Application of an iron-based metal-organic framework material in separation of methane / ethane / propane mixed gas

Abstract

This invention discloses the application of an iron-based metal-organic framework (MOF) in the separation of methane / ethane / propane mixed gases. The MOF is a porous material formed by the self-assembly of iron trioxide clusters as secondary building blocks and pyrazole carboxylic acid ligands. This material serves as an adsorbent for the separation of mixed gases containing methane, ethane, and propane. In fixed-bed adsorption processes, this material exhibits significantly stronger adsorption affinity and capacity for ethane and propane than for methane. When adsorbing simulated natural gas, it can achieve complete retention of ethane and propane in the adsorption bed, obtaining methane with a purity of up to 99.9% in one step. The adsorbent material of this invention possesses excellent separation selectivity, high adsorption capacity, outstanding cycling stability (no performance degradation after five cycles), and mild regeneration performance, showing broad industrial application prospects in natural gas purification, coalbed methane recovery, and associated gas treatment in oil fields.

Description

Technical Field

[0001] This invention belongs to the field of gas adsorption and separation technology, specifically relating to the application of an iron-based metal-organic framework material in the separation of methane / ethane / propane mixed gases, particularly in the purification of natural gas, recovery of coalbed methane, and treatment of associated gas in oil fields. Background Technology

[0002] Metal-organic frameworks (MOFs) are a class of crystalline porous materials formed by the self-assembly of inorganic metal cluster nodes and organic ligands through coordination bonds. With their modular composition, programmable topology, and systematically tunable pore environment, MOFs exhibit exceptional functional designability. Their single-crystal nature makes it possible to resolve the structure-property relationships of materials at the atomic scale, thus providing an ideal platform for achieving high-precision separation of C1-C3 hydrocarbons through the synergistic effect of surface chemistry and confinement.

[0003] Currently, strategies for improving the gas separation performance of metal-organic frameworks (MOFs) mainly focus on precise control of pore size, functional modification of pore surfaces, and dynamic adjustment of framework flexibility. Against this backdrop, cage-like MOFs, such as IRMOs, HKUST-1, ZIFs, and the UiO series, have attracted significant attention due to their combination of ultra-high porosity and unique guest adaptability. Compared to traditional straight-channel MOFs, cage-like MOFs possess large-sized ordered cavities connected by narrow windows. This structural feature makes them ideal carriers for implementing "dual-function" strategies: on the one hand, the narrow windows can utilize the molecular sieve effect to achieve precise recognition of molecules of specific sizes, thus ensuring separation selectivity; on the other hand, the spacious internal cavities provide ample space, ensuring high adsorption capacity and effectively resolving the trade-off between separation selectivity and adsorption capacity. Further research has shown that introducing functional groups or Lewis acid-base sites on the surface of the cage cavity at the atomic scale can enhance the interaction between the host and guest. The resulting "synergistic optimization of pore size and pore chemical environment" strategy can significantly improve separation efficiency while maintaining high pore volume and has achieved breakthroughs in many gas separation challenges.

[0004] While the aforementioned strategies have demonstrated great potential in the design and application of cage-like metal-organic frameworks, a clear consensus has yet to be reached regarding the separation mechanism of multi-component hydrocarbon mixtures such as methane / ethane / propane, particularly the dynamic behavior of different molecules within the cage cavity and the host-guest recognition mechanism. Current research still relies on methods such as molecular simulation to elucidate the binding sites and diffusion pathways of guest molecules. Furthermore, how to construct ideal separation materials with high stability, high selectivity, and high adsorption capacity under mild and economical synthetic conditions by precisely controlling the cage structure and its surface chemical environment remains a technical challenge for researchers in this field.

[0005] Therefore, developing a novel metal-organic framework material that can achieve efficient separation in complex hydrocarbon mixtures and possesses excellent structural stability and regeneration performance is of great significance for promoting the development of industrial methane purification and light hydrocarbon recovery technologies. Summary of the Invention

[0006] To address the aforementioned technical problems in existing technologies, the present invention aims to provide an application of iron-based metal-organic framework materials in the separation of methane / ethane / propane mixed gases, with the goal of obtaining high-purity methane from natural gas in one step and effectively recovering ethane and propane. Compared with isomorphic Zr-MOF, the iron-based MOF material of the present invention exhibits 4.7 times and 3.3 times longer breakthrough times for ethane and propane, respectively; compared with the classic MIL-101(Fe), the propane adsorption capacity is increased by approximately 70%, and the separation window is significantly broadened.

[0007] The technical solution adopted in this invention is as follows: An application of an iron-based metal-organic framework material in the separation of methane / ethane / propane mixed gases, wherein the iron-based metal-organic framework material is a porous material formed by the self-assembly of iron trioxide clusters as secondary building blocks and pyrazole carboxylic acid ligands.

[0008] Furthermore, the pyrazole carboxylic acid ligand is 4-pyrazole carboxylic acid.

[0009] Furthermore, the preparation method of the iron-based metal-organic framework material is as follows: S1: Dissolve the trivalent Fe salt and pyrazole carboxylic acid ligand in an organic solvent, stir thoroughly until homogeneous, transfer the mixture to a reaction vessel, seal the reaction vessel, and transfer it to an oven to heat and react at 70-100℃ for 18-36 hours; S2: After the reaction in step S1 is completed, the iron-based metal-organic framework material is obtained through post-processing.

[0010] Furthermore, the trivalent Fe salt is a nitrate, sulfate, or chloride salt of Fe, and the molar ratio of the trivalent Fe salt to the pyrazole carboxylic acid ligand is 1:0.8-2, preferably 1:1-1.2.

[0011] Further, in step S1, the organic solvent is N,N'-dimethylformamide, and the volume of the organic solvent, in terms of the amount of trivalent Fe salt, is 3-8 L / mol, preferably 4-5 L / mol.

[0012] Furthermore, in step S1, the heating reaction temperature is 80-90℃, and the heating reaction time is 20-25 h.

[0013] Further, in step S2, the post-processing steps are: cooling to room temperature, centrifuging to collect the precipitate, washing it several times with DMF and methanol to remove residual impurities, and drying it to obtain the iron-based metal-organic framework material.

[0014] Furthermore, the separation is carried out in a fixed-bed adsorption column, which includes passing the methane / ethane / propane mixed gas through an adsorption bed filled with the iron-based metal-organic framework material, selectively adsorbing ethane and propane to obtain a methane-rich gas stream.

[0015] Furthermore, the adsorption separation temperature is 20-30℃ and the pressure is 0.8-1.2 bar.

[0016] Furthermore, it also includes a step of regenerating the iron-based metal-organic framework material after adsorption saturation: purging the adsorption bed with an inert gas at 70-100°C to desorb and recover ethane and propane.

[0017] Compared with the prior art, the present invention has the following significant advantages: 1) Excellent Separation Selectivity: This invention is the first to apply a specific structured iron-based MOF material to the separation of a CH4 / C2H6 / C3H8 ternary system. Experiments show that this material has a much stronger adsorption capacity for ethane and propane than for methane. When processing simulated natural gas with a CH4 / C2H6 / C3H8 volume ratio of 85 / 10 / 5, complete retention of ethane and propane can be achieved in the adsorption bed, with the breakthrough time / mass of ethane and propane delayed to 75 min / g and 190 min / g, respectively, thereby obtaining methane with a purity of up to 99.9% in one step, demonstrating excellent separation performance.

[0018] 2) High adsorption capacity: Under conditions of 298 K and 1 bar, the material exhibits an adsorption capacity of up to 59.65 cm⁻¹ for propane. 3 / g, the adsorption capacity for ethane is 41.39 cm⁻¹. 3 / g, which has a significant competitive advantage among similar light hydrocarbon separation materials, ensuring the treatment capacity per unit volume of adsorbent.

[0019] 3) Excellent renewability and stability: After five adsorption-desorption cycle experiments, the breakthrough curve and adsorption capacity retention rate of this material are still above 99.5%, the structure is stable, the performance is not degraded, and it has excellent potential for industrial application.

[0020] 4) Mild regeneration conditions: Adsorption-saturated iron-based MOF materials can be completely regenerated by purging with an inert gas under mild conditions of 70-100°C, and high-purity ethane and propane (e.g., propane purity >99.82%) can be recovered. This process has low energy consumption, is simple to operate, and is economical.

[0021] 5) Simple process and strong adaptability: The separation method provided by this invention is based on a mature fixed-bed adsorption process. It is simple to operate, easy to scale up, and can be directly applied to industrial scenarios such as natural gas purification, coalbed methane recovery, and associated gas treatment in oil fields. Attached Figure Description

[0022] The following detailed description, in conjunction with the accompanying drawings and embodiments of the present invention, will provide further details.

[0023] Figure 1 The powder X-ray diffraction (PXRD) pattern of the iron-based MOF material in Example 1 of this invention is compared with the simulated pattern to confirm its phase structure.

[0024] Figure 2 The PXRD spectra of the iron-based MOF material in Example 1 of this invention after treatment under different conditions (air, water, acetone, 100°C) confirm its stability.

[0025] Figure 3 The images show the single-component adsorption isotherms of CH4, C2H6, and C3H8 for the iron-based MOF material used in the embodiments of this invention at 298 K and 273 K, demonstrating its high adsorption capacity for heavy hydrocarbons.

[0026] Figure 4 In this embodiment of the invention, the dynamic breakthrough curve of the iron-based MOF material for the ternary CH4 / C2H6 / C3H8 mixture (85 / 10 / 5, v / v / v) demonstrates the separation effect of obtaining high-purity methane in one step.

[0027] Figure 5 In this embodiment of the invention, the desorption and regeneration curves of the adsorption-saturated iron-based MOF material at different temperatures (70, 80, 90, 100°C) are shown.

[0028] Figure 6 The cyclic dynamic breakthrough test results of the iron-based MOF material in this embodiment of the invention demonstrate its excellent cyclic stability. Detailed Implementation

[0029] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0030] Example 1: Preparation of iron-based MOF materials The iron-based MOF material used in this invention can be synthesized using methods found in existing technologies. The specific steps are as follows: 3.2 mmol of ferric nitrate nonahydrate and 3.2 mmol of 4-pyrazolecarboxylic acid were dissolved in 16 mL of N,N'-dimethylformamide. The mixture was stirred at room temperature for 30 minutes to ensure homogeneity, and then reacted in an 80°C oven for 2 days. After the reaction was complete, the mixture was cooled to room temperature, and the yellow-green precipitate was collected by centrifugation. The precipitate was washed repeatedly with DMF and methanol to remove residual impurities, and then dried in air to obtain the iron-based MOF material. The sample was activated in a vacuum oven at 100°C for 12 h to remove impurities.

[0031] X-ray diffraction analysis confirmed the following crystallographic parameters of the iron-based MOF material in Example 1: space group Fm-3m, cell parameters a=b=c=31.6818(14) Å, α=β=γ=90°, cell volume V=31800(4) Å. 3 It has a multi-level cage structure, including a ~5.0 Å tetrahedral cage, a ~10.5 Å hexahedral cage and a ~15.0 Å polyhedral cage, and the cages are connected by a ~2.6 Å triangular window and a 6.2×6.2 Å square window.

[0032] Example 2: Confirmation of the material's structure and stability The iron-based MOF material prepared in Example 1 was subjected to PXRD testing, and the results are as follows: Figure 1 As shown, the experimental spectrum and the simulated spectrum are in high agreement, proving that the material is the target phase with high purity.

[0033] The iron-based MOF material was immersed in air, water, and acetone for one week, and then heat-treated at 100°C for one week before PXRD testing was performed again. Figure 2 As shown, the diffraction peak positions of all treated samples did not shift significantly compared to the original samples, proving that the material has good physicochemical and hydrothermal stability and can meet the requirements for long-term use under industrial conditions.

[0034] Example 3: Single-component gas adsorption performance test The activation method of the iron-based MOF sample in Example 1 is as follows: The iron-based MOF sample is placed in a vacuum oven and activated at 100°C for 12 hours to remove impurities such as solvents in the pores.

[0035] The single-component static adsorption isotherms of CH4, C2H6, and C3H8 for the activated iron-based MOF sample of Example 1 were tested at 298 K and 273 K. The experimental results are as follows: Figure 3 As shown, under conditions of 298 K and 1.0 bar, the adsorption capacity of the sample for C3H8 reached as high as 59.65 cm⁻¹. 3 / g, the adsorption capacity for C2H6 is 41.39 cm⁻¹. 3 / g, while the adsorption capacity for CH4 is only 14.98 cm⁻¹.3 / g. In the low-pressure region (0-0.04 bar), the adsorption capacity for C3H8 is approximately 24 times that for CH4. This indicates that the material has a much stronger adsorption affinity and capacity for ethane and propane than for methane, laying a solid foundation for efficient separation.

[0036] Example 4: Dynamic Penetration Separation Experiment (Core Application Verification of the Invention) Column packing and activation: The iron-based MOF material prepared in Example 1 was pressed into tablets, crushed, and sieved (40-60 mesh). 0.4 g of the tablets was weighed and packed into a fixed bed column with an inner diameter of 3 mm and a length of 250 mm. The column was activated by purging with helium at 100°C for 12 hours.

[0037] Breakthrough experiment: A natural gas mixture with a CH4 / C2H6 / C3H8 volume ratio of 85 / 10 / 5 was introduced into the adsorption column at a flow rate of 1.5 sccm at 20°C and 1 bar to conduct a dynamic breakthrough experiment. The gas composition covered the typical composition range of natural gas and associated gas from oil fields. The experimental results are as follows: Figure 4 As shown.

[0038] according to Figure 4 As shown, CH4 was eluted first with a delay time of 5 min / g; C2H6 broke through at 75 min / g; and C3H8 broke through at 190 min / g. Before 75 min / g, only CH4 was detectable in the outlet gas stream, achieving one-step high-purity purification of CH4. The CH4 purity was measured to be 99.9%, making it suitable for direct output as a qualified natural gas product. The calculated dynamic adsorption capacities of C2H6 and C3H8 were 13.79 cm⁻¹. 3 / g and 16.02 cm 3 / g.

[0039] Regeneration and desorption experiments: After adsorption saturation, the adsorption column was regenerated by purging with argon gas at 5 ml / min at 70, 80, 90, and 100°C. The experimental results are as follows: Figure 5 As shown.

[0040] according to Figure 5 As shown, the desorption rate increases with increasing temperature. At 100°C, ethane and propane can be rapidly and completely desorbed. Integral calculations show that, under regeneration conditions at 90°C, propane with a purity of over 99.82% (5.84 cm⁻¹) can be recovered. 3 / g) and ethane with a purity of 99.5% or higher (1.88 cm 3Even at a low temperature of 70°C, iron-based MOFs can achieve relatively complete desorption, and can be regenerated using industrial waste heat, significantly reducing operating energy consumption. At 100°C, both ethane and propane desorb rapidly due to the excessively high temperature. However, because the material adsorbs a larger amount of propane, the desorption curve for ethane almost coincides with the baseline.

[0041] During regeneration, although ethane and propane begin desorption simultaneously, their adsorption affinities on the iron-based MOF material differ (propane is stronger, ethane is weaker), leading to significant differences in desorption kinetics: ethane desorbs first in large quantities from the iron-based MOF material surface, forming a high-concentration "desorption front," while the desorption peak of propane lags significantly. To separate ethane and propane during desorption from the iron-based MOF material, a segmented collection method is used: when ethane concentration is dominant in the initial stage of desorption, the system switches to an "ethane collection tank"; once the ethane concentration decreases and propane becomes the main component, it switches back to a "propane collection tank," thus obtaining high-purity ethane and propane products respectively.

[0042] The two values ​​mentioned above are 5.84 cm. 3 / g and 1.88 cm 3 / g represents the volume of propane and ethane that can be recovered per gram of iron-based MOF material after regeneration, under standard conditions. The detection requires the use of a gas flow meter and a gas chromatograph: During regeneration, all gas discharged from the adsorption column outlet at a specific stage (such as the propane collection stage) is introduced into the gas flow meter, and the total gas volume at room temperature is recorded. Simultaneously, a sample is taken from the gas stream and injected into the gas chromatograph for analysis to obtain the propane volume percentage. The total volume of propane that can be recovered during regeneration can then be calculated.

[0043] Cyclic stability experiment: The above "adsorption-regeneration-desorption" process was repeated 5 times, with the desorption temperature at 100°C. The experimental results are as follows: Figure 6 As shown, the breakthrough curves of the five cycles almost completely overlap, and the propane adsorption capacity retention rate is above 99.5%, indicating that the material maintains excellent separation performance after repeated use and has the reliability for long-term industrial operation.

[0044] Comparative Example 1: Comparison of different metal centers (Zr-MOF) This Comparative Example 1 aims to illustrate the effect of the metal center on separation performance. The preparation method of the Zr-MOF material in Comparative Example 1 is the same as that in Example 1, except that the metal precursor is replaced by an equimolar amount of zirconium oxychloride octahydrate (ZrOCl2·8H2O), and the same molar amount of 4-pyrazole carboxylic acid ligand is used to synthesize the comparative material Zr-MOF under the same conditions.

[0045] Under identical conditions (25°C, 1 bar, 1.5 mL / min), the dynamic breakthrough performance of Zr-MOF for a ternary gas mixture of CH4 / C2H6 / C3H8 (85 / 10 / 5, v / v / v) was tested. The breakthrough order of Zr-MOF was consistent with that of Fe-MOF, but the key performance indicators differed significantly: the breakthrough time for CH4 was approximately 4 min / g, for C2H6 it was approximately 16 min / g, and for C3H8 it was approximately 60 min / g.

[0046] In comparison, the breakthrough times of the Fe-MOF of this invention for C2H6 and C3H8 (75 min / g and 190 min / g, respectively) are 4.7 times and 3.3 times that of Zr-MOF. This indicates that, despite having the same organic ligand linkage, the iron-centered MOF provides superior pore confinement effects or host-guest interaction sites, thus exhibiting stronger "restriction" capabilities and a higher dynamic separation window for ethane and propane. Comparative Example 1 demonstrates that the selection of iron as the metal center in this invention is crucial for achieving excellent separation performance.

[0047] Comparative Example 2: Comparison of different organic ligands (Fe-BDC) Comparative Example 2 presents the application performance of a typical MOF material (MIL-101(Fe)) reported in the literature in natural gas separation. According to the report by Qin et al. (ACS Applied Materials & Interfaces, 2022, 14(40), 45444-45450), the single-component adsorption capacities of this MOF material for the ternary mixture of CH4 / C2H6 / C3H8 at 298 K and 1 bar are 0.32 mmol / g (CH4), 1.64 mmol / g (C2H6), and 6.46 mmol / g (C3H8), respectively. The selectivity of C3H8 and C2H6 over CH4 was further verified by transient breakthrough experiments.

[0048] Under the same test conditions (25°C, 1 bar), the present invention synthesized MIL-101(Fe) according to the method described in the literature and conducted verification tests. The results showed that the adsorption capacity of this material for C3H8 was approximately 35 cm⁻¹. 3 / g, which is far lower than the 59.65 cm⁻¹ of the Fe-MOF of this invention. 3 / g. More importantly, dynamic breakthrough experiments showed that due to the larger pore size (approximately 29-34 Å) and pore window of MIL-101(Fe), its molecular sieve effect and confinement effect on C2H6 and C3H8 were weaker. Although C3H8 still broke through last, the breakthrough time interval between C2H6 and CH4 was significantly shortened (CH4 breakthrough time approximately 3 min / g, C2H6 breakthrough time approximately 10 min / g), making it impossible to obtain high-purity methane within a wide time window.

[0049] Comparative Example 2 demonstrates that the 4-pyrazole carboxylic acid ligand selected in this invention, through its unique microporous cage-like structure (window size ~2.6-6.2 Å) with iron clusters, is key to achieving the synergistic effect of efficient kinetic separation and thermodynamic selective adsorption of C1-C3 alkanes.

[0050] In summary, this invention successfully achieved the one-step extraction of high-purity methane from natural gas (a CH4 / C2H6 / C3H8 mixture) under mild conditions of ambient temperature and pressure by applying an iron-based MOF material with a specific multi-level cage structure to the separation of natural gas. It also enabled the efficient recovery of high-value-added ethane and propane. Compared to Comparative Examples 1-2, the isomorphic Zr-MOF obtained by replacing Fe with Zr exhibited C2H6 and C3H8 breakthrough times that of Fe-MOF, which were only 1 / 4.7 and 1 / 3.3, respectively. MIL-101(Fe), obtained by replacing 4-pyrazole carboxylic acid with terephthalic acid, lost its effective molecular confinement ability due to its excessively large pore size, failing to provide a sufficient separation window.

[0051] The iron-based MOF material with a specific multi-level cage structure of the present invention exhibits excellent separation selectivity, high adsorption capacity, outstanding cycle stability and mild regeneration performance, and has excellent industrial application prospects in the fields of natural gas purification, coalbed methane recovery and associated gas treatment in oil fields.

[0052] The contents described in this specification are merely an enumeration of the implementation forms of the inventive concept, and the scope of protection of this invention should not be regarded as limited to the specific forms described in the embodiments.

Claims

1. An application of an iron-based metal-organic framework material in the separation of methane / ethane / propane mixed gases, characterized in that, The iron-based metal-organic framework material is a porous material formed by the self-assembly of iron trioxide clusters as secondary building units and pyrazole carboxylic acid ligands.

2. The application as described in claim 1, characterized in that, The pyrazole carboxylic acid ligand is 4-pyrazole carboxylic acid.

3. The application as described in claim 1, characterized in that, The preparation method of the iron-based metal-organic framework material is as follows: S1: Dissolve the trivalent Fe salt and pyrazole carboxylic acid ligand in an organic solvent, stir thoroughly until homogeneous, transfer the mixture to a reaction vessel, seal the reaction vessel, and transfer it to an oven to heat and react at 70-100℃ for 18-36 hours; S2: After the reaction in step S1 is completed, the iron-based metal-organic framework material is obtained through post-processing.

4. The application as described in claim 3, characterized in that, The trivalent Fe salt is a nitrate, sulfate, or chloride salt of Fe, and the molar ratio of the trivalent Fe salt to the pyrazole carboxylic acid ligand is 1:0.8-2, preferably 1:1-1.

2.

5. The application as described in claim 3, characterized in that, In step S1, the organic solvent is N,N'-dimethylformamide, and the volume of the organic solvent, in terms of the amount of trivalent Fe salt, is 3-8 L / mol, preferably 4-5 L / mol.

6. The application as described in claim 3, characterized in that, In step S1, the heating reaction temperature is 80-90℃, and the heating reaction time is 20-25 h.

7. The application as described in claim 3, characterized in that, In step S2, the post-processing steps are as follows: cool to room temperature, centrifuge to collect the precipitate, wash several times with DMF and methanol to remove residual impurities, and dry to obtain the iron-based metal-organic framework material.

8. The application as described in claim 1, characterized in that, The separation is carried out in a fixed-bed adsorption column, which involves passing the methane / ethane / propane mixed gas through an adsorption bed filled with the iron-based metal-organic framework material, selectively adsorbing ethane and propane to obtain a methane-rich gas stream.

9. The application as described in claim 8, characterized in that, The adsorption separation temperature is 20-30℃, and the pressure is 0.8-1.2 bar.

10. The application as described in claim 8, characterized in that, It also includes a step of regenerating the iron-based metal-organic framework material after adsorption saturation: purging the adsorption bed with an inert gas at 70-100°C to desorb and recover ethane and propane.

Images