CN108164549B - COFs materials based on flexible modules and their preparation methods and uses - Google Patents

Abstract

The invention belongs to the field of organic porous material adsorbents, and in particular relates to a COFs material constructed based on flexible modules and a preparation method and application thereof. The present invention provides a COFs material constructed based on flexible modules, the structure of which is shown in formula I. The present invention also provides a preparation method of the above-mentioned COFs material constructed based on flexible modules, and its use as an iodine adsorbent. A series of COFs materials with large lattice size prepared by the flexible module of the present invention all show extremely high iodine enrichment ability, and have great practical prospects in the field of enrichment and separation of conventional and radioactive iodine.

Description

COFs materials based on flexible modules and their preparation methods and uses

technical field

The invention belongs to the field of organic porous material adsorbents, and in particular relates to a COFs material constructed based on flexible modules and a preparation method and application thereof.

Background technique

Covalent Organic Frameworks (COFs) are a class of porous crystalline materials prepared by connecting organic molecular building blocks through covalent bonds. COFs have the characteristics of large specific surface area, excellent chemical and biological stability, easy functionalization and highly ordered periodic sequence, etc. important application value. Although the research on COFs has made great progress in the past ten years, especially in the past five years, there is still great difficulty in how to easily and effectively prepare COFs with regular structure and high crystallinity. In order to obtain a good crystal structure, COFs generally use relatively rigid monomer modules and special bonding methods. The molecular chain after bonding is usually linear or nearly linear as a whole, such as boron double oxygen bonds, carbon nitrogen double bonds. , carbon-carbon double bonds, alkyne bonds, etc., this design method greatly limits the composition and structure types of COFs. Using a variety of flexible building blocks to prepare COFs creates the possibility to enrich the structural diversity and complexity of COFs. In particular, covalent organic framework materials based on soft modules have greater potential stretchability and more diverse stacking methods because the flexible modules have greater rotational freedom than rigid monomers. However, due to the large rotational freedom of flexible modules, it is difficult to obtain regularly arranged spatial structures and high crystallinity for COFs prepared from flexible modules. Therefore, the construction of highly crystalline and novel covalent organic frameworks based on flexible modules remains a challenging research direction.

For the practical application of materials, high crystallinity and regular and orderly structure arrangement will greatly increase the application performance of materials in the fields of optics, electricity and so on. However, in many cases, good crystallinity does not necessarily mean good application properties, such as the application of materials in the fields of adsorption, separation, and catalysis. Therefore, how to effectively control the crystallinity of covalent organic framework materials and thereby adjust their related application properties is a research topic of great practical significance, especially for the large-scale crystals with important development value based on flexible modules. Lattice-sized COFs materials are more important, and this research may bring unprecedented performance and application prospects to COFs materials.

Iodine-129 is the most important radioactive pollutant in airborne radioactive nuclear waste, because of its extremely long radioactive half-life (1.57×10 ⁷ years), volatility and biocompatibility, it is extremely harmful to the ecological environment and human health. big hazard. Therefore, the design and preparation of suitable materials for efficient capture and storage of iodine is crucial for both public safety and nuclear energy safety. People are used to using inorganic composite adsorbents such as natural or synthetic molecular sieves as iodine adsorbents, but the adsorption capacity of such materials for iodine is not ideal, and the adsorption capacity is relatively low. Porous materials are one of the hotspots in the research of iodine adsorption materials due to their superior specific surface area, porosity, and relatively regular pore structure. These mainly include inorganic porous materials, metal-organic frameworks (MOFs), and porous organic polymers (POPs). The loading rate of iodine by inorganic porous materials is usually between 8% and 175%, which is relatively low. At the same time, the poor recycling ability of inorganic porous materials limits their application in iodine capture. Metal-organic frameworks exhibit superior iodine uptake ability than inorganic adsorbents, however, due to their poor moisture and humidity stability and acid-base stability, MOFs are difficult to use in practical nuclear exhaust gas (containing a large amount of water vapor). ) capture and storage of iodine.

SUMMARY OF THE INVENTION

The present invention provides a COFs material constructed based on flexible modules, the structure of which is shown in formula I:

wherein, R ₁ and R ₂ are independently -H or -OH. Preferably, both R ₁ and R ₂ are -H. Preferably, both R ₁ and R ₂ are -OH.

The present invention also provides the above-mentioned preparation method of the COFs material constructed based on the flexible module, comprising the following steps:

a. Preparation of 2,4,6-tri-p-formylphenoxy-1,3,5-triazine: dissolve p-hydroxybenzaldehyde in mixed solvent, add alkali at 0～5℃, stir for 10～ 30min; slowly add acetone-dissolved cyanuric chloride solution dropwise, continue to react at 0-5°C for 0.5-2h; then heat up to reflux for 1-5h, cool naturally, pour the reacted solution into distilled water, filter, solid Washed with alkaline solution and dried in vacuo to obtain 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO);

b. Preparation of COFs based on flexible modules: ultrasonically sonicated the substituted benzidine and mixed solvent, TPT-CHO and mixed solvent for 2-10 min respectively to form a uniform dispersion, and then slowly added to the TPT-CHO dispersion Acetic acid, sealed, and left to react at 60-100 °C for 2-5 days to obtain COFs materials based on flexible modules.

In the above-mentioned preparation method of COFs materials based on flexible modules, the mixed solvent in step a is a mixed solvent of acetone and water; the volume ratio of acetone to water is 5:1 to 1:2.

In the above-mentioned preparation method of COFs materials based on flexible modules, the alkali is any one of NaOH, KOH, CsOH, Ca(OH) ₂ , Sr(OH) ₂ or Ba(OH) ₂ ; The molar ratio of p-hydroxybenzaldehyde to alkali is 1:1～1:2.

In the above-mentioned preparation method of COFs material based on flexible module construction, the molar ratio of p-hydroxybenzaldehyde to cyanuric chloride in step a is 5:1 to 3:1.

In the above-mentioned preparation method of COFs material based on flexible module construction, the molar ratio of alkali to p-hydroxybenzaldehyde in the alkali solution described in step a is 3:2 to 3:1; the alkali solution is saturated NaHCO ₃ Either solution, 8-16% Na ₂ CO ₃ solution or 5-10% NaOH solution.

In the above-mentioned preparation method of COFs material based on flexible module construction, the mixed solvent described in step b is mesitylene (1,3,5-trimethylbenzene)/dioxane solvent; The volume ratio of the six rings is 3:1 to 1:3.

In the above-mentioned preparation method of COFs material based on flexible module construction, the molar ratio of the substituted benzidine described in step b to TPT-CHO is 7:4 to 6:5.

In the above-mentioned preparation method of COFs material based on flexible module construction, the concentration of acetic acid in step b is 5-15M; the molar ratio of acetic acid and TPT-CHO is 3:1-15:1.

The reaction formula of the above-mentioned preparation method of COFs materials based on flexible modules is:

wherein, R ₁ and R ₂ are independently -H or -OH. Preferably, both R ₁ and R ₂ are -H.

The present invention also provides the use of the above-mentioned COFs material constructed based on the flexible module as an iodine adsorbent.

The invention creatively uses a large flexible monomer containing a triazine ring and an ether bond as a building block to prepare a COFs material by an aldol-amine condensation reaction. This in-situ nitrogen loading method makes the product structure with abundant nitrogen atoms, which can greatly increase the loading rate of iodine. At the same time, the prepared COFs have good crystal shape and large lattice size, which are extremely rare in the field of COFs constructed with flexible modules. The present invention, through theoretical calculation and experimental research, for the first time deeply discusses the influence of the hydrogen bond content in the COFs material layer on the crystallinity and iodine adsorption performance of the flexible module constructed COFs. It is found that the hydrogen bond reduces the rotational freedom of the building unit, The partial structure of COFs is locked, thereby effectively increasing the crystallinity of the material. Therefore, the crystallinity of the product COFs can be effectively controlled by controlling the content of hydrogen bonds in the system. However, the existence of hydrogen bonds also occupies a large number of adsorption active sites, which makes the material's capture performance for iodine significantly decrease. However, a series of COFs materials with large lattice size prepared by flexible modules in the present invention all show extremely high iodine enrichment capacity, and the maximum adsorption capacity is as high as 5.43 g/g, which has never been reported, which is the highest among all types of adsorption materials at present. This feature makes it have great practical prospects in the field of enrichment of radioactive iodine in the reprocessing of radioactive nuclear waste.

Description of drawings

Fig. 1 Solid NMR spectra (a, b) and X-ray photoelectron spectra (c, d) of the covalent organic framework materials TPT-BD COF (compound 1) and TPT-DHBD COF (compound 2) provided by the present invention .

Fig. 2 Powder X-ray diffraction patterns of the covalent organic framework materials TPT-BD COF (compound 1) and TPT-DHBD COF (compound 2) provided by the present invention and the computational simulation structures of Materials Studio.

Fig. 3 Infrared spectra of the covalent organic framework materials TPT-BD COF (compound 1) and TPT-DHBD COF (compound 2) provided by the present invention before and after irradiation with a total dose of 10 ⁵ Gy of gamma rays.

Figure 4. The adsorption time curves of compounds 1-5 for iodine.

Figure 5. Desorption curves of compounds 1-5 for iodine.

Fig. 6 Infrared spectra of the covalent organic framework materials TPT-BD COF (compound 1) and TPT-DHBD COF (compound 2) provided by the present invention and three reaction raw materials.

Fig. 7 Infrared spectra of compounds 1-5.

Fig. 8 Infrared spectra of the covalent organic framework material TPT-BD COF (compound 1) of the present invention before and after adsorption of iodine.

Figure 9. Thermogravimetric curves of compound 2 (TPT-DHBD COF) before and after iodine adsorption.

Figure 10 Cyclic adsorption experiment of compound 1 (TPT-BD COF).

Detailed ways

The preparation method of COFs material based on flexible module construction includes the following steps:

a. Preparation of 2,4,6-tri-p-formylphenoxy-1,3,5-triazine: dissolve p-hydroxybenzaldehyde in mixed solvent, add alkali at 0～5℃, stir for 10～ 30min; slowly add acetone-dissolved cyanuric chloride solution dropwise, continue to react at 0-5°C for 0.5-2h; then heat up to reflux for 1-5h, cool naturally, pour the reacted solution into distilled water, filter, solid Washed with alkaline solution and dried in vacuo to obtain 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO);

b. Preparation of COFs based on flexible modules: ultrasonically sonicated the substituted benzidine and mixed solvent, TPT-CHO and mixed solvent for 2-10 min respectively to form a uniform dispersion, and then slowly added to the TPT-CHO dispersion Acetic acid, sealed, and left to react at 60-100 °C for 2-5 days to obtain COFs materials based on flexible modules.

In the above-mentioned preparation method of COFs materials based on flexible modules, the mixed solvent in step a is a mixed solvent of acetone and water; the volume ratio of acetone to water is 5:1 to 1:2.

In the above-mentioned preparation method of COFs materials based on flexible modules, the alkali is any one of NaOH, KOH, CsOH, Ca(OH) ₂ , Sr(OH) ₂ or Ba(OH) ₂ ; The molar ratio of p-hydroxybenzaldehyde to alkali is 1:1～1:2.

In the above-mentioned preparation method of COFs material based on flexible module construction, the molar ratio of p-hydroxybenzaldehyde to cyanuric chloride in step a is 5:1 to 3:1.

In the above-mentioned preparation method of COFs material based on flexible module construction, the molar ratio of alkali to p-hydroxybenzaldehyde in the alkali solution described in step a is 3:2 to 3:1; the alkali solution is saturated NaHCO ₃ Either solution, 8-16% Na ₂ CO ₃ solution or 5-10% NaOH solution.

In the above-mentioned preparation method of COFs material based on flexible module construction, the mixed solvent described in step b is mesitylene (1,3,5-trimethylbenzene)/dioxane solvent; The volume ratio of the six rings is 3:1 to 1:3.

In the above-mentioned preparation method of COFs material based on flexible module construction, the molar ratio of the substituted benzidine described in step b to TPT-CHO is 7:4 to 6:5.

In the above-mentioned preparation method of COFs material based on flexible module construction, the concentration of acetic acid in step b is 5-15M; the molar ratio of acetic acid and TPT-CHO is 3:1-15:1.

Example 1 Preparation of 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO)

Weigh 7.45 g of p-hydroxybenzaldehyde and dissolve it in a mixed solvent of acetone and water (1:1 v/v, 100 mL), add 2.48 g of NaOH under ice bath conditions, and stir for 30 min. Slowly add 50 mL of acetone-dissolved cyanuric chloride (3.68 g) solution dropwise into the reactor, stir under an ice bath for 1 h, reflux at 80° C. for 2 h, and cool naturally. The reacted solution was poured into 300 mL of distilled water to form a large amount of white solid. After filtration, the solid was washed 2-3 times with 10% Na ₂ CO ₃ solution and dried in vacuo. The obtained white solid powder was recrystallized from ethyl acetate to obtain TPT-CHO as a white crystalline powder with a yield of 88.9%.

¹ H NMR (400 MHz, CDCl ₃ ) δ: 9.99 (s, 3H), 7.92 (d, J=8.7 Hz, 6H), 7.32 (d, J=8.5 Hz, 6H).

Example 2 Preparation of covalent organic framework material TPT-BD COF (compound 1)

Weigh out 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO) (88.4mg, 0.2mmol) and benzidine (BD) (55.2mg, 0.3mmol) respectively It was placed in a 10 mL glass vial, each solvent was added with mesitylene/dioxane (1:1 v/v, 1 mL), and ultrasonicated for 5 min to form a uniform dispersion. Then, the TPT-CHO dispersion was added to the 3,3'-dihydroxybenzidine dispersion, and finally acetic acid (6M, 0.2 mL) was slowly added, sealed, and allowed to stand at 85°C for 4d to obtain a yellow crystalline solid product. rate 61.3%. Molecular formula ((C ₁₄ H ₉ N ₂ O) _n ), elemental analysis (% calculated/measured): C 76.01/72.80, H4.10/4.35, N12.66/11.36, O 7.23/11.49).

Example 3 Preparation of covalent organic framework material TPT-DHBD COF (compound 2)

Weigh out 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO) (88.4 mg, 0.2 mmol) and 3,3'-dihydroxybenzidine (DHBD) ( 64.8 mg, 0.3 mmol) were placed in 10 mL glass vials, respectively, and the solvent mesitylene/dioxane (1:1 v/v, 1 mL) was added to each, and ultrasonicated for 8 min to form a uniform dispersion. Then, the TPT-CHO dispersion was added to the 3,3'-dihydroxybenzidine dispersion, and finally acetic acid (6M, 0.3 mL) was slowly added, sealed, and allowed to stand at 70 °C for 5 d to obtain a yellow crystalline solid product. rate 62.5%. Molecular formula ((C ₁₄ H ₉ N ₂ O ₂ ) _n ), elemental analysis (% calculated/measured): C70.87/65.46, H3.83/4.05, N 11.81/9.87, O 13.49/20.62).

From the analysis in Figure 1a,b, it can be seen that sharp characteristic resonance peaks of imino carbon appear at 154ppm in both TPT-BD COF and TPT-DHBD COF spectra, confirming the existence of imine bonds in the material structure. Figure 1c,d are the N 1s core layer spectra measured by X-ray photoelectron spectroscopy (XPS), TPT-BD COF (compound 1) and TPT-DHBD COF (compound 2) at 399.4 eV, 398.7 eV and 399.4 eV, The peak at the binding energy of 399.0eV belongs to the peaks of the C=N bond and imine bond of the triazine ring. There are only two peaks in the spectrum, and their areas are basically the same, indicating that only the triazine ring nitrogen atom and the imine are present in the material. Nitrogen atoms are present in a 1:1 ratio to match the structure. It can be seen that the present invention successfully prepares the COFs material combined by aldimine condensation.

Example 4 Preparation of covalent organic framework material TPT-DHBD ₂₅ COF (compound 3)

Benzidine (BD) (82.8 mg, 0.45 mmol) and 3,3'-dihydroxybenzidine (DHBD) (32.4 mg, 0.15 mmol) were placed in a 20 mL glass pressure bottle, and 2 mL of mesitylene/dioxygen was added. Hexacyclic (1:1v/v) mixed solvent was sonicated for 8min to form a uniform dispersion. Then take the monomer 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO) (176.8mg, 0.4mmol) and also disperse it in 2mL of mesitylene/dioxane (1:1 v/v) to form a homogeneous dispersion. Then the TPT-CHO dispersion was transferred to a pressure-resistant bottle, mixed by ultrasonic for 20 s, then slowly added acetic acid (6M, 0.6 mL), sealed, and allowed to stand at 90 °C for 3 d to obtain a yellow crystalline solid product with a yield of 59.8 %. Molecular formula ((C ₅₆ H ₃₆ N ₈ O ₅ ) _n ), elemental analysis (% calculated/measured): C 74.66/71.19, H 4.03/4.46, N 12.44/11.21, O 8.88/13.14).

Example 5 Preparation of covalent organic framework material TPT-DHBD ₅₀ COF (compound 4)

Benzidine (BD) (55.2 mg, 0.3 mmol) and 3,3'-dihydroxybenzidine (DHBD) (64.8 mg, 0.3 mmol) were placed in a 20 mL glass pressure-resistant bottle, and 2 mL of mesitylene/dioxygen was added. Hexacyclic (1:1 v/v) mixed solvent was sonicated for 5 min to form a uniform dispersion. Then take the monomer 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO) (176.8mg, 0.4mmol) and also disperse it in 2mL of mesitylene/dioxane (1:1 v/v) to form a homogeneous dispersion. Then the TPT-CHO dispersion was transferred to a pressure-resistant bottle, mixed by ultrasonic for 20 s, and then slowly added acetic acid (6M, 0.4 mL), sealed, and allowed to stand at 80 °C for 4 d to obtain a yellow crystalline solid product with a yield of 78.4 %. Molecular _formula (( _C56H36N8O6 ) _n ), elemental analysis (% calculated/measured): _C73.35 /69.34, H 3.96/4.27, N _12.22 /10.87, O 10.47/15.52).

Example 6 Preparation of covalent organic framework material TPT-DHBD ₇₅ COF (compound 5)

Benzidine (BD) (27.6 mg, 0.15 mmol) and 3,3'-dihydroxybenzidine (DHBD) (97.2 mg, 0.45 mmol) were placed in a 20 mL glass pressure bottle, and 2 mL of mesitylene/dioxygen was added. Hexacyclic (1:1 v/v) mixed solvent was sonicated for 6 min to form a uniform dispersion. Then take the monomer 2,4,6-tri-p-formylphenoxy-1,3,5-triazine (TPT-CHO) (176.8mg, 0.4mmol) and also disperse it in 2mL of mesitylene/dioxane (1:1 v/v) to form a homogeneous dispersion. Then the TPT-CHO dispersion was transferred to a pressure-resistant bottle, mixed by ultrasonic for 20s, and then slowly added acetic acid (6M, 0.5mL), sealed, and allowed to stand at 75°C for 5d to obtain a yellow crystalline solid product with a yield of 68.7 %. Molecular formula ((C ₅₆ H ₃₆ N ₈ O ₇ ) _n ), elemental analysis (% calculated/measured): C 72.10/67.29, H 3.89/4.21, N 12.01/10.87, O 12.00/17.63).

Example 7 Powder X-ray Diffraction (PXRD) and theoretical simulation experiments of compound 1 and compound 2

Powder X-ray diffraction (PXRD) is one of the main characterization methods for the structure of crystalline materials. Since the PXRD main diffraction peaks of most COF materials appear in the 2θ range of 2-5°, the testing range of the X-ray diffractometer used in the present invention is 2-40°. The exact structures of the materials TPT-BD COF and TPT-DHBD COF were determined by the comparative study of PXRD experimental test values and theoretical simulation values.

PXRD showed that TPT-DHBD COF (compound 1) had multiple distinct diffraction peaks (Fig. 2a), and its main diffraction peak was caused by the (100) plane, located at 2.27°. Other clearly visible peaks are located at 2θ of 4.07°, 4.69°, 6.22°, 8.13°, 10.18°, attributed to (110), (200), (210), (220), (xxx) crystal planes, respectively.

α＝β＝90°,andγ＝120°,相关系数为R_P＝2.80％和R_WP＝3.91％。Pawley精修值与实验观测值能很好的匹配。材料TPT-BD COF在2θ为2.27、4.03、4.53、6.16、8.12°处有较为明显的衍射峰(图2b)，他们分别由(100)、(110)、(200)、(210)、(220)晶面产生。结构模拟显示实验值各晶面的衍射峰与AA堆叠方式基本相同，PawleyPawley精修的晶胞参数为

α＝β＝90°，γ＝120°,相关系数为R_P＝4.48％和R_WP＝6.11％。通过PXRD对比发现TPT-DHBD COF的XRD衍射峰强度大，图谱受基线干扰小，而TPT-BD COF其(100)衍射峰峰强较弱，受基线干扰严重，且在2θ为21.5°附近有较强的无定型峰的出现。说明TPT-DHBD COF比TPT-BD COF具有更好的结晶度，这可能是由于TPT-DHBD COF中形成了氢键的缘故。The crystal structure of TPT-DHBD COF (compound 2) was simulated using Materials Studio 7.0 software, and the results showed that the simulated PXRD pattern was similar to the experimental value in the way of AA stacking. The unit cell parameters of the Pawley refinement are

α=β=90°, and γ=120°, the correlation coefficients are R _P =2.80% and R _WP =3.91%. The Pawley refinement values are well matched to the experimental observations. The material TPT-BD COF has obvious diffraction peaks at 2.27, 4.03, 4.53, 6.16, and 8.12° at 2θ (Fig. 2b), which are represented by (100), (110), (200), (210), ( 220) crystal planes are generated. The structural simulation shows that the diffraction peaks of each crystal plane in the experimental value are basically the same as the AA stacking method, and the unit cell parameters refined by PawleyPawley are:

α=β=90°, γ=120°, the correlation coefficients are R _P =4.48% and R _WP =6.11%. Through PXRD comparison, it is found that the XRD diffraction peak intensity of TPT-DHBD COF is high, and the pattern is less disturbed by the baseline, while the (100) diffraction peak of TPT-BD COF is weaker and seriously disturbed by the baseline, and there is a 2θ near 21.5°. The appearance of stronger amorphous peaks. It shows that TPT-DHBD COF has better crystallinity than TPT-BD COF, which may be due to the formation of hydrogen bonds in TPT-DHBD COF.

Example 8 γ-ray irradiation experiment of compound 1 and compound 2

From the designed structure, it can be seen that the series of covalent organic framework materials TPT-DHBD _X COF prepared by the present invention have nitrogen-rich structure (C=N) and rich π-π conjugated system, and such a structural characteristic is that it has a structure with iodine. Interactions offer many possibilities. In order to investigate the stability of the material to withstand ray irradiation in a possible real application environment, the present invention uses a γ-ray radiation source of ⁶⁰ Co to irradiate compound 1 and compound 2 with a total dose of 10 ⁵ Gy of γ-ray, The radiation stability of the material is demonstrated by comparing the infrared spectra before and after irradiation.

The infrared spectrum of Fig. 3 shows that the material hardly changes before and after irradiation, indicating that the material has good radiation stability and can at least withstand the irradiation of γ-rays with a total dose of 10 ⁵ Gy. This indicates that the prepared covalent organic framework materials have potential applications in the field of radioactive iodine capture (such as ¹²⁹ I and ¹³¹ I). In addition, the prepared materials also have good thermal stability, especially TPT-BD COF (compound 1), which only loses 7% in weight when the temperature is from 25 to 400 °C. The prepared materials are therefore very suitable for adsorption-desorption applications of iodine vapors in practical environments.

Example 9 Adsorption experiment of compounds 1-5 on iodine

To evaluate the material's ability to enrich iodine vapor, the prepared powders of compounds 1 to 5 were exposed to a closed system containing excess iodine at a constant temperature of 75°C and ambient pressure, which was similar to typical The nuclear fuel reprocessing environment is close. The adsorption capacity of the material for iodine under different adsorption time was detected by gravimetric method. The specific operation is as follows: 10 mg of adsorbent powder is placed in a 2 mL open glass tube, and the glass tube is put into a glass vial containing 500 mg of iodine, sealed, and the adsorption experiment is carried out under normal pressure at 75°C. After a period of time (0-48h), the sample was taken out, cooled and weighed, and its adsorption capacity at different times was calculated. Calculate the adsorption capacity of the adsorbent for iodine through the increase in weight: _Cu = (W ₂ -W ₁ )/W ₁ ×100wt%; where _Cu is the adsorption capacity, and W ₁ , W ₂ are the mass of the material before and after adsorption of iodine vapor .

The results in Figure 4 show that in the first 6h, the adsorption capacity increased rapidly in an almost linear trend, reaching more than 70% of the total adsorption capacity; by 12h, the adsorption basically approached equilibrium; after the adsorption time exceeded 32h, the iodine The loadings hardly changed, indicating that the adsorption had reached equilibrium. During the adsorption process, the color of the material continued to deepen, and finally changed from yellow to dark black. The material after adsorption of iodine was denoted as TPT-DHBD _X COFs@I ₂ . From the adsorption curve, we can see that the rates of the first and last stages of adsorption are very different, which indicates that the iodine enrichment is the combined effect of chemisorption and physical adsorption. The adsorption kinetics study found that chemisorption is the main mode of action and has a faster adsorption kinetics. Under this condition, the amounts of 1 g of compounds 1 to 5 that can adsorb iodine are about 5.43 g/g, 3.88 g/g, 4.63 g/g, 4.30 g/g, and 4.12 g/g, respectively. It can be seen that the compounds 1 to 5 series of materials have ultra-high iodine saturation adsorption capacity, especially the material TPT-BD COF (compound 1) enrichment of iodine can reach up to 5.43g/g, which is higher than all reported iodine. adsorption materials, such as MOFs, amorphousPOPs and other COFs materials. More importantly, the gram-level TPT-DHBD COF (compound 2) prepared by us can still enrich iodine as high as 4.03 g/g, which creates conditions for the large-scale production and practical application of this type of material. It has great potential application value in the field of enrichment and removal of radioactive iodine in specific environments such as nuclear accident sites.

The infrared spectrum analysis in Fig. 8 shows that the peaks of the material before and after the adsorption of iodine have shifted greatly. For example, the C=C bond and CH bond in the benzene ring of TPT-BD COF are located at 1499cm ^-1 and 815cm ^-1 , and move to 1492cm ^- 1. ¹ , 805cm ^-1 ; the C=N double bond of the triazine ring moves from 1566cm ^-1 and 1363cm ^-1 to 1562cm ^-1 and 1359cm ^-1 ; especially the peak of imine bond, moves from 1622cm ^-1 to 1631cm ^-1 , there is a huge change. The rest of the materials have similar laws, indicating that the iodine adsorption may occur simultaneously on the imine bond, triazine ring and benzene ring of the material.

Example 10 Desorption experiment of iodine by compounds 1 to 5

The desorption experiment of iodine was carried out at 125 °C (Fig. 5). The operation process was as follows: 30 mg of the material TPT-DHBD _X COFs@ _I2 after adsorbed iodine vapor was placed in a 2 mL open glass tube, and it was placed in an open glass tube. In a glass vial (50 mL), the desorption experiment was carried out at 125 °C under normal pressure. The desorption efficiency of iodine is calculated by Er=(30–Wt)/W _X ×100wt%, where Er represents the desorption efficiency and Wt represents the thermal release of TPT-DHBD _X COFs@ _I2 within the corresponding time (0-420 min). Mass, W _X represents the content of iodine in 30 mg TPT-DHBD _X COFs@ _I2 .

Experiments show that all materials can release more than 80% of iodine within 40min, with a faster desorption rate. After 6h, the desorption could basically reach equilibrium, and almost all the iodine was desorbed (see Table 1). The iodine loading of TPT-DHBD COF (compound 2) was calculated by TGA characterization (Fig. 9), and the loadings were 4.77 g/g (400 °C), 4.24 g/g (350 °C), 3.77 g/g, respectively. (250°C), this result is basically consistent with the value measured by gravimetry, and a small difference may be due to the incomplete release of some iodine at the temperature of calculating the iodine loading. The reusability experiment showed that the TPT-BD COF (compound 1) could maintain an iodine adsorption capacity of 515 wt% (up to 96% recovery) after the first cycle was completed. After the third cycle, it still retained 472 wt% iodine adsorption capacity (87.9% recovery) (Figure 10). It can be seen that the adsorption of iodine vapor by the series of COF materials prepared by the present invention is an efficient and reversible process, which has great operability in the enrichment and storage of volatile iodine.

Table 1 Variation of desorption amount of iodine with time (125℃)

A series of COFs materials with large lattice size prepared by the flexible module of the present invention all show extremely high iodine enrichment capacity, and the maximum adsorption capacity is as high as 5.43 g/g, which has never been reported, which is the highest among all types of adsorption materials at present. , this feature makes it have great practical prospects in the field of enrichment of radioactive iodine in the reprocessing of radioactive nuclear waste.

Claims (4)

1. The COFs material constructed based on the flexible module has a structure shown in a formula I:

wherein R is₁、R₂Independently is-H or-OH.

2. The COFs materials constructed based on flexible modules according to claim 1, wherein: r₁、R₂Are all-H.

3. The COFs materials constructed based on flexible modules according to claim 1, wherein: r₁、R₂Are all-OH.

4. Use of the COFs material constructed based on flexible modules as claimed in claim 1 to 3 as an iodine adsorbent.

Images