Ind. Eng. Chem. Res. 1993,32, 2525-2529
2525
Ethylbenzene Dealkylation and Realkylation over Pore Size Regulated MFI Zeolite Jagannath Das, Yajnavalkya S. Bhat, and A n a n d B. Halgeri' Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391 346, Gujrat, India
The dealkylation and realkylation of ethylbenzene over silica-deposited MFI zeolite have been studied. The relationship between para selectivity enhancement and extent of silica deposition is established. The effects of reaction conditions like variation in reaction temperature and weight hourly space velocity on the high para selectivity feature of modified zeolite are examined. A study of the kinetics of the reaction in the temperature range 523-573 K is carried out. The estimated activation energy is similar to that reported for toluene disproportionation over MFI zeolite. Introduction The conversion of ethylbenzene over MFI zeolite takes place mainly by dealkylation and realkylation (Olson and Hagg, 1984). Amelse (1988) has reported that biphenylalkane intermediate cannot form within the pores of MFI zeolite and that the disproportionation occurs by dealkylation and realkylation. This has been confirmed by an isotope tracer technique. Tsai et al. (1992) have studied disproportionation of n-propylbenzene as a method to explore the relationship between intrinsic reaction mechanism and internal pore systems of zeolites. They concluded that with MFI zeolite, disproportionation reaction proceeds through a monomolecular s N 1 reaction mechanism (dealkylation/realkylation). Hence, disproportionation of ethylbenzene which takes place via dealkylation/ realkylation over MFI zeolite is important from the point that one of the products of the reaction,p-diethylbenzene, finds application in the recovery of p-xylene from a Cgraffinate mixture by the adsorption process (Jasara and Bhat, 1988). On MFI zeolite of smaller crystal size, the disproportionation of ethylbenzene results in a near thermodynamic equilibrium composition of p-diethylbenzene. The treatment of MFI zeolite with different agents by means of several procedures has enhanced the para selectivity, making it higher than that of thermodynamic value (Kaeding et al., 1981; Young et al., 1982). Among the several procedures employed for modifying the MFI zeolite, the chemical vapor deposition (CVD) of silica, employingtetraethyl orthosilicate which involvesblocking of nonselective external surfaces and the pore mouth sites with silica without altering the internal structure, is gaining importance (Niwa et al., 1986; Wang et al., 1988; Halgeri et al., 1991). The present work is addressed to the study of ethylbenzene disproportionation over silylated MFI zeolite. It also encompasses the effects of various parapleters like extent of silica incorporation, reaction temperature, and weight hourly space velocity (WHSV) onp-diethylbenzene selectivityand kinetics of the reaction. Experimental Section MFI zeolite with an Si/Al ratio of 80 was synthesized using a template (tetrapropylammonium bromide) as per the procedure outlined in the literature (Argauer and Landolt, 1972). The zeolite sample was characterized by XRD for MFI phase purity, IR for pentasil structure, SEM for crystallite size and morphology, MAS-NMR for aluminum incorporation in the framework, and TPD of
* T o whom correspondence should be addressed.
Table I. Performance Comparison of MFI Zeolite with and without Modification for Ethylbenzene Disproportionation*
ethylbenzene conversion (wt %) selectivity to products (%) benzene diethylbenzene othersd diethylbenzene composition ( 76 ) Para meta ortho
catalyst silylatedb MFI zeolitec MFI zeolite 12.83 12.5
41.31 56.66 2.03
38.72 54.72 6.56
33.43 99.42 63.96 0.58 2.61 a Temperature = 573 K, HdHC = 2. * WHSV = 2.9 h-1. WHSV = 5.8 h-l. Lighters, xylenes, ethyltoluenes, trimethylbenzenes.
ammonia for acidity. The proton form of MFI zeolite was obtained by calcination of as-synthesized material for 10 h in air a t 813 K followed by repeated ion exchange with a 1M aqueous solution of ammonia nitrate and a further calcination at 783 K for 10 h. The proton form of the zeolite was pressed without binder, crushed, and sieved. The size fraction between 0.25 and0.50mm was used for catalytic experiments. These experiments were carried out in a continuous down flow integral reactor a t atmospheric pressure. The chemical vapor deposition was done in situ using tetraethyl orthosilicate followingthe procedure reported elsewhere (Wang et al., 1988). A Varian Vista 6000 gas chromatograph equipped with an LB-550 capillary column and flame ionization detector was employed for the product analysis. Analytical grade ethylbenzene constituted the feed in all the experiments. Results and Discussion The terms conversion and selectivity used in this section are defined as follows: EB conversion, % = wt 5% EB in feed - wt % EB in product w t % EB in feed DEB selectivity, 76 = w t 5% DEB in product w t % EB conversion
PDEB selectivity, % = wt wt% ' %PDEB DEB ininp:oduct roduct Selectivity Enhancement by Silylation. Table I summarizes the results of ethylbenzene disproportionation
QSSS-SSS5/93/2632-2525$04.00/0 0 1993 American Chemical Society
2526 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
over MFI zeolite with and without silica deposition. At nearly the same conversion level of around 13%, p-diethylbenzene selectivity increased from 33.43 % with MFI zeolite to 99.42% with silylated MFI zeolite. The enhancement in para selectivity was due to the pore size regulation achieved by vapor deposition of the bulky molecule, tetraethyl orthosilicate, followed by its calcination at 813 K. As the molecule size of the alkoxy compound is larger than that of the zeolite pore opening, on decomposition the deposition of silica takes place on the external surface and pore mouth entrance. The internal structure remains unaffected, but the pore opening size is reduced (Niwa et al., 1986). The effects of pore opening size reduction on the diffusivity of aromatic molecules inside the zeolite were monitored by a standard test reaction involving probe molecules ethylbenzene and m-xylene (Halgeri et al., 1991). In the case of ethylbenzene realkylation, the alkyl group already present in the benzene ring activates the ortho and para positions. Due to a space constraint inside the zeolite channel, realkylation of ethyl group takes place only at the para position. While o-diethylbenzene can form on the external surface or pore mouth entrance sites, because of steric factors it is not formed even on these sites. The meta isomer is formed from isomerization of para product. The pore size regulation by silica covers the pore mouth entrance and the external surface sites. The extent of meta isomer formation decreased with increase in silica deposition. Hence, p-diethylbenzene is formed as primary product, and its further isomerization to the meta form is suppressed by the zeolite modification. This is in accordance with the literature. Paparatto et al. (1987) reported that the para isomer formed selectively inside ZSM-5 channels while isomerization proceeded just on the external surfaces and that the improvement in para selectivity by the modification was due to the inactivation of the acid sites on the external surfaces. On the other hand, Kim et al. (1989) have suggested that the improvement in para selectivity by the modification of HZSM-5 with oxides was due to the suppression of the isomerization of the primarily produced para isomer. A similar conclusion was also reported (Lonyi et al., 1989). It has also been reported that selective poisoning of the external surface of zeolite crystallites will improve the shape selectivity of the para isomer (Anderson et al., 1979; Yashima et al., 1981; Nunan et al., 1984). Paparatto et al. (1989) have dealt with the role of the external surface sites in reaction with ZSM-5 catalysts. According to them, on the zeolite samples having larger external surface area the selectivity to para isomer becomes lower because of the higher extent of the isomerization reaction. The same results can be interpreted in terms of diffusional limitations within the zeolite channels. Kaeding et al. (1981), Young et al. (1982), and Kaeding et al. (1984) have investigated, beside toluene ethylation with ethylene, the alkylation with methanol and toluene disproportionation, comparing the performances obtained on ZSM-5 in proton form and modified by adding P,B and polymeric materials or by coke deposition during the experimental runs. To explain the increase in selectivity to para isomers due to these modifications, the authors suggest a mechanism based on reduction in dimensions of pore opening and channels which favor the formation and diffusion of the smallest isomers. However, the interaction of the modifying agents with the acid sites on the external surface, which reduces the non-shape-selective isomerization, is not excluded. Vayssilov et al. (1993) have reported para-selective alkylation of toluene with methanol over ZSM-5 zeolites.
W
W
I
1
/ r’
2 g
+
/
BENZENE /DEB
1
W
a 0
1
I
I
I
4
8
12
16
20
S i 0 2 DEPOSITEO (wt.%) Figure 1. Para selectivity enhancement with progressive silylation. Temperature = 573 K, WHSV = 2.87 h-1, HdHC = 2.
The reactions of toluene alkylation and xylene isomerization are considered both on the external and internal surface catalytic centers of the zeolite crystals. Uguina et al. (1992, 1993) have studied Mg and Si as ZSM-5 modifier agents for selective toluene disproportionation. For both agents, para selectivity corresponding to the primary product was loo%, p-xylene being the unique isomer able to leave the modified pore structure due to the coupled effects of steric constraints enhanced by the modifier agents and the fast internal isomerization. The further decline of para selectivity with conversion is caused by the primary product isomerization on the external acid sites. The best para selectivity/activity relationship obtained with the silicon modification has been assigned to the deposition of this agent on the external zeolite surface, which is in good agreement with kinetic and adsorption-diffusion measurements. Murakami (1989) has investigated the superselective catalysis by CVD zeolites. This study reports deposition of an ultrathin layer of silica to cover the external surface of zeolites, and this layer of silica is effective in controlling the pore opening size without changing the internal structure. Our modified catalyst is prepared by a procedure similar to that of Murakami. The results of our standard test, involving two probe molecules of known dimension, can be best explained by considering external surface modification during silylation. Figure 1illustrates the enhancement in para selectivity with progress in silica deposition. The zeolite deposited with 16 w t % silica showed a very high selectivity of about 100% at around 13% ethylbenzene conversion. This zeolite was selected for further studies, because it produced very pure p-diethylbenzene which can be straightway used as a desorbent in p-xylene separation process. Effects of Temperature and WHSV on Para Selectivity. The effects of reaction temperature on the para selectivity feature of silylated MFI zeolite are presented in Figure 2. With the raise in reaction temperature from 523 to 623 K, ethylbenzene conversion increased from 5% to 28%, while the selectivity to diethylbenzene decreased from 58.13%to 33.01% and benzene selectivity increased from 48.48 % to 53.2 5%. The change in p-diethylbenzene selectivity was from 100% to 98%. With increase in reaction temperature, the purity of para isomer in the diethylbenzenes decreased.
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2527 Reaction Mechanism
10011
Y
Y
Y
n
n
p - D E B SELECTIVITY
'
fH3
fH3
CH2
6 CH2
& *Ot
60
-
I
;
e
W
- 36
CH3
r
I
0
+
1
tCH2-CH-j
y
&
CH2=CH2
Qy-CH3
3
\
40
- 2 YI:0
-
-1
Reaction Scheme fH3
523
I
I
I
548
573
603
:
743
I
623
TEMPERATURE ( K ) Figure 2. Effect of temperature on performance parameter. WHSV = 2.87 h-1, HdHC = 2. p-DEB SELECTIVITY
It
h
s
Y
Ew 8 0 -
3
CH2 - CH,
w
E
-I
I
Figure 4. Reaction scheme and mechanism of ethylbenzene disproportionation over silylated MFI zeolite.
2I
Table 11. Kinetic Constants of Ethylbenzene Disproportionation at Different Temperatures
0
60-
-3
W V
3 40-
z
a
4
r
\
-2 BENZENE/ D EB
temp, K 523 548 573
m
8
12
16
20
WHSV ( < ' I Figure 3. Effect of WHSV on performanceparameter. Temperature = 573 K, HdHC = 2.
The effects of WHSV variation on the catalytic performance of silica-deposited MFI zeolite are shown in Figure 3. With variation in WHSV from 1.74 to 17.4 h-l, the conversion of ethylbenzene changed from 18.215% to 2.65 % ,benzene selectivity increased, and p-diethylbenzene selectivity changed from 98 5% to 100 % . Here again there is an effect on the purity of p-diethylbenzene at lower WHSV. Reaction Mechanism. The reaction scheme and mechanism of ethylbenzene disproportionation over silylated zeolite are presented in Figure 4. Kinetic Study. Kinetic runs were carried out in the region free of interparticle diffusion effect. This was established by the experiments carried out with constant W/F (see eq 1) but with varying weight of the catalyst and liquid feed rates to the reactor. There are two types of diffusion processes to be accounted for in the case of a zeolite-catalyzed reaction: (i) between the macropores of catalyst particles and (ii) micropores inside the zeolite channels. The experiments for finding the interparticle diffusion region show only the absence of macropore diffusion. As the channel dimensions of MFI alumino-
k,mol h-1 g of catalyst-' atm-1 2.00 x 103 5.50 X 1V 11.16 X 103
silicate are comparable to those of reactant ethylbenzene and products p-diethylbenzene and benzene, micropore diffusional resistance cannot be avoided. Hence, the kinetic parameters computed here include the diffusional effects. Similar kinetic studies using MFI zeolite have been published for toluene ethylation (Lee et al., 1985), isomerization of ethylbenzene and rn-xylene (Hsu et al., 19881, and disproportionation of toluene (Chang et al., 1987; Nayak and Riekert, 1986). Ethylbenzene disproportionation was carried out at different temperatures, viz., 523,548, and 573 K, in order to obtain the kinetic data. At each temperature, W/F was varied to achieve different ethylbenzene conversions. Assuming a first-order dependence of ethylbenzene concentration with rate, a model similar to that of Nayak and Riekert (1986) proposed for toluene disproportionation was adopted: reaction rate = -r = dx/d(W/F) = k(1- x ) (1) where r is the rate of ethylbenzene conversion, x is conversion,and W/F is space time expressed as g of catalyst h mol-I. On integration, eq 1 becomes In 1/(1- x ) = k(W/F) (2) A plot of In 1/ (1- x ) versus W/F yields the kinetic constant k. Such plots at temperatures 523, 548, and 573 K are given in Figure 5. Table I1 presents the kinetic constants a t different temperatures.
2528 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 n 76.
Table 111. Activation Energy Value Comparison for Ethylbenzene and Toluene Dieproportionation activation energy, ref reaction studied temp, K kJ/mol 84 Beltrame et al. toluene disproportionation 513-573 ethylbenzene 523-573 85.7 present work disproportionation
7 c c
is due to enhanced extent of dealkylation while realkylation came down. The change in reaction temperature from 523 to 623 K and WHSV from 17.4 to 1.74 h-l affected p-diethylbenzene purity (from 100% to 98%). The estimated activation energy assuming first-order dependence was 85.72 kJ/mol. This value is close to the value that was reported for toluene disproportionation over MFI zeolite in the literature.
W
C d
W/F Figure 5. Plot of In 1/(1 - x ) vs W/F at different temperatures. - 4 .4r
I
Acknowledgment We are grateful to Dr. I. S. Bhardwaj, Director (R&D), for encouragement during this investigation and also for permission to publish this paper.
Literature Cited
-6.41 1.72
I
1.79
I
1.86
1 1.93
I , 1ij3 T Figure 6. Arrhenius plot for ethylbenzene dealkylation and realkylation over pore size regulated MFI zeolite.
The kinetic constant k is related to activation energy E by the relationship
k = k, exp(-EIRT) (3) where ko is the frequency factor, R is the gas constant, and T is the temperature. By plotting In k against 1/T, one can calculate activation energy from the slope. Such a plot is shown in Figure 6. The activation energy estimated by least-squares fitting is given in Table I11along with the reported activation energy values for toluene disproportionation over MFI zeolite. The activation energy for ethylbenzene disproportionation is close to the value reported for toluene disproportionation (Beltrame et al., 1985).
Conclusion A very high para product selectivity during ethylbenzene dealkylation and realkylation can be achieved by pore opening size regulation of MFI zeolite. The zeolite deposited with 16 wt % silica converted 12.5% ethylbenzene with 99.42% p-diethylbenzene selectivity at 573 K and WHSV = 2.9 h-l. With increase in reaction temperature, ethylbenzene conversion and benzene selectivity increased and diethylbenzene selectivity went down. This
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Received for review January 22, 1993 Revised manuscript received June 10, 1993 Accepted June 14, 1993. a Abstract published in Advance ACS Abstracts, August 15, 1993.
