Patent Application
20250128203
Efficient carbon dioxide (CO2) separation technologies are essential for mitigating climate change. Electrochemically mediated carbon capture (EMCC) using redox-tunable sorbents has emerged as a promising alternative to traditional thermochemical methods due to its versatility and energy efficiency. The undesirable linear free-energy relationship between redox potential and CO2 binding affinity in existing EMCC chemistries, however, makes it fundamentally challenging to optimize key sorbent properties independently via chemical modifications. Thus, new redox-tunable sorbents for CO2 separation technologies are needed to address these challenges.
In some aspects, the presently disclosed subject matter provides a compound of formula (I):
wherein:
In certain aspects, the electron donating group is selected from alkoxide, hydroxyl, alkoxyl, amine, amide, ester, and alkyl. In certain aspects, the electron withdrawing group is selected from halogen, cyano, nitro, haloalkyl, ammonium, carbonyl, and sulfonyl.
In some aspects, R1 and R2 are each independently selected from H and branched or straightchain C1-C8 alkyl; R3, R4, and R6 are each independently selected from H, branched or straightchain C1-C4 alkyl, halogen, C1-C4 alkoxyl, nitro, and —C(═O)—OR7, wherein R7 is branched or straightchain C1-C4 alkyl; R5 is selected from H, branched or straightchain C1-C4 alkyl, halogen, C1-C4 alkoxyl, nitro, —C—OR8, wherein R8 is selected from an amino acid, a substituted amino acid, and —NHR9, wherein R9 is —CHR10—C(═O)—O—R11, wherein R10 is H or —CH2—OH, and Ru is branched or straightchain C1-C4 alkyl, and —C(═O)—OR12, wherein R12 is selected from H and branched or straightchain C1-C4 alkyl; provided that (i) at least one of R1, R2, R3, R4, R5, and R6 is not H, and (ii) R4 and R5 cannot both be Br at the same time if R1, R2, R3, and R6 are each H; and acceptable salts thereof.
In certain aspects, R1 and R2 are each H. In other aspects, R1 and R2 are each branched or straightchain C1-C8 alkyl. In particular aspects, R1 and R2 are each —CH2CH2—CH3 or —CH2CH(CH2CH3)(CH2CH2CH2CH3).
In certain aspects, one or more of R3, R4, R5, and R6 is selected from branched or straightchain C1-C4 alkyl, C1-C4 alkoxyl, halogen, nitro, and —C(═O)—OR12, wherein R12 is H or branched or straightchain C1-C4 alkyl.
In certain aspects, (i) R3 and R6 are each the same and are not H; (ii) R4 and R5 are the each same and are not H; or (iii) R3, R4, R5, and R6 are each the same and are not H.
In certain aspects, (i) R3 is halogen and R4, R5, and R6 are each H; (ii) R4 is halogen and R3, R5, and R6 are each H; (iii) R5 is —C(═O)—OR12, wherein R12 is H or branched or straightchain C1-C4 alkyl, R3 and R6 are each H, and R4 is H or halogen; or (iv) R6 is nitro and R5 is H, R3 is H or C1-C4 alkoxyl, and R4 is H or —C(═O)—OR12 wherein R12 is H or branched or straightchain C1-C4 alkyl.
In particular aspects, the compound of formula (I) is selected from:
In other aspects, the compound of formula (I) is selected from:
In certain aspects, the compound of formula (I) comprises a compound of formula (Ia):
wherein: R8 is —NHR9, wherein R9 is —CHR10—C(═O)—O—R11, wherein R10 is H or —CH2—OH, and R11 is branched or straightchain C1-C4 alkyl. In particular aspects, the compound of formula (Ia) is selected from:
In certain aspects, the presently disclosed subject matter provides a sorbent material comprising a compound of formula (I), or in certain aspects, a compound of formula (Ia).
In other aspects, the presently disclosed subject matter provides a process for capturing CO2 from a gas sample, the process comprising contacting the gas sample with a compound of formula (I), or a sorbent material comprising a compound of formula (I).
In certain aspects, the process comprises an electrochemically mediated carbon capture (EMCC) process. In particular aspects, the process comprises (i) electro-reduction of the sorbent to form an adduct with CO2; and (ii) oxidising the adduct to liberate CO2 regenerate the sorbent.
In certain aspects, the gas sample is selected from ambient air, i.e., direct air capture, ventilated air, and a stream of gas, e.g., flue gas or other emissions from a point source, such as an industrial source, such as a chemical plant, petroleum production facility, a cement plant, power generation, and transportation, and from enclosed spaces, for example, in buildings and in the cabin space of cars, trains, airplanes, spacecraft, submarines, and the like.
In certain aspects, the process is a fixed-bed process or a flow-based process.
In other aspects, the presently disclosed subject matter provides a system for separating carbon dioxide from ambient air or a stream of gases, the system comprising the compound of formula (I) or a sorbent material comprising a compound of formula (I).
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.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
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:
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.
In some embodiments, the presently disclosed subject matter provides a compound of formula (I):
wherein:
In certain embodiments, the electron donating group is selected from alkoxide, hydroxyl, alkoxyl, amine, amide, ester, and alkyl. In certain aspects, the electron withdrawing group is selected from halogen, cyano, nitro, haloalkyl, ammonium, carbonyl, and sulfonyl.
In particular embodiments, R1 and R2 are each independently selected from H and branched or straightchain C1-C8 alkyl; R3, R4, and R6 are each independently selected from H, branched or straightchain C1-C4 alkyl, halogen, C1-C4 alkoxyl, nitro, and —C(═O)—OR7, wherein R7 is branched or straightchain C1-C4 alkyl; R5 is selected from H, branched or straightchain C1-C4 alkyl, halogen, C1-C4 alkoxyl, nitro, —C—OR8, wherein R8 is selected from an amino acid, a substituted amino acid, and —NHR9, wherein R9 is —CHR10—C(═O)—O—Ru, wherein R10 is H or —CH2—OH, and Ru is branched or straightchain C1-C4 alkyl, and —C(═O)—OR12, wherein R12 is selected from H and branched or straightchain C1-C4 alkyl; provided that (i) at least one of R1, R2, R3, R4, R5, and R6 is not H, and (ii) R4 and R5 cannot both be Br at the same time if R1, R2, R3, and R6 are each H; and acceptable salts thereof.
As used herein, the term “alkyl” refers to a univalent group derived from an alkane by removal of a hydrogen atom from any carbon atom and having the chemical formula of —CnH2n+1. The groups derived by removal of a hydrogen atom from a terminal carbon atom of unbranched alkanes form a subclass of normal alkyl (n-alkyl) groups H(CH2)n. The groups RCH2, R2CH (R≠H), and R3C (R≠H) are primary, secondary, and tertiary alkyl groups, respectively.
An alkyl can be a straightchain (i.e., unbranched) or branched acyclic hydrocarbon having the number of carbon atoms designated (i.e., C1-10 means one to ten carbons, including 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 carbons). In particular embodiments, the term “alkyl” refers to C1-20 inclusive, including 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbons. In other embodiments, the alkyl can be a C1-C4 alkyl, including 1, 2, 3, and 4 carbons. In yet other embodiments, the alkyl can be a C1-C6 alkyl, including 1, 2, 3, 4, 5, and 6 carbons. In even yet other embodiments, the alkyl can be a C1-C8 alkyl, including 1, 2, 3, 4, 5, 6, 7, and 8 carbons.
“Lower alkyl” refers to an alkyl group having 1 to about 8 carbon atoms (i.e., a C1-8 alkyl), e.g., 1, 2, 3, 4, 5, 6, 7, or 8 carbon atoms. “Higher alkyl” refers to an alkyl group having about 10 to about 20 carbon atoms, e.g., 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In certain embodiments, “alkyl” refers to straight-chain alkyls. In other embodiments, “alkyl” refers to branched alkyls. In certain other embodiments, “alkyl” refers to straight-chain and/or branched alkyls. “Branched” refers to an alkyl group in which a lower alkyl group, such as methyl, ethyl, or propyl, is attached to a linear alkyl chain.
Representative alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, neopentyl, n-hexyl, sec-hexyl, 2-ethylhexyl, n-heptyl, n-octyl, n-decyl, n-undecyl, and dodecyl.
Alkyl groups can optionally be substituted (a “substituted alkyl”) with one or more substituents, which can be the same or different. Such substituent groups include, but are not limited to, alkyl, substituted alkyl, cycloalkyl, halogen, acyl, carboxyl, oxo, aryl, substituted aryl, alkoxyl, hydroxyl, nitro, amino, alkylamino, dialkylamino, sulfate, cyano, and mercapto.
The terms “halo,” “halide,” or “halogen” as used herein refer to fluoro, chloro, bromo, and iodo groups. Additionally, terms such as “haloalkyl,” are meant to include monohaloalkyl and polyhaloalkyl. For example, the term “halo(C1-C4)alkyl” is mean to include, but not be limited to, trifluoromethyl, 2,2,2-trifluoroethyl, 4-chlorobutyl, 3-bromopropyl, and the like.
The terms “alkoxyl” or “alkoxy” are used interchangeably herein and refer to a saturated (i.e., alkyl-O—) or unsaturated (i.e., alkenyl-O— and alkynyl-O—) group attached to the parent molecular moiety through an oxygen atom, wherein the terms “alkyl,” “alkenyl,” and “alkynyl” are as previously described and can include C1-20 inclusive, linear, branched, or cyclic, saturated or unsaturated oxo-hydrocarbon chains, including, for example, methoxyl, ethoxyl, propoxyl, isopropoxyl, n-butoxyl, sec-butoxyl, tert-butoxyl, and n-pentoxyl, neopentoxyl, n-hexoxyl, and the like.
As used herein, an “alkoxide” group is —O−.
As used herein, a “hydroxyl” group is —OH.
The terms “alkoxyl” or “alkoxy” are used interchangeably herein and refer to a saturated (i.e., alkyl-O—) or unsaturated (i.e., alkenyl-O— and alkynyl-O—) group attached to the parent molecular moiety through an oxygen atom, wherein the terms “alkyl,” “alkenyl,” and “alkynyl” are as previously described and can include C1-20 inclusive, linear, branched, or cyclic, saturated or unsaturated oxo-hydrocarbon chains, including, for example, methoxyl, ethoxyl, propoxyl, isopropoxyl, n-butoxyl, sec-butoxyl, tert-butoxyl, and n-pentoxyl, neopentoxyl, n-hexoxyl, and the like.
The term “amine” refers to the —NH2 group and also refers to a nitrogen containing group as is known in the art derived from ammonia by the replacement of one or more hydrogen radicals by organic radicals. For example, the terms “acylamino” and “alkylamino” refer to specific N-substituted organic radicals with acyl and alkyl substituent groups respectively.
The amino group is —NR′R″, wherein R′ and R″ are typically selected from hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl.
As used herein, an “amide” is —NH—C(═O)—R.
As used herein, an “ester” is —O—C(═O)—R.
The terms “halo,” “halide,” or “halogen” as used herein refer to fluoro, chloro, bromo, and iodo groups. Additionally, terms such as “haloalkyl,” are meant to include monohaloalkyl and polyhaloalkyl. For example, the term “halo(C1-4)alkyl” is mean to include, but not be limited to, trifluoromethyl, 2,2,2-trifluoroethyl, 4-chlorobutyl, 3-bromopropyl, and the like.
As used herein, “cyano” refers to —C≡N.
As used herein, “nitro” refers to —NO2.
As used herein, “ammonium” refers to —N+R3.
As used herein, “carbonyl” refers to —C(═O)—R.
The term “carboxyl” refers to the —COOH group. Such groups also are referred to herein as a “carboxylic acid” moiety.
As used herein, “sulfonyl” refers to —SO3R.
As used herein, the term “amino acid” includes moieties having a carboxylic acid group and an amino group. The term amino acid thus includes both natural amino acids (including proteinogenic amino acids) and non-natural amino acids. The term “natural amino acid” also includes other amino acids that can be incorporated into proteins during translation (including pyrrolysine and selenocysteine). Additionally, the term “natural amino acid” also includes other amino acids, which are formed during intermediary metabolism, e.g., ornithine generated from arginine in the urea cycle. The natural amino acids are summarized immediately herein below:
The natural or non-natural amino acid may be optionally substituted. In one embodiment, the amino acid is selected from proteinogenic amino acids. Proteinogenic amino acids include glycine, alanine, valine, leucine, isoleucine, aspartic acid, glutamic acid, serine, threonine, glutamine, asparagine, arginine, lysine, proline, phenylalanine, tyrosine, tryptophan, cysteine, methionine and histidine. The term amino acid includes alpha amino acids and beta amino acids, such as, but not limited to, beta alanine and 2-methyl beta alanine. The term amino acid also includes certain lactam analogues of natural amino acids, such as, but not limited to, pyroglutamine. The term amino acid also includes amino acids homologues including homocitrulline, homoarginine, homoserine, homotyrosine, homoproline and homophenylalanine.
The terminal portion of the amino acid residue or peptide may be in the form of the free acid i.e., terminating in a —COOH group or may be in a masked (protected) form, such as in the form of a carboxylate ester or carboxamide. In certain embodiments, the amino acid or peptide residue terminates with an amino group. In an embodiment, the residue terminates with a carboxylic acid group —COOH or an amino group —NH2. In another embodiment, the residue terminates with a carboxamide group. In yet another embodiment, the residue terminates with a carboxylate ester.
As disclosed hereinabove, the term “amino acid” includes compounds having a —COOH group and an —NH2group. A substituted amino acid includes an amino acid which has an amino group which is mono- or di-substituted. In particular embodiments, the amino group may be mono-substituted. (A proteinogenic amino acid may be substituted at another site from its amino group to form an amino acid which is a substituted proteinogenic amino acid). The term substituted amino acid thus includes N-substituted metabolites of the natural amino acids including, but not limited to, N-acetyl cysteine, N-acetyl serine, and N-acetyl threonine. The term substituted amino acid also includes acetyl-L-leucylglycine.
For example, the term “N-substituted amino acid” includes N-alkyl amino acids (e.g., C1-6 N-alkyl amino acids, such as sarcosine, N-methyl-alanine, N-methyl-glutamic acid and N-tert-butylglycine), which can include C1-6 N-substituted alkyl amino acids (e.g., N-(carboxy alkyl) amino acids (e.g., N-(carboxymethyl)amino acids) and N-methylcycloalkyl amino acids (e.g., N-methylcyclopropyl amino acids)); N,N-di-alkyl amino acids (e.g., N,N-di-C1-6 alkyl amino acids (e.g., N,N-dimethyl amino acid)); N,N,N-tri-alkyl amino acids (e.g., N,N,N-tri-C1-6 alkyl amino acids (e.g., N,N,N-trimethyl amino acid)); N-acyl amino acids (e.g., C1-6 N-acyl amino acid); N-aryl amino acids (e.g., N-phenyl amino acids, such as N-phenylglycine); N-amidinyl amino acids (e.g., an N-amidine amino acid, i.e., an amino acid in which an amine group is replaced by a guanidino group).
The term “amino acid” also includes amino acid alkyl esters (e.g., amino acid C1-6 alkyl esters); and amino acid aryl esters (e.g., amino acid phenyl esters).
For amino acids having a hydroxy group present on the side chain, the term “amino acid” also includes O-alkyl amino acids (e.g., C1-6 O-alkyl amino acid ethers); O-aryl amino acids (e.g., O-phenyl amino acid ethers); O-acyl amino acid esters; and O-carbamoyl amino acids.
For amino acids having a thiol group present on the side chain, the term “amino acid” also includes S-alkyl amino acids (e.g., C1-6 S-alkyl amino acids, such as S-methyl methionine, which can include C1-6 S-substituted alkyl amino acids and S-methylcycloalkyl amino acids (e.g., S-methylcyclopropyl amino acids)); S-acyl amino acids (e.g., a C1-6 S-acyl amino acid); S-aryl amino acid (e.g., a S-phenyl amino acid); a sulfoxide analogue of a sulfur-containing amino acid (e.g., methionine sulfoxide) or a sulfoxide analogue of an S-alkyl amino acid (e.g., S-methyl cysteine sulfoxide) or an S-aryl amino acid.
In other words, the presently disclosed subject matter also includes derivatives of natural amino acids, such as those mentioned above which have been functionalized by simple synthetic transformations known in the art (e.g., as described in “Protective Groups in Organic Synthesis” by T W Greene and P G M Wuts, John Wiley & Sons Inc. (1999)), and references therein.
Examples of non-proteinogenic amino acids include, but are not limited to: citrulline, hydroxyproline, 4-hydroxyproline, β-hydroxyvaline, ornithine, β-amino alanine, albizziin, 4-amino-phenylalanine, biphenylalanine, 4-nitro-phenylalanine, 4-fluoro-phenylalanine, 2,3,4,5,6-pentafluoro-phenylalanine, norleucine, cyclohexylalanine, α-aminoisobutyric acid, α-aminobutyric acid, α-aminoisobutyric acid, 2-aminoisobutyric acid, 2-aminoindane-2-carboxylic acid, selenomethionine, lanthionine, dehydroalanine, 7-amino butyric acid, naphthylalanine, aminohexanoic acid, pipecolic acid, 2,3-diaminoproprionic acid, tetrahydroisoquinoline-3-carboxylic acid, tert-leucine, tert-butylalanine, cyclopropylglycine, cyclohexylglycine, 4-aminopiperidine-4-carboxylic acid, diethylglycine, dipropylglycine and derivatives thereof wherein the amine nitrogen has been mono- or di-alkylated.
In certain embodiments, R1 and R2 are each H. In other embodiments, R1 and R2 are each branched or straightchain C1-C8 alkyl. In particular embodiments, R1 and R2 are each —CH2CH2—CH3 or —CH2CH(CH2CH3)(CH2CH2CH2CH3).
In certain embodiments, one or more of R3, R4, R5, and R6 is selected from branched or straightchain C1-C4 alkyl, C1-C4 alkoxyl, halogen, nitro, and —C(═O)—OR12, wherein R12 is H or branched or straightchain C1-C4 alkyl.
In certain embodiments, (i) R3 and R6 are each the same and are not H; (ii) R4 and R5 are the each same and are not H; or (iii) R3, R4, R5, and R6 are each the same and are not H. In such embodiments, the compounds of formula (I) are referred to as symmetric isoindigos.
In certain embodiments, (i) R3 is halogen and R4, R5, and R6 are each H; (ii) R4 is halogen and R3, R5, and R6 are each H; (iii) R5 is —C(═O)—OR12, wherein R12 is H or branched or straightchain C1-C4 alkyl, R3 and R6 are each H, and R4 is H or halogen; or (iv) R6 is nitro and R5 is H, R3 is H or C1-C4 alkoxyl, and R4 is H or —C(═O)—OR12, wherein R12 is H or branched or straightchain C1-C4 alkyl. In such embodiments, the compounds of formula (I) are referred to as nonsymmetric isoindigos.
In particular embodiments, the compound of formula (I) is a symmetric isoindigo and is selected from:
In other embodiments, the compound of formula (I) is a nonsymmetric isoindigo and is selected from:
In certain embodiments, the compound of formula (I) comprises a compound of formula (Ia):
wherein: R8 is —NHR9, wherein R9 is —CHR10—C(═O)—O—R11, wherein R10 is H or —CH2—OH, and R11 is branched or straightchain C1-C4 alkyl. In particular embodiments, the compound of formula (Ia) is selected from:
In certain embodiments, the presently disclosed subject matter provides a sorbent material comprising a compound of formula (I), or in certain embodiments, a compound of formula (Ia).
In other embodiments, the presently disclosed subject matter provides a process for capturing CO2 from a gas sample, the process comprising contacting the gas sample with a compound of formula (I), or a sorbent material comprising a compound of formula (I).
In certain embodiments, the process comprises an electrochemically mediated carbon capture (EMCC) process. In particular embodiments, the process comprises (i) electro-reduction of the sorbent to form an adduct with CO2; and (ii) oxidising the adduct to liberate CO2 regenerate the sorbent.
In certain embodiments, the gas sample is selected from ambient air, i.e., direct air capture, ventilated air, and a stream of gas, e.g., flue gas or other emissions from a point source, such as an industrial source, such as a chemical plant, petroleum production facility, a cement plant, power generation, and transportation, and from enclosed spaces, for example, in buildings and in the cabin space of cars, trains, airplanes, spacecraft, submarines, and the like.
In certain embodiments, the process is a fixed-bed process or a flow-based process.
In other embodiments, the presently disclosed subject matter provides a system for separating carbon dioxide from ambient air or a stream of gases, the system comprising the compound of formula (I) or a sorbent material comprising a compound of formula (I).
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, ±20%, in some embodiments ±15%, 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.
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 matter. 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 matter. The 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.
The presently disclosed subject matter demonstrates a new design paradigm for EMCC sorbents, which breaks the undesirable scaling relationship by leveraging intramolecular hydrogen bonding in isoindigo derivatives. The redox potentials of isoindigos can be anodically shifted by more than 350 mV to impart sorbents with high oxygen stability without compromising CO2 binding, culminating in an EMCC system with minimised parasitic reactions. With a vast synthetic space presented, the presently disclosed subject matter provides a generalisable strategy to finetune interactions between redox-active organic molecules and CO2, thereby addressing a longstanding challenge in developing effective carbon capture methods driven by non-conventional stimuli.
Carbon capture from stationary emitters or directly from the ambient environment, followed by sequestration or utilisation, is critical to mitigating climate change. Mac Dowell et al., 2017; Deutz and Bardow, 2021; Bui et al., 2018. Wet chemical scrubbing methods for carbon dioxide (CO2) separation known in the art, however, are technically and economically challenged by various inherent limitations, including high energy consumption for sorbent regeneration, thermal degradation, complexity in heat integration when retrofitting existing infrastructures, process equipment corrosion, and fugitive emission of volatile toxic sorbents to the environment. Chu, 2009; Sanz-Pérez et al., 2016. Alternatively, electrochemically mediated carbon capture (EMCC) has emerged as a promising technology. Sharifian et al., 2021; Liu et al., 2022; Rheinhardt et al., 2017; Renfrew et al., 2020; Barlow et al., 2022; Diederichsen et al., 2022.
In EMCC, reversible CO2 capture and release is modulated by switching electrochemical potentials. Therefore, they can be operated isothermally at ambient pressure, powered by renewable energy sources, and modularly designed to accommodate the multiscale nature of carbon capture needs. Among the EMCC mechanisms explored to date, one popular strategy is to use redox-active organic compounds as CO2 carriers (redox-tunable Lewis bases), with quinones being the most studied class of molecules. Bell et al., 1988; Mizen and Wrighton, 1989; Nagaoka et al., 1992. Electro-reduction of these molecules generates nucleophiles that form adducts with electrophilic CO2, which can be later oxidised to liberate pure CO2 while regenerating the sorbents (
The past two decades have witnessed steady research progress in developing EMCC processes using redox-tunable sorbents. Rheinhardt et al., 2017; Barlow et al., 2022; Rahimi et al., 2022; Scovazzo et al., 2003; Gurkan et al., 2015; Voskian et al., 2019; Diederichsen et al., 2021; Li et al., 2022. Several bench-scale prototypes have been demonstrated for fixed-bed and flow-based CO2 separations attributed to materials and device-level engineering efforts. Gurkan et al., 2015; Voskian and Hatton, 2019; Diederichsen et al., 2021. In contrast, molecular-level design principles for precise sorbent property tuning remain largely unestablished beyond the simplistic method of structural substitution with electron-donating and withdrawing groups, despite their central role in EMCC.
The lack of reliable sorbent chemistry has intrinsically hindered existing EMCC processes. A practical EMCC system usually requires chemical modification of redox-tunable sorbents to improve key properties including, but not limited to, solubility, processability, and stability against impurities. For example, anodically shifting the redox potential will enhance the robustness of activated sorbent against molecular oxygen (O2), a common gas stream impurity, for improved carbon capture efficiency. Nevertheless, the CO2 binding affinity of existing redox-tunable Lewis base sorbents, such as quinones, is highly susceptible to chemical modifications, which decreases dramatically as the redox potential shifts anodically. Simeon et al., 2022; Barlow and Yang, 2022; Bui et al., 2022.
The seminal work from W. L. Bell et al. introduced a method to calculate the CO2 binding constant (KCO
A minimum log KCO
Importantly, it is challenging to overcome the coupling between E1/2 and log KCO
The presently disclosed subject matter presents a new class of redox-tunable CO2 carriers based on isoindigo compounds, which can successfully overcome the undesirable coupling between redox potential and CO2 binding strength (
Flow-based separation prototypes also have been demonstrated to evaluate the EMCC performance of the isoindigo sorbents, which can achieve CO2 capacity utilisation efficiencies up to approximately 80% and energy consumptions as low as 127.3 kJ mol−1 per CO2 capture/release cycle. With intrinsic 02 stability, high structural tunability, and synthetic feasibility, isoindigos are promising to serve as the next-generation EMCC sorbents. Moreover, the presently disclosed subject matter demonstrates a generalisable strategy to overcome the intrinsic linear free-energy limits in redox-active organic species that can be broadly applied to EMCC and beyond.
1.4.1 Redox-Tunable CO2 Absorption of Isoindigo Isoindigo and its derivatives have been extensively utilised as core building blocks in organic semiconductors, Wang et al., 2014; Bogdanov and Mironov, 2021; Kiss et al., 2021; Wei et al., 2021, but have rarely been explored as redox molecules for organic electrodes. Without wishing to be bound to any one particular theory, it is thought that isoindigos bearing α,β-unsaturated 1,4-diketone functionalities to be redox-active and can complex with CO2 at the oxygen centres in the reduced state. A series of electrochemical experiments was conducted to validate the redox-driven interaction between isoindigo (IId) and CO2.
First, we performed bulk electrolysis of IId using 0.25 M lithium perchlorate (LiClO4) in dimethyl sulfoxide (DMSO) as the supporting electrolyte under N2 or CO2 atmosphere (
Since the discovery of the EMCC mechanism using quinones in 1988, Bell et al., 1988; Mizen and Wrighton, 1989, to our best knowledge, the widely accepted electrochemically generated quinone-CO2 carbonate adduct is still a proposed structure and has not been confirmed in an EMCC process by non-ambiguous characterisations. This lack of confirmation is probably due to the poor stability of the adducts and the transient bonding nature between the reduced sorbent and CO2. On the contrary, crude NMR spectra suggest bulk electro-reduction of IId under CO2 can yield the proposed adduct with full conversion (IId was fully consumed), indicating the high selectivity of this chemistry and the sufficient stability of the adduct, which we postulate to be the result of intramolecular hydrogen bonding. Noticeably, the IId-CO2 solution obtained from bulk electrolysis can be stably stored in ambient air with high water content. After 53 days, less than 9 mol % of IId-CO2 oxidised back to the neutral IId form (
To confirm the redox activity of IId, we measured its cyclic voltammetry (CV) using 0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) in DMF as the supporting electrolyte (
As a Lewis acid, CO2 can withdraw the electron density from IId·− to promote the second electron transfer, giving rise to an anodically shifted second reduction wave. Further, the oxidation peak also shifts anodically and becomes quasi-reversible, corroborating the formation of the IId-CO2 adduct, which requires more energy for CO2 desorption.
This preliminary finding encouraged us to expand the chemical scope of isoindigos and search for more qualified EMCC compounds. Isoindigos can be synthesised through the condensation reaction between 2-oxindoles and isatins. This reaction allows the modular design of redox sorbents, where the two building units can be modified separately and integrated into isoindigos in the last step, thereby addressing the synthetic barriers when engineering EMCC sorbents (Section 1.8). The presently disclosed subject matter, in part, demonstrates the structural modification of isoindigo at the 5, 6, and N-positions with 21 examples (
The redox behaviours of derivatised isoindigos were examined under N2 via CV. All of the isoindigos tested displayed redox couples typical to stepwise two-electron transfer, underscoring the good electrochemical reversibility of these molecules (
Among the 21 examples of isoindigos, only IId, 6BIId, and 66DBIId exhibit less well-defined shapes for the second redox wave. This result is probably due to the rotational isomerisation of the one-electron reduced isoindigo radical anions. As shown in
Ideal EMCC sorbents shall have redox potentials more positive than the oxygen reduction potential (−1.35 V vs. Fc+/Fc in DMF,
To verify the above hypothesis, we show that EWG substituent at 5-position is more effective in facilitating electro-reduction than that at 6-position. This increase in effectiveness arises because the former is in the ortho-position of Ha and more effective in pulling away the electron density, thereby inducing a stronger hydrogen bonding (
Counterintuitively, adding electron-donating groups (EDGs), such as methoxy shows a negligible influence on E1/2(IId/IId·−), as evidenced by IId vs. 55DMIId (both show E1/2 of −1.29 V vs. Fc+/Fc, Table 1), and 5NIId vs. 5M5NIId (both show E1/2 of −1.08 V vs. Fc+/Fc, Table 3). This observation is likely due to the charge-transfer effect that lowers the energy level of the molecule, offsetting the electronic effect from EDGs. More particularly, isoindigo species are electron acceptors (n-type organic semiconductors), Wei et al., 2021, where charge transfer can be induced between the electron-deficient isoindigo rings and the electron-rich methoxy group. UV-vis absorption spectra suggest an optical bandgap of 1.90-1.98 eV for most isoindigos with or without chemical modification (Table 3 and
The interplay between substituent groups and intramolecular hydrogen bonding in isoindigos and
1H NMR (H )
1H NMR (Hb)
) in
/Fc)
H NMR were recorded on the neutral molecules in DMSO-d6 using solvent residual peak as the internal reference for calibration.
Bond length was obtained from DFT-optimized structures.
cResults from the non-symmetric oxindole ring with the EWG-substituent.
indicates data missing or illegible when filed
After understanding the effect of molecular structures on the redox potentials of isoindigos, we further studied their CO2 binding behaviours. All isoindigos in this Example exhibited anodically shifted potential for the second electron transfer process under CO2, confirming their ability to form CO2 adducts upon electro-reduction. Importantly, through the combined effect of EWGs substitution and hydrogen bonding (a) discussed above, all isoindigos with unsubstituted Hb display E1/2 values anodic to oxygen reduction under CO2, implying favourable stability of isoindigo sorbents against O2.
In all previous reports on redox-tunable CO2 sorbents, anodically shifted redox potential always comes with significantly diminished CO2 binding affinity (Table 2). A key finding of this Example, however, is that hydrogen atom (Hb) on the lactam-N of isoindigos can induce intermolecular hydrogen bonding with the complexed CO2 molecule to thermodynamically stabilise the CO2 adduct (
To further demonstrate the role of hydrogen bonding (b) on CO2 adduct stabilisation, we synthesised two examples with N-substitutions to eliminate this hydrogen bonding (NNDPr66DBIId and NNDEHIId). Compared with 6,6′-dibromo substituted 66DBIId (log KCO
To visualize the success in breaking the scaling relationship between redox potential and CO2 binding affinity, the change in KCO
We further tested the bimolecular rate constant (kbimolecular) for the reaction between isoindigo radical anion and CO2 (
At relatively low CO2 concentrations (20% or 10%), the CV curves of isoindigos usually exhibit a positively shifted second reduction peak compared to that under N2; however, most do not completely emerge into the first (
As a final note, CO2 complexation can be frustrated when introducing extremely strong EWGs or strong hydrogen bonding acceptors to isoindigo (5N6MCIId and 6CIIdNa). Detailed analysis is included in Section 1.10 and
Distinct from previously reported redox-tunable CO2 carriers, isoindigos possess intramolecular hydrogen bonding in both inactivated (neutral) and activated (reduced) forms, accounting for their unique EMCC properties. Therefore, we selected the seven most representative structures from our library and carefully compared various parameters, such as NMR spectra, density functional theory (DFT)-optimised bond lengths, redox potentials, and CO2 binding constants, to investigate the interplay between substituent groups and hydrogen bonding on the thermodynamic properties of isoindigos. Key data are summarised in Table 1 (also see
The electron density of Ha at 4-position and Hb at N-position can be regarded as indicators of the bond strength of a and b, respectively. Specifically, 1H NMR spectra reveal a chemical shift of 9.06 ppm for Ha and 10.89 ppm for Hb in unmodified IId, corresponding to a DFT-optimised bond length of 1.963 Å for a and 1.954 Å for b. Introducing electron-withdrawing bromo groups at 5-position (55DBIId) downfield shifts Ha to 9.32 ppm and Hb to 11.11 ppm, resulting in an enhanced hydrogen bonding of 1.939 Å for a and 1.946 Å for b, respectively. Interestingly, the electronics and hydrogen bonding of each oxindole ring can be independently tuned in nonsymmetric isoindigos. Using the nonsymmetric 5BIId as an example, the chemical shifts of Ha and Hb are 9.07 and 10.96 ppm at the non-substituted side, corresponding to a DFT-optimised bond length of 1.962 Å for a and 1.957 Å for b, which is very close to IId. In contrast, Ha and Hb shift downfield to 9.31 and 11.05 ppm at the bromo-substituted side, corresponding to an enhanced hydrogen bonding of 1.936 Å for a and 1.943 Å for b.
Moreover, introducing the same EWG at different positions modulates the strength of hydrogen bonding (a) differently. For example, with the same dibromo-substitution, the chemical shift of Ha in 66DBIId (8.99 ppm) is upfield to that of 55DBIId (9.32 ppm). This substitution results in a weakened hydrogen bonding (a) of 1.961 Å in 66DBIId than that of 1.939 Å in 55DBIId, explaining the more negative reduction potential of 66DBIId (−1.14 V vs. Fc+/Fc) than 55DBIId (−1.09 V vs. Fc+/Fc). The above results strongly suggest that, in addition to the electronic effect of substituent groups, intramolecular hydrogen bonding (a) also is vital in facilitating the reduction of isoindigo.
Although decreasing the electron density of isoindigos by introducing EWGs at 5,6-positions can effectively facilitate their reduction, counterintuitively, the reduced isoindigos exhibit negligible decay in CO2 affinities, underscoring the importance of intramolecular hydrogen bonding (b). Using DFT calculation, we found that the nucleophilicity of the oxygen centre indeed weakens when EWG is introduced, as suggested by the increased length of the carbonate C—O bond (c). For instance, the bond length of c on the oxindole ring with —COOMe group is increased by 1.2 pm compared to that on the non-substituted ring in 6MCIId. As mentioned above, however, hydrogen bonding (b) strengthens with stronger or increasing number of EWG substituents to keep the bond length of c nearly constant. Moreover, the DFT-optimised structure suggests that hydrogen bonding (b) is precluded in NNDPr66DBIId, and the carbonate bends out-of-plane to the reduced isoindigo rings due to steric repulsion. Thus, the bond length of c in NNDPr66DBIId is increased by 1.8 pm compared to 66DBIId, giving rise to a significant drop in KCO
The collective information above confirms conclusions from the Example. First, hydrogen bonding (a) can reduce the electron density at the redox centre and facilitate reduction. Second, hydrogen bonding (a) can be tuned by substituent groups, where EWG at 5-position is more effective than 6-position to shorten the bond length of a and hence facilitate reduction. Third, hydrogen bonding (b) stabilises the complexed CO2 when EWGs are introduced.
It is important to note that the effect of intramolecular hydrogen bonding on CO2 complexation has been briefly studied in prior works using quinones. It was observed, however, that hydrogen bonding occupies the CO2 binding sites of quinones and diminishes the ability for CO2 capture. Nagaoka et al., 1992; Schimanofsky et al., 2022.
Therefore, this Example presents the first demonstration that intramolecular hydrogen bonding can facilitate CO2 adduct formation and break the intrinsic linear free-energy relationship of EMCC chemistries. This characteristic is attributed to the unique chemical structure of isoindigo that allows free rotation of the oxindole rings in the reduced state as supported by DFT simulation, breaking the intramolecular hydrogen bonding (a) to create space for CO2 complexation, which is further enhanced by the intramolecular hydrogen bonding (b) through the amide functionality.
Breaking the correlation between chemical modification and CO2 binding affinity greatly enhances the degree of freedom in finetuning sorbent properties. For instance, by installing EWGs, such as methyl carboxylate, E1/2 of 66DMCIId under CO2 can be positively shifted by 300 mV compared to that of unmodified IId to impart 02 stability, while the log KCO
Unmodified IId has a moderate solubility in DMF (approximately 230 mM), which needs to be improved for practical use in flow-based EMCC systems. Liu et al., 2022. To facilitate chemical functionalisation, we introduced a carboxyl group to the 6-position of isoindigos, which can be easily connected with amino acids through amidation reactions. As a proof of concept, we utilised glycine methyl ester and serine methyl ester as solubility enhancers and three nonsymmetric isoindigos were prepared (6AIIdGly, 6AIIdSer, and 6B6AIIdSer). 6B6AIIdSer features halogen substitution on one oxindole ring to tune redox potentials and amino acid ester functionalisation on the other oxindole ring to enhance solubility. The solubilities of 6AIIdGly, 6AIIdSer, and 6B6AIIdSer increase to 606, 830, and 568 mM in DMF, respectively (Table 7), likely due to the enhanced molecular interaction between the polar functional groups (amide and carboxylate) and DMF solvent. This hypothesis is supported by the fact that 6AIIdSer is more soluble than 6AIIdGly due to the additional hydroxyl group from the serine moiety.
Conventional amine scrubbing sorbents have raised environmental concerns due to their biotoxicity. del Rio et al., 2019; Rohr et al., 2013. This Example demonstrates that introducing amino ester functionalities into isoindigos substantially improves their biocompatibility with mammalian cells. Unmodified IId shows a LC50 (lethal concentration that causes 50% cell death) of 11.2, 50.4, and 15.8 g mL−1 for NIH3T3/GFP mouse fibroblasts, U2OS.EGFP human osteosarcoma cells, and MCF10A human breast epithelial cells, respectively, after 48 h cell culture (
Before evaluating the CO2 capture performance of isoindigos in EMCC devices, we assessed the influence of electrolytes and common gas stream impurities, such as water and O2, on their CO2 binding properties. Using 55DBIId as an example, the electrochemical behaviours remained almost unaffected up to a high O2 content (16% CO2 and 20% O2,
We also studied the influence of supporting salt on the redox and CO2 binding behaviours of isoindigos (
Based on the CV peak potentials for CO2 capture and release, we estimated the theoretical minimum energy requirement for CO2 separation using isoindigo sorbents, which ranges from 9.8 to 27.6 kJ mol−1 CO2 (
The release/capture efficiency, defined as the ratio of the total amount of CO2 released and captured in each cycle, is averaged to be 97%, indicating the good reversibility of the sorbent. Considering a relatively constant electrochemical (Coulombic) efficiency of the EMCC prototype at approximately 84%, the major loss should be attributed to two factors: (1) our CC-CV protocol where the CO2 adduct was not fully oxidised; (2) the crossover of sorbents and counter electrolytes caused by membrane swelling, which limits all current nonaqueous redox-flow electrochemical systems. 6MCIId also was evaluated at a higher percentage of CO2 removal (
In addition to 6MCIId, we evaluated the performance of other isoindigo sorbents, such as 55DBIId (
6B6AIIdSer exhibits a much lower oxidation potential for CO2 release, likely due to its higher solubility. Moreover, 6B6AIIdSer shows excellent cycling stability with negligible voltage decay over more than 40 cycles and approximately 200 hours of operation with a degradation rate of 2%.
We further tested the EMCC performance of 6MCIId using simulated flue gas (10% CO2+3% O2 balanced in N2) (
As a final note, we evaluated the CO2 capture capability of 6MCIId under low CO2 concentration and its CO2 release capability under pure CO2. Using 1% CO2 with 0.3% O2 as the feed, we observe an early-stage energy consumption of 224.2 kJ mol−1 CO2 and a single pass CO2 removal of greater than 90% (
In this Example, we focused on exploring the fundamental chemistry of isoindigos as redox-active CO2 carriers and their potential to overcome the linear free-energy relationship that limits the structural modification of EMCC sorbents. The flow-based prototype in this Example, however, is not an ultimate design for practical systems but a proof-of-concept demonstration to evaluate the performance of isoindigo at the lab scale. We believe future efforts can substantially improve the performance by optimising the electrolytes, electrodes, and membranes of EMCC devices.
This Example demonstrates the rational design of a new class of bifunctional redox-tunable CO2 carriers based on isoindigo and derivatives thereof. The unique intramolecular hydrogen bonding in isoindigo moieties enables a wide range of chemical modifications to facilitate electro-reduction, tune solubility, and preclude parasitic reactions without compromising their CO2 binding ability. With coupled experimental and computational studies, we provide an in-depth analysis of the structure-function relationships of isoindigos as EMCC sorbents. Compared to existing EMCC sorbents, the presently disclosed isoindigo compounds have the following advantages: 1) a nearly constant log KCO
In addition to flow-based EMCC, we envisage that isoindigos also can find applications as solid adsorbents in fixed-bed systems, due to their descent charge mobility and the abundant methods in synthesising and processing isoindigo-based polymers developed by the community of organic semiconductors. The presently disclosed subject matter paves the way for engineering more reliable EMCC systems by breaking the fundamental barriers of the scaling relationship between redox potential and CO2 binding strength when designing redox-tunable CO2 sorbents.
NMR spectra were collected on a Bruker AMX400 (400 MHz) spectrometer. Chemical shifts were reported in parts per million (ppm). Residual solvent peak was used as an internal reference.
UV-vis absorption spectra were recorded on a Thermo Scientific Genesys 10S UV-vis spectrophotometer.
To a mixture of isatin (1.106˜20 mmol, 1.0 equiv) and 2-oxindole (1.106˜20 mmol, 1.0 equiv) in acetic acid (˜87 equiv) was added 37% HCl solution (1 vol % of acetic acid). The suspension was heated at reflux for 1 to 3 days under Ar atmosphere before being cooled to room temperature (most reactions were completed in 1 day except for those substrates with strong electron-withdrawing groups which required further reaction time). The mixture was filtered and washed with copious water, ethanol, and ethyl acetate (EA). The product was dried in the vacuum oven at 60° C. for 15 h to afford the desired isoindigos in 50˜97% yield.
The target was synthesised by the general procedure starting with 5-methoxyisatin (209 mg, 1.18 mmol) and 5-methoxy-2-oxindole (192.6 mg, 1.18 mmol) in a day to afford the desired product (324 mg, 85%). 1H NMR (400 MHz, DMSO) δ 10.69 (s, 2H), 8.85 (d, J=2.6 Hz, 2H), 6.97 (dd, J=8.5, 2.7 Hz, 2H), 6.75 (d, J=8.5 Hz, 2H), 3.73 (s, 6H).
The target was synthesised by the general procedure starting with isatin (2.94 g, 20 mmol) and 2-oxindole (2.66 g, 20 mmol) in a day to afford the desired product (5.01 g, 96%). 1H NMR (400 MHz, DMSO) δ 10.89 (s, 2H), 9.06 (d, J=7.9 Hz, 2H), 7.34 (t, J=7.6 Hz, 2H), 6.96 (t, J=7.8 Hz, 2H), 6.84 (d, J=7.7 Hz, 2H). 13C NMR (101 MHz, DMSO) δ 168.92, 144.03, 133.27, 132.54, 129.25, 121.65, 121.07, 109.46.
The target was synthesised by the general procedure starting with 5-fluoroisatin (195 mg, 1.18 mmol) and 5-fluoro-2-oxindole (178.3 mg, 1.18 mmol) in a day to afford the desired product (177 mg, 50%). 1H NMR (400 MHz, DMSO) δ 11.02 (s, 2H), 9.00 (dd, J=11.4, 2.7 Hz, 2H), 7.25 (td, J=8.7, 2.8 Hz, 2H), 6.85 (dd, J=8.6, 4.9 Hz, 2H).
The target was synthesised by the general procedure starting with isatin (323.7 mg, 2.2 mmol) and 5-bromo-2-oxindole (466.5 mg, 2.2 mmol) in a day to afford the desired product (691 mg, 92%). 1H NMR (400 MHz, DMSO) δ 11.05 (s, 1H), 10.96 (s, 1H), 9.31 (d, J=1.8 Hz, 1H), 9.07 (d, J=8.0 Hz, 1H), 7.52 (dd, J=8.3, 2.0 Hz, 1H), 7.37 (t, J=7.6 Hz, 1H), 6.98 (t, J=7.8 Hz, 1H), 6.83 (dd, J=16.1, 8.0 Hz, 2H).
The target was synthesised by the general procedure starting with 6-bromoisatin (250 mg, 1.106 mmol) and 2-oxindole (147 mg, 1.106 mmol) in a day to afford the desired product (261 mg, 69%). 1H NMR (400 MHz, DMSO) δ 11.06 (s, 1H), 10.95 (s, 1H), 9.04 (d, J=7.2 Hz, 1H), 9.00 (d, J=8.9 Hz, 1H), 7.36 (s, 1H), 7.18 (d, J=7.1 Hz, 1H), 6.98 (d, J=8.6 Hz, 2H), 6.84 (d, J=7.5 Hz, 1H).
The target was synthesised by the general procedure starting with 6-bromoisatin (533 mg, 2.36 mmol) and 6-bromo-2-oxindole (500 mg, 2.36 mmol) in a day to afford the desired product (776 mg, 78%). 1H NMR (400 MHz, DMSO) δ 11.10 (s, 2H), 8.99 (d, J=8.7 Hz, 2H), 7.18 (dd, J=8.7, 2.0 Hz, 2H), 6.99 (d, J=2.0 Hz, 2H).
The target was synthesised by the general procedure starting with 5-bromoisatin (2.132 g, 9.44 mmol) and 5-bromo-2-oxindole (2 g, 9.44 mmol) in a day to afford the desired product (3.87, 97%). 1H NMR (400 MHz, DMSO) δ 11.11 (s, 2H), 9.32 (d, J=2.0 Hz, 2H), 7.55 (dd, J=8.3, 2.1 Hz, 2H), 6.83 (d, J=8.3 Hz, 2H).
The target was synthesised by the general procedure starting with 5-nitroisatin (227 mg, 1.18 mmol) and 2-oxindole (157.1 mg, 1.18 mmol) in three days to afford the desired product (339 mg, 93%). 1H NMR (400 MHz, DMSO) δ 11.63 (s, 1H), 11.03 (s, 1H), 10.12 (d, J=2.3 Hz, 1H), 9.08 (d, J=8.1 Hz, 1H), 8.29 (dd, J=8.7, 2.4 Hz, 1H), 7.40 (t, J=7.6 Hz, 1H), 7.02 (dd, J=15.7, 8.0 Hz, 2H), 6.88 (d, J=7.6 Hz, 1H).
The target was synthesised by the general procedure starting with 5-nitroisatin (227 mg, 1.18 mmol) and 5-methoxy-2-oxindole (192.6 mg, 1.18 mmol) in three days to afford the desired product (309 mg, 78%). 1H NMR (400 MHz, DMSO) δ 11.61 (s, 1H), 10.84 (s, 1H), 10.14 (d, J=2.3 Hz, 1H), 8.83 (d, J=2.6 Hz, 1H), 8.29 (dd, J=8.7, 2.4 Hz, 1H), 7.08-6.98 (m, 2H), 6.79 (d, J=8.5 Hz, 1H), 3.75 (s, 3H).
The target was synthesised by the general procedure starting with isatin (1.47 g, 10 mmol) and methyl 2-oxindole-6-carboxylate (1.91 g, 10 mmol) in a day to afford the desired product (2.93 g, 92%). 1H NMR (400 MHz, DMSO) δ 11.08 (s, 1H), 10.94 (s, 1H), 9.15 (d, J=8.4 Hz, 1H), 9.07 (d, J=8.0 Hz, 1H), 7.57 (dd, J=8.4, 1.7 Hz, 1H), 7.38 (td, J=7.7, 1.2 Hz, 1H), 7.34 (d, J=1.6 Hz, 1H), 7.02-6.94 (m, 1H), 6.85 (d, J=7.3 Hz, 1H), 3.87 (s, 3H).
The target was synthesised by the general procedure starting with 6-bromoisatin (1.13 g, 5 mmol) and methyl 2-oxindole-6-carboxylate (956 mg, 5 mmol) in a day to afford the desired product (1.74 g, 87%). 1H NMR (400 MHz, DMSO) δ 11.12 (s, 1H), 11.09 (s, 1H), 9.14 (d, J=8.4 Hz, 1H), 9.02 (d, J=8.7 Hz, 1H), 7.57 (dd, J=8.4, 1.7 Hz, 1H), 7.33 (d, J=1.6 Hz, 1H), 7.20 (dd, J=8.7, 2.0 Hz, 1H), 7.01 (d, J=2.0 Hz, 1H), 3.87 (s, 3H).
The target was synthesised by the general procedure starting with 5,6-difluoroindoline-2,3-dione (366.2 mg, 2 mmol) and 5,6-difluoroindolin-2-one (338.3 mg, 2 mmol) in three days to afford the desired product (345 mg, 52%). 1H NMR (400 MHz, DMSO) δ 11.20 (s, 2H), 9.21 (dd, J=13.3, 8.7 Hz, 2H), 6.90 (dd, J=10.2, 7.1 Hz, 2H).
The target was synthesised by the general procedure starting with 5-nitroisatin (960.7 mg, 5 mmol) and methyl 2-oxindole-6-carboxylate (956 mg, 5 mmol) in three days to afford the desired product (1.77 g, 97%). 1H NMR (400 MHz, DMSO) δ 11.68 (s, 1H), 11.22 (s, 1H), 10.10 (s, 1H), 9.15 (d, J=8.4 Hz, 1H), 8.30 (dd, J=8.7, 2.1 Hz, 1H), 7.57 (d, J=8.5 Hz, 1H), 7.33 (s, 1H), 7.03 (d, J=8.7 Hz, 1H), 3.87 (s, 3H).
The target was synthesised by the general procedure starting with methyl isatin-6-carboxylate (615.5 mg, 3 mmol) and methyl 2-oxindole-6-carboxylate (573.6 mg, 3 mmol) in three days to afford the desired product (1.1 g, 97%). 1H NMR (400 MHz, DMSO) δ 11.18 (s, 2H), 9.17 (d, J=8.2 Hz, 2H), 7.58 (d, J=7.9 Hz, 2H), 7.33 (s, 2H), 3.87 (s, 6H).
Methyl 2-oxindole-6-carboxylate (2 g, 10.4 mmol) was suspended in a mixture of methanol (20 mL) and 1 M NaOH (aq, 21 mL). The mixture was heated at reflux for 3 hours and allowed to cool down to room temperature. The solution was neutralised with 1 M HCl (aq) at 0° C. and a beige colour precipitate was formed. The precipitate was collected by filtration and washed with water and methanol. The crude product was dried in vacuo at 60° C. to give a beige solid (1.8 g, 97%), which was used in the next step without further purification. 1H NMR (400 MHz, DMSO) δ 12.85 (s, 1H), 10.49 (s, 1H), 7.56 (d, J=7.6 Hz, 1H), 7.33-7.30 (2H), 3.55 (s, 2H).
The target was synthesised by the general procedure starting with isatin (1.47 g, 10 mmol) and 2-oxindoline-6-carboxylic acid (1.77 g, 10 mmol) in a day to afford the desired product (2.55 g, 83%). 1H NMR (400 MHz, DMSO) δ 13.10 (s, 1H), 11.05 (s, 1H), 10.93 (s, 1H), 9.13 (d, J=8.4 Hz, 1H), 9.07 (d, J=8.1 Hz, 1H), 7.55 (dd, J=8.4, 1.7 Hz, 1H), 7.46-7.23 (m, 2H), 7.04-6.93 (m, 1H), 6.85 (d, J=7.2 Hz, 1H).
The target was synthesised by the general procedure starting with 6-bromoisatin (2.26 g, 10 mmol) and 2-oxindoline-6-carboxylic acid (1.77 g, 10 mmol) in a day to afford the desired product (3.52 g, 91%). 1H NMR (400 MHz, DMSO) δ 13.15 (s, 1H), 11.11 (s, 1H), 11.10 (s, 1H), 9.11 (d, J=8.4 Hz, 1H), 9.01 (d, J=8.7 Hz, 1H), 7.54 (dd, J=8.4, 1.6 Hz, 1H), 7.31 (d, J=1.3 Hz, 1H), 7.18 (dd, J=8.7, 2.0 Hz, 1H), 6.98 (d, J=1.9 Hz, 1H).
6CIId (306.3 mg, 1 mmol) was added portionwise to a solution of NaOH (0.2 M, 5 mL) in methanol at 0° C. The mixture was stirred for another 15 min at 0° C. before it was concentrated in vacuo to dryness to afford a brown solid (328 mg, >99%). 1H NMR (400 MHz, DMSO) δ 10.84 (s, 2H), 9.05 (d, J=8.1 Hz, 1H), 8.93 (d, J=8.3 Hz, 1H), 7.40 (d, J=8.3 Hz, 1H), 7.35-7.24 (m, 2H), 6.95 (t, J=7.9 Hz, 1H), 6.83 (d, J=7.8 Hz, 1H).
To a mixture of 6CIId (1.23 g, 4 mmol) and 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) (1.64 g, 4.32 mmol) in anhydrous DMF (25 mL) was added N,N-diisopropylethylamine (DIPEA) (12 mmol, 2.1 mL) dropwise. The mixture was stirred in Ar for 30 min before glycine methyl ester hydrochloride (540 mg, 4.32 mmol) in DMF (5 mL) was added. The reaction was monitored by TLC and stirred for another 15 h at room temperature. Upon completion of the reaction, the dark solution was poured onto a saturated NaHCO3 aqueous solution (300 mL). The precipitate was collected by filtration, washed repeatedly with saturated NaHCO3(aq), water, 0.2 M HCl (aq), cold methanol and dichloromethane, and dried in vacuo to give a dark red solid. The crude product was recrystallised from ethanol to afford a dark red powder (1.284 g, 85%). 1H NMR (400 MHz, DMSO) δ 11.12 (s, 1H), 10.94 (s, 1H), 9.18-8.98 (m, 3H), 7.47 (dd, J=8.4, 1.6 Hz, 1H), 7.37 (td, J=7.7, 0.9 Hz, 1H), 7.29 (d, J=1.4 Hz, 1H), 7.03-6.94 (m, 1H), 6.85 (d, J=7.6 Hz, 1H), 4.02 (d, J=5.8 Hz, 2H), 3.67 (s, 3H). 13C NMR (101 MHz, DMSO) δ 170.26, 168.91, 168.81, 165.89, 144.50, 144.01, 136.57, 134.87, 133.22, 132.20, 129.65, 128.93, 124.24, 121.57, 121.27, 119.94, 109.67, 108.17, 51.75, 41.24.
To a mixture of 6CIId (1.85 g, 6 mmol) and 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) (2.46 g, 6.48 mmol) in anhydrous DMF (30 mL) was added N,N-diisopropylethylamine (DIPEA) (3.15 mL, 18 mmol) dropwise. The mixture was stirred in Ar for 30 min before L-serine methyl ester hydrochloride (1.16 g, 6.48 mmol) in DMF (5 mL) was added. The reaction was monitored by TLC and stirred for another 15 h at room temperature. Upon completion of the reaction, the dark solution was poured onto a saturated NaHCO3 aqueous solution (300 mL). The precipitate was collected by filtration, washed repeatedly with saturated NaHCO3(aq), water, 0.2 M HCl (aq), cold methanol and dichloromethane, and dried in vacuo to give a dark red solid. The crude product was recrystallised from ethanol to afford a dark red powder (1.39 g, 57%). 1H NMR (400 MHz, DMSO) δ 11.12 (s, 1H), 10.93 (s, 1H), 9.12 (d, J=8.4 Hz, 1H), 9.07 (d, J=7.9 Hz, 1H), 8.70 (d, J=7.3 Hz, 1H), 7.49 (dd, J=8.5, 1.4 Hz, 1H), 7.37 (t, J=7.2 Hz, 1H), 7.30 (s, 1H), 6.98 (t, J=7.6 Hz, 1H), 6.85 (d, J=7.6 Hz, 1H), 5.09 (t, J=6.1 Hz, 1H), 4.54 (dd, J=12.6, 5.5 Hz, 1H), 3.80 (t, J=5.7 Hz, 2H), 3.66 (s, 3H). 13C NMR (101 MHz, DMSO) δ 170.98, 168.98, 168.85, 165.90, 144.54, 143.96, 136.75, 134.90, 133.29, 132.27, 129.68, 128.89, 124.25, 121.61, 121.33, 120.28, 109.73, 108.38, 60.96, 55.74, 51.95.
To a mixture of 6B6CIId (2.31 g, 6 mmol) and 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) (2.46 g, 6.48 mmol) in anhydrous DMF (30 mL) was added N,N-diisopropylethylamine (DIPEA) (3.15 mL, 18 mmol) dropwise. The mixture was stirred in Ar for 30 min before L-serine methyl ester hydrochloride (1.16 g, 6.48 mmol) in DMF (5 mL) was added. The reaction was monitored by TLC and stirred for another 15 h at room temperature. Upon completion of the reaction, the dark solution was poured onto a saturated NaHCO3 aqueous solution (300 mL). The precipitate was collected by filtration, washed repeatedly with saturated NaHCO3(aq), water, 0.2 M HCl (aq), cold methanol and dichloromethane, and dried in vacuo to give the pure solid product in dark red (2.14 g, 73%). 1H NMR (400 MHz, DMSO) δ 11.16 (s, 1H), 11.09 (s, 1H), 9.11 (d, J=8.4 Hz, 1H), 9.02 (d, J=8.7 Hz, 1H), 8.71 (d, J=7.4 Hz, 1H), 7.49 (dd, J=8.4, 1.6 Hz, 1H), 7.30 (d, J=1.4 Hz, 1H), 7.20 (dd, J=8.6, 2.0 Hz, 1H), 7.01 (d, J=1.9 Hz, 1H), 5.07 (t, J=6.1 Hz, 1H), 4.53 (dd, J=12.7, 5.5 Hz, 1H), 3.81 (s, 2H), 3.66 (s, 3H).
To a mixture of 66DBIId (158 mg, 0.377 mmol) and K2CO3 (261 mg, 1.89 mmol) in DMF (5 mL) under Ar was added propyl bromide (0.2 mL). The mixture was heated at 100° C. under Ar for 18 h. The reaction mixture was cooled and evaporated to dryness in vacuo. The crude solid was partition between dichloromethane and water. The organic phase was dried over Na2SO4, filtered, and dried in vacuo. The crude product was then purified by column chromatography on silica gel (230˜400 mesh, hexane/CH2Cl2=1:3) to give a red colour solid (104 mg, 55%). 1H NMR (400 MHz, DMSO) δ 9.05 (d, J=8.6 Hz, 2H), 7.41 (d, J=1.9 Hz, 2H), 7.25 (dd, J=8.6, 1.9 Hz, 2H), 3.74 (t, J=7.0 Hz, 4H), 1.62 (dd, J=14.6, 7.6 Hz, 4H), 0.90 (t, J=7.4 Hz, 6H).
To a mixture of isoindigo (2.62 g, 10 mmol) and K2CO3 (6.9 g, 50 mmol) in DMF (100 mL) was added 1-bromo-2-ethylhexane (3.8 mL, 22 mmol) under Ar atmosphere. The mixture was heated at 100° C. for 1 day. After the reaction was cooled to room temperature, the mixture was poured onto 1 L of water and a dark red precipitate was formed. The solid was filtered, washed with a copious amount of water and cold methanol, and dried in vacuo. The crude product was purified by column chromatography on silica gel (230-400 mesh, hexane/CH2Cl2=1:1) to give a dark red solid (1.58 g, 32%). 1H NMR (400 MHz, DMSO) δ 9.10 (d, J=7.6 Hz, 2H), 7.44 (t, J=7.7 Hz, 2H), 7.03 (dd, J=17.1, 8.0 Hz, 4H), 3.66 (d, J=7.6 Hz, 4H), 2.40 (d, J=10.6 Hz, 4H), 1.82 (s, 2H), 1.44-1.07 (m, 14H), 0.86 (dt, J=13.8, 7.2 Hz, 10H).
Electrochemical measurements were conducted using a BioLogic VSP potentiostat (BioLogic Science Instruments). For the cyclic voltammetry measurements, glassy carbon (Ø=3 mm) was used as the working electrode, platinum wire was used as the counter electrode, silver wire was used as the pseudo-reference electrode and ferrocene was used as the internal reference. In a standard cyclic voltammetry measurement, the molecule of interest (2.5 mM) was dissolved in anhydrous DMF with 100 mM NBu4PF6 as the supporting salt. Typical cyclic voltammograms were collected at a scan rate of −50 mV s−1. To study the effects of ionic species on the redox behaviour of the sorbent molecules, 100 mM LiClO4, NaClO4, KClO4, NBu4ClO4, sodium triflate (NaOTf), or sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) was used as the supporting electrolyte salt.
IId2− and IId-CO2 adduct were synthesised by constant current bulk electrolysis of IId in DMSO-d6 under N2 or CO2 atmosphere. The setup consisted of a three-neck round-bottom flask for the working, counter, and reference electrodes, respectively. Carbon felt (CT GF030; Fuel Cell Store) was used as the working electrode and a silver wire was used as the reference electrode. The counter electrode was separated from the isoindigo solution with a fritted electrode chamber (MR-1196; Bioanalytical Systems). A piece of carbon felt coated with LiFePO4 (50 mg) was used as the counter electrode, which was immersed in a neat electrolyte (1 mL) without isoindigo. LiClO4 (250 mM) in DMSO-d6 was used as the electrolyte. In a standard condition, IId (26.2 mg, 0.1 mmol) was stirred and reduced at a constant current of 1.5 mA in DMSO-d6 (2.5 mL, 250 mM LiClO4) under continuous N2 or CO2 bubbling. The product was used for NMR analysis without further purification.
The Nafion™ membranes (Nafion™ 115; Ionpower) were pretreated accordingly a previously reported method. Li et al., 2022. The membrane was boiled in 3% hydrogen peroxide for 1 h. The membranes were then boiled in 0.25 M sulfuric acid solution for 1 h and cleaned in boiling deionised water for 30 min (twice). Subsequently, the membranes were boiled in 0.25 M sodium hydroxide solution for 1 h and cleaned in boiling deionised water for 30 min (twice). Finally, the membranes were dried under vacuum at 80° C. for at least 1 d and stored in anhydrous DMF.
In a typical experiment, the sorbent tank (20 mL scintillation vial with septum cap) was continuously bubbled with CO2 feed gas (balanced by N2) with the flow rate controlled by a mass flow controller (Alicat). For testing under simulated flue gas conditions, a gas mixture of 10% CO2+3% O2 balanced by N2 was used. An infrared-based CO2 sensor (SprintIR-W 100%) was connected at the gas exit to monitor the CO2 concentration continuously. The tank headspace was filled with plastic beads (McMaster-Carr) to limit the mixing time in the headspace. The Fc tank was kept air free. A two-channel peristaltic pump (Masterflex) circulated the sorbent and the Fc electrolyte to a commercial flow cell (Scribner) at a flow rate of 10 mL min−1. The flow cell utilised graphite plates with 5 cm2 interdigitated flow fields pressed against two pieces of carbon paper electrodes (Sigracet 28 AA) at each side of the graphite electrodes to distribute the liquid flow. A Nafion™ 115 membrane was sandwiched by two pieces of polypropylene and placed between the carbon paper electrodes. The cell was kept sealed by Kalrez fluoropolymer elastomer gaskets (0.02 inches thick). CO2 capture was conducted in a constant current mode while the release was carried out following a constant current/constant voltage protocol (that is, oxidizing the adduct at a constant current until a specific cut-off voltage followed by a constant voltage hold (the cut-off current for the constant voltage hold was 5% of the value used in the constant current oxidation). The cycling protocols of each experiment are provided in the corresponding figure captions.
For experiments using 6MCIId sorbent, we added 10 mM 6MCIId and 4 mM ferrocenium tetrafluoroboronate (FcBF4) into the CE tank to mitigate sorbent crossover and facilitate the reduction of the oxidised ferrocenium in the counter electrolyte, respectively. For less soluble 55DBIId and 66DBIId, the sorbent was dispersed in DMAc to form a slurry catholyte.
The Gaussian16 code, Gaussian 16 Rev. C.01, 2016, was used in the DFT calculations with the B3LYP functional, Becke, 1993; Lee et al., 1988, and the 6-31++g(d,p) basis set. Clark et al., 1983; Hariharan and Pople, 1973; Hehre et al., 1972. The conductor-like polarizable continuum model (CPCM) was used to simulate the solvation effect of DMF solvent at room temperature. Barone and Cossi, 1998. To simulate the processes in solvated environment, free energies excluding translational contributions at 298 K are calculated. Benchmark of calculated redox potentials of the first and second electron transfer steps against measured potentials and comparison of functionals can be found in our previous work. Li et al., 2022. Vibrational frequencies were calculated for optimized structures by determining the second derivative of the energy with respect to the nuclear coordinates and then transforming to mass-weighted coordinates.
The equilibrium redox potential of the Lewis base sorbents (abbreviated as B) was calculated as (G(Bred)−G(Box))/(−|e|), where G(Bred) and G(Box) are the Gibbs free energies of the oxidized and reduced base, respectively. The CO2 binding energy (Ebinding) was calculated as Ebinding=GCO2@B−GB−GCO2, where the first term on the right side of the equation is the Gibbs free energy of CO2 adsorbed on the sorbent molecule, the second and third terms are the Gibbs energies of the isolated B (any charge state) and CO2, respectively.
NIH3T3/GFP fibroblast cell line was kindly provided by Dr. Yun Chen at Johns Hopkins University and cultured in Dulbecco's Modified Eagle's Medium (DMEM; Gibco) supplemented with 10% fetal bovine serum (FBS; Hyclone) and 1% penicillin/streptomycin (Pen/Strep; Gibco). U2OS.EGFP cells were kindly provided by Dr. J Keith Joung at Massachusetts General Hospital and cultured in DMEM supplemented with 10% FBS, 2 mM GlutaMAX (Gibco), and 1% Pen/Strep. MCF10A epithelial cell line was kindly provided by Dr. Konstantinos Konstantopoulos at Johns Hopkins University and cultured in DMEM/F-12 (1:1) supplemented with 5% New Zealand horse serum (Gibco), 20 ng mL−1 EGF (PeproTech), 0.5 g mL−1 hydrocortisone (Sigma Aldrich), 100 ng mL−1 cholera toxin (Sigma Aldrich), 10 g mL−1 insulin (Gibco), and 1% Pen/Strep. All cells were cultured in a humidified incubator at 5% CO2 and 37° C.
Cell cytotoxicity was examined by alamarBlue HS Cell Viability Reagent (Invitrogen) according to manufacturer's protocol. Rampersad, 2012; Kumar et al., 2018. Briefly, cells were plated in 96 wellplates (Falcon) at a density of 20,000 per well for overnight. Testing chemicals dissolved in DMF were sterilised by passing through 0.2 m DMSO-safe Acrodisc syringe filters (Pall Laboratory) and then added into culture medium at predetermined concentrations. All testing conditions contained 1% DMF as the solvent background. The cells were incubated with the chemicals for 48 hours, washed once with Dulbecco's Phosphate Buffered Saline (DPBS; Gibco), and then incubated with fresh culture medium containing alamarBlue reagent for 4 hours in a humidified incubator at 37° C. and 5% CO2. Absorbance was measured at 570 nm by a SpectraMax microplate reader, with 600 nm as a reference wavelength. The cell viability was calculated by relative absorbance to control groups. Data were presented as mean±standard deviation (SD). The dose-response curves were fitted using a four-parameter logistic function to calculate the cytotoxic median lethal concentration (LC50). Cell morphology was monitored throughout the assay and images were acquired with an EVOS cell imaging system (Thermo Fisher).
AThe first half-wave potential.
BThe second half-wave potential cathodic to the first one.
1H NMR
1H NMR
AFc+/Fc is used as the internal reference.
BΔEpeak(2) was calculated based on the difference between the second reduction peak potentials (IId●−/IId2−) in CO2 and N2.
Through Knoevenagel condensation between 2-oxindoles and isatins, electron-donating or withdrawing groups (EDGs or EWGs) can be easily encoded into the aromatic systems of isoindigos at 5 or 6-position. The N-substituent groups can be introduced subsequently via SN2 reactions. This ease in the synthesis and modification of isoindigo moieties allows us to quickly build a vast library of EMCC sorbent candidates with, in this Example, 21 representative compounds in total, where the redox potential, CO2 binding constant, and solubility can be easily tuned via molecular engineering. Specifically, we explored a variety of substituents including electron-donating methoxy and electron-withdrawing fluoro, bromo, carboxyl, methyl carboxylate, amide, and nitro groups at 5 or 6-position of isoindigo to study the electronic and steric impact on their redox potentials and CO2 affinities. N-substituents with aliphatic chains also have been probed to verify the role of intramolecular hydrogen bonding in CO2 complexation. This design gives rise to a non-exhaustive library of isoindigos.
The CO2 binding constant (KCO2) was calculated by Eq 1:
where F is the Faraday constant, R is the ideal gas constant, T is the temperature, ΔEpeak(2) is the difference between the second reduction peak potentials (IId·−/IId2−) in CO2 and N2 atmospheres, and [CO2] is the concentration of dissolved CO2 (0.198 M in DMF under 100% CO2 headspace at 298 K).
In electrochemistry, the Cottrell equation describes the current response (i) when the potential is a step function in time (t):
where i is current, n is the number of electrons transferred per molecule, A is the area of the electrode, C*0 is the bulk concentration of the redox-active isoindigo molecule, D is the diffusion coefficient, and t is time. The expected current for a diffusion limited electrochemical process following an ECE (electron transfer, chemical reaction, electron transfer) mechanism at a plane electrode under diffusion-controlled conditions is:
where n1 and n2 are the number of electrons transferred in the first and second step, respectively, and kf is the forward reaction rate constant of the chemical reaction. Here, to calculate kf after the first electron transfer, n2 and n1 are set to 1. Eq 3 then becomes:
The integrated Cottrell equation relates total charge Q with time for chronoamperometry:
Then, the constant k can be calculated, here k is assumed constant:
where n is the number of electrons reduced per molecule.
To calculate kf, the constant k is calculated using Eq 5 using potentiostatic measurements. Plugging in Eq 6, and rearranging Eq 4 gives:
The data for determining kf is shown in
where [CO2]=0.0396 M.
We further designed an isoindigo with extremely strong EWG substituents to test the tolerance of the intramolecular hydrogen bonding (b) for promoting CO2 binding. 5N6MCIId, with a nitro substitution at 5-position in one ring and a methyl carboxylate substitution at 6′-position in the other ring, exhibits the most anodically shifted reduction potential under N2 (−0.93 V vs. Fc+/Fc) among all isoindigos in this work. Due to the non-symmetrical structure in 5N6MCIId, the strong electron-withdrawing carboxylate and nitro group can work synergistically to frustrate the CO2 binding in one oxindole ring. The carboxylate group withdraws the electron density from the O atom within the ring connecting it, while the nitro group in the adjacent ring creates steric hindrance and dipole-dipole repulsion to inhibit CO2 binding (
In addition to EDG and EWG substituents, we also study the effect of ionic substituents on the isoindigo moiety. We synthesised an isoindigo (6CIIdNa) bearing sodium carboxylate group at 6-position (
The decay of the flow-based EMCC prototype may come from many different factors, such as crossover of the redox-active electrode molecules, decomposition of the electrolyte or carbon paper, or degradation of the CO2 carrier. For 6B6AIIdSer (
where rd is the degradation rate, Q0 is the initial energy capacity of the sorbent tank, Qn is the last-cycle energy capacity of the sorbent tank, and n is the cycle number.
Using −1.3 V as the cut-off potential, the energy capacity of the flow cell decreased to 7 mAh in the last cycle tested. The initial energy capacity of 6B6AIIdSer (0.2 M, 5 mL) is 53.6 mAh. Therefore, the estimated degradation rate is approximately 4.62%.
All publications, patent applications, patents, and other references mentioned in the specification are indicative of the level of those skilled in the art to which the presently disclosed subject matter pertains. All publications, patent applications, patents, and other references are herein incorporated by reference to the same extent as if each individual publication, patent application, patent, and other reference was specifically and individually indicated to be incorporated by reference. It will be understood that, although a number of patent applications, patents, and other references are referred to herein, such reference does not constitute an admission that any of these documents form part of the common general knowledge in the art.
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.
This application claims priority to U.S. Application No. 63/592,345, filed Oct. 23, 2023, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63592345 | Oct 2023 | US |
