Boron-containing ZSM-5 molecular sieve and preparation method thereof

Abstract

The invention discloses a boron-containing ZSM-5 molecular sieve and a preparation method thereof. The skeleton of the boron-containing ZSM-5 molecular sieve contains boron, aluminum and silicon, and the absolute value of the difference between the crystal bulk phase silicon-aluminum ratio measured by XRF and the crystal surface silicon-aluminum ratio measured by EDS is less than or equal to 1. The preparation method of the boron-containing ZSM-5 molecular sieve comprises the following steps: mixing a boron source, a first silicon source, a seed crystal and water, performing pretreatment, adding a second silicon source, an aluminum source and an inorganic additive, performing uniform mixing, and performing crystallization, solid-liquid separation, washing, ion exchange and drying to obtain the boron-containing ZSM-5 molecular sieve. According to the invention, the boron-containing ZSM-5 molecular sieve with uniform distribution of aluminum inside and outside the crystal can be prepared in the absence of a template agent and an organic additive.

Description

Technical Field

[0001] This invention relates to a boron-containing ZSM-5 molecular sieve and its preparation method, belonging to the field of catalytic materials technology. Background Technology

[0002] ZSM-5 molecular sieves, due to their unique three-dimensional cross-channel structure and excellent acidity and stability, are widely used in catalytic cracking / pyrolysis, alkylation, disproportionation, isomerization, and methanol conversion. Modifying ZSM-5 molecular sieves is an important way to further improve their reactivity. Introducing heteroatoms during the synthesis of ZSM-5 molecular sieves can effectively modulate their acidity distribution and is one of the most widely studied modification methods.

[0003] Currently, numerous heteroatom-substituted ZSM-5 molecular sieves, such as those substituted with Ga, B, Mg, Zn, Fe, and Ti, have been successfully synthesized. The introduction of boron can effectively modulate the acidity of ZSM-5 molecular sieves, resulting in higher reaction selectivity in reactions such as methanol-to-olefins (MTO) and methanol-to-propylene (MTP). ZSM-5 molecular sieves with a boron-only framework (B-ZSM-5) exhibit extremely low MTO conversion rates due to their weak acidity. ZSM-5 molecular sieves with both aluminum and boron in their framework (B-Al-ZSM-5) possess moderately strong acidity and have attracted considerable attention from researchers.

[0004] CN103708497A discloses a nanoparticle-stacking B-Al-ZSM-5 zeolite catalyst for methanol-to-olefins (MTP) and its preparation method and application. By introducing a boron source and MFI-type zeolite seeds into the ZSM-5 zeolite synthesis system (silicon source, aluminum source, alkali source, template agent, and water), the synthesized nanoparticle-stacking B-Al-ZSM-5 zeolite exhibits high crystallinity and controllable crystal size. When used in the MTP reaction, it can improve the diffusion rate, enhance resistance to carbon deposition, and improve the acid properties of the ZSM-5 molecular sieve, which is beneficial for enhancing propylene selectivity and extending catalyst lifetime.

[0005] CN103301880A discloses a method for preparing ZSM-5 molecular sieves containing heteroatoms and their application in the dehydrogenation of methanol to formaldehyde. Tetrapropylammonium bromide is dissolved in water, followed by the sequential addition of 1,6-hexanediamine, NaOH, and boric acid. The mixture is stirred until dissolved, and then silica is added while stirring. The reaction solution is poured into a crystallization vessel. After crystallization, the product is filtered and washed with deionized water until neutral. After drying, ZSM-5 raw powder containing heteroatoms is obtained. The raw powder is calcined at 550°C with air for 2 hours to remove the template agent, yielding the molecular sieve. This technique produces ZSM-5 molecular sieves containing heteroatoms containing boron with high crystallinity and good reproducibility using mixed template agents. Furthermore, boron-containing ZSM-5 molecular sieves with a silicon-to-boron ratio of 5.5–10 exhibit high catalytic activity and formaldehyde selectivity.

[0006] CN104098110A discloses a method for preparing B-A1-ZSM-5 zeolite with controllable particle size and its application. The method uses boron, aluminum, silicon, alkali, inorganic salts, and a template agent as raw materials. Seed crystals are added to the reaction system, followed by crystallization in a crystallization reactor, filtration, washing, drying, and calcination to obtain B-A1-ZSM-5 zeolite with a particle size of 100 nm-15 μm. This technology has a simple synthesis process and allows for the recycling of waste liquid during the synthesis process, thereby reducing wastewater discharge, raw material consumption, and production costs. By adding one of ZSM-5, BZSM-5, or Silicalite-1 nano-zeolite seed crystals to the reaction system, the particle size of B-A1-ZSM-5 zeolite can be effectively adjusted. When this B-A1-ZSM-5 zeolite is used to catalyze the synthesis of propylene from methanol, it exhibits high propylene selectivity, methanol conversion rate, and catalyst lifetime.

[0007] The synthesis of B-A1-ZSM-5 still requires the introduction of a template agent, but the introduction of a template agent often increases the synthesis cost. Furthermore, the introduction of a template agent leads to the discharge of wastewater containing organic matter, and the high-temperature calcination removal of the template agent results in high energy consumption and air pollution. Therefore, template-free methods for preparing B-A1-ZSM-5 still need further development.

[0008] CN114751426A discloses a method for preparing B-Al-ZSM-5 molecular sieves and their applications. This method, without adding organic ammonium, directly introduces a high-concentration system of boron source, MFI-type molecular sieve seed crystal aid, and solid silicon-aluminum source into an alkaline solution. Following hydrothermal crystallization, filtration, washing, and drying, it efficiently synthesizes B-Al-ZSM-5 molecular sieves with a high silicon-to-aluminum ratio. The advantages of this technology are high raw material utilization, high single-reactor yield, low cost, and no nitrogen oxide emissions. However, this technology still introduces alcohol compounds and polymers as additives, and the introduction of organic additives can still cause environmental pollution.

[0009] The introduction of boron often affects the synthesis of ZSM-5 molecular sieves. Therefore, the synthesis of B-Al-ZSM-5 usually requires the introduction of template agents or organic additives.

[0010] Wei Zhou et al. compared the synthesis of B-Al-ZSM-5 without a template agent with the synthesis using a seed crystal method (Journal of Solid State Chemistry 179(2006)855-865), and found that the seed crystal method for synthesizing B-Al-ZSM-5 easily produces impure phases. The study also pointed out that the template-free synthesized B-Al-ZSM-5 has more defect sites, resulting in poor hydrothermal stability and a tendency for structural collapse during high-temperature hydrothermal treatment.

[0011] In summary, existing organic template-free synthesis systems for B-Al-ZSM-5 molecular sieves still suffer from problems such as narrow synthesis phase regions, easy generation of impure phases, and poor hydrothermal stability. Furthermore, traditional organic template-free ZSM-5 molecular sieve synthesis methods often result in silicon-rich internal crystals and aluminum-rich surface crystals, with uneven aluminum distribution throughout the crystal. The aluminum-rich surface also makes the ZSM-5 molecular sieve surface more susceptible to secondary reactions, leading to poorer reaction selectivity. Summary of the Invention

[0012] To address at least one of the aforementioned technical problems, the present invention aims to provide a boron-containing ZSM-5 molecular sieve and its preparation method. The boron-containing ZSM-5 molecular sieve of the present invention features a uniform aluminum distribution throughout the crystal structure.

[0013] To achieve the above objectives, the first aspect of the present invention provides a boron-containing ZSM-5 molecular sieve, wherein the framework of the boron-containing ZSM-5 molecular sieve contains boron, aluminum and silicon, and the absolute value of the difference between the bulk silicon-aluminum ratio (i.e., SiO2 / Al2O3 molar ratio) of the boron-containing ZSM-5 molecular sieve measured by XRF (X-ray fluorescence spectrometry) and the surface silicon-aluminum ratio measured by EDS (energy dispersive X-ray spectroscopy) is ≤1.

[0014] According to a specific embodiment of the present invention, preferably, the silicon-to-aluminum ratio of the bulk phase of the boron-containing ZSM-5 molecular sieve is 16-50, and the silicon-to-aluminum ratio on the crystal surface is 15-51.

[0015] According to a specific embodiment of the present invention, preferably, the boron-aluminum ratio (i.e., B2O3 / Al2O3 molar ratio) of the boron-containing ZSM-5 molecular sieve is 0.2-0.6. This boron-silicon ratio can be determined using inductively coupled plasma atomic emission spectrometry (ICP).

[0016] According to a specific embodiment of the present invention, preferably, the crystallinity of the boron-containing ZSM-5 molecular sieve is 85-100%. The boron-containing ZSM-5 molecular sieve of the present invention has the characteristic of high crystallinity.

[0017] According to a specific embodiment of the present invention, preferably, the total acid content of the boron-containing ZSM-5 molecular sieve, measured by pyridine-IR desorption at 150℃, is 120-220 μmol / g, and the strong acid content, measured by pyridine-IR desorption at 350℃, is 30-55 μmol / g. The boron-containing ZSM-5 molecular sieve of the present invention has the characteristic of low strong acid content.

[0018] According to a specific embodiment of the present invention, preferably, the crystallinity decay rate of the boron-containing ZSM-5 molecular sieve after hydrothermal aging at 800℃ for 2 hours is 10-20%. The boron-containing ZSM-5 molecular sieve of the present invention has the characteristic of good hydrothermal stability. The formula for calculating the crystallinity decay rate is: (crystallinity of boron-containing ZSM-5 molecular sieve - crystallinity of boron-containing ZSM-5 molecular sieve after hydrothermal aging at 800℃ for 2 hours) ÷ crystallinity of boron-containing ZSM-5 molecular sieve × 100%.

[0019] A second aspect of the present invention provides a method for preparing the above-mentioned boron-containing ZSM-5 molecular sieve, comprising the following steps:

[0020] (1) After mixing and pretreating the boron source, the first silicon source, the seed crystal and water, a first mixed system is obtained;

[0021] (2) After the first mixture system, the second silicon source, the aluminum source and the inorganic additive are mixed evenly, the second mixture system is obtained;

[0022] (3) After crystallizing the second mixed system, it is then subjected to at least solid-liquid separation, washing, ion exchange and drying to obtain the boron-containing ZSM-5 molecular sieve;

[0023] Wherein, the first silicon source is an alkaline silicon source, the second silicon source is one or a combination of alkaline silicon source, neutral silicon source and acidic silicon source, and the inorganic additive is an inorganic ammonium salt.

[0024] In the above preparation method, preferably, in step (1), the boron source includes one or a combination of several of boric acid (H3BO3), sodium borate (Na3BO3), and boron oxide (B2O3).

[0025] In the above preparation method, preferably, in step (1), the first silicon source includes water glass, etc.

[0026] In the above preparation method, preferably, in step (1), the seed crystal is a molecular sieve with an MFI crystal structure, and the weight ratio of the amount of seed crystal to the total amount of SiO2 in the first silicon source and the second silicon source is (0.01-0.10):1.

[0027] In the above preparation method, preferably, in step (1), the pretreatment is carried out under stirring conditions, the temperature of the pretreatment is 40-70℃, and the time is 1-4h.

[0028] In the above preparation method, preferably, in step (2), the second silicon source includes one or a combination of several of water glass, silica gel, silica sol and fumed silica.

[0029] In the above preparation method, preferably, in step (2), the aluminum source includes one or a combination of several of sodium aluminate, aluminum sulfate, aluminum chloride and aluminum nitrate.

[0030] In the above preparation method, preferably, in step (2), the inorganic additive includes one or a combination of several of the following: ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0031] In the above preparation method, preferably, in step (2), the weight ratio of the amount of inorganic additive to the total amount of SiO2 in the first silicon source and the second silicon source is (0.02-0.10):1.

[0032] In the above preparation method, preferably, in steps (1) and (2), the weight ratio of SiO2 in the first silicon source to SiO2 in the second silicon source is (0.5-1.7):1.

[0033] In the above preparation method, preferably, in step (2), the molar ratio of each substance in the second mixed system satisfies Na2O:SiO2:Al2O3:B2O3:H2O=(0.07-0.14):1:(0.013-0.042):(0.006-0.03):(14-30).

[0034] In the above preparation method, preferably, in step (1) and / or step (2), an alkaline source such as sodium hydroxide or an acid such as concentrated sulfuric acid may be added to make the molar ratio of each substance in the second mixed system meet the above range.

[0035] In the above preparation method, preferably, in step (3), the crystallization is a two-stage crystallization; the temperature of the first stage crystallization is 110-140℃ and the time of the first stage crystallization is 2-6h; the temperature of the second stage crystallization is 150-190℃ and the time of the second stage crystallization is 12-36h.

[0036] In the above preparation method, in step (3), the ion exchange can be carried out using conventional techniques in the art, such as acid exchange or ammonium salt exchange. The present invention does not impose any special restrictions on the conditions of ion exchange.

[0037] The third aspect of the present invention provides a boron-containing ZSM-5 molecular sieve, which is prepared by the above-described method for preparing boron-containing ZSM-5 molecular sieve.

[0038] The technical solution of the present invention has at least the following advantages and beneficial effects:

[0039] The introduction of boron effectively modulates the acidity of B-Al-ZSM-5 molecular sieves, resulting in higher reaction selectivity in certain catalytic reactions. However, the synthesis of B-Al-ZSM-5 molecular sieves typically requires the introduction of template agents or organic additives, leading to increased costs and environmental pollution. Even when synthesizing B-Al-ZSM-5 without template agents, problems remain, including a narrow synthesis phase region, the formation of impure phases, poor hydrothermal stability, and uneven aluminum distribution within the crystal. This invention, through pretreatment with alkaline silicon and boron sources, the use of two silicon sources, the introduction of inorganic additives, and a two-stage crystallization method involving both low and high temperatures, achieves high-quality synthesis of boron-containing ZSM-5 molecular sieves (i.e., B-Al-ZSM-5 molecular sieves) without template agents or organic additives. The B-Al-ZSM-5 molecular sieve of this invention features low preparation cost, high crystallinity, low strong acid content, good hydrothermal stability, and uniform aluminum distribution throughout the crystal. Attached Figure Description

[0040] Figure 1 The XRD patterns of molecular sieves B-Al-ZSM-5-5 and DZM-2 prepared in Example 5 and Comparative Example 2 are shown.

[0041] Figure 2 The XRD patterns of molecular sieves B-Al-ZSM-5-5 and DZM-2 prepared in Example 5 and Comparative Example 2 after hydrothermal aging at 800℃ for 2 hours are shown. Detailed Implementation

[0042] To provide a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the present invention will now be described in detail below, but this should not be construed as limiting the scope of the invention.

[0043] Analytical methods

[0044] The sample phases were characterized using a D / Max-3C X-ray diffractometer (XRD) manufactured by Rigaku Corporation of Japan, with a phase range of 22.5-25 μm. 0The peak area of ​​the five-finger peak determines the crystallinity of the sample. Test conditions include: X-ray CuK... α The voltage was set to 40kV, the current to 20mA, and the scanning speed to 2° / min.

[0045] The bulk silicon-to-aluminum ratio of the sample was determined using a ZSXPrimus X-ray fluorescence spectrometer (XRF) manufactured by Rigaku Corporation of Japan. The EZ mode was used for scanning, which means that the instrument automatically adjusts the test conditions according to different elements to obtain the spectral peaks of each element in the sample, including the spectral peak information of all elements after fluorine in the periodic table (including fluorine elements). The accompanying software automatically normalizes and calculates all the scanned spectral peak data to obtain the content of each element.

[0046] The surface silicon-to-aluminum ratio of the samples was determined using an X-ray energy dispersive spectrometer (EDAXXM2 60S) attached to a Zeiss Ultra-Plus field emission scanning electron microscope (SEM) from Germany.

[0047] The boron-to-aluminum ratio of the sample was determined using an Agilent 5110 inductively coupled plasma optical emission spectrometer (ICP). Before testing, 50 mg of the sample was stirred overnight in a mixture of 1 mL aqua regia and 100 μL hydrofluoric acid. The hydrofluoric acid was then removed from the mixture in an 80°C oil bath. The hydrofluoric acid-free mixture was then diluted, filtered through an aqueous filter, and brought to a final volume of 10 mL to obtain the test solution.

[0048] The pyridine-infrared (Py-IR) acidity of the sample was determined using a Bruker TENS0R27 Fourier transform infrared spectrometer from Germany. The specific method included: pressing approximately 10 mg of the powder sample into a thin sheet, fixing it in the infrared cell, and first purifying it under vacuum (350℃, 1×10⁻⁶). -3 After 2 hours of cooling to room temperature to adsorb pyridine, the temperature was then programmed to the determination temperature (fixed points were 150℃ and 350℃) for vacuum desorption (1×10⁻⁶). -3 (pa) 60 minutes, record 1700-1400cm -1 Infrared spectra of the region. The total acid content (L and Brønsted acid) of the sample was measured after desorption at 150℃, and the strong acid content (L and Brønsted acid) was measured after desorption at 350℃. In the infrared spectra at each desorption temperature, the concentration at 1540 cm⁻¹... -1 The peak at 1450 cm⁻¹ represents the Brønsted acid site. -1 The peak at the L acid site is used to calculate the amount of B acid and L acid by measuring the peak area. The sum of the two is the amount of acid measured at that desorption temperature.

[0049] raw material

[0050] 1. Industrial ZSM-5 molecular sieve, SiO2 / Al2O3 = 33, Catalyst Division, Lanzhou Petrochemical Company, China National Petroleum Corporation;

[0051] 2. NaOH, boron oxide, boric acid, sodium borate, aluminum chloride, aluminum nitrate, concentrated sulfuric acid (18.4 mol / L), reagents from China National Pharmaceutical Group.

[0052] 3. Water glass (density 1.266 g / ml, SiO2 content 250 g / l, Na2O content 88 g / l), aluminum sulfate (Al2O3, 90 g / l), silica gel (reduced by 11.86%), catalyst plant of Lanzhou Petrochemical Company, China National Petroleum Corporation;

[0053] 4. Silica sol (SiO2 content 40%), Shandong Baite New Materials Co., Ltd.

[0054] 5. Silica (reduced by 13.04%), Zhuzhou Xinglong New Materials Co., Ltd.;

[0055] 6. Ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium sulfate, ammonium chloride, ammonium nitrate, reagents from China National Pharmaceutical Group.

[0056] Examples and Comparative Examples

[0057] The present invention is illustrated below by way of examples and comparative examples, but the present invention is not limited to these examples. Of course, various modifications can be made within the scope of the present invention.

[0058] Example 1

[0059] 6.695 g of boric acid was dissolved in 920.2 g of water. After the boric acid was fully dissolved, 729.29 g of water glass and 24.08 g of industrial ZSM-5 molecular sieve were added with stirring. The mixture was heated to 45 °C and pretreated with stirring for 3.5 h. Then, 111.23 g of silica was added and stirred for 30 min. After that, 155.93 g of aluminum sulfate and 7.22 g of ammonium chloride were slowly added and stirred for 1 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H2O. =0.12:1:0.027:0.0135:22; The resulting system was then transferred to a reactor and heated to 110℃ for 4 hours to crystallize, followed by further heating to 175℃ for 24 hours to crystallize; After crystallization, the crystallization product was filtered and washed; Subsequently, the prepared molecular sieve was mixed according to the weight ratio of molecular sieve dry basis: water: NH4Cl = 1:6:0.3, heated to 85℃, stirred at a constant temperature for 45 minutes, and after filtration, washing, and drying, B-Al-ZSM-5-1 was obtained.

[0060] Example 2

[0061] 6.09 g of boron oxide was dissolved in 959.0 g of water. After the boron oxide was fully dissolved, 698.77 g of water glass and 3.41 g of industrial ZSM-5 molecular sieve were added with stirring. The mixture was heated to 70 °C and stirred for 1 h for pretreatment. Then, 230.23 g of silica gel was added and stirred for 60 min. After that, 179.92 g of aluminum sulfate and 34.09 g of ammonium sulfate were slowly added and stirred for 0.7 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H2O=0. The ratio of 0.07:1:0.022:0.0154:16 was then transferred to a reactor, heated to 140℃ for 2 hours of crystallization, and then further heated to 165℃ for 28 hours of crystallization. After crystallization, the crystallization product was filtered and washed. Subsequently, the prepared molecular sieve was mixed with water at a weight ratio of 1:7:0.4 (dry basis of molecular sieve): 0.022:0.0154:16, heated to 90℃, and stirred at a constant temperature for 30 minutes. After filtration, washing, and drying, B-Al-ZSM-5-2 was obtained.

[0062] Example 3

[0063] 11.08 g of sodium borate was dissolved in 1080.27 g of water. After the sodium borate was fully dissolved, 787.48 g of water glass and 10.0 g of industrial ZSM-5 molecular sieve were added with stirring. The mixture was heated to 50 °C and pretreated with stirring for 2.0 h. Then, 236.24 g of silica sol was added and stirred for 45 min. After that, 36.67 g of aluminum chloride and 17.50 g of ammonium phosphate were slowly added and stirred for 1.2 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H 2O = 0.11:1:0.033:0.027:24; the resulting system was then transferred to a reactor, heated to 130℃ for 5.0 h of crystallization, and then further heated to 190℃ for 12 h of crystallization; after crystallization, the crystallization product was filtered and washed; then the prepared molecular sieve was mixed according to the weight ratio of molecular sieve dry basis: water: NH4NO3 = 1:5:0.5, heated to 75℃, stirred at a constant temperature for 40 min, and after filtration, washing and drying, B-Al-ZSM-5-3 was obtained.

[0064] Example 4

[0065] 5.47 g of boric acid was dissolved in 1999.44 g of water. After the boric acid was fully dissolved, 723.43 g of water glass, 12.86 g of industrial ZSM-5 molecular sieve, and 11.43 g of sodium hydroxide were added with stirring. The mixture was heated to 60 °C and pretreated with stirring for 2.5 h. Then, 324.16 g of silica gel was added and stirred for 75 min. 42.60 g of aluminum nitrate and 8.57 g of diammonium hydrogen phosphate were slowly added, and the mixture was stirred for 1.1 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2. O3:H2O=0.09:1:0.014:0.0062:20; the resulting system was then transferred to a reactor, heated to 120℃ for 3.0 h for crystallization, and then further heated to 180℃ for 20 h for crystallization; after crystallization, the crystallization product was filtered and washed; then the prepared molecular sieve was mixed according to the weight ratio of molecular sieve dry basis: water: NH4NO3=1:8:0.2, heated to 80℃, stirred at a constant temperature for 60 min, and after filtration, washing and drying, B-Al-ZSM-5-4 was obtained.

[0066] Example 5

[0067] 7.60 g of boron oxide was dissolved in 2524.44 g of water. After the boron oxide was fully dissolved, 1116.96 g of water glass and 20.14 g of industrial ZSM-5 molecular sieve were added with stirring. The mixture was heated to 40 °C and pretreated with stirring for 4.0 h. Then, 209.53 g of silica was added and stirred for 112 min. After that, 173.92 g of aluminum sulfate and 16.11 g of ammonium nitrate were slowly added and stirred for 1.4 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H2. O = 0.13:1:0.018:0.0162:29; the resulting system was then transferred to a reactor and heated to 125℃ for 6 hours of crystallization, followed by further heating to 150℃ for 36 hours of crystallization; after crystallization, the crystallization product was filtered and washed; the prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis of molecular sieve): 94℃, hydrochloric acid solution was added, pH was adjusted to 2.3, and the mixture was stirred at a constant temperature for 75 minutes. After filtration, washing, and drying, B-Al-ZSM-5-5 was obtained.

[0068] Comparative Example 1

[0069] The molecular sieve prepared in this comparative example has the same silicon-to-aluminum ratio as in Example 5, except that no boron source is added, resulting in the preparation of Al-ZSM-5 molecular sieve.

[0070] Add 1116.96g of water glass and 20.14g of industrial ZSM-5 molecular sieve to 2524.44g of water with stirring. Heat to 40℃ and pre-treat with stirring for 4.0h. Then add 209.53g of silica and stir for 112min. Next, slowly add 173.92g of aluminum sulfate and 16.11g of ammonium nitrate, and stir for 1.4h to homogenize the system. The molar ratio of each substance in the system is Na₂O:SiO₂:Al₂O₃:H₂O = 0.1. The ratio of 3:1:0.018:29 was used to obtain the system. The resulting system was then transferred to a reactor and heated to 125°C for 6 hours to crystallize. The temperature was then further increased to 150°C for 36 hours to crystallize. After crystallization, the crystallization product was filtered and washed. The prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis of molecular sieve): 94°C. Hydrochloric acid solution was added to adjust the pH to 2.3. The mixture was stirred at a constant temperature for 75 minutes. After filtration, washing, and drying, DZM-1 was obtained.

[0071] Comparative Example 2

[0072] The B-Al-ZSM-5 molecular sieve used in this comparative example had the same silicon-to-aluminum ratio as that in Example 5, and its preparation method was carried out in accordance with the contents described in Journal of Solid State Chemistry 179(2006)855-865.

[0073] 7.60 g of boron oxide was dissolved in 1847.20 g of water. After the boron oxide was fully dissolved, 173.92 g of aluminum sulfate and 55.5 ml of concentrated sulfuric acid were added. Then, 2039.67 g of water glass was added dropwise to the solution. After stirring evenly, 20.14 g of industrial ZSM-5 molecular sieve was added to obtain a mixed system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H2O = 0.13:1:0.018:0.0162:29. The resulting system was then transferred to a reaction vessel and heated to 150 °C for crystallization for 36 h. After crystallization, the crystallization product was filtered and washed. The prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis of molecular sieve): 94 °C. Hydrochloric acid solution was added to adjust the pH to 2.3. The mixture was stirred at a constant temperature for 75 min. After filtration, washing, and drying, DZM-2 was obtained.

[0074] Comparative Example 3

[0075] The comparative example uses the same B-Al-ZSM-5 molecular sieve with the same silicon-to-aluminum ratio as Example 5, but without the addition of inorganic additives.

[0076] 7.60 g of boron oxide was dissolved in 2524.44 g of water. After the boron oxide was fully dissolved, 1116.96 g of water glass and 20.14 g of industrial ZSM-5 molecular sieve were added with stirring. The mixture was heated to 40 °C and pretreated with stirring for 4.0 h. Then, 209.53 g of silica was added and stirred for 112 min. Finally, 173.92 g of aluminum sulfate was slowly added and stirred for 1.4 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H2O. =0.13:1:0.018:0.0162:29; The resulting system was then transferred to a reactor and heated to 125℃ for 6 hours of crystallization, followed by further heating to 150℃ for 36 hours of crystallization; After crystallization, the crystallization product was filtered and washed; The prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis of molecular sieve): 94℃, hydrochloric acid solution was added, pH was adjusted to 2.3, and the mixture was stirred at a constant temperature for 75 minutes. After filtration, washing, and drying, DZM-3 was obtained.

[0077] Comparative Example 4

[0078] The comparative example uses the same B-Al-ZSM-5 molecular sieve with the same silicon-to-aluminum ratio as Example 5, except that only the first silicon source—water glass—is added, and the second silicon source—silica—is not added.

[0079] 7.60 g of boron oxide was dissolved in 1874.53 g of water. After the boron oxide was fully dissolved, 2039.67 g of water glass, 55.45 ml of concentrated sulfuric acid, and 20.14 g of industrial ZSM-5 molecular sieve were added with stirring. The mixture was heated to 40 °C and pretreated with stirring for 4.0 h. Then, 173.92 g of aluminum sulfate and 16.11 g of ammonium nitrate were slowly added, and the mixture was stirred for 1.4 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3:H2O = The ratio of 0.13:1:0.018:0.0162:29 was then used. The resulting system was transferred to a reactor and heated to 125°C for 6 hours to crystallize, followed by further heating to 150°C for 36 hours. After crystallization, the crystallization product was filtered and washed. The prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis of molecular sieve): 94°C. Hydrochloric acid solution was added to adjust the pH to 2.3, and the mixture was stirred at a constant temperature for 75 minutes. After filtration, washing, and drying, DZM-4 was obtained.

[0080] Comparative Example 5

[0081] The comparative example uses the same B-Al-ZSM-5 molecular sieve with the same silicon-to-aluminum ratio as Example 5, except that only the second silicon source—silica—is added, and the first silicon source—water glass—is not added.

[0082] 7.60 g of boron oxide was dissolved in 4311.17 g of water. After the boron oxide was fully dissolved, 20.14 g of industrial ZSM-5 molecular sieve was added with stirring. The mixture was heated to 40 °C and pretreated with stirring for 4.0 h. Subsequently, 463.18 g of silica and 98.81 g of sodium hydroxide were added. After stirring for 112 min, 173.92 g of aluminum sulfate and 16.11 g of ammonium nitrate were slowly added. The mixture was then stirred for 1.4 h to homogenize the system. The molar ratio of each substance in the system was Na2O:SiO2:Al2O3:B2O3. The H2O ratio was 0.13:1:0.018:0.0162:29. The resulting system was then transferred to a reactor and heated to 125°C for 6 hours of crystallization, followed by further heating to 150°C for 36 hours of crystallization. After crystallization, the crystallization product was filtered and washed. The prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis). The mixture was heated to 94°C, hydrochloric acid solution was added, and the pH was adjusted to 2.3. The mixture was stirred at a constant temperature for 75 minutes. After filtration, washing, and drying, DZM-5 was obtained.

[0083] Comparative Example 6

[0084] The comparative example uses the same B-Al-ZSM-5 molecular sieve with the same silicon-to-aluminum ratio as Example 5, except that no pretreatment step is performed.

[0085] 7.60 g of boron oxide was dissolved in 2524.44 g of water. After the boron oxide was fully dissolved, 1116.96 g of water glass and 20.14 g of industrial ZSM-5 molecular sieve were added with stirring, followed by 209.53 g of silica. After stirring for 112 min, 173.92 g of aluminum sulfate and 16.11 g of ammonium nitrate were slowly added, and the mixture was stirred for 1.4 h to homogenize the system. The molar ratio of the substances in the system was Na₂O:SiO₂:Al₂O₃:B₂O₃:H₂O = 0.1. The ratio of the molecular sieve to water was 3:1:0.018:0.0162:26. The resulting system was then transferred to a reactor and heated to 125°C for 6 hours to crystallize, followed by further heating to 150°C for 36 hours to crystallize. After crystallization, the crystallization product was filtered and washed. The prepared molecular sieve was then mixed with water at a weight ratio of 1:9 (dry basis). The mixture was heated to 94°C, hydrochloric acid solution was added, and the pH was adjusted to 2.3. The mixture was stirred at a constant temperature for 75 minutes. After filtration, washing, and drying, DZM-6 was obtained.

[0086] The analysis and evaluation results of the above embodiments and comparative examples are as follows.

[0087] Figure 1 XRD patterns of the molecular sieves B-Al-ZSM-5-5 and DZM-2 prepared in Example 5 and Comparative Example 2 are shown. Figure 1As can be seen, the B-Al-ZSM-5-5 prepared in the embodiments of the present invention has a higher diffraction peak intensity compared with DZM-2. Calculations show that the crystallinity of B-Al-ZSM-5-5 and DZM-2 are 90% and 69%, respectively, indicating that the method of the present invention is more advantageous for obtaining B-Al-ZSM-5 molecular sieves with higher crystallinity under template-free conditions.

[0088] Figure 2 XRD patterns of molecular sieves B-Al-ZSM-5-5 and DZM-2 prepared in Example 5 and Comparative Example 2 after hydrothermal aging at 800℃ for 2 hours are presented. Figure 2 As can be seen, the B-Al-ZSM-5-5 prepared in this embodiment of the invention exhibits higher diffraction peak intensity after hydrothermal aging compared to DZM-2. Calculations show that the crystallinity of B-Al-ZSM-5-5 and DZM-2 after hydrothermal aging are 79% and 43%, respectively. The molecular sieve of this embodiment has a lower crystallinity decay rate, indicating that the B-Al-ZSM-5 molecular sieve of this embodiment has higher hydrothermal stability.

[0089] Table 1 shows the crystallinity of the molecular sieves in each embodiment and comparative example, the crystallinity after hydrothermal aging at 800℃ for 2 hours, and the crystallinity decay rate.

[0090] Table 1

[0091]

[0092] The results of measuring the bulk silicon-to-aluminum ratio (using XRF) and surface silicon-to-aluminum ratio (using EDS) of the molecular sieves prepared in the above examples and comparative examples, as well as the boron-to-aluminum ratio of the molecular sieves, are shown in Table 2.

[0093] Table 2

[0094]

[0095]

[0096] As shown in Table 2, the silicon-to-aluminum ratio (S / A ratio) of the molecular sieves prepared in Comparative Examples 1-6 was significantly higher than that measured by EDS than by XRF. EDS primarily measures the elemental distribution on the crystal surface, indicating that the molecular sieves prepared in Comparative Examples 1-6 are characterized by high silicon content within the crystals and high aluminum content on the surface. In contrast, the absolute values ​​of the differences between the S / A ratio measured by EDS and the S / A ratio measured by XRF for the B-Al-ZSM-5 molecular sieves in each embodiment of this invention are all less than 1, indicating that the B-Al-ZSM-5 molecular sieve crystals in each embodiment of this invention have a uniform aluminum distribution.

[0097] The pyridine-infrared (Py-IR) acidity of the molecular sieves prepared in the above examples and comparative examples was determined, and the results are shown in Table 3.

[0098] Table 3

[0099]

[0100] Table 3 presents the pyridine-infrared acid content data of the molecular sieves prepared in the above embodiments and comparative examples, where the acid content at 150℃ and 350℃ represents the total acid content and the strong acid content, respectively. As can be seen from Table 3, the B-Al-ZSM-5 molecular sieves of each embodiment of the present invention have a lower strong acid content.

[0101] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A boron-containing ZSM-5 molecular sieve, wherein, The framework of the boron-containing ZSM-5 molecular sieve contains boron, aluminum and silicon, and the absolute value of the difference between the silicon-aluminum ratio of the bulk phase of the crystal and the silicon-aluminum ratio of the crystal surface determined by XRF and EDS is ≤1.

2. The boron-containing ZSM-5 molecular sieve according to claim 1, wherein, The boron-containing ZSM-5 molecular sieve has a bulk silicon-to-aluminum ratio of 16-50 and a surface silicon-to-aluminum ratio of 15-51.

3. The boron-containing ZSM-5 molecular sieve according to claim 1, wherein, The boron-aluminum ratio of the boron-containing ZSM-5 molecular sieve is 0.2-0.

6.

4. The boron-containing ZSM-5 molecular sieve according to claim 1, wherein, The boron-containing ZSM-5 molecular sieve has a crystallinity of 85-100%.

5. The boron-containing ZSM-5 molecular sieve according to claim 1, wherein, The total acid content of the boron-containing ZSM-5 molecular sieve, measured by pyridine-infrared desorption at 150℃, is 120-220 μmol / g, and the strong acid content, measured by pyridine-infrared desorption at 350℃, is 30-55 μmol / g.

6. The boron-containing ZSM-5 molecular sieve according to claim 4, wherein, The crystallinity of the boron-containing ZSM-5 molecular sieve decreased by 10-20% after hydrothermal aging at 800℃ for 2 hours.

7. A method for preparing a boron-containing ZSM-5 molecular sieve according to any one of claims 1-6, comprising the following steps: (1) After mixing and pretreating the boron source, the first silicon source, the seed crystal and water, a first mixed system is obtained; (2) After the first mixture system, the second silicon source, the aluminum source and the inorganic additive are mixed evenly, the second mixture system is obtained; (3) After crystallizing the second mixed system, it is then subjected to at least solid-liquid separation, washing, ion exchange and drying to obtain the boron-containing ZSM-5 molecular sieve; Wherein, the first silicon source is an alkaline silicon source, the second silicon source is one or a combination of alkaline silicon source, neutral silicon source and acidic silicon source, and the inorganic additive is an inorganic ammonium salt.

8. The preparation method according to claim 7, wherein, In step (1), the boron source includes one or a combination of boric acid, sodium borate and boron oxide.

9. The preparation method according to claim 7, wherein, In step (1), the first silicon source comprises water glass.

10. The preparation method according to claim 7, wherein, In step (1), the seed crystal is a molecular sieve with an MFI crystal structure, and the weight ratio of the amount of seed crystal to the total amount of SiO2 in the first silicon source and the second silicon source is (0.01-0.10):

1.

11. The preparation method according to claim 7, wherein, In step (1), the pretreatment is carried out under stirring conditions, the temperature of the pretreatment is 40-70℃, and the time is 1-4h.

12. The preparation method according to claim 7, wherein, In step (2), the second silicon source includes one or a combination of several of water glass, silica gel, silica sol and silica.

13. The preparation method according to claim 7, wherein, In step (2), the aluminum source includes one or a combination of several of sodium aluminate, aluminum sulfate, aluminum chloride and aluminum nitrate.

14. The preparation method according to claim 7, wherein, In step (2), the inorganic additive includes one or a combination of several of ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium phosphate, diammonium hydrogen phosphate and ammonium dihydrogen phosphate.

15. The preparation method according to claim 7, wherein, In step (2), the weight ratio of the amount of inorganic additive to the total amount of SiO2 in the first silicon source and the second silicon source is (0.02-0.10):

1.

16. The preparation method according to claim 7, wherein, In steps (1) and (2), the weight ratio of SiO2 in the first silicon source to SiO2 in the second silicon source is (0.5-1.7):

1.

17. The preparation method according to claim 7, wherein, In step (2), the molar ratio of each substance in the second mixed system satisfies Na2O:SiO2:Al2O3:B2O3:H2O=(0.07-0.14):1:(0.013-0.042):(0.006-0.03):(14-30).

18. The preparation method according to claim 7, wherein, In step (3), the crystallization is a two-stage crystallization; the temperature of the first stage crystallization is 110-140℃ and the time of the first stage crystallization is 2-6h; the temperature of the second stage crystallization is 150-190℃ and the time of the second stage crystallization is 12-36h.

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