A method for the direct electrochemical epoxidation of propene with oxygen to produce propylene oxide

Abstract

This invention provides a method for the direct electrochemical epoxidation of oxygen and propylene to prepare propylene oxide. Carbon black oxide is used as a two-electron oxygen reduction catalyst. The two-electron oxygen reduction catalyst is sprayed onto hydrophobic carbon paper to prepare a gas diffusion electrode as the cathode. PBS buffer is used as the cathode electrolyte, and TS-1 titanium-silicon molecular sieve is added to the cathode electrolyte as an epoxidation catalyst. Propylene is directly fed into the chamber (1) of a flow electrolytic cell. Under the action of a peristaltic pump, the cathode electrolyte in the cathode electrolytic cell (2) is pumped into the flow electrolytic cell. In the chamber (1), the cathode electrolyte and gas in the chamber (1) of the flow electrolytic cell flow into the cathode electrolytic cell (2) under the action of a peristaltic pump. A titanium mesh is used as the anode and sulfuric acid is used as the anode electrolyte to carry out the oxygen evolution reaction. Oxygen is introduced into the cathode. When an external current is applied, the anode undergoes the OER reaction and the cathode undergoes the two-electron ORR reaction. The H2O2 generated in situ acts as an oxidant and epoxidizes propylene to propylene oxide under the catalysis of the epoxidation catalyst. The cold trap collection chamber (3) is used to collect the volatile reaction product propylene oxide.

Description

Technical Field

[0001] This invention belongs to the field of propylene oxide preparation technology, specifically relating to a method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene. Background Technology

[0002] Propylene oxide (PO) is one of the most important raw materials in the plastics industry, used to produce polyurethane, polyester and propylene glycol. Through further processing, it can be used to manufacture nonionic surfactants, pesticide emulsifiers, defoamers and other products. Its derivatives have been widely used in the automotive, construction, textile, cosmetic, pharmaceutical and fire protection industries. Nearly a hundred downstream products have been produced, making it an important raw material for fine chemical products.

[0003] Currently, the main industrial production routes for propylene oxide fall into three categories: the chlorohydrin method, the co-oxidation method, and the direct hydrogen peroxide oxidation method. The chlorohydrin method is the most mature process, offering advantages such as a short production flow, low investment, high selectivity, and high product yield. However, its main raw material is chlorine, and the hypochlorous acid produced during production causes severe corrosion to equipment, generating not only large amounts of wastewater but also over two tons of lime slag. The co-oxidation method mainly includes the ethylbenzene co-oxidation method and the isobutane co-oxidation method. Both processes produce corrosive substances that damage equipment. They suffer from drawbacks such as long reaction flows, complex processes, high requirements for raw material and equipment quality, numerous intermediate products, high investment costs, and high COD values ​​in wastewater. Both co-oxidation methods are co-production processes, and the co-products can share some of the costs. While the tert-butanol produced by the isobutane co-oxidation method can be converted into methyl tert-butanol ether for gasoline blending, methyl tert-butanol ether poses risks of polluting water bodies and harming human health.

[0004] Most current electrochemical methods for preparing propylene oxide are carried out in slow-reacting H+ cells, resulting in low yields and rates. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of the embodiments of the present invention and to briefly describe some preferred embodiments.

[0006] As one aspect of the present invention, the present invention provides a method for the direct electrochemical epoxidation of oxygen and propylene to prepare propylene oxide, wherein carbon black oxide is used as a two-electron oxygen reduction catalyst, the two-electron oxygen reduction catalyst is sprayed onto hydrophobic carbon paper to prepare a gas diffusion electrode as a cathode, PBS buffer solution is used as the cathode electrolyte, and TS-1 titanium silicon molecular sieve is added to the cathode electrolyte as an epoxidation catalyst. Propylene is directly introduced into the chamber of a flow electrolytic cell. Under the action of a peristaltic pump, the cathode electrolyte in the cathode electrolytic cell is pumped into the chamber of the flow electrolytic cell. The cathode electrolyte and gas in the chamber of the flow electrolytic cell flow into the cathode electrolytic cell under the action of a peristaltic pump. A titanium mesh is used as the anode, and sulfuric acid is used as the anolyte to carry out the oxygen evolution reaction. Oxygen is introduced into the cathode. When an external current is applied, the anode undergoes the OER reaction, and the cathode undergoes the two-electron ORR reaction. The H2O2 generated in situ acts as an oxidant, and under the catalysis of the epoxidation catalyst, propylene is epoxidized to propylene oxide. A cold trap collection chamber is used to collect the volatile reaction product propylene oxide.

[0007] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention, the concentration of the PBS buffer is 1 mol / L and the concentration of the anolyte is 0.1 mol / L.

[0008] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention: the two-electron oxygen reduction catalyst is prepared as follows: 50 mg of carbon black is weighed and placed in a round-bottom flask, 37.5 ml of concentrated HNO3 is measured and prepared into 50 ml of 12 M HNO3 solution as an oxidant, and the reaction is carried out under reflux at 85 °C for 12 h, the solid is separated by high-speed centrifugation at 7000 rpm for 5 min, and the solution is washed until the pH is neutral.

[0009] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention: the epoxidation catalyst is prepared as follows: First, 17.00g of tetrapropylammonium hydroxide solution is weighed and added to a 200ml beaker, then 30.44g of tetraethyl orthosilicate and 0.25g of tetrabutyl titanate are added dropwise, and finally 21.45g of water is added as a solvent. After mixing evenly, the mixture is continuously irradiated with ultraviolet light for 1 hour to form a transparent solution. Hydrothermal crystallization is then carried out at 170℃ for 72 hours, followed by centrifugation and washing at 8000rpm. The precipitate is dried overnight at 80℃. -1 The heating rate was adjusted, and the catalyst was calcined at 550℃ for 16 hours to obtain a white powdery epoxidation catalyst.

[0010] As a preferred embodiment of the method for the direct electrochemical epoxidation of oxygen and propylene to prepare propylene oxide according to the present invention, the loading of the two-electron oxygen reduction catalyst on the gas diffusion electrode is 0.5 mg·cm⁻¹. -2 .

[0011] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention: oxygen is introduced into the cathode at a flow rate of 30 sccm.

[0012] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention: the propylene is directly fed into the chamber of the flow electrolysis cell at a flow velocity of 2 sccm.

[0013] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention: the cathode electrolyte in the cathode electrolytic cell is pumped into the chamber of the flow electrolytic cell under the action of a peristaltic pump, wherein the flow rate of the cathode electrolyte is 0.4 L / h.

[0014] As a preferred embodiment of the method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to the present invention, the temperature of the cathode electrolyte in the cathode electrolytic cell is maintained at 30°C.

[0015] The beneficial effects of this invention: This invention provides a method for preparing propylene oxide via direct electrochemical epoxidation of propylene with oxygen. It couples the two-electron oxygen reduction reaction with the propylene epoxidation reaction while achieving high PO selectivity through optimization of the reaction apparatus and conditions. In-situ production of propylene oxide using hydrogen peroxide reduces reaction costs and minimizes the problem of further decomposition due to excessively high local hydrogen peroxide concentrations. As shown in Table 1, compared with other methods, the yield of this invention can reach 0.54 mmol·h⁻¹. -1 cm -2 This reactor method can significantly increase the yield of PO. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0017] Figure 1 XRD diffraction patterns of CB and O-CB.

[0018] Figure 2High-resolution C1s XPS spectra of (a) CB and (b) O-CB; high-resolution O1s XPS spectra of (c) CB and (d) O-CB.

[0019] Figure 3 (a) Nitrogen adsorption-desorption isotherms of O-CB; (b) Pore size distribution of O-CB.

[0020] Figure 4 XRD diffraction patterns of TS-1 and Silicalite.

[0021] Figure 5 RRDE test performance of CB and O-CB: disk current density and ring current (a) 0.1M KOH and (c) 0.05M H2SO4; H2O2 selectivity, Faraday efficiency and electron transfer number (b) 0.1M KOH (d) 0.05M H2SO4.

[0022] Figure 6 RRDE test performance of CB and O-CB: disk current density and ring current (a) 0.1M PBS and (c) 0.1M Na2SO4; H2O2 selectivity, Faraday efficiency and electron transfer number (b) 0.1M PBS (d) 0.1M Na2SO4.

[0023] Figure 7 The RRDE performance of O-CB in 0.1M PBS at different pH values ​​was tested: (a) loop current; (b) H2O2 selectivity and Faraday efficiency; (c) disk current density; (d) electron transfer number.

[0024] Figure 8 This is a schematic diagram of the reaction in the integrated system of electrosynthesis of H2O2 and propylene epoxidation.

[0025] Figure 9 This is a schematic diagram of the electrolysis system in cell H.

[0026] Figure 10 The pH of the cathode electrolyte changes with reaction time when the anolyte is (a) H2SO4, (b) KOH and (c) Na2SO4.

[0027] Figure 11 (a) Electrolysis curves of different cathode electrolytes after 1 hour of reaction when the anode is H2SO4; (b) PO yield and selectivity in different cathode electrolytes.

[0028] Figure 12(a) Electrolysis curve of PBS electrolyte without TS-1 for 1 h; (b) Gas chromatogram of the reaction solution (inset shows the result with TS-1 added); (c) PO yield and selectivity obtained from different control experiments; (d) CV curves of O-CB in 0.1M PBS saturated with different gases.

[0029] Figure 13 Schematic diagram of the reaction of the flow cell integrated system with (a) G-bubbling and (b) G-Lmixture feeding methods.

[0030] Figure 14 This is a voltage-time graph from the stability test in Example 2.

[0031] Figure 15 This is a graph showing the PO selectivity and yield during the long-term stability test in Example 2.

[0032] Figure 16 The graph shows the PO selectivity and yield of different reaction apparatuses in Comparative Example 1.

[0033] Figure 17 The graph shows the PO selectivity and yield at different cathode electrolyte flow rates in Comparative Example 2.

[0034] Figure 18 The graph shows the selectivity and yield of PO under different current densities in Comparative Example 3.

[0035] Figure 19 The graph shows the selectivity and yield of PO under different cathode electrolyte pH values ​​in Comparative Example 4. Detailed Implementation

[0036] To make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples.

[0037] (1) Pretreatment of CB: Weigh a sufficient amount of carbon black powder (XC-72R) and place it in a small porcelain boat. Place it in a high-temperature tube furnace for deoxidation treatment at a rate of 40 ml / min. -1 Ar gas was introduced at a rate of 50°C / min for 50 min, and the heating rate was maintained at 5°C / min. -1 The temperature was raised to 750℃ and held for 2 hours. After cooling to room temperature, the pretreated CB was obtained, collected, and stored in a vacuum drying oven.

[0038] (2) Preparation of the two-electron catalyst O-CB: Weigh 50 mg of the pretreated CB obtained in step (1) and place it in a round-bottom flask. Measure 37.5 ml of concentrated HNO3 and prepare 50 ml of 12 M HNO3 aqueous solution as the oxidant. Assemble a reflux system in a fume hood and reflux the reaction at 85 °C for 12 h. Stir continuously during the reaction to ensure that the CB is uniformly and completely oxidized. After the reaction, centrifuge at 10,000 rpm to separate the solid. The centrifugation speed is set to 7,000 rpm and the time is 5 min. Wash thoroughly with deionized water and ethanol multiple times to remove residual impurities and wash the solution to neutral pH. After washing, dry in a vacuum oven at 60 °C for 12 h to obtain the O-CB catalyst.

[0039] (3) Preparation of gas diffusion electrode (O-CB / CP): YLS-30T carbon paper was used as the GDL substrate. Catalyst ink was sprayed onto the carbon paper surface with the MPL side using ultrasonic spraying. The catalyst was uniformly dispersed on the carbon paper surface, and the spraying area was 4 cm². 2 The catalyst loading was 0.5 mg·cm³. -2 The spraying time is 710 seconds.

[0040] (4) Preparation of titanium-silicon molecular sieve (TS-1): First, 17.00 g of tetrapropylammonium hydroxide solution was weighed and added to a 200 ml beaker. Then, 30.44 g of tetraethyl orthosilicate and 0.25 g of tetrabutyl titanate were added dropwise. Finally, 21.45 g of water (18.2 MΩ·cm) was added as a solvent. After mixing evenly, the mixture was continuously irradiated with ultraviolet light (500 W, mercury lamp) for 1 h, and stirred continuously during irradiation to ensure complete hydrolysis. After irradiation, a transparent solution was formed, and hydrothermal crystallization was carried out at 170 °C for 72 h. The synthesized product was washed multiple times with deionized water by centrifugation at 8000 rpm for 5 min. After washing, it was dried overnight at 80 °C. Finally, it was dried at 4 °C·min. -1 The heating rate was adjusted, and the template was removed by calcination in a muffle furnace at 550℃ for 16 hours, finally yielding white powder TS-1.

[0041] Material structure analysis and characterization: XRD tests were performed on Cabot carbon black powder (XC-72R) before and after oxidation to analyze the changes in the material's phase structure. The results are as follows: Figure 1As shown. The XRD pattern of pretreated CB shows a distinct "bun-shaped" diffraction peak at 24.8°, corresponding to the (002) crystal plane of C, and a weaker bulging diffraction peak at 43.3°, corresponding to the (100) crystal plane of C. These "bun-shaped" diffraction peaks indicate that the original CB has an amorphous composition and is an amorphous structure. This high disorder of CB leads to its unstable chemical properties and high reducibility, which is beneficial for subsequent oxidation treatment. The oxidized O-CB material also shows two bulging diffraction peaks at 24.8° and 43.3°, but no other strong peaks appear. The above results indicate that nitric acid oxidation does not change the amorphous structure of the original CB.

[0042] XPS characterization was performed to investigate the elemental composition and active site information of the catalyst after oxidation. Figure 2 (ab) are the C1s spectra of CB and O-CB, respectively. The results of peak fitting show that the two materials have similar graphitic carbon and defect carbon contents, indicating that oxidation treatment does not cause obvious carbon defects. Figure 2 (cd) are the O1s spectra of CB and O-CB, respectively. It can be seen that both materials have three peaks around 531.35 eV, 532.5 eV, and 534 eV, which correspond to C=O, COC / COOH, and OH adsorbed on the surface in the materials, respectively. - Or H2O. The contents of various oxygen-containing functional groups are summarized in Table 1. It can be seen that the C=O content in the oxidized material is significantly increased, which is conducive to the formation of 2e-. - ORR reaction.

[0043] Table 1. Bonding distribution and elemental content of O element in CB and O-CB materials.

[0044] CB 33.00 24.80 0.75 1.63 O-CB 30.50 48.60 1.59 5.53

[0045] Subsequently, the pore structure of the O-CB material was analyzed, and the results were obtained through BET testing. Figure 3 As shown, O-CB exhibits a hierarchical porous structure with a high proportion of mesopores, which facilitates O2 mass transfer. The specific surface area, pore volume, and average pore size of the material were then calculated using the BJH algorithm, and the results are summarized in Table 2. It can be seen that O-CB possesses a large BET specific surface area, which is one of the reasons for its excellent material properties.

[0046] Table 2. Nitrogen adsorption-desorption parameters of O-CB

[0047]

[0048] The synthesized TS-1 was characterized by XRD tests, such as... Figure 4As shown, obvious characteristic peaks appear at 7.9°, 8.7°, 23°, 23.9° and 24.4°. After comparison with Silicalite (PDF#40696), it is proven that it has the typical MFI topology of molecular sieves, where the Ti:Si molar ratio is 1:100.

[0049] The specific surface area and pore size distribution of TS-1 were characterized using BET. The BET surface area and pore volume of TS-1 were 397 m² / m³. 2 ·g -1 and 0.22cm 3 ·g -1 The average pore size was 3.16 nm, and the results are summarized in Table 3.

[0050] Table 3 Nitrogen adsorption-desorption parameters of TS-1

[0051]

[0052] Electrochemical Performance Testing: One of the limiting factors for the in-situ utilization of electrogenerated H₂O₂ is that the epoxidation reaction must take place in an electrolyte, making the choice of electrolyte crucial. The electrolyte not only affects the efficiency of H₂O₂ production, but the electrolyte salts also significantly interfere with the epoxidation reaction of H₂O₂ and propylene, directly impacting the production efficiency of the target product (PO). Due to this specificity and unknown nature, four typical 2e-ORR electrolytes (H₂SO₄, Na₂SO₄, PBS, and KOH) were investigated, including acidic, neutral, alkaline, and PBS buffer solutions.

[0053] First, the 2e-ORR activity and selectivity of O-CB in different electrolytes were tested using LSV. Carbon black oxide was chosen for this experiment because the OO bond in *H₂O₂ or *OOH is more easily broken under alkaline conditions than under acidic conditions, ultimately forming water. For carbon-based materials, the interaction between the *OOH radical and the catalyst surface is weak, resulting in weak adsorption of intermediates on the catalyst surface. Therefore, the intermediates are easily desorbed from the surface, ensuring the stability of the key covalent bond (OO bond) during H₂O₂ formation, rather than its breakage. Thus, the pH of the electrolyte has a relatively small impact on selectivity for carbon-based materials. First, O₂-saturated KOH was used as the electrolyte (note: all electrolytes were uniformly 0.1M concentration during RRDE testing). The RRDE was maintained at 1600 rpm, and LSV scans were performed on CB before and after oxidation. The results are as follows: Figure 5 As shown in (ab), the original CB also has a good onset potential (0.73V vs. RHE) and limiting current density (2.1mA·cm). -2However, its average H2O2 selectivity of less than 70% makes it unsuitable as an excellent 2e-electrode. - The ORR catalyst, O-CB, after oxidation treatment, exhibited a significant performance improvement, with both the ORR onset potential and selectivity increased. Combined with material characterization, the performance improvement was attributed to the introduction of numerous oxygen-containing functional groups. These results indicate that O-CB possesses high ORR activity (0.78 V vs. RHE) and a high limiting current density (2.8 mA·cm⁻¹) in KOH. -2 It has high H2O2 selectivity (average above 85%), enabling efficient production of H2O2 under alkaline conditions. Figure 5 (cd) shows the LSV test curves of the two materials in H2SO4 electrolyte. It can be seen that both CB and O-CB exhibit large overpotentials (at least 0.6V) in acidic electrolyte. O doping also enhances the activity and selectivity of CB, ultimately achieving an average H2O2 selectivity of nearly 70%. The lower selectivity in acidic environments is due to the large overpotential further driving the ORR reaction until H2O is generated, thus significantly reducing the selectivity and formation rate of H2O2. This is particularly evident under acidic conditions because, under the negative charge of acid, a large amount of H+ accumulates on the catalyst surface. + This causes the locally generated H2O2 molecules to be further reduced to H2O, as shown in the following reaction equation:

[0054] H2O2 + 2e - +2H + →2H2O

[0055] Figure 6 The tests were conducted in a neutral pH environment. A comparison of LSVs in two different electrolyte environments also showed that the 2-electron ORR performance of O-CB was superior to that of the original CB in all electrolyte environments, exhibiting nearly 80% H2O2 selectivity in both environments. The difference lies in the fact that, as the reaction voltage increases, the H2O2 current and selectivity of O-CB in the Na2SO4 environment decrease significantly. Above 0.25V (vs. RHE), its selectivity even falls below that of the original CB material. This significant decrease in selectivity may be due to the unstable pH environment on the catalyst surface, with locally high concentrations of H2O2. + This will cause H2O2 molecules to continue to be reduced to H2O. The strong oxidizing effect of nitric acid means that although a large number of oxygen-containing functional groups can be incorporated into the carbon black surface, their types are uncontrollable. Introducing a large amount of O elements will generate more surface carbonyl groups, leading to a decrease in selectivity at high voltages.

[0056] Figure 7The LSV test was performed on O-CB in PBS buffer at different pH values. The results showed that O-CB material was active for electrochemical synthesis of H2O2 under different pH conditions (pH 5-8), and its performance was optimal at pH 6, with the highest H2O2 partial current reaching about 0.12 mA. This provides a prerequisite for subsequent regulation of the electrolyte environment of the epoxidation reaction.

[0057] In summary, the O-CB material exhibits excellent 2-electron ORR performance in four different electrolytes and PBS buffers with different pH values. It shows the highest H2O2 selectivity (close to 90%) in KOH and PBS solutions, while the H2O2 selectivity in H2SO4 and Na2SO4 is between 70-80%.

[0058] Schematic diagram of the electrocatalytic propylene epoxidation reaction: Besides the design of the electrocatalyst, developing a well-configured electrochemical device for the electrosynthesis of H2O2 is another important factor for achieving large-scale practical application in this field. This invention utilizes the TS-1 molecular sieve catalyst, combined with the electrochemical generation of H2O2 and the propylene epoxidation reaction, to design an integrated electrochemical system. A schematic diagram of the reaction system is shown below. Figure 8 As shown, this system can produce H2O2 at low cost and utilize it in situ, efficiently producing the more valuable PO. To utilize low-cost O2 for the propylene epoxidation reaction, the entire reaction system consists of three catalysts: an lrO2 catalyst for the anodic reaction, oxidizing water to O2 at low overpotentials and exhibiting excellent OER performance in various electrolytes; an O-CB electrocatalyst for efficiently reducing O2 to H2O2; and a heterogeneous catalyst TS-1 for the propylene epoxidation reaction. When an external current is applied, the OER reaction occurs at the anode, and the 2-electron ORR reaction occurs at the cathode. The TS-1 catalyst uses the in-situ generated H2O2 as an oxidant to epoxidize propylene to PO. In this way, the catalytic system can produce PO from propylene, O2, and green electricity without requiring any expensive chemicals, making it a simple (integrated), energy-efficient (O2 as a reactant, utilizing green electricity), and environmentally friendly (no toxic chemicals or harmful byproducts) process.

[0059] Investigation of reaction prerequisites and reaction mechanism: The feasibility and reaction mechanism of the reaction were investigated in an H-type electrolytic cell. A schematic diagram of the electrolysis system is shown below. Figure 9 As shown.

[0060] In the H cell, to prevent the cathode products from reacting at the anode, a Nafion 117 membrane (proton exchange membrane) was used to separate the cathode and anode electrolytes. The reaction was tested using a three-electrode system with an IrO2-loaded Ti mesh electrode (electrode area 1.5 cm²). 2The TS-1 molecular sieve catalyst is used as the anode to initiate the OER reaction at a low overpotential. The cathode is a pre-coated O-CB / CP gas diffusion electrode used for the 2-electron ORR reaction, with an Ag / AgCl electrode serving as the reference electrode. The TS-1 molecular sieve catalyst is uniformly dispersed in the cathode electrolyte before the reaction. During the reaction, the electrolyte is continuously stirred to prevent catalyst settling and reducing the effective catalytic area. Propylene is introduced at a gas flow rate of 2 sccm, and O2 is bubbled at a gas flow rate of 30 sccm.

[0061] First, the anolyte was screened. Acidic, neutral, and alkaline electrolytes were selected because pH stability is crucial in the epoxidation process; significant pH changes make it impossible to determine the true performance of the catalyst. Each anolyte was paired with one of the four catholytes for electrolysis to investigate the trend of cathodic pH changes over time. The results are as follows: Figure 10 As shown in the figure, when the catholyte is H2SO4 and KOH, the pH does not change significantly over time regardless of the electrolyte chosen for the anode. This is because strong acid and strong base electrolytes themselves possess a large amount of H+. + or OH - Compared to the significant amount of proton migration and consumption during the reaction, the pH change is minimal. The more significant change is in the neutral environment; it can be seen that only when the anode is H₂SO₄ and the cathode is PBS buffer can the neutral environment be maintained even after a prolonged reaction.

[0062] After determining the anolyte environment, four types of catholytes were investigated at 50 mA·cm⁻¹. -2 A polarization test was conducted for 1 hour at a given current, and the yield of PO was measured. The polarization curves and the yield and selectivity of PO are shown below. Figure 11 As shown, in this catalytic system, due to the continuous purging of O2 and propylene, some PO rapidly volatilizes or is carried into the gas phase with the fast-flowing gas. In order to collect as much product as possible, the gas outlet is connected to a cold trap collection device to receive most of the volatilized product. Figure 11 (ab) shows the electrolysis curves, which reveal that the reaction voltage remains relatively stable without fluctuations. This indicates that the synthesized catalyst O-CB exhibits good electrochemical stability in different electrolyte environments, and the epoxidation reaction does not affect the voltage. Figure 11 As can be seen in (c), propylene oxide was detected in the electrolyte after all the reactions, which indicates the feasibility of the integrated system of electrosynthesis of H2O2 and epoxidation. O2 is the only oxidant in the whole reaction. The reaction equation is shown below. The fact that PO collected in the cold trap accounts for the majority indicates the problem of severe product volatilization.

[0063]

[0064] In both acidic and alkaline environments, the yield and selectivity of PO were low, indicating that the propylene epoxidation reaction is pH-dependent and exhibits good epoxidation performance only under neutral conditions. Na₂SO₄, on the other hand, showed a low PO yield due to pH changes. These results suggest that the propylene epoxidation reaction needs to be carried out in PBS, which has a buffering effect.

[0065] Next, we will explore the reaction mechanism. Figure 12 (a) Electrolysis chromatogram obtained after 1 hour of polarization reaction without the addition of TS-1 catalyst. The organic matter in the reaction solution was detected, and the gas chromatogram results are as follows: Figure 12 As shown in (b) (the inset is a chromatogram obtained with TS-1 added), the specific yield data are presented in [the relevant section]. Figure 12 In (c), it can be seen that only dissolved propylene can be detected in the reaction solution without TS-1, while adding TS-1 yields a large amount of propylene oxide. This indicates that the production of PO is inseparable from the catalytic effect of TS-1. Other comparative experimental results are as follows: Figure 12 As shown in (c), when no external current is applied and only O2 and C3H6 are introduced, PO is not generated even in the presence of TS-1, indicating that O2 does not directly oxidize propylene. The experimental results of directly adding H2O2 show that epoxidation only occurs when H2O2, C3H6, and TS-1 are present simultaneously. Figure 12 (d) shows the CV curves of O-CB in nitrogen, oxygen and propylene saturated electrolyte, which can be seen that propylene is not reduced at the O-CB cathode.

[0066] Therefore, the reaction mechanism of this catalytic system is as follows: O2 passes through the O-CB electrode via 2e - ORR is converted into H2O2. H2O2 diffuses into the solution and reacts with propylene on the TS-1 surface to synthesize PO. The strongly acidic anolyte can continuously transfer protons to compensate for the consumption of the cathode and maintain pH stability over a long period of time.

[0067] Example 2:

[0068] Flow cell yield testing and propylene feed method study: Flow cell was used to further increase PO production; the reaction flow diagram is shown below. Figure 13 As shown. Figure 13In (a), specifically, the cathode-side bipolar plate is placed flat on a table and a positioning screw is installed. Hydrophobic carbon paper coated with O-CB is placed above the flow channel of the plate, and conductive copper foil is attached to both sides. A gasket is placed on top, followed by the chamber plate of the middle flow electrolysis cell. Another gasket is placed, and then a proton exchange membrane is placed to isolate the anode and cathode reaction sites. Another gasket is used to separate the proton exchange membrane from the cathode reaction sites, and then the lrO2 catalyst for the anode OER reaction is placed. Finally, the anode-side bipolar plate is placed on top. One side of the cathode-side plate's interface is connected to oxygen, and the other side is connected to the atmosphere to release excess oxygen.

[0069] like Figure 13 As shown in Figure a, the cathode electrolytic cell 2, which contains the cathode electrolyte, is connected to four pipelines. Two pipelines are connected to the flow electrolytic cell chamber 1, one pipeline is used to pump the cathode electrolyte into the flow electrolytic cell chamber 1 under the action of a peristaltic pump, and another pipeline sends the cathode electrolyte and H2O2 generated by the ORR reaction into the cathode electrolytic cell 2. The third pipeline is used to introduce propylene into the cathode electrolytic cell 2 to facilitate the epoxidation reaction with the H2O2 introduced into the cathode electrolytic cell 2 and TS-1 dissolved in the electrolyte. The last pipeline is connected to the downstream cold trap device to collect the volatilized product propylene oxide. Figure 13 In step a, the main site for the synthesis of propylene oxide is in cathode electrolytic cell 2.

[0070] like Figure 13 (b) The O-CB catalyst prepared in Example 1 was used to efficiently reduce O2 to H2O2. A gas diffusion electrode (O-CB / CP) was prepared according to the method in Example 1 as the cathode, and 20 ml of 1 mol / L PBS buffer was used as the cathode electrolyte. 0.1 g of TS-1 titanium-silicon molecular sieve prepared in Example 1 was added to the cathode electrolyte as an epoxidation catalyst for the propylene epoxidation reaction. A titanium mesh was used as the anode, and 20 ml of 0.1 mol / L sulfuric acid was used as the anolyte for the oxygen evolution reaction. When an external current was applied, the anode underwent the OER reaction, and the cathode underwent the 2-electron ORR reaction. The TS-1 catalyst used the in-situ generated H2O2 as an oxidant to epoxidize propylene to PO. Figure 13 As shown in (b), the flow cell reactor is assembled as follows: Specifically, the bipolar plate on the cathode side is placed flat on a table and a positioning screw is installed. Hydrophobic carbon paper coated with O-CB is placed above the flow channel of the plate, and conductive copper foil is attached to both sides. A gasket is placed on top, followed by the chamber plate of the middle flow electrolysis cell. Another gasket is placed, followed by a proton exchange membrane to isolate the anode and cathode reaction sites. Another gasket is used to separate the proton exchange membrane from the cathode reaction site, and then the IrO2 catalyst for the anode OER reaction is placed. Finally, the bipolar plate on the anode side is placed on top. One side of the cathode side plate is connected to oxygen, and the other side is connected to the atmosphere to release excess oxygen.

[0071] like Figure 13 As shown in Figure b, the cathode electrolytic cell 2, which contains the cathode electrolyte, has three pipelines. Two of these pipelines connect to the chamber 1 of the flow electrolytic cell, one is used to pump the cathode electrolyte into the chamber 1 of the flow electrolytic cell using a peristaltic pump, and the other pipeline introduces the cathode electrolyte, the product propylene oxide, and the unreacted gaseous propylene into the cathode electrolytic cell 2. The last pipeline connects to a subsequent cold trap device to collect the volatilized product propylene oxide. In addition to the two pipelines connected to the cathode electrolytic cell 2 described above, the chamber 1 of the flow electrolytic cell also has a third pipeline for introducing propylene into the chamber 1 of the flow electrolytic cell. This allows the propylene to react with the in-situ generated H2O2 and TS-1 dissolved in the electrolyte in the chamber to undergo a propylene epoxidation reaction. Figure 13 In step b, the main site for the synthesis of propylene oxide is chamber 1 of the flowing electrolytic cell.

[0072] In chamber 1 of the flow electrolyzer, the H2O2 generated in situ undergoes an epoxidation reaction after being catalyzed by TS-1 and coming into full contact with the propylene that flows directly into the chamber.

[0073] The cathode electrolyzer 2 is used to hold the cathode electrolyte so that it can be circulated in the flow cell and to collect the product propylene oxide as much as possible.

[0074] Cold trap collection chamber 3 is used to collect the volatile reaction product propylene oxide (PO).

[0075] The electrolytic cell is supplied with O2 (30 sccm) on one side and cathode electrolyte (0.4 L / h) on the other. Propylene (2 sccm) is directly introduced into chamber 1 of the flow electrolytic cell (G-L mixture) to form a stable three-phase interface, facilitating continuous reaction. The cathode electrolyte is 20 ml of 1 mol / L PBS buffer (pH 7), circulating between the reactor and the electrolytic cell. The gas phase outlet of the electrolytic cell is connected to a cold trap collection device to collect as much volatile propylene oxide as possible. The anolyte is H2SO4 for the oxygen evolution reaction with a low overpotential. Constant current testing is performed on an electrochemical workstation CS310MA at a current density of 80 mA·cm⁻¹. -2 The experiment lasted for 3 hours.

[0076] Product Analysis: For the detection of the test solution, a portion of the reaction solution needs to be aspirated and filtered through a 0.22μm syringe filter (Nylon 66, Jinteng) to remove solid particles. Using a GC9790plus gas chromatograph and a flame ionization detector (FID), the sample is injected into the injection port of an HP-5 (30m × 320μm × 0.25μm) column. After measuring the peak areas of the product and internal standard, the specific yield is calculated using the following formula. The quantitative detection of the following products is similar.

[0077] The yield of PO was determined using the internal standard method, with chemically inert and water-soluble ethylene glycol dimethyl ether chosen as the internal standard. The key to quantification using the internal standard method lies in determining the relative correction factor between the internal standard and the analyte. The calculation formula is shown below. Based on the relative correction factor, the amount of product in the test solution can be calculated, as shown in the following formula:

[0078] f = (As / ms) / (Ar / mr)

[0079] mi = f × Ai / (As / ms)

[0080] In the formula, f is the relative correction factor, As and ms are the masses of the added internal standard peak area, Ar and mr are the masses of the added product peak area, Ai is the peak area of ​​the product in the test solution, and mi is the mass of the product in the test solution. To determine f, a standard sample needs to be prepared, as follows: Add 10 μL of PO and 10 μL of internal standard to 20 mL of electrolyte. Take 0.8 μL of the standard sample for gas chromatography detection, repeat multiple times, and take the average value to calculate f.

[0081] The PO yield in this embodiment is 0.54 mmol·h. -1 ·cm -2

[0082] To evaluate the stability of the G-Lmixture system, an electrochemical stability test was conducted for up to 35 hours under the aforementioned optimal reaction conditions. The voltage-time and yield results are as follows: Figure 14 , 15 As shown, it is noteworthy that the overall voltage remained stable between 2 and 2.5 V throughout the long electrolysis process, indicating that the G-Lmixture system can stably produce PO. PO production is continuous and stable, with an average production rate of 0.54 mmol·h⁻¹·cm⁻¹. -2 A total of 3.220 mmol of propylene oxide was produced within 6 hours and 8.568 mmol of propylene oxide was produced within 35 hours, showing relatively high yield and PO selectivity.

[0083] Comparative Example 1: Under the condition that other factors remain unchanged, experiments were conducted to compare two aeration methods for propylene: one involving a gas-buffing electrolytic cell containing cathode electrolyte (G-bubbling), and the other involving a gas-buffing electrolytic cell containing cathode reaction chamber in a flow cell (G-Lmixture), as well as the commonly used H cell. The results were compared at 80 mA·cm⁻¹. -2 Electrolysis reactions with different catalytic systems were carried out for 3 hours under a certain current.

[0084] Production volume is as follows Figure 16 As shown, compared with the common H-tank, this reactor has a significant increase in PO production (p < 0.05).

[0085] Furthermore, when the distribution method is changed to G-L mixture, better yield is achieved (approximately 0.61 mmol po·h). -1 By optimizing the reaction structure of the flow cell, the yield of PO can be greatly increased. The local high concentration of H2O2 on the surface of the GDL electrode can oxidize propylene to PO more quickly, indicating that changing the gas inlet method to form a stable three-phase reaction interface has a significant effect on improving the forward reaction.

[0086] Comparative Example 2: Under G-Lmixture aeration mode, the effect of the flow rate of the cathode electrolyte PBS on the reaction was investigated at 80 mA·cm⁻¹. -2 The catalytic system was subjected to an electrolysis reaction for 3 hours under a current of [current value missing].

[0087] The yields at different electrolyte flow rates are as follows: Figure 17 As shown, when the flow rate is too low, TS-1 will settle to the bottom, thus failing to exert sufficient catalytic activity; when the flow rate is too high, it will reduce the residence time of propylene, resulting in a decrease in yield. When the flow rate is between 0.2 and 0.8 L / h... -1 During this period, PO production did not change significantly. Considering overall energy consumption, 0.4 L·h -1 This is the optimal cathode flow rate condition.

[0088] Comparative Example 3: Under G-Lmixture aeration mode, the effect of current density on the reaction was investigated. Electrolysis reaction of the catalytic system was carried out for 3 hours under different current densities.

[0089] The output at different current densities is as follows: Figure 18 As shown, when the current density decreases, the rate of H2O2 production at the cathode decreases, resulting in lower PO yield. When the current density is too high, the selectivity of the two-electron ORR decreases, the electrolyte submerges the electrode, reducing the three-phase interface of the reaction and thus reducing O2 mass transfer and reaction yield. At a current density of 80 mA·cm⁻¹, the reaction yield is significantly reduced. -2 The yield of PO reaches its maximum value under these conditions, which are the optimal reaction conditions.

[0090] Comparative Example 4: Under G-Lmixture aeration mode, the effect of cathode electrolyte pH on the reaction was investigated. Electrolysis reaction of the catalytic system was carried out for 3 hours under different cathode electrolyte pH conditions.

[0091] The yields at different pH values ​​are as follows Figure 19As shown, when the electrolyte is H2SO4 and KOH, the selectivity for PO production is less than 20%. Other byproducts (propylene glycol, propionaldehyde, etc.) were detected in the electrolyte, and only a small amount of PO was obtained. In PBS buffer, the yield of PO reached its highest value between pH 6 and 7. In acidic buffer, the catalytic performance of O-CB decreased, resulting in a decrease in H2O2 production. The reason for the decrease in production in alkaline environment may be due to the inhibitory effect of alkali metals in the solution on the propylene epoxidation reaction on TS-1.

[0092] Table 4 is a comparison table of PO output between this work and the other work.

[0093] Table 4

[0094]

[0095]

[0096] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene, characterized in that: Carbon black oxide was used as a two-electron oxygen reduction catalyst. The two-electron oxygen reduction catalyst was sprayed onto hydrophobic carbon paper to prepare a gas diffusion electrode as the cathode. PBS buffer was used as the cathode electrolyte. TS-1 titanium silicon molecular sieve was added to the cathode electrolyte as an epoxidation catalyst. Propylene was directly fed into the chamber (1) of the flow electrolysis cell. Under the action of a peristaltic pump, the cathode electrolyte in the cathode electrolysis cell (2) was pumped into the chamber (1) of the flow electrolysis cell. The cathode electrolyte and gas in the chamber (1) of the flow electrolysis cell flowed into the cathode electrolysis cell (2) under the action of a peristaltic pump. A titanium mesh was used as the anode and sulfuric acid was used as the anode electrolyte to carry out the oxygen evolution reaction. Oxygen was introduced into the cathode. When an external current was introduced, the anode underwent an OER reaction and the cathode underwent a two-electron ORR reaction. The H2O2 generated in situ was used as an oxidant. Under the catalysis of the epoxidation catalyst, propylene was epoxidized to propylene oxide. The cold trap collection chamber (3) was used to collect the volatile reaction product propylene oxide. The two-electron oxygen reduction catalyst is prepared as follows: Weigh 50 mg of carbon black and put it into a round-bottom flask. Measure 37.5 ml of concentrated HNO3 and prepare 50 ml of 12 M HNO3 solution as an oxidant. Reflux the reaction at 85 °C for 12 h. Centrifuge at 7000 rpm to separate the solid for 5 min. Wash the solution and adjust the pH to neutral. The cathode electrolyte in the cathode electrolysis cell (2) is pumped into the chamber (1) of the flow electrolysis cell under the action of a peristaltic pump, wherein the flow rate of the cathode electrolyte is 0.4 L / h; The temperature of the cathode electrolyte in the cathode electrolytic cell (2) is maintained at 30°C; The propylene is directly fed into the chamber (1) of the flow electrolysis cell, and the gas flow rate of propylene is 2 sccm. 0.1 g of TS-1 titanium-silicon molecular sieve was added to 20 mL of cathode electrolyte as an epoxidation catalyst; Oxygen is introduced into the cathode at a flow rate of 30 sccm. The concentration of the PBS buffer is 1 mol / L, and the concentration of the anolyte is 0.1 mol / L.

2. The method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to claim 1, characterized in that: The epoxidation catalyst is prepared as follows: First, 17.00 g of tetrapropylammonium hydroxide solution is weighed and added to a 200 ml beaker. Then, 30.44 g of tetraethyl orthosilicate and 0.25 g of tetrabutyl titanate are added dropwise. Finally, 21.45 g of water is added as a solvent. After mixing thoroughly, the mixture is continuously irradiated with ultraviolet light for 1 hour to form a transparent solution. Hydrothermal crystallization is then carried out at 170 °C for 72 hours. The solution is washed by centrifugation at 8000 rpm, and the precipitate is dried overnight at 80 °C. -1 The heating rate was adjusted, and the catalyst was calcined at 550℃ for 16 hours to obtain a white powdery epoxidation catalyst.

3. The method for preparing propylene oxide by direct electrochemical epoxidation of oxygen and propylene according to claim 1 or 2, characterized in that: The loading of the two-electron oxygen reduction catalyst on the gas diffusion electrode is 0.5 mg·cm³. -2 .

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