On the Limitations To Establish the Contribution of the Different

3400

Ind. Eng. Chem. Res. 1997, 36, 3400-3415

On the Limitations To Establish the Contribution of the Different Reaction Mechanisms from Selectivity Data, During Cracking of Long-Chain Linear Paraffins Avelino Corma,*,† Pablo J. Miguel,‡ and Antoni V. Orchille´ s‡ Instituto de Tecnologıá Quı´mica, UPV-CSIC, Universidad Polite´ cnica de Vale` ncia, Avenida de los Naranjos s/n, 46022 Vale` ncia, Spain, and Departament d’Enginyeria Quı´mica, Universidad de Vale` ncia, Dr. Molinear 50, 46100 Burjassot, Vale` ncia, Spain

n-Decane, n-dodecane, and n-tetradecane have been cracked at very short time on stream on ZSM-5, BETA, and USY zeolites. It is shown that different reaction schemes can be used to explain the initial selectivity values obtained for the different products. In all cases an “uncertainty band” was found which makes it impossible to determine with accuracy the separated contribution of the mono- and bimolecular cracking to the overall cracking. Nevertheless, it has been possible to establish the selectivities for the hydrogen transfer reactions. Finally, the influence of the zeolite structure, chain length, time on stream, and level of conversion on the product distribution and the extent of the reactions involved has been studied. Introduction Recent studies have shown that the paraffin cracking occurs through two mechanisms well differentiated (Haag and Dessau, 1984; Corma et al., 1985; Lombardo and Hall, 1988; Abbot and Wojciechowski, 1989; Lukyanov et al., 1994; Wielers et al., 1991). The first mechanism (A) is bimolecular in nature and involves a hydride transfer step from a neutral molecule to a surface carbenium ion type molecule to form the adsorbed carbenium ion of the former, followed by its β-scission:

RH + R1+ f R+ + R1H R+ f reaction product The second mechanism (B) is monomolecular in nature and involves the attack of a proton on a C-C bond of the paraffin, forming a pentacoordinated carbocation, and this will crack, giving a paraffin and the complementary adsorbed carbenium ion:

RH + H+ f RH2+ f R1+ + R2H In the first attempts (Haag and Dessau, 1984; Wielers et al., 1991) the contribution of mechanisms A and B was established from the selectivity to branched paraffin products which were considered to be only produced from mechanism A and from those considered to come from mechanism B: H2, CH4, C2H6. We consider that this was only a first approximation to the problem since mechanism A does not always produce branched products and mechanism B also produces paraffins other than CH4 and C2H6. More recently (Bamwenda et al., 1994a; 1995; Zhao et al., 1993), a full reaction network for paraffin cracking has been constructed, and from this the contribution of mechanisms A and B for the cracking of C6 and C7 paraffins has been established by means * To whom correspondence should be addressed. Telephone: 34 6 3877800. Fax: 34 6 3877809. e-mail: itq@ upvnet.upv.es. † Universidad Polite ´ cnica de Vale`ncia. ‡ Universidad de Vale ` ncia. S0888-5885(96)00730-0 CCC: $14.00

of the mass balance equations and the initial selectivity results obtained for different reaction products. While in a first approximation one may conclude that this is a definitive work, it is necessary to question this since more than one network may exist, which can explain the selectivity results. Furthermore, it also has to be considered that if a true reaction step is not independent with respect to the rest of the reactions of the network, a system of equations will always be obtained, wherein the matrix of the coefficients is singular (its determinant cancels out) and consequently it has no solution. In a previous work (Corma et al., 1996b), by using the initial selectivities and following the suggestion of Smith and Missen (1979), we constructed potential reaction networks for n-heptane cracking, wherein all the steps or chemical reactions were independent. It appears that, among all, several reaction networks were possible, and they were giving different contributions of the mono- and bimolecular cracking mechanism. In this work, we attempt to establish a general methodology for discussing the contribution of the different cracking mechanisms on long-chain n-paraffins, and from that we attempt to discuss the effect of the hydrocarbon chain length, level of conversion, nature of the catalyst, and time on stream at which the reaction is carried out. Experimental Section Materials. n-Decane, n-dodecane, and n-tetradecane (99+%) were obtained from Aldrich and were used without further purification. N2 (99.999% purity) was used as a carrier gas. USY-24.46 (unit cell 24.46 Å), USY-24.31 (unit cell 24.31 Å), ZSM-5, and BETA catalysts were prepared using standard methods. Details of preparation and characteristics of the samples are presented elsewhere (Corma et al., 1994). The final samples were pelletized, crushed, and sieved, and particles with 0.30-0.50 mm were used. Reaction Procedure. The experiments were carried out at 500 °C and atmospheric pressure in a fixed-bed continuous glass reactor. The reactor was heated in an electric furnace. The catalyst was diluted with SiO2 (BASF), and the length of the bed was maintained © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3401 Table 1. Initial Molar Selectivities and Product Type for n-Decane Crackinga USY-24.46 product

ISj

hydrogen methane ethane ethylene propane propylene n-butane i-butane n-C4d i-butene n-pentane i-pentane n-C5d b-C5d n-hexane b-C6 n-C6d b-C6d n-heptane b-C7 C7d benzene toluene n-octane b-C8 ArC8H10 n-nonane b-C9 ArC9H12 ArC10H14 coke CnHmb coke CnHmc

0.0096 0.0015 (0.0000) 0.0000

∑ISjCjd ∑ISjHjd

10.0011 22.0022

USY-24.31 type

0.0099 0.0106 0.0628 0.2859 0.6534 0.1462 0.2411 0.2448 0.1508 0.0536 0.1739 0.0878 0.1663 0.0301 0.0819 0.0155 0.0456 0.0102 0.0113 0.0015 0.009 0.0058 0.0024

1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2U 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U

0.0044 0.0095

1+2U 1U 1U 1U 2S

BETA

ZSM-5

ISj

type

ISj

type

ISj

type

0.0722 0.0105 0.0152 0.0501 0.2435 0.6438 0.1347 0.1353 0.2842 0.1516 0.0919 0.1254 0.0929 0.1755 0.0584 0.0722 0.0316 0.0774 0.0197 0.0107 0.0027 0.0002 0.0007 0.0057 0.0002 0.0021 0.0205

1+2S 1+2S 1+2S 1+2S 1+2S 1S 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1+2U 1+2S 1U

0.0987 0.0315 0.0224 0.1328 0.4219 0.6531 0.1945 0.2376 0.2452 0.1662 0.0783 0.0864 0.0592 0.1273 0.0273 0.0191 0.0091 0.0304 0.0064 0.0061

1+2S 1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 1U 1U 1U 1U 1U

0.0026 0.0216 0.0009

1+2S 1+2S 1U

0.0232 0.0079

1+2S 1U

0.2007 0.0256 0.0985 0.2418 0.2427 1.0251 0.1425 0.0221 0.3692 0.2106 0.1138 0.0059 0.0588 0.1142 0.0823 0.0000 0.0029 0.0084 0.0465 0.0023 0.0000 0.0000 0.0000 0.0068 0.0000 0.0000 0.0103

1+2S 1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 2U 1U 1+2S 1U 1+2S 2S 2S 2S 1U 2S 2S 1U

0.0023 0.0000 (0.0000) 0.0000

1+2U 2U 2S

0.0107 0.0009 (0.0154) 0.0206

1+2S 1+2S 1+2S

0.0000

2S

9.9993 21.9990

10.0009 22.0020

10.0010 22.0020

a Product type: 1, primary; 2, second; S, stable; U, unstable. b Coke weight selectivity. c Coke molar selectivity with n/m ) 0.8. initial molar selectivity for product j; CJ, HJ, number of carbon or hydrogen atoms, respectively, in product j.

constant. Samples were taken at the outlet of the reactor at a programmed time following the procedure described elsewhere (Corma et al., 1996b). The N2/ reactant molar ratio was always 9, and the flow of hydrocarbon was 7.7 × 10-6 mol s-1. The samples were separated in a 100 m Supelco-Petrocol capillary column and in a Porapack Q + silica packed column and analyzed by GC using two detectors (TCD and FID). Blank experiments were done using ground glass in the place of the catalyst under reaction conditions. The products obtained from these experiments were assumed to be the result of thermal cracking. In order to analyze acid catalysts, thermal yields over ground glass were subtracted from the yields from the acid catalysts. Results and Discussion Initial Product Selectivity and Reaction Network. In Tables 1-3 the initial selectivities for the primary reaction products in the cracking of n-decane, n-dodecane, and n-tetradecane obtained at time on stream zero, i.e., in the absence of decay, are given. The changes in selectivity with the chain length of the hydrocarbon and the structure of the catalyst have been studied in another work (Corma et al., 1996a). From the observed product distribution a reaction scheme has been established, which consider (a) all possibilities for protolytic cracking in the reactant molecule, (b) all hydride transfer possibilities for the secondary carbenium ions which are smaller than the reactant molecule, (c) β-scission-alkylation reactions; (d) hydrogen transfer reactions, and (e) other reactions with a secondary

d

ISj,

character which are established to obtain (or to produce) the intermediate species in the above reactions. Among these we include charge isomerization, branching isomerization, and adsorption-desorption reactions. As an example, we present in Figure 1 all possible reactions which can be drawn for n-decane cracking, while in Table 4 we present a listing of all chemical species (Smith and Missen, 1979) which participtate in the reaction network of the three reactants considered. In agreement with the mass balance equations, the selectivity to any of the species in the network can be calculated by the following expression: R

νijRSi ) Sj; ∑ i)1

j ) 1, 2, ..., k, ..., S (species present in the network) (1)

where νij is the stoichiometric coefficient of species j in reaction i, RSi represents the selectivity of reaction i, and Sj is the selectivity of species j, whose value is -1 for the reactant (RRS, reactant reaction selectivity), is 0 if it is a positively charged species, or is given in Tables 1-3 if it is an observed reaction product. In matricial terms, eq 1 can be written as

ΓTRS ) S

(2)

in where Γ is the R × S matrix of the stoichiometric coefficients, RS is the reaction selectivity vector, and S is the species selectivity vector.

3402 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 2. Initial Molar Selectivities and Product Type for n-Dodecane Crackinga USY-24.46 product

d

ISj

hydrogen methane ethane ethylene propane propylene n-butane i-butane n-C4d i-butene n-pentane i-pentane n-C5d b-C5d n-hexane b-C6 n-C6d b-C6d n-heptane b-C7 C7d benzene toluene n-octane b-C8 ArC8H10 n-nonane b-C9 ArC9H12 n-decane ArC10H14 n-undecane coke CnHmb coke CnHmc ∑ISjCjd ∑ISjHjd

11.9992 25.9990

USY-24.31 type

0.0093 0.0086 0.0657 0.2586 0.7304 0.1386 0.2296 0.3909 0.2186 0.0583 0.2072 0.1109 0.2079 0.0343 0.0894 0.0226 0.0765 0.0127 0.0218 0.0107 0.0011 0.0069 0.0090 0.0043 0.0056 0.0035

1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2U 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1U 1+2U 1U

0.0118 0.0012 0.0033

1U 1U 1U

(0.0000) 0.0000

2S

BETA

ZSM-5

ISj

type

ISj

type

ISj

type

0.0814 0.0088 0.0093 0.0425 0.1593 0.7034 0.1411 0.2173 0.4456 0.2376 0.0727 0.1451 0.1189 0.2404 0.0447 0.0898 0.0372 0.0949 0.0181 0.0254 0.0141 0.0004 0.0012 0.0086 0.0044 0.0046 0.0039 0.0003 0.0031 0.0016 0.0012

1+2S 1+2S 1+2S 1+2S 1+2S 1S 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1U 1+2S 1U 1U 1+2U 1U 1+2U

0.1215 0.0299 0.0105 0.1364 0.3328 0.8270 0.1924 0.3521 0.3547 0.2318 0.0826 0.1002 0.0761 0.1603 0.0280 0.0221 0.0111 0.0379 0.0084 0.0080 0.0051 0.0031 0.0275 0.0043 0.0012 0.0286 0.0017

1+2S 1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1U 1+2S 1U

0.2312 0.0216 0.0493 0.2509 0.1857 1.2622 0.1576 0.0193 0.4354 0.2883 0.0965 0.0049 0.0752 0.1595 0.0761 0.0000 0.0039 0.0097 0.0767 0.0011 0.0000 0.0000 0.0000 0.0547 0.0000 0.0000 0.0226

1+2S 1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 2U 1U 1+2S 1U 1+2S 2S 2S 2S 1U 2S 2S 1U

0.0118 0.0002 0.0015

1+2S 1U 1+2S

0.0000 0.0027

2S 1U

(0.0163) 0.0262

1+2S

(0.0000) 0.0000

2S

12.0005 26.0016

11.9998 25.9995

11.9994 25.9988

a Product type: 1, primary; 2, secondary; S, stable; U, unstable. b Coke weight selectivity. c Coke molar selectivity with n/m ) 0.8. ISj, initial molar selectivity for product j; Cj, Hj, number of carbon or hydrogen atoms, respectively, in product j.

In order to solve eq 2, it should occur that the RS matrix is not singular, and this only occurs if it is generated through a scheme of independent reactions. In agreement with the literature (Smith and Missen, 1979; Aris and Mah, 1963), the maximum number of independent reactions which can be generated with a set of species is given by the equation:

number of reactions ) number of species - range A (3) where A is a matrix with as many columns as chemical species involved in the reaction network and as many raws as elements existing in the system. In the scheme presented in Figure 1, 72 reactions are given, and in agreement with Table 4, in this scheme 68 species take part. Thus it is clear that in this case there can only exist a maximum of 65 independent reactions, in such a way that, in the 72 reactions presented in Figure 1, seven of them should be obtained as a lineal combination of the others. This, indeed, occurs since all the reactions corresponding to hydride transfer wherein lineal carbenium ions participate (from C2+ to C8+) are a lineal combination of the rest. For instance, it can seen that reaction [39*] is generated by the combination [9*] - [16*] - [19*] + [22*] - [23*] + [24*] + [25*] - [27*] - [28*] - [43*] + [45*] - [46*] [47*] + 2[48*] + [49*] - [50*] - [52*] + [53*] - [54*] + [58*]. It is then clear that if one wants to use the conservation equation for each chemical species and the selectiv-

ity data for each product, in order to calculate the selectivities for each of the reactions, it is absolutely necessary to use independent reactions in the reaction scheme. If this were not done, the system of lineal equations obtained would be unresolvable. In our case, we have generated with the 68 species in Table 4 a possible network in which only 65 reactions or steps participate, all of them indepedently. In the network named as 1 in Figure 1, it is considered that the lineal paraffins are only coming from monomolecular cracking and not through hydride transfer. Such a situation will produce the maximum and minimum selectivities of n-decane toward the mono- and bimolecular reactions, respectively. On the other hand, and also with the reactions presented in Figure 1, it is possible to generate other different schemes where all reactions are independent and can be used to solve the system of equations which originate the conservation equations. In this way, network 2 (see Figure 1) can be obtained considering that all lineal paraffins, with the exception of CH4, are formed via hydride transfer and not through a monomolecular cracking. As can be seen, in this network 62 species intervene, since primary carbocations C3+-C8+ have disappeared, and, consequently, there will only be 59 step of independent reactions. This scheme, contrary to network 1, will produce for n-decane maximal and minimal selectivities toward bimolecular and monomolecular reactions, respectively. By means of the same methodology, we have proposed for n-dodecane and n-tetradecane networks 3 and 5

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3403 Table 3. Initial Molar Selectivities and Product Type for n-Tetradecane Crackinga USY-24.26 product

d

hydrogen methane ethane ethylene propane propylene n-butane i-butane n-C4d i-butene n-pentane i-pentane n-C5d b-C5d n-hexane b-C6 n-C6d b-C6d n-heptane b-C7 C7d benzene toluene n-octane b-C8 ArC8H10 n-nonane b-C9 ArC9H12 n-decane ArC10H14 n-undecane n-dodecane coke CnHmb coke CnHmc ∑ISjCjd ∑ISjHjd

14.0011 30.0022

USY-24.31 type

ISj 0.0068 0.0079 0.0701 0.2366 0.7536 0.1297 0.2016 0.4524 0.2580 0.0626 0.2145 0.1404 0.2789 0.0451 0.1173 0.0411 0.1335 0.0188 0.0376 0.0279 0.0016 0.0087 0.0134 0.0053 0.0079 0.0084

1+2S 1+2S 1+2S 1+2S 1+2U 1+2S 1+2S 1U 1U 1U 1+2U 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1U 1+2U 1U

0.0152 0.0058 0.0041 0.0011

1U 1U 1U 1U

(0.0000) 0.0000

2S

BETA

ZSM-5

ISj

type

ISj

type

ISj

type

0.0882 0.0049 0.0054 0.0261 0.1404 0.7318 0.1442 0.2229 0.5198 0.2768 0.0681 0.1699 0.1616 0.3138 0.0374 0.1168 0.0452 0.1506 0.0125 0.0355 0.0336 0.0006 0.0036 0.0075 0.0095 0.0111 0.0043 0.0011 0.0067 0.0026 0.0021 0.0012

1+2S 1+2S 1+2S 1+2S 1+2S 1S 1+2S 1+2S 1U 1U 1S 1+2S 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1U 1+2S 1U 1U 1+2U 1U 1+2U 1U

0.1244 0.0203 0.0079 0.1385 0.3010 0.9145 0.1913 0.3806 0.4298 0.2937 0.0899 0.1150 0.1087 0.2244 0.0355 0.0327 0.0197 0.0684 0.0088 0.0141 0.0156 0.0034 0.0313 0.0047 0.0019 0.0301 0.0023 0.0003 0.0123 0.0014 0.0017 0.0007

1+2S 1+2S 1+2S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 1U 1U 1U 1U 1U 1U 1+2S 1+2S 1U 1U 1+2S 1U 1U 1+2S 1U 1+2S 1U

0.2538 0.0161 0.0358 0.2524 0.1427 1.3927 0.1262 0.0104 0.5011 0.3435 0.0878 0.0034 0.1215 0.2082 0.0729 0.0000 0.0089 0.0249 0.0705 0.0026 0.0000 0.0000 0.0000 0.0659 0.0000 0.0000 0.0545

1+2S 1+2S 1S 1+2S 1+2S 1U 1+2S 1+2S 1U 1U 1U 1+2S 1U 1U 1U 2U 1U 1+2S 1U 1+2S 2S 2S 2S 1U 2S 2S 1U

0.0000 0.0333

2S 1U

0.0189 0.0052

1U 1U

(0.0172) 0.0321

1+2S

(0.0000) 0.0000

2S

14.0002 30.0006

14.0015 30.0030

14.0009 30.0018

a Product type: 1, primary; 2, secondary; S, stable; U, unstable. b Coke weight selectivity. c Coke molar selectivity with n/m ) 0.8. ISj, initial molar selectivity for product j; Cj, Hj, number of carbon or hydrogen atoms, respectively, in product j.

wherein the selectivity to the monomolecular reactions is maximum, while networks 4 and 6 represent a situation in which the reactant selectivity of the reactant toward the monomolecular reactions is minimal. Any reaction step not considered in Figure 1 or in those similar for n-dodecane and n-tetradecane which involve the species given in Table 4 can be obtained as a lineal combination. In this way a propagation reaction, which can also be called the chain process (Cumming and Wojciechowski, 1996), such as

R

BRS )

+

nC10 +

+ nC8+

can be generated by means of the following combination of reactions included in network 1:

[6] - [41] + [47] + [12] + [19] - [24] This means that while several reaction networks which are able to explain the product distribution may exist, all of them will represent an intermediate situation between the following two extremes: The first one corresponds to a maximum contribution of the monomolecular cracking and can be defined by means of the monomolecular reaction selectivity (MRS): R

MRS )

where νikM represents the stoichiometric coefficient for the disappearance of the reactant (k) in the monomolecular reaction i. In the second extreme situation the contribution of the monomolecular cracking will be minimum (MRS MIN). This implies a maximum in the contribution of the bimolecular cracking, defined by means of the variable bimolecular reaction selectivity (BRS):

νikMRSi ∑ i)1

(4)

νikBRSi ∑ i)1

(5)

where νikB is the stoichiometric coefficient for the disappearance of the reactant (k) in the bimolecular reaction i. The band between the two extreme situations defines an “uncertainty band”, whose size will depend on the catalyst and the reaction conditions. Only in very specific cases, such as the cracking of isobutane, does the uncertainty band disappear (Corma et al., 1996b), the real situation being in most of the cases included in between the two limiting situations. In Table 5 a summary of the calculation carried out, taking into account the six networks considered in Figure 1, is given. Thus, when studying the influence of the n-paraffin chain length on the participation of the mono- and bimolecular mechanisms on the USY-24.31

3404 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3405



Figure 1. Possible reaction steps for n-decane cracking on zeolites and numeration of those in each network considered.

of the reactant reaction selectivity (RRS) calculated, the summatory of the products between the reaction selectivities and the stoichiometric coefficient of the reactant, and the selectivity of the Bro¨nsted sites (H+S) are to any practical effect -1 and 0, respectively. Furthermore, the calculated selectivity to isobutene (calcd i-C4dS) is practically the same as the experimental value. The effective hydrogen transfer selectivities calculated by R

EHTS ) Figure 2. Influence of n-alkane chain length over monomolecular reaction selectivity (MRS) in n-alkane cracking over USY-24.31 catalyst. MRS data for n-heptane from Corma et al., 1996b.

catalyst (Figure 2), it is found that the uncertainty band decreases when increasing the hydrocarbon chain length. Meanwhile, the values of the MRS corresponding to the extreme situations also decrease, especially the maximum values, with the chain length. The observed tendency indicates that there is a decrease in the relative contribution of the monomolecular cracking with respect to the bimolecular cracking when increasing the paraffin chain length, and this can be explained by taking into account that when increasing the carbon chain length the number of potential carbenium ions able to go through hydride transfer increases, as well as the number of potential β-scission cracking positions per hydrocarbon molecule. In Table 5 it can also be shown that in extreme situations the selectivity of the monomolecular reactions (MRS) follows the order ZSM-5 > BETA > USY-24.31 > USY-24.46, while the selectivity of the bimolecular reactions (BRS) follows the reverse order. This behavior confirms that bimolecular reactions are space-demanding reactions, and therefore the structure and internal void space and acid strength of the zeolite strongly affect their selectivity (Haag and Dessau, 1984). As a consequence of the method used (Smith and Missen, 1979), it can also be observed that, in any network of independent reactions, the number of species must always be equal to the number of reactions plus 3. This fact makes it necessary to eliminate three mass balance equations (eq 1), which can later on be used to test the solutions obtained. In our case, we have not considered the equations corresponding to the reactant, isobutene, and the species corresponding to the acid sites, and it can be observed in Table 5 that the values

NEHTiRSi ∑ i)1

(6)

are given in Table 5. In eq 6, NEHTi (number of effective hydrogen transfer of reaction i) is defined as the number of hydrogen molecules which are transferred in reaction i and whose value is zero for all reactions except for the hydrogen transfer ones, and therefore it does not depend on the particular network considered but on the initial selectivity of aromatics and coke. It can be observed that this parameter increases when increasing the alkane chain length and follows the order BETA > USY-24.46 > USY-24.31 > ZSM-5. This is a logical and expected behavior, taking into account the framework Si/Al ratio as well as the pore geometry of the catalysts (Guerzani and Abbot, 1993; Corma and Orchille´s, 1989). Cracking of 2′-Methylpentane. We have also applied the methodology presented here to the experimental results reported by others. We have selected for this the initial selectivity data given by Zhao et al. (1993) for the cracking of 2-methylpentane on a HY zeolite. In Figure 3 it is shown a possible reaction scheme involving 40 species and 40 reactions, among which must be several which are not independent. More specifically, reactions [17], [18], and [20] can be obtained as a lineal combination of the others. Reaction [40] is written in this way in order to explain the experimental C/H observed in the coke deposited on the catalyst. If one accepts that the coke comes from the olefins generated during the first reaction steps (Guisnet and Magnoux, 1992), reaction [40] must be considered only in stoichiometric terms. In this way, reaction [40] establishes that per each 53.6 carbon and 105 hydrogen atoms deposited on the catalyst as coke, 1.1 extra molecules of paraffins are generated. With reaction [40], it is not possible to explain fully the excess of paraffins observed experimentally. The excess can be due to hydride transfer reactions ([17]-

3408 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 4. Numeration for Molecular and Ionic Participant Species in Proposed Reaction Networks n-C10

n-C12

n-C14

species

[Netw.#1]

[Netw.#2]

[Netw.#3]

[Netw.#4]

[Netw.#5]

[Netw.#6]

hydrogen methane ethane ethylene propane propylene n-butane i-butane n-C4d i-butene n-pentane i-pentane n-C5d b-C5d n-hexane b-C6 n-C6d b-C6d n-heptane b-C7 C7d benzene toluene n-octane b-C8 ArC8H10 n-nonane b-C9 ArC9H12 n-decane ArC10H14 n-undecane n-dodecane n-tetradecane coke H+ (acid sites)

1 2 3 4 5 6 7 8 9 for check 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 reactant 29 a

1 2 3 4 5 6 7 8 9 for check 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 reactant 29

1 2 3 4 5 6 7 8 9 for check 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 reactant

1 2 3 4 5 6 7 8 9 for check 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 reactant

30 for check

30 for check

32 for check

32 for check

1 2 3 4 5 6 7 8 9 for check 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 reactant 33 for check

1 2 3 4 5 6 7 8 9 for check 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 reactant 33 for check

31

31

33

33

34

34

+

32

+ +

33 +

34 32

34

+

+

34

36

36

35

37

35

33

37

35

38

36

36

34

38

36

39

37

37

+ +

35

35

39

40

38

35

40

37

41

38

39

36

41

38

42

39

40

37

42

39

43

40

+

+ +

41

+ + +

+

n-C7

+

+ +

43

44

42

38

44

40

45

41

43

39

45

41

46

42

44

40

46

42

47

43

45

47

48

46

41

48

43

49

44

47

42

49

44

50

45

48

43

50

45

51

46

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3409 Table 4 (Continued) n-C10 species +

n-C12

n-C14

[Netw.#1]

[Netw.#2]

[Netw.#3]

[Netw.#4]

[Netw.#5]

[Netw.#6]

49

44

51

46

52

47

50

45

52

47

53

48

51

46

53

48

54

49

53

47

55

49

56

50

54

48

56

50

57

51

55

49

57

51

58

52

56

50

58

52

59

53

57

51

59

58

52

60

53

61

54

59

53

61

54

62

55

60

54

62

55

63

56

61

55

63

56

64

57

+

+

+

52

n-C8+ +

+

ArC8H11+ +

n-C9+ + +

ArC9H13

+

54

+

55

60

64

n-C10+ +

+

ArC10H15

+

65

62

56

65

57

66

58

63

57

66

58

67

59

64

58

67

59

68

60

65

59

68

60

69

61

69

61

70

70

62

71

62

71

63

72

63

+

n-C11+ + +

73

n-C12

+

+ +

n-C13+

72

64

74

64

73

65

75

65

76

66

77

67

78

68

79

69

80

70

+

n-C14

+

+

a

Not used in the network.

[25]) and also to another reaction such as (Cumming and Wojciechowski): + 2MP +

+

That was not considered in the reaction scheme due to the fact that this is a lineal combination of others ([19] + [32] - [33] - [7] - [16] - [35] - [36]). On the other hand, the set of reactions presented in Figure 3 will also allow us to find schemes of independent reactions and to calculate the maximum and

minimum values for the selectivities of the initiation (monomolecular) and the chain propagation (bimolecular) reactions. This is shown in Figure 4 for the monomolecular reactions as a function of the reaction temperature. The two curves are the limits in which the real selectivities for these types of reactions will occur. It can be seen that the relative contribution of the monomolecular reactions in the extreme situations increases when increasing the reaction temperature. With respect to Figure 4, it can be pointed out that the initial selectivity values for initiation monomolecular reactions calculated by Zhao et al., which are 0.281,



Figure 3. Possible reaction steps for 2-methylpentane cracking on zeolites.

0.429, and 0.669, are located in the band delimited by the two curves. However, we believe that these values correspond to a particular reaction scheme, and there are other reaction schemes as valid as the one considered by those authors from the point of view of explaining the observed product distribution which will give different values than those above. Thus, we have to conclude that all calculations and conclusions established with any particular reaction scheme should be considered with caution. Influence of Conversion on Molar Selectivities. Up until now initial product selectivities, i.e., selectivities when conversion tends to zero, have been consid-

ered. Our purpose in this section is to test if the methodology developed here can also be used in situations where the degree of conversion is not close to zero. For this purpose we have measured the selectivities to the reaction products at different levels of conversion working at “zero” time on stream, i.e., time on stream at 0 s (see the Experimental Section), on n-decane cracking on a 12-membered-ring zeolite such as BETA. The results obtained are given in Table 6. The reaction scheme shown in Figure 1 has been satisfactorily applied, and the selectivities of each of the reactions in networks 1 and 2 have been calculated and validated with the variables RRS, H+S, and i-C4dS.

3412 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 5. Initial Molar Selectivities of Reactant for Monomolecular and Bimolecular Mechanisms and Effective Hydrogen Transfer in n-Alkanes Cracking over Zeolite Catalysts catalyst

-MRSa -BRS -RRS calc. i-C4dS H+S -MRS -BRS -RRS calcd i-C4dS H+ S EHTS

network

value

1 1 1 1 1 2 2 2 2 2 1

MAX MIN

1-9b 29-35

MIN MAX

49, 65 1-9, 46-59 1-2 16-29 43, 59 1-2, 40-53 60-65

exptl i-C4dS -MRS -BRS -RRS calcd i-C4dS H+ S -MRS -BRS -RRS calcd i-C4dS H+ S EHTS

exptl i-C4dS

USY-24.26

USY-24.31

n-Decane 0.4823 0.5177 1.0000 0.1505 9.465 × 10-8 0.0099 0.9901 1.0000 0.1505 -1.246 × 10-8 0.0666

0.6359 0.3643 1.0002 0.1523 -4.879 × 10-8 0.0827 0.9175 1.0002 0.1523 -7.953 × 10-8 0.0159

0.1508 3 3 3 3 3 4 4 4 4 4

MAX MIN

MIN MAX

exptl i-C4dS -MRS -BRS -RRS calcd i-C4dS H+ S -MRS -BRS -RRS calcd i-C4dS H+ S EHTS

reaction i

0.1516

n-Dodecane 0.4480 0.5523 1.0003 0.2197 1.322 × 10-7 0.0093 0.9910 1.0003 0.2197 7.916 × 10-9 0.0861 0.2186

5 5 5 5 5 6 6 6 6 6

MAX MIN

MIN MAX

0.2376

n-Tetradecane 0.4237 0.5763 1.0000 0.2577 5.437 × 10-8 0.0068 0.9932 1.0000 0.2577 -2.627 × 10-7 0.1125 0.2580

0.5180 0.4823 1.0003 0.2384 -1.961 × 10-7 0.0902 0.9101 1.0003 0.2384 -1.188 × 10-7 0.0315

0.4444 0.5557 1.0001 0.2771 8.836 × 10-8 0.0931 0.9070 1.0001 0.2771 -2.530 × 10-7 0.0723 0.2768

BETA

ZSM-5

0.7049 0.2952 1.0001 0.1662 9.674 × 10-8 0.1302 0.8699 1.0001 0.1662 4.785 × 10-8 0.2389

0.9594 0.0406 1.0000 0.2104 1.977 × 10-7 0.2263 0.7737 1.0000 0.2104 7.683 × 10-9 0.0000

0.1662

0.2106

0.5948 0.4051 0.9999 0.2317 -4.761 × 10-8 0.1514 0.8485 0.9999 0.2317 2.608 × 10-8 0.2960 0.2318 0.5518 0.4482 1.0000 0.2932 2.503 × 10-8 0.1447 0.8553 1.0000 0.2932 -1.403 × 10-7 0.3328 0.2937

0.9747 0.0253 1.0000 0.2885 3.283 × 10-7 0.2528 0.7472 1.0000 0.2885 -9.965 × 10-8 0.0000 0.2883 0.9836 0.0164 1.0000 0.3433 -1.248 × 10-7 0.2699 0.7301 1.0000 0.3433 -1.471 × 10-7 0.000 0.3435

a

MRS: monomolecular reaction selectivity. BRS: bimolecular reaction selectivity. RRS: reactant reaction selectivity. H+S: Brønsted sites selectivity. b In Figure 1.

Figure 4. Effect of temperature on monomolecular reaction selectivity (MRS) in 2-methylpeane cracking over HY catalyst. Data from Zhao et al. (1993). (b, MRS MSX; O, MRS MIN).

Looking at the values from Table 7 and as occurs with the initial selectivities, there are a wide band of selectivities wherein the mono- and bimolecular reac-

tions move. If one takes the results corresponding to the network where the contribution of the monomolecular reactions is maximum, it can be seen that the relative contribution of the monomolecular reactions decreases when increasing the level of conversion. This is consistent with an increase in the coverage of the catalyst surface by carbenium ions when increasing the level of conversion. On the contrary, when the network corresponds to a minimum contribution of the monomolecular reactions (only methane and hydrogen are produced via monomolecular cracking), it can also be seen that they increase when increasing the level of conversion. This behavior which appears as illogical is not so if one takes into account that H2 and CH4 are primary plus secondary stable products (Tables 1-3); i.e., H2 and CH4 also come from the products. In this sense, with the reaction scheme used, we are assigning all H2 and CH4 produced to the cracking of the reactant molecule and, therefore, we produce an unrealistic increase of the selectivities for these reactions. At this point one should ask, how is it possible that a reaction scheme which does not take into account secondary reactions can be adequated to reproduce situations wherein the level of conversion is high and consequently the contribution of secondary reactions is important?

Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3413 Table 6. Molar Selectivities and Product Type for n-Decane Cracking over BETA Catalyst as a Reactant Conversion Function in the Absence of Deactivation (T.O.S. ) 0 s)a molar selectivities Sj for conversion (X) product

type

0.0000

0.4127

0.6841

0.9082

0.9880



0.0987 0.0315 0.0224 0.1328 0.4219 0.6531 0.1945 0.2376 0.2452 0.1662 0.0783 0.0864 0.0592 0.1273 0.0273 0.0191 0.0091 0.0304 0.0064 0.0061

0.0993 0.0324 0.0212 0.1454 0.4484 0.6393 0.1857 0.2522 0.2409 0.1535 0.0797 0.0895 0.0556 0.1209 0.0261 0.0202 0.0081 0.0284 0.0060 0.0062

0.1008 0.0405 0.0225 0.1550 0.4698 0.6433 0.1923 0.2732 0.2194 0.1439 0.0765 0.0944 0.0462 0.1011 0.0234 0.0189 0.0062 0.0222 0.0051 0.0060

0.1080 0.0611 0.0291 0.2042 0.5272 0.6057 0.1963 0.2958 0.2025 0.1209 0.0699 0.1064 0.0282 0.0636 0.0180 0.0170 0.0025 0.0097 0.0035 0.0048

0.1237 0.1059 0.0338 0.2271 0.5741 0.4438 0.2181 0.3941 0.1212 0.0733 0.0674 0.1116 0.0142 0.0329 0.0148 0.0158 0.0012 0.0044 0.0021 0.0033

1+2S 1+2S 1U

0.0026 0.0216 0.0009

0.0032 0.0247 0.0008

0.0041 0.0310 0.0007

0.0077 0.0474 0.0005

0.0126 0.0684 0.0001

1+2S 1U

0.0232 0.0079

0.0251 0.0022

0.0321 0.0008

0.0394 0.0002

0.0548 0.0000

1+2S 1+2S 1+2S

0.0107 0.0009 (0.0154) 0.0206

0.0114 0.0011 (0.0182) 0.0244

0.0130 0.0011 (0.0200) 0.0268

0.0161 0.0013 (0.0254) 0.0340

0.0233 0.0019 (0.0451) 0.0605

10.0009 22.0020

9.9965 21.9934

9.9965 21.9942

9.9991 21.9984

10.0000 22.0006

∑SjCj ∑SjHj

a Product type: 1, primary; 2, secondary; S, stable; U, unstable. b Coke weight selectivity. c Coke molar selectivity with n/m ) 0.8. S , j molar selectivity for product j; Cj, Hj, number of carbon or hydrogen atoms, respectively, in product j.

Table 7. Influence of the Conversion (X) on Molar Selectivities of Reactant for Monomolecular and Bimolecular Mechanisms and Effective Hydrogen Transfer in n-Decane Cracking over BETA Zeolite in the Absence of Deactivation (T.O.S. ) 0 s). conversion (X)

-MRS -BRS -RRS calcd i-C4dS H+S -MRS -BRS -RRS calcd i-C4dS H+S EHTS

network

value

reaction i

1 1 1 1 1 2 2 2 2 2 1

MAX MIN

1-9 29-35

MIN MAX

49, 65 1-9, 46-59 1-2 16-29 43, 59 1-2, 40-53 60-65

exptl i-C4dS

0.0000

0.4127

0.6841

0.9082

0.9880

0.7049 0.2952 1.0001 0.1662 9.674 × 10-8 0.1302 0.8699 1.0001 0.1662 4.785 × 10-8 0.2389

0.7031 0.2971 1.0002 0.1549 1.162 × 10-7 0.1317 0.8685 1.0002 0.1549 -1.064 × 10-7 0.2697

0.6877 0.3129 1.0006 0.1463 1.448 × 10-7 0.1413 0.8593 1.0006 0.1463 -1.444 × 10-7 0.3243

0.6779 0.3222 1.0001 0.1214 5.250 × 10-8 0.1691 0.8310 1.0001 0.1214 5.111 × 10-8 0.4377

0.6570 0.3433 1.0003 0.0741 5.960 × 10-8 0.2296 0.7707 1.0003 0.0741 -1.781 × 10-7 0.6645

0.1662

The answer can be that this is a consequence of Smith and Missen’s method used. For instance, if one takes the reaction scheme reported in Figure 1, he can see that while the secondary recracking reactions are not directly represented, they are included in an implicit way since they can be generated as a lineal combination of the reactions present in the scheme. For instance, a secondary reaction of the type + H2

can be obtained by the combination [1*] - [32*] + [56*], or if we consider a secondary recracking of the type

0.1535

0.1439

0.1209

0.0733

+

it can be obtained by [30*] - [35*] + [20*] + [43*]. Thus, all recracking reactions result from lineal combinations of the reactions given in Figure 1, and in this sense they do not have to be included there nor are they necessary to fit the experimental data. In other words, even if secondary reactions occur, one will not be able to evaluate their importance from the experimental selectivity data. Finally, the hydrogen transfer reactions, which are considered in the reaction scheme of Figure 1, strongly increase when increasing the level of conversion, as can be seen from the changes in the EHTS parameter in

3414 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 8. Molar Selectivities and Product Type for n-Decane Cracking over BETA Catalyst as a T.O.S. Function for a Constant Conversion Level (X ) 0.80)a molar selectivities Sj for T.O.S. (s) product

type

0

6

18

90

330



0.1038 0.0483 0.0251 0.1663 0.4935 0.6476 0.1951 0.2761 0.2180 0.1366 0.0731 0.1013 0.0380 0.0783 0.0217 0.0179 0.0044 0.0165 0.0045 0.0056

0.1020 0.0413 0.0229 0.1570 0.4603 0.6251 0.1913 0.2740 0.2228 0.1466 0.0791 0.1013 0.0462 0.0939 0.0255 0.0236 0.0053 0.0195 0.0051 0.0064

0.1007 0.0388 0.0226 0.1399 0.4332 0.6186 0.1885 0.2688 0.2277 0.1551 0.0858 0.1013 0.0504 0.1051 0.0288 0.0258 0.0063 0.0225 0.0059 0.0072

0.1001 0.0379 0.0222 0.1357 0.4023 0.6166 0.1804 0.2570 0.2394 0.1692 0.0900 0.1013 0.0550 0.1180 0.0336 0.0279 0.0068 0.0241 0.0076 0.0079

0.1001 0.0379 0.0222 0.1357 0.3920 0.6166 0.1742 0.2429 0.2520 0.1791 0.0911 0.1013 0.0588 0.1238 0.0355 0.0279 0.0066 0.0250 0.0094 0.0080

1+2S 1+2S 1U

0.0052 0.0385 0.0006

0.0044 0.0317 0.0007

0.0040 0.0278 0.0008

0.0034 0.0241 0.0011

0.0028 0.0229 0.0015

1+2S 1U

0.0346 0.0005

0.0328 0.0005

0.0311 0.0005

0.0293 0.0006

0.0274 0.0006

1+2S 1+2S 1+2S

0.0140 0.0011 (0.0216) 0.0289

0.0140 0.0019 (0.0198) 0.0265

0.0140 0.0020 (0.0178) 0.0239

0.0140 0.0022 (0.0127) 0.0170

0.0140 0.0025 (0.0090) 0.0120

9.9998 22.0000

10.0003 22.0008

10.0002 22.0010

10.0005 22.0008

9.9998 21.9992

∑SjCjd ∑SjHjd

a Product type: 1, primary; 2, secondary; S, stable; U, unstable. b Coke weight selectivity. c Coke molar selectivity with n/m ) 0.8. molar selectivity to product j; Cj, Hj, number of carbon or hydrogen atoms, respectively, in product j.

Table 7. This behavior increases the saturation of olefins, decreasing their amount as can be seen with the isobutene selectivity (i-C4dS in Table 7). Influence of the Time on Stream on Selectivity. The procedure used to calculate the molar selectivity at a level of conversion different than zero can also be used to calculate the variation produced in the molar selectivity when changing time on stream (T.O.S.), keeping constant the level of conversion. In this way the selectivity curves allowed one to determine the molar selectivities corresponding to the cracking of n-decane on the BETA zeolite at a conversion of 0.8, and they are given in Table 8. These values have been used to calculate the selectivity of the reactions corresponding to networks 1 and 2 in Figure 1. The results of this analysis are shown in Figure 5 where the selectivities of the bimolecular reactions are represented in the two extreme situations, i.e., maximum and minimum contribution to the mechanism, as well as the effective selectivity for the hydrogen transfer. Both curves corresponding to the BRS increase when increasing the T.O.S. or what is the same when increasing the amount of coke deposited on the catalyst. Meanwhile, the EHTS curve presents a different behavior. To this respect it should be pointed out that the data in Table 8 relative to coke do not really represent the coke deposited on the catalyst but instantaneous selectivities. Logically, a catalyst producing 0.8 conversion at 330 s should contain more coke deposited than another sample of catalyst which produces the same conversion at 6 s of time on stream.

d

Sj,

Figure 5. Effect of TOS at constant conversion level (X ) 80%) on bimolecular reaction selectivity (BRS) and effective hydrogen transfer selectivity (EHTS) in n-decane cracking over BETA catalyst. (b, BRS MAX; O, BRS MIN; 9, EHTS).

From Figure 5, and despite the high uncertainty band observed in this case, it can be deduced that the catalyst deactivation does not seem to have a notorious effect on the relative contribution of the mono- and bimolecular mechanisms. However, the EHTS clearly decreases, this being in agreement with the increase in the olefin/ paraffin ratio in the C3 (from 1.31 to 1.57), C4 (from 0.75


to 1.03), and C5 (from 0.67 to 0.95) fractions, as well as with the decrease of aromatics and coke. The effect observed on this catalyst seems contradictory to what was found in other works (Corma et al., 1996a; Gates et al., 1979) where it was said that the hydrogen transfer reactions are favored with the catalyst deactivation as a consequence of the faster poisoning of the strongest acid sites. The different behavior observed on the BETA zeolite make us think that the decay not only will influence the relative proportion of the strongest acid sites but also will change the adsorption capacity for the products responsible for the hydrogen transfer reactions and those responsible for coke, as well as their accessibility to the active sites when coke itself is deposited. Conclusions In conclusion, when a reaction mechanism wants to be established on the bases of the experimental selectivity data, the reaction considered should be independent. If this is not so, when the conservation laws are applied, then a system of linearly dependent equations will be obtained, and there will not be a solution. This procedure when applied to the cracking of paraffins generates several independent reaction schemes, in principle equally probable, and this makes it impossible to establish wich accuracy the mono- and bimolecular mechanism of cracking by means of the selectivity data. Only if one finds a method to establish the percentage of a given product which proceeds via monomolecular and bimolecular reactions it is possible to derive the mechanism of the cracking reaction from initial selectivity results. One possibility is to consider the cracking of molecules with less than 5 carbon atoms, as for instance isobutane, which can produce H2 and CH4. Since these two products cannot be made via bimolecular cracking, it should then be possible to establish a distinction between the selectivity of mono- and bimolecular reactions. Literature Cited Abbot, J.; Wojciechowski, B. W. The mechanism of paraffin reactions on HY zeolite. J. Catal. 1989, 115, 1. Aris, R.; Mah, R. H. S. Independence of Chemical Reactions. Ind. Eng. Chem. Fundam. 1963, 2 (2), 90. Bamwenda, G. R.; Zhao, Y.; Wojciechowski, B. W. The influence of reaction temperature on the cracking mechanism of 2-methylhexane. J. Catal. 1994a, 148, 595. Bamwenda, G. R.; Zhao, Y.; Wojciechowski, B. W. The inactivity of extra Framework Aluminum in the cracking of 2,3-Dimethylbutane on USHY. J. Catal. 1994b, 150, 243. Bamwenda, G. R.; Zhao, Y.; Groten, W. A.; Wojciechowski, B. W. The effects of EFAl extraction of 2-methylpentane cracking over steamed HY. J. Catal. 1995, 157, 209.

Corma, A.; Orchille´s, A. V. Formation of products responsible for Motors and Research octane of gasolines produced by cracking. The implication of framework Si/Al ratio and operation variables. J. Catal. 1989, 115, 551. Corma, A.; Planelles, J.; Sańchez-Marıń, J.; Toma´s, F. The role of different types of acid sites in the cracking of alkanes on zeolite catalysts. J. Catal. 1985, 93, 30. Corma, A.; Miguel, P. J.; Orchille´s, A. V. Influence of hydrocarbon chain length and zeolite structure on the catalyst activity and deactivation for n-alkanes cracking. Appl. Catal. A 1994, 117, 29. Corma, A.; Miguel, P. J.; Orchille´s, A. V. Product selectivity effects during cracking of alkanes at very short and longer times on stream. Appl. Catal. A 1996a, 138, 57. Corma, A.; Miguel, P. J.; Orchille´s, A. V. Can macroscopic parameters, such conversion and selectivity, distinguish among different cracking mechanisms on acid catalysts? J. Catal. 1996b, submitted. Cumming, K. A.; Wojciechowski, B. W. Hydrogen transfer, coke formation and catalyst decay and their role in the chain mechanism of catalytic cracking. Catal. Rev.-Sci. Engl. 1996, 38 (1), 101. Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. Guerzani, F. N.; Abbot, J. Catalytic cracking of a hydrocarbon mixture on combinations of HY and HZSM-5 zeolites. J. Catal. 1993, 139, 289. Guisnet, M.; Magnoux, P. Composition of the carbonaceous compounds responsible for zeolite deactivation. NATO ASI Ser., Ser. C 1992, 352, 437. Haag, W. O.; Dessau, R. M. Duality of mechanism for acid catalysed paraffin cracking. 8th Int. Cong. Catal. (Proc.) 1984, 2, II305. Lombardo, E. A.; Hall, W. K. The mechanism of isobutane cracking over amorphous and crystalline aluminosilicates. J. Catal. 1988, 112, 565. Lukyanov, D. B.; Shtral, V. I.; Khadzhiev, S. N. A kinetic model for the hexane cracking reaction H-ZSM-5. J. Catal. 1994, 146, 87. Smith, W. R.; Missen, R. W. What is Chemical Stoichiometry? Chem. Eng. Ed. 1979, 13, 26. 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. Zhao, Y.; Bamwenda, G. R.; Wojciechowski, B. W. Cracking Selectivity Patterns in the presence of chain mechanism. The cracking of 2-methylpentane. J. Catal. 1993, 142, 465.

Received for review November 18, 1996 Revised manuscript received May 9, 1997 Accepted May 12, 1997X IE9607309

X Abstract published in Advance ACS Abstracts, July 1, 1997.

On the Limitations To Establish the Contribution of the Different

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