Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

2.1 Introduction
2.1 Introduction
Ever since Haruta et al. [1‐3] disproved the prejudice of gold being a beautiful but essentially inert
metal in catalysis, a huge run on gold started – first with the in‐depth investigation of the Au
catalyzed gas phase CO oxidation proceeding already at room temperature or even below. The use of
gold as a potent heterogeneous oxidation catalyst was extended to selective liquid phase oxidations
resulting in an immense amount of literature [4‐10]. But the origin of the fascination of gold is also its
strongest burden: its low availability and high price (apart from the color and the gold‐digger myths).
Despite the tremendous knowledge on gold catalyzed reactions, there are only few examples where
gold is industrially competitive [11]. On the other hand, the smaller siblings of gold – silver and
copper – have long been applied for important catalytic processes in industry, silver being used in
methanol oxidation as well as ethylene epoxidation and copper e.g. for methanol synthesis. However
in academics, both silver and copper received far less attention and thus have not made it to step out
of the shadow of gold especially in selective liquid‐phase oxidations. Considering that silver is roughly
2 orders and copper even 4 orders of magnitude less expensive than gold makes industrial
applications much more likely. Establishing new processes on the background of the discussion on
“Green Chemistry” is certainly important and the price of a catalyst is a crucial factor in making a
process sustainable. With respect to CO oxidation, both silver [12‐15] and copper [16‐19] have
proven their ability to catch up with gold. Applications of heterogeneous copper and silver catalysts
in the total oxidation of harmful organic compounds in wastewater effluents [20‐23] underline their
potential in liquid phase oxidations. Recently, the potential of the coinage metals mainly in gas phase
reactions was summarized in two reviews [4, 10]. This chapter will concentrate on liquid‐phase
oxidations and compare heterogeneous silver and copper catalysis with the broad field of gold
catalysis. The intention is to show the niches and individual strengths of silver and copper but also
supplying a short overview over important features of liquid‐phase gold catalysis. Opportunities for
future research will be described, benefitting from the broad knowledge on gold catalysis. Focus has
been laid on selective oxidations by intrinsically heterogeneous catalysts. Thus, total oxidations and
immobilized complexes being an extensive topic especially for Cu catalysis will only be covered
where it supports the argumentation. After providing a short summary of synthesis techniques
towards the different catalysts the most important selective oxidation reactions will be described
where one or more of the coinage metals play a significant role as a catalyst material. A discussion
with respect to important parameters for catalyst synthesis, differences between the catalytic
properties and performance of the metals and finally opportunities for future work will complete this
chapter.
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