Ind. Eng. Chem. Res. 2006, 45, 1571-1574
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Highly Selective Synthesis of Diphenylmethane with Acidic Ionic Liquids Shuhan Cui, Bin Lu,* Qinghai Cai, Xijin Cai, Xuemei Li, Xue Xiao, Lijie Hou, and Yuanyuan Han Department of Chemistry, Harbin Normal UniVersity, No. 50 Hexing Road, Nangang District, Harbin 150080, People’s Republic of China
Synthesis of diphenylmethane (DPM) from benzene and benzyl chloride with acidic ionic liquids, such as (C2H5)3NHCl-AlCl3, (C2H5)3NHCl-ZnCl2, (C2H5)3NHCl-FeCl3, and (C2H5)3NHCl-CuCl2, as catalysts was investigated. The results showed that (C2H5)3NHCl-AlCl3 is a highly efficient catalyst. Both the yield and the selectivity of DPM reached up to 100% in the presence of (C2H5)3NHCl-AlCl3 as catalyst, when the reaction was carried out at ambient conditions. Introduction Diphenylmethane (DPM) and substituted diphenylmethanes are important chemicals used as pharmaceutical intermediates and other fine chemicals.1 DPM has industrially been synthesized by Friedel-Crafts reaction of benzene with benzyl chloride in the liquid phase, using homogeneous acids, such as AlCl3, FeCl3, and H2SO4,2 as catalysts. There are several problems in the application of these catalysts, such as high toxicity, corrosion, difficulties in separation or recovery, disposal problems due to a large number of acidic effluents, etc. Hence, some solid acids used in this reaction as alternatives have been developed, for example, HY zeolite,3 H-ZSM-5,4 heteropolyacid salts,5 sulfated ZrO2,6 ion-exchanged clays,7 and Fe-containing MCM-41.8 Although these solid acidic catalysts are harmless to the environment and health, they showed a poor activity or higher performing temperature in the benzylation reaction. Therefore, the development of a new and highly efficient catalyst for the synthesis of diphenylmethane is of commercial importance and academic interest. In the present investigation, an efficient benzylation process of benzene and benzyl chloride to DPM with industrial significance was realized. In this process, an acidic ionic liquid was used as the catalyst; a DPM yield and selectivity of 100% were achieved under ambient conditions. This result is very rare in the reported literature on the synthesis of DPM. Moreover, highly selective synthesis of organic compounds is among the most important targets pursued in green chemistry. Experimental Section The ionic liquids used as catalysts, (C2H5)3NHCl-AlCl3, (C2H5)3NHCl-FeCl3, (C2H5)3NHCl-ZnCl2, and (C2H5)3NHClCuCl2, were synthesized according to a literature method.9 In a typical run, appropriate amounts of benzene, benzyl chloride, and catalyst were charged in a glass reactor equipped with a condenser and magnetic stirrer. The reaction mixture was heated to a certain temperature in a water bath by an electric heater, while the reaction was carried out above room temperature. After several hours, the reactor with the reaction solution was cooled to room temperature. The catalyst was separated out, and then the liquid samples were analyzed by GC (Shimadzu, GC-14C) and GC-MS. Results and Discussion Influence of Various Reaction Conditions on the Yield and Selectivity. When alkylation reaction of benzene with benzyl * To whom correspondence should be addressed. Tel.: 86-45188060580. Fax: 86-451-88060580. E-mail:
[email protected].
Figure 1. Influence of the catalyst amount on the conversion and yield. Benzene/benzyl chloride ) 4/1 (mol/mol), T ) room temperature, t ) 3 h, and Et3NCl/AlCl3 ) 1/2 (mol/mol).
chloride was carried out in the presence of an ionic liquid, Et3NHCl-AlCl3, DPM was formed. As shown in Figure 1, the conversion of benzyl chloride or yield of DPM increased with the amount of catalyst used in the system. It reached a maximum of 100% with 1.0 g of catalyst and 50 mL of benzene used (molar ratio of catalyst to benzene 0.0025/0.56), and then the yield remained constant while the amount of catalyst increased continuously. The turnover number of the DPM formation is 56.9 with a catalyst and benzene ratio of 0.0025/0.56, showing that the ionic liquid, Et3NHCl-AlCl3, is a very efficient catalyst for the synthesis of DPM with a conversion and yield of 100% via alkylation of benzene with benzyl chloride. At that time, the inherently atomic economy of this reaction was achieved. The effect of the benzene/benzyl chloride molar ratio on the catalytic activity of the catalyst is shown in Figure 2. When the ratios are changed from 3/1 to 4/1 by keeping the amount of benzene constant, the yield and conversion greatly increase from 60.6% to 100%. Hereafter, they hold a constant value of 100% in the range of the ratio from 4/1 to 15/1; that is, the selectivity to DPM was found to be independent of the ratio in that range. It remained constant at 100%. The dependence of the yield and selectivity of DPM on the reaction temperature was investigated in the temperature range of 20-80 °C using the chloroaluminated ionic liquid as the catalyst. The yield and the selectivity are found to be independent of the reaction temperature with a reaction time of 3 h, as well as a time of 0.5 h; both are 100% in the whole range of temperatures from 20 to 80 °C (Figure 3). For this synthesis
10.1021/ie0510268 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
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Figure 2. Influence of the molar ratio of benzene to benzyl chloride on the conversion and yield. Wcatal ) 1 g, T ) room temperature, t ) 3 h, and Et3NCl/AlCl3 ) 1/2 (mol/mol).
Figure 3. Influence of the reaction temperature on the selectivity and yield. Benzene/benzyl chloride ) 4/1 (mol/mol), Wcatal ) 1 g, t ) 3 h (9) and 0.5 h (0), and Et3NCl/AlCl3 ) 1/2 (mol/mol).
reaction, the reaction heat is ∆H° ) -37.0 kJ/mol, which is calculated from the formation heats of various substances in the following reaction: This value reveals that the synthesis
reaction is an exothermic one. In terms of the thermodynamics, therefore, the elevation of the reaction temperature is of a great disadvantage to the generation of DPM, but the reaction was carried out in an open system, so the raised temperature could result in acceleration of the release of HCl gas formed in the reaction process, which promoted the reaction shift to the direction of producing DPM. As a result, a high conversion and yield of DPM were obtained. Figure 4 depicts the conversion and the yield at room temperature as a function of the reaction time. The yield increased linearly to about 35%, 70%, and 100% at reaction times of 0.17 h (10 min), 0.33 h (20 min), and 0.5 h, respectively, and then remained constant with further increases in the reaction time. However, the selectivity remained at 100% throughout this process.
Figure 4. Influence of the reaction time on the conversion and yield. Benzene/benzyl chloride ) 4/1 (mol/mol), Wcatal ) 1 g, T ) room temperature, and Et3NCl/AlCl3 ) 1/2 (mol/mol).
Figure 5. Influence of the AlCl3/(C2H5)3NHCl molar ratio on the selectivity (4) and yield (9). Benzene/benzyl chloride ) 4/1 (mol/mol), Wcatal ) 1 g, T ) room temperature, and t ) 1 h.
It was reported that chloroaluminated ionic liquids are designated as basic when the molar ratio of AlCl3 is smaller than 0.5. A melt is referred to as neutral at an AlCl3 ratio of exactly 0.5, where only the anion AlCl4- is present. Finally, an acidic chloroaluminated melt is one in which the AlCl3 ratio is larger than 0.5. In such acidic melts, the anions Al2Cl7- and Al3Cl10- exist.10 Therefore, the yield of DPM is near zero at AlCl3/(C2H5)3NHCl ) 1/1, because the catalyst is neutral at that ratio (Figure 5). The yield increased with increasing acidity of the catalyst due to the increase in the ratio of AlCl3 to (C2H5)3NHCl. When the ratio was increased from 1.5/1 (the molar ratio of AlCl3 is 0.6) to 2/1 (0.67), the yield increased from 2.8% to 100%. While the ratio continuously increased to 3/1, the yield and selectivity decreased to 95.4%, which was probably ascribed to the coreaction taking place in the presence of a stronger acidic catalyst at an AlCl3 molar ratio of 0.75. It is seen from the above results that the catalyst possesses an efficient catalytic activity. The conversion of the benzyl chloride and selectivity of DPM are all 100% under ambient conditions, whereas, when the reactants and product were decanted out at the end of the reaction and the catalyst was reused for the second run, the yield of DPM was merely 0.9% with the reused catalyst. This finding indicated that the catalyst
Ind. Eng. Chem. Res., Vol. 45, No. 5, 2006 1573 Table 1. Catalytic Performance of Alkylation Reaction with Various Ionic Liquids catalyst
feedstock
(C2H5)3NHCl-AlCl3 (C2H5)3NHCl-ZnCl2 (C2H5)3NHCl-FeCl3 (C2H5)3NHCl-CuCl2 (C2H5)3NHCl-AlCl3 (C2H5)3NHCl-AlCl3
C6H6 + PhCH2Cl C6H6 + PhCH2Cl C6H6 + PhCH2Cl C6H6 + PhCH2Cl C6H5CH3 + PhCH2Cl C6H4(CH3)2 + PhCH2Cl
conversion selectivity yield (%) (%) (%) 100 2.1 2.1 3.3 100 12.8
100 100 100 100 100 100
100 2.1 2.1 3.3 100 12.8
was rapidly deactivated due to its sensitivity to moisture and dissolving elution of the catalyst accompanied by the decanting solution.11 Catalytic Activity of Various Catalysts. Besides the chloroaluminated ionic liquid, other similar ionic liquids, such as (C2H5)3NHCl-ZnCl2, (C2H5)3NHCl-FeCl3, and (C2H5)3NHClCuCl2, were also used for the alkylation reaction. The catalytic performance of these catalysts in the benzylation of benzene with benzyl chloride is depicted in Table 1. As can be seen from Table 1, (C2H5)3NHCl-ZnCl2, (C2H5)3NHCl-FeCl3, and (C2H5)3NHCl-CuCl2 are found to have a very low activity at the same conditions as compared to (C2H5)3NHCl-AlCl3. The percentage conversions are merely 2.1%, 2.1%, and 3.3% in the presence of the former three catalysts, respectively, and the percentage conversion is 100% with (C2H5)3NHCl-AlCl3 as the catalyst. However, in terms of selectivity, they are also found to be very selective (100%) in the benzylation of benzene. The low activities of the three catalysts might be explained on the basis of their weaker acidity than that of the latter. Benzylation of Substituted Benzene. When toluene and xylene were used as alternatives of benzene, the benzylation reaction of them with benzyl chloride was carried out in the presence of (C2H5)3NHCl-AlCl3. The conversion of benzyl chloride in the benzylation of toluene is found to be as high as that of benzene at the same conditions, and 100% selectivity of the corresponding product is also found (Table 1). And for the xylene system, the conversion and yield are only 12.8% under the same conditions. This phenomenon may have to do with the steric effect of xylene. Mechanism Consideration. The benzylation of benzene with benzyl chloride in the presence of Bro¨nsted or Lewis acids as catalysts appears to proceed via an electrophilic intermediate.12,13 In this process, the acidic species Al2Cl7- first interacts with the benzyl chloride to produce electrophilic benzylcarbenium and the species AlCl4-, and then the carbenium ion attacks the benzene ring to generate an intermediate complex. The complex interacts with AlCl4- to produce DPM, HCl gas, and AlCl3, as follows:14,15
C6H5CH2Cl + Al2Cl7- f C6H5CH2+ + 2AlCl4-
(1)
C6H5CH2+ + C6H5-H f H-C6H5+-CH2C6H5
(2)
H-C6H5+-CH2C6H5 + AlCl4- f C6H5-CH2-C6H5 + HCl + AlCl3 (3) AlCl3 + AlCl4- f Al2Cl7-
(4)
When using a Lewis acidic ionic liquid as the catalyst, a phenomenon in the field of acid/base chemistry of ionic liquids deserves to be mentioned. This is the so-called superacidity of protons in ionic liquids. It is generated when a strong mineral acid is dissolved in the acidic chloroaluminated ionic liquid, according to a report.16 The superacid properties of protons have
been explained by reactions between the dissolved HCl and the acidic species in the melt, which release nude protons with extremely low solvation and therefore very high reactivity, as shown in the following equation:17
HCl + Al2Cl7- f [H+]nonsolvated + 2AlCl4The molar ratio of AlCl3 employed in the catalyst of the system is 0.67. Thus, the strong Lewis acids and the superacidic protons, coexist in the system, since HCl as a byproduct can be formed in the reaction process. The presence of the nonsolvated H+ as a strong Bro¨nsted acid, together with a Lewis acid, mutually accelerates the alkylation reaction. The reaction rate is, therefore, so fast that the reaction is complete in a short time. On the other hand, the strong polarity and electrostatic field of the chloroaluminated ionic liquid used as the catalyst or reaction medium may stabilize the intermediate carbenium ion, which would make it possible that the carbon charged positively has enough time to attack the benzene ring to generate the target product.18 In the meantime, the polyalkylated benzenes were not present in the reaction product, which may be ascribed to the steric factor.14 This is the reason that the reaction possesses a high conversion and yield. Conclusions It is demonstrated that (C2H5)3NHCl-AlCl3 catalyzed the benzylation of benzene and benzyl chloride efficiently, which presented the synthesis of DPM with a high conversion of the feedstock and selectivity of the target product as compared with those in other reports at room temperature. Moreover, it is considered that a high catalytic activity of the ionic liquid (C2H5)3NHCl-AlCl3 as the catalyst is attributed to the formation of nonsolvated H+ and stabilizing action of the chloroaluminated ionic liquid to the intermediate carbenium in terms of mechanism consideration. The high conversion, especially selective synthesis of diphenylmethane, is the final target pursued by green chemistry. Acknowledgment We greatly acknowledge the financial support of this work by the Natural Science Foundation and Science and Technology Project Foundation of Heilongjiang Province, People’s Republic of China (Grant Nos. TB2005-16 and GC03A404). Literature Cited (1) Okada, S. I.; Tanaka, K.; Nakahara, Y.; Nakagawa, N. Selective Friedel-Crafts Alkylation on a Vermiculite, a highly Active Natural Clay Mineral with Lewis Acid Sites. Bull. Chem. Soc. Jpn. 1992, 65, 2833. (2) Yang, K. G.; Hua, R. M.; Wang, H.; Xu, B. Q. Benzylation of Aromatic Compounds with Benzyl Chloride Catalyzed by Nafion/SiO2 Nanocomposite Catalyst. Chin. Chem. Lett. 2005, 16 (4), 527. (3) Coq, B.; Gourves, V.; Figueras, F. Benzylation of toluene by benzyl chloride over protonic zeolites. Appl. Catal., A 1993, 100, 69. (4) Choudhary, V. R.; Jana, S. K.; Kiran, B. P. Alkylation of benzene by benzyl chloride over H-ZSM-5 zeolite with its framework Al completely or partially substituted by Fe or Ga. Catal. Lett. 1999, 59, 217. (5) Yusuke, I.; Mayumi, O.; Kazuo, U. Alkali metal salts and ammonium salts of Keggin-type heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions. Appl. Catal., A 1995, 132, 127. (6) Koyande, S. N.; Jaisswal, R. G.; Jayaram, R. V. Reaction Kinetics of Benzylation of Benzene with Benzyl Chloride on Sulfate-Treated Metal Oxide Catalysts. Ind. Eng. Chem. Res. 1998, 37, 908. (7) Cseri, T.; Bekassy, S.; Figueras, F.; Rizner, S. Benzylation of aromatics on ion-exchanged clays. J. Mol. Catal. A 1995, 98, 101. (8) He, N.; Bao, S.; Xu, Q. Fe-containing mesoporous molecular sieves materials: very active Friedel-Crafts alkylation catalysts. Appl. Catal., A 1998, 169, 29.
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(9) Sherif, F. G.; Shyu, L. J.; Greco, C. C. Linear alxylbenzene formation using low-temperature ionic liquid. U.S. Patent 5,824,832, 1998. (10) Seddon, K. R. Ionic Liquids for Clean Technology. Technol. Biotechnol. 1997, 68, 351. (11) Qiao, C. Z.; Zhang, Y. F.; Zhang, J. C.; Li, C. Y. Activity and stability investigation of [BMIM][AlCl4] ionic liquid as catalyst for alkylation of benzene with 1-dodecene. Appl. Catal., A 2004, 276, 61. (12) Singh, A. P.; Bhattarchaya, D.; Sharma, S. A novel catalytic method for the alkylation of benzene to diphenylmethane over H-ZSM-5 zeolite catalysts. J. Mol. Catal. 1995, 102, 139. (13) Chaube, V. D. Benzylation of benzene to diphenylmethane using zeolite catalysts. Catal. Commun. 2004, 5, 321. (14) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. ReV. 1999, 99, 2071. (15) Boon, J. A.; Levisky, J. A.; Pflug, J. L.; Wilkes, J. S. FriedelCrafts reactions in ambient-temperature molten salts. J. Org. Chem. 1986, 51, 480.
(16) Karpinski, Z. J.; Osteryoung, R. A. Determination of equilibrium constants for the tetrachloroaluminate ion dissociation in ambient-temperature ionic liquids. Inorg. Chem. 1984, 23, 1491. (17) Smith, G. P.; Dworkin, A. S.; Pigni, R. M.; Zingg, S. P. Bro¨nsted superacidity of hydrochloric acid in a liquid chloroaluminate. Aluminum chloride-1-ethyl-3-methyl-1H-imidazolium chloride. J. Am. Chem. Soc. 1989, 111, 525. (18) Qiao, K.; Deng, Y. Alkylations of benzene in room temperature ionic liquids modified with HCl. J. Mol. Catal. A 2001, 171, 81.
ReceiVed for reView September 13, 2005 ReVised manuscript receiVed January 9, 2006 Accepted January 9, 2006 IE0510268
