Process for the electrochemical preparation of hydrogen peroxide

#The present invention provides a process for the electrochemical preparation of hydrogen peroxide in an electrolysis cell the structure of which is substantially analogous to that of a fuel cell.

#It is known that hydrogen peroxide can be prepared by the so-called anthraquinone cyclic process. This large-scale industrially applied process has the disadvantages, on the one hand, that the hydrogen peroxide produced has to be concentrated to a concentration in the region of mostly 50 to 70 wt. % and, on the other hand, that the transport of such solutions to the site of use is costly. Since in any case, in many fields of use of hydrogen peroxide, only dilute solutions are used, there is a keen interest in preparing hydrogen peroxide on site. At the same time, in many applications there is an interest in preparing hydrogen peroxide on demand and using it immediately, without the need for a device to store highly concentrated hydrogen peroxide.

#In order to make hydrogen peroxide available on site and on demand, electrochemical processes are receiving renewed interest. Most of the currently known processes are based on the cathodic reduction of oxygen and the use of an alkali metal hydroxide as the electrolyte. Alkaline hydrogen peroxide solutions are obtained, wherein the molar ratio of, for example, NaOH to hydrogen peroxide is in the range 2.3 to 1 to about 1 to 1. A review of currently known processes is provided by P. C. Foller and R. T. Bombard in Journal of Applied Electrochemistry 25 (1995), 613-627. The Dow Chemical Co., for example, operates a trickle bed cell with a trickle bed located at the cathode, wherein the particles in the trickle bed consist of graphite particles with a coating of polytetrafluoroethylene and carbon black. This process operates at room temperature and atmospheric pressure and the catholyte flow is controlled by the anolyte via a diaphragm. Platinized titanium is used as the anode. A molar solution of caustic soda is used as the electrolyte. The disadvantage of this process is that it can be run only at low current densities, which means that the investment costs for this type of plant are high. The current efficiency decreases greatly as the current density increases.

#The present invention will be further understood with reference to the accompanying drawings, wherein:

#FIG. 1 shows a schematic diagram of a device for the electrochemical preparation of hydrogen peroxide.

#FIG. 2 is a diagram which demonstrates the variation in concentration of the aqueous hydrogen peroxide solution with the thickness of the membrane.

#FIG. 1 shows a schematic diagram of a device for the electrochemical preparation of an aqueous hydrogen peroxide solution from oxygen and hydrogen. The electrolysis cell (1) is a typical fuel cell, the structure of which is familiar to a person skilled in the art; K indicates the cathode compartment, A indicates the anode compartment and MEA indicates a membrane electrode assembly or unit. The cathode and anode are connected to a source of power (2). Oxygen is supplied to the cathode compartment via line (5) and hydrogen is supplied to the anode compartment via line (6). Water from a water storage tank (9) is introduced into line (5) by means of a high-pressure pump or some other device, for example a nebuliser; in heat exchanger (3), the oxygen/water vapour mixture is heated to the desired reaction temperature. In a similar manner, the hydrogen stream in line (6) can be moistened with water, wherein water is introduced from a storage tank (10) via a high-pressure pump or nebuliser or the like and the gas/water mixture is heated to the desired temperature in another heat exchanger (4). The product streams emerging from the fuel cell contain, on the cathode side, aqueous hydrogen peroxide and unreacted oxygen and, on the anode side, water and unreacted hydrogen. The gas/liquid mixtures produced can be separated in a separating device (11 or 12). The H2O2 concentration can be increased in particular by supplying water vapour at a temperature of 180° C. to the O2 stream.

#FIG. 2 gives the variation in concentration of hydrogen peroxide in g/l with the thickness of the membrane. Comparison examples VB1 to VB3 are trials in each of which a commercial membrane from a different manufacturer was used, the thickness of these being in the range 40 to 50 μm. In contrast, in the examples according to the invention, B1 and B2, the thickness of the membrane was 180 and 170 μm respectively. The H2O2 concentration can be increased by increasing the thickness of the membrane several times, as shown in the figure. The process according to the invention can be performed at normal or elevated temperature. The H2O2 concentration can also be increased by increasing the pressure. At a pressure of about 30 bar, the H2O2 concentration increases to well above 10 wt. %. In the comparison examples and examples the following commercially available membranes were used: VB1: Nafion® 112, VB2: Gore 40 μm, VB3: PallR1010; B1: Nafion® 117, B2: PallBCM4010.

#By using a thick membrane in accordance with the invention in the MEU of a fuel cell, a substantially higher concentration of H2O2 can be obtained than when using a thin membrane under given operating conditions such as pressure, temperature, current density and degree of moistening.