Patent Application
20240123428
The disclosure relates to catalytic materials containing oxidized amorphous carbon impregnated with boron and methods of producing the same, as well as methods for the oxidative dehydrogenation of alkanes using the catalytic materials.
C3 and C4 olefins, such as propylene (propene), 1-butene, isobutene and butadiene, are widely used starting materials in the industrial synthesis of a variety of important chemical products. The principal industrial method for producing C3 and C4 olefins is steam cracking, a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. The products obtained by steam cracking depend on the composition of the feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and furnace residence time. For example, a feed composition that primarily contains ethane (ethane cracking) would result in high ethylene yields, while a feed composition including larger hydrocarbons, such as naptha (naptha cracking), would result in a larger yield of C3 and C4 olefins.
In recent years, the demand for C3 and C4 olefins has outstripped supply from traditional cracker units, and this trend is expected to accelerate. This trend is primarily due to the availability of cheap shale gas, prompting many chemical companies to convert their naphtha crackers into ethane crackers, thus shifting production towards ethylene and away from longer chain C3 and C4 olefins. Accordingly, the demand for C3 and C4 olefins is growing faster than can be supplied by cracking.
Because C3 and C4 olefin production by conventional steam cracking has not kept pace with rising demand, several alternative “on-purpose” olefin production technologies that convert short chain alkanes to the corresponding olefins have been developed. Examples include the catalytic dehydrogenation (DH) of short chain alkanes, such as propane, to the corresponding olefin, such as propene, using a supported CrOx/Al2O3 catalyst (“CATOFIN®” (Lummus)), a Pt/Sn alloy supported on Al2O3 (“OLEFLEX™” (UOP)), or Pt/Sn supported on Zn-aluminate with co-fed steam (“STAR®” (Uhde)) (see Sattler et al., Chem. Rev., 2014, 114 (20), 10613-10653).
These and other currently used on-purpose dehydrogenation technologies are energy intensive, because the dehydrogenation reaction is highly endothermic. Furthermore, because they require high temperature conditions, they result in substantial catalyst deactivation due to the formation of coke. Thus, they require continuous catalyst regeneration. In addition, these processes may require substantially reduced pressure to shift the dehydrogenation equilibrium towards the desired products, further contributing to the high production costs associated with these methods.
Oxidative dehydrogenation (ODH), the catalytic dehydrogenation of feedstock alkanes in the presence of oxygen, is an intriguing alternative to conventional dehydrogenation that addresses each of the disadvantages of current DH technology. When oxygen is co-fed to act as a reactant, the reaction thermodynamics are altered such that the resulting net reaction is exothermic. Accordingly, the reaction can proceed at much lower reaction temperatures, resulting in decreased energy costs and increased catalyst stability. Oxygen in the feed stream also eliminates coke formation on the catalyst surface and thus creates no need for catalyst regeneration.
Despite these purported advantages, industrial-scale ODH processes have not been implemented, due to poor control of unwanted side-reactions (mainly the over-oxidation of olefin to CO and CO2), which results in low olefin selectivity at conversions necessary for industrial implementation. For example, existing catalysts for propane ODH typically provide ˜50-60% selectivity to propene at 10% propane conversion, with the byproducts largely made up of CO and CO2. As a result, even after more than 30 years of research into catalysis development for ODH (almost entirely focused on supporting vanadium oxide on amorphous oxide supports (e.g., SiO2, Al2O3, TiO2, CeO2, ZrO2) and structured oxides (e.g., MCM-41, SBA-15)), ODH has not been successfully used in the industrial-scale production of C3 and C4 olefins.
In recent years, lye Hermans et al. have disclosed the use of various boron-containing materials as high performing catalysts for the ODH of alkanes (see, e.g., U.S. Pat. Nos. 10,011,540; 10,125,059 and 10,407,364). More specifically, bulk boron materials, such as hexagonal boron nitride (h-BN), exhibit high selectivity towards propylene, and are currently recognized as the benchmark catalyst for the ODH of propane. However, such materials are expensive to synthesize, making them unlikely candidates for large-scale, industrial applications.
Accordingly, there is a need in the art for improved methods and catalysts for the oxidative dehydrogenation of C2-C5 alkanes to the corresponding olefins that exhibit the improved selectivity of bulk boron materials, while potentially being less expensive to implement in large-scale industrial settings.
We disclose herein new and improved catalytic materials and methods for catalyzing the oxidative dehydrogenation of C2-C5 alkanes or C2-C5 alkylbenzene to the corresponding C2-C5 olefins or styrene. The improved methods use catalysts containing an oxidized amorphous carbon support having its surface impregnated with boron. Selectivity for the desired olefin reaction product is comparable to h-BN catalysts, while exhibiting a significantly shorter induction period.
Accordingly, in a first aspect, the disclosure encompasses a catalytic material comprising oxidized amorphous carbon impregnated with boron.
In some embodiments, the catalytic material comprises a plurality of 3-coordinate boron species bound to the oxidized amorphous carbon, wherein the plurality of 3-coordinate boron species comprise B(OB)(OH)2, B(OB)2(OH), B(OB)3, B(OB)2(OC), B(OB)(OC), and B(OC)3. In some such embodiments, the majority of 3-coordinate boron species are clustered. In some embodiments, between 60 and 90% of the plurality of 3-coordinate boron species are B(OB)(OH)2, B(OB)2(OH), and B(OB)3. In some embodiments between 40% and 70% of the plurality of 3-coordinate boron species are B(OB)(OH)2 and B(OB)2(OH).
In some embodiments, (i) the catalytic material comprises from 25 wt % to 40 wt % boron, based on the total weight of the catalytic material, (ii) the catalytic material comprises a complete layer of boron covering the oxidized amorphous carbon, (iii) the boron is impregnated onto the surface of the oxidized amorphous carbon, (iv) the oxidized amorphous carbon comprises, at its unimpregnated surface, one or more moieties selected from the group consisting of a carboxyl, a phenolic hydroxyl, a carbonyl, an anhydride, and a —C—O, (v) the oxidized amorphous carbon is oxidized activated carbon, (vi) the catalytic material is capable of catalyzing oxidative dehydrogenation (ODH) of an alkane or an alkyl group (e.g. ethane, propane, n-butane, isobutane, or any combination thereof), (vii) wherein the catalytic material has an ODH induction period of less than 12 hours, (viii) the catalytic material is thermally stable (e.g. at a temperature equal to or less than 600° C.), or any combination thereof.
In a second aspect, the disclosure encompasses a method of producing a catalytic material comprising impregnating oxidized amorphous carbon with boron.
In some such embodiments, impregnating the surface of the oxidized amorphous carbon with boron is performed by contacting the oxidized amorphous carbon with excess boric acid solution.
In some embodiments, the method further comprises oxidizing amorphous carbon to produce the oxidized amorphous carbon, calcining the impregnated oxidized amorphous carbon, thermally treating the impregnated oxidized amorphous carbon, or any combination thereof
In some such embodiments, oxidizing amorphous carbon to produce the oxidized amorphous carbon is completed by contacting amorphous carbon with an oxidizing agent under conditions suitable for producing the oxidized amorphous carbon. In some such embodiments, the oxidizing agent is nitric acid, ozone, or hydrogen peroxide.
In some such embodiments, thermally treating the impregnated oxidized amorphous carbon is completed by heating the impregnated oxidized amorphous carbon under an inert atmosphere.
In some embodiments, the method produces the catalytic material disclosed in the first aspect.
In a third aspect, the disclosure encompasses a method of making one or more desired chemical products. The method includes the step of contacting a heterogeneous catalyst including the catalytic material as described above with oxygen and one or more liquid or gaseous reactants. The heterogeneous catalyst catalyzes the oxidative dehydrogenation of the one or more liquid or gaseous reactants to form the one or more desired chemical products.
In some embodiments, the one or more liquid or gaseous reactants comprise an alkane or a hydrocarbon comprising an alkyl group, and the one or more desired chemical products comprise one or more olefins or one or more hydrocarbons comprising an alkenyl group.
In some embodiments, the one or more liquid or gaseous reactants comprise a C2-C5 alkane or C2-C5 alkylbenzene.
In some embodiments, the one or more liquid or gaseous reactants comprise ethane, propane, n-butane, isobutane, or any combination thereof.
In some embodiments, the one or more desired chemical products comprise ethene, propene, isobutene, 1-butene, 2-butene, butadiene, or any combinations thereof. In some such embodiments, the one or more liquid or gaseous reactants comprise propane and the one or more desired chemical produces comprises propene. In some embodiments, the one or more liquid or gaseous reactants comprise ethylbenzene.
In some embodiments, (i) the contacting step occurs at a temperature of from 400° C. to 800° C., (ii) the oxygen and one or more liquid or gaseous reactants are in a reactant stream that is contacted with the heterogeneous catalyst, (iii) the heterogeneous catalyst has an induction period of less than 12 hours, (iv) the heterogeneous catalyst has a selectivity for the one or more desired chemical products of 70% or greater, or any combination thereof.
In some embodiments, the reactant stream comprises from 0% to 70% nitrogen by volume.
In some embodiments, the heterogeneous catalyst has an induction period of eight hours or less.
In some embodiments, the heterogeneous catalyst has the selectivity for the one or more desired chemical products of 85% or greater.
Further features and advantages of the disclosed methods and catalytic materials will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The disclosure will be better understood and features, aspects and advantages other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such detailed description makes reference to the following drawings.
While the disclosed methods and catalysts may include various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are described in detail herein. However, the description of specific embodiments herein is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
This disclosure is based on our discovery that the use of a boron-impregnated oxidized amorphous carbon catalyst facilitates improved oxidative dehydrogenation of alkanes, such as propane, to desired olefins, such as propene. Specifically, the disclosed methods exhibit high selectivity towards the desired product while decreasing the production of unwanted byproducts, such as CO and CO2. Furthermore, the process occurs at relatively low temperatures, and the catalyst is stable over time, and does not need to be frequently regenerated. Furthermore, the disclosed catalysts exhibit a significantly improved induction period as compared to other boron-containing catalysts, and would be significantly less expensive to implement in large scale, industrial applications than other boron-containing catalysts.
This disclosure is not limited to the particular methodology, protocols, materials, and reagents described, as these may vary. Furthermore, the terminology used in this disclosure describes particular embodiments only, and is not intended to limit the scope of the present invention, which will be limited only by the language of the appended claims.
As used in this disclosure and in the appended claims, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. The terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably. The terms “comprising”, “including”, and “having” can also be used interchangeably.
Unless defined otherwise, all technical and scientific terms used in this disclosure, including element symbols, have the same meanings as commonly understood by one of ordinary skill in the art. Chemical compound names that are commonly used and recognized in the art are used interchangeably with the equivalent IUPAC name. For example, ethene is the same as ethylene, propene is the same as propylene, butene is the same as butylene, 2-methylpropane is the same as isobutane, and 2-methylpropene is the same as isobutene.
The following abbreviations are used throughout this disclosure: BN, boron nitride; B/OAC, boron-impregnated oxidized activated carbon; DH, dehydrogenation; h-BN, hexagonal form of boron nitride; NDC, Norit Darco 12×40 activated carbon; OAC, oxidized activated carbon; ODH, oxidative dehydrogenation; ODHP, oxidative dehydrogenation of propane; P, partial pressure for a given gas; S, selectivity for a given product; WHSV−1, inverse weight-hour-space-velocity; % X, conversion for a given reactant.
All publications and patents specifically mentioned in this disclosure are incorporated by reference herein for all purposes, including for describing and disclosing the chemicals, instruments, statistical analysis and methodologies that are reported in the publications that might be used in connection with the disclosed methods and devices. All references cited in this disclosure are indicative of the level of skill in the art.
As demonstrated in the Examples, amorphous carbon may be oxidized by contacting the amorphous carbon with an oxidizing agent, such as nitric acid. The method used to oxidize the carbon is not limited to this process, and any method of oxidizing amorphous carbon could be used. For example, U.S. Pat. No. 4,371,454 discloses methods used to oxidize carbon to create spherical from pitch and amorphous carbon.
In another non-limiting example, ozone could be used to oxidize the amorphous carbon. Ozone oxidation forms hydroxyl, carbonyl, and —C—O— surface moieties, with —CH species being consumed and resulting in —COOH species (see Mawhinney, D. B. and Yates Jr. J. T. Carbon, 2001, 39, 1167-1173).
In yet another non-limiting example, hydrogen peroxide could be used to oxidize the amorphous carbon. Schlogl and Su (Phys. Chem. Chem. Phys. 2015, 17, 1567-1571) showed that H202 can successfully oxidize carbon nanotubes (CNTs), while inducing lower oxygen functionalities (carboxylic groups and anhydrides) than HNO3. Carbon oxidation was conducted by autoclaving reduced CNT, water, and H2O2 at 100° C. for 4 hours (reduced CNT is HCl treated commercially available CNT).
The Examples also demonstrate that boron may be impregnated onto the oxidized amorphous carbon surface by exposing the surface to boron, such with boric acid in water solution. However, other methods for exposing the surface with boron and related boron oxide moieties may be used.
The Examples also demonstrate that the catalytic materials prepared from impregnation of oxidized amorphous carbon is an active and selective catalyst for ODH. Moreover the catalytic material exhibits equal propylene selectivity and improved productivity (kgpropylene kgcat−1 hr−1) as compared to h-BN.
The Examples are illustrative only, and do not limit the scope of the invention in any way. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and the following examples and fall within the scope of the appended claims. Definitions
As used herein, the term “amorphous carbon” refers to a carbon material without long-range crystalline order. The term “oxidized amorphous carbon” refers to amorphous carbon that undergo an oxidation reaction, which can be defined as the loss of electrons (or gain of oxygen, or loss of hydrogen) during a reaction by a molecule, atom, or ion. In some embodiment, oxidized amorphous carbon comprises one or more moieties selected from the group consisting of a carboxyl, a phenolic hydroxyl, a carbonyl, an anhydride, and a —C—O—.
As used herein, the term “carboxyl” refers to the functional group of the formula —C(O)OH
As used herein, the term “phenolic hydroxyl” refers to the functional group of the formula —C6H5OH
As used herein, the term “carbonyl” refers to the functional group of the formula —C(O)—
As used herein, the term “anhydride” refers to the functional group of the formula —C(O)OC(O)—
In some embodiments, the oxidized amorphous carbon is oxidized activated carbon. As used herein, the term “activated carbon” refers to a type of purified, powdered charcoal that is treated physically or chemically to generate microfissures that vastly increase its adsorptive surface area available for adsorption or chemical reactions.
In some embodiments, the amorphous carbon is oxidized by an “oxidizing agent.” As used herein, the term “oxidizing agent” refers to chemicals that have the ability to oxidize other compounds. In some embodiments, the oxidizing agent is nitric acid, ozone, or hydrogen peroxide.
As used herein, the term “impregnated” or “impregnation” refers to a procedure to load a given porous support (e.g. activated carbon) with a metal or metalloid component in (i) the solid-state way (i.e. physical mixture of both components in solid state) or (ii) more commonly via wet impregnation (i.e. physical mixture of the support in solid state and the metal or metalloid component dissolved in a liquid solution), in some embodiments, the boron is impregnated onto the surface of the oxidized amorphous carbon. In some embodiments, impregnating the surface of the oxidized amorphous carbon with boron is performed by contacting the oxidized amorphous carbon with excess boric acid.
As used herein, the term “3-coordinate boron species” refers to boron-containing group wherein the boron atom is bonded to three moieties and possesses a hybridization state of sp2. The “3-coordinate boron species” typically have a molecular geometry of trigonal planar. The 3-coordinate boron species may be represented by OH, OB, or OC moieties. It should be understood that the moieties represented as OB or OC may have the boron or carbon atom bound to additional atoms (e.g., 2 or 3 additional atoms) to form the catalytic material. OH moieties may be typically found at the outer surface of the catalytic material. OC moieties may typically be found at the interface between the amorphous carbon and a layer of boron covering the oxidized amorphous carbon. OB moieties may typically be found within a layer of boron covering the oxidized amorphous carbon. The 3-coordinate boron species may comprise B(OB)(OH)2, B(OB)2(OH), B(OB)3, B(OB)2(OC), B(OB)(OC), B(OC)3, or any combination thereof.
In some embodiments, the majority of 3-coordinate boron species are clustered. As used herein, the term “ clustered” or “clusters” refers to a 3-carbon species that exhibit B—O—B bonds, e.g., B(OB)(OH)2, B(OB)2(OH), or B(OB)3. In some embodiments, between Clustered 3-coordinate boron species may form part of a cluster extending from the amorphous carbon or layer of boron covering the amorphous carbon having a multiplicity of layers of boron.
In some embodiments, between 60 and 90% of the plurality of 3-coordinate boron species are B(OB)(OH)2, B(OB)2(OH), and B(OB)3. In some embodiments, between 65 and 85% or between 70% and 80% of the plurality of 3-coordinate boron species are B(OB)(OH)2, B(OB)2(OH), and B(OB)3.
In some embodiments, between 40 and 70% of the plurality of 3-coordinate species are B(OB)(OH)2 and B(OB)2(OH). In some embodiments, between 45 and 65%, between 50 and 65%, or between 55 and 65% of the plurality of 3-coordinate boron species are B(OB)(OH)2 or B(OB)2(OH).
In some embodiments, the catalytic material comprises from 25 wt % to 40 wt % boron, based on the total weight of the catalytic material. In some embodiments, the catalytic material comprises from 25 wt % to 35 wt % boron, based on the total weight of the catalytic material.
In some embodiments, the catalytic material comprises a complete layer of boron covering the oxidized amorphous carbon. As used herein, the term “a complete layer of boron” refers to a layer containing 3-coordinate boron species comprising B(OB)(OH)2, B(OB)2(OH), B(OB)3, B(OB)2(OC), B(OB)(OC), and B(OC)3, wherein the layer substantially surrounds the amorphous carbon. A complete layer of boron can surround and protect the underlying amorphous carbon from decomposition or combustion at elevated reaction temperatures. In contrast, an incomplete layer of boron may leave substantial portions of amorphous carbon vulnerable to decomposition or combustion at reaction temperatures.
As used herein, the term “oxidative dehydrogenation” refers to an oxidative process of dehydrogenating hydrocarbons (e.g. alkanes and hydrocarbons comprising an alkyl group) to corresponding olefinic counterparts in the presence of oxygen. When oxygen is co-fed with one of more liquid of gaseous reactant to act as a reactant, the reaction thermodynamics are altered such that the resulting net reaction is exothermic. Accordingly, the reaction can proceed at much lower reaction temperatures, resulting in decreased energy costs and increased catalyst stability. Oxygen in the feed stream also eliminates coke formation on the catalyst surface and thus creates no need for catalyst regeneration.
As used herein, the term “hydrocarbon” refers to an organic compound containing only hydrogen and carbon atoms. In some embodiments, the catalytic material is capable of catalyzing oxidative dehydrogenation (ODH) of an alkane or an alkyl group.
As used herein, the term “alkane” refers to saturated hydrocarbons having a formula of CnH2n+2, wherein n is a positive integer greater than or equal to 1. Examples of alkanes include, but are not limited to methane, ethane, n-propane, iso-propane, butane, etc. As used herein, the term “alkyl” refers to a saturated, straight or branched hydrocarbon chain radical. In some embodiment, the number of carbon atoms in an alkane or alkyl moiety is indicated by the prefix “Cx-Cy”, wherein x is the minimum and y is the maximum number of carbon atoms in the substituent. Representative examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, etc.
As used herein, the term “olefin” refers to an unsaturated hydrocarbon containing at least one “C═C” double bond. The term “alkenyl” as used herein, refers to a straight or branched hydrocarbon chain containing at least one carbon-carbon double bond. Non-limiting examples of alkenyl include buta-1,3-dienyl, ethenyl, 2-propenyl, 2-methyl-2-propenyl, 3-butenyl, 4-pentenyl, 5-hexenyl, etc.
As used herein, the term “alkylbenzene” refers to benzene attached to at least one alkyl group as described herein. Non-limiting examples of alkylbenzene refers to methylbenzene, ethylbenzene, propylbenzene, etc. In some embodiments, the one or more liquid or gaseous reactants comprise an alkylbenzene, such as ethylbenzene.
As used herein, the term “induction period” refers to the initial slow phase of a chemical reaction which later accelerates. In some embodiments, the catalytic material has an ODH induction period of less than 12 hours. In some embodiments, the catalytic material has an ODH induction period of less than 11, 10, 9, or 8 hours.
As used herein, the term “thermally stable” refers to the ability of a compound or material to resist the action of heat and to maintain its physical and chemical properties. In some embodiments, the catalytic material is thermally stable at a temperature equal to or less than 600° C. In some embodiments, the catalytic material is thermally stable at a temperature equal to or less than 550° C., or equal to or less than 500° C.
As used herein, the term “inert atmosphere” refers to a reaction atmosphere which is composed mainly of chemically inactive gases. Common chemically inert gases include but are not limited to argon and nitrogen.
Bulk boron materials, such as hexagonal boron nitride (h-BN), are highly selective catalysts for the oxidative dehydrogenation of propane (ODHP). Previous attempts to improve the productivity of these systems involved the immobilization of boron on silica and resulted in less selective catalysts. Here, we report that acid-treated, activated carbon-supported boron prepared via incipient wetness impregnation with boric acid (B/OAC) exhibits equal propylene selectivity and improved productivity (kgpropylene kgcat−1 hr−1 ) as compared to h-BN. Characterization of the fresh and spent catalysts with infrared, Raman, X-ray photoelectron, and solid-state NMR spectroscopies reveals the presence of oxidized/hydrolyzed boron that is clustered on the surface of the support. Introduction.
Boron-containing materials have been shown to be high performing for the oxidative dehydrogenation (ODH) of light alkanes to light olefins.[1-6] In particular, bulk and nanotubular hexagonal boron nitride (h-BN) exhibit benchmark-high selectivity towards propylene for the ODH of propane (e.g., ca. 85% at 9% conversion), significantly outperforming previous state-of-the-art silica-supported vanadium oxide ODH catalysts (61% at 9% conversion).11 The enhanced performance of h-BN for ODH is attributed to an amorphous oxidized/hydrolyzed boron layer [B2(OH)2xO 3−x, x=0-3] which resides on the BN sheets.[2,7-10] Solid-state NMR spectroscopy and other surface characterization techniques have shown that the amorphous oxidized/hydrolyzed boron layer is primarily composed of 3-coordinate boron oxide/hydroxide species (B2(OH)2xO3−x, x=0-3) that form under reaction conditions.[2,9,11,12] Ultra-high field 11B NMR spectroscopy revealed that the oxidized/hydrolyzed boron layer grows off the BN sheets through BN2O groups, where the O atom is bridging to the oxidized/hydrolyzed boron layer.[12] Notably, h-BN does not appear to deactivate but features an induction period for optimal catalytic performance, suggesting that the oxidized/hydrolyzed boron layer provides the active species. [1,5,8,11] This is in agreement with previous quantum chemical calculations that suggested clustering of boron oxide/hydroxide species are required for ODH.[10]
Further investigation of oxidized/hydrolyzed boron oxide species for ODH have been conducted through the synthesis and characterization of silica-supported boron oxide materials (B/SiO2).[5,13] Detailed characterization of B/SiO2 ODH catalysts showed that the majority of boron is composed of clustered 3-coordinate boron oxide/hydroxide species [B2(OH)2xO3−x, x=0-3], likely representing similar species observed in h-BN ODH catalysts.[5,12,13] Alternatively, an MCM-22 zeolite isomorphously substituted with boron primarily contains B(OSi)3 species isolated in the zeolite framework and is inactive for ODH. This observation further supports the hypothesis that the clustering of oxidize/hydrolyzed boron gives rise to its ODH catalytic activity.[12,14] However, B/SiO2 catalysts show worse selectivity for ODH of propane to propylene than h-BN (B/SiO2: 76.5%, h-BN: 87.4%;
Here, the Examples demonstrate the use of activated carbon, treated with nitric acid and impregnated with boric acid as a boron source (referred to as B/OAC,
Activated carbons have long been used as catalyst supports.[15-24] Further, amorphous carbon itself has shown significant activity and selectivity for ODH reactions (i.e. ethylbenzene to styrene, isobutane to isobutene, 2-butanol to 2-butanone, and ethanol to acetaldehyde), while structured, stable carbons (i.e. carbon nanotubes, carbon nanofibers, nanodiamonds, onion-like carbons, etc.) have been used for more challenging substrates like propane.[25-32] However, amorphous carbon-based catalysts have not been explored for propane ODH, likely due to the combustion of the material at the required reaction conditions (500° C.).[33-36] Structured carbon surfaces are often functionalized by chemical treatments to induce thermal stability under oxidative conditions (
The Examples demonstrate the unprecedented selectivity to alkenes in ODH reactions is not exclusive to bulk boron materials but can be achieved by amorphous carbon-supported boron oxide. Boron-coated oxygen-functionalized carbon (ca. 28 wt % B) catalyzes propane ODH and produces near identical product distribution trends to h-BN (
B/OAC and bulk boron catalysts, namely h-BN, only differ in their rates of propane consumption, productivity, and induction periods (
Due to the reduced induction period and the hypothesis that fresh B/OAC is nearer to the stable active phase for propane ODH than h-BN and B/SiO2, spectroscopic characterization is necessary for the advancement of boron-based catalysts. Fresh and spent B/OAC catalysts were characterized through a combination of solid-state NMR, infrared (IR), Raman, and X-ray photoelectron spectroscopies (XPS). XPS spectra of the B is region of fresh and spent B/OAC catalysts both display a broad, slightly asymmetric signal centered at ca. 194-195 eV, in the general region for boron oxide/hydroxide type species (
It was previously reported that ODHP catalytic activity was not reliant on the surface area of the boron-containing catalyst, but rather due to differences in active site densities and/or electronic interactions between the catalyst surface and the bulk structure.15 For example, for vanadium oxide ODHP catalysts, it was found that support material can exert variable electronic influence on the active VO4 sites on the surface and affect the observed rate. Here, we attribute the superior observed rate and productivity of B/SiO2 to electronic interactions between the active site and support.
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of fresh and spent B/OAC, OAC (no boron), and H3B03 (boric acid) were recorded to further probe surface structure and the incorporation of boron onto the carbon support (
DRIFTS spectra of B/OAC and OAC differ significantly in the low wavenumber regime, where vibrations at 1220 cm−1 and a broad peak between 1290 and 1580 cm−1 are seen in both fresh and spent B/OAC, while OAC exhibits peaks centered at ca. 1240, 1580, and 1756 cm−1 (
Magic-angle spinning (MAS) 11B solid-state NMR spectroscopy (ex-situ) was used to further probe structure within fresh and spent B/OAC heterogenous catalysts and identify the type of boron species formed on the surface of the carbon support. All solid-state NMR spectroscopy experiments were performed at B0=14.1 T with 25 kHz MAS. Quantitative single-pulse (recycle delay >5×T1, 10° tip-angle) 11B solid-state NMR spectra of the fresh and spent catalysts show near identical NMR signals in the typical region of 3-coordinate BO3 containing species (
A simultaneous analytical fit of the 11B 3Q-MQMAS and single-pulse direct excitation NMR spectra reveals that there are five unique boron species with quadrupolar coupling constants (CQ) of 2.65 MHz and diso(11B) of (blue fit) 18.3 ppm, (orange fit) 17.0 ppm, (brown fit) 15.0 ppm, (green fit) 13.6 ppm and (pink fit) 11.5 ppm (
1H 11B dipolar-based HETCOR NMR experiments were performed to identify boron species in close-spatial proximity to hydrogen atoms (
Next, homonuclear 11B dipolar double-quantum-single-quantum (DQ-SQ) correlation expeirments were performed to identify boron species in close spatial proximity to other boron atoms.[12,67,68] As mentioned above, the 1D single-pulse and 2D 3Q-MQMAS spectra revealed all boron species take the general formula of BO3. Therefore, the 11B dipolar DQ-SQ experiments will reveal boron species with B—O—B bonds as two 11B spins are required to generate the observed DQ-coherence.[69] 2D 11B dipolar DQ-SQ spectra of fresh B/OAC were recorded with either 0.32 or 1.6 ms of total BR2½ homnuclear dipolar recouplinel applied to the 11B spins (
[a]Populations were determined from the quantiative (recyle delay > 5×T1) 10° tip-angle single pulse spectra.
[b]The color of the highlighted boron atom corresponds to the color of the analyitical fits given in FIG. 5.
[c]All boron species have CQ = 2.65 MHz and h = 0.
aKey corresponds to B atoms in FIG. 21A and 21B.
bdiso(11B) calculated via a calibration curve with the following equation: δiso = −0.8845σ + 86.704.[20]
cDFT calculated 11B CQ were corrected via a calibration curve with the following equation: CQ, corr. = 0.8987CQ, calc..[20]
dTwo of the same boron species exist in each structure (see FIG. S16); a and b correspond to the two different units (average diso(11B), CQ and h of the two are given in FIG. S16 simulated spectra).
Oxidized activated carbon-supported boron (B/OAC) was shown to be an active and highly selective catalyst for the oxidative dehydrogenation of propane to propylene. The B/OAC catalyst yielded excellent selectivity towards propylene and is the most selective supported boron catalysts reported thus far, where it exhibited nearly the same product distribution as h-BN. Furthermore, B/OAC is significantly cheaper than h-BN, making it potentially economically more viable. Reactivity data indicated that B/OAC did not experience support interactions that result in reduced selectivity, as in B/SiO2, likely as a result of a boron overlayer stabilizing and coating the carbon support. Molecular level characterization through XPS, IR, Raman and solid-state NMR spectroscopies revealed that the B/OAC material under atmospheric conditions contains clustered oxidized/hydrolyzed boron species residing on the surface of the carbon support. Spectroscopic analysis of B/OAC before and after catalysis reveals minimal differences, suggesting that while the boron structure may change under reaction conditions, the material reverts back to its initial state upon removal of heat and reactants. 2D 11B MQMAS solid-state NMR spectra of the fresh and spent catalysts revealed five unique boron species: B(OB)x(OH)3−x (x=1-2), B(OB)3, B(OB)2(OC), B(OB)(OC)2 and B(OC)3. 2D 1H—11B HETCOR and 11B homonuclear correlation solid-state NMR spectra aided in the assignment of all NMR signals. Lastly, analytical fits of quantitative 11B solid-state NMR spectra illustrated that B(OB)x(OH)3−x (x=1-2) and B(OB)3 species make up ca. 80% of all boron, confirming that the majority of boron is clustered on the surface of the carbon support. This work demonstrates that carbon-supported boron oxide is a promising catalyst for the oxidative dehydrogenation of propane while confirming mechanistic hypotheses.
Materials Synthesis. The materials B/OAC were synthesized via wetness impregnation. Prior to impregnation, 0.5 g activated carbon (Norit Darco 12×40, specific surface area of 600 m2 g−1) was oxidized by stirring the carbon with 100 mL of HNO3 (56% v/v) solution at 60° C. for 24 hours. The mixture was filtered and washed with DI water until the pH was 7. The carbon was dried under static conditions overnight at 110° C. For impregnation, a solution of boric acid (H3BO3, Sigma Aldrich) in 18 milliQ water was prepared with sufficient water to allow for dissolution of boric acid. Impregnation was performed under stirring and mild heating at 50° C. overnight then the mixture was heated at 110° C. to evaporate excess water. The impregnated sample and excess boric acid was transferred to a tube reactor where it was thermally treated to 600° C. at 1° C. min−1 under N2 and held at 600° C. for 3 h. The impregnation was repeated again for a total of two impregnations.
Catalytic Testing. Carbon-supported boron catalysts were tested for their catalytic performance as prepared. Typically, about 200 mg of catalyst were loaded into quartz reactor tube (9 mm diameter) and supported on a bed of quartz wool in the center prior to being loaded into a split tube furnace. A diluent was not used due to the good thermal conductivity of activated carbon. Flow rates of propane (instrument grade, Matheson), oxygen (UHP, Airgas), and nitrogen (UHP, Airgas) were controlled using three mass flow controllers (Bronkhorst) calibrated to each individual gas to allow total flowrates of 40-200 mL min−1. Catalysts were heated to reaction temperature under a flow of nitrogen prior to the introduction of the reaction mixture. The reactor effluent was passed through a thermoelectrically-cooled liquid-gas separator to condense formed water before being analyzed by an Inficon Micro GC Fusion equipped with three columns (Rt-Molsieve 5a, Rt-U Bond, and Rt-Alumina Bond/Na2SO4) with individual thermal conductivity detectors (TCD). The carbon balance of each data point closes within 2%.
where Fcarbon out=of all carbon products from reactor in mol gcat−1, Falkane inflow of all alkane into reactor in mol gcat−1
where Fproduct out=flow of product from reactor in mol gcat−1, Fcarbon out=flow of all carbon products from reactor in mol gcat−1
Inverse weight-hour-space-velocity, WHSV−1(kgcat s molalkane−1):
where Mcat=mass of catalyst loaded in reactor (kg), (V/n)STP=24.5 (L/mol) at 298.15 K, (1 atm, R=8.206*10−2 L atm K−1 mol−1), Ftot=total flow of all inlet gases (L s−1), Nalkane=mol percent alkane in gas feed (mol %)
Raman. In-situ Raman measurements were conducted with a Renishaw InVia Raman spectrometer with a 785 nm excitation laser. All spectra were collected using a 1200 lines mm -1 grating and were taking with aa range of 100-2000 cm−1 and a dispersion of 1.36565 cm−1 pixel -1 . Each sample was loaded in a high-temperature Linkam CCR1000 cell and heated at 15° C./min under air to 500° C., stopping every 100° C. to record Raman spectra. The spectra are reported without background subtraction.
Infrared Spectroscopy. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were recorded by averaging 128 scans with a resolution of 8 cm−1. A DiffuslR accessory (PIKE Technologies) was used. The accessory was flushed with dry air at a flow rate of 20 mL min−1 and heated at a rate of 10° C. min−1 up to a temperature of 500° C.
XPS. XPS measurements were done via Thermo Scientific XPS using a micro-focused monochromated Al K-alpha X-ray source with a flood gun to reduce surface charging. Samples conditions were 10 -7 mbar pressure and room temperature. C 1s, O 1s, and B 1s regions were scanned using a 50 ev pass energy, a 50 ms dwell time, a 400 μm spot size, and a 0.2 ev energy step size. The number of scans per element was adjusted to improved signal-to-noise. All surface compositions, and peak integration/deconvolution were done via Avantage (Thermo Scientific) Software package.
ICP. Elemental analysis was performed by inductively coupled plasma—optical emission spectroscopy (ICP-OES) measurements on an Agilent 5110 VDV at the Water Science and Engineering Laboratory at the University of Wisconsin-Madison. Prior to analysis, 12-20 mg of B/OAC sample was leached using three consecutive acid treatments of 5 mL of 100% aqua regia. Leached samples were diluted by a factor of 20 using nano-pure H2O. Samples were measured using both external standard curve and standard addition methods. The external standards were made using diluted aqua regia that was used to leach a bare carbon support. This treated aqua regia ensured a closer matrix match to the samples. Standard addition was done using a 6-point curve including the diluted neat sample. Three B wavelength were averaged to determine the concentrations, 208.956 nm, 249.678 nm, and 249.772 nm. All samples were measured in triplicate to ensure accuracy. Both methods returned the same results within the error margin.
Water Isotherm. H2O pulse adsorption was performed on a Micromeritics Autochem II 2920 instrument. For this experiment, between 100 to 200 mg of sample was calcined at 773 K (10 K/min) for 1 h under flowing air and subsequently cooled (10 K/min) to room temperature under He (all flowrates were 50 mL/min unless stated otherwise). The sample was then pulsed with He saturated with water vapor at 40° C. with a 5 mL STP loop until no more uptake of water was observed. The water pulsing end point was denoted by the consecutive appearance of constant area peaks as monitored by a TCD detector. Afterwards, the total volume of water adsorbed per sample was calculated by determining the amount of unadsorbed water and subtracting this amount to the total amount of water pulsed.
Differential Scanning calorimetry. Differential scanning calorimetry experiments were conducted on a TGA/DSC1 system (Mettler-Toledo). Samples were heated in 70 uL Alumina crucibles covered with a lid with a pinhole using the above heating method in the figure (bottom right). Two glass transition temperatures (Tg) for B/OAC were determined via the midpoint method at 61.18° C. and 80.36° C. Both B/OAC and borosilicate show major inflection points at 390.2° C. and 399.09° C. respectively.
Solid-State NMR Spectroscopy. All solid-state NMR spectroscopy experiments were conducted on a 14.1 T (n0(1H)=600 MHz) Bruker wide-bore magnet spectrometer equipped with a Bruker Avance II console and a 2.5 mm HXY magic-angle spinning (MAS) NMR probe configured in double resonance mode. A 1H high-pass (n0(1H)=600 MHz) or 31P low-pass (n0(31P)=243 MHz) filter was applied to the 1H or X preamplifiers, respectively, to better isolate the two channels. Fresh and spent B/OAC catalysts were center packed between powdered Teflon into 2.5 mm NMR rotors in a N2 filled glovebox and the rotors were spun with N2 gas to minimize sample hydration/air exposure. The magnetic field strength was immediately calibrated following removal of each sample to 1H chemical shifts of neat tetramethylsilane with adamantane as a secondary chemical shift reference (diso(1H)=1.82 ppm). 11B chemical shifts were referenced using previously reported IUPAC relative NMR frequencies (BF3·Et2O). [1] Bruker topspin 3.6.1 was used to process all NMR spectra. Analytical simulations of the 11B single-pulse and MQMAS spectra were performed using the ssNake NMR software.[2]
All experimental parameters (number of scans, recycle delay, t1 TD points, h dwell (Dh), dipolar recoupling durations and total experimental times) are given in Table 3. 1H and 11B longitudinal relaxation constants (Ti) were measured using a saturation recovery experiment. 1H radio frequency (RF) pulses were directly calibrated on the fresh and spent B/OAC materials. All 1Hp/2 pulses were 2.5 ms in duration, corresponding to a 100 kHz RF field. Direct excitation single-pulse 11B NMR experiments were recorded with a repetition delay of greater than 5×T1 and a 10° tip angle (1.67 ms in duration, corresponding to a 8.3 kHz RF field and 16.6 kHz central-transition nutation frequency) to ensure the spectra were quantitative. The same experiment was performed on a rotor filled entirely with adamantane to subtract out 11B NMR signals coming from the stator and other probe components. 100 kHz RF field of SPINA1-64 i H heteronuclear decoupling was performed during the acquisition of all 11B spectra)
11B Single-Pulse
11B Single-Pulse
aRecycle delay > 5 × T1.
2D split-ti triple-quantum multiple-quantum MAS (3Q-MQMAS) experiments were recorded with previously described pulse sequences.[4-6] 0Q→3Q excitation and 3Q→1Q reconversion pulse lengths were 4.0 and 1.6 ms in duration, respectively. 100 kHz RF field of SPINA1-64 1H heteronuclear decoupling was performed throughout the entire experiment. The 11B isotropic dimension spectral width was scaled by (2.125) 1 to account for faster chemical shift evolution under 3Q coherence. The 11B isotropic dimension was calibrated from an 11B MQMAS of a sodium borate glass with identical transmitter offsets. The isotropic dimension of the 11B MQMAS of the sodium borate glass was referenced by setting the 4-coordinate BO4 signal to the same shift observed in the direct dimension since the CQ<0.5 MHz (thus QIS is negligible). 2D 11B→1H D-RINEPT heteronuclear correlation experiments were performed with previously described pulse sequences.[7,8] The symmetry based SR421 heteronuclear dipolar recoupling scheme was applied to the 1H spins (50 kHz RF field) to re-introduce 1H—11B dipolar couplings.[9] 100 kHz RF field of SPINA1-64 1heteronuclear decoupling was performed during the indirect acquisition of 11B. Central-transition (CT) selective 11B p/2 and p pulse lengths were 15 and 30 ms in duration, respectively, corresponding to a 8.3 kHz RF field and 16.6 kHz CT nutation frequency. 2D 11B DQ-SQ homonuclear correlation experiments were performed with previously described NMR pulse sequences.[10-12] A CT selective p pulse was applied during h evolution to ensure only CT DQ coherence between two 11B spins were selected during phase cycling.[10] The bracket BR 21 homonuclear dipolar recoupling scheme was used to generate DQ coherence directly from Z-magnetization.[12] Each p pulse in the BR21 homonuclear dipolar recoupling block was 40 ms in duration, corresponding to a 6.25 kHz RF field and 12.5 kHz CT nutation frequency. 100 kHz RF field of SPINA1-64 1H heteronuclear decoupling was performed throughout the entire experiment. A schematic illustration of all pulses sequences is given in
Density Functional Theory Calculations. Period plane-wave density functional theory (DFT) calculations were performed in CASTEP [13] (version 2017 R2) with the gauge including projected augmented-wave (GIPAW) approach [14]. Plane-wave DFT geometry optimizations and NMR calculations utilized the Generalized Gradient Approximation (GGA) with the Perdew-Burke-Ernzerhof exchange-correlation functional,[15] Tkatchenko-Scheffler (TS) dispersion corrections[16], On-the-Fly ultrasoft pseudopotential [17,18] and zeroth-order regular approximation (ZORA) relativistic treatment.[19] A 0.07 Å−1 k-point spacing and 630 eV kinetic energy cutoff was used for all calculations.
The phenyl borate systems were constructed in the open-source Avogadro molecular builder and visualization tool application prior to all DFT calculations.[19] For DFT, the structure was located in the middle of a 20 Å×20 Å×20 Å lattice that was surrounded by vacuum to ensure completer isolation between multiple species as the calculations were periodic (see CIF files). The calculated 11B shielding (s) values were converted to isotropic chemical shift (disc) using a previously published calibration curve (experimentally determined shifts versus DFT calculated shielding values for samples with known crystal structures) utilizing identical DFT parameters.[20] The calculated 11B CQ values were corrected via a previously published calibration curve, similar to that for converting shielding to shift.[20]
In this example, we demonstrate that boron on carbon compositions made using amorphous carbon that is not oxidized are not viable catalysts. In each case, the composition was unstable and the entirety of the composition combusted to CO2 during catalysis. Combustion of the resulting structure indicates the instability of the material under ODH reaction conditions.
In a first attempt at making such a composition, incipient wetness impregnation (IWI) of dehydrated Norit Darco 12×40 activated carbon (NDC) was performed with triisopropyl borate. The NDC was not oxidized before impregnation, and the entirety of the resulting structure combusted to CO2 during catalysis.
The NDC was crushed up and thermally treated at 120° C. in an ambient catalyst oven overnight with triisopropyl borate in a purge box under N2 flow. Calcination was then performed at 600° C. in a muffle furnace.
In a second attempt, wetness impregnation of dehydrated Norit Darco 12×40 was performed with boric acid solution. Again, the NDC was not oxidized before impregnation, and the resulting structure burned during catalysis.
NDC was crushed up and thermally treated at 120° C. in an ambient catalyst oven overnight. Subsequently, wetness impregnation with boric acid solution was conducted overnight. The NDC was stirred in solution on a hot plate at 80° C. and the solution and carbon were then dried in a catalyst oven until the water evaporated. Calcination was then performed at 600° C. in a muffle furnace. We also attempted a post-impregnation thermal treatment at 600° C. under N2.
In a third attempt, wetness impregnation of reduced Norit Darco 12×40 was performed with boric acid solution. Once again, the resulting structure burned during catalysis.
The NDC was dispersed in DI water and adjusted to pH 9-10 using NaCO3. NaBH4 was added, and the mixture was heated at 80° C. for 1 hr on a hot plate. The mixture was then filtered and washed with DI water, and dried overnight in a catalyst oven (120° C.).
Wetness impregnation with boric acid solution was conducted overnight. The NDC was stirred in solution on a hot plate at 80° C., then the solution and carbon were dried in a catalyst oven until water evaporated. The resulting composition was thermally treated at 600° C. under N2.
This example demonstrates that using oxidized amorphous carbon is essential to the thermal stability of the disclosed catalysts.
This application claims benefit of priority to U.S. Provisional Application 63/144,561, filed Feb. 2, 2021, the contents of which is incorporated by reference in its entirety.
This invention was made with government support under 1916775 awarded by the National Science Foundation and under DE-SC0017918 awarded by the US Department of Energy. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/014872 | 2/2/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63144561 | Feb 2021 | US |
