Modification Of Water Adsorption Performance Of Metal Organic Framework UiO-66

Adsorption heat storage is a technology that can realize energy storage and release by utilizing a reversible adsorption process.It can effectively utilize lowgrade energy such as solar energy,industrial waste heat,and geothermal energy,and has the advantages of energy conservation and environmental protection.Therefore,the research and promotion of efficient adsorption heat storage technology are of great significance for improving energy utilization efficiency and reducing carbon emissions.The performance of adsorbents/adsorbates directly affects the efficiency of adsorption heat storage.Water vapor has become the most commonly used adsorbent due to its advantages of being inexpensive,non-toxic,and harmless,as well as high evaporation enthalpy.UiO-66 is a metal-organic framework with excellent water,thermal,and chemical stability,which has shown good application prospects in the field of adsorption heat storage.However,its low hydrophilicity and saturated adsorption capacity limited its application.The main purpose of this article is to modulate the water adsorption performance of UiO-66 and to study the effects and mechanisms of various regulatory methods on the morphology,structure,and water adsorption properties of UiO-66.The main contents and conclusions are as follows:Firstly,the effects of chromium doping on the morphology,structure,and water adsorption properties of UiO-66 were studied,and the effects of reaction scale and microwave-assisted rapid synthesis on the regulatory effect of chromium doping were explored.The results indicate that moderate Cr doping can significantly improve the pore volume and water adsorption properties of UiO-66.When the reaction scale is small,the hydrophilicity and saturated water adsorption capacity of Cr-doped UiO-66 samples are significantly improved.The water uptake of Cr-doped UiO-66 samples reached 0.26 g/g and 0.58 g/g at P/P0=0.3 and P/P0=0.9.respectively,which were 2.0 and 1.6 times higher than those of undoped UiO-66 sample.The heat storage capacity of Cr-doped UiO-66 has also been improved,that is.the Cr-doped sample with the best hydrophilicity achieved an average desorption heat of 584.5J/g in 10 adsorption and desorption cycles,which is 11.5%higher than undoped UiO-66 sample.When the reaction scale is increased.the sample yield increased several times,but the improvement effect of Cr doping on the water adsorption performance of UiO-66 decreased.Microwave-assisted synthesis method shortened the preparation time of Cr-doped UiO-66 from 24 hours to 2 hours and considerably improved these samples’ saturated water adsorption capacity.The highest water adsorption capacity of the Cr-doped UiO-66 sample prepared by the microwave-assisted method can reach 0.59 g/g at P/P0=0.9.Secondly,the water adsorption performance of UiO-66 was modulated using p-aminobenzoic acid(PABA),p-nitrobenzoic acid(PNBA),and p-hydroxybenzoic acid(PHBA)as regulators,and their regulation mechanisms were studied.The results indicate that introducing these organic carboxylic acids with single carboxylic acid coordination sites into the structure of UiO-66 can increase the defects in it while introducing hydrophilic functional groups,resulting in improved pore volume and water adsorption performance of UiO-66.Among them,PABA and PHBA have improved the adsorption capacity and hydrophilicity of UiO-66 saturated water adsorption capacity,respectively,while PNBA has improved both,with the most significant effect.The highest water uptake of PNBA modulated UiO66 samples at P/P0=0.3 and P/P0=0.9 were 0.24 g/g and 0.58 g/g,respectively,which were 60.0%and 40.0%higher than those of unmodulated UiO-66 samples,respectively.The heat storage capacity of PNBA modulated UiO-66 sample has also been significantly improved,the PNBA modulated sample with the best water adsorption performance achieved an average desorption heat of 658.6 J/g in 10 adsorption and desorption cycles,which is 1.4 times that of the unregulated UiO66 sample.Finally,the organic ligands of pyridine-2,5-dicarboxylic acid(H2PyDC),2amino-1,4-benzene dicarboxylic acid(ABDC),picolinic acid(PA),4-aminobenzoic acid(ABA)were introduced into defective UiO-66 through post-synthesis ligand exchange(PSE)to modulate its water adsorption performance.PSE combines defect regulation with the introduction of new ligands.The former is beneficial for increasing the specific surface area and pore volume of UiO-66,while the latter introduces more hydrophilic water points into the UiO-66 structure,which is beneficial for improving the hydrophilicity of UiO-66.The PSE-regulated UiO-66 samples exhibited good hydrophilicity and water adsorption capacity.Among them,the ABDC-ligands exchanged UiO-66 sample had the highest water uptake at P/P0=0.9(0.50 g/g).25.0%higher than the LD-UiO-66 sample without defect and PSE.The PyDC2-ligands exchanged UiO-66 sample had the highest water adsorption capacity at P/P0=0.3(0.23 g/g).