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Vapour phase reaction of toluene with ethyl acetate over Al-MCM-41 molecular sieves
Catalysis Communications 7 (2006) 47–51 www.elsevier.com/locate/catcom
Vapour phase reaction of toluene with ethyl acetate over Al-MCM-41 molecular sieves Kandaiyan Shanmugapriya, Muthiahpillai Palanichamy, Velayutham Murugesan
*
Department of Chemistry, Anna University, Chennai 600 025, India Received 24 May 2005; accepted 31 August 2005 Available online 26 October 2005
Abstract Al-MCM-41 molecular sieves of Si/Al ratios 25, 50 and 150 were synthesised hydrothermally. The vapour phase reaction of toluene with ethyl acetate was studied over these catalysts at 175, 200, 225, 250, 275 °C. The products were p-xylene, ethyl benzene (EB), p-ethyl toluene (p-ET), m-ethyl toluene (m-ET), p-methyl acetophenone (p-MAP), o-methyl acetophenone (o-MAP). Transalkylation of toluene to p-xylene, ethylation of toluene to p-ET and isomerisation of the latter to m-ET were observed as the major reactions with minor acylation of toluene. The transalkylation and isomerisation were found to be strongly influenced by the feed ratio and WHSV, respectively. The study of time-on-stream indicated blocking of particularly strong acid sites by coke, which was suggested advantageous in order to reduce such site dependent transalkylation and isomerisation. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Toluene; Ethyl acetate; Al-MCM-41; p-Ethyl toluene
1. Introduction Ethylation of toluene with ethylene or ethanol is a reaction of high industrial importance as the product, p-ethyl toluene, on subsequent dehydrogenation gives methylstyrene, which is the monomer for the production of polymethylstyrene. This polymer has more commercial importance than polystyrene in terms of high flash point, glass transition temperature and low specific gravity. Xylene, obtained as by product in this reaction, is an important starting material for the production of synthetic fibres, plasticizers and resins [1]. Chlorinated alumina and aluminium chloride have been commonly employed industrial acid catalysts for ethylation of toluene. The use of these catalysts is connected with undesirable corrosion and pollution problems. Kaeding et al. [2] studied the reaction of toluene with ethylene over HZSM-5 zeolite, wherein meta-isomer was obtained as the major product. The same reaction over HZSM-5 modified with *
Corresponding author. Tel.: +91 44 22200660/22203144; fax: +91 44 22200660. E-mail address: [email protected] (V. Murugesan). 1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.08.011
phosphorous and a variety of metal oxides gave 90–99% of para-ethyl toluene. Cejka et al. [3] investigated toluene alkylation over HZSM-5 zeolite and compared the effect of coke deposition and surface silylation on the para-selectivity. It was concluded that a decrease in the transport rates of xylenes as the decisive factor for an enhancement of the paraselectivity when secondary isomerization of para-isomers on the external surface played a minor role. Paparatto et al. [4] studied toluene ethylation over modified HZSM-5 zeolite and reported the prevalence of para-isomer and the absence of ortho-isomer virtually in the reaction products. Cheralathan et al. [5] reported ethylation of toluene with ethanol over aluminophosphate based molecular sieves and attributed the yield of meta-ethyl toluene to the high acidity of the catalysts. There are also few studies concerning the selective conversion over MCM-41 material in the alkylation of toluene [6–8]. In all the conventional methods different alkylating agents such as alkenes, alkyl halides and alcohols have been used in the ethylation of toluene over various catalysts in the liquid as well as in the vapour phase. But these reagents possess several disadvantages, for example, olefins
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K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51
gives polyolefins that blocks the active sites of the catalysts, alkyl halides are hazardous as they produce hydrogen halide vapours and alcohols yield water that clusters around the Bronsted acid sites reducing the chemisorption of the alkylating agents [9]. Recently, we have reported alkyl acetates as active alkylating agents over Al-MCM-41 molecular sieves [10,11]. It is suggested that ester adsorption is stronger than the alcohol on the Bronsted acid sites of the catalysts. In continuation of this work, we have investigated ethylation of toluene with ethyl acetate as alkylating agent in the vapour phase over Al-MCM-41 molecular sieves. 2. Experimental All the experimental details and the catalytic performance are as described elsewhere [10]. The physico-chemical characterization of the catalysts is found similar to our earlier reports [10,11]. The acidity measurements alone are produced here for better understanding of readers. The acidity of the calcined materials was recorded on a Nicolet Avatar 360 FT-IR spectrophotometer equipped with a high temperature vacuum chamber. Approximately 10 mg of the sample was taken in the sample holder and dehydrated at 500 °C for 6 h under vacuum (10 5 mbar). The sample was then cooled to room temperature. Then pyridine was adsorbed at room temperature. The physically adsorbed pyridine was removed by heating the sample at 150 °C under vacuum (10 5 mbar) for 30 min, the removed material was cooled to room temperature and the
spectrum was recorded. The acidity was calculated using the extinction coefficients of the bands of Bronsted and Lewis acid site adsorbed pyridine. 3. Results and discussion 3.1. Characterisation The acidity of the calcined materials was measured by FT-IR spectroscopy using pyridine as probe (Fig. 1). The samples give the expected bands due to Lewis acid bound (1450, 1575 and 1623 cm 1), Bronsted acid bound (1545 and 1640 cm 1) and both Lewis and Bronsted acid bound pyridine (1490 cm 1). These data coincide with those reported by Climent et al. [12]. The acidity of the catalysts was calculated using the extinction coefficients of the bands of Bronsted and Lewis acid site adsorbed pyridine [13] and the results are presented in Table 1. 3.2. Effect of temperature The vapour phase reaction of toluene with ethyl acetate was studied at 175, 200, 225, 250, 275 °C over Al-MCM-41 (25), Al-MCM-41 (50) and Al-MCM-41 (150). The WHSV and the feed ratio (toluene:ethyl acetate) were at 2.67 h 1 and 1:3, respectively. The products were found to be p-xylene, ethylbenzene (EB), p-ethyltoluene (p-ET) and methyltoluene (m-ET) as the major products and p-methyl acetophenone (p-MAP), o-methyl acetophenone (o-MAP) as the minor ones (Scheme 1). The toluene conversion
Fig. 1. Bronsted and Lewis acidity of Al-MCM-41: (a) Si/Al = 25, (b) Si/Al = 50, and (c) Si/Al = 150.
K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51 Table 1 Bronsted and Lewis acidity values for mesoporous molecular sieves Catalyst
423 K
Al-MCM-41 (25) Al-MCM-41 (50) Al-MCM-41 (150) a
B.A.a
L.A.a
7.9 7.1 3.5
9.1 9.0 5.1
Acidity (lmol py/g catalyst).
C2H5
CH3
CH3
CH3COOC2H5 + Catalyst
CH3 EB
p-Xylene CH3
CH3
CH3
CH3 COCH3
+
+
+ C2H5 COCH3
C2H5 p-ET
m-ET
p-MAP
o-MAP
Scheme 1. Reaction of toluene with ethyl acetate.
and the products selectivity are presented in Table 2. Toluene conversion increased from 175 to 225 °C but at 250 °C and above it decreased. As the catalyst appeared black at and above 250 °C the decrease in conversion above 250 °C was attributed to coke formation. The activity of the catalyst at 225 °C followed the order Al-MCM-41
49
(25) > Al-MCM-41 (50) > Al-MCM-41 (150). Although this order appeared to be same as the acidity of the catalysts, the activity difference was not much. Therefore, the hydrophilic and hydrophobic property of the catalysts also should influence toluene conversion. Since toluene and ethyl acetate are hydrophobic, they may not be brought close to the acidic sites of hydrophilic Al-MCM-41 (25). Hence, the conversion over this catalyst was not much higher than the other catalysts. The temperature (225 °C), might not be sufficient to expel water that resides on the Bronsted acid sites of the mesopores. Clustering of water around the Bronsted acid sites of the mesopores has already been reported in the literature [12]. Generally, water may not be completely lost from the Bronsted acid sites which is the main active sites for ethylation in this study. In the TPD study, the absorbance of OH2 bend of water lying just above 1600 cm 1 will not become zero even when pyridine desorption was made to occur at 400 °C. Ethyl acetate, chemisorbed on the Bronsted acid sites of the catalyst surface was to yield ethyl cation. In addition, it might be partly protonated over less acidic sites to yield species for electrophilic acylation. Ethylbenzene can react with ethyl cation or partly protonated ester by remaining either in the chemisorbed state on the Bronsted acid sites or in the vapour state. Since the feed ratio was at 1:3, and ethyl acetate is more prone to chemisorption on the Bronsted acid sites at its carbonyl oxygen of the ester, most of toluene might be in the vapour state without much adsorption. Hence, the mechanism of electrophilic reaction between ethyl cation or partly protonated ester and ethylbenzene might be Eley–Rideal type [14] to yield ring alkylated and acylated products like p-ET and p-MAP and/or m-MAP, respectively. In addition, cracking of toluene to benzene was also observed as the trans alkylated product, p-xylene, was obtained. The yield of ethylbenzene might occur via ethylation of benzene rather than cracking of ethyltoluene
Table 2 Effect of temperature on toluene conversion and product selectivity Catalysts
Temperature (°C)
Toluene conversion (%)
p-Xylene
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
o-MAP
Al-MCM-41 (25)
175 200 225 250 275
53.9 56.3 65.6 60.9 57.1
20.3 21.4 22.6 23.3 25.0
18.6 19.9 20.8 23.2 24.1
34.8 32.6 31.8 30.3 26.8
18.6 21.0 22.6 23.2 24.1
4.8 3.3 2.2 0 0
2.9 1.8 0 0 0
Al-MCM-41 (50)
175 200 225 250 275
55.4 59.5 61.2 57.0 53.8
17.9 19.0 20.9 23.0 24.1
17.4 18.9 20.6 23.1 23.6
36.7 33.5 32.7 28.6 24.1
19.9 22.7 23.4 25.3 28.2
4.5 3.8 2.4 0 0
3.6 2.1 0 0 0
Al-MCM-41 (150)
175 200 225 250 275
50.8 55.7 58.9 53.6 47.8
16.2 18.0 18.6 19.4 20.1
19.0 21.1 22.9 23.7 24.2
39.6 35.2 33.8 30.9 27.1
20.5 23.8 24.7 26.0 28.6
3.3 1.9 0 0 0
1.4 0 0 0 0
Note. Toluene:ethyl acetate (feed ratio) = 1:3; WHSV = 2.63 h 1.
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K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51
to ethylbenzene, as the ethyl group can be easily cleaved than the methyl group when present together in the same ring. The selectivity to p-xylene increased with increase in temperature, as transalkylation is a reaction of high activation energy demanding. But the increase was not much as the active sites could be better used for the chemisorption of ethyl acetate rather than toluene. The same trend was also observed over Al-MCM-41 (50) and Al-MCM-41 (150). But among the catalysts, Al-MCM-41 (25) showed little enhanced selectivity because of its higher density of acid sites. Again the selectivity to p-xylene over AlMCM-41 (50) and Al-MCM-41 (150) was not higher than Al-MCM-41 (25), hence, transalkylation might also be influenced by hydrophilic and hydrophobic property of the catalysts. As benzene was not observed in this study, it might be ethylated electrophilically as and when it was formed during transalkylation. The increase in selectivity of ethylbenzene with increase in temperature and nearly the same selectivity to p-xylene at all the temperatures very well confirmed transalkylation of toluene and immediate ethylation of released benzene. Similar trend of increase with increase in temperature was observed over all the catalysts. The selectivity of p-ET decreased with increase in temperature, but a reverse trend was observed for m-ET. Again, the magnitude of decrease in the selectivity of pET was found to be nearly equal to the increase of the same to m-ET. Hence, m-ET might be exclusively formed by the isomerisation of p-ET. Such isomerization was also reported already [15]. Since the active sites were already used for chemisorption of ethyl acetate, it might be expected that most of the isomerisation of p-ET to m-ET might occur on the same sites over which the formation of p-ET occurred. But such sites were required to be more acidic than the others. The same trend in selectivity for p-ET and m-ET was also observed over all the three catalysts. Since, the acidic sites of Al-MCM-41 (150) might be stronger than those of the other catalysts, it might be expected to give more isomerisation. But contrary to this, it showed less isomerisation, as the active sites were few and might be used for chemisorption of mostly ethyl acetate. But, the difference in the selectivity to the ethyltoluene isomers was not much significant to correlate with their density and strength of acid sites. As mentioned above, both p- and o-methyl acetophenone were the least formed products. They were observed at low temperature but not at high temperatures.
Since isomerization of p-ET to m-ET is a time-dependant process, it could be avoided by running the reaction at high WHSV. Again, since the transalkylation depends on the availability of free acidic sites, this reaction may also be avoided by running the reaction with high ethyl acetate content in the feed. These concepts were tested and the results are discussed. 3.3. Effect of the feed ratio The effect of feed ratio on conversion and products selectivity was studied over Al-MCM-41 (25) at 225 °C with the feed rate 1.5 ml/h (Table 3). The feed ratio was varied from 1:1 to 1:5. As expected, the conversion decreased at the feed ratio 1:5. It might be due to dilution of toluene in the vapour phase. Such dilution effect on conversion was also reported [16]. The selectivity to p-xylene decreased with increase in the ethyl acetate content in the feed in line with our view, but transalkylation was not completely avoided. Again, it is to be mentioned that at the feed ratio 1:5, there was a sudden fall in p-xylene selectivity as expected. The preferential adsorption of ethyl acetate on the Bronsted acid sites was also evident by the very high selectivity to p-ET at the feed ratio 1:5. Due to excess of ethyl acetate content in the feed isomerisation of p-ET to m-ET also decreased, and it could also be possible to avoid transalkylation with still high ethyl acetate content in the feed. The selectivity of ethyl benzene decreased with increase in the ethyl acetate content in the feed as expected. The increase of selectivity to p-ET with increase in ethyl acetate (EA) content also substantiates decrease of isomerisation and transalkylation. 3.4. Effect of WHSV The effect of WHSV on conversion and products selectivity was studied over Al-MCM-41 (25) with a feed ratio 1:5 at 225 °C. The WHSV was varied as 1.75, 2.68 and 3.58 h 1, and the results are presented in Table 4. The conversion nearly remained the same with increase in WHSV. But the selectivity to p-xylene and EB were not brought to zero with increase in WHSV illustrating still existence of transalkylation. The increase in p-ET selectivity with the consequent decrease of the same for m-ET was also observed. But, the isomerisation of p-ET was not just avoided at 3.58 h 1. Hence, both the increase in ethyl acetate content in the feed and WHSV showed a high increase in the
Table 3 Effect of feed ratio on toluene conversion and product selectivity Catalysts
Feed ratio
Toluene conversion (%)
p-Xylene
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
Al-MCM-41 (25)
1:1 1:3 1:5
40.7 65.6 57.1
23.1 22.6 7.1
20.0 20.8 6.4
30.4 31.8 65.0
25.1 22.6 19.2
1.4 2.2 2.3
Note. Temperature = 225 °C.
K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51
51
Table 4 Effect of WHSV on toluene conversion and product selectivity Catalysts
WHSV (h 1)
Toluene conversion (%)
p-Xylene
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
Al-MCM-41 (25)
1.75 2.62 3.5
55.4 57.1 54.3
10.9 7.1 6.3
9.8 6.4 6.2
51.7 65.0 70.9
25.2 19.2 14.7
2.4 2.3 1.9
Note. Toluene:ethyl acetate (feed ratio) = 1:5; temperature = 225 °C.
Table 5 Effect of time-on-stream on toluene conversion and product selectivity Catalysts
Al-MCM-41 (25)
Time (h)
1 2 3 4 5
Toluene conversion (%)
54.3 46.9 42.2 40.5 39.0
p-Xylene
6.3 5.8 5.1 4.2 3.0
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
6.2 5.6 4.9 4.0 2.8
70.9 74.5 79.7 86.1 91.1
14.7 12.4 9.3 5.0 3.1
1.9 1.7 1.0 0.7 0
Note. Toluene:ethyl acetate (feed ratio) = 1:5; temperature = 225 °C; WHSV = 3.58 h 1.
selectivity to p-ET, but isomerisation and transalkylation were not completely suppressed. 3.5. Effect of time-on-stream The effect of time-on-stream on conversion and products selectivity was studied over Al-MCM-41 (25) with WHSV 3.58 h 1 and feed ratio 1:5. The study was conducted for 5 h of stream and the results are presented in Table 5. Conversion decreased with increase in time-on-stream, illustrating gradual increase in the blocking of active sites by coke. It was also evident by the gradual decrease in the selectivity to p-xylene and ethylbenzene. The selectivity to p-ET increased monotonously with increase in time-on-stream whereas the same to m-ET decreased because of gradual decrease in the availability of strong acid sites for isomerisation. Hence, from the time-on-stream study it could be suggested that partial blocking of the more acidic sites might favour enhanced selectivity to p-ET. 4. Conclusion The study reveals that Al-MCM-41 molecular sieves are active catalysts for vapour phase ethylation of toluene. Ethyl acetate is capable of both alkylating and acylating aromatics over solid acid catalysts like Al-MCM-41 molecular sieves. Transalkylation of toluene and isomerisation of p-ET can be dramatically decreased by increasing ethyl acetate content in the feed and WHSV, respectively. In addition, partial blocking of more active sites can also be a convenient route to reduce transalkylation and isomerisation.
Acknowledgements The authors are grateful to University Grants Commission, New Delhi for financial support under Department Research Scheme (DRS). One of the authors, K. Shanmugapriya, is thankful to CSIR for the award of SRF fellowship. References [1] J. Roth, Chem. Eng. News 8 (1987) 13. [2] W.W. Kaeding, L.B. Young, C. Chu, J. Catal. 89 (1984) 267. [3] J. Cejka, N. Zilkova, B. Wichterlova, G.E. Mirth, J.A. Lercher, Zeolites 17 (1996) 265. [4] G. Paparatto, E. Moretti, G. Leofanti, F. Gatti, J. Catal. 105 (1987) 227. [5] K.K. Cheralathan, C. Kannan, M. Palanichamy, V. Murugesan, Ind. J. Chem. 39A (2000) 921. [6] J.M. Valtierra, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. A 158 (1997) L1. [7] M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, K.B. Lal, Appl. Catal. A 242 (2003) 347. [8] R. Savidha, A. Pandurangan, Appl. Catal. A 276 (2004) 39. [9] V. Umamaheswari, M. Palanichamy, Banumathi Arabindoo, V. Murugesan, Ind. J. Chem. A 39 (2000) 1241. [10] K. Shanmugapriya, M. Palanichamy, Banumathi Arabindoo, V. Murugesan, J. Catal. 224 (2004) 347. [11] K. Shanmugapriya, R. Anuradha, M. Palanichamy, Banumathi Arabindoo, V. Murugesan, J. Mol. Catal. A 221 (2004) 145. [12] M.J. Climent, A. Corma, S. Iborra, S. Miquel, J. Primo, F. Rey, J. Catal. 183 (1999) 76. [13] C.A. Emies, J. Catal. 141 (1993) 347. [14] L.K. Rikho, A.O. Krause, Ind. Eng. Chem. Res. 34 (1995) 1172. [15] J. Cejka, B. Wichterlova, S. Bednarova, Appl. Catal. A 79 (1991) 215. [16] V. Umamaheswari, M. Palanichamy, V. Murugesan, J. Catal. 210 (2002).
Vapour phase reaction of toluene with ethyl acetate over Al-MCM-41 molecular sieves Kandaiyan Shanmugapriya, Muthiahpillai Palanichamy, Velayutham Murugesan
*
Department of Chemistry, Anna University, Chennai 600 025, India Received 24 May 2005; accepted 31 August 2005 Available online 26 October 2005
Abstract Al-MCM-41 molecular sieves of Si/Al ratios 25, 50 and 150 were synthesised hydrothermally. The vapour phase reaction of toluene with ethyl acetate was studied over these catalysts at 175, 200, 225, 250, 275 °C. The products were p-xylene, ethyl benzene (EB), p-ethyl toluene (p-ET), m-ethyl toluene (m-ET), p-methyl acetophenone (p-MAP), o-methyl acetophenone (o-MAP). Transalkylation of toluene to p-xylene, ethylation of toluene to p-ET and isomerisation of the latter to m-ET were observed as the major reactions with minor acylation of toluene. The transalkylation and isomerisation were found to be strongly influenced by the feed ratio and WHSV, respectively. The study of time-on-stream indicated blocking of particularly strong acid sites by coke, which was suggested advantageous in order to reduce such site dependent transalkylation and isomerisation. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Toluene; Ethyl acetate; Al-MCM-41; p-Ethyl toluene
1. Introduction Ethylation of toluene with ethylene or ethanol is a reaction of high industrial importance as the product, p-ethyl toluene, on subsequent dehydrogenation gives methylstyrene, which is the monomer for the production of polymethylstyrene. This polymer has more commercial importance than polystyrene in terms of high flash point, glass transition temperature and low specific gravity. Xylene, obtained as by product in this reaction, is an important starting material for the production of synthetic fibres, plasticizers and resins [1]. Chlorinated alumina and aluminium chloride have been commonly employed industrial acid catalysts for ethylation of toluene. The use of these catalysts is connected with undesirable corrosion and pollution problems. Kaeding et al. [2] studied the reaction of toluene with ethylene over HZSM-5 zeolite, wherein meta-isomer was obtained as the major product. The same reaction over HZSM-5 modified with *
Corresponding author. Tel.: +91 44 22200660/22203144; fax: +91 44 22200660. E-mail address: [email protected] (V. Murugesan). 1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.08.011
phosphorous and a variety of metal oxides gave 90–99% of para-ethyl toluene. Cejka et al. [3] investigated toluene alkylation over HZSM-5 zeolite and compared the effect of coke deposition and surface silylation on the para-selectivity. It was concluded that a decrease in the transport rates of xylenes as the decisive factor for an enhancement of the paraselectivity when secondary isomerization of para-isomers on the external surface played a minor role. Paparatto et al. [4] studied toluene ethylation over modified HZSM-5 zeolite and reported the prevalence of para-isomer and the absence of ortho-isomer virtually in the reaction products. Cheralathan et al. [5] reported ethylation of toluene with ethanol over aluminophosphate based molecular sieves and attributed the yield of meta-ethyl toluene to the high acidity of the catalysts. There are also few studies concerning the selective conversion over MCM-41 material in the alkylation of toluene [6–8]. In all the conventional methods different alkylating agents such as alkenes, alkyl halides and alcohols have been used in the ethylation of toluene over various catalysts in the liquid as well as in the vapour phase. But these reagents possess several disadvantages, for example, olefins
48
K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51
gives polyolefins that blocks the active sites of the catalysts, alkyl halides are hazardous as they produce hydrogen halide vapours and alcohols yield water that clusters around the Bronsted acid sites reducing the chemisorption of the alkylating agents [9]. Recently, we have reported alkyl acetates as active alkylating agents over Al-MCM-41 molecular sieves [10,11]. It is suggested that ester adsorption is stronger than the alcohol on the Bronsted acid sites of the catalysts. In continuation of this work, we have investigated ethylation of toluene with ethyl acetate as alkylating agent in the vapour phase over Al-MCM-41 molecular sieves. 2. Experimental All the experimental details and the catalytic performance are as described elsewhere [10]. The physico-chemical characterization of the catalysts is found similar to our earlier reports [10,11]. The acidity measurements alone are produced here for better understanding of readers. The acidity of the calcined materials was recorded on a Nicolet Avatar 360 FT-IR spectrophotometer equipped with a high temperature vacuum chamber. Approximately 10 mg of the sample was taken in the sample holder and dehydrated at 500 °C for 6 h under vacuum (10 5 mbar). The sample was then cooled to room temperature. Then pyridine was adsorbed at room temperature. The physically adsorbed pyridine was removed by heating the sample at 150 °C under vacuum (10 5 mbar) for 30 min, the removed material was cooled to room temperature and the
spectrum was recorded. The acidity was calculated using the extinction coefficients of the bands of Bronsted and Lewis acid site adsorbed pyridine. 3. Results and discussion 3.1. Characterisation The acidity of the calcined materials was measured by FT-IR spectroscopy using pyridine as probe (Fig. 1). The samples give the expected bands due to Lewis acid bound (1450, 1575 and 1623 cm 1), Bronsted acid bound (1545 and 1640 cm 1) and both Lewis and Bronsted acid bound pyridine (1490 cm 1). These data coincide with those reported by Climent et al. [12]. The acidity of the catalysts was calculated using the extinction coefficients of the bands of Bronsted and Lewis acid site adsorbed pyridine [13] and the results are presented in Table 1. 3.2. Effect of temperature The vapour phase reaction of toluene with ethyl acetate was studied at 175, 200, 225, 250, 275 °C over Al-MCM-41 (25), Al-MCM-41 (50) and Al-MCM-41 (150). The WHSV and the feed ratio (toluene:ethyl acetate) were at 2.67 h 1 and 1:3, respectively. The products were found to be p-xylene, ethylbenzene (EB), p-ethyltoluene (p-ET) and methyltoluene (m-ET) as the major products and p-methyl acetophenone (p-MAP), o-methyl acetophenone (o-MAP) as the minor ones (Scheme 1). The toluene conversion
Fig. 1. Bronsted and Lewis acidity of Al-MCM-41: (a) Si/Al = 25, (b) Si/Al = 50, and (c) Si/Al = 150.
K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51 Table 1 Bronsted and Lewis acidity values for mesoporous molecular sieves Catalyst
423 K
Al-MCM-41 (25) Al-MCM-41 (50) Al-MCM-41 (150) a
B.A.a
L.A.a
7.9 7.1 3.5
9.1 9.0 5.1
Acidity (lmol py/g catalyst).
C2H5
CH3
CH3
CH3COOC2H5 + Catalyst
CH3 EB
p-Xylene CH3
CH3
CH3
CH3 COCH3
+
+
+ C2H5 COCH3
C2H5 p-ET
m-ET
p-MAP
o-MAP
Scheme 1. Reaction of toluene with ethyl acetate.
and the products selectivity are presented in Table 2. Toluene conversion increased from 175 to 225 °C but at 250 °C and above it decreased. As the catalyst appeared black at and above 250 °C the decrease in conversion above 250 °C was attributed to coke formation. The activity of the catalyst at 225 °C followed the order Al-MCM-41
49
(25) > Al-MCM-41 (50) > Al-MCM-41 (150). Although this order appeared to be same as the acidity of the catalysts, the activity difference was not much. Therefore, the hydrophilic and hydrophobic property of the catalysts also should influence toluene conversion. Since toluene and ethyl acetate are hydrophobic, they may not be brought close to the acidic sites of hydrophilic Al-MCM-41 (25). Hence, the conversion over this catalyst was not much higher than the other catalysts. The temperature (225 °C), might not be sufficient to expel water that resides on the Bronsted acid sites of the mesopores. Clustering of water around the Bronsted acid sites of the mesopores has already been reported in the literature [12]. Generally, water may not be completely lost from the Bronsted acid sites which is the main active sites for ethylation in this study. In the TPD study, the absorbance of OH2 bend of water lying just above 1600 cm 1 will not become zero even when pyridine desorption was made to occur at 400 °C. Ethyl acetate, chemisorbed on the Bronsted acid sites of the catalyst surface was to yield ethyl cation. In addition, it might be partly protonated over less acidic sites to yield species for electrophilic acylation. Ethylbenzene can react with ethyl cation or partly protonated ester by remaining either in the chemisorbed state on the Bronsted acid sites or in the vapour state. Since the feed ratio was at 1:3, and ethyl acetate is more prone to chemisorption on the Bronsted acid sites at its carbonyl oxygen of the ester, most of toluene might be in the vapour state without much adsorption. Hence, the mechanism of electrophilic reaction between ethyl cation or partly protonated ester and ethylbenzene might be Eley–Rideal type [14] to yield ring alkylated and acylated products like p-ET and p-MAP and/or m-MAP, respectively. In addition, cracking of toluene to benzene was also observed as the trans alkylated product, p-xylene, was obtained. The yield of ethylbenzene might occur via ethylation of benzene rather than cracking of ethyltoluene
Table 2 Effect of temperature on toluene conversion and product selectivity Catalysts
Temperature (°C)
Toluene conversion (%)
p-Xylene
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
o-MAP
Al-MCM-41 (25)
175 200 225 250 275
53.9 56.3 65.6 60.9 57.1
20.3 21.4 22.6 23.3 25.0
18.6 19.9 20.8 23.2 24.1
34.8 32.6 31.8 30.3 26.8
18.6 21.0 22.6 23.2 24.1
4.8 3.3 2.2 0 0
2.9 1.8 0 0 0
Al-MCM-41 (50)
175 200 225 250 275
55.4 59.5 61.2 57.0 53.8
17.9 19.0 20.9 23.0 24.1
17.4 18.9 20.6 23.1 23.6
36.7 33.5 32.7 28.6 24.1
19.9 22.7 23.4 25.3 28.2
4.5 3.8 2.4 0 0
3.6 2.1 0 0 0
Al-MCM-41 (150)
175 200 225 250 275
50.8 55.7 58.9 53.6 47.8
16.2 18.0 18.6 19.4 20.1
19.0 21.1 22.9 23.7 24.2
39.6 35.2 33.8 30.9 27.1
20.5 23.8 24.7 26.0 28.6
3.3 1.9 0 0 0
1.4 0 0 0 0
Note. Toluene:ethyl acetate (feed ratio) = 1:3; WHSV = 2.63 h 1.
50
K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51
to ethylbenzene, as the ethyl group can be easily cleaved than the methyl group when present together in the same ring. The selectivity to p-xylene increased with increase in temperature, as transalkylation is a reaction of high activation energy demanding. But the increase was not much as the active sites could be better used for the chemisorption of ethyl acetate rather than toluene. The same trend was also observed over Al-MCM-41 (50) and Al-MCM-41 (150). But among the catalysts, Al-MCM-41 (25) showed little enhanced selectivity because of its higher density of acid sites. Again the selectivity to p-xylene over AlMCM-41 (50) and Al-MCM-41 (150) was not higher than Al-MCM-41 (25), hence, transalkylation might also be influenced by hydrophilic and hydrophobic property of the catalysts. As benzene was not observed in this study, it might be ethylated electrophilically as and when it was formed during transalkylation. The increase in selectivity of ethylbenzene with increase in temperature and nearly the same selectivity to p-xylene at all the temperatures very well confirmed transalkylation of toluene and immediate ethylation of released benzene. Similar trend of increase with increase in temperature was observed over all the catalysts. The selectivity of p-ET decreased with increase in temperature, but a reverse trend was observed for m-ET. Again, the magnitude of decrease in the selectivity of pET was found to be nearly equal to the increase of the same to m-ET. Hence, m-ET might be exclusively formed by the isomerisation of p-ET. Such isomerization was also reported already [15]. Since the active sites were already used for chemisorption of ethyl acetate, it might be expected that most of the isomerisation of p-ET to m-ET might occur on the same sites over which the formation of p-ET occurred. But such sites were required to be more acidic than the others. The same trend in selectivity for p-ET and m-ET was also observed over all the three catalysts. Since, the acidic sites of Al-MCM-41 (150) might be stronger than those of the other catalysts, it might be expected to give more isomerisation. But contrary to this, it showed less isomerisation, as the active sites were few and might be used for chemisorption of mostly ethyl acetate. But, the difference in the selectivity to the ethyltoluene isomers was not much significant to correlate with their density and strength of acid sites. As mentioned above, both p- and o-methyl acetophenone were the least formed products. They were observed at low temperature but not at high temperatures.
Since isomerization of p-ET to m-ET is a time-dependant process, it could be avoided by running the reaction at high WHSV. Again, since the transalkylation depends on the availability of free acidic sites, this reaction may also be avoided by running the reaction with high ethyl acetate content in the feed. These concepts were tested and the results are discussed. 3.3. Effect of the feed ratio The effect of feed ratio on conversion and products selectivity was studied over Al-MCM-41 (25) at 225 °C with the feed rate 1.5 ml/h (Table 3). The feed ratio was varied from 1:1 to 1:5. As expected, the conversion decreased at the feed ratio 1:5. It might be due to dilution of toluene in the vapour phase. Such dilution effect on conversion was also reported [16]. The selectivity to p-xylene decreased with increase in the ethyl acetate content in the feed in line with our view, but transalkylation was not completely avoided. Again, it is to be mentioned that at the feed ratio 1:5, there was a sudden fall in p-xylene selectivity as expected. The preferential adsorption of ethyl acetate on the Bronsted acid sites was also evident by the very high selectivity to p-ET at the feed ratio 1:5. Due to excess of ethyl acetate content in the feed isomerisation of p-ET to m-ET also decreased, and it could also be possible to avoid transalkylation with still high ethyl acetate content in the feed. The selectivity of ethyl benzene decreased with increase in the ethyl acetate content in the feed as expected. The increase of selectivity to p-ET with increase in ethyl acetate (EA) content also substantiates decrease of isomerisation and transalkylation. 3.4. Effect of WHSV The effect of WHSV on conversion and products selectivity was studied over Al-MCM-41 (25) with a feed ratio 1:5 at 225 °C. The WHSV was varied as 1.75, 2.68 and 3.58 h 1, and the results are presented in Table 4. The conversion nearly remained the same with increase in WHSV. But the selectivity to p-xylene and EB were not brought to zero with increase in WHSV illustrating still existence of transalkylation. The increase in p-ET selectivity with the consequent decrease of the same for m-ET was also observed. But, the isomerisation of p-ET was not just avoided at 3.58 h 1. Hence, both the increase in ethyl acetate content in the feed and WHSV showed a high increase in the
Table 3 Effect of feed ratio on toluene conversion and product selectivity Catalysts
Feed ratio
Toluene conversion (%)
p-Xylene
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
Al-MCM-41 (25)
1:1 1:3 1:5
40.7 65.6 57.1
23.1 22.6 7.1
20.0 20.8 6.4
30.4 31.8 65.0
25.1 22.6 19.2
1.4 2.2 2.3
Note. Temperature = 225 °C.
K. Shanmugapriya et al. / Catalysis Communications 7 (2006) 47–51
51
Table 4 Effect of WHSV on toluene conversion and product selectivity Catalysts
WHSV (h 1)
Toluene conversion (%)
p-Xylene
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
Al-MCM-41 (25)
1.75 2.62 3.5
55.4 57.1 54.3
10.9 7.1 6.3
9.8 6.4 6.2
51.7 65.0 70.9
25.2 19.2 14.7
2.4 2.3 1.9
Note. Toluene:ethyl acetate (feed ratio) = 1:5; temperature = 225 °C.
Table 5 Effect of time-on-stream on toluene conversion and product selectivity Catalysts
Al-MCM-41 (25)
Time (h)
1 2 3 4 5
Toluene conversion (%)
54.3 46.9 42.2 40.5 39.0
p-Xylene
6.3 5.8 5.1 4.2 3.0
Selectivity of products (%) EB
p-ET
m-ET
p-MAP
6.2 5.6 4.9 4.0 2.8
70.9 74.5 79.7 86.1 91.1
14.7 12.4 9.3 5.0 3.1
1.9 1.7 1.0 0.7 0
Note. Toluene:ethyl acetate (feed ratio) = 1:5; temperature = 225 °C; WHSV = 3.58 h 1.
selectivity to p-ET, but isomerisation and transalkylation were not completely suppressed. 3.5. Effect of time-on-stream The effect of time-on-stream on conversion and products selectivity was studied over Al-MCM-41 (25) with WHSV 3.58 h 1 and feed ratio 1:5. The study was conducted for 5 h of stream and the results are presented in Table 5. Conversion decreased with increase in time-on-stream, illustrating gradual increase in the blocking of active sites by coke. It was also evident by the gradual decrease in the selectivity to p-xylene and ethylbenzene. The selectivity to p-ET increased monotonously with increase in time-on-stream whereas the same to m-ET decreased because of gradual decrease in the availability of strong acid sites for isomerisation. Hence, from the time-on-stream study it could be suggested that partial blocking of the more acidic sites might favour enhanced selectivity to p-ET. 4. Conclusion The study reveals that Al-MCM-41 molecular sieves are active catalysts for vapour phase ethylation of toluene. Ethyl acetate is capable of both alkylating and acylating aromatics over solid acid catalysts like Al-MCM-41 molecular sieves. Transalkylation of toluene and isomerisation of p-ET can be dramatically decreased by increasing ethyl acetate content in the feed and WHSV, respectively. In addition, partial blocking of more active sites can also be a convenient route to reduce transalkylation and isomerisation.
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