments with all three alkenes present, ca. 2-3 w t % of each C, alkene was added to the feed. When each alkene was observed individually, the amount added was ca. 4-5 wt % . To keep the catalyst in its active sulfided state, sulfur was added to the feed at 200 parts per million by weight. The heavy catalytic naphtha used in the second part of this study contained about 15 mol % alkenes and 0.2 w t % sulfur. Procedure. After the sulfiding was completed, a flow of feed at 2 LHSV was begun. After this initial 22 h contact period, a 2-h reaction test was conducted during which the product was collectedat 280 K for GC analysis (usinga 100-mSupelcoPetrocol D H column). At the end of this 2-h test, the reaction conditions were changed and another 22-h line-outperiod was initiated before the next 2-h reaction test period. This experimental procedure was repeated throughout the study. The temperature range for the model compound experiments was chosen so as to keep the combined conversion for hydrogenation and skeletal isomerization generally below 15% . Higher temperature operationduring the HCN experiments provided a range of desulfurization of up to 90%.
Results and Discussion
C,Alkenes. Three different reactions of the C7 alkenes were observed double bond isomerization, skeletal isomerization, and hydrogenation. Cyclization products were not observed in appreciable amounts. The individual pure component experiments revealed that each alkene had a unique product spectrum. Thus, isoalkenesand isoalkanes were generated only from isoalkenes, not from normal alkenes or cycloalkenes. The product yields produced from both the individual compounds and the mixture of the three compounds were similar quantitatively as well. Table 3 summarizes the hydrogenation and skeletal isomerization products of each alkene. The skeletal isomerization products, always relatively small, never amounted to more than 25% of those from hydrogenation. The temperature dependence of the distribution of remaining alkenes is shown in Figures 1-3 for the normal alkene, isoalkene, and cycloalkene reactants, respectively. The dashed lines in Figures 1-3 represent the equilibrium distributions calculated by minimizing the Gibbs free energy using thermodynamic data estimated from computational quantum chemical calculations using the AM1 Hamiltonian as found in the GAMESS ab initio program. Figure 1shows that at the lowest reaction temperature of 408 K, the predominant normal alkenes were the 1-and 2-heptene isomers. As the reaction temperature increased, the fraction of 1-heptenedropped below O i l and the 2-cis/ trans-heptene isomer abundance diminished due to secondary conversion to 3-cisltrans-heptene. Note that the isomers appeared t o move toward their predicted equi-
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440 460 480 500 Temperature, K Figure 1. Distribution of unconverted normal alkenes. (*) 1-heptene,( 0 )t-3-heptene,(m)c-3-heptene,( 0 )t-2-hepkne,(e) c-Pheptene, (---)equilibrium value for indicatedheptene isomer.
300
11
420
l
.-
$ 1 '5 0.4
Temperature, K Figure 2. Distribution of unconverted isoalkenes: (*) 2-m-1hexene, (H) 2-m-2-hexene,(e)5-m-l-hexene,( 0 )5-m-2-hexene, (0) 2-m-3-hexene,(---) equilibrium value for indicated m-hexene isomer.
librium values as the reaction temperature increased. Note also, as the thermodynamic calculations indicate, the trans isomers for both 2- and 3-heptene were more abundant than their respective cis isomers. Figure 2 shows that 2-m-2-hexene was the predominant iso-C, alkene isomer. This is as predicted thermodynamically, although deviations appear at the higher reaction temperatures. The other species are close to their equilibrium values as the reaction temperature approaches 477 K, although there is still a higher than predicted amount of the starting isoalkene, 2-m-l-hexene, present in the reaction mixture. The cycloalkene mixture was almost exclusively l-mcyclohexene at the low reaction temperatures, although small amounts of the 3-m and 4-m isomers appeared at the higher reaction temperatures (Figure 3). The three isomers were still approaching their predicted equilibrium fractions at 477 K. The rates of hydrogenation of the three alkenes are shown in Figure 4. At all of the reaction temperatures, these rates aligned in the order of normal alkenes (heptenes) > isoalkenes (m-hexenes) > cycloalkenes (mcyclohexenes). This is consistent with other studies where branched alkenes have already been shown to react via direct hydrogenation over sulfided catalysts more slowly than normal alkenes." The activation energies declined in the order cycloalkenes (47 kcal/mol) > isoalkenes (36 kcal/mol) > normal alkenes (29 kcal/mol).
Energy & Fuels, Vol. 8, No.1 , l W
C7 Alkene Conversion and Reactivity
E
200 -
440 460 480 500 Temperature, K Figure 3. Distribution of unconverted cycloalkenes: (u) l-mcyclohexene, ( 0 ) 3-m-cyclohexene, ( 0 )Cm-cyclohexene, (---) equilibrium value for indicated m-cyclohexene isomer.
10
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501
= 5I -
--___--
I
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4 0 ' 460 ' 480 ' 500 Temperature, K Figure 4. Rates of hydrogenation for (u) heptenes, ( 0 ) mhexenes, and (*) m-cyclohexenes. o'%AO
'
420
The order of hydrogenation rates is evidently due to steric factors. Consider again the isomeric distributions of the alkenes shown in Figures 1-3. At the lowest reaction temperature, 1-heptene (monosubstituted) and 2-heptene (disubstituted); 2-m-2-hexene (trisubstituted) and 2-m1-hexene (disubstituted); and 1-m-cyclohexene (trisubstituted) were the predominant isomers for their respective alkenes. The steric hindrance at the olefin center on these molecules declines in the order cycloalkenes > isoalkenes > normal alkenes. This is also the order for increasing hydrogenation reactivity. As the reaction temperature increased, the order of reactivity remained the same; however, the rates for the normal and isoalkenes converged. The isomeric distributions show that this was primarily due to the normal alkene rate change, since the isoalkene distribution is relatively unchanged as the temperature increased. In the n-alkene distribution, most of the 1-heptene was replaced by disubstituted 2- and 3-cisltrans-heptene. This increase in substitution for the normal alkene mixture created greater steric hindrance, and its hydrogenation reactivity was thus reduced relative to that for the isoalkenes. Heavy Catalytic Naphtha. The trends delineated above for the model compound studies were also followed (11) Frye,C.G.;Barger,B.D.;Brennen,H.M.;Coley,J.R.;Gutbarlet,
L. C. Id.Eng. Chem. R o d . Res. and Dev. 1963,2,40.
-
. .
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,
-
+
,
I
520 530 540 550 560 Temperature; K Figure 8. Reactivity of C7 alkenes in heavy catalytic naphtha: ( 0 )normal alkenes, (e)isoalkenes. 500
510
.
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8 20
n
:
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*
0
0
4
1
lo 910
520
530
540
550
560
Temperature, K
Figure6. Reactivityof alkenesin heavycatalyticnaphtha baed on carbon chain length ( 0 )CS alkenes, (e)C7 alkenes, (0)C g alkenes. during reaction of the heavy catalytic naphtha over the same catalyst under similar reaction conditions. This is demonstrated in Figure 5, where the conversion of C7 alkenes in HCN is shown for the range of 20-9096 desulfurization. At all temperatures studied, the normal alkenes were more reactive than the isoalkenes. The activation energies for hydrogenation of the normal and isoalkenes were 26 and 33 kcal/mol, respectively. These values compare very well with the activation energies determined from the model compound experiments. The kinetics of cycloalkenes conversion were unavailable because of analytical chemistry limitations involving the complexities of the HCN. However, the complexities of the HCN did allow for the development of another trend. This is shown in Figure 6 as a plot of the observed alkene conversion vs. temperature, parametric in carbon number. The trend CS > C7 > CS was observed over the entire temperature range.
Conclusions The three C7 alkenes, 1-heptene,2-m-1-hexene,and l-mcyclohexene, underwent double bond isomerization and hydrogenation when reacted a t temperatures from 408 to 477 K and a pressure of 2 MPa. Only the iso- and cycloalkenes reacted by skeletal isomerization. The rates
150 Energy & Fuels, Vol. 8, No. 1, 1994
of hydrogenation of the three alkenes aligned in the following order: normal alkenes (heptenes) > isoalkenes (m-hexenes) > cycloalkenes (m-cyclohexenes). The distributions of the unconverted alkenes generally approached predicted equilibrium values as the reaction temperature increased. The alkenes in HCN showed the same order of reactivity and similar activation energies as in the model compound studies. The reactions of the HCN also revealed
Goddard and Kukes the carbon chain length dependence of the reactivities as Cs > C7 > Cs.
Acknowledgment. The authors thank K. T. McBride, L. M. Green and M.T. Klein (University of Delaware) for helpful discussions; A. J. Novak for operating the pilot plant; and K. B. Anderson for the GC/MS work.
