Method for preparing polyamide ceramic composite nanofiltration membrane by taking long-chain alkane as organic phase solvent

Abstract

The invention relates to the technical field of membrane separation, in particular to a method for synthesizing a polyamide nanofiltration membrane on a water / oil interface by adopting a reverse interfacial polymerization process and adopting a long-chain alkane solvent as an organic phase solvent, and belongs to a novel process technology for nanofiltration membrane preparation. Aiming at the reverse interfacial polymerization characteristic adopted when a ceramic membrane is used as a polyamide nanofiltration membrane carrier, C8-C16 n- / isomeric long-chain alkane is creatively adopted as an organic phase solvent, and specific oscillation operation is matched, so that the high-viscosity long-chain alkane is effectively loaded into membrane pores. Compared with light n-alkanes such as n-hexane and the like used in the prior art, the solubility of acyl chloride monomers is remarkably improved. The improvement ensures that the ceramic membrane carrier can load sufficient acyl chloride organic phase during reverse interfacial polymerization, and provides sufficient oil phase monomer source for subsequent reaction immersed in water phase.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a method for synthesizing polyamide nanofiltration membranes at the water / oil interface using a reverse interfacial polymerization process with long-chain alkane solvents as organic phase solvents. This method belongs to a novel process technology for nanofiltration membrane preparation. Background Technology

[0002] Nanofiltration (NF) is a pressure-driven membrane with separation performance between reverse osmosis and ultrafiltration, and a molecular weight cutoff range of 200-1000 Da. It is mainly used to remove organic matter and pigments from surface water, partially desalinate it, and selectively extract and concentrate active ingredients in the food and pharmaceutical industries. Therefore, the strong demand for freshwater resources has greatly promoted the development of NF membranes.

[0003] Currently, the mainstream preparation process for organic nanofiltration membranes is forward interfacial polymerization (i.e., the support is first loaded with an aqueous amine solution and then immersed in an acyl chloride organic phase). The reason for choosing this process is that commonly used organic polymer supports (such as polyethersulfone PES and polysulfone PSF) are usually quite thin (only a few hundred micrometers), and their limited storage space makes it difficult to effectively load and maintain a sufficient amount of organic phase (oil phase). Secondly, amine monomers (such as piperazine PIP and m-phenylenediamine MPD) have high solubility and stability in the aqueous phase, while acyl chloride monomers (such as TMC) have relatively limited solubility in the organic phase and are easily hydrolyzed. Therefore, adopting a forward configuration with a "small aqueous phase volume and a large oil phase volume," where the support is first loaded with an aqueous phase and then immersed in a large amount of oil phase for reaction, helps ensure a sufficient supply of aqueous monomers inside the support, while the large amount of oil phase outside can also provide a relatively sufficient amount of TMC to participate in the reaction. Conversely, attempts to perform reverse interfacial polymerization on traditional organic supports (i.e., immersing an amine aqueous phase into a support after loading an acyl chloride organic phase) face fundamental difficulties: the weak support is difficult to load with enough organic phase (small oil phase volume), and when a large amount of aqueous phase is immersed (large aqueous phase volume), the limited supply of TMC monomers inside the support is insufficient, and the diffusion path of aqueous monomers into the oil phase is long and the concentration gradient is easily diluted, resulting in the inability to form a uniform and dense high-performance polyamide layer.

[0004] Ceramic supports, due to their unique physicochemical properties, provide an ideal platform for overcoming this limitation. On one hand, the thickness (2–3 mm) of ceramic supports (typically composed of porous structures such as TiO2, Al2O3, SiO2, or ZrO2) is significantly greater than that of organic supports (hundreds of micrometers), providing ample internal space sites for loading more reactive phases (whether aqueous or oil-based). On the other hand, the surface of ceramic supports typically exhibits both hydrophilic and oleophilic properties (i.e., amphiphilicity), enabling them to effectively adsorb and load aqueous solutions as well as effectively wet and load organic solutions—an advantage unmatched by traditional organic polymer supports. These characteristics allow ceramic supports not only to efficiently perform traditional forward interfacial polymerization but also to be perfectly suited for reverse interfacial polymerization processes—their thick porous structure and amphiphilic surface can load sufficient amounts of TMC organic phase, providing a sufficient source of oil-based monomers for subsequent immersion in the aqueous phase.

[0005] Existing technologies have disclosed the preparation of polyamide separation layers via reverse interfacial polymerization on ceramic supports. However, the organic solvents used in these processes are all light n-alkanes such as hexane. The inventors discovered that when using reverse interfacial polymerization, due to the "small oil phase and large water phase," the solubility of light n-alkanes, represented by hexane, in acyl chlorides is insufficient. This results in an insufficient number of organic monomers participating in the interfacial polymerization reaction, which severely restricts the selective separation performance of the membrane (it is difficult to achieve both water flux and rejection rate).

[0006] Therefore, there is an urgent need to optimize and improve the existing method for preparing polyamide nanofiltration membranes by reverse interfacial polymerization on ceramic supports. Summary of the Invention

[0007] To address the problems in preparing polyamide nanofiltration membranes via reverse interfacial polymerization on ceramic membranes, this invention uses C8-C16 normal / isomeric long-chain alkanes as organic phase solvents, breaking through existing technical bottlenecks, synergistically improving membrane separation performance (water flux and salt rejection rate), and meeting the needs of industrial applications.

[0008] Specifically, this invention provides a method for preparing polyamide ceramic composite nanofiltration membranes using long-chain alkanes as organic phase solvents, comprising the following steps:

[0009] S1. Provide ceramic membrane;

[0010] S2. After loading the ceramic membrane into the organic phase monomer solution, remove the ceramic membrane and dry it; the organic phase monomer solution uses C8-C16 normal / isomeric long-chain alkanes as the organic phase solvent;

[0011] S3. The dried ceramic membrane is immersed in an aqueous monomer solution to carry out a reverse interfacial polymerization reaction to form a polyamide separation layer.

[0012] S4. Remove the reacted ceramic membrane and perform heat treatment to obtain a polyamide ceramic composite nanofiltration membrane.

[0013] Preferably, in step S1, the ceramic membrane is one of an alumina ceramic tube ultrafiltration membrane, a zirconia ceramic tube ultrafiltration membrane, a titanium dioxide ceramic tube ultrafiltration membrane, or a silicon carbide ceramic tube ultrafiltration membrane with a pore size of 5 nm to 1 μm.

[0014] Preferably, in step S2, the organic monomer concentration in the organic monomer solution is 0.1-0.5 wt%, and the organic monomer is an organic compound containing a polyacrylamide chloride group, selected from one or more of pyromellitic trimethylolpropionate chloride, phthaloyl chloride, or pyromellitic tetramethylolpropionate chloride.

[0015] Preferably, in step S2, no surfactant or auxiliary stabilizer is added to the organic phase monomer solution to maintain a non-polar state.

[0016] Preferably, in step S2, after the ceramic membrane is immersed in the organic phase monomer solution for loading, it is sealed with plastic wrap, and then placed in a constant temperature shaker at 20-50°C for 10-50 minutes. After that, it is taken out and dried in the air for 10-60 minutes, and the organic solvent on the inner surface of the membrane is allowed to dry naturally.

[0017] Preferably, in step S3, the aqueous monomer solution is a mixed solution of an aqueous monomer and an alkaline solution, and the mass concentration of the aqueous monomer is 0.1–6 wt%; the aqueous monomer is an organic compound containing multiple amino groups, selected from one or more of piperazine, m-phenylenediamine, ethylenediamine, diethylenetriamine, polyethyleneimine, or dopamine; and the alkaline solution is a NaOH solution with a mass concentration of 0.1–0.7 wt%.

[0018] Preferably, in step S3, the interfacial polymerization reaction takes 10 to 360 seconds; after the reaction is completed, the membrane surface is rinsed with pure water to remove residual monomers and byproducts.

[0019] Preferably, in step S4, the heat treatment temperature is 40–160°C and the heat treatment time is 20–45 min.

[0020] The present invention also provides a polyamide ceramic composite nanofiltration membrane prepared using long-chain alkanes as organic phase solvents, which is prepared by the above method.

[0021] The polyamide ceramic composite nanofiltration membrane prepared by the present invention using long-chain alkanes as organic phase solvent can be applied in salt solution retention.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] This invention addresses the reverse interfacial polymerization characteristics of ceramic membranes used as polyamide nanofiltration membrane carriers. It innovatively employs C8-C16 normal / isomeric long-chain alkanes as organic phase solvents, coupled with specific shaking operations, to ensure effective loading of high-viscosity long-chain alkanes into the membrane pores. Compared to the light normal-chain alkanes such as hexane used in existing technologies, this significantly improves the solubility of acyl chloride monomers. This improvement ensures that the ceramic membrane carrier can load a sufficient amount of acyl chloride organic phase during reverse interfacial polymerization, providing a sufficient source of oil phase monomers for subsequent immersion in the aqueous phase. Consequently, the resulting polyamide separation layer is more uniform and dense, effectively improving the membrane's water flux and salt rejection rate, overcoming the bottleneck of difficulty in simultaneously achieving high water flux and salt rejection rate in existing technologies. Attached Figure Description

[0024] Figure 1 SEM characterization image of the membrane sample prepared in Example 1 of this invention (left - cross-section, right - surface). Detailed Implementation

[0025] Example 1

[0026] A method for preparing polyamide ceramic composite nanofiltration membranes using reverse interfacial polymerization with long-chain (normal / isomeric) alkanes as the organic phase, specifically including the following preparation steps:

[0027] 1. Ceramic membrane pretreatment

[0028] First, wrap the outer surface of the ceramic membrane tube with PTFE tape (Teflon tape), then wrap it tightly with transparent tape, and coat both sides with A / B glue. After solidification, it is ready for use.

[0029] 2. Preparation of polyamide ceramic composite nanofiltration membrane

[0030] (1) The pretreated ceramic membrane was immersed in a 0.1 wt% solution of trimesoyl chloride with n-(iso)octane (n / i-C8) as the organic solvent and sealed with plastic wrap. Then it was placed in an air bath constant temperature shaker and shaken at 30°C for 30 min. After that, it was taken out and dried in the air for 35 min, and the organic solvent on the inner surface of the membrane was allowed to dry naturally.

[0031] (2) The dried ceramic membrane was immersed in a solution of 0.1 wt% aqueous monomer piperazine and 0.3 wt% NaOH for interfacial polymerization for 120 s, and then the ceramic membrane was removed.

[0032] (3) Heat-treat the ceramic membrane in an oven at 140°C for 30 minutes to obtain a polyamide ceramic composite nanofiltration membrane.

[0033] Example 2

[0034] The preparation steps in this embodiment are the same as in Example 1, except that the organic solvent is n / i-C16 hexadecane, the concentration of the organic phase monomer pyromellitic chloride is 0.5 wt%, and the shaking time in the gas bath constant temperature shaking box is 60 min.

[0035] Example 3

[0036] The preparation steps in this embodiment are the same as in Example 1, except that the mass concentration of the aqueous piperazine monomer solution is 6wt%, and the interfacial polymerization reaction time is 360s, the heat treatment temperature is 60℃, and the heat treatment time is 45min when preparing the polyamide ceramic composite nanofiltration membrane.

[0037] Example 4

[0038] The preparation steps in this embodiment are the same as in Example 1. The difference is that when preparing the polyamide ceramic composite nanofiltration membrane, the heat treatment temperature is 40°C, the heat treatment time is 20 min, and the interfacial polymerization reaction time is 10 s.

[0039] Example 5

[0040] The preparation steps in this embodiment are the same as in Example 1, except that the organic solvent is toluene, the concentration of the organic phase monomer pyromellitic acid chloride is 0.4 wt%, the heat treatment temperature is 70 °C, and the heat treatment time is 10 min.

[0041] Example 6

[0042] The preparation steps in this embodiment are the same as in Example 1, except that the organic solvent is dichloromethane, the organic phase monomer is pyromellitic chloride, the organic phase monomer concentration is 0.5wt%, the heat treatment temperature is 60℃, and the treatment time is 5min.

[0043] Comparative Example 1

[0044] Compared to Example 1, instead of using long-chain alkanes as organic solvents for interfacial polymerization, short-chain alkanes (n-hexane), commonly used in the laboratory, were used as organic solvents for reverse interfacial polymerization on the surface of the ceramic film.

[0045] (1) Immerse the pretreated ceramic membrane in a 0.1 wt% solution of trimesoyl chloride using n-hexane or cyclohexane as the organic solvent, and seal it with plastic wrap. Then place it in an air bath constant temperature shaker and shake it at 30°C for 30 min. Remove it and dry it in the air for 35 min, allowing the organic solvent on the inner surface of the membrane to air dry naturally.

[0046] (2) The dried ceramic membrane was immersed in a solution of 0.1 wt% aqueous piperazine and 0.3 wt% NaOH and subjected to interfacial polymerization for 120 s. The ceramic membrane was then removed.

[0047] (3) Heat treatment of the ceramic membrane in an oven at 140°C for 30 minutes to obtain a polyamide ceramic composite nanofiltration membrane.

[0048] Comparative Example 2

[0049] The difference from Example 1 is that the ceramic membrane was subjected to hydroxyl activation treatment (immersion in deionized water) before interfacial polymerization. All other parameters are the same.

[0050] Comparative Example 3

[0051] The difference from Example 1 is that the ceramic membrane used for interfacial polymerization is a flat sheet membrane without any coating treatment. The interfacial polymerization reaction is carried out only on one side of the flat sheet membrane; all other parameters and steps are the same.

[0052] Comparative Example 4

[0053] The difference from Example 1 is that the 30-minute oscillation in the air bath constant temperature shaker is replaced by a 30-minute static soaking at the same temperature. All other parameters and steps are the same.

[0054] Comparative Example 5

[0055] The difference from Example 1 is that the aqueous piperazine solution does not contain NaOH solution, while the other parameters and steps are the same.

[0056] Comparative Example 6

[0057] The difference from Example 1 is that the aqueous piperazine solution does not contain NaOH solution, the concentration of the organic phase monomer pyromellitic chloride is 0.5%, and the other parameters and steps are the same.

[0058] Test methods

[0059] Test conditions: The nanofiltration performance of the designed membrane was evaluated using a cross-flow filtration device, with an effective measurement area of ​​11.3 cm². 2 The feed solution was a 1000 ppm salt solution containing monovalent / divalent salts at a flow rate of 30 L / h, at room temperature. All tests were pre-pressurized at 5 bar for 40 min, and then tested at 4 bar after the system stabilized. Monovalent / divalent salts included any one or more combinations of sodium sulfate, magnesium sulfate, sodium chloride, and magnesium chloride.

[0060] The permeability (J, LMH / bar) of a composite membrane reflects its permeability, while the rejection rate (R, %) reflects its separation performance. Both can be calculated using the following equations:

[0061]

[0062] In the formula: Δv(L) is the feed solution volume, A(m 2) represents the membrane area, t(h) represents the effective permeation time, p(bar) represents the transmembrane pressure, and C p (g / L) and C f (g / L) represents the concentrations in the permeate solution and the feed, respectively.

[0063] Test Results

[0064] 1. Membrane retention performance and permeation flux testing

[0065] (1) The retention rate and permeation flux of the composite nanofiltration membranes prepared in Examples 1-4 and the comparative examples were tested, and the results are shown in Table 1 and Table 2 below:

[0066] Table 1. Retention rate test results of composite nanofiltration membranes

[0067]

[0068] Table 2. Permeation flux test results of composite nanofiltration membranes

[0069]

[0070]

[0071] As can be seen from Tables 1 and 2, the polyamide nanofiltration membrane prepared by reverse interfacial polymerization can maintain good permeation flux while also protecting against Mg. 2+ The membrane exhibits high ion rejection (MgCl2, MgSO4). This is because, during the RIP process, the amine concentration in the reaction zone is much higher than the acyl chloride concentration. This results in a large number of unreacted amine groups remaining on the membrane surface after the reaction. These amine groups are easily protonated in aqueous solution, forming a positively charged polyamide separation layer with a more uniform charge distribution. These positively charged membranes are effective for separating divalent / monovalent ions (such as MgCl2, MgSO4). 2+ / Li+) has higher selectivity because the Donnan effect repels divalent cations, while conventional negatively charged membranes attract divalent cations.

[0072] As shown in Comparative Example 5 in Table 1, the membrane's retention performance is significantly reduced. This is because HCl is generated inside the ceramic channels during the reaction. If the aqueous phase is neutral / acidic, HCl will rapidly protonate the PIP within the channels, deactivating it. The limited diffusion space of the ceramic support exacerbates the local pH drop (in traditional forward polymerization, HCl can diffuse into a large amount of the aqueous phase). In other words, under neutral or acidic conditions (pH ≤ 7), the rapid protonation and deactivation of PIP within the channels leads to incomplete reaction and discontinuous membrane. Simultaneously, the TMC is rapidly hydrolyzed by the infiltrated water, resulting in insufficient cross-linking and a sharp drop in desalination rate.

[0073] Figure 1This is a SEM image of the membrane sample prepared in Example 1 of this application. As can be seen from the image, the membrane prepared by reverse IP is significantly smoother and flatter. Traditional TFC nanofiltration membranes prepared using the PIP-TMC system typically exhibit a relatively rough surface morphology, often forming a distinct "ridge-and-valley" structure. In contrast, the surface of the membrane layer prepared by reverse IP (organic phase first contacts the substrate) tends to be smoother and "valley-free," approximating a continuous membrane structure. This difference indicates that in traditional IP, the heterogeneous polymerization reaction occurs intensely near the water droplet, easily producing local protrusions; RIP, by adjusting the phase sequence and diffusion path at the reaction interface, achieves uniform growth of the polyamide layer, resulting in a more uniform polyamide layer that exhibits a macroscopically smooth structure on SEM.

[0074] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for preparing polyamide ceramic composite nanofiltration membranes using long-chain alkanes as organic phase solvents, characterized in that... Includes the following steps: S1. Provide ceramic membrane; S2. After loading the ceramic membrane into the organic phase monomer solution, remove the ceramic membrane and dry it; the organic phase monomer solution uses C8-C16 normal / isomeric long-chain alkanes as the organic phase solvent; S3. The dried ceramic membrane is immersed in an aqueous monomer solution to carry out a reverse interfacial polymerization reaction to form a polyamide separation layer. S4. Remove the reacted ceramic membrane and perform heat treatment to obtain a polyamide ceramic composite nanofiltration membrane.

2. The method according to claim 1, characterized in that, In step S1, the ceramic membrane is one of the following: an alumina ceramic tube ultrafiltration membrane, a zirconia ceramic tube ultrafiltration membrane, a titanium dioxide ceramic tube ultrafiltration membrane, or a silicon carbide ceramic tube ultrafiltration membrane with a pore size of 5 nm to 1 μm.

3. The method according to claim 1, characterized in that, In step S2, the organic phase monomer solution has a mass concentration of 0.1 to 0.5 wt%, and the organic phase monomer is an organic compound containing a polyacrylamide chloride group, selected from one or more of pyromellitic trimethylolpropionate chloride, phthaloyl chloride, or pyromellitic tetramethylolpropionate chloride.

4. The method according to claim 1, characterized in that, In step S2, no surfactants or auxiliary stabilizers are added to the organic phase monomer solution to maintain a non-polar state.

5. The method according to claim 1, characterized in that, In step S2, after the ceramic membrane is immersed in the organic phase monomer solution for loading, it is sealed with plastic wrap and then placed in a constant temperature shaker at 20-50℃ for 10-50 minutes. After that, it is taken out and dried in the air for 10-60 minutes, and the organic solvent on the inner surface of the membrane is allowed to dry naturally.

6. The method according to claim 1, characterized in that, In step S3, the aqueous monomer solution is a mixed solution of aqueous monomer and alkaline solution, and the mass concentration of the aqueous monomer is 0.1-6 wt%. The aqueous monomer is an organic compound containing multiple amino groups, selected from one or more of piperazine, m-phenylenediamine, ethylenediamine, diethylenetriamine, polyethyleneimine, or dopamine. The alkaline solution is a NaOH solution with a mass concentration of 0.1-0.7 wt%.

7. The method according to claim 1, characterized in that, In step S3, the interfacial polymerization reaction takes 10 to 360 seconds; after the reaction is completed, the membrane surface is rinsed with pure water to remove residual monomers and byproducts.

8. The method according to claim 1, characterized in that, In step S4, the heat treatment temperature is 40–160°C and the heat treatment time is 20–45 min.

9. A polyamide-ceramic composite nanofiltration membrane prepared using long-chain alkanes as the organic phase solvent, characterized in that... Prepared using the method described in claim 1.

10. The application of the polyamide ceramic composite nanofiltration membrane prepared using long-chain alkanes as organic phase solvent as described in claim 9 in salt solution retention.

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