Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CO + 1/2 O 2
Figure 1‐1: Catalytic oxidation reactions.
CHAPTER 1
O
OH 2
O
Au/TiO 2
Pd-Bi/Al 2O 3
CO 2
*
O
ligand sphere is often an effective and relatively simple way of tailoring catalyst properties. In some
cases, homogenous catalysts are highly active already in minimal concentrations which make a
catalyst separation step dispensable. Zirconocenes (“Kaminsky catalysts”) used in olefin
polymerization [9] are an often cited example for catalysts with such remarkable activities. Within
oxidation catalysis cobalt salts are used initiating the radical autoxidation of hydrocarbons. In
combination with manganese salts and a bromide source inexpensive cobalt salts form the basis of
the AMOCO process producing terephthalic acid from p‐xylene [10]. Due to the nature of a radical
autoxidation only small quantities of catalyst are necessary to achieve high conversions and – in
terms of figures – enormous turn‐over frequencies. When catalysts are less active and production
costs or mandatory end product qualities require separation of the catalyst, heterogeneous catalysts
are advantageous. Grafting of homogeneous catalysts is a possibility [11] but the usually observed
higher stability and the significantly lower production costs make this a key research area of
inherently heterogeneous catalysts – minerals, oxides, (supported) nanoparticles, metal foams etc.
In order to achieve high efficacy, selectivity is often regarded to be more important and
harder to achieve than activity. Selectivity in terms of heterogeneously catalyzed oxidation reactions
includes the selective formation of one partial oxidation product, the chemoselective oxidation of
only one oxidizable moiety within a molecule (though to several different products, e.g. the selective
side‐chain oxidation of alkyl aromatic compounds, cf. Chapter 4) and the selective oxidation of one
type of compound within a mixture of several (e.g. CO oxidation in H2 containing atmosphere [12])
[13]. Due to the capability of many noble metals to adsorb and activate molecular oxygen, supported
noble metal nanoparticles were especially successful and play a key role in selective oxidation
OH
OH
2
O
OH
*
Co/Mn/Br
O 2
Fe phthalocyanine ∗
H 2O 2
O
HO
O
OH
OH
O
O
O
∗
O
OH