Monovalent alkali metal ion separation composite membrane, preparation method and application thereof

Abstract

This invention discloses a monovalent alkali metal ion separation composite membrane, its preparation method, and its application, belonging to the field of ion separation membrane technology. This invention prepares a monovalent alkali metal ion separation composite membrane by combining heteropolyacids with two-dimensional materials. By selecting heteropolyacids with appropriate coordination energies to different monovalent alkali metal ions, the heteropolyacids and Li... + The absolute value of the difference between the coordination energy and the binding energy of hydrated lithium ions is smaller, thus enabling Li+ to be transported in the membrane at a higher rate than other monovalent alkali metal ions, achieving the goal of separating monovalent alkali metal ions with a high separation ratio.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of ion separation membranes, and more particularly relates to a monovalent alkali metal ion separation composite membrane and a preparation method and application thereof. BACKGROUND

[0002] Driven by the rapid growth of sustainable energy demand for lithium, the exploitation of lithium has become crucial. Extracting lithium from aquatic sources such as salt lakes requires efficient and environmentally friendly separation technologies. Membrane separation technology has the advantages of low energy consumption, environmental friendliness, and no phase change in lithium extraction, which makes it an effective method for high-efficiency separation of lithium ions from salt lake brine. Membrane separation technology basically solves the key challenge of removing calcium and magnesium ions from salt lake brine. However, due to the small differences in radius and dehydration energy between monovalent cations present in salt lake brine, the removal of other monovalent cation (such as sodium ion, potassium ion) impurities is still a big problem that membrane separation technology has not yet overcome. Through further research and development, separation membranes based on new two-dimensional materials are expected to solve the bottleneck of existing membrane separation technology, providing important support for the efficient use of lithium resources and the development of new energy industry.

[0003] As a biomimetic target of ion separation membrane, KcsA shows extremely high monovalent cation selectivity due to its sub-nanometer channel and precise coordination of ions with its carbonyl groups. Inspired by this, various oxygen-containing organic ligand designs, such as crown ethers or sulfonates, have appeared in the existing technology (J. Membr. Sci. 2019, 591, 117345; Nat. Mater. 2019, 18, 76-81.) for ion separation membranes to achieve the separation of monovalent cations 1-3 . Crown ethers can selectively surround and bind specific ions through their cyclic structure, with high selectivity. The structure of crown ethers can be adjusted to its ion selectivity and affinity by changing the size of the ring and the type of substituents. This diversity allows crown ethers to be designed and synthesized to meet different separation needs. However, the synthesis of crown ethers is costly, and the hydrolytic aging of crown ethers often damages the nanochannel for ion transport, thereby weakening or even reversing the separation selectivity between monovalent cations. The introduction of sulfonates in the existing technology (Angew. Chem. Int. Ed. 2021, 60, 22265-22269.) can adjust the affinity of ion sieving materials and enhance the selectivity for certain specific ions. This helps to achieve efficient separation in complex mixtures, but the permeability of lithium ions in the separation process is not high. Most two-dimensional membranes, such as graphene oxide, molybdenum disulfide, metal-organic frameworks, and covalent organic framework membranes, all exhibit better potassium ion permeability, which is not conducive to lithium extraction, especially from low-lithium-content salt lakes. In summary, it is crucial to construct a separation membrane with high separation ratio of lithium ions based on two-dimensional materials. SUMMARY

[0004] The separation of monovalent alkali metal ions has always been a challenge in the field of membrane separation. In view of the challenge of difficult separation of monovalent cations in the prior art, the inventors first studied the influencing factors of the transmission rate of Li + , K + , Na + in two-dimensional layered membranes, and found that two-dimensional nanochannels such as vermiculite (V), graphene oxide (GO) and layered double hydroxide (LDH, magnesium aluminum hydrotalcite) all exhibit non-selective but fast transmission of monovalent cations. The transmission rate of monovalent alkali metal ions in two-dimensional layered membranes is affected by the hydration binding energy of the ions themselves on the one hand, and the interaction force between the ions and the interlayer material on the other hand;

[0005] For the purpose of separating monovalent alkali metal ions, we hope that the difference between the interaction force of Li + and the interlayer material and the binding energy of hydrated lithium ions is as small as possible to accelerate the transmission of Li + ; at the same time, we also hope that the difference between the interaction force of other monovalent alkali metal ions M + and the interlayer material and the binding energy of hydrated lithium ions is as large as possible to slow down the transmission of M + .

[0006] Based on this, the present application seeks suitable two-dimensional materials or modified two-dimensional materials, so that the difference between the binding energy of the modified two-dimensional material and Li + and the binding energy of hydrated lithium ions is as small as possible, and the difference between the binding energy of the material and other monovalent alkali metal ions and the binding energy of hydrated cations is as large as possible, so that a high separation ratio can be achieved.

[0007] [Monovalent alkali metal ion separation composite membrane]

[0008] The first aspect of the present application provides a monovalent alkali metal ion separation composite membrane, which comprises a two-dimensional material and a heteropoly acid (HPA) distributed between the layers of the two-dimensional material; the two-dimensional material is selected from vermiculite, graphene oxide or layered double hydroxide (LDH);

[0009] The exchange barrier of the heteropoly acid and the monovalent alkali metal ion should satisfy the following relationship:

[0010]

[0011] Wherein, is the exchange barrier of Li + coordinated with the heteropoly acid and Li + combined with water:

[0012]

[0013] wherein, is Li + coordination energy with the heteropoly acid; is Li + hydration energy with water;

[0014]

[0015] and E HPA are the Gibbs free energies of the lithium ion-heteropoly acid complex, the bare lithium ion, and the heteropoly acid, respectively;

[0016]

[0017] wherein, and are the Gibbs free energies of the hydrated lithium ion, the bare lithium ion, and water, respectively;

[0018] is M + coordinately bound to the heteropoly acid + and M + bound to water;

[0019]

[0020] wherein, is M + coordination energy with the heteropoly acid; is M + hydration energy with water;

[0021]

[0022] and E HPA are the Gibbs free energies of the M ion-heteropoly acid complex, the bare M ion, and the heteropoly acid, respectively;

[0023]

[0024] wherein, and are the Gibbs free energies of the hydrated M ion, the bare M ion, and water, respectively;

[0025] M + is selected from K + or Na + .

[0026] Preferably, M + is K + .

[0027] As described herein, "heteropoly acid" refers to a highly symmetrical polyanion formed by linking various different types of oxide ions (such as phosphorus, silicon, vanadium, molybdenum, tungsten, etc.) through oxygen bridges. The general formula of these compounds is X m M n O p q- wherein X is usually a heteroatom such as phosphorus, silicon, arsenic, etc., and M is a transition metal element such as molybdenum or tungsten. The structure is usually formed by a complex network of several metal oxide clusters (such as molybdenum oxide clusters or tungsten oxide clusters) surrounding a central heteroatom (such as phosphorus or silicon).

[0028] From the foregoing, it can be seen that:

[0029]

[0030] The greater the absolute value of the difference, the higher the separation ratio of Li + / M + . Preferably, the absolute value of the difference is greater than 1 eV, more preferably, the absolute value of the difference is greater than 1.5 eV, and most preferably, the absolute value of the difference is greater than 1.6 eV.

[0031] As a preferred embodiment of the first aspect of the present application, a monovalent alkali metal ion separation composite membrane is provided, the composite membrane comprising a heteropoly acid and a two-dimensional material; the heteropoly acid is selected from phosphotungstic acid, Keggin-type silicotungstic acid or phosphomolybdic acid; the two-dimensional material is selected from vermiculite, graphene oxide or layered double hydroxide (LDH); the monovalent alkali metal ion separation composite membrane is used for the separation of monovalent alkali metal ions Li + from K + or Na + .

[0032] As a preferred embodiment of any of the embodiments of the first aspect of the present application, the mass percentage of the heteropoly acid in the total mass of the casting solution is not higher than 10%, preferably not higher than 5%, and most preferably 0.5-5%.

[0033] As a preferred embodiment of any of the embodiments of the first aspect of the present application, the two-dimensional material is vermiculite, and the heteropoly acid is Keggin-type phosphotungstic acid (PTA). Experiments show that the highest separation ratio of Li + / M + is obtained when vermiculite is used and PTA is used as the heteropoly acid.

[0034] As a preferred embodiment of any of the embodiments of the first aspect of the present application, the heteropoly acid and the two-dimensional material are connected by non-covalent bonds or by covalent bonds. In the example of non-covalent bond connection, at least hydrogen bond interaction exists between the heteropoly acid and the two-dimensional material.

[0035] As a preferred embodiment of any of the first aspect of the present application, the thickness of the composite membrane is 0.5-5 μm. Preferably, the thickness of the composite membrane is 0.5-2 μm.

[0036] As a preferred embodiment of any of the first aspect of the present application, the lithium ion permeability: potassium ion permeability of the composite membrane is (2-18): 1. The higher the lithium ion permeability, the higher the lithium ion transmission rate of the composite membrane.

[0037] As a preferred embodiment of any of the first aspect of the present application, the lithium ion permeability of the composite membrane is 0.3-1.3 mol m -2 h -1 .

[0038] As a preferred embodiment of any of the first aspect of the present application, the electric dialysis lithium ion flux of the composite membrane is not less than 2.96 mol m -2 h -1 .

[0039] As a preferred embodiment of any of the first aspect of the present application, the lithium / potassium ion two-component separation ratio of the composite membrane is not less than 3, preferably not less than 5, and most preferably not less than 6, under the condition of an applied electric field of 0.8 V / cm.

[0040] [Method for preparing monovalent alkali metal ion separation composite membrane]

[0041] The second aspect of the present application provides a method for preparing a monovalent alkali metal ion separation composite membrane according to any of the first aspect of the present application, comprising:

[0042] Providing a two-dimensional material dispersion liquid with heteropoly acid dissolved as a casting solution; the heteropoly acid is selected from phosphotungstic acid, Keggin-type silicotungstic acid or phosphomolybdic acid, and the two-dimensional material is selected from vermiculite, graphene oxide or layered double hydroxide; in the casting solution, the mass concentration of the two-dimensional material is 0.02-0.2 mg / mL, and the mass percentage of the heteropoly acid is 0.5-10%;

[0043] Forming the monovalent alkali metal ion separation composite membrane from the casting solution.

[0044] As a preferred embodiment of any of the second aspect of the present application, the casting solution is placed and stably maintained at 2-8°C for 4-12 hours before forming the monovalent alkali metal ion separation composite membrane.

[0045] As a preferred embodiment of any of the second aspect of the present application, the heteropoly acid is Keggin-type phosphotungstic acid.

[0046] As a preferred embodiment of any of the second aspect of the present application, the two-dimensional material is vermiculite.

[0047] Applications of monovalent alkali metal ion separation composite membranes

[0048] The third aspect of this invention provides the application of the monovalent alkali metal ion separation composite membrane according to any embodiment of the first aspect of this invention in the separation of monovalent alkali metal ions, wherein the monovalent alkali metal ions include Li + It also includes selections from K + Or Na + One or two of the following. Preferably, the monovalent alkali metal ion comprises or is Li. + and K + .

[0049] As a preferred embodiment of any of the third aspects of the present invention, the application is performed in an applied electric field. Preferably, the applied electric field is 0.27V / cm to 0.8V / cm, and most preferably 0.8V / cm.

[0050] Beneficial effects

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] (1) This invention prepares a monovalent alkali metal ion separation composite membrane by combining heteropolyacids with two-dimensional materials. By selecting heteropolyacids with appropriate coordination energies to different monovalent alkali metal ions based on the magnitude of the exchange barrier, the heteropolyacids can react with Li... + The difference between the coordination energy and the binding energy of hydrated lithium ions is smaller, thus enabling rapid exchange of lithium ions between layers and forming rapid lithium ion transport channels in the membrane, allowing Li... + The transport rate in the membrane is higher than that of other monovalent alkali metal ions, thus achieving the goal of separating lithium ions with a high separation ratio.

[0053] (2) In this invention, the heteropolyacid is selected from phosphotungstic acid, Keggin-type silicotungstic acid, or phosphomolybdic acid, and the two-dimensional material is selected from vermiculite, graphene oxide, and layered bimetallic hydroxides. In this system, the transport rate of lithium ions is higher than that of potassium ions. Preferably, a monovalent alkali metal ion separation composite membrane prepared by combining Keggin-type phosphotungstic acid (PTA) with a two-dimensional material is used. Among many heteropolyacids, Keggin-type phosphotungstic acid and Li + The difference between the coordination energy and the binding energy of hydrated lithium ions is smaller, leading to rapid exchange of lithium ions between layers and the formation of rapid lithium ion transport channels in the membrane, thus enabling Li... + The transport rate in the membrane is higher than that of other monovalent alkali metal ions (potassium or sodium ions), thus achieving the goal of separating lithium ions with a high separation ratio.

[0054] (3) In the monovalent alkali metal ion separation composite membrane of the present application, the Keggin type phosphotungstic acid reaches the ideal purpose of separating monovalent alkali metal ions within a specific range of mass percentage; in some embodiments, the lithium / potassium ion two-component separation ratio reaches 6, indicating that the composite membrane has excellent monovalent alkali metal ion separation performance.

[0055] (4) The preparation process of the present application is simple, and the preparation conditions are low, which is conducive to the large-scale production and application of the monovalent alkali metal ion separation composite membrane; in addition, the monovalent alkali metal ion separation composite membrane with high separation ratio of the present application is expected to be applied in various lithium resource exploitation and extraction fields (including salt lake lithium extraction, lithium extraction using beneficiation wastewater, etc.), and improve the utilization efficiency of lithium resources. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 It is a structural schematic diagram of the monovalent alkali metal ion separation composite membrane of the present application.

[0057] Figure 2 It is a SEM cross-sectional view of PTA@V-3, D-PTA@V-5, PTA@GO-3 and PTA@LDH-3 composite membranes.

[0058] Figure 3 It is a positive osmosis test data graph of HPA@2D-x composite membrane: (a) the influence of PTA doping amount in PTA@V-x composite membrane on Li + permeability, Li + / K + ideal separation ratio (permeability ratio); (b) the influence of heteropoly acid type on Li + permeability, Li + / K + ideal separation ratio (permeability ratio): PTA@V-3, PMA@V-3, STA@V-1, D-PTA@V-5 prepared in Example 2; (c) the influence of PTA doping in PTA@GO-3 in Example 3 and PTA@LDH-3 in Example 4 on Li + / K + ideal separation ratio (permeability ratio).

[0059] Figure 4 It is a data graph of electrodialysis test of PTA@V-3 composite membrane under different applied electric fields, and the insert is the separation ratio of pure vermiculite membrane and PTA@V-3 composite membrane.

[0060] Figure 5 It is the 24-hour positive osmosis test data of PTA@V-3 composite membrane.

[0061] Figure 6Lithium-ion recognition in heteropolyacid-functionalized two-dimensional membranes. (a) Ion exchange equilibrium of three states during transport in heteropolyacid-functionalized two-dimensional membranes: hydrated ions (M... + @H2O), heteropolyacid ligand ions (M + @HPA) and ions (M) interacting with the two-dimensional membrane channel wall + @2D); (b) With M + @H2O and M + @The increase in the exchange barrier (ΔE) between HPAs leads to an exponential increase in ion permeability; (c) Li of vermiculite membranes functionalized with four heteropolyacids + / K + The difference between ideal selectivity and the calculated exchange barrier (where, and (all negative); (d) Li of three PTA-functionalized two-dimensional films + / K + Ideal selectivity, and the calculated exchange barriers for lithium-ion transport in three two-dimensional film materials (where, It is a negative value. and (All are positive values). Detailed Implementation

[0062] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0063] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0064] Reagent manufacturer:

[0065] The vermiculite was purchased from Shijiazhuang Chenxing Industrial Co., Ltd., and it is thermally expandable vermiculite.

[0066] The graphene oxide was purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., and was freeze-dried.

[0067] Phosphotungstic acid was purchased from Tianjin Xiens Biochemical Technology Co., Ltd.

[0068] Keggin-type silicotungstic acid was purchased from Tianjin Xiens Biochemical Technology Co., Ltd.

[0069] Phosphomolybdic acid was purchased from Tianjin Xiens Biochemical Technology Co., Ltd.

[0070] The metal ions are analytical grade metal chlorides.

[0071] Synthesis of layered double hydroxide (Mg-Al LDH) nanosheets:

[0072] Dissolve 20 mmol Mg(N03)2-6H20, 10 mmol Al(N03)3-9H20 and 26 mmol hexamethylenetetramine in 80 mL water, place the mixture in a high-pressure reactor and heat at 140 °C, centrifuge the precipitate three times in water and ethanol (10000 rpm, 10-15 min) and dry at room temperature. Add a mixture of 300 mL NaN03 and 5 mM HN03 in hydrochloric acid, shake in a shaker under nitrogen atmosphere for 1 day, centrifuge three times in water and ethanol (10000 rpm, 10-15 min). Add 100 mL formamide, shake in a shaker under nitrogen atmosphere for 2 days, filter to remove unexfoliated particles and obtain the formamide dispersion of Mg-Al LDH nanosheets.

[0073] Well-Dawson type phosphotungstic acid preparation: Dissolve 10 g Na2W04-2H20 and 35 mL water, heat to boiling, slowly add 15 mL of 85% H3P04, add 1 mL H202 under stirring. Reflux the resulting solution for 3 h, cool, add 10 g KCl solid, stir for 1 h, suction filter. Recrystallize with a small amount of water, filter, cool in ice water, suction filter. Collect the crystals and dry.

[0074] As used herein, the term“about” is used to provide flexibility to a given term, measurement, or value. The degree of flexibility of a particular variable will be readily determined by one of skill in the art.

[0075] Concentrations, amounts, and other numerical data can be presented herein in a range format. It is to be understood that such range format is used only for convenience and brevity and should be construed as having been followed to the right of the recited range to include any and all derived or sub-ranges therein. For example, a numerical range of“about 1 to about 4.5” should be interpreted to include not only the explicitly recited limits of about 1 to about 4.5, but also include individual values and sub-ranges within the indicated range, such as 2, 3, 4, and 1-3, 2-4, etc. The same applies to ranges reciting only one numerical value, such as“less than about 4.5”, which should be interpreted to include all of the above-referenced values and ranges. In addition, it is intended that every maximum numerical limitation given throughout this specification includes every minimum numerical limitation

[0076] Example 1 Calculation of exchange barrier

[0077] In the present invention, heteropoly acids capable of coordinating with monovalent metal ions are selected as ligands. In order to understand the lithium ion selectivity of the heteropoly acid functionalized two-dimensional membrane from a molecular point of view, a diffusion model based on the interactions of cations with recognition sites and water is established. For the cation transport of the perlite-like membrane with interlayer water, the coordination of cations with heteropoly acid ligands and the exchange between interlayer water and the interaction between cations and the capillary walls of the two-dimensional channel should be considered.

[0078] The compensation between the coordination and hydration of cations is considered as the following exchange equilibrium (equation (9)), as shown in equation (10), the ion permeability (k) obtained by experiment increases exponentially with the corresponding exchange barrier (ΔE) (equation (11)):

[0079]

[0080] k∝exp(ΔE)(10)

[0081]

[0082] The competition between the coordination process and the hydration process becomes an obstacle to the migration of cations, so the lower the exchange barrier (|ΔE|), the easier the transport of cations in the heteropoly acid functionalized two-dimensional membrane.

[0083] Heteropoly acids include two types: Keggin type and Wells-Dawson type, such as Keggin type phosphotungstic acid (H3PW 12 O 40 , PTA), phosphomolybdic acid (Keggin type, H3PMo 12 O 40 , PMA), silicotungstic acid (Keggin type, H4SiW 12 O 40 , STA) and Wells-Dawson type phosphotungstic acid (α-K6P2W 18 O 62 , D-PTA). Keggin type heteropoly acid anion X + nM 12 O 40 - (wherein M generally refers to tungsten or molybdenum, and X can be almost any other cation, such as phosphorus, silicon, etc.) is composed of a central heteroatom (X) and four groups of three M-O octahedrons. The formula of Wells-Dawson type heteropoly acid anion is X + nM 18 O 62 - which can be regarded as the combination of two Keggin units. The heteropoly acid nanocage has a cavity which can coordinate with monovalent cations (with a diameter generally between ).

[0084] The functionalized two-dimensional nanochannels are constructed from nanosheets such as vermiculite (V), graphene oxide (GO), and layered double hydroxides (LDH), which all exhibit non-selective but fast monovalent cation transport, the interaction between cations and the two-dimensional channel capillary wall is much smaller than the coordination between cations and heteropolyacid ligands, so the specific numerical influence of the interaction between cations and the two-dimensional channel capillary wall can be ignored in the heteropolyacid selection of the composite membrane of the present application.

[0085] By functionalizing the two-dimensional nanochannels with different types of heteropolyacids, the interaction between monovalent cations and heteropolyacids and interlayer water is studied to determine the system that can be effectively used for the separation of monovalent alkali metal ions.

[0086] Therefore, in order to achieve the purpose of high separation ratio of monovalent alkali metal ions, the selection of heteropolyacids should meet formula (1):

[0087]

[0088] wherein, is the exchange potential barrier of Li+ coordinated with heteropolyacids and Li+ combined with water:

[0089]

[0090] wherein, is the coordination energy of Li + combined with heteropolyacids; is the hydration energy of Li + combined with water;

[0091]

[0092] and E HPA are the Gibbs free energies of lithium ion-heteropolyacid complex, bare lithium ion, and heteropolyacid, respectively;

[0093]

[0094] wherein, and are the Gibbs free energies of hydrated lithium ion, bare lithium ion, and water, respectively;

[0095] is the exchange potential barrier of M + combined with heteropolyacids and M + combined with water:

[0096]

[0097] wherein, is the coordination energy of M+ Coordination energy with heteropoly acid; M + Hydration energy with water;

[0098]

[0099] and E HPA are the Gibbs free energies of M ion-heteropoly acid complex, bare M ion and heteropoly acid, respectively;

[0100]

[0101] wherein, and are the Gibbs free energies of hydrated M ion, bare M ion and water, respectively;

[0102] M + is selected from K + or Na + .

[0103] Specifically, the calculation method: all first-principles calculations are completed by the Vienna Ab initio Simulation Package (VASP). The ion-electron interaction is described by the projector augmented wave (PAW) method. The interaction of the electron exchange-correlation energy is treated by the generalized gradient approximation (GGA) combined with the Perdew-Burke-Ernzerhof (PBE) method. In this work, the Gamma-centered k-point grid is used, and the cutoff energy is set to 520 eV. The energy convergence criterion and the force convergence criterion are set to 1 × 10-5eV and 0.01 eV / Å, respectively. The model of the metal hydration energy in water is constructed using the Packing function in Material Studio, and the calculation conditions are the same as above. Li + and PTA system as an example:

[0104]

[0105]

[0106]

[0107]

[0108] E PTA = -463.0112 eV According to the formula:

[0109]

[0110]

[0111]

[0112]

[0113] According to the formula:

[0114]

[0115]

[0116] The same method is used to calculate

[0117]

[0118] The results show that the PTA strategy is adopted, Indicating that the transmission rate of lithium ions between the layers of two-dimensional materials is greater than that of potassium ions and sodium ions.

[0119] On this basis, the application studies and calculates the transmission rates of Li + , K + in different heteropoly acid systems:

[0120] In the PMA system, Indicating that the transmission rate of lithium ions between the layers of two-dimensional materials is greater than that of potassium ions.

[0121] In the STA system, Indicating that the transmission rate of lithium ions between the layers of two-dimensional materials is greater than that of potassium ions.

[0122] In the D-PTA system, Indicating that the transmission rate of lithium ions between the layers of two-dimensional materials is greater than that of potassium ions.

[0123] The following will be verified by experiments, and the high-efficiency separation composite membrane of monovalent alkali metal ions is implemented according to the above scheme.

[0124] Example 2 HPA@V composite membrane

[0125] 1. Preparation of the membrane

[0126] The embodiment provides a method for preparing a high-separation-ratio monovalent alkali metal ion separation composite membrane by doping inorganic materials (taking heteropoly acid as the inorganic material and taking vermiculite (V) as a two-dimensional material), and specific steps are as follows:

[0127] Step 1: Preparation of the casting solution

[0128] The monolayered nanosheets of vermiculite are prepared by ion intercalation method, which comprises the following steps:

[0129] A certain amount of heat-expandable vermiculite powder is dissolved in saturated sodium chloride solution, heated at 100°C and stirred at 1000 rpm for 48 hours. The obtained sodium-exchanged vermiculite filter cake is washed with deionized water, heated and stirred in 2M lithium chloride solution for 24 hours. The obtained lithium-exchanged vermiculite filter cake is dispersed in deionized water and ultrasonicated for 30 minutes. Finally, the colloidal solution is obtained by centrifuging the lithium-exchanged vermiculite filter cake for 10 times at 5000 rpm for 20 minutes each time, and the monolayered nanosheet dispersion solution is obtained by adding an appropriate amount of water. In this embodiment, the lithium-exchanged vermiculite filter cake is dispersed in deionized water and ultrasonicated for 30 minutes, centrifuged at 3000 rpm for 10 minutes to remove the precipitate, and centrifuged at 10000 rpm for 10 minutes to remove the supernatant, for a total of 8 times of centrifugation to obtain the colloidal solution.

[0130] The monolayered nanosheet dispersion solution (0.02 mg / ml) prepared in step one is doped with heteropoly acid at a certain mass percentage to obtain a casting solution with a total mass of 20 g, wherein the mass percentage of heteropoly acid in the total mass of the casting solution is 0.5-10 wt%. The above solution is mixed uniformly and kept stable at 4°C for 8 hours to obtain the HPA@V-x casting solution, wherein HPA represents heteropoly acid, V represents monolayered nanosheet of vermiculite, and x represents the mass percentage of heteropoly acid in the total mass of the casting solution.

[0131] Step two: preparation of high separation ratio monovalent cation separation membrane

[0132] The porous nylon-66 membrane is placed on a suction filtration device, and the HPA@V-x casting solution obtained in step one is added and suction filtered to obtain the monovalent alkali metal ion separation composite membrane. The thickness of the monovalent alkali metal ion separation composite membrane can be 0.5-5 μm, and in this embodiment, the thickness of the monovalent alkali metal ion separation composite membrane is about 0.5 μm.

[0133] In one embodiment, other conditions are the same as the foregoing, except that the monolayered nanosheet of vermiculite with a concentration of 0.02 mg / ml is used to form a membrane by suction filtration, and the obtained membrane is denoted as V membrane.

[0134] In one embodiment, other conditions are the same as the foregoing, except that the monolayered nanosheet of vermiculite with a concentration of 0.02 mg / ml is used, and the heteropoly acid used is PTA, and the addition amount of PTA accounts for 0.5% of the total mass of the casting solution, and the monovalent alkali metal ion separation composite membrane obtained is denoted as PTA@V-0.5 membrane.

[0135] In one embodiment, the other conditions are the same as the foregoing, except that the monolayer vermiculite nanosheet with a concentration of 0.02 mg / ml is used, the heteropoly acid is PTA, the mass percentage of PTA in the total mass of the casting solution is 1%, and the obtained monovalent alkali metal ion separation composite membrane is recorded as PTA@V-1 membrane.

[0136] In one embodiment, the other conditions are the same as the foregoing, except that the monolayer vermiculite nanosheet with a concentration of 0.02 mg / ml is used, the heteropoly acid is PTA, the mass percentage of PTA in the total mass of the casting solution is 3%, and the obtained monovalent alkali metal ion separation composite membrane is recorded as PTA@V-3 membrane. As shown in FIG. 3, it is a structural schematic diagram of the inorganic material doped high separation ratio monovalent alkali metal ion separation composite membrane of the application. Figure 1 Figure 2 As shown in FIG. 4, the PTA@V-3 membrane presents a typical layered stacking structure and is well stacked.

[0137] In one embodiment, the other conditions are the same as the foregoing, except that the monolayer vermiculite nanosheet with a concentration of 0.02 mg / ml is used, the heteropoly acid is PTA, the mass percentage of PTA in the total mass of the casting solution is 5%, and the obtained monovalent alkali metal ion separation composite membrane is recorded as PTA@V-5 membrane.

[0138] In one embodiment, the other conditions are the same as the foregoing, except that the monolayer vermiculite nanosheet with a concentration of 0.02 mg / ml is used, the heteropoly acid is PTA, the mass percentage of PTA in the total mass of the casting solution is 10%, and the obtained monovalent alkali metal ion separation composite membrane is recorded as PTA@V-10 membrane.

[0139] In one embodiment, the other conditions are the same as the foregoing, except that the monolayer vermiculite nanosheet with a concentration of 0.02 mg / ml is used, the heteropoly acid is PMA, the mass percentage of PMA in the total mass of the casting solution is 3%, and the obtained monovalent alkali metal ion separation composite membrane is recorded as PMA@V-3 membrane.

[0140] In one embodiment, the other conditions are the same as the foregoing, except that the monolayer vermiculite nanosheet with a concentration of 0.02 mg / ml is used, the heteropoly acid is STA, the mass percentage of STA in the total mass of the casting solution is 1%, and the obtained monovalent alkali metal ion separation composite membrane is recorded as STA@V-1 membrane.

[0141] ​In one embodiment, other conditions are the same as the foregoing, except that monolayered vermiculite nanosheets with a concentration of 0.02 mg / ml are used, the heteropoly acid is D-PTA, and the mass percentage of D-PTA in the total mass of the casting solution is 5%, and the obtained monovalent alkali metal ion separation composite membrane is denoted as D-PTA@V-5 membrane. As shown in FIG. 6b, the SEM shows that the D-PTA@V-5 composite membrane also presents a typical layered stacking structure; as shown in FIG. 6c, in the positive osmosis test, the D-PTA@V-5 composite membrane has a better lithium ion permeability than potassium ion. Figure 2 b, the SEM shows that the D-PTA@V-5 composite membrane also presents a typical layered stacking structure; as shown in FIG. 6c, in the positive osmosis test, the D-PTA@V-5 composite membrane has a better lithium ion permeability than potassium ion. Figure 3 b, the SEM shows that the D-PTA@V-5 composite membrane also presents a typical layered stacking structure; as shown in FIG. 6c, in the positive osmosis test, the D-PTA@V-5 composite membrane has a better lithium ion permeability than potassium ion.

[0142] 2. Membrane characterization

[0143] The composite membranes with different doping ratios obtained in step two were dried, and according to the SEM cross-sectional images of some of the composite membranes, the stacking of the composite membranes with different ratios was known.

[0144] 3. Performance test of the membrane

[0145] The composite membranes with different doping ratios were subjected to single-ion system positive osmosis test and binary-ion system electrodialysis test.

[0146] (1) Positive osmosis test:

[0147] The positive osmosis test was performed using an H-type sample cell. The composite membrane was sealed in the middle of the sample cell with a clamp. 0.1M aqueous chloride salt solution (lithium chloride, sodium chloride or potassium chloride) was placed on the feed side, and the membrane faced the feed side. An equal volume of deionized water was added to the permeate side and stirred. The conductivity of the solution on the permeate side was recorded once every minute by a conductivity meter (SevenDirect SD30). The calculation formula of the ion permeability in the single-ion system is:

[0148]

[0149] C t represents the ion concentration of the permeate side solution at the sampling time (mol L -1 ), C0represents the initial ion concentration of the permeate (mol L -1 ), V represents the volume of deionized water on the permeate side (L), A represents the effective membrane area (m -2 ), and t represents the test time interval (h). Three different samples of each type of membrane were tested to obtain the average value of the permeability and evaluate the standard deviation.

[0150] Selectivity in single-ion system The calculation formula is:

[0151]

[0152] M + represents Na+ and K + ;

[0153] (2) Electrodialysis test:

[0154] As an example, the PTA@V-3 membrane obtained in Step two was used to separate four chambers for electrodialysis, together with two commercial anion exchange membranes (FAA-50, Fumatech GmbH). The PTA@V-3 membrane was placed in the middle, facing the anode chamber. The binary ion system electrodialysis test was carried out at an external electric field of 0.2-0.8 V / cm, respectively. The cathode chamber and the anode chamber were filled with 200 mL of 0.3 M sodium sulfate solution. The dilution chamber was filled with 200 mL of 0.05 M lithium sulfate / potassium sulfate solution, and the concentration chamber was filled with 200 mL of 0.0025 M sulfuric acid solution. The mass concentration of lithium ions and potassium ions in the concentration chamber was analyzed by ion chromatography (ECOIC, Metrohm). Before the test, a series of electrolyte solutions with different concentrations were prepared, and a calibration curve was obtained by ion chromatography. The formula for calculating the ion permeability of the binary ion system is as follows:

[0155]

[0156]

[0157] representing the mass permeability (g m -2 h -1 ). P i representing the molar permeability (mol m -2 h -1 ), p t the ion mass concentration in the concentration chamber (ppm) at the sampling time, p0 represents the initial ion mass concentration (ppm), M i represents the atomic molar mass (g mol -1 ), A represents the effective membrane area (m -2 ), and t represents the test time interval (h).

[0158] The separation factor of the binary ion system is calculated as follows:

[0159]

[0160]

[0161] Similarly, three different membrane samples were tested to obtain the average value of the permeability and evaluate the standard deviation.

[0162] As Figure 3As shown in 3a and 3b, it can be seen that the preferred doped inorganic material for the HPA@Vx composite membrane prepared in this embodiment is PTA, and the preferred doping mass percentage of the PTA@Vx composite membrane is 3%. The ion permeability of each membrane under the same conditions in the single-component forward osmosis experiment is:

[0163] Table 1. Ion permeability of single-component forward osmosis experiments of composite membranes

[0164]

[0165]

[0166] The results showed that in the single-component forward osmosis ion permeability experiment, when the PTA content in the casting solution was between 0% and 10% by mass, the lithium ion permeability was higher than that of potassium ions. However, when the PTA content in the casting solution was 0%, 0.5%, and 10% by mass, the ratio of lithium ion to potassium ion permeability was less than 3. Conversely, when the PTA content in the casting solution was 1%, 3%, and 5% by mass, the ratio of lithium ion to potassium ion permeability was greater than 3, reaching its highest value at 3% PTA content. When the PTA content in the casting solution was 0% and 3% by mass, the lithium ion permeability was higher than that of sodium ion permeability. However, when the PTA content in the casting solution was 0% by mass, the difference between lithium ion and sodium ion permeability was very small. At 3% PTA content, the ratio of lithium ion to sodium ion permeability reached over 8.

[0167] The ion separation ratio in the two-component electrodialysis experiment under the same conditions as PTA@V-3 and V membranes was:

[0168] Table 2. Ion separation ratio of the composite membrane in two-component electrodialysis (0.8V / cm applied electric field)

[0169] Membrane Separation factor for lithium / potassium ions V 1.43 PTA@V-3 6.07

[0170] like Figure 4 As shown, the separation factor of the PTA@V-3 composite membrane in the binary ion system gradually increases with the increase of the applied electric field; under the condition of an applied electric field of 0.8 V / cm, the separation factor of lithium / potassium ions is as high as 6.07 (Table 2), and under this condition, the lithium ion flux of the PTA@V-3 composite membrane in electrodialysis is 2.96 mol / m³. -2 h -1 .

[0171] 4. Stability test of PTA@V-3 composite membrane

[0172] The stability of the composite membrane was evaluated by subjecting it to a single-ion system forward osmosis test for up to 24 hours as described in step four.

[0173] As Figure 5 shown, the lithium ion permeability and lithium / potassium ion separation ratio of the PTA@V-3 composite membrane remain stable in the 24-hour test.

[0174] Example 3 HPA@GO composite membrane

[0175] The present example provides a method for preparing a monovalent cation separation composite membrane with high separation ratio by doping inorganic materials (taking PTA as the inorganic material and graphene oxide as the two-dimensional material).

[0176] The freeze-dried graphene oxide nanosheets were dispersed in deionized water (0.02 mg / ml) and stirred overnight. The dispersion was centrifuged several times to remove the precipitate to obtain a colloid, and an appropriate amount of water was added to obtain a single-layer graphene oxide nanosheet dispersion, which was mixed with phosphotungstic acid at a certain mass percentage (3 wt%). The mixture was suction filtered to obtain the PTA@GO-3 composite membrane. The above characterization and performance test were performed.

[0177] As Figure 2 c shows, SEM shows that the PTA@GO-3 composite membrane is well stacked.

[0178] As Figure 3 c shows, the separation performance of the PTA@GO-3 composite membrane prepared in the present example in the mono-ion positive osmosis system can be obtained. Among them, the GO membrane without doping PTA cannot separate Li + and K + in the single-component system, while the PTA@GO-3 composite membrane doped with 3% PTA has a Li + / K + separation ratio of about 4 (lithium ion permeability: potassium ion permeability is 4:1), and the separation ratio is significantly improved after doping.

[0179] Example 4 HPA@LDH composite membrane

[0180] The present example provides a method for preparing a monovalent alkali metal ion separation composite membrane with high separation ratio by doping inorganic materials (taking PTA as the inorganic material and layered double hydroxide as the two-dimensional material).

[0181] The formamide dispersion solution (0.02 mg / ml) of Mg-Al layered double hydroxide nanosheets was mixed with phosphotungstic acid at a certain mass percentage (3 wt%), and suction filtered to obtain the PTA@LDH-3 cation separation membrane. The above characterization and performance test were performed.

[0182] As Figure 2 d shows, SEM shows that the PTA@LDH-3 composite membrane is tightly stacked.

[0183] AsFigure 3 As shown in Figure c, the separation performance of the PTA@LDH-3 composite membrane prepared in this embodiment in a unary ion system can be observed. Specifically, the LDH membrane without PTA doping is almost unable to separate Li in a single-component system. + and K + The PTA@LDH-3 composite film with 3% PTA doping exhibits superior performance in a single-component system compared to Li. + / K + The separation ratio is approximately 3 (lithium ion penetration: potassium ion penetration is 3:1), and the separation ratio is significantly improved after doping.

[0184] The results obtained under the theoretical guidance of Example 1 are combined with the experiments in Example 2, and the analysis results are as follows:

[0185] With Li + and K + Taking the selectivity in PTA as an example, the exchange barrier between lithium ions and PTA was calculated in Example 1. It is -1.90 eV, and its absolute value is compared to The absolute value is 1.62 eV lower ( (-3.52 eV), while in the experiment of Example 2, it was demonstrated that Li + / K + The selectivity (penetration ratio) can reach up to 16.

[0186] According to the method calculated in Example 1, as M... + @H2O and M + The increase in the exchange barrier (ΔE) between @HPA membranes affects the Mg content of the PTA@V-3 membrane. 2+ K + Na + Ca 2+ Li + Ion penetration increases exponentially.

[0187] In the D-PTA@V system, as calculated in Example 1, The absolute value is only greater than The absolute value is 0.34 eV lower, while the experiment in Example 2 showed that Li + / K + Selectivity (penetration ratio) is only 2.23. Figure 6 c). Therefore, the selectivity between monovalent cations in functionalized vermiculite membranes mainly depends on the exchange barrier between coordination and hydration processes. Li + The exchange barrier is less than K + When exchanging barriers, Li + The transmission rate is greater than K + Li + and K+ The greater the difference in exchange barrier, i.e. The greater the absolute value of the difference, the higher the selectivity of Li + / K + .

[0188] When the same percentage of heteropoly acid is used to prepare a composite membrane (PTA 3%), the calculation results show that Li + The exchange barrier for lithium ions entering the channel of vermiculite for transmission is negative and the lowest among the three materials, so lithium ions are more likely to enter the channel of vermiculite formed by the two-dimensional material Figure 6 d), indicating that vermiculite as a two-dimensional material has a synergistic effect in the lithium ion recognition of the heteropoly acid functionalized membrane.

[0189] Therefore, to construct a lithium ion recognition membrane, it is necessary to reduce the exchange barrier between the lithium ion hydration state and its coordination state with heteropoly acid and its interaction state with two-dimensional nanosheets. In summary, the experimental verification results show that among the four heteropoly acids, the most preferred heteropoly acid ligand for the construction of a monovalent alkali metal ion separation membrane system constructed by vermiculite / GO / LDH two-dimensional materials is PTA, which is consistent with the guidance in Example 1.

[0190] In addition to the above examples, other heteropoly acid materials can also be replaced for doping, such as Keggin-type silicotungstic acid, phosphomolybdic acid, and other configurations of heteropoly acid, etc., as long as they meet the principle of the difference in exchange barrier of Li + and M + , they can be used for the construction of a monovalent alkali metal ion separation composite membrane.

[0191] The above content is a schematic description of the present application and its embodiments, which is not restrictive. The embodiments shown in the examples are only one of the embodiments of the present application, and the actual embodiments are not limited thereto. Therefore, if a person skilled in the art is inspired thereby, without departing from the spirit of the present application, similar embodiments and examples can be designed without creative design, which shall fall within the protection scope of the present application.

Claims

1. Use of a monovalent alkali metal ion separation composite membrane in monovalent alkali metal ion separation, characterized in that, said monovalent alkali metal ion comprises Li + , further comprising one or both of K + , or Na + . the monovalent alkali metal ion separation composite membrane comprises a two-dimensional material and a heteropoly acid distributed between layers of the two-dimensional material; the two-dimensional material is selected from vermiculite, graphene oxide or layered double hydroxide; an exchange barrier of the heteropoly acid and the monovalent alkali metal ion should satisfy the following relationship: ∣ ∣＜∣ ∣(1) wherein, Li coordinated with heteropoly acid + and Li combined with water + exchange barrier: (2) wherein, is Li + coordination energy with heteropoly acid; is Li + hydration energy with water; , and are the Gibbs free energies of the lithium ion-heteropolyacid complex, the bare lithium ion, and the heteropolyacid, respectively; wherein, , and are the Gibbs free energies of hydrated lithium ions, bare lithium ions, and water, respectively; M coordinated with a heteropoly acid + and M combined with water + exchange barrier: wherein, is M + coordination energy of the ion with the heteropoly acid; is M + hydration energy of the ion with water; , and are the Gibbs free energies of the M ion-heteropolyacid complex, the bare M ion, and the heteropolyacid, respectively; wherein, , and are the Gibbs free energies of hydrated M ions, bare M ions, and water, respectively; M + selected from K + or Na + ; The lithium ion permeation rate:potassium ion permeation rate of the monovalent alkali metal ion separation composite membrane is (2-18): 1; the lithium ion permeation rate of the monovalent alkali metal ion separation composite membrane is 0.3-1.3 mol m -2 h -1 .

2. Use of a monovalent alkali metal ion separating composite membrane in the separation of monovalent alkali metal ions, characterized in that, the monovalent alkali metal ion separation composite membrane comprises a two-dimensional material and a heteropoly acid distributed between layers of the two-dimensional material; the heteropoly acid is selected from phosphotungstic acid, Keggin-type silicotungstic acid or phosphomolybdic acid, and the two-dimensional material is selected from vermiculite, graphene oxide or layered double hydroxide; said monovalent alkali metal ion comprises Li + , further comprising one or both of K + , or Na + . The lithium ion permeation rate:potassium ion permeation rate of the monovalent alkali metal ion separation composite membrane is (2-18): 1; the lithium ion permeation rate of the monovalent alkali metal ion separation composite membrane is 0.3-1.3 mol m -2 h -1 .

3. Use according to claim 2, characterized in that, the two-dimensional material is selected from vermiculite, and the heteropoly acid is Keggin-type phosphotungstic acid.

4. Use according to claim 2 or 3, characterized in that, The mass percentage of the heteropoly acid in the monovalent alkali metal ion separation composite membrane is not higher than 10%.

5. Use according to claim 4, characterized in that, The thickness of the monovalent alkali metal ion separation composite membrane is 0.5-5 μm.

6. Use according to claim 5, characterized in that, The lithium / potassium ion two-component separation ratio of the monovalent alkali metal ion separation composite membrane is not lower than 6 under the condition of an external electric field of 0.8 V / cm.

7. Use according to claim 2, characterized in that, The preparation method of the monovalent alkali metal ion separation composite membrane comprises the following steps: a two-dimensional material dispersion liquid in which a heteropoly acid is dissolved is provided as a casting solution; the heteropoly acid is selected from phosphotungstic acid, Keggin-type silicotungstic acid or phosphomolybdic acid, and the two-dimensional material is selected from vermiculite, graphene oxide or layered double hydroxide; in the casting solution, the mass concentration of the two-dimensional material is 0.02-0.2 mg / mL, and the mass percentage of the heteropoly acid is 0.5-10%; the casting solution is made into a monovalent alkali metal ion separation composite membrane.

8. Use according to claim 7, characterized in that, Before the monovalent alkali metal ion separation composite membrane is made, the casting solution is placed at 2-8 ℃ for 4-12 hours.

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