Energy & Fuels 1994,8, 1394-1400
1394
Catalytic Hydrocracking Reaction Pathways, Kinetics, and Mechanisms of n-Alkylbenzenes Carlonda L. Russell and Michael T. Klein* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Richard J. Quann and Jeffrey Trewella Mobil Research and Development Corporation, Paulsboro, New Jersey 08066 Received June 23, 1994@
The hydrocracking reaction pathways and kinetics of alkyl aromatic structural elements present in typical petroleum feedstocks were probed through experiments at 350 "C and 1000 psig of hydrogen pressure with a Shell NiW/USY zeolite catalyst. Experimental results for C4-Cl5 alkylbenzenes provided insight into their respective alkane-like and aromatic-like character, as well as the effect of side-chain length on the kinetics and product selectivities. Initial product selectivities indicated that the expected prototypical ring dealkylation to benzene was accompanied by other significant reactions. Reactions in the side chain produced various shorter-chain alkylbenzenes, including toluene. Ring closure of the side chain was also significant, producing hydroaromatic bicyclic compounds. The overall reaction rate was first order and generally increased with increasing alkyl side-chain length.
Introduction Catalytic hydrocracking is used extensively in petroleum-refining processes to produce high-quality gasoline, diesel and jet fuels, and lubricants from petroleum stocks such as residua, cycle oils and gas oils. Conversion of aromatics in these feeds yields lower molecular weight species with increased WC ratios. This task requires the catalyst to be dual-functional, having a metal component for hydrogenation and an acid component for cracking. While hydrocracking catalysts will often have very strong acid and moderate hydrogenation functions, the relative strengths can be varied to produce desired product spectra. In general, amorphous cracking catalysts are used in processes where heteroatom removal and aromatic hydrogenation are most desired, while zeolite-containing catalysts are used for significant molecular weight reduction. Both short- and long-chain alkyl aromatics are prevalent in virgin feeds to hydr0crackers.l The literature reflects the careful attention paid to short-chain aromatics, such as C1-C3 alkylbenzenes.2 These studies have helped develop the presupposition that alkyl aromatics primarily undergo dealkylation at the ring, yielding benzene and the corresponding paraffin product. Much less attention has been devoted t o the hydrocracking reaction pathways and kinetics of long-chain alkyl aromatics. The hydrocracking of n-decylbenzene over a mild cracking catalyst, NiW/SizOd203, revealed
* To whom all correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, October 1, 1994. (1)Sullivan, R. F., Boduszynski, M. M., Fetzer, J. C. Molecular Transformation in Hydrotreating and Hydrocracking. Energy Fuels, 1989,3,603-612. (2) Mochida, I.; Yoneda, Y. Linear Free Energy Relationships in Heterogeneous Catalysis: Dealkylation of Alkylbenzene on Cracking Catalysts. J. Catal. 1964,3,386-392. 0887-0624/94/2508-1394$04.50/0
side-chain cyclization, producing bicyclic compound^.^ It was speculated that this type of reaction was responsible for PNA buildup during processing of residuumderived vacuum gas oi1s.l More recently, FCC-motivated studies (PH,= 0) of the catalytic cracking of n-heptylbenzene revealed evidence of both side-chain dealkylation and cyclization.* This comprehensive study provided selectivities and proposed mechanisms for catalytic cracking, but clearly the roles of hydrogen and a metal component in the catalysts were not addressed. In short, the literature provides a vital foundation for future work aimed at extending both the length of the alkyl side chains and the effect of the catalyst metal function and process hydrogen. This motivated the present work aimed at the determination of the effects of alkyl side-chain length on the primary pathways and kinetics of C4-C15 alkyl benzene hydrocracking.
Experimental Section Materials. Ethylbenzene and n-butylbenzene (Aldrich Chemical, 99%), n-nonylbenzene (Alfa Chemicals, 97%), npentylbenzene, n-heptylbenzene, and n-pentadecylbenzene (Wiley Organics, 97%),and n-dodecylbenzene (TCI, 98%), 1000 psig of hydrogen (Matheson Gas Products), and hydrogen sulfide (Matheson Gas Products, 90% N&O% H2S) were all obtained from commercial sources and used as received. All reactants and purities were confirmed by mass spectroscopy. The Shell NiW/USY zeolite catalyst was provided by Mobil Research and Development Corp. (3) Sullivan, R. F.; Egan, C. J.; Langlois, G. E. Hydrocracking of Alkylbenzenes and Polycyclic Aromatic Hydrocarbons on Acidic Catalysts. Evidence for Cyclization of the Side Chains. J . Catal. 1964,3, 183-195. (4)Coma, A Miguel, P. J.; Orchilles, A. V.; Koermer, G. S. Cracking of Long-chain Alkyl Aromatics on USY Zeolite Catalysts. J . Catal. 1992,135,45-59.
0 1994 American Chemical Society
Energy & Fuels, Vol. 8, No. 6, 1994 1395
Hydrocracking Pathway of ndlkylbenzenes The catalyst, received in pellet form, was sulfided under a stream of 10 w t % H2S in nitrogen for 2 hours at 404 "C (760 OF). The catalyst was sulfided in one gram batches, and then mixed together t o ensure uniformity. The pellets were then crushed and sieved to a very fine mesh to reduce any possible mass-transfer limitations. The crushed catalyst was stored under cyclohexane in a nitrogen atmosphere t o prevent air contamination. Reactions. The reactions were carried out in a 1 L batch autoclave. The autoclave allowed for reactant injection at time zero, periodic liquid sampling, and precise temperature and pressure measurements. Cyclopentane was chosen as the solvent because its cracking would not hinder product identification. The reactor was loaded with 500 cm3of cyclopentane, 10 g of catalyst, and 1 g of carbon disulfide. After loading, the reactor was purged of air with hydrogen and heated to reaction temperature. An initial reactor loading of 500 cm3 cyclopentane and 100 psi of hydrogen resulted in a total pressure of 1800 psig at 350 "C. In the loading device, 20 g of reactant was pressurized with hydrogen to provide driving force t o ensure rapid injection of contents and simultaneous pressurization of the reactor to a total of 2800 psig. Microbatch reactors of 12 cm3 capacity were also used to complement the autoclave data. The microbatch reactors permitted study of "neat" reactions, Le., reactions in the absence of the cyclopentane solvent. The microreactor was loaded with the reactant (between0.1 and 0.684 g) and catalyst according to a reactant-to-catalyst ratio of approximately 10. A 1/2 cm diameter steel ball was added to aid in the mixing. Carbon disulfide (50pL) was added t o ensure that the catalyst remained in the sulfide form throughout the reaction. The reactor was pressurized with 1100 psig of hydrogen at room temperature. The reactor was vertically shaken for 30 s and then submerged into a fluidized sandbath for the reaction duration ranging from 10 to 90 min. After the desired reaction time had elapsed, the reactor was removed from the sand bath, immobilized, and submerged into an ice bath to quench the reaction. After a sufficient cooling period, the hydrogen was released from the microbatch reactor. A precisely measured external standard, biphenyl, was then carefully added to the reactor, and the entire contents were collected in methylene chloride. The catalyst was allowed t o settle and the sample then prepared for analysis. In all cases, the microbatch reactor trends are consistent with autoclave data. Thermal cracking experiments were carried out at the same reaction conditions, minus only the catalyst, to determine the extent of background pyrolysis. Analytical Chemistry. Product identification was by GCMS using a Hewlett-Packard 5890 instrument and, when possible, by coinjection of the products in a Hewlett Packard 5880A gas chromatograph. The GC was equipped with flame ionization detector and a 60 ft HP silica gel capillary column (id. 0.1 cm). Response factors, which measured the extent of the linear relationship between the weight ratios and the area ratios of the product to standard, were determined frequently for the reactant and products using biphenyl as an external standard. The response factor for all products was 1.00 f 0.05. The response factors for products which could not be obtained were therefore estimated t o be equal to 1.0. Product selectivities were determined by dividing the products' molar yields by the conversion of the reactant.
Results and Discussion Thermal Cracking. Experiments with n-pentadecylbenzene (PDB) at the reaction conditions listed above, except for the absence of the catalyst, revealed the role of background thermal cracking. A PDB conversion of -2% was observed after 90 min; as developed below, this pyrolysis conversion amounted to less than 3% of
c1
iE c1
.o 1
-1
1
0)
2a
2
Initial Pentadecylbenzene Concentration moVL
Figure 1. Elucidation of long-chain alkylbenzene reaction
order.
E
8
0.0021
al
A
CI
m
K
,B
A
A A
A
alw
'I! 9 0.001
0=
.-
'L-5E Q,
6
0.000
A
L 0
2
4
6
8 10 12 14 16
Carbon Number in Side-Chain
Figure 2. Effects of alkyl side-chainlength on overall reaction kinetics of alkylbenzene hydrocracking.
the hydrocracking conversion over the reaction time range of interest. The molar yield of the major thermal cracking product, toluene, at these reaction conditions was less than 2% of the toluene formation from the hydrocracking reaction. Thermal cracking was therefore subsequently neglected. Kinetics of Reactant Conversion. The overall reaction order for PDB hydrocracking was deduced by following the dependence of the pseudo-first-order rate constant on the initial reactant concentration. To this end, several hydrocracking experiments were conducted where the initial concentration of PDB ranged from 0.03 to 0.20 mom. The reaction order is contained in the slope of the log-log plot of the apparent first-order rate constant versus initial reactant concentration in Figure 1. The least-squares fit of 0.0002 for the slope = 1- n, where n is the reaction order, indicates that the overall reaction is essentially first order over the concentration range examined. The effect of side-chain length on the overall kinetics of alkylbenzene hydrocracking is shown in Figure 2. The kinetics for ethylbenzene (EB), butylbenzene (BB), pentylbenzene (PB), heptylbenzene (HB), nonylbenzene (NB), dodecylbenzene (DDB), and pentadecylbenzene (PDB) reveal a trend where the pseudo-first-order rate constant generally increases with increasing side-chain length. This increase is somewhat linear from EB to HB. The increase subsides at NB. Indeed, the pseudofirst-order rate constants for DDB and PDB are slightly lower than those for HB and NB. The details of the product distribution provide interpretation clues. These are summarized in Figure 3ac, which are plots of selectivity versus conversion for
1396 Energy &Fuels, Vol. 8, No. 6, 1994 A
z
0 0
Russell et al. Table 1. Major Products and Selectivities from C4-Cl5 Benzene Hydrocracking at 40%Conversion
Nonylbenzene Dodecylbenzene Pentadecylbenzene
0.2
1
0.0
0.7
0.2 0.4 0.6 0.8
1.0
Converslon
.
A
o+r 3e 0.6 :
-
0.5 :
on
0.4:
2z
0.2
0 0
Nonylbenzene Dodecylbenzene Pentadecylbenzene
A
-
0.0
0.2
0.4 0.6
0.8
1.0
Converslon 1
E
0.7
A 0 0
.-
-b: c
d
0.62
0.35
0.38
toluene
0.10
0.07
0.08
ethylbenzene
0.04
0.08
0.06
Nonylbenzene Dodecylbenzene Pentadecylbenzene
0.4
u.v
0.0
benzene
0
a
0-
propylbenzene
-0
0.003
0.002
butylbenzene
-
0.05
0.04
tetralin
0.03
0.16
0.12
indane
0.03
0.14
0.11
1-methylindane
0.12
0.03
0.03
&
0.05
0.02
0.02
0 8 *o&
0 o .: o ~ ” ~ ” ’ ‘ l ’
0 0 A! 0 0.1
5
a c 1 5 H 3 1
products
s 1 . , , , , 1 , ) , 1I ($ 0.0
d
a C g H l 9
0.6
0.2
0.4 0.6
0.8
1.0
Conversion
0-
m t D
2-methylindane
Figure 3. (a) Selectivity of benzene for NB, DDB, and PDB hydrocracking. (b) Selectivity of t h e total short-chain alkyl aromatic products from NB, DDB, a n d PDB hydrocracking. (c) Selectivity t o t h e total ring-closure products i n NB, DDB, a n d PDB.
the product classes of NB, DDB and PDB. Figure 3a shows that NB had a slightly lower selectivity to benzene than did DDB or PDB. On the other hand, Figure 3b shows that the selectivity to the total shortchain alkyl-aromatic products were essentially equal. The most significant difference was in the selectivity to the total ring closure products. The nearly equal selectivities observed from DDB and PDB were less than observed from NB. It thus seems likely that a drop in ring closure rate for DDB and PDB caused their overall rate to drop below the values for HB and NB. Steric hindrance brought about by the proximity of the catalyst pore walls is a likely cause for the decrease in closure reaction rates for the longer-chain molecules. Hydrocracking Products: General Trends. Butylbenzene (BB), nonylbenzene (NB), and pentadecylbenzene (PDB) were selected as representative and therefore subjected to detailed study of their product spectra. The major products from their hydrocracking reactions are summarized in Table 1. For all three reactants, the product in highest selectivity was benzene. This provided evidence of the dominance of acid (carbenium ion) cracking chemistry. The benzene selectivity at 40% conversion was 0.62, 0.34, and 0.37 for BB, NB, and PDB, respectively. The higher selectivity to benzene for BB reflects its dearth of reactivity options relative to the other reactants. Self-alkylation reactions produced the several bicyclic
compounds listed in Table 1. Methylindanes were the most abundant ring closure products for BB. Tetralin and indane were the major ring closure products formed from NB and PDB. Reactions in the side chain produced short-chain alkylbenzenes, such as toluene and ethylbenzene. Selectivities to toluene were 0.1, 0.07, and 0.08 for BB, NB, and PDB, respectively. The selectivity to ethylbenzene was 0.05 f 0.03 for all three reactants. A variety of minor products also formed. The longchain paraffins, pentadecane and tetradecane, evolved from PDB, and nonane evolved from NB. However, the selectivities for the ostensibly complementary pairs (e.g., pentadecane and benzene from PDB) were not equivalent, with the aromatic always found in higher yields. Pentadecane most likely underwent appreciable secondary reactions. Cracking in the side chain of NB and PDB t o form long chain (’(24) alkylbenzenes was also operative. For instance, NB yielded pentyl- to octylbenzenes (s I0.008) and PDB gave pentyl- to nonylbenzenes (s I 0.006). Several pentanes and hexanes were identified strictly by the molecular weight from mass spectroscopy. Their fragmentation patterns did not allow for confident assignment of the structure of any individual isomer. Methylcyclopentane and cyclohexane were among these products. The other C5s and C6s were most likely straight-chain and branched parafins that resulted from cracking and isomerization reactions of the longer straight-chain dealkylation products.
Energy & Fuels, Vol. 8, No. 6, 1994 1397
Hydrocracking Pathway of ndlkylbenzenes
I
1.2
,
0.2
100 Tlme, min
0
200
3
0.06
0)
s
0.04
-2
Q
P
200
0.08
Q)
-
100
Tlme, min
3 0.06 L
1
0
0.08
F
,
P
0.02
0.04 0.02 0.00
0.00 100 Time, mln
0
200
0
-Tlme, mln 100
200
Figure 4. (a-d) Nonylbenzene hydrocracking molar yields. Scheme 1. Apparent Side-ChainMigration Isomers of n-AlkylbenzeneHydrocracking.
r
h e&
e &-k+=
*&
Table 2. Liquid Product Recovery Indices as an Indication of the Loss of Identifiable Products as Gases
av product recovery index (%) av aromatic ring balance (%I
93.5
96.4
79.0
97.0
105.0
116.0
BB hydrocracking also appeared to involve minor alkylation of the side chain to form pentyl-, hexyl-, heptyl-, and octylbenzenes. The highest yield that any of these products achieved was 0.006, initially for pentylbenzene, 0.005 for heptylbenzene, and 0.0006 for octylbenzene. The pentyl- to octylbenzenes were potential contributors to other observed products such as benzene, toluene, and tetralin. However, the concentration of these species was so low that their contribution was negligible over the reaction time. Several isomers of NB and PDB were also identified. Their mass spectra were consistent with apparent sidechain migration, as in Scheme 1. The yields of these isomers increased and then became constant over the course of the reaction. The initial selectivities for the individual isomers were comparatively low, at values (0.01. The side chain for BB also isomerized to give 1-methylpropylbenzene at an initial selectivity of 0.012. These isomerization reactions summarized by eq 1could be an important first step in the overall ring dealkylation, since they would allow p-scission to produce a secondary alkylcarbenium ion. The liquid product recovery indices (PRI), defined as the total weight of identified products divided by the initial reactant loading, are shown in Table 2 for BB, NB, and PDB. The PRI for BB hydrocracking was consistently high with an average of 93.5% closure. The PRI range for NB was from 96.8 t o 80%with an average
of 90.3%. The PDB PRI started high at 96.4% for early reaction times but steadily decreased over time to 64.3%, giving an average of 79%. The average aromatic ring balances for BB, NB, and PDB were 97,105, and 116%, respectively. These high numbers suggest that aromatic coke contributed very little to the unmeasurable material. It thus seems reasonable that most of the unidentified mass went to the formation of gases. Note that since the gases would be a direct result from alkyl side-chain cracking, it is consistent that PDB would produce more gases due to its longer chain. Pathway Analysis. The reaction path analysis of the representative reactant, nonylbenzene, serves to illustrate the qualitative network behavior of all of the alkylbenzenes studied, whose kinetics differed only in the values of rate parameters discerned. Figure 4a-d summarize the initial rates. The rate of reactant disappearance of Figure 4a is matched to within 99% closure by the initial rates of product formation. Benzene formed most rapidly, followed in descending order of initial rate by the ring closure products, tetralin and indane. 1-and 2-Methylindane were much less abundant. The short-chain alkylbenzenes, toluene and ethylbenzene, appeared with comparatively smaller initial rates, but the yields grew to a point where they were significant products. The Delplot analysis5 sorted products according to their rank, i.e., order of appearance in the network. A nonzero y intercept in a plot of selectivity versus conversion is found for primary products, while a zero intercept is indicative of secondary or higher products. The Delplot of Figure 5a shows that benzene was a clear primary product of NB hydrocracking. Figure 5b reveals that tetralin and indane were primary products that underwent secondary reaction. Both methylindane isomers were minor primary products. Figure 5c reveals that the short-chain alkylbenzenes, toluene, ethylbenzene, and butylbenzene, were also primary prod( 5 ) Bhore, N. A,; Klein, M. T.; Bischoff, K. B. The Delplot Technique: a New Method for Reaction Pathway Analysis. Ind. Eng. Chem. Res. 1990, 29,313.
Russell et al.
1398 Energy & Fuels, Vol. 8, No. 6, 1994
10.6 0.5
1 e
5 0.3
:::I.
.
,
I
,
,
_I
Benzene
I
,
0.0 0.0 0.2 0.4 0.6 0.8 1.0
Figure 6. Proposed reaction network for n-alkylbenzene
Conversion
hydrocracking. Rate constant values are given in Table 2.
(4
A A secondary, n - a l k a n s n-alkene-n-carbenium ion
0.2 x
;0.1 -
A A A
A A
A
0 : : 0 0
A A 0
f 0
*.a
.
I
0.
A A
A
0
Toluene Ethylbenzene Propyl benzene Butyl benzene
6 I
tertiary ‘I i’carbenium ion
\u f
Cracked products
Figure 7. Example of alkane hydrocracking mechanism inferred by Coonradt and Garwood (1964).
e
Table 3. First-OrderRate Constant Values from Network in Figure 6 (k x los) k BB (n = 4) NB (n = 9) PDB ( n = 15)
0.3 I >. 0.21: x,
1 A *: :
,
*,
lBJ
A 0
0.1
Tetralin lndan 2-Methyl indan 1-Methyl indan
99 o f 2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion
(c)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 Amin)
4.55 0.96 0.41 -
0.31 1.70 0.56 0.24 21.1 27.6 12.2 2.7E-1
6.88 1.39 0.40 0.30 3.72 0.65 0.45 3.19 19.4 2.3 3.24 1.78 1.38 1.17 1.3E-0
5.32 0.78 0.44 1.01 1.98 -0 -0 1.11 17.8 11.7 16.4 0.21 4.83 2.85 1.3E-0
Figure 5. Delplots for nonylbenzene hydrocracking.
ucts. Note, however, the increase in toluene selectivity with conversion, which is characteristic of a secondary formation route. That is, toluene was likely both primary and secondary. Finally, the zero intercept for propylbenzene suggests it was a minor product of secondary or higher reactions. The increase in selectivity to toluene was attributed to secondary reactions. A likely source of these reactions likely involved the reaction of butylbenzene, which reacts further to toluene and ethylbenzene. Table 1 confirms that toluene and ethylbenzene were indeed products from reaction of butylbenzene. The ringclosure products, tetralin and indane, likely contributed as well. Both tetralin and methylindane participate in ring-opening reactions, forming butylbenzene.6 Thus, the pathways comprising the reactions of ring opening products to butylbenzene, which reacts further to toluene, ethylbenzene, and benzene, also belong in the network. The foregoing reaction path information is summarized in the overall network of Figure 6 . The reactant has primary pathways to benzene, Cl-C4 alkylbenzenes, tetralin, indane, and 1-and 2-methylindanes. Significant secondary reactions include the reactions of the key intermediate, butylbenzene, to (6) Korre, S. Ph.D. Thesis, University of Delaware, 1993.
produce benzene, toluene, ethylbenzene, tetralin, indane, and 1-and 2-methylindanes. Quantitative kinetics analysis used the network of Figure 6 and the data of Figure 4 to obtain the rate constants of Figure 6 . The rate constants for the reactant disappearance and product formation were obtained by minimizing a n objective function that was the sum of the square of the difference of the experimental and calculated yields. The MLSL algorithm was used to optimize the parameter^.^ Table 3 summarizes the kinetics of each step for each reactant. The model fit for NB appears as the solid lines in Figures 4a-d. The parameters provide an excellent model fit for benzene and all of the ring-closure products. Mechanisms. Acid-catalyzed cracking of hydrocarbons such as those described in this study involves the chemistry of carbenium ions. A possible mechanism leading to the products observed involves the elementary steps of proton attack on the aromatic ring followed by dealkylation of a n alkylcarbenium ion, whose subsequent deprotonation to an olefin completes the acid catalyzed cycle;s the olefin is hydrogenated quickly at the conditions employed here. In parallel to this dealkylation route, the general hydrocracking mecha(7) Stark, S. Ph.D. Thesis, University of Delaware, 1993. (8) Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions; Academic Press, Inc.: New York, 1981.
Energy & Fuels, Vol. 8, No. 6, 1994 1399
Hydrocracking Pathway of ndlkylbenzenes
W
-u
--
-
wL-u+
@
Cracked Products
Figure 8. Mechanism for benzene formation.
0Figure 9. Proposed hydrocracking mechanism for toluene and ethylbenzene formation,
+ Re +RHt
e R"+v+RH++-~
Re + v + R H + *
S+H2 ---
+-+*-
P, CY++
+RH
+
Isom.
+Cracked Products Figure 10. Proposed hydride abstraction to form toluene and ethylbenzene.
nism developed by9 for n-alkanes is also likely. This includes the dehydrogenation of the side chain t o form olefinic intermediates that are in turn subject t o protonation, yielding carbenium ions that undergo isomerization, hydrogen transfer and ,!?-scissionreactions. The essence of this scheme is shown in Figure 7. Application of these concepts to the alkylbenzenes studied here explains the observed product spectra well. The data and thermodynamical considerations suggest that side-chain dealkylation was likely preceded by alkyl side-chain isomerization, such as that shown in Scheme 1. Proton attack at the ring, followed by ,!?-scission, would thus lead to benzene and a secondary alkylcarbenium ion. This carbenium ion could then abstract a hydride from another molecule, form as an olefin by proton elimination, or isomerize and undergo ,!?-scission to form an olefin and another carbenium ion. Olefins would be hydrogenated as result of high hydrogen pressures. This scheme is summarized in Figure 8. Two parallel mechanisms can lead to the formation of the alkylbenzene products: the traditional hydrocracking mechanism involving dehydrogenation to olefinic intermediates and a cycle involving hydride abstraction by previously formed carbenium ions. Figures 9 and 10 illustrate these alternatives. The elementary steps in Figure 9 begin with the dehydrogenation of the alkyl side chain to produce an olefinic intermediate. The (9)Coonradt, H.L.;Ganvood, W. E. Mechanism of Hydrocracking: Reactions of Paraffins and Olefins. Ind. Eng. Chem. Process Des. Deu. 1964,3,38.
double bond is then protonated to produce a carbenium ion, which can participate in hydride transfer or undergo /3-scission to produce another carbenium ion and an olefin. The identity of the species that retains the ionic charge depends on the energetics of the reaction. For example, the ,!?-scissionin Figure 9a produces a benzylic carbenium ion and is thus more favorable than the /3-scission producing a methylcarbenium ion, and after hydrogenation of the olefin intermediate, butylbenzene. The subsequent formation of toluene requires hydride abstraction by the benzylic carbenium ion. The only significant p-scission step in Figure 9b leads to the formation of styrene (ultimately ethylbenzene) and an alkylcarbenium ion. Note that the formation of a primary carbenium ion in Figure 9b may not be literal. There are other views that provide different possibilities. For example, WeitkamplO has suggested the notion of a concerted hydride shift with the ,!?-scission. Alternatively, a prior isomerization in the side chain would allow for the formation of a secondary carbenium ion after scission. Another possibility for the ,!?-scission includes prior nonclassical cyclopropane intermediates, which have been proposed for alkane cracking.l' The second type of mechanism for toluene and ethyl benzene formation is illustrated in Figure 10. Hydride (10)Weitkamp, J. The influence of Chain Length in Hydrocracking and Hydroisomerization of n-Alkanes. Hydrocracking and Hydrotreating; ACS Symposium Series; American Chemical Society: Washington, DC, 1975;Vol. 20,p 28. (11)Sie, S. T.Acid-Catalyzed Cracking of Paraffinic Hydrocarbons. Ind. Eng. Chem. Res. 1992,31,1881-1889.
1400 Energy & Fuels, Vol. 8, No. 6,1994
Figure 11. (a) Elementary steps for butylbenzene ring closure. (b) Elementary steps for pentylbenzene ring closure. (c) Elementary steps for ring closure followed by dealkylation for reactants with n z 3.
transfer between an existing carbenium ion, R+, and a reactant molecule gives a reactant-derived carbenium ion and R-H. For the long-chain molecules, essentially all of the hydrogens on the chain are possible candidates for abstraction. Thermochemically, the hydrogen on the a-carbon is most favored, whereas the hydrogens on the terminal carbon are least favored. Note the slight positive reaction path degeneracy contributions to the still slow reaction at the terminal carbon. Formation of the ion at the a-position followed by p-scission results in styrene (ethylbenzene) and an alkylcarbenium ion. Hydride abstraction from the y-position is more significant than that at the remaining 13 carbon positions because its p-scission leads to the formation of the stabilized benzylcarbenium ion. This rationalization is consistent with toluene and ethylbenzene having the highest selectivities among the alkylbenzene products, the balance of which does not enjoy the stability of the benzylic carbenium ion at any point in the elementary step formation sequence. The observed selectivities to the ring-closure products tracked the ability of carbenium ions to form in the side chain. This suggests that self-alkylation of the ring through side-chain carbenium ions was the dominant mechanism. Formation of an indane compound requires location of the carbenium ion at the y-position. In contrast, formation of a tetralin compound specifies the 6 position for location of the carbenium ion. The length of the side chain for each reactant influenced the associated ability to form these required carbenium ions. Thus, during reaction of butylbenzene, the carbenium ion formed at the y-position with high selectivity because the d-position would lead t o an unstable primary carbenium ion. Subsequent ring closure, hydrogen rearrangement to give a very stable tertiary carbenium ion, and deprotonation to methylindane complete the mechanism shown in Figure l l a . The d-position methyl group remains on the methylindane molecule because its unfavorable dealkylation would
Russell et al. result in the formation of a highly unstable methylcarbenium ion. In short, Figure l l a serves to rationalize why butylbenzene ring closure resulted in methylindane and very little tetralin or indane. The foregoing reasoning can be applied to the higher alkylbenzenes as well. Carbenium ion formation at the y-position of pentylbenzene leads to ethylindane, while formation at the d-position leads to methyltetralin. Both of these routes involve comparatively stable secondary carbenium ions, as shown in Figure l l b . The pathways for the C7 to CIS alkyl benzenes involved one additional step made possible by the “excess” chain length beyond that required for the indane or tetralin ring itself. For n > 3 in Figure l l c , thermodynamic considerations drive ring closure to be followed by dealkylation. For example, the carbenium ion formed at the d-position in Figure l l c closes to give the intermediate cyclic carbenium ion. Competitive with deprotonation to afford the a-alkyltetralin, a 1,3 hydride shift makes possible a subsequent p-scission of the “excess” alkyl group to afford isotetralin, which undergoes rapid isomerization t o tetralin, and the corresponding alkylcarbenium ion. Evidently an alkyl group with at least three carbon atoms can sustain the carbenium ion and make the kinetics of this pathway dominant. This rationalizes the observed formation of unsubstituted tetralin and indan for C, to CIS alkylbenzenes.
Conclusions 1. Long-chain alkylbenzene hydrocracking comprises three reaction families: ring dealkylation to form benzene, reactions in the side chain to form short-chain alkyl aromatics, and ring closure, resulting in hydroaromatic bicyclic compounds. 2. The overall reactivity of alkylbenzene hydrocracking is affected by the length of the side chain. The rate increases as side-chain length increases to a carbon number of about nine. The slight decrease in apparent rate constant for reactant molecules larger than CSalkylbenzene appears to be due to the increasing steric hindrance difficulty for the ring-closure reaction to occur with the longer-chain molecules. 3. The most significant effect of side-chain length was on the selectivity to and within the ring-closure reaction family. Specifically, ring closure of butylbenzene resulted in methylindanes, whereas ring closure of pentylbenzene resulted in methyltetralins and ethylindane. In contrast, the C ~ - C I alkylbenzenes ~ gave unsubstituted tetralin and indane. These product distributions were directed by the availability and pathways of comparatively stable secondary carbenium ion reactions.
Acknowledgment. We thank D. Kalaygian for his help in conducting experiments. The authors acknowledge the financial support of Mobil Research and Development Corp. (Paulsboro) and the State of Delaware, as authorized by the State Budget Act of Fiscal years 1990-93.
