One-step synthesis of benzyl acetate by gas phase acetoxylation of
toluene over highly active and selective Pd–Sb–TiO2 catalysts{
A. Benhmid, K. V. Narayana, A. Martin* and B. Lu¨cke
Institut fu¨r Angewandte Chemie Berlin-Adlershof e.V.{, Richard-Willsta¨tter-Str. 12, D-12489 Berlin,
Germany. E-mail: a.martin@aca-berlin.de; Fax: 149-30-63 92 44 54; Tel: 149-30-63 92 43 06
Received (in Cambridge, UK) 28th April 2004, Accepted 24th June 2004
First published as an Advance Article on the web 2nd August 2004
Significantly high benzyl acetate selectivity of 85% at a
monometallics while the combination of Pd and Sb proved to
have an amazing effect on their catalytic properties. Pd alone
supported on TiO2 (i.e. 5% Pd/TiO2 catalyst) displayed very much
less activity (Xtoluene ~ 2.4%; YBA ~ 2.2%) and at the same time
Sb alone supported on TiO2 (i.e. 8% Sb/TiO2 catalyst) was also
found to exhibit poor performance (Xtoluene ~ 2%; YBA ~ 1.7%).
However, the combination of both Pd and Sb in appropriate
amounts significantly enhanced the activity of the present Pd and
Sb catalysts. The activity of 5% Pd–8% Sb–TiO2 catalyst was nearly
16–18 times higher (Xtoluene ~ 37%; YBA ~ 32%) than that of TiO2
supported monometallic either on Pd or Sb catalysts. This
observation clearly indicated the existence of significant synergy
between Pd and Sb of the present catalysts (Fig. 1).
toluene conversion of ca. 93% was achieved for the first time
in a single step by gas phase acetoxylation over highly active
and selective Pd–Sb–TiO2 catalysts.
Acetoxylation is an industrially important reaction for producing
benzyl esters from alkyl benzenes using carboxylic acids in an
oxidising atmosphere. No suitable vapour phase method is
available to date for the direct conversion of alkyl benzenes in
general and toluene to benzyl acetate in particular. Benzyl acetate
(BA) is used chiefly in perfumery, food and also in the chemical
industry (notably as a solvent for cellulose acetate). The majority of
the work reported so far on the acetoxylation of methyl aromatics
(e.g. toluene to BA) was carried out under liquid phase conditions
and in batch reactors, e.g.1–4 Attempts have also been made over
the past few decades to develop suitable vapour phase processes for
the direct acetoxylation of toluene.5,6 Eberson et al.7 achieved only
very low yields of acetoxylated products (y 1% per pass) even with
the addition of various promoters (e.g. Au, Bi, Ag–Bi, Au–Bi etc.)
to Pd catalysts. Very recently, Komatsu et al.8 reported on the gas
phase acetoxylation of toluene over SiO2 supported different
intermetallic Pd compounds like Pd2Ge, Pd5Ga2, Pd3Pb, Pd3Bi
etc.; a maximum yield of BA of only around 7% could be achieved.
In this communication, we disclose a first report on vapour
phase synthesis of BA in significantly higher yields and explore the
influence of Pd loading as well as time-on-stream of the best Pd–
Sb–TiO2 catalyst.
It is obvious from Fig. 2 that an increase in Pd content has a
highly pronounced promotional effect on the catalytic performance
of the catalysts. The conversion of toluene has been observed to
increase continuously from 16% to 92.6% with increase in Pd
loading from 0.5 to 20 wt%, while it has no considerable effect on
BA selectivity, which remained more or less constant at around
85%. Benzaldehyde is found to be the major by-product and the
balance is total oxidation products (COx). It was also proved in
separate experiments that COx is formed mainly by the oxidative
decomposition of acetic acid but not from toluene as the reaction
temperature is very low. It is also known that the decomposition of
acetic acid is relatively much easier compared to toluene. The moles
of toluene converted per gram catalyst per hour vary in the range
from 9.6 6 1024 to 5.5 6 1023 moles g21 h21
.
Two types of TiO2 (anatase) supported catalysts (mono- and
bimetallic) were prepared by impregnation. Pd or Sb anatase
supported catalysts were prepared using PdCl2 or SbCl3. The
bimetallic catalysts were synthesised in a first step by impregnation
of SbCl3 onto the anatase carrier, followed by oven drying and
calcination at 400 uC for 3 h. The second step involves the
impregnation of PdCl2 to the above solid in a desired amount
followed by drying of the resulting solid mass in an oven at 120 uC
for 16 h as described elsewhere.9 The loading of Pd varied in the
range from 0.5 to 20 wt% with constant Sb loading (8 wt%).
The catalytic tests were carried out in a fixed bed stainless steel
reactor. 1 ml of catalyst is loaded in the reactor and the reaction is
performed at 2 bars in presence of air. The catalyst was activated
in situ under airflow at 300 uC for 2 h prior to the activity tests. The
organic feed mixture of toluene and acetic acid in the mole ratio of
1 : 4 was pumped to the reactor using an HPLC pump. The
product stream was analysed by on line-GC.
Fig. 1 Synergetic effect between Pd and Sb in Pd–Sb–TiO2 catalysts.
Surface areas and pore volumes were drastically decreased from
161 to 42 m2 g21 and 0.215 to 0.055 cm3 g21 with an increase in Pd
loading from 0.5 to 20 wt%. The XRD reflections correspond to
metallic Pd and PdO phases that could be seen in the (fresh)
catalysts only with higher Pd loadings of 10 wt% and above. TEM
analysis of fresh catalysts showed an increase in the Pd particle size
from v 5 nm to about 20 nm with increasing Pd loading.
Our preliminary catalytic tests show poor results on
{ Electronic supplementary information (ESI) available: supporting
{ A member of the EU-funded Coordination Action of Nanostructured
Catalytic Oxide Research and Development in Europe (CONCORDE).
Fig. 2 Influence of Pd loading on the activity and selectivity of Pd–Sb–TiO2
catalysts (p ~ 2 bar, T ~ 210 uC, (mole ratios) toluene : acetic acid : O2 :
inert gas ~ 1 : 4 : 3: 16, GHSV ~ 2688 h21 (STP), residence time ~ 1.34 s).
2 1 1 8
C h e m . C o m m u n . , 2 0 0 4 , 2 1 1 8 – 2 1 1 9
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
Benhmid
