An alkyl chain-modified zeolite-doped dimethicone mixed matrix membrane, and a preparation method and applications thereof

Abstract

The application discloses an alkyl chain modified zeolite doped polydimethylsiloxane mixed matrix membrane and a preparation method and application thereof, and the mixed matrix membrane comprises a polydimethylsiloxane matrix, wherein MFI type zeolite is doped in the polydimethylsiloxane matrix, and the MFI type zeolite is grafted with alkyl chains with carbon number C1-C 16 The application constructs a novel mixed matrix membrane for separating azeotropic mixture by doping MFI zeolite with adjustable pore chemical properties into a PDMS matrix. The MFI zeolite provides adjustable transmission channels for organic molecules by grafting alkyl chains with different carbon numbers (C1, C4, C8) to capture the organic molecules into the zeolite pores, and enhances the preferential adsorption of the organic molecules and the subsequent diffusion through the zeolite pores. The mixed matrix membrane enhances the affinity for DMC molecules and reduces the transmission resistance of DMC through the membrane when separating a dimethyl carbonate / methanol azeotropic mixture, and exhibits excellent permeation flux.

Description

Technical Field

[0001] This invention relates to an alkyl chain modified zeolite-doped polydimethylsiloxane mixed matrix membrane, its preparation method and application, belonging to the field of membrane separation technology. Background Technology

[0002] The efficient separation of organic-organic azeotropic mixtures is crucial for industries such as pesticides, petrochemicals, and pharmaceuticals. Dimethyl carbonate (DMC) is a green chemical product that has attracted much attention in recent years. It can be used in carbonylation and methylation reactions to obtain a crude product, a DMC-methanol (MeOH) azeotropic mixture. Current techniques for separating DMC-methanol azeotropic mixtures, such as azeotropic distillation, pressure swing distillation, and low-temperature crystallization, suffer from high energy consumption and high equipment costs. In contrast, pervaporation membrane technology shows great potential for separating DMC-methanol azeotropic mixtures due to its cleanliness, energy efficiency, and high efficiency. Membranes used for DMC-methanol separation can be divided into two main categories: methanol-selective membranes and DMC-selective membranes. From an energy consumption perspective, pervaporation is more suitable for preferentially removing small components in the mixture because the latent heat of vaporization of methanol is much higher than that of DMC (1073.6 vs 381.4 kJ / mol). Therefore, DMC-selective membranes have a greater advantage in practical industrial applications.

[0003] Polydimethylsiloxane (PDMS), a commercially available membrane material, is one of the most studied DMC selective membranes. Increasing the membrane's permeate flux is an effective way to enhance feedstock handling capacity during the separation process. The separation layer affects the permeate flux of composite membranes. Thinner membrane layers can achieve higher flux but are also more prone to aggregate formation and structural defects within the polymer chains, thus reducing membrane robustness and separation factor. Another promising approach is to incorporate microporous fillers into the polymer to form hybrid matrix membranes (MMMs). However, research on using MMMs to improve the selective permeate performance of DMC remains limited. The potential of effective fillers for developing DMC permeate membranes remains to be explored. Compared to the widely studied methanol selective membranes, the design and customization of DMC selective membrane materials face more challenges in two aspects: (i) fewer polymers and inorganic fillers preferentially permeate DMC rather than methanol; and (ii) the molecular dynamic diameter of DMC is larger than that of methanol, resulting in lower diffusion selectivity through the membrane. Summary of the Invention

[0004] The present invention aims to provide an alkyl chain-modified zeolite-doped polydimethylsiloxane (DMC) mixed matrix membrane, its preparation method, and its applications. By grafting alkyl chains with different carbon numbers onto MFI-type zeolite, tunable transport channels are provided for organic molecules to capture them into the zeolite pores, enhancing preferential adsorption and subsequent diffusion through the zeolite pores. This mixed matrix membrane exhibits enhanced affinity for DMC molecules and reduced transport resistance of DMC through the membrane when separating dimethyl carbonate (DMC) / methanol azeotropic mixtures, demonstrating excellent permeation flux.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] An alkyl chain-modified zeolite-doped polydimethylsiloxane mixed matrix membrane, wherein the mixed matrix membrane comprises a polydimethylsiloxane matrix doped with MFI-type zeolite, and the MFI-type zeolite is grafted with C1-C2 carbon atoms. 16 alkyl chain.

[0007] The preparation method of the above-mentioned alkyl chain modified zeolite-doped polydimethylsiloxane mixed matrix film includes the following steps:

[0008] S1. Prepare MFI type zeolite, then disperse the MFI type zeolite in a solution containing silane coupling agent, heat the reaction, and then centrifuge, wash and dry to obtain alkyl chain modified zeolite;

[0009] S2. Alkyl chain modified zeolite and crosslinking agent are dispersed in polydimethylsiloxane solution to prepare casting solution. After film formation, the film is dried to obtain mixed matrix film.

[0010] Preferably, in step S1, the MFI type zeolite is prepared by dispersing tetraethoxysilane in an aqueous solution of tetrapropylammonium hydroxide to obtain a gel, heating it and discarding the supernatant, and then centrifuging, washing, calcining and grinding it.

[0011] Preferably, the molar ratio of tetraethoxysilane, tetrapropylammonium hydroxide and water is (2-7):(90-105):(200-300).

[0012] Preferably, the heating conditions are: 150-180℃, 48-96h;

[0013] The calcination conditions are: 500-650℃, 8-24h.

[0014] Preferably, in step S1, the ratio of MFI-type zeolite to silane coupling agent is (1-5) g:(1-3) mmol; wherein,

[0015] The silane coupling agent is any one of methyltriethoxysilane, isobutyltriethoxysilane, and octyltriethoxysilane.

[0016] Preferably, in step S1, the heating reaction conditions are: 90-120℃, 20-48h;

[0017] The drying conditions are: 100-150℃.

[0018] Preferably, in step S2, the concentration of the polydimethylsiloxane solution is 3-10 wt%.

[0019] The amount of alkyl chain modified zeolite added is 5-65% of the total mass of alkyl chain modified zeolite and polydimethylsiloxane;

[0020] The mass ratio of crosslinking agent to polydimethylsiloxane is 1:(8-15).

[0021] Preferably, in step S2, a coating method is used to form a film, which is then exposed to air for 1-5 hours and then dried at 60-100°C for 12-40 hours.

[0022] The above-mentioned alkyl chain modified zeolite-doped polydimethylsiloxane mixed matrix membrane is used in the separation of dimethyl carbonate and methanol.

[0023] The beneficial effects of this invention are as follows:

[0024] A hybrid matrix membrane was prepared by incorporating zeolites with tunable pore chemistry (MFI-type zeolites grafted with alkyl chains of different carbon numbers (C1, C4, C8)) into the reference membrane material polydimethylsiloxane (PDMS), thereby providing tunable transport channels for organic molecules. The organophilic alkyl chains can trap organic molecules within the zeolite pores, but due to steric hindrance, they can also hinder the diffusion of organic molecules through the zeolite, thus facilitating the effective separation of dimethyl carbonate / methanol azeotropic mixtures and exhibiting excellent permeation flux in the separation of dimethyl carbonate / methanol azeotropic mixtures. Attached Figure Description

[0025] Figure 1 A schematic diagram illustrating how MFI zeolite filler modulates the pore chemistry in a polydimethylsiloxane (PDMS) membrane to achieve selective transport of DMC and methanol molecules.

[0026] Figure 2 Scanning electron microscope (SEM) images of (a) pristine Si-MFI and MFI with tunable pore chemistry (b: MTES-MFI, c: IBTES-MFI, d: OTES-MFI);

[0027] Figure 3 (a) X-ray diffraction (XRD) spectra of Si-MFI and functionalized MFI (MTES-MFI, IBTES-MFI, OTES-MFI), and (b) Fourier transform infrared (FTIR) spectra.

[0028] Figure 4 (a) Nitrogen adsorption curves, (b) BET specific surface areas of Si-MFI and functionalized MFI (MTES-MFI, IBTES-MFI and OTES-MFI);

[0029] Figure 5 Scanning electron microscope (SEM) images of the surface (ae) and cross section (fj), respectively, for (a, f) polydimethylsiloxane (PDMS) film, 30 wt% Si-MFI / PDMS film (b, g) and 30 wt% functionalized MFI / PDMS mixed matrix film (MMMs) ((c, h) MTES-MFI, (d, i) IBTES-MFI, (e, j) OTES-MFI);

[0030] Figure 6 X-ray diffraction (XRD) spectra of (a) pure PDMS film and four MFI / PDMS mixed matrix films (MMMs) and (b) attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra;

[0031] Figure 7 Adsorption curves of (a) pure polydimethylsiloxane (PDMS) membrane, (b) MTES-MFI / PDMS, (c) IBTES-MFI / PDMS and (d) OTES-MFI / PDMS mixed matrix membranes (MMMs) in dimethyl carbonate (DMC) and methanol;

[0032] Figure 8 Low-frequency nuclear magnetic resonance (LF-NMR) signals of PDMS membranes, Si-MFI / PDMS MMMs, and MTES-MFI / PDMS MMMs in (a) methanol and (b) dimethyl carbonate (DMC);

[0033] Figure 9 The pervaporation performance of (a) pure PDMS membrane, PDMS MMMs containing four MFI fillers with a loading of 30 wt%, and (b) MTES-MFI / PDMS MMMs with different loadings (operating conditions: 30 wt% DMC / methanol, 40 °C) was evaluated.

[0034] Figure 10 SEM images of the surface and cross-section of MTES-MFI / PDMS MMMs with different loadings (a, d: 10 wt%; b, e: 30 wt%; c, f: 50 wt%).

[0035] Figure 11To determine the pervaporation performance of MTES-MFI / PDMS MMM under different feed DMC concentrations (operating temperature: 40℃): (a) permeation flux; (b) separation factor and DMC content in permeate;

[0036] Figure 12 Pervaporation performance of 50wt% MTES-MFI / PDMS MMM at different operating temperatures (feed concentration: 30wt% DMC / methanol): (a) permeation flux and separation factor; (b) Arrhenius plot;

[0037] Figure 13 Long-term pervaporation performance of 50wt% MTES-MFI / PDMS MMM for separating 30wt% DMC / methanol at 40℃. Detailed Implementation

[0038] In this embodiment, a novel mixed matrix membrane for separating azeotropic mixtures was constructed by incorporating MFI zeolite with tunable pore chemistry into a PDMS matrix. Figure 1 As shown, MFI zeolite provides tunable transport channels for organic molecules by grafting alkyl chains with different carbon numbers (C1, C4, C8). The purpose of designing organophilic alkyl chains is to capture organic molecules into the zeolite pores, thereby enhancing preferential adsorption of organic molecules and subsequent diffusion through the zeolite pores. Simultaneously, the steric hindrance effect of the grafted alkyl chains on the entry and diffusion of organic molecules was explored. Through a combination of systematic characterization and pervaporation processes, the key role of zeolite pore chemistry in the physicochemical properties, molecular transport properties, and DMC-methanol separation performance of alkyl-grafted MFI / PDMS hybrid matrix membranes was revealed.

[0039] raw material

[0040] Vinyl-terminated polydimethylsiloxane (PDMS, RTV 615, Huazhisheng, Shenzhen), polytetrafluoroethylene (PTFE, average pore size: 100 nm, Beijing Haicheng Shijie), dimethyl carbonate (DMC, Maclean), and deionized water (Wahaha, Hangzhou). Methanol, n-heptane, tetraethoxysilane (TEOS), and tetrapropylammonium hydroxide (TPAOH, 25% aqueous solution) were purchased from China National Pharmaceutical Chemical Reagent Co., Ltd. Methyltriethoxysilane (MTES), isobutyltriethoxysilane (IBTES), and octyltriethoxysilane (OTES) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All solvents were not further purified.

[0041] Synthesis of zeolite packings with tunable pore chemistry

[0042] For the typical MFI-type zeolite synthesis, a certain amount of TPAOH is directly added to deionized water. After the mixed solution becomes clear (solution A), TEOS is added dropwise to solution A. After stirring at approximately 800 rpm for about 6 hours, a homogeneous gel with a molar ratio of 5.0 TEOS:96 TPAOH:250 H2O is finally formed. This gel is transferred to a hydrothermal autoclave and placed in a high-temperature oven preheated to 160°C for 72 hours. After the reaction is complete, the supernatant is discarded, and the solid particles are washed several times with deionized water by centrifugation. Then, the solid particles are transferred to a muffle furnace and calcined at 550°C for at least 8 hours to remove the organic template from the pores. Finally, a white powder is obtained by mechanical grinding, which is used for the preparation of alkylation-modified and mixed matrix membranes (MMMs).

[0043] MFI particles were functionalized to obtain tunable pore chemistry as follows: First, 2 mmol of a silane coupling agent (MTES, IBTES, or OTES) was mixed with 15 g of n-heptane solution. Then, 2 g of the prepared MFI-type zeolite powder was dispersed into the mixed solution, and the mixture was stirred and refluxed overnight in an oil bath at 100 °C. After centrifugation, the functionalized particles were washed three times with n-heptane, and then the remaining solvent was removed in a vacuum oven at 120 °C. The obtained MFI samples with tunable pore chemistry were named MTES-MFI, IBTES-MFI, and OTES-MFI, respectively.

[0044] Membrane preparation

[0045] A 5 wt% PDMS solution was prepared by uniformly dispersing a certain amount of PDMS monomer in 15 ml of n-heptane solution. Then, different masses (MFI loading = M...) were... MFI / (M MFI +M PDMS MFI or functionalized MFI particles (wt%) were added to the above PDMS solution, ultrasonicated, and stirred for 1 hour to form a homogeneous suspension. Subsequently, a crosslinking agent (monomer to crosslinking agent mass ratio of 10:1) was added to ensure sufficient crosslinking. The solution with appropriate viscosity was coated onto a PTFE substrate using a doctor blade coating method. Finally, fresh MMM was exposed to air for 2 hours and then transferred to an oven at 80°C to dry for at least 12 hours to obtain pristine MFI / PDMS and functionalized MFI / PDMS MMM.

[0046] Characterization

[0047] The structure and morphology of the MMM were analyzed using a field emission scanning electron microscope (Phenom Pharos G2, Netherlands) at 10 kV. The crystal structure of the film was determined using X-ray diffraction (XRD, Miniflex 600, Rigaku, Japan) at a 2θ range of 5°–50° and a scan rate of 20° / min. Fourier transform infrared spectroscopy (ATR-FTIR, PerkinElmer, USA) was used at 500–4000 cm⁻¹. -1 The chemical functional groups of the membrane and powder were analyzed within the specified wavelength range. The N2 isotherm was measured at 77 K using a fully automated Bruner-Emmett-Teller (BELSOR-MAX, Microtrac-BEL, Japan) adsorption system to calculate the surface area of ​​the packing material. The T2 inversion spectrum of the membrane was measured using low-field nuclear magnetic resonance spectroscopy (LF-NMR, Bruker minispec MQ60, Germany).

[0048] The solubility and diffusion coefficients of DMC and methanol in PTFE-free membranes were measured using a gravimetric method. Membrane samples were identical in size and shape for more accurate comparison. The membranes were immersed separately in pure methanol and pure DMC solvents at a constant temperature (25°C). After a period of time, residual organic solvents on the surface of the membrane samples were quickly wiped away with lint-free paper. When the weight of the membrane sample no longer increased, it indicated that solvent absorption had reached equilibrium.

[0049] Pervaporation separation of DMC-methanol mixture

[0050] This experiment used a pervaporation apparatus with an effective membrane area of ​​4.9 cm². 2 The feedstock organic mixture was delivered by a peristaltic pump and circulated on the membrane surface. A vacuum pump was used to evacuate the downstream side of the membrane, maintaining the downstream pressure below 200 Pa. The concentrations of DMC on both the feedstock and permeate sides were measured using gas chromatography (Agilent 8860GC, USA). Results for each membrane sample were measured at least three times. The separation factor (β), permeate flux (J), and normalized flux (J') (normalized membrane thickness 8 μm) were calculated as follows:

[0051] J = M / At,

[0052]

[0053] M represents the total weight of permeate collected during the operating time, in kg; t represents the operating time of the pervaporation process, in h; A represents the effective membrane area, in m². 2 ; l is the membrane thickness in μm; in addition, x and y represent the mass fraction of DMC (D) or methanol (M) on the feed side and the permeate side, respectively.

[0054] Properties of Functionalized MFI and MFI Particles

[0055] Silanes with alkyl chains of different lengths were selected to modulate the pore chemistry of the MFI zeolite filler. The morphologies of pristine Si-MFI and MFI particles with tunable pore chemistry are shown below. Figure 2 As shown, they remain almost unchanged. The average particle size of these particles is approximately 500 nm.

[0056] The crystal structure of MFI particles with tunable pore chemistry was studied by X-ray diffraction (XRD), such as... Figure 3 As shown in region a. Diffraction peaks in the 5° to 50° range confirm that the synthesized MFI powder has a good crystal structure, and the functionalized MFI particles retain their crystal structure. The chemical properties of the MFI powder were measured using Fourier transform infrared spectroscopy (FTIR). Figure 3 As shown in region b, 1081cm -1 The absorption peak at 2965 cm⁻¹ is attributed to the stretching vibrations of the Si-O-Si backbone. A peak at 2965 cm⁻¹ was observed in the spectrum of the functionalized MFI sample. -1 and 2856cm -1 The absorption peaks at these locations were identified as stretching vibrations of the -CH3 and -CH2- groups, respectively. The study found that as the number of carbon atoms in the alkyl chain on the functionalized MFI particles increased, the peak intensities of the -CH3 and -CH2- groups increased compared to the original MFI. This indicates that the pore chemistry of the MFI particles has been successfully regulated using silane coupling agents.

[0057] like Figure 4 As shown in region a, the nitrogen adsorption curves of the four powders exhibit type I isotherms in the relative pressure range of 0–1.0, suggesting a microporous crystal structure. It was observed that when the pore chemistry of the MFI powders was adjusted using silane coupling agents with alkyl chains of different lengths, the nitrogen adsorption capacity of the functionalized MFI powders decreased compared to the original MFI powder. This is attributed to the alkyl chains occupying the pore structure of the MFI. The BET specific surface area measurements of the four MFI powders are shown below. Figure 4 As shown in region b, the longer the silane chain of MFI-functionalized silane, the lower the BET specific surface area, which is due to the reduction in adsorption sites and pore volume.

[0058] Properties of hybrid matrix membranes

[0059] Pure polydimethylsiloxane (PDMS) membranes and PDMS mixed matrix membranes (MMMs) containing four different MFI fillers were prepared, with the filler loading at 30 wt%. The surface and cross-sectional morphologies of pure PDMS membranes, Si-MFI / PDMS membranes, and functionalized MFI / PDMS membranes (coated on polytetrafluoroethylene (PTFE) substrates) are shown below. Figure 5 As shown, in all membranes, the dense separation layer exhibited good interfacial adhesion to the porous PTFE layer. The thickness of the separation layer was approximately 8 μm. Furthermore, Si-MFI and alkyl chain-functionalized MFI particles were observed to be uniformly dispersed within the crosslinked PDMS layer.

[0060] like Figure 6 As shown, the physicochemical properties of the PDMS-based films were characterized and measured by X-ray diffraction (XRD) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The pure PDMS film exhibits a broad peak at approximately 12°, indicating that the PDMS polymer has an amorphous structure. When Si-MFI and functionalized MFI were incorporated into the PDMS film, characteristic peaks of MFI appeared, suggesting that the crystalline structure of MFI was preserved. The FTIR spectra of PDMS and MMMs are shown below. Figure 6 As shown in region b. 1097cm -1 800cm -1 and 1262cm -1 The peaks at 2963 cm⁻¹ represent the deformation vibrations of the Si-O-Si backbone and the Si-C bond, respectively. -1 The peak at that point represents the asymmetric CH stretching vibration of the CH3 group in the side chain.

[0061] Adsorption tests and low-frequency nuclear magnetic resonance (LF-NMR) characterization of the membrane

[0062] The adsorption capacity of a membrane sample is determined by measuring its mass change after immersion in different solvents, and its affinity for the solvent can be assessed accordingly. Figure 7 As shown in Table 1, the adsorption curves of functionalized PDMS MMMs and PDMS membranes in methanol and dimethyl carbonate (DMC) solvents were compared. It was observed that the adsorption curves of all mentioned membranes for DMC significantly exceeded those for methanol, indicating that the hydrophobic PDMS-based membranes have a significant affinity for nonpolar DMC molecules. Specifically, as shown in Table 1, MTES-MFI / PDMS MMMs exhibited the highest adsorption capacity for DMC, and the adsorption order for DMC was: MTES-MFI / PDMS > PDMS > IBTES-MFI or OTES-MFI / PDMS. MTES-MFI / PDMS MMMs also showed a slight increase in methanol adsorption. The enhanced adsorption of DMC by MTES-MFI / PDMS MMMs was much greater than the enhanced adsorption of methanol, confirming that the MTES-MFI packing material preferentially adsorbs DMC rather than methanol. However, the longer alkyl chains (C4 or C8) in IBTES or OTES partially block the pore structure of the MFI filler, resulting in a lower adsorption capacity in PDMS MMMs compared to pure PDMS membranes.

[0063] Table 1 is based on Figure 7 Calculated adsorption results of different membranes

[0064]

[0065] To further investigate the adsorption behavior of PDMS-based membranes in organic solvents, the transverse relaxation time (T2) distribution was analyzed using low-frequency nuclear magnetic resonance (LF-NMR) technology. The hydrogen protons in the membrane or solvent are represented by the relative intensity of the T2 signal peak. The T2 relaxation times of three membranes in DMC or methanol solvents were studied. Figure 8 As shown, high T 2l The signal peak position (1000-10000 ms) corresponds to the free solvent molecules. T 2a The signal peak (less than 1000 ms) is the 1H signal peak of solvent molecules adsorbed inside the membrane pores or on the membrane surface. In PDMS films, Si-MFI / PDMS, and MTES-MFI / PDMS MMMs, it was found that in DMC solvent, the T1 signal peak of MTES-MFI / PDMS MMMs was significantly higher. 2a Signal peak intensity as a percentage of total intensity (T) 2a +T 2l The percentage of ) was significantly higher than the other two membranes, and the MTES-MFI / PDMS MMM had a stronger affinity for DMC than for methanol.

[0066] Pervaporation performance

[0067] To further understand the impact of functionalized MFI / PDMS MMMs on pervaporation performance, they were compared with those of pristine MFI / PDMS and pure PDMS membranes. Figure 9 As shown in region a, under the same test conditions, the thickness-normalized flux of functionalized MFI / PDMS MMMs is higher than that of pure PDMS membranes. This result indicates that the addition of nonpolar alkyl chain functionalized MFI packing promotes the preferential adsorption of nonpolar DMC molecules on the membrane surface. Conversely, the separation performance of the original MFI / PDMS MMM is lower than that of the pure PDMS membrane. This is due to the presence of residual hydroxyl groups on the surface and in the pores of the MFI particles. With increasing carbon number in the functionalized alkyl chains, the permeate flux and separation factor show a decreasing trend. This phenomenon is due to the longer alkyl chains reducing the inherent pore size and hindering the passage of organic molecules (especially DMC) through the MFI packing due to steric hindrance.

[0068] The effect of MTES-MFI loading on membrane performance was further investigated, such as... Figure 9 As shown in region b, it can be seen that the flux gradually increases and the separation factor also improves with the increase of MTES-MFI loading. The thickness-normalized flux and separation factor of 50wt% MTES-MFI / PDMSMMM are 14.0 kg / m².2 h and 3.6 are 1.8 times and 1.1 times that of pure PDMS membranes, respectively. On the one hand, a 50 wt% loading of MTES-MFI packing material can provide relatively continuous diffusion channels in the MMM because the dispersed packing material is tightly connected and defect-free in the PDMS matrix, thereby reducing the transport resistance of the MTES-MFI / PDMS membrane layer. On the other hand, the MTES-MFI / PDMS MMM has a stronger affinity for DMC molecules than for methanol molecules, which can improve the separation factor.

[0069] In addition, the surface and cross-sectional morphology of MTES-MFI / PDMSMMMs with loadings of 10-50 wt% were observed by scanning electron microscopy (SEM). Figure 10 As the loading of MTES-MFI filler increases, the particles are uniformly dispersed in the membrane layer without agglomeration, and the membrane surface becomes rougher. Furthermore, no pinholes were observed on the surface and cross-section of the 50wt% MTES-MFI / PDMS MMM, indicating that even with a filler loading as high as 50wt%, the functionalized MFI filler still exhibits good interfacial compatibility with the polymer matrix.

[0070] The separation performance under several key operating conditions (such as feed concentration, operating temperature, and long-term measurement) was systematically studied to explore the practical application feasibility of 50wt% MTES-MFI / PDMS MMM. Figure 11 As shown, the permeation flux, separation factor, and DMC concentration in the permeate all exhibit linear changes as the DMC concentration varies from 10 wt% to 50 wt%. Because the membrane has a stronger affinity for the DMC solvent, a higher DMC feed concentration causes the polymer matrix to swell, thereby increasing the free volume of the polymer network, which helps to promote the diffusion process.

[0071] Figure 12 The effect of feed temperature on the pervaporation performance of 50 wt% MTES-MFI / PDMS MMMs was demonstrated. Figure 12 As shown in region a, the permeation flux increases with increasing operating temperature, while the separation factor decreases. On one hand, with increasing feed temperature, both the pressure differential of the feed components and the migration of polymer chains increase simultaneously, leading to a decrease in transport resistance and promoting the diffusion of permeating molecules through the membrane. On the other hand, because methanol has a smaller molecular dynamic diameter than DMC, the methanol flux increases faster than the DMC flux when the operating temperature increases, resulting in a slight decrease in the separation factor. The temperature dependence of DMC and methanol fluxes was further investigated using the Arrhenius equation. Activation energy (E) a The higher the value, the more sensitive the component flux is to the operating temperature. For example... Figure 12As shown in region b, for 50wt% MTES-MFI / PDMS MMM, between 30°C and 60°C, the E of DMC a Value lower than methanol's E a The value (9.8 vs 19.9 KJ / mol) indicates that during pervaporation, a portion of the methanol flux is more sensitive to the operating temperature.

[0072] like Figure 13 As shown, the long-term separation performance of a 50 wt% MTES-MFI / PDMS membrane for a 30 wt% DMC / methanol azeotropic mixture at 40 °C was investigated. Over 150 h of continuous operation, the 50 wt% MTES-MFI / PDMS MMM exhibited stable permeation flux and separation factor. Overall, the PDMS MMM demonstrates high permeation flux and good stability, showing great potential for practical applications in the separation of organic azeotropes.

[0073] Comparison of separation performance with literature

[0074] Over the past two decades, numerous reports have been published on the use of PDMS-based mixed matrix membranes (MMMs) for the separation of DMC / methanol azeotropic mixtures via pervaporation processes, as listed in Table 2. Alkylated modified microporous zeolites have demonstrated a stronger affinity for DMC than for methanol, providing a rapid transport pathway for DMC. In this application, the optimal MTES-MFI / PDMS MMM with a loading up to 50 wt% outperforms state-of-the-art membranes in DMC / methanol separation.

[0075] Table 2 Comparison of pervaporation performance of DMC / methanol azeotropic mixtures separated by PDMS-based mixed matrix membranes

[0076]

[0077] Reference 1: L. Wang, X. Han, J. Li, L. Qin, D. Zheng, Preparation of modified mesoporous MCM-41 silica spheres and its application in pervaporation, Powder Technol. 231 (2012) 63 - 69. Reference 2: Z. Liu, W. Lin, Q. Li, Q. Rong, H. Zu, M. Sang, Separation of dimethyl carbonate / methanol azeotropic mixture by pervaporation with dealcoholized room temperature-vulcanized silicone rubber / nanosilica hybrid active layer, Sep. Purif. Technol. 248 (2020) 116926.

[0078] Reference 3: L. Wang, X. Han, J. Li, D. Zheng, L. Qin, Modified MCM-41 silica spheres filled polydimethylsiloxane membrane for dimethyl carbonate / methanol separation via pervaporation, J. Appl. Polym. Sci. 127(6) (2013) 4662 - 4671.

[0079] Reference 4: L. Wang, X. Han, J. Li, X. Zhan, J. Chen, Hydrophobic nano-silica / polydimethylsiloxane membrane for dimethyl carbonate-methanol separation via pervaporation, Chem. Eng. J. 171(3) (2011) 1035 - 1044.

[0080] In summary, this application proposes a strategy to enhance the azeotropic separation performance of polydimethylsiloxane (PDMS) membranes by adjusting the pore chemistry of the packing material. Compared with the performance of long-alkyl-chain functionalized MFI (C4 or C8) / PDMS mixed matrix membranes (MMMs), short-alkyl-chain (C1) functionalized MFI packing material, at a loading of 50 wt%, not only enhances the membrane's affinity for dimethyl carbonate (DMC) relative to methanol but also reduces the transport resistance of DMC through the membrane. Therefore, a 50 wt% MTES-MFI / PDMS MMM exhibits a separation efficiency of 11.5 kg / m³ when separating a 30 wt% DMC / methanol azeotropic mixture at 40 °C. 2 With a permeation flux of h and a separation factor of 3.6, it shows great potential for practical application.

[0081] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. The application of an alkyl chain-modified zeolite-doped polydimethylsiloxane mixed matrix membrane in the separation of dimethyl carbonate and methanol, characterized in that, The mixed matrix membrane comprises polydimethylsiloxane matrix doped with MFI type zeolite, wherein the MFI type zeolite is grafted with alkyl chains having a carbon number of C1. The preparation method of the hybrid matrix membrane includes the following steps: S1. Prepare MFI type zeolite, then disperse the MFI type zeolite in a solution containing methyltriethoxysilane, heat the reaction, and then centrifuge, wash and dry to obtain alkyl chain modified zeolite; S2. Alkyl chain modified zeolite and crosslinking agent are dispersed in polydimethylsiloxane solution to prepare casting solution. After film formation, the film is dried to obtain mixed matrix film.

2. The application according to claim 1, characterized in that, In step S1, the MFI type zeolite is prepared by dispersing tetraethoxysilane in an aqueous solution of tetrapropylammonium hydroxide to obtain a gel, heating it and discarding the supernatant, and then centrifuging, washing, calcining and grinding it.

3. The application according to claim 2, characterized in that, The molar ratio of tetraethoxysilane, tetrapropylammonium hydroxide and water is (2-7):(90-105):(200-300).

4. The application according to claim 2, characterized in that, In the preparation method of MFI type zeolite, the heating conditions are: 150-180℃, 48-96h; the calcination conditions are: 500-650℃, 8-24h.

5. The application according to claim 1, characterized in that, In step S1, the ratio of MFI type zeolite to methyltriethoxysilane is (1-5) g:(1-3) mmol.

6. The application according to claim 1, characterized in that, In step S1, the heating reaction conditions are: 90-120℃, 20-48h; the drying conditions are: 100-150℃.

7. The application according to claim 1, characterized in that, In step S2, the concentration of the polydimethylsiloxane solution is 3-10 wt%; the amount of alkyl chain modified zeolite added is 5-65% of the total mass of alkyl chain modified zeolite and polydimethylsiloxane; and the mass ratio of crosslinking agent to polydimethylsiloxane is 1:(8-15).

8. The application according to claim 1, characterized in that, In step S2, a coating method is used to form a film. After the film is formed, it is exposed to air for 1-5 hours and then dried at 60-100℃ for 12-40 hours.

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