A method for revealing desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory

Abstract

The present application relates to a kind of method based on density functional theory reveals the desulfurization mechanism of porous adsorbent Zn-MOF-74, using Multiwfn software calculation and experiment is combined, the method for researching the high-efficiency adsorption mechanism of Zn-MOF-74 porous adsorbent SO2, Zn-MOF-74 porous material prepared by using solvothermal method shows great superiority in desulfurization, using the quantum chemistry method based on density functional theory, at atomic molecular level by conceiving search, electrostatic potential, van der waals potential, binding energy, weak interaction force analysis, the calculation of molecular pore free volume, combined with experimental results, reveals the high-efficiency adsorption mechanism of Zn-MOF-74 porous material;The adsorption behavior of SO2 in the channel inside adsorbent is explained quantitatively and qualitatively in the form of image, and a theoretical basis for the application of quantum chemistry theory in the field of chemical technology is laid.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, and in particular to a method for revealing the desulfurization mechanism of the porous adsorbent Zn-MOF-74 based on density functional theory. Background Technology

[0002] SO2 is a difficult-to-treat pollutant with a wide impact, high toxicity, and large content in air pollution problems. Even trace amounts of SO2 can deactivate CO2 absorbents or denitrification catalysts, thereby reducing other flue gas purification processes. Metal-organic frameworks (MOFs) are a class of materials prepared by using organic compounds as ligands and metal salt ions through a series of methods to control pore size. They are porous zeolite materials that are currently the subject of intense research in the field of materials science. Due to their porous structure, MOFs have significantly better performance than other competitive adsorbents. They have high adsorption capacity, selectivity, and stability for SO2, and using them as adsorbents can achieve the goal of reducing sulfur pollution.

[0003] Quantum chemistry techniques based on density functional theory can be used to visually observe the specific adsorption behavior of SO2 in an adsorption system, facilitating the revelation of the efficient desulfurization mechanism of porous materials and providing qualitative and quantitative explanations of the rationality of adsorption sites and behavior. The reaction processes and results obtained through experimental methods are no longer sufficient for molecular-level mechanism research. In recent years, quantum chemistry and computational chemistry methods have become powerful tools in fields such as novel materials research and surface adsorption, opening up a new avenue for deeper research into adsorption behavior. Summary of the Invention

[0004] This invention provides a method for revealing the desulfurization mechanism of the porous adsorbent Zn-MOF-74 based on density functional theory. The adsorption properties of SO2-Zn-MOF-74 are calculated on a server using density functional theory in Multiwfn software. The method qualitatively and quantitatively analyzes the efficient adsorption mechanism of Zn-MOF-74 with a huge specific surface area prepared by solvothermal method from the perspective of quantum chemistry. This lays a theoretical foundation for revealing the adsorption mechanism of new materials and the widespread application of quantum chemical theory.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] A method for revealing the desulfurization mechanism of the porous adsorbent Zn-MOF-74 based on density functional theory includes the following steps:

[0007] 1) Preparation of Zn-MOF-74 porous adsorbent by solvothermal method;

[0008] 2) The adsorption performance of Zn-MOF-74 porous adsorbent for SO2 was tested experimentally;

[0009] 3) Constructing a porous Zn-MOF-74 material model: Selecting the unit cell information of Zn-MOF-74 from the CCDS crystal library and expanding the cell, extracting independent units for hydrogen replenishment at the end bond sites, and using CP2K for structure and unit cell optimization;

[0010] 4) Calculate the adsorption properties of the Zn-MOF-74 porous material, including the following steps:

[0011] a) Select Multiwfn as the simulation software;

[0012] b) SO2 was added to the optimized Zn-MOF-74 model to perform a conformational search to find the optimal adsorption site. Based on density functional theory, the SO2 adsorption position and molecular orientation were explained by calculating and analyzing electrostatic potential and van der Waals potential in the CP2K module. The range and type of weak interaction forces were analyzed, and finally the binding energy and pore free volume were calculated.

[0013] Furthermore, the solvothermal method for preparing Zn-MOF-74 porous adsorbent includes:

[0014] 1) Mix 0.15–0.20 g of zinc nitrate hexahydrate and 0.05–0.10 g of 2,5-dihydroxyterephthalic acid evenly;

[0015] 2) Add 8-10 ml of organic solvent DMF and 1-2 ml of water, and mix thoroughly;

[0016] 3) Transfer the mixed mixture to a stainless steel-lined autoclave, seal it, and heat it to 120-150℃ for 3-5 days;

[0017] 4) The product obtained by heating is filtered and washed with 20-30 ml of DMF until the supernatant is clear;

[0018] 5) The product is then repeatedly washed with 20-30 ml of methanol in a glass vial, once every 2-3 hours for 3-5 consecutive days, to remove the DMF organic solvent from the precipitate. Finally, the product is dried under vacuum to obtain Zn-MOF-74 porous material.

[0019] Furthermore, the prepared Zn-MOF-74 porous adsorbent is first activated by heating under a nitrogen atmosphere before adsorption testing experiments are conducted.

[0020] Furthermore, the experiment to test the adsorption performance of the Zn-MOF-74 porous adsorbent was conducted by placing it in a quartz fixed-bed reactor at room temperature and pressure to test SO2 adsorption.

[0021] Furthermore, in step 3), the structure and cell optimization using CP2K involves using CP2K code and optimizing the geometry using PBE functionals and DFT-D3 dispersion correction, and combining the DZVP-MOLOPT-SR-GTH basis set with the Geodecker-Teter-Hutter pseudopotential.

[0022] Furthermore, the optimal adsorption site is the SO2 adsorption location in the system with the lowest energy obtained after conformational search of different adsorption systems.

[0023] Furthermore, the adsorption system constructed by adding SO2 to the Zn-MOF-74 model was analyzed using the IGM independent gradient model at the location of the optimal adsorption site, and the system was divided into adsorbent and adsorbate segments for segmental investigation.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] This invention uses the quantum chemical simulation software Multiwfn to theoretically calculate the properties of the constructed unit cell and structure, namely the distribution of surface electrostatic potential and van der Waals potential, and the volume of free channels. By searching for the optimal adsorption sites, the weak interaction forces and binding energies are calculated. Combined with experimental results, this invention reveals a method for the efficient adsorption mechanism of Zn-MOF-74, laying a theoretical foundation for revealing the adsorption mechanism of novel materials and the widespread application of quantum chemical theory.

[0026] Compared with traditional single experimental methods, the method based on density functional theory calculations described in this invention for analyzing the efficient adsorption mechanism of Zn-MOF-74 porous materials has the following significant advantages: Through theoretical chemical calculations, the surface chemical properties of the porous adsorbent are analyzed at the atomic and molecular level, including the electrostatic potential, van der Waals potential, and pore free volume, to explain adsorption sites and molecular orientation. Combined with quantum chemical techniques, the optimal adsorption sites are found using conceptual search. The range and type of weak interactions during SO2 adsorption are analyzed graphically using an independent gradient model, and the adsorption energy is quantitatively calculated considering basis set overlap errors. The adsorption behavior of Zn-MOF-74 on SO2 is analyzed qualitatively and quantitatively. By combining experimental results with theoretical calculations, the efficient desulfurization mechanism of Zn-MOF-74 is revealed. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the SO2 adsorption and desorption isotherms of Zn-MOF-74 described in this invention at 293K and 1 bar.

[0028] Figure 2 This is a schematic diagram of the optimal adsorption sites of SO2 on Zn-MOF-74 as described in this invention.

[0029] Figure 3 This is a schematic diagram of the electrostatic potential distribution on the Zn-MOF-74 isosurface described in this invention.

[0030] Figure 4 This is a schematic diagram of the electrostatic potential distribution on the SO2 isosurface and the dyed target described in this invention.

[0031] Figure 5 This is a schematic diagram of the van der Waals potential distribution on the Zn-MOF-74 isosurface described in this invention.

[0032] Figure 6 This is a schematic diagram of the weak interaction force analysis within the SO2-MOF system described in this invention.

[0033] Figure 7 This is a scatter plot of δg vs sign(λ2)ρ in the SO2-MOF system described in this invention.

[0034] Figure 8 This is a schematic diagram of the free volume within the molecular channels of Zn-MOF-74 described in this invention. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0036] This invention discloses a method for revealing the desulfurization mechanism of the porous adsorbent Zn-MOF-74 based on density functional theory, comprising the following steps:

[0037] 1) Preparation of Zn-MOF-74 porous adsorbent by solvothermal method;

[0038] 0.17g of zinc nitrate hexahydrate and 0.08g of 2,5-dihydroxyterephthalic acid were uniformly mixed. Then, 9ml of organic solvent DMF and 1ml of water were added and mixed evenly. The mixture was transferred to a 28ml stainless steel-lined autoclave, and the autoclave was sealed and heated to 120℃ for 3 days. The product obtained by heating was filtered and washed with 20ml of DMF until the supernatant was clear. The product was then repeatedly washed with 20ml of methanol in a glass vial every 2 hours for 3 consecutive days. Finally, after vacuum drying, a stable Zn-MOF-74 porous material was obtained. The Zn-MOF-74 porous material was a stable Zn-MOF-74 porous material prepared by a solvothermal method at 120℃ and normal pressure.

[0039] 2) The adsorption performance of Zn-MOF-74 porous adsorbent for SO2 was tested experimentally;

[0040] The adsorbent's activity was tested using an adsorption performance testing device for a flue gas analyzer. Before gas adsorption, activation was performed by heating under a nitrogen atmosphere. This process removed the water of crystallization from the sample material, making it easier to obtain active sites on the metal coordination sites. It also improved the adsorption performance and chemical reactivity of the molecules. The entire adsorption process was carried out in a quartz fixed-bed reactor with an inner diameter of 0.8 cm containing the adsorbent. The SO2 adsorption and desorption isotherms of Zn-MOF-74 at room temperature are shown below. Figure 1 As shown, the absorption capacity at 293K and 1 bar is as high as 8.16 mmol / g, which is much higher than that of traditional adsorbents.

[0041] 3) Constructing a porous Zn-MOF-74 material model: Selecting the unit cell information of Zn-MOF-74 from the CCDS crystal library and expanding the cell, extracting independent units for hydrogen replenishment at the end bond sites, and using CP2K for structure and unit cell optimization;

[0042] First, the unit cell information of Zn-MOF-74 was selected from the CCDC crystal library and loaded into the Multiwfn program. Using CP2K code, the geometry was optimized using PBE functionals and the DFT-D3 dispersion correction proposed by Grimme. The DZVP-MOLOPT-SR-GTH basis set was combined with the Geodecke-Teter-Hutter pseudopotential, with a plane wave cutoff energy of 450 Ry and a relative cutoff energy of 50 Ry. For energy calculations, the non-metallic atoms C, N, and O used the more precise TZVP-MOLOPT-SR-GTH basis set. Since most of the interactions in the adsorption system are weak, the effects of dispersion correction and basis set overlap error were also considered in the calculation. The constructed and optimized model helps to reveal the high-efficiency adsorption performance and surface properties of Zn-MOF-74 unit cells prepared by the solvothermal method.

[0043] 4) Calculate the adsorption properties of the Zn-MOF-74 porous material, including the following steps:

[0044] a) Select Multiwfn as the simulation software;

[0045] b) SO2 was added to the optimized Zn-MOF-74 model to perform conformational search to find the best adsorption site. Based on density functional theory, the SO2 adsorption position and molecular orientation were explained by calculating and analyzing electrostatic potential and van der Waals potential in the CP2K module. The range and type of weak interaction forces were analyzed, and finally the binding energy and pore free volume were calculated.

[0046] The calculated results of the proposed search site, electrostatic potential energy, van der Waals potential energy, weak interaction force analysis, binding energy and free volume were used to analyze the adsorption performance of Zn-MOF-74 for SO2.

[0047] By applying Abcluster and incorporating optimized Zn-MOF-74 unit cell information, and randomly distributing a large number of disordered SO2 atoms around the adsorbent channels while ensuring that their positions and rotation directions are different, the lowest energy point of each adsorption system was identified. After determining the location, 50 more disordered SO2 atoms were introduced into the vicinity of this point. The final energy difference was less than 1 × 10⁻⁶. -9 kcal / mol was considered the optimal adsorption site; after conformational search, the optimal adsorption sites of the SO2-Zn-MOF-74 adsorption system were as follows: Figure 2 As shown, it can be clearly observed that SO2 molecules preferentially adsorb around Zn metal atoms and gather around the material wall. The O end preferentially approaches the metal atoms. Considering that most of the interaction forces are weak, the rationality of the adsorption site is proven by the subsequent electrostatic potential and van der Waals potential.

[0048] The electrostatic potential was calculated by introducing the optimized unit cell into a surface with an electron density of 0.001au and a grid spacing of 0.25 Bohr.

[0049] The calculated average electrostatic potential energy of the Zn-MOF-74 surface is 47.46 kcal / mol, with the maximum electrostatic potential energy occurring near the Zn atoms and benzene rings, resulting in 75.43 kcal / mol. Substituting the calculated electron density information into a VMD visualization program and rendering it in Tachyon format, the electrostatic potential distribution of the Zn-MOF-74 surface can be obtained using the BWR staining method. Figure 3 As shown, the adsorbent surface is clearly dark overall, indicating that the electrostatic potential energy on the adsorbent surface is mostly positive. Therefore, it attracts small molecules with negative electrostatic potential energy. The area around the Zn atoms and benzene ring is significantly darker than the surrounding area, indicating a higher electrostatic potential energy than other regions of the material, suggesting strong adsorption capacity in this region. Electrostatic potential energy analysis of the SO2 molecule surface was also performed, yielding an S-terminal electrostatic potential energy of 29.58 kcal / mol and an O-terminal electrostatic potential energy of -21.97 kcal / mol. Figure 4 As shown, the O end of the SO2 molecule is light-colored, indicating that the O end of the SO2 molecule is negatively charged. When it adsorbs with Zn-MOF-74, the O end preferentially adsorbs on the material surface and is preferentially close to the Zn metal atoms, satisfying the condition of electrostatic potential complementarity, so as to achieve the principle of minimum energy and stability. The adsorption sites caused by electrostatic potential complementarity are consistent with the conformation search results.

[0050] When using Multiwfn for graphical analysis of the van der Waals potential energy of Zn-MOF-74, a grid spacing of 0.3 Bohr was used. The analysis was performed through graphical simulation. Figure 5 It can be clearly observed that, considering only van der Waals interactions, this type of MOF most tends to adsorb small molecules onto the corners of the framework, because... Figure 5 As shown by the isosurface, the van der Waals potential is most negative in this region. Calculations show that the minimum van der Waals potential energy is -2.52 kcal / mol, and the minimum site appears around Zn atoms. This indicates that the van der Waals potential energy of Zn-MOF-74 will pull SO2 molecules to the vicinity of metal atoms, which is consistent with the results obtained from the conformation search. Through the study of electrostatic potential energy and van der Waals potential energy, it can be found that the weak interaction force of Zn-MOF-74 is the direct cause of the dominant SO2 adsorption site.

[0051] For the Zn-MOF-74 and SO2 adsorption system, an IGM independent gradient model was used to analyze the optimal adsorption site location. The system was divided into adsorbent and adsorbate segments for analysis. This method mainly analyzed the type and range of weak interactions between the two segments in the SO2-MOF adsorption system. The changes in the system were observed intuitively through images and scatter plots. The process is similar to redefining the function using the system's wavefunction information and distinguishing the regions with different forces within the system using specific numerical values. The calculated results were then rendered using a VMD visualization program. Figure 6 It can be clearly seen that the weak interaction force has a disc-shaped range, and most of it consists of dispersive forces with slight hydrogen bonding. Because of its proximity to metal atoms and the benzene ring, the forces are primarily van der Waals forces and electrostatic potential. A scatter plot of δg vs sign(λ2)ρ is also provided, displaying δg_intra and δg_inter with dark and light dots respectively on the same plot. Figure 7 As shown, this corresponds to Zn-MOF-74. We can see that the horizontal axis sign(λ2)ρ has a large number of black dots in the positive region. According to the color scale diagram, this indicates the presence of steric hindrance in the adsorption system. There is a sharp peak at a position where sign(λ2)ρ is approximately -0.06. Since the electron density at this position is not very high and it is light-colored, it is considered to be the hydrogen bonding between SO2 and the benzene ring of MOF. This is also part of the reason for the strong adsorption capacity of the adsorbent. In the region where sign(λ2)ρ is negative, there are still a large number of black scattered dots. Since the electron density at these points is relatively high, it is inferred that these are chemical bonds present in the system.

[0052] When studying the adsorption of Zn-MOF-74 with SO2, since most intermolecular forces are weak, the basis set overlap error (BSSE) must be considered. Therefore, the Gaussian mixture plane wave basis set method used in CP2K requires BSSE correction using the counterpoise method when calculating weak interaction energies. The correction formula is as follows:

[0053] E_BSSE=[E(A)-EAB(A)]+[E(B)-EAB(B)] (1)

[0054] Here, E(A)-EAB(A) represents the energy of fragment A calculated alone minus the energy of fragment A under the AB basis functions, and the same applies to fragment B; the calculation of EAB(A) requires the use of ghost atoms, and B will be set as a ghost atom. A ghost atom refers to a position that only has a basis function but no atomic charge and spin multiplicity.

[0055] Calculations revealed that the binding energy of Zn-MOF-74 with SO2 at the optimal adsorption site is -18.02 kcal / mol. The quasi-molecular density is obtained by superimposing the electron densities of each atom in its free state. This represents the position of an atom within a molecule, but before the electron density has relaxed due to bonding. The region within a certain iso-surface of the quasi-molecular density is considered the intramolecular region, while the region outside this iso-surface can be considered the molecular pore. The larger the pore volume, the larger the reaction space, the more adsorption sites it can provide, and the more suitable it is for SO2 adsorption. When constructing the unit cell of Zn-MOF-74, its molecular pores are visualized and rendered as follows: Figure 8 The product of the three side lengths of the unit cell model is 3956.42 Angstrom. 3 The calculated free volume is 2590.45 Angstrom. 3 It accounts for 65.58% of the total volume, which also indicates that Zn-MOF-74 has the highest free volume ratio, which is much higher than that of traditional adsorbents, and can provide more adsorption sites.

[0056] The above embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the above embodiments. Unless otherwise specified, the methods used in the above embodiments are conventional methods.

Claims

1. A method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory, characterized in that, Includes the following steps: 1) Preparation of Zn-MOF-74 porous adsorbent by solvothermal method; 2) The adsorption performance of Zn-MOF-74 porous adsorbent for SO2 was tested experimentally; 3) Constructing a porous Zn-MOF-74 material model: Selecting the unit cell information of Zn-MOF-74 from the CCDS crystal library and expanding the cell, extracting independent units for hydrogen replenishment at the end bond sites, and using CP2K for structure and unit cell optimization; 4) Calculate the adsorption properties of the Zn-MOF-74 porous material, including the following steps: a) Select Multiwfn as the simulation software; b) SO2 was added to the optimized Zn-MOF-74 model to perform a conformational search to find the optimal adsorption site. Based on density functional theory, the SO2 adsorption position and molecular orientation were explained by calculating and analyzing electrostatic potential and van der Waals potential in the CP2K module. The range and type of weak interaction forces were analyzed, and finally the binding energy and pore free volume were calculated.

2. The method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory according to claim 1, characterized in that, The steps for preparing Zn-MOF-74 porous adsorbent by the solvothermal method include: 1) Mix 0.15–0.20 g of zinc nitrate hexahydrate and 0.05–0.10 g of 2,5-dihydroxyterephthalic acid evenly; 2) Add 8-10 ml of organic solvent DMF and 1-2 ml of water, and mix thoroughly; 3) Transfer the mixed mixture to a stainless steel-lined autoclave, seal it, and heat it to 120-150℃ for 3-5 days; 4) The product obtained by heating is filtered and washed with 20-30 ml of DMF until the supernatant is clear; 5) The product is then repeatedly washed with 20-30 ml of methanol in a glass vial, once every 2-3 hours for 3-5 consecutive days, to remove the DMF organic solvent from the precipitate. Finally, the product is dried under vacuum to obtain Zn-MOF-74 porous material.

3. The method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory according to claim 1, characterized in that, The prepared Zn-MOF-74 porous adsorbent was first activated by heating under a nitrogen atmosphere before adsorption testing experiments were conducted.

4. The method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory according to claim 1, characterized in that, The experiment to test the adsorption performance of the Zn-MOF-74 porous adsorbent involved placing it in a quartz fixed-bed reactor at room temperature and pressure to test SO2 adsorption.

5. The method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory according to claim 1, characterized in that, In step 3), the structure and cell optimization using CP2K involves using CP2K code and optimizing the geometry using PBE functionals and DFT-D3 dispersion correction, and combining the DZVP-MOLOPT-SR-GTH basis set with the Geodecker-Teter-Hutter pseudopotential.

6. The method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory according to claim 1, characterized in that, The optimal adsorption site is the SO2 adsorption location in the system with the lowest energy obtained after conformational search of different adsorption systems.

7. The method for revealing the desulfurization mechanism of porous adsorbent Zn-MOF-74 based on density functional theory according to claim 1, characterized in that, The adsorption system constructed by adding SO2 to the Zn-MOF-74 model was analyzed using the IGM independent gradient model at the location of the optimal adsorption site, and the system was divided into adsorbent and adsorbate for segmental investigation.

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