±
[1] S. Chandrasekhar, inHandbook of Liquid Crystals(Eds: D. Demus, J.Goodby, G. W. Gray, H.-W. Spiess, V. Vill), Wiley-VCH, Weinheim1998, Vol. 2B, Ch. VIII.
[2] N. Boden, R. J. Bushby, J. Clements, M. Jesudason, P. F. Knowles, G. Williams,Chem. Phys. Lett.1988,152, 94. N. Boden, R. J. Bushby, J. Clements, M. Jesudason, P. F. Knowles, G. Williams,Chem. Phys. Lett.
1989,154, 613.
[3] D. Adam, P. Schuhmacher, J. Simmerer, L. Häussling, K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf, D. Haarer,Nature1994,371, 141.
[4] C.-Y. Liu, H.-L. Pan, M. A. Fox, A. J. Bard,Science1993,261, 897.
[5] D. Markovitsi, F. Rigaut, M. Mouallem, J. Malthte,Chem. Phys. Lett.
1987,135, 236.
[6] T. Christ, B. Glüsen, A. Greiner, A. Kettner, R. Sander, V. Stümpflen, V.
Tsukruk, J. Wendorff,Adv. Mater.1997,9, 48.
[7] B. Mourey, J. N. Perbet, M. Hareng, S. Le Berre,Mol. Cryst. Liq. Cryst.
1982,84, 193.
[8] C. Destrade, P. Foucher, H. Gasparoux, N. H. Tinh, A. M. Levelut, J. Malthte,Mol. Cryst. Liq. Cryst.1984,106, 121.
[9] H. Mori, Y. Itoh, Y. Nishiura, T. Nakamura, Y. Shinagawa,Jpn. J. Appl.
Phys.1997,36, 143.
[10] C. Vauchier, A. Zann, P. L. Barny, J. C. Dubois, J. Billard,Mol. Cryst. Liq.
Cryst.1981,66, 103.
[11] T. S. Perova, J. K. Vij,Adv. Mater.1995,7, 919.
[12] K. Ichimura, inOrganic Photochromic and Thermochromic Compounds (Eds: J. C. Crano, R. Guglielmetti), Kluwer Academic/Plenum, New York 1999, Vol. 2, Ch. I.
[13] K. Ichimura,Chem. Rev.2000,100, 1847.
[14] K. Anderle, R. Birenheide, M. J. A. Werner, J. H. Wendorff,Liq. Cryst.
1991,9, 691.
[15] A. Natansohn, P. Rochon, M. Pezolet, P. Audet, D. Brown, S. To, Macromolecules1994,27, 2580.
[16] T. Fischer, L. Läsker, J. Stumpe, S. G. Kostromin, J. Photochem.
Photobiol. A1994,80, 453.
[17] W. M. Gibbons, P. J. Shannon, S.-T. Sun, B. J. Swetlin,Nature1991,351, [18] M. Schadt, H. Seiberle, A. Schuster,49. Nature1996,381, 212.
[19] K. Ichimura, S. Morino, H. Akiyama,Appl. Phys. Lett.1998,73, 921.
[20] S. Furumi, M. Nakagawa, S. Morino, K. Ichimura, H. Ogasawara,Appl.
Phys. Lett.1999,74, 2438.
[21] N. Kawatsuki, T. Yamamoto, H. Ono,Polym. J.1999,31, 630.
[22] M. Kidowaki, T. Fujiwara, K. Ichimura,Chem. Lett.1999, 641.
[23] M. Kidowaki, Doctoral Thesis, Tokyo Institute of Technology, Japan, March2000.
[24] M. Kidowaki, T. Fujiwara, K. Ichimura,Chem. Lett.1999, 643.
[25] We employed here a dichroic ratio (DRNPL) exhibiting azimuthal orienta- tional order of azobenzene moieties. The value is defined here asDRNPL
= (A1±A2)/A0, whereA1andA2stand for absorbances at maximum ab- sorption wavelength due to thep±p* transition of theE-isomer of the azo- benzene, monitored by probing polarized light with the electric vectors parallel with and perpendicular to the incident plane of the oblique non- polarized light, respectively.A0indicates absorbance before photoirradia- tion.
[26] K. Ichimura, M. Han, S. Morino,Chem. Lett.1999, 85.
[27] DLC (2) of 2,3,6,7,10,11-hexakis(4-octyloxybenzoyloxy)triphenylene ex- hibits NDand Drphases (K 153 C Dr168 C ND244 C I). N. H. Tinh, H.
Gasparoux, C. Destrade,Mol. Cryst. Liq. Cryst.1981,68, 101.
[28] In the case of irradiation with linearly polarized light, the dichroic ratio (DRLPL) indicates a value of (A1±A2)/A0, whereA1andA2are the maxi- mum absorbances of the azobenzene monitored by probing light with the electric vector perpendicular to and parallel with that of the linearly polar- ized actinic light, respectively, in order to compare the value ofDRNPL. [29] The optical phase difference indicates a difference between phase veloci-
ties of vertically and horizontally polarized light (s- andp-polarized light) with respect to the oriented DLC film.
[30] A. M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, N. H. Tinh,J.
Phys.1981,42, 147.
[31] C. D. Favre-Nicolin, J. Lub, P. Sluis,Adv. Mater.1996,8, 1005.
Core±Shell Microspheres of a Catalytically Active Rhodium Complex Bound to a Polyelectrolyte- Coated Latex**
By Stefan Mecking* and Ralf Thomann
Materials constructed by binding transition metal com- plexes to defined polymeric architectures are a subject of im- mense interest. A major motivation is their application to ca- talysis. The high activities and selectivities of homogeneous catalysts based on soluble transition metal complexes
[1]are opposed by a usually tedious separation from the products after the reaction, often involving thermal strain on the reac- tants. Thus, the development of materials combining the ad- vantages of classical homogeneous and heterogeneous cata- lysts remains a challenge. Most approaches to immobilizing metal complexes have employed microporous organic or inor- ganic solids.
[2]However, a limited diffusion of the substrates within these materials is disadvantageous. Also, the cross- linked copolymers used mostly possess a heterogeneous struc- ture consisting of domains of strongly differing composition.
[3]As a different strategy, binding metal complexes onto struc- tures with dimensions in the range of colloids (1±1000 nm) is finding increasing attention. Structures based on the self-orga- nization of surfactants
[4]or surfactant ligands
[5]are highly or- dered, however due to their dynamic nature aggregation num- bers can fluctuate and such structures can be shear sensitive.
Very recently, catalytically active metal centers have been bound successfully to dendrimers.
[6]However, the well-de- fined molecular structure of suitable dendrimers requires te- dious synthesis.
[7]By contrast, polymer lattices are readily ob- tained by aqueous emulsion polymerization, yielding monodisperse particles in the range of ca. 50±1000 nm in di- ameter.
[8]By microemulsion polymerization, particles as small as 10 nm have already been obtained.
[9]Polymer lattices pos- sess an easily accessible outer surface (e.g., for ideal spheres of Æ 200 nm ca. 30 m
2/g solids). Highly charged lattices stabi- lized by ionic groups located on the particle surface can bind molecules of opposite charge e. g., the binding of multiple layers of synthetic high molecular weight polyelectrolytes to polymer lattices has recently been shown to yield versatile structures of nanometer-scale dimensions.
[10]Simple binding of a polyelectrolyte to a charged particle represents a convenient means of arranging charged moieties
Adv. Mater.2000, 12,No. 13, July 5 ÓWILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/1307-0953 $ 17.50+.50/0
953
COMMUNICA TIONS
______________________
±
[*] Dr. S. Mecking, Dr. R. ThomannInstitut für Makromolekulare Chemie und Freiburger Materialforschungszentrum
der Albert-Ludwigs-Universität Freiburg Stefan-Meier-Str. 31, D-79104 Freiburg (Germany) E-mail: mecking@uni-freiburg.de
[**] We thank R. Muelhaupt for his generous support of our work. W. Richter- ing, M. Soddemann, and F. Horn are acknowledged for fruitful discussions on polymer lattices. Financial support by the Deutsche Forschungsge- meinschaft is gratefully acknowledged. A generous loan of rhodium chloride was provided by Degussa-Hüls AG. AAS analysis were carried out by S. Hirth-Walter, and31P NMR spectra were obtained by D. Hunk- ler. S.M. thanks G. Moerber for excellent technical assistance.
First publ. in: Advanced Materials 12 (2000), pp. 953-956
Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/4353/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-43537
at the particle surface (Scheme 1). We report the preparation of structurally well-defined microparticles by the electrostatic
binding of a low molecular weight rhodium complex to a poly- electrolyte layer assembled on the surface of a latex particle, and application of the resulting materials to hydroformylation as a catalytic reaction. The core±shell structure of the micro- spheres is demonstrated by means of electron microscopy.
Only a few approaches for binding catalytically active metal complexes to lattices have been reported previously.
[11]Most often, a large proportion of the metal centers was located in the interior of the latex.
Highly charged monodisperse lattices with particle sizes of 100±400 nm diameter, prepared by copolymerization of sty- rene and potassium p-styrenesulfonate with K
2S
2O
8as an ini- tiator, were utilized.
[12,13]Coating with excess poly(diallyldi- methylammonium chloride) (PDADMAC), followed by repeated ultracentrifugation/decantation/redispersion cycles, for complete removal of excess PDADMAC and added NaCl electrolyte, yielded coated lattices.
[10]Charge inversion of the lattices by coating with the cationic polyelectrolyte is demon- strated by a reversal of the f-potential.
To ensure effective binding to the charged latex, an ad- sorbed molecule should itself be multiply charged. As in catal- ysis temporary dissociation of a ligand from the metal center can occur, it should be advantageous for each ligand to be multiply charged. For this reason the well-known complex [(H)Rh(CO)(NaTPPTS)
3],
[14]containing three tri-sulfonated NaTPPTS ligands was employed. In a typical experiment,
3 mmol complex/g latex (solids) were bound to a latex of di- ameter 280 nm. Complete adsorption is evidenced by atomic absorption spectroscopy (AAS) of supernatant solutions from ultracentrifugation of the complex-loaded lattices (detection
limit 0.1 ppm Rh, corresponding to 2 % of the amount of rho- dium present). Saturation experiments monitored by AAS re- vealed that the loading employed corresponds to approxi- mately 50 % of the capacity of the latex. Latex particles were visualized by electron microscopy (Fig. 1, transmission elec- tron microscopy [TEM] image, lower left). Energy-dispersive X-ray analysis (EDX) shows an even distribution of rhodium on the latex particles (Fig. 1, upper left). Microtome cuts of single latex particles embedded in an epoxy resin were investi- gated. TEM clearly shows that the particle surface is covered by material of high electron density (Fig. 1, lower right). The increased electron density of this layer is due to a large extent to rhodium, as rhodium-specific element specific imaging (ESI; Fig. 1 upper right inset) and also comparison with a rho- dium-free latex confirm. Microtome sectioning of spherical particles yields layers of spheres, including caps. A simple vis- ual inspection of the images shown gives a wrong, much too large, impression of the layer thickness if this point is not tak- en into account. Application of simple geometric principles to the data obtained yields a calculated layer thickness of less than 10 nm (see Experimental section). In the coating and loading procedures, no significant coagulation of the latex was observed. By centrifugation and redispersion cycles, the parti- cles could also be transferred to methanol as a dispersing me- dium, yielding stable lattices. This is of particular interest as the general applicability to catalytic reactions is greatly ex- panded, methanol being a much more versatile solvent than water for organic reactants and also gases such as hydrogen.
As a first probe for the catalytic activity and stability of the materials prepared, the hydroformylation of methyl acry- late
[15](Eq. 1) was investigated.
(1)
In agreement with literature results
[15]and comparative ex- periments with a non-immobilized catalyst, methyl 2-formyl- propionate (M2FP) is obtained as the major product along with small amounts of methyl 4-oxobutyrate (M3FP).
[16]In ex- periments with different reaction times, varying from 1.5±15 h, a steady activity of ca. 30 TO/h (TO = turnovers) was observed.
[17,18]By comparison, under the same conditions non-latex bound [(H)Rh(CO)(NaTPPTS)
3] displayed an ini- tial activity of 50 TO/h (1.5 h reaction time), decreasing to an average 18 TO/h in a 15 h experiment. These similar rates show that the immobilized catalyst is readily accessible to the substrates. The presence of chloride ions originating from PDADMAC can potentially result in a decrease of catalyst performance in hydroformylation reactions. To this end, the above catalysis experiments revealed no disadvantageous ef- fect.
[19]Under the reaction conditions employed, i.e., 60 C and high-speed stirring (1300 rpm) in methanol as a reaction medium, usually little if any coagulation was observed.
[20]Pro-
954
ÓWILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/1307-0954 $ 17.50+.50/0 Adv. Mater.2000, 12,No. 13, July 5COMMUNICA TIONS
Scheme 1. Schematic representation of polyelectrolyte adsorption on a charged latex particle.
ducts were efficiently removed from the latex after the reac- tion by centrifugation.
[21]AAS of the supernatant solutions reveals that with the system described some undesired leach- ing of rhodium occurs at long reaction times (£20 %; 1 ppm).
Accordingly, in recycling experiments a significantly reduced catalytic activity is observed. However, catalysis occurs by la- tex-bound metal complexes, as evidenced by the steady cata- lytic activity observed in experiments over different reaction times (vide supra), independent of any leaching of rho- dium.
[22]Leaching may occur via loss of phosphine ligands and formation of carbonyl complexes during the catalytic ex- periment.
[23]In accordance with the above, prolonged stirring of the metal-loaded latex under reaction conditions in the ab- sence of substrates caused no leaching to be observed.
Whereas leaching in hydroformylation and the relatively low loading of the model system described demand improvement, the good stability achieved under practically relevant condi- tions and good activity by comparison to reaction in homoge- neous solution show the suitability of this approach.
In summary, the adsorption of a catalytically active metal complex on microparticles via electrostatic interactions of sul- fonated phosphine ligands with a polycation layer has been demonstrated. Structurally well-defined core±shell particles are obtained by this convenient, versatile approach.
Experimental
Materials and General Considerations: The complex [(H)Rh(CO) (NaTPPTS)3] [14,24] was prepared according to literature procedures. A CO/H2
1:1 mixture (Messer-Griesheim; H299.999 %, CO 99.994 %) was used as received.
Analytical grade methanol, deionized water (Milli-Q plus grade) and all other sol- vents and reagents were degassed prior to use. Gas chromatographic analyses were
performed on a Chrompack 9001 unit equipped with a 25 m CP-Wax 52 CB col- umn. Solids contents of lattices were determined using a Sartorius MA30 moisture analyzer. TEM investigations were carried out on a LEO 912 Omega apparatus using an acceleration voltage of 120 kV. For EDX, an Oxford Instruments module was employed. ESI experiments were performed using the five-window method (SIS-software). Sectioning (ca. 40 nm) of the embedded lattices was performed with a Reichert & Jung Ultracut E microtome at ambient conditions. Sectioning of spheres leads to a variety of structures, ranging from small sphere caps to equator- ial sections with the same diameter as the original latex. Measuring the thickness of the shell, the three-dimensional character of the sections must be taken into ac- count. Depending on the diameter of the latex and the thickness of the sections, the observed apparent shell thickness is too high. The apparent increase in the shell thickness can be calculated using geometrical sphere equations. It is smallest for equatorial sections; for the equatorial sections prepared it is about 4 nm. To es- timate the thickness of the rhodium-containing shell the mean value taken from 100 equatorial sections was determined and corrected with this value.z-potentials were measured on a Malvern Mastersizer without additional electrolytes.
Latex Preparation: Anionic lattices were prepared by aqueous emulsion co- polymerization of styrene and potassiump-styrene sulfonate using K2S2O8as an initiator according to literature procedures [12]. Optionally, divinylbenzene (5 mol.-%) was added as a crosslinking agent. The resulting lattices were dia- lyzed against water. For coating with the polyelectrolyte, the latex was reacted with an excess of PDADMAC (Aldrich, averageMwca. 300 000 g/mol). Typi- cally, a latex was diluted to a solids content of 6 %. PDADMAC and NaCl were added corresponding to a concentration of 5 mM in cationic polyelectrolyte (calculated with respect to monomer units) and 0.3 M in NaCl. The mixture was agitated for 30 min, and then ultracentrifugated (15 000 g), decanted, and the residue redispersed in water. The washing procedure was repeated at least three times, monitoring removal of excess electrolyte.
For adsorption of the metal complex for catalysis experiments, in a typical experiment a solution of 4mmol [(H)Rh(CO)(NaTPPTS)3] in a few milliliters of water was added to 70 mL of a polyelectrolyte-coated latex (solids content 2.6 %, particle sizeÆ280 nm) with gentle shaking. After 30 min, the latex was ultracentrifugated, the supernatant decanted, and the solid residue was redis- persed in methanol.
Hydroformylation Reactions: These were carried out in a 200 mL stainless steel reactor (Büchi AG, Ulster), equipped with a mechanical stirrer and a heat- ing/cooling jacket controlled by a thermocouple dipping into the reaction solu- tion. Immediately after the catalytic reaction, the reaction mixture was ultra- centrifugated, and the supernatant was analyzed by gas chromatography using dimethyl succinate as an internal standard. The identity of the products methyl 2-formylpropionate and methyl 4-oxobutyrate was confirmed by1H and13C NMR spectroscopy (one-dimensional and 2D correlated spectroscopy) of iso-
Adv. Mater.2000, 12,No. 13, July 5 ÓWILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/1307-0955 $ 17.50+.50/0
955
COMMUNICA TIONS
Fig. 1. Electron microscopy imaging of Rh-loaded, PDADMAC-coated latex particles. Lower left: TEM. Insets on upper left: EDX C Kaand Rh Lg1. Lower right:
TEM of microtome cuts. Inset on upper right: ESI of microtome cuts.
lated samples. Blank experiments showed no activation of the steel reactor em- ployed for methyl acrylate hydroformylation.
Received: February 23, 2000 Final version: April 11, 2000
±
[1] Applied Homogeneous Catalysis with Organometallic Compounds (Eds:B. Cornils, W. A. Herrmann), Wiley-VCH, Weinheim1996.
[2] a) J. H. Clark, A. P. Kybett, D. J. Macquarrie,Supported Reagents, VCH, Weinheim 1992. b) C. U. Pittman, in Comprehensive Organometallic Chemistry (Eds: G. Wilkinson, F. G. A. Stone, E. W. Abel), Pergamon Press, Oxford1982, Vol. 8, pp. 553±611.
[3] F. de Dardel, T. V. Arden, in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. (Eds: W. Gerhartz, E. Barbara), VCH, Weinheim1992, Vol. A14, pp. 393±459.
[4] a) D. F. Evans, H. Wennerstroem,The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, VCH, Weinheim1994. b) G.
Oehme, in Aqueous-Phase Organometallic Catalysis (Eds: B. Cornils, W. A. Herrmann), Wiley-VCH, Weinheim1998, pp. 193±206.
[5] a) B. Fell, G. Papadogianakis,J. Mol. Catal.1991,66, 141. b) H. Ding, B. E. Hanson, T. Bartik, B. Bartik,Organometallics1994,13, 3761.
[6] a) J. W. J. Knapen, A. W. van der Made, J. C. de Wilde, P. W. N. M. van Leeuwen, P. Wijkens, D. M. Grove, G. van Koten,Nature1994,372, 659.
b) H. Brunner,J. Organomet. Chem.1995,500, 39. c) P. Bhyrappa, J. K.
Young, J. S. Moore, K. S. Suslick,J. Am. Chem. Soc.1996,118, 5708. d) D.
Seebach, R. E. Marti, T. Hintermann,Helv. Chim. Acta1996,79, 1710.
e) M. T. Reetz, G. Lohmer, R. Schwickardi,Angew. Chem. Int. Ed. Engl.
1997,36, 1526;Angew. Chem.1997,109, 1559. f) H.-F. Chow, C. C. Mak, J. Org. Chem.1997,62, 5116. g) C. Koellner, B. Pugin, A. Togni,J. Am.
Chem. Soc. 1998, 120, 10 274. h) N. J. Hovestad, E. B. Eggeling, H. J. Heidbuechel, J. T. B. H. Jastrzebski, U. Kragl, W. Keim, D. Vogt, G. van Koten,Angew. Chem. Int. Ed.1999,38, 1655;Angew. Chem.1999, 111, 1763. j) N. Brinkmann, D. Giebel, G. Lohmer, M. T. Reetz, U. Kragl, J. Catal.1999,183, 163. k) G. Oosterom, R. J. van Haaren, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van Leeuwen,Chem. Commun.1999, 1119.
[7] Recent reviews: a) M. Fischer, F. Vögtle,Angew. Chem. Int. Ed.1999,38, 884; Angew. Chem.1999,111, 934. b) A. W. Bosman, H. M. Janssen, E. W. Meijer,Chem. Rev.1999,99, 1665.
[8] a)Emulsion Polymerization and Emulsion Polymers(Eds: P. A. Lovell, M. S. El-Aasser), Wiley, Chichester1997. b) Wäûrige Polymerdispersionen (Ed: D. Distler), Wiley-VCH, Weinheim1999.
[9] a) M. Antonietti, R. Basten, S. Lohmann,Macromol. Chem. Phys.1995, 196, 441. b) F. Candau, Y. S. Leong, G. Pouyet, S. Candau,J. Colloid Inter- face Sci.1984,101, 167.
[10] a) E. Donath, D. Walther, V. N. Shilov, E. Knippel, A. Budde, K. Lowack, C. A. Helm, H. Moehwald,Langmuir1997,13, 5294. b) G. B. Sukhoru- kov, E. Donath, H. Lichtenfeld, E. Knippel, M. Knippel, A. Budde, H.
Moehwald,Colloids Surf. A1998,137, 253. c) E. Killmann, D. Bauer, A.
Fuchs, O. Portenlaenger, R. Rehmet, O. Rustemeier,Prog. Colloid Polym.
Sci.1998,111, 135. d) F. Caruso, H. Lichtenfeld, E. Donath, H. Moehwald, Macromolecules1999,32, 2317. e) F. Caruso, A. S. Susha, M. Giersig, H.
Moehwald,Adv. Mater.1999,11, 950. Also see f) S. W. Keller, S. A. John- son, E. S. Brigham, E. H. Yonemoto, T. E. Mallouk,J. Am. Chem. Soc.
1995,117, 12 879. g) N. G. Hoogeveen, M. A. Cohen Stuart, G. J. Fleer, M. R. Boehmer,Langmuir1996,12, 3675.
[11] a) A. M. van Herk, K. H. van Streun, J. van Welzen, A. L. German,Br.
Polymer J.1989,21, 125. b) H. Turk, W. T. Ford,J. Org. Chem.1991,56, 1253. c) W. T. Ford, R. D. Badley, R. S. Chandran, S. Hari Babu, M. Hassanein, S. Srinivasan, H. Turk, H. Yu, W. Zhu, inPolymer Latexes, ACS Symp. Series 492, ACS, Washington, DC1992, pp. 422±431.
[12] K. Furusawa, W. Norde, J. Lyklema,Kolloid Z. Z. Polymer.1972,250, 908.
[13] By this procedure, lattices with surface charges in the range of ca.
6±25mC/cm2are obtained.
[14] W. A. Herrmann, J. A. Kulpe, W. Konkol, H. Bahrmann,J. Organomet.
Chem.1990,389, 85.
[15] a) G. Fremy, E. Monflier, J.-F. Carpentier, Y. Castanet, A. Mortreux, Angew. Chem. Int. Ed. Engl.1995,34, 1474;Angew. Chem.1995,107, 1608. b) G. Fremy, E. Monflier, J.-F. Carpentier, Y. Castanet, A. Mortreux, J. Catal.1996,162, 339 (average rate reported in toluene±water biphasic system with [Rh(acac)(CO)2]/NaTPPTS: 250 TO/h; in toluene solution with [Rh(acac)(CO)2]/PPh3: 220 TO/h). c) Y. Takegami, Y. Watanabe, H. Masada,Bull. Chem. Soc. Jpn.1967,40, 1459. d) M. Tanaka, T. Haya- shi, I. Ogata,Bull. Chem. Soc. Jpn.1977,50, 2351.
[16] iso/nca.30 at 60 C, 50 bar CO/H2. In some cases, formation of small amounts of methyl propionate, resulting from the hydrogenation of methyl acrylate, was also observed.
[17] Reaction conditions: methanol as a dispersant (90 mL); 4mmol rhodium;
115 mmol methyl acrylate; 60 C; 50 bar combined CO/H2(1:1) pressure.
[18] TOºturnoversºmol(aldehydes)/mol(rhodium).
[19] In agreement, 31P and 1H NMR analysis of an aqueous solution of [(H)Rh(CO)(NaTPPTS)3] in the presence of excess PDADMAC revealed no change in the catalyst precursor complex structure.
[20] In cases where small amounts of coagulate were observed, this was usually found on the stirrer and the upper limit of the liquid level in the reactor.
Presumably, it was formed by evaporation of solvent, and thus does not represent an intrinsic problem.
[21] In view of potential applications, filtration on a suitable membrane will be a more convenient method of separation than centrifugation.
[22] Catalysis by leached sodium to a small extent cannot be excluded.
[23] Application of complexes with multidentate ligands may prove to be ad- vantageous, as they coordinate more efficiently to the metal center:
a) W. A. Herrmann, C. W. Kohlpaintner, H. Bahrmann, W. Konkol, J. Mol. Catal.1992,73, 191. b) W. A. Herrmann, C. W. Kohlpaintner, R. B. Manetsberger, H. Bahrmann, H. Kottmann,J. Mol. Catal. A1995, 97, 65. c) A. J. Sandee, V. F. Slagt, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van Leeuwen,Chem. Commun.1999, 1633.
[24] J. P. Arhancet, M. E. Davis, J. S. Merola, B. E. Hanson,J. Catal.1990, 121, 327.
Synthesis of Monodisperse Spherical Silicon Dicarbodiimide Particles**
By Ya-Li Li, Edwin Kroke,* Alexander Klonczynski, and Ralf Riedel*
Sol±gel synthesis has been established as an important route to new materials with unconventional structure and morphol- ogy, for example, fine powders, films, and fibers. This method has the advantages of flexible processing, high purity and homogeneity of synthesized materials, as well as much lower processing temperatures compared to conventional tech- niques.
[1±3]One successful application of the sol±gel process is the synthesis of spherical micro- or nanoparticles based on the so-called ªStöber processº. In 1965, Stöber et al. found that spherical silica particles can be obtained by hydrolysis of tet- raethoxysilane (TEOS) in strongly basic water/alcohol mix- tures.
[4]The particles were formed in solutions containing ex- cess water (H
2O/TEOS ratios of 20:1 and above) and at a high pH value. At low pH in highly concentrated solutions silica gels are normally formed. In order to control the growth of the respective gels or colloidal products, the sol±gel behavior of alkoxides associated with the hydrolysis and condensation of alkoxides has been investigated in detail over the past two decades. The ceramics obtained from alkoxide precursors are almost exclusively oxides.
Recently, a non-oxidic sol±gel process was developed as a novel anhydrous technique to produce Si-based ternary ce- ramics and pure SiC. This process is based on the reaction of chlorosilanes with bis(trimethylsilyl)carbodiimide.
[5±9]Com-
956
ÓWILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/1307-0956 $ 17.50+.50/0 Adv. Mater.2000, 12,No. 13, July 5COMMUNICA TIONS
±
[*] Dr. E. Kroke, Dr. Y.-L. Li, A. Klonczynski, Prof. R. Riedel Technische Universität DarmstadtFachbereich Materialwissenschaft, Fachgebiet Disperse Feststoffe Petersenstrasse 23, D-64287 Darmstadt (Germany)
[**] The authors gratefully acknowledge the Deutsche Forschungsge- meinschaft (DFG), Bonn (Germany), for financial support of the present work. Y.-L. Li thanks the Alexander von Humboldt Foundation for a re- search fellowship.
