Ind. Eng. Chem. Res. 1994,33, 223-234
223
Kinetic Modeling of Ethene and Propene Aromatization over HZSM-5 and GaHZSM-5 Dmitri B. Lukyanov,t N. Suor Gnep, and Michel R. Guisnet Catalyse en Chimie Organique (Chimie 7), URA CNRS 350, Universitk de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
A kinetic model for ethene and propene aromatization over HZSM-5 and GaHZSM-5 is developed. This model describes olefin oligomerization and cracking on zeolite catalytic sites (ZCS), diene formation via hydrogen transfer on ZCS and via dehydrogenation on gallium species, diene cyclization on ZCS, and formation of cyclic diolefins and aromatics via hydrogen transfer on ZCS and via dehydrogenation on gallium species. The rate constants of various reaction steps are compared, and the contribution of gallium in formation of dienes and aromatics is estimated. It is shown that aromatics formation accelerates olefin conversion due t o the olefin consumption, on one hand, and inhibits olefin conversion due to the partial blocking of the zeolite catalytic sites, on the other hand. Because of this, both the increase and the decrease in olefin conversion over GaHZSM-5 can be observed (in comparison with HZSM-5), depending on the feed olefin and on the reaction conditions. Oligomerization
1. Introduction
C2’
Consideration of aromatization reactions of various feedstocks (alcohols, olefins, paraffins, etc.) over ZSM5-type catalysts leads to the conclusion that all reactions proceed via formation and subsequent transformation of light olefins into aromatics (Haag, 1984; Weisz, 1986; Guisnet et al., 1992). The reaction pathway for aromatics formation from light olefins over HZSM-5 (see Figure 1) includes two main stages: (i) olefin interconversion and (ii) olefin aromatization (Haag, 1984). Both stages are complex acid catalyzed reactions proceedingvia carbenium ion mechanisms. Olefin interconversion includes olefin isomerization, oligomerization, and cracking steps (Garwood, 1983). Isomerization steps are not shown in Figure 1,since it has been established (Haag, 1984; Quann et al., 1988) that under the conditions of olefin aromatization olefin isomers are in equilibrium. Olefin aromatization involves,as in the case of catalysis by strong acids, a number of polymerization, isomerization, cyclization, and hydrogen-transfer steps, lumped under the term “conjunct polymerization” (Ipatieff and Pines, 1936; Pines, 1981). Following the proposed mechanisms of olefin conjunct polymerization (Pines, 1981) and taking into account the results of studies of olefin aromatization over HZSM-5 (Vedrine et al., 1980;Gnep et al., 1987;Quann et al., 1988), one can conclude that the formation of olefinic carbenium ions (dienesadsorbed on protonic acid sites) via hydrogentransfer reactions between olefins is the first step in the sequence of reaction steps leading to aromatics formation (see Figure 1). Once formed, olefinic carbenium ions can be easily transformed into cyclic olefins via cyclization reactions. After that a number of hydrogen-transfer steps between cyclic olefins and cyclic diolefins, on one hand, and carbenium ions, on the other hand, occur resulting in aromatics formation. Simultaneously with the formation of aromatics the formation of light paraffins is observed (Quann et al., 1988). This is a consequence of the hydrogen-transfer mechanisms in which the formation of hydrogen-deficient hydrocarbons (dienes, cyclic diolefins, and aromatics) is balanced by the formation of paraffins (Poutsma, 1976; Pines, 1981). In a number of papers (Shibata et al., 1986; Gnep et al., 1988, 1989; Meriaudeau et al., 1989; Le Van Mao et al., 1991; Inui et al., 1992) it has been shown that selectivity toward aromatics can be increased significantly if olefin aromatization is carried out over gallium-containingZSM-5 0888-5885/94/26~3-0223$04.50/0
Hydride transfer
4
- C5’
D6-DlO
1
I Cyclization Hydride transfer
Hydride transfer
-
A6 A10 ALKYLBENZENES
CYCLIC DIOLEFINS
CYCLIC OLEFINS
Figure 1. Schematicrepresentation of olefin aromatization pathway over HZSM-5.
catalysts (GaHZSM-5, HZSM-5 + Ga203, Ga-silicate of MFI structure). Although all investigators agree that gallium speciesprovide a new route for aromatics formation via dehydrogenation reaction steps, the exact mechanism of gallium action is not yet established (for discussion see review papers by Guisnet et al. (1992) and Ono (1992)). It is also not clear in dehydrogenation of which hydrocarbons gallium species are involved: in dehydrogenation of olefins into dienes, or in dehydrogenation of naphthenes into aromatics, or in dehydrogenation of both the olefins and naphthenes. In order to get further insight into mechanisms of olefin aromatization over HZSM-5 and GaHZSM-5 catalysts, quantitative information on the rates of various reaction steps is necessary. Such information is not available in the literature and it cannot be easily obtained because of the complexity of the aromatization reaction. As has been already shown for the methanol to hydrocarbon reaction (Luk’yanovet al., 1988;Luk’yanov, 1989),kinetic modeling can be used as a tool for investigation of complex reactions and for quantitative estimation of the role of various reaction steps in these reactions. Recently Luk’yanovand Shtral (1991) described a kinetic model for light olefin aromatization reaction over HZSM-5 zeolites differing in the aluminum content and pretreatment conditions. The proposed kinetic model was based on the simplified reaction scheme (direct cyclization of olefins was considered) and was used for analysis of the reactivities of C2=Clo’ olefins and for quantitative comparisonof the catalytic activities of various HZSM-5 zeolites in the different reaction steps. Since the simplified reaction scheme was 0 1994 American Chemical Society
224 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
used, comparison of the rates of different reaction steps was not performed. In this paper we propose a kinetic model for light olefin aromatization over HZSM-5 and GaHZSM-5 zeolites, based on the reaction scheme shown in Figure 1 (in the case of GaHZSM-5catalyst the dehydrogenation of olefins, cyclic olefins, and diolefins is considered as well). We also report quantitative data on the rates of various reaction steps over HZSM-5, and discuss the role of gallium species in the formation of dienes from olefins and aromatic hydrocarbons from naphthenes. For the determination of the kinetic parameters we use the experimental data on ethene and propene conversion over HZSM-5 (&/A1= 40) and GaHZSM-5 prepared by an ion exchange method (4 wt 5% of Ga). These data were reported previously by Gnep et al. (1988) and Doyemet (1989),and were obtained under non-deactivatingconditions (catalyst time on stream was 2-3 min) in a flow reactor at 530 "C and hydrocarbon pressure of 1 bar. 2. Kinetic Model
As has been already mentioned above, aromatization reactions of various feedstocks over ZSM-5 catalysts proceed via formation of light olefins and their subsequent aromatization. It seems unquestionablethat the sequence of olefin aromatization steps as well as the rate constants of these steps should be the same for different feedstocks (although the rates of aromatization steps could be different because of different concentrationsof olefins and other hydrocarbons which could compete with olefins for the catalytic sites). In this connection, it was of interest to develop a kinetic model for light olefin aromatization reaction that could meet two requirements: (i) to describe quantitatively the main pathways of olefin aromatization reaction and (ii) to form the basis for other kinetic models for aromatization reactions of different feedstocks (e.g., for methanol to gasoline conversion or for liquefied petroleum gas (LPG) aromatization process). In formulating the kinetic model, it was also kept in mind that this model should be detailed enough to be used (if necessary) for mathematical modelingof the aromatization processes, i.e., to allow prediction of both the quantity and the quality (composition) of the aromatic products. The procedure of kinetic model formulation is discussed below. 2.1. Components of the Kinetic Model. As is clear from the Introduction, olefin aromatization is a complex heterogeneous reaction that involves many individual reaction species and many more chemical reactions (one species can participate in many reactions). Therefore, the system is far too complex to consider chemical transformations of every individual hydrocarbon. In this work the reduction of the number of the reaction species was conducted by lumping all isomers of the same carbon number into a single component. Previously it has been shown (Luk'yanov et al., 1985a,b; Quann and Krambeck, 1991) that such a lumping results in kinetic models that can be used successfully in the design and development of industrial processes over ZSM-5catalysts. In this way, 42 components, which represent all reaction species involved in the light olefin aromatization reaction, were formed: 9 olefins C2=-C10=, 10 paraffins Cl-Clo, hydrogen, 7 dienes D4-Dl0, 5 alkylcyclohexenes X6-xl0, 5 alkylcyclohexadienes Y6-yl0, and 5 alkylbenzenes AgA10 (subscript denotes the number of carbon atoms in the component molecule). The selected components, as will be shown below, allow description in detail of olefin aromatization pathways over ZSM-5 catalysts. These
components allow as well extension of the proposed kinetic model for other aromatization processes, some of which are of commercial interest. An example of such an extension for propane aromatization reaction, recently reported in brief by Lukyanovet al. (1993),will be discussed in detail in a future publication. It is worthwhile to mention that selection of the wider components would result in a kinetic model that could not be applied for the problem of mathematical modeling and optimizing of commercial processes. Moreover, the wider lumping would be in contradiction with the wellknown difference in the reactivities of olefins and paraffins of different molecular weight. Because of these two reasons, wider components were not considered in this study. 2.2. Reaction Scheme. The kinetic model for ethene and propene aromatization has been derived on the basis of the set of adsorption and reaction steps shown below. In this scheme Z denotes protonic acid sites of the zeolite and 2, corresponds to gallium species which are active in the dehydrogenation reaction steps. Adsorption of paraffins and hydrogen on Z and Z, catalytic sites has been considered to be negligible. 1.hydrocarbon adsorption on zeolite protonic acid sites (31 steps):
C,=
+Z
D,
+Z
X,
Z
-
-
Ks1
+ + + -
C,-Z
(2 In I10)
(1)
Kd
D,Z
(4 In 5 10)
(2)
X,Z
(6 In 5 10)
(3)
Y,Z (6 In 5 10)
(4)
(6 In I10)
(5)
K&3
Kd
Y,
Z
K.6
Z
A,
A,Z
2. hydrocarbon adsorption on gallium catalytic sites (31 steps):
-
c,=+ z, c,=z, (2 In I10)
-
(6)
K7.
D,
+ Z,
+ Z,
KS.8
X, Y,
+ Z,
Kd
A,
+ Z,
D,Z,
(4 In I10)
(7)
X,Z,
(6 In I10)
(8)
Y,Z,
(6 In I 10)
(9)
A,Z,
(6 In I10)
(10)
&IO
3. olefin oligomerizationand cracking on zeolite protonic acid sites (16 steps):
-
koL(n,m)
c,=+cm=z c,+,=z kcR(n,m)
(2 In,m 5 8 ;4 In
+ m 510)
(11)
4. diene formation via hydrogen transfer between two
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 225 olefin molecules on zeolite protonic acid sites (63 steps):
+ C,'Z
-
kDdm)
+ C,
(4 In I 10; 2 I m I10) (12) 5. interaction of olefins with dienes on zeolite protonic acid sites to form higher molecularweight dienes (15 steps): C,=
C,=
+ D,Z
-
D,Z
&DOL
D,+,Z
kDCR
+
(2 In I 6; 4 S m I 8; 6 In m I10) (13) 6. diene cyclization on zeolite protonic acid sites (5 steps):
-
kw(n)
D,Z X,Z ( 6 5 n 510) (14) 7. aromatics formation via hydrogen transfer between olefins and cyclic olefins (or cyclic diolefins) on zeolite protonic acid sites (90 steps):
X,
+ C,'Z
-
kdm)
-
Y,Z
+ C,
(6 In I10; 2 Im I10) (15)
k.dm)
Y,+C,'Z
A,Z+C,
(6In510;2ImI10) (16)
8. cracking of C5+paraffins on zeolite protonic acid sites (39 steps):
-
kca(n)
C,+Z C,
+Z
-
kcc(n,m)
C,'Z
C,'Z+H,
+ C,,
(5 I n I 1 0 )
(17)
(5 I n I 10; 2 I m 5 9) (18)
9. diene formation via olefin dehydrogenation on gallium catalytic sites (7 steps):
-
In agreement with previous kinetic studies of olefin reactions over ZSM-5 (Luk'yanovand Shtral, 1991;Quam and Krambeck, 1991),the rate constants of many reaction steps are considered as functions of the number of carbon atoms in the components; e.g., the rate constant koL(n,m) depends on the number of carbon atoms in the molecules of both olefins (C,' and Cmp)participating in the olefin oligomerization step (11). This is done in order to reflect the effect of the hydrocarbon molecular weight on the hydrocarbon reactivity and, in the case of reaction 18, to reflect as well the different probabilities of formation of Cz'-Cg' olefins in the paraffin protolytic cracking steps. 2.3. Rate Expressions. For derivation of the rate equations two simplifying assumptions were introduced right from the beginning: Rates of hydrocarbon adsorption and desorption are much higher than the rates of hydrocarbon transformation. In other words, adsorption equilibrium is considered to be established. Adsorption constants are independent of hydrocarbon molecular weight. The equations for the rates of the reaction steps were derived on the basis of the mass action law (Temkin, 19791, as is illustrated below by the equation for the rate of olefin oligomerization-cracking steps: roL = ~
a l ~ o ~ ~ ~ , -~ K,~kc~(n,m)P~,+,-[Zl ~ ~ c n ~ c m . ~ ~ l
(22) where roL is the net rate of reaction 11;Kal is the adsorption constant for olefins on zeolite protonic acid sites; koL(n,m) and IzcR(n,m) are the rate constants for oligomerization of C,' and C,' olefins and for cracking of Cn+,' olefin, respectively; Pc,-, PCm-9 and Pcn+m- are the partial pressures of C,', Cm5,and C,+,= olefins, respectively; [Zl is the steady-state concentration of vacant zeolite protonic acid sites. It should be noted that reactions over zeolite and gallium catalytic sites were treated independently. Steady-state concentrations of Z and Z, were determined as
kDpi
C,'Z, D,Z, + H, (4 In I10) (19) 10. aromatics formation via dehydrogenation of cyclic olefins and diolefins on gallium catalytic sites (10 steps):
-
kYFl
X,Z,
Y,Z,
k m
Y,Z,
A,Z,
+ H, + H,
(6 I n I 10) (6 I n I 10)
(20) (21)
In the above scheme only six-membered rings for cyclic olefins are considered. This is done in order to decrease the number of components in the kinetic model, although we understand that formation of six-membered rings from dienes can proceed via formation and following isomerization of five-membered rings, especially in the case of hexadiene cyclization (Gnep et al., 1987). We consider the steps of diene cyclization as irreversible steps. This is also done in order to simplify the kinetic model. In the above scheme one can see the steps of protolytic cracking of CS+ paraffins which are the products of hydrogen transfer steps. These steps are introduced in the reaction scheme, since the analysis of the kinetic model for hexane cracking reaction over HZSM-5 (Luk'yanov et al., 19921, performed during this work, has demonstrated that cracking rates of CS+ paraffins (and, probably, of C5 paraffins) should be relatively high under reaction conditions used in this study.
With numeration of all the components from 1 (CZ' olefin) to 42 (A10 alkylbenzene) and of the reaction steps from 1 (CZ' + CZ'Z Cr'Z) to 245 (YloZ, AloZ, + Hz), we can represent the kinetic model as a set of 42 equations describing the rates of transformations of 42 components in 245 reaction steps:
-
-
245
where Ri is the rate of transformation of the ith component; vi8 is the stoichiometric coefficient of the ith component in the sth reaction step; rs is the rate of the sth reaction step. For kinetic modeling the following reactor model has been used: dCi/dr = RiMi (1 I i 5 42)
(26)
where Ci is the weight fraction of the ith component in the reaction mixture; 7 is the contact time (h); Rj is the rate of transformation of the ith component (mol/(g h)); Mi is the molecular weight of the ith component (g/mol). For
226 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
numerical integration of the system of differential equations, the Gear method was applied. 2.4. Estimation of Kinetic Parameters. The reaction scheme used in this study involves 31 reversible and 214 irreversible reaction steps. This results in a tremendous number of rate constants (276)which cannot be estimated on the basis of any reasonable number of experiments. To reduce complexity, C4=-C1o= olefins were considered as species of the same reactivity in all reaction steps except the steps of olefin cracking. In addition, it has been assumed that the reactivity of cyclic olefins and diolefins in the steps of hydrogen transfer is independent of their molecular weight. Both assumptions are in agreement with the generally accepted carbenium ion mechanisms of olefin transformation over acidic catalysts, and are further supported by the results of the previous kinetic study of olefin aromatization over ZSM-5 (Luk'yanov and Shtral, 1991). With GaHZSM-5 catalyst, an additional assumption was made that the rates of the dehydrogenation steps are independent of the hydrocarbon molecular weight. In our opinion, this was the only reasonable assumption that could be made in the absence of any quantitative data on dehydrogenation rates of different hydrocarbons over gallium active species in ZSM-5. In consequence of the above made assumptions 12 rate constants were used for olefin oligomerizationand cracking steps (reaction ll),16 rate constants were used for acidcatalyzed olefin aromatization steps (reactions 12-16), and 3 rate constants were used for gallium-catalyzed dehydrogenation steps (reactions 19-21). The values of these rate constants were determined by means of comparison of the mathematical modeling results with the experimental dependence of the product distribution on contact time (WHSV-') for ethene and propene reactions at 530 "C. It should be noted that these experimental data are not sufficient for determination of the rate constants of paraffin protolytic cracking steps (reactions 17 and 181, since these steps do not play an appreciable role in olefin aromatization reaction. Therefore, the rate constant values for the paraffin cracking steps were estimated using the experimental data on c5-C~paraffin reactions, and will be discussed (together with these data) in a future publication on kinetic modeling of the paraffin aromatization reaction over ZSM-5 catalysts. Estimation of the rate constants of olefin transformation steps was carried out step by step. A t first, simulations were performed at relatively low contact time values in order to estimate the values of the rate constants of olefin oligomerization and cracking steps in reaction 11, and to describe quantitatively olefin distribution before the beginning of the aromatization reaction. Since reaction 11 is a reversible reaction, one could expect that thermodynamic data would allow calculation of the rate constants of the reverse (cracking) steps from the rate constants of the forward (oligomerization)steps. In reality, as has been shown by Quann and Krambeck (1991), the situation is strongly complicated by two factors: (i) the standard Gibbs free energy of the isomer groups (components of the kinetic model) cannot be computed definitely, since it is not exactly known what isomers of C6=-C10= olefins can be formed in the channels of the medium-pore ZSM-5; (ii)the thermodynamic lumping does not necessarily coincide with a kinetic one. Because of these two reasons, the thermodynamic data on C2'-Clo' olefins available in the literature (Stull et al., 1969;Alberty, 1983)were not incorporated directly into the kinetic model, but were used at the beginning of the estimation procedure for computation of the initial rate constant values.
I
0
0.004
0 008 I
0 * 012
Contact time (h)
0
O s
002
0.004
0 006 I
Contact time (h) Figure 2. Ethene (A)and propene (B)oligomerization over HZSM5. Experimental data (points) and calculated curves for the concentrations of reaction products as functions of contact time.
In agreement with the previous study (Luk'yanov and Shtral, 1991)it has been confirmed that 1 2 rate constants (for details see the above-mentioned paper) allow quantitative description of olefin interaction in 16 oligomerization and cracking steps. A sharp increase in olefin reactivity from ethene to butenes observed previously was also confirmed. Figure 2 demonstrates the degree of agreement between experimental data, obtained at low and moderate olefin conversions, and simulated curves, obtained with the zero values of all rate constants except the rate constants of olefin oligomerization and cracking steps. Since the description of olefin distribution before the beginning of the aromatization process was obtained, rate constants of the other reaction steps were introduced and the estimation procedure was continued. During this procedure the effect of different rate constants on olefin conversion and product distribution was investigated. These results are discussed in the following sections of the paper. 3. Kinetic Modeling Results
3.1. Effect of Aromatics Formation on Ethene and Propene Conversion over HZSM-5.In order to derive a primary description of light olefin aromatization reaction over HZSM-5 zeolite, we have assumed that the rates of diene cyclization steps (reaction 14) and aromatics formation steps (reactions 15 and 16) are much higher than the rates of diene formation steps (reaction 12). It should be noted that this assumption was used only a t the beginning of the estimation procedure (in order to simplify it) and was eliminated in the final description of the olefin aromatization reaction. Figures 3 and 4 compare kinetic modeling results, corresponding to the primary description (curves l), with the experimental data on ethene and propene transformations plotted as function of contact time. Figure 3 shows that
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 227 I
100 I
7 Y
3
Y
Y
0
0
0
0,Ol
0.02
0.03
0.04
30t
n 7i
A
2ot 0
0,05
20
40
60
Conversion ( x )
Contact time ( h )
-
100 I
30
E! 201
5 Y
O
V c V 0
1
100
n.I/ iI /I I
l 0 0
B 201
B
80
20
40
60
80
100
Conversion ( X I 0
0
0.01
0.02
0.03
0.04
0.05
Contact time ( h ) Figure 3. Ethene (A) and propene (B)aromatization over HZSM-5. Experimental data (points)and calculatedcurves for olefin conversion as a function of contact time. Curve 1, primary description; curve 2, rate constants of diene formation are increased by a factor of 2.5; curve 3, adsorption constant for aromatics is increased by a factor of 30; dashed line, rate constants of diene formation are equated to zero.
2\ /
Contact time ( h )
I
0.03 0.04 0.05 Contact time ( h ) Figure 4. Ethene (A) and propene (B)aromatization over HZSM-5. Experimental data (points) and calculated curves for aromatics concentration as a functionof contacttime. For curves refer to Figure 3. 0
0.01
0.02
the dependence of propene conversion on contact time is described with good accuracy, but for ethene transformation simulated conversion is too high as compared with the experimental one at the contact time value of 0.048 h. From Figure 4 it follows that agreement between
Figure 5. Ethene aromatization over HZSM-5. Experimental data (points) and calculated curves for the concentrations of C A S paraffins (A) and aromatics (B)as functions of ethene conversion (curve 2 coincides with curve 3). For curves refer to Figure 3.
simulated and experimental data on aromatics formation attained in the primary description (curves 1) was qualitative rather than quantitative. From Figure 3B it is evident that aromatics formation has a very strong effect on propene conversion at conversions higher than 60% (comparecurve 1 with the dashed line that corresponds to the hypothetical case when aromatics formation does not occur). This result can be easily understood. With propene as feed, olefins are rapidly equilibrated (see Figures 2B and 3B) in the absence of aromatization reaction, and further transformation of propene does not occur. Aromatization reaction (through diene formation) withdraws olefins from the reaction mixture, and in consequence of this, propene transformation proceeds further. Simulations of ethene conversion performed in the absence of aromatization reaction have demonstrated (see Figure 3A, dashed curve) that, with ethene as feed, olefin oligomerization reaction stops at ethene conversionof approximately 87 5%. Because of this, the effect of aromatics formation on ethene conversion is observed only at very high conversions and is less pronounced than in the case of propene transformation. Figures 5 and 6 show simulated and experimental data on the concentrations of aromatics and C& paraffins plotted vs olefin conversion. Presented data (see curves 1)have ledus to the conclusionthat aromatization reaction starts at lower conversions of feed olefin than is predicted in the primary description. Therefore, the rate constants of diene formation steps were increased by a factor of 2.5, and good agreement was attained between simulated and experimental data plotted in coordinates of concentration vs conversion (see Figures 5 and 6, curves 2). After that we plotted aromatics concentration as function of contact time (see Figure 4,curves 2), and found that simulated and experimental data were in agreement only at very low contact time values, while at high contact time values simulated aromatics concentration was much higher than the experimental one. We also considered the dependence
228 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
0
20
40
60
80
100
Conversion ( x ) 0
20
40
60
80
100
Conversion ( X I
x 2.3 -
a V c
9
1
V 0
0 0
20
40
60
80
100
Conversion ( X ) Figure 6. Propene aromatization over HZSM-5. Experimental data (points) and calculated curves for the concentrations of C& paraffins (A) and aromatics (B) aa functions of propene conversion (curve 2 coincides with curve 3). For curves refer to Figure 3.
of olefin conversion on contact time (see Figure 3, curves 21, and found an increase in conversions of both ethene and propene. Obviously, this increase was due to the effect of aromatics formation on the olefin conversion. Having in mind all results presented above, we have concluded that aromatics, once formed, inhibit transformations of olefins. This conclusion was not entirely unexpected to us, since the inhibiting effect of aromatics on the hydrocarbon transformation on acid sites was reported previouslyby several authors (Gnep and Guisnet, 1977; Guisnet and Fouche, 1991;Namba et al., 1986;Chen et al., 1988). In order to introduce inhibition into the kinetic model, we have been increasing the aromatics adsorption constant step by step (in the primary description the adsorption constant for aromatics was equal to the adsorption constant for olefins). Finally, with the increase of adsorption constant by a factor of 30, the desired description of the experimental data on both ethene and propene reactions was attained (see Figures 3-6, curves 3). It should be noted that in this description the adsorption constants for dienes, cyclic olefins, and cyclic diolefins were 5 times lower than the adsorption constant for aromatics. The observed difference between olefin and aromatics adsorption constants seems to be too high to be explained only by the difference in the basicity of olefins and alkylbenzenes. Therefore, we would like to propose, in agreement with the previous explanations of the inhibiting effect of aromatics on mordenites (Chen et al., 1988; Guisnet and Fouche, 19911, the following explanation of the results presented above. From our point of view, inhibition of olefin transformations results not only from the competitive adsorption of aromatics and olefins on acid sites, but also (and,maybe, mainly) from the blocking of zeolite catalytic sites and channels by sterically large aromatic moleculesof low diffusivity. Such molecules,e.g., 1,3,5-trimethylbenzene, can be formed in the channel intersections of HZSM-5, but they are not able to move
Figure 7. Final description of propene aromatization over HZSM5. Experimental data (pointa) and calculated curves for the total concentration of dienes and cyclic olefins( 0 )and for the concentration of cyclic diolefins (0) t u functions of propene conversion.
along the channels. Some of these molecules can be transformed into isomerswhich can escapefrom the zeolite; e.g., 1,3,5-trimethylbenzenecan be transformed into 1,2,4trimethylbenzene, and the other molecules can be considered as coke precursors. Formation of sterically large aromatic molecules in the channel intersections should limit the diffusion of olefins in the zeolite channels, causing a decrease in the apparent rate of olefin transformation. 3.2. Comparison of the Rate Constants of Various Reaction Steps over HZSM-5. From stoichiometry of the aromatization process it follows that the molar ratio between paraffins and aromatics produced in the hydrogen-transfer steps should be 3:l. We compared the experimental molar concentrations of paraffins and aromatic hydrocarbons at different propene conversionsand found out that this ratio was equal to 3 only at high propene conversions (above 80 % ), while at moderate conversions this ratio changed in the range between 8 (le = 32%) and 3.8 ( x = 62%). This observation means that not all unsaturated hydrocarbons (dienes, cyclic olefins, and diolefins), which are produced in the initial steps of the aromatization reaction, are immediately converted into aromatics. In order to estimate quantitatively the concentrations of dienes, cyclic olefins, and cyclic diolefins, we have assumed that (i) the average molecule of these hydrocarbons contains 7 carbon atoms (in agreement with the composition of the product aromatic hydrocarbons) and that (ii) the total concentration of dienes and cyclic olefins is 5 times higher than the concentration of cyclic diolefins. The second assumption was based on the data reported by Quann et al. (1988) on the concentrations of C,HZ,Z and CnHzn-4hydrocarbons formed in the course of propene conversion on ZSM-5 catalyst. With these assumptions we obtained estimations of the concentrations of unsaturated hydrocarbons in the reaction mixture at different levels of propene conversion (see Figure 7), and used these estimations in the procedure of determination of the rate constants of various reaction steps of the aromatization process. Analysis of mathematical modeling results and experimental data has shown that 16 rate constants allow the quantitative description of the ethene and propene aromatization process over HZSM-5: three rate constants were used for the steps of diene formation in reaction 12; one rate constant was used for the steps of diene interaction with olefins in the forward reaction 13 and one rate constant was used for the steps of diene cracking in the backward reaction 13; five rate constants were used for the steps of diene cyclization in reaction 14;and six rate constants were used for the steps of aromatics formation in the consequent reactions 15and 16. Figures 7-11 demonstrate the degree of agreement between experimental
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 229
Y
8
z
z
40 I
I
i 30-
1
A
20-
U
c
8 10-
8
V
0. 0
0.05
0.10
0.15
0.20
0.25
Conversion
Contact t i m e ( h )
I
(x)
50
-g
e
5
40
CI
1
-
5
30
U
20
c
8 0
0.01
0.02
0,011
0.03
0 0
0,
Contact t i m e ( h )
Figure 8. Final description of ethene (A) and propene (B)aromatization over HZSM-5. Experimental data (points) and calculated curves for the concentrations of aromatics and C r C s paraffins as functions of contact time.
I
10
/
o /
20
40 60 Conversion ( % )
80
100
Figure 10. Final description of ethene aromatization over HZSM-5 (a) and GaHZSM-5 (0)zeolites. Experimental data (points) and calculated curves for the concentrations of C&a paraffins (A) and aromatics (B)as functions of ethene conversion.
I
IUU
0
0 0
0.05
0.10
0.15
0.20
0.25
contoct t i m e ( h )
20
40 60 Conversion ( x )
80
100
50
Y
-g
40
U
1
30
. 3
CI
0
20 OI
c U
3
10
J
B 0 0
0
0.05
0,lO
0,15
0.20
0,25
Contact t i m e ( h )
Figure 9. Final description of ethene (A) and propene (B)aromatization over HZSM-5).( and GaHZSM-5 (0) zeolites.Experimental data (pointa) and calculated curves for olefin conversionas a function of contact time.
and simulated data attained in the final description of ethene and propene aromatization over HZSM-5 zeolite. Table 1 shows the relative values of rate constants of various hydrogen-transfer steps over HZSM-5. From
20
40 60 Conversion ( x )
80
100
Figure 11. Final description of propene aromatization over HZSM-5 ( 0 )and GaHZSM-5 (0) zeolites. Experimental data (points) and calculated curves for the concentrations of CzCa paraffins (A) and aromatics (B)as functions of propene conversion.
presented data it follows that olefin reactivity in hydrogen transfer increases from ethene to butenes and remains constant with further increase of olefin molecular weight. The observed difference in olefin reactivities is in agreement with the previously reported data (Luk’yanov and Shtral, 1991)and can be explained, from our point of view,
230 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 1. Effect of Molecular Weight on Olefin Reactivity in the Hydrogen-Transfer Steps on HZSM-5 (C,,=, C,' = Olefins; C, = Paraffins; D, = Dienes; X, = Cyclic Olefins; Y, = Cyclic Diolefins; A, = Alkylbenzenes)
---
reaction stea
+Z',C x, +Z',C Y, + Z',C C,'
D,Z + C, Y,Z + c, A,Z + C,
re1 rate const value" m=2 m=3 m24 1.0 9.8 13 36 356 482 108 1068 1446
"Rate constant value for the interaction of C,' adsorbed ethene has been taken as unity.
olefins with
Table 2. Qualitative Correlation between the Rate Constants of Olefin Aromatization Steps Shown in Figure 1 reaction steps olefin oligomerization olefin cracking diene formation diene cyclization cyclic diolefin formation aromatics formation
rate constants slightly lower than olefin cracking rate constants the highest 10-100 times lower than olefin oligomerization rate constants the same as olefin cracking rate constants the same as olefin oligomerization rate constants slightly higher than olefin oligomerization rate constants
by the difference in the probabilities of formation and/or existence of primary, secondary, and tertiary carbenium ions, which are generally accepted to be intermediate species in the olefin transformations on zeolites. Actually, only a primary carbenium ion can be formed from ethene, a secondary ion from propene, and secondary and tertiary olefins. = ions from C ~ = - C ~ O Table 1demonstrates that the rate of hydrogen transfer from one olefin to the other is about 35 times lower than the rate of hydrogen transfer from cyclic olefin to olefin. The rate of the latter reaction in turn is 3 times lower than the rate of hydrogen transfer from cyclic diolefin to olefin. The obtained results correlate well with the increasing stability of the carbenium ions, which are the products of the examined sequence of hydrogen-transfer reactions. In order to determine the slowest reaction step in the olefin aromatization reaction, we have compared the rate constants of various reaction steps shown in Figure 1.The rate constants of diene cyclization were compared with the rate constants of olefin cracking (these constants are of the same dimension), and were found to be of the same order of magnitude. Taking into account that olefin cracking proceeds somewhat faster than olefin oligomerization, as has been reported by Haag (1984) for 1-heptene cracking and propene oligomerization reactions, one can conclude that diene cyclization steps are relatively rapid in comparison with olefin oligomerization steps. Oligomerization of olefins in turn has been found in this work to be faster than diene formation via hydrogen transfer between olefins. For example, comparison of the rate constant of dimerization of two Cd= olefins with the rate constant of hydrogen transfer between these olefins has demonstrated that the dimerization rate is about 30 times higher than the rate of hydrogen transfer. The latter result is of special interest, since it provides data on the rates of interaction of the same molecules on the same catalytic site in two different reactions: formation of a C-C bond and hydride abstraction. To represent the obtained kinetic results in an easy and convenient manner, we have summarized them in Table 2 (comparisonof the rate constants of different dimensions
was discussed above). Presented data demonstrate that the steps of diene formation via hydrogen transfer between olefins are the slowest reaction steps in the olefin aromatization reaction over HZSM-5. 3.3. Effect of Gallium on the Rates of Formation of Dienes and Aromatics. In order to estimate quantitatively the effect of gallium on the rates of various reaction steps of olefin transformation,we have compared the rate constants of the kinetic model for ethene and propene aromatization over HZSM-5 and GaHZSM-5 zeolites. The procedure of determination of the rate constant values for GaHZSM-5catalyst is discussedbriefly below. A t the beginning of this procedure we have compared ethene and propene conversions and olefin product distribution, observedon HZSM-5and GaHZSM-5zeolites at low contact time values. The comparison has shown that under these conditions (when aromatics are not yet formed) both the conversion and product distribution are practically the same for these two zeolites. This result means that Ga species introduced into HZSM-5 by the ion exchange method (i) do not participate in the initial steps of ethene and propene transformation and (ii) do not change the catalytic (acidic) sites of the parent HZSM5. The second conclusion is further supported by the identity of ammonia temperature programmed desorption curves found earlier for HZSM-5 and GaHZSM-5 samples considered in this work, and by similar activities of these two samples in the model acid catalyzed reactions of n-heptane cracking and cyclohexeneisomerization (Gnep et al., 1988). From conclusion i it follows that Ga species are inactive not only in the olefin oligomerization and cracking steps, but also in the other acid catalyzed reaction steps of the aromatization process. Actually, the results of this work (see Table 2) demonstrate that acid catalyzed olefin oligomerization and cracking steps are at least as fast as acid catalyzed olefin aromatization steps. Since that is so, it is difficult to imagine that Ga species, which are inactive in the acid-catalyzed olefin oligomerization and cracking steps, will participate in the acid catalyzed olefin aromatization steps. Taking into account these considerations and conclusion ii on the invariability of acidic sites, we have concluded that the rate constant values for the acid catalyzed reaction steps, determined for HZSM5, can be also used in the kinetic description of the acid catalyzed reaction steps over GaHZSM-5 catalyst. Recent analysis of the literature data on aromatization reactions over Ga-containing HZSM-5 zeolites (Guisnet et al., 1992; Ono, 1992) has led us to the conclusion that the effect of Ga on the olefin aromatization results mainly from Ga participation in the steps of olefin dehydrogenation into dienes, as was proposed by Ono et al. (1989) and Meriaudeau et al. (1989), and/or in the steps of naphthene dehydrogenation, as was proposed by Gnep et a1.(1988). In order to test this conclusion, we investigated the effect of the rate constants of the above-mentioned reaction steps on the concentrations of the main hydrocarbon products of the aromatization process (aromatics and c2-C~paraffins). Figures 12 and 13 compare the kinetic modeling results with the experimental data on ethene aromatization over GaHZSM-5 zeolite. From presented data it follows that assumption on Ga participation only in olefin dehydrogenation (Figure 12) or only in naphthene dehydrogenation (Figure 13) does not allow the obtaining quantitative description of experimental data on both aromatics and
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 231
c
50
-b!
40
Y
3
2
c
0
30-
3
.e Y
Y 0
b
20
20-
c e, V 0 c
C V
10
u
0 0
10o
20
40
60
Conversion
s
A
3
30
s
1
40-
v
.e
c e,
-5 Y
Y
b
5
80
m
,
100
Conversion
(%)
I
(%)
50
il
40
-b! -
13
40-
z
c
0
2
B
Y
30-
1
.e Y
i
L
g 20e, V
0 c
100
0
o
20
40
Conversion
60
80
100
m
0
paraffin formation on GaHZSM-5. The same conclusion was drawn from consideration of the results of kinetic modeling of the propene reaction. Since that is so, the rate constants for olefin and naphthene dehydrogenation were introduced in the kinetic model simultaneously.After that the final description of ethene and propene aromatization reaction over GaHZSM-5 was obtained (see Figures 9-11). In this description one rate constant was used for the steps of olefin dehydrogenation in reaction 19 and one rate constant was used for the steps of dehydrogenation of cyclic olefins and cyclic diolefins in reactions 20 and 21 (rate constants k y ~ and l k m were considered to be equal, since the available experimental data did not allow separation of them). Comparison of these rate constants has shown that dehydrogenation of cyclic olefins and cyclic diolefins proceeds 8 times faster than the dehydrogenation of olefins. It is worthwhile to mention that the correctness of the obtained estimations of the dehydrogenation rate constants is further supported by the data on hydrogen formation over GaHZSM-5 catalyst (see Table 3). It was of interest to compare quantitatively the contributions of Ga species and zeolite catalytic sites in the formation of dienes and aromatics on GaHZSM-5catalyst. The results of kinetic modeling performed with this aim are shown in Figures 14and 15, where curves 1correspond to the formation of dienes and aromatics on the zeolite protonic acid sites via hydrogen-transfer reactions and curves 2 correspond to the formation of these hydrocarbons on the gallium species via dehydrogenation reactions. From presented data one can draw two conclusions. The first one is that the contribution of gallium to the formation of dienes (Figure 14) is several times higher than its
40
Conversion
(%)
Figure 12. Ethene aromatization over GaHZSM-5. Experimental data (points) and calculated curves for the concentrations of CZCS paraffins (A) and aromatics (B)as functions of ethene conversion. Curve 1, final description corresponding to reaction on HZSM-5; curves 2 and 3, rate constants of diene formation via olefin dehydrogenation are equal to 1.8 and 18 mol/(g h), respectively.
,
20
60
80
100
(%)
Figure 13. Ethene aromatization over GaHZSM-5. Experimental data (points) and calculated curves for the concentrations of c 2 - C ~ paraffins (A) and aromatics (B)as functions of ethene conversion. Curve 1, final description corresponding to reaction on HZSM-5; curves 2 and 3, rate constants of aromatics formation via dehydrogenation of cyclic olefins and diolefins are equal to 30 and 300 mol/(g h), respectively. Table 3. Experimental and Simulated Data on Hydrogen Formation over GaHZSM-5. at Ethene and Propene Conversion of 85% feed olefin ethene propene
hydrogen concn, wt % exptl simuld 2.0 2.3 1.5 1.4
a With HZSM-5catalyst, both the experimental and the simulated hydrogen concentrations were always below 0.05 wt %.
contribution to the formation of aromatics (Figure 15). The second conclusion is that, with ethene as feed, the gallium contribution in the formation of both the dienes and aromatics is higher than in the case of propene conversion. For example, Figure 14 shows that, at olefin conversion of 30 % and with ethene as feed, about 85 % of dienes are formed via the dehydrogenation route, while with propene as feed, this route produces about 70% of dienes. In conclusion we would like to draw attention to the effect of gallium on the olefin conversion. Figure 9 shows that in propene aromatization reaction GaHZSM-5 is slightly more active than HZSM-5, while in ethene aromatization reaction the latter catalyst is more active. This unexpected result is nicely predicted by the kinetic model and can be explained by the different effect of aromatics formation on ethene and propene conversion. With propene as feed, an increase in the rate of aromatics formation due to the effect of gallium should be followed (see section 3.1) by an acceleration of the olefin transformation due to the olefin consumption, on one hand, and by an inhibition of the olefin transformation due to
232 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 16
4u 20
00
40
60
0
Conversion (I)
20 40 Conversion ( % I
60
16
5
-
12}
B
3
0 0
20
Conversion
40
60
0
20
Conversion
(%)
40
60
(%)
Figure 14. Simulated curves for diene concentration as a function of ethene (A) and propene (B) conversion (rate constants of diene cyclization steps are equated to zero). Curve 1, diene formation is considered to occur only through hydrogen-transfer route (rate constants of diene formation via dehydrogenation are equated to zero); curve 2, diene formation is considered to occur only through dehydrogenation route (rate constants of diene formation via hydrogen transfer are equated to zero).
Figure 15. Simulated curves for aromatics concentration as a function of ethene (A) and propene (B)conversion (diene formation proceeds via both hydrogen-transfer and dehydrogenation steps). Curve 1, aromatics formation is Considered to occur only through hydrogen-transfer route (rate constants of aromatics formation via dehydrogenation are equated to zero); Curve 2, aromatics formation is considered to occur only through dehydrogenation route (rate constants of aromatics formation via hydrogen transfer are equated to zero).
the blocking of the zeolite catalytic sites and channels, on the other hand. From Figure 9B it appears that the accelerating effect is stronger than the inhibiting one. With ethene as feed, the accelerating effect is much less pronounced (see Figure 3A) and, in consequence of this, the inhibiting effect overcomes (see Figure 9A). It is worthwhile to note that previous studies of propene aromatization over HZSM-5 and GaHZSM-5, performed by Shibata et al. (1986), have demonstrated that gallium introduction in HZSM-5 is followed by a significant increase in the olefin conversion. In contrast to this finding, the results of the present work demonstrate that the propene conversion is only slightly higher on the GaHZSM-5 sample than on the HZSM-5 one (see Figure 9B). This contradiction can be explained if one takes into consideration (i) the difference in the propene initial partial pressure (0.2 bar in the experiments of Shibataet al. (1986) and 1 bar in our experiments) and (ii) the complex character of the effect of aromatics formation on the rate of olefin transformation. Actually, a decrease in the propene partial pressure leads to a decrease in the partial pressure of the reaction products. In consequence of this, the inhibiting effect of the aromatic products should be much less pronounced in the experiments of Shibata et al. (1986) in comparison with the experiments considered in this work. Kinetic modeling results shown in Figure 16 support this conclusion and demonstrate furthermore that, with the olefin initial partial pressure of 0.2 bar, the GaHZSM-5 sample should be more active than the HZSM-5 one in both the ethene and the propene aromatization reactions.
4. Conclusions
The following conclusions can be drawn: (i)A kinetic model for ethene and propene aromatization over HZSM-5 and GaHZSM-5 is developed. This model describes olefin oligomerization and cracking on zeolite catalytic sites, diene formation via hydrogen-transfer steps on zeolite catalytic sites and via dehydrogenation steps on gallium active species, diene cyclization on zeolite catalytic sites, and formation of cyclic diolefins and aromatics via hydrogen-transfer steps on zeolite catalytic sites and via dehydrogenation steps on gallium active species. The rate constants of various reaction steps were compared, and the steps of hydrogen transfer between olefins resulting in diene formation were found to be the slowest reaction steps. (ii) The role of gallium, added to HZSM-5, in the olefin aromatization reaction is considered, and a conclusion is drawn that gallium species are active in the dehydrogenation of olefins into dienes and of naphthenes into aromatics. The contributions of gallium species and zeolite catalytic sites in the formation of dienes and aromatics are estimated. It is shown that, with ethene as feed, about 85% of dienes and 55 % of aromaticsare formed on gallium active species. In the case of propene conversion, these species are responsible for the formation of 70 % of dienes and 25% of aromatics. (iii) The effect of aromatics formation on the olefin transformation is considered. It is shown that an increase in the rate of aromatics formation is followed by an acceleration of the olefin conversion due to the olefin
Ind. Eng. Chem. Res., Vol. 33, No. 2 , 1994 233
'80
*
I
1
A
0
0.05
0.10
0.15
0.20
0.25
Contact time ( h )
kcH(n)= rate constant of protolytic cracking of C-H bond in C, paraffin (mol/(g bar h)) kc&, rn) = rate constant of cracking of C,+,= olefin resulting in formation of C,' and C,' olefins (mol/(g h)) kDc(n) = rate constant of cyclization of D, diene (mol/(g h)) k D C R = rate constant of diene cracking (mol/(g h)) kDF(m) = rate constant of formation of D, diene via hydrogen transfer from C,' olefin to C,= olefin adsorbed on zeolite protonic acid site (mol/(g bar h)) k D F l = rate constant of diene formation via olefin dehydrogenation (mol/(g h)) kDoL = rate constant of interaction between olefin and diene adsorbed on zeolite protonic acid site (mol/(g bar h)) kOL(n,m ) = rate constant of oligomerization of C,' and C,= olefins (mol/(g bar h)) kyF(m) = rate constant of cyclic diolefin formation via hydrogen transfer from cyclic olefin to C," olefin adsobed on zeolite protonic acid site (mol/(g bar h)) kyF1 = rate constant of cyclic diolefin formation via dehydrogenation of cyclyc olefinson gallium active species (mol/ (g h)) Mi = molecular weight of the ith component (g/mol) PA,, Pc,., PD,, Px,, Py, = partial pressure of alkylbenzene A,, olefin C,=, diene D,, cyclic olefin X,, and cyclic diolefin Y,, respectively (bar) Ri = rate of transformation of the ith component (mol/(gh)) r, = rate of the sth reaction step (mol/(g h)) WHSV = weight hour space velocity (h-l) Z = zeolite protonic acid site [Z] = steady-state concentration of vacant zeolite protonic acid sites Z, = gallium active species [Z,] = steady-state concentration of gallium active species
Y7 B
2o 0 0
0.05
0.10
0.15
0.20
0.25
Contact time ( h ) Figure 16. Simulated curves for ethene (A) and propene (B) conversions.as functions of contact time on HZSM-5 (curve 1) and GaHZSM-5 (curve 2). Feed: olefin and nitrogen in a molar ratio 1:4.
consumption, on one hand, and by an inhibition of the olefin conversion due to the partial blocking of the zeolite catalytic sites, on the other hand. Because of the complex character of this effect, both the increase and the decrease in the olefin conversion over GaHZSM-5 can be observed (in comparison with HZSM-5), depending on the feed olefin and on the reaction conditions.
Acknowledgment We thankV. I. Shtral for the preparation of the program for computer simulations. Also, we thank the reviewers for their helpful comments on the manuscript. D.B.L. gratefully acknowledges the French Ministry of Research and Technology for the fellowship that allowed him to work at the University of Poitiers beginning November 1992.
Nomenclature
Ci = weight fraction of the ith component in the reaction mixture Kal,Ka2, Ka3, Ka4, Kas = constants of adsorption on zeolite protonic acid sites of olefins, dienes, cyclic olefins, cyclic diolefins, and aromatics, respectively (bar-l) Kas, Ka7, Ka8, Kae, Kale = constants of adsorption on gallium active species of olefins, dienes, cyclic olefins, cyclic diolefins, and aromatics, respectively (bar-') k m ( m ) = rate constant of aromatics formation via hydrogen transfer from cyclic diolefin to C,' olefin adsorbed on zeolite protonic acid site (mol/(g bar h)) kml= rate constant of aromatics formation via dehydronation of cyclic diolefins on gallium active species (mol/(g h)) kcc(n,rn)= rate constant of protolytic cracking of C-C bond in C, paraffin resulting in formation of C,= olefin (mol/(g bar h))
Greek Symbols stoichiometric coefficient of the ith component in the sth reaction step 7 = contact time (h)
u, =
Literature Cited Alberty, R. A. Extrapolation of Standard Chemical Thermodynamic Properties of Alkene Isomer Groups to Higher Carbon Numbers. J. Phys. Chem. 1983,87,4999. Chen, J.-K.; Martin, A. M.; Kim, Y. G.; John, V. T. Competitive Reaction in Intrazeolitic Media. Znd. Eng. Chem. Res. 1988,27, 401. Doyemet, Y. J. Aromatisation de 1'Ethane et du Propane sur des Zeolithes du Type MFI. Ph.D. Thesis, University of Poitiers, France, 1989. Garwood,W. E. Conversion of Cz-Cloto Higher Olefinsover Synthetic Zeolite ZSM-5. ACS Symp. Ser. 1983,218,383. Gnep, N.S.;Guisnet, M. Ethylbenzene Isomerization on Platinum Fluorided Alumina. 11. Kinetic Study. Bull. SOC.Chim. Fr. 1977, 435. Gnep, N.S.; Doyemet, J. Y.; Seco,A. M.; Ramoa Ribeiro, F.; Guisnet, M. Conversion of Light Alkanes into Aromatic Hydrocarbons. 1-Dehydrocyclodimeriation of Propane on PtHZSM-5 Catalysts. Appl. Catal. 1987,35,93. Gnep, N.S.;Doyemet, J. Y .;Seco, A. M.; Ramoa Ribeiro, F.; Guisnet, M. Conversion of Light Alkanes to Aromatic Hydrocarbons. 11. Role of Gallium Speciesin Propane Transformation on GaHZSM-5 Catalysts. A p p K a t a l . 1988,43, 155. Gnep, N.S.;Doyemet, J. Y.;Guisnet, M. Conversionof Light Alkanes to Aromatic Hydrocarbons. 3. Aromatization of Propane and Propene on Mixtures of HZSM-5 and of Gaz03. Stud. Surf. Sci. Catal. 1989,46,153. Guisnet, M.; Fouche, V. Isomerization of n-Hexane on Platinum Dealuminated Mordenite catalysts. 111.Influence of Hydrocarbon Impurities. Appl. Catal. 1991,71, 307. Guisnet, M.; Gnep, N. S.; Alario, F. Aromatization of Short Chain Alkanes on Zeolite Catalysts. Appl. Catal. 1992,89,1. Haag, W.0.Acid Catalysiswith Medium Pore Zeolites. In Proceedings of the 6 t h International Zeolite Conference; Olson, D. H., Bisio, A., Eds.; Butterworths: Surrey, UK, 1984;p 466.
234 Ind. Eng.Chem. Res., Vol. 33, No. 2, 1994 Inui, T.; Nagata, H.; Matsuda, H.; Kim, J.-B.; Ishihara, Y. Selective Synthesis of Aromatic Hydrocarbons from Propylene on GalliumSilicate Catalysts. Ind. Eng. Chem. Res. 1992, 31, 995. Ipatieff, V. N.; Pines, H. Propylene Polymerization Under High Pressure and Temperature with and without Phosphoric Acid. Ind. Eng. Chem. 1936,28, 684. Le Van Mao, R.; Dufresne,L.; Yao, J.; Ly, D. The Concept of Hydrogen Back Spillover (HBS). Towards the Development of HighPerformance Catalysts for the Aromatization of Light Alkanes and Olefins. In Preprints of Symposium on Alkylation, Aromatization, Oligomerization, and Isomerization of Short-Chain Hydrocarbons over Heterogeneous Catalysts; ACS Division of Petroleum Chemistry, Inc.: New York, 1991; Vol. 36, p 716. Luk’yanov, D. B. Analysis of the Mechanism of the Methanol to Hydrocarbon Conversion and Comparison of Catalysts Using a Kinetic Model. Kinet. Catal. 1989, 30, 186 (Engl. transl.). Luk’yanov,D. B.; Shtral, V. I. Use of a Kinetic Model for Investigation of Light Olefin Aromatization Reactions over HZSM-5 Zeolites. In Preprints of Symposium on Alkylation, Aromatization, Oligomerization,andlsomerizationof Short-ChainHydrocarbons over Heterogeneous Catalysts;ACS Division of Petroleum Chemistry, Inc.: New York, 1991; Vol. 36, p 693. Luk’yanov, D. B.; Timoshenko, V. I.; Slinko, M. G.; Vinogradov, V. V. Kinetic Model for the Conversionof Methanol to Hydrocarbons on Zeolite Catalysts. Dokl. Phys. Chem. Proc. Acad. Sci. USSR 1985a, 192 (Engl. transl.). Luk’yanov, D. B.; Timoshenko, V. I.; Slinko, M. G.; Vinogradov, V. V.; Nefedov, B. K. Kinetic Model of the Reaction of Methanol Conversionto Hydrocarbons on High-SilicaZeoliteCatalysts. Sou. Chem. Ind. (Khim. Promst) 1985b, 517 (Engl. transl.). Luk’yanov, D. B.; Slinko, M. G.; Vinogradov, V. V. Use of a Mathematical Experiment when Investigating the Conversion of Methanol to Hydrocarbons on ZSM-5High-Silicon Zeolites. Dokl. Phys. Chem., Proc. Acad. Sci. USSR 1988, 451 (Engl. transl.). Luk’yanov, D. B.; Shtral, V. I.; Khadzhiev, S. N. A Kinetic Model for Hexane Cracking Reaction over HZSM-5. In Extended Abstracts and Program of the Ninth International Zeolite Conference;Higgins, J. B., von Ballmoos, R., Treacy, M. M. J., Eds.; Butterworth-Heinemann: Stoneham, MA, 1992; RP 36. Lukyanov, D. B.; Gnep, N. S.; Guisnet, M. A Kinetic Model for Propane Aromatization Reaction over ZSM-5 Catalysts. In Book of Abstracts; Europacat-1, Montpellier; 1993; Montpellier Languedoc-Roussillon Technopole: Montpellier, 1993; p 187. Meriaudeau, P.; Sapaly, G.; Naccache, C. On the Role of Ga, Pt, PtCu ang PtGa in HZSM-5 for the Cyclization of Propane into Aromatics. Stud. Surf. Sci. Catal. 1989, 49, 1423.
Namba, S.; Sato, K.; Fujita, K.; Kim, J. H.; Yashima, T. Shape Selective Cracking of Octane in the Presence of Another Hydrocarbon on HZSM-5. In Proceedings of the Seventh International Zeolite Conference; Murakami, Y., Iijima, A,, Ward, J. W., Eds.; Kodansha: Tokyo, 1986; p 661. Ono, Y. Transformation of Lower Alkanes into Aromatic Hydrocarbons over ZSM-5 Zeolites. Catal. Rev.-Sci. Eng. 1992,34,179. Ono, Y.; Nakatani, H.; Kitagawa, H.; Suzuki, E. The Role of Metal Cations in the Transformation of Lower Alkanes into Aromatic Hydrocarbons. Stud. Surf. Sci. Catal. 1989,44, 279. Pines, H. Chemistry of Catalytic Hydrocarbon Conversions; Academic Press: New York, 1981; Chapter 1. Poutsma, M. L. Mechanistic Considerations of Hydrocarbon Transformations Catalyzed by Zeolites. ACS Monogr. 1976,171, 437. Quann, R. J.; Krambeck, F. J. Olefin Oligomerization Kinetics over ZSM-5. In Chemical Reactions in Complex Mixtures. The Mobil Workshop; Sapre, A. V., Krambeck, F. J., Eds.; Van Nostrand Reinhold New York, 1991; p 143. Quann, R. J.; Green, L. A.; Tabak, S. A.; Krambeck, F. J. Chemistry of Olefin Oligomerizationover ZSM-5 Catalysts. Ind. Eng. Chem. Res. 1988, 27, 565. Shibata, M.; Kitagawa, H.; Sendoda, Y.; Ono, Y. Transformation of Propene into Aromatic Hydrocarbons over ZSM-5 Zeolites. In Proceedings of the Seventh International Zeolite Conference; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Kodansha: Tokyo, 1986; p 717. Stull, D. R.; Westrum, E. F.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; John Wiley: New York, 1969. Temkin, M. I. The Kinetics of Some Industrial Heterogeneous Catalytic Reactions. Adv. Catal. 1979,28,173. Vedrine, J. C.; Dejaifve, P.; Garbowski, E. D.; Derouane, E. G. Aromatics Formation from Methanol and Light Olefins Conversions on HZSM-5 Zeolite: Mechanism and Intermediate Species. Stud. Surf. Sci. Catal. 1980,5, 29. Weisz, P. B. The Remarkable Active Site: A1 in SiOz. Znd. Eng. Chem. Fundam. 1986,25,53. Received f o r review June 4, 1993 Reuised manuscript received October 21, 1993 Accepted November 2, 1993@
* Abstract published in Advance ACS Abstracts, January 1, 1994.
