Alkali-doped moF-74, gas sensitive material, gas sensitive sensor and preparation method and application thereof

Abstract

The application discloses alkali metal doped MOF-74, a gas sensitive material, a gas sensitive sensor and a preparation method and application thereof, and belongs to the technical field of gas sensitive sensors. The application takes magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid as raw materials, in-situ adds alkali metal salt doping, takes triethylamine (TEA) as a deprotonating agent, optimizes a reaction process, rapidly performs synthesis of the alkali metal doped MOF-74 at room temperature, realizes miniaturization of the MOF-74 without losing crystallinity and high porosity, and is favorable for improving the gas sensitive performance of the material. The alkali metal doped MOF-74 is used for the gas sensitive sensor, has excellent selectivity and high sensing response to CO2 gas, and can meet the demand of CO2 gas detection. Moreover, the preparation period is short, heating is not needed, and the method is favorable for large-scale production.

Description

Technical Field

[0001] This application relates to the field of gas sensor technology, and in particular to alkali metal-doped MOF-74, gas-sensitive materials, gas-sensitive sensors, their preparation methods and applications. Background Technology

[0002] Carbon dioxide (CO2) is the most abundant greenhouse gas in the atmosphere, and its concentration has been gradually increasing due to deforestation and the burning of fossil fuels during the industrial age. Low concentrations of CO2 are non-toxic, while high concentrations can irritate the body's respiratory center. High concentrations of CO2 in enclosed spaces can lead to a reduction in breathable oxygen, resulting in respiratory illnesses such as asthma, allergies, and dizziness. When CO2 concentrations are too high, plants wither and their leaves turn yellow. The continued rise in CO2 in the environment will seriously affect human health and plant development and growth, causing significant environmental changes. Therefore, developing real-time CO2 gas detection technologies and sensors is crucial for reducing global greenhouse gas emissions.

[0003] Among them, mass-based gas sensors can operate at room temperature. When the sensitive material adsorbs the target gas, the change in sensor characteristics depends only on the mass of the adsorbed gas, and not on the change in the electrical properties of the sensitive material. The most common type is the quartz crystal microbalance sensor. Quartz crystal microbalance sensors have attracted increasing attention due to their high accuracy and small element size. If a material that can effectively adsorb CO2 is designed and synthesized, then mass-based gas sensors can be used to detect CO2. Metal-organic frameworks (MOFs), as a new type of inorganic-organic hybrid material, have broad application prospects in gas separation and storage, catalysis, drug delivery, and sensing. MOF-74 is a honeycomb material composed of divalent metal ions and 2,5-dihydroxyterephthalic acid. Each metal atom is coordinated with three O atoms of the carboxylic acid and two O atoms of the -OH group. Simultaneously, a sixth atom is occupied by hydrogen peroxide or solvent molecules. Nevertheless, heating or vacuum treatment can easily remove water or solvent molecules, thus providing open unsaturated metal sites. It is considered an excellent material for capturing carbon dioxide. However, the design and preparation of economical MOFs remains a challenge. The most widely used method for MOF preparation is still hydrothermal / solvothermal synthesis. Under solvothermal conditions, the thermally unstable functional groups of the ligands coordinate with metal ions, leading to unwanted solids and inhibiting MOF assembly. Therefore, although many MOFs have been reported, chemically functional MOFs are very limited. Large-scale preparation of MOF materials is a major challenge for the practical application of quartz crystal microbalance CO2 sensors. Summary of the Invention

[0004] Given the long synthesis cycle of MOF-74 materials in existing technologies, and the potential for agglomeration and large particle size in conventional hydrothermal / solvothermal reaction methods, hindering large-scale production, it is necessary to provide alkali metal-doped MOF-74, gas-sensitive materials, gas sensors, their preparation methods, and applications. By introducing triethylamine as a deprotonating agent into the MOF-74 preparation process, simultaneously adding alkali metal salts in situ for doping, and optimizing the reaction process, MOF-74 can be rapidly synthesized at room temperature. This achieves miniaturization of MOF-74 without sacrificing crystallinity and high porosity, thus improving the material's gas-sensing performance. Using alkali metal-doped MOF-74 in gas sensors exhibits excellent selectivity and high sensing response for CO2 gas, meeting the requirements for CO2 gas detection. Furthermore, the preparation cycle is short, requiring no heating, which is beneficial for large-scale production.

[0005] A first aspect of this application provides a method for preparing alkali metal-doped MOF-74, comprising the following steps:

[0006] Magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid were added to a first solvent, followed by the sequential addition of an alkali metal salt and triethylamine. The mixture was stirred and reacted to prepare the alkali metal-doped MOF-74. The alkali metal salt included at least one of lithium nitrate, sodium nitrate, or potassium nitrate.

[0007] In some embodiments, the first solvent is a mixed solution of DMF, ethanol and water; in the mixed solution of DMF, ethanol and water, the volume ratio of DMF, ethanol and water is (12-18):(0.8-1.2):1; optionally, the volume ratio of DMF, ethanol and water is (14-16):1:1.

[0008] In some embodiments, the mass ratio of magnesium nitrate hexahydrate to 2,5-dihydroxyterephthalic acid is (3.5-4.5):1; optionally, the mass ratio of magnesium nitrate hexahydrate to 2,5-dihydroxyterephthalic acid is (4.1-4.2):1.

[0009] In some embodiments, the mass ratio of magnesium nitrate hexahydrate to the alkali metal salt is (15-25):1, and optionally, the mass ratio of magnesium nitrate hexahydrate to the alkali metal salt is (18-20):1.

[0010] In some embodiments, the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate is (80-130) mL:1g; optionally, the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate is (104-112) mL:1g.

[0011] In some embodiments, the stirring conditions include: a reaction temperature of room temperature; a stirring speed of 750-1000 r / min; and a stirring time of 2-3 h.

[0012] In a second aspect of this application, an alkali metal-doped MOF-74 obtained by the aforementioned preparation method is provided.

[0013] A third aspect of this application provides the application of the aforementioned alkali metal-doped MOF-74 in gas-sensitive materials.

[0014] In a fourth aspect, this application provides a gas-sensitive material, including the aforementioned alkali metal-doped MOF-74.

[0015] A fifth aspect of this application provides a gas sensor comprising the aforementioned gas-sensitive material.

[0016] A sixth aspect of this application provides a method for preparing a gas-sensitive sensor, characterized by comprising the following steps: mixing the gas-sensitive material and a second solvent to obtain a solution of the gas-sensitive material; coating the solution of the gas-sensitive material onto the electrode surface of a quartz crystal microbalance sensor, and drying it to prepare the gas-sensitive sensor.

[0017] In some embodiments, the mass-to-volume ratio of the gas-sensitive material to the second solvent is 1 mg:(1-5) mL; the solvent includes water.

[0018] The seventh aspect of this application provides the application of the aforementioned alkali metal-doped MOF-74, the aforementioned gas-sensitive material, or the aforementioned gas-sensitive sensor in carbon dioxide gas detection.

[0019] The technical solution of this application has the following advantages:

[0020] 1. This application uses magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid as raw materials, with in-situ addition of alkali metal salt doping, triethylamine (TEA) as a deprotonating agent, and optimizes the reaction process to synthesize alkali metal-doped MOF-74 through a metal-organic framework assembly reaction at room temperature. This significantly shortens the reaction time and allows the reaction to occur at room temperature without heating, solving the problems of long synthesis cycle and large grain size caused by high-temperature reaction growth in MOF-74. Furthermore, it enables the miniaturization of MOF-74 material without losing its crystallinity and high porosity, which is beneficial for improving the gas-sensing properties of the material.

[0021] 2. The gas sensor prepared using the gas-sensitive material of this application has excellent selectivity and high sensing response to CO2 gas, which can meet the needs of CO2 gas detection. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 This is a scanning electron microscope image of MOF-74 prepared in Comparative Example 1;

[0024] Figure 2 This is the X-ray diffraction pattern of the alkali metal-doped MOF-74 prepared in Example 1;

[0025] Figure 3 These are scanning electron microscope images of alkali metal-doped MOF-74 from Examples 1-3, wherein (a) is the alkali metal-doped MOF-74 obtained by adding lithium nitrate in Example 1; (b) is the alkali metal-doped MOF-74 obtained by adding sodium nitrate in Example 2; and (c) is the alkali metal-doped MOF-74 obtained by adding potassium nitrate in Example 3.

[0026] Figure 4 This is a scanning electron microscope image of MOF-74 prepared by a conventional solvothermal method;

[0027] Figure 5 These are performance test results of the gas sensors prepared in Examples 1-3 and Comparative Example 1, where (a)-(d) are dynamic response curves of the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 at 25°C to 1000ppm carbon dioxide, respectively.

[0028] Figure 6 The graphs show the performance test results of the gas sensors prepared in Examples 1-3 and Comparative Example 1. (a) is a graph showing the response of the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 to 1000 ppm carbon dioxide at 20-65°C as a function of temperature; (b) is a graph showing the selectivity test results of the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 to ethanol, acetone, methanol, nitrogen dioxide, methane, hydrogen, ammonia and carbon dioxide at 20°C.

[0029] Figure 7 The graph shows the performance test results of the gas sensors prepared in Examples 1-3 and Comparative Example 1. In the graph, (a)-(d) are the repeatability test results of the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 for 5 consecutive tests at 20°C on 10000ppm carbon dioxide. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this application clearer and to provide a more thorough and comprehensive understanding of the disclosure, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. The described embodiments are only a part of the embodiments of this application, and not all of them.

[0031] The implementation of this application will be described in detail below with reference to the accompanying drawings. This embodiment is implemented based on the technical solution of this application, and provides detailed implementation methods and specific operation processes, but the protection scope of this application is not limited to the following embodiments.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0033] the term

[0034] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

[0035] In this application, the terms "multiple" or "various" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" or "at least one" means one or more of two.

[0036] In this application, terms such as "optional" and "particular" are used to describe the purpose and indicate differences in content, but should not be construed as limiting the scope of protection of this application.

[0037] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0038] In this application, when numerical intervals (i.e., numerical ranges) are mentioned, unless otherwise specified, the distribution of selectable numerical values ​​within the numerical interval is considered continuous, and includes the two endpoints of the numerical interval (i.e., the minimum and maximum values), as well as every numerical value between these two endpoints. Unless otherwise specified, when a numerical interval refers only to integers within that numerical interval, including the two endpoint integers of the numerical range, as well as every integer between the two endpoints, is equivalent to directly listing every integer. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be merged. In other words, unless otherwise specified, the numerical ranges disclosed herein should be understood to include any and all subranges included therein. The "numerical value" in the numerical interval can be any quantitative value, such as a number, percentage, ratio, etc. The term "numerical interval" can be broadly included to include numerical interval types such as percentage intervals, ratio intervals, and proportion intervals.

[0039] In this application, the term "room temperature" generally refers to 4-35°C, and preferably 20 ± 5°C. In the embodiments of this application, room temperature refers to 20-30°C.

[0040] In this application, unless otherwise specified, the temperature parameters are permitted to be either constant-temperature treatment or variations within a certain temperature range. It should be understood that the constant-temperature treatment allows temperature fluctuations within the precision range of the instrument control. Fluctuations are permitted within ranges such as ±5℃, ±4℃, ±3℃, ±2℃, and ±1℃.

[0041] In this application, if the unit of a data range is only followed by the right endpoint, it indicates that the units of the left and right endpoints are the same. For example, 2-5 h means that the units of the left endpoint "2" and the right endpoint "5" are both h (hours).

[0042] A first aspect of this application provides a method for preparing alkali metal-doped MOF-74, comprising the following steps:

[0043] Magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid were added to a first solvent, followed by the sequential addition of an alkali metal salt and triethylamine. The mixture was stirred and reacted to prepare the alkali metal-doped MOF-74. The alkali metal salt included at least one of lithium nitrate, sodium nitrate, or potassium nitrate.

[0044] It should be noted that this application uses magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid as raw materials, adds alkali metal salt doping in situ, uses triethylamine (TEA) as a deprotonating agent, and optimizes the reaction process to synthesize alkali metal-doped MOF-74 through a metal-organic framework assembly reaction at room temperature. The reaction time is greatly shortened, and the reaction is carried out at room temperature without heating, solving the problems of long synthesis cycle and large grain size caused by high-temperature reaction growth of MOF-74. Furthermore, it enables the miniaturization of MOF-74 material without losing its crystallinity and high porosity, which is beneficial for improving the gas-sensing performance of the material.

[0045] Understandably, triethylamine, as a deprotonating agent, can accelerate the deprotonation of 2,5-dihydroxyterephthalic acid, which is beneficial for the binding of metal ions with deprotonated organic ligands. This allows for the preparation of MOF-74 under milder conditions. At the same time, the shortened reaction cycle and room temperature reaction are also conducive to the preparation of smaller MOF-74, achieving miniaturization without losing crystallinity and high porosity, which is beneficial for improving the gas-sensing performance of the material.

[0046] It is worth noting that the in-situ addition of alkali metal salt doping in this application can effectively improve the gas sensitivity of MOF-74 and enhance its sensing performance for carbon dioxide gas. Compared to preparing MOF-74 first and then doping with alkali metal salt, in-situ addition can increase the doping degree of alkali metal salt and effectively penetrate into the internal structure of MOF-74, thereby improving the overall performance of MOF-74. In particular, the addition of triethylamine in this application results in MOF-74 particles with smaller, nanoscale sizes and greater steric hindrance, making in-situ alkali metal salt doping even more important.

[0047] In some embodiments, the first solvent is a mixed solution of DMF, ethanol and water; wherein the volume ratio of DMF, ethanol and water in the mixed solution is (12-18):(0.8-1.2):1.

[0048] Optionally, the volume ratio of DMF, ethanol and water is (14-16):1:1; specific volume ratios of DMF, ethanol and water can be 12:0.8:1, 12:1:1, 12:1.2:1, 14:1:1, 15:1:1, 16:1:1, 18:0.8:1, 18:1:1, etc.

[0049] In some embodiments, the mass ratio of magnesium nitrate hexahydrate to 2,5-dihydroxyterephthalic acid is (3.5-4.5):1; optionally, the mass ratio of magnesium nitrate hexahydrate to 2,5-dihydroxyterephthalic acid is (4.1-4.2):1. Specifically, the mass ratio of magnesium nitrate hexahydrate to 2,5-dihydroxyterephthalic acid can be 3.5:1, 4:1, 4.1:1, 4.17:1, 4.2:1, 4.3:1, 4.4:1, 4.5:1, etc.

[0050] In some embodiments, the mass ratio of magnesium nitrate hexahydrate to the alkali metal salt is (15-25):1. Optionally, the mass ratio of magnesium nitrate hexahydrate to the alkali metal salt is (18-20):1. Specifically, the mass ratio of magnesium nitrate hexahydrate to the alkali metal salt can be 15:1, 16:1, 17:1, 18:1, 19:1, 20:1, 21:1, 22:1, 23:1, 24:1, 25:1, etc.

[0051] In some embodiments, the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate is (80-130) mL:1g; optionally, the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate is (104-112) mL:1g. Specifically, the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate can be 80 mL:1g, 90 mL:1g, 100 mL:1g, 104 mL:1g, 105 mL:1g, 106 mL:1g, 107 mL:1g, 108 mL:1g, 112 mL:1g, 120 mL:1g, 130 mL:1g, etc.

[0052] In some embodiments, the conditions for the stirring reaction include: a reaction temperature of room temperature; a stirring speed of 750-1000 r / min; and a stirring time of 2-3 h.

[0053] Understandably, room temperature generally refers to 4-35°C, preferably 20 ± 5°C. In the embodiments of this application, room temperature refers to 20-30°C, preferably 20-25°C. Furthermore, a faster stirring speed is beneficial for improving the mixing and dispersion effect, accelerating the deprotonation of 2,5-dihydroxyterephthalic acid, facilitating the binding of metal ions to deprotonated organic ligands, and increasing the doping degree of alkali metal salts. In the embodiments of this application, the stirring speed is 750-1000 r / min, preferably 800-850 r / min, specifically 750 r / min, 800 r / min, 850 r / min, 900 r / min, 950 r / min, 1000 r / min, etc.

[0054] Understandably, alkali metal salts can be added in solid form or first prepared as a solution before addition. For example, the alkali metal salt can be dissolved in DMF to prepare a solution before addition, and the mass-to-volume ratio of the alkali metal salt to DMF can be (0.01-0.05) g: 10 mL.

[0055] Understandably, adding all of the triethylamine to the reaction system relatively quickly is beneficial to improving reaction efficiency, for example, adding it within 10 seconds, specifically within 2-3 seconds.

[0056] Understandably, in some of these embodiments, the reaction time can be reduced from the conventional 20 hours to 2 hours.

[0057] Understandably, the reaction may be followed by solid-liquid separation, washing, and drying steps.

[0058] Understandably, there are no special limitations on the conditions for solid-liquid separation, drying, and washing described above. Common solid-liquid separation methods (centrifugation, filtration, etc.), common drying methods (such as heating drying, vacuum drying, freeze drying, natural drying, etc.), and common washing methods can be used.

[0059] For example, washing conditions may include: washing with DMF and methanol, centrifuging 2-4 times (3 times); drying may include vacuum drying; vacuum drying conditions may include: heating to 60-80°C at a rate of 5°C / min, holding at that temperature for 10-12 hours, and then cooling naturally; furthermore, before heating and drying, the step of evacuating the oven to a vacuum level of 0.1-0.3 MPa may be included.

[0060] In a second aspect of this application, an alkali metal-doped MOF-74 obtained by the aforementioned preparation method is provided.

[0061] A third aspect of this application provides the application of the aforementioned alkali metal-doped MOF-74 in gas-sensitive materials.

[0062] In a fourth aspect, this application provides a gas-sensitive material, including the aforementioned alkali metal-doped MOF-74.

[0063] A fifth aspect of this application provides a gas sensor comprising the aforementioned gas-sensitive material.

[0064] Understandably, the gas sensor prepared from the gas-sensitive material of this application has excellent selectivity and high sensing response to CO2 gas, which can meet the needs of CO2 gas detection.

[0065] A sixth aspect of this application provides a method for preparing a gas-sensitive sensor, characterized by comprising the following steps: mixing the gas-sensitive material and a second solvent to obtain a solution of the gas-sensitive material; coating the solution of the gas-sensitive material onto the electrode surface of a quartz crystal microbalance sensor, and drying it to prepare the gas-sensitive sensor.

[0066] Understandably, common coating methods can be used to coat the gas-sensitive material solution onto the electrode surface of the quartz crystal microbalance sensor, such as drop coating, spin coating, and spray coating. In the drop coating step, a pipette with a volume of 0.1-2.5 μl can be used; drying can be achieved by baking or natural drying, with a drying temperature of 60-80℃. The coating amount of the gas-sensitive material solution is determined based on achieving the required gas-sensitive performance and is not specifically limited.

[0067] In some embodiments, the mass-to-volume ratio of the gas-sensitive material to the second solvent is 1 mg:(1-5) mL; the solvent includes water.

[0068] The seventh aspect of this application provides the application of the aforementioned alkali metal-doped MOF-74, the aforementioned gas-sensitive material, or the aforementioned gas-sensitive sensor in carbon dioxide gas detection.

[0069] Sources of reagents and instruments

[0070] Quartz crystal microbalance: purchased from Beijing Chenjing Electronics Co., Ltd., with a fundamental frequency of 10 MHz, composed of AT-cut quartz crystals (8 mm in diameter) and covered with silver electrodes (5 mm in diameter) on both sides.

[0071] 2,5-Dihydroxyterephthalic acid (molecular formula: C8H6O6) (purity ≥98.0%): purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0072] Magnesium nitrate (molecular formula: Mg(NO3)2·6H2O) (purity ≥98.0%): purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0073] Anhydrous ethanol: purchased from Chengdu Kelong Chemical Reagent Factory;

[0074] DMF: Purchased from Chengdu Kelong Chemical Reagent Factory;

[0075] Triethylamine: Purchased from Chengdu Kelong Chemical Reagent Factory;

[0076] Carbon dioxide detector: purchased from Mansfield, UK;

[0077] Frequency counter: Purchased from Keysight Technologies, Inc., USA;

[0078] Electric thermostatic drying oven: purchased from Kangheng Instruments Co., Ltd.;

[0079] Pipettes: Purchased from Shanghai Guangzheng Medical Instruments Co., Ltd.;

[0080] Where specific experimental steps or conditions are not specified in the embodiments, they can be performed according to the conventional experimental steps or conditions described in the literature in this field. All raw materials or instruments used are commercially available conventional products, including but not limited to those used in the embodiments of this application.

[0081] The following are specific examples.

[0082] Example 1

[0083] This embodiment provides a method for preparing a gas sensor.

[0084] (1) The preparation method of alkali metal-doped MOF-74 is as follows:

[0085] 0.168 g of 2,5-dihydroxyterephthalic acid and 0.7 g of Mg(NO3)2·6H2O were added to 75 mL of a mixed solvent of DMF / ethanol / water (the volume ratio of DMF, ethanol and water was 15:1:1) to obtain a mixed solution.

[0086] Dissolve 0.037 g of lithium nitrate in 10 mL of DMF and add it to the mixed solution. Then quickly add 0.5 mL of triethylamine (within 3-5 seconds) while stirring vigorously (800-850 r / min). Continue stirring at room temperature for 2 hours.

[0087] After the reaction was complete, the precipitate was collected by centrifugation and redispersed in fresh DMF. After filtration, yellow crystals were obtained. The crystals were then washed three times with DMF and methanol, filtered, and immersed in methanol, with the methanol being replaced daily for three consecutive days. After filtration, the crystals were dried under vacuum at 80 °C for 6 hours to obtain alkali metal-doped MOF-74 (alkali metal Li-doped MOF-74).

[0088] (2) Preparation of gas sensor: 1 mg of alkali metal doped MOF-74 prepared was used as gas sensing material and dispersed in 1 mL of deionized water to obtain a dispersion of 1 mg / mL. Then, the dispersion was coated on the electrode surface of a quartz crystal microbalance (electrode diameter is 5 mm, drop volume is 2.5 μl) using a pipette. The coated sensor was placed in a constant temperature drying oven at 60℃ and dried for 2 h to form a sensing layer, thus obtaining a gas sensor.

[0089] The X-ray diffraction pattern of the alkali metal-doped MOF-74 prepared in Example 1 is shown below. Figure 2 As shown, the scanning electron microscope image is as follows: Figure 3 As shown.

[0090] Example 2

[0091] This embodiment provides a method for preparing a gas sensor, which differs from Embodiment 1 only in that lithium nitrate is replaced with sodium nitrate to obtain alkali metal-doped MOF-74 (alkali metal Na-doped MOF-74), and this alkali metal-doped MOF-74 is used as the gas-sensitive material to construct a gas sensor according to the method of Embodiment 1.

[0092] Example 3

[0093] This embodiment provides a method for preparing a gas sensor, which differs from Embodiment 1 only in that lithium nitrate is replaced with potassium nitrate to obtain alkali metal-doped MOF-74 (alkali metal K-doped MOF-74), and this alkali metal-doped MOF-74 is used as the gas-sensitive material to construct a gas sensor according to the method of Embodiment 1.

[0094] Comparative Example 1

[0095] This comparative example provides a method for fabricating a gas sensor, the steps of which are as follows:

[0096] (1) Preparation of MOF-74 material: 0.168 g of 2,5-dihydroxyterephthalic acid and 0.7 g of Mg(NO3)2·6H2O were added to 75 mL of a mixed solvent of DMF, ethanol and water (the volume ratio of DMF, ethanol and water was 15:1:1) to obtain a mixed solution.

[0097] Add 0.5 mL of triethylamine to the mixed solution quickly (within 3-5 seconds) while stirring vigorously (800-850 r / min), and then continue stirring at room temperature for 2 hours.

[0098] After the reaction was complete, the precipitate was collected by centrifugation and redispersed in fresh DMF. After filtration, yellow crystals were obtained. The crystals were then washed three times with DMF and methanol, filtered, and then immersed in methanol, with the methanol changed daily for three consecutive days. After filtration, the crystals were dried under vacuum at 80 °C for 6 hours to obtain MOF-74.

[0099] (2) Preparation of gas sensor: Refer to step (2) of Example 1.

[0100] The MOF-74 scanning electron microscope image obtained in step (1) of this comparative example is shown below. Figure 1 As shown.

[0101] In addition, MOF-74 was prepared by a conventional solvothermal method. The specific process was as follows: 2,5-dihydroxyterephthalic acid (0.111 g) and magnesium nitrate hexahydrate (0.475 g) were dissolved in 75 mL of a mixed solvent of DMF, ethanol and water (the volume ratio of DMF, ethanol and water was 15:1:1). The solution was then placed in an oven at 125 °C for 20 hours. Before gas testing, the sample was dried under vacuum at 80 °C for 12 hours.

[0102] Scanning electron microscope image of MOF-74 prepared by conventional solvothermal method is shown below. Figure 4 As shown.

[0103] It can be seen that the alkali metal-doped MOF-74 prepared in Example 1 has good crystallinity and is smaller in size than the MOF-74 prepared by Comparative Example 1 and conventional solvothermal method. It can be miniaturized without losing crystallinity and high porosity.

[0104] Test Example 1: Response of a gas sensor to carbon dioxide gas

[0105] The gas-sensing performance of the gas sensors in Examples 1-3 and Comparative Example 1 was tested at room temperature (20°C) using a carbon dioxide detector and a frequency counter. First, the gas sensor was exposed to a nitrogen atmosphere for 10-15 minutes. Then, a specific concentration of the gas to be tested was injected into the chamber through a dynamic gas mixing system. After the gas reacted fully with the sensor, the device was exposed to a nitrogen atmosphere again, thereby obtaining the gas-sensing response curve.

[0106] like Figure 5 As shown, the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 were tested at 20°C for 1000 ppm carbon dioxide. The response values ​​of the gas sensors in Examples 1-3 were greater than those in Comparative Example 1. In particular, the response value of the gas sensor in Example 1 reached 95, indicating that the gas sensor prepared in this application can detect CO2 gas very sensitively at room temperature and has excellent response characteristics. Furthermore, the response characteristics to CO2 gas were effectively improved after doping with alkali metal salts.

[0107] Test Example 2: Response of Gas Sensor at Different Temperatures

[0108] The gas-sensing performance of the gas sensors in Examples 1-3 and Comparative Example 1 was tested at 20-65°C using a carbon dioxide detector and a frequency counter. First, the gas sensor was exposed to a nitrogen atmosphere for 10-15 minutes. Then, a specific concentration of the gas to be tested was injected into the chamber through a dynamic gas mixing system. After the gas reacted fully with the sensor, the device was exposed to a nitrogen atmosphere again to obtain the gas-sensing response.

[0109] like Figure 6As shown in (a), the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 were tested at 1000 ppm carbon dioxide at 20-65℃. It can be seen that the response values ​​of the gas sensors in Examples 1, 2, 3 and Comparative Example 1 decrease with increasing temperature, showing a negative Arrhenius temperature dependence, which proves that the optimal operating temperature of the above gas sensors is 20℃.

[0110] Test Example 3: Selectivity of Gas Sensor for Carbon Dioxide Gas

[0111] Consistent with the conditions of Test Example 1, eight interfering gases, including carbon dioxide, were tested at the optimal operating temperature of 20°C.

[0112] like Figure 6 As shown in (b), the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 showed significantly higher responses to carbon dioxide gas than the other seven interfering gases, indicating that the gas sensors prepared in this application exhibit outstanding selectivity for carbon dioxide gas.

[0113] Test Example 4: Repeatability of the Gas Sensor's Response to Carbon Dioxide Gas

[0114] Consistent with the conditions of Test Example 1, the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 were subjected to five cycles of testing in a 10,000 ppm carbon dioxide atmosphere at 20°C.

[0115] like Figure 7 As shown in the figure, the gas sensor prepared in this application has good repeatability and can achieve repeated testing multiple times, which has good practical application value.

[0116] Based on the above performance test results, the optimal operating temperature of the gas sensors prepared in Examples 1-3 of this application is 20°C, and they exhibit good response to carbon dioxide gas, along with excellent selectivity and good repeatability. In particular, Example 1 achieves a response of 95% to 1000 ppm carbon dioxide gas at the optimal operating temperature. Comparing Comparative Example 1 and Example 1, it can be seen that although the preparation method of MOF-74 without alkali metal doping in Comparative Example 1 is simpler, its sensing performance for carbon dioxide gas is not as good as that of the gas sensor prepared in Example 1. Therefore, this application provides a simple preparation method for a carbon dioxide gas sensor with a short synthesis cycle, and this gas sensor has superior sensing performance for carbon dioxide gas.

[0117] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0118] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for preparing alkali metal-doped MOF-74, characterized in that, Includes the following steps: Magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid were added to a first solvent, followed by the sequential addition of an alkali metal salt and triethylamine. The mixture was stirred and reacted at room temperature to prepare the alkali metal-doped MOF-74. The alkali metal salt included at least one of lithium nitrate, sodium nitrate, or potassium nitrate. The mass ratio of the magnesium nitrate hexahydrate to the alkali metal salt is (15-25):

1.

2. The preparation method according to claim 1, characterized in that, The first solvent is a mixed solution of DMF, ethanol and water; And / or in the first solvent, the volume ratio of DMF, ethanol and water is (12-18):(0.8-1.2):1; And / or the mass ratio of the magnesium nitrate hexahydrate to the 2,5-dihydroxyterephthalic acid is (3.5-4.5):1; And / or the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate is (80-130) mL:1g; And / or the conditions for the stirring reaction include: stirring speed of 750-1000 r / min; stirring time of 2-3 h.

3. The preparation method according to claim 2, characterized in that, The volume ratio of DMF, ethanol and water is (14-16):1:1; And / or the mass ratio of the magnesium nitrate hexahydrate to the 2,5-dihydroxyterephthalic acid is (4.1-4.2):1; And / or the mass ratio of the magnesium nitrate hexahydrate to the alkali metal salt is (18-20):1; And / or the volume-to-mass ratio of the first solvent to the magnesium nitrate hexahydrate is (104-112) mL:1g.

4. Alkali metal-doped MOF-74 prepared by the preparation method according to any one of claims 1-3.

5. The application of alkali metal-doped MOF-74 prepared by the preparation method according to any one of claims 1-3 or the alkali metal-doped MOF-74 according to claim 4 in gas-sensitive materials.

6. A gas-sensitive material, characterized in that, This includes alkali metal-doped MOF-74 prepared by the preparation method according to any one of claims 1-3 or alkali metal-doped MOF-74 according to claim 4.

7. A gas-sensitive sensor, characterized in that, Includes the gas-sensitive material as described in claim 6.

8. The method for preparing the gas sensor according to claim 7, characterized in that, Includes the following steps: The gas-sensitive material and the second solvent are mixed to obtain a solution of the gas-sensitive material; The gas-sensitive material solution was coated onto the electrode surface of the quartz crystal microbalance sensor and dried to prepare the gas-sensitive sensor.

9. The preparation method according to claim 8, characterized in that, The mass-to-volume ratio of the gas-sensitive material to the second solvent is 1 mg:(1-5) mL; the second solvent includes water.

10. The application of the alkali metal-doped MOF-74 obtained by the preparation method according to any one of claims 1-3, the alkali metal-doped MOF-74 according to claim 4, the gas-sensitive material according to claim 6, the gas-sensitive sensor according to claim 7, or the gas-sensitive sensor obtained by the preparation method according to claim 8 or 9 in carbon dioxide gas detection.

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