Design And Preparation Of Adsorbents For CO₂ Capture And Low-carbon Olefins Adsorption

Metal-organic frameworks（MOFs）as new type of porous materials have attracted attention in recent years and show the great potential in gas separations like CO₂ capture and light hydrocarbons separation.By loading amines on MOFs,the CO₂ capacity could be effectively improved,while problems of the amine molecular diffusion,amine efficiency,and the stability of small molecular amines need to be addressed before practical application.In the separation of light paraffins/olefins,introducing transition metal ions to MOFs can enhance the ethylene adsorption capacity,but such adsorbents have problems of non cost-effective and low selectivity.To solve these problems,this work prepared the mixed alkanolamine-polyethylenimine adsorbents via wet impregnation.This work selected mesoporous silica and MIL-101（Cr）as supports,and polyethyleneimine（PEI）and hydroxyl-containing additives as amines.In addition,the Cu⁺π-complex adsorbents were prepared by double solvent methods with mesoporous silica and MIL-101（Cr）as supports.Hydroxyl groups improved the dispersion behavior of PEI and the amine efficiency of solid amine adsorbents.And the C₂H₄/C₂H₆ adsorption selectivity of adsorbents increased by loading Cu⁺.The main conclusions are summarized as follows:（1）Adding the additive Span80 into MIL-101（Cr）/PEI solid amine adsorbents can increase the flexibility of PEI segments and the number of contact sites of CO₂ with amine groups,and enable the CO₂ adsorption capacity to increase by about 40%.However,the promotion effect is weakened with increasing temperature.The addition of diethanolamine（DEA）could significantly improve the performance of MIL-101（Cr）/PEI in CO₂adsorption.The thermogravimetric adsorption test of CO₂ at 35℃showed that the CO₂ adsorption capacity of MIL-101（Cr）/PEI increased by～57%,up to 3.29 mmol/g after adding DEA.Although small molecule DEA easily decomposed,the adsorption capacity of MIL-101（Cr）/PEI/DEA maintained at 2.47 mmol/g after 10 adsorption-desorption cycles.（2）We have successfully prepared solid mixed amine adsorbents with mesoporous silica HP260 as supports,achieved co-loading of PEI and low molecular weight amine species containing hydroxyl groups.The results showed that two amine additives of DEA and 3-amino-1,2-propanediol（AP）were introduced into PEI respectively to reduce the CO₂ diffusion resistance and increase the active CO₂ adsorption sites.DEA had a more significant effect on the CO₂ adsorption performance of adsorbents than AP probably due to the difference in interaction force between DEA/AP and PEI,steric hindrance and CO₂affinity of DEA/AP.When the weight ratio of HP260/PEI/DEA reached to the optimal value of 2:1:1,the CO₂ capacity increased by～53%,achieving up to 2.93 mmol/g at 35℃in comparison to silica/PEI adsorbents.Their amine efficiency was improved from 0.17mol CO₂/mol N to 0.40 mol CO₂/mol N.The breakthrough test in a simulated flue gas（10%CO₂/N₂）at 35℃showed that moisture can increase the adsorption capacity of CO₂.Under humid conditions,the HP260/PEI/DEA CO₂ breakthrough capacities q_b and q₅₀were 1.57 and 2.49 mmol/g,respectively.Such adsorbents exhibits good reversibility and the CO₂ capacity is nearly stable after two adsorption-desorption cycles.（3）MOFs(Ni₂（dobdc）and Zn（ox）_0.5（trz）)were successfully prepared for C₂H₄/C₂H₆separation.At 17 bar,the C₂H₄ adsorption capacity of Ni₂（dobdc）and Zn（ox）_0.5（trz）were2.56 and 3.16 mmol/g,and C₂H₆ adsorption capacity were 2.09 and 3.03 mmol/g,respectively.C₂H₄/C₂H₆ IAST adsorption selectivity of Ni₂（dobdc）and Zn（ox）_0.5（trz）were 1.45 and 1.04,respectively.Cu⁺loadedπ-complex adsorbents were successfully prepared for C₂H₄/C₂H₆ separation.The C₂H₄ adsorption capacity of Cu⁺@C803 was 2.5mmol/g and the introduction of Cu⁺increased the separation selectivity from 1.16 to 1.29.The adsorption capacities of C₂H₄ and C₂H₆ of MIL-101（Cr）were 13.96 and 14.23mmol/g at 17 bar,respectively.Besides,the adsorption capacities increase with the increase of pressure,and the C₂H₄/C₂H₆ selectivity was 0.89 at 17 bar.After introducing Cu⁺,although the adsorption capacity reduced to 11.60 mmol/g with the increasing of C₂H₄ diffusion resistance,the IAST ideal selectivity of C₂H₄/C₂H₆ increased up to 1.63.