First-principles Study On The Microscopic Mechanism Of CO₂ Capture By Alkylethylenediamine-Functionalized M₂ (dobpdc) Series

With the progress of industry and rapid development of economy,excessive consumption of fossil fuels directly leads to the continuous growth of CO₂ concentration in the atmosphere,which aggravates the environmental problems brought by the greenhouse effect.Carbon capture and sequestration(CCS)is an effective strategy to reduce CO₂emissions.Therefore,the design and synthesis of potential CO₂ capture materials is essential.Among them,the organic amine functionalized M₂(dobpdc)(M=Mg,Zn,et al;H₄(dobpdc)=4,4?-dihydroxy-(1,1?-biphenyl)-3,3?-dicarboxylic acid)materials have become research hotspots due to their excellent characteristics of high CO₂ adsorption capacity and selectivity at low pressure,and low regeneration energy.At present,there have been a series of experimental studies on this kind of materials,but the theoretical research is still lacking.In addition,the structure-activity relationship between the material and amine structure,metal center et al has not been established,and the formation mechanism of the adsorption products has not been reported yet.The process of CO₂ capture in such materials is complex,and it is difficult to reveal the complete reaction path at the atomic level through experimental methods,while the calculation based on first principles is an effective mean to investigate the microscopic reaction mechanism.Therefore,the adsorption properties and microscopic reaction mechanism of CO₂ adsorption on M₂(dobpdc)(M=Mg,Sc-Zn)series modified by five alkylethylenediamines(the primary-primary amine:1,1-dimethyl-1,2-ethylenediamine;the primary-secondary amine:N–methylethylenediamine,N–ethylethylenediamine,N–isopropylethylenediamine;the secondary-secondary:N,N'–dimethylethylenediamine)have been investigated using van der Waals(vd W)corrected density functional theory(DFT-D3)calculations.The results show that the diamine molecules tend to interact with the unsaturated metal sites of M₂(dobpdc)via the N atom in the end with small steric hindrance,and after capturing CO₂,they prefer to form the highly ordered ammonium carbamates,which are consistent with the phenomenons observed in the experiment.With the increase of substituents,the binding strength of amines increases,however the CO₂ adsorption capacity decreases.The binding energies of CO₂ show a strong metal dependence.On the whole,when M is Sc and Mg,the material has a strong CO₂ adsorption capacity.These can provide important guidance for the selection of carbon capture materials.The Bader charge analyses demonstrate that CO₂is a strong electron acceptor,and its gaining electrons are mainly from the diamine,with a small amount from the metal atom bonded with CO₂.The results of mechanism show that the formation process of the ammonium carbamate includes two steps,that is,the formation of the N-coordinated zwitterion intermediate and the rearrangement of the intermediate.Firstly,nucleophilic addition between CO₂ and the metal-bound amine,and proton transfer simultaneously occur.This process is the rate-determining step,with higher energy barriers(0.6-1.6 e V).Then,the N-coordinated zwitterion intermediate formed during the nucleophilic addition is quickly rearranged into the O-coordinated ammonium carbamate product.This process is easy,with lower barriers(<0.6 e V),which can promote the whole reaction toward the product.This results in a suddenly abundant adsorption of CO₂,which can provide favorable evidence for the unusual step-shaped adsorption isotherms in the experiment.The barriers of the rate-determining step follow the order of 2°/2°>1°/2°>1°/1°.Moreover,the barriers of the rate-determining step have a positive correlation with the magnitude of magnetic moment accumulated on systems.This paper shows that the CO₂adsorption capacity can be tailored by the judicious choice of M²⁺type and the diamine structure,and the calculated results can provide important theoretical guidance for developing highly efficient CO₂ capture materials.In addition,unveiling the microscopic mechanism can provide an in-depth understanding of the micro process of CO₂ capture,and is of great significance for the design and optimization of materials.