Chemistry Reference
In-Depth Information
Fig. 7.29
Oxidation of cyclohexanone
with immobilised Ru-bipy complex.
The supported catalyst was capable of the oxida-
tion of cyclohexane (and of other hydrocarbons)
with
t
-BuOOH to give the alcohol and ketone. The
ratio of ketone to alcohol was higher than that
found with the homogeneous complex, a result that
was explained by the competitive adsorption of
the alcohol on the pore surface, excluding alkane,
thus being preferentially oxidised to ketone. A
further interesting fact is that the turnover number
decreases with loading and is generally higher than
that found with the homogeneous catalyst. No
explanation was given, but this may indicate that
higher dispersion of the metal centres may minimise
the formation of inactive dimeric or oligomeric
species, as postulated by Thomas
et al
. with the Co-
based system discussed next. No leaching was
observed either by 'hot filtration' or by atomic
absorption.
Thomas
et al
. have published details of a supported
trimeric Co species that is also an effective stable
catalyst for the oxidation of cyclohexane [57].
They supported the complex shown in Fig. 7.30 by
functionalising MCM-41 with bromopropyl trime-
thoxysilane and then treating this with an aqueous
solution of glycine to give the supported amino acid
ligand. Complexation with the Co precursor then
was carried out and the expected change in structure
of the Co complex (caused by replacement of a bridg-
ing OH with a bridging carboxylate) was shown by
in situ extended x-ray adsorption fine structure
(EXAFS) studies.
Catalytic activity was good, again using
t
-BuOOH
as primary oxidant, and steady rates of oxidation of
ca. 200 turnovers per hour could be sustained for
several days (giving a turnover number of ≥10 000).
Ketone/alcohol ratios were remarkably high, reach-
ing 3.3 after prolonged reaction times. Again, leach-
ing could not be observed by either 'hot filtration' or
by atomic absorption studies.
Fig. 7.30
Structure of supported trimeric Co complex.
3.4 Base catalysis (other than oxidations)
The subject of base catalysis has been investigated by
a number of authors. A few papers deal with inor-
ganic bases supported on micelle-templated silicas
but the majority deal with nitrogen bases, ranging
from simple amines to the much more basic guani-
dines as potential replacements for bases such as
alkali metal hydroxides in synthesis. The activity of
the latter types of catalysts in the base-catalysed
epoxidation reaction has been described already.
Alkali-containing MCM-41 materials were pre-
pared and evaluated as basic catalysts by Kloetstra &
van Bekkum [74]. They used an Al/Si material
and ion-exchanged the desired metal cation into
the structure, replacing the H and Na present
from the synthesis. The Na-exchanged material dis-
played reasonable activity in the Knoevenagel reac-
tion of benzaldehyde and ethyl cyanoacetate, a
carbon acid with a p
K
of around 11. This relatively
facile reaction is one of the most commonly
studied reactions in such systems and is not parti-
cularly demanding. Conversions and selectivities
were good, in reasonable times. Direct comparison
with other catalytic systems is made difficult, despite
