A method for catalytic selective oxidation esterification of 1,2-propanediol to synthesize methyl lactate

Abstract

The application discloses a method for synthesizing methyl lactate by catalyzing selective oxidation esterification of 1,2-propanediol, which comprises the following steps: contacting oxygen, 1,2-propanediol and a catalyst in a reactor, and reacting to obtain methyl lactate; the catalyst is composed of a metal oxide carrier and gold elements loaded on the surface of the metal oxide carrier; in the catalyst, the mass of the gold elements is 0.1-2 wt% of the mass of the metal oxide carrier; and the metal oxide carrier is at least one selected from SiO2, ZnO, gamma-Al2O3, TiO2, CeO2, MgO and Sm2O3.

Description

Technical Field

[0001] This application relates to a method for the selective oxidative esterification of 1,2-propanediol to synthesize methyl lactate, which belongs to the field of chemical engineering. Background Technology

[0002] Lactic acid, a naturally occurring organic acid, is widely used in the food, pharmaceutical, and cosmetic industries, and is the main monomer of polylactic acid (PLA), a biodegradable plastic. With increasing environmental awareness and the emergence of the "green chemistry" concept, the demand for lactic acid is rising year by year. Traditional lactic acid production methods mainly rely on the fermentation of starch or glucose, a process that is not only costly but also suffers from low production efficiency and difficulties in product separation. Meanwhile, 1,2-propanediol, a major byproduct of biodiesel, is primarily used as a solvent and polymer material, resulting in low commercial added value. Therefore, converting 1,2-propanediol into more economically valuable and environmentally friendly chemicals, such as lactic acid, is of significant importance and has great development potential.

[0003] The catalytic oxidation of 1,2-propanediol is mostly carried out in alkaline aqueous systems. In practical applications, the product needs to undergo separation processes such as acidification, concentration, and refining, which are costly and generate a large amount of inorganic salt byproducts, limiting the industrial application of this catalytic system (ACS Sustainable Chem. Eng. 2023, 11, 7274-7287). In contrast, in alcohol systems, lactic acid directly reacts with the alcohol through esterification, allowing for direct separation and purification via distillation, reducing the complex post-processing. Currently, most catalysts used in the catalytic oxidation of 1,2-propanediol are gold-based, formed by loading gold nanoparticles onto supports using various preparation methods. The particle size plays a crucial role in the reaction. However, gold nanoparticle catalysts still suffer from limited catalytic efficiency and metal utilization. Especially in highly selective catalytic reactions, further reduction in metal particle size may pose challenges to catalyst activity and stability. Summary of the Invention

[0004] In supported catalysts, when the size of the supported metal particles is reduced to the size of a single atom, both their specific surface area and surface free energy increase dramatically, allowing for full utilization of the catalytic active sites and improving the utilization rate of the catalyst's metal atoms. The single-atom gold catalyst provided by this invention can efficiently and selectively catalyze the oxidative esterification of 1,2-propanediol to prepare methyl lactate.

[0005] This invention aims to develop a highly efficient multiphase single-atom gold catalyst to selectively oxidize the primary hydroxyl groups in 1,2-propanediol to ester groups, thereby preparing methyl lactate. 1,2-propanediol can be obtained through microbial fermentation and biotransformation of biomass feedstocks. Utilizing the sugars in plant feedstocks (such as corn and sugarcane) or waste materials (such as rice husks and bagasse), these carbohydrates can be converted into 1,2-propanediol through enzymatic hydrolysis and fermentation.

[0006] According to one aspect of this application, a method for preparing single-atom gold and its application in the selective oxidative esterification of 1,2-propanediol to prepare methyl lactate are provided. 1,2-propanediol is used as the substrate, and a catalytic system consisting of single-atom gold as the active neutral component and a metal oxide as the support is dispersed in 1,2-propanediol. Oxygen is introduced, and the reaction yields methyl lactate with high selectivity. This method features high yield, recyclability, and environmental friendliness, and has broad application prospects.

[0007] According to one aspect of this application, a method for synthesizing methyl lactate by selective oxidative esterification of 1,2-propanediol using catalysis is provided, comprising the following steps:

[0008] In a reactor, oxygen, 1,2-propanediol and catalyst are contacted and reacted to obtain methyl lactate;

[0009] The catalyst is composed of a metal oxide support and gold elements supported on the surface of the metal oxide support;

[0010] In the catalyst, the mass of gold element is 0.1 to 2 wt% of the mass of the metal oxide support;

[0011] The metal oxide support is selected from at least one of SiO2, ZnO, γ-Al2O3, TiO2, CeO2, MgO, and Sm2O3.

[0012] The catalyst is obtained through the following steps:

[0013] Au(OH)3 was mixed with water, oxygen was introduced, the mixture was heated and NaOH was added, the mixture was refluxed, and then mixed with a metal oxide support. The mixture was stirred, allowed to stand, evaporated, calcined under a nitrogen atmosphere, and dried under vacuum to obtain the catalyst.

[0014] The molar ratio of NaOH to Au(OH)3 is 3 to 9, based on the molar amount of the metal element.

[0015] The reflux temperature is 50–120°C;

[0016] The reflux time is 3–12 hours;

[0017] The stirring time is 0.5 to 12 hours;

[0018] The settling time is 1 to 24 hours;

[0019] The evaporation temperature is 60–150°C;

[0020] The evaporation time is 5–48 hours;

[0021] The calcination temperature is 150–300°C;

[0022] The calcination time is 1 to 12 hours;

[0023] The vacuum drying temperature is 50–110°C;

[0024] The vacuum drying time is 3 to 24 hours.

[0025] The single-atom gold prepared by this method has high catalytic activity and can interact with the supported metal oxide. It exhibits high product selectivity, is easy to separate from the reaction system, and has excellent cycle stability.

[0026] The partial pressure of the oxygen is 0.1–3 MPa;

[0027] Optionally, the partial pressure of the oxygen is 0.5 to 2 MPa.

[0028] The reaction temperature is 30–180°C;

[0029] Optionally, the reaction temperature is 50–140°C.

[0030] The reaction time is 1–24 hours;

[0031] Optionally, the reaction time is 4 to 12 hours.

[0032] The mass ratio of 1,2-propanediol to catalyst is 1:1 to 38.

[0033] The beneficial effects that this application can produce include:

[0034] 1) This invention provides a method for preparing a single-atom gold catalyst. Due to the high catalytic activity of single-atom gold and its interaction with the support, high selectivity for methyl lactate can be achieved.

[0035] 2) The catalyst system used in this method has good stability, can selectively oxidize 1,2-propanediol, and has a high product yield. The catalyst preparation method is simple, the process is convenient, and it is easy to recover, which is in line with the concept of green chemistry.

[0036] 3) The reaction conditions of this invention are mild, the catalyst system used is simple, and the catalyst can still maintain good stability after being recycled five times, and the yield of the product remains above 78%. Detailed Implementation

[0037] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0038] Unless otherwise specified, the raw materials and catalysts used in the embodiments of this application were all purchased commercially. Unless otherwise specified, the testing methods were all conventional methods, and the instrument settings were all as recommended by the manufacturers.

[0039] The analysis method in the embodiments of this application is as follows:

[0040] The conversion of 1,2-propanediol and the selectivity of methyl lactate were both calculated based on the number of carbon moles:

[0041]

[0042] Example 1:

[0043] 1 g of Au(OH)3 powder (0.1 wt% of the carrier) was suspended in 30 mL of deionized water. Oxygen was introduced, and the mixture was heated to 80 °C. NaOH powder was then added, and the mixture was refluxed for 8 h to obtain a transparent solution. The mass of the gold element was calculated based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 3 (based on the metal element). 1 g of SiO2 was added to the solution, and the mixture was stirred for 3 h, allowed to stand for 2 h, evaporated at 120 °C for 12 h, calcined at 200 °C for 9 h under a nitrogen atmosphere, and then vacuum dried at 80 °C for 7 h to obtain Au-O. x -Na-(OH) y / SiO2 catalyst.

[0044] Add 5 mmol of 1,2-propanediol and Au-O x -Na-(OH) y The SiO2 catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of the catalyst to 1,2-propanediol was 1:38). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 0.1 MPa of oxygen was introduced, and the temperature was raised to 30 °C. The reaction was carried out at this temperature for 1 h. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and a sample was taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 78%, and the selectivity of methyl lactate was 83%.

[0045] Example 2:

[0046] 1 g of a 0.5 wt% carrier powder was suspended in 30 mL of deionized water. Oxygen was introduced, and the mixture was heated to 90 °C. NaOH powder was then added, and the mixture was refluxed for 7 h to obtain a transparent solution. The mass of the gold element was calculated based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 4 (based on the metal element). 1 g of γ-Al₂O₃ was added to the solution, and the mixture was stirred for 5 h, allowed to stand for 5 h, evaporated at 110 °C for 18 h, calcined at 220 °C for 7 h under a nitrogen atmosphere, and then vacuum dried at 70 °C for 5 h to obtain Au-O. x -Na-(OH) y / γ-Al2O3 catalyst.

[0047] Add 5 mmol of 1,2-propanediol and Au-O x -Na-(OH) y The γ-Al₂O₃ catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of the catalyst to 1,2-propanediol was 1:8). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 0.5 MPa of oxygen was introduced, and the temperature was raised to 60 °C. The reaction was carried out at this temperature for 2 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and samples were taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 89%, and the selectivity of methyl lactate was 86%.

[0048] Example 3:

[0049] 1 g of a carrier powder (1.0 wt%) was suspended in 30 mL of deionized water. Oxygen was introduced, and the mixture was heated to 120 °C. NaOH powder was then added, and the mixture was refluxed for 3 h to obtain a transparent solution. The mass of the gold element was based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 5 (based on the metal element). 1 g of TiO2 was added to the solution, stirred for 0.5 h, and allowed to stand for 24 h. The solution was then evaporated at 60 °C for 48 h, calcined at 150 °C for 12 h under a nitrogen atmosphere, and finally vacuum dried at 50 °C for 24 h to obtain Au-O. x -Na-(OH) y / TiO2 catalyst.

[0050] Add 5 mmol of 1,2-propanediol, Au-O x -Na-(OH) yTiO2 catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of catalyst to 1,2-propanediol was 1:4). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 1.0 MPa of oxygen was introduced, and the temperature was raised to 90 °C. The reaction was carried out at this temperature for 4 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and samples were taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 95%, and the selectivity of methyl lactate was 88%.

[0051] Example 4:

[0052] 1 g of a carrier powder (1.5 wt%) was suspended in 30 mL of deionized water. Oxygen was introduced, and the mixture was heated to 50 °C. NaOH powder was then added, and the mixture was refluxed for 12 h to obtain a transparent solution. The mass of the gold element was based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 6 (based on the metal element). 1 g of ZnO was added to the solution, and the mixture was stirred for 12 h, allowed to stand for 1 h, evaporated at 150 °C for 5 h, calcined at 300 °C for 1 h under a nitrogen atmosphere, and then vacuum dried at 110 °C for 3 h to obtain Au-O. x -Na-(OH) y / ZnO catalyst.

[0053] Add 5 mmol of 1,2-propanediol and Au-O x -Na-(OH) y ZnO catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of catalyst to 1,2-propanediol was 1:2.5). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 1.5 MPa of oxygen was introduced, and the temperature was raised to 120 °C. The reaction was carried out at this temperature for 8 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and samples were taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 99%, and the selectivity of methyl lactate was 95%.

[0054] Example 5:

[0055] 1 g of Au(OH)3 powder (1.5 wt% of the carrier) was suspended in 30 mL of deionized water. Oxygen was bubbled through the solution, and the temperature was raised to 60 °C. NaOH powder was then added, and the mixture was refluxed for 10 h to obtain a transparent solution. The mass of the gold element was calculated based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 7 (based on the metal element). 1 g of MgO was added to the solution, and the mixture was stirred for 9 h, allowed to stand for 8 h, evaporated at 140 °C for 24 h, calcined at 280 °C for 3 h under a nitrogen atmosphere, and then vacuum dried at 100 °C for 12 h to obtain Au-O. x -Na-(OH)y / MgO catalyst.

[0056] Add 5 mmol of 1,2-propanediol and Au-O x -Na-(OH) y The MgO catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of the catalyst to 1,2-propanediol was 1:2). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 2.0 MPa of oxygen was introduced, and the temperature was raised to 150 °C. The reaction was carried out at this temperature for 16 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and samples were taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 96%, and the selectivity of methyl lactate was 87%.

[0057] Example 6:

[0058] 1 g of Au(OH)3 powder (2.0 wt% of the carrier) was suspended in 30 mL of deionized water. Oxygen was introduced, and the mixture was heated to 100 °C. NaOH powder was then added, and the mixture was refluxed for 5 h to obtain a transparent solution. The mass of the gold element was calculated based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 8 (based on the metal element). 1 g of Sm2O3 was added to the solution, and the mixture was stirred for 7 h, allowed to stand for 12 h, evaporated at 80 °C for 36 h, calcined at 180 °C for 10 h under a nitrogen atmosphere, and then vacuum dried at 90 °C for 15 h to obtain Au-O. x -Na-(OH) y / Sm2O3 catalyst.

[0059] Add 5 mmol of 1,2-propanediol, Au-O x -Na-(OH) y The Sm₂O₃ catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of the catalyst to 1,2-propanediol was 1:1.5). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 2.5 MPa of oxygen was introduced, and the temperature was raised to 150 °C. The reaction was carried out at this temperature for 16 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and samples were taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 93%, and the selectivity of methyl lactate was 84%.

[0060] Example 7:

[0061] 1 g of Au(OH)3 powder (2.0 wt% of the carrier) was suspended in 30 mL of deionized water. Oxygen was introduced, and the mixture was heated to 100 °C. NaOH powder was then added, and the mixture was refluxed for 9 h to obtain a transparent solution. The mass of the gold element was calculated based on the mass of the gold itself, and the molar ratio of NaOH to Au(OH)3 was 9 (based on the metal element). 1 g of CeO2 was added to the solution, and the mixture was stirred for 1 h, allowed to stand for 18 h, evaporated at 100 °C for 32 h, calcined at 250 °C for 5 h under a nitrogen atmosphere, and then vacuum dried at 60 °C for 18 h to obtain Au-O. x -Na-(OH) y / CeO2 catalyst.

[0062] Add 5 mmol of 1,2-propanediol, Au-O x -Na-(OH) y The CeO2 catalyst and 10 mL of methanol were added to a 50 mL reactor (the mass ratio of the catalyst to 1,2-propanediol was 1:1). The reactor was closed, and the air inside was replaced with oxygen six times. Then, 3.0 MPa of oxygen was introduced, and the temperature was raised to 180 °C. The reaction was carried out at this temperature for 24 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Ethyl heptanoate (internal standard) was added, and samples were taken for gas chromatography analysis. The conversion rate of 1,2-propanediol and the selectivity of methyl lactate were calculated. The conversion rate of 1,2-propanediol was 96%, and the selectivity of methyl lactate was 85%.

[0063] Example 8:

[0064] The catalyst was prepared and the selective oxidative esterification of 1,2-propanediol was carried out according to the conditions of Example 4. The difference from Application Example 4 was that after the reaction was completed, the catalyst was separated by centrifugation and washed with deionized water by centrifugation 5 times. The catalyst was then recycled for selective oxidative esterification of 1,2-propanediol. The catalyst was recycled 5 times in this way. The results are shown in Table 1.

[0065] Table 1. Effect of catalyst on selective oxidative esterification and recycling of 1,2-propanediol

[0066]

[0067] As shown in Table 1, the prepared catalyst maintained high catalytic activity for the selective oxidative esterification of 1,2-propanediol after five cycles, with the yield of methyl lactate remaining above 89%. The catalysts provided in the other examples also maintained high catalytic activity under the same conditions, with the yield of methyl lactate remaining above 78% after five cycles.

[0068] Comparative Example 1

[0069] Au / ZnO catalyst was prepared by the sol-gel method: 1 g of HAuCl4·4H2O (1.5 wt% of the support) was added to deionized water containing polyvinyl alcohol (1 wt%, 1.2 mL). After stirring for 10 min, 7.5 mL of NaBH4 solution (0.1 M) was added dropwise, and stirring continued for 1 h. The solution was acidified to pH 1 by adding hydrochloric acid dropwise, and 1 g of ZnO support was added under vigorous stirring. After stirring for 2 h, the mixture was allowed to stand overnight. The precipitate was filtered, washed, and dried under vacuum at 110 °C for 3 h to obtain the Au / ZnO catalyst.

[0070] The oxidative esterification of 1,2-propanediol was carried out under the reaction conditions described in Example 4, except that Au / ZnO prepared by deposition precipitation was used as the catalyst. The conversion rate of 1,2-propanediol was 72%, and the selectivity for methyl lactate was 35%.

[0071] Comparative Example 2

[0072] Au / TiO2 catalyst was prepared using a urea deposition-precipitation method: 1.0 wt% HAuCl4·4H2O, 0.02 mol urea, 1 g TiO2 support, and 50 mL deionized water were weighed and added to a round-bottom flask wrapped with aluminum foil. The suspension was vigorously stirred in an 80°C water bath for 6 h and aged overnight at room temperature. The precipitate was filtered, washed, dried at 110°C for 5 h, and then calcined in flowing air at 400°C for 5 h to obtain the Au / TiO2 catalyst.

[0073] The oxidative esterification of 1,2-propanediol was carried out under the reaction conditions described in Example 3, except that Au / TiO2 prepared by deposition precipitation was used as the catalyst. The conversion rate of 1,2-propanediol was 78%, and the selectivity for methyl lactate was 25%.

[0074] The catalyst support used in this invention can interact with the supported metal, and due to the size effect, the prepared single-atom gold catalyst has high catalytic activity and selectivity, mild reaction conditions, simple operation, easy catalyst separation, strong stability, and broad application prospects.

[0075] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for the selective oxidative esterification of 1,2-propanediol to synthesize methyl lactate, characterized in that, Includes the following steps: In a reactor, oxygen, 1,2-propanediol and catalyst are contacted and reacted to obtain methyl lactate; The catalyst is composed of a metal oxide support and gold elements supported on the surface of the metal oxide support; In the catalyst, the mass of gold element is 0.1 to 2 wt% of the mass of the metal oxide support; The metal oxide support is selected from at least one of SiO2, ZnO, γ-Al2O3, TiO2, CeO2, MgO, and Sm2O3.

2. The method according to claim 1, characterized in that, The catalyst is obtained through the following steps: Au(OH)3 was mixed with water, oxygen was introduced, the mixture was heated and NaOH was added, the mixture was refluxed, and then mixed with a metal oxide support. The mixture was stirred, allowed to stand, evaporated, calcined under a nitrogen atmosphere, and dried under vacuum to obtain the catalyst.

3. The method according to claim 2, characterized in that, The molar ratio of NaOH to Au(OH)3 is 3 to 9, based on the molar amount of the metal element. The reflux temperature is 50–120°C; The reflux time is 3–12 hours; The stirring time is 0.5 to 12 hours; The settling time is 1 to 24 hours; The evaporation temperature is 60–150°C; The evaporation time is 5–48 hours; The calcination temperature is 150–300°C; The calcination time is 1 to 12 hours; The vacuum drying temperature is 50–110°C; The vacuum drying time is 3 to 24 hours.

4. The method according to claim 1, characterized in that, The partial pressure of the oxygen is 0.1–3 MPa; Preferably, the partial pressure of the oxygen is 0.5 to 2 MPa.

5. The method according to claim 1, characterized in that, The reaction temperature is 30–180°C; Preferably, the reaction temperature is 50–140°C.

6. The method according to claim 1, characterized in that, The reaction time is 1–24 hours; Preferably, the reaction time is 4 to 12 hours.

7. The method according to claim 1, characterized in that, The mass ratio of 1,2-propanediol to catalyst is 1:1 to 38.

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