3356
Ind. Eng. Chem. Res. 1996, 35, 3356-3361
Alkylation of Toluene with Ethanol Jerzy Walendziewski* and Janusz Trawczyn ´ ski Institute of Chemistry and Technology of Petroleum and Coal, Wrocław Technical University, ul. Gdan´ ska 7/9, 50-344 Wrocław, Poland
A series of Y and ZSM-5 zeolite based catalysts was prepared. Zeolites were cation exchanged and formed with 50% of aluminum hydroxide as a binder, and the obtained catalysts were finally thermally treated. Activity tests in alkylation of toluene with ethanol were carried out in the temperature range of 325-400 °C, in nitrogen or hydrogen stream, and a pressure up to 3 MPa. The feed consisted of toluene and ethanol mixed in a mole ratio 1/1 or 2/1. The obtained results showed that among the studied catalysts the highest activity in the alkylation reaction was attained by ZSM-5 zeolite based catalyst with a moderate acidity and medium silica to alumina ratio, i.e., ∼50. Activity and selectivity of the most active catalyst as well as conversion of the feed components were similar to those reported in other papers. The content of p-ethyltoluene in alkylation products attained ca. 60%. Introduction Ethyltoluene isomers, especially p-ethyltoluene (pET), are products of great industrial significance. Dehydrogenation of p-ethyltoluene allows one to obtain p-methylstyrene, the monomer for poly(p-methylstyrene), a product which can find appreciable application in the near future. The best method of p-ET synthesis is alkylation of toluene with ethylene or ethanol. In both reactions ethylene is an alkylation agent (Levesque and Dao, 1989; Keading et al., 1984). However, in the case of ethanol it is first dehydrated to ethylene at relatively low temperature. Lately, application of ethanol, because of its availability from bioprocesses, gained more attention (Levesque and Dao, 1989). In 1970 Yashima et al. stated that metal-exchanged Y zeolite catalyzes alkylation of toluene with up to 50% selectivity to p-ET, while the equilibrium content of p-ET in ET mixtures is 33.7% (Keading et al., 1984). Zeolites like Y, X, and mordenite are active in alkylation reaction but catalyst aging due to coking is severe, leading to deactivation in a few hours. In the next years the application of ZSM-5 zeolite with a good shape selectivity attracted great interest (Lee and Park, 1993). According to Bhandarkar and Bhatia (1994), mediumpore-size zeolites ZSM-5 show high stability, low coke formation, and longer life. It was suggested that the activity of ZSM-5 zeolites can be proportional to Bro¨nsted sites concentration (Haag et al., 1984). Numerous modifications of ZSM-5 zeolite, e.g., enlargement of the crystal size (Chen et al., 1979), impregnation with phosphorus and boron (Young et al., 1982), and thermal treatment at high temperature, were proposed. According to Lonyi et al. (1991), H-ZSM-5 zeolites containing catalysts modified with P, B, and Mg show higher selectivity than unmodified ones, and alkylation reaction takes place inside ZSM-5 zeolite pores in the para-position. They also stated that the diffusivity and acidity of a catalyst affect p-selectivity. Because of steric constraints, a primary alkylation reaction within the pores of ZSM-5 is in the paraposition; however, p-alkyltoluene can isomerize to other isomers (Paparatto et al., 1987). It was found that p-ethyltoluene is mainly formed when isomerization * Author to whom correspondence is addressed. Tel/fax: (+48-71)22-15-80. E-mail:
[email protected]. pl.
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reaction is suppressed by introducing of inorganic additives, which diminish the number of strong acid sites (Chandevar et al., 1984). It is evident that no shape-selective reactions take place on the external surface of zeolite crystals. Therefore, catalysts without strong acid sites or catalysts which have external crystal surfaces covered with, e.g., silica, phosphorus, boron, or acid sites poisoned by large organic bases, gave increased paraselectivity. According to Vinek et al. (Vinek et al., 1989), phosphorus treatment results in a decrease in the strength of both Bro¨nsted and Lewis acid sites, while Cai et al. (Cai et al., 1985) stated that when H-ZMS-5 catalyst was modified with phosphorus, both Lewis and Bro¨nsted acid sites of various strengths were decreased, and when modified by magnesium, the stronger acid sites were eliminated while the weaker Lewis acid sites increased. During alkylation of toluene with ethanol, side reactions also occur, such as formation of carbonaceous deposits, which result in fast catalyst deactivation. Deactivation of alkylation catalysts can be suppressed in the presence of a hydrogen stream (Keading et al., 1984). Most of the results presented in the literature on toluene alkylation with ethanol were obtained by using zeolite in parent, powder form, i.e., without forming and using of a binder. Therefore, in this paper alkylation of toluene with ethanol studies over shaped zeolite catalysts were carried out and the effects of process variables such as process temperature, gas pressure, time on stream, and toluene to ethanol mole ratio were determined. Experimental Techniques Catalysts Preparation. Characteristics of the ZSM-5 zeolite samples are given in Table 1. Preparation of ZSM-5 Zeolite Samples. Producer Intytut Chemii Przemysłowej, Warsaw, Poland, included the following operations: (a) slurrying of zeolite in water for 1 h, (b) dropping of ammonium acetate solution to the zeolite suspension, cation exchange by mixing of the suspension for 3 h, (c) filtering of the slurry, (d) double washing out of the slurry with hot water, (e) drying of the zeolite at ambient temperature overnight and 24 h at 110 °C. Exchange operation was made two times. Preparation of Ni-Y and H-Y Zeolites. Na-Y zeolite, produced by Inowrocławskie Zakłady Chemic© 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3357 Table 1. Chemical Composition of the Applied Samples of ZSM-5 and Y Zeolites and Total Acidity of the Final Catalysts
sample
Na content (wt %)
SiO2/Al2O3 mole ratio (mol/mol)
Na contenta (wt %)
total acidityb (mmol of NH3/g)
Z-1, ZSM-5 Z-2, ZSM-5 Z-3, ZSM-5 Z-4, ZSM-5 Z-5, ZSM-5 H-Y Ni-Y
4.2 2.0 1.35 1.2 2.3 10.4 10.4
22 48.9 73.8 83.5 42 4.9 4.9
0.457 0.08 0.07 0.1 0.3 1.2 2.1
1.03 0.718 0.454 0.398 0.759 1.32 0.84
a
After ion exchange. b Final catalyst.
zne, contained 10.41 wt % of sodium and a 4.9 SiO2/ Al2O3 mole ratio attained. Preparation of Ni-Y zeolite was quite similar to that before and included the following operations: (a) slurrying of zeolite in water for 1 h, (b) dropping of a nickel acetate solution to the zeolite slurry (the concentration and volume of the applied nickel salt solution were allowed to obtain ca. 150% overabundance of Ni2+ cation in relation to the sodium content in zeolite), (c) mixing of the slurry of zeolite with a nickel salt solution for 3 h, (d) filtering of the slurry, (e) washing out of the slurry with hot water, (f) drying of the zeolite in ambient temperature overnight and 24 h at 110 °C, (g) calcination of zeolite in air in the temperature range of 250550 °C (6 h). The zeolite-exchanging operation was repeated four times. In the result Ni-Y zeolite containing 8.8% wt Ni and 2.1% wt Na was obtained. In the case of H-Y zeolite ammonium acetate instead of nickel acetate was applied and the obtained zeolite sample was not calcined. The zeolite-exchanging operation was repeated four times, and after the fifth operation zeolite was calcined under an air stream saturated with water vapor (200 mmHg) in the same temperature range. It was stated that the final product contains 1.2 wt % of sodium (88% exchange of sodium). All zeolite samples were mixed with the aluminum hydroxide powder (50/50), peptized with a 1% nitric solution, and formed using a piston extruder. The obtained catalyst extrudates were dried overnight at ambient temperature and 24 h at 110 °C and calcined at 500 °C for 5 h. Supports and Catalysts Characterization. (a) Compacted bulk densities were obtained from measurements of the mass of 100 cm3 of catalysts after they had been compacted. (b) The pore size distributions in the pore radius range 1.5-100 nm were obtained from benzene adsorptiondesorption isotherms made with a McBain-Bacr balance. On the basis of these results, pore volumes, surface areas, and mean pore radii of the supports and catalysts were calculated. (c) Total pore volumes were calculated from the volume of water adsorbed by catalyst samples. (d) Surface acidities were measured by temperatureprogrammed desorption of ammonia in the range of temperature 180-550 °C, at temperature increases of 10 °C/min. (e) The crushing strengths were obtained from measurements of the force necessary to crush extrudates along the radial direction. Physicochemical properties of catalysts are given in Table 2, and catalyst acidities are characterized in Figure 1.
Figure 1. Acid site strength distribution for the selected catalysts determined by the ammonia thermodesorption method. Table 2. Physicochemical Properties of Zeolite Based Catalysts for Alkylation of Toluene with Ethanol catalyst property
H-Y
Z-1
Z-2
Z-3
0.622 0.645 0.665 0.636 bulk density, kg/dm3 pore volume, dm3/kg 0.713 0.710 0.595 0.684 mean pore radius, nm 3.4 3.05 3.8 3.4 specific surface area, m2/g 348 383 155 260.8 acidity, mmol of NH3/g 0.84 1.03 0.718 0.455 pore volume distribution (volume of pores, dm3/ kg for pores) 1.5-3 nm 0.145 0.378 0.101 0.230 3-5 nm 0.110 0.073 0.092 0.076 5-10 nm 0.11 0.054 0.050 0.067 10-30 nm 0.088 0.05 0.036 0.045 30-100 nm 0.025 0.031 0.040 0.030 1.5-100 nm 0.479 0.585 0.301 0.45
Method of Catalysts Activity Testing. Investigations of catalytic activity were carried out in a fixedbed down-flow reactor packed with 4 cm3 of ground catalysts (0.25-0.6 mm fraction). A mixture of ethanol and toluene in a mole ratio 1:1 or 1:2 was fed to the reactor with LHSV ) 3 h-1 under a nitrogen or hydrogen pressure of mainly 1 MPa, as well as 2 and 3 MPa and in the temperature range 325-400 °C. Liquid products of the reaction were analyzed by a gas chromatography method using a capillary column (60 m, silicon oil OV-17) in a Chrom 4 apparatus. Identification of reaction products was carried out using gas chromatography and mass spectroscopy. Toluene conversion was calculated according to the formula:
toluene conversion ) (content of toluene in the feed content of toluene in the product) × 100%/ content of toluene in the feed
∑
∑
Most of the activity measurements were started after 2 h from the beginning of the activity test, under a
3358 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
nitrogen pressure of 1 MPa. Four hours were necessary for determination of the catalyst activity in one process parameter set. Only in the case of the experiments presented in Table 5 were hydrogen and nitrogen pressures (from 1 to 3 MPa) applied in order to estimate their influence on feed component conversion. For catalysts of low activity and selectivity, i.e., Ni-Y and H-Y, activity measurements were made two times. In the case of zeolite ZSM-5 based catalysts activity tests were repeated at least three times or even more. In all the experiments quite satisfied repeatability was obtained, taking into account ethyl alcohol and toluene conversion (ca. 2%) as well as feed component and alkyltoluene contents in reaction products (below 1%). Oxidative regeneration of catalysts was made by using nitrogen with 2% oxygen content as the combustion agent, in the temperature range 400-500 °C for 3-5 h. Gas hourly space velocity (GHSV) was changed in the range 300-800 Nm3/m3 h, i.e., 1.2-3.2 dm3/h, depending on the temperature in the catalyst bed. The end of the regeneration process was stated by determination of CO2 absence in flue gases (NaOH solution in the presence of a pH indicator). Results and Discussion Catalysts Properties. Characteristics of the applied ZSM-5 and Y zeolites are presented in Table 1. The ZSM-5 zeolites differed mainly in sodium and alumina content, and such compositions resulted in silica to alumina mole ratios from 22 up to 83. Two-step exchanging of the zeolites with an ammonium acetate solution makes it possible to lower the sodium content so the hydrogen-exchanged zeolite forms, after drying and calcination, contained an appreciably lower quantity of sodium in comparison to the fresh samples, from 0.1 up to ca. 0.5 wt %. The final catalysts (after forming with aluminum hydroxide and thermal treatment) presented similar propertiessbulk density and total porositysbut somewhat different porous structure (Table 2) as well as total acidity and acid site strength distribution (Table 1 and Figure 1). It is necessary to underline similar or lower acidity of zeolite H-ZSM-5 based catalysts in comparison to the acidity of zeolite H-Y and Ni-Y based ones. Among the H-ZSM-5 zeolites containing catalysts, the highest acidity was presented by this one, which was prepared by using a Z-1 zeolite sample of the highest exchanged sodium quantity (1.03 mmol of NH3/g). Low sodium zeolite samples, Z-3 and Z-4, were characterized by the lowest acidity, while Z-2 and Z-5 samples were characterized by medium strength acidity. In the whole temperature range of ammonia desorption H-ZSM-5 zeolite based catalysts in comparison to zeolite H-Y and Ni-Y based ones presented a more uniform acid site strength distribution. On the other hand, the difference between Ni-Y and H-Y based catalysts was found in the total acidity, 1.32 for Ni-Y and 0.84 mmol of NH3/g for the H-Y form (Table 1), while their pore structure was almost the same. Similarly, the pore size distributions of zeolite Z-4 and Z-5 based catalysts was similar to the pore structure determined for the Z-3 catalyst. Catalysts Activity. The preliminary studies of Ni-Y zeolite based catalysts showed their very low activity in the alkylation process of toluene with ethanol. Table 3 presents results of the prepared catalyst activity measurements obtained after 2 h of the activity tests. This means that it was the initial alkylation activity of the catalysts. The liquid products of alkylation contained only a small quantity of ethyltoluene and other
Table 3. Chemical Composition of Alkylation Products of Toluene with Ethyl Alcohola composition of alkylation products (%) catalyst Ni-Y H-Y
H-ZSM-5, Z-1 H-ZSM-5, Z-2
H-ZSM-5, Z-3 H-ZSM-5, Z-4 H-ZSM-5, Z-5
reaction ethyl ethyl alkylation temp (°C) ether alcohol toluene products 350 350 (reg) 350 375 400 350 (reg) 375 (reg) 400 (reg) 350 375 350 375 400 350 (reg) 375 (reg) 400 (reg) 350 375 375
4.8 12.4 3.8 2.5 1.8 7.2 3.5 4.4 5.7 5.4 3.7 4.8 5.0 5.6 6.5 4.1 5.0 5.4 3.1
2.1 4.9 1.1 1.0 0.2 2.8 2.6 2.2 0.5 0.8 0.8 0.6 0.2 3.0 1.2
78.8 82.2 78.3 71.0 82.2 76.8 77.3 81.2 88.2 84.6 48.9 48.4 48.7 58.7 55.2 60.2 89.1 90.4 87.5
15.5 15.5 25.5 15.7 13.3 16.6 11.9 5.6 9.2 46.5 46.1 46.3 35.5 38.3 35.7 2.9 3.1 9.4
a Notes: total pressure 1 MPa, nitrogen; ethyl alcohol to toluene mole ratio ) 1:1; reg, product composition obtained over catalysts after oxidative regeneration.
alkyltoluene isomers. In the presence of this catalyst the main products of the reaction were ethylene, ethyl ether, and water (part of them, especially ethylene, in effluent gas) and a small quantity of ethyl alcohol. Fast catalyst deactivation was observed. Application of zeolite H-Y made it possible to obtain slightly better results. In this case the products of toluene alkylation contained from 12 to over 25% of alkyltoluene isomers, but the content of ethyl ether was still too high. An increase in the reaction temperature from 350 to 375 and 400 °C resulted first in an increase in conversion and alkyltoluene content up to 25.5% and then in a decrease to ca. 16%. Similar results were obtained after regeneration of the H-Y catalyst (by coke combustion); however, regenerated catalysts were less active and contents of alkyltoluenes in the process products were lower. The highest toluene conversion over H-Y catalyst was found at a temperature of 375 °C (Table 3). The obtained results, conversion level, are similar to those obtained by Bhandarkar and Bhatia (1994), but they stated that the distinct maximum of conversion of toluene to alkyltoluenes there is at lower temperature, 325-350 °C. Among the ZSM-5 zeolite based catalysts only the Z-2 one exhibited high activity (Table 3). Over 46 wt % of the liquid products obtained in its presence were alkyltoluenes (at the beginning step of the activity tests). Only a small quantity of unreacted ethyl alcohol and up to 5% of the light components (mainly ethyl ether) was found in the liquid product of the process. Taking into account the mole ratio of toluene to ethanol (1:1), one can calculate that the theoretical content of monoethyltoluene in hydrocarbon reaction product (without water) should attain a value of 100%. Since application of the fresh Z-2 catalyst allowed one to obtain 48.7% conversion of toluene to ethyltoluene isomers (mainly monoethyltoluene) and only a small quantity of ethyl alcohol was found in the reaction product, then the remaining part of the alcohol, ca. 50%, was transformed to ethylene and ethyl ether. After regeneration of the Z-2 catalyst (by coke combustion method), the alkylation activity of the catalysts and the yield of alkylates in
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3359
Figure 2. Influence of time on stream on toluene conversion by using Z-2 catalyst, T ) 375 °C, LHSV ) 3 h-1: CN1, 1 MPa nitrogen pressure, toluene/ethyl alcohol mole ratio ) 1; CN1R, 1 MPa nitrogen pressure, toluene/ethyl alcohol mole ratio ) 1, regenerated; CN2, 1 MPa nitrogen pressure, toluene/ethyl alcohol mole ratio ) 2; CH2, 1 MPa hydrogen pressure, toluene/ethyl alcohol mole ratio ) 2.
products were clearly lower in comparison to the activity obtained over the fresh catalyst. It is worth underlining here two important observations. Up until now catalysts for toluene alkylation with alcohol, which also are presented and discussed in the literature, do not possess the stable activity (Bhandarkar and Bhatia, 1994). In the course of the 24 h tests they lost their activity very fast (Figure 2). For low toluene to ethyl alcohol mole ratios, toluene conversion diminished almost 2 times in the case of the fresh and almost 50% in the case of the regenerated catalyst. A higher stability was presented by the Z-2 catalyst in the case of a higher toluene to ethyl alcohol mole ratio, 2, and hydrogen as feed component. Comparison of the alkylation activity of Y and ZSM-5 based catalysts as a function of temperature is shown in Figure 3. For all the catalysts the highest activity was attained at a temperature of ∼375 °C. It is also evident that conversion of toluene over regenerated catalyst was 10% lower than in the test using the fresh one. Analysis of the reaction gas products was made only from time to time. It was found that in the case of catalysts with low alkylation activity the main components, besides nitrogen, were ethyl ether, less than 0.5%, and ethylene, up to 3%. On the other hand, gas products obtained in the presence of catalysts with higher alkylation activity, e.g., Z-2, contained only trace quantities of both side products. Ethylene is the alkylation agent, and therefore it is consumed in this reaction in the presence of active catalysts. The chemical composition of the product obtained by using the selected catalyst is presented in Table 4. From Ni-Y to Z-2 zeolite based catalysts better and better results were obtained. Application of the Ni-Y based catalysts, with the highest acidity, resulted not only in low conversion but also in low selectivity of reaction. Results of chromatographic analysis showed
Figure 3. Influence of reaction temperature on conversion of toluene by using H-Y as well as fresh and regenerated H-ZSM-5 catalysts. Table 4. Chemical Compositions of Liquid Products Obtained in the Alkylation Process of Toluene with Ethanol over Prepared Catalysts (Process Parameters the Same as in Table 3) catalyst
hydrocarbon
Z-2 Z-2, reg HHH-Y1 Ni-Y H-Y (95%)a ZSM-5 ZSM-5
ethanol 0.44 0.31 0.34 ethyl ether 0.72 0.52 0.51 heptane benzene 0.34 0.04 toluene 42.4 45.19 58.8 ethylbenzene 1.6 0.27 p-xylene 3.6 0.63 0.36 1-ethyl-3-methylbenzene 13.16 8.19 7.89 1-ethyl-4-methylbenzene 5.34 8.27 7.92 1-ethyl-2-methylbenzene 7.28 7.67 6.98 trimethylbenzene 0.46 methylisopropylbenzene 0.68 0.47 diethylbenzene 0.85 0.37 methylpropylbenzene ethyldimethylbenzene 2.86 1.1 1,3-diethyl-5-methylbenzene 3.85 4.77 3.31 2,4-diethyl-1-methylbenzene 1.39 1.74 1.18 diethylmethylbenzene 3.61 4.13 3.21 1,4-diethyl-2-methylbenzene 3.64 4.19 2.6 triethylbenzene 0.6 0.84 0.17 dimethyldiethylbenzene 0.4 0.49 triethylmethylbenzene 6.76 9.96 5.79 ∑compounds 38 21 21
0.44 0.52 44.63 0.811 1.09 12.5 30.8 3.79 0.68 0.61 2.64
0.23 0.34 1.05 54.7 12.3 24.3 2.7 0.37 2.07
0.72 0.77 2.5
14
12
a
Zeolite and alumina binder contents of 95% and 5%, respectively.
that the liquid product of the reaction contained 38 individual chemicals in which, besides the desired product p-ethyltoluene, polysubstituted products like tetraethylbenzene, ethylmethylbenzene, and so on are present in considerable quantities. A similar composition was found for products obtained using H-Y zeolite based catalyst; however, in this case only 21 compounds were found. The highest concentration in the liquid product was stated for p,m,o-ethyltoluene (∼27%) and diethyltoluene (∼15%). Because alumina is a good catalyst of the dehydration reaction and catalyzes dehydration of ethanol to ethyl ether, then one series
3360 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 Table 5. Chemical Composition of Alkylation Products of Toluene with Ethyl Alcohol over Z-2, H-ZSM-5 Zeolite Based Catalyst in Nitrogen and Hydrogen Stream Flow (LHSV ) 3 h-1, T ) 375 °C, Toluene to Ethyl Alcohol Molar Ratio ) 2) parameters: pressure (MPa), time (h) 1, 4, H2 (N2) 1, 8, H2 (N2) 1, 12, H2 (N2) 2, 16, H2 (N2) 2, 20, H2 (N2) 3, 24, H2 (N2)
composition of alkylation products (%) ethyl ether
ethyl alcohol
toluene
2.4 (2.0) 1.2 (1.9) 2.2 (1.8) 1.8 (1.7) 1.6 (1.3) 1.6 (1.9)
0.7 (0.7) 0.8 (0.7) 0.5 (0.4) 0.6 (0.8) 0.4 (0.5) 0.3 (0.6)
63.3 (63.2) 65.3 (65.7) 64.4 (68.0) 66.7 (70.1) 67.8 (70.9) 68.7 (71.7)
alkylation toluene products convn (wt %) 33.6 (34.1) 32.8 (32.4) 32.9 (29.8) 30.8 (27.4) 30.2 (27.3) 29.3 (25.8)
20.8 (21.0) 18.4 (17.8) 19.5 (15.0) 16.6 (12.4) 15.2 (11.4) 14.1 (10.4)
of experiments was carried out with H-Y1 catalyst which contained only a small quantity (5%) of alumina as a binder. The obtained results of the experiments revealed very similar compositions of products obtained in the presence of both H-Y catalysts, with various binder contents. This means that increased alumina contents in H-Y catalyst have some advantageous influence on the alkylation activity and detrimental influence on the selectivity (Table 4). In comparison to the product obtained in the presence H-Y1 catalyst (5% alumina), products obtained over H-Y (50% alumina) contained a higher content of alkylation products but other than ethyltoluenes. Paraselectivity was similar. Appreciable higher selectivity to alkylation products and ethyltoluene was obtained in further experiments in which Z-2 catalyst was applied. It is evident from data given in Table 4 that more than 47% of the liquid product was ethyltoluene and the content of the other alkyl derivatives amounted to only 7%. Unfortunately, even in the case of H-ZSM-5 zeolite almost 35% of ethyltoluene consisted of less desired (o,m)-ethyltoluenes. From the paraselectivity point of view, even less advantageous was the composition of product obtained in the presence of regenerated Z-2 catalyst in which (o,m)-toluenes made ca. 38%. Results presented in Table 4 clearly indicate a higher paraselectivity of Z-2 catalyst in comparison to the H-Y one. While, p,m,oethyltoluene contents in reaction products obtained by using H-Y catalyst were quite the same, ca. 8%, Z-2 catalyst enabled one to obtain products with p-ethyltoluene content 30.8% while the concentrations of o- and m-isomers were only 3.8 and 12.5%. Further alkylation experiments were carried out using a toluene to ethanol mole ratio of 2:1 and under hydrogen and nitrogen streams. Results of these studies are given in Table 5. It is known that in some refinery processes, such as hydrocarbon isomerization, hydrogen presence in reagent streams results in hydrogenation of the large molecular coke precursor and improvement of catalyst life. Therefore, in order to improve catalyst stability under alkylation process conditions nitrogen was replaced by hydrogen. Because of quite different feed compositions (80 wt % toluene and 20 wt % ethanol), toluene conversion was lower and the obtained liquid reaction products contained a lower quantity of alkyltoluenes, 33-35%, in the beginning of the activity tests. Stability tests (24 h of continuous run on stream, results in Figure 2) pointed out that, in comparison to tests in nitrogen stream, the activity of Z-2 catalyst in hydrogen stream was more stable and the alkylation activity almost did not decrease before 12 h. One can conclude that there is some limited advantageous influence of hydrogen pressure on catalyst lifetime, but no correlation between hydrogen pressure (up to 3 MPa) and catalysts activity was found. Because
Table 6. Comparison of Ethyltoluene Selectivity Obtained with Reported Values process param Bhandarkar and alkyltoluene Paparatto et al. Lonyi et al. and this composition (1989) (1991) Bhatia (1994) paper temperature, °C W/F {kg cat/ kmol/h] SiO2/Al2O3 toluene to ethanol mole ratio conversion p-ethyltoluene (mol %) m-ethyltoluene (mol %) o-ethyltoluene (mol %) others a
350 30
400 42.5
400 17.5
375 21.2
25 4
15 5
50 4.0
49 1 (2)a
20.5 57
56.5
15.2 52.6
44.2 (21) 56.6b
40
40.8
39.0
23.0
3
3.0
2.3
7.0
6.1
13.4
b
Toluene to alcohol mole ratio 1 (or 2). Composition of alkyltoluenes (without toluene, ethyl alcohol, and ethyl ether).
of higher toluene to ethanol mole ratios, a higher quantity of ethyl alcohol reacted with toluene. As a result a lower quantity of ethylene and only trace amounts of ethyl ether as well as a higher content of monoethyltoluenes was obtained. Results presented in this work only in the limited range are comparable to those obtained in other studies (Table 6). In relation to the test parameters presented in these studies our activity measurements were carried at the shorter reaction time, 21.2 kg of cat‚h/kmol, but in the higher temperature range and in the presence of nitrogen stream (1 MPa). Selectivity of the process to ethyltoluene isomers in our experiments was lower, the content of other products of alkylation attained 6-7%, but the content of p-ethyltoluene in p,m,o-ethyltoluene mixtures was relatively high and attained ca. 60%. In order to diminish the content of undesired compounds in the reaction product (especially the quantity of diand triethyltoluenes), it is reasonable to increase the toluene to ethanol mole ratio. Furthermore, there is some effect, but rather small, of the high content of alumina (used as a binder in this study) on catalyst activity. Because of lower toluene to ethyl alcohol mole ratios in our studies than in other ones (and a higher content of ethyl alcohol), relatively higher toluene conversion could be obtained at similar process parameters. On the other hand, in comparison to reported studies, our alkyltoluene products contained a similar quantity of p-ethyltoluene, a lower quantity of m-ethyltoluene, and an appreciably higher quantity of o-ethyltoluene as well as other alkyltoluenes. Then application of a higher toluene to alcohol mole ratio gives higher process selectivity than this one obtained in studies with lower reactant (toluene/ethyl alcohol) mole ratio. Physicochemical properties of ZSM-5 zeolites presented in Table 1 do not allow to find reasons of high activity of Z-2 catalyst in comparison to other catalysts. The fresh zeolite which was applied for its preparation contained a small sodium quantity, its silica to alumina ratio had medium value, and it was similar to the value for zeolite in Z-3 catalyst. It is worth indicating that there is no distinct influence of catalyst acidity on the alkylation of toluene with ethanol. Both catalyst with the highest total acidity, Ni-Y, and H-Y and Z-4 catalysts of the lowest one do not present as high alkylation activity as Z-2 catalyst of moderate acidity. On the basis of these results, one can state that catalysts for the alkylation process of toluene with ethanol should
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3361
possess moderate acidity and silica to alumina mole ratio ∼50. Strong acid sites content also does not influence catalyst alkylation activity and selectivity. Catalyst Z-1 with the highest total acidity (amongst the H-ZSM-5 based catalysts) and especially high acidity in the range of the strongest acid sites presented very low alkylation activity, appreciably lower in comparison to Z-2 and H-Y catalysts. Unfortunately, because of the low alkylation activity of Z-1 catalyst no analyses of chemical composition of products were made; therefore, it is not possible to estimate its alkylation selectivity. It is surprising that only one zeolite sample (used for preparation of Z-2 catalyst) enables one to prepare a catalyst with high alkylation activity, while activity of the catalysts which were prepared by using the remaining zeolite samples was very low. Further studies, e.g., crystal structure determination of the original and exchanged zeolite samples, are necessary in order to explain this phenomenon. Conclusions 1. H-ZSM-5 zeolites with medium acidity (ca. 0.8 mmol of NH3/g) and medium silica to alumina mole ratio (ca. 50) are efficient alkylation catalyst with high activity and selectivity to p-ethyltoluene (ca. 60%). 2. There is no detrimental influence of alumina binder on the alkylation activity of H-ZSM-5 zeolites. 3. There is some advantageous influence of hydrogen presence on H-ZSM-5 zeolite based alkylation catalyst stability; however, the pressure value effect on catalyst stability was not stated.
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Received for review October 31, 1995 Revised manuscript received June 10, 1996 Accepted July 1, 1996X IE950659R X Abstract published in Advance ACS Abstracts, September 1, 1996.
