Fly ash-based strong hydrophobic honeycomb 4A molecular sieve adsorbent and low-temperature CO2 trapping method

Abstract

The invention discloses a fly ash-based strong hydrophobic honeycomb 4A molecular sieve adsorbent and a low-temperature CO2 trapping method. The preparation method of the 4A molecular sieve adsorbent comprises the following steps: S1, mixing decarbonized fly ash with sodium hydroxide, and calcining; s2, mixing the calcined product with water, then adding sodium tungstate, stirring and then aging; s3, carrying out hydrothermal crystallization on the aged slurry, washing and drying to obtain strong hydrophobic 4A molecular sieve powder; s4, mixing the 4A molecular sieve powder, alkaline silica sol, hydroxypropyl methyl cellulose, polyethylene oxide, glass fibers and kapok, adding water for pugging, and performing extrusion molding; and S5, drying the formed wet blank, and calcining to obtain the product. Surface silanol groups (-SiOH) are eliminated through tungsten doping, a water molecule adsorption path is blocked, and hydrophobic modification is achieved. According to the method, the fly ash is adopted to completely replace a silicon-aluminum source, and the step of acid pickling / aluminum supplementing is omitted. One-step extrusion molding is adopted, and compared with the prior art (secondary dipping roasting), the process period is shortened.

Description

Technical Field

[0001] The invention relates to a fly ash-based highly hydrophobic honeycomb 4A molecular sieve adsorbent and a low-temperature CO2 capture method, belonging to the technical field of solid waste resource utilization and carbon capture. Background Art

[0002] Fly ash is a major solid waste generated by coal-fired power plants. Large accumulations of fly ash pose a threat to human health and pollute the environment. Currently, my country faces challenges with high fly ash accumulation and low comprehensive utilization rates. Zeolite 4A is a three-dimensional framework composed of silicon-oxygen and aluminum-oxygen tetrahedra, consisting of an 8-membered ring connected by six similar cavities. Its free cavities have a diameter of 4.12 Å. As an ideal adsorbent and desiccant, zeolite 4A is commonly used for gas adsorption and separation. Given its rich silicon and aluminum content, converting fly ash into zeolite 4A for low-temperature CO2 capture is a viable "waste-to-waste" approach.

[0003] The silicon-aluminum ratio of the raw materials is an important condition for the preparation of molecular sieves. Many works adjust the silicon-aluminum ratio by adding additional silicon and aluminum to prepare molecular sieves from fly ash. Chinese patent application CN 107758681A discloses a method for preparing 4A zeolite molecular sieve from fly ash and diatomaceous earth. The method adjusts the silicon-aluminum ratio of the raw materials to prepare 4A zeolite molecular sieve by grinding and sieving fly ash and diatomaceous earth. Chinese patent application CN 113277527A discloses a method for preparing NaX and NaA molecular sieves by acid washing and high-temperature alkali activation of fly ash to obtain a silicon source, and adding an aluminum source, a sodium source and NaX seeds thereto. In addition, Chinese patent application CN 115432713A provides a method for preparing molecular sieves from fly ash by adding sodium metaaluminate. The addition of additional silicon and aluminum sources is bound to increase the cost of synthesis and preparation, which is not conducive to large-scale preparation.

[0004] Flue gas CO2 capture and direct air capture are effective carbon reduction technologies for mitigating global warming. However, due to low CO2 concentrations (flue gas: 10-15%, air: 400 ppm), these technologies currently face challenges such as low carbon capture yields and high costs. Using a low-temperature adsorption (TSA) process can combine the advantages of zeolite molecular sieves' high low-temperature adsorption capacity with low capture energy consumption. Christopher W. Jones, Sean MW Wilson, and others reported a zeolite molecular sieve ultra-low-temperature adsorption (TSA) process that consumes significantly less energy than the current conventional TSA process (Song Min Gyu; Rim Guanhe; Kong Fanhe; Priyadarshini Pranjali; Rosu Cornelia; Lively, Ryan P.; Jones, Christopher W., Cold-Temperature Capture of Carbon Dioxide with Water Coproduction from Air Using Commercial Zeolites). Industrial&Engineering Chemistry Research 2022, 61 (36), 13624-13634.Wilson, Sean MW, The potential of direct air capture using absorbents incold climates. iScience 2022, 25 (12), 105564.). In addition, during the CO2 capture process in a humid environment, zeolite molecular sieves experience competitive adsorption of H2O, which greatly reduces the adsorption performance of zeolite molecular sieves. Julien Grand et al. prepared a surface-free silanol molecular sieve by incorporating tungsten into the MFI framework, effectively improving the hydrophobicity of the material (Grand, J.; Talapaneni, SN; Vicente, A., One-pot synthesis of silanol-free nanosized MFI zeolite. Nature Materials 2017, 16 (10), 1010-1015.). Currently, there are few related studies and most commercial zeolite powders are used as adsorption materials. With reference to the above work, in order to consider the subsequent application potential and achieve the green chemical goal of "waste treatment with waste", the present invention is proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a fly ash-based strongly hydrophobic honeycomb 4A molecular sieve adsorbent, and to use it for low-temperature CO2 adsorption in a humid environment, thereby realizing high-value-added utilization of solid waste fly ash and alleviating CO2 emission problems, thereby achieving the effect of "treating waste with waste".

[0006] The preparation method of the fly ash-based highly hydrophobic honeycomb 4A molecular sieve adsorbent provided by the present invention comprises the following steps: S1, mixing the decarbonized fly ash with sodium hydroxide and calcining; S2, mixing the product calcined in step S1 with water, then adding sodium tungstate, stirring and then aging; S3, hydrothermally crystallizing the aged slurry obtained in step S2, washing and drying to obtain a highly hydrophobic 4A molecular sieve powder; S4, mixing the 4A molecular sieve powder, alkaline silica sol, hydroxypropyl methylcellulose, polyethylene oxide, glass fiber and kapok, adding water to knead the mud and then extruding into strips; S5. The wet molded blank obtained in step S4 is dried and then calcined to obtain a honeycomb-shaped 4A molecular sieve adsorbent.

[0007] In the preparation method of the present invention, in step S1, the mass ratio of the fly ash to the sodium hydroxide is 1:1-1:2, so that the silicon and aluminum elements in the fly ash are directly converted into a 4A molecular sieve precursor, eliminating the acid washing / aluminum filling steps required in the prior art, reducing raw material costs, and generating zero waste residue. The calcination temperature is 550-700° C. and the calcination time is 2-4 hours.

[0008] In the preparation method of the present invention, in step S2, water is added at a liquid-to-solid ratio of 3-9:1; Sodium tungstate is added at a tungsten-silicon ratio of 0.02-0.3:1 to completely eliminate the surface silanol groups (-SiOH) by replacing the skeleton silicon sites through [WO4] tetrahedrons, thereby blocking the hydrogen bonding sites of water molecules; After stirring for 0.5-6 hours, the aging is carried out for 12-24 hours.

[0009] In the preparation method of the present invention, in step S3, the hydrothermal crystallization temperature is 70-100° C. and the time is 12-48 hours, which reduces energy consumption and avoids skeleton collapse.

[0010] In the preparation method of the present invention, in step S4, the mass ratio of the 4A molecular sieve powder, the alkaline silica sol, the hydroxypropyl methylcellulose, the polyethylene oxide, the glass fiber and the kapok is 50-80:6-12:1-3:1-3:3-8:1-2; The present invention uses alkaline silica sol to form a low-temperature bonding skeleton, uses kapok fiber to construct through-holes (the holes are formed by pyrolysis during calcination after forming), and uses glass fiber to improve compressive strength; The vacuum degree of the extrusion molding is 0.08-0.1 MPa, and the extrusion pressure is 14-16 MPa.

[0011] In the preparation method of the present invention, in step S5, the drying temperature is 60-100° C. and the drying time is 1-6 hours; The honeycomb 4A molecular sieve adsorbent is obtained by heating the temperature to 350-650°C at a heating rate of 1-5°C / min and calcining for 3-8 hours. Slow heating can effectively prevent structural collapse and pore defects of the embryo: the embryo is prone to pore collapse during rapid heating, while slow heating can gradually eliminate residual moisture and organic binder inside the embryo, avoiding rapid vaporization and cracking.

[0012] The honeycomb-shaped 4A molecular sieve adsorbent prepared by the present invention has a honeycomb structure, a pore diameter of 1-3 mm, and a wall thickness of 1-3 mm.

[0013] Based on the honeycomb 4A molecular sieve adsorbent, the present invention also provides a low-temperature CO2 capture method, including the step of using the honeycomb 4A molecular sieve adsorbent for capture.

[0014] Preferably, the capture is performed according to the following steps: The honeycomb 4A molecular sieve adsorbent is placed in a fixed bed reactor, and CO2-containing gas is introduced at a temperature of -60°C to 0°C with a flow rate of 0.1-2 L / min, and adsorption is carried out for 60-180 min. Zeolite molecular sieves exhibit unique advantages in low-temperature adsorption. The kinetic energy of gas molecules is reduced in low-temperature environments, the van der Waals force of zeolite pores on polar molecules such as CO2 is enhanced, and the regular micropores of zeolite are more likely to form multi-layer adsorption.

[0015] Preferably, after adsorption saturation, the fixed bed reactor is heated to 200-300° C., and N 2 is switched to purge desorption for 20-90 min at a flow rate of 0.1-2 L / min.

[0016] The present invention synthesizes a highly hydrophobic honeycomb 4A molecular sieve from fly ash that can be directly deployed in a reactor without adding additional silicon and aluminum. Using this adsorbent for low-temperature CO2 capture can significantly increase the adsorption capacity of the zeolite molecular sieve under low CO2 concentrations and reduce process energy consumption. It also effectively improves the adsorption performance of the zeolite molecular sieve in a humid environment, and has practical application potential.

[0017] This invention eliminates surface silanol groups (-SiOH) through tungsten doping, blocking the water adsorption pathway and achieving hydrophobic modification. This invention uses fly ash to completely replace the silicon and aluminum source, eliminating the acid washing and aluminum filling steps. This invention utilizes a one-step extrusion process, shortening the process cycle compared to existing technologies (two-stage impregnation and roasting). BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The present invention is a flow chart of preparing 4A molecular sieve adsorbent by using fly ash.

[0019] Figure 2 4A is a SEM image of the highly hydrophobic 4A molecular sieve powder prepared in Example 1 of the present invention.

[0020] Figure 3 This is a physical picture of the highly hydrophobic honeycomb 4A molecular sieve prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0022] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0023] Example 1: Preparation of honeycomb 4A adsorbent using fly ash as silicon and aluminum source Process such as Figure 1 shown.

[0024] 1. Preparation of 4A molecular sieve from fly ash The fly ash was first placed in a muffle furnace and calcined at 650°C for 2 h for decarbonization. 10 g of the decarbonized fly ash and 14 g of sodium hydroxide were added to a crucible and mixed evenly. The mixture was placed in a muffle furnace and heated to 650°C at 5°C / min for 2 h. The calcined mixture was crushed and placed in a reactor and mixed with water at a mass ratio of 1:6. 0.91 g of sodium tungstate (tungsten-silicon ratio 0.04:1) was added. After mixing, it was stirred at room temperature for 2 h and then aged at room temperature for 12 h. The aged material was sealed and hydrothermally heated at 90°C for 18 h. The hydrothermal product was washed to neutrality, filtered, and dried at 100°C for 3 h to obtain a highly hydrophobic 4A molecular sieve powder ( Figure 2 ), it can be seen that the sample has regular cubic single crystals, which are consistent with the crystal morphology characteristics of 4A zeolite, indicating that 4A zeolite molecular sieve was successfully prepared using fly ash as raw material.

[0025] 2. Mixing and kneading Mix 35g of 4A molecular sieve prepared in step 1, 5g of alkaline silica sol, 1g of hydroxypropyl methylcellulose, 1g of polyethylene oxide, 4g of glass fiber, and 0.6g of kapok. Add the mixed materials to a slurry mill and mix and extrude them. Slowly add water during the slurry milling process until the materials are agglomerated. During the slurry milling process, control the material temperature at around 30°C and the slurry mill speed at 20 r / min.

[0026] 3. Extrusion molding and drying activation The material after mud kneading is placed in an extruder for extrusion molding. The temperature is controlled at 30 ° C, the extrusion pressure is about 15 MPa, and the vacuum degree is 0.08 MPa. The extrusion die is a circular specification with a diameter of 49 mm, a circular channel structure, a pore diameter of 2 mm, and a wall thickness of 1 mm. The embryo is cut into a length of 100 mm to obtain a honeycomb 4A molecular sieve adsorbent wet blank; the wet blank is placed in an oven at 60 ° C for 1 hour; the semi-dry blank is placed in a tubular furnace for activation under a nitrogen atmosphere, and the temperature is raised to 550 ° C at a rate of 5 ° C / min and calcined for 3 hours to obtain a strongly hydrophobic honeycomb 4A molecular sieve, such as Figure 3 shown.

[0027] 4. Low temperature dry and wet air DAC test The adsorbent was placed in a custom-built fixed-bed reactor at a temperature of -20°C, a gas flow rate of 120 mL / min, and a test gas of 400 ppm CO₂ / N₂ for 120 minutes. After the adsorbent reached saturation, the reactor was removed and heated to 300°C. A N₂ purge was then applied at a flow rate of 200 mL / min for 30 minutes to achieve desorption. Breakthrough curves were measured under dry gas and 80% relative humidity (RH). The CO₂ concentration at the fixed-bed outlet was recorded using an infrared gas analyzer, and the CO₂ adsorption capacity was calculated. The results are shown in Table 1.

[0028] Example 2: Preparation of honeycomb 4A adsorbent using fly ash as silicon and aluminum source Process such as Figure 1 shown.

[0029] 1. Preparation of 4A molecular sieve from fly ash The fly ash was first placed in a muffle furnace and calcined at 650°C for 2 h to decarbonize. 10 g of the decarbonized fly ash and 16 g of sodium hydroxide were added to a crucible and mixed evenly. The mixture was placed in a muffle furnace and heated to 650°C at 5°C / min and calcined for 2 h. The calcined mixture was crushed and placed in a reactor and mixed with water in a mass ratio of 1:6. 0.68 g of sodium tungstate (tungsten-silicon ratio 0.03:1) was added. After mixing, it was stirred at room temperature for 2 h and then aged at room temperature for 12 h. The aged material was sealed and hydrothermally heated at 90°C for 18 h. The hydrothermal product was washed to neutrality, filtered, and dried at 100°C for 3 h to obtain a highly hydrophobic 4A molecular sieve powder.

[0030] 2. Mixing and kneading Mix 40g of 4A molecular sieve prepared in step 1, 6g of alkaline silica sol, 2g of hydroxypropyl methylcellulose, 2g of polyethylene oxide, 4g of glass fiber, and 0.55g of kapok. Add the mixed material to a slurry mill and mix and extrude. Slowly add water during the slurry milling process until the material is agglomerated. During the slurry milling process, control the material temperature at around 30°C and the slurry mill speed at 20 r / min.

[0031] 3. Extrusion molding and drying activation The kneaded material was placed in an extruder for extrusion molding at a controlled temperature of 30°C, an extrusion pressure of approximately 15 MPa, and a vacuum of 0.08 MPa. The extrusion die was a circular, 49 mm diameter die with a circular pore structure of 2 mm in diameter and a wall thickness of 1 mm. The embryo was cut into 100 mm lengths to obtain a honeycomb 4A molecular sieve adsorbent wet billet. The wet billet was then dried in an oven at 60°C for 1 hour. The semi-dry billet was activated in a tubular furnace under a nitrogen atmosphere and calcined at 650°C at a rate of 5°C / min for 4 hours to obtain a highly hydrophobic honeycomb 4A molecular sieve.

[0032] 4. Low temperature dry and wet air DAC test The adsorbent was placed in a custom-built fixed-bed reactor at a temperature of -20°C, a gas flow rate of 120 mL / min, and a test gas of 400 ppm CO₂ / N₂ for 120 minutes. After the adsorbent reached saturation, the reactor was removed and heated to 250°C. A N₂ purge was then applied for 30 minutes at a flow rate of 200 mL / min to achieve desorption. Breakthrough curves were measured under dry gas and 80% relative humidity (RH). The CO₂ concentration at the fixed-bed outlet was recorded using an infrared gas analyzer, and the CO₂ adsorption capacity was calculated. The results are shown in Table 1.

[0033] Example 3: Preparation of honeycomb 4A adsorbent using fly ash as silicon and aluminum source Process such as Figure 1 shown.

[0034] 1. Preparation of 4A molecular sieve from fly ash The fly ash was first placed in a muffle furnace and calcined at 650°C for 2 h to decarbonize. 10 g of the decarbonized fly ash and 14 g of sodium hydroxide were added to a crucible and mixed evenly. The mixture was placed in a muffle furnace and heated to 650°C at 5°C / min and calcined for 2 h. The calcined mixture was crushed and placed in a reactor and mixed with water in a mass ratio of 1:6. 1.37 g of sodium tungstate (tungsten-silicon ratio 0.06:1) was added. After mixing, it was stirred at room temperature for 2 h and then aged at room temperature for 12 h. The aged material was sealed and hydrothermally heated at 100°C for 18 h. The hydrothermal product was washed to neutrality, filtered, and dried at 100°C for 3 h to obtain a highly hydrophobic 4A molecular sieve powder.

[0035] 2. Mixing and kneading Mix 50g of 4A molecular sieve, 8g of alkaline silica sol, 3g of hydroxypropyl methylcellulose, 2.5g of polyethylene oxide, 7.5g of glass fiber, and 1g of kapok prepared in step 1. Add the mixed materials to a clay mill and mix and extrude them. Slowly add water during the clay milling process until the materials are agglomerated. During the clay milling process, control the material temperature at around 30°C and the clay mill speed at 20 r / min.

[0036] 3. Extrusion molding and drying activation The kneaded material was placed in an extruder for extrusion molding at a controlled temperature of 30°C, an extrusion pressure of approximately 15 MPa, and a vacuum of 0.08 MPa. The extrusion die was a circular extrusion die with a diameter of 49 mm, a circular pore structure with a pore diameter of 2 mm and a wall thickness of 1 mm. The embryo was cut into 100 mm lengths to obtain a honeycomb 4A molecular sieve adsorbent wet billet. The wet billet was then dried in an oven at 60°C for 1 hour. The semi-dry billet was activated in a tubular furnace under a nitrogen atmosphere and calcined at 450°C at a rate of 5°C / min for 7 hours to obtain a highly hydrophobic honeycomb 4A molecular sieve.

[0037] 4. Low temperature dry and wet air DAC test The adsorbent was placed in a custom-built fixed-bed reactor at a temperature of -20°C, a gas flow rate of 120 mL / min, and a test gas of 400 ppm CO₂ / N₂ for 120 minutes. After the adsorbent reached saturation, the reactor was removed and heated to 200°C. A N₂ purge was then applied for 30 minutes at a flow rate of 200 mL / min to achieve desorption. Breakthrough curves were measured under dry gas and 80% relative humidity (RH). The CO₂ concentration at the fixed-bed outlet was recorded using an infrared gas analyzer, and the CO₂ adsorption capacity was calculated. The results are shown in Table 1.

[0038] Comparative Example 1: Preparation of honeycomb 4A molecular sieve using chemical raw materials 1. Preparation of 4A molecular sieve from chemical raw materials 23g of sodium silicate and 16g of sodium aluminate were placed in a container and mixed, followed by the addition of 24g of sodium hydroxide and 220ml of deionized water, and stirred at room temperature for 30min. After stirring, the sealed container was crystallized at 90°C for 4h, the crystallized product was filtered and washed to neutrality, and then dried to obtain 4A molecular sieve.

[0039] 2. Mixing and kneading Mix 35g of 4A molecular sieve prepared in step 1, 5g of alkaline silica sol, 1g of hydroxypropyl methylcellulose, 1g of polyethylene oxide, 4g of glass fiber, and 0.55g of kapok. Add the mixed material to a clay mill and mix and extrude. Slowly add water during the clay milling process until the material is agglomerated. During the clay milling process, control the material temperature at around 30°C and the clay mill speed at 20 r / min.

[0040] 3. Extrusion molding and drying activation The kneaded material was placed in an extruder for extrusion molding at a controlled temperature of 30°C, an extrusion pressure of approximately 15 MPa, and a vacuum of 0.08 MPa. The extrusion die was a circular, 49 mm diameter die with a circular pore structure of 2 mm in diameter and a wall thickness of 1 mm. The embryo was cut into 100 mm lengths to obtain a honeycomb-shaped 4A molecular sieve adsorbent wet billet. The wet billet was then dried in an oven at 60°C for 1 hour. The semi-dried billet was activated in a tubular furnace under a nitrogen atmosphere and calcined at 550°C at a rate of 5°C / min for 4 hours to obtain the honeycomb 4A molecular sieve.

[0041] 4. Low temperature dry and wet air DAC test The adsorbent was placed in a custom-built fixed-bed reactor at a temperature of -20°C, a gas flow rate of 120 mL / min, and a test gas of 400 ppm CO₂ / N₂ for 120 minutes. After the adsorbent reached saturation, the reactor was removed and heated to 300°C. A N₂ purge was then applied at a flow rate of 200 mL / min for 30 minutes to achieve desorption. Breakthrough curves were measured under dry gas and 80% relative humidity (RH). The CO₂ concentration at the fixed-bed outlet was recorded using an infrared gas analyzer, and the CO₂ adsorption capacity was calculated. The results are shown in Table 1.

[0042] Comparative Example 2 The process is basically the same as Example 1, except that the 4A molecular sieve obtained in step 1 is not formed, and the low-temperature adsorption and room-temperature desorption in step 4 are directly carried out. The results are shown in Table 1.

[0043] 1. Preparation of 4A molecular sieve from fly ash The fly ash was first placed in a muffle furnace and calcined at 650°C for 2 h to decarbonize. 10 g of the decarbonized fly ash and 14 g of sodium hydroxide were added to a crucible and mixed evenly. The mixture was placed in a muffle furnace and heated to 650°C at 5°C / min and calcined for 2 h. The calcined mixture was crushed and placed in a reactor and mixed with water in a mass ratio of 1:6. 0.91 g of sodium tungstate (tungsten-silicon ratio 0.04:1) was added. After mixing, it was stirred at room temperature for 2 h and then aged at room temperature for 12 h. The aged material was sealed and hydrothermally heated at 90°C for 18 h. The hydrothermal product was washed to neutrality, filtered, and dried at 100°C for 3 h to obtain a highly hydrophobic 4A molecular sieve powder.

[0044] 2. Low temperature dry and wet air DAC test The adsorbent was placed in a custom-built fixed-bed reactor at a temperature of -20°C, a gas flow rate of 120 mL / min, and a test gas of 400 ppm CO₂ / N₂ for 120 minutes. After the adsorbent reached saturation, the reactor was removed and heated to 200°C. A N₂ purge was then applied for 30 minutes at a flow rate of 200 mL / min to achieve desorption. Breakthrough curves were measured under dry gas and 80% relative humidity (RH). The CO₂ concentration at the fixed-bed outlet was recorded using an infrared gas analyzer, and the CO₂ adsorption capacity was calculated. The results are shown in Table 1.

[0045] Comparative Example 3 The process is basically the same as Example 1, except that sodium tungstate is not added in step 1.

[0046] The results are shown in Table 1.

[0047] 1. Preparation of 4A molecular sieve from fly ash The fly ash was first placed in a muffle furnace and calcined at 650°C for 2 h to decarbonize. 10 g of the decarbonized fly ash and 14 g of sodium hydroxide were added to a crucible and mixed evenly. The mixture was placed in a muffle furnace and heated to 650°C at 5°C / min and calcined for 2 h. The calcined mixture was crushed and placed in a reactor and mixed with water in a mass ratio of 1:6. After mixing, it was stirred at room temperature for 2 h and then aged at room temperature for 12 h. The aged material was sealed and hydrothermally heated at 90°C for 18 h. The hydrothermal product was washed to neutrality, filtered, and dried at 100°C for 3 h to obtain 4A molecular sieve powder.

[0048] 2. Mixing and kneading Mix 35g of 4A molecular sieve prepared in step 1, 5g of alkaline silica sol, 1g of hydroxypropyl methylcellulose, 1g of polyethylene oxide, 4g of glass fiber, and 0.6g of kapok. Add the mixed materials to a slurry mill and mix and extrude them. Slowly add water during the slurry milling process until the materials are agglomerated. During the slurry milling process, control the material temperature at around 30°C and the slurry mill speed at 20 r / min.

[0049] 3. Extrusion molding and drying activation The kneaded material was placed in an extruder for extrusion molding at a controlled temperature of 30°C, an extrusion pressure of approximately 15 MPa, and a vacuum of 0.08 MPa. The extrusion die was a circular, 49 mm diameter die with a circular pore structure of 2 mm in diameter and a wall thickness of 1 mm. The embryo was cut into 100 mm lengths to obtain a honeycomb 4A molecular sieve adsorbent wet billet. The wet billet was then dried in an oven at 60°C for 1 hour. The semi-dry billet was activated in a tubular furnace under a nitrogen atmosphere and calcined at 550°C at a rate of 5°C / min for 3 hours to obtain a highly hydrophobic honeycomb 4A molecular sieve.

[0050] 4. Low temperature dry and wet air DAC test The adsorbent was placed in a custom-built fixed-bed reactor at a temperature of -20°C, a gas flow rate of 120 mL / min, and a test gas of 400 ppm CO₂ / N₂ for 120 minutes. After the adsorbent reached saturation, the reactor was removed and heated to 300°C. A N₂ purge was then applied at a flow rate of 200 mL / min for 30 minutes to achieve desorption. Breakthrough curves were measured under dry gas and 80% relative humidity (RH). The CO₂ concentration at the fixed-bed outlet was recorded using an infrared gas analyzer, and the CO₂ adsorption capacity was calculated. The results are shown in Table 1.

[0051] Table 1 Low temperature DAC performance, mechanical properties and cycle performance of Examples 1-3 and Comparative Examples 1-3

[0052] The energy consumption of the traditional TSA or PSA process for CO2 capture is high. In addition, most adsorbents used for CO2 capture have high preparation costs and cannot be directly applied to the reactor. The present invention uses fly ash to obtain a highly hydrophobic honeycomb 4A adsorbent that can be directly deployed in the reactor through extrusion molding without adding additional silicon and aluminum, which is highly consistent with the circular economy development strategy. The obtained adsorbent can achieve a CO2 capture capacity of 2.07 mmol / g and a CO2 capture capacity of 1.258 MWh / t in the low-temperature capture DAC process. CO2 Ultra-low energy consumption.

Claims

1. A method for preparing a fly ash-based highly hydrophobic honeycomb 4A molecular sieve adsorbent, comprising the following steps: S1, mixing the decarbonized fly ash with sodium hydroxide and calcining; S2, mixing the product calcined in step S1 with water, then adding sodium tungstate, stirring and then aging; S3, hydrothermally crystallizing the aged slurry obtained in step S2, washing and drying to obtain a highly hydrophobic 4A molecular sieve powder; S4, mixing the 4A molecular sieve powder, alkaline silica sol, hydroxypropyl methylcellulose, polyethylene oxide, glass fiber and kapok, adding water to knead the mud and then extruding into strips; S5. The wet molded blank obtained in step S4 is dried and then calcined to obtain a honeycomb-shaped 4A molecular sieve adsorbent.

2. The preparation method according to claim 1, wherein: In step S1, the mass ratio of the fly ash to the sodium hydroxide is 1:1-1:2; The calcination temperature is 550-700° C. and the calcination time is 2-4 hours.

3. The preparation method according to claim 1 or 2, characterized in that: In step S2, water is added at a liquid-to-solid ratio of 3-9:1; Add the sodium tungstate according to the tungsten-silicon ratio of 0.02-0.3:1; After stirring for 0.5-6 hours, the aging is carried out for 12-24 hours.

4. The preparation method according to any one of claims 1 to 3, characterized in that: In step S3, the hydrothermal crystallization temperature is 70-100° C. and the time is 12-48 hours.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step S4, the mass ratio of the 4A molecular sieve powder, the alkaline silica sol, the hydroxypropyl methylcellulose, the polyethylene oxide, the glass fiber and the kapok is 50-80:6-12:1-3:1-3:3-8:1-2; The vacuum degree of the extrusion molding is 0.08-0.1 MPa, and the extrusion pressure is 14-16 MPa.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step S5, the drying temperature is 60-100° C. and the drying time is 1-6 hours; The honeycomb 4A molecular sieve adsorbent is obtained by heating to 350-650° C. at a heating rate of 1-5° C. / min and calcining for 3-8 hours.

7. The honeycomb 4A molecular sieve adsorbent prepared by the method according to any one of claims 1 to 6, having a honeycomb structure.

8. A low-temperature CO2 capture method, comprising the step of using the honeycomb 4A molecular sieve adsorbent according to claim 7 for capture.

9. The method according to claim 8, characterized in that: Follow the steps below to perform the capture: The honeycomb 4A molecular sieve adsorbent is placed in a fixed bed reactor, and CO2-containing gas is introduced at a temperature of -60°C to 0°C at a flow rate of 0.1-2 L / min for adsorption for 60-180 min.

10. The method according to claim 9, characterized in that: After adsorption saturation, the fixed bed reactor was heated to 200-300° C., and N 2 was switched to purge desorption for 20-90 min at a flow rate of 0.1-2 L / min.

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