Method for researching MOF (Metal Organic Framework) synthesis process by using low-field nuclear magnetic imaging device

Abstract

The invention discloses a method for researching an MOF (Metal Organic Framework) synthesis process by utilizing a low-field nuclear magnetic imaging device, and belongs to the technical field of material synthesis process analysis. The method comprises the following steps: firstly, preparing a metal source solution and an organic ligand solution; one of the solutions is selected for low-field nuclear magnetic resonance relaxation time measurement, and imaging sequence parameters are optimized; mixing the two solutions, and immediately putting the mixed solution into a low-field nuclear magnetic imaging device; and carrying out continuous relaxation time weighted imaging and layered relaxation time measurement on the reaction process by using the optimized imaging sequence. The lossless, in-situ, visual and quantitative monitoring of the metal-organic framework material in the nucleation, growth and sedimentation process is realized by analyzing the change of image signal intensity in time and space and the change trend of relaxation time (such as T2 value) of different liquid layers along with time. According to the method, required equipment is simple, operation is convenient and fast, and an innovative research means is provided for deeply understanding the MOF synthesis mechanism and optimizing the synthesis process.

Description

Technical Field

[0001] This invention belongs to the field of material synthesis process analysis technology, specifically relating to a method for in-situ, non-destructive, and real-time monitoring of the synthesis process of metal-organic framework materials using low-field nuclear magnetic resonance imaging technology. Background Technology

[0002] Metal-organic frameworks (MOFs) are crystalline porous materials with periodic network structures formed by the self-assembly of metal ions or clusters with organic ligands through coordination bonds. They have broad application prospects in gas storage, separation, catalysis, and drug delivery. The performance of MOFs mainly depends on their crystal structure, size, and morphology, which are directly affected by their synthesis process (such as nucleation rate and growth mechanism).

[0003] Currently, common methods for studying MOF synthesis include intermittent sampling combined with offline characterization techniques such as X-ray diffraction and scanning electron microscopy. These methods cannot continuously observe the same reaction system, potentially missing crucial transient intermediate information, and the sampling process is prone to interfering with the reaction progress. Although online techniques such as dynamic light scattering and UV-Vis spectroscopy have some applications, they are difficult to effectively probe opaque or high-concentration systems, and cannot provide spatially resolved information within the reactor.

[0004] Low-field nuclear magnetic resonance imaging (NMR) technology possesses advantages such as being non-destructive, having high penetration, and providing spatial resolution. Currently, it is mainly used for imaging the distribution of water or oil in food, petroleum cores, and biological tissues. However, there are no reports on its innovative application to monitoring complex chemical processes such as MOF synthesis, especially in reflecting the formation and sedimentation behavior of solid products through changes in parameters such as relaxation time. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for studying the MOF synthesis process based on a low-field nuclear magnetic resonance imaging device. This method enables in-situ, real-time, and visual monitoring of the MOF synthesis reaction, intuitively displaying the dynamic changes in product formation and sedimentation, and providing an effective means for understanding the synthesis mechanism and optimizing the synthesis process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for studying the MOF synthesis process using a low-field nuclear magnetic resonance imaging device, characterized by comprising the following steps: S1. Solution preparation: Prepare precursor solution A containing a metal source and precursor solution B containing an organic ligand, respectively; the solvent is methanol, ethanol, water or a mixture thereof; S2. NMR parameter pre-calibration and sequence optimization: Take an appropriate amount of solution A or B and place it in the sample tube of the low-field NMR imaging device. Perform magnetic field homogenization and measure its longitudinal relaxation time T1 and transverse relaxation time T2 respectively. Based on the relaxation characteristics, screen and set the pulse sequence parameters required for imaging. S3. In-situ reaction and real-time monitoring: Remove the sample tube from the magnet, quickly mix solutions A and B, and then immediately place the mixture in the center of the magnet of the low-field NMR imaging device; S4. Data Acquisition and Imaging: Start the optimized imaging sequence in step S2, and perform layered or three-dimensional relaxation time data acquisition and weighted imaging on the system during the reaction process to continuously acquire a series of layered relaxation time changes and nuclear magnetic resonance images; S5. Process Analysis and Interpretation: Analyze the image sequence and interpret the nucleation, growth and sedimentation process of MOF materials based on the changes in signal intensity and relaxation time; among them, the generated MOF solid particles appear as low signal dark areas in T2 weighted images and high signal bright areas in T1 weighted images because their relaxation time is significantly shorter than that of the liquid phase.

[0007] Further, the metal source in step S1 is a nitrate, chloride, or acetate of zinc, copper, zirconium, or iron; the organic ligand is an imidazole or carboxylic acid compound.

[0008] Furthermore, the pulse sequence mentioned in step S2 is a CPMG sequence, SE sequence, or IR sequence, used to obtain T2 and T1 values ​​and their weighted images.

[0009] Furthermore, in step S4, the generation kinetics of MOF are analyzed by quantitatively measuring the change curve of T2 or T1 values ​​in a specific region of the image over time.

[0010] Furthermore, the monitoring in step S4 includes stratified relaxation time measurement: multiple regions of interest are set in the vertical direction within the reactor, and the transverse relaxation time T2 or longitudinal relaxation time T1 of each region is measured and recorded in real time and quantitatively using CPMG or IR sequences.

[0011] Furthermore, the process analysis in step S5 includes plotting and analyzing kinetic curves showing the changes in T2 or T1 values ​​of different liquid layers over time. These curves are closely related to the reaction progress. The slow increase in T2 or T1 values ​​in the upper layer (clear liquid zone) indicates that the number of small particles or crystal nuclei in the solution has decreased due to sedimentation. The sharp drop in T2 or T1 values ​​in the middle layer (reaction zone) reflects the rapid increase in the concentration of solid particles in this region, and the rapid formation and growth of crystals. The rapid decrease and eventual stabilization of the T2 or T1 value in the bottom layer (precipitation zone) at extremely low levels indicates that a dense MOF precipitate layer has been formed and is stabilizing. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the system structure of the present invention; Figure 2 This is a flowchart of the method of the present invention; Figure 3 This is a physical image of the nuclear magnetic resonance imaging analyzer used in this invention; Figure 4 This is a T2-weighted imaging image of the initial stage of the ZIF-8 synthesis process in Example 1; Figure 5 This is a T2-weighted image taken 8 hours after the ZIF-8 synthesis process in Example 1. Figure 6 The curves show the changes in T2 values ​​over time for the upper, middle, and lower layers during the ZIF-8 synthesis process in Example 1. Detailed Implementation

[0013] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The following embodiments use typical ZIF-8 synthesis as an example, but the scope of protection of the present invention is not limited thereto.

[0014] Example 1: Monitoring the synthesis process of ZIF-8 using low-field nuclear magnetic resonance imaging

[0015] Solution preparation:

[0016] Solution A: Add 0.025 mol of zinc nitrate hexahydrate to 200 mL of anhydrous methanol and stir magnetically until completely dissolved.

[0017] Solution B: Add 0.1 mol of 2-methylimidazole to 200 mL of anhydrous methanol and stir magnetically until completely dissolved.

[0018] NMR parameter pre-calibration and sequence optimization:

[0019] Take 10 mL of solution B and place it in a dedicated NMR sample tube, then place it in the center of the magnet of a 0.5T low-field NMR imaging device; Perform automatic or manual field shimming to optimize magnetic field uniformity; The average T2 value of solution B was measured using a CPMG sequence (approximately 900 ms). Based on this, the T2-weighted imaging parameters (echo time TE = 100 ms, repetition time TR = 1000 ms) were set to ensure that the solid particles and liquid had a clear signal contrast.

[0020] In-situ reaction and real-time monitoring:

[0021] Remove the sample tube from the magnet, quickly pour all of solution A into solution B and mix thoroughly; Immediately place the mixed reaction sample tube back into the center of the imager magnet, ensuring that the position is consistent with that during shimming.

[0022] Data acquisition and imaging:

[0023] The preset T2-weighted multilayer imaging and T2 mapping sequence was initiated. Starting from the start of the reaction (t=0), data was acquired every 5 minutes for the first 2 hours, every 30 minutes from the 2nd to the 8th hour, and every 60 minutes from the 8th to the 24th hour. The acquired data was used to reconstruct cross-sectional images and to quantitatively calculate the T2 values ​​of the upper, middle, and lower preset regions of the reaction tube.

[0024] Process Analysis and Results:

[0025] like Figure 4 As shown, the image of the initial stage of the reaction (first 10 minutes) has a uniform and bright signal, corresponding to a homogeneous liquid phase; About 10 minutes later, the overall signal of the image darkened, indicating that ZIF-8 nanocrystals began to nucleate and grow, and their rapid relaxation characteristics caused signal attenuation. As the reaction proceeds (1–8 hours), the signal in the lower dark region further decreases, especially in the bottom layer, clearly reflecting the continuous formation of ZIF-8 crystals and their gravity sedimentation process. Figure 5 ); After the reaction was completed (24 hours), the image showed a clear two-layer structure: the upper layer was a clear liquid with a strong signal, and the lower layer was a dense ZIF-8 precipitate with a very weak signal.

[0026] By quantitatively analyzing the change in average signal intensity in the underlying region over time, the generation dynamics of ZIF-8 can be obtained.

[0027] Quantitative analysis of relaxation time:

[0028] like Figure 6 As shown, the curves of T2 values ​​changing over time are for three layers.

[0029] Upper layer: The T2 value slowly decreased from approximately 900 ms to 350 ms within 2 hours after the reaction started, and then gradually increased to 1500 ms, indicating that this region was initially affected by Zn. 2+ The introduction of ZIF-8 particles shortens the relaxation time, but the relaxation time subsequently increases as the particles settle. Middle layer: The T2 value dropped rapidly from 900 ms within 1 hour of the start of the reaction and two-phase separation occurred, reflecting the rapid and large-scale formation of ZIF-8 crystals in this region. The increased particle concentration led to a significant acceleration of relaxation. Bottom layer: The T2 value dropped rapidly in the early stage of the reaction and two phases appeared. After about 3 hours, the low T2 phase stabilized at about 30 ms, which quantitatively confirmed the formation and stability of the dense precipitate layer.

[0030] Example 2

[0031] The difference between this embodiment and Embodiment 1 is that, in the parameter adjustment stage, the average T1 value of solution A is measured using an IR sequence, and the T1-weighted imaging parameters are set accordingly, so that solid particles and liquid form a significant signal contrast (the solid has a shorter T1 and a stronger signal).

[0032] The beneficial effects of this invention are as follows: This invention is the first to apply low-field nuclear magnetic resonance imaging (NMR) technology to the monitoring of MOF synthesis processes, achieving undisturbed, in-situ, and visualized tracking of the reaction process. The method uses relatively simple equipment and is less expensive than high-field MRI. It can intuitively provide spatially resolved information about the internal conditions of the reactor, making it particularly suitable for studying the physicochemical processes of MOF nucleation, growth, phase separation, and sedimentation. This is of great significance for guiding the controllable synthesis of MOF materials.

Claims

1. A method of investigating MOF synthesis processes with a low-field nuclear magnetic imaging device, characterized in that, The method comprises the following steps: S1. Preparing metal source solution A and organic ligand solution B respectively; S2. Taking solution A or B, performing shimming operation in a low-field nuclear magnetic imaging device, measuring relaxation time, and optimizing imaging sequence parameters accordingly; S3. Mixing solution A and B, and immediately returning to the low-field nuclear magnetic imaging device; S4. Continuously imaging and measuring layered relaxation time of the reaction process using the optimized imaging sequence; S5. Analyzing spatial distribution characteristics of image signal intensity and dynamic curves of relaxation time of different layers to analyze the synthesis process of MOF.

2. The method of claim 1, wherein, The layered relaxation time measurement in step S4 refers to setting multiple regions of interest along the vertical direction in the reactor, and using CPMG sequence or similar quantitative sequence to calculate and output the transverse relaxation time T2 value of each region in real time.

3. The method of claim 2, wherein, By plotting the T2 value of different layers such as supernatant, middle suspension and bottom precipitate area with time, the reaction process can be quantitatively characterized: the slow rise of the T2 value of the upper layer reflects the settlement of small particles; the sharp decrease of the T2 value of the middle layer indicates the rapid generation and growth of crystals; and the stable T2 value of the bottom layer at a low level indicates the formation of a dense precipitate layer.

4. The method of claim 1, wherein, The solvent in step S1 is methanol, ethanol or water.

5. The method of claim 1, wherein, The metal source in step S1 is zinc nitrate, zinc acetate, copper nitrate or zirconium chloride; and the organic ligand is 2-methylimidazole, trimesic acid or terephthalic acid.

6. The method of claim 1, wherein, The imaging sequence in steps S2 and S4 is CPMG sequence or spin echo sequence or inversion recovery sequence.

7. The method of claim 1, wherein, In step S5, the MOF solid appears as a dark area with signal intensity lower than that of the liquid phase in the T2 weighted image, and the appearance, thickening and signal weakening of the dark area can indicate the nucleation, growth and settlement process of MOF.

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