A full-silica molecular sieve, a preparation method and application thereof

Abstract

The present application relates to a kind of full silicon molecular sieve and its preparation method and application, alkali source, template agent, silicon source are added into water and mixed uniformly, then template agent is added in guiding gel, obtain reaction mixture, using variable temperature section crystallization, after rapid heat treatment calcination, full silicon Silicalite-1 Molecular sieve is prepared, the molecular sieve prepared can be used for selective separation of small molecule gas and VOCs gas adsorption purification.Compared with prior art, in the synthesis process, guiding gel is used to provide a large number of original crystal nucleus for synthesis system, can improve the crystallization rate of molecular sieve, shorten crystallization time. Meanwhile, template agent is added in guiding gel, the original crystal nucleus of molecular sieve in it induces the crystallization of molecular sieve, can greatly reduce the dosage of template agent in synthesis reaction solution, reduce cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of synthesis of all-silica molecular sieves, and in particular to an all-silica molecular sieve, a preparation method therefor and an application thereof. BACKGROUND

[0002] Silicalite-1 molecular sieve is an all-silica molecular sieve with MFI framework topology, which was developed by Mobil Corporation in the 1980s. The MFI framework topology has two intersecting ten-membered ring channels with pore sizes of 0.54 nm x 0.56 nm and 0.51 nm x 0.54 nm, respectively. Due to its special channel structure and moderate pore size, the MFI-type molecular sieve is widely used in the fields of catalysis, separation and adsorption purification, and has good shape-selective selectivity for small molecule alkanes and aromatic hydrocarbons. Among them, Silicalite-1 molecular sieve has higher hydrothermal stability and lipophilicity due to the absence of aluminum in the framework, and has great application potential in the fields of catalysis, separation and adsorption purification. However, the synthesis of Silicalite-1 molecular sieve usually requires the addition of a large amount of TPA + template agent for guiding preparation, and the crystallization time is long and the synthesis cost is high. Therefore, low-cost preparation such as reducing the content of template agent and reducing energy consumption has become a research hotspot for Silicalite-1 molecular sieve.

[0003] Patent CN110668460A uses tetrapropylammonium hydroxide and tetrapropylammonium bromide as a double template to synthesize Silicalite-1 molecular sieve, although the amount of tetrapropylammonium hydroxide is reduced, but the total amount of template agent is high (template agent: silicon source = 0.25-0.3:1), and the preparation cost is still high. Patent CN109368654A uses mesoporous silica as a silicon source and tetrapropylammonium hydroxide as a template to prepare nano Silicalite-1 molecular sieve by using a vapor-assisted crystallization method, although the amount of template agent is saved, but the crystallization time is long (24-100h), the synthesis process requires strict water amount, and the preparation process is complicated. Patent CN108002396B successfully prepared Silicalite-1 molecular sieve by greatly reducing the amount of tetrapropylammonium bromide through a special seed-assisted method, but the seed preparation process is complex, requires ball milling and alkali treatment, and the molecular sieve crystallization time is long (1-3d). Patent CN105858672B successfully prepared Silicalite-1 molecular sieve using tetraethyl salt as a template, but mineralizing agent needs to be added in the preparation process and the crystallization time is long (3-30d). In summary, most of the existing researches use TPA + as a template to prepare Silicalite-1 molecular sieve, and the existing synthesis methods for reducing the amount of template agent have a complicated preparation process, which greatly limits the industrial application.

[0004] Furthermore, molecular sieves synthesized using organic templates often require template removal before use to ensure unobstructed pores. High-temperature calcination (typically at 550°C in air) at a low heating rate (1-20°C / min) for 2-5 hours is the most common demolding method. However, this demolding process is a strongly exothermic reaction, often leading to problems such as damage to the molecular sieve framework, cell shrinkage, and intergranular cracks. Moreover, the long calcination time results in significant energy consumption. Therefore, developing new demolding methods is a research hotspot in this field.

[0005] Patent CN116040637A utilizes the ionic liquid 1-butyl-3-methylimidazolium chloride for thermal extraction at 110–130°C to remove the template agent from mesoporous silica. However, this method requires a large amount of extractant and necessitates washing and centrifugation, making the process cumbersome and generating significant wastewater. Patent CN115216052B uses organic solvents to remove the template agent from hierarchical porous MOF materials through etching at 25–100°C. This method requires large amounts of organic solvents, polluting the environment and incurring high costs. Patent CN1613768A uses hydrogen peroxide as an oxidant to oxidize and remove the organic template agent from mesoporous molecular sieves under hydrothermal conditions, but this process uses a strong oxidant, causing environmental pollution. Patent CN108778997B utilizes air to disperse molecular sieve powder or suspension into an aerosol, and then passes it through a hot gas stream heated to 950-1050K for rapid calcination. The calcination time is 1.5-2.5s, which can effectively remove 29-75% of the template agent in the molecular sieve. However, this method requires a large amount of air to disperse the molecular sieve. Summary of the Invention

[0006] The purpose of this invention is to provide an all-silica molecular sieve, its preparation method and application, and to reduce costs.

[0007] The objective of this invention can be achieved through the following technical solution: a method for preparing an all-silicon molecular sieve, wherein an alkali source, a template agent, and a silicon source are added to water and mixed evenly, and then a guiding gel with a template agent is added to obtain a reaction mixture, which is subjected to variable temperature segmented crystallization, and the all-silicon molecular sieve is obtained after filtration, washing, drying, and calcination.

[0008] Preferably, the method for preparing the guiding gel is as follows: adding an alkali source, a template agent, and a silicon source to water and stirring until the mixture is uniform, and then aging at room temperature to obtain the guiding gel.

[0009] More preferably, the molar ratio of the components of the guiding gel is alkali source: template agent: silicon source: water = 0~0.3: 0.05~0.3: 1: 10~70, and the aging time is 12~48h.

[0010] More preferably, the molar ratio of the components of the guiding gel is alkali source: template agent: silicon source: water = 0.05~0.2: 0.1~0.25: 1: 20~50.

[0011] More preferably, in the guiding gel, the alkaline source includes one or more of NaOH, KOH, ammonia, and tetrapropylammonium hydroxide; the template agent includes one or more of tetrapropylammonium bromide or tetrapropylammonium hydroxide; and the silicon source includes one or more of silica sol, water glass, tetraethyl orthosilicate, or silica.

[0012] More preferably, the alkali source in the guiding gel is NaOH or tetrapropylammonium hydroxide.

[0013] More preferably, the silicon source is silica sol or tetraethyl orthosilicate.

[0014] In this invention, regarding the addition of the alkali source and the template agent, tetrapropylammonium hydroxide can be selected as both the alkali source and the template agent. In this case, the tetrapropylammonium hydroxide in the system is split into two ions, wherein the tetrapropylammonium ion serves as the template agent and the hydroxide ion serves as the alkali source, and the ratio of the two ions is 1:1.

[0015] Preferably, the preparation method of the all-silica molecular sieve includes the following steps:

[0016] 1) Add the alkali source, template agent, and silicon source to deionized water and stir until uniformly mixed. Then, age at room temperature to obtain the directed gel.

[0017] 2) Add the alkali source, template agent, and silicon source to deionized water and stir until they are mixed evenly. Then add the directed gel prepared in step 1) to the mixed solution and stir until they are mixed evenly. Perform segmented crystallization. Raise the crystallization temperature to a certain temperature and crystallize for a certain time at a low heating rate. Then rapidly raise the crystallization temperature to crystallize. After filtration, washing, drying, and rapid heat treatment, calcination is performed to obtain the all-silica Silicalite-1 molecular sieve.

[0018] Preferably, the variable-temperature segmented crystallization process includes a first stage of low-temperature crystallization and a second stage of crystallization;

[0019] The heating rate of the first stage of low-temperature crystallization is 0.5-3℃ / min, the crystallization temperature is 50-90℃, and the crystallization time is 1-5h; the heating rate of the second stage of crystallization is 4-8℃ / min, the crystallization temperature is 100-200℃, and the crystallization time is 8-16h.

[0020] More preferably, the crystallization temperature of the first stage of low-temperature crystallization is 60-80℃, the heating rate is 0.5-2℃ / min, and the crystallization time is 2-4h.

[0021] More preferably, the crystallization temperature of the second crystallization stage is 110-170°C, and the crystallization time is 10-14 hours.

[0022] Preferably, the alkali source, template agent, and silicon source are added to water and mixed evenly to obtain the synthesis system. The molar ratio of the reactants in the synthesis system is alkali source: template agent: silicon source: water = 0.05~0.4: 0.05~0.3: 1: 10~100.

[0023] More preferably, the molar ratio of reactants in the synthesis system is alkali source: template agent: silicon source: water = 0.1~0.35: 0.05~0.2: 1: 20~100.

[0024] More preferably, the amount of the guiding gel added is 3 to 15% of the weight of the synthesis system.

[0025] More preferably, the amount of the guiding gel added is 5-10% of the weight of the synthesis system.

[0026] Preferably, when the alkali source, template agent, and silicon source are added to water and mixed evenly, the alkali source includes one or more of NaOH, KOH, ammonia, and tetrapropylammonium hydroxide; the template agent includes one or more of n-butylamine, triethylamine, tetrapropylammonium bromide, and tetrapropylammonium hydroxide; and the silicon source includes one or more of silica sol, water glass, tetraethyl orthosilicate, or silica.

[0027] This invention also adds TPA to the guiding gel. + The template agent, in which the original crystal nuclei of the molecular sieve induce the crystallization of the molecular sieve, can greatly reduce the TPA in the synthesis system. + The amount of template agent used, or even the use of inexpensive organic amines such as n-butylamine or triethylamine as template agents, can reduce costs.

[0028] More preferably, the silicon source is silica sol or tetraethyl orthosilicate.

[0029] Preferably, the roasting process includes one or more rapid heat treatment roasting processes;

[0030] Alternatively, the roasting process may include roasting via a rapid heat treatment process followed by a conventional high-temperature roasting process.

[0031] More preferably, the rapid heat treatment calcination process has a heating and cooling rate of 5-15℃ / s, a calcination temperature of 400-700℃, and a calcination time of 0.5-5min.

[0032] More preferably, the conventional high-temperature roasting process has a roasting temperature of 250-550℃ and a roasting time of 0.5-3h.

[0033] More preferably, the roasting process is 1-2 times of rapid heat treatment roasting, or 1 time of rapid heat treatment roasting followed by conventional high-temperature roasting.

[0034] More preferably, the rapid heat treatment calcination process has a heating and cooling rate of 6-10℃ / s, a calcination temperature of 500-700℃, and a calcination time of 1-3min.

[0035] More preferably, the conventional high-temperature roasting process has a roasting temperature of 300-500℃ and a roasting time of 1-2 hours.

[0036] This invention uses a rapid thermal calcination process to demold molecular sieves. The calcination process is highly efficient and low-energy, and the prepared Silicalite-1 molecular sieve has few defects in its framework and is enriched in micropores. It exhibits excellent performance in the selective separation of small molecule gases and the adsorption and purification of VOCs gases.

[0037] An all-silica molecular sieve, prepared by the above method, has a specific surface area higher than 340 m². 2 / g, total pore volume higher than 0.20cm³ 3 / g, microporous specific surface area higher than 285m² 2 / g, micropore volume higher than 0.14cm³ 3 / g, which can be used for the adsorption and separation of small molecule alkanes and aromatics.

[0038] The molecular sieve prepared by this invention has good dispersibility, high crystallinity, and low cost.

[0039] One application of the above-mentioned all-silica molecular sieve is to use the all-silica molecular sieve for the selective separation of small molecule gases and the adsorption and purification of VOCs gases.

[0040] Preferably, the all-silica molecular sieve is used for the adsorption and separation of small molecule alkanes and aromatics in high-humidity gases.

[0041] More preferably, the all-silica molecular sieve is formed into a granular or honeycomb-shaped adsorbent for adsorption and separation of small molecule alkanes and aromatics in a fixed bed or zeolite rotor.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. This invention utilizes a directed gel to provide a large number of initial crystal nuclei for the synthesis system during the synthesis process, which can improve the crystallization rate of the molecular sieve and shorten the crystallization time. Simultaneously, a template agent is added to the directed gel, and the initial crystal nuclei of the molecular sieve within it play an inductive role in the crystallization of the molecular sieve. This significantly reduces the amount of template agent required in the synthesis reaction solution, thereby lowering costs.

[0044] 2. The formation of molecular sieve crystal nuclei is due to the depolymerization and rearrangement of silicate ions in the gel system under alkaline conditions. Low temperatures are more conducive to the formation of crystal nuclei. A low heating rate can slow down the hydrolysis and condensation process, allowing for more complete formation of crystal nuclei, which is more conducive to the formation of uniform small crystals, while shortening the crystallization time. This invention utilizes variable-temperature segmented crystallization, employing a low heating rate during the low-temperature stage to prepare molecular sieve crystals with uniform size, small particle size, and uniform dispersion.

[0045] 3. Conventional high-temperature calcination often uses a relatively low heating rate to reduce cell shrinkage and framework collapse during the calcination process. This invention uses a faster heating rate to quickly decompose the template agent in the molecular sieve channels, which can effectively alleviate the framework defect problem generated during the calcination process. The pore structure characterization of the prepared molecular sieve by a physical adsorption instrument shows that it has few framework defects, is enriched in micropores, and exhibits excellent adsorption performance for small molecule alkanes and aromatics.

[0046] 4. This invention utilizes inexpensive n-butylamine or a small amount of TPA by adding a guiding gel. + Using a template agent, Silicalite-1 molecular sieves with high dispersibility and uniform size were rapidly prepared by temperature-segmented crystallization at a low temperature crystallization rate. At the same time, a rapid heat treatment process was used to effectively remove the template agent from the molecular sieve in a short time.

[0047] 5. This invention can utilize inexpensive n-butylamine, triethylamine, or a very small amount of TPA. + This invention enables the direct and rapid synthesis of all-silica Silicalite-1 molecular sieves with small particle size and high uniform dispersion using template agents. The preparation process is simple, uses inexpensive raw materials, and is cost-effective.

[0048] 6. The preparation process of this invention is simple, which greatly reduces the cost of template agents, shortens the crystallization time, and reduces the energy consumption of template agent calcination, thus realizing the low-cost preparation of Silicalite-1 molecular sieve.

[0049] 7. This invention provides a low-cost method for preparing all-silica molecular sieves by introducing TPA. + The template agent's directed gel and low-temperature crystallization stages employ a low heating rate and rapid heat treatment calcination process, using inexpensive raw materials. This has led to the development of a simple, efficient, and low-energy preparation method for Silicalite-1 molecular sieves. Furthermore, the pore structure characterization of the prepared Silicalite-1 molecular sieves by a physical adsorption instrument has demonstrated that it has few framework defects, rich micropores, and excellent adsorption and separation performance for small molecule alkanes and aromatics. Attached Figure Description

[0050] Figure 1 The XRD pattern of the all-silica molecular sieve prepared in Example 1;

[0051] Figure 2 The image shows the SEM pattern of the all-silica molecular sieve prepared in Example 1.

[0052] Figure 3 The N2 adsorption-desorption isotherm is shown for the all-silica molecular sieve prepared in Example 1. Detailed Implementation

[0053] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0054] In the following examples and comparative examples, the BET specific surface area of ​​the molecular sieve was determined by N2 adsorption using an ASAP2020 HD88 physical adsorption instrument.

[0055] The particle size and morphology of molecular sieves were analyzed using a Hitachi S-4800 cold field emission high-resolution scanning electron microscope from Japan, with an operating voltage of 2KV and a working distance of 5-10nm.

[0056] The prepared all-silica molecular sieve tablets were crushed into 20-30 mesh particles. The selective separation of small molecule gases and the adsorption and purification performance of VOCs were tested in a micro fixed bed device at room temperature and pressure. The two-component mixed gas of small molecule gases was at a concentration of 50% / 50%, and the concentration of VOCs was 500-1500 ppm.

[0057] With a 95% adsorption removal rate as the breakthrough point, the adsorption capacity is calculated using the following formula:

[0058]

[0059] In the formula: q is the equilibrium adsorption amount of the gas, g / g;

[0060] F represents the gas flow rate, in ml / min;

[0061] t is the adsorption time, in minutes;

[0062] C0 represents the concentration in the intake air, in ppm.

[0063] C i The concentration of the exhaust gas after i min of adsorption is in ppm.

[0064] m is the amount of adsorbent loaded, in grams;

[0065] t s ν is the time to adsorption equilibrium, in minutes.

[0066] The formula for calculating the separation coefficient is as follows:

[0067]

[0068] In the formula: f is the separation coefficient;

[0069] q1 is the adsorption amount of component 1 in the mixed gas, in g / g;

[0070] q2 represents the adsorption amount of component 2 in the gas mixture, in g / g;

[0071] The volume fraction of component 1 in the gas mixture, expressed as %.

[0072] The volume fraction of component 2 in the gas mixture, in %;

[0073] Example 1

[0074] Preparation of the directional gel: Weigh 0.2g NaOH and dissolve it in 59.82g deionized water. After stirring evenly, add 16.24g tetrapropylammonium hydroxide (25wt%) solution and stir evenly. Then add 24.0g silica sol (25wt%) and stir evenly. After aging at room temperature for 24h, the directional gel is obtained. The molar ratio of the gel composition is SiO2:NaOH:TPAOH:H2O=1:0.05:0.2:50.

[0075] Weigh 0.5g NaOH and dissolve it in 45g deionized water. After complete dissolution, add 1.66g tetrapropylammonium bromide and dissolve it completely. Then add 1.37g n-butylamine and dissolve it completely. Slowly add 30.0g silica sol (25wt%) and stir until homogeneous. Add 7.8g of the prepared directed gel and stir until homogeneous. The molar ratio of the system components is SiO2:TPABr:n-butylamine:NaOH:H2O = 1:0.05:0.15:0.1:30. The resulting mixture is placed in a hydrothermal reactor and heated to 80℃ at a heating rate of 1℃ / min for 3h. Then, the temperature is increased to 170℃ at a heating rate of 5℃ / min for 14h. After the reaction, filter, wash, and dry to obtain Silicalite-1 molecular sieve. Silicalite-1 molecular sieves were calcined at 600℃ for 1 min by heating at a rate of 10℃ / s, followed by rapid cooling to room temperature within 1 min, and then treated with a conventional calcination process at 350℃ for 2 h. The XRD pattern of the prepared all-silica Silicalite-1 molecular sieve is shown below. Figure 1 As shown, the SEM image is as follows Figure 2 As shown, the particle size is approximately 1.5 μm, and the crystals are uniformly dispersed. The N₂ adsorption-desorption isotherm is shown below. Figure 3 As shown, the specific surface area of ​​BET is 355.85 m².2 / g, with a microporous specific surface area of ​​310.56m². 2 / g, total pore volume is 0.22cm³ 3 / g, micropore volume is 0.18cm³ 3 / g. Adsorption properties are shown in Table 1.

[0076] Example 2

[0077] Preparation of the directional gel: Weigh 0.8g NaOH and dissolve it in 36.0g deionized water. After stirring evenly, add 13.31g tetrapropylammonium bromide. After complete dissolution, add 48.0g silica sol (25wt%). After stirring evenly, age at room temperature for 24h to obtain the directional gel. The molar ratio of the gel composition is SiO2:NaOH:TPABr:H2O=1:0.1:0.25:20.

[0078] Weigh 0.5g NaOH and dissolve it in 45g deionized water. After complete dissolution, add 1.83g n-butylamine. After complete dissolution, slowly add 30.0g silica sol (25wt%) and stir until homogeneous. Add 11.5g of the prepared directed gel and stir until homogeneous. The molar ratio of the system components is SiO2:n-butylamine:NaOH:H2O = 1:0.2:0.35:100. The resulting mixture is placed in a hydrothermal reactor and heated to 60℃ at a heating rate of 0.5℃ / min for 4 hours to crystallize. Then, the temperature is increased to 170℃ at a heating rate of 4℃ / min for 12 hours to crystallize. After the reaction, the mixture is filtered, washed, and dried to obtain Silicalite-1 molecular sieve. The Silicalite-1 molecular sieve is calcined at 500℃ at a heating rate of 8℃ / s for 1 minute, and then rapidly cooled to room temperature within 1 minute. This rapid heat treatment calcination is then repeated once more. The prepared all-silica Silicalite-1 molecular sieve had a particle size of approximately 1.0 μm and uniform crystal dispersion. Its BET specific surface area was 358.11 m². 2 / g, with a microporous specific surface area of ​​306.39m². 2 / g, total pore volume is 0.21cm³ 3 / g, micropore volume is 0.17cm³ 3 / g. Adsorption properties are shown in Table 1.

[0079] Example 3

[0080] Preparation of the guiding gel: The gel composition is as in Example 1, and the aging time is 32h.

[0081] Weigh 1.0 g NaOH and dissolve it in 45 g deionized water. After complete dissolution, add 3.33 g tetrapropylammonium bromide and dissolve it completely. Then add 2.53 g triethylamine and dissolve it completely. Slowly add 30.0 g silica sol (25 wt%) and stir until homogeneous. Add 8.0 g of the prepared directed gel and stir until homogeneous. The molar ratio of the system components is SiO2:TPABr:triethylamine:NaOH:H2O = 1:0.1:0.2:0.2:30. The resulting mixture is placed in a hydrothermal reactor and heated to 80 °C for 2 h at a heating rate of 0.5 °C / min. Then, it is heated to 200 °C for 10 h at a heating rate of 8 °C / min. After the reaction, filter, wash, and dry to obtain Silicalite-1 molecular sieve. Silicalite-1 molecular sieves were calcined at 650℃ for 1 min by heating at a rate of 10℃ / s, followed by rapid cooling to room temperature within 1 min, and then treated with a conventional calcination process at 500℃ for 1 h. The prepared all-silica Silicalite-1 molecular sieve had a particle size of approximately 1.8 μm and uniformly dispersed crystals. Its BET specific surface area was 345.85 m². 2 / g, with a microporous specific surface area of ​​290.39m². 2 / g, total pore volume is 0.20cm³ 3 / g, micropore volume is 0.15cm³ 3 / g. Adsorption properties are shown in Table 1.

[0082] Example 4

[0083] Preparation of the guiding gel: The gel composition is as in Example 1, and the aging time is 48h.

[0084] Weigh 0.75 g NaOH and dissolve it in 90 g deionized water. After complete dissolution, add 3.33 g tetrapropylammonium bromide and dissolve it completely. Then add 0.91 g n-butylamine and dissolve it completely. Slowly add 30.0 g silica sol (25 wt%) and stir until homogeneous. Add 8.1 g of the prepared directed gel and stir until homogeneous. The molar ratio of the system components is SiO2:TPABr:n-butylamine:NaOH:H2O = 1:0.1:0.1:0.15:50. The resulting mixture is placed in a hydrothermal reactor and heated to 80 °C at a heating rate of 1 °C / min for 3 h, then heated to 130 °C at a heating rate of 5 °C / min for 14 h. After the reaction, filter, wash, and dry to obtain Silicalite-1 molecular sieve. Silicalite-1 molecular sieves were calcined at 500℃ for 3 min at a heating rate of 6℃ / s, followed by rapid cooling to room temperature within 1 min, and then this rapid heat treatment calcination was repeated once. The prepared all-silica Silicalite-1 molecular sieve had a particle size of approximately 0.9 μm and uniform crystal dispersion. The BET specific surface area was 363.72 m². 2 / g, with a microporous specific surface area of ​​305.51m². 2 / g, total pore volume is 0.23cm³ 3 / g, micropore volume is 0.17cm³ 3 / g. Adsorption properties are shown in Table 1.

[0085] Example 5

[0086] Preparation of the directional gel: Weigh 20.31g of tetrapropylammonium hydroxide (25wt%) solution, dissolve it in 20.775g of deionized water, stir evenly, add 24.0g of silica sol (25wt%), stir evenly, and age at room temperature for 48h to obtain the directional gel. The molar ratio of the gel composition is SiO2:TPAOH:H2O=1:0.25:30.

[0087] Weigh 0.75 g NaOH and dissolve it in 22.5 g deionized water. After complete dissolution, add 1.66 g tetrapropylammonium bromide. After complete dissolution, slowly add 30.0 g silica sol (25 wt%) and stir until homogeneous. Add 5.5 g of the prepared directed gel and stir until homogeneous. The molar ratio of the system components is SiO2:TPABr:NaOH:H2O = 1:0.05:0.15:20. The resulting mixture is placed in a hydrothermal reactor and heated to 80 °C for 4 h at a heating rate of 0.5 °C / min. Then, it is heated to 110 °C for 14 h at a heating rate of 5 °C / min. After the reaction, the mixture is filtered, washed, and dried to obtain Silicalite-1 molecular sieve. The Silicalite-1 molecular sieve is calcined at 600 °C for 1 min at a heating rate of 10 °C / s, and then rapidly cooled to room temperature within 1 min. This rapid heat treatment calcination is then repeated once more. The prepared all-silica Silicalite-1 molecular sieve had a particle size of approximately 1.1 μm and uniform crystal dispersion. Its BET specific surface area was 360.02 m². 2 / g, with a microporous specific surface area of ​​301.25m². 2 / g, total pore volume is 0.22cm³ 3 / g, micropore volume is 0.17cm³ 3 / g. Adsorption properties are shown in Table 1.

[0088] Comparative Example 1

[0089] 25.38 g of tetrapropylammonium hydroxide was weighed and dissolved in 26 g of deionized water. After complete dissolution, 30.0 g of silica sol (25 wt%) was slowly added and stirred until homogeneous. The molar ratio of the system components was SiO2:TPAOH:H2O = 1:0.25:30. The resulting mixture was placed in a hydrothermal reactor and crystallized at 170 °C for 24 h. After the reaction, the mixture was filtered, washed, and dried to obtain Silicalite-1 molecular sieve. Then, it was calcined at 550 °C for 5.0 h to prepare all-silica Silicalite-1 molecular sieve with a particle size of approximately 1.5 μm and a BET specific surface area of ​​353.15 m². 2 / g, with a microporous specific surface area of ​​266.57m². 2 / g, total pore volume is 0.23cm³ 3 / g, micropore volume is 0.13cm³ 3 / g. Adsorption properties are shown in Table 1.

[0090] Comparative Example 2

[0091] Molecular sieve preparation was performed as in Comparative Example 1. Silicalite-1 molecular sieves were calcined at 600℃ for 1 min by heating at a rate of 10℃ / s, then rapidly cooled to room temperature within 1 min, and subsequently treated with a conventional calcination process at 350℃ for 2 h. All-silica Silicalite-1 molecular sieves were obtained, with a particle size of approximately 1.3 μm and uniform crystal dispersion. The BET specific surface area was 359.23 m². 2 / g, with a microporous specific surface area of ​​298.23m². 2 / g, total pore volume is 0.22cm³ 3 / g, micropore volume is 0.16cm³ 3 / g. Adsorption properties are shown in Table 1.

[0092] Comparative Example 3

[0093] The system for preparing the directed gel and molecular sieve was as described in Example 1. The resulting mixture was placed in a hydrothermal reactor and crystallized at 170°C for 24 hours. The subsequent calcination process was a conventional calcination at 550°C for 5.0 hours, yielding an all-silica Silicalite-1 molecular sieve with a particle size of approximately 2.3 μm and uniform crystal dispersion. The BET specific surface area was 350.27 m². 2 / g, with a microporous specific surface area of ​​261.63m². 2 / g, total pore volume is 0.22cm³ 3 / g, micropore volume is 0.13cm³ 3 / g. Adsorption properties are shown in Table 1.

[0094] The selective separation of CO2 / CH4 and C2H6 / CH4 and the adsorption and purification performance of VOCs gas in the all-silica molecular sieve samples of Examples 1-5 and Comparative Examples 1-3 were evaluated, and the results are shown in Table 1.

[0095] Table 1 Performance Evaluation Results

[0096]

[0097] It can be seen that the all-silica Silicalite-1 molecular sieve prepared by using a low-temperature-rate variable-temperature crystallization process assisted by a guided gel and a rapid heat treatment calcination process significantly improves the selective separation of small molecule gases and the adsorption and purification performance of VOCs. The all-silica molecular sieve preparation process of this invention is simple, greatly reduces raw material costs and process energy consumption, and improves product performance, showing good prospects for industrial application.

[0098] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing an all-silica molecular sieve, characterized in that, An alkali source, a template agent, and a silicon source are added to water and mixed evenly. Then, a directed gel containing a template agent is added to obtain a reaction mixture. The mixture is then subjected to variable-temperature segmented crystallization and calcined to obtain the all-silicon molecular sieve. The method for preparing the directed gel is as follows: adding an alkali source, a template agent, and a silicon source to water and stirring until the mixture is homogeneous, and then aging at room temperature to obtain the directed gel; The variable-temperature segmented crystallization process includes a first stage of low-temperature crystallization and a second stage of crystallization; The heating rate of the first stage of low-temperature crystallization is 0.5-3℃ / min, the crystallization temperature is 50-90℃, and the crystallization time is 0-5h; the heating rate of the second stage of crystallization is 4-8℃ / min, the crystallization temperature is 100-200℃, and the crystallization time is 8-16h. The roasting process includes one or more rapid heat treatment roasting processes. Alternatively, the roasting process may include roasting via a rapid heat treatment process followed by a conventional high-temperature roasting process. The rapid heat treatment calcination process has a heating and cooling rate of 5-15℃ / s, a calcination temperature of 400-700℃, and a calcination time of 0.5-5min. The conventional high-temperature roasting process involves a roasting temperature of 250-550℃ and a roasting time of 0.5-3 hours. The all-silica molecular sieve is an all-silica Silicalite-1 molecular sieve.

2. The method for preparing all-silica molecular sieves according to claim 1, characterized in that, The molar ratio of the components of the guiding gel is alkali source: template agent: silicon source: water = 0~0.3: 0.05~0.3: 1: 10~70, and the aging time is 12~48h.

3. The method for preparing all-silica molecular sieves according to claim 1, characterized in that, In the guiding gel, the alkaline source includes one or more of NaOH, KOH, ammonia, and tetrapropylammonium hydroxide; the template agent includes one or more of tetrapropylammonium bromide or tetrapropylammonium hydroxide; and the silicon source includes one or more of silica sol, water glass, tetraethyl orthosilicate, or silica.

4. The method for preparing all-silica molecular sieve according to claim 1, characterized in that, The alkali source, template agent, and silicon source were added to water and mixed evenly to obtain the synthesis system. The molar ratio of the reactants in the synthesis system was alkali source: template agent: silicon source: water = 0.05~0.4:0.05~0.3:1:10~100. The amount of the guiding gel added is 3-15% of the weight of the synthesis system.

5. The method for preparing all-silica molecular sieves according to claim 1, characterized in that, When the alkali source, template agent, and silicon source are added to water and mixed evenly, the alkali source includes one or more of NaOH, KOH, ammonia, and tetrapropylammonium hydroxide; the template agent includes one or more of n-butylamine, triethylamine, tetrapropylammonium bromide, and tetrapropylammonium hydroxide; and the silicon source includes one or more of silica sol, water glass, tetraethyl orthosilicate, or silica.

6. An all-silica molecular sieve, characterized in that, The all-silica molecular sieve prepared by any one of claims 1 to 5 has a specific surface area higher than 340 m². 2 / g, total pore volume higher than 0.20cm³ 3 / g, microporous specific surface area higher than 285m² 2 / g, micropore volume higher than 0.14cm³ 3 / g.

7. An application of the all-silica molecular sieve according to claim 6, characterized in that, The all-silicon molecular sieve is used for the selective separation of small molecule gases and the adsorption and purification of VOCs gases.

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