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

Abstract

The invention discloses alkali metal doped MOF-74, a gas-sensitive material, a gas-sensitive sensor and a preparation method and application of the gas-sensitive sensor, and belongs to the technical field of gas-sensitive sensors. Magnesium nitrate hexahydrate and 2, 5-dihydroxyterephthalic acid are used as raw materials, alkali metal salt is added in situ for doping, triethylamine (TEA) is used as a deprotonation agent, a reaction process is optimized, alkali metal doped MOF-74 is rapidly synthesized at room temperature, miniaturization of MOF-74 is realized on the premise that crystallinity and high porosity are not lost, and the preparation method is simple and easy to implement. The gas-sensitive property of the material is favorably improved. 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 requirements of CO2 gas detection. The preparation period is short, heating is not needed, and large-scale production is facilitated.

Description

Technical Field

[0001] The present application relates to the technical field of gas sensors, and in particular to alkali metal-doped MOF-74, gas-sensitive materials, gas sensors, and preparation methods and applications thereof. Background Art

[0002] Carbon dioxide (CO2) is the most abundant greenhouse gas in the atmosphere, and its concentration has gradually increased due to deforestation and the burning of fossil fuels during the industrial era. Low concentrations of CO2 are non-toxic, while high concentrations can stimulate the body's respiratory center. High concentrations of CO2 in confined spaces can lead to a decrease in breathable oxygen, which can cause respiratory diseases such as asthma, allergies, and dizziness. When CO2 concentrations are too high, plants will wilt and their leaves will turn yellow. The continued rise in CO2 in the environment will seriously affect human health and the development and growth of plants, causing significant changes to the environment. Therefore, the development of real-time CO2 gas detection technology and sensors is crucial to reducing global greenhouse gas emissions.

[0003] Among them, mass-based gas sensors are able to work at room temperature. When the sensitive material adsorbs the target gas, the change in the sensor characteristics is only related to the mass of the adsorbed gas, and does not depend on the change in the electrical properties of the sensitive material. The most common is the quartz crystal microbalance sensor. Quartz crystal microbalance sensors have attracted more and more attention due to their high precision and small component size. If a class of materials 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 materials, have broad application prospects in gas separation and storage, catalysis, drug delivery, and sensing. MOF-74 is a honeycomb material composed of a combination of divalent metal ions and 2,5-dihydroxyterephthalic acid. Each metal atom coordinates with three O atoms of the carboxylic acid and two O atoms of the -OH group. At the same time, the sixth atom is occupied by hydrogen peroxide or solvent molecules. Nevertheless, heating or vacuum treatment can easily remove water molecules or solvent molecules, resulting in open unsaturated metal sites. It is considered to be an excellent material for capturing carbon dioxide. However, how to design and prepare economical MOFs remains a problem. The most widely used method for preparing MOFs is still hydrothermal / solvothermal synthesis. Under solvothermal conditions, the thermally unstable functional groups of the ligands will coordinate metal ions, resulting in unwanted solids and inhibiting MOF assembly. Therefore, although many MOFs have been reported, MOFs with chemical functions are very limited. The large-scale preparation of MOF materials is the main challenge for the practical application of quartz crystal microbalance CO2 sensors. Summary of the invention

[0004] Based on the fact that the MOF-74 material synthesis cycle in the prior art is long, and the conventional hydrothermal / solvothermal reaction method may cause MOF-74 material agglomeration, large particles, and difficulty in large-scale production, it is necessary to provide alkali metal-doped MOF-74, gas-sensitive materials, gas-sensitive sensors, and preparation methods and applications thereof. By introducing triethylamine as a deprotonating agent in the preparation process of MOF-74, adding alkali metal salt doping in situ, and optimizing the reaction process, MOF-74 can be quickly synthesized at room temperature, and the miniaturization of MOF-74 is achieved without losing crystallinity and high porosity, which is beneficial to improving the gas-sensitive properties of the material. The alkali metal-doped MOF-74 is used in gas sensors, which have excellent selectivity and high sensing response to CO2 gas, and can meet the needs of CO2 gas detection. In addition, the preparation cycle is short, no heating is required, and it is conducive to large-scale production.

[0005] In a first aspect of the present application, a method for preparing alkali metal-doped MOF-74 is provided, comprising the following steps:

[0006] Magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid are added to a first solvent, and an alkali metal salt and triethylamine are added in sequence, and the mixture is stirred for reaction to prepare the alkali metal-doped MOF-74; the alkali metal salt includes 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 the magnesium nitrate hexahydrate to the 2,5-dihydroxyterephthalic acid is (3.5-4.5):1; optionally, the mass ratio of the magnesium nitrate hexahydrate to the 2,5-dihydroxyterephthalic acid is (4.1-4.2):1.

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

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

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

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

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

[0014] In a fourth aspect of the present application, a gas-sensitive material is provided, comprising the aforementioned alkali metal-doped MOF-74.

[0015] In a fifth aspect of the present application, a gas sensor is provided, comprising the aforementioned gas-sensitive material.

[0016] The sixth aspect of the present application provides a method for preparing a gas sensor, which is characterized in that it includes 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 on the electrode surface of a quartz crystal microbalance sensor, and drying to prepare the gas sensor.

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

[0018] The seventh aspect of the present 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, 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 metal-organic framework assembly reaction at room temperature. The reaction time can be greatly shortened, and the reaction is carried out at room temperature without heating, which solves the problems of long MOF-74 synthesis cycle and large grain growth due to high-temperature reaction, and realizes the miniaturization of MOF-74 materials without losing crystallinity and high porosity, which is conducive to improving the gas-sensing performance of the material.

[0021] 2. The gas sensor prepared using the gas-sensitive material of the present application has excellent selectivity and high sensing response to CO2 gas, and can meet the needs of CO2 gas detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

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

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

[0025] Figure 3 are scanning electron microscope images of alkali metal-doped MOF-74 of 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; (c) is the alkali metal-doped MOF-74 obtained by adding potassium nitrate in Example 3;

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

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

[0028] Figure 6 1 is a performance test result diagram of the gas sensors prepared in Examples 1-3 and Comparative Example 1, wherein (a) is a curve diagram 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 selectivity test result 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 1 is a performance test result diagram of the gas sensors prepared in Examples 1-3 and Comparative Example 1, wherein (a)-(d) are respectively the repeatability test results of the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1, which were tested 5 times continuously for 10000 ppm carbon dioxide at 20°C. DETAILED DESCRIPTION

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

[0031] The implementation of the present application is described in detail below in conjunction with the accompanying drawings. This embodiment is implemented based on the technical solution of the present application, and a detailed implementation method and specific operation process are given, but the protection scope of the present application is not limited to the following embodiments.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0033] the term

[0034] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0035] In the present application, "plurality", "multiple" and the like, unless otherwise specified, refer to a number greater than or equal to 2. For example, "one or more" or "at least one" means one or greater than or equal to two.

[0036] In this application, "optional", "particularly" and the like are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this application.

[0037] In the present application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0038] In the present application, when it comes to a numerical interval (i.e., a numerical range), unless otherwise specified, the distribution of the optional numerical values ​​in the numerical interval is considered to be continuous, and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical interval, and each numerical value between the two numerical endpoints. Unless otherwise specified, when the numerical interval only refers to the integers in the numerical interval, it includes the two endpoint integers of the numerical range, and each integer between the two endpoints, which is equivalent to directly listing each integer. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined. In other words, unless otherwise specified, the numerical range disclosed herein should be understood to include any and all sub-ranges included therein. The "numerical value" in the numerical interval can be any quantitative value, such as a number, a percentage, a ratio, etc. "Numerical interval" allows broadly including numerical interval types such as percentage intervals, ratio intervals, and ratio intervals.

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

[0040] In this application, unless otherwise specified, the temperature parameter is allowed to be either a constant temperature treatment or to vary within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument. Fluctuations within the range of ±5°C, ±4°C, ±3°C, ±2°C and ±1°C are allowed.

[0041] In this application, when referring to the unit of a data range, if there is a unit only after the right endpoint, it means 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] In a first aspect of the present application, a method for preparing alkali metal-doped MOF-74 is provided, comprising the following steps:

[0043] Magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid are added to a first solvent, and an alkali metal salt and triethylamine are added in sequence, and the mixture is stirred for reaction to prepare the alkali metal-doped MOF-74; the alkali metal salt includes 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 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, which solves the problems of long MOF-74 synthesis cycle and large grain growth due to high temperature reaction, and realizes the miniaturization of MOF-74 materials without losing crystallinity and high porosity, which is beneficial to improving the gas-sensing properties of the materials.

[0045] It is understandable that the addition of triethylamine as a deprotonating agent can accelerate the deprotonation of 2,5-dihydroxyterephthalic acid, which is beneficial to the combination of metal ions and deprotonated organic ligands, and the preparation of MOF-74 under milder conditions. At the same time, the shortening of the reaction cycle and the reaction at room temperature are also conducive to the preparation of smaller MOF-74, achieving miniaturization without losing crystallinity and high porosity, which is beneficial to improving the gas-sensing properties of the material.

[0046] It should be noted that the in-situ addition of alkali metal salt doping in this application can effectively improve the gas sensitivity of MOF-74 and improve the sensing performance of carbon dioxide gas. Compared with preparing MOF-74 first and then doping with alkali metal salts, in-situ addition can increase the doping degree of alkali metal salts, and can effectively enter the internal structure of MOF-74, thereby improving the comprehensive performance of MOF-74. In particular, the addition of triethylamine in this application prepares a smaller particle size of MOF-74, which is nanoscale and has greater steric hindrance, so in-situ addition of alkali metal salt doping is more important.

[0047] 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.

[0048] Optionally, the volume ratio of DMF, ethanol and water is (14-16):1:1; the specific volume ratio 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 the magnesium nitrate hexahydrate to the 2,5-dihydroxyterephthalic acid is (3.5-4.5):1; optionally, the mass ratio of the magnesium nitrate hexahydrate to the 2,5-dihydroxyterephthalic acid is (4.1-4.2):1. Specifically, the mass ratio of the magnesium nitrate hexahydrate to the 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 the magnesium nitrate hexahydrate to the alkali metal salt is (15-25):1. Optionally, the mass ratio of the magnesium nitrate hexahydrate to the alkali metal salt is (18-20):1. Specifically, the mass ratio of the magnesium nitrate hexahydrate to the alkali metal salt may 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 mass ratio of the first solvent to the magnesium nitrate hexahydrate is (80-130) mL:1 g; optionally, the volume mass ratio of the first solvent to the magnesium nitrate hexahydrate is (104-112) mL:1 g. Specifically, the volume mass ratio of the first solvent to the magnesium nitrate hexahydrate can be 80 mL:1 g, 90 mL:1 g, 100 mL:1 g, 104 mL:1 g, 105 mL:1 g, 106 mL:1 g, 107 mL:1 g, 108 mL:1 g, 112 mL:1 g, 120 mL:1 g, 130 mL:1 g, etc.

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

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

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

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

[0056] It is understandable that in some of the embodiments, the reaction time can be shortened from the traditional 20 hours to 2 hours.

[0057] It is understandable that the stirring reaction may further include solid-liquid separation, washing and drying steps.

[0058] It is understandable that the conditions for the above-mentioned solid-liquid separation, drying and washing are not particularly limited, and 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, the washing conditions may include: washing with DMF and methanol, and centrifuging 2-4 times (may be 3 times); drying may include vacuum drying; the vacuum drying conditions may be: heating to 60-80°C at a rate of 5°C / min, keeping warm for 10-12h, and cooling naturally; further, before heating and drying, the step of evacuating the vacuum degree of the oven to 0.1-0.3Mpa may be included.

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

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

[0062] In a fourth aspect of the present application, a gas-sensitive material is provided, comprising the aforementioned alkali metal-doped MOF-74.

[0063] In a fifth aspect of the present application, a gas sensor is provided, comprising the aforementioned gas-sensitive material.

[0064] It is understandable that the gas sensor made of the gas-sensitive material of the present application has excellent selectivity and high sensing response to CO2 gas, and can meet the needs of CO2 gas detection.

[0065] The sixth aspect of the present application provides a method for preparing a gas sensor, which is characterized in that it includes 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 on the electrode surface of a quartz crystal microbalance sensor, and drying to prepare the gas sensor.

[0066] It is understandable that the solution of the gas-sensitive material can be coated on the electrode surface of the quartz crystal microbalance sensor by common coating methods, such as drip coating, spin coating, spray coating, etc. Among them, a 0.1-2.5 μl pipette can be used in the drip coating step; drying can be done by drying, natural drying, etc., and the drying temperature can be 60-80°C. The coating amount of the solution of the gas-sensitive material is based on the requirements for achieving gas-sensitive performance and is not particularly limited.

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

[0068] In a seventh aspect of the present application, 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 is provided.

[0069] Sources of reagents and instruments

[0070] Quartz crystal microbalance: purchased from Beijing Chenjing Electronics Co., Ltd., with a base frequency of 10 MHz, composed of AT-cut quartz crystal (8 mm in diameter) 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 Company, UK;

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

[0078] Electric constant temperature drying oven: purchased from Kangheng Instrument Co., Ltd.

[0079] Pipette: purchased from Shanghai Guangzheng Medical Instrument Co., Ltd.

[0080] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the art can be used. The raw materials or instruments used are all conventional products that can be obtained commercially, including but not limited to the raw materials or instruments used in the examples of this application.

[0081] The following are specific embodiments.

[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 comprises the following steps:

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

[0087] After the reaction is completed, the precipitate is collected by centrifugation and redispersed in fresh DMF, and yellow crystals are obtained after filtration. Then, it is washed with DMF and methanol three times, filtered and immersed in methanol, and the methanol is replaced once a day for 3 consecutive days. After filtration, it is vacuum dried at 80 ° C for 6 hours to obtain alkali metal doped MOF-74 (alkali metal Li doped MOF-74).

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

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

[0090] Example 2

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

[0092] Example 3

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

[0094] Comparative Example 1

[0095] This comparative example provides a method for preparing a gas sensor, the steps 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] Quickly add 0.5 mL of triethylamine to the mixed solution (add within 3-5 s) while stirring vigorously (speed 800-850 r / min), and then continue stirring at room temperature for 2 hours.

[0098] After the reaction is completed, the precipitate is collected by centrifugation and redispersed in fresh DMF, and yellow crystals are obtained after filtration. Then, it is washed with DMF and methanol three times, filtered and immersed in methanol, and the methanol is changed once a day for 3 consecutive days. After filtration, it is vacuum dried 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 scanning electron microscopy image of MOF-74 prepared in step (1) of this comparative example is as follows: Figure 1 shown.

[0101] In addition, MOF-74 was prepared by conventional solvothermal method. The specific process is 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), and then placed in an oven at 125 °C for 20 hours. Before gas testing, the sample was heated and dried under vacuum conditions at 80 °C for 12 hours.

[0102] The scanning electron microscope image of MOF-74 prepared by conventional solvothermal method is as follows Figure 4 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 in Comparative Example 1 and the conventional solvothermal method, and can be miniaturized without losing crystallinity and high porosity.

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

[0105] The gas-sensing performance of the gas sensors of Examples 1-3 and Comparative Example 1 was tested at room temperature of 20°C using a carbon dioxide detector and a frequency counter. The gas sensors were first exposed to a nitrogen atmosphere for 10-15 minutes, and then a specific concentration of the test gas was injected into the chamber through a dynamic gas distribution system. After the gas fully reacted with the sensor, the device was exposed to a nitrogen atmosphere to obtain a gas-sensitive response curve.

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

[0107] Test Example 2 Response of gas sensor at different temperatures

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

[0109] like Figure 6As shown in (a), the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 were tested with 1000ppm carbon dioxide at 20-65°C. 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°C.

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

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

[0112] like Figure 6 As shown in (b), the responses of the gas sensors prepared in Examples 1, 2, 3 and Comparative Example 1 to carbon dioxide gas are significantly higher than those of the other seven interfering gases, indicating that the gas sensors prepared in the present application exhibit outstanding selectivity to 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 cycle tests in a 10000 ppm carbon dioxide atmosphere at 20°C.

[0115] like Figure 7 As shown, it can be seen that the gas sensor prepared in the present application has good repeatability, can realize continuous multiple repeatability tests, and has good practical application value.

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

[0117] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described 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 above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A method for preparing alkali metal-doped MOF-74, characterized in that: The steps include: Magnesium nitrate hexahydrate and 2,5-dihydroxyterephthalic acid are added to a first solvent, and an alkali metal salt and triethylamine are added in sequence, and the mixture is stirred for reaction to prepare the alkali metal-doped MOF-74; the alkali metal salt includes at least one of lithium nitrate, sodium nitrate or potassium nitrate.

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 mass ratio of the magnesium nitrate hexahydrate to the alkali metal salt is (15-25):1; and / or the volume mass ratio of the first solvent to the magnesium nitrate hexahydrate is (80-130) mL:1 g; And / or the stirring reaction conditions include: reaction temperature is room temperature; stirring speed is 750-1000 r / min; stirring time is 2-3h.

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 mass ratio of the first solvent to the magnesium nitrate hexahydrate is (104-112) mL:1 g.

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

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

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

7. A gas sensor, characterized in that: Comprising the gas-sensitive material as claimed in claim 6.

8. The method for preparing the gas sensor according to claim 7, characterized in that: The following steps are involved: Mixing the gas-sensitive material with a second solvent to obtain a solution of the gas-sensitive material; The gas sensor is prepared by coating the solution of the gas-sensitive material on the electrode surface of a quartz crystal microbalance sensor and drying the solution.

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

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

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