CN112802558B - Method for predicting performance of covalent organic framework for extracting lithium from salt lake based on molecular simulation - Google Patents
Method for predicting performance of covalent organic framework for extracting lithium from salt lake based on molecular simulation Download PDFInfo
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Abstract
The invention provides a method for predicting the performance of a covalent organic framework for extracting lithium from a salt lake based on molecular simulation, which comprises the following steps: setting a water box containing water molecules, magnesium ions and lithium ions, combining the water box with a COF separation membrane, constructing a COF brine model box, optimizing the structure of the COF brine model box, and setting the density of the COF brine model box according to the actual brine density; and (II) carrying out molecular dynamics simulation on the set COF brine model box, counting the transmission speeds of magnesium ions and lithium ions in the pore canal of the COF separation membrane after the simulation is finished, and calculating the separation factor according to the transmission speeds of the magnesium ions and the lithium ions. The method for predicting the magnesium-lithium separation performance by using the molecular simulation method provided by the invention can rapidly judge the effect of the COF magnesium-lithium separation in a limited time.
Description
Technical Field
The invention belongs to the technical field of molecular simulation, and relates to a method for predicting the performance of a covalent organic framework for extracting lithium from a salt lake based on molecular simulation.
Background
The lithium ion battery is used as a new energy source which is environment-friendly, high in efficiency and reusable, and is widely applied to the aspects of military field, rail transit and the like. Lithium resources are an important component in lithium ion batteries, and the demand thereof is rising by about 8% per year, resulting in a continuous rise in the price thereof. It has been ascertained that 70% of lithium resources are all stored in salt lakes worldwide, so that the extraction of lithium from salt lakes is a main way to obtain lithium resources at home and abroad. The reserve of lithium resources in salt lakes in China is the second world, but the problem of high magnesium-lithium ratio restricts the acquisition of lithium resources, so how to realize magnesium-lithium separation becomes a key problem. In all methods for extracting lithium from salt lakes, membrane separation is the most green technology, accords with the environmental policy in new period of China, but the separation performance of the current commercial membrane is not high, and the cost of magnesium-lithium separation is increased. Therefore, it is important to design a novel membrane material with high flux and high magnesium-lithium separation performance.
In recent years, covalent Organic Framework (COF) materials are receiving widespread attention from researchers. The membrane material is considered to be an ideal membrane material for salt separation because of the characteristics of a highly ordered pore canal structure, adjustable pore diameter, easy functional modification and the like. COFs are numerous and nano-sized materials, however, and if trial and error is used in experiments to attempt magnesium-lithium separation performance, significant effort and money are expended both synthetically and characteristically.
Molecular simulation refers to simulating or simulating microscopic behaviors of molecular motion by using a theoretical method and a computing technology, and equivalent actions among atoms to motion of a particle system by using proper simplifying conditions, so that a complex quantum mechanical equation is avoided. The motion of atoms follows newton's second law and the particle system as a whole follows the hamiltonian principle. In contrast, the atomic calculations performed entirely from quantum mechanics are called "first principles (ab into) calculations". The first principle calculation is high in accuracy, but is complex in calculation, and large-scale simulation is difficult to realize. Molecular simulation greatly expands the application range of atomic computer simulation while ensuring the precision. First principles calculations are typically tens, hundreds of atoms, whereas molecular modeling can even achieve millions, even tens of millions, of atoms of operations. The molecular simulation is not only efficient but also can effectively save the cost, so that the molecular simulation is widely applied to the subjects of medicine, chemical industry, materials and the like.
CN111307684a discloses a molecular simulation method for calculating gas permeability in micro-nano pores, and giant regular monte carlo simulation is performed in the high-pressure region H to balance initial conditions; performing molecular dynamics simulation to enable gas molecules in the H region to continuously flow to the L region through the pores under the action of pressure difference; after the system flow reaches equilibrium, the statistics is carried out on the molecular number N of the gas passing through the pores by adopting a statistical method. The invention adopts an unbalanced molecular dynamics simulation method, the proposed modeling method can simulate the flow of gas in micro-nano pores under different temperature and pressure conditions, and the gas molecular number is counted by a statistical method to directly calculate the permeability.
CN108959844a discloses a method for evaluating the flow properties of a polymer material by using a molecular simulation method, comprising: constructing a single-chain structure of the polymer to be detected according to the repeating units of the polymer to be detected; constructing a bulk phase structure model of the polymer according to the structural parameters, the formula components and the like of the polymer; sequentially performing primary relaxation of the configuration, primary screening of the overall optimal configuration of the NVT ensemble, secondary screening of the overall optimal configuration of the NPT ensemble, and cooling annealing simulation of the NVT and NPT ensemble on the bulk structure model to fully balance the configuration and the density of the model and obtain second structure data; performing full dynamic balance on the second structural data to obtain dynamic track data; and (3) carrying out stress autocorrelation function analysis on the dynamic track data, testing the effectiveness of the viscosity data, and obtaining the reliable shear viscosity data of the polymer to be tested.
CN104899356A discloses a quantitative analysis method for metalloporphyrin MOFs material CO 2 /CH 4 A method for separating efficiency. The method of the invention quantitatively analyzes the metalloporphyrin MOFs material CO based on quantum chemical density functional theoretical calculation and Monte Carlo molecular simulation 2 /CH 4 Is not limited, and the separation efficiency of the same is improved. Quantitative analysis of CO by determining the interaction energy, heat of adsorption, of probe molecules with different metalloporphyrin ligands 2 Phase with metalloporphyrin MOFs materialsInteraction conditions, finally by CO 2 /CH 4 Calculation of adsorption Selectivity CO characterizing different metalloporphyrin MOFs materials 2 /CH 4 Separation efficiency and characteristics. The method comprises cluster model construction, structural optimization of stable configuration, calculation of partial charge and CO 2 /CH 4 Calculation of separation coefficient, calculation of adsorption energy and adsorption heat, and CO 2 /CH 4 Analysis and characterization of separation efficiency.
At present, the work of researching the separation performance of magnesium and lithium on the COF through molecular simulation is not reported yet.
Disclosure of Invention
Aiming at the defects existing in the prior art, the invention aims to provide a method for predicting the performance of a covalent organic framework for extracting lithium from a salt lake based on molecular simulation.
To achieve the purpose, the invention adopts the following technical scheme:
the invention provides a method for predicting the performance of a covalent organic framework for extracting lithium based on molecular simulation, which is characterized by comprising the following steps:
setting a water box containing water molecules, magnesium ions and lithium ions, combining the water box with a COF separation membrane, constructing a COF brine model box, optimizing the structure of the COF brine model box, and setting the density of the COF brine model box according to the actual brine density;
and (II) carrying out molecular dynamics simulation on the set COF brine model box, counting the transmission speeds of magnesium ions and lithium ions in the pore canal of the COF separation membrane after the simulation is finished, and calculating the separation factor according to the transmission speeds of the magnesium ions and the lithium ions.
According to the invention, the molecular dynamics simulation is adopted to effectively predict the separation factor of the COF separation membrane for magnesium-lithium separation, and the separation effect of the COF separation membrane for magnesium-lithium separation can be rapidly judged in a limited time. The method not only can effectively reduce cost consumption caused by experiments, but also can simulate experiments which cannot be accurately performed and completed due to the limitations of related equipment and materials and the like, and also provides theoretical reference for the design research and development and structural optimization of the COF material, thereby shortening the research and development period.
As a preferable technical scheme of the invention, in the step (I), the water molecules are constructed by adopting an SPC/E model or a TIP4P model.
Preferably, the force field parameters of the COF brine model box are dreading or CVFF.
Preferably, the metering unit of the COF brine model box selects real format.
In the step (i), as a preferred technical solution of the present invention, the structure optimization process of the COF brine model box includes: the initial structure is subjected to a temperature raising and reducing cycle under the NVT ensemble, and the duration is set to be 0.5-2 ns, for example, 0.5ns, 0.6ns, 0.7ns, 0.8ns, 0.9ns, 1.0ns, 1.1ns, 1.2ns, 1.3ns, 1.4ns, 1.5ns, 1.6ns, 1.7ns, 1.8ns, 1.9ns or 2.0ns, but the invention is not limited to the recited values, and other non-recited values within the range of values are equally applicable.
In the present invention, the full term of NVT ensemble is a canonical ensemble (canonical ensemble), which is a representative of the Monte Carlo method simulation process. Assuming N particles are in a box of volume V, they are placed in a huge thermal reservoir of constant temperature T. At this time, the total energy (E) and the system pressure (P) may fluctuate around a certain average value. The balance system is a closed system, is in thermal contact with a large heat source, and has the advantages of heat balance achieved by energy exchange, equal temperature, enough large heat reservoir and definite temperature. The characteristic function is the helmholtz free energy F (N, V, T). The concept of NVT ensembles is well known to those skilled in the art and will not be described in detail with respect to its deep theory.
As a preferable technical scheme of the invention, in the step (I), the heating and cooling circulation process comprises the following steps: from T 1 Heating to T 2 Then from T 2 Cooling to T 1 。
Preferably T 1 Set to 300K.
Preferably T 2 Is set to 400 to 1000K may be, for example, 400K, 450K, 500K, 550K, 600K, 650K, 700K, 750K, 800K, 850K, 900K, 950K or 1000K, but is not limited to the values recited, and other non-recited values within the range are equally applicable, preferably 500 to 800K.
In a preferred embodiment of the present invention, in the step (i), the COF separation film is provided with a water box on both sides thereof, and the thickness of the water box is > 2nm, for example, 1.0nm, 1.1nm, 1.2nm, 1.3nm, 1.4nm, 1.5nm, 1.6nm, 1.7nm, 1.8nm, 1.9nm or 2.0nm, but the water box is not limited to the listed values, and other non-listed values within the range of the values are equally applicable.
The brine concentration of the COF brine model box is preferably 0.5 to 1mol/L, for example, 0.5mol/L, 0.55mol/L, 0.6mol/L, 0.65mol/L, 0.7mol/L, 0.75mol/L, 0.8mol/L, 0.85mol/L, 0.9mol/L, 0.95mol/L or 1mol/L, but the present invention is not limited to the above-mentioned values, and other non-mentioned values within the above-mentioned values are equally applicable.
Preferably, the boundary condition adopted by the COF brine model box is a three-dimensional periodic boundary.
In a preferred embodiment of the present invention, in the step (i), the density setting process of the COF brine model box includes: under NPT system, the pressure of the film passing is set to P 1 The temperature is set as T 3 The operation time is set as H 1 So that each particle in the water box is extruded into the pore canal of the COF separation film under the action of the film passing pressure, and then the film passing pressure is adjusted to P 2 Keeping the temperature unchanged, and adjusting the operation time to H 2 。
In the present invention, the full term NPT ensemble is an isothermal and isobaric ensemble. I.e. the particle number (N), the pressure (P), the temperature (T) are determined. Typically in a monte carlo simulation. There may be fluctuations in its total energy (E) and system volume (V). The system is a constant temperature hot bath in the case of a movable system wall. The characteristic function is the Gibbs free energy G (N, P, T). The concept of NPT ensemble is well known to those skilled in the art and will not be described in detail with respect to its deep theory.
Preferably, P 1 The pressure is set to 5 to 50 atmospheres, and for example, 5 atmospheres, 10 atmospheres, 15 atmospheres, 20 atmospheres, 25 atmospheres, 30 atmospheres, 35 atmospheres, 40 atmospheres, 45 atmospheres, or 50 atmospheres are used, and the values are also applicable.
Preferably T 3 Set to 300K.
Preferably H 1 The value is set to 0.5 to 2ns, for example, 0.5ns, 0.6ns, 0.7ns, 0.8ns, 0.9ns, 1.0ns, 1.1ns, 1.2ns, 1.3ns, 1.4ns, 1.5ns, 1.6ns, 1.7ns, 1.8ns, 1.9ns or 2.0ns, but the present invention is not limited to the above-mentioned values, and other values not mentioned in the above-mentioned numerical ranges are equally applicable.
Preferably, P 2 Set to 1 atmosphere.
Preferably H 2 The value is set to 0.5 to 2ns, for example, 0.5ns, 0.6ns, 0.7ns, 0.8ns, 0.9ns, 1.0ns, 1.1ns, 1.2ns, 1.3ns, 1.4ns, 1.5ns, 1.6ns, 1.7ns, 1.8ns, 1.9ns or 2.0ns, but the present invention is not limited to the above-mentioned values, and other values not mentioned in the above-mentioned numerical ranges are equally applicable.
In the step (II), the molecular dynamics simulation is performed by using LAMMPS software, and simulation parameters to be controlled include bond length and bond angle of water molecules, interatomic force and simulation step length, and framework atoms in the COF separation membrane are ensured to be fixed in the simulation process.
Preferably, the bond length and bond angle of the water molecules are fixed by using a vibration algorithm.
Preferably, the interatomic forces are described by van der Waals forces calculated using LJ12-6 potential energy and electrostatic forces calculated using PPPM algorithm.
Preferably, the PPPM algorithm has an accuracy of 10 -4 ～10 -6 For example, it may be 10 -4 、10 -4.5 、10 -5 、10 -5.5 Or 10 -6 But are not limited to, the recited values, and other non-recited values within the range of values are equally applicable.
Preferably, the simulation step size is set to 1-2 fs, for example, 1.0fs, 1.1fs, 1.2fs, 1.3fs, 1.4fs, 1.5fs, 1.6fs, 1.7fs, 1.8fs, 1.9fs or 2.0fs, but not limited to the recited values, and other non-recited values within the range of values are equally applicable.
As a preferred embodiment of the present invention, in the step (II), the molecular dynamics simulation is performed under non-equilibrium conditions.
Preferably, the molecular dynamics simulation process specifically includes:
and applying an external force to the water box at one side of the COF separation membrane along the direction perpendicular to the surface of the COF separation membrane, and recording movement track data of water molecules, magnesium ions and lithium ions in the water box at one side of the COF separation membrane under the drive of a membrane passing pressure difference.
Preferably, the movement track of the water molecules, the magnesium ions and the lithium ions is recorded every 0.1 to 1ps, for example, 0.1ps, 0.2ps, 0.3ps, 0.4ps, 0.5ps, 0.6ps, 0.7ps, 0.8ps, 0.9ps or 1.0ps, but is not limited to the recited values, and other non-recited values within the range of the values are equally applicable.
Preferably, the molecular dynamics simulation process is performed for 1 to 50ns, for example, 1ns, 5ns, 10ns, 15ns, 20ns, 25ns, 30ns, 35ns, 40ns, 45ns or 50ns, but not limited to the recited values, and other non-recited values within the range of values are equally applicable.
In a preferred embodiment of the present invention, the pressure difference between the film layers is calculated according to the formula (1) and is controlled to 5 to 500MPa, and for example, 5MPa, 10MPa, 50MPa, 100MPa, 150MPa, 200MPa, 250MPa, 300MPa, 350MPa, 400MPa, 450MPa or 500MPa may be used, but the pressure difference is not limited to the values listed, and other values not listed in the numerical range are equally applicable.
Wherein,ΔP represents the pressure difference across the membrane, n ion Represents the particle number of magnesium ions and lithium ions, f ion Indicating a constant force applied to magnesium ions and lithium ions, water represents the particle number of water molecules, f water Represents a constant force applied to water molecules, a represents an area of the COF separation film; f (f) ion And f water Satisfy formula (2):
wherein m is ion Represents the molecular weight of magnesium ions and lithium ions, m water Represents the molecular weight of water molecules.
In the step (ii), the formula of calculation of the separation factor is shown in formula (3):
wherein,and->Represents the film speed of magnesium ion and lithium ion respectively,/-, respectively>Representing the magnesium ion concentration in the water box on the feed side; />The lithium ion concentration in the water box on the feed side is shown.
But are not limited to, the recited values, and other non-recited values within the range of values are equally applicable.
Compared with the prior art, the invention has the beneficial effects that:
the non-equilibrium molecular dynamics method provided by the invention predicts the separation factor of magnesium-lithium separation in the COF separation membrane, not only can effectively reduce cost consumption caused by experiments, but also can simulate experiments which cannot be accurately performed and completed due to the limitations of related equipment and materials and the like, and also provides theoretical reference for the design research and development of COF materials and the structural optimization, and shortens the research and development period.
Detailed Description
The technical scheme of the invention is further described by the following specific embodiments.
Example 1
The embodiment provides a method for predicting the performance of a covalent organic framework for extracting lithium from a salt lake based on molecular simulation, which specifically comprises the following steps:
(1) The settings include H 2 O、MgCl 2 And LiCl 2 The water box was combined with TPE-COF separation membrane and COF brine model box was constructed. The water molecules select SPC/E model, the force field parameters select DREIDING, the measuring unit selects real format, the structure of the COF brine model box is optimized, then the temperature is increased from 300K to 500K under NVT system, and then the temperature is reduced from 500K to 300K, and an annealing circulation process is carried out.
(2) The COF brine model cartridge consisted of two TPE-COF separation membranes and two water cartridges on each side (serving as feed and permeate sides, respectively) with three-dimensional periodic boundaries throughout the system. Wherein the water boxes on both sides respectively contain 1898H 2 O, 17 MgCl 2 And 17 LiCl, the brine concentration of the water box was maintained at 1mol/L. In order to make the brine density approach more reasonable, firstly, under the NPT system, the film passing pressure is set to 10 atmospheres, the temperature is set to 300K, and the operation is carried out for 1ns, so that each particle in the water box is extruded into a pore canal of the TPE-COF separation film under the action of the film passing pressure, then the film passing pressure is adjusted to 1 atmosphere, the temperature is kept unchanged, and the operation is carried out for 1ns, so that the density of the water box is kept consistent with the actual condition.
(3) Non-equilibrium state molecular dynamics simulation is carried out on the whole system by adopting LAMMPS software, and a vibration algorithm is used for fixing the long bond angle of water molecule bonds and all atomsThe force of the intermediate reaction is described by van der Waals force calculated using LJ12-6 potential energy and electrostatic force with an accuracy of 10 -5 The simulation step size is set to 2fs, and the framework atoms in the TPE-COF separation membrane are ensured to be fixed in the simulation process. And applying external force to the water box at one side of the TPE-COF separation membrane along the direction perpendicular to the surface of the TPE-COF separation membrane, and under the drive of the membrane-passing pressure difference, water molecules, magnesium ions and lithium ions in the water box at one side of the TPE-COF separation membrane pass through the membrane, and recording movement track data of the water molecules, the magnesium ions and the lithium ions once every 1ps, wherein the whole molecular dynamics simulation process is carried out for 20ns.
The film-passing pressure difference was calculated according to formula (1), and the film-passing pressure difference was controlled at 50MPa.
Wherein ΔP represents the pressure difference across the membrane, n ion Represents the particle number of magnesium ions and lithium ions, f ion Indicating a constant force applied to magnesium ions and lithium ions, water represents the particle number of water molecules, f water Represents the constant force exerted on the water molecules, A represents the area of the TPE-COF separation membrane; f (f) ion And f water Satisfy formula (2):
after the simulation reaches a steady state, the simulation is analyzed by a self-compiled program to obtain Mg 2+ At a speed of 0.03/ns, li + The speed was 0.265/ns.
(4) Calculating a separation factor using formula (3):
wherein,and->Represents the film speed of magnesium ion and lithium ion respectively,/-, respectively>Representing the magnesium ion concentration in the water box on the feed side; />The lithium ion concentration in the water box on the feed side is shown.
According to the formula, the separation factor of the TPE-COF separation membrane is calculated to be 9.
Example 2
The embodiment provides a method for predicting the performance of a covalent organic framework for extracting lithium from a salt lake based on molecular simulation, which specifically comprises the following steps:
(1) The settings include H 2 O、MgCl 2 And LiCl 2 The water box was combined with JUL-COF separation membrane and a COF brine model box was constructed. The water molecules select SPC/E model, the force field parameters select DREIDING, the measuring unit selects real format, the structure of the COF brine model box is optimized, then the temperature is increased from 300K to 500K under NVT system, and then the temperature is reduced from 500K to 300K, and an annealing circulation process is carried out.
(2) The COF brine model cartridge consisted of two layers JUL-COF separation membranes and two water cartridges on each side (serving as feed and permeate sides, respectively) with three-dimensional periodic boundaries throughout the system. Wherein the water boxes on both sides respectively contain 1898H 2 O, 17 MgCl 2 And 17 LiCl, the brine concentration of the water box was maintained at 1mol/L. In order to make the brine density more reasonable, firstly, under the NPT system, the pressure of the passing film is set to 10 atmospheres, the temperature is set to 300K, and the operation is carried out for 1ns, so that each particle in the water box is extruded into the pore canal of the JUL-COF separation film under the action of the passing film pressure, then the passing film pressure is adjusted to 1 atmosphere, the temperature is kept unchanged,the operation is carried out for 1ns, so that the density of the water box is consistent with the actual situation.
(3) Non-equilibrium state molecular dynamics simulation is carried out on the whole system by adopting LAMMPS software, a vibration algorithm is used for fixing the long bond angle of a water molecule bond, all interatomic acting forces are described by Van der Waals force and electrostatic force, wherein the Van der Waals force is calculated by adopting LJ12-6 potential energy, and the electrostatic force is calculated by adopting the precision of 10 -5 The simulation step size is set to be 1fs, and framework atoms in the JUL-COF separation membrane are ensured to be fixed in the simulation process. Along the direction vertical to the surface of the JUL-COF separation film, external force is applied to the water box at one side of the JUL-COF separation film, and under the drive of film passing pressure difference, water molecules, magnesium ions and lithium ions in the water box at one side of the JUL-COF separation film pass through the film, and movement track data of the water molecules, the magnesium ions and the lithium ions are recorded once every 1ps, and the whole molecular dynamics simulation process is carried out for 15ns.
The film-passing pressure difference was calculated according to formula (1), and the film-passing pressure difference was controlled at 50MPa.
Wherein ΔP represents the pressure difference across the membrane, n ion Represents the particle number of magnesium ions and lithium ions, f ion Indicating a constant force applied to magnesium ions and lithium ions, water represents the particle number of water molecules, f water Represents a constant force applied to water molecules, and A represents an area of a JUL-COF separation film; f (f) ion And f water Satisfy formula (2):
after the simulation reaches a steady state, the simulation is analyzed by a self-compiled program to obtain Mg 2+ At a speed of 0.065/ns, li + The speed was 0.13/ns.
(4) Calculating a separation factor using formula (3):
wherein,and->Represents the film speed of magnesium ion and lithium ion respectively,/-, respectively>Representing the magnesium ion concentration in the water box on the feed side; />The lithium ion concentration in the water box on the feed side is shown.
According to the formula, the separation factor of the JUL2-COF separation membrane is calculated to be 2.
Example 3
The embodiment provides a method for predicting the performance of a covalent organic framework for extracting lithium from a salt lake based on molecular simulation, which specifically comprises the following steps:
(1) The settings include H 2 O、MgCl 2 And LiCl 2 Combining the water box with a CCOF-COF separation membrane and constructing a COF brine model box. The water molecules select SPC/E model, the force field parameters select DREIDING, the measuring unit selects real format, the structure of the COF brine model box is optimized, then the temperature is increased from 300K to 500K under NVT system, and then the temperature is reduced from 500K to 300K, and an annealing circulation process is carried out.
(2) The COF brine model cartridge consisted of two CCOF-COF separation membranes and water cartridges on both sides (serving as feed side and permeate side, respectively) with three-dimensional periodic boundaries throughout the system. Wherein the water boxes on both sides respectively contain 1898H 2 O, 17 MgCl 2 And 17 LiCl, the brine concentration of the water box was maintained at 1mol/L. To make the brine density more reasonable, the brine density is firstly improved under the NPT systemThe pressure of the passing film is set to be 10 atmospheres, the temperature is set to be 300K, and the operation is carried out for 1ns, so that each particle in the water box is extruded into a pore canal of the CCOF-COF separation film under the action of the pressure of the passing film, then the pressure of the passing film is regulated to be 1 atmosphere, the temperature is kept unchanged, and the operation is carried out for 1ns, so that the density of the water box is kept consistent with the actual condition.
(3) Non-equilibrium state molecular dynamics simulation is carried out on the whole system by adopting LAMMPS software, a vibration algorithm is used for fixing the long bond angle of a water molecule bond, all interatomic acting forces are described by Van der Waals force and electrostatic force, wherein the Van der Waals force is calculated by adopting LJ12-6 potential energy, and the electrostatic force is calculated by adopting the precision of 10 -5 The simulation step size is set to 1fs, and the skeleton atoms in the CCOF-COF separation membrane are ensured to be fixed in the simulation process. And applying external force to the water box at one side of the CCOF-COF separation membrane along the direction vertical to the surface of the CCOF-COF separation membrane, and performing membrane passing of water molecules, magnesium ions and lithium ions in the water box at one side of the CCOF-COF separation membrane under the drive of membrane passing pressure difference, wherein the motion track data of the water molecules, the magnesium ions and the lithium ions are recorded once every 1ps, and the whole molecular dynamics simulation process is performed for 15ns.
The film-passing pressure difference was calculated according to formula (1), and the film-passing pressure difference was controlled at 50MPa.
Wherein ΔP represents the pressure difference across the membrane, n ion Represents the particle number of magnesium ions and lithium ions, f ion Indicating a constant force applied to magnesium ions and lithium ions, water represents the particle number of water molecules, f water Represents a constant force applied to water molecules, A represents the area of a CCOF-COF separation membrane; f (f) ion And f water Satisfy formula (2):
analysis was performed with a self-compiled program after the simulation reached steady state,obtaining Mg 2+ At a speed of 0.046/ns, li + The speed is 0.315/ns.
(4) Calculating a separation factor using formula (3):
wherein,and->Represents the film speed of magnesium ion and lithium ion respectively,/-, respectively>Representing the magnesium ion concentration in the water box on the feed side; />The lithium ion concentration in the water box on the feed side is shown.
According to the formula, the separation factor of the CCOF-COF separation membrane is calculated to be 6.8.
The applicant declares that the above is only a specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and it should be apparent to those skilled in the art that any changes or substitutions that are easily conceivable within the technical scope of the present invention disclosed by the present invention fall within the scope of the present invention and the disclosure.
Claims (25)
1. A method for predicting the performance of a covalent organic framework for lithium extraction based on molecular modeling, said method comprising:
setting a water box containing water molecules, magnesium ions and lithium ions, combining the water box with a COF separation membrane, constructing a COF brine model box, optimizing the structure of the COF brine model box, and setting the density of the COF brine model box according to the actual brine density;
two sides of the COF separation membrane are respectively provided with a water box which is respectively used as a feeding side and a permeation side;
the structure optimization process of the COF brine model box comprises the following steps: the initial structure carries out a heating and cooling cycle under the NVT ensemble, and the duration is set to be 0.5-2 ns;
(II) carrying out molecular dynamics simulation on the set COF brine model box, counting the transmission speeds of magnesium ions and lithium ions in the pore canal of the COF separation membrane after the simulation is finished, and calculating separation factors according to the transmission speeds of the magnesium ions and the lithium ions;
the molecular dynamics simulation process specifically comprises the following steps:
applying an external force to a water box on one side of the COF separation membrane along the direction perpendicular to the surface of the COF separation membrane, and recording movement track data of water molecules, magnesium ions and lithium ions in the water box on one side of the COF separation membrane under the drive of a membrane passing pressure difference;
the calculation formula of the separation factor is shown in formula (1):
wherein,and v Li + Represents the film speed of magnesium ion and lithium ion respectively,/-, respectively>Representing the magnesium ion concentration in the water box on the feed side; c (C) Li + The lithium ion concentration in the water box on the feed side is shown.
2. The method of claim 1, wherein in step (i), the water molecules are constructed using an SPC/E model or a TIP4P model.
3. The method of claim 1, wherein in step (i), the force field parameters of the COF brine model box are selected from the group consisting of dreading and CVFF.
4. The method of claim 1, wherein in step (i), the unit of measure of the COF brine model cartridge is in real format.
5. The method of claim 1, wherein in step (i), the temperature ramp-up and ramp-down cycle comprises: from T 1 Heating to T 2 Then from T 2 Cooling to T 1 。
6. The method of claim 5, wherein T 1 Set to 300K.
7. The method of claim 5, wherein T 2 Is set to 400-1000K.
8. The method of claim 7, wherein T 2 Setting 500-800K.
9. The method according to claim 1, wherein in step (i), the thickness of the water box is > 2nm.
10. The method of claim 1, wherein in step (i), the COF brine model cartridge has a brine concentration of 0.5 to 1mol/L.
11. The method of claim 1, wherein in step (i), the COF brine model box uses a boundary condition that is a three-dimensional periodic boundary.
12. The method of claim 1, wherein in step (i), saidSetting the density of the COF brine model box includes: under NPT system, the pressure of the film passing is set to P 1 The temperature is set as T 3 The operation time is set as H 1 So that each particle in the water box is extruded into the pore canal of the COF separation film under the action of the film passing pressure, and then the film passing pressure is adjusted to P 2 Keeping the temperature unchanged, and adjusting the operation time to H 2 。
13. The method of claim 12, wherein P 1 Setting the pressure to be 5-50 atmospheres.
14. The method of claim 12, wherein T 3 Set to 300K.
15. The method of claim 12, wherein H 1 Is set to 0.5-2 ns.
16. The method of claim 12, wherein P 2 Set to 1 atmosphere.
17. The method of claim 12, wherein H 2 Is set to 0.5-2 ns.
18. The method of claim 1, wherein in step (ii), the molecular dynamics simulation is performed by LAMMPS software, and the simulation parameters to be controlled include bond length and bond angle of water molecules, interatomic forces, and simulation steps, and the framework atoms in the COF separation membrane are ensured to be fixed during the simulation.
19. The method of claim 18, wherein the bond length and bond angle of the water molecule are fixed using a shaker algorithm.
20. The method of claim 18, wherein the interatomic forces are described by van der waals forces calculated using LJ12-6 potential energy and electrostatic forces calculated using PPPM algorithm.
21. The method of claim 20, wherein the PPPM algorithm has a precision of 10 -4 ～10 -6 。
22. The method of claim 18, wherein the simulation step size is set to 1-2 fs.
23. The method according to claim 1, wherein in the step (ii), the movement trace of water molecules, magnesium ions and lithium ions is recorded every 0.1-1 ps in the molecular dynamics simulation process.
24. The method of claim 1, wherein in step (ii), the molecular dynamics simulation is performed for 1 to 50ns.
25. The method according to claim 1, wherein the pressure difference across the membrane is calculated according to formula (2) and is controlled to be 5 to 500MPa;
wherein ΔP represents the pressure difference across the membrane, n ion Represents the particle number of magnesium ions and lithium ions, f ion Represents a constant force applied to magnesium ions and lithium ions, n water Represents the particle number of water molecules, f water Represents a constant force applied to water molecules, a represents an area of the COF separation film; f (f) ion And f water Satisfy formula (3):
wherein m is ion Represents the molecular weight of magnesium ions and lithium ions, m water Represents the molecular weight of water molecules.
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