Applied Fundamental Research Of Modified Mesoporous Materials In Carbon Dioxide Capture

In recent years,the sharply atmospheric CO2 concentration has increased dramatically due to the wide-scale consumption of fossil fuels,which leads to the aggravation of environmental problems such as greenhouse effect,thus,the emission reduction of CO2 has become a global focus of attention.The adsorption method has the advantages of high product purity,fast mass transfer rate and low energy consumption,and showed a broad application prospect in the field of CO2 capture.Adsorbent is the key to determine the CO2 adsorption efficiency.However,the current CO2 adsorbents generally have the drawbacks of low adsorption capacities at low pressures,very sensitive to adsorption conditions and insufficient stability,which limit the further promotion of the adsorption method.Therefore,it is urgent to develop superior CO2 adsorbents with high adsorption capacity,high selectivity and great stability.Based on the above background,this work was focused on the key issues in the development of high-efficiency CO2 adsorbents,a series of mesoporous adsorbents with nitrogen rich active sites were prepared in order to improve the CO2 adsorption performance,meanwhile,the CO2 adsorption mechanism was carried out.The main research results obtained in this work are as follows:(1)A new type of porous magnesium carbonate adsorbent was synthesized based on MgO and CO2,and CO2 storage was realized in the material synthesis stage.The porous magnesium carbonate was further modified by polyethyleneimine(PEI)and tetraethylenepentamine(TEPA),and a new amine-modified CO2 adsorbent was synthesized.By investigating the CO2 adsorption behavior of the amine-modified adsorbent under different amine loading,adsorption temperature,and gas flow and flue gas components,it was found that the amine modification significantly improved the CO2 adsorption effect.The optimal PEI loading was 20 wt.%,and the highest CO2 adsorption capacity was 1.07 mmol/g(75?).Compared with unmodified adsorbent,the CO2 adsorption capacity was increased by 2.96 times,and showed good regeneration stability.It is found that the CO2 adsorption process of amine-modified porous magnesium carbonate follows the acid-base reaction mechanism,and the moisture in flue gas can promote the CO2 adsorption of amine-modified adsorbent.(2)Aiming at the problem that amine-modified adsorbent was prone to amine exudation,the ordered nitrogen-doped mesoporous carbon adsorbents were prepared by one-step hydrothermal self-assembly method using Pluronic F127 as soft template and melamine or urea as nitrogen source,and the stability of the adsorbent was effectively improved by introducing nitrogen-containing groups into the carbon skeleton in situ.The product showed a large specific surface area(421-498.6 m2/g),a highly ordered pore structure(p6mm)and high nitrogen content.Nitrogen atoms were uniformly doped in the carbon skeleton in the form of pyridine-N,pyrrolic-N,quaternary-N and pyridine-N-oxide,which significantly improves the CO2 adsorption performance of the carbon adsorbent by increasing the polarity of mesoporous carbon.It is found that the adsorption behavior of CO2 on ordered nitrogen doped mesoporous carbon follows the multi-scale synergistic reaction mechanism of nitrogen group and pore structure.The NOMC-M adsorbent has a maximum CO2 adsorption capacity of 3.55 mmol/g,and exhibits excellent regeneration stability in the continuous CO2 adsorption/desorption cyclic test.Various adsorption isotherm models were used to fit the adsorption isotherms of CO2 and N2,further combined with the ideal adsorption solution theory(IAST),the adsorption selectivity of the ordered nitrogen-doped mesoporous carbon for CO2/N2 was simulated and predicted.(3)In view of the possible collapse of the mesoporous structure caused by the addition of nitrogen sources,highly ordered nitrogen-doped mesoporous carbon adsorbents(NOMCs)were synthesized through amino acid catalyzed hydrothermal self-assembly method by using meta-aminophenol as the carbon and nitrogen homologues.L-lysine was used to effectively control the direction of the polymerization reaction,increase the reaction rate,and act as a nitrogen source to simultaneously participate in the self-assembly process and doped into the carbon skeleton.The effects of the nitrogen content of the adsorbents on the structure and surface properties of NOMCs were investigated using various characterization methods.The results showed that NOMCs exhibited high specific surface area(351-435 m2/g)and uniform pore size distribution,and can maintain an orderly two-dimensional hexagonal p6mm mesoporous structure under high nitrogen content conditions.The synergistic effect between the pore structure and the nitrogen content of the adsorbent was an important factor affecting the CO2 adsorption performance.Thus,the NOMC-L-0.5 adsorbent with appropriate nitrogen content and highly ordered mesoporous structure showed the highest CO2 adsorption capacity and excellent regeneration stability.Based on the Clausius-Clapeyron equation,it was found that the adsorption process of CO2 on adsorbent was physical adsorption.The Dual-site Langmuir(DSL)model had the most accurate fitting results for adsorption behavior of CO2 and N2 on NOMCs(R2>0.999),and the CO2/N2 adsorption selectivity of NOMC-L-0.5 in typical coal-fired power plant flue gas components predicted by the I AST model was as high as 43.2,which provides guidance for the preparation of adsorbents and the optimal design of adsorption processes.