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Para-selective alkylation of toluene with methanol over ZSM-5 zeolites
Applied Catalysis A: General, 94 (1993)
117
117-130
Elsevier Science Publishers B.V., Amsterdam APCAT
A2398
Pura-selective alkylation of toluene with methanol over ZSM-5 zeolites A kinetic model Georgi Vayssilov, Marin Yankov and Abdul Hamid Department
of Organic Chemistry,
University
of Sofia, I J. Bourchier
Avenue,
Sofia 1126
(Bulgaria) (Received 25 February 1992, revised manuscript received 10 September 1992)
Abstract The composition of the product of toluene methylation over ZSM-5 zeolites can be determined by a method developed in this work based on mass-balance and kinetic equations for the investigated process. The reactions of toluene alkylation and xylene isomerization are considered both on the external and the internal surface catalytic centers of the zeolite crystals. The influence of the intracrystalline diffusion of the reagents and products is qualitatively taken into account after comparison of the diffusion coefficients of the substances. An equation for the optimal reaction conditions is obtained. The values of the catalytic parameters are estimated using experimental data from the literature. Keywords: alkylation; kinetic model; selectivity (para-xylene);
zeolites.
INTRODUCTION
The investigation of different approaches for the production of xylenes has been the subject of numerous experiments in recent years. Special attention has been given to the increase of p-xylene yield in such processes because of its application in the polymer industry. One of the most studied reactions has been toluene alkylation with methanol in the presence of heterogeneous catalysts. During the 1980s various types of molecular sieves - H-ZSM zeolites, their cation-exchanged forms and SAP0 -were used as catalysts in this process [l-12]. It is well known that these catalysts consist of pores with a definite size, of the order of the molecular diameter, and that the intracrystalline diffusion of products and the selectivity of processes can be governed by this pore size [ 13-161. Correspondence to: Dr. G. Vayssilov, Department of Organic Chemistry, Bourchier Avenue, Sofia 1126, Bulgaria. Fax. (+359-2)622808.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers
B.V.
University
All rights reserved.
of Sofia, 1 J.
118
G. Vayssilov et al./Appl. Catal. A 54 (1993) 117-130
Another route to obtainp-xylene is by o- or m-xylene isomerization over the same catalysts. Unfortunately, only about 25% of the initial substance can be converted to p-isomer by this reaction since less than 25% of the thermodynamic equilibrium mixture of xylenes consists ofp-xylene (in the temperature range 300-1000 K) [2], while intracrystalline diffusion controlled toluene methylation leads to more than 90% p-xylene [2-4,121. However, xylene isomerization takes place simultaneously with toluene alkylation and changes the products’ distribution. A very interesting problem in this field is the influence of the external surface of the zeolite crystals on the activity and selectivity of the reaction [ 171. Chen [4]; Praenkel and Levy [8] and Fraenkel et al. [ 181 have shown that large crystal zeolite samples result in higher p-selectivity than small crystal samples (with a larger external surface). This fact suggests that undesirable processes occur on the external active sites, e.g.p-xylene isomerization to other xylenes. This assumption was confirmed by a number of experiments using catalysts with an external surface covered by phosphorus or silane compounds. Some of the catalysts showed a very high selectivity to p-xylene [ 2-4,8,12]. In most of the papers on this subject, experimental results have been published describing the activity and p-xylene selectivity as being dependent on catalyst characteristics or reaction conditions. Suitable catalysts and conditions were looked for according to these criteria. Another approach aimed at finding an optimal catalytic system was to attempt to describe quantitatively the processes occurring during the catalytic transformation. However, up-to date far fewer theoretical model papers have been published than experimental ones. Wei [ 191 applied his mathematical theory to the selectivity of alkylation and isomerization carried out in zeolite pores, neglecting completely the external surface of the catalysts. One of the most valuable conclusions in his work is that as a result of selective diffusion more than 90% of the xylene product leaves the catalyst channels as p-isomer. A diffusion-kinetic model of the reaction both in the pores and on the external active centers has been developed by Hashimoto et al. [20]. This method showed good agreement with experimental data presented in ref. 20 but its application to each catalytic system requires additional experiments in order to determine the specific rate constants for the catalyst used. In the present work a kinetic model for the description of the processes occurring in toluene alkylation with methanol over a zeolite catalyst (H-ZSM5 ) accompanied by xylene isomerization is proposed. It takes into account conclusions from previous methods [ 19,201. Moreover, this method provides the possibility of investigating the influence of the reaction conditions (such as space velocity and catalyst amount) and the effect of the characteristics of the catalyst on the products distribution. For the sake of convenience some reasonable assumptions and simplifications are proposed.
119
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130 KINETIC MODEL
Mass-balance equations In order to determine the conversion and selectivity of the discussed processes, the mass-balance equation can be used for each of the components of the system: z+div(vCi)
=I(
+qj).
Here Ci is the concentration of substance i, v is the flow velocity, and qj is the rate of the chemical reaction in which i is consumed ( - ) or produced ( + ). In our investigation a fixed-bed reactor with a steady feed flow was used. The flow is unidirectional (marked by X) with u, = u, = 0 and u, = u = const. It should be noted that the catalytic process becomes stationary after z,=x,/u (normally r, M l-100 s) and the derivatives dCi/a t are equal to zero. In this way eqn. (1) gets the form
The main chemical reactions occurring in the system toluene-methanolzeolite are o-CH, PhCH3 + H,O m-CH3PhCH, +HzO p-CHB PhCH3 + Hz 0
Izom,
o-CH, PhCH3 + zeolite .k
mo
m-CH, PhCH3 + zeolite
o-CH,PhCH,
+ zeolite ep-CH,PhCH,
+ zeolite
m-CH,PhCH,
+ zeolite*p-CH:,PhCH,, pm
+ zeolite
Toluene disproportionation is 20-1000 times slower (for different types of zeolites) than alkylation and xylene isomerization [ 31. Under usual reaction conditions methanol can also undergo some transformations but its products can hardly change the isomeric xylene distribution. So, these side reactions affect only on the effective feed composition. All these transformations are performed on the external as well as on the
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
120
internal surface catalytic centers of the zeolite and both the reaction rates of the methylation and the isomerization are proportional to the number of active sites. These rates are of the first order with respect to the toluene and xylene concentrations [ 3,9]. Thus eqn. (2) leads to the following expressions for each compound:
act U
= - (k,+k,+kJnC,
dC, ’ ax
= -(k,+k,+kp)nC, (3)
where n = a,s, + Oisiis the number of the surface catalytic centers, s, and Siare the areas of the external and internal surfaces of the catalyst, o, and ai are the surface concentrations of the active sites (the number of the centers per unit area of the zeolite surface). It should be noted that all rate constants k in eqns. (3) are independent of the catalyst characteristics such as area, crystals size and number of active sites. The values of these constants should be the same as for catalysts with comparable properties. Influence of diffusion In the processes occurring on the internal acid centers the diffusion of the reagents and products in the zeolite channels plays an important role. Both toluene and methanol diffuse faster than the products and their diffusion does not influence the reaction rates. p-Xylene has a similar diffusion coefficient D,,=5.1X10-12cm2/s [21] in H-ZSM-5 at about 300 K. For the other reaction products (o- and m-xylenes) the coefficients are, however, more than 200 times lower than forp-xylene - D,,= 2.4 x lo-i4 cm2/s and D,,= 1.6 x lo-l4 cm2/s [ 211. At a higher temperature (393 K) this ratio is more than 1000 [ 111. Assuming that the rate constants k,, k, and kp are of the same order, then the diffusion determines the selectivity of the transformation in the catalyst. The parameter ~=r’/2D, where 2~ is the average thickness of the zeolite crystals, allows the estimation of the time required for a molecule of the product to leave the zeolite’s internal site. This means that p-xylene produced on the internal active sites moves out much faster than the other products and after zP (for the species used in this work 2rz 1 pm and 7P FZ250 s) from the beginning of the process its transfer becomes stationary in each zeolite crystal. On the other
121
G. Vayssilov et al.fAppl. Catal. A 94 (1993) 117-130
hand, o- and m-xylene concentrations in the catalyst channels become very high and isomerization top-xylene takes place. Thus, almost the entire amount of the reagents in the zeolite crystal is transformed into p-xylene. This effect is shown quantitatively in ref. 19 by analysis of xylene isomerization and diffusion in the pores of the molecular sieves. Accordingly eqn. (3) for the reaction space out of the zeolite crystals are changed as follows:
ac, 5T
=-
GX
(k, +k,
+kp)n,C,-kp&Ct
=k, n, G + kpon, C,,
v
(4)
ac,, ’ ax = k, n, Ct + kpmn, C,, %
k&J,-
=
’ ax
(lz,,+$,)n,C,,x
The equation for C, has a solution C,(x)=Co
a=i[(k,,+k,+kp)n,+kpni]
exp( -c2yx),
(5)
where C, (0) = Co. Using eqn. (5) the following solutions for the concentrations of the reaction products are obtained \
C,,(x) =
=
P
Co~~[exp(-ux)-exp(-pz)l
i(Iz,. +&An,,
%l =
G,(x)
1 (Y, =-kpn
F[l-exp(
= co
V
-(xx)]
-;[l-exp(
-px)]
[
Ik,n,,
E,
=tk,,n..
V
C,,(x)
= co
Gl -
[
amp
ik,n,,
+ (6) 1
[1-exp(
-ax)]
E,--ikpmne”pP--a
-F(l-exp(
-/?x)]
1 ,
Flow velocity and contact time The value of v (in cm/s) can be calculated using the ideal gas equation (for
1‘22
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
p= 1 atm) and by knowing the experimental parameter molar velocity dn/ dt=p*dV/dt Ax RTAn v=z=spdt=S2.06Sp
T
,AV dt
Here S (cm’) is the cross-section area of the reactor, AV is the volume of the liquid reagent mixture introduced in the reactor for At, and in the constant p is the density of this liquid while Xi and Mi are the p*=p/M*=p/CXiMi molar part and mass of the substances in the mixture. Taking into account that x,/v =
xcs 82.06 Tp*(AV/At)
1 =82.06
V‘? Tp* (AV/At)
=y0
(7)
where V, is the volume of the catalyst and 8 is the contact time (8= l/LHSV), eqn. (5) gets the form Ct=Coexp(
-a’y@,
a’=o!v.
(8)
When the determination of the contact time by WHSV is used [ 13,= m,/ (Am/ At) 1, then yw= (p/p,)7 (p, denotes the catalyst powder density in the reactor). After transformation of eqns. (6)) all concentrations in the product obtained obviously depend only on 8, T and a number of constants. This was observed by Kaeding et al. by compensation of the effect of pressure with WHSV at a constant contact time (Table 10 in ref. 2). Determination
of kinetic parameters
By eqns. (5 ) - (8) the kinetic parameters of the catalytic system considered could be determined using experimental data for conversion and product distribution under various conditions. It should be taken into account that in order to obtain completely accurate numerical results, a great number of catalytic experiments are required. Furthermore, these experiments should be carried out under the same conditions excluding only one parameter - velocity, amount of the catalyst, its surface area or other characteristics. Since temperature influences both the coefficient y and all rate constants k, it is less convenient to allow this reaction parameter to vary. Some additional difficulties arise in distinguishing the contributions of different basic parameters to the results obtained. More particularly, the determination of the zeolite external surface area and catalytic centers distribution is rather complicated since they depend strongly on the crystal size and manner of synthesis of the zeolite. However, a realistic evaluation of such parameters was made for H-ZSM-5 and some other zeolites by the benzene filled-
123
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
pore method [ 22,231 and by measuring the ion-exchange capacity [ 241 (Table 1). The determination of the coefficient (YI,and hence &, is demonstrated as the simplest example. The results for the alkylation of toluene with methanol over H-ZSM-5 treated with trimethylphosphite (TMPT) [12] (T=500 K, X, : X, = 1: 2 ) were used. By means of this treatment almost the entire external surface of the zeolite crystals and some of the pores were covered, so that n, = 0 and cxi is approximately equal to a’. By transforming eqn. (8) to: ln(C,/Co)=
-a’@
(9)
and knowing the value of y one obtains (Y’ = cub for the investigated catalyst. This result is shown in Fig. 1. The least squares method leads to cub= 1.4 x 10 -’ s-l and lz, =0.28 g/mol s. The value of n= 0.5 mmol/g for a H-ZSM-5 (Si/Al ratio about 30) sample is taken from ref. 25. The other kinetic parameters in the following section were determined by fitting with experimental data [ 21. TABLE 1 External surface area and catalytic centers distribution for ZSM-5 zeolites Zeolite
SifA1
Crystal dimensions
s (m*/g)
& (m/g)
H-ZSM-5 Na-ZSM-5
33.6 35
7X4.5X2.5/*m l-3.4 pm
367
11.2
n,ln
Ref.
0.19
22 23
Fig. 1. Determination of (Y’ using eqn. (9) and experimental data for the methylation of toluene over H-ZSM-5 treated with TMPT [12] (T=500 K, Xi:X,=1:2).
G. Vayssiloo et al./Appl. Catal. A 94 (1993) 117-130
124 RESULTS AND DISCUSSION
The equations obtained in the previous section allow us to analyze the influence of some characteristics of the catalyst and reaction conditions on toluene conversion and the distribution of the products. Influence of the catalyst characteristics The concentration of the active centers of the catalyst is one of the most important characteristics of zeolites [ 261, since these centers take part in all catalytic transformations. This concentration can be changed both by the surface area s and the surface concentration of the catalytic sites 0. Fig. 2 shows the dependence of the yield of isomeric xylenes on the surface area of the zeolite used. The calculations were carried out for H-ZSM-5 treated with H3P04 at 873 K and LHSV=20 h-l. The curves in Fig. 2 show that xylene concentrations increase almost linearly with s in the range 200-500 m2/g. As already mentioned in the previous section, the external surface area s, is an important factor for the selectivity of the catalyst. Since the ratio s,/s can be varied for each zeolite either by the crystal size [ 171 or by selective external surface passivation [ 41, some results for the xylene isomer distribution as dependent on this ratio are given in Fig. 3. The three lines in Fig. 3 correspond to three values of the total surface area of the zeolite used, i.e. s= 300,350 and 400 m2/g. The sample with s= 300 m2/g is the most selective to p-xylene and the amount of m-xylene is less than in the other cases. The ratio s,/s changes
0
F
,11,1~1111,~,,,,,,,,,,,,1,,,,,,,,,,, 100
200
surface
300
area
400
500
[sq.m./g]
e
1 i0'0
Fig. 2. Concentration of xylenes in the product of toluene methylation as a function of the total surface area of the catalyst at LHSV 20 h-l. Dashed lines are for p-xylene at 10 (- - -), 20 (- - -) and 30 (--) h-‘.
G. Vayssilou et al./Appl. Catal. A 94 (1993) I1 7-130
125
m Fig. 3. Xylene isomer distribution in the product for s= 300 (0 ), 350 ( 0 ) and 400 ( A ) versus ratio .s,/s= 1 X 10e3-1 X 10-l. A shows the equilibrium point.
LHSV Fig. 4. p-Xylene concentration (- - - ) fimol/m*.
m2/g
(h-l) in the product versus LHSV for a=0.3
(+-),
0.7 (- - -) and 1.3
along the lines from 1 x low3 (points close to lOO%p-isomer) to 1 x lo-‘. None of the calculated points reach that of equilibrium xylene distribution. The activity and selectivity of the process also depend on another parameter of the catalyst - o, as can be seen in Fig. 4 where some results of the calculation of p-xylene yield versus LHSV are given for o= 0.3,0.7 and 1.3 pmol/m2. It is interesting that the maximum of the p-isomer concentration in the product remains constant for different values of o but it moves to higher LHSV for higher o. On the other handp-selectivity calculated by the next equation [9] p-selectivity = [C,, - C,, (equilibrium) ] / [ 100 - C,, (equilibrium) ]
(10)
decreases with the rise of the surface concentration of active sites D as seen in
G. Vayssilou et al./Appl. Catal. A 94 (1993) 117-130
126
707,,,,,,...1,,,,,,,,,,,,,~,,,,,1~,~,,"'T 0.2
0.5
0
[/-Lmol/sq.m] 0.E
1.1
’
Fig. 5. Selectivity to p-xylene (calculated from eqn. 10) as a function of the active sites concentrationofthecatalystcrforLHSV=lO (-- -),20 (---) and30 (---) h-l.
Fig. 5. This behavior of p-xylene selectivity is due to the increase of o- and misomer production and the dependence can be connected with the aluminum content in the catalyst because CJincreases with the rise of the Al/Si ratio. Moreover, the investigations of Borade et al. [ 271 and Namba et al. [ 281 show different Al/Si ratios on the external surface of the zeolite crystals and in the bulk; in the paper of Namba et al. a method of decreasing the aluminum content on the surface is proposed. This may be another way of controlling the product distribution. Influence of reaction conditions The change of LHSV and consequently the contact time is one of the mildest manners of controlling the catalytic processes. In Fig. 6 a typical case of how the yield of o-, m- andp-xylene is dependent on LHSV is shown. As expected, the p-xylene concentration in the product has a maximum while the other isomers decrease continuously with the rise of space velocity. Thus, the selectivity of toluene alkylation to p-xylene decreases with the contact time; the behavior of the selectivity [calculated by eqn. (10) ] is shown in Fig. 7 for three zeolite samples with a=0.3,0.7 and 1.3 pmol/m. p-Selectivity for various values of toluene conversion is given in Fig. 8. In accordance with experimental data [9] p-selectivity strongly depends on conversion especially at higher conversion. Looking at the curves shown in Fig. 8 it can be seen that a choice can be made for each catalyst between higher selectivity and higher conversion.
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
LHSV
127
[k’]
Fig. 6. Concentration of o-, m- andp-xylene in the product of toluene methylation for passivated H-ZSM-5 zeolite (s = 360 m*/g, s, = 6.5 m’/g ).
versus LHSV
30
Fig. 7. Selectivity top-xylene (calculated from eqn. 10) as a function of the contact time for u= 0.3 (---),0.7 (- - -) and 1.3 (- - -) pmol/m*.
Optimization of the process The knowledge of the explicit equation for the concentrations of the different substances in the product allows optimal reaction conditions to be established. Thus, from the first derivative of C,, (19) in respect to contact time 8, the optimal contact time can be calculated: 8 opt
= IQ’P’_,’
lncv’ ’
p’=
pv.
G. Vayssilov et al.fAppl. Catal. A 94 (1993) 117-130
128 100 _
---
--
--__
4
.
--__
-_
80-
\
-. ‘.
-.
E5 8
\ -\
.+
‘, ‘\
60
‘\
\
-ii
\
\
\
\
40 s "h x
20
b 0 0
111111,,,(,,1,,,,,,,,,,,,~,,,,,,,,,,,,,,,,,,,,,, toKene ‘“convE-sion 80
100
Fig. 8. Selectivity to p-xylene (calculated from eqn. 10) as a function of the toluene conversion fors,/s=1xIO-1(~),2~10-2(---)and4x10-3(---).
\ 3000
\
\
,
-. -. ZOO0
1000
e
‘.
-.
O,,,,,.,,,,,,,,,~,,,,,,,~,~,~,~,,,,,,
160
260
surface
area
360
[sq.m./g]
460
Fig. 9. Optimal contact time 0,,, versus total surface area s for s,/s= 1 X 10-l (---), (- - -) and4x10m3 (- - -).
2 X lo-*
&,, is the maximum of the curve ofp-xylene concentration in Fig. 6. The values of &,, are for different catalyst samples with total surface area s=200-500 m2/g and s,/s between 1x 10-l and 4 x 10P3 are shown in Fig. 9. &,,, for the last sample is more than three times higher than the first one (solid line). CONCLUSION
We have developed a kinetic model for the determination of the composition of the aromatic product from the catalytic methylation of toluene. The nu-
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
129
merical results obtained in the previous section show that the behaviour of the process can be quantitatively predicted by the proposed model under various reaction conditions. However, some limitations of the method should be pointed out: - In the simplification of eqns. (3) the assumption is made that o- and m-xylene concentrations in the reaction space can be neglected. This assumption is reasonable for catalysts with a small external surface of crystals (usually after passivation) and for short contact times. - In the present model of the process the feed composition can vary in narrow limits (toluene : methanol from 1: 2 to 2 : 1) since very high methanol concentrations lead to the alkylation of xylenes, while very low ones may alter the equation for the alkylation rate. Within these limits, the influence of the initial composition of the mixture on the products distribution is weaker than that of the other reaction conditions. It should be stressed that the method proposed here is quite appropriate and applicable to most selective catalysts. LIST OF SYMBOLS
concentration of substance; i-t, toluene; m, methanol; ox, oxylene, mx, m-xylene, px, p-xylene (cm”/s) diffusion coefficient of i in zeolite channels (g/mol s)rate constant for methylation of toluene to xylenes; i=o, o-xylene; m, m-xylene; p, p-xylene (g/m01 s)rate constant for isomerization of i-xylene toj-xylene; i,j=o, m, P molecular mass of the reagent i-t, m (mol/g) number of the surface catalytic centers (ni, internal; n,, external) cross-section area of the reactor (cm2) (m”/g) surface area of the catalyst (ai, internal; s,, external) volume of liquid feed introduced in the reactor (cm3) volume of the catalyst (cm3) (cm/s) flow velocity molar part of the reagent i=t,m in the feed ( g/cm3 ) density of the liquid feed ( mol/m2) surface concentration of the active sites contact time (h) the optimal contact time for p-isomer. (h)
(mol/l)
Di ki kij Mi n
G. Vayssilov et al.jAppl.
130
Catal. A 94 (1993) 117-130
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117-130
Elsevier Science Publishers B.V., Amsterdam APCAT
A2398
Pura-selective alkylation of toluene with methanol over ZSM-5 zeolites A kinetic model Georgi Vayssilov, Marin Yankov and Abdul Hamid Department
of Organic Chemistry,
University
of Sofia, I J. Bourchier
Avenue,
Sofia 1126
(Bulgaria) (Received 25 February 1992, revised manuscript received 10 September 1992)
Abstract The composition of the product of toluene methylation over ZSM-5 zeolites can be determined by a method developed in this work based on mass-balance and kinetic equations for the investigated process. The reactions of toluene alkylation and xylene isomerization are considered both on the external and the internal surface catalytic centers of the zeolite crystals. The influence of the intracrystalline diffusion of the reagents and products is qualitatively taken into account after comparison of the diffusion coefficients of the substances. An equation for the optimal reaction conditions is obtained. The values of the catalytic parameters are estimated using experimental data from the literature. Keywords: alkylation; kinetic model; selectivity (para-xylene);
zeolites.
INTRODUCTION
The investigation of different approaches for the production of xylenes has been the subject of numerous experiments in recent years. Special attention has been given to the increase of p-xylene yield in such processes because of its application in the polymer industry. One of the most studied reactions has been toluene alkylation with methanol in the presence of heterogeneous catalysts. During the 1980s various types of molecular sieves - H-ZSM zeolites, their cation-exchanged forms and SAP0 -were used as catalysts in this process [l-12]. It is well known that these catalysts consist of pores with a definite size, of the order of the molecular diameter, and that the intracrystalline diffusion of products and the selectivity of processes can be governed by this pore size [ 13-161. Correspondence to: Dr. G. Vayssilov, Department of Organic Chemistry, Bourchier Avenue, Sofia 1126, Bulgaria. Fax. (+359-2)622808.
0926-860X/93/$06.00
0 1993 Elsevier Science Publishers
B.V.
University
All rights reserved.
of Sofia, 1 J.
118
G. Vayssilov et al./Appl. Catal. A 54 (1993) 117-130
Another route to obtainp-xylene is by o- or m-xylene isomerization over the same catalysts. Unfortunately, only about 25% of the initial substance can be converted to p-isomer by this reaction since less than 25% of the thermodynamic equilibrium mixture of xylenes consists ofp-xylene (in the temperature range 300-1000 K) [2], while intracrystalline diffusion controlled toluene methylation leads to more than 90% p-xylene [2-4,121. However, xylene isomerization takes place simultaneously with toluene alkylation and changes the products’ distribution. A very interesting problem in this field is the influence of the external surface of the zeolite crystals on the activity and selectivity of the reaction [ 171. Chen [4]; Praenkel and Levy [8] and Fraenkel et al. [ 181 have shown that large crystal zeolite samples result in higher p-selectivity than small crystal samples (with a larger external surface). This fact suggests that undesirable processes occur on the external active sites, e.g.p-xylene isomerization to other xylenes. This assumption was confirmed by a number of experiments using catalysts with an external surface covered by phosphorus or silane compounds. Some of the catalysts showed a very high selectivity to p-xylene [ 2-4,8,12]. In most of the papers on this subject, experimental results have been published describing the activity and p-xylene selectivity as being dependent on catalyst characteristics or reaction conditions. Suitable catalysts and conditions were looked for according to these criteria. Another approach aimed at finding an optimal catalytic system was to attempt to describe quantitatively the processes occurring during the catalytic transformation. However, up-to date far fewer theoretical model papers have been published than experimental ones. Wei [ 191 applied his mathematical theory to the selectivity of alkylation and isomerization carried out in zeolite pores, neglecting completely the external surface of the catalysts. One of the most valuable conclusions in his work is that as a result of selective diffusion more than 90% of the xylene product leaves the catalyst channels as p-isomer. A diffusion-kinetic model of the reaction both in the pores and on the external active centers has been developed by Hashimoto et al. [20]. This method showed good agreement with experimental data presented in ref. 20 but its application to each catalytic system requires additional experiments in order to determine the specific rate constants for the catalyst used. In the present work a kinetic model for the description of the processes occurring in toluene alkylation with methanol over a zeolite catalyst (H-ZSM5 ) accompanied by xylene isomerization is proposed. It takes into account conclusions from previous methods [ 19,201. Moreover, this method provides the possibility of investigating the influence of the reaction conditions (such as space velocity and catalyst amount) and the effect of the characteristics of the catalyst on the products distribution. For the sake of convenience some reasonable assumptions and simplifications are proposed.
119
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130 KINETIC MODEL
Mass-balance equations In order to determine the conversion and selectivity of the discussed processes, the mass-balance equation can be used for each of the components of the system: z+div(vCi)
=I(
+qj).
Here Ci is the concentration of substance i, v is the flow velocity, and qj is the rate of the chemical reaction in which i is consumed ( - ) or produced ( + ). In our investigation a fixed-bed reactor with a steady feed flow was used. The flow is unidirectional (marked by X) with u, = u, = 0 and u, = u = const. It should be noted that the catalytic process becomes stationary after z,=x,/u (normally r, M l-100 s) and the derivatives dCi/a t are equal to zero. In this way eqn. (1) gets the form
The main chemical reactions occurring in the system toluene-methanolzeolite are o-CH, PhCH3 + H,O m-CH3PhCH, +HzO p-CHB PhCH3 + Hz 0
Izom,
o-CH, PhCH3 + zeolite .k
mo
m-CH, PhCH3 + zeolite
o-CH,PhCH,
+ zeolite ep-CH,PhCH,
+ zeolite
m-CH,PhCH,
+ zeolite*p-CH:,PhCH,, pm
+ zeolite
Toluene disproportionation is 20-1000 times slower (for different types of zeolites) than alkylation and xylene isomerization [ 31. Under usual reaction conditions methanol can also undergo some transformations but its products can hardly change the isomeric xylene distribution. So, these side reactions affect only on the effective feed composition. All these transformations are performed on the external as well as on the
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
120
internal surface catalytic centers of the zeolite and both the reaction rates of the methylation and the isomerization are proportional to the number of active sites. These rates are of the first order with respect to the toluene and xylene concentrations [ 3,9]. Thus eqn. (2) leads to the following expressions for each compound:
act U
= - (k,+k,+kJnC,
dC, ’ ax
= -(k,+k,+kp)nC, (3)
where n = a,s, + Oisiis the number of the surface catalytic centers, s, and Siare the areas of the external and internal surfaces of the catalyst, o, and ai are the surface concentrations of the active sites (the number of the centers per unit area of the zeolite surface). It should be noted that all rate constants k in eqns. (3) are independent of the catalyst characteristics such as area, crystals size and number of active sites. The values of these constants should be the same as for catalysts with comparable properties. Influence of diffusion In the processes occurring on the internal acid centers the diffusion of the reagents and products in the zeolite channels plays an important role. Both toluene and methanol diffuse faster than the products and their diffusion does not influence the reaction rates. p-Xylene has a similar diffusion coefficient D,,=5.1X10-12cm2/s [21] in H-ZSM-5 at about 300 K. For the other reaction products (o- and m-xylenes) the coefficients are, however, more than 200 times lower than forp-xylene - D,,= 2.4 x lo-i4 cm2/s and D,,= 1.6 x lo-l4 cm2/s [ 211. At a higher temperature (393 K) this ratio is more than 1000 [ 111. Assuming that the rate constants k,, k, and kp are of the same order, then the diffusion determines the selectivity of the transformation in the catalyst. The parameter ~=r’/2D, where 2~ is the average thickness of the zeolite crystals, allows the estimation of the time required for a molecule of the product to leave the zeolite’s internal site. This means that p-xylene produced on the internal active sites moves out much faster than the other products and after zP (for the species used in this work 2rz 1 pm and 7P FZ250 s) from the beginning of the process its transfer becomes stationary in each zeolite crystal. On the other
121
G. Vayssilov et al.fAppl. Catal. A 94 (1993) 117-130
hand, o- and m-xylene concentrations in the catalyst channels become very high and isomerization top-xylene takes place. Thus, almost the entire amount of the reagents in the zeolite crystal is transformed into p-xylene. This effect is shown quantitatively in ref. 19 by analysis of xylene isomerization and diffusion in the pores of the molecular sieves. Accordingly eqn. (3) for the reaction space out of the zeolite crystals are changed as follows:
ac, 5T
=-
GX
(k, +k,
+kp)n,C,-kp&Ct
=k, n, G + kpon, C,,
v
(4)
ac,, ’ ax = k, n, Ct + kpmn, C,, %
k&J,-
=
’ ax
(lz,,+$,)n,C,,x
The equation for C, has a solution C,(x)=Co
a=i[(k,,+k,+kp)n,+kpni]
exp( -c2yx),
(5)
where C, (0) = Co. Using eqn. (5) the following solutions for the concentrations of the reaction products are obtained \
C,,(x) =
=
P
Co~~[exp(-ux)-exp(-pz)l
i(Iz,. +&An,,
%l =
G,(x)
1 (Y, =-kpn
F[l-exp(
= co
V
-(xx)]
-;[l-exp(
-px)]
[
Ik,n,,
E,
=tk,,n..
V
C,,(x)
= co
Gl -
[
amp
ik,n,,
+ (6) 1
[1-exp(
-ax)]
E,--ikpmne”pP--a
-F(l-exp(
-/?x)]
1 ,
Flow velocity and contact time The value of v (in cm/s) can be calculated using the ideal gas equation (for
1‘22
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
p= 1 atm) and by knowing the experimental parameter molar velocity dn/ dt=p*dV/dt Ax RTAn v=z=spdt=S2.06Sp
T
,AV dt
Here S (cm’) is the cross-section area of the reactor, AV is the volume of the liquid reagent mixture introduced in the reactor for At, and in the constant p is the density of this liquid while Xi and Mi are the p*=p/M*=p/CXiMi molar part and mass of the substances in the mixture. Taking into account that x,/v =
xcs 82.06 Tp*(AV/At)
1 =82.06
V‘? Tp* (AV/At)
=y0
(7)
where V, is the volume of the catalyst and 8 is the contact time (8= l/LHSV), eqn. (5) gets the form Ct=Coexp(
-a’y@,
a’=o!v.
(8)
When the determination of the contact time by WHSV is used [ 13,= m,/ (Am/ At) 1, then yw= (p/p,)7 (p, denotes the catalyst powder density in the reactor). After transformation of eqns. (6)) all concentrations in the product obtained obviously depend only on 8, T and a number of constants. This was observed by Kaeding et al. by compensation of the effect of pressure with WHSV at a constant contact time (Table 10 in ref. 2). Determination
of kinetic parameters
By eqns. (5 ) - (8) the kinetic parameters of the catalytic system considered could be determined using experimental data for conversion and product distribution under various conditions. It should be taken into account that in order to obtain completely accurate numerical results, a great number of catalytic experiments are required. Furthermore, these experiments should be carried out under the same conditions excluding only one parameter - velocity, amount of the catalyst, its surface area or other characteristics. Since temperature influences both the coefficient y and all rate constants k, it is less convenient to allow this reaction parameter to vary. Some additional difficulties arise in distinguishing the contributions of different basic parameters to the results obtained. More particularly, the determination of the zeolite external surface area and catalytic centers distribution is rather complicated since they depend strongly on the crystal size and manner of synthesis of the zeolite. However, a realistic evaluation of such parameters was made for H-ZSM-5 and some other zeolites by the benzene filled-
123
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
pore method [ 22,231 and by measuring the ion-exchange capacity [ 241 (Table 1). The determination of the coefficient (YI,and hence &, is demonstrated as the simplest example. The results for the alkylation of toluene with methanol over H-ZSM-5 treated with trimethylphosphite (TMPT) [12] (T=500 K, X, : X, = 1: 2 ) were used. By means of this treatment almost the entire external surface of the zeolite crystals and some of the pores were covered, so that n, = 0 and cxi is approximately equal to a’. By transforming eqn. (8) to: ln(C,/Co)=
-a’@
(9)
and knowing the value of y one obtains (Y’ = cub for the investigated catalyst. This result is shown in Fig. 1. The least squares method leads to cub= 1.4 x 10 -’ s-l and lz, =0.28 g/mol s. The value of n= 0.5 mmol/g for a H-ZSM-5 (Si/Al ratio about 30) sample is taken from ref. 25. The other kinetic parameters in the following section were determined by fitting with experimental data [ 21. TABLE 1 External surface area and catalytic centers distribution for ZSM-5 zeolites Zeolite
SifA1
Crystal dimensions
s (m*/g)
& (m/g)
H-ZSM-5 Na-ZSM-5
33.6 35
7X4.5X2.5/*m l-3.4 pm
367
11.2
n,ln
Ref.
0.19
22 23
Fig. 1. Determination of (Y’ using eqn. (9) and experimental data for the methylation of toluene over H-ZSM-5 treated with TMPT [12] (T=500 K, Xi:X,=1:2).
G. Vayssiloo et al./Appl. Catal. A 94 (1993) 117-130
124 RESULTS AND DISCUSSION
The equations obtained in the previous section allow us to analyze the influence of some characteristics of the catalyst and reaction conditions on toluene conversion and the distribution of the products. Influence of the catalyst characteristics The concentration of the active centers of the catalyst is one of the most important characteristics of zeolites [ 261, since these centers take part in all catalytic transformations. This concentration can be changed both by the surface area s and the surface concentration of the catalytic sites 0. Fig. 2 shows the dependence of the yield of isomeric xylenes on the surface area of the zeolite used. The calculations were carried out for H-ZSM-5 treated with H3P04 at 873 K and LHSV=20 h-l. The curves in Fig. 2 show that xylene concentrations increase almost linearly with s in the range 200-500 m2/g. As already mentioned in the previous section, the external surface area s, is an important factor for the selectivity of the catalyst. Since the ratio s,/s can be varied for each zeolite either by the crystal size [ 171 or by selective external surface passivation [ 41, some results for the xylene isomer distribution as dependent on this ratio are given in Fig. 3. The three lines in Fig. 3 correspond to three values of the total surface area of the zeolite used, i.e. s= 300,350 and 400 m2/g. The sample with s= 300 m2/g is the most selective to p-xylene and the amount of m-xylene is less than in the other cases. The ratio s,/s changes
0
F
,11,1~1111,~,,,,,,,,,,,,1,,,,,,,,,,, 100
200
surface
300
area
400
500
[sq.m./g]
e
1 i0'0
Fig. 2. Concentration of xylenes in the product of toluene methylation as a function of the total surface area of the catalyst at LHSV 20 h-l. Dashed lines are for p-xylene at 10 (- - -), 20 (- - -) and 30 (--) h-‘.
G. Vayssilou et al./Appl. Catal. A 94 (1993) I1 7-130
125
m Fig. 3. Xylene isomer distribution in the product for s= 300 (0 ), 350 ( 0 ) and 400 ( A ) versus ratio .s,/s= 1 X 10e3-1 X 10-l. A shows the equilibrium point.
LHSV Fig. 4. p-Xylene concentration (- - - ) fimol/m*.
m2/g
(h-l) in the product versus LHSV for a=0.3
(+-),
0.7 (- - -) and 1.3
along the lines from 1 x low3 (points close to lOO%p-isomer) to 1 x lo-‘. None of the calculated points reach that of equilibrium xylene distribution. The activity and selectivity of the process also depend on another parameter of the catalyst - o, as can be seen in Fig. 4 where some results of the calculation of p-xylene yield versus LHSV are given for o= 0.3,0.7 and 1.3 pmol/m2. It is interesting that the maximum of the p-isomer concentration in the product remains constant for different values of o but it moves to higher LHSV for higher o. On the other handp-selectivity calculated by the next equation [9] p-selectivity = [C,, - C,, (equilibrium) ] / [ 100 - C,, (equilibrium) ]
(10)
decreases with the rise of the surface concentration of active sites D as seen in
G. Vayssilou et al./Appl. Catal. A 94 (1993) 117-130
126
707,,,,,,...1,,,,,,,,,,,,,~,,,,,1~,~,,"'T 0.2
0.5
0
[/-Lmol/sq.m] 0.E
1.1
’
Fig. 5. Selectivity to p-xylene (calculated from eqn. 10) as a function of the active sites concentrationofthecatalystcrforLHSV=lO (-- -),20 (---) and30 (---) h-l.
Fig. 5. This behavior of p-xylene selectivity is due to the increase of o- and misomer production and the dependence can be connected with the aluminum content in the catalyst because CJincreases with the rise of the Al/Si ratio. Moreover, the investigations of Borade et al. [ 271 and Namba et al. [ 281 show different Al/Si ratios on the external surface of the zeolite crystals and in the bulk; in the paper of Namba et al. a method of decreasing the aluminum content on the surface is proposed. This may be another way of controlling the product distribution. Influence of reaction conditions The change of LHSV and consequently the contact time is one of the mildest manners of controlling the catalytic processes. In Fig. 6 a typical case of how the yield of o-, m- andp-xylene is dependent on LHSV is shown. As expected, the p-xylene concentration in the product has a maximum while the other isomers decrease continuously with the rise of space velocity. Thus, the selectivity of toluene alkylation to p-xylene decreases with the contact time; the behavior of the selectivity [calculated by eqn. (10) ] is shown in Fig. 7 for three zeolite samples with a=0.3,0.7 and 1.3 pmol/m. p-Selectivity for various values of toluene conversion is given in Fig. 8. In accordance with experimental data [9] p-selectivity strongly depends on conversion especially at higher conversion. Looking at the curves shown in Fig. 8 it can be seen that a choice can be made for each catalyst between higher selectivity and higher conversion.
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
LHSV
127
[k’]
Fig. 6. Concentration of o-, m- andp-xylene in the product of toluene methylation for passivated H-ZSM-5 zeolite (s = 360 m*/g, s, = 6.5 m’/g ).
versus LHSV
30
Fig. 7. Selectivity top-xylene (calculated from eqn. 10) as a function of the contact time for u= 0.3 (---),0.7 (- - -) and 1.3 (- - -) pmol/m*.
Optimization of the process The knowledge of the explicit equation for the concentrations of the different substances in the product allows optimal reaction conditions to be established. Thus, from the first derivative of C,, (19) in respect to contact time 8, the optimal contact time can be calculated: 8 opt
= IQ’P’_,’
lncv’ ’
p’=
pv.
G. Vayssilov et al.fAppl. Catal. A 94 (1993) 117-130
128 100 _
---
--
--__
4
.
--__
-_
80-
\
-. ‘.
-.
E5 8
\ -\
.+
‘, ‘\
60
‘\
\
-ii
\
\
\
\
40 s "h x
20
b 0 0
111111,,,(,,1,,,,,,,,,,,,~,,,,,,,,,,,,,,,,,,,,,, toKene ‘“convE-sion 80
100
Fig. 8. Selectivity to p-xylene (calculated from eqn. 10) as a function of the toluene conversion fors,/s=1xIO-1(~),2~10-2(---)and4x10-3(---).
\ 3000
\
\
,
-. -. ZOO0
1000
e
‘.
-.
O,,,,,.,,,,,,,,,~,,,,,,,~,~,~,~,,,,,,
160
260
surface
area
360
[sq.m./g]
460
Fig. 9. Optimal contact time 0,,, versus total surface area s for s,/s= 1 X 10-l (---), (- - -) and4x10m3 (- - -).
2 X lo-*
&,, is the maximum of the curve ofp-xylene concentration in Fig. 6. The values of &,, are for different catalyst samples with total surface area s=200-500 m2/g and s,/s between 1x 10-l and 4 x 10P3 are shown in Fig. 9. &,,, for the last sample is more than three times higher than the first one (solid line). CONCLUSION
We have developed a kinetic model for the determination of the composition of the aromatic product from the catalytic methylation of toluene. The nu-
G. Vayssilov et al./Appl. Catal. A 94 (1993) 117-130
129
merical results obtained in the previous section show that the behaviour of the process can be quantitatively predicted by the proposed model under various reaction conditions. However, some limitations of the method should be pointed out: - In the simplification of eqns. (3) the assumption is made that o- and m-xylene concentrations in the reaction space can be neglected. This assumption is reasonable for catalysts with a small external surface of crystals (usually after passivation) and for short contact times. - In the present model of the process the feed composition can vary in narrow limits (toluene : methanol from 1: 2 to 2 : 1) since very high methanol concentrations lead to the alkylation of xylenes, while very low ones may alter the equation for the alkylation rate. Within these limits, the influence of the initial composition of the mixture on the products distribution is weaker than that of the other reaction conditions. It should be stressed that the method proposed here is quite appropriate and applicable to most selective catalysts. LIST OF SYMBOLS
concentration of substance; i-t, toluene; m, methanol; ox, oxylene, mx, m-xylene, px, p-xylene (cm”/s) diffusion coefficient of i in zeolite channels (g/mol s)rate constant for methylation of toluene to xylenes; i=o, o-xylene; m, m-xylene; p, p-xylene (g/m01 s)rate constant for isomerization of i-xylene toj-xylene; i,j=o, m, P molecular mass of the reagent i-t, m (mol/g) number of the surface catalytic centers (ni, internal; n,, external) cross-section area of the reactor (cm2) (m”/g) surface area of the catalyst (ai, internal; s,, external) volume of liquid feed introduced in the reactor (cm3) volume of the catalyst (cm3) (cm/s) flow velocity molar part of the reagent i=t,m in the feed ( g/cm3 ) density of the liquid feed ( mol/m2) surface concentration of the active sites contact time (h) the optimal contact time for p-isomer. (h)
(mol/l)
Di ki kij Mi n
G. Vayssilov et al.jAppl.
130
Catal. A 94 (1993) 117-130
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