Titanium-based water-absorbing MOF material with strong hydrophilicity, preparation method thereof and low-humidity water-absorbing application

Abstract

The application discloses a titanium-based water-absorbing MOF material with strong hydrophilicity, a preparation method of the MOF material and low-humidity water-absorbing application, and the MOF material is a porous material with a three-dimensional network structure, and a structural general formula is Ti8O8(OH)4X6, wherein X represents one or more para-bicarboxylic acid organic ligands; the titanium-based MOF material can meet the demand of low-humidity water-absorbing application, and can be used as an adsorbent for water vapor adsorption driven refrigeration application. The preparation process introduces the organic ligand with a hydrophilic site into the MOF material structure through a doping modification method, increases the density of the hydrophilic site in the structure, improves the affinity of the pore channel to water molecules, and thus improves the water adsorption amount under 10%-20% RH. The material has excellent water adsorption performance, the preparation method is simple and reliable, provides new inspiration and ideas for the control of the hydrophilicity of the material, and is helpful to the development of low-humidity water-absorbing materials.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of environmental and energy materials, and relates to a metal organic framework material and a preparation method thereof, in particular to a titanium-based water-absorbing MOF material with strong hydrophilicity, a preparation method thereof, and low-humidity water-absorbing application. BACKGROUND

[0002] Metal organic framework materials (MOFs) are a kind of porous materials formed by self-assembly of metal clusters and organic ligands. Due to the infinite arrangement and combination of metal nodes and organic ligands, MOFs exhibit unique tunability in structure and chemical properties. MOF materials with different topological structures and different pore surfaces exhibit different water adsorption performance curves. Compared with traditional water adsorption materials such as zeolites, silica gels and hygroscopic salts, MOFs have strong affinity for water molecules, can be desorbed at low temperature, are stable in humid environments, and most importantly, their pore structures can be designed and modified as needed. These unique advantages make MOF materials useful for atmospheric water collection, water adsorption driven refrigeration / heat, dehumidification, etc. in low-humidity environments. Among them, water adsorption driven refrigeration, which involves energy consumption, is a very important and urgent problem to be solved.

[0003] Fossil fuels and other non-renewable resources are widely used in heating and cooling systems, and harmful fluorochlorocarbons are used as working fluids for compressors to participate in refrigeration cycles, which is not conducive to environmental protection. Therefore, adsorption driven heat pumps and water chillers using water as the working fluid have attracted widespread attention. This adsorption refrigeration method can use clean energy such as solar energy and industrial waste heat to recycle the adsorbent, and the regeneration temperature is low, greatly reducing energy consumption. Most importantly, it uses environmentally friendly media (water, ethanol, etc.) as the working fluid, which greatly meets the requirements of green and sustainable development.

[0004] At present, traditional water adsorbents used commercially, such as hygroscopic salts (lithium chloride, lithium bromide, etc.), silica and zeolites, have been used in adsorption heat pumps and water chillers. However, these adsorbents have obvious shortcomings. Hygroscopic salts are prone to decomposition, have a short service life, and have the risk of corroding equipment; silica gel has weak hydrophilicity, resulting in insufficient adsorption capacity at low relative humidity; and zeolites have very strong hydrophilicity, requiring extremely high regeneration temperatures of 150℃ or higher. These are not conducive to reducing the cost budget and energy consumption of refrigeration units.

[0005] MOFs, due to their easily tunable composition and structure, offer the potential to flexibly control their water adsorption properties, potentially overcoming the drawbacks of these materials and meeting the requirements for adsorbents in practical applications. However, to date, few MOFs have been found that are well-suited to the various requirements and exhibit excellent water adsorption performance in refrigeration processes. Most MOFs exhibit low adsorption capacity in the 10%–20% relative humidity range, resulting in a trade-off between adsorption capacity and adsorption range, making it difficult to simultaneously meet these requirements. Furthermore, the harsh reaction conditions and complex preparation processes of most MOFs hinder cost and resource efficiency. Summary of the Invention

[0006] In response to the problems and shortcomings in the above-mentioned background technology, the present invention provides a titanium-based water-absorbing MOF material with strong hydrophilicity, its preparation method and low-humidity water absorption application. The preparation method controls the reaction temperature and reaction time to achieve the doping of organic ligands without destroying the MOF framework structure, thereby changing the number and distribution of functional sites in the MOF pore cage, thereby changing the strength of the interaction between water molecules and the framework, thereby achieving the improvement of its hydrophilicity and the regulation of water adsorption performance, which effectively promotes the practical application of MOFs materials for water vapor adsorption-driven refrigeration.

[0007] The present invention adopts the following technical solutions:

[0008] A titanium-based water-absorbing MOF material with strong hydrophilicity. The MOF material is composed of regularly connected titanium oxide clusters and organic ligands. It is a porous material with a three-dimensional network structure. The general structural formula is Ti8O8(OH)4X6, where X represents one or more para-dicarboxylic acid organic ligands.

[0009] Furthermore, the organic ligand is a para-dicarboxylic acid organic ligand having a hydrophilic functional site or a hydrophilic group, selected from 2-aminoterephthalic acid (BDC-NH2), 2,5-pyridinedicarboxylic acid (H2-2,5-pdc), 2,5-pyrimidinedicarboxylic acid (H2-2,5-pmdc), 2,5-pyrazinedicarboxylic acid (H2-2,5-pydc), 2,5-pyridazinedicarboxylic acid (H2-2,5-pddc), and 1,2,4,5-piperazinedicarboxylic acid (H2-1,2,4,5-tdc).

[0010] Furthermore, the titanium-based water-absorbing MOF material with strong hydrophilicity can be used as an adsorbent material to capture a large amount of water molecules in the air under low humidity conditions. The water adsorption capacity of the MOF material is as high as 0.37 g –1 , the low humidity condition is 10%–20% RH.

[0011] The preparation method of the titanium-based water-absorbing MOF material with strong hydrophilicity comprises the following steps:

[0012] Step 1: Add the first organic ligand to a mixed solution of N,N-dimethylformamide and methanol, dissolve it by ultrasonication, then add the titanium-containing reagent and continue ultrasonication to mix it evenly. Then transfer the solution to a reactor and place it in a preheated oven to react for at least 16 hours. The reaction product is thoroughly washed, solvent exchanged, and vacuum dried to obtain a preliminary MOF crystal material.

[0013] Step 2: The preliminary MOF crystal material and the second organic ligand obtained in step 1 are added to a mixed solution of N,N-dimethylformamide and methanol in proportion, and after ultrasonic mixing, the mixture is transferred to a reactor and placed in a preheated oven to react for at least 24 hours. The reaction product is fully washed, solvent exchanged, and vacuum dried to obtain the titanium-based water-absorbing MOF material with strong hydrophilicity;

[0014] Wherein, the first organic ligand and the second organic ligand are respectively selected from one or more of 2-aminoterephthalic acid (BDC-NH2), 2,5-pyridinedicarboxylic acid (H2-2,5-pdc), 2,5-pyrimidinedicarboxylic acid (H2-2,5-pmdc), 2,5-pyrazinedicarboxylic acid (H2-2,5-pydc), 2,5-pyridazinedicarboxylic acid (H2-2,5-pddc), and 1,2,4,5-piperazinedicarboxylic acid (H2-1,2,4,5-tdc), and are different types of organic ligands.

[0015] Furthermore, in the mixed solution in step one, the volume ratio of N,N-dimethylformamide and methanol is 1:1, the molar ratio of the first organic ligand and the titanium-containing reagent is 4:1, and the titanium-containing reagent is tetrabutyl titanate; in step two, the molar ratio of the MOF crystal material and the second organic ligand is 1:6, and in the mixed solution, the volume ratio of N,N-dimethylformamide and methanol is 1:1 or 9:1.

[0016] Furthermore, the ultrasonication time in step 1 and step 2 is 20-30 minutes, and the oven temperature is 140-160°C.

[0017] Furthermore, the sufficient washing is washing with N,N-dimethylformamide and methanol at least three times each; the solvent exchange requires soaking the material in one or more of methanol, ethanol, and acetone, replacing the solution with fresh one every 4 hours, and repeating at least eight times; the vacuum drying is vacuum drying the material after solvent exchange at room temperature and 65-150°C for 12 hours each.

[0018] The titanium-based water-absorbing MOF material with strong hydrophilicity can meet the needs of low-humidity water absorption applications, and can especially be used as an adsorbent in refrigeration products driven by water vapor adsorption.

[0019] Based on practical application needs and existing research challenges, this invention selects para-dicarboxylic acid ligands with hydrophilic functional sites or groups, specifically one or more suitable functionalized organic ligands, and incorporates them into existing titanium-based MOF materials to create a new MOF material. The hydrophilicity of the material's structural framework is systematically controlled. Because the pore structure of this material contains various hydrophilic groups and sites, the structure's affinity for water molecules is greatly enhanced, improving the spatial utilization of water molecules in the pores. Furthermore, the material can absorb up to 0.37 g water in a low-humidity environment of 20% RH. –1 , which is significantly better than similar materials that have been publicly reported. In addition, the reaction solvent used in the preparation of this type of material is of a single type, which greatly simplifies the process operation, and the doped and modified crystalline material maintains the structure and morphology of the initial MOF material without damaging the initial material, laying a solid foundation for further improvement of water adsorption performance. This regulation strategy achieves the strong hydrophilicity and high low-pressure water adsorption capacity that are difficult to obtain simultaneously in MOFs materials, providing new inspiration and ideas for the regulation of water adsorption curves. Coupled with its simple and reliable preparation method, it has great industrial application potential in water adsorption refrigeration.

[0020] The beneficial effects of the present invention are:

[0021] (1) The present invention selects suitable organic ligands and metal clusters to form MOF materials, wherein the organic ligands all have hydrophilic groups or hydrophilic sites. Thus, there are a large number of rich water adsorption sites inside the structure of the synthesized material. In addition, the combination of hydrophilic groups and hydrophilic sites reduces the occupation of the pore space by the hydrophilic groups and enhances the interaction between the pores of the material and the water molecules. It can also easily capture water vapor in the air under a low humidity environment of 10%-20% RH. Among them, under the humidity condition of 20% RH, the water adsorption capacity reaches 0.37g –1 , which is significantly better than similar materials that have been publicly reported.

[0022] (2) The present invention adopts a unique preparation method of doping modification to obtain a target material with enhanced hydrophilicity, partially replacing the original ligand with an organic ligand rich in hydrophilic nitrogen sites, while the crystal structure of the material remains intact and is not destroyed. On this basis, the water adsorption performance is further regulated, so that the water adsorption capacity in a low humidity environment is increased by two times compared with the unmodified state.

[0023] (3) The present invention uses a single type of reaction solvent in the process of preparing the relevant MOF materials, which greatly simplifies the process operation and saves production costs.

[0024] (4) The MOF material in the present invention greatly meets the multi-faceted requirements of adsorbent materials for water adsorption refrigeration under low humidity conditions, and achieves strong hydrophilicity and high low-pressure water adsorption capacity that are difficult to obtain simultaneously in MOFs materials, providing new inspiration and ideas for the regulation of water adsorption curves, and has great industrial application potential in water adsorption refrigeration. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a structural diagram of the organic ligand involved in the present invention.

[0026] Figure 2 Schematic diagram of the microscopic crystal structure of the material in Example 1.

[0027] Figure 3 This is the PXRD pattern of the material in Example 1.

[0028] Figure 4 This is the H NMR spectrum of the material in Example 1.

[0029] Figure 5 This is the 77K nitrogen isothermal full adsorption curve of the material in Example 1.

[0030] Figure 6 This is the isothermal water adsorption curve (298K) of the material in Example 1.

[0031] Figure 7 Schematic diagram of the microscopic crystal structure of the material in Example 2.

[0032] Figure 8 This is the PXRD pattern of the material in Example 2.

[0033] Figure 9 This is the 77K nitrogen isothermal full adsorption curve of the material in Example 2.

[0034] Figure 10 This is the isothermal water adsorption curve (298K) of the material in Example 2. DETAILED DESCRIPTION

[0035] The contents of the present invention will be further illustrated below with reference to examples, but these examples do not limit the scope of protection of the present invention. Based on the technical solution of the present invention, various modifications or variations that can be made by those skilled in the art without creative work are still within the scope of protection of the present invention.

[0036] Example 1

[0037] BDC-NH2 was dissolved in a mixture of 3.5 mL of N,N-dimethylformamide and 3.5 mL of methanol, and a uniform solution was obtained by ultrasonic treatment for 20 minutes. Then, 1.5 mmol of tetrabutyl titanate was added and ultrasonic treatment was performed for 5 minutes. Subsequently, the reaction kettle was placed in an oven at 150°C, and heating was continued for 16 hours. After cooling, the reaction product was filtered, and the obtained solid was washed with N,N-dimethylformamide and methanol for at least three times, respectively, and dried in air. The homogeneous crystalline material obtained in the above step was exchanged in methanol for at least eight times at an interval of at least 4 hours, and then vacuum dried to obtain the activated metal-organic framework material Ti8O8(OH)4(BDC-NH2)6.

[0038] BDC-NH2 was dissolved in a mixture of 3.5 mL of N,N-dimethylformamide and 3.5 mL of methanol, and a uniform solution was obtained by ultrasonic treatment for 20 minutes. Then, 1.5 mmol of tetrabutyl titanate was added and ultrasonic treatment was performed for 5 minutes. Subsequently, the reaction kettle was placed in an oven at 150°C, and heating was continued for 16 hours. After cooling, the reaction product was filtered, and the obtained solid was washed with N,N-dimethylformamide and methanol for at least three times, respectively, and dried in air. The homogeneous crystalline material obtained in the above step was exchanged in methanol for at least eight times at an interval of at least 4 hours, and then vacuum dried to obtain the activated metal-organic framework material Ti8O8(OH)4(BDC-NH2)6. 1-y (H2-2,5-pmdc) y ,0<y<1)

[0039] The organic ligand used in Ti8O8(OH)4(X)6 and other similar organic ligands used in the present application are shown in Figure 1 , and a schematic diagram of the microcrystalline structure of the material is shown in Figure 2 , the structure is doped with nitrogen-containing ligands to introduce more hydrophilic sites. The PXRD characterization spectrum data are shown in Figure 3 , and it can be seen from Figure 3 that the material has good crystallinity and phase purity.

[0040] In order to characterize the ligand composition of Ti8O8(OH)4(X)6, it was digested with hydrochloric acid and dissolved in a deuterated reagent for hydrogen nuclear magnetic resonance test, and the obtained hydrogen nuclear magnetic resonance spectrum is shown in Figure 4 , and calculation shows that y is 0.05.

[0041] In order to characterize the pore volume of Ti8O8(OH)4(X)6, 77K nitrogen isothermal adsorption test was performed on Ti8O8(OH)4(X)6, and the obtained nitrogen full adsorption curve is shown in Figure 5 , and calculation shows that the specific surface area of Ti8O8(OH)4(X)6 is 1290 m2 / g, and the pore volume is 0.58 cm 3 / g.

[0042] To characterize the water adsorption performance of Ti8O8(OH)4(X)6, isothermal water adsorption test was performed on Ti8O8(OH)4(X)6. 30 mg of Ti8O8(OH)4(X)6 sample was taken, and the test temperature was set to 25℃, and the isothermal water adsorption curve was obtained. The isothermal water adsorption curve is shown in Figure 6 , which shows that the material has a very high water adsorption capacity under low humidity conditions.

[0043] Example 2:

[0044] 6 mmol of BDC-NH2 was completely dissolved in a mixed solution of 3.5 mL of N,N-dimethylformamide and 3.5 mL of methanol, and a uniform solution was obtained by ultrasonic treatment for 15 minutes, and then 1.5 mmol of tetrabutyl titanate was added and ultrasonic treated for 5 min. Subsequently, the reaction kettle was placed in a 150℃ oven, and heating was continued for 16 h. After cooling, the reaction was filtered, and the obtained solid was washed with N,N-dimethylformamide and methanol at least three times, and dried in air. The homogeneous crystal material obtained in the above step was exchanged in methanol at least eight times by solvent exchange method, with an interval of at least 4 hours each time, followed by vacuum drying, to obtain the undoped metal-organic framework material Ti8O8(OH)4(BDC-NH2)6. The schematic diagram of the microcrystal structure of the material is shown in Figure 7 , and the PXRD characterization spectrum is shown in Figure 8 .

[0045] To characterize the pore volume of Ti8O8(OH)4(BDC-NH2)6, 77K nitrogen isothermal full adsorption test was performed on Ti8O8(OH)4(BDC-NH2)6, and the test results are shown in Figure 9 , and the specific surface area of Ti8O8(OH)4(BDC-NH2)6 was calculated to be 1270 m2 / g, and the pore volume was 0.54 cm 3 / g, which is comparable to Ti8O8(OH)4(X)6.

[0046] To characterize the water adsorption performance of Ti8O8(OH)4(BDC-NH2)6, and compare with the doped and modified Ti8O8(OH)4(X)6, isothermal water adsorption test was performed on Ti8O8(OH)4(BDC-NH2)6. About 30 mg of Ti8O8(OH)4(BDC-NH2)6 sample was taken, and the test temperature was set to 25℃, and the isothermal water adsorption curve at this temperature was obtained Figure 10From the adsorption curve, it can be seen that due to the relatively weak affinity of the pores for water molecules, the water adsorption of Ti8O8(OH)4(BDC-NH2)6 mainly occurs in the relative humidity range of 20%–30%. At 20% relative humidity, the water adsorption capacity of Ti8O8(OH)4(BDC-NH2)6 is only 0.12gg –1 , much lower than Ti8O8(OH)4(X)6.

[0047] The above comparative experiments show that by doping and modifying MOF to obtain a structure rich in hydrophilic sites, the water adsorption capacity of MOF materials in low-humidity environments can be significantly improved, the water adsorption performance can be improved in a targeted manner, and the application range of the material in refrigeration can be expanded.

Claims

1. A titanium-based water-absorbing MOF material with strong hydrophilicity, characterized in that: The MOF material is a porous material composed of titanium oxide clusters and organic ligands regularly connected and having a three-dimensional network structure, and the general structural formula is Ti8O8(OH)4X6, wherein X represents a variety of para-dicarboxylic acid organic ligands; The preparation method thereof comprises: Step 1: Add the first organic ligand to a mixed solution of N,N-dimethylformamide and methanol, dissolve it by ultrasonication, then add the titanium-containing reagent and continue ultrasonication to mix it evenly. Then transfer the solution to a reactor and place it in a preheated oven to react for at least 16 hours. The reaction product is thoroughly washed, solvent exchanged, and vacuum dried to obtain a preliminary MOF crystal material. Step 2: The preliminary MOF crystal material and the second organic ligand obtained in step 1 are added to a mixed solution of N,N-dimethylformamide and methanol in proportion, and after ultrasonic mixing, the mixture is transferred to a reactor and placed in a preheated oven to react for at least 24 hours. The reaction product is fully washed, solvent exchanged, and vacuum dried to obtain the titanium-based water-absorbing MOF material with strong hydrophilicity; The first organic ligand is 2-aminoterephthalic acid (BDC-NH2), and the second organic ligand is selected from one or more of 2,5-pyridinedicarboxylic acid (H2-2,5-pdc), 2,5-pyrimidinedicarboxylic acid (H2-2,5-pmdc), 2,5-pyrazinedicarboxylic acid (H2-2,5-pydc), 2,5-pyridazinedicarboxylic acid (H2-2,5-pddc), and 1,2,4,5-piperazinedicarboxylic acid (H2-1,2,4,5-tdc).

2. The titanium-based water-absorbing MOF material with strong hydrophilicity according to claim 1, characterized in that The MOF material can be used as an adsorbent material to capture a large amount of water molecules in the air under low humidity conditions. The water adsorption capacity of the MOF material is as high as 0.37 g –1 , the low humidity is 10%–20%RH.

3. A method for preparing a titanium-based water-absorbing MOF material with strong hydrophilicity as claimed in claim 1, characterized in that the steps include: Step 1: Add the first organic ligand to a mixed solution of N,N-dimethylformamide and methanol, dissolve it by ultrasonication, then add the titanium-containing reagent and continue ultrasonication to mix it evenly. Then transfer the solution to a reactor and place it in a preheated oven to react for at least 16 hours. The reaction product is thoroughly washed, solvent exchanged, and vacuum dried to obtain a preliminary MOF crystal material. Step 2: The preliminary MOF crystal material and the second organic ligand obtained in step 1 are added to a mixed solution of N,N-dimethylformamide and methanol in proportion, ultrasonically mixed and then transferred to a reactor and placed in a preheated oven to react for at least 24 hours. The reaction product is fully washed, solvent exchanged, and vacuum dried to obtain the titanium-based water-absorbing MOF material with strong hydrophilicity.

4. The method for preparing a titanium-based water-absorbing MOF material with strong hydrophilicity according to claim 3, characterized in that: In step 1, the volume ratio of N,N-dimethylformamide and methanol in the mixed solution is 1:1, the molar ratio of the first organic ligand and the titanium-containing reagent is 4:1, and the titanium-containing reagent is tetrabutyl titanate; in step 2, the molar ratio of the MOF crystal material and the second organic ligand is 1:6, and in the mixed solution, the volume ratio of N,N-dimethylformamide and methanol is 1:1 or 9:

1.

5. The method for preparing a titanium-based water-absorbing MOF material with strong hydrophilicity according to claim 3, characterized in that: The ultrasonic time in step 1 and step 2 is 20-30 minutes, and the oven temperature is 140-160°C.

6. The method for preparing a titanium-based water-absorbing MOF material with strong hydrophilicity according to claim 3, characterized in that: The thorough washing is washing with N,N-dimethylformamide and methanol at least three times each; the solvent exchange requires soaking the material in one or more of methanol, ethanol, and acetone, replacing the solution with fresh solution every 4 hours, and repeating at least eight times; the vacuum drying is vacuum drying the material after solvent exchange at room temperature and 65-150°C for 12 hours each.

7. Use of the MOF material according to any one of claims 1 to 2 or the MOF material prepared by the method according to any one of claims 3 to 6, characterized in that: The MOF material is used as an adsorbent in refrigeration products driven by water vapor adsorption.

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