A bimetallic MOF membrane and a preparation method and application thereof

Abstract

The application relates to a bimetallic MOF film and a preparation method and application thereof, in particular to a bimetallic MIL-100(In) / Cr film material and a preparation method thereof, and application in toluene / methylcyclohexane separation. The application provides a bimetallic MOF film, namely a MIL-100(In) / Cr film, which combines the difference between the electronic cloud distribution of toluene and methylcyclohexane, surrounds the concept of "push-pull electrons", is based on a metal post-replacement strategy, constructs an intelligent recognition MOF film with a DA type (electron donor-acceptor type pore structure) MOF pore structure, and realizes the double electrostatic attraction recognition ability of aromatic hydrocarbon-toluene.

Description

Technical Field

[0001] This invention relates to a bimetallic MOF membrane, its preparation method and application, specifically to a bimetallic MIL-100(In) / Cr membrane material and its preparation method, as well as its application in the separation of toluene / methylcyclohexane. Background Technology

[0002] Toluene (TOL) and methylcyclohexane (MCH), as typical aromatic / aliphatic hydrocarbon separation systems, play a crucial role in the petrochemical and new energy fields. In integrated refining and chemical processes, efficient separation of aromatics is essential for catalytic reforming and preventing coking in subsequent cracking units. Simultaneously, in hydrogen storage technology based on liquid organic hydrogen carriers (LOHC), the hydrogenation of toluene to methylcyclohexane is the core hydrogen storage step. Since the hydrogenation reaction cannot achieve complete conversion of aromatics, unreacted toluene must undergo rigorous separation and recycling to ensure the overall process efficiency and the stability of the hydrogen storage cycle. However, toluene and methylcyclohexane have extremely similar boiling points (toluene 110.6 °C, methylcyclohexane 101.2 °C) and very small differences in molecular kinetic diameter (toluene approximately 0.53 nm, methylcyclohexane approximately 0.65 nm), making their efficient separation highly complex. Currently, industrial separation of aromatic and aliphatic hydrocarbon systems mainly relies on distillation, extractive distillation, crystallization, and traditional adsorption methods. However, these traditional technologies have significant limitations: traditional distillation often requires more than 300 theoretical plates, making it a typical energy-intensive process; low-temperature crystallization has poor separation efficiency; and traditional zeolite, carbon materials, or polymer adsorption technologies suffer from limited separation mechanisms and extremely high energy consumption during material regeneration. To meet the major strategic needs of national energy sectors such as deep naphtha refining and low-carbon hydrogen storage, breakthroughs in key aromatic hydrocarbon separation technologies are urgently needed to support the improvement of refining and chemical processes.

[0003] Pervaporation membrane separation technology, as a novel energy-saving alternative, consumes only one-third or even less energy than traditional distillation, attracting widespread attention from academia and industry. Among numerous membrane materials, metal-organic frameworks (MOFs) exhibit great potential due to their highly tunable pore size and chemical microenvironment. However, existing MOF membrane separation research still faces significant challenges for aromatic / aliphatic hydrocarbon systems with similar physicochemical properties and minimal differences in molecular dynamic diameter. On the one hand, traditional separation mechanisms mainly rely on molecular size sieving, and single pore size control is insufficient for effective differentiation; on the other hand, the chemical environment within the pores of conventional MOF materials is relatively homogeneous, lacking specific recognition capabilities for the π-electron structure of aromatic rings, resulting in limited adsorption selectivity for aromatic and aliphatic hydrocarbons. Taking MIL-100(In) as an example, although it has a dual-pore structure and multi-level pore windows (0.77nm and 1.5nm), its small pores still cannot effectively sieve toluene (0.53 nm) and methylcyclohexane (0.65 nm). At the same time, the active sites in the pores are uniformly distributed and lack the ability to directionally recognize the π-electron system of aromatic rings, making it difficult to achieve selective transport of toluene.

[0004] Therefore, there is an urgent need to develop a novel MOF membrane material that retains the advantages of MOF pore structure and constructs a multi-synergistic separation mechanism with both size sieving and molecular recognition functions by regulating the electronic structure and active site distribution within the pores, so as to achieve efficient separation of the toluene / methylcyclohexane system. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a bimetallic MOF film—MIL-100(In) / Cr film—which, by combining the differences in electron cloud distribution between toluene and methylcyclohexane, focuses on "pushing..." The "electron-pulling" concept, based on a post-metal substitution strategy, constructs a structure with D A smart recognition MOF membrane with an A-type (electron donor-acceptor pore structure) MOF pore structure enables dual electrostatic attraction recognition of aromatic hydrocarbons, including toluene.

[0006] Specifically, the present invention provides a bimetallic MOF film comprising a porous substrate and a bimetallic MIL-100(In) / Cr film layer supported on the porous substrate; the bimetallic MIL-100(In) / Cr film layer has a MIL-100 topological framework structure, wherein the metal center in the framework structure is composed of the host metal In³ + and doped metal Cr³ + Together they form a bimetallic MIL-100(In) / Cr film with an interior composed of electron-deficient Cr³⁺. + Sites and electron-rich In³ +Electron donor-acceptor type pore structure composed of sites.

[0007] Preferably, the doped metal Cr³ + The proportion of the core metal in the bimetallic MIL-100(In) / Cr film is 10% to 80%.

[0008] Preferably, the porous substrate is one or more of an alumina support, a zirconia support, or a silicon oxide support; and / or, the porous substrate has a diameter of 20-40 mm, a thickness of 1.5-2 mm, an average pore size of 80-120 nm, and a porosity of 20%-60%.

[0009] The present invention also provides a method for preparing the above-mentioned bimetallic MOF membrane, which includes the following steps: (1) substrate film formation: a precursor solution containing a host metal salt and an organic ligand is added to a mixed solvent containing N,N-dimethylformamide, ethanol and water to form a precursor solution, and a dense MIL-100(In) membrane is synthesized in situ on a porous substrate by a homogeneous hydrothermal reaction; (2) metal post-displacement: the porous substrate with the MIL-100(In) membrane on its surface obtained in step (1) is immersed in a solution containing the displacement metal Cr³ + In an alcohol solution, a metal substitution reaction is carried out to make Cr³ + Partially replaces In³ in the MIL-100(In) skeleton + (3) Washing and activation: After the displacement reaction is completed, the membrane material sample is taken out, and after washing and activation with alcohol washing aid, the bimetallic MOF membrane is obtained.

[0010] Preferably, in step (1), the molar ratio of the main metal salt, organic ligand, N,N-dimethylformamide, ethanol and water in the precursor solution is 1:0.5-2:2-5:2-5:0.5-2.

[0011] Preferably, in step (2), the Cr³⁺-containing metal is used. + In the alcoholic solution, the chromium source is selected from one or a combination of two of chromium nitrate and chromium sulfate; and / or, the alcohol solvent is selected from one or a combination of methanol, ethanol, or isopropanol; and / or, the chromium-containing substituted metal Cr³ + The concentration of the alcohol solution is 0.025~0.5 mol / L.

[0012] Preferably, in step (2), the temperature of the metal displacement reaction is 50℃~85℃, and the reaction time is 3~24 hours.

[0013] Preferably, in step (3), the alcohol washing aid is selected from one or more combinations of methanol, ethanol or isopropanol; and / or, after washing, the bimetallic MOF membrane is stored in anhydrous alcohol solvent to maintain its pore wetting and activation state.

[0014] This invention also provides an application of the above-mentioned bimetallic MOF membrane in the separation of toluene and methylcyclohexane. The bimetallic MOF membrane material is used as a pervaporation separation membrane. The electron donor-acceptor type pore structure and unsaturated metal defect sites inside the bimetallic MIL-100(In) / Cr membrane layer are used to conduct electrostatic attraction and coordination interaction with toluene molecules. Through the specific recognition and preferential adsorption of toluene, the pervaporation separation of the toluene and methylcyclohexane mixture is achieved.

[0015] Preferably, the operating temperature of the pervaporation separation is 25–65°C; the feed ratio of toluene to methylcyclohexane is 10:90–90:10.

[0016] Compared with the prior art, the present invention has the following advantages: (1) Precisely constructing specific DA-type bimetallic channels to achieve innovative control of material structure. This invention is the first to utilize a post-metal substitution strategy with In³ + The main metal, Cr³ + A specific bimetallic MIL-100(In) / Cr framework structure was successfully constructed using a doped metal. This structure not only maintains the uniform micropore size distribution of the material, but also, due to the Cr³⁺... + The introduction of this compound induced ligand deficiency defects within the framework, significantly increasing the specific surface area (up to approximately 1170 m² / g). This defect-rich and microporous bimetallic network structure provides extremely abundant transport channels and active adsorption sites for the subsequent pervaporation separation process.

[0017] (2) Based on the "push-pull electron" effect of electronegativity difference, specific and efficient recognition of toluene is achieved. This invention achieves a breakthrough separation effect for toluene / methylcyclohexane, which have extremely similar physicochemical properties. This invention specifically selects Cr³⁺... + As a doped metal, it has irreplaceable and targeted advantages: compared to conventional Fe³⁺ + or Al³ + Cr³ + It has a more moderate electronegativity (Pauling electronegativity is approximately 1.66), which is similar to that of the host metal In³. + There is a significant difference between them (electronegativity is approximately 1.78). During the separation process, electron-deficient Cr³⁺… + The site can be strongly electrostatically attracted to the electron-rich π-electron cloud on the toluene ring, while the electron-rich In³+ The site readily interacts with the electron-deficient hydrogen atoms on the periphery of the benzene ring. Meanwhile, Cr³ + The numerous unsaturated metal sites induced by doping serve as active centers, further enhancing the strong coordination and electrostatic coupling with toluene molecules. This dual approach, based on molecular-level "specific chemical recognition" and "mass transfer path optimization due to ligand deficiency," significantly reduces the diffusion resistance of toluene, enabling the membrane material to exhibit extremely strong preferential adsorption capacity for toluene, successfully breaking the "trade-off" limit in the traditional toluene / methylcyclohexane separation mechanism.

[0018] (3) It possesses excellent long-term operational stability and extremely high potential for industrial application. The bimetallic MIL-100(In) / Cr membrane provided by this invention exhibits extremely excellent physicochemical stability. Despite the introduction of ligand deficiency defects in the structure, the membrane material still maintains excellent thermal stability up to 357 °C. In a continuous liquid pervaporation separation test conducted in a harsh high-temperature organic solvent environment for up to 120 hours, the membrane structure did not swell or deteriorate, and the concentration of toluene on the permeate side remained consistently at a high purity level of 98 wt.%, without any performance degradation. Furthermore, the metal post-displacement process adopted in this invention not only successfully avoids the phase separation problem easily caused by in-situ co-growth of bimetals, but also has simple operation and mild reaction conditions, showing significant advantages in reducing material costs and improving environmental friendliness, and has extremely broad prospects for large-scale industrial preparation and application. Attached Figure Description

[0019] Figure 1 The XRD patterns of MIL-100(In) / Cr particles before and after replacement in Example 1 are shown below. Figure 2 The images show the SEM images and EDS spectra of MIL-100(In) / Cr particles before and after replacement in Example 1. Figure 3 SEM images of the MIL-100(In) / Cr membrane before and after replacement in Example 1; Figure 4 The nitrogen adsorption-desorption isotherm curves of MIL-100(In) / Cr before and after replacement in Example 1 at 77 K. Figure 5 Thermogravimetric analysis (TGA) curves of MIL-100(In) / Cr particles before and after replacement in Example 1; Figure 6 The UV spectra of MIL-100(In) / Cr particles before and after replacement in Example 1; Figure 7 XPS spectra of MIL-100(In) / Cr particles before and after replacement in Example 1; Figure 8 The vapor profiles of the MIL-100(In) / Cr membrane against toluene / methylcyclohexane before and after replacement in Example 1 are shown. Figure 9 The graph shows the separation performance of the MIL-100(In) / Cr-0.05 membrane replaced in Example 1 for toluene / methylcyclohexane. Figure 10 Long-term stability test of the MIL-100(In) / Cr-0.05 membrane replaced in Example 1 against toluene / methylcyclohexane. Detailed Implementation

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention, and not all embodiments. Any other embodiments obtained by those skilled in the art based on these drawings without creative effort are within the protection scope of the present invention.

[0021] Example 1 like Figure 1 As shown, this embodiment provides a method for preparing a MIL-100(In) / Cr membrane, the specific steps of which are as follows: (1) A dense, defect-free MIL-100(In) film was prepared on an alumina support by an in-situ solvothermal method. Specifically, 0.588 g of 1,3,5-benzenetricarboxylic acid was added to 17.5 mL of N,N-dimethylformamide and sonicated for 10 minutes until completely dissolved. Then, 17.5 mL of ethanol was added under stirring at room temperature and stirring was continued for 10 minutes to obtain a homogeneous ligand solution. 0.619 g of indium chloride was added to 5 mL of deionized water and sonicated for 5 minutes until completely dissolved to form a transparent liquid. This metal solution was slowly poured into the ligand solution and stirred for 10 minutes to mix evenly. Finally, the above-mentioned synthesis solution was transferred to a stainless steel high-pressure reactor with a PTFE liner and heated to 120°C. o The reaction was carried out in an electrically heated blast oven at C for 24 hours. After the reaction was completed, the mixture was cooled to room temperature for later use.

[0022] (2) Immerse the substrate membrane in 0.05 mol·L⁻¹ -1 A MIL-100(In) / Cr-0.05 membrane was prepared in a Cr(NO3)3·9H2O ethanol solution. The system was then placed at 80°C under sealed conditions. o The reaction was heated in a constant temperature water bath for 12 h.

[0023] (3) After the reaction is complete, allow the system to cool naturally to room temperature, remove the membrane sample, and rinse it repeatedly with anhydrous ethanol to thoroughly remove unreacted ions and impurities adsorbed on the membrane pores and surface. Before conducting subsequent pervaporation tests, all membrane samples are kept in anhydrous ethanol to maintain their pore wettability and activation.

[0024] Example 2 The only difference between this embodiment and Example 1 is that the concentration of Cr(NO3)3·9H2O in the Cr(NO3)3·9H2O ethanol solution in step (2) is 0.025 mol·L. -1 A MIL-100(In) / Cr-0.025 membrane was prepared.

[0025] Example 3 The only difference between this embodiment and Example 1 is that the concentration of Cr(NO3)3·9H2O in the Cr(NO3)3·9H2O ethanol solution in step (2) is 0.0375 mol·L. -1 A MIL-100(In) / Cr-0.0375 membrane was prepared.

[0026] Example 4 The only difference between this embodiment and Example 1 is that the concentration of Cr(NO3)3·9H2O in the Cr(NO3)3·9H2O ethanol solution in step (2) is 0.0625 mol·L. -1 A MIL-100(In) / Cr-0.0625 membrane was prepared.

[0027] Example 5 The only difference between this embodiment and Example 1 is that the concentration of Cr(NO3)3·9H2O in the Cr(NO3)3·9H2O ethanol solution in step (2) is 0.075 mol·L. -1 A MIL-100(In) / Cr-0.075 membrane was prepared.

[0028] Comparative Example 1 The only difference between this comparative example and Example 1 is the absence of step (2), which produces the MIL-100(In) membrane.

[0029] Comparative Example 2 The only difference between this comparative example and Example 1 is that the concentration of Cr(NO3)3·9H2O in the Cr(NO3)3·9H2O ethanol solution in step (2) is 0.7 mol·L. -1 MIL-100(In) / Cr-0.7 membrane was prepared.

[0030] Comparative Example 3 The only difference between this comparative example and Example 1 is that the constant temperature water bath temperature in step (2) is replaced with 90°C.

[0031] Comparative Example 4 The only difference between this comparative example and Example 1 is that Cr(NO3)3·9H2O in step (2) is replaced with CrCl3.

[0032] Comparative Example 5 The only difference between this comparative example and Example 1 is that Cr(NO3)3·9H2O in step (2) is replaced with Fe(NO3)3·9H2O. 。

[0033] Comparative Example 6 The only difference between this comparative example and Example 1 is that Cr(NO3)3·9H2O in step (2) is replaced with Al(NO3)3·9H2O.

[0034] Characterization results like Figure 1 As shown, MIL-100(In) prepared in Comparative Example 1 and MIL-100(In) / Cr-0.05 prepared in Example 1 were characterized by XRD to investigate the Cr content. 3+ The effect of doping on its crystallinity and structure. The characteristic diffraction peaks of MIL-100(In) / Cr-0.05 at 3.8° and 10.7° are in high agreement with the simulated standard MIL-100(In) spectrum, indicating that the prepared material maintains the typical crystal structure of MIL-100(In), and Cr... 3+ The introduction of [something] did not significantly affect the integrity of its skeleton.

[0035] like Figure 2 As shown, EDS elemental surface mapping analysis was used to analyze MIL-100(In) prepared in Comparative Example 1 and MIL-100(In) / Cr-0.05 prepared in Example 1. Almost no characteristic signal of Cr was detected in pure MIL-100(In) without chromium doping, while a clear characteristic peak signal of Cr was observed in MIL-100(In) / Cr-0.05, directly confirming that Cr was successfully introduced into the MIL-100(In) framework. Furthermore, Cr exhibited a highly uniform spatial distribution, with no obvious local enrichment or segregation observed. Comparing the Cr distribution maps with those of In revealed that they showed almost identical spatial distribution profiles, strongly demonstrating that Cr... 3+Uniformly dispersed within the MIL-100(In) / Cr-0.05 framework, it ensures the uniformity of material structure and properties, providing a reliable material basis and performance guarantee for its subsequent application as a functional filler in pervaporation separation membranes.

[0036] like Figure 3 As shown, the morphological changes of the MIL-100(In) prepared in Comparative Example 1 and the MIL-100(In) / Cr-0.05 sample prepared in Example 1 were compared and analyzed by SEM. The MIL-100(In) film exhibits a typical irregular octahedral crystal structure with clear outlines and a smooth surface. The crystal morphology of the MIL-100(In) / Cr-0.05 film did not change significantly, maintaining a complete irregular octahedral morphology, with no obvious defects or secondary phase precipitation on the surface, indicating that Cr... 3+ The introduction of [the substance] did not damage the main crystal structure of MIL-100(In).

[0037] like Figure 4 As shown, the doped sample exhibits a "hysteresis loop" and displays typical Type I and Type IV composite isotherm characteristics, indicating that the material possesses both microporous and mesoporous structures. This may stem from the difference in ionic radii between In and Cr, making perfect isomorphic substitution difficult to achieve during coordination assembly and easily introducing structural defects. The presence of these structural defects macroscopically manifests as a higher specific surface area and larger pore volume for MIL-100(In) / Cr compared to pure MIL-100(In). The BET specific surface area and total pore volume of MIL-100(In) are 10¹⁹ m², respectively. 2 / g and 0.45 cm 3 The specific surface area and total pore volume of MIL-100 / Cr-0.05 were increased to 1170 m² / g. 2 / g and 0.74 cm 3 / g. Furthermore, these structural defects contribute to the generation of numerous unsaturated metal sites within the framework. This material, with its abundant internal pore structure, is expected to provide more transport channels and adsorption sites for subsequent pervaporation separation processes, potentially significantly improving the membrane's separation performance.

[0038] like Figure 5 As shown, Cr was systematically evaluated by thermogravimetric analysis (TGA) of MIL-100(In) prepared in Comparative Example 1 and MIL-100(In) / Cr-0.05 prepared in Example 1. 3+ The effect of doping on the thermal stability of materials. Comparative analysis shows that after Cr... 3 +The TGA behavior of the replaced samples was basically consistent with the typical MIL-100(In) thermal decomposition process reported in the literature, indicating that the main framework structure of the material was maintained. The thermogravimetric process of all samples can be divided into three stages. The first stage (room temperature - 180°C) o The weight loss in C) is mainly attributed to the removal of physically adsorbed moisture from the material surface and pores; the second stage of weight loss occurs at 180°C. o C-357 o Near C, all samples showed a slight mass loss, which mainly corresponds to the removal of coordinated water molecules from the metal-oxygen cluster, possibly accompanied by the removal of a small amount of residual carboxylic acid ligands that did not participate in coordination. When the temperature exceeds 357°C... o After C, the material enters the third stage, in which the skeleton structure begins to undergo significant thermal decomposition. This indicates that the overall thermal stability and skeleton structure robustness of the MIL-100(In) / Cr-0.05 material are maintained, providing an important guarantee for the practical application of the material under higher temperature conditions.

[0039] like Figure 6 As shown, the UV-Vis absorption spectra and corresponding bandgap energies of MIL-100(In) / Cr-0.05 prepared in Comparative Example 1 and Example 1 reveal the regulatory effect of MIL-100(In) / Cr-0.05 doping on the electronic structure and bandgap of the materials. Figure 6 As shown in Figure a, MIL-100(In) exhibits strong absorption in the ultraviolet region, with the main absorption peak located near 300 nm. This peak can be attributed to ligand-to-metal charge transfer transition (LMCT). Furthermore, the broad absorption band appearing in the 300-400 nm range may originate from the spin-allowed dd transition of In(III) ions in the MIL-100(In) framework. Simultaneously, the spectrum of MIL-100(In) / Cr-0.05 shows a new absorption peak in the 400-700 nm visible light region, with a gradual red shift at the absorption edge, extending the absorption range from the original ultraviolet region to the visible light region. This series of changes indicates that Cr... 3+ The introduction of [a specific technology] effectively modulates the electronic structure of the material and significantly broadens its photoresponse range.

[0040] Figure 6 b shows (αhν) after transformation using the Tauc plot method. 2The relationship between Cr and photon energy (hν) was plotted, and the optical band gap values ​​of MIL-100(In) and MIL-100(In) / Cr-0.05 were calculated based on this. It can be clearly observed from the figure that the band gap value of MIL-100(In) / Cr-0.05 is lower than that of MIL-100(In), specifically 4.061 and 4.051 eV. This phenomenon indicates that Cr... 3+ The successful incorporation of [a specific substance] effectively modulated the electronic band structure of MIL-100(In). The narrowing of the band gap directly reduced the energy threshold required for ligand-to-metal charge transfer, making it easier for electrons to transition from the highest occupied molecular orbital of the organic ligand to the lowest unoccupied molecular orbital of the metal cluster under photoexcitation conditions. The improved electron transition efficiency means that more electrons can successfully transfer to the metal center. With the enhancement of the charge transfer process, the metal center gains more additional electrons, resulting in a significant increase in its electron cloud density.

[0041] XPS was used to reveal the surface chemical states and internal charge transfer mechanisms of MIL-100(In) prepared in Comparative Example 1 and MIL-100(In) / Cr-0.05 prepared in Example 1. Figure 7 As shown, except for pure MIL-100(In), characteristic signals of four elements—C, O, In, and Cr—were detected in the full XPS spectra of all doped samples, further confirming the presence of these elements in the prepared samples. In the In 3d XPS spectrum shown in 7b, the MIL-100(In) sample exhibits two characteristic peaks at 452.8 eV and 445.3 eV, which are attributed to In, respectively. 3+ 3D 3 / 2 and 3D 5 / 2 Spin-orbit splitting peaks. With Cr 3+ The introduction of Cr causes the In 3d XPS peak in the bimetallic MIL-100(In) / Cr-0.05 sample to shift towards a lower binding energy. This reverse shift indicates that Cr 3+ Doping may trigger a redistribution of the electron cloud density at the metal center, and the aforementioned changes in electronic structure may be attributed to the combined effects of bandgap modulation and electronegativity differences. As previously mentioned ( Figure 6 ), Cr 3+Doping leads to a gradual decrease in the band gap of the material, making it easier for electrons to transition from organic ligands to the metal cluster, and accelerating the transition rate, resulting in more electrons migrating to the metal center. According to the Pauling scale, In has a higher electronegativity (1.78) than Cr (1.66), and electrons tend to shift towards In sites, increasing the electron cloud density at In centers and causing XPS to move towards lower binding energies. Furthermore, the relative abundance of the two characteristic peaks in the O 1s XPS spectrum changes with increasing doping concentration: the MO bond (531.9 eV) gradually decreases, while the In-OH / oxygen vacancy (533.6 eV) gradually increases. This trend may be related to the introduced structural defects. These losses lead to partial unsaturated coordination of the metal centers, resulting in more oxygen vacancy defects. The formation of these defects helps to build more unsaturated metal sites in the MIL-100(In) / Cr-0.05 framework, thus providing more active centers for adsorption.

[0042] MIL-100(In) and MIL-100(In) / Cr-0.05 powders were selected at 298 K, and single-component vapor adsorption experiments on toluene and methylcyclohexane were conducted to evaluate the differences in selective adsorption behavior between the two. Figure 8 The adsorption rate curves of MIL-100(In) and MIL-100(In) / Cr-0.05 for toluene and methylcyclohexane are shown. For undoped MIL-100(In), the adsorption rates for toluene and methylcyclohexane are similar, with internal diffusion rate constants of 0.004 and 0.006, respectively, indicating that the adsorption kinetics of the original membrane for these two substances are not significantly different. However, with the introduction of Cr... 3+ Subsequently, the internal diffusion rate constant of toluene in the pores of MIL-100(In) / Cr-0.05 significantly increased to approximately 0.04218, indicating a substantial reduction in its transport resistance within the pores, thereby significantly accelerating the adsorption rate of toluene. This value is much higher than the internal diffusion rate constant of methylcyclohexane (approximately 0.002), suggesting that the adsorption of methylcyclohexane by MIL-100(In) / Cr-0.05 is significantly inhibited, and the difference in adsorption rates between the two is greatly amplified. This significant differentiation at the kinetic level is one of the reasons why the MIL-100(In) / Cr-0.05 membrane exhibits excellent separation selectivity for the toluene / methylcyclohexane system.

[0043] To more intuitively verify this mechanism, specific adsorption capacity data for toluene and methylcyclohexane from MIL-100(In) / Cr-0.05 were further presented to elucidate the basis for its significant separation selectivity for both. Figure 8 As shown in b, the adsorption capacity difference between the two is significant: the adsorption capacity of MIL-100(In) / Cr-0.05 toluene reaches 134 mg·g. -1The adsorption capacity of methylcyclohexane is only 93 mg·g. -1 The adsorption capacity of MIL-100(In) toluene is 110 mg·g. -1 The adsorption capacity of methylcyclohexane is only 77 mg·g. -1 Furthermore, the overall trend of the adsorption isotherms shows that toluene maintains a high adsorption plateau across a wide range of relative pressures, and its adsorption capacity increases rapidly in the low-pressure region, exhibiting typical micropore-filling behavior. This clearly demonstrates the material's preferential adsorption characteristics for toluene. In stark contrast, the adsorption curve for methylcyclohexane remains at a low plateau, with slow growth in adsorption capacity throughout the entire test pressure range, indicating a non-preferential adsorption state. This is consistent with the results obtained from previous adsorption kinetic analysis.

[0044] In order to evaluate Cr 3+ The effect of doping on the separation performance of MOF membranes was investigated by testing MIL-100(In) membranes and MIL-100(In) / Cr-0.05 membranes at 65 °C. o C. Pervaporation separation performance of toluene / methylcyclohexane 90:10 feed. The results show that Cr 3 + Doping strategies significantly modulate the mass transfer properties of the film. For example... Figure 9 As shown, the flux of the MIL-100(In) membrane is 0.5 kg·m. -2 ·h -1 The separation factor was 1.4, while Cr 3+ After doping, both the permeation flux and separation factor of the membrane showed an increasing trend, exhibiting superior separation performance: the flux reached as high as 2.1 kg·m⁻². -2 ·h -1 The separation factor reached 5.3, achieving a simultaneous improvement in permeability and selectivity. This excellent result also indirectly verifies the feasibility of this post-metal replacement method.

[0045] In the field of organic solvent pervaporation separation, the long-term operational stability of membrane materials is a key evaluation indicator for their practical industrial application. This study systematically evaluated the long-term performance stability of the MIL-100(In) / Cr-0.05 membrane in the continuous separation of a toluene / methylcyclohexane mixed system. Figure 10 As shown, at a feed temperature of 65°C o C. Under the condition of a toluene / methylcyclohexane feed ratio of 90:10, the membrane operated continuously for 120 hours, and its permeation flux remained stable at 2.11 ± 0.1 kg·m³ throughout the test. -2 ·h -1Within the specified range, the separation factor remained around 5.4 ± 0.1, and the toluene concentration on the permeate side remained around 98 wt.%, without significant decay or fluctuation. This excellent long-term operational stability indicates that even under prolonged continuous operation in a high-temperature organic solvent environment, the MIL-100(In) / Cr-0.05 membrane maintains its structural integrity and separation function stability, without performance degradation due to solvent swelling, thermal stress, or chemical corrosion. These results not only validate the practical application potential of the MIL-100 / Cr-0.05 membrane in aromatic / cycloalkanes separation but also provide important experimental evidence and feasibility support for the subsequent transition of membrane separation technology from laboratory scale to industrial scale-up.

[0046] Table 1. MIL-100(In) and MIL-100 / Cr-X films prepared in Examples 1-5 and Comparative Examples 1-6 at 65°C o C. Pervaporation separation performance of p-toluene / methylcyclohexane with a 90:10 feed ratio Table 1 shows the MIL-100(In) and MIL-100 / Cr-X films prepared in Examples 1-5 and Comparative Examples 1-4 at 65 °C. o C. Pervaporation separation performance of p-toluene / methylcyclohexane with a 90:10 feed ratio. The MIL-100(In) prepared in Comparative Example 1 has a pore size of 0.77 nm, but this is insufficient for effective sieving of toluene (0.53 nm) and methylcyclohexane (0.65 nm). Furthermore, the uniform distribution of active sites within the pores lacks the ability to directionally recognize the π-electron system of aromatic rings, making selective transport of toluene difficult. Therefore, efficient separation of the p-toluene / methylcyclohexane system cannot be achieved. The MIL-100 / Cr-0.7 prepared in Comparative Example 2, due to Cr... 3+Excessive concentration, despite the 1.5 nm pore size allowing more molecules to enter the channels, hinders molecular desorption due to the high micropore ratio, leading to retention and impeding subsequent effective diffusion and adsorption. This results in a simultaneous decrease in permeation flux and separation performance. Furthermore, these molecules tend to aggregate non-specifically at the pore openings, masking active sites in the MOF channels and weakening the membrane material's specific recognition of toluene molecules, thus reducing the separation factor. In Comparative Example 3, the MIL-100 / Cr-0.0.05 exhibits a significantly increased entropy contribution (TΔS) due to the increased replacement temperature, driving ions to migrate deeper into the lattice or internal channels to reach thermodynamic stability. During this process, deeply migrating dopant ions may occupy key molecular transport channels or windows in the bulk phase, further exacerbating the pore volume contraction effect. For larger molecules like methylcyclohexane, this physical space compression may prevent them from passing through completely, resulting in a sharp decrease in flux. For smaller toluene molecules, the transport path becomes more tortuous, leading to a reduction in both flux and separation factor. The MIL-100 / Cr-0.0.05 prepared in Comparative Example 4, due to the use of CrCl3, an inert Lewis acid, has a higher Cr content. 3+ The coordination exchange kinetics of Cr are slow, resulting in significantly lower reactivity. This fundamental difference leads to Cr... 3+ It cannot effectively replace the In in the MIL-100(In) skeleton. 3+ The substitution reaction is difficult to occur. In Comparative Example 5, the MIL-100 / Fe-0.0.05 mixture exhibits good performance due to the presence of Fe³⁺. + With In³ + They share certain similarities in fundamental properties such as electronegativity and ionic radius. Under the same post-substitution conditions, Fe³⁺ + It can also enter some metal node sites in the MIL-100(In) framework, but its ability to modulate the electronic structure within the channels is significantly weaker than that of Cr³⁺. + The system failed to achieve selective separation of toluene and methylcyclohexane using a bimetallic membrane. Comparative Example 6 prepared MIL-100 / Al-0.0.05, despite Al³... + While capable of participating in partial metal site replacement, chromium's lack of d-orbital participation and limited defect regulation prevent it from achieving electronic structure reconstruction and synergistic adsorption enhancement within the MIL-100(In) channels, thus hindering the acquisition of a bimetallic membrane for selective separation of toluene and methylcyclohexane. Therefore, chromium failed to successfully dope into the MIL-100(In) framework, preventing the attainment of the desired MIL-100(In) / Cr bimetallic membrane. The results of the above examples and comparative examples demonstrate that, within the concentration range (0.025~0.5 mol / L), temperature range (50℃~85℃), and specific chromium source conditions defined in this invention patent, a bidirectional improvement in membrane flux and separation factor can be achieved.

Claims

1. A bimetallic MOF film, characterized in that, It includes a porous substrate and a bimetallic MIL-100(In) / Cr film layer supported on the porous substrate; the bimetallic MIL-100(In) / Cr film layer has a MIL-100 topological framework structure, wherein the metal center in the framework structure is composed of the host metal In³ + and doped metal Cr³ + Together they form a bimetallic MIL-100(In) / Cr film with an interior composed of electron-deficient Cr³⁺. + Sites and electron-rich In³ + Electron donor-acceptor type pore structure composed of sites.

2. The bimetallic MOF film according to claim 1, characterized in that, The doped metal Cr³ + The proportion of the core metal in the bimetallic MIL-100(In) / Cr film is 10% to 80%.

3. The bimetallic MOF film according to claim 1, characterized in that, The porous substrate is one or more of an alumina support, a zirconia support, or a silicon oxide support; and / or the porous substrate has a diameter of 20-40 mm, a thickness of 1.5-2 mm, an average pore size of 80-120 nm, and a porosity of 20%-60%.

4. A method for preparing a bimetallic MOF film as described in any one of claims 1 to 3, characterized in that, The process includes the following steps: (1) Substrate film formation: A precursor solution containing a host metal salt and an organic ligand is added to a mixed solvent containing N,N-dimethylformamide, ethanol, and water to form a precursor solution. A dense MIL-100(In) film is synthesized in situ on a porous substrate by a homogeneous hydrothermal reaction; (2) Post-metal replacement: The porous substrate with the MIL-100(In) film on its surface obtained in step (1) is immersed in a solution containing the replacement metal Cr³ + In an alcohol solution, a metal substitution reaction is carried out to make Cr³ + Partially replaces In³ in the MIL-100(In) skeleton + (3) Washing and activation: After the displacement reaction is completed, the membrane material sample is taken out, and after washing and activation with alcohol washing aid, the bimetallic MOF membrane is obtained.

5. The method according to claim 4, characterized in that, In step (1), the molar ratio of the main metal salt, organic ligand, N,N-dimethylformamide, ethanol and water in the precursor solution is 1:0.5-2:2-5:2-5:0.5-2.

6. The method according to claim 4, characterized in that, In step (2), the Cr³-containing metal is... + In the alcoholic solution, the chromium source is selected from one or a combination of two of chromium nitrate and chromium sulfate; and / or, the alcohol solvent is selected from one or a combination of methanol, ethanol, or isopropanol; and / or, the chromium-containing substituted metal Cr³⁺ + The concentration of the alcohol solution is 0.025~0.5 mol / L.

7. The method according to claim 4, characterized in that, In step (2), the temperature of the metal displacement reaction is 50℃~85℃, and the reaction time is 3~24 hours.

8. The method according to claim 4, characterized in that, In step (3), the alcohol washing aid is selected from one or more combinations of methanol, ethanol or isopropanol; and / or, after washing, the bimetallic MOF membrane is stored in anhydrous alcohol solvent to maintain its pore wetting and activation state.

9. The application of a bimetallic MOF membrane as described in any one of claims 1 to 3 in the separation of toluene and methylcyclohexane, characterized in that, Using the bimetallic MOF membrane material as a pervaporation separation membrane, the electron donor-acceptor type pore structure and unsaturated metal defect sites inside the bimetallic MIL-100(In) / Cr membrane layer are utilized to conduct electrostatic attraction and coordination interaction with toluene molecules. Through the specific recognition and preferential adsorption of toluene, the pervaporation separation of the toluene and methylcyclohexane mixture is achieved.

10. The application according to claim 9, characterized in that, The operating temperature for the pervaporation separation is 25–65°C; the feed ratio of toluene to methylcyclohexane is 10:90–90:10.

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