The Study Of CO₂ Adsorption From Flue Gas With Polyamine-based Protic Ionic Liquid Functionalized Mesoporous Silica

As global industrialization development,the large-scale combustion of fossil fuels has led to the increasing concentration of CO2 in the atmosphere,resulting in greenhouse effect and climate change.Emissions of CO2 from flue gas are the main cause of the increasing atmospheric CO2 concentration,so how to remove CO2 efficiently and with low energy consumption in flue gas has become the focus of research at home and abroad.Compared with the conventional absorption and separation technologies for CO2 capture,the adsorption method has the advantages of low corrosion,low energy consumption and high compatibility with existing equipment.It is regarded as a CO2 separation technology with great prospect of industrial application due to its advantages.Among the many adsorbents,solid amines have been widely studied for their advantages such as large adsorption capacity and fast adsorption rate,but their amines are easily leached and flue gas impurities like SO2 can react with amines irreversibly,resulting in the reduction of adsorption capacity or even inactivation.In recent years,the ionic liquid with very low saturated vapor pressure,excellent thermal stability,and adjustable structure has become the hotspot of CO2 capture,but the traditional ionic liquid has complex synthetic steps,high cost,and low CO2 absorption capacity,limiting the wide range of industrial applications.Therefore,in order to develop an adsorbent with high CO2 adsorption capacity,high stability,and strong impurity gas tolerance,this paper would like to prepare ionic liquid complexes by synthesizing novel polyamine-based protic ionic liquids and their functionalized mesoporous molecular sieves(SBA-15)solid adsorbents,to study their dynamic CO2 adsorption capacity under simulated flue gas conditions,and to screem out the solid adsorbents with highest CO2 adsorption capacity.The effect of flue gas impurities on CO2 adsorption capacity of the optimized adsorbent is also studied.In addition,this paper investigates the CO2 adsorption capacity of squarate-calcium metal-organic framework(Ca(C4O4)(H2O))and the effect of water vapor on its adsorption performance.Firstly,we prepared novel polyamine-based protic ionic liquids(triethylenetetrammonium nitrate([TETA][NO3])and tetraethylenepentammonium nitrate([TEPA][NO3]))by acid-base neutralization of inexpensive organic amines and inorganic acids in a one-step process.Then,the polyamine-based protic ionic liquids funcetionalized SBA-15 hybrid adsorbents was synthesized by wet impregnation.The hybrid adsorbents have high CO2 adsorption capacity as well as fast CO2 adsorption rate.The hybrid adsorbents still maintained the two-dimensional hexagonal pore structure of the support,and with the increasing of loading amount the specific surface area,pore volume,and average pore size were gradually decreased and the CO2 adsorption capacity showed a tendency of increasing first and then decrease.The optimized impregnation of[TETA][NO3]and[TEPA][NO3]was 66 wt%.In the temperature range of 298-348 K?the maximum CO2 adsorption capacity of[TETA][NO3]and[TEPA][NO3]modified SBA-15 were 2.12 and 2.15 mmol/g,respectively,at 333 K.The intraparticle diffusion model was applied to calculate the CO2 adsorption rates.The[TETA][NO3]and[TEPA][NO3]modified SBA-15 adsorbents have fast CO2 adsorption rates of 131 ×10-3 and 147 ×10-3 mmol/g·s0.5,respectively.It was three times higher than other ionic liquid or amine-modified solid sorbents.In addition,both adsorbents possessed good long-term cyclic stability.The density functional theory(DFT)calculations showed that CO2 preferentially reacted with the primary amine-N(3)H2 in[TETA][NO3]and secondary amine-N(2)H in[TEPA][NO3]to form carbamate.Secondly,the effect of the presence of trace impurity gases(H2O,SO2,and NO)in the flue gas on the CO2 adsorption performance of[TETA][NO3]and[TEPA][NO3]functionalized SBA-15 composites was studied.The results showed that the CO2 adsorption capacity and capture rate of the two hybrid materials increased by about one-third in the presence of trace amounts of H2O.This is attributed to additional CO2 capture pathways with amine groups that result from the humid environment.The additional pathway was further explained with FT-IR spectroscopy and DFT calculations,which reveal that in the presence of H2O the CO2 molecules react with amine groups of the protic ionic liquids to form stable zwitterionic bicarbonate ions.On the other hand,the sorbents have strong resistance to SO2 and NO over the tested concentration range of 0?500 ppm,with limited impact on CO2 capture.For the high SO2 tolerance of the composites,the effect of the introduction of nitrate on the SO2 tolerance was investigated using triethylenetetramine and tetraethylenepentamine functionalized SBA-15 without nitrate as the control groups.The results showed that the SO2 tolerance of the nitrate-containing sorbents was significantly higher than that of sorbents without nitrate over multiple capture cycles in the presence of 500 ppm.The significant improvement in SO2 resistance was further explained with DFT calculation,which revealed the binding energy and net charge transfer amounts between SO2 and amine groups were decreased after introducing the nitrate into the amine.To further investigate the regenerability and structure stability of nitrate-containing sorbents for CO2 capture under realistic conditions,the long-term operating performance was evaluated under 150 ppm SO2,150 ppm NO,100%RH,15%CO2,balance N2.The CO2 adsorption capacities and the structural properties of nitrate-containing sorbent were nearly unchanged over 8 adsorption/regeneration cycles.Finally,we have studied the CO2 adsorption performance of a squarate-calcium metal-organic framework(Ca(C4O4)(H2O))with high stability and water vapor resistance.The accessible pore size of Ca(C4O4)(H2O)is 3.4 A,which is larger than the kinetic diameter of CO2(3.3 A)but smaller than the kinetic diameter of N2(3.65 A)suggesting great size sieving potential for CO2/N2 separation.Ca(C4O4)(H2O)exhibited excellent separation performance for CO2/N2(15/85 vol)with adsorption selectivity values of 25 at 1 bar and 298 K.Additionally,Ca(C4O4)(H2O)showed strong interaction with CO2 based on the high isosteric heat of 49.9 kJ/mol.The strong affinity between CO2 and Ca(C4O4)(H2O)was further explained with DFT calculations,which exhibited that CO2 was bound tightly via hydrogen bonds between the oxygen atoms in CO2 and hydrogen atoms in H2O,as well as ?-? interactions between CO2 and the aromatic ligand in the framework.Furthermore,the cyclic adsorption/regeneration experiments under 100%RH,15 vol%CO2,and balance N2 demonstrated that Ca(C4O4)(H2O)possessed the excellent tolerance to water vapor and recyclability.However,its dynamic CO2 adsorption capacity is relative low,only 1.11 mmol/g,which cannot effectively capture CO2 from flue gas streams.