A novel route to produce 4-t-butyltoluene by t-butylation of toluene with t-butylalcohol over mesoporous Al-MCM-41 molecular sieves

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1 Applied Catalysis A: General 286 (2005) A novel route to produce 4-t-butyltoluene by t-butylation of toluene with t-butylalcohol over mesoporous Al-MCM-41 molecular sieves M. Selvaraj a, *, S.H. Jeon a, J. Han b, P.K. Sinha c, T.G. Lee a, ** a Research Institute of New Energy and Environmental System, Yonsei University, Seoul , South Korea b Environment & Process Technology Division, KIST, Seoul , South Korea c CWMF, BARC Facilities, Govt. of India, Kalpakkam , India Received 16 August 2004; received in revised form 16 February 2005; accepted 23 February 2005 Available online 8 April 2005 Abstract t-butylation of toluene with t-butylalcohol (t-buoh) as alkylating agent was conducted under liquid phase reaction conditions over Al- MCM-41 with different Si/Al ratios for highly selective synthesis of 4-t-butyltoluene. For the reactions, the influences of various reaction parameters such as reaction temperature, time and t-buoh to toluene ratio are discussed. With increasing the Si/Al ratios of Al-MCM-41 catalysts from 21 to 104, the conversion of toluene, and the yield and selectivity of 4-t-butyltoluene decreased because the number of Brønsted acid sites in Al-MCM-41catalysts is found to decrease almost linearly with increasing ratios of Si/Al. The conversion and selectivity is increased in Al-MCM-41(21) with each cycling of reaction. The Si/Al ratio of 21 is found to be more suitable for the t-butylation of toluene to highly selective synthesis of 4-t-butyltoluene. Thus the selectivity of 4-t-butyltoluene is higher in Al-MCM-41(21) than that in other Al- MCM-41 samples. # 2005 Elsevier B.V. All rights reserved. Keywords: Al-MCM-41; Catalytic activity; Brønsted acidity; Conversion of toluene; Selectivity of 4-t-butyltoluene 1. Introduction The alkylations of aromatic hydrocarbons with olefins/ different types of alcohols are applied on a large scale in the chemical industry [1]. Among para-dialkylated aromatics, p-xylene, p-diisopropylbenzene, p-ethyltoluene, p-dietylbenzene, p- and m-cymenes and 4-tert-butyltoluene are very important in fine chemical and petrochemical industries [2 6]. The above products are alkylated-aromatic products. 4-t- Butyltoluene is produced by alkylation of toluene with anyone alkylating agent such as isobutylene, diisobutylene, MTBE or t-buoh. The product has been used as an intermediate product to produce 4-t-butylbenzoic acid and 4-t-butylbenzaldehyde; they are especially used in perfumery and in the fields of plastics and resins [7,8]. * Corresponding author. Tel.: ; fax: ** Co-corresponding author. addresses: selvarajman25@yahoo.com (M. Selvaraj), teddy.lee@yonsei.ac.kr (T.G. Lee). Ioffe et al. [9] studied different catalytic systems such as AlCl 3,AlCl 3 CH 3 NO 2, sulfuric acid and polyphosphoric acid in the liquid phase for alkylation of toluene by C 4 - alcohols. By the above reaction, low yield and selectivity of para isomer was obtained. These alkylations are still performed with catalysts in chemical industries. Often such catalysts are strong mineral acids or Lewis acids (e.g. HF, H 2 SO 4, and AlCl 3 ), which are highly toxic and corrosive. They are dangerous to handle and to transport as they corrode storage and disposal containers. Often the products need to be separated from the acid with a difficult and energy consuming process. Finally, it occurs frequently that these acids are neutralized at the end of the reaction and, therefore, the correspondent salts have to be disposed. Similar problems arise when free bases are used as catalysts. In order to avoid these problems many efforts have been devoted to the search of solid acid catalysts that are more selective, safe, environmentally friendly, regenerable, and reusable and which do not have to be destroyed after reaction X/$ see front matter # 2005 Elsevier B.V. All rights reserved. doi: /j.apcata

2 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) Thus, some solid acid catalysts are used for t-butylation of toluene. t-butylation of toluene reaction was carried out using toluene with MTBE, t-buoh and t-butyl chloride in the presence of activated clay, silica-alumina and iron sulphate catalysts under liquid phase reaction conditions [10,11]. The 4-t-butyltoluene was synthesized over NiY zeolite as catalyst, but, by the above reactions, the yield and selectivity of the products ( p, m and o-isomers) were very low because the activity of the catalysts is very low [12]. From the above solid acid catalysts, low yield and selectivity of para isomer was obtained, because all the catalysts have small surface areas along with different structures. The performance of the zeolite is also limited by diffusional constraints associated with smaller pores. These materials suffer from limited thermal stability as well as negligible catalytic activity due to framework neutrality. Moreover, the need for present day heterogeneous catalysts in processing hydrocarbons with high molecular weights has made researchers search for better systems. These limitations were overcome after the discovery of mesoporous materials [13,14]. Incorporation of aluminum [15] and other metals [16,17] into their mesoporous structure has therefore been investigated in order to introduce solid-state acidity and thereby catalytic function. The typical characteristics of Al-MCM- 41, viz. highly ordered mesoporosity, large surface area, high thermal stability and mild acidity, give the possibility of applying these materials as catalysts in the synthesis and conversion of large molecules. Corma et al. [15] first reported the details of synthesis and characterization of aluminum incorporating mesoporous materials. However, the catalytic activity of the material was low in comparison to the usual silica-alumina catalyst and also the thermal stability was poor. Busio et al. [18] also reported synthesis and characterization of the Al- MCM-41, wherein incorporation of an excess of aluminum (Si/Al = 10) formed an impure crystal-phase tridimite, and the Lewis acid site prevailed because of the octahedral non-framework aluminum, accompanied with the collapse of the structure. Selvaraj et al. [2,3,19 23] reported the details of synthesis and characterization of some mesoporous solid acid catalysts. These have been used for isopropylation of toluene, ethoxylation of b-naphthol, selfcondensation of acetophenone and intermolecular cyclization of ethanolamine; from those reactions, good conversion and product selectivity are obtained. Particularly, alkylation of toluene was carried out over mesoporous solid acid catalysts by the best candidates [3,5,6]. We have applied the mesoporous catalysts for highly selective synthesis of 4-t-butyltoluene. In the present study, Al-MCM-41 with Si/Al ratios equal to 21, 42, 62, 83 and 104 were synthesized and characterized according to the published method [22 24] using cetyltrimethylammonium bromide as template under hydrothermal conditions. The materials have been used as catalysts for the higher selectivity of 4-t-butyltoluene produced by the alkylation of toluene using t-buoh as the alkylating reagent. Owing to its low cost and extensive use in industries, t-buoh was chosen instead of isobutene. The effects of reaction temperature, time and t-buoh to toluene ratio on the selectivity of 4-t-butyltoluene were investigated. 2. Experimental 2.1. Materials The syntheses of Al-MCM-41 materials were carried out by hydrothermal method using sodium metasilicate (Na 2 SiO 3 5H 2 O), cetyltrimethylammonium bromide (C 16 H 33 (CH 3 ) 3 N + Br), aluminum sulphate (Al 2 (SO 4 ) 3 18H 2 O), sulfuric acid (H 2 SO 4 ). In order to study the formation of 4-tbutyltoluene by t-butylation of toluene, the reagents t-buoh ((CH 3 ) 3 C OH), toluene (C 7 H 8 O) and decane were used. All chemicals (AR grade) were purchased from Aldrich & Co., USA Synthesis and characterization of Al-MCM-41 Al-MCM-41 with Si/Al ratios equal to 21, 42, 62, 83 and 104 were synthesized and characterized; acidity measurements were done according to the published method [22 24] t-butylation of toluene experimental procedure for liquid phase catalytic reaction The Al-MCM-41 catalyst (0.2 g freshly calcined catalyst kept at 400 8C was used) was added into a mixture of t-buoh/toluene (various mmol ratios) with 100 ml of decane as solvent. Each reaction was carried out in a stirred batch autoclave reactor (100 ml, Autoclave Engineers) at reaction temperatures between C for different times (h). The reactor was flushed twice with nitrogen to replace air. Alkylation reactions were carried out at the autogeneous pressure. The reactor was cooled down to 0 8C and the reaction products were recovered from the reactor. The samples of the reaction mixture were withdrawn periodically from the closed reactor and analysed on a CHROMPACK 9002 gas chromatograph equipped with a CP Sil 5 CB column (25 m 0.53 mm) and an FID detector. The temperature program was held at 60 8C (5 min), increased from 60 to 220 8C with a slope of 5 8C/min and held at 220 8C during 5 min isothermally. The products of the reaction were identified on a GC/MS QP5000 (Shimadzu) with EI and capillary column (HP-1, 50 m 0.2 mm 0.33 mm); carrier gas was helium (1 ml/ min). Temperature program: from 50 8C with a gradient of 5 8C/min to 240 8C was used.

3 46 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) Results and discussion The synthesized Al-MCM-41 catalysts have been used for t-butylation of reaction with t-buoh to highly selective synthesis of t-butyltoluene. The reaction mechanism is discussed Mechanism of t-butylation of toluene The t-butylation of toluene with t-buoh is an electrophilic substitution reaction on the aromatic ring. t- Butylation reactions catalyzed by acids or solid acid zeolites are commonly considered to proceed via carbenium ion mechanisms [3]. t-buoh reacts with solid acid catalyst to form isobutene, along with removal of water (Eq. (1)). Isobutene is protonated by the catalyst to form t-butyl carbocation (Eq. (2)). The carbocation further reacts with toluene in the presence of the catalyst to form 4-t-butyltoluene and 3-t-butyltoluene Eq. (3). Either 4-t-butyltoluene or 3-tbutyltoluene reacts with the carobocation over the catalyst to form 3,5-di-t-butyltoluene (Eq. (4)). Excess isobutylene further reacts with each molecule over the catalyst to form oligomers (Eq. (5)) while the oligomers (R-alkyl groups) react with excess toluene in the presence of the catalyst to form alkyltoluenes with longer alkyl chains (Eq. (6)). The remaining oligomers further react with water over the catalyst to form alcohols (Eq. (7)). All the above products (all equations) are obtained with respect to the catalytic properties along with optimal reaction conditions. The reaction of toluene with t-buoh is given in Fig. 1. Fig. 1. Reaction scheme of t-butylation of toluene.

4 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) In all cases, the main reaction products have been identified as 4-t-butyltoluene and 3-t-butyltoluene. 2-t- Butyltoluene was present in the reaction products only in trace amounts. The main product is logically 4-t-butyltoluene because para-position is favored by the influence of the steric hindrance of the methyl group on one side and voluminous t-bu group due to the structure of mesoporous Al-MCM-41 catalysts. The formation of 2-t-butyltoluene is hindered by orthoposition of methyl and voluminous t-bu group. The same steric effect allows only the formation of 3,5-di-tbutyltoluene, where all alkyl groups are in the metaposition. 3,5-Di-t-butyltoluene was also found in the reaction products, but only in trace amounts obtained over Al-MCM-41 catalysts. The other products have been identified by GC MS as alkyltoluenes with longer alkyl chains. These products were formed by alkylation of toluene with lower oligomers of isobutene (preferentially by dimers). The effects of various parameters on the t- butylation of toluene reaction are discussed Selectivity of 4-t-butyltoluene The reaction of t-butylation of toluene was carried out in the presence of various Si/Al ratios of Al-MCM-41 catalysts. Maximum conversion of toluene to the extent of 86.2 and 92.3% 4-t-butyltoluene selectivity was obtained when the reaction was carried out in the presence of Al-MCM-41(21). The conversion of toluene and selectivity of 4-t-butyltoluene in the presence of Al-MCM-41(21) are higher, due to the high aluminum content, the high hydrothermal stability and also the higher number of Brønsted acid sites from creation of negative charges on the pore walls, which is attributed to the incorporation of Al trivalent ions in place of tetrahedral Si in the structure. The number of acid sites for the different catalysts follow the order; Al-MCM-41(21) > Al- MCM-41(42) > Al-MCM-41(62) > Al-MCM-41(83) > Al-MCM-41(104), as obtained from TPD and FTIRpyridine treatment. This reaction is activated on the m- and the p-positions by the presence of the methyl group. The selectivity of 4-t-butyltoluene is higher than those of 3-tbutyltoluene, 2-t-butyltoluene and 3,5-di-t-butyltoluene due to steric hindrance of the methyl group at the para-position. The 3-t-butyltoluene and 3,5-di-t-butyltoluene are thermodynamically more stable [3]. Thus, the conversion of toluene and selectivity of 4-t-butyltoluene are higher in Al-MCM- 41(21) than in the other Al-MCM-41 catalysts. The results are shown in Table Variation of reaction time with different Si/Al ratios of Al-MCM-41 The liquid phase reaction of t-butylation of toluene was carried out at various reaction times with 2:1 mmol ratio of t- BuOH to toluene and 100 ml of decane as solvent at 175 8C reaction temperature, in the presence of Al-MCM-41 with different Si/Al ratio catalysts. Lower reaction time (<1 h) does not favor the formation of 4-t-butyltoluene because surface activity of the catalysts is insufficient to react with reactants. Then conversion of toluene, yield and selectivity Table 1 t-butylation of toluene: variation of reaction time with different Si/Al ratios of Al-MCM-41 Catalysts Time (h) Conversion of toluene (%) Yield of the products (%) 4-t-BT selectivity 4-t-BT 3-t-BT Others Al-MCM-41(21) Al-MCM-41(42) Al-MCM-41(62) Al-MCM-41(83) Al-MCM-41(104) Reaction conditions: 0.2 g of catalyst; reaction temperature (T) = 1758C; 1:2 mmol ratio of t-butylalcohol to toluene; 100 ml of n-decane; 4-t-BT, 4-tbutyltoluene; 3-t-BT, 3-t-butyltoluene; others 2-t-butyltoluene, 2,5-di-t-butyltoluene and oligomers.

5 48 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) Table 2 t-butylation of toluene: variation of reaction temperature with different Si/Al ratios of Al-MCM-41 Catalysts Temperature (8C) Conversion of toluene (%) Yield of the products (%) 4-t-BT selectivity 4-t-BT 3-t-BT Others Al-MCM-41(21) Al-MCM-41(42) Al-MCM-41(62) Al-MCM-41(83) Al-MCM-41(104) Reaction conditions: 0.2 g of catalyst; reaction time = 2 h; 1:2 mmol ratio of t-butylalcohol to toluene; 100 ml of n-decane; 4-t-BT, 4-t-butyltoluene; 3-t-BT, 3-tbutyltoluene; others 2-t-butyltoluene, 2,5-di-t-butyltoluene and oligomers. of 4-t-butyltoluene increase with increasing reaction time up to 2 h over different Al-MCM-41 (different Si/Al ratios) catalysts in the same reaction conditions. But the conversion, yield and selectivity decrease with increasing Si/Al ratios, because the acid sites on the catalyst surface are decreased with decreasing aluminum content. The results are shown in Table 1. When the reaction time is increased (>3 h), the conversion of toluene increased, but the yield and selectivity Table 3 t-butylation of toluene: variation of mmol ratio (t-butylalcohol/toluene) with different Si/Al ratios of Al-MCM-41 Catalysts t-ba/toluene mmol ratio Conversion of toluene (%) Yield of the products (%) 4-t-BT selectivity 4-t-BT 3-t-BT Others Al-MCM-41(21) 1: : : : Al-MCM-41(42) 1: : : : Al-MCM-41(62) 1: : : : Al-MCM-41(83) 1: : : : Al-MCM-41(104) 1: : : : Reaction conditions: 0.2 g of catalyst; reaction temperature (T) = 1758C; reaction time = 2 h; 100 ml of n-decane; 4-t-BT, 4-t-butyltoluene; 3-t-BT, 3-tbutyltoluene; others 2-t-butyltoluene, 2,5-di-t-butyltoluene and oligomers.

6 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) Fig. 2. Variation of reaction time with (a) conversion of toluene (%) and (b) selectivity of 4-t-butyltoluene (%) at different temperature over Al-MCM- 41(21) material. Fig. 3. Variation of reaction time with (a) conversion of toluene (%) and (b) selectivity of 4-t-butyltoluene (%) on different mmol ratio (t-butanol/ toluene) over Al-MCM-41(21) material. of 4-t-butyltoluene decreased, because 4-t-butyltoluene is gradually transformed to 3-t-butyltoluene, while the other by-products, namely 2-t-butyltoluene, 3,5-di-t-butyltoluene and oligomers slightly increased. The conversion of toluene and selectivity of 4-t-butyltoluene are higher in Al-MCM-41(21) than those of other Al-MCM-41 catalysts due to the greater chemisorption of reactants on the catalyst surface pores due to the higher number of Brønsted acid sites at 175 8C for 2 h; the results are shown in Table 1. So the optimum reaction time was found to be 2 h for the highly selective synthesis of 4-t-butyltoluene. Such higher yield and selectivity of 4-t-butyltoluene and conversion of toluene using Al-MCM-41(21) depict its superiority in performance compared to other Al- MCM Variation of reaction temperature with different Si/Al ratios of Al-MCM-41 The t-butylation of toluene was carried out at various reaction temperatures with 2:1 mmol ratio of t-buoh to toluene and 100 ml of decane as solvent for 2 h reaction time in the presence Al-MCM-41 with different Si/Al ratio catalysts. The results are shown in Table 2. When the temperature was increased up to 175 8C at the same reaction conditions, the conversion of toluene, yield and selectivity of 4-t-butyltoluene increased. After the reaction temperature of 175 8C, the conversion increases, but the yield and selectivity of 4-t-butyltoluene decrease, because the selectivity of 3-t-butyltoluene and of other by-products namely 2-t-butyltoluene, 3,5-di-t-butyltoluene and oligo-

7 50 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) The liquid phase reaction of t-butylation of toluene was carried out at various reaction times with 2:1 mmol ratio of t- BuOH to toluene and 100 ml of decane as solvent at different reaction temperatures in the presence of Al-MCM-41(21). When the reaction time was increased to 2 h at the temperature from 125 to 175 8C in the same reaction conditions, the conversion of toluene increases while the yield and selectivity of 4-t-butyltoluene increases, but, after 2 h, the yield and selectivity decreased. The results are shown in Fig. 2. When the reaction time (>4 h) along with temperature (>175 8C) are increased, selectivity of 3-tbutyltoluene increased and yields of other by-products as 2- t-butyltoluene and 3,5-di-t-butyltoluene also increased, because of debutylation of 4-t-butyltoluene with formation of toluene and isobutylene Variation with t-butanol to toluene ratio Fig. 4. Variation with run of the catalysts with (a) conversion of toluene (%) and (b) selectivity of 4-t-butyltoluene (%) in the presence of different Al- MCM-41 materials. mers increase by transalkylating of 4-t-butyltoluene, while the 4-t-butyltoluene has low thermodynamical stability at higher reaction temperature (>175 8C). The conversion of toluene, yield and selectivity of 4-t-butyltoluene are higher over Al-MCM-41(21) than that the values of other Al- MCM-41 catalysts due to the increased catalytic activity along with the higher number of Brønsted acid sites on the surface of the catalyst while most of converted reactants are favored into 4-t-butyltoluene at 175 8C for 2 h. So the optimum reaction temperature was found to be 175 8C for the highly selective synthesis of 4-t-butyltoluene. When the reaction temperature is further increased to 200 8C, the conversion of toluene and yield and selectivity of 4-tbutyltoluene decreased. This may be due to the debutylation of 4-t-butyltoluene with formation of toluene and isobutylene. The t-butylation of toluene was carried out at 175 8C reaction temperature with various mmol ratios of t-buoh to toluene and 100 ml of decane as solvent for 2 h reaction time in the presence Al-MCM-41 with different Si/Al ratio catalysts; the results are shown in Table 3. The conversion of toluene, yield and selectivity of t-butyltoluene decreased at 1:1 mmol ratio of t-buoh to toluene. This may be due to the t-buoh is insufficient to react with toluene. As the 2 mmol of t-buoh is increased to 1 mmol of toluene, the conversion of toluene, yield and selectivity of 4-t-butyltoluene increase. This may be due to equilibrating of the two reactants on the Brønsted acid sites of the inner side surface of catalyst. As the 2 mmol of toluene is increased to 1 mmol of t-buoh, the conversion increased, but the yield and selectivity of 4-tbutyltoluene the 3-t-butyltoluene decreased, while other byproducts slightly increased. When 4 mmol of toluene was combined with 1 mmol of t-buoh, the conversion of toluene, yield and selectivity of 4-t-butyltoluene and 3-tbutyltoluene decreased, but the other products of oligomers increased. In all the cases, 4-t-butyltoluene was obtained as the major product along with small amounts of 3-tbutyltoluene products. In addition, trace amounts of other alkylated products like 3,5-di-t-butyltoluene, 2-t-butyltoluene and oligomers were also observed. At a reaction temperature of 175 8C for reaction time at 2 h over different Si/Al ratios of Al-MCM-41 catalysts, the highest conversion of toluene and highest selectivity of 4-t-butyltoluene were obtained at t-buoh to toluene ratio of 2:1, while the conversion and yield and selectivity increased with high aluminum content such results are shown in Table 3. Generally, as the molar ratio of toluene is increased with t- BuOH, the conversion of toluene decreased, and as the molar ratio of t-buoh is increased with toluene, the conversion of toluene decreased. This may be due to coking of the catalyst by unsaturation of reactant and catalyst pores; then there would be fast diffusion without reaction from the catalyst active sites. Hence the optimal molar ratio of t-buoh to toluene is 2:1. Thus, the optimum conditions for obtaining

8 M. Selvaraj et al. / Applied Catalysis A: General 286 (2005) maximum conversion of toluene (86.2%) and highest selectivity of 4-t-butyltoluene (92.3%) can be summarized as follows: Catalyst, Al-MCM-41 with Si/Al = 21; reaction temperature = 175 8C, time = 2 h and t-buoh to toluene ratio = 2:1. The liquid phase reaction of t-butylation of toluene was carried out at various reaction times with different mmol ratios of t-buoh to toluene and 100 ml of decane as solvent at 175 8C in the presence Al-MCM-41(21); the results are given in Fig. 3. The conversion of toluene and selectivity of 4-t-butyltoluene increase but the yield and selectivity decrease with different time in the series 1:1 < 1:4 < 1:2 < 2:1 mmol ratios of t-buoh to toluene. The conversion and selectivity are higher in 2:1 mmol ratios for 2 h than those of other mmol ratios due to equilibrium of the reactants with the greater chemisorption on the Brønsted acid sites of catalyst surfaces. The yields of other products such as oligomers in 1:1 mmol ratio, 3-t-butyltoluene and 2,5-di-t-butyltoluene in 1:2 and 1:4 ratios are slightly higher than those of 2:1 mmol ratio due to the favorable catalyst surface and reaction conditions Recyclability All Al-MCM-41 catalysts were reused for the t- butylation of toluene at 175 8C with 2 h reaction time and 2:1 mmol ratio of t-buoh to toluene for the highly selective synthesis of t-butyltoluene; the results have been depicted in Fig. 4. No loss of catalytic activity was observed after 4 runs. Instead, its conversion of toluene, yield and selectivity of t- butyltoluene increased with each cycling in Al-MCM- 41(21). But the conversion, yield and selectivity decreased in other Al-MCM-41 catalysts at the same reaction conditions. The results are shown in Fig. 4. This may be due to decreasing of the catalytic activity along with dehydration of Brønsted acid sites on the surface of the catalyst because of the catalysts having less aluminum content. 4. Conclusions The selective synthesis of 4-t-butyltoluene was carried out over Al-MCM-41 catalysts with different optimal conditions. The Si/Al-molar ratios are increased, the conversion of toluene, yield and selectivity of 4-tbutyltoluene decreased. When the Al-MCM-41(21) was reused for the t-butylation of toluene with 2:1 mmol ratio of t-buoh to toluene at 175 8C reaction temperature and at 2 h reaction time, the conversion of toluene and selectivity of 4- t-butyltoluene increased in only Al-MCM-41(21) with each cycling. The conversion and selectivity of t-butyltoluene are higher in recyclable Al-MCM-41(21) than the values of other Al-MCM-41 catalysts due to no loss of catalytic activity after the recycling process. 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