Ind. Eng. Chem. Res. 1993,32,403-408
403
Acid-Catalyzed Cracking of Paraffinic Hydrocarbons. 3. Evidence for the Protonated Cyclopropane Mechanism from Hydrocracking/ Hydroisomerization Experiments S. Tiong Sie KoninklijkeIShell-Laboratorium,Amsterdam (Shell Research B. V.),P.O. Box 3003, 1003 A A Amsterdam, The Netherlands
The mechanism for acid-catalyzed cracking proposed in the first paper of this series, which assumes a protonated dialkylcyclopropane carbonium ion as intermediate instead of a classical carbenium ion, has been applied to rationalize experimental results obtained in the hydrocracking/ hydroisomerization of normal paraffins. The new mechanism is shown to be capable of explaining many characteristics of the hydrocracking process which cannot be understood via the classical theory, such as the virtual absence of methane and ethane as cracked products, the relatively low production of propane as compared to the higher hydrocarbons, and characteristic patterns in the branching of the cracked fragments. The new mechanism also makes it clear why crackability increases so rapidly with increasing number of carbon atoms above n-heptane and why n-hexane and lower hydrocarbons are so difficult to crack. An explanation is also offered for the ease with as contrasted with the difficulty which high selectivities can be achieved in the isomerization of CFJCS, of isomerizing C7+ without significant cracking.
Introduction In the first paper of this series (Sie, 1992)we discussed a number of problems associated with the established theory of acid-catalyzed cracking, which incited us to formulate a new mechanism. In the new mechanism a nonclassical carbonium ion, viz., a protonated dialkylcyclopropane,is assumed to be a reaction intermediate,which following hydride shifts and relocation of electrons fragmenta into a tertiary carbenium ion (yielding a branched paraffin upon hydride transfer) and an olefin as cracked producta. This protonated cyclopropane intermediate takes the place of the classical carbenium ion which undergoes @-scissionin the established theory (Sie, 1992). In the second paper of this series it was shown that the new mechanism is capable of explaining many features of the catalytic cracking process which up to now have not been or have only poorly been understood (Sie, 1993). In the present paper the resulta of hydrocracking and hydroisomerization experiments will be analyzed. The proposed cracking mechanism is rather similar to that of skeletal isomerization of paraffins in that both assume the intermediacy of a protonated cyclopropane species. It is therefore postulated that hydroisomerization and hydrocrackingdo not onlytake place as consecutivereactions (cracking following isomerization of the feed paraffin or isomerization of products formed by cracking), but may also occur as parallel reactions which share a common intermediate, as is shown in the simplified Scheme I for the hydroisomerization and hydrocracking of a normal paraffin. An important aspect of the reaction mechanism shown in Scheme I is that the value of n should be at least 1. A value of 0 for n implies that the initial classical carbenium ion to be formed from the parent paraffin molecule would be a primary one, which is very unlikely on energetic grounds. For skeletal isomerization the value of m should likewise be 1or greater, as discussed in the first paper of this series. To make cracking according to the proposed mechanism possible, the value of m should be at least 3. If m is 2, the carbon atom of the alkyl group in 8-position to the cyclopropane ring will be a primary carbon atom, and it is very unlikely on energetic grounds that such a carbon atom can release the hydride ion to be shifted
Scheme I. Simplified Reaction Scheme for Hydroieomerization and Hydrocracking of a Normal Paraffin by the Protonated Cyclbpropane Mechanism
1
n2i
/ H I H H H fH: H-IC -H : H I n @ H H :HI,
I-c-c-c-/c/
CLASSICAL CARBENIUM ION
I H
‘H
[Hj H H [HI H-/C I-C-C-/C -H ;HIn I H :HI, ~
\
/C\ H I H H ISOMERIZED PRODUCT
/HI H H H-/C/-C-C-H ;HIn I H
+
H H [HI H-C-C-1CI-H H H [Hjm.2
/C\
H I H H
CRACKEDPRODUCTS
toward the cyclopropane ring carbon, as required for cracking (cf. part 1 of this series of papers).
Hydrocracking of Normal Paraffins Effect of Chain Length on Reactivity. The strong effect of chain length on the reactivity of paraffins under mild hydrocracking conditions is a feature which cannot be explained by the classical cracking theory. Nevertheless, this effect is of importance, for one thing because it forms the basis of the selectiveproduction of hydrocarbons in the middle-distillate range by the hydrocracking of heavy paraffins produced from synthesis gas, thus constituting an efficient route from synthesis gas to middle distillates (Sie et al., 1988).
Q888-5085/93/ 2632-Q403$Q4.OOIQ 0 1993 American Chemical Society
404 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993
I
Ln 2.1r 1 - x
0
0
1.8
1.5 1.2
0.9
0.6
Table I. Influence of Chain Length on the Reactivity of Normal Paraffins in Hydroisomerization/HydrocrecLing (Experimental Data from Weitkamp (1975). Catalyst, Pt/Ca/Y; T = 275 OC. Rates in lo-* mol of Paraffin per gram of Catalyst per hour) pred contribution" hydropred calcd obsd hydroconv selectivity for conv isomerization cracking rate isomerization, % feed rate 0.13 100 n-Cs 0.13 0.13 0 0.38 53 0.20 0.18 n-CT 0.38 0.62 42 0.26 0.36 n-Ca 0.53 38 0.54 0.87 n-C9 1.1 0.33 0.46 0.90 1.36 34 n-CI1 1.3 0 Italicized figures are taken aa basis for prediction of the rates for other hydrocarbons.
0.3
0
0.2
0.4
0.6
0.8 1.0 TIME, h
Figure 1. Formation of cracked products in the hydrocracking of n-heptane, n-octane, and n-nonane represented by a first-order reaction plot (Hoffmann et al., 1987).
According to the classical theory of acid-catalyzed cracking, the crackability of normal paraffins of different chain lengths should be about proportional to the number of secondarycarbon atoms, i.e., the total number of carbon atoms minus 2. This means that except for the lowest paraffins the crackability should not be too different from one that is proportional to molecular weight. In actual fact, however, it has been found that the depth of hydrocracking under identical conditions increases very sharply in the range of carbon numbers from 6 to 10 (Archibald et al., 1960). In other studies which compared the hydrocrackingof n-Ca and n-Cl6, also alarge difference in reactivity was observed in that conditions which caused the cetane to be almost completely cracked led to only 50% conversion of the octane (Flinn et al., 1960). The protonated cyclopropane mechanism predicts a much steeper increase of the reactivity for hydrocracking with increasing carbon number than the classical theory. If the simplifying assumption is made that there is no effect of the size of the alkyl groups of the protonated dialkylcyclopropane intermediate on the chances of its being formed and its probability to crack, so long as the condition is satisfied that the values of n and m in Scheme I are at least 1 and 3, respectively, it can easily be deduced that the relative molar reactivities should be proportional to the total carbon number minus 6. This dependence of crackability on carbon number, which has already been discussed in part 2 of this series of papers dealing with catalytic cracking,explains the sharp increase of the depth of hydrocracking with carbon number in the experiments mentioned above. Figure 1 is a first-order plot of the crackingof n-heptane, n-octane, and n-nonane (Hoffmann et al., 1987). It can be inferred that the reactivities relative to n-heptane are 2.0 for n-octane and 2.3 for n-nonane. The expected relative reactivitiesaccording to the proposed mechanism are 2 and 3 for n-octane and n-nonane, respectively, and the order of magnitude is therefore correctly predicted. In the above prediction of the relative reactivities preisomerization of the feed paraffin is not taken into account, and the cracking rates are thus supposed to be attributable to hydrocracking of normal paraffins only. This is obviously too simple a representation of the actual situation.
If one considers not the rate of cracking in terms of the formation of cracked fragments, but the conversion of the feed paraffin in terms of its rate of disappearance irrespective of the reaction route, a reactivity is obtained which more truly refers to the normal paraffin feed molecule itself. Table I lists some data on the reactivities of normal paraffins with carbon numbers between 6 and 11 for hydroisomerizationlhydrocracking (Weitkamp, 1975). In this table we have attempted to interpret the experimentally observed conversion rates accordingto the protonated cyclopropane mechanism. With the same simplifyingassumption as in hydrocracking, viz., that the length of the alkyl groups attached to the cyclopropane ring has no effect as long as the condition is satisfied that n and m are at least 1 (cf. Scheme I), it follows that the reactivity for isomerization should be proportional to the carbon number of the feed paraffin minus 4. (Note that this is in accordance with a zero reactivity for skeletal isomerization of n-butane.) Assuming that in the case of n-hexane the observed conversion is attributable to isomerization only, it is possible to calculate the contribution of isomerization of the other normal paraffiis on the basis of proportionality with N - 4, if N is the number of carbon atoms of the normal paraffin molecule. By subtracting this contribution from the observed conversion rate of n-heptane, one may deduce the hydrocracking contribution in the case of this paraffii, and on this basis calculatethe hydrocracking rates for the other normal paraffins, assuming it to be proportional with N - 6. By adding together the contributions of hydroisomerization and hydrocracking, total conversion rates may be computed. Table I shows that the predicted conversion rates are in satisfactory agreement with the experimentallyobserved rates. From the computed contributionsof hydroisomerization and hydrocracking it is possible to determine the selectivity for either reaction. The figures for isomerizationselectivity in Table I show that this selectivity decreases with increasing length of the paraffin chain. Carbon Number Distribution in the Products Obtained by Hydrocracking of Normal Paraffins. Figures 2-4 show the carbon number spectra of the products obtained by primary hydrocracking of the normal paraffins from n-heptane through n-hexadecane over a Pt/Ca/Y-zeolite catalyst (Weitkamp, 1975, 1978). The figures show the virtual absence of C1 and C2,as well as that of the hydrocarbons that would be obtained by splitting off C1 or CZfrom the feed paraffins. The other hydrocarbon fragments except for Ca and C N are ~ produced in about equimolar amounts, with a slight
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 405 Table 11. Carbon Number Distribution in Products from Hydrocracking of n-Dodecane (mo1/100 mol of Clz Cracked) (Experimental Data from Schulz and Weitkamp (1972). T = 285-300 OC; P = 50 bar)
CRACKED PRODUCT, mol I 100 mol
too
o
n-C,
A
n-C9 '20
H , ~
catalyst cracking conversion, % ' C1
Pt/Ca/Y
Pd/Mn/H-Y
Pd/H-Y
56
50
30
c3
7.0 31.8 41.9 42.9 41.0 30.6 5.9
0.2 17.5 55.0 51.6 44.2 33.7 18.6 2.1
0.2 20.9 56.8 52.2 43.1 33.4 16.1 2.3
c2 c4
c5 c6 c7
CARBON NUMBER
OF CRACKED PRODUCTS
Figure 2. Carbon number distribution in products obtained by hydrocracking of n-heptane, n-nonane, and n-undecane (Weitkamp, 1975).
n
CRACKED PRODUCT, mol I100 mol
i20r
ioo/
t
CS CQ ClO c11
Table 111. Carbon Number Distribution in Products from Hydrocracking of n-Hexadecane and n-Docosane (mo1/100 mol Cracked) (Experimental Data from Coonradth and Garwood (1964). Catalyst, Pt/Silica-Alumina) ~~
feed T, OC conversion, 76
0 n - C 8 H,8
rn
n-CmH22 T n-C,,H,
c1 c2
c3 c 4 c5 c6
c7
2
1
3
4
5 6 7 8 9 '10 ii CARBON NUMBER OF CRACKED PRODUCTS
Figure 3. Carbon number distribution in producta obtained by hydrocracking of n-octane, n-decane, and n-dodecane (Weitkamp, 1975). CRACKED PROWCT. mol I 1OOmol
40
30 30
20
0
r
i
n-C14Hm
-
-
n-C16 371 53.1 1 1 7 19 21 17 22
4
6 8 10 12 CARBON NUMBER OF CRACKED PROWCTS
Figure 4. Carbon number distribution in products obtained by hydrocrackingof n-tetradecane and n-hexadecane (Weitkamp, 1978).
tendency for a higher concentration of hydrocarbon fragments in the middle range ( N is the carbon number of the feed paraffin). C3 and C N - ~are produced in significant amounts, which are, however clearly lower than the amounts of hydrocarbon fragments in between. The striking feature of lower C S and C N - production ~ cannot be explained by the classical carbenium ion theory, but becomes plausible with the present mechanism. Referring to Scheme I, and considering that n and m should have values of at least 1 and 3, respectively, one readily infers that C3 can only be formed as one type of fragment, viz., a linear one, whereas each higher fragment lower than C N -can ~ be formed as two species, viz., an isoparaffin and a linear fragment. C N - is~ formed as an isoparaffin only.
feed T, "C conversion, % C8 c9
ClO c11 c 1 2 c13+
n-C16 371 53.1 24 22 19 18 13 5
n-czo 336 94.3 27 23 20 16 12 33
Thus, the molar amounts of C3 and CN-3 should be half of the amounts of hydrocarbon fragments with values in between. With the simplifying assumption of equal chances of cracking for any arbitrary combination of n and m values satisfying the condition stated above (within the limit set by the total carbon number of the feed paraffin), it follows that equal molar amounts of fragments with carbon numbers between 4 and N - 4 should be obtained. The fact that the distributions in this range are not entirely flat, but tend to exhibit a slight maximum in the middle, means that the assumption of equal chances is somewhat too simple and that the length of the alkyl groups of the protonated cyclopropane species has some effect if it is ~ is not very short. The fact that C3 and C N - production half, but even less than half of that of the intermediate fragments, has probably the same cause. The lower production of C3 and C N - and ~ the about equimolar production of intermediate fragments is a feature which is difficult to explain by the classical theory but is made entirely plausible by the present mechanism. This feature appears to be quite common: it can also be noticed in hydrocracking experimentswith other catalysts, as demonstrated in Table I1 for other zeolitic catalysts (Schulz and Weitkamp, 1972) and in Table I11 for a nonzeolitic catalyst and a higher paraffin, viz., Cm (Coonradt and Garwood, 1964). Other studies on hydrocracking of n-hexadecane over sulfided Nihilica-alumina and Pt/ silica-alumina Catalysts also showed the characteristic lower production of C3 and C13 and the almost equimolar amounts of fragmentsin between, when secondary cracking was absent (Langlois et al., 1966; Langlois and Sullivan, 1969). Product distributions with similar characteristics were also observed in hydrocracking of Fischel-Tropsch paraffins over a bifunctional catalyst (Sie et al., 1988). n-Heptane is the only paraffin for which the lower production of C3 is not observed (cf. Figure 2). This can
0 2
n-Czo 336 94.3
