WO2016050859A2 - Method for preparing a catalyst support and a catalyst - Google Patents

Method for preparing a catalyst support and a catalyst Download PDF

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Publication number
WO2016050859A2
WO2016050859A2 PCT/EP2015/072589 EP2015072589W WO2016050859A2 WO 2016050859 A2 WO2016050859 A2 WO 2016050859A2 EP 2015072589 W EP2015072589 W EP 2015072589W WO 2016050859 A2 WO2016050859 A2 WO 2016050859A2
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Prior art keywords
carrier
catalyst
hydrogen peroxide
support
phosphate
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PCT/EP2015/072589
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French (fr)
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WO2016050859A3 (en
Inventor
Frédérique DESMEDT
Pierre Miquel
Yves VLASSELAER
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Solvay Sa
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Publication of WO2016050859A3 publication Critical patent/WO2016050859A3/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/08—Silica
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/48—Silver or gold
    • B01J23/52—Gold
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/14—Phosphorus; Compounds thereof
    • B01J27/16—Phosphorus; Compounds thereof containing oxygen, i.e. acids, anhydrides and their derivates with N, S, B or halogens without carriers or on carriers based on C, Si, Al or Zr; also salts of Si, Al and Zr
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/14—Phosphorus; Compounds thereof
    • B01J27/185—Phosphorus; Compounds thereof with iron group metals or platinum group metals
    • B01J27/1856—Phosphorus; Compounds thereof with iron group metals or platinum group metals with platinum group metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61—Surface area
    • B01J35/613—10-100 m2/g
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61—Surface area
    • B01J35/615—100-500 m2/g
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/63—Pore volume
    • B01J35/638—Pore volume more than 1.0 ml/g
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/64—Pore diameter
    • B01J35/647—2-50 nm
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • B01J37/0207—Pretreatment of the support
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • B01J37/0209—Impregnation involving a reaction between the support and a fluid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/03—Precipitation; Co-precipitation
    • B01J37/031—Precipitation
    • B01J37/035—Precipitation on carriers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/16—Reducing
    • B01J37/18—Reducing with gases containing free hydrogen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/20—Sulfiding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/28—Phosphorising
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B15/00—Peroxides; Peroxyhydrates; Peroxyacids or salts thereof; Superoxides; Ozonides
    • C01B15/01—Hydrogen peroxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B15/00—Peroxides; Peroxyhydrates; Peroxyacids or salts thereof; Superoxides; Ozonides
    • C01B15/01—Hydrogen peroxide
    • C01B15/029—Preparation from hydrogen and oxygen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/06—Washing

Definitions

  • the present invention relates to a method for preparing a catalyst support and a catalyst, and to the use of said catalyst namely for the direct synthesis of hydrogen peroxide.
  • Hydrogen peroxide is a highly important commercial product widely used as a bleaching agent in the textile or paper manufacturing industry, a disinfecting agent and basic product in the chemical industry and in the peroxide compound production reactions (sodium perborate, sodium percarbonate, metallic peroxides or percarboxyl acids), oxidation (amine oxide manufacture), epoxidation and hydroxylation (plasticizing and stabilizing agent manufacture).
  • the most common method to produce hydrogen peroxide is the Auto Oxidation (AO) process.
  • AO Auto Oxidation
  • hydrogen and oxygen react to form hydrogen peroxide by the alternate oxidation and reduction of alkylated anthraquinones in organic solvents.
  • a significant disadvantage of this process is that it is costly and produces a significant amount of by-products that must be removed from the process.
  • AO Auto Oxidation
  • the initial selectivity is much better but is unstable and wouldn't be stabilized by o-phosphoric acid addition. Moreover, the conversion rate is low, so the final H202 concentration remains below a value acceptable at industrial scale.
  • US 2008/0305033 relates to a catalyst effective for the direct reaction of hydrogen and oxygen to form hydrogen peroxide and which includes particles of gold and/or palladium deposited upon an acid- washed support which is preferably Si02, Ti02, A1203 or Fe203. Acid washing is preferably performed using dilute nitric acid at ambient temperature and for about 3 hours.
  • Raman analysis indicated the presence of an inorganic phosphate on the carrier surface (probably titanium phosphate) and the elementary analysis confirms the presence of P after the catalyst impregnation and reduction.
  • Titanium phosphate being water insoluble, this could explain why said catalyst is indeed stable over time.
  • sulfuric acid Ti sulfate being also water insoluble.
  • sulfuric acid since in the presence of hydrogen peroxide, sulfuric acid generates Caro's acid, which is very corrosive, the use of sulfuric acid is preferably avoided in the frame of the present invention.
  • the present invention relates to a method for preparing a catalyst support comprising the following steps:
  • catalyst support intend to designate a support for a catalytically active specie, generally a metal.
  • this support comprises a carrier i.e. a solid material on which Ti02 is precipitated.
  • the carrier of the invention comprises SiO 2 (silicon dioxide or silica) and even more preferably, it consists essentially of Si02.
  • the carrier used in the invention has a large specific surface area of for example above 20 m /g calculated by the BET method, preferably greater than 100 m /g.
  • the specific surface area of the carrier does not exceed 500 m /g, preferably not 300 m /g, even more preferably not 200 m 2 /g.
  • the pore volume of the carrier can be for example in the range 0.1 to 3 ml/g, preferably 0.5 to 2 ml/g.
  • the pore size of the carrier is from 5 to 20 nm, more preferably from 10 to 15 nm.
  • the mean particle size of the support ranges from 50 ⁇ to a few mm, preferably from 60 to 210 ⁇ for slurry catalysts. In the case of fix bed catalysts, the particle size ranges preferably from 500 ⁇ to 5 mm.
  • the carrier can essentially be amorphous like silica gel or can be comprised of an orderly structure of mesopores, such as MCM-41, MCM-48, SBA-15, or a crystalline structure, like a zeolite.
  • Silica gel gives good results in DS of hydrogen peroxide.
  • the precipitation of Ti02 on the carrier may be accomplished by a variety of techniques known in the art.
  • One such method involves impregnating (preferably at room temperature - atmospheric pressure) the carrier with a precursor of Ti02, optionally followed by drying.
  • the Ti02 precursor may include any suitable Ti hydroxide, alkoxide, halide or oxyhalide.
  • the precursor of Ti02 is a Ti alkoxide, preferably Ti butoxide, used in a concentration calculated to reach the desired Ti content (for instance between 1 and 20 Wt, preferably between 2 and 15 Wt; more preferably between 5 and 12 Wt).
  • This Ti butoxide impregnation is preferably done in 1-butanol as solvent and is preferably done under agitation (200 rpm mechanical stirring). After impregnation, the carrier is preferably dried at 90°C during 24 hours.
  • the precursor is converted, for example by hydrolysis followed by heat treatment, to titanium oxide, which is precipitated onto the carrier.
  • the heat treatment is a calcination, preferably under static air.
  • the so obtained Ti02 is mainly amorphous with some rays proving the presence of small amounts of anathase.
  • This treatment comprises a first step comprising contacting, preferably at room temperature - atmospheric pressure - under mechanical stirring (200 rpm), the carrier with an aqueous o-phosphoric acid solution.
  • aqueous o-phosphoric acid solutions containing between 75 and 95 Wt, typically about 85 Wt, give good results especially when using water as solvent.
  • 1.7 g o-phosphoric acid solution and 50 ml of water can be added.
  • the contact time is comprised between 3 and 240 h, preferably between 5 and 120 h, more preferably between 10 and 96 h.
  • the treatment comprises a second step which comprises evaporating at least part of the o-phosphoric acid solution in order to precipitate a phosphate on said carrier.
  • the temperature during the evaporation step is comprised between 20 and 200°C, preferably between 50 and 150°C and even more preferably between 70 and 130°C. It is generally performed at atmospheric pressure.
  • the duration of the evaporation step is comprised between 2 and 200 h, preferably between 5 and 100 h, more preferably between 7 and 72 h.
  • the evaporation conditions are in fact preferably chosen so that at least 25 Wt, preferably at least 50 Wt and even more preferably, at least 80 Wt of the solution is evaporated. In a preferred embodiment, substantially all the solution is evaporated and the carrier is hence substantially dry.
  • the amount of precipitated phosphate is such that the Ti/P molar ratio is comprised between 10 and 0.1, preferably between 7.5 and 0.25, and more preferably between 5 and 0.5.
  • the present invention also relates to a method for preparing a catalyst, said method comprising depositing (supporting) at least one catalytically active metal selected from elements in Groups 7 to 11, on the support obtained by the method described above.
  • the catalytically active metal which may be used in the present invention can be selected by a person skilled in the art according to the intended use of the catalyst.
  • the catalyst comprises at least one metal selected from among the platinum group comprised of ruthenium, rhodium, palladium, osmium, iridium, platinum, or any combination of these metals.
  • the catalyst comprises palladium or a combination of palladium with another metal (for example rhodium, platinum or gold). Palladium gives good results especially in the frame of hydrogen peroxide DS.
  • the amount of catalytically active metal supported on the carrier can vary in a broad range, but is preferably comprised from 0.001 to 10 wt. , more preferably from 0.1 to 5 wt. % and most preferably from 0.3 to 3 wt. , each based on the total weight of the carrier.
  • the catalytically active metal is preferably present at least partly in reduced form.
  • a metal in reduced form means metal atoms having the oxidization level 0 or lower, such as Pd° or Pd hydride.
  • the deposition of the catalytically active metal onto the support can be performed using any of the known preparation techniques for supported metal catalyst, e.g. impregnation, adsorption, ionic exchange, etc.
  • the metal can be deposited by dipping the carrier in a solution of metal halides followed by reduction.
  • the catalysts of the invention do not require calcination to be effective, which is advantageous from an energetic point of view.
  • the support is contacted with a solution of palladium chloride.
  • the product is recovered, for example by filtration.
  • the metal deposited on the support is preferably (at least partially) reduced, for example by using hydrogen at high temperature.
  • This hydrogenation step can be carried out for example at a temperature from 100°C to 300°, preferably from 120°C to 200°C for 1 to 10 hours, preferably for 2 to 6 hours.
  • the hydrogen is preferably dilutes with an inert gas, for instance with nitrogen, typically about 5 times.
  • the catalysts according to the invention are suitable for catalyzing various reactions, including for example oxidation reactions as oxidation of propene, ethylene, phenol in the presence of hydrogen peroxide.
  • the catalysts are used for catalyzing the synthesis of hydrogen peroxide, either by the AO route or by DS, in particular for catalyzing the DS of hydrogen peroxide.
  • the invention is also directed to the use of the catalyst obtained by the process described above in production of hydrogen peroxide.
  • hydrogen and oxygen are reacted continuously over the catalyst in the presence of a liquid solvent in a reactor to generate a liquid solution of hydrogen peroxide.
  • the catalyst is used for the direct synthesis of hydrogen peroxide preferably in a three-phase system: the catalyst (solid) is put in a solvent (liquid medium comprising water or an alcohol like methanol or ethanol or acetonitrile) and the gases (H 2 , O 2 and an inert gas) are bubbled in the suspension, preferably in presence of at least one stabilizing additive (for instance a halide and / or an inorganic acid: see below).
  • a solvent liquid medium comprising water or an alcohol like methanol or ethanol or acetonitrile
  • the gases H 2 , O 2 and an inert gas
  • at least one stabilizing additive for instance a halide and / or an inorganic acid: see below.
  • the above DS process for producing hydrogen peroxide may comprise reacting hydrogen and oxygen in the presence of the catalyst in a reactor.
  • the process of this invention can be carried out in continuous, semi-continuous or discontinuous mode, by the conventional methods, for example, in a stirred tank reactor with the catalyst particles in suspension, a basket-type stirred tank reactor, a fixed-bed reactor, etc.
  • the catalyst can be separated by different known processes, such as filtration if the catalyst in suspension is used, which would afford the possibility of its subsequent reuse.
  • the amount of catalyst used is that necessary to obtain a concentration of 0.01 to 10 wt. % catalyst (regarding the total weight of solvent and catalyst) and preferably 0.02 to 5 wt.
  • the concentration of the obtained hydrogen peroxide solution according to the invention is generally higher than 2 wt. , preferably higher than 4 wt. %, most preferably higher than 7 wt. %. In some cases, it may even exceed 10 wt. %.
  • the catalysts of the invention are unfortunately also over-hydrogenation and decomposition catalysts of the peroxide formed. It is consequently advantageous for the liquid phase of the three-phase system in which the synthesis is carried out, to contain a compound capable of selectively poison the hydrogen peroxide decomposition and over-hydrogenation sites present on the surface of the catalyst.
  • Halide ions are good representatives of these compounds. Their optimum concentration must be determined by means of laboratory tests within the capability of the person skilled in the art.
  • Chloride, bromide and iodide ions are suitable to inhibit the decomposition sites of the catalyst.
  • the bromide ion has given the best results, especially when present in a concentration between 0.05 and 3 mmol/1 of liquid phase and, preferably, between 0.1 and 2 mmol/1.
  • the liquid phase it is preferable for the liquid phase to contain a small quantity of an acid (like orthophosphoric acid) with the aim of inhibiting spontaneous non-catalytic decomposition of the hydrogen peroxide.
  • an acid like orthophosphoric acid
  • This quantity is advantageously small not only for corrosion issues but also because when the acid concentration is too high, the solubility of the gases in the liquid phase decreases and the acid can react with the hydrogen peroxide present.
  • suitable acids mention may be made of nitric acid and orthophosphoric acid.
  • Orthophosphoric acid is preferred.
  • the catalysts of the invention allow using the inorganic acid in an amount of 0.5 M/l or less, even of 0.1 M/l or less which enables lowering the costs and corrosion problems while still providing good synthesis results.
  • the temperature of the reaction is normally chosen at a value of between -5 and 50 °C and, preferably, between 0 and 20 °C.
  • the pressure chosen is greater than atmospheric pressure and is generally between 1 and 150 bar and, preferably, between 25 and 100 bar.
  • Example 1 support preparation
  • the alcohol was then evaporated under vacuum (80°C - 140mbar) 200 ml of demineralized water were slowly introduced using a syringe pump and the hole was left aging at room temperature overnight.
  • the solid was then filtered under vacuum and washed with 1 1 of demineralized water. The washed solid was dried at 90°C during 24 hours. It was calcined at 400°C during 6 hours under static air.
  • the solution of ZrOC12 was slowly introduced in the suspension at 50°C.
  • the pH was maintained between 8.4 - 8.5 by adding drops of the solution NH40H 25%Wt.
  • the washed solid was dried one night at 100°C and then calcined 3 hours at 600°C.
  • An aqueous solution of palladium chloride was prepared with the amount of Pd necessary in order to obtain the desired loading of the metal on the catalyst. Typically the total volume of the solution for 20 g of carrier was 24 ml.
  • the catalyst was dried at 95°C for 24 hours.
  • the Pd was reduced under influence of hydrogen, diluted with nitrogen, for a duration and at the
  • an aqueous solution of Pd chloride and Au chloride (AuC13) was prepared and also acidified in order to improve the solubility of the precursors.
  • the reactor was cooled at the temperature indicated in Table 2 and the working pressure was set (by introduction of nitrogen) at the value also indicated in Table 2.
  • the reactor was flushed all the time of the reaction with the following mix of gases: Hydrogen (3.6% Mol) / Oxygen (55.0% Mol) / Nitrogen (41.4% Mol).
  • the total gas flow was 2708mlN/min
  • Liquid samples were taken to measure hydrogen peroxide and water concentration.
  • Hydrogen peroxide was measured by redox titration with cerium sulfate. Water was measured by Karl-Fisher titration.
  • Catalysts based on titanium oxide show a low to very low selectivity. They are moreover unstable. The maximum of productivity obtained with such type of catalyst is 3 to 8 times less than what could be obtained with catalysts based on Ti/Si (Comparative Examples C6, C7 and Examples 10 and 11).
  • Zr oxide on silica treated with o-phosphoric acid shows poorer performances than the untreated one (C5).
  • the treated catalyst shows a higher conversion rate but a lower selectivity already from the beginning.
  • a slight improvement is observed in presence of 0.1M H 3 PO 4 in liquid phase (C9 vs C8): the conversion is slightly higher and the selectivity is more stable.
  • there is no real improvement when comparing the catalyst pre-treated with the acid (C9) and the catalyst untreated but used in presence of the acid (C5): they both have a similar behavior in terms of selectivity loss.
  • the catalyst based on Ti Ox/silica doesn't show any significant
  • the carrier treatment with o-phosphoric acid before the Pd impregnation is more than an acid absorption as the result is sensitively different.
  • the o-phosphoric acid is not only impregnated on the surface but interacts with the carrier (chemically bonded) and enhances the surface acidity.
  • the proof is that the catalyst even without addition of free o- phosphoric acid is more active (generates a higher H202 concentration) than the catalyst based on TiOx/silica used with acidity.

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  • Organic Chemistry (AREA)
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Abstract

Method for preparing a catalyst support and a catalyst The present invention relates to a method for preparing a catalyst support, said method comprising the following steps: - precipitating TiO2 on a carrier - treating the so obtained carrier with o-phosphoric. It also relates to a method of preparation of a catalyst using said support and to the use of said catalyst namely for the direct synthesis of hydrogen peroxide.

Description

Method for preparing a catalyst support and a catalyst
This application claims priority to EP application N°14187557.5 filed on October 2, 2014, the whole content of this application being incorporated herein by reference for all purposes The present invention relates to a method for preparing a catalyst support and a catalyst, and to the use of said catalyst namely for the direct synthesis of hydrogen peroxide.
Hydrogen peroxide is a highly important commercial product widely used as a bleaching agent in the textile or paper manufacturing industry, a disinfecting agent and basic product in the chemical industry and in the peroxide compound production reactions (sodium perborate, sodium percarbonate, metallic peroxides or percarboxyl acids), oxidation (amine oxide manufacture), epoxidation and hydroxylation (plasticizing and stabilizing agent manufacture). Commercially, the most common method to produce hydrogen peroxide is the Auto Oxidation (AO) process. In this process, hydrogen and oxygen react to form hydrogen peroxide by the alternate oxidation and reduction of alkylated anthraquinones in organic solvents. A significant disadvantage of this process is that it is costly and produces a significant amount of by-products that must be removed from the process.
One highly attractive alternative to the Auto Oxidation (AO) process (because it can be called a "green" process) is the production of hydrogen peroxide directly by reacting hydrogen and oxygen in the presence of metal catalysts supported on various oxides such as silica as a catalyst carrier.
However, in this "direct synthesis" (DS) process, when a catalyst based on silica as carrier is used, the reaction product, i.e., hydrogen peroxide is generally not efficiently produced since the production of water as a by-product is very high and even higher than the hydrogen peroxide production after a certain period of time.
Therefore, mixed catalysts were developed wherein metal oxides, sulfates and phosphates were supported (precipitated) on silica to form a carrier for an active metal generally comprising palladium: see for instance W0 2013/068243 (Zr oxide on silica), WO 2013/068340 (Nb and Ta oxides on silica) and copending application WO 2014/072169 (sulfates and phosphates of alkaline-earth metals on silica) all in the name of the Applicant. Although all these catalysts have a high selectivity and a good mechanical resistance, it has been found however that their selectivity decreases over time.
In literature, it has also been proposed to use silica impregnated with Ti oxide in order to reach high productivity in hydrogen peroxide DS.
However, in the case of the titanosilicate, the selectivity remains very low
(max. 30%). The high productivity is obtained thank to a high conversion rate.
In the case of the silica impregnated with Ti oxide, the initial selectivity is much better but is unstable and couldn't be stabilized by o-phosphoric acid addition. Moreover, the conversion rate is low, so the final H202 concentration remains below a value acceptable at industrial scale.
US 2008/0305033 relates to a catalyst effective for the direct reaction of hydrogen and oxygen to form hydrogen peroxide and which includes particles of gold and/or palladium deposited upon an acid- washed support which is preferably Si02, Ti02, A1203 or Fe203. Acid washing is preferably performed using dilute nitric acid at ambient temperature and for about 3 hours.
We have now discovered that by treating a carrier based on Ti oxide precipitated on silica with o-phosphoric acid in given conditions, the initial selectivity is lower but it remains more stable, especially when the reaction is conducted in the presence of o-phosphoric acid, while the conversion rate is increased by a factor three (roughly). The final H202 concentration is then very high (higher than 100 g/kg).
This founding was surprising because if such a treatment was indeed successful for Nb/silica based catalysts (see the Examples of WO 2013/068340), it wasn't for Zr/silica based ones while Nb and Zr belong to the same period of the periodic table which isn't the case for Nb and Ti.
Raman analysis indicated the presence of an inorganic phosphate on the carrier surface (probably titanium phosphate) and the elementary analysis confirms the presence of P after the catalyst impregnation and reduction.
Titanium phosphate being water insoluble, this could explain why said catalyst is indeed stable over time. A similar effect might be observed with sulfuric acid (Ti sulfate being also water insoluble). However, since in the presence of hydrogen peroxide, sulfuric acid generates Caro's acid, which is very corrosive, the use of sulfuric acid is preferably avoided in the frame of the present invention.
Thus, the present invention relates to a method for preparing a catalyst support comprising the following steps:
- precipitating Ti02 on a carrier comprising silica - treating the so obtained carrier with o-phosphoric in order to precipitate a phosphate on said carrier.
The terms "catalyst support" intend to designate a support for a catalytically active specie, generally a metal. According to the invention, this support comprises a carrier i.e. a solid material on which Ti02 is precipitated.
The carrier of the invention comprises SiO2 (silicon dioxide or silica) and even more preferably, it consists essentially of Si02.
In one embodiment, the carrier used in the invention has a large specific surface area of for example above 20 m /g calculated by the BET method, preferably greater than 100 m /g. Generally, the specific surface area of the carrier does not exceed 500 m /g, preferably not 300 m /g, even more preferably not 200 m2/g.
The pore volume of the carrier can be for example in the range 0.1 to 3 ml/g, preferably 0.5 to 2 ml/g.
Preferably, the pore size of the carrier is from 5 to 20 nm, more preferably from 10 to 15 nm.
Typically, the mean particle size of the support ranges from 50 μιη to a few mm, preferably from 60 to 210 μιη for slurry catalysts. In the case of fix bed catalysts, the particle size ranges preferably from 500 μιη to 5 mm.
The above given preferred ranges give good results in DS of hydrogen peroxide.
The carrier can essentially be amorphous like silica gel or can be comprised of an orderly structure of mesopores, such as MCM-41, MCM-48, SBA-15, or a crystalline structure, like a zeolite. Silica gel gives good results in DS of hydrogen peroxide.
The precipitation of Ti02 on the carrier may be accomplished by a variety of techniques known in the art. One such method involves impregnating (preferably at room temperature - atmospheric pressure) the carrier with a precursor of Ti02, optionally followed by drying. The Ti02 precursor may include any suitable Ti hydroxide, alkoxide, halide or oxyhalide.
In a preferred embodiment, the precursor of Ti02 is a Ti alkoxide, preferably Ti butoxide, used in a concentration calculated to reach the desired Ti content (for instance between 1 and 20 Wt, preferably between 2 and 15 Wt; more preferably between 5 and 12 Wt). This Ti butoxide impregnation is preferably done in 1-butanol as solvent and is preferably done under agitation (200 rpm mechanical stirring). After impregnation, the carrier is preferably dried at 90°C during 24 hours.
The precursor is converted, for example by hydrolysis followed by heat treatment, to titanium oxide, which is precipitated onto the carrier.
Generally, the heat treatment is a calcination, preferably under static air.
Calcination at about 400°C during 6 hours gives good results especially with impregnated Ti butoxide.
According to XRD (X Ray Diffraction) analysis, the so obtained Ti02 is mainly amorphous with some rays proving the presence of small amounts of anathase.
According to the invention, the carrier on which Ti02 has been
precipitated is treated with o-phosphoric acid in given conditions in order to precipitate a phosphate on it. This treatment comprises a first step comprising contacting, preferably at room temperature - atmospheric pressure - under mechanical stirring (200 rpm), the carrier with an aqueous o-phosphoric acid solution. Typically, aqueous o-phosphoric acid solutions containing between 75 and 95 Wt, typically about 85 Wt, give good results especially when using water as solvent. Typically, for 15 g carrier, 1.7 g o-phosphoric acid solution and 50 ml of water can be added.
Generally, the contact time is comprised between 3 and 240 h, preferably between 5 and 120 h, more preferably between 10 and 96 h.
According to the invention, the treatment comprises a second step which comprises evaporating at least part of the o-phosphoric acid solution in order to precipitate a phosphate on said carrier.
Generally, the temperature during the evaporation step is comprised between 20 and 200°C, preferably between 50 and 150°C and even more preferably between 70 and 130°C. It is generally performed at atmospheric pressure.
Generally, the duration of the evaporation step is comprised between 2 and 200 h, preferably between 5 and 100 h, more preferably between 7 and 72 h.
The evaporation conditions are in fact preferably chosen so that at least 25 Wt, preferably at least 50 Wt and even more preferably, at least 80 Wt of the solution is evaporated. In a preferred embodiment, substantially all the solution is evaporated and the carrier is hence substantially dry. Generally, the amount of precipitated phosphate is such that the Ti/P molar ratio is comprised between 10 and 0.1, preferably between 7.5 and 0.25, and more preferably between 5 and 0.5.
The present invention also relates to a method for preparing a catalyst, said method comprising depositing (supporting) at least one catalytically active metal selected from elements in Groups 7 to 11, on the support obtained by the method described above.
The catalytically active metal which may be used in the present invention can be selected by a person skilled in the art according to the intended use of the catalyst. In one preferred embodiment of the present invention, the catalyst comprises at least one metal selected from among the platinum group comprised of ruthenium, rhodium, palladium, osmium, iridium, platinum, or any combination of these metals. In a more preferred embodiment, the catalyst comprises palladium or a combination of palladium with another metal (for example rhodium, platinum or gold). Palladium gives good results especially in the frame of hydrogen peroxide DS.
The amount of catalytically active metal supported on the carrier can vary in a broad range, but is preferably comprised from 0.001 to 10 wt. , more preferably from 0.1 to 5 wt. % and most preferably from 0.3 to 3 wt. , each based on the total weight of the carrier.
In the catalyst according to the invention, the catalytically active metal is preferably present at least partly in reduced form. In the context of that embodiment of the present invention, a metal in reduced form means metal atoms having the oxidization level 0 or lower, such as Pd° or Pd hydride.
The deposition of the catalytically active metal onto the support can be performed using any of the known preparation techniques for supported metal catalyst, e.g. impregnation, adsorption, ionic exchange, etc. For the
impregnation, it is possible to use any kind of inorganic or organic salt of the metal to be impregnated that is soluble in the solvent used. Suitable salts are, for example, halides such as chloride, acetate, nitrate, oxalate, etc. For example, the metal can be deposited by dipping the carrier in a solution of metal halides followed by reduction. Generally, the catalysts of the invention do not require calcination to be effective, which is advantageous from an energetic point of view.
In a preferred embodiment of the invention, the support is contacted with a solution of palladium chloride. After the metal has been deposited on the support material, the product is recovered, for example by filtration. Subsequently, the metal deposited on the support is preferably (at least partially) reduced, for example by using hydrogen at high temperature. This hydrogenation step can be carried out for example at a temperature from 100°C to 300°, preferably from 120°C to 200°C for 1 to 10 hours, preferably for 2 to 6 hours. The hydrogen is preferably dilutes with an inert gas, for instance with nitrogen, typically about 5 times.
The catalysts according to the invention are suitable for catalyzing various reactions, including for example oxidation reactions as oxidation of propene, ethylene, phenol in the presence of hydrogen peroxide. Preferably the catalysts are used for catalyzing the synthesis of hydrogen peroxide, either by the AO route or by DS, in particular for catalyzing the DS of hydrogen peroxide.
Hence, in another aspect, the invention is also directed to the use of the catalyst obtained by the process described above in production of hydrogen peroxide. In the process of the invention, hydrogen and oxygen (as purified oxygen or air) are reacted continuously over the catalyst in the presence of a liquid solvent in a reactor to generate a liquid solution of hydrogen peroxide. The catalyst is used for the direct synthesis of hydrogen peroxide preferably in a three-phase system: the catalyst (solid) is put in a solvent (liquid medium comprising water or an alcohol like methanol or ethanol or acetonitrile) and the gases (H2, O2 and an inert gas) are bubbled in the suspension, preferably in presence of at least one stabilizing additive (for instance a halide and / or an inorganic acid: see below).
The above DS process for producing hydrogen peroxide may comprise reacting hydrogen and oxygen in the presence of the catalyst in a reactor. The process of this invention can be carried out in continuous, semi-continuous or discontinuous mode, by the conventional methods, for example, in a stirred tank reactor with the catalyst particles in suspension, a basket-type stirred tank reactor, a fixed-bed reactor, etc. Once the reaction has reached the desired conversion levels, the catalyst can be separated by different known processes, such as filtration if the catalyst in suspension is used, which would afford the possibility of its subsequent reuse. In the case of the stirred tank reactor, the amount of catalyst used is that necessary to obtain a concentration of 0.01 to 10 wt. % catalyst (regarding the total weight of solvent and catalyst) and preferably 0.02 to 5 wt. %. The concentration of the obtained hydrogen peroxide solution according to the invention is generally higher than 2 wt. , preferably higher than 4 wt. %, most preferably higher than 7 wt. %. In some cases, it may even exceed 10 wt. %.
In addition to their catalytic properties for the reaction of direct synthesis of the hydrogen peroxide, the catalysts of the invention are unfortunately also over-hydrogenation and decomposition catalysts of the peroxide formed. It is consequently advantageous for the liquid phase of the three-phase system in which the synthesis is carried out, to contain a compound capable of selectively poison the hydrogen peroxide decomposition and over-hydrogenation sites present on the surface of the catalyst. Halide ions are good representatives of these compounds. Their optimum concentration must be determined by means of laboratory tests within the capability of the person skilled in the art. This concentration must be sufficient in order to achieve poisoning the majority of the decomposition sites of the catalyst and, at the same time, not too high in order to avoid as much as possible the oxidation reaction of the halide ion by the hydrogen peroxide. Chloride, bromide and iodide ions are suitable to inhibit the decomposition sites of the catalyst. The bromide ion has given the best results, especially when present in a concentration between 0.05 and 3 mmol/1 of liquid phase and, preferably, between 0.1 and 2 mmol/1.
As can be seen from the Examples, it is preferable for the liquid phase to contain a small quantity of an acid (like orthophosphoric acid) with the aim of inhibiting spontaneous non-catalytic decomposition of the hydrogen peroxide. This quantity is advantageously small not only for corrosion issues but also because when the acid concentration is too high, the solubility of the gases in the liquid phase decreases and the acid can react with the hydrogen peroxide present. As suitable acids, mention may be made of nitric acid and orthophosphoric acid. Orthophosphoric acid is preferred. The catalysts of the invention allow using the inorganic acid in an amount of 0.5 M/l or less, even of 0.1 M/l or less which enables lowering the costs and corrosion problems while still providing good synthesis results.
The temperature of the reaction is normally chosen at a value of between -5 and 50 °C and, preferably, between 0 and 20 °C.
The pressure chosen is greater than atmospheric pressure and is generally between 1 and 150 bar and, preferably, between 25 and 100 bar.
The invention will now be illustrated in more detail by way of the following examples which are not intended to limit its scope but merely to describe some preferred embodiments thereof. Example 1: support preparation
Precipitation of Ti oxide on silica
In a glass reactor equipped with a mechanical stirrer and a nitrogen inlet, 50 g of silica (Pore volume = 1.2 ml/g; Surface area = 250 m2/g; Pore diameter = 14 nm; Mean Particle Size = 120 microns), 400 g 1-butanol and 37g of titanium butoxide were introduced.
They were put under stirring (200 rpm) and under nitrogen atmosphere during 3 hours at room temperature.
The alcohol was then evaporated under vacuum (80°C - 140mbar) 200 ml of demineralized water were slowly introduced using a syringe pump and the hole was left aging at room temperature overnight.
The solid was then filtered under vacuum and washed with 1 1 of demineralized water. The washed solid was dried at 90°C during 24 hours. It was calcined at 400°C during 6 hours under static air.
Precipitation of Zr oxide on silica
In a beaker of 1 L, 400cc of demineralized water were introduced.
Mechanical stirring (around 200 rpm) was started and a pH electrode was introduced in the solution.
Some drops of an aqueous solution of NH40H 25 Wt were added to reach a pH of 8.5 and the silica was also added. The suspension was then heated at 50°C.
An aqueous solution of 15g zirconium oxichloride (ZrOC12) in 25 ml of demineralized water was prepared.
With a syringe pump, the solution of ZrOC12 was slowly introduced in the suspension at 50°C. At the same time, the pH was maintained between 8.4 - 8.5 by adding drops of the solution NH40H 25%Wt.
After addition of the solution of ZrOC12, the hole was left under stirring during one hour at 50°C. The suspension was aged at room temperature during 20 minutes, then filtered under vacuum and the solid was washed with demineralized water.
The washed solid was dried one night at 100°C and then calcined 3 hours at 600°C.
Treatment of the precipitated silica with H3PO4
In a beaker, 15 g of the precipitated silica, 1.7 g of o-phosphoric acid 85% and 50ml of demineralized water were introduced. They were left under magnetic stirring (200 rpm) during 48 hours at room temperature. The suspension was heated at 95°C during 24 hours in order to evaporate the liquid, to precipitate a phosphate on the solid and to dry it.
Example 2: catalyst preparation
An aqueous solution of palladium chloride was prepared with the amount of Pd necessary in order to obtain the desired loading of the metal on the catalyst. Typically the total volume of the solution for 20 g of carrier was 24 ml.
Some drops of HC1 (from 4 to 20) were added to the suspension and the medium was heated at 70°C under magnetic stirring until all the precursor salts had been dissolved.
The solution was added to the support and well mixed until all the liquid phase has been adsorbed by the support (incipient wetness).
The catalyst was dried at 95°C for 24 hours. The Pd was reduced under influence of hydrogen, diluted with nitrogen, for a duration and at the
temperature indicated in Table 1. Pd and Au concentrations were measured by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry).
For the Pd/Au catalyst, instead of an aqueous solution of palladium chloride, an aqueous solution of Pd chloride and Au chloride (AuC13) was prepared and also acidified in order to improve the solubility of the precursors.
Example 3: direct synthesis of hydrogen peroxide
In a HC276 reactor, methanol, hydrogen bromide, o-phosphoric acid and catalyst were introduced in the amounts indicated in Table 2.
The reactor was cooled at the temperature indicated in Table 2 and the working pressure was set (by introduction of nitrogen) at the value also indicated in Table 2.
The reactor was flushed all the time of the reaction with the following mix of gases: Hydrogen (3.6% Mol) / Oxygen (55.0% Mol) / Nitrogen (41.4% Mol). The total gas flow was 2708mlN/min
When the gas phase out was stable (which was checked by GC (Gas Chromatography) on line), the mechanical stirrer was started at 1200 rpm.
GC on line analyzed every 15 minutes the gas phase out.
Liquid samples were taken to measure hydrogen peroxide and water concentration.
Hydrogen peroxide was measured by redox titration with cerium sulfate. Water was measured by Karl-Fisher titration.
The results were expressed and calculated as follows: Selectivity, % = H2O2 cone. / (H2O2 cone. + H20 cone.)
Conversion, % = H2 consumed/H2 fed
Productivity = H2O2 produced (g)/(duration of the test (h) x (Pd used in the reactor (g))
The main parameters of the catalyst preparation are summarized in Table 1 below, and the experimental conditions and results obtained are summarized in Table 2 below.
The following conclusions can be made:
Comparative Examples CI and C2:
Catalysts based on TS1, especially when tested in the presence of a halide
(Br) and o-phosphoric acid in the liquid phase, show a high productivity (236 g H2O2 / (g Pd x h)) which is totally comparable to the one obtained with the catalyst based on TiOx/silica. However, this high productivity is mainly due to a very high conversion (nearly 80% of the hydrogen used) and not to a high selectivity (only 30%). However, conversion is not an issue since the outgas can be recycled and re-enriched before being sent back to the reactor. Selectivity is the key point since the main by-product obtained is water which can't be valorized and so represents a loss.
Comparative Examples C3 and C4:
Catalysts based on titanium oxide show a low to very low selectivity. They are moreover unstable. The maximum of productivity obtained with such type of catalyst is 3 to 8 times less than what could be obtained with catalysts based on Ti/Si (Comparative Examples C6, C7 and Examples 10 and 11).
Comparative Examples C5, C8 and C9:
Zr oxide on silica treated with o-phosphoric acid (C9) shows poorer performances than the untreated one (C5). The treated catalyst shows a higher conversion rate but a lower selectivity already from the beginning. For the catalyst based on Zr oxide on silica treated with o-phosphoric acid, a slight improvement is observed in presence of 0.1M H3PO4 in liquid phase (C9 vs C8): the conversion is slightly higher and the selectivity is more stable. However, there is no real improvement when comparing the catalyst pre-treated with the acid (C9) and the catalyst untreated but used in presence of the acid (C5): they both have a similar behavior in terms of selectivity loss.
Comparative Examples C6, C7 and Examples 10 and 11:
If we compare the catalyst based on TiOx/silica (Comparative Examples
C6 & C7) with the one based on Ti phosphate /silica (Examples 10 & 11 which are according to the invention), we see that the catalyst of the present invention show a more stable selectivity.
The catalyst based on Ti Ox/silica doesn't show any significant
improvement when it is used in presence of 0,1M o-phosphoric acid (C7 vs C6). The H202 concentrations obtained are roughly equivalent. There is a slight improvement of the stability of the selectivity but the final value remains in the same range as without any acid addition.
The situation is totally different for the catalyst of the invention (11 vs 10).
The addition of H3P04 (0,1M) leads to an improvement of both activity
(improvement of the final concentration of roughly 10%) and stability of the selectivity (nearly the same during the entire test).
We can conclude therefrom that the carrier treatment with o-phosphoric acid before the Pd impregnation is more than an acid absorption as the result is sensitively different. The o-phosphoric acid is not only impregnated on the surface but interacts with the carrier (chemically bonded) and enhances the surface acidity. The proof is that the catalyst even without addition of free o- phosphoric acid is more active (generates a higher H202 concentration) than the catalyst based on TiOx/silica used with acidity.
The addition of o-phosphoric acid is still necessary to enhance the selectivity stability of such type of catalyst. We suppose that the catalyst based on Ti phosphate/silica doesn't express enough Bronsted acidity at its surface to avoid the addition of free acid in liquid phase as it is the case for Nb
phosphate/silica based catalyst (see WO 2013/068340). It is described in literature that this compound (Nb phosphate) has a very high Bronsted acidity, higher than the one of niobic acid. This is not the case for the Ti phosphate based compounds. They are described as efficient catalysts for oxidation reaction but they are not described as solids developing Bronsted acidity.
However, the effect of the free acid in liquid phase is magnified in the case of the catalyst based on Ti phosphate/silica: the selectivity is high and more stable than for the catalyst based on TiOx/silica in presence of free acid. We assume that there is a synergetic effect between the phosphate precipitated on the surface of the catalyst and the acid in liquid phase, but we have no explanation for this phenomenon.
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.
Table 1
Figure imgf000014_0001
Table 2
Figure imgf000015_0001

Claims

C L A I M S
1. A method for preparing a catalyst support, said method comprising the following steps:
- precipitating Ti02 on a carrier comprising SiO2
- treating the so obtained carrier with o-phosphoric in order to precipitate a phosphate on said carrier.
2. The method according to claim 1, wherein the carrier is SiO2.
3. The method according to claim 1 or 2, wherein the carrier has a specific surface area of above 20 m /g calculated by the BET method, but which does not exceed 500 m2/g.
4. The method according to any of the preceding claims, wherein the pore volume of the carrier is in the range of 0.1 to 3 ml/g.
5. The method according to any of the preceding claims, wherein the pore size of the carrier is from 5 to 20 nm.
6. The method according to any of the preceding claims, wherein the precipitation of Ti02 on the carrier is accomplished by impregnating the carrier with a precursor of Ti02, optionally followed by drying, and then converting this precursor, for example by hydrolysis followed by heat treatment, to titanium oxide.
7. The method according to any of the preceding claims, wherein the treatment with o-phosphoric in order to precipitate a phosphate on the carrier comprises a first step comprising contacting the carrier with an aqueous o- phosphoric acid solution.
8. The method according to the preceding claim, wherein the treatment with o-phosphoric in order to precipitate a phosphate on the carrier comprises a second step which comprises evaporating at least part of the o-phosphoric acid aqueous solution.
9. The method according to the preceding claim, wherein the temperature during the evaporation step is comprised between 20 and 200°C.
10. The method according to claim 8 or 9, wherein the duration of the evaporation step is comprised between 2 and 200 h.
11. A method for preparing a catalyst, said method comprising supporting at least one catalytically active metal selected from elements in Groups 7 to 11, on a support obtained by a method according to any of the preceding claims.
12. The method according to the preceding claim, wherein the catalytically active metal is palladium.
13. The method according to claim 11 or 12, wherein the amount of catalytically active metal supported on the carrier is from 0.001 to 10 wt. , based on the total weight of the carrier.
14. The method according to any of claims 11 to 13, wherein after the metal has been deposited on the carrier, the product is recovered, washed and dried and subsequently, the metal deposited on the support is (at least partially) reduced.
15. Use of a catalyst obtained by a method according to any of claims 11 to 14, for catalyzing oxidation reactions in the presence of hydrogen peroxide, or for catalyzing the synthesis of hydrogen peroxide, either by the auto-oxidation route or by direct synthesis.
16. Use according to the preceding claim, wherein the catalyst is used for the direct synthesis of hydrogen peroxide.
17. Use according to the preceding claim, wherein said synthesis is realized in a three-phase system wherein the catalyst (solid) is put in a solvent (liquid medium comprising water or an alcohol like methanol) and gases (H2, O2 and an inert gas) are bubbled in the so obtained suspension, preferably in presence of at least one stabilizing additive.
18. Use according to the preceding claim, wherein the stabilizing additive comprises a halide, preferably bromide.
19. Use according to claim 17 or 18, wherein the stabilizing additive comprises an inorganic acid, preferably o-phosphoric acid.
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