Methods and systems for the production of a glycol via electricity driven hydrogen peroxide

Abstract

A method of making a mesoporous single metal atom catalyst is provided, the method comprising: (a) forming a layer of silica on surfaces of a zeolitic imidazolate framework (ZIF) material to provide a silica coated ZIF material; (b) pyrolyzing the silica coated ZIF material to provide a pyrolyzed silica coated ZIF material; removing the layer of silica from the pyrolyzed silica coated ZIF material to provide an etched ZIF material; and oxidizing the etched ZIF material to provide a mesoporous single metal atom catalyst. The mesoporous single metal atom catalysts and electrolyzers and cascade systems comprising the catalysts are also provided. Methods of using the electrolyzers and cascade systems are also provided.

Description

Atty. Dkt. No.00100-0404-PCT METHODS AND SYSTEMS FOR THE PRODUCTION OF A GLYCOL VIA ELECTRICITY DRIVEN HYDROGEN PEROXIDE CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 694,268 that was filed September 13, 2024, the entire contents of which are incorporated herein by reference. BACKGROUND

[0002] Propylene glycol, recognized for its very low toxicity, is a sustainable chemical widely utilized across a broad spectrum of industrial applications. The chemistry involves the epoxidation of propylene-to-propylene oxide, followed by the hydrolysis of this oxide using a strong acid at high temperatures. The initial step can be carried out either through the chlorohydrin process under harsh conditions or by reacting with H2O2derived from the anthraquinone process, both of which are highly energy-intensive and struggle with low selectivity. To address these challenges, electrifying these chemical processes could serve as a sustainable solution, utilizing renewable electricity. However, the electrochemical production of propylene glycol in a single step process has thus far been limited by low selectivity and Faradaic efficiencies (FES), since it can lead to a variety of oxidation products. SUMMARY

[0003] Mesoporous single metal atom catalysts are provided, embodiments of which have been found to be highly active in the 2-electron oxygen reduction reaction (ORR) in acidic electrolytes. Methods of making and using the catalysts in electrolyzers are also provided, including methods and electrolyzers for the electrosynthesis of H2O2. The electrosynthesized H2O2 may be used directly (i.e., without purification) in a downstream reactor configured to oxidize an olefin (e.g., propylene) using the H2O2 to produce a glycol (e.g., propylene glycol). The related electrolyzers, cascade systems, and methods are also encompassed by the present disclosure.

[0004] By way of illustration, the Example below demonstrates a strategy that improved the catalytic activity and stability of certain Co single-atom catalysts (SACs) for the 2- electron oxygen reduction reaction (ORR) in acidic conditions. This approach was initiated with an emphasis on both inner- and outer-sphere ORR mechanisms, which are notablyAtty. Dkt. No.00100-0404-PCT affected by the electrolyte pH—an aspect often overlooked in the field. It has been widely believed that catalysts featuring porosity high surface areas would be better suited for the 4- electron ORR, rather than the 2-electron pathway, due to their superior mass transport properties. Contrary to this belief, it was found that a mesoporous catalyst structure with an average size of ~3.9 nm unexpectedly and substantially enhanced the 2-electron ORR pathway, especially in acidic media. This optimal structure effectively modulated the interaction between Co active sites and key H2O2 intermediates. A 1.93 Vcell at 300 mA cm-2was observed for over 50 h with ~89% of Faradaic efficiency (FE) in the first electrolysis stage of the cascade system. Building on this, a continuous fixed-bed reactor was developed that directly produced propylene glycol with ~99% selectivity, achieving an electron utilization efficiency of 72% for propylene glycol at 300 mA cm-2. Energy analysis revealed a total energy cost of 26.8 GJ per tonne of pure propylene stream, which is a 30% reduction relative to the 38.6 GJ tonne-1required in the chemical engineering process. This corresponds to Aspen simulation results for the thermochemical reactor setup and the separation costs of the generated propylene glycol solution from water and others to obtain a pure product stream.

[0005] An embodiment 1 is a method of making a mesoporous single metal atom catalyst, the method comprising: (a) forming a layer of silica on surfaces of a zeolitic imidazolate framework (ZIF) material to provide a silica coated ZIF material; (b) pyrolyzing the silica coated ZIF material to provide a pyrolyzed silica coated ZIF material; removing the layer of silica from the pyrolyzed silica coated ZIF material to provide an etched ZIF material; and oxidizing the etched ZIF material to provide a mesoporous single metal atom catalyst.

[0006] An embodiment 2 is according to embodiment 1, wherein the ZIF material comprises Co and the metal of the mesoporous single metal atom catalyst comprises Co.

[0007] An embodiment 3 is according to any of embodiments 1-2, wherein the mesoporous single metal atom catalyst is characterized by a Brunauer-Emmett-Teller (BET) surface area of at least 1300 m2 / g.

[0008] An embodiment 4 is according to any of embodiments 1-3, wherein the mesoporous single metal atom catalyst is characterized by an average mesopore diameter in a range of from 3 nm to 5 nm and a dVp / dDp value at the average mesopore diameter value of at least 0.15 cm3 / g nm.Atty. Dkt. No.00100-0404-PCT

[0009] An embodiment 5 is according to any of embodiments 1-4, wherein the mesoporous single metal atom catalyst comprises isolated metal atoms dispersed within a carbon matrix, the carbon matrix comprising carbon, nitrogen, and oxygen, wherein the carbon matrix defines a plurality of mesopores.

[0010] An embodiment 6 is according to embodiment 5, further wherein: the isolated metal atoms are present at an amount of from 1 weight% to 5 weight%; the oxygen is present at an amount of from 1 atomic% to 10 atomic%; at least some of the isolated metal atoms are coordinated to nitrogen; at least some of the oxygen is bound to carbon forming epoxide groups; and the isolated metal atoms are oxidized.

[0011] An embodiment 7 is according to any of embodiments 1-6, wherein the mesoporous single metal atom catalyst is in the form of a plurality of nanoparticles.

[0012] An embodiment 8 is a mesoporous single metal atom catalyst comprising isolated metal atoms dispersed within a carbon matrix, the carbon matrix comprising carbon, nitrogen, and oxygen, wherein the carbon matrix defines a plurality of mesopores, and further wherein the mesoporous single metal atom catalyst is characterized by a BET surface area of at least 1300 m2 / g.

[0013] An embodiment 9 is according to embodiment 8, wherein the metal comprises Co.

[0014] An embodiment 10 is according to any of embodiments 8-9, characterized by an average mesopore diameter in a range of from 3 nm to 5 nm and a dVp / dDp value at the average mesopore diameter value of at least 0.15 cm3 / g nm.

[0015] An embodiment 11 is according to any of embodiments 8-10, further wherein: the isolated metal atoms are present at an amount of from 1 weight% to 5 weight%; the oxygen is present at an amount of from 1 atomic% to 10 atomic%; at least some of the isolated metal atoms are coordinated to the nitrogen; at least some of the oxygen is bound to the carbon forming epoxide groups; and the isolated metal atoms are oxidized.

[0016] An embodiment 12 is according to any of embodiments 8-11, in the form of a plurality of nanoparticles.

[0017] An embodiment 13 is according to embodiment 12, wherein the metal is Co.

[0018] An embodiment 14 is an electrolyzer configured to produce H2O2, the electrolyzer comprising: (a) a cathode comprising the mesoporous single metal atom catalyst of any of embodiments 8-13; (b) an acidic catholyte in contact with the cathode; and (c) an anode inAtty. Dkt. No.00100-0404-PCT electrical communication with the cathode such that, upon delivery of O2 to the cathode and generation of a potential difference between the cathode and the anode, the mesoporous single metal atom catalyst catalyzes electroreduction of the O2 via a 2-electron pathway to produce an output stream comprising H2O2.

[0019] An embodiment 15 is a method of producing H2O2, the method comprising delivering O2 to the electrolyzer of embodiment 14 and generating the potential difference between the cathode and the anode.

[0020] An embodiment 16 is according to embodiment 15, wherein the method is characterized by a Faradic efficiency of at least 80% towards H2O2 at a current density of 300 mA / cm2.

[0021] An embodiment 17 is a cascade system configured to oxidize an olefin, the cascade system comprising: (a) an electrolyzer comprising (i) a cathode comprising a mesoporous single metal atom catalyst comprising isolated metal atoms dispersed within a carbon matrix, the carbon matrix comprising carbon, nitrogen, and oxygen, wherein the carbon matrix defines a plurality of mesopores, and further wherein the mesoporous single metal atom catalyst is characterized by a BET surface area of at least 1300 m2 / g; (ii) an acidic catholyte in contact with the cathode; and (iii) an anode in electrical communication with the cathode such that, upon delivery of O2 to the cathode and generation of a potential difference between the cathode and the anode, the mesoporous single metal atom catalyst catalyzes electroreduction of the O2 via a 2-electron pathway to produce an output stream comprising H2O2; and (b) a reactor in fluid communication with the electrolyzer, the reactor configured to oxidize an olefin to a glycol using the H2O2 of the output stream.

[0022] An embodiment 18 is according to embodiment 17, wherein the output stream comprising the H2O2is delivered directly from the electrolyzer to the reactor.

[0023] An embodiment 19 is a method of oxidizing an olefin, the method comprising delivering O2 to the electrolyzer of the cascade system of any of embodiments 17-18, generating the potential difference between the cathode and the anode, and delivering an olefin to the reactor of the cascade system of any of embodiments 17-18.

[0024] An embodiment 20 is according to embodiment 19, wherein the olefin is propylene and the glycol is propylene glycol.Atty. Dkt. No.00100-0404-PCT

[0025] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Illustrative embodiments of the disclosure will hereafter be described with reference to the accompanying drawings.

[0027] FIGS.1A-1E. Thermodynamic studies of 2-electron ORR electrocatalyst for H2O2production. FIG.1A: Pourbaix diagram for oxygen reduction mechanisms. All potentials were expressed relative to the standard hydrogen electrode (SHE). FIG.1B: ORR mechanisms on the a comparative non-metal catalyst (O-CNT; left) and comparative metal- based compound electrocatalysts for H2O2electrosynthesis (Co SAC; right). Carbon catalysts mostly catalyze via the outer-sphere pathway at high pH levels, but they exhibit large overpotential at low pH. In contrast, Co SACs have direct adsorption sites, enabling them to exhibit activity for 2-electron ORR in both acidic and alkaline conditions. FIGS.1C-1E: Comparison of the oxygen reduction performances in various pH electrolytes. Polarization curves at 1600 rpm and calculated H2O2generation currents at the ring electrode for both representative catalysts in (FIG.1C) 0.1M KOH (pH~13, alkaline), (FIG.1D) 0.1 M K2SO4 (pH~7, neutral), and (FIG.1E) 0.1 M HClO4(pH~1, acidic) electrolytes.

[0028] FIGS.2A-2I. The synthesis and structural characterization of Meso-CoNC(O), an illustrative mesoporous single metal atom catalyst according to the present disclosure. FIG. 2A: Schematic synthesis process of Meso-CoNC(O) by silica coating, pyrolysis for carbonization, etching silica off, and surface oxidation. FIGS.2B-2C: Low-resolution TEM images of Meso-CoNC(O) (FIG.2B) and comparative CoNC(O) (FIG.2C). Scale bars, 100 nm. FIG.2D: High-resolution HAADF-STEM image of Meso-CoNC(O). FIG.2E Co K-edge k3-weighted FT-EXAFS spectra in R-space for Meso-CoNC(O), comparative CoNC(O), and Co foil. FIGS.2F-2H: N2 adsorption-desorption isotherms (FIG.2F), mesopore size distributions (FIG.2G), and BET surface area and area determined range of meso- macropores (FIG.2H) of the above two catalysts. Vp: pore volume; Dp: pore diameter. FIG. 2I: Adsorption amounts of Rhodamine B dependent on time with Meso-CoNC(O) and comparative CoNC(O) catalysts, determined by UV-Vis absorbance measurements at 554 nm.Atty. Dkt. No.00100-0404-PCT

[0029] FIGS.3A-3G. Electrochemical characterization of Meso-CoNC(O). FIGS.3A-3B: ORR polarization curves (solid lines) of RRDE at 1600 rpm and their H2O2detection current densities (dotted lines) at the ring electrode in 0.1 M HClO4 (FIG.3A), and H2O2 selectivity (FIG.3B). FIG.3C: Mass activity of Meso-CoNC(O) and comparative CoNC(O) and other comparative 2-electron ORR catalysts. FIG.3D: Long-term stability tests of Meso-CoNC(O) using chronoamperometry measurements at 0.3 V vs. RHE. FIG.3E: Electric double-layer capacitance (Cdl) measured by cyclic voltammogram. FIG.3F: Cdl normalized by the BET surface area. FIG.3G: The redox behavior of the [Fe(CN)6]3- / 4-couple at different concentrations (0 to 150 mM) on both Meso-CoNC(O) and comparative CoNC(O).

[0030] FIGS.4A-4F. Cascade production of propylene glycol using an electricity driven acidic H2O2solution. FIG.4A: Schematic illustration of the cascade reactor system for propylene glycol production developed in the present disclosure (dashed box) as compared to conventional systems (top). P: propylene; PO: propylene oxide; PG: propylene glycol. FIG. 4B: Comparison of electrochemical performance of Meso-CoNC(O) and comparative CoNC(O) in flow cell with 1 mm gap using 0.5 M H2SO4. FIG.4C: Electron-to-propylene glycol efficiency at high current densities, with Meso-CoNC(O) as an electrocatalyst and titanium silicalite-1 (TS-1) as a thermochemical catalyst. FIG.4D: The corresponding propylene glycol production rates at high current densities. FIG.4E: Stability test for the continuous production of propylene glycol stream at 300 mA cm-2. FIG.4F: Comparison of specific energy consumption via the conventional chemical engineering process and that of the present cascade process using electricity driven H2O2. DETAILED DESCRIPTION

[0031] In one aspect, a method of making a mesoporous single metal atom catalyst is provided that comprises (a) forming a layer of silica on surfaces of a zeolitic imidazolate framework (ZIF) material to provide a silica coated ZIF material; (b) pyrolyzing the silica coated ZIF material to provide a pyrolyzed silica coated ZIF material; (c) removing the layer of silica from the pyrolyzed silica coated ZIF material to provide an etched ZIF material; and (d) oxidizing the etched ZIF material to provide a mesoporous single metal atom catalyst. An embodiment of such a method is illustrated in FIG.2A, in which particles of ZIF-67 / ZIF-8 (the zeolitic imidazolate framework material) are coated with a silica layer, pyrolyzed, etched, and oxidized to form an illustrative mesoporous single metal atom catalyst (Meso- CoNC(O)). A comparative material synthesized using the same steps, except for (a) and (c),Atty. Dkt. No.00100-0404-PCT is labeled CoNC(O). As described in the Example, below, another comparative material synthesized using the same steps, except for (a), (c), and (d), is referred to herein as CoNC.

[0032] The ZIF material used to synthesize the present mesoporous single metal atom catalysts is a type of metal-organic framework material comprising tetrahedrally-coordinated metal ions (e.g., transition metal ions such as Co, Zn, and combinations thereof) connected by imidazolate linkers. Various ZIF materials may be used, including combinations of different types of materials, as was used in the Example, below, to provide the illustrative mesoporous catalyst shown in FIG.2A. The metal of the selected ZIF material corresponds to the metal of the present mesoporous catalyst.

[0033] The layer of silica may be formed by exposing the selected ZIF material to a solution comprising a silica precursor (e.g., tetraethyl orthosilicate) under conditions to convert the silica precursor to silica on surfaces of the ZIF material. Other components may be included in the solution, e.g., solvent (e.g., water), base (e.g., NaOH), surfactant (e.g., cetyltrimethylammonium bromide). The conditions include parameters such as the concentrations of the solution components, coating temperature, and coating time. Illustrative values for these parameters are provided in the Example, below. As also demonstrated in the Example, these parameters, particularly coating time, may be adjusted to tune the thickness of the silica layer, which in turn, affects the porosity / surface area / average mesopore size of the present mesoporous catalysts.

[0034] The pyrolysis may be carried out using sufficiently high temperatures so as to decompose molecules of the ZIF material and form a carbon-rich matrix in which the metal of the mesoporous single metal atom catalyst is dispersed. Illustrative pyrolysis temperatures and other conditions (e.g., pyrolysis time, pyrolysis atmosphere) are provided in the Example, below. The layer of silica may then be removed by etching, including by exposing the pyrolyzed silica coated ZIF material to concentrated base while heating for a period of time. Illustrative concentrations, bases, etching temperatures, and etching times are provided in the Example, below. Oxidation may be carried out by exposing the etched ZIF material to oxygen (e.g., air) while heating for a period of time. Illustrative oxidation temperatures and oxidation times are provided in the Example, below.

[0035] The resulting mesoporous single metal atom catalysts are porous materials having high surface areas and large populations of mesopores. Surface area may be quantified using Brunauer-Emmett-Teller (BET) analysis as described in the Example, below. InAtty. Dkt. No.00100-0404-PCT embodiments, the present mesoporous catalysts are characterized by a BET surface area of at least 1300 m2 / g, at least 1350 m2 / g, at least 1400 m2 / g, at least 1450 m2 / g, or a range of between any of these values. By contrast, comparative single metal atom catalysts have significantly lower BET surface areas. (See FIGS.2F, 2H.) As shown in FIG.2G, the population of mesopores in the present mesoporous catalysts is significantly greater than in comparative single metal atom catalysts. In embodiments, the average diameter of the mesopores in the present mesoporous catalysts is in a range of from 3 nm to 5 nm and the dVp / dDp at the average diameter value is at least 0.15 cm3 / g nm. As a result, in embodiments, the present mesoporous catalysts are characterized by a mesopore-macropore surface area of at least 200 m2 / g, at least 250 m2 / g, at least 300 m2 / g, at least 350 m2 / g, at least 400 m2 / g, or a range of between any of these values. (See FIG.2H.) By contrast, comparative single metal atom catalysts exhibit significantly lower values. As shown in FIG. 2I, the increased surface area and increased mesopore population of the present mesoporous catalysts greatly increases the mass transfer through the catalysts as compared to comparative single metal atom catalysts.

[0036] Regarding the composition of the mesoporous single metal atom catalysts, the material may be characterized as having single (i.e., isolated) metal atoms (as distinguished from a collection of multiple metal atoms, e.g., metal nanoparticles) homogenously distributed within a carbon matrix. (See FIG.2D.) The amount of metal in the present mesoporous catalysts may be, e.g., from 1 to 5 weight%, from 1.5 to 4.5 weight%, or from 2 to 4 weight% (weight of metal / (total weight of catalyst)). The carbon matrix generally comprises other atoms in addition to carbon, particularly nitrogen (N), and due to the oxidation step, oxygen (O). As a result, the present mesoporous catalysts comprise both metal-N coordination sites (see FIG.2E) and epoxide (C-O-C) functional groups (as determined from Fourier Transform Infrared Spectroscopy (FTIR), data not shown). The amount of oxygen in the present mesoporous catalysts may be, e.g., from 1 to 10 atomic% as determined using X-ray photoelectron spectroscopy (XPS, data not shown). X-ray absorption near-edge structure (XANES) analysis may be used as described in the Example below to confirm that the metal atoms are oxidized, i.e., electron deficient.

[0037] The morphology of the mesoporous single metal atom catalysts is generally particulate in nature, e.g., the catalysts may be characterized as a plurality of nanoparticles as shown in FIG.2B. These nanoparticles are spherical but have a faceted surface. The average diameter of the nanoparticles may be, e.g., from 50 nm to 200 nm, from 75 nm to 175 nm, orAtty. Dkt. No.00100-0404-PCT from 90 nm to 150 nm. The nanoparticles are distinguished from other nanostructures, e.g., nanorods, nanotubes.

[0038] The mesoporous single metal atom catalysts may be characterized by other properties, including electric double-layer capacitance (Cdl) capacitance. In embodiments, the present mesoporous catalysts are characterized by a Cdlof at least 200 F / g, of at least 225 F / g, of at least 250 F / g, or a range of between any of these values. (See FIG.3E.) By contrast, comparative single metal atom catalysts that have significantly lower Cdlvalues. Relatedly, in embodiments, the present mesoporous catalysts are characterized by a Cdl / BET surface area ratio of at least 0.15 F / m, at least 0.16 F / m, at least 0.17 F / m, at least 0.18 F / m, or a range of between any of these values. (See FIG.3F.) These values indicate that a large portion of the surface area of the present mesoporous catalysts is accessible to facilitate a desired electrochemical reaction (e.g., ORR). By contrast, comparative single metal atom catalysts that have significantly lower Cdl / BET surface area ratios.

[0039] In another aspect, an electrolyzer comprising any of the disclosed mesoporous single metal atom catalysts is also provided. An illustrative embodiment of an electrolyzer 400 is shown in FIG.4A. The electrolyzer 400 comprises a cathode 402 comprising any of the disclosed mesoporous single metal atom catalysts (e.g., Meso-CoNC(O)); an anode 404 in electrical communication with the cathode 402; and a separator 406 between the cathode 402 and the anode 404. Oxygen (O2) is delivered to the cathode 402 via an inlet. The cathode 402 may be provided by a catalyst ink comprising any of the disclosed mesoporous single metal atom catalysts, the catalyst ink coated onto a gas diffusion layer through which the O2 flows. Aqueous acidic electrolytes are used for the catholyte and the anolyte, to maintain an acidic pH (e.g., pH less than 4, pH less than 3, pH less than 2, or a range between any of these values, including pH from 1 to 4). The anode 404 may comprise a catalyst configured to catalyze an oxidation reaction at the anode 404, e.g., water oxidation. Upon generating a potential difference between the cathode and the anode, H2O2 is produced at the cathode 402 via the 2-electron ORR pathway, which may be collected as an output stream 409 comprising the H2O2 (e.g., consisting of the H2O2 in the acidic catholyte). Thus, a method for the electrosynthesis of H2O2using the electrolyzer 400 is also encompassed by the present disclosure.

[0040] The present electrolyzers configured for the electrosynthesis of H2O2 and the corresponding methods of using such electrolyzers to generate acidic H2O2, may beAtty. Dkt. No.00100-0404-PCT characterized by high H2O2 selectivities (e.g., at least 90%, at least 95%, at least 99%, or a range between any of these values (all at 0.3 V versus RHE; see FIG.3D)); high H2O2activities (e.g., at least 30 A / g cat, at least 32 A / g cat, at least 35 A / g cat, or a range between any of these values (all at 0.5 V versus RHE; see FIG.3C)); and high Faradic efficiencies (FEs) (e.g., at least 80%, at least 85%, at least 90%, or a range between any of these values (all at 300 mA / cm2; see FIG.4B)).1. As described herein, these high values are particularly surprising since the increased surface area and mesopore population would have been expected to facilitate the 4-electron ORR pathway, resulting in the production of water, not H2O2.

[0041] The H2O2 produced from the present electrolyzers may be used in any application in which a source of H2O2is desired. For example, the H2O2may be used as a feedstock for a downstream catalytic process such as oxidizing an olefin (e.g., propylene) to a glycol (e.g., propylene glycol). That is, any of the disclosed electrolyzers may be a component of a cascade system such as the illustrative cascade system 408 shown in FIG.4A. Such cascade systems and methods of using the systems are also encompassed by the present disclosure.

[0042] The illustrative cascade system 408 further comprises a thermocatalytic reactor 410 configured to oxidize the olefin using the output stream 409 comprising acidic H2O2 from the electrolyzer 400. The thermocatalytic reactor 410 may comprise a catalyst (e.g., titanium silicalite-1) capable of catalyzing the olefin oxidation reaction. The output stream 409 from the electrolyzer 400 may be fed directly (i.e., without any intervening processing step) into the thermocatalytic reactor 410. The olefin (e.g., propylene, P) may be fed into the thermocatalytic reactor 410 via another inlet. The output stream 412 from the thermocatalytic reactor 410 comprises the glycol (e.g., propylene glycol, PG). As demonstrated in the Example, below, the output stream 412 may consist of the glycol as the only product (although some water and acid may be present). The cascade system 408 may be characterized as a continuous cascade system since O2 may be continuously fed into the electrolyzer 400, the output stream 409 comprising acidic H2O2may be continuously fed into the thermocatalytic reactor 410, and the output stream 412 comprising the glycol may be continuously collected therefrom.

[0043] The present cascade systems configured to oxidize an olefin using acidic H2O2 may be characterized by high electron-to-glycol efficiencies (e.g., at least 70%, at least 73%, at least 75%, or a range between any of these values (all at 100 mA / cm2; see FIG.4C)); andAtty. Dkt. No.00100-0404-PCT high glycol production rates (e.g., at least 5 mmol / hr, at least 5.5 mmol / hr, at least 6 mmol / hr, or a range between any of these values (all at 500 mA / cm2; see FIG.4D). EXAMPLE

[0044] Introduction

[0045] Integrating electrochemical routes with chemical reactions presents a promising approach in the development of more sustainable processes in the industry for propylene glycol production. The traditional chemical process for glycol, using anthraquinone-based hydrogen peroxide, faces limitations due to high energy consumption and the utilization of hazardous substances. This Example demonstrates a continuous cascade system that incorporates H2O2 electrosynthesis with thermochemical reactors, achieving selective production of propylene glycol at industrial-relevant rates. Acidic H2O2streams were directly upgraded from electrolyzers without purification. The findings revealed that direct adsorption sites and porosity significantly influenced the oxygen reduction mechanism, which can operate via inner- or outer-sphere reaction pathways. To optimize this process, an acid-stable, mesoporous cobalt single-atom electrocatalyst was developed that achieved unexpectedly high activity and selectivity, exhibiting a 1.93 Vcellat 300 mA cm-2for over 50 h with ~89% Faradic efficiency. This robust acidic O2 to H2O2 electrocatalytic system, which avoids unwanted ion crossover issues, achieved an electron utilization efficiency of 72% for the selective production of propylene glycol at industrial-relevant current densities, within continuous combined cascade reactor systems.

[0046] Methods

[0047] Preparation of electrocatalysts

[0048] All reagents used in this Example were obtained from suppliers and used without further purification. To synthesize ZIF-67 / ZIF-8, 453.4 mg of Co(NO3)2∙6H2O and 9.040 g of Zn(NO3)2∙6H2O were dissolved in 400 mL of methanol to form a clear solution. This was followed by adding 984 mg of 2-methylimidazole, which was dissolved in another 400 mL of methanol. After continuous stirring for 2 minutes to ensure thorough mixing, the mixture was left undisturbed at room temperature for 24 h. The resulting light purple precipitates were then centrifuged, washed three times with methanol, and dried under vacuum at 80 °C. The resulting ZIF-67 / ZIF-8 powders were first coated with a silica layer. Typically, 300 mg of the ZIF powder was dispersed in methanol, followed by the addition of a 0.01 M NaOH solutionAtty. Dkt. No.00100-0404-PCT containing 125 mg of cetyltrimethylammonium bromide (CTAB). Subsequently, 1.2 mL of tetraethyl orthosilicate, diluted in 6 mL of methanol, was added dropwise to the solution, and the mixture was stirred for 30 minutes. The core-shell nanoparticles were then separated by filtration, dried under vacuum at 80 °C overnight, and subjected to a heat treatment at 1000 °C for 2 h under an Ar atmosphere in a tube furnace. The silica layer was then removed by etching with a 6 M NaOH solution at 90 °C for 24 h, followed by thorough washing until the filtrate was neutral. The dried powder (200 mg) was then oxidized in air at 280 °C for 1 h to produce the oxidized cobalt-based catalyst, denoted as Meso-CoNC(O). For the comparative CoNC and CoNC(O) samples, 2 g of the as-synthesized ZIF-67 / ZIF-8 powder was first heated at 1000 °C for 2 hours under an Ar atmosphere to obtain CoNC. The subsequent steps were identical to the synthesis of CoNC(O), which were the oxidized CoNC samples.

[0049] Material characterization

[0050] The X-ray diffraction (XRD) spectra of the synthesized hydroxide samples were recorded using a Rigaku D / MAX 2500 with Cu Kα radiation (λ = 0.1541 nm) at 40 kV and 200 mA. The scan range was from 20 to 80 degrees at a rate of 2 degrees / min with a step size of 0.013 degrees. The high-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) results were obtained on a JEM-2100F (JEOL) microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Thermo Fisher Scientific K-Alpha+ XPS system at the Busan Center of the Korea Basic Science Institute. The X-ray source was a monochromated Al Kα (1486.6 eV, X-ray energy: 12 kV, 72 W) with a spot size of 400 μm. Peaks were deconvoluted using XPS Peak 4.1 software. X-ray absorption fine structure (XAFS) measurements were conducted on the 7D beamline of the Pohang Light Source-II (PLS-II, 3 GeV), using a Si(111) double crystal monochromator detuned by 30% to reduce higher harmonic generation. All spectra were acquired in transmission mode, processed to extract absorbance, and analyzed using the ATHENA program within the ARTEMIS software suite integrated with IFEFFIT. Soft X-ray absorption spectroscopy (sXAS) was performed on the 6A beamline of PAL-II. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics 3-FLEX surface characterization analyzer after degassing the samples under vacuum at 170 °C for 12 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, pore volume was determined at a relative pressure (P / Po) of 0.99, and pore size distribution was analyzed using nonlocal density functional theory (NLDFT) for pores up to 2 nm and the Barrett-Joyner-HalendaAtty. Dkt. No.00100-0404-PCT (BJH) method for pores ranging from 2 to 250 nm, with microporosity evaluated by t-plot analysis.

[0051] Electrochemical measurements

[0052] The electrochemical properties were investigated using Autolab potentiostat (PGSTAT302N) in a three-electrode cell equipped with the ring-disk electrode (RRDE) setup (AFE8R4GCPT, Pine Instruments Corporation). A saturated calomel electrode (HI-5412, Hanna) was used as a reference electrode, and the electrolytes were 0.05 M H2SO4(Sigma- Aldrich) and 0.1 M KOH (99.99%, Sigma-Aldrich). The catalyst inks were prepared by dispersing the 5 mg of synthesized catalysts in 2000 μL of isopropanol, then 50 μL of Nafion solution (5 wt%, Ion Power) was added. After the sonication of this suspension, working electrodes were prepared through spin-coating process. Prior to ink dropping, the working electrode was polished mechanically with alumina suspension (Buehler) and rinsed with deionized water to eliminate the impurities.4.02 μL of the catalyst ink was then drop-casted onto a disk electrode of the RRDE tip spinning at an initial rate of 100 rpm and advanced to 400 rpm to achieve uniform electrode coverage. The electrode catalyst loading was 50 μg / cm2. Prior to any electrochemical measurements, all the potentials were converted to the RHE scale by calibration in a H2-saturated electrolyte against a Pt electrode. Then, H2O2 productivity and selectivity were obtained by polarization curves in O2-saturated conditions between 0.05 V and 1.05 V at a scan rate of 10 mV / s with 1600 rpm rotating, while holding the potential of ring electrode at 1.2 V. The ORR current was corrected by subtracting the current obtained in an Ar-saturated electrolyte from that measured in O2-saturated conditions. All measured ring currents were also corrected using the collection efficiency (N) of RRDE setup to get the overall current density as all H2O2 generated were detected. The H2O2 selectivity was calculated by the equation: Selectivity of H2O2(%) = 200 × (Ir / N) / (Id+ Ir / N) where Irand Iddenote the ring and disk current, respectively. The kinetic current (Ik) was calculated by the following equation: 1 / Im = 1 / Il + 1 / Ik where Imindicates the measured current and Ilis the limiting current. The limiting current was obtained from the Levich equation with the calculated total electron transfer number (n) from the RRDE setup, as it was difficult to determine the value of limiting current for the carbonaceous catalysts:Atty. Dkt. No.00100-0404-PCT Il = 0.62 n F A Do2 / 3ω1 / 2υ-1 / 6Co where F, A, Do, ω, υ and Co indicate the Faraday constant (96,485 C mol-1), geometric area of the disk electrode (0.19625 cm2), diffusion coefficient of O2 in the electrolyte at 298 K (1.85 × 10-5cm2s-1), electrode rotation speed (rad s-1), kinematic viscosity of O2 (0.89 × 10-2cm2s-1) and O2concentration (1.21 × 10-6mol cm-3).

[0053] Peroxide reduction reaction (PRR), measurements were carried out under the same conditions as for OER measurements in a three-electrode system using RRDE. The PRR polarization curves were recorded with 10 mV / s and 1600 rpm rotation speed in Ar- saturated 0.05 M H2SO4 and 0.1 M KOH solutions with 1.3 and 10 mM H2O2 electrolytes, respectively. To investigate the kinetics of outer-sphere electron transfer process, cyclic voltammetry (CV) was also conducted from -0.5 V to 1.0 V (vs. Ag / AgCl) at a scan rate of 200 mV s-1in an Ar-saturated solution of 0.5 M K2SO4+ x mM of K4[Fe(CN)6] (Sigma- Aldrich) (x = 1, 2, 5, 20, 50, 75, 100, and 150).

[0054] To quantify the amount of generated H2O2 and achieve high peroxide concentrations at industrial-relevant production rates, electrosynthesis was first performed in a flow cell using custom-made cells. The two electrodes with the same conditions were placed on opposite sides of one 1 mm thick spacer with 0.5 cm wide by 2.0 cm channels, separated by a Nafion 212 membrane (1.0 × 2.5 cm2) between spacer and anode electrode. Around 0.5 mg cm-2of catalyst was loaded onto a gas diffusion layer (GDL) via air-brush coating. The GDL was a JNT30-A6H carbon paper. Around 1.0 mg cm-2of commercial IrO2 catalyst (Fuel Cell store) was loaded onto a titanium felt GDL (Fuel Cell store) electrode as the anode for water oxidation. The cathode and anode electrolytes were circulated with 0.5 or 0.05 M H2SO4. The flow rates of both sides were roughly 1.5 ml min-1controlled using a peristaltic pump, and 40-200 sccm of O2 gas was supplied through the flow field of the titanium bipolar plate.

[0055] Propylene oxidation in thermochemical reactors using the electricity driven H2O2

[0056] The oxidation reaction of propylene to oxide or glycol is an exothermic process, typically conducted at 40-60 °C. Thus, a fixed-bed reactor column was equipped with an outer jacket that allowed the circulation of a large volume of water to maintain a constant temperature. The reactor featured a tube 3 / 4 inches in diameter, fitted with a thermocouple, an inlet for the gaseous propylene supply, and a second inlet connected to the liquid reagents, notably a supply tank for H2O2 sourced from the initial electrolyzer in the cascade system.2 gAtty. Dkt. No.00100-0404-PCT of a commercial titanium silicalite-1 (TS-1) catalyst (ACS Materials) was packed in the reactor column, using quartz wool and ceramic beads as pre- and post-packing layers, respectively. Before starting the reaction, the reactor was purged of air and any undesired O2 gas in the H2O2supply from the electrolyzer system using nitrogen. To continuously generate propylene glycol, 40 sccm of propylene was fed into the reactor, and an acidic H2O2 solution was supplied at a rate of 1.5 mL min-1. With each initial setup using the TS-1 catalyst, the catalyst layer was activated by running the propylene oxidation process for about one hour at 40-60 °C. The gas and liquid byproducts were not collected but were discarded during this activation. This step facilitated the partial adsorption of substances by the TS-1 catalyst, aiding in its activation. All experimental results were conducted after this activation phase, following a rinse of the catalyst with fresh deionized water.

[0057] Product analysis

[0058] The flow cell was first stabilized by applying each potential for 10 min before collecting liquid products. After electrolysis, the generated H2O2concentration was detected by titration with potassium permanganate (0.1 M KMnO4, Sigma-Aldrich). The Faradaic efficiency (FE) of H2O2in flow cell systems was calculated by the equation: 2MnO4- + 5H2O2+ 6H+→ 2Mn2++ 5O2+ 8H2O

[0059] Sulfuric acid (1 M H2SO4) was used as the H+source. The FE for H2O2 production was calculated from the following equation: FE (%) = × 100 (%)

[0060] For quantification of the generated propylene glycol,1H NMR spectra were collected on a Bruker 400 MHz spectrometer to quantify the liquid products. To describe the electron efficiency for propylene glycol production, the ETP was defined as follows: ETP (%) = FE × Selectivity (in thermocatalysis) × Yield of H2O2(%) = × 100 (%) = × 100 (%)

[0061] The ETP is defined as an alternative to FE, which is commonly used in electrocatalysis to evaluate the selectivity of the desired electrochemical pathways. Here, the ETP is a measure of the overall electron utilization efficiency for propylene glycol productionAtty. Dkt. No.00100-0404-PCT and is determined by the following variables: (1) the FE of the H2O2 production in the first reactor; (2) the H2O2utilization efficiency for propylene oxidation in the second reactor; and (3) the ethylene glycol selectivity during propylene oxidation using H2O2 in the second process.

[0062] Results

[0063] Mechanistic origins of 2-electron oxygen reduction pathway for H2O2 electrosynthesis

[0064] Electrochemical reduction of O2to H2O2, while maintaining high selectivity even at industrial-relevant current densities, was crucial for the cascade strategy for selective propylene glycol production. The ORR can proceed through either a 4-electron reaction (line 1), generating H2O, or a 2-electron reaction (line 2), producing H2O2 (FIG.1A). Both ORR processes are pH-dependent, with equilibrium potentials following the Nernst equation and shifting by -59.1 mV / pH. However, the 2-electron pathway becomes more complicated under alkaline conditions. Considering that H2O2 is a weak acid (pKa = 11.63), it is unstable in electrolytes at pH > 11.63, dissociating into HO2-. Consequently, the ORR to form HO2- involves two electrons but only one proton, resulting in a different slope of -29.6 mV / pH (line 2’ in FIG.1A).

[0065] The rate-determining step in ORR is the initial electron transfer to O2, leading to the formation of the superoxide radical anion (∙O2-) since the nonpolar O2 molecule is stable. Notably, this first electron transfer can occur through an outer-sphere reaction, or an inner- sphere reaction. For the outer-sphere electron transfer reaction, the process can be described as (O2)sol+ e- → (O2-)sol(line 3). This mode of electron transfer is believed to contribute to the so-called facile kinetics of ORR, which is independent on the catalysts surface. Conversely, in the inner-sphere electron transfer, the equilibrium potential shifts by -ΔGads / F, where ΔGads represents the adsorption energy of bulk O2- to (O2-)ad on the catalyst surfaces. This mode of electron transfer can be described as (O2)sol+ e- → (O2-)ad(line 3’). The stronger the attractive interaction, the more positively the equilibrium potential shifts.

[0066] The minimum required overpotential is determined by this equilibrium potential difference between the target 2-electron ORR process and this first electron transfer step. At pH = 13, this overpotential is relatively small, between line 2’ and line 3, indicating that catalysts lacking specific interaction between catalyst and O2can still demonstrate activity through outer-sphere electron transfer. This is why under alkaline conditions almost anyAtty. Dkt. No.00100-0404-PCT electronically conducting carbon material can produce H2O2 (FIG.1B). However, at low pH, inner-sphere electron transfer can effectively reduce the overpotential, which increases as the pH decreases, by directly adsorbing and stabilizing O2 and intermediates at the active site. Although both inner- and outer-sphere electron transfer mechanisms might coexist, the contribution of each pathway is significantly influenced by environmental factors, including the catalyst type and the electrolyte pH. This analysis reveals an approach to enhancing the electrocatalytic inner-sphere electron transfer mechanism in acidic conditions by introducing direct adsorption sites suitable for selective reduction of O2to H2O2.

[0067] The ORR performance was evaluated for three comparative catalysts, a non-metal carbon catalyst (O-CNT), and two cobalt single atom catalysts containing electron-deficient cobalt sites (Co SAC (1) and Co SAC (2)) in three different solutions: 0.1 M HClO4, 0.1 M K2SO4, and 0.1 M KOH, using a rotating ring-disk electrode (RRDE). FIG.1C shows that both O-CNT and Co SAC (2) exhibited high current and remarkably lower overpotentials (with O-CNT being ~40 mV lower at 1 mA cm-2) with high 2-electron selectivity (~90%) in alkaline conditions. This fast kinetics of O-CNT illustrates why oxidized carbon materials have been used as catalysts for H2O2 synthesis in alkaline conditions. However, as the pH decreased, the performance disparity between the two catalysts began to increase significantly. In neutral pH electrolyte, the onset potential of O-CNT shifted in a negative direction by ~250 mV, while Co SAC (1) still showed similar activity compared to the alkaline electrolyte (FIG.1D). In acidic conditions, Co SAC (1) exhibited better performance, whereas O-CNT showed negligible ORR activity, indicating an overpotential difference of ~600 mV between them (FIG.1E).

[0068] A key observation was that the two Co SACs maintained similar ORR activity across different electrolyte pH levels, consistently aligning with DFT predictions. However, contrary to simulations that indicated O functional groups should serve as optimal adsorption sites, the reality showed that those functional groups provided only a minor enhancement in the outer-sphere electron transfer. To summarize, with reference to FIG.1A, with decreasing pH , the overpotential for ORR through the outer-sphere pathway increased significantly, leading to negligible H2O2production. Therefore, the inventors adopted a new approach to designing catalysts with high activity and selectivity for 2-electron ORR in acidic conditions. Specifically, catalysts were designed to include numerous direct adsorption sites with optimal binding energy on the surface, along with pore structures to maximize their utilization.Atty. Dkt. No.00100-0404-PCT

[0069] Electrocatalyst preparation and characterization

[0070] In view of the thermodynamical analysis and findings described above, mesoporous Co SACs were designed to maximize the active site density by exposing the originally inaccessible Co sites and to improve mass transport. The synthesis process is illustrated in FIG.2A. Zeolitic imidazolate framework (ZIF-67 / ZIF-8) particles undergo size reduction after heat treatment, as the metal Zn sites sublimate during the high-temperature process, along with the loss of some C and N. By adding a rigid coating (silica shell) to the ZIF-67 / ZIF-8 surface, these silica shells helped maintain particle shape during pyrolysis, providing structural support. In addition, shrinkage caused by the loss of Zn, C, and N resulted in an outward rather than inward reduction, pulling the internal structure toward the outer silica layer and leading to the creation of mesoporous structures (SiO2@Meso-CoNC). After pyrolysis, the silica layer was removed by etching, and the Meso-CoNC sample was subjected to air heat treatment to produce the partially oxidized and electron-deficient Meso- CoNC(O).

[0071] FIG.2B shows the transmission electron microscopy (TEM) image of Meso- CoNC(O), with many noticeable pores within the particles. To highlight the distinct advantage of the porous morphology, a non-mesoporous comparative sample, partially oxidized CoNC(O), was prepared using the same procedure but without applying any silica coating (FIG.2C). This control sample exhibited a high proportion of Co sites that were inaccessible, being confined within the particle structure. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) demonstrated that individual Co atoms were uniformly distributed throughout the carbon matrix (FIG.2D). The optimized Meso-CoNC(O) contained a high concentration of single cobalt atoms (2.1 wt%). The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) curves for both Meso- CoNC(O) and comparative CoNC(O) showed only a primary peak at 1.4 Å, indicative of Co– N coordination, indicating that the pore structures were generated without any agglomeration (FIG.2E).

[0072] To improve the 2-electron selectivity, both catalysts were subjected to air heat- treatment oxidation, resulting in comparable oxygen contents from 1 at% to 9 at% by using X-ray photoelectron spectroscopy (XPS) analysis. Fourier-transform infrared spectroscopy (FTIR) showed that the predominant oxygen species were epoxides (C-O-C) in both catalysts. The electronic structures of the Co center before and after the partial oxidation wereAtty. Dkt. No.00100-0404-PCT examined using X-ray absorption near-edge structure (XANES) analysis. The white line in the Co K-edge spectrum of Meso-CoNC(O) showed an upshift to higher binding energy and higher absorption-edge energy compared to Meso-CoNC, indicating that the electron- deficient Co sites were successfully generated.

[0073] The Meso-CoNC(O) catalyst displayed an increased Brunauer-Emmett-Teller (BET) surface area of 1410 m2g-1compared to 1260 m2g-1for comparative CoNC(O), featuring numerous mesopores across its surface (FIGS.2F-2H). This significant increase in surface area was attributed to the altered surface morphology and the presence of mesopores within the structure.

[0074] To verify the structural advantages of Meso-CoNC(O), its adsorption ability and diffusion kinetics were evaluated for dye molecules, Rhodamine B, compared to the comparative CoNC(O) using UV-Vis spectroscopy (FIG.2I). The results showed that Meso- CoNC(O) exhibited a much faster dye adsorption rate and a 1.5 times higher capacity than comparative CoNC(O). These results provide an indicator of mass transport within the catalyst samples. Consequently, catalysts were synthesized that were identical in all surface properties, such as the amount, electronic state of Co, and surface functional groups, but differed in their pore structures.

[0075] Electrocatalytic performances and experimental investigation

[0076] The ORR activity of the Meso-CoNC(O) catalyst was investigated under acidic conditions (FIGS.3A-3B). Meso-CoNC(O) exhibited a remarkable H2O2 selectivity at 92% at 0.3 V vs. RHE, outperforming that of comparative CoNC(O) (84%) and comparative CoNC (26%) (FIG.3B). In addition, Meso-CoNC(O) exhibited a superior activity e of 34 A gcatal-1at 0.5 V vs. reversible hydrogen electrode (RHE) in the 2-electron ORR pathway. This result was attributed to its electron-deficient Co sites and particularly, its mesoporous structure, which synergistically enhanced selectivity and activity in H2O2 production. As shown in FIG.3C, this performance was also better than all other reported catalysts for 2- electron ORR in acidic media. It is also noted that the porosity of Meso-CoNC(O) can be adjusted by varying the thickness of the silica coating, which is itself dependent on the coating time. Specifically, Meso-CoNC(O) treated using a 1-hour silica coating time exhibited the highest performance, indicating that mesopore structure with an average size of 3.9 nm was optimal for efficient mass transport via the 2-electron ORR pathway. TheseAtty. Dkt. No.00100-0404-PCT results are surprising as it was previously believed that increasing porosity and surface area would facilitate the 4-electron ORR pathway, leading to H2O, rather than H2O2.

[0077] The catalytic stability of Meso-CoNC(O) in acidic conditions was evaluated using chronoamperometry measurements at 0.3 V vs. RHE on the RRDE (FIG.3D). Meso- CoNC(O) showed a nearly constant current and maintained high selectivity for H2O2of ~90% over 50 h, confirming its robust catalytic durability in acidic media. The predominant degradation mechanism is demetallation. Prior simulation results suggested that the free energy barrier for demetallation at single-atom Fe sites with O2 adsorbates is negative, while at Co sites, it is positive. This indicates that Co sites are more resistant to demetallation, demonstrating that O2 and acidic media do not significantly influence Co demetallation. Additionally, Meso-CoNC(O) features already partially oxidized carbon surfaces that favor the selective 2-electron pathway, resulting in minimal further carbon oxidation and negligible loss in catalytic activity.

[0078] It was observed that Meso-CoNC(O) displayed inferior catalytic performance for H2O2 production under alkaline conditions as compared to comparative CoNC(O), since the reduced overpotential for outer-sphere electron transfer allowed not only the engineered Co sites but also various carbon sites and functional groups to participate in the ORR. This broader participation may accelerate the (2+2) mechanism, potentially decreasing H2O2selectivity. To investigate how mesoporous structures effectively prevent the further reduction of H2O2to water in the (2+2)-electron ORR mechanism in acidic media, peroxide reduction kinetics was examined. Meso-CoNC(O) revealed a lower current for peroxide reduction than comparative CoNC(O), indicating that the mesopore structure aided in repelling the generated H2O2 away from the active sites in acidic electrolyte. This action enhanced the selectivity for the 2-electron pathway.

[0079] As noted above, previous research has often overlooked the influence of the operating environment, such as electrolyte pH. The findings described in this Example indicate that in alkaline conditions, both the high surface area and porosity adversely impact the 2-electron pathway for H2O2 production under alkaline conditions, even though they are advantageous for the 4-electron ORR. However, under acidic conditions, it was found that a multitude of adsorption sites and increased surface area due to the mesoporous structure do facilitate effective mass transport. The addition of extra supporting salts (K2SO4) further enhanced the selectivity for H2O2 in acidic media. The decreased spatial thickness of theAtty. Dkt. No.00100-0404-PCT electric double layer as the electrolyte ionic strength increased prevented further reduction of H2O2intermediates.

[0080] For a quantitative analysis of the effect of the mesoporous structure, total capacitance (Cdl) was measured, largely tracking the BET area, with Meso-CoNC(O) ~238 F g-1, and comparative CoNC(O) ~175 F g-1(FIG.3E). Cdlreflected the electrochemically accessible active surface area, largely affected by the morphology. Thus, the comparison of Cdlto BET surface area ratio indicated the portion of the physical surface area participating in the electrochemical reaction. The notably higher value of Meso-CoNC(O) compared to comparative CoNC(O) demonstrates that the mesoporous structure played a crucial role in electrolyte wetting into the micropores and facilitates the utilization of active sites (FIG.3F).

[0081] Finally, to establish a clear correlation between the desired mesoporous structure and the selectivity mechanism for 2-electron ORR, the redox behavior of Fe(CN)63- / 4-, a well- known probe for outer-sphere electron transfer reactions, was analyzed (FIG.3G). Meso- CoNC(O) demonstrated ~11% lower capacity for outer-sphere reactions than CoNC(O) when correlating the redox charge with the concentration of Fe(CN)63- / 4-species. All these findings suggest that while the mesoporous structure resulted in reduced electron donation capability via the outer-sphere pathway, the large Cdl / BET area ratio was beneficial in increasing the utilization of total active sites. Such interfacial behavior in Meso-CoNC(O) is likely to aid in the selective reduction of reactant O2 molecules while inhibiting further reduction to water, whether through outer- or inner-sphere reactions.

[0082] Propylene oxidation to glycol using electrosynthesized H2O2via a cascade catalysis

[0083] A cascade catalytic process was developed, utilizing electrochemically generated H2O2to produce propylene glycol (FIG.4A, dashed box). The initial stage of this process involved an electrocatalytic reactor designed for the electrosynthesis of H2O2, which was then directly supplied to the subsequent thermochemical catalytic reactor without purification. In the second stage, propylene was selectively oxidized to propylene glycol using the acidic H2O2stream as the oxidizer in the thermochemical catalytic reactor.

[0084] H2O2electrosynthesis was first conducted in a flow cell reactor featuring a 1 mm gap, using 0.5 M H2SO4 electrolyte and the Meso-CoNC(O) catalysts. As shown in FIG. 4B, Meso-CoNC(O) achieved a high current density of 300 mA cm-2for H2O2production at a relatively low cell voltage of 1.93 Vcell, with FE ~89% to H2O2. In comparison, comparativeAtty. Dkt. No.00100-0404-PCT CoNC(O) exhibited FEs of ~80% for H2O2 production at 100 mA cm-2and ~72% at 300 mA cm-2, significantly lower than that of Meso-CoNC(O). The electrocatalytic performance of Meso-CoNC(O) in diluted electrolytes (0.05 M H2SO4) was also evaluated, used in the subsequent continuous cascade catalysis operation at pH~1. Under this condition, Meso- CoNC(O) achieved a high current density of 500 mA cm-2for H2O2 electrosynthesis at 2.45 Vcell.

[0085] Subsequently, the cascade production of propylene glycol in a custom-made fixed-bed reactor was demonstrated, in which 2 g of commercial TS-1 catalyst was positioned in the reactor column. The successful synthesis of propylene glycol in the cascade reactor system was confirmed by1H NMR. In addition, because of the relatively small amount of generated H2O2, no methanol additive was needed to enhance the solubility of propylene. Consequently, propylene glycol was the only product detected in the liquid phase with no other byproducts, although the presence of a certain amount of water and sulfuric acid was detected. Thus, costly downstream separation and purification processes are not necessary.

[0086] The overall catalytic performance was determined by calculating the electron-to- propylene glycol efficiency (ETP; see above). This efficiency was derived from the FE to H2O2 formation, selectivity in thermocataylsis, and the H2O2 utilization efficiency. Under optimized conditions, a maximum ETP of up to ~ 76% at 100 mA cm-2was achieved, while ETP slightly decreased at high applied current densities (FIG.4C). The propylene glycol production rate increased with the current density and reached a maximum of 6.4 mmol h-1(FIG.4D). To achieve a concentrated propylene glycol solution, the electrode area within the flow cell was extended from 1 to 5 cm2in a single modular unit, and a 5.1 wt% propylene glycol solution was successfully obtained. This design allows for future scalability by stacking multiple units.

[0087] The stability test on the cascade reactor system for the continuous production of propylene glycol at 300 mA cm-2was carried out over 80 h. As shown in FIG.4E, continuous production of propylene glycol was achieved with negligible degradation in selectivity and activity. Consequently, it was successfully demonstrated that propylene glycol stream can be produced, coupling with the outperformed acidic H2O2electrosynthesis in a cascade system at industrial-relevant production rates.

[0088] Techno-economic analysis based on the cascade reactor system results estimated a plant-gate levelized cost for propylene production, using Meso-CoNC(O) in the firstAtty. Dkt. No.00100-0404-PCT electrolyzer and TS-1 as the thermochemical catalyst in the second fixed-bed reactor. The cost was estimated to be half the market price, at US$1315 compared to US$2600 tonne-1on a 100 wt% basis. The electrochemical ORR-to-H2O2 using Meso-CoNC(O) required 26.8 GJ per ton of propylene glycol (7.5 GJ ton-1for electricity driven H2O2synthesis, 4.9 GJ ton-1for operating the thermochemical reactor, 13.9 GJ ton-1for separation process, and 0.5 GJ ton-1for the unreacted propylene separation for recirculation) (FIG.4F). This is in comparison to 38.6 GJ per ton of propylene glycol for a conventional process for producing propylene oxide using anthraquinone-based H2O2with subsequent hydrolysis for glycol generation (29.9 GJ ton-1for H2O2 from the anthraquinone process, 3.3 GJ ton-1for producing propylene oxide using H2O2from anthraquinone process, and 4.9 GJ ton-1for hydration and final separation, and 0.5 GJ ton-1of propylene recycling).

[0089] Conclusions

[0090] In summary, the cascade system designed for the highly selective conversion of O2-to-propylene glycol achieved a cascade efficiency of 70-76%, operating at a production rate of 6.4 mmol h-1, and attained a maximum propylene glycol concentration of 5.1 wt%. This integrated electrochemical and thermochemical approach offers a streamlined process, directly upgrading acidic H2O2 streams from electrolyzers without purification and selectively producing the valuable chemical, propylene glycol. To enhance the efficiency of this integrated system, this Example focused on generating H2O2 intermediates in the first process that were well-suited for subsequent conversion in the second process, thereby boosting productivity in acidic H2O2 electrosynthesis. These findings highlight the importance of direct adsorption sites on the catalyst surface and the optimization of cobalt site utilization to accelerate the inner-sphere ORR pathway. By adjusting the porosity of catalysts, H2O2electrosynthesis was enhanced. Notably, the Meso-CoNC(O) catalyst achieved a cell voltage of 1.93 V at 300 mA cm-2, with ~89% FE for H2O2 in an acidic medium. This innovative cascade process design marks a significant step toward the sustainable production of H2O2and other high-value chemicals, aligning with future decarbonization efforts.

[0091] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”Atty. Dkt. No.00100-0404-PCT

[0092] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

[0093] If not already included, all numeric values of parameters in the present disclosure are proceeded by the term “about” which means approximately. This encompasses those variations inherent to the measurement of the relevant parameter as understood by those of ordinary skill in the art. This also encompasses the exact value of the disclosed numeric value and values that round to the disclosed numeric value.

[0094] Unless otherwise indicated, and in recognition of the inherent nature of the techniques described herein, throughout the present disclosure, terms and phrases such as “absence,” “free,” “does not comprise,” etc. encompass, but do not require a perfect absence of the referenced entity.

[0095] Unless otherwise indicated, the term “type” as used herein refers to chemical formula such that a single type means the same chemical formula and different type means different chemical formula. Similarly, use of “more” as in “one or more types” refers to use of different types of the relevant entity.

[0096] Unless otherwise indicated, throughout the present disclosure, terms such as “comprising” and the like may be replaced with terms such as “consisting” and the like.

Claims

Atty. Dkt. No.00100-0404-PCT WHAT IS CLAIMED IS:

1. A method of making a mesoporous single metal atom catalyst, the method comprising: (a) forming a layer of silica on surfaces of a zeolitic imidazolate framework (ZIF) material to provide a silica coated ZIF material; (b) pyrolyzing the silica coated ZIF material to provide a pyrolyzed silica coated ZIF material; (c) removing the layer of silica from the pyrolyzed silica coated ZIF material to provide an etched ZIF material; and (d) oxidizing the etched ZIF material to provide a mesoporous single metal atom catalyst.

2. The method of claim 1, wherein the ZIF material comprises Co and the metal of the mesoporous single metal atom catalyst comprises Co.

3. The method of claim 1, wherein the mesoporous single metal atom catalyst is characterized by a Brunauer-Emmett-Teller (BET) surface area of at least 1300 m2 / g.

4. The method of claim 1, wherein the mesoporous single metal atom catalyst is characterized by an average mesopore diameter in a range of from 3 nm to 5 nm and a dVp / dDp value at the average mesopore diameter value of at least 0.15 cm3 / g nm.

5. The method of claim 1, wherein the mesoporous single metal atom catalyst comprises isolated metal atoms dispersed within a carbon matrix, the carbon matrix comprising carbon, nitrogen, and oxygen, wherein the carbon matrix defines a plurality of mesopores.

6. The method of claim 5, further wherein: the isolated metal atoms are present at an amount of from 1 weight% to 5 weight%; the oxygen is present at an amount of from 1 atomic% to 10 atomic%; at least some of the isolated metal atoms are coordinated to nitrogen; at least some of the oxygen is bound to carbon forming epoxide groups; and the isolated metal atoms are oxidized.Atty. Dkt. No.00100-0404-PCT 7. The method of claim 1, wherein the mesoporous single metal atom catalyst is in the form of a plurality of nanoparticles.

8. A mesoporous single metal atom catalyst comprising isolated metal atoms dispersed within a carbon matrix, the carbon matrix comprising carbon, nitrogen, and oxygen, wherein the carbon matrix defines a plurality of mesopores, and further wherein the mesoporous single metal atom catalyst is characterized by a BET surface area of at least 1300 m2 / g.

9. The mesoporous single metal atom catalyst of claim 8, wherein the metal comprises Co.

10. The mesoporous single metal atom catalyst of claim 8, characterized by an average mesopore diameter in a range of from 3 nm to 5 nm and a dVp / dDp value at the average mesopore diameter value of at least 0.15 cm3 / g nm.

11. The mesoporous single metal atom catalyst of claim 10, further wherein: the isolated metal atoms are present at an amount of from 1 weight% to 5 weight%; the oxygen is present at an amount of from 1 atomic% to 10 atomic%; at least some of the isolated metal atoms are coordinated to the nitrogen; at least some of the oxygen is bound to the carbon forming epoxide groups; and the isolated metal atoms are oxidized.

12. The method of claim 11, in the form of a plurality of nanoparticles.

13. The mesoporous single metal atom catalyst of claim 12, wherein the metal is Co.

14. An electrolyzer configured to produce H2O2, the electrolyzer comprising: (a) a cathode comprising the mesoporous single metal atom catalyst of claim 8; (b) an acidic catholyte in contact with the cathode; and (c) an anode in electrical communication with the cathode such that, upon delivery of O2to the cathode and generation of a potential difference between the cathode and the anode, the mesoporous single metal atom catalyst catalyzes electroreduction of the O2 via a 2-electron pathway to produce an output stream comprising H2O2.Atty. Dkt. No.00100-0404-PCT 15. A method of producing H2O2, the method comprising delivering O2 to the electrolyzer of claim 14 and generating the potential difference between the cathode and the anode.

16. The method of claim 15, wherein the method is characterized by a Faradic efficiency of at least 80% towards H2O2at a current density of 300 mA / cm2.

17. A cascade system configured to oxidize an olefin, the cascade system comprising: (a) an electrolyzer comprising (i) a cathode comprising a mesoporous single metal atom catalyst comprising isolated metal atoms dispersed within a carbon matrix, the carbon matrix comprising carbon, nitrogen, and oxygen, wherein the carbon matrix defines a plurality of mesopores, and further wherein the mesoporous single metal atom catalyst is characterized by a BET surface area of at least 1300 m2 / g; (ii) an acidic catholyte in contact with the cathode; and (iii) an anode in electrical communication with the cathode such that, upon delivery of O2 to the cathode and generation of a potential difference between the cathode and the anode, the mesoporous single metal atom catalyst catalyzes electroreduction of the O2 via a 2-electron pathway to produce an output stream comprising H2O2; and (b) a reactor in fluid communication with the electrolyzer, the reactor configured to oxidize an olefin to a glycol using the H2O2 of the output stream.

18. The cascade system of claim 17, wherein the output stream comprising the H2O2 is delivered directly from the electrolyzer to the reactor.

19. A method of oxidizing an olefin, the method comprising delivering O2 to the electrolyzer of the cascade system of claim 17, generating the potential difference between the cathode and the anode, and delivering an olefin to the reactor of the cascade system of claim 17.

20. The method of claim 19, wherein the olefin is propylene and the glycol is propylene glycol.

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