- Email: [email protected]
Hydroformylation of 7-tetradecene using Rh-TPPTS in a microemulsion
Applied Catalysis A: General 236 (2002) 173–178
Hydroformylation of 7-tetradecene using Rh-TPPTS in a microemulsion Marco Haumann a , Hülya Yildiz b , Herbert Koch c , Reinhard Schomäcker b,∗ b
a Department of Chemistry, University of Cape Town, Cape Town, South Africa Institut für Technische Chemie, Technische Universität Berlin, Sekretarait TC 8, Strasse des 17 Juni 135, Berlin, Germany c Sasol Germany GmbH, Olefins and Surfactants, Marl, Germany
Received 16 October 2001; received in revised form 10 May 2002; accepted 13 May 2002
Abstract Water soluble rhodium catalyst complexes are highly active in a microemulsion system stabilized by technical grade surfactants of the alkyl-polyglycolether type. At temperatures around 120 ◦ C and pressures of 100 bar internal alkenes are hydroformylated with high regioselectivity. 7-Tetradecene as model alkene was effectively hydroformylated in microemulsions with high reaction rates yielding 2-hexyl-nonal. Under the reaction conditions applied the equilibria between different catalytically active complexes are shifted towards the unmodified HRh(CO)3 . Easy catalyst recycling of the water soluble modified complex is possible when the equilibrium is shifted back under work-up conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Microemulsion; Hydroformylation; Internal alkene; Water soluble; Rhodium branched aldehyde
1. Introduction The hydroformylation or oxo synthesis is one of the most important applications of homogeneous catalysis on an industrial scale [1,2]. Since, the establishment of two phase catalysis in the Ruhrchemie/Rhˆone-Poulenc process (RCH/RP) loss of catalyst as the major disadvantage of homogeneous catalysis is negligible, since the catalyst is immobilized within the water phase. The only drawback of this low-pressure-oxo (LPO) process is its limitation for propene and butene. In a recent paper, we have reported the hydroformylation of 1-dodecene in a microemulsion using the water soluble Rh-TPPTS catalyst complex [3]. High activi∗ Corresponding author. Tel.: +49-30-3-14-24973; fax: +49-30-3-14-79552. E-mail address: [email protected] (R. Schomäcker).
ties and good selectivities towards terminal aldehydes can be achieved under moderate reaction conditions (80 ◦ C, 80 bar). At higher temperatures the terminal double bond is shifted towards the thermodynamically favored internal positions. GC–MS experiments have shown that these internal alkenes are converted into their corresponding aldehydes. The reactivity towards hydroformylation decreases dramatically from terminal alkenes over internal alkenes to branched internal alkenes as depicted in Fig. 1 [4]. In this work, the terminal alkene 1-dodecene was replaced by 7-tetradecene to study the hydroformylation of an internal alkene in more detail. Due to the lower reactivity of the internal alkene higher temperatures and syngas pressures have to be applied. Under these conditions the equilibrium between ligand modified and unmodified catalyst complexes can be shifted towards the unmodified
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 2 8 4 - 3
174
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
Fig. 1. Relative activities of alkenes in hydroformylation.
rhodium species. −L
+CO
HRh(CO)L3 HRh(CO)L2 HRh(CO)2 L2 +L
−CO
−L
−L
+L
+L
internal alkene. The pH of the aqueous phase was 7, the overall rhodium concentration was 200 ppm with a ligand/metal ratio of 4 (for most experiments) and an alkene/metal ratio of 2500.
HRh(CO)2 L HRh(CO)3
The formation of unmodified rhodium species should result in increased activity as well as lower selectivity towards linear aldehydes.
3. Results
2. Experimental setup
Due to the lower reactivity of 7-tetradecene compared to 1-dodecene studied in [3] the reaction temperature has to be higher than 75 ◦ C. Therefore, the more hydrophilic surfactants Marlipal O13/80 and O13/100 were chosen, forming single phase microemulsions at 100 and 120 ◦ C, respectively. Hydroformylation experiments were carried out at 100 bar syngas pressure and temperatures between 60 and 120 ◦ C (Fig. 2). The conversion of 7-tetradecene at 80 ◦ C was significantly lower compared to 1-dodecene. In Table 1 the results are summarized. The selectivity data clearly show that the rhodium catalyst mainly converted the internal double bond into the branched aldehyde 2-hexyl nonanal (2-HN). From the rate constants given in Table 1 the activation energy was calculated to be 70 kJ/mol. This value
The setup was identical to the previous one used for the hydroformylation of 1-dodecene [3]. The phase behavior of microemulsions using 1-dodecene has been studied in detail and it is known from the literature that the general pattern observed with 1-dodecene can also be applied when using 7-tetradecene as alkene [5]. Single phase microemulsions can be formed between 60 ◦ C and 120 ◦ C by the use of surfactants with different degrees of ethoxylation. For the system Rh-TPPTS/1-dodecene the influence of stirrer speed, catalyst composition and acidity of the aqueous catalyst phase has been studied in detail [4]. The optimal process parameters of these investigations were applied for the conversion of the
3.1. Variation of temperature and estimation of activation energy
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
175
Fig. 2. Hydroformylation of 7-tetradecene at various temperatures. 100 bar, 200 ppm rhodium, L/M = 4, Marlipal O13/80.
is almost 16 kJ/mol higher than the one determined for the hydroformylation of 1-dodecene (54.5 kJ/mol) under the same conditions [3]. This is a result of the lower reactivity of internal alkenes compared to the corresponding terminal alkenes reported in the literature (see Fig. 1).
observed and this was further enhanced at 110 ◦ C (see Table 2). At 110 ◦ C the reaction rate was studied as a function of pressure between 60 and 130 bar. The overall rate constant kobs was determined using data fitting software tools derived by Hugo at TU Berlin from the rate equation
3.2. Influence of total syngas pressure
dcolefin = kobs (colefin )n dt with kobs = kr ccatalyst (pCO )x (pH2 )y −
At 80 ◦ C the pressure dependence was studied between 60 and 100 bar. As expected a strong increase in reaction velocity is observed with increasing pressure, however on a lower level compared to the conversion of 1-dodecene. On increasing the pressure from 80 to 100 bar at 80 ◦ C a marked acceleration in rate was
A first order dependence with regard to the olefin was observed in all experiments. In Table 2 the results for the pressure dependence are summarized. The high regioselectivity of the CO insertion has been verified by GC–MS measurements done
Table 1 Hydroformylation of 7-tetradecene as a function of temperature at 100 bar syngas pressure Temperature (◦ C)
Pressure (bar)
Conversion (after 10 h)
Isomerization/2-Hna
kobs (10−3 min−1 )
60 80 100 120
100 100 100 100
0.09 0.56 0.80 0.96
0.5 0.3 0.3 0.4
0.2 1.2 3.0 8.0
200 ppm rhodium, L/M = 4. a Ratio between isomeric aldehydes and main product 2-hexyl-nonanal.
176
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
Table 2 Hydroformylation of 7-tetradecene as a function of total syngas pressure at two different temperatures Temperature (◦ C)
Pressure (bar)
Conversion (after 10 h)
Isomerization/2-Hna
kobs (10−3 min−1 )
80 80 80 110 110 110 110
60 80 100 60 80 100 130
0.09 0.21 0.56 0.12 0.16 0.73 0.89
0.5 0.5 0.3 0.3 0.5 0.5 0.4
0.2 0.6 1.4 0.6 1.0 3.5 4.5
200 ppm rhodium, L/M = 4. a Ratio between isomeric aldehydes and main product 2-hexyl-nonanal.
at Infracor, Marl. Main product is the aldehyde 2-hexyl-nonanal which is derived from 7-tetradecene without isomerization. A plot of the observed k-values against the syngas pressure shows an exponential growth shape for both 80 ◦ C and 110 ◦ C as depicted in Fig. 3. At higher syngas pressure the reaction seems to be further accelerated. This nonlinear dependence and the fast conversion at 110 ◦ C raised the question whether the active species is still the water soluble rhodium complex or an unmodified rhodium carbonyl complex, generated by complete loss of ligands.
3.3. Variation of ligand excess To answer the question whether modified or unmodified rhodium species are present under more severe reaction conditions a series of control experiments were carried out. In the first one the standard microemulsion composition using 200 ppm rhodium was chosen without addition of ligand, thus creating an unmodified rhodium complex. In a second experiment a small amount of rhodium (ca. 10 ppm) was added to pure tetradecene resulting in a homogeneous complex under reaction conditions. The results are shown in Fig. 4.
Fig. 3. Pressure dependence of the observed rate constants kobs for 80 ◦ C and 110 ◦ C.
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
177
Fig. 4. Variation of ligand excess. 100 ◦ C, 100 bar, Marlipal O13/80.
In both experiments the rhodium complex seems to be unmodified. The experiment without addition of TPPTS showed almost the same activity as the experiments with the standard excess of ligand (L/M = 3 and 4). Furthermore, the second experiment revealed that small amounts of unmodified rhodium (∼10 ppm) result in almost the same conversion as 200 ppm ligand modified rhodium in a microemulsion system. With this result the upcurved plot of kobs versus pressure may be explained by an additional term in the rate law y x 0.7 )(p 1 ) + k like kobs = kmod (pCO unmod (pCO )(pH2 ). H2 When the ligand excess was increased to a L/M ratio of 8, a significantly lower conversion was observed. Under these conditions the rhodium complex is water soluble only and included within the water core of the reverse micelles. This water soluble complex is much less reactive than the unmodified one. After cooling down and depressurizing the reaction vessel the upper organic phase has been analyzed by AAS for traces of rhodium. From the measurements at L/M = 8 no significant loss of rhodium into the organic phase could be detected for the microemulsion experiments. Still the additional use of an ultrafiltration process offers the possibility of simple and complete catalyst recovery.
4. Conclusion and outlook Under the more severe reaction conditions applied for the conversion of internal alkenes the active species seems to be an unmodified rhodium carbonyl complex when working with small excess of ligand. Small amounts (10 ppm) of this unmodified complex account for both the high activity and the high regioselectivity at elevated temperatures. The unmodified complex can be transferred back into the modified one by lowering the reaction temperature and the syngas pressure, thus immobilizing the rhodium catalyst in the water phase. Formation of the unmodified complex can be suppressed when working with ligand excess significantly higher than 4:1. The drawback of lower reaction rates can be overcome by increasing the temperature. Due to the regioselectivity of the conversion application of Rh-catalysed hydroformylation in fine chemical production are interesting. The advantage of the use of the microemulsion as reaction medium for these hydroformylation of hydrophobic olefines is the simple catalyst recovery by temperature induced phase separation and ultrafiltration if necessary [7]. When terminal aldehydes are the desired products, the
178
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
rhodium system in a microemulsion is not applicable. Use of less reactive cobalt complexes or dual catalyst systems are favorable. These catalyst complexes give an isomerization of the internal alkenes prior to the hydroformylation reaction [6]. Acknowledgements This work was supported by Sasol Germany GmbH, Olefins and Surfactants and the Graduate College “Synthetic, mechanistic and reaction-engineering aspects of metal containing catalysts” at TU Berlin. The authors are grateful to referee #1 for fruitful and constructive discussions.
References [1] B. Cornils, in: J. Falbe (Ed.), New Syntheses with Carbon Monoxide, Springer, Berlin, 1980, Chapter 1. [2] H. Bahrmann, H. Bach, Ullmann’s Encylcopedia and Industrial Chemistry, 5th Edition, Vol. A18, 1991, p. 321 [3] M. Haumann, H. Koch, P. Hugo, R. Schomäcker, Appl. Catal. A Gen. 225 (1/2) (2002) 239. [4] C. Botteghi, R. Ganzerla, M. Lenarda, G. Moretti, J. Mol. Catal. 40 (1987) 129. [5] M. Lade, Ph.D. thesis, Technische Universitaet Berlin, Berlin, 2000. [6] (a) M. Beller, J.G.E. Krauter, J. Mol. Catal. A 143 (1999) 31; (b) M. Beller, B. Zimmermann, H. Geissler, Chem. Eur. J. 5 (4) (1999) 1301. [7] K.V. Schubert, E.W. Kaler, Ber. Bunsenges. Phys. Chem. 100 (3) (1996) 190.
Hydroformylation of 7-tetradecene using Rh-TPPTS in a microemulsion Marco Haumann a , Hülya Yildiz b , Herbert Koch c , Reinhard Schomäcker b,∗ b
a Department of Chemistry, University of Cape Town, Cape Town, South Africa Institut für Technische Chemie, Technische Universität Berlin, Sekretarait TC 8, Strasse des 17 Juni 135, Berlin, Germany c Sasol Germany GmbH, Olefins and Surfactants, Marl, Germany
Received 16 October 2001; received in revised form 10 May 2002; accepted 13 May 2002
Abstract Water soluble rhodium catalyst complexes are highly active in a microemulsion system stabilized by technical grade surfactants of the alkyl-polyglycolether type. At temperatures around 120 ◦ C and pressures of 100 bar internal alkenes are hydroformylated with high regioselectivity. 7-Tetradecene as model alkene was effectively hydroformylated in microemulsions with high reaction rates yielding 2-hexyl-nonal. Under the reaction conditions applied the equilibria between different catalytically active complexes are shifted towards the unmodified HRh(CO)3 . Easy catalyst recycling of the water soluble modified complex is possible when the equilibrium is shifted back under work-up conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Microemulsion; Hydroformylation; Internal alkene; Water soluble; Rhodium branched aldehyde
1. Introduction The hydroformylation or oxo synthesis is one of the most important applications of homogeneous catalysis on an industrial scale [1,2]. Since, the establishment of two phase catalysis in the Ruhrchemie/Rhˆone-Poulenc process (RCH/RP) loss of catalyst as the major disadvantage of homogeneous catalysis is negligible, since the catalyst is immobilized within the water phase. The only drawback of this low-pressure-oxo (LPO) process is its limitation for propene and butene. In a recent paper, we have reported the hydroformylation of 1-dodecene in a microemulsion using the water soluble Rh-TPPTS catalyst complex [3]. High activi∗ Corresponding author. Tel.: +49-30-3-14-24973; fax: +49-30-3-14-79552. E-mail address: [email protected] (R. Schomäcker).
ties and good selectivities towards terminal aldehydes can be achieved under moderate reaction conditions (80 ◦ C, 80 bar). At higher temperatures the terminal double bond is shifted towards the thermodynamically favored internal positions. GC–MS experiments have shown that these internal alkenes are converted into their corresponding aldehydes. The reactivity towards hydroformylation decreases dramatically from terminal alkenes over internal alkenes to branched internal alkenes as depicted in Fig. 1 [4]. In this work, the terminal alkene 1-dodecene was replaced by 7-tetradecene to study the hydroformylation of an internal alkene in more detail. Due to the lower reactivity of the internal alkene higher temperatures and syngas pressures have to be applied. Under these conditions the equilibrium between ligand modified and unmodified catalyst complexes can be shifted towards the unmodified
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 2 8 4 - 3
174
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
Fig. 1. Relative activities of alkenes in hydroformylation.
rhodium species. −L
+CO
HRh(CO)L3 HRh(CO)L2 HRh(CO)2 L2 +L
−CO
−L
−L
+L
+L
internal alkene. The pH of the aqueous phase was 7, the overall rhodium concentration was 200 ppm with a ligand/metal ratio of 4 (for most experiments) and an alkene/metal ratio of 2500.
HRh(CO)2 L HRh(CO)3
The formation of unmodified rhodium species should result in increased activity as well as lower selectivity towards linear aldehydes.
3. Results
2. Experimental setup
Due to the lower reactivity of 7-tetradecene compared to 1-dodecene studied in [3] the reaction temperature has to be higher than 75 ◦ C. Therefore, the more hydrophilic surfactants Marlipal O13/80 and O13/100 were chosen, forming single phase microemulsions at 100 and 120 ◦ C, respectively. Hydroformylation experiments were carried out at 100 bar syngas pressure and temperatures between 60 and 120 ◦ C (Fig. 2). The conversion of 7-tetradecene at 80 ◦ C was significantly lower compared to 1-dodecene. In Table 1 the results are summarized. The selectivity data clearly show that the rhodium catalyst mainly converted the internal double bond into the branched aldehyde 2-hexyl nonanal (2-HN). From the rate constants given in Table 1 the activation energy was calculated to be 70 kJ/mol. This value
The setup was identical to the previous one used for the hydroformylation of 1-dodecene [3]. The phase behavior of microemulsions using 1-dodecene has been studied in detail and it is known from the literature that the general pattern observed with 1-dodecene can also be applied when using 7-tetradecene as alkene [5]. Single phase microemulsions can be formed between 60 ◦ C and 120 ◦ C by the use of surfactants with different degrees of ethoxylation. For the system Rh-TPPTS/1-dodecene the influence of stirrer speed, catalyst composition and acidity of the aqueous catalyst phase has been studied in detail [4]. The optimal process parameters of these investigations were applied for the conversion of the
3.1. Variation of temperature and estimation of activation energy
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
175
Fig. 2. Hydroformylation of 7-tetradecene at various temperatures. 100 bar, 200 ppm rhodium, L/M = 4, Marlipal O13/80.
is almost 16 kJ/mol higher than the one determined for the hydroformylation of 1-dodecene (54.5 kJ/mol) under the same conditions [3]. This is a result of the lower reactivity of internal alkenes compared to the corresponding terminal alkenes reported in the literature (see Fig. 1).
observed and this was further enhanced at 110 ◦ C (see Table 2). At 110 ◦ C the reaction rate was studied as a function of pressure between 60 and 130 bar. The overall rate constant kobs was determined using data fitting software tools derived by Hugo at TU Berlin from the rate equation
3.2. Influence of total syngas pressure
dcolefin = kobs (colefin )n dt with kobs = kr ccatalyst (pCO )x (pH2 )y −
At 80 ◦ C the pressure dependence was studied between 60 and 100 bar. As expected a strong increase in reaction velocity is observed with increasing pressure, however on a lower level compared to the conversion of 1-dodecene. On increasing the pressure from 80 to 100 bar at 80 ◦ C a marked acceleration in rate was
A first order dependence with regard to the olefin was observed in all experiments. In Table 2 the results for the pressure dependence are summarized. The high regioselectivity of the CO insertion has been verified by GC–MS measurements done
Table 1 Hydroformylation of 7-tetradecene as a function of temperature at 100 bar syngas pressure Temperature (◦ C)
Pressure (bar)
Conversion (after 10 h)
Isomerization/2-Hna
kobs (10−3 min−1 )
60 80 100 120
100 100 100 100
0.09 0.56 0.80 0.96
0.5 0.3 0.3 0.4
0.2 1.2 3.0 8.0
200 ppm rhodium, L/M = 4. a Ratio between isomeric aldehydes and main product 2-hexyl-nonanal.
176
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
Table 2 Hydroformylation of 7-tetradecene as a function of total syngas pressure at two different temperatures Temperature (◦ C)
Pressure (bar)
Conversion (after 10 h)
Isomerization/2-Hna
kobs (10−3 min−1 )
80 80 80 110 110 110 110
60 80 100 60 80 100 130
0.09 0.21 0.56 0.12 0.16 0.73 0.89
0.5 0.5 0.3 0.3 0.5 0.5 0.4
0.2 0.6 1.4 0.6 1.0 3.5 4.5
200 ppm rhodium, L/M = 4. a Ratio between isomeric aldehydes and main product 2-hexyl-nonanal.
at Infracor, Marl. Main product is the aldehyde 2-hexyl-nonanal which is derived from 7-tetradecene without isomerization. A plot of the observed k-values against the syngas pressure shows an exponential growth shape for both 80 ◦ C and 110 ◦ C as depicted in Fig. 3. At higher syngas pressure the reaction seems to be further accelerated. This nonlinear dependence and the fast conversion at 110 ◦ C raised the question whether the active species is still the water soluble rhodium complex or an unmodified rhodium carbonyl complex, generated by complete loss of ligands.
3.3. Variation of ligand excess To answer the question whether modified or unmodified rhodium species are present under more severe reaction conditions a series of control experiments were carried out. In the first one the standard microemulsion composition using 200 ppm rhodium was chosen without addition of ligand, thus creating an unmodified rhodium complex. In a second experiment a small amount of rhodium (ca. 10 ppm) was added to pure tetradecene resulting in a homogeneous complex under reaction conditions. The results are shown in Fig. 4.
Fig. 3. Pressure dependence of the observed rate constants kobs for 80 ◦ C and 110 ◦ C.
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
177
Fig. 4. Variation of ligand excess. 100 ◦ C, 100 bar, Marlipal O13/80.
In both experiments the rhodium complex seems to be unmodified. The experiment without addition of TPPTS showed almost the same activity as the experiments with the standard excess of ligand (L/M = 3 and 4). Furthermore, the second experiment revealed that small amounts of unmodified rhodium (∼10 ppm) result in almost the same conversion as 200 ppm ligand modified rhodium in a microemulsion system. With this result the upcurved plot of kobs versus pressure may be explained by an additional term in the rate law y x 0.7 )(p 1 ) + k like kobs = kmod (pCO unmod (pCO )(pH2 ). H2 When the ligand excess was increased to a L/M ratio of 8, a significantly lower conversion was observed. Under these conditions the rhodium complex is water soluble only and included within the water core of the reverse micelles. This water soluble complex is much less reactive than the unmodified one. After cooling down and depressurizing the reaction vessel the upper organic phase has been analyzed by AAS for traces of rhodium. From the measurements at L/M = 8 no significant loss of rhodium into the organic phase could be detected for the microemulsion experiments. Still the additional use of an ultrafiltration process offers the possibility of simple and complete catalyst recovery.
4. Conclusion and outlook Under the more severe reaction conditions applied for the conversion of internal alkenes the active species seems to be an unmodified rhodium carbonyl complex when working with small excess of ligand. Small amounts (10 ppm) of this unmodified complex account for both the high activity and the high regioselectivity at elevated temperatures. The unmodified complex can be transferred back into the modified one by lowering the reaction temperature and the syngas pressure, thus immobilizing the rhodium catalyst in the water phase. Formation of the unmodified complex can be suppressed when working with ligand excess significantly higher than 4:1. The drawback of lower reaction rates can be overcome by increasing the temperature. Due to the regioselectivity of the conversion application of Rh-catalysed hydroformylation in fine chemical production are interesting. The advantage of the use of the microemulsion as reaction medium for these hydroformylation of hydrophobic olefines is the simple catalyst recovery by temperature induced phase separation and ultrafiltration if necessary [7]. When terminal aldehydes are the desired products, the
178
M. Haumann et al. / Applied Catalysis A: General 236 (2002) 173–178
rhodium system in a microemulsion is not applicable. Use of less reactive cobalt complexes or dual catalyst systems are favorable. These catalyst complexes give an isomerization of the internal alkenes prior to the hydroformylation reaction [6]. Acknowledgements This work was supported by Sasol Germany GmbH, Olefins and Surfactants and the Graduate College “Synthetic, mechanistic and reaction-engineering aspects of metal containing catalysts” at TU Berlin. The authors are grateful to referee #1 for fruitful and constructive discussions.
References [1] B. Cornils, in: J. Falbe (Ed.), New Syntheses with Carbon Monoxide, Springer, Berlin, 1980, Chapter 1. [2] H. Bahrmann, H. Bach, Ullmann’s Encylcopedia and Industrial Chemistry, 5th Edition, Vol. A18, 1991, p. 321 [3] M. Haumann, H. Koch, P. Hugo, R. Schomäcker, Appl. Catal. A Gen. 225 (1/2) (2002) 239. [4] C. Botteghi, R. Ganzerla, M. Lenarda, G. Moretti, J. Mol. Catal. 40 (1987) 129. [5] M. Lade, Ph.D. thesis, Technische Universitaet Berlin, Berlin, 2000. [6] (a) M. Beller, J.G.E. Krauter, J. Mol. Catal. A 143 (1999) 31; (b) M. Beller, B. Zimmermann, H. Geissler, Chem. Eur. J. 5 (4) (1999) 1301. [7] K.V. Schubert, E.W. Kaler, Ber. Bunsenges. Phys. Chem. 100 (3) (1996) 190.