Alkali metal modified zeolite adsorbent material, method for preparing the same, and use thereof

Abstract

The application discloses an alkali metal modified zeolite adsorption material and a preparation method and application thereof, and comprises the following steps: (1) a precursor solution is added drop by drop into pretreated zeolite, and it is ensured that the precursor solution is completely adsorbed by the zeolite, then the impregnated zeolite is obtained by sealing and standing; (2) the impregnated zeolite is dried and calcined, then H2 is introduced to carry out activation treatment, and the alkali metal modified zeolite is obtained; (3) the alkali metal modified zeolite is subjected to hydrothermal aging treatment, and the hydrothermal stable alkali metal modified zeolite is obtained. The material can adapt to a series connection working mode of a carbon hydrogen trap and a three-effect catalytic conversion device, the preparation process is simple and controllable, the cost is low, and the material is suitable for large-scale industrial production and automobile cold start HCs emission control.

Description

Technical Field

[0001] This invention belongs to the field of automotive exhaust purification materials technology, and particularly relates to an alkali metal modified zeolite, its preparation method and application. Background Technology

[0002] With increasingly stringent global environmental regulations, controlling hydrocarbon (HC) emissions from vehicle exhaust has become a key research direction for the internal combustion engine industry. During engine cold starts, the combustion chamber temperature is low (typically <300℃), resulting in poor fuel atomization and incomplete combustion of the air-fuel mixture. Simultaneously, the three-way catalytic converter (TWC), a core component of exhaust aftertreatment, requires an ignition temperature above 300℃ to achieve catalytic activity, leading to the direct emission of large amounts of unpurified HCs during cold starts. Data shows that HC emissions during cold starts account for 60%-80% of total vehicle HC emissions, with key components including acetylene, acetaldehyde, and 1-butene, with 1-butene accounting for over 60%, posing significant hazards to the atmospheric environment and human health.

[0003] Hydrocarbon traps (HCTs) are core devices for addressing HCs emissions during engine cold starts, and their performance directly depends on the low-temperature adsorption capacity, high-temperature desorption efficiency, and hydrothermal stability of the adsorption material. Currently, commonly used adsorption materials include activated carbon, silica gel, and zeolite molecular sieves. Activated carbon is prone to thermal decomposition above 200℃ and exhibits poor thermal stability; silica gel has low selectivity for polar molecules, making it difficult to specifically capture HCs; while ZSM-5 zeolite, due to its adjustable silica-alumina ratio, excellent thermal stability (no structural decomposition above 500℃), and shape selectivity (straight pore size 0.53nm×0.56nm, sinusoidal pore size 0.55nm×0.55nm), has become the preferred HCT adsorption material.

[0004] However, unmodified H-ZSM-5 zeolite has significant drawbacks in practical applications:

[0005] 1. Insufficient adsorption performance: The adsorption capacity for typical HCs during cold starts (especially 1-butene, molecular dynamic diameter 0.50 nm; acetaldehyde, molecular dynamic diameter 0.48 nm) is limited, with an adsorption saturation time of only about 20 min, which cannot cover the HC emission cycle during engine cold starts (usually 30-60 min); and the adsorption of acetylene (molecular dynamic diameter 0.33 nm) is mainly chemical adsorption, which makes subsequent desorption difficult and easily causes blockage of adsorption sites.

[0006] 2. Poor hydrothermal stability: During actual engine operation, the ZSM-5 zeolite in the HCT is exposed to high temperatures (>500℃) and water vapor for extended periods, making it prone to hydrothermal aging (HTA). The core of this process is the hydrolytic breakage of the Si-O(H)-Al bonds in the zeolite framework induced by high-temperature water vapor. Al atoms detach from the framework to form extra-framework Al (EFAL), leading to zeolite pore collapse, a decrease in specific surface area (the specific surface area of ​​unmodified H-ZSM-5 decreased from 378.6 m² / g to 352.1 m² / g after hydrothermal aging), and an adsorption capacity degradation exceeding 30%.

[0007] 3. Competitive Adsorption Interference: H2O (a strongly polar molecule with a large dipole moment) and CO2 (a quadrupole molecule) contained in engine exhaust gases easily compete with HCs for adsorption sites in zeolites. Under high humidity conditions, H2O can occupy more than 40% of the adsorption sites, significantly reducing the HCs capture efficiency of zeolites.

[0008] Therefore, developing a ZSM-5 zeolite modification technology with high adsorption capacity, high hydrothermal stability, and suitability for cold start scenarios has become an urgent problem to be solved in this field. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide an alkali metal modified zeolite adsorbent material with high adsorption capacity, high hydrothermal stability and suitable for cold start scenarios, as well as its preparation method and application.

[0010] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0011] A method for preparing an alkali metal modified zeolite adsorbent material includes the following steps:

[0012] (1) Add the precursor solution dropwise to the pretreated zeolite to ensure that the precursor solution is completely adsorbed by the zeolite, and then seal and let stand to obtain the impregnated zeolite.

[0013] The precursor solution is prepared by dissolving nitrate powder containing alkali metal ions in water of equal volume to the saturated water absorption capacity of the zeolite.

[0014] The alkali metal is one of potassium, sodium, or lithium, and the alkali metal loading is 1 wt.%-5 wt.%.

[0015] (2) The impregnated zeolite is dried and then calcined, and then activated by H2 to obtain alkali metal modified zeolite; the calcination temperature is 500~600℃;

[0016] When the alkali metal is potassium or sodium, the following step (3) is also included:

[0017] (3) The alkali metal modified zeolite was subjected to hydrothermal aging treatment to obtain hydrothermally stable alkali metal modified zeolite.

[0018] The hydrothermal aging treatment involves first heating to 750-790K under inert gas protection and activating for 0.5-1.5 hours, followed by introducing water vapor and maintaining a constant temperature of 750-790K for 10-15 hours.

[0019] As a further improvement, the zeolite in step (1) is H-ZSM-5 zeolite with Si / Al = 23-383.

[0020] As a further improvement, the pretreatment process of zeolite in step (1) includes calcining the zeolite at 400~500℃ for 3~5h.

[0021] As a further improvement, the method for determining the saturated water absorption in step (1) is as follows: weigh the pretreated zeolite, slowly add deionized water and stir continuously until there are no visible free droplets on the surface of the zeolite, and record the amount of deionized water consumed, which is the saturated water absorption of the zeolite.

[0022] As a further improvement, the sealing and standing time in step (1) is 20~30h.

[0023] As a further improvement, when the alkali metal is potassium, the potassium loading is controlled to be 1 wt.%-3 wt.%.

[0024] As a further improvement, the calcination in step (2) is carried out at 500~600℃ for 5~7h; the activation treatment temperature is the same as the calcination temperature, and the activation treatment time is 1.5~2.5h.

[0025] As a further improvement, in the isothermal treatment process described in step (3), the ratio of water vapor to nitrogen flow rate is 1~1.2:10, and the water vapor flow rate is 4~5 mL / min.

[0026] The present invention provides an alkali metal modified zeolite adsorbent material, which is obtained by the preparation method described above.

[0027] The present invention also provides an application of the alkali metal modified zeolite adsorbent material in the field of hydrocarbon capture during engine cold start.

[0028] This invention provides a zeolite material that is synergistically regulated by alkali metal ion modification and hydrothermal aging, and its application in the capture of hydrocarbons (HCs) during engine cold start. The core of this invention is to solve the problem of excessive HC emissions caused by the three-way catalytic converter (TWC) not reaching the ignition temperature during engine cold start.

[0029] The main improvements of this invention are as follows:

[0030] 1. Synergistic regulation strategy of alkali metal loading and hydrothermal aging: For the first time, a medium-to-low loading (1wt.%-3wt.%) K + After precise hydrothermal aging treatment (773 K, steam flow rate 4.44 mL / min, treatment time 12 h), modified ZSM-5 zeolite can achieve "controlled dealumination". During this process, partial dealumination of the zeolite framework generates silanol nests (i.e., Si-O- enriched regions formed by framework dealumination), which serve as new adsorption sites, enhancing hydrogen bonding with acetaldehyde and induced dipole interaction with 1-butene; simultaneously, it avoids high K loading. + The resulting pore blockage and excessive dealuminization leading to skeletal collapse ultimately increased the 1-butene adsorption capacity by 25%-35% and the desorption peak temperature by 9K-10K, thus balancing adsorption capacity and desorption efficiency.

[0031] 2. Differentiated modification design based on exhaust gas scenarios: Designing dedicated modification schemes for different engine exhaust gas components: K + / Na + Modified ZSM-5 zeolite utilizes its adsorption preference for polar molecules to enhance the capture of polar HCs such as acetaldehyde, making it suitable for typical humidity exhaust gas scenarios; Li + Modified ZSM-5 zeolite with the help of Li + With its small radius (0.076 nm) and high charge density, it achieves selective adsorption of CO2 through the interaction of a strong local electric field with the CO2 quadrupole moment (adsorption capacity of 1.83 mmol / g at 298 K and 0.06 kPa), and the H2O adsorption capacity is less than 0.00002 mmol / g. It is suitable for high CO2 and humid exhaust gas scenarios (such as ethanol gasoline engines) and has outstanding resistance to water vapor interference.

[0032] 3. Precision optimization of the equal-volume impregnation process: By measuring the saturated water absorption of H-ZSM-5 zeolite (20g of pretreated zeolite has a saturated water absorption of 7mL), the volume of the precursor solution is ensured to be perfectly matched with the water absorption characteristics of the zeolite, thus solving the problem of uneven distribution of alkali metal ions in the traditional impregnation process. Combined with the post-treatment steps of calcination at 550℃ (decomposing alkali metal nitrates into metal oxides) and high-temperature activation with H2, the density of adsorption active sites is further improved, ensuring the stability and consistency of material performance.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. Excellent and highly targeted adsorption performance: 1wt.%-3wt.% K +After hydrothermal aging, the modified ZSM-5 zeolite exhibits an adsorption capacity of over 0.11 mmol / g for 1-butene and an adsorption saturation time extended to over 35 min, which can completely cover the peak HC emission period during engine cold starts. The adsorption capacity for acetaldehyde is increased by over 40%, and the adsorption capacity for acetylene is reasonably controlled to avoid subsequent desorption difficulties, thus achieving differentiated and efficient capture of typical HCs during cold starts.

[0035] 2. High hydrothermal stability: After hydrothermal aging treatment, the stability under medium and low loads (K) is maintained. + The modified ZSM-5 zeolite exhibits a specific surface area decrease of <5% and maintains a pore volume of 0.149-0.155 cubic centimeters / g. After 50 cycles under NEDC conditions (referring to the operating conditions specified in GB 18352.6-2016 "Limits and Measurement Methods for Pollutant Emissions from Light-Duty Vehicles (China Stage VI)"), the HCs capture efficiency decreases by <5%, making it suitable for the high-temperature and high-humidity hydrothermal environment of long-term engine operation and ensuring a long service life.

[0036] 3. Strong scene adaptability: K + / Na + Modified ZSM-5 zeolite is suitable for exhaust gas scenarios with normal humidity. Li + Modified ZSM-5 zeolite is suitable for high CO2 and humid exhaust gas scenarios, and can meet the exhaust gas treatment needs of different vehicle types (traditional gasoline vehicles and ethanol gasoline vehicles), with a wide range of applications.

[0037] 4. High industrialization potential: The preparation process requires no special equipment, and the raw materials (H-ZSM-5 zeolite, alkali metal nitrates) are low in cost; the process parameters are clear and easy to control repeatedly; the production cost is 15%-20% lower than that of existing modified ZSM-5 zeolite, and it has the conditions for large-scale industrial production. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 It is for H-ZSM-5 and different K + X-ray diffraction (XRD) pattern of K-ZSM-5 zeolite with loading;

[0040] Figure 2 Spectra of NH3-TPD temperature-progression desorption for H-ZSM-5, K-ZSM-5 (without hydrothermal aging treatment) and K-ZSM-5 (with hydrothermal aging treatment);

[0041] Figure 3 For different K + Adsorption kinetics curves of K-ZSM-5 with varying loading for acetylene, acetaldehyde, and 1-butene, where Figure (a) is for acetylene, Figure (b) is for acetaldehyde, and Figure (c) is for 1-butene;

[0042] Figure 4 For different K + Temperature-programmed spectra of the catalytic performance of modified ZSM-5 zeolite with and without hydrothermal aging treatment, based on loading and hydrothermal aging treatment, are shown for acetylene, acetaldehyde, and 1-butene, respectively. Figure (a) shows acetylene, Figure (b) shows acetaldehyde, and Figure (c) shows 1-butene.

[0043] Figure 5 For K + Adsorption isotherms of modified ZSM-5 zeolite for four hydrocarbons (acetylene, ethylene, acetaldehyde, and 1-butene) and carbon dioxide / water vapor are shown in Figure (a) for K2-ZSM-5, Figure (b) for K8-ZSM-5, and Figure (c) for K... 32 -ZSM-5; K2, K8, K 32 Corresponding to K doping respectively + The silica-alumina ratios of the ion-modified ZSM-5 zeolite are 383, 95, and 23, corresponding to 2, 8, and 32 K ion numbers.

[0044] Figure 6 for Na + Adsorption isotherms of modified ZSM-5 zeolite for four hydrocarbons (acetylene, ethylene, acetaldehyde, and 1-butene) and carbon dioxide / water vapor are shown in Figure (a) for Na2-ZSM-5, Figure (b) for Na8-ZSM-5, and Figure (c) for Na... 32 -ZSM-5; Na2, Na8, Na 32 Corresponding to Na doping + The silica-alumina ratio of the ion-modified ZSM-5 zeolite is 383, 95, and 23.

[0045] In the attached diagram, H-ZSM-5 represents hydrogen-form ZSM-5 zeolite molecular sieve (unmodified), and K-ZSM-5 represents K... + Modified ZSM-5 zeolite, but without hydrothermal aging treatment. K-ZSM-5 HTA represents K zeolite that has undergone hydrothermal aging treatment. + Modified ZSM-5 zeolite. Detailed Implementation

[0046] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0047] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0048] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0049] This invention uses zeolite as a matrix, loads alkali metal ions by an equal-volume impregnation method, and then calcines, activates at high temperature, and ages hydrothermally to obtain hydrothermally stable alkali metal modified zeolite.

[0050] In some specific embodiments, the preparation method of the alkali metal modified zeolite adsorbent material of the present invention includes the following steps:

[0051] (1) Add the precursor solution dropwise to the pretreated zeolite, stirring continuously during the addition process to ensure that the precursor solution is completely adsorbed by the zeolite and that there is no liquid residue; then seal and let stand at room temperature to promote the full diffusion of alkali metal ions in the pores of the zeolite and ensure uniform ion distribution to obtain the impregnated zeolite.

[0052] In some embodiments, the zeolite is preferably ZSM-5 zeolite, such as H-ZSM-5 zeolite molecular sieve. H-ZSM-5 zeolite with a Si / Al ratio of 23-383 is preferred. Using H-ZSM-5 zeolite with a Si / Al ratio of 23-383 as the matrix, this range of Si / Al ratio is achieved by replacing 2, 8, or 32 Si atoms in the ZSM-5 framework with Al atoms (corresponding to Si / Al ratios of 383, 95, and 23, respectively). Furthermore, Al substitution is preferentially performed at the T12 site in the ZSM-5 framework—among the 12 crystallographic T sites in the ZSM-5 framework, the T12 site exhibits superior binding stability with alkali metal ions, which can improve the structural stability of the modified material.

[0053] In some embodiments, the zeolite pretreatment process includes calcining the zeolite at 400-500°C for 3-5 hours to remove impurities and moisture.

[0054] In some embodiments, the preparation of the precursor solution includes: accurately weighing the corresponding alkali metal nitrate powder according to the target alkali metal ion loading, dissolving it in water with a volume equal to the saturated water absorption of the zeolite (i.e., the volume of the precursor solution is equal to the saturated water absorption of the zeolite), stirring until completely dissolved, and obtaining a homogeneous alkali metal ion precursor solution.

[0055] The determination of saturated water absorption is as follows: a certain amount of pretreated zeolite is weighed, deionized water is slowly added dropwise while stirring continuously until no visible free droplets are visible on the surface of the zeolite. The amount of deionized water consumed is recorded, which is the saturated water absorption of the zeolite.

[0056] In some embodiments, the alkali metal ion is K + Na + Or Li + One of them has an alkali metal ion loading (mass percentage of alkali metal ions to zeolite) of 1 wt.%-5 wt.%.

[0057] Alkali metal ion selection and loading control: Selecting different alkali metal ions based on the exhaust gas scenario: K + The loading rate is controlled at 1wt.%-3wt.% to enhance the capture of 1-butene and acetaldehyde; Na + Modification properties and K + Similar, suitable for low-cost production scenarios; Li + When the loading is 5 wt.%, it has excellent selective adsorption effect on CO2 and is suitable for high CO2 exhaust gas scenarios.

[0058] The equal-volume impregnation method uses alkali metal nitrates (KNO3, NaNO3, or LiNO3) as the metal source. The volume of the precursor solution is equal to the saturated water absorption capacity of the zeolite, and then it is allowed to stand at room temperature. The equal-volume impregnation process requires no special equipment, is simple to operate, and is easy to achieve industrial-scale mass production.

[0059] In some embodiments, during the equal-volume impregnation process, the sample is sealed and left to stand at room temperature for 20-30 hours.

[0060] (2) After the impregnated zeolite is dried, it is calcined and then H2 is introduced for high-temperature activation treatment to obtain alkali metal modified zeolite.

[0061] In some embodiments, the drying is performed at 100-120°C for 10-15 hours.

[0062] In some embodiments, the calcination is carried out by raising the temperature to 500-600°C at a rate of 4-8°C / min and calcining at this temperature for 5-7 hours. Calcination can decompose nitrates such as KNO3 into oxides such as K2O.

[0063] In some embodiments, the temperature for high-temperature activation treatment by introducing H2 is maintained at the same temperature as the calcination temperature, and the high-temperature activation treatment time is 1.5~2.5h.

[0064] This high-temperature activation treatment can effectively regulate the chemical microenvironment within the zeolite channels at high temperatures, promoting the stable existence of potassium ions in a highly dispersed and highly active form on the zeolite framework. Its effects include:

[0065] a. Maintain a highly dispersed state of active potassium species to avoid the formation of an inert phase.

[0066] KNO3, after calcination, forms K2O, which readily reacts with CO2 and other substances in air to transform into ineffective phases such as carbonates, thus reducing the efficiency of potassium species entering the zeolite framework. Using an H2 atmosphere provides a stable reducing / inert environment, effectively suppressing the aforementioned side reactions, ensuring that K2O remains active and undergoes solid-phase acid-base interactions with framework acid sites to form framework-coordinated potassium. + .

[0067] b. Gently remove any remaining material from the pores to maintain the stability of the skeletal structure.

[0068] After impregnation and drying, zeolite may retain trace amounts of organic matter, moisture, or nitrate intermediates in its pores. Calcination in air can easily trigger severe oxidation, leading to localized overheating and causing dealumination or damage to the pore structure. A H2 atmosphere can remove these residues through gentle hydrogenolysis and desorption, avoiding the thermal shock caused by oxidative calcination and maintaining the stability of the zeolite crystal structure.

[0069] c. Moderately activate the surface to increase K + Migration and exchange efficiency

[0070] High-temperature H2 can form a suitable number of hydroxyl defects or weak oxygen vacancies on the zeolite surface, increasing the number of exchangeable sites and enhancing K. + The ability to migrate and diffuse within the pores and promote uniform and stable coordination sites at the skeletal acid sites enhances the adsorption performance of the material.

[0071] (3) The alkali metal modified zeolite was subjected to hydrothermal aging treatment to obtain hydrothermally stable alkali metal modified zeolite.

[0072] In some embodiments, a two-step "activation-steam treatment" method is adopted. The specific conditions of the hydrothermal aging treatment are as follows: the modified alkali metal zeolite is placed in a tubular reactor, nitrogen is introduced as a protective gas, the reaction system is heated to 750~790K and activated for 0.5~1.5h to remove adsorbed water on the zeolite surface; while keeping the nitrogen flow rate constant, steam is introduced (the ratio of steam to nitrogen flow rate is 1~1.2:10, and the steam flow rate is 4~5mL / min), and the zeolite is treated at a constant temperature of 750~790K for 10~15h to form a "microporous-mesoporous" composite structure, thus obtaining hydrothermally stable alkali metal modified zeolite. The mesoporous structure formed after hydrothermal aging increases the mesopore content by 15%-20%, improving the diffusion efficiency of large molecular weight HCs such as 1-butene. Simultaneously, the specific surface area remains at 367.2 m² / g-380.6 m² / g, and the pore volume remains at 0.149 cm³ / g-0.155 cm³ / g, ensuring a sufficient number of adsorption sites. This solves the diffusion limitation problem of large molecular weight 1-butene in traditional microporous ZSM-5.

[0073] In applications, K + / Na + The adsorption of HCs is enhanced through electrostatic interactions (with the C=O functional group in the acetaldehyde molecule) and induced dipole interactions (with the π electron cloud of the -C=C- bond in the 1-butene molecule); Li + Selective adsorption of CO2 is achieved through the interaction between a strong local electric field and the CO2 quadrupole moment. After hydrothermal aging, the number of Brønsted acid sites in zeolite decreases but their intensity increases, while the number of Lewis acid sites increases, further enhancing the interaction strength between molecules and adsorption sites.

[0074] In some embodiments, alkali metal-modified ZSM-5 zeolite, when the alkali metal ion is K... + At that time, K + The loading was medium to low (1 wt.%-3 wt.%). After hydrothermal aging, the zeolite showed a 25%-35% increase in 1-butene adsorption capacity compared to unmodified H-ZSM-5 under the conditions of 293 K, 600 ppm C2H2 and C2H4O concentrations in the mixed gas, and 1800 ppm C4H8 (1-butene) concentration. The adsorption saturation time was extended from 20 min to over 35 min, the desorption peak temperature was 9 K-10 K higher than that of unmodified H-ZSM-5 (512 K), the specific surface area remained at 367.2 m² / g-380.6 m² / g, and the pore volume remained at 0.149 cm³ / g-0.155 cm³ / g.

[0075] In some embodiments, alkali metal-modified ZSM-5 zeolite, when the alkali metal ion is Li... + At 298 K and a relative pressure of 0-0.06 kPa, the zeolite exhibited an adsorption capacity of 1.83 mmol / g for CO2, less than 0.00002 mmol / g for H2O, less than 0.006 mmol / g for C2H2, and less than 0.004 mmol / g for C2H4.

[0076] In some embodiments, alkali metal-modified ZSM-5 zeolite, when the alkali metal ion is Na... + At that time, high loading (5wt.%) Na + Modified ZSM-5 zeolite exhibits an adsorption capacity of over 1.21 mmol / g for H2O, less than 0.01 mmol / g for C4H8, and maintains an adsorption capacity of 0.68 mmol / g - 0.78 mmol / g for acetaldehyde under conditions of 298 K and relative pressure of 0-0.06 kPa.

[0077] In some embodiments, the application of the alkali metal modified zeolite in the field of cold start hydrocarbon capture in engines involves filling the alkali metal modified zeolite into a cordierite honeycomb carrier (pore density of 400 cpsi) to form the core adsorption unit of a hydrocarbon trap (HCT); the HCT is connected in series with a three-way catalytic converter (TWC) in the vehicle exhaust system, with the HCT located upstream of the TWC.

[0078] The HCT process is as follows: During the engine cold start phase (exhaust gas temperature < 200℃), alkali metal modified zeolite captures HCs in the exhaust gas through its low-temperature adsorption properties; when the engine runs until the exhaust gas temperature rises to 300℃-400℃, the HCs desorb from the zeolite and enter the TWC with the exhaust gas for catalytic conversion. Among them, 1wt.%-3wt.% K + After hydrothermal aging, the modified ZSM-5 zeolite achieves an HCs capture efficiency of over 85% under NEDC conditions (referring to the conditions specified in GB 18352.6-2016 "Limits and Measurement Methods for Pollutant Emissions from Light-Duty Vehicles (China VI)"). After 50 cycles under these conditions, the capture efficiency decreases by less than 5%.

[0079] When the volume fraction of CO2 in the engine exhaust is greater than 5%, Li is selected. + Modified zeolite was used as the adsorption unit for HCT. When the relative humidity of the exhaust gas was >60%, a K1wt.%-3wt.% was selected. + Or Na + Modified zeolite is used as the adsorption unit of HCT, and the exhaust gas needs to be pretreated and dried, or the zeolite can be used to selectively capture acetaldehyde by adsorbing polar molecules.

[0080] In summary, this invention utilizes alkali metal ions (K... + Na + Li + Through precise loading, parameter optimization of the equal-volume impregnation process, and controllable hydrothermal aging, a modified ZSM-5 zeolite material with excellent performance and strong adaptability to various scenarios was successfully prepared. Among these, the medium-low loading K... + After hydrothermal aging, modified ZSM-5 zeolite significantly improved its adsorption-desorption performance for typical HCs during cold start; Li + Modified ZSM-5 zeolite possesses excellent selective CO2 adsorption capacity and resistance to water vapor interference. The material's preparation process is simple and controllable, low-cost, and exhibits stable performance, enabling large-scale industrial production. Furthermore, it effectively adapts to the series operation mode of hydrocarbon traps (HCTs) and three-way catalytic converters (TWCs), providing an efficient and reliable solution for HCs emission control during automotive cold starts. It has significant application value and promising prospects in the field of automotive exhaust purification.

[0081] Example 1: 1wt.% K + Preparation of modified ZSM-5 zeolite

[0082] 1. Pretreatment: Weigh 20g of H-ZSM-5 zeolite (Si / Al=30), place it in a muffle furnace, and calcine it at 450℃ for 4h to remove impurities and moisture adsorbed on the zeolite surface; after natural cooling, transfer it to a drying oven at 60℃ for later use.

[0083] 2. Determination of saturated water absorption: Place the pretreated H-ZSM-5 zeolite in a clean beaker, slowly add deionized water dropwise, stirring for 5 minutes after each 1 mL drop is added, until no visible free droplets are visible on the surface of the zeolite. Record the amount of deionized water consumed as 7 mL, which is the saturated water absorption of the H-ZSM-5 zeolite.

[0084] 3. Preparation of precursor solution: Based on 1 wt.% K + To calculate the loading, accurately weigh 0.517 g of KNO3 powder (99.9% purity) and dissolve it in 7 mL of deionized water. Stir magnetically for 30 min until the KNO3 is completely dissolved to obtain a homogeneous and transparent KNO3 solution. + Precursor solution.

[0085] 4. Isovolume impregnation: The K mixture prepared in step 3... + The precursor solution was added dropwise to 20g of pretreated H-ZSM-5 zeolite, with continuous stirring using a glass rod during the addition process to ensure that the precursor solution was completely adsorbed by the zeolite and that no liquid residue remained. The beaker was then sealed with plastic wrap and left to stand at room temperature (25℃) for 24 hours to promote K... + It diffuses fully within the pores of the zeolite.

[0086] 5. Drying and calcination: The sample impregnated in step 4 was transferred to a porcelain boat and placed in a forced-air drying oven to dry at 100℃ for 12 hours to remove moisture from the sample; then the porcelain boat was placed in a muffle furnace and heated to 550℃ at a heating rate of 5℃ / min, and calcined at this temperature for 6 hours to decompose KNO3 into K2O; then H2 was introduced at a flow rate of 30mL / min for high-temperature activation treatment for 2 hours, and after natural cooling, 1wt.% K-ZSM-5 zeolite was obtained.

[0087] 6. Hydrothermal aging: The 1 wt.% K-ZSM-5 zeolite obtained in step 5 was placed in the quartz tube of a tubular reactor, and N2 was introduced at a flow rate of 40 mL / min as a protective gas; the reaction system was heated to 773 K at a heating rate of 10 °C / min and activated for 1 h; while keeping the temperature and N2 flow rate constant, water vapor (provided by an HSG-02 water vapor generator) was introduced at a flow rate of 4.44 mL / min and treated at 773 K for 12 h; after the sample was naturally cooled to room temperature, hydrothermally stable 1 wt.% K-ZSM-5-HTA zeolite was obtained.

[0088] After hydrothermal aging treatment, K in this embodiment + The modified ZSM-5 zeolite exhibited a specific surface area decrease of <5% and maintained a pore volume of 0.149-0.155 cm³ / g. This sample showed an adsorption capacity of over 0.11 mmol / g for 1-butene and a 40% increase in acetaldehyde adsorption capacity compared to the unmodified H-ZSM-5.

[0089] 1 wt.%, 3 wt.%, and 5 wt.% K-ZSM-5 were prepared using the equal-volume impregnation method according to Example 1. Figure 1 Curves ① H-ZSM-5 (zeolite standard spectrum), ② 1wt.% K-ZSM-5, ③ 3wt.% K-ZSM-5, ④ 5wt.% K-ZSM-5; Characteristic diffraction peaks of ZSM-5 are marked (2θ=7.9°, 8.8°, 23.1°, 23.9°, matching the International Zeolite Association (IZA) standard spectrum. Figure 1 It can be seen that curves ①-④ all show the ZSM-5 characteristic peak, proving that K + The load did not damage the zeolite framework structure; with K + As the loading increases (①→④), the characteristic peak intensity decreases from 3200 a.u. to 2800 a.u. (23.1° peak), because K + Enhanced X-ray scattering after entering the channel to verify K + Successfully loaded inside the zeolite (not attached to the surface), laying the structural foundation for subsequent improvement of adsorption performance.

[0090] Example 2: 3wt.% Na + Preparation of modified ZSM-5 zeolite

[0091] The preparation steps are the same as in Example 1, except that:

[0092] In step 3, based on 3 wt.% Na + Loading calculation: 1.551 g of NaNO3 powder (99.9% purity) was accurately weighed and dissolved in 7 mL of deionized water to prepare Na... + Precursor solution;

[0093] After hydrothermal aging in step 6, 3 wt.% Na-ZSM-5-HTA zeolite was obtained. The adsorption capacity of this sample for acetaldehyde reached 0.108 mmol / g, which was 32% higher than that of unmodified H-ZSM-5, and the specific surface area after hydrothermal aging was 375.2 m² / g.

[0094] Example 3: 5wt.% Li + Preparation of modified ZSM-5 zeolite

[0095] The preparation steps are the same as in Example 1, except that:

[0096] In step 3, based on 5 wt.% Li + Loading calculation: 2.075 g of LiNO3 powder (99.9% purity) was accurately weighed and dissolved in 7 mL of deionized water to prepare Li. + Precursor solution;

[0097] Without the need for the hydrothermal aging treatment in step 6, 5 wt.% Li-ZSM-5 zeolite was directly obtained. This sample exhibited excellent resistance to water vapor interference, with a CO2 adsorption capacity of 1.83 mmol / g and an H2O adsorption capacity of 0.000015 mmol / g at 298 K and a relative pressure of 0.06 kPa.

[0098] Performance testing

[0099] 1. Experimental Method Description:

[0100] Adsorption breakthrough experiments were conducted in a fixed-bed reaction system using a stainless steel tubular reactor (10 mm inner diameter, 500 mm length). Temperature was controlled by an external heating device and monitored in real-time via thermocouples. The inlet gas consisted of a mixture of acetylene (C2H2, 600 ppm), acetaldehyde (C2H4O, 600 ppm), and 1-butene (C4H8, 1800 ppm), with high-purity nitrogen as the carrier gas at a flow rate of 60 mL / min. The experimental temperature was maintained at 293 K. The exhaust gas components were monitored in real-time using an online gas chromatograph (FID detector), and adsorption breakthrough curves were recorded at different time points to calculate adsorption saturation time and adsorption capacity.

[0101] Temperature-programmed desorption (TPD) experiments used the same apparatus as the breakthrough experiments. After adsorption saturation, the sample was first purged under a nitrogen atmosphere for 30 min, and then heated to 773 K at a rate of 2 K / min. During the heating process, the desorption signal was recorded in real time by gas chromatography to obtain the desorption peak temperature and desorption curve.

[0102] The N2 adsorption-desorption experiments were conducted using a specific surface area analyzer (test temperature 77.3 K). The samples were pre-degassed under vacuum at 573 K for 6 h. The obtained isotherms were used to calculate the specific surface area (BET method) and pore size distribution (BJH method). The results, combined with temperature-programmed desorption and adsorption breakthrough experimental data, were used to comprehensively evaluate the adsorption capacity, hydrothermal stability, and pore structure characteristics of the zeolite material.

[0103] 2. Adsorption breakthrough experiments, temperature-programmed desorption (TPD) experiments, and N2 adsorption-desorption experiments were conducted on the modified ZSM-5 zeolite samples prepared in the examples and comparative examples to test their adsorption performance and hydrothermal stability. The results are shown in Table 1 (data source: experimental measurements and GCMC simulations):

[0104] Table 1

[0105]

[0106] As shown in Table 1, the medium-low loading (1wt.%-3wt.%) K prepared by this invention + / Na + After hydrothermal aging, modified ZSM-5 zeolite showed significantly better adsorption capacity, adsorption saturation time, and hydrothermal stability for 1-butene compared to unmodified H-ZSM-5; Li + Modified ZSM-5 zeolite exhibits excellent selective CO2 adsorption capacity and resistance to water vapor interference, making it suitable for application needs in different exhaust gas scenarios.

[0107] Comparative Experiment 1: No hydrothermal aging treatment

[0108] Hydrothermal aging group: Example 1;

[0109] Non-hydrothermal aging group: The difference from Example 1 is that step 6 is omitted.

[0110] The performance comparison results are shown in Table 2:

[0111] Table 2

[0112]

[0113] As can be seen from Table 2, hydrothermal aging does not "destroy" the structure, but rather optimizes the adsorption-desorption matching through "controllable dealumination and mesopore formation".

[0114] Comparative Experiment 2: Effects of Different Hydrothermal Aging Conditions on Performance

[0115] Optimized group (different from Example 1 in that the K loading rate is 3 wt.%): 3 wt.% K-ZSM-5-HTA (773 K, water vapor 4.44 mL / min, 12 hours);

[0116] Deterioration Group 1: 3wt.% K-ZSM-5-HTA (High-temperature degradation: 873K, the rest is the same as the optimization group);

[0117] Deterioration group 2: 3wt.% K-ZSM-5-HTA (long-term degradation: 773K, 24h, the rest is the same as the optimization group).

[0118] The performance comparison results are shown in Table 3:

[0119] Table 3

[0120]

[0121] As can be seen from Table 3, the hydrothermal conditions (773K, 12 hours) set in Example 1 are the critical point between "controllable modification" and "structural degradation". Deviating from these conditions will lead to a sharp drop in zeolite performance, proving that the precise control of hydrothermal parameters is the key to this technology.

[0122] Comparative Experiment 3: Non-activation treatment after calcination

[0123] High-temperature activation treatment group: 5 wt.% K-ZSM-5, the difference from Example 1 is that the K loading is 5 wt.%;

[0124] Non-activated treatment group: Compared with the high-temperature activated treatment group, it was directly cooled after calcination at 550℃ for 6 hours, without H2 high-temperature activation treatment.

[0125] The performance comparison results are shown in Table 4:

[0126] Table 4

[0127]

[0128] Table 4 shows that the high-temperature activation treatment with H2 after calcination promotes the existence of potassium ions in a highly dispersed and highly active form, which can prevent K from agglomerating in the form of oxides and ensure its uniform distribution in the zeolite channels.

[0129] Comparative Experiment 4: K + Modified vs Na + modified

[0130] K + Group: 1 wt.% K-ZSM-5-HTA (Example 1);

[0131] Na + Group: 1 wt.% Na-ZSM-5-HTA (preparation method same as K) + Group, only replace KNO3 with NaNO3).

[0132] The performance comparison results are shown in Table 5:

[0133] Table 5

[0134]

[0135] Table 5 shows that in a cold start environment with humid exhaust gases, K + Modified ZSM-5 exhibits superior hydrocarbon adsorption selectivity and resistance to water vapor interference compared to Na. + Modification, proof of K + It is an alkali metal cation that is more suitable for cold start scenarios.

[0136] Depend on Figure 2 It is evident that the high-temperature peak is particularly prominent in H-ZSM-5 zeolite, indicating an abundance of Brønsted acid sites, typically originating from bridged hydroxyl groups (Si–OH–Al) in the framework. Due to the relatively complete framework structure and high framework aluminum content of H-ZSM-5, a strong high-temperature desorption peak can be observed. The introduction of potassium ions significantly weakens the intensity of the high-temperature peak, and the temperature corresponding to the highest point of the desorption curve decreases, implying a reduction in both the number and intensity of Brønsted acid sites in K-ZSM-5. This is because potassium ions replace the original protons (H+), thereby weakening the original Brønsted acidity. After hydrothermal treatment, the NH3 desorption signal of the sample weakens again, and the areas of both the low-temperature and high-temperature peaks decrease, with the weakening of the high-temperature peak being more significant. This is because under high-temperature and high-humidity conditions, aluminum atoms in the zeolite framework are prone to migration or loss, a phenomenon known as "dealuminization." This process reduces or redistributes the original framework aluminum, weakens or eliminates some bridged hydroxyl groups, and thus significantly reduces the number of Brønsted acid sites.

[0137] Depend on Figure 3 It can be observed that H-ZSM-5 exhibits a faster adsorption rate for acetaldehyde and 1-butene in the early stage of adsorption, but K-ZSM-5 zeolite shows a greater saturation adsorption capacity. On K-ZSM-5 zeolite, the total adsorption of acetylene decreases with increasing potassium ion loading; conversely, the saturation adsorption capacity of acetaldehyde and 1-butene increases with increasing potassium mass fraction, with the highest adsorption capacity observed on 5 wt.% K-ZSM-5.

[0138] Depend on Figure 4 It can be seen that K +After hydrothermal treatment, the temperatures corresponding to the desorption peaks of K-ZSM-5 samples with different loading levels were all increased, indicating that the hydrothermal aging process significantly enhanced the interaction strength between gas molecules and active sites. Among the comparisons of different loading levels, the 1% K-ZSM-5 showed the most significant rightward shift in desorption peak temperatures, with the temperatures corresponding to the desorption peaks of acetylene (532 K), acetaldehyde (493 K), and 1-butene (522 K) increasing by 9 K, 10 K, and 9 K, respectively, compared to before hydrothermal treatment. The 5% K-ZSM-5 also showed a significant increase in desorption peak temperatures, with the temperatures corresponding to the desorption peaks of the three components being 534 K, 493 K, and 523 K, respectively. The 3% K-ZSM-5 showed the smallest increase in desorption peak temperature.

[0139] Depend on Figure 5-6 It can be seen that for K + and Na + With modification, as the number of alkali metal cations increases (i.e., the silica-alumina ratio decreases), zeolites preferentially adsorb water molecules, resulting in a significant increase in water adsorption capacity. The adsorption capacity for C2H4O remains essentially unchanged, while the adsorption capacity for other hydrocarbons shows a decreasing trend. The adsorption capacities of Na2-ZSM-5 zeolite for C2H4O, C4H8, and H2O are 0.78 mmol / g, 0.11 mmol / g, and 0.09 mmol / g, respectively; while the adsorption capacity per gram of Na... 32 -ZSM-5 can adsorb 0.68 mmol C2H4O, 1.21 mmol H2O, and only 0.007 mmol C4H8. This is because water molecules are polar molecules with extremely strong dipole moments. The increased number of cations provides more strong adsorption sites, and the small size of water molecules gives them an absolute advantage in the adsorption competition with hydrocarbons. Due to the presence of water molecules, the adsorption of other components by zeolite is inhibited. Meanwhile, the polar carbonyl group (C=O) in acetaldehyde readily interacts strongly with alkali metal cations, allowing acetaldehyde to maintain a large adsorption capacity.

[0140] Application Examples

[0141] The 1 wt.% K-ZSM-5-HTA zeolite prepared in Example 1 was used as an adsorbent material and filled into a cordierite honeycomb support (size: Φ100mm×150mm, pore density 400cpsi) with a filling amount of 60% of the support volume to form the core adsorption unit of the hydrocarbon trap (HCT). The HCT was installed in series with a three-way catalytic converter (TWC, Pt-Rh-Pd three-way catalyst, size matched with HCT) in the exhaust system of a 1.5L gasoline engine. The HCT was located upstream of the TWC, with a distance of 30cm between them to ensure that the desorbed HCs could completely enter the TWC for catalytic conversion.

[0142] A cold start emission test was conducted on the exhaust system under NEDC conditions (referring to the operating conditions specified in GB 18352.6-2016 "Limits and Measurement Methods for Pollutant Emissions from Light-Duty Vehicles (China VI)") (ambient temperature 25℃, engine initial temperature was the same as ambient temperature). The test results are as follows:

[0143] 1. 60 seconds before cold start (exhaust gas temperature < 200℃, HCT in adsorption stage): HCs emissions drop from 120mg before HCT installation to 18mg, HCs capture efficiency reaches 85%;

[0144] 2. After the engine runs for 120 seconds (exhaust gas temperature rises to 350℃, HCT enters the desorption stage): the amount of HCs desorbed reaches 98% of the adsorption amount, and the catalytic conversion rate of the desorbed HCs after entering TWC reaches 99%;

[0145] 3. During the cold start phase of the vehicle (first 5 minutes): the total HCs emissions are reduced by 72%, which is far below the requirement of ≤30mg for HCs emissions during cold start in the China VI emission standard.

[0146] 4. After 50 cycles of use, the HCs collection efficiency decreases by less than 5%.

[0147] This application example demonstrates that the alkali metal modified ZSM-5 zeolite of the present invention can effectively adapt to the tandem working mode of HCT and TWC, realize the closed-loop treatment of "adsorption-desorption-catalytic conversion" of HCs during cold start, and meet the practical application requirements of automotive exhaust purification.

[0148] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention should fall within the protection scope of the present invention.

Claims

1. A method for preparing an alkali metal-modified zeolitic adsorbent material, characterized by, The method comprises the following steps: (1) adding the precursor solution drop by drop into the pretreated zeolite, ensuring that the precursor solution is completely adsorbed by the zeolite, and then sealing and standing to obtain the impregnated zeolite; The precursor solution is prepared by dissolving alkali metal nitrate powder in water with a volume equal to the saturated water absorption amount of the zeolite; The alkali metal is one of potassium, sodium or lithium, and the alkali metal loading is 1wt.%-5wt.%; The zeolite is H-ZSM-5 zeolite with Si / Al=23-383; (2) drying the impregnated zeolite, calcining, and then activating by passing in H2 to obtain the alkali metal modified zeolite; The calcination temperature is 500-600℃, and the activation temperature is the same as the calcination temperature, and the activation time is 1.5-2.5h; When the alkali metal is potassium or sodium, the following step (3) is further included: (3) hydrothermal aging treatment of the alkali metal modified zeolite to obtain a hydrothermally stable alkali metal modified zeolite; The hydrothermal aging treatment is first heated to 750-790K under inert gas protection and activated for 0.5-1.5h, and then water vapor is introduced, and the temperature is kept at 750-790K for 10-15h.

2. The method of claim 1, wherein the alkali metal modified zeolitic adsorbent material is prepared by the steps of: The pretreatment process of the zeolite in step (1) comprises calcining the zeolite at 400-500℃ for 3-5h.

3. The method of claim 1, wherein the alkali metal modified zeolitic adsorbent material is prepared by the steps of: The method for measuring the saturated water absorption amount in step (1) is as follows: weighing the pretreated zeolite, slowly adding deionized water and continuously stirring until no free droplets are visible on the surface of the zeolite, and recording the deionized water consumption, which is the saturated water absorption amount of the zeolite.

4. The method of claim 1, wherein the alkali metal modified zeolitic adsorbent material is prepared by the steps of: The sealing and standing time in step (1) is 20-30h.

5. The method of claim 1, wherein the alkali metal modified zeolitic adsorbent material is prepared by the steps of: When the alkali metal is potassium, the potassium loading is controlled to be 1wt.%-3wt.%.

6. The method of claim 1, wherein the alkali metal modified zeolitic adsorbent material is prepared by the steps of: The calcination in step (2) is calcination at 500-600℃ for 5-7h.

7. The preparation method of the alkali metal modified zeolite adsorbent material according to claim 1, characterized in that, During the constant temperature treatment in step (3), the flow rate ratio of water vapor to nitrogen is 1-1.2:10, and the water vapor flow rate is 4-5mL / min.

8. An alkali metal modified zeolitic adsorbent material characterized in that, The method for measuring the saturated water absorption amount in step (1) is as follows: weighing the pretreated zeolite, slowly adding deionized water and continuously stirring until no free droplets are visible on the surface of the zeolite, and recording the deionized water consumption, which is the saturated water absorption amount of the zeolite.

9. The application of the alkali metal modified zeolite adsorbent material of claim 8 in the field of engine cold start hydrocarbon capture.

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