Synthesis, Structure and Catalytic Activity of ... - Jacobs University

Ch.1 Introduction
_____________________________________________________________
catalytic oxidation reactions. Their closely packed arrays of oxide anions can
be practically viewed as discrete fragment models of the extended metal oxide
lattices known for being efficient heterogeneous catalysts. 2,4,5,6,7,13
The basic mechanism involved in POM catalyzed oxidation reactions with
molecular oxygen is shown in eq. A; where POMox and POMred are the
oxidized and reduced forms of the POM respectively. The substrate is first
oxidized by the POM which is then regenerated by the oxidant that is often
oxygen to form water. Oxidation of the substrate is often accompanied by
proton transfer to the consequently reduced catalyst (eq. A):
S + [POM]ox + nH + → Hn[POM]red + P
Hn[POMred] + n/4 O2 → [POM]ox + n/2 H2O (eq. A)
Such a mechanism is thermodynamically favorable under the condition that
the reduction potentials of the substrate, POM and molecular oxygen
increase in the following order:
E(S) ≤ E(POM) ≤ E(O2)
This mechanism is usually followed in the formation of carboxylic acids from
the corresponding aldehydes (methacrylic acid from methacrolein) and the
dehydrogenation of alcohols and aldehydes and carboxylic acids to form C=C
and C=O bonds (isobutyric acid transformation to methacrylic acid). 1,2
Phosphomolybdovanadates were employed first by Kozhevnikov and Matveev
as secondary catalysts in the Wacker process for the transformation of
ethylene to acetaldehyde; H5PV2Mo10O40 was found out to be the most
efficient in the reoxidation of the primary catalyst PdCl2 according to (eq. B)
as a replacement of the halide containing CuCl2. 14
Pd 0 + [POM]ox + 2H + → H2[POM]red + Pd II
H2[POM]red + 1/2 O2 → [POM]ox + H2O (eq. B)
Neumann et al. later tried to utilize this catalyst in the direct oxidative
dehydrogenation of various organic substrates such as 9,10-
dihydroanthracene, dehydrogenation of benzylic alcohol or
13