2548
Ind. Eng. Chem. Res. 1993,32, 2548-2554
Kinetics of Toluene Alkylation with Methanol over Mg-Modified ZSM-5 Jose L. Sotelo,’ Maria A. Uguina, Jose L. Valverde, and David P. Serrano Chemical Engineering Department, Faculty of Chemistry, Complutense University of Madrid, 28040 Madrid, Spain
The kinetics of toluene alkylation with methanol has been studied over both unmodified and Mgmodified ZSM-5 catalysts. On the basis of the product distribution obtained over the unmodified zeolite, the following secondary reactions have been considered: methanol dehydration, toluene disproportionation and xylene dealkylation. Power law and Langmuir-Hinshelwood-HougenWatson (LHHW) models have been tested for the main toluene alkylation reaction, the best fits to the experimental data being achieved with two models: a second-order power law model and a model based on the Rideal-like mechanism. T h e first one has been applied to the results obtained with the Mg-modified catalyst, including the effect of the diffusion rate, due to its simpler expression. Over this catalyst p-xylene is the only isomer present in the primary product, hence rn- and o-xylene formation takes place by p-xylene isomerization on the external surface catalytic sites. Therefore, the kinetic model developed for the Mg-modified zeolite takes into account, as well as diffusional effects, the influence of p-xylene isomerization over the external zeolite surface, reproducing the experimental product distribution with an average relative error of 6.8 % .
Introduction There is considerable industrial demand for p-xylene because of its importance as raw material for terephthalic acid and dimethyl terephthalate manufacture and for the synthesis of vitamins and other pharmaceuticals. The selective preparation of p-xylene by alkylation of toluene with methanol has potential interest as an alternative route to the conventional separation of the xylene isomers by adsorption (Parex process) or to toluene disproportionation. p-Xylene can be selectively obtained by alkylation or disproportionation of toluene on ZSM-5 type zeolites due to their shape selectivity, which can be enhanced by modification with several agents (Young et al., 1982; Cavallaro et al., 1987; Uguina et al., 1992). Several authors have related the para selectivity exhibited by ZSM-5 zeolites to the length of intrazeolitic diffusion paths and to the tortuosity of the channel system, being therefore controlled by the type of modification and the crystal size. Product reentry into the zeolite pores was assumed to explain the lower para selectivity observed with small zeolite crystals and at high conversions (Wei, 1982; Olson and Haag, 1984). A second approach considers that p-xylene is preferentially produced inside the zeolite channels due to diffusional constraints, while acid sites located on the external surface are responsible for isomerization reactions (Uguina et al., 1992;Valverde, 1991). Fraenkel(1990) has proposed a reaction model in accordance with this mechanism, assuming a first-order kinetics, which allows him to couple easily the rates of the intrinsic reaction and the intracrystalline diffusion. Most of the kinetic models based on these approaches assume that all methanol is converted through the main alkylation reaction and methanol diffusion inside the zeolite crystals is relatively fast. Therefore, the secondary reaction of methanol dehydration forming light hydrocarbons is not taken into account. Generally, in these models toluene conversion into xylene isomers is considered a first-order reaction, a simplification which makes possible coupling between the diffusion and reaction rates. Experimental data reported in previous work show that methanol is completely converted in the temperature range
* To whom correspondence should be addressed. ~m8-5aa519312632-2548$04.0010
normally used (673-873 K) (Young et al., 1982;Bhat et al., 1989; Valverde, 1991). Recently, Bhat et al. (19891, and Mantha et al. (19911, have proposed a kinetic model using the Langmuil-Hinshelwood approach, where the secondary reactions of methanol to any products other than xylenes and trimethylbenzenes are ignored. Likewise, Vayssilov et al. (1993) have developed a power law kinetic model where only the reactions of toluene alkylation and xylene isomerization have been considered. These authors have included both external and internal surface catalytic centers of the zeolite crystals to explain the observed product distribution, although they have used experimental data obtained at an unique temperature (500 K). The aim of the present paper is to develop a kinetic model for toluene alkylation with methanol on both unmodified and Mg-modified ZSM-5 zeolite based on the product distribution, considering the extension of secondary reactions such as methanol dehydration, toluene disproportionation, and xylene dealkylation, in contrast with earlier literature works, which include only the main reaction in the kinetic analysis. This is a more realistic kinetic model which might be applied to reproduce the product distribution obtained in industrial reactors operating at high conversions.
Experimental Section ZSM-5 zeolite was synthesized by hydrothermal crystallization using ethanol as template, as described in a previous paper (Costa et al., 19871, obtaining a product which a SiOZ/Al203molar ratio of 58and 1009%crystallinity (determined by XRD). The average crystal size of the zeolite (5.4pm) was measured by laser granulometry using a Cilas 715 granulometer, being in good agreement with the value observed in SEM micrographs. The acid form of the zeolite was obtained by ion exchange with an HC1 solution followed by calcination at 600 OC for 6 h. The zeolite was pelletized using montmorillonite as binder (35 wt % 1. The zeolite modification was carried out by impregnation with an aqueous solution of magnesium acetate, yielding a catalyst with 1.1 w t % of magnesium after calcination in air. The amount of magnesium over the catalyst was optimized in previous experiments (Sotelo et al., 1991). The catalytic experiments were carried out at atmospheric pressure in a fixed bed, continuous downflow, 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32,No. 11,1993 2649 100
100
1 :i
-
80
h
80
M
v
+e
60 d
E
P
2
D
a
g .e
40
0
B 20
1 0 650
1 700
750
800
T (K)
0
I
I
Figure 1. Unmodified ZSMd zeolite. Influence of reaction temperature (W/FT, = 108.0 g h/mol; ToVMeOH = 2).
I
Figure 3. Unmodified ZSM-5 zeolite. Influence of ToVMeOH ratio (T= 723 K,W/FT, = 5.4 g Wmol).
100
Table I. Product Distribution and Reaction Parameters over Unmodified ZSM-6 Obtained at 723 K and Different Space-Time Values
IP 80 h
ormration conditions
J
c
2 60
E
n
.-e5
I
Tol/MeOH
methane ethane ethylene propane propene dimethyl ether
40
o
d
20
c4
0 0
20
60 W/Fdg.h/mol)
40
80
100
120
Figure 2. Unmodified ZSM-5 zeolite. Influence of space-time (T = 723 K,Tol/MeOH = 2).
stainless-steel reactor (i.d. 0.02 m). Prior to the reaction, the catalyst (0.01-4g) was treated with a nitrogen stream a t the desired reaction temperature for 1h. The liquid toluene and methanol mixture was fed with a Sage syringe pump and passed through a preheater to vaporize the charge. The gaseous and liquid products were analyzed separately with a Hewlett-Packard 5880 A gas chromatograph, using a 5% SP-1200/5bentone 34 on Supelcoport column for liquids and a Porapak Q column for gases. For the liquid product, an homogeneous phase was achieved by addition of known amounts of n-propanol. The aging of both unmodified and Mg-modified ZSM-5 zeolites was checked to be negligible during the time necessary to collect the reaction products in the kinetic measurements.
Results and Discussion Unmodified ZSM-6Zeolite. Previously to the kinetic study, experiments were carried out over the unmodified ZSM-5 zeolite varying reaction temperature T (673-798 K), space-time W/FT, (2.7-108 g h/mol referred to the toluene feed rate) and toluene/methanol molar ratio in the feed ToUMeOH (1-4). The corresponding results are shown in Figures 1-3,and the following conclusions can be derived
methanol benzene toluene ethylbenzene p-xylene m-xy1ene o-xylene ethyltoluene trimethylbenzene water
C SX
P. YX
2.7 10.8 108.0 2 2 2 product distribution (mol %) 0.6 0.5 3.5 0.1 0.2 2.1 5.2 3.2 2.0 0.5 1.0 1.3 0.5 0.1 0.1 0.1 1.0 0.4 50.7 0.0 6.0 2.7 1.0 0.8 0.6 29.8
0.1
0.1
1.9 17.5 45.2 29.6 0.2 0.3 3.8 2.9 7.2 6.8 2.6 3.0 1.8 0.3 0.7 0.9 31.5 29.6 reaction parameters ( % ) 18.0 27.7 49.6 84.4 74.6 40.1 61.3 27.9 23.0 15.8 21.4 21.6
TolueneConversion,C. It increaseswith temperature, space-time, and the methanol proportion in the feed (lower ToVMeOH ratio). Selectivity toward Xylene, S,. S, decreases as temperature and space-time are higher, but it remains nearly constant when Tol/MeOH is varied. This loss of selectivity is due to secondary reactions such as xylene dealkylation to toluene and xylene alkylation forming trimethylbenzene (Li et al., 1992). Para Selectivity, P, (p-Xylene Proportion in the Total of Xylenes). It increases at lower space-times, i.e., a t lower toluene conversions. The alkylation of toluene is expected to yield p-xylene and o-xylene as primary products based in mechanistic considerations (Csicsery, 1969; Cavallaro et al., 19871, but because of the stereospecificity of ZSM-5, p-xylene is formed selectively, being the main isomer in the primary product (product
2560 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
just outside of the zeolite channels): from Figure 2 the extrapolated P, at W / F T =~ 0 is nearly 80 5%. Xylene Yield, Y,.When temperature is varied, xylene yield reaches a maximum a t =723 K due to the combined effects of a higher extension for the main toluene alkylation reaction and for the conversion of methanol through the above mentioned secondary reactions. Higher space-times and lower Tol/MeOH ratio favour the xylene yield (Figures 2 and 3). Table I shows the product distribution obtained at 723 K and different space-time values. I t is observed that benzene yield is favored at higher temperatures and s p a c e times, due mainly to toluene disproportionation. Likewise, methanol is almost completely converted in all experiments, even at the lowest space-time. The production of light hydrocarbons was proportional to the methanol content in the feed. The hydrocarbons produced in this way, mainly ethane and ethylene, are responsible for the formation of some aromatic compounds, such as ethylbenzene and ethyltoluene through secondary alkylation reactions. Kinetic Model. The channel dimensions of ZSM-5 are comparable with the kinetic diameter of toluene and xylene, so micropore diffusion may influence the overall reaction rate. The dependence of the toluene alkylation rate upon diffusion has been determined, approximately, by calculating the observable Weisz modulus, assuming a first-order reaction rate:
(ii) gaseous hydrocarbons were substituted by an hypothetic one with two carbon atoms; and (iii) compounds as trimethylbenzene, ethyltoluene, or ethylbenzene were ignored because of their small concentration in the reaction product. In the kinetic model here developed, xylene dealkylation and toluene disproportionation were supposed to be irreversible pseudo-first-order reactions. These assumptions are supported by the continuously increasingbenzene concentration in the reaction product with space-time, so the equilibrium of the disproportionation reaction has not been reached yet. This approach has been used by other authors (Do and Bhaskar, 1990). Hence
An n-order kinetic expression will be admitted for the methanol decomposition reaction = kgp,"' (8) Two types of rate equation have been used for the toluene alkylation reaction: r2
Power law model:
a = (rl)o&2~dDTeCTs
(1) (Tl)obs being the observed rate of the toluene alkylation reaction, L the catalyst diffusional half width (2.7 X 10-6 m),p,thecatalystdensity(1.41 X 1@g/m3),D~,thetoluene ~ toluene concentration in effective diffusivity, and C T the the external surface. Xiao and Wei (1992) have proposed an unified diffusion theory which can be used to describe the configurational diffusion regime taking place in zeolites. These authors have applied the theory to predict the activation energy and the preexponential factor for the diffusion coefficient of different hydrocarbons in ZSM-5 zeolite, a good concordance with experimental data being observed. Therefore, we have used this model to evaluate the toluene diffusion coefficient through the ZSM-5 channel system at 723 K, a value of 5 X lo-' m2/h being obtained. The observed toluene alkylation rate was determined at 723 K and Tol/MeOH = 2, from the slope of the toluene conversioncurve at the lowest space-time,yielding a (P1)obs value of 6.69 X le2 mol/g h ( C T ~= 9.53 mol/m3). From these results a Weisz modulus of 0.14 is obtained, leading to an effectiveness factor higher than 0.95; i.e., the intracrystalline diffusional influence on the reaction rate can be neglected. In accordance with the product distribution, the system can be described with the following reactions:
Main Reaction toluene alkylation Secondary Reactions methanol dehydration:
T+M
toluene disproportionation: xylene dealkylation:
r2
M
x
rl
X +W
T
Rideal-like mechanism (Fraenkel, 1990): M +1
rli
W + CH21
*
T + CH21 X1 (2)
(3)
rs
T s '/zB + '/2X
-
Dual-mechanism:
(15)
vi
'/2GH + W
r4
Langmuir-Hinshelwood-Hougen-Watson models:
+ '/,GH
(4)
(5)
where three simplifications have been made: (i) the three xylene isomers were grouped in a single component (X);
rd
(16)
Xl*X+l (17) In both cases different kinetic equations can be derived for rij and ri{ according to the rate-controlling step considered (Froment and Bischoff, 1990). Several reaction orders have been tested, yielding eight different power law models (eqs 6-9) and 16 power law + LHHW models (eqs 6-8 and the corresponding equation for the rate-controlling step in the dual or Rideal-like mechanism).
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2661 100
F, = regression sum of square/parameter number residual sum of square/degree of freedom
I
p-SELECTIVIM
where ne is the number of parameters in the model. A given regression is considered to be meaningful when F, is larger than F(n,, (mn - ne), 1 - a). In this work we considered an a value of 0.05. In Table 11, models which satisfy the preceding two conditions are shown. For the power law rate expressions the best fit is achieved with model no. 2 (ml = 1, m2 = 1 and m3 = 2; 4 = 4.65). Among the LHHW models, the Rideal-like mechanism, controlled by the surface reaction (model no. 23):
TOLUENE CONVERSION
[pw -
