Preparation method of small crystal zsm-5 molecular sieve catalyst and application thereof in butene catalytic cracking reaction

Abstract

The application belongs to the technical field of molecular sieve catalytic materials, and particularly relates to a preparation method of small-grain ZSM-5 molecular sieve catalyst and application of the small-grain ZSM-5 molecular sieve catalyst in a butene catalytic cracking reaction. The method uses white carbon black as a silicon source, an aluminum compound as an aluminum source, and an aqueous solution of tetrapropylammonium hydroxide as an organic template solution. The silicon source, the aluminum source, and the organic template solution are directly ground and mixed, and then placed in a stainless steel synthesis kettle. Crystallization is carried out at 160-200 DEG C for 48-96 hours. After centrifugal washing, separation, drying, and calcination, the small-grain ZSM-5 molecular sieve catalyst is obtained. The method does not add ZSM-5 seeds, and does not additionally add water during the preparation process. The obtained molecular sieve has an MFI topological structure, a grain size of 130-200 nm, and a silicon aluminum atomic ratio Si / Al of 50-300. When the catalyst is used in a 1-butene catalytic cracking reaction for preparing ethylene and propylene, the catalyst has good butene conversion performance and low-carbon olefin selectivity.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve catalytic materials technology, specifically to a method for preparing a small-crystal ZSM-5 molecular sieve catalyst and its application in the catalytic cracking of butene to produce low-carbon olefins. Background Technology

[0002] With the development of industries such as oil refining, ethylene cracking, methanol-to-olefins (MTO), and catalytic cracking, the output of by-product C4 hydrocarbons is continuously increasing. Butene, as an important component of C4 olefins, is widely available and relatively inexpensive. However, in actual industrial processes, some C4 hydrocarbon resources are still mainly used as fuel components or low-value-added feedstocks, failing to achieve full high-value utilization. Therefore, developing efficient butene conversion technologies is of great significance for improving the utilization rate of C4 resources, expanding the sources of low-carbon olefins, and optimizing the structure of integrated refining and chemical industries. Ethylene and propylene are among the most important basic chemical raw materials in the modern petrochemical industry, widely used in the production of polyethylene, propylene oxide, acrylonitrile, and other bulk chemicals. Catalytic cracking of butene to produce ethylene, propylene, and other low-carbon olefins is one of the important pathways to achieve high-value utilization of C4 olefins.

[0003] The catalytic cracking of butene typically requires an acidic catalyst. Molecular sieve catalysts have attracted widespread attention in butene cracking due to their regular pore structure, tunable acidic centers, high hydrothermal stability, and good shape-selective catalytic performance. Among them, ZSM-5 molecular sieve, with its MFI topology, has pore sizes that match the molecular dynamics diameter of low-carbon hydrocarbons, allowing for some degree of control over reactant conversion pathways and product distribution. Therefore, it is considered a typical catalytic material for the catalytic cracking of butene to low-carbon olefins.

[0004] However, existing ZSM-5 molecular sieves still have some shortcomings in the catalytic cracking of butene. Conventional ZSM-5 molecular sieves typically have crystallite sizes in the micrometer range (>1 μm). The long micropores severely restrict diffusion, resulting in high diffusion resistance and long residence times for butene molecules and cracking products within these micrometer-sized channels. This leads to further side reactions within the channels, such as hydrogen transfer, aromatization, oligomerization, cyclization, or coking, which would otherwise be rapidly desorbed. This not only reduces the selectivity for target low-carbon olefins like ethylene and propylene but also generates large polycyclic aromatic hydrocarbons (coking precursors) that clog the channels, causing rapid catalyst deactivation. Therefore, reducing the molecular sieve crystallite size and shortening the diffusion path are important ways to improve the catalytic cracking performance of butene.

[0005] Studies have shown (see references: CN201010171630.7, CN117658169A) that reducing the grain size of ZSM-5 can significantly shorten the diffusion paths of reactants and products in systems such as naphtha cracking and ethanol conversion to ethylene. For butene catalytic cracking, small-grained ZSM-5 molecular sieves, due to their shorter diffusion paths, larger external surface area, and more accessible acidic sites, are beneficial for improving the mass transfer efficiency of reactants and products. This is expected to maintain a high butene conversion while improving the selectivity for low-carbon olefins such as ethylene and propylene, and also improving catalyst stability.

[0006] Currently, small-grained ZSM-5 molecular sieves are typically prepared using hydrothermal synthesis. This method generally requires a large amount of water as a solvent and often relies on the addition of seed crystals to induce nucleation and control grain size. While these methods can yield ZSM-5 molecular sieves within a certain particle size range, they still suffer from problems such as high solvent consumption, high mother liquor discharge, long crystallization cycles, complex operation steps, and strong dependence on seed crystals. Furthermore, while existing solid-phase grinding methods for synthesizing ZSM-5 molecular sieves can reduce solvent usage, they still have significant shortcomings in controlling grain size. For example, the sample synthesized in the patent published by Zhejiang University (CN201210118788.7) is still a relatively large micron-sized molecular sieve. In existing technologies, seed crystals still need to be added to achieve grain size control, or additional components such as mineralizers and alkalinity adjusters are required.

[0007] Therefore, it is necessary to develop a simple, seedless, and green preparation method for small-crystal ZSM-5 molecular sieves suitable for wide-range control of silicon-aluminum ratio, and to apply it to butene catalytic cracking reaction to improve the efficiency of high-value utilization of butene resources and the selectivity of low-carbon olefin products. Summary of the Invention

[0008] First, the present invention aims to overcome the shortcomings of the prior art and provide a method for preparing small-crystal ZSM-5 molecular sieve catalysts, so as to solve the problems of strong dependence on seed crystals, need to add alkali sources or mineralizers and other auxiliary agents, serious waste of water resources, and complex process in the existing preparation process.

[0009] Secondly, this invention provides a method for preparing small-grained ZSM-5 molecular sieves by solid-phase grinding without adding ZSM-5 seed crystals, alkali sources, mineralizers, or additional water. This method also features a wide range of adjustable silicon-aluminum ratios.

[0010] Furthermore, this invention also provides the application of the small-crystal ZSM-5 molecular sieve in the catalytic cracking of butene to produce low-carbon olefins, in order to solve the problem of insufficient selectivity for low-carbon olefins caused by the large crystal size of existing catalysts.

[0011] To solve the above-mentioned technical problems, the present invention is implemented as follows:

[0012] In a first aspect, the present invention provides a method for preparing a small-crystal ZSM-5 molecular sieve catalyst, which comprises the following raw materials: silica as a silicon source, an aluminum-containing compound as an aluminum source, and tetrapropylammonium hydroxide aqueous solution as an organic template agent; the preparation method includes the following steps:

[0013] (1) The silica, aluminum-containing compound and tetrapropylammonium hydroxide aqueous solution are directly ground and mixed to obtain a precursor mixture;

[0014] (2) Place the precursor mixture obtained in step (1) in a synthesis vessel and crystallize it at 160-200°C for 48-96 hours to obtain the crystallized product;

[0015] (3) The crystallized product obtained in step (2) is washed, dried and calcined to obtain the target product, small-crystal ZSM-5 molecular sieve catalyst.

[0016] In the preparation method, no ZSM-5 seed crystals, no alkali source, and no mineralizing agent are added to the raw materials; no additional water is added in step (1) except for the water introduced by the tetrapropylammonium hydroxide aqueous solution.

[0017] Furthermore, the aluminum-containing compound is one or a mixture of two or more of nano-alumina, aluminum isopropoxide, aluminum sulfate, and aluminum nitrate.

[0018] Further, in step (1), the silica is calculated as SiO2, the aluminum-containing compound is calculated as Al2O3, and the molar ratio of each raw material is: SiO2: Al2O3: tetrapropylammonium hydroxide: H2O = 100~600 : 1 : 8~15 : 135~255.

[0019] Furthermore, the silicon-to-aluminum atomic ratio (Si / Al) of the small-crystal ZSM-5 molecular sieve catalyst is 50–300.

[0020] Further, in step (1), the direct grinding time is 5 to 120 minutes; in step (2), the crystallization temperature is 165°C and the crystallization time is 60 hours; in step (3), the drying temperature is 100 to 140°C and the calcination temperature is 500 to 600°C.

[0021] Further, in step (1), the direct grinding time is 10 to 60 minutes; in step (3), the drying temperature is 110°C; and the calcination temperature is 550°C.

[0022] Secondly, the present invention also provides a small-crystal ZSM-5 molecular sieve catalyst prepared by the above preparation method, wherein the small-crystal ZSM-5 molecular sieve catalyst has an MFI topology, a crystal size of 130-200 nanometers, and a silicon-aluminum atomic ratio of Si / Al of 50-300.

[0023] Thirdly, the present invention also provides an application of the above-mentioned small-crystal ZSM-5 molecular sieve catalyst in the catalytic cracking of butene to produce low-carbon olefins. The small-crystal ZSM-5 molecular sieve catalyst is shaped, pulverized, and then loaded into a fixed-bed reactor, where it is activated at 450–650°C for 6–12 hours under an activating gas atmosphere. After activation, 1-butene feedstock is introduced, and the reaction proceeds at a weight hourly space velocity (WHSV) of 5–20 h⁻¹. -1 The butene catalytic cracking reaction is carried out at a pressure of 0.1–0.5 MPa and a reaction temperature of 450–600 °C, so that 1-butene is cracked into ethylene and propylene.

[0024] Furthermore, the activating gas is high-purity nitrogen.

[0025] Furthermore, the reaction temperature is 550°C, the reaction pressure is 0.1 MPa, and the weight hourly space velocity (WHSV) is 15 h⁻¹. -1 .

[0026] This invention uses silica as the silicon source, aluminum-containing compounds (nano-alumina, aluminum isopropoxide, aluminum sulfate, or aluminum nitrate) as the aluminum source, and tetrapropylammonium hydroxide aqueous solution as the organic template agent. The mixture is directly ground and mixed in a stainless steel synthesis reactor, then crystallized at 160–200°C for 48–96 hours. After centrifugation, washing, drying, and calcination, a small-crystal ZSM-5 molecular sieve catalyst is obtained. Due to its small crystallite size of 130–200 nm and adjustable silicon-to-aluminum atomic ratio (Si / Al), the obtained ZSM-5 molecular sieve catalyst can shorten the diffusion path of reactants and products in the molecular sieve channels during the catalytic cracking reaction of butene, reducing secondary reactions and carbon deposition tendency, thereby improving the selectivity of 1-butene cracking to produce ethylene, propylene, and other low-carbon olefins.

[0027] The technical concept of this invention is mainly reflected in the following aspects:

[0028] (1) The synthesis of existing small-crystal ZSM-5 mostly relies on seed-induced nucleation. This invention can obtain nanoscale molecular sieves with a grain size of 130-200 nm without adding ZSM-5 seeds.

[0029] (2) No additional water is added during the preparation process of this invention. It relies solely on the water introduced from the aqueous solution of tetrapropylammonium hydroxide (molar ratio of H2O / SiO2 is about 135-255:100-600), which is a low-water solid-phase grinding method, unlike the large amount of solvent used in traditional hydrothermal synthesis.

[0030] (3) The present invention enables the silicon source, aluminum source and template agent to come into full contact through direct grinding, eliminating the steps of stirring, aging and aging in traditional hydrothermal synthesis.

[0031] (4) The present invention can achieve a wide range of Si / Al control from 50 to 300.

[0032] (5) The small-crystal ZSM-5 molecular sieve obtained in this invention is used for the catalytic cracking of 1-butene to produce ethylene and propylene. The small crystals shorten the diffusion path and reduce secondary reactions, thereby improving the selectivity of low-carbon olefins. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the preparation process of the small-crystal ZSM-5 molecular sieve of the present invention;

[0034] Figure 2 The XRD patterns are of the products of Embodiment 1 and Comparative Example 1 of the present invention;

[0035] Figure 3 This is a SEM image of the product obtained in Example 1 using aluminum nitrate as the aluminum source and Si / Al=300;

[0036] Figure 4 This is a SEM image of the product obtained in Example 2 using aluminum isopropoxide as the aluminum source and Si / Al=200;

[0037] Figure 5 This is a SEM image of the product obtained in Example 3 using aluminum sulfate as the aluminum source and Si / Al=100;

[0038] Figure 6 This is a SEM image of the product obtained in Example 4 using nano-alumina as the aluminum source and Si / Al=50.

[0039] Figure 7 The NH3-TPD diagrams are for the Si / Al = 50-300 products obtained in Examples 1-4.

[0040] Figure 8 The graph shows the changes in 1-butene catalytic cracking conversion rate and the selectivity of ethylene and propylene products as a function of reaction time for the products of Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0041] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0042] Example 1

[0043] This embodiment provides a method for preparing small-crystal ZSM-5 molecular sieves.

[0044] (1) Preparation of precursor mixture: using silica (specific surface area of ​​150 m²) 2 The silicon source was silica (7-40 nm in particle size), the aluminum source was aluminum nitrate, and the organic template agent was tetrapropylammonium hydroxide aqueous solution. The raw materials were weighed according to a molar ratio of SiO2:Al2O3:tetrapropylammonium hydroxide:H2O = 600:1:13.5:228.6, where SiO2 was derived from silica, Al2O3 from aluminum nitrate, and H2O from water introduced from the tetrapropylammonium hydroxide aqueous solution; no additional water was added during the preparation process. Silica, aluminum nitrate, and a 40% tetrapropylammonium hydroxide aqueous solution were mixed and directly ground for 30 minutes to ensure thorough and uniform mixing of the silicon source, aluminum source, and organic template agent, yielding a precursor mixture.

[0045] (2) Crystallization treatment: The precursor mixture obtained in step (1) is transferred to a stainless steel synthesis reactor, sealed, and crystallized at 165°C for 60 hours to obtain the crystallized product. In this embodiment, ZSM-5 seed crystals are not added during the preparation process.

[0046] (3) Post-processing: The crystallized product obtained in step (2) is centrifuged and washed until the washing liquid is close to neutral. The resulting solid product is then dried at 110°C and calcined at 550°C to obtain a small-crystal ZSM-5 molecular sieve catalyst.

[0047] Appendix Figure 1 This is a schematic diagram of the green preparation process for this small-crystal ZSM-5 molecular sieve. (Attached) Figure 2 The X-ray diffraction (XRD) pattern of the product in Example 1 shows that the product has a typical MFI structure and no other impurities, exhibiting high crystallinity. (See attached image.) Figure 3 The image shows a scanning electron microscope (SEM) image of the product from Example 1. The image indicates that the obtained product is a pure phase, spherical in shape, with an average grain size of approximately 130 nm, belonging to nanoscale small-grained molecular sieves.

[0048] (4) Evaluation of 1-butene catalytic cracking reaction: Weigh 1g of the small-crystal ZSM-5 molecular sieve catalyst obtained in step (3), pulverize it into 20-40 mesh particles after extrusion molding, and pack it into a fixed-bed reactor. Use high-purity nitrogen as the activation gas and activate it at 550℃ for 10 hours.

[0049] After activation, 1-butene feedstock was introduced into the fixed-bed reactor, and the reaction was carried out at a temperature of 550℃, a pressure of 0.1 MPa, and a weight hourly space velocity of 15 h⁻¹.-1 Under certain conditions, butene is subjected to catalytic cracking reaction to convert 1-butene into low-carbon olefin products including ethylene and propylene.

[0050] Figure 8 The reaction results show that the small-crystal ZSM-5 molecular sieve catalyst prepared in this example can be used for the catalytic cracking of 1-butene to produce low-carbon olefins. The 1-butene conversion rate after 12 hours of reaction is 64%, and it exhibits good butene conversion performance and low-carbon olefin selectivity. The overall selectivity of ethylene and propylene after 12 hours of reaction is 56%.

[0051] Example 2

[0052] This embodiment is basically the same as Embodiment 1, except that: aluminum isopropoxide is used as the aluminum source, the target silicon-aluminum atomic ratio Si / Al is 200, and the molar ratio of the feed is SiO2:Al2O3:tetrapropylammonium hydroxide:H2O = 400:1:13.5:228.6.

[0053] The resulting product is a small-crystal ZSM-5 molecular sieve with an average crystal size of approximately 180 nm.

[0054] Example 3

[0055] This embodiment is basically the same as Embodiment 1, except that: aluminum sulfate is used as the aluminum source, the target silicon-aluminum atomic ratio Si / Al is 100, and the molar ratio of the feed is SiO2:Al2O3:tetrapropylammonium hydroxide:H2O = 200:1:13.5:228.6.

[0056] The resulting product is a small-crystal ZSM-5 molecular sieve with a crystal size of 180 nm.

[0057] Example 4

[0058] This embodiment is basically the same as Embodiment 1, except that: the aluminum source is nano-alumina, the target silicon-aluminum atomic ratio Si / Al is 50, and the molar ratio of the feed is SiO2:Al2O3:tetrapropylammonium hydroxide:H2O = 100:1:13.5:228.6.

[0059] The resulting product is a small-crystal ZSM-5 molecular sieve with a crystal size of 170 nm.

[0060] Figure 7 The acid content characterization results of the Si / Al=50-300 products obtained in Examples 1-4 show that as the silicon-aluminum atomic ratio increases, the acid content measured by NH3-TPD gradually decreases. This indicates that the method can achieve the technical effect of wide-range control of the silicon-aluminum ratio.

[0061] Comparative Example 1

[0062] This comparative example uses a commercially available ZSM-5 molecular sieve catalyst with an MFI topology ( Figure 2 As shown in the figure, its silicon-to-aluminum atomic ratio (Si / Al) is 300, and its average grain size is about 2 μm. It undergoes 1-butene catalytic cracking reaction under the same reaction conditions as in Example 1.

[0063] Figure 8 Comparing the reaction results of this comparative example with those of the small-crystal ZSM-5 molecular sieve catalyst obtained in Example 1, it can be found that both have similar butene conversion rates, approximately 65% ​​(Comparative Example 1) and 64% respectively after 12 hours of reaction. However, the overall selectivity of the target product in the example is approximately 12 percentage points higher than that in the comparative example. Under the same silicon-aluminum atomic ratio, the difference in butene cracking performance between the catalyst obtained in Example 1 and the commercial ZSM-5 catalyst in Comparative Example 1 indicates that the small-crystal structure prepared by solid-phase milling without seeding has a positive effect on improving the distribution of butene cracking products. This may be achieved by shortening the diffusion path, reducing the residence time of generated propylene and ethylene in the channels, thereby reducing the possibility of side reactions such as deep cracking, hydrogen transfer, aromatization, and coking, and thus improving the selectivity of low-carbon olefins.

[0064] In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. In the description of this specification, the terms "one embodiment," "some embodiments," "embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the 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. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0065] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a small-crystal ZSM-5 molecular sieve catalyst, characterized in that, The preparation method comprises the following raw materials: silica as a silicon source, an aluminum-containing compound as an aluminum source, and tetrapropylammonium hydroxide aqueous solution as an organic template agent; the preparation method includes the following steps: (1) The silica, aluminum-containing compound and tetrapropylammonium hydroxide aqueous solution are directly ground and mixed to obtain a precursor mixture; (2) Place the precursor mixture obtained in step (1) in a synthesis vessel and crystallize it at 160-200°C for 48-96 hours to obtain the crystallized product; (3) The crystallized product obtained in step (2) is washed, dried and calcined to obtain the target product, small-crystal ZSM-5 molecular sieve catalyst. In the preparation method, no ZSM-5 seed crystals, no alkali source, and no mineralizing agent are added to the raw materials; no additional water is added in step (1) except for the water introduced by the tetrapropylammonium hydroxide aqueous solution.

2. The method for preparing the small-crystal ZSM-5 molecular sieve catalyst according to claim 1, characterized in that: The aluminum-containing compound is one or a mixture of two or more of nano-alumina, aluminum isopropoxide, aluminum sulfate, and aluminum nitrate.

3. The method for preparing the small-crystal ZSM-5 molecular sieve catalyst according to claim 1, characterized in that: In step (1), the precipitated silica is calculated as SiO2, the aluminum-containing compound is calculated as Al2O3, and the molar ratio of each raw material is: SiO2: Al2O3: tetrapropylammonium hydroxide: H2O = 100~600 : 1 : 8~15 : 135~255.

4. The preparation method according to claim 3, characterized in that: The silicon-to-aluminum atomic ratio (Si / Al) of the small-crystal ZSM-5 molecular sieve catalyst is 50–300.

5. The method for preparing the small-crystal ZSM-5 molecular sieve catalyst according to claim 4, characterized in that: In step (1), the direct grinding time is 5 to 120 minutes; in step (2), the crystallization temperature is 165°C and the crystallization time is 60 hours; in step (3), the drying temperature is 100 to 140°C and the calcination temperature is 500 to 600°C.

6. The method for preparing the small-crystal ZSM-5 molecular sieve catalyst according to claim 5, characterized in that: In step (1), the direct grinding time is 10 to 60 minutes; in step (3), the drying temperature is 110°C; and the calcination temperature is 550°C.

7. The small-crystal ZSM-5 molecular sieve catalyst prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The small-crystal ZSM-5 molecular sieve catalyst has an MFI topology, a grain size of 130–200 nm, and a silicon-to-aluminum atomic ratio (Si / Al) of 50–300.

8. The application of the small-crystal ZSM-5 molecular sieve catalyst as described in claim 7 in the catalytic cracking of butene to produce low-carbon olefins, characterized in that: The small-crystal ZSM-5 molecular sieve catalyst was shaped, pulverized, and then loaded into a fixed-bed reactor. It was activated at 450–650°C for 6–12 hours under an activating gas atmosphere. After activation, 1-butene feedstock was introduced, and the reaction was carried out at a weight hourly space velocity (WHSV) of 5–20 h⁻¹. -1 The butene catalytic cracking reaction is carried out at a pressure of 0.1–0.5 MPa and a reaction temperature of 450–600 °C, so that 1-butene is cracked into ethylene and propylene.

9. The application of the small-crystal ZSM-5 molecular sieve catalyst according to claim 8 in the catalytic cracking of butene to produce low-carbon olefins, characterized in that: The activating gas is high-purity nitrogen.

10. The application of the small-crystal ZSM-5 molecular sieve catalyst according to claim 9 in the catalytic cracking of butene to produce low-carbon olefins, characterized in that: The reaction temperature was 550°C, the reaction pressure was 0.1 MPa, and the weight hourly space velocity (WHSV) was 15 h⁻¹. -1 .

Images