Dry Gel Conversion Synthesis And Loading Of Metal Organic Frameworks Adsorbents

The spontaneous adsorption behavior of the adsorbent for water vapor can be widely used in fields of indoor humidity adjustment,adsorption heat pumps?AHPs?,rotary wheel dehumidification,and water harvesting from air.Compared with traditional water vapor adsorbents such as silica gel and molecular sieves,metal organic framework?MOF?adsorbents have the advantages of large adsorption capacity,low regeneration energy consumption and adjustable adsorption curve,which show broad application prospect.How to realize the convenience synthesis and the efficient loading of MOF adsorbent is a hot and difficult topic for researchers at present.In this work,selecting MIL-100?Fe?and MIL-160 as very promising MOF adsorbents,and taking the potential applications of the above-mentioned adsorbents as the background,the dry gel conversion?DGC?synthesis of MOF adsorbents and their loading on glass fiber paper and melamine foam were investigated.The relationship between the structure and the adsorption/desorption performance of the new adsorbents was discussed in detail.The main content was as follows:Firstly,MIL-100?Fe?adsorbent with high crystallinity was prepared by DGC method in solvent-free and glass bottles using ferric nitrate nonahydrate and trimesic acid as raw materials.The effect of crystallization temperature on the crystal phase,morphology,pore structure and the adsorption/desorption property of the adsorbent was systematically investigated.The results of XRD analysis showed that under the new process,the dry gel could crystallize into MIL-100?Fe?adsorbent at the lowest temperature of 140?,and the yield was as high as 85%.Compared with the crystallization temperature of 160?and the yield of 76%of MIL-100?Fe?adsorbent synthesized by conventional hydrothermal method,the crystallization temperature of the one prepared by the DGC method decreased and the yield increased.SEM and pore structure analysis indicated that MIL-100?Fe?adsorbent transformed from dry gel particles had a small crystal size and could be accumulated in an orderly way,so the product had a more obvious hierarchical porous structure and larger pore volume.The test results manifested that MIL-100?Fe?adsorbent synthesized by DGC method had a vapor saturation adsorption capacity of up to 0.78 g/g and a desorption activation energy of only 42.2 k J/mol at 25?and 60%RH,which was better than the adsorption/desorption performance of the one synthesized by traditional hydrothermal process.After 50adsorption-desorption cycles,MIL-100?Fe?adsorbent owned excellent cycle stability.The novel MIL-100?Fe?adsorbent can be used for indoor air humidity regulation.Secondly,in order to improve the adsorption capacity of MIL-100?Fe?at low humidity,using the hierarchical porous MIL-100?Fe?as matrix,and impregnating the hygroscopic salt Li Cl into the pores of the matrix,Li Cl@MIL-100?Fe?composite adsorbent for AHPs was prepared by incipient wetness impregnation method.The effect of Li Cl loading on the crystal structure,morphology,chemical composition,pore structure,adsorption/desorption performance and cycle stability of the composite was discussed.The leakage test showed that the Li Cl maximum loading was 32.9wt%on the premise of ensuring long-term exposure to high humidity environment without leakage.XRD results indicated that the introduction of Li Cl caused the diffraction peak of the composite material to be significantly weakened,but did not destroy the internal structure of the matrix.SEM-EDS analysis displayed that Li Cl which distributed evenly in the composite mainly entered the pores of the matrix,and a small amount remained outside.The dynamic water vapor adsorption test?DVS?showed that the saturated adsorption capacity of the composite adsorbent was significantly higher than that of pure MIL-100?Fe?fewer than 30%RH.The adsorption kinetics curve manifested that the diffusion coefficient of water molecules on the surface of the composite adsorbent was1.393×10^-10cm²/s,which was much higher than that of pure MIL-100?Fe?(0.566×10^-10cm²/s).Thirdly,using glass fiber paper as matrix,a convenient DGC in situ synthesis strategy was proposed to prepare glass fiber-based MIL-100?Fe?monolithic adsorbent suitable for rotary wheel dehumidification.The effect of the reaction sol concentration on the crystalline phase,morphology,pore structure,adsorption/desorption performance and the cycling stability of the monolith was discussed in detail.The pore structure analysis indicated that the higher the concentration of the reactant sol,the higher the porosity of the product.The results of SEM-EDS analysis showed that MIL-100?Fe?crystals which evenly dispersed in the surface and voids of glass fiber,and XRD analysis revealed that MIL-100?Fe?adsorbent could be conveniently synthesized in the reaction sols with molar ratios of 1 Fe?III?/0.67 BTC/x H2O?x=93-185?.Among them,the MIL-100?Fe?monolith obtained at x=93 exhibited a high loading?85.7%?,large BET specific surface area and pore volume?1441 m²/g and 0.80 cm³/g,respectively?,and high saturated adsorption ratio?0.496 g/g?at 298 K and 60%RH.Fourthly,using ferrous chloride tetrahydrate and trimesic acid and sodium hydroxide?Na OH?as raw materials,new DGC strategy for the rapid synthesis of the glass fiber based MIL-100?Fe?monolith at low temperature in a constant temperature and humidity chamber was presented.The effects of crystallization temperature and time on the crystal structure,loading,porosity,morphology and water vapor adsorption/desorption performance of MIL-100?Fe?monolith were studied.XRD and SEM analysis indicated that the MIL-100?Fe?crystal could be conveniently synthesized on the glass fiber substrate at a low temperature of50-70°C and in a short time of 2-6 h.As the crystallization temperature rose,the crystal loading decreased,and the crystal loading of the synthesized product was highest?77.7%?at50?for 6 h.SEM showed that the particle size of MIL-100?Fe?crystals was about 500 nm.During the solid-phase crystallization process,the small crystals were aggregated into spheres of about 2.5 um,which were evenly distributed in the voids of the glass fiber paper.The MIL-100?Fe?monolithic adsorbent obtained at 50°C for 6 h had the largest BET specific surface area?1260 m²/g?and pore volume?0.57 cm³/g?,and its water vapor adsorption capacity was also the largest?0.581 g/g?.In addition,its regeneration temperature were lower than 80?,and the desorption activation energy was 51.6 k J/mol.Finally,the DGC method was extended to the synthesis of MOF MIL-160 mololith,which was combined with melamine foam to prepare a foam-based MIL-160 monolithic adsorbent,and its potential application in water harvesting from air was explored.The effects of curing condition,dry gel composition and crystallization temperature on the crystal structure,porosity,morphology and water vapor adsorption/desorption performance of the monolithic adsorbent were studied.XRD results showed that after curing at 25°C and 90%RH,the dry gel could crystallize into perfect MIL-160 crystals.Properly increasing the amount of Na OH could reduce the crystallization temperature of the dry gel conversion,and it could crystallize at a minimum of 60°C.The BET test showed that the BET specific surface area and pore volume of the monolith were between 1116?1292 m²/g and 0.48?0.54 cm³/g,respectively,and the average pore diameter was about 0.6 nm.Under the condition that the molar ratio of dry gel was Al³⁺:FDCA:Na OH=1:1:2 at 60°C,MIL-160 crystals could be synthesized directly on the foam substrate to prepare monolithic adsorbent.DVS curve showed that under 20%RH,the vapor saturation adsorption capacity of the monolithic adsorbent was 0.281 g/g.DTG analysis indicated that the desorption activation energy of the monolithic adsorbent was 68.9 k J/mol.The cycle test showed that the foam-based MIL-160monolithic adsorbent had good cycle stability.