Process for the manufacture of hydrogen peroxide

Details Process for the manufacture of hydrogen peroxide Process for the manufacture of hydrogen peroxide

1999-04-15

2002-04-23

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Peroxide

Reexamination Certificate

active

06375920

ABSTRACT:

DESCRIPTION
The present invention relates to a process for preparing a hydrogen peroxide solution having a hydrogen peroxide content of not less than 2.5% by weight by continuous reaction of hydrogen and oxygen over a catalyst comprising palladium as active component.
Customary industrial processes for preparing hydrogen peroxide are the electrolysis of acidic ammonium sulfate solutions, the oxidation of isopropyl alcohol and the anthraquinone process. The direct synthesis of hydrogen peroxide from the elements over transition metal catalysts is known, but has not found commercial use to date.
There are several reasons for this. For instance, hydrogen and oxygen form explosive gas mixtures if the level of hydrogen in the gas mixture is above 5% by volume. On the other hand, the rate of formation of hydrogen peroxide on using hydrogen-oxygen mixtures outside the explosive range is generally too low to ensure reasonable space-time yields. In addition, an excessively high level of oxygen in the reaction gas can speed up the oxidative degradation of the catalysts.
The selectivity of the reaction is a further problem. For instance, the reaction to form the hydrogen peroxide has to compete with that to form water. In addition, the catalysts which are suitable for forming hydrogen peroxide from the elements also catalyze the degradation reaction of hydrogen peroxide in the presence of excess hydrogen in accordance with the following reaction equation: H
2
O
2
+H
2
→2H
2
O. This selectivity problem can be solved by using continuous processes at high flow rates. However, this has the consequence that, at a low reaction rate, the hydrogen peroxide concentrations obtained in the effluent become too low for commercial use of this process. In addition, excessively high flow rates give rise to catalyst abrasion problems, leading to reduced on-stream times and hence likewise to commercial disadvantages for this process.
U.S. Pat. No. 4,009,252 discloses an optimum ratio of O
2
to H
2
within the range from 1.5:1 to 20:1, i.e. in the explosive range, for the formation of hydrogen peroxide from hydrogen and oxygen over palladium catalysts. The reaction is carried out as a batch process and the space-time yields which are obtained are unsatisfactory.
WO 92/04277 describes the reaction of hydrogen with oxygen in a tubular reactor packed with an aqueous suspension of catalyst. The gas mixtures used are preferably located within the explosive range. A high flow rate of the reaction medium (>1 m/sec) is used to ensure that the reaction gas is completely dispersed in the reaction medium in the form of small bubbles, preventing an explosive reaction of the reaction gas. The hydrogen peroxide concentrations obtained after a single pass through the reaction zone are below 1% by weight. Higher yields can only be obtained by repeated passes through the reaction zone. The fact that the catalyst is used in the form of a suspension turns out to be a problem because this necessitates elaborate filtration and recycling measures, with inevitable losses of the catalyst. Reaction tubes which are suitable for the required reaction pressure—pressures of above 80 bar are disclosed in the illustrative embodiments—are comparatively costly. The cited process must therefore be considered very costly. A similar process with improved fluidization of the hydrogen and oxygen streams is described in WO 96/05138.
U.S. Pat. No. 5,500,202 and EP-A 579 109 describe a continuous process for preparing hydrogen peroxide by reacting H
2
/O
2
gas mixtures over a stationary pulverulent catalyst (particle sizes within the range 10 &mgr;m to 250 &mgr;m) in a trickle bed reactor. To reduce the risk of explosion due to the large gas volume which is customary for a trickle bed reactor, nitrogen is added as an inert gas to the reaction gas. However, this entails additional costs. The aqueous hydrogen peroxide solutions obtained in this way only have concentrations within the range from 3 to 5%. The conversion based on hydrogen is merely within the range of 25-35%. No data are provided about catalyst life. Reactors having a maximum internal diameter of 30 mm are recommended because of the considerable heat production. Industrial use would therefore require the installation of several thousands of these tubular reactors, which entails high capital expenditure costs.
U.S. Pat. No. 4,336,238 and U.S. Pat. No. 4,336,239 describe the reaction of hydrogen and oxygen to form hydrogen peroxide over palladium catalysts in organic solvents or solvent mixtures in the presence or absence of water. The process described makes it possible to reduce the proportion of hydrogen in the reaction gas, but the hydrogen peroxide concentrations of more than 2.4% by weight which are obtained on using reaction gas mixtures comprising less than 5% by volume of hydrogen are too low for commercial application. The use of organic solvents has an advantageous effect on catalyst life. However, over a run of 285 hours catalyst activity decreases to 69% of the original value, which is still too low for industrial application. U.S. Pat. No. 4,389,390 describes a similar process in which the catalyst which has become detached from the support is recovered by activated carbon filters. A further advantage of this process is that removal of the catalyst from the reaction medium reduces the tendency of the hydrogen peroxide to decompose. However, no hydrogen hydrogen peroxide solutions having a concentration of above 2.1% by weight are obtained in continuous operation.
The literature proposes various solutions for the problem of catalyst deactivation in the preparation of hydrogen peroxide from the elements. For instance, U.S. Pat. No. 5,352,645 and WO 92/04976 describe special solid supports composed of spray-dried, colloidal silica gel. The use of superacidic oxides as support materials is proposed in U.S. Pat. No. 5,236,692 and EP-A 437 836. This avoids the customary acid content in the reaction medium. EP-A 627 381 teaches the use of niobium, tantalum, molybdenum or tungsten oxides as support materials which are characterized by high acid resistance. U.S. Pat. No. 5,292,496 describes the use of cerium-comprising support materials to avoid the use of halogen as a stabilizer in the reaction medium. However, in the references cited, hydrogen peroxide is always prepared by batch or semicontinuous processes which have little to recommend them for industrial use. In addition, the short reaction times do not provide any information about catalyst life.
The use of catalyst monoliths comprising palladium as active component is described by Kosak in Catalysis of Organic Reactions (eds. Scaros and Prunier), Marcel Dekker Inc., New York 1995, p. 115 et seq., for the preparation of hydrogen peroxide from the elements. The reaction is carried out batchwise in an aqueous reaction medium at a comparatively high pressure of 144 bar and a molar ratio of O
2
:H
2
=4.7, i.e. in the explosive range.
It is an object of the present invention to provide a process for preparing hydrogen peroxide from hydrogen and oxygen to prepare hydrogen peroxide solutions having a hydrogen peroxide content of above 2.5% by weight even from hydrogen-oxygen mixtures outside the explosive range (O
2
:H
2
>20:1). The catalysts used shall have long on-stream times.
We have found that this object is achieved by a continuous process comprising the reaction of hydrogen and oxygen in water and/or C
1
-C
3
-alkanols as reaction medium over catalytic structures comprising palladium as active component.
The present invention accordingly provides a process for preparing a hydrogen peroxide solution having a hydrogen peroxide content of not less than 2.5% by weight by continuous reaction of hydrogen and oxygen over a catalyst comprising palladium as active component, which comprises performing the reaction in water and/or a C
1
-C
3
-alkanol as reaction medium over a catalytic structure.
Catalytic structures are catalysts in which the catalytically active component is situated on the surface of specif

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