Molecular Sieve

Energy & Fuels 2009, 23, 51–54

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Benzylation of Anisole Catalyzed by MoCl5 or MoCl5/Molecular Sieve System Qiaoxia Guo,* Lianshan Li, Liwei Chen, Yanqing Wang, Shenyong Ren, and Baojian Shen* State Key Laboratory of HeaVy Oil Processing, Department of Applied Chemistry, Faculty of Chemical Science and Engineering, China UniVersity of Petroleum, Beijing 102249, China ReceiVed August 20, 2008. ReVised Manuscript ReceiVed NoVember 1, 2008

MoCl5 was found as a new catalyst for the benzylation of anisole. In comparison to the general Lewis acid, such as FeCl3, ZnCl2, and AlCl3, the reactivity of MoCl5 was higher than AlCl3 but lower than FeCl3 under the same reaction conditions. When MoCl5 was supported on HZSM-5 or TS-1 molecular sieve, the yield of this reaction was increased from 45% to 99% or 98%, and the selectivity of para/ortho was promoted from 1:1 to 1.9:1 or 2.2:1, respectively. A mechanism of anisole benzylation over MoCl5 was proposed, which was different from the traditional Friedel-Crafts alkylation mechanism.

1. Introduction Alkylation of aromatic compounds is one of the important reactions used in the chemical industry.1,2 For example, alkylanisole derivatives are valuable intermediates for the manufacture of pharmaceuticals, antioxidant, etc. It is usually obtained by Friedel-Crafts alkylation reactions. The reactions are usually catalyzed by strong mineral acids or Lewis acids (e.g., HF, H2SO4, AlCl3,3,4 FeCl3,5 NbPO4,6,7 Ga(NO3)3,8 and [Mo(CO)4Br2]29) to give complex reaction mixtures. Polyalkylation, isomerization, transalkylation, dealkylation, and polymerization all occur under the normal reaction conditions.3 Alkyl groups normally activate the aromatic ring toward eletrophilic substitution reactions. The formation of polyalkylated products is always observed in the Friedel-Crafts alkylation, leading to decreased yields, laborious separation procedures, and energy-consuming processes. In addition, disposal of the reaction residue that was generated by usual workup will cause additional environmental problems. Thus, an increase the selectivity is attractive for the * To whom correspondence should be addressed. Telephone and Fax: +86-10-89733369. E-mail: [email protected]. (1) Franck, H. G.; Stadelhofer, J. W. Industrial Aromatic Chemistry; Springer: Berlin, Germany, 1988. (2) Perego, C.; Ingallina, P. Recent advances in the industrial alkylation of aromatics: New catalysts and new processes. Catal. Today 2002, 73, 3–22. (3) Olah, G. A. Friedel-Crafts Chemistry; Wiley: New York, 1973. (4) Du, W.-H.; An, Zh.-W.; Xu, M.-L.; Li, Zh.-B.; Ma, D.-H.; Wang, F.-Sh.; Kong, F.-L. Regio- and stereoselective reaction of 1-acyl-4chlorocyclohexane with substituted aromatics. Chin. J. Synth. Chem. 1997, 5 (2), 205–208. (5) Commandeur, R.; Berger, N.; Jay, P.; Kervenal, J. Synthesis of dielectric liquids by improved Friedel-Crafts condensation. U.S. Patent 5,186,864, 1993. (6) Pereira, C. C. M.; Lachter, E. R. Alkylation of toluene and anisole with 1-octen-3-ol over niobium catalysts. Appl. Catal., A 2004, 226, 67– 72. (7) de la Cruz, M. H. C.; da Silva, J. F. C.; Lachter, E. R. Catalytic activity of niobium phosphate in the Friedel-Crafts reaction of anisole with alcohols. Catal. Today 2006, 118, 379–384. (8) Berrichi, Z. E.; Louis, B.; Tessonnier, J. P.; Ersen, O.; Cherif, L.; Ledoux, M. J.; Pham-Huu, C. One-pot synthesis of Ga-SBA-15: Activity comparison with Ga-post-treated SBA-15 catalysts. Appl. Catal., A 2007, 316, 219–225. (9) Malkov, A. V.; Davis, S. L.; Mitchell, W. L.; Kocˇovsky´, P. Molybdenum(II)-catalyzed alkylation of electron-rich aromatics with allylic acetates. Tetrahedron Lett. 1997, 38 (27), 4899–4902.

chemist. To avoid these problems, many efforts have been devoted to the search of solid acid and other catalysts (e.g., zeolites,10-13 sulfated zirconia,14 clays exchanged by metallic ions,15-17 acid-treated clay18 and graphite,19 supercritical CO2,20 heteropolyacid,21 and phosphotungstic acid [H3PW12O40]22). Previous studies revealed that the major factor influencing the activity and selectivity is the acidity and channel size of the solid acid catalysts. (10) Coq, B.; Gourves, V.; Figueras, F. Benzylation of toluene by benzyl chloride over protonic zeolites. Appl. Catal., A 1993, 100, 69–75. (11) Aguilar, J.; Melo, F. V.; Sastre, E. Alkylation of biphenyl with methanol over Y zeolites. Appl. Catal., A 1998, 175, 181–189. (12) Corma, A.; Martıńez-Soria, V.; Schnoeveld, E. Alkylation of benzene with short-chain olefins over MCM-22 zeolite: Catalytic behaviour and kinetic mechanism. J. Catal. 2000, 192, 163–173. (13) Vinu, A.; Sawant, D. P.; Ariga, K.; Hartmann, M.; Halligudi, S. B. Benzylation of benzene and other aromatics by benzyl chloride over mesoporous AlSBA-15 catalysts. Microporous Mesoporous Mater. 2005, 80, 195–203. (14) Clark, J. H.; Monks, G. L.; Nightingale, D. J.; Price, P. M.; White, J. F. A new solid acid-based route to linear alkylbenzenes. J. Catal. 2000, 193, 348–350. (15) Brown, C. M.; Barlow, S. J.; McQuarrie, D. J.; Clark, J. H.; Kybett, A. P. Catalysts comprising metal compounds supported on a clay or hydrous silicate and their use. European Patent 0 352 878 A1, 1990. (16) Cseri, T.; Be´ka´ssy, S.; Figueras, F.; Rizner, S. Benzylation of aromatics on ion-exchanged clays. J. Mol. Catal. A: Chem. 1995, 98, 101– 107. (17) Cativiela, C.; Garcıá, J. I.; Garcıá-Matres, M.; Mayoral, J. A.; Figueras, F.; Fraile, J. M.; Cseri, T.; Chiche, B. Clay-catalyzed FriedelCrafts alkylation of anisole with dienes. Appl. Catal., A 1995, 123, 273– 287. (18) Chitnis, S. R.; Sharma, M. M. Alkylation of aniline with R-methylstyrene and separation of close boiling aromatic amines through reaction with R-methylstyrene, using acid-treated clay catalysts. React. Funct. Polym. 1997, 33, 1–12. (19) Sereda, G. A. Alkylation on graphite in the absence of Lewis acids. Tetrahedron Lett. 2004, 45, 7265–7267. (20) Chateauneuf, J. E.; Nie, K. An investigation of a Friedel-Crafts alkylation reaction in homogeneous supercritical CO2 and under subcritical and splitphase reaction conditions. AdV. EnViron. Res. 2000, 4, 307–312. (21) Okumura, K.; Yamashita, K.; Hirano, M.; Niwa, M. The active and reusable catalysts in the benzylation of anisole derived from a heteropoly acid. J. Catal. 2005, 234, 300–307. (22) Castro, C.; Corma, A.; Primo, J. On the acylation reactions of anisole using R,β-unsaturated organic acids as acylating agents and solid acids as catalysts: A mechanistic overview. J. Mol. Catal. A: Chem. 2002, 177, 273–280.

10.1021/ef800680p CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

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In this paper, the synergetic effects of the MoCl5 and molecular sieve (HZSM-5 and TS-1), and the Lewis acidity of FeCl3, ZnCl2, or AlCl3 are investigated by using the alkylation reaction of benzene derivatives. The influence to product yields and selectivities were discussed, which have no report previously. A mechanism of anisole alkylation over MoCl5 and molybdenum-modified molecular sieve was proposed.

Guo et al. Table 1. Benzylation of Anisole Using Different Alkylation Reagents conditions

benzyl chloride benzyl alcohol benzyl bromide

room temperature, 1 h room temperature, 3 h 40 °C, 3 h

a

3. Results and Discussion 3.1. Effects of Alkylation Reagents. First of all, a widely available alkylation reagent, benzyl chloride, was employed to react with anisole. In the reaction, 0.16 mL (1.5 mmol) of anisole was added to the mixture of 0.4119 g (1.5 mmol) of MoCl5 and 15 mL of hexane (solvent) at room temperature, and then 0.17 mL (1.5 mmol) of benzyl chloride was added; after usual workup, it provided monobenzyl anisole in 45% GC combined yield. The isolated yield was 23%. The radio of para/ortho was 1:1 (see eq 1).

As shown in Table 1, when benzyl alcohol was used as an alkylation reagent instead of benzyl chloride, the GC yield of monobenzyl anisole was sharply decreased to 11% (isolated yield was 7.5%). This result indicated that benzyl alcohol is a poor alkylation reagent for this reaction. To our surprise, when benzyl bromide was employed instead of benzyl alcohol, the yield of monobenzyl anisole was also very low (GC yield was

45 (23) 11 (7.5) 12

GC yields (isolated yields are in the parentheses).

Table 2. Benzylation of Different Substrates with Benzyl Chloride

2. Experimental Section 2.1. Preparation of Catalysts. All reactions were carried out under nitrogen. Metal chlorides were stored and handled under a nitrogen atmosphere. Molybdenum pentachloride (MoCl5) was supplied by the Alfa Aesar company and used as received. A total of 1 mmol of MoCl5 was dissolved into a CH2ClCH2Cl solution, and then it was impregnated to support on 1 g of HZSM-5 or TS-1 by the incipient wetness impregnation method, followed by evaporation to dryness under vacuum, because of the poor solubility of MoCl5 in CH2ClCH2Cl. Thus, the above procedure repeated many times until MoCl5 completely dissolved in CH2ClCH2Cl and loaded in the molecular sieves. 2.2. General Alkylation Reaction Procedures. To a suspension of MoCl5 (1 mmol) and 10 mL of hexane was added anisole (1 mmol) and benzyl chloride (1 mmol), and then the mixture was stirred at room temperature for 1 h. The reaction was quenched with 3 N HCl aqueous solution, and organic products were extracted with Et2O. Combined organic extracts were washed with H2O, NaHCO3 (20% aqueous solution), H2O, and brine and then dried over anhydrous Na2SO4. Filtration, evaporation, and column chromatography on silica gel with petroleum ether/ether (50:1) as an eluent afforded the desired compounds. The products were analyzed by gas chromatography (GC), GC-mass spectrometry (MS), and nuclear magnetic resonance (NMR). 2.3. Product Analysis. 1H (300 MHz) and 13C (100 MHz) NMR spectra were recorded for CDCl3 [containing 1% tetramethylsilane (TMS)] solution at 25 °C on a JNM-LA300 FT-NMR (JEOL Ltd.) spectrometer. GC analysis was performed on SHIMADZU GC14B equipped with a fused silica capillary column SHIMADZU CBP1-M25-025 and SHIMADZU C-R8A-Chromatopac integrator. GC analysis was carried out using dodecane as internal standards.

percent yielda

alkylation regent

substrate toluene anisole phenol benzoic acid a

percent yielda

conditions room room room room

temperature, temperature, temperature, temperature,

3h 1h 24 h 24 h

17 (10) 45 (23) no reaction no reaction


12%). The authors speculate that the formed bromide anion (Br-) may reacted with the catalyst MoCl5 in the system to form MoBrCl4 or other molybdenum halides of chloride and bromide, and these halide have a low reactivity in this reaction. 3.2. Effects of the Substituent of the Substrate. The alkylation reaction yields of benzyl chloride to the substrates with different substituents are listed in Table 2. It indicated that toluene and anisole were alkylated with benzyl chloride to give benzyl toluene and benzyl anisole (Table 2, runs 1 and 2). Whereas, it had no reaction when phenol and benzoic acid were used as the substrates. The alkylation reaction activity (yield) sequence (Table 2, runs 1, 2, and 4) of toluene, anisole, and benzoic acid is easy to understand according to a classical Friedel-Crafts-type acidcatalyzed benzene alkylation mechanism, where the benzylation of an aromatic compound is easier if the electron-donating groups are present in the aromatic ring and difficult if an electron-withdrawing group appears.3 Here, anisole (with an electron-donating group MeO-) showed the highest yield among the three, and benzoic acid have no reaction because of the electron-withdrawing group of -COOH. Kocˇovsky´ et al.9 also reported that increasing the electron density of the aromatic ring had a beneficial effect on the reactivity. Similar effects have also been reported in the niobium acid and niobium phosphate on the benzylation of anisole and toluene with benzyl alcohol.6,7,23 However, it is very surprise for the phenol case. No reaction was observed, although -OH is a known electron-donating group. This may be due to the oxyphilic molybdenum species, which had no catalytic reactivity. The same kind of reaction may also occur for the benzoic acid case. 3.3. Effects of Different Lewis Acids. The alkylation reaction of benzyl chloride with anisole was also used to study the catalytic activities of several conventional Lewis acid catalysts, such as ZnCl2, AlCl3, and FeCl3 (Table 3). It indicated that the reactivity of MoCl5 was higher than AlCl3 and ZnCl2 but lower than FeCl3 under the conditions used here. 3.4. Synergetic Effects of MoCl5 and Molecular Sieve (HZSM-5 and TS-1). As the results showed in Tables 1-3, all of the cases gave low yield and bad selectivity of the monobenzyl anisole. The attempt to resolve this issue has led to our recent research on the application of zeolite (HZSM-5) and titanium silicate molecular sieve (TS-1). HZSM-5 zeolite is a widely used solid acid catalyst and a well-known shape(23) Moraes, M.; Pinto, W. de S. F.; Gonzalez, W. A.; Carmo, L. M. P. M.; Pastura, N. M. R.; Lachter, E. R. Benzylation of toluene and anisole by benzyl alcohol catalyzed by niobic acid: Influence of pretreatment temperature in the catalytic activity of niobic acid. Appl. Catal., A 1996, 138, L7–L12.

Benzylation of Anisole Catalyzed by MoCl5

Energy & Fuels, Vol. 23, 2009 53

Table 3. Benzylation of Anisole with Benzyl Chloride Catalyzed by Different Lewis Acids catalyst MoCl5 AlCl3 FeCl3 ZnCl2 a

percent yielda

conditions room room room room

temperature, temperature, temperature, temperature,

1 1 3 5

h h h h

45 (23) 35 58 14


catalyst MoCl5 MoCl5/HZSM-5 (mechanical mixed) MoCl5/HZSM-5 (impregnated) MoCl5/TS-1 (impregnated) a

Table 4. Benzylation of Anisole with Benzyl Chloride Catalyzed by the MoCl5/Molecular Sieve System catalyst

conditions

percent yielda

HZSM-5 MoCl5 MoCl5/HZSM-5 (mechanical mixed) MoCl5/HZSM-5 (impregnated) MoCl5/TS-1 (impregnated)

room temperature, 24 h room temperature, 1 h room temperature, 3 h

no reaction 45 (23) 100

1:1 1:1

room temperature, 5 h

99

1.9:1

room temperature, 24 h

98

2.2:1

a

Table 5. Benzylation of Anisole with Benzyl Bromide Catalyzed by the MoCl5/Molecular Sieve System

para/ortho ratio


selective catalyst. The TS-1 is also reported to be an efficient catalyst for the formation of the carbon-carbon bond in the Mukaiyama-type aldol reaction of silyl enol ethers and aldehydes.24 First, mechanical mixed MoCl5 and HZSM-5 was used as the catalyst. The yield of monoalkylated product was enhanced to 100%, but the selectivity was not improved. Then, impregnated MoCl5/HZSM-5 or MoCl5/TS-1 was used as catalysts. The combined product yields increased from 45 to 99 and 98%, and the selectivity of para/ortho was promoted from 1:1 to 1.9:1 and 2.2:1, respectively (see Table 4). Experimental results indicated that HZSM-5 itself had no observed reactivity on this alkylation reaction. Because of the poor solubility of MoCl5 in hexane [CH3(CH2)4CH3], the alkylation reactions (as in Tables 1-3) were carried out under a suspension of MoCl5. The dispersion of MoCl5 is not good; thus, it influences the activity of MoCl5 and leads to the low yield of the benzylation product. When the MoCl5/molecular sieve system was used as the catalyst, the good reactivity of MoCl5 can then be interpreted. The adsorption of MoCl5 on the surface (outside and internal) of the molecular sieve resulted in a well dispersion of MoCl5, and the porous structure of the molecular sieve made it possible for the diffusion of reactants to MoCl5 sites; i.e., the exposure degree of catalytically active sites was increased. Another possible reason of the increased activity may be attributed to the joint-aid acidity of MoCl5 with ZSM-5 or TS-1. For the increasing para selectivity in the MoCl5/HZSM-5 or MoCl5/TS-1 catalyst system, it appears to be the shape-selective effect25-28 in the medium pore size ZSM-5 or TS-1. The pore dimensions of ZSM-5 are 0.51 × 0.55 and 0.53 × 0.56 nm, and the pore dimension of TS-1 is 0.56 × 0.54 nm. The diameter of the substrate is close to the pore size of HZSM-5 or TS-1. Thus, it had more space around the para position of the substrate (such as anisole) when it was adsorbed in the pore of ZSM-5 (24) Sasidharan, M.; Raju, S. V. N.; Srinivasan, K. V.; Paul, V.; Kumar, R. Titanium silicate molecular sieve, TS-1, catalysed C-C bond formation in Mukaiyama type aldol reactions. Chem. Commun. 1996, 129–130. (25) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape SelectiVe Catalysis in Industrial Applications; Marcel Dekker: New York, 1989. (26) Csiczery, S. M. Catalysis by shape selective zeolitessScience and technology. Pure Appl. Chem. 1986, 58, 841–856. (27) Jacobs, P. A.; Martens, J. A. Introduction to acid catalysis with zeolites in hydrocarbon reactions. Stud. Surf. Sci. Catal. 1991, 58, 445– 496. (28) Weitkamp, J. New directions in zeolite catalysis. Stud. Surf. Sci. Catal. 1991, 65, 21–46.

conditions 40 40 40 40

°C, °C, °C, °C,

3h 4h 21 h 4h

percent yielda

para/ortho ratio

12 68 71 43

1.3:1 2.2:1 2.2:1 1.9:1

GC yields.

or TS-1, and thus, the alkylation reagent would have an opportunity to attack the para position of anisole. The ortho position is too close to the wall of the molecular sieve to take the reaction, i.e., did not permit the formation of the ortho isomer, whose critical diameters are significantly larger. Moreover, molecules such as para-dialkylbenzenes diffuse much more rapidly in a medium pore system of ZSM-5 or TS-1 than their ortho or meta isomers. The product was easy to remove from the catalyst. For example, Olson et al.29 reported, in 1981, that the diffusion rate of para-xylene in ZSM-5 is more than 103 times greater than ortho-xylene and meta-xylene. As a result, the selectivity of para/ortho was promoted from ca. 1:1 to ca. 2:1. As for the reason of why it still showed so much orthoposition product, the authors think that the MoCl5 adsorbed on the outside surface of the molecular sieve have more freedom space than that in the pore, while the outside MoCl5 showed higher reactivity. No meta-position product formation was observed. The alkylation of anisole by benzyl bromide were further investigated by using the MoCl5/HZSM-5 or MoCl5/TS-1 catalyst system. The results are shown in Table 5. An obvious increase in the yields and para/ortho selectivities compared to the MoCl5 catalyst system was observed, although the yields of the MoCl5/molecular sieve system were not so high as the anisole/benzyl chloride case. These results indicated that MoCl5 and molecular sieve (HZSM-5 and TS-1) showed synergetic effects in the alkylation reaction. 3.5. Reaction Mechanism. Electrophilic alkylation of aromatics catalyzed by Lewis acid is commonly considered as proceeding via a carbenium-ion-type mechanism. Lachter et al.6 indicated that the Bro¨nsted acid sites (NbOH or POH) in the niobium phosphate are capable of generating a carbenium ion intermediate (allylic cation) from allylic alcohol under the conditions employed, which could readily undergo electrophilic substitution with the aromatics. The initial carbenium ion intermediate could possibly be generated by protonation of the alcohol under appropriate reaction conditions. Another possibility is the coordination of the allylic alcohol with the Lewis acid sites (coordinatively unsaturated Nb5+ sites), which makes possible the electrophilic substitution of the aromatic compounds. Choudhary and Jana30 reported a redox mechanism in that the metal chloride species present in the catalysts have redox properties, which are expected to play important roles in the benzylation reaction over InCl3, GaCl3, FeCl3, and ZnCl2 supported on commercial clays or on high silica mesoporous MCM-41 catalysts. To investigate the reaction mechanism of the present system, the reactant-adding sequence was studied. In the first experiment, first, anisole was added in the mixture of hexane and molyb(29) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. H. Crystal structure and structure-related properties of ZSM-5. J. Phys. Chem. 1981, 85, 2238–2243. (30) Choudhary, V. R.; Jana, S. K. Benzylation of benzene and substituted benzenes by benzyl chloride over InCl3, GaCl3, FeCl3 and ZnCl2 supported on clays and Si-MCM-41. J. Mol. Catal. A: Chem. 2002, 180, 267–276.

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Scheme 1. Proposed Reaction Mechanism for Benzylation of Anisole with MoCl5

Guo et al. Table 6. Influence of the Catalyst Amount of MoCl5 anisole/benzyl chloride/MoCl5

conditions

percent yielda

1:1:1 1:1:0.1

room temperature, 1 h room temperature, 7 h

45 (23) 40

a

denum(V) chloride and then benzyl chloride was added. The reaction takes place smoothly. However, in the second experiment, when benzyl chloride was added in the mixture of hexane and molybdenum(V) chloride first, then anisole was added. No product was observed even it stayed for 24 h. This means that the reaction followed a different mechanism between molybdenum(V) chloride and that of traditional Lewis acid. On the basis of the above result, we proposed the reaction mechanism as follows (Scheme 1): Because of the oxyphilic property and big size of molybdenum, first, the π electron of anisole coordinate to MoCl5 may give intermediate 1, then HCl was eliminated to give molybdenum tetrachloride complex 2, and then the complex reacted with benzyl chloride to give the benzylation product and regenerated molybdenum(V) chloride. This indicated that the reaction was a catalytic reaction. The evolution of HCl was observed by Waldvogel for the MoCl5-mediated oxidative (31) Waldvogel, S. R. The reaction pattern of the MoCl5-mediated oxidative aryl-aryl coupling. Synlett 2002, 4, 622–624.


aryl-aryl coupling reaction.31 In the present work, complex 2 was not obtained, but similar complexes were reported by Anorgan in the reaction of benzene or naphthalene with TaCl5 or NbCl5.32 Table 6 listed the reaction results of anisole with benzyl chloride under the different amounts of MoCl5. These results support that it is a catalytic reaction by MoCl5. 4. Conclusions Molybdenum pentachloride can catalyze the benzylation of anisole with benzyl chloride under extremely mild conditions. A moderate yield and a selectivity of a 1:1 para/ortho ratio were obtained. When MoCl5/HZSM-5 or MoCl5/TS-1 was used as the catalyst instead of MoCl5, the yields of this reaction were increased from 45% to 99% and 98% and the selectivity of para/ ortho was promoted from 1:1 to 1.9:1 and 2.2:1, respectively. The mechanism study indicated that the reaction of anisole with benzyl chloride by molybdenum pentachloride was a catalytic reaction. It followed a different reaction mechanism from the traditional carbenium ion mechanism. EF800680P (32) Funk, H.; Niederla¨nder, K. Uber die einwirkung von niob- und tantalpentachlorid auf organische verbindungen(II). Ber. 1928, 61, 1385– 1388.

Molecular Sieve

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