Process for the preparation of a hydrophobic h-type al-beta zeolite catalyst and its use

Abstract

The application discloses a preparation method of a pore hydrophobic H-type Al-beta zeolite catalyst and application thereof. The method adopts triethoxyfluorosilane and tetraethyl orthosilicate as a silicon source, and directly in-situ hydrothermally synthesizes fluorine-based functionalized Al-beta zeolite through a template-free route. The strong hydrophobicity and electron absorption property of the fluorine group are utilized to simultaneously enhance the pore hydrophobicity and acid strength of the Al-beta zeolite catalyst, weaken the adsorption of water molecules on the acid sites, fundamentally inhibit the occurrence of a hydrolysis side reaction of lactide, and realize one-step efficient synthesis of lactide in a lactic acid liquid phase. The pore hydrophobic H-type Al-beta zeolite catalyst synthesized by the application can exhibit a satisfactory lactide yield, which can reach 84.80%.

Description

Technical Field

[0001] This invention relates to the hydrophobic modification technology of Al-beta zeolite and the field of catalysis, specifically to a method for controlling the hydrophobicity of the pores of Al-beta zeolite as a Brønsted acid catalyst and its application in the catalytic synthesis of lactic acid from lactide. Background Technology

[0002] Ring-opening polymerization of lactide is the main synthetic route for preparing high molecular weight polylactic acid (PLA). Therefore, achieving efficient lactide synthesis is of great significance for the large-scale development of the PLA industry. Michiel Dusselier et al. (Science, 2015, 349, 78–80) proposed utilizing the shape selectivity and abundant Brønsted acidic sites of Al-beta zeolite channels to catalyze the conversion of lactic acid to lactide under aromatic solvent reflux dehydration conditions, achieving a yield of up to 68%. However, this process suffers from untimely and incomplete dehydration, failing to avoid side reactions between lactide and water. Furthermore, the adsorption of water molecules at the Brønsted acidic sites easily deactivates the lactide, leading to a decrease in lactide yield. The wettability of the catalyst has a significant impact on molecular diffusion during the catalytic process. Enhancing the hydrophobicity near the Brønsted acidic sites in the Al-beta zeolite channels can weaken the adsorption of water molecules at these sites, fundamentally inhibiting the hydrolysis of lactide and simultaneously improving catalyst stability.

[0003] Organofunctionalization is a commonly used method for modifying molecular sieves. It involves introducing organic groups into the molecular sieve through the modification of organosilanes. It is mainly divided into post-synthetic modification and in-situ synthesis. Post-synthetic modification refers to reacting organic groups with the silanol groups on the zeolite surface after zeolite synthesis. In this method, the organosilanes are mostly modified on the surface of the molecular sieve, making it difficult for them to enter the framework and easily clogging the pores, hindering the diffusion of reactant molecules within the sieve channels. In-situ synthesis uses organosilanes as a silicon source. During the crystallization process of zeolite, the organic groups are directly attached to the pore surface or framework of the zeolite molecular sieve, synthesizing organofunctionalized zeolite molecular sieves, effectively avoiding pore clogging. Currently, alkylsilanes are commonly used as modifying agents. However, during the high-temperature calcination process to remove the template agent, the Si–C bonds on the zeolite pore surface are easily broken, destroying the alkyl groups modified within the zeolite pores (Chemical Communications, 2014, 50, 2012–2014). Template-free synthetic routes for molecular sieves can retain modified alkyl groups, significantly improving the hydrophobicity of the sieve channels. For example, Xu et al. (Xu Hua, Zhejiang University, 2019) first obtained alkyl-modified silica solid powder by co-hydrolysis of alkylsilane and tetraethyl orthosilicate, and then used it as a silicon source to synthesize phenyl and methyl-functionalized beta zeolites via a hydrothermal method according to the formula 16.34 SiO2 / 1.0 Al2O3 / 4.39 Na2O / 185.11 H2O. The synthesized zeolites exhibited improved hydrophobicity and maintained their toluene adsorption capacity for a longer period. However, template-free methods introduce Na+ into the zeolite framework. + When zeolites are used as Brønsted acid catalysts, they require ion exchange to convert Na-type zeolites to H-type zeolites. This process still needs to be carried out at high temperatures (>500 °C), inevitably damaging the modified alkyl groups in the pores and affecting the hydrophobicity within the pores. Therefore, how to effectively improve the hydrophobicity within the pores of zeolites when used as Brønsted acid catalysts remains an urgent problem to be solved. Summary of the Invention

[0004] The purpose of this invention is to address the limitations of current technologies by proposing a method for preparing a hydrophobic H-type Al-beta zeolite catalyst and its application. This method uses triethoxyfluorosilane and tetraethyl orthosilicate as silicon sources, and directly synthesizes fluorine-functionalized Al-beta zeolite in situ via a template-free route using direct hydrothermal treatment. The strong hydrophobicity and electron-withdrawing properties of the fluorine group simultaneously enhance the hydrophobicity of the Al-beta zeolite catalyst's pores and acid strength, reducing the adsorption of water molecules at acidic sites and fundamentally inhibiting the hydrolysis side reaction of lactide, thus achieving a one-step, highly efficient liquid-phase synthesis of lactide from lactic acid. The hydrophobic H-type Al-beta zeolite catalyst synthesized by this invention achieves a lactide yield of up to 84.80%.

[0005] The technical solution provided by this invention is as follows: A method for preparing a hydrophobic H-type Al-beta zeolite catalyst includes the following steps: (1) Sodium hydroxide and sodium aluminate solids are mixed and dissolved in water, heated to 40–50 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane are added to mixture A in sequence and stirred for 8–12 h to obtain reaction gel. In step (1), the molar ratio is sodium hydroxide: sodium aluminate: silicon source: water = (9.61–11.63):1: (34.2–51.5): (1650–2350); The silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element accounts for 5–20% of the molar amount of silicon. (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir evenly, and place in a stainless steel crystallization kettle with a polytetrafluoroethylene liner. Crystallize at 120–140 °C for 18–168 h. After washing and drying, the modified Al-beta zeolite is obtained. In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.0–11.0% of the total mass of silicon dioxide (from a silicon source) and aluminum oxide (from sodium aluminate) in the system; (3) The modified Al-beta zeolite was placed in an ammonium chloride solution for ion exchange, and after centrifugation and drying, it was calcined in a muffle furnace to obtain a hydrophobic H-type Al-beta zeolite catalyst. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 0.5–1 mol / L, the solid-liquid mass ratio is controlled at 1:30–1:50, the reaction is carried out at 70–90 °C for 4–6 h, and the ion exchange-drying process is repeated 3–4 times. The calcination conditions in step (3) are: calcination at 500–600 ℃ for 4–9 h, with a heating rate of 0.5–1 ℃ / min.

[0006] The hydrophobic H-type Al-beta zeolite prepared by the method is used for the catalytic reaction of lactic acid to lactide synthesis.

[0007] Specifically, the steps include: adding lactic acid solution, substance Q, and hydrophobic H-type Al-beta zeolite catalyst to a reactor and refluxing for 1–7 h to obtain lactide; In this process, 0.2–3.0 g of lactic acid solution and 0.1–2.0 g of hydrophobic H-type Al-beta zeolite catalyst are added to every 10 mL of substance Q; the mass concentration of the lactic acid solution is 50–90%; substance Q is toluene or xylene, wherein the xylene is o-xylene, m-xylene, or p-xylene.

[0008] The essential features of this invention are: This invention adds triethoxyfluorosilane as a common silicon source to the conventional silicon source—tetraethyl orthosilicate—while simultaneously enhancing the hydrophobicity of the pores and the acid strength of the Al-beta zeolite catalyst. Triethoxyfluorosilane is an organosilane reagent with excellent thermal stability. The Si–F bond in this molecule remains stable at high temperatures (>500 °C), and the fluorine group has strong hydrophobicity and electron-withdrawing properties. This invention utilizes its strong hydrophobicity to weaken the adsorption of water molecules at Brønsted acidic sites and its electron-withdrawing properties to enhance the binding effect of Al-beta zeolite Brønsted acidity, thus achieving a one-step, efficient synthesis of lactide from lactic acid in the liquid phase.

[0009] The beneficial effects of this invention are: This invention utilizes triethoxyfluorosilane and tetraethyl orthosilicate as silicon sources to prepare hydrophobically modified Al-beta zeolite via in-situ hydrothermal synthesis. Taking advantage of the high-temperature stability of the Si–F bonds in the triethoxyfluorosilane molecule, the prepared Al-beta zeolite is further subjected to ion exchange and calcination to obtain a hydrophobic H-type Al-beta zeolite catalyst. The fluorine modification not only enhances the acidity of the Al-beta zeolite and improves its catalytic activity, but also weakens the adsorption of product water at Brønsted active sites in the hydrophobic H-type Al-beta zeolite catalyst, inhibiting the hydrolysis of lactide and thus increasing the yield of lactide.

[0010] The beneficial effects of this invention are as follows: After modification, the total acidity of the hydrophobic H-type Al-beta zeolite catalyst increased from 2.34 mmol / g to 3.45 mmol / g. In particular, the acidity of Brønsted acids was significantly improved, increasing from 53.74 μmol / g to 232.25 μmol / g. Furthermore, the acid strength of both moderately strong and strong acids was significantly enhanced.

[0011] In the H-type Al-beta zeolite catalyst with hydrophobic pores, the amount of physically adsorbed water desorbed decreased from 10.3 wt% to 5.5 wt%; at the same time, the amount of water vapor adsorbed also decreased significantly, further indicating that fluorine modification can improve the hydrophobicity of molecular sieve pores.

[0012] Currently, Michiel Dusselier et al. (Science, 2015, 349, 78–80) used Al-beta zeolite to catalyze the production of lactic acid (90% concentration) into lactide, achieving a lactide yield of 68%. Compared to that work, the hydrophobic H-type Al-beta zeolite catalyst synthesized in this invention exhibits a satisfactory lactide yield of up to 84.80% in the catalytic reaction of lactic acid (90% concentration) into lactide. Attached Figure Description

[0013] Figure 1 SEM images of the synthesized products from Examples 1–5 and the comparative examples are shown.

[0014] Figure 2 The XRD patterns are those of the synthesized products from Examples 1–5 and the comparative examples.

[0015] Figure 3 NH3-TPD characterization of the synthesized products of Example 2 and the comparative example.

[0016] Figure 4 The images show the Py-FTIR spectra of the synthesized products from Example 2 and the comparative example.

[0017] Figure 5 The images show the water vapor adsorption curves of the synthesized products in Example 2 and the comparative example. Detailed Implementation

[0018] The present invention is described in detail through the following embodiments, but the embodiments are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0019] The following test methods are used in various embodiments of the present invention: XRD analysis was performed using a Bruker D8 Focus instrument from Germany to characterize the phase composition of the products. The instrument parameters were: Cu Kα radiation, tube current 200 mA, tube voltage 40 kV, scan rate 6° / min, and scan range 5–40°.

[0020] The NH3-TPD assay was performed using an Auto Chem 2920 instrument from Micron Instruments, Inc. (USA), to characterize the acidity and acid strength of molecular sieves. The assay consisted of three main processes: pretreatment, NH3 adsorption, and temperature-programmed desorption. The specific steps were as follows: 30–40 mg of dried molecular sieve sample was weighed and placed in a reaction tube. The sample was pretreated by heating from room temperature to 300 °C at a rate of 10 °C / min. He gas was then used to purge the sample for 1 h (30 mL / min). The sample was cooled to 50 °C and NH3 / He gas was passed through at 30 mL / min for 1 h until saturation. The He gas was then switched to 30 mL / min and purge for 1 h to remove weakly physically adsorbed NH3 until the signal stabilized. Finally, the sample was desorbed at 800 °C under a He atmosphere at a rate of 10 °C / min, and the extracted gas was detected using a TCD detector.

[0021] The Py-FTIR assay was performed using a Thermo Fisher Nicolet iS50 from Thermo Fisher Scientific, used to characterize the acidity of catalysts, distinguish between Brønsted and Lewis acid types, and determine their relative abundance. The test conditions were as follows: a certain amount of powder sample was taken, fixed in the infrared cell, and first activated under vacuum at 350 °C (1 × 10⁻⁶). -3 After 2 hours of adsorption at 50 °C, the sample was cooled to room temperature, and the spectral density was scanned to obtain background data. After adsorbing pyridine at 50 °C for half an hour, vacuum desorption (1 × 10⁻⁶) was performed by programming the temperature to setpoint temperatures (200 °C and 350 °C). -3 Pa) for half an hour, and record 1700–1400 cm. -1 Infrared spectrum in the wavenumber region. The formula for calculating acid content is C = K(Brønsted / Lewis)*I(Brønsted / Lewis)*R*R / W / 4*1000; where K(Brønsted / Lewis) is the absorption coefficient of Brønsted acid or Lewis acid, which are 1.88 and 1.42 respectively, I(Brønsted / Lewis) is the peak area of ​​Brønsted acid or Lewis acid, W is the tablet weight (in mg), and R is the tablet diameter, which is 1.3 cm.

[0022] The TG test was performed using a NETZSCH STA 449 F3 instrument from NETZSCH GmbH, Germany, primarily used to determine the water content in molecular sieve materials. The specific steps were as follows: Before testing, 0.4 mL of water was mixed with 20 mg of sample and stirred at 1000 rpm for 1 h at 90 °C to allow for complete water absorption. The mixture was then dried in a 60 °C oven for 16 h to ensure complete drying. The testing procedure was as follows: 5–10 mg of sample was weighed into a crucible, placed in the instrument, and the parameters were set. Under a nitrogen atmosphere, the temperature was increased from 30 °C to 800 °C at a rate of 10 °C / min, and the weight loss ratio of the sample was recorded.

[0023] The water vapor adsorption test was performed using a 3 Flex instrument from Micron Instruments, Inc. (USA). After degassing the molecular sieve sample at 300 °C for 12 hours, the water vapor adsorption isotherm was obtained at 25 °C.

[0024] The Al-beta zeolite seed crystals involved in this invention are known materials, and their preparation methods are detailed in the literature (Modulation of wettability of molecular sieve membrane and coating surface and its application in enhancing the separation process of liquid mixtures, Li Yun, 2019, Tianjin University).

[0025] Example 1: The specific operation is as follows (1) Sodium hydroxide and sodium aluminate solids were mixed and dissolved in water, heated to 40 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane were added to mixture A in sequence and stirred for 8 h to obtain reaction gel. Wherein, the molar ratio is n (Sodium hydroxide): n (Sodium aluminate): n (Silicon source): n (Water) = 9.61 : 1 : 34.2 : 1650; the silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element is 5% of the molar amount of silicon; (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir for 40 minutes until homogeneous, and then place it in a stainless steel crystallization kettle with a polytetrafluoroethylene liner. Crystallize at 120 °C for 120 h. After washing and drying, the modified Al-beta zeolite is obtained. In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.0% of the total mass of silicon dioxide (from a silicon source) and aluminum oxide (from sodium aluminate) in the system; (3) The modified Al-beta zeolite was placed in an ammonium chloride solution for ion exchange and then centrifuged and dried. After repeating the ion exchange-drying process three times, the H-type Al-beta zeolite catalyst with hydrophobic pores was obtained by calcination in a muffle furnace. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 0.5 mol / L, the solid-liquid mass ratio is controlled at 1:30, and the reaction is carried out at 70 °C for 4 h. The calcination conditions in step (3) are: calcination at 600 ℃ for 4 h, with a heating rate of 0.5 ℃ / min.

[0026] (4) Thermogravimetric analysis was performed on the H-type Al-beta zeolite catalyst with hydrophobic pores: Before 300 °C, the weakly adsorbed water bound to the molecular sieve surface began to desorb. Therefore, the weight loss of the sample before 300 °C was recorded and measured to be 9.2 wt%.

[0027] (5) Catalytic testing of the hydrophobic H-type Al-beta zeolite catalyst: 1.0 g of 90% lactic acid solution and 10 mL of toluene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask. 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask, and the mixture was refluxed (140 °C) for 3 h. After the reaction, the product was analyzed by high-performance liquid chromatography according to the analytical method reported by Michiel Dusselier et al. (Science, 2015, 349, 78–80) (the following examples are the same and will not be repeated). The lactide yield was calculated to be 76.54%.

[0028] Example 2: The specific operation is as follows (1) Sodium hydroxide and sodium aluminate solids were mixed and dissolved in water, heated to 40 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane were added to mixture A in sequence and stirred for 8 h to obtain reaction gel. Wherein, the molar ratio is n (Sodium hydroxide): n (Sodium aluminate): n (Silicon source): n (Water) = 11.25 : 1 : 40.9 : 1653; The silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element accounts for 10% of the molar amount of silicon. (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir for 40 minutes until homogeneous, then place in a stainless steel crystallization kettle with a polytetrafluoroethylene liner and crystallize at 140 °C for 18.5 h. After washing and drying, the modified Al-beta zeolite is obtained; In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.3% of the total mass of silicon dioxide (from silicon source) and aluminum oxide (from sodium aluminate) in the system; (3) The modified Al-beta zeolite was placed in an ammonium chloride solution for ion exchange, centrifuged and dried, and the ion exchange-drying process was repeated 3 times. After calcination in a muffle furnace, a hydrophobic H-type Al-beta zeolite catalyst was obtained. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 1 mol / L, the solid-liquid mass ratio is controlled at 1:50, and the reaction is carried out at 80 °C for 6 h. The calcination conditions in step (3) are: calcination at 550 ℃ for 6 h, with a heating rate of 1 ℃ / min.

[0029] (4) The thermogravimetric test was the same as in Example 1. The weight loss was measured to be 5.5 wt%.

[0030] (5) The catalytic process was the same as in Example 1. The calculated yield of lactide was 84.80%.

[0031] Example 3: The specific operation is as follows (1) Sodium hydroxide and sodium aluminate solids were mixed and dissolved in water, heated to 45 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane were added to mixture A in sequence and stirred for 10 h to obtain reaction gel. Wherein, the molar ratio is n (Sodium hydroxide): n (Sodium aluminate): n (Silicon source): n (Water) = 10.2 : 1 : 45.4 : 1720; the silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element accounts for 15% of the molar amount of silicon. (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir for 40 minutes until homogeneous, and then place it in a stainless steel crystallization kettle with a polytetrafluoroethylene liner. Crystallize at 140 °C for 21 h. After washing and drying, the modified Al-beta zeolite is obtained. In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.7% of the total mass of silicon dioxide (from silicon source) and aluminum oxide (from sodium aluminate) in the system; (3) The modified Al-beta zeolite was placed in an ammonium chloride solution for ion exchange, centrifuged and dried, and the ion exchange-drying process was repeated 3 times. After calcination in a muffle furnace, a hydrophobic H-type Al-beta zeolite catalyst was obtained. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 0.5 mol / L, the solid-liquid mass ratio is controlled at 1:40, and the reaction is carried out at 80 °C for 5 h. The calcination conditions in step (3) are: calcination at 500 ℃ for 9 h, with a heating rate of 1 ℃ / min.

[0032] (4) The thermogravimetric test was the same as in Example 1. The weight loss was measured to be 7.8 wt%.

[0033] (5) The catalytic process was the same as in Example 1. The calculated yield of lactide was 80.09%.

[0034] Example 4: The specific operation is as follows (1) Sodium hydroxide and sodium aluminate solids were mixed and dissolved in water, heated to 50 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane were added to mixture A in sequence and stirred for 12 h to obtain reaction gel. Wherein, the molar ratio is n (Sodium hydroxide): n (Sodium aluminate): n (Silicon source): n (Water) = 10.99 : 1 : 51.5 : 2350; the silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element accounts for 20% of the molar amount of silicon. (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir for 40 minutes until homogeneous, and then place it in a stainless steel crystallization kettle with a polytetrafluoroethylene liner for crystallization at 140 °C for 24 h. After washing and drying, the modified Al-beta zeolite is obtained; In step (2), the amount of Al-beta zeolite seed crystals added accounts for 11.0% of the total mass of silicon dioxide (from a silicon source) and aluminum oxide (from sodium aluminate) in the system; (3) The modified Al-beta zeolite was placed in an ammonium chloride solution for ion exchange, centrifuged and dried, and the ion exchange-drying process was repeated 4 times. After calcination in a muffle furnace, a hydrophobic H-type Al-beta zeolite catalyst was obtained. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 1 mol / L, the solid-liquid mass ratio is controlled at 1:50, and the reaction is carried out at 90 °C for 5 h. The calcination conditions in step (3) are: calcination at 550 ℃ for 6 h, with a heating rate of 1 ℃ / min.

[0035] (4) The thermogravimetric test was the same as in Example 1. The weight loss was measured to be 9.7 wt%.

[0036] (5) The catalytic process was the same as in Example 1. The calculated yield of lactide was 74.79%.

[0037] Example 5: The specific operation is as follows (1) Sodium hydroxide and sodium aluminate solids were mixed and dissolved in water, heated to 45 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane were added to mixture A in sequence and stirred for 10 h to obtain reaction gel. Wherein, the molar ratio is n (Sodium hydroxide): n (Sodium aluminate): n (Silicon source): n (Water) = 11.63 : 1 : 48.7 : 2000; the silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element accounts for 10% of the molar amount of silicon. (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir for 40 minutes until homogeneous, and then place it in a stainless steel crystallization kettle with a polytetrafluoroethylene liner for crystallization at 120 °C for 168 h. After washing and drying, the modified Al-beta zeolite is obtained; In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.8% of the total mass of silicon dioxide (from a silicon source) and aluminum oxide (from sodium aluminate) in the system; (3) The modified Al-beta zeolite was placed in an ammonium chloride solution for ion exchange, centrifuged and dried, and the ion exchange-drying process was repeated 4 times. After calcination in a muffle furnace, a hydrophobic H-type Al-beta zeolite catalyst was obtained. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 1 mol / L, the solid-liquid mass ratio is controlled at 1:50, and the reaction is carried out at 90 °C for 6 h. The calcination conditions in step (3) are: calcination at 550 ℃ for 6 h, with a heating rate of 0.5 ℃ / min.

[0038] (4) The thermogravimetric test was the same as in Example 1. The weight loss was measured to be 6.4 wt%.

[0039] (5) The catalytic process was the same as in Example 1. The calculated yield of lactide was 81.98%.

[0040] Example 6: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 1, except that step (5) of the catalytic test was performed as follows: 1.8 g of 50% lactic acid solution and 10 mL of toluene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (140 °C) for 3 h. The lactide yield was calculated to be 82.83%.

[0041] Example 7: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 2, except that step (5) of the catalytic test was performed as follows: 1.0 g of 90% lactic acid solution and 10 mL of toluene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (140 °C) for 1 h. The lactide yield was calculated to be 74.06%.

[0042] Example 8: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 3, except that the catalytic test in step (5) was as follows: 0.2 g of 90% lactic acid solution and 10 mL of toluene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.1 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to it. The mixture was then refluxed (140 °C) for 5 h. The lactide yield was calculated to be 72.58%.

[0043] Example 9: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 4, except that step (5) of the catalytic test was performed as follows: 3.0 g of 90% lactic acid solution and 10 mL of toluene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 2.0 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (140 °C) for 3 h. The lactide yield was calculated to be 79.24%.

[0044] Example 10: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 5, except that step (5) of the catalytic test was performed as follows: 1.0 g of 90% lactic acid solution and 10 mL of toluene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (140 °C) for 7 h. The lactide yield was calculated to be 75.67%.

[0045] Example 11: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 1, except that step (5) of the catalytic test was performed as follows: 1.0 g of 90% lactic acid solution and 10 mL of p-xylene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (168 °C) for 3 h. The lactide yield was calculated to be 82.11%.

[0046] Example 12: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 1, except that step (5) of the catalytic test was performed as follows: 1.0 g of 90% lactic acid solution and 10 mL of m-xylene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (169 °C) for 3 h. The lactide yield was calculated to be 80.74%.

[0047] Example 13: The specific operation is as follows The H-type Al-beta zeolite catalyst with hydrophobic pores was prepared according to the method in Example 1, except that step (5) of the catalytic test was performed as follows: 1.0 g of 90% lactic acid solution and 10 mL of o-xylene (used as a solvent and dehydrating agent) were added to a 25 mL three-necked flask, and 0.5 g of the hydrophobic H-type Al-beta zeolite catalyst was accurately weighed and added to the flask. The mixture was then refluxed (174 °C) for 3 h. The lactide yield was calculated to be 79.10%.

[0048] Comparison: The specific steps are as follows (1) Sodium hydroxide and sodium aluminate solids were mixed and dissolved in water, heated to 40 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate was added to mixture A and stirred for 8 h to obtain a reaction gel. Wherein, the molar ratio is n (Sodium hydroxide): n (Sodium aluminate): n (Silicon source): n (Water) = 11.25 : 1 : 40.9 : 1653; the silicon source in the system is entirely provided by tetraethyl orthosilicate; (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir for 40 minutes until homogeneous, then place in a stainless steel crystallization kettle with a polytetrafluoroethylene liner and crystallize at 140 °C for 18.5 h. After washing and drying, Al-beta zeolite is obtained; In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.3% of the total mass of silicon dioxide (from tetraethyl orthosilicate) and aluminum oxide (from sodium aluminate) in the system; (3) Al-beta zeolite was placed in ammonium chloride solution for ion exchange, centrifuged and dried, and the ion exchange-drying process was repeated 3 times. After calcination in a muffle furnace, H-type Al-beta zeolite catalyst was obtained. The ion exchange conditions in step (3) are as follows: the concentration of ammonium chloride solution is 1 mol / L, the solid-liquid mass ratio is controlled at 1:50, and the reaction is carried out at 80 °C for 6 h. The calcination conditions in step (3) are: calcination at 550 ℃ for 6 h, with a heating rate of 1 ℃ / min.

[0049] (4) The thermogravimetric test was the same as in Example 1. The weight loss was measured to be 10.3 wt%.

[0050] (5) The catalytic process was the same as in Example 1. The calculated yield of lactide was 70.24%.

[0051] SEM images of the products obtained in Examples 1–5 and the comparative examples are shown below. Figure 1 As shown, the synthesized products all exhibit a uniform spherical morphology with a particle size of approximately 200 nm, indicating that the fluorine group modification has no significant effect on the morphology of the molecular sieve. Furthermore, the synthesized molecular sieves have a smaller particle size, providing a shorter path for the diffusion of reactants and products during the catalytic reaction.

[0052] The XRD patterns of the products prepared in Examples 1–5 and the comparative examples are as follows: Figure 2 As shown, both the modified and unmodified products exhibit characteristic diffraction peaks belonging to Beta zeolite, proving that the synthesized samples all belong to the pure phase BEA* topology.

[0053] The NH3-TPD results of the products prepared in Example 2 and the comparative example are as follows: Figure 3 As shown, all samples exhibited three characteristic peaks in their NH3-TPD spectra, corresponding to weak acid regions in the 30–300 ℃ range, moderately strong acid regions in the 300–500 ℃ range, and strong acid regions in the 500–700 ℃ range, respectively. The acid content corresponding to various acid strengths of the molecular sieves was quantitatively calculated using the TCD external standard method, and the specific results are shown in Table 1. The total acid content of the modified Al-beta zeolite increased from 2.34 mmol / g to 3.45 mmol / g. Simultaneously, the temperatures corresponding to the maximum peak intensities of the moderately strong acid and strong acid characteristic peaks were significantly increased, indicating that fluorine modification can significantly enhance the acid strength of moderately strong and strong acids in Al-beta zeolite.

[0054] Table 1: Calculation results of NH3-TPD acidity of the synthesized products in Example 2 and the comparative example.

[0055] The Py-FTIR results of the products prepared in Example 2 and the comparative example are as follows: Figure 4 As shown, 1610 and 1450 cm -1 The characteristic peak at this location belongs to the Lewis acidic site, located at 1545 cm⁻¹. -1 The peak at 1490 cm⁻¹ belongs to the Brønsted acidic site, while the peak at 1490 cm⁻¹ belongs to the Brønsted acidic site. -1 The peaks at the specified locations are attributed to the acidic sites of Brønsted and Lewis acids. Compared to the unmodified product prepared in the comparative example, the modified product prepared in Example 2 exhibited a significantly increased total acid content, with the Brønsted acid content increasing from 53.74 µmol / g to 232.25 µmol / g. Specific acid content results are shown in Table 2.

[0056] Table 2: Calculation results of Py-FTIR acidity of the synthesized products in Example 2 and the comparative example

[0057] The weight loss of the products obtained in Examples 1–5 and the comparative examples before 300 °C is shown in Table 3. The weight loss of the unmodified product obtained in the comparative example was 10.3 wt%, while the weight loss of the modified products obtained in Examples 1–5 was significantly increased, indicating that fluorine modification improved the hydrophobicity of the Al-beta zeolite channels.

[0058] Table 3: Weight loss of the synthesized products from Examples 1–5 and Comparative Examples before 300 °C

[0059] The water vapor adsorption curves of the products obtained in Example 2 and the comparative example are as follows: Figure 5 As shown, the modified product obtained in Example 2 adsorbed less water vapor than the unmodified product obtained in the comparative example, indicating that the fluorine modification effectively prevented water vapor from entering the pore structure of the zeolite during dynamic adsorption, further proving that the hydrophobicity of the molecular sieve pores was improved after modification.

[0060] Table 4 shows the yields of lactic acid to lactide produced by different examples and comparative examples under the same catalytic reaction conditions. Compared with the catalytic performance of the comparative example products, the fluorinated modified products obtained in the examples showed an improved lactide yield.

[0061] Table 4: Lactide yield of the synthesized products from Examples 1–5 and comparative examples

[0062] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing a hydrophobic H-type Al-beta zeolite catalyst, characterized in that: Includes the following steps: (1) Sodium hydroxide and sodium aluminate solids are mixed and dissolved in water, heated to 40–50 °C and stirred until homogeneous to obtain mixture A. Tetraethyl orthosilicate and triethoxyfluorosilane are added to mixture A in sequence and stirred for 8–12 h to obtain reaction gel. In step (1), the molar ratio is sodium hydroxide: sodium aluminate: silicon source: water = (9.61–11.63) : 1 : (34.2–51.5) : (1650–2350); The silicon source in the system is provided by tetraethyl orthosilicate and triethoxyfluorosilane, and the fluorine element accounts for 5–20% of the molar amount of silicon. (2) Add Al-beta zeolite seed crystals to the reaction gel obtained in step (1), stir evenly, and place in a stainless steel crystallization kettle with a polytetrafluoroethylene liner. Crystallize at 120–140 °C for 18–168 h. After washing and drying, the modified Al-beta zeolite is obtained. In step (2), the amount of Al-beta zeolite seed crystals added accounts for 10.0–11.0% of the total mass of silicon dioxide and aluminum oxide in the system; the silicon dioxide comes from a silicon source and the aluminum oxide comes from sodium aluminate. (3) The modified Al-beta zeolite was placed in ammonium chloride solution for ion exchange and reacted at 70–90 °C for 4–6 h. After centrifugation and drying, it was calcined in a muffle furnace to obtain a hydrophobic H-type Al-beta zeolite catalyst. The concentration of ammonium chloride solution is 0.5–1 mol / L, and the solid-liquid mass ratio is controlled at 1:30–1:

50. The calcination conditions in step (3) are: calcination at 500–600 ℃ for 4–9 h, with a heating rate of 0.5–1 ℃ / min.

2. The method for preparing the hydrophobic H-type Al-beta zeolite catalyst as described in claim 1, characterized in that: Repeat the ion exchange-drying process 3–4 times.

3. The application of the hydrophobic H-type Al-beta zeolite prepared by the method of claim 1, characterized in that it is used for the catalytic reaction of lactic acid to lactide synthesis.

4. The application as described in claim 3, characterized by comprising the following steps: adding lactic acid solution, substance Q and hydrophobic H-type Al-beta zeolite catalyst to a reactor and performing a reflux reaction for 1–7 h to obtain lactide; in, For every 10 mL of substance Q, add 0.2–3.0 g of lactic acid solution and 0.1–2.0 g of hydrophobic H-type Al-beta zeolite catalyst; the mass concentration of the lactic acid solution is 50–90%; substance Q is toluene or xylene.

5. The application as described in claim 4, wherein the xylene is o-xylene, m-xylene, or p-xylene.

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