Co3 (HCOO) 6 hexagonal flaky crystal as well as preparation method and application thereof

Abstract

The invention provides a Co3 (HCOO) 6 hexagonal flaky crystal as well as a preparation method and application thereof. The preparation method is a solvothermal method and comprises the following steps that cobalt salt is taken and dissolved in an organic solvent, and a metal solution is obtained after the cobalt salt is fully dissolved; adding formic acid to obtain a mixed solution; carrying out water bath heating on the mixed solution, and carrying out a reaction under the assistance of stirring; and after the reaction is finished, naturally cooling to room temperature, washing a reaction product, and drying in a drying oven to obtain the Co3 (HCOO) 6 hexagonal flaky crystal. According to the method, under the condition that no additive is added, the crystal morphology can be regulated and controlled only by regulating and controlling MOF synthesis parameters, the growth direction of Co3 (HCOO) 6 is controlled, and the regular hexagonal morphology is obtained. The crystal can be applied to the field of CO2 / CH4 adsorption separation, and has better CO2 / CH4 gas adsorption separation performance.

Description

Technical Field

[0001] The invention relates to the technical field of CO2 separation, and in particular to a Co3(HCOO)6 hexagonal plate-like crystal and a preparation method and application thereof. Background Art

[0002] At present, chemical separation mainly adopts heat-driven separation technology (such as distillation). The energy consumption related to the chemical separation process accounts for about 50% of industrial energy consumption and 10-15% of the world's total energy consumption. In addition, this process releases a large amount of carbon dioxide and other harmful gases, which have a serious impact on the environment. Therefore, it is very urgent to develop energy-saving and environmentally friendly alternative technologies to reduce the energy consumption required in the chemical separation process in the chemical industry, reduce the release of harmful gases, and reduce pollution to the environment.

[0003] In the process of carbon dioxide capture, CO2 must be purified first. Nowadays, most of the CO2 gas separation technologies that have been applied in the industrial field include amine absorption, cryogenic distillation, etc. However, although the amine absorption technology is commercially mature and can effectively remove CO2 from industrial flue gas, the large amount of energy required for solvent regeneration and the use of inhibitors to control corrosion and oxidative degradation caused by residual solvents in the flue gas flow; cryogenic distillation is gradually considered undesirable due to its high energy consumption. Compared with the separation methods of distillation and amine absorption, adsorption separation technology is considered to be more promising for the industrialization of carbon dioxide capture. Due to its advantages such as no phase change in separation operation and low production cost, it is widely regarded by researchers as a new technology that is expected to replace traditional amine absorption technology to achieve efficient separation of mixed gases. It is a high-tech technology with the potential to achieve pollution-free clean production and meets the requirements of sustainable development.

[0004] Metal-organic framework (MOFs) materials are a class of porous materials formed by the coordination and connection of metal ions or ion clusters with organic ligands. They have the advantages of rich chemical composition, large specific surface area, adjustable pores, and excellent chemical and hydrothermal stability. In recent years, the research focus of metal-organic framework materials has gradually shifted from crystal synthesis to performance exploration. More and more metal-organic framework materials have begun to emerge and achieve outstanding results in the fields of adsorption, separation, catalysis, sensing, fluorescence, and drug delivery. This type of material has become the focus of continuous attention in the scientific community due to its potential application value. Compared with ordinary MOFs, MOF flake crystals have the characteristics of ultra-thin thickness, larger specific surface area, and more exposed active sites, which has attracted widespread attention from researchers. MOF flake crystals can be prepared by top-down and bottom-up synthesis methods. The top-down synthesis method uses layered metal organic framework materials as precursors, and uses strategies such as ultrasonic peeling and ball milling peeling to achieve interlayer peeling to prepare flaky crystals. However, this mechanical peeling method is easy to damage the morphology and structure of MOF flaky crystals. The bottom-up synthesis method starts from the reaction raw materials and prepares nanosheets in situ through chemical reactions.

[0005] Co3(HCOO)6 (also known as Co-fa) is completely based on Co 2+ and formate (HCOO - ), is a solid 3D microporous metal organic framework (MOF) material consisting of The one-dimensional channel is composed of zigzag segments. The literature (Chem. Eur. J. 2011, 17, 12076-12083) reports a Co3 (HCOO) 6 membrane with CO2 / CH4 adsorption performance. The preparation method comprises the following steps: 0.158 g of cobalt nitrate hexahydrate is dissolved in 15 ml of N, N-dimethylformamide under stirring for 30 minutes to fully dissolve it; then 37.5 μl of formic acid is added, and the solution is transferred to a polytetrafluoroethylene microwave reactor, and the temperature is controlled at 100°C and heated for 20 seconds. After the reaction is completed, it is naturally cooled to room temperature, centrifuged and the reaction product is washed 3 times to obtain Co3 (HCOO) 6 seeds. The suspension made of the seeds is deposited on the carrier to form a seed layer, and then placed vertically in an autoclave lined with polytetrafluoroethylene, and the autoclave contains a synthesis solution of 0.177 g of cobalt nitrate hexahydrate, 137.8 μl of formic acid and 15 ml of N, N-dimethylformamide. The seed crystals were grown again at 100 °C for 1 day. After the reaction, the membrane was washed once with N,N-dimethylformamide and three times with methanol, and finally activated and dried. At 298K and 1 bar, the adsorption capacities of CO2 and CH4 of the material were 39.24 cm 3 / g and 16.86cm 3 / g, and the adsorption capacity of CO2 is 2.3 times that of CH4. However, this method does not regulate the morphology of Co3(HCOO)6 crystals, and the adsorption and separation performance of CO2 / CH4 needs to be improved.

[0006] MOF materials are still common technical difficulties in the controllable preparation of oriented crystals and crystal films. In the process of MOF crystal morphology and orientation experiments, researchers often achieve their regulation by introducing additional deprotonating agents, surfactants or growth regulators. However, this brings the additional difficulty of completely and thoroughly removing these additives. In previous reports, there is no research and practical precedent for the preparation of Co3(HCOO)6 hexagonal flake crystals under solvothermal conditions. Summary of the invention

[0007] In order to solve the above technical problems, the purpose of the present invention is to provide a Co3(HCOO)6 hexagonal plate crystal and a preparation method thereof. The method uses cobalt salt and formic acid as main raw materials and can achieve the growth of Co3(HCOO)6 hexagonal plate crystals under solvent thermal conditions.

[0008] The present invention also aims to provide a Co3(HCOO)6 hexagonal plate-like crystal for use in the field of CO2 / CH4 gas adsorption separation.

[0009] To achieve the above object, the present invention provides a method for preparing Co3(HCOO)6 hexagonal plate-like crystals, which comprises the following steps:

[0010] S1, dissolving a cobalt salt in an organic solvent to obtain a metal solution after it is fully dissolved; and then adding formic acid to obtain a mixed solution;

[0011] S2, heating the mixed solution in a water bath, and stirring to react;

[0012] S3. After the reaction is completed, the mixture is naturally cooled to room temperature, the reaction product is washed and dried to obtain Co3(HCOO)6 hexagonal plate-like crystals.

[0013] According to a specific embodiment of the present invention, preferably, the temperature of the water bath heating is room temperature-100°C, more preferably 25-90°C. In the synthesis process, since the reaction of cobalt and formic acid is an exothermic process, the water bath heating is added to provide heat, so that the overall temperature of the synthetic liquid will slowly rise and get closer and closer to the boiling point of water (100°C), and the temperature difference with water is getting smaller and smaller, so the crystal first undergoes explosive nucleation at high temperature. Since the temperature difference with water is getting smaller and smaller, the heat transfer is weakened, and the liquid cannot continue to heat up, so that the crystal grows slowly after the explosive nucleation, which is conducive to the formation of nano Co3 (HCOO) 6 crystals. Therefore, the overall reaction temperature of the present invention is relatively low, and the competitive mode of explosive nucleation and slow growth of Co3 (HCOO) 6 crystals is conducive to the generation of nano crystals, and uniform and smaller Co3 (HCOO) 6 crystals are obtained. In addition, the addition of organic acid is a direct component of the co-fa metal organic framework, and the thermal stability of the metal organic framework is 300°C. The present invention controls the reaction temperature at a relatively low level, and can maintain or form a hexagonal structure during the heating process, without causing carbonization of the organic ligand (formic acid), and without causing the collapse of the co-fa metal organic framework and losing the hexagonal structure.

[0014] According to a specific embodiment of the present invention, preferably, the water bath heating time is 1-48h, more preferably 2-40h. The crystallization reaction time of the present invention is relatively short, because the crystallization time is an important factor affecting the grain size and morphology. The present invention can avoid a significant increase in the grain size by controlling the crystallization within a reasonable time range, which is conducive to the formation of nano-grains.

[0015] According to a specific embodiment of the present invention, preferably, the stirring speed is 210-400rpm, more preferably 300-350rpm. In the present invention, continuous stirring during heating can transfer heat to the center of the synthetic liquid more evenly and quickly, and can also strengthen the heat transfer between the liquid and the solid wall, so that the solution is heated more evenly; if not stirred, the synthetic liquid may be heated unevenly, so that the external solution may be overheated, while the internal part is still cold. In addition, stirring can fully mix the reactants and increase the reaction rate.

[0016] According to a specific embodiment of the present invention, preferably, the drying temperature is room temperature-80°C.

[0017] According to a specific embodiment of the present invention, preferably, the drying time is 6-24 hours.

[0018] In the above preparation method, the drying can be performed in an oven.

[0019] According to a specific embodiment of the present invention, preferably, the cobalt salt includes one or a combination of two or more of cobalt chloride hexahydrate, cobalt nitrate hexahydrate, cobalt acetate hexahydrate, cobalt oxalate, etc.

[0020] According to a specific embodiment of the present invention, preferably, the organic solvent comprises N,N-dimethylformamide and / or N,N-diethylformamide.

[0021] According to a specific embodiment of the present invention, preferably, the washing solvent includes one or a combination of two or more of methanol, ethanol, n-butanol, isopropanol, N,N-dimethylformamide, etc.

[0022] According to a specific embodiment of the present invention, preferably, the raw materials of the mixed solution satisfy the following molar ratio range: cobalt salt: formic acid = 1: 5-1: 200, cobalt salt: organic solvent = 1: 50-1: 400. More preferably, the raw materials of the mixed solution satisfy the following molar ratio range: cobalt salt: formic acid = 1: 10-1: 200, cobalt salt: organic solvent = 1: 50-1: 390. Further preferably, the raw materials of the mixed solution satisfy the following molar ratio range: cobalt salt: formic acid = 1: 80-1: 180, cobalt salt: organic solvent = 1: 350-1: 390.

[0023] In the preparation process, cobalt nitrate is first dissolved in an organic solvent such as DMF and hydrolyzed into cobalt ions and nitrate ions, and then the cobalt ions react with formic acid to form a pink precipitate. Among them, sufficient cobalt ions react with formic acid to easily obtain a thermodynamically stable product, and formic acid can fully deprotonate and react with Co 2+ Complete coordination, but excess formic acid can easily lead to the formation of other components. In addition, there is an interaction between the molecules of Co3(HCOO)6 crystals and organic solvents such as DMF, which can form a precursor similar to Co3(HCOO)6·DMF. Therefore, excess organic solvents will also affect the synthesis of pure Co3(HCOO)6 crystals.

[0024] The present invention also provides a Co3(HCOO)6 hexagonal flaky crystal, which is obtained by the above preparation method; the Co3(HCOO)6 crystal is pink.

[0025] According to a specific embodiment of the present invention, preferably, the crystal has a regular hexagonal morphology.

[0026] According to a specific embodiment of the present invention, preferably, the particle size of the crystal is 0.3-6 μm, preferably 0.3-4 μm; wherein the particle size is the face diagonal of the hexagonal crystal.

[0027] According to a specific embodiment of the present invention, preferably, the thickness of the crystal is 300-500 nm.

[0028] According to a specific embodiment of the present invention, preferably, the specific surface area of ​​the crystal is 240-350m 2 / g.

[0029] According to a specific embodiment of the present invention, preferably, the pore volume of the crystal is 0.1008-0.1157cm 3 / g; wherein the pore volume refers to the total pore volume per unit mass of porous solid.

[0030] The present invention also provides application of the above-mentioned Co3(HCOO)6 hexagonal plate-like crystals in the field of CO2 / CH4 gas adsorption separation.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) Compared with the Co3(HCOO)6 material prepared by the traditional solvent thermal method, the present invention adopts the solvent thermal method of water bath heating and stirring throughout the process, and can achieve the regulation of crystal morphology by adjusting the MOF synthesis parameters without adding any additives, and control the growth direction of Co3(HCOO)6 so that it grows preferentially along the (10-1) crystal plane to obtain a regular hexagonal morphology. The preparation method is simple and easy to operate. The prepared Co3(HCOO)6 hexagonal plate-like crystals have the characteristics of high morphological regularity, good crystallinity, uniform size, large specific surface area and good thermal stability. When heated in air or exposed to moisture, its framework can also be well maintained.

[0033] (2) The hexagonal flaky Co3(HCOO)6 crystals of the present invention have a weak adsorption capacity for CH4, a high CO2 / CH4 equilibrium adsorption ratio, and better CO2 / CH4 gas adsorption and separation performance. Compared with particles with irregular morphology, the CO2 / CH4 adsorption and separation capacity is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a scanning electron microscope image of the Co3(HCOO)6 flake crystals prepared in Example 1.

[0035] Figure 2 This is a scanning electron microscope image of the Co3(HCOO)6 flake crystals prepared in Example 2.

[0036] Figure 3 This is a scanning electron microscope image of the Co3(HCOO)6 flake crystals prepared in Example 3.

[0037] Figure 4 This is a scanning electron microscope image of the irregular Co3(HCOO)6 crystal prepared in Comparative Example 1.

[0038] Figure 5 The scanning electron microscope image (a) and the flattened XRD pattern (b) of the Co3(HCOO)6 plate-like crystals prepared in Example 4.

[0039] Figure 6 This is the XRD pattern of the Co3(HCOO)6 flake crystals prepared in Example 5.

[0040] Figure 7 Nitrogen adsorption curves of Co3(HCOO)6 flake crystals of different sizes at 77K; (a) is a crystal with a size of 2 μm prepared in Example 6; (b) is a crystal with a size of 4 μm prepared in Example 2.

[0041] Figure 8 This is the thermogravimetric diagram of the Co3(HCOO)6 flake crystals prepared in Example 7.

[0042] Fig. 9 CO2 and CH4 adsorption isotherms of Co3(HCOO)6 flake crystals of different sizes; (a) is a crystal with a size of 2 μm prepared in Example 8; (b) is a crystal with a size of 4 μm prepared in Example 2.

[0043] Fig.10 These are the CO2 and CH4 adsorption isotherms of the irregular Co3(HCOO)6 crystals prepared in Comparative Example 2. DETAILED DESCRIPTION

[0044] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0045] Example 1

[0046] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0047] 0.2910g of cobalt salt and 5ml of N,N-dimethylformamide were placed in a glass bottle and mixed, and then ultrasonically vibrated to fully dissolve; then 0.38ml of formic acid was added, and the water bath temperature was controlled at 90℃ and heated for 2h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 40℃ for 8h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0048] Repeat the above steps and increase the heating time to 12 h to obtain a similar product.

[0049] Repeat the above steps and increase the heating time to 24 h to obtain a similar product.

[0050] Repeat the above steps and increase the heating time to 36 h to obtain a similar product.

[0051] The morphology of the experimental samples was observed using a scanning electron microscope. Figure 1 As shown, the particle size of the hexagonal Co3(HCOO)6 crystals obtained by heating for 2 hours in this embodiment is 0.3 μm and the thickness is 300 nm.

[0052] Example 2

[0053] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0054] 0.2910g of cobalt salt and 15ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 3.02ml of formic acid was added, the water bath temperature was controlled at 30℃, and heated for 12h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 50℃ for 18h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0055] Repeat the above steps and increase the heating temperature to 50°C to obtain a similar product.

[0056] Repeat the above steps and increase the heating time to 70°C to obtain a similar product.

[0057] Repeat the above steps and increase the heating time to 90°C to obtain a similar product.

[0058] The morphology of the experimental samples was observed using a scanning electron microscope. Figure 2 As shown, the particle size of the hexagonal Co3(HCOO)6 crystals obtained by heating at 70°C in this embodiment is 4 μm and the thickness is 500 nm.

[0059] Figure 7 The nitrogen adsorption experimental results of Co3(HCOO)6 hexagonal plate crystals of different sizes at 77K are given by Figure 7 As can be seen from (b) in the figure, the adsorption rate of the Co3(HCOO)6 crystals with a particle size of 4 μm prepared at 70°C in this example increases rapidly under lower pressure, and the Co3(HCOO)6 crystals show a typical type I curve.

[0060] The specific surface area of ​​the 4 μm Co3(HCOO)6 crystals prepared in this example is 241.41 m 2 / g.

[0061] Fig. 9The adsorption isotherms of CO2 and CH4 of hexagonal flake Co3(HCOO)6 crystals of different sizes are shown in Figure 2. The adsorption isotherms of hexagonal flake Co3(HCOO)6 crystals with a particle size of 4 μm synthesized in this embodiment are shown in Figure 3. Fig. 9 As shown in (b), the two gases reach adsorption saturation at a pressure of 10 bar. The slope of CO2 in the low-pressure zone is steeper than that of CH4, indicating that the crystal has a higher affinity for CO2. The adsorption amounts of CO2 and CH4 are 31.47 cm 3 / g and 9.19cm 3 / g, the adsorption capacity of CO2 is 3.4 times that of CH4.

[0062] Example 3

[0063] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0064] 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 6.80ml of formic acid was added, the water bath temperature was controlled at 80℃, and heated for 40h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0065] Repeat the above steps and reduce the amount of N,N-dimethylformamide to 25 ml to obtain a similar product.

[0066] Repeat the above steps and reduce the amount of N,N-dimethylformamide to 20 ml to obtain a similar product.

[0067] Repeat the above steps and reduce the amount of N,N-dimethylformamide to 15 ml to obtain a similar product.

[0068] The morphology of the experimental samples was observed using a scanning electron microscope. Figure 3 As shown, the crystals obtained in this embodiment under the condition that the amount of N,N-dimethylformamide used is 15 ml are Co3(HCOO)6 hexagonal plate-like crystals with smooth crystal surface, regular shape and uniform size, with a particle size of 3 μm and a thickness of 500 nm.

[0069] Example 4

[0070] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0071] 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 6.8ml of formic acid was added, the water bath temperature was controlled at 80℃, and heated for 18h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0072] Spread the flake crystals flat on aluminum foil or glass and observe the morphology of the experimental samples with a scanning electron microscope. Figure 5 As shown, the crystals obtained in this example have smooth and flat developed crystal faces, regular shapes, and uniform sizes, which are laid flat on aluminum foil paper, and XRD shows that the crystals have strong orientation.

[0073] Example 5

[0074] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0075] 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 6.8ml of formic acid was added, the water bath temperature was controlled at 80℃, and heated for 20h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0076] like Figure 6 As shown, the hexagonal flake crystals obtained experimentally are consistent with the simulated peaks, and characteristic peaks appear at corresponding positions. The strongest XRD peak at 9.78° corresponds to the (10-1) crystal plane of the crystal, indicating that pure phase Co3(HCOO)6 crystals were successfully prepared.

[0077] Example 6

[0078] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0079] 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 6.8ml of formic acid was added, the water bath temperature was controlled at 80℃, and heated for 22h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0080] The results of the nitrogen adsorption experiment of the 2 μm Co3(HCOO)6 crystals prepared in this example at 77K are as follows: Figure 7As shown in (a): At lower pressure, the adsorption rate increases rapidly and the Co3(HCOO)6 crystal shows a typical type I curve.

[0081] The specific surface area of ​​the 2 μm Co3(HCOO)6 crystals prepared in this example is 262.44 m 2 / g, pore volume 0.1008cm 3 / g.

[0082] Example 7

[0083] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0084] 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 6.8ml of formic acid was added, the water bath temperature was controlled at 80℃, and heated for 24h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0085] Thermogravimetric analysis results of the synthesized materials are shown in Figure 8 As shown. Figure 8 It can be seen that: at a temperature of 200°C, the continuous weight loss is 14wt%, which corresponds to the solvent molecules in the pores. Therefore, at this temperature, the skeleton is not decomposed; when the temperature reaches about 300°C, the skeleton of the Co3(HCOO)6 crystal is decomposed, indicating that the Co3(HCOO)6 crystal prepared in this embodiment has good thermal stability.

[0086] Example 8

[0087] This embodiment provides a Co3(HCOO)6 hexagonal plate-like crystal, and the preparation method thereof comprises:

[0088] 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide were placed in a glass bottle and mixed, then ultrasonically vibrated to fully dissolve; then 6.8ml of formic acid was added, the water bath temperature was controlled at 80℃, and heated for 26h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain Co3(HCOO)6 hexagonal plate crystals.

[0089] The CO2 and CH4 adsorption isotherms of the hexagonal Co3(HCOO)6 crystals with a particle size of 2 μm synthesized in this example are as follows: Fig. 9As shown in (a), the two gases reach adsorption saturation at a pressure of 10 bar. The slope of CO2 in the low-pressure zone is steeper than that of CH4, indicating that the crystal has a higher affinity for CO2. The adsorption amounts of CO2 and CH4 are 36.97 cm 3 / g and 5.59cm 3 / g, the CO2 adsorption capacity is 6.6 times that of CH4. Compared with the crystals with a particle size of 4 μm synthesized in Example 2 in (b), the size is reduced, the CO2 adsorption capacity of the crystals is increased, and the methane adsorption capacity is reduced, but this change is not significant, and the adsorption difference is not large, indicating that the separation performance of Co3(HCOO)6 crystals for CO2 / CH4 is relatively stable within a certain size range.

[0090] Comparative Example 1

[0091] This comparative example provides a Co3(HCOO)6 crystal, the preparation method of which includes:

[0092] Take 0.2910g of cobalt salt and 30ml of N,N-dimethylformamide and mix them in a glass bottle, then perform ultrasonic oscillation to make them fully dissolved; then add 7.65ml of formic acid, control the water bath temperature at 80℃, and heat for 40h. After the reaction is completed, cool it naturally to room temperature, centrifuge and wash the reaction product 3 times, and dry it in an oven at 60℃ for 22h to obtain irregularly shaped Co3(HCOO)6 crystals. The morphology of the experimental samples was observed by scanning electron microscope. The morphology is as follows Figure 4 As shown, compared with Example 3, the crystals obtained in this comparative example have no regular shape and are not uniform in size.

[0093] Comparative Example 2

[0094] This comparative example provides a Co3(HCOO)6 crystal, the preparation method of which includes:

[0095] 0.2910g of cobalt salt and 35ml of N,N-dimethylformamide were mixed in a glass bottle, and then ultrasonically vibrated to fully dissolve them; then 6.8ml of formic acid was added, and the water bath temperature was controlled at 80℃ and heated for 26h. After the reaction was completed, it was naturally cooled to room temperature, centrifuged and washed 3 times, and dried in an oven at 60℃ for 22h to obtain irregularly shaped Co3(HCOO)6 crystals.

[0096] The adsorption isotherms of CO2 and CH4 were measured for the irregularly shaped Co3(HCOO)6 crystals. Fig.10 shown. Fig.10 The results shown in the figure show that the adsorption capacities of CO2 and CH4 for irregularly shaped Co3(HCOO)6 crystals at 298K and 10bar are 40.55cm 3 / g and 29.23cm 3 / g, the CO2 adsorption capacity is 1.4 times that of CH4. In comparison, it is much lower than the equilibrium adsorption ratio of CO2 / CH4 of 6.6 in Example 8, indicating that the hexagonal flaky Co3(HCOO)6 crystals are more beneficial for CO2 / CH4 separation.

[0097] Comparative Example 3

[0098] This comparative example provides a Co3(HCOO)6 crystal, which is synthesized according to the literature (Chem.Eur.J.2011,17,12076-12083), and specifically includes the following steps:

[0099] Take 0.158g of cobalt salt and dissolve it in 15ml of N,N-dimethylformamide under stirring for 30 minutes to fully dissolve it; then add 37.5μl of formic acid, transfer the solution to a polytetrafluoroethylene microwave reactor, and heat it at 100°C for 20 seconds; after the reaction is completed, naturally cool to room temperature, centrifuge and wash the reaction product 3 times to obtain Co3(HCOO)6 seeds; deposit the suspension made of the seeds on a carrier to form a seed layer, and then place it vertically in an autoclave lined with polytetrafluoroethylene, which contains a synthetic solution of 0.177g of cobalt nitrate hexahydrate, 137.8μl of formic acid and 15ml of N,N-dimethylformamide; the seeds are secondary grown at 100°C for 1 day; after the reaction is completed, wash the membrane once with N,N-dimethylformamide and three times with methanol, and finally activate and dry.

[0100] At 298K and 1 bar, the adsorption capacities of CO2 and CH4 of the material prepared in this comparative example are 39.24 cm 3 / g and 16.86cm 3 / g, the CO2 adsorption capacity is 2.3 times that of CH4, which is much lower than the equilibrium adsorption ratio of CO2 / CH4 of 6.6 in Example 8. In comparison, Example 8 has a weaker adsorption capacity for CH4 and has better CO2 / CH4 adsorption and separation capacity, indicating that hexagonal flaky Co3(HCOO)6 crystals are more beneficial for CO2 / CH4 separation.

[0101] The morphology and CO2 / CH4 adsorption separation performance of the Co3(HCOO)6 crystals prepared in the examples and comparative examples are summarized and compared below. The results are shown in Table 1:

[0102] Table 1. Preparation process parameters, crystal size and adsorption test results of examples and comparative examples

[0103]

[0104]

[0105] It can be seen from the above Examples 1-8 and Comparative Examples 1-3 that the technical solution of the present invention provides a method for preparing Co3(HCOO)6 crystals with simple process and convenient operation, which can achieve the regulation of crystal morphology by regulating MOF synthesis parameters without adding any additives, thereby obtaining a regular hexagonal flake morphology, thereby having better CO2 / CH4 adsorption and separation ability.

Claims

1. A method for preparing Co3(HCOO)6 hexagonal plate-like crystals, comprising the following steps: S1, dissolving a cobalt salt in an organic solvent to obtain a metal solution after it is fully dissolved; and then adding formic acid to obtain a mixed solution; S2, heating the mixed solution in a water bath, and stirring to react; S3, after the reaction is completed, the mixture is naturally cooled to room temperature, the reaction product is washed and dried to obtain Co3(HCOO)6 hexagonal plate-like crystals; in, The raw materials of the mixed solution satisfy the following molar ratio range: cobalt salt: formic acid = 1:5-1:200, cobalt salt: organic solvent = 1:50-1:

400.

2. The preparation method according to claim 1, wherein The water bath is heated at a temperature of room temperature to 100°C; Preferably, the water bath heating time is 1-48h.

3. The preparation method according to claim 1, wherein The stirring speed is 210-400 rpm.

4. The preparation method according to claim 1, wherein The cobalt salt includes one or a combination of two or more of cobalt chloride hexahydrate, cobalt nitrate hexahydrate, cobalt acetate hexahydrate, and cobalt oxalate.

5. The preparation method according to claim 1, wherein The organic solvent includes N,N-dimethylformamide and / or N,N-diethylformamide.

6. The preparation method according to any one of claims 1 to 5, wherein: The raw materials of the mixed solution meet the following molar ratio ranges: cobalt salt: formic acid = 1:10-1:200, cobalt salt: organic solvent = 1:50-1:

390.

7. A Co3(HCOO)6 hexagonal plate-like crystal obtained by the preparation method according to any one of claims 1 to 6.

8. The hexagonal plate-like crystal according to claim 7, wherein: The particle size of the crystals is 0.3-6 μm, preferably 0.3-4 μm; Preferably, the thickness of the crystal is 300-500 nm.

9. The hexagonal plate-like crystal according to claim 7, wherein: The specific surface area of ​​the crystal is 240-350m 2 / g; Preferably, the pore volume of the crystal is 0.1008-0.1157cm 3 / g.

10. Use of the Co3(HCOO)6 hexagonal plate-like crystals according to any one of claims 7 to 9 in the field of CO2 / CH4 gas adsorption separation.

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