Preparation And Co₂ Adsorption Performance Of Functionalized Metal-organic Frameworks

CO₂ capture and sequestration is considered to be the most efficient way to control greenhouse effect.Adsorption is one of the effective postcombustion methods of CO₂ sequestration owing to its low energy requirements and a relatively simple technology.Adsorption technology is a technique which focuses on porous structured,so to develop new porous materials efficient to adsorb CO₂ is the key point.As porous materials,metal-organic frameworks（MOFs）is considered as one of the promising physical adsorbent for CO₂ capture.To further enhance CO₂ adsorption capacity and higher selectivity for CO₂/N₂,some MOFs were modified by postsynthetic method to obtain novel modified MOFs.These materials andthe parent composite components were characterized using X-ray powder diffraction（XRD）,thermo-gravimetricanalysis（TGA）,N₂ adsorption-desorption isotherm analysis（BET）,FT-IR and SEM/EDX techniques.CO₂ adsorption on these composites was evaluated at different temperatures under wide pressure range.The main contents and results are summarized as follows:Three types of Mg/DOBDC samples modified by EDA,TETA or TEPA were synthesized for CO₂ adsorption.Their CO₂ capture performance was investigated by dynamic adsorption experiments at 60℃.It was found that the aminated Mg/DOBDC was an efficient adsorbent for CO₂ capture.Among them,the sample aminated by TEPA exhibited the highest adsorption capacity with the longest breakthrough time of CO₂.Before saturation,its CO₂ adsorption capacity was up to 6.06 mmol/g.In addition,among the adsorbents of Mg/DOBDC modified by 30 wt%,40 wt%and 50 wt%TEPA,Mg/DOBDC-40 had the highest adsorption capacity.Moreover,CO₂ adsorption on the TEPA-Mg/DOBDC adsorbent was promoted by water vapor,and the adsorption capacity was enhanced to 8.31 mmol/g.It was showed that the water vapor was beneficial to the adsorption of CO₂ on the TEPA-Mg/DOBDC.The adsorption capacity of the TEPA-Mg/DOBDC adsorbent dropped only 3%after 5 consecutive adsorption/desorption cycles.A typical metal-organic framework HKUST-1 was modified by doping it with alkali metals（Li,Na and K）,which was further used to investigate the CO₂ adsorption capacities of these materials.The results showed that the CO₂ storage capacity of HKUST-1 doped with moderate quantities of Li+,Na+ and K+,individually,was greater than that of unmodified HKUST-1.The highest CO₂ adsorption uptakeof 8.64 mmol/g was obtained with 1K-HKUST-1,and it was ca.11%increase in adsorption capacity at 298 K and 18 bar as compared with HKUST-1.The adsorption/desorption cycle experiment revealed that the adsorption performance of 1K-HKUST-1 was fairly stable,without obvious deterioration in the adsorption capacity of CO₂ after 10 cycles.New composites of UiO-66 and graphene oxide（GO）were synthesized by hydrothennal method.The effect of graphite oxide（GO）content on the crystal structure,surface morphology and pore structure of the composites was investigated.At the same time,tthe CO₂ adsorption performance of UiO-66 and UiO-66/GO composites were compared.The influence of the structure and chemical properties on the CO₂ capture performance was studied.Through the analysis of the experimental results,it was concluded that with the increase of the amount of GO,the particle size of UiO-66/GO composites decreases,and the shape of particles becomes more and more irregular.The specific surface area of UiO-66/GO composites increases first and then decreases with the increase of GO content.When the amount of GO was 5%,the BET specific surface area of the UiO-66/GO-5 material was increased by 40%,up to 1184 m/g.Low pressure and high pressure CO₂ static adsorption experiment results showed that the composite UiO-66/GO-5 exhibited the maximum CO₂ uptake of 3.37 mmol/g at 298 K and 1 bar,which increased by 48%in comparison with that of the UiO-66;10.85 mmol/g at 298 K and 14 bar.To further enhance CO₂/N₂ selectivity,on the basis of the last chapter,the UiO-66-NH2 and the UiO-66-NH2/GO composites were prepared by hydrothermal method.The UiO-66-NH2/GO composites surface area,total pore volume and micropore volume were 1052 m2/g,0.345 cm3/g and 0.286 cm3/g,an increase of 28%,46%and 34%than the UiO-66-NH2.Compared with the UiO-66-NH2,the thermal stability of the UiO-66-NH2/GO composites was also significantly increased.Chemical stability test results showed that the UiO-66-NH2/GO composite could exist stably in water,acid and alkali environment,the structure did not collapsed.Low pressure CO₂ static adsorption experiment results showed that the high CO₂ adsorption capacity of UiO-66-NH2/GO composites was determined by the high specific surface area and pore volume as well as the additional reaction adsorption sites in the composites.Compared to the UiO-66,the UiO-66/GO and the UiO-66-NH2,UiO-66-NH2/GO showed the highest CO₂ adsorption heat and CO₂/N₂ selectivity.The adsorption/desorption cycle experiment revealed that the adsorption performance of the UiO-66-NH2/GO composites was fairly stable,without noticeable degradation in the adsorption capacity of CO₂ after 6 cycles.