Mulberry-shaped grade pore ZSM-5 molecular sieve single crystal derivative as well as synthesis method and application thereof

Abstract

The invention relates to a series of mulberry-shaped grade pore ZSM-5 molecular sieve single crystal derivatives and a synthesis method and application thereof. The preparation method comprises the following steps: under the conditions of no mesoporous template agent and no etching treatment, firstly, by adjusting zeolite nucleation and growth kinetics, constructing a mulberry-shaped hierarchical pore ZSM-5 molecular sieve single crystal; then carrying out ammonium exchange treatment to prepare mulberry-shaped H-type grade pore ZSM-5 molecular sieve single crystals; and then loading rare earth elements into the mixture by adopting an impregnation method to obtain the mulberry-shaped rare earth loaded H-type grade pore ZSM-5 molecular sieve single crystal. According to the preparation method, the dual gain effects of enhancing diffusion of a grade pore structure and improving acidity of the rare earth metal La are realized by utilizing the acidity modulation effect of the rare earth element La and the synergistic modification effect of carbon deposit species, so that on one hand, the problem of micropore blockage caused by traditional molecular sieve impregnation is solved; on the other hand, the MTO catalytic life and the propylene selectivity of the bifunctional catalyst are remarkably improved by utilizing low-load rare earth elements.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of rare earth materials and molecular sieves, in particular to a mulberry-like hierarchical pore ZSM-5 molecular sieve single crystal derivative and a synthesis method and application thereof. BACKGROUND

[0002] ZSM-5 molecular sieve has a three-dimensional 10-membered ring cross-channel structure, and has become a very important catalyst in the field of methanol to olefins (MTO) catalysis due to the advantages of low cost, wide variable range of silicon-aluminum ratio, high hydrothermal stability, etc. However, the microporous ZSM-5 molecular sieve has inherent diffusion resistance due to the restriction of its pore opening (<2 nm), which brings a series of adverse effects, including excessive side reactions, carbon deposition and catalyst deactivation, etc.

[0003] The methods for shortening the diffusion path in the prior art include reducing the zeolite particle size, constructing zeolites with hierarchical pore structure, etc., which can improve the transport diffusion performance and active site accessibility, thereby obtaining better catalytic performance. In addition, by introducing a second component (such as lanthanum oxide, cerium oxide, etc.) into the molecular sieve, the acid type and strength of the molecular sieve can be redistributed, and the generation rate of carbon deposition species can be slowed down. The introduction of the second component can also promote the decomposition of the carbon deposition precursors generated during the reaction, thereby improving the service life of the catalyst.

[0004] Hierarchical pore molecular sieves can be prepared by using hard templates (such as carbon), soft templates (such as polymers, organosilane surfactants) or nanocrystal self-assembly, etc. However, the first two methods need to use expensive and highly customized mesoporous template agents, and the third method needs very harsh synthesis conditions (such as variable temperature crystallization or adding seed orientation). In addition, the molecular sieves synthesized by the nanocrystal self-assembly method mostly do not have a single crystal structure, which leads to reduced catalytic performance. For a long time, how to construct hierarchical porous molecular sieve single crystals only by adjusting the zeolite nucleation and growth kinetics has been a key issue of researchers' attention, because this approach is not only simple but also has potential industrial feasibility. SUMMARY

[0005] To solve the above problems existing in the prior art, the present application provides a series of ZSM-5 molecular sieve single crystal derivatives with mesoporous, microporous hierarchical pore structure, which includes three different substances, namely hierarchical pore ZSM-5 molecular sieve single crystal, H-type hierarchical pore ZSM-5 molecular sieve single crystal and rare earth loaded H-type hierarchical pore ZSM-5 molecular sieve single crystal, each of which has a mulberry-like structure and a hierarchical pore structure.

[0006] Further, the mulberry-like structure is specifically a crystal with an overall particle size of 700 nm to 900 nm formed by ordered assembly of nanocrystal structure units with a diameter of 70 nm to 90 nm.

[0007] As a matching, the application also provides a preparation method of the above-mentioned various grade hole ZSM-5 molecular sieve monocrystal, comprising: mixing and stirring uniformly water, an aluminum source, an alkali source, an organic structure directing agent, and a silicon source to obtain a precursor solution, crystallizing the precursor solution, and then filtering, washing, and drying in sequence, thereby obtaining a solid product, and calcining the solid product in an air atmosphere to obtain a mulberry-shaped grade hole ZSM-5 molecular sieve monocrystal; performing ammonium exchange treatment on the mulberry-shaped grade hole ZSM-5 molecular sieve monocrystal, and calcining to obtain a mulberry-shaped H-type grade hole ZSM-5 molecular sieve monocrystal; and loading a rare earth element onto the mulberry-shaped H-type grade hole ZSM-5 molecular sieve monocrystal, and calcining to obtain a mulberry-shaped rare earth-loaded H-type grade hole ZSM-5 molecular sieve monocrystal.

[0008] In the above scheme, the aluminum source is selected from at least one of aluminum isopropoxide, aluminum hydroxide, sodium aluminate, aluminum nitrate, and aluminum sulfate, and is preferably sodium aluminate; the alkali source is selected from at least one of sodium hydroxide and ammonia water, and is preferably sodium hydroxide; the organic structure directing agent is selected from at least one of tetrapropylammonium hydroxide and tetraethylammonium hydroxide, and is preferably tetrapropylammonium hydroxide; and the silicon source is selected from at least one of silica sol and tetraethyl orthosilicate, and is preferably tetraethyl orthosilicate.

[0009] In the above scheme, in the process of preparing the precursor solution, the molar ratio of the silicon source (calculated as SiO2), the aluminum source (calculated as Al2O3), the organic structure directing agent, the alkali source, and water is 95-105:1-5:25-35:5-10:5000-6000, and is preferably 100:2:30:7:5462.

[0010] In the above scheme, the crystallization reaction temperature of the precursor solution is 150-180°C, and the crystallization reaction time is 24-48h.

[0011] In the above scheme, the specific process of the ammonium exchange treatment is as follows: the mulberry-shaped grade hole ZSM-5 molecular sieve monocrystal is added to an ammonium nitrate solution, fully soaked and stirred, and then dried after being filtered; the soaking, filtering, and drying processes are repeated multiple times, and finally the solid product is calcined at a high temperature to obtain the mulberry-shaped H-type grade hole ZSM-5 molecular sieve monocrystal.

[0012] In the above scheme, the rare earth element is loaded onto the mulberry-shaped H-type grade hole ZSM-5 molecular sieve monocrystal by an impregnation method, the impregnation solution used is a lanthanum nitrate aqueous solution, and the loaded rare earth element is lanthanum (La).

[0013] As a supplement, the specific process of the impregnation method is as follows: the mulberry-shaped H-type grade hole ZSM-5 molecular sieve monocrystal is mixed with the impregnation solution, stirred until the water evaporates, and then dried, followed by calcining the solid product, to finally obtain the mulberry-shaped rare earth-loaded H-type grade hole ZSM-5 molecular sieve monocrystal.

[0014] In the above scheme, the atmosphere of each step of calcination is air atmosphere, and the calcination temperature is controlled between 500-650℃.

[0015] In the above scheme, the mass percentage of rare earth elements in the mulberry-like rare earth loaded H-type hierarchical pore ZSM-5 molecular sieve monocrystal is 2wt%-20wt%.

[0016] The application also provides application of the above hierarchical pore ZSM-5 molecular sieve monocrystal derivative in preparation of olefins in a catalytic MTO reaction.

[0017] Based on a relatively simple crystal dynamics regulation strategy, the mulberry-like hierarchical pore ZSM-5 molecular sieve monocrystal and its derivative synthesized by orderly aggregation of nanocrystals are synthesized without the participation of mesoporous templates, by only fine regulation of the water amount, aluminum concentration and other parameters in the precursor solution. Compared with the existing similar products or methods, the progress of the application mainly lies in the following points: (1) Without relying on any mesoporous template and etching treatment, the mulberry-like hierarchical pore ZSM-5 molecular sieve monocrystal with mesoporous and microporous structures is prepared, which not only reduces the synthesis cost but also simplifies the synthesis steps, and the whole method has many advantages such as simplicity, economy and reliability.

[0018] (2) The hierarchical pore ZSM-5 molecular sieve monocrystal synthesized by the application is formed by orderly assembly of 70nm-90nm nanocrystal structural units to form a whole particle size of 700nm-900nm mulberry-like structure. This unique structure significantly improves the specific surface area and external specific surface area of the molecular sieve monocrystal, not only has the advantage of short diffusion path of nanocrystal (70nm-90nm), but also eliminates the disadvantage of difficult recovery of nanocrystal because of the large size (700nm-900nm) of the whole.

[0019] (3) The rare earth metal La is loaded on the mulberry-like H-type hierarchical pore ZSM-5 molecular sieve monocrystal by impregnation, and the synergistic modification effect of the rare earth La element is utilized to realize the dual gain effect of hierarchical pore structure enhancement diffusion and rare earth metal lanthanum improving acidity. Not only does it solve the problem of micropore blockage that may be caused by traditional molecular sieve impregnation, but also significantly improves the catalytic life and propylene selectivity of the bifunctional catalyst by using low-loading rare earth. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The XRD comparison chart of different molecular sieve samples prepared in Examples 1-4.

[0021] Figure 2 The SEM photo of Hier-ZSM-5 prepared in Example 1.

[0022] Figure 3 TEM photograph of Hier-ZSM-5 prepared for Example 1.

[0023] Figure 4 SEM photograph of C-ZSM-5 prepared for Comparative Example 1.

[0024] Figure 5 MTO catalytic life chart of different molecular sieve samples in Examples 1-4 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0025] In order for those skilled in the art to have a clearer understanding and recognition of the technical solutions, objectives and beneficial effects of the present application, the present application is described in detail below through several examples. It is emphasized that the embodiments of the present application are not limited to the listed examples, and the present application can also have many other embodiments, and any improvement or modification to the technical solutions of the present application based on these embodiments shall fall within the protection scope of the present application.

[0026] The raw materials used in the present application are all ordinary commercially available, and are all analytical pure if not otherwise specified.

[0027] The present application provides a simple, economical and reliable synthesis method of mulberry-like hierarchical pore ZSM-5 molecular sieve single crystal derivatives. The method can synthesize mulberry-like hierarchical pore ZSM-5 molecular sieve single crystals with mesoporous and microporous structures through precise control of the alkalinity, water amount, molar ratio of silicon source and aluminum source of the precursor solution under the premise of not relying on any mesoporous template agent and etching treatment. Then, the mulberry-like H-type hierarchical pore ZSM-5 molecular sieve single crystals are prepared by ammonium exchange treatment. Finally, the mulberry-like rare earth-loaded H-type hierarchical pore ZSM-5 molecular sieve single crystals are obtained by loading the rare earth metal element lanthanum (La) on the mulberry-like H-type hierarchical pore ZSM-5 molecular sieve single crystals through the impregnation method, and the dual gain effect of enhanced diffusion and improved acidity is ultimately realized. The entire synthesis process is as follows: (1) The weighed deionized water, aluminum source, alkali source, organic structure directing agent and silicon source are added to the reactor in proportion, and stirred uniformly to obtain a precursor solution.

[0028] In this step, the selection and amount of the raw materials are very critical. The aluminum source is selected from at least one of isopropyl aluminum, aluminum hydroxide, sodium aluminate, aluminum nitrate and aluminum sulfate, the alkali source is selected from at least one of sodium hydroxide and ammonia water, the organic structure directing agent is selected from at least one of tetrapropylammonium hydroxide (TPAOH) and tetraethylammonium hydroxide (TEAOH), and the silicon source is selected from at least one of silica sol and tetraethyl orthosilicate.

[0029] Preferably, the aluminum source is sodium aluminate, the alkali source is sodium hydroxide, the organic structure directing agent is TPAOH, and the silicon source is tetraethyl orthosilicate.

[0030] During the mixing process, the amount ratio of each raw material needs to be accurately controlled. Specifically, the molar ratio of the silicon source (calculated as SiO2), the aluminum source (calculated as Al2O3), the organic structure directing agent, the alkali source, and deionized water is 95-105:1-5:25-35:5-10:5000-6000, and preferably 100:2:30:7:5462.

[0031] (2) The precursor solution is transferred to a hydrothermal crystallization kettle, and a hydrothermal crystallization reaction is performed by heating. After cooling, the solid sample is obtained by filtering and washing with deionized water multiple times (for example, 3 times, 5 times), and drying.

[0032] The hydrothermal crystallization reaction temperature is 150-180°C, such as 150°C, 160°C, 170°C, 180°C, etc. The hydrothermal crystallization reaction time at different temperatures is different, and is usually between 24-48h, such as 24h, 36h, 48h, etc. The solid sample drying temperature is controlled between 60-100°C, which can be 60°C, 80°C, 100°C, etc.

[0033] (3) The dried solid sample is transferred to a muffle furnace, and the organic structure directing agent is removed by high-temperature calcination to obtain a raspberry-like hierarchical pore ZSM-5 molecular sieve single crystal. The molecular sieve single crystal is formed by orderly assembling about 70-90nm nanocrystal structural units to form a raspberry-like structure with a whole particle size of about 700-900nm.

[0034] The calcination atmosphere is air, and the calcination temperature is controlled between 500-650°C, such as 500°C, 550°C, 600°C, 650°C, etc.

[0035] (4) The raspberry-like hierarchical pore ZSM-5 molecular sieve single crystal prepared is added to an ammonium nitrate aqueous solution and stirred for ammonium exchange, followed by suction filtration and drying. The above ammonium exchange-suction filtration-drying process is repeated multiple times (for example, 2 times, 3 times, 5 times), and then the dried solid sample is transferred to a muffle furnace for high-temperature calcination to obtain a raspberry-like H-type hierarchical pore ZSM-5 molecular sieve single crystal, which is named Hier-ZSM-5.

[0036] wherein the concentration of the aqueous ammonium nitrate solution is 0.5-1 mol / L, for example 0.5 mol / L, 0.65 mol / L, 0.8 mol / L, 1.0 mol / L. The mixing and stirring temperature is controlled at 60-80°C, for example 60°C, 70°C, 80°C. The sample drying temperature is controlled at 60-100°C, for example 60°C, 80°C, 100°C. The calcination atmosphere is air, and the calcination temperature is controlled at 500-650°C, for example 500°C, 550°C, 600°C, 650°C.

[0037] (5) A proper amount of rare earth metal compound (for example, lanthanum La) is dissolved in water, and then Hier-ZSM molecular sieve monocrystals are fully impregnated, and stirred until the water evaporates, and then dried. Finally, the dried sample is transferred to a muffle furnace for high-temperature calcination, to obtain raspberry-like rare earth-loaded H-type hierarchical pore ZSM-5 molecular sieve monocrystals, which are named xLa / Hier-ZSM-5 (x represents the rare earth loading amount).

[0038] wherein the stirring temperature is controlled at 40-60°C, for example 40°C, 50°C, 60°C. The sample drying temperature is controlled at 60-100°C, for example 60°C, 80°C, 100°C. The calcination atmosphere is air, and the calcination temperature is controlled at 500-650°C, for example 500°C, 550°C, 600°C, 650°C. The loading amount of the rare earth element lanthanum (i.e. the mass of metal lanthanum / the mass of Hier-ZSM-5) is controlled at 2wt%-20wt%, for example 2wt%, 3.8wt%, 7.6wt%, 9.5wt%, 11.6wt%, 15.3wt%, 18.8wt%, 20wt%, etc.

[0039] Example 1 Sodium hydroxide is used as the alkali source, sodium aluminate is used as the aluminum source, TPAOH is used as the structure directing agent, and tetraethyl orthosilicate is used as the silicon source. According to the molar ratio of 100 SiO2:2 Al2O3:30 TPAOH:7 NaOH:5462 H2O, deionized water is weighed into a beaker, and then the aluminum source, the alkali source, the organic structure directing agent, and the silicon source are sequentially added and stirred for 6 hours to obtain a molecular sieve precursor solution.

[0040] The above-mentioned molecular sieve precursor solution is transferred to a hydrothermal crystallization kettle, heated to 180°C, and hydrothermally crystallized for 24 hours. After filtration, the obtained solid sample is washed with water by centrifugation for 3 times, and then placed in an oven at 80°C for overnight drying. The solid sample is taken out of the oven, transferred to a muffle furnace, heated to 550°C, and calcined for 7 hours. After natural cooling, raspberry-like hierarchical pore ZSM-5 molecular sieve monocrystals are obtained.

[0041] The raspberry-like hierarchical pore ZSM-5 molecular sieve monocrystals are added into an aqueous solution of ammonium nitrate with a concentration of 1 mol / L, the obtained mixture is heated to 80°C and ammonium exchanged for 2 hours, followed by suction filtration and drying (drying temperature 80°C). The ammonium exchange-suction filtration-drying is repeated for 3 times, and the solid sample after the last drying is transferred to a muffle furnace and heated to 550°C for calcination for 7 hours, and the raspberry-like H-type hierarchical pore ZSM-5 molecular sieve monocrystals are obtained after natural cooling, which are named as Hier-ZSM-5.

[0042] Example 2 With sodium hydroxide as an alkali source, sodium aluminate as an aluminum source, TPAOH as a structure directing agent, and tetraethyl orthosilicate as a silicon source, deionized water is weighed into a beaker according to a molar ratio of 100SiO2:2Al2O3:30TPAOH:7NaOH:5462H2O, and then the aluminum source, the alkali source, the organic structure directing agent, and the silicon source are sequentially added, and stirred for 6 hours to obtain a molecular sieve precursor solution.

[0043] The above-mentioned molecular sieve precursor solution is transferred to a hydrothermal crystallization kettle, heated to 180°C for hydrothermal crystallization reaction for 24 hours. After filtration, the obtained solid sample is washed with water by centrifugation for 3 times, and then placed in an oven at 80°C for drying overnight. The solid sample is taken out from the oven, transferred to a muffle furnace, heated to 550°C for calcination for 7 hours, and the raspberry-like hierarchical pore ZSM-5 molecular sieve monocrystals are obtained after natural cooling.

[0044] The raspberry-like hierarchical pore ZSM-5 molecular sieve monocrystals are added into an aqueous solution of ammonium nitrate with a concentration of 1 mol / L, the obtained mixture is heated to 80°C and ammonium exchanged for 2 hours, followed by suction filtration and drying (drying temperature 80°C). The ammonium exchange-suction filtration-drying is repeated for 3 times, and the solid sample after the last drying is transferred to a muffle furnace and heated to 550°C for calcination for 7 hours, and the raspberry-like H-type hierarchical pore ZSM-5 molecular sieve monocrystals are obtained after natural cooling, which are named as Hier-ZSM-5.

[0045] 0.1558 g of lanthanum nitrate hexahydrate is dissolved in 2 mL of deionized water, and then 1 g of Hier-ZSM-5 molecular sieve monocrystals is added, the obtained mixture is heated to 50°C and stirred for 3 hours, and the water is evaporated during the stirring. The solid sample thus obtained is transferred to an oven at 80°C for drying for 3 hours, and then placed in a muffle furnace and heated to 550°C for calcination for 7 hours, and the raspberry-like rare earth-loaded H-type hierarchical pore ZSM-5 molecular sieve monocrystals are obtained after natural cooling.

[0046] It can be calculated that the loading amount of the rare earth metal element La in this embodiment is 5wt%, and the corresponding product is recorded as 5La / Hier-ZSM-5.

[0047] Example 3 This example is basically the same as Example 2, except that the weight of lanthanum nitrate hexahydrate is adjusted from 0.1558 g to 0.3117 g. The loading of the rare earth metal element La in the corresponding product is 10 wt%, denoted as 10La / Hier-ZSM-5.

[0048] Example 4 This example is basically the same as Example 2, except that the weight of lanthanum nitrate hexahydrate is adjusted from 0.1558 g to 0.4675 g. The loading of the rare earth metal element La in the corresponding product is 15 wt%, denoted as 15La / Hier-ZSM-5.

[0049] Comparative Example 1 C-ZSM-5 zeolite was synthesized by a conventional hydrothermal method using sodium hydroxide as an alkali source, sodium aluminate as an aluminum source, TPAOH as a structure directing agent, and tetraethyl orthosilicate as a silicon source. The structure directing agent, the aluminum source, and water were mixed and stirred until completely dissolved, and then the silicon source and the alkali source were slowly added. The mixture was continuously stirred for 6 hours to obtain a zeolite precursor solution.

[0050] The zeolite precursor solution was transferred to a hydrothermal crystallization kettle and heated to 180°C for hydrothermal crystallization reaction for 48 hours. After filtration, the obtained solid sample was washed with water by centrifugation for 3 times, and then placed in an oven at 80°C overnight for drying. The solid sample was taken out of the oven and transferred to a muffle furnace, heated to 550°C for calcination for 7 hours, and naturally cooled to obtain a conventional microporous ZSM-5 zeolite.

[0051] The obtained conventional microporous ZSM-5 zeolite was added to an aqueous ammonium nitrate solution with a concentration of 1 mol / L, and the obtained mixture was heated to 80°C for ammonium exchange for 2 hours, followed by suction filtration and drying (drying temperature 80°C). The ammonium exchange-suction filtration-drying was repeated for 3 times, and the solid sample after the last drying was transferred to a muffle furnace, heated to 550°C for calcination for 7 hours, and naturally cooled to obtain an H-type conventional microporous ZSM-5 zeolite, denoted as C-ZSM-5.

[0052] Comparative Example 2 C-ZSM-5 zeolite was synthesized by a conventional hydrothermal method using sodium hydroxide as an alkali source, sodium aluminate as an aluminum source, TPAOH as a structure directing agent, and tetraethyl orthosilicate as a silicon source. The structure directing agent, the aluminum source, and water were mixed and stirred until completely dissolved, and then the silicon source and the alkali source were slowly added. The mixture was continuously stirred for 6 hours to obtain a zeolite precursor solution.

[0053] The above-mentioned molecular sieve precursor solution was transferred into a hydrothermal crystallization kettle, heated to 180°C and hydrothermally crystallized for 48 hours. After filtration, the obtained solid sample was washed with water by centrifugation for 3 times, and then dried in an oven at 80°C overnight. The solid sample was taken out of the oven, transferred into a muffle furnace, heated to 550°C and calcined for 7 hours, and then naturally cooled to obtain a conventional microporous ZSM-5 molecular sieve.

[0054] The conventional microporous ZSM-5 molecular sieve prepared was added into an aqueous solution of ammonium nitrate with a concentration of 1 mol / L, and the obtained mixture was heated to 80°C and ammonium exchanged for 2 hours, followed by suction filtration and drying (drying temperature: 80°C). The ammonium exchange-suction filtration-drying was repeated for 3 times, and the solid sample after the last drying was transferred into a muffle furnace, heated to 550°C and calcined for 7 hours, and then naturally cooled to obtain an H-type conventional microporous ZSM-5 molecular sieve, which was named as C-ZSM-5.

[0055] 0.3117 g of lanthanum nitrate hexahydrate was dissolved in 2 mL of deionized water, and then 1 g of C-ZSM-5 molecular sieve was added. The obtained mixture was heated to 50°C and stirred for 3 h, and at the same time the water was evaporated. The thus obtained solid sample was transferred into an oven at 80°C and dried for 3 h, and then placed in a muffle furnace and heated to 550°C and calcined for 7 hours, and then naturally cooled to obtain a rare earth-loaded H-type conventional microporous ZSM-5 molecular sieve.

[0056] It can be calculated that the loading amount of the rare earth metal element La in the product of the present example is 10 wt%, and the corresponding product is recorded as 10La / C-ZSM-5.

[0057] In order to fully understand the performance of the products prepared in each example, the following tests were carried out on the samples respectively: (1) X-ray diffraction (XRD) test.

[0058] The XRD test results of each molecular sieve single crystal sample prepared in Examples 1-4 are shown in Figure 1 As can be seen from the figure, each sample shows a typical MFI structure diffraction peak, which indicates that the incorporation of La element does not change the crystal structure of ZSM-5.

[0059] (2) Scanning electron microscope (SEM) test.

[0060] The SEM test results of the Hier-ZSM-5 sample prepared in Example 1 are shown in

[0061] Figure 2 ​The SEM test results of the Hier-ZSM-5 sample prepared in Example 1 are shown in FIG. 2. As can be seen from the figure, the Hier-ZSM-5 sample is in a raspberry structure formed by assembling a plurality of nanoparticles, wherein the size of the nanoparticle structural unit is 70-90 nm, and the overall size of the crystal is about 700-900 nm. In addition, there are abundant intercrystalline mesopores in the Hier-ZSM-5 sample, which indicates that the hierarchical-pore ZSM-5 zeolite is successfully synthesized.

[0062] The SEM test results of the C-ZSM-5 sample prepared in Comparative Example 1 are shown in FIG. 3. As can be seen from the figure, the sample has good dispersibility, and the crystal size is 2-3 μm, without hierarchical-pore structure. Figure 4

[0063] (3) Transmission electron microscopy (TEM) test.

[0064] The TEM test results of the Hier-ZSM-5 sample prepared in Example 1 are shown in FIG. 2. As can be seen from the figure, the Hier-ZSM-5 sample is in a raspberry structure formed by assembling a plurality of nanoparticles, wherein the size of the nanoparticle structural unit is 70-90 nm, and the overall size of the crystal is about 700-900 nm. In addition, there are abundant intercrystalline mesopores in the Hier-ZSM-5 sample, which indicates that the hierarchical-pore ZSM-5 zeolite is successfully synthesized. Figure 3 Figure 3 The TEM test results of the Hier-ZSM-5 sample prepared in Example 1 are shown in FIG. 2. As can be seen from the figure, the Hier-ZSM-5 sample is in a raspberry structure formed by assembling a plurality of nanoparticles, wherein the size of the nanoparticle structural unit is 70-90 nm, and the overall size of the crystal is about 700-900 nm. In addition, there are abundant intercrystalline mesopores in the Hier-ZSM-5 sample, which indicates that the hierarchical-pore ZSM-5 zeolite is successfully synthesized.

[0065] (4) Specific surface area test.

[0066] The nitrogen adsorption-desorption isotherms of the zeolite samples prepared in Examples 1-4 and Comparative Examples 1-2 were recorded by using a JW-BK300C specific surface area analyzer at 77 K. Before measurement, each sample was vacuum degassed at 300°C for 12 hours. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and the micropore surface area and volume were obtained by the t-plot method. The relevant test results are shown in Table 1.

[0067] Table 1 Comparison table of pore data of the zeolite samples prepared in Examples 1-4 and Comparative Examples 1-2

[0068] As can be seen from the table, due to the existence of the raspberry structure and the hierarchical-pore structure, the Hier-ZSM-5 zeolite monocrystal sample prepared in Example 1 has higher specific surface area and external specific surface area than the conventional microporous C-ZSM-5 sample prepared in Comparative Example 1. Compared with Examples 1 and Comparative Example 1, the specific surface area of the rare earth-loaded H-type ZSM-5 zeolite samples in Examples 2-4 and Comparative Example 2 slightly decreases, which is because the La species blocks part of the micropore channels of the zeolite.

[0069] (5) MTO catalytic performance test.​​

[0070] The catalytic performance of the molecular sieve samples prepared in Examples 1-4 and Comparative Examples 1-2 in the methanol to olefins reaction was tested in a fixed bed equipped with a quartz tube reactor. Each of the molecular sieve samples was activated at 500°C for 2h before the reaction, with a nitrogen flow rate of 90 mL / min; then cooled to 490°C to start the MTO reaction, with a mass space velocity of 3.2h -1 , and the test results are shown in Table 1. The lifetime of the molecular sieve catalyst is defined as the duration of the MTO reaction in which the methanol conversion is higher than 50%. Figure 5

[0071] As can be seen from the figure, the introduction of La element significantly improves the reaction lifetime of the molecular sieve catalyst, and the molecular sieve sample of Example 1 exhibits a longer catalytic lifetime than Comparative Example 1 due to the presence of the raspberry-like structure and hierarchical pore structure. It is worth noting that under the same La loading (10wt% La), the lifetime of the raspberry-like hierarchical pore molecular sieve single crystal sample is improved more significantly than that of the conventional microporous molecular sieve sample, thereby confirming that the presence of hierarchical pores can effectively improve the diffusion limitation of the molecular sieve catalyst.​

Claims

1. A single-crystal derivative of ZSM-5 molecular sieve having a mulberry-like structure and a hierarchical pore structure, characterized in that: The derivatives include mulberry-shaped graded pore ZSM-5 molecular sieve single crystals, mulberry-shaped H-type graded pore ZSM-5 molecular sieve single crystals, and mulberry-shaped rare earth-loaded H-type graded pore ZSM-5 molecular sieve single crystals.

2. The derivative as claimed in claim 1, characterized in that: The mulberry-like structure is a crystal with an overall particle size of 700nm to 900nm formed by the orderly assembly and aggregation of nanocrystalline structural units with a diameter of 70nm to 90nm.

3. The method for preparing the molecular sieve single-crystal derivative according to any one of claims 1-2, characterized in that... The method includes: mixing silicon source, aluminum source, organic structure directing agent, alkali source and water in proportion to prepare a precursor solution; heating the precursor solution to crystallize and then separating the solid and liquid components; calcining the obtained solid product to obtain mulberry-shaped graded porous ZSM-5 molecular sieve single crystals; subjecting the mulberry-shaped graded porous ZSM-5 molecular sieve single crystals to ammonium exchange treatment; calcining to obtain mulberry-shaped H-type graded porous ZSM-5 molecular sieve single crystals; loading rare earth elements onto the mulberry-shaped H-type graded porous ZSM-5 molecular sieve single crystals using an impregnation method; and calcining to obtain mulberry-shaped rare earth-loaded H-type graded porous ZSM-5 molecular sieve single crystals.

4. The method as described in claim 3, characterized in that: The aluminum source is selected from at least one of aluminum isopropoxide, aluminum hydroxide, sodium aluminate, aluminum nitrate, and aluminum sulfate; the alkali source is selected from at least one of sodium hydroxide and ammonia water; the organic structure directing agent is selected from at least one of tetrapropylammonium hydroxide and tetraethylammonium hydroxide; and the silicon source is selected from at least one of silica sol and tetraethyl orthosilicate.

5. The method as described in claim 4, characterized in that: The aluminum source is sodium aluminate, the alkali source is sodium hydroxide, the organic structure directing agent is tetrapropylammonium hydroxide, and the silicon source is tetraethyl orthosilicate. The molar ratio of silicon source (calculated as SiO2), aluminum source (calculated as Al2O3), organic structure directing agent, alkali source, and water is 95~105:1~5:25~35:5~10:5000~6000, preferably 100:2:30:7:5462.

6. The method as described in claim 3, characterized in that: The hydrothermal crystallization reaction temperature of the precursor solution is 150–180℃, and the hydrothermal crystallization reaction time is 24–48h.

7. The method as described in claim 3, characterized in that... The ammonium exchange process includes: adding mulberry-shaped graded porous ZSM-5 molecular sieve single crystals into ammonium nitrate solution for thorough soaking, filtering and drying the solid, repeating the soaking, filtering and drying process multiple times, and finally calcining the solid product to obtain mulberry-shaped H-type graded porous ZSM-5 molecular sieve single crystals.

8. The method as described in claim 3, characterized in that The rare earth element loading process specifically includes: mixing mulberry-shaped H-type graded pore ZSM-5 molecular sieve single crystals with rare earth impregnation solution, stirring and heating to evaporate the water, and then calcining the obtained solid product to obtain mulberry-shaped rare earth loaded H-type graded pore ZSM-5 molecular sieve single crystals. The rare earth impregnation solution is specifically an aqueous solution of lanthanum nitrate, and the rare earth loading is 2wt% to 20wt%.

9. The method according to any one of claims 3-8, characterized in that: The atmosphere used in each roasting step is air, and the roasting temperature is 500-650℃.

10. The application of the molecular sieve single-crystal derivative according to any one of claims 1-2 in the catalytic MTO reaction to prepare olefins.

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