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Alkylation of benzene with methanol on zeolites: Infrared spectroscopy studies
Alkylation of benzene with methanol on zeolites: Infrared spectroscopy studies Jan Rakoczy
Institute of Organic Chemistry and Technology, Technical University of Cracow, Krak6w, Poland Tadeusz Romotowski
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krak6w, Poland Chemisorption of CH3OH and CD3OD on NaY and HY zeolites was investigated by i.r. spectroscopy. The reaction "in situ" of methoxy groups with benzene was observed. It was found that the first step of methylation of the benzene ring is the chemisorption of methanol on the acid sites of the zeolites. Then, methoxy groups react with the aromatic ring. Keywords: Methylation aromatic rings on faujasites; i.r. spectroscopy
INTRODUCTION
EXPERIMENTAL
The mechanism of the methylation of aromatic rings on zeolite catalysts was for the first time reported by Venuto and Landis.1 The analogous mechanism was presented by Kaeding et al. 2 for the case of the methylation of toluene, whereas coadsorption of toluene and methanol on HZSM-5 zeolites was examined by Mirth and Lercher. ~ This reaction was assumed to proceed via the oxonium ion that was formed as a result of chemisorption of methanol on the Br6nsted acid sites. Mirth and Lercher suggested that methylation of the toluene rings results from the coadsorption of both methanol and toluene on acid sites, but they do not explain what is the course of the further stages of the reaction. Two papers 4'5 presented the different mechanism of the discussed reaction. The methylation was in accordance with the Rideal-type mechanism. Methoxy groups reacted with the aromatic ring diffusing from a gas phase (Figure 1). The purpose of the present paper was the investigation of the methylation mechanism of the aromatic ring on the hydrogen form of zeolites by i.r. spectroscopy. The reaction of methanol with benzene catalyzed by the Br6nsted acid sites was observed "in situ." The obtained results confirm the hypothetical mechanism of the methylation of aromatic compounds described on the basis of catalytic studies. 4'5
The investigations were carried out using the Specord 75 IR spectrometer working on-line with a vacuum system. Such a construction allowed the reagents to be adsorbed and desorbed and also the measurement of the pressure. Benzene, methanol (analytically pure), and methanol substituted by deuterium CD3OD were applied. NaY zeolite with a Si/A1 ratio of 2.5 prepared by the Inowroetaw Sodium Factory and its hydrogen form 4 (5.14)HNaY with a 72% exchange degree Na + to H + were used as a catalyst. Before the main experiment, each sample was calcinated at 360 ° under vacuum conditions.
Address reprint requests to Dr. Rakoczy at the Institute of Organic Chemistry and Technology, Technical University of Cracow, ul. Warszawska 24, 31-155 Krakbw, Poland. Received 27 April 1992; accepted 17 November 1992 © 1993 Butterworth-Heinemann 256
ZEOLITES, 1993, Vol 13, April~May
RESULTS Stability of methoxy groups in HNaY and NaY zeolites Methanol adsorption on hydrogen and sodium forms of Y zeolite was carried out at 140°C under 20 Torr, and, afterward, the measurement cell was outgassed. Under the above conditions, no chemisorption of methanol on the sodium form of Y zeolite was observed (Figure2). On the other hand, as a result of methanol sorption on the (5.14)HNaY zeolite, two bands at 2980 and 2850 cm-1 appeared. In the case of CD~OD, two other bands, namely, at 2130 and 2070 cm -1, were observed. At 260°C, all the bands were stable and their intensities did not change in spite of the constant outgassing of the measurement cell. The methanol chemisorption caused reduction of the OH groups' intensity.
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski
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Figure I Hypothetical mechanism methylation of the aromatic rings on zeolites
The effect of temperature on the intensity of the 2130 and 2070 cm -1 bands corresponding to C-D bonds of CDsOD chemisorbed on zeolite is presented in Figure 3. With the increase of temperature from 140 to 308°C, the intensity of the 2130 and 2070 cm -I bands decreases with a simultaneous increase of the intensity o f the 2675 and 2615 cm -I bands (deuterium-exchanged of Br6nsted sites). However, the chemisorbed methoxy groups on the (5.14)HNaY zeolite were present up to 308°C. A similar experiment was carried out for the investigation of the stability of the complexes: zeolite - CH3OH (Figure 4). As a result of the methanol chemisorption, the spectrum of the zeolite changed. The intensity of the 3640 and 3540 cm- l bands of the OH groups clearly decreased and two new bands at 2980 and 2850 cm-I characteristic of C - H bonds of methyl groups, appeared (Figure 4a). Despite the 37
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Figure 3 The effect of the temperature outgassing on the absorption bands of the Z-O-CD3 group: (a) (5.14)HNaY, t = 133°C; (b) after absorption CD3OD at 142°C; (c) t = 260°C; (d) t = 286°C; (e) t = 308°C.
continuous outgassing of the cell and the temperature increase up to 371°C, these two bands did not disappear; however, their intensity reduced markedly (Figure 4d). The weaker band at 2850 cm-1 was not recorded yet at 326°C (Figure 4c).
Reaction of methoxy groups with benzene T h e (5.14)HNaY zeolite, containing methoxy groups, was contacted with benzene with a simultaneous temperature increase until the maxima of the C - H or C-D bonds of the methoxy groups disappeared. As a result of the reaction of benzene with zeolite containing Z-O-CD~ groups, the bands at 2130 and 2070 cm -~ had already disappeared at 260°C (Figure 5j~. After the outgassing of the sample at 135°C, the OH groups in the zeolite were restored
(Figure 5g). I
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Figure 2 CD3OD chemisorption of NaY zeolite: (a) zeolite after activation at temperature 368°C, t = 144°C, p - 3 x 10 -3 Torr; (b) CD3OD adsorption, t = 144°C, PCD3OD = 24 Torr; (c) after outgassing at t = 145°C, p = 5 x 10 -3 Torr.
The same results were observed during the studies on the methoxy groups' activity that was formed by chemisorption of CH3OH on the HNaY zeolite (Figure 6). Under benzene pressure of 23 Torr, the increase of temperature in the measurement cell to 253°C caused the 2980 and 2850 cm -x bands to disappear entirely and the Br6nsted acid sites to reappear.
ZEOLITES, 1993, Vol 13, April/May
257
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski 37 I
37
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Figure 4 The effect of the t e m p e r a t u r e outgassing on con c e n tration of m e t h o x y groups in ( 5 . 1 4 ) H N a Y zeolite: (a) after absorption CH3OH; t = 144°C; p = 8.5 x 10-3 Torr; (b) t = 256°C;
(c) t = 326°C; (d) t = 371°C.
The analogous results referring to adsorption of methanol on such zeolites as faujasite and ZSM-5 have been described by many authors. 6--12 The methoxonium ions observed by some of these authors 6's'lh12 undergo transformation to methoxy species above 150°C. The stability of methoxyl groups separated after the outgassing of water and methanol excess from the zeolite (Z) results from the fact that dissociation of the Z-O-CH3 group with the formation of CH2: (double-radical) or their transformation into the hydrocarbon deposit requires high activation energy. The "a" spectrum in Figure 4 shows that in the range of 3500-3000 cm -1 absorbance of methoxylated zeolite was markedly higher in comparison with absorbance of the hydrogen form. OH stretching vibrations occur in this part of the spectrum and are seen as a wide band. They can derive from physically absorbed methanol, 6"]'~ methoxonium 12 ions, or water molecules bonded with hydrogen bonds. 14 Since the above band was rather weak and decayed with temperature increase, one can assume that methoxy groups dominated in the zeolite after chemisorption of methanol at 150°C. When benzene was introduced into the measurement cell, the Z-O-CD3 and Z-O-CH3 bands dis-
38
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DISCUSSION Attempts for the chemisorption of methanol on the hydrogen forms of the Y zeolite proved that the methanol-Br6nsted acid site complex was very stable. For example, the bands of the CD3 groups of the chemisorbed deuterium-substituted methanol were observed in the i.r. spectra even at 308°C and under the 7.0 x 10 - 2 T o r t (Figure 3e). In the case of CH3OH, the 2980 cm -l band atrributed to the methoxy group was still observed at 371 °C (Figure 4d). The high stability of the Br6nsted acid site methanol complex is confirmed by the data listed in Tables 1 and 2. The tables present results of the absorption measurements of O - H , C-H, and C-D bonds in the range of 3800-2400 cm-1 in relation to the time of the outgassing of the zeolite samples. Table 1 shows that the intensity of the 2980 and 2850 cm -I bands (attributed to C - H bonds of the methyl groups) is constant even when the hydrogen form of the methoxylated zeolite was heated to 256.5°C under 6.0 x 10 -3 Torr. Moreover, the reproduction of hydroxyl groups was not observed. In the case of the application of d e u t e r i u m substituted methanol (Table 2), the intensity of the CD3 groups was constant; however, a small increase of O-D concentration in relation to the time was observed. Hence, one can assume that as a result of methanol chemisorption the methyoxylation reaction of zeolite OH groups takes place (Figure 1).
258
ZEOLITE& 1993, Vol 13, April/May
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Figure 5 Reaction of Z - O - C D 3 g r o u p s with benzene: (a) H N a Y , t = 135°C, p 2 x 10 -2 Tort; (b) C D 3 O D a b s o r p t i o n at 135°C, Pco3oo = 22 Torr; (c) after outgassing CD3OD, t = 140°C, p = 6 x 10- 2 Torr; (d) t = 140 o C, Pc6H s = 10 Torr; (e) t = 210 o C, PcsH s = 9.5 Torr; (f) t = 260°C, PCeHe ----- 8 Torr; (g) zeolite after outgassing benzene, t = 135°C, p = 1 x 10 -2 Torr.
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski 38 I
33 . . . .
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Table 2 The effect of the outgassing time of the measurement cell on the absorbance of the bands of the i.r. spectra of (5.14)HNaY zeolite that was chemisorbed by CD3OD
-1
I. /7
Outgassing time (min)
A2eTs
A2els
A213o
A2o7o
0 15 25 40 50
0.17 0.18 0.19 0.20 0.20
0.13 0.14 0.14 0.15 0.15
0.02 0.02 0.02 0.02 0.02
0.04 0.04 0.04 0.04 0.04
OD g r o u p s
CDz groups
Outgassing was carried o u t u n d e r 7.0 x 10 -2 Torr pressure and at the temperature of 260°C
Figure 6 Reaction of methoxy groups with benzene: (a) H N a Y after activation, t = 140°C; (b) zeolite after methanol chemisorption and outgassing measurement cell, t = 142°C; (c)t 140.5°C, 24 T o r t ; (d) t = 196~C, PC6H 6 = 24 T o r t ; (e) t = 253°C, Pc6H s = 23 T o r r ; (f) zeolite after outgassing benzene, t = 140.5°C, p = 2 x 10 -2 Torr.
PC6H s =
appeared even at 260°C, although they were stable at this temperature in the absence of benzene. This phenomenon can be explained by a hypothesis that the alkylation reaction of the benzene ring takes place. Moreover, the observation of the intensities of the OH maximum (Figure 6a and J') showed that the Br6nsted acid sites recover entirely in consequence of the reaction between the methoxyl groups with benzene. The absorbances at 3650 and 3540 cm-l of the OH groups were 0.50 and 0.33, respectively, for the initial hydrogen form (Figure 6a). After the Table 1 The effect of the outgassing time of the measurement cell on the absorbance of the bands of the i.r. spectra of (5.14)HNaY zeolite that was chemisorbed by methanol
groups
Outgassing time (rain)
A3640
A3s40
A29eo
A2eso
0 20 40 55
0.25 0.38 0.28 0.38
0.25 0.39 0.26 0.35
0.13 0.12 0.12 0.11
0.05 0.04 0.05 0.05
OH g r o u p s
CH 3
Outgassing was carried out under 6.0 x 10 -3 Torr pressure and at the temperature of 256.5°C
chemisorption of methanol and the reaction between methoxyl groups and benzene (Figure 6J), the absorbances of the rebuih hydroxyl groups were 0.47 and 0.34. The above results confirm the methylation mechanism presented in Figure I. The first step in the alkylation reaction of aromatic rings on the catalytically active zeolite is the chemisorption of methanol on the acid Br6nsted sites. As a result of the above, the formed methoxy groups react, in the second step, with the rings of the aromatic compounds. The sodium form of zeolite is not active in the methylation reaction because of the lack of strong acid sites. On the other hand, the recovery of the OH groups, observed in Figure 5, as a result of the reaction of Z-O-CD3 and benzene, is not the additional confirmation of the proposed mechanism. The parallel experiment proved that even at the ambient temperature an isotopic exchange between the deuteriumexchanged Br6nsted sites and benzene proceeded quickly enough to rebuild the OH groups of the zeolite. The mechanism proposed by the authors of the present paper differs from the one suggested by Mirth and Lercher." " Since the methoxonium ion is the intermediate form of the methoxylation of Br6nsted acid sites, under the conditions of the alkylation reaction at temperatures above 150°C, both species of methanol and, consequently, two parallel alkylation mechanisms undoubtedly take place. -
--
3
15
CONCLUSION l. A result of the chemisorption of CH3OH and CDaOD on the hydrogen-form Y zeolite is the formation of methoxy groups. These groups are stable at the temperature range 300-370°C. On the sodium-form Y zeolite (when strong Br6nsted acid sites are not present), no chemisorbed methanol was found. 2. When methoxy groups come into contact with benzene, they have already disappeared even at temperature 260°C. 3. On the basis of the above findings, it is possible to come to the conclusion that methylation of aromatic rings follows when methoxy groups react with aromatic compounds.
ZEOLITES, 1993, Vol 13, April/May
259
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski
REFERENCES 1 Venuto, P.B. and Landis, P.S. Adv. Catal. 1968, 18, 326 2 Kaeding, W.W., Chu, C., Young, L.B., Weinstein, B., Butter, S.A.J. Catal. 1981, 67(1), 159 3 Mirth, G. and Lerher, J.A., J. Catal. 1991, 132, 244 4 Rakoczy, J. and Sulikowski, B. React. Kinet. Catal. Lett. 1988, 36(1) 241 5 Rakoczy, J. React. Kinet. Catal. Lett., in press 6 Salvador, P. and Kladnig, W. J. Chem. Soc., Faraday Trans. / 1977, 73, 1153 7 Kubelkova, L., Novakova, J. and Jiru, P. in Structure and Reactivity of Modified Zeo/ites (Eds. P.A. Jacobs et al.) Elsevier, Amsterdam, 1984, p. 217
260
ZEOLITES, 1993, Vol 13, A p r i l / M a y
8 Forester, T.R. and Howe, R.F.J. Am. Chem. Soc. 1987, 109, 5076 9 Zibtek, M. and Beresifiska, I. Zeolites 1985, 5, 245 10 Bosacek, V. and Twaruzkova, Z. Coll. Czech. Chem. Commun. 1971, 36, 551 11 Kubelkova, L., Novakova, J. and Dolejsek, Z. J. Catal. 1987, 108, 208 12 Kubelkova, L., Novakova, J. and Nedomova, K. J. Catal. 1990, 124, 441 13 Mirth, G., Lerher, J.A., Anderson, M.W. and Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 86(17) 3039 14 Jentys, A., Warecka, G., Derewinski, M. and Lercher, J.A.J. Phys. Chem. 1989, 93, 4837 15 Mirth, G. and Lerher, J.A.J. Phys. Chem. 1991, 95, 3736.
Institute of Organic Chemistry and Technology, Technical University of Cracow, Krak6w, Poland Tadeusz Romotowski
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krak6w, Poland Chemisorption of CH3OH and CD3OD on NaY and HY zeolites was investigated by i.r. spectroscopy. The reaction "in situ" of methoxy groups with benzene was observed. It was found that the first step of methylation of the benzene ring is the chemisorption of methanol on the acid sites of the zeolites. Then, methoxy groups react with the aromatic ring. Keywords: Methylation aromatic rings on faujasites; i.r. spectroscopy
INTRODUCTION
EXPERIMENTAL
The mechanism of the methylation of aromatic rings on zeolite catalysts was for the first time reported by Venuto and Landis.1 The analogous mechanism was presented by Kaeding et al. 2 for the case of the methylation of toluene, whereas coadsorption of toluene and methanol on HZSM-5 zeolites was examined by Mirth and Lercher. ~ This reaction was assumed to proceed via the oxonium ion that was formed as a result of chemisorption of methanol on the Br6nsted acid sites. Mirth and Lercher suggested that methylation of the toluene rings results from the coadsorption of both methanol and toluene on acid sites, but they do not explain what is the course of the further stages of the reaction. Two papers 4'5 presented the different mechanism of the discussed reaction. The methylation was in accordance with the Rideal-type mechanism. Methoxy groups reacted with the aromatic ring diffusing from a gas phase (Figure 1). The purpose of the present paper was the investigation of the methylation mechanism of the aromatic ring on the hydrogen form of zeolites by i.r. spectroscopy. The reaction of methanol with benzene catalyzed by the Br6nsted acid sites was observed "in situ." The obtained results confirm the hypothetical mechanism of the methylation of aromatic compounds described on the basis of catalytic studies. 4'5
The investigations were carried out using the Specord 75 IR spectrometer working on-line with a vacuum system. Such a construction allowed the reagents to be adsorbed and desorbed and also the measurement of the pressure. Benzene, methanol (analytically pure), and methanol substituted by deuterium CD3OD were applied. NaY zeolite with a Si/A1 ratio of 2.5 prepared by the Inowroetaw Sodium Factory and its hydrogen form 4 (5.14)HNaY with a 72% exchange degree Na + to H + were used as a catalyst. Before the main experiment, each sample was calcinated at 360 ° under vacuum conditions.
Address reprint requests to Dr. Rakoczy at the Institute of Organic Chemistry and Technology, Technical University of Cracow, ul. Warszawska 24, 31-155 Krakbw, Poland. Received 27 April 1992; accepted 17 November 1992 © 1993 Butterworth-Heinemann 256
ZEOLITES, 1993, Vol 13, April~May
RESULTS Stability of methoxy groups in HNaY and NaY zeolites Methanol adsorption on hydrogen and sodium forms of Y zeolite was carried out at 140°C under 20 Torr, and, afterward, the measurement cell was outgassed. Under the above conditions, no chemisorption of methanol on the sodium form of Y zeolite was observed (Figure2). On the other hand, as a result of methanol sorption on the (5.14)HNaY zeolite, two bands at 2980 and 2850 cm-1 appeared. In the case of CD~OD, two other bands, namely, at 2130 and 2070 cm -1, were observed. At 260°C, all the bands were stable and their intensities did not change in spite of the constant outgassing of the measurement cell. The methanol chemisorption caused reduction of the OH groups' intensity.
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski
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Figure I Hypothetical mechanism methylation of the aromatic rings on zeolites
The effect of temperature on the intensity of the 2130 and 2070 cm -1 bands corresponding to C-D bonds of CDsOD chemisorbed on zeolite is presented in Figure 3. With the increase of temperature from 140 to 308°C, the intensity of the 2130 and 2070 cm -I bands decreases with a simultaneous increase of the intensity o f the 2675 and 2615 cm -I bands (deuterium-exchanged of Br6nsted sites). However, the chemisorbed methoxy groups on the (5.14)HNaY zeolite were present up to 308°C. A similar experiment was carried out for the investigation of the stability of the complexes: zeolite - CH3OH (Figure 4). As a result of the methanol chemisorption, the spectrum of the zeolite changed. The intensity of the 3640 and 3540 cm- l bands of the OH groups clearly decreased and two new bands at 2980 and 2850 cm-I characteristic of C - H bonds of methyl groups, appeared (Figure 4a). Despite the 37
34
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Figure 3 The effect of the temperature outgassing on the absorption bands of the Z-O-CD3 group: (a) (5.14)HNaY, t = 133°C; (b) after absorption CD3OD at 142°C; (c) t = 260°C; (d) t = 286°C; (e) t = 308°C.
continuous outgassing of the cell and the temperature increase up to 371°C, these two bands did not disappear; however, their intensity reduced markedly (Figure 4d). The weaker band at 2850 cm-1 was not recorded yet at 326°C (Figure 4c).
Reaction of methoxy groups with benzene T h e (5.14)HNaY zeolite, containing methoxy groups, was contacted with benzene with a simultaneous temperature increase until the maxima of the C - H or C-D bonds of the methoxy groups disappeared. As a result of the reaction of benzene with zeolite containing Z-O-CD~ groups, the bands at 2130 and 2070 cm -~ had already disappeared at 260°C (Figure 5j~. After the outgassing of the sample at 135°C, the OH groups in the zeolite were restored
(Figure 5g). I
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Figure 2 CD3OD chemisorption of NaY zeolite: (a) zeolite after activation at temperature 368°C, t = 144°C, p - 3 x 10 -3 Torr; (b) CD3OD adsorption, t = 144°C, PCD3OD = 24 Torr; (c) after outgassing at t = 145°C, p = 5 x 10 -3 Torr.
The same results were observed during the studies on the methoxy groups' activity that was formed by chemisorption of CH3OH on the HNaY zeolite (Figure 6). Under benzene pressure of 23 Torr, the increase of temperature in the measurement cell to 253°C caused the 2980 and 2850 cm -x bands to disappear entirely and the Br6nsted acid sites to reappear.
ZEOLITES, 1993, Vol 13, April/May
257
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski 37 I
37
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Figure 4 The effect of the t e m p e r a t u r e outgassing on con c e n tration of m e t h o x y groups in ( 5 . 1 4 ) H N a Y zeolite: (a) after absorption CH3OH; t = 144°C; p = 8.5 x 10-3 Torr; (b) t = 256°C;
(c) t = 326°C; (d) t = 371°C.
The analogous results referring to adsorption of methanol on such zeolites as faujasite and ZSM-5 have been described by many authors. 6--12 The methoxonium ions observed by some of these authors 6's'lh12 undergo transformation to methoxy species above 150°C. The stability of methoxyl groups separated after the outgassing of water and methanol excess from the zeolite (Z) results from the fact that dissociation of the Z-O-CH3 group with the formation of CH2: (double-radical) or their transformation into the hydrocarbon deposit requires high activation energy. The "a" spectrum in Figure 4 shows that in the range of 3500-3000 cm -1 absorbance of methoxylated zeolite was markedly higher in comparison with absorbance of the hydrogen form. OH stretching vibrations occur in this part of the spectrum and are seen as a wide band. They can derive from physically absorbed methanol, 6"]'~ methoxonium 12 ions, or water molecules bonded with hydrogen bonds. 14 Since the above band was rather weak and decayed with temperature increase, one can assume that methoxy groups dominated in the zeolite after chemisorption of methanol at 150°C. When benzene was introduced into the measurement cell, the Z-O-CD3 and Z-O-CH3 bands dis-
38
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DISCUSSION Attempts for the chemisorption of methanol on the hydrogen forms of the Y zeolite proved that the methanol-Br6nsted acid site complex was very stable. For example, the bands of the CD3 groups of the chemisorbed deuterium-substituted methanol were observed in the i.r. spectra even at 308°C and under the 7.0 x 10 - 2 T o r t (Figure 3e). In the case of CH3OH, the 2980 cm -l band atrributed to the methoxy group was still observed at 371 °C (Figure 4d). The high stability of the Br6nsted acid site methanol complex is confirmed by the data listed in Tables 1 and 2. The tables present results of the absorption measurements of O - H , C-H, and C-D bonds in the range of 3800-2400 cm-1 in relation to the time of the outgassing of the zeolite samples. Table 1 shows that the intensity of the 2980 and 2850 cm -I bands (attributed to C - H bonds of the methyl groups) is constant even when the hydrogen form of the methoxylated zeolite was heated to 256.5°C under 6.0 x 10 -3 Torr. Moreover, the reproduction of hydroxyl groups was not observed. In the case of the application of d e u t e r i u m substituted methanol (Table 2), the intensity of the CD3 groups was constant; however, a small increase of O-D concentration in relation to the time was observed. Hence, one can assume that as a result of methanol chemisorption the methyoxylation reaction of zeolite OH groups takes place (Figure 1).
258
ZEOLITE& 1993, Vol 13, April/May
\\\x\\
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Figure 5 Reaction of Z - O - C D 3 g r o u p s with benzene: (a) H N a Y , t = 135°C, p 2 x 10 -2 Tort; (b) C D 3 O D a b s o r p t i o n at 135°C, Pco3oo = 22 Torr; (c) after outgassing CD3OD, t = 140°C, p = 6 x 10- 2 Torr; (d) t = 140 o C, Pc6H s = 10 Torr; (e) t = 210 o C, PcsH s = 9.5 Torr; (f) t = 260°C, PCeHe ----- 8 Torr; (g) zeolite after outgassing benzene, t = 135°C, p = 1 x 10 -2 Torr.
Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski 38 I
33 . . . .
I
28x100cm . . . .
Table 2 The effect of the outgassing time of the measurement cell on the absorbance of the bands of the i.r. spectra of (5.14)HNaY zeolite that was chemisorbed by CD3OD
-1
I. /7
Outgassing time (min)
A2eTs
A2els
A213o
A2o7o
0 15 25 40 50
0.17 0.18 0.19 0.20 0.20
0.13 0.14 0.14 0.15 0.15
0.02 0.02 0.02 0.02 0.02
0.04 0.04 0.04 0.04 0.04
OD g r o u p s
CDz groups
Outgassing was carried o u t u n d e r 7.0 x 10 -2 Torr pressure and at the temperature of 260°C
Figure 6 Reaction of methoxy groups with benzene: (a) H N a Y after activation, t = 140°C; (b) zeolite after methanol chemisorption and outgassing measurement cell, t = 142°C; (c)t 140.5°C, 24 T o r t ; (d) t = 196~C, PC6H 6 = 24 T o r t ; (e) t = 253°C, Pc6H s = 23 T o r r ; (f) zeolite after outgassing benzene, t = 140.5°C, p = 2 x 10 -2 Torr.
PC6H s =
appeared even at 260°C, although they were stable at this temperature in the absence of benzene. This phenomenon can be explained by a hypothesis that the alkylation reaction of the benzene ring takes place. Moreover, the observation of the intensities of the OH maximum (Figure 6a and J') showed that the Br6nsted acid sites recover entirely in consequence of the reaction between the methoxyl groups with benzene. The absorbances at 3650 and 3540 cm-l of the OH groups were 0.50 and 0.33, respectively, for the initial hydrogen form (Figure 6a). After the Table 1 The effect of the outgassing time of the measurement cell on the absorbance of the bands of the i.r. spectra of (5.14)HNaY zeolite that was chemisorbed by methanol
groups
Outgassing time (rain)
A3640
A3s40
A29eo
A2eso
0 20 40 55
0.25 0.38 0.28 0.38
0.25 0.39 0.26 0.35
0.13 0.12 0.12 0.11
0.05 0.04 0.05 0.05
OH g r o u p s
CH 3
Outgassing was carried out under 6.0 x 10 -3 Torr pressure and at the temperature of 256.5°C
chemisorption of methanol and the reaction between methoxyl groups and benzene (Figure 6J), the absorbances of the rebuih hydroxyl groups were 0.47 and 0.34. The above results confirm the methylation mechanism presented in Figure I. The first step in the alkylation reaction of aromatic rings on the catalytically active zeolite is the chemisorption of methanol on the acid Br6nsted sites. As a result of the above, the formed methoxy groups react, in the second step, with the rings of the aromatic compounds. The sodium form of zeolite is not active in the methylation reaction because of the lack of strong acid sites. On the other hand, the recovery of the OH groups, observed in Figure 5, as a result of the reaction of Z-O-CD3 and benzene, is not the additional confirmation of the proposed mechanism. The parallel experiment proved that even at the ambient temperature an isotopic exchange between the deuteriumexchanged Br6nsted sites and benzene proceeded quickly enough to rebuild the OH groups of the zeolite. The mechanism proposed by the authors of the present paper differs from the one suggested by Mirth and Lercher." " Since the methoxonium ion is the intermediate form of the methoxylation of Br6nsted acid sites, under the conditions of the alkylation reaction at temperatures above 150°C, both species of methanol and, consequently, two parallel alkylation mechanisms undoubtedly take place. -
--
3
15
CONCLUSION l. A result of the chemisorption of CH3OH and CDaOD on the hydrogen-form Y zeolite is the formation of methoxy groups. These groups are stable at the temperature range 300-370°C. On the sodium-form Y zeolite (when strong Br6nsted acid sites are not present), no chemisorbed methanol was found. 2. When methoxy groups come into contact with benzene, they have already disappeared even at temperature 260°C. 3. On the basis of the above findings, it is possible to come to the conclusion that methylation of aromatic rings follows when methoxy groups react with aromatic compounds.
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Alkylation of benzene with methanol on zeolites: J. Rakoczy and T. Romotowski
REFERENCES 1 Venuto, P.B. and Landis, P.S. Adv. Catal. 1968, 18, 326 2 Kaeding, W.W., Chu, C., Young, L.B., Weinstein, B., Butter, S.A.J. Catal. 1981, 67(1), 159 3 Mirth, G. and Lerher, J.A., J. Catal. 1991, 132, 244 4 Rakoczy, J. and Sulikowski, B. React. Kinet. Catal. Lett. 1988, 36(1) 241 5 Rakoczy, J. React. Kinet. Catal. Lett., in press 6 Salvador, P. and Kladnig, W. J. Chem. Soc., Faraday Trans. / 1977, 73, 1153 7 Kubelkova, L., Novakova, J. and Jiru, P. in Structure and Reactivity of Modified Zeo/ites (Eds. P.A. Jacobs et al.) Elsevier, Amsterdam, 1984, p. 217
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8 Forester, T.R. and Howe, R.F.J. Am. Chem. Soc. 1987, 109, 5076 9 Zibtek, M. and Beresifiska, I. Zeolites 1985, 5, 245 10 Bosacek, V. and Twaruzkova, Z. Coll. Czech. Chem. Commun. 1971, 36, 551 11 Kubelkova, L., Novakova, J. and Dolejsek, Z. J. Catal. 1987, 108, 208 12 Kubelkova, L., Novakova, J. and Nedomova, K. J. Catal. 1990, 124, 441 13 Mirth, G., Lerher, J.A., Anderson, M.W. and Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 86(17) 3039 14 Jentys, A., Warecka, G., Derewinski, M. and Lercher, J.A.J. Phys. Chem. 1989, 93, 4837 15 Mirth, G. and Lerher, J.A.J. Phys. Chem. 1991, 95, 3736.