Method of producing hydrogen peroxide

The invention relates to a method of producing hydrogen peroxide by direct reaction between hydrogen and oxygen in an aqueous reaction medium, in which method hydrogen and oxygen are contacted with a catalyst suspended into the reaction medium, and the catalyst is a catalytically active surface material deposited on a support of porous silica, alumina or non-fluorinated carbon having a BET surface less than about 150 m.sup.2 /g, wherein pores with a diameter exceeding about 10 nm constitute more than about 50% of the total pore volume. The invention also relates to a catalyst useful in the claimed method.

The present invention relates to a method of producing hydrogen peroxide by 
direct reaction of hydrogen and oxygen in an aqueous medium in the 
presence of a catalyst. The invention also relates to a catalyst suitable 
for use in such a method. 
Production of hydrogen peroxide by direct reaction between hydrogen and 
oxygen can be performed by contacting hydrogen and oxygen with a catalyst 
in an aqueous reaction medium as described in, for example, the U.S. Pat. 
Nos. 4,681,751, 4,772,458, and 5,128,114, and EP 627381. 
However, it is hard to obtain high concentrations of hydrogen peroxide 
which is assumed to be due to the fact that the same catalyst that 
promotes formation of hydrogen peroxide also catalyses its decomposition 
into water and oxygen. 
In order to obtain a selective catalyst the U.S. Pat. No. 5,338,531 and 
Chuang et al, "Selective Oxidation of Hydrogen to Hydrogen Peroxide", 
Studies in Surface Science and Catsysis, Vol. 72, pp 33-41 disclose use of 
a palladium catalyst on support of fluorinated carbon. Such supports are, 
however, comparatively expensive and may also cause problems with foaming, 
cladding and formation of dry deposits in the reactor. 
It is an object of the present invention to solve the problem of providing 
a process of producing hydrogen peroxide directly from hydrogen and oxygen 
with high selectivity, thus enabling preparation of aqueous solutions of 
hydrogen peroxide at high concentrations. It is another object of the 
invention to provide a selective catalyst suitable for such a process. 
In catalytic processes it is generally considered favourable to use a 
catalyst with a large active surface which can be provided by depositing 
the active material on a porous support of, for example, adsorbent carbon. 
However, it has now surprisingly been found that the selectivity of a 
catalyst for preparation of hydrogen peroxide can be improved if the 
active material is deposited on a porous support which is hydrophilic, has 
a comparatively small surface area and mainly having pores with a 
comparatively large diameter. 
Thus, the present invention relates to a method of producing hydrogen 
peroxide by direct reaction between hydrogen and oxygen in an aqueous 
reaction medium. The hydrogen and the oxygen are contacted with a catalyst 
suspended in the reaction medium, the catalyst comprising a catalytically 
active material deposited on a support of porous silica, alumina or 
non-fluorinated carbon having a BET surface less than about 150 m.sup.2 
/g, wherein pores with a diameter exceeding about 10 nm constitutes more 
than about 50% of the total pore volume. 
The invention also relates to a catalyst suitable for use in such a 
process, which catalyst comprises a catalytically active material 
deposited on a support of porous silica, alumina or non-fluorinated carbon 
having a BET surface less than about 150 m.sup.2 /g, wherein pores with a 
diameter exceeding about 10 nm constitutes more than about 50% of the 
total pore volume. 
The most preferred support material is non-fluorinated carbon, particularly 
carbon black. 
The support material is preferably not hydrophobic and it is particularly 
preferred if the catalyst is in the form of particles sinking in water. 
The BET surface area of catalyst support is preferably less than about 120 
m.sup.2 /g, most preferably less than about 100 m.sup.2 /g, but does 
preferably exceed about 10 m.sup.2 /g, most preferably about 20 m.sup.2 
/g. Preferably pores with a diameter exceeding about 10 nm, most 
preferably exceeding about 20 nm constitutes more than about 50%, most 
preferably about 80% of the total pore volume. 
Without being bound to any specific theory, it is assumed that low specific 
surface area and a low amount of small pores minimizes accumulation of 
hydrogen peroxide in the catalyst where there is a deficiency of hydrogen 
which inevitably leads to its decomposition to form water. Such 
accumulation may also lead to dissolution of the catalytically active 
material. Further, it is assumed that carbon as such promotes 
decomposition of hydrogen peroxide, which decomposition can be minimized 
by using a support with a small specific surface. 
In catalytic processes it is generally considered favourable if the active 
material is deposited on the support in the form of as small particles as 
possible, thus maximizing the active surface of the catalyst. However, 
according to the present invention it has been found tat fairly large 
particles of the active material does not result in any substantial loss 
in productivity, but may involve higher stability of the catalyst. The 
particle size can be expressed as the surface area of the active material 
which suitably is from about 25 to about 500 m.sup.2 /g active material, 
preferably from about 30 to about 100 m.sup.2 /g active material. 
The catalyst is preferably in the form of particles with an average 
diameter from about 1 to about 100 .mu.m, most preferably from about 5 to 
about 50 .mu.m or from about 20 to about 50 .mu.m. 
Preferably the catalyst contains from about 0.1 to about 10% by weight, 
most preferably from about 0.3 to about 8% by weight of the catalytically 
active material. The catalyst can be prepared by impregnating a support 
with a solution or a colloid of the active material as described in, for 
example, U.S. Pat. No. 5,338,531. 
The catalytically active material suitably comprises one or more precious 
metals, preferably selected from group VIII metals or gold, most 
preferably palladium, platinum or mixtures thereof. Most preferably the 
active material is a mixture of from about 90 to 100% by weight of 
palladium and from 0 to about 10% by weight of platinum. 
The aqueous reaction medium is suitably acidic and does preferably contain 
from about 0.01 to about 1 moles/liter of free hydrogen ions, most 
preferably from about 0.02 to about 0.2 moles/liter of free hydrogen ions. 
The acid may for example be supplied in the form of sulfuric acid, 
phosphorous acid or perchloric acid which preferably is present in an 
amount from about 0.01 to about 1 mole/liter, most preferably from about 
0.02 to about 0.2 moles/liter. Further, the reaction medium also suitably 
contains one or several halide ions such as bromide, chloride, or iodide, 
of which bromide being particularly preferred. The halogenide is 
preferably present in an amount from about 1 to about 1000 ppm by weight, 
most preferably from about 2 to about 100 ppm by weight, and may be 
supplied in the form of alkali metal salts such as sodium, potassium or 
mixtures thereof or as the corresponding acids. 
The process is suitably carried out by continuously feeding hydrogen and 
oxygen in gas form to a pressurised reaction vessel containing a slurry of 
catalyst particles in the reaction medium. The oxygen may be supplied as 
substantially pure gas or in the form of an oxygen containing gas such as 
air. The gas phase in the reactor suitably contains an excess of oxygen, 
preferably from 0 to about 75 mol % or from 0 to about 25 mol %. The 
reaction is favoured by a high content of hydrogen, suitably above about 
0.1 mol %, preferably above about 1 mol %, but for safety reasons it is 
preferred not to exceed the detonation limit at about 19 mol % and most 
preferred not to exceed the explosion limit at about 5 mol %. The pressure 
is suitably maintained from about 10 to about 200 bars, preferably from 
about 30 to about 100 bars, while the temperature suitably is maintained 
from about 0 to about 100.degree. C., preferably from about 20 to about 
70.degree. C. In order to achieve sufficient mass transport it is 
preferred that the reaction medium is agitated or pumped around, or that 
the gas is injected in the bottom of the reaction vessel. The hydrogen 
peroxide formed dissolves in the reaction medium which continuously is 
withdrawn from the reaction vessel through a filter on which the catalyst 
is retained. The hydrogen peroxide can be separated from the reaction 
medium with conventional unit operations such as evaporation, distillation 
or combinations thereof. The reaction medium can then be recycled to the 
reaction vessel, optionally after addition of make up chemicals such as 
sulfuric acid, alkali metal bromide etc. 
An embodiment of the invention is further described through the following 
Example, which however not should be interpreted as limiting the scope of 
the invention. If not otherwise stated all contents and percentages refer 
to parts or percent by weight.

EXAMPLE 
Catalysts according to the invention containing 3% by weight of Pd were 
prepared by impregnating carbon black support particles (Elftex 465 and 
Black Pearl 3700, both from Cabot Corporation) with a Pd containing 
citrate colloid and reducing the Pd with hydrogen as described in U.S. 
Pat. No. 5,338,531. A commercial catalyst containing 5.2% by weight of Pd 
on carbon (Johnson Matthey, Type 39, batch 19) was used as a reference. 
The kinetics and the selectivity for the catalysts were compared by 
preparing hydrogen peroxide in an autoclave with 40 ml aqueous reaction 
medium containing catalyst particles in an amount corresponding to 0.09 g 
Pd per liter solution, which medium was agitated at 1700 rpm. The reaction 
medium also contained 1% by weight sulfuric acid and 5 ppm NaBr. Hydrogen 
and oxygen were fed to keep a pressure of 97 bars and a hydrogen 
concentration of about 3% by volume in the autoclave head space. The 
temperature was 35.degree. C. Each catalyst test was performed as a number 
of batchruns with batch tines ranging between 3 and 16 hours. After each 
batch run, the catalyst was separated from the formed peroxide by 
filtration and returned to the autoclave for reuse without make up of 
fresh catalyst. 
The total weight increase and the final hydrogen peroxide concentration 
were determined and the selectivity was calculated according to the 
formula: 
EQU % selectivity=n.sub.p /(n.sub.p +n.sub.w) 
where n.sub.p and n.sub.w represent formed moles of hydrogen peroxide and 
water respectively. 
For the Cabot carriers, the size of the palladium crystallites were 
determined using X-ray diffraction and transmission electron microscopy. 
All other data were determined from the suppliers. 
The results appear in the table below: 
______________________________________ 
Black Johnson Matthey 
Pearl 5.2 wt % Pd, 
Support Elftex 465 
3700 Type 39/Batch 19* 
______________________________________ 
BET area of support (m.sup.2 /g) 
84 43 950 
% pore vol. &gt; 10 (nm) 
50 98 66 
Equiv. Pd diam. (nm) 
8-15 8-15 1-2 
Pd surface (m.sup.2 /liter soln.) 
3-6 3-6 22-45 
Avg. selectivity (%) 
69 73 64 
Avg. production (g H.sub.2 O.sub.2 /l, h 
7.4 8.9 8.0 
Avg. final conc. H.sub.2 O.sub.2 (wt %) 
7.6 8.5 6.9 
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*Comparative example 
The results clearly show that decreasing BET area promotes the selectivity 
despite higher final peroxide concentration. It is also surprisingly 
found, that the activity in terms of produced hydrogen peroxide per liter 
and hour is not proportional to the total palladium surface exposed.