Chemistry Reference
In-Depth Information
Iron(III)(acac)
3
has been supported inside the
pores of MCM-41 and used as a catalyst for the
oxidation of phenol [120], giving catechol (66%),
hydroquinone (27%) and benzoquinone (7%).
They found that the oxidation proceeded best
with aqueous hydrogen peroxide and in water as
solvent.
Rao
et al
. [108] have published preliminary details
of a Cu-based system for the oxidation of phenol.
Their work involves the incorporation of Cu(II)
acetate dimer into the cubic Al-containing MCM-48.
The oxidation of phenol was studied in buffered
aqueous medium at ambient temperature using air
as oxidant. Conversion was 36% and the product
was 'predominantly' catechol. Turnover numbers are
(at 37) modest but nonetheless are almost 10 times
higher than with unsupported copper acetate.
Further work on a related system (Cu ion-
exchanged into Al-MCM-41) also has been published
[121]. The authors found that this was the most
effective method for the attachment of Cu and that
MCM-41 was a superior support to a range of
other (amorphous) materials and zeolites. Their
results were obtained from the system benzene/air/
ascorbic acid. The latter is used as a reductant for the
Cu species. The oxidation of benzene to phenol is
considerably more difficult than that of phenol to
diphenolics, and the yields of phenol obtained were
never higher than 1.5%. Although this work indi-
cates that such an oxidation is possible in principle,
an efficient separation method (coupled to higher
conversions) would need to be found before the
method could be of value. Vanadium-containing
MCM-41 also has been found to be an active catalyst
for the oxidation of benzene with dilute hydrogen
peroxide. Phenol was the only liquid product [122].
Immobilised metallophthalocyanines also have
been developed that are capable of the oxidation of
phenols using hydrogen peroxide as solvent [123].
In particular the iron-containing tetrasulfophthalo-
cyanine was superior to the other catalysts and to
homogeneous versions of the catalyst. Curiously, the
dimeric form of the organometallic species was more
active than the monomeric species, which is the
opposite of what is normally found.
The efficient coupling of phenols is an increasingly
important area of research, especially with the in-
creasing importance of ligands such as binaphthyls
in asymmetric catalysis (see Refs 124 and 125).
Corma
et al
. [126] have published details of MCM-
41-supported Fe and Cu catalysts that are excellent
for the oxidative coupling of 2-naphthol to the
(racemic) 1,1¢-binaphthol, which is one of the most
important of the binaphthyl systems in catalysis.
Their results (with metal-exchanged MCM-41
catalysts) show that Fe gives 99% conversion of
the naphthol and 100% selectivity towards the de-
sired product under an air atmosphere. Cop-
per-exchanged materials display significantly lower
selectivities (Fig. 7.28). A comparison with many
other systems indicates that the Fe-MCM catalyst is
the optimum system for this reaction. The authors
also show that, even with a pore size of 35 Å, diffu-
sion of the product out of the pore system is not the
limiting factor.
Alkane oxidation
The oxidation of simple, unfunctionalised alkanes
is a difficult task and some progress has been made
in the oxidation of models such as cyclohexane.
The products of this oxidation are the alcohol and
ketone, which can be oxidised further (together) to
give caprolactone, a key intermediate for both nylon
and poly(caprolactone). For example, a supported
Ru-bipy complex (Fig. 7.29) has been prepared
[127], and its activity in the oxidation of cyclo-
hexane has been studied (Fig. 7.29).
Fig. 7.2
8
Preparation of binaphthols
using mesoporous catalysts.
