WO2016153969A1

WO2016153969A1 - Catalysts and related methods for photocatalytc production of h2o2 and thermocatalytic reactant oxidation

Info

Publication number: WO2016153969A1
Application number: PCT/US2016/023044
Authority: WO
Inventors: Kimberly A. GRAY; Justin M. NOTESTIN; Todd R. EATON
Original assignee: Northwestern University
Current assignee: Northwestern University
Priority date: 2015-03-20
Filing date: 2016-03-18
Publication date: 2016-09-29
Anticipated expiration: 2017-09-20
Also published as: US20200095126A1; US20160272495A1; US10807868B2; US10508032B2

Abstract

Catalysts, catalytic systems and related synthetic methods for in situ production of H₂O₂ use thereof in reaction with oxidizable substrates.

Description

Catalysts and Related Methods for Photocatalytic Production of H₂0₂ and

Thermocatalytic Reactant Oxidation

[0001] This application claims priority to and the benefit of application serial no. 62/136,073 filed March 20, 2015, the entirety of which is incorporated herein by reference.

[0002] This invention was made with government support under DE-SC0006718 awarded by the Department of Energy. The government has certain rights in the invention. Background of the Invention.

[0003] Approximately 3.5 million metric tons of hydrogen peroxide (H₂0₂) are produced globally each year for use in waste treatment, pulp and paper bleaching, and the chemical industry. H₂0₂ is an environmentally-friendly, atom-efficient, and selective oxidant for chemical synthesis. Unfortunately, current industrial-scale synthesis is problematic. Direct synthesis from H₂ and 0₂ has been largely unsuccessful and is inherently dangerous. H₂0₂ is environmentally unfriendly, expensive, and difficult to carry out at all but the largest scale. Moreover, H₂0₂ must be diluted before shipping.

[0004] Accordingly, there remains an ongoing effort in the art to develop a safe, efficient and economical method for H₂0₂ production, to utilize the benefits and advantages associated with this oxidant.

Summary of the Invention.

[0005] In light of the foregoing, it is an object of the present invention to provide catalyst material(s) and related method(s) for the production of H₂0₂ and use in selective oxidation reactions, thereby overcoming various deficiencies and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all its respects, to every aspect of this invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of this invention.

[0006] It can be an object of the present invention to provide in situ H₂0₂ production without dilution and transportation safety issues associated with the prior art.

[0007] It can also be an object of the present invention to provide a catalyst material whereby photocatalytic properties, for H₂0₂ production, and thermocatalytic properties, for substrate oxidation, can be independently tuned and optimized. [0008] It can also be an object of the present invention, alone or in conjunction with one or more of the preceding objectives, to provide photo- and thermocatalytic functions with a single catalyst material.

[0009] Other objects, features, benefits and advantages of the present invention will be apparent from this summary and the following descriptions of various preferred embodiments, and will be readily apparent to those skilled in the art having knowledge of various H₂O₂ production and related oxidation techniques. Such objects, features, benefits and advantages will be apparent from the above as taken into conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom.

[0010] The present invention can be directed to a catalytic system comprising a photocatalyst for production of H₂O₂ from O₂ and a proton donor component, such a

photocatalyst comprising a particulate T1O₂ core component and a S1O₂ shell component coupled to such a core component, such a photocatalyst comprising T1O₂ surface areas; a thermocatalyst for reactant oxidation with produced H₂0₂, such a thermocatalyst adjacent to such a

photocatalyst and comprising a S1O₂ component and a transition metal moiety coupled thereto; a reaction medium comprising O₂, a proton donor component and an oxidizable reactant component; and ultra-violet radiation introduced to such a reaction medium for a time and at a wavelength sufficient to produce H₂0₂ and oxidize such a reactant component therewith.

[0011] In certain embodiments, a proton donor component of such a system can be selected from methanol, ethanol, propanols, butanols, ethylene glycol, propylene glycol, glycerol, and carbohydrates. Regardless, such an oxidizable reactant component can be selected from alkenes, including but not limited to styrene and other styrenic compounds, cycloalkenes, allyl chloride, allyl alcohol and related such unsaturated reactants. Without limitation, a transition metal moiety of such a system can be selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn moieties. In certain embodiments, such a transition metal moiety can be Ti. In certain such embodiments, such an alkene reactant component can be propylene.

[0012] Such a catalytic system can comprise both a photocatalyst and a thermocatalyst provided on such a T1O₂ core component, with such a transition metal moiety coupled to such S1O₂ shell component. In certain embodiments, such a transition metal can be Ti, and such an oxidizable reactant component can be propylene. Regardless, such a system can further comprise a catalyst to regenerate a proton donor component. For instance and without limitation, where such a proton donor source is isopropanol and a by-product of H₂0₂ production is acetone, such a system can comprise a hydrogenation catalyst to reduce acetone and regenerate isopropanol.

[0013] In part, the present invention can also be directed to a composition comprising a nanoparticulate Ti0₂ core component; a Si0₂ shell component coupled to such a core component; and a transition metal moiety coupled to such a shell component, such a composition comprising pores comprising Ti0₂ surface areas. In certain embodiments, such a transition metal moiety can be selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn moieties. Regardless, such a shell component can have a thickness dimension of about 1.5- about 3.5 nm.

[0014] In part, the present invention can also be directed to a method for concomitant H₂0₂ production and reaction. Such a method can comprise providing a reaction medium comprising a catalyst composition of the sort discussed above or illustrated elsewhere herein, a proton donor component for photoreduction of 0₂, and an oxidizable reactant component; introducing 0₂ to such a reaction medium; and irradiating such an oxygenated reaction medium with ultra-violet light of a wavelength and for a time sufficient to produce H₂0₂ and oxidize such a reactant component within such a reaction medium. In certain embodiments, such a proton donor component can be selected from alcohols. In certain other embodiments, without regard to proton donor component, such an oxidizable reactant component can, as discussed above, be selected from various alkenes. As a separate consideration, such a transition metal moiety can be selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn moieties. Without limitation, such an alkene can be propylene, and such a method can comprise oxidation thereof to propylene oxide. Regardless, as discussed above and illustrated elsewhere herein, such a method can comprise regeneration of such a proton donor source.

[0015] In part, the present invention can also be directed to a method of using a single catalyst for hydrogen peroxide production and alkene oxidation. Such a method can comprise providing a reaction medium comprising a single catalyst composition comprising a nanoparticulate Ti0₂ core component, a Si0₂ shell component coupled to such a core component and a transition metal moiety coupled to such a shell component, a proton donor component and an alkene component, such a composition comprising a photocatalytic Ti0₂-Si0₂ component and a thermocatalytic transition metal-Si0₂ component and pores comprising Ti0₂ surface areas; introducing 0₂ to such a reaction medium; and irradiating such an oxygenated reaction medium with ultra-violet light of a wavelength and for a time sufficient to produce hydrogen peroxide and oxidize such an alkene component within such a reaction medium. Proton donor

components and transition metals can be selected from those discussed above or illustrated elsewhere herein. Without limitation, such an alkene component can be selected from cyclo- and straight-chain alkenes. In certain such embodiments, such an alkene can be propylene, which can be oxidized to propylene oxide.

Detail Description of the Drawings.

[0016] Figure 1 : TEM image of Si0₂@Ti0₂ core-shell material, in accordance with certain non-limiting embodiments of this invention.

[0017] Figure 2: N₂ adsorption isotherms for Ti0₂ core (^A) and Si0₂@Ti0₂ (·) materials. In the region below P/Po=0.06 there is less than monolayer coverage for the core-shell material, indicating the presence of microporosity. This adsorption phenomenon is absent for the Ti0₂ core material.

[0018] Figure 3 : TEM images of the Si0₂@Ti0₂ material with silica shell thickness between 1.9 and 3.0 nm.

[0019] Figure 4: Epoxide yields after 2 hours for control reactions for the combined photo-/thermo-catalytic system and Ti- Si0₂@Ti0₂ catalyst: (A) standard conditions (isopropanol, 0₂, UV light and 65°C), (B) standard conditions except at room temperature, (C) standard conditions except no UV illumination, (D) standard conditions except using dodecane as solvent (i.e., less effective proton donor and hole scavenger than IP A for H₂0₂ synthesis), (E) standard conditions except N₂ is bubbled to remove 0₂, and (F) standard conditions using Ti-Si0₂ material only, showing that it does not respond to the 365-nm UV light.

[0020] Figure 5: Epoxide yields vs. time for the thermocatalytic epoxidation of c/5-cyclooctene on Ti-Si0₂ for (·) 3.5 mmol H₂0₂ (50 wt%, aq.) added at once at the beginning of the reaction and (O) 0.2 mmol H₂0₂ (4.0 M dry in MeCN) added continuously at a rate of 0.1 mmol h^"1 over the course of the reaction, mimicking the in situ H₂0₂ production of the combined photo-/thermo-catalytic system. The yields for the combined photo-/thermo-catalytic system is also shown ( I ).

[0021] Figure 6: Catalytic activity over an extended period of time, as shown through c/5-cyclooctene ( ) and cyclooctene epoxide (♦) production, epoxide selectivity (O) and mass balance (·). Yields are reported per gram of photocatalyst.

[0022] Figure 7: UV light affect on reaction, as shown through Acetone cyclooctene epoxide (♦), and H₂0₂ ( * , right axis) production per gram of photocatalyst.

Detailed Description of Certain Embodiments.

[0023] As relates to one or more non-limiting embodiments of this invention, reference is made to Scheme 1. A Ti0₂ surface is partially coated with silica (hereon referred to as Si0₂@Ti0₂), and the resulting material is used to synthesize H₂0₂ photocatalytically from 0₂ (in the air) and a proton donor/hole scavenger. In this case protons are generated and

photo-generated holes are scavenged by the concurrent photocatalytic oxidation of isopropanol to acetone, but this can be achieved using many other sources, including other alcohols. If desired, the co-product acetone could be reduced with H₂ in a second step using existing art, thereby recycling the proton source and hole scavenger. The H₂0₂ generated in situ then migrates to an epoxidation site {e.g., either on the same or another catalyst particle) for use as an oxidant in alkene epoxidati on—for instance, either, cz^'s-cyclooctene to cyclooctane oxide or 1- octene to octane oxide. An example of such an epoxidation catalyst is Ti-Si0₂. Alternatively, the components necessary for performing these tasks can be combined onto a single catalyst particle by first overcoating a Ti0₂ core with silica, then grafting dispersed Ti sites onto the silica (hereon referred to as Ti-Si0₂@Ti0₂).

n

Scheme 1 : Combined photo- and thermo-catalytic system for hydrogen peroxide production on Si0₂@Ti0₂ (left) and subsequent consumption via alkene epoxidation on Ti-Si0₂ (right). Note that the Ti-Si0₂ site may exist on the surface of the Si0₂@Ti0₂ material, as in Ti-Si0₂@Ti0₂.

[0024] Performing H₂0₂ synthesis and epoxidation in such a combined photo/thermo system offers the following advantages over conventional H₂0₂ synthesis and epoxidation reactions: (i) Si0₂@Ti0₂ is a more active photocatalyst than Ti0₂ for H₂0₂ synthesis, (ii) the H₂0₂ does not need to be purified and concentrated, (iii) the H₂0₂ is not diluted with H₂0 (as is commercial H₂0₂), which is known to inhibit the epoxidation step, (iv) the epoxidation occurs at higher rates than the case where excess aqueous H₂0₂ is added at the start of the reaction.

[0025] Performance of the present invention is demonstrated through the following comparisons (Table 1, below, summarizes the catalytic performance of the materials):

Si0₂@Ti0₂ gives higher yields of H₂0₂ at a given time than the Ti0₂ core. Table 1 entry 1 gives the performance of the Ti0₂ core material, while entry 2 gives the performance of Si0₂@Ti0₂. Si0₂@Ti0₂ yields 14 times greater H₂0₂ than the Ti0₂ core alone. This is unexpected because the Si0₂ shell covers part of the active Ti0₂ surface. The net H₂0₂ production on Si0₂@Ti0₂ is equivalent to 21 mM H₂0₂ h^"1, which is 6 times greater than the best-performing previously reported photocatalysts for H₂0₂ synthesis (3.4 mM H₂0₂ h^"1). A TEM image of the Si0₂@Ti0₂ catalyst is shown in Figure 1, which shows that the catalyst comprises a Ti0₂ core with a ~2 nm Si0₂ shell.

[0026] Addition of Ti-Si0₂ to a reactor with Si0₂@Ti0₂ provides a catalyst system that can perform the thermocatalytic epoxidation reaction using in situ generated H₂0₂, as illustrated in Scheme 1. Entry 3 of Table 1 demonstrates that this physical mixture greatly increases the epoxide yield (compared to entries 1 & 2). Consequently, the H₂0₂ yield is reduced due to consumption of H₂0₂ by the epoxidation reaction. Producing the H₂0₂ in situ means that it does not need to be produced elsewhere, purified, concentrated, diluted, and finally transported to the reactor responsible for epoxidation. Instead, the H₂0₂ is not only produced on site, it is synthesized in the very same reactor as the epoxidation reaction.

[0027] The photo/thermo-catalytic system with in situ H₂0₂ production leads to higher rates of epoxidation when compared to conventional methods of epoxidation. Entry 4 of Table 1 displays the catalytic performance of Ti-Si0₂ for a conventional thermocatalytic epoxidation reaction, when an excess of aqueous H₂0₂ is added at the beginning of the reaction and there is no photocatalytic cycle taking place to produce H₂0₂. The epoxide yield for the photo/thermo-catalytic system (entry 3) is 7 times greater than the conventional epoxidation system (entry 4). This is unexpected because the conventional system contains a much higher instantaneous concentration of H₂0₂ (3.5 mmol) than is measured in the combined photo/thermo- catalytic system (0.42 mmol, using the H₂0₂ yield from entry 2).

[0028] The photo/thermo-catalytic system with in situ H₂0₂ productions is active for other epoxidations. Table 1 entry 5 displays the catalytic performance for the combined system using 1-octene as the substrate. The rates of 1-octene epoxidation are 20 times higher for the combined photo/thermo-catalytic system than for a conventional thermocatalytic reaction

(entry 6). 1-octene is a model reaction for the epoxidation of propylene, which is currently

practiced at industrial scale over Ti-Si0₂ catalysts with added H₂0₂ by various industrial

concerns.

[0029] The separate features of the Si0₂@Ti0₂ photocatalyst and Ti-Si0₂

thermocatalyst can be combined onto a single catalyst particle to perform tandem photo- and

thermo-catalytic functions. By grafting Ti onto the silica-overcoated Ti0₂ core, a material

capable of performing both catalytic reaction cycles can be obtained. Table 1 entry 7 displays the performance of single component catalyst Ti-Si0₂@Ti0₂. While not achieving epoxide

yields as high as the combined photo-/thermo-catalytic system (entry 3) this material still yields more epoxide than either Ti0₂ or Si0₂@Ti0₂, due to addition of tetrahedral Ti on the silica shell.

The performance of this material may be improved by optimizing the Ti0₂ morphology, the shell structure, or the Ti loading.

[0030] This system is not limited to Ti-Si0₂ as the epoxidation catalyst. Table 1, entries 8 and 9 display catalytic results for the combined photo-/thermo-catalytic system when using Ta-Si0₂ and Nb-Si0₂ as the thermocatalysts, respectively. As would be understood by

those skilled in the art and made aware of this invention, most catalysts that use conventional

H₂0₂ can also be utilized herewith.

Table 1 : Product yields after 2 hours of reaction in photo-/thermo-catalytic system for various catalysts.

Acetone H₂0₂ Epoxide

Entry Photocatalyst Thermocatalyst ^ammol g^{"1 a}mmol g^{"1 a}mmol g^{"1 b}mmol g^"1

1 Ti0₂ core - 42 3 0.4 ^en/a

2 Si0₂@Ti0₂ - 22 42 0.5 ^en/a

3 Si0₂@Ti0₂ Ti-Si0₂ 37 16 12 7 c4 - Ti-Si0₂ ^dn/a ^dn/a ^dn/a 0.8 f5 Si0₂@Ti0₂ Ti-Si0₂ 42 9 3 2 c'^f6 - Ti-Si0₂ ^dn/a ^dn/a ^dn/a 0.1 g Ti-Si0₂@Ti0₂ n/a 14 13 3 ^en/a

8 Si0₂@Ti0₂ Ta-Si0₂ 48 25 14 3 9 Si0₂@Ti0₂ Nb-SiQ₂ 32 6 8 6

Conditions: 5 mg photocatalyst (if present), 15 mg thermocatalyst (if present), 10 mL isopropanol, 1.15 mmol c/^'s-cyclooctene, 65°C, 365 nm UV, 0₂ bubbled every 30 minutes

Yield per gram of photocatalyst material. ^bYield per gram of thermocatalyst material. ^c3.5 mmol H₂0₂ (aq) added. ^dNot applicable; no photocatalyst used. ^eNot applicable; no separate thermocatalyst used. ^fl-octene as alkene substrate g Ti- Si0₂@Ti0₂ acting as thermocatalyst and photocatalyst

[0031] Table 2 summarizes physical characterization data from the materials examined. All of the Ti0₂ based materials have edge energies of 3.2 eV, indicating that the

addition of a silica shell does not change the photo-response of the Ti0₂ core material. The edge energy of Ti-Si0₂ of 3.7 eV indicates that this material contains predominantly isolated Ti

cations, with some clustered, non-tetrahedral Ti sites. Addition of the silica shell increases the surface area of the Ti0₂ core, likely due to the formation of micropores, as indicated from the N₂ isotherm (Figure 2). The crystal size does not significantly change with the addition of the silica shell. TEM images confirm the formation of a thin silica shell (Figure 3). Taken together the characterization data show that the core-shell materials comprise a crystalline anatase Ti0₂ core with a transparent ~2 nm microporous silica shell.

Table 2: Physical characterization data of materials

Surface Crystallite

E_s ^a Area" size⁰

Material eV mV nm

Ti0₂ core 3.2 43 23

core-shell 3.2 57 27

Ti-core-shell 3.2 56 27

Ti-Si0₂ 3.7 589 - Estimated from DRUV-visible spectra transformed with a tauc plot.

Estimated from N₂ physisorption using the BET method.

Estimated from XRD; only anatase Ti0₂ is observed

[0032] As illustrated by Table 1 entries 1-3, addition of the silica shell

unexpectedly enhances H₂0₂ production, while addition of Ti to the shell surface increases the epoxidation activity. To prove that the reaction proceeds as illustrated in Scheme 1, several

control reactions were performed with the combination Ti-Si0₂@Ti0₂ catalyst (Figure 4).

Removing any component of the combined photo-/thermo-catalytic system reduces or eliminates production of epoxide, confirming that the reaction proceeds through the photocatalytic

production of hydrogen peroxide from isopropanol and the subsequent epoxidation of cis- cyclooctene via hydrogen peroxide. Comparing the combined photo/thermocatalytic systems for both alkenes (entries 3 and 5 from Table 1) to conventional thermocatalytic reactions (entries 4 and 6 from Table 1) shows that epoxidation rates for the combined system are unexpectedly 7 times greater for cz^'s-cyclooctene and 20 times greater for 1-octene. Finally, Figure 5 shows that slowly adding dry H₂0₂ over the course of the reaction increases epoxide yields vs. the case where H₂0₂ is added all at once, but still does not reach the productivity of the combined photo- /thermo-catalytic system. Thus, without limitation to any one theory or mode of operation, a combined photo-/thermo-catalytic system may improve epoxide yields by slowly synthesizing H₂0₂ in parallel with its consumption. Further, and also without limitation, comparing yields of H₂0₂ and acetone, the Si0₂@Ti0₂ material may be improving H₂0₂ yields by inhibiting the unproductive, subsequent decomposition of H₂0₂, as opposed to accelerating the production of H₂0₂.

[0033] Accordingly, the present invention describes catalytic materials and method(s) for the photocatalytic production of H₂0₂ in the same reactor as the subsequent consumption of H₂0₂ in a useful chemical transformation (e.g., alkene epoxidation). Such methods avoid H₂0₂ purification and transport, and greatly simplify the total production process. A representative photocatalyst described herein for H₂0₂ synthesis exhibits significantly higher synthesis rates than those previously reported, and the epoxidation rates for a combined system are higher than those observed for a conventional system, due to the intimate coupling of H₂0₂ generation and consumption.

Examples of the Invention.

[0034] The following non-limiting examples and data illustrate various aspects and features relating to the catalyst materials and/or methods of the present invention, including unitary photo- and thermocatalytic nanoparticles, as are available through the synthetic methodologies described herein. In comparison with the prior art, the present methods and catalyst materials provide results and data, which are surprising, unexpected and contrary thereto. While the utility of this invention is illustrated through the use of several catalyst materials, proton donor and reactant components which can be used therewith, it will be understood by those skilled in the art that comparable results are obtainable with various other catalyst materials, proton donor and reactant components, as are commensurate with the scope of this invention. Example 1

[0035] Synthesis of SiO?(¾TiO? core-shell. Anatase Ti0₂ nanoparticles were used as received from Sigma-Aldrich (<25 nm particles, "Ti0₂ core"). 2 g of the Ti0₂ core, 50 mL ethanol (KOPTEC, 200 proof), 3.2 mL H40H solution (56.6 w/w%) were added to a plastic container, sonicated for 30 minutes to disperse the oxide particles and equilibrate the pH. The mixture was transferred to a shaker plate, shaking at 200-250 rpm, and 1 mL of

tetraethoxyorthosilicate (TEOS, Sigma-Aldrich) was added in 0.1 mL increments every

20 minutes while shaking the vial in between. The mixture was allowed to shake overnight then centrifuged to collect solids. The solids were resuspended in 50 mL of 18 ΜΩ deionized water then centrifuged to wash away excess TEOS and H₄OH. This washing step was repeated 5 times. Finally the solids were collected and dried in a drying oven at 150°C for 12 to 15 hours.

[0036] Various other methods for Si0₂@Ti0₂ synthesis can be utilized, including the use of other core materials, Si0₂ precursors, and deposition conditions, as would be understood by those skilled in the art.

Example 2

[0037] Synthesis of Ti-SiO^fajTiO?. The core-shell material of Example 1 was dried under vacuum (<200 mTorr) at 200°C for 12 to 15 hours to remove physisorbed water. In a separate round-bottomed flask titanocene dichloride (Ci₀Hi₀Cl₂Ti, Aldrich 97%) was dissolved in freshly-distilled anhydrous toluene. In an Ar-filled glovebox the dried core-shell support was added to the round-bottomed flask and sealed. The round-bottomed flask was then removed from the glovebox and attached to a condenser and refluxed under N₂ for 48 hours. The reflux occurred without stirring to avoid grinding the core-shell particles. The material was then collected via vacuum filtration where it was washed with 200 mL toluene, 400 mL acetonitrile, and allowed to dry at room temperature overnight.

[0038] Various other methods of adding Ti to Si0₂@Ti0₂ can be utilized, including the use of other precursors and deposition methods as would be understood by those skilled in the art. Likewise, as would be understood in the art, other catalytically active cations can be coupled to the Si0₂ surface, such as thermocatalytic transition metals of the sort described elsewhere herein, through use of, for instance, corresponding metallocene starting materials. Example 3

[0039] Synthesis of Ti-SiO?. Mesoporous silica (Si0₂, Selecto, 540 m² g^"1, 2.2 nm average pore radius, 100-200 μιη particles) was dried under vacuum (<200 mTorr) at 500°C for 12 to 15 hours and stored under N₂ before use. 4-tert-butyldimethoxycalix[4]-arene

(C4₆H₆₀O4, "CxMe₂") was synthesized by methylating 4-tert-butylcalix[4]arene (C₄4H₅₆04, "Cx", Sigma- Aldrich) with methyl iodide, following a literature procedure. Titanium-tert-butyl- calix[4]-arene chloride (TiCx) was synthesized by adding T1CI4 (1.0 M in tolene, Sigma- Aldrich) to a solution of CxMe₂ in toluene and refluxed for 48 hours. The dried silica was then added to the TiCx solution in an Ar-filled glovebox, then removed from the glovebox and refluxed in toluene for >72 hours. During reflux N₂ was continuously bubbled through the reaction solution and vented through the top of the condenser to remove HC1. The TiCx-Si0₂ material was collected via vacuum filtration and repeatedly washed with toluene until the filtrate was clear (approximately 400 mL per g of material). The material was washed further with THF then water and dried at room temperature under vacuum. The final titanium loading was 200 μπιοΐ g^"1 or 1 wt% Ti, as determined by ICP-AES.

Example 4

[0040] Synthesis of Ta-SK Mesoporous silica (Si0₂, Selecto, 540 m² g^"1, 2.2 nm average pore radius, 100-200 μπι particles) was dried under vacuum (<200 mTorr) at 500°C for 12 to 15 hours and stored under N₂ before use. To obtain tantalum-fert-butyl-calix[4]-arene (TaCx), TaCl₅ was refluxed in toluene with 1 equivalent of Cx to form TaCx. The dried silica was then added to the TaCx solution in an Ar-filled glovebox, then removed from the glovebox and refluxed in toluene for >72 hours. During reflux N₂ was continuously bubbled through the reaction solution and vented through the top of the condenser to remove HC1. The TaCx-Si0₂ material was collected via vacuum filtration and repeatedly washed with toluene until the filtrate was clear (approximately 400 mL per g of material). The material was washed further with THF then water and dried at room temperature under vacuum.

Example 5

[0041] Synthesis of Nb-SiO?. Mesoporous silica (Si0₂, Selecto, 540 m² g^"1, 2.2 nm average pore radius, 100-200 μπι particles) was dried under vacuum (<200 mTorr) at 500°C for 12 to 15 hours and stored under N₂ before use. To obtain niobium-tert-butyl-calix[4]-arene (NbCx), NbCl₅ was refluxed in toluene with 1 equivalent of Cx to form NbCx. The dried silica was then added to the NbCx solution in an Ar-filled glovebox, then removed from the glovebox and refluxed in toluene for >72 hours. During reflux N₂ was continuously bubbled through the reaction solution and vented through the top of the condenser to remove HC1. The NbCx-Si0₂ material was collected via vacuum filtration and repeatedly washed with toluene until the filtrate was clear (approximately 400 mL per g of material). The material was washed further with THF then water and dried at room temperature under vacuum.

[0042] As indicated above, various other thermocatalysts can be utilized, including most transition metal cations supported on Si0₂, and many other materials useful in catalytic oxidation with H₂0₂.

Example 6

[0043] Catalytic reactions. Prior to all catalytic reactions, the materials described above were calcined at 600°C for 2 hours in static air. For the combined photo-/thermo-catalytic reaction system 5 to 10 mg of photocatalyst (Ti0₂ core, Si0₂@Ti0₂ or Ti-Si0₂@Ti0₂) and 0 to 70 mg of thermocatalyst (Ti-Si0₂, Ta-Si0₂ or Nb-Si0₂) were added to a 20 mL reaction vial with 10 mL isopropanol, 1.15 mmol alkene (czs-cyclooctene or 1-octene) and 150 uL dodecane as an internal standard. A hole was punched into the reaction vial septum to allow the fitting of a quartz test tube to hold the 365 nm pen-ray UV lamp. Prior to reaction the reaction mixture was sonicated for 5 minutes to disperse any aggregated particle and then allowed to heat up to 65°C for 30 minutes. The UV lamp was allowed to warm-up for 30 minutes prior to starting the reaction. 0₂ was bubbled through the reaction solution for 5 minutes prior to starting the reaction, and then for 1 minute at each sampling interval (0, 30, 60, 90, 120 minutes).

[0044] The reaction was started by introducing the pen-ray lamp to the reaction mixture and the vessel was agitated using a Glas-Col shaker plate. 150 μΐ_^ aliquots were collected into GC vials with ~1 mg Ag powder to decompose any H₂0₂ and avoid overoxidation. Acetone, cz^'s-cyclooctene and cyclooctene oxide were quantified by GC-FID. H₂0₂ was quantified by iodometry (see below). For thermal reactions, 10 to 30 mg of thermocatalyst were added to a 20 mL reaction vial with 10 mL isopropanol, 1.15 mmol alkene, 150 uL dodecane as internal standard, sonicated for 5 minutes, allowed to heat up to 65°C for 30 minutes, and then 3.5 mmol H₂0₂ (either from aqueous 50 wt% H₂0₂ or 4.0 M H₂0₂ in MeCN dried via MgS0₄) is added to initiate the reaction. Alternatively, in one control reaction H₂0₂ (dried, in MeCN) is added at a rate of 100 μηιοΐ h^"1 continuously over the course of the reaction with a syringe pump.

Example 7

[0045] Catalyst activity can be demonstrated through extended reaction. A physical mixture of Si0₂@Ti0₂ photocatalyst and Ti-SiC^ thermocatalyst was run with 60 mM cyclooctene in isopropanol. With reference to Figure 6, conversion of alkene reaches 79% after 60 hours with 98% selectivity towards cyclooctene epoxide (97% mass balance, shown in black, where mass balance is ([Alkene]_t+[Epoxide]_t)/[Alkene]imtiai)- O2 is bubbled through periodically.

Example 8

[0046] Reaction can be affected via the illumination source. With reference to Example 1, reaction conditions were as follows: 5 mg Si0₂@Ti0₂, 15 mg Ti-Si0₂, 10 mL isopropanol, 1.15 mmol czs-cyclooctene, 65°C, 365 nm UV (between 0-1, 2-3, 4-5 hours, etc.); O2 bubbled every 30 minutes. As shown in Figure 7, periods of illumination produce hydrogen peroxide and acetone. Hydrogen peroxide is consumed during dark periods, while no acetone is produced. As long as sufficient hydrogen peroxide is present, cyclooctene epoxide formation continues during light and dark cycles.

Example 9

[0047] In accordance with certain embodiments of this invention, hydrogenation can be used to regenerate a starting alcohol proton donor, as shown in Scheme 2.

Scheme 2.

[0048] Acetone hydrogenation was performed to demonstrate the recovery of isopropanol from the photocatalytic oxidation system. A reaction solution was prepared with 1.1 mmol acetone, 0.4 mmol cyclooctene oxide, and dodecane as internal standard in 50 mL isopropanol. Prior to hydrogenation, Pt on activated carbon ("Pt/C", 1 wt% Pt, Aldrich) was heat treated to 400°C for 4 hours under flowing He, ~1 mL min in a u-tube reactor. The Pt/C catalyst was sealed within the u-tube until use for hydrogenation. The reaction solution and 230 mg of Pt/C was loaded and sealed into a high-pressure Parr reactor, de-gassed with N₂, then charged with 10 bar H₂. Hydrogenation occurred at 10 bar H₂, 50°C, with stirring at 600 rpm. After 18 hours the reactor was cooled, de-pressurized, and an aliquot was collected for GC analysis: no epoxide was consumed under these conditions, whereas 100% of the acetone was

hydrogenated.

Example 10

[0049] Iodometric titration of H?Q?. H₂0₂ was quantified by iodometry adapting previously described literature techniques. After 2 hours of reaction 200 [iL aliquots were added to 1 mL of 50 v/v% H₂S0₄ and N₂ was bubbled through the solution for 10 minutes to remove 0₂. Then 1 w/w% KI (aq) was added to form I₂, which was a yellow solution. The solution was titrated with 0.1 mM Na₂S₂0₃ until the solution color was faintly yellow, then 0.1 mL starch indicator was added, forming a dark purple color, and the solution was further titrated until colorless. The Na₂S₂0₃ titrant was standardized with solutions of KI₃.

Example 1 1

[0050] Characterization. All catalyst samples were calcined for 2 hours at 600°C prior to characterization.

[0051] TEM images were obtained using a JEOL 21 OOF transmission electron microscope. Catalyst samples were suspended in methanol, sonicated for 5 minutes, then platinum TEM grids were dipped into the suspension and dried at room temperature before imaging.

[0052] Diffuse-reflectance UV-visible spectra were collected with a UV-3600 Shimadzu spectrophotometer with a Harrick Praying Mantis accessory for powder measurements and polytetrafuoroethylene as the baseline reference. Reflectance spectra were converted with the Kebulka-Munk transformation and edge energies were taken from Tauc plots of the spectra.

[0053] N₂ physisorption measurements were performed on a Micromeritics ASAP 2010. Prior to analysis, powder samples were dried at 200°C under vacuum (<5 μπι Hg).

Surface areas were calculated by the BET method.

[0054] X-ray diffraction (XRD) spectra were collected with a Rigaku X-ray diffractometer from 20 to 60° 2Θ, with Cu Ka radiation. The slit width, dwell time and slit widths were kept constant for all materials. Crystallite sizes were estimated using the Scherrer equation for the (101) anatase reflection at a Bragg angle of 25.2° 2Θ, assuming a shape factor of 0.9 with a lower detection limit of approximately 5 nm.

[0055] H₂0₂ is widely used as a "green" oxidant for chemical synthesis, because it is efficient, selective, and has only water as a byproduct. In situ H₂0₂ generation for selective oxidation has long been a goal of catalysis, specifically so for propylene epoxidation. As demonstrated, above, this invention provides an excellent catalyst for H₂0₂ production from 0₂ and a proton donor. In addition, H₂0₂ production can be coupled to a thermocatalyst for epoxidation, thus preventing the need to purify, concentrate, then dilute H₂0₂ for sale and transport. This invention also demonstrates oxidation of 1-octene, a model substrate for epoxidation of linear alkenes, indicating to those skilled in the art, broader applicability to other synthetic processes, including the production of propylene oxidation.

Claims

We claim:

1. A catalytic system comprising:

a photocatalyst for production of H₂0₂ from 0₂ and a proton donor component, said photocatalyst comprising a particulate Ti0₂ core component and a Si0₂ shell component coupled to said core component, said photocatalyst comprising Ti0₂ surface areas;

a thermocatalyst for reactant oxidation, said thermocatalyst adjacent to said photocatalyst and comprising a Si0₂ component and a transition metal moiety coupled thereto;

a reaction medium comprising 0₂, a proton donor component and an oxidizable reactant component; and

ultra-violet radiation introduced to said reaction medium for a time and at a wavelength sufficient to produce H₂0₂ and oxidize said reactant component therewith.

2. The system of claim 1 wherein said proton donor component is selected from alcohols.

3. The system of claim 1 wherein said oxidizable reactant component is selected from alkenes.

4. The system of claim 1 wherein said transition metal moiety is selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn moieties.

5. The system of claim 4 wherein said transition metal moiety is Ti.

6. The system of claim 5 wherein said alkene is propylene.

7. The system of claim 1 wherein said photocatalyst and said thermocatalyst are provided on said Ti0₂ core component, said transition metal moiety coupled to said Si0₂ shell component.

8. The system of claim 7 wherein said transition metal is Ti and said oxidizable reactant is propylene.

9. The system of claim 2 wherein said alcohol is isopropanol.

10. The system of claim 9 wherein acetone is a by-product of said H₂0₂ production.

11. The system of claim 10 comprising a hydrogenation catalyst to reduce said acetone and regenerate said isopropanol.

12. A composition comprising a nanoparticulate Ti0₂ core component; a Si0₂ shell component coupled to said core component; and a transition metal moiety coupled to said shell component, said composition comprising pores comprising Ti0₂ surface areas.

13. The composition of claim 12 wherein said transition metal moiety is selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn moieties.

14. The composition of claim 13 wherein said transition metal moiety is Ti.

15. The composition of claim 12 wherein said Si0₂ shell component has a thickness dimension of about 1.5 - about 3.5 nm.

16. The composition of claim 15 wherein said transition metal is Ti.

17. A method for concomitant H₂0₂ production and reaction, said method

comprising:

providing a reaction medium comprising a composition of claim 12, a proton donor component, and an oxidizable reactant component;

introducing 0₂ to said reaction medium; and

irradiating said oxygenated reaction medium with ultra-violet light of a wavelength and for a time sufficient to produce H₂0₂ and oxidize said reactant component within said reaction medium.

18. The method of claim 17 wherein said proton donor component is selected from alcohols.

19. The method of claim 17 wherein said oxidizable reactant component is selected from alkenes.

20. The method of claim 17 wherein said transition metal moiety is selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn moieties.

21. The method of claim 20 wherein said transition metal is Ti.

22. The method of claim 21 wherein said alkene is propylene, and said propylene is oxidized to propylene oxide.

23. The method of claim 18 wherein said alcohol is isopropanol.

24. The method of claim 23 wherein acetone is a by-product of said H₂0₂ production.

25. The method of claim 24 comprising hydrogenation of said acetone and regeneration of said isopropanol.

26. A method of using a single catalyst for hydrogen peroxide production and alkene oxidation, said method comprising:

providing a reaction medium comprising a single catalyst composition comprising a nanoparticulate Ti0₂ core component; a Si0₂ shell component coupled to said core component; and a transition metal moiety coupled to said shell component, said composition comprising pores comprising Ti0₂ surface areas, a proton donor component and an alkene component, said catalyst composition comprising a photocatalytic Ti0₂-Si0₂ component and a thermocatalytic transition metal-Si0₂ component;

introducing 0₂ to said reaction medium; and

irradiating said oxygenated reaction medium with ultra-violet light of a wavelength and for a time sufficient to produce hydrogen peroxide and oxidize said alkene within said reaction medium.

27. The method of claim 26 wherein said proton donor component is selected from alcohols.

28. The method of claim 26 wherein said proton donor component is isopropanol.

29. The method of claim 26 wherein said alkene is selected from cyclo- and straight- chain alkenes.

30. The method of claim 29 wherein said alkene is selected from octenes and propylene.

31. The method of claim 26 wherein said transition metal is selected from V, Ti, Cr, Mn, Co, Cu, Zn, Mo, Nb, Ta, W, Os, Re, Ir, and Sn.

32. The method of claim 31 wherein said transition metal is Ti.

33. The method of claim 32 wherein said alkene is propylene, and said propylene is oxidized to propylene oxide.

34. The method of claim 26 wherein said irradiation is intermittent to control hydrogen peroxide production and reactant oxidation.