Process for the direct synthesis of hydrogen peroxide

Details Process for the direct synthesis of hydrogen peroxide Process for the direct synthesis of hydrogen peroxide

2001-06-01

2003-10-07

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Peroxide

Reexamination Certificate

active

06630118

ABSTRACT:

The present invention relates to a bimetallic catalyst and a process for the direct synthesis of hydrogen peroxide from hydrogen and oxygen which uses said catalyst.
Hydrogen peroxide is a commercially important compound which is widely used as bleach in the textile and paper industry, as a biocide in the environmental field and in oxidation processes in the chemical industry.
Examples of these processes are those using titanium silicalite as catalysts, such as the epoxidation of olefins (EP-100,119), the ammoximation of carbonyl compounds (U.S. Pat. No. 4,794,198), the oxidation of ammonia to hydroxylamine (U.S. Pat. No. 5,320,819) and the hydroxylation of aromatic hydrocarbons (U.S. Pat. No. 4,369,783).
The industrial production of aqueous solutions of H
2
O
2
by means of a complex two-step process, is known. In this process a solution of an anthraquinone, such as butylanthraquinone or ethylanthraquinone, in an organic medium immiscible with water is first hydrogenated and then oxidized with air to produce H
2
O
2
which is subsequently extracted in aqueous phase.
This procedure is expensive owing to the high investment costs necessary for the complex production unit involved and the necessity of separating and disposing of the by-products generated during the oxidation phase and purifying and reintegrating the anthraquinone solution before being re-used.
Processes for the direct synthesis of hydrogen peroxide from H
2
and O
2
have been proposed in the art for over-coming these disadvantages. These processes generally use a catalytic system consisting of a noble metal, particularly metals of the platinum group or their mixtures, in the form of salts or as supported metals.
For example, U.S. Pat. Nos. 4,772,458 and 4,832,938 describe a process for the synthesis of aqueous solutions of H
2
O
2
using a catalyst based on palladium and/or platinum supported on carbon, in the presence of hydrogen ions and bromide ions. Quantities of acids ranging from 2.5 to 10 g/liter are generally required for obtaining high concentrations of H
2
O
2
.
The use of high quantities of acids creates serious problems linked to the dissolution of the active phase (metal) of the catalyst in the reaction medium, with the consequent instability of both the catalyst and hydrogen peroxide solution produced.
In addition, under these conditions, the solutions of H
2
O
2
produced are difficult to use owing to the high acid content. These processes also operate under critical conditions as concentrations of H
2
higher than 5% (17% or over) are used with respect to the reaction mixture and consequently fall within the explosivity limit of H
2
/O
2
mixtures.
Patent application EP-492.064 describes a process for the synthesis of hydrogen peroxide from hydrogen and oxygen which uses a catalyst based on palladium supported on a halogenated resin, in particular a brominated styrene/divinylbenzene resin.
The reaction is carried out in water, in the presence of an acid selected from sulfuric, phosphoric or nitric acid. Operating according to this process however concentrations of H
2
O
2
of about 0.58% are obtained.
Patent application EP-504,741 describes a process for the synthesis of H
2
O
2
from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on an acid or superacid carrier selected from oxides of molybdenum, zirconium or tungsten.
Operating according to this process, hydrogen peroxide is obtained in concentrations of not more than 1%.
U.S. Pat. No. 5,320,921 describes a process for the synthesis of H
2
O
2
from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on a heteropolyacid made insoluble in water. The reaction is carried out in water in the presence of bromide ions (0.5 mmoles/liter of sodium bromide). Concentrations of H
2
O
2
equal to about 1.1% are obtained.
These processes of the known art are consequently characterized by a low productivity and selectivity of the reaction and by the production of H
2
O
2
solutions which are too dilute for an economic industrial exploitation.
Italian patent application MI 98-A-01843, filed by the Applicant, relates to a process for the direct synthesis of hydrogen peroxide using a catalyst based on palladium and/or platinum co-impregnated on a sulfonated activated carbon.
It has now been found that by carrying out the reaction between hydrogen and oxygen in the presence of a bimetallic catalyst prepared by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, it is possible to overcome the drawbacks of the known art described above. This preparation method also allows the activity and selectivity of the catalyst, the concentration of hydrogen peroxide or consumption of hydrogen, to be modulated in relation to the process requirements and availability of raw materials, for example low cost hydrogen.
In particular, the use of these catalysts gives the following advantages:
(i) the possibility of using extremely low quantities of halides (<10
−4
moles/l) and free acids (H
+
<10
−2
moles/l) in the reaction medium. This has beneficial effects on the stability of the catalytic system and H
2
O
2
solutions obtained, thus allowing the possibility of directly using the above solutions in oxidation processes. The presence of only traces of halides and acidity is not a disadvantage for direct use in the above reactions as the possible introduction of salts or acidity in downstream processes is minimized.
(ii) the production of hydrogen peroxide solutions in adequate concentrations for direct use and economically valid in oxidation processes, generally ranging from 2% to 10% by weight.
(iii) the possibility of carrying out the reaction under high safety conditions. In fact, below 4% by volume of hydrogen means operating well outside the explosivity range of inert H
2
—O
2
mixtures.
In accordance with this, the present invention relates to a bimetallic catalyst obtained by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, and a process for the synthesis of hydrogen peroxide by the direct reaction of hydrogen with oxygen, in a solvent medium containing a halogenated promoter and an acid promoter, in the presence of said catalyst.
In particular, the catalyst used for the purposes of the present invention is obtained by:
(a) preparation of the solutions or suspensions of precursors of the single metal components of the catalytic system;
(b) dispersion of the solutions or suspensions obtained in (a) in sequence on a carrier;
(c) treatment of the catalyst with a reducing agent of the metal and drying at 120-140° C. between one dispersion and another.
In the preparation of said catalyst, steps (b) and (c) can be repeated once or several times.
The metal components of the catalyst are selected from those of the platinum group. Palladium and platinum are preferably used.
In these catalysts the palladium is normally present in a quantity ranging from 0.01 to 5% by weight and the platinum in a quantity ranging from 0.01 to 1% by weight, with an atomic ratio between platinum and palladium ranging from 0.1/99.9 to 50/50.
The palladium is preferably present in a quantity ranging from 0.4 to 2% by weight and the platinum in a quantity ranging from 0.05 to 0.5% by weight, with an atomic ratio between platinum and palladium ranging from 1/99 to 30/70.
The dispersion of the active components on the carrier can be effected by means of precipitation, impregnation or adsorption starting from solutions of their salts or soluble complexes selected from acetates, halides and nitrates.
The reduction of the catalyst components to the metal state can be carried out by means of thermal and/or chemical treatment with reducing substances such as hydrogen, sodium formiate, sodium citrate, using preparative methods well known in the art.
The inert carrier can consist of activated carbon, silica, alumina, silica-alumina, zeolites and other materials well known in the art. Activated ca

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