MILSTEIN DAVID (IL)
CENTRE NAT RECH SCIENT (FR)
YEDA RES & DEV (IL)
| US20170283447A1 | 2017-10-05 |
ANGEW. CHEM. INT., vol. 47, 2008, pages 3245 - 3249
ANGEWANDTE CHEMIE INTERNATIONAL EDITION, vol. 49, no. 40, 2010, pages 7229 - 7233
"Molecular Electrocatalysis for Alcohol Oxidation: Insights and Challenges for Catalyst Design", ACS APPL. ENERGY MATER., vol. 3, no. 1, 2020, pages 38 - 46
| 24 CLAIMS 1. A use of an acceptor-less dehydrogenation catalyst for an electrocatalytic oxidation of an alcohol to an ester or/and an acid under electrochemical conditions, the acceptor-less dehydrogenation catalyst being represented by the following formula (I) : ( I) wherein : Z is a heteroatom selected from the group consisting of C and N ; M is selected from the group consisting of Fe, Co, Ni, Ru, Rh , Pd, Os, Pt, Ir and Mn ; Li , L2, and L3 are anionic and neutral ligands independently selected from the group consisting of H, CO, PRaRbRc, P(ORa)(ORb)(ORc), AsRaRbRc, SbRa,Rb, Rc, SRaRb, a nitrile group (RaCN or CN) , N2, CS, a heteroaryl group, ORa, N(Ra)2, OCORa and a halogen group; Ra, Rb, Rc which are the same or different independently represent H, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group having 6 to 1 2 carbon atoms, a halogen atom , a heterocyloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 3 to 12 carbon atoms, an alkylcycloalkyl group having from 3 to 1 0 carbon atoms and an alkylaryl group having from 6 to 12 carbon atoms; Ri represents -CH ; Xi represents -CH2; or Ri and Xi are linked together to form an aromatic ring having 6 carbon atoms; X2 represents -CH2 or a simple bond linking Xi to R2; R’i represents -CH ; X’i represents -CH2; or R’i and X’i are linked together to form an aromatic ring having from 3 to 6 carbon atoms; X’2 represents -CH2 or a simple bond linking X’i to R’2; R2 and R’2 which are the same or different independently represent PRdRe, P(ORd)(ORe) , P(ORd)(Re) , an amine group (NRdRe), an imine, an oxazoline, a sulfide (SRd) , a sulfoxide (S( = O)Rd), OCORd, and ORd; Rd, Re which are the same or different independently represent H, an alkyl group having from 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group having from 6 to 1 2 carbon atoms, a heterocyloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 3 to 12 carbon atoms, an alkylcycloalkyl group having from 3 to 1 0 carbon atoms and an alkylaryl group having from 6 to 1 2 carbon atoms, Rs represents H, an alkyl group having from 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group, a heterocycloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 6 to 10 carbon atoms, an alkylcycloalkyl group having from 3 to 10 carbon atoms, an alkylaryl group having from 6 to 12 carbon atoms and a halogen . 2. The use according to claim 1 , wherein : Z is N ; M is Ru or Mn , Li , L2, and L3 are anionic and neutral ligands independently selected from the group consisting of H, CO, PRaRbRc, and a halogen ; Ra, Rb, Rc which are the same or different represent H, an alkyl group having from 1 to 6 carbon atoms, an aryl group having from 6 to 1 0 carbon atoms, a heteroaryl group having from 3 to 1 2 carbon atoms and a halogen ; R1 represents -CH ; Xi represents -CH2; or Ri and Xi are linked together to form an aromatic ring having 6 carbon atoms; X2 represents -CH2 or a simple bond linking Xi to R2; R’i represents -CH ; X’i represents -CH2; or R’i and X’i are linked together to form an aromatic ring having 6 carbon atoms; X’2 represents -CH2 or a simple bond linking X’i to R’2; R2 and R’2 which are the same or different independently represent PRdRe, an amine group (NRdRe), and a sulfide (SRd) ; Rd, Re which are the same or different represent H, an alkyl group having from 1 to 6 carbon atoms, an aryl group having from 6 to 1 0 carbon atoms and a heteroaryl group having from 3 to 12 carbon atoms; R3 is H. 3. The use according to claim 1 or 2, wherein : Z is N ; M is Ru or Mn , Li , L2, and L3 are anionic and neutral ligands independently selected from the group consisting of H, CO, PRaRbRc, and a halogen ; Ra, Rb, Rc which are the same or different represent H, an alkyl group selected from methyl, ethyl, propyl and butyl , an aryl group selected from phenyl, benzyl and a halogen selected from F, Cl and Br; Ri represents -CH ; Xi represents -CH2; or Ri and Xi are linked together to form an aromatic ring having 6 carbon atoms; X2 represents -CH2 or a simple bond linking Xi to R2; R’i represents -CH ; X’i represents -CH2; or R’i and X’i are linked together to form an aromatic ring having 6 carbon atoms; X’2 represents -CH2 or a simple bond linking X’i to R’2; 27 R2 and R’2 which are the same or different independently represent PRdRe, an amine group (NRdRe), Rd, Re, which are the same or different represent H, an alkyl group selected from methyl , ethyl, propyl, i-propyl, butyl, t-butyl, sec-butyl or an aryl group selected from phenyl and benzyl, R3 is H. 4. The use according to any one of claims 1 to 3, wherein the acceptorless dehydrogenation catalyst of formula ( I) is represented by one of the following structures: (XIV) (XI I I) 28 5. The use according to any one of claims 1 to 4, wherein the alcohol is electrocatalytic oxidized at least to an ester. 6. The use according to any one of claims 1 to 5, wherein the electrocatalytic oxidation is carried out in a homogeneous phase. 7. The use according to claim 6, wherein the acceptor-less dehydrogenation catalyst is solubilized in the alcohol which is oxidized, the oxidation being realized without additional organic solvent. 8. The use according to claim 6, wherein the acceptor-less dehydrogenation catalyst is solubilized in an additional organic solvent. 9. The use according to any one of claims 1 to 4, wherein the use is carried out in a heterogeneous phase, and the acceptor-less dehydrogenation catalyst is fixed on a conductive support. 10. The use according to claim 9, wherein the use is carried out under heterogeneous conditions and the alcohol is oxidized at least to ester. 11. The use according to claim 9 or 10, wherein solvent is water and the pH is between 7 and 14. 12. The use according to any one of claims 1 to 12, wherein the alcohol is oxidized at least to 30 FE% (Faradaic efficiency) to an ester with these 29 conditions: 0.1 M LiCI, 0.1 M LiOH in EtOH, 1 mM catalyst, 0.3 V vs Ag/AgNOs (0.01 M in 0.1 M TBAPFe in CH3CN), glassy carbon working electrode, separated counter electrode compartment. 13. The use according to any one of claims 1 to 12, wherein alcohol is ethanol and ethanol is oxidized to ethyl acetate. 14. The use according to any one of claims 1 to 13, wherein the electrocatalytic oxidation is conducted : - at ambient temperature, without heating , or - at temperature inferior to 60°C. 15. The use according to any one of claims 1 to 14, wherein the organometallic catalyst is in contact with a working solution comprising the alcohol and comprising a base chosen in the group comprising MOH, MOR (R = alkyl, benzyl), MOtBu (with M = Li, Na, K), or neutral organic bases such as lutidine, pyridine, DBU ( 1 ,8-Diazabicyclo[5.4.0]undec-7- ene), TBD (Triazabicyclodecene) or other guanidine bases, trialkyl amines, or phosphorous bases, such as Verkade’s proazaphosphatranes, or phosphazenes. 16. The use according to claim 1 5, wherein a constant current between 1 and 10 mA is applied. 17. The use according to any one of claims 1 to 16, wherein an anodic half-cell reaction is coupled with a cathodic half-cell reaction , the cathodic half-cell reaction being an electrochemical reduction of CO2 to CO. 18. The use according to any one of claims 1 to 1 7, wherein the electrocatalytic oxidation takes place in a flow cell. |
ACCEPTOR-LESS DEHYDROGENATION CATALYSTS
TECHN ICAL FI ELD
The invention relates to the dehydrogenation process of alcohols in the presence of acceptor-less dehydrogenation catalysts under electrochemical conditions.
BACKGROUND ART
Various thermal methods are known for the dehydrogenation/oxidation of alcohols to aldehydes/ketones, acids and esters.
Dehydrogenation/oxidation of alcohols can be performed in the presence of an oxidant/hydrogen-acceptor via transfer hydrogenation . Catalytic transfer hydrogenation reactions is well-known using molecular catalysts. These processes mainly use acetone as both the solvent and the hydrogen-acceptor, mainly focusing on the oxidation of secondary alcohols that undergo facile transfer hydrogenation . Other co- oxidants/hydrogen-acceptors might be suitable olefins. For example, using Rh-based transfer hydrogenation catalysts, the Grutzmacher group showed that ethanol can be oxidized and reformed to ethyl acetate in the presence of ketones or alkenes (Angew. Chem . Int. 2008, 47, 3245-3249).
In recent years, transfer hydrogenation catalysts were proposed as possible catalyst candidates for the electrochem ical oxidation of alcohols. In 201 0, Grutzmacher et al have described a fuel cell operating in a strongly basic media (2M KOH) for the oxidation of ethanol to acetate (CH 3 COO ) , using as the anode catalyst a molecular [Rh(OTf)(trop2NH)(PPh 3 )] complex, deposited on a conductive carbon support (Angewandte Chemie International Edition 2010, 49 (40), 7229- 7233). In a 2020 review paper, Cook et al have presented the current State-of-the-Art for molecular electrocatalysis capable of alcohol oxidation , illustrated by three case studies; namely a copper/nitroxyl radical cooperative catalyst system (case 1 ), noble metal-hydrides with proximal amine group (case 2) for transfer hydrogenation , nickel hydrides with P 2 N 2 ligands (case 3) (Molecular Electrocatalysts for Alcohol Oxidation: Insights and Challenges for Catalyst Design, ACS Appl. Energy Mater. 2020, 3 (1), 38-46).
[0001 ] Nevertheless, published results for the molecular electrocatalytic oxidation of alcohols is limited to low turnover numbers (<5), substrates that undergo facile transfer hydrogenation (e.g . isopropanol and secondary alcohols in general) and at unknown or high overpotential (>1 .2 V) . Hence, the reactivity is in general limited to products obtainable via transfer hydrogenation (2-electron oxidation products), or activated alcohols (e.g . benzyl alcohol) without the possibility to oxidize reform simple aliphatic alcohols, e.g . to their corresponding esters under electrocatalytic conditions.
SUMMARY OF THE INVENTION
Various embodiments are directed to addressing the effects of one or more of the problems set forth above. The following presents a simplified summary of embodiments in order to provide a basic understanding of some aspects of the various embodiments. This summary is not an exhaustive overview of these various embodiments. It is not intended to identify key/critical elements or to delineate the scope of these various embodiments. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
The present invention arises from the unexpected finding by the inventors that ruthenium and manganese-based catalysts of formula ( I) are active in electrochemical oxidation if ethanol.
Thus, the present invention provides the use of an acceptorless dehydrogenation catalyst for an electrocatalytic oxidation of an alcohol to an ester or/and an acid under electrochemical conditions, the acceptor-less dehydrogenation catalyst being represented by the general formula (I) below:
( I) wherein :
Z is a heteroatom selected from the group consisting of C and N ;
M is selected from the group consisting of Fe, Co, Ni, Ru, Rh , Pd, Os, Pt, Ir and Mn ;
Li , L 2 , and L 3 are anionic and neutral ligands independently selected from the group consisting of H, CO, PR a RbRc, P(OR a )(ORb)(OR c ), AsR a RbRc, SbR a ,Rb, R c , SR a Rb, a nitrile group (R a CN or CN) , N 2 , CS, a heteroaryl group, OR a , N(R a ) 2 , OCOR a and a halogen group;
R a , R b , R c which are the same or different independently represent H, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, a halogen atom , a heterocyloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 3 to 12 carbon atoms, an alkylcycloalkyl group having from 3 to 1 0 carbon atoms and an alkylaryl group having from 6 to 12 carbon atoms;
Ri represents -CH ;
Xi represents -CH 2 ; or Ri and Xi are linked together to form an aromatic ring having 6 carbon atoms;
X 2 represents -CH 2 or a simple bond linking Xi to R 2 ;
R’i represents -CH ;
X’i represents -CH 2 ; or R’i and X’i are linked together to form an aromatic ring having from 3 to 6 carbon atoms;
X’ 2 represents -CH 2 or a simple bond linking X’i to R’ 2 ; R2 and R’2 which are the same or different independently represent PR d R e , P(ORd)(OR e ) , P(OR d )(R e ) , an amine group (NR d R e ), an imine, an oxazoline, a sulfide (SRd) , a sulfoxide (S( = O)Rd), OCORd, and ORd;
Rd, R e which are the same or different independently represent H, an alkyl group having from 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group having from 6 to 1 2 carbon atoms, a heterocyloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 3 to 12 carbon atoms, an alkylcycloalkyl group having from 3 to 1 0 carbon atoms and an alkylaryl group having from 6 to 1 2 carbon atoms,
R3 represents H, an alkyl group having from 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group, a heterocycloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 6 to 10 carbon atoms, an alkylcycloalkyl group having from 3 to 10 carbon atoms, an alkylaryl group having from 6 to 12 carbon atoms and a halogen .
DETAI LED DESCRI PTION OF THE INVENTION
Definitions
As used herein , the term “alkyl” refers to linear or branched chain, saturated, hydrocarbon groups having preferably 1 to 1 0 carbon atoms. By way of example, the term “C1 -6 alkyl” refers to an alkyl group containing 1 to 6 carbon atoms. Non limiting example of linear or branched alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, sec-butyl, pentyl, and hexyl.
As used herein , the term “heteroalkyl” refers to a linear or branched alkyl chain preferably having 2 to 6 carbon atoms in the chain, one or more of which has been replaced by a heteroatom selected from the group consisting of O, N, and S. Examples of heteroalkyl groups include, but are not limited to, methoxyethane, dimethyl ether, diethyl ether, and trimethylamine.
As used herein , the term “cycloalkyl” refers to an unsubstituted or substituted cyclic hydrocarbon group having preferably 3 to 6 carbon atoms. It includes monocyclic, fused, and polycyclic rings. Examples of cycloalkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl.
The term “heterocycloalkyl” refers to an unsubstituted or substituted monocyclic or polycyclic ring preferably having 3 to 6 carbon atoms, one or more of which has been replaced by a heteroatom selected from the group consisting of O, N, and S. Examples of heterocycloalkyl groups include, but are not limited to, oxirane, oxetane, hydrofurane, hydropyrane, thiirane, thiethane, hydrothiophene, hydrothiopyrene, aziridine, azetidine, pyrrolidine, piperidine, imidazole, oxazole, and piperazine.
The term “alkylcycloalkyl” refers to a linear or branched alkyl group as defined above substituted by a cycloalkyl group as defined above.
The term “aryl” refers to a substituted or unsubstituted aromatic monocyclic or polycyclic hydrocarbon group having preferably 6 to 12 carbon atoms. Examples of aryl groups include, but are not limited to, phenyl, biphenyl, and naphthyl.
The term “heteroaryl” refers to an unsubstituted or substituted monocyclic or polycyclic aromatic ring having preferably 3 to 12 carbon atoms in each ring , one or more of which has been replaced by a heteroatom selected from the group consisting of O, N, and S. Examples of heteroaryl groups include, but are not limited to, furan , benzofurane, thiophene, pyrrole, pyrazole, pyridine, thiazole, imidazole, pyrimidine, indole, quinoline, isoquinoline oxazole, isoxazole, pyrazine, triazole, thiadiazole, tetrazole, and pyrazole.
The term “arylalkyl” refers to a linear or branched alkyl group as defined above substituted by an aryl group as defined above. Examples of arylalkyl groups include, but are not limited to benzyl.
As intended herein, the term “heteroatom” preferably refers to an atom selected from the group consisting of N, O, S, P, Mn, Ru.
As intended herein the term “halogen” refers to atoms selected from the group consisting
Acceptor-less dehydrogenation catalyst
Preferably, the acceptor-less dehydrogenation catalyst according to the present embodiments is a ruthenium complex or a manganese complex. More preferably the acceptor-less dehydrogenation catalyst according to the present embodiments is a pyridyl ruthenium complex or a pyridyl manganese complex.
The present embodiments preferably relate to a compound of formula ( I) wherein :
Z is N ;
M is Ru or Mn ,
Li , L 2 , and L 3 are anionic and neutral ligands independently selected from the group consisting of H, CO, PR a RbRc, and a halogen ;
R a , Rb, Rc which are the same or different represent H, an alkyl group having from 1 to 6 carbon atoms, an aryl group having from 6 to 1 0 carbon atoms, a heteroaryl group having from 3 to 1 2 carbon atoms and a halogen ;
Ri represents -CH ;
Xi represents -CH 2 ; or Ri and Xi are linked together to form an aromatic ring having 6 carbon atoms;
X 2 represents -CH 2 or a simple bond linking Xi to R 2 ;
R’i represents -CH ;
X’i represents -CH 2 ; or R’i and X’i are linked together to form an aromatic ring having 6 carbon atoms;
X’ 2 represents -CH 2 or a simple bond linking X’i to R’ 2 ;
R 2 and R’ 2 which are the same or different independently represent PRdRe, an amine group (NR d R e ), and a sulfide (SR d ) ;
R d , R e which are the same or different represent H, an alkyl group having from 1 to 6 carbon atoms, an aryl group having from 6 to 1 0 carbon atoms and a heteroaryl group having from 3 to 12 carbon atoms;
R 3 is H.
The present embodiments preferably relate to a compound of formula ( I) wherein :
Z is N ;
M is Ru or Mn , Li , L 2 , and L 3 are anionic and neutral ligands independently selected from the group consisting of H, CO, PR a RbR c , and a halogen ;
R a , Rb, Rc which are the same or different represent H, an alkyl group selected from methyl, ethyl, propyl and butyl , an aryl group selected from phenyl, benzyl and a halogen selected from F, Cl and Br;
Ri represents -CH ;
Xi represents -CH 2 ; or Ri and Xi are linked together to form an aromatic ring having 6 carbon atoms;
X 2 represents -CH 2 or a simple bond linking Xi to R 2 ;
R’i represents -CH ;
X’i represents -CH 2 ; or R’i and X’i are linked together to form an aromatic ring having 6 carbon atoms;
X’ 2 represents -CH 2 or a simple bond linking X’i to R’ 2 ;
R 2 and R’ 2 which are the same or different independently represent PRdRe, an amine group (NR d R e ),
Rd, R e , which are the same or different represent H, an alkyl group selected from methyl , ethyl, propyl, i-propyl, butyl, t-butyl, sec-butyl or an aryl group selected from phenyl and benzyl,
R 3 is H.
In an embodiment, the compound of formula (I) is of the following formula (I I) below:
( H) wherein R 2 , R’ 2 , Li , L 2 , and L 3 are as defined above.
Preferably, Li , L 2 , and L 3 are independently selected from the group consisting of H, CO, PPh 3 , and a halogen selected from F, Cl and Br;
Preferably, R 2 and R’ 2 independently represent N(Et) 2 , N(i-Pr) 2 , P(i- Pr) 2 , P(t-Bu) 2 , N H(t-Bu), NH(i-Pr), and PPh 2 .
In another embodiment, the compound of formula (I I I) as defined above is of the following formula ( IV) below:
(H I) wherein R 2 and R’ 2 are as defined above. Preferably, R 2 and R’ 2 independently represent N(Et) 2 , N(i-Pr) 2 , P(i-Pr) 2 , P(t-Bu) 2 , NH(t-Bu), NH(i-Pr), and PPh 2 .
In an embodiment, the compound of formula (I) is of the following formula (IV) below:
( IV) wherein Z, M, X 2 , X’ 2 , R 2 , R’ 2 , Li , L 2 , and L 3 are as defined above.
In a preferred embodiment of the compound of formula (IV) : Z is N ; M is Ru ;
X 2 and X’ 2 independently represent -CH 2 or a simple bond.
Li , L 2 , and L 3 are independently selected from the group consisting of H, CO, PPh 3 , and a halogen selected from F, Cl and Br;
R 2 and R’ 2 independently represent N(Et) 2 , N(i-Pr) 2 , P(i-Pr) 2 , P(t- BU) 2 , NH(t-Bu) , NH(i-Pr) , and PPh 2 .
In an embodiment, the compound of formula (I) as defined above is of the following formula (V) below: wherein M, R 2 , R’ 2 , Li , L 2 , and L 3 are as defined above.
Preferably, in the compound of formula (V)
M is Ru
Li , L 2 , and L 3 are independently selected from the group consisting of H, CO, PPh 3 , and a halogen selected from F, Cl and Br;
R 2 and R’ 2 independently represent N(Et) 2 , N(i-Pr) 2 , P(i-Pr) 2 , P(t- BU) 2 , NH(t-Bu) , NH(i-Pr) , PPh 2 , P(i-Pr) 2 .
In an embodiment, the compound of formula (I) as defined above is of the following formula (VI) below:
(VI I) wherein M, R 2 , R’2, Li , L 2 , and L 3 are as defined above.
Preferably, in the compound of formula (VI I)
M is Ru
Li , L 2 , and L 3 are independently selected from the group consisting of H, CO, PPh 3 , and a halogen selected from F, Cl and Br;
R 2 and R’ 2 independently represent N(Et) 2 , N(i-Pr) 2 , P(i-Pr) 2 , P(t- BU) 2 , NH(t-Bu) , NH(i-Pr) , and PPh 2 .
In an embodiment, the compound of formula (I) as defined above is of the following formula (VI I) :
(VI I) wherein R 2 and R’ 2 are as defined above.
Preferably, R 2 and R’ 2 independently represent N(Et) 2 , N(i-Pr) 2 , P(i- Pr) 2 , P(t-Bu) 2 , N H(t-Bu), NH(i-Pr), and PPh 2 .
Preferably, the acceptor-less dehydrogenation catalyst according to the present embodiments is selected from the group consisting of a compound of formula (IX), a compound of formula (X) and a compound of formula (XI), a compound of formula (XII), a compound of formula (XIII), a compound of formula (XIV), a compound of formula (XV), and a compound of formula (XVI) represented below:
The compound of formula (IX) is also known as Ru PNP. The compound of formula (X) is also known as Ru PNN. The compound of formula (XI) is also known Ru PNNH. The compound of formula (XI I) is also known as RuAcridinel . The compound of formula (XI I I) is also known as RuAcridine2. The compound of formula (XV) is also known as Mn PNP. The compound of formula (XVI) is also known as Mn PNNH.
Electrocatalytic oxidation
In particular embodiments, the alcohol is electrocatalytic oxidized at least to an ester.
In particular embodiments, the electrocatalytic oxidation is carried out in a homogeneous phase.
In other particular embodiments, the acceptor-less dehydrogenation catalyst is solubilized in the alcohol which is oxidized, the oxidation being realized without additional organic solvent.
In other particular embodiments, the acceptor-less dehydrogenation catalyst is solubilized in an additional organic solvent.
In particular embodiments, the use is carried out in a heterogeneous phase, and the acceptor-less dehydrogenation catalyst is immobilized on a conductive support.
In particular embodiments, the use is carried out under heterogeneous conditions and in that the alcohol is oxidized at least to ester.
Under heterogenous conditions, advantageously, the solvent is water and the pH between 7 and 14. In some particular embodiments, the alcohol is advantageously oxidized at least to 30 FE% (Faradaic Efficiency) to an ester with these conditions: 0.1 M LiCI, 0.1 M LiOH in EtOH, 1 mM catalyst, 0.3 V vs Ag/AgNO 3 (0.01 M in 0.1 M TBAPFe in CH3CN) , glassy carbon working electrode, separated counter electrode compartment.
In some particular embodiments, the alcohol is advantageously oxidized at least to 50 FE% (Faradaic Efficiency) to an ester with these conditions: 0.1 M LiOH in 10 % w/w EtOH in H2O, catalytic ink comprised of 0.2 mg/cm 2 catalyst, 1 mg/cm 2 carbon black (xc72r) , 5 |iL/cm 2 Nation ® (5 % w/w) deposited on Toray paper, 0.3 V vs Ag/AgCI, cathode compartment separated by an anion exchange membrane (Sustanion) .
In some particular embodiments, the alcohol is advantageously oxidized at least at 25 FE% (Faradaic Efficiency) to an ester with these conditions: 0.2 M LiBF4 in 10 % w/w EtOH in H2O, catalytic ink comprised of 0.2 mg/cm 2 catalyst, 1 mg/cm 2 carbon black (xc72r) , 5 |iL/cm 2 Nation ® (5 % w/w) deposited on a conducting support (Toray paper), 3 mA constant current electrolysis, separated cathode compartment.
In some embodiments, alcohol is ethanol and ethanol is oxidized to ethyl acetate.
In some particular embodiments, the electrocatalytic oxidation is conducted:
- at ambient temperature, without heating , or
- at temperature inferior to 60°C.
In some embodiments, the organometallic catalyst is in contact with a working solution comprising the alcohol and comprising a base chosen in the group comprising MOH, MOR (R = alkyl, benzyl) , MOtBu (with M = Li, Na, K) , or neutral organic bases such as lutidine, pyridine, DBU ( 1 ,8-Diazabicyclo[5.4.0]undec-7-ene), TBD (Triazabicyclodecene) or other guanidine bases, trialkyl amines, or strong phosphorous bases, such as Verkade’s proazaphosphatranes and phosphazenes.
In some particular embodiments, the organometallic catalyst is PNN, the base being KOH or LiOH, 1 mM of organometallic catalyst being solubilized in the working solution comprising 0.1 M of the base.
In some embodiments, a constant current of 3 mA is applied.
In some particular embodiments, an anodic half-cell reaction is coupled with a cathodic half-cell reaction , the cathodic half-cell reaction being an electrochemical reduction of CO2 to CO.
Advantageously, the electrocatalytic oxidation takes place in a flow cell.
Particular embodiment
In a particular embodiment, the acceptor-less dehydrogenation catalyst used for an electrocatalytic oxidation of an alcohol to an ester or/and an acid under electrochemical conditions, is represented by the following structure of any one of the formulae F1 , F2 or F3:
M being selected from the group consisting of Fe, Co, Ni, Ru , Rh , Pd, Os, Pt, Ir, Mn ;
L and X being anionic and neutral ligands independently selected from the group consisting of H, CO, PR a RbRc, P(OR a )(ORb)(OR c ) , AsR a RbR c , SbR a , Rb, R c , SR a Rb, a nitrile group (R a CN or CN) , N 2 , CS, a heteroaryl group, OR a , N(R a ) 2 , OCOR a and a halogen group; the indices n and m of the ligands X and L are equal to 1 , 2 or 3;
R a , Rb, R c which are the same or different independently represent H, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group having 6 to 1 2 carbon atoms, a halogen atom , a heterocyloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 3 to 12 carbon atoms, an alkylcycloalkyl group having from 3 to 1 0 carbon atoms and an alkylaryl group having from 6 to 12 carbon atoms
Z being selected from the group consisting of C, N ;
R’ being an organic substituent of the aromatic ring ;
R being an organic substituent, typically selected from the group consisting of 'Pr, f Bu, Ph, Et, Me, Bn, H. In some particular embodiments, X is an anionic ligand.
In some embodiments, L is a neutral ligand.
In some embodiments, in the formulae F1 :
E’ and E which are the same or different independently represent PRdR e , P(ORd)(OR e ) , P(OR d )(R e ) , an amine group (NR d R e ), an imine, an oxazoline, a sulfide (SRd) , a sulfoxide (S( = O)Rd), OCORd, and ORd;
Rd, R e which are the same or different independently represent H, an alkyl group having from 1 to 6 carbon atoms, a cycloalkyl group having from 3 to 6 carbon atoms, an aryl group having from 6 to 1 2 carbon atoms, a heterocyloalkyl group having from 3 to 6 carbon atoms, a heteroaryl group having from 3 to 12 carbon atoms, an alkylcycloalkyl group having from 3 to 1 0 carbon atoms and an alkylaryl group having from 6 to 1 2 carbon atoms,
In some particular embodiments, the catalyst is represented by the structure of any one of the formulae RuPNN, Ru PNP, RuPNN H,
RuAcridinel , or RuAcridine2 :
The present invention will be understood and appreciated more fully from the following non-limiting Figures and Examples.
DESCRI PTION OF THE FIGURES
Figure 1
Figure 1 shows the structure of three acceptor-less dehydrogenation catalyst according to the invention for an electrocatalytic oxidation of an alcohol to an ester or/and an acid under electrochemical conditions.
Figure 2
Figure 2 shows the structure of five Ruthenium acceptor-less dehydrogenation catalysts according to the invention for an electrocatalytic oxidation of an alcohol to an ester or/and an acid under electrochemical conditions.
Figure 3
Figure 3 shows the principles of electrochemical activation of acceptorless dehydrogenation catalysts, in homogenous system (separate cell) .
Figure 4
Figure 4 is 1 H-NMR of electrolytic solution , experimental conditions being 0.1 M LiOH in pure EtOH, 1 -5 mM catalyst, T = 25°C, V app iied = 0.3 V vs SCE.
Figure 5
Figure 5 shows the principles of electrochemical activation of acceptorless dehydrogenation catalysts, in heterogeneous system (flow-cell) .
Figure 6
Figure 6 is a charge versus time curve obtained in a flow cell, the experimental conditions being 0.1 M LiOH in 10 wt% EtOH in H2O, 0.2 mg . cm -2 catalysts, T = 25°C, V app iied = 0.3 V vs SCE.
Figure 7
Figure 7 is 1 H-NMR (D2O) after reaction using the conditions 0.1 M LiOH, EtOH, 0.3V vs SCE, I mM cat, T = 25°C, homogenous conditions, AD catalysts being Ru PNN. Figure 8
Figure 8 is a charged passed elapsed diagram under conditions of Figure 7, TON = 17, TOF = 3,4 h 1
Figure 9
Figure 9 is a charged passed diagram using the conditions 0.2 mg. cm 2 cat, 0.3 V vs SCE, 10 wt% EtOH in H2O, 0.1M LiOH, heterogeneous conditions, TON = 160, TOF = 60 h 1 , TON EIOAC = 47, TON OAc = 113, AD catalyst being RuPNN.
Figure 10
Figure 10 represents the normalized maximum rate generated by the tested catalysts (pA/mM) (RuPNN (5mM), RuPNNH (4 mM), RuPNP (4 mM), MnPNP (4 mM), MnPNNH (4 mM)) in function of the potential peak (V vs Fc 0/+ ) at low base loading (in THF, 0.1M TBAPF 6 , 1M EtOH, 10 mM NaOEt).
Figure 11
Figure 11 represents the normalized maximum rate generated by the tested catalysts (pA/mM) (RuPNN (5mM), RuPNNH (4 mM), RuPNP (4 mM), MnPNP (4 mM), MnPNNH (4 mM)) in function of the potential peak (V vs Fc 0/+ ) at low base loading (in THF, 0.1 M TBAPF 6 , 1M EtOH, 20 mM NaOEt).
EXAMPLE
EXAMPLE 1
A group of ruthenium pincer complexes such as are shown to be active in the electrochemical oxidation of ethanol.
The reaction set-up used is a three-electrode assembly comprising of an inert glassy carbon working electrode, a platinum mesh counter-electrode separated from working compartments by a ceramic frit, and either a SCE (sat. Calomel) or an Ag/AgNOs (0.01 M AgNOs, 0.1 M TBAP in CH3CN) reference electrode. In this homogeneous three- electrode set-up, the molecular catalyst (in general 1 mM) is directly solubilized in the working solution . The working solution generally comprises the alcohol (ethanol) and 0.1 M of base (e.g . KOH, LiOH, NaOH), both as electrolyte and co-substrate.
The products formed are analyzed by ionic chromatography, GC/MS and 1 H-NMR.
As an example, using the PNN-complex in 0.1 M KOH at 0.3 V vs. SCE, 6.2 mM acetate can be formed after 3h , corresponding to turnovernumbers > 6.
To the knowledge of the inventors, this is the first example for the electrochemical activation of AD-dehydrogenation catalysts for primary aliphatic alcohols. Nevertheless, the product formed (acetate) is reminiscent to reaction pathways also accessible via transferhydrogenation catalysts. Tuning the conditions (e.g . 0.1 M LiOH), the unique reactivity of AD-catalysts can be explored. Under these conditions (still 0.3 V vs. SCE) , > 7mM of ethyl acetate can be formed (together with 1 1 mM acetate) . To the knowledge of the inventors, this is the first example of molecular electrochemical reforming of ethanol to ethyl acetate with an AD-catalysts.
Although homogenous conditions are advantageous in terms of few necessary infrastructure needed, as well as the possibility to analyze the reaction (mechanism) in detail, for practical applications, immobilizing the catalyst on an electrode surface might be more desirable.
[0036] A catalytic ink was fabricated by mixing the catalyst with a conducting support (e.g . carbon black, carbon nanotubes, graphene) and a binder (e.g . Nation®), suspended in a solvent (e.g. acetone, THE, ethanol) .
The mixture is sonicated shortly and then deposited on an electrode support (e.g . Freudenberg paper, Toray paper, carbon cloth) via hot drop casting (in general, 1 0 °C below the boiling point of the employed solvent) .
Catalysts loadings on the final electrode are 0.2 mg/cm 2 .
A 1 0 cm 2 electrode (Freudenberg paper as support) was fabricated an inserted into an electrochemical flow cell.
The anolyte (and catholyte) compartment including tubings had a volume of 1 00 mL.
A Sustainion® anion exchange membrane was used to separate catholyte and anolyte compartments.
A 10 cm 2 commercial Pt/Ti alloy was used as the cathode.
The anolyte solution was recycled and flown through the flow cell at a rate of 1 L/h.
Electrolysis was conducted in 0.1 M LiOH in 1 0wt% ethanol in water at 0.3 V vs. Ag/AgCI (cell potential of. 1 .74 V) for 3 h at 25 °C.
GC/MS and IC confirmed the formation of 2 mM ethylacetate (turnovernumber > 47) and 4.6 mM acetate (turnovernumber > 1 1 0) with faradaic efficiencies around >90% under un-optimized conditions.
Importantly, the low cell potential has to be noted as well as a low overpotential at the anode of approximately 520 mV.
These results demonstrate the possibility of immobilizing the catalysts successfully, increasing catalyst lifetime and upscaling the reaction conditions.
The invention demonstrates that for the first time acceptor-less dehydrogenation catalysts can be activated electrochemically and that, moreover, their thermal chemistry can be directly translated into electrochemical schemes, i.e. the same products can be obtained under thermal and electrochemical set-ups.
Compared to the few examples of heterogenous ethanol reforming to ethyl acetate for example, catalyst loading is extremely low (as well as the transition metal content). In addition , given that AD- catalysts can be activated electrochemically, their broad range of applications can be electrified.
Finding suitable molecular electrocatalysts for alcohol reformation is a remarkable challenge. Most reported cases of molecular electrocatalytic alcohol oxidation are limited to non-preparative studies, secondary alcohols known to be good transfer hydrogenation targets and or low turnover number <5. An example of performing molecular electrocatalytic alcohol oxidation by the Gruzmacher group using a transfer hydrogenation catalyst, is not able to access the same chemical space than under thermal activation schemes.
Using electrochemistry instead of thermal activation has several advantages including , cheap reagents (electrons) , safety (avoidance of high temperature and pressure, as well as explosive/highly reactive reactives), control and scalability (flow-application and cell-stacks) . Hence, being able to translate a field of classical thermal chemistry (hydrogenation/dehydrogenation chemistry) into electrochemistry is highly advantageous for all applications of that field.
Using catalytic electrochemistry for the oxidation of ethanol to ethyl acetate allows the production under highly atom and energy efficient conditions. Indeed, using ethanol as the starting material, the only byproduct formed is formally H2 in the form of protons and electrons. It could thus replace common oxidation procedures using stoichiometric amounts of oxidants or procedures that liberate H 2 under refluxing conditions.
If the stability and activity of the employed catalysts can be increased, the present method might be interesting to synthesize a variety of esters from readily available alcohol feedstock under controlled and safe conditions.
A commercial electrolyzer for organic synthesis might be fabricated that would allow the preparation of oxidized compounds under highly energy efficient and safe conditions. Adopting a flow cell approach , such an electrolyzer could range from lab scale production for synthetic purposes to large scale acid/ester production from cheap primary resources, in particular for chemical industry, pharmaceutical industry and cosmetic industry.
The possibility to apply the proposed technology to hydrogen storage/release applications has tremendous potential, in particular for LOHC (Liquid Organic Hydrogen Carrier) .
The chemical industry is responsible for around 25% of global industrial energy consumption and thus for around 12.5% of total energy consumption today. Replacing thermal activation schemes in chemistry with electrocatalytic methodologies pledges to bring a long several advantages, such as safety, scalability, atom efficiency, reaction control and finally energy efficiency. Indeed, in electrochemical transformations the energy input for a given reaction can be controlled and monitored finely, offering the opportunity to make electrochemistry a key player in a sustainable economy of the future. Finding potent molecular electrocatalysts for the reversible oxidation/reduction of alcohol/carbonyl substrates is thus a remarkable challenge. EXAMPLE 2
Several Acceptor-less alcohol dehydrogenation catalyst according to the invention have been tested for their activity in electrochemical alcohol oxidation using cyclic voltammetry.
1. The following conditions have been tested :
In all cases, catalytic currents are observed and both ruthenium and manganese-based catalyst are shown to be active. Comparison of activity in the presence and absence of catalyst clearly demonstrate the positive effect of all probed catalysts, both in terms of current (rate) and potential (energy efficiency).
2. Electrochemical alcohol oxidation in the presence of the tested catalysts have been compared to uncatalyzed alcohol oxidation in the presence of 1 M EtOH, 20 mM NaOET in 0.1 M TBAPF 6 in THE.
All tested catalyst clearly showed important catalytic activity, both in terms of potential, as well as rate (current), compared to the uncatalyzed alcohol oxidation , which proceed at significant hig her potentials and lower currents. 3. The maximum currents generated by the tested catalysts have been compared in THF, 0.1M TBAPFe, 1 M EtOH, 10 mM NaOEt (Figure 10), and in THF, 0.1M TBAPF 6 , 1 M EtOH, 20 mM NaOEt (Figure 11) normalized by their respective concentrations.
The positive effect of co-substrate (base) is apparent from the increase of peak currents, as well as the shift toward less positive potentials.