Activity of gold supported catalysts strongly depends on the type and composition of support, which determine the size of Au nanoparticles (Au NPs), gold-support interaction influencing gold properties, interaction with the reactants and, in this way, the reaction pathway. The aim of this study was to use two types of zeolites: the three dimensional HBeta and the layered two-dimensional MCM-36 as supports for gold, and modification of their properties towards the achievement of different properties in oxidation of glucose to gluconic acid with molecular oxygen and hydrogen peroxide. Such an approach allowed establishment of relationships between the activity of gold catalysts and different parameters such as Au NPs size, electronic properties of gold, structure and acidity of the supports. The zeolites were modified with (3-aminopropyl)-trimethoxysilane (APMS), which affected the support features and Au NPs properties. Moreover, the modification of the zeolite lattice with boron was applied to change the strength of the zeolite acidity. All modifications resulted in changes in glucose conversion, while maintaining high selectivity to gluconic acid. The most important findings include the differences in the reaction steps limiting the reaction rate depending on the nature of the oxidant applied (oxygen vs. H2O2), the important role of porosity of the zeolite supports, and accumulation of negative charge on Au NPs in catalytic oxidation of glucose.
Four different methods for gold deposition on Beta zeolite, namely impregnation, ion-exchange, deposition-reduction, and grafting on (3-aminopropyl)trimethoxysilane functionalized support, were applied to investigate their influence on textural/structural changes in the zeolite support and its surface acidity. The as-prepared materials were fully characterized by XRD, N2 physisorption, ICP-OES, XPS, TEM, and pyridine adsorption. The obtained results indicated that bifunctional redox–acidic materials prepared within this work were characterized not only by different gold loading and gold particle size, but also different textural parameters and acidity. All these features were strongly affected by the procedure applied for gold deposition. The introduction of Au into Beta zeolite by ion exchange caused a significant decrease in the Si/Al ratio in the zeolite framework. The size of Au particles determined the textural parameters of the zeolite and the number of Lewis acid sites (LAS). The Brønsted acid sites (BAS) number was decreased if (3-aminopropyl)trimethoxysilane or NaBH4 were used in the procedure of gold deposition. The highest BAS/LAS ratio was achieved for the sample prepared by ion exchange in the ammonium form of Beta zeolite. The presented results permit making a proper choice of the gold modification procedure for the preparation of bifunctional (redox–acidic) materials, addressed to a desired application.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.