Selective hydrogenation of hydrogen peroxide in the
epoxidation effluent of the HPPO process
Gema Blanco-Brieva a, M. Pilar de Frutos-Escrigb, Hilario Martínb, Jose
M. Campos-Martina* and Jose L. G. Fierro a*
aSustainable
Energy and Chemistry Group.
Instituto de Catálisis y
Petroleoquímica, CSIC, Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain.
http://www.icp.csic.es/eqs/
bCentro
de Tecnología Repsol, A-5, Km. 18, 28935 Móstoles, Madrid, Spain.
*CORRESPONDING AUTHOR FOOTNOTE
Dr. J. M. Campos-Martin, e-mail: j.m.campos@icp.csic.es
Prof. Dr. J. L. G. Fierro, e-mail: jlgfierro@icp.csic.es
Fax: +34 915854760
1
Research Highlights

H2O2 can be selectively converted by hydrogenation in presence of
Propylene Oxide

H2O2 can be completely converted in 20 min or less in presence of
platinum catalyst.

The formation of byproducts from PO depends on the reaction
temperature, the catalyst amount and the quantity of hydrogen in the gas
phase.
Graphical Abstract
2
ABSTRACT
This work describes selective H2O2 hydrogenation in the exit stream of the
epoxidation reactor employed in the Hydrogen Peroxide-Propylene Oxide
(HPPO) process. Pd/Al2O3 and Pt/Al2O3 catalysts were employed for this
purpose. The effect of the reaction temperature, catalyst amount and hydrogen
partial pressure on catalyst performance were investigated. It was found that
the Pt catalyst is much more active than its Pd counterpart. Under optimized
reaction conditions, the hydrogen peroxide present in the exit stream can be
completely hydrogenated with the Pt catalyst with a reaction time of no longer
than 20 min and an almost negligible amount of byproducts derived from
propylene oxide.
Keywords: Epoxidation, hydrogen peroxide, selective hydrogenation, HPPO
3
Introduction
Propylene oxide is a highly reactive chemical used as an intermediate for the
production of numerous commercial materials. It reacts readily with compounds
containing active hydrogen atoms, such as alcohols, amines, and acids.
Therefore, propylene oxide is used worldwide to produce such versatile
products as: Polyether polyols (polyglycol ethers), Propylene glycols and
Propylene glycol ethers. Propene oxide is currently produced using two different
types of commercial processes: the chlorohydrin process and the hydroperoxide
process. In 1999, the production capacity was distributed evenly between these
two processes; however, because of the environmental impacts of the
chlorohydrin process, the most recently built plants are all using hydroperoxide
process [1].
An interesting alternative is the epoxidation of propylene with hydrogen
peroxide. This epoxidation process produces PO with very high selectivity (95%
or higher) and, theoretically, excretes only H2O as a by-product [1]. Its
commercialization has been hindered, however, largely by the supply of H2O2.
The alternative proposed is the integration of the for H2O2 synthesis with the
propylene epoxidation process catalyzed by titanium silicalite (TS-1) [2-9].
Propylene oxide production based on the direct on-site synthesis of hydrogen
peroxide is abbreviated as HPPO (Hydrogen Peroxide-Propylene Oxide).
The epoxidation of propylene with hydrogen peroxide on titanium catalysts is
very effective [10-16], but 100% conversion of hydrogen peroxide cannot be
achieved in an industrial-scale reactor. As a consequence, small amounts of
hydrogen peroxide still remain when the reaction mixture exits the epoxidation
4
reactor. Hydrogen peroxide cannot be introduced into the PO purification step,
however, because its decomposition produces oxygen, which can cause
serious safety problems. A simple way to solve this drawback is to decompose
the residual hydrogen peroxide at the exit of the epoxidation reactor once
unreacted propylene has been separated. Thermal decomposition cannot be
used because an increase in the reaction temperature yields propylene oxide
by-product formation that makes this process not economical. An alternative is
the catalytic decomposition at low temperatures [17], at low temperatures the
by-products can be minimized and the oxygen gas generated by hydrogen
peroxide decomposition can then simply be removed with an inert gas flow;
however, the production of oxygen, despite controlled conditions, is still a safety
risk.
An interesting alternative involves hydrogenation of the hydrogen peroxide, a
reaction that produces only water without the formation of possible flammable
atmospheres. However, investigations of the hydrogenation of H2O2 over
different catalyst are scarce [18-20]. Hydrogen peroxide destruction is strongly
influenced by the oxidation state of the metal employed. In general, noble
metals in their metal forms are capable of hydrogenating H2O2 [8, 19, 21-23]
because they are more catalytically active. The presence of different halide
anions (F-, Cl-, Br- and I-) in the medium or in the catalyst, depending upon the
concentration of the halide anions, also enhance or hinder hydrogen peroxide
destruction. The cations associated with halide anions have, however, little or
no influence on H2O2 destruction. For example, chloride or bromide anions
drastically
inhibit
rapid
H2O2
destruction
but
promote
slower
H2O2
hydrogenation [21].
5
The rate of hydrogen peroxide hydrogenation is nearly independent of the
reaction time, and consequently, the rate is also independent of the H2O2
concentration (a zero order reaction with respect to H2O2 concentration) [21].
The reaction rate is, however, strongly influenced by the reaction temperature;
as expected, the hydrogenation rate increases with increasing temperature.
Some theoretical studies showed that Pd and Pt are the most selective metals
for the complete reduction of oxygen to water because they can efficiently
catalyze both O–O bond scission and O–H bond formation [24].
The aim of this work was to study the selective catalytic hydrogenation of
hydrogen peroxide in a solution that simulates the epoxidation reactor exit
stream of an HPPO process. This research focused on minimizing the formation
of byproducts derived from the organic compounds present in the stream.
Experimental Methods
Catalysts
Alumina-supported palladium and platinum catalysts were employed in this
work. Pd/Al2O3 and Pt/Al2O3 (0.5 wt.% metal loading) shaped as cylindrical
pellets (3.2 x 3.2 mm) were purchased from Johnson Matthey. These catalysts
are commercially distributed in the reduced (metallic) state. For comparative
purposes, cylindrical pellets (3.2 x 3.2 mm) of bare -alumina were also
employed.
Catalyst Characterization
Textural properties were determined from the adsorption-desorption isotherms
of nitrogen recorded at 77 K with a Micromeritics TriStar 3000. The specific area
6
was calculated by applying the BET method to the range of relative pressures
(P/P0) of the isotherms between 0.03 and 0.3 and taking a value of 0.162 nm 2
for the cross-section of adsorbed nitrogen molecules at 77 K.
Powder X-ray diffraction (XRD) patterns were recorded in the 0.5–10º 2θ range
using a step mode (0.05, 5 s) with a Seifert 3000 XRD diffractometer equipped
with a PW goniometer with Bragg–Brentano θ/2θ geometry, an automatic slit,
and a bent graphite monochromator.
X-ray photoelectron spectra (XPS) were acquired with a VG Escalab 200R
spectrometer equipped with a hemispherical electron analyzer and a Mg K (h
= 1253.6 eV) non-monochromatic X-ray source. The samples were degassed in
the pretreatment chamber at room temperature for 1 h prior to being transferred
into the instrument’s ultra-high vacuum analysis chamber. The Si2p, O1s, S2p
and C1s signals were scanned several times at a pass energy of 20 eV to
obtain good signal-to-noise ratios and good resolution. The binding energies
(BE) were referenced to the BE of the C1s line at 284.9 eV. The invariance of
the peak shapes and widths at the beginning and end of the analyses indicated
constant charge throughout the measurements. Peaks were fitted by a nonlinear least squares fitting routine using a properly weighted sum of the
Lorentzian and Gaussian component curves after background subtraction [25].
Hydrogen Peroxide Hydrogenation
The Pd/Al2O3 and Pt/Al2O3 catalysts were evaluated in the hydrogenation of
hydrogen peroxide. The catalytic tests were performed in a high pressure stirred
reactor (Autoclave Engineers) equipped with a falling basket. In a typical run,
the catalyst (H2O2/metal = 400/1 by weight) was put into a basket without
7
contact with the liquid phase (300 g). The effluent of the epoxidation reactor in
an HPPO process after elimination of propylene was simulated in methanol
(600 ppm acetaldehyde, 12.74 wt.% propylene oxide, 72.7 wt.% methanol, 0.49
wt.% 1-methoxy-2-propanol, 0.78 wt.% 2-methoxy-1-propanol, 0.59 wt.% 1,2propanediol, 10.70 wt.% H2O, and 2 wt.% H2O2) [6].
The reactor was pressurized to 1.5 MPa and purged with N2 and hydrogen was
then fed until the addition of the desired amount (0.5-0.2 mol). Finally, the
pressure was increased up to 3.0 MPa with nitrogen, and the reaction mixture
was heated up to the reaction temperature. To start the reaction, the basket that
contained the catalyst was lowered until it was in contact with the reaction
mixture.
The experimental procedure to take samples is not so simple due to the
volatility of PO. The reactor is under pressure and between 313 and 333 K, if
we take directly sample from the reactor the PO is loss without control. For this
reason, we take the samples in a closed stainless steel recipient under
pressure, and then we cool down the sample in an ice bath. When it is cold, we
depressurize slowly and take the sample. This procedure takes some time and
we are not able to take samples with a frequency lower than 5 minutes. The
hydrogen peroxide and water concentrations were measured by iodometric and
Karl-Fischer standard titrations, respectively. The concentration of the organic
compounds was determined by CG-FID using an Agilent 6850 instrument fitted
with a DB-WAX capillary column.
8
Results and Discussion
The characterization results indicated no differences in the textural properties of
both catalyst samples (Table 1). XRD analysis (not shown here) showed
diffraction peaks for γ-alumina without any diffraction line originating from the
metal species. This result confirms the high dispersion of platinum, most
probably as very small clusters with a size of less than 3 nm supported on the
alumina substrate.
The chemical state of the platinum and palladium was determined from the Xray photoelectron spectra. The Pd3d and Pt4d core-levels showed the
characteristic spin–orbit splitting of these levels. The most intense (Pd3d5/2 and
Pt4f7/2) components of the doublet were located at lower binding energies and
the least intense (Pd3d3/2 and Pt4f5/2) at higher binding energies. Chemical
information can be extracted from each of these components, but attention was
only paid to the most intense ones (Pd3d5/2 and Pt4f7/2). Fresh and used
samples showed the presence of one unique component corresponding to welldispersed metallic species on alumina (Pd3d5/2 at 336.0 eV, and Pt4f7/2 at 314.0
eV).
The activity of the Pt/Al2O3 catalyst was tested in the selective H2O2
hydrogenation and compared with that of Pd/Al2O3 under the same operative
conditions, and the results are shown in Figure 1. The thermal decomposition of
hydrogen peroxide can be ruled out because is very small as we showed in a
previous report [17]. Both catalysts were active; however, the Pt/Al2O3 catalyst
exhibited a higher hydrogen peroxide conversion, reaching total conversion in
9
90 min compared to its Pd counterpart, who reached only 64% conversion in
120 min. Accordingly, Pt/Al2O3 was used for further studies.
Quantitative XPS data (Table 2) revealed that the surface Pd(Pt)/Al ratio is
higher for Pd than for Pt in both the fresh and used catalysts. These results
indicate that metal exposure is higher in the Pd catalyst than in its Pt
counterpart, suggesting that the Pd is more highly dispersed. The Pt catalyst is,
however, clearly more active in the hydrogen peroxide hydrogenation than its
Pd counterpart. Thus, Pt has a higher intrinsic activity than palladium for this
reaction. It has been demonstrated that supported PdO catalysts have lower
H2O2 decomposition/hydrogenation activity than the corresponding Pd 0
catalysts [18], and this effect is attributed to the higher propensity of H 2O2 to
adsorb onto the Pd0 surface compared to the PdO surface [18]. Similar studies
for platinum were not found in the literature.
The Pt/Al surface atomic ratio of the Pt catalyst (Table 2) does not change after
use in the reaction at different temperatures and is very close to the value
determined for the fresh catalyst. This result indicates that the Pt catalyst is a
robust system and is particularly well suited for the target reaction under the
reaction conditions selected in this work.
It is known that operating conditions may help in controlling the reaction
network, which plays a major role in driving the reaction towards high selectivity
for hydrogen peroxide hydrogenation. The influence of reaction temperature
(313-333 K) was tested by checking the behavior of the Pt/Al2O3 catalyst. An
increase in reaction temperature from 313 to 323 K resulted in an increase in
the hydrogen peroxide conversion, but a further increase in temperature to 333
K did not lead to a corresponding further increase in the conversion (Figure 2).
10
This behavior can be attributed to mass transfer limitations. At this temperature,
the reaction rate is very high and conversion of hydrogen peroxide is limited by
several diffusion steps: gas-liquid, liquid-pellet, and pellet surface-active side.
Hydrogen peroxide is not the only molecule that can be hydrogenated under
these reaction conditions; propylene oxide and acetaldehyde can also be
hydrogenated. We observed the conversion of both compounds (Figure 2). PO
hydrogenation results in the formation of 2-propanol (no 1-propanol was
detected), while acetaldehyde yields ethanol. The hydrogenation rate of these
products increases as the temperature increases from 313 to 333 K. The
conversion of PO is minimal for all temperatures, while the conversion of
acetaldehyde is fairly high; this result is interesting because in the downstream
PO purification steps, the separation of acetaldehyde is difficult to manage.
No variation was observed in the rest of the compounds present in the reaction
mixture (1-methoxy-2-propanol, 2-methoxy-1-propanol, 1,2-propanediol) as a
function of the temperature employed. The best results were obtained at 323 K,
but the outlet of the epoxidation reactor in the HPPO process is usually at 333 K
[15, 16]. To avoid the need to introduce a heat exchange unit, a reaction
temperature of 333 K, which gave quite good but not the best results, was
selected for further evaluation
After the selection of the reaction temperature, the effect of the quantity of
hydrogen in the gas phase was studied. The amount of hydrogen was varied
from an excess (0.5 mol) to a near-stoichiometric quantity (0.2 mol). The
hydrogen peroxide conversions observed using different amounts of hydrogen
are shown in Figure 3. Hydrogen peroxide conversion did not depend on the
amount of hydrogen fed. Different behavior was observed for the hydrogenation
11
of PO and acetaldehyde, however. The hydrogenation of PO and acetaldehyde
increased as the amount of hydrogen in the reactor increased. This effect may
be due to the different hydrogenation rates of the compounds. The
hydrogenation rate is clearly higher for hydrogen peroxide when close to a
stoichiometric quantity of hydrogen is employed, and all hydrogen peroxide is
consumed, while only a small amount of PO and acetaldehyde is hydrogenated.
When an excess of hydrogen is used, however, a greater amount of PO and
acetaldehyde is hydrogenated. The concentration of other compounds present
in the reactor did not change with the amount of hydrogen fed.
Next, the effect of the amount of catalyst employed was studied. The
H2O2/metal weight ratio was varied between 200/1 to 800/1. The results
obtained for weight ratios of 200/1 and 400/1 were nearly the same, indicating
that the use of a catalyst amount greater than 400/1 implies the use of an
excess of catalyst.
A comparison of reactions using H2O2/metal weight ratios of 400/1 and 800/1 is
shown in Figure 4. The hydrogen peroxide conversion rate decreases when the
amount of catalyst introduced is lower, but in both cases, the complete
conversion of hydrogen peroxide was reached. The hydrogenation of PO and
acetaldehyde was also affected by the amount of catalyst used (Figure 4), but
the changes are less marked because the hydrogenation rate of these
compounds is slow, however the final amount converted is similar for both
quantities of catalyst employed.
12
Conclusions
The selective hydrogenation of hydrogen peroxide present in the exit stream of
the epoxidation reactor in an HPPO process can be performed with an aluminasupported platinum (or palladium) catalyst without significant formation of
byproducts derived from propylene oxide. Using optimal reaction conditions
(333 K, 0.2 mol of H2, H2O2/metal = 400/1) and an appropriate catalyst, the
hydrogen peroxide present in the exit stream can be completely converted in 20
min or less.
Increases in the reaction temperature, the quantity of hydrogen in the gas
phase, and the catalyst amount resulted in a greater H2O2 conversion rate, but
they also increased the formation of byproducts from PO. The loss of PO
selectivity and an increased byproduct formation are not desirable from an
industrial point of view because separation/purification units have to be added
to the plant hardware, resulting in an increase in process costs and a reduced
process economy.
Acknowledgements
The authors acknowledge financial support from Repsol (Spain) and the
MICINN (Spain) through the PSE-310200-2006-2, FIT-320100-2006-88 and
ENE2007-07345-C03-01/ALT
projects.
GBB
gratefully
acknowledges
a
fellowship granted by Repsol.
13
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15
16
Table 1
Textural properties of the catalysts employed.
BET surface area
Pore Volume
Pore diameter
(m2/g)
(ml/g)
(nm)
Pd/Al2O3
99.1
0.24
10
Pt/Al2O3
97.7
0.24
10
Catalyst
Table 2
XPS analysis data for fresh and used catalysts
Catalyst
Binding Energy (eV) of
Pd3d5/2 or Pt4f7/2 Levels
Surface Atomic Ratio
(Pt or Pd)/Al
Pd/Al2O3 fresh
336.0
0.0083
Pd/Al2O3 used
336.0
0.0120
Pt/Al2O3 fresh
314.0
0.0041
Pt/Al2O3 used
314.2
0.0042
17
100
90
% H2O2 Conversion
80
70
60
50
40
30
20
Pd/Al2O3
10
Pt/Al2O3
0
0
20
40
60
80
100
120
Time / min
Figure 1
Performance of Pt/Al2O3 catalyst (H2O2/metal = 400/1) for the
hydrogen peroxide hydrogenation compared with Pd/Al2O3 at 313 K and 0.2 mol
H2.
18
100
3.5
313 K
323 K
333 K
90
3.0
2.5
70
% PO Conversion
% H2O2 Conversion
80
60
50
40
30
333K
323K
313K
20
10
2.0
1.5
1.0
0.5
0
0.0
0
10
20
30
40
50
0
10
Time / min
20
30
40
50
Time / min
0.25
313 K
323 K
333 K
wt. % Acetaldehyde
0.20
0.15
0.10
0.05
0.00
0
10
20
30
40
50
Time / min
Figure 2 Influence of the temperature in the hydrogenation of hydrogen
peroxide, PO and acetaldehyde with Pt/Al2O3 catalyst (H2O2/metal =
400/1, 0.5 mol H2).
19
100
3.5
0.2 mol H2
90
0.3 mol H2
3.0
0.4 mol H2
2.5
70
% PO Conversion
% H2O2 Conversion
80
60
50
40
0.2 mol H2
30
0.3 mol H2
20
0.4 mol H2
10
0.5 mol H2
0.5 mol H2
2.0
1.5
1.0
0.5
0
0.0
0
5
10
15
20
25
0
5
10
15
20
25
Time / min
Time / min
0.12
wt. % Acetaldehyde
0.10
0.08
0.06
0.2 mol H2
0.04
0.3 mol H2
0.02
0.4 mol H2
0.5 mol H2
0.00
0
5
10
15
20
25
Time / min
Figure 3 Influence of the amount of hydrogen in the hydrogenation of
hydrogen peroxide, PO and acetaldehyde with at 333 K and Pt/Al2O3
catalyst (H2O2/metal = 400/1).
20
100
0.5
H2O2:Pt = 400:1
90
H2O2:Pt = 800:1
0.4
70
% PO Conversion
% H2O2 Conversion
80
60
50
40
30
H2O2:Pt
20
0.3
0.2
0.1
400:1
800:1
10
0
0.0
0
5
10
15
20
25
30
0
5
Time / min
10
15
20
25
30
Time / min
0.10
H2O2:Pt = 400:1
wt. % Acetaldehyde
0.09
H2O2:Pt = 800:1
0.08
0.07
0.06
0.05
0.04
0.03
0
5
10
15
20
25
30
Time / min
Figure 4
Effect of the variation of the catalyst amount in the hydrogenation
of hydrogen peroxide PO and acetaldehyde at 333 K with Pt/Al2O3
catalyst (H2O2/metal = 400/1, 0.2 mol H2).
21