Chemistry Letters Vol.36, No.9 (2007)
1171
tium content gave maximum performance in terms of both con-
version and selectivity. Addition of strontium can modify the
chemical, ionic, or electrical properties of transition metals
and leads to suppress the particle growth and sintering.10 Posi-
tive promotion effect of the alkaline metals, including Na, Rb,
and Cs, and Ba has been demonstrated particularly in the epox-
idation of ethylene by silver catalyst.11,4b Lowering of apparent
activation energy for the olefin epoxidation has been pointed out
by the presence of cesium as silver promotor.4a
Total PO
PO inner
PO outer
Unlike membrane reactor system, the conventional fixed
bed reactor gave only low yield of PO (less than 0.5%) despite
of increased loading of catalysts. Combustion of propylene oxide
and propylene was predominant yielding large amount of CO2
and ring-opened products of PO and their isomerized by-prod-
ucts. The advantage of segregated feeding of propylene and
O2 is it can prevent successive reactions including further oxida-
tion of PO.
In conclusion, a novel membrane reactor process to
manufacture PO was presented. Use of Ag–Sr catalysis loaded
in porous alumina membrane combined with a segregated feed
of O2 and propylene realized O2 epoxidation of propylene with
notable PO selectivity.
Figure 2. Selectivity for propylene oxide at inner and outer side
of the membrane tube as a function of time. AgSr content:
6.5 wt % in Al2O3 tube.
surface have demonstrated a well-dispersed and homogeneous
distribution of Ag and Sr. Double layered ꢀ-Al2O3 tube was
used as the supports of the membranes D and E. In this case,
the top surface of the ꢀ-Al2O3 tube was coated with smaller size
alumina particles (ꢁ 0:05 mm).
Table 1 shows the numerical data of the propylene epoxida-
tion by a series of membrane reactors. Noteworthy, the major re-
action products are PO and CO2 in the present reaction system.
The segregated feeding of substrate and oxidant has resulted in
better conversion and selectivity than cofeeding of two reac-
tants.8 The conversion increased by elevation of temperature,
yet resulted in decrease of selectivity due to enhanced CO2 for-
mation. The optimized temperature with respect to PO yield
ranged 220–240 ꢂC. The highest selectivity up to 57.4% of PO
was attained by the membrane A at 240 ꢂC with propylene con-
version of 6.5%, resulting 3.7% of PO yield. PO was mostly
found from inner tube side, but nearly 25% of it was found from
shell side (Figure 2). This ratio depends on the metal loading that
affects the permeation of O2 and PO. When the metal loading is
extremely low, amount of PO found at inner and outer tube side
became closer. The conversion and PO selectivity decreased
with increase of metal loading (membranes A, B and C). This
trend is accompanied by the increase of O2 consumption leading
to an enhanced formation of CO2 and other C3 mono-oxygenates
by successive oxidation of PO. Large amount of silver enhances
particle agglomeration or clustering which could decrease the
number of Ag0 sites accessible by propylene.9 It is essential to
control the feed rate of O2 lower than 5.0 mLꢃminꢄ1 in order
to prevent further oxygenation of hydrocarbon while keeping
better conversion and selectivity.
In the cases of membranes D, E, the permeation of gases was
retarded by the presence of narrow channels at the top layer. In
these cases, propylene conversion as well as oxygen consump-
tion is apparently low. In addition, PO selectivity is decreased
owing to the successive oxidation of PO yielding large amount
of CO2 and ring-opened products of PO. Thus, facile permeation
of reactants and transportation of product across the membrane
are essentially important for the membrane epoxidation system.
A similar epoxidation (267 ꢂC) examined by the membrane
loaded only with silver resulted in significantly poor PO selectiv-
ity (0.7%) and yielded mainly CO2 (96.5%). Coloading of stron-
tium combined with silver has remarkably improved the PO
selectivity by promotion of the catalytic activity of silver. By
changing Ag/Sr ratio, it appeared that around 25 wt % of stron-
References and Notes
1
2
Degussa Science Newsletter, June, 2004, pp. 11–13.
a) M. Eissen, J. O. Metzger, E. Schmidt, U. Schneidewind,
Arpe, Industrial Organic Chemistry, 4th ed., Wiley-VCH,
Weinheim, 2003, p. 267.
3
4
5
a) S. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Shoji,
D. Farrusseng, J. C. Jalibert, C. Mirodatos, C. Guizard, Catal.
´
45. d) M. J. Alfonso, M. Menendez, J. Santamaria, Chem.
6
7
8
For example, co-feeding of O2 and propylene mixture (from the
tube side) at 240 ꢂC into the membrane impregnated with 13%
of Ag and Sr has led to 1.9% conversion, with 30 and 66%
selectivity for PO and CO2, respectively. While in a segregated
feeding of substrate and O2 gave 3.4% of propylene conversion
with 42 and 54.8% selectivity for PO and CO2, respectively.
9
11 A. Ayame, Y. Uchida, H. Ono, M. Miyamoto, T. Sato, H.
12 Supporting Information is available electronically on the
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