Ind. Eng. Chem. Res. 1995,34, 516-523
516
Kinetic Modeling of Propane Aromatization Reaction over HZSM-5 and GaHZSM-5 Dmitri B. Lukyanov,*9+N. Suor Gnep, and Michel R. Guisnet Catalyse en Chimie Organique, URA CNRS 350, Universiti de Poitiers, 40 Avenue d u Recteur Pineau, 86022 Poitiers, France
A detailed kinetic model for a propane aromatization reaction over HZSM-5 and GaHZSM-5 is developed. Kinetic modeling results show that propane transformation over HZSM-5 occurs
via protolytic cracking and hydrogen transfer routes. The contributions of both routes in propane conversion are established. Rate constants of propane transformation steps are found to be a t least 1000 times lower than the rate constants of diene formation steps, which, in turn, are the slowest among the acid-catalyzed olefin aromatization steps. Gallium introduced into ZSM-5 catalyst is active in dehydrogenation of propane into propene, of olefins into dienes, and of naphthenes into aromatics. At the same time, gallium species catalyze propane transformation into methane and ethene hydrogenation into ethane. Both latter reactions appear to be the main reasons for the limit to aromatics selectivity over GaHZSM-5 catalysts. 1. Introduction
-
Propane transformation into olefins
BTX aromatic hydrocarbons, namely, benzene, toluene, and xylenes, are now obtained by catalytic reforming of naphthas. Zeolite-catalyzed aromatization of lower cost liquefied petroleum gas (LPG), mainly composed of propane and butane, provides a new attractive means of producing BTX aromatics (Mowry et al., 1985a, 1985b). In this connection, it is not surprising that much attention has been paid to the aromatization of light paraffins, in particular of propane, over zeolite catalysts. Zeolites with MFI pore structure have been generally chosen since it is wellknown (Rollmann and Walsh, 1982; Derouane, 1985; Guisnet and Magnoux, 1989) that their deactivation by coking is relatively slow in comparison with zeolites with other pore structures. The recent reviews by Seddon (19901, Guisnet et al. (19921, and Ono (1992) demonstrate that the highest yields of aromatic hydrocarbons are obtained with ZSM-5 catalysts modified by gallium or zinc. Gallium-containing catalysts appear to be more preferable for industrial application because of their higher stability under process operating conditions (Seddon, 1990). In agreement with the literature data (Guisnet et al., 1992; Ono, 1992), one can represent the reaction of propane aromatization over ZSM-5 catalysts as a twostage process: (i) propane transformation into light olefins and (ii)aromatization of light olefins (see Figure 1). Both stages are complex heterogeneous reactions. With HZSM-5 catalysts, the first stage includes two routes (Kitagawa et al., 1986; Gnep et al., 1987): protolytic cracking route (11, resulting in formation of the primary products (hydrogen, methane, ethene, propene), and hydrogen transfer route (2), resulting in formation of propene and another paraffin. The second stage, olefin aromatization, proceeds (Vedrine et al., 1980; Pines, 1981; Gnep et al., 1987; Quann et al., 1988) through a number of acid-catalyzed oligomerization, cracking, cyclization, and hydrogen transfer steps (Figure 1). In consequence of the hydrogen transfer mechanisms (steps 4, 6, and 7 in Figure 1>,aromatics formation is balanced by formation of paraffins (Poutsma, 1976; Pines, 1981; Quann et al., 1988). These
' Permanent address: Karpov Institute of Physical Chemistry, U1. Obukha 10, Moscow 103064, Russia.
Ht,Ga
(1) C3' I
C3 + Cn*
H2 + Cf
C3+
Cn + C3+
=
C3' + Ht I
Olefin aromatization
r/rl-
(5)
1
Ht
c -
Aromatics
Cyclic diolefins
H",sp'
Cyclic olefins
Figure 1. Pathway of propane aromatization reaction over HZSM-5 and GaHZSM-5.
product paraffins undergo cracking reactions into light olefins and nonaromatizable CI and C2 paraffins decreasing, in this way, the maximum possible yield of aromatic products on HZSM-5 catalysts. At present it is well established (for review, see papers by Seddon (19901,Guisnet et al. (19921, and Ono (1992)) that both the rate of propane transformation and the selectivity toward aromatics can be increased significantly if the propane aromatization reaction is carried out over gallium-containing ZSM-5 catalysts (GaHZSM5, HZSM-5 Ga203, gallium silicate of MFI structure). All investigators agree that gallium species provide new dehydrogenation routes for propane transformation into propene, as was proposed by Inui et al. (1987)and Gnep et al. (1988a, 1988b), and for olefin transformation into aromatics, as was proposed by Inui et al. (19861, Shibata et al. (19861, and Kitagawa et al. (1986). Such a catalytic action of gallium species allows qualitative explanation of both the increase in propane conversion and the increase in aromatics selectivity. But it is not yet clear by what reactions the main undesired products (methane, ethane) are produced over GaHZSM-5 catalysts and what are the reasons for an apparent limit t o aromatics selectivity over these catalysts (Seddon, 1990). In this paper we propose a kinetic model for a propane aromatization reaction over HZSM-5 and GaHZSM-5 catalysts, based on the reaction scheme shown in Figure
+
0888-5885/95/2634-0516~Q9.QO/Q 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 517
1. We use the obtained kinetic modeling results t o discuss quantitatively the role of various reaction steps in propane transformation into olefins and in formation of desired (aromatics) and undesired (methane, ethane) products. We consider as well the role played by zeolite acid sites and gallium active species in the propane aromatization reaction and discuss on this basis the reasons for the limit t o aromatics selectivity over GaHZSM-5 catalysts. For determination of the kinetic parameters, we have used the experimental data on conversion of CZ,CS, and Cq paraffins over HZSM-5 (Si/ Al = 27) and GaHZSM-5 prepared by an impregnation method (4 wt % of Ga). These data were reported previously by Aittaleb (1992) and were obtained under nondeactivating conditions (catalyst time on stream was 2-3 min) in a flow reactor at 530 "C and hydrocarbon pressure of 1 bar. 2. Kinetic Model
The kinetic model for the propane aromatization reaction developed in this work represents an extension of the recently reported kinetic model for light olefin aromatization (Lukyanov et al., 1994). The extension was done by means of addition of the new reaction steps, which have been found to be essential in the propane aromatization reaction. The procedure and principles of the kinetic model formulation were discussed in detail previously (Lukyanov et al., 1994)and, as a consequence of this, are not considered in this paper. 2.1. Components of the Kinetic Model. In agreement with the earlier work (Lukyanov et al., 1994), the number of reaction species was reduced by lumping all isomers of the same carbon number into a single component. In this way, 42 components, which represent all reaction species involved in the propane aromatization reaction, were formed: 9 olefins C2=-C10=, 10 paraffins Cl-Clo, hydrogen, 7 dienes D4-Dl0, 5 alkylcyclohexenes &-Xlo, 5 alkylcyclohexadienes YSYlo, and 5 alkylbenzenes &-A10 (subscript index denotes the number of carbon atoms in the component molecule). 2.2. Reaction Scheme. The reaction scheme used for the development of the kinetic model for propane aromatization includes 62 steps of hydrocarbon adsorption and 276 steps of hydrocarbon transformation on zeolite (Z) and gallium (Z,) catalytic sites. All adsorption steps and most of the reaction steps were used previously in the kinetic model for light olefin aromatization reaction (Lukyanov et al., 1994). The list of these reaction steps is as follows: olefin oligomerization and cracking on zeolite acid sites (16 steps); diene formation via hydrogen transfer between two olefin molecules on zeolite acid sites (63 steps); interaction of olefins with dienes on zeolite acid sites to form higher molecular weight dienes (15 steps); diene cyclization on zeolite acid sites (5 steps); aromatics formation via hydrogen transfer between olefins and cyclic olefins (or cyclic diolefins) on zeolite acid sites (90 steps); diene formation via olefin dehydrogenation on gallium catalytic sites (7 steps); aromatics formation via dehydrogenation of naphthenes (cyclic olefins, diolefins) on gallium catalytic sites (10 steps); cracking of CS+paraffins on zeolite acid sites (39 steps). The new reaction steps that were introduced in this work in order to describe quantitatively the pathways of the propane aromatization reaction are the following. paraffins on zeolite acid 1. Transformation of sites (6 steps):
c,+zc,+z-
kcc(n,m)
kcH(n)
C,=Z
+ H,
(2 In
I4)
cm=z + cn-m( n = 3, 4;m = 2, 3)
(1) (2)
2. Paraffin transformation on gallium catalytic sites (17 steps):
cn
kCHl(n)
+ g
'
kH&)
C,'Z,
+ H,
c, + z,- k c c W c,-,-z, + c, -
(2 I n I 10) (3)
(3 I n I 10)
(4)
3. Hydrogen transfer between propane and olefins on zeolite acid sites (8 steps):
c, + c,=z
-c,=z+ cn ( n kHT(n)
= 2; 4 I n I 10) (5)
Reactions 1, 2, and 5, as well as forward reaction 3 were introduced into the kinetic model in agreement with the literature data (Kitagawa et al., 1986; Inui et al., 1987; Gnep et al., 1987, 1988a, 1988b). Reaction 4 and backward reaction 3 were introduced in accordance with the results of kinetic modeling performed in the course of this work (see section 3.3). The dependence of the rate constants of the reaction steps shown above on the number of carbon atoms in the components reflects the well-known effect of the hydrocarbon molecular weight on hydrocarbon reactivity. 2.3. Rate Expressions. The equations for the rates of the reaction steps were derived on the basis of the mass action law (Temkin, 19791, and the reactions over zeolite and gallium catalytic sites were treated independently (for details see the paper by Lukyanov et al. (1994)). The kinetic model for the propane aromatization reaction consisted of a set of 42 equations describing transformation of 42 components in 276 reaction steps. For kinetic modeling the plug flow reactor model was used, and for 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 (276 reaction steps) results in a tremendous number of rate constants, which cannot be estimated on the basis of any reasonable number of experiments. In the previous paper, we showed (Lukyanov et al., 1994) that the number of rate constants can be decreased significantly and that 30 rate constants are sufficient to describe in detail the olefin aromatization pathways: 12 rate constants were used for olefin oligomerization and cracking steps, 16 rate constants for acid-catalyzed olefin aromatization steps, and 2 rate constants for gallium-catalyzed dehydrogenation steps. In this study, the values of the rate constants of the acid-catalyzed olefin oligomerization, cracking, and aromatization steps were obtained by correcting the values of these rate constants determined earlier (Lukyanov et al., 1994) for the olefin aromatization reaction over HZSM-5 of a S U A ratio of 40. This correction was carried out in accordance with the previously reported (Olson et al., 1980; Haag et al., 1984; Luk'yanov and Shtral, 1991) linear dependence of HZSM-5 catalytic activity on the HZSM-5 aluminum content. The rate constants of gallium-catalyzed aromatization steps were estimated in this work on the
518 Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995 12
6o
I
c
0
0.2
0.4
0.6
Contact time (h)
0
0.2
0.4
0.6
Contact time (h)
Figure 3. Experimental data (points) and calculated curves for propane conversion as function of contact time: catalysts, HZSM-5 ( 0 )and GaHZSM-5 (0);temperature, 530 "C; propane pressure, 1 bar.
0
0.2
0.4
0.6
Conversion ( X )
Contact time (h)
Figure 2. Propane aromatization over HZSM-5. Experimental data (points) and calculated curves for the concentrations of light parafins (A), light olefins (B), aromatics, and Cg+ aliphatic hydrocarbons (C) as functions of contact time (temperature, 530 "C; propane pressure, 1 bar).
basis of the data on propane aromatization over a GaHZSM-5 catalyst. For description of the acid-catalyzed transformation of CZ-C~paraffins (reactions 1and 2), six rate constants have been used. These rate constants were estimated by means of comparison of the mathematical modeling results with the experimental dependence of the product distribution on contact time (WHSV-l) for CZ,C3, and C4 paraffin reactions over HZSM-5 catalyst. The rate constants of the hydrogen transfer steps (reaction 5) were estimated in a similar manner (details are given in section 3.1). In this case two rate constants were used, since all C4=-C10= olefins were considered to be of the same reactivity. To reduce the number of the rate constants of p a r d i n reactions over gallium catalytic sites, we have assumed that the rate constants of transformation of Cdt hydrocarbons are independent of the hydrocarbon molecular weight. This assumption results in six rate constants for the reversible reaction (3) and in two rate constants for the irreversible reaction (4). The values of these rate constants were estimated on the basis of the previously reported (Aittaleb, 1992) experimental data on C r C 4 paraffin conversion over GaHZSM-5 catalyst. Figure 2 illustrates the rate constant estimation procedure and shows the degree of agreement attained finally between experimental and simulated data on the propane aromatization reaction over HZSM-5. Figures 3-5 compare experimental and simulated data on propane conversion and product distribution over HZSM-5 and GaHZSM-5 catalysts. From presented
Conversion ( X I
Conversion ( X )
Figure 4. Propane aromatization over HZSM-5 (0)and GaHZSM-5 (0) a t 530 "C. Experimental data (points) and calculated curves for the concentrations of hydrogen (A), methane (B), and ethane (C) as functions of propane conversion (propane pressure, 1 bar).
data it follows that the proposed kinetic model describes with good accuracy the main pathways of propane aromatization over these catalysts. Hence, this model can be used for clarification of the role of various reaction steps and, consequently, of different catalytic sites in the propane aromatization reaction. These points are discussed in the following sections of the paper. 3. Kinetic Modeling Results 3.1. Initial Steps of Propane Transformation over HZSM-6. Kinetic modeling has demonstrated, in agreement with the previously made proposal ( E t a gawa et al., 1986),the existence of two routes of propane transformation: (i) in the steps of protolytic cracking and (ii) in the steps of hydrogen transfer between propane and carbenium ions generated by product
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 519 I
12 I
ci
0
10
20
30
Converslon
-
40
0
50
10
(XI
20
30
Converslon
40
50
40
50
(X)
I
I
E!
Y
- . . I
E
2 0
u
a o 0
10
20
30
50
40
Converslon ( X I
Y
0
20
10
30
Converslon
(X)
Figure 6. Experimental data (points) and calculated curves for the concentrations of ethane (A) and butanes (B)as functions of propane conversion over HZSM-5 a t 530 "C (propane pressure, 1 bar): curve 1,final description; curve 2, rate constants of hydrogen transfer route ( k ~ ~ ( equal n ) ) to zero.
Converslon ( X I Figure 5. Propane aromatization over HZSM-5 (0)and GaHZSM-5 ( 0 )a t 530 "C. Experimental data (points) and calculated curves olefins (A), CS+ aliphatic for the concentrations of Cz'-C4' hydrocarbons (B), and aromatics (C) as functions of propane conversion (propane pressure, 1 bar). Table 1. Rate Constants (mol/(gbar h)) of Initial Steps of Propane Transformation over HZSM-5 and GaHZSM-5 Catalysts GaHZSM-5 reaction step
- c1+ CZ' CS-H~+C~= c3
HZSM-5 34.6 12.6
total 57.2 100.4
H+
Ga
34.6 12.6
22.6 87.8
olefins. The rate constants of the first route shown in Table 1 demonstrate that cracking of the C-C bond occurs 2.7 times faster than cracking of the C-H bond. The similarity of the ratio between the rates of cracking of C-C and C-H bonds in the propane molecule was reported previously by Guisnet et al. (1992) for a number of HZSM-5 catalysts with a different Si/Al ratio. The rate constants of the hydrogen transfer route of propane transformation were determined on the basis of the experimental data on propane conversion (Figure 3) and on ethane and butane concentrations (Figures 2A and 6). From data in Figure 6 it is clear that this route plays a very important role in formation of ethane and butanes: at propane conversions below lo%, formation of these products occurs only via this route, and at higher propane conversions, this route is responsible for formation of about 50% of ethane and butanes. Estimations of the rate constants show that ethene is about 11 times less reactive in the hydrogen transfer steps than butenes. This result is in agreement with the previously reported (Lukyanov et al., 1994) difference in CZ= and Cq= olefin reactivities (13 times) in the hydrogen transfer olefin aromatization steps and can be explained by the difference in probabilities of formation orfand existence of different carbenium ions, which
iq0t-..:r' 1 20
V
0 0
0.2
Contact tlme
0,4
0-6
(h)
Figure 7. Experimental data (points) and calculated curves for propane conversion over HZSM-5 a t 530 "C (propane pressure, 1 bar). For curves, refer to Figure 6.
are generally accepted (e.g., Chen and Haag (1988)) t o be intermediate species in hydrogen transfer reactions over zeolite catalysts. It was of interest to compare quantitatively the contributions of the protolytic cracking and hydrogen transfer routes in the overall propane conversion over HZSM-5. For this we performed kinetic modeling of propane conversion in the absence of the hydrogen transfer steps (the rate constants of these steps were equated to zero). Propane conversion simulated in this way is shown in Figure 7 (curve 2) together with propane conversion simulated on the basis of the both routes (curve 1). Presented data indicate that the contribution of the hydrogen transfer route to the overall propane conversion increases with increasing conversion and that at conversions of 20-40% this contribution is about 20%. 3.2. Propane Aromatization Reaction over HZSM-5.As has been already mentioned in the Introduction, the steps of propane transformation into olefins are followed by the steps of olefin aromatization (see Figure 1). It has been shown (Lukyanov et al., 1994) that among these aromatization steps the steps of diene formation via hydrogen transfer between olefins and carbenium ions (steps 4 in Figure 1)are the slowest. The results of this study demonstrate that the rate constants of these slowest aromatization steps are lo3lo4 times higher (depending on the carbenium ions
520 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 6 H.
Converslon ( X )
-v
~
20 30 IrO 50 Conversion ( X I Figure 8. Experimental data (points) and calculated curves for the concentrations of methane (A) and ethene (B)as functions of propane conversion over GaHZSM-5 at 530 "C (propane pressure, 1bar): curves 1,final description; curves 2, methane and ethene formation considered to occur only on zeolite acid sites (rate constants of methane formation over gallium catalytic sites (kccl(n))equal to zero).
0
10
involved in these steps) than the sum of the rate constants of propane transformation into methane and hydrogen (the absolute values of the latter rate constants are given in Table 1). Thus, one can conclude, in agreement with the previously made statements (Gnep et al., 1988a, 1988b), that the steps of propane activation are the limiting steps in propane aromatization over HZSM-5 catalysts. 3.3. Initial Steps of Propane Transformation and Formation of Methane and Ethane over GaHZSM-5. In order to estimate quantitatively the effect of gallium on the rates of various reaction steps shown in Figure 1, we have determined the rate constants of these steps over GaHZSM-5 catalyst. In agreement with previous kinetic and TPD studies of GaHZSM-5 catalysts prepared in the absence of hightemperature reduction treatment (Gnep et al., 1988b; Kitagawa et al., 1986;Lukyanov et al., 19941, it has been assumed that introduction of gallium into HZSM-5 does not change the acid active sites of the catalyst. Therefore, the rate constant values for the acid-catalyzed reaction steps determined in this work for the HZSM-5 catalyst were also used (without any modification) in the kinetic description of the acid-catalyzed reaction steps over GaHZSM-5 catalyst. At the beginning of the rate constant estimation procedure we have introduced into the kinetic model, in agreement with the literature data (Inui et al., 1987; Gnep et al., 1988a, 198813; Guisnet et al., 1992; Ono, 19921, gallium-catalyzed steps of dehydrogenation of propane and product C4+ paraffins. We performed kinetic modeling of the propane aromatization reaction and found that the kinetic model describes with good accuracy the formation of hydrogen and propene, but not the formation of methane and ethene (see Figure 8, curves 2). So the reaction step of propane transformation into methane and ethene over gallium catalytic sites was introduced into the kinetic model and the description of methane and ethene formation (see Figure 8, curves 1) was improved (the discrepancy between
Converslon ( X ) Figure 9. Experimental data (points) and calculated curves for ethane concentration as function of propane conversion over GaHZSM-5 a t 530 "C (propane pressure, 1 bar): curve 1, final description; curve 2, ethane formation considered to occur only on zeolite acid sites (rate constant of ethane formation via ethene hydrogenation equal to zero).
simulated and experimental methane concentrations observed at propane conversions of 40-50% is discussed in the next section). On the basis of this result, we have concluded that gallium active species participate not only in the propane dehydrogenation steps but also in the steps of propane transformation into methane and ethene. Comparison of the rate constants of these reaction steps over gallium catalytic sites is made in Table 1. The above conclusion is further supported by previously reported data on propane transformation over pure Gas03 (Gnep et al., 1988a),which demonstrate that the rate of formation of ethene and methane is about 2.5 times lower than the rate of formation of propene and hydrogen. It seems probable that the ability of gallium species t o catalyze propane transformation into methane results from the strong electron-acceptor properties of these species (Kazansky et al., 1989). If this is so, then the mechanism of this reaction can be the same as was proposed recently by Ono (1992) for zinc species in ZnHZSM-5 catalysts. Table 1 shows that the total rate of the initial steps of propane transformation into methane (plus ethene) and hydrogen (plus propene) on GaHZSM-5 is about 3.3 times higher than on HZSM-5. In consequence of this, one could expect that the role of the hydrogen transfer route in propane transformation over GaHZSM-5 should be less pronounced than in the case of HZSM-5. Kinetic modeling confirms this supposition and demonstrates that over GaHZSM-5 the contribution of the hydrogen transfer route to the overall propane conversion is always below 5% (in the case with HZSM-5 this contribution is about 20%). As has been already shown (see section 3.11, the hydrogen transfer route of propane transformation over HZSM-5 is responsible, to a large extent, for the formation of ethane. Since this route does not play an appreciable role in propane transformation over GaHZSM-5, one could expect that the concentration of ethane would be much lower than in the case with HZSM-5. Contrary to this expectation, the experimental data shown in Figures 4C and 9 demonstrate that the ethane concentration over GaHZSM-5 is relatively high, and it is much higher than predicted by kinetic modeling (compare experimental data with curve 2 in Figure 9). On the basis of this discrepancy, we have concluded that with the GaHZSM-5 catalyst ethane is produced not only on the zeolite acid sites but also on the gallium catalytic sites. It has been assumed that the new route of ethane formation includes the steps of ethene hydrogenation by molecular hydrogen, which is the product of the other steps of the propane aromati-
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 521 zation reaction. We performed kinetic modeling under this assumption and obtained good agreement between experimental and simulated data on ethane formation (see Figure 9, curve 1). It is important to note that the assumption of the ability of gallium species to catalyze olefin hydrogenation reactions has been recently confirmed by Barre et al. (1993a), who demonstrated that ethene and propene can be hydrogenated by molecular hydrogen over pure Ga203 at 530 "C. In consequence of these results, the steps of paraffin dehydrogenation over gallium catalytic sites are considered in the kinetic model (see reaction scheme, steps 3) as reversible reaction steps. It seems probable that it is the reversibility of the dehydrogenation steps that is responsible for a decrease in aromatics selectivity observed by Meitzner et al. (1993) over a GaHZSM-5 catalyst in the experiments with propane in the mixture with molecular hydrogen. From the results presented above, it follows that gallium species incorporated in ZSM-5 catalysts are active not only in the desired dehydrogenation reactions of hydrocarbons, as is considered generally (Seddon, 1990; Guisnet et al., 1992; Ono, 19921,but also catalyze the formation of the main undesired products, namely, methane and ethane. It seems probable that the ratio between the rate constants of desired and undesired gallium-catalyzed reactions can be a strong function of the state of gallium in the catalyst. If that is so, then the change in this ratio can be one of the reasons for the improved aromatics selectivityreported by a number of authors (e.g., Changyu et al. (1988), Price and Kanazirev (1990), Barre et al. (1993b), and Meitzner et al. (1993)) for gallium-containing ZSM-5 catalysts reduced during hydrogen pretreatment or in the course of propane aromatization. 3.4. Propane Aromatization Reaction over GaHZSM-5. The quality of the description of the propane aromatization reaction over GaHZSM-5 catalyst is illustrated by the data shown in Figures 3-5. The kinetic modeling results demonstrate, in agreement with the previous conclusion (Lukyanov et al., 19941, that quantitative description of aromatics formation over GaHZSM-5 catalysts can be obtained if two gallium-catalyzed reactions (dehydrogenationof olefins into dienes (as proposed by Kitagawa et al. (19861, Shibata et al. (19861, Ono et al. (19891, and Meriaudeau et al. (1989)) and dehydrogenation of naphthenes into aromatics (as proposed by Gnep et al. (1988a, 1988b, 1989)) are taken into consideration. It is confirmed also that the rate constant of dehydrogenation of naphthenes (cyclic olefins, cyclic diolefins) is about 8 times higher than the rate constant of olefin dehydrogenation. It was of interest t o compare quantitatively the contributions of gallium and zeolite catalytic sites in the formation of dienes and aromatics on GaHZSM-5 catalyst. The results of kinetic modeling performed with this aim are shown in Figure 10, where curves 1and 3 correspond to the formation of dienes and aromatics on the zeolite acid sites via hydrogen transfer reactions and curves 2 and 4 correspond to the formation of these hydrocarbons on the gallium species via dehydrogenation reactions. From the presented data it is clear that gallium plays a very important role in the propane aromatization reaction over a GaHZSM-5 catalyst: about 90% of dienes and more than 50% of aromatics are formed over gallium catalytic sites. Concluding, we compare simulated and experimental data on the composition of aromatic hydrocarbons
:*E 4
L
e 4
U
S8 O0
5
/I
10
15
20
Conversion (Z)
$ 7
L
+ c
E
SO
0
5
10
15
20
Conversion ( X ) Figure 10. Calculated curves for the concentrationsof dienes (A) and aromatics (B)as functions of propane conversion: curve 1, diene formation considered to occur only in hydrogen transfer reactionsover zeolite acid sites (rate constants of diene cyclization steps equated to zero);curve 2, diene formation considered to occur only in dehydrogenationreactions over gallium catalytic sites (rate constants of diene cyclization steps equated to zero); curve 3, aromatics formation considered to occur only in hydrogen transfer reactions over zeolite acid sites; curve 4, aromatics formation consideredto occur only in dehydrogenationreactions over gallium catalytic sites. Table 2. Experimental and Simulated Composition of BTX Aromatics Formed over GaHZSM-5at a Propane Conversion of 41% hydrocarbon benzene toluene xylenes
concentration,wt % experimental simulated 9.2 9.2 3.0
8.6 10.5 5.2
produced over GaHZSM-5. These data are shown in Table 2 and demonstrate that simulated concentrations of toluene and xylenes are somewhat higher and that of benzene is somewhat lower than the experimentally observed concentrations of these products. Taking into consideration this discrepancy as well as the discrepancy between simulated and experimental data on methane and hydrogen formation, observed a t propane conversions of 40-50% (see Figure 4A,B), we assume that gallium catalytic sites should be active in the hydrogenolysis of methyl-substituted aromatics. This assumption is further supported by the recently reported data (Barre et al., 1993b) on the significant hydrogenolysis activity of GazOfiZSM-5 catalysts pretreated by hydrogen. 4. Conclusions 4.1. Kinetic Model. A kinetic model that describes in detail the pathways of the propane aromatization reaction over HZSM-5 and GaHZSM-5is developed. On the basis of kinetic modeling results, the role of various reaction steps and different catalytic sites in propane transformation and product distribution is clarified. 4.2. Propane Reaction over HZSM-5. It is demonstrated that propane transformation over HZSM-5 occurs via two routes. Route A protolytic cracking of C-C and C-H bonds in the propane molecule, resulting in formation of methane ethene and hydrogen propene, respectively. The ratio between the rate constants of methane and hydrogen formation is 2.7.
+
+
522 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
Route B: hydrogen transfer between propane and carbenium ions formed by product olefins on zeolite acid sites. The contribution of this route to the overall propane conversion is about 20% (at propane conversions of 20-40%). Route B is responsible to a large extent for the formation of ethane and butanes. The rate constants of the steps of propane transformation into olefins are found to be at least 1000 times lower than the rate constants of diene formation steps, which, in turn, have been shown (Lukyanov et al., 1994) to be the slowest among the acid-catalyzed olefin aromatization steps. 4.3. Propane Reaction over GaHZSM-5. With catalysts used in this study, the initial rate of propane transformation over GaHZSM-5is found to be 3.3 times higher than over HZSM-5. The obtained results demonstrate that gallium species are active in methane and hydrogen formation: the ratio between the rate constants of formation of these products over gallium catalytic sites is found t o be 0.26 (for zeolite protonic acid sites this ratio is equal to 2.7). On GaHZSM-5 catalyst, the role of the hydrogen transfer route decreases, and the contribution of this route to propane conversion is always below 5%. Gallium added to HZSM-5 contributes to the aromatization reaction catalyzing dehydrogenation of olefins into dienes and of naphthenes (cyclic olefins and diolefins) into aromatics. It is shown that the contribution of gallium catalytic sites in diene formation is about 90%, and in formation of aromatics, it is somewhat higher than 50%. 4.4. Formation of Undesired Products, Methane and Ethane, over GaHZSM-5. Comparison of kinetic modeling results with the experimental data has shown that methane is produced in the steps of propane transformation catalyzed by both the zeolite and the gallium catalytic sites. An assumption is made that at high propane conversion a portion of the methane is produced over gallium catalytic sites via hydrogenolysis of methyl-substituted aromatic products. Formation of ethane occurs mainly in the gallium-catalyzed steps of ethene hydrogenation by molecular hydrogen, which is a product of the dehydrogenation steps of propane aromatization reaction. The reaction steps of methane formation over zeolite and gallium catalytic sites and the steps of ethene hydrogenation over gallium catalytic sites appear to be the main reasons for the limit to aromatics selectivity over GaHZSM-5 catalysts.
Acknowledgment D.B.L. gratefully acknowledges the French Ministry of Research and Technology for the fellowship that allowed him t o work at the University of Poitiers in the course of 9 months in 1992-1993.
Nomenclature k c c ( n , m ) = rate constant of protolytic cracking of C-C bond
in C, paraffin resulting in formation of C,= olefin (mol/ (g bar h)) kccl(n) = rate constant of C, paraffin transformation into methane over gallium catalytic sites (moV(g bar h)) k c & ) = rate constant of protolytic cracking of C-H bond
in C, paraffin (mol/(gbar h)) kc~l(n= ) rate constant of dehydrogenation of C, paraffin over gallium catalytic sites (moV(g bar h))
k d n ) = rate constant of C,= olefin hydrogenation by
molecular hydrogen on gallium catalytic sites (mol/(gbar h)) k ~ & ) = rate constant of hydrogen transfer between propane and C,= olefin adsorbed on zeolite acid site (moll (g bar h)) WHSV = weight hour space velocity (h-l) Z = zeolite acld site Z, = gallium catalytic site
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Received for review December 9, 1993 Revised manuscript received August 17, 1994 Accepted September 24, 1994@ IE930631K Abstract published in Advance A C S Abstracts, December 1, 1994. @
