A method and application for achieving stable migration of Lewis acids into the molecular sieve framework with the assistance of boron atoms.

Abstract

This invention belongs to the field of molecular sieve preparation, and provides a method and application for the stable migration of Lewis acids into the molecular sieve framework with the assistance of boron atoms. The steps are as follows: A post-treatment method is used, with boron-containing molecular sieves as the silicon source, followed by the addition of a zirconium source, a structure directing agent, and water. After uniform mixing, hydrothermal crystallization is performed, followed by centrifugation, washing, drying, and calcination to obtain Zr-G molecular sieves. An acid treatment method is then used, where the Zr-G molecular sieves are thoroughly treated in an acid solution at 100°C, followed by centrifugation, washing, drying, and calcination to obtain Zr-M molecular sieves. This invention achieves efficient introduction of Lewis acids into the molecular sieve framework, solving the problems of weakly acidic surface-bonded species mainly formed during post-synthesis of Lewis acid sites, which are prone to aggregation, loss, and lack of confinement effects. This opens up new avenues for the comprehensive application of Lewis acid molecular sieves in sustainable catalysis.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve preparation, specifically relating to a method and application for achieving stable migration of Lewis acids into the molecular sieve framework with the assistance of B atoms. Background Technology

[0002] Lewis acid (L-acid) molecular sieves are characterized by electron-deficient metal centers such as zirconium (Zr) and tin (Sn), playing a crucial role in sustainable catalysis. They can activate oxygen-containing substances in reactions such as biomass commercialization through coordination interactions. Traditional methods involve directly synthesizing and introducing metals to construct Lewis acid sites (Maura Koehle, et al. Lewisacidic zeolite Beta catalyst for the...).

[0003] Meerwein–Ponndorf–Verley reduction of furfural[J].Catal.Sci.Technol.,2016,6:3018-3026), but due to the restriction of heteroatoms on the molecular sieve framework, direct synthesis methods usually require a long synthesis cycle or harsh synthesis conditions. Solid-state ion exchange (I. Hermans, et al. Simple and Scalable Preparation of Highly Active Lewis Acidic Sn-β[J]. Angew. Chem. Int. Ed., 2012, 51(47): 11736-11739.) and post-synthesis (Daniel T. Bregante, et al. Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts[J]. ACS Catal. 2018, 8(4): 2995–3010) can circumvent the above problems. However, the state of metal species on the surface of molecular sieves is unstable, and they are prone to enrichment to form oxide clusters or loss. They cannot maintain a stable loading of metal species well and lack confinement effect. Recent studies have shown that by removing Ge from BEC zeolites to create cavities, it is possible to directionally construct metal active sites (Y. Román-Leshkov, et al. Selective active site placement in Lewis acid zeolites and implications for catalysis of oxygenated compounds[J]. Chem. Sci., 2020, 11(37): 10225-10235.). However, this method is limited to specific zeolite systems and lacks universality.

[0004] Therefore, developing a universal and simple strategy to achieve efficient anchoring of L-acid sites in the molecular sieve framework remains a challenge. Summary of the Invention

[0005] To address existing challenges, this invention provides a method for achieving stable migration of Lewis acids into the molecular sieve framework with the assistance of boron atoms.

[0006] The technical solution of the present invention is as follows:

[0007] A method for achieving stable migration of Lewis acids into the molecular sieve framework with the assistance of boron atoms includes the following steps:

[0008] (1) Synthesis of grafted Lewis acid molecular sieve: A post-treatment method was adopted, using B-containing molecular sieve as silicon source, and then adding zirconium source, structure directing agent and water, and mixing them evenly to form a first mixture; the first mixture was hydrothermally crystallized, centrifuged, washed, dried and calcined to obtain Zr-G molecular sieve;

[0009] The structure directing agent is tetrabutylammonium hydroxide (abbreviated as TBAOH); the zirconium source is zirconium sulfate (Zr(SO4)2).

[0010] The addition ratio of silicon source, zirconium source, structure directing agent and water in the first mixture is as follows: the silicon source is calculated as SiO2, the zirconium source is calculated as ZrO2, and the molar ratio of SiO2, ZrO2, structure directing agent and water is 1:0.02:0.1:10.

[0011] (2) Synthesis of migrating Lewis acid molecular sieve: Zr-G molecular sieve was treated in an acid solution at 100℃ for 24h by acid treatment, centrifugation, washing, drying and calcination to obtain Zr-M molecular sieve.

[0012] The acid solution is a 2 mol / L HNO3 solution;

[0013] The ratio of acid solution to molecular sieve is 1:100.

[0014] The specific implementation steps of step (1) are as follows:

[0015] Using boron-containing molecular sieves as the silicon source, zirconium source, structure directing agent and water were added and stirred for 2 hours to form a first mixture. The first mixture was crystallized at 120℃ for 2 days. After cooling at room temperature, the sample was obtained by centrifugation and washed with water until pH=7. Then it was dried at 80℃ for 12 hours. The dried sample was calcined at 550℃ for 6 hours at a heating rate of 2℃ / min to obtain Zr-G molecular sieves.

[0016] The specific implementation steps of step (2) are as follows:

[0017] Zr-G molecular sieve was treated in acid solution at 100℃ for 24h, cooled at room temperature, centrifuged to obtain the sample, washed with water until pH=7; then dried at 80℃ for 12h; the dried sample was calcined at 550℃ for 6h at a heating rate of 2℃ / min to obtain Zr-M molecular sieve.

[0018] The beneficial effects of this invention are as follows: This invention successfully achieves the efficient introduction of Lewis acids into the molecular sieve framework using the assistance of boron atoms, solving the problems of weakly acidic surface-bonded species mainly forming in the subsequent synthesis of Lewis acid sites, which are prone to aggregation, loss, and lack of confinement effects. The boron atoms provide pre-occupancy, and during acid treatment, they shed to generate a large number of silanol nests, providing suitable sites for Zr migration. After migration, the Zr species transforms from a weakly acidic surface-grafted species into a strongly Lewis acidic four-coordinate Zr species, which can stably exist in the molecular sieve framework. This invention opens up new avenues for the comprehensive application of Lewis acid molecular sieves in sustainable catalysis. Attached Figure Description

[0019] Figure 1 The infrared spectrum of pyridine adsorption of the sample prepared in Example 1 is shown.

[0020] Figure 2 The infrared spectrum of pyridine adsorption of the sample prepared in Example 2 is shown.

[0021] Figure 3 The infrared spectrum of pyridine adsorption for the sample prepared in Comparative Example 3. Detailed Implementation

[0022] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0023] Example 1

[0024] Boric acid, tetraethyl orthosilicate, a 40% tetrabutylammonium hydroxide aqueous solution, and water were mixed evenly and stirred for 12 hours to obtain a mixture with a molar composition of 10SiO2:1B2O3:3.6TBAOH:120H2O. The mixture was placed in a 25mL hydrothermal reactor lined with polytetrafluoroethylene and statically crystallized at 140℃ for 5 days. After centrifugation, washing, and drying, B-SPP molecular sieve was obtained.

[0025] Using B-SPP as the silicon source, a 40% (w / w) aqueous solution of tetrabutylammonium hydroxide, zirconium sulfate, and water were added and stirred for 2 hours to obtain a mixture with a molar composition of 1SiO2:0.02Zr(SO4)2:0.1TBAOH:10H2O. This mixture was then placed in a 25 mL polytetrafluoroethylene-lined hydrothermal reactor and crystallized at 120 °C for 48 hours. After centrifugation, washing, drying, and calcination, Zr-B-SPP-G molecular sieve M1-G was obtained. M1-G was treated with 2M HNO3 aqueous solution at 100 °C for 24 hours, followed by centrifugation, washing, drying, and calcination to obtain Zr-B-SPP-A molecular sieve M1. The pyridine adsorption infrared spectra of M1-G and M1 are shown below. Figure 1 As shown, the acid treatment successfully introduced Zr species into the molecular sieve framework and brought about more Lewis acids.

[0026] Example 2

[0027] Boric acid, a 25% (w / w) tetraethylammonium hydroxide aqueous solution, and water were mixed evenly and stirred for 30 min. Then, silica was added and stirred for 10 h to obtain a mixture with a molar composition of 5SiO2:0.16B2O3:0.65TEAOH:57.5H2O. The mixture was placed in a 25 mL polytetrafluoroethylene-lined hydrothermal reactor and dynamically crystallized at 140 °C for 6 days. After filtration, washing, and drying, B-BEA molecular sieve was obtained.

[0028] Using B-BEA as the silicon source, a 40% (w / w) aqueous solution of tetrabutylammonium hydroxide, zirconium sulfate, and water were added and stirred for 2 hours to obtain a mixture with a molar composition of 1SiO2:0.02Zr(SO4)2:0.1TBAOH:10H2O. This mixture was placed in a 25 mL polytetrafluoroethylene-lined hydrothermal reactor and crystallized at 120 °C for 48 hours. After centrifugation, washing, drying, and calcination, Zr-B-BEA-G molecular sieve M2-G was obtained. M2-G was treated with 2M HNO3 aqueous solution at 100 °C for 24 hours, followed by centrifugation, washing, drying, and calcination to obtain Zr-B-BEA-A molecular sieve M2. The pyridine adsorption infrared spectra of M2-G and M2 are shown below. Figure 2 As shown, the acid treatment successfully introduced Zr species into the molecular sieve framework and brought about more Lewis acids.

[0029] Comparative Example 1

[0030] Using B-SPP as the silicon source, a 40% (w / w) aqueous solution of tetrabutylammonium hydroxide, zirconium sulfate, and water were added and stirred for 2 hours to obtain a mixture with a molar composition of 1SiO2:0.01-0.04Zr(SO4)2:0.1TBAOH:10H2O. The mixture was placed in a 25 mL hydrothermal reactor lined with polytetrafluoroethylene and crystallized at 120 °C for 48 hours. After centrifugation, washing, drying, and calcination, Zr-B-SPP-G molecular sieve D1-xG (where x represents the Zr content, x = 0.01, 0.02, 0.04) was obtained. D1-xG was treated with 2M NO3 aqueous solution at 100 °C for 24 hours. After centrifugation, washing, drying, and calcination, Zr-B-SPP-A molecular sieve D1-x was obtained.

[0031] Comparative Example 2

[0032] Using B-SPP as the silicon source, a 40% (w / w) aqueous solution of tetrabutylammonium hydroxide, zirconium sulfate, and water were added and stirred for 2 hours to obtain a mixture with a molar composition of 1SiO2:0.02Zr(SO4)2:0.1TBAOH:10H2O. The mixture was placed in a 25 mL hydrothermal reactor lined with polytetrafluoroethylene and crystallized at 120 °C for 48 hours. After centrifugation, washing, drying, and calcination, Zr-B-SPP-G molecular sieve D2-G was obtained. D2-G was treated with 1-6 M HNO3 aqueous solution at 100 °C for 24 hours. After centrifugation, washing, drying, and calcination, Zr-B-SPP-A molecular sieve D2-y was obtained (where y represents the nitric acid concentration, y = 1, 2, 6, 12).

[0033] Comparative Example 3

[0034] Tetrabutylammonium hydroxide (40% by mass) aqueous solution and water were mixed evenly and stirred for 12 h to obtain a mixture with a molar composition of 5SiO2:1.8TBAOH:60H2O. The mixture was placed in a 25 mL hydrothermal reactor lined with polytetrafluoroethylene and crystallized at 120 °C for 72 h. After centrifugation, washing and drying, Si-SPP molecular sieve was obtained.

[0035] Using Si-SPP as the silicon source, a 40% (w / w) aqueous solution of tetrabutylammonium hydroxide, zirconium sulfate, and water were added and stirred for 2 hours to obtain a mixture with a molar composition of 1SiO2:0.02Zr(SO4)2:0.1TBAOH:10H2O. This mixture was then placed in a 25 mL polytetrafluoroethylene-lined hydrothermal reactor and crystallized at 120 °C for 48 hours. After centrifugation, washing, drying, and calcination, Zr-SPP-G molecular sieve D3-G was obtained. D3-G was treated with 2M HNO3 aqueous solution at 100 °C for 24 hours, followed by centrifugation, washing, drying, and calcination to obtain Zr-SPP-A molecular sieve D3. The pyridine adsorption infrared spectra of D3-G and D3 are shown below. Figure 3 As shown, the acid treatment successfully introduced Zr species into the molecular sieve framework and brought about more Lewis acids.

[0036] Application Example 1

[0037] The catalysts obtained in Examples 1-2 and Comparative Examples 1-3 were used as catalysts for the transfer hydrogenation of cyclohexanone to cyclohexanol in the MPV reaction, and the activity of the catalysts was evaluated.

[0038] 1. Catalytic performance evaluation method: A 0.002M cyclohexanone isopropanol solution was used as the reaction substrate. A sealed glass bottle was used as the reaction vessel. The catalyst, cyclohexanone and isopropanol were added to the bottle respectively, with a molar ratio of cyclohexanone to catalyst of 8. The glass bottle was placed in an oil bath at 100°C and reacted for 8 hours.

[0039] 2. Test conditions: Gas chromatography was used to analyze the content of each substance in the solution. The test results are shown in Table 1 below.

[0040] Table 1. Catalyst activity evaluation results in Example 1

[0041]

[0042] The catalyst in Example 1 uses B-SPP molecular sieve as the parent material. During the synthesis process, acid treatment causes B to detach, resulting in a large number of silanol nests, which capture more Lewis acidic Zr species, allowing cyclohexanone to react almost completely in the MPV reaction.

[0043] Table 2 Catalyst Activity Evaluation Results (Example 2)

[0044]

[0045] In Example 2, the catalyst used B-BEA as the parent material. During the synthesis process, acid treatment caused B to detach, resulting in a large number of silanol nests, which captured more Lewis acidic Zr species, thus achieving a significant improvement in reactivity. The versatility of this invention was verified by expanding the types of molecular sieves used.

[0046] Table 3. Catalyst activity evaluation results for Comparative Example 1

[0047]

[0048] The results of Comparative Example 1 show that when the Zr content is too low, the amount of L-acid introduced into the zeolite is insufficient to provide the catalytic sites required for the MPV reaction; when the Zr content is too high, the reactivity is low, and the Zr species in the zeolite tend to aggregate to form ZrO2 clusters with Brønsted acid, which is not conducive to the occurrence of the MPV reaction. Therefore, a Zr content of y = 0.02 is a suitable condition for catalyzing the MPV reaction.

[0049] Table 4. Catalyst activity evaluation results for Comparative Example 2

[0050]

[0051] The results of Comparative Example 2 show that when the nitric acid concentration is low, the driving force provided by the acid is insufficient for all Zr species to migrate into the framework; when the nitric acid concentration is too high, it will damage the molecular sieve structure, which is not conducive to the reaction. Therefore, a nitric acid concentration of 2M is a suitable treatment condition for driving Zr migration.

[0052] Table 5. Results of catalyst activity evaluation in Comparative Example 3

[0053]

[0054] The reaction results of Comparative Example 3 show that the molecular sieve, which lacks B species and is only acid-treated, captures fewer Lewis acid species due to the small number of silanol nests, and therefore cannot exert good activity in the MPV reaction.

Claims

1. A method for Lewis acid stabilized migration into a molecular sieve framework by B atom assistance, characterized in that, The method comprises the following steps: (1) synthesis of grafted Lewis acid molecular sieve: a post-treatment method is adopted, a B-containing molecular sieve is used as a silicon source, a zirconium source, a structure directing agent and water are added, and a first mixed solution is formed after mixing uniformly; the first mixed solution is hydrothermally crystallized, centrifuged, washed, dried and calcined to obtain a Zr-G molecular sieve; (2) synthesis of migrated Lewis acid molecular sieve: an acid treatment method is adopted, the Zr-G molecular sieve is treated in an acid solution at a temperature of 100 ℃ for 24 h, centrifuged, washed, dried and calcined to obtain a Zr-M molecular sieve.

2. The method for realizing stable migration of Lewis acid into the framework of a molecular sieve assisted by B atoms according to claim 1, characterized in that, in step (1), in step (1), the structure directing agent is tetrabutylammonium hydroxide; the zirconium source is zirconium sulfate; the addition ratio of the silicon source, the zirconium source, the structure directing agent and water in the first mixed solution is that the silicon source is calculated as SiO2, the zirconium source is calculated as ZrO2, and the molar ratio of SiO2, ZrO2, the structure directing agent and water is 1:0.02:0.1:

10.

3. The method for realizing stable migration of Lewis acid into the framework of a molecular sieve assisted by B atoms according to claim 2, characterized in that, the specific implementation steps of step (1) are as follows: a B-containing molecular sieve is used as a silicon source, a zirconium source, a structure directing agent and water are added, stirring is performed for 2 h to form a first mixed solution; the first mixed solution is crystallized at a temperature of 120 ℃ for 2 d; cooling is performed at room temperature, centrifugal separation is performed to obtain a sample, water washing is performed until the pH value is 7; then drying is performed at a drying temperature of 80 ℃ for 12 h; the dried sample is calcined at a temperature of 550 ℃ for 6 h, and the temperature rising speed is 2 ℃ / min to obtain a Zr-G molecular sieve.

4. The method for realizing stable migration of Lewis acid into the framework of a molecular sieve assisted by B atoms according to claim 1, characterized in that, the specific implementation steps of step (2) are as follows: the Zr-G molecular sieve is treated in an acid solution at a temperature of 100 ℃ for 24 h, cooling is performed at room temperature, centrifugal separation is performed to obtain a sample, water washing is performed until the pH value is 7; then drying is performed at a drying temperature of 80 ℃ for 12 h; the dried sample is calcined at a temperature of 550 ℃ for 6 h, and the temperature rising speed is 2 ℃ / min to obtain a Zr-M molecular sieve.

5. The method for realizing stable migration of Lewis acid into the framework of a molecular sieve assisted by B atoms according to claim 4, characterized in that, in step (2), the acid solution is an HNO3 solution with a concentration of 2 mol / L; the addition ratio of the acid solution and the Zr-G molecular sieve is that the mass ratio of the molecular sieve and the acid solution is 1:

100.

6. Application of the Zr-M molecular sieve obtained by any one of the methods in claims 1-5 in an MPV reaction.

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