Method for force field development of mof materials containing unsaturated metal sites for hydrogen adsorption

Abstract

The application discloses a force field development method for hydrogen adsorption of metal organic framework (MOF) containing unsaturated metal sites, which firstly carries out calculation simulation of hydrogen adsorption isotherm of Cu-BTC (metal-organic framework material), and focuses on development of the force field. By constructing an accurate molecular model and corresponding force field parameters, hydrogen adsorption behavior of Cu-BTC under different pressure conditions is successfully simulated. In the calculation research, the DFT and molecular simulation combination method is adopted to develop the force field, and the interaction potential between hydrogen molecules and the pore surface of Cu-BTC is corrected in combination with experimental data. The results show that the developed force field can accurately predict the hydrogen adsorption isotherm in Cu-BTC, and has good consistency with the experimental results, thereby providing a theoretical basis for further development of MOF materials for hydrogen storage.

Description

Technical Field

[0001] This invention relates to the fields of computational chemistry and nanocomposite materials, specifically to a method for developing a force field for hydrogen adsorption in MOF materials containing unsaturated metal sites. Background Technology

[0002] Metal-organic frameworks (MOFs) have become popular materials in quantum screening and gas separation research due to their highly tunable pore structure and functionalized metal sites (such as open copper sites, Cu-OMS). MOFs not only possess large specific surface areas and pore volumes, but also allow for optimization of hydrogen molecule adsorption and separation performance by adjusting the coordination environment of the metal sites. MOFs with open copper sites (Cu-OMS) exhibit strong adsorption capacity in hydrogen, making them ideal research subjects.

[0003] Traditional classical force fields (such as UFF and DREIDING) have limitations in describing the interactions between metal sites (especially open copper sites) and hydrogen molecules in MOFs. These classical force fields cannot accurately capture the complex interactions between metal sites and gas molecules, especially the strong interactions between metal and gas molecules. Therefore, it is necessary to develop specific force field models to accurately describe the interactions between hydrogen and open copper sites in MOFs.

[0004] To more accurately simulate the adsorption behavior of hydrogen in MOFs, we utilize quantum mechanical calculations (such as DFT) to obtain the interaction between gas molecules and metal sites. Based on these quantum mechanical calculations, we developed new force field models that can more accurately describe the interaction between hydrogen and open copper metal sites. These newly developed force field models not only improve the accuracy of the simulation but also effectively avoid many important interactions that classical force fields cannot capture, especially the many-body polarization effect in metal-gas interactions. By developing a dedicated force field applicable to hydrogen molecules and MOFs, we can subsequently perform simulation screening on a large number of MOF materials to identify those that perform well under actual separation conditions. This method provides an efficient and economical approach for the design and optimization of hydrogen separation materials.

[0005] Therefore, the development of hydrogen force fields stems not only from the scientific need for hydrogen isotope separation, but also from the functionalization and manipulation of metal-organic framework materials, improvements in computational force fields, and the advancement of high-throughput screening technologies. These factors have collectively driven the research progress in hydrogen isotope separation technology and MOF materials. Summary of the Invention

[0006] A method for developing a force field to study the adsorption of hydrogen by MOF materials containing unsaturated metal sites through theoretical calculations is proposed. This method accurately describes the special interaction between hydrogen and unsaturated metal sites and simulates and verifies the hydrogen adsorption behavior of Cu-BTC under different pressure conditions.

[0007] To achieve the above objectives, this invention provides a method for developing a force field to study the adsorption of hydrogen in MOF materials containing unsaturated metal sites, the main contents of which include the following steps:

[0008] 1. Model Structure Selection: Choose a suitable metal-organic framework (MOF) as the model, such as Cu-BTC. The structure needs to have experimentally validated good performance and provide sufficient symmetry and computability for DFT calculations.

[0009] 2. DFT calculations: Periodic calculations were performed on the MOF structure using density functional theory (DFT) to study the interaction between hydrogen and open copper metal sites (Cu-OMS). Adsorption configurations were calculated and the adsorption positions of hydrogen molecules at the metal sites were optimized.

[0010] 3. Interaction Analysis: The interaction properties between hydrogen and Cu-OMS are analyzed using DFT results to determine the optimal adsorption configuration. This step also includes decomposing the interaction energy.

[0011] 4. Establishing a force field model: Based on the DFT calculation results, a force field model is developed. The interaction between hydrogen and Cu-OMS is modeled, typically using a combination of van der Waals (vdW) and Coulomb potentials, with appropriate cutoff distances set for the interaction between hydrogen and unsaturated Cu. Furthermore, the Morse potential may be used to fit an accurate description of the interaction between the metal site and the adsorbed molecules.

[0012] 5. Force Field Validation: The accuracy of the force field is validated by applying it to molecular simulations of MOFs and comparing the results with experimental data. It is necessary to ensure that the force field can correctly predict experimental results such as adsorption isotherms, and that the simulation results match the experimental data.

[0013] Through these steps, force field development can not only accurately describe the interaction between hydrogen and metal-organic frameworks, but also provide a reliable tool for subsequent material screening and performance prediction. Attached Figure Description

[0014] Figure 1 The process of developing a force field for H2 based on unsaturated (Cu) sites in the metal-organic framework material Cu-BTC;

[0015] Figure 2The cutoff distance of the simulated adsorption isotherm of H2 in Cu-BTC at 77K is compared with that of the experiment.

[0016] Figure 3 The simulated adsorption isotherm of H2 in Cu-BTC at 77K is compared with the experimental results. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0018] Example 1

[0019] 1. Model Structure Selection: First, an unsaturated metal (Cu) site gene fragment needs to be constructed. This study aims to explore the interaction mechanism between Cu-BTC and gas molecules (H2). Therefore, we selected the widely used Cu-BTC as the reference material. The main reasons for choosing this material are its ease of synthesis and experimental verification, the ability to easily obtain high-quality samples in experiments, and the capacity to more accurately measure adsorption isotherms, thereby verifying the accuracy of the calculation results. Furthermore, the structure has high symmetry, facilitating truncation and calculation. Figure 1 As shown, by truncating the terminal saturated C atoms and saturating them with H atoms, a model cluster containing two unsaturated metal Cu atoms—Cu2(COO)4(C6H5)4—was extracted as the model cluster for study.

[0020] 2. DFT calculations: Optimization calculations were performed on the hydrogen molecule and the extracted Cu cluster separately. The optimization and quantization calculations of the MOF cluster were performed using the Gaussian09 software package, with b3lyp as the functional, a mixed basis set, 6-31G* for nonmetals, and lanl2dz for metals, along with the metal pseudopotential lanl2dz. After optimization, the hydrogen molecule was placed directly above the Cu cluster molecule, and both were set to rigidity. Configuration optimization was then performed. After optimization, the position was maintained; this position represents the optimal relative position between the H2 molecule and the model cluster. Using the center of mass of the H2 molecule as the reference point, the distance *r* between it and the nearest adjacent Cu atom was changed, thus shifting its distance from the Cu atoms. Gradually increase to Furthermore, the binding energy was calculated using the single-point energy at each distance. At each distance, we calculated the binding energy between them based on different single-point energies and metallic potential energies. The formula for calculating the binding energy is (1):

[0021] E int =E (cluster+X) -E (cluster) -E (X) ……(1)

[0022] Among them, E intE represents the binding energy between a metal cluster and a guest molecule. (cluster+X) Representing metal clusters and object groups

[0023] The sum of the single-point energies of the particles; E (cluster) E represents the single-point energy of a metal cluster. (X) This represents the single-point energy of H2.

[0024] 3. Analysis of interactions: Using the energy decomposition program developed by our research group, the OMS force was calculated. Based on the energy decomposition results, the deduction formula is (2):

[0025] E oms =E DFT -E vdw -E coul ……(2)

[0026] E OMS E represents the OMS force. coul Represents Coulomb force, E vdw Representing van der Waals forces, i.e. intermolecular forces, the relationship between OMS forces and distance was used to calculate the binding energy for both periodic structure models and cluster models. The total force was decomposed using the energy decomposition program developed by our research group. The calculated binding energy was subtracted to obtain the OMS forces, as shown in Table 1. The trend between OMS forces and distance was fitted, and the fitting formula is (3):

[0027]

[0028] Since the extracted Cu cluster contains two unsaturated Cu molecules, two r values, r1 and r2, are used for a more accurate fit. r1 represents the distance between the unsaturated Cu molecules closest to the H2 molecules, and r2 represents the distance between the unsaturated Cu molecules farther from the H2 molecules. The fitting formula then becomes (4):

[0029]

[0030] The interaction represented by the formula occurs between the metal atom and the central site located at the HH bond. In the formula, R is the distance parameter corresponding to the location of the Morse potential minimum, D0 is the potential energy minimum, R0 is the distance corresponding to the atomic interaction potential energy minimum, and α is the width of the potential well, fitted from a distance of... The OMS force was then fitted using Origin, and initial values ​​were assigned to D0, R0, and α respectively.

[0031] Table 1. Energy Disassembly Forces Between Cu-BTC Clusters and H2

[0032]

[0033] Note: Cu1 represents the unsaturated Cu closest to the H2 molecule, and Cu2 represents the unsaturated Cu farther from the H2 molecule; OMS-H2 represents the interaction force between the OMS site and H2, E coul Represents Coulomb force, E vdw Represents van der Waals force, and DFT represents the total force calculated through quantization.

[0034] 4. Establish a force field model: Fit the relationship between the interaction force between the OMS site and H2 and the distances between Cu1 and Cu2. The fitting starts from a distance of... The OMS force was then fitted using Origin, with initial values ​​assigned to D0, R0, and α respectively. The parameter fitting results are shown in the table below:

[0035] Table 2 Fitting parameters of Cu-BTC clusters and H2OMS interaction forces

[0036]

[0037] 5. Force Field Verification: For H2 gas, a virtual atom model was used, with LJ and atomic charge parameters derived from the TraPPE force field. In this force field, H2 is modeled as a rigid molecule with a three-point model and an H bond angle of 180°. The virtual atom is located in the middle of the HH bond, and only the virtual atom H2_COM is an LJ interaction site. The LJ parameter is ε / K. b =36.7K and The point charge at each LJ site is q. H =0.468e and q COM = -0.936e. The specific charges and LJ parameters of the adsorbed molecules are shown in the table below.

[0038] Table 2. Charge and LJ force field parameters of the hydrogen model

[0039]

[0040] Based on the fitted force field parameters, this study uses GCMC simulation to predict the adsorption isotherm of H2 in Cu-BTC. The potential energy parameters of the MOF framework atoms are taken from the universal force field (UFF), while the potential energy parameters of the guest molecules are... The experimental surface area and pore volume of the Cu-BTC material used in the verification experiment were 2095 m²... 2 / g and 0.76cm 3 / g, compared to the theoretical value of 2164m 2 / g and 0.82cm 3The g / g ratio is very close, indicating that the material has good activation properties. This experiment used the laboratory-developed high-throughput computational software HT-CADSS (High-throughput-based Complex Adsorption and Diffusion Simulation Suite), employing grand canonical ensemble Monte Carlo simulation (GCMC) to calculate the hydrogen adsorption capacity of the MOF material at 77 K. The cutoff setting was also tested; see [link to documentation]. Figure 2 The cutoff settings are as follows: As shown in the figure, the cutoff distance is The time value is closest to the experimental value, see Figure 3 .

[0041] This experiment, through meticulous theoretical calculations and simulations, not only revealed the interaction mechanism between Cu-MOF and H2 molecules, but also provided important theoretical support for the application of MOF materials in gas adsorption and separation. Future research will further explore the interactions between different MOF structures, different gas molecules, and under different conditions to promote the optimization of MOF material performance and the expansion of its application scope. The results show that the developed force field can accurately predict the adsorption isotherm of hydrogen in Cu-BTC, and exhibits good agreement with experimental results.

Claims

1. A method for developing a force field for hydrogen adsorption in MOF materials containing unsaturated metal sites through theoretical calculations, characterized in that... By employing a combination of DFT and molecular simulation methods, force fields were developed to accurately describe the specific interactions between hydrogen and unsaturated metal sites, and the hydrogen adsorption behavior of Cu-BTC under different pressure conditions was simulated and verified.

2. A prediction method according to claim 1, characterized in that, Includes the following steps: A force field method for hydrogen unsaturated metal sites (OMS) was developed, employing a multi-scale computation approach that combines density functional theory (DFT) and molecular simulations. The following is an overview of the method: (1) DFT calculation of model MOF: First, periodic DFT calculation is performed on MOF with copper propeller wheel unit (such as Cu-BTC) to study the interaction between H2 and Cu-OMS. (2) Force Field Development: A new force field was developed based on DFT results to more accurately describe the interaction between hydrogen isotopes and Cu-OMS. The interaction was modeled by combining van der Waals (vdW) and Coulombic potentials, with a specific cutoff distance set. The Morse potential was used to describe the Cu-H2 interaction, and the calculation results were in high agreement with the DFT results. (3) Optimization and Validation: The accuracy of the force field was verified by comparing the simulated adsorption isotherms with experimental data, demonstrating its ability to accurately predict the behavior of hydrogen in MOFs. The interaction between the adsorbate and Cu-OMS plays a crucial role in determining separation performance; ignoring these interactions may lead to incorrect material identification. This approach demonstrates the importance of combining quantum mechanical calculations with molecular simulations for force field development, providing a reliable framework for the design of hydrogen-adsorbed MOFs.

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