Process for the preparation of polyester nanofiltration membranes based on hydroxyethylhexahydro-s-triazine

Abstract

This invention discloses a method for preparing a polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine (HET). The method mainly includes dissolving HET in ultrapure water to form an aqueous solution; dissolving 1,3,5-pyromellitic acid chloride in n-heptane to form an organic solution; and sequentially soaking a polyacrylonitrile-based membrane in the above aqueous and organic solutions. The polyester nanofiltration membrane is obtained by reacting the hydroxyl groups in HET with the acyl chloride groups of 1,3,5-pyromellitic acid chloride at the phase interface. The introduction of HET brings abundant hydroxyl groups, achieving high water flux. Simultaneously, unreacted acyl chloride groups hydrolyze to generate carboxyl groups, which are then deprotonated to generate negatively charged carboxylate groups, giving the membrane negative charge and enabling effective retention of anionic dyes. The membrane exhibits an anionic dye rejection rate greater than 99.5% and a dye / salt selectivity as high as 371.25. The nanofiltration membrane prepared by this invention has broad application prospects for the resource-based treatment of saline wastewater in pharmaceutical, dyeing, and food industries.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane separation technology, and in particular to a method for preparing a polyester nanofiltration membrane of hydroxyethyl hexahydrotriazine. Background Technology

[0002] Industrial wastewater from the pharmaceutical, dyeing, and food industries often contains high concentrations of dyes and salts. Due to its high color intensity, strong toxicity, and recalcitrant nature, this dye wastewater poses a serious threat to ecosystems, biological health, and human life, and requires costly treatment. Therefore, efficiently separating dyes and salts from wastewater and recycling the recovered resources to reduce wastewater treatment costs is crucial for wastewater resource utilization and achieving sustainable development.

[0003] Various membrane materials, including polyamide (PA), polyester (PE), polyesteramide (PEA), and covalent organic frameworks (COF), have been applied to the preparation of nanofiltration membranes for dye / salt separation. Compared with traditional polyamide nanofiltration membranes, polyester membranes exhibit superior chlorine resistance due to the absence of amide bonds in their molecular chains, which are easily attacked by chlorine. The flux and selectivity of nanofiltration membranes are significantly limited; current research mainly focuses on improving water flux, while the separation performance of low molecular weight dyes is often neglected. This research bias restricts the comprehensive application of membrane technology in efficient dye separation and wastewater treatment. From the perspective of the separation mechanism of nanofiltration membranes, steric hindrance and the Donan effect are the most fundamental factors affecting the rejection rate. Polyester nanofiltration membranes not only enhance the hydrophilicity of the membrane surface and improve water flux by introducing more abundant hydroxyl groups, but also enhance the rejection capacity for dye molecules through the combined Donan effect and steric hindrance effect.

[0004] [References]

[0005] [1]Fini MN,Montesantos N,Maschietti M,et al.Performance evaluation of membrane filtration for treatment of H2S scavenging wastewater fromoffshore oil and gas production[J].Separation and Purification Technology,2021,277:119641.

[0006] [2] Ding L, Huang Y, Shi L, et al. Tris-phenol phosphine oxide-based polyester loose nanofiltration membranes with a three-dimensional structure for efficient dye / salt separation[J]. Separation and Purification Technology, 2025, 354: 128962.

[0007] [3] Zhao R, Jin P, Zhu J, et al. Amino acid-based loose polyamide nanofiltration membrane with ultrahigh water permeance for efficient dye / salt separation[J]. Journal of Membrane Science, 2023, 673: 121477.

[0008] [4] Li Q, Liao Z, Fang X, et al. Tannic acid assisted interfacial polymerization based loose thin-film composite NF membrane for dye / salt separation[J]. Desalination, 2020, 479: 114343.

[0009] [5] Fang Y X, Lin Y F, Xu Z L, et al. A novel clover-like COFs membrane fabricated via one-step interfacial polymerization for dye / salt separation[J]. Journal of Membrane Science, 2, 673: 121470. Summary of the Invention

[0010] In view of the above-mentioned prior art, the present invention proposes a method for preparing a polyester nanofiltration membrane based on hydroxyethyl hexahydro-s-triazine (HET). [1] Polyester nanofiltration membranes were prepared by interfacial polymerization with 1,3,5-trimethylbenzene chloride (TMC) to improve the separation performance of dyes / salts.

[0011] To address the aforementioned technical problems, this invention proposes a method for preparing a polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine, mainly comprising: dissolving hydroxyethyl hexahydrotriazine in ultrapure water to form an aqueous solution; dissolving 1,3,5-tristyrene chloride in n-heptane to form an organic solution; and sequentially immersing a polyacrylonitrile-based membrane in the above aqueous and organic solutions. The polyester nanofiltration membrane is obtained by reacting the hydroxyl groups in hydroxyethyl hexahydrotriazine with the acyl chloride groups of 1,3,5-tristyrene chloride at the phase interface. The specific steps are as follows:

[0012] Step 1, Pretreatment of polyacrylonitrile film: Wash the polyacrylonitrile film three times with ultrapure water, air dry at room temperature, and then fix it in a plastic mold;

[0013] Step 2, Immersion in aqueous solution: Dissolve hydroxyethyl hexahydrotriazine in ultrapure water to prepare an aqueous solution of 0.2-0.5% by mass, and adjust the pH to 13 with sodium hydroxide solution to obtain the aqueous solution. Add the aqueous solution to the mold described in Step 1. The amount of aqueous solution used is proportional to the area of ​​the polyacrylonitrile base film at 10 mL / 25 cm². 2 After soaking and reacting for 10 minutes, the film is removed and the surface liquid is removed. The polyacrylonitrile-based film after soaking and reacting is then fixed into the mold.

[0014] Step 3, Polyester Synthesis: Dissolve 1,3,5-pyromellitic trimethylol chloride in n-heptane to obtain an organic phase solution with a mass percentage of 0.1%. Add the organic phase solution to a mold with a polyacrylonitrile base film fixed thereon after the treatment in Step 2. The amount of organic phase solution used is proportional to the area of ​​the polyacrylonitrile base film at a ratio of 5-10 mL / 25 cm². 2 The membrane was soaked for 3-10 minutes, then removed and air-dried at room temperature. It was then washed three times with ethanol and ultrapure water to obtain the polyester nanofiltration membrane.

[0015] Furthermore, in the preparation method described in this invention, wherein:

[0016] In step one, the molecular weight cutoff of the polyacrylonitrile base film is 10 kDa, and the plastic mold has a base film exposed area of ​​5×5 cm.

[0017] In step two, the hydroxyethyl hexahydrotriazine solution has a mass percentage of 0.3%. The sodium hydroxide solution has a molar concentration of 3M.

[0018] In step three, the ratio of the amount of organic phase solution used to the area of ​​the polyacrylonitrile-based membrane is 5 mL / 25 cm². 2 The soaking reaction time is 5 minutes.

[0019] The polyester nanofiltration membrane prepared according to this invention is stored in ultrapure water.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] (1) Hydroxyethyl hexahydrotriazine (HET) was selected as the aqueous monomer, which can react with 1,3,5-trimethylammonium chloride (TMC) at room temperature and pressure. As a hydroxyl monomer, HET has lower reactivity with the acyl chloride group in TMC than traditional amines, thereby effectively reducing the reaction rate, promoting uniform growth of the membrane, and facilitating the formation of a looser active layer to increase water transport pathways.

[0022] (2) The water flux of the membrane prepared by this invention reaches 43.97–76.02 Lm. -2 h -1 bar -1 The membrane exhibits a rejection rate of over 99.5% for anionic dyes, a NaCl rejection rate of 13.68–29.54%, and a dye / salt selectivity as high as 178.57–371.25. The polyester nanofiltration membrane prepared in this invention can be used in pharmaceutical, printing and dyeing, and food industries, as well as in wastewater treatment, demonstrating good separation performance and water flux, and high resource utilization efficiency.

[0023] (3) The materials used are readily available, the preparation process is simple, the preparation process is short, and the operation is convenient, which is conducive to large-scale industrial production. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the preparation and reaction mechanism of the polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine proposed in this invention;

[0025] Figure 2 These are scanning electron microscope images of the surface of the NPE-2 film prepared in Example 2;

[0026] Figure 3 This is a scanning electron microscope image of the surface of the control membrane 1 prepared in Comparative Example 1;

[0027] Figure 4 The water contact angle diagrams are for the membranes prepared in Examples 1-4 and Comparative Examples 1-2.

[0028] Figure 5 The graphs show the dye rejection performance and water flux of the membranes prepared in Examples 1-4 and Comparative Membranes 1-2. Detailed Implementation

[0029] The design concept for preparing polyester nanofiltration membranes based on hydroxyethyl hexahydrotriazine (HET) is as follows: HET is used as the aqueous solution monomer, and TMC is used as the organic solution monomer. These monomers are polymerized on a polyacrylonitrile-based membrane via interfacial polymerization to form the polyester nanofiltration membrane. The introduction of HET brings abundant hydroxyl groups, achieving high flux. Simultaneously, the unreacted acyl chloride groups hydrolyze to generate carboxyl groups, which then deprotonate to form negatively charged carboxylate groups, giving the polyester membrane an electronegativity that effectively retains anionic dyes. For example... Figure 1 As shown, the preparation of this polyester nanofiltration membrane mainly includes: dissolving HET in ultrapure water and adjusting the pH value with sodium hydroxide solution to obtain an aqueous phase solution; dissolving TMC in n-heptane to form an organic phase solution; and sequentially soaking a polyacrylonitrile-based membrane in the above aqueous and organic phase solutions. The polyester nanofiltration membrane is obtained by the reaction between the hydroxyl groups in HET and the acyl chloride groups of TMC at the phase interface. Because HET has different diffusion coefficients in water and TMC in the organic phase, the membrane tends to form more readily in the organic phase. This method achieves the preparation of a HET-based polyester nanofiltration membrane while simultaneously improving the membrane's hydrophilicity. The water flux of this polyester nanofiltration membrane reaches 43.97–76.02 Lm. -2 h -1 bar -1 The membrane exhibits a rejection rate of over 99.5% for anionic dyes, a NaCl rejection rate of 13.68–29.54%, and a separation factor of 178.57–371.25. When the nanofiltration membrane prepared by this invention is applied to the resource utilization treatment of saline wastewater in pharmaceutical, printing and dyeing, and food industries, it demonstrates high water flux and dye / salt resource utilization efficiency.

[0030] The separation performance of the prepared polyester nanofiltration membrane was evaluated mainly by characterizing the membrane's salt rejection rate, dye rejection rate, salt and dye separation factor, and water flux.

[0031] Retention rate calculation formula: R = (1 - C) p / C f )×100%, where R represents the retention rate, C p and C f These are the concentrations (ppm) of dye or salt in the permeate and feed solutions, respectively. Unless otherwise specified, the feed solutions are 1000 ppm NaCl aqueous solution and 100 ppm Congo red aqueous solution, respectively.

[0032] Separation factor calculation formula: S=(1-R) salt ) / (1-R dye )×100%, where Rsalt R represents the salt rejection rate. dye This represents the dye rejection rate. Unless otherwise specified, the ratio of salt to dye in the feed solution is 10:1 (i.e., 1000 ppm of salt solution mixed with 100 ppm of dye solution).

[0033] Water flux L m -2 h -1 bar -1 Defined as: the volume of water passing through the effective membrane area per unit pressure per unit time under certain operating pressure conditions. Unless otherwise specified, the test pressure of the membrane is 4 bar.

[0034] Specific embodiments of the present invention are given below. These specific embodiments are only used to further illustrate the present invention and do not limit the scope of protection of the claims of this application.

[0035] Example 1

[0036] The preparation of polyester nanofiltration membranes based on hydroxyethyl hexahydrotriazine involves the following steps:

[0037] Step 1, Pretreatment of polyacrylonitrile base film: The polyacrylonitrile base film with a molecular weight cutoff of 10kDa is washed three times with ultrapure water, dried at room temperature, and then fixed in a plastic mold with a base film exposed area of ​​5×5cm.

[0038] Step 2, soaking in aqueous solution: Dissolve HET in ultrapure water to prepare a 0.2% (w / w) aqueous solution of hydroxyethyl hexahydrotriazine, and adjust the pH to 13 with a 3M sodium hydroxide solution to obtain an aqueous solution; take 10 mL of this aqueous solution and add it to the polyacrylonitrile-based membrane in the mold from Step 1 to soak it. After 10 min, take out the polyacrylonitrile-based membrane, remove the surface water droplets with a rubber roller, and fix the membrane back into the mold.

[0039] Step 3, reaction synthesis of polyester: Dissolve TMC in n-heptane to prepare an organic phase solution with a mass percentage of 0.1%. Take 5 ml of this organic phase solution and quickly add it to the mold in Step 2 where the polyacrylonitrile base membrane treated in Step 2 is fixed. After soaking and reacting for 5 min, take it out and air dry it at room temperature. Wash it three times with ethanol and ultrapure water respectively to obtain a polyester nanofiltration membrane, abbreviated as NPE-1 membrane.

[0040] The NPE-1 membrane prepared in Example 1 was tested and found to have a NaCl rejection rate of 13.68%, a Congo red rejection rate of 99.68%, a separation factor of 267.86, and a water flux of 76.02 L / m³. -2 h -1 bar -1 .

[0041] Example 2

[0042] The preparation process of the polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine is basically the same as that in Example 1, except that in step two, the mass concentration of HET is changed from 0.2% to 0.3%, and the final polyester nanofiltration membrane is referred to as NPE-2 membrane. Figure 2 This is a scanning electron microscope image of the surface of the NPE-2 membrane prepared in Example 2.

[0043] The NPE-2 membrane prepared in Example 2 was tested and found to have a NaCl rejection rate of 14.93%, a Congo red rejection rate of 99.78%, a separation factor of 371.25, and a water flux of 68.18 L / m³. -2 h -1 bar -1 .

[0044] Example 3

[0045] The preparation process of the polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine is basically the same as that in Example 1, except that in step 2, the mass concentration of HET is changed from 0.2% to 0.4%, and the final polyester nanofiltration membrane is referred to as NPE-3 membrane.

[0046] The NPE-3 membrane prepared in Example 3 was tested and found to have a NaCl rejection rate of 25.44%, a Congo red rejection rate of 99.63%, a separation factor of 243.38, and a water flux of 51.94 L / m³. -2 h -1 bar -1 .

[0047] Example 4

[0048] The preparation process of the polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine is basically the same as that in Example 1, except that in step two, the mass concentration of HET is changed from 0.2% to 0.5%, and the final polyester nanofiltration membrane is referred to as NPE-4 membrane.

[0049] The NPE-4 membrane prepared in Example 3 was tested and found to have a NaCl rejection rate of 29.54%, a Congo red rejection rate of 99.51%, a separation factor of 178.57, and a water flux of 43.97 L / m³. -2 h -1 bar -1 .

[0050] Comparative Example 1

[0051] The preparation process of the polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine is basically the same as that in Example 1, except that in step two, the mass concentration of HET is changed from 0.2% to 0.1%. The final polyester nanofiltration membrane is referred to as control membrane 1. Figure 3This is a scanning electron microscope image of the surface of the control membrane 1 prepared in Comparative Example 1.

[0052] Tests showed that, compared to membrane 1, the NaCl rejection rate was 12.93%, the Congo red rejection rate was 99.15%, the separation factor was 93.75, and the water flux was 90.27 L / m³. -2 h -1 bar -1 .

[0053] Comparative Example 2

[0054] The preparation process of the polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine is basically the same as that in Example 1, except that in step 2, the mass concentration of HET is changed from 0.2% to 1.0%. The final polyester nanofiltration membrane is referred to as control membrane 2.

[0055] Tests showed that, compared to membrane 3, the NaCl rejection rate was 30.67%, the Congo red rejection rate was 99.20%, the separation factor was 142.86, and the water flux was 22.37 L / m³. -2 h -1 bar -1 .

[0056] Figure 4 The water contact angle diagrams for NPE1 membrane, NPE2 membrane, NPE3 membrane, NPE4 membrane, control membrane 1, and control membrane 2 show that HET introduces hydroxyl groups, which improves the hydrophilicity of the active layer of the membrane. This is because the increased concentration of the aqueous monomer HET provides more abundant hydroxyl groups, thereby improving the hydrophilicity of the membrane and reducing the water contact angle.

[0057] Figure 5 The Congo red rejection rate and water flux of NPE1 membrane, NPE2 membrane, NPE3 membrane, NPE4 membrane, control membrane 1 and control membrane 2 are shown. Table 1 lists the NPE1 membrane, NPE2 membrane, NPE3 membrane, NPE4 membrane, control membrane 1, control membrane 2, polyacrylonitrile-based membrane, and polyester (THPPO / TMC) membrane. [2] Polyamide (Arg / TMC) [3] Polyesteramide (TAIP / TMC) [4] and covalent organic frameworks (MPD-TFB / Nylon) [5] Water flux, Congo red rejection rate, NaCl rejection rate, and separation factor. Figure 5 As shown in Table 1, under lower aqueous monomer concentrations, the water flux is high, but the membrane rejection rate is low, resulting in poor membrane dye / salt selectivity. As the HET concentration increases, the degree of membrane cross-linking is higher, resulting in high membrane rejection rate and excellent separation factor, while also exhibiting high water flux. However, with further increases in HET concentration, the membrane rejection rate is low, the selectivity is poor, and the water flux is too small.

[0058] Table 1

[0059]

[0060]

[0061] This invention provides a method for preparing a polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine. The materials used are readily available, the preparation process is simple, the preparation flow is short, and the operation is convenient. Membrane separation performance is mainly evaluated by the membrane's salt rejection rate, dye rejection rate, salt and dye separation factor, and water flux. Among these, the separation factor is the core indicator for evaluating membrane separation performance. Compared to salt rejection rate and dye rejection rate alone, it more comprehensively reflects the membrane's actual separation capability in complex systems. Currently, mainstream membrane materials all have significant limitations in terms of performance balance. While polyacrylonitrile-based membranes exhibit ultra-high water flux, their separation factor is very small, and the Congo red rejection rate is only 98.05%. Although polyester (THPPO / TMC) achieves a dye rejection rate of 99.3%, its low water flux and separation factor severely limit its wastewater treatment efficiency. Although polyamide (Arg / TMC) and covalent organic framework (MPD-TFB / Nylon) have large water flux, there is still room for optimization in Congo red rejection rate and separation factor, and their overall performance still needs to be further improved.

[0062] In the preparation method of this invention, HET is introduced as an aqueous monomer to participate in interfacial polymerization, thereby achieving the preparation of a polyester nanofiltration membrane with excellent comprehensive performance, including high Congo red dye rejection rate, high separation factor, and high flux. The obtained polyester nanofiltration membrane has a Congo red dye rejection rate greater than 99.5%, and can reach as high as 99.78%. Due to the hydrolysis of unreacted acyl chloride groups to generate carboxyl groups, which are then deprotonated to generate negatively charged carboxylate groups, the polyester membrane becomes negatively charged. At the same time, the introduction of hydroxyl groups improves the hydrophilicity of the membrane, achieving a simultaneous improvement in the rejection rate and flux of the composite nanofiltration membrane.

[0063] In this invention, through a comparison of Examples 1-4 and Comparative Examples 1-2, it can be concluded that the membrane performance changes with the concentration of the aqueous monomer HET. Under lower HET concentration conditions, due to insufficient supply of hydroxyl functional groups in the reaction system, the crosslinking degree of the polyester separation layer decreases, making it difficult to form a continuous and dense network structure, such as... Figure 3 As shown, this structural defect results in membrane materials having a large water flux, but with low retention rate and poor selectivity, such as... Figure 5 As shown in Table 1, with the continuous increase of HET concentration, the rate of monomer diffusion to the interface increases, and the reaction rate at the interface also increases further, resulting in a thicker and denser separation layer. The membrane has good dye rejection rate and separation factor, but at the same time, due to the greater obstruction to water transport, the water flux decreases, as shown in Table 1. Figure 5 As shown in Table 1. However, in the presence of excessively high concentrations of aqueous HET monomers, the diffusion of monomers in the aqueous phase affects the efficiency of monomer migration to the interface, reducing membrane quality and formation efficiency, resulting in low membrane rejection, poor selectivity, and further reducing the membrane's water flux, such as... Figure 5 As shown in Table 1. Therefore, in the preparation method of this invention, the concentration of the aqueous monomer HET was optimized, and the mass percentage of the HET aqueous solution was limited to 0.2-0.5%, thereby obtaining a polyester nanofiltration membrane with high dye rejection rate and excellent separation factor, while also having high water flux. It can be used for the resource recovery treatment of saline wastewater in pharmaceutical, printing and dyeing, and food industries.

[0064] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many changes under the guidance of the present invention without departing from the spirit of the present invention, and these changes are all within the protection scope of the present invention.

Claims

1. A method for preparing a polyester nanofiltration membrane based on hydroxyethyl hexahydrotriazine, characterized in that, Hydroxyethyl hexahydrotriazine was dissolved in ultrapure water to form an aqueous solution; 1,3,5-tristyrene chloride was dissolved in n-heptane to form an organic solution; a polyacrylonitrile-based membrane was sequentially immersed in the above aqueous and organic solutions, and a polyester nanofiltration membrane was prepared by reacting the hydroxyl groups in hydroxyethyl hexahydrotriazine with the acyl chloride groups of 1,3,5-tristyrene chloride at the phase interface; the specific steps of this method are as follows: Step 1, Pretreatment of polyacrylonitrile film: Wash the polyacrylonitrile film three times with ultrapure water, air dry at room temperature, and then fix it in a plastic mold; Step 2, Immersion in aqueous solution: Dissolve hydroxyethyl hexahydrotriazine in ultrapure water to prepare an aqueous solution of 0.2-0.5% by mass, and adjust the pH to 13 with sodium hydroxide solution to obtain the aqueous solution. Add the aqueous solution to the mold described in Step 1. The amount of aqueous solution used is proportional to the area of ​​the polyacrylonitrile base film at 10 mL / 25 cm². 2 After soaking and reacting for 10 minutes, the film is removed and the surface liquid is removed. The polyacrylonitrile-based film after soaking and reacting is then fixed into the mold. Step 3, Polyester Synthesis: Dissolve 1,3,5-pyromellitic trimethylol chloride in n-heptane to obtain an organic phase solution with a mass percentage of 0.1%. Add the organic phase solution to a mold with a polyacrylonitrile base film fixed thereon after the treatment in Step 2. The amount of organic phase solution used is proportional to the area of ​​the polyacrylonitrile base film at a ratio of 5~10 mL / 25cm². 2 After soaking and reacting for 3-10 minutes, the membrane is removed, air-dried at room temperature, and washed three times with ethanol and ultrapure water respectively to obtain the polyester nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that, In step one, the molecular weight cutoff of the polyacrylonitrile base film is 10 kDa, and the plastic mold has a base film exposed area of ​​5×5 cm.

3. The preparation method according to claim 1, characterized in that, In step two, the hydroxyethyl hexahydrotriazine solution has a mass percentage of 0.3%.

4. The preparation method according to claim 1, characterized in that, In step two, the molar concentration of the sodium hydroxide solution is 3M.

5. The preparation method according to claim 1, characterized in that, In step three, the ratio of the amount of organic phase solution used to the area of ​​the polyacrylonitrile-based membrane is 5 mL / 25 cm². 2 The soaking reaction time is 5 minutes.

6. The preparation method according to claim 1, characterized in that, The prepared polyester nanofiltration membrane was stored in ultrapure water.

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