Ind. Eng. Chem. Res. 2010, 49, 2091–2095
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Alkylation of Toluene with t-Butyl Alcohol over Zeolite Catalysts Hong-jun Dong† and Li Shi*,‡ State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, China Meilong Road 130, Shanghai 200237, People’s Republic of China, and Petroleum Processing, East China UniVersity of Science and Technology, China Meilong Road 130, Shanghai 200237, People’s Republic of China
Alkylation of toluene with tert-butylalcohol was investigated over three zeolite catalysts (HY, Hβ, and HMCM22). The acid properties of zeolites were characterized by FT-IR. The fact that the reaction was influenced by the mount of acid was discussed emphatically, and the reaction temperature on conversion of toluene and selectivity for 4-tert-butyl toluene was also studied. The main products have been identified as 4-tertbutyltoluene and 3-tert-butyltoluene. The best result was obtained when temperature was 473 K, the space velocity was 4.0 h-1, and the mole ratio of toluene to tert-butylalcohol was 6. Zeolite HMCM-22, with the highest total acid and the lowest ratio of the total B acid to the total L acid, possessed high activity for toluene conversion and high paraselectivity over the three types of zeolite catalysts. 1. Introduction Acid-catalyzed reactions such as Friedel-Crafts alkylation are one of the important processes in organic synthesis, fine chemical production as well as in petrochemical processes. The alkylated-aromatic products, particularly the 4-tert-butyltoluene, have commercial importance as intermediates for production of 4-tert-butylbenzoic acid and 4-tert-butylbenzaldehyde, which find applications as modifiers in alkylated resins, in production of fragrances, pharmaceuticals, and polymerization regulators for polyesters.1 4-tert-Butyltoluene is industrially produced by alkylation of toluene with homogeneous catalysts. But the use of these homogeneous catalysts such like sulphuric acid, phosphoric acid, and boron triflouride gives rise to many problems concerning handling, safety, corrosion, and waste disposal. To avoid these problems, many efforts have been undertaken to search for solid acid catalysts that are more selective, safety, and eco-friendly.2 There are three kinds of methods of producing 4-tertbutyltoluene by alkylation of toluene with isobutylene, methyltert-butylether and tert-butanol in the liquid phase and in the presence of activated clay or silica-alumina catalysts at 423-503 K. The use of Ni-Y zeolite has been published, but catalytic activity and paraselectivity to desire 4-tert-butyltoluene was very low.3 Mravec et al.4 recently studied this and found that zeolite H-MOR (with Si/Al ) 17.5) showed the best catalytic activity (59% conversion) and paraselectivity (near 90%) after 8 h in their study. Sebastian at al.5 investigated the influence of acidity on regioselective tert-butylation of toluene over high silica mordenite catalysts in the vapor phase and in the temperature range 413-433 K. They obtained the best results over H-MOR zeolite with conversion of toluene 18% and selectivity to 4-TBTO 67% at 433 K and at the molar ratio TBA /toluene equal1:8 and at WHSV ) 3. Kostrab et al.6 studied tert-butylation of toluene with tert-butanol in the liquid phase over large-pore mordenite zeolite (H-MOR with Si/Al ) 10.5) and over cerium-modified parent zeolite with 1-6 wt % cerium at 453 K and at autogenous pressure. Nonmodified H-MOR was catalytic active with 66% conversion of toluene and paraselectivity near 84% at 453 K after 8 h. Cerium * To whom correspondence should be addressed. E-mail: yyshi@ ecust.edu.cn. † State Key Laboratory of Chemical Engineering. ‡ Petroleum Processing.
modification of parent mordenite by impregnation decreased catalytic activity but enhanced and retained constant high paraselectivity. The study dealt with an extensive investigation of the tertbutylation of toluene over three types zeolites (HY, H-BEA, HMCM-22) with three-dimensional pore structure. In the paper, the fact that the reaction is influenced by acid properties of zeolites was studied emphatically, and the paraselectivity of these zeolites as well as the influences of reaction temperature was also discussed. 2. Experimental Section 2.1. Catalysts and Chemicals. Large-pore Y zeolite with SiO2/Al2O3 ratio of 15, Hβ with SiO2/Al2O3 ratio of 6 and MCM-22 with SiO2/Al2O3 ratio of 10 are procured from China Petroleum & Chemical Catalyst Company. All the chemicals used in this study were of analytical reagent grade and obtained from Shanghai Chemical Reagent Company. The zeolite was ion-exchanged and converted into H-form after the organic template was removed. The sample was mixed with adhesive γ- Al2O3 (weight ratio 0.5:0.5), blended with the proper amount of 10% HNO3 solution, and squeezed to strips. The catalyst was calcined at 823 K for 3 h in air, crushed, and screened into 20-40 meshes to be used. 2.2. Characterization. The products of the reaction were identified on a GC/MS (GC 6890-MS 5973 N, Agilent) with EI and capillary column (HP-5MS 30 m × 0.25 mm × 0.25 µm); carrier gas was helium (1 mL/min). Temperature program is from 333 K with a gradient of 283 K /min to 493K. For Infrared spectra of chemisorbed pyridine, self-supported thin wafers were prepared, evacuate at 653 K for 2 h and cool to 353 K. Then pyridine vapor was introduced into the cell and the sample was equilibrated for 30 min. Subsequently, the spectra were recorded after evacuation of the sample (for 2 h) at various desorption temperatures. All the spectra are measured using a Nicolet 60 SXB spectrometer with 4 cm-1 resolution, scanning 32 times. The wavenumber range was from 4000 cm-1 to 400 cm-1. There were two kinds of acid variety. The absorption peak of Brønsted acid (B) is at 1540 cm-1 and Lewis acid (L) is at 1450 cm-1. 2.3. Alkylation of Toluene. Alkylation of toluene was carried out in a down flow fixed-bed reactor. The zeolite was placed at the center of the reactor supported by quartz wool.
10.1021/ie901080t 2010 American Chemical Society Published on Web 01/19/2010
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Scheme 1. t-Butylation Reaction over Solid Acid Catalysts
The reactor was placed vertically in a two-zone furnace controlled by temperature programmers. Liquid reactant mixture containing toluene + TBA was fed with a high-precision syringe pump. The conversion of toluene and 4-tert-butyltoluene (PTBT) selectivity were calculated according to the following equations Conversion of toluene (wt %) ) fraction of toluene converted(wt %) × 100 PTBT selectivity (wt %) ) PTBT in prod (wt %) × 100 tert - butyl toluenes in prod (wt %)
∑
3. Results and Discussion The reaction had been investigated over three commercial zeolites. The catalytic reaction mechanism was as shown in Scheme 1. In all cases, the main reaction products have been identified as 4-tert-butyltoluene (PTBT) and 3-tert-butyltoluene (3-TBT); however, 2-tert-butyltoluene was not found. 3, 5-di-tert-Butyltoluene was also found in the products, but only in the trace amounts over H-Y and HMCM-22 zeolites. The main product was logically 4-tert-butyltoluene because the para position was favored by the influence of steric hindrance of the methyl group on one side, the great mass of tert-butyl group on the other side, as well as the shape-selective action attributable to proper structure of zeolite catalysts. The other products have been identified as alkyltoluenes with longer alkyl chain. These products were formed by alkylation of toluene with lower oligomers of isobutylene. 3.1. Effect of Reaction Temperature on HY Zeolite. Figure 1. showed the influence of reaction temperature on activity and selectivity over HY zeolite under the suitable reaction conditions. There was a marginal increase in the conversion of toluene with increasing reaction temperature. The conversion of toluene
(38%) was high in 453- 493 K, but it dropped to 32% at higher temperatures. However, the conversion rate of TBA was high at all temperatures. For the PTBT selectivity, it had initially increased within 433-493K temperature zones, while it decreased significantly above 473K.The maximum of the PTBT selectivity was obtained at 493K after 2 h’s reaction. Increasing the reaction temperature above 493 K led to a steep fall in its value, which was the result of nonselective reactions that consumed the alkylating agent, thus reducing its availability for the reaction. It was known that butene was formed through dehydration of TBA on acidic catalysts which can oligomerize to C8 and C12 olefins and then crack to low boilers. Because these reactions were dominant at higher reaction temperatures, a fall in the toluene conversion as well as butylation selectivity were expected.
Figure 1. tert-Butylation of toluene with TBA on HY catalyst at different reaction temperatures. Reaction time 2 h, space velocity 4 h-1, toluene: TBA ) 6:1.
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010
Figure 2. tert-Butylation of toluene with TBA on HY catalyst at different space velocities. Reaction time 2 h, reaction temperature 473 K, toluene: TBA ) 6:1.
Figure 3. PTBT selectivity on different toluene:TBA mole ratios. Reaction temperature 473 K, space velocity 4 h-1.
3.2. Effect of Space Velocity on HY Zeolite. Space velocity was an important parameter, as it not only influences the conversion of toluene, it also has an effect on paraselectivity. Figure 2 showed the influence of space velocity on activity and the selectivity over HY zeolite under the suitable reaction conditions. With the space velocity’s increase, toluene conversion and selectivity of tert-butyl toluene first increased to the maximum when space velocity was 4 h-1, and then decreased after 4 h-1. When some amount of zeolites was packed in the fixed-bed reactor under certain circumstances as the space velocity increased, thus increasing fluid line rate and lowering the outside diffusion of resistance, the rate of toluene conversion increased. If the space velocity increased further and the contact time was cut down, the conversion rate of toluene decreased. In addition, the space velocity also affects the extent of side reaction, leading to a selective p-tert-butyl toluene change. 3.3. Influence of Toluene:TBA Mole Ratio. As can be seen from Figure 3, conversion of TBA was complete at all toluene to TBA mole ratios. The effect of excessive TBA was observed at toluene: TBA mole ratios of 1:2. Unlike mordenite catalysts,7 where toluene conversion increased with higher alcohol content in the feed, there was a reduction in the toluene conversion on HY zeolites. The PTBT selectivity was the best at toluene: TBA mole ratios of 6:1, and it showed that the increase in the amount
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Figure 4. tert-Butylation of toluene with TBA on three different zeolites catalyst at different reactions times. Reaction temperature 473 K, space velocity 4 h-1, toluene:TBA ) 6:1.
of tert-butyl alcohol would reduce the PTBT selectivity. At higher TBA concentrations, the oligomerization of isobutylene that is formed on dehydration of TBA is in competition with the main alkylation reaction. The lower utilization of the alkylating agent leads to a steep fall in the alkylation selectivity with higher TBA contained in the feeds. 3.4. Alkylation of Toluene: Comparison of Three Different Zeolites. When the comparison was made among the three different zeolites (Figure 4), the most active catalyst for the reaction was HMCM-22. There was a rise in the toluene conversion on zeolites, reaching a maximum after 4 h on stream. This kind of phenomenon was reported by Karge et al.8 for large pore zeolites, which was termed as the induction period. As the reaction proceeded further, the conversion of toluene decreased over Hβ and HMCM-22 zeolites to various extents, but the drop was steeper on Hβ zeolite than it on HMCM-22 zeolite. The toluene conversion on HY zeolite stayed comparatively steady at around 30-32% and then started to decrease with time passing. The main reaction of toluene conversion was influenced by the amount of acid sites. Table 1 show that HY zeolite was characterized with higher acidity compared to Hβ; it therefore must have more acid sites in the internal surface than in the internal surface of Hβ zeolites. Figure 5 showed PTBT selectivity on the three different zeolites. Over all the catalysts, the initial PTBT selectivity increased slowly. The maximum selectivity was obtained after 4 h on steam for MCM-22 zeolites, after it began to drop with time, and was much higher than that over the HY and Hβ zeolites. It was known that the diffusion rate of molecules within the pores played an important role in the case of zeolites and was influenced by the ratio of the pore diameter to molecular dimensions. HMCM-22 had narrower pore system than the HY and Hβ zeolites;9 therefore, the PTBT had the highest probability of diffusing out of the pores of MCM-22, whereas 3-TBT was expected to have lower diffusion rates because of larger molecular dimensions. Hence, HMCM-22 showed the highest selectivity of PTBT. Compared to Hβ, the selectivity was relatively higher on HY zeolite. It was that the isomerization took place more easily on the external surface, where it was not sterically hindered. The amount of 3-TBT was thermodynamically favored on the external surface of Hβ zeolite though both catalysts had approximately the same pore diameter. Because HY had a much
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Table 1. Acidic Properties of Different Zeolitesa C (× 10-4mol g-1)
a
catalysts
T
TL
TB
SL
SB
WL
WB
TB:TL
HMCM-22 HY Hβ
49.48 47.88 12.25
22.31 19.03 4.11
27.17 28.85 8.14
8.80 7.99 1.77
3.43 3.31 0.85
13.51 11.04 2.34
23.74 25.54 7.28
1.22 1.52 1.98
T, total acid; TL, total L acid; TB, total B acid; SL, strong L acid; SB, strong B acid; WL, weak L acid; WB, weak B acid.
Figure 5. tert-Butylation of toluene with TBA on three different zeolites catalyst at different reaction times. Reaction temperature 473 K, space velocity 4 h-1, toluene:TBA ) 6:1.
Figure 7. Spectra of acidic properties at 723 K desorption: (a) HMCM-22, (b) HY, (c) Hβ.
on HMCM-22 zeolite was higher, because the total acid of HMCM-22 zeolite was more than those of HY and Hβ zeolites. It was obvious that the amount of weak acid was more than that of the strong acid, so alkylation of toluene was favored in the absence of the strong acid. From the ratio of TB and TL we can see that as ratio of total B and total L acid increased, the PTBT selectivity was decreased. Therefore, increasing the amount of the weak acid and reducing ratio of the total B and total L acid contributed to increase the amount of reactive acid sites, thus improving the capacity of catalysts. 4. Conclusions
Figure 6. Spectra of acidic properties at 473 K desorption: (a) HMCM-22, (b) HY, (c) Hβ.
lower aluminum content, which was reflected in lower concentration of acid sites at the external surface, and as a result many 3,5-di-tert-butyltoluene were embedded in the channels of zeolite because of its large size. The formation of the latter products gave Hβ easier deactivation than when on HY zeolite. The results of infrared ray spectra of the acidity characterization of HY and Hβ, HMCM-22 zeolites were given in Figures 6 and 7 and Table 1. Pyridine bound to Brønsted (1540 cm-1) and Lewis sites (1450 cm-1) and the common C-C stretching band corresponding to TB and TL could be clearly seen in these spectra. The variation in total acid, B acid, L acid, and relative TB/TL ratio on three different zeolites with the evacuation temperature was given in Table 1. Over the three zeolites, the total acid was the highest and the TB/TL ratio was lowest on the HMCM-22 zeolite. For the reaction, the toluene conversion
Alkylation of toluene with tert-butyl alcohol had been investigated on three large pore zeolites, viz., Hβ, HY, and HMCM-22.The main reaction products had been identified as 4-tert-butyltoluene and 3-tert-butyltoluene. 2-tert-Butyltoluene was not found. The main product of the reaction, 4-tertbutyltoluene is kinetically favored. The steric hindrance of the methyl group on one side and the voluminous tert-butyl group on the other side favored its formation. For the HY zeolite, the conversion of toluene reached highest at 473k after 2 h, and the PTBT selectivity was the best at toluene:TBA mole ratios of 6:1 and space velocity 4 h-1, but it would fall at the higher temperatures. The conversion of toluene over HMCM-22 was higher than it on Hβ, HY zeolites, because total acid was the highest and the TB/TL ratio was the lowest on the HMCM-22 zeolite. The PTBT selectivity on HMCM-22 was the highest over the three catalysts. Note Added after ASAP Publication: The author affiliations have been revised since the ASAP publication of this paper on January 19, 2010. The version with the corrected author affiliations was reposted to the Web January 29, 2010.
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(7) Tawada, S.; Sugi, Y.; Kubota, Y.; Imada, Y.; Hanaoka, T.; Matsuzaki, T.; Nakajima, K.; Kunimori, K.; Kim, J.-H. Ceria-modification of Hmordenites The deactivation of external acid sites in the isopropylation of biphenyl and the isomerization of 4,40-diisopropylbiphenyl. Catal. Today 2000, 60, 243. (8) Karge, H. G.; Ernst, S.; Weihe, M.; Weib, U.; Weitkamp, J. A comparative study of the acidity of various zeolites using the differential heats of ammonia adsorption as measured by high-vacuum micro-calorimetry. Stud. Surf. Sci. Catal. 1994, 84, 1805. (9) Pai, S.; Gupta, U.; Chilukuri, S. Butylation of toluene: Influence of zeolite structure and acidity on 4-tert-butyltoluene selectivity. J. Mol. Catal. A: Chem. 2007, 265, 109.
ReceiVed for reView July 10, 2009 ReVised manuscript receiVed September 13, 2009 Accepted December 30, 2009 IE901080T
