CN111004397B - Metal organic framework molecular material of electron-rich system and application thereof in photocatalytic reduction of heavy metal ions - Google Patents
Metal organic framework molecular material of electron-rich system and application thereof in photocatalytic reduction of heavy metal ions Download PDFInfo
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Abstract
The invention relates to a novel metal organic framework molecular material of an electron-rich system, which has the molecular formula as follows: c27H21CdN9O7The chemical formula is [ Cd (TIPA) (NO)3)2]·H2O, abbreviated Cd-TIPA (TIPA = tris- (4-imidozolylphenyl) amine). The compound belongs to a monoclinic system,C2space group, type utp topology. The synthesis method specifically comprises the following steps: adding Cd (NO)3)2·6H2Mixing O and TIPA in a mixed solvent of distilled water and N, N-dimethylformamide uniformly, transferring the mixture into a reaction kettle to react for 70 to 80 hours at the temperature of between 90 and 100 ℃, cooling the mixture to room temperature, separating out crystals, washing and drying the crystals to obtain the compound. The novel metal organic framework molecular material can be used as a photocatalyst to catalyze and reduce heavy metal ions, particularly Cr (VI) and Mn (VII).
Description
Technical Field
The invention belongs to the technical field of preparation of metal organic framework compounds, and particularly relates to a novel metal organic framework molecular material of an electron-rich system, a synthetic method and application of the novel metal organic framework molecular material as a photocatalyst in catalytic reduction of heavy metal ions, particularly Cr (VI) and Mn (VII).
Background
In recent years, rapid development of chemical industry has led to increasingly severe water pollution, especially in the widespread use of heavy metals in electroplating, painting processes, mining, and electronic product manufacturing. However, most heavy metals are highly toxic and carcinogenic, and pose a great threat to the environment and human health. Wherein the hexavalent chromium ion Cr (VI) is identified as one of the most toxic species in the heavy metal pollution, and the environmental protection agency regulates the content of hexavalent chromium ion in drinking waterShould not exceed 0.1
(δ = 0.1 ppm). Therefore, for the safety of drinking water and the health of human beings, it is urgent to detect and reduce trace Cr (VI) ions in water rapidly and efficiently. Various methods have been developed to remove cr (vi) from water, such as chemical precipitation, biological adsorption, ion exchange, electrolysis, adsorption, electrochemical reduction, and the like. However, these conventional methods have disadvantages of high cost, secondary pollution, complicated manufacturing processes, and low efficiency. It is known that chromium is present in the natural environment in mainly two oxidation states, cr (vi) and cr (iii), of which trivalent chromium has low toxicity to humans and is an essential trace element for humans. The most efficient method for removing hexavalent chromium is not absorption but degradation/reduction to trivalent chromium cr (iii). Therefore, it is reasonable and feasible to develop a photocatalytic technology for reducing Cr (VI) ions with high toxicity into Cr (III) ions with low toxicity.
The metal-organic frameworks (MOFs) have the characteristics of various structures, high porosity, large volume, tunable microenvironment and the like, are widely applied to the fields of gas storage, magnetism, optical materials, chemical sensing and the like, and particularly provide a good platform for researching catalytic reaction mechanisms on a molecular level in the field of catalysis. With the rapid development and deep research of MOFs, photocatalytic MOFs have been rapidly developed in recent years and are mainly applied to water decomposition and CO2Reduction, degradation of organic and inorganic contaminants, etc. To achieve Cr with a specific reduction6+The photocatalytic function of the ion, MOF, is primarily based on the physicochemical properties of the organic ligands in the MOF structure. In recent years, tris (4-imidazolylphenyl) amine (TIPA) has received much attention due to its excellent properties, such as: 1) TIPA is a strong electron donor in the initial state; 2) it can stabilize the charge transfer state; 3) the material has good hole transmission capability, and the conductivity of the material can be improved; 4) has the characteristics of large volume and various coordination modes. The present application contemplates TIPA and D10The incorporation of the metal structure in a framework will result in excellent metal-ligand charge transfer and fluorescence properties.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a novel electron-rich double-interpenetration metal organic framework molecular material, a synthesis method and application thereof as a photocatalyst in catalytic reduction of heavy metal ions, particularly Cr (VI) and Mn (VII).
In order to achieve the purpose, the invention adopts the following technical scheme:
a metal organic framework molecular material of an electron-rich system has a molecular formula as follows: c27H21CdN9O7，Has a chemical formula of [ Cd (TIPA)]·(NO3)2·H2O, abbreviated Cd-TIPA (TIPA = tris- (4-imidozolylphenyl) amine). The compound belongs to a monoclinic system,C2space group, type utp topology. The material can be used as a catalyst to efficiently carry out photocatalytic reduction on heavy metal ions Cr (VI) and Mn (VII).
The invention provides a method for synthesizing the metal organic framework molecular material of the electron-rich system, which synthesizes Cd (NO)3)2·6H2And mixing the O and the TIPA in the solvent uniformly, transferring the mixture into a reaction kettle, reacting at 90-100 ℃ for 70-80 hours, cooling to room temperature, separating out crystals, washing and drying to obtain the compound.
Further, in the method for synthesizing the electron-rich metal-organic framework molecular material, the Cd (NO) is3)2·6H2The mass ratio of O to TIPA is 1.5-2: 1.
further, in the method for synthesizing the electron-rich metal-organic framework molecular material, the solvent is preferably a mixture of distilled water and N, N-dimethylformamide.
The invention also provides application of the metal organic framework molecular material of the electron-rich system as a catalyst in photocatalytic reduction of heavy metal ions.
More preferably, the electron-rich metal-organic framework molecular material is applied to photocatalytic reduction of heavy metal ions such as Cr (VI) or Mn (VII) as a catalyst.
Compared with the prior art, the invention has the following beneficial effects:
1) the metal organic framework material can accurately analyze the crystal structure characteristics through X-ray single crystal diffraction, and provides theoretical support for further conjecturing the interaction between the active center and a reaction substrate and researching the catalytic reaction mechanism of the metal organic framework material;
2) the metal organic framework material has the characteristics of an n-type semiconductor, has a LUMO energy level (-1.30V vs. NHE) with high reduction potential and a HOMO energy level (1.37V vs. NHE) with high oxidation potential, can simultaneously realize the reduction of heavy metal and the oxidation of water, and provides a development space for future practical application;
3) the invention is beneficial to developing a photoreduction reaction synthesis strategy with the characteristics of atom economy, high efficiency and the like. The catalytic system abandons the traditional method with high cost, secondary pollution, complex manufacturing process and low efficiency, directly carries out reduction catalytic reaction on heavy metal ions in the sewage under the irradiation of visible light, and accords with the development concept of green chemistry;
4) the invention realizes high-efficiency catalysis by designing and adjusting reasonable matching of three-dimensional and electronic effects between the catalytic active sites and the substrate. The catalytic process is heterogeneous, the catalyst can be recovered through filtration or centrifugal separation, the cyclic utilization is realized, and the catalytic efficiency is not obviously reduced after the repeated cycle, so that the multifunctional metal organic framework material provides a good theoretical basis for the development of the fields of sewage treatment, environmental protection and the like in China.
Drawings
FIG. 1 (a) is a schematic diagram of the unit cell structure of compound Cd-TIPA; (b) a coordination environment diagram of a Cd (1) atom; (c) a two-dimensional plane structure diagram of a compound Cd-TIPA; (d) schematic representation of the 2D (6, 3) framework of the compound Cd-TIPA; (e) the three-dimensional network structure of the compound Cd-TIPA (color code: Cd, green; C, grey; O, red; N, blue, with partial hydrogen atoms omitted for clarity);
FIG. 2 (a) a two-dimensional interpenetrating structure of a Cd-TIPA framework; (b) a simplified schematic of a two-dimensional interpenetrating frame;
FIG. 3 is an infrared spectrum of compound Cd-TIPA;
FIG. 4 XRD spectrum of compound Cd-TIPA;
FIG. 5 (a) excitation (black left) and emission (red right) spectra of TIPA at room temperature; (b) excitation (black left) and emission (red right) spectra of CdTIPA compounds at room temperature; (c) the fluorescence lifetime curve of the CdTIPA compound;
FIG. 6 (a) the UV-visible diffuse reflectance spectrum of compound CdTIPA; (b) a tauc curve; (c) 0.1M Na2SO4Mott-Schottky plot of the compound CdtIPA in aqueous solution (inset: energy plot of VB and CB levels of CdtIPA);
FIG. 7 (a) is a diagram showing UV-VIS absorption spectra of the photocatalytic reduction of Cr (VI) by Cd-TIPA as a catalyst under different irradiation times of visible light (the inset is a diagram showing Cr (VI) solution in different reaction times under visible light); (b) time-corresponding-ln (C)t/C0) Linear plot, calculating the rate constant of the cr (vi) reduction reaction;
FIG. 8 (a) UV-VIS absorption spectrum of photo-reduced Cr (VI) with only white light; (b) the UV-VIS absorption spectrum of photo-reduced Cr (VI) with ligand TIPA only;
FIG. 9 (a) UV-VIS absorption spectra of the photocatalytic reduction of Mn (VII) with Cd-TIPA as catalyst after different times of irradiation with visible light (inset: pictures of Mn (VII) solution at different reaction times under visible light); (b) time-corresponding-ln (C)t/C0) Linear plot, rate constants for mn (vii) reduction reactions were calculated;
FIG. 10 (a) PXRD pattern of the catalyst after three cycles of catalysis by compound Cd-TIPA; (b) photo-reduction Cr (VI) ion reaction cycle test chart.
Detailed Description
The technical solution of the present invention is further described in detail with reference to the following examples, but the scope of the present invention is not limited thereto.
Example 1
A process for synthesizing the new dual-interpenetrating organic-metal frame molecular material of electron-rich system includes ligand and catalyst [ Cd (TIPA) (NO)3)2]·H2The synthesis of O specifically comprises the following steps:
1) synthesis of ligand tris- (4-imidazolilylphenyl) amine (TIPA)
The synthesis can be carried out in particular by reference to the literature (S.Z. Luo, X.L. Mi, L. Zhang, et al. Functionalized chinese medicinal compositions as high hly effective microorganisms for microbial addition to nitroalcohols [ J ]. Angew Chem Int Ed., 2006, 45: 3093-.
2) Compound [ Cd (TIPA)) (NO3)2]·H2Synthesis of O
The compound Cd-TIPA is prepared by self-assembly under hydrothermal conditions, and specifically comprises the following components: weighing Cd (NO)3)2·6H2O42 mg (0.14 mmol) and TIPA 23 mg (0.05 mmol) are put in a mixed solvent (the mixed solvent is formed by mixing distilled water and N, N-dimethylformamide DMF with the volume ratio of 2: 1), placed and stirred for 12h at room temperature to be uniformly mixed, then transferred into a high-pressure reaction kettle, slowly heated in a temperature-controlled oven, reacted for 75 h at 95 ℃, slowly cooled to room temperature, and light yellow crystals are separated out, washed by water and dried to obtain the compound Cd-TIPA, wherein the yield is 70 percent (based on TIPA ligand).
The chemical formula of the compound Cd-TIPA is as follows: c27H21CdN9O7Elemental analysis and ICP (%): calculated for compound: c, 46.60; h, 3.04; n, 18.11; cd, 16.15. Experimental values: c, 46.65; h, 3.10; n, 18.01; cd, 16.10.
Crystal structure analysis: the compound Cd-TIPA was subjected to crystal test and analysis, and the results are shown below.
As can be seen in table 1: the compound CdTIPA belongs to a monoclinic system,C2and (4) space group. As can be seen from FIG. 1a, the unit cell of the compound CdTIPA consists of one crystallographically independent Cd (II) atom, one TIPA ligand and two nitrate ions. Wherein the crystallographically independent Cd (1) atom adopts hepta-coordination to form a twisted pentagonal bipyramid structure, which coordinates with the nitrogen atoms of the three TIPA ligands and the two oxygen atoms of the nitrate ions, respectively, as shown in fig. 1 b.
Each TIPA ligand is taken as a tridentate ligand, and the centers of three Cd (II) are connected through three imidazolyl to generate a (6, 3) connected honeycomb two-dimensional(2D) Network (fig. 1 c). From a topological point of view, each cd (ii) atom is also linked to three TIPA ligands. Thus, both the cd (ii) atom and the TIPA ligand may be defined as a triple junction. Due to NO3 -Ions act only as terminal ligands and are therefore not considered for the moment in topology analysis. On this basis, the topology of the CdTIPA was analyzed in detail using a TOPOS program, which has a topology network of type utp (fig. 1 d). Here, the TIPA ligands in the utp network were arranged alternately up and down to form a concavo-convex utp network. More importantly, the 2D layers are mutually obliquely inserted to form a three-dimensional oblique multilayer structure, and the channel size of the 3D network structure is 20.88 a 16.77 a (fig. 1 e). The dihedral angle of the two interpenetrating 2D layers is 60.58 a, and the intermolecular interactions significantly enhance the structural stability of the framework (fig. 2).
TABLE 1 crystallographic data for compound Cd-TIPA
Infrared spectrum: the infrared spectrum of the compound CdTIPA is shown in figure 3. As can be seen from the figure: 3400cm−1The left and right broadband absorption peaks are attributed to H2And O stretching and contracting vibration. 1516 cm−1Characteristic absorption peaks of (a) are attributed to the imidazole group in the TIPA ligand. In addition, 1383 cm−1Characteristic absorption peak of (A) is ascribed to NO3 -Ions.
Powder diffraction of radiation: the X-ray powder diffraction pattern of the compound CdtiPA is shown in FIG. 4, the Simulated pattern represents the pattern obtained by simulation, and the Experimental pattern represents the pattern measured by experiment. The comparison shows that the peak types and the peak positions of the two are basically completely matched, which indicates that the sample is pure and has few impurities.
Photoluminescence: as shown in fig. 5 b: the compound CdTIPA was excited at λ = 381 nm, corresponding to an emission wavelength of 463 nm, respectively. Due to Cd2+D of (A)10The structure is difficult to oxidize or reduce, and these emission bands are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature. Thus, are combined withThe emission of the CdTIPA can be substantially attributed to the pi → pi or pi → n electronic transition centered on the TIPA ligand resulting in luminescence, since a similar emission is also observed in the TIPA ligand (fig. 5a), a slight blue shift attributable to the ligand coordination with the metal. At the same time, we also investigated the fluorescence lifetime of compound CdTIPA (fig. 5 c). The lifetime curve of CdTIPA can be well fitted to a bi-exponential functionI = A 1 exp(-t/τ1) + A 2 exp(-t/τ2) (whereinIIt is the intensity of the light emitted,tis time, τ1And τ2Is a fast and slow component of the luminous lifetime,A 1andA 2 referred to as a pre-factor). The formula shows that the luminescence lifetime of CdTIPA is about 5.76 mus at room temperature. The above indicates that the stable polymeric material may be a suitable candidate for a latent photoactive material.
Ultraviolet-visible diffuse reflectance spectrum:
the optical absorption properties of the compound Cd-TIPA were investigated using uv-vis diffuse reflectance spectroscopy, as shown in fig. 6 a. It was found that a predominantly broadband absorption of Cd-TIPA was observed at about 250-500 nm. Here, the bandgap of Cd-TIPA was calculated using the Kubelka-Munk function:
αhν = A(hν-Eg)n/2
wherein α, hv, a and Eg represent absorption coefficient, photon energy, constant and band gap, respectively. In the function, the value of n is determined by the type of optical transition in the semiconductor (n =1 for direct transition; n =4 for indirect transition), and n =4 for compound Cd-TIPA for indirect transition. Thus, the bandgap Eg of Cd-TIPA is 2.67 eV (FIG. 6 b). In addition, the energy level change of the compound Cd-TIPA is verified through a test curve. As shown, the flat band potential of the compound Cd-TIPA was measured to be-1.50 eV (vs. Ag/AgCl). The conduction band potential (ECB) is calculated by the following formula: eCB = E (vs. Ag/AgCl) + 0.2V. Therefore, the conduction band potential value of the compound Cd-TIPA is-1.30V. According to formula EVB = ECB + EgCombining ultraviolet-visible absorption spectrum and electrochemical test result to obtain E of the compound Cd-TIPAVBThe value was 1.37eV (FIG. 6 c). The results show that E of this compoundThe b potential is more negative than the standard redox potential (0.51 eV vs. NHE) of Cr (VI)/Cr (III), indicating that the photo-generated electrons are easily transferred from the compound Cd-TIPA to Cr (III)2O7 2-On ion, the photocatalyst is suitable for Cr2O7 2-Photoreduction of (1).
Example 2: experiment of photocatalytic application
Photocatalytic reduction reaction of heavy metal ions: selecting potassium dichromate (K)2Cr2O7) And potassium permanganate (KMnO)4) Photocatalytic tests were performed under visible light as the source of Cr (VI) and the source of Mn (VII). The specific operation is as follows: 5 mg of the compound Cd-TIPA was dispersed in 5 mL of an aqueous Cr (VI) or Mn (VII) solution at an initial concentration of 0.5 mol/L, and the mixed solution was exposed to a 10W white light lamp. After various irradiation times, the solution was removed from the suspension and centrifuged to remove the catalyst for analysis. The photocatalytic process of the compound was studied by ultraviolet-visible absorption spectroscopy.
Study of photocatalytic Properties
At present, the reductive transformation from Cr (VI) to Cr (III) is the most promising method for remedying Cr (VI) contamination. Previous studies have shown that increasing the acid concentration in the solution greatly increases the rate of cr (vi) reduction in aqueous solution, but this is against the original purpose of green chemistry and the pursuit of more environmentally friendly conditions is currently still the most important task. In this study, Cr (VI) was completely reduced within 8 min only in the presence of the compound Cd-TIPA, as shown in FIG. 7 a. At 351 nm, the absorption of cr (vi) decreases in order as the color changes from yellow to colorless, indicating a decrease in the concentration of cr (vi) ions. Meanwhile, the absorbance under different illumination time is converted into the reduction ratio of Cr (VI) ions, and the reduction ratio is represented by-ln (C)t/C0) The apparent rate constant values were calculated over time (fig. 7 b), indicating that the catalytic effect of the catalyst depends only on the reaction conditions employed. The apparent rate constant of the catalyst was calculated to be 0.22 min-1. It is evident that the composite material has significant catalytic activity for cr (vi) reduction. By way of comparison, when the photocatalytic reaction was carried out using only white light irradiation without the presence of a catalyst, cr (vi) was found to be hardly reduced;when TIPA alone was used as catalyst, the Cr (VI) concentration was abnormal and slightly increased within 16 min (see FIG. 8).
In addition, reduction of high valence manganese has also been investigated. In the present study, Mn (VII) can be reduced by more than 95% within 12 min in the presence of a compound Cd-TIPA. As shown in FIG. 9a, the absorbance of Mn (VII) at 526nm and 546 nm decreased sequentially with the increase of the reaction time, and the color also changed from purple to red, then to yellow brown, and finally to a colorless transparent solution, indicating that reduction of Mn (VII) occurred. It is noted here that the reaction has a clear isotonic spot at 476 nm, indicating that the redox reaction proceeds smoothly without the formation of multiple products. The Mn oxidation state changes from heptavalent to hexavalent, and finally divalent, as a result of combining the uv-visible absorption spectrum with the color change of the solution. Meanwhile, based on the absorbance of Mn (VII) ions at 526nm, the absorbance at different irradiation times is converted into the reduction ratio of Mn (VII), and the reduction ratio is expressed as-ln (C)t/C0) As ordinate, and time as abscissa, the apparent rate constant value was calculated to be 0.29 min-1 (FIG. 9 b). It is evident that this compound also has a significant photocatalytic activity for mn (vii) reduction.
The stability and reusability of catalysts have been considered as important aspects for large-scale applications. Therefore, the same compound was used for multiple catalytic reactions to understand the recoverability of Cd-TIPA in photocatalytic reduction of Cr (VI). After each cycle, the catalyst was separated by centrifugation. The resulting catalyst was washed with water, dried at 50 ℃ for 12h, and then the recovered photocatalyst was used in the next cycle. As shown in FIG. 10, the catalytic activity of the compound Cd-TIPA for removing Cr (VI) was tested by a number of cycles (FIG. 10). From the above experimental results it is clear that: even after the third catalytic cycle, the Cd-TIPA retained its original crystal structure with no significant reduction in conversion. The results show that the catalyst has higher stability and heterogeneous catalytic capability.
Claims (5)
1. A metal organic framework molecular material of an electron-rich system has a molecular formula as follows: c27H21CdN9O7Of the formula [ Cd (TIPA) ]]·(NO3)2·H2O, which belongs to the monoclinic system,C2space group, topology type utp; the metal organic framework molecular material is used as a catalyst for photocatalytic reduction of heavy metal ions.
2. A process for the synthesis of the electron-rich metal-organic framework molecular material of claim 1, wherein Cd (NO) is introduced3)2·6H2Mixing O and TIPA in a solvent uniformly, transferring the mixture into a reaction kettle, reacting at 90-100 ℃ for 70-80 hours, cooling to room temperature, separating out crystals, washing and drying to obtain the compound I;
the solvent is formed by mixing distilled water and N, N-dimethylformamide.
3. Process for the synthesis of a metal-organic framework molecular material of an electron-rich system according to claim 2, characterized in that said Cd (NO)3)2·6H2The mass ratio of O to TIPA is 1.5-2: 1.
4. the use of the metal-organic framework molecular material of an electron-rich system as defined in claim 1 as a catalyst for the photocatalytic reduction of heavy metal ions.
5. The use of the electron-rich metal-organic framework molecular material of claim 4 as a catalyst in the photocatalytic reduction of heavy metal ions, wherein the heavy metal ions are Cr (VI) or Mn (VII).
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| KR20180010562A (en) * | 2016-07-21 | 2018-01-31 | 삼성전자주식회사 | Functional Material including Metal Organic Framework, Method of Preparing the same, and Photochemical Sensor including the same |
| CN107619417A (en) * | 2017-08-29 | 2018-01-23 | 中国科学院福建物质结构研究所 | A kind of cadmium organic coordination compound with photocatalytic activity and preparation method and application |
| CN109320731B (en) * | 2018-10-15 | 2021-04-20 | 重庆师范大学 | Cadmium-organic coordination polymer Cd5-MOF, and preparation method and application thereof |
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