Patent Application
20210170377
Decades of research devoted to zeolite and zeotype catalysts have focused on understanding how the unique pore structure of these materials enables size- and shape-selective catalysis. The rates and selectivities toward specific, desired catalytic transformations depends critically on the dispersive interactions between the micropores of the zeolite catalyst and the reactants contained within these spaces. These interactions provide a basis to modify rates and selectivities by stabilizing surface intermediates and transition states along reaction coordinates.
Zeolite catalysts that contain framework-substituted Lewis acidic heteroatoms (e.g., Ti, Sn, Nb, Ta) catalyse stereoselective mono- and di-saccharide isomerization, aldol condensation, alcohol dehydration, Baeyer-Villiger oxidation, and alkene epoxidation reactions. For example, the discovery that Sn-substituted zeolite *BEA (Sn-BEA) activates ketones for Baeyer-Villiger oxidation with hydrogen peroxide (H2O2) catalysed a renaissance of work with Sn-BEA for use in other reactions (e.g., glucose isomerization, ethanol dehydration). Titanium-substituted silicalite-1 (TS-1) was developed in the 1980's and has found industrial application as a catalyst for propylene epoxidation with H2O2, which inspired the development of subsequent design principles for epoxidation reactions. The presence of Brønsted acid sites (e.g., those formed at framework Al or B atoms)) catalyze secondary reactions that are detrimental to the desired chemistry, even when Al atoms remain in trace quantities (Si:Al=10-150) (J. Catal., 2018, 368, 145). In the context of alkene epoxidation, the desired epoxides formed by primary reaction pathways readily undergo undesirable ring opening hydrolysis over Brønsted acids, which decreases yields. Therefore, researchers invested significant effort into developing synthetic methods that avoid forming these deleterious sites through direct hydrothermal synthesis or through the post-synthetic removal of adventitious Al atoms.
The zeolite faujasite (FAU) is an important structure used in 95% of the catalyst market (by mass), due to its widespread use in fluid catalytic cracking units. The synthesis of Al-free metal-substituted FAU (M-FAU) materials have not been reported, despite the widespread use of this framework in the oil and gas industries. Current methods for the synthesis of M-FAU either rely on the post-synthetic modification of Al-FAU or direct hydrothermal synthesis in the presence of AlNaO2. For example, Trejda et al. synthesized Nb- and Ta-substituted FAU hydrothermally in the presence of sodium aluminate (needed to crystalize the structure) yet, this approach produced in M-Al-FAU materials with a Si:Al ratio equal to 2.3 (Catal. Today, 2010, 158, 170). Methods for the post-synthetic modification of Al-FAU rely on steaming at high temperatures or acid treatments to partially remove Al from framework positions and create vacant silanol nest ((SiOH)4) defects that can be substituted with the desired metal atoms. In all previous reports, a significant amount of Al (Si:Al ratios=19-150) remains, which is detrimental for a number of relevant reactions and leads to significant reductions in yields and selectivities. Consequently, the advent of synthetic methods to produce Al-free M-FAU would be impactful for numerous industrially practiced catalytic reactions including alkene epoxidation and biomass upgrading.
The problem is millions of metric tons of zeolites are synthesized annually for production of chemicals; yet, there is no process for creating Al-free FAU for desired applications. Accordingly, there is a need for a process that allows these inexpensively obtained zeolites to be modified to perform new catalytic reactions that are relevant for more efficient production of chemicals and fuels from fossil resources or renewable biomass resources and as selective adsorbent materials.
This disclosure shows that sequential treatments of Al-FAU in concentrated HNO3 removes nearly all of the Al within these materials and increase Si:Al ratios from 15 to values greater than 900. The incorporation of base and early-transition metals into the FAU framework preserves the crystallinity of these materials and is confirmed using a combination of techniques including X-ray diffraction, N2 volumetric adsorption, diffuse-reflectance UV-vis spectroscopy, 29Si nuclear magnetic resonance spectroscopy, and infrared spectroscopy. Infrared spectra of pyridine adsorbed within M-FAU synthesize by these methods show the presence of Lewis acid sites and undetectable numbers of Brønsted acid sites, within the sensitivity of the methods used.
Accordingly, this disclosure provides a modified zeolite comprising dealuminated faujasite that has a crystalline framework and micropores, wherein a metal heteroatom (M) is integrated into a dealuminated node of the dealuminated faujasite via a M-O—Si linkage;
wherein the modified zeolite has a silicon to aluminum (Si:Al) mole ratio of about 200 or greater and a silicon to integrated metal (Si:M) mole ratio of about 15 or greater.
In various embodiments, the nodes comprise one or more integrated metals (M) other than aluminum, such as titanium (see
This disclosure also provides a method for forming the modified zeolite as described above comprising:
Additionally, this disclosure provides a method for catalyzing an oxidation reaction comprising:
contacting the modified zeolite catalyst as described above, an oxidizing agent and a substrate under suitable catalytic reaction conditions;
wherein the substrate and oxidizing agent have a sufficiently appropriate size to enter a micropore of the modified zeolite for catalysis, wherein the substrate undergoes an oxidation reaction at an integrated metal heteroatom inside the micropore that is accessible for catalyzing the oxidation reaction.
The following drawings form part of the specification and are included to further demonstrate certain embodiments or various aspects of the invention. In some instances, embodiments of the invention can be best understood by referring to the accompanying drawings in combination with the detailed description presented herein. The description and accompanying drawings may highlight a certain specific example, or a certain aspect of the invention. However, one skilled in the art will understand that portions of the example or aspect may be used in combination with other examples or aspects of the invention.
represents a quasi-equilibrated step, while
represents a kinetically relevant step.
Six gaussian curves were chosen to fit the infrared spectra of M-FAU in order to capture the proper curvature and yield a R2 value >0.995. Fitting procedures using three-to-five gaussian curves yielded nearly identical values of $m.
Heteroatom framework-substituted zeolites are important materials that enable shape- and size-selective catalysis. The efficacy of these materials for desired catalytic reactions depends critically on dispersive interactions between the microporous void of the zeolite and the reactant molecules stabilized within it.
Synthesis of zeolites that contain aluminum is well-developed and relatively cost effective, however, the synthesis of these same structures when they contain other catalytically active elements (e.g., titanium, tin, niobium, tantalum, hafnium, zirconium, tungsten, molybdenum, etc.) is more time consuming, less reliable, provide lower yields, gives lower densities of active metal atoms per gram catalyst, requires expensive synthesis additives (called structure directing agents), or the methods simply do not exist. A pre-existing example is the synthesis of two widely used zeolite catalysts-titanium beta (Ti-BEA) and tin beta (Sn-BEA). These catalysts can be created directly by hydrothermal methods, however, the dealumination of a standard aluminum beta (Al-BEA) by extended thermal treatments in concentrated acids (e.g., nitric, sulfuric, or hydrochloric acids) can remove the aluminum atoms leaving behind point defects. The adsorption of Ti or Sn metal complexes onto these point defects from a liquid solvent (e.g., organic phases) followed by drying and oxidation at elevated temperatures (>400 C) integrates these metal atoms into the crystal structure in a catalytically active form.
Here, we develop a post-synthetic method to synthesize base and transition metal-substituted (Ti, Nb, Ta, and Sn) FAU with ultralow Al contents (Si:Al >900), which is confirmed using X-ray diffraction, elemental analysis, and N2 volumetric adsorption and 29Si MAS-NMR, DRUV-vis, and IR spectroscopic characterization. Ti-FAU selectively stabilized styrene (C8H8) within its pores during epoxidation with H2O2. Turnover rates for C8H8 epoxidation within Ti-FAU are 2- and 7-fold greater than in analogous Ti-BEA and Ti—SiO2, respectively; yet, turnover rates of H2O2 decomposition are similar for all three materials. Consequently, Ti-FAU gives greater rates and selectivities for this reaction than common Ti-bearing silicates. The mechanism for epoxidation remains constant for all Ti-silicates examined (i.e., Ti-FAU, Ti-BEA, and Ti—SiO2). Therefore, the improved performance of Ti-FAU reflects differences in activation free energies for epoxidation that show an enthalpic preference in Ti-FAU relative to Ti—SiO2 and an entropic gain relative to Ti-BEA. These results demonstrate the synthesis of M-FAU with ultralow Al contents are useful for catalytic reactions involving bulky reactants that cannot occur in smaller pore zeotype materials (Ti-MFI), that exhibit deactivation due to changes in Ti-atom coordination (e.g., Ti—SiO2), and that are prone to losses catalyzed by residual Brønsted acid sites (e.g., epoxidations, oxidations, and isomerization reactions).
The following definitions are included to provide a clear and consistent understanding of the specification and claims. As used herein, the recited terms have the following meanings. All other terms and phrases used in this specification have their ordinary meanings as one of skill in the art would understand. Such ordinary meanings may be obtained by reference to technical dictionaries, such as Hawley's Condensed Chemical Dictionary 14th Edition, by R. J. Lewis, John Wiley & Sons, New York, N.Y., 2001.
References in the specification to “one embodiment”, “an embodiment”, etc., indicate that the embodiment described may include a particular aspect, feature, structure, moiety, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, moiety, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, moiety, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect or connect such aspect, feature, structure, moiety, or characteristic with other embodiments, whether or not explicitly described.
The singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to “a compound” includes a plurality of such compounds, so that a compound X includes a plurality of compounds X. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as “solely,” “only,” and the like, in connection with any element described herein, and/or the recitation of claim elements or use of “negative” limitations.
The term “and/or” means any one of the items, any combination of the items, or all of the items with which this term is associated. The phrases “one or more” and “at least one” are readily understood by one of skill in the art, particularly when read in context of its usage. For example, the phrase can mean one, two, three, four, five, six, ten, 100, or any upper limit approximately 10, 100, or 1000 times higher than a recited lower limit.
As will be understood by the skilled artisan, all numbers, including those expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth, are approximations and are understood as being optionally modified in all instances by the term “about.” These values can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the descriptions herein. It is also understood that such values inherently contain variability necessarily resulting from the standard deviations found in their respective testing measurements. When values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value without the modifier “about” also forms a further aspect.
The terms “about” and “approximately” are used interchangeably. Both terms can refer to a variation of ±5%, ±10%, ±20%, or ±25% of the value specified. For example, “about 50” percent can in some embodiments carry a variation from 45 to 55 percent, or as otherwise defined by a particular claim. For integer ranges, the term “about” can include one or two integers greater than and/or less than a recited integer at each end of the range. Unless indicated otherwise herein, the terms “about” and “approximately” are intended to include values, e.g., weight percentages, proximate to the recited range that are equivalent in terms of the functionality of the individual ingredient, composition, or embodiment. The terms “about” and “approximately” can also modify the endpoints of a recited range as discussed above in this paragraph.
As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. It is therefore understood that each unit between two particular units are also disclosed. For example, if 10 to 15 is disclosed, then 11, 12, 13, and 14 are also disclosed, individually, and as part of a range. A recited range (e.g., weight percentages or carbon groups) includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, all language such as “up to”, “at least”, “greater than”, “less than”, “more than”, “or more”, and the like, include the number recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio. Accordingly, specific values recited for radicals, substituents, and ranges, are for illustration only; they do not exclude other defined values or other values within defined ranges for radicals and substituents. 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.
This disclosure provides ranges, limits, and deviations to variables such as volume, mass, percentages, ratios, etc. It is understood by an ordinary person skilled in the art that a range, such as “number1” to “number2”, implies a continuous range of numbers that includes the whole numbers and fractional numbers. For example, 1 to 10 means 1, 2, 3, 4, 5, . . . 9, 10. It also means 1.0, 1.1, 1.2. 1.3, . . . , 9.8, 9.9, 10.0, and also means 1.01, 1.02, 1.03, and so on. If the variable disclosed is a number less than “number10”, it implies a continuous range that includes whole numbers and fractional numbers less than number10, as discussed above. Similarly, if the variable disclosed is a number greater than “number10”, it implies a continuous range that includes whole numbers and fractional numbers greater than number10. These ranges can be modified by the term “about”, whose meaning has been described above.
One skilled in the art will also readily recognize that where members are grouped together in a common manner, such as in a Markush group, the invention encompasses not only the entire group listed as a whole, but each member of the group individually and all possible subgroups of the main group. Additionally, for all purposes, the invention encompasses not only the main group, but also the main group absent one or more of the group members. The invention therefore envisages the explicit exclusion of any one or more of members of a recited group. Accordingly, provisos may apply to any of the disclosed categories or embodiments whereby any one or more of the recited elements, species, or embodiments, may be excluded from such categories or embodiments, for example, for use in an explicit negative limitation.
The term “contacting” refers to the act of touching, making contact, or of bringing to immediate or close proximity, including at the cellular or molecular level, for example, to bring about a physiological reaction, a chemical reaction, or a physical change, e.g., in a solution, in a reaction mixture.
An “effective amount” refers to an amount effective to bring about a recited effect, such as an amount necessary to form products in a reaction mixture. Determination of an effective amount is typically within the capacity of persons skilled in the art, especially in light of the detailed disclosure provided herein. The term “effective amount” is intended to include an amount of a compound or reagent described herein, or an amount of a combination of compounds or reagents described herein, e.g., that is effective to form products in a reaction mixture. Thus, an “effective amount” generally means an amount that provides the desired effect.
The term “substantially” as used herein, is a broad term and is used in its ordinary sense, including, without limitation, being largely but not necessarily wholly that which is specified. For example, the term could refer to a numerical value that may not be 100% the full numerical value. The full numerical value may be less by about 1%, about 2%, about 3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, about 10%, about 15%, or about 20%.
Wherever the term “comprising” is used herein, options are contemplated wherein the terms “consisting of” or “consisting essentially of” are used instead. As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the aspect element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the aspect. In each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” may be replaced with either of the other two terms. The disclosure illustratively described herein may be suitably practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.
This disclosure provides methods of making the compounds and compositions of the invention. The compounds and compositions can be prepared by any of the applicable techniques described herein, optionally in combination with standard techniques of organic synthesis. Many techniques such as etherification and esterification are well known in the art. However, many of these techniques are elaborated in Compendium of Organic Synthetic Methods (John Wiley & Sons, New York), Vol. 1, Ian T. Harrison and Shuyen Harrison, 1971; Vol. 2, Ian T. Harrison and Shuyen Harrison, 1974; Vol. 3, Louis S. Hegedus and Leroy Wade, 1977; Vol. 4, Leroy G. Wade, Jr., 1980; Vol. 5, Leroy G. Wade, Jr., 1984; and Vol. 6; as well as standard organic reference texts such as March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th Ed., by M. B. Smith and J. March (John Wiley & Sons, New York, 2001); Comprehensive Organic Synthesis. Selectivity, Strategy & Efficiency in Modern Organic Chemistry. In 9 Volumes, Barry M. Trost, Editor-in-Chief (Pergamon Press, New York, 1993 printing); Advanced Organic Chemistry, Part B: Reactions and Synthesis, Second Edition, Cary and Sundberg (1983).
The formulas and compounds described herein can be modified using protecting groups. Suitable amino and carboxy protecting groups are known to those skilled in the art (see for example, Protecting Groups in Organic Synthesis, Second Edition, Greene, T. W., and Wutz, P. G. M., John Wiley & Sons, New York, and references cited therein; Philip J. Kocienski; Protecting Groups (Georg Thieme Verlag Stuttgart, New York, 1994), and references cited therein); and Comprehensive Organic Transformations, Larock, R. C., Second Edition, John Wiley & Sons, New York (1999), and referenced cited therein.
As used herein, the term “substituted” or “substituent” is intended to indicate that one or more (for example, 1-20 in various embodiments, 1-10 in other embodiments, 1, 2, 3, 4, or 5; in some embodiments 1, 2, or 3; and in other embodiments 1 or 2) hydrogens on the group indicated in the expression using “substituted” (or “substituent”) is replaced with a selection from the indicated group(s), or with a suitable group known to those of skill in the art, provided that the indicated atom's normal valency is not exceeded, and that the substitution results in a stable compound.
A “solvent” as described herein can include water or an organic solvent. Examples of organic solvents include hydrocarbons such as toluene, xylene, hexane, and heptane; chlorinated solvents such as methylene chloride, chloroform, and dichloroethane; ethers such as diethyl ether, tetrahydrofuran, and dibutyl ether; ketones such as acetone and 2-butanone; esters such as ethyl acetate and butyl acetate; nitriles such as acetonitrile; alcohols such as methanol, ethanol, and tert-butanol; and aprotic polar solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and dimethyl sulfoxide (DMSO). Solvents may be used alone or two or more of them may be mixed for use to provide a “solvent system”.
The phrase “one or more” is readily understood by one of skill in the art, particularly when read in context of its usage. For example, one or more metals means each of the one or more metals can be different.
Zeolites are microporous, aluminosilicate minerals that can be used as catalysts for chemical reactions. Faujasite is an example of a zeolite. The faujasite framework consists of sodalite cages which are connected through hexagonal prisms. The pore, which is formed by a 12-membered ring, has a relatively large diameter of 7.4 Å. The supercage comprises an inner cavity of the has a diameter of 12 Å and is surrounded by 10 sodalite cages.
The term “node” refers to a point at which lines or pathways intersect or branch; e.g., a central or connecting point. As used herein, the node refers to the point where a metal atom may or may not be positioned within the crystalline framework of the zeolite. The node may comprise an aluminum or titanium atom or another metal atom as disclosed herein. The node may also comprise a void in place of, for example, an aluminum atom that was positioned at the node, thereby filling the void.
Early transition metals are defined as metals on the left side of the periodic table from group 3 to group 7. Late transition metals are defined as metals on the right side of the d-block, from group 8 to 11. Other metals can be selected from group 12-14.
This disclosure provides a modified zeolite comprising dealuminated faujasite (FAU) that has a crystalline framework and micropores, wherein a metal heteroatom (M) is integrated into a dealuminated node of the dealuminated faujasite via a M-O—Si linkage;
wherein the modified zeolite has a silicon to aluminum (Si:Al) mole ratio of about 200 or greater and a silicon to integrated metal (Si:M) mole ratio of about 15 or greater.
In additional embodiments, the micropores have diameters of about 1 nm to about 2 nm. In other embodiments, the diameters are about 0.4 nm, about 0.6 nm, about 0.8 nm, about 1.5 nm, about 2.5 nm, or about 3 nm.
In further embodiments, the mole ratio of silicon to aluminum is greater than 200. In other embodiments, the ratio is about 10, about 15, about 20, about 25, about 50, about 100, about 150, about 200, about 250, about 300, about 350, about 400, about 450, about 500, about 550, about 600, about 650, about 700, about 750, about 800, about 850, about 900, about 950 or more, about 1000, about 1100, about 1200, about 1300, about 1400, about 1500, about 2000, about 3000, or about 4000.
In some other embodiments, the mole ratio of silicon to aluminum is about 500 to about 2000, about 750 to about 1000, about 900 to about 1500, or about 950 to about 2000. It is noted that sufficiently high Si:Al ratio is required to reduce or eliminate deleterious side products from reactions that occur at Brønsted acid sites associated with the Al atoms. For example, when performing epoxidations diol formation become problematic when the Si:Al ratio is not sufficiently high.
In various embodiments, the modified zeolite is dealuminated: mordenite (MOR), chabazite (CHA), Socony Mobil-five (MFI), or Mobil Composition of Matter-twenty-two (MWW). It is noted that zeolites are designated by their three-letter codes, as known to persons of ordinary skill in the art.
In additional embodiments, the nodes comprise one or more integrated metals (M), early transition metals, late transition metals, or other metals/metaloids. In other embodiments, the early transition metals, late transition metals, lanthanides, or other metals are exposed at the inner surface of the micropores.
In additional embodiments, the early transition metals are one or more of titanium, tin, niobium, iron, cerium, lanthanum, tantalum, zirconium, hafnium, molybdenum, and tungsten. In other embodiments, the other metals are tin and/or germanium.
In further embodiments, the early transition metals are integrated as M-O—Si linkages, or the early transition metals are bonded to siliceous nests as M-O—Si linkages, as can be determined by diffuse reflectance UV-VIS spectroscopy, XRD, and FTIR spectroscopy. In some other embodiments, the early transition metals are essentially free of M-O-M linkages as can be determined by diffuse reflectance UV-VIS and Raman spectroscopy.
In yet other embodiments, the zeolite is essentially free of silanol as can be determined by infrared spectroscopy. In some embodiments, the (SiOH)4 (formed by dealumination) is consumed when a metal (other than aluminum) is nested or bonded to the silanol moieties. In some preferred embodiments, the dealuminated zeolite is FAU, the integrated metal is titanium, and the mole ratio of silicon to aluminum is about 200 to about 2000.
This disclosure also provides a modified faujasite zeolite comprising a crystalline framework, micropores, and nodes in the crystalline framework comprising integrated titanium, wherein the mole ratio of silicon to aluminum in the modified faujasite is 900 or more and the modified faujasite comprises less aluminum compared to an unmodified faujasite zeolite.
This disclosure additionally provides a modified faujasite zeolite comprising a dealuminated crystalline framework, micropores, and nodes in the crystalline framework void of aluminum, wherein the mole ratio of silicon to aluminum in the modified faujasite is 900 or more and the modified faujasite comprises less aluminum compared to an unmodified faujasite zeolite.
Also, this disclosure provides a method for forming a modified zeolite comprising:
In some embodiments, the contacting is at the reflux temperature (or lower) of the mineral acid or organic acid solutions. In other embodiments, the mineral acid is nitric acid, sulfuric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, or a combination thereof. In yet other embodiments, the organic acid is acetic acid, formic acid, oxalic acid, citric acid, ethylenediaminetetraacetic acid, other chelating organic acid complexes, or a combination thereof. In various embodiments, the micropores have diameters of about 1 nm to about 2 nm, or about 0.5 nm to about 3 nm.
In other embodiments, the method comprises integrating heteroatoms into the nodes of the dealuminated zeolite. In additional embodiments, the modified zeolite is formed at a second temperature of greater than 400° C. In some embodiments the first temperature or second temperature is independently about 200° C. to about 400° C., about 400° C. to about 600° C., about 600° C. to about 800° C., about 450° C., about 500° C., or about 550° C.
In other embodiments, the method further comprises integrating a metal heteroatom into a dealuminated node of the dealuminated faujasite. In various embodiments, the heteroatoms are integrated by anhydrous liquid-phase grafting. In additional embodiments, the integrated heteroatoms are early transition metals, late transition metals, or other metals. In other various embodiments, the early transition metals, late transition metals, or other metals are exposed at the surface of the micropores. In further embodiments, the parent zeolite is faujasite (FAU), the early transition metals are titanium, and the mole ratio of silicon to aluminum is about 200 to about 2000.
Additionally, this disclosure provides a method for catalyzing a chemical reaction comprising contacting a molecule and a zeolite catalyst under suitable catalytic reaction conditions,
wherein the zeolite catalyst comprises a dealuminated zeolite having a crystalline framework, micropores, and nodes in the crystalline framework integrated with one or more integrated metals, and the zeolite catalyst has a mole ratio of silicon to aluminum of greater than 150; and
the molecule has a sufficiently small size to enter the micropores and undergo the catalytic chemical reaction.
In some other embodiments, the early transition metals, late transition metals, or other metals are exposed at the surface of the micropores. In yet other embodiments, the micropores have diameters of about 1 nm to about 2 nm. In further embodiments, the contacting comprises an oxidizing agent, the catalyzed chemical reaction affords an oxidized molecule, and the turnover rate of the catalyzed chemical reaction is at least two-times greater than a corresponding zeolite catalyst that has a mole ratio of silicon to aluminum of less than 150 or is not dealuminated FAU.
In additional embodiments, the molecule is an olefin, the oxidizing agent is a peroxide, the crystalline framework of the zeolite catalyst is FAU, the early transition metal is titanium, and the mole ratio is about 200 to about 2000. In some embodiments, oxidizing agent is a peroxide or hydroperoxide.
In yet other embodiments, the suitable catalytic reaction conditions comprise the contacting at the early transition metals exposed at the surface of the micropores, a polar aprotic solvent, and a catalytic reaction temperature of about −10° C. to about 80° C. In some embodiments the solvent is water or a polar protic solvent. In other embodiments, the reaction temperature is about 20° C. to about 60° C., about 25° C. to about 55° C., about 55° C. to about 155° C., or about 100° C. to about 200° C.
Heteroatom Substituted Zeolite FAU with Ultralow Al Contents for Liquid-Phase Oxidation Catalysis. Comparisons of the rates, selectivities, and yields for styrene (C8H8) epoxidation with H2O2 over Ti-FAU to that in other Ti-silicates demonstrates advantages of locating Ti active sites within the supercages of the FAU structure. Turnover rates for styrene epoxidation over Ti-FAU are 2- and 7-fold greater than those in Al-free Ti-BEA (0.65 nm pore diameter) and Ti—SiO2 (5.4 nm pore diameter), respectively. Rates of H2O2 decomposition (2.7±0.9 (mmol H2O2)(mol Ti·s)−1) remain constant for all structures, and therefore, do not depend on the characteristic dimensions of these Ti-silicate catalysts. Consequently, Ti-FAU gives greater selectivities and greater turnover rates for styrene epoxidation than Ti-BEA and Ti—SiO2. Mechanistic interpretation of epoxidation rates measured as a function of reactant concentrations show that all Ti-silicates irreversibly activate H2O2 to form Ti—OOH intermediates that react with C8H8 in a kinetically relevant step to form styrene oxide. Measured activation enthalpies for C8H8 epoxidation with pore size among these materials (e.g., Ti—SiO2 (37±4 kJ mol−1)>Ti-FAU (22±2 kJ mol−1) >Ti-BEA (9±2 kJ mol−1)), which shows that the micropores of BEA enthalpically stabilize C8H8 epoxidation transition states relative to the Ti—OOH reactive intermediate. Apparent activation entropies, however, become increasingly negative with decreasing pore size (−155 to −234 J mol−1 K−1) suggesting that the dispersive interactions between C8H8 epoxidation transition states and the pore walls of BEA results in the significant loss of translational and vibrational entropy. Consequently, the greater rates and selectivities for C8H8 epoxidation within Ti-FAU arise from the balance of enthalpy-entropy compensation effects that depend on the pore structure of the silicate. The 1.2 nm voids of FAU give enthalpic benefits for C8H8 epoxidation, relative to Ti—SiO2, yet does not incur the same entropic losses suffered within Ti-BEA. Collectively, these methods and data provide a pathway to synthesize Al-free M-FAU materials, from synthetic or natural Al-FAU, that can be used for adsorption, separations, or catalysis, and which can allow for the access of other Ti-zeolite structures through inter-zeolitic transformations.
Crystallinity of FAU is Maintained During Post-Synthetic Modification. Titanium, niobium, tantalum, and tin framework-substituted FAU (M-FAU) were synthesized through the post-synthetic modification of H+-form Al-FAU.
aMeasured by EDXRF.
bDetermined by DRUV-vis spectroscopy by extrapolating the linear portion of the leading edge of the corresponding Tauc plot (FIG. 3).
cEstimated by taking the ratio of the intensity for the 10.3° to the 6.3° diffraction features and assuming perfect 100% crystallinity for Al—FAU.
dCalculated using N2 adsorption isotherms (FIG. 2a).
eQuantified using 29Si MAS-NMR (FIG. 4).
fDetermined from FTIR spectra of dehydrated M—FAU (FIG. 5a)
gNMR features for Si atoms residing as Si(OAl)(OSi)3 and Si(OSi)3OH overlap and precludes determination.
hν(O—H) of H+ bound to Si—O—Al moieties overlap with ν(O—H) of hydrogen-bonded SiOH, which prevents quantification
Post-Synthetic Modification Does Not Form Mesopores. Nitrogen adsorption isotherms (77 K;
The surface area of M-FAU was estimated using BET theory and yielded equivalent values for the three samples tested (740-800 m2 g−1, Table 1), which further suggests that post-synthetic modification of Al-FAU does not modify the long-range order of the framework or create sufficiently large defects that change the internal surface area.
Metal Sites Within FAU Framework are Highly Disperse. Optical band gaps reports on the speciation (and dispersion) of semi-conducting and insulating solids.
Grafting of Metal Ions Leads to Reduction in the Number of SiOH. The fraction of Si atoms that exist as SiOH within each M-FAU can provide indirect evidence for the formation and elimination of (SiOH)4 upon dealumination and subsequent metal-ion incorporation.
The fraction of Si atoms that reside as Si(OSi)3OH (ϕNMR) is described by the fraction of Q3 sites to the sum of Q3 and Q4 sites
where AQ
A statistical distribution of (SiOH)4 within the framework before and after the substitution of M atoms (assuming a final Si:M ratio of ˜40; Table 1) suggests a 35% reduction in the total number of (SiOH)4, which qualitatively agrees with the measured changes in ϕNMR between Si-FAU and M-FAU (˜44% reduction). Values of ϕNMR, however, do not directly represent the density of (SiOH)4 (i.e., SiOH formed upon dealumination), because these values encompass all Si atoms that exist as SiOH including those at point defects and on the external surface of FAU particles.
Metal Ions Within M-FAU Occupy (SiOH)4 and Are Lewis Acidic. Isolated SiOH and hydrogen-bonded (SiOH)4 moieties in M-FAU materials possess distinct ν(O—H) that can be used to yield semiquantitative estimates for the relative density of (SiOH)4 groups.
Relative densities of isolated SiOH and (SiOH)4 among M-FAU samples are estimated by normalizing each infrared spectra to the total number of framework bonds, which is assumed to be constant among these samples and is represented by the intensity of the ν(Si—O—Si) (1865 cm−1) within this study. Peak fitting (Section S3.2) of the ν(O—H) region allows for the deconvolution of isolated SiOH (3740 cm−1) and (SiOH)4 groups (3300-3740 cm−1); where the ratio of the cumulative area of ν(O—H) for (SiOH)4 (A(SiOH)
Table 1 shows values of ϕIR decrease from 3.4±0.2 for Si-FAU to ˜2±0.1 for M-FAU. The decrease in ϕIR (˜40%) for Ti- and Sn-FAU is quantitatively consistent with the expected loss of (SiOH)4 (˜35%) upon metal-atom incorporation. Nb- and Ta-FAU are pentacoordinate and possess a pendant —OH, which obviates how changes in ν(O—H) solely result in the loss of (SiOH)4; however, the general trend in decreasing ϕIR a for these materials suggests the incorporation of Nb and Ta atoms into (SiOH)4. Collectively, the data and interpretation from X-ray diffraction, diffuse reflectance, UV-vis, N2 volumetric adsorption, 29Si MAS-NMR, and IR spectroscopy experiments suggest that the post-synthetic modification procedure presented here first generates a nearly siliceous FAU material with a number of (SiOH)4 nests equal to the original number of Al atoms, and second, substitutes the desired M atoms (M=Ti, Nb, Ta, Sn) into the zeolite framework.
Heteroatom-substituted zeolites often act as solid Lewis acid catalysts, where the efficacy of these materials depends strongly on the electron affinity (described colloquially as the “Lewis acid strength”) of the active site. Pyridine molecules bound to Brønsted acid sites form pyridinium ions that possess vibrational modes distinct from pyridine molecules bound to Lewis acid sites, and these differences provides a means to discriminate between different types of acid sites within solid materials.
Al-FAU clearly possesses the greatest density of Brønsted acid sites among these materials. Within Ti-, and Sn-FAU the feature at 1540 cm−1 is nearly indistinguishable from baseline, which suggests these materials do not possess spectroscopically observable densities of Brønsted acid sites.
Reaction Pathways for Alkene Epoxidation with Hydrogen Peroxide. Zeolites and mesoporous silicates bind reactants by charge transfer at active sites but also permit the selective stabilization of surface intermediates through combinations of van der Waals and specific interactions among the extended zeolite surface, the solvent molecules and the reactive species. The extent of stabilization depends on both the size and shape of the confining pore and the reactive species contained within these spaces.
The importance of ultralow Al contents is exemplified in reactions that include Al-FAU to simulate a Si:Al of 150. The presence of Al-FAU results in no measurable C8H8O formation; yet, rather forms 1-phenyl-1,2-ethanediol (from C8H8O ring opening) and 1-phenylethanol (from C8H8 hydration over H+ sites). Therefore, the differences in C8H8 epoxidation catalysis and the corresponding H2O2 selectivity must relate to how the stability of C8H8-derived intermediates depend on the chemical characteristics of the Ti-silicate catalyst.
Turnover rates for (C8H8) epoxidation with hydrogen peroxide (H2O2) were measured as a function of C8H8 and H2O2 concentration to provide insight as to the mechanism for alkene epoxidation and reconcile the differences in rates and selectivities for C8. Notably, the Ti-FAU sample used within these kinetic measurements was synthesized to contain 0.3%, by weight, Ti atoms to avoid artifacts that may arise from internal concentration gradients (i.e., to satisfy the Madon-Boudart criterion).
All Ti-based catalysts exhibit two kinetic regimes that differ in how epoxidation rates depend on the concentrations of reactants. At low [C8H8]:[H2O2] (<1), turnover rates increase linearly with [C8H8] and do not vary with [H2O2] (when [H2O2] is >5-10−3 M), which suggests that active sites are saturated with reactive species derived from H2O2 (e.g., Ti—OOH). At low values of [H2O2] (<5-10−3 M), turnover rates over Ti-BEA and Ti-FAU show a first-order dependence on both [C8H8] and [H2O2], which suggests that active sites are saturated with solvent molecules, rather than an intermediate derived from the reactants. At high [C8H8]:[H2O2](>10), turnover rates are independent of [C8H8], indicating that the identity of the most abundant reactive intermediate (MARI) under these conditions is derived from C8H8 (e.g., styrene oxide; C8H8O). The dependence of C8H8O formation on reactant concentrations are identical with prior findings within our group for the epoxidation of cyclohexene, styrene, 1-octene, and sulfoxidation of 2,5-dimethylthiophene over groups 4 and 5-substituted zeolite BEA.
r
E
=k
4[C8H8][Ti—OOH] (3)
where ki is the rate constant for step I in
where Ki is the equilibrium constant for step I, [L] is the total number of Ti atoms loaded into the reactor, and the five terms within the denominator correspond to Ti active sites that are occupied by solvent molecules, adsorbed C8H8, adsorbed H2O2, Ti—OOH intermediates, and adsorbed C8H8O, respectively.
Reaction conditions where turnover rates depend linearly on [C8H8] and are independent of [H2O2] result in active sites that are saturated with Ti—OOH intermediates and reduces equation 4 to yield
which matches the experimental observations within
Equation 6 is consistent with the independence of epoxidation turnover rates on [C8H8] at high [C8H8]:[H2O2] within
Thermochemical Analysis Shows Transition State Stabilization. Transition state theory postulates that the rate of reaction depends on the stability of an activated complex (i.e., a transition state) relative to the stability of the stable intermediate immediately preceding it along a reaction trajectory (Scheme 1). In the context of alkene epoxidation, Ti—OOH—C8H8‡ represents the transition state for C8H8 epoxidation which forms transiently upon reaction between Ti—OOH reactive intermediates with proximate C8H8.
Within the tenets of transition state theory, turnover rates for C8H8 epoxidation, under conditions that result in Ti—OOH MARI, are given by
where kB is the Boltzmann constant, h is Planck's constant, T is the absolute temperature, and KE‡ is the transition state equilibrium constant for epoxidation. KE‡ depends on the thermodynamic stability of the transition state relative to the reference state and takes the form
where ΔHApp‡ and ΔSApp‡ are the apparent activation enthalpy and entropy for epoxidation, respectively.
Epoxidation within Ti-FAU results in intermediate values of ΔHApp‡ and ΔSApp‡, which results in favorable enthalpic stabilization due to the surrounding supercage relative to Ti—SiO2; yet, provides enough flexibility for Ti—OOH—C8H8‡ as compared to Ti-BEA. Ti—SiO2 possesses a greater dependence on temperature than Ti-FAU, which suggests that at a high enough temperature the rates should be greater on Ti—SiO2. The isokinetic point (i.e., the temperature at which the rates of C8H8 epoxidation are equal) between Ti-FAU and Ti-FAU, however, lies outside the solvent temperature window (CH3CN has a boiling point of 82° C. at atmospheric pressure), such that Ti-FAU will always possess a greater rate than Ti—SiO2 within these types of reactors. Additional evidence for the selective stabilization of bulky aromatic transition states (i.e., for 2,4-dimethylstyrene) within Ti-FAU relative to other Ti-silicates is provided in Example 5 below.
Conclusions. Multiple treatments of Al-FAU (Si:Al=15) in HNO3 removes nearly all of the Al atoms to produce siliceous FAU (Si:Al >900). These treatments are necessary to remove the adventitious Al atoms, that may act as deleterious sites during zeolite catalysis (e.g., within sugar isomerization, alcohol upgrading, alkene epoxidation). The liquid-phase grafting of metal chlorides and alkoxides leads to the isomorphic substitution of metal (M=Ti, Nb, Ta, Sn) atoms into the framework of FAU. Ti-FAU catalysts efficiently activate H2O2 to form Ti—OOH intermediates that are active for alkene epoxidation. In the case of styrene epoxidation, Ti-FAU possesses rates of epoxidation that are greater than Ti-BEA and Ti—SiO2 by factors of 2 and 7, respectively. Rates of H2O2 decomposition, however, are invariant with the characteristic pore diameter of the Ti-silicate catalyst. Differences in catalysis are not due to differences in the mechanism for epoxidation; yet, reflect differences in the stability of the transition states for C8H8 epoxidation. Specifically, C8H8 epoxidation transition states are enthalpically stabilized within Ti-FAU relative to Ti—SiO2 and also possess greater entropic freedom than within Ti-BEA, which results in the lowest free energies within the bounds of solvent stability. The work presented here serves as an exemplary example for how zeolite framework topology can be chosen to selectively stabilize desired surface intermediates. The synthetic protocols established here will enable the design of new materials in the quest for the rational development of catalysts.
The following Examples are intended to illustrate the above invention and should not be construed as to narrow its scope. One skilled in the art will readily recognize that the Examples suggest many other ways in which the invention could be practiced. It should be understood that numerous variations and modifications may be made while remaining within the scope of the invention.
Heteroatom-substituted FAU (M-FAU) were prepared through the post-synthetic modification of W-form Al-FAU (Zeolyst, CBV 720; Si:Al 15). Al-FAU was treated in HNO3 (Macron Chemicals, 68-70 wt. %, 20 cm3 gAl-FAU−1) at reflux for 18 hours with the intent to remove framework Al by forming soluble Al(NO3)3. The solids were recovered by vacuum filtration, washed with additional HNO3 (5 cm3 gAl-FAU−1), and deionized H2O (17.8 MΩ cm; 25 cm3 gAl-FAU−1). Note that concentrated HNO3 can easily cause chemical burns and should be handled carefully. These washed solids were then dried at 823 K (5 K min−1) for 6 h in flowing air (Airgas, Ultra-zero grade; 100 cm3 min−1) to produce a partially-dealuminated FAU with a Si:Al of ˜200 (estimated by energy dispersive X-ray fluorescence, see below). The partial dealumination may result from the redeposition of Al into the FAU framework during filtration at room temperature. To further remove Al atoms, these dried solids were then subjected to a second dealumination sequence identical to that described above to produce Si-FAU with a Si:Al greater than 900.
Ti and Sn atoms were incorporated into the FAU framework through the liquid-phase grafting of TiCl4 (Sigma-Aldrich, 99.9%) or SnCl4.5H2O (Sigma-Aldrich, 98%) in dichloromethane (DCM, Fisher Chemicals, Certified ACS Stabilized, 25 cm3 gSi-FAU−1). Nb- and Ta-substituted FAU were prepared by refluxing Nb(OEt)5 (Sigma Aldrich, 99.95%) or Ta(OEt)5 (Sigma-Aldrich, 99.98%) in isopropanol (Fisher Chemical, Optima, 25 cm3 gSi-FAU−1). In all cases, the suspensions containing Si-FAU and MCl4 or M(OEt)5 were kept under an argon atmosphere using standard Schlenk technique for at least 6 h. Prior to introduction of the solvent and the MClx or M(OEt)5 precursor, the Si-FAU was dehydrated at 473 K under vacuum (<5 Pa) for 3 h to desorb any residual H2O that may hydrolyze the metal precursors. Dehydration of Si-FAU is particularly important when using the TiCl4 precursor, which readily hydrolyzes to form oligomeric and bulk TiOx aggregates. The solvent and other volatile components were removed via rotary evaporation and the recovered solids were heated in flowing air (100 cm3 min−1) to 823 K at 5 K min−1 and held for 6 h to yield bright, white-colored solids in all cases.
Titanium substituted BEA (Ti-BEA) and Ti-grafted SiO2 (Ti—SiO2) materials were used within a previous study (ACS Catal., 2018, 8, 2995), which presents the detailed chemical and physical characterization of these materials. The titanium silicalite-1 (TS-1) material was synthesized according to the recommended procedure from the International Zeolite Association's Synthesis Commission to contain 0.3 wt. % Ti. The relevant characterization data is presented within the Supporting Information (
Briefly, Ti-BEA was prepared by the post-synthetic modification of commercial Al-BEA (Zeolyst, CP814E). Al-BEA was contacted with HNO3 at reflux for 18 h with the intent of forming soluble Al(NO3)3. The solids were recovered by vacuum filtration and washed thoroughly with H2O prior to dehydration at 823 K (6 h; 5 K min−1) in flowing air (100 cm3 min−1), which produced Si-BEA (Si:Al >1200). Ti atoms were incorporated into Si-BEA through the liquid-phase incorporation of TiCl4 in DCM at reflux. Volatile components were removed via rotary evaporation and recovered solids were treated at 823 K (5 K min−1) for 6 h in flowing air (100 cm3 min−1) to produce Ti-BEA.
Ti—SiO2 was synthesized through the grafting of titanium 1,3-dimethoxy-tert-butylcalix[4]arene (Ti-dmCalix) onto SiO2 (Selecto Scientific, 32-62 m particle size, 5.4 nm pore diameter) through reflux in toluene in an Ar atmosphere. Prior to grafting, SiO2 was dehydroxylated at 573 K under vacuum (<5 Pa) for 10 h to produce isolated SiOH and minimize any Ti—O—Ti oligomer formation. Ti-dmCalix-grafted SiO2 was recovered by vacuum filtration and treated at 823 K (5 K min−1) for 6 h in air (100 cm3 min−1) to produce Ti—SiO2.
TS-1 was synthesized hydrothermally in hydroxide media. In short, 150 mg of titanium (IV) butoxide (TBOT; Sigma-Aldrich, 97%) was dissolved in 27.7 g of tetraethylorthosilicate (TEOS; Sigma-Aldrich, 98%) in a polypropylene container to form a homogeneous solution and was subsequently cooled to 273 K. Separately, a mixture of 28.7 g of tetrapropylammonium hydroxide (TPAOH; Sachem, 40% in H2O) and 50.5 g of H2O was cooled to 273 K and was slowly added (over ˜1 min) to the solution of TBOT and TEOS, which yielded a biphasic mixture. This solution was then warmed to 298 K and stirred for 12 h to produce a homogeneous solution, which indicates complete hydrolysis of the TBOT and TEOS. The cover was then removed to evaporate the ethanol and butanol formed through hydrolysis and produce a solution with the approximate composition of 1 Si:0.0033 Ti:0.43 TPAOH:28.3 H2O. This solution was then loaded into a Teflon-lined stainless-steel autoclave (Parr instruments, 125 cm3) that contained 5% (relative to SiO2 in the gel) TS-1 from a previous synthesis. This autoclave was heated to 443 K while rotating (30 rpm) in a convection oven for 3 days. The resulting solids were recovered by centrifugation, washed with H2O, and dried for 16 h at 373 K. The dried solids were then heated in flowing air (100 cm3 min−1) at 823 K (1 K min−1) for 10 h to produce TS-1.
The metal contents of all M-FAU were determined using energy dispersive X-ray fluorescence. Finely-ground M-FAU samples were loaded into a polypropylene sample holder (2.45 cm aperture) which was sealed with ultralene film. These samples were loaded into a spectrometer (Shimadzu, EDX-7000) whose sample chamber was purged with He (Airgas, Ultra-zero grade) prior to measurement. Spectra were obtained between 0 and 30 keV (500 scans), and the relative intensities of the element-specific fluorescence features and their associated calibration factors were used to determine the percent, by mass, of each element within the sample.
The crystallinity and contraction/expansion of the FAU framework was measured through X-ray diffraction. Samples were loaded onto a polypropylene holder and X-ray diffractograms were collected on a diffractometer (Siemens/Bruker, D5000) with Cu Kα radiation (0.15418 nm) under ambient conditions.
The surface area and pore-size distributions of M-FAU were determined by N2 adsorption. Gas-phase N2 adsorption isotherms (77 K) were collected on a volumetric adsorption instrument (Micromeritics, 3Flex). Samples (50-100 mg) were pelletized and sieved to retain particles between 250 and 500 μm in diameter. These samples were degassed by heated under vacuum (<0.7 Pa, 673 K) for 6 h prior to adsorption measurements. Pore size distributions were determined from N2 adsorption isotherms using a cylindrical pore model with non-local density functional theory (NLDFT) in the 3Flex software.
The presence of highly-disperse M atoms (and absence of bulk or oligomeric MOx domains) was inferred by the band edge energies, which were measured using diffuse reflectance UV-vis spectroscopy. Total reflectance spectra were measured under ambient conditions using a diffuse-reflectance accessory (Harrick, Cricket) with a UV-Vis-NIR spectrophotometer (Agilent, CARY 5). Prior to measurement, samples were intimately mixed with magnesium oxide (MgO; Sigma-Aldrich, 99.995%) in a 1:10 ratio by mass.
Infrared (IR) spectra of adsorbed pyridine (Sigma-Aldrich, 99.8%) were used to confirm the presence of Lewis acid sites within M-FAU and to detect Brønsted acid sites associated with remaining framework Al atoms. See
The fraction of Si atoms that exist as Si(OSi)3OH (ϕNMR) within M-FAU samples were determined using 29Si magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. MAS-NMR spectra (4,000 scans) were collected on a spectrometer (Varian, Unity Inova 300 MHz; 7.05 T), operating at 59.6 MHz Larmor frequency, equipped with a 4 mm MAS probe (Varian-Chemagnetics, double-resonance HX, APEX) under ambient conditions. M-FAU samples (˜70 mg) were loaded into 4.0 mm outer diameter zirconia rotors that were spun at 10 kHz. Powdered octakis(trimethylsiloxy)silsesquioxane (Q8M8) was used for 29Si chemical shift referencing (Q8M8 has a chemical shift of 11.45 ppm relative to tetramethylsilane (TMS) at 0 ppm). See
For all kinetic measurements, a Ti-FAU sample with 0.3 wt. % Ti atoms was used to avoid artifacts from internal mass-transfer restrictions. Rates of styrene (C8H8; Sigma-Aldrich, 99%) and 2,4-dimethylstyrene (C10H12; Sigma-Aldrich, 97%) epoxidation were measured in batch reactors (100 cm3, three-neck round bottom flasks) equipped with reflux condensers to minimize evaporative losses. See
While specific embodiments have been described above with reference to the disclosed embodiments and examples, such embodiments are only illustrative and do not limit the scope of the invention. Changes and modifications can be made in accordance with ordinary skill in the art without departing from the invention in its broader aspects as defined in the following claims.
All publications, patents, and patent documents are incorporated by reference herein, as though individually incorporated by reference. No limitations inconsistent with this disclosure are to be understood therefrom. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention.
This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/944,412, filed Dec. 6, 2019, which is incorporated herein by reference.
This invention was made with government support under Grant No. W911NF-18-1-0100 awarded by the U.S. Army Research Office. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62944412 | Dec 2019 | US |
