TWI846324B - Nanofiltration membrane and manufacturing method thereof - Google Patents

Abstract

A method for manufacturing a nanofiltration membrane includes the following operations. First, a polybenzimidazole solution is cast on a substrate to form a polybenzimidazole liquid membrane on the substrate. Next, a pH of a coagulation bath is adjusted from 1 to 13. The polybenzimidazole liquid membrane is then soaked into the coagulation bath for phase inversion, thereby forming a polybenzimidazole nanofiltration membrane.

Description

Nanofiltration membrane and manufacturing method thereof

The present invention relates to a nano-filter membrane, in particular to a polybenzimidazole nano-filter membrane and a method for manufacturing the same.

In recent years, with the rapid growth in demand for personal electronic devices, the research and development of lithium-ion batteries as energy storage devices has been continuously upgraded because of their advantages such as light weight, high energy storage, high reactivity and non-toxicity. This has brought about the urgency of efficient methods for extracting Li ⁺ from saline solutions (including seawater and groundwater) to meet market demand. However, most saline solutions have a higher magnesium to lithium mass ratio, which makes it challenging to separate Li ⁺ from ^Mg2+ . To date, advances in membrane technology have become a viable option for ion separation applications.

Nanofiltration (NF) is one of the most promising membrane separation processes for water treatment and desalination. It is a pressure-driven process that selectively rejects certain components due to various repulsive forces such as physical sieving, the Gibbs-Donnan effect, and electrostatic repulsion. These mechanisms favor Li ⁺ / ^Mg2+ separations because most multivalent Mg2 ⁺ cations can be rejected while still allowing Li ⁺ cations to pass through the membrane pores. Unlike reverse osmosis (RO), NF can be performed at low to moderate pressures while still providing higher selectivity than ultrafiltration (UF). Additionally, it can retain valuable minerals in the water that are needed by humans or for specific applications that cannot be achieved by reverse osmosis.

One aspect of the present invention is to provide a method for manufacturing a nanofiltration film capable of separating lithium ions and magnesium ions. The method for manufacturing the nanofiltration film comprises the following operations. First, a polybenzimidazole solution is cast on a substrate to form a polybenzimidazole liquid film on the substrate. Then, the pH value of the coagulation bath is adjusted to 1 to 13. The polybenzimidazole liquid film is then immersed in the coagulation bath for phase inversion, thereby forming a polybenzimidazole nanofiltration film.

According to certain embodiments of the present invention, the concentration of the polybenzimidazole solution is 15 wt % to 25 wt %.

According to certain embodiments of the present invention, the method for manufacturing the nanofiltration membrane further comprises immersing the polybenzimidazole nanofiltration membrane in a glycerol aqueous solution.

According to certain embodiments of the present invention, the volume ratio of glycerol to water in the glycerol aqueous solution is 1:1.

According to certain embodiments of the present invention, the method for manufacturing the nanofiltration membrane further comprises immersing the polybenzimidazole nanofiltration membrane in a surface modification solution to form a surface-modified polybenzimidazole nanofiltration membrane, wherein the surface modification solution comprises a hyperbranched polyethyleneimine polymer.

According to certain embodiments of the present invention, the hyperbranched polyethyleneimine polymer has a molecular weight of 10,000 g/mole to 50,000 g/mole.

According to certain embodiments of the present invention, the method for manufacturing the nanofiltration membrane further comprises immersing the surface-modified polybenzimidazole nanofiltration membrane in an isopropyl alcohol aqueous solution.

Another aspect of the present invention is to provide a nano-filter membrane capable of separating lithium ions and magnesium ions, such as the nano-filter membrane manufactured by the above-mentioned nano-filter membrane manufacturing method.

Another aspect of the present invention is to provide a nanofiltration membrane capable of separating lithium ions and magnesium ions. The nanofiltration membrane comprises a polybenzimidazole porous nanofiltration membrane body having the following structure: , .

According to certain embodiments of the present invention, the nanofiltration membrane further comprises a hyperbranched polyethyleneimine polymer located in the porous structure of the polybenzimidazole porous nanofiltration membrane body.

In order to make the description of the disclosure more detailed and complete, the following provides an illustrative description of the implementation and specific embodiments of the present invention; however, this is not the only form of implementing or using the specific embodiments of the present invention. The embodiments disclosed below can be combined or replaced with each other under beneficial circumstances, and other embodiments can be added to one embodiment without further recording or description.

In order to make the description of the present disclosure more detailed and complete, reference may be made to the attached drawings and various embodiments described below, in which the same numbers in the drawings represent the same or similar elements.

As used herein, "about", "approximately" or "roughly" generally refers to a numerical value with an error or range of about 20%, preferably about 10%, and more preferably about 5%. If not explicitly stated in the text, the numerical values mentioned are deemed to be approximate values, that is, the error or range indicated by "about", "approximately" or "roughly".

Unless otherwise clearly indicated in the context, singular terms used herein include plural referents. By referring to a specific reference such as "one embodiment", a specific feature, structure or characteristic is indicated in at least one of the embodiments of the present invention, so that when such a phrase "in one embodiment" appears in various places by a specific reference, it is not necessary to refer to the same embodiment. Furthermore, in one or more embodiments, these specific features, structures, or characteristics can be combined with each other as appropriate.

One aspect of the present invention is to provide a method for manufacturing a nanofiltration film, which utilizes a nonsolvent induced phase separation (NIPS) method to manufacture an asymmetric flat nanofiltration film, because this method is simple, environmentally friendly and can produce uniform nanofiltration films. FIG. 1 shows a method 10 for manufacturing a nanofiltration film according to multiple embodiments of the present invention. FIG. 2 shows a schematic diagram of a certain stage in the method for manufacturing a nanofiltration film according to multiple embodiments of the present invention. FIG. 3 shows a schematic diagram of a certain stage in the method for manufacturing a nanofiltration film according to multiple embodiments of the present invention. As shown in FIG. 1, method 10 includes step 110, step 120, and step 130.

Please refer to FIG. 1 and FIG. 2 at the same time. In step 110, a polybenzimidazole (PBI) solution 210 is cast on a substrate 220 to form a polybenzimidazole liquid film 230 on the substrate 220. Specifically, the polybenzimidazole solution 210 can be cast on the substrate 220 using a casting blade to form the polybenzimidazole liquid film 230.

In some embodiments, the concentration of the polybenzimidazole solution 210 is 15 wt % to 25 wt %. More specifically, the polybenzimidazole solution 210 includes polybenzimidazole as a solute and N,N-dimethylacetamide (DMAc) as a solvent. In addition, the polybenzimidazole solution 210 also includes lithium chloride (LiCl) as a stabilizer. In some embodiments, the substrate 220 includes polyamide (or nylon). For example, the substrate 220 includes nonwoven 6,6-nylon.

Please refer to FIG. 1 and FIG. 3 at the same time. In step 120, the pH value of the coagulation bath is adjusted to 1 to 13. In some embodiments, the coagulation bath 240 mainly includes deionized water (DI). For example, hydrochloric acid (HCl) can be added to the deionized water to adjust the coagulation bath 240 to acidity (1≤pH＜7), for example, the pH value of the coagulation bath 240 can be 1, 2, 3, 4, 5 or 6. For another example, sodium hydroxide (NaOH) can be added to the deionized water to adjust the coagulation bath 240 to alkalinity (7＜pH≤13), for example, the pH value of the coagulation bath 240 can be 8, 9, 10, 11, 12 or 13.

Please refer to FIG. 1 and FIG. 3 at the same time. In step 120, the polybenzimidazole liquid membrane 230 is immersed in a coagulation bath 240 for phase inversion, thereby forming a polybenzimidazole nanofiltration membrane 250. In some embodiments, step 120 is performed at room temperature, such as 25±0.5°C. It can be understood that the generated polybenzimidazole nanofiltration membrane 250 is a porous flat membrane. In some embodiments, the molecular weight cut off (MWCO) of the polybenzimidazole nanofiltration membrane 250 is 1845 to 1911 Dalton (Da).

In some embodiments, the method 10 for manufacturing a nanofiltration membrane further comprises soaking the polybenzimidazole nanofiltration membrane 250 in a glycerol aqueous solution. Then, the polybenzimidazole nanofiltration membrane 250 is subjected to a drying process. In some embodiments, the volume ratio of glycerol to water in the glycerol aqueous solution is 1:1. This is to prevent the pores of the polybenzimidazole nanofiltration membrane 250 from collapsing during the subsequent drying process.

In some embodiments, the method 10 for manufacturing a nanofiltration membrane further comprises immersing the polybenzimidazole nanofiltration membrane 250 in a surface modification solution to form a surface modified polybenzimidazole nanofiltration membrane. Specifically, the surface modification solution comprises ethanol as a solvent and a hyperbranched polyethyleneimine (HPEI) polymer as a solute. For example, the concentration of HPEI in the surface modification solution can be 1wt%. It is understood that the hyperbranched polyethyleneimine polymer can be used as a pore sealing agent. The method for manufacturing the nanofiltration membrane disclosed herein uses an ethanol solution of hyperbranched polyethyleneimine to achieve green and environmentally friendly surface modification because the toxicity of ethanol is much lower than other organic solvents, such as acetonitrile, N-methyl-2-pyrrolidone (NMP) and 2-propanol.

In some embodiments, the hyperbranched polyethyleneimine polymer has a molecular weight of 10,000 g/mole to 50,000 g/mole. For example, the molecular weight of the hyperbranched polyethyleneimine polymer can be 15,000 g/mole, 20,000 g/mole, 25,000 g/mole, 30,000 g/mole, 35,000 g/mole, 40,000 g/mole or 45,000 g/mole. According to many embodiments, when the molecular weight of the hyperbranched polyethyleneimine polymer is greater than a certain value, such as 50,000 g/mole, it is difficult to adhere to the surface of the polybenzimidazole nanofiltration membrane 250, and it is difficult to play the role of surface modification. On the contrary, when the molecular weight of the hyperbranched polyethyleneimine polymer is less than a certain value, such as 10000 g/mole, the hyperbranched polyethyleneimine polymer cannot effectively seal the defective pores of the polybenzimidazole nanofiltration membrane 250. In some embodiments, the molecular weight cutoff (MWCO) of the surface-modified polybenzimidazole nanofiltration membrane is 288KD.

In some embodiments, the method 10 for manufacturing the nanofiltration membrane further includes immersing the surface-modified polybenzimidazole nanofiltration membrane in an isopropyl alcohol aqueous solution. The purpose of this is to clean the unreacted polymer on the surface of the surface-modified polybenzimidazole nanofiltration membrane. Then, the surface-modified polybenzimidazole nanofiltration membrane is subjected to a drying process.

It should be understood that although the above method 10 shows many steps, actions and/or features, not all of the above operations, actions and/or features are required, and there may be other operations, actions and/or features that are not shown. In some implementations, the order of the above operations and/or actions may be different from that shown in Figure 1. In addition, in some implementations, the actions shown may be further divided into multiple sub-actions, while in other implementations, some of the actions shown may be performed simultaneously with each other.

Another aspect of the present invention is to provide a nanofiltration membrane, which is a nanofiltration membrane made by the nanofiltration membrane manufacturing method 10 as described above, and the nanofiltration membrane can improve the separation efficiency of lithium ions and magnesium ions. More specifically, the nanofiltration membrane includes a polybenzimidazole porous nanofiltration membrane body, which has the following structure: or .

When the nanofiltration membrane undergoes phase change in an acidic (1≤pH＜7) coagulation bath during the manufacturing process, the resulting polybenzimidazole porous nanofiltration membrane has structure.

When the nanofiltration membrane undergoes phase change in an alkaline (7＜pH≤13) coagulation bath during the manufacturing process, the resulting polybenzimidazole porous nanofiltration membrane has structure.

When the nanofiltration membrane undergoes phase change in a neutral (pH=7) coagulation bath during the manufacturing process, the resulting polybenzimidazole porous nanofiltration membrane has structure.

In some embodiments, the nanofiltration membrane further comprises a hyperbranched polyethyleneimine polymer located on the polybenzimidazole porous nanofiltration membrane body and in the porous structure of the polybenzimidazole porous nanofiltration membrane body.

The following experimental examples are used to illustrate some aspects of the present disclosure in detail so that a person skilled in the art can implement the present disclosure. The experimental examples described below are intended to enable a person skilled in the art to better understand the present disclosure and its technical effects. The experimental examples and comparative examples herein are not intended to limit the present disclosure.

Experimental Example 1: The polybenzimidazole nanofiltration membranes (represented by Examples 1, 2, 3, 4 and 5, respectively) produced by phase inversion under acid-base conditions with pH values of 2, 5, 7, 9 and 12 in the coagulation bath were subjected to a solute rejection test, wherein the solute was magnesium chloride (MgCl ₂ ).

FIG. 4 shows the separation performance of various polybenzimidazole nanofiltration membranes manufactured under different pH conditions according to various embodiments of the present invention. As shown in FIG. 4, it can be seen from the curves in the figure that the magnesium chloride rejection rates of Example 1, Example 2, Example 3, Example 4 and Example 5 are 6.69±0.96%, 5.14±0.59%, 4.38±0.31%, 6.63±0.88% and 9.28±0.36%, respectively, among which Example 3 has the lowest solute rejection rate. For the polybenzimidazole nanofiltration membrane generated by phase inversion in an acidic coagulation bath, the magnesium chloride rejection rate of Example 1 is greater than that of Example 2, and the magnesium chloride rejection rate of Example 2 is greater than that of Example 3. The polybenzimidazole nanofilter membrane generated by phase inversion in an alkaline coagulation bath has a magnesium chloride rejection rate of Example 5 greater than that of Example 4, and the magnesium chloride rejection rate of Example 4 is greater than that of Example 3. In addition, the polybenzimidazole nanofilter membrane generated by phase inversion in an alkaline coagulation bath has a magnesium chloride rejection rate greater than that of the polybenzimidazole nanofilter membrane generated by phase inversion in an acidic coagulation bath.

In addition, in this experimental example, the thickness of the polybenzimidazole nanofiltration film varies with the pH value of the coagulation bath. For example, the film thicknesses of Example 1, Example 2, Example 3, Example 4, and Example 5 are 36.4±1.5 microns, 33.4±1.3 microns, 26.1±1.1 microns, 34.2±1.5 microns, and 37.31±1.1 microns, respectively. The reason is that the polybenzimidazole liquid film is instable when the amino groups are protonated or deprotonated during the phase inversion process in the coagulation bath. This situation may lead to delayed demixing, which causes the film thickness to become thicker and thicker during the extended phase inversion process. Furthermore, the surface pore structure of the polybenzimidazole nanofilter membrane also varies with the pH value of the coagulation bath. For example, the surface of Example 5 forms longer finger-like macrovoids than the other examples (Examples 1 to 4) because the coagulation bath with a higher pH value contains more OH ^- groups, which are attracted by the positively charged polybenzimidazole nanofilter membrane, causing the non-solvent (i.e., water in the coagulation bath) ^to invade and form longer finger-like macrovoids.

Experimental Example 2: The polybenzimidazole nanofiltration membranes of Examples 1, 2, 3, 4 and 5 were surface modified with a hyperbranched polyethyleneimine polymer with a molecular weight of 2000 g/mole (represented by Examples 6, 7, 8, 9 and 10, respectively), and then subjected to a solute rejection test, wherein the solute was magnesium chloride (MgCl ₂ ).

FIG5 shows the separation performance of various polybenzimidazole nanofiltration membranes after surface modification according to various embodiments of the present invention. Compared with FIG4, the magnesium chloride rejection rate of Examples 6 to 10 as shown in FIG5 is significantly increased, because the surface-modified polybenzimidazole nanofiltration membranes of Examples 6 to 10 have different surface pore sizes (or molecular weight cutoff) and thicknesses. As can be seen from the curve in Figure 5, the magnesium chloride rejection rates of Example 6, Example 7, Example 8, Example 9 and Example 10 are 97.70±4.40%, 75.90±4.84%, 67.91±2.95%, 79.90±4.51% and 89.54±4.53%, respectively, among which Example 8 has the lowest solute rejection rate, and Example 6 and Example 10 have the highest magnesium chloride rejection rate.

Experimental Example 3: The polybenzimidazole nanofiltration membrane of Example 5 was surface-modified with hyperbranched polyethyleneimine polymers with molecular weights of 2000 g/mole, 25000 g/mole and 75000 g/mole (represented by Examples 11, 12 and 13, respectively), and then subjected to a solute rejection test, wherein the solute was magnesium chloride (MgCl ₂ ).

FIG6 shows the separation performance of the polybenzimidazole nanofiltration membranes generated in a coagulation bath at a pH of 12 according to various embodiments of the present invention after surface modification with hyperbranched polyethyleneimine polymers of different molecular weights. As shown in FIG6, from the curves in the figure, it can be seen that the magnesium chloride rejection rates of Example 5, Example 11, Example 12 and Example 13 are 9.28%, 93.25±0.71%, 97.16±1.06% and 96.14±0.39%, respectively, among which the magnesium chloride rejection rate of Example 12 is the highest, which means that the magnesium chloride retention performance of Example 13 is the best. Compared with Example 11, the surface-modified polybenzimidazole nanofiltration membrane obtained by surface modification using a hyperbranched polyethyleneimine polymer having a molecular weight of 25000 g/mole (Example 12) can more effectively seal the surface defect holes of the polybenzimidazole nanofiltration membrane. However, since the molecular weight of the hyperbranched polyethyleneimine polymer used in Example 13 is too large, it cannot enter the surface pores of the polybenzimidazole nanofiltration membrane. As mentioned above, the hyperbranched polyethyleneimine polymer that cannot enter the surface pores of the polybenzimidazole nanofiltration membrane will be carried away by the isopropyl alcohol aqueous solution.

Experimental Example 4: Using Example 12, a nanofiltration test was performed to separate lithium ions and magnesium ions in saline solutions with concentrations of 500ppm, 1000ppm and 2000ppm, wherein lithium chloride (LiCl) and magnesium chloride (MgCl ₂ ) with a weight ratio of 1:10 were used to simulate the saline solution.

FIG. 7 shows the rejection rate of the nanofiltration test in saline solutions of different concentrations according to Example 12 of the present invention. As shown in FIG. 7 , the rejection rate decreases with the increase of the concentration of the saline solution, which is a common concentration polarization effect of nanofiltration membranes. Despite the concentration polarization effect, the rejection rate of magnesium ions is still maintained at a high level of 95.9% to 93.23% for saline solutions with a concentration of 500 ppm to 2000 ppm. On the contrary, when the concentration of the saline solution increases from 500 ppm to 1000 ppm, the retention rate of lithium ions will significantly decrease from 43.6% to 27.44%. Furthermore, when the feed concentration increases to 2000 ppm, the retention rate of lithium ions will continue to decrease to 13.87%. Therefore, although the retention rates of these two ions will decrease with the increase of the concentration of the saline solution, due to their huge differences in size and valence, the nanofiltration membrane disclosed in the present invention improves the separation efficiency of lithium ions/magnesium ions.

In this experimental example, the separation factor (S _Li,Mg ) of lithium ions relative to magnesium ions is measured using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (Thermo, iCAP 7000) and is obtained by calculating a specific formula for the separation factor. For example, when S _Li,Mg is equal to 1, it means that there is no separation between lithium ions and magnesium ions. When S _Li,Mg is greater than 1, it means that lithium ions pass through the nanofilter faster than magnesium ions. Therefore, the larger the value of S _Li,Mg , the more ideal it is for separating lithium ions from magnesium ions.

Experimental Example 5: The separation factors (S _Li,Mg ) of lithium ions relative to magnesium ions of Example 12 were compared with those of eight commercially available membranes (Comparative Examples 1 to 8). The results are recorded in Table 1 below.

Table I S _Li,Mg Comparative Example 1 NF-90 (Filmtec™, Dow) 2.1 Comparison Example 2 DL-2540 (GE Osmonics) 3.5 Comparison Example 3: Desal-DK (GE Osmonics) 3.22 Comparative Example 4 PEI-TMC 20.0 Comparative Example 5 NF-IL-2 8.12 Comparative Example 6 PA-BPEI-EDTA 9.2 Comparison Example 7 RIP-0.250 9.22 Comparative Example 8 PIP-MWCNTs/PEI/PES 7.13 Embodiment 12 15.15

It can be seen from Table 1 above that the separation performance of lithium ions and magnesium ions of the surface-modified polybenzimidazole nanofiltration membrane of Example 12 of the present disclosure is significantly higher than that of other commercially available membranes.

In summary, the manufacturing method of the nano-filter film disclosed in the present invention is simple and environmentally friendly, and can produce uniform nano-filter films. The nano-filter film produced by this method can also improve the separation efficiency of lithium ions and magnesium ions.

Although the present invention has been disclosed in the above embodiments, the above is only the preferred embodiment of the present invention and is not intended to limit the present invention. Any person skilled in the art can make various equivalent changes and modifications without departing from the spirit and scope of the present invention, which should all fall within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined in the attached patent application.

10: Methods

110: Steps

120: Step

130: Step

210:Polybenzimidazole solution

220: Base material

230:Polybenzimidazole liquid membrane

240: Coagulation bath

250:Polybenzimidazole nanofiltration membrane

In order to make the above and other purposes, features, advantages and embodiments of the present invention more clearly understandable, the detailed description of the attached figures is as follows: Figure 1 shows a method for manufacturing a nanofiltration membrane according to multiple embodiments of the present invention. Figure 2 shows a schematic diagram of a certain stage in the method for manufacturing a nanofiltration membrane according to multiple embodiments of the present invention. Figure 3 shows a schematic diagram of a certain stage in the method for manufacturing a nanofiltration membrane according to multiple embodiments of the present invention. Figure 4 shows the ATR-FTIR spectra of various polybenzimidazole nanofiltration membranes manufactured under different pH conditions according to multiple embodiments of the present invention. Figure 5 shows the separation performance of various polybenzimidazole nanofiltration membranes after surface modification according to multiple embodiments of the present invention. FIG. 6 shows the separation performance of the polybenzimidazole nanofiltration membrane generated in a coagulation bath with a pH value of 12 according to various embodiments of the present invention after surface modification with hyperbranched polyethyleneimine polymers of different molecular weights. FIG. 7 shows the rejection rate of the nanofiltration test in saline solutions of different concentrations according to Example 12 of the present invention.

Domestic storage information (please note in the order of storage institution, date, and number) None Foreign storage information (please note in the order of storage country, institution, date, and number) None

10: Methods

110: Steps

120: Steps

130: Steps

Claims (9)

A method for manufacturing a nanofiltration film comprises: casting a polybenzimidazole solution on a substrate to form a polybenzimidazole liquid film on the substrate; adjusting a pH value of a coagulation bath to 1 to 13; and immersing the polybenzimidazole liquid film in the coagulation bath for phase inversion to form a polybenzimidazole nanofiltration film. The method for manufacturing a nanofiltration film as described in claim 1, wherein the concentration of the polybenzimidazole solution is 15wt% to 25wt%. The method for manufacturing the nanofiltration membrane as described in claim 1 further comprises immersing the polybenzimidazole nanofiltration membrane in a glycerol aqueous solution. The method for manufacturing a nanofiltration membrane as described in claim 3, wherein the volume ratio of glycerol to water in the glycerol aqueous solution is 1:1. The method for manufacturing the nanofiltration membrane as described in claim 1 further comprises immersing the polybenzimidazole nanofiltration membrane in a surface modification solution to form a surface modified polybenzimidazole nanofiltration membrane, wherein the surface modification solution comprises a hyperbranched polyethyleneimine polymer. A method for manufacturing a nanofiltration membrane as described in claim 5, wherein the hyperbranched polyethyleneimine polymer has a molecular weight of 10,000 g/mole to 50,000 g/mole. The method for manufacturing the nanofiltration membrane as described in claim 5 further comprises immersing the surface-modified polybenzimidazole nanofiltration membrane in an isopropyl alcohol aqueous solution. A nanofiltration membrane, comprising: a nanofiltration membrane produced by a nanofiltration membrane production method as described in any one of items 1 to 7 of the patent application scope. A nanofiltration membrane includes: a polybenzimidazole porous nanofiltration membrane body having a structure as follows:

or

; and a hyperbranched polyethyleneimine polymer located on the surface of the polybenzimidazole porous nanofiltration membrane, wherein the hyperbranched polyethyleneimine polymer has a molecular weight of 10000 g/mole to 50000 g/mole.

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