Selective hydrogenation of hydrogen peroxide in the epoxidation effluent of the HPPO process

Gema Blanco-Brievaa, M. Pilar de Frutos-Escrigb, Hilario Martínb, Jose M. Campos-Martina* and Jose L. G. Fierroa*

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

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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

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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

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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

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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].

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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

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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 non-

linear 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

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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.% 1,2-

propanediol, 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.

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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 X-

ray 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 well-

dispersed 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

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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 Pd0 catalysts

[18], and this effect is attributed to the higher propensity of H2O2 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).

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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

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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.

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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 alumina-

supported 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.

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Table 1 Textural properties of the catalysts employed.

CatalystBET surface area

(m2/g)Pore Volume

(ml/g)Pore diameter

(nm)

Pd/Al2O3 99.1 0.24 10

Pt/Al2O3 97.7 0.24 10

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

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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.

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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).

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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).

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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).

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