Synthesis And Performance Of Zinc-based MOFs@Polymer Composite Materials For Carbon Dioxide Adsorption

To achieve the“dual carbon”goal,the development of efficient and low-cost carbon capture technology has become the top priority.Among the separation technologies,adsorption separation has the advantages of high separation efficiency,low energy consumption,and no additional pollution.Metal-organic framework（MOFs）has attracted wide attention due to its adjustable pore structure,large CO₂ adsorption capacity,and high separation selectivity.However,these issues such as poor stability,water competition adsorption,and powder shaping have not been effectively resolved.This article will focus on the synthesis of low-cost and high-stability zinc-based MOFs@polymer composite.The main research contents are as follows:（1）For the cement kiln flue gases with CO₂ concentration higher than 15vol%,two pillared layered Zn-MOFs,Zn₂（Atz）₂（IPA）and Zn₂（Datz）₂（IPA）,were synthesized using hydrophobic isophthalic acid（IPA）as the organic ligands,Zn-3-amino-1,2,4-triazole（Zn-Atz）,and Zn-3,5-diamino-1,2,4-triazole（Zn-Datz）as the layers.During the synthesis process,the crystal structure was re-regulated by changing the types of solvent and the temperature of crystal growth.Test results show that the CO₂ adsorption capacities of Zn₂（Atz）₂（IPA）and Zn₂（Datz）₂（IPA）were 2.42 and 2.54mmol·g^-1 at 298 K and atmospheric pressure,respectively,which was higher than the value(1.79 and 2.39 mmol·g^-1)reported in the literature;the CO₂adsorption heats of Zn₂（Atz）₂（IPA）and Zn₂（Datz）₂（IPA）were 26.3 and 28.4k J·mol^-1,respectively,which was relatively low in MOFs;the water adsorption capacity of Zn₂（Datz）₂（IPA）is abnormally low,only 1.13 mmol·g^-1at 75%RH,which results in the retention of 80%CO₂ adsorption capacity in multi-component penetration experiment with wet flue gas（15 vol%CO₂/85vol%N₂,75%RH）.Furthermore,to achieve the efficient molding of powder,Zn₂（Datz）₂（IPA）nanocrystals were loaded in the amino-functionalization poly（acrylate）（AFP）by in-situ crystallization technique and obtained the Zn₂（Datz）₂（IPA）@AFP composite.When the crystal loading rate is 67.3 wt.%,the CO₂ adsorption capacity is 83.1%of the original powder,indicating that the CO₂ adsorption efficiency of Zn₂（Datz）₂（IPA）crystals obtained by in-situ crystallization is 23%higher than that of the original powder.（2）For the coal fired flue gases with CO₂ concentration of around 10～14vol%,CALF-20@poly（acrylate）composite material with the characteristics of carbophilic and hydrophobic is designed and synthesized using poly（acrylate）as the hydrophobic layer and Zn₂（1,2,4-triazolate）₂（oxalate）（CALF-20）as the carbophilic layer.Non-uniform crystallization was induced by the limited space provided by micrometer-sized pores in poly（acrylate）and the nanoscale rough surface,nanoscale Zn C₂O₄ crystals were obtained as the seeds of CALF-20 to reduce the nucleation barrier and obtain the CALF-20nanoparticles.Compared with the original CALF-20 with an average size of1.8μm,the crystals in poly（acrylate）have an average size of 170 nm.After the nano-crystallization,the increased surface energy and lattice defects provide more CO₂ adsorption sites,making the CO₂ adsorption capacity of the composite at 313 K and 0.1 bar reached 2.15 mmol·g^-1,and the adsorption efficiency of the crystal has been increased by 32%.In addition,the preferential adsorption of CO₂ and the blocking effect of hydrophobic shell weaken the competition adsorption of H₂O.Thus,in a multi-component competition experiment（10 vol%CO₂/90 vol%N₂,90%RH）,the composite material maintained 97.1%CO₂ adsorption capacity of dry conditions.（3）For natural gases post-combustion flue gas with CO₂ concentration of around 3～9 vol%,Zn₂（3-methyl-1,2,4-triazolate）₂（oxalate）（Zn₂（Mtz）₂Ox）crystals were synthesized by adjusting the crystal growth conditions.The results showed that the CO₂ adsorption capacity of the crystals obtained in glycol solvent reached 1.54 mmol·g^-1 at 313 K and 0.05 bar,which was better than the value reported in the literature.Then,Zn₂（Mtz）₂Ox@poly（acrylate）composite was synthesized by in-situ crystallization technique,and the crystal size was two orders of magnitude smaller than that of the powder.The results showed that the CO₂ adsorption capacity was further increased to 1.68mmol·g^-1 at 313 K and 0.05 bar.Compared with the Zn₂（Mtz）₂Ox-EG powder,the CO₂/N₂ adsorption ratio of the composite was increased by 1.15 times.In the simulated flue gas with high humidity（5 vol%CO₂/95 vol%N₂,90%RH,313 K）,the composite can still retain 83.6%of the CO₂ adsorption capacity under anhydrous conditions.（4）For the confined space with CO₂ concentration of around 0.5 vol%,Zn₂（Atz）₂Ox and Zn₂（Atz）₂Ox@AFP composite were synthesized by using Atz containing amino group as the organic ligand instead of the Mtz.During the preparation of composite,the strong adsorption property of the amine-functionalized AFP carrier attracted the Zn²⁺and oxalate ions to aggregate at the pore surface of carrier.The increased local supersaturation of the precursor and the orientation force of the pore surface of carrier resulted in the transformation of the Zn₂（Atz）₂Ox crystal from three-dimensional（3D）bulk crystals to 2D nanosheets.Compared to the Zn₂（Atz）₂Ox powder,the CO₂adsorption capacity of the composite reached 1.4 mmol·g^-1 at 298 K and 0.5vol%,and its adsorption efficiency increased by 59.4%.In addition,the Zn₂（Atz）₂Ox@AFP showed an inhibitory effect on N₂ adsorption,the IAST selectivity for 0.5 vol%CO₂/99.5 vol%N₂ mixture reached 59786.More importantly,compared with Nb OFFIVE-1-Ni and TIFSIX-3-Ni,which also have excellent CO₂ adsorption capacity at low CO₂ concentration,the composite exhibits the highest hydrothermal stability,and can still retain94.3%of CO₂ adsorption after being soaked in water at 403 K for 7 days.