Methane is an abundant hydrocarbon resource that is often underutilized because of its low boiling point and chemical inertness. Thus, technologies for converting methane to liquid chemicals such as methanol would enable better utilization of this low-carbon resource.1-3 Current methane valorization technologies rely on an indirect process involving initial steam reforming to H2 and CO. The reforming step requires capital-intensive facilities that are not amenable to remote deployment.4 Consequently, spontaneously released natural gas at oil wells is being flared at massive scales.5,6 The development of mild, direct methane-to-methanol processes (Scheme 1) that can operate portably is expected to stem flaring as well as expand the versatility of natural gas.7,8
While many homogeneous and heterogeneous systems have been investigated for methane-to-methanol conversion,1,8,9 simple PtII chloride salts in water, PtIIClx(H2O)(4-x)(2-x) (denoted collectively as PtII), offer unique advantages. The catalytic cycle (
This system has the following advantages: first, the organometallic activation of methane offers superior selectivity for mono-oxidation compared to catalysts that operate via radical intermediates;8,10-12 second, while most homogeneous catalysts that do organometallic activation require impractical8 concentrated acid media for boosting the catalytic rate and selectivity,13,14 PtII operates in water. Along with the relatively low reaction temperature (>100° C.), these advantages position PtII chloride salts, often referred to as “Shilov's catalyst,” as privileged agents for methane-to-methanol conversion under mild conditions.
A critical drawback of Shilov's catalyst, as originally reported, is its requirement for a stoichiometric PtIV oxidant, which is economically impracticable.15 The key to developing an alternative oxidation strategy for this catalytic system is to achieve precise control over the driving force (thermodynamics) and/or rate (kinetics) of the oxidation reaction. In view of the catalytic cycle, there are two distinct approaches to the problem. First, PtIV may be directly replaced by an alternative oxidant that can oxidize the PtII—CH3 intermediate (
The inherent difficulty of fine-tuning oxidation using chemical reagents, has, presumably, contributed to the limited success in replacing stoichiometric PtIV. Notably, oxidants such as heteropoly acids, CuCl2, FeCl3, and Br2 were identified as kinetically competent toward oxidation of PtII—CH3 (
The present disclosure relates to a process for oxidizing a compound, comprising:
wherein the anion is chloride, fluoride, bromide, iodide, a carboxylate, nitrate, perchlorate, phosphate, or sulfate;
the initial concentration of PtII species is about 1 mM to about 10 M;
R1 is C1-C20 alkyl, C3-C12 cycloalkyl, C5-C10 heterocyclyl, C6-C12 aryl, or C5-C12 heteroaryl; and
R2 is H, —OH, —C(═O)H, or —C(═O)OH.
In some embodiments, the electrical potential is applied; and the electrical potential is adjusted to maintain the concentration of PtII species at about 95% to about 105% of the initial concentration.
In certain embodiments, electrical current is applied; and the electrical current is adjusted to maintain the concentration of PtII species at about 95% to about 105% of the initial concentration.
In some embodiments, the reaction mixture is contained within a reaction vessel comprising a working electrode, and a counter electrode, and, optionally, a reference electrode.
In certain embodiments, the reaction vessel further comprises a PtII sensing electrode.
In some embodiments, the reaction vessel is a flow reaction vessel.
In certain embodiments, the concentration of PtII species is measured potentiometrically.
In some embodiments, the concentration of PtII species is measured with a PtII sensing electrode. For example, the PtII sensing electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, a Pt wire electrode, and a platinized Pt/H2 electrode.
In certain embodiments, the working electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, an Hg/HgSO4 electrode, an Ag/AgCl electrode, a Pt wire electrode, a platinized Pt/H2 electrode, a calomel electrode, a fluorine-doped tin oxide electrode, an indium-doped tin oxide electrode, a glassy carbon electrode, a carbon black electrode, a pyrolytic graphite electrode, a graphite electrode, a carbon nanotube electrode, and a boron-doped diamond electrode. For example, the working electrode is a Pt foil electrode.
In some embodiments, the reference electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, an Hg/HgSO4 electrode, an Ag/AgCl electrode, a Pt wire electrode, a platinized Pt/H2 electrode, a calomel electrode, a fluorine-doped tin oxide electrode, an indium-doped tin oxide electrode, a glassy carbon electrode, a carbon black electrode, a pyrolytic graphite electrode, a graphite electrode, a carbon nanotube electrode, and a boron-doped diamond electrode. For example, the reference electrode is an Ag/AgCl electrode.
In certain embodiments, the counter electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, an Hg/HgSO4 electrode, an Ag/AgCl electrode, a Pt wire electrode, a platinized Pt/H2 electrode, a calomel electrode, a fluorine-doped tin oxide electrode, an indium-doped tin oxide electrode, a glassy carbon electrode, a carbon black electrode, a pyrolytic graphite electrode, a graphite electrode, a carbon nanotube electrode, and a boron-doped diamond electrode. For example, the counter electrode is a Pt mesh electrode.
In some embodiments, the counter electrode is immersed in a solution of an electron acceptor. For example, the electron acceptor is a proton or vanadyl sulfate. Alternatively, the counter electrode is an oxygen-consuming electrode.
In certain embodiments, the anion is chloride, fluoride, acetate, nitrate, perchlorate, phosphate, or sulfate. For example, the anion is chloride.
In some embodiments, the chloride is a constituent of a salt selected from the group consisting of NaCl, KCl, LiCl, CsCl, RbCl, MgCl2, CaCl2, BaCl2, NH4Cl, and HCl. For example, the salt is NaCl.
In certain embodiments, the source of PtII species is selected from the group consisting of K2PtCl4, Na2PtCl4, Li2PtCl4, H2PtCl4, (NH4)2PtCl4, K2PtBr4, Na2PtBr4, Li2PtBr4, H2PtBr4, (NH4)2PtBr4, K2Pt(CN)4, Na2Pt(CN)l4, Li2Pt(CN)4, H2Pt(CN)4, (NH4)2Pt(CN)4, K2PtCl6, Na2PtCl6, Li2PtCl6, H2PtCl6, (NH4)2PtCl6, Pt(NH3)4Cl2, Pt(NH3)4(NO3)2, Pt(NH3)4(OH)2, Pt(NH3)4Cl4, and PtO2. For example, the source of PtII species is K2PtCl4.
In some embodiments, the Bronsted acid is selected from the group consisting of H2SO4, HCl, HNO3, H3PO4, HClO4, and a carboxylic acid. For example, the Bronsted acid is HCl.
In certain embodiments, the temperature is about 20° C. to about 500° C. For example, the temperature is about 150° C. to about 300° C.
In some embodiments, electrical current is applied at a constant current.
In certain embodiments electrical potential is applied under constant potential conditions.
In some embodiments, the compound of formula R1-R2 is an alkane or a cycloalkane.
In certain embodiments, the compound of formula R1-R2 is methane.
In certain embodiments the compound of formula R1-R2 is oxidized to an alcohol. For example, the alcohol is a diol or a polyol.
Overview
The present disclosure relates to an electrochemical solution to the problem of sustained aqueous PtII-catalyzed methane-to-methanol conversion, exploiting the unparalleled control over oxidation rate and driving force that electrochemistry affords. While direct electrooxidation of the fleeting PtII—CH3 intermediate is unfeasible due to the small fraction of reaction solution volume in contact with the electrode surface, the electrochemical oxidation of PtII could be carried out at precisely controlled rates to enable stable and continuous PtII-catalyzed methane oxidation (
For convenience, before further description of the present invention, certain terms employed in the specification, examples and appended claims are collected here. These definitions should be read in light of the remainder of the disclosure and understood as by a person of skill in the art. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by a person of ordinary skill in the art.
In order for the present invention to be more readily understood, certain terms and phrases are defined below and throughout the specification.
The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.
The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.
As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e., “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.
As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.
It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited.
In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.
“Alkyl” refers to a fully saturated cyclic or acyclic, branched or unbranched carbon chain moiety having the number of carbon atoms specified, or up to 30 carbon atoms if no specification is made. For example, alkyl of 1 to 8 carbon atoms refers to moieties such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, and octyl, and those moieties which are positional isomers of these moieties. Alkyl of 10 to 30 carbon atoms includes decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl, tricosyl and tetracosyl. In certain embodiments, a straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone (e.g., C1-C30 for straight chains, C3-C30 for branched chains), and more preferably 1 to 20. Alkyl groups may be optionally substituted with one or more substituents, for example, halogen, alkyl, cycloalkyl, hydroxyl, amino, heterocyclyl, alkoxy, and the like.
“Alkane” refers to a fully saturated cyclic or acyclic, branched or unbranched carbon chain molecule having the number of carbon atoms specified, or up to 30 carbon atoms if no specification is made. For example, alkane of 1 to 8 carbon atoms refers to moieties such as methane, ethane, propane, butane, pentane, hexane, heptane, and octane, and those molecules which are positional isomers of these molecules. Alkane of 10 to 30 carbon atoms includes decane, undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, heptadecane, octadecane, nonadecane, eicosane, heneicosane, docosane, tricosane and tetracosane. In certain embodiments, a straight chain or branched alkane has 30 or fewer carbon atoms in its backbone (e.g., C1-C30 for straight chains, C3-C30 for branched chains), and more preferably 20 or fewer. Alkanes may be optionally substituted with one or more substituents, for example, halogen, alkyl, cycloalkyl, hydroxyl, amino, heterocyclyl, alkoxy, and the like.
“Cycloalkane” means mono- or bicyclic or bridged or spirocyclic, or polycyclic saturated carbocyclic rings, each having from 3 to 20 carbon atoms. Preferred cycloalkanes have from 3-12 carbon atoms in their ring structure. Cycloalkanes may be optionally substituted with one or more substituents, for example, halogen, alkyl, hydroxyl, amino, heterocyclyl, alkoxy, and the like.
The terms “alkoxyl” or “alkoxy” as used herein refers to an alkyl group, as defined above, having an oxygen moiety attached thereto. Representative alkoxyl groups include methoxy, ethoxy, propoxy, tert-butoxy, and the like.
The terms “amine” and “amino” are art-recognized and refer moieties that can be represented by the formulae:
wherein R1, R2 and R3 each independently represent an alkyl, an aryl, a cycloalkyl, or a heterocyclyl.
The term “aryl” as used herein includes 3- to 12-membered substituted or unsubstituted single-ring aromatic groups in which each atom of the ring is carbon (i.e., carbocyclic aryl) or where one or more atoms are heteroatoms (i.e., heteroaryl). Preferably, aryl groups include 6- to 12-membered rings The term “aryl” also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings wherein at least one of the rings is aromatic. Heteroaryl groups include substituted or unsubstituted aromatic 3- to 12-membered ring structures, more preferably 5- to 12-membered rings, whose ring structures include one to four heteroatoms. Heteroaryl groups include, for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine, pyridazine and pyrimidine, and the like. Aryl and heteroaryl can be monocyclic, bicyclic, or polycyclic.
The term “halo”, “halide”, or “halogen” as used herein means halogen and includes, for example, and without being limited thereto, fluoro, chloro, bromo, iodo and the like, in both radioactive and non-radioactive forms. In a preferred embodiment, halo is selected from the group consisting of fluoro, chloro and bromo.
The terms “heterocyclyl” or “heterocyclic group” refer to 3- to 12-membered ring structures, more preferably 5- to 12-membered rings, whose ring structures include one to four heteroatoms. Heterocycles can be monocyclic, bicyclic, spirocyclic, or polycyclic. Heterocyclyl groups include, for example, thiophene, thianthrene, furan, pyran, isobenzofuran, chromene, xanthene, phenoxathiin, pyrrole, imidazole, pyrazole, isothiazole, isoxazole, pyridine, pyrazine, pyrimidine, pyridazine, indolizine, isoindole, indole, indazole, purine, quinolizine, isoquinoline, quinoline, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, pteridine, carbazole, carboline, phenanthridine, acridine, pyrimidine, phenanthroline, phenazine, phenarsazine, phenothiazine, furazan, phenoxazine, pyrrolidine, oxolane, thiolane, oxazole, piperidine, piperazine, morpholine, lactones, lactams such as azetidinones and pyrrolidinones, sultams, sultones, and the like. The heterocyclic ring can be substituted or unsubstituted.
The term “substituted” refers to moieties having substituents replacing a hydrogen on one or more carbons of the backbone. It will be understood that “substitution” or “substituted with” includes the implicit proviso that such substitution is in accordance with permitted valence of the substituted atom and the substituent, and that the substitution results in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. As used herein, the term “substituted” is contemplated to include all permissible substituents of organic compounds. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and non-aromatic substituents of organic compounds. The permissible substituents can be one or more and the same or different for appropriate organic compounds. It will be understood by those skilled in the art that substituents can themselves be substituted, if appropriate. Unless specifically stated as “unsubstituted,” references to chemical moieties herein are understood to include substituted variants. For example, reference to an “aryl” group or moiety implicitly includes both substituted and unsubstituted variants.
Structures depicted herein are also meant to include compounds that differ only in the presence of one or more isotopically enriched atoms. For example, compounds produced by the replacement of a hydrogen with deuterium or tritium, or of a carbon with a 13C- or 14C-enriched carbon are within the scope of this invention.
For purposes of this invention, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 67th Ed., 1986-87, inside cover.
Identification of a Suitable Electrode for PtII-Catalyzed Electrochemical Methane Oxidation Reaction (EMOR).
The electrochemical mediation scheme put forward above (
Electro-Oxidation of PtII/IV at the Elevated Temperatures Required for Methane Activation by PtII.
These experiments were conducted above the boiling point of water and were, therefore, carried out in a high-pressure electrochemical cell (
Pt electrodes were also capable of sustained and efficient PtII/IV oxidation. Bulk electrolysis of a stirred solution was conducted at 130° C. by applying a constant potential below 1.1 V. After chronoamperometry at 0.874, 0.924 and 0.974 V for 77, 40 and 17 min, respectively, half of the PtII ions in the initial solution were converted to PtIV ions as determined by UV-Vis analysis. At all three potentials examined, PtIV was generated with 100% Faradaic efficiency (Table 1).
Sustained methane oxidation catalysis will lead to a progressive rise in methanol concentration in the reactor over time. Thus, in addition to supporting facile PtII/IV oxidation, the electrode must be inert towards further oxidation of the CH3OH product. This is a particular concern for Pt, which is the standard electrocatalyst for oxidation of CH3OH to CO2.35 Indeed, in 0.5 M H2SO4 at 130° C., addition of 30 mM CH3OH gives rise to the well-known anodic features associated with CH3OH electro-oxidation (
Sustained Methane Oxidation Catalysis Via Dynamic Electrochemical Control of the PtII:PtIV Ratio.
The above studies provide the basis for carrying out continuous methane-to-methanol oxidation catalysis via electrochemical regeneration of PtIV (
Careful choice of the applied current is critical for sustained catalysis. In order to maintain a constant PtII:PtIV ratio over the course of the reaction, the rate of PtII oxidation at the electrode must match the rate of methane oxidation catalysis in the solution. At a fixed rate of PtII/IV oxidation, a simple mathematical derivation shows that any small difference between the two rates will increase exponentially over time (see Example 4). Thus, the applied current must constantly match the rate of catalysis to maintain a steady ratio of PtII:PtIV. To achieve this matching, it is necessary to monitor [PtII] and adjust the current (i) accordingly. In order to achieve this, the open-circuit potential (OCP) of the working compartment was employed as an in situ probe of the instantaneous PtII:PtIV ratio in solution. In the reactor, the PtII and PtIV ions exist in various ligated states (PtIIClx(H2O)(4-x)(2-x), PtIVClx(H2O)(6-x)(4-x)), each pair of which has different redox potentials. Assuming that [Cl−] is constant, the following modified form of the Nernst equation may be derived as shown in Scheme 2:
where E0″ and n represents the weighted average of the redox potentials and chloride stoichiometries, respectively. Thus, using the equation in Scheme 2, the instantaneous PtII:PtIV ratioscan be estimated potentiometrically. EC can be determined from the initial OCP reading and the known initial PtII:PtIV ratio.
The potential required for electrolysis (ECP, CP=chronopotentiometry) equals the equilibrium electrode potential (OCP) plus the magnitude of overpotential (η) applied. By definition, η is the difference between the applied potential (ECP) and EOCP, as marked with green arrows in
Independent quantification of the PtII:PtIV ratio at the end of the EMOR confirmed the power of in situ current modulation. At the end of each reaction, [PtII] and [PtIV] in the working compartment was measured by UV-Vis spectroscopy. Despite a wide variation in reaction time (5-29 h) and consequently turnover number (see below), UV-Vis analysis confirmed that the final PtII% (19-23%) values were all similar (Table 1). These values are somewhat lower than the initial PtII% (30%), reflecting the preference to err on the side of lower PtII% to prevent irreversible Pt0 deposition (see below). Interestingly, despite the agreement in final PtII% values, ΔOCP (=OCPlast−OCPfirst), which should reflect the final PtII% according to the equation in Scheme 2, was more negative for longer reactions by up to 14 mV. This may be due to decreasing [Cl−] in the reaction solution as a result of CH3Cl formation. Despite this additional long-term effect, changes in the OCP between constant-current intervals provided a faithful indication of whether the PtII% was increasing or decreasing, allowing for appropriate adjustment of i. Together, these results demonstrate that the PtII% can indeed be maintained over long time durations of catalysis through dynamically-controlled electrochemical oxidation.
Careful control of the PtII:PtIV ratio during the reaction is essential for another reason: PtIV ions suppress the irreversible decomposition of PtII to Pt0.15,39 Indeed, at the end of all of the EMOR trials, the bulk reaction solutions contained no visible Pt0 precipitates. Only a few adventitious Pt0 deposits were observed on the reactor surfaces and crevices where mass transport was restricted and replenishment of PtIV was impeded (see Example 6). Although an extensive discussion of Pt0 deposition mechanisms is beyond the scope of the current work, the present results are consistent with the proposal that maintenance of a sufficient concentration of PtIV prevents Pt0 formation.
Analysis of methane oxidation products from the EMOR reactor. Operation of the EMOR reactor using the feedback modulation procedure described above allowed for continuous functionalization of methane (Table 2 and
aThe length of time the reactor was at the designated temperature, which spanned from ~80 minutes after the start of heating to the time at which the reactor was removed from the oil bath.
biave was calculated by dividing the total charge passed by the reaction time.
cΔOCP is the difference between the first and last OCP readings (=OCPlast − OCPfirst).
dThe hydrated form of formaldehyde, which is the predominant form of formaldehyde in the acidic pH employed.
eThe TONs were determined from dividing the μmol of product by the average of the initial and final μmol of PtII for each reaction. The TOFs were obtained by dividing the TON by the time duration of each reaction. The total number of turnovers were calculated by assuming that all oxidation reactions were catalyzed by PtII: the total number of oxidizing equivalent were calculated as (μmolCH3OH + μmolCH3Cl + 2 * μmolCH2(OH)2 + 3 * μmolHCOOH + 4 * μmolCO2) and this sum was divided by the average μvmolPtII to determine total TON. For CH3X-specific turnovers, only (μmolCH3OH + μmolCH3Cl) was divided by μmolPtII.
In all cases, CH3OH is observed as the majority product in 69-72% yield (Table 2). Appreciable quantities of CH3Cl are also observed with a yield that decreases from 24 to 13% as the reaction time increases (Table 2). Small amounts of overoxidized products (CH2(OH)2, HCOOH and CO2) were observed in less than 20% combined yield. Taking these overoxidized products to represent PtII-catalyzed oxidation of CH3OH by 1, 2 and 3-equivalents of PtIV, respectively, the overall Faradaic efficiencies were in excess of 90% in all cases (Table 3).
The per-PtII turnover numbers could not be rigorously determined due to minor fluctuations in [PtII] over the course of the reaction (see above), but approximate values were calculated from the known initial and final PtII amounts. For the longest trial, TON values of 6 and 9 for monofunctionalized products (CH3X═CH3OH and CH3Cl) and total oxidation events were obtained, respectively (Table 2). The TOF for CH3X, estimated to be 0.2-0.3 h−1, showed a decreasing trend with increasing reaction time due to the overoxidation of CH3OH. In contrast, the TOF for total oxidation events was relatively constant at ca. 0.3 h−1 for different reaction times. Together, these observations demonstrate that electrochemical re-oxidation effectively sustains PtII-based methane functionalization catalysis.
Combining the four trials in Table 2,
An electrochemical approach for continuous methane-to-methanol conversion using aqueous PtII catalysts has been establishes. Cl-adsorbed Pt surfaces were shown to be competent for the inner-sphere two-electron oxidation of PtII to PtIV while inert toward parasitic oxidation of the methanol product. In situ potential measurements and current modulation allowed us to carry out continuous steady-state catalysis by maintaining the PtII:PtIV ratio. While the test reactors were run up to 30 h, further reactor engineering to incorporate automatic real-time current modulation, enhance solution mixing, and rigorously separate the anode and cathode compartments should allow for extended operation. Moreover, integration of an oxygen-consuming counter electrode will enable net aerobic methane-to-methanol conversion. Examples of oxygen-consuming electrodes are disclosed in the following U.S. patents: U.S. Pat. Nos. 10,202,700; 9,163,318; 9,118,082; and 4,603,118; which are each incorporated herein by reference in their entirety.
While additional challenges must be overcome in order to realize practical PtII-catalyzed methane conversion,15 it is believed that the electrochemical approach developed here will enable continued progress toward practical technologies for aerobic methane valorization.
The present disclosure relates to a process for oxidizing a compound, comprising:
wherein the anion is chloride, fluoride, bromide, iodide, a carboxylate, nitrate, perchlorate, phosphate, or sulfate;
the initial concentration of PtII species is about 1 mM to about 10 M;
R1 is C1-C20 alkyl, C3-C12 cycloalkyl, C5-C10 heterocyclyl, C6-C12 aryl, or C5-C12 heteroaryl; and
R2 is H, —OH, —C(═O)H, or —C(═O)OH.
In some embodiments, the electrical potential is applied; and the electrical potential is adjusted to maintain the concentration of PtII species at about 95% to about 105% of the initial concentration.
In certain embodiments, electrical current is applied; and the electrical current is adjusted to maintain the concentration of PtII species at about 95% to about 105% of the initial concentration.
In some embodiments, the reaction mixture is contained within a reaction vessel comprising a working electrode, and a counter electrode, and, optionally, a reference electrode.
In certain embodiments, the reaction vessel further comprises a PtII sensing electrode.
In some embodiments, the reaction vessel is a flow reaction vessel.
In certain embodiments, the concentration of PtII species is measured potentiometrically.
In some embodiments, the concentration of PtII species is measured with a PtII sensing electrode. For example, the PtII sensing electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, a Pt wire electrode, and a platinized Pt/H2 electrode.
In certain embodiments, the working electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, an Hg/HgSO4 electrode, an Ag/AgCl electrode, a Pt wire electrode, a platinized Pt/H2 electrode, a calomel electrode, a fluorine-doped tin oxide electrode, an indium-doped tin oxide electrode, a glassy carbon electrode, a carbon black electrode, a pyrolytic graphite electrode, a graphite electrode, a carbon nanotube electrode, and a boron-doped diamond electrode. For example, the working electrode is a Pt foil electrode.
In some embodiments, the reference electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, an Hg/HgSO4 electrode, an Ag/AgCl electrode, a Pt wire electrode, a platinized Pt/H2 electrode, a calomel electrode, a fluorine-doped tin oxide electrode, an indium-doped tin oxide electrode, a glassy carbon electrode, a carbon black electrode, a pyrolytic graphite electrode, a graphite electrode, a carbon nanotube electrode, and a boron-doped diamond electrode. For example, the reference electrode is an Ag/AgCl electrode.
In certain embodiments, the counter electrode is selected from the group consisting of a Pt foil electrode, a Pt mesh electrode, an Hg/HgSO4 electrode, an Ag/AgCl electrode, a Pt wire electrode, a platinized Pt/H2 electrode, a calomel electrode, a fluorine-doped tin oxide electrode, an indium-doped tin oxide electrode, a glassy carbon electrode, a carbon black electrode, a pyrolytic graphite electrode, a graphite electrode, a carbon nanotube electrode, and a boron-doped diamond electrode. For example, the counter electrode is a Pt mesh electrode.
In some embodiments, the counter electrode is immersed in a solution of an electron acceptor. For example, the electron acceptor is a proton or vanadyl sulfate. Alternatively, the counter electrode is an oxygen-consuming electrode.
In certain embodiments, the anion is chloride, fluoride, acetate, nitrate, perchlorate, phosphate, or sulfate. For example, the anion is chloride.
In some embodiments, the chloride is a constituent of a salt selected from the group consisting of NaCl, KCl, LiCl, CsCl, RbCl, MgCl2, CaCl2, BaCl2, NH4Cl, and HCl. For example, the salt is NaCl.
In certain embodiments, the source of PtII species is selected from the group consisting of K2PtCl4, Na2PtCl4, Li2PtCl4, H2PtCl4, (NH4)2PtCl4, K2PtBr4, Na2PtBr4, Li2PtBr4, H2PtBr4, (NH4)2PtBr4, K2Pt(CN)4, Na2Pt(CN)l4, Li2Pt(CN)4, H2Pt(CN)4, (NH4)2Pt(CN)4, K2PtCl6, Na2PtCl6, Li2PtCl6, H2PtCl6, (NH4)2PtCl6, Pt(NH3)4Cl2, Pt(NH3)4(NO3)2, Pt(NH3)4(OH)2, Pt(NH3)4Cl4, and PtO2. For example, the source of PtII species is K2PtCl4.
In some embodiments, the Bronsted acid is selected from the group consisting of H2SO4, HCl, HNO3, H3PO4, HClO4, and a carboxylic acid. For example, the Bronsted acid is HCl.
In certain embodiments, the temperature is about 20° C. to about 500° C. For example, the temperature is about 150° C. to about 300° C.
In some embodiments, electrical current is applied at a constant current.
In certain embodiments electrical potential is applied under constant potential conditions.
In some embodiments, the compound of formula R1-R2 is an alkane or a cycloalkane.
In certain embodiments, the compound of formula R1-R2 is methane.
In certain embodiments the compound of formula R1-R2 is oxidized to an alcohol. For example, the alcohol is a diol or a polyol.
Although methods and materials similar or equivalent to those described herein can be used in the practice of the present invention, suitable methods and materials are described below. The invention is further described in the following examples, which do not limit the scope of the invention described in the claims.
Chemicals and Materials
Potassium tetrachloroplatinate (K2PtCl4, 99.9% metals basis) was purchased from Strem chemicals. Sodium hexachloroplatinate hexahydrate (Na2PtCl6.6H2O, 31.3% Pt), platinum foil (0.025 mm thick), mesh wire (99.9% metals basis), and silver wire (1.0 mm dia., 99.999%) were purchased from Alfa Aesar. Glassy carbon disk (3 mm dia.) and platinum disk (2 mm dia.) electrodes and Hg/Hg2SO4 (in sat. K2SO4; 0.64 V vs SHE) reference electrode were purchased from CH Instruments. Fluorine-doped tin oxide (FTO) (TEC15, ˜7 Ω/sq) was purchased from Hartford Glass Co. Inc. (Hartford City, Ind.). Nafion 117 (178 μm thick) and Nafion HP (20 μm thick; PTFE-reinforced) were purchased from Ion Power Inc., and polybenzimidazole membranes (55 μm thick) were purchased from PBI Performance Products Inc. Ceramic fritted glass tubes for the home-made double-junction Ag/AgCl reference electrode were purchased from Pine Instruments. Methane (UHP GR 4.0) was purchased from Airgas. All solutions were prepared with ultrapure water (Milli-Q Type 1; resistivity=18 MΩ-cm).
Electrochemical Methods
Electrochemical experiments were performed using a Biologic VMP3 or CHI760E potentiostat, with the latter showing more stable responses for high temperature experiments. Glassy carbon and platinum disk electrodes were polished successively with 1 μm, 0.3 μm, and 0.05 μm alumina slurry on a soft polishing cloth, with >5 min. of sonication in Milli-Q water in between. At room temperature, the counter compartment was separated from the working solution by a Nafion 117 (˜180 μm thick) membrane and a Pt mesh was used as the counter electrode. Room temperature cyclic voltammetry and bulk electrolysis were performed under ambient conditions.
All potential values in the manuscript are referenced to the Standard Hydrogen Electrode (SHE). Current values were reported as current densities in most cases, normalized by the surface area of the electrode. For glassy carbon and FTO electrodes, the geometric surface areas were used. For Pt electrodes, the electrochemically active surface area (“real surface area”) was determined by integrating the hydrogen underpotential deposition (H-UPD) region and dividing by the known capacitance for surface-adsorbed H (210 μC/cm2) (
High-Temperature Electrochemistry
The reactor and its operation. A modified Parr reactor (
While omitted in the schematic diagram of the reactor (
This was done in order to reduce the working solution volume for easier stirring and less amount of Pt salts needed. A custom-made PTFE piece was placed between the glass liner and the glass cell to fill the void space between the two and hold the working electrode, reference electrode and counter compartments in their respective positions. At the end of a high-temperature experiment, the solution volume decreased from 23 mL to 18-20 mL from evaporation and droplets of liquid condensed on the inner surfaces of the reactor. These were collected separately in the analysis (see below).
To set up the reactor, the working solution (23 mL) and counter solution were first degassed with Ar or N2. After the various parts of the reactor were assembled and the reactor sealed, the headspace was purged with Ar or N2 by three vacuum-refill cycles. For EMOR, the headspace was filled at room temperature with 500 psi of methane with at least three pressurization-vent cycles.
The solution was constantly stirred at 200 rpm with a spinfin stir bar, which has the advantage of having a relatively stationary footprint. Since the reactor walls prevented visual confirmation of effective stirring, the following procedure was used to ensure convective transport in all reactor runs: after reactor assembly and setup, the electrode was polarized at 1.06 V vs SHE and the chronoamperometric trace was recorded. Then, stirring rate was gradually increased to 200 rpm. If the current increased due to convective mass transport (e.g.,
After confirming stirring, the reactor was placed in an oil bath and heated to 130° C. The actual CH4 pressure during reactor operation (130° C.) is estimated at 675 psi according to the ideal gas law. During heating, the open circuit potential of the electrode was monitored and showed a steady and reproducible increase (
The working electrode (WE). A platinum wire or a platinum foil (for measurements of PtII electro-oxidation faradaic efficiencies and EMOR) was used as the working electrode. They were cleaned before and after each experiment by several cycles of potential sweep in 0.5 M H2SO4 between 1.14 V and −0.4 V vs SHE until a reproducible cyclic voltammogram was obtained with characteristic hydrogen underpotential deposition and surface oxide formation features. Generally, little change was observed before and after each experiment (
The reference electrode (RE). For the reference electrode, a double-junction Ag/AgCl reference electrode was used. A clean silver wire (1.0 mm dia., 99.999%) was polished with fine-grit sandpaper and sonicated in 3% HNO3 and Milli-Q water for 10 min. each. Then, it was galvanostatically oxidized at 10 μA/cm2 for >24 hr in 0.5 M H2SO4 and 10 mM NaCl, with a graphite counter electrode separated from the working solution by a Nafion 117 membrane. The resulting AgCl-coated wire was encased in a glass tube closed at one end with a ceramic frit, which was encased in another larger glass tube with a ceramic frit tip. The potential of the reference electrode fabricated as such was −0.333 V vs Hg/Hg2SO4, or +0.307 V vs SHE at room temperature. Potentials at high temperature was also converted to the SHE scale by adding 0.307 V. While redox potential can vary with temperature,40 it was observed that using this conversion value leads to background Pt H-UPD wave potentials that coincide between room and 130° C. data (
The counter electrode (CE) and counter compartment. The counter electrode was a Pt mesh separated from the working solution with H+-conducting membranes. For the EMOR trials that took several hours and had considerable amount of charge passed, it was necessary to prevent the reduction of H+ to H2 at the counter electrode because H2 was found to diffuse into the working solution and reduce the Pt ions to metallic Pt0. ˜3 M of vanadyl sulfate ((VIVO)(SO4)) was dissolved into the counter compartment electrolyte (0.5 M H2SO4 and 10 mM NaCl) to function as a surrogate electron acceptor; the blue VIVO2+ ions are reduced to VIII ions at potentials more positive of H+ reduction, thus functioning as the terminal oxidant in the system. With the vanadyl ions, no H2 was detected in the headspace GC analysis.
As for the H+-conducting membrane that separates the working and counter solutions, the temperature and presence of Pt ions necessitated the simultaneous employment of two materials. The H+-conducting membrane stack consisted of alternating layers of Nafion HP (˜20 m thick, PTFE-enhanced) and polybenzimidazole membranes; the Nafion is chemically stable towards Pt ions, but has a low operation temperature (up to around 80° C.). Specifically, the glass transition temperature of Nafion is 110° C.,41 and at 130° C. loss of ionic conductivity for the thicker Nafion 117 or slow electrolyte leakage for the thinner Nafion HP was observed. On the other hand, the polybenzimidazole retains its performance at high temperature, but having aryl C—H bonds that can be activated by PtII, seemed to be reactive towards Pt ions so that Pt ions deposit as black Pt0 on the membrane (
Carbon electrodes. Carbon looked promising at first, as a glassy carbon electrode shows a clear electrochemical oxidation wave in the presence of PtII ions (
In order to rationalize these observations, the following hypothesis was put forth, based on the series of CV acquired successively (
As for the current decay at the higher potential of 1.39 V (
From the fact that deactivation was partially reversed upon negative polarization of the electrode to ˜0.5 V vs SHE (
For carbon electrodes, bulk electrolysis was also attempted. In spite of the decay in current density during oxidation, if a high-surface area carbon electrode (e.g. graphite felt or carbon paper) was used, bulk conversion of PtII ions to PtIV ions could be achieved (
Fluorine-doped tin oxide (FTO) electrodes. FTO is a cheap and commonly used electrode material with optical transparency and high chemical stability. In particular, it has been shown to be remarkably robust in highly acidic and oxidizing environment.2 Therefore, the ability of FTO to effect electrochemical oxidation of PtII to PtIV was investigated. As shown in
Pt Electrodes. Additional Information for the Interpretation of PtII CVs.
Is the amount of dissociated Cl− enough to suppress oxide formation? The PtIICl42− ion undergoes slow acid hydrolysis in aqueous solutions with a rate constant of 4×10−5 s−1.0.5 Therefore, after 5 min., a freshly prepared 1 mM K2PtIICl4 solution will have generated 0.01 mM Cl−, and after an hour, 0.13 mM. Such a small concentration of Cl− turns out to be enough to suppress oxide formation, especially at lower potentials (
Suppression of PtIV reduction in the presence of 10 mM Cl−. It has been argued above, from the suppression of oxide formation, that the surface of Pt electrode adsorbs Cl− from hydrolysis of the PtIICl42− ions even without additional Cl− ions. However, the PtIV reduction wave was suppressed only when more Cl− was added. This is because Cl− adsorption depends on the electrode potential. The Cl− adsorption isotherm determined with radioactive Cl− (
As a side note, PtIV reduction was observed if more negative potentials were reached (
While Cl− ions adsorb to Pt electrodes and suppress oxide formation, at high potentials (above 1.1 V vs RHE) oxide formation resumes (
Acquisition of current-overpotential relationship (Tafel plot). The raw data for the Tafel plot is shown in
Assessment of Pt0-catalyzed non-electrochemical oxidation of CH3OH.
To account for oxidation of CH3OH catalyzed by PtII alone, control experiments were performed in parallel with ampules that do not contain the metallic Pt pieces. Following 3 hours at 130° C., 2.8±0.2 mM CH3OH was oxidized in the presence of Pt metal, whereas the same amount, 2.7±0.4 mM, was oxidized in the absence of metallic Pt. Therefore, it was concluded that Faradaic or non-Faradaic overoxidation of the methanol product is negligible on Pt electrodes during EMOR.
In order to select the electrolyte environment for carrying out the proposed EMOR, the effect of electrolyte composition on the catalytic activity of PtII for methane oxidation and undesired methanol oxidation was explored.
Choice of acid and the effect of its concentration. The pH of the solution should be acidic in order to prevent hydrolytic degradation of the platinum ions at elevated temperatures, unless there is a high concentration of Cl−.30 Conveniently, the low pH requirement (i.e. high concentration of H3O+ ions and conjugate base anions) automatically makes the solution electrically conductive, which is a prerequisite for electrochemistry. Sulfuric acid was chosen as in many other works because it is chemically very stable and low-cost, and are expected to interfere minimally with the C—H activation step of PtII.51 The concentration of sulfuric acid showed a small yet measurable effect on the rate of PtII-catalyzed oxidation of methane to methanol and further oxidation of methanol (
Effect of chloride concentration. The presented data highlight that Cl− is essential for inhibiting CH3OH oxidation at the Pt surface. However, Cl− ions are also known to inhibit the C—H activation step in
Therefore, initially (before the information about electrode passivation by Cl− was obtained) attempts were made to decrease [Cl−] as much as possible by exploiting the fact that the electrochemical oxidation of PtII to PtIV proceeds even in the absence of extra Cl−. A solution of 3 mM PtII and 7 mM PtIV having a net “negative” Cl− concentration was prepared by generating the PtIV ions by bulk electrolysis of a solution of K2PtIICl4 without any added Cl− (cf. PtIV from Na2PtIVCl6 has two more Cl− than PtII). With the expectation that this will increase the fraction of PtIICl2(H2O)2 among the PtII ions in the solution and accelerate the overall rate for methane oxidation, the solution was tested for reaction with methane. Surprisingly, this solution actually showed slower production of methanol compared to a solution containing equal concentrations of PtII and PtIV but PtIVCl62− as the PtIV ions (
On the other hand, adding in 10 mM of Cl− decreased the rate of methane-to-methanol conversion only slightly. The reduction in the overall rate was more pronounced at 100 mM of Cl−, but was less than an inverse first order, as the rate decreased by only ˜⅓ for a 10-fold increase in [Cl−]; the reaction order in Cl− depends on the range of [Cl−].55 The effect of [Cl−] on methanol oxidation (
Given the 100% Faradaic efficiency of PtII/IV oxidation at the Pt electrode, the molar rate of PtII oxidation, rox, is directly proportional to the applied current (i):
where F is Faraday's constant and V is the volume of the reaction solution. The denominator contains a factor of 2 to account for the two electrons required for each PtII/IV oxidation reaction. The rate of methane oxidation catalysis, rcat, is first-order in [PtII]:
r
cat
=k
obs[PtII] (2)
where kobs is the observed pseudo-first order rate constant under the CH4 pressure and temperature conditions employed. For every catalytic turnover, an equivalent of PtIV is reduced to PtII, and, thus, rcat has a positive contribution to d[PtII]/dt. On the other hand, rox has a negative contribution to d[PtII]/dt. Overall, the following is obtained:
For a fixed value of applied current, rox is time-invariant, thus integration yields:
[PtII]=Cek
Upon solving for the integration constant C using the initial conditions, the following is obtained:
[PtII]=([PtII]t=0−rox/kobs)ek
If rox exactly equals the rate of PtII-catalyzed C—H functionalization (rox=kobs[PtII]t=0), the time-dependent exponential term in equation 6 will go to zero and [PtII] will remain constant over time. However, even very small differences between rox and kobs[PtII]t=0 will result in a non-zero exponential term that will cause the [PtII] and, thus, the PtII:PtIV ratio, to rapidly deviate from its initial value over time. If rox is constantly re-adjusted to match rcat, however, [PtII] can be maintained at a steady-state. Therefore, these equations highlight the need to constantly modulate the rate of PtII electro-oxidation, rox.
The required η is determined by the current density (j) required for steady-state catalysis, and j equals the required current (i) divided by the electrode area (A). Since i depends on the reactor solution volume (V) and the catalytic rate constant (kobs) (eq. 1 and 4 above), the magnitude of η will also depend on these parameters.
While enlarging A will decrease η and, thus, the electrical energy input, it will also increase electrode cost and may increase the rate of parasitic Pt0-catalyzed CH3OH oxidation. It is important to underscore that in the disclosed reactors, η was quite small (<50 mV) even when the electrode was sufficiently small as to observed negligible surface-mediated CH3OH oxidation (see Assessment of Pt0-catalyzed non-electrochemical oxidation of CH3OH in Section 2). Also, because the rate of PtII electro-oxidation at any η is proportional to [PtII], the [PtII] can be increased to increase the overall rate of catalysis without requiring additional overpotential.
As stated above, PtII decomposes to Pt0 when the oxidant, PtIV, is depleted. In the reactors where the PtII:PtIV ratio was constantly monitored and controlled, no Pt0 was visible in the well-stirred portion of the working solution. However, in areas of poor convection, Pt0 formation was observed. First, there were some specks of grey Pt0 on the upper parts of the glass cell wall where droplets of the reaction solution had splashed (
The amount of Pt ions that deposited as Pt0 was calculated from the difference in total μmol of PtII and PtIV ions before and after the reaction. Dividing this by the initial μmol of Pt ions, obtain the % loss of Pt ions for each EMOR trial can be obtained, as shown in Table 2 and the concentration scale-up trial in Table 5. As shown in Table 6, the amount of the irreversible Pt0 deposition increases with increasing reactor operation time. The higher concentration trial showed negligible Pt0 loss, which may be due to the higher PtIV concentration overall.
Explanation for normalization of product concentration by iave in
Concentration scale-up trial. All of the EMOR reactor experiments reported in this disclosure were done with identical reaction solution composition ([PtII]=3 mM, [PtIV]=7 mM, [Cl−]=10 mM). In order to gain further understanding of the system, it was attempted to scale up the concentrations of all species (PtII, PtIV and Cl−) by 5 times. The reactor was run for 10.5 hr for straightforward comparison with a reactor run for the same length of time with the default concentrations. The result is shown in Table 5, and here are some differences that were observed for the higher concentration trial:
Simulation details. The concentrations of various methane oxidation products were calculated numerically with the simple mechanism in
Δt was set to 0.0093 hr, a sufficiently small value that showed no difference in the simulation when it was increased or decreased. With the given pressure and temperature, [CH4] was set to 44 mM.57 This is an approximate value because the equation for calculating Henry's constant at different temperatures was only validated in the range T=273-361 K, while the reactor was run at 403 K. PCH4 and [CH4] was considered to be constant throughout the reactor run because the amount of methane that was converted to products in the EMOR reactors (<400 μmol for the longest reactor run) was negligible compared to the amount of methane in the large headspace (˜200 mmol). Then, the parameters k1-k6 were adjusted until a good fit with experimental reactor data was achieved. The fitted parameters are given in Table 4. To emphasize, the fitted parameters are not true rate constants but apparent values, and that they are crude estimations as the data-to-parameter ratio is low and the reaction mechanism (
CH4 vs. CH3OH oxidation. In the literature, there are two cases that explicitly report experimentally assessed selectivity of aqueous PtII chloride salt for CH4 vs CH3OH (which is not necessarily identical to the selectivity of RCH3 vs RCH2OH). The experiments were reproduced by the inventors. The different relative rates are summarized in Table 7. Parenthetically, a model PtII complex in trifluoroethanol, (N—N)PtII(CH3)(TFE) (N—N═ArN=C(CH3)—C(CH3)═NAr, TFE=trifluoroethanol), showed relative rates of C—H activation of kCH4/kCH3OH=0.77.12
aAuthors mention possibility of Pt0 formation during the reaction.
For the estimation experiment, two identical high-pressure NMR tubes were charged with the same solution of 3 mM PtII+7 mM PtIV in the 0.5 M H2SO4+10 mM NaCl electrolyte. One contained 7.5 mM of CH3OH while the other did not. The tube without CH3OH (Tube 1) was pressurized with 100 psi of CH4, while the tube containing CH3OH (Tube 2) was pressurized with 100 psi of Ar. Another heavy-walled NMR tube was charged with blank electrolyte containing internal standards and pressurized with 100 psi of CH4 (Tube 3). The three heavy-walled NMR tubes were heated together in an oil bath for 1 hr and 10 min at 130° C., then quantitated for the amount of CH3OH and compared with the initial CH4 or CH3OH concentration. The initial CH4 concentration in Tube 1 was estimated from Tube 3, which showed [CH4]=8.6 mM before heating and 5.7 mM after heating due to reduced solubility of methane at elevated temperatures; it is difficult to determine the exact CH4 concentration in Tube 1 because the constricted geometry of the tube slows down gas/liquid equilibration. As the table shows, from 5.7-8.6 mM of methane 1.1 mM of net methanol formation was observed (13-19% of initial CH4), and from 7.5 mM of methanol 1.2 mM of net oxidation was observed (16% of initial CH3OH). Taking the ratio of the relative reacted amounts, kCH4/kCH3OH is estimated to be 0.8-1.2. These results are summarized in Table 8.
CH3OH, CH2(OH)2, HCOOH oxidation. Sealed glass ampules containing solutions of PtII, PtIV and the different substrates in 0.5 M H2SO4+10 mM NaCl electrolyte were heated at 130° C. The decrease in substrate concentrations for different time duration are compared in Table 9.
a For CH2(OH)2, some CH3OH was present initially because they were added as a polymerization inhibitor in the concentrated formaldehyde bottle.
Possible explanations for the discrepancy between the rates. The rate constants derived from simulation and stoichiometric reactions outside the EMOR reactor are all apparent or observed rate constants (kobs) which are extrinsic values that depend on the reaction conditions employed. While this precludes a direct comparison between the two sets of rate constants, comparison of the ratios of these rate constants, i.e. selectivities, can be made.
The comparison shows that the selectivity of PtII for CH4 over CH3OH was similar (k1/k2=0.8-1.2 vs. 0.6 for EMOR-simulated vs. non-EMOR estimation), but rates of further oxidation of CH3OH showed greater discrepancies (k2/k3=0.2 vs.>1 and k3/k4=0.2 vs>>1). These differences may point to Pt0-catalyzed oxidation of CH2(OH)2 and HCOOH. While Cl-adsorption effectively suppresses the oxidation of CH3OH, it is unknown whether it will be equally effective in suppressing the oxidation of CH2(OH)2 and HCOOH. This implies that the simulation-derived rate constants for PtII-catalyzed oxidation of CH2(OH)2 and HCOOH may have been overestimations.
As explained earlier, methane oxidation products in the reactor freely migrate to other parts of the reactor such as the reference and counter compartments via vaporization. The counter compartment contained a high (3 M) concentration of vanadyl sulfate, which has an oxidation potential capable of oxidizing methanol. It is, therefore, important to estimate the degree of product oxidation, if any, that occurs due to the vanadyl ions. The reactor was set up in the usual way except that the working solution was blank electrolyte spiked with 4.6 mM of CH3OH (total 105 μmol) without any Pt ions. The cell was pressurized with CH4 as usual and heated at 130° C. for 37 hr. After 37 hr, 0.7 μmol of CH2(OH)2 and 2.5 μmol of CO2 were recorded. This amounts to ˜0.004 μmol of CH2(OH)2 and ˜0.015 μmol of CO2 from 1 mM of CH3OH per hour. From this, it is estimated that ˜2% of the total CH2(OH)2 and ˜10% of the total CO2 formed in the EMOR reactors may be attributed to oxidation by vanadyl ions in the counter compartment. As both CH2(OH)2 and CO2 are minor products in the reactor trials, this contribution in the analysis was ignored.
Faradaic efficiency (FE) is defined by the mols of product of electron transfer divided by the mols of electrons that were passed through the circuit.
Bulk electrolysis of PtII to PtIV at 130° C. 22 or 23 mL solutions of 5 mM of K2PtCl4, 5 mM Na2PtCl6 and 10 mM NaCl in 0.5 M H2SO4 were oxidized with stirring at a Pt foil working electrode. A pure PtII solution was not used because of its tendency towards disproportionation and Pt0 precipitation at elevated temperatures. The [PtIV] at the end was measured by UV-Vis spectroscopy to calculate the μmol of PtIV generated (ΔPtIV). The [PtIV] at the end was measured by UV-Vis spectroscopy to calculate the μmol of PtIV generated (ΔPtIV) using the following equation: FE=2*ΔPtIV/(μmol of e−).
At the three different potentials that were tested, the Faradaic efficiencies were ˜100%. See Table 1 for the results. That additional error arises from the difficulty of measuring the solution volume (reduced due to evaporation within the reactor) accurately after the reaction.
Faradaic efficiency of EMOR reactors. In the presence of methane, PtIV in the solution is consumed by reacting with methane or products from methane oxidation. The overall Faradaic efficiency was calculated by summing up the μmols of the methane oxidation products multiplied by the number of oxidized equivalents according to
FE=2*(nCH3OH+nCH3Cl+2*nCH2(OH)2+3*nHCOOH+4*nCO2+ΔPtIV)/ne−
where ni denotes the mols of species i. Solutions in the working compartment, in the reference electrode compartment, and droplets condensed on the inner surfaces of the reactor were separately collected and analyzed by NMR to determine the concentrations of CH3OH, CH2(OH)2 and HCOOH. These were multiplied by the respective solution volumes, and combined. As noted above, NMR quantitation of the counter compartment solution could not be carried out due to the high concentration of paramagnetic vanadium species. The headspace gas was analyzed for CH3Cl and CO2 (vide supra). The result is shown in Table 3.
Because the FE for PtII electro-oxidation is ˜100%, the FE for the EMOR reactors should also be ˜100%. Indeed, close to 100% FE values were observed. The missing FE may be accounted for by the products in the counter compartment that were not quantitated. Also, a significant margin of error is expected, as there are several sources of potential errors, e.g. NMR and GC measurements, solution volume estimation, possible deviation of gas solubility from that in pure water, etc.
Quantitation of the PtII and PtIV ions was performed with UV-vis spectroscopy (Cary 50, Agilent). PtIVCl62− ions in aqueous solutions show a strong absorption at 262 nm, where PtIICl42− ions absorb little.5 PtIICl42− ions show an absorption maximum at 214 nm, but this peak is often covered up under the broad absorbance of PtIVCl62− in mixed solutions.* On the other hand, the total concentration of Pt ions could be determined by reaction with SnCl243 which gives rise to a strong absorbance at 404 nm. Therefore, the concentration of PtII and PtIV was determined by measuring the absorbance at 262 nm and the total concentration using the following equation, where εPtIV and εPtII denote the extinction coefficients of PtII and PtIV at 262 nm, and d denotes the dilution factor:
[PtIV]=(A262 nm/d−εPtIV*[Pt]total)/(εPtIV−εPtII)
[PtII]=[Pt]total−[PtIV]
Importantly, PtIICl42− and PtIVCl62− ions undergo hydrolysis over time, and the species with less Cl− coordination has different values of extinction coefficient. Therefore, each sample was diluted in 1 M HCl and irradiated with a 4W UV lamp (252 or 365 nm) for >5 min. for complete anation prior to measurement of the 262 nm absorbance.44
Determination of ε for [Ptn]total. Both PtII and PtIV (PtIV is reduced to PtII prior to complexation) undergo complexation with SnIICl3− to give a strong orange-red color.45 For accurate determination of the extinction coefficient at 404 nm, a Beer's plot was constructed with solutions of PtII and PtIV whose Pt concentrations were determined by ICP-MS. Stock solutions of PtII and PtIV were prepared from K2PtCl4 and Na2PtCl6, respectively. These stock solutions were diluted to three different concentrations with a 1 M SnCl2+3 M HCl solution and reacted for >5 min. The background subtracted absorbance was then plotted against the concentration determined by ICP-MS to give the extinction coefficient (
Determination of εPtIV and εPtII at 262 nm. Freshly prepared stock solutions of K2PtCl4 and Na2PtCl6 were serially diluted in 1 M HCl and measured (
*Note I: During the course of the work, it was discovered that the second absorption maximum of PtIICl42− ions at 230 nm, though lower in extinction coefficient (7.2×103 cm−1 M−1),46 is suitable for determination of [PtII] because absorption by PtIVCl62− ions hits a minimum at this wavelength. An alternative quantitation protocol that uses the absorbance at 230 nm and 262 nm showed identical results to the protocol described above that uses absorbance at 262 nm and absorbance at 404 nm from the SnCl3− complex of Pt ions.
**Note II: The exact value of the extinction coefficients may slightly vary from the true values as the spectrometer was not calibrated with external standards (e.g. εPtIV at 262 nm=2.45×103 M−1 cm−1 according to ref. 47). However, this does not compromise the validity of the results because the same spectrometer was used throughout the studies and linearity of response was confirmed in the absorbance range (A=0.1-0.7) where measurement was carried out.
CH3OH, CH2(OH)2 and HCOOH. These solution-phase products were determined by NMR (Varian 500 MHz or Bruker 500/600 MHz instruments) with various solvent suppression techniques to suppress the H2O peak (presaturation, excitation sculpting or wet). The sample solution was mixed with 25 vol % of D2O solution containing acetic acid internal standard (caution: prolonged storage of this internal standard solution compromises the measured concentration via slow H/D exchange of CH3COOH in D2O), then adjusted to ˜2 M total acid concentration by the addition of 8 M D2SO4. This was done because the peak position of CH2(OH)2 (hydrated form of formaldehyde, which is the predominant form in 0.5 M H2SO4) was close to that of the solvent water; lowering the pH shifted the water peak more downfield and allowed us to observe and integrate the CH2(OH)2 peak (representative spectrum in
For determining methane oxidation products in the reactor, solutions were collected from the working compartment, reference compartment, and droplets condensed on the inner walls. All of them contained some product because the high temperature of the reactor causes product migration via vaporization. The extent of migration was greater with longer reactor operations. As for small amounts of products in the counter compartment (counter compartment volume (3 mL)<<working compartment volume (23 mL)), the high concentration of paramagnetic vanadyl ions precluded their determination by NMR and were therefore excluded.
CO2 and CH3Cl. These gaseous products were determined by gas chromatography (GC) measurement (SRI instruments, model 8610C) of the reactor headspace gas after the reactor has cooled down to room temperature. CO2 was calibrated by serial dilution of a commercial calibration gas (Product no. X08AR98C33A0000, Airgas) with Ar (
In order to assess reaction rates of non-electrochemical catalysis by PtII (i.e. PtIV are stoichiometric oxidants and no re-oxidation of PtII occurs), solutions of PtII+PtIV were heated in the presence of substrate in heavy-walled NMR tubes or glass ampules.
To measure the rate of methane oxidation, heavy-walled NMR tubes (Norell, item no. S-5-500-HW-7) were charged with solutions of PtII and PtIV, pressurized to 100 psi of methane, and manually agitated for >2 min. to allow gas-liquid mixing and dissolve methane. The tubes were placed in a stirred oil bath and heated to 130° C. After a set time (typically ˜1.5 hr), the tubes were cooled down and the solution was withdrawn and analyzed by NMR.
To measure the oxidation rate of methanol, formaldehyde, and formic acid, solutions of PtII and PtIV containing the substrate were flame-sealed in scored glass ampules (Kimble Chase, 1 mL, item no. 12010L-1), placed in an aluminum heating block with silicone oil, and heated to 130° C.
A flow electrochemical reactor is an advantageous system, which can be utilized in electrochemical oxidation of methane in view of the presently disclosed process for maintaining catalytically active PtII in the methane oxidation system over prolonged periods of time. The necessary aspects of the flow electrochemical reactor involve:
A schematic representation of an appropriate electrochemical flow reactor design is shown in
All of the U.S. patents and U.S. patent application publications cited herein are hereby incorporated by reference. In case of conflict, the present specification, including definitions, will control.
Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.
This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/819,046, filed Mar. 15, 2019.
Number | Date | Country | |
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62819046 | Mar 2019 | US |