Ind. Eng. Chem. Res. 1996,34,1517-1528
1617
KINETICS, CATALYSIS, AND REACTION ENGINEERING Alkylation of Benzene or Toluene with MeOH or C2H4 over ZSM-5 or /?Zeolite: Effect of the Zeolite Pore Openings and of the Hydrocarbons Involved on the Mechanism of Alkylation Panagiotis G. Smirniotis Chemical Engineering Department, University of Cincinnati, Cincinnati, Ohio 45221 -0171
Eli Ruckenstein" Chemical Engineering Department, State University of New York at Buffalo,Buffalo, New York 14260
A comparative study of the alkylation of benzene or toluene with MeOH or CzH4 over a medium (ZSM-5) and a large pore (p) zeolite of comparable acidities was carried out. It was observed that the reaction temperature in combination with the structure of the zeolite plays an important role in the reactions that take place. The maximum yield of either the primary or secondary alkylation products may occur at an intermediate temperature, which is lower over /3 zeolite.
Due to its pore structure, p zeolite favors secondary alkylation reactions and also disproportionation reactions of the generated alkylaromatics to a higher extent than ZSM-5 does. MeOH generates both primary and secondary alkylation products, while CzH4 favors oligomerization reactions, primary alkylation reactions, and particularly, disproportionation reactions. Toluene is more reactive than benzene. The aromatidalkylating agent molar ratio plays an important role in the relative importance of the reactions that take place. The size of the pores of the zeolite in combination with the sizes of the aromatic hydrocarbons and the alkylating agents employed determines whether the alkylation occurs via a Langmuir-Hinshelwood (LH) or Rideal-Eley (RE) mechanism. When a LH mechanism occurs, the alkylation rate passes through a maximum with respect to the concentration of the aromatic hydrocarbon employed; no such maximum occurs for the RE mechanism.
1. Introduction The alkylation of benzene or toluene with light hydrocarbons over acidic catalysts for production of alkylaromatics is a process of industrial significance (Weitkamp, 1985). The alkylation over solid zeolite catalysts gained increased interest compared to the highly contaminating and corrosive liquid catalysts (hydrofluoric or sulfuric acids) usually employed. Initially, protonated or rare earth exchanged faujasites were used in the alkylation of benzene with various olefins (Venuto e t al., 1966; Mortikov and Marchenko, 1980) or with C2H4 (Morita et al., 1973; Weitkamp, 1985). ZSM-5 has been used as an alkylation catalyst for the methylation of toluene with methanol (Kaeding et al., 1981; Weitkamp, 1985; Mirth and Lercher, 1991; Sotelo et al., 1993). Other molecular sieves have provided beneficial effects in alkylation reactions as well. Some typical examples are mordenite in the alkylation of benzene with ethylene and propylene (Becker et al., 1973), EU-1 zeolite in the methylation of toluene with methanol (Rao et al., 1989), zeolite 8, in the alkylation of benzene with 2-propanol (Reddy et al., 1993), KZ-1 zeolite in the alkylation of toluene with methanol (Rane and Chakrabarty, 1993), and silicoaluminophosphates in the alkylation of benzene with propylene (Ivanova et al., 1993). The main advantage of zeolites as alkylating catalysts, in comparison with the nonzeolitic catalysts, is
* Author to whom correspondence should be addressed.
that they can significantly influence the product selectivity via their microporous structure. A typical example is the significant increase of the para selectivity of dialkylbenzenes, achieved by impregnating the zeolites with various metals which modify their effective pore openings. For instance, the employment of phosphorus in ZSM-5 increases the p-xylene selectivity in the alkylation of toluene with methanol (Kaeding et al., 1981), and the impregnation with Mg or Ni also increases the para selectivity (Kaeding et al., 1984; Kaeding, 1985; Sotelo et al., 1993). In the present study we examine the performance of ZSM-5 and zeolite p in the alkylation of benzene or toluene with methanol or ethylene. The purpose is to compare the behavior of a 10-ring pore member zeolite such as ZSM-5 (medium pore size) with that of a 12ring pore member such as ,8 zeolite (large pore size), under identical conditions. While faujasites have also been employed in alkylation reactions, their low resistance to cokin limited their industrial use. The supercages (12 ) of the X and Y faujasites favor the accumulation and enlargement of coke precursors, which leads to a rapid loss of activity. In contrast, p zeolite possesses smaller channel intersections (of about 9 8)and comparable channel diameters (straight channels of about 7.3 x 6.5 fi and tortuous channels of about 5.5 x 5.5 fi for zeolite p, and about 7.4 x 7.4 fi for the faujasites). Benzene and toluene have been chosen because in the coming years their prices are expected to decrease since environmental regulations demand
1
0888-5885l95I2634-1517~Q9.0Ql~0 1995 American Chemical Society
1518 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995
lower proportions of the latter aromatics in the gasoline pool. Hence, a catalyst that could utilize this fraction of the refinery stream for the production of useful higher polybranched aromatics would be of value. Methanol and ethylene have been selected as alkylating agents because they are representatives of alcohols and alkenes, respectively. We found that the reaction temperature in combination with the structural characteristics of the zeolites controls the types of reactions that occur. A maximum in the alkylation rate exists at some intermediate temperature, which is smaller over /3 zeolite. The aromatidalkylating agent molar ratio is an important parameter which affects the relative extent of the alkylation reactions. The pore size of the zeolite plays an important role in the alkylation mechanism. The alkylation of bulky aromatics with relatively large alkylating agents over small or medium pore zeolites obeys the Langmuir-Hinshelwood (LH) mechanism, which implies the adsorption of both the aromatic and the alkylating agent molecules on the catalyst surface. In contrast, the alkylation of relatively small aromatics over large pore zeolites follows the Rideal-Eley (RE) kinetics, which implies the reaction of the adsorbed alkylating agent with gas phase aromatic molecules.
Table 1. Acidity (in m o l n-Butylamindgcat) and Strength Distribution of ZSM-5 and fi Zeolite Determined by the n-ButylamineTitration Method acidity at HO= pK. values of ZSM-5 Bzeolite
6.8
2.0
0.8
-3.0
-8.2
-10.1
-12.0
4.72 4.08
1.96 1.12
0.76 0.60
0.64 0.52
0.44 0.36
0.12 0.08
0.00 0.00
flow quartz reactor. The reactor was placed into a temperature programmable controlled furnace. The temperature was varied between 100 and 600 "C, and the total pressure was atmospheric. The temperature was varied a t a rate of 10 "C/min. In all the experiments, 100 mg (on a dry basis) of catalyst was loaded on a quartz frit located in the middle of the reactor. The reactants employed in the present study were benzene (Aldrich, 99+%), toluene (Aldrich, 99+%), methanol (Aldrich, 99+%), and ethylene (Matheson, 99.9%). The liquid hydrocarbons were introduced into the bed through a heated line with a liquid infusion pump at the appropriate flow rate. High-purity He (99.995%)was employed as the carrier gas. Gas flow controllers were employed for the regulation of the C2H4 and He flow rates. In the present study the aromatidalkylating agent molar ratios were varied in the range 0.5-3.0. After each experiment, the catalyst was regenerated by 2. Experimental Section heating in air at 550 "C for 2 h. All catalysts acquired 2.1. Synthesis and Preparation of the Catalysts. their initial activity aRer this step. Fresh catalysts were Zeolite /? was synthesized hydrothermally from an used for each feed mixture. aluminosilicate gel by the method of Wandliger et al. The analysis of the products was performed with a (1967). The nominal molar composition of the gel was gas chromatograph (HP 5890, series 11) attached to a ~~S~O~:A~Z~~:~.~N~Z with ~ :tetraeth~ T E A O Hmass : ~ ~spectrometer O H Z O (HP 5971 11). The GC unit was ylammonium hydroxide (TEAOH) as the template. The equipped with a PoraPLOT wide bore capillary column steps of the zeolite synthesis have been presented in (0.53 mm i.d. 28 m length with a film thickness of 20 detail earlier (Smirniotis and Ruckenstein, 1993). Its pm), a molecular sieve (5A) packed column, and a highSiO2/A12O3 molar ratio was 28, as determined by atomic performance capillary column (HP-1,cross-linked methabsorption spectroscopy (AAS). ZSM-5 was synthesized yl silicone, 0.2 mm i.d., 12 m length with a film hydrothermally from an aluminosilicate gel by the thickness of 0.33 pm). A PC was attached to the GCI method (example 4)of Chen et al. (1978). The nominal MS unit for data acquisition and storage. The reactor molar composition of the gel was 30SiOz:Alz03:24NazO: effluent stream was transferred to the GC through a 3TPA-Br:1200Hz0, with tetrapropylammonium bromide heated line (170 "C). (TPA-Br)as the template; details concerning the preparation steps were presented elsewhere (Smirniotis and 3. Results and Discussion Ruckenstein, 1994). The SiOz/A12O3 molar ratio of In the present study a comparison between the ZSM-5 was 24. Both zeolites were highly crystalline, performances of ZSM-5 and /? zeolite in the alkylation as determined by X-ray diffraction (MID). of benzene or toluene with either methanol or ethylene For the preparation of the acidic forms of the zeolites, was carried out. The effect of temperature on the a calcination step a t 510 "C for 5 h in 0 2 was first product yields was examined in the range of 100-600 employed in order t o burn the template occluded in the "C, under atmospheric pressure. The effect of the pores. After grinding and sieving (150 mesh size sieve), aromatidalkylating agent molar ratio, which was varied each of the zeolites (2 g) was cation exchanged under between 0.5 and 3.0, was also investigated. All the reflux in 1L of 0.5 M NH&l solution at 95 "C for 4 h. yields and selectivities reported, with the exception of The N&+/zeolites were transformed to their protonated the time on stream behavior, are at 5 min on stream form by calcination in air at 450 "C for 1h. hence for the initial stages of reaction where the 2.2. Acidity and Acidic Strength Distribution. catalysts are free of coke. The yield of species i is The alkylation of aromatics with light hydrocarbons is defined as a reaction that requires relatively strong acidic sites. The acidity and the acidic strength distribution of Yi= ZSM-5 and ,0 zeolite were determined by the butylamine titration method (Tanabe, 1970). This method, which where X is the conversion of the alkylating agent and determines both the Br~nstedand the Lewis acidities, Si is the selectivityof species i defined as the molar ratio is based on the color change of indicators with various of species i to all the products generated multiplied by pKa values. From these values, the Ho of the catalyst surface can be obtained; the number of acid sites for 100. The acid strength distribution and the acidity, meaeach HOcan be determined by butylamine titration. All sured in mmol n-butylaminelg cat, is presented in Table the indicators were purchased from Aldrich, and ben1. Both zeolites change the colors of all indicators with zene (Aldrich, 99+%) was employed as solvent. pK, 2 -10.1. However, they are not superacids, since, 2.3. Catalytic Tests. The catalytic runs were as considered by Tanabe et al. (19891, the value of Ho = performed in a 60 cm long and 0.7 cm i.d. vertical plug
xs,
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Table 2. Conversion Data over ZSMd and B Zeolite for the Alkylation of Benzene and Toluene with Methanol and Ethylene versus Temperature ZSM-5 p zeolite 100 200 270 350 420 470 520 550 580 600 100 200 270 350 420 470 "C "C "C "C "C "C "C "C "C "C "C "C "C "C "C "C (a)Benzene with Methanol (BenzeneMeOH Molar Ratio = 1, WHSV = 3.8 h-l, 5 min on Stream) MeOH 5.0 15.1 66.3 100 100 100 100 93.2 100 100 100 100 100 benzene 1.2 6.7 24.7 34.9 36.5 37.6 38.5 23.7 41.3 54.7 59.7 61.6 66.5 (b) Benzene with Ethylene (Benzene/CzH4Molar Ratio = 1,WHSV = 3.7h-l, 5 min on Stream) 5.1 6.2 23.7 94.8 71.8 40.1 39.3 18.1 23.8 43.3 57.8 64.3 52.8 CzH4 benzene 4.2 4.8 15.9 65.7 39.1 20.9 18.7 16.3 20.3 27.2 35.7 26.7 24.2 (c) Toluene with Methanol (TolueneMeOH Molar Ratio = 1, WHSV = 4.3 h-l, 5 min on Stream) 10.8 35.1 100 100 100 100 100 100 100 100 100 100 100 MeOH toluene 2.4 31.8 86.1 89.1 87.5 87.8 88.3 4.2 88.3 70.8 68.3 62.4 63.6 (d) Toluene with Ethylene (Toluene/Cz& Molar Ratio = 1, WHSV = 4.2 h-l. 5 min on Stream) Cz& 2.1 34.5 35.2 91.3 100 10076.4 1.1 93.3 78.8 79.4 82.2 79.8 toluene 0.4 22.1 24.7 23.1 37.9 51.4 40.8 44.0 43.2 48.0 57.5 46.3
-12.0 is the upper limit of superacidity. As suggested by Jacobs (1984),for alkylation reactions to occur the alkylation catalyst should have values of HOsmaller than -6.63. Both zeolites employed in the present study satisfy the above requirement. ZSM-5 is slightly more acidic than /? zeolite. 3.1. Temperature Effect. The reaction temperature can significantly alter the extent of the various reactions which take place during the alkylation of aromatics. The effect of temperature in the alkylation of benzene or toluene with either MeOH or C2H4, for an aromatidalkylating agent molar ratio equal to unity, is as follows: 3.1.1. Alkylation of Benzene with Methanol. "he effect of the reaction temperature on the yields of the
550 "C
100 64.2 54.0 23.7
80.2 42.1
alkylation products for the alkylation of benzene with methanol over ZSM-5 and /3 zeolite is presented in Figure 1(the conversion data of both the aromatics and the alkylating agents over the zeolites at various temperatures are presented in Table 2). At about 350 "C a broad maximum in the yield of C1-CG paraffins and olefins occurs. "he maximum over ZSM-5 is larger than that over /? zeolite due to the higher capability of ZSM-5 to generate light olefins from methanol (Hutchings et al., 1994). This maximum probably occurs because of the decrease of the activity of the catalysts for temperatures higher than about 400 "C, as a result of the deactivation of the strong active sites by coke formation. Indeed, the catalysts were black for temperatures above about 400 "C. At low temperatures the yield of toluene,
1520 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
which is a product of the primary alkylation, over ZSMd is low but increases monotonically for temperatures higher than about 300 "C. In contrast, the toluene yield over zeolite ,b is significantlyhigher, namely, about 3576, even a t the lowest temperature employed (100 "C) and increases for temperatures higher than 300 "C. A similar trend was observed during the alkylation of benzene with propylene over ZSM-12 as a function of temperature, a case in which the cumene selectivity (product of the primary alkylation) increased with temperature (Pradhan et al., 1990). The yield of C8 aromatics, which are products of the secondary alkylation reactions (reaction scheme I), passes through a maximum over ZSM-5 at a temperature of about 350 "C (Figure la). (Note: Ethylene (*),
which acts as the alkylating agent of benzene, is generated from methanol according to the traditional methanol to olefins reaction pathway over acidic zeolites.) The yields of the o- and p-xylenes also pass through a maximum over /3 zeolite at a lower temperature than those over ZSM-5. The yield of ethylbenzene, which is a product of the primary alkylation of benzene with ethylene (reaction scheme I), is very low at any temperature over /3 zeolite in comparison with ZSM-5 at 5 min on stream (Figure 1). The Cg aromatic yields over ZSM-5 are significantly lower than those of the C8 compounds. This is due to the space constraint that the pores of ZSM-5 impose. In contrast, the Cg aromatic yields over /3 zeolite are larger and possess a very steep maximum a t the relatively low temperature of 270 "C. The above observations indicate that /3 zeolite shifts the alkylation pathway (reaction scheme I) toward the Cg aromatics, while ZSM-5 favors the primary alkylation. Aromatics with C210 were not detected with any of the zeolites under the conditions employed. The conversion of benzene is lower than that of MeOH a t any temperature since the secondary alkylation reactions and the oligomerization reactions at which methanol participates consume additional MeOH molecules. 3.1.2. Alkylation of Benzene with Ethylene. The alkylation of benzene or aromatics with ethylene over acidic zeolites takes place via the generally accepted carbenium ion mechanism. On the basis of this mechanism, the ethylene is protonated over the Bransted acidic sites to a carbenium ion (tricoordinated carbon atom). Its electrophilic attack on the aromatic n-electrons generates a mono- or polyalkylbenzenium ion. The desorption of the latter intermediate and loss of the proton results in the gas phase alkylated aromatic and the restoration of the Bransted acid site. There have been numerous controversial discussions regarding the mechanism followed by olefins during the alkylation of aromatics over acidic zeolites. In the early work of Becker et al. (1973)it was suggested that the alkylation of benzene with ethylene or propylene over the protonated mordenite follows Langmuir-Hinshelwood (LH) kinetics. This implies that both the alkylating agent and the benzene molecules are adsorbed on the catalyst surface on adjacent sites before they react. A similar
reaction mechanism was considered by Morita et al. (1973) for the alkylation of benzene with ethylene over Lay zeolite. It was assumed that the reaction proceeds according to the Langmuir-Hinshelwood (LH) scheme, the rate-determining step being the reaction between the benzene adsorbed on the aprotonic sites (La) and the ethylene adsorbed on the protonic sites. However, this scheme implies the assumption that two neighboring species react, even though there is a repulsive electrostatic interaction between them. Venuto et al. (1966) suggested that the alkylation of aromatics with light olefins over acidic X and Y faujasites follows the Rideal-Eley (RE) mechanism. On the basis of the latter scheme, gas phase aromatic molecules react with adsorbed carbenium ions formed from olefins over the acidic sites. This reaction scheme explains fairly well the gas phase alkylation of aromatics with olefins but contradicts the experimental evidence that benzene is easily adsorbed on acidic sites (Venuto and Cattanach, 1968). An attempt to identlfy the reaction mechanisms involved in the alkylation of aromatics with methanol and ethylene will be presented later in this paper. The effect of the reaction temperature on the product yields for the alkylation of benzene with ethylene over ZSM-5 and zeolite ,b is presented in Figure 2. The total yield of c 1 - C ~ paraffins and olefins passes through a maximum over both zeolites. However, the yield is higher over ZSM-5 than over ,b zeolite. The higher extent of ethylene oligomerization reactions over the Bransted sites of ZSM-5 is probably responsible for this higher value. A maximum in the C2H4 conversion was observed (Table 2). The ethylbenzene yield passes through a pronounced maximum at 350 "C and is larger for ZSM-5 than ,f3 zeolite (Figure 2b,d) in the initial 3 h; but it acquires higher values over ,f3 zeolite for longer times on stream. A different trend is observed for the toluene generated whose yield increases with temperature, the increase being much steeper over ,b zeolite. This result is surprising since ethylbenzene and not toluene is expected to be generated over acidic zeolites from benzene and ethylene. A possible reaction mechanism is provided by the following reaction scheme.
In the first step of reaction scheme 11, the generally accepted electrophilic attack on the n-electrons of benzene by the ethylcarbenium ion (Weitkamp, 1985) takes place and results in the formation of three ethylbenzenium cations (a, b, and c). The desorption of the intermediate a, which is the most stable among the three intermediates, leads, after the loss of a proton, to the gas phase ethylbenzene and restoration of the Bransted acid site. This constitutes the main reaction at relatively low temperatures.
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1621 80 I
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100 200 300 400 500 600 700
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Figure 2. Yields of products for the alkylation of benzene with ethylene over ZSM-5 (top figures) and zeolite B (bottom figures) versus temperature (benzendMeOH molar ratio = 111, WHSV = 3.7 h-l, 5 min on stream): (a, c) total lumped products (b, d) aromatic products except toluene.
It seems that the increase in temperature favors a secondary reaction pathway, which starts from the intermediate b. The gas phase benzene is attracted by the positive charge of the latter intermediate, located on the second carbon atom from the edge of the alkyl chain. A protonated cyclopropane (PCP) intermediate which involves two aromatic rings is thus generated. Protonated cyclopropanes have been suggested as possible intermediates involved in the B type rearrangement (rearrangements in which the number of ramifications increases or decreases) for the isomerization of long chain parafins (Weitkamp, 1982;Gianneto et al., 1986). In the present case, the positive charge is distributed among the two carbon atoms of the alkyl chain and one carbon atom of the aromatic ring. The disproportionation of the above bulky intermediate results in a gas phase toluene molecule and an adsorbed methylbenzenium ion which can further lose a proton, thus restoring the Bransted site and generating another (the second) toluene molecule. The above reaction pathway explains the relatively large toluene generation at elevated temperatures over B zeolite (Figure 2c). ZSM-5, which has smaller pores than ,b zeolite, cannot accommodate the bulky C12 intermediate, and therefore the toluene generation over ZSM-5 is indeed much smaller. The yields of the xylenes and Cg aromatics (Figure 2) are low for both zeolites regardless of the reaction temperature. This indicates that disproportionation reactions between toluene and ethylbenzene, which are the main aromatics generated and which could lead to xylenes and Cg aromatics, occur only to a very small extent. 3.13. Alkylation of Toluene with Methanol. The alkylation of toluene with methanol for production of
CZ8 aromatics is an important industrial reaction due to the relatively low cost of the reactants and the high value of the products. Many researchers tried to modify the acidic zeolites in order to alter the selectivities of the alkylaromatics produced. A typical example is the impregnation of ZSM-5 with various metals which significantly increases the selectivity ofp-xylene, which is the most valuable xylene (Kaeding et al., 1981;Sotelo et al., 1993). The kinetic studies of Sotelo et al. (1993) regarding the alkylation of toluene with MeOH over Mgmodified ZSM-5 have shown that the reaction follows the Rideal-Eley mechanism; the methanol is adsorbed on the catalyst surface while the toluene reacts from the gas phase with the adsorbed alkylating agent. In contrast, the in-situ IR studies of the surface species during the methylation of toluene over HZSM-5 in the range 100-400 "C (Mirth and Lercher, 1991) have shown that this reaction follows Langmuir-Hinshelwood (LH) kinetics. From that study it was concluded that methanol and toluene form a coadsorption complex over the Bransted acidic sites. The yields of all products for the alkylation of toluene with methanol as a function of temperature are presented in Figure 3 for ZSM-5 and /? zeolite. With both zeolites, the generation Of c1-C~and olefins is at a low level; slightly higher yields are detected over ZSM-5. The benzene generated, which is a product of toluene transalkylation, is in small amounts over both zeolites for temperatures below 400 "C. However, a t higher temperatures the benzene yield increases. The yield of Cs aromatics passes through a maximum which over ZSM-5 is at about 400 "C, while for /? zeolite it is shiRed to a lower temperature. The maximum of the Cg aromatic yield is higher over /3 zeolite than over ZSM-5 probably because of the larger pore sizes of the former
1622 Ind. Eng. Chem. Res., Vol. 34, No. 5 , 1995 30 b) 0
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Figure 3. Yields of products for the alkylation of toluene with methanol over ZSM-5 (top figures) and zeolite ,8 (bottom figures) versus temperature (toluene/MeOH molar ratio = 1/1,WHSV = 4.3 h-l, 5 min on stream): (a, c) total lumped products, (b, d) aromatic products except benzene. 80
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Figure 4. Yields of products for the alkylation of toluene with ethylene over ZSM-5 (top figures) and zeolite ,8 (bottom figures) versus temperature (toluene/MeOH molar ratio = 1/1,WHSV = 4.2 h-l, 5 min on stream): (a, c) total lumped products, (b, d) aromatic products except benzene.
zeolite. The concentrations of the xylenes presented in Figure 3b, d have the equilibrium values indicated by
Kaeding et al. (1981). According t o the equilibrium values, the selectivity ofp-xylene is about the same as
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1523
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Figure 5. Product yields for the alkylation of benzene over ZSMd (top figures) and over /3 zeolite (bottom figures) for various benzene/ alkylation agent molar ratios at 350 "C (5 min on stream): (a, c) the alkylation agent is MeOH (WHSV of MeOH = 1.1h-l); (b, d) the alkylation agent is C2& (WHSV of C2H4 = 1.0 h-l).
that of o-xylene, while the selectivity of m-xylene is approximately equal to the sum of the selectivities of the other two xylenes. This is as expected since no impregnation with metals that could change the shape selectivity of the zeolite pores was employed. The ethylbenzene yield over ZSM-5, which is the product of the alkylation of the benzene (generated by toluene transalkylation) with ethylene is higher than that over /3 zeolite (compare Figure 3b and d). A similar trend was observed for the direct alkylation of benzene with methanol (Figure l b and d) and for the direct alkylation of benzene with C2H4 to ethylbenzene (see Figure 2b and d). Hence, when methanol is employed as the alkylating agent of toluene, the ethylene, which is generated more easily from MeOH over ZSM-5 than over /3 zeolite, alkylates the benzene produced to ethylbenzene. The yield of the cf3aromatics passes through a broad maximum, which occurs at a lower temperature over /3 zeolite. The yields of C g aromatics are comparable over ZSM-5 and /3 zeolite. By comparing Figures 1and 3 one can conclude that the aromatic yield when MeOH is employed as the alkylating agent increases with the number of methyl groups of the aromatic molecules employed as feed. A similar trend has been noted earlier (Bauer et al., 1990) indicating that the reactivity increases with the size of the aromatic molecule employed.
3.1.4. Alkylation of Toluene with Ethylene. The
c1-c6 yield of paraffins and olefins for the alkylation
of toluene with C2H4 over ZSM-5 and /3 zeolite passes through a maximum (Figure 4a, c). The maximum is located at a lower temperature over /3 zeolite than over ZSM-5. The transalkylation of toluene takes place to a higher extent than when MeOH was employed, and a larger amount of benzene is generated. The rate of toluene transalkylation is higher over /3 zeolite than over ZSM-5, especially at high temperatures. In contrast to ZSM-5, for which the cf3aromatic yield increases with temperature but remains relatively constant above 250 "C, the cf3aromatic yield over /3 zeolite passes through a maximum at about 200 "C. The yields of xylenes and ethylbenzene follow the same trends. The yields of the three xylenes are approximately equal to those corresponding to the equilibrium. The Cg aromatic yield over /3 zeolite remains at a very low level at all temperatures; for ZSM-5 the C g yield is somewhat higher. The reactions involved in the alkylation of benzene and toluene were the primary and secondary alkylation reactions, toluene transalkylation, dealkylation of aromatics (disproportionation reaction), and oligomerization reactions. The characteristics of the zeolite in combination with the reaction temperature significantly alter the reactions involved in the alkylation of aromatics.
1524 Ind. Eng.Chem. Res., Vol. 34,No.5, 1995
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50
1
8
40
a
c
s
30
z! a 'j; 20 10 0 1 2 3 4 5 6 7 8 9
Carbon Number
'
7
8arbonNumber
8
9
Figure 6. Product yields for the alkylation of toluene over ZSM-5 (top figures) and over zeolite (bottom figures) for various toluene/ alkylation agent molar ratios at 350 "C (5 min on stream): (a, c) the alkylation agent is MeOH (WHSV of MeOH = 1.1h-l); (b, d) the alkylation agent is C2H4 (WHSV of C2H4 = 1.0 h-l).
3.2. Effect of the Aromatic/Alkylating Agent Molar Ratio. For the alkylation of benzene with MeOH over ZSM-5, the C2-C5 yields of paraffins and olefins decrease and those of the primary and secondary alkylation products increase with increasing benzene1 MeOH molar ratio (Figure 5a). Over zeolite /3 the yields of C2-C5 paraffins and olefins are lower than those over ZSM-5 and decrease with increasing benzene/MeOH molar ratio. The increase of the benzene/MeOH molar ratio results in an increase of the primary alkylation and decrease of the secondary alkylation reactions (Figure 5c). As expected when C2H4 is employed, the yields of C2-C5 p a r a i n s and olefins are higher (Figure 5b, d) over both zeolites. The increase of the benzene/ C2H4 molar ratio increases the primary alkylation products over ZSM-5 but decreases the primary alkylation reactions and increases the disproportionation reactions over /3 zeolite. The yield of alkylated aromatics (toluene, xylenes, and C g aromatics when benzene is used as feed, and benzene, xylenes, and C g aromatics when toluene is used) is higher with MeOH as the alkylating agent. For the alkylation of toluene, the increase of the toluene/MeOH molar ratio increases the products of primary alkylation over ZSM-5; over /3 zeolite the primary alkylation reaction decreases while the products of secondary alkylation and toluene transalkylation increase with increasing toluene/MeOH molar ratio.
When (32% is employed as the alkylating agent, the CaC5 yields of paraffins and olefins are higher than when MeOH is used (Figure 6b, d) over both zeolites. This indicates that during the alkylation of aromatics, the rate of ethylene oligomerization to higher hydrocarbons is higher than the rate of transformation of MeOH to light hydrocarbons. Zeolite p favors disproportionation reactions to a higher extent than ZSM-5 at all toluene/ c2H4 ratios. 3.3. Effect of Time on Stream. The effect of the time on stream on the selectivities of primary and secondary products of alkylation during the alkylation of benzene and toluene is presented in Figures 7 and 8, respectively. The benzene conversion over both zeolites (Figure 7)increases after a few minutes on stream. This is because of the decrease of the consumption of the alkylating agent with time on stream in the oligomerization reactions, due to the selective deactivation of some very strong acid sites during the initial stages of the reaction. A similar trend has been observed during the methylation of toluene over HZSM-5 (Bauer et al., 1990) and over KZ-1 zeolite (Rane and Chakrabarty, 1993). For the alkylation of benzene with MeOH over ZSM-5 and /3 zeolite at similar levels of conversion, the selectivities of the primary alkylation product (toluene) are comparable. The selectivities for the secondary alkylation products (c28aromatics) increase with time on stream and are higher than for the primary ones over
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1525 100 a)
d
.se
E
8 V
I
I 0
0
10
20
30
40
Time, h 100
d
8
8
r
2
'E
.-8
g
0 u5
V
I
Conversion
I
W C8,9Amn'elka 0 Ccnvenion
"
0
A Elhylbm" 0 C7.8,9 Aromakr
10
20
30
40
Time, h
Figure 7. (a) Benzene conversion and primary and secondary alkylation product selectivities for the alkylation of benzene with MeOH (filled symbols) and C2H4 (open symbols) over ZSM-5 versus time on stream at 350 "C (benzenehlkylating agent molar ratio = 1,WHSV = 3.8 h-' for MeOH plus benzene and WHSV = 3.7 h-l for C2H4 plus benzene). (b) Benzene conversion and primary and secondary alkylation product selectivities for the alkylation of benzene with MeOH (filled symbols) and C f i (open symbols) over B zeolite versus time on stream at 350 "C (benzendalkylating agent molar ratio = 1,WHSV = 3.8 h-l for MeOH plus benzene and WHSV = 3.7 h-' for CzH4 plus benzene).
both zeolites. With C2H4 as the alkylating agent, the selectivity of the primary product (ethylbenzene), at almost the same conversion, is higher over /3 zeolite than over ZSM-5 for reaction times longer than 3 h. At shorter reaction times the selectivity over ZSM-5 is the largest one. The selectivities of the products of disproportionation reactions (xylenes and C7, Cg aromatics) are comparable over both zeolites. In the alkylation of toluene with MeOH over ZSM-5 and /3 zeolite (Figure 8a and b, respectively), at comparable levels of conversion, the /3 zeolite generates less primary alkylation products (CS aromatics) but more secondary alkylation products (Cg aromatics) than ZSM5. With ethylene as feed, the toluene conversion is low over both zeolites (about 50%). For times on stream shorter than about 10 h, the selectivities of the primary alkylation products (Cg aromatics) over ,I3 zeolite are low but increase later, while the selectivities for secondary reaction products (CS, Cg aromatics which result from disproportionation and dealkylation reactions) are relatively high. In contrast, ZSM-5 favors equally the primary and secondary alkylation products (Figure 8a). As shown in Figures 7 and 8 the time on stream performance of ZSM-5 and ,I3 zeolite at 350 "C is satisfactory; operation at temperatures above 400 "C results, however, in a rapid decrease of the catalyst activity. Regarding the aromatic molecule employed in the alkylation reaction, one can note that the alkylation of
0
10
20
30
40
Time, h
Figure 8. (a) Toluene conversion and primary and secondary product selectivities for the alkylation of toluene with MeOH (filled symbols) and toluene conversion, primary alkylation products and secondary reaction product selectivities (disproportionation and dealkylation) for the alkylation of toluene with CzH4 (open symbols) over ZSM-5 versus time on stream at 350 "C (toluene/ alkylating agent molar ratio = 1,WHSV = 4.3 h-' for MeOH plus toluene and WHSV = 4.2 h-l for CzH4 plus toluene ). (b) Toluene conversion and primary and secondary product selectivities for the alkylation of toluene with MeOH (filled symbols) and toluene conversion and primary alkylation product and secondary reaction product selectivities (disproportionation and dealkylation) for the alkylation of toluene with C2H4 (open symbols) over /? zeolite versus time on stream at 350 "C (toluendalkylating agent molar ratio = 1,WHSV = 4.3 h-' for MeOH plus toluene and WHSV = 4.2 h-l for C2H4 plus toluene).
benzene with CzH4 favors the primary products, while its alkylation with MeOH favors both the primary and the secondary alkylation products over both zeolites. With toluene, the primary products are favored when MeOH is employed over both zeolites, while the products of disproportionation reactions are favored when C2€& is employed, again over both zeolites. 3.4. Mechanisms of Benzene and Toluene Alkylation. There have been numerous controversial discussions about the mechanism of alkylation of aromatics with light hydrocarbons over acidic zeolites. As a typical example one can consider the alkylation of benzene with ethylene. According to Venuto et al. (1966) and Weitkamp (1985) the alkylation of benzene with ethylene over acidic faujasites and ZSM-5, respectively,obeys the Rideal-Eley (RE) mechanism. In contrast, Becker et al. (1973) and Morita et al. (1980) suggested that the alkylation of benzene with ethylene over Wmordenite or alkali faujasites follows Langmuir-Hinshelwood (LH) kinetics. Another example of contradictory opinions is the methylation of toluene with MeOH. In the work of Mirth and Lercher (1991)it was shown that the alkylation obeys the LH mechanism over HZSM-5, while Rakoczy and Sulikowski (1988) and Sotelo et al. (1993)
1626 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
suggested that the methylation of toluene over HZSM5, protonated faujasites, or Mg/ZSM-5 is consistent with the RE mechanism. In what follows it will be shown that there are conditions under which each of the two mechanisms can occur. The pore size of the zeolite in combination with the sizes of the aromatic hydrocarbon and of the alkylating agent are the parameters that determine the alkylation mechanism. For a relatively small pore size zeolite, one can consider that the LH mechanism occurs. In this case, the alkylating agent adsorbed on the catalyst surface reads with the aromatic molecule which is adsorbed on an adjacent site. However, when a large pore size zeolite is employed, the aromatic molecule can easily diffuse in the zeolite pores and react from the gas phase with the adsorbed alkylating agent. The mechanism is in this case of the RE type. Our experiments regarding the alkylation of toluene with CZH4 over ZSM-5 (medium pore size) and the alkylation of benzene with MeOH over @ zeolite (large pore size) support the above conclusion. For LH kinetics, the rate of alkylation is given by the following reaction rate equation (in which one assumes that the surface reaction is rate determining):
Table 3. Values of the Parameters of Eqs 2 and 3 for the AUrylation of Toluene (WHSV of C& = 1.0 h-l) with C a over ZSM-5 and the Alkylation of Benzene (WHSV of C B O H = 1.1 h-9 with MeOH over /? Zeolite, respectively (T= 270 "C, Atmospheric Total Pressure, CA = 9.0 x 10-3 mom, 5 min on Stream)
k, x lo5 catalyst ZSM-5 (LH) j3 zeolite (RE)
(moYg cat s) 7.74 1507.3
KA (IJmol) 10.01 32.67
KB (IJmol) 1032.5 420.9
goodness of fit (%) 85.6 98.8
.
4
.
3-
t 2 -
0
..
0
0
0
0
1-
where CA is the concentration of the alkylating agent and CB is the concentration of the aromatic. The corresponding rate expression for the RE mechanism is given by
U
0
0.
1.Q
0.5
1.5
2.0
3
C x 10 (molesfliter)
(3) The adsorption of the alkylating agent (MeOH or C2H4) on the Brmsted sites is a relatively fast step (Morita et al., 1973; Mirth and Lercher, 1991). Hence, what determines whether the LH or RE mechanism is acting depends upon the state of the aromatic hydrocarbon before it reacts with the adsorbed alkylating agent. In the present experiments, the concentrations of Cz& and MeOH were kept constant at a relatively high level (CA =9 x mom) in comparison with the concentrations of toluene and benzene which were significantly lower and were varied so that the alkylating agenuaromatic molar ratio was in the range 5-50. Since the concentration of the alkylating agent (Cp3 is constant and much higher than that of toluene or benzene (CB),eqs 2 and 3 acquire the following forms (4)
ne
Figure 9. Rate of toluene alkylation with CzH4 (open symbols) over ZSMd (WHSV of CZ& = 1.0h-l) and alkylation rate at the reactor entrance of benzene with MeOH (filled symbols) over j3 zeolite (WHSV of MeOH = 1.1 h-l) at 270 "C versus the concentration of the aromatic at the reactor entrance (Ck).The concentration of either CZ& or MeOH was C A = ~ 9.0 10-3 m o m x- C B (5 ~ min time on stream).
"C as a function of the concentration of toluene or benzene a t the entrance of the reactor, respectively, are presented in Figure 9. The rate of alkylation of toluene with CZH4 passes through a broad maximum for CB~," w mom. This indicates that the alkylation of toluene with CzH4 over ZSM-5 (medium pore size zeolite)follows the Langmuir-Hinshelwood mechanism because eq 4 indeed has a maximum for (7) The maximum alkylation rate is given by
and
The constants in eqs 4 and 5 have been determined from the experimental data using the equation
(6) The rates of the alkylation of toluene with CzH4 over ZSM-5 and of benzene with MeOH over ,8 zeolite at 270
In contrast to the LH mechanism, the RE mechanism (eq 3) does not provide a maximum. The values of the parameters of eq 2 for the alkylation of toluene with ethylene over ZSM-5, determined by nonlinear squares fitting, are given in Table 3. The goodness of the fit is relatively high indicating that the kinetic data obtained indeed correspond to LH kinetics. The attempt to fit these data to eqs 3 or 5, which correspond to the RE mechanism, was completely unsuccessful. In contrast,
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1527 the data for the benzene alkylation with MeOH over /3 zeolite (large pore zeolite), which do not present a maximum (Figure 9), could be satisfactorily represented by the Rideal-Eley mechanism (see Table 3). The adsorption constant KBfor toluene alkylation with C2H4 over ZSM-5 is larger than that for benzene alkylation with MeOH over /3 zeolite. This indicates, as expected, that toluene is adsorbed much more strongly than benzene on the catalyst surface. Hence, the zeolite pore openings in combination with the sizes of the alkylating agent and the aromatic hydrocarbon employed in the alkylation determine the reaction mechanism.
4. Conclusions From the present study the following conclusions can be drawn: 1. The zeolite structure in combination with the reaction temperature determines the types of reactions that occur. Zeolite B favors secondary alkylation reactions, while ZSM-5 favors primary ones. This is a result of the larger pores of ,8 zeolite which allow the formation of bulky reaction intermediates. Disproportionation reactions among the alkylaromatics are favored to a greater extent by ,8 zeolite than ZSM-5. For both zeolites the yields of most of the alkylated products pass through maxima at an intermediate temperature, which is lower for ,8 zeolite. Operation at high temperatures should be avoided because the activity of the catalysts decreases with time on stream due to coke deactivation. 2. The aromatidalkylating agent molar ratio significantly affects the relative extents of the primary and secondary alkylation reactions. It was found that with benzene as feed, MeOH favors both the primary and secondary alkylation reactions, while C2H4 particularly favors the primary ones. When toluene was employed, the yield of the alkylated aromatic products was higher than for benzene with MeOH. The secondary products are favored when toluene is alkylated with C2H4. 3. The size of the pores of the zeolites in combination with the sizes of the aromatic hydrocarbons and of the alkylating agent determines the alkylation mechanism. For aromatic molecules comparable with the pores of the zeolites, the alkylation follows the LangmuirHinshelwood (LH) mechanism. In contrast, with relatively large pore zeolites and for aromatic molecules small enough compared to the size of the pores of the zeolites a Rideal-Eley (RE) mechanism occurs.
Nomenclature AI, A'1: constants in eqs 4 and 5 Az: constant in eqs 4 and 5 CA: concentration of the alkylating agent, mol L-l Cg: concentration of the aromatic employed in the alky-
lating reaction, mol L-l
F: feed rate of the aromatics, mol s-l k,: surface reaction rate constant, mol g cat-' s-l KA: adsorption constant of A species, mol-l L KB:adsorption constant of B species, mol-l L ?-A: rate of alkylation, moles g cat-l s-l Si: selectivity of species i defined as the ratio of the moles of i product to the moles of all the products multiplied by 100 W. weight of the catalyst, g X: mole conversion of the alkylating agent x : mole conversion of the aromatic Y,:yield of species i
Subscripts f: exit conditions e: entrance conditions
Literature Cited Bauer, F.; Dermietzel, J.; Jockisch, W. Diffusion Effects on the Kinetics of Toluene Methylation and Xylene Isomerization on HZSM-5 Zeolites. In Catalysis and Adsorption by Zeolites; Studies in Surface Science and Catalysis 65; Elsevier: Amsterdam, 1990; pp 305-313. Becker, K. A.; Karge, H. G.; Streubel, W.-D. Benzene Alkylation with Ethylene and Propylene over H-Mordenite as Catalyst. J . Catal. 1973,28, 403-413. Chen, N. Y.; Miale, J. N.; Reagan, W. J. Preparations of Zeolites. U S . Patent 4,112,056, Sept. 5, 1978. Gianneto, G. E.; Perot, G. R.; Guisnet, M. R. Hydroisomerization and Hydrocracking of n-Alkanes. 1. Ideal Hydroisomerization PtHY Catalysts. Znd. Eng. Chem. Prod. Res. Dev.1986, 25, 481-490. Hutchings, G. J.; Johnson, P.; Lee, D. F.; Warwick, A.; Williams, C. D.; Wilkinson, M. The Conversion of Methanol and Other 0-Compounds to Hydrocarbons over Zeolite ,!I.J . Catal. 1994, 147, 177-185. Ivanova, I. I.; Dumont, N.; Gabelica, Z.; Nagy, J. B.; Derouane, E.; Ghighy, F.; Ivashkiva, 0. E.; Dmitruk, E. V.; Smirnov, A.; Romanovsky, B. V. Alkylation of Benzene with Propylene over SAPO-5, SAPO-11 and SAPO-37. In Proc. of the 9th Inter. Zeolite Conference, Montreal; Von Ballmoos, R., et al., eds.; Butterworth-Heinemann: Boston, 1993; Vol. 11, p 449-456. Jacobs, P. A. In Characterization of Heterogeneous Catalysts; Dekker: New York, 1984; p 367. Kaeding, W. W.; Chu, C.; Young, L. B.; Weistein, B.; Butter, S. A. Selective Alkylation of Toluene with Methanol to Produce Puraxylene. J . Catal. 1981, 67, 159-174. Kaeding, W. W.; Yong, L. B.; Chu, C.-C. Shape-Selective Reactions with Zeolite Catalysts. IV.Alkylation of Toluene with Ethylene to Produce p-Ethyltoluene. J . Catal. 1984, 89, 267-273. Kaeding, W. W. Shape-selective Reactions with Zeolite Catalysts. V. Alkylation or Disproportionation of Ethylbenzene to Produce p-Diethylbenzene. J . Catal. 1986,95, 512-519. Mirth, G.; Lercher, J. A. In-situ IR Spectroscopic Study of the Surface Species during Methylation of Toluene over HZSM-5. J . Catal. 1991, 132, 244-252. Morita, Y.; Matsumoto, H.; Kimura, T.; Kato, F.; Takayasu, M. Adsorptive and Catalytic Properties of Lay Zeolite in the Alkylation of Benzene with Ethylene. Bull. Jpn. Pet. Inst. 1973, 15 (l),37-44. Mortikov, E. S.; Marchenko, L. S. On the Mechanism of Benzene Alkylation with Olefins on Zeolite Catalysts. In Proc. of the 5th Conference of Zeolites, Naples, Italy, 1980; Heyden: Chichester, 1980; pp 696-704. Pradham, A. R.; Rao, B. S.; Shiralkar, V. P. Isopropylation of Benzene over Large Pore Zeolites. In Catalytic and Adsorption by Zeolites; Studies in Surface Science and Catalysis 65; Elsevier; Amsterdam, 1990; Vol. 99, pp 347-356. Rakoczy, J.; Sulikowski, B. Alkylation of Toluene with Methanol on Zeolites: Evidence for Rideal Type Mechanism. React. Kinet. Cutul. Lett. 1988,36 (l),241-246. Rane, S. J.; Chakrabarty, D. K. Shape Selective Reactions of some Organic Compounds on the Zeolite KZ-1. Appl. Catal. 1993, 93, 191-202. Rao, G. N.; Kumar, R.; Ratnasamy, P. Shape Selectivity of Zeolite EU-1 in Reactions of Aromatic Hydrocarbons. Appl. Catal. 1989,49,307-318. Reddy, K. S. N.; Rao, B. S.; Shiralkar, V. P. Alkylation of Benzene with Isoproponal over Zeolite Beta. Appl. Catal. 1993,95,5363.
Smirniotis, P. G.; Ruckenstein, E. Comparison between Zeolite and y-AlzO3 Supported Pt for &forming Reactions. J . Catal. 1993,140,526-542. Smirniotis, P. G.; Ruckenstein, E. Comparison of the Performance of ZSM-5, B zeolite, Y, USY, and Their Composites in the Catalytic Cracking of n-Octane, 2,2,4-Trimethylpentane, and 1-Octene. Znd. Eng. Chem. Res. 1994,33, 800-813. Sotelo, J. L.; Uguina, M. A.; Valverde, J. L.; Serrano, D. P. Kinetics of Toluene Alkylation with Methanol over Mg-Modified ZSM5. Ind. Eng. Chem. Res. 1993,32, 2548-2554.
1628 Ind.Eng.Chem. Res., Vol. 34, No. 5, 1995 Tanabe, K. Solid Acids and Bases: Their Catalytic Properties; Kodansa: Tokyo, and Academic Press: New York, 1970. Tanabe, R;Misono, M.; Ono, Y.; Hatori, Y. In New Solid Acids and Bases: Their Catalytic Properties; Studies in Surface Science and Catalysis 51; Elsevier: Amsterdam, 1989; p 206. Venuto, P. B.; Hamilton, L. A.; Landis, P. S. Organic'hactions Catalyzed by Crystalline Aluminosilicates 11. Alkylation Reactions: Mechanistic and Aging Considerations. J . Catal. 1966,5,484-493. Venuto, P.; Cattanach, J. Molecular Sieves. SOC.Chem.Znd.1968, 117-125. Wandliger, L. R.; Kerr, G. T.; Rosinski, E. J. Catalytic Composition of a Crystalline Zeolite. U.S.Patent 3,308,069,March 7,1967. Weitkamp, J. Isomerization of Long-chain n-Alkanes on a WCaY Zeolite Catalyst. Znd. Eng. Chem. Prod. Res. Dev. 1982, 21, 550-558.
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Received for review October 24, 1994 Revised manuscript received February 23, 1995 Accepted March 10, 1995@ IE940614A
Abstract published in Advance ACS Abstracts, April 15, 1995. @
