Selective functionalization of methane to methanol and other oxygenates remains economically appealing but scientifically challenging. This is largely due to methane's inert C—H bond. In addition, the partially oxygenated products are more reactive than methane and prone to overoxidation forming CO2 as main products. Despite enormous efforts for over a century, the direct conversion of methane to methanol is still hindered due to a trade-off between CH4 conversion and methanol selectivity. For direct methane functionalization to replace the energy intensive industrial two-step methanol production process, new catalysts need to mitigate such trade-offs and achieve both high yield and selectivity.
From a chemistry perspective, methane can be activated either via insertion of a metal atom into the C—H bond or via hydrogen atom transfer (HAT). The former organometallic approach often requires a homogeneous metal complex catalyst, which limits its application in scalable fuel production. The latter often utilizes reactive species such as ·OH radicals to activate methane to produce methyl radicals (·CH3). The enzyme methane monooxygenase (MMO) combines HAT with an additional control of molecular transport to achieve highly selective methanol formation. Biomimicry of MMO using Fe/Cu-exchanged zeolites has also been explored, although room temperature conversion with high yields has not yet been achieved.
Photochemical methane oxidation reactions generate ·OH radicals directly from low-cost and abundant H2O and oxygen using inorganic catalysts at room temperature and thus are promising for large scale fuel production. Previous reported photocatalysts usually consist of semiconductors such as TiO2, ZnO, BiVO4, and WO3 and metal cocatalysts such as Pd, Au, Ag, and Au—Cu. However, overoxidation of methanol to CO2 is still the limiting factor in achieving high oxygenates yields and selectivity, especially for commercialized TiO2. On bare TiO2, methanol is readily oxidized by holes and surface trapped ·OH radicals on the surface (
Despite advances in alkane oxidation research, there is still a scarcity of methods that are both effective, with high yields under room temperature conditions, and selective for specific oxidation products such as, for example, methanol and other industrially useful products without overoxidation. An ideal method would use low-cost and abundant solvents and oxidants, would be scalable, and would not require a homogeneous metal catalyst. These needs and other needs are satisfied by the present disclosure.
In accordance with the purpose(s) of the present disclosure, as embodied and broadly described herein, the disclosure, in one aspect, relates to a method for oxidizing alkanes to produce industrially useful solvents and other compounds. In a further aspect, the method includes the steps of contacting an alkane or mixture of alkanes with a core-shell nanoparticle and an oxidant to produce a mixture and then irradiating the mixture with UV and/or visible light. The methods are selective for desired products and do not produce overoxidized species such as, for example, carbon dioxide. In a still further aspect, the methods are scalable and can be conducted for a short time under relatively mild conditions. In an aspect, the core-shell nanoparticle includes a metal-oxide containing semiconductor core, an amorphous, radiation transparent shell, and optional metal nanoparticle dopants in the shell.
Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. In addition, all optional and preferred features and modifications of the described embodiments are usable in all aspects of the disclosure taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.
Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or can be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The high activation barrier of the C—H bond in methane, combined with the high propensity of methanol and other liquid oxygenates toward overoxidation to CO2, have historically posed significant scientific and industrial challenges to the selective and direct conversion of methane to energy-dense fuels and chemical feedstocks. Herein is disclosed a unique core-shell nanostructured photocatalyst, silica encapsulated TiO2 decorated with AuPd nanoparticles (TiO2@SiO2—AuPd), for alkane oxidation with high yields and high selectivity. In one aspect, the core-shell catalytic particles include a metal oxide core optionally coated with a nanoscopic shell that selectively prevents methanol overoxidation on its surface and possesses high selectivity and yield of oxygenates even at high UV intensity, without greatly hindering alkane conversion. This transport selective architecture is composed of an amorphous layer, which can contain SiO2, and can be decorated with metal nanoparticles such as, for example, AuPd nanoparticles (SiO2—AuPd). While it is noted that AuPd nanoparticles have a well-known role in methanol formation, without wishing to be bound by theory, the metallic nanoparticle decorations serve a second role in this photocatalytic architecture by allowing for the diffusion of species necessary for methane and/or other alkane oxidation.
In one aspect, disclosed herein is a method for oxidizing an alkane, the method including at least the steps of contacting a composition including the alkane with a core-shell nanoparticle and an oxidant to produce a mixture and irradiating the mixture to produce one or more oxidized alkane species. In a further aspect, the alkane can be a C1-C6 linear, branched, or cyclic alkane, or can be a mixture of different C1-C6 linear, branched, or cyclic alkanes. In some aspects, the alkane can be methane or ethane, although other alkanes are also contemplated and should be considered disclosed. Further in this aspect, the C—H activation mechanism is very similar among all alkanes. In an aspect, the composition can include at least 5 vol % of the alkane, or from about 20 to about 100 vol % of the alkane, optionally from about 20 to about 50 vol % of the alkane, from about 40 to about 60 vol % of the alkane, or from about 50 to about 100 vol % of the alkane. In an alternative aspect, the method can be carried out with an alkane partial pressure of from about 0.1 to about 200 bar, from about 6 to about 200 bar, from about 1 to about 150 bar, or from about 6 to about 30 bar.
In one aspect, the oxidant can be O2, H2O2, N2O, or a combination thereof. In some aspects, when the oxidant is O2, the O2 partial pressure can be expressed in terms of ratio of alkane (e.g. CH4 or another alkane) partial pressure to O2 partial pressure. In an aspect, the alkane to O2 ratio can be about 100:0.5, or about 100:1, or about 2:1.
In any of these aspects, the mixture can further include a solvent, such as, for example, water. The solvent can be present in a bench-scale reaction in an amount of from about 1 to about 1000 mL, from about 20 mL to about 500 mL, from about 75 to about 150 mL, or from about 75 to about 100 mL, about 100 to about 125 mL, or from about 125 to about 150 mL. In some aspects, when the reaction is scaled up, for every 1 gram of catalyst (i.e. core-shell nanoparticles) used, from about 2 L to about 100 L of solvent can be used, or from about 2 L to about 50 L of solvent can be used.
In an aspect, a core of the core-shell nanoparticle includes at least one semiconductor, including, but not limited to TiO2, SrTiO3, ZnO, BiVO4, In2O3, carbon nitride, and combinations thereof. Oxide-containing semiconductors and other semiconductors having a band gap of from about 2 to about 4 eV not listed herein are also contemplated and should be considered disclosed. Without wishing to be bound by theory, any oxide semiconductor generating holes that can react with water to form OH radicals and/or electrons can be useful as part or all of the composition of the core.
In a further aspect, a shell of the core-shell nanoparticle includes at least one oxide transparent to UV or visible radiation. Further in this aspect, the shell may be amorphous. In still another aspect, the shell can be hydrophilic. In some aspects, the at least one oxide is or includes SiO2. In a further aspect, any hydrophilic, amorphous, and UV or visible light transparent oxide is contemplated for the disclosed shells. In another aspect, the shell has a thickness of from about 0.5 nm to about 20 nm, or from about 0.5 nm to about 10 nm, or from about 1 nm to about 8 nm. In some aspects, the shell thickness is about 5 nm. In another aspect, the thickness of the shell layer correlates with the type of oxide in the shell as well as its pore size and structure.
In some aspects, the shell further includes a dopant or “decoration” such as, for example, gold, platinum, palladium, copper, ruthenium, rhenium, or any combination thereof, including, but not limited to, combinations such as AuPd and CuPd. In one aspect, the dopant can be present in an amount of from about 0.1 to about 20 wt %, or from about 0.1 to about 10 wt %, from about 1 to about 5 wt %, or from about 2 to about 8 wt % relative to the weight of the nanoparticles. Without wishing to be bound by theory, having a metal nanoparticle or decoration loading above about 50% may block light absorption and interfere with the disclosed reactions. In some aspects, in the disclosed methods, the core-shell nanoparticles can be present in an amount of at least about 5 mg, or of at least about 10 mg. In one aspect, when the oxidant is O2, any metal nanoparticle that can dissociate O2 can be used for the oxidant.
In one aspect, an exemplary nanoparticle can include a TiO2 core, an SiO2 shell, and a dopant consisting of a combination of gold and palladium. Further in this aspect, the oxidant can be O2. Another exemplary nanoparticle can include a TiO2 core, an SiO2 shell, and no dopant. Further in this aspect, the oxidant can be H2O2.
In an aspect, the alkane can be methane and the one or more oxidized alkane species can be formic acid, formaldehyde, methanol, methyl hydroperoxide, carbon dioxide, or any combination thereof. Further in this aspect, the amount of methanol produced is at least 2, 4, 6, or 8 times greater than the amount of carbon dioxide produced. Still further in this aspect, the amount of carbon dioxide produced can be less than 2 mmol per grams of catalyst (i.e., core-shell nanoparticle) per hour relative to a total amount of oxidized alkanes produced. In a still further aspect, the alkane can be ethane and the one or more oxidized alkane species can be acetic acid, acetaldehyde, ethanol, or any combination thereof.
In one aspect, irradiation can be accomplished with UV or visible light having a wavelength of from about 320 to about 780 nm. In another aspect, irradiation can be accomplished using a xenon lamp or daylight. In still another aspect, irradiation can be accomplished using UV light having a wavelength of about 365 nm. In another aspect, the light can have a flux of greater than about 10 mW/cm2, of from about 10 to about 1000 mW/cm2, of from about 100 to about 500 mW\cm2, of from about 500 to about 1000 mW/cm2, of from about 130 to about 470 mW/cm2, or of about 130 to 200 mW/cm2, about 200 to about 350 mW/cm2, or about 350 to about 470 mW/cm2.
In one aspect, the method can be carried out under mild conditions for a short time period and is scalable. In some aspects, the method can be carried out as a batch process or as a continuous process. In a further aspect, the method can be carried out from about 0 to about 70° C., from about 5 to about 70° C., from about 15 to about 70° C., from about 15 to about 45° C., from about 45 to about 70° C., from about 22 to about 28° C., from about 22 to about 25° C., or from about 25 to about 28° C. In some aspects, the method can be carried out as a continuous process for from about 10 minutes to about 24 hours, or from about 15 minutes to about 6 hours, or for about an hour.
Many modifications and other embodiments disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosures are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. The skilled artisan will recognize many variants and adaptations of the aspects described herein. These variants and adaptations are intended to be included in the teachings of this disclosure and to be encompassed by the claims herein.
Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure.
Any recited method can be carried out in the order of events recited or in any other order that is logically possible. That is, unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.
All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.
While aspects of the present disclosure can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present disclosure can be described and claimed in any statutory class.
It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.
Prior to describing the various aspects of the present disclosure, the following definitions are provided and should be used unless otherwise indicated. Additional terms may be defined elsewhere in the present disclosure.
As used herein, “comprising” is to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more features, integers, steps, or components, or groups thereof. Moreover, each of the terms “by,” “comprising,” “comprises,” “comprised of,” “including,” “includes,” “included,” “involving,” “involves,” “involved,” and “such as” are used in their open, non-limiting sense and may be used interchangeably. Further, the term “comprising” is intended to include examples and aspects encompassed by the terms “consisting essentially of” and “consisting of.” Similarly, the term “consisting essentially of” is intended to include examples encompassed by the term “consisting of.
As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a catalyst,” “an oxidant,” or “an alkane,” includes, but is not limited to, mixtures or combinations of two or more such catalysts, oxidants, or alkanes, and the like.
It should be noted that ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms a further aspect. For example, if the value “about 10” is disclosed, then “10” is also disclosed.
When a range is expressed, a further aspect includes from the one particular value and/or to the other particular value. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g. the phrase “x to y” includes the range from ‘x’ to ‘y’ as well as the range greater than ‘x’ and less than ‘y.’ The range can also be expressed as an upper limit, e.g. ‘about x, y, z, or less’ and should be interpreted to include the specific ranges of ‘about x,’ ‘about y’, and ‘about z’ as well as the ranges of ‘less than x’, less than y’, and ‘less than z’. Likewise, the phrase ‘about x, y, z, or greater’ should be interpreted to include the specific ranges of ‘about x,’ ‘about y,’ and ‘about z’ as well as the ranges of ‘greater than x,’ greater than y,’ and ‘greater than z.’ In addition, the phrase “about ‘x’ to ‘y’”, where ‘x’ and ‘y’ are numerical values, includes “about ‘x’ to about ‘y’”.
It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a numerical range of “about 0.1% to 5%” should be interpreted to include not only the explicitly recited values of about 0.1% to about 5%, but also include individual values (e.g., about 1%, about 2%, about 3%, and about 4%) and the sub-ranges (e.g., about 0.5% to about 1.1%; about 5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range.
As used herein, the terms “about,” “approximate,” “at or about,” and “substantially” mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, it is generally understood, as used herein, that “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about,” “approximate,” or “at or about” whether or not expressly stated to be such. It is understood that where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.
As used herein, the term “effective amount” refers to an amount that is sufficient to achieve the desired modification of a physical property of the composition or material. For example, an “effective amount” of a catalyst refers to an amount that is sufficient to achieve the desired improvement in the property modulated by the formulation component, e.g. achieving the desired level of production of methanol relative to the amount of methane originally present in the reaction mixture. The specific level in terms of wt % or vol % in a composition required as an effective amount will depend upon a variety of factors including the amount and type of alkane to be converted, amount and type of metal particles decorating the shell of the core-shell particle, wavelength and photon flux of irradiation to which the reaction mixture is exposed, and desired end products.
As used herein, the terms “optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
Unless otherwise specified, temperatures referred to herein are based on atmospheric pressure (i.e. one atmosphere).
Now having described the aspects of the present disclosure, in general, the following Examples describe some additional aspects of the present disclosure. While aspects of the present disclosure are described in connection with the following examples and the corresponding text and figures, there is no intent to limit aspects of the present disclosure to this description. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the present disclosure.
The present disclosure can be described in accordance with the following numbered Aspects, which should not be confused with the claims.
Aspect 1. A method for oxidizing an alkane, the method comprising:
Aspect 2. The method of aspect 1, wherein the alkane comprises a C1-C6 linear, branched, or cyclic alkane.
Aspect 3. The method of aspect 1, wherein the alkane comprises methane or ethane.
Aspect 4. The method of aspect 1, wherein the composition comprises at least about 5 vol % of the alkane.
Aspect 5. The method of aspect 4, wherein the composition comprises from about 20 to about 100 vol % of the alkane.
Aspect 6. The method of aspect 1, wherein the method is conducted with an alkane partial pressure of from about 0.1 to about 200 bar.
Aspect 7. The method of aspect 1, wherein the oxidant comprises O2, H2O2, N2O, or any combination thereof.
Aspect 8. The method of aspect 7, wherein the method is conducted with a ratio of alkane partial pressure to O2 partial pressure of from 100:0.5 to about 2:1.
Aspect 9. The method of aspect 1, wherein the mixture further comprises a solvent.
Aspect 10. The method of aspect 9, wherein the solvent comprises water.
Aspect 11. The method of aspect 9, wherein the solvent is present at from about 2 L to about 100 L of solvent per gram of core-shell nanoparticles.
Aspect 12. The method of aspect 1, wherein a core of the core-shell nanoparticle comprises at least one semiconductor.
Aspect 13. The method of aspect 12, wherein the semiconductor comprises an oxide with a band gap of from about 2 to about 4 eV.
Aspect 14. The method of aspect 12, wherein the at least one semiconductor comprises TiO2, SrTiO3, ZnO, BiVO4, In2O3, carbon nitride, or any combination thereof.
Aspect 15. The method of aspect 1, wherein a shell of the core-shell nanoparticle comprises at least one oxide transparent to UV or visible radiation.
Aspect 16. The method of aspect 15, wherein the at least one oxide comprises SiO2.
Aspect 17. The method of aspect 15, wherein the shell has a thickness of from about 0.5 nm to about 20 nm.
Aspect 18. The method of aspect 16, wherein the thickness is about 5 nm.
Aspect 19. The method of aspect 1, wherein a shell of the core-shell nanoparticle further comprises a dopant.
Aspect 20. The method of aspect 19, wherein the dopant comprises gold, platinum, palladium, copper, rhenium, ruthenium, or any combination thereof.
Aspect 21. The method of aspect 19, wherein the dopant is present in an amount of from about 0.1 wt % to about 10 wt % relative to the total weight of the nanoparticles.
Aspect 22. The method of aspect 21, wherein the dopant is present at about 5 wt % relative to the total weight of the nanoparticles.
Aspect 23. The method of aspect 1, wherein the core-shell nanoparticles are present in an amount of about 5 mg.
Aspect 24. The method of aspect 1, wherein the mixture is irradiated using light.
Aspect 25. The method of aspect 1, wherein the light has a wavelength from about 320 nm to about 780 nm.
Aspect 26. The method of aspect 24, wherein the mixture is irradiated using 365 nm UV light.
Aspect 27. The method of aspect 24, wherein the light has a flux greater than about 10 mW/cm2.
Aspect 28. The method of aspect 27, wherein the light has a flux of from about 130 to about 470 mW/cm2.
Aspect 29. The method of aspect 1, wherein the method is carried out at a temperature of from about 0 to about 70° C.
Aspect 30. The method of aspect 1, wherein the method is carried out as a batch process or a continuous process.
Aspect 31. The method of aspect 30, wherein the method is carried out as a continuous process for from about 10 minutes to about a 24 hours.
Aspect 32. The method of aspect 19, wherein a core of the core-shell nanoparticle comprises TiO2, wherein the shell of the core-shell nanoparticle comprises SiO2, and wherein the dopant comprises gold and palladium.
Aspect 33. The method of aspect 19, wherein the oxidant comprises O2.
Aspect 34. The method of aspect 1, wherein a core of the core-shell nanoparticle comprises TiO2, wherein a shell of the core-shell nanoparticle comprises SiO2, and wherein the oxidant comprises H2O2.
Aspect 35. The method of aspect 1, wherein the alkane comprises methane and the one or more oxidized alkane species comprises formic acid, formaldehyde, methanol, methyl hydroperoxide, carbon dioxide, or any combination thereof.
Aspect 36. The method of aspect 35, wherein an amount of methanol produced is at least 4 times greater than an amount of carbon dioxide produced.
Aspect 37. The method of aspect 35, wherein an amount of carbon dioxide produced is less than 2 mmol per grams of core-shell nanoparticle per hour relative to a total amount of oxidized alkane species produced.
Aspect 38. The method of aspect 1, wherein the alkane comprises ethane and the one or more oxidized alkane species comprises acetic acid, acetaldehyde, ethanol, or any combination thereof.
Aspect 39. An oxidized alkane produced by the method of aspect 1.
The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary of the disclosure and are not intended to limit the scope of what the inventors regard as their disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric.
Herein, a core-shell type photocatalytic architecture for methane oxidation with high yields and selectivity has been developed. In this architecture, the TiO2 core is coated with a nanoscopic shell that selectively blocks methanol without greatly hindering methane conversion. This transport selective architecture is composed of an amorphous SiO2 layer with decorated AuPd nanoparticles (SiO2—AuPd). While the intended purpose of the incorporation of AuPd nanoparticles in the design was because of its well-known role in methanol formation, it serves a second role in this photocatalytic architecture by allowing for the diffusion of species necessary for methane oxidation. Under a UV flux of 130 mW/cm2, TiO2@SiO2—AuPd produced 15.4 mmol/gcat·h of liquid oxygenates with 94.5% selectivity at 9.65 bar total pressure of CH4 and O2. At this reaction condition, its SiO2-free counterparts (AuPd/TiO2) produced CO2 as the major product. Due to the protective silica layer, the high oxygenates selectivity can be maintained at various reaction conditions, which enables the use of higher UV flux (470 mW/cm2) to produce 21.3 mmol/gcat·h of oxygenates with 80% selectivity. The working principle of the catalyst was further elucidated by a series of systematic studies varying the catalyst structure and reaction conditions. It is also shown that this core-shell catalyst design is generalizable for selective oxidation of other alkanes.
The silica shell was prepared using a modified Stöber method on P25 TiO2 (denoted as TiO2@SiO2) with tunable thickness. AuPd colloids were loaded onto TiO2@SiO2 (denoted as TiO2@SiO2—AuPd), followed by calcination in air at 350° C. (
Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction confirmed that the TiO2 nanoparticles were fully encapsulated by a uniform amorphous SiO2 shell with about 5 nm thickness (
Photocatalytic experiments were performed at room temperature (25±3° C.) in a batch reactor with 6.9 bar CH4 and 2.75 bar O2 (
aEntry 8: reaction is performed without light illumination (dark condition).
bEntry 9: 6.9 bar Ar gas instead of CH4 (no CH4 in the system).
cEntry 10: no photocatalyst used.
aLight intensity: 0.13 W/cm2, irradiation area: 19.625 cm2, light incident time: 1 h
bAQY(%) = [n(CH3OOH) + n(CH3OH) × 3 + n(HCHO) × 5 + n(HCOOH) × 7 + n(CO2) × 9] × 100%/N(photons), where n(CH3OOH), n(CH3OH), n(HCHO), n(HCOOH) and n(CO2) represent the mole numbers of produced CH3OOH, CH3 OH, HCHO, HCOOH and CO2 molecules, respectively. For example, the AQY for TiO2@SiO2—AuPd is calculated as: (13.7 + 63.8 × 3 + 65.2 ×+ 11.7 × 7 + 8.9 × 9) × 10−6 × 6.02 × 1023 × 100%/1.7 × 1022 = 2.45%.
The critical role of amorphous SiO2 shell in mitigating methanol overoxidation in photochemical methane conversion was further demonstrated by decreasing the water volume (FIG. 2B3), as the presence of water is known to stabilize methanol and prevent its overoxidation to CO2. After decreasing water volume from 100 to 20 mL, the oxygenates selectivity over AuPd/TiO2 decreased from 52.7% to 18.3% with CO2 as the dominant product. Remarkably, TiO2@SiO2—AuPd largely maintained the high selectivity toward oxygenates (82.6%). Here, a 100 mL water volume was used for all following studies.
The silica shell thickness is an essential parameter for the catalyst design. With 6.9 bar of methane and 2.75 bar of oxygen, a 5 nm thick silica shell produces the optimal oxygenates selectivity and yields (
To uncover the role of silica shell and the importance of precise structural design of TiO2@SiO2—AuPd, the role of each component was investigated individually.
The key species involved in this photochemical process could be water that reacts with photogenerated h+ or oxygen that reacts with photogenerated e−, and ·OH/O2·− radicals that diffuse out of the silica shell (
Considering that such oxygen diffusion, not observed on the TiO2@SiO2, was enabled by loading AuPd nanoparticles on the silica shell, it is hypothesized that AuPd enables oxygen to permeate the silica shell by dissociating the O2 molecule. AuPd nanoparticles supported on metal oxides have previously been reported to facilitate O2 molecule dissociation. In addition, it is known that the transport of atomic oxygen species through oxide thin film at room temperature is dominated by a field-induced drift, which is generated by the chemisorption of reactive oxygen species. Although no direct evidence was obtained for such seemingly counterintuitive oxygen dissociation process, it is the most plausible rationale for the series of results presented above.
We further confirmed the effect of the SiO2 shell and AuPd by measuring the radical generation with different catalysts (
As evidenced from the results presented, in accordance with previous studies in photochemical reactions, the primary working principle of TiO2@SiO2—AuPd can be hypothesized (
Enclosing the photocatalyst with a water permeable shell is a generalizable pathway to increase the selectivity of various photo-oxidative reactions, which could have a broad impact in the field of clean energy. The potential of the disclosed strategy was demonstrated in the selective transformation of ethane, which is another major component of natural gas.
In summary, an encapsulated photocatalyst with transport selective architectures was designed as a generalizable strategy to achieve high selectivity and activity simultaneously in photochemical alkane oxidation reactions. The transport selective architecture employs a nanometer thick, water permeable oxide shell to prevent photogenerated holes from overoxidizing the oxygenates products and AuPd nanoparticles to enable the diffusion of oxygen as electron scavenger. It is believed that this strategy demonstrates the power of precise nanostructure design for photocatalysis and can be widely applied to catalytic reactions where the decoupling of surface photochemical processes and solution chemical processes is desirable.
All reagents were commercially obtained without purification. Titanium (IV) oxide (P25), Tetraethyl orthosilicate (TEOS), gold chloride trihydrate (HAuCl3·3H2O), palladium chloride (PdCl2), sodium borohydride (NaBH4), polyvinyl pyrrolidone (PVP, Mw=130,000), ammonia solution (28-30%), hydrochloric acid (37%), ethanol, dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich. Methane (99.999% research purity), oxygen (99.999%) and argon (99.999%) were purchased from Airgas. Deionized (DI) water with a resistivity of 18.2 MΩcm−1 was used in all experiments.
TiO2@SiO2 particles are synthesized using modified procedures from earlier work. In a typical synthesis run, 50 mg of commercial TiO2 (P25) powder was dispersed in 25 mL of a mixed solvent of water and ethanol (volume ratio of H2O:EtOH=1:4). A certain amount of TEOS was added to suspension. After sonication for 15 mins, 0.5 mL ammonium hydroxide solution (28-30%) was added into the mixture to catalyze the hydrolysis of TEOS. The reactor was stirred at room temperature overnight. The TiO2@SiO2 samples were collected via centrifuge and washed twice using DI water. TiO2@SiO2 sample were then calcinated under 350° C. for 2 hours in air, unless specified. For the TiO2@SiO2 (550° C.) sample, the synthesis procedure was the same but had a different calcination condition: 550° C. 4 h in air. The thickness of SiO2 was well-controlled by tuning the added TEOS amount; 100 μL TEOS was added to obtain SiO2 shell with ˜5 nm thickness.
The AuPd nanoparticles were synthesized using procedures from previous work. Typically, 12.6 mg HAuCl3·3H2O and 5.67 mg PdCl2 (Au and Pd molar ratio at 1:1) and 11.6 mg PVP (Mw=130,000) were dissolved in 400 mL water. After stirring for 30 minutes, 3.2 mL freshly made NaBH4 (0.1 M) aqueous solution was injected into the above solution. After stirring for another 2 hours, the AuPd colloid was concentrated to 100 mL and stored for future use. The concentration of AuPd NPs solution was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES).
100 mg as-synthesized TiO2@SiO2 powder was dispersed in 30 mL water under sonication. A controlled amount of the AuPd colloid solution was added dropwise to the above solution (the volume of AuPd nanoparticle solution was calculated by the loading and the concentration of AuPd nanoparticle solution). The mixture was stirred at room temperature overnight. The TiO2@SiO2—AuPd samples were collected by centrifuge and then annealed in air at 350° C. for 2 h. The SiO2 shell thickness was 5 nm for TiO2@SiO2—AuPd, unless otherwise specified.
The synthesis procedure was same as the synthesis of TiO2@SiO2—AuPd, except using TiO2 or SiO2 as supporting materials.
Transmission electron microscopy (TEM) was performed on a FEI Tecnai transmission electron microscope with an acceleration voltage of 200 kV. High resolution TEM, scanning transmission electron microscopy (STEM) and energy dispersive spectrometer were performed on a FEI Titan electron microscope with an accelerating voltage of 300 kV. X-ray diffraction (XRD) data were collected on an Empyrean X-ray diffractometer from PANalytical B.V. with Cu Kα (λ=1.5418 Å). The molar ratio of Au, Pd and mass loading of AuPd co-catalysts were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis (Thermo Scientific ICAP 6300 Duo View Spectrometer). Nitrogen sorption isotherms were measured using an Anton PaarAutosorb iQ3 system. UV-vis diffuse reflectance spectra (UV-DRS) were measured by an Agilent Cary 6000i UV/Vis/NIR spectrometer and transformed into absorption spectra via Kubelka-Munk transformation. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) experiments at Au L3-edges and Pd K-edge were carried out in the fluorescence mode at the beamline 20-ID of Advanced Photon Source at Argonne National Laboratory. The incident beam was monochromatized by using a Si (111) fixed-exit, double-crystal monochromator, a harmonic rejection mirror was applied to cut off the harmonics at high X-ray energy. Data reduction, data analysis, and EXAFS fitting (Table 4) were performed with the Athena and Artemis software packages.
The photocatalytic reactions were performed in a 250 mL batch photoreactor equipped with a quartz window to allow for light irradiation (
The photoreactor was directly connected to a gas chromatograph (SRI instrument MG #5), equipped with a thermal conductivity detector (TCD), a flame ionization detector (FID) and a methanizer for the gas product analysis of CO2, CO, and ethane. The liquid oxygenate products were analyzed using nuclear magnetic resonance spectroscopy (NMR) and the colorimetric method. CH3OH, CH3OOH and HCOOH were quantified via 1H-NMR on a Varian Inova 600 MHz NMR equipped with a water suppression system. Typically, 0.63 mL of liquor was mixed with 0.17 mL of D2O to prepare a solution for NMR measurements. Dimethyl sulfoxide (DMSO) was used as an internal standard. The formaldehyde (HCHO) amount was determined by the colorimetric method. First, 15 g ammonium acetate, 0.3 mL of acetic acid, and 0.2 mL pentane-2,4-dione were dissolved in 100 mL water, to make the reagent solution. Then, 0.5 mL of the sample liquor was mixed with 2 mL of water and 0.5 mL of reagent solution. The mixture was kept at 35° C. for 1 hour in a water bath and measured by UV-vis absorption spectroscopy at 412 nm (Agilent Cary 6000i UV/Vis/NIR spectrometer). The concentration of HCHO in the sample liquor was determined by the calibration curve using a series of standard HCHO solutions.
The methane conversion and oxygenates selectivity in this process are calculated according to the following equations:
The initial methane amount in the system is calculated according to the following equation (V is the volume of the headspace of the reactor):
The apparent quantum yield (AQY) was calculated according to the following equation:
where N(electrons) and N(photons) represent the number of reacted electrons and the number of incident photons, respectively. N(photons)=IAt/Eλ, where I, A, t and Eλ represent incident light intensity (W/cm2), irradiation area (cm2), light incident time (s) and photo energy (J), respectively. For the calculation of N(electrons), an approach following reported work was used: N(electrons)=n(CH3OOH)+n(CH3OH)×3+n(HCHO)×5+n(HCOOH)×7+n(CO2)×9, where n(CH3OOH), n(CH3OH), n(HCHO), n(HCOOH) and n(CO2) represent the mole numbers of produced CH3OOH, CH3OH, HCHO, HCOOH and CO2 molecules, respectively.
To study the reusability of the catalyst, the solid catalyst was separated by centrifugation after each reaction run. The catalysts were re-used in the next run after drying at 90° C. overnight under vacuum and annealed at 300° C. in air to remove any adsorbed organic species.
In the 13CH4 isotopic experiments, 10 mg TiO2@SiO2—AuPd photocatalyst were dispersed in 20 mL H2O and degassed for 30 min to completely remove air. 1.73 bar (25 psi) 13CH4 (99 atom % 13C, Sigma Aldrich), 5.17 bar (75 psi) 12CH4 and 2.75 bar O2 were added to the photoreactor. The reaction was carried for 1 h under light irradiation (130 mW/cm2). The liquid products were collected and measured by 1H-NMR (Varian Inova 600 MHz).
In the 18O2 isotopic experiments, 10 mg TiO2@SiO2—AuPd photocatalyst was dispersed in 100 mL H2O and degassed for 30 min to completely remove air. 8.27 bar (120 psi) CH4 and 1.38 bar (20 psi) 18O2 (99 atom % 18O, Sigma Aldrich) were added to the photoreactor. The reaction was carried for 4 h under light irradiation (130 mW/cm2). The products were measured by GC-MS (Agilent 7890B GC with Agilent 5977A MS).
Analysis of Photogenerated Superoxide Anion Radicals (O2·−)
2,3-Bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt (XTT salt) was used as the indicator for the O2·− radical measurements. XTT is reduced by O2·− to form the orange-colored XTT-formazan, which was measured using a UV-vis spectrophotometer at 470 nm. Typically, 10 mg of catalyst was dispersed in 100 mL of a 0.1 mM XTT aqueous solution in dark under stirring. Argon was purged through the reactor several times to remove air. 6.90 bar (100 psi) argon (99.999%, Airgas) and 0.34 bar (5 psi) O2 (21% O2 balanced with Ar, Airgas) were added into the reactor. After irradiation for 10 mins, the reactant solution was collected by centrifugation and then used for UV-Vis measurements.
The hydroxyl radicals (·OH) production was measured by photoluminescence (PL) experiments using coumarin as a probe molecule. Coumarin reacts with ·OH to form 7-hydroxycoumarin (7-HC), which gives high fluorescence at around 454 nm (
TON is calculated via the following formula:
As there are no well-established techniques for precisely measuring the number of active sites in heterogeneous photocatalysts, the number of moles of catalysts is used to represent the moles of active sites according to the reported work. It is worth noting that the calculated TON using this method is a lower limit for TON since the number of active sites in moles is lower than the amount of catalyst in moles.
The mole numbers of total products after 3 h reaction for 10 mg TiO2@SiO2—AuPd catalysts is 339.2 μmol (
It has been demonstrated that SiO2 is catalytically inert for the reaction (
We first find that the total yields as well as the O2·− radicals (or ·OOH radicals) production are strongly suppressed by the SiO2 shell, which is alleviated by the addition of AuPd (
Given that AuPd nanoparticles supported on metal oxides has been reported to facilitate O2 dissociation, the primary role of AuPd on SiO2 is to activate oxygen molecules to atomic oxygen species (O) so that it can penetrate the silica shell and scavenge electron from TiO2. This is herein called “O transport mechanism,” which is discussed herein and shown in
However, an alternative process was also explored. Water, instead of O2, could serve as an electron scavenger, reacting with photogenerated electrons to form ·H atoms (reaction A2 below), which can diffuse through SiO2 shell via spillover. These ·H atoms then react with O2 to generate ·OOH radicals, which could be promoted by AuPd nanoparticles. Herein, this is called the “H transport mechanism” as shown in
First, considering the H-transport mechanism, the key reactions with electrons and holes on TiO2 surface:
H2O+h+→H++·OH, water oxidation (A1)
H2O+e−→·H+OH· (or H++e−→H), water reduction (A2)
Consider now that H atoms and ·OH radicals diffuse from the TiO2 surface through SiO2 layer to be released to the bulk water solution or find AuPd catalyst surface.
·H+O2→·OOH (facilitated by AuPd) (A3)
·H+·OH→H2O (side reaction that eliminates ·H and ·OH) (A4)
·H+·H→H2 (hydrogen gas production) (A5)
H2+½O2→H2O (in the presence of O2 and AuPd catalyst, H2 and O2 will form H2O) (A6)
·OH+CH4→·CH3+H2O (A7)
As methane is also blocked by the SiO2 shell, it cannot be activated directly by holes on TiO2 surface. The major contributor for methane activation is ·OH and ·OOH radicals, whose quantities are directly correlated with the total product yields as well as the final product distribution.
Now let us consider the O-transport mechanism. From numerous previous works, for TiO2—AuPd, the electron scavenging process is through oxygen reacting with photogenerated electrons. In this proposed mechanism for TiO2@SiO2—AuPd, the electron scavenging is carried out with oxygen. The potential reactions are as follows:
O2→2O (in the presence of AuPd; O transports through SiO2 to reach TiO2 surface) (B1)
H2O+h+→H++·OH, water oxidation on TiO2 surface (B2)
2O+e−→O2−, on TiO2 surface (B3)
For AuPd/TiO2, the product yields and distributions are almost identical for 0.55 and 2.75 bar of O2, suggesting that concentration of O2 is not the limiting factor of the total yields (
As the SiO2 thickness is increased to 5 nm, the O2 partial pressure dependence of the total yield emerges (
The combination of experimental results for different catalyst structures and reaction conditions suggests that in many cases the hole reaction with water (H2O+h+→H++·OH) could be the rate limiting step, especially in the absence or very small thicknesses of the SiO2 shell around the TiO2 core. The data also seem to suggest that H transport through the SiO2 layer is unlikely a key mechanism for these photocatalytic reactions. Instead, it is likely O2 dissociation on AuPd into O atoms and O transport through the SiO2 shell as the key electron scavenging mechanism. Finally, depending on the thickness of the SiO2 shell, O-transport could become competitive with hole-water reaction as the rate limiting step. For the experimental conditions investigated here, it is the combination of these two rate-limiting steps that control the total reaction yields, whereas the selectivity for partial oxygenation of CH4 is controlled by the selective transport through the SiO2 shell.
It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.
This application claims the benefit of U.S. Provisional Application No. 63/320,721, filed on Mar. 17, 2022, which is incorporated herein by reference in its entirety.
This invention was made with Government support under contract DE-FE0031867 awarded by the Department of Energy and under contract N00014-17-1-2918 awarded by the Office of Naval Research. The Government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2023/063432 | 3/1/2023 | WO |
Number | Date | Country | |
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63320721 | Mar 2022 | US |