Catalytic oxidation of alkyl aromatics using a novel silica supported Schiff base
complex
Ian C. Chisem,a John Rafelt,a M. Tantoh Shieh,b Janet Chisem (née Bovey),a James H. Clark,*a† Roshan
Jachuck,b Duncan Macquarrie,a Colin Ramshawb and Keith Scottb
a Department of Chemistry, University of York, Heslington, York, UK YO10 5DD
b Department of Chemical and Process Engineering, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, UK NE1 7RU
A new heterogeneous catalyst based on a chemically
modified mesoporous silica gel and possessing immobilised
chromium ions has been prepared and successfully applied
to the aerial oxidation of alkyl aromatics at atmospheric
pressure and in the absence of solvent.
The catalytic oxidation of ethylbenzene (used as a model
substrate) was carried out in neat substrate using 1.5 g catalyst
and air as the consumable source of oxygen. The reaction was
performed in a baffled glass reactor with overhead stirring and
fitted with a Dean–Stark trap to facilitate the removal of water
from the reactor. After a short induction period the conversion
rate in the first 5 h of operation was ca. 5% h21, corresponding
to a frequency of 1225 turnovers h21 per catalytic site
(assuming 0.10 mmol g21 loading of the active site). After the
first 5 h, the rate of conversion dropped significantly; this is
attributed to the inefficient removal of water from the reaction
system, and poor adsorption of ethylbenzene onto the catalyst
surface as the acetophenone concentration increases. In addi-
tion, catalyst reuse studies were performed by recycling 1.5 g of
catalyst twice without any regeneration or conditioning by
decanting the liquid from the reactor and adding fresh substrate
and then repeating the experiment. The results shown in Fig. 1
demonstrate that the recycled catalyst retains its activity and the
catalytic rate is equal to that of the fresh catalyst. Furthermore,
the induction period observed with the use of fresh catalyst was
eliminated when the catalyst was recycled. This suggests that
poor product desorption when the catalyst is not saturated with
the product was the cause of the induction periods observed.
The oxidation of organic substrates provides routes to a wide
range of functionalised molecules. Traditional methods involve
the use of large quantities of poisonous high oxidation state Cr,
Mn and Os reagents.1 Lower oxidation state metals such as CoII,
MnII and CuII in AcOH may be used2 with O2 as the consumable
oxidant. However, the conditions are often harsh, the reagent
mixture is corrosive (bromide is used as a promoter), and the
chemistry is rarely selective. Environmentally acceptable
catalytic oxidations that operate under moderate conditions in
the liquid phase with high selectivity are clearly desirable. A
range of supported reagents has been used in the liquid phase
oxidation of organic substrates. Advantages of supported
reagents include ease of handling, use and recovery, low
toxicity and the avoidance of solvents. However, in oxidations,
supported reagents have generally acted as stoichiometric
reagents, making their large scale use difficult and expensive.
We have proven the possibility of developing genuinely
catalytic supported reagents which are active in some oxida-
tions.3,4 More recently, catalysts based on chemically modified
silicas, which have higher catalytic site densities, have been
prepared.5,6 We now report a novel heterogeneous catalyst for
the liquid phase partial oxidation of alkyl aromatic substrates
based on a chemically modified mesoporous silica gel which
possesses significantly enhanced activity. The material strongly
binds chromium ions and is robust, reusable and active in the
oxidation of ethylbenzene and methyl aromatics.
The preparation of the chromium catalyst (CHRISS) is shown
in Scheme 1. Salicylaldehyde (1 equiv.) was added to excess
absolute EtOH, to which 3-aminopropyl(trimethoxy)silane (1
equiv.) was added. The solution instantly became yellow due to
imine formation. Chromium(III) acetate (0.5 equiv.) was then
added to the solution, and the mixture stirred for a further 30
min to allow the new ligands to complex the chromium. The
silica (Kieselgel 100) was then added and the mixture stirred
overnight. The final product was washed with water, EtOH and
finally Et2O until the washings were colourless. Further drying
of the solid product was carried out on a rotary evaporator at
70 °C for 2 h. The loading achieved is ca. 0.10 mmol g21
[determined by atomic absorption spectroscopy (AAS)]. The
catalyst has an average pore size of 100 Å and a particle size of
30–140 mm. The infrared spectrum of the free ligand (i.e. prior
to complexation with the metal) shows a band at 1642 cm21
attributed to the C§N stretching vibration of the imine. This is
reduced to 1593 cm21 upon complexation of Cr3+. Apart from
bands in the 250–450 nm region, the diffuse reflectance UV
spectrum of the catalyst shows a band at ca. 600 nm
corresponding to d-d transition for the metal complex. The band
is shifted to higher wavelength by about 30 nm from the
corresponding band in pure Cr(OAc)3 or Cr(OAc)3 physisorbed
on silica, consistent with a change in ligand environment.
EtO
EtO
Si
N
EtO
HO
chromic
acetate
EtO
EtO
Si
N
EtO
O
Cr3+
O
EtO
EtO
N
Si
EtO
SiO2
N
O
Cr3+
O
N
Scheme 1
Chem. Commun., 1998
1949