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Preparation of mesoporous solids containing encapsulated metal species of technological interest

The advent of mesoporous silicas such as MCM-41 has provided new opportunities for research into supported metal catalysis. The loading of metals into framework structures and particularly into the pores of porous molecular sieves, has long been of interest because of their potential catalytic activity. The larger pore sizes of mesoporous silicas allow for the possibility of preparing dispersions of metal/metal oxide particles within the pores while still allowing access by reagent molecule. In standard fabrication processes, metal particle size is not well controlled. Most studies of metal/metal oxide loaded mesoporous silicas report metal/metal oxide particles larger than the pore diameter. In the present work, new methods of preparing metal loaded systems have been developed to facilitate a more even dispersion of the metal. The metals were loaded as readily decomposed organometallic complexes so the chemistry involved in forming the metal/metal oxide particles minimized possible sintering effects. New synthetic routes were developed to facilitate loading into the pores or the framework of the silica matrix. An optimised acid synthetic route for preparing MCM-41 was used throughout. The acid concentration, surfactant concentration, reaction time and stirring rate, all of which have strong influence on the morphology and structure of the mesoporous materials under acidic synthetic conditions, were optimised to give systems of maximum surface area while exhibiting regular pore structures and good stability. The refined reaction conditions of high stirring speeds, low surfactant concentration, high acid concentrations and short reactions times developed in the present work represent important improvements in synthetic methodology giving stable, well ordered, small pore, thick walled and high surface area products. Single metal and mixed metal organometallic complexes were loaded into the preformed mesoporous supports by a modified imbibing method and then decomposed to prepare metal and mixed metal nanoparticles within the structures. However, loading by this method caused disordering and destruction of the MCM-41 porous supports, the broadening of the XRD diffraction lines, and the significant reduction of pore size, pore volume and specific surface area. A major contribution of the current work has been the development of two methods for controlling the loading of metals into the structures during synthesis. These methods offer the significant advantages of loading without structural effects and yielding producing systems where surface area increased rather than decreased with loading. By the M method, a hydrophobic organometallic complex was incorporated within the micelle system formed by the surfactant, cetyltrimethylammonium bromide. This micelle system containing the organometallic complex forms the template for the pores during the sol-gel formation of the mesoporous solids. Incorporation of the metal within the micelle facilitates the dispersion of the metal throughout the surfactant phase filling the pores of final precursor material. Calcining to remove the organic material favours leaving the metal or metal oxide in the form of small particles widely and uniformly dispersed in the mesoporous channels. By the T method, organometallic molecules are dissolved in the Si(OEt)4 of the synthesis mixture. Calcining the organic material from the resulting precursor favours leaving the metal or metal oxide in a widely and uniformly dispersed state in the walls of mesopores. The synthetic methods developed were extended to prepare mixed metal nanoparticles of defined composition using single-source mixed metal organometallic compounds. In this way novel mixed metal (Mo/Fe and Mo/Co) phases with compositions determined by the stoichiometry of the initial mixed metal complex were produced and characterised. The nature of the metal/metal oxide particles was studied by XRD and TEM. At the metal loadings obtained (ranging up to 4%) crystalline metal and metal oxide particles should be readily detected. The absence of well defined XRD peaks characteristic of the metal or metal oxides in any of the samples provided evidence that the metals after calcining were well dispersed, with particles sizes less than a few nm and too small to give diffraction signals. TEM studies revealed a clearly defined hexagonal pore structure with pore diameters of approximately 2 nm but gave no evidence of metal particles larger than the resolving power of the instrument (approximately 1 nm).
Type of thesis
Xu, S. (2011). Preparation of mesoporous solids containing encapsulated metal species of technological interest (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/5953
University of Waikato
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