Low silica-alumina ratio x-type zeolite molecular sieve, preparation method and use thereof

Abstract

The present disclosure provides a low-silica-alumina-ratio X-type zeolite molecular sieve, which, relative to its total weight, comprises: 70-90 wt% of raw X-type zeolite molecular sieve, 10-30 wt% of crystal-transformed X-type zeolite molecular sieve and no more than 1.5 wt% of amorphous phase; said low-silica-alumina-ratio X-type zeolite molecular sieve has a SiO2 / A12O3 molar ratio of 2.0-2.15, and a crystal size in the range of 4-6 μm and of 400-600 nm. The present disclosure also provides a method for preparing said low-silica-alumina-ratio X-type zeolite molecular sieve and the use thereof. The method of the present disclosure employs readily available raw materials and features simple steps, and the prepared zeolite molecular sieve exhibits excellent adsorption performance.

Description

LOW SILICA- ALUMINA RATIO X-TYPE ZEOLITE MOLECULAR SIEVE, PREPARATION METHOD AND USE THEREOFFIELD OF THE INVENTION

[0001] The present disclosure relates to the field of molecular sieve adsorbents. Specifically, the present disclosure relates to a low-silica-alumina-ratio X-type zeolite molecular sieve, a preparation method and use thereof.BACKGROUND

[0002] Molecular sieve materials are crystalline aluminosilicates with ordered pore structures, good hydrothermal stability, and tunable functionalities, finding widespread applications in fields such as industrial catalysis, adsorption separation, and ion exchange. X-type zeolites are silicate crystals with a FAU-type framework structure, having a basic structure composed of SiO4 and A1O4 tetrahedras that form a three-dimensional network through shared oxygen atoms, and belonging to one type of faujasite. Based on the SiCE / AECE molar ratio, X-type zeolite molecular sieves can be further classified into low-silica-alumina-ratio (SiCE / AECE = 2.0-2.2), medium- silica-alumina-ratio (SiCE / AECE = 2.0-2.4), and high-silica-alumina-ratio (SiCE / AECE = 2.4-3.0) X-type zeolites. Among these, low-silica-alumina-ratio zeolites contain a relatively high amount of negative charges due to the high aluminum lattice, accompanied by a greater number of charge-balancing cations within the lattice. This results in enhanced adsorption capacity and selective adsorption performance, thereby demonstrating broad application prospects. Currently, various research on reducing the silica-alumina ratio of X-type zeolites have been reported.

[0003] Generally, artificially synthesized X-type zeolites are powdery and need to be mixed with a binder, followed by operations like shaping, activation and so on, to obtain shaped molecular sieves with a certain mechanical strength before they can be further applied in industrial units. In industrial applications, the effective component of the shaped material is the molecular sieve, while the binder primarily serves to bond the molecular sieves, imparting a certain mechanical strength to the shaped molecular sieves. While imparting the material with a certain mechanical strength, the binder also reduces the effective composition of the shaped material; simultaneously, it may block some pores, causing a reduction in the dynamic performance of the molecular sieves, thereby degrading the performance of the shaped molecular sieve material in industrial applications.SUMMARY

[0004] In view of the above problems in the art, the present disclosure provides a low-silica- alumina-ratio X-type zeolite molecular sieve, a preparation method and use thereof. The molecular sieve possesses excellent micropore volume and adsorption capacity, and featuressimple preparation and low cost, thus effectively solving or at least mitigating one or more problems existing in the art.

[0005] Therefore, according to one aspect of the present disclosure, there provides a low-silica- alumina-ratio X-type zeolite molecular sieve, which, relative to its total weight, comprises: 70-90 wt% of raw X-type zeolite molecular sieve, 10-30 wt% of crystal -transformed X-type zeolite molecular sieve and no more than 1.5 wt% of amorphous phase; said low-silica-alumina-ratio X- type zeolite molecular sieve has a SiCh / AECh molar ratio of 2.0-2.15, and a crystal size in the range of 4-6 pm and of 400-600 nm.

[0006] According to another aspect of the present disclosure, there provides a method for preparing the aforementioned low-silica-alumina-ratio X-type zeolite molecular sieve, comprising the following steps:1) mixing raw X-type zeolite molecular sieve with a binder uniformly, followed by shaping, drying, and calcining;2) subjecting the shaped molecular sieve obtained after the calcination in step 1) to an in-situ crystallization treatment in an inorganic alkaline solution; and3) washing, drying, and calcining the shaped molecular sieve material after the in-situ crystallization treatment.

[0007] According to a further aspect of the present disclosure, there provides use of the aforementioned low-silica-alumina-ratio X-type zeolite molecular sieve as a CO2 adsorbent.

[0008] In the present disclosure, by mixing the raw X-type zeolite molecular sieve with a binder, followed by shaping, drying, and calcining, and then subjecting it to an in-situ crystallization treatment using an inorganic alkaline solution to convert the binder into molecular sieve, the finally obtained X-type zeolite molecular sieve exhibits a high crystallinity, and a low level of amorphous phase impurities (<1.5 wt%), and its SiCWAECh molar ratio can be as low as the theoretical value of 2.0. It also possesses excellent micropore volume and CO2 adsorption capacity, thus showing great application potential in the CO2 adsorbent market. Additionally, the raw materials for preparing the low-silica-alumina-ratio X-type zeolite molecular sieve are readily available, and the preparation process is simple, allowing for widespread applications in industrial manufacturing processes.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present disclosure is described in conjunction with the accompanying drawings to enable those skilled in the art to better understand various features and advantages of the present disclosure, wherein:

[0010] FIG. l is a scanning electron micrograph of the spherical parent molecular sieves preparedin Example 3;

[0011] FIG. 2 is a scanning electron micrograph of the target product molecular sieves prepared in Example 3;

[0012] FIG. 3 is an X-ray diffraction pattern of the target product molecular sieves prepared in Example 3 and the raw molecular sieve powder used;

[0013] FIG. 4 is a scanning electron micrograph of a commercially available binderless X-type molecular sieve adsorbent 1;

[0014] FIG. 5 is a scanning electron micrograph of a binderless X-type molecular sieve adsorbent 2 prepared according to the prior art.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] To further enable those skilled in the art to understand the present disclosure, the specific embodiments of the present disclosure are described in detail below. However, it should be understood that the described embodiments of the present disclosure are merely exemplary, and the present disclosure is not limited to these embodiments.

[0016] According to one aspect of the present disclosure, there provides a low-silica-alumina- ratio X-type zeolite molecular sieve, which, relative to its total weight, comprises: 70-90 wt% of raw X-type zeolite molecular sieve, 10-30 wt% of crystal -transformed X-type zeolite molecular sieve and no more than 1.5 wt% of amorphous phase; the low-silica-alumina-ratio X-type zeolite molecular sieve has a SiCh / AECh molar ratio of 2.0-2.15, and a crystal size in the range of 4-6 pm and of 400-600 nm.

[0017] Preferably, the low-silica-alumina-ratio X-type zeolite molecular sieve comprises 80-90 wt% of the raw X-type zeolite molecular sieve, 10-20 wt% of the crystal -transformed X-type zeolite molecular sieve, and no more than 1.0 wt% of the amorphous phase.

[0018] Preferably, the low-silica-alumina-ratio X-type zeolite molecular sieve has a SiCWAECE molar ratio of 2.0-2.11.

[0019] Preferably, the low-silica-alumina-ratio X-type zeolite molecular sieve has a specific surface area of 720-870 m2 / g, a micropore surface area of 666-865 m2 / g, and a micropore volume of 0.26-0.34 cm3 / g.

[0020] Preferably, the low-silica-alumina-ratio X-type zeolite molecular sieve has a specific surface area of 830-864 m2 / g, a micropore surface area of 750-855 m2 / g, and a micropore volume of 0.28-0.32 cm3 / g.

[0021] The raw X-type zeolite molecular sieve used in the present application is a commercially available conventional X-type zeolite. Preferably, the raw X-type zeolite molecular sieve is selected from NaX zeolites or KNaX zeolites, having a SiCWAECE molar ratio of 2.0-2.5and a crystal size in the range of 4-6 gm.

[0022] In some embodiments, the crystal-transformed X-type zeolite molecular sieve is formed by in-situ crystallization treatment of a binder, which is a clay -type binder capable of being converted into a molecular sieve structure, for example, selected from kaolin, montmorillonite, dickite, nacrite, firestone, halloysite, or mixtures thereof, preferably halloysite. Said binders are all commercially available.

[0023] The amorphous phase is the residue of the binder after the in-situ crystallization.

[0024] The low-silica-alumina-ratio X-type zeolite molecular sieve of the present disclosure exhibits two characteristic crystal size ranges, namely 4-6 pm and 400-600 nm, respectively. By comparing the scanning electron micrographs of the molecular sieves before and after the in-situ crystallization, it can be found that the molecular sieves on the adsorbent containing the binder have a crystal size of about 4-6 pm, with almost no molecular sieves of smaller crystal sizes. Therefore, it can be concluded that the molecular sieves with larger crystal size are the raw molecular sieves, and the molecular sieves with smaller crystal size are derived from the in-situ crystallization of the binder. The molecular sieves with smaller crystal size have a static adsorption capacity comparable to that of the molecular sieves with larger crystal size, but exhibit a higher dynamic mass transfer capability, leading to a better performance in industrial applications.

[0025] According to another aspect of the present disclosure, there provides a method for preparing the low-silica-alumina-ratio X-type zeolite molecular sieve, comprising the following steps:1) mixing raw X-type zeolite molecular sieve with a binder uniformly, followed by shaping, drying, and calcining;2) subjecting the shaped molecular sieve obtained after the calcination in step 1) to an in-situ crystallization treatment in an inorganic alkaline solution; and3) washing, drying, and calcining the shaped molecular sieve material after the in-situ crystallization treatment.

[0026] In some embodiments, in step 1), the raw X-type zeolite molecular sieve and the binder are mixed uniformly at a weight ratio of 70:30 to 90: 10, preferably 80:20 to 90: 10. The raw X- type zeolite molecular sieve and the binder are as described above.

[0027] In step 1), any shaping method well-known to those skilled in the art can be used to shape the uniform mixture of the raw X-type zeolite molecular sieve and the binder, including but not limited to ball rolling and extrusion.

[0028] Equipment for ball rolling can be a polishing machine, a Nauta mixer, or an Eirich mixer. During ball rolling, the uniformly mixed and compacted raw material is placed into therotating equipment, and water is sprayed while rolling to cause the raw material to adhere and agglomerate into pellets. In total, water is added in an amount of 30-45 wt%, preferably 37-39 wt%, relative to the total solid weight. The pellets formed by ball rolling are dried and calcined to obtain the product.

[0029] Extrusion is performed with an industrially common extruder. During extrusion, the raw materials are firstly mixed uniformly, and then an appropriate amount of water is added so that the loss on ignition (LOI) of the material is between 30% and 45%. The mixed material is then transferred to the extruder, and a suitable extrusion die is selected for extrusion. The extruded adsorbent is dried and calcined to obtain the product.

[0030] When ball rolling is employed, the shaped molecular sieve is preferably a spherical shape with a diameter of 1-3 mm. When extrusion is employed, the shaped molecular sieve is preferably a strip shape with a diameter of 1.5-3.5 mm and a length of 2-5 mm.

[0031] In some embodiments, in step 1), the drying temperature is 90-270°C, the drying time is 0.3-12 h; and the calcining temperature is 300-650°C, the calcination time is 0.2-4 h.

[0032] In some embodiments, in step 2), the inorganic base is selected from sodium hydroxide, potassium hydroxide, or a mixture thereof.

[0033] In some embodiments, in step 2), the concentration of the inorganic alkaline solution is 0.4-4 mol / L, preferably 1-4 mol / L, more preferably 3-4 mol / L.

[0034] In some embodiments, the in-situ crystallization treatment is performed either under static conditions or with stirring. Preferably, the in-situ crystallization treatment is performed with stirring.

[0035] In some embodiments, the liquid-solid ratio of the inorganic alkaline solution to the shaped molecular sieve for the in-situ crystallization treatment is 1-6 L / kg, preferably 1-4 L / kg; the treatment temperature is 80-100°C, preferably 88-96°C; and the treatment time is 2-24 h, preferably 4-12 h.

[0036] In some embodiments, in step 3), the drying temperature is 120-250°C, the drying time is 0.4-3 h; and the calcining temperature is 450-600°C, the calcination time is 0.2-2 h.

[0037] According to a further aspect of the present disclosure, there provides use of the aforementioned low-silica-alumina-ratio X-type zeolite molecular sieve as a CO2 adsorbent.

[0038] The low-silica-alumina-ratio X-type zeolite molecular sieve of the present disclosure exhibits a 2 torr CO2 adsorption capacity, as measured at 25°C, of 39.7 mL / g or more, even up to 47.4 mL / g.

[0039] The CO2 characteristic diffusion coefficient of the low-silica-alumina-ratio X-type zeolite molecular sieve of the present disclosure is 2.7* ICT6s '-3.39 / 106s '.

[0040] The molecular sieve provided by the present disclosure avoids pore blockage by thebinder and enhances the dynamic performance of the adsorbent through in-situ crystallization of the binder into low-silica-alumina-ratio X-type molecular sieve of smaller crystal size; meanwhile, for molecular sieve of the same topology structure, the low-silica-alumina-ratio X- type molecular sieve of smaller crystal size exhibits superior dynamic performance compared to that of larger crystal size, further enhancing the dynamic performance of the molecular sieve, thereby demonstrating more excellent performance in industrial applications such as CCE adsorption. For example, the low-silica-alumina-ratio X-type zeolite molecular sieve of the present disclosure can be used as an adsorbent in CCE pressure swing adsorption / temperature swing adsorption processes, or in the purification process of any other CCE-containing gases.

[0041] In the present application, the descriptions of the various features or embodiments can be combined with each other provided such combinations are not mutually contradictory, and all such combinations shall fall within the scope claimed in the present application.

[0042] The terms "comprise", "comprising", and "including" as used in the present application cover both the cases consisting solely of the listed elements and the cases including other elements in addition to the listed elements.

[0043] In the specification and claims of the present application, numbers expressing temperature, quantity, concentration, percentage, and the like shall in all cases be understood as modified by the term "about".

[0044] Unless otherwise defined, all scientific and technical terms used in the present application have the same meanings as commonly understood by those skilled in the art to which this disclosure belongs. When definitions of terms in the present application conflict with the meanings commonly understood by those skilled in the art to which this disclosure belongs, the definitions described in the present application shall prevail.EXAMPLES

[0045] The following examples are provided in conjunction with the accompanying drawings to further illustrate the present disclosure, so that those skilled in the art can fully understand the objectives, features, and effects of the present disclosure. However, the protection scope of the present disclosure is not limited by the following examples.

[0046] Main raw materials and performance parameter testing equipment in the examples are listed below:Raw X-type zeolite molecular sieve: a commercial NaX zeolite with a SiCWAECE molar ratio of 2.06 and a crystal size range of 4-6 pm;Halloysite: a commercial product with a SiCWAECE molar ratio of 1.84;SiCE / AECE molar ratio is determined by using a PE-8000 ICP-OES instrument from PerkinElmer;CO2 adsorption capacity is determined by using a Bel-Max adsorption instrument from Bel Japan. The procedure of determination is as follows: firstly, an appropriate amount of adsorbent is weighed and placed in the sample chamber, the sample is then heated to 400°C under vacuum and held at that temperature for 6 h, followed by in-situ vacuum activation in the adsorption instrument for 4 h. After the completion of activation, CO2 is introduced at 25°C and the adsorption isotherm is recorded; the CO2 adsorption capacity can be obtained from the adsorption isotherm.

[0047] CO2 characteristic diffusion coefficient is determined by using a BSD DVS adsorption instrument from Beishide, Beijing. The procedure of determination is as follows: firstly, an appropriate amount of adsorbent is weighed and placed in the sample chamber of the equipment, the sample is then heated to 300°C under a flowing nitrogen atmosphere and held at that temperature for 6 h, then a CO2 / N2 mixture gas with a CO2 concentration of 2000 ppm is introduced, and the curve of sample weight change over time is recorded; the diffusion coefficient can be calculated by applying a suitable model equation, based on this curve.

[0048] BET specific surface area, micropore surface area, and micropore volume are determined by using a 3Flex adsorption instrument from Micromeritics, USA. The procedure of determination is as follows: firstly, an appropriate amount of adsorbent is weighed and placed in the sample chamber, the sample is then heated to 400°C under vacuum and held at that temperature for 6 h, followed by in-situ vacuum activation in the adsorption instrument for 4 h. After the completion of activation, N2 is introduced at -196°C and the adsorption isotherm is recorded; the BET specific surface area, micropore surface area, and micropore volume can be calculated based on the adsorption isotherm and model equations.PREPARATION EXAMPLE 1

[0049] 1) Appropriate amounts of the raw X-type zeolite molecular sieves and halloysite were weighed at a weight ratio of 85 : 15, then transferred to a mixer for thorough mixing. The uniformly mixed mixture was transferred to a compactor, where an appropriate amount of water was added while stirring and compacting. Once the material was compacted to a dough-like state, the mixture was transferred to an extruder for extrusion. The extruded strips were collected, dried at 150°C for 45 min, and calcined at 600°C for 20 min to obtain strip-shaped parent molecular sieves.

[0050] Characterization and analysis indicated that the strip-shaped parent molecular sieve had a 2 torr CO2 adsorption capacity at 25°C of 34.4 mL / g; a BET specific surface area of 757 m2 / g, a micropore surface area of 735 m2 / g, and a micropore volume of 0.28 cm3 / g.

[0051] 2) 8 g of the strip-shaped parent molecular sieves were weighed, placed into 48 mL of a NaOH solution with a concentration of 0.9 mol / L, and held at 90°C for 14 h for an in-situ crystallization treatment;

[0052] 3) The molecular sieves after the completion of in-situ crystallization were washed with deionized water until the washing solution became neutral; the washed molecular sieves were dried at 200°C for 30 min and calcined at 550°C for 30 min to obtain the target product molecular sieves.

[0053] Characterization and analysis indicated that the target product molecular sieve had a SiCWAECh molar ratio of 2.0; a 2 torr CO2 adsorption capacity at 25°C of 42.4 mL / g, which was 23.3% higher than that of the strip-shaped parent molecular sieve; a BET specific surface area of 855 m2 / g, a micropore surface area of 848 m2 / g, and a micropore volume of 0.31 cm3 / g, which were 12.9%, 15.4%, and 10.7% higher than that of the strip-shaped parent molecular sieve, respectively.PREPARATION EXAMPLE 2

[0054] The preparation was carried out according to the method described in Example 1, with the difference being that the concentration of the NaOH solution was 3.6 mol / L.

[0055] Characterization and analysis indicated that the target product molecular sieve had a SiO2 / A12Os molar ratio of 2.06; a 2 torr CO2 adsorption capacity at 25°C of 45.1 mL / g, which was 31.1% higher than that of the strip-shaped parent molecular sieve.PREPARATION EXAMPLE 3

[0056] The preparation was carried out according to the method of Example 2, with the difference being that molecular sieves prepared by ball rolling, i.e., spherical parent molecular sieves, were used.

[0057] FIG. 1 shows a scanning electron micrograph of the spherical parent molecular sieves; it can be concluded that their sizes are relatively uniform, within the range of 4-6 pm.

[0058] Characterization and analysis indicated that the spherical parent molecular sieve had a 2 torr CO2 adsorption capacity at 25°C of 34.8 mL / g; a BET specific surface area of 763 m2 / g, a micropore surface area of 748 m2 / g, and a micropore volume of 0.28 cm3 / g.

[0059] FIG. 2 shows a scanning electron micrograph of the finally obtained target product molecular sieves; it can be seen that smaller crystal particles with a size range of 400-600 nm, i.e., the molecular sieves generated by in-situ crystallization of the binder, are attached to the original spherical parent molecular sieves.

[0060] Characterization and analysis indicated that the target product molecular sieve had a SiCL / ALOs molar ratio of 2.11; a 2 torr CO2 adsorption capacity at 25°C of 45.5 mL / g, which was 30.7% higher than that of the spherical parent molecular sieve; a BET specific surface area of 864 m2 / g, a micropore surface area of 855 m2 / g, and a micropore volume of 0.32 cm3 / g, which were 13.2%, 14.3%, and 14.3% higher than that of the spherical parent molecular sieve, respectively.

[0061] The CO2 characteristic diffusion coefficients of the spherical parent molecular sieve andthe target product molecular sieve were further characterized, being 2.26* 1CF6s ' and 3.39>< 10"6s ', respectively; the CO2 characteristic diffusion coefficient of the target product molecular sieve was 50% higher than that of the spherical parent molecular sieve.

[0062] FIG. 3 is the X-ray diffraction patterns of the product of Example 3 and the raw molecular sieve. As can be seen from FIG. 3, the molecular sieve after in-situ crystallization has a high crystallinity and contains almost no amorphous phase; after in-situ crystallization, no other impurity peaks appear in the pattern of Example 3, indicating successful crystal transformation of halloysite into the X-type molecular sieves.PREPARATION EXAMPLE 4

[0063] The preparation was carried out according to the method of Example 3, with the difference being that the in-situ crystallization treatment was performed with stirring.

[0064] Characterization and analysis indicated that the target product molecular sieve had a SiO2 / A12Os molar ratio of 2.02; a 2 torr CO2 adsorption capacity at 25°C of 46.5 mL / g, which was 33.6% higher than that of the spherical parent molecular sieve.COMPARATIVE EXAMPLE 1

[0065] Comparative Example 1 is a commercially available binderless X-type molecular sieve adsorbent. FIG. 4 is a scanning electron micrograph of the molecular sieve adsorbent of Comparative Example 1. As can be seen from FIG. 4, the molecular sieve adsorbent of Comparative Example 1 mainly consists of larger crystal particles with a size of 5-10 pm, which is significantly different from the bimodally distributed molecular sieves of Example 3. Furthermore, characterization and analysis indicated that the molecular sieve adsorbent of Comparative Example 1 had a SiCWAECh molar ratio of 2.17.COMPARATIVE EXAMPLE 2

[0066] Comparative Example 2 is a binderless X-type molecular sieve adsorbent prepared according to the prior art. The molecular sieve adsorbent of Comparative Example 2 was prepared according to the method disclosed in CN107159105A, i.e., obtained by shaping a mixture of 13X- type molecular sieve, kaolin, a shaping aid, and a pore-forming agent, followed by activation, aging in an alkaline solution, crystal transformation in an alkaline solution, and activation. Characterization and analysis indicated that it had a 2 torr CO2 adsorption capacity at 25°C of 41.5 mL / g, which was lower than that of the samples in the examples of the present disclosure. FIG. 5 is a scanning electron micrograph of the molecular sieve adsorbent of Comparative Example 2. As can be seen from FIG. 5, the molecular sieve adsorbent of Comparative Example 2 mainly consists of larger crystal particles with a size of 4-8 pm and some smaller particles with insufficientcrystallization. Compared with Example 3, the molecular sieve adsorbent of Comparative Example 2 has a relative crystallinity of 95%, indicating a higher amount of amorphous phase.

Claims

CLAIMS:

1. A low-silica-alumina-ratio X-type zeolite molecular sieve, relative to its total weight, comprising: 70-90 wt% of raw X-type zeolite molecular sieve, 10-30 wt% of crystal-transformed X-type zeolite molecular sieve and no more than 1.5 wt% of amorphous phase; wherein the low- silica-alumina-ratio X-type zeolite molecular sieve has a SiCh / AhCh molar ratio of 2.0-2.15, and a crystal size in the range of 4-6 pm and of 400-600 nm.

2. The low-silica-alumina-ratio X-type zeolite molecular sieve of claim 1, wherein it comprises 80-90 wt% of the raw X-type zeolite molecular sieve, 10-20 wt% of the crystal- transformed X-type zeolite molecular sieve, and no more than 1.0 wt% of the amorphous phase.

3. The low-silica-alumina-ratio X-type zeolite molecular sieve of claim 1 or 2, wherein it has a SiCh / AhCh molar ratio of 2.0-2.11.

4. The low-silica-alumina-ratio X-type zeolite molecular sieve of any one of claims 1 to 3, wherein it has a specific surface area of 720-870 m2 / g, a micropore surface area of 666-865 m2 / g, and a micropore volume of 0.26-0.34 cm3 / g.

5. The low-silica-alumina-ratio X-type zeolite molecular sieve of any one of claims 1 to 4, wherein it has a specific surface area of 830-864 m2 / g, a micropore surface area of 750-855 m2 / g, and a micropore volume of 0.28-0.32 cm3 / g.

6. The low-silica-alumina-ratio X-type zeolite molecular sieve of any one of claims 1 to 5, characterized in that the raw X-type zeolite molecular sieve is selected from NaX zeolites or KNaX zeolites having a SiCWAbCh molar ratio of 2.0-2.5 and a crystal size in the range of 4-6 pm.

7. The low-silica-alumina-ratio X-type zeolite molecular sieve of any one of claims 1 to 6, wherein the crystal-transformed X-type zeolite molecular sieve is formed by in-situ crystallization treatment of a binder selected from kaolin, montmorillonite, dickite, nacrite, firestone, halloysite, or mixtures thereof, preferably halloysite.

8. A method for preparing the low-silica-alumina-ratio X-type zeolite molecular sieve of any one of claims 1 to 7, comprising the following steps:1) mixing raw X-type zeolite molecular sieve with a binder uniformly, followed by shaping, drying, and calcining;2) subjecting the shaped molecular sieve obtained after the calcination in step 1) to an in-situ crystallization treatment in an inorganic alkaline solution; and3) washing, drying, and calcining the shaped molecular sieve material after the in-situ crystallization treatment.

9. The method of claim 8, wherein in step 1), the raw X-type zeolite molecular sieve and the binder are mixed at a weight ratio of 70:30 to 90: 10, preferably 80:20 to 90: 10, with the raw X-type zeolite molecular sieve and the binder defined in claims 6 and 7, respectively.

10. The method of claim 8 or 9, wherein in step 1), the drying temperature is 90-270°C, the drying time is 0.3-12 h; and the calcining temperature is 300-650°C, the calcination time is 0.2-4 h.

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