Chemistry of inorganic compounds – Oxygen or compound thereof – Peroxide
Reexamination Certificate
2000-03-20
2002-05-14
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Peroxide
Reexamination Certificate
active
06387346
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on German Application DE 199 12 733.6, filed Mar. 20, 1999, which disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to a process for producing hydrogen peroxide by direct synthesis. In the process hydrogen and oxygen are converted in the presence of a heterogeneous catalyst containing at least one noble metal, in particular palladium, as its catalytically active component, in the presence or absence of a solvent. Use of a catalyst according to the invention provides advantages over the use of previously known catalysts with regard to H
2
selectivity and/or the attainable H
2
O
2
concentration.
BACKGROUND OF THE INVENTION
The direct synthesis of hydrogen peroxide by converting hydrogen and oxygen in an acidic aqueous medium in the presence of a noble metal supported catalyst is known from various references, for example, European Patent Disclosure EP-B 0 272 830. In the process described in this reference and also in the processes referred to therein, an aqueous reaction medium is used which, for the sake of inhibiting the decomposition of hydrogen peroxide formed contains a strong acid, such as H
2
SO
4
and/or HCI. Pd on activated charcoal, or other heterogeneous catalysts containing Pd and/or Pt, catalyze the composition. By adding a bromide promoter, the selectivity is increased. This process has various problems, among them selectivity that is too low and/or too low an attainable H
2
O
2
concentration, and/or a low space-time yield, and sometimes also a high catalyst discharge; as a result, the industrial expense required for recovering the catalyst increases, and the H
2
selectivity drops as the service life of the catalyst increases.
Many references are accordingly directed to furnishing improved catalysts for this process. In the process of European Patent Disclosure EP-A 0 366 419, a gas mixture containing H
2
and O
2
is passed through a catalyst bed disposed in a trickle-bed reactor, while at the same time an aqueous phase containing H
2
SO
4
and HCI is trickled in parallel flow over the catalyst bed. Although high selectivity is attained in this process by using a noble metal catalyst bonded to a hydrophobic support, under the usual pressure and temperature conditions, nevertheless the high selectivity is at the disadvantageous cost of a very low H
2
O
2
concentration (0.15 to 0.3%). To obtain commercial H
2
O
2
solutions, complicated concentration and/or distillation steps must follow, which thus reduce the economy of the process.
In the process according to Japanese Patent Disclosure JP-A 7-241473, with a conventional reaction sequence, hydrogen and oxygen are reacted in an acidic aqueous medium in the presence of catalyst particles with a particle size below 50 nm that are applied to hydrophobic support particles. The catalyst particles are gold particles, while the hydrophobic support particles are hydrophobic or hydrophobized organic and inorganic substances. The production of the hydrophobic support that contains gold particles includes precipitation of a basic gold salt onto the hydrophobic vehicle from an aqueous solution that contains HAuCI
4
and a reduction and/or calcination step for converting the gold compound to elemental gold. In a trickle-bed reactor, a 4.8 wt. % aqueous hydrogen peroxide solution is obtained with 80% selectivity.
A further trickle-bed process is taught by EP-A 0 579 109. Here an aqueous phase in parallel flow with the gas mixture containing the H
2
and O
2
is trickled over a catalyst bed based on a noble metal that is bonded to either activated charcoal, an inorganic oxide, or a sulfate. It is essential to adhere to a certain volume ratio of the gas phase to the liquid phase. With good selectivity for H
2
, an aqueous hydrogen peroxide solution with a content of approximately 5 wt. % is obtained. An increase in the H
2
O
2
concentration is successfully achieved in the process described in German Patent Application DE 198 16 297.9, in which a gas mixture containing hydrogen and oxygen, which is substantially saturated or supersaturated with water vapor, is delivered to the reactor, and the aqueous phase is only then formed by condensation. In this process, known catalysts are used, in particular those having one or more elements of Group VIII and/or Group I of the periodic system. The catalytically active elements are typically bonded to a particulate or monolithic support.
In the postprocessing of the example in EP-A 0 579 109, it has been demonstrated that if a catalyst based on 2 wt. % Pd on activated charcoal is used with the aqueous hydrogen peroxide, a high palladium discharge is brought about, which shortens the service life of the catalyst.
In German Patent Disclosure DE-A41 27 918, a palladium/gold alloy with a content of 5-95 wt. % gold on a solid support such as carbon or aluminum oxide is used as the catalyst for the described reaction, in order to obtain a high formation speed, high yield and high selectivity. The production of the catalyst requires a two-stage process. As has been demonstrated, in this case also there is a need to increase the H
2
selectivity and the H
2
O
2
concentration.
SUMMARY OF THE INVENTION
The object of the present invention is accordingly to provide an improved process of this general type that leads to higher H
2
selectivity and/or to higher H
2
O
2
concentration. In a preferred embodiment, ways are also to be shown in which the noble metal discharge from the reactor can be reduced and the aqueous hydrogen peroxide and thus the service life of the catalyst can be increased.
The invention comprises a process for producing hydrogen peroxide by direct synthesis, in which hydrogen and oxygen are converted in the presence of a heterogeneous catalyst containing at least one noble metal, in particular palladium, as its catalytically active component, in the presence or absence of a solvent. The catalyst used has a catalytically active component that substantially comprises palladium or at least two metals selected from the platinum group (Group VIII) and Group I of the Periodic Table of Elements and which has been produced by spray pyrolysis or flame pyrolysis, including the conversion of a solution or suspension, containing compounds of the one or more catalytically active metals in a suitable elemental ratio in dissolved form, into a gas-supported particle collective. introduction thereof into a spray pyrolysis or flame pyrolysis reactor, the compounds of the catalytically active metals being converted substantially into these metals or alloys thereof, and separation of the solid particles formed from the gas stream.
The process according to the invention can be performed using the catalytically active component, that is, the palladium or noble metal alloy obtained by spray or flame pyrolysis, or a combination of catalytically active components. As an alternative, and generally preferably, these catalytically active metals or alloys are used in supported form. The supports can be particulate materials, such as powders, extrudates, granulates, or other molded bodies made from a powdered material. For producing the supported catalysts that contain the catalytically active component or components, known processes can be employed. For instance, it is possible for the powder of the catalytically active component produced by spray pyrolysis or preferably by flame pyrolysis to be mixed with a powdered support material, plasticized, and deformed, and then for the molded bodies to be solidified by calcination. It is also possible for the metal powder to be mixed with a support or carrier and made into tablets. It is additionally possible for the powdered catalytic component or components to be dispersed in a suitable liquid and impregnated into an already prefabricated molded support. Then, depending on the type of procedure employed, either a catalyst largely saturated with metal or a so-called eggshell catalyst, in which the metal is located only in the outermost zone of the support
Bertsch-Frank Birgit
Hemme Ina
Katusic Stipan
Rollmann Jürgen
Von Hoppel Lukas
Degussa-Huls AG
Langel Wayne A.
Pillsbury & Winthrop LLP
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