Ind. Eng. Chem. Res. 1992,31,1881-1889
1881
Acid-Catalyzed Cracking of Paraffinic Hydrocarbons. 1. Discussion of Existing Mechanisms and Proposal of a New Mechanism S.Tiong Sie Koninklijke/Shell-Laboratorium,Amsterdam (Shell Research B. V.),P.O.Box 3003, 1003 AA Amsterdam, The Netherlands
The generally accepted theory of acid-catalyzed cracking of paraffinic hydrocarbons is a carbenium ion mechanism according to which a classical carbenium ion is formed from a hydrocarbon, which then undergoes @-scissionto give a carbenium ion of lower carbon number and an olefin. It is shown that this theory fails to provide a satisfactory explanation for certain characteristic features of catalytic cracking and hydrocracking. Furthermore, the basic premises of this theory, which was derived from an analogy with that of radical cracking, appear to be questionable. A new mechanism is now proposed which has some features in common with the mechanism of acid-catalyzed isomerization of paraffins, viz., the intermediacy of a nonclassical carbonium ion of cyclopropyl structure. This new theory provides an explanation for many of the characteristic features of cracking processes.
Introduction The generally accepted theory of acid-catalyzed cracking dates back to 1949, when Greensfelder, Voge, Good, and Thomas proposed their carbenium ion (then called carbonium ion) theory (Greensfelder et d., 1949; Thomas, 1949). This theory has found widespread acceptance and is still being quoted in most literature on catalytic cracking and hydrocracking. This acceptance is probably best illustrated by the opinion of Hansford, who stated not too long ago (Hansford, 1983) "its success has endured few challenges now for some 35 years, and it seems unlikely that a better theory as applied to catalytic cracking will ever replace it." The only principal new element added to the theory of acidic cracking of Paraffins since is the proposal made by Haag and Dessau that cracking may also occur by a new reaction path involving, instead of a tricoordinated carbenium ion, a pentacoordinated carbonium ion as an intermediate. The latter type of cracking, characterized by the formation of Hz, C1, and Cz,which products are normally essentially absent in acid-catalyzed cracking, is claimed to occur in experiments performed at relatively high temperatures using hexanes as feed (Haag and Dessau, 1984). In the following discussion we will focus primarily on the classical carbenium ion theory as this supposedly describes the more general mechanism of acidic cracking operative under the relatively mild conditions required for catalytic cracking or hydrocracking of high-molecularweight hydrocarbons. The cracking of lighter hydrocarbons by the mechanism of Haag and Dessau will be briefly discussed in a later part of this paper. The Classical Carbenium Ion Theory Cracking of Normal Paraffins. In the theories of Greensfelder,Voge, Good, and Thomas cracking proceeds via a carbenium ion as an intermediate, which undergoes a @-scissionto form an olefin fragment and a carbenium ion fragment. The latter ion can be transformed to a hydrocarbon fragment by hydride tramfer from a paraffin molecule, which then becomes a carbenium ion. In this way a reaction cycle is created. The initial carbenium ions may be formed from a feed paraffin molecule by, for instance, hydride abstraction or by dehydrogenation and proton addition to the olefinic molecule formed. This classical mechanism is shown in Scheme I for the cracking of a normal paraffin. Our main objection to this theory concerns the @-scission. As can be seen in Scheme I, a scission of the C-C
Scheme I. Cracking of a Normal Paraffin by the Classical Carbenium Ion Mechanism of Greensfelder, Voge, Good, and Thomas
1
WOLEFIN
HYDRIDE TRANSFER
\\ \ CRACKED PRODUCTS
\
H /Hi 'd-c-~c; -H H ;HJm ".PARAFFIN
Table I. Heats of Formation of Alkylcarbenium Ions (Franklin, 1968) carbenium ion type of ion AH*,kcal/mola methyl 258 prjmary ethyl 225 PrFW n-propyl 218 PrpW n-butyl 211 Primary secondary isopropyl 194 secondary sec-butyl 190 tertiary tert-butyl 174 1 kcal = 4.18 kJ.
bond in the @-positionrelative to the positively charged carbon atom of the carbenium ion will in the first instance produce a primary carbenium ion. A reaction in which a secondary or tertiary carbenium ion ends up in a primary ion will have such a high energy of activation that it is not likely to proceed at any significant rate under relatively mild conditions. From Table I, which lists the heats of formation of some alkylcarbenium ions, it can be inferred that the energy differences between a primary and a secondary ion are larger than 20 kcal/mol (1 k d = 4.18 kJ). A difference of 20 kcal/mol between the activation energies of reactions transforming the original ion into a primary and a secondary ion would correspond with a difference in reaction rate by a factor of 2 X 108 at a temperature level of 250 OC (a temperature that is not uncommon in the hydrocracking of paraffins). If notwithstanding the above argument a primary carbenium ion is at a l l formed in the first instance, there is
0888-588519212631-1881$03.Oo/O 0 1992 American Chemical Society
1882 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
no plausible reason why this should not be an ethyl (or even a methyl) ion; i.e., in Scheme I the value of m may be any number including zero. Methyl and ethyl ions will ultimately end up as methane and ethane, and it is wellknown from practice that acid cracking under mild conditions is characterized by the virtual absence of these cracked fragments. Greensfelder et al. (1949)concede that methane and C2 can be formed according to their mechanism and even state “of course, some methane and C2 are formed, which need not be ascribed to thermal cracking, but to adequate activation of much less favored types of cracking”. The predominance of C3and higher fragments in the product of acid-catalyzed cracking of paraffins is rationalized by pointing at the higher stability of secondary and tertiary carbenium ions so that the enthalpy of the overall reaction will favor the formation of these ions, which can only exist in C3 and higher. However, this is in principle more a thermodynamic than a mechanistic argument, which is in our opinion not valid unless the cracking reaction is a reversible one (which is hardly the case) or unless the activation energy of the rate-determining step is in some way lowered with increasing reaction heat (which is often observed). It is often assumed that the primary carbenium ion, once formed, will rapidly transform (if allowed by the length of the carbon chain) into a secondary or tertiary ion by a hydrogen shift (a hydride shift, or proton detachment to form an olefin followed by reattachment of the proton to the other carbon atom of the double bond) (Haensel, 1951; Voge, 1958). While this is an energetically favorable rearrangement, which is very likely to occur as a secondary step, it does not eliminate the difficulty involved in the formation of the primary ion by the @-scission,unless the scission and the rearrangement steps occur in some concerted fashion. Starting from a normal paraffin and a straight-chain secondary carbenium derived from it, @-scissionshould result in a primary carbenium ion with a shorter chain, which is still a straight one, however (see Scheme I). Hydride transfer will produce a normal paraffin as product from this carbenium ion. The other cracked fragment is a linear a-olefin, which will be hydrogenated to a linear paraffin, too, in hydrocracking. Cracking of Branched Paraffins and Formation of Branched Products. According to the @-scissionmechanism discussed above, the primary products from cracking of a normal paraffin should be linear. However, practical experience in catalytic cracking as well as in hydrocracking teaches that the products are to a large degree branched. Although skeletal isomerization of primary products in a secondary reaction is not to be ruled out, it cannot explain the fact that the degree of isomerization quite often exceeds the thermodynamicequilibrium value. For instance, in the hydrocracking of normal paraffins from Clo upward over sulfided W/silica-alumina or W/fluorided clay catalysts the iso/normal ratio in the butanes was found to be 3 times higher than the thermodynamic equilibrium ratio, while in the pentane fraction the ratio was higher by a factor of 2 (Archibald et al., 1960). Similarly, in hydrocracking of normal hexadecane it proved possible to obtain iso/normal ratios of 3.5 in the butanes, of 6.5 in the pentanes, and of 12.6 in the hexanes, whereas the equilibrium ratios under the prevailing conditions are 0.6,2.3,and 3.8, respectively (Flinnet al., 1960). In catalytic cracking, too, the iso/normal ratios in the paraffinic products generally exceed the thermodynamic equilibrium ratios (Coonradt and Garwood, 1964). In catalytic cracking of n-decane over
Scheme 11. Cracking of Mono-,Di-, and Tribranched Paraffins According to the Classical Carbenium Ion
Mechanism
a silica-alumina catalyst (in the presence of hydrogen, but without a hydrogen activation function on the catalyst) the iso/normal ratio in the C4-C7paraffins was as high as 10 (Langlois and Sullivan, 1969). Iso/normal ratios above the thermodynamic equilibrium ratios were observed for the paraffin products formed in many more recent studies on normal paraffm hydrocracking, to be discussed in more detail later. The excess branching is therefore an intrinsic feature, rather than an incidental occurrence. It proves that at least some of the branched paraffins are primary products of the cracking and not a result of postisomerbation. This is particularly true in the case of C4,since under typical hydrocracking conditions n-butane cannot be isomerized, as will be discussed later. The formation of branched products in hydrocracking may be understood if skeletal isomerization of the straight-chain feed paraffm is a prerequisite for cracking according to the classical carbenium ion mechanism. Schulz and Weitkamp, for instance, proposed that isomerization of normal paraffms necessarily precedes hydrocracking (Schulz and Weitkamp, 1972;Weitkamp, 1976). An argument against this concept is that isomerization produces in the first instance a monobranched isomer, from which most likely a tertiary carbenium ion is formed. It can readily be inferred from Scheme I1 that in this case the @-scissionshould again result in the initial formation of a primary carbenium ion. Hence, the preisomerization of the most abundant monomethyl isomer products does not reduce the energy barrier for @-scission,but even aggravates the problem since the tertiary ion is lower in energy than the secondary ion in Scheme I. It can be inferred from Scheme I1 that the only way to avoid the formation of a primary carbenium ion by the @-scissionmechanism is the presence of a tertiary carbon or a quarternary carbon in the @-positionto the carbon atom carrying the positive charge. This implies that the isoparaffin amenable to cracking must be at least a dibranched and preferably a tribranched one, with the branched carbon atoms separated by one other carbon atom. It has been suggested that formation of multibranched isomers from the feed and cracking are consecutive reactions (Steijns et al., 1981). Cracking of a normal paraffm must thus proceed through the stages of formation of monobranched isomers, dibranched isomers, and finally cracked products. While this hypothesis may not have the energy barrier problem referred to above if the dibranched isomer has the right structure, it cannot be reconciled with a number of facts, as will be shown below. If the assumption is made that isomerization is a very fast reaction compared to cracking in the above consecutive scheme, there should be no difference in observed cracking
Ind. Eng. Chem. Res., Vol. 31, NO. 8, 1992 1883 Table 11. Distribution of Dimethylhexanes (%) Formed in the Hydroisomerization/Crackingof n -Octane (Experimental Data from Vansina et al. (1983); T = 192.5-237.5 O C : P = 5-100 bar: Catalyst, Pd/USY) conversion of n-Cg, % dimethylhexanes 9.1 47.3 58.9 72.5 93.2 equilib" 17.5 16.8 18.4 17.2 15.3 292 10.3 6.8 14.3 14.6 14.7 14.7 18.9 2,3 28.9 28.2 29.6 29.2 30.7 2,4 33.7 20.1 27.5 21.5 20.9 21.3 2,5 20.9 11.4 12.8 9.4 9.8 10.8 393 5.6 7.8 7.2 7.6 7.3 6.6 394 10.6
" Thermodynamic equilibrium composition at 227 "C, calculated on the basis of data from API Project 44 (1953). rate between isomeric hydrocarbons. Significant differences in crackability of isomeric hydrocarbons have been reported, however. For instance, over a silica-zirconiaalumina catalyst 2,2,4-trimethylpentane cracked more rapidly than n-octane at 550 OC, while 2,7-dimethyloctane was more readily cracked over the same catalyst at 500 "C than n-dodecane, notwithstanding its lower carbon number (Voge, 1958). A branched nonadecane, viz., 2,6,10,14tetramethylpentadee, was found to crack nearly 5 times as fast as n-octadecane over a rare-earth-exchanged Xzeolite at 510 OC. A heptamethylnonane, on the other hand, cracked far less rapidly than its straight-chain isomer, n-hexadecane, under the same conditions (Nace, 1969). The latter results cannot be explained by differences in carbon level on the catalyst since the observed levels should have the opposite effect. In hydrocracking, too,signifcant differences in cracking behavior between isomeric paraffins have been observed. For instance,Archibald et al. reported more rapid cracking over sulfided W/fluorided clay catalyst at 375 "C for 2,2,4-timethylpentane than for n-octane, but 2,2,3-trimethylbutane and a branched isododecane (mainly 2,2,4,6,6-pentamethylheptane) cracked less readily than the corresponding normal paraffins under the same conditions (Archibald et al., 1960). Differences in crackability between 2,2,4-trimethylpentaneand n-octane, with faster cracking of the former isomer, have also been observed over a Ni/silica-alumina catalyst at 300-350 "C. 2,2,3-Trimethylbutane, on the other hand, cracked less readily than its straight-chain isomer, n-heptane, at 350 "C (Ciapetta and Hunter, 1953). If in contrast to the above premise the isomerization to the required di- or multibranched paraffins would be slow relative to the cracking of the latter, one should expect that the concentration of the specific isomers suitable for cracking will be abnormally low in the product. This is not supported by experimental findings. For instance, in the distribution of the dimethylhexanes formed in the hydroisomerization/crackingof n-octane, the only isomer amenable to cracking with formation of a secondary ion is the 2,4-isomer (cf. Scheme 11) and the data reported in Table I1 do not show an abnormally low concentration of this isomer. Likewise, the distribution of dimethyloctanes formed in the hydroisomerization of n-decane provides no evidence at all for preferential cracking away of the suitable Structures 2,4- and 3,bdimethylodane, as can be seen from the data listed in Table 111. Examination of the isomer distribution in the cracked producta from hydrocrackingof n-dodecane does not reveal abnormally low concentrations of 2,4dimethylpentane and 2,4-dimethylhexane,which should be removed preferentially by secondary cracking,if these structures are so much more crackable than the others (see Table IV).
Table 111. Distribution of Dimethyloctanes (%) Formed in the Hydroisomerization/Cracking of n -Decane (Experimental Data from Steijns et al. (1981); T = 186.7-215 "C; P = 6.5-101.7 bar: Catalyst. PtAJSY) dimethylconversion of n-Clo, % octanes 16.0 30.1 41.8 49.8 50.2 98.6 3.5 3.3 3.3 3.3 3.3 292 3.1 10.3 11.6 13.0 10.9 10.3 2,3 11.8 19.3 16.1 15.0 16.5 18.1 2,4 15.7 25.5 25.9 23.5 21.8 2,5 + 3,5 23.3 23.3 16.3 15.5 14.6 15.4 15.6 2,6 14.6 11.3 10.6 12.0 12.0 12.1 12.2 2,7 + 3,6 3.8 4.5 5.0 4.7 5.1 3,3 3.7 9.3 9.3 8.9 8.4 9.8 8.3 3,4 2.0 2.6 2.7 2.7 2.9 4,4 2.3 0.0 3.3 2.3 2.2 1.8 2.2 4,5 ~
Table IV. Distribution of C7 and CBIsomers (%) in Reaction Product of n -Dodecane Cracking (Experimental Data from Schulz and Weitkamp (1972); Catalyst, Pd/H/Y Zeolite; P = 40 bar) temp/conversion C, and C g 275 "C/ 300 "C/ 350 "C/ equilib isomers formed 16% 30% 63% comDosn' n-heptane 7.3 11.4 12.2 11.0 2-Me-hexane 36.9 35.3 34.3 21.3 3-Me-hexane 32.9 32.4 32.8 19.3 3-Et-pentane 1.6 1.6 1.9 2.7 2,3-di-Me-pentane 11.7 10.5 10.3 23.9 2,4-di-Me-pentane 8.6 7.6 7.1 5.5 2,2-di-Me-pentane 0.5 0.5 0.5 7.4 3,3-di-Me-pentane 0.3 0.4 0.5 7.3 2,2,3-tri-Me-butane 0.2 0.3 0.4 1.6 10.3 11.8 11.9 n-octane 6.4 2-Me-heptane 24.1 23.2 23.0 14.8 3-Me-heptane 28.3 28.4 29.6 17.5 4-Me-heptane 9.2 10.1 9.7 5.7 3-Et-hexane 7.0 2,3-di-Me-hexane 6.0 5.3 4.9 3.3 2,4-di-Me-hexane 12.3 10.7 9.8 10.4 2,5-di-Me-hexane 11.1 9.0 7.8 8.9 2.1 2.2 3.6 3,I-di-Me-hexane 2.0 2,2-di-Me-hexane 0.3 0.5 0.6 5.2 3,3-di-Me-hexane 0.2 0.3 0.3 4.7 2-Me-3-Et-pentane 0.1 0.1 0.1 0.9 Thermodynamic equilibrium composition at 327 "C,calculated from API Project 44 data (1953).
Irrespective of whether the formation of dibranched isomers or their cracking is rate-limiting, the concept of exclusive cracking of specific dibranched structures according to the classical theory should result in similar product distributions from different isomeric feed paraffins. The experiments mentioned earlier in comparing cracking rates of isomers provide ample evidence that this is not the case: significantly different distributions of cracked products were observed. Another argument against the mechanism of @-scission from di- or multibranched structures is that the olefin fragment formed in catalytic cracking must be a branched one (cf. Scheme 11). This is contrary to the relatively low degree of branching generally observed in the light olefin fraction, as will be discussed in the following papers of this series. Analogy and Differences with Radical Cracking. The classical theory of acid-catalyzed cracking was originally conceived on the analogy of the theory of thermal cracking, where free radicals are intermediate species which crack by a @-scissionmechanism (Rice and Teller, 1938, 1939; Koasiakoff and Rice, 1943). The free radical cracking of a normal paraffin can be represented as in Scheme I if an electron is added to the positively charged carbon, thus
1884 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 Table V. C-H Bond Dissociation Energies of Alkanes in Radical Formation (Nonhebel and Walton. 1974) alkane radical formed type of radical AH, kcal/mol methane methyl prfmary 104 ethane ethyl Primary 98 isopropyl secondary 94 propane isobutane tert-butyl tertiary 91
transforming the carbenium ions into the corresponding free radicals. While this analogy is tempting in ita simplicity, it can in our opinion hardly be correct, since the energy barriers involved are quite different. The formation of a primary alkyl radical as cracked fragment from a secondary radical is energetically not so disfavored as the formation of a primary carbenium ion from a secondary one. As can be inferred from the data in Table V, the order of stability of alkyl radicals is the same as that of carbenium ions, viz., tertiary > secondary > primary, but the energy differences are much smaller: between 2 and 4 kcal/mol (except for the methyl radical, which needs 6 kcal/mol more to be formed than the ethyl radical). At a temperature which is sufficiently high for overcoming the C-H bond dissociation energies (cf. Table V) so that the initiation of the free radical chain reaction of cracking is sufficient to allow the latter reaction to proceed at a practical rate, the energy differences of 2-4 kcal/mol should not play a very decisive role. At a temperature level of 600 "C, differences in activation energy of 2 and 4 kcal/mol correspond with differences in reaction rate by factors of 3 and 10, respectively. This is much smaller than the factor of 2 X lo8 mentioned above for carbenium ion reactions. If energy differences between secondary and primary carbenium ions were not 80 crucial, product distributions in acid-catalyzed cracking would not principally deviate from those in thermal cracking. However, the characteristics of the two cracking processes are quite different, as has already been recognized more than half a century ago (Egloff et al., 1939). These differences can be summarized as follows (Haensel, 1951). Thermal cracking: major product is ethene with much methane and ethane; many n-a-olefins; few branched aliphatics. Catalytic cracking: little methane, ethane, and ethene; major product is C3 to C,; few n-a-olefins above C,; aliphatics mostly branched. If a primary carbenium ion were to be formed by a @-scissionas shown in Scheme I, there is no reason why it should not undergo a second @-scissionsince the energy barrier for this is not too great. The olefin product split off would be ethene. This process may be repeated, resulting in very high yields of ethene from cracking of a heavy paraffin, analogous to what is achieved by thermal cracking. When starting from straight-chain paraffins, thermal cracking producea predominantly straighbchain fragments, in accordance with the @-scissioncracking mechanism. This is in strong contrast with acid-catalyzed cracking as discussed above disproving again the validity of the 8scission concept in acidic cracking. Another argument brought forward to support the 8scission concept in carbenium ion cracking is that "it represents the only manner of molecular splitting that will give a neutral olefin and a smaller carbonium ion without the rearrangement of carbon and hydrogen atoms during the process. Only electrons are shifted, and thus the principle of least motion for elementary chemical reactions is maintained- (Voge, 1958). This is an argument borrowed from radical cracking where the principle of least motion
Scheme 111. Mechanism of Acid-Catalyzed Isomerization of a Normal Paraffin
1
HYDRIDE ABSTRACTIONITRANSFER n 2 1
{Hj H H H [H]
H-ICI-C-C-C-:C!-H \Hjn@ H
CLASSICAL CARBENIUM ION
H [HI,
H I H H
is also valid (Teller and Rice, 1938, 1939). Although the importance of this principle seems debatable, the notion that the &scission is the only poesibility which satisfies this principle is not true. We will show later that alternative reaction mechanisms can be formulated which come very close to this "principle of least motion". Other reasons to doubt the validity of the classical acidcracking mechanism of Greensfelder, Voge, Good, and Thomas is that it does not provide an explanation for the strong dependence of cracking rate on chain length in the case of catalytic cracking of normal paraffins. This was already recognized by Voge, who stated "The increase in rate of cracking with increasing carbon number is too great to be accounted for by the increased number of carboncarbon bonds or the increased number of secondary hydrogens susceptible to attack, and must therefore result from some other influence ..." (Voge, 1958). Likewise, we found this theory incapable of rationalizing the very strong dependence of cracking rate on chain length, which is generally observed in hydrocracking, as well as some other characteristics of this process, like the low formation of C3as compared with the formation of fragments of higher carbon number.
A New Proposed Mechanism for Acid-Catalyzed Cracking To try to circumvent the above-mentioned problems, we propose a reformulation of the classical carbenium ion theory. The main difference is that the intermediacy of a nonclassical carbonium ion instead of a classical carbenium ion is postulated, very much in the same way as in the skeletal isomerization of paraffms (Condon, 1958; Brouwer, 1968). The mechanism of the latter reaction is discussed below. Mechanism of Hydroisomerization of Paraffins. The mechanism of skeletal isomerization of paraffins is shown in Scheme I11 (Condon, 1958; Brouwer, 1968). It involves the intermediacy of carbonium ions of a nonclassical type,viz., the protonated cyclopropane structure. Isomerization of the carbon chain occurs by breakage of either of the two bonds linking the bridged methylene group with the carbon atoms of the main chain. Strong evidence for this mechanism is the finding by Brouwer and Oelderik that n-butane is not isomerized by HF-SbF5 to
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 188s any significant extent under conditions where n-pentane and n-hexane are rapidly converted into isopentane and isohexanes, respectively (Brouwer and Oelderik, 1968a,b). This can be understood since for butane m = 0 in reaction Scheme I11 so that rupture of the only bond in the protonated methylcyclopropane that produces the isobutane skeleton must lead to a primary carbenium ion, which is energetically very unfavorable. Breakage of the other two bonds of the cyclopropane ring is possible, but will in this case result in a secondary carbenium ion with a straight chain. This will lead to either the original butane molecule or a n-butane molecule in which two carbon atoms have exchanged their position in the chain. Using 'W-labeled n-butane, Brouwer was able to show the occurrence of this exchange of carbon atom positions in n-butane (Brouwer, 1968). Moreover, the rate of this exchange reaction proved to be as expected on the basis of observed isopentane formation from n-pentane. This can be considered to constitute almost conclusive evidence for the involvement of a protonated cyclopropane structure as an intermediate in paraffin isomerization. Further evidence for this isomerization mechanism has been obtained more recently in isomerization experiments with 'Clabeled n-butane and also n-pentane and n-hexane over Pt/silica-alumina catalysts at 300 "C (Chevalier et al., 1977). Under these conditions, too, which are more representative of the conditions of practical hydroisomerization and hydrocracking processes than the experiments at low temperatures with the HF-SbF5 superacid mentioned earlier, very little skeletal isomerization ' ! did occur. of n-butane was observed, but scrambling of Y Moreover, the rate of transformation of [lJW] butane into [2-'W]butane was found to correlate very well with the rate of transformation of n-pentane into isopentane in experiments with different catalysts. Studies on the isomerization of normal paraffins with 6-15 C-atoms over a Pt/Ca-Y catalyst showed that the rate of formation of the 2-methyl isomers is relatively low. This effect can be shown to be consistent with the protonated cyclopropane mechanism but cannot be explained by a classical mechanism via alkyl and hydride shifts (Weitkamp, 1982). A Protonated Cyclopropane Structure a8 Reaction Intermediate in Acid-Catalyzed Cracking. If the intermediacy of a nonclassical carbonium ion as opposed to a classical carbenium ion in isomerization is accepted, there is no reason why this nonclassical ion should not play a role in cracking as well. A reaction mechanism based on this concept is outlined in Scheme IV. In this scheme, the protonated cyclopropane ring with two alkyl groups is shown without any further indication of the preferred positions of the extra H and the positive charge. From quantum-mechanicalcalculations carried out on protonated unsubstituted cyclopropane, it may be concluded that edge-protonated or corner-protonated structures are energetically favored over a face-protonated one (Brouwer and Hogeveen, 1972). Thus the extra H will be preferentially located near two adjacent carbon atoms or one carbon atom of the cyclopropane ring. With a dialkylsubstituted propane ring,it may be expected that the extra H will have a preference to be near the unsubstituted carbon of the cyclopropane ring and to leave the positive charge preferentially at the substituted carbon atoms. The reasons for this is that the electron-donating effect of the alkyl groups will tend to stabilize this structure more than the alternative structure in which the extra H is near a substituted carbon and the positive charge on the unsubstituted one (analogous to the high stability of a tertiary
Scheme IV. Proposed Mechanism of Acid-Catalyzed Cracking of a Normal Paraffin :HI H-/
H
H H
H H !HI
c i-c- c-c-c-c- /c -H ;H;,,@ H H H H LH;,,
cussIcAL CARBENIUM ION
"31
i
carbenium ion as compared to that of a secondary one). One structure among the alternative resonance structures of the protonated dialkylcyclopropane is shown in Scheme IV (third line). A l,&hydride shift from a C-atom of an alkyl group to the C-atom of the cyclopropane ring to which this group is linked, combined with a 1,2-hydride shift between two C-atoms of the cyclopropane ring and bond scission, leads to the tertiary carbenium ion and the linear olefin of line 5 in Scheme IV. Hydride transfer from a feed paraffin molecule to this tertiary ion produces an isoparaffii, by which the reaction cycle is closed. An important consequence of this reaction mechanism is that the tertiary carbenium ion should have at least four carbon atoms; i.e., the value of n in Scheme IV should be at least 1. A value of zero for n implies that the initial carbenium ion would be a primary one, which is very unlikely on energetic grounds. The olefinic fragment should have at least three carbon atoms; i.e., m'should be at least 1. A value of zero for m' would imply a hydride shift from a primary carbon atom, which is energetically very unfavorable, for the same reaeon why hydride abstraction from a primary carbon atom to form a primary carbenium ion is unlikely to occur. The 1,bhydride shift and subsequent transformations can be regarded as a nucleophilic attack of a hydride ion originating from a &carbon of the alkyl group on the nearest corner atom of the cyclopropane ring, with the alkyl group minus H as leaving group. The occurrence of these reactions as principal cracking mechanism is made more plausible when the structure of the cyclopropyl cation is considered in a three-dimensional form. Figure 1shows a likely three-dimensional structure suggested on the basis of general chemical considerations. It can be seen that the H atom of the Bcarbon atom can be in a favorable position for nucleophilic attack on the positively charged carbon atom of the cyclopropane ring. Figure 2 shows the positions of the atoms before and after
1886 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 H
H
H
"
H
c-c-\ ~c-c-c-c fH H c A@ /
H
R-C= C-R, ,
I \,
\
I
c'
HI $H
H
-
R-c
i;'y
--:c-R'
\\
I'
,c;
H
\
H
-
Y
?
R-C;--C-R >I
2
/c\
H
H
Figure 3. Conventional representationof resonance Structures of the protonated dialkylcyclopropane.
Figure 1. Three-dimensionalrepresentationof the protonated dialkylcyclopropane intermediate.
H
H
Figure 2. Relocation of electrons during cracking.
the process. It can be seen that the positions of the atoms involved hardly change; only the binding electrons are relocated. Hence, it appears that the presently proposed cracking mechanism also satisfies the "principle of least motion" used before to support the classical theory. The transformation of the classical secondary carbenium ion into the protonated dialkylcyclopropanestructure is not expected to involve a very high energy barrier since the energy contents of the two ions should not be too different on the basis of the following arguments. Theoretical considerations indicate that two forms of protonated olefine are stable, viz., one in which the proton is attached to one of the carbon atoms of the unsaturated bond with the other carbon atom surrounded by a sextet of electrons and bearing a positive charge, Le., the classical carbenium ion, and another one where the proton interacts with the *-electrons and is located in a position between the two carbon atoms, bridging these two atoms (s-complex) (Dewar, 1949;Dewar and Dougherty, 1975). Physical and chemical evidence indicates that cyclopropane has an "olefinic character", behaving very much like an olefin in many situations. Hence, the protonated cyclopropanering may be compared to a protonated olefin, i.e., a secondary carbenium ion, in energy terms. The protonated cyclopropane ring may also be viewed as a r-complex between an olefin and an alkyl ion instead of the above proton. In a more classical representation, the bridging of the carbon atoms of the olefinic bond by the carbon atom of the alkyl ion forms (partial) bonds of the cyclopropane ring. The actual protonated cyclopropane structure is a hybrid of resonance structures in which each of the ring carbons can assume the role of the carbon atom of the alkyl ion, as depicted in Figure 3.
The resonance may be expected to contribute considerably to the stability of the protonated cyclopropane structure, thus compensating for the inherent strain in the three-membered ring. That resonance is indeed capable of greatly enhancing the stability of this ring is evident from the known high stability of the more unsaturated cyclopropenylium cation, which should have aromatic character. The triphenyl derivative of this cation is so stable that salts of it have been synthesized (cf. Nenitzescu, 1968). Protonated cyclopropane structures have been deduced from the stereochemistry of Wagner-Meerwein-type rearrangements of norbornyl derivatives, and the evidence is further strengthened by experiments with carbon isotopes (Roberta et al., 1954). It is of interest to note that these rearrangements occur at relatively low temperatures (below 50 "C) without the need for superacids (acetic acid is a suitable medium). The existence of protonated cyclopropane has been demonstrated in the gas phase in mass spectrometric experiments (Rylander and Meyerson, 1956) and the occurrence of this C3H7+ion (which is different from the isopropyl ion) has been reported in the mass spectra of nearly all large aliphatic hydrocarbons. The ionization potential to form this C3H7+ion has been determined to be 7.90 V, which is not much higher than the ionization potential of 7.43V needed to form the isopropyl ion (a classical secondary carbenium ion). This difference corresponds with a difference in energy of about 11 kcal/mol, which is much less than the energy difference between a primary and a secondary carbenium ion (Rylander and Meyerson, 1956). This energy difference between the protonated cyclopropane ion and a secondary carbenium ion should become even smaller when there is an accumulation of alkyl groups on the ring carbons. The protonated dialkylcyclopropane ion assumed as intermediate in the cracking reaction should therefore be much more stable than the unsubstituted ion, for an analogous reason why a dialkyl carbocation (i.e., a secondary carbenium ion) is much more stable than a methyl ion (cf. Table I). The next step in the cracking mechanism of Scheme IV tranforms the protonated dialkylcyclopropane ion into a tertiary carbenium ion, which is a relatively stable species. The overall reaction is in effect the transformation of a secondary carbenium ion into a tertiary one, and from the viewpoint of the ions alone there is a positive driving force for the reaction. Thus the proposed mechanism allows cracking to take place by a reaction path which leads from a higher to a lower energy level without having to pass high-energy mountains. Consequences of the New Mechanism. According to the protonated cyclopropane mechanism the primary products of cracking of a normal paraffin are a branched paraffin and a linear olefin. This may explain why the iso/normal ratio in the light paraffins produced may be higher than the thermodynamic equilibrium ratio, while the ratio in the light olefine is below the equilibrium value. The mechanism provides a direct and simple explanation why C1 and Czare not formed, since the lightest product that can be made is C3(cf. Scheme IV with minimal values
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1887 Scheme V. Mechanism of n -Hexane Cracking via a Pentacoordinated Carbonium Ion as Intermediate H H H H H H H - C -C - C - C- C -C - H H H H H H H
"-HEXANE
Table VI. Yields (%) of Saturated Hydrocarbons in Cracking of n-Hexane over H-ZSM-6 (mo1/100 mol Cracked) (Experimental Data by Haag and Dessau (198& T = 450 O C ; 10% Conversion) hydrocarbon Dress.. bar producta formed 0.1 0.2 1 HZ nd" nd 4.8 methane 10.3 7.3 3.3 27.8 21.1 8.6 ethane propane 46.0 52.4 66.5 n-butane 9.8 12.0 20.4 isobutane 2.1 2.8 8.0 n-pentane nd nd 2.3 ~
H
H'
/ \ \
H H
';I?
+
H
+
"nd = not determined.
H H H H H-C-C-C-C-H H H H H
H H H H H 0 C-C-C-C-C-H H H H H H
1
H
H-H
+ -Ha
METHANE + PENTENE-I
H H H H H H H-C-C-C-C-C-C-H H O H H H H
HVDROOEN HEXENE.1
t
1
~~
-Ha
n-0UTANE t ETHENE
of n and m'; i.e., n = 1and m'= 1). Since a C3 fragment can only be produced in one form, whereas C4and higher can each be made in two forms, it can be expected that fewer moles of C3 are formed than of C4and higher. This can indeed be observed, particularly in hydrocracking. The new mechanism also provides a clear explanation why the cracking reactivity should sharply increase with increasing carbon number in the range from 6 to 10 (Archibald et al., 1960). The smallest paraffm chain that can be cracked according to this mechanism is C,, and the chances of bond breakage will show a much sharper increase with carbon numbers above 7 than proportional with the number of carbon atoms. n-Hexadecane, for instance, cracks at a rate which is not a fador of 2, but an order of magnitude faster than the rate of octane cracking (Egloff et al., 1939; Flinn et al., 1960; Voge, 1958; Nace, 1969). More detailed comparisons of the theoretical predictions with experimental results of catalytic cracking and hydrocracking studies will be presented in the following papers of this series.
Cracking of Normal Paraffins below Heptane and of Highly Branched Paraffins Cracking of n -Hexane. The proposed mechanism is not applicable to the cracking of n-hexane and lower paraffm. These hydrocarbons are indeed not easily cracked by acid catalysis. Under more forcing conditions, however, particularly when use is made of zeolites of high intrinsic acid strengths a t relatively high temperatures, cracking of n-hexane does take place but leads to an entirely different product spectrum. A mechanism for this mode of cracking has been proposed by Haag and Dessau and is shown in Scheme V. This mechanism (Haag and Dessau, 1984) involves a pentacoordinated carbonium ion (as opposed to a tricoordinated carbenium ion) as intermediate, which is formed by proton addition to the paraffin rather than by hydride abstraction. The carbonium ion splits in the way shown in Scheme V, yielding lower paraffins including methane and primary carbenium ions as cracked fragments. The latter ions may form an olefin by losing a proton. Also molecular hydrogen can be formed plus an
Table VIJ. Initial Selectivities (mol per mol of n-Heptane Feed) in Cracking of n -Heptane over H-USY and HighSilica H-ZSM-5 As Reported by Corma et al. (1985) (T=47OoC;P=1bar) hydrocarbon H-USY H-ZSM-5 methane 0.015 0.016 ethane 0.043 0.022 ethene 0.145 0.390 propane 0.625 0.435 propene 0.467 0.200 n-butane 0.185 0.351 isobutane 0.100 0.312 total butenes 0.364 0.062 0.088 0.034 0.023 0.055 0.004
olefin as end product like in simple dehydrogenation. The formation of a primary carbenium ion need not be a problem in this case, since the pentacoordinated carbonium ion is probably also a species of high energy content, which can only be formed with strong Bronsted acids at relatively high temperature (at low temperatures superacids are required). In the cracking cycle, they are also formed by proton exchange with the highly reactive primary carbenium ion. The different product spectrum as compared with the more usual acid-catalyzed cracking can be seen in Table VI. The substantial formation of hydrogen, methane, and ethane and the low isobutaneln-butane ratio are clearly differentfrom the more established mode of acid-catalyzed cracking. Similar product distributions from hexane cracking over different zeolites have been observed in more recent studies (Wielers et al., 1991). On the basis of the protonated cyclopropane mechanism it is understandable that the characteristics of the pentacoordinated carbonium ion cracking are so prominent only with hexanes and lighter hydrocarbons: with higher paraffins the former mechanism provides an easier path for cracking. Cracking of n-heptane, which is just allowed by this mechanism, may be a borderline case. This is suggested by the data listed in Table VII,which shows that with the H-USY catalyst the product spectrum still has many characteristics of the more common mode of cracking (low methane, ethane, and ethylene, isobutane/ n-butane ration above l),whereas with H-ZSM-5 catalyst features of the other mode of cracking are apparent (formation of some C1 and C2,low isobutaneln-butane ratio). Even with such paraffms as n-hexane the Occurrence of some cracking by the protonated cyclopropane mechanism need not be completely excluded. At high conversions, the olefin fragments formed may give rise to formation of some heavier paraffins by polymerization or alkylation, which heavier paraffins may be subsequently cracked. Some support for this may be derived from experiments of Santilli, who reported that co-feeding propene with n-
1888 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
hexane c a w changes in the product spectrum obtained during cracking over ZSM-5at 370 "C (Less C1 and C2, more C,-Cd, while some C7+producta were detected in the product (Santilli, 1990). Cracking of Highly Branched Paraffins. The cracking of moderately branched paraffins according to the protonated cyclopropane mechanism may proceed in very much the same way as with normal paraffii if the carbon number is sufficiently high. Formation of the initial carbenium ion, which will be a tertiary one, will be easier. However, with multibranched isomers the formation of the protonated cyclopropane ring may not be possible in certain cases, for instance, when there is a succession of quaternary and tertiary carbon atoms in the backbone chain. To allow cracking by the present mechanism, preisomerization to a more suitable structure will be required, but this may be difficult if the protonated cyclopropane structure is also essential in isomerization. This may be a reason why 2,2,3-trimethylbutane and 2,2,4,6,6pentamethylheptanewere more difficult to crack than their corresponding normal isomers, as mentioned earlier (Archibald et al., 1960). The already mentioned low cracking rate of 2,2,3-trimethylpentane as compared to that of noctane (Ciapetta and Hunter, 1953) may have a similar cause. In contrast to the latter observation, 2,2,4-trimethylpentane is more reactive than n-octane in both catalytic cracking and hydrocracking (Voge, 1958; Ciapetta and Hunter, 1953; Archibald and Hunter, 1960). The product spectrum obtained from the two isomers is also quite different: whereas the product formed from n-octane contains significant amounts of propane, n-butane, isopentane, and isobutane (the producta to be expected from the protonated cyclopropane mechanism), 2,2,4-trimethylpentane cracking gives only iso-C4 in very high selectivity (Ciapetta and Hunter, 1953). The almost exclusive production of isobutane from this isooctane has also been observed by others (Vansina et al., 1983). It is in principle possible that isomerization of 2,2,4-trimethylpentane to other octane isomers is followed by the usual mode of acid-catalyzed cracking, and the occurrence of isomerization to form other tribranched, dibranched, monobranched, and even linear C8 isomers in significant concentrations besides cracking has been observed with a Pt/USY catalyst at 80-160 "C (Vansina et al., 1983). However, in view of the abnormal distribution of cracked products, it is also possible that cracking of 2,2,4-trimethylpentane represents a special case because the structure is particulary suited to &scission (cf. lower line of reaction Scheme II) without the energy barrier problems discussed earlier. According to this mode of cracking, isobutane and isobutene are the only products (hence, 100% isobutane in the case of hydrocracking) so that the cracking may be regarded as the reversal of the alkylation reaction whereby 2,2,4-trimethylpentane is formed from isobutene and isobutane. The above obmvations of hydroiaomerization/cracking of isooctane over a Pt/USY catalyst at 80-160 "C are in line with the behavior of the 2,2,4-trimethylpentyl cation in superacids at -80 "C (Brouwer and Hogeveen, 1970). This cation was found to rapidly reach equilibrium with the other trimethylpentyl ions and appears to cleave rapidly even at this low temperature to give the tert-butyl ion as the sole product. Relation between Hydroisomerization and Hydrocracking. Since hydroisomerization and hydrocracking of paraffins can occur over the same catalysts under rather similar conditions so that they can manifest themselves
Scheme VI. Mechanistic Relation between Paraffin Isomerization and Hydrocracking :H) H H H [ H ] H-~c~-c-c-c-Ic/-H ;Hjn H
n-PARhFFIN
H ;Him
H
"?I
H
H H
:H:
H
IH)
H-ICi-C-C-IC!-H m*' , [H;" 1 H \H:,,, .C\ I
,
H I H H ISOMERIZED PRODUCT
\ [Hi H H
H
H [H\
;HJn
H
H LHjm.2
H-~CI-C-C-H + H-C-C-ICI-H 1
.C\
H
H I H H
CRACUEDPROCUCTS
in one and the same experiment, it is logical to mume that there is a connection between them. Such a close relation was already recognized by Weitkamp, who speculated that the two reactions occur in series, with isomerization preceding hydrocracking (Weitkamp, 1975,1978). This sequence was deduced from the experimental finding that more severe conditions giving rise to higher conversions tend to increase the extent of cracking. As already mentioned, it has even been suggested that isomerization to di- or multibranched isomers is a prerequisite for hydrocracking (Steijns et al., 1981). According to the present hypothesis, hydroisomerization and hydrocracking do not n d y occur in the indicated sequence, but they may occur in parallel, sharing a common reaction intermediate. This is illustrated in Scheme VI. Of course, in practice sequential reactions may also take place: an isomerized paraffin may be cracked but a cracked linear hydrocarbon fragment too may be isomerized. Starting from a given paraffin and aiming at high conversions, reactions therefore follow the paths of a complex network involving parallel as well as sequential reactions. A factor of importance in this respect is the relative reactivity of feed and product paraffins as a function of branching and of carbon number. It is likely that the activation energy for the reaction pathway that leads to cracking is higher than the one which leads to isomerization from the common intermediate. This means that higher temperatures will favor cracking more than isomerization. Catalysts of very high acidity which can catalyze the carbenium ion reactions at low temperature therefore seems to offer the best prospects for maximizing the selectivity in the isomerization of paraffins.
Conclusions The classical mechanism for acid cracking as formulated by Greensfelder, Voge, Good, and Thomas assumes a 8scission to be operative in analogy with thermal cracking. It is felt that this analogy is untenable and that there are strong objections to it from a theoretical point of view. The mechanism fails to account for a large number of experimental facts. Therefore, a new mechanism has been proposed which assumes a protonated cyclopropane as a reaction intermediate, just as in the mechanism of isomerization of
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1889 paraffins. This new mechanism not only casta a new light on the relation between hydroisomerization and hydrocracking, but it is also capable of providing a logical explanation for many experimental facts which have hitherto not been explained in a satisfactory way, such as the low formation of C1and C2,the strong branching in the cracked paraffim fractions, and the strong increase of cracking reactivity with carbon numbers above 7. A more detailed and more quantitative interpretation of experimental resulta obtained in catalytic cracking as well as hydrocracking by the new mechanism will be presented in the following papers of this series.
Literature Cited API Project 44. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds; Rosehi, F. D., Pitzer, K. S., Amett, R. L., Braun, R. M., Pimentel, G. C.; Camegie Press: Pittsburgh, PA, 1953. Archibald, R. B.; Greensfelder, B. S.; Holzman, G.; Rowe, D. H. Catalytic Hydrocracking of Aliphatic Hydrocarbons. Znd. Eng. Chem. 1960,52, 745-750. Brouwer, D. M. HF-SbF5 catalysed Isomerization of n-Butane-1-13C. Recl. Trau. Chim. Pays Bas 1968,87,1435-1444. Brouwer, D. M.; Oelderik, J. M. HF-SbF5 catalysed Isomerization of 2-Methyl-pentane. Recl. Trav. Chim. Pays Bas 1968a, 87, 721-736. Brouwer, D. M.; Oelderik, J. M. Isomerization of Paraffins with HF-SbF5. Kinetics and Mechanism of the Hydride-ion Transfer and Rearrangement Steps in Alkylcarbonium Ion Reactions. American Chemical Society Division of Petroleum Chemietry, San Francisco Meeting, April 24,1968b. Brouwer, D. M.; Hogeveen, H. The Importance of Orbital Orientation as Rate-controlling Factor in Intramolecular Reactions of Carbonium Ions. Recl. Trav. Chim. Pays Bas 1970,89,211-224. Brouwer, D. M.; Hogeveen, H. Electrophilic Subtitutions at Alkanea and in Alkylcarbonium Ions. Progress in Physical Organic Chemistrv: Streitwiwr, A. Jr., Taft, R. W., Eds.; Wiley: New York, 19f2; Vol. 9, pp 197-240. Chevalier. F.: Guinet. M.: Mauzel. R.Tracer Studies of the Isomerization of Paraffii on Bifunctional Catalysts. Proceedings of the 6th International Congress on Catalysis; Bond, G.C., Wells, P. B., Tomkins, F. C., Eds.; The Chemical Society: London, 1977; VOl. 1,pp 476-487. Ciapetta, F. G.; Hunter, J. B. Isomerization of Saturated Hydrocarbons in Presence of Hydrogenation-Cracking Catalysts. Znd. Eng. Chem. 1953,45,147-165. Condon, F. E. Catalytic Isomerization of Hydrocarbons. In Catalysis; Emmett, P. H., Ed.; Reinhold New York, 1958; Vol. VI, Chapter 2, p 121. Coonradt, H. L.; Garwood, W. E. Mechanism of Hydrocracking. Ind. Eng. Chem. Process Des. Dev. 1964,3,38-45. Corma, A.; Monton, J. B.; Orchilles, A. V. Cracking of n-Heptane on a HZSM-5 Zeolite. The influence of Acidity and Pore Structure. Appl. Catal. 1985,16,59-74. Dewar, M. J. S. The Electronic Theory of Organic Chemistry; Oxford University Press: Oxford, UK, 1949; pp 211-213. Dewar, M. J. S.; Dougherty, R. C. The PMO Theory of Organic Chemistry; Plenum Press: New York, 1975; p 288. Egloff, G.; Morrell, J. C.; Thomas, L.; Bloch, H. S. The Catalytic Cracking of Aliphatic Hydrocarbons. J. Am. Chem. SOC.1939,61, 3371-3580. Flinn, R.A.; Larson, 0. A.; Beuther, H. The Mechanism of Catalytic Hydrocracking. Znd. Eng. Chem. 1960,52,153-156. Franklin, J. L. Thermodynamic Aspects. Carbonium Zons; Olah,G. A., Schleyer, P. von R.,E&.; Interscience: New York, 1968; Vol. I, p 85.
Greensfelder, B. S.; Voge, H. H.; Good, G.M. Catalytic and Thermal Cracking of Pure Hydrocarbons. Znd. Eng. Chem. 1949, 41, 2573-2584. Haag, W. C.; Derrsau, R. M. Duality of Mechanism in Acid-Catalyzed Paraffin Cracking. Proceedings of the 8th Congress on Catalysis; Verlag Chemie: Weinheim, 1984, Vol. 2, pp 11-305-11-313. Haensel, V. Catalytic Cracking of Pure Hydrocarbons. Frankenburg, W. G., Komarewsky, V. I., Rideal, E. K., E&.; Advances in Catalysis 3; Academic Press: New York, 1951; pp 179-197. Hanaford, R. C. Development of the Theory of Catalytic Cracking. Davis, B. H., Hettinger, W. P., Jr., Us.; ACS Symposium Series 222; American Chemical Society: Washington, DC, 1983; p 252. Kossiakoff,A.; Rice, F. 0. Thermal Decomposition of Hydrocarbons, Resonance Stabilization and Isomerization of Free Radicals. J. Am. Chem. SOC.1943,65,590-595. Langlois, G. E.; Sullivan, R. F. Chemistry of Hydrocracking. Symposium on Refining Petroleum for Chemicals. American Chemical Society, Division of Petroleum Chemistry New York City Meeting, Sept 7-12, 1969; Preprints Vol. 14, No. 3, pp 18-39. Nace, D. M., Catalytic Cracking over Crystalline Aluminwilicatea. Znd. Eng. Chem. Prod. Res. Dev. 1969,8, 24-38. Nenitzescu, C. D. Aromatically stabilized Cyclic Cations. In Carbonium ZOIU); Olah,G. A., Schleyer, P. von R.,E&.; Interscience: New York, 1968; Vol I. pp 19-20. Nonhebel, D. C.; Walton, J. C. Free-radical Chemistry; University Press: Cambridge, 1974; p 103. Rice, F. 0.; Teller, E. The Role of Free Radicals in Elementary Organic Reactions. J. Chem. Phys. 1938,6,489-496. Rice, F. 0.; Teller, E. Corrections to Paper 'The Role of Free Radicals in Organic Reactions". J. Chem. Phys. 1939, 7, 199. Roberta, J. D.; Lee, C. C.; Saunders, W. H., Jr. Rearrangements in Carbonium Ion-Type Reactions of Cl4-Labelled Norbornyl Derivatives. J. Am. Chem. SOC.1954, 76,4501-4510. Rylander, P. N.; Meyerson, S. Organic Ions in the Gas Phase. I. The Cationated Cyclopropane Ring. J. Am. Chem. SOC.1956, 78, 5799-5802. Santilli, D. S. Mechanism of Hexane Cracking in ZSM-5. Appl. Catal. 1990,60,137-141. Schulz, H. F.; Weitkamp, J. H. Hydrocracking and Hydroisomerization of n-Dodecane. Ind. Eng. Chem. R o d . Res. Dev. 1972,11,46-53. Steijns, M.; Froment, G.;Jacobs, P.; Uytterhoeven, J.; Weitkamp, J. Hydrohmerization and Hydrocracking. 2. Product Distributions from n-Decane and n-Dodecane. Znd. Eng. Chem. Prod. Res. Dev. 1981,20,654-660. Thomas, C. L. Chemistry of Cracking Catalysts. Znd. Eng. Chem. 1949,41,2564-2573. Vansina, H.; Baltanas, M. A.; Froment, G. F. Hydroisomerization and Hydrocracking. 4. Product Distribution from n-Octane and 2,2,4-trimethylpentane. Znd. Eng. Chem. R o d . Res. Dev. 1983, 22,526-531. Voge, H. H. Catalytic Cracking. In Catalysis; Emmett, P. H., Ed.; Reinhold New York, 1958; Vol. VI, Chapter 5, pp 407492. Weitkamp, J. The Influence of C h i n Length in Hydrocracking and Hydroisomerization of n-Alkanes. Ward, J. W., Qader, S. A., Ed.; ACS Sympoeium Series 20; American Chemical Society: Washington, DC, 1975; pp 1-27. Weitkamp, J. Hydrocracken, Cracken und Isomerisieren von Kohlenwaeeerstoffen. Erdoel Kohle, Erdgas, Petrochem. 1978, 31, 13-22. Weitkamp, J. Isomerization of Long-chain n-Alkanes on a Pt/CaY Zeolite Catalyst. Znd. Eng. Chem. Prod. Res. Dev. 1982, 21, 56&558.
Wielers, A. F. H.; Vaarkamp, M.; Post, M. F. M. Relation between Properties and Performance of Zeolites in Paraffin Cracking. J. Catal. 1991,127, 51-66.
Received for reuiew July 1, 1991 Revised manuscript received March 4, 1992 Accepted April 13, 1992
