This application claims priority to European application No 11170357.5 filed on June 17, 2011, the whole content of this application being incorporated herein by reference for all purposes.

The present invention relates to a process for the manufacture of hydrogen peroxide. In particular, it relates to the manufacture of hydrogen peroxide by direct synthesis from reacting hydrogen and oxygen.

The standard large scale production method for hydrogen peroxide involves the use of anthraquinone as an intermediate. Such process requires very large amounts of organic solvents and thus cannot be considered as strictly environmentally friendly. Such process is also energy-consuming. And last but not least, such process typically requires huge plants, which is not compatible with on-site and on-demand production. There has thus been considerable study over the last years to develop alternative processes for the manufacture of hydrogen peroxide.

In particular, there were attempts to develop processes based on direct oxidation of hydrogen by oxygen, even if such processes imply the use of potentially explosive mixtures of hydrogen and oxygen. For instance, patent US 3,361,533 published in 1968 discloses a process for the manufacture of hydrogen peroxide by contacting hydrogen and oxygen with a solid catalyst in a liquid medium, the solid catalyst containing conveniently palladium either alone or alloyed or mixed with a minor proportion of one or more other metals, especially gold or platinum. Similarly, a more recent example is US 6,958,138 Bl, published in 2005, which discloses direct manufacture of hydrogen peroxide from hydrogen and oxygen in the presence of supported single-, bi- or multi- metal catalysts, said multi-metal catalysts consisting of a majority metal and several minority metals. Another example is given by international patent application WO 2007/007075 which discloses the use of supported Au/Pd bimetallic catalysts for the direct synthesis of ¾(¾, the catalyst support having been acid washed prior to metal deposition.

Further bi-metallic catalysts for the direct synthesis of hydrogen peroxide are disclosed in US 6,387,346, US 2006/0252947, US 2005/0201925 and by G. Bernardotto, et al. in Applied Catalysis A: General 358 (2009) 129-135. Despite all these efforts, there still remains a need for an improved and commercially viable catalyst as well as to an improved and commercially viable direct reaction process for the direct synthesis of hydrogen peroxide.

The purpose of the present invention is to provide an improved process for the formation of hydrogen peroxide by direct synthesis. In particular, the purpose of the present invention is to provide a process exhibiting an increased ¾(¾ productivity and/or a reduced hydrogenation activity (i.e. to avoid hydrogenation of H2O2 into H2O) and/or an increased selectivity to ¾(¾.

The present invention therefore relates to a process for the manufacture of hydrogen peroxide by direct synthesis comprising converting hydrogen and oxygen to hydrogen peroxide in the presence of a heterogeneous supported catalyst comprising at least gold, palladium and platinum as catalytically active component, wherein the weight ratios palladium to platinum (Pd/Pt) and gold to platinum (Au/Pt) are both equal to or higher than 1 :1, and wherein gold is present in an amount of at least 0.2% by weight, preferably at least 0.3% by weight of the catalyst support.

Indeed, it has been surprisingly found that reacting hydrogen and oxygen in the presence of a supported catalyst comprising at least palladium, gold, and platinum in afore-said ratios and amounts allows the preparation of hydrogen peroxide with an increased productivity and/or a reduced hydrogenation activity and/or increased selectivity to hydrogen peroxide.

Thus, one of the essential features of the present invention resides in the use of supported catalysts comprising at least gold, palladium and platinum, wherein the Au/Pt and Pd/Pt weight ratios are both equal to or higher than 1 : 1 , in particular higher than 1 :1. The Au/Pt and Pd/Pt weight ratios may vary widely, but are typically, independently of one another (preferably both), higher than 1 :1, equal to or higher than 1.2: 1, with preference equal to or higher than 1.5:1, with particular preference equal to or higher than 2: 1, with higher preference equal to or higher than 4:1, with especial preference equal to or higher then 6:1, for example equal to or higher than 8:1. The Au/Pt and Pd/Pt weight ratios are commonly equal to or lower than 500:1, very often equal to or lower than 100: 1, frequently equal to or lower than 50:1, for example equal to or lower then 25:1.

Usually, in the process of the present invention, the (Au+Pd)/Pt weight ratio is from 2.5:1 to 100:1, especially from 3:1 to 75:1, particularly from 4:1 to 50:1, more particularly from 10:1 to 40: 1, such as from 5:1 to 40:1, most particularly from 15:1 to 30:1, such as from 7:1 to 30:1. In the process of the invention, the Pd/Au weight ratio is typically equal to or lower than 50:1, preferably equal to or lower than 40:1, more preferably equal to or lower than 30:1, most preferably equal to or lower than 25:1, such as equal to or lower than 5:1, 4:1, 3:1 or 2:1. The Pd/Au weight ratio is generally equal to or higher than 1 :3, often equal to or higher than 1 :2.5, more often equal to or higher than 1:2, most often equal to or higher thanl:1.5. A Pd/Au weight ratio about 1 :1 may for instance give good results.

The catalysts used in the process of the present invention generally comprise a total amount of noble metals from 1 to 30 % by weight of the catalyst support, especially from 2 to 20 wt%, more especially from 4 to 10 wt%. In such catalysts, the palladium metal is in general present in an amount equal to or higher than 0.05% by weight of the catalyst support, preferably equal to or higher than 0.1 wt%, more preferably equal to or higher than 0.2 wt%, such as equal to or higher than 0.3 wt%, equal to or higher than 0.4 wt%, equal to or higher than 0.5 wt%, equal to or higher than 0.6 wt%, equal to or higher than 0.7 wt% or equal to or higher than about 1 wt%. The gold metal is in general present in an amount equal to or higher than 0.3 wt% of the catalyst support, preferably equal to or higher than 0.4 wt%, more preferably equal to or higher than 0.5 wt%, most preferably equal to or higher than 0.6 wt%, with higher preference equal to or higher than 0.7 wt%, with highest preference equal to or higher than 0.8 wt%, for instance equal to or higher than about 1 wt%. The gold and palladium metals are commonly present each, independently of one another, in an amount equal to or lower than 9.5% by weight of the catalyst support, especially equal to or lower than 8 wt%, particularly equal to or lower than 5 wt%. The total amount of gold and palladium is commonly from 0.25 to 9.9% by weight of the catalyst support, in particular from 0.5 to 7.5 wt%, more particularly from 1 to 5 wt%. The platinum is usually present in a total amount from 0.001 to 1 % by weight of the catalyst support, preferably from 0.05 to 0.8 wt%, such as from 0.05 to 0.5 wt%, more preferably from 0.1 to 0.7 wt%, such as from 0.1 to 0.3 wt%.

The catalysts used in the process of the present invention are in general trimetallic catalysts comprising gold, palladium and platinum. Anyway, the catalysts of the present invention may further comprise at least one additional noble metal, for instance at least one additional noble metal selected from the group consisting of silver, rhodium, iridium, ruthenium, osmium, and rhenium. Said additional noble metal may for example be present in an amount from 0 to 1 % by weight of the catalyst support.

In the process of the present invention, the catalyst support to or in which the at least three noble metals present as catalytically active component are bound may be any type of support known in the art. Said catalyst support may for example be selected from the group consisting of carbon supports, oxide supports, and silicate supports, in particular from carbon supports, AI2O ₃ , T1O2, CeC>2, Zr0 ₂ , Fe2C>3, S1O2, silica-alumina and zeolites or any mixture thereof. Suitable carbon supports are for instance graphite, carbon black, glassy carbon, activated carbon, highly oriented pyrolytic graphite (HOPG), single-walled and multi-walled carbon nanotubes, especially activated carbon. The support may be porous or non-porous, preferably porous. Said catalyst support may be used directly to deposit the catalytically active component, or may first be pretreated, prior to noble metal deposition. An example of pretreatment is acid washing of the support that may be carried out using a mineral acid such as hydrochloric acid or nitric acid. Preferably the acid is dilute nitric acid, and supports are treated for example for 3 hours at ambient temperature.

Deposition of gold, palladium and platinum onto the catalyst support to manufacture the catalyst may be performed by any method known in the art, especially starting from metallic gold, palladium and platinum, and/or starting from gold, palladium and platinum precursors.

In a first specific embodiment of the present invention, the noble metals present as catalytically active component may be deposited onto the catalyst support in the form of metal oxides or metal ions by any method known in the art, to form a catalyst precursor, for instance by incipient wetness method. The at least three noble metals, i.e. gold, palladium and platinum, may be deposited simultaneously or sequentially, conveniently simultaneously. The catalyst precursor is then transformed into the corresponding metals via at least one of a heat treatment, chemical reduction in the presence of a reducing agent, or electrochemical reduction (electrodeposition). Heat treatment is typically conducted at a temperature from 250 to 600°C, preferably from 300 to 500°C, more preferably from 350 to 450°C, for example around 400°C. Heat treatment may be conducted under any type of atmosphere such as oxygen or oxygen containing atmosphere, reducing atmosphere, or inert atmosphere, for instance under oxygen, air, nitrogen, argon, hydrogen or mixtures thereof. Chemical reduction reactions and electrodeposition are well known in the art. Suitable reducing agents are known in the art and may for instance be selected from the group consisting of H ₂ , NaH, LiH, LiAlH ₄ , NaBH ₄ , CaH ₂ , SnCl ₂ ,

diisobutylaluminium hydride (DIBAL-H), sodium citrate, disodium citrate, trisodium citrate, sodium formate, formic acid, and hydrazine, commonly H ₂ or NaBH ₄ .

In a second specific embodiment of the present invention, the noble metals may be deposited onto the catalyst support by direct deposition of metallic gold, palladium and platinum, used in the form of a dispersion in an aqueous or organic medium, generally in the form of an aqueous dispersion, advantageously in the form of a colloidal dispersion.

It is also possible to combine both the first and second specific

embodiments described above.

In the process of the present invention, the respective amounts and weight ratios of gold, palladium and platinum may of course be adapted to the nature of the catalyst support and reaction conditions.

Any suitable additive may also be further added to the medium during the preparation of the catalyst, for instance in at least one of the mediums comprising the noble metals and/or noble metal precursors. Suitable additives are for instance dispersants, stabilizers and templating agents.

After deposition of the gold and palladium onto the catalyst support, the supported catalyst may be recovered by any suitable separation method, such as filtration, decantation and/or centrifugation. Said supported catalyst may further be washed and dried, for instance at about 50 to 100°C, under air or under protective atmosphere, typically under air. The supported catalyst may also be further heat treated and/or reduced.

The process of the present invention may be conducted in any type of suitable reactor known in the art, especially in a stirred reactor, for example into an autoclave equipped with stirring means, a loop reactor or a tube reactor. The catalyst may be present into the reactor as a fixed bed or as a fluid bed.

The present process is usually conducted at a temperature equal to or higher than -10°C, often equal to or higher than 0°C, more often equal to or higher than 1°C. The process is typically conducted at a temperature equal to or lower than 90°C, in particular equal to or lower than 50°C, more particularly equal to or lower than 30°C. For instance, the process may be carried out a temperature between 1 and 50°C, especially between 2 and 20 °C,

advantageously between 2 and 10°C. An inert gas such as carbon dioxide, nitrogen, helium or argon may also be present with oxygen and/or hydrogen gas, or the oxygen and/or hydrogen gas may be first saturated with water vapor. The hydrogen to oxygen molar ratio is in general from 10:1 to 1:100, with preference from 5:1 to 1:50, with higher preference from 1 :1 to 1 : 15, for example around 1 :7.

The total pressure into the reactor (as measured at 20°C) is in general from 0.1 to 20 MPa, preferably from 1 to 10 MPa, more preferably from higher than 1 to 6 MPa, for instance around 4 to 5 MPa.

The duration of the reaction between oxygen and hydrogen may vary widely and may be adapted to the reaction conditions. Suitable durations are for instance from 0.5 to 400 minutes, particularly from 1 to 360 minutes, more particularly from 2 to 300 minutes, such as from 1 to 120 minutes or from 2 to 60 minutes.

In the present invention the hydrogen and oxygen are commonly reacted in the presence of a liquid such as water, organic solvents and mixtures thereof. Especially suitable organic solvents are water miscible solvents such as methanol, ethanol, isopropyl alcohol, acetone and glycols such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol etc.

In a certain embodiment of the present invention the reaction medium may further comprise any other additional components which may contribute to a desired result or avoid an undesired result. For example, bromide ions, preferably in the form of hydrogen bromide (HBr), sodium bromide or potassium bromide, most preferably HBr, may be added. It has surprisingly been found that bromide ions reduce the tendency for ¾0 formation at higher temperatures, causing an increase in yield due to decreased decomposition (thus stabilisation) of the ¾(¾. The amount of bromide ions added calculated as HBr can be between 0.1 to 30 mg/kg of the reaction medium, more preferably between 5 and 20 mg/kg of the reaction medium and most preferably around 10 mg/kg of the reaction medium.

In a further embodiment it has surprisingly been found that the addition of an inorganic acid may also induce a stabilisation of the hydrogen peroxide formed. Preferred inorganic acids are sulphuric acid, nitric acid, hydrochloric acid and ortho-phosphoric acid. The amount of inorganic acid can be between 0.01 and 0.5 M, more preferably between 0.02 and 0.2 M and most preferable around 0.06 M. In a preferred embodiment, ortho-phosphoric acid is added at a concentration of 0.06 M. Bets results with respect to yield and selectivity are obtained by adding both, bromide ions and an inorganic acid to the reaction medium, in particular HBr and ortho-phosphoric acid.

The process of the present invention may be carried out batchwise, for instance in an autoclave, but it can also be operated in a continuous or semi- continuous mode.

The process according to the invention is advantageous in that it has demonstrated improved yields (¾(¾ productivity) and/or higher selectivity towards the preparation of hydrogen peroxide by direct reaction of hydrogen and oxygen and/or reduced hydrogenation activity (i.e. hydrogenation of ¾(¾ into ¾0). The process of the present invention is also environmentally friendly, especially as it can be used without organic solvents, especially without non- environmentally friendly solvents. It can also be operated under mild operating conditions, with a reduced energy usage. The process of the present invention is especially suitable for on-site / on-demand production of hydrogen peroxide, among others due the small size of the production devices.

The present invention also relates to hydrogen peroxide obtainable by the process described above.

In view of the above, the present invention also relates to a supported catalyst suitable for hydrogen peroxide direct synthesis obtained by reacting hydrogen and oxygen, said catalyst comprising a catalyst support and at least gold, palladium and platinum as catalytically active component, wherein the weight ratios Pd/Pt and Au/Pt are both equal to or higher than 1 : 1 and wherein the amount of gold is equal to or higher than 0.2% by weight, preferably equal to or higher than 0.3% by weight of the catalyst support. In a preferred

embodiment, in said supported catalyst, the (Au+Pd)/Pt, Pd/Au, Au/Pt and Pd/Pt weight ratios are as defined above.

The catalysts according to the present invention exhibit surprisingly a long life and recyclability, while allowing the preparation of ¾(¾ by direct synthesis with an improved productivity and/or decreased hydrogenation activity (and so increase the total selectivity to hydrogen peroxide).

The present invention also relates to the use of such supported catalysts for the direct synthesis of hydrogen peroxide.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The present invention is further illustrated below without limiting the scope thereto.

Examples

Except otherwise mentioned, examples were conducted according to the following procedures.

Preparation of the catalysts

The catalysts were prepared by impregnation of suitable support materials : CeC> ₂ (<5 micron particulate (99.9% purity) from Aldrich), Τί(¾ (mainly anatase, P25 from Degussa), Zr(¾ (<5 micron particulate (99%+ purity) from Aldrich), Nb ₂ 0 ₅ (99.99% metal basis, from Aldrich) carbon (G60 activated carbon from Aldrich), and S1O ₂ (SBA-15 commercially available from Claytec Inc. and from Acros Organics).

An incipient wetness method using aqueous solutions of PdCl ₂ (Johnson

Matthey), HAuCi ₄ .3H ₂ 0 (Johnson Matthey) and/or PtCl ₂ (Johnson Matthey) was employed. The paste formed was ground and dried at 80 °C or 110 °C for 16 hours and calcined in static air, typically at 400°C for 3 hours.

H2Q2 direct synthesis and hydrogenation - batch tests

Catalyst testing was performed using a Parr Instruments stainless steel autoclave with a nominal volume of 50 ml and a maximum working pressure of 14 MPa. The autoclave was equipped with an overhead stirrer (0 - 2000 rpm) and provision for measurement of temperature and pressure.

For the synthesis of hydrogen peroxide 10 mg of the supported catalyst were charged in an autoclave containing 5.6 g methanol and 2.9 g water. The autoclave was purged three times with CO ₂ (3 MPa) and then filled with 5% H ₂ /CO ₂ and 25% O ₂ /CO ₂ to give a hydrogen to oxygen ratio of 1 :2, at a total pressure of 3.7 MPa at 20 °C. The reaction medium was cooled down to 2°C and stirring (1200 rpm) was started on reaching the desired temperature (2°C).

Experiments were carried out for 30 min.

Hydrogen peroxide hydrogenation was carried out using the conditions described for the direct synthesis with the addition of 4wt% H ₂ 0 ₂ ( 50wt%, Aldrich) to the solvent in place of water (0.68g H ₂ 0 ₂ , 2.22g H ₂ 0 and 5.9g MeOH)The experiment is carried out in the absence of 25%0 ₂ /CC> ₂ . The wt% of H ₂ O ₂ hydrogenated was determined by titrating aliquots of the fresh solution and the solution after reaction with acidified Ce(S0 ₄ )2 (0.0288 M) in the presence of two drops of ferroin indicator.

H?Q? direct synthesis and hydrogenation - semi-continuous tests

In a 380 cc Hastelloy B22 reactor, methanol (150 g), Hydrogen bromide

(lOppm), 0.06M ortho-phosphoric acid and catalyst (0.36 g) are introduced.

The reactor is cooled to 5°C and the working pressure is at 50 bars (obtained by introduction of nitrogen).

The reactor is flushed all the time of the reaction with the mix of gases:

Hydrogen (3.6% Mol) / Oxygen (25.0% Mol) / Nitrogen (71.4% Mol). The total flow is 2567 mlN/min. The oxygen / hydrogen ration is 7.2: 1.

When the gas phase out is stable (GC on line), the mechanical stirrer is started at 1500 rpm.

GC on line analyzes every 10 minutes the gas phase out.

Liquid samples are taken to measure hydrogen peroxide and water concentration. Hydrogen peroxide is measured by redox titration with cerium sulfate.

Water is measure by Karl-Fisher.

The selectivity is calculated following the fomula:

Selectivity in H ₂ 0 ₂ (%) = N mol H ₂ 0 ₂ formed / (N mol H ₂ 0 ₂ formed + N mol water formed)

Reference Examples 1-4 and Examples 5-10 (Au/Pd/Pt on CeQ ₂ ):

Table 1 below illustrates hydrogen peroxide productivity and

hydrogenation activity for various catalytic active components based on Au, Pd and/or Pt on Ce0 ₂ support. Those trials have been performed in batch configuration and without any addition of bromide or inorganic acidity.

Table 1

Ex. Catalyst Productivity ^* Hydrogenation

(molH ₂ 0 ₂ /kg- Activity ^* cat/h) (molH ₂ 0 ₂ /kg- cat/h)

1 2.50%Au-2.50%Pd/CeO ₂ 68 145 (7%)

2 2.50% Au-2.50%Pt/CeO ₂ 20 109 (5%)

3 2.50%Pd-2.50%Pt/CeO ₂ 138 182 (9%)

4 4.80%Pd-0.20%Pt/CeO ₂ 125 192 (9%)

5 0.83%Au-3.33%Pd-0.83%Pt/CeO ₂ 138 66 (3%)

6 2.475 %Au-2.475%Pd-0.05%Pt/CeO ₂ 63 46 (2%) 7 2.45%Au-2.45%Pd-0.10%Pt/CeO ₂ 109 76 (4%)

8 2.00% Au-2.00%Pd- 1.00%Pt/CeO ₂ 115 93 (5%)

9 2.3%Au-2.5%Pd-0.2%Pt/CeO ₂ 155 94 (5%)

10 2.5%Au-2.3%Pd-0.2%Pt/CeO ₂ 86 11 (0.5%)

Productivity denoted as moles H ₂ 0 ₂ produced per kg catalyst per hour

Hydrogenation activity denoted as moles H ₂ 0 ₂ destroyed per kg catalyst per hour, based on 4wt% starting [H ₂ 0 ₂ ]. Number in brackets denoted at percent H ₂ 0 ₂ destroyed during experiment.

Examples 11 and 12: Au/Pd/Pt on C

Table 2 below illustrates the comparison between a trimetallic catalyst and a classical bimetallic catalyst (Pd and Au) on active carbon. Those trials have been performed in semi-continuous configuration with addition of bromide ions and ortho-phosphoric acid.

Table 2

As clearly demonstrate in this example, the use of a trimetalic catalyst enhances the selectivity of the reaction to hydrogen peroxide and decreases the final concentration of water in the reaction medium.

Examples 13 and 14: Au/Pd/Pt on Ce0 ₂

Table 3 below illustrates the importance of the addition of bromide in the reaction medium for enhancing the selectivity of the reaction. Those trials have been performed in semi-continuous configuration with addition of ortho- phosphoric acid.

Table 3

Examples 15 and 16: Au/Pd/Pt on C

Table 4 below illustrates the importance of using the right bromide content in the reaction medium for enhancing the selectivity and the conversion rate of the reaction. Those trials have been performed in semi-continuous configuration with addition of bromide and ortho-phosphoric acid. Table 4

Examples 17 to 19: Au/Pd/Pt on several supports

Table 5 below illustrates the use of catalyst prepared on several supports (CeC>2 or C) and with different ratio Au/Pt and Pd/Pt. Those trials have been performed in semi-continuous configuration with addition of bromide and ortho- phosphoric acid.

Table 5

Catalyst A Catalyst B Catalyst C

Au, %Wt 2.40 4.60 3.75

Catalyst Pd, %Wt 2.40 0.20 0.63

Pt, %Wt 0.20 0.20 0.63

Support C Ce0 ₂ Ce0 ₂

Methanol g 149.51 151.16 150.04

HBr ppm 10 10 10