Ordered mesoporous materials as model supports to study catalyst preparation

Abstract

Catalysts are indispensable to modern-day society because of their prominent role in petroleum refining, chemical processing, and the reduction of environmental pollution. The catalytically active component often consists of small metal (oxide) particles that are supported on a carrier such as silica or alumina. These particles are generally introduced via impregnation of the support with a precursor-containing solution followed by drying. Subsequent thermal treatment in air converts the precursor into the desired metal oxide, or metal when followed by high-temperature reduction with hydrogen. The choice of precursor and the experimental conditions applied during the preparation largely affect the final dispersion of the active phase. This thesis describes the use of ordered mesoporous materials as model supports, in particular SBA-15. It is demonstrated that by combining ordered mesoporous supports with (3D)-TEM and bulk characterisation techniques such as XRD and Nitrogen physisiorption fundamental insight can be obtained into the effects of the individual steps in preparation on the final dispersion of the metal (oxide) particles and the implications thereof for catalysis. The research focussed on Ni and Co ex nitrate catalysts as these are amongst the most widely applied metals and their preparation using nitrate precursors is attractive, but generally disappointing as poor metal dispersions are obtained. The outcome of a comprehensive study carried out to identify the reasons for this are described and it is demonstrated that the poor dispersions are caused by severe sintering and redistribution during the air calcination step used to convert the nitrate precursor into the respective oxide. The factors that play an important role are discussed and a novel method is presented that comprises of decomposition in NO/He atmosphere, which allows preparation of supported nickel oxide and cobalt oxide particles of 3 to 5 nm at loadings of 24 to 37 wt% and 16 to 23 wt%, respectively. The validity of this method for different systems is demonstrated and an explanation for the role of NO is proposed. Moreover, the relevance of this new method for catalysis is proven using the hydrogenation of Soybean oil and the Fischer-Tropsch synthesis of syngas into hydrocarbons over silica supported Ni and Co catalysts, respectively. It is shown that the activity of the NO/He treated catalysts is superior compared to the catalysts treated with air calcination. Finally, the results of our study on the use of ordered mesoporous materials as host for homogeneous PCP and SCS-pincer Pd catalysts are presented. It is demonstrated that the complexes were successfully tethered to the support via a trialkoxysilane moiety without destruction of the ordered mesoporous support. The activity of the SBA-15 modified with PCP-pincer Pd-complex as Lewis acid in the aldol condensation between methyl isocyanoacetate and benzaldehyde is discussed together with recycling results that show the true heterogeneous nature of the catalyst in this reaction.