I n d . Eng. Chem. Res. 1994,33,2913-2923
2913
Catalytic Cycles and Selectivity of Hydrocarbon Cracking on Y-Zeolite-BasedCatalysts? George Yaluris: b s t a m J. Madon3 Dale F. Rudd: and James A Dumesic'*' Center for Clean Industrial a n d Treatment Technologies, Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706, a n d Engelhard Corporation, 101 Wood Avenue, Iselin, New Jersey 08830
Environmental concerns have created the need for selective catalysts that increase the yield of desirable products (e.g.,isobutylene from hydrocarbon cracking units) or reduce the production of polluting byproducts. The development of selective catalysts may be facilitated by understanding the chemical factors controlling the rates of the various catalytic cycles available to the reactants and products. We have developed a kinetic model based on carbenium and carbonium ion surface chemistry for isobutane cracking and extended it to 2-methylhexane cracking over USY-based catalysts. Catalytic cycles for isobutane cracking that include initiation reactions lead to olefin production, while cycles that include hydride ion transfer reactions lead to paraffin production. The overall chemistry of the major catalytic cycles is the same for isobutane and 2-methylhexane cracking, although additional reaction pathways are available for the larger 2-methylhexane molecule. Paraffins and olefins with three or more carbon atoms can be produced from 2-methylhexane by cycles that include both initiation and hydride ion transfer reactions and the paraffin to olefin ratio cannot be greater than 1. By allowing us to build catalytic cycles, such models help identify similarities and differences in reactivity patterns for various reactants.
Introduction Recent changes in environmental legislation (EPA, 1990) have created the need for more selective fluid catalytic cracking (FCC) catalysts. An understanding of catalytic selectivity in FCC is necessary to minimize the formation of unwanted byproducts while increasing yields of hydrocarbons that when used in reformulated gasoline will help reduce pollution. Catalysts may be required to selectively form isobutylene and isopentenes but give reduced yields of larger olefins and aromatics. In response to such needs, several groups have recently developed kinetic models that primarily provide ways of predicting product distributions for catalytic cracking and other industrially important processes which involve complex hydrocarbon feedstocks (Baltanas et al., 1989; Feng et al., 1993; Hillewaert et al., 1988; Liguras and Allen, 1989a,b; Lox and Froment, 1993; Willems and Froment, 1988a,b). For example, Froment and coworkers (Feng et al., 19931, have developed fundamental kinetic models for catalytic cracking in which reaction networks are generated by computer algorithms, and rate constants of each elementary step are calculated as the product of the number of single events and the single event rate constant. This approach to kinetic modeling utilizes single event rate constants, independent of the feedstock, that may be obtained for each catalyst by experiments with typical key hydrocarbons. However, such models do not allow us to obtain a unified description of the rates of the catalytic cracking cycles for various hydrocarbons over different catalysts. This information is necessary to facilitate the search for selective catalysts by helping to understand how catalytic and chemical factors affect reaction pathways that
* Author to whom correspondence
should be addressed. Presented at the Symposium on Catalytic Reaction Engineering for Environmentally Benign Processes at the American Chemical Society National Meeting, San Diego, CA, March 13-18, 1994. University of Wisconsin. 5 Engelhard Corp.
*
0888-5885/94/2633-2913$04.50/0
are available to reactants and products. Such relationships may be effectively studied by microkinetic analyses which lead t o a more thorough kinetic understanding of catalytic cycles for reactions at various conversions, temperatures, pressures, and over various catalysts (Dumesic et al., 1993). This approach may help identify strategies to develop cleaner and more efficient processes. We recently presented a kinetic model that may be used to probe the catalytic cycles involved in the cracking of isobutane (Yaluris et al., 1994a,b). This model is based on well-known concepts in carbocation chemistry and provides information on the dependence of the rates of catalytic cycles on conversion, temperature, and catalyst acidity. In the present paper, we briefly review this model for isobutane cracking and then extend it to describe and compare acid catalyzed reactions of 2-methylhexane with those of isobutane.
Experimental Section We used FCC catalysts made via the Engelhard in situ technology (Haden and Dzierzanowski, 1970;Brown et al., 1985). Briefly, the catalysts were pretreated as follows: USY-C was calcined at 840 K for 2 h, USY-S was steamed for 2 h a t 1060 K, USY-S1 was steamed for 2 h a t 1030 K, and USY-S2 was steamed for 5 h at 1060 K. We used the first two catalysts in the isobutane study and the latter two catalysts for 2-methylhexane cracking. Table 1lists the physical properties of these catalysts. A mixture of 25% isobutane in He was used in the isobutane cracking study, whereas 10% 2-methylhexane in He was used to study 2-methylhexane cracking. Experiments were carried out in a Pyrex plug flow reactor; details of the experiments and the analyses have been given elsewhere (Yaluris et al., 1994a). We neglected thermal cracking reactions in the case of isobutane cracking but had t o account for them in the case of 2-methylhexane cracking. We studied thermal reactions in reactors containing only quartz rings. We determined total catalytic acidity by thermogravimetric and infrared measurements of the extents of
0 1994 American Chemical Society
2914 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 Table 1. Properties of Catalysts catalyst USY-c USY-s 221 zeolite surface area (m2/g) 274 349 total surface area (m2/g) 418 31 zeolite contenta (%) 38 24.43 24.30 unit cell size (A) 6.64 20.56 AlFb 27.9 SUAl 8.3 Brensted sites GmoYg) 532 52 81 Lewis sites @moVg) 304
USY-s1 USY-s2 233 203 371 332 32 28 24.27 24.33 9.85 3.43 18.5 55.0 129 15.4 164 37.2
a From BET measurements assuming surface area of pores ‘2 nm is mainly due to the Y-zeolite. Number of framework Al atoms per unit cell.
pyridine adsorption a t 473 K. The relative numbers of Lewis and Bransted acid sites were determined from diffise-reflectance infrared spectra of adsorbed pyridine at 473 K. We have given details of these measurements elsewhere (Chen et al., 1992). The number of Brensted acid sites is lower than the number of framework Al cations. The causes of this behavior have been discussed extensively elsewhere (Biaglow et al., 1991; Biaglow et al., 1993; Biaglow et al., 1994).
Parameter Estimation for the Kinetic Model The reaction mechanisms that describe the catalytic cracking of isobutane and 2-methylhexane consist of a large number of reaction steps. Our reaction mechanism for isobutane cracking consists of 21 reaction steps and accounts for 12 major products, whereas the reaction mechanism for 2-methylhexane cracking has 33 reaction steps and accounts for 16 major products. Kinetic analyses require estimates of preexponential factors and activation energies for these steps that are consistent with thermodynamics. We estimated preexponential factors using transition state theory. Rate constants are given by the equation (Benson, 1976)
(1) where k~ is the Boltzmann constant, h is the Planck constant, AS* is the standard entropy change, and AW is the enthalpy change from reactants t o the activated complex. Since a rate constant is described by the Arrhenius equation, we use the following simplified equation for the preexponential factor, A:
We have given elsewhere (Yaluris et al., 1994a) a detailed description of the procedure used to estimate the entropies of all surface species and transition states at 773 K. We ensured thermodynamic consistency of the activation energies by using the Evans-Polanyi correlation to estimate the activation energies from the enthalpy changes of reaction (Dumesic et al., 1993):
Ea =E,,
+a M
(3)
where E , and a are constants for a given family of reactions. For simplicity, we set the value of a to 0.5 for all reactions. In cases where equation 3 predicted a negative value, we set Ea equal t o zero and the activation energy of the opposite step equal to the heat of the endothermic reaction. We have presented else-
where (Yaluris et al., 1994a) the estimation of the heats of formation for all gas phase species and gas phase carbenium ions. Since there is no information for the heats of formation of the surface species, we introduced (Yaluris et al., 1994a; Rekoske et al., 1993) a parameter, AH+,that represents the heat of stabilization of a carbenium ion relative to a proton on the catalyst. This parameter is an average value of the corresponding heats of stabilization of the individual carbenium ions, and small changes in its value represent substantial changes in the Brensted acid strength. Using the estimated gas phase heats of formation of all species and AH+,which accounts for adsorption on the catalytic surface, the heats of all reactions included in the reaction scheme can be estimated as a function of AH+ To limit the number of the adjustable parameters in odel, a common Evans-Polanyi constant may be assi Our ed to reactions with similar chemistry. Therefore, for the carbenium and carbonium ion chemistry employed here, we defined the following reaction families to formulate the Evans-Polanyi correlations in isobutane and 2-methylhexane cracking: (1) C-C bond protolysis reactions, involving rearrangements between a primary and a tertiary carbenium ion, (2) C-C bond protolysis reactions with no carbenium ion rearrangement, (3) C-C bond protolysis reactions involving rearrangement of a primary carbenium ion to a secondary carbenium ion, (4) olefin adsorptioddesorption reactions, (5) isomerization of tertiary carbenium ions to secondary ions, (6),!?-scissionreactions where isomerization of a secondary to a tertiary carbenium ion occurs, (7) &scission reactions involving no rearrangement of the resulting carbenium ion, (8),!?-scission reactions involving rearrangement of a primary carbenium ion to a secondary ion, (9) oligomerization reactions involving rearrangements between tertiary and primary isobutyl carbenium ions. Since the chemistry of ethylene formation is not well understood, the ethylene formation steps were not included in the above families. We also did not constrain the hydride ion transfer steps to be in a single family because of the importance of these steps in the models.
3
Isobutane Cracking We have presented a detailed kinetic analysis of isobutane cracking elsewhere (Yaluris et al., 1994a,b). Here, we summarize the main aspects of that work in order to logically extend our model to 2-methylhexane cracking. Figure 1shows the reaction mechanism used in these studies. This scheme accounts for the formation of hydrogen, methane, ethylene, all paraffins and olefins with 3 and 4 carbon atoms, isopentane, and 2-methyl-2-butene. Since kinetic analyses cannot distinguish between different possible carbenium ion initiation processes (Dwyer and Rawlence, 1993;Haag and Dessau, 1984; Lombard0 and Hall, 1988; McVicker et al., 19831, we assumed, for simplicity that isobutane cracking is initiated via protolysis. The reaction then proceeds via a series of isomerization, oligomerization, p-scission, and hydride ion transfer reactions and is terminated by desorption of surface carbenium ions as olefins (Shertukde et al., 1992; Zhao et al., 1993; Corma et al., 1994). The hydride ion transfer step is written in our reaction scheme as an Eley-Rideal step where a surface carbenium ion reacts with a gas phase parafin or with
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2916 0.6 1
J
1.
2.
3. 4.
5.
6.
7.
10
+ = = A (
+=
&
+
A
8.
9.
10. 11.
12.
13. 14.
15.
16. 17.
18.
19.
20.
21.
A + &
=
A+/L
Figure 1. Reaction scheme for isobutane cracking.
a paraffin which is weakly adsorbed as an adjacent precursor state (Madon, 1991). The choice of such a reaction step to describe hydride ion transfer is kinetically equivalent to a reaction with an adsorbed paraffin on a nonspecified site, provided that such sites do not become saturated. These sites cannot be Bransted or Lewis acid sites (Corma et al., 1990; Corma et al.,1989; Madon, 1991). Step 2 of this mechanism belongs to family 2; steps 4, 13, and 15-18 belong to family 4; steps 6, 9, 11,and
15
20
25 30 35 40 Conversion (%) Figure 2. Simulated and exDerimental distributions of CX. c4, ana C5 species for isobutane hacking over USY-Cat 773 K'and various conversions. Distributions are based on the total amounts of C3,C4, and c5 species in the product stream. Points represent experimental data.
14 belong to family 5; steps 7 and 10 belong to family 6; step 12 belongs to family 7; and steps 5 and 8 belong to family 9. Our experimental data (Yaluris et al., 1994a) and results presented by other workers (Abbot and Wojciechowski, 1988b; Corma et al., 1985; Kramer et al., 1985;McVicker et al., 1983; Ono and Kanae, 1991; Shertukde et al., 1992; Shigeishi et al., 1991) indicate that isomerization and desorption processes are equilibrated at the reaction conditions employed here. Therefore, we set the Evans-Polanyi constants of families 4 and 5 t o zero. In addition, we found the Evans-Polanyi constants for families 6 and 7 to be kinetically insignificant and were set at a constant value. We first used the data from catalyst USY-C to calibrate the model with 9 adjustable parameters for 130 dependent responses. These parameters were AH+, the preexponential factor of the step producing ethylene, and the Evans-Polanyi constants of families 2 and 9, and steps 1, 3, 19, 20, and 21. Next, we fitted the 39 dependent responses for catalyst USY-S using the AH+ and the Evans-Polanyi constant of step 3, as adjustable parameters with only very minor changes in the values of the other parameters. The value of the preexponential factor for the step producing ethylene was 2.78 x 10l2,the same value as estimated for catalyst USY-C. The number of sites for each catalyst in the plug-flow reactor was set equal to the number of measured Bransted acid sites. We obtained good agreement between the experimental data and the results of the model for both catalysts. The model predicts the trends of complex product distributions at different conversions and temperatures, and for different catalysts, using a limited number of kinetic parameters that are related t o the fundamental surface chemistry operative during catalytic cracking. For example, the model predicts (Figure 2) that as conversion increases, the fraction of C3 species in the CS-C~product stream increases a t the expense of Cq species. It also predicts paraffin and olefin selectivities a t different conversions, temperatures, and catalyst pretreatments (Yaluris et al., 1994a). We found discrepancies between the model predictions and the experimental data only for olefin production rates at conversions higher than 30%, and the model generally overestimates the amounts of olefins produced at these conversions. For the purposes of this analysis, the paraffin to olefin ratio is defined as the ratio of product paraffins to olefins containing three or more carbon atoms (Yaluris et al., 1994b). A comparison between
2916 Ind. Eng. Chem. Res., Vol. 33, No. 12,1994
1
25 30 35 40 Conversion (%) Figure 3. Simulated and experimental paraffin to olefin ratios for isobutane cracking over catalysts USY-Cand USY-S at 773 K and various conversions. Parafins and olefins with three or more carbon atoms are counted. Points represent experimental data. 10
15
20
the experimentally observed paraffin to olefin ratio and the ratio predicted by the model (Figure 3) for USY-C and USY-S catalysts shows excellent agreement at lower conversions. The agreement is not as good at higher conversions because the partial pressures of olefins in the product stream increase at high conversions and our model does not include reactions that consume olefins such as coke formation. The mechanism for isobutane cracking, shown in Figure 1, consists of a number of different catalytic cycles. The nature of the catalytic cycles that take place during isobutane cracking does not change when the experimental conditions or the catalyst change; however, the relative rates of these cycles are altered, resulting in different activities and selectivities. Figure 4 shows a schematic diagram of the catalytic cycles for catalyst USY-C a t 11%conversion and 773 K. We form a catalytic cycle by starting at a particular surface species, move along the reaction lines that connect the various surface species, and finally return to the original surface species. Intersecting line segments separate reaction lines into sections representing reactants and products. Gas phase reactants are written next to the reaction line that connects the participating surface species reactants. Arrows associated with the intersecting line segments indicate the allowable directions for reaction. Two arrows on a reaction line indicate a reversible process, and two arrows of equal length indicate an equilibrated process. Dashed lines indicate the slowest processes, while solid lines indicate faster processes. The fastest processes are shown with thick solid lines. Since a reaction can be part of more than one catalytic cycle, the total rates of different segments of a cycle may not necessarily be equal. However, the steady state approximation holds for all surface species. Specifically, the net rate of production of a particular surface species by all reaction lines leading to that species is equal to the net rate of consumption of that surface species by all reaction lines departing from it. The identification and interactions of the many pathways that constitute the various cycles (Figure 4) may lead to a better understanding of the cracking process. For example, the initiatioddesorption cycle that produces olefins, starts with an initiation reaction, which may be followed by an isomerization process, and ends with a desorption process. In such a cycle for propylene formation, a surface proton reacts with isobutane and yields methane and C3+. The surface carbenium ion then transfers a proton to the surface and
e C9+ species
Figure 4. Catalytic cycles for isobutane cracking over catalyst USY-C at 773 K and 11% conversion at the reactor exit.
desorbs as propylene. The roles of different catalytic cycles may change at different conversions. For example, the initiatioddesorption cycle described here is dominated by other cycles at ca. 2% conversion, and the net result is propylene consumption. A hydride ion transfer cycle that becomes important includes the protonation of propylene to C3+. These cations then react with isobutane to produce propane and isobutyl cations via hydride ion transfer. The isobutyl cations subsequently transfer a proton to the surface and desorb as C4 olefins. As the concentration of olefins in the product stream increases with increasing conversion, additional olefins are produced by an oligomerization//3-scission cycle. During this cycle an isobutyl cation formed by other cycles (e.g., by initiation) oligomerizes with gas phase isobutylene to form CS+species, which yield propylene and isopentyl cations via p-scission. These isopentyl cations may react to give isopentane or isopentene. They may also further oligomerize with gas phase isobutylene to produce Cg+ species which crack to give (26' and propylene. The c6+intermediate krther cracks and the cycle is closed when a propyl carbenium ion desorbs aRer yielding the original Bransted acid site. Since in this cycle Cq species are replaced by C3 and C5 species, this cycle has a major impact on the distribution of C3, Cq, and C5 species in the product stream. The rates of the processes that constitute the catalytic cycles are shown in Figure 5 at different reactor lengths or conversions. Initiation reactions are irreversible, and their rates are not a strong function of conversion. Initiation steps lead to olefin production when combined with desorption processes in the initiatioddesorption cycle. Hydride ion transfer reactions are also irreversible and their rates are negligible at low conversion.
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2917 0.016
initiation
r
'a
,- 0.012
I.
-.-.-Oligomerization p - scission
.-b
c)
..e.
m m
a... .e
Q
3 0.008
.... .... (... ....-
.
2.
.e*.
Desorption Hydride Transfer
0
....I
..# ..a.
3.
...*'
4.
5.
0"'
0
'
-.*-.
0.2
6. ,
,
I
0.4
,
,
,
1
0.6
,
,
,
,
,
0.8
,
,
, 1
PFR Fractional Distance
Figure 5. Turnover frequencies with respect to reactor length for isobutane cracking over catalyst USY-C at 773 K and 11%
7.
a. 9.
conversion.
However, as conversion increases, the olefin concentration in the product stream increases and the equilibrium coverage of the surface by carbenium ions increases. This results in significant increases, in the rates of hydride ion transfer reactions with increasing conversion. Accordingly, the importance of the corresponding cycles increases, leading to increased paraffin production with increasing conversion. The rates of oligomerization reactions depend on the olefin concentration in the gas stream and the surface coverages by carbenium ions. Therefore, the rates of these processes also increase with conversion, but a t a slower pace than the hydride ion transfer reactions. Figure 5 demonstrates the autocatalytic nature of catalytic cracking. The observed activity is a function of the two processes that consume isobutane, namely initiation and hydride ion transfer. At very low conversions (less than 1%)most of the activity is due t o the initiation reactions. However, the rates of processes that depend on surface coverage by carbenium ions, such as hydride ion transfer reactions, increase significantly as conversions increase. These processes dominate the performance of the catalyst at conversions higher than 4%.
10.
11
12
13. 14.
IS. 16. 17. 18.
19.
20.
21.
22. 23.
2-MethylhexaneCracking 24.
While the simplicity of a small hydrocarbon like isobutane allows us to readily describe pathways in catalytic cracking, it is important and useful to determine the reaction chemistry of longer carbon chains. Indeed, the utility of kinetic analysis as a tool to probe factors that control catalyst performance depends on the ability to extend the kinetic model to different reaction systems. We have, therefore, attempted to extend our model for isobutane cracking to study 2-methylhexane cracking over two steamed Y-zeolite-based catalysts. Figure 6 shows the mechanism that we have used to describe the kinetics of 2-methylhexane cracking. The chemistry of this reaction sequence is essentially the same as the chemistry of isobutane cracking. Hydrogen and methane are produced by protolytic cracking of C-H and C-C bonds (steps 1 and 6, respectively), forming surface isoheptyl and hexyl cations, respectively. Propylene, isobutylene, 1-butene, truns-2-butene, cis-2butene, and 2-methyl-2-butene are produced by deprotonation and subsequent desorption reactions of the corresponding carbenium ions and are analogous to reactions of isobutane cracking (steps 22-28). Propyl-
25. 26 27.
28. 29.
30. 31.
32.
33.
Figure 6. Reaction scheme for 2-methylhexane cracking.
ene and CS+species, which can react to form isopentane and 2-methyl-2-butene, are produced by the same set
2918 Ind. Eng. Chem. Res., Vol. 33, No. 12,1994
of oligomerizatiodP-scission reactions that take place during isobutane cracking (steps 19-21). Our reaction scheme is able to account for all the Cs species produced without the need for additional oligomerizatiodP-scission reactions. In agreement with our data for isobutane cracking, our experimental data indicate that the formation of higher olefins is negligible. Therefore, we do not include desorption steps for the larger carbenium ions. Since the surface concentration of primary carbenium ions on Y zeolites is expected to be negligible, we include ethylene formation from 2-methylhexane as an irreversible, nonelementary step (step 8). The main differences between the mechanisms for isobutane and 2-methylhexane cracking are due to the longer carbon chain of 2-methylhexane which leads to additional reaction pathways. For example, methane may be produced by cracking at the terminal C-C bond, which leaves a primary carbenium ion on the surface that rearranges to a tertiary ion (step 5). The longer carbon chain makes the formation of higher paraffins like ethane (step 9), propane (steps 10 and ll),isobutane (step 131, and n-butane (step 14) via initiation reactions possible. With the exception of ethane, these paraffins are also formed by hydride ion transfer reactions. Compared to isobutane cracking, the species that undergo /I-scission and hydride ion transfer reactions are different for 2-methylhexane, but the chemistry of these reactions does not change. In particular, isobutane and isohexane, in addition to propane, n-butane and isopentane, are formed by hydride ion transfer from a tertiary position on the reactant paraffin to a surface carbenium ion (steps 29-33). Propylene, isobutylene, and 1-butene can be produced by /I-scission of hexyl and isoheptyl cations (steps 15-18). Isomerization reactions involving different carbenium ions may participate in 2-methylhexane cracking (steps 2-4,7, and 12),but the essential chemistry of the process remains unchanged from isobutane cracking. Several products (hydrogen, methane, and propane) may be produced by more than one initiation reaction pathways. When isomerization reactions of the resulting carbenium ions are taken into account (steps 2-4, 7, and 12, respectively), some of these steps are not linearly independent. For example, hydrogen can be produced by protolytic cracking of the tertiary or any of the secondary C-H bonds. However, unless information is available that allows us to distinguish chemically between the two reactions, the optimization procedure cannot distinguish between the two processes. For this reason, and since the protolysis of a secondary C-H bond is not considered to be important for hydrogen production over Y-zeolites, we only include hydrogen formation by protolysis of the tertiary C-H bond (step 1)(Abbot and Wojciechowski, 1987; Abbot and Wojciechowski, 1989; Brenner and Emmett, 1982). A similar assumption is not necessary for production of methane and propane (steps 5 and 6, and 10 and 11,respectively). Initiation steps 5, 6 and 11belong to the same families with other reactions (steps 9, 14, and 13, respectively) and they are assigned a common Evans-Polanyi constant with these steps. Therefore, it may be possible to distinguish the initiation reactions producing methane and propane from each other by their chemical similarity with other linearly independent initiation processes. We estimated the kinetic parameters for 2-methylhexane cracking using the methods outlined earlier for
Table 2. Standard Entropy Changes and Preexponential Factors Estimated at 773 K for Reaction Steps in 2-MethylhexaneCracking preexponential factors ( s -or ~ s-l Torr-') AS (cal AS*(cal mol-' K-l) mol-' K-l) forward reverse -24.7 8.5 x lo4 2.7 x lo6 step 1 -6.89 2.18 1.09 2.8 x 1013 9.3x 1012 step 2 2.18 2.8 x 1013 9.3 1012 1.09 step 3 1.09 2.18 2.8 1013 9.3 1012 step 4 step 5 -24.7 8.5 x lo4 8.9 x lo6 -4.66 step 6 8.5 x lo4 6.8x lo6 -24.7 -4.14 -0.26 1.4 1013 1.8 1013 step 7 -0.52 8.5 x 104 o step 8 33.0 -24.7 8.5 x lo4 6.3x lo6 -3.98 step 9 step 10 8.5 x lo4 1.5 x lo6 -24.7 -5.66 -24.7 step 11 8.5 x lo4 5.6 x lo6 -3.75 1.0 x 1013 2.6 x 1013 step 12 -0.95 -1.91 -24.7 8.5 104 1.1 io6 -5.06 step 13 8.5 x lo4 1.8 x lo6 -24.7 -1.52 step 14 36.3 step 15 2.9 x 10l6 4.3 x lo6 14.8 step 16 2.9 x 10l6 6.4 x lo6 14.8 35.5 2.9 x 10l6 6.1 x lo5 14.8 40.2 step 17 35.6 step 18 2.6 x 10l6 5.7 x lo6 14.7 step 19 3.0 x 10l6 1.6 x lo5 -23.4 -38.4 -0.16 1.5 x 1013 1.8 1013 step 20 -0.33 step 21 1.2 x 104 14.9 3.0 x 1016 43.5 step 22 1.9 105 1.8 x 1016 14.0 37.1 step 23 2.1 x 10'6 1.1 x 106 14.3 38.5 step 24 7.9 104 2.1 x 1016 14.3 39.1 14.3 step 25 2.1 x 10l6 2.5 x lo6 36.8 14.3 2.1 x 10l6 2.8 x lo6 step 26 36.6 14.5 2.4 x 10l6 3.5 x lo6 step 27 36.4 14.7 step 28 2.6 x 10l6 2.4 x lo5 37.3 -2.66 step 29 -27.7 1.9 x 104 7.1 104 -2.05 step 30 4.9 104 -25.8 1.4 105 step 31 9.4x 104 -24.5 1.2 105 -0.43 -2.04 step 32 1.4 x 104 4.0 104 -28.2 -1.52 step 33 -27.9 1.7 x 104 3.7 104
isobutane cracking. Table 2 gives the results for estimates of preexponential factors. We estimated activation energies using the Evans-Polanyi correlation and assigning to reactions of similar chemistry a common Evans-Polanyi constant according to the families defined in the Parameter Estimation for the Kinetic Model section. Steps 5 and 9 for 2-methylhexane cracking belong to family 1; steps 6 and 14 belong t o family 2; steps 11and 13 belong to family 3; steps 2228 belong to family 4; steps 2-4, 7, 12 and 20 belong to family 5; steps 21 and 17 belong to families 6 and 7, respectively; steps 16 and 18 belong to family 8; and steps 15 and 19 belong to family 9. We did not include step 10 in a family because it does not involve rearrangement of a primary to a tertiary carbenium ion through the formation of a secondary carbenium ion. The ethylene formation step (step 8) was also not included in any family since its chemistry is not well understood. Families 2, 4-7, and 9 have reactions in both mechanisms, and estimates of the Evans-Polanyi constants from analyses of isobutane cracking were useful for estimating activation energies for 2-methylhexane cracking. As in the case of isobutane, the remaining unknown Evans-Polanyi constants were adjustable parameters in our model. Our estimates of the preexponential factors based on reasonable assumptions about surface mobility are only first approximations. Since we do not treat preexponential factors as adjustable parameters, our uncertainties in predicting these values from transition state theory must be compensated by changes in activation energies. Therefore, as we extrapolate our kinetic model from isobutane to 2-methylhexane,we cannot expect the
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2919 Table 3. Enthalpy Changes and Activation Energies for Catalyst USY-S1 Estimated at 773 K for Reaction Steps in 2-Methylhexane Cracking
Table 4. Enthalpy Changes and Activation Energies for Catalyst USY-S2 Estimated at 773 K for Reaction Steps in 2-Methylhexane Cracking
catalyst USY-S1, AH+ = 166.7 kcal mol-'
catalyst USY-S2, AH+ = 167.6 kcal mol-l
~~~
A",
step 1 step 2 step 3 step 4 step 5 step 6 step 7 step 8 step 9 step 10 step 11 step 12 step 13 step 14 step 15 step 16 step 17 step 18 step 19 step 20 step 21 step 22 step 23 step 24 step 25 step 26 step 27 step 28 step 29 step 30 step 31 step 32 step 33
(kcal mol-') -7.5 -18.2 -18.2 -18.2 -22.7 -2.7 -20.0 21.6 -19.7 -17.6 0 -17.6 0.03 1.92 2.52 36.8 22.5 21.1 -19.3 18.0 3.38 18.4 34.5 20.8 18.0 18.4 34.4 36.1 -19.9 -2.28 -18.0 -0.47 0.75
Eo
Ea,for
(kcal mol-') 37.4 0 0 0 46.3 39.7 0 23.1 46.3 70.0 31.7 0 31.7 39.7 35.1 29.8 20.0 29.8 35.1 0 18.7 0 0 0
(kcal mol-') 33.6 0 0 0 34.9 38.4 0 33.9 36.5 61.2 31.7 0 31.7 40.7 36.3 48.2 31.2 40.4 25.4 18.0 20.4 18.4 34.5 20.8 18.0 18.4 34.4 36.1 12.6 24.1 11.4 26.0 22.9
0 0 0 0
22.5 25.2 20.4 26.2 22.5
AH-
Ea,,,
(kcal mol-1) 41.1 18.2 18.2 18.2 57.7 41.1 20.0 12.3 56.2 78.9 31.7 17.6 31.7 38.8 33.8 11.4 8.77 19.3 44.7 0 17.1 0 0 0 0
0 0 0 32.5 26.3 29.4 26.4 22.2
values of the Evans-Polanyi constants to remain completely unchanged. In addition, the value of the AH+ parameter is an average of the enthalpies of stabilization of surface carbenium ions of different sizes and structures with respect to the enthalpy of stabilization of the proton. Since we do not allow different values of AH+ for different carbenium ions, the values of EvansPolanyi constants for surface reactions involving different carbenium ions are only approximations. The experimental data indicate that gas phase cracking, probably by radical processes, occurs along with catalytic cracking of 2-methylhexane. Therefore, we used experimental data from blank experiments to correct the catalytic cracking data. The catalytic bed is a small portion of the total reactor volume, and most of the gas phase contribution to the reaction occurs upstream of the catalytic bed. Hence, the product streams of gas phase cracking processes, measured in experiments with empty reactors at flow rates equal to those used in catalytic studies, were used in our analyses as additions to the 2-methylhexane feed to the catalyst bed. We used the steamed catalyst USY-S1 to calibrate the kinetic model for 2-methylhexane cracking and fitted the 102 dependent responses using the following 12 adjustable parameters: AH+ and the Evans-Polanyi constants of families 1,3, and 8, and steps 1,8,10,2933. For simplicity, the preexponential factor for step 8 was assigned the same value as the initiation steps. During our simulations we determined that initiation steps 10 and 11for propane formation, which with step 12 are linearly dependent, could not be distinguished from each other, and the Evans-Polanyi constant of
step 1 step 2 step 3 step 4 step 5 step 6 step 7 step 8 step 9 step 10 step 11 step 12 step 13 step 14 step 15 step 16 step 17 step 18 step 19 step 20 step 21 step 22 step 23 step 24 step 25 step 26 step 27 step 28 step 29 step 30 step 31 step 32 step 33
(kcal mol-') -6.60 -18.2 -18.2 -18.2 -21.9 -1.82 -20.0 21.6 -18.8 -16.7 0.89 -17.6 0.92 2.81 2.52 36.8 22.5 21.1 -19.3 18.0 3.38 17.5 33.6 19.9 17.2 17.6 33.5 35.2 -19.9 -2.28 -18.0 -0.47 0.75
Eo
(kcal mol-') 35.8 0 0 0 46.1 39.7 0 24.7 46.1 70.0 35.6 0 35.6 39.7 35.1 29.8 20.0 29.8 35.1 0 18.7 0 0 0 0 0 0 0
22.5 25.2 21.5 26.2 22.5
Ea,for
Ea,,
(kcal mol-')
(kcal mol-')
32.5 0 0 0 35.1 38.8 0 35.5 36.6 61.6 36.1 0 36.1 41.1 36.3 48.2 31.2 40.4 25.4 18.0 20.4 17.5 33.6 19.9 17.2 17.6 33.5 35.2 12.6 24.1 12.5 26.0 22.9
39.1 18.2 18.2 18.2 57.0 40.7 20.0 13.9 55.5 78.3 35.2 17.6 35.2 38.3 33.8 11.4 8.77 19.3 44.7 0 17.1 0
0 0 0 0 0 0
32.5 26.3 30.5 26.4 22.2
step 10 was set to a high value. Table 3 shows the resulting activation energies. We then fitted the 153 dependent responses available for the USY-S2 catalyst using the following 6 adjustable parameters: AH+ and Evans-Polanyi constants of families 1and 3, and steps 1, 8 and 31. As in the case of isobutane cracking, we assumed the Evans-Polanyi constants for oligomerization, p-scission, and hydride ion transfer reactions to be catalyst independent. We allowed the parameter of step 31, which is a hydride ion transfer step, to change to better fit the ratio of isobutane versus n-butane produced. Table 4 shows the resulting activation energies.
Results and Discussion Our kinetic models for isobutane and 2-methylhexane cracking provide good descriptions of experimental data collected at various temperatures and conversions over different USY-zeolite-based catalysts. These models predict the trends of complex product distributions using a limited number of kinetic parameters that relate to the fundamental surface chemistry of carbenium and carbonium ions. Figures 7 and 8 show comparisons between experimental data and model predictions for 2-methylhexane cracking over USY-S1 and USY-S2 catalysts. For USYS1, the model accurately predicts the experimental rates for both paraffin and olefin formation and captures the essential trends of the experimental data. We also obtain good agreement for site time yields of paraffins over USY-S2, and for site time yields of olefins a t
2920 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
0.05
E
i=
0.04
al
L
iii cn
E
f
n
0.03 0.02 5
10
15 20 25 Conversion (Oh)
30
Figure 7. Simulated and experimental paraffin and olefin site time yields for 2-methylhexane cracking over USY-S1 at 773 K and various conversions. Points represent experimental data.
-p E
0.025 L
I 0.1
0.02
0.08
0.015
0.06
; 1 0.01
0.04
0.005
0.02
F
v
2
i’ v,
i=
3 ‘D 2 cn
6
4
8
10
12
14
16
Conversion (%)
Figure 8. Simulated and experimental paraffin and olefin site time yields for 2-methylhexane cracking over USY-S2 at 773 K and various conversions. Points represent experimental data.
0
/ /
0 2
\
/
5
cn
$
~
15 20 25 30 Converslon (%) Figure 9, Simulated and experimental paraffin to olefin ratios for 2-methylhexane cracking over USY-S1 and USY-S2 a t 773 K and various conversions. Paraffins and olefins with three or more carbon atoms are counted. Points represent experimental data.
5
10
conversions above 5-6%. At conversions below 5%, USY-S2 is very active in the formation of olefins. We tentatively explain this observation by invoking a catalytic site that is very active for olefin production but is rapidly poisoned by coke formation at higher conversions. High rates of olefin formation may occur via cracking of cation radicals formed on electron acceptor sites (Stamires and Turkevich, 1964; McVicker et al., 1983). In general, the model predicts changes in the paraffin t o olefin ratio (Figure 9) with conversion well. This ratio increases only slightly with conversion for USY-S1, while it increases appreciably at low conversions for USY-S2. Olefin desorption reactions are shown by our kinetic analysis to have essentially the same activation energies for catalysts USY-S and USY-S1 (Yaluris et al., 1994a);
Figure 10. Catalytic cycles for 2-methylhexane cracking over catalyst USY-S1 at 773 K and 9.2% conversion at the reactor exit.
two catalysts that have undergone similar steaming pretreatments. In addition, activation energies of hydride ion transfer reactions maintain the trends reported for isobutane cracking. Values found for these steps in 2-methylhexane cracking are about 4 kcal/mol lower than values for similar steps reported for isobutane cracking, suggesting that the preexponential factors are underestimated for 2-methylhexane. The activation energy for hydrogen formation by initiation reactions during 2-methylhexane cracking over catalyst USY-S1 is lower by ca. 6 kcal/mol compared to the value reported for isobutane cracking over USY-S (Yaluris et al., 1994a). This change is probably due to hydrogen production from coke producing processes which are not taken into account by this model. The rate of such processes has been reported to increase as the molecular weight of the reactant increases (Gates et al., 1979). Figure 10 shows a schematic diagram of the catalytic cycles operative during the catalytic cracking of 2methylhexane for catalyst USY-S1 a t 773 K and 9.2% conversion. We have grouped several surface and gas phase species together for simplicity, and some cycles presented here are, therefore, a combination of several cycles of similar chemistry. Furthermore, since surface CS+ species are produced and consumed in a single oligomerization/,&scissionreaction pathway (steps 19211, we have not included this species in the schematic diagram shown in Figure 10. The nomenclature in this figure is the same as for Figure 4. Differences between the catalytic cycles for isobutane and 2-methylhexane are due to the larger carbon chain length of the 2-methylhexane molecule which increases the number of available reaction pathways. Propylene is produced by the initiatioddesorption cycle that starts with the protolytic cracking of 2methylhexane, forming n-butane or isobutane and leaving a propyl cation on the surface. This cation can then desorb as propylene and restore the Br~nstedacid site. However, in contrast to isobutane cracking, these cycles
also produce paraffins with three or more carbon atoms resulting in 1:l paraffin to olefin ratio. Another difference from isobutane cracking is that propylene production by this cycle is not dominated by other cycles that consume propylene. The main pathway for the formation of propyl cations which are consumed by hydride ion transfer now involves j3-scission reactions of isoheptyl cations. On the other hand, some of the initiation/ desorption cycles shown in Figure 10 that lead to C4 olefin production are dominated by hydride ion transfer cycles. These cycles that produce isobutylene and l-butene are dominated by cycles that can include isomerization, @-scission,and hydride ion transfer reactions, and they eventually lead to the formation of the corresponding paraffins or isomerized olefins. An important difference between the chemistry of isobutane and 2-methylhexane cracking is the role of hydride ion transfer cycles. During isobutane cracking, hydride ion transfer reactions result in the formation of isobutyl cations which desorb as isobutylene, isomerize, or take part in oligomerization reactions. In the case of 2-methylhexane cracking, hydride ion transfer reactions result in the formation of isoheptyl cations which are consumed by j3-scission reactions. For example, a hydride ion transfer/@-scissioncycle may start with the adsorption of isobutylene to form an isobutyl cation. Hydride ion transfer with 2-methylhexane then yields isobutane and an isoheptyl cation. Since the latter does not desorb at an appreciable rate, it undergoes @-scission,yielding a C4 olefin and a propyl cation on the surface. The cycle closes with propylene formation by the deprotonation of the propyl cation. During this cycle one reactant paraffin (2-methylhexane) and one product olefin (isobutylene) are replaced by one product paraffin (isobutane) and two product olefins ((3.4 and C3). Therefore, the hydride ion transferlj3-scission cycles lead to both paraffin and olefin formation a t a 1:l ratio. Another difference between isobutane and 2-methylhexane cracking is the existence of initiation/@-scission cycles for 2-methylhexane. These cycles consist of initiation reactions that produce isoheptyl and hexyl cations that are sufficiently large to undergo /?-scission reactions. Hence, these cycles produce two olefins for each 2-methylhexane molecule consumed. Because of the increased significance of the various @-scissioncycles for 2-methylhexane cracking, their contribution to olefin production is substantially increased compared to the initiation/desorption cycles. For example, for a conversion of about 10% the contribution of the j3-scission cycles t o the TOF of olefin production is ca. 65% at the exit of the reactor for catalyst USY-S1, compared to a value of ca. 15% for catalyst USY-S in the case of isobutane cracking. The distribution of C3, C4, Cg, and c6 species in the product stream depends on the rates of the catalytic cycles presented here. Since the cycles that produce C3 and C4 species a t a 1:l ratio dominate the reaction, the distribution of C3, Cq, Cg, and CS species does not change appreciably with conversion. This result is in agreement with the experimental data which indicate that the fraction of C3 species in the c 3 - C ~product stream changes only slightly from 0.503 t o 0.494 for a change in conversion from 9.2 to 24.6% over USY-S1. The rates of the constituent processes for the various catalytic cycles are shown in Figure 11 versus the fractional position in the PFR for catalyst USY-S1 a t 773 K and 9.2% conversion. Important similarities exist
-Initiation
.-
A
#a ._ a
-
u)
a a u
.
D
Oligomerization
0
Desorption Hydride Transfer
0.04
I ---p - scission
0.03
-
@
I 0 I
e
*-.
0
s
..-/I*. . .
g 0.02 -
LL
e
/.-.
4 I,...
0.01
-
# I
.-....-# -.-e-.,@
&
r
0
d 0
0.2 0.4 0.6 0.8 PFR Fractional Distance
1
Figure 11. Turnover frequencies with respect to reactor length for 2-methyihexane cracking over catalyst USY-S1 at 773 K and 9.2%conversion.
between the results shown in this figure and the results for isobutane cracking shown in Figure 5. Initiation rates change relatively little with increasing 2-methylhexane conversion. In addition, the autocatalytic nature of catalytic cracking causes the rates of coverage dependent processes like hydride ion transfer and @+cission reactions to increase substantially as 2-methylhexane conversion increases; a result of an increase in the surface coverage by carbenium ions. However, important differences also exist between isobutane and 2methylhexane cracking. For example, initiation reactions for 2-methylhexane cracking produce Ci to C4 paraffins and have a more significant contribution to the total activity for 2-methylhexane cracking. This contribution to the total TOF is 37% for catalyst USYS1 at the exit of the reactor at 9.2% conversion, whereas it is only 22% for isobutane cracking activity over USYS. This difference is caused by a larger number of C-C bonds that can break protolytically for 2-methylhexane cracking (Abbot and Wojciechowski, 1988a; Abbot and Wojciechowski, 19891, especially since it is easier to break internal C-C bonds than those close to the terminal carbon (Abbot and Wojciechowski, 1989; Mirodatos and Barthomeuf, 1988). Our analysis seems to confirm this for catalyst USY-S1. Almost 90% of the paraffins produced over this catalyst by initiation reactions are C3 and C4 paraffins. Although the rates of hydride ion transfer and @-scission reactions increase with conversion, they are quite significant even at low conversion for 2-methylhexane. This is in contrast with our analyses of isobutane cracking since hydride transfer reactions become negligible at low conversions where surface coverage by carbenium ions is low. However, because of radical cracking processes in the reactor section above the catalyst, the 2-methylhexane feed stream contains olefins that become equilibrated with the catalytic surface a t the reactor inlet. Thus, carbenium ion coverage a t the reactor inlet results in hydride ion transfer reactions becoming important earlier than expected in the reactor. This agrees with suggestions (Stamires and Turkevitch, 1964; McVicker et al., 1983; Scherzer, 1989) that catalytic cracking may be initiated by protonation on Brgnsted sites of olefins produced by radical cracking of the reactants. Because paraffins and olefins are produced from initiatioddesorption, initiation/j3-scission,and hydride ion transferlp-scission cycles, the paraffin to olefin ratio is a function of the rates of these cycles. As shown in Figure 11,the rates of the hydride ion transfer reactions
2922 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
increase when conversion increases, and therefore increasing conversion favors hydride ion transfer/@scission cycles. These cycles produce olefins and paraffins at a 1:l ratio, which is the same ratio produced by initiatioddesorption cycles. In contrast, the initiatiod @-scissioncycles produce two olefins per 2-methylhexane molecule consumed. Assuming that significant hydrogen transfer does not occur to form coke or aromatics, the paraffin to olefin ratio for species with three or more carbon atoms should always be less than 1, in agreement with the conclusions reached by others (Abbot and Wojciechowski, 1987). As shown in Figure 9, the paraffin to olefin ratio changes only slightly with increasing conversion for catalyst USY-S1, while it changes significantly for catalyst USY-S2. The behavior of the paraffin to olefin ratio with conversion depends on which of the initiation cycles is dominant at the inlet of the reactor. If the initiatioddesorption cycles are dominant at the reactor entrance (catalyst USY-Sl), then the initial value of the paraffin to olefin ratio is close t o 1. As conversion increases, contributions of the hydride ion transfer/@scission cycles increase this ratio only slightly to the limiting value of 1. If, however, the initiation/@-scission cycles are dominant at the reactor entrance (catalyst USY-SP), then the initial value of the paraffin to olefin ratio is substantially lower than 1. Subsequent contributions from the hydride ion transfer/@-scissioncycles as conversion increases increase the value of this ratio toward its upper limit.
Conclusions We have developed kinetic models for isobutane and 2-methylhexane cracking, based on carbonium and carbenium ion surface chemistry, gas phase thermodynamic data, transition state theory for preexponential factors, and the Evans-Polanyi correlation for activation energies. The parameters of these models, which are related to catalyst properties, provide information about factors controlling catalytic activity and selectivity. We have used these models t o probe catalytic cycles that occur during cracking a t different temperatures and conversions over various USY-zeolite-based catalysts. Catalytic cycles for isobutane cracking that include initiation reactions are independent of conversion and lead to olefin production. Cycles that include hydride ion transfer, oligomerization, and @-scission reactions become more important at higher conversions. Since the hydride ion transfer cycles lead t o paraffin production, higher conversions result in lower olefin selectivity. We found important differences in the types of catalytic cycles occurring during 2-methylhexane cracking due t o the larger number of reaction pathways available for the larger hydrocarbon molecule. For example, paraffins and olefins with three or more carbon atoms may be produced by cycles that include both initiation and hydride ion transfer reactions. The paraffin to olefin ratio cannot be greater than 1, and the effect of conversion on this value is smaller than for isobutane cracking. Thus kinetic models based on fundamental surface chemistry that control the catalytic process may be used to identify similarities and differences in the reactivities of different hydrocarbon molecules. This approach provides strategies for developingnew catalysts that are able to effectively adjust rates of the different catalytic cycles to increase the selectivity of desirable products
and in some cases reduce production of polluting byproducts. Accordingly, this approach should find important applications in the development of catalysts for clean manufacturing.
Acknowledgment We would like t o thank Stan Koziol for carrying out the kinetic measurements and Gale Hodge for the FTIR work. This work was partially supported by funds provided by Engelhard Corp. and the Office of Basic Energy Sciences of the U. S. Department of Energy (DEFG02-84ER13183). This work was also supported in part by the U. S. Environmental Protection Agency and the Center for Clean Industrial and Treatment Technologies.
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