Propylene epoxidation with hydrogen peroxide over palladium containing titanium silicalite

Article abstract of DOI:10.1016/S1381-1169(98)00265-9

The epoxidation of propylene to propylene oxide with H₂O₂ was studied over palladium impregnated and reduced titanium silicalite (TS-1), over merely impregnated titanium tilicalite and over untreated titanium silicalite. The use of such catalytic systems in the epoxidation of propylene with a H₂-O₂ mixture motivated us to assess the influence of operating conditions and the effect of the Pd loading on the epoxidation capability of the titanium silicalite catalyst. Concerning the operating conditions TS-1 was found to be very active even at temperatures as low as 10°C. Lowering the H₂O₂ concentrations to only 2 wt.% of H₂O₂ caused the PO yield to increase slightly over TS-1 and 1% Pd/TS-1. TS-1 catalysts that were merely impregnated with [Pd(NH₃)₄]Cl₂ were less active than the catalysts that were reduced after impregnation, though the latter is more active in the decomposition of H₂O₂. The deactivation of TS-1 after impregnation with [Pd(NH₃)₄]Cl₂ was probably caused by the blocking of the Ti sites by ammonia, since the impregnation with PdCl₂ did not cause any decrease in activity. Reducing the catalyst removes the ammonia and improves the catalytic performance of the Pd loaded catalyst.

Full text of DOI:10.1016/S1381-1169(98)00265-9

Ž
.
Journal of Molecular Catalysis A: Chemical 141 1999 215–221
Propylene epoxidation with hydrogen peroxide over palladium
containing titanium silicalite
)
W. Laufer, R. Meiers, W. Holderich
¨
Chemical Technology and Heterogeneous Catalysis, UniÕersity of Technology RWTH Aachen, Worringer Weg 1, 52074 Aachen, Germany
Abstract
The epoxidation of propylene to propylene oxide with H2O2 was studied over palladium impregnated and reduced
Ž
.
titanium silicalite TS-1 , over merely impregnated titanium tilicalite and over untreated titanium silicalite. The use of such
catalytic systems in the epoxidation of proplyene with a H2–O2 mixture motivated us to assess the influence of operating
conditions and the effect of the Pd loading on the epoxidation capability of the titanium silicalite catalyst. Concerning the
operating conditions TS-1 was found to be very active even at temperatures as low as 108C. Lowering the H2O2
concentrations to only 2 wt.% of H2O2 caused the PO yield to increase slightly over TS-1 and 1% PdrTS-1. TS-1 catalysts
w
Ž
. x
that were merely impregnated with Pd NH3 4 Cl2 were less active than the catalysts that were reduced after impregnation,
though the latter is more active in the decomposition of H2O2. The deactivation of TS-1 after impregnation with
w
Ž
. x
Pd NH3 4 Cl2 was probably caused by the blocking of the Ti sites by ammonia, since the impregnation with PdCl2 did not
cause any decrease in activity. Reducing the catalyst removes the ammonia and improves the catalytic performance of the Pd
loaded catalyst. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Propylene epoxidation; Hydrogen peroxide; Palladium; Titanium silicalite
1. Introduction
aqueous H2O2 solution by the in situ formation
of H2O2. Two different approaches are em-
ployed for the formation of in situ hydrogen
The development of new routes for the syn-
thesis of propylene oxide has been attracting
much interest over the recent years 1 . The
invention of titanium silicalite-1 TS-1 by
Taramasso et al. 2 offered new opportunities
for the epoxidation of olefins with hydrogen
peroxide. Especially the epoxidation of propy-
lene has been studied in some detail and has
been found to proceed at high conversion rate
and high selectivities under mild conditions in
w x
peroxide. The first approach 4 makes use of
w x
Ž
.
the conventional anthraquinone AQ process
for the production of hydrogen peroxide by
feeding propylene and oxygen to the oxidation
Ž
.
w x
Ž
.
stage of anthrahydroquinone AHQ . Contacting
AHQ with oxygen leads to the formation of
hydrogen peroxide, which is then consumed in
the oxidation of propylene to PO catalyzed by
TS-1. The second concept is based on the oxida-
tion of propylene by a H2–O2 gas mixture over
a precious metal containing titanium silicalite
w x
the liquid phase 3 . Due to economic considera-
tions it has been tried to substitute the use of
w
x
5–8 . Such catalytic systems have also been
tested for the oxidation of alkanes, for the hy-
)
Corresponding author
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 98 00265-9

(
)
216
W. Laufer et al.rJournal of Molecular Catalysis A: Chemical 141 1999 215–221
droxylation of benzene to phenol and for the
5508C in air. As determined by ICP analysis the
resulting SirTi-ratio was 37.
w
x
oxidation of phenol 4,9 . Hydrogen peroxide is
directly synthesized from H2 and O2 at the
precious metal sites of the bifunctional catalyst
and consumed as oxidant at the Ti-sites.
w
Ž
.
x
Ž
.
A Pd NH3 4 NO3 2 solution was prepared
Ž
.
by dissolving 10 g of Pd NO3 2 in 400 g of an
aqueous 25 wt.% ammonia-solution. After the
mixture had been stirred for 3 d at 508C, the
solution was filtered and the resulting concen-
tration of Pd was determined by ICP-analysis
For the epoxidation of propylene with a H2–
w
x
O2 mixture Sato et al. 5–7 and Mueller et al.
w x
8 employ a palladium impregnated TS-1 cata-
Ž
. Ž .
lyst suspended in solvents such as water or
methanol. The reported conversions for the oxi-
dation of propylene with a H2–O2 mixture are
below 2% and the selectivity suffers from the
hydrogenation of propylene to propane. We have
proved that the in situ generation of hydrogen
peroxide at the precious metal site is the rate
21.6 mgrml . Pd NH3 4Cl2 was prepared by
dissolving PdCl2 in aqueous 25 wt.%
ammonia-solution.
Pd is supported on TS-1 by suspending 5 g of
TS-1 in 20 g of deionized water and adding the
w
Ž
.
x
Ž
.
appropriate amount of Pd NH3 4 NO3 2 or
Ž
.
Pd NH3 4Cl2 to the solution. For the impregna-
tion with PdCl2 acetone was used instead of
water. The mixture was stirred for 1 d at 808C.
The solid is recovered by evaporating the sol-
vent under vacuum at 508C and dried at 608C
for 1 d. As determined by ICP analysis the
resulting catalyst were loaded with 0.1, 0.5, 1 or
2 wt.% Pd.
w
x
determining step in this reaction 10 . Enhanc-
ing the catalytic performance of the catalyst
must therefore focus on the impregnation and
reduction procedure. We report now how reac-
tion conditions, the impregnation and reduction
of a PdrTS-1 catalyst affects the synthesis of
PO with hydrogen peroxide. The objective of
this study is to assess the loss of epoxidation
capacity of the TS-1 catalyst and the decompo-
sition of H2O2 by the Pd loading.
Reduction was carried out with 5 vol.% H2
q95 vol.% N2 or pure N2 at a heating rate of 1
Krmin from room temperature to 1508C. Under
the pure nitrogen atmosphere the catalyst was
autoreduced by the thermal decomposition of
the NH3 ligands.
Catalytic tests were performed as batch reac-
tions in a 75 ml stainless steel autoclave lined
with a teflon beaker. 0.075 g TS-1 catalyst, 2 g
methanol, 2 g deionized water and 1.2 g aque-
ous 30 wt.% H2O2 solution were charged to the
reactor and a stirring bar was placed inside the
beaker.
After the autoclave was cooled down in an
ice bath, 8 g propylene were filled into the
reactor by condensation. The reaction was
started by immersing the autoclave into an oil
bath. The stirring bar, which was driven by a
magnetic stirrer, provided the necessary agita-
tion. Standard reaction temperature was 508C
and the standard reaction time were 3 h. At the
end of the reaction the autoclave was cooled
down again. A sample was withdrawn over a
filter and mixed with a defined amount of methyl
2. Experimental
TS-1 was prepared according to the proce-
dure described in the original TS-1 patent by
w x
Taramasso et al. 2 . 499 g of tetraethylorthosili-
cate were stirred under a CO2-free atmosphere
and 15 g of tetraethyltitanate were added fol-
lowed by dropwise addition of 800 g of a 25
wt.% solution of tetrapropylammonium hydrox-
ide. After the alcohol had been distilled off
under vacuum at 508C, 868 g of distilled water
were added to the solution. The mixture was
transferred to an autoclave, heated at 1758C and
stirred under autogenous pressure for 10 d. Af-
ter cooling down to room temperature, the crys-
talline product was separated from the liquid by
centrifugation, washed with water, dried for 12
h at 1208C and finally calcined for 10 h at

(
)
W. Laufer et al.rJournal of Molecular Catalysis A: Chemical 141 1999 215–221
217
tert-butyl ether, which acted as external stan-
dard. GC analysis was performed on a Carlo
Erba Z 2300 gas chromatograph, using a flame
ionization detector and 4 m column containing
Carbowax 1500 as stationary phase.
The decomposition reaction of H2O2 was
carried out similar to the epoxidation reaction
except that no propylene was charged to the
reactor. Reaction temperature was kept at 508C
for 0.5 h. H2O2 was determined by iodiometric
titration.
Fig. 2. Epoxidation of proylene with H2O2 over TS-1 at different
temperatures. Conditions: 0.075 g TS-1, 1.2 g aq. 30 wt.%
H2O2-solution, 8 g propylene, 2 g MeOH, 2 g H2O, 3 h.
For the intended use of the PdrTS-1 catalyst
in the oxidation of propylene with a H2–O2
mixture we were interested in the effect of
reaction temperature on the catalytic perfor-
mance of TS-1, because the direct synthesis of
3. Results and discussion
The catalytic activity of palladium free TS-1
was assessed by running the epoxidation reac-
tion at different reaction times. After 3 h 99%
of the hydrogen peroxide was consumed with a
PO-yield of 67% based on H2O2 added. The
hydrogen peroxide that did not react with propy-
lene decomposed to water and oxygen.
w
x
H2O2 is usually carried out at 0–108C 11–13
whereas epoxidation with H2O2 is performed at
w x
40–508C 3 . The catalytic results of tempera-
ture variation is presented in Fig. 2. In accor-
w x
dance to literature data 3 the maximum PO
Ž
.
The selectivity of propylene oxide based on
yields 67% were obtained at temperatures in
the range of 40–508C. Higher temperatures lead
to a decrease in the PO yield which can be
attributed to the enhanced decomposition of
H2O2. Lowering the reaction temperature re-
sults only in a slight decrease of the PO yield.
Even at 108C a PO yield of 55% was obtained.
Since the TS-1 catalyst is also very active at
subambient temperatures it is possible to con-
duct the direct oxidation of propylene with a
H2–O2 mixture at those temperatures, too.
Ž
all organic products molar amount of produced
.
PO to molar amount of the all organic products
Ž
.
was 97% Fig. 1 . Propylene glycol, 1-methoxy-
2-propanol and 2-methoxy-1-propanol were the
main by-products. All identified by-products
were formed from propylene oxide by epoxide
ring opening and reaction with a nucleophile.
As the results in Fig. 1 indicate the standard
reaction time of 3 h is sufficient to complete the
reaction.
With respect to the oxidation of propylene
with a H2–O2 mixture it is furthermore interest-
ing to study the effect of H2O2 concentrations
on the catalytic performance of TS-1. When a
H2–O2 mixture is used as the oxidant, the
H2O2 formed in situ reacts with the propylene
to yield PO thus leading to very low concentra-
tions of H2O2 in the solution. We have investi-
gated the influence of the initial H2O2 concen-
trations for the epoxidation of propylene with
H2O2 by varying the H2O2 concentration from
2 to 15 wt.%. The epoxidation was catalyzed
over TS-1 and over 1% PdrTS-1. The latter of
Fig. 1. Epoxidation of proylene with H2O2 over TS-1. Condi-
tions: 0.075 g TS-1, 1.2 g aq. 30 wt.% H2O2-solution, 8 g
propylene, 2 g MeOH, 2 g H2O, 508C.

(
)
218
W. Laufer et al.rJournal of Molecular Catalysis A: Chemical 141 1999 215–221
For the loss of epoxidation capacity one can
imagine two different reasons:
1. The Pd loading enhances the H2O2 decom-
position
2. The epoxidation activity itself is negatively
affected by the Pd impregnation.
In order to examine both possibilities we
conducted epoxidations reactions and decompo-
sition tests with four different catalytic systems:
Ø unloaded TS-1
Fig. 3. Epoxidation of proylene with H2O2 over TS-1 and 1%
PdrTS-1.
w
Ž
. x
Ø Pd NH3 4 Cl2 in solution with TS-1
Ž
x
Ø TS-1 impregnated with 1% Pd Pd was pro-
Ž
.
.
the two catalysts was loaded with 1% Pd
vided in the form of Pd NH3 4 Cl2
Ž
Ø TS-1 impregnated with 1% Pd Pd was pro-
Ž
w
Ž
.
x
.
Pd NH3 4 Cl2 and autoreduced at 1508C un-
w
Ž
.
x
.
der nitrogen. For the palladium free TS-1 we
observed the PO yield to decrease with increas-
ing H2O2 concentrations: from 75% PO yield
for 2 wt.% H2O2 to 65% PO yield for 15 wt.%
vided in the form of Pd NH3 4 Cl2 and
subsequently autoreduced at 1508C.
The results of the epoxidation reaction is
shown in Fig. 4 while the results of the decom-
position tests are presented in Fig. 5.
Ž
.
H2O2 Fig. 3 . The 1% PdrTS-1 catalyst seems
to be less affected by the H2O2 concentrations:
the PO yield decreases from 43% to 39%. The
fact that the PO yield increases with lower
H2O2 concentrations should also favour the
generation of PO in the oxidation of propylene
with a H2–O2 mixture. However, the palladium
loading caused the PO yield to decrease by ca.
40% compared to the unloaded TS-1. This re-
sult motivates a more detailed inspection of the
possible causes for the decrease in epoxidation
capacity.
The results in Fig. 4 show that the H2O2
conversion was not affected by the different
catalytic systems whereas the PO yields exhib-
ited a strong dependence on the catalysts em-
ployed. As expected the highest PO yield, 67%,
was obtained with the untreated TS-1 catalyst.
The PO yield dropped to 43% for the impreg-
nated and reduced 1% Pd catalyst. The merely
impregnated 1% Pd catalyst achieved only a PO
yield of 37%. This result is rather surprising as
the decomposition tests demonstrate that the
Ž
w
Ž
. x
.
Fig. 4. Epoxidation of propylene with H2O2 over various catalytic systems. Conditions: 0.075 g catalyst q1.7 mg Pd NH3 Cl2 , 1.2 g
4
H2O2 , 8 g propylene, 2 g MeOH, 2 g H2O, 3 h, 508C.

(
)
W. Laufer et al.rJournal of Molecular Catalysis A: Chemical 141 1999 215–221
219
Ž
w
Ž
. x
.
Fig. 5. Decomposition of H2O2 over various catalytic systems. Conditions: 0.075 g catalyst q1.7 mg Pd NH3 Cl2 , 1.2 g H2O2 , 2 g
4
MeOH, 2 g H2O, 0.5 h, 508C.
reduced 1% Pd catalyst decomposes H2O2 more
than three times faster than the merely impreg-
nated catalyst. More H2O2 was decomposed by
the reduced catalysts due to the presence of
metallic Pd, which is well known for catalyzing
The decrease in epoxidation capacity can be
attributed to the blocking of the active sites by
the ammonia solution and the Pd precursor
w
Ž
.
x
w x
Pd NH3 4 Cl2. Thiele and Roland 15 ob-
served also that the TS-1 activity decreased
drastically after ammonia treatment and was
regained by calcining the TS-1 at 5508C. They
ascribe this effect to the deprotonation of the
titanium site by a base, which causes blocking
of the catalytic activity for epoxidation:
w
x
the H2O2 decomposition 14 . Since the reduced
1% PdrTS-1 catalysts performs better in the
epoxidation reaction in spite of its H2O2 de-
composition ability, we must assume that the
epoxidation capacity of the impregnated 1%
PdrTS-1 was lowered drastically and that the
loss in epoxidation capacity surpasses the de-
composition effect by far. The Pd precursor
itself lowers the PO yield only by 9% when
brought into solution with TS-1, which is most
probably caused by the decomposition of H2O2
by the Pd ions in solution.
SiO Ti–OH2 qMX
4
| SiO Ti–OHyMqqHX
4
Calcining the ammonia treated TS-1 drives
off ammonia as does the autoreduction at 1508C
and thus shifts back the equilibrium back to the
Fig. 6. Epoxidation of propylene with H2O2 over 1% PdrTS-1 using different Pd precursors. Conditions: 0.075 g catalyst, 1.2 g H2O2 , 8 g
propylene, 2 g MeOH, 2 g H2O, 3 h, 508C. Reduction of impregnated catalysts: 1508C under 5% H2r95% N2.

(
)
220
W. Laufer et al.rJournal of Molecular Catalysis A: Chemical 141 1999 215–221
Fig. 7. Epoxidation of propylene with H2O2 over TS-1 catalysts loaded with different amounts of Pd. Conditions: 0.075 g catalyst, 1.2 g
H2O2 , 8 g propylene, 2 g MeOH, 2 g H2O, 3 h, 508C. Conversion H2O2: 100%.
w
Ž
. x
left. According to this concept impregnation
with an ammonia-free Pd precursor should yield
a more active catalyst for the epoxidation reac-
tion. For this purpose TS-1 was impregnated
with PdCl2 dissolved in HCl and reduced with
5% H2rN2 at 1508C. As a reference catalyst
TS-1 was impregnated with Pd NH3 4 Cl2 and
reduced in the same way, leading also to a 1%
PdrTS-1 catalyst. As shown in Fig. 6 the am-
monia free catalyst, prepared with PdCl2, is
indeed superior and yields 66% PO compared to
45% for the catalyst impregnated with
Pd NH3 4 Cl2 and autoreduced at 1508C under
nitrogen yielding catalyst with 0.1, 0.5, 1 and 2
wt.% Pd. Catalysts were tested in the merely
impregnated and in reduced form for the epoxi-
dation reaction with H2O2 under standard reac-
tion conditions.
w
Ž
. x
As demonstrated by the results presented in
Fig. 7 the merely impregnated and the reduced
catalyst exhibited two different trends with in-
creasing Pd loading. The PO yield decreased
almost linearly over the impregnated form while
the PO yield levels off at 1–2% Pd over the
reduced form. Due to these different relations
the difference in PO yield was most pro-
nounced, i.e., 22%, for the 2% PdrTS-1 cata-
lyst and was almost neglectable for the 0.1%
PdrTS-1 catalyst. These results clearly show
that the removal of the ammonia from the Ti
sites becomes more important for higher Pd
loadings.
w
Ž
. x
Pd NH3 4 Cl2. The catalyst impregnated with
PdCl2 is even as active as the untreated TS-1
catalysts, which indicates that H2O2 decomposi-
tion on the Pd surface of this catalyst is rather
negligible for the epoxidation reaction.
The best reduction method of catalyst for the
direct oxidation of propylene with O2–H2 is the
Ž
autoreduction under N2 decomposition of
w
x.
NH3-ligand 16,17 . Unfortunately the catalyst
impregnated with PdCl2 must be reduced under
H2rN2.
4. Conclusions
Apart from the effect of the Pd precursor on
the propylene epoxidation we have investigated
the relationship between the amount of Pd load-
ing and catalytic activity. It is known from the
oxidation of propylene with a H2–O2 mixture
that PO yield increases digressively with Pd
The impregnation of TS-1 with Pd tetramine
ligands leads to a significant loss of the catalyst’s
activity, which is probably due the blocking of
the active Ti site by ammonia. The activity can
partially be restored by reducing the impreg-
nated catalyst and thus removing the ammonia.
The reduced catalyst is still less active as the
w
x
loadings ranging from 0.01 to 2 wt.% Pd 5–7 .
For this purpose TS-1 was impregnated with

(
)
W. Laufer et al.rJournal of Molecular Catalysis A: Chemical 141 1999 215–221
221
w x
5
M. Sato, T. Miyake, JP Patent Application 4 352771 to
Tosoh, 1992.
A. Sato, M. Oguri, M. Tokumaru, T. Miyake, JP Patent
Application 269029 to Tosoh, 1996.
untreated TS-1 because of the decomposition of
H2O2 by palladium.
w x
6
w x
7
A. Sato, M. Oguri, M. Tokumaru, T. Miyake, JP Patent
Application 269030 to Tosoh, 1996.
Acknowledgements
w x
8
U. Muller, P. Lingelbach, P. Baßler, W. Harder, E. Karsten,
¨
V. Kohl, J. Dembowski, N. Rieber, M. Fischer, German
Patent Application DE 44 25 672 A1 to BASF, 1996.
T. Tatsumi, K. Yuasa, H.J. Tominaga, J. Chem. Soc., Chem.
Commun., 1992, 1446.
Financial support of the Hoechst is gratefully
acknowledged. The authors express their sincere
w x
9
w
x
10 R. Meiers, R.P. Schulz, W. Holderich, Epoxidation of propy-
¨
thanks to Prof. Kuhlein, Dr. Dingerdissen and
¨
lene and direct synthesis of hydrogen peroxide by hydrogen
and oxygen, J. Chem. Soc., Chem. Comm., submitted.
Dr. Schulz for helpful and stimulating discus-
sions.
w
w
x
11 L.W. Grosser, J.-A.T. Schwarz, U.S. Patent 4,832,938 to Du
Pont, 1989.
x
12 H. Nagashima, Y. Ishiuchi, Y. Hiramatsu, M. Kawakami,
Europ. Patent Application 539 846 A1 to Mitsubishi Gas
Chemical, 1992.
13 M. Kawakami, Y. Ishiuchi, H. Nagshima, T. Tomita, Y.
Hiramatsu, Europ. Patent Application 623 552 A1 to Mit-
subishi Gas Chemical, 1994.
14 J. Schirrmann, S.Y. Delavarenne, Hydrogen Peroxide in or-
References
w
w
x
w x
1
Ullmanns Encyclopedia of Industrial Chemistry, 5th edn.,
Vol. 22, VCH Weinheim, 1993, p. 239.
x
w x
2
M. Taramasso, G. Perego, B. Notari, U.S. Patent 4,410,501
to Snamprogetti, 1983.
Ž
.
ganic chemistry, Elfa Oyxchemie Handels Arau CH , 1979.
w
w
x
x
Ž
.
15 G.F. Thiele, E. Roland, J. Mol. Catal. A 117 1997 351.
w x
3
Ž
.
M.G. Clerici, G. Belussi, U. Romano, J. Catal. 129 1991
159.
Ž
.
16 W.M.H. Sachtler, Z. Zhang, in: D. Eley, P. Weisz Eds. ,
Advances in Catalysis, Vol. 39, Academic Press, New York,
1993, p. 129.
w x
4
P. Ingallina, M.G. Clerici, L. Rossi, G. Belussi, Science and
technology in catalysis, Stud. Surf. Sci. Catal. 92 1995 31.
Ž
.
w
x
Ž
.
17 P. Albers, J. Kiwi, J. Mol. Catal. 58 1990 115.