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S-alkylisothiouronium salts as phase-transfer catalysts in p-xylene oxidation by oxygen
Journal M2945
of Molfxulur Cata&sti,
73 (1992) Lll-L13
Lll
S-akylisothiouronium salts as phasetransfer catalysts in p-xylene oxidation by oxygen M. Harustiak
and A. Kaszonyi
Department of Organic Technology, Slovak Technical University, 812 37 Bratislava (Czechoslovakia) (Received November 20, 1991; accepted March 9, 1992)
The industrial importance of terephthalic acid during recent decades has inspired scientists in their efforts to seek more efficient catalytic reaction systems for its preparation by p-wlene oxidation. In commercial processes acetic acid is the most frequently used solvent. This oxidation, which has a free-radical reaction mechanism, is catalyzed by cobalt compounds together with bromine compounds (precursors of bromine radicals), usually CoBr,. When water is used instead of corrosive acetic acid as the solvent, a two-phase (aqueous and organic) reaction system is formed. However, cobalt bromide catalyst is dissolved in the aqueous phase only, and the oxidation of p-xylene does not proceed. We have found [l-2] that in the presence of catalytic amounts of lipophilic quaternary ammonium or phosphonium compounds, cobalt bromide possesses high catalytic activity in such a twophase system. The analogy between quaternary ammonium or phosphonium compounds (which are known to be phase-transfer catalysts) and S-all@sothiouronium salts was the reason why we studied the application of the latter in some phase-transfer catalytic reactions. We observed moderate catalytic efficiency in the substitution reaction of benzyl chloride with NaNOa, poor efficiency in the bromination of dodecanol with HBr. The subject of this study is the good phase-transfer catalytic efficiency of lipophilic S-alkylisothiouroni~ cations in the above-mentioned CoBracatalyzed oxidation of p-xylene in the presence of a separate aqueous phase. According to our knowledge this is the Grst example of the successful application of S-a&ylisothiouronium salts as phase-transfer catalysts. Experimental
S-alkylisothiouronium bromides with C4-C1a linear -1 groups were prepared [3] by alkylation of thiourea with corresponding alkyl bromides:
0304~6102j92/$6.00
Q 1992 - Elsevier Sequoia. All rights reserved
L12
H2N H2N,
,CS
+
‘*?\
+ .CS-R .;;/
R-Br+
IW
Bf
H2N
The reaction rates of p-xylene oxidation were measured by following the oxygen consumption. All reaction components and both catalysts were put into a 50 ml glass-lined reactor connected via a flexible metal capillary to the apparatus for measuring the oxygen consumption under a constant pressure of 0.8 MPa. The reaction mixture was mixed by shaking the reaction vessel in a thermostatted oil both, using a vibrator. The reaction rates were measured over the range of the speed of agitation, where transport phenomena do not limit reaction rates. Initial (YJ and maximum (r& reaction rates in Table 1 were calculated from two almost linear parts of the S-shaped plots of oxygen consumed versus time. Parameter N represents the total amount of oxygen consumed (in moles of oxygen per mol of starting hydrocarbon) by the time the reaction spontaneously stops.
Results and discussion Experimental data ofp-xylene oxidation in the presence of water, catalyzed by CoBr, and S-aIl@sothiouronium bromides with C&& alkyl groups (as an additive catalyst) are given in Table 1. Plots of the amount of oxygen consumed versus time showed S-shaped curves, which is typical for catalytic oxidation with a free-radical reaction mechanism. The main role of cobalt compounds is in catalytic decomposition of hydroperoxides and peroxy TABLE 1 The influence of the length of the alkyl group of additive catalyst on its catalytic activity Catalyst
1x104 (mol dmm3 s-‘)
r,,x 103 (mol dme3 s-l)
N (mol OJmol PX)
none S-butylisothiouronium bromide
0 0.4
0 0.5
0 0.14
S-hexyljsoth_iouronium bromide
1.1
1.0
0.23
S-octylisothiouronium bromide
2.4
1.4
0.54
S-decylisothiouronium bromide
2.4
1.6
0.57
S-dodecylisothiouronium bromide
2.4
1.9
0.63
Conditions: p-xylene 60 mmol, water 111 mmol, CoBrz 0.2 mmol, additive catalyst 0.4 mmol, pressure 0.8 MPa, react. temp. 140 “C.
L13
compounds formed during the reaction [2]. Bromine radicals, formed by the reaction of hydroperoxides with Br- , act as initiators. As we showed previously [l-2], lipophilic quaternary ammonium or phosphonium cations are active in the transfer of Br- from the aqueous to the organic phase. A higher concentration of Br- in the organic phase causes the formation of catalytic Co-Br species, and the oxidation reaction can proceed. Moreover, quaternary onium cations help to transfer new portions of Br- (as the precursors of bromine radicals) into the organic phase, because some of the bromine radicals reacts with other radicals in termination steps of the reaction, and stabile bromine compounds are formed. The oxidation reaction with S-alkylisothiouroniumcations as an additive catalyst has the same features as the reaction in which quaternary onium compounds are used. One can propose that their role is analogous to the role of quaternary onium cations, i.e., they act as phase-transfer catalysts for Br- anions. Their catalytic activity depends on their lipophilicity, represented by the length of the alkyl groups. The catalytic efficiency of S-(nbutyl)isothiouronium bromide is low, caused by its unfavourable distributing coefficient between the aqueous and organic phases, which does not allow a substantial increase in the Br- concentration in the organic phase. Longer alkyl groups make S-all@sothiouronium cations more lipophilic, and a greater catalytic effect was observed. S-alkylisothiouronium compounds with Clz or longer alkyl groups are able to form and stabilize micelles in multiphase systems. Therefore, their micellar catalytic effect (similar to that reported in ref. 4) should be proposed in addition to their phase-transfer catalytic activity. The advantage of S-all@sothiouronium salts (in comparison with quaternary onium phase-transfer catalysts) is that their preparation is based on the alkylation of thiourea instead of the alkylation of more expensive, higher tertiary amines or phosphines. References 1 M. Hronec and M. Harustiak,React. Kin&. Catal. L&t., 27 (1985) 231. 2 M. Hsrustiak, M. Hronec and J. Ilavsky, J. Mol. CataL, 53 (1989) 209. 3 S. McKenzie, in H. H. Reid (ed.), Organic Compounds of Sdphur, Selenium, and Tellurium, Vol. 1, The Chemical Society, London, 1970, chap. 5 and references therein. 4 M. Harustiak, M. Hronec, J. Ilavsky and S. Witek, C&x?. L&t., 2 (1988) 391.
of Molfxulur Cata&sti,
73 (1992) Lll-L13
Lll
S-akylisothiouronium salts as phasetransfer catalysts in p-xylene oxidation by oxygen M. Harustiak
and A. Kaszonyi
Department of Organic Technology, Slovak Technical University, 812 37 Bratislava (Czechoslovakia) (Received November 20, 1991; accepted March 9, 1992)
The industrial importance of terephthalic acid during recent decades has inspired scientists in their efforts to seek more efficient catalytic reaction systems for its preparation by p-wlene oxidation. In commercial processes acetic acid is the most frequently used solvent. This oxidation, which has a free-radical reaction mechanism, is catalyzed by cobalt compounds together with bromine compounds (precursors of bromine radicals), usually CoBr,. When water is used instead of corrosive acetic acid as the solvent, a two-phase (aqueous and organic) reaction system is formed. However, cobalt bromide catalyst is dissolved in the aqueous phase only, and the oxidation of p-xylene does not proceed. We have found [l-2] that in the presence of catalytic amounts of lipophilic quaternary ammonium or phosphonium compounds, cobalt bromide possesses high catalytic activity in such a twophase system. The analogy between quaternary ammonium or phosphonium compounds (which are known to be phase-transfer catalysts) and S-all@sothiouronium salts was the reason why we studied the application of the latter in some phase-transfer catalytic reactions. We observed moderate catalytic efficiency in the substitution reaction of benzyl chloride with NaNOa, poor efficiency in the bromination of dodecanol with HBr. The subject of this study is the good phase-transfer catalytic efficiency of lipophilic S-alkylisothiouroni~ cations in the above-mentioned CoBracatalyzed oxidation of p-xylene in the presence of a separate aqueous phase. According to our knowledge this is the Grst example of the successful application of S-a&ylisothiouronium salts as phase-transfer catalysts. Experimental
S-alkylisothiouronium bromides with C4-C1a linear -1 groups were prepared [3] by alkylation of thiourea with corresponding alkyl bromides:
0304~6102j92/$6.00
Q 1992 - Elsevier Sequoia. All rights reserved
L12
H2N H2N,
,CS
+
‘*?\
+ .CS-R .;;/
R-Br+
IW
Bf
H2N
The reaction rates of p-xylene oxidation were measured by following the oxygen consumption. All reaction components and both catalysts were put into a 50 ml glass-lined reactor connected via a flexible metal capillary to the apparatus for measuring the oxygen consumption under a constant pressure of 0.8 MPa. The reaction mixture was mixed by shaking the reaction vessel in a thermostatted oil both, using a vibrator. The reaction rates were measured over the range of the speed of agitation, where transport phenomena do not limit reaction rates. Initial (YJ and maximum (r& reaction rates in Table 1 were calculated from two almost linear parts of the S-shaped plots of oxygen consumed versus time. Parameter N represents the total amount of oxygen consumed (in moles of oxygen per mol of starting hydrocarbon) by the time the reaction spontaneously stops.
Results and discussion Experimental data ofp-xylene oxidation in the presence of water, catalyzed by CoBr, and S-aIl@sothiouronium bromides with C&& alkyl groups (as an additive catalyst) are given in Table 1. Plots of the amount of oxygen consumed versus time showed S-shaped curves, which is typical for catalytic oxidation with a free-radical reaction mechanism. The main role of cobalt compounds is in catalytic decomposition of hydroperoxides and peroxy TABLE 1 The influence of the length of the alkyl group of additive catalyst on its catalytic activity Catalyst
1x104 (mol dmm3 s-‘)
r,,x 103 (mol dme3 s-l)
N (mol OJmol PX)
none S-butylisothiouronium bromide
0 0.4
0 0.5
0 0.14
S-hexyljsoth_iouronium bromide
1.1
1.0
0.23
S-octylisothiouronium bromide
2.4
1.4
0.54
S-decylisothiouronium bromide
2.4
1.6
0.57
S-dodecylisothiouronium bromide
2.4
1.9
0.63
Conditions: p-xylene 60 mmol, water 111 mmol, CoBrz 0.2 mmol, additive catalyst 0.4 mmol, pressure 0.8 MPa, react. temp. 140 “C.
L13
compounds formed during the reaction [2]. Bromine radicals, formed by the reaction of hydroperoxides with Br- , act as initiators. As we showed previously [l-2], lipophilic quaternary ammonium or phosphonium cations are active in the transfer of Br- from the aqueous to the organic phase. A higher concentration of Br- in the organic phase causes the formation of catalytic Co-Br species, and the oxidation reaction can proceed. Moreover, quaternary onium cations help to transfer new portions of Br- (as the precursors of bromine radicals) into the organic phase, because some of the bromine radicals reacts with other radicals in termination steps of the reaction, and stabile bromine compounds are formed. The oxidation reaction with S-alkylisothiouroniumcations as an additive catalyst has the same features as the reaction in which quaternary onium compounds are used. One can propose that their role is analogous to the role of quaternary onium cations, i.e., they act as phase-transfer catalysts for Br- anions. Their catalytic activity depends on their lipophilicity, represented by the length of the alkyl groups. The catalytic efficiency of S-(nbutyl)isothiouronium bromide is low, caused by its unfavourable distributing coefficient between the aqueous and organic phases, which does not allow a substantial increase in the Br- concentration in the organic phase. Longer alkyl groups make S-all@sothiouronium cations more lipophilic, and a greater catalytic effect was observed. S-alkylisothiouronium compounds with Clz or longer alkyl groups are able to form and stabilize micelles in multiphase systems. Therefore, their micellar catalytic effect (similar to that reported in ref. 4) should be proposed in addition to their phase-transfer catalytic activity. The advantage of S-all@sothiouronium salts (in comparison with quaternary onium phase-transfer catalysts) is that their preparation is based on the alkylation of thiourea instead of the alkylation of more expensive, higher tertiary amines or phosphines. References 1 M. Hronec and M. Harustiak,React. Kin&. Catal. L&t., 27 (1985) 231. 2 M. Hsrustiak, M. Hronec and J. Ilavsky, J. Mol. CataL, 53 (1989) 209. 3 S. McKenzie, in H. H. Reid (ed.), Organic Compounds of Sdphur, Selenium, and Tellurium, Vol. 1, The Chemical Society, London, 1970, chap. 5 and references therein. 4 M. Harustiak, M. Hronec, J. Ilavsky and S. Witek, C&x?. L&t., 2 (1988) 391.