WO2017118887A1

WO2017118887A1 - Systems and methods for the co-production of hydrogen and alkylene oxides

Info

Publication number: WO2017118887A1
Application number: PCT/IB2016/057682
Authority: WO
Inventors: Te Chang; Subramanian SANKARAN
Original assignee: Sabic Global Technologies B.V.
Priority date: 2016-01-05
Filing date: 2016-12-15
Publication date: 2017-07-13

Abstract

Methods of co-producing hydrogen and alkylene oxides are described. One method includes obtaining a reaction mixture that includes water, solvent, alkene reactants, water splitting photocatalyst and an epoxidation catalyst; and exposing the reaction mixture to ultraviolet light, visible light, or both ultraviolet and visible light in a wavelength range of 300 to 700 nm to produce H₂ and alkylene oxides.

Description

SYSTEMS AND METHODS FOR THE CO-PRODUCTION OF HYDROGEN AND

ALKYLENE OXIDES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit to U.S. Provisional Application No. 62/274,891 entitled "Systems and method for the Co-production of Hydrogen and Alkylene Oxides", filed January 5, 2016.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention generally concerns methods of co-producing hydrogen and alkylene oxides by exposing an aqueous methanol alkene reactant mixture to ultraviolet in the presence of a photocatalyst to produce the hydrogen and an alkylene oxide.

B. Description of Related Art

[0003] Hydrogen production from photocatalytic water splitting offers enormous potential benefits for the energy section and chemical sectors, however, the solar-to-hydrogen energy conversion efficiency is low. One of the main limitations of most photocatalysts is the rapid recombination of photo-generated electron/hole pairs as well as the reverse reaction to water. One approach to improve photocatalyst includes adding a metallic region as a reduction site on the Ti0₂ semiconductor surface for improved hydrogen generation. Another limitation to water splitting reactions is that high concentrations of flammable hydrogen (H₂) and oxygen (0₂) concentrations greater than 4.2% (LFL, lower flammable limit) and 0₂ greater than 4% (MOC, minimum oxygen concentration). When coproduced oxygen species are not removed, elevated oxygen partial pressure and oxygen species concentrations can hinder the water splitting reaction and decrease hydrogen generation. To control the amount of oxygen released, sacrificing reagents such as glycerin or glycols, have been used to consume oxygen generated in the system. Hence, the cost of hydrogen production is tied to the cost of sacrificing reagent. The oxidation of sacrificing reagent can also produces polluting volatile organic compounds (VOCs) and contaminates the water pool for electrolysis. Various attempts to avoid the flammable region of the gas mixture and reduce oxygen partial pressure include: (1) use of oxygen sacrificing reagent to reduce free oxygen concentration in hydrogen, (2) addition of sweeping inert gas, such as steam, carbon dioxide, or nitrogen to reduce hydrogen concentration in the reactor exit gas, (3) use of air sweeping to maintain H₂ below the LFL, e.g., less than 4%, and/or (4) use of a membrane reactor to produce and separate hydrogen and oxygen simultaneously. However, the above methods have significant drawbacks and challenges in both economic terms and technical feasibility. Thus, the separation of oxygen and hydrogen at photocatalytic site remains costly and difficult.

SUMMARY OF THE INVENTION

[0004] A discovery has been made that solves many of the problems associated with generation of hydrogen from photocatalytic water splitting reactions. The premise of the discovery lies in the addition of an alkene to react with the oxygen species generated in situ to form alkylene oxides. Such a reaction limits the recombination of hydrogen and oxygen to form water, thereby pushing the equilibrium of the reaction to the formation of hydrogen. This discovery provides an elegant, safe, and cost effective method for co-producing hydrogen and alkylene oxides. The method combines an epoxidation catalyst with water splitting photocatalysts to scavenge coproduced oxygen species during the hydrogen generation from the water splitting reaction. Notably, the invention provides for 1) substantially reducing or elimination of explosion hazards of hydrogen-oxygen mixture, 2) the reduction or elimination of costly sacrificing reagents and 3) the production of the economically valuable alkene oxides {e.g., propylene oxide).

[0005] Methods, processes, and systems for co-producing hydrogen (H₂) gas and alkylene oxides via in situ photocatalytic water splitting and epoxidation reactions are described. A method can include (a) obtaining a reaction mixture comprising water, methanol, alkene reactants, water splitting photocatalyst and an epoxidation catalyst; and (b) exposing the reaction mixture to ultraviolet light, visible light, or both ultraviolet and visible light in a wavelength range of 300 to 700 nm to produce the H₂ gas and the alkylene oxide. Alkene reactants can include ethylene, propylene, butylene, allyl alcohol, or a mixture thereof. In a particular embodiment, propylene is used as the alkene reactant. The water splitting photocatalyst and epoxidation catalyst can be provided as a slurry or in the form of a panel {e.g., a panel of a photoreactor). In another aspect, a process can include (a) feeding a 5 to 70% water-methanol solution feed and a propylene feed into a photocatalytic reactor that includes a catalytic surface, the catalytic surface can include a photocatalytic water splitting catalyst and an epoxidation catalyst to form a reaction mixture; (b) exposing the reaction mixture to ultraviolet and visible light in a wavelength range of 300 to 700 nm, at a pressure of 1 to 200 psig, and at a temperature of between 10 °C to 50 °C; (c) collecting and/or isolating hydrogen gas produced by the splitting of water and alkylene oxide produced by the epoxidation of the alkene. In other aspects, the process and/or method can include collecting a crude alkylene oxide- water-methanol solution from the photocatalytic reactor; isolating the alkylene oxide from the water-methanol; recycling the water-methanol solution and further refining the isolated alkylene oxide. Recycling the water-methanol solution can include removing impurities form the water-methanol solution and re-introducing the purified water- methanol solution into the photocatalytic reactor. The process can include regenerating the catalytic surface by adding new catalyst and/or purging deactivated catalyst.

[0006] The water splitting photocatalyst can include titanium dioxide and the titanium dioxide can be anatase, rutile, or both. In some aspects, the titanium dioxide water splitting catalyst can include a noble metal (e.g., palladium (Pd), platinum (Pt), gold (Au), or any combination thereof. The particle size of the water splitting photocatalyst can range from 1 to 50 nm, 2 to 25 nm, 3 to 50 nm, or form clusters of particles having a size of 100 to 500 nm, or 100 to 200 nm or any range or value there between. When provided as a slurry, the amount of the slurry is at most 10 weight percent of the water splitting photocatalyst. The epoxidation catalyst can be micro- or mesoporous titanium zeolite that includes titanium (e.g., titanium silicalite catalyst (TS-1), titanosilicates with MWW topology, or metal loaded (e.g., vanadium) titanium dispersed MCM-41 zeolites). The average particle size of the epoxidation catalyst can range from 100 to 500 nm, or 100 to 200 nm. In some aspects, the water splitting photocatalyst and the epoxidation catalyst can be bound together (e.g., using a silica binder) forming a catalytic particle having an average particle size of 5 to 50 microns of a catalytic surface. Particle size can be determined using a scanning electron microscope or laser diffraction techniques. A commercial instrument such as Malvern Mastersizer (Malvern, United Kingdom) can be used to determine particle size distribution

[0007] The water-solvent can include acetonitrile and/or alcohols such as methanol, n- propanol, butanols or any combination thereof. The water-solvent solution feed into can contain 5% to 70 wt.% methanol, or at 20-30 wt.% methanol. The alkene feed can be at more than 2 times the molar H₂O₂ generation rate in the photocatalytic reactor. The methanol- water solution can be buffered with 10 to 1000 ppm of electrolytes to maintain a pH between 5.0 and 7.2 with ammonium, sodium, or potassium salts of dihydrogen or mono-hydrogen phosphates. The epoxidation catalysts can be micro- or mesoporous Ti containing zeolites, such as titanium silicalite (TS-1), titanosilicate with MWW topology (TIMWW), or metal loaded (e.g., vanadium)-titanium dispersed MCM-41. Photocatalytic water splitting catalysts can have noble metal (e.g., Pd, Pt, or Au) nanoparticles, deposited on titania (Ti0₂) of an anatase and rutile mixtures. Both photoactive and epoxidation catalyst crystals size can range from 100-500 nm, preferably in 100-200 nm for enhanced activity.

[0008] Reaction conditions to produce hydrogen and the alkylene oxide can include a pressure of 1 to 200 psig (0.006 MPa to 1.4 MPa), a temperature of 10 °C to 60 °C. The reaction mixture can include 5 to 70 wt.% methanol or 20 to 30 wt.% methanol. The alkene reactant can be supplied at rate that is 2 times the molar hydrogen peroxide (H₂0₂) production by the photocatalyst. The reaction mixture can be buffered to a pH of 5.0 7.2. Buffering agents can include ammonium, sodium, or potassium salt mixtures of dihydrogen phosphates and mono-hydrogen phosphates. In some aspects, the solution is buffered with proper amounts of phosphoric acid and potassium or sodium hydroxide. Buffering the reaction mixture can inhibit the epoxide from ring-opening and/or stabilize the in situ generated peroxide species.

[0009] In some aspects a system for producing hydrogen and an alkylene oxide is described. A system can include (a) a light source; (b) a water-methanol feed source; (c) an alkene feed source; (d) a photocatalytic reactor that includes a catalytic surface, the photocatalytic reactor being (i) in fluid communication with the water-methanol feed source and the alkene feed source, and (ii) configured to provide for the light source to illuminate the catalytic surface. The system can further include (e) a product processing module that is fluidly connected to the photocatalytic reactor to remove gas products, liquid products, or gas and liquid products from the photocatalytic reactor. The product processing module can include a hydrogen gas and alkene gas separator, configured to separate the alkene gas and hydrogen gas, recycle the alkene gas back to the photocatalytic reactor, and collect the hydrogen gas. In some aspects, the product processing module can include an alkylene oxide separator configured to separate the alkylene oxide from the water-methanol solution, recycle water-methanol back to the photocatalytic reactor and collect the alkylene oxide product. The system can also include an alkylene oxide refining module configured to purify the alkylene oxide product and/or a water-methanol refining module configured to purify the separated water-methanol prior to recycling to the photocatalytic reactor. In some embodiments, the system can further include a catalyst regenerator configured to periodically remove the catalytic surface from the photocatalytic reactor and (i) regenerate the catalytic surface and return all or part of the catalytic surface to the photocatalytic reactor, and/or (ii) identify non-regenerative catalytic surfaces and purge the non-regenerative or defective catalytic surface. [0010] In the context of the present invention, 60 embodiments are described. Embodiment 1 describes a method of co-producing hydrogen and alkylene oxides comprising: (a) obtaining a reaction mixture comprising water, solvent, alkene reactants, water splitting photocatalyst and an epoxidation catalyst; and (b) exposing the reaction mixture to ultraviolet light, visible light, or both ultraviolet and visible light in a wavelength range of 300 to 700 nm to produce H₂ and alkylene oxides. Embodiment 2 is the method of embodiment 1, wherein the water splitting photocatalyst and epoxidation catalyst are provided as a slurry or in the form of a panel. Embodiment 3 is the method of embodiment 2, wherein the slurry is at most 10 weight percent water splitting photocatalyst. Embodiment 4 is the method of any one of embodiments 1 to 4, wherein the alkene is ethylene, propylene, butene, butylene, allyl alcohol, or a mixture thereof. Embodiment 5 is the method of embodiment 4, wherein the alkene is propylene. Embodiment 6 is the method of any one of embodiments 1 to 5, wherein the water splitting photocatalyst is a titanium dioxide (Ti0₂) water splitting photocatalyst. Embodiment 7 is the method of embodiment 6, wherein the Ti0₂ is in an anatase phase, a rutile phase, or mixture thereof. Embodiment 8 is the method of any one of embodiments 6 to 7, wherein the Ti0₂ water splitting catalyst comprises a noble metal. Embodiment 9 is the method of embodiment 8, wherein the noble metal is palladium (Pd), platinum (Pt), or gold (Au). Embodiment 10 is the method of any one of embodiments 1 to 9, wherein the water splitting photocatalyst has an average particle size of 1 to 500 nm, preferably 1 to 50 nm. Embodiment 11, is the method of embodiment 10, wherein the average particle size is 100 to 200 nm. Embodiment 12 is the method of any one of embodiments 1 to 11, wherein the epoxidation catalyst is a micro- or mesoporous titanium containing zeolites. Embodiment 13 is the method of embodiment 12, wherein the epoxidation catalyst is titanium silicate, titanosilicate, or a vanadium-titanium dispersion. Embodiment 14 is the method of any one of embodiments 12 to 13, wherein the average particle size of the epoxidation catalyst is 100 to 500 nm. Embodiment 15 is the method of embodiment 14, wherein the average particle size is 100 to 200 nm. Embodiment 16 is the method of any one of embodiments 1 to 15, wherein the water splitting photocatalyst and the epoxidation catalyst are bound together to form a catalytic particle or catalyst surface. Embodiment 17 is the method of embodiment 16, wherein the average particle size of the catalyst is 5 to 50 μιη. Embodiment 18 is the method of any one of embodiments 16 to 17, wherein the catalytic particle or catalyst surface comprises a silica binder. Embodiment 19 is the method of any one of embodiments 1 to 18, wherein the reaction performed at a pressure of 1 to 200 psig. Embodiment 20 is the method of any one of embodiments 1 to 19, wherein the reaction is performed at a temperature of 10 to 60 °C. Embodiment 21 is the method of any one of embodiments 1 to 20, wherein the solvent comprises acetonitrile, alcohols or both. Embodiment 22 is the method of embodiment 21, wherein the reaction mixture comprises 5 to 70% methanol or 20 to 30% methanol. Embodiment 23 is the method of any one of embodiments 1 to 22, wherein the alkene is supplied at 2 times molar H₂0₂ production of the photocatalyst. Embodiment 24 is the method of any one of embodiments 1 to 23, wherein the reaction mixture is buffered to a pH of 5.0 to 7.2. Embodiment 25 is the method of embodiment 24, wherein the buffer comprises ammonium, sodium, or potassium salt mixtures of dihydrogen phosphates and mono-hydrogen phosphates. [0011] Embodiment 26 is a process for co-producing hydrogen and alkylene oxides comprising: (a) feeding a 5 to 70% water-methanol solution feed and an alkene feed into a photocatalytic reactor comprising a catalytic surface, the catalytic surface comprising a photocatalytic water splitting catalyst and an epoxidation catalyst forming a reaction mixture; (b) exposing the reaction mixture to ultraviolet and visible light at a pressure of 1 to 200 psig, and at a temperature of between 10 to 60 °C; (c) collecting and/or isolating hydrogen gas produced by the splitting of water and alkene oxides produced by the epoxidation of the alkene. Embodiment 27 is the process of embodiment 26, further comprising removing H₂ and alkene gas, separating the hydrogen gas and the alkene gas, and recycling the isolated alkene gas to the photocatalytic reactor. Embodiment 28 is the process of any one of embodiments 26 to 27, further comprising collecting a crude alkene oxide solution from the photocatalytic reactor, isolating the alkylene oxides from the water-methanol solution, recycling the water-methanol solution, and further refining the isolated alkylene oxides. Embodiment 29 is the process of embodiment 28, wherein recycling the water-methanol solution further comprises removing impurities from the water-methanol solution and re- introducing the purified water-methanol solution into the photocatalytic reactor. Embodiment 30 is the process of any one of embodiments 26 to 29, further comprising periodically regenerating the catalytic surface. Embodiment 31 is the process of embodiment 30, wherein regenerating the catalytic surface comprises adding new catalyst and purging defective catalyst. Embodiment 32 is the process of any one of embodiments 26 to 31, wherein the alkene is ethylene, propylene, butylene, ally alcohol, or a mixture thereof. Embodiment 33 is the process of embodiment 32, wherein the alkene is propylene. Embodiment 34 is the process of any one of embodiments 26 to 33, wherein the water splitting photocatalyst is a titanium dioxide (Ti0₂)water splitting photocatalyst. Embodiment 35 is the process of embodiment 34, wherein the Ti0₂ of the water splitting catalyst is in an anatase form, a rutile form, or an anatase and rutile mixture. Embodiment 36 is the process of any one of embodiments 34 to 35, wherein the Ti0₂ water splitting catalyst comprises a noble metal. Embodiment 37 is the process of embodiment 36, wherein the noble metal is palladium (Pd), platinum (Pt), or gold (Au). Embodiment 38 is the process of any one of embodiments 26 to 37, wherein the average particle size of the water splitting photocatalyst is 100 to 500 nm. Embodiment 39 is the process of embodiment 38, wherein the average particle size is 100 to 200 nm. Embodiment 40 is the process of any one of embodiments 26 to 39, wherein the epoxidation catalyst is a micro- or mesoporous titanium containing zeolite. Embodiment 41 is the process of embodiment 40, wherein the epoxidation catalyst is a titanium silicate, a titanosilicate, or a vanadium-titanium dispersion. Embodiment 42 is the process of any one of embodiments 26 to 41, wherein the average particle size of the epoxidation catalyst is 100 to 500 nm. Embodiment 43 is the process of embodiment 42, wherein the average particle size is 100 to 200 nm. Embodiment 44 is the process of any one of embodiments 26 to 43, wherein the water splitting photocatalyst and the epoxidation catalyst are bound together to form the catalytic surface. Embodiment 45 is the process of embodiment 44, wherein the catalytic surface comprises a silica binder. Embodiment 46 is the process of any one of embodiments 26 to 45, wherein the catalytic surface is a catalytic particle or a catalytic panel. Embodiment 47 is the process of embodiment 46, wherein the catalytic particle is in a slurry. Embodiment 48 is the process of embodiment 47, wherein the slurry comprises at most 10 weight percent water splitting photocatalyst. Embodiment 49 is the process of any one of embodiments 44 to 49, wherein the average particle size of the catalytic particle is 5 to 50 μιη. Embodiment 50 is the process of any one of embodiments 26 to 49, wherein the water-methanol solution is 20 to 30% methanol. Embodiment 51 is the process of any one of embodiments 26 to 50, wherein the alkene feed is supplied at 2 times molar H₂0₂ production of the photocatalyst. Embodiment 52 is the process of any one of embodiments 26 to 51, wherein the methanol solution is buffered to a pH of 5.0 to 7.2. Embodiment 53 is the process of embodiment 52, wherein the buffer comprises ammonium, sodium, or potassium salt mixtures of dihydrogen phosphates and mono-hydrogen phosphates.

[0012] Embodiment 54 is a system for producing hydrogen and alkylene oxide comprising: (a) a light source; (b) a water-methanol feed source; (c) an alkene feed source; (d) a photocatalytic reactor comprising a catalytic surface, the photocatalytic reactor being (i) in fluid communication with the water-methanol feed source and the alkene feed source, and (ii) configured to provide for the light source to illuminate the catalytic surface. Embodiment 55 is the system of embodiment 54, further comprising (e) a product processing module that is fluidly connected to the photocatalytic reactor to remove gas products, liquid products, or gas and liquid products from the photocatalytic reactor. Embodiment 56 is the system of embodiment 55, wherein the product processing module comprises a hydrogen gas and alkene gas separator, wherein the alkene gas and hydrogen gas are separated and the alkene gas is recycled to the photocatalytic reactor and the hydrogen gas is collected as a photocatalytic reactor product. Embodiment 57 is the system of any one of embodiments 55 to 56, wherein the product processing module comprises an alkylene oxide separator, wherein the alkylene oxide is separated from the water-methanol solution, and the water-methanol is recycled to the photocatalytic reactor and the alkylene oxide is collected as a photocatalytic reactor product. Embodiment 58 is the system of embodiment 57, further comprising an alkylene oxide refining module to further purify the alkylene oxide product. Embodiment 59 is the system of embodiment 58, further comprising a water-methanol refining module to purify the separated water-methanol prior to recycling to the photocatalytic reactor. Embodiment 60 is the system of any one of embodiments 54 to 59, further comprising a catalyst regenerator configured to periodically remove the catalytic surface from the photocatalytic reactor and (i) regenerate the catalytic surface and return all or part of the catalytic surface to the photocatalytic reactor, and/or (ii) identify non-regenerative catalytic surfaces and purge the non-regenerative or defective catalytic surface.

[0013] The following includes definitions of various terms and phrases used throughout this specification.

[0014] The term "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%. [0015] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0016] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

[0017] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0018] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising", "including", "containing", or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more", "at least one", and "one or more than one."

[0019] The terms "wt.%", "vol.%", or "mol.%" refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component.

[0020] The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0021] The methods, processes, and systems of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase "consisting essentially of," in one non-limiting aspect, a basic and novel characteristic of the methods, processes, and systems of the present invention are their abilities to catalyze the formation of hydrogen gas and alkylene oxides from water, methanol and alkene reactants.

[0022] Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings.

[0024] FIG. 1A is a schematic of an embodiment of a water splitting/epoxidation system having a catalytic surface.

[0025] FIG. IB is schematic of an embodiment of a water splitting/epoxidation system that includes catalyst particles.

[0026] FIG. 1C is a schematic of an embodiment of a water splitting/epoxidation system with a light source in the reaction unit.

[0027] FIG. 2 is a schematic of a mechanism of the water splitting and epoxidation reaction.

[0028] FIG. 3 is a schematic of a water splitting/epoxidation system in combination with product processing units.

[0029] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0030] A discovery has been made that resolves the flammability issue when a mixture of hydrogen and oxygen is generated from a water splitting reaction. The discovery is premised on the idea of a new cohesive catalytic system that does not generate free oxygen during photocatalytic water splitting for hydrogen production. The alkene in the feed can serve as a reactant and/or as an oxygen diluent in case undesirable oxygen release occurs. Without wishing to be bound by theory it is believed that the contact of water with the photocatalyst produces hydrogen gas and hydrogen peroxide (H₂O₂) in situ as shown in reaction scheme (I). H₂0 i_2L_^ __^ H₂ + H₂0₂

water splitting catalyst

(I).

The in situ hydrogen peroxide can be absorbed onto the surface of the epoxidation catalyst {e.g., a titanium containing zeolite) and reacted with the alkene reactant to form the alkylene oxide as shown in reaction scheme (II):

where Ri, R₂, R₃, R₄ are each individually hydrogen or alkyl groups having a carbon number of 1 to 5. The generated water can then be used to produce more hydrogen peroxide and hydrogen so that the overall reaction is as shown in reaction scheme (III):

(III) where Ri, R₂, R₃, R₄ are each individually hydrogen or alkyl groups having a carbon number of 1 to 5. When hydrogen peroxide is reacted away from liquid phase, the concentrations of peroxide precursor species, superoxide, hydroxyl radicals and oxygen, are low on the photocatalyst surfaces for improved electron and charge transfer between catalyst valance band and conduction band for the production of hydrogen. A. Systems and Methods for Producing Hydrogen and Alkylene Oxides

[0031] Systems and methods to produce hydrogen and alkene oxides from a reaction mixture that includes water and an alkene are described. The reaction mixture can be contacted with a water splitting photocatalyst and an epoxidation photocatalyst. The photocatalyst can be separate catalysts or combined as one photocatalyst. The resulting product stream can include hydrogen, water and alkylene oxides. Some or all of the product stream can be separated, collected, transported, recycled to the reaction unit, further processed, or combinations thereof. These and other non-limiting aspects of the present invention are discussed in further detail in the following sections with reference to the Figures. It should be understood that all units or modules described in the Figures also includes valves, thermocouples, controllers (automated or manual controllers), computers or any other equipment deemed necessary to control or operate the units.

1. System and Method for Producing A Product Stream That Includes Hydrogen and Alkylene Oxides

[0032] Referring to FIGS. 1A and IB, a non-limiting representation of a water splitting/alkene epoxidation system 100 of the present invention is provided. The systems each include a photocatalyst 102 and an epoxidation catalyst 104, a light source 106, and reactor unit 108 that can be used to hold aqueous solutions of water and solvent (e.g., alcohols or acetonitrile) 110. Although not shown, the system 100 can also include one or more inlets for the aqueous methanol solution 110 and alkene reactant and at least one or more outlets for produced hydrogen and oxygen formed during the water splitting reaction (See, for example FIGS. 1C and 3). In one embodiment, the photocatalyst 102 and the epoxidation catalyst 104 can be coated onto the interior surfaces of reactor unit 108 (See, FIG. 1A), preferably with the photoactive layer 102 contacting the water/methanol solution 110 and the epoxidation catalyst in contact with the alkene reactant. As shown in FIG. 1A, the catalysts 102 and 104 are on the interior surface of the reactor unit 108. In a particular aspect, reactor unit 108 is a panel shaped fixed-bed reactor with photoactive and epoxidation catalyst mixtures coated surfaces that are submerged in a portion of the methanol-water solution. Alternatively, and in another embodiment, the photocatalyst 102 and the epoxidation catalyst are combined into one catalyst 112 and then coated on the wall of the reactor unit 108 or placed into the water as particles (See, for example, FIG. IB). In certain instances, a plurality of photocatalysts 102, 104, and/or 112 can be used to maximize hydrogen and alkylene oxide production. For example, the reactor unit can be a slurry reactor design with active catalysts suspended in the reaction mixture solution and a lighting arrangement maximizing the light irradiation received by suspended slurry catalysts.

[0033] In either instance, the reactor unit 108 can be a transparent, translucent or even opaque such as those that can magnify light (e.g., opaque container having a pinhole(s)). The photocatalyst 102 can be used to split water to produce H₂ and 0₂. The light source 106 can include either one of or both of visible and (400-600 nm) and ultraviolet light (280-400). The light can excite the photoactive catalyst 102 or titanium dioxide layer in a combined photocatalyst/epoxidation catalyst 1 12 to excite an electron in the valence band 1 14 to the conductive band 1 16. The excited electrons (e^") leave a corresponding hole (h⁺) when the electrons move to the conductive band. The excited electrons (e^") are used to reduce some of the hydrogen ions to form hydrogen gas, and the holes (h⁺) are used to oxidize oxygen ions to oxygen gas. Some of the hydrogen ions react with the oxygen to form peroxide ion (-OOH ). The epoxidation catalyst catalyzes the reaction of the in situ generated peroxide ion with the alkene to form alkylene oxides as shown in the reaction schematic shown in FIG. 2. Notably, the system 100 does not require the use of an external bias or voltage source. Further, the efficiency of the system 100 allows for one to avoid or use minimal amounts of a sacrificial agent such as methanol, ethanol, propanol, butanol, isobutanol, methyl tert-butyl ether, ethylene glycol, propylene glycol, glycerol, oxalic acid, or any combination thereof. In certain aspects, however, 5 to 70 w/v%, or preferably 20 to 30 w/v%, of a sacrificial agent can be included in the aqueous solution. The presence of the sacrificial agent can increase the efficiency of the system 100 by further reducing the likelihood of hole/electron recombination via oxidation of the sacrificial agent by the hole rather than recombination with the excited electron. Preferred sacrificial agent methanol is used. In addition to being capable of catalyzing water splitting without an external bias or voltage, the photocatalysts of the present invention may be included in an anode of an electrochemical cell capable of forming oxygen and hydrogen by electrolysis of water and the epoxidation catalyst can be suspended in solution. In a non-limiting example, light energy may be provided to a photocell and from the light energy a voltage between the anode and the cathode is produced and water molecules are split to form hydrogen and oxygen. The method can be practiced such that the hydrogen production rate from water can be modified as desired by subjecting the system to various amounts of light energy or light flux. For example, the photoactive catalyst 102 can be used as the anode in a transparent container containing an aqueous solution and used in a water splitting system.

[0034] In some embodiments, system 100 can include an internal light source 106 positioned in the reaction unit 108. Referring to FIG. 1C, reaction unit 108 (e.g., a quartz or glass reactor) can include light source 106 (e.g., 200 W UV lamp) position in reaction unit 108, a gas line 1 18 positioned proximate the bottom of the reaction unit 108, a gas/liquid line 120 positioned proximate the top of the reaction unit 108. Gas line 1 18 can include sparging ring 122 and gas/liquid line 120 can include frit ring 124. In a non-limiting example, the water/solvent feed solution can be prepared by mixing 80% water and 20% solvent (e.g., methanol) with the addition of potassium dihydrogen phosphate (KH₂PO₄) salt to 500 ppm concentration. Reaction mixture 126 can include water/solvent solution and catalyst (e.g. 3 to 5 wt% of catalyst).

[0035] Reaction mixture 126 can be radiated with UV light (e.g., for 1 hour), the water/solvent solution enters the reaction unit 108 through gas/liquid line 120 and the alkene feed enters the reaction unit 108 through gas line 1 18. The product stream 128 can be withdrawn into a gas-liquid separation vessel 130 operated at ambient pressure. The continuous gas and liquid streams can be sampled for analyses. The gas/liquid line 120 can keep catalyst particles inside the reactor. The alkylene oxide can be recovered from additional separation steps that also separate and recycle unreacted alkene from product gases as described in Section 2 below and FIG. 3.

[0036] Reaction conditions in reactor unit 108 can include a temperature of 10 °C to 60 °C, or 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 60 °C, or any value or range there between. The reaction pressure can range from atmospheric to 1.4 MPa, or 0.005 MPa, 0.01 MPa, 0.1 MPa, 0.5 MPa, 0.75 MPa, 1.0 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, or any value or range there between. The alkene reactants can be in the gas phase and can further include a carrier gas. The carrier gas can be an inert gas (e.g., helium or argon) or nitrogen. A weighted hourly space velocity (WHSV) of the alkene flow to system 100 can be 0.02 h^"1 to 15 h^"1. In some embodiments, the flow of the gaseous alkene reactant can be sufficient to provide agitation of the reaction mixture. Such reaction conditions provide a propylene conversion from 2% to 50%. 2. Processing of the Product Stream

[0037] Referring to FIG. 3, a schematic of a system 300 for the co-production of hydrogen and alkylene oxide via photocatalytic water splitting and epoxidation reactions and processing of the product stream is depicted. Alkene reactant feed 302 and aqueous methanol feed 304 can enter system 100 and form a reaction mixture. In system 100, the reaction mixture can be exposed to light source 106 and produce a crude product stream 306. Crude product stream 306 can include hydrogen gas, alkylene oxide, unreacted starting materials, and by-products. The hydrogen gas and alkylene oxide are generated through contact with the water splitting photocatalyst 102 to generate hydrogen peroxide in situ, which then reacts with the alkene reactant in the presence of the epoxidation catalyst 104 to generate alkylene oxides (See, for example, FIGS. 1 A, IB and 2). The crude product steam 306 can exit reactor unit 108 of system 100 and enter product processing module 307. Processing module 307 can include gas/liquid separation unit 308 and hydrogen/alkene separation unit 314, and alkylene oxide/solvent separation unit 320. In separation unit 308, gaseous crude product stream 310 and liquid crude product stream 312 can be separated from the crude product mixture 306. Separation unit 308 can be any gas/liquid separation unit known in the art {e.g., a flash drum, a break-pot, a knock-out drum, a knock-out pot, a compressor suction drum or, compressor inlet drum, or the like). Gaseous crude product stream 310 can include hydrogen gas and gaseous alkenes. Gaseous alkenes can include C₂ to C₅ alkenes {e.g., ethene, propylene, butene, butylene, pentene, etc.).

[0038] The gaseous crude product stream 310 can exit separation unit 308 and enter gas/gas separation unit 314 {e.g., hydrogen/alkene distillation unit). In gas/gas separation unit 314, hydrogen is separated from the gaseous alkenes to produce hydrogen product stream 316 and gaseous alkene stream 318. Hydrogen product stream 316 can be collected, stored, transported, used in other processing units, or any combination thereof. A portion of gaseous alkene product stream 318 can be recycled to reactor unit 108. As shown, the portion of gaseous alkene stream 318 is combined with alkene reactant stream, however, it should be understood that gaseous alkene stream 318 enter reactor unit 108 directly or can enter reactor unit 108 and/or be combined with gaseous reactant stream 302. In some embodiments gaseous alkene stream 318 is not recycled, but collected, stored, transported, sold, used in other processing units or any combination thereof [0039] Crude liquid product stream 312 can be an aqueous alcoholic solution of alkene oxides. In some embodiments, crude liquid product stream includes by-products generated from ring opening of the alkylene oxide to form C₂+ glycols and glycol ethers with alcohols. Crude liquid product stream 312 can enter liquid/liquid separation unit 320 (e.g. alkene oxide/solvent distillation unit). In liquid/liquid separation unit 320, the alkylene oxides are separated from the aqueous solvent mixture to produce crude alkylene oxides stream 322 and crude solvent stream 324. Crude alkylene oxides stream 322 exits liquid/liquid separation unit 320 and enters alkylene oxides purification unit 326. In alkylene oxides purification unit 326, the alkylene oxides are purified to produce the alkylene oxides product stream 328. Purified alkylene oxides product stream can be collected, stored, transported, used in other processing units, or any combination thereof.

[0040] Crude solvent stream 324 can enter solvent purification unit 330. In solvent purification unit 330, aqueous methanol is separated from heavier carbon-based by-products formed during the catalysis reaction to form purified aqueous methanol stream 332 and solvent by-product stream 334. Purified aqueous methanol stream 332 can be recycled to reactor unit 108 of system 100 to continue the catalytic cycle. In some embodiments, methanol stream 336 is combined with purified aqueous methanol stream 332 to increase the amount of methanol in the reactor unit 108.

[0041] Spent water splitting photocatalyst and/or spent epoxidation catalyst 338 can exit reactor unit 108 and enter catalyst regeneration unit 340. In catalyst regeneration unit 340, the catalysts 102, 104 or the combined catalyst 1 12 can be regenerated using known catalysts regeneration methods. The regenerated catalyst 342 can be provided to the reactor unit 108. New catalyst 344 can be combined with the regenerated catalyst 342 to reactor unit 108. In some embodiments, only new catalyst 344 is supplied to the reactor unit 108. C. Reaction Mixture and Product Stream

1. Reaction Mixture

[0042] The reaction mixture can include a mixture of water, methanol, and alkene. The water, methanol and alkenes are available from numerous commercial vendors and/or can be made through refining of hydrocarbons. a. Water and methanol mixture

[0043] The water and methanol mixture can include from 5 vol.% to 70 vol.%, 10 vol.% to 50 vol.%, 20 vol.% to 30 vol.%, or 5 vol.%, 10 vol.%, 15 vol.%, 20 vol.%, 25 vol.%, 30 vol.%, 35 vol.%, 40 vol.%, 45 vol.%, 50 vol.%, 55 vol.%, 60 vol.%, 65 vol.%, 70 vol.% or any value or range there between. In addition to the methanol and water, the water and methanol mixture can be buffered with 10 ppm to 1000 ppm of electrolytes to maintain a pH at 5.0-7.2, which is slightly acidic, but near neutral. The preferred buffer electrolytes are ammonium, sodium, or potassium salts of dihydrogen phosphates and mono-hydrogen phosphates. The buffered methanol-water solution may preferably be prepared by adding desired amount of phosphoric acid and ammonium, potassium, or sodium hydroxide solution to the water-methanol mixture. b. Reactant alkene

[0044] The alkene reactant can be any alkene having 1 to 5 carbon atoms that is capable of undergoing an epoxidation reaction, however, alkenes having greater than 5 carbon atoms can be used. Non-limiting examples of the alkene reactant includes, ethene, propylene, butene, isobutene, ally alcohol. In some aspects, the alkene reactant does not include any other compounds. In other aspects the alkene is a mixture of alkenes. In some aspects, the reactant feed includes propylene, allyl alcohol, or both. In some instances, the alkene can be combined with an inert carrier gas (e.g., helium or argon) and provided to the aqueous alcoholic mixture. In certain embodiments, the alkene is dissolved in water-alcohol solvent and provided as an aqueous solution. The rate of the alkene reactants can be controlled such that the amount of alkene reactant is 1.5 to 2.5, or 2 times the molar amount of hydrogen peroxide being produced through the photocatalytic water splitting reaction. c. Catalysts

[0045] The catalysts can be prepared by processes known to those having ordinary skill in the art, for example the catalyst can be prepared by any one of the methods comprising liquid-liquid blending, solid-solid blending, or liquid-solid blending (e.g., any of precipitation, co-precipitation, impregnation, complexation, gelation, crystallization, microemulsion, sol-gel, solvothermal, hydrothermal, sonochemical, or combinations thereof).

The weight ratio of water splitting photocatalysts to epoxidation catalysts can range of 0.2 to 4, or preferably 1 to 2, or 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1., 2.2, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, or any value or range there between. The overall solid concentration of the catalysts in the reaction mixture is at most 10 wt.%, or 5 wt.%, 3 wt.%, 2 wt.% or 1 wt.%, preferably 3 wt.% to 5 wt.%. [0046] The water splitting photocatalyst can include any suitable material, such as, for example metal oxides, electroconductive materials, and/or the like, capable of producing hydrogen and oxygen from water. Non-limiting examples of photocatalysts include titanium dioxide and metal loaded titanium dioxide catalyst. The titanium dioxide can be anatase phase, rutile phase, brookite phase, or any combination thereof. In some aspects, anatase titanium dioxide is used. The metal loaded titanium dioxide catalyst can include noble metals. Non-limiting examples of noble metals include palladium (Pd), platinum (Pt), gold (Au), silver (Ag), Osmium (Os), ruthenium (Ru), and rhodium (Rh), oxides or alloys thereof. In a particular aspect, anatase and/or rutile titanium dioxide can be loaded with nanoparticles palladium (Pd), platinum (Pt), gold (Au). The photocatalysts can be configured as a thin layer or film coated on an interior surface of the reactor (e.g., via spray coating, drop casting, and/or the like, on, for example, an interior surface of panels inside of the reactor), suspended particles (e.g., a slurry), and/or the like. Particularly, photocatalysts can have thicknesses or particle size tailored to harvest light with maximum efficiency. In some aspects, the water splitting photocatalyst can have an average particle size of 1 to 500 nm, 5 to 50 nm, or 3 to 10 nm, or about 5.5 nm. In some embodiments the water splitting photocatalyst can form clusters having a particle size of 100 to 500 nm, or 100 to 200 nm, or 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm or any range or value there between.

[0047] The epoxidation catalyst can be any catalyst capable of catalyzing an epoxidation reaction (e.g., photocatalysts and/or liquid phase catalysts) with an alkene (olefin). Non- limiting examples of epoxidation catalysts include micro- or mesoporous titanium containing zeolites, such as titanium silicalite (TS-1), titanosilicate with MWW topology (TEVIWW), or metal loaded titanium dispersed MCM-41 material. Non-limiting examples of metals include vanadium or oxides thereof. The epoxidation catalyst can be purchased from a commercial site or made using known methods for making catalysts. [0048] In some embodiments, the water splitting photocatalyst and the epoxidation catalyst can be bound together (e.g., silica binder). Binders can be used to bind the water splitting and epoxidation catalyst together to form filterable catalyst particles to facilitate slurry reactor applications. Silica binders can also be used to bind photo and epoxidation catalysts onto photo panel surfaces. An average particle size of the combined water splitting/epoxidation catalyst can be from 1 to 50 micron, 10 to 40 micron, 20 to 30 micron, or 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 micron. In some embodiments, the combined catalyst is bound to the interior surface of the reactor.

2. Product Stream

[0049] The product stream can include hydrogen, alkene oxides, water, methanol, alkenes and, in some instances, epoxide ring opening by-products. As described above the hydrogen and alkene oxides can be separated from the product stream. The produced alkene oxides can include ethylene oxide, propylene oxide, butylene oxide, cis-butene oxide, glycidyl, and combinations thereof. These alkylene oxides can be isolated, sold or used in a variety of chemical applications. For example, propylene oxide can be used to make polyether polyols, propylene glycols, and propylene glycol ethers. Propylene oxides can also be used in the manufacture of flame retardants, modified carbohydrates, synthetic lubricants, oil field drilling chemicals, textile surfactants and the like.

EXAMPLES

[0050] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Prophetic Example 1

(Co-production of Hydrogen, and propylene oxide)

[0051] Aeroxide® P25 titanium dioxide (Ti0₂, Evonik Degussa, Germany) powder will be deposited with 0.2% Pd and 0.1% Au, calcined at 300 °C. The prepared photo catalyst will be mixed with titanium silicalite-1 (TS-1, (ACS Material®, U.S.A.) at 50/50 mixture.

Silica binder will be added to the catalyst mixture to about 20 wt% and worked into a spray dry slurry solution. The catalyst slurry will be spray dried to obtain average size particles about 20 microns. The spray dried catalysts will then calcined at 400 °C for use. [0052] The reaction feed solution will be prepared by mixing 80% water and 20% methanol with the addition of potassium dihydrogen phosphate (KH₂P0₄) salt to 500 ppm concentration. A glass reactor (100 mL, See, for Example FIG. 1C, 108) used in the proposed experiment is equipped with 200W UV lamp 106 as a light source, a 1 micron gas sparging ring 124 at the bottom and a 5 micron frit ring 122 at the top for gas and liquid effluent withdrawn. The starting slurry solution 126 in the reactor can be prepared by stirring and suspending previously spray dried photo catalytic/TS-1 catalysts (4.75 gm) into a feed water/methanol solution (80 gm) to have a solid slurry suspension (5.6 wt%).

[0053] After the starting, the catalyst slurry solution 126 will be radiated with UV light for one hour, the feed solution and propylene feed are started and maintained at rates of 23 gm/hr and 19 gm/hr, respectively. The reactor is operated at 5 psig pressure and ambient temperature warming up to 30 °C. The gas and liquid effluent 128 will be withdrawn into a gas-liquid separation vessel 130 operated at ambient pressure. The continuous gas and liquid streams are sampled for analyses. The expected mass flows and component mole compositions are given in Table 1. The productivities of the photo catalytic/TS-1 catalyst are expected at 0.204 gm PO/gm cat/hr and 0.013 gm H₂/gm cat/hr. The gas effluent has low oxygen concentration less than 2 vol% and hydrogen near 6 vol%. Propylene oxide is recovered from additional separation steps that also separate and recycle unreacted propylene from product gases.

Table 1

*propylene glycol monomethyl ethers: CH₃CH(OH)CH₂OCH₃ and CH₃CH(OCH₃)CH₂(OH).

Claims

1. A method of co-producing hydrogen and alkylene oxides comprising:

(a) obtaining a reaction mixture comprising water, solvent, alkene reactants, water splitting photocatalyst and an epoxidation catalyst; and

(b) exposing the reaction mixture to ultraviolet light, visible light, or both ultraviolet and visible light in a wavelength range of 300 to 700 nm to produce H₂ and alkylene oxides.

2. The method of claim 1, wherein the water splitting photocatalyst and epoxidation catalyst are provided as a slurry or in the form of a panel.

3. The method of claim 2, wherein the slurry is at most 10 weight percent water splitting photocatalyst.

4. The method of claim 1, wherein the alkene is ethylene, propylene, butene, butylene, allyl alcohol, or a mixture thereof.

5. The method of claim 4, wherein the alkene is propylene.

6. The method of claim 1, wherein the water splitting photocatalyst is a titanium dioxide (Ti0₂) water splitting photocatalyst.

7. The method of claim 6, wherein the Ti0₂ water splitting catalyst comprises a noble metal, preferably palladium (Pd), platinum (Pt), or gold (Au).

8. The method of claim 7, wherein the water splitting photocatalyst has an average particle size of 1 to 500 nm, preferably 1 to 50 nm.

9. The method of claim 1, wherein the epoxidation catalyst is a micro- or mesoporous titanium containing zeolites.

10. The method of claim 9, wherein the epoxidation catalyst is titanium silicate, titanosilicate, or a vanadium-titanium dispersion.

11. The method of claim 10, wherein the average particle size of the epoxidation catalyst is 100 to 500 nm, preferably 100 to 200 nm.

12. The method of claim 1, wherein the water splitting photocatalyst and the epoxidation catalyst are bound together to form a catalytic particle or catalyst surface.

13. The method of claim 1, wherein the reaction performed at a pressure of 1 to 200 psig and/or a temperature of 10 to 60 °C.

14. The method of claim 1, wherein the solvent comprises acetonitrile, alcohols or both.

15. The method of claim 14, wherein the reaction mixture comprises 5 to 70% methanol or 20 to 30% methanol.

16. The method of claim 1, wherein the alkene is supplied at 2 times molar H₂0₂ production of the photocatalyst.

17. The method of claim 1, wherein the reaction mixture is buffered to a pH of 5.0 to 7.2.

18. The method of claim 17, wherein the buffer comprises ammonium, sodium, or potassium salt mixtures of dihydrogen phosphates and mono-hydrogen phosphates.

19. A process for co-producing hydrogen and alkylene oxides comprising:

(a) feeding a 5 to 70% water-methanol solution feed and an alkene feed into a photocatalytic reactor comprising a catalytic surface, the catalytic surface comprising a photocatalytic water splitting catalyst and an epoxidation catalyst forming a reaction mixture;

(b) exposing the reaction mixture to ultraviolet and visible light at a pressure of 1 to

200 psig, and at a temperature of between 10 to 60 °C;

(c) collecting and/or isolating hydrogen gas produced by the splitting of water and alkene oxides produced by the epoxidation of the alkene.

20. A system for producing hydrogen and alkylene oxide comprising:

(a) a light source;

(b) a water-methanol feed source;

(c) an alkene feed source; (d) a photocatalytic reactor comprising a catalytic surface, the photocatalytic reactor being (i) in fluid communication with the water-methanol feed source and the alkene feed source, and (ii) configured to provide for the light source to illuminate the catalytic surface.