PDMS-ROF cross-linked membrane based on beta-ketoenamine structure as well as preparation method and application of PDMS-ROF cross-linked membrane

Abstract

The invention relates to a PDMS-ROF cross-linked membrane based on a beta-ketoenamine structure as well as a preparation method and application of the PDMS-ROF cross-linked membrane, and belongs to the field of high polymer material and membrane separation. The preparation method comprises the following steps: adding an aldehyde monomer solution into a PDMS solution of which a branched chain or a main chain is connected with amino groups to obtain a PDMS-ROF membrane casting solution with a beta-keto-enamine structure, blade-coating the PDMS-ROF membrane casting solution on a hydrophobic base membrane, and drying to obtain the PDMS-ROF membrane based on the beta-keto-enamine structure. The invention provides a PDMS-ROF membrane with high pervaporation performance based on a beta-ketoenamine structure and a preparation method of the PDMS-ROF membrane. According to the PDMS-ROF membrane disclosed by the invention, through Schiff base reaction between amino and aldehyde group and subsequent irreversible enol-ketone tautomerization synthesis, the preparation method simultaneously realizes rapid crosslinking and formation of a high-rigidity structure; the obtained PDMS-ROF membrane has an excellent separation factor and also has a considerable total flux.

Description

Technical Field

[0001] The present invention relates to the field of polymer materials and membrane separation, and more specifically, to a PDMS-ROF cross-linked membrane based on a β-ketoenamine structure, a preparation method and applications thereof, and in particular to a preparation method of a polydimethylsiloxane-rubber organic framework (PDMS-ROF) cross-linked membrane based on a β-ketoenamine structure and applications thereof in pervaporation dealcoholization. The membrane is suitable for separation processes such as pervaporation purification of bio-alcohols. Background Art

[0002] Bioethanol is a green, renewable bioenergy source derived from biomass fermentation. With its widespread availability and environmental friendliness, it has recently been recognized as an excellent alternative to fossil fuels. To improve bioethanol production efficiency and product purity, timely separation of ethanol from the fermentation broth is a key factor in its production. Membrane-based pervaporation (PV) technology utilizes the chemical potential difference between upstream and downstream components of the membrane as the driving force for mass transfer. With its advantages of ease of operation, low energy consumption, and environmental friendliness, PV has become an advanced method for separating and purifying ethanol in recent years.

[0003] Polydimethylsiloxane (PDMS) is a mainstream dealcoholization membrane material due to its high hydrophobicity and stability, but it generally requires cross-linking to achieve membrane-forming properties. Conventional cross-linking, based on condensation reactions between silanol groups or self-cross-linking between silicon and hydrogen atoms, typically requires hours or even days to achieve a stable cross-linked state. This lengthy cross-linking cycle not only severely limits the production efficiency of PDMS membranes, but also poses a significant challenge to precisely controlling the degree of cross-linking. Qin Peiyong's team reported in The Ultrafast and Continuous Fabrication of a Polydimethylsiloxane Membrane by Ultraviolet-Induced Polymerization a method for ultrafast continuous preparation of PDMS pervaporation membranes by ultraviolet-induced functionalized PDMS polymerization, but the prepared membranes were not very rigid. Wang Yan's team reported in High-Performance and Scalable Organosilicon Membranes for Energy-Efficient Alcohol Purification a high-performance and scalable organosilicon membrane. By reacting highly active trimesoyl chloride (TMC) with amino PDMS, they achieved rapid and controllable cross-linking of the PDMS material, but the prepared membranes were still not rigid enough.

[0004] In addition to optimizing the controllable cross-linking of the PDMS system, improving the chain rigidity of the PDMS cross-linked network is also one of the important key factors affecting the membrane separation performance. By increasing the rigidity of the PDMS chain, the stability of the microporous structure of the cross-linked network can be enhanced, and the free volume of the PDMS network can be optimized to a certain extent, thereby significantly improving the ethanol selectivity of the membrane. For example, Jin Wanqin's team reported a method in PDMS with Tunable Side Group Mobility and Its Highly Permeable Membrane for Removal of Aromatic Compounds to increase the rigidity of the PDMS chain by introducing benzene rings, thereby reducing the spatial steric hindrance of large molecules passing through the membrane and achieving selective transport of molecules. However, the prepared membrane cannot be cross-linked quickly. The defects of existing cross-linking and modification strategies can be summarized as follows: it is difficult to achieve both rapid cross-linking and high rigidity, dynamic bonds limit the durability of the material, and the complex process hinders industrial application.

[0005] In recent years, rubber organic frameworks (ROFs), which combine soft rubber polymer segments with rigid framework structures, have provided new ideas for cross-linking modification of PDMS. Mihail Barboiu's team reported in Rubbery organic frameworks (ROFs) toward ultrapermeable CO2-selective membranes that ROFs constructed with reversible chemical bonds combine soft polytetrahydrofuran rubber polymer segments with rigid framework structures to obtain CO2 capture membranes with both good selectivity and permeability. The presence of dynamic imine bonds provides the material with the ability to dynamically reorganize, but the presence of dynamic bonds also makes the resulting membrane structure unstable. Summary of the Invention

[0006] The present invention uses a reversible Schiff base reaction and an irreversible enol-ketone isomerization reaction to rapidly crosslink a highly rigid and stable PDMS-ROF network, addressing the existing technical problem of simultaneously achieving both rapid crosslinking and high rigidity in pervaporation membrane preparation.

[0007] According to a first aspect of the present invention, a method for preparing a PDMS-ROF membrane based on a β-ketoenamine structure is provided, comprising the following steps:

[0008] (1) adding an aldehyde monomer solution to a PDMS solution having amino groups attached to the side chains and / or main chains, causing the aldehyde groups to undergo a reversible Schiff base reaction with the amino groups to obtain an intermediate, and then the enol on the intermediate is irreversibly tautomerized to a ketone, thereby obtaining a PDMS-ROF casting solution having a β-ketoenamine structure;

[0009] (2) The casting solution obtained in step (1) is coated on the hydrophobic base film, and after drying, a PDMS-ROF film based on the β-ketoenamine structure is obtained.

[0010] Preferably, the aldehyde monomer is at least one of 2-hydroxy-1,3,5-benzenetricarboxaldehyde, 2,4-dihydroxy-1,3,5-benzenetricarboxaldehyde and 1,3,5-triformylphloroglucinol.

[0011] Preferably, the ratio of the amount of amino groups in the PDMS having amino groups connected to the side chains and / or main chains to the amount of aldehyde groups on the aldehyde monomer is (3.0-9.0):1.

[0012] Preferably, the hydrophobic base membrane is a polyvinylidene fluoride membrane or a polytetrafluoroethylene membrane.

[0013] According to another aspect of the present invention, a prepared PDMS-ROF membrane based on a β-ketoenamine structure is provided.

[0014] Preferably, the thickness of the PDMS-ROF membrane based on the β-ketoenamine structure is 5-6 μm.

[0015] According to another aspect of the present invention, there is provided application of the PDMS-ROF membrane based on the β-ketoenamine structure in membrane separation.

[0016] Preferably, the membrane separation is pervaporation.

[0017] Preferably, the membrane separation is used for separation of alcohol-water mixture.

[0018] Preferably, the alcohol in the alcohol-water mixture is methanol, ethanol, isopropanol or n-butanol.

[0019] In general, the above technical solution conceived by the present invention, compared with the existing technology, solves the problems of difficulty in achieving both rapid cross-linking and high rigidity in the traditional PDMS cross-linking process and the complex process, and can achieve the following beneficial effects:

[0020] (1) The present invention provides a PDMS-ROF membrane with high pervaporation performance based on a β-ketoenamine structure and a preparation method thereof. The preparation process successfully crosslinks the membrane via a rapid, reversible Schiff base reaction between amino and aldehyde groups to obtain a ROF network with dynamic imine bonds. On this basis, the enol-imine structure in the network undergoes irreversible enol-keto tautomerism through proton transfer, resulting in a β-ketoenamine structure with greater stability and rigidity. This preparation method simultaneously achieves rapid crosslinking and the formation of a highly rigid structure.

[0021] (2) The present invention provides a method for preparing a PDMS-ROF membrane based on a β-ketoenamine structure, which rapidly crosslinks to form a PDMS-ROF network via a reversible Schiff base reaction and an irreversible enol-keto isomerization reaction. The PDMS-ROF network, formed by PDMS with amino acids attached to the backbone or MAPDMS with multiple amino groups on the side chains and Tp, combines the good permeability of a rubbery polymer with the excellent selectivity of a framework structure.

[0022] (3) The present invention simplifies the synthesis process, shortens the PDMS cross-linking time, imparts additional rigidity to the PDMS segments, and simultaneously regulates the cross-linking density of the PDMS-ROF network. The present invention exhibits a simple process and excellent bio-alcohol separation performance, showing broad application prospects in membrane separations (e.g., pervaporation bio-alcohol purification).

[0023] (4) The PDMS-ROF membrane prepared by the preparation method of the β-ketoenamine structure-based PDMS-ROF membrane provided by the present invention has excellent pervaporation performance. Compared with the traditional silanol cross-linked membrane, it has an excellent separation factor (the separation factor is 9.67 when separating ethanol) and also has a considerable total flux (the total flux when separating bioethanol is 1.82 kg m - 2 h -1 ). The flux for separation of various alcohols is greater than 1.2 kg m -2 h -1 .

[0024] (5) The PDMS-ROF membrane prepared by the preparation method of the PDMS-ROF membrane based on the β-ketoenamine structure provided by the present invention has good universality, especially excellent performance in the pervaporation purification of methanol, ethanol, isopropanol and n-butanol. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the formation mechanism of the PDMS-ROF network according to the present invention.

[0026] Figure 2 This is the SEM cross-sectional image of the PDMS-ROF composite membrane with a β-ketoenamine structure.

[0027] Figure 3 The performance of four PDMS-ROF networks according to the present invention for methanol removal by pervaporation is compared with that of other polymer-based pervaporation membranes.

[0028] Figure 4 The performance of four PDMS-ROF networks according to the present invention for ethanol removal by pervaporation is compared with that of other polymer-based pervaporation membranes.

[0029] Figure 5 The performance of four PDMS-ROF networks according to the present invention for isopropanol removal by pervaporation is compared with that of other polymer-based pervaporation membranes.

[0030] Figure 6 The performance of four PDMS-ROF networks according to the present invention for the pervaporation removal of n-butanol is compared with that of other polymer-based pervaporation membranes. DETAILED DESCRIPTION

[0031] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0032] Examples 1-5

[0033] The present invention provides a method for preparing a PDMS-ROF cross-linked membrane based on a β-ketoenamine structure, comprising the following steps:

[0034] (1) Preparation of base membrane: Polyvinylidene fluoride (PVDF) powder was dried under vacuum at 80°C for 24 hours before use. PVDF, triethyl phosphate (TEP), polyvinyl pyrrolidone (PVP) and N-methyl pyrrolidone (NMP) were mixed in a mass ratio of 16:30:6:48 and stirred at 80°C for 48 hours to obtain a uniform casting solution. The casting solution was scraped onto the surface of the polyester non-woven fabric with a 120μm stainless steel scraper. It was then transferred to a water coagulation bath for non-solvent induced phase inversion (NIPs). In order to further remove TEP, PVP and NMP in the membrane, the deionized water was replaced every 6 hours for two days. Finally, the PVDF base membrane was freeze-dried for use.

[0035] (2) Preparation of PDMS-ROF casting solution: A 0.075 wt% dichloromethane solution of Tp (1,3,5-triformylphloroglucinol) was added in batches to a 10 wt% dichloromethane solution of branched MAPDMS with multiple amino groups. The content of amino groups on MAPDMS was 0.5 mmol / g, and the mass ratio of MAPDMS to Tp was 136:1. At this time, the molar ratio of amino groups to aldehyde groups was 4.8:1. At this time, the addition of Tp solution was stopped to obtain a PDMS-ROF casting solution with a ketoenamine structure. Figure 1According to the formation mechanism of the PDMS-ROF network of the present invention, the network is first successfully formed through a rapid reversible Schiff base reaction between the amino group of MAPDMS and the aldehyde group of Tp to obtain a ROF network with a dynamic imine bond. On this basis, the enol-imine structure in the network realizes irreversible enol-keto tautomerism through proton transfer, thereby obtaining a β-ketoenamine structure with stronger stability and rigidity.

[0036] (2) Preparation of PDMS-ROF composite membrane: Use a 150 μm thick stainless steel scraper to scrape the degassed PDMS-ROF casting solution onto the surface of the PVDF base membrane, and then heat it at 40 °C for 12 h to ensure that the solvent is completely evaporated to obtain the PDMS-ROF composite membrane. Figure 2 This is an SEM cross-sectional image of a PDMS-ROF composite membrane having a β-ketoenamine structure. The thickness of the cross-sectional film of the PDMS-ROF composite membrane is 5.03 μm.

[0037] The present invention also explores the effects of different mass ratios of amine monomers to aldehyde monomers on the performance of the membrane, as follows:

[0038] The present invention relates to a Schiff base reaction between a Tp monomer and MAPDMS and subsequent enol-keto tautomerism. Therefore, the mass ratio of Tp to MAPDMS affects the molar ratio of amino groups to aldehyde groups, thereby affecting the crosslinking density of the PDMS-ROF network and, in turn, its separation performance. Table 1 shows the performance of PDMS-ROF membranes prepared according to different molar ratios of amino groups to aldehyde groups during pervaporation purification of ethanol at 60°C (the amino content in the MAPDMS is 0.5 mmol / g, and the amount of MAPDMS used in preparing the solution is 1 g). The total flux and separation factor were tested using a pervaporation apparatus with a feed concentration of 5 wt% ethanol solution. A pre-run of 1.5 hours was performed before the test to stabilize the membrane performance. The test conditions were 60°C, and the downstream pressure of the membrane was maintained below 100 Pa.

[0039] Table 1

[0040]

[0041] Example 1, Examples 6-8: Effects of different aldehyde monomer systems on membrane properties

[0042] The aldehyde monomers of the present invention are crosslinking agents for crosslinking MAPDMS. PDMS-ROF membranes were prepared according to Specific Embodiments 1, 2, and 3 using four different aldehyde monomers (trimethylenediamine (Tb), 2-hydroxy-1,3,5-benzenetricarboxaldehyde (HTP), 2,4-dihydroxy-1,3,5-benzenetricarboxaldehyde (DTP), and Tp). All four aldehyde monomers were prepared as 0.075 wt% dichloromethane solutions. The remaining steps of this example were the same as those of Example 1. Table 2 shows the performance of PDMS-ROF membranes prepared with different aldehyde monomers for pervaporation purification of ethanol at 60°C.

[0043] Table 2

[0044]

[0045] Example 9: The PDMS-ROF composite membrane prepared in Example 1 was used to test the pervaporation performance of four alcohols (methanol, ethanol, isopropanol, and n-butanol). The feed concentrations were adjusted according to the water solubility of the alcohols, with methanol, ethanol, and isopropanol at 5 wt %, and n-butanol at 1 wt %. The remaining steps were the same as in Example 1. Table 3 shows the pervaporation performance of the PDMS-ROF membrane prepared with Tp monomer at 60°C.

[0046] Table 3

[0047]

[0048] Example 1, Example 10: Effects of different amine monomer systems on membrane performance

[0049] The aldehyde monomer Tp of the present invention is a crosslinking agent for crosslinking PDMS. PDMS-ROF membranes were prepared according to Example 1 using two amine monomers: APDMS with amino groups terminated in the main chain and MAPDMS with multiple amino groups in the side chains. Both amine monomers were prepared as 10 wt% solutions in dichloromethane. The remaining steps in this example were the same as those in Example 1. Table 4 shows the performance of PDMS-ROF membranes prepared using Tp-crosslinked amine monomers for pervaporation purification of ethanol at 60°C.

[0050] Table 4

[0051]

[0052] Comparative Example

[0053] The performance of the membranes prepared in Examples 1, 6-8 was compared with that of other polymer-based pervaporation membranes for removing methanol, ethanol, isopropanol and n-butanol. Figure 3-6 The above comparison fully verifies the technical progress and creativity of the present invention in the field of pervaporation membranes.

[0054] Depend on Figure 3It can be seen that the total flux of the PDMS-ROF membrane prepared according to Examples 1, 6-8 when removing methanol reached 1.92-3.02 kg m -2 h -1 , far exceeding the total flux reported in existing literature (0.5-1.5 kg m -2 h -1 ), while the separation factor is maintained at around 6, and the separation performance is quite impressive.

[0055] Depend on Figure 4 It can be seen that the PDMS-ROF membrane prepared according to Example 1 has a flux of 1.82 kg m -2 h -1 The separation factor reached 9.67, which is the most advantageous among polymer-based pervaporation membranes. Meanwhile, the PDMS-ROF membranes prepared according to Examples 6-8 achieved a flux of 2.62-2.86 kg m for ethanol removal. -2 h -1 , far exceeding the total flux reported in most existing literature (0.3-2.5 kg m -2 h -1 ).

[0056] Depend on Figure 5 It can be seen that the total flux of the PDMS-ROF membrane prepared according to Examples 1, 6-8 when removing isopropanol reached 1.21-2.83 kg m -2 h -1 , far exceeding the total flux reported in existing literature (0.3-1.5 kg m -2 h -1 ), while the separation factor is maintained at around 8.4-10.3, and the separation performance is quite impressive.

[0057] Depend on Figure 6 It can be seen that the flux of the PDMS-ROF membranes prepared according to Examples 1 and 7 for removing n-butanol reached 1.92 and 3.91 kg m -2 h -1 The separation factors reached 40.67 and 38.25, which are dominant among polymer-based pervaporation membranes. Meanwhile, the PDMS-ROF membranes prepared according to Examples 6 and 8 have an advantage in the total flux when removing n-butanol by pervaporation, with total fluxes of 2.47 and 3.62 kg m -2 h -1 .

[0058] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a PDMS-ROF membrane based on a β-ketoenamine structure, characterized in that: The following steps are involved: (1) adding an aldehyde monomer solution to a PDMS solution having amino groups attached to the side chains and / or main chains, causing the aldehyde groups to undergo a reversible Schiff base reaction with the amino groups to obtain an intermediate, and then the enol on the intermediate is irreversibly tautomerized to a ketone, thereby obtaining a PDMS-ROF casting solution having a β-ketoenamine structure; (2) The casting solution obtained in step (1) is coated on the hydrophobic base film, and after drying, a PDMS-ROF membrane based on the β-ketoenamine structure is obtained.

2. The method for preparing a PDMS-ROF membrane based on a β-ketoenamine structure according to claim 1, wherein: The aldehyde monomer is at least one of 2-hydroxy-1,3,5-benzenetricarboxaldehyde, 2,4-dihydroxy-1,3,5-benzenetricarboxaldehyde and 1,3,5-triformylphloroglucinol.

3. The method for preparing a PDMS-ROF membrane based on a β-ketoenamine structure according to claim 1, wherein: The ratio of the amount of amino groups in the PDMS having amino groups connected to the side chains and / or main chains to the amount of aldehyde groups on the aldehyde monomer is (3.0-9.0):

1.

4. The method for preparing a PDMS-ROF membrane based on a β-ketoenamine structure according to claim 1, wherein: The hydrophobic base membrane is a polyvinylidene fluoride membrane or a polytetrafluoroethylene membrane.

5. A PDMS-ROF membrane based on a β-ketoenamine structure prepared by the method according to any one of claims 1 to 4.

6. The PDMS-ROF membrane based on β-ketoenamine structure according to claim 5, characterized in that The thickness of the PDMS-ROF film based on the β-ketoenamine structure is 5-6 μm.

7. Use of the PDMS-ROF membrane based on β-ketoenamine structure as claimed in claim 5 or 6 in membrane separation.

8. The use according to claim 7, characterized in that The membrane separation is pervaporation.

9. The use according to claim 7, characterized in that The membrane separation is used for separation of alcohol-water mixture.

10. The use according to claim 9, characterized in that The alcohol in the alcohol-water mixture is methanol, ethanol, isopropanol or n-butanol.

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