Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

Chapter 7
Concluding Remarks
7.1 Conclusions and summary
This thesis covered several topics related to the oxidation of organic compounds over heterogeneous
catalysts in the liquid phase. Special focus was put on using oxygen as the least expensive oxidizing
agent. Within the coinage metals the dominating role of gold catalysts was compared to the
performance of silver and copper catalysts for the most widely studied liquid‐phase oxidation
reactions. Synthesis strategies are important; for gold, standard impregnation techniques are
unsuited, while silver catalysts might require the incorporation of subsurface‐oxygen species to be
active. Copper catalysts are widely active for different reactions both in the metallic form and
oxidized form. In the latter case, isolated copper species usually give the best performance in terms
of conversion, selectivity and stability when compared to oxidic copper nanoparticles. A pronounced
difference in the reactivity between gold on the one hand and copper and silver on the other hand is
that the latter two can catalyze the anaerobic oxidation of alcohols which gives rise to new
applications in catalysis. Gold catalysts appear to be less efficient in oxidizing hydrocarbons which
usually proceeds via a radical (aut)oxidation mechanism though there is some discrepancy in the
open literature. Here, either copper (e.g. for benzene hydroxylation) or silver (e.g. for the side‐chain
oxidation of alkyl aromatic compounds) are better suited candidates. The potential of silver catalysts
for alcohol oxidation was thoroughly investigated. By using a simple screening approach, silver
supported on SiO2 in combination with CeO2 nanoparticles was found to be an effective catalyst for
the aerobic oxidation of alcohols. The calcination procedure for the silver catalyst had a strong
influence on the catalytic performance: relatively short calcination times at 500 °C gave the best
catalyst. This might be due to incorporation of silver‐oxygen species as suggested by EXAFS, TEM and
XRD measurements. The collaborative effect between Ag/SiO2 and CeO2 was not related to leaching
but likely to direct physical contact, CeO2 showing to be well‐dispersed on the SiO2 support after the
reaction. Interestingly, both Pd as well as Au catalysts were also promoted by the simple addition of
CeO2 nanoparticles when applied under the same reaction conditions. When used in combination
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