SELECTIVE OXIDATION OF METHANE

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made with government support under grant DE- AR0000952 awarded by the Department of Energy. The government has certain rights in the invention.

BACKGROUND

Selective conversion of methane to liquid hydrocarbons represents a promising approach toward efficient utilization of natural gas. Nisbet and Bousquet, 2014. The present industrial route for such conversions relies on a two-step process by first reforming methane to generate synthesis gas (CO and H2) at elevated temperatures (greater than 500 °C), and then reacting CO with H2 to form methanol or other liquid products. Choudhary and Choudhary, 2008; Tang et al., 2014. This process, however, is energy -intensive and economically nonviable for distributed sources such as flare gas. Buzcu-Guven and Harriss, 2012. More robust technologies toward direct conversion of methane into condensed energy carriers are needed to facilitate transportation and storage. Julian-Duran et al., 2014.

SUMMARY

In some aspects, the presently disclosed subject matter provides a catalyst composite comprising a copper dimer having two copper atoms bridged by an oxygen atom, wherein each copper atom is coordinated to two nitrogen atoms of a carbon nitride substrate.

In certain aspects, the carbon nitride substrate comprises one or more of graphitic carbon nitride, a-carbon nitride, -carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof. In particular aspects, the carbon nitride substrate comprises graphitic carbon nitride. In more particular aspects, the graphitic carbon nitride has a form selected from a film, a sphere, a nanotube, a nanorod, a nanosized powder, and combinations thereof.

In certain aspects, the catalyst composite comprises a loading of Cu of about 0.35 wt% of the total weight of the composite. In particular aspects, the catalyst composite has an N Is X-ray photoelectron spectroscopy (XPS) spectrum exhibiting a binding energy ranging from about 397 eV to about 408 eV comprising four peaks centered at about 398.6 eV, about 399.4 eV, about 401.0 eV, and about 404.5 eV. In particular aspects, the catalyst composite has a Cu 2p X-ray photoelectron spectroscopy (XPS) spectrum exhibiting peaks at 932.5 eV and 952.3 eV. In particular aspects, the catalyst composite has an Absorption Near Edge Spectroscopy (XANES) Cu K-edge spectrum exhibiting a pre-edge transition at about 8,984 eV.

In certain aspects, the two copper atoms have an oxidation state between +1 and +2. In particular aspects, the two copper atoms have an oxidation state of about +1.63 and about +1.72.

In certain aspects, the catalyst composite has an average distance between the two copper atoms is between about 0.18 nm to about 0.38 nm. In particular aspects, the average distance between the two copper atoms is about 0.28 nm ± 0.02 nm.

In other aspects, the presently disclosed subject matter provides a method for preparing the catalyst composite of claim 1, the method comprising:

(a) providing a copper-dimer organometallic precursor;

(b) mixing the copper dimer organometallic precursor with a carbon nitride substrate to form a mixture; and

(c) heating the mixture to form a catalyst composite comprising a copper dimer having two copper atoms bridged by an oxygen atom, wherein each copper atom is coordinated to two nitrogen atoms of a carbon nitride substrate.

In certain aspects, the copper-dimer organometallic precursor comprises an (oxalato)(bipyridine)copper(II) complex (Cu2(bpy)2(μ-ox)]C1). In particular aspects, the Cu2(bpy)2(μ-ox)]C1 is prepared by reacting copper chloride (CuC1) with 2, 2’, -bipyridine and oxalic acid.

In certain aspects, the carbon nitride substrate comprises one or more of graphitic carbon nitride, a-carbon nitride, -carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof. In particular aspects, the carbon nitride substrate comprises graphitic carbon nitride. In more particular aspects, the graphitic carbon nitride is prepared by calcination of urea. In certain aspects, the method for preparing the catalyst composite comprises heating the mixture from about 50 °C to about 250 °C for about 10 hours at a rate of about 2 °C min' ¹ .

In other aspects, the presently disclosed subject matter provides a method for oxidizing methane, the method comprising contacting methane with the presently disclosed catalyst composite in the presence of an oxidizing agent.

In certain aspects, the oxidizing agent comprises H2O ₂ . In particular aspects, the method comprises thermocatalytic oxidation of CHr. In more particular aspects, the methane, catalyst composite, and oxidizing agent are maintained at a predetermined pressure and temperature for a period of time.

In certain aspects, the oxidizing agent comprises O ₂ . In particular aspects, the method comprises photocatalytic oxidation of CH4. In more particular aspects, the methane, catalyst composite, and oxidizing agent are irradiated with visible light at a predetermined pressure and temperature for a period of time.

In certain aspects, the method of oxidizing CH4 further comprises forming one or more methyl oxygenates. In particular aspects, the one or more methyl oxygenates are selected from methanol (CH3OH) and methyl hydroperoxide (CH3OOH). In more particular aspects, the method further comprises reducing the methyl hydroperoxide (CH3OOH) to methanol (CH3OH).

Certain aspects of the presently disclosed subject matter having been stated hereinabove, which are addressed in whole or in part by the presently disclosed subject matter, other aspects will become evident as the description proceeds when taken in connection with the accompanying Examples and Figures as best described herein below.

BRIEF DESCRIPTION OF THE FIGURES

Having thus described the presently disclosed subject matter in general terms, reference will now be made to the accompanying Figures, which are not necessarily drawn to scale, and wherein:

FIG. la, FIG. lb, FIG. 1c, FIG. Id, FIG. le, FIG. If, FIG. 1g, and FIG Ih illustrate the synthesis of the presently disclosed Cu2@C ₃ N ₄ catalysts. (FIG. la) Scheme of a representative synthetic route. (FIG. lb, FIG. 1c) Characterization of the Cu-dimer precursor, [Cu2(bpy)2(μ-ox)]C1 complex using FTIR (FIG. lb) and UV-vis DRS. (FIG. Id) Comparison of FTIR spectra for CU2@C3N4 and g-C ₃ N ₄ . (FIG. le, FIG. If, FIG. 1g) Representative HAADF-STEM images of Cu2@C ₃ N ₄ , with the insets showing line-scanning intensity profiles of Cu dimers. (FIG. Ih) Statistical distribution of the Cu-Cu distance in the Cu dimers derived from the STEM images;

FIG. 2a, FIG. 2b, FIG. 2c, FIG. 2d, FIG. 2e, and FIG. 2f show the characterization of the presently disclosed CU2@C3N4 catalysts. (FIG. 2a) XPS spectrum at the N Is edge and the corresponding deconvolution. (FIG. 2b) XANES spectra and (FIG. 2c) k ² -weighted EXAFS spectra at the Cu K edge, with Cu foil, C112O. CuO and Cu-TPP (one Cu coordinated with for N atoms) as the reference. (FIG. 2d) Fitting of the EXAFS spectrum with consideration of both monomeric and dimeric Cu sites. (FIG. 2e) The simulated structure model of dicopper-oxo center. (FIG. 2f) Geometric parameters of the dicopper-oxo center determined for CU2@C3N4;

FIG. 3a, FIG. 3b, FIG. 3c, FIG. 3d, and FIG. 3e show thermocatalytic oxidation of CH4 with H2O ₂ . (FIG. 3a) Yields and productivity of methyl oxygenates at different reaction temperatures. (FIG. 3b) Comparisons of product yields and productivity over different catalysts. (FIG. 3c) Correlation between productivity of methyl oxygenates and gain factor for different catalysts. (FIG. 3d) Simulated pathways for the reaction between CH4 with H2O ₂ on the CU2@C3N4 catalysts, with the middle inset illustrating the electron distribution of the CH4 molecule being activated on the bridging oxygen site. Energy barriers also are given for the associated molecular transformations. (FIG. 3e) The DFT calculated free energy diagram for the Cu2@C ₃ N ₄ -catalyzed partial oxidation of CH4 with H2O ₂ . Three stages consisting of H2O ₂ activation, CH4 activation and methyl oxygenates formation are distinguished with different colors. The error bars presented in (FIG. 3a, FIG. 3b, and FIG. 3c) indicate the statistical distribution derived from three independent measurements;

FIG. 4a, FIG. 4b, FIG. 4c, FIG. 4d, FIG. 4e, and FIG. 4f show the photocatalytic oxidation of CH4 with O ₂ . (FIG. 4a) Yields and productivity of methyl oxygenates as a function of reaction time at 0.1 MPa CH4 and 0.1 MPa O ₂ . (FIG. 4b) CH4 conversions and productivity of methyl oxygenates at different CHr and O ₂ partial pressures. (FIG. 4c) EPR spectra recorded for the various control experiments using DMPO as the radical trapping agent. (FIG. 4d, FIG. 4e) In situ irradiation XPS spectra collected at the O Is (FIG. 4d) and N Is (e) edges. (FIG. 41) Schematic illustration of the photocatalytic oxidation of CH4 with O ₂ catalyzed by CU2@C ₃ N ₄ . The values “-1.45 and 1.31 eV” label the estimated position of dicopper-oxo states in the band structure of g-C3N4, as determined by performing Tauc plot analysis on the UV-vis DRS and UPS spectra of CU2@C3N4. The error bars shown in (FIG. 4a, FIG. 4b) indicate the statistical distribution derived from three independent measurements;

FIG. 5a and FIG. 5b show the thermogravimetric analysis (TGA) profiles showing the weight loss and the corresponding first derivative values of the dimeric copper complex (FIG. 5a) and the pristine CU2@C3N4 with ligands (FIG. 5b) when burned in air;

FIG. 6 shows the XRD patterns of CU2@C3N4 and g-C3N4 ;

FIG. 7 shows an XPS spectrum of Cu 2p edge for CU2@C3N4;

FIG. 8 shows the Cu K-edge XANES spectra of CU2@C3N4, Cu foil, C112O, CuO and copper tetraphenylporphyrin (Cu-TPP). Inset: the molecular structure of Cu-TPP which involves one Cu coordinated to four N via Cu-N bonding;

FIG. 9a and FIG. 9b show alternative Cu-dimer configurations simulated by using DFT and comparison of the corresponding EXAFS fitting to experimental spectra;

FIG. 10 is the k-space fitting analysis of EXAFS spectrum for Cu2@C ₃ N4 with consideration of both monomeric and dimeric Cu sites. This fitting analysis corresponds to FIG. 2d. The corresponding fitting parameters are summarized in Table 1;

FIG. 1 la and FIG. 1 lb show the Bader charge analysis for the oxidation state of Cu in CU2@C ₃ N4. The error bars in FIG. 11b indicate the statistical distribution of the computed Cu charges. Fitting the Bader charge of Cu derived from DFT calculations into a calibration curve established based on the references of metallic Cu, CU2O and CuO gives an oxidation state of +1.67, which is consistent with experimental results based on XPS and XANES;

FIG. 12 is a representative NMR spectrum collected for the methane oxidation products using CU2@C3N4 catalysts and H2O ₂ as the oxidizer;

FIG. 13 is a calibration curve for the analysis of CH3OOH using NMR. The error bars indicate the statistical distribution derived from three independent measurements; FIG. 14 is a calibration curve for the analysis of CH3OH using NMR. The error bars indicate the statistical distribution derived from three independent measurements;

FIG. 15 is a calibration curve for the GC analysis of CO ₂ . The error bars indicate the statistical distribution derived from three independent measurements;

FIG. 16 shows the room temperature reduction of CH3OOH with NaBHr to form CH3OH;

FIG. 17a and FIG. 17b show the CH4 conversion and product selectivity of the thermocatalytic oxidation of methane using H2O ₂ and Cu2@C3Nr. (FIG. 13a) Dependence on reaction temperature and (FIG. 13b) time (at 50 °C). The results indicate that evaluated temperature and prolongated reaction time lead to the overoxidation of CH4 to CO ₂ , which is the thermodynamically most stable product. The error bars indicate the statistical distribution derived from three independent measurements;

FIG. 18 show the cycling test of Cu2@C ₃ N ₄ for thermocatalytic oxidation of CH ₄ with H2O ₂ . The error bars indicate the statistical distribution derived from three independent measurements;

FIG. 19a, FIG. 19b, FIG. 19c, and FIG. 19d show the characterization of the spent Cu2@C3Nr catalyst after the 6-h durability test. (FIG. 19a) Representative HAADF-STEM images with Cu dimers highlighted using red circles. (FIG. 19b) k ² - weighted EXAFS spectra at the Cu K edge, with Cu foil, C112O, CuO and Cu-TPP (one Cu coordinated with for N atoms) being used as the references. (FIG. 19c, FIG. 19d) Fitting of the EXAFS spectrum with consideration of both monomeric and dimeric Cu sites;

FIG. 20 is an NMR spectrum collected for the control experiment using bare g-C3N4. No oxidation product was detected;

FIG. 21a, FIG. 21b, FIG. 21c, FIG. 2 Id, and FIG. 21 e. show (FIG. 21a, FIG. 21b) XANES and (FIG. 21c) EXAFS spectra collected at the Cu K edge for CUI@C ₃ N4. CU foil, Cu2, CuO and Cu-TPP (1 Cu coordinated with 4 N atoms) also were shown as references. (FIG. 2 Id) The optimized structure of CUI@C ₃ N ₄ based on DFT calculations. (FIG. 21 e) Fitting parameters for the EXAFS spectrum of CUI@C ₃ N ₄ ; FIG. 22 is the calibration curve of H2O ₂ quantified by the titration of Ce(SO4)2. The error bars indicate the statistical distribution derived from three independent experimental measurements;

FIG. 23 shows the comparisons of H2O ₂ consumption and gain factors (denoted as mol of CH3OH and CH3OOH divided by total mol of H2O ₂ consumed) over different catalysts. The error bars indicate the statistical distribution derived from three independent experimental measurements;

FIG. 24 is the free energy diagram for the second reaction pathway of methane partial oxidation to CH3OH on fresh Cu2@C ₃ N ₄ catalysts;

FIG. 25a and FIG. 25b are the EPR spectra of the radicals *OOH in methanol (FIG. 21a) and »OH in H2O (FIG. 21b) at different conditions with DMPO as the radical trapping agent, showing that the presence of Cu2@C N4 enhances the cleavage of H2O ₂ to »OOH and *OH;

FIG. 26 is the comparison of the barriers for H2O ₂ cleavage to *OOH and •OH catalyzed by different catalysts;

FIG. 27 shows the in situ EPR characterization of thermal catalytic CH4 selective oxidation by H2O ₂ on CU2@C3N4 with DMPO as radical trapping agent;

FIG. 28a, FIG. 28b, and FIG. 28c show (FIG. 28a, FIG. 28b) Experimental set-up of photocatalytic CH4 oxidation by O ₂ . (FIG. 28c) Schematic illustration of the process for photocatalytic CH4 oxidation by O ₂ ;

FIG. 29 shows the product selectivity of photocatalytic CH4 oxidation with O ₂ over CU2@C3N4 as a function of reaction time. The error bars indicate the statistical distribution derived from three independent experimental measurements;

FIG. 30 shows the product selectivity of photocatalytic CHr oxidation by O ₂ over CU2@C ₃ N ₄ as function of CH4/O ₂ ratios. The error bars indicate the statistical distribution derived from three independent experimental measurements;

FIG. 31 shows the in situ irradiation XPS characterizations of Cu 2p edge on Cu2@C3Nr under dark and visual light;

FIG. 32a and FIG. 32b show the (FIG. 32a) UV-vis DRS results of C ₃ N ₄ and hydrated CU2@C3N4 and (FIG. 32b) corresponding Tauc plots of C.3N4 and hydrated Cu2@C3Nr; and

FIG. 33 shows the UPS results of C ₃ N ₄ and hydrated Cu2@C ₃ N ₄ . DETAILED DESCRIPTION

The presently disclosed subject matter now will be described more fully hereinafter with reference to the accompanying Figures, in which some, but not all embodiments of the inventions are shown. Like numbers refer to like elements throughout. The presently disclosed subject matter may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Indeed, many modifications and other embodiments of the presently disclosed subject matter set forth herein will come to mind to one skilled in the art to which the presently disclosed subject matter pertains having the benefit of the teachings presented in the foregoing descriptions and the associated Figures. Therefore, it is to be understood that the presently disclosed subject matter is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.

I. OXO DICOPPER ANCHORED ON CARBON NITRIDE FOR SELECTIVE OXIDATION OF METHANE

In some embodiments, the presently disclosed subject matter provides dimeric copper centers supported on graphitic carbon nitride (denoted herein as CU2@C ₃ N4) as advanced catalysts for the partial oxidation of CH4. The copperdimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photo-catalytic reaction conditions, with hydrogen peroxide (H2O ₂ ) and oxygen (O ₂ ) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH4 with O ₂ achieves greater than 10% conversion and greater than 98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol gCu' ¹ h' ¹ .

A comprehensive comparison to the literature results under similar reaction conditions indicate that the presently disclosed results represent the highest activity for partial oxidation of methane, with improvement factors of at least greater than 10. The presently disclosed copper-dimer catalysts were first evaluated for thermal oxidation of methane using H2O ₂ as the oxidizer and then further applied for photocatalytic oxidation of methane with O ₂ .

Accordingly, in some embodiments, the presently disclosed subject matter provides a catalyst composite comprising a copper dimer having two copper atoms bridged by an oxygen atom, wherein each copper atom is coordinated to two nitrogen atoms of a carbon nitride substrate.

In certain embodiments, the carbon nitride substrate comprises one or more of graphitic carbon nitride, a-carbon nitride, 0-carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof. In particular embodiments, the carbon nitride substrate comprises graphitic carbon nitride. In more particular embodiments, the graphitic carbon nitride has a form selected from a film, a sphere, a nanotube, a nanorod, a nanosized powder, and combinations thereof.

In certain embodiments, the catalyst composite comprises a loading of Cu of about 0.35 wt% of the total weight of the composite, including about 0.25 wt% to about 0.5 wt%, including about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40. 41, 42, 43, 44, 45, 46, 47, 48, 49, and 50 wt%.

In particular embodiments, the catalyst composite has an N Is X-ray photoelectron spectroscopy (XPS) spectrum exhibiting a binding energy ranging from about 397 eV to about 408 eV comprising four peaks centered at about 398.6 eV, about 399.4 eV, about 401.0 eV, and about 404.5 eV. In particular embodiments, the catalyst composite has a Cu 2p X-ray photoelectron spectroscopy (XPS) spectrum exhibiting peaks at 932.5 eV and 952.3 eV. In particular embodiments, the catalyst composite has an Absorption Near Edge Spectroscopy (XANES) Cu K-edge spectrum exhibiting a pre-edge transition at about 8,984 eV.

In certain embodiments, the two copper atoms have an oxidation state between +1 and +2, including about +1, +1.1, +1.2, +1.3, +1.4, +1.5, +1.6, +1.7, +1.8, +1.9, and +2. In particular embodiments, the two copper atoms have an oxidation state of about +1.63 and about +1.72.

In certain embodiments, the catalyst composite has an average distance between the two copper atoms is between about 0. 18 nm to about 0.38 nm, including about 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, and 0.38 nm. In particular embodiments, the average distance between the two copper atoms is about 0.28 nm ± 0.02 nm.

In other embodiments, the presently disclosed subject matter provides a method for preparing the catalyst composite of claim 1, the method comprising:

(a) providing a copper-dimer organometallic precursor;

(b) mixing the copper dimer organometallic precursor with a carbon nitride substrate to form a mixture; and (c) heating the mixture to form a catalyst composite comprising a copper dimer having two copper atoms bridged by an oxygen atom, wherein each copper atom is coordinated to two nitrogen atoms of a carbon nitride substrate.

In certain embodiments, the copper-dimer organometallic precursor comprises an (oxalato)(bipyridine)copper(II) complex (Cu2(bpy)2(μ-ox)]C12). In particular embodiments, the Cu2(bpy)2(μ-ox)]C1 is prepared by reacting copper chloride (CuC1 ₂ ) with 2,2’, -bipyridine and oxalic acid.

In certain embodiments, the carbon nitride substrate comprises one or more of graphitic carbon nitride, a-carbon nitride, β-carbon nitride, cubic carbon nitride, pseudocubic carbon nitride, and combinations thereof. In particular embodiments, the carbon nitride substrate comprises graphitic carbon nitride. In more particular embodiments, the graphitic carbon nitride is prepared by calcination of urea.

In certain embodiments, the method for preparing the catalyst composite comprises heating the mixture from about 50 °C to about 250 °C for about 10 hours at a rate of about 2 °C min ¹ .

In other embodiments, the presently disclosed subject matter provides a method for oxidizing methane, the method comprising contacting methane with the presently disclosed catalyst composite in the presence of an oxidizing agent.

In certain embodiments, the oxidizing agent comprises H2O ₂ . In particular embodiments, the method comprises thermocatalytic oxidation of CH4. In more particular embodiments, the methane, catalyst composite, and oxidizing agent are maintained at a predetermined pressure and temperature for a period of time.

In certain embodiments, the oxidizing agent comprises O ₂ . In particular embodiments, the method comprises photocatalytic oxidation of CHr. In more particular embodiments, the methane, catalyst composite, and oxidizing agent are irradiated with visible light at a predetermined pressure and temperature for a period of time.

In certain embodiments, the predetermined pressure for either the thermocatalytic oxidation or photocatalytic oxidation of CH4 is about 3 MPa, including about 1 MPa to about 5 MPa, including 1, 2, 3, 4, and 5 MPa. Likewise, in certain embodiments, the predetermined temperature is about 50 °C, including about 35 °C to about 65 °C, including 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, and 65 °C. Further, in some embodiments, the period of time is between about 30 min to about 120 min, including about 30, 45, 60, 75, 90, 105, and 120 min.

In certain embodiments, the method of oxidizing CH4 further comprises forming one or more methyl oxygenates. In particular embodiments, the one or more methyl oxygenates are selected from methanol (CH3OH) and methyl hydroperoxide (CH3OOH). In more particular embodiments, the method further comprises reducing the methyl hydroperoxide (CH3OOH) to methanol (CH3OH).

Following long-standing patent law convention, the terms “a,” “an,” and “the” refer to “one or more” when used in this application, including the claims. Thus, for example, reference to “a subject” includes a plurality of subjects, unless the context clearly is to the contrary (e.g., a plurality of subjects), and so forth.

Throughout this specification and the claims, the terms “comprise,” “comprises,” and “comprising” are used in a non-exclusive sense, except where the context requires otherwise. Likewise, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing amounts, sizes, dimensions, proportions, shapes, formulations, parameters, percentages, quantities, characteristics, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term “about” even though the term “about” may not expressly appear with the value, amount or range. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are not and need not be exact, but may be approximate and/or larger or smaller as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art depending on the desired properties sought to be obtained by the presently disclosed subject matter. For example, the term “about,” when referring to a value can be meant to encompass variations of, in some embodiments, ± 100% in some embodiments ± 50%, in some embodiments ± 20%, in some embodiments ± 10%, in some embodiments ± 5%, in some embodiments ±1%, in some embodiments ± 0.5%, and in some embodiments ± 0.1% from the specified amount, as such variations are appropriate to perform the disclosed methods or employ the disclosed compositions.

Further, the term “about” when used in connection with one or more numbers or numerical ranges, should be understood to refer to all such numbers, including all numbers in a range and modifies that range by extending the boundaries above and below the numerical values set forth. The recitation of numerical ranges by endpoints includes all numbers, e.g., whole integers, including fractions thereof, subsumed within that range (for example, the recitation of 1 to 5 includes 1, 2, 3, 4, and 5, as well as fractions thereof, e g., 1.5, 2.25, 3.75, 4.1, and the like) and any range within that range.

EXAMPLES

The following Examples have been included to provide guidance to one of ordinary skill in the art for practicing representative embodiments of the presently disclosed subject mater. In light of the present disclosure and the general level of skill in the art, those of skill can appreciate that the following Examples are intended to be exemplary only and that numerous changes, modifications, and alterations can be employed without departing from the scope of the presently disclosed subject mater. The synthetic descriptions and specific examples that follow are only intended for the purposes of illustration, and are not to be construed as limiting in any manner to make compounds of the disclosure by other methods.

EXAMPLE 1

Oxo Dicopper Anchored on Carbon Nitride for Selective Oxidation of Methane

1.1 Overview

Provided herein are dimeric copper centers supported on graphitic carbon nitride (denoted herein as CU2@C3N4) as advanced catalysts for the partial oxidation of CH4. These catalysts are synthesized by immobilization of a copper-dimer organometallic complex on C ₃ N ₄ , with dicopper-oxo centers forming via mild calcinations. The derived CU2@C3N4 catalysts are characterized by combining scanning transmission electron microscopy (STEM), X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), with the derived atomic structures of copper centers further confirmed by computational modeling based on density functional theory (DFT) calculations. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photo-catalytic reaction conditions, with hydrogen peroxide (H2O ₂ ) and oxygen (O ₂ ) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH4 with O ₂ achieves greater than 10% conversion and greater than 98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol gCu' ¹ h ⁴ . Mechanistic studies reveal that the high reactivity of CU2@C3N4 for partial oxidation of methane can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C ₃ N ₄ substrate, which not only facilitate the heterolytic scission of the C-H bond, but also promote H2O ₂ and O ₂ activation in thermo- and photo-catalysis, respectively.

1.2 Background

Selective conversion of methane (CH4) into value-added chemicals represents a significant challenge for the efficient utilization of rising hydrocarbon sources. Direct, partial oxidation of methane to methyl oxygenates has received intensive attention in the recent years. Farrell and Linic, 2016; Hammond et al., 2012. Early studies used transition copper exchanged zeolites to catalyze the reaction between CH4 and O ₂ , and employed a two-step chemical looping process to subsequentially activate O ₂ and desorb the products. Woertink et al., 2009; Pappas et al., 2018; Groothaert et al., 2005. Despite the achievement of high selectivities, these reactions suffer from low CH4 conversions (typically less than 0.03%) and reaction rates (less than 30 pmol gcata ¹ h ¹ ). Narsimhan et al., 2016; Dinh et al., 2019; Koishybay and Shantz, 2020. Later on, partial oxidation of methane in a single step was demonstrated by using non-O2 oxidizers, such as oleum, Palkovits et al., 2009), selenic acid, Jones et al., 2004) and H2O ₂ , Shan et al., 2017; Sushkevich et al., 2017; Grundner et al., 2015, but the cost associated with these oxidizing agents restricts their practical implementations. Agarwal et al., 2017. More recent efforts have thus turned to in- s ll generation of H2O ₂ from O ₂ by using selective oxygen reduction catalysts, such as Au-Pd containing zeolites. Jin et al., 2020.

Alternatively, photo-excitation using visible light is proposed to be advantageous with near-room temperature activation of CH4, thereby mitigating the concern of over oxidation to form CO ₂ upon heating. Song et al., 2019a; Song et al., 2019b. The reported photocatalytic oxidation of methane, however, is still limited by relatively low methane conversions (less than 1%) and productivities (between about 0.001 to about 150 mmol gcata h ^-1 ), Song et al., 2019b, because the commonly used photocatalysts have quite large bandgaps (e.g., approximately 3.2 eV for TiO2, Song et al., 2020, and approximately 3.4 eV for ZnO, Song et al., 2019a) and may only activate methane via the Fenton or homolytic mechanisms that have relatively sluggish kinetics. Szecsenyi et al., 2018a.

In this respect, graphitic carbon nitride (g-C3N4) represents a promising photocatalytic substrate with a modest band gap in the range of 2.7-2.9 eV. Wen et al., 2017; Xu and Gao, 2012; Su et al., 2010. Its abundant nitrogen sites have been shown to be capable of anchor atomically dispersed transition metal sites. Shi et al., 2020. It thus becomes interesting to investigate the potential coordination of active Cu sites on g-C3N4 and examine their synergies in the partial oxidation of methane. 1.3 Scope

The presently disclosed subject matter demonstrates Cu2@C3N4 as highly efficient catalysts for the partial oxidation of methane. The dimeric copper catalysts were synthesized by supporting an (oxalato)(bipyridine)copper(II) complex, [Cu2(bpy)2(μ -ox)]C12, on g-C3N4 and then applying a mild thermal treatment in air (FIG. la). The derived catalysts contained dicopper-oxo centers anchoring on g- C ₃ N ₄ via four Cu-N bonds (two for each copper atom), as characterized by using STEM, XPS and XAS, and also confirmed with atomistic simulations. The obtained copper-dimer catalysts were first evaluated for thermal oxidation of methane using H2O ₂ as the oxidizer, and then further applied for photocatalytic oxidation of methane with O ₂ . Mechanisms governing the observed catalytic enhancements toward selective oxidation of methane were interpreted via combining computational simulation of the reaction pathways, spin-trapping EPR analysis of possible radical intermediates and in situ XPS measurements under light irradiation. 1.4 Results and Discussion

1.4. 1 Synthesis and Characterization of CU2@CIN4

The copper-dimer precursor [Cu2(bpy)2(μ-ox)]C1 ₂ was first prepared by a complexation reaction of copper chloride (CuCh). 2,2,-bipyridine and oxalic acid. Reinoso et al., 2003. The g-CxXU substrate was grown by calcination of urea at 550 °C. Martin et al., 2014. CU2@C3N4 catalysts were synthesized by self-assembly of the dimeric copper complex on g-ChN-i. Zhao et al., 2018, and then treating the mixture in air at 250 °C to immobilize the copper species (FIG. la). The loading of Cu was determined to be 0.35 wt% by using inductively coupled plasma mass spectrometry (ICP-MS).

The complexation of pyridine, Cu ²⁺ and oxalate (C2O4 ² ) to form an organometallic compound was confirmed by using Fourier transform infrared spectroscopy (FTIR). The hydroxyl (O-H) and carbonyl (C=O) stretching features around 3,450 cm' ¹ and 1,670 cm' ¹ , respectively, which are associated with oxalic acid disappeared after the reaction. This disappearance was accompanied with the blue shift of the characteristic band (attributed to the asymmetric stretching of the pyridyl ring) of 2,2’-pyridine at ca. 1,580 cm' ¹ to ca. 1,650 cm' ¹ , a consequence of its chelation with Cu ²⁺ (FIG. lb). Gerasimova and Katsyuba, 2013 Correspondingly, the d-d transition of Cu ²⁺ at about 650 nm to about 700 nm had a blue shift of 48 nm in the ultraviolet-visible diffuse reflectance spectroscopy (UV- vis DRS) patterns (FIG. 1c). The FTIR spectra of g-C3N4 and CU2@C3N4 exhibited the stretching vibration modes characteristic of the -NH group around 3,180 cm' ¹ , C- N heterocycles in the wavelength range of 1,100-1,650 cm' ¹ and the breathing mode of tri-.s-tri azine units at 810 cm' ¹ (FIG. Id). Compared to [Cu2(bpy)2(μ-ox)]C12 CU2@C3N4 presented no infrared features associated with the dimeric copper complex, indicating the complete removal of organic ligands during the immobilization process. This observation was further confirmed with thermogravimetric analysis (TGA) (FIG. 5). The [Cu2(bpy)2(μ-ox)]C12/g-C ₃ N ₄ mixture lost approximately 6% of its initial weight upon annealing in air at up to 250 °C, which is close to the expectation estimated based on the ligand content (approximately 80 wt%) of |Cu2bpy)tip-ox)C1 ₂ and its ratio relative to the carbon nitride substrate (approximately 8%) used in the synthesis. X-ray diffraction (XRD) patterns collected for the CU2@C3N4 catalysts only show the (001) and (002) peaks associated with g-C ₃ N ₄ , with the absence of copper metal or oxide features indicating the highly dispersed nature of copper species (FIG. 6).

Atomic structures of the dimeric copper moieties were resolved by using aberration correction high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging (FIG. le, FIG. If. and FIG. Ig). The collected STEM images exhibit a large number of adjacent, paired bright dots (labeled with red circles, less than 0.35 nm in size for each dot) distributed on a substrate of lower contrasts. These small bright dots can be attributed to atomically dispersed Cu considering their much higher Z contrast (M = 65 for Cu) than C ₃ N ₄ (M = 12 or 14). Line-profile scanning for approximately 100 pairs of such bright dots give an average distance of 2.8 (± 0.2 A) (FIG. Ih). This average distance is much shorter than the value (5.2 A) for the two copper atoms within [Cu2(bpy)2(p- ox)]Ch, again confirming the reconstruction and condensation of the copper-dimer moieties as a result of the removal of organic ligands in the synthesis.

The chemical nature of the Cu dimers in CmV.C3N 1 was probed by using X- ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The N Is XPS spectrum exhibits a broad feature with the binding energy ranging from 397 eV to 408 eV (FIG. 2a). This feature can be deconvolved into four peaks centered at 398.6 eV, 399.4 eV, 401.0 eV, and 404.5 eV, which can be assigned to pyridinic (C-N=C), tertiary (-N<), pyrrolic (-NH) nitrogen and π-π* transition of C=N or uncondensed terminal amine groups in g-C3N4, respectively. Mane et al., 2017; Chen et al., 2017. The Cu 2p spectrum shows two peaks at 932.5 eV and 952.3 eV, which are characteristic of Cu(I) or Cu° (FIG. 7). Fox et al., 2008.

The XPS analysis, however, as well as the corresponding Auger electron spectrum (AES), was unable to explicitly determine the oxidation state of Cu due to the reduced signal-to-noise ratio associated with the low copper content in the catalysts. The copper oxidation state in Cu2@:C3N4 was better resolved by using X- ray Absorption Near Edge Spectroscopy (XANES) (FIG. 2b). The Cu K-edge spectrum exhibits a pre-edge transition at 8,984 eV, which falls between the peaks associated with CU2O (8,983 eV) and CuO (8,986 eV). This indicates an intermediate oxidation state between +1 and +2 for Cu in CU2@C3N4. Noticeably, these results do not support a mixture of Cu(I) and Cu(II), as the 1 s→3d transition at 8,977 eV, a feature characteristic of Cu ²⁺ (as shown for the references CuO and copper tetraphenylporphyrin (Cu-TPP) in FIG. 8), Fox et al., 2008, is absent in the spectrum of Cu2@C ₃ N4. The partial oxidation state (between +1 and +2) of Cu within the copper dimers supported on g-C ₃ N4 can be viewed as a result of the semiconducting nature of the substrate. This characteristic is distinguished from the extensively studied copper-exchanged zeolites, in which the dicopper-oxo centers ([Cu-O-Cu] ²⁺ ) anchor on the Al sites with localized negative charges and have an oxidation state of +2 for both Cu atoms. Groothaert et al., 2005; Xie et al., 2021; Tsai et al., 2014.

The atomic structure of the Cu dimers was resolved by combining extended X-ray absorption fine structure (EXAFS) analysis and atomistic modeling based on DFT calculations. FIG. 2c compares the k ² -weighted Cu K edge EXAFS spectra for CU2@C ₃ N4, CU foil, C112O, CuO and Cu-TPP (with single-atom Cu ²⁺ coordinating to four pyrrolic N, FIG. 8). The Cu2@C ₃ N4 catalyst exhibits first-shell scattering at 1.62 A in R space (prior to phase correction), which is proximate to the values, 1.59 and 1.55 A, found for Cu-TPP and CuO, respectively. This characteristic is distinct from that for C112O and Cu, the first-shell scattering of which locates at 1.50 and 2.30 A, respectively. From these observations, the primary scattering pair at 1.62 A in the R-space spectrum of Cu2@C ₃ N4 was tentatively assigned to be Cu-N or Cu-0 bonding. To fit the EXAFS spectrum, a total of 9 possible Cu-dimer configurations was postulated, based on which DFT calculations were performed to relax the structures to determine bonding distances and angles (FIG. 9 and Table 1). Considering the presence of minor Cu monomers observed in the STEM images (FIG. le, FIG. If, and FIG. 1g), a linear combination of Cu monomers and dimers was applied to fit the EXAFS spectrum. Various possible Cu-dimer configurations have been considered, with the corresponding EXAFS spectra compared to the experimental results to identify the best fit (FIG. 2d, FIG. 2e; also see FIG. 10 and FIG. 11). It is estimated that 72.4% of the Cu atoms are in the dimeric configuration, close to the value (approximately 70%) derived from statistical analysis of the STEM images. The determined copper-dimer structure comprises two Cu atoms bridged by an O atom, with the Cu-0 bonds having lengths of 1.76 A and 1.79 A and an included angle (zCu-O-Cu) of 99.6° (FIG. 2f, Table 1). Each Cu atom is coordinated to two N atoms on the C ₃ Nr framework, with the bonding distance varying from 1.90 to 1.99 A and the bonding angle (ZN-Cu-N) being 82° for Cuα and 110° for Cup (FIG. 2f). Noticeably, the identified configuration best fitted to the EXAFS spectrum also has the lowest (most negative) formation energy among the various configurations, in line with the expectation for stable atomic structures in the real catalysts (Table 2). The combined EXAFS analysis and DFT calculations resolved the Cu-Cu distance in the Cu dimers to be approximately 2.71 A (Table 1), which is in agreement with the average Cu-Cu distance measured from the STEM images (FIG. th). It is noted that this value is much smaller than the Cu-Cu distance (4.10 A) associated with the dicopper-oxo center ([Cu-O-Cu] ²⁺ ) in Cu-ZSM-5. Tsai et al., 2014. Furthermore, Bader charge analysis based on DFT calculation show that the Cu atoms in the Cu dimers have oxidation states of +1.63 and +1.72 (FIG. 11), resembling the results derived from XANES spectra (see the above discussion for FIG. 2b).

To calculate the formation energies of potential Cu dimer structures, the following equation was used:

E( ormation energy) = ESystem) - E S ubst rate) - a*E(Cu) - bx0.EO ₂ ) Here, A’(System) represents the total free energy of the system, ( Substrate), E(Cu) and E(O ₂ ) represent the free energies of the support, Cu atoms and oxygen gas, and a and b are the numbers of Cu and O atoms involved in the considered structure.

From the above discussion, it can be seen that the dimeric copper centers in CU2@C3N4 have distinct atomic structures and electronic chemical properties compared to their counterparts confined in zeolites. Without wishing to be bound to any one particular theory, it is thought that their non-integer oxidation state (intermediate between +1 and +2) and reduced cluster size (smaller Cu-Cu distance as compared to Cu-ZSM-5) would lend them exquisite catalytic performance for selective oxidation of methane. Xie et al., 2021.

1.4.2 Thermocatalytic oxidation of methane with H2O2

The Cu2@CiN4 catalysts were first evaluated for the thermocatalytic oxidation of CH4 (FIG. 12, FIG. 13, FIG. 14, FIG. 15, and FIG. 16). This evaluation was conducted using a continuous stirred-tank reactor (CSTR) filled with 0.2 mM of H2O ₂ and 0.1 MPa of CH4 (see 1.5 Methods). Methyl oxygenates (CH3OH and CH3OOH) were found to be the primary products, with the yield achieving 0. 14% within 30 min of reaction at 30 °C (FIG. 3a). As previously reported, the generated CH3OOH can be facilely reduced to CH3OH under ambient conditions (FIG. 16). Agarwal et al., 2017; Ab Rahim et al., 2013.

The yield of methyl oxygenates increased to 0.37% at 70 °C, corresponding to the increase of productivity from 51.6 to 129.7 mmol gcu ¹ h ¹ . Albeit the increase of reaction rate, the rise of reaction temperature is accompanied with the increase of CO ₂ selectivity from 0.8% at 30 °C to 5.0% at 70 °C (FIG. 17). Similarly, elongated operations also led to the yield of more CO ₂ (FIG. 17). The cyclability tests showed that the Cu2@C ₃ N ₄ catalyst was stable throughout the methane oxidation reaction with H2O ₂ . In five consecutive measurements by refilling CH4 and H2O ₂ , the CU2@C ₃ N4 catalyst exhibited indiscernible change in reactivity and product distribution, with the productivity of methyl oxygenates found to be consistent at approximately 70 mmol gcu' ¹ h' ¹ at 50 °C (FIG. 18). Furthermore, the atomic structure of dicopper-oxo centers was confirmed to remain intact after reaction by performing HADDF-STEM imaging and EXAFS analysis on the spent CU2@C ₃ NI catalyst after 6 h of reaction (FIG. 19 and Table 3).

Considering that the bare g-C ₃ N4 substrate is inactive for CH4 oxidation (FIG. 20), the dicopper-oxo centers can be identified as the active sites in Cu2@C ₃ N4. Considering that monomeric Cu in copper exchanged zeolites, Woertink et al., 2009; Kulkami et al., 2018; Yashnik et al., 2020, or metal-organic frameworks, Zheng et al., 2019, also has been discussed to be active for CH4 oxidation, comparative studies were performed on a single-atom control (Cu1@C ₃ N4) using the same g-C3N4 substrate. This catalyst was prepared by using a vapor-migration strategy, Qu et al., 2018, with the Cu loading also controlled to be at approximately 0.35 wt%, with the single-atom dispersion confirmed by using XAS (FIG. 21). Catalytic studies showed that CUI@C ₃ N ₄ was barely active for methane oxidation, delivering a yield of only 0.03% (versus 0.2% by Cu2@C ₃ N ₄ ) for methyl oxygenates at 50 °C (FIG. 3b). The low reactivity of CUI@C ₃ N ₄ indicates that Cu monomers, if present in the CU2@C3N4 catalysts, would not make significant contributions to the observed high methane partial oxidation activity, and also underlines the necessity of having dicopper-oxo centers for catalyzing the partial oxidation of methane. The activity of CU2@C3N4 also is substantially enhanced as compared to copper-exchanged zeolites. A comparative study of Cu-ZSM5 with a Si/Al ratio of 11.5 and full exchange (Cu/Al approximately 0.51) using similar reaction conditions only delivered a productivity of 25.5 mmol gcu' ¹ h' ¹ for methyl oxygenates at 50 °C, as compared to 74.4 mmol gcu ¹ h ¹ for Cu2@C ₃ N ₄ at this temperature (FIG. 3b). Note that the copper species in this Cu-ZSM-5 catalyst also is predominantly present in the form of dicopper-oxo centers, Xie et al., 2021, similar to that identified in Cu2@C ₃ N ₄ (as shown in FIG. 21). These results indicate that the dimeric Cu supported on carbon nitride is much more reactive for the oxidation of methane with H2O ₂ than their counterparts confined in zeolites.

In the partial oxidation of methane with H2O ₂ , the efficiency of utilizing the peroxide oxidizer (instead of producing O ₂ through a disproportionation reaction) is an important metric for evaluating the performance of catalysts. Agarwal et al., 2017; Ravi et al., 2019. This metric is usually assessed by comparing the “gain factor” that is defined as the molar ratio between the produced methyl oxygenates (CH3OH and CH3OOH) and the consumed H2O ₂ . Agarwal et al., 2017.

Post-reaction titration of the concentration of residual hydrogen peroxide using cerium sulfate, Lu et al., 2018, (FIG. 22) showed that the CU2@C3N4 catalyst had a gam factor of 0.19 (FIG. 3c and FIG. 23). In comparison, the gain factor was determined to be only 0.03 and 0.06 for CUI@C3N4 and Cu-ZSM5-11.5, respectively. It is interesting that the gain factor exhibited dependence on the copper loading in the dimer catalyst. A CU2@C3N4 catalyst of reduced loading (0.25 wt%) had a gam factor of 0.12, which is lower than that for the normal catalyst with 0.35 wt% of copper. Moreover, correlation between the productivity of methyl oxygenates and the gain factor gives rise to a linear relationship, underscoring its meaning of describing the reactivity between methane and H2O ₂ on a given catalyst (FIG. 3c). Agarwal et al., 2017; Xing et al., 2021. To understand the enhanced reactivity of CU2@C3N4 for methane partial oxidation, DFT calculations were performed to simulate the reaction pathways on the dicopper-oxo centers (FIG. 3d, FIG. 3e; also see FIG. 24 and Table 4). It is predicted that the reaction starts with sequential activation of two H2O ₂ molecules on the copper-dimer centers through radical mechanisms. Szecsenyi et al., 2018a; Szecsenyi et al., 2018b; Osadchii et al., 2018. The first hydrogen peroxide molecule is dissociated via H2O ₂ —> -OOH + *H, where the hydrogen adsorbs on the bridging oxygen and the -OOH radical migrates onto Cu _K to become a peroxyl (*OOH) adsorbate. The second hydrogen peroxide undergoes H2O ₂ — > -OH + *OH with the hydroxyl group adsorbing on Cup and the -OH radical recombines with the *H on the bridging oxygen site to form a H2O molecule. The involvement of -OOH and .OH in H2O ₂ activation was corroborated by the observation of these radicals in the electron paramagnetic resonance (EPR) spectroscopic studies by using 5,5’- dimethyl-l-pyrroline-N-oxide (DMPO) as the radical trap (FIG. 25). Schneider et al., 2020.

The generation of radicals is the rate limiting factor in both cases of H2O ₂ activation, which is predicted to have a kinetic barrier of 0.17 (for OOH) or 0.56 (for OH) eV. Noticeably, these barriers are substantially lower than the corresponding values found for the single-atom Cu sites (1.3 and 1.5 eV, FIG. 26) and the dicopper-oxo centers confined in zeolites (0.58 and 0.81 eV in Cu-ZSM-5), Hammond et al., 2012; Hori et al., 2018, in line with the higher gain factor and enhanced utilization of H2O ₂ as observed on the CU2@C3N4 catalysts (FIG. 3c). The enhanced H2O ₂ activation on Cu2@C3N4 could be ascribed to the π-conjugated heterocyclic rings and the semiconducting nature of the C ₃ N ₄ substrate, which is known for accommodation of charge transfer and able to supply electrons to the dicopper-oxo center for stabilization of the oxygenated adsorbates. Zhou et al., 2019; Xiao et al., 2020; Guo et al., 2021; Tong et al., 2018. The C ₃ N ₄ supported Cu dimers are thus believed to be more advantageous than their zeolitic counterparts for catalyzing the redox chemistries being examined here.

Table 4. Energy barriers and enthalpies for elementary steps during the thermal catalytic CH4 selective oxidation by H2O ₂ to CH3OH on fresh Cu dimer of Cu2@C ₃ N ₄ .

Following the activation of H2O ₂ , methane is introduced to the dicopper oxo center with one of the C-H bond attacked by the bridging oxygen (FIG. 3d and FIG. 3e). This C-H bond dissociation has a modest energy barrier of 0.61 eV (vs. approximately 0.71 eV in the case of Cu-ZSM-5). Pappas et al., 2018; Kulkami et al., 2018; Yashnik et al., 2020. While the generated H adsorbs on the bridging oxygen, the methyl group migrates on to the adjacent Cu sites. Accordingly, the C-H bond dissociation is believed to be heterolytic instead of homolytic or the Fenton type, as no CH3 radicals were observed using EPR (FIG. 27). Agarwal et al., 2017; Szecsenyi et al., 2018a; Ab Rahim et al., 2013. The heterolytic dissociation of C-H bond is believed to be essential for partial oxidation of methane at high selectivities, as the other two activation mechanisms via CH3 radicals are ty pically accompanied with over oxidation to form substantial amounts of CO ₂ . Compared to the case in Cu-ZSM-5, Ab Rahim et al., 2013, the Cu dimers supported on g-CsNr have shorter Cu-0 bond length (1.77 A vs 1.88 A) and smallerZCu-O-Cu (99 6° vs 135°), which are believed to sterically favor the heterolytic cleavage of the C-H bond and facilitate the transer of the -CH3 group. Noticeably, the -CH3 group can adsorb on either Cu _α or Cuβ, where the reaction bifurcates into two possible pathways. On the one hand, *CH3 on Cu _α recombine with the *OOH on this site to form *CH3OOH. On the other hand, it also can recombine with *OH on Cup to form *CH3OH.

Desorption of these adsorbates gives rise to the corresponding methyl oxygenates. While the rate is limited by the *CH3 + *OH *CH3OH recombination on Cup (with a barrier of 0.72 eV), the highest barrier for the CH3OOH pathway is found to be the desorption of *CH3OOH (0.52 eV). Overall, the CH3OOH pathway associated with Cu _α is energetically more favorable than the CH3OH pathway with Cup, explaining the experimentally observed much higher yield of CH3OOH than CH3OH. The pathways as revealed in FIG. 3d emphasize the synergy among the two Cu atoms and the bridging O in catalyzing the complex reaction involving multiple molecules (e.g., CH4 + 2H2O ₂ —> CH3OOH + 2H2O), which is a unique feature of the carbon nitride supported dimeric copper centers. An analogous reaction mechanism also was proposed in the partial oxidation of methane with H2O ₂ catalyzed by Au-Pd colloids. Agarwal et al., 2017.

1.4.3 Photocatalytic oxidation of methane with O2

Despite the selective oxidation of methane obtained with CU2@C3N4, the thermocatalytic reaction still relies on the use of H2O ₂ as oxidant, which is not readily available in industry. Moreover, the low CH4 conversions (less than 1%) also limits the potential of this process for practical implementations. Considering that g- C ₃ N ₄ is a semiconductor (with a bandgap of 2.7-2.9 eV, Wen et al., 2017; Xu and Gao, 2012) with demonstrated photocatalytic applications, Su et al., 2010, photocatalysis was evaluated to overcome the limitation of thermocatalytic reactions.

Photocatalytic oxidation of methane was carried out at 50 °C by applying near-edge excitation (300 W Xenon lamp equipped with a 420-nm bandpass filter) and using O ₂ as the oxidant (FIG. 28). Without being bound to any one particular theory, it is thought that photoexcitation can efficiently activate O ₂ and generate the oxygenates (*OOH and *OH). mimicking and improving the role that H2O ₂ played in the reaction. Song et al., 2019a; Song et al., 2019b; Luo et al., 2021 .

The photocatalytic reaction gave much higher conversions of methane than the thermocatalytic process, reaching 1.3% at 1 h (FIG. 4a). The methane conversion increases with time, reaching approximately 13. 1% at 6 h, where the products were found to be still dominated by CH3OOH and CH3OH (98.9% selectivity, FIG. 29). The productivity of methyl oxygenates reached the peak value of 249.7 mmol gcu ¹ h ^-1 at 2 h, representing an improvement factor of approximately 3.6 as compared to the thermocatalytic reaction. Further improvement of the productivity was obtained by raising the partial pressure of methane ( CH). AS PCI 14 increased from 0. 1 to 1 MPa (at P02 = 0.1 MPa, while the total pressure was kept constant at 3 MPa), the productivity escalated from 184.3 to 709.8 mmol gcu' ¹ h’ ¹ , albeit with the methane conversion reducing from 13.1 to 5.1 (at 6 h, FIG. 4b and FIG. 30). The improvement of productivity at higher PCH 4 can be ascribed to the increased concentration of dissolved methane in the aqueous solution. Jin et al., 2020. The low conversion of methane at high P cH 4 was likely limited by the inadequacy of oxygen. At P02 = 0.5 MPa and cH4 = 1 MPa, a methane conversion of 10.1% was obtained with greater than 98% selectivity toward methyl oxygenates, corresponding to an even higher productivity of 1399.3 mmol gcu' ¹ h- ¹ . A comprehensive comparison to the literature results under similar reaction conditions indicate that this activity represents the highest activity for partial oxidation of methane, with improvement factors of at least greater than 10 (Table 5). The photocatalytic oxidation of methane with O ₂ was confirmed by conducting control experiments under various conditions (Table 6). In particular, the Cu2@C ₃ N ₄ catalyst was found to be inactive in darkness (while the other conditions were kept the same), ruling out the involvement of thermocatalytic reaction between CH4 and O ₂ in the photocatalytic studies. The photocatalytic activity of bare g-C ₃ N ₄ also was nearly negligible, underlining the role of Cu dimers in catalyzing the related molecular transformations. The generation of active peroxide species in situ during the photocatalytic reaction was confirmed by performing EPR spectroscopic studies by also using DMPO as the radical trapping agent (FIG. 4c). The spectra recorded under visible light irradiation, in both cases with and without methane, show the fingerprints of -OOH radicals, which can be assigned to the spin of unpaired electrons on oxygen. Hammond et al., 2012; Song et al., 2019a; Xing et al., 2021; Luo et al, 2021.

Similar to the findings from photocatalytic studies, such signals were not observed from the controls in the absence of O ₂ , Cu2@C ₃ N ₄ , or light. These OOH radicals are likely derived from the thermal activation of H2O ₂ (as observed in the thermocatalytic studies, FIG. 25), which was produced from photocatalytic reduction of O ₂ in situ. Shiraishi et al, 2014; Moon et al, 2017; Rao and Hay on, 1975. It thus becomes evident that CU2@CIN4 is not only a good thermocatalyst for partial oxidation of methane with H2O ₂ , but also an exceptional photocatalyst when the oxidant is replaced by O ₂ .

Table 6. Catalytic performance of multiple control experiments for the photocatalysis of selective CH4 oxidation by O ₂ .

In addition to the reduction of O ₂ to peroxides, the photon excitation also is believed to enhance the methane activation. This characteristic was revealed by using in situ irradiation X-ray photoelectron spectroscopy (ISI-XPS), Zhang et al., 2020, to examine charge transfer between the dimeric copper center and the C ₃ N ₄ substrate (see 1.5 Methods). As shown in FIG. 4d, the XPS spectra collected on hydrated Cu2@C ₃ N ₄ in darkness exhibited two O Is peaks at ca. 533.0 and 531.7 eV, which can be assigned to the oxygen binding to Cu, i.e., -Cu-OH and -Cu-O-Cu-, respectively. Akhavan et al., 2011; Bojestig et al., 2020.

Under light irradiation (between about 400 to about 500 nm), both of these two peaks had a blue shift of approximately 0.5 eV. Similar observations were obtained at the Cu 2p edge (FIG. 31). Meanwhile, a red shift of the N Is peak associated with C ₃ N ₄ was observed, from 398.8 eV in darkness to 398.2 eV under light irradiation (FIG. 4e). Such phenomena consistently point to the transfer of holes (rather than electrons) from the g- C ₃ N ₄ substrate to the dicopper-oxo center, where CH4 is activated and oxidized to form *CH ₃ . Meanwhile, the excited electrons in the g-C3N4 substrate lead to the reduction of O ₂ and formation of H2O ₂ , which then migrates or diffuses onto the dicopper-oxo center and gets activated to form *OOH or *OH. In the following, these oxygen species recombine with *CH3 to form methyl oxygenates, as in the case of thermocatalytic reactions (FIG. 4f and FIG. 32- FIG. 33). Similar phenomena of charge transfer induced catalytic enhancements have previously been reported in photocatalysis using T1O ₂ based photocatalysts. Dong et al., 2013; Wang et al., 2018; Xie et al., 2018.

1.5 Methods

1.5.1 Materials and Chemicals

The following chemicals were purchased and used as-received without further purification: Copper(II) chloride dihydrate (CuC1 2H2O, ACS grade, Sigma Aldrich), 2, 2, -bipyridine (C10H8N2, reagent grade, Sigma Aldrich), oxalic acid (HO ₂ CCO ₂ H, reagent grade, Alfa Aesar), urea (NH2CONH2, ACS grade, Sigma Aldrich), dicyandiamide (NH2C(=NH)NHCN, ACS grade, Sigma Aldrich), copper(II) acetylacetonate (Cu(acac)2, ACS reagent, Sigma Aldrich), oleylamine (CH3(CH ₂ )7CH=CH(CH ₂ )8NH2, >98%, Sigma Aldrich), ethanol (C2H5OH, HPLC grade, Fisher Scientific), methanol (CH3OH, HPLC grade, Fisher Scientific), deionized water (18.2 MΩ) was collected from an ELGA PURELAB flex apparatus.

1.5.2 Synthesis of copper dimer complex

Solutions A, B and C were prepared by ultrasonically dispersion method, respectively. The detailed preparation process was as follows: Solution A: 1.6 mmol 272 mg CuC1 ₂ 2H2O was ultrasonically dispersed in 20 mL deionized water; Solution B: 1.6 mmol 248 mg 2, 2, -bipyridine was ultrasonically dispersed in 10 mL methanol; Solution C: 0.8 mmol 100 mg oxalic acid was ultrasonically dispersed in 10 mL deionized water; Subsequently, adding solution B and solution C to solution A drop by drop respectively and kept stirring for 1 h. Finally, the light-blue solid was obtained by centrifugation, washing with water and methanol for three times and drying in vacuum. Reinoso et al., 2003.

1.5.3 Synthesis of g-CiN 4

20 g of urea was placed to an alumina crucible (100 mL). Subsequently, the crucible was sealed with multiple layers of tin foil and put into a muffle furnace with the heating program from 50 °C to 550 °C for 2 h at the rate of 20 °C min' ¹ . The obtained powder was further subjected to the above calcination operation, with the difference being that the heating rate was kept at 5 °C min' ¹ and the retention time at 550 °C was 3 h. Finally, the yellowish-white powder was obtained.

1.5.4 Synthesis of CuSyfC3N 4

Solution A: 0.5 g g-C3N4 was ultrasonically dispersed in 50 mL methanol solution; Solution B: 42 mg copper dimer was ultrasonically dispersed in 5mL methanol solution; Solution B was added dropwise added to solution A and was stirred at room temperature for 24 h, and the obtained solid was calcined in muffle furnace with the heating program from 50 °C to 250 °C for 10 h at the rate of 2 °C min' ¹ . Finally, the blue-yellow solid was obtained.

1.5.5 Synthesis of Cui@g-C3N4 (Wu et al., 2019)

3 g dicyandiamide and 340 mg CuC1 ₂ 2H2O were grounded to be-well mixed, then spread in an alumina crucible (100 mL) with a cap covered. The crucibles were places in a muffle furnace, and gradually heated to 550 °C for 8 hours with the ramping rate of 5 °C min' ¹ and then cooled down.

1.5.6 Material Characterization

X-Ray Diffraction (XRD) patterns were obtained from a PANalytical X’Pert ³ X- ray diffractometer equipped with a Cu K« radiation source (z = 1.5406 A). Nitrogen adsorption measurements were measured on a Micromeritics ASAP 2010 instrument with the samples degassed under vacuum at 300°C for 4 h. Specific surface area (SSA) was calculated using the Brunauer-Emmett-Teller (BET) theory. The Cu contents were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a PerkinElmer Elan DRC II Quadrupole ICP-MS after dissolution of the samples in aqua regia. High angle annular dark field (HAADF) STEM images were acquired using a JEOL TEM/STEM ARM 200CF (equipped with an Oxford X-max 100TLE windowless X-ray detector) at a 22-mrad probe convergence angle and a 90-mrad inner-detector angle. The analysis of surface elements was performed on X-ray photoelectron spectroscopy (XPS), Thermo Fisher Scientific Escalab 250Xi spectrometer with Al Ka radiation as the excitation source. Fourier Transform Infrared Spectroscopy were carried out on ThermoNicolet Nexus 670. Diffuse reflectance ultraviolet-visible (UV-Vis) spectra were collected on a Shimadzu UV-2450 spectrometer equipped with an integrating sphere attachment using BaSO4 as the reference. FTIR Spectrometer Ultraviolet photoelectron spectroscopy (UPS) measurements were performed on an ESCALAB 250 UPS instrument with a He lα gas discharge lamp operating at 21.22 eV and a total instrumental energy resolution of 90-120 meV.

XAS experiments were performed at the 10-BM beamline at the Advanced Photon Source (APS) at Argonne National Laboratory. Samples were pressed into a stainless-steel sample holder. All measurements were performed at the Cu K edge (8.9789 keV) in transmission mode in fast scan from 250 eV below the edge to 800 eV above the edge. Spectra processing, including background removal and normalization were performed on ATHENA module in Demeter package. The extraction of structural parameters and fitting of the DFT optimized models of fresh and spent Cui V.C3N4 samples were performed on ARTEMIS module. For the optimized structure, EXAFS data were fit from k = 2.7 to 10 A- 1 (dk=2) and R = 1 - 3.2 A with a Hanning window.

Electron Paramagnetic Resonance (EPR) measurements were performed on a Bruker EMX EPR spectrometer at X-band frequency (9.46 GHz). 5,5 ’-Dimethyl- 1- pyrroline-N-oxide (DMPO) was used as the spin-trapping agent, which can capture the radicals *CH3, »OOH and »OH. As for the detection of *OOH and »OH, methanol and DI H2O were used respectively, due to the DMPO-OOH is not stable in H2O, would be quickly converted to DMPO-OH.

The in-situ irradiation X-Ray photoelectron spectroscopy (ISI-XPS) was carried out on AXIS SUPRA (Kratos Analytical Inc, Shimadazu) coupled with a continuous tunable wavelength light optical fiber (PLS-EM 150, Beijing Perfectlight Co. Ltd.) . The wavelength of irradiation light was set at 400-500 nm to mimicking the visible light. The measurement setup is developed to monitor the photoelectron transfer process. Before measurement, the hydrated Cu 1@g-C.3N4 was obtained by pretreatment of fresh Cu1@g- C ₃ N ₄ by water.

1.5.7 Thermocatalyti c meas uremen ts

The selective methane oxidation was performed in a high-pressure Parr reactor. 0.2 mmol H2O ₂ dissolved in 10 mL deionized H2O was used as the oxidizing agent. 50 mg of catalyst powder was added to the aqueous solution. After evacuating the air left in reactor by flowing methane (0.1 MPa) and purging for five times, the system then was pressurized with argon to 3 MPa. The solution was vigorously stirred at 1500 rpm, meanwhile heated to 50 °C. Both temperature and pressure were well controlled and kept constant during catalysis. The reaction time of all experiments was strictly controlled at certain time (e.g., 30 mins, 1 or 2 h) after the temperature of solution reaches a pre-set temperature. After the reaction, the reactor was set in an ice bath to cool down immediately, the solution was kept being stirred at 1500 rpm.

Upon completely cooling the reaction down to ice bath temperature, the gas components (i.e., CH4, CO ₂ ) were injected and determined with gas chromatograph equipped with a BID detector (GC-2010 plus, Shimadzu). Before analysis, the gas in the autoclave was used to sweep the GC lines for 20 s. The solution consisting of liquid products was filtered from catalyst powder. The liquid products, including CH3OOH, CH3OH and others, were quantitatively analyzed with 'H-NMR. Typically, 0.7 mL of sample and 0. 1 mL of D2O were placed in an NMR tube along with 4,4-dimethyl-4- silapentane-1 -sulfonic acid (DSS, 5=0 ppm) as the internal standard. During NMR measurements, a solvent suppression program was run to minimize the signal originating from H2O. A typical 'H-NMR spectrum is provided in FIG. 12. The identified oxygenated products were methanol (5=3.34 ppm) and methyl hydroperoxide (5=3.85 ppm). Ratios of peak areas of methanol or methyl peroxide to peak area of DSS were calculated. The products are determined by using the standard curves of methanol and methyl hydroperoxide as provided in FIG. 13, FIG. 14.

The H2O ₂ concentration was measured by a traditional cerium sulfate Ce(SO4 )2 titration method based on the mechanism that a yellow solution of Ce ⁴⁺ would be reduced by H2O ₂ to colorless Ce ³⁺ (2Ce ⁴⁺ + H2O ₂ —> 2Ce ³⁺ + 2H ⁺ + O ₂ ). Thus, the concentration of Ce ⁴⁺ before and after the reaction can be measured by ultraviolet-visible spectroscopy. The wavelength used for the measurement was 316 nm. The standard curve of H2O ₂ is provided in FIG. 21.

7.5.5 Photocatalytic studies

The photocatalytic methane oxidation reaction tests were conducted in a 50-rnL batch-reactor equipped with a quartz window to allow light irradiation. Typically, 50-mg catalyst was dispersed in 10-mL deionized water by ultrasonication for 10 min. Then the mixture was added into the reaction cell, and the reaction cell was placed in the batch- reactor. The batch-reactor was purged with 0.1-MPa CH4 and 0. 1-MPa O ₂ for five times to exhaust air, then the reactor was pressurized with argon to 3 MPa. To study the influence of different CH4 or O ₂ partial pressure on the photocatalytic reaction, 0.5- or 1- MPa CH4 with 0.1-MPa O ₂ or 0. 1-MPa CH4 with 0.5 MPa O ₂ also were applied. Subsequently, the reactor was stirred at 50 °C under the light irradiation provided by a 300-W xenon lamp (MC-XS500, Testmart), equipped with a 420-nm optical filter (Ceaulight), the light intensity was controlled at 100 mW/cm. A thermocouple was inserted into the solution to directly detect the temperature of the liquid solution. During the process, the temperature was maintained at 50 °C. After the reaction, the reactor was cooled in an ice bath to a temperature below 10 °C. The analysis of products followed the same protocol as shown above. The conversion of CH4, the selectivity of products, and the mass reaction rate were calculated according to the following equations:

1.5.9 Computational Methods

In this work, all the simulations were carried out for the direct synthesis of CH3OOH and CH3OH within the framework of the spin-polarized generalized gradient approximation with the Perdew-Burke-Emzerh, Perdew, et al., 1996, of functional in the VASP code. Kresse and Furthmuller, 1996. The cutoff energy of plane-wave basis expansion was set to 400 eV. The approach of project-augmented-wave (PAW), Kresse and Joubert, 1999, was exploited to describe the interaction between core-electron and valence electron. The Methfessel-Paxton-approach with a fermi smearing width of 0. 1 eV was used to determine partial occupancies on electronic states. Electronic convergence was set to 10 ⁵ eV, and geometries were converged to less than 0.05 eV/A. All the possible surfaces were constructed with 2 x 2 x 1 Monkhorst-Pack k-point mesh sampling which is well tested. Chow and Vosko, 1980. The effect of vdW interaction is significant for the reaction mechanism in our precious study. Yao et al., 2019; Wei et al., 2019. Therefore, the DFT-D3 method of Grimme et al., 2011, was utilized to calculate all the energetics and structures of the intermediates and transition states. All the surfaces were relaxed with a 15 A vacuum region. The transition states (TSs) were searched using the method called a constrained optimization scheme. Liu and Hu, 2003; Zhang et al., 1999. The TSs were confirmed by two rules: (i) all forces on atoms vanish; (ii) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom. Vibrational frequency analyses were performed to confirm the integrity of TSs. Cortright and Dumesic, 2001.

1.6 Summary

The presently disclosed subject matter provides novel dimeric copper catalysts for the partial oxidation of methane. These catalysts were synthesized by immobilization of a copper-dimer organometallic complex on graphitic carbon nitride. Dicopper-oxo centers were characterized as anchoring on this substrate via Cu-N bonding. The derived CU2@C ₃ N4 catalysts were first examined for thermocatalytic oxidation of methane with H2O ₂ , and then studied for photocatalytic reactions with O ₂ being used as the oxidant. Enhanced catalytic activities were demonstrated in both cases as compared to the other reported catalysts under similar reaction conditions, achieving improvement factors of more than an order of magnitude. Synergy of the bridging oxygen, the two copper sites and the semiconducting C ₃ N ₄ substrate has been revealed to promote H2O ₂ and O ₂ activation and the heterolytic scission of CH4. The presently disclosed subject matter highlights the great potential of carbon nitride supported dimeric copper centers in catalyzing redox chemical reactions.

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Although the foregoing subject matter has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be understood by those skilled in the art that certain changes and modifications can be practiced within the scope of the appended claims.