Enantioselective Synthesis Of Amino Acids By Electrocatalytic Reduction Of CO₂ On Chiral Cu Nanomaterials

Carbon dioxide reduction products ranging from small molecules to biomolecules are highly diverse in natural systems.Electrocatalytic reduction of carbon dioxide to produce a variety of products is one of the most efficient methods to understand nature and convert carbon dioxide into usable energy or industrial chemicals.The Cu and Cu-based alloys are the only family of catalysts for the electroreduction of CO₂ to multicarbon products,i.e.,the C₁-C₂ molecules and C_3-4 aliphatic alcohols.In addition,the products of CO₂ reduction with C-N bonds are limited to C₁-C₂ species（urea,methylamine,ethylamine and acetamide）.The C₃₊products with various functional groups（C-N and C=O bonds）,especially the biomolecules,have never been reported.The lack of product variety and low efficiency in the formation of C₃₊products is due to the high reaction energy barrier on the catalyst.The strategy of this thesis is to create chiral kink sites with high Miller index on Cu surfaces to activate unconventional C₃₊production pathway of electrocatalytic reduction of CO₂ to amino acids.The energy barrier of the reactions would be decreased by electrostatic interaction and coordination effect due to geometrically arranged low-coordinated metal species in chiral kink sites.Meanwhile,enantioselectivity of amino acids would be achieved by chiral atomic surfaces due to the different affinity of amino acids enantiomers with difference geometric configuration.In this thesis,（i）chiral nanostructured copper films（CCFs）with chiral kink sites were prepared along with electroreduction of Cu ions interacted with chiral molecules dissolved in electrolyte on conductive substrate;（ii）enantoselectivity of CCFs for various amino acids and saccharides were found experimentaly,which was considered to be due to the different adsorption energy of the chiral kink sites to the enantiomers;（iii）enatimeric excess C₃₊amino acids were synthesized via electrocatalytic synthesis from carbon dioxide and NH₃ using CCFs as electrodes,due to the chiral kink sites on CCFs was more conducive to decreasing the free energy of3-hydroxypyruvate,and improving amino acids formation.Chapter 1.Introduction.Firstly,basics of carbon dioxide reduction and recent research progress of carbon dioxide electro-reduction were introduced.Secondly,the synthesis methods of amino acid,chiral nanomaterials with enantioselectivity were summarized.Finally,the challenges in the electro-reduction of carbon dioxide were clarified and the significance of the topic and research strategies were proposed.Chapter 2.Synthesis and chiral responses of CCFs.CCFs with chiral surfaces were synthesized via electrodeposition with His as both a structure directing agent and symmetry breaking agent on a Ni substrate.The formation of a ligand-to-metal charge transfer-type bonding between imidazole group of L-His and Cu ions induced the formation of fusiform nanoparticles arranged on CCFs.These nanoparticles were surrounded by the polycrystalline nanobuds with chiral kink sites.The reconstructed atomic model of high-angle annular dark-field scanning transmission electron microscopy of CCFs showed that Cu（653）^S is the most possible chiral kink sites on CCFs.The adsorption energies of L-His on Cu（651）^S,Cu（653）^S and Cu（17,5,1）^S are lower than other eight chiral Cu kink sites,which also indicated the Cu（653）^S is the most possible chiral kink sites induced by L-His.L-His adsorption energies on Cu（653）^S are lower than its D-His adsorption energies indicating the formation of this chiral kink sites on the L-CCF surfaces would be possibly induced via L-His adsorption.The optical activity of CCFs was characterized by diffused reflection circular dichroism（DRCD）.Finally,it was confirmed that no chiral residue was on CCFs surfaces by X-ray photoelectron spectroscopy（XPS）,high performance liquid chromatography（HPLC）,etc.Chapter 3.Enantioselectivity of CCFs for amino acids and saccharides.The behaviours of chiral inorganic surfaces of CCFs with simultaneous enantioselectivity for both amino acids and saccharides was studied by differential pulse voltammetry（DPVs）.DPVs showed that the oxidation currents for both L-amino acids and D-ribose on L-CCFs synthesized with L-His as a chirality-inducing agent were higher than those of their antipodes,indicating specific enantioselectivity on the same Cu chiral surface.Adsorption energies of ribose and amino acid enantiomers on the S-enantiomorphs of chiral kink sites suggested that stronger interactions between both L-amino acids and D-ribose with chiral kink sites was the sources of specific enantioselectivity.Such universal enantioselectivity could be attributed to that an unit of chiral kink sites Cu（653）^S is large enough to contain one saccharide or amino acid molecule,even arginine possessing the longest carbon chain in 21 kinds of proteinogenic amino acids,which allows all these chiral molecules interact with a minimum repetitive unit of Cu（653）^S with high efficiency and universal enantioselectivity.According to these results,possible chiral products from carbon dioxide reduction were preliminarily screened.Chapter 4.Synthesis of amino acid by carbon dioxide electroreduction on the chiral surface of CCFs.Carbon dioxide electroreduction was carried out with CCFs as the working electrode using NH₄HCO₃ aqueous solution as both the electrolyte and nitrogen source.Various amino acids,ethanol and formic acid were produced.The main product of amino acid was serine with an ee greater than 90%and Faraday efficiency of 1.2%.It was confirmed that these products were synthetized from carbon dioxide reduction by isotope experiments.It has been observed from analysis of carbon dioxide reduction products by in situ PIMS that the 3-hydropyruvic acid is the important intermediate in serine formation.H₂CO-COCO*was generated via the addition reaction of OH₂CCO*with CO*;3-hydropyruvic acid was then generated from hydrolysis of H₂CO-COCO*;and finally,serine was synthesized via amination with ammonia to form 3-hydropyruvic acid.Free energies of all intermediates was found to be lower on the Cu（653）^S than on the achiral Cu（111）,which makes the reaction on Cu（653）^S thermodynamic favorable.The configuration of intermediates can be stabilized by the interaction between chiral kink sites and the carbonyl group of the intermediate,which decrease the activation energy for serine formation.Asymmetric carbon dioxide reduction by CCFs was achieved by the lower adsorption energy of L-His than D-His on Cu（653）^S.In this thesis,the formation of the CCFs,asymmetric electrocatalytic carbon dioxide reduction to amino acids and the effect of chiral surfaces on electrochemical carbon dioxide reduction were studied.It was found that the configuration of three-carbon intermediates and transition states on the chiral surfaces of catalysts was one of the important reasons for improving the formation of C₃₊products with C-N bond.It provides an experimental and theoretical evidence for carbon dioxide reduction to C₃₊,and provides an important reference for the future research of electrochemical carbon dioxide reduction in chemistry and biology fields.