Chapter 14
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Micellar Catalysis as a Clean Alternative for Selective Epoxidation Reactions J. H. M. Heijnen, V. G. de Bruijn, L. J. P. van den Broeke, and J. T. F. Keurentjes Department of Chemical Engineering and Chemistry, Process Development Group, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
In the search for more environmentallyfriendlyprocesses, the use of micellar structures opens up a range of new possibilities. One of the most exciting applications is the incorporation of homogeneous catalysts in micelles to perform reactions: micellar catalysis. Micellar catalysis can be used to perform for example epoxidation reactions. In this way organic solvents, in which epoxidations are usually performed, are replaced by aqueous surfactant solutions. In this study propylene and 1-octene have been successfully epoxidized by hydrogen peroxide, catalyzed by micelle -incorporated porphyrin catalysts. Furthermore, several experimental techniques have been used to get more insight in the micelle-catalyst system.
© 2002 American Chemical Society In Clean Solvents; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Introduction In micellar catalysis, a nonpolar homogeneous catalyst is solubilized in an aqueous micellar system, where it converts nonpolar reactants to polar products. To study micellar catalysis, the epoxidation of propylene to propylene oxide has been chosen as a model reaction. Hydrogen peroxide is used as the oxidizing agent: A CH —CH=CH
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3
2
+
H 0 2
2
CH —CH — C H 3
2
+
H0 2
The principle of micellar catalysis of propylene to propylene oxide is schematically shown in Figure 1. Propylene gas is bubbled through an aqueous solution, containing the micelle-incorporated epoxidation catalyst and diffuses from the gas phase through the aqueous phase into the micelles. In the micelles, the catalyst converts propylene and hydrogen peroxide to propylene oxide and water. In-situ extraction of polar propylene oxide will reduce its further oxidation, increasing the reaction selectivity. The use of water instead of organic solvents will avoid the emission of the latter. Due to its high heat capacity, water will allow for safer process operation, since boiling conditions are not easily reached. Furthermore, it is known that an organized medium affects reaction rate and selectivity of many organic reactions (1,2) as well as metal catalyzed reactions (3-6). Another advantage is that, once incorporated into a micelle, the homogeneous nonpolar catalyst can be easily separated from the polar products, e.g. by micellar enhanced ultrafiltration (7).
Figure 1. The principle of micellar catalysis. The goal of this research is to design a process for propylene epoxidation by hydrogen peroxide based on micellar catalysis. Three levels of design can be identified. The first level is the design of the homogeneous epoxidation catalyst. Catalyst performance can be optimized by modification of the ligand and the central transition metal atom (8-10). At the second level, the micelle-catalyst combination is designed for optimal contacting of catalyst and reactants on a molecular scale. Due to the high polarity gradient in the micelle, the nonpolar
In Clean Solvents; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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propylene can be brought into close contact with the polar hydrogen peroxide. By altering substituents on the catalyst ligand, the catalyst polarity can be modified, resulting in a different location in the micelle. Furthermore, the surfactant can be designed by changing the type, length and sequence of the surfactant blocks. Finally, reactant contacting on a macroscale, operating conditions, and product separation are optimized in a process design. Our main focus has been on micelle-catalyst design and process design.
Experimental The epoxidation catalysts used are shown in Figure 2. Ti(Silsesquioxane) [{(c-C H9)7Si 0 }Ti(IV)(ri -C H5)] was kindly supplied by the Schuit Institute of Catalysis at Eindhoven University of Technology. Mn(Salen)Cl [(+)-N N,Bis(3,5-ditert-tubylsalicylidene)-1,2-cyclohexanediamino-manganese(III)chloride] and Mn(TPP)Cl [5,10,15,20-tetrakis(phenyl)porphyrin Manganese(III) chloride] were purchased from Fluka and Mn(TDCPP)Cl [5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin Manganese(III) chloride] was purchased from Frontier Scientific. Propylene (99+%, lecture bottle), acetonitrile, dichloromethane, «-octane and iso-octane were obtained from Aldrich. Hydrogen peroxide (50 wt% in water) and MPEG (polyethylene glycol monomethyl ether) were obtained from Acros Chimica and imidazole and 1octene were obtainedfromMerck. The surfactants used are listed in Table I. 5
5
7
12
5
?
R
R
2
1 R
R=cyclopentyl R
t-Bu
t-Bu
3
Figure 2. Selected epoxidation catalysts. 1 Ti(Silsesquioxane); 2 Mn(Salen)Cl; 3 Mn(TPP)Cl; 4 Mn(TDCPP)Cl.
In Clean Solvents; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Table I. Selected surfactants
a
Surfactant
Structure
Triton X-100 Brij35 SDS CTAB Pluronic P84 Pluronic L64 Pluronic PI 03 Pluronic F127
C(CH ) -CH -
