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Liquid phase alkylation of anisole by benzyl alcohol catalyzed on alumina-supported niobia
Catalysis Communications 8 (2007) 1650–1654 www.elsevier.com/locate/catcom
Liquid phase alkylation of anisole by benzyl alcohol catalyzed on alumina-supported niobia Marcus H.C. de la Cruz a, Mona A. Abdel-Rehim b, Angela S. Rocha b, Joa˜o F.C. da Silva a, Arnaldo da Costa Faro Jr. b, Elizabeth R. Lachter a,* a
Departamento de Quı´mica Orgaˆnica, Instituto de Quı´mica, UFRJ, Ilha do Funda˜o, CT, Rio de Janeiro, RJ, CEP 21949-900, Brazil Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica, UFRJ, Ilha do Funda˜o, CT, Rio de Janeiro, RJ, CEP 21949-900, Brazil
b
Received 27 October 2006; received in revised form 12 January 2007; accepted 16 January 2007 Available online 24 January 2007
Abstract The catalytic activity of Nb2O5, Al2O3 and alumina-supported niobia were evaluated in the alkylation reaction of anisole with benzyl alcohol in liquid phase. Alumina exhibited low activity and poor selectivity for alkylates. In contrast, high activities and selectivities to alkylated products were obtained with the niobia and niobia-alumina. The latter could be re-used after drying at 773 K. The catalysts were characterized by conventional techniques such as XRD, FTIR of adsorbed pyridine and nitrogen physisorption. Alumina presents only Lewis acidity, nevertheless niobium based catalysts present Brønsted and Lewis acid sites. The acidity trends are compatible with the activity patterns. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Niobic acid; Alkylation; Friedel-Crafts reaction; Solid acid catalyst; Niobia-alumina
1. Introduction Alkylation of aromatic hydrocarbons is one of the most important reactions used in industrial scale. The production of cumene as intermediate in the manufacture of phenol, the production of long chain alkylbenzenes as intermediates for detergents, the benzylation of benzene and toluene by benzyl chloride and benzyl alcohol for producing dielectric fluids are excellent examples. In general, for these processes, acid catalysts (e.g. AlCl3, H2SO4, HF or H3PO4) have been used, in spite of their low selectivity and high corrosiveness [1]. There is considerable interest in replacing strongly acidic, homogeneous, corrosive and polluting catalysts in the manufacture of various industrial products with environmentally clean heterogeneous solid acid catalysts and processes. The uses of solid catalysts
*
Corresponding author. Fax: +55 021x25627944. E-mail address: [email protected] (E.R. Lachter).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.01.019
such as zeolites and clays in the alkylation of aromatic compounds have been studied [1]. The benzylation of toluene with benzyl alcohol over iron-promoted sulfated zirconia systems was evaluated. Along with the benzylated product, benzyl ether, formed by the self condensation of benzyl alcohol, was also obtained as main product [2]. A nafion-silica composite was evaluated as catalyst in the toluene reaction with benzyl alcohol. The final products were found to be dibenzyl ether, the isomers of benzyltoluenes and dibenzyltoluenes and small amounts of unidentified by-products [3]. Benzylation of anisole with benzyl alcohol was carried out with H3PO4–WO3–Nb2O5-derived catalysts. The authors found that catalytic performance depended significantly on calcination temperature [4]. On the other hand, efforts have also been made to develop new applications of niobium compounds in chemical industries [5–7]. Our group has been interested in the alkylation reaction catalyzed by niobium compounds [8– 11]. Depositing the niobium oxide on the surface of a
M.H.C. de la Cruz et al. / Catalysis Communications 8 (2007) 1650–1654
suitable support, with large surface area and easily controlled textural properties is an interesting possibility [12,13]. In the present paper, we describe a comparison between Nb2O5, Al2O3 and alumina-supported niobia catalysts in the Friedel-Crafts alkylation of anisole with benzyl alcohol. 2. Experimental
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of 0.32 mm and film of 1 lm. The temperature was programmed from 353 K to 553 K at 20 K min1, with 2 mL min1 of hydrogen as carrier gas. The substrate and product contents were followed as a function of time. The identification of the products was carried-out by gas-chromatography–mass spectrometry analysis (GC– MS) on a HP 6890 instrument, using a DB-5 (30 m) fused silica column with helium as carrier gas in the same temperature program used in the GC VARIAN.
2.1. Catalysts 3. Results and discussion Bulk niobium oxide was prepared by calcination at 773 K for 2 h of niobic acid (Nb2O5 Æ nH2O), supplied by CBMM (Araxa´, Brazil). The c-alumina was obtained by calcination of a commercial boehmite supplied by Condea Chemie (Pural SB) at 773 K for 3 h. The alumina-supported niobia, Nb2O5/Al2O3, was prepared by multiple incipient wetness impregnation of c-alumina with niobium pentaethoxide in n-hexane solution as described previously [14]. The final niobium content is 17.2 wt.% niobia that represents around 72.6% of the theoretical monolayer of niobia on the alumina support used. 2.2. Characterization Surface areas and pore volumes were determined from nitrogen adsorption at the normal boiling temperature of liquid nitrogen, 77 K, in a volumetric apparatus, Micromeritics ASAP 2010C. Before the analyses all samples were pre-treated in situ under vacuum at 673 K. The catalysts were analyzed by X-ray diffraction on a Phillips DW-1710 diffractometer using Cu Ka radiation, k = 1.5418. FTIR measurements were carried-out in a Nicolet Magna 760 spectrophotometer. Self-supported wafers of the samples, containing 10 mg cm2, were evacuated at 105 Torr and 723 K for 16 h. After cooling to room temperature, the spectrum was recorded. The samples were then exposed to 0.5 Torr pyridine vapors and, after evacuation at 423 K until the vacuum returned to its base level, a second spectrum was recorded. The bands of the adsorbed pyridine were obtained by subtracting the spectra recorded after and before pyridine introduction.
3.1. Characterization The X-ray diffractograms of the catalysts are shown in Fig. 1. The gamma alumina presents broad peaks, however the niobia presents a crystalline structure characteristic of the hexagonal Nb2O5 phase (TT-phase) [15,16]. The Nb2O5/Al2O3 catalyst presents a similar pattern to that of alumina. This suggests that the niobia aggregates are well-dispersed on the alumina surface. The textural properties of the catalysts are presented in Table 1. The decrease in surface area when niobia was deposited on alumina may be largely explained simply by the increase in catalyst density due to incorporation of the niobia and is accompanied by a decrease of pore diameter. As expected the surface area of well-crystallized niobia is lower than those of alumina and niobia-alumina. The alumina, niobia and niobia-alumina were analyzed by FTIR of adsorbed pyridine. The spectra were obtained after adsorption of 0.5 Torr of pyridine vapor and evacuation at 423 K. Fig. 2 shows the bands of adsorbed pyridine species obtained from the spectra recorded after and before adsorption. Pyridine is a basic probe molecule widely used for the study of the acidity of materials, for the reason that it permits the identification of Brønsted and Lewis acid sites on surfaces. In the spectra shown in Fig. 2, we can verify that there is pyridine adsorbed on Lewis sites on all catalysts, from the presence of the bands around 1600–1625 cm1 (m8a C–C), 1575 cm1 (m8b C–C), 1490 cm1 (m19a C–N)
2.3. General alkylation procedure The alkylation reactions were carried-out by stirring a suspension of the catalyst (pre-treated at 773 K for 2 h in stove) with a mixture of an aromatic compound, anisole, (150 mmol) and alkylating agent, benzylic alcohol (10 mmol). The reaction temperature was 433 K. The catalytic activity and selectivity were measured by analyzing the products by gas chromatography. The analyzes of the products, substrate and alkylating agents were performed using a VARIAN model 3800 gas-chromatograph equipped with a flame ionization detector and capillary column VA-5 with 30 m, diameter
Fig. 1. XRD patterns of the catalysts.
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Table 1 Chemical and textural properties of the catalysts Catalyst
SA/m2g
Al2O3 Nb2O5/Al2O3 Nb2O5
224 177 40
a b
1a
APD/nmb 9.8 9.2 10.9
Surface area determined by BET method. Average pore diameter.
and 1450 cm1 (m19b C–N). Alumina presents a band attributed to m8a at 1621 cm1and the same vibration mode for the niobia appears between 1609 and 1621 cm1. Niobiaalumina presents two bands in this region indicating that both alumina-associated and niobia-associated sites exist on its surface. The band around 1540 cm1, attributed to pyridine adsorbed on Brønsted acid sites, is observed in the spectrum of niobia and niobia-alumina, which indicates that these catalysts present Brønsted acidity but alumina does not. 3.2. Catalytic activity in benzylation reaction Benzyl-anisoles are the products of benzylation of aromatic hydrocarbons with benzyl alcohol catalyzed by solid acids and dibenzyl-ether is a product of the auto-etherification of the benzyl alcohol. The reaction schemes for anisole benzylation and auto-etherification of benzylic alcohol are presented in Scheme 1. The results for the reaction of anisole with benzyl alcohol on all catalysts are presented in Table 2. They show that, after 300 min of reaction, the alumina converts only 33% of the benzyl alcohol and the main product was dibenzylether. The low activity of alumina is in agreement with the absence of Brønsted acid sites, as demonstrated in pyridine adsorption experiments. However, the Lewis acid sites are responsible for ether formation, as shown in the accepted mechanism presented in Scheme 2 [17].
Scheme 1. Possible reactions in alkylation of anisole with benzyl alcohol. (a) Benzylation of anisole; (b) auto-etherification of benzyl alcohol.
When the reaction was catalyzed by niobia the conversion was very high, 100% after 180 min, and the main products were from mono-alkylation. The niobia-alumina is extremely active in the alkylation reaction of anisole with benzylic alcohol and very selective to benzyl-anisole (Table 2, Entry 3). These results may be explained by the presence of Brønsted acid sites on both niobia and niobia-alumina. One important difference between these niobia-based catalysts is their surface area. Niobia deposited on alumina presented a higher surface area than pure niobia, so the number of Brønsted acid sites per gram of catalysts may be larger on the former. Pure niobia was tested previously in the same reaction at 353 K, in order to compare the result, so obtained, with that of heteropoly acid [4]. The authors argued that Nb2O5 is substantially inactive. We believe the difference shown in Ref. [4] and the presents results can possibly be attributed to the distinction reaction conditions. Fig. 3 shows results for a kinetic study for alkylation of anisole with benzyl alcohol. A maximum conversion is achieved at 90 min for niobia-alumina. Niobia converts 100% only at 180 min and alumina attains 33% of conversion after a linear rise with the time. The re-utilization of niobia-alumina for the same reaction was investigated. After the first test, the catalyst was pre-treated at 773 K and the catalytic test was performed
Fig. 2. FTIR spectra of pyridine adsorbed and evacuated at 423 K of the catalysts: (a) alumina; (b) niobia-alumina; and (c) niobia.
M.H.C. de la Cruz et al. / Catalysis Communications 8 (2007) 1650–1654
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Table 2 Conversion and selectivity for anisole alkylation with benzyl alcohol (BzOH) Entry
Catalyst
Time (min)
Conversion (%)
Products distribution (%) Alkylated
Al2O3a Nb2O5a Nb2O5/Al2O3a Nb2O5/Al2O3b
1 2 3 4 a b
300 180 90 100
33 100 100 100
ortho
para
0.0 27.4 29.2 32.4
0.0 41.2 44.9 45.9
Ether
Dialkylated
96.2 16.6 20.3 15.4
3.8 14.7 6.3 6.3
Anisole/BzOH = 150/10; reflux; 1 g of catalyst, pre-treated at 773 K. Anisole/BzOH = 150/10; reflux; 0.85 g of reused catalyst, pre-treated at. 773 K.
Scheme 2. Mechanism of auto-etherification of benzyl alcohol on Lewis acid sites of alumina.
Fig. 3. Anisole benzylation with benzyl alcohol activity for all catalysts.
Fig. 4. Benzylation activity, re-utilization of the niobia-alumina catalyst.
in the same way as before. The result is presented in Table 2, Entry 4, and Fig. 4 shows the trend with the time. The activity of the regenerated niobia-alumina catalyst is identical to that of the fresh catalyst. Therefore it is demonstrated that these catalysts have potential industrial applications in environmentally friendly processes.
4. Conclusions Alumina-supported niobia is an efficient catalyst for Friedel-Crafts alkylation of anisole with benzylic alcohol. This catalyst presents high activity and selectivity and may be reusable, essential characteristics for industrial use.
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Dibenzylether is the main product on the alumina, which presents only Lewis acidity. However, the alkylated products are dominant with niobia-alumina, such as with niobium oxide catalysts, in which Brønsted acidity was present, as characterized by the IR spectra of adsorbed pyridine. This trend is in agreement with the proposition that Brønsted sites on catalysts surfaces are responsible for the formation of the benzyl cation ðC6 H5 CHþ 2 Þ, which attacks the anisole ring in the course of the Friedel-Crafts alkylation. Acknowledgements The authors wish thank CBMM (Companhia Brasileira de Metalurgia e Minerac¸a˜o) for supplying the samples of niobic acid, FUJB (Fundac¸a˜o Universita´ria Jose´ Bonifa´cio) and CNPq (Conselho Nacional de Pesquisa e Desenvolvimento) for financial support. PETROBRAS is also thanked for the XRD and FTIR measurements. References [1] K. Tanabe, W.F. Ho¨lderich, Appl. Catal. A 181 (1999) 399.
[2] H. Suja, C.S. Deepa, K. Sreejarani, S. Sugunan, React. Kinet. Catal. Lett. 79 (2003) 373. [3] P. Beltrame, G. Zuretti, Appl. Catal. A 283 (2005) 33. [4] K. Okumura, K. Yamashita, M. Hirano, M. Niwa, J. Catal. 234 (2005) 300. [5] I. Nowak, M. Ziolek, Chem. Rev. 99 (1999) 3603. [6] M. Ziolek, Catal. Today 78 (2003) 28. [7] K. Tanabe, Catal. Today 78 (2003) 65. [8] M. Moraes, W. de, S.F. Pinto, W.A. Gonzalez, L.M.P.M. Carmo, N.M.R. Pastura, E.R. Lachter, Appl. Catal. A 138 (1996) L7–L12. [9] M. Morais, E.F. Torres, L.M.P.M. Carmo, N.M.R. Pastura, W.A. Gonzalez, A.C.B. dos Santos, E.R. Lachter, Catal. Today 28 (1996) 17. [10] M.H.C. de La Cruz, J.F.C. da Silva, E.R. Lachter, Appl. Catal. A 245 (2003) 377. [11] C.C.M. Pereira, E.R. Lachter, Appl. Catal. A 266 (2004) 67. [12] C.L.T. da Silva, V.L.L. Camorim, J.L. Zotin, M.L.R.D. Pereira, A.C. Faro Jr., Catal. Today 57 (2000) 209. [13] M.A. Abdel-Rehim, A.C.B. dos Santos, V.L.L. Camorim, A.C. Faro Jr., in: Proceedings of the 12th Brazilian Congress on Catalysis, Angra dos Reis, vol. 1, 2003, p. 243. [14] M.A. Abdel-Rehim, A.C.B. dos Santos, V.L.L. Camorim, A.C. Faro Jr., Appl. Catal. A 305 (2006) 211. [15] V.S. Braga, J.A. Dias, S.L.C.L. Dias, J.L. de Macedo, Chem. Mater. 17 (2005) 690. [16] A.L. Petre, J.A. Perdigo´n-Melo´n, A. Gervasini, A. Auroux, Catal. Today 78 (2003) 377. [17] B. Shi, B.H. Davis, J. Catal. 157 (1995) 359.
Liquid phase alkylation of anisole by benzyl alcohol catalyzed on alumina-supported niobia Marcus H.C. de la Cruz a, Mona A. Abdel-Rehim b, Angela S. Rocha b, Joa˜o F.C. da Silva a, Arnaldo da Costa Faro Jr. b, Elizabeth R. Lachter a,* a
Departamento de Quı´mica Orgaˆnica, Instituto de Quı´mica, UFRJ, Ilha do Funda˜o, CT, Rio de Janeiro, RJ, CEP 21949-900, Brazil Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica, UFRJ, Ilha do Funda˜o, CT, Rio de Janeiro, RJ, CEP 21949-900, Brazil
b
Received 27 October 2006; received in revised form 12 January 2007; accepted 16 January 2007 Available online 24 January 2007
Abstract The catalytic activity of Nb2O5, Al2O3 and alumina-supported niobia were evaluated in the alkylation reaction of anisole with benzyl alcohol in liquid phase. Alumina exhibited low activity and poor selectivity for alkylates. In contrast, high activities and selectivities to alkylated products were obtained with the niobia and niobia-alumina. The latter could be re-used after drying at 773 K. The catalysts were characterized by conventional techniques such as XRD, FTIR of adsorbed pyridine and nitrogen physisorption. Alumina presents only Lewis acidity, nevertheless niobium based catalysts present Brønsted and Lewis acid sites. The acidity trends are compatible with the activity patterns. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Niobic acid; Alkylation; Friedel-Crafts reaction; Solid acid catalyst; Niobia-alumina
1. Introduction Alkylation of aromatic hydrocarbons is one of the most important reactions used in industrial scale. The production of cumene as intermediate in the manufacture of phenol, the production of long chain alkylbenzenes as intermediates for detergents, the benzylation of benzene and toluene by benzyl chloride and benzyl alcohol for producing dielectric fluids are excellent examples. In general, for these processes, acid catalysts (e.g. AlCl3, H2SO4, HF or H3PO4) have been used, in spite of their low selectivity and high corrosiveness [1]. There is considerable interest in replacing strongly acidic, homogeneous, corrosive and polluting catalysts in the manufacture of various industrial products with environmentally clean heterogeneous solid acid catalysts and processes. The uses of solid catalysts
*
Corresponding author. Fax: +55 021x25627944. E-mail address: [email protected] (E.R. Lachter).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.01.019
such as zeolites and clays in the alkylation of aromatic compounds have been studied [1]. The benzylation of toluene with benzyl alcohol over iron-promoted sulfated zirconia systems was evaluated. Along with the benzylated product, benzyl ether, formed by the self condensation of benzyl alcohol, was also obtained as main product [2]. A nafion-silica composite was evaluated as catalyst in the toluene reaction with benzyl alcohol. The final products were found to be dibenzyl ether, the isomers of benzyltoluenes and dibenzyltoluenes and small amounts of unidentified by-products [3]. Benzylation of anisole with benzyl alcohol was carried out with H3PO4–WO3–Nb2O5-derived catalysts. The authors found that catalytic performance depended significantly on calcination temperature [4]. On the other hand, efforts have also been made to develop new applications of niobium compounds in chemical industries [5–7]. Our group has been interested in the alkylation reaction catalyzed by niobium compounds [8– 11]. Depositing the niobium oxide on the surface of a
M.H.C. de la Cruz et al. / Catalysis Communications 8 (2007) 1650–1654
suitable support, with large surface area and easily controlled textural properties is an interesting possibility [12,13]. In the present paper, we describe a comparison between Nb2O5, Al2O3 and alumina-supported niobia catalysts in the Friedel-Crafts alkylation of anisole with benzyl alcohol. 2. Experimental
1651
of 0.32 mm and film of 1 lm. The temperature was programmed from 353 K to 553 K at 20 K min1, with 2 mL min1 of hydrogen as carrier gas. The substrate and product contents were followed as a function of time. The identification of the products was carried-out by gas-chromatography–mass spectrometry analysis (GC– MS) on a HP 6890 instrument, using a DB-5 (30 m) fused silica column with helium as carrier gas in the same temperature program used in the GC VARIAN.
2.1. Catalysts 3. Results and discussion Bulk niobium oxide was prepared by calcination at 773 K for 2 h of niobic acid (Nb2O5 Æ nH2O), supplied by CBMM (Araxa´, Brazil). The c-alumina was obtained by calcination of a commercial boehmite supplied by Condea Chemie (Pural SB) at 773 K for 3 h. The alumina-supported niobia, Nb2O5/Al2O3, was prepared by multiple incipient wetness impregnation of c-alumina with niobium pentaethoxide in n-hexane solution as described previously [14]. The final niobium content is 17.2 wt.% niobia that represents around 72.6% of the theoretical monolayer of niobia on the alumina support used. 2.2. Characterization Surface areas and pore volumes were determined from nitrogen adsorption at the normal boiling temperature of liquid nitrogen, 77 K, in a volumetric apparatus, Micromeritics ASAP 2010C. Before the analyses all samples were pre-treated in situ under vacuum at 673 K. The catalysts were analyzed by X-ray diffraction on a Phillips DW-1710 diffractometer using Cu Ka radiation, k = 1.5418. FTIR measurements were carried-out in a Nicolet Magna 760 spectrophotometer. Self-supported wafers of the samples, containing 10 mg cm2, were evacuated at 105 Torr and 723 K for 16 h. After cooling to room temperature, the spectrum was recorded. The samples were then exposed to 0.5 Torr pyridine vapors and, after evacuation at 423 K until the vacuum returned to its base level, a second spectrum was recorded. The bands of the adsorbed pyridine were obtained by subtracting the spectra recorded after and before pyridine introduction.
3.1. Characterization The X-ray diffractograms of the catalysts are shown in Fig. 1. The gamma alumina presents broad peaks, however the niobia presents a crystalline structure characteristic of the hexagonal Nb2O5 phase (TT-phase) [15,16]. The Nb2O5/Al2O3 catalyst presents a similar pattern to that of alumina. This suggests that the niobia aggregates are well-dispersed on the alumina surface. The textural properties of the catalysts are presented in Table 1. The decrease in surface area when niobia was deposited on alumina may be largely explained simply by the increase in catalyst density due to incorporation of the niobia and is accompanied by a decrease of pore diameter. As expected the surface area of well-crystallized niobia is lower than those of alumina and niobia-alumina. The alumina, niobia and niobia-alumina were analyzed by FTIR of adsorbed pyridine. The spectra were obtained after adsorption of 0.5 Torr of pyridine vapor and evacuation at 423 K. Fig. 2 shows the bands of adsorbed pyridine species obtained from the spectra recorded after and before adsorption. Pyridine is a basic probe molecule widely used for the study of the acidity of materials, for the reason that it permits the identification of Brønsted and Lewis acid sites on surfaces. In the spectra shown in Fig. 2, we can verify that there is pyridine adsorbed on Lewis sites on all catalysts, from the presence of the bands around 1600–1625 cm1 (m8a C–C), 1575 cm1 (m8b C–C), 1490 cm1 (m19a C–N)
2.3. General alkylation procedure The alkylation reactions were carried-out by stirring a suspension of the catalyst (pre-treated at 773 K for 2 h in stove) with a mixture of an aromatic compound, anisole, (150 mmol) and alkylating agent, benzylic alcohol (10 mmol). The reaction temperature was 433 K. The catalytic activity and selectivity were measured by analyzing the products by gas chromatography. The analyzes of the products, substrate and alkylating agents were performed using a VARIAN model 3800 gas-chromatograph equipped with a flame ionization detector and capillary column VA-5 with 30 m, diameter
Fig. 1. XRD patterns of the catalysts.
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Table 1 Chemical and textural properties of the catalysts Catalyst
SA/m2g
Al2O3 Nb2O5/Al2O3 Nb2O5
224 177 40
a b
1a
APD/nmb 9.8 9.2 10.9
Surface area determined by BET method. Average pore diameter.
and 1450 cm1 (m19b C–N). Alumina presents a band attributed to m8a at 1621 cm1and the same vibration mode for the niobia appears between 1609 and 1621 cm1. Niobiaalumina presents two bands in this region indicating that both alumina-associated and niobia-associated sites exist on its surface. The band around 1540 cm1, attributed to pyridine adsorbed on Brønsted acid sites, is observed in the spectrum of niobia and niobia-alumina, which indicates that these catalysts present Brønsted acidity but alumina does not. 3.2. Catalytic activity in benzylation reaction Benzyl-anisoles are the products of benzylation of aromatic hydrocarbons with benzyl alcohol catalyzed by solid acids and dibenzyl-ether is a product of the auto-etherification of the benzyl alcohol. The reaction schemes for anisole benzylation and auto-etherification of benzylic alcohol are presented in Scheme 1. The results for the reaction of anisole with benzyl alcohol on all catalysts are presented in Table 2. They show that, after 300 min of reaction, the alumina converts only 33% of the benzyl alcohol and the main product was dibenzylether. The low activity of alumina is in agreement with the absence of Brønsted acid sites, as demonstrated in pyridine adsorption experiments. However, the Lewis acid sites are responsible for ether formation, as shown in the accepted mechanism presented in Scheme 2 [17].
Scheme 1. Possible reactions in alkylation of anisole with benzyl alcohol. (a) Benzylation of anisole; (b) auto-etherification of benzyl alcohol.
When the reaction was catalyzed by niobia the conversion was very high, 100% after 180 min, and the main products were from mono-alkylation. The niobia-alumina is extremely active in the alkylation reaction of anisole with benzylic alcohol and very selective to benzyl-anisole (Table 2, Entry 3). These results may be explained by the presence of Brønsted acid sites on both niobia and niobia-alumina. One important difference between these niobia-based catalysts is their surface area. Niobia deposited on alumina presented a higher surface area than pure niobia, so the number of Brønsted acid sites per gram of catalysts may be larger on the former. Pure niobia was tested previously in the same reaction at 353 K, in order to compare the result, so obtained, with that of heteropoly acid [4]. The authors argued that Nb2O5 is substantially inactive. We believe the difference shown in Ref. [4] and the presents results can possibly be attributed to the distinction reaction conditions. Fig. 3 shows results for a kinetic study for alkylation of anisole with benzyl alcohol. A maximum conversion is achieved at 90 min for niobia-alumina. Niobia converts 100% only at 180 min and alumina attains 33% of conversion after a linear rise with the time. The re-utilization of niobia-alumina for the same reaction was investigated. After the first test, the catalyst was pre-treated at 773 K and the catalytic test was performed
Fig. 2. FTIR spectra of pyridine adsorbed and evacuated at 423 K of the catalysts: (a) alumina; (b) niobia-alumina; and (c) niobia.
M.H.C. de la Cruz et al. / Catalysis Communications 8 (2007) 1650–1654
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Table 2 Conversion and selectivity for anisole alkylation with benzyl alcohol (BzOH) Entry
Catalyst
Time (min)
Conversion (%)
Products distribution (%) Alkylated
Al2O3a Nb2O5a Nb2O5/Al2O3a Nb2O5/Al2O3b
1 2 3 4 a b
300 180 90 100
33 100 100 100
ortho
para
0.0 27.4 29.2 32.4
0.0 41.2 44.9 45.9
Ether
Dialkylated
96.2 16.6 20.3 15.4
3.8 14.7 6.3 6.3
Anisole/BzOH = 150/10; reflux; 1 g of catalyst, pre-treated at 773 K. Anisole/BzOH = 150/10; reflux; 0.85 g of reused catalyst, pre-treated at. 773 K.
Scheme 2. Mechanism of auto-etherification of benzyl alcohol on Lewis acid sites of alumina.
Fig. 3. Anisole benzylation with benzyl alcohol activity for all catalysts.
Fig. 4. Benzylation activity, re-utilization of the niobia-alumina catalyst.
in the same way as before. The result is presented in Table 2, Entry 4, and Fig. 4 shows the trend with the time. The activity of the regenerated niobia-alumina catalyst is identical to that of the fresh catalyst. Therefore it is demonstrated that these catalysts have potential industrial applications in environmentally friendly processes.
4. Conclusions Alumina-supported niobia is an efficient catalyst for Friedel-Crafts alkylation of anisole with benzylic alcohol. This catalyst presents high activity and selectivity and may be reusable, essential characteristics for industrial use.
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M.H.C. de la Cruz et al. / Catalysis Communications 8 (2007) 1650–1654
Dibenzylether is the main product on the alumina, which presents only Lewis acidity. However, the alkylated products are dominant with niobia-alumina, such as with niobium oxide catalysts, in which Brønsted acidity was present, as characterized by the IR spectra of adsorbed pyridine. This trend is in agreement with the proposition that Brønsted sites on catalysts surfaces are responsible for the formation of the benzyl cation ðC6 H5 CHþ 2 Þ, which attacks the anisole ring in the course of the Friedel-Crafts alkylation. Acknowledgements The authors wish thank CBMM (Companhia Brasileira de Metalurgia e Minerac¸a˜o) for supplying the samples of niobic acid, FUJB (Fundac¸a˜o Universita´ria Jose´ Bonifa´cio) and CNPq (Conselho Nacional de Pesquisa e Desenvolvimento) for financial support. PETROBRAS is also thanked for the XRD and FTIR measurements. References [1] K. Tanabe, W.F. Ho¨lderich, Appl. Catal. A 181 (1999) 399.
[2] H. Suja, C.S. Deepa, K. Sreejarani, S. Sugunan, React. Kinet. Catal. Lett. 79 (2003) 373. [3] P. Beltrame, G. Zuretti, Appl. Catal. A 283 (2005) 33. [4] K. Okumura, K. Yamashita, M. Hirano, M. Niwa, J. Catal. 234 (2005) 300. [5] I. Nowak, M. Ziolek, Chem. Rev. 99 (1999) 3603. [6] M. Ziolek, Catal. Today 78 (2003) 28. [7] K. Tanabe, Catal. Today 78 (2003) 65. [8] M. Moraes, W. de, S.F. Pinto, W.A. Gonzalez, L.M.P.M. Carmo, N.M.R. Pastura, E.R. Lachter, Appl. Catal. A 138 (1996) L7–L12. [9] M. Morais, E.F. Torres, L.M.P.M. Carmo, N.M.R. Pastura, W.A. Gonzalez, A.C.B. dos Santos, E.R. Lachter, Catal. Today 28 (1996) 17. [10] M.H.C. de La Cruz, J.F.C. da Silva, E.R. Lachter, Appl. Catal. A 245 (2003) 377. [11] C.C.M. Pereira, E.R. Lachter, Appl. Catal. A 266 (2004) 67. [12] C.L.T. da Silva, V.L.L. Camorim, J.L. Zotin, M.L.R.D. Pereira, A.C. Faro Jr., Catal. Today 57 (2000) 209. [13] M.A. Abdel-Rehim, A.C.B. dos Santos, V.L.L. Camorim, A.C. Faro Jr., in: Proceedings of the 12th Brazilian Congress on Catalysis, Angra dos Reis, vol. 1, 2003, p. 243. [14] M.A. Abdel-Rehim, A.C.B. dos Santos, V.L.L. Camorim, A.C. Faro Jr., Appl. Catal. A 305 (2006) 211. [15] V.S. Braga, J.A. Dias, S.L.C.L. Dias, J.L. de Macedo, Chem. Mater. 17 (2005) 690. [16] A.L. Petre, J.A. Perdigo´n-Melo´n, A. Gervasini, A. Auroux, Catal. Today 78 (2003) 377. [17] B. Shi, B.H. Davis, J. Catal. 157 (1995) 359.