WO1998002379A2

WO1998002379A2 - Reactor and method for synthesis of hydrogen peroxide

Info

Publication number: WO1998002379A2
Application number: PCT/US1997/013465
Authority: WO
Inventors: James A. Mcintyre; Keith R. Plowman; David J. Moll; Von G. Landon
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1996-07-15
Filing date: 1997-07-11
Publication date: 1998-01-22
Anticipated expiration: 1999-01-15
Also published as: AU3902097A; WO1998002379A3

Abstract

This invention is a chemical reactor and method for synthesis of hydrogen peroxide from molecular hydrogen and oxygen. The chemical reactor comprises a polymeric solution-diffusion membrane having a hydrogen contact side and an oxygen contact side and chambers for placing the relevant reactants in contact with the membrane. Conditions are provided such that a preferred flux of molecular hydrogen across the polymeric solution-diffusion membrane is at least five times greater than flux of the molecular oxygen across the membrane. The chemical reactor and method may be used to controllably synthesize hydrogen peroxide directly from molecular hydrogen and oxygen outside of the explosive range, without the use of organic solvents or complex equipment for ionic and electrical transport.

Description

REACTOR AND METHOD FOR SYNTHESIS OF HYDROGEN PEROXIDE

This invention relates to a method and apparatus for synthesis of hydrogen peroxide by reaction of molecular hydrogen and oxygen. The trend in commodities today is for materials and processes which are

"environmentally friendly". One such material is hydrogen peroxide. Hydrogen peroxide has many potential applications in, for example, chemical oxidation processes. One especially large field of use is as a bleaching agent in the pulp and paper industry. The demand for hydrogen peroxide is expected to grow at a rapid rate for many years. However, the current commercially practiced processes for synthesis of hydrogen peroxide are inefficient and have many disadvantages. As such it would be advantageous to develop a more efficient process for production of this commodity.

Most hydrogen peroxide is manufactured by a well-known anthraquinone process through successive reduction and oxidation reactions. Among the disadvantages of this method are that it requires the addition of numerous organic solvents, forms many unwanted by-products, and requires various separation steps. Another method for forming hydrogen peroxide is by catalytic reaction of hydrogen and oxygen with supported or homogeneous platinum group metal catalysts suspended or dissolved in aqueous solutions. However, this method requires bringing hydrogen and oxygen into a dangerous, potentially explosive, mixture together at high pressures (for optimum performance, usually greater than 7000 kPa), constituting a serious safety hazard. One method for avoiding excessive mixing of hydrogen and oxygen is cathodic reduction of oxygen in an alkali metal hydroxide solution. However, this process requires input of significant amounts of electrical energy and use of corrosion resistant equipment. A second method for avoiding excessive mixing of hydrogen and oxygen is to use fuel and reactor cells such as Proton Exchange

Membrane (PEM) fuel cells. However, since such cells typically require formation and conductance of electrons and ions across the fuel cell by means of an electrochemical potential, they typically require complex catalytic, ionic, and electrical equipment. This equipment is generally inappropriate for large scale manufacturing operations. Similarly, a method for producing hydrogen peroxide without excessive mixing of the hydrogen and oxygen using a palladium metal membrane is unsuitable. For example, such a membrane is expensive, susceptible to poisoning, requires relatively higher temperatures for satisfactory hydrogen fluxes, and may be lifetime limited due to hydrogen embrittlement. Therefore, alternative methods for production of hydrogen peroxide are desired. It would be an advantage to have a reactor and method wherein molecular hydrogen

(H₂) and molecular oxygen (O₂) may be controUably, but directly, reacted to form hydrogen peroxide outside of the explosive range, without the use of organic solvents or complex equipment for ionic and electrical transport. The chemical reactor of the invention disclosed herein comprises: (a) a polymeric solution-diffusion membrane having a hydrogen contact side and an oxygen contact side;

(b) a hydrogen supply chamber for placing molecular hydrogen in contact with the hydrogen contact side of the polymeric solution-diffusion membrane; and (c) an oxygen supply chamber for placing molecular oxygen in contact with the oxygen contact side of the polymeric solution-diffusion membrane.

The polymeric solution-diffusion membrane is positioned between the hydrogen supply chamber and the oxygen supply chamber such that the hydrogen contact side of the polymeric solution-diffusion membrane faces, and is operatively connected to, the hydrogen supply chamber and the oxygen contact side of the polymeric solution-diffusion membrane faces, and is operatively connected to, the oxygen supply chamber.

It is a further aspect of this invention to provide a method for synthesizing hydrogen peroxide (H₂O₂) by reacting hydrogen and oxygen using the chemical reactor described herein. An advantage of this method is that hydrogen peroxide may be produced at attractive rates using a considerably simplified process when compared to existing technology.

FIG. 1 is one embodiment of the invention. It illustrates oxygen and water entering the reactor through an inlet into an oxygen supply chamber 1 , and hydrogen entering the reactor through an inlet into a hydrogen supply chamber 2. A polymeric solution-diffusion membrane 4 having a hydrogen contact side and an oxygen contact side 3 separates the two chambers. The hydrogen contact side of the polymeric solution-diffusion membrane 4 faces the hydrogen supply chamber 2 and the oxygen contact side 3 comprises an oxygen reducing catalyst and faces the oxygen supply chamber 1. Two outlets 5 for withdrawal of product and excess gas are located at opposite ends of the reactor from the two inlets. FIG. 2(a) depicts a cross section of a reactor containing "serpentine" channels

(depicted in FIG. 2(b)) for this invention. In this figure, the polymeric solution-diffusion membrane 4 additionally includes a wet-proofed carbon paper 6 positioned between the oxygen contact side 3 and the remainder of the polymeric solution-diffusion membrane 4. Serpentine channels refer to a continuously connected series of oxygen supply chambers 1 on the oxygen contact side 3 and a corresponding continuously connected series of hydrogen supply chambers on the hydrogen contact side 4.

FIG. 3 depicts hydrogen permeance verses TBBA-PC membrane thickness. FIG. 4 depicts permeation properties of TBBA-PC/PTMSP composite membranes of various PTMSP and TBBA-PC thicknesses. FIG.'s 3 and 4, therefore, demonstrate the relationship between membrane thickness and flux rate of molecular hydrogen through the membrane. The chemical reactor of this invention requires a polymeric solution-diffusion membrane. The function of the polymeric solution-diffusion membrane is to permeate molecular hydrogen through the membrane at a controlled flux rate such that the hydrogen may react with oxygen safely and selectively. The polymeric solution-diffusion membrane must also inhibit excessive oxygen transport across the membrane. "Flux" as used herein shall mean the flow rate of a permeating species per unit cross-sectional area of the polymeric solution-diffusion membrane (that is (standard cc)/(cm² sec), wherein "standard" is equal to 0° C and 760 mmHg pressure). The polymeric solution-diffusion membrane must be reasonably stable and substantially non-reactive in the presence of a reducing substance (hydrogen), oxidizing substances (oxygen and hydrogen peroxide), and also substantially insoluble in water. Specific examples of acceptable materials for use as polymeric solution-diffusion membranes will be set forth below.

The polymeric solution-diffusion membrane has a hydrogen contact side and an oxygen contact side. The hydrogen contact side of the polymeric solution-diffusion membrane is positioned such that it faces, and operatively connects to, a hydrogen supply chamber. The oxygen contact side of the polymeric solution-diffusion membrane is positioned such that it faces, and operatively connects to, an oxygen supply chamber. "Operatively connects" means that each chamber is positioned with respect to the polymeric solution-diffusion membrane such that a relevant composition (for example, hydrogen or oxygen) can be placed in contact with its respective contact side of the polymeric solution- diffusion membrane. "Chamber" includes any vessel, space, zone, or the like, capable of substantially containing and facilitating contact between any relevant composition and an appropriate surface of the polymeric solution-diffusion membrane. Thus, a hydrogen supply chamber provides an effective environment for introducing, containing, and placing hydrogen, or a hydrogen containing mixture, in contact with the hydrogen contact side of the polymeric solution-diffusion membrane. Similarly, the oxygen supply chamber provides an effective environment for introducing, containing, and placing oxygen, or an oxygen containing mixture, in contact with the oxygen contact side of the polymeric solution- diffusion membrane. In addition, each chamber desirably has at least one opening for supply and/or removal of relevant composition(s), reaction products, or both. It may also be useful to utilize a "serpentine" channel arrangement, similar to that set forth in "FIG. 2", wherein the hydrogen and/or oxygen chambers on respective sides of the membrane consist of a series of connected parallel channels with alternating flow directions. With such a serpentine channel, it is preferred that the channels on one side of the membrane are aligned in parallel with respective channels on the other side of the membrane.

Furthermore, more than one opening per chamber may also be provided wherein one opening is an inlet for introducing a relevant composition into its respective chamber and one opening is an outlet for removing reaction products and/or unreacted relevant compositions.

The chemical reactor may further comprise a means for supplying the hydrogen to the hydrogen supply chamber and a means for supplying the oxygen to the oxygen supply chamber. Each of these means may be any conventional system or apparatus that transports relevant compositions from a source of the compositions into the respective hydrogen supply or oxygen supply chambers. In its simplest form, each means may be a pump and a conduit, inlet, or passageway operatively connected to a source of the composition such that the relevant composition is pumped from its source, through the conduit, and into its respective chamber. The chemical reactor may further comprise a similar type of means to recover hydrogen peroxide from the oxygen supply chamber.

The oxygen contact side of the membrane preferably comprises an oxygen reducing catalyst. Examples of beneficial oxygen reducing catalysts include: silver, gold, bismuth, palladium, cobalt (see, for example, Putten et al., J. Chem. Soc, Chem. Commun. 477 (1986), niobium-titanium, lanthanum-manganese mixtures, indium-tin oxide mixtures, praeseodymium-indium oxide mixtures, metal phthalocyanines (see, for example, Cook et al., 137 [No. 6] J. Electrochem. Soc. 2007 (1990), metal porphyrins (see, for example, Chan et al., 105 J. Am. Chem. Soc. 3713-14 (1983)), and anthraquinone-based catalysts (see, for example, Degrand, 169 J. Electroanal. Chem. 259-68 (1984)). Platinum and copper may also be used in combination with the above catalysts. Preferred oxygen reducing catalysts comprise at least palladium.

Catalysts may be in the form of pure materials, or supported on non-catalytic materials. They may be applied directly to the membrane surface or they may be supported on a porous sheet structure which is placed in contact with the membrane. Methods for incorporating and depositing catalysts on membranes are well known in the art and a skilled artisan is capable of optimizing these deposition methods to form an oxygen contact side of the membrane comprising an oxygen reducing catalyst. Examples of such deposition methods are disclosed in: A. B. Stiles, "Catalyst Manufacture, Laboratory and Commercial Preparations", Marcel Dekker, Inc., New York, ISBN 0-8247-7055-2, 1983; C. N. Satterfield, "Heterogeneous Catalysis in Practice", McGraw-Hill Book Company, New York, ISBN 0-07- 054875-7, 1980; A. J. Appleby and F. R. Foulkes, "Fuel Cell Handbook", Van Nostrand Reinhoid, New York, ISBN 00-442-31926-6, 1989; K. Kinoshita, "Electrochemical Oxygen Technology", John Wiley & Sons, Inc., New York, ISBN 0-471-57043-5, 1992; Nidola et al., U.S. Pat. No. 4,364,803 (1982); and Takenaka et al., U.S. Pat. No. 4,328,086 (1982). In order for this invention to function desirably, the polymeric solution-diffusion membrane must allow molecular hydrogen to diffuse from the hydrogen contact side to the oxygen contact side at a rate at least equal to that required for maintaining an acceptable minimum rate of reaction with oxygen. However, the polymeric solution-diffusion membrane must also prevent excessive flow of hydrogen to the oxygen contact side, since such excessive flow can create a danger of an explosion, or necessitate venting or recycle of large volumes of undesirably mixed gases, adding complicated and costly steps to the synthesis reaction. A mechanism for diffusion of the molecular hydrogen through the membrane is driven by a difference in thermodynamic activity existing at separate sides of the membrane. The activity is equivalent to the concentration difference and leads to diffusion in the direction of decreasing activity (that is diffusion towards the oxygen contact side). See, generally, Kesting and Fritzsche, "Polymeric Gas Separation Membranes", pp. 19-59 (1993). This mechanism is in contrast to electrochemical mechanisms wherein hydrogen is transported in an atomic or ionic (not molecular) form through a medium.

The ability of the polymeric solution-diffusion membrane to deliver the required flux of molecular hydrogen is also dependent on the partial pressure difference across the polymeric solution-diffusion membrane as well as the polymeric solution-diffusion membrane's permeance and thickness. Partial pressure is defined as the mole fraction of a particular gas multiplied by the total pressure of the gas mixture (strictly, only when the ideal gas law holds) of the gas across the membrane. Thus, if a membrane is being used to hold gases in separate chambers, it is possible for the gases to mix even when there is a total pressure differential across the membrane. That is, even if the pressure in one chamber, containing at the start "gas A" is larger than that in the other chamber "gas B", since the partial pressure of gas B is larger in Chamber B than Chamber A, it will diffuse against the total pressure gradient into Chamber A. The concept of selectivity in the membrane may be used to inhibit that from occurring. That is, a membrane material that has a high selectivity to hydrogen versus oxygen, for example, will allow the controlled transport of hydrogen toward the oxygen side, while minimizing the transport of oxygen to the hydrogen side. Furthermore, total pressure differential across the membrane may be used to increase the rate of transport of a desired gas (for example hydrogen) across the membrane since it has the effect of increasing the effective partial pressure of the desired gas.

"Permeance" of a particular gas is equal to the flux divided by the partial pressure difference of the gas across the polymeric solution-diffusion membrane (that is (standard cc)/(cm² sec cm Hg)). Differences in permeance result not only from diffusivity (mobility) differences of the molecular hydrogen and oxygen, but also from differences in the physicochemical interactions of the gases within the membrane. Permeance may also be affected by conditions such as temperature, water vapor pressure, swelling and thickness of the polymeric solution-diffusion membrane, and presence of other gases or contaminants. A preferred molecular hydrogen permeance range for obtaining the desired hydrogen peroxide selectivities is from 1x10^s to 2.5x10^'4 (standard cc)/(cm²-sec cm Hg) and more preferably from 2.5x10⁵ to 1.5x10^"4 (standard cc)/(cm²-sec cm Hg). The relationship of membrane thickness and permeance is specifically illustrated in FIG. 3, where the permeance of hydrogen through a tetrabromobisphenol A polycarbonate (TBBA-PC) membrane is shown to decrease with increasing membra«te thickness.

Therefore, as mentioned previously, adequate control of flux is a necessary property of the polymeric solution-diffusion membrane. To avoid excessive mixing of oxygen into the hydrogen stream, a polymeric solution-diffusion membrane that allows molecular hydrogen to transport preferentially relative to oxygen is desired. Excessive flow of oxygen into the hydrogen stream is typically uneconomical because excessive oxygen in the hydrogen means that large volumes of mixed gas must either be lost via venting, or recycled, for example, by controlled reaction of the hydrogen and oxygen. As one of skill in the art will recognize, flux in the polymeric solution-diffusion membrane is dependent upon the conditions in which the membrane is being operated. However, it has now been discovered that for optimal hydrogen peroxide synthesis from hydrogen and oxygen, a polymeric solution-diffusion membrane which, under conditions of operation of the chemical reactor, provides a flux of molecular hydrogen which is at least five times greater than the flux of oxygen through the membrane is preferred. More preferably, the flux of molecular hydrogen should be at least ten times greater than the flux of oxygen through the membrane. Most preferably, the flux of molecular hydrogen should be at least fifteen times greater than the flux of oxygen through the membrane.

Since there are many materials and combinations of materials (mixtures, blends, multilayers) that have acceptable flux rates, selection of a polymeric solution-diffusion membrane for obtaining the described fluxes should be within the skill in the art when combined with the teachings provided herein. Preferably, the polymeric solution-diffusion membrane is an organic polymeric-based membrane. An example of a common organic, polymeric-based membrane is perfluorosulfonic acid (PFSA). For a discussion of PFSA polymers, and methods of preparing such polymers, see De Vellis et al., U.S. Patent No. 4,846,977, col. 5, lines 1 -36. See also Kirk -Othmer, "Perfluorinated-lonomer Membranes," Encyclopedia of Chemical Technology, pp. 591 - 98 (1984) and A. Eisenberg and H. Yeager, "Perfluorinated lonomer Membranes", ACS Symposium Series No. 180 (1982). An example of a commercially available PFSA polymer is NAFION™ (E.I. du Pont de Nemours and Company). Preferred examples of polymeric-based membranes useful in this invention include polymeric-based membranes which comprise at least one compound selected from the following: tetrabromobisphenol A polycarbonate (TBBA-PC); 9,9-bis(3,5-dibromo-4- hydroxyphenyl)fluorene (TBBHPF); and poly(vinylidene difluoride) (PVDF). Alternative polymeric-based membranes which may be useful comprise materials such as sulfonated styrene grafts on a polytetrafluoroethylene backbone (commercially available from RAI

Research Corporation as RAIPORE™ membranes) and crosslinked sulfonated copolymers of vinyl compounds (commercially available from Ionics, Inc., as TYPE CR™ membranes). The structure or morphology of the polymeric solution-diffusion membrane may be homogeneous, composite, or asymmetric. Preferably, membranes are utililzed that are thin enough to allow acceptable hydrogen transport rates, yet are robust enough to be handled for reactor fabrication and operations. In light of the disclosure herein, the skilled in the art should be able to prepare a membrane in any of the aforementioned forms. Several examples of methods for forming such membranes are disclosed in a patent issued to The Dow Chemical Company, U.S. Patent No. 4,818,254, col. 4, line 63, through col. 7, line 23. In the case of a homogeneous membrane, the entire thickness of the membrane may serve as the polymeric solution-diffusion membrane. Since a homogeneous polymeric solution-diffusion membrane may be relatively thin or highly deformable, it may be desirable to provide support to the membrane. There are well known ways to produce membranes to do this. In one embodiment, the peripheral area of the membrane is affixed to a framing structure which supports the outer edge of the membrane. The membrane can be affixed to the framing structure by a clamping mechanism, adhesive, chemical bonding, or other techniques known by one of skill in the art. The membrane affixed to the frame can then be sealingly engaged in the conventional manner in a vessel so that the membrane surface inside the framing support separates two otherwise non-communicating compartments in the vessel. The skilled artisan will recognize that the structure which supports the membrane can be an integral part of the vessel or even the outer edge of the membrane.

In many embodiments, however, it is preferable that the polymeric solution-diffusion membrane is asymmetric ("anisotropic") or composite, and most preferably asymmetric. Asymmetric and composite membranes will typically have at least one dense solution- diffusion region and at least one generally porous or less dense region which offers additional mechanical support. Asymmetric membranes are generally continuous and the two regions are typically composed of the same materials, a thin region which is typically a discriminating layer (i.e discriminates one gas from another), and a thicker microporous supporting layer. Preparation of such a membrane is described in Example 2(b), below. In a composite membrane, a layer of membrane (that is "discriminating layer") is typically supported on a layer of porous substrate or structure. Generally, the porous substrate or structure is of a different composition than the membrane itself and is chosen to have very high permeability for at least one of the gases. Examples of porous substrates/supports are carbon paper (for example Toray TGPH-120™) and supports for reverse osmosis membranes such as disclosed in U. S. Patent 4,277,344, assigned to FilmTec Corporation. Additionally, a high permeability material such as poly[1 -trimethylsilyl- 1 -propyne] ("PTMSP") may be added to the porous support to enhance overall flux, provide a surface with enhanced capability to impede the transportation of undesirable fluid components through the composite structure, or to minimize the effect of pinhole leaks in the selective (or "discriminating") layer.

The porous supporting layer is characterized in that it does not greatly impede the transport of molecular hydrogen when the hydrogen is placed in contact with the porous layer. Generally, the selectivity of the support layer doesn't matter and is typically very low. However, it cannot be chosen indiscriminately since it also does have some impact on both the flux and selectivity of the composite. This is illustrated in FIG. 4 for TBBA-PC/PTMSP composites. Note that the permeance and selectivity vary as a function of the thickness of both the support layer (PTMSP) and the discriminating layer (TBBA-PC). Thus, in order to obtain both the desired flux and selectivity, one must choose specific thickness combinations.

To prepare a composite polymeric solution-diffusion membrane, a homogeneous, thin, membrane can be formed on the support surface. Alternatively, it can be formed independently and thereafter adhered to a porous support after formation. One reactor configuration is for the oxygen contact side of the polymeric solution-diffusion membrane to comprise the porous support having an oxygen reducing catalyst on one side of the porous support and the remainder of the polymeric solution-diffusion membrane on the opposite side of the porous support, such as depicted in "FIG. 2". Alternatively, the porous support can be the surface upon which the membrane is cast. In a preferred embodiment, the porous supporting layer is a very porous polymer membrane. Illustrative of such polymeric supporting layers are cellulose ester and microporous polysulfone membranes. Such membranes are commercially available under the trade names MILLIPORE™, PELLICON™ and DIAFLOW™. Where such supporting membranes are thin or highly deformable, a frame may also be necessary to adequately support the semi-permeable membrane. The polymeric solution-diffusion membrane may be utilized in many different structural forms. Typically, the membrane is in the form of a flat sheet, however one preferred embodiment is to use a hollow fiber form of the polymeric solution-diffusion membrane. For example, a microporous, hollow fiber of a polymer such as polysulfone, cellulose acetate, or some other cellulose ester is useful as a supporting layer. A layer of polymeric solution-diffusion membrane is then coated on the inside or outside surface of the hollow fiber. Polysulfone hollow fibers are most preferred for this application. Hollow fiber membranes can be formed by spinning fibers from an appropriate solution of a selected polymer in a water-miscible solvent. Such spinning is well known to those skilled in the art, and the formation of hollow fibers which are homogeneous, asymmetric, or composite membranes, require the adaptation of the hereinbefore described procedures to the hollow fiber form of the membrane. In light of the disclosure herein, such adaptations are well within the skill of the art. The solution-diffusion region of the hollow fiber may occur at or in the vicinity of the outside external surface, at or in the vicinity of the inside internal surface, at some region between both the external and internal surfaces, or a combination thereof. The solution-diffusion region in such membranes may be a dense region, a region of non- continuous porosity, a region resembling a closed cell foam, or a region with an enhanced free-volume state. Such asymmetric hollow fiber membranes are conveniently formed by casting or extruding the polymer in blends of solvent and optional non-solvent for the polymer in the general manner described in the prior art. Illustrative patents describing typical preparation of asymmetric membranes from polymers include the following U.S. Patents: 4,955,995; 4,772,392; 4,486,202; and 4,329,157.

The polymeric solution-diffusion membranes of this invention may be subjected to treatments with heat or by stretching in order to modify properties of the membranes. Optionally, the polymeric solution-diffusion membranes may be subjected to other treatments known to one of skill in the art such as solvent annealing, etching, irradiating, cross-linking, fluorinating, sulfonating, plasma treating, and the like. In one preferred embodiment, the polymeric solution-diffusion membrane is heat annealed before use. The membrane is exposed to temperatures above the beta transition and below the glass transition temperature of the membrane for a period of time to partially density the polymer. This procedure can optionally be performed under vacuum. For example, with tetrabromo bisphenol A polycarbonate, temperatures between 185° C and 230°C are preferred. Another aspect of this invention is a method of using the chemical reactor for synthesis of hydrogen peroxide. As described, supra, with respect to the function of the chemical reactor, the method comprises placing molecular hydrogen in contact with the hydrogen contact side of the polymeric solution-diffusion membrane and placing oxygen in contact with the oxygen contact side of the polymeric solution-diffusion membrane. With the chemical reactor disclosed herein, particularly beneficial hydrogen peroxide synthesis occurs when conditions are provided sufficient to have a flux of molecular hydrogen through the polymeric solution-diffusion membrane which is at least five times greater than the flux of oxygen through the membrane. More preferably, the flux of molecular hydrogen should be at least ten times greater than the flux of oxygen through the membrane, and most preferably, at least fifteen times greater than flux of oxygen through the membrane. When the molecular hydrogen is contacted with the hydrogen contact side, the hydrogen permeates through the polymeric solution-diffusion membrane in molecular form to the oxygen contact side and placed in contact with the oxygen at an interface between the oxygen contact side and the oxygen. The hydrogen then reacts with the oxygen to form a reaction product comprising hydrogen peroxide. As described above, preferably the oxygen contact side comprises an oxygen reducing catalyst (for example palladium). The oxygen reducing catalyst at the oxygen contact side is chosen to enhance the rate of the reaction between hydrogen and oxygen, and to provide high selectivity to produce hydrogen peroxide rather than water. Although the oxygen may be provided to the oxygen contact side of the membrane as a stream of pure oxygen gas, a preferred method of introducing oxygen to the oxygen contact side is as a component in a mixture such as air. It is also preferable for the oxygen to be introduced in a mixture with water. The water helps dilute the hydrogen peroxide product, thereby reducing its potential decomposition. The water may also assist in the removal of the heat of reaction. It is desirable that, when the oxygen is introduced in a mixture with water, the concentration of oxygen is high enough such that at least bubbles, or even pockets, of oxygen are present in the water (in contrast to substantially all of the oxygen being dissolved in the water). It is most preferred to further include additives in the oxygen feed stream which are optimized for increasing hydrogen peroxide selectivity. Such additives, for example, may include sulfuric acid (H₂SO₄) and hydrogen bromide (HBr). Such additives are known in the art for increasing selectivity to hydrogen peroxide. See T.Z. Pospelova et al., "Palladium-Catalysed Synthesis of Hydrogen Peroxide from the Elements," Russian Journal of Physical Chemistry 35(2), 143 (1961 ). This method of chemical synthesis may, if desired, be conducted at an elevated temperature. Generally, the temperature should not exceed a temperature at which any one of the materials of the chemical reactor (for example the membrane), or product (for example hydrogen peroxide) significantly decompose or degrade. This temperature, and the significance of chemical reactor degradation, vary according to the composition of the polymeric solution-diffusion membrane. Generally, the temperature is maintained at less than 75° C, however, one of skill in the art is capable of selecting an appropriate temperature as other conditions in the chemical reactor may be varied. For example, by placing a gaseous H₂ feed in contact with the hydrogen contact side and placing a gaseous O₂ and liquid H₂O feed mixture in contact with the oxygen contact side having palladium deposited thereon, H₂O₂ synthesis is favorable using a tetrabromobisphenol A polycarbonate (TBBA-PC) polymeric solution-diffusion membrane at a temperature of from 0°C to 50°C. Preferably, the synthesis is conducted at a temperature from 5°C to 20°C. A temperature in this range not only favors H₂O₂ synthesis, but is also well below the temperature at which the chemical reactor will begin to degrade. In addition, the method of the invention is typically conducted at a pressure of from ambient (taken as 100 kPa) to 14,000 kPa ( 2030 psi). It is preferred that a pressure differential between each side of the composite membrane does not exceed 415 kPa ( 60 psi) to avoid damage to the membrane. Robust membranes, however, may allow the use of higher differential pressures from increased hydrogen pressures. Elevated hydrogen pressures can enhance the flux of hydrogen relative to that of oxygen. Generally, increased pressure provides an increased mass transfer rate of the reactants. A particularly preferred pressure is from 750 kPa ( 109 psi) to 6,800 kPa ( 986 psi). Finally, it is preferable to remove any reaction products from the oxygen contact side of the polymeric solution-diffusion membrane. This isolates desirable reaction products and minimizes undesirable side reactions and decomposition of hydrogen peroxide. This may be accomplished using any means known by those of skill in the art. A simple method for reaction product removal is for the product to be swept up at the surface of the oxygen contact side of the polymeric solution-diffusion membrane into a continually flowing oxygen feed stream containing liquid water.

Examples

The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the use of the invention.

EXAMPLE 1

(a) Carbon Paper and Catalyst Composite Preparation Carbon paper (Toray TGPH-120™, obtained from E-TEK, Inc., Natick, MA) for applying a catalyst was prepared by coating a dispersion of (poly)tetrafluoroethylene (PTFE), specifically DuPont's TeflonT-30™, using a volume of dispersion containing the equivalent of 10-18 mg of PTFE per centimeter squared (cm²) of carbon paper. The paper was dried in an oven under vacuum at 325°C to melt the PTFE and disperse it over the whole surface of the carbon paper. The resulting treated carbon paper was hydrophobic (as demonstrated by water repulsion). The catalyst was prepared by mixing 1.25 gram (g) of 20 weight percent (wt%) Pd on carbon (obtained from E-TEK, Inc.) with 5g glycerol, 1.25 g water, and 8.3 g of a 5 wt% solution of NAFION™ in an alcohol/water solution (obtained from Aldrich Chemical, Milwaukee, Wl) to obtain a catalyst paint. The catalyst paint was applied to the wet proofed carbon paper using an artist's brush. Only a thin coating was applied each time. It was then dried in the oven under vacuum at 135°C, cooled to room temperature ( 25^CC), and the painting continued until all the paint was transferred onto the carbon paper to obtain a carbon paper composite with the required Pd loading. The composite was then dried at 135°C for 45 minutes under vacuum. The carbon paper/catalyst composite was then heat pressed at 2.25 MPa, and a plate temperature of 140°C to form a carbon paper composite having an approximate size of 36 x 11 cm.

(b) TBBA-PC Asymmetric Membrane Preparation

A TBBA-PC (tetrabromobisphenol A polycarbonate) asymmetric membrane was prepared by forming a 32 wt% solution of TBBA-PC in N-methylpyrrolidone at 58°C. The solution was degassed in a vacuum oven and was cast on a PYREX™ glass plate heated to 75°C. The resulting cast film and the plate was immediately immersed in water having a temperature between 10°C to 25°C and left in the water for between 1 to 2 hours. The resulting asymmetric membrane was air dried and then dried in vacuum at 60°C. The thickness of the membrane was 0.2 mm and the membrane had one side that was comparatively more dense than its opposite side which was comparatively more microporous. Permeation rates of hydrogen and oxygen through the membrane were measured using standard techniques, as described and referenced in J. Comyn, Editor, "Polymer Permeability", Elsevier Applied Science Publishers, London/New York, 1985, ISBN 0-85334-322-5. The hydrogen flux through the membrane ranged from 1x10⁵ to 1x10^"6 (standard cc)/(cm²-sec). This hydrogen flux was as high as 15 times greater than the oxygen flux through the membrane.

EXAMPLE 2 H₂O₂ Synthesis Using a Polymeric-based Solution-diffusion Membrane

(a) A TBBA-PC asymmetric membrane (prepared as described in Example 1(b)) and a carbon paper/catalyst composite (prepared as described in Example 1 (a)), and having a Pd catalyst loading of 0.6 mg/cm², was sandwiched together in a reactor similar to that depicted in "FIG. 2". The reactor consisted of metal plates having milled flow channels arranged in a "serpentine" pattern, the flow channels being used to supply hydrogen to one side of the membrane, and oxygen and water to the other side of the membrane. The area of membrane exposed to the serpentine channels on each face of the membrane was 15 in² ( 100 cm²). The dense side (as opposed to the microporous, hydrogen contact, side) of the asymmetric TBBA-PC film was placed against the exposed carbon paper side of the carbon paper/catalyst composite. The catalyst side of the composite thus faced the oxygen/water flow channel. The aqueous "water" solution also contained additives which increase the selectivity for producing hydrogen peroxide of 0.01 M H₂SO₄ and 0.008M HBr. This aqueous solution was added to the oxygen feed stream at a rate of 0.25 mlJmin., while the molecular oxygen was added at a rate of 0.5 L/min. The outflowing liquid on the oxygen contact side of the polymeric solution-diffusion membrane was analyzed for hydrogen peroxide. The concentration of hydrogen peroxide was 1.35 wt%.

(b) A second experiment using a membrane containing a catalyst layer with a Pd loading of 0.13 mg/cm² (made using 5% Pd on Vulcan XC-72 carbon using similar procedures as in "1 (a)") produced 0.58 wt% hydrogen peroxide at an aqueous solution feed flow rate of 1 mL/min. (oxygen flow of 0.250 Umin), and 1.85 wt% hydrogen peroxide at an aqueous solution feed flow rate of 0.125 mUmin. (oxygen flow of 0.250 Umin). The experiment was conducted under the same pressure and differential pressure as used in "2(a)", above.

(c) A third experiment using a membrane containing a catalyst layer with a Pd loading of 6 mg/cm² (made from 20 wt% Pd on Vulcan XC-72 carbon using similar procedures as in "1 (a)") produced 0.65 wt% hydrogen peroxide under the same conditions used in "2(b)", above.

(d) Another experiment was done using the membrane and conditions of "2(a)", above, except that the reactor was operated at a pressure of 150 psig ( 1034 kPa) for H₂ and 100 psig ( 690 kPa) for O₂ with a differential pressure of 50 psig ( 345 kPa). The concentration of peroxide was measured at 0.55 wt% on a continuous basis. The concentration reached that level over a period of 10 hours and maintained it at that level. In each of the examples "2(a)" through "2(d)", above, the H₂ diffusion through the membrane ranged from 7 to 30 standard cc /min for the 15 in² ( 100 cm²) of area exposed to the gas supply channels. Analysis of the exiting gas on the oxygen side showed that 80% of the hydrogen diffusing through the membrane completely reacted to form water or hydrogen peroxide.

Other embodiments of the invention will be apparent to the skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and example be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.

Claims

What is claimed is:

1. A chemical reactor comprising:

(a) a polymeric solution-diffusion membrane having a hydrogen contact side and an oxygen contact side;

(b) a hydrogen supply chamber for placing hydrogen in contact with the hydrogen contact side of the polymeric solution-diffusion membrane; and

(c) an oxygen supply chamber for placing oxygen in contact with the oxygen contact side of the polymeric solution-diffusion membrane; wherein the polymeric solution-diffusion membrane is positioned between the hydrogen supply chamber and the oxygen supply chamber such that the hydrogen contact side of the polymeric solution-diffusion membrane faces, and is operatively connected to, the hydrogen supply chamber and the oxygen contact side of the polymeric solution-diffusion membrane faces, and is operatively connected to, the oxygen supply chamber.

2. The chemical reactor of Claim 1 wherein the oxygen contact side comprises an oxygen reducing catalyst.

3. The chemical reactor of Claim 2 wherein the oxygen reducing catalyst comprises palladium.

4. The chemical reactor of Claim 1 wherein the polymeric solution-diffusion membrane comprises a material selected from tetrabromobisphenol A polycarbonate, 9,9-bis(3,5- dibromo-4-hydroxyphenyl)fluorene, and poly(vinylidene difluoride).

5. A method for synthesis of hydrogen peroxide using the chemical reactor of Claim 1 , wherein the method comprises:

(a) placing molecular hydrogen in contact with the hydrogen contact side of the polymeric solution-diffusion membrane;

(b) placing molecular oxygen in contact with the oxygen contact side of the polymeric solution-diffusion membrane; and

(c) permeating the molecular hydrogen through the polymeric solution-diffusion membrane to an interface between the oxygen contact side of the polymeric solution-diffusion membrane and the molecular oxygen; wherein conditions are provided sufficient to have a flux of molecular hydrogen through the polymeric solution-diffusion membrane which is at least five times greater than flux of the molecular oxygen through the membrane. b. i ne metnoα of Claim 5 wherein the molecular oxygen in step (D; IS mixed witn water.

7. The method of Claim 5 wherein the molecular oxygen in step (b) is mixed with additives which increase the selectivity for producing hydrogen peroxide .

8. The method of Claim 5 wherein the conditions are provided sufficient to have a flux of molecular hydrogen which is at least ten times greater than the flux of molecular oxygen through the membrane.

9. The method of Claim 5 wherein a temperature of between 0° C to 50° C is maintained.

10. The method of Claim 5 wherein the molecular hydrogen and/or oxygen is contacted with the polymeric solution-diffusion membrane by means of a serpentine channel.

ABSTRACT OF THE DISCLOSURE

This invention is a chemical reactor and method for synthesis of hydrogen peroxide from molecular hydrogen and oxygen. The chemical reactor comprises a polymeric solution- diffusion membrane having a hydrogen contact side and an oxygen contact side and chambers for placing the relevant reactants in contact with the membrane. Conditions are provided such that a preferred flux of molecular hydrogen across the polymeric solution- diffusion membrane is at least five times greater than flux of the molecular oxygen across the membrane. The chemical reactor and method may be used to controUably synthesize hydrogen peroxide directly from molecular hydrogen and oxygen outside of the explosive range, without the use of organic solvents or complex equipment for ionic and electrical transport.