2896
Ind. Eng. Chem. Res. 1999, 38, 2896-2901
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Model for Skeletal Isomerization of n-Butene over ZSM-22 R. Byggningsbacka,* N. Kumar, and L.-E. Lindfors Laboratory of Industrial Chemistry, Process Chemistry Group, A° bo Akademi University, Biskopsgatan 8, FIN-20500 A° bo, Finland
A kinetic model for n-butene transformation over ZSM-22 was developed. The effects of temperature, partial pressure, and deactivation on the formation of isobutene and byproducts were included in the model. The model was able to predict n-butene transformation over ZSM22 reasonably well, although several assumptions were made in order to reduce the number of adjustable parameters. The major assumptions were that isobutene is only produced through monomolecular reactions and byproducts are only produced in reactions between two n-butene molecules or n-butene and isobutene. Introduction There has been an increasing interest in the skeletal isomerization of n-butene to isobutene over zeolites, since isobutene can be used in the reaction with methanol to produce MTBE (methyl tert-butyl ether). Branched ethers such as MTBE have become an important and highly demanded part of reformulated gasoline, since they enhance the octane number. As the current supply of isobutene from catalytic cracking is insufficient to meet the increasing demand for MTBE, considerable interest has been devoted to producing isobutene from n-butene (Butler and Nicolaides, 1993; Houzvicka et al., 1997). To the best of our knowledge, there has been no published material so far on kinetic modeling of nbutene to isobutene over ZSM-22 zeolites. ZSM-22 is one of the most selective catalysts in skeletal isomerization of 1-butene (Simon et al., 1994; Byggningsbacka et al., 1997), because of its one-dimensional channel system of 10-membered rings with channel diameters of 4.5 × 5.5 Å and no cavities (Kokotailo et al., 1985). Furthermore, ZSM-22 is much more active than other shape selective skeletal isomerization catalysts such as ZSM35 (Byggningsbacka et al., 1998) and SAPO-11. Another advantage of ZSM-22 over ZSM-35 is that ZSM-22 is selective in skeletal isomerization of 1-butene to isobutene within a very short time on stream whereas ZSM-35 needs to be modified by coke deposits for a longer time in order to obtain its shape selectivity (Byggningsbacka et al., 1998).
Experimental Section ZSM-22 was synthesized according to the method described by Byggningsbacka et al. (1997). After wash* To whom correspondence should be addressed. Phone: +358 2 215 4555. Fax: +358 2 215 4479. E-mail:
[email protected].
ing, filtration, and drying, the synthesized zeolite was pressed, crushed, and sieved to particle sizes of 0.1250.250 mm. The as-synthesized form of the zeolites was obtained after the organic template was removed in a calcination step at 550 °C in a flow of nitrogen for 6 h followed by air for an additional 10 h. The as-synthesized form of ZSM-22 was used without further modification, as the activity of ZSM-22 did not increase after ion exchange in a NH4Cl solution and subsequent calcination (Byggningsbacka et al., 1998). The catalytic activity of ZSM-22 in skeletal isomerization of 1-butene (99.0% purity, AGA) to isobutene was investigated using a fixed-bed microreactor system operating at close to atmospheric pressure. The reactant was diluted with nitrogen (99.999% purity, AGA) to regulate the partial pressure of 1-butene. Although 1-butene was the reactant fed to the reactor, n-butene (1-butene, cis-2-butene, and trans-2-butene) can be considered the real reactant, as double-bond isomerization is much faster than skeletal isomerization or any of the other reactions. The fixed-bed reactor was modeled as an isothermal pseudohomogeneous plug flow reactor. The flow through the reactor was considered to be constant, since skeletal isomerization of n-butene is a monomolecular reaction and the dominating byproducts (propene and pentenes) are produced after dimerization followed by cracking. The products from the reactor were analyzed with a gas chromatograph (Varian 3700) equipped with a flame-ionization detector (FID). A capillary column (50 m × 0.32 mm i.d. fused-silica PLOT Al2O3-KCl) was used to separate the products. The experimental data used in the modeling was collected from over 280 experiments. The temperature in the experiments was varied between 350 and 500 °C, the partial pressure of 1-butene was varied between 0.4 and 1.3 atm, and the WHSV of 1-butene was varied between 50 and 1600 h-1. Products were analyzed between 10 and 300 min time on stream (TOS).
10.1021/ie990123w CCC: $18.00 © 1999 American Chemical Society Published on Web 06/12/1999
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2897 Table 1. Typical Product Distribution Obtained from 1-Butene Transformation over ZSM-22 at Temperature ) 400 °C, Partial Pressure of n-Butene ) 0.42 atm, and TOS ) 10 min product
yield (mol %)
selectivity (mol %)
ethene propane propene isobutane n-butane n-butene isobutene pentenes hexenes heptenes octenes
0.2 0.1 5.1 0.2 1.1 55.5 31.6 3.8 0.3 0.3 1.8
0.5 0.2 11.5 0.4 2.4 71.0 8.6 0.7 0.6 4.1
Table 2. Estimated Values for the Parameters at 450 °C (Tmean ) 723.15 K) and Their Standard Errors parameter
parameter value
standard error
A1,mean (atm s-1) Ea1,mean (J mol-1) A2,mean (s-1) Ea2,mean (J mol-1) A3,mean (s-1) Ea3,mean (J mol-1) A′1,mean ∆Hr1,mean (J mol-1) A′2,mean (atm-1) ∆Hr2,mean (J mol-1) a (min-1) b
134 93 700 9.26 31 600 24.0 75 800 0.664 -1630 0.473 -103 000 0.0375 0.309
11.7 3090 0.86 3010 2.57 5480 0.00886 1210 0.0605 2670 0.0123 0.0333
Reaction Steps A simplified reaction scheme was used in the model. The major assumptions were that isobutene is only produced through a monomolecular mechanism and the byproducts are produced in the reactions between two n-butene molecules or n-butene and isobutene. Although there has been intensive discussion as to whether isobutene is produced through monomolecular or bimolecular reactions, the authors of several papers published recently have been in favor of the monomolecular mechanism (Asensi et al., 1996; Houzvicka et al., 1996; Meriaudeau et al., 1997; Byggningsbacka et al., 1998). The reaction between two isobutene molecules was not included in the model, as the constrained space inside the channel system of 10-membered-ring zeolites prevents this reaction (Buchanan et al., 1996). A typical product distribution obtained from n-butene transformation over ZSM-22 is presented in Table 1. The reaction steps in the monomolecular skeletal isomerization of n-butene (n-C4) to isobutene (iso-C4) catalyzed by Bro¨nsted acid sites (H+) can be written as
n-C4 + H+ S n-C4+ n-C4+ S iso-C4+ (rate-limiting step) iso-C4+ S iso-C4 + H+ n-C4 S iso-C4 where n-C4+ and iso-C4+ denote carbenium ion intermediates. The skeletal isomerization step can be considered to be rate limiting, since theoretical calculations by Boronat et al. (1998) demonstrated that the activation energy of this step is much higher than those of both n-butene adsorption and isobutene desorption.
Figure 1. n-Butene (O) transformation to isobutene (×) and byproducts (*) as a function of space time. The initial partial pressures of n-butene in the experiments were 0.52, 0.9, and 1.3 atm. The experiments were performed at 400 °C, and products were analyzed after 10 min TOS. The points in the figures are experimental values, and the lines are calculated values.
The reaction between two n-butene molecules to byproducts (B) can be written as
n-C4 + H+ S n-C4+ n-C4+ + n-C4 w B+ (rate-limiting step) B+ S B + H+ n-C4 + n-C4 w B The reaction between n-butene and isobutene molecules
2898 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999
Figure 2. n-Butene (O) transformation to isobutene (×) and byproducts (*) as a function of space time. Products were analyzed after 10, 60, 180, and 300 min TOS. The experiments were performed at 400 °C, and the initial partial pressure of n-butene was 0.84 atm. The points in the figures are experimental values, and the lines are calculated values.
to byproducts (B) can be written as
n-C4 + H+ S n-C4+ n-C4+ + iso-C4 w B+ (rate-limiting step) B+ S B + H+ n-C4 + iso-C4 w B
isobutene can also be formed in bimolecular reactions, but for the sake of simplicity this was neglected in the calculations. The contribution of the bimolecular reaction in the formation of isobutene over ZSM-22 is nevertheless low because the shape selective properties of the zeolite prevent the formation of highly branched dimers such as 2,2,4-trimethylpentene. Kinetic Equations The following rate equations (eqs 1-3) were derived according to the reaction mechanisms above. The equi-
or
iso-C4 + H+ S iso-C4+ iso-C4+ + n-C4 w B+ (rate-limiting step) B+ S B + H+
r1 )
k1K2(pn-butene - pisobutene/K1) 1 + K2phydrocarbons
(1)
k2K2p2n-butene r2 ) 1 + K2phydrocarbons
(2)
n-C4 + iso-C4 w B depending on whether it is the n-butene or the isobutene which reacts with the Bro¨nsted acid site. The dimerization steps can be considered rate limiting, since theoretical calculation by Rigby et al. (1997) demonstrated that the activation energies of these steps are much higher than those of both adsorption of n-butene and isobutene and desorption of byproducts. Some
r3 )
k3K2pn-butenepisobutene 1 + K2phydrocarbons
(3)
librium constant K2 was obtained by simplifying ∑Kjpj to only one constant, multiplied by the total partial pressure of hydrocarbons. The diffusion of the reactant and the products inside the zeolite crystals was not explicitly included in the model. Nevertheless, most of
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2899
Figure 3. n-Butene (O) transformation to isobutene (×) and byproducts (*) as a function of space time. The temperature in the experiments was 350, 400, 450, and 500 °C. Products were analyzed after 10 min TOS, and the initial partial pressure of n-butene was 0.9 atm. The points in the figures are experimental values, and the lines are calculated values.
the activity was probably fairly close to the pore mouth, as the reactant and products were of similar size to that of the pores of the zeolite. The effect of temperature on the rate and equilibrium constants was assumed to obey the laws of Arrheniusand van’t Hoff, respectively:
kj ) Aj,mean exp(-Eaj,meanq)
j ) 1-3
Kk ) A′k,mean exp(-∆Hrk,meanq) q)
[
]
1 1 1 R Tr Tmean
(4)
k ) 1, 2 (5)
tion, since the coke formation is probably a result of oligomerization of alkenes. Most of the deactivation occurred immediately when the catalyst was exposed to the reactant, followed by a much slower decay. The rapid initial deactivation was probably more dependent on the partial pressure and temperature, but the slower decay could be rather well modeled by eq 7. The following differential equations were used to calculate the changes in partial pressure of n-butene, isobutene, and byproducts as a function of space time (τ):
(6)
The deactivation as a result of coke formation was taken into consideration using the following empirical equation:
r(t) )
rt)0 (1 + at)b
(7)
This equation was used for both isobutene and byproducts, since the changes in the shape selective properties as a function of TOS over ZSM-22 were small. The effect of temperature, WHSV, and partial pressure on the rate of deactivation was ignored, although these parameters could have a considerable effect on the rate of deactiva-
dpn-butene ) -r1 - 2r2 - r3 dτ
(8)
dpisobutene ) r1 - r3 dτ
(9)
dpbyproducts ) 2(r2 + r3) dτ
(10)
The space time (τ) in the experiments was calculated using the following equation:
τ)
(
)( )( )
Tin pr m F(V˙ in,nitrogen + V˙ in,1-butene) Tr pin
(11)
2900 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999
Since the flows of nitrogen and 1-butene were regulated by mass flow controllers at ambient temperature and pressure (Tin ) 298 K and pin ) 1 atm), the flow through the reactor bed was corrected with the measured reaction temperature (Tr) and pressure (pr). The density (F) of the ZSM-22 catalyst bed was approximately 548 kg/ m3. The differential equations (eqs 8-10) were solved numerically using the backward difference method (Henrici, 1962) implemented in the software LSODE (Hindmarch, 1983). The object function was minimized using the Levenberg-Marquardt method (Marquardt, 1963). The methods were available in the MODEST software (Haario, 1994). Results and Discussion The estimated parameter values obtained are summarized in Table 2. The values for the activation energies are lower than what might be expected according to theoretical calculations (Boronat et al., 1998). This is an indication that the reactions might be diffusion limited, which is fairly reasonable, as the channels of ZSM-22 are of molecular size. The effect of the initial partial pressure of n-butene, on n-butene transformation to isobutene and byproducts as a function of space time (τ), is presented in Figure 1. The initial partial pressures of n-butene in the experiments were 0.52, 0.9, and 1.3 atm. The results presented in Figure 1 were obtained at 400 °C, and the products were analyzed after 10 min TOS. The selectivity for isobutene increased with decreasing partial pressure of n-butene. The model predicted the effect of partial pressures on n-butene transformation to isobutene and byproducts rather well over the investigated temperature range. The predictions were more accurate at low space times when the conversion of n-butene was low and the dominating byproducts were propene and pentenes. The effect of deactivation on the n-butene transformation to isobutene and byproducts as a function of space time is presented in Figure 2. Products were analyzed after 10, 60, 180, and 300 min TOS. The results presented in Figure 2 were obtained at 400 °C, and the initial partial pressure of n-butene was 0.84 atm. The equation used for describing the effect of deactivation was able to predict the effect of deactivation well, not only at the temperature and partial pressure presented in Figure 2 but also over the entire investigated temperature and pressure ranges. The effect of temperature, on n-butene transformation to isobutene and byproducts as a function of space time, is presented in Figure 3. The temperatures in the experiments were 350, 400, 450, and 500 °C. The results presented in Figure 3 were obtained using 0.9 atm initial partial pressure of n-butene, and the products were analyzed after 10 min TOS. The selectivity for isobutene increased with increasing temperature. Conclusions The developed model describes the effect of temperature, partial pressure, and deactivation on skeletal isomerization of n-butene to isobutene and the formation of byproducts rather well, although several assumptions were made in order to simplify the model. The model is more accurate at low conversions, when the formation of byproducts other than propene and
pentenes is minor. It is possible to describe deactivation over ZSM-22 using only two parameters, since the shape selectivity of this particular zeolite does not change much as a function of time on stream. The model is able to predict the effect of deactivation reasonably well, although possible effects of temperature, partial pressure, and space time on the rate of deactivation were not taken into consideration. The assumption that isobutene is produced only through a monomolecular mechanism did not cause any problems in the prediction of isobutene formation. Acknowledgment Financial support from the Finnish Graduate School in Chemical Engineering (GSCE) is gratefully acknowledged. Nomenclature Aj,mean ) frequency factor in Arrhenius equation A′k, mean ) pre-exponential factor in van’t Hoff equation a ) deactivation parameter b ) deactivation parameter Eaj,mean ) activation energy in Arrhenius equation ∆Hrk,mean ) reaction enthalpy in van’t Hoff equation Kk ) equilibrium constants kj ) rate constants m ) catalyst mass phydrocarbons ) partial pressure of hydrocarbons pin ) pressure at the mass flow controllers pi ) partial pressures (i ) n-butene, isobutene, byproducts, and hydrocarbons) pr ) total pressure measured in the reactor R ) gas constant (8.3143 J/(mol K) ri ) reaction rates Tin ) temperature at the mass flow controllers Tr ) temperature measured in the reactor Tmean ) mean temperature used in the parameter estimation (Tmean ) 723 K) t ) time on stream Vin,i ) volume flows through mass flow controllers (i ) 1-butene and nitrogen) F ) bulk density of the catalyst bed τ ) space time Abbreviations B ) byproducts B+ ) carbenium ion of byproducts H+ ) Bro¨nsted acid site iso-C4 ) isobutene iso-C4+ ) carbenium ion of isobutene n-C4 ) n-butene n-C4+ ) carbenium ion of n-butene TOS ) time on stream WHSV ) weight hourly space velocity
Literature Cited Asensi, M. A.; Corma, A.; Martinez, A. Skeletal isomerization of 1-butene on MCM-22 zeolite catalyst. J. Catal. 1996, 158, 561. Boronat, M.; Viruela, P.; Corma, A. Theoretical study of the mechanism of zeolite-catalyzed isomerization reactions of linear butenes. J. Phys. Chem. A 1998, 102, 982. Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O. Mechanistic considerations in acid-catalyzed cracking of olefins. J. Catal. 1996, 158, 279. Butler, A. C.; Nicolaides, C. P. Catalytic skeletal isomerization of linear butenes to isobutene. Catal. Today 1993, 18, 443. Byggningsbacka, R.; Lindfors, L.-E.; Kumar, N. Catalytic activity of ZSM-22 zeolites in the skeletal isomerization reaction of 1-butene. Ind. Eng. Chem. Res. 1997, 36 (8), 2990.
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2901 Byggningsbacka, R.; Kumar, N.; Lindfors, L.-E. Comparative study of the catalytic properties of ZSM-22 and ZSM-35/Ferrierite zeolites in the skeletal isomerization of 1-butene. J. Catal. 1998, 178, 611. Haario, H. MODEST User’s Guide; Profmath Oy: Helsinki, Finland, 1994. Henrici, P. Discrete variable methods in ordinary differential equations; Wiley: New York, 1962. Hindmarsh, A. C. ODEPACK-A Systematized collection of ODEsolvers. In Scientific Computing; Stepleman, R., et al., Eds.; IMACS/North-Holland: Amsterdam, 1983; p 55. Houzvicka, J.; Diefenbach, O.; Ponec, V. The role of bimolecular mechanism in the skeletal isomerization of n-butene to isobutene. J. Catal. 1996, 164, 288. Houzvicka, J.; Hansildaar, S.; Ponec, V. The shape selectivity in the skeletal isomerization of n-butene to isobutene. J. Catal. 1997, 167, 273. Kokotailo, G. T.; Schlenker, J. L.; Dwyer, F. G.; Valyocsik, E. W. The framework topology of ZSM-22: A high silica zeolite. Zeolites 1985, 5, 349.
Marquardt, D. W. An algorithm for least-squares estimation on nonlinear parameters. SIAM J. 1963, 11, 431. Meriaudeau, P.; Naccache, C.; Le, H. N.; Vu, T. A.; Szabo, G. Selective skeletal isomerization of n-butenes over ferrierite catalyst: further studies on the possible mechanisms. J. Mol. Catal. A 1997, 123, L1. Rigby, A. M.; Kramer, G. J.; van Santen, R. A. Mechanisms of hydrocarbon conversion in zeolites: A quantum mechanical study. J. Catal. 1997, 170, 1. Simon, M. W.; Suib, S. L.; O’Young, C.-L. Synthesis and characterization of ZSM-22 zeolites and their catalytic behavior in 1-butene isomerization reactions. J. Catal. 1994, 147, 484.
Received for review February 18, 1999 Revised manuscript received April 28, 1999 Accepted May 5, 1999 IE990123W
