A Nb-Beta molecular sieve and its preparation method and application

Abstract

This invention discloses an Nb-Beta molecular sieve, its preparation method, and its application. Metallic niobium is immobilized onto a dealubilized Beta molecular sieve via a solid-state ion exchange method, resulting in a niobium oxide-supported Nb-Beta molecular sieve. The niobium species form nested bonds with the hydroxyl groups of the dealubilized Beta molecular sieve, creating a twisted Si-O-NbO structure. x It exists in the form of Nb-Beta molecular sieve, in which NbO x The diffraction peaks are located at 28.42°–28.47°, and the silicon-niobium molar ratio is 10:1–15:1. The Nb-Beta molecular sieve prepared in this invention is mixed with cyclohexene, acetonitrile, and hydrogen peroxide solution and reacted at a reaction temperature of 60°C and a reaction pressure of 0.1 MPa for 2 hours, achieving a cyclohexene conversion rate of over 90% and an epoxycyclohexane selectivity of over 75%. The Nb-Beta molecular sieve prepared in this invention, as a catalyst, improves the cyclohexene conversion rate and epoxycyclohexane selectivity, and significantly shortens the reaction time.

Description

Technical Field

[0001] This invention relates to the field of catalytic materials, and more particularly to an Nb-Beta molecular sieve, its preparation method, and its application. Background Technology

[0002] Cyclohexane oxide is a very important fine chemical intermediate. Its molecule contains epoxy groups, making it chemically reactive. Under acid or base catalysis, it readily undergoes ring-opening and reacts with nucleophiles such as amines, alcohols, phenols, and carboxylic acids to produce compounds with higher added value. Currently, the main methods for synthesizing cyclohexane oxide are the haloalcohol method and the epoxidation method. The haloalcohol method is complex, causes severe equipment corrosion, and produces numerous byproducts. The epoxidation method is currently the most researched and widely used method for preparing cyclohexane oxide. Depending on the oxygen source used, the epoxidation method can be divided into peroxyacid epoxidation, molecular oxygen epoxidation, and hydrogen peroxide epoxidation. Among these, the peroxyacid method generates a large amount of organic waste acid, causing serious environmental pollution; molecular oxygen has relatively low reactivity and requires a highly efficient catalyst to achieve good epoxidation results; hydrogen peroxide has high reactivity and produces only water as the sole byproduct, making it an environmentally friendly oxygen source. Therefore, the hydrogen peroxide epoxidation method has received widespread attention in recent years. The hydrogen peroxide epoxidation process involves the catalytic oxidation of cyclohexene by hydrogen peroxide molecules to form cyclohexene oxide. The catalyst is the core of this process, and its activity directly determines the effectiveness of the cyclohexene epoxidation reaction. Based on their phase, catalysts can be classified into homogeneous and heterogeneous catalysts. Compared to homogeneous catalysts, heterogeneous catalysts offer advantages such as ease of recovery and reusability, but they also face challenges such as fixed catalytic active sites, hindering the full realization of their activity. Therefore, developing highly active heterogeneous catalysts is of great significance for the cyclohexene epoxidation reaction.

[0003] Zeolite molecular sieves are an important class of heterogeneous catalysts, widely used in petrochemical, fine chemical, and other fields. Titanium silicate molecular sieves, by doping titanium atoms into the molecular sieve topology to form unique framework-coordinated titanium Lewis acid active centers, exhibit significant catalytic activity for liquid-phase selective oxidation reactions such as olefin epoxidation, phenol hydroxylation, and aldehyde-ketone ammonium oximeation. Ti-Beta molecular sieves are a typical example of titanium silicate zeolite molecular sieves. They are zeolite molecular sieves formed by doping titanium atoms into a BEA topology with a twelve-membered ring channel structure. In the cyclohexene epoxidation reaction at 60℃ for 2 hours, they can achieve a cyclohexene conversion rate of approximately 10%. Multi-level porous nanocrystalline Ti-Beta molecular sieves can be prepared using a nanocrystal seed-assisted synthesis method, which greatly eliminates internal diffusion resistance. However, in the cyclohexene epoxidation reaction at 60℃ for 2 hours, the cyclohexene conversion rate can only be increased to 21%, indicating that the catalytic performance of the framework-coordinated titanium active centers is insufficient (Inorg. Chem. 2022, 61:4887-4894). Summary of the Invention

[0004] Purpose of the invention: The present invention aims to provide a highly efficient Nb-Beta molecular sieve catalyst and its preparation method, as well as the application of Nb-Beta molecular sieve in the preparation of cyclohexane oxide.

[0005] Technical solution: The Nb-Beta molecular sieve of this invention is composed of niobium oxide immobilized on a dealubilized Beta molecular sieve. Niobium species form bonds with the hydroxyl groups in the dealubilized Beta molecular sieve, and the niobium oxide exists as a twisted Si-O-NbO structure. x The bonds exist in the form of Nb-Beta molecular sieves, where NbO x The diffraction peaks are located at 28.42° to 28.47°, and the silicon-niobium molar ratio is 10:1 to 15:1.

[0006] The method for preparing Nb-Beta molecular sieves according to the present invention includes:

[0007] (1) Weigh the Beta molecular sieve raw powder, exchange it with ammonium salt, and then calcine it at high temperature to obtain H-Beta molecular sieve;

[0008] (2) H-Beta molecular sieve was acid-washed and dealuminized, and then vacuum dried to obtain Si-Beta molecular sieve rich in hydroxyl groups.

[0009] (3) The Si-Beta molecular sieve rich in hydroxyl groups was mixed with niobium salt, ground until homogeneous, placed in a high pressure vessel for crystallization, washed and dried, and calcined at high temperature to obtain Nb-Beta molecular sieve.

[0010] Preferably, the silicon-to-aluminum ratio of the Beta molecular sieve raw powder in step (1) is 25:1 to 40:1, and the average particle size is 50 to 150 nm; the ammonium salt is any one of ammonium nitrate, ammonium carbonate, ammonium bicarbonate, and ammonium sulfate, and the concentration of the ammonium salt aqueous solution is 5 to 10 mol / L; the ion exchange temperature is 40 to 90 °C, and the ion exchange time is 6 to 24 h; the calcination atmosphere is air or nitrogen, the calcination temperature is 400 to 600 °C, the heating rate is 2 to 3 °C / min, and the calcination time is 4 to 10 h.

[0011] Preferably, the acid used in step (2) is any one of nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, and citric acid, and the concentration of the acid used is 10-15 mol / L; the volume ratio of H-Beta molecular sieve to acid used is 1:10-1:20; the pickling temperature is 20-120℃, and the pickling time is 6-24h; the vacuum drying temperature is 60-200℃, and the drying time is 8-24h; the silicon-to-aluminum ratio in the Si-Beta molecular sieve obtained by pickling is 500:1-1000:1.

[0012] Preferably, the niobium salt used in step (3) is any one of niobium ethanol, niobium oxalate, niobium chloride, and niobium acetate. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt is 10:1 to 15:1. The crystallization temperature is 140 to 180°C. The crystallization time is 12 to 48 h. The calcination atmosphere is air or nitrogen. The calcination temperature is 500 to 600°C. The heating rate is 2 to 3°C / min. The calcination time is 4 to 10 h.

[0013] This invention relates to the application of the Nb-Beta molecular sieve prepared in the preparation of cyclohexane oxide. Cyclohexane oxide is prepared by stirring Nb-Beta molecular sieve with cyclohexene, acetonitrile, and hydrogen peroxide solution. The mass ratio of Nb-Beta molecular sieve to cyclohexene to acetonitrile is 1:1:20 to 1:20:100, the mass fraction of the hydrogen peroxide aqueous solution is 30% to 50%, the molar ratio of hydrogen peroxide to cyclohexene is 1:10 to 5:1, the reaction temperature is 50 to 100 °C, the reaction pressure is 0.1 to 5 MPa, and the reaction time is 2 to 8 h.

[0014] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: 1. The present invention uses a solid-state ion exchange method to immobilize metallic niobium onto a dealuminized Beta molecular sieve, obtaining a niobium oxide-supported Nb-Beta molecular sieve catalyst, wherein the niobium species form a nested bond with the hydroxyl groups of the dealuminized Beta molecular sieve, in the form of a twisted Si-O-NbO x The bond form exists on Nb-Beta molecular sieves, NbO xThe diffraction peaks are located at 28.42°~28.47°, and the silicon-niobium molar ratio is 10:1~15:1; 2. Nb-Beta molecular sieves are used as catalysts in the preparation of cyclohexane oxide. Under the reaction temperature of 60℃ and the reaction pressure of 0.1MPa for 2h, the cyclohexene conversion rate can reach more than 90% and the cyclohexane oxide selectivity can reach more than 75%, which successfully improves the catalytic performance of cyclohexene epoxidation reaction and significantly shortens the reaction time. Attached Figure Description

[0015] Figure 1 This is the X-ray diffraction pattern of the Nb-Beta molecular sieve in Example 1 of the present invention;

[0016] Figure 2 This is the X-ray diffraction pattern of the Nb-Beta molecular sieve in Example 2 of the present invention;

[0017] Figure 3 This is the X-ray diffraction pattern of the Nb-Beta molecular sieve in Example 3 of the present invention;

[0018] Figure 4 This is the X-ray diffraction pattern of the Nb-Beta molecular sieve in Example 4 of the present invention;

[0019] Figure 5 This is the X-ray diffraction pattern of the Nb-Beta molecular sieve in Example 5 of the present invention;

[0020] Figure 6 This is the X-ray diffraction pattern of the Nb-Beta molecular sieve in Example 6 of the present invention;

[0021] Figure 7 The X-ray diffraction pattern of the Si-Beta molecular sieve in Comparative Example 1 of this invention is shown below.

[0022] Figure 8 This is the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2 of this invention;

[0023] Figure 9 The UV-Vis spectrum of the Nb-Beta molecular sieve in Example 1 of this invention;

[0024] Figure 10 This is the UV-Raman spectrum of the Nb-Beta molecular sieve in Example 1 of the present invention. Detailed Implementation

[0025] The invention will now be further described with reference to the accompanying drawings.

[0026] Example 1

[0027] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 25:1 and an average particle size of 50nm and add it to 20mL of ammonium nitrate solution with a concentration of 5mol / L for ion exchange. Stir at 80℃ for 12h, collect the solid by centrifugation, wash and dry it, and calcine it at 550℃ for 6h in air atmosphere at a rate of 2.5℃ / min to obtain H-beta molecular sieve.

[0028] (2) 2g of the obtained H-Beta molecular sieve was added to a 10mol / L nitric acid solution for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:10. The mixture was stirred at 80℃ for 12h, the solid was collected by centrifugation, and the supernatant was washed until pH=7. The supernatant was dried under vacuum at 150℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-to-aluminum ratio of Si-Beta molecular sieve was 500:1.

[0029] (3) Si-Beta molecular sieve powder and niobium ethanol were placed in a mortar. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt was 10:1. The mixture was ground to homogeneity using a pestle and then transferred to a high-pressure reactor. It was crystallized at 140°C for 24 hours, washed and dried, and then calcined at 550°C for 6 hours in air at a rate of 2.5°C / min to obtain Nb-Beta molecular sieve with a silicon-niobium molar ratio of 10:1.

[0030] The X-ray diffraction pattern of the Nb-Beta molecular sieve in this embodiment is as follows: Figure 1 As shown, NbO x The diffraction peak is located at 28.43°. This is compared to the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2. Figure 8 Compared to the Nb-Beta molecular sieve in this embodiment, NbO x The diffraction peak shifted to the right at a high angle of about 0.03°, indicating that NbO x It has been distorted.

[0031] Example 2

[0032] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 30:1 and an average particle size of 100nm and add it to 20mL of ammonium carbonate solution with a concentration of 5mol / L for ion exchange. Stir at 90℃ for 12h, collect the solid by centrifugation, wash and dry it, and calcine it at 600℃ for 6h in air atmosphere at a rate of 3℃ / min to obtain H-beta molecular sieve.

[0033] (2) 2g of the obtained H-Beta molecular sieve was added to 20mL of 10mol / L hydrochloric acid solution for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:15. The mixture was stirred at 100℃ for 12h, the solid was collected by centrifugation, and the supernatant was washed until pH=7. The supernatant was dried in vacuum at 100℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-to-aluminum ratio of Si-Beta molecular sieve was 750:1.

[0034] (3) Si-Beta molecular sieve powder and niobium oxalate were placed in a mortar. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt was 10:1. The mixture was ground into homogeneity using a pestle and then transferred to a high-pressure reactor. It was crystallized at 150°C for 24 hours, washed and dried, and then calcined at 600°C for 6 hours in air at a rate of 3°C / min to obtain Nb-Beta molecular sieve with a silicon-niobium molar ratio of 10:1.

[0035] The X-ray diffraction pattern of the Nb-Beta molecular sieve in this embodiment is as follows: Figure 2 As shown, NbO x The diffraction peak is located at 28.45°. This is compared to the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2. Figure 8 Compared to the Nb-Beta molecular sieve in this embodiment, NbO x The diffraction peak shifted to the right at a high angle of about 0.05°, indicating that NbO x It has been distorted.

[0036] Example 3

[0037] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 35:1 and an average particle size of 150nm and add it to 20mL of ammonium sulfate solution with a concentration of 8mol / L for ion exchange. Stir at 90℃ for 24h, collect the solid by centrifugation, wash and dry it, and calcine it at 600℃ for 6h in air atmosphere at a rate of 2℃ / min to obtain H-beta molecular sieve.

[0038] (2) 2g of the obtained H-Beta molecular sieve was added to 20mL of sulfuric acid solution with a concentration of 13mol / L for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:10. The mixture was stirred at 80℃ for 24h, the solid was collected by centrifugation, and the supernatant was washed until pH=7. The mixture was dried in vacuum at 120℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-to-aluminum ratio of Si-Beta molecular sieve was 1000:1.

[0039] (3) Si-Beta molecular sieve powder and niobium chloride were placed in a mortar. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt was 10:1. The mixture was ground into homogeneity using a pestle and then transferred to a high-pressure reactor. It was crystallized at 160°C for 36 hours, washed and dried, and then calcined at 600°C for 6 hours in air at a rate of 2°C / min to obtain Nb-Beta molecular sieve with a silicon-niobium molar ratio of 10:1.

[0040] The X-ray diffraction pattern of the Nb-Beta molecular sieve in this embodiment is as follows: Figure 3 As shown, NbO x The diffraction peak is located at 28.42°. This is compared to the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2. Figure 8 Compared to the Nb-Beta molecular sieve in this embodiment, NbO x The diffraction peak shifted to the right at a high angle of approximately 0.02°, indicating that NbO x It has been distorted.

[0041] Example 4

[0042] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 40:1 and an average particle size of 50nm and add it to 20mL of ammonium bicarbonate solution with a concentration of 8mol / L for ion exchange. Stir at 60℃ for 12h, collect the solid by centrifugation, wash and dry it, and calcine it at 550℃ for 6h in a nitrogen atmosphere at a rate of 3℃ / min to obtain H-beta molecular sieve.

[0043] (2) 2g of the obtained H-Beta molecular sieve was added to 20mL of 13mol / L phosphoric acid solution for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:10. The mixture was stirred at 80℃ for 6h, the solid was collected by centrifugation, and the supernatant was washed until pH=7. The mixture was dried in vacuum at 180℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-to-aluminum ratio of Si-Beta molecular sieve was 500:1.

[0044] (3) Si-Beta molecular sieve powder and niobium acetate were placed in a mortar. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt was 10:1. The mixture was ground into homogeneity using a pestle and then transferred to a high-pressure reactor. It was crystallized at 170°C for 24 hours, washed and dried, and then calcined at 550°C for 6 hours in a nitrogen atmosphere at a rate of 3°C / min to obtain Nb-Beta molecular sieve with a silicon-niobium molar ratio of 10:1.

[0045] The X-ray diffraction pattern of the Nb-Beta molecular sieve in this embodiment is as follows: Figure 4 As shown, NbO x The diffraction peak is located at 28.42°. This is compared to the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2. Figure 8 Compared to the Nb-Beta molecular sieve in this embodiment, NbO x The diffraction peak shifted to the right at a high angle of approximately 0.02°, indicating that NbO x It has been distorted.

[0046] Example 5

[0047] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 25:1 and an average particle size of 50nm and add it to 20mL of ammonium nitrate solution with a concentration of 10mol / L for ion exchange. Stir at 80℃ for 24h, collect the solid by centrifugation, wash and dry it, and calcine it at 600℃ for 6h in air atmosphere at a rate of 2℃ / min to obtain H-beta molecular sieve.

[0048] (2) 2g of the obtained H-Beta molecular sieve was added to 20mL of citric acid solution with a concentration of 15mol / L for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:15. The mixture was stirred at 60℃ for 12h, the solid was collected by centrifugation, and the supernatant was washed until pH=7. The mixture was dried in vacuum at 150℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-aluminum ratio of Si-Beta molecular sieve was 750:1.

[0049] (3) Si-Beta molecular sieve powder and niobium ethanol were placed in a mortar. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt was 10:1. The mixture was ground into homogeneity using a pestle and then transferred to a high-pressure reactor. It was crystallized at 180°C for 24 hours, washed and dried, and then calcined at 600°C for 8 hours in air at a rate of 3°C / min to obtain Nb-Beta molecular sieve with a silicon-niobium molar ratio of 10:1.

[0050] The X-ray diffraction pattern of the Nb-Beta molecular sieve in this embodiment is as follows: Figure 5 As shown, NbO x The diffraction peak is located at 28.44°. This is compared to the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2. Figure 8 Compared to the Nb-Beta molecular sieve in this embodiment, NbO x The diffraction peak shifted to the right at a high angle of approximately 0.04°, indicating that NbO x It has been distorted.

[0051] Example 6

[0052] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 25:1 and an average particle size of 50nm and add it to 20mL of ammonium nitrate solution with a concentration of 10mol / L for ion exchange. Stir at 80℃ for 12h, collect the solid by centrifugation, wash and dry it, and calcine it at 500℃ for 6h in a nitrogen atmosphere at a rate of 3℃ / min to obtain H-beta molecular sieve.

[0053] (2) 2g of the obtained H-Beta molecular sieve was added to a 15mol / L nitric acid solution for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:20. The mixture was stirred at 80℃ for 12h, the solid was collected by centrifugation, and the supernatant was washed until pH=7. The supernatant was dried under vacuum at 150℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-to-aluminum ratio of Si-Beta molecular sieve was 1000:1.

[0054] (3) Si-Beta molecular sieve powder and niobium ethanol were placed in a mortar. The silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt was 15:1. The mixture was ground into homogeneity using a pestle and then transferred to a high-pressure reactor. It was crystallized at 140°C for 24 hours, washed and dried, and then calcined at 500°C for 6 hours in a nitrogen atmosphere at a rate of 3°C / min to obtain Nb-Beta molecular sieve with a silicon-niobium molar ratio of 15:1.

[0055] The X-ray diffraction pattern of the Nb-Beta molecular sieve in this embodiment is as follows: Figure 6 As shown, NbO x The diffraction peak is located at 28.47°. This is compared to the X-ray diffraction pattern of Nb₂O₅ in Comparative Example 2. Figure 8 Compared to the Nb-Beta molecular sieve in this embodiment, NbO x The diffraction peak shifted to the right at a high angle of approximately 0.07°, indicating that NbO x It has been distorted.

[0056] Comparative Example 1

[0057] (1) Weigh 2g of Beta molecular sieve with a silicon-to-aluminum ratio of 25:1 and an average particle size of 50nm and add it to 20mL of ammonium bicarbonate solution with a concentration of 5mol / L for ion exchange. Stir at 80℃ for 12h, collect the solid by centrifugation, wash and dry it, and calcine it at 550℃ for 6h in air atmosphere at a rate of 2.5℃ / min to obtain H-Beta molecular sieve.

[0058] (2) 2g of the obtained H-Beta molecular sieve was added to a 10mol / L nitric acid solution for acid washing and dealuminization. The volume ratio of H-Beta molecular sieve to acid used was 1:10. The mixture was stirred at 80℃ for 12h, and the solid was collected by centrifugation. The supernatant was washed until pH=7 and then dried under vacuum at 150℃ for 12h to obtain dealuminized Si-Beta molecular sieve. The silicon-to-aluminum ratio of the Si-Beta molecular sieve was 500:1.

[0059] Comparative Example 2

[0060] 1.793 g of niobium oxalate was weighed and added to 30 mL of a deionized water / ethanol mixture (volume ratio 3:1), and dissolved completely at 60 °C. The mixture was transferred to an autoclave and crystallized at 180 °C for 24 h. The solid was collected by centrifugation, washed, and dried. The solid was ground into a fine powder and placed in a tube furnace, where it was calcined at 350 °C for 3 h under a N2 atmosphere at a rate of 3 °C / min to obtain solid Nb₂O₅.

[0061] Comparative Example 3

[0062] Weigh 2g of Si-Beta molecular sieve from Comparative Example 1 and mix it with 0.44g of Nb2O5 solid from Comparative Example 2. Grind the mixture thoroughly in a mortar to obtain a mixed sample of Si-Beta and Nb2O5.

[0063] Test 1

[0064] This test example illustrates the reaction effects of the molecular sieve obtained by the method provided in this invention and the molecular sieve obtained by the comparative method on the cyclohexene epoxidation reaction.

[0065] The catalyst samples prepared in the above examples and comparative examples were mixed evenly in a three-necked flask equipped with a reflux condenser at a mass ratio of sample:cyclohexene:acetonitrile = 1:10:20. The mixture was heated to 60°C, and then 30% hydrogen peroxide was added at a molar ratio of cyclohexene:hydrogen peroxide = 1:1 under stirring. The reaction was carried out at a reaction temperature of 60°C and a reaction pressure of 0.1 MPa for 4 hours. The distribution of each product was determined by using an OV-17 capillary column on a Shimadzu GC-2014C chromatograph. The results are shown in Table 1.

[0066] in:

[0067]

[0068]

[0069] Table 1

[0070] catalyst Cyclohexene conversion rate (%) Selectivity of cyclohexane oxide (%) Example 1 90.2 76.1 Example 2 91.1 77.4 Example 3 90.8 76.6 Example 4 90.7 75.2 Example 5 90.5 75.5 Example 6 90.3 75.8 Comparative Example 1 1.2 23.1 Comparative Example 2 8.3 42.6 Comparative Example 3 8.8 35.8

[0071] As shown in Table 1, the Nb-Beta molecular sieve prepared by the method of the present invention has high catalytic activity. When used in the cyclohexene epoxidation reaction, the cyclohexene conversion rate and the cyclohexane oxide selectivity are significantly higher than those of the Si-Beta molecular sieve in Comparative Example 1, Nb2O5 in Comparative Example 2, and the mixture of Si-Beta molecular sieve and Nb2O5 in Comparative Example 3.

[0072] Test 2

[0073] The Nb-Beta molecular sieve samples prepared in Examples 1 to 6 above were reacted according to Test 1, centrifuged and dried, and then subjected to cyclohexene epoxidation reaction according to the reaction conditions of Test 1. The results after 5 cycles are shown in Table 2.

[0074] Table 2

[0075]

[0076]

[0077] As shown in Table 2, the Nb-Beta molecular sieve prepared by the method of the present invention has high stability.

[0078] The embodiments described in this invention are all preferred solutions, but this invention is not limited to the above embodiments. Simple modifications to this invention without departing from the scope of this invention also fall within the protection scope of this invention.

Claims

1. A method for preparing Nb-Beta molecular sieves, characterized in that, The method includes: (1) Weigh the Beta molecular sieve raw powder, exchange it with ammonium salt, and then calcine it at high temperature to obtain H-Beta molecular sieve; wherein the silicon-aluminum ratio of the Beta molecular sieve raw powder is 25:1~40:1 and the average particle size is 50~150 nm. (2) H-Beta molecular sieve was acid-washed and dealuminized, and vacuum dried to obtain Si-Beta molecular sieve rich in hydroxyl groups; dealuminized Si-Beta molecular sieve was obtained, and the silicon-to-aluminum ratio of Si-Beta molecular sieve was 500:1~1000:

1. (3) The Si-Beta molecular sieve rich in hydroxyl groups is mixed with niobium salt, ground until homogeneous, placed in an autoclave for crystallization, washed and dried, and calcined at high temperature to obtain Nb-Beta molecular sieve; wherein, the niobium salt is any one of niobium ethanol, niobium oxalate, niobium chloride and niobium acetate; Nb-Beta molecular sieves are formed by immobilizing niobium oxide on a dealuluminated Beta molecular sieve. Niobium species form bonds with the hydroxyl groups in the dealuluminated Beta molecular sieve, and the niobium oxide exists in a twisted Si-O-NbO structure. x The bonds exist in the form of Nb-Beta molecular sieves, wherein NbO x The XRD diffraction peaks are located at 28.42°~28.47°, and the silicon-niobium molar ratio is 10:1~15:1; In step (3), the silicon-niobium molar ratio of Si-Beta molecular sieve to niobium salt is 10:1~15:1, the crystallization temperature is 140~180℃, the crystallization time is 12~48 h, the calcination atmosphere is air or nitrogen, the calcination temperature is 400~600℃, the heating rate is 2~3℃ / min, and the calcination time is 4~10 h.

2. The method for preparing Nb-Beta molecular sieve according to claim 1, characterized in that, In step (1), the ammonium salt is any one of ammonium nitrate, ammonium carbonate, ammonium bicarbonate and ammonium sulfate, the concentration of the ammonium salt aqueous solution is 5~10 mol / L; the ion exchange temperature is 40~90℃, the ion exchange time is 6~24 h; the calcination atmosphere is air or nitrogen, the calcination temperature is 500~600℃, the heating rate is 2~3℃ / min, and the calcination time is 4~10 h.

3. The method for preparing Nb-Beta molecular sieve according to claim 1, characterized in that, In step (2), the acid used for pickling is any one of nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid and citric acid, the concentration of the acid is 10~15 mol / L, the volume ratio of H-Beta molecular sieve to the acid used is 1:10~1:20, the pickling temperature is 20~120℃, the pickling time is 6~24 h, the vacuum drying temperature is 60~200℃, and the drying time is 8~24 h.

4. The application of the Nb-Beta molecular sieve prepared by the method according to claim 1 in the preparation of cyclohexane oxide.

5. The application according to claim 4, characterized in that, Epoxycyclohexane was obtained by stirring Nb-Beta molecular sieve with cyclohexene, acetonitrile, and an aqueous solution of hydrogen peroxide.

6. The application according to claim 5, characterized in that, The mass ratio of Nb-Beta molecular sieve:cyclohexene:acetonitrile is 1:1:20~1:20:100, the mass fraction of hydrogen peroxide aqueous solution is 30%~50%, the molar ratio of hydrogen peroxide to cyclohexene is 1:10~5:1, the reaction temperature is 50~100℃, the reaction pressure is 0.1~5 MPa, and the reaction time is 2~8 h.

Images