A low-impurity high-silica-alumina ratio mordenite molecular sieve and a preparation method thereof

Abstract

The application belongs to the technical field of inorganic nonmetallic porous material preparation, and discloses a low-impurity-crystal high-silicon-aluminum-ratio mordenite molecular sieve and a preparation method thereof. The method first prepares an initial silicon-aluminum gel from water, sodium hydroxide, sodium aluminate and a first part of silica sol, and pre-crystallizes in the presence of a mordenite guide seed liquid to obtain a semi-crystallized MOR core slurry containing MOR crystal nuclei, mother liquor and incompletely crystallized silicon-aluminum gel. Then, the sodium aluminate, a hydroxyl carboxylic acid salt aluminum source complexing agent and a second part of silica sol are prepared into a complexed silicon-aluminum feeding solution, which is added into the semi-crystallized MOR core slurry in batches and aged, and then subjected to continuous crystallization, solid-liquid separation, washing, drying and calcination to obtain the low-impurity-crystal high-silicon-aluminum-ratio mordenite molecular sieve. The method is conducive to inhibiting the generation of ZSM-5 impurity crystals and quartz impurities, and improving the crystal phase purity and silicon-aluminum distribution uniformity of the high-silicon-aluminum-ratio MOR product.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic porous material preparation technology, specifically relating to a low heterocrystal, high silica-to-alumina ratio mordenite molecular sieve and its preparation method. Background Technology

[0002] Mordenite molecular sieves are a type of crystalline aluminosilicate molecular sieve with a regular pore structure, often referred to as MOR-type molecular sieves, where MOR is the framework structure code for mordenite. These molecular sieves possess a one-dimensional main channel structure, good thermal stability, and tunable acidity, making them highly valuable for applications in catalytic cracking, alkylation, isomerization, adsorption separation, and ion exchange. With increasing demands for hydrothermal stability, selectivity, and service life in industrial catalysis and adsorption processes, high-silica-to-alumina ratio MOR-type mordenite molecular sieves have attracted more attention. However, during hydrothermal synthesis, as the silicon content increases, the polymerization, nucleation, and crystal growth of silicon-alumina species become more difficult to coordinate, easily leading to problems such as uneven crystallization, insufficient crystallinity, localized silicon enrichment, and the formation of secondary crystalline phases.

[0003] In the prior art, CN102602958B discloses a method for preparing mesoporous mordenite zeolite, which obtains mordenite molecular sieves by hydrothermal crystallization after mixing silicon source, aluminum source, alkali source and dealugenized mordenite seed crystals; CN114349020A discloses a high-silica mordenite zeolite and its preparation method, which obtains hydrophobic MOR zeolite by acid treatment, high-temperature steam calcination, and re-acid treatment using Na-MOR as raw material. The above technologies improve the performance of mordenite zeolite from the perspectives of seed-guided synthesis and post-treatment modification, respectively, but still have shortcomings: the former is still difficult to effectively coordinate nucleation, silicon-aluminum species rearrangement and crystal growth in high silicon-aluminum ratio systems, and is easily affected by local alkalinity, viscosity and silicon-aluminum ratio distribution during scale-up preparation, resulting in ZSM-5 impurities or silica-like secondary phases; the latter mainly relies on post-treatment dealugenization or surface conditioning, which has a long process and may cause framework defects, non-framework aluminum residues or uneven acidity distribution. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a low-impregnation, high-silica-to-alumina ratio mordenite zeolite molecular sieve and its preparation method. This method first constructs an initial silica-alumina gel suitable for MOR nucleation, and pre-crystallizes it under the action of a mordenite-guided seed solution to form a semi-crystallized MOR nucleus paste. Subsequently, a complexed silica-alumina feed solution containing a hydroxycarboxylate-based aluminum source complexing agent is introduced. Through batch feeding, aging, and further crystallization, a high-silica-to-alumina ratio mordenite zeolite molecular sieve is obtained. This method can reduce the risk of ZSM-5 impurities and quartz impurities in high-silica systems, and improve the crystal phase purity and silica-alumina distribution uniformity of the product.

[0005] A method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve includes the following steps: S1. Water, sodium hydroxide, sodium aluminate and the first part of silica sol are mixed to obtain an initial silica-alumina gel. Mordenite seed crystal solution is added to the initial silica-alumina gel for pre-crystallization to obtain a semi-crystallized MOR core slurry that shows the characteristic peak of mordenite MOR as detected by XRD. The semi-crystallized MOR core slurry is a slurry system containing MOR crystallized solid phase, mother liquor and incompletely crystallized silica-alumina species formed by pre-crystallization. S2. Sodium aluminate, hydroxycarboxylate aluminum source complexing agent and water are mixed and subjected to complexation and dispersion treatment to obtain complexed aluminum source solution. The complexed aluminum source solution is mixed with the second part of silica sol to obtain complexed silicon-aluminum feed solution. S3. The complexed silicon-aluminum feed solution is added to the semi-crystallized MOR core slurry in batches, and each batch is aged after addition to obtain a high silicon-aluminum ratio MOR crystallization precursor slurry. S4. The high silicon-to-alumina ratio MOR crystallization precursor slurry is crystallized. After crystallization, it is subjected to solid-liquid separation, washing, drying and calcination to obtain a low impurity crystal high silicon-to-alumina ratio mordenite molecular sieve. The semi-crystallized MOR core paste is not subjected to solid-liquid separation, washing, or drying before entering step S3.

[0006] More preferably, in step S1, the initial silica-alumina gel has a SiO2 / Al2O3 molar ratio of 23 to 28 based on oxides, and the SiO2 contained in the first part of the silica sol accounts for 35% to 45% of the total SiO2 contained in the first part of the silica sol and the second part of the silica sol.

[0007] More preferably, in step S2, the molar ratio of SiO2 / Al2O3 in the complexed silicon-aluminum feed solution, calculated as oxides, is 120-150; the molar composition of the high silicon-aluminum ratio MOR crystallization precursor slurry obtained after feeding in step S3 satisfies: a SiO2 / Al2O3 molar ratio of 40-60, a Na2O / SiO2 molar ratio of 0.08-0.18, and a H2O / SiO2 molar ratio of 12-25.

[0008] More preferably, the mordenite seed solution is prepared by dispersing mordenite seed powder in an aqueous sodium hydroxide solution, wherein the molar ratio of SiO2 / Al2O3 of the mordenite seed powder (calculated as oxides) is 10-25, the concentration of the aqueous sodium hydroxide solution is 0.05-0.50 mol / L, and the mass of the mordenite seed powder accounts for 5-20 wt% of the total mass of the mordenite seed solution.

[0009] More preferably, the silicalite seed powder does not have the ZSM-5 crystal phase characteristic peak as detected by XRD.

[0010] More preferably, in step S1, the pre-crystallization temperature is 125–145°C, and the pre-crystallization time is 4–10 h; the relative crystallinity of the MOR obtained by sampling the semi-crystallized MOR core slurry is 8%–35% as determined by XRD peak height method.

[0011] More preferably, in step S2, the hydroxycarboxylate aluminum source complexing agent is either sodium gluconate or sodium citrate, and the ratio of the amount of the hydroxycarboxylate aluminum source complexing agent to the amount of aluminum source in the complexed silicon-aluminum feed solution converted to Al2O3 is 0.05 to 0.35:1. The sodium aluminate, hydroxycarboxylate aluminum source complexing agent, and water are stirred at 50 to 80°C for 20 to 60 minutes for complexation and dispersion treatment.

[0012] More preferably, in step S3, the complexed silicon-aluminum feed solution is added to the semi-crystallized MOR core paste in 3 to 6 batches under stirring conditions. The sum of the amounts added in each batch is the total amount of the complexed silicon-aluminum feed solution. After each batch of complexed silicon-aluminum feed solution is added, it is aged at 120 to 145°C for 20 to 60 minutes.

[0013] A low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve prepared using the preparation method described above.

[0014] More preferably, the low heterocrystal, high silica-to-alumina ratio mordenite molecular sieve has a SiO2 / Al2O3 molar ratio of 25–45 as determined by X-ray fluorescence spectroscopy, a relative crystallinity of MOR relative to the standard mordenite reference sample as determined by XRD peak height method of not less than 85%, a ZSM-5 heterocrystal peak height not exceeding 3% of the peak height of the main characteristic peak of MOR, and a crystalline silica secondary phase characteristic peak height not exceeding 2% of the peak height of the main characteristic peak of MOR; the ratio of the outer surface Si / Al molar ratio determined by X-ray photoelectron spectroscopy to the bulk Si / Al molar ratio determined by X-ray fluorescence spectroscopy is 0.85–1.15.

[0015] The beneficial effects of this invention are: This invention employs a continuous control method—"low silica-alumina ratio pre-nucleation—complexed silica-alumina feeding—batch aging and recombination—continued crystallization"—to achieve staged control over the nucleation and crystal growth process of high silica-alumina ratio mordenite molecular sieves. Compared to methods that directly prepare high silica-alumina ratio gels and crystallize them in one step, this invention first introduces a mordenite seed solution, confirmed to be of pure phase, into the initial silica-alumina gel, which is more suitable for the formation of the mordenite MOR crystal phase. This allows the system to form MOR dominant crystal nuclei in the early stages, reducing the possibility of competition for nucleation by impurities such as ZSM-5 in the high silica system. A complexed aluminum source solution is pre-formed using sodium aluminate and hydroxycarboxylate-based aluminum source complexing agents, and then combined with the second part of silica sol to form a complexed silica-alumina feeding solution. This ensures that the added high silica material still contains aluminum species that can participate in framework rearrangement, avoiding the precipitation of silica or quartz impurities due to localized silica enrichment. Furthermore, in this invention, the complexed silica-alumina feed solution is added to the semi-crystallized MOR nucleus slurry in batches, and aging is performed after each batch. This allows the added silica-alumina species to gradually disperse, recombine, and participate in subsequent crystallization growth in an environment where existing MOR nuclei and incompletely crystallized silica-alumina gel coexist, rather than instantaneously forming localized oversaturated regions in the system. Thus, the various steps of this invention form an interconnected crystal phase guidance and material recombination relationship, which can increase the silica-alumina ratio of the product while suppressing the formation of ZSM-5 impurities and quartz impurities, resulting in mordenite molecular sieves with high crystallinity, low impurity levels, and a relatively uniform distribution of silica-alumina on the outer surface and in the bulk phase. Detailed Implementation

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] In the following examples and comparative examples, the silicon source is industrial silica sol, calculated based on a SiO2 content of 30 wt%; the aluminum source is sodium aluminate, calculated based on the effective NaAlO2 component; the alkali source is sodium hydroxide, calculated based on pure product; the water is deionized water; and the hydroxycarboxylate aluminum source complexing agent is anhydrous sodium gluconate or anhydrous sodium citrate. If industrial raw materials with different effective contents are used, adjustments are made according to their effective component content. The mordenite guided seed solution is prepared by dispersing mordenite seed powder (without detected ZSM-5 characteristic peaks by XRD) in a 0.20 mol / L sodium hydroxide aqueous solution, with the seed powder accounting for 10 wt% of the total mass of the seed solution. In the following examples and comparative examples, the gel molar composition is calculated based on a total SiO2 provided by the silica sol of 10 mol; the seed powder is not included in the main silica-alumina gel molar composition, and the water and sodium hydroxide in the seed solution are calculated as H2O / SiO2 and Na2O / SiO2 based on the actual amount added. Unless otherwise specified, the washing was performed until the pH of the filtrate was 8.5–9.5, the drying conditions were 110℃ for 12 hours, and the calcination conditions were 580℃ for 4 hours.

[0018] Example 1: This example provides a method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve, comprising the following steps: S1. Add 579.4 g of deionized water and 43.94 g of sodium hydroxide to a reactor equipped with a stirrer. Stir until completely dissolved, then slowly add 32.07 g of sodium aluminate and continue stirring until dissolved. Subsequently, under stirring conditions, slowly add 901.3 g of the first part of silica sol to obtain the initial silica-alumina gel. The first part of silica sol provides 4.5 mol of SiO2, and the initial silica-alumina gel has a SiO2 / Al2O3 molar ratio of 23 (based on oxides).

[0019] 81.1 g of the aforementioned mordenite seed solution was added to the initial silica-alumina gel, and after stirring for 30 min, the mixture was pre-crystallized at 125 °C for 4 h in a closed hydrothermal reactor to obtain a semi-crystallized MOR core slurry. After pre-crystallization, a small amount of slurry sample was taken, and after solid-liquid separation, washing, and drying at 110 °C, the relative crystallinity of the solid-phase MOR was determined to be 10.3% by XRD peak height method. The semi-crystallized MOR core slurry was not subjected to solid-liquid separation, washing, or drying before entering the subsequent feeding step.

[0020] S2. Mix 7.51g sodium aluminate, 0.50g sodium gluconate, and 100.0g deionized water, and stir at 50℃ for 20min to obtain a complexed aluminum source solution. Then, mix the complexed aluminum source solution with 1101.5g of the second part of silica sol to obtain a complexed silicon-aluminum feed solution. The second part of silica sol provides 5.5mol of SiO2. The SiO2 / Al2O3 molar ratio of the complexed silicon-aluminum feed solution (calculated as oxides) is 120, and the ratio of the amount of sodium gluconate to the amount of aluminum source in the complexed silicon-aluminum feed solution converted to Al2O3 is 0.05:1.

[0021] S3. Adjust the temperature of the semi-crystallized MOR core slurry obtained in step S1 to 120℃. In a closed hydrothermal reactor equipped with a stirrer and a pressure-resistant feeding port, add the complexed silicon-aluminum feed solution obtained in step S2 to the semi-crystallized MOR core slurry in three batches, with the same amount added in each batch. After each batch is added, age at 120℃ for 20 minutes to obtain a high silicon-aluminum ratio MOR crystallization precursor slurry. After feeding, the molar composition of the high silicon-aluminum ratio MOR crystallization precursor slurry satisfies the following conditions: SiO2 / Al2O3 molar ratio is 41.4, Na2O / SiO2 molar ratio is 0.08, and H2O / SiO2 molar ratio is 12.

[0022] S4. The high silica-to-alumina ratio MOR crystallization precursor slurry is heated to 160°C and crystallized for another 18 hours. After crystallization, it is cooled to room temperature, and then subjected to solid-liquid separation, washing, drying, and calcination to obtain a low-impurity, high silica-to-alumina ratio mordenite zeolite molecular sieve.

[0023] Example 2: This example provides a method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve, comprising the following steps: S1. Add 590.0 g of deionized water and 40.00 g of sodium hydroxide to a reactor equipped with a stirrer. Stir until completely dissolved, then slowly add 20.49 g of sodium aluminate and continue stirring until dissolved. Subsequently, slowly add 701.0 g of the first part of silica sol under stirring conditions to obtain the initial silica-alumina gel. The first part of silica sol provides 3.5 mol of SiO2, and the initial silica-alumina gel has a SiO2 / Al2O3 molar ratio of 28 (based on oxides).

[0024] 63.1 g of the aforementioned mordenite seed solution was added to the initial silica-alumina gel, and after stirring for 30 min, the mixture was pre-crystallized at 145 °C for 10 h in a closed hydrothermal reactor to obtain a semi-crystallized MOR core slurry. After pre-crystallization, a small amount of slurry sample was taken, and after solid-liquid separation, washing, and drying at 110 °C, the relative crystallinity of the solid-phase MOR was determined to be 33.2% by XRD peak height method. The semi-crystallized MOR core slurry was not subjected to solid-liquid separation, washing, or drying before entering the subsequent feeding step.

[0025] S2. Add 88.26g of sodium hydroxide to 2450.0g of deionized water, stir to dissolve, then add 7.10g of sodium aluminate and 3.91g of anhydrous sodium citrate, and stir at 80℃ for 60min to obtain a complexed aluminum source solution; subsequently, mix the complexed aluminum source solution with 1301.8g of the second part of silica sol to obtain a complexed silicon-aluminum feed solution. The second part of silica sol provides 6.5mol of SiO2, the SiO2 / Al2O3 molar ratio of the complexed silicon-aluminum feed solution (calculated as oxides) is 150, and the ratio of the amount of sodium citrate to the amount of aluminum source in the complexed silicon-aluminum feed solution converted to Al2O3 is 0.35:1.

[0026] S3. Adjust the temperature of the semi-crystallized MOR core slurry obtained in step S1 to 145℃. In a closed hydrothermal reactor equipped with a stirrer and a pressure-resistant feeding port, add the complexed silicon-aluminum feed solution obtained in step S2 to the semi-crystallized MOR core slurry in 6 batches, with the same amount added in each batch. After each batch is added, age at 145℃ for 60 minutes to obtain a high silicon-aluminum ratio MOR crystallization precursor slurry. After feeding, the molar composition of the high silicon-aluminum ratio MOR crystallization precursor slurry satisfies the following conditions: SiO2 / Al2O3 molar ratio is 59.4, Na2O / SiO2 molar ratio is 0.18, and H2O / SiO2 molar ratio is 25.

[0027] S4. The high silica-to-alumina ratio MOR crystallization precursor slurry is heated to 168°C and crystallized for another 30 hours. After crystallization, it is cooled to room temperature, and then subjected to solid-liquid separation, washing, drying, and calcination to obtain a low-impurity, high silica-to-alumina ratio mordenite zeolite molecular sieve.

[0028] Example 3: This example provides a method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve, comprising the following steps: S1. Add 650.0 g of deionized water and 35.00 g of sodium hydroxide to a reactor equipped with a stirrer. Stir until completely dissolved, then slowly add 26.23 g of sodium aluminate and continue stirring until dissolved. Subsequently, under stirring conditions, slowly add 801.1 g of the first part of silica sol to obtain the initial silica-alumina gel. The first part of silica sol provides 4.0 mol of SiO2, and the initial silica-alumina gel has a SiO2 / Al2O3 molar ratio of 25 (based on oxides).

[0029] 72.1 g of the aforementioned mordenite seed solution was added to the initial silica-alumina gel, and after stirring for 30 min, the mixture was pre-crystallized at 135 °C for 7 h in a closed hydrothermal reactor to obtain a semi-crystallized MOR core slurry. After pre-crystallization, a small amount of slurry sample was taken, and after solid-liquid separation, washing, and drying at 110 °C, the relative crystallinity of the solid-phase MOR was measured to be 22.4% using the XRD peak height method. The semi-crystallized MOR core slurry was not subjected to solid-liquid separation, washing, or drying before proceeding to the subsequent feeding steps.

[0030] S2. Add 51.76g of sodium hydroxide to 1126.4g of deionized water, stir to dissolve, then add 7.29g of sodium aluminate and 1.94g of anhydrous sodium gluconate, and stir at 65℃ for 40min to obtain a complexed aluminum source solution. Then mix the complexed aluminum source solution with 1201.7g of the second part of silica sol, and continue stirring for 20min to obtain a complexed silicon-aluminum feed solution. The obtained complexed silicon-aluminum feed solution should be prepared and used immediately. The second part of silica sol provides 6.0mol of SiO2. The SiO2 / Al2O3 molar ratio of the complexed silicon-aluminum feed solution (calculated as oxides) is 135, and the ratio of the amount of sodium gluconate to the amount of aluminum source in the complexed silicon-aluminum feed solution converted to Al2O3 is 0.20:1.

[0031] S3. Adjust the temperature of the semi-crystallized MOR core slurry obtained in step S1 to 135℃. In a closed hydrothermal reactor equipped with a stirrer and a pressure-resistant feeding port, add the complexed silicon-aluminum feed solution obtained in step S2 to the semi-crystallized MOR core slurry in four batches, with the same amount added in each batch. After each batch is added, age at 135℃ for 40 minutes to obtain a high silicon-aluminum ratio MOR crystallization precursor slurry. After feeding, the molar composition of the high silicon-aluminum ratio MOR crystallization precursor slurry satisfies the following conditions: SiO2 / Al2O3 molar ratio is 48.9, Na2O / SiO2 molar ratio is 0.13, and H2O / SiO2 molar ratio is 18.

[0032] S4. The high silica-to-alumina ratio MOR crystallization precursor slurry is heated to 165°C and crystallized for another 24 hours. After crystallization, it is cooled to room temperature, and then subjected to solid-liquid separation, washing, drying, and calcination to obtain a low-impurity, high silica-to-alumina ratio mordenite zeolite molecular sieve.

[0033] Comparative Example 1: The difference between this comparative example and Example 3 is that: semi-crystallized MOR core paste is not prepared, complexed silica-alumina feed solution is not prepared, and batch feeding and post-feeding aging are not performed; the total amount of other raw materials, the final gel molar composition and post-treatment conditions are consistent with those of Example 3.

[0034] Specifically, 1776.4 g of deionized water and 86.76 g of sodium hydroxide were added to a reactor equipped with a stirrer. After stirring and dissolving, 33.52 g of sodium aluminate and 1.94 g of anhydrous sodium gluconate were added, and stirring was continued until uniformly dispersed. Subsequently, 2002.8 g of silica sol was slowly added under stirring conditions, followed by 72.1 g of mordenite seed crystal solution. Stirring was continued for 30 min to obtain a high silica-to-alumina ratio MOR crystallization precursor slurry. In the high silica-to-alumina ratio MOR crystallization precursor slurry, the silica sol provided 10 mol of SiO2, the SiO2 / Al2O3 molar ratio was 48.9, the Na2O / SiO2 molar ratio was 0.13, and the H2O / SiO2 molar ratio was 18.

[0035] The obtained high silica-to-alumina ratio MOR crystallization precursor slurry was transferred into a closed hydrothermal reactor and directly heated to 165℃ for crystallization for 24 hours. After crystallization, it was cooled to room temperature, separated into solid and liquid phases, and washed, dried, and calcined under the general conditions described above to obtain the mordenite molecular sieve sample of Comparative Example 1.

[0036] Comparative Example 2: Except for extending the pre-crystallization process in step S1 until the low-silicon MOR is basically completely crystallized, and then redispersing it after solid-liquid separation and washing, the composition of the feed liquid, the feeding method, the final crystallization conditions and the post-treatment conditions of this comparative example are the same as those of Example 3.

[0037] Specifically, the initial silica-alumina gel was prepared according to the raw material dosage and feeding method in step S1 of Example 3, and mordenite seed crystal solution was added. The difference is that this comparative example did not terminate after 7 hours of pre-crystallization, but continued crystallization at 135°C for 24 hours to obtain a low-silicon MOR crystallization slurry. After crystallization, samples were taken, and after solid-liquid separation, washing, and drying at 110°C, the relative crystallinity of the solid-phase MOR was measured to be 87.6% by XRD peak height method. Subsequently, the obtained low-silicon MOR crystallization slurry was subjected to solid-liquid separation and washed under the above general conditions to obtain a low-silicon MOR filter cake.

[0038] The obtained low-silicon MOR filter cake was re-added to the reactor after being converted to dry weight. Deionized water and sodium hydroxide aqueous solution were added for dispersion, so that the amount of SiO2 provided by the low-silicon MOR solid phase in the redispersed slurry was 4.0 mol. The molar ratios of SiO2 / Al2O3, Na2O / SiO2, and H2O / SiO2 in the total system after subsequent feeding were 48.9, 0.13, and 18, respectively, resulting in a low-silicon MOR redispersed slurry. Subsequently, a complexed silica-alumina feed solution was prepared according to step S2 of Example 3, and the complexed silica-alumina feed solution was added to the low-silicon MOR redispersed slurry in four batches according to step S3 of Example 3. After each batch was added, the slurry was aged at 135°C for 40 min. After feeding, the resulting high-silicon-alumina ratio MOR crystallization precursor slurry had a SiO2 / Al2O3 molar ratio of 48.9, a Na2O / SiO2 molar ratio of 0.13, and an H2O / SiO2 molar ratio of 18.

[0039] Finally, crystallization and post-treatment were carried out according to the crystallization conditions in step S4 of Example 3 to obtain the mordenite molecular sieve sample of Comparative Example 2.

[0040] Comparative Example 3: Except for step S2, in which anhydrous sodium gluconate was not added and a complexed aluminum source solution was not formed, the initial gel composition, pre-crystallization conditions, silicon-aluminum ratio of the feed solution, feeding method, final crystallization conditions and post-treatment conditions of this comparative example were all consistent with those of Example 3.

[0041] Specifically, semi-crystallized MOR core paste was prepared according to step S1 of Example 3. In step S2, 52.12g of sodium hydroxide was added to 1126.4g of deionized water, stirred and dissolved, and then 7.29g of sodium aluminate was added. The mixture was stirred at 65°C for 40 min to obtain an aluminum source dispersion. Subsequently, the aluminum source dispersion was mixed with 1201.7g of the second part of silica sol, and stirred for another 20 min to obtain a common silica-alumina feed solution. The common silica-alumina feed solution was prepared and used immediately. The second part of silica sol provided 6.0 mol of SiO2, and the SiO2 / Al2O3 molar ratio of the common silica-alumina feed solution (based on oxides) was 135.

[0042] Subsequently, following step S3 of Example 3, the ordinary silica-alumina feed solution was added to the semi-crystallized MOR core slurry in four batches. After each batch was added, the slurry was aged at 135°C for 40 minutes. After the feed was completed, the resulting high silica-alumina ratio MOR crystallization precursor slurry had a SiO2 / Al2O3 molar ratio of 48.9, a Na2O / SiO2 molar ratio of 0.13, and a H2O / SiO2 molar ratio of 18. Finally, crystallization and post-treatment were performed according to step S4 of Example 3 to obtain the mordenite molecular sieve sample of Comparative Example 3.

[0043] Comparative Example 4: Except for step S3, in which the complexed silicon-aluminum feed solution is added all at once and no batch feeding or aging after each batch feeding is performed, the preparation conditions of the semi-crystallized MOR nucleus paste, the composition of the complexed silicon-aluminum feed solution, the final crystallization conditions and the post-treatment conditions are the same as those in Example 3.

[0044] Specifically, a semi-crystallized MOR core slurry was prepared according to step S1 of Example 3, and a complexed silicon-aluminum feed solution was prepared according to step S2 of Example 3. The temperature of the semi-crystallized MOR core slurry was then adjusted to 135°C. In a closed hydrothermal reactor equipped with a stirrer and a pressure-resistant feed port, all the complexed silicon-aluminum feed solution was added to the semi-crystallized MOR core slurry at once. After addition, stirring was continued for 10 minutes. No batch feeding or aging after feeding was performed, resulting in a high silicon-aluminum ratio MOR crystallization precursor slurry.

[0045] After feeding, the resulting high-silicon-to-alumina MOR crystallization precursor slurry had a SiO2 / Al2O3 molar ratio of 48.9, a Na2O / SiO2 molar ratio of 0.13, and a H2O / SiO2 molar ratio of 18. Subsequently, crystallization and post-treatment were performed according to the crystallization conditions in step S4 of Example 3 to obtain the mordenite molecular sieve sample of Comparative Example 4.

[0046] Performance testing To verify the crystal phase purity, high silica-to-alumina ratio characteristics, MOR crystallinity, and uniformity of silica-alumina distribution of the low heteromorphic high silica-to-alumina ratio mordenite molecular sieve obtained in this invention, XRD, XRF, and XPS tests were performed on the samples obtained in Examples 1-3 and Comparative Examples 1-4. Unless otherwise specified, all test results are the average of three parallel test results.

[0047] 1. XRD phase analysis and impurity crystal peak intensity ratio test The samples obtained in Examples 1-3 and Comparative Examples 1-4 were dried to constant weight at 110°C, ground, and passed through a 200-mesh sieve. Crystal phase analysis was performed using a powder X-ray diffractometer. The test conditions were: Cu Kα rays, tube voltage 40 kV, tube current 40 mA, scanning range 2θ = 5°–40°, and scanning step size 0.02°. XRD tests were performed according to JY / T 0587-2020 "General Rules for Polycrystalline X-ray Diffraction Methods".

[0048] The MOR characteristic peak of the standard mordenite zeolite reference sample was used as the peak position reference; the characteristic peak of the ZSM-5 reference sample with less overlap with the MOR main crystalline phase peak was used as the basis for judging ZSM-5 impurities; and the quartz characteristic peak at 2θ=26.6°±0.2° was used as the basis for judging quartz impurities. After background subtraction of the XRD patterns of each sample, the peak heights of the MOR main characteristic peak, the ZSM-5 impurity characteristic peak, and the quartz impurity peak were recorded. The MOR main characteristic peak was selected from the standard mordenite zeolite reference sample with clear peak shape, high intensity, and less overlap with the ZSM-5 and quartz impurity peaks. When the quartz characteristic peak overlapped with the adjacent MOR diffraction peak, the peak shape fitting or the peak contribution of the standard mordenite zeolite reference sample at the same position was used to calculate the intensity ratio of the quartz impurity peak.

[0049] 2. MOR relative crystallinity test The relative crystallinity of MOR was calculated using the XRD peak height method. A standard mordenite reference sample was selected, and the sum of its MOR characteristic peak heights was measured under the same XRD testing conditions as the sample to be tested. This sum of peak heights was used as the 100% baseline. After testing the example and comparative samples under the same conditions, the same MOR characteristic peaks were selected, and their peak heights were summed.

[0050] The relative crystallinity of MOR is calculated using the following formula: MOR relative crystallinity / % = Sum of peak heights of MOR characteristic peaks of sample / Sum of peak heights of MOR characteristic peaks of standard mordenite reference sample × 100%.

[0051] 3. Product SiO2 / Al2O3 molar ratio test X-ray fluorescence spectrometry (XRF) was used to determine the SiO2 and Al2O3 contents in the samples. Before testing, the samples were dried to constant weight at 110℃, ground and mixed thoroughly, and then prepared using a pellet method. Calibration curves for Si and Al elements were established before testing. XRF testing was performed according to the general requirements for XRF spectrometry in GB / T 16597-2019, "General Rules for X-ray Fluorescence Spectrometry in Metallurgical Products".

[0052] The SiO2 / Al2O3 molar ratio of the product is calculated using the following formula: SiO2 / Al2O3 molar ratio = (SiO2 mass percentage / 60.08) / (Al2O3 mass percentage / 101.96).

[0053] 4. Test of Si / Al ratio on outer surface and Si / Al ratio in bulk phase The Si / Al atomic ratio on the outer surface of the samples was determined using X-ray photoelectron spectroscopy (XPS). Before testing, the samples were dried at 110℃, pressed into pellets, and placed on the sample stage for testing under vacuum. Al Kα rays were used as the excitation source to acquire the full spectrum and high-resolution spectra of Si 2p and Al 2p. The binding energy was corrected using C 1s = 284.8 eV, and the Si / Al atomic ratio on the outer surface was calculated based on the peak area and sensitivity factor. XPS testing was performed according to GB / T 19500-2025 "General Rules for X-ray Photoelectron Spectroscopy Analysis of Surface Chemical Analysis". The bulk Si / Al atomic ratio was converted from the SiO2 / Al2O3 molar ratio measured by XRF. Bulk Si / Al atomic ratio = SiO2 / Al2O3 molar ratio / 2.

[0054] The ratio of Si / Al on the outer surface to Si / Al in the bulk phase is calculated using the following formula: The ratio of Si / Al on the outer surface to Si / Al in the bulk phase = the atomic ratio of Si / Al on the outer surface measured by XPS / the atomic ratio of Si / Al in the bulk phase converted by XRF.

[0055] The closer the Si / Al ratio of the outer surface to the Si / Al ratio of the bulk phase is to 1, the less obvious the silicon-rich or aluminum-poor phenomenon appears on the outer surface of the sample relative to the bulk phase. This can be used to evaluate whether there is obvious silicon-rich or aluminum-poor phenomenon on the sample surface.

[0056] The results are shown in Table 1 below.

[0057] Table 1 Performance test results of the samples obtained from the examples and comparative examples

[0058] As shown in Table 1, the SiO2 / Al2O3 molar ratios of the samples obtained in Examples 1-3 ranged from 31.6 to 43.2, indicating that the present invention can obtain high Si / Al ratio mordenite molecular sieves. Meanwhile, the relative crystallinity of MOR in Examples 1-3 was all higher than 88%, the peak intensity ratio of ZSM-5 impurity crystals and the peak intensity ratio of quartz impurity phases were low, and the Si / Al ratio of the outer surface to the bulk phase was close to 1. This indicates that the present invention first establishes dominant MOR crystal nuclei through semi-crystallized MOR nuclei, and then uses complexed Si / Al feed liquid to add and age the feed in batches, so that the added Si / Al species are gradually dispersed, reorganized and participate in the growth of MOR crystals, thereby reducing the risk of local silicon enrichment and competitive crystal phase formation. Compared with Example 3, Comparative Example 1 showed a significant increase in impurity peaks and quartz impurity phases during direct crystallization with a single feed, indicating that direct crystallization of the high-silicon system is prone to crystallization runaway. Comparative Example 2 showed an increase in the Si / Al ratio on the outer surface to the Si / Al ratio in the bulk phase after complete crystallization, indicating that the added species are more easily enriched on the surface. Comparative Example 3 showed an increase in quartz impurity phases without the addition of a complexing agent, indicating that the complexing agent helps disperse and slowly release aluminum species. Comparative Example 4 showed an increase in impurity phases during a single feed, indicating that batch feeding and aging play an important role in stabilizing the crystallization process of high-silicon MOR.

[0059] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0060] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve, characterized in that, Includes the following steps: S1. Water, sodium hydroxide, sodium aluminate and the first part of silica sol are mixed to obtain an initial silica-alumina gel. Mordenite seed crystal solution is added to the initial silica-alumina gel for pre-crystallization to obtain a semi-crystallized MOR core slurry that shows the characteristic peak of mordenite MOR as detected by XRD. The semi-crystallized MOR core slurry is a slurry system containing MOR crystallized solid phase, mother liquor and incompletely crystallized silica-alumina species formed by pre-crystallization. S2. Sodium aluminate, hydroxycarboxylate aluminum source complexing agent and water are mixed and subjected to complexation and dispersion treatment to obtain complexed aluminum source liquid. The complexed aluminum source liquid is mixed with the second part of silica sol to obtain complexed silicon-aluminum feed liquid. S3. The complexed silicon-aluminum feed solution is added to the semi-crystallized MOR core slurry in batches, and each batch is aged after addition to obtain a high silicon-aluminum ratio MOR crystallization precursor slurry. S4. The high silicon-to-alumina ratio MOR crystallization precursor slurry is crystallized. After crystallization, it is subjected to solid-liquid separation, washing, drying and calcination to obtain a low impurity crystal high silicon-to-alumina ratio mordenite molecular sieve. The semi-crystallized MOR core paste is not subjected to solid-liquid separation, washing, or drying before entering step S3.

2. The method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve according to claim 1, characterized in that, In step S1, the initial silica-alumina gel has a SiO2 / Al2O3 molar ratio of 23 to 28 based on oxides, and the SiO2 contained in the first part of the silica sol accounts for 35% to 45% of the total SiO2 contained in the first part of the silica sol and the second part of the silica sol.

3. The method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve according to claim 2, characterized in that, In step S2, the SiO2 / Al2O3 molar ratio of the complexed silicon-aluminum feed solution, calculated as oxides, is 120–150; the molar composition of the high silicon-aluminum ratio MOR crystallization precursor slurry obtained after feeding in step S3 satisfies the following: SiO2 / Al2O3 molar ratio is 40–60, Na2O / SiO2 molar ratio is 0.08–0.18, and H2O / SiO2 molar ratio is 12–25.

4. The method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve according to claim 1, characterized in that, The mordenite seed solution is prepared by dispersing mordenite seed powder in an aqueous sodium hydroxide solution. The molar ratio of SiO2 / Al2O3 of the mordenite seed powder (based on oxides) is 10-25, the concentration of the aqueous sodium hydroxide solution is 0.05-0.50 mol / L, and the mass of the mordenite seed powder accounts for 5-20 wt% of the total mass of the mordenite seed solution.

5. The method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve according to claim 4, characterized in that, The silicate zeolite seed powder was found to have no ZSM-5 phase characteristic peaks by XRD analysis.

6. The method for preparing a low-impregnation, high-silica-to-alumina mordenite molecular sieve according to claim 1, characterized in that, In step S1, the pre-crystallization temperature is 125-145℃ and the pre-crystallization time is 4-10h; the relative crystallinity of the MOR obtained by sampling the semi-crystallized MOR core slurry is 8%-35% as determined by XRD peak height method.

7. The method for preparing a low-heteromorphic, high-silica-alumina ratio mordenite zeolite molecular sieve according to claim 3, characterized in that, In step S2, the hydroxycarboxylate aluminum source complexing agent is either sodium gluconate or sodium citrate. The ratio of the amount of the hydroxycarboxylate aluminum source complexing agent to the amount of aluminum source in the complexed silicon-aluminum feed solution converted to Al2O3 is 0.05 to 0.35:

1. The sodium aluminate, hydroxycarboxylate aluminum source complexing agent, and water are stirred at 50 to 80°C for 20 to 60 minutes for complexation and dispersion treatment.

8. The method for preparing a low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve according to claim 3, characterized in that, In step S3, the complexed silicon-aluminum feed solution is added to the semi-crystallized MOR core paste in 3 to 6 batches under stirring conditions. The sum of the amounts added in each batch is the total amount of the complexed silicon-aluminum feed solution. After each batch of complexed silicon-aluminum feed solution is added, it is aged at 120 to 145°C for 20 to 60 minutes.

9. A low-impure-crystal, high-silica-alumina-ratio mordenite zeolite molecular sieve prepared by the preparation method according to any one of claims 1 to 8.

10. The low heterocrystal, high silica-to-alumina ratio mordenite zeolite molecular sieve according to claim 9, characterized in that, The low-heteromorphic, high-silica-alumina-ratio mordenite molecular sieve has a SiO2 / Al2O3 molar ratio of 25–45 as determined by X-ray fluorescence spectroscopy. The relative crystallinity of the MOR relative to the standard mordenite reference sample, as determined by XRD peak height method, is not less than 85%. The peak height of the ZSM-5 heteromorphic peak is not more than 3% higher than the peak height of the main characteristic peak of the MOR, and the peak height of the characteristic peak of the crystalline silica secondary phase is not more than 2% higher than the peak height of the main characteristic peak of the MOR. The ratio of the outer surface Si / Al molar ratio determined by X-ray photoelectron spectroscopy to the bulk Si / Al molar ratio determined by X-ray fluorescence spectroscopy is 0.85–1.15.

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