Chinese Journal of Chemical Engineering, 16(3) 357—360 (2008) Blanc Reaction of Aromatic Compounds Catalyzed by Ionic Liquids* FANG Yanxiong (方岩雄)**, DENG Yunquan (邓运泉), REN Qinggang (任清刚), HUANG Jingping (黄敬平), ZHANG Saidan (张赛丹), HUANG Baohua (黄宝华) and ZHANG Kun (张焜) Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China Abstract Ionic liquids have been used as catalysts for Blanc reaction of toluene. The effects of reaction tempera- ture, reaction time and dosage of the ionic liquid catalyst have been investigated, and the catalytic performance of different ionic liquid catalysts for toluene chloromethylation was also studied. The reaction was found to proceed under mild conditions with excellent conversion (up to 90%) in the absence of Lewis acids. The ionic liquids could be recycled and reused without loss of their catalytic activities. Keywords chloromethylation, Blanc reaction, ionic liquid, aromatic compound, toluene, phase transfer 1 INTRODUCTION Blanc chloromethylation reaction of aryl compounds with chloromethyl methyl ether in chloroaluminate ionic Blanc reaction, or chloromethylation reaction, is liquids prepared from AlCl3 and 1-alkylpyridinium or one of the most fundamental reactions in synthetic 1-methyl-3-alkylimidazolium halide ([Rmin]X) qua- chemistry, and has been documented in previous arti- ternary ammonium salts [16]. The use of toxic chloro- cles [1-7]. It has been widely applied in synthesis of a methyl methyl ether and AlCl3 made the process less variety of specialty and fine chemicals, e.g. pharma- benign. Wang et al. [17] reported Blanc chloromethy- ceuticals, agrochemicals, dyes, flavors and fragrances, lation reaction of aryl compounds with paraformalde- monomers, additives and modifier of polymer, in which hyde using the reusable ionic liquid 1-ethyl-3- methylimidazolium tetrafluoroborate [emim]BF4 as chloromethyl group can easily be changed to CH2OH, promoter. But [emim]BF4 was much more expensive CHO, CH2CN, CH2NH2, CH3, CH2R, etc. Blanc reaction is normally performed in polar and difficult to prepare than [Rmin]X. In the present paper, instead of using chloromethyl methyl ether and solvents e.g. CCl4, dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc) or N-methyl pyr- AlCl3, 1-alkyl-3-methylimidazolium bromide has been rolidone (NMP), and these reactions are usually car- employed as a novel catalyst for the Blanc reaction of ried out in a heterogeneous medium in the presence of toluene (Fig. 1) with formaldehyde and hydrogen chloride for the first time. The performance of eight Lewis acid, e.g. ZnCl2 or SnCl4 [1, 2]. The use of Lewis acids is important to activate formaldehyde. N-alkyl-3-methyl imidazolium ionic liquids for However, stoichiometric amount of Lewis acid to sub- preparation of methylbenzyl chloride has been inves- strate is required. These Lewis acid catalysts, in gen- tigated either. eral, are not recommended for the inherent problems of corrosiveness, high susceptibility to water, diffi- 2 EXPERIMENTAL culty in catalyst recovery, environmental hazards and waste control after the reaction. 2.1 General remarks Recently, the research on ionic liquids (ILs) has been escalating at an overwhelming rate [8-10]. These All the used chemicals were from commercial liquids have captivated us owing to their remarkable sources without any pretreatment. All reagents were of properties. They have been used as reaction solvents analytical grade. Toluene, paraformaldehyde and hy- and phase-transfer catalysts/agents, since they are drochloric acid were purchased from the local market. N-quats [11]. It would be worthwhile to employ ionic Alkyl halides and N-methyl-imidazole were purchased liquids as PTC in the Blanc reaction, for these ionic from ABCR GmbH & Co. KG. The ionic liquid liquids could also activate the anions and thus result in [C12mim]Br was synthesized according to the litera- an enhanced reaction rate [12]. The use of ionic liquids ture procedures [18, 19]. The reaction product was would also have the advantages over conventional reac- analyzed by GC (Agilent GC6890). Ionic liquid can tion conditions if the product could be decanted from be reused after removal of water at 70°C for 12 h un- the solvent and the ionic liquid may be recycled. The der vacuum (1 Pa). ionic liquid catalyst may substitute the Lewis acids [13-15], and the use of high toxic chloromethyl methyl ether as chloromethylification reagent and environ- mentally unfavorable halogenated solvents could be avoided. Blanc reactions with ionic liquids as catalysts were seldom reported [16, 17]. Qiao and Deng reported Figure 1 Chloromethylation of toluene Received 2007-09-24, accepted 2008-03-19. * Supported by the National Natural Science Foundation of China (20376015), the Natural Science Foundation of Guangdong Province (32491) and Guangzhou Project of Science & Technology (2004J1-C0151). ** To whom correspondence should be addressed. E-mail: [email protected] 358 Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 2.2 Chloromethylation of toluene methyl product was investigated using ionic liquid of [C12mim]Br. It could be seen clearly that appreciable Toluene (2.76 g, 0.03 mol), paraformaldehyde change was observed when the reaction temperature - (1.50 g, 0.05 mol), 37% hydrochloric acid (5 ml) and was increased from 35 to 100°C (entries 1 5). The conversion of toluene increased with raising reaction ionic liquid [C12mim]Br (0.40 g, 0.0012 mol) were added in a 25-ml flask equipped with reflux condenser temperature first and then kept almost constant be- - and oil-bath. The total volume of the reaction mixture tween 65°C 80°C. The yield would decrease when the was about 10.2 ml. The aqueous phase was consisted temperature was over 80°C. Another influencing fac- of IL, paraformaldehyde and HCl, and the organic tor was that the concentration of hydrogen chloride in phase was of toluene and paraformaldehyde. Hydro- the system decreased with increasing temperature, gen chloride gas was bubbled into the flask at the flow leading to the decrease of conversion. rate of 2 ml·min－1. The mixture was stirred for 10 h at The effect of reaction time was investigated us- - 65°C. Then, the mixture was decanted to isolate the ing [C12mim]Br ionic liquid (entries 4, 6 9). The chloromethylation products. conversion of toluene increased with prolonging reac- tion time. When the reaction time was 12 h, the reac- tion was almost completed and the yield for 3 RESULTS AND DISCUSSION monochloromathylation product reached up to 90%. Furthermore, the effect of dosage of the ionic 3.1 Chloromethylation of toluene liquid on the catalytic performance was also investi- gated (entries 4, 10-14). The conversion of toluene Blanc reaction of toluene with ionic liquid as increased rapidly from 0 to 1/100 [n(IL)/n(substrate)], catalyst has been investigated, as shown in Table 1. and the conversion would almost keep constant when The reactions in ionic liquids afforded excellent yields the dosage of ionic liquid was more than 2/100[n(IL)/ with higher selectivity in relatively shorter time. n(substrate)]. It is because N-alkyl-3-methyl imida- The reaction was found to proceed smoothly with zolium ionic liquid is also a kind of surfactant. When high conversion rate, and the product was confirmed the surfactant molecules inhabit all interface, super- by 1H NMR analysis where no decomposition or fluous surfactant molecules are useless. From the ef- by-products were observed. The effect of reaction fect of dosage of ionic liquid, the ionic liquid acted as temperature on the yield and selectivity of monochloro- a phase transfer catalyst. Table 1 Blanc reaction of toluene catalyzed by ionic liquids① IL/substrate Conversion② Selectivity/% Entry Ionic liquid (mol ratio) T/°C Time/h /% mono-chloro-product ortho-product para-product 1 [C12mim]Br 4/100 30 10 ＜1 0 0 0 2 [C12mim]Br 4/100 50 10 25 ＞99 49 51 3 [C12mim]Br 4/100 65 10 70 ＞99 53 47 4 [C12mim]Br 4/100 80 10 71 ＞99 50 50 5 [C12mim]Br 4/100 100 10 38 ＞99 51 49 6 [C12mim]Br 4/100 65 4 40 ＞99 55 45 7 [C12mim]Br 4/100 65 6 47 ＞99 48 52 8 [C12mim]Br 4/100 65 8 54 ＞99 54 46 9 [C12mim]Br 4/100 65 12 90 ＞99 54 46 10 0/100 65 10 12 ＞99 54 46 11 [C12mim]Br 1/100 65 10 66 ＞99 54 46 12 [C12mim]Br 2/100 65 10 69 ＞99 51 49 13 [C12mim]Br 8/100 65 10 73 ＞99 52 48 14 [C12mim]Br 15/100 65 10 69 ＞99 50 50 15 [C2mim]Br 4/100 65 10 39 ＞99 50 50 16 [C3mim]Br 4/100 65 10 38 ＞99 51 49 17 [C4mim]Br 4/100 65 10 41 ＞99 52 48 18 [C5mim]Br 4/100 65 10 57 ＞99 52 48 19 [C6mim]Br 4/100 65 10 50 ＞99 55 45 20 [C7mim]Br 4/100 65 10 49 ＞99 49 51 21 [C8mim]Br 4/100 65 10 64 ＞99 55 45 ① Reaction condition: toluene 0.03 mol; paraformaldehyde 0.05 mol; hydrochloric acid 5 ml. ② Conversion was determined based on GC results. Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 359 Finally, other ionic liquids have been tried for the easily recycled and reused for subsequent reactions. reaction system (entries 4, 15-21). Among all investi- The recycling process involved the product layer was gated ionic liquids, [C12mim]Br demonstrated the best decanted and the ionic liquid layer was concentrated performance. It was also found that the longer length to remove water under vacuum. The reaction cata- of alkyl chain of ionic liquids (being more lipophilic) lyzed by the recycled [C12mim]Br ionic liquid was could affect favorably the conversion of toluene.