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Diffusion and reaction in micro- and mesopores Christoph Schüth Diffusion in micro- and mesopores is in many cases limiting for mass transfer and reaction rates in porous natural or synthetic materials. Accessibility and reactivity of micro- and meso-pore domains should be a function of the pore sizes as well as pore polarities. In natural po-rous media both parameters can be assumed to show a distribution, depending on the type of the porous material. Moreover, educts and products of a reaction may show different po-larities resulting in a distribution of diffusivities with an impact on overall reaction rates. The objective of this work is the synthesis and characterization of well-defined porous mate-rials with different pore sizes and polarities containing catalytically active sites, and study their accessibility with various methods. The catalytic hydrodehalogenation of chlorinated hydrocarbons (trichloroethylene (TCE) and chlorobenzene) and the hydrogenation of ben-zene serve as model reactions resulting in different product distributions in terms of polarity and molecular size. The use of well-characterized model solids should allow to relate diffu-sion rates and reactivities of the different materials to pore sizes and pore polarities. In the first step, the synthesis and first characterization of metal complexes will be performed. The selection of the metal complexes is predefined by the model reactions as described earlier. Pincer complexes (M-1a-c,2a-c) which are accessible by various procedures will be synthesized. In general a number of organic and organometallic transformations have to be performed to generate complexes M-1a-c,2a-c. Their special features are the different donor atoms which coordinate to the metal, the functional groups at the donor atoms and very important for this project the hydrolysable group at the end of the spacer. These features allow to adjust the metal complexes for a special application in catalysis. (R'O) 3Si X [M] X M-1a-c, M-2a-c Scheme 1 X = SR (1), NR 2 (2) R = Me (a), Ph (b), Cy (c) [M] = Rh, Pd, Ni, Ru complex fragments R' = Me, Et Spacer e.g. C 6-alkane or short PEG chain It is planned to perform the dehalogenation reactions with the SCS-Pd complexes (Pd- 1). Catalysts of this type are highly stable in air and water and are known to cleave carbon-chlorine bonds. This will be important for field applications planned at a later date of this project. To fine tune the active center with respect to electron density and steric hindrance R will be varied from methyl (Pd-1a), phenyl (Pd-1b) to cyclo-hexyl (Pd- 1a). All complexes will be screened for their catalytic activity in dehalogenation and hydrogenation reactions. - 144 -
Those complexes which performed best in the reactivity screening will be incorporated into porous oxides. The porous oxides will be prepared by sol-gel processing. This technique allows the creation of oxide networks by progressive hydrolysis and condensation reactions of molecular precursors (Scheme 2). Therefore it is an ideal and versatile method to incorporate well defined metal complex catalysts into the network. The parameters which influence the sol-gel reaction allow to control the morphology and properties of the materials. As in this project the pore size and the polarity of the matrix are of interest only those parameters will be varied which have an influence on this properties. The following educts and parameters will be considered: (1) The alkoxysilanes tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), methyltrimethoxysilane (MTMOS) and phenyltrialkoxysilane (PhTMOS); (2) The metal complex catalysts developed; (3) the solvents THF, H2O, ethanol, ethylene glycol, glycerine; (4) pH; (5) templates (tensides, organic polymers etc.). k H 2O + l Si(OR') 4 + m RSi(OR') 3 + n Scheme 2 (R'O) 3Si 1. pH X 2. Solvent 3. Templates [M] 4. Temperature 5. Pressure X 6. Stirring speed M-1a-c,2a-c MOS and TEOS are network builders. Their different alkoxy groups allow to control the ki-netic of the hydrolysis. If only these are used in the reaction the pore size of the materials depends on the pH. The amount of alkyltrimethoxysilanes will have an influence on the pore sizes and on the polarity. It is further assumed that the Alkyl group will have an additional effect on the diffusion of the organic molecules into the porous material. For the generation of larger pore sizes template additives might have to be used. Due to the thermal sensitivity of the metal complexes calcination has to be excluded. Thus the additives will have to be ex-tracted by extensive solvent processing. During the mixing of the organic and inorganic reaction partners the organic part of the metal complex catalyst will be surrounded by the non-polar organic functions R. This will ensure that in the final material the catalyst will be on the non-polar surface of the pores. When the main properties of the solids and the resulting processes limiting or enabling the access of diffusing compounds into micro- and mesopores are known, this knowledge could be used to synthesize tailored materials for a specific application. This could be catalysts with high and sustained reactivities in aqueous systems by selectively providing access to reactive sites. In groundwater remediation, there is especially a need for destructive methods as can be achieved by catalysis, because no secondary waste streams are produced. Besides the fundamental research, this project has therefore also a very applied background, with the potential for a commercial utilization of the results. - 145 - Si X X [M] X
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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Annual Report 2006 Faculty 11 Mater
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Preface The year 2006 of the Depart
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difference of numerical and analyti
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Institute of Materials Science Phys
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Research Projects Bulk Amorphous Al
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el Dsoki, C.; Landersheim, V. ; Kau
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Das, J.; Kim, K. B.; Xu, W.; Wei, B
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Staff Members Head Prof. Dr. Jürge
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Zhou L.; Rixecker G.; Aldinger F.;
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concerned with the synthesis and ch
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Publications Fleissner, A.; Schmid,
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• Surface analysis The UHV-surfac
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T. Mayer, U. Weiler, E. Mankel and
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Thin films The Thin Film division w
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Berky, W.; Gottschalk, S.; Elliman,
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Guest Scientists Research Projects
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Hesse, S.; Zimmermann, J.; von Segg
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Structure Research The research in
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Research Projects Functional Advanc
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Dincer, I.; Elerman, Y.; Elmali, A;
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hexakis(imidazole)nickel(II)bis(for
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Chemical Analytics The chemical ana
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Collaborations The above mentioned
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Hoffmann, P.; Non-Invasive Identifi
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Ferroelectrics For this class of ma
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Materials Modelling The research ac
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Materials for Renewable Energies In
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Joint Research Laboratory Nanomater
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Collaborative Research Center (SFB)
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B7 P.I.: Prof. H. v. Seggern / Dr.
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The glass transition temperature (T
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transition allows to investigate at
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Relative density 1.00 0.95 0.90 0.8
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Fig. 2 shows the time dependencies
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Dielectric interface trap engineeri
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∆V C q th eff 12 −2 p ( VGS = 0
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Fig. 3: Schematic of the photovolta
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synthesis and analysis of interface
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Fig. 2: Resistivity vs temperature
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Fig. 2: X-ray appearance near-edge
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Capacity [mAhg -1 ] 700 600 500 400
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Texture investigations and field em
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screening effects. Gold coating of
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Figure 2 shows a selected region of
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The spectrum shows that apart from
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The effect of crystallinity on the
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