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Benzylation of benzene by benzyl chloride over Fe-modified ZSM-5 and H-β zeolites and Fe2O3 or FeCl3 deposited on micro-, meso- and macro-porous supports
Microporous and Mesoporous Materials 56 (2002) 65–71 www.elsevier.com/locate/micromeso
Benzylation of benzene by benzyl chloride over Fe-modified ZSM-5 and H-b zeolites and Fe2O3 or FeCl3 deposited on micro-, meso- and macro-porous supports Vasant R. Choudhary *, Suman K. Jana, Ajit S. Mamman Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India Received 26 October 2001; received in revised form 20 May 2002; accepted 23 May 2002
Abstract A number of Fe-containing solid catalysts, such as Fe-modified ZSM-5 type zeolites (Fe2 O3 /H-ZSM-5, SO2 4 /Fe2 O3 / H-ZSM-5, H-FeMFI and H-FeAlMFI), Fe-modified H-b zeolites (Fe2 O3 /H-b and SO2 4 /Fe2 O3 /H-b), Fe2 O3 supported on meso-porous Si-MCM-41, silica gel or macro-porous silica–alumina commercial catalyst carrier (SA-5205), and FeCl3 impregnated on 13X zeolite, Si-MCM-41, silica gel or commercial clays––montmorillonite K10 (Mont K10) or kaolin, have been compared for their performance in the benzylation of benzene by benzyl chloride (80 C). Among these catalysts, the Fe2 O3 /H-b (or H-ZSM-5) and FeCl3 /Mont K10 (or Si-MCM-41) are found to be highly promising ones for the benzylation, even in the presence of moisture. These catalysts can also be reused in the reaction but with reduced activity. No direct relationship is observed between the acidity (measured in terms of ammonia chemisorbed at 50 or 200 C) and the benzylation activity of the catalysts. The benzylation activity is controlled mainly by the redox properties of the catalyst. The selectivity for diphenyl methane in the benzylation was found to vary from catalyst to catalyst. 2002 Elsevier Science Inc. All rights reserved. Keywords: Benzylation of benzene; Fe-containing solid catalysts
1. Introduction Liquid phase Friedel–Crafts type reactions (e.g. benzylation of aromatic compounds), using homogeneous acid catalysts are practiced very commonly in the organic synthesis [1]. The most commonly used acid catalyst for these reactions is
* Corresponding author. Tel.: +91-20-589-3041/0765; fax: +91-20-589-3041/3355. E-mail addresses: [email protected], [email protected] (V.R. Choudhary).
anhydrous AlCl3 . This catalyst, however, poses several problems, such as its use in more than the stoichiometric amounts, difficulty in its separation and recovery; disposal of spent catalyst, corrosion, high toxicity and high moisture sensitivity (which demands moisture-free reactants and solvent and moisture-free atmosphere for handling the catalyst). It is, therefore, of great practical importance to replace AlCl3 and other similar homogeneous catalysts by reusable and easily separable catalysts (such as heterogeneous solid catalysts), having high activity for Friedel–Crafts type reactions and little or no moisture sensitivity. Earlier studies
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V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
indicated that highly acidic solid catalysts, such as HY [2], H-ZSM-5 [2,3] and sulfated ZrO2 [4] show poor activity in the benzylation of benzene and other aromatic compounds. Clark et al. [5] reported high activity of thermally activated clayzic catalyst (ZnCl2 supported on montmorillonite K10 (Mont K10)) for the benzylation of aromatic compounds. However, this clayzic catalyst showed low selectivity (80%) for the monoalkylated product [5] and moreover it is sensitive to moisture [6]. Recently, the FeCl3 /Mont K10 [7], sulfated Fe2 O3 –ZrO2 [4], Fe/MCM-41 [8,9] and H-FeMFI zeolite [3] catalysts are reported to show high activity in the benzylation of benzene. However, we have observed that Fe-containing solid catalysts also show high activity for the self-condensation of benzyl chloride [10]. The present investigation was therefore undertaken to study a large number of Fe-containing micro-, meso- and macro-porous solid catalysts for their activity and selectivity in the benzylation of benzene by benzyl chloride in the presence or absence of moisture in the reaction mixture.
2. Experimental 2.1. Fe-containing micro-, meso- and macro-porous solid catalysts The ZSM-5 type zeolite catalysts: H-ZSM-5 (Si=Al ¼ 30), H-FeMFI (Si=Fe ¼ 16:5) and HFeAlMFI (Si=Fe ¼ 28:1 and Si=Al ¼ 26:2) were prepared by the procedure similar to that described earlier [3]. The Fe2 O3 (5 wt.%)/H-ZSM-5 (Si=Al ¼ 30:0), Fe2 O3 (5 wt.%)/H-b (Si=Al ¼ 17:0), Fe2 O3 (10 wt.%)/Si-MCM-41, Fe2 O3 (10 wt.%)/silica gel and Fe2 O3 (10 wt.%)/SA-5205 catalysts were prepared by impregnating ferric nitrate from its aqueous solution on H-ZSM-5, H-b, SiMCM-41 (high silica meso-porous MCM-41, surface area ¼ 1180 m2 g1 ), silica gel (Fuji Division, surface area ¼ 280 m2 g1 ) or low surface area macro-porous silica–alumina commercial catalyst carrier (SA-5205 having chemical composition: 11.8% SiO2 and 86.5% Al2 O3 , surface area < 0:01 m2 g1 , pore volume ¼ 0:35 cm3 g1 and average pore diameter 200 lm, obtained from M/s
NORTON Co., USA) by incipient wetness technique, drying the impregnated mass in an air oven at 100 C for 7 h and then calcining in air at 450 C for 4 h. The sulfated Fe2 O3 /H-ZSM-5 and Fe2 O3 / H-b (with a SO2 4 loading of 6 wt.%) catalysts were prepared by impregnating ammonium sulfate from its aqueous solution on Fe2 O3 (5 wt.%)/H-ZSM-5 and Fe2 O3 (5 wt.%)/H-b by incipient wetness technique, then drying the impregnated mass in an air oven at 100 C for 7 h and then calcining in air at 450 C for 4 h. The FeCl3 /13X, FeCl3 /Si-MCM41, FeCl3 /silica gel, FeCl3 /kaolin and FeCl3 /Mont K10 (all with the FeCl3 loading of 10 wt.%) catalysts were prepared by impregnating anhydrous ferric chloride from its acetonitrile solution on 13X (Alltech), Si-MCM-41, silica gel (Fuji), kaolin (Aldrich) and Mont K10 (Aldrich) respectively, by incipient wetness technique and evaporating the solvent in vacuum oven at 80 C for 12 h. The Fe-containing catalysts and catalyst supports were characterized for their acidity measured in terms of the ammonia chemisorbed at 50 and 200 C. The chemisorption of ammonia at a particular temperature in the temperature study is defined as the amount of ammonia retained on the catalyst at that temperature when the catalyst (0.3 g) pre-saturated with ammonia was swept with a pure inert gas (moisture-free N2 ) at a flow rate of 20 cm3 min1 for a period of 0.5 h. 2.2. Catalytic reaction Benzylation of benzene by benzyl chloride over the different Fe-containing micro-, meso- and macro-porous catalysts was carried out in a magnetically stirred glass reactor (capacity: 25 cm3 ) fitted with a reflux condenser, having a low dead volume, mercury thermometer and arrangement for continuously bubbling moisture-free N2 (30 cm3 min1 ) through the liquid reaction mixture. The reaction was started by injecting benzyl chloride in the reaction mixture containing benzene and catalyst at the reaction temperature (at 80 C). The course of the reaction was followed by measuring quantitatively the HCl evolved in the reaction by acid–base titration. The experimental procedure and product analysis are given earlier [13–15]. The poly benzyl chloride (which is formed
V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
by the condensation of benzyl chloride) was isolated from the reaction mixture by the procedure given elsewhere [14].
3. Results and discussion The Fe-containing catalysts (viz. H-FeMFI (ferro-silicate of ZSM-5 type), H-FeAlMFI (ferroalumino silicate of ZSM-5 type)), Fe2 O3 deposited
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on H-ZSM-5, H-b, Si-MCM-41 (meso-porous high silica MCM-41), silica gel or macro-porous silica–alumina catalyst carrier (SA-5205), sulfated Fe2 O3 deposited on H-ZSM-5 or H-b zeolite and anhydrous FeCl3 impregnated on 13X molecular sieve, silica gel, kaolin clay, Mont K10 clay or SiMCM-41 are compared for their performance in the benzylation of benzene at 80 C in Tables 1 and 2. The benzylation activity of the catalysts is expressed in terms of turnover rate (TOR) for the
Table 1 Results of the benzylation of benzene by benzyl chloride over different micro-, meso- and macro-porous Fe2 O3 -containing solid catalysts at 80 C (reaction mixture ¼ 13 ml dry benzene þ 1 ml benzyl chloride þ 0:1 g catalyst) Fe-containing catalyst
Types of pores (pore size)
Time (min) for benzyl chloride conversion 50%
H-ZSM-5 H-FeAlMFI H-FeMFI Fe2 O3 /H-ZSM-5 Fe2 O3 /H-ZSM-5a Sulfated-Fe2 O3 /HZSM-5 H-b Fe2 O3 /H-b Sulfated-Fe2 O3 /H-b Fe2 O3 /Si-MCM-41 Fe2 O3 /silica gel Fe2 O3 /SA-5205 Fe2 O3 /SA-5205a Fe2 O3 /Si-MCM-41a Fe2 O3 (unsupported)b Silica gel, Si-MCM41 or SA-5205 a b
Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.7 nm) Micro-pores (0.7 nm) Micro-pores (0.7 nm) Meso-pores (3.0 nm) Micro-pores (<1.0 nm) Macro-pores (ffi200 lm) Macro-pores (ffi200 lm) Meso-pores (3.0 nm) – –
Induction period (min)
90% No reaction upto 2 h
Selectivity (%)
TOR for half the reaction (mmol g1 s1 )
Diphenyl methane
Poly benzyl chloride
–
–
–
30.3
104.0
5.5
90.0
10.0
2.2
6.7
25.0
0.9
89.0
11.0
5.8
2.5
8.5
<0.1
85.0
15.0
8.3
3.1
9.3
0.7
84.6
15.4
6.7
4.7
15.0
0.4
69.9
30.1
4.4
–
–
–
5% conversion upto 2 h 3.3
11.5
0.2
89.0
11.0
6.3
4.0
12.4
1.1
68.5
31.5
5.2
4.4
12.3
0.5
84.0
16.0
2.4
4.9
13.5
0.8
83.0
17.0
2.1
3.8
11.5
0.3
81.0
19.0
2.4
5.1
13.3
1.5
80.0
20.0
2.0
5.3
13.4
1.5
83.3
16.7
2.0
16.1
29.8
7.3
79.5
20.5
0.6
–
–
–
Less than 1% conversion in 2 h
When benzene saturated with water was used as the substrate (concentration of water: 0.37 mol%). Amount of catalyst: 0.01g.
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Table 2 Results of the benzylation of benzene by benzyl chloride over supported FeCl3 (10.0 wt.%) catalysts at 80 C (reaction mixture ¼ 13 ml dry benzene þ 1 ml of benzyl chloride þ 0:1 g catalyst) Catalyst
FeCl3 /13X FeCl3 /silica gel FeCl3 /kaolin FeCl3 /Mont K10 FeCl3 /Mont K10a FeCl3 /Si-MCM-41 FeCl3 /Si-MCM-41a FeCl3 (unsupported)b Mont K10
Time (min) required for benzyl chloride conversion 50%
90%
37.1 4.2 3.8 1.8 2.7 2.2 3.3 2.8
46.8 13.0 9.6 6.5 7.5 7.1 8.4 7.8
Induction period (min)
32.6 0.2 0.9 0.1 1.1 0.3 1.4 0.8
Selectivity (%)
TOR for half the reaction (mmol g1 s1 )
Diphenyl methane
Poly benzyl chloride
90.1 91.4 90.6 91.0 90.0 92.0 91.1 89.0
9.9 8.6 9.4 9.0 10.0 8.0 8.9 11.0
0.6 5.0 5.5 11.7 7.8 9.6 6.4 7.5
4% conversion in 2 h
a
When benzene saturated with water (0.37 mol% water) was used as substrate. b Amount of catalyst: 0.01g.
half the reaction (i.e. for 50% conversion of benzyl chloride), as the millimoles of benzyl chloride converted per gram of iron per second. The results on the catalyst carriers or supports used in the preparation Fe-containing catalysts are also included in Tables 1 and 2. The catalysts are also compared for their acidity (measured in terms of the ammonia chemisorbed at two different temperatures (50 and 200 C) in Table 3. From the comparison (Tables 1–3), following important observations could be made: • The silica gel, Si-MCM-41, SA-5205, 13X and kaolin supports without Fe2 O3 (or FeCl3 ), show Table 3 Data on acidity of the catalysts Catalyst
Ammonia chemisorbed (mmol g1 ) 50 C
200 C
H-b Fe2 O3 (5%)/H-b H-ZSM-5 H-FeMFI H-FeAlMFI Fe2 O3 (5%)/H-ZSM-5 SO2 4 (6%)/Fe2 O3 (5%)/ H-ZSM-5 Mont K10 FeCl3 (10%)/Mont K10 FeCl3 (10%)/Si-MCM-41
2.15 2.09 0.84 0.78 0.70 0.81 1.73
1.0 0.84 0.36 0.29 0.31 0.32 0.96
0.41 1.23 0.94
0.3 – –
almost no catalytic activity in the benzene benzylation reaction. Even the highly acidic supports, such as H-ZSM-5, H-b and Mont K10, show little activity in the benzene benzylation reaction. However, a partial or complete substitution of Al by Fe in the H-ZSM-5 (which results in a decrease in the acidity) causes a drastic increase in the catalytic activity, the benzylation activity being much higher for a complete substitution of Al by Fe. • The TOR for all the Fe2 O3 containing catalysts (Table 1) is much higher than that for the Fe2 O3 without any support. However, in case of the supported FeCl3 catalysts (Table 2), the FeCl3 / Mont K10 and FeCl3 /Si-MCM-41 catalysts show higher activity but the other catalysts show lower activity than that of the unsupported FeCl3 . • Among the supported Fe2 O3 catalysts, the Fe2 O3 /H-ZSM-5 shows the highest activity with almost no reaction induction period. The order for the benzene benzylation activity of the different catalysts is as follows: Fe2 O3 =H-ZSM-5 > Fe2 O3 /H-b > sulfated Fe2 O3 /H-b > sulfated Fe2 O3 /H-ZSM-5 > Fe2 O3 /SA-5205 > Fe2 O3 /Si-MCM-41 > Fe2 O3 /silica gel. Because of its higher channel diameter, the Fe2 O3 supported on H-b zeolite as compared to that on H-ZSM-5 is more preferable for the benzylation reactions involving larger size molecules.
V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
• The sulfatation of Fe2 O3 (which causes an increase in the Lewis acidity [11]) supported on H-ZSM-5 or H-b zeolite has resulted in the decrease in both the activity and selectivity (for diphenyl methane) in the benzene benzylation. • Among the supported FeCl3 catalysts, FeCl3 / Mont K10 shows the highest activity with a negligibly small reaction induction period. The FeCl3 deposited on kaolin, Si-MCM-41 or silica gel also shows high benzene benzylation activity with a small reaction induction period. However, the FeCl3 /13X shows the lowest activity because of the very high induction period (32.6 min). • The use of moist benzene (the benzene saturated with water) instead of dry benzene has caused only a small decrease in the activity of the Fe2 O3 /H-ZSM-5, Fe2 O3 /SA-5205, Fe2 O3 / Si-MCM-41, FeCl3 /Mont K10 and FeCl3 /SiMCM-41 catalysts because of the small increase in the induction period. This shows that these catalysts have only a little moisture sensitivity for the benzylation. • Among the Fe-containing catalysts, the supported FeCl3 catalysts show higher selectivity for the diphenyl methane in the benzylation. • The H-ZSM-5 has higher acidity but very much lower benzylation activity than the H-FeMFI, H-FeAlMFI and Fe2 O3 /H-ZSM-5 catalysts. Similarly, the H-b zeolite also has higher acidity but much lower benzylation activity than that of the Fe2 O3 /H-b, H-FeMFI, Fe2 O3 /HZSM-5, FeCl3 /Mont K10 and FeCl3 /Si-MCM41. Sulfatation of the Fe2 O3 /H-ZSM-5 has resulted in a large increase in its acidity but a decrease in its benzylation activity. Thus, in general, there is no direct relationship between the acidity and benzylation activity of the catalysts. The observations lead to the conclusion that in the benzene benzylation over the Fe-containing catalysts, the acidity of the catalysts does not decide their catalytic activity. This is consistent with the fact that catalysts show only a very little moisture sensitivity in the benzene benzylation. The presence of moisture in the reaction mixture causes a small increase in the reaction induction period;
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after the induction period, the reaction proceeds with almost the same rate as that observed in the absence of moisture. Hence, when the highly active Fe-containing catalysts are used for the benzylation, there is no need to thoroughly dry aromatic substrate and/or solvent. This is of great practical importance. The H-ZSM-5, silica gel, Si-MCM-41, kaolin and Mont K10 supported Fe2 O3 or FeCl3 catalysts are more active in the benzylation than the Fe3þ exchanged Mont K10 catalyst reported elsewhere [12]. The Fe-containing catalysts are also more active but somewhat less selective in the benzene benzylation than the corresponding Gaor In-containing catalysts reported earlier by us [3,13,14]. The lower selectivity of the Fe-containing catalysts is attributed mostly to their higher activity for the condensation of benzyl chloride [9,15]. Results showing the reuse of the Fe2 O3 /H-b, FeCl3 /Si-MCM-41 and FeCl3 /Mont K10 catalysts (which are highly promising catalysts for the benzylation reaction) catalysts are presented in Fig. 1. The results reveal that these catalysts can be reused in the benzylation. However, there is an appreciable loss in the activity in the reuse of these catalysts. This is expected mostly because of the leaching of the active catalyst component (i.e. iron) during the benzylation reaction. Further work is necessary to strongly bind the active component on the support. As indicated earlier, the high activity of the Fe-containing catalysts in the benzene benzylation is not essentially controlled by their acidity. This is consistent with that observed earlier for the metal cation-exchanged Mont K10 [12]. It seems to be controlled mostly by their redox properties (Fe3þ þ e () Fe2þ ). A following redox mechanism is, therefore, suggested for the benzylation over the Fe-containing catalysts. C6 H5 CH2 Cl þ Fe3þ () C6 H5 CH2 Clþ þ Fe2þ ð1Þ C6 H5 CH2 Clþ () C6 H5 CHþ 2 þ Cl
ð2Þ
Fe2þ þ Cl () Fe3þ þ Cl
ð3Þ
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Fig. 1. Benzylation of benzene by benzyl chloride (at 80 C) over the fresh and reused Fe2 O3 /H-b, FeCl3 /Si-MCM-41 and FeCl3 /Mont K10 catalysts. þ C6 H5 CHþ 2 þ C6 H5 –H ! C6 H5 –CH2 –C6 H5 þ H
ð4Þ Hþ þ Cl ! HCl
ð5Þ
This mechanism is somewhat similar to that proposed by Cseri et al. [12] for the benzylation over the Fe3þ -exchanged Mont K10. 4. Conclusions Following important conclusions can be drawn from the present investigation on the benzylation of benzene by benzyl chloride over the different supported Fe2 O3 and FeCl3 catalysts: 1. The Fe2 O3 or FeCl3 supported on H-ZSM-5, Hb, macro-porous silica–alumina, meso-porous Si-MCM-41, silica gel or commercial clays (kaolin or Mont K10) is highly active catalyst for the benzylation of benzene. The most promising supported Fe2 O3 and FeCl3 catalysts are Fe2 O3 / H-ZSM-5 (or H-b) and FeCl3 /Mont K10.
2. The supported Fe2 O3 or FeCl3 catalyst shows high benzene benzylation activity even in the presence of moisture in the reaction mixture and hence does not demand moisture-free aromatic substrate and/or solvent to be used in the benzylation reaction. The supported Fe2 O3 or FeCl3 catalyst can be reused in the reaction but with reduced activity. 3. The activity of the Fe-containing catalysts for the benzene benzylation is not directly related to their acidity; it is controlled mainly by their redox properties.
Acknowledgements Suman K. Jana is grateful to CSIR, New Delhi, for awarding Senior Research Fellowship.
References [1] G.A. Olah, Friedel–Crafts and Related Reactions, Wiley– Interscience, New York, 1963.
V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71 [2] B. Cog, V. Gourves, F. Figueras, Appl. Catal. A: Gen. 100 (1993) 69. [3] V.R. Choudhary, S.K. Jana, B.P. Kiran, Catal. Lett. 59 (1999) 217. [4] S.N. Koyande, R.G. Jaiswal, R.V. Jayaram, Ind. Eng. Chem. Res. 37 (1998) 908. [5] J.H. Clark, A.P. Kybatt, D.J. Macquarrie, S.J. Barlow, P.J. Landon, J. Chem. Soc., Chem. Commun. (1989) 1353. [6] S.J. Barlow, T.W. Bastock, J.H. Clark, S.R. Cullen, J. Chem. Soc., Perkin Trans. (1994) 411. [7] S.G. Pai, A.R. Bajpai, A.B. Deshpande, S.D. Samant, Synth. Commun. 27 (1997) 2267. [8] J. Cao, N. He, C. Li, J. Dong, Q. Xu, Stud. Surf. Sci. Catal. 117 (1998) 461.
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[9] N. He, S. Bao, Q. Xu, Appl. Catal. A: Gen. 169 (1998) 29. [10] S.K. Jana, B.P. Kiran, V.R. Choudhary, in: V. Murugesan, B. Arabindoo, M. Palanichamy (Eds.), Recent Trends in Catalysis, Narosa Publishing House, New Delhi, 1999, p. 106. [11] K. Arata, Sekiyu Gakkaishi 39 (1996) 185. [12] T. Cseri, S. Bekassy, S. Figueras, S. Rizner, J. Mol. Catal. A: Chem. 98 (1995) 101. [13] V.R. Choudhary, S.K. Jana, B.P. Kiran, J. Catal. 192 (2000) 257. [14] V.R. Choudhary, S.K. Jana, B.P. Kiran, Catal. Lett. 64 (2000) 223. [15] V.R. Choudhary, S.K. Jana, M.K. Chaudhari, J. Mol. Catal. A: Chem. 170 (2001) 251.
Benzylation of benzene by benzyl chloride over Fe-modified ZSM-5 and H-b zeolites and Fe2O3 or FeCl3 deposited on micro-, meso- and macro-porous supports Vasant R. Choudhary *, Suman K. Jana, Ajit S. Mamman Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India Received 26 October 2001; received in revised form 20 May 2002; accepted 23 May 2002
Abstract A number of Fe-containing solid catalysts, such as Fe-modified ZSM-5 type zeolites (Fe2 O3 /H-ZSM-5, SO2 4 /Fe2 O3 / H-ZSM-5, H-FeMFI and H-FeAlMFI), Fe-modified H-b zeolites (Fe2 O3 /H-b and SO2 4 /Fe2 O3 /H-b), Fe2 O3 supported on meso-porous Si-MCM-41, silica gel or macro-porous silica–alumina commercial catalyst carrier (SA-5205), and FeCl3 impregnated on 13X zeolite, Si-MCM-41, silica gel or commercial clays––montmorillonite K10 (Mont K10) or kaolin, have been compared for their performance in the benzylation of benzene by benzyl chloride (80 C). Among these catalysts, the Fe2 O3 /H-b (or H-ZSM-5) and FeCl3 /Mont K10 (or Si-MCM-41) are found to be highly promising ones for the benzylation, even in the presence of moisture. These catalysts can also be reused in the reaction but with reduced activity. No direct relationship is observed between the acidity (measured in terms of ammonia chemisorbed at 50 or 200 C) and the benzylation activity of the catalysts. The benzylation activity is controlled mainly by the redox properties of the catalyst. The selectivity for diphenyl methane in the benzylation was found to vary from catalyst to catalyst. 2002 Elsevier Science Inc. All rights reserved. Keywords: Benzylation of benzene; Fe-containing solid catalysts
1. Introduction Liquid phase Friedel–Crafts type reactions (e.g. benzylation of aromatic compounds), using homogeneous acid catalysts are practiced very commonly in the organic synthesis [1]. The most commonly used acid catalyst for these reactions is
* Corresponding author. Tel.: +91-20-589-3041/0765; fax: +91-20-589-3041/3355. E-mail addresses: [email protected], [email protected] (V.R. Choudhary).
anhydrous AlCl3 . This catalyst, however, poses several problems, such as its use in more than the stoichiometric amounts, difficulty in its separation and recovery; disposal of spent catalyst, corrosion, high toxicity and high moisture sensitivity (which demands moisture-free reactants and solvent and moisture-free atmosphere for handling the catalyst). It is, therefore, of great practical importance to replace AlCl3 and other similar homogeneous catalysts by reusable and easily separable catalysts (such as heterogeneous solid catalysts), having high activity for Friedel–Crafts type reactions and little or no moisture sensitivity. Earlier studies
1387-1811/02/$ - see front matter 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 4 4 2 - 0
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V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
indicated that highly acidic solid catalysts, such as HY [2], H-ZSM-5 [2,3] and sulfated ZrO2 [4] show poor activity in the benzylation of benzene and other aromatic compounds. Clark et al. [5] reported high activity of thermally activated clayzic catalyst (ZnCl2 supported on montmorillonite K10 (Mont K10)) for the benzylation of aromatic compounds. However, this clayzic catalyst showed low selectivity (80%) for the monoalkylated product [5] and moreover it is sensitive to moisture [6]. Recently, the FeCl3 /Mont K10 [7], sulfated Fe2 O3 –ZrO2 [4], Fe/MCM-41 [8,9] and H-FeMFI zeolite [3] catalysts are reported to show high activity in the benzylation of benzene. However, we have observed that Fe-containing solid catalysts also show high activity for the self-condensation of benzyl chloride [10]. The present investigation was therefore undertaken to study a large number of Fe-containing micro-, meso- and macro-porous solid catalysts for their activity and selectivity in the benzylation of benzene by benzyl chloride in the presence or absence of moisture in the reaction mixture.
2. Experimental 2.1. Fe-containing micro-, meso- and macro-porous solid catalysts The ZSM-5 type zeolite catalysts: H-ZSM-5 (Si=Al ¼ 30), H-FeMFI (Si=Fe ¼ 16:5) and HFeAlMFI (Si=Fe ¼ 28:1 and Si=Al ¼ 26:2) were prepared by the procedure similar to that described earlier [3]. The Fe2 O3 (5 wt.%)/H-ZSM-5 (Si=Al ¼ 30:0), Fe2 O3 (5 wt.%)/H-b (Si=Al ¼ 17:0), Fe2 O3 (10 wt.%)/Si-MCM-41, Fe2 O3 (10 wt.%)/silica gel and Fe2 O3 (10 wt.%)/SA-5205 catalysts were prepared by impregnating ferric nitrate from its aqueous solution on H-ZSM-5, H-b, SiMCM-41 (high silica meso-porous MCM-41, surface area ¼ 1180 m2 g1 ), silica gel (Fuji Division, surface area ¼ 280 m2 g1 ) or low surface area macro-porous silica–alumina commercial catalyst carrier (SA-5205 having chemical composition: 11.8% SiO2 and 86.5% Al2 O3 , surface area < 0:01 m2 g1 , pore volume ¼ 0:35 cm3 g1 and average pore diameter 200 lm, obtained from M/s
NORTON Co., USA) by incipient wetness technique, drying the impregnated mass in an air oven at 100 C for 7 h and then calcining in air at 450 C for 4 h. The sulfated Fe2 O3 /H-ZSM-5 and Fe2 O3 / H-b (with a SO2 4 loading of 6 wt.%) catalysts were prepared by impregnating ammonium sulfate from its aqueous solution on Fe2 O3 (5 wt.%)/H-ZSM-5 and Fe2 O3 (5 wt.%)/H-b by incipient wetness technique, then drying the impregnated mass in an air oven at 100 C for 7 h and then calcining in air at 450 C for 4 h. The FeCl3 /13X, FeCl3 /Si-MCM41, FeCl3 /silica gel, FeCl3 /kaolin and FeCl3 /Mont K10 (all with the FeCl3 loading of 10 wt.%) catalysts were prepared by impregnating anhydrous ferric chloride from its acetonitrile solution on 13X (Alltech), Si-MCM-41, silica gel (Fuji), kaolin (Aldrich) and Mont K10 (Aldrich) respectively, by incipient wetness technique and evaporating the solvent in vacuum oven at 80 C for 12 h. The Fe-containing catalysts and catalyst supports were characterized for their acidity measured in terms of the ammonia chemisorbed at 50 and 200 C. The chemisorption of ammonia at a particular temperature in the temperature study is defined as the amount of ammonia retained on the catalyst at that temperature when the catalyst (0.3 g) pre-saturated with ammonia was swept with a pure inert gas (moisture-free N2 ) at a flow rate of 20 cm3 min1 for a period of 0.5 h. 2.2. Catalytic reaction Benzylation of benzene by benzyl chloride over the different Fe-containing micro-, meso- and macro-porous catalysts was carried out in a magnetically stirred glass reactor (capacity: 25 cm3 ) fitted with a reflux condenser, having a low dead volume, mercury thermometer and arrangement for continuously bubbling moisture-free N2 (30 cm3 min1 ) through the liquid reaction mixture. The reaction was started by injecting benzyl chloride in the reaction mixture containing benzene and catalyst at the reaction temperature (at 80 C). The course of the reaction was followed by measuring quantitatively the HCl evolved in the reaction by acid–base titration. The experimental procedure and product analysis are given earlier [13–15]. The poly benzyl chloride (which is formed
V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
by the condensation of benzyl chloride) was isolated from the reaction mixture by the procedure given elsewhere [14].
3. Results and discussion The Fe-containing catalysts (viz. H-FeMFI (ferro-silicate of ZSM-5 type), H-FeAlMFI (ferroalumino silicate of ZSM-5 type)), Fe2 O3 deposited
67
on H-ZSM-5, H-b, Si-MCM-41 (meso-porous high silica MCM-41), silica gel or macro-porous silica–alumina catalyst carrier (SA-5205), sulfated Fe2 O3 deposited on H-ZSM-5 or H-b zeolite and anhydrous FeCl3 impregnated on 13X molecular sieve, silica gel, kaolin clay, Mont K10 clay or SiMCM-41 are compared for their performance in the benzylation of benzene at 80 C in Tables 1 and 2. The benzylation activity of the catalysts is expressed in terms of turnover rate (TOR) for the
Table 1 Results of the benzylation of benzene by benzyl chloride over different micro-, meso- and macro-porous Fe2 O3 -containing solid catalysts at 80 C (reaction mixture ¼ 13 ml dry benzene þ 1 ml benzyl chloride þ 0:1 g catalyst) Fe-containing catalyst
Types of pores (pore size)
Time (min) for benzyl chloride conversion 50%
H-ZSM-5 H-FeAlMFI H-FeMFI Fe2 O3 /H-ZSM-5 Fe2 O3 /H-ZSM-5a Sulfated-Fe2 O3 /HZSM-5 H-b Fe2 O3 /H-b Sulfated-Fe2 O3 /H-b Fe2 O3 /Si-MCM-41 Fe2 O3 /silica gel Fe2 O3 /SA-5205 Fe2 O3 /SA-5205a Fe2 O3 /Si-MCM-41a Fe2 O3 (unsupported)b Silica gel, Si-MCM41 or SA-5205 a b
Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.5–0.6 nm) Micro-pores (0.7 nm) Micro-pores (0.7 nm) Micro-pores (0.7 nm) Meso-pores (3.0 nm) Micro-pores (<1.0 nm) Macro-pores (ffi200 lm) Macro-pores (ffi200 lm) Meso-pores (3.0 nm) – –
Induction period (min)
90% No reaction upto 2 h
Selectivity (%)
TOR for half the reaction (mmol g1 s1 )
Diphenyl methane
Poly benzyl chloride
–
–
–
30.3
104.0
5.5
90.0
10.0
2.2
6.7
25.0
0.9
89.0
11.0
5.8
2.5
8.5
<0.1
85.0
15.0
8.3
3.1
9.3
0.7
84.6
15.4
6.7
4.7
15.0
0.4
69.9
30.1
4.4
–
–
–
5% conversion upto 2 h 3.3
11.5
0.2
89.0
11.0
6.3
4.0
12.4
1.1
68.5
31.5
5.2
4.4
12.3
0.5
84.0
16.0
2.4
4.9
13.5
0.8
83.0
17.0
2.1
3.8
11.5
0.3
81.0
19.0
2.4
5.1
13.3
1.5
80.0
20.0
2.0
5.3
13.4
1.5
83.3
16.7
2.0
16.1
29.8
7.3
79.5
20.5
0.6
–
–
–
Less than 1% conversion in 2 h
When benzene saturated with water was used as the substrate (concentration of water: 0.37 mol%). Amount of catalyst: 0.01g.
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Table 2 Results of the benzylation of benzene by benzyl chloride over supported FeCl3 (10.0 wt.%) catalysts at 80 C (reaction mixture ¼ 13 ml dry benzene þ 1 ml of benzyl chloride þ 0:1 g catalyst) Catalyst
FeCl3 /13X FeCl3 /silica gel FeCl3 /kaolin FeCl3 /Mont K10 FeCl3 /Mont K10a FeCl3 /Si-MCM-41 FeCl3 /Si-MCM-41a FeCl3 (unsupported)b Mont K10
Time (min) required for benzyl chloride conversion 50%
90%
37.1 4.2 3.8 1.8 2.7 2.2 3.3 2.8
46.8 13.0 9.6 6.5 7.5 7.1 8.4 7.8
Induction period (min)
32.6 0.2 0.9 0.1 1.1 0.3 1.4 0.8
Selectivity (%)
TOR for half the reaction (mmol g1 s1 )
Diphenyl methane
Poly benzyl chloride
90.1 91.4 90.6 91.0 90.0 92.0 91.1 89.0
9.9 8.6 9.4 9.0 10.0 8.0 8.9 11.0
0.6 5.0 5.5 11.7 7.8 9.6 6.4 7.5
4% conversion in 2 h
a
When benzene saturated with water (0.37 mol% water) was used as substrate. b Amount of catalyst: 0.01g.
half the reaction (i.e. for 50% conversion of benzyl chloride), as the millimoles of benzyl chloride converted per gram of iron per second. The results on the catalyst carriers or supports used in the preparation Fe-containing catalysts are also included in Tables 1 and 2. The catalysts are also compared for their acidity (measured in terms of the ammonia chemisorbed at two different temperatures (50 and 200 C) in Table 3. From the comparison (Tables 1–3), following important observations could be made: • The silica gel, Si-MCM-41, SA-5205, 13X and kaolin supports without Fe2 O3 (or FeCl3 ), show Table 3 Data on acidity of the catalysts Catalyst
Ammonia chemisorbed (mmol g1 ) 50 C
200 C
H-b Fe2 O3 (5%)/H-b H-ZSM-5 H-FeMFI H-FeAlMFI Fe2 O3 (5%)/H-ZSM-5 SO2 4 (6%)/Fe2 O3 (5%)/ H-ZSM-5 Mont K10 FeCl3 (10%)/Mont K10 FeCl3 (10%)/Si-MCM-41
2.15 2.09 0.84 0.78 0.70 0.81 1.73
1.0 0.84 0.36 0.29 0.31 0.32 0.96
0.41 1.23 0.94
0.3 – –
almost no catalytic activity in the benzene benzylation reaction. Even the highly acidic supports, such as H-ZSM-5, H-b and Mont K10, show little activity in the benzene benzylation reaction. However, a partial or complete substitution of Al by Fe in the H-ZSM-5 (which results in a decrease in the acidity) causes a drastic increase in the catalytic activity, the benzylation activity being much higher for a complete substitution of Al by Fe. • The TOR for all the Fe2 O3 containing catalysts (Table 1) is much higher than that for the Fe2 O3 without any support. However, in case of the supported FeCl3 catalysts (Table 2), the FeCl3 / Mont K10 and FeCl3 /Si-MCM-41 catalysts show higher activity but the other catalysts show lower activity than that of the unsupported FeCl3 . • Among the supported Fe2 O3 catalysts, the Fe2 O3 /H-ZSM-5 shows the highest activity with almost no reaction induction period. The order for the benzene benzylation activity of the different catalysts is as follows: Fe2 O3 =H-ZSM-5 > Fe2 O3 /H-b > sulfated Fe2 O3 /H-b > sulfated Fe2 O3 /H-ZSM-5 > Fe2 O3 /SA-5205 > Fe2 O3 /Si-MCM-41 > Fe2 O3 /silica gel. Because of its higher channel diameter, the Fe2 O3 supported on H-b zeolite as compared to that on H-ZSM-5 is more preferable for the benzylation reactions involving larger size molecules.
V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
• The sulfatation of Fe2 O3 (which causes an increase in the Lewis acidity [11]) supported on H-ZSM-5 or H-b zeolite has resulted in the decrease in both the activity and selectivity (for diphenyl methane) in the benzene benzylation. • Among the supported FeCl3 catalysts, FeCl3 / Mont K10 shows the highest activity with a negligibly small reaction induction period. The FeCl3 deposited on kaolin, Si-MCM-41 or silica gel also shows high benzene benzylation activity with a small reaction induction period. However, the FeCl3 /13X shows the lowest activity because of the very high induction period (32.6 min). • The use of moist benzene (the benzene saturated with water) instead of dry benzene has caused only a small decrease in the activity of the Fe2 O3 /H-ZSM-5, Fe2 O3 /SA-5205, Fe2 O3 / Si-MCM-41, FeCl3 /Mont K10 and FeCl3 /SiMCM-41 catalysts because of the small increase in the induction period. This shows that these catalysts have only a little moisture sensitivity for the benzylation. • Among the Fe-containing catalysts, the supported FeCl3 catalysts show higher selectivity for the diphenyl methane in the benzylation. • The H-ZSM-5 has higher acidity but very much lower benzylation activity than the H-FeMFI, H-FeAlMFI and Fe2 O3 /H-ZSM-5 catalysts. Similarly, the H-b zeolite also has higher acidity but much lower benzylation activity than that of the Fe2 O3 /H-b, H-FeMFI, Fe2 O3 /HZSM-5, FeCl3 /Mont K10 and FeCl3 /Si-MCM41. Sulfatation of the Fe2 O3 /H-ZSM-5 has resulted in a large increase in its acidity but a decrease in its benzylation activity. Thus, in general, there is no direct relationship between the acidity and benzylation activity of the catalysts. The observations lead to the conclusion that in the benzene benzylation over the Fe-containing catalysts, the acidity of the catalysts does not decide their catalytic activity. This is consistent with the fact that catalysts show only a very little moisture sensitivity in the benzene benzylation. The presence of moisture in the reaction mixture causes a small increase in the reaction induction period;
69
after the induction period, the reaction proceeds with almost the same rate as that observed in the absence of moisture. Hence, when the highly active Fe-containing catalysts are used for the benzylation, there is no need to thoroughly dry aromatic substrate and/or solvent. This is of great practical importance. The H-ZSM-5, silica gel, Si-MCM-41, kaolin and Mont K10 supported Fe2 O3 or FeCl3 catalysts are more active in the benzylation than the Fe3þ exchanged Mont K10 catalyst reported elsewhere [12]. The Fe-containing catalysts are also more active but somewhat less selective in the benzene benzylation than the corresponding Gaor In-containing catalysts reported earlier by us [3,13,14]. The lower selectivity of the Fe-containing catalysts is attributed mostly to their higher activity for the condensation of benzyl chloride [9,15]. Results showing the reuse of the Fe2 O3 /H-b, FeCl3 /Si-MCM-41 and FeCl3 /Mont K10 catalysts (which are highly promising catalysts for the benzylation reaction) catalysts are presented in Fig. 1. The results reveal that these catalysts can be reused in the benzylation. However, there is an appreciable loss in the activity in the reuse of these catalysts. This is expected mostly because of the leaching of the active catalyst component (i.e. iron) during the benzylation reaction. Further work is necessary to strongly bind the active component on the support. As indicated earlier, the high activity of the Fe-containing catalysts in the benzene benzylation is not essentially controlled by their acidity. This is consistent with that observed earlier for the metal cation-exchanged Mont K10 [12]. It seems to be controlled mostly by their redox properties (Fe3þ þ e () Fe2þ ). A following redox mechanism is, therefore, suggested for the benzylation over the Fe-containing catalysts. C6 H5 CH2 Cl þ Fe3þ () C6 H5 CH2 Clþ þ Fe2þ ð1Þ C6 H5 CH2 Clþ () C6 H5 CHþ 2 þ Cl
ð2Þ
Fe2þ þ Cl () Fe3þ þ Cl
ð3Þ
70
V.R. Choudhary et al. / Microporous and Mesoporous Materials 56 (2002) 65–71
Fig. 1. Benzylation of benzene by benzyl chloride (at 80 C) over the fresh and reused Fe2 O3 /H-b, FeCl3 /Si-MCM-41 and FeCl3 /Mont K10 catalysts. þ C6 H5 CHþ 2 þ C6 H5 –H ! C6 H5 –CH2 –C6 H5 þ H
ð4Þ Hþ þ Cl ! HCl
ð5Þ
This mechanism is somewhat similar to that proposed by Cseri et al. [12] for the benzylation over the Fe3þ -exchanged Mont K10. 4. Conclusions Following important conclusions can be drawn from the present investigation on the benzylation of benzene by benzyl chloride over the different supported Fe2 O3 and FeCl3 catalysts: 1. The Fe2 O3 or FeCl3 supported on H-ZSM-5, Hb, macro-porous silica–alumina, meso-porous Si-MCM-41, silica gel or commercial clays (kaolin or Mont K10) is highly active catalyst for the benzylation of benzene. The most promising supported Fe2 O3 and FeCl3 catalysts are Fe2 O3 / H-ZSM-5 (or H-b) and FeCl3 /Mont K10.
2. The supported Fe2 O3 or FeCl3 catalyst shows high benzene benzylation activity even in the presence of moisture in the reaction mixture and hence does not demand moisture-free aromatic substrate and/or solvent to be used in the benzylation reaction. The supported Fe2 O3 or FeCl3 catalyst can be reused in the reaction but with reduced activity. 3. The activity of the Fe-containing catalysts for the benzene benzylation is not directly related to their acidity; it is controlled mainly by their redox properties.
Acknowledgements Suman K. Jana is grateful to CSIR, New Delhi, for awarding Senior Research Fellowship.
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