Mixed-solvothermal Synthesis And Loading Of Metal-organic Frameworks As Desiccants

Porous adsorbents（desiccants）can be widely used in adsorption heat pump,rotary wheel dehumidification,indoor air humidity control and water harvest.In these areas,traditional inorganic porous desiccants such as silica gel,zeolite,aluminum phosphate molecular sieve have carried out extensive scientific research and application.However,these desiccants have some disadvantages,such as low equilibrium adsorption capacity（aluminum phosphate）,high desorption temperature（zeolite）and low adsorption capacity under low humidity（silica gel）,which restrict their application.In recent years,a kind of metal-organic framework（MOF）,which is consisted of metal ions（or metal clusters）and organic ligands,has shown broad application prospects in water vapor adsorption due to its high specific surface area,excellent adsorption capacity,adjustable pore size and low desorption temperature.How to conveniently synthesize MOFs as desiccants and loading MOFs on the substrate is a crucial and difficult task of current research.In this study,the synthesis of MOFs as desiccants by mixed-solvothermal method and the loading MOFs on glass fiber paper and aluminum foil were studied.The main research contents are as follows:（1）The synthesis of MIL-101（Cr）as desiccant was investigated with chromium nitrate（Cr（NO₃）·9H₂O）and terephthalic acid（PTA）as raw materials,N,N-dimethylformamide（DMF）and water（H₂O）as mixed solvent.The effects of DMF/H₂O volume ratio on the crystal phase,morphology,pore structure and the adsorption/desorption property of MIL-101（Cr）were systematically investigated.XRD results showed that MIL-101（Cr）could be synthesized by mixed-solvothermal process at 140°C,which was much lower than that by traditional hydrothermal method（220°C）.Besides,at the volume ratio of 0.20DMF/H₂O,reaction temperature at 160°C,the mixed-solvothermal method had a higher yield（83.3%）than the traditional hydrothermal method（53.3%）,SEM observation illuminated that the MIL-101（Cr）crystals synthesized by the new method were uniform,complete and fine（about 200 nm）,while those synthesized by the traditional method were large and unevenly distributed（400～800 nm）.The results of BET and static adsorption displayed that compared with the traditional hydrothermal method（2318.6 m²/g and 0.921 g/g,respectively）,the products synthesized by the new process had higher specific surface area（2453.1 m²/g）and saturated adsorption capacity（0.964 g/g）.Accordingly,the desorption activation energy of the latter（45.08 k J/mol）was slightly higher than that of the former（43.93 k J/mol）.The desiccant can be used for indoor air humidity control and rotary wheel dehumidification.（2）In order to improve the adsorption capacity of MIL-101（Cr）at low humidity,the MIL-101（Cr）-halide salt composite desiccant was prepared by impregnating Ca Cl₂ and Li Cl solutions into the MIL-101（Cr）matrix by the initial wet impregnation method.The effects of Ca Cl₂ and Li Cl loading on the crystal phase,morphology and pore structure were discussed.The results showed that the leakage of hygroscopic salt in MIL-101（Cr）could be effectively prevented by controlling the hygroscopic salt concentration.The suitable salt concentration was25 wt%Ca Cl₂ and 15 wt%Li Cl,and the corresponding loading was 52.8 wt%and 35.5 wt%,respectively.XRD results revealed that the diffraction peak intensity of the composite decreased obviously due to the introduction of hygroscopic salt.BET results indicated that the hygroscopic salt mainly entered into the micropores of MIL-101（Cr）,resulting in the decrease of specific surface area,pore volume and pore size of the composite.The results of DVS water vapor adsorption displayed that the water vapor adsorption isotherm of the composite moved to the direction of low relative pressure.When the relative pressure（P/P₀）was 0.3,the equilibrium adsorption capacity of Ca25 and Li15 was 0.437 g/g and 0.446 g/g,respectively,which was much higher than that of MIL-101（Cr）（0.03 g/g）.In addition,due to the presence of hygroscopic salt,the composite had higher water vapor diffusion rate and lower desorption activation energy.MIL-101（Cr）-halide salt composite desiccant has potential application in adsorption heat pump.（3）Based on the framework structure of CAU-10-H and Al-fumarate,the mixed Al-MOFs containing CAU-10-H and Al-fumarate MOFs were synthesized by mixed-solvothermal method with the same aluminum source（Al₂（SO₄）₃·18H₂O）and different organic ligands（isophthalic acid（PIA）,fumaric acid（FA））.The effects of PIA/FA molar ratio on the crystal phase,pore structure and morphology of Al-MOFs were investigated.The results indicated that the mixed Al-MOFs could be easily synthesized by the mixed-solvothermal process in one-pot.By adjusting the PIA/FA molar ratio in the precursor solution,the water vapor adsorption isotherm,specific surface area,pore size and desorption activation energy of Al-MOFs could can be easily adjusted between the limits of CAU-10-H and Al-fumarate.The Al-MOFs showed different crystal phases and morphologies.FT-Raman spectroscopy observed the existence of double ligands in the mixed Al-MOFs.XRD and ¹HNMR could quantitatively analyze the content of ligands in the product.The actual composition in the product slightly deviated from that of the precursors,which might be related to the differences in crystallization rates between CAU-10-H and Al-fumarate in the mixture.The one-pot mixed-solvothermal process can provide guidance for adjusting the water adsorption curve of MOFs to apply to a wider range of water vapor adsorption field.（4）In-situ synthesis of Al-fumarate coating on aluminum foil was studied by mixed-solvothermal process using aluminum foil as substrate,Al₂（SO₄）₃·18H₂O and fumaric acid（FA）as raw materials.The effects of the DMF/H₂O volume ratio,FA molar content and the crystallization temperature on the coating mass fraction were discussed emphatically.The crystal structure,pore structure and the morphology of the MOF coating were characterized.And its water vapor adsorption/desorption performance was evaluated.XRD analysis showed that Al-fumarate coating could be prepared by mixed-solvothermal method under a wide range of conditions.By controlling DMF/H₂O ratio of 5/30～30/5,fumaric acid molar content of 2～8mmol,crystallization temperature of 90～120°C,the cauliflower Al-fumarate crystals with coating thickness of 60～450μm could be obtained.The results of water vapor adsorption displayed that the Al-fumarate coating had similar adsorption curve compared with Al-fumarate powder.DTG results also exhibited that the coating had lower desorption activation energy and desorption temperature.In addition,the Al-fumarate coating had good cycle stability.After 50adsorption desorption cycles,the adsorption capacity of the coating to water vapor remained unchanged.The Al-fumarate MOF coating has a promising application prospect in aluminum finned tube heat exchanger in AHPs（adsorption heat pumps）.（5）Using glass fiber paper as substrate,Al₂（SO₄）₃·18H₂O and fumaric acid（FA）as raw materials,sodium hydroxide（Na OH）as deprotonating agent,the low-temperature in-situ synthesis of glass fiber-based Al-fumarate monolithic desiccant was studied.The effects of the FA/H₂O molar ratio and crystallization temperature and time on the crystal phase,pore structure,morphology and the water vapor adsorption/desorption property of Al-fumarate were systematically discussed.XRD results showed that the monolithic desiccant could be easily synthesized under the mole ratios of FA/H₂O from 0.016～0.0215,the crystallization temperatures from 40～70°C and the crystallization time from 30～120 min.The results indicated that under the conditions of a mole ratio of FA to H₂O of 0.02,reaction temperature of 50°C,and reaction time of 60 min,the obtained Al-fumarate monolith had high loading percentage（76%）,large BET surface area（740.15 m²/g）and high adsorption capacity（0.3906g/g）.SEM observation discovered that the Al-fumarate desiccant synthesized by in-situ crystallization was closely combined with the glass fiber to form a compact monolithic desiccant with about thickness of 245μm.The results of adsorption kinetics showed that the early adsorption rate of Al-fumarate monolithic desiccant was faster than that of Al-fumarate powder.DTG results also observed that Al-fumarate monolithic desiccant had lower desorption activation energy（55.8 k J/mol）and desorption temperature.Glass fiber-based Al-fumarate monolithic desiccant is suitable for rotary wheel dehumidification.Finally,MOF-303 monolithic desiccant was prepared at low temperature using glass fiber paper as substrate,Al（NO₃）₃·9H₂O,3,5-pyrazole dicarboxylic acid（PZDC）as raw materials and Na OH as deprotonating agent.The effects of reactant ratio and crystallization temperature on crystal structure and porosity were studied.The results showed that the perfect MOF-303crystal could be synthesized at 70°C and the molar ratio of 1Al³⁺:1PZDC:3Na OH.The yield of MOF-303 prepared by this method was as high as 97.7%,which was much higher than that of traditional hydrothermal method（34.8%）.Under these conditions,MOF-303 crystal could be directly synthesized on glass fiber substrate,so as to prepare MOF-303 monolithic desiccant.The loading percentage of monolithic desiccant was as high as 53%.The DVS curve showed that the saturated adsorption capacity of the monolithic desiccant was 0.151 g/g at 20%relative humidity.The MOF-303 monolithic desiccant is suitable for water harvest in arid desert area.