Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 550-558
550
CATALYST SECTION Isomerization of Long-chain n -Alkanes on a Pt/CaY Zeolite Catalyst Jens Weitkamp Engler-Bunte-Instltute, Division of Federal Republic of Germany
Gas, Oil, and Coal, University of Karlsruhe, Richard- Wilstiitter-Alee 5, ~7-7500 Karlsruhe
1,
Isomerization of heavier petroleum fractions is of interest for pour point lowering of jet fuel, diesel fuel, and lubricants. I n this study the pure n-alkanes with 6 to 15 carbon atoms were isomerized on a Pt/CaY zeolite catalyst in the presence of hydrogen. A detailed analysis of the i-alkanes formed was achieved by capillary GLC. Up to ca. 40 % conversion isomerizationis virtually free from hydrocracking, but at elevated conversion severe hydrocracking occurs. At low conversion monobranched i-alkanes, Le., methyl, ethyl, propyl, and butyl isomers, are formed exclusively. Dimethyl isomers are formed in consecutive reactions. The rate of formation of 2-methyl isomers is surprisingly low. This effect is shown to be consistent with a branching mechanism via protonated cyclopropanes while it cannot be explained by a classical mechanism via alkyl and hydride shifts.
Introduction Isomerization of light paraffins has been practiced extensively in the petroleum refining industry. Objectives are either the manufacture of isobutane from n-butane or the enhancement of octane rating of the C5 and/or C6 gasoline fraction. Isomerization of n-paraffins in heavier petroleum fractions such as jet fuel, diesel fuel, or lubricants is of interest for improvement of low-temperature properties such as pour point or cloud point (Breimer et al., 1957; Gibson et al., 1960; Orkin, 1969; Schenk et al., 1956). So far, no processes are known for isomerization of jet fuel or diesel fuel, but skeletal isomerization is probably one of the major reactions in severe lube oil hydrotreatment (Bull and Marmin, 1980). The unfavorable influence of n-alkanes on low-temperature properties is easily understood from their relatively high melting points (Figure 1). In the carbon number range of, e.g., jet fuel a single methyl branching results in a melting point depression of 40 to 70 O C if branching occurs in the 3- through 7-position. It is also seen from Figure 1 that branching in the 2-position is considerably less effective. Based on a vast amount of experimental work with C4 to C6 alkanes various mechanisms of isomerization have been advanced. In a simplified manner they can be classified into isomerization over metals, especially platinum (Anderson, 1973; Gault, 1981), bimolecular mechanisms providing dimerization followed by rearrangements and cleavage (Bolton and Lanewala, 1970; Karabatsos et al., 19611, and monomolecular mechanisms providing the formation of alkylcarbenium ions, skeletal rearrangements of the latter, and formation of product alkanes from rearranged carbenium ions. A t least three modes of interconversion of alkanes and alkylcarbenium ions can be discerned. (i) Protonation of a C-H bond in the alkane gives a nonclassical carbonium ion which decomposes into molecular hydrogen and a carbenium ion. (ii) Hydride abstraction from the alkane can also be effected by a Lewis acid, e.g., a carbenium ion. 0196-4321/82/1221-0550$01.25/0
(iii) In the presence of a dehydrogenation/hydrogenation catalyst the alkane can form alkenes which are readily protonated to give alkylcarbenium ions. Recently, there has been considerable debate as to which of the above modes is operative in noble metal loaded zeolites (Chick et al., 1977; Kouwenhoven, 1973). A major difficulty in isomerization of alkanes with more than six carbon atoms is their pronounced tendency to cleave (Ciapetta and Hunter, 1953; Weitkamp, 1975). Acidic catalysts lacking a hydrogenation component, e.g., Si02-A1203(Plank et al., 1957) or REHX zeolite (Nace, 1969) as well as bifunctional catalysts with a weak hydrogenation component, e.g., sulfided Ni/Si02-A1,03 (Flinn et al., 1960) or WSz/Si02-A1,03 (Archibald et al., 1960) fail to isomerize long-chain n-alkanes to any significant extent. On the other hand, high selectivities for isomerization of, e.g., n-decane, n-dodecane, or n-hexadecane could be achieved up to medium degrees of conversion with several platinum containing bifunctional catalysts. Examples are Pt/Si02-A1203 (Coonradt and Garwood, 1964), Pt/CaY (Schulz and Weitkamp, 1972), and Pt/ultrastable Y zeolite (Jacobs et al., 1980; Steijns et al., 1981). According to Coonradt and Garwood (1964), these selectivity changes are due to the relative strength of hydrogenation/dehydrogenation activity and acidity of the catalyst. If the dehydrogenation activity is high, then the steady-state concentrations of n-alkenes are relatively high, i.e., close to equilibrium. The long-chain n-alkenes are claimed to displace rearranged carbenium ions from the acid sites by competitive sorption/desorption thus preventing &scission of branched carbenium ions. Very little is known on the details of product distributions in isomerization of long-chain alkanes. Iijima et al. (1970) reported on isomerization of heptanes and octanes over Pd/Si02-A120,. Schulz and Weitkamp (1972) observed a low rate of formation of 2-methylundecane when n-dodecane was isomerized on Pt/CaY. However, no straightforward interpretation of the effect could be given. In a preliminary note (Weitkamp, 1981), it was outlined 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev.,Vol. 21, No. 4, 1982 551 $ -54
-
-30
-26
-IO
-5
.6
.lo
1L
15
100
I
I
I
I
220
240
t
c
a
-801
200
1
9
10
11
12
13
C A R B O N NUMBER
Figure 1. Differences of melting points of methyl isomers CmHzm+2 and corresponding n-alkane.
n -hexane n-hep tane n-octane n -nonane n-decane n -undecane n-dodecane n-tridecane n-tetradecane n-pentadecane
purity, w t % 99.98 99.75 99.81 (2-M-Hp 0.02%; 3-M-Hp 0.02%; ~-M-HD 0.005%) 99.32 99.87 99.41 99.58 (3-M-Un 0.27%; 5-M-Un 0.05%) 99.54 (2-M-Do 0.03%; 3-M-DO 0.01%: 4-M-DO 0.01%) 99.51 98.61 (2-M-Te 0.18%; 3-M-Te 0.15%)
that low rates of formation of 2-methyl isomers are consistent with a branching mechanism via protonated cyclopropanes. The present paper gives systematic product distributions of n-alkane isomerization including all members of the homologous series from n-hexane through n-pentadecane. These data shed new light on the branching mechanism via carbocations. Experimental Section Experiments were conducted in a small-scale pressure apparatus with a saturator in which flowing hydrogen was loaded with vapors of the feed hydrocarbon, and a fixed bed reactor. Details are given elsewhere (Pichler et al., 1972). The Pt content of the Pt/CaY zeolite, its %/A1 molar ratio, total content of Ca2+and Na+ cations per Al, and Ca2+/(Ca2++ Na+) exchange level amounted to 0.5 w t %, 2.50,0.97 equivjmol, and 86%, respectively, according to the manufacturer (Union Carbide Corp.). Presumably, the catalyst contained a binder. A particle size ranging from 0.2 to 0.3 mm was applied. The following pretreatment procedure was chosen: heating in N2 (5 dm3/h) at 100 kPa up to 400 "C; cooling in the same purge of N2 to below 50 OC; heating in H2 (5 dm3/h) at 100 kPa up to 400 OC. Typically, the mass of dry catalyst was 1.00 g. Purity of hydrogen was at least 99.993% by volume. Purities of the liquid feed hydrocarbons as determined by capillary GLC are listed in Table I. Unspecified impurities were other n-alkanes, alkenes, or naphthenes which did not interfere in the chromatograms with the i-alkanes formed. n-Octane, n-dodecane, n-tridecane, and n-pentadecane contained some small amounts of their respective i-alkanes; cf. Table I. In these cases measured product distributions were corrected assuming no conversion of the impurities. However, no corrections were made for the somewhat time-dependent concentrations of the impurities in the gaseous effluent from the saturator. Temperature-programmed capillary GLC with a flame ionization detector was used for product analysis. Length and internal diameter of the stainless steel columns were
280
Figure 2. Isomerization and hydrocracking conversion of n-+ridecane (Pn.c1gH, = 14 P a ; PH2 = 3.9 MPa; W/Fn.cla~% = 775 g h mol-').
Table I. Punty of Feed Hydrocarbons feed
260
R E A C T I O N T E M P E R A T U R E , 'C
m
>i too
99 2-M-Do 3-M-DO 4-M-Do 5-M-Do 6-M-Do 3-E-Un 4-E-Un 5-E-Un 6-E-Un 4-P-De 5-P-De 5-B-NO
7.0 15.1 18.4 22.8 23.4 1.9 3.4 3.6 2.0 0.7 1.4 0.3
2-M-De 3-M-De 4-M-De 5-M-De 3-E-NO 4-E-NO 5-E-NO 4-P-OC
11.6 22.3 25.9 30.0 2.5 4.5 2.1 1.1
n-C14H30 2 00 14 3.9 460 2.9 > 99 2-M-Tr 5 . 8 3-M-Tr 12.1 4-M-Tr 15.5 5-M-Tr 20.4 6-N1-Tr 7-M-Tr} 31.0c 3-E-DO 1.5 4-E-DO 3.2 5-E-DO 3.7 6-E-Do 3.6 4-P-Un 0.7 5-P-Un 1 . 5 6-P-Un 0.6 5-B-De 0.4
2-M-Un 3-M-Un 4-M-Un 5-M-Un 6-M-Un 3-E-De 4-E-De 5-E-De 4-P-NO 5-P-NO
9.4 17.8 22.2 24.7 13.3 2.9 4.0 4.2 0.8 0.7
n-C15H32
190 7 3.9 84 0 2.2 > 99 2-M-Te 3-M-Te 4-M-Te 5-M-Te 6-M-Te 7-M-Te 3-E-Tr 4-E-Tr 5-E-Tr 6-E-Tr 7-E-Tr 4-P-DO 5-P-DO 6-P-Do 5-B-Un 6-B-Un
1
6.4 12.0 13.5 16.9 17.2 17.4 1.5 3.1 3.3 4.7d 0.8 1.6 1.6 -e -e
z ' - C ~ & , + ~+ a Smb: Selectivity of monobranched i-alkanes, defined as monobranched i-C,H,,+,/(monobranched multiply branched i-CmHZm+,), Separation of 3-M-Hp and 3-E-Hx incomplete, Estimated percentages are 49.0% 3-M-Hp and 1.4% 3-E-Hx. 6-M-Tr and 7-M-Tr could n o t be separated by GLC. By analogy with the other carbon numbers it is assumed that there is twice as much 6-M-Tr as 7-M-Tr. Separation of 6-E-Tr and 7-E-Tr incomplete. Estimated percentages are 3.1% 6-E-Tr and 1.6% 7-E-Tr. e Presumably, 5-B-Un and 6-B-Un were present in a total amount of > ethyl > propyl > butyl isomers.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Table 111. Isomerization of n-C,,H, through n-CI5H3,. Distribution of Methyl Isomers in mol % a t High Degrees of Conversion feed
T, "C P n C ,H
,m+
* kPa
P H I , MPa W/FnC,H,,,,+Z~ g
XnC,H,,+,~
overall, %
XnCmHzm+2,iso., % SM,n %
n-C12H26 260 16 3.9 810 99.1 3.1 30
n-C 13H28
2-M-Un 20.8 3-M-Un 22.9 4-M-bn 19.1 5-M-Un 23.6 6-M-Un 13.6
2-M-Do 18.9 3 - M - b 20.6 4-M-DO 17.4 5-M-Do 22.1 6-M-Do 21.0
260 14 3.9 7 45 99.5 4.5 29
0
5 2 LL
2
2
E
+ -
0
20
40
60
BO
100
X ~ - C , , H ~ , , , O v e r a l l , '1-
Figure 4. Distribution of the methyldecanes formed in isomerization of n-undecane (see legend of Figure 3 for reaction conditions).
Relative rates of formation of individual methyl isomers from a given n-alkane are particularly interesting, especially from C9Hzoonward. The following general results are disclosed. (i) The 2-methyl isomers are formed at a surprisingly low rate. E.g., the rate of formation of 2methyldecane from n-undecane is one-half compared to that of 3-methyldecane. (ii) Rates of formation increase slightly in the order 3-methyl < 4-methyl < 5-methyl etc. (iii) A low rate of formation is again encountered for the isomer with the methyl branching in the center position of the main chain if the carbon number of the latter (m - 1) is odd, i.e., if the overall carbon number m is even. Examples are the formation of 5-methylnonane from ndecane and 6-methylundecanefrom n-dodecane. Note that in these cases center branching is approximately half as fast as expected according to rule no. ii. Statistically, effect no. iii is a trivial one since the center position in a main chain with an odd carbon number exists only once while any other position exists twice. Table I1 reveals similar effects for the ethyl isomers from CllHZ4upward as well as for the propyl isomers in the range C14H30 and C15H32. The principal influence of the overall conversion on the distribution of the methyl isomers is shown in Figure 4. As in Figure 3, CllH24 was chosen as an example. It is evident that no. i and ii of the above-described effects are weakened and eventually cease to exist as the overall conversion is raised. Near 100% conversion (where most of the undecane is hydrocracked, cf. Figure 2) the concentrations of the methyldecanes approximatelyequal each other with a slight preference of 3-methyldecane. Table I11 gives the distributions of the methyl isomers formed from n-dodecane through n-pentadecane at high degrees of conversion. 6-Methylundecane and most probably 7-methyltridecane occur in concentrations
2-M-Tr 17.6 3-M-Tr 19.2 4-M-Tr 16.5 5-M-Tr 18.6
2-M-Te 14.3 3-M-Te 17.3 4-M-Te 15.4 5-M-Te 18.4 6-M-Te 16.9 7-M-Te 17.7
i-CmHlm+2. N o separation of 6-M-Tr and
3
2-METHVLDECANE
n -C ISH 32 240 7 3.9 1520 95.1 23.1 28
1 28.1b
Selectivity of methyl isomers, defined as methyl isomers C,H,,+,/total 7-M-Tr by GLC.
0
n-C,,H,, 250 14 3.9 675 92.4 35.4 30
zot
lol
A
3-ETHYLNONANE 4-ETHYLNONANE 5-ETHYLNONANE
i
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554
Ind. Eng. Chem. Prod. Res. Dev., Vo:. 21, No. 4, 1982
Table IV. Isomerization of n-C,H,, through n-C9H2,. Product Distributions in mol %
-
_ . I _ . _ _ _ _
feed
-
T , "C Pn-CmH 21n+9
kP a
XitC,H,,+,>
overall iso., "z
P H ? MPa 14'/Ffl-C,flE12m+2, g h mol-'
-YnCJ?,H
2 n7+,
n-C "H ,h 300
__ - -___--.-___-___
230 3.7 83 72.0 68.2
n-C9Hzo 275 2 30 3.7 138 76.4 67.1
I
2-M-Hx 33.2 3-M-Hx 36.8 3-E-Pn 3.0 2,2-DM-Pn 6.5 2,3-DM-Pn 9.0 2,4-DM-Pn 6.3 3,3-DM-Pn 4.5 2,2,3-TM-Bu 0 . i
N o t resolved.
n-C *H19 27 5 230 3.7 292 82.6 67.0
Both diastereomers.
2-M-Hp 3-M-Hp/. a 3-E-Hx 4-M-Hp 2,2 -DM-Hx 2 , 3-DM-Hx 2,4-DM-Hx 2 ,5-DM-Hx 3,3-DM-Hx 3,4-DM-Hx 3-E-2-M-Pn 2.2,3 -TM-Pn 2,2,4-TM-Pn 2,3,3-TM -Pn 2,3,4-TM-Pn
23.8 31.3
10.5 5.4 5.1 9.9 6.9 3.9 2.3 0.7 0.1
