- Email: [email protected]
Friedel-Crafts acylation of aromatics catalysed by supported ionic liquids
Applied Catalysis A: General 215 (2001) 185–190
Friedel-Crafts acylation of aromatics catalysed by supported ionic liquids M.H. Valkenberg, C. deCastro, W.F. Hölderich∗ Department of Chemical Technology and Heterogeneous Catalysis, University of Technology, RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany Received 11 September 2000; received in revised form 6 February 2001; accepted 6 February 2001
Abstract Different ionic liquids were used as catalysts for Friedel-Crafts acylation reactions. Supported chloroferrate ionic liquids were tested in liquid and in gas phase reactions. The catalysts, consisting of the ionic liquid and charcoal as a carrier, are easy to prepare and show interesting catalytic properties. Comparisons between different ionic liquids in the liquid phase are presented, as well as reactions in the gas phase. Furthermore, possible reasons for the deactivation of the catalyst will be discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Friedel-Crafts reactions; Acylation; Ionic liquids; Immobilisation; Supported liquid phase
1. Introduction One of the main tasks of contemporary industrial chemistry is the search for catalysts for the production of fine and intermediate chemicals [1]. Friedel-Crafts acylation reactions lead to aromatic ketones, which are important intermediates in a very wide field of chemistry, including pharmacologicals, dyes, fragrances and agrochemicals, such as fungicides, herbicides and insecticides. Many of the industrial processes still use HF or AlCl3 as catalysts for this kind of reaction, producing a high amount of contaminated waste. Considering the ecological and economical problems associated with waste management in most civilised countries [2], an alternative, salt free synthesis would be of high interest [3]. Even though current research tends to concentrate on zeolite catalysed synthesis [4–7], ionic liquids
(ILs) have been presented as alternative catalysts as well [8–11]. They are known to have interesting and, more importantly, tuneable properties. Depending on the organic cation and inorganic anion the ILs consist of, they can have very different physical and chemical properties. The organic cation determines the solubility, the density and the viscosity of the liquids. Even when working with a single cation, e.g. the imidazolium cation, the properties of the IL are highly variable through the different possibilities of alkyl groups. The Lewis-acidity of ILs is dependent on the metal halide chosen and the ratio of metal halide to organic base [12,13]. By using Lewis-acidic ionic liquids supported on solid supports, we introduce a new kind of catalyst for the acylation of aromatic compounds.
2. Experimental ∗ Corresponding author. Tel.: +49-241-806-560; fax: +49-241-888-291. E-mail address: [email protected] (W.F. Hölderich).
To ensure reaction conditions free of water, all experimental steps have to be carried out under an inert
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 5 3 1 - 2
186
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
atmosphere. Before impregnation, the silica supports were calcined at 550◦ C for 3 h and then stored under argon. The charcoal was dried for 6 h at 150◦ C at 10 Pa and stored under argon. The 1-methyl-3-butyl-imidazolium chloride ([bmim]Cl) used for the preparation of the ionic liquids was kindly provided by Elementis Specialties, Durham, UK. Iron chloride and activated charcoal were purchased from Merck AG. The silica support FK700 was kindly provided by Degussa-Hüls AG. Toluene, m-xylene, anisol, acetyl chloride, acetic anhydride and dichloromethane were purchased from Fluka in 99% purity and used without further purification. All of the ionic liquids tested contained 1-methyl3-butyl-imidazolium chloride as the organic base and a two-fold molar excess of metal halide. 2.1. 1-Methyl-3-butyl-imidazolium chloroferrate (Fe-IL) In a round bottomed flask equipped with a magnetic stirrer and a gas inlet valve, 32.44 g anhydrous FeCl3 (0.2 mol) were slowly added to 17.47 g (0.1 mol) 1-methyl-3-butyl-imidazolium chloride. To ensure complete reaction, the mixture was left stirring over night. 1-Methyl-3-butyl-imidazolium chloroaluminate (Al-IL) and 1-methyl-3-butyl-chlorostannate were prepared analogously to the Fe-IL. 2.2. Preparation of the immobilised Fe-IL In a round bottomed flask equipped with a magnetic stirrer and a gas inlet valve, an excess of ionic liquid was slowly added to the support. Either FK 700 amorphous silica, or granular activated charcoal supports were used. To ensure complete homogenisation, the mixture was left stirring over night. The excess ionic liquid was removed by 24 h extraction with boiling dichloromethane in a Soxhlet apparatus. The catalyst was dried at 10 Pa and stored under argon. The reactions in discontinuous liquid phase (batch reactions) were carried out in round-bottomed flask equipped with a reflux condenser, gas-inlet valve, and sampling exit. A magnetic stirrer equipped with a thermostat and a silicon oil bath were used to maintain
the reaction temperature and ensure the homogeneity of reactants. The catalyst was weighed in the reactor and then the reactants were added according to the proportion desired. Undecane was added as an internal standard to follow the reaction kinetics and to calculate the mass balance. Samples were taken periodically and analysed by gas chromatography in order to quantify conversion and selectivity of the reaction. In all reactions presented here, a ratio of 1 mol acetylating agent to 5 mol of aromatic compound was used. As standard reaction temperature 100◦ C was chosen, the experiments were usually stopped after 1 h reaction time. The continuous liquid phase reactor system consisted of a stainless steel tube of 100 mm length and 6 mm width. The catalyst was transferred to the reactor and the endings were closed with a metallic net to prevent the loss of catalyst. A peristaltic pump pushed the solution containing the aromatic compound, the acetylating agent, and the internal standard through the reactor with an adjusted weight hourly space velocity (WHSV). The reaction temperature was controlled by a thermostatic silicon oil bath. Samples were taken periodically and analysed by GC in order to follow the reaction course. The continuous gas phase reactor system consisted of a 1 m long coiled tube. The catalyst was introduced into the tube and stayed at the lower part. The reaction mixture was fed into the reactor and vaporised at the beginning of the tube. The reaction mixture was pumped by a peristaltic pump and a flow of dry nitrogen was used to carry the vaporised reactants through the reactor. The products were collected in a cooling trap from where samples were periodically withdrawn to follow the reaction course by GC. 2.3. Analytics For BET-measurements, an ASAP 2000 machine from Micromeritics was used. The concentrations of Fe, Al, Sn and Si in the samples were determined by ICP-AES, using a Spectro-Flame D machine from Spectro. CHN analysis was performed on a Elementar Vario EL. GC analysis was done on a Siemens RGC 202, using a 30 m SE 54 column from Hewlett-Packard. GC–MS analysis were performed on a Varian 3400 CX GC and a Varian Saturn 3 mass spectrometer.
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
187
Scheme 1. General reaction scheme of Friedel-Crafts acylation reactions.
3. Results and discussion Literature on Friedel-Crafts acylation reactions (see Scheme 1) still shows aluminium(III)chloride as “catalyst” for this kind of reaction, even though a two- to three-fold molar excess of this metal halide is needed in order to allow complete conversion. Experiments with pure chloroaluminate ionic liquids in liquid phase reactions displayed the same need for high amounts of catalyst (see Table 1). A comparison with two other Lewis-acidic metal halides, iron(III)chloride and tin(II)chloride quickly proved both of them to be alternative catalysts. These metal halides were chosen for reasons of price, availability and easy handling. The ionic liquids prepared with them displayed good catalytic properties when
used in acylation reactions. Table 1 shows conversion and selectivity found for a number of experiments catalysed by these three examined ILs. In all of the reactions, the Al-IL led to the lowest conversions. The ionic liquid containing tin(II)chloride gave better conversions and similar selectivities, but the best results were obtained with an IL containing iron(III)chloride. Here, the conversions were by far the highest, while the selectivities obtained were, with the exception of the acylation of m-xylene, in the same range as with the Al-IL and Sn-IL. Therefore, further experiments concentrated on the Fe-IL catalyst. In the liquid phase acylation of m-xylene with acetyl chloride catalysed by Fe-IL, we found a comparably low selectivity of about 80%.
Table 1 Acylation of aromatics: batch reactions over non-immobilised ILsa Ionic liquid
Reaction
Molar ratio (catalyst:aromatic compound)
Al-IL
Mesitylene + AcCl Anisole + Ac2 O Anisole + Ac2 O m-Xylene + AcCl Toluene + AcCl Benzene + AcCl
1:205 1:205 1:45 1:205 1:205 1:205
68.05 0.31 8.26 3.52 0 0
98.08 94.12 96.1 96.35 0 0
Sn-IL
Anisole + Ac2 O Anisole + Ac2 O m-Xylene + AcCl
1:205 1:45 1:205
2.48 19.65 3.56
90.41 94.41 94.94
Fe-IL
Mesitylene + AcCl Anisole + Ac2 O Anisole + Ac2 O m-Xylene + AcCl m-Xylene + AcCl Toluene + AcCl Benzene + AcCl
1:205 1:205 1:45 1:205 1:45 1:205 1:205
94.74 90.17 100.00 33.80 45.6 0.54 0
94.9 96.64 98.13 79.39 83.12 87.92 0
a
Conversion (%)
T = 100◦ C, reaction time: 1 h, molar ratio aromatic compound/acetylating agent = 5:1.
Selectivity to main products (%)
188
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
Table 2 Behaviour of conversion and selectivity with temperature in the acylation of m-xylene with acetyl chloridea T (◦ C)
Conversion (%)
Selectivity
20 40 60 80 100
3.05 7.28 10.53 28.91 33.80
59.49 54.07 59.14 70.26 79.39
a Catalyst: Fe-IL, N = 0.6, reaction time: 1 h, molar ratio: 0.2 mmol IL:9 mmol AcCl:45 mmol m-xylene.
Scheme 2. Consecutive reaction.
Interestingly, the selectivity to the main product increased with temperature (see Table 2). Considering this to be a rather unusual behaviour, we concentrated on the by-products of the reaction. We found the most important by-product, as identified by GC–MS, to be 1-(1-chlorovinyl)-2,4-dimethyl-benzene, leading us to assume that the consecutive reaction shown in Scheme 2 takes place. The HCl concentration in the liquid is crucial for this reaction, which could explain the high rate of this reaction at lower temperatures. Here, the HCl is not removed from the mixture fast enough, and a higher amount of the 2,4-dimethyl-acetophenone is used in the consecutive reaction, leading to a lower selectivity towards the main product. The water formed in this reaction leads to a non-reversible deactivation of the catalyst, so it has be avoided. Furthermore, this con-
secutive reaction explains the low selectivity towards the main product. The chlorostannate ionic liquid was highly viscous with a melting point close to room temperature. The high viscosity of these melts hindered the immobilisation process described in the experimental part. For this reason, as well as the higher conversions achieved with the Fe-IL, further immobilisation experiments concentrated on the chloroferrate liquids only. The chloroferrate ionic liquids were immobilised on different silica supports, as well as on charcoal. Reactions in batch and continuous liquid phase resulted in lower conversions but higher selectivities than homogeneously catalysed reactions. These results looked promising, but ICP-AES analysis of the Fe-content of the used catalysts and product solutions displayed a strong leaching of the ionic liquid. Table 3 displays the results for batch reactions, reactions in the continuous liquid phase showed the same correlation between conversion and the amount of iron in the solution. It has to be assumed that the leaching of the ionic liquid in liquid phase reactions is due to a strong complex formation between the metal halide and the carboxylic groups of the reactants. This is not countered by the formation of a stable covalent bond to the oxygen of hydroxyl-groups on the surface of the supports, as has been assumed for and immobilised chloroaluminate ILs and described in detail in the relevant publications [14,15]. The supported Fe-IL can be more correctly described as an supported liquid phase (SLP) catalyst [16,17]. The leaching in the liquid phase decreases the practical use of the catalyst, especially since it is probably caused by one of the starting materials. In view of the high amount of leaching, no further reactions were carried out in liquid phase, the question if a part of the reaction is catalysed heterogeneously was not addressed.
Table 3 Leaching in liquid phase reactionsa Reaction
Catalyst
Conversion (%)
Iron in solution (ppm)
Anisole + Ac2 O m-Xylene + AcCl m-Xylene + AcCl, 2nd run
Fe-IL on MCM 41 Fe-IL on MCM 41 Fe-IL on MCM 41
6.54 15.01 4.62
135.35 56.05 20.25
a
Catalyst: Fe-IL on MCM 41, 0.1 g, N = 0.6, reaction time: 1 h, 45 mmol m-xylene:9 mmol AcCl.
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
189
Fig. 1. Reactions in the gas phase: yield of 2,4-dimethoxy-acetophenon at different temperatures.
To avoid this problem of leaching, the feasibility of reactions in the gas phase was tested. As a test reaction, the acylation of m-xylene was chosen. This compound is less activated than the anisole, but the lower boiling point of the main product (228◦ C in comparison to 258◦ C) facilitated reactions at lower temperatures. Reactions with acetic anhydride as acetylating agent led to conversions of less than 1%, so the work focused on reactions with acetyl chloride. There, conversions of up to 5% and product selectivities of more than 98% could be obtained. The consecutive reaction shown in Scheme 2 was completely avoided, the main by-product proved to be the expected ortho-acylated product. Acylation reactions in the gas phase were tested at temperatures ranging from 200 to 300◦ C (see Fig. 1). Conversions reached were clearly below those in liquid phase reactions, but more than 99% of the recovered products were the 2,4-dimethyl-acetophenone. As seen in Fig. 1, a deactivation takes place. This, we attribute to the formation of polyaromatic products which do not desorb easily from the catalyst. Their presence could be proven by washing the catalyst with dichloromethane after the reaction and a following analysis of the washing solution. Here, various polyaromatic compounds resulting from condensation reactions of starting material and products could be detected via GC and GC–MS. The formation of these products decreases with increasing temperature, probably due to the faster desorption of
the products and coke precursors. A detailed analysis of possible by- and consecutive-products of acetylation reactions can be found in the literature [18]. Unfortunately, the gas phase reactions proved to have some serious drawbacks. As shown in Fig. 1, the conversion seems to increase at the beginning of the reaction. Since no activation of the catalyst is necessary, this is unexpected. We found the cause for this behaviour in the strong adsorption of the different reactants on the catalyst surface. Since the substances commonly used as external standards show the same adsorption behaviour, no correct data for conversion and selectivity can be achieved until an equilibrium between ad- and de-sorption is reached. To avoid this problem, the standard was used only to allow calculation of the exact molar ratios of the starting materials at the beginning of the reaction. The conversion was calculated from the amount of product found in ratio to the amount of product calculated for total conversion. By using cooling traps filled with liquid nitrogen a complete recovery of the aromatic compound and the products could be ensured. These data were used to calculate the conversion and the selectivities to different products. The second problem encountered in the gas phase reactions was the impossibility of reaching a satisfying mass balance when working with acetyl chloride. Calculations based on an external standard, as well as weight measurements showed a loss of up to 10 wt.%
190
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
Acknowledgements
Scheme 3. Decomposition of the acetyl chloride.
of the total mass of reactants. Since GC samples proved that the product solution contained nearly exclusively the aromatic compounds, meaning the starting material and the products, the most probable explanation for the loss seems to be a decomposition of the acetyl chloride. No acetic acid was found, indicating that this decomposition is not due to water in the reaction system. A cooling trap filled with sodium hydroxide on the other hand improved the mass balance and allowed recovery of up to 98% of the mass of starting materials. That means that a decomposition of the acetyl chloride to ketene and HCl takes place on the surface of the catalyst (see Scheme 3).
This work was carried out as a part of the BRITE-Euram project BE 906-3745. The authors are grateful to the European Commission for the funding of this work. Our thanks also go to our consortium partners in the project for stimulating and helpful discussions. References [1] [2] [3] [4] [5]
[6]
4. Conclusions The use of iron chloride-based ionic liquids in the Friedel-Crafts acylation of aromatic compounds can be considered as an interesting new alternative to the existing homogeneous catalysts. It is possible to obtain high conversions and very high selectivities in a variety of reactions. The immobilisation of these catalysts, however, leads to some serious problems. Experiments in liquid phase resulted in a high amount of leaching, leading to the question, whether a heterogeneous reaction takes place at all. This leaching problem can be avoided when working in the gas phase. Here a deactivation through heavier products takes place, but optimisation of the reaction conditions should allow to improve the yields. Work on this project will continue. We expect to be able to improve the results by using different types of catalysts in the future.
[7] [8]
[9] [10]
[11] [12] [13] [14] [15] [16] [17] [18]
W.-F. Hölderich, Stud. Surf. Sci. Catal. 75 (1993) 127–163. J.N. Armor, Appl. Catal. A: General 189 (1999) 153–162. J. H Clark, Green Chem. 1 (1999) 1–8. K. Tanabe, W.F. Hölderich, Appl. Catal. A: General 181 (1999) 399–434. M. Spagnol, L. Gilbert, et al., in: Proceedings of the Fourth International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 1996. K. Smith, Z. Zhenhua, P.K.G. Hodgson, J. Mol. Catal. A: General 134 (1998) 121–128. D. Das, S. Cheng, Appl. Catal. A: General 201 (1999) 159– 168. J.S. Wilkes, in: G. Mamantov, R. Marassi (Eds.), NATO ASI Series C: Mathematical and Physical Sciences, Vol. 202, D. Reichel Publishing Company, 1987, pp. 405–416. J.A. Boon, J.A. Levisky, J.L. Pflug, J.S. Wilkes, J. Org. Chem. 51 (1986) 480–483. E. Benazzi, Y. Chauvin, A. Hirschauer, N. Ferrer, H. Olivier, J.Y. Bernhard, US Patent 5,693,585 (1996), to Institute Français du Petrole. C.J. Adams, M.J. Earle, G. Roberts, K.R. Seddon, Chem. Commun. (1998) 2097–2098. K.R. Seddon, J. Chem. Technol. Biot. 68 (1997) 351–356. T. Welton, Chem. Rev. 99 (1999) 2071–2084. C. deCastro, E. Sauvage, M.H. Valkenberg, W.F. Hölderich, J. Catal. 196 (2000) 86–94. M.H. Valkenberg, C. deCastro, W.F. Hölderich, Top. Catal., in press. M. Makkee, B.A.A.L. van Setten, J.A. Mouljin, Stud. Surf. Sci. Catal. 116 (1995) 667–673. J. Villadsen, H. Livbjerg, Catal. Rev. Sci. Eng. 17 (1978) 203–227. D. Rohan, C. Canaff, E. Fromentin, M. Guisnet, J. Catal. 117 (1998) 296–305.
Friedel-Crafts acylation of aromatics catalysed by supported ionic liquids M.H. Valkenberg, C. deCastro, W.F. Hölderich∗ Department of Chemical Technology and Heterogeneous Catalysis, University of Technology, RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany Received 11 September 2000; received in revised form 6 February 2001; accepted 6 February 2001
Abstract Different ionic liquids were used as catalysts for Friedel-Crafts acylation reactions. Supported chloroferrate ionic liquids were tested in liquid and in gas phase reactions. The catalysts, consisting of the ionic liquid and charcoal as a carrier, are easy to prepare and show interesting catalytic properties. Comparisons between different ionic liquids in the liquid phase are presented, as well as reactions in the gas phase. Furthermore, possible reasons for the deactivation of the catalyst will be discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Friedel-Crafts reactions; Acylation; Ionic liquids; Immobilisation; Supported liquid phase
1. Introduction One of the main tasks of contemporary industrial chemistry is the search for catalysts for the production of fine and intermediate chemicals [1]. Friedel-Crafts acylation reactions lead to aromatic ketones, which are important intermediates in a very wide field of chemistry, including pharmacologicals, dyes, fragrances and agrochemicals, such as fungicides, herbicides and insecticides. Many of the industrial processes still use HF or AlCl3 as catalysts for this kind of reaction, producing a high amount of contaminated waste. Considering the ecological and economical problems associated with waste management in most civilised countries [2], an alternative, salt free synthesis would be of high interest [3]. Even though current research tends to concentrate on zeolite catalysed synthesis [4–7], ionic liquids
(ILs) have been presented as alternative catalysts as well [8–11]. They are known to have interesting and, more importantly, tuneable properties. Depending on the organic cation and inorganic anion the ILs consist of, they can have very different physical and chemical properties. The organic cation determines the solubility, the density and the viscosity of the liquids. Even when working with a single cation, e.g. the imidazolium cation, the properties of the IL are highly variable through the different possibilities of alkyl groups. The Lewis-acidity of ILs is dependent on the metal halide chosen and the ratio of metal halide to organic base [12,13]. By using Lewis-acidic ionic liquids supported on solid supports, we introduce a new kind of catalyst for the acylation of aromatic compounds.
2. Experimental ∗ Corresponding author. Tel.: +49-241-806-560; fax: +49-241-888-291. E-mail address: [email protected] (W.F. Hölderich).
To ensure reaction conditions free of water, all experimental steps have to be carried out under an inert
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 5 3 1 - 2
186
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
atmosphere. Before impregnation, the silica supports were calcined at 550◦ C for 3 h and then stored under argon. The charcoal was dried for 6 h at 150◦ C at 10 Pa and stored under argon. The 1-methyl-3-butyl-imidazolium chloride ([bmim]Cl) used for the preparation of the ionic liquids was kindly provided by Elementis Specialties, Durham, UK. Iron chloride and activated charcoal were purchased from Merck AG. The silica support FK700 was kindly provided by Degussa-Hüls AG. Toluene, m-xylene, anisol, acetyl chloride, acetic anhydride and dichloromethane were purchased from Fluka in 99% purity and used without further purification. All of the ionic liquids tested contained 1-methyl3-butyl-imidazolium chloride as the organic base and a two-fold molar excess of metal halide. 2.1. 1-Methyl-3-butyl-imidazolium chloroferrate (Fe-IL) In a round bottomed flask equipped with a magnetic stirrer and a gas inlet valve, 32.44 g anhydrous FeCl3 (0.2 mol) were slowly added to 17.47 g (0.1 mol) 1-methyl-3-butyl-imidazolium chloride. To ensure complete reaction, the mixture was left stirring over night. 1-Methyl-3-butyl-imidazolium chloroaluminate (Al-IL) and 1-methyl-3-butyl-chlorostannate were prepared analogously to the Fe-IL. 2.2. Preparation of the immobilised Fe-IL In a round bottomed flask equipped with a magnetic stirrer and a gas inlet valve, an excess of ionic liquid was slowly added to the support. Either FK 700 amorphous silica, or granular activated charcoal supports were used. To ensure complete homogenisation, the mixture was left stirring over night. The excess ionic liquid was removed by 24 h extraction with boiling dichloromethane in a Soxhlet apparatus. The catalyst was dried at 10 Pa and stored under argon. The reactions in discontinuous liquid phase (batch reactions) were carried out in round-bottomed flask equipped with a reflux condenser, gas-inlet valve, and sampling exit. A magnetic stirrer equipped with a thermostat and a silicon oil bath were used to maintain
the reaction temperature and ensure the homogeneity of reactants. The catalyst was weighed in the reactor and then the reactants were added according to the proportion desired. Undecane was added as an internal standard to follow the reaction kinetics and to calculate the mass balance. Samples were taken periodically and analysed by gas chromatography in order to quantify conversion and selectivity of the reaction. In all reactions presented here, a ratio of 1 mol acetylating agent to 5 mol of aromatic compound was used. As standard reaction temperature 100◦ C was chosen, the experiments were usually stopped after 1 h reaction time. The continuous liquid phase reactor system consisted of a stainless steel tube of 100 mm length and 6 mm width. The catalyst was transferred to the reactor and the endings were closed with a metallic net to prevent the loss of catalyst. A peristaltic pump pushed the solution containing the aromatic compound, the acetylating agent, and the internal standard through the reactor with an adjusted weight hourly space velocity (WHSV). The reaction temperature was controlled by a thermostatic silicon oil bath. Samples were taken periodically and analysed by GC in order to follow the reaction course. The continuous gas phase reactor system consisted of a 1 m long coiled tube. The catalyst was introduced into the tube and stayed at the lower part. The reaction mixture was fed into the reactor and vaporised at the beginning of the tube. The reaction mixture was pumped by a peristaltic pump and a flow of dry nitrogen was used to carry the vaporised reactants through the reactor. The products were collected in a cooling trap from where samples were periodically withdrawn to follow the reaction course by GC. 2.3. Analytics For BET-measurements, an ASAP 2000 machine from Micromeritics was used. The concentrations of Fe, Al, Sn and Si in the samples were determined by ICP-AES, using a Spectro-Flame D machine from Spectro. CHN analysis was performed on a Elementar Vario EL. GC analysis was done on a Siemens RGC 202, using a 30 m SE 54 column from Hewlett-Packard. GC–MS analysis were performed on a Varian 3400 CX GC and a Varian Saturn 3 mass spectrometer.
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
187
Scheme 1. General reaction scheme of Friedel-Crafts acylation reactions.
3. Results and discussion Literature on Friedel-Crafts acylation reactions (see Scheme 1) still shows aluminium(III)chloride as “catalyst” for this kind of reaction, even though a two- to three-fold molar excess of this metal halide is needed in order to allow complete conversion. Experiments with pure chloroaluminate ionic liquids in liquid phase reactions displayed the same need for high amounts of catalyst (see Table 1). A comparison with two other Lewis-acidic metal halides, iron(III)chloride and tin(II)chloride quickly proved both of them to be alternative catalysts. These metal halides were chosen for reasons of price, availability and easy handling. The ionic liquids prepared with them displayed good catalytic properties when
used in acylation reactions. Table 1 shows conversion and selectivity found for a number of experiments catalysed by these three examined ILs. In all of the reactions, the Al-IL led to the lowest conversions. The ionic liquid containing tin(II)chloride gave better conversions and similar selectivities, but the best results were obtained with an IL containing iron(III)chloride. Here, the conversions were by far the highest, while the selectivities obtained were, with the exception of the acylation of m-xylene, in the same range as with the Al-IL and Sn-IL. Therefore, further experiments concentrated on the Fe-IL catalyst. In the liquid phase acylation of m-xylene with acetyl chloride catalysed by Fe-IL, we found a comparably low selectivity of about 80%.
Table 1 Acylation of aromatics: batch reactions over non-immobilised ILsa Ionic liquid
Reaction
Molar ratio (catalyst:aromatic compound)
Al-IL
Mesitylene + AcCl Anisole + Ac2 O Anisole + Ac2 O m-Xylene + AcCl Toluene + AcCl Benzene + AcCl
1:205 1:205 1:45 1:205 1:205 1:205
68.05 0.31 8.26 3.52 0 0
98.08 94.12 96.1 96.35 0 0
Sn-IL
Anisole + Ac2 O Anisole + Ac2 O m-Xylene + AcCl
1:205 1:45 1:205
2.48 19.65 3.56
90.41 94.41 94.94
Fe-IL
Mesitylene + AcCl Anisole + Ac2 O Anisole + Ac2 O m-Xylene + AcCl m-Xylene + AcCl Toluene + AcCl Benzene + AcCl
1:205 1:205 1:45 1:205 1:45 1:205 1:205
94.74 90.17 100.00 33.80 45.6 0.54 0
94.9 96.64 98.13 79.39 83.12 87.92 0
a
Conversion (%)
T = 100◦ C, reaction time: 1 h, molar ratio aromatic compound/acetylating agent = 5:1.
Selectivity to main products (%)
188
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
Table 2 Behaviour of conversion and selectivity with temperature in the acylation of m-xylene with acetyl chloridea T (◦ C)
Conversion (%)
Selectivity
20 40 60 80 100
3.05 7.28 10.53 28.91 33.80
59.49 54.07 59.14 70.26 79.39
a Catalyst: Fe-IL, N = 0.6, reaction time: 1 h, molar ratio: 0.2 mmol IL:9 mmol AcCl:45 mmol m-xylene.
Scheme 2. Consecutive reaction.
Interestingly, the selectivity to the main product increased with temperature (see Table 2). Considering this to be a rather unusual behaviour, we concentrated on the by-products of the reaction. We found the most important by-product, as identified by GC–MS, to be 1-(1-chlorovinyl)-2,4-dimethyl-benzene, leading us to assume that the consecutive reaction shown in Scheme 2 takes place. The HCl concentration in the liquid is crucial for this reaction, which could explain the high rate of this reaction at lower temperatures. Here, the HCl is not removed from the mixture fast enough, and a higher amount of the 2,4-dimethyl-acetophenone is used in the consecutive reaction, leading to a lower selectivity towards the main product. The water formed in this reaction leads to a non-reversible deactivation of the catalyst, so it has be avoided. Furthermore, this con-
secutive reaction explains the low selectivity towards the main product. The chlorostannate ionic liquid was highly viscous with a melting point close to room temperature. The high viscosity of these melts hindered the immobilisation process described in the experimental part. For this reason, as well as the higher conversions achieved with the Fe-IL, further immobilisation experiments concentrated on the chloroferrate liquids only. The chloroferrate ionic liquids were immobilised on different silica supports, as well as on charcoal. Reactions in batch and continuous liquid phase resulted in lower conversions but higher selectivities than homogeneously catalysed reactions. These results looked promising, but ICP-AES analysis of the Fe-content of the used catalysts and product solutions displayed a strong leaching of the ionic liquid. Table 3 displays the results for batch reactions, reactions in the continuous liquid phase showed the same correlation between conversion and the amount of iron in the solution. It has to be assumed that the leaching of the ionic liquid in liquid phase reactions is due to a strong complex formation between the metal halide and the carboxylic groups of the reactants. This is not countered by the formation of a stable covalent bond to the oxygen of hydroxyl-groups on the surface of the supports, as has been assumed for and immobilised chloroaluminate ILs and described in detail in the relevant publications [14,15]. The supported Fe-IL can be more correctly described as an supported liquid phase (SLP) catalyst [16,17]. The leaching in the liquid phase decreases the practical use of the catalyst, especially since it is probably caused by one of the starting materials. In view of the high amount of leaching, no further reactions were carried out in liquid phase, the question if a part of the reaction is catalysed heterogeneously was not addressed.
Table 3 Leaching in liquid phase reactionsa Reaction
Catalyst
Conversion (%)
Iron in solution (ppm)
Anisole + Ac2 O m-Xylene + AcCl m-Xylene + AcCl, 2nd run
Fe-IL on MCM 41 Fe-IL on MCM 41 Fe-IL on MCM 41
6.54 15.01 4.62
135.35 56.05 20.25
a
Catalyst: Fe-IL on MCM 41, 0.1 g, N = 0.6, reaction time: 1 h, 45 mmol m-xylene:9 mmol AcCl.
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
189
Fig. 1. Reactions in the gas phase: yield of 2,4-dimethoxy-acetophenon at different temperatures.
To avoid this problem of leaching, the feasibility of reactions in the gas phase was tested. As a test reaction, the acylation of m-xylene was chosen. This compound is less activated than the anisole, but the lower boiling point of the main product (228◦ C in comparison to 258◦ C) facilitated reactions at lower temperatures. Reactions with acetic anhydride as acetylating agent led to conversions of less than 1%, so the work focused on reactions with acetyl chloride. There, conversions of up to 5% and product selectivities of more than 98% could be obtained. The consecutive reaction shown in Scheme 2 was completely avoided, the main by-product proved to be the expected ortho-acylated product. Acylation reactions in the gas phase were tested at temperatures ranging from 200 to 300◦ C (see Fig. 1). Conversions reached were clearly below those in liquid phase reactions, but more than 99% of the recovered products were the 2,4-dimethyl-acetophenone. As seen in Fig. 1, a deactivation takes place. This, we attribute to the formation of polyaromatic products which do not desorb easily from the catalyst. Their presence could be proven by washing the catalyst with dichloromethane after the reaction and a following analysis of the washing solution. Here, various polyaromatic compounds resulting from condensation reactions of starting material and products could be detected via GC and GC–MS. The formation of these products decreases with increasing temperature, probably due to the faster desorption of
the products and coke precursors. A detailed analysis of possible by- and consecutive-products of acetylation reactions can be found in the literature [18]. Unfortunately, the gas phase reactions proved to have some serious drawbacks. As shown in Fig. 1, the conversion seems to increase at the beginning of the reaction. Since no activation of the catalyst is necessary, this is unexpected. We found the cause for this behaviour in the strong adsorption of the different reactants on the catalyst surface. Since the substances commonly used as external standards show the same adsorption behaviour, no correct data for conversion and selectivity can be achieved until an equilibrium between ad- and de-sorption is reached. To avoid this problem, the standard was used only to allow calculation of the exact molar ratios of the starting materials at the beginning of the reaction. The conversion was calculated from the amount of product found in ratio to the amount of product calculated for total conversion. By using cooling traps filled with liquid nitrogen a complete recovery of the aromatic compound and the products could be ensured. These data were used to calculate the conversion and the selectivities to different products. The second problem encountered in the gas phase reactions was the impossibility of reaching a satisfying mass balance when working with acetyl chloride. Calculations based on an external standard, as well as weight measurements showed a loss of up to 10 wt.%
190
M.H. Valkenberg et al. / Applied Catalysis A: General 215 (2001) 185–190
Acknowledgements
Scheme 3. Decomposition of the acetyl chloride.
of the total mass of reactants. Since GC samples proved that the product solution contained nearly exclusively the aromatic compounds, meaning the starting material and the products, the most probable explanation for the loss seems to be a decomposition of the acetyl chloride. No acetic acid was found, indicating that this decomposition is not due to water in the reaction system. A cooling trap filled with sodium hydroxide on the other hand improved the mass balance and allowed recovery of up to 98% of the mass of starting materials. That means that a decomposition of the acetyl chloride to ketene and HCl takes place on the surface of the catalyst (see Scheme 3).
This work was carried out as a part of the BRITE-Euram project BE 906-3745. The authors are grateful to the European Commission for the funding of this work. Our thanks also go to our consortium partners in the project for stimulating and helpful discussions. References [1] [2] [3] [4] [5]
[6]
4. Conclusions The use of iron chloride-based ionic liquids in the Friedel-Crafts acylation of aromatic compounds can be considered as an interesting new alternative to the existing homogeneous catalysts. It is possible to obtain high conversions and very high selectivities in a variety of reactions. The immobilisation of these catalysts, however, leads to some serious problems. Experiments in liquid phase resulted in a high amount of leaching, leading to the question, whether a heterogeneous reaction takes place at all. This leaching problem can be avoided when working in the gas phase. Here a deactivation through heavier products takes place, but optimisation of the reaction conditions should allow to improve the yields. Work on this project will continue. We expect to be able to improve the results by using different types of catalysts in the future.
[7] [8]
[9] [10]
[11] [12] [13] [14] [15] [16] [17] [18]
W.-F. Hölderich, Stud. Surf. Sci. Catal. 75 (1993) 127–163. J.N. Armor, Appl. Catal. A: General 189 (1999) 153–162. J. H Clark, Green Chem. 1 (1999) 1–8. K. Tanabe, W.F. Hölderich, Appl. Catal. A: General 181 (1999) 399–434. M. Spagnol, L. Gilbert, et al., in: Proceedings of the Fourth International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 1996. K. Smith, Z. Zhenhua, P.K.G. Hodgson, J. Mol. Catal. A: General 134 (1998) 121–128. D. Das, S. Cheng, Appl. Catal. A: General 201 (1999) 159– 168. J.S. Wilkes, in: G. Mamantov, R. Marassi (Eds.), NATO ASI Series C: Mathematical and Physical Sciences, Vol. 202, D. Reichel Publishing Company, 1987, pp. 405–416. J.A. Boon, J.A. Levisky, J.L. Pflug, J.S. Wilkes, J. Org. Chem. 51 (1986) 480–483. E. Benazzi, Y. Chauvin, A. Hirschauer, N. Ferrer, H. Olivier, J.Y. Bernhard, US Patent 5,693,585 (1996), to Institute Français du Petrole. C.J. Adams, M.J. Earle, G. Roberts, K.R. Seddon, Chem. Commun. (1998) 2097–2098. K.R. Seddon, J. Chem. Technol. Biot. 68 (1997) 351–356. T. Welton, Chem. Rev. 99 (1999) 2071–2084. C. deCastro, E. Sauvage, M.H. Valkenberg, W.F. Hölderich, J. Catal. 196 (2000) 86–94. M.H. Valkenberg, C. deCastro, W.F. Hölderich, Top. Catal., in press. M. Makkee, B.A.A.L. van Setten, J.A. Mouljin, Stud. Surf. Sci. Catal. 116 (1995) 667–673. J. Villadsen, H. Livbjerg, Catal. Rev. Sci. Eng. 17 (1978) 203–227. D. Rohan, C. Canaff, E. Fromentin, M. Guisnet, J. Catal. 117 (1998) 296–305.