CN113694740B - Positively charged nanofiltration membrane prepared from ionized monomers and preparation method thereof - Google Patents
Positively charged nanofiltration membrane prepared from ionized monomers and preparation method thereof Download PDFInfo
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- B01D67/0002—Organic membrane manufacture
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- B01D61/02—Reverse osmosis; Hyperfiltration ; Nanofiltration
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- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
- B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
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- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
- B01D69/12—Composite membranes; Ultra-thin membranes
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Abstract
The invention belongs to the technical field of membrane materials, and particularly relates to a positively charged nanofiltration membrane prepared from ionized monomers and a preparation method thereof. The preparation method comprises the steps of coating the ionized monomer solution on the porous support base membrane, then continuously coating the acyl chloride monomer solution to enable the ionized monomer and the acyl chloride monomer to generate polymerization reaction on the porous support base membrane, and drying to obtain the positively charged nanofiltration membrane. The preparation method is simple, does not involve any nano material or complex operation process, and meanwhile, the annular quaternary ammonium ion main body has strong tension, the amino group has extremely low steric hindrance, and can strengthen the amidation reaction activity, so that the nanofiltration membrane is ultrathin and free of defects, the efficient interception of magnesium chloride is realized, and the membrane separation performance is excellent.
Description
Technical Field
The invention belongs to the technical field of membrane materials, and particularly relates to a positively charged nanofiltration membrane prepared from ionized monomers and a preparation method thereof.
Background
Nanofiltration is a novel membrane separation technology with high efficiency, low energy consumption and simple operation, and plays an increasingly important role in the aspects of hard water softening, heavy metal separation, industrial wastewater treatment, resource recycling and the like. However, the "trade-off" effect present with nanofiltration membranes (i.e., the tendency of the membrane to exhibit this trade-off in permeate flux and selectivity) limits membrane separation efficiency. In order to efficiently separate increasingly complex practical application systems, a nanofiltration membrane with higher membrane permeation flux and solute selectivity is urgently needed to be obtained. One effective approach is to reduce the thickness of the membrane, reduce the mass transfer distance of the membrane, and thus increase the membrane flux. Interfacial polymerization is the most important method for preparing commercial nanofiltration membranes, however, at present, most of the aqueous monomers used for interfacial polymerization are neutral or amphoteric monomers, the polarity is relatively low, and the diffusion rate in organic-phase n-hexane is high, so that interfacial polymerization film formation occurs in a wide range, the film thickness is large, and the mass transfer resistance is increased. Commercial nanofiltration membranes such as NF90 and NF270 of Dow corporation, UTC series membranes of Toray corporation, and DK and DL series membranes of Utility electric company are all 200nm or more in thickness.
CN108816061A discloses a preparation method of a folded graphene nanofiltration membrane, and specifically discloses a method for preparing a graphene oxide membrane by using AAO as a base membrane and performing suction filtration on a graphene oxide dispersion liquid, and then transferring the graphene oxide membrane to waterA nanofiltration membrane with the thickness less than 100nm is obtained on the porous support basement membrane, and the membrane permeability reaches 21Lm -2 h -1 bar -1 The retention rate of sodium sulfate is kept above 95%. Although the film thickness is reduced to 100nm, the flux is obviously improved, the cost of the graphene oxide is too high, and the operation process is difficult to amplify, so that the technical scheme is difficult to have commercial value. CN112108018A discloses a preparation method of a silicon dioxide/polyimide loose nanofiltration membrane, and the thickness of the membrane is only 30nm. Firstly, diamine monomer and diacid anhydride monomer are reacted to obtain polyamic acid, then silicon dioxide nano particles and imidization reagent are introduced into the polyamic acid to form uniform and stable casting solution, and then the uniform and stable casting solution is coated and formed into a film by a phase inversion method to obtain the high-flux polyimide nanofiltration membrane, wherein the water flux of the membrane is 80Lm at 0.4MPa -2 h -1 The rejection rate of sodium sulfate is more than 89%, and the hydrophilicity and the thermal stability are both superior to those of a pure polyimide nanofiltration membrane. However, this method involves many steps such as monomer dissolution, casting solution preparation, defoaming, film scraping, phase inversion, heat treatment, and the operation is cumbersome.
In addition, the preparation of the ultra-thin nanofiltration membrane with positive charge by direct interfacial polymerization of monomers is rarely reported in patents and literatures. The positively charged nanofiltration membrane is widely used for heavy metal removal, hard water softening, basic dye separation, lithium extraction in salt lakes and the like. CN113019146A discloses a preparation method of an ultrahigh flux composite nanofiltration membrane, which is to add hyperbranched polyester molecules into a piperazine water solution and then carry out interfacial polymerization with a polybasic acyl chloride monomer to prepare the nanofiltration membrane. The super-esterified polyester has controllable slow release effect on piperazine, the prepared film has the thickness of 8.34nm and the pure water flux of 51.62Lm under 0.2MPa -2 h -1 bar -1 The rejection rate for sodium sulfate was 98.01%. However, this method requires the introduction of a third phase of monomer while the prepared membrane remains an electronegative membrane. CN112844046A discloses a method for preparing a positively charged nanofiltration membrane, which comprises preparing a base membrane by a phase inversion method, sequentially coating a polybasic amino compound and a polybasic acyl chloride compound on the surface of the base membrane for interfacial polymerization to prepare a primary layer, and finally placing the primary layer in a mixed solution of a polyfunctional amine compound and a crosslinking agent for immersionAnd (5) soaking to obtain the positively charged nanofiltration membrane. The polyfunctional amine compound is at least one selected from polyethyleneimine, polyvinylamine and polypropyleneamine. The optimal flux of the membrane is 27.78Lm at 0.5MPa -2 h -1 The retention rate of calcium chloride was 98.25%. However, the invention involves two interfacial polymerization processes, increasing the membrane thickness and crosslink density, and the membrane flux is not high.
In summary, the prior art still lacks a simple method for preparing a positively charged ultrathin nanofiltration membrane.
Disclosure of Invention
Aiming at the defects or improvement requirements of the prior art, the invention provides a simple and quick preparation method of a positive-charged ultrathin nanofiltration membrane, which is suitable for the prior production process. Meanwhile, the reaction degree of amido bonds is increased by the amino group with low steric hindrance on the quaternary ammonium salt, and the ultrathin defect-free positive nanofiltration membrane with the thickness of 15-40nm is finally prepared.
In order to achieve the above object, according to one aspect of the present invention, there is provided a method for preparing a positively charged nanofiltration membrane from an ionized monomer, comprising the steps of coating an ionized monomer solution on a porous support base membrane, then continuing to coat an acyl chloride monomer solution, allowing the ionized monomer and the acyl chloride monomer to undergo a polymerization reaction on the porous support base membrane, and drying to obtain the positively charged nanofiltration membrane; the structure of the ionized monomer is shown as a general formula (1):
wherein, R is a functional group which at least contains one quaternary ammonium ion and has a cyclic structure, and X is Cl, br or I.
Preferably, the cyclic structure of R is any one of a diazacycle, a triazacycle, a tetraazacycle, a bipyridine, piperazine, pyrazine, imidazole and a azaspiro ring.
Preferably, R is any one of formula (2) to formula (9), wherein,
preferably, the acid chloride monomer is any one of 1,3,5-benzenetricarboxychloride, 1,2,4,5-benzenetetracarboxyl chloride, 1,3,5-benzenetrisulfonyl chloride and cyanuric chloride.
Preferably, the ionized monomer solution has a mass concentration of 0.3 to 4wt% and a pH of 10 to 13.
Preferably, the mass concentration of the acid chloride monomer is 0.1 to 0.6wt%.
Preferably, the solvent of the ionized monomer solution is water, and the solvent of the acid chloride monomer solution is an organic solvent.
Preferably, the porous support base membrane is one or a mixture of polyvinylidene fluoride, polysulfone, polyethersulfone and polyacrylonitrile membrane.
Preferably, the drying temperature is 30-70 ℃ and the drying time is 10-30 minutes.
According to another aspect of the invention, the positively charged nanofiltration membrane prepared by the preparation method is provided.
The invention has the following beneficial effects:
(1) The method can prepare the ultrathin nanofiltration membrane with the thickness of 15-40nm only by replacing the traditional water-phase monomer with the ionized quaternary ammonium salt monomer, has simple preparation method, does not relate to any nano material or complex operation process, and conforms to the existing process for preparing the nanofiltration membrane.
(2) The quaternary ammonium salt monomer designed by the invention has low diffusion rate in organic phase n-hexane, but the low-steric-hindrance terminal amino group has high reaction activity, and meanwhile, the cyclic quaternary ammonium ion main body has strong tension and can promote the activity of amidation reaction, so that the nanofiltration membrane is thin and free of defects, and the efficient interception (> 97%) of magnesium chloride is realized.
(3) At present, the preparation of small molecular monomers of positively charged nanofiltration membranes by one-step interfacial polymerization method is not reported, and most of the small molecular monomers arePrepared by interfacial polymerization of polyethyleneimine water-phase monomers, and such membranes have relatively low flux (typically about 15-30Lm at 0.6 MPa) -2 h -1 ) The invention adopts the same membrane preparation method to prepare the ultra-high flux positively charged nanofiltration membrane (100-130 Lm) - 2 h -1 ) And the technology has remarkable technical progress.
(4) The positive-charged ultrathin nanofiltration membrane prepared by the invention has excellent membrane separation performance. Under 0.6MPa, the flux to magnesium chloride can reach 130Lm -2 h -1 The retention rate is more than 97%. In addition, the separation layers of the nanofiltration membrane prepared by the invention are connected through stable chemical bonds (amido bonds), so the membrane structure is stable, the pressure resistance is good, the good separation performance is kept after the nanofiltration membrane continuously works for 120 hours under 0.2-0.6MPa, and the difficult problems of poor high pressure resistance and long-term stability of the ultrathin membrane are solved.
Drawings
FIG. 1 is a schematic diagram of the synthesis route of the interfacial reaction of example 1 of the present invention.
FIG. 2 is a schematic view of interfacial polymerization of example 1 of the present invention and comparative example 1, wherein a in FIG. 2 is comparative example 1 and b in FIG. 2 is example 1;
FIG. 3 is a TEM image of film thickness of comparative example 1 and example 1 of the present invention, wherein a in FIG. 3 is comparative example 1 and b in FIG. 3 is example 1;
fig. 4 is the results of the pressure stability and long-term operation stability tests of example 1 of the present invention, wherein a in fig. 4 is the pressure stability test and b in fig. 4 is the long-term operation stability test.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Examples
Preparation examples
The ionized monomer is prepared by a one-step quaternization reaction (as shown in formula a below). Adding a tertiary nitrogen-containing molecule and a quaternizing agent into ethanol according to a molar ratio of 1. Using formula (2) as an example (as shown in formula b below), 1,4-diazidobicyclo [2.2.2] octane and 2-bromoethylamine hydrobromide are added into ethanol according to the molar ratio of 1, stirred and refluxed at 70 ℃ for 8h, filtered by suction, and dried in vacuum, so as to obtain 1,4-diazidobicyclo [2.2.2] octane quaternary ammonium salt monomer. The yield was 92%, the reaction principle was as follows:
example 1
A positively charged nanofiltration membrane prepared from ionized monomers is prepared by the following method:
coating the aqueous solution of 1,4-diamineethyl-1,4-azidobicyclo [2.2.2] octane ammonium bromide salt on the polysulfone support base film, wherein the mass fraction is 0.5wt%, the pH value is 12, and the soaking time is 5 minutes. And then coating the 1,3,5-benzene trimethyl acyl chloride normal hexane solution on the membrane with the mass fraction of 0.3wt% for 1 minute, so that the ionized monomer and the acyl chloride monomer are subjected to polymerization reaction on the porous support base membrane. And (3) drying the prepared membrane in a 50 ℃ oven for 10min to obtain the positive-charged ultrathin high-flux nanofiltration membrane.
Examples 2-8 differ from example 1 in the ionized monomer, as detailed in table 1.
Examples 9-10 differ from example 1 in the porous support base film, as detailed in table 1.
Example 11 differs from example 1 in the mass concentration of ionized monomer, as detailed in table 1.
Example 12 differs from example 1 in the ionized monomer wetting time, as detailed in table 1.
Example 13 differs from example 1 in the mass concentration of 1,3,5-benzenetricarbonyl chloride, as detailed in table 1.
Example 14 differs from example 1 in the drying temperature, as detailed in table 1.
Examples 15-16 differ from example 1 in the acid chloride monomer. Example 15 the acid chloride monomer was 1,2,4,5-benzenetetracarboxylic acid chloride and example 16 the acid chloride monomer was 1,3,5-benzenetrisulfonyl chloride for details in Table 2
Comparative example
Comparative example 1
Comparative example 1 differs from example 1 in that comparative example 1 used a conventional commercial aqueous monomer piperazine to prepare a nanofiltration membrane. The monomer was purchased from national pharmaceutical group chemical reagents, ltd, and is detailed in table 1.
Comparative example 2
Comparative example 2 differs from example 1 in that comparative example 2 used an ionized monomer 1,4-diamineethyl-1,1,4,4-tetramethylammonium bromide salt to prepare a nanofiltration membrane. The monomer was purchased from national pharmaceutical group chemical reagents, ltd, and is detailed in table 1.
Comparative example 3
Comparative example 3 differs from example 1 in that comparative example 3 uses 1-methyl-3-bromoethylamine-1,4-azidobicyclo [2.2.2] octane ammonium bromide to prepare a nanofiltration membrane as detailed in table 1.
Comparative example 4
Comparative example 4 differs from example 1 in that comparative example 4 uses terephthaloyl chloride as the acid chloride monomer, which is available from national chemical group, ltd, as detailed in table 2.
Table 1 table of parameters of examples
TABLE 2 continuation of the parameters of the examples
Test examples and comparative examples
Nanofiltration membranes prepared in examples 1 to 16 and comparative examples 1 to 4 were subjected to nanofiltration performance tests.
The nanofiltration performance test method is as follows. In the invention, the flux (F) and the rejection rate (R) are two important parameters for measuring the separation performance of the nanofiltration membrane. The result of the invention is that the cross-flow nanofiltration instrument is used for testing, and the testing conditions are as follows: 1000ppm of magnesium chloride solution, 0.6MPa,25 ℃. Taking the ultrathin high-flux nanofiltration membrane prepared in the example 1 as an example, the long-term stability and the separation performance of the membrane under different pressures are tested, and other test conditions are unchanged.
The rejection rate is the ratio of solute to solute which is trapped by the membrane after passing through the nanofiltration membrane. The specific calculation formula is as follows:
wherein, C f And C p Feed solution and permeate solution solute concentrations, respectively. The concentration of magnesium chloride is determined by a conductivity meter.
Flux refers to the volume (V) of water passing through the membrane per unit area (S) per unit time (t) at a certain operating pressure. The specific calculation formula is as follows:
the test results are shown in table 3 below.
TABLE 3 test results table
The reaction principle of the present invention is exemplified by example 1 and comparative example 1. FIG. 2 is a schematic view of interfacial polymerization of example 1 of the present invention and comparative example 1, wherein a in FIG. 2 is comparative example 1 and b in FIG. 2 is example 1.
As can be seen from FIG. 2, the interfacial polymerization occurs at the interface of two phases, in which the aqueous phase monomer diffuses into the organic phase solvent n-hexane, and then condenses with the organic phase monomer to form a film. At present, the monomer piperazine for preparing the commercial nanofiltration membrane is a neutral low-polarity monomer, and the diffusion rate of the monomer piperazine in a nonpolar solvent n-hexane is higher than that of a polar monomer. For the monomer 1,4-diamineethyl-1,4-azidobicyclo [2.2.2] octane ammonium bromide in example 1, the monomer polarity is very large due to the ionization characteristic, the diffusion rate in n-hexane is low, the interfacial polymerization reaction region is shortened, and the prepared film thickness is greatly reduced.
FIG. 3 is TEM image of film thickness of comparative example 1 and example 1 of the present invention, wherein a in FIG. 3 is comparative example 1 and b in FIG. 3 is example 1.
As can be seen from FIG. 3, the film thickness of example 1 was reduced from 180nm to 20nm as compared with that of comparative example 1. The reduction of the film thickness shortens the mass transfer path of water molecules, and meanwhile, the ionized monomer is combined with the water molecules through electrostatic interaction, so that the hydrophilicity is excellent, and the increase of the film flux is promoted together.
FIG. 1 is a schematic diagram of the synthetic route of the interfacial reaction of example 1 of the present invention.
As can be seen from fig. 1, when the interfacial polymerization reaction zone is reduced, the probability of defects is increased, and it is more difficult to prepare a defect-free nanofiltration membrane. The invention therefore particularly selects ionised monomers having very little steric hindrance of the amino group, so that the highly reactive amino group compensates for the low diffusion rate disadvantage. The invention also discovers that the amino group with low steric hindrance can generate amidation reaction of secondary amine, further increases the amidation reaction degree, and is beneficial to constructing a defect-free ultrathin nanofiltration membrane.
In addition, the ionized monomer enables the nanofiltration membrane to be finally charged with positive charges, and high rejection rate of magnesium chloride is kept, while the commercial piperazine-trimesoyl chloride nanofiltration membrane is a negative electric membrane and low in rejection rate of magnesium chloride. Therefore, the positively charged nanofiltration membrane can be applied to the occasions of heavy metal removal, hard water softening, lithium extraction in salt lakes and the like.
In conclusion, the ionized monomer designed by the invention has high polarity, high reaction activity, good hydrophilicity and large positive charge density, can endow the membrane with the advantages of ultra-thin, hydrophilic and high positive charge density in the interfacial polymerization process, and is beneficial to preparing the positive charge ultra-thin high-flux nanofiltration membrane.
Fig. 4 is the results of the pressure stability and long-term operation stability tests of example 1 of the present invention, wherein a in fig. 4 is the pressure stability test and b in fig. 4 is the long-term operation stability test.
As can be seen from fig. 4, compared with the ultra-thin nanofiltration membrane prepared from the existing nano materials such as graphene oxide, molybdenum disulfide, and covalent organic metal framework, the nanofiltration membrane prepared by covalent bond connection is still based on interfacial polymerization, so that the stability of the membrane is good. The membrane maintained excellent separation performance even at 120h continuous operation and different operating pressures.
Further analyzing the data in table 3, it can be seen that the positive ultrathin nanofiltration membrane of the invention has made great technical progress in three layers of monomer design, membrane thickness, and membrane performance.
Firstly, as can be seen from comparison between examples 1 to 8 and comparative example 1, the flux of the nanofiltration membrane prepared by using the ionized monomer is much higher than that of the weakly polar monomer used in comparative example 1 under the same membrane preparation conditions, and the technical progress is obvious. On one hand, the thickness of the nanofiltration membrane prepared by the ionized monomer is much smaller than that of the nanofiltration membrane prepared by the weak polar monomer; on the other hand, ionized monomers are a class of monomers with strong hydrophilicity, and can endow the membrane with better hydrophilicity.
Second, as can be seen from the comparison of examples 1-8 with comparative examples 2-3, not all of the ionized monomers can be directly interfacially polymerized to form a film. The monomer needs to have a large tensile ring structure, the amino group needs to have low steric hindrance and be a primary amine. This is because the diffusion rate of the ionized monomer is slow, and strong reactivity is required to compensate for the low diffusion rate, so as to prepare a defect-free nanofiltration membrane. The monomer amino groups in examples 1-8 are all at the edge of the main structure, with small steric hindrance and high reactivity.
Third, as can be seen from the comparison of examples 1-8, the membrane flux is higher when the monomer has a spatially twisted configuration (as in examples 1-3). The reason is that when the molecules in the non-planar twisted folding configuration are polymerized at the interface, a large number of intermolecular gaps can be generated, the free volume of the film is increased, and the mass transfer of water molecules is enhanced. When the monomer has a plurality of positive charge sites, the thickness of the membrane is thinner, and the retention rate of magnesium chloride is higher. This is because the quaternary ammonium salt ion is a source of strong polarity of the ionized monomer, and the more the quaternary ammonium ion, the stronger the polarity of the monomer, the slower the diffusion rate into n-hexane, and the thinner the film. Meanwhile, the quaternary ammonium ions enhance the positive charge density of the membrane surface, enhance the rejection of magnesium chloride and increase the retention rate.
Fourthly, as can be seen from comparison between example 1 and examples 9 to 10, different porous support membranes are used as the base membrane to achieve better effects, which indicates that the method for preparing the positively charged ultrathin high-flux nanofiltration membrane can be applied to different commercialized base membranes.
Fifth, as can be seen from the comparison of example 1 with examples 11-14, the membrane flux exhibited a small decrease with increasing aqueous phase concentration and soaking time, increasing organic phase concentration, and increasing drying temperature. This is because the degree of crosslinking of the membrane is increased with the increase in monomer concentration, reaction time, and drying temperature, and the membrane is made denser, and the membrane flux is reduced, but still much higher than that of comparative example 1.
Sixth, as can be seen from the comparison of example 1 with examples 15-16, good technological advances were achieved with different acid chloride monomers. As the functionality of the acid chloride monomer is increased from 3 to 4, the membrane becomes thicker and a small decrease in flux occurs because the degree of cross-linking of the membrane increases with increasing functionality and the water mass transfer resistance increases. As can be seen from example 1 and comparative example 4, when the functionality of the acid chloride monomer was 2, a defect-free nanofiltration membrane could not be formed because the functionality of the aqueous phase monomer was also 2, only a linear product was formed during the polycondensation reaction, the degree of crosslinking was insufficient, and it was difficult to form a defect-free polyamide membrane.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (6)
1. A preparation method of a positively charged nanofiltration membrane prepared from ionized monomers is characterized in that ionized monomer solution is coated on a porous support base membrane, then acyl chloride monomer solution is coated continuously, so that the ionized monomers and the acyl chloride monomers have polymerization reaction on the porous support base membrane, and the positively charged nanofiltration membrane can be obtained after drying; the structure of the ionized monomer is shown as the general formula (1):
wherein X is Cl, br or I;
r is any one of formula (2) to formula (9), wherein,
moreover, the thickness of the positively charged nanofiltration membrane is 15-40nm;
the mass concentration of the ionized monomer solution is 0.3-4wt%, and the pH value is 10-13;
the mass concentration of the acyl chloride monomer solution is 0.1-0.6wt%.
2. The method according to claim 1, wherein the acid chloride monomer is any one of 1,3,5-benzenetricarbochloride, 1,2,4,5-benzenetetracarboxyl chloride, 1,3,5-benzenetrisulfonyl chloride, and cyanuric chloride.
3. The method according to claim 1, wherein the solvent of the ionized monomer solution is water and the solvent of the acid chloride monomer solution is an organic solvent.
4. The method of claim 1, wherein the porous support base membrane is a blend of one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, and polyacrylonitrile membranes.
5. The method according to claim 1, wherein the drying is carried out at a temperature of 30 to 70 ℃ for 10 to 30 minutes.
6. The positively charged nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 5.
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