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Tert-butylation of phenol with isobutanol over magnesium- aluminium hydrotalcites
T.S.R. Prasada Rao and G. Murali Dhar (Editors)
563
Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved
Tert-butylation of phenol aluminium hydrotalcites
with
isobutanol
over
magnesium-
A. H. Padmasri, V. Durga Kumari and P. Kanta Rao* Catalysis & Physical Chemistry Section, Indian Institute of Chemical Technology, Hyderabad - 500 007, India Magnesium-aluminium hydrotalcites are prepared with Mg/AI atomic ratios 27 and characterized by XRD, DTA, surface area measurements and acidity-basicity measurements. The tert-butylation of phenol is carried out on the calcined magnesium-aluminium hydrotalcites using isobutanol in the temperature range of 350500~ The major products of this reaction are O-tert-butyl phenol and 2-tert-butyl phenol. Along with the normal alklyated products O-butenyl phenol and 2-butenyl phenols are also obtained. A correlation between the product distribution and the nature of acid-base properties of the catalyst is studied. I. I N T R O D U C T I O N Tert-butylation is an important reaction as the products of this reaction, tertbutyl phenols are used as antioxidants, in the synthesis of various agrochemicals, flagrance compounds, thermoresistant polymers and protecting agents for plastics. The other products namely O-butyl and butenyl phenols are used in the synthesis of flavouring, fragrance compounds and pharmaceutical intermediates, chromans[ 1]. Butylation of phenol is reported on acidic catalysts HY zeolite, A1CI3, ionexchange resins and on superacid catalysts like Nation-H[2-5] in liquid phase using an alkene, isobutene or an alcohol, isobutanol/tert-butanol and on zeolite catalysts [6-7] with isobutanol/tert-butanol in vapour phase. Further MTBE is also reported for butylation of phenol in vapour phase[8]. However, tert-butylation of phenol by isobutanol has not been studied extensively in vapour phase. Many conventiorL~' acid and base catalysts are being replaced by more eco-friendly solid catalysts. In the present investigation calcined magnesiumaluminium hydrotaleites are used for the tert-butylation of phenol using isobutanol. The aim of the present work is to correlate the acid-base properties of hydrotalcites with the obtained product distribution. Magnesium-aluminium hydrotalcite is a naturally occurring anionic clay that decomposes reversibly upon calcination at high temperatures to form high surface area basic mixed oxide. To generate the hydrotalcite structure, AI3+ cations replace *For c~~ndenc~
564 some of the Mg 2§ cations in the brucite, Mg(OH)2 and the net positive charge produced is balanced by the inerlayered carbonate. In addition to carbonate, water molecules are also present.The synthetic hydrotalcite-like compounds have a general formula [Ml-x nMxm(OH)2]x"§ Ax/~.mH20 where M n & M m are divalent and trivalent metal ions respectively. 'A' is the interexchangeable anion with charges from -1 to -3 and x = 0.1-0.33. The structure can accommodate wide variations in the Mg2§ 3§ ratio, type of anions and different 2+ and 3+ cations[9-11 ]. 2. E X P E R I M E N T A L
2.1. Preparation and characterization Hydrotalcites with Mg/AI atomic ratios of 2-7 were prepared by coprecipitation under super saturation conditions [ 11] by adding a mixture of aqueous NaOH and Na2CO3 to a solution containing metal nitrates at room temperature with vigorous stirring maintaining the pH 9-10. The resulting heavy slurry was aged at 65~ for 1 hour with stirring. The precipitate was filtered and washed with warm distilled water until the Na contents of the resulting solid was below 0.1% and dried in an oven at 110~ overnight. Thus obtained hydrotacites were calcined in air at 500~ for 8h. Table 1 shows the characteristics of the hydrotalcites prepared by varying the Mg/AI ratio between 2 to 7. The surface area of the calcined catalysts varied between 90 to 142m2g"1. The maximum surface area is obtained on hydrotalcite with Mg/AI ratio=4. Acidity measured by n-butylamine titration decreased with increasing Mg/AI ratio. Basicity measured by benzoic acid titration increased with increasing Mg/AI ratio from 2 to 4.5 and there is no further increase with increase in Mg/AI ratio. Table 1 Characteristics of Hydrotalcites Total acidity (in Total basicity (in Catalyst Mg:AI BET atomic surface m.moles of n- m.moles of Pharea butyl amine g~ COOH g~ cat.) ratio (m2g'l) cat.) 0.38 A1203 0:1 220 0.90 4.33 HT-I 2:1 120 0.65 4 92 HT-2 3:1 90 0.51 510 HT-3 4:1 142 0.53 512 HT-4 4.5:1 110 0.52 4 70 HT-5 5:1 120 0.53 512 HT-6 7:1 130 0.44 5 12 ....M~O 1"0 110 0.18 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
XRD (Phase) uncalcined calcined Bohemite T-A1203 Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Brucite M~O .
.
.
.
.
.
.
565 The hydrotalcites were characterized by XRD recorded on a Phillips PW 1051 diffractometer using CuKct radiation. The surface areas of the samples were determined by BET method on a high vacuum system made of glass. DTA patterns taken on a Leeds and Northup DTA unit up to 600~ in air gave two characteristic endotherms which have been reported previously [ 12,13] that below 200~ only the interstitial water is lost and on further heating to 450-500~ dehydroxylation is complete and carbonate is lost as CO2.
2.2 Apparatus and procedure About 1 g of the catalyst was loaded supported on glass wool diluted with ceramic beeds in a flow type reactor mounted vertically in a cylindrical furnace and the temperature was measured by a moving Cr-AI thermocouple placed in a centrally located thermowell. The catalyst was then calcined at 500~ in air for 8h. It was further activated at the calcination temperature in N2 flow for about 4h before each experimental run. The reaction mixture (Ph-OH and iso-BuOH) was introduced through a calibrated B.Braun secura syringe pump to give a controlled feed rate and the products were collected every hour using an ice-trap and analyzed by a gasschromatograph using 10% SE-30 column with flame ionization detector. The products were then confirmed by GC-MS analysis.
Feed: Motar
rotio ( 1 : 7 ]
(~-OH:
isoBuOH)
4-
2mr
h "1
o-
/,ml
h -1
60
O m,,
~n ty'
1,0
14.1
z o L)
20
I
i
Y-At203 HT-1 HT-3 HT-5 HT-6 MgO CATALYST
Figure 1. Change in the conversion of phenol in the tert-butylation reaction over different hydrotalcites at 450~
566 3. R E S U L T S A N D D I S C U S S I O N
3.1. t-butylation of phenol The t-butylation of phenol over A1203, MgO and hydrotalcites is studied using a feed mixture of phenol and isobutanol (Ph-OH : iso-BuOH = 1:7) to understand the catalytic activity of these catalysts at 450~ varying the contact time. The results are shown in Figure 1. It can be seen from the figure that butylation activity is low on pure A1203 and MgO compared to hydrotalcites. On introducing AI3+ into magnesia lattice the butylation activity has shown a characteristic variation. The maximum butylation activity is seen on Mg/AI ratio = 4-5. A similar pattern is observed at both the contact times studied. Figure 2a shows the effect of feed rate on t-butylation of phenol over hydrotalcite. These results show that a feed rate of 4 ml h1 appears to be ideal for tbutylation. A phenol:alcohol ratio of 1:7 appears to be more suitable with respect to the activity of the catalysts (Figure 2b). t-butylation of phenol with isobutanol is studied in the temperature range of 350-500~ over A1203, MgO and hydrotalcites and is shown in Figure 3. The butylation activity is low on Al2Os in the temperature range studied and this may be seen as due to the alcohol side reactions. The high temperature activity of MgO may be due to the basicity available only at and above 450~ [ 14,15]. A linear increase in butylation activity is seen on hydrotalcites. 50
80
o - 1"5 ( $ - O H z c)
40
9 - 1:7($-OH. iso-BuOH)
|
iso-BuOH)
3o
60
U3
,,,
40
Z
o u
(J
20
Io
.< 0
~
I
s
-_I
8
I
~o
I
~2
FEED EATE ( ml h "1)
Figure 2a. Effect of feed rate on tertbutylation of phenol over HT-3(Mg/AI = 4) at 450~
~-At203
HT1
HT3
MgO
CATALYST
Figure 2b. Effect of molar ratio on tert-butylation of phenol over hydrotalcites at 4 ml h"1and 450 ~
567
&-'/-At203 =-HT 1 O-HT 3 B-HT 5 o-HT 6
e-MgO 0 -
z o u3 iz
ta >,
t,0-
Z O (.3
20-
0
w
350
I
z.O0
z.50
500
TEMPERATURE(*C)
Figure 3. Effect of temperature on tert-butylation of phenol over hydrotalcites at a feed (0-OH" iso-BuOH = 1"7) rate of 4 ml h~.
The main products of t-butylation of phenol on hydrotalcites are O- and 2butenyl phenols and O- and 2-tert-butyl phenols. The other dialkenyl and alkyl phenols are also formed in trace amounts. However, the nature of the products differed depending on the acid-base properties of hydrotalcites. Figure 4 shows the product distribution at different temperatures. O- and 2-tert-butyl phenols are the only products on A1203 at 400~ On MgO, O-butenyl and 2-butenyl phenols are obtained in equal amount at 400~ and with increase in temperature the formation of 2-butyl phenol is also observed. The product distribution on hydrotalcites differed from that of A1203 and MgO. On HT with Mg/AI ratio = 2, only O- alkylation is observed. On HT with Mg/AI ratio = 4-6, C-alkylation is also taking place along with O-alkylation. 2-butenyl phenol is formed only above 450~ There is no pronounced effect of Mg/Al ratio on the selectivity of this product.
568 80 - 1= O-butenyt phenol 2 =O-tert-butyt phenol
60
l,
2
phenol
3:2-kUtt~ /.- 2-tert-butyt phenol
T=500oc
2
3 2
&020 _,
1
2
1
~
i
T : 1,50oc 80
Z, _
2
,,-,
>. 60 i.,,.
2
2
3
2
~,1,0 u .J
w
m
1
20
I.
1
/.
1
I,
i !1
T= 1.00oC
8060
t,
2 2 2
2
1
3
1.0 20
i i i
Y-At203 HT-1
HT-3 HT-S CATALYST ---,-.
n
HT-6
MgO
Figure 4. Change in selectivities in the tert-butylation of phenol over hydrotalcites at a feed (~-OH 9 iso-BuOH = 1"7) rate of 4 ml h "t. The time on stream ofbutylation activity studied over HT-3 (Mg/AI ratio = 4) in the temperature range of 350-500~ is shown in Figure 5. A steady activity is observed at 350~ and 400~ The activity is seen increasing at 450~ indicating that more alcohol is available for butylation with time. A high steady activity of this reaction is observed on HT-3 at 500~
569
70
60 500~ 50
W
A v
z
0
....,,
r~ w ~> z 0
/.50~ ,
40 30'
2o
4 k . ~
4 O0
10 0
~A
A w
350~ I
60
I
120
I
180
T I ME | MIN )
Figure 5. Change in conversion of phenol in the tert-butylation reaction with time on stream at a feed (~-OH : iso-BuOH - 1:7) rate of 4 ml h 1 over HT-3 (Mg:AI = 4).
The product distribution in the t-butylation of phenol with isobutanol may be explained based on the nature of acid-base strengths and the mode of adsorption of phenol. It has been reported in the literature that phenol is adsorbed horizontally on acid-catalysts like AIzO3 and a vertical mode of adsorption is proved on basic catalysts like MgO [16,17]. The horizontal adsorption of phenol results in O-alkylation and also C-alkylation at ortho and para positions which are close to the surface of the catalyst whereas in the vertical adosrption mode, only the ortho selectivity is observed. The extent of C-alkylation depends on the strength of the acid site. However, a combined participation of acid-base properties is also reported in the methylation of phenol over hydrotalcites [18]. Hence, a scheme depicting the correlation between the acid-base properties of the catalysts with the product distribution in the t-butylation of phenol is shown below as:
570 Catalyst
Temperature Active site
'y-Al203
low high
acid acid
Mode adsorption phenol horizontal horizontal
Hydrotalcite
low
acid & base
horizontal
O-tert-butyl phenol O-butenyl phenol
high
acid & base
horizontal/vertical
O-tert-butyl phenol O-butenyl phenol 2-tert-butyl phenol 2-butenyl phenol
low
weak acid/ horizontal strong base
O-butenyl phenol 2-butenyl phenol
high
weak acid/ strong base
O-butenyl phenol 2-tert-butyl phenol 2-butenyl phenol
MgO
of Product of
horizontal/vertical
O-tert-butyl phenol 2-tert-butyl phenol
4. CONCLUSIONS In the tert-butylation of phenol it is found that hydrotalcites are more active than the pure oxides, MgO and y-A1203. The formation of the O-alkylated products on these basic catalysts indicate the presence of weak acidic sites. It was also quite interesting to observe the formation of the alkenyl phenols along with the normal products whose mass differed by 2 units from the normal alkyl phenols. The distribution of the products varied with reaction temperature and the acid-base properties of the catalysts. ACKNOWLEDGEMENTS One of the authors A.H. Padmasri, JRF thanks the U.G.C, New Delhi for awarding fellowship.
571 REFERENCES
.
.
.
.
o
9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Barbara Elvers, Stephen Hawkins, Gail Schulz (eds.), Ullmann's Encylopedia of Industrial Chemistry, 5th edn., VoI.A. 19, p.325, p.341. Corma, Avelino; Garcia, Hermenegildo; Primo, Jaime; J.Chem.Res., Synop. 40 (1988) 1. V.A. Koshchii, B.Ya. Kozlikovskii, A.A. Matyusha, Zh. Org. Khim. 24(7) (1988) 1508. D.V. Milc'henko, E.L.Savel'eva, A.Yu. Milc'henkoYuffa, A.Ya Neitekhimiya, 29(3) (1989) 336. A. Rajeev Rajadhyaksha and D. Dilip Chaudhari, Ind. Eng. Chem. Res. 26(7) (1987) 1276. Xu. Peimo; Feng. Bing; Chen. Shaozhan, Huadong Huadong Xueynan Xuebao, 14(4) (1988) 476. D. Claren'ce Chang, D. Stuart Hellring (Mobil Oil, Corp.), U.S. Us.5,288,927 (C1 568-789; C07(37/14), 22 Feb. 1994, Appl. 11,574, 01 Feb.1993, 4 pp. K. Gokul Chandra, M.M. Sharma, Catal. Letters, 19(4) (1993) 309. S. Miyata, Clays Clay Miner., 23 (1975) 369. W.T. Reichle, J. Catal., 94 (1985) 547. F. Cavani, F. Trifiro and Vaccari, Catal. Today, 11 (1991) 173. S. Miyata, Clays Clay Miner., 28 (1980) 50. W.T. Reichle, S.Y. Kang and D.S. Everhardt, J. Catal., 101 (1986) 352. V. Durgakumari, G. Sreekanth and S. Narayanan, Research on Chemical Intermediates, 14 (1990) 223. H. Hattori, Materials Chemistry and Physics, 18 (1988) 533. K. Tanabe and T. Nishizaki, in G.C. Bond, P.B. Wells and F.C. Tompkins (eds.), Proceedings of the 6th Int. Conf. on Catalysis, The Chemical Soc., London, 1977, Vol.2, p.863. K. Tanabe, B. Imelik, C. Naccache, G. Coudurier, Y. B~enTaarit and J.C. Vedrin (eds.), Catalysis by Acids and Bases, Elsevier, Amsterdam, 1985, p. 1. S. Velu and C.S. Swamy, Appl. Catal., 119 (1994) 241.
563
Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved
Tert-butylation of phenol aluminium hydrotalcites
with
isobutanol
over
magnesium-
A. H. Padmasri, V. Durga Kumari and P. Kanta Rao* Catalysis & Physical Chemistry Section, Indian Institute of Chemical Technology, Hyderabad - 500 007, India Magnesium-aluminium hydrotalcites are prepared with Mg/AI atomic ratios 27 and characterized by XRD, DTA, surface area measurements and acidity-basicity measurements. The tert-butylation of phenol is carried out on the calcined magnesium-aluminium hydrotalcites using isobutanol in the temperature range of 350500~ The major products of this reaction are O-tert-butyl phenol and 2-tert-butyl phenol. Along with the normal alklyated products O-butenyl phenol and 2-butenyl phenols are also obtained. A correlation between the product distribution and the nature of acid-base properties of the catalyst is studied. I. I N T R O D U C T I O N Tert-butylation is an important reaction as the products of this reaction, tertbutyl phenols are used as antioxidants, in the synthesis of various agrochemicals, flagrance compounds, thermoresistant polymers and protecting agents for plastics. The other products namely O-butyl and butenyl phenols are used in the synthesis of flavouring, fragrance compounds and pharmaceutical intermediates, chromans[ 1]. Butylation of phenol is reported on acidic catalysts HY zeolite, A1CI3, ionexchange resins and on superacid catalysts like Nation-H[2-5] in liquid phase using an alkene, isobutene or an alcohol, isobutanol/tert-butanol and on zeolite catalysts [6-7] with isobutanol/tert-butanol in vapour phase. Further MTBE is also reported for butylation of phenol in vapour phase[8]. However, tert-butylation of phenol by isobutanol has not been studied extensively in vapour phase. Many conventiorL~' acid and base catalysts are being replaced by more eco-friendly solid catalysts. In the present investigation calcined magnesiumaluminium hydrotaleites are used for the tert-butylation of phenol using isobutanol. The aim of the present work is to correlate the acid-base properties of hydrotalcites with the obtained product distribution. Magnesium-aluminium hydrotalcite is a naturally occurring anionic clay that decomposes reversibly upon calcination at high temperatures to form high surface area basic mixed oxide. To generate the hydrotalcite structure, AI3+ cations replace *For c~~ndenc~
564 some of the Mg 2§ cations in the brucite, Mg(OH)2 and the net positive charge produced is balanced by the inerlayered carbonate. In addition to carbonate, water molecules are also present.The synthetic hydrotalcite-like compounds have a general formula [Ml-x nMxm(OH)2]x"§ Ax/~.mH20 where M n & M m are divalent and trivalent metal ions respectively. 'A' is the interexchangeable anion with charges from -1 to -3 and x = 0.1-0.33. The structure can accommodate wide variations in the Mg2§ 3§ ratio, type of anions and different 2+ and 3+ cations[9-11 ]. 2. E X P E R I M E N T A L
2.1. Preparation and characterization Hydrotalcites with Mg/AI atomic ratios of 2-7 were prepared by coprecipitation under super saturation conditions [ 11] by adding a mixture of aqueous NaOH and Na2CO3 to a solution containing metal nitrates at room temperature with vigorous stirring maintaining the pH 9-10. The resulting heavy slurry was aged at 65~ for 1 hour with stirring. The precipitate was filtered and washed with warm distilled water until the Na contents of the resulting solid was below 0.1% and dried in an oven at 110~ overnight. Thus obtained hydrotacites were calcined in air at 500~ for 8h. Table 1 shows the characteristics of the hydrotalcites prepared by varying the Mg/AI ratio between 2 to 7. The surface area of the calcined catalysts varied between 90 to 142m2g"1. The maximum surface area is obtained on hydrotalcite with Mg/AI ratio=4. Acidity measured by n-butylamine titration decreased with increasing Mg/AI ratio. Basicity measured by benzoic acid titration increased with increasing Mg/AI ratio from 2 to 4.5 and there is no further increase with increase in Mg/AI ratio. Table 1 Characteristics of Hydrotalcites Total acidity (in Total basicity (in Catalyst Mg:AI BET atomic surface m.moles of n- m.moles of Pharea butyl amine g~ COOH g~ cat.) ratio (m2g'l) cat.) 0.38 A1203 0:1 220 0.90 4.33 HT-I 2:1 120 0.65 4 92 HT-2 3:1 90 0.51 510 HT-3 4:1 142 0.53 512 HT-4 4.5:1 110 0.52 4 70 HT-5 5:1 120 0.53 512 HT-6 7:1 130 0.44 5 12 ....M~O 1"0 110 0.18 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
XRD (Phase) uncalcined calcined Bohemite T-A1203 Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Hydrotalcite MgO Brucite M~O .
.
.
.
.
.
.
565 The hydrotalcites were characterized by XRD recorded on a Phillips PW 1051 diffractometer using CuKct radiation. The surface areas of the samples were determined by BET method on a high vacuum system made of glass. DTA patterns taken on a Leeds and Northup DTA unit up to 600~ in air gave two characteristic endotherms which have been reported previously [ 12,13] that below 200~ only the interstitial water is lost and on further heating to 450-500~ dehydroxylation is complete and carbonate is lost as CO2.
2.2 Apparatus and procedure About 1 g of the catalyst was loaded supported on glass wool diluted with ceramic beeds in a flow type reactor mounted vertically in a cylindrical furnace and the temperature was measured by a moving Cr-AI thermocouple placed in a centrally located thermowell. The catalyst was then calcined at 500~ in air for 8h. It was further activated at the calcination temperature in N2 flow for about 4h before each experimental run. The reaction mixture (Ph-OH and iso-BuOH) was introduced through a calibrated B.Braun secura syringe pump to give a controlled feed rate and the products were collected every hour using an ice-trap and analyzed by a gasschromatograph using 10% SE-30 column with flame ionization detector. The products were then confirmed by GC-MS analysis.
Feed: Motar
rotio ( 1 : 7 ]
(~-OH:
isoBuOH)
4-
2mr
h "1
o-
/,ml
h -1
60
O m,,
~n ty'
1,0
14.1
z o L)
20
I
i
Y-At203 HT-1 HT-3 HT-5 HT-6 MgO CATALYST
Figure 1. Change in the conversion of phenol in the tert-butylation reaction over different hydrotalcites at 450~
566 3. R E S U L T S A N D D I S C U S S I O N
3.1. t-butylation of phenol The t-butylation of phenol over A1203, MgO and hydrotalcites is studied using a feed mixture of phenol and isobutanol (Ph-OH : iso-BuOH = 1:7) to understand the catalytic activity of these catalysts at 450~ varying the contact time. The results are shown in Figure 1. It can be seen from the figure that butylation activity is low on pure A1203 and MgO compared to hydrotalcites. On introducing AI3+ into magnesia lattice the butylation activity has shown a characteristic variation. The maximum butylation activity is seen on Mg/AI ratio = 4-5. A similar pattern is observed at both the contact times studied. Figure 2a shows the effect of feed rate on t-butylation of phenol over hydrotalcite. These results show that a feed rate of 4 ml h1 appears to be ideal for tbutylation. A phenol:alcohol ratio of 1:7 appears to be more suitable with respect to the activity of the catalysts (Figure 2b). t-butylation of phenol with isobutanol is studied in the temperature range of 350-500~ over A1203, MgO and hydrotalcites and is shown in Figure 3. The butylation activity is low on Al2Os in the temperature range studied and this may be seen as due to the alcohol side reactions. The high temperature activity of MgO may be due to the basicity available only at and above 450~ [ 14,15]. A linear increase in butylation activity is seen on hydrotalcites. 50
80
o - 1"5 ( $ - O H z c)
40
9 - 1:7($-OH. iso-BuOH)
|
iso-BuOH)
3o
60
U3
,,,
40
Z
o u
(J
20
Io
.< 0
~
I
s
-_I
8
I
~o
I
~2
FEED EATE ( ml h "1)
Figure 2a. Effect of feed rate on tertbutylation of phenol over HT-3(Mg/AI = 4) at 450~
~-At203
HT1
HT3
MgO
CATALYST
Figure 2b. Effect of molar ratio on tert-butylation of phenol over hydrotalcites at 4 ml h"1and 450 ~
567
&-'/-At203 =-HT 1 O-HT 3 B-HT 5 o-HT 6
e-MgO 0 -
z o u3 iz
ta >,
t,0-
Z O (.3
20-
0
w
350
I
z.O0
z.50
500
TEMPERATURE(*C)
Figure 3. Effect of temperature on tert-butylation of phenol over hydrotalcites at a feed (0-OH" iso-BuOH = 1"7) rate of 4 ml h~.
The main products of t-butylation of phenol on hydrotalcites are O- and 2butenyl phenols and O- and 2-tert-butyl phenols. The other dialkenyl and alkyl phenols are also formed in trace amounts. However, the nature of the products differed depending on the acid-base properties of hydrotalcites. Figure 4 shows the product distribution at different temperatures. O- and 2-tert-butyl phenols are the only products on A1203 at 400~ On MgO, O-butenyl and 2-butenyl phenols are obtained in equal amount at 400~ and with increase in temperature the formation of 2-butyl phenol is also observed. The product distribution on hydrotalcites differed from that of A1203 and MgO. On HT with Mg/AI ratio = 2, only O- alkylation is observed. On HT with Mg/AI ratio = 4-6, C-alkylation is also taking place along with O-alkylation. 2-butenyl phenol is formed only above 450~ There is no pronounced effect of Mg/Al ratio on the selectivity of this product.
568 80 - 1= O-butenyt phenol 2 =O-tert-butyt phenol
60
l,
2
phenol
3:2-kUtt~ /.- 2-tert-butyt phenol
T=500oc
2
3 2
&020 _,
1
2
1
~
i
T : 1,50oc 80
Z, _
2
,,-,
>. 60 i.,,.
2
2
3
2
~,1,0 u .J
w
m
1
20
I.
1
/.
1
I,
i !1
T= 1.00oC
8060
t,
2 2 2
2
1
3
1.0 20
i i i
Y-At203 HT-1
HT-3 HT-S CATALYST ---,-.
n
HT-6
MgO
Figure 4. Change in selectivities in the tert-butylation of phenol over hydrotalcites at a feed (~-OH 9 iso-BuOH = 1"7) rate of 4 ml h "t. The time on stream ofbutylation activity studied over HT-3 (Mg/AI ratio = 4) in the temperature range of 350-500~ is shown in Figure 5. A steady activity is observed at 350~ and 400~ The activity is seen increasing at 450~ indicating that more alcohol is available for butylation with time. A high steady activity of this reaction is observed on HT-3 at 500~
569
70
60 500~ 50
W
A v
z
0
....,,
r~ w ~> z 0
/.50~ ,
40 30'
2o
4 k . ~
4 O0
10 0
~A
A w
350~ I
60
I
120
I
180
T I ME | MIN )
Figure 5. Change in conversion of phenol in the tert-butylation reaction with time on stream at a feed (~-OH : iso-BuOH - 1:7) rate of 4 ml h 1 over HT-3 (Mg:AI = 4).
The product distribution in the t-butylation of phenol with isobutanol may be explained based on the nature of acid-base strengths and the mode of adsorption of phenol. It has been reported in the literature that phenol is adsorbed horizontally on acid-catalysts like AIzO3 and a vertical mode of adsorption is proved on basic catalysts like MgO [16,17]. The horizontal adsorption of phenol results in O-alkylation and also C-alkylation at ortho and para positions which are close to the surface of the catalyst whereas in the vertical adosrption mode, only the ortho selectivity is observed. The extent of C-alkylation depends on the strength of the acid site. However, a combined participation of acid-base properties is also reported in the methylation of phenol over hydrotalcites [18]. Hence, a scheme depicting the correlation between the acid-base properties of the catalysts with the product distribution in the t-butylation of phenol is shown below as:
570 Catalyst
Temperature Active site
'y-Al203
low high
acid acid
Mode adsorption phenol horizontal horizontal
Hydrotalcite
low
acid & base
horizontal
O-tert-butyl phenol O-butenyl phenol
high
acid & base
horizontal/vertical
O-tert-butyl phenol O-butenyl phenol 2-tert-butyl phenol 2-butenyl phenol
low
weak acid/ horizontal strong base
O-butenyl phenol 2-butenyl phenol
high
weak acid/ strong base
O-butenyl phenol 2-tert-butyl phenol 2-butenyl phenol
MgO
of Product of
horizontal/vertical
O-tert-butyl phenol 2-tert-butyl phenol
4. CONCLUSIONS In the tert-butylation of phenol it is found that hydrotalcites are more active than the pure oxides, MgO and y-A1203. The formation of the O-alkylated products on these basic catalysts indicate the presence of weak acidic sites. It was also quite interesting to observe the formation of the alkenyl phenols along with the normal products whose mass differed by 2 units from the normal alkyl phenols. The distribution of the products varied with reaction temperature and the acid-base properties of the catalysts. ACKNOWLEDGEMENTS One of the authors A.H. Padmasri, JRF thanks the U.G.C, New Delhi for awarding fellowship.
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