Abstract
MCM-41 is an ordered mesoporous material, displaying a honeycomb-like structure of uniform mesopores (3 nm in diameter) running through a matrix of amorphous silica. Because of the high porosity (pore volume » 1.0 ml g-1) and concomitant large surface area (approximately 1,000 m2 g-1) MCM-41 is in principle ideally suited
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to be used as a support material for heterogeneous catalysts, as it offers the possibility to apply (precursors of) active phases in a highly dispersed fashion. Unfortunately there is one pronounced drawback associated with MCM-41, viz. its limited stability towards a number of catalyst precursors. Therefore new methods have been explored to apply catalytically active phases, notably nickel / nickel oxide and molybdenum oxide, inside the mesopores of MCM-41. The results of these research efforts have been compiled in this thesis.
For the application of well-dispersed nickel (oxide) nanoparticles inside the mesopores use has been made of the favourable properties of an aqueous solution containing a chelated nickel precursor, viz. nickel citrate. During drying after incipient wetness impregnation the viscosity of such a solution increases tremendously, thus immobilising the nickel precursor inside the mesopores. Immobilisation is further brought about by hydrogen bonding interactions between the nickel citrate precursor complexes and the pore walls of the all-silica MCM-41 support material. Upon calcination the precursor complexes decompose and nickel oxide nanoparticles are formed, which are well-dispersed and situated exclusively inside the mesopores of the support. Reduction yields metallic nickel nanoparticles. During all the catalyst preparation processes the unique structure of the support material is well-retained.
Two new methods have been developed for the application of molybdenum oxide inside the mesopores of MCM-41. Both methods rely on incipient wetness impregnation of all-silica MCM-41 with a suitable (aqueous) molybdenum precursor solution. The first molybdenum precursor used consists of trivalent molybdenum chloride complexes, which have been obtained by electrochemical reduction. The other precursor is obtained by (slowly) adding ammonium heptamolybdate (AHM) to a 1 : 1 solution of hydrochloric acid and water, yielding MoO2Cl2-complexes. Catalyst preparation results, after drying and calcination, in MoO3/MCM-41 catalysts exhibiting unprecedented high loadings ánd dispersions of MoO3. Moreover, the favourable support properties are completely retained when one of these two precursors is used. Strikingly different results are obtained when a common catalyst preparation procedure is followed, using an aqueous solution of AHM: in that case the support structure is completely destroyed.
MCM-41 supported and reference molybdenum oxide catalysts have been tested in the catalytic oxidation of ethane. A much higher activity was obtained for MoO3 catalysts supported by MCM-41, compared to Aerosil-200 supported and bulk molybdenum oxide catalysts, thus corroborating the very high dispersion of MoO3 inside the mesopores of the MCM-41 support. A kinetic characterisation indicated that catalysis takes place via a so-called pseudo-redox mechanism and that (internal) diffusion limitations do not occur with the MCM-41 supported catalysts. Moreover, catalyst preparation with MoO2Cl2 as a precursor yielded more active catalysts than preparation with a trivalent precursor when MCM-41 was the support material. With Aerosil-200 an MoO2Cl2-precursor yields relatively large MoO3 platelets of a low dispersion, situated next to the support.
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