New mechanistic insight in the gold-based propene epoxidation

Abstract

Propene oxide is a very important intermediate for the synthesis of commercial products, including adhesives, paints, and cosmetics. The gas-phase epoxidation of propene over Au/Ti-based catalysts is an intriguing scientific topic, not only because of the industrial importance of the production of propene oxide, but also because this reaction offers fundamental insights into the working principles of Au-based catalysis. The current methods used for propene oxide synthesis combine several reaction steps and have economic or environmental problems. Au/Ti-based catalysts are able to produce propene oxide with high selectivity out of propene, hydrogen and oxygen in a simple process since a single reactor is needed. However, a fundamental understanding of the reaction mechanism is necessary for developing more efficient Au/Ti-based propene epoxidation catalysts. The aim of this PhD thesis was to shed further insight into the mechanism of Au/Ti-based catalysts for the epoxidation of propene with O2 and H2. The first effort of the research has focused on the synthesis of well-defined Au/Ti-SBA-15 catalysts. Different methods have been used for the synthesis of the Ti-SBA-15 supports. Au/Ti-SBA-15 prepared by grafting Ti in the SBA-15 structure contains a higher absolute amount of tetrahedral Ti than the ones prepared by a hydrothermal method of Ti-SBA-15 support. The catalytic activity of Au/Ti-SBA-15 catalysts was found to be dependent on the amount and dispersion of Ti as well on the Au particle size. Further more, the epoxidation activity of Au/Ti-SBA-15 catalysts was improved by performing a post-synthesis NH4NO3 treatment. Formation of a large number of surface hydroxyl groups together with ammonium species on the surface support have a beneficial effect on the homogeneous deposition of Au. The superior activity of treated Au/Ti-SBA-15 materials is related to the increased number of Au adsorption sites. Several spectroscopic techniques were used for characterization of the resulted catalysts and for understanding of the reaction mechanism. In-situ XAFS spectroscopy provided evidence on the adsorption of propene on Au nanoparticles. The results indicated that the adsorption of propene is a key step in the epoxidation mechanism over Au/Ti-based catalysts. Furthermore, steady state isotopic kinetic analysis performed on Au/TiO2 and Au/Ti-SBA-15 was used to investigate if the support oxygen is playing a role in the production of propene oxide. The results obtained from our studies were discussed together with the literature reports in order to understand the related propene epoxidation mechanism.