HYDROISOMERIZATION OF OLEFINS C . G . F R Y E , 6. D . B A R G E R , H . M . B R E N N A N , J . A N D L . C. G U T B E R L E T
R . COLEY,
Research and Development Department, American Oil Go., Whiting, Ind.
When olefins are hydrogenated over metallic catalysts, such as nickel, the normal reaction products are paraffins of the same skeletal structure as the starting olefins. If the catalyst also possesses strong paraffin isomerization capabiliiies, the product i s a mixture of paraffins which i s limited b y the paraffin isomer equilibrium. The latter reaction i s pictured as occurring in two steps-olefin saturation followed by paraffin isomerization. It i s now possible to hydrogenate normal olefins to isoparaffins beyond the limit of paraffin equilibrium. Using sulfided nickel on silica-alumina as the catalyst, normal pentene has been hydrogenaied mainly to isopentane. A reaction mechanism i s suggested, involving carbonium ion intermediates, in which the tertiary carbonium ion i s selectively hydrogenated. These results demonstrate the wide scope of reaction included in the field of catalysis. When the free energy difference i s favorable, the implementation of a reaction-no matter how complex-depends only on developing the proper catalyst. of olefins usually leads to paraffiis of the same skeletal structure (5). However, shifts in structure can result from isomerization either of the olefin before hydrogenation or of the paraffin after hydrogenation ( 7 , 9 ) . I n these cases. where hydrogenation rate of the olefin isomers are about the same, the final compositim is limited by the equilibrium distribution of the olefins or of the paraffins. Paraffin isomerization (7) in the presence of hydrogen has frequently been referred to as hydroisomerization although no net h)drogen consumption occurs. I n this article, the term hydroisomerization of olefins is used to distinguish a new reaction in which olefins are hydrogenated primarily to isoparaffins. This type of reaction is discussed in recent patent literature ( 7 0 ) . The ratio of iso- to n-paraffins in the product is in excess of the thermodynamic iso-to-normal equilibrium ratios of either the olefins, or the paraffins. The reaction requires a catalyst with both acidity and hydrogenation acti\ity. The acidity and hydrogenation activity have been studied separately. To gain insight into the mechanism of reaction. various olefins and paraffins have been subjected to hydroisomerization conditions.
taining 570 nickel. The catalysts \yere dried for 2 hours a t 400' F. and then calcined for 6 hours at 1000' F. Both catalysts Lvere pretreated a t atmospheric pressure and 750' F. first Xvith floxving hydrogen and then for 1 hour with a mixture of 8% hydrogen sulfide in hydrogen. The amount of hydrogen sulfide used \vas a t least twice the stoichiometric quantity needed to convert nickel to nickel sulfide. Sext the catalyst )vas cooled to reaction temperature in the presence of the hydrogen sulfide-hydrogen mixture. The system was pressured with hydrogen (100 to 1000 p.s.i.) and the flow of hydrogen and liquid feed started simultaneously. Liquid and gaseous products were separated in a dry iceacetone cooled condenser. The liquid \vas weighed and analyzed by gas chromatography. The gaseous product was metered through a \vet test meter and analyzed by mass spectrometry. Material balances. including hydrogen consumption, ranged from 95 to lOjYc. based on hydrocarbon feed. While the major part of the product consisted of Ca to C, hydrocarbons, a trace of methane (less than 0.1 wt. %) was obtained and material above Ci. particularly C I Ohydrocarbon, was detected but not analyzed quantitatively.
Experimental
Process Studies
YDROGEKATION
The folloi\ing hydrocarbons from Phillips Petroleum Corp. were used : 1- and 2-Pentene, technical grade 2-Methyl-2-butene, pure grade Isopentane, pure grade 3-Methylpentane, pure grade Methylcyclopentane, technical grade (repurified to 99.8% purity) Except Lvhere the effect of sulfur concentration was being investigated, carbon disulfide was added to these reagents to about 0.8 wt. 70sulfur level. Electrolytic hydrogen was passed over palladium on charcoal and dried over Drierite before use. Experiments were performed in a flow s>-stemusing 3 to 6 ml. of catalyst. The reactor was 3/,-inch i.d. and 14 inches long Xvith a '/,-inch 0.d. axial thermocouple well. Catalysts were prepared by impregnating either Nalco HA silica-alumina cracking catalyst or Davison Grade 62 silica gel \vith aqueous nickel acetate to give a final product con40
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
The effect of temperature and pressure on the ratio iso- to npentane from the hydroisomerization of n-pentene is shown in Figure 1. In these experiments, the catalyst was sulfided nickel on silica-alumina. Results obtained from 1-pentene were not measurably different from those obtained from 2pentene, and both are included. For comparison, equilibrium ratios of isopentenes to n-pentenes and of iso- to npentanes are also given. At all conditions. the ratio of isoto n-pentane is greater than the equilibrium ratio of the olefins or paraffins. At 1000 p.s.i., the observed ratio is about 10, or t\vo and one-half times the equilibrium ratio of the paraffins. The observed ratio increases markedly with decreased pressure to about 25 at 100 p.s.i. At all pressures, it decreases slightly \vith increasing temperature. The change with temperature is in direct proportion to the change in the equilibrium ratio of iso- to n-olefins and paraffins. Thus, for any given pressure. the ratio (iso- t o n-parafin)obs./(iso- to n-paraffin)eq. = constant
\Vhen hydrogen is in excess: the effect of hydrogen rate on product distribution is relatively small a t all conditions investigated. For example, at 615' F.: 1000 p.s.i.. and 5 LHSV, the ratio iso- to n-pentane increased from 7.5 to 10 when hydrogen was increased from 2.0 to 8.5 moles per mole of olefin. Conversion of olefins \vas 10070 in both cases. Effect of Sulfur
T o obtain a high ratio of iso- to n-pentane, the nickel hydrogenation component of the catalyst must be maintained in a sulfided state. Sulfiding can be accomplished by addition of a compound, such as carbon disulfide, which is readily converted to hydrogen sulfide under reaction conditions. After about 3 hours on stream, the results of hydroisomerizing 1pentene containing varying amounts of carbon disulfide a t 1000 p.s.i,, 600' F.: and 6 moles of hydrogen per mole of olefin are : Sulfur, \vt. yo Iso- to n-pentane
0 1.1
0.01 2.7
0.1
11.1
1.o 10.6
For comparison, the iso-to-normal equilibrium ratio is 5.4 for the olefins and 3.2 for the paraffins. The critical dependence of the iso- to n-pentane ratio on sulfur when the sulfur content of the feed is increased from 0.01 to 0.1 wt. yo suggests a change in catalyst composition. The increase in the iso-to-normal ratio is believed to occur when reduced nickel is converted to nickel sulfide. This interpretation is supported by the published equilibria for the nickel-nickel sulfide system (2) which show that, under the conditions of the experiment, when sulfur is increased above about 0.017G, reduced nickel is converted to Xi&. Thus, Xi3& may be the preferred hydrogenation component. The low ratio of iso- to n-pentane obtained when the feed is sulfurfree shows that the catalyst, although initially sulfided, must be continually exposed to a sulfiding atmosphere to prevent reduction of the preferred catalytic species. Effect of Catalyst Acidity
The effect of the acidity of the support on product distribution has been determined by comparing results from a sulfided-nickel on silica catalyst, a nonacidic support ( 6 ) , with results from a sulfided-nickel on silica-alumina catalyst (Table I ) . \\'ith the silica support. double-bond shift. hydrogenation, and disproportionation (products of higher and lower carbon number than the starting olefin) are all evident, but there is no evidence of structural isomerization. Over the silica-alumina supported catalyst, hydrogenation reactions are about 10 times faster. The unconverted 1-pentene is a t thermodynamic equilibrium with 2-pentene. Similarly, as a group. the branched olefins are also a t equilibrium. However, equilibrium between the branched and unbranched olefins is not attained. The rate constant for the disproportionation reaction is many times more rapid with the silicaalumina support. The structural isomerization to iso-olefins and the increased rate of disproportionation are characteristics of increased catalyst acidity ( 4 , 8 ) .
products. O n the silica-alumina catalyst the reactivities of these olefins are quite different, as shown in Table 111. The ratio iso-to-n-pentane is 50 from 2-methyl-2-butene whereas the ratio is only 7 from 1-pentene. The first order rate constants for hydrogenation are 52 for the iso-olefin as compared with 27 for the n-olefin. The possibility that disproportionation involves mainly branched structures is suggested from the five times greater rate constant for 2-methyl-2-butene than for I-pentene. Reactivity of Paraffins and Naphthenes
Paraffins and naphthenes are relatively unreactive over sulfided nickel on silica-alumina under the conditions used for hydroisomerizing olefins, as shown in Table IV. Isopentane isomerizes only slightly ; 3-methylpentane is somelvhat more isomerized ; methylcyclopentane undergoes some ring opening and cracking. Consequently, isomerization of paraffins is negligible under hydroisomerization conditions and need not be considered in a mechanism of olefin hydroisomerization.
Table 1.
Effect of Catalyst Support on Olefin Hydroisomerization (520' F., 1000 P A . , 10 V,/hr./V,)
Sulfided 'VVtcXel On SilzcaOn Silzca Alumina Feed 50% I-Pentene 50 7" 2-Pentene I-P~ntpne M't. %
Products
...
Isopentane n-Pentane 3-Methyl-1-butene 2-Methyl-1-butene 2-Methyl-2-butene 1-Pentene 2-Pentene
.,. ...
...
12.8 80.2
+ C6 + c; cz
Rate constant (mmolrs/hr. g. catalyst) Hydrogenation Disproportionation
I
27.0
0.3
4.2 10.4
7.7 0.57
69.0 23.0
0.2
c 4
N
37.6 4.0 0.8 3.1 8.5 4.4
6.5
20
I-
I
2 5 0 PSI
u"
. 15 C
N
i U
10
-
.-I Olefin Reactivity 5 -
The relative reactivities of n-pentene and 2-methyl-2-butene were determined on nonacidic and acidic catalysts. The results with sulfided nickel on silica are shown in Table 11. Hydrogenation of 2-methyl-2-butene is slower on this catalyst than the hydrogenation of n-pentene ; however, the branched olefin is converted much more extensively to disproportionation
OL
1
I
400
500
600
TEMPERATURE,
Figure 1 .
I
O F .
Hydroisornerization of n-pentenes VOL. 2
NO. 1
MARCH 1963
41
Reaction Mechanism Table II.
Reactivity of Olefins with Sulfided Nickel on Silica (520’ F., 1000 p s i , 10 V,/hr./V,)
Feed
50%
7-Pentene 50%
2-Pentene W’t.
Products
lso-Pentane n-Pentane 3-Methyl-1-butene 2-Methyl-1-butene 2-Methyl-2-butene 1-Pentme 2-Pentene
yo
...
4.3
6.5
... 2.1 21 . o
... ... ...
69.2
12.8 80.2 0.2 0.3
c, + C4
+
c6 C7 Rate constant (rhmoles/hr. g. catalyst) Hydrogenation Disproportionation Table 111.
2-Methyl2-butene
. .
... 1.1 2.3
5 .O 4.0
7.7 0.57
Reactivity of Olefins with Sulfided Nickel on Silica-A I um i na (500’ F., 1000 psi., 10 V,/hr./V,)
Feed 2-Methyl7-Pentene 2-bufene w t . 70
Products
Isopentane n-Pentane Isopentene n-Pentene CB C4
C6
++
c 7
Rate constant (mmoles/hr. g. catalyst) Hydrogenation Disproportionation Table IV.
15.0 3.9 14.2 60 4 2.2 4.3
29.2 0.6 35.6 6 2 10 0 18.4
27.0
52.0 49.0
9.0
Reactivity of Paraffin and Naphthene with Sulfided Nickel on Silica-Alumina
(600’ F., 500 p.s.i., 1.0 Vo/hr./Ve) 3-MdhylMethylcycloIsopentane fentane pentane Feed Product Feed Product Feed Product rlnalysis. Wt. To
ISO-C~HIO Trace 0.4 Iso-C~H~~ 100.0 9 9 . 8 3.2 0.4 n-CsH,? 0.2 Trace 2-Methylpentane 0.5 4.3 0.6 3-Methylpentane 99.3 92.0 0.3 n-CaH11 0.2 0.5 0.2 0.2 Methylcyclopen tane 99.8 95.8 Cyclohexane 1.7 Rate constant (mmoles/hr. g. catalyst) Isomerization 0.4 cracking 0.03 0.7
+
T h e above observations suggest the mechanism for the hydroisomerization of olefins shown in Figure 2. T h e reaction of an olefin with hydrogen may follow either of two reaction paths. Direct hydrogenation ( k l and kl’) may occur, giving a paraffin of the same skeletal structure as the starting olefin. Regardless of support, this reaction is fast when reduced metal catalystssuch as nickel-arc used. Although nickel sulfide is much less active than nickel on a n inert support such as silica, it also preferentially catalyzes direct hydrogenation. T h e relative reactivity of olefins for direct hydrogenation also depends on structure of the reacting molecule. Branched olefins-such as 2-methyl-2-butene-react more sloivly than olefins in which the double bond is unhindered; Thus, a n isomerization of n-olefin to iso-olefin followed by direct hydrogenation will not yield iso-paraffins preferentially. T h e alternate reaction path involves the formation of a carbonium ion on the catalyst surface ( k and ~ k e ’ ) . This is favored by the use of a n acidic catalyst-such as silica-alumina -and a weak hydrogenation component. T h e carbonium ion then undergoes isomerization (k3) to give a mixture of secondary and tertiary carbonium ions. At equilibrium, the concentration of the tertiary carbonium ion is favored and far exceeds that of the secondary carbonium ion ( 3 ) . Reaction between the carbonium ion and a hydride ion (k4 and kd’) leads to formation of the paraffin. Because the adsorbed tertiary carbonium ion is favored thermodynamically, isoparaffin is preferentially formed from either straight- or branched-chain olefins when this reaction path is followed. With both silica and silica-alumina as the support, a hydrogenation component which is too active (nickel instead of sulfided nickel) lowers the ratio of iso- to n-paraffins because of increased reaction via the direct hydrogenation route ( 7 ) . However, with sulfided nickel on silica-alumina, the carbonium ion route is the predominant reaction and is not limited by either olefin or paraffin equilibrium but appears to depend on the carbonium ion equilibrium and the relative reactivities of the carbonium ions involved. T h e experimental results reported can be obtained if the ratio of tertiary carbonium ions to secondary carbonium ions on the catalyst surface is high and the rate of hydride ion transfer to the two species is about the same, or if the rate of hydride ion transfer to the tertiary carbonium ion is exceptionally high, or by a n appropriate combination of these two factors. T h e hydride ion and also the protons required for carbonium ion formation, must come ultimately from molecular hydrogen. Either the secondary or the tertiary carbonium ion may react with a molecule of feed olefin to yield a dimer: which subsequently hydrocracks to give what appear to be disproportionation products. literature Cited
c-c=c-c
k3
=
bl!
c-d-c -C
+
C-CFC-C
C-GC-C-C
Figure 2. 42
Mechanism of olefin hydroisomerization
l & E C PR0DU:CT
RESEARCH A N D DEVELOPMENT
(1) Ciapetta. F. G., Ind. Enq. Chem. 45; 164 (1953). (2) Emmett, P. H., “Catalysis,” Vol. 5 , p. 451, Reinhold, New York, 1957. (3) Evans, A. G., Trans. Faraday Soc. 42, 719 (1946). (4) Evering, B. L.: D‘OuvilIe, E. L., Lien, A. P.: \Vaugh, R. C., Ind. En?. Chem. 45, 582 (1953). (5) Gaschke, M., Spector, M., Heineman. H.: Paper 46, Second International Congress on Catalysis, Paris, July 1960. (6) Johnson, O., J . Phys. Chem. 59, 827 (1955). (7) Pines, H., Benov, G., J . A m . Chem. Soc. 82, 2483 (1960). Zbid.,82, 2471 (1960). (8) Pines, H., Haag, \V. 0.. (9) Pines, H., Marechal, J.. Postl, W. S.,Ibid., 77, 6390 (1955). (10) Shell Internationale Research Maatschappij N. V., Brit. Patent 878,035 (July 11, 1960). RECEIVED for review October 10, 1962 ACCEPTED January 11, 1963 Division of Petroleum Chemistry, 142nd Meeting, .4CS, Atlantic City, N. J., September 1962.
