Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

2.2.1 Synthesis of gold catalysts
2.2 Catalyst synthesis for oxidation reactions
Gold has attracted much interest and high efforts were put into developing efficient gold catalysts
which undoubtedly contributed to the success of this noble metal. The most prevalent method to
synthesize supported noble metal catalyst, i.e. (dry) impregnation usually affords undesirable large
particles when used for Au catalysts. Therefore, other techniques were developed in order to
synthesize Au nanoparticles on various supports such as deposition‐precipitation, adsorption of
colloidal Au, vacuum deposition, grafting and reduction, etc. An extensive overview over these
techniques was given in two reviews [4, 24]. Catalysts featuring metallic copper and silver particles
can be prepared similarly; e.g. colloidal methods where pre‐reduced metal nanoparticles are
adsorbed on a support.
2.2.2 Synthesis of copper catalysts
The most prominent types of Cu catalysts feature Cu as (1) homodisperse ionic Cu species, (2)
supported (potentially metallic) Cu nanoparticles and (3) (mixed) oxides or other bulk ionic materials
(e.g. phosphates [25]). Note that often several Cu species are abundant in one catalyst, e.g. both
CuxO particles and isolated Cu ions (Figure 2‐2). Therefore the categorization into different catalyst
types and hence synthesis techniques shall only be understood as a rough classification.
Copper catalysts with isolated Cu centers are most abundant and catalytically active in many
oxidation reactions and often more stable against leaching than oxidic Cu nanoparticles. These
species are frequently synthesized by ion exchange of zeolites [26‐30], similar porous materials [31,
32] and other minerals with ion exchanger properties like clays [33] or hydroxyapatite [34]. The ion
exchange procedure is straightforward and therefore industrially applicable; the parent material is
treated with a solution of the desired ion followed by filtration and drying. A higher concentration of
the active metal can be obtained by repeating this treatment several times [27, 31] though high
metal concentrations and/or calcination treatment can afford Cu domains which are often undesired
due to the aforementioned leaching issue. In principal, supports modified by organic linkers can also
be ion‐exchanged in order to graft Cu complexes [35, 36] but this class of catalysts will not be
covered here.
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