Mesoporous silica SBA-15 supported Cobalt oxide: an investigation of the structure, morphology and catalytic activity of various composites

Mesoporous silica supported cobalt oxide composites have attracted enormous attention due to their wide range of applications, especially in the catalysis field. The focus of this thesis concerns the preparation of mesoporous silica SBA-15 supported cobalt oxide species and the evaluation of their catalytic activities for liquid phase oxidation of various alcohols and olefins. The impact of different variables such as preparation method, metal precursor, metal loading and dopant was investigated on the morphology and dispersion of the active particles, the overall structure of the resulting composite materials and their catalytic activity. The first section of the thesis concerns the use of Co(NO3)2.6H2O as a precursor. Various amounts of Co(NO3)2.6H2O were deposited on SBA-15 using the “two-solvent” method. This method accommodates cobalt oxide species exclusively inside the pores of SBA-15. Analogous composite materials were also prepared using the more conventional methods of impregnation and adsorption. Irrespective of preparation method, all the composite materials were black. The two-solvent method resulted in the formation of crystalline cobalt oxide (Co3O4) nanorods which filled adjacent mesopores to form patches incorporating all the cobalt available. In the case of composites prepared via impregnation cobalt oxide particles formed both on the external and internal surfaces of the SBA-15, also incorporating all the available cobalt. When adsorption was used as a preparation method, cobalt oxide species were predominantly formed inside the pores of SBA-15, however not all the cobalt available in solution was incorporated. All the composites were characterised by N2 adsorption-desorption, XRD, XPS, TPR, ICP-MS, FTIR, DR UV-vis, SEM, TEM, STEM and elemental mapping. The size of the crystalline Co3O4 patches was measured using the XRD diffraction patterns and Scherrer equation. The lattice fringes of Co3O4 were also observed in HRTEM images. The catalytic activity of the composite materials in liquid phase oxidation of various substrates was determined by GC and GC-MS. It was found that for the composite materials prepared by the “two-solvent” method, the catalytic activity varied in reverse proportion to the cobalt loading and that the composite with the lowest amount of cobalt exhibited the highest catalytic activity. This was attributed to the morphology of the cobalt oxide species and the way they were dispersed throughout the SBA-15. As the cobalt loading increased, more pores became blocked due to the elongation of the Co3O4 nanorods inside the pores and, thus, a lower proportion of the cobalt was accessible by the reactants. The composite prepared via adsorption also demonstrated high catalytic activity as a result of the relatively good dispersion of Co3O4 and, hence, its accessibility by the reactants. The composite prepared via impregnation exhibited the lowest catalytic activity compared to the analogue composites with the same cobalt loading prepared either by the two-solvent method or adsorption. This is attributed to the formation of large cobalt clusters (poor dispersion) mostly on the external surfaces of SBA-15 on the aperture of the pores. The composites prepared via the two-solvent method could be reused up to at least three times without significant changes in their catalytic activities. In the second section of the thesis SBA-15 was loaded with various amounts of CoCl2.6H2O via the two-solvent method, impregnation and adsorption. Regardless of the preparation method a blue coloration with intensity proportional to the cobalt loading was observed for the composites. The suite of characterisation techniques (see above) was applied to investigate the morphology, dispersion and chemistry of the incorporated cobalt as well as the structure of the composites. The cobalt oxide species were found to exist as an amorphous phase which was highly dispersed throughout the SBA-15, presumably due to the interaction between Co2+ ions and the silica surface. A comparison of composites shows that the micropores became progressively more blocked as the cobalt loading was increased, but the mesopore volume was reduced only marginally. For the series of composites prepared via the two-solvent method, an unexpected morphological transformation was observed when the cobalt loading was 30 wt% or beyond. This transformation happened during the calcination step of the preparation and resulted in the collapse of the highly ordered mesoporous channels of SBA-15. It is suggested that the interaction between incorporated Co2+ ions and the oxygen of the silica walls at high cobalt loadings, is what leads to this transformation during calcination. The catalytic activity of these composite materials was evaluated for liquid phase oxidation of cyclohexanol. For the series prepared by the two-solvent method, it was found that the composite with lowest cobalt loading again showed the highest catalytic activity. However, the composite which experienced the morphological transformation (Co 30 wt%) showed only slightly lower catalytic activity compared to other composites. This suggests that although the surface area decreased substantially, the catalytically active species remained accessible to the reactants. It was confirmed that the oxidation reaction did not occur in the absence of either cobalt or oxidant tert-butylhydroperoxide, TBHP. With this series of catalysts, it was found that a small amount of cobalt could ‘leak’ into solution, indicating that the catalytic activity for this series of composites might best be thought of as a mixed ‘heterogeneous-homogeneous’ system. Since ceria has been widely employed as a dopant, both for bulk and supported metal oxides with promising results on their catalytic activity, the main objective of the third section was to investigate the effect of ceria as a promoter on the morphology, dispersion, crystallinity, leakage and catalytic activity of the SBA-15 supported cobalt oxide species. Two series of composites were prepared via the two-solvent method using Co(NO3)2.6H2O and CoCl2.6H2O with 5 wt% loading as cobalt precursors and Ce(NO3)3.6H2O with 0.5 wt% loading as a dopant. It was found that the ceria-doped composite prepared from Co(NO3)2.6H2O exhibited better dispersion of cobalt oxide species, with both Co3O4 single nanorods and less extensive Co3O4 patches, than its non-doped analogue. Here the better dispersion of the Co3O4 nanorods, the synergistic effect between cobalt oxide and ceria and the improvement in redox properties result in higher catalytic activity of the composite in liquid phase oxidation of various alcohols and olefins. Ceria-doped composites prepared from Co(NO3)2.6H2O was reusable up to four times without significant change in its catalytic activity. However for the composites prepared from CoCl2.6H2O (5 wt%) and Ce(NO3)3.6H2O (0.5 wt%), the addition of the dopant did not improve the crystallinity of the cobalt oxide phase. The XRD patterns did not indicate any trace of phase separated cobalt-cerium mixed metal oxide. The dispersion of the cobalt oxide species in this specific case appeared to be the same as for the undoped analogue. However the ceria-doped composites demonstrated better catalytic activity. This can be attributed to the improvement in the redox properties of Co2+/Co3+ due to the presence of ceria. Ceria-doped composites prepared from CoCl2.6H2O still exhibited some slight ‘leakage’. The effect of the Co:Ce ratio on the morphology and catalytic activity of the composites was also investigated. These composites were prepared via the two-solvent method using CoCl2.6H2O (5 wt%) and Ce(NO3)3.6H2O (0.5-2 wt%) as metal precursors. The highest catalytic activity was obtained from composites with 0.5 wt% and 1.5 wt% cerium loadings. It is speculated that these two compositions facilitate better synergy between cobalt oxide species and ceria. An in-depth study on the liquid phase oxidation of benzyl alcohol was carried out to investigate the mechanism of the oxidation reaction. The amount of TBHP added was not stoichiometrically sufficient to facilitate the ‘double oxidation’ of the substrate through benzaldehyde and then to benzoic acid. Moreover, after the completion of the reaction TBHP was still observed (by GC) to be present. This suggests that aerial oxygen was also behaving as an oxidant and that TBHP behaved more as an initiator to the catalytic reaction. This was confirmed by carrying out the reaction under an inert atmosphere (N2) and also in the absence of the TBHP with negligible conversion in both cases. In summary, it was found that various preparation methods result in the formation of cobalt oxide species with different morphology and dispersion. The two-solvent method clearly facilitates the formation of the cobalt oxide species inside the pores of the SBA-15 and prevents their aggregation into inactive clusters on the external surface of the support. The nature of the cobalt precursor, more specifically the nature of the counter ion, determines the nature of the cobalt oxide species, its crystallinity and its interaction with the support. This, in turn, impacts the overall structure of the composite. The amount of cobalt loading also affects the morphology of the active species, the overall structure of the composite, the degree of pore blockage and thus the availability of the active sites. Ceria, as a dopant, was found to improve the dispersion of the Co3O4 nanorods prepared from Co(NO3)2.6H2O) and the redox properties of the cobalt oxide species (prepared using either Co(NO3)2.6H2O or CoCl2.6H2O as cobalt precursor) supported on SBA-15.  (...)