Method for pyrolysis of lithium hexafluorophosphate using a lewis acid material as catalyst

Abstract

The application discloses a Lewis acid material catalyzed lithium hexafluorophosphate pyrolysis method, and constructs PF6 based on Lewis acid site gamma-Al2O3 nanosheet ‑ A complete hydrolysis thermal catalytic system, in the thermal catalytic system, gamma-Al2O3 nanosheet with rich Lewis acid sites is used as a catalyst, and inorganic acid is used to provide a positive ion supplier, wherein unsaturated coordinated Al III sites are catalytic active centers, the strong Lewis acidity of Al III sites promotes the pyrolysis of PF6 ‑ has strong combination, which is helpful for fixing PF6 ‑ and promotes the pyrolysis of PF6 ‑ is more susceptible to attack of nucleophilic substances such as water, meanwhile, the polar cation in water can further weaken the structure stability of PF6 ‑ , reduces the reaction condition, and enables gamma-Al2O3 to effectively promote the pyrolysis of PF6 ‑ under the thermal catalytic medium; the application provides technical support for the difficult disposal of lithium hexafluorophosphate in lithium battery production wastewater by constructing Lewis material mediated thermal catalysis.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a method for pyrolyzing lithium hexafluorophosphate catalyzed by Lewis acid materials. Background Technology

[0002] In recent years, with the popularization of the new energy industry, the lithium battery industry, which requires new energy, has developed rapidly. Lithium battery electrolyte, as one of the four major materials in lithium batteries, is the carrier of ion transport in lithium batteries, playing a role in conducting lithium ions between the positive and negative electrodes. Lithium hexafluorophosphate (LiPF6) electrolyte has advantages such as good ionic conductivity and good electrochemical stability, and is currently the most widely used lithium-ion electrolyte. Both the production process of LiPF6 and the disposal of waste lithium batteries generate high-fluoride and high-phosphorus wastewater with LiPF6 as the main pollutant. If not properly treated, the highly stable LiPF6 will persistently release fluoride and phosphate ions into water bodies, causing serious harm to surface water and even triggering environmental problems such as eutrophication and red tides.

[0003] Due to its unique electronic structure, LiPF6 exhibits strong Lewis basicity and high stability and biotoxicity. Therefore, traditional biochemical processes are hampered by its high biotoxicity, making effective disposal difficult. While traditional physicochemical methods, such as adding alkaline substances like calcium hydroxide, can partially decompose LiPF6, they cannot achieve complete hydrolysis and suffer from problems like large reagent consumption and high solid waste generation. Theoretically, the high temperature and pressure conditions of thermocatalysis can lower the hydrolysis energy barrier of LiPF6, further promoting its hydrolysis. γ-Al₂O₃, due to its excellent thermodynamic and chemical stability and abundant Lewis acid sites, is widely used in thermocatalysis. Theoretically, γ-Al₂O₃, rich in Lewis acid sites, can efficiently bind Lewis-based PF6. - This allows for the complete hydrolysis of LiPF6.

[0004] Based on this, the present invention constructs a new technology for the pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid-rich materials, aiming to fundamentally solve the environmental problems such as excessive fluorine and phosphorus caused by the difficult decomposition of LiPF6, and to provide a new approach for the treatment of high-fluorine and high-phosphorus wastewater in the lithium-ion battery industry, which has broad practical significance. Summary of the Invention

[0005] To address the challenges of treating lithium hexafluorophosphate (LiPF6) in wastewater from the lithium battery industry and the difficulty in achieving deep hydrolysis of LiPF6, this invention provides a novel thermocatalytic hydrolysis technology using Lewis acid materials. By constructing a thermocatalytic system mediated by materials rich in Lewis acid sites, its thermal stability is reduced, PF bonds are activated, and the thermocatalytic hydrolysis efficiency of LiPF6 is improved, achieving deep treatment of LiPF6 and effectively avoiding the problem of excessive fluoride and phosphorus content in water caused by incomplete hydrolysis of LiPF6.

[0006] To achieve the above objectives, the technical solution provided by this invention is: a method for pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid materials, wherein the pyrolysis method is based on constructing PF6 using γ-Al2O3 nanosheets with Lewis acid sites. - The complete hydrolysis thermocatalytic system reduces the thermal stability of lithium hexafluorophosphate, activates the PF bond, and adds an inorganic acid as an additional polarizing cation, namely H. + The source of lithium hexafluorophosphate is obtained through catalytic pyrolysis, and the specific steps are as follows:

[0007] Step 1, Acidification treatment: Add lithium hexafluorophosphate simulated wastewater to the magnetically stirred reactor, and then add inorganic acid to adjust the pH of the lithium hexafluorophosphate simulated wastewater to 2-3.

[0008] Step 2, Catalytic pyrolysis: Measure 40-60 ml of acidified lithium hexafluorophosphate simulated wastewater and add it to a magnetically stirred reactor. Then add 0.5-1 wt% of Lewis acid-rich γ-Al2O3 nanosheet catalyst based on the mass of the lithium hexafluorophosphate simulated wastewater. Turn on the magnetically stirred reactor and heat it up. After heating, keep the temperature and carry out the thermal catalytic hydrolysis reaction.

[0009] Preferably, in step 2, the temperature range for heating inside the magnetically stirred reactor is 80–120°C, and the holding time is 4–6 hours.

[0010] Preferably, in step 2, the γ-Al₂O₃ nanosheets rich in Lewis acid sites provide catalytic active sites, and the inorganic acid provides polarized cations, namely H₂O. + .

[0011] Preferably, the inorganic acid added in step 1 is one or more of hydrochloric acid, sulfuric acid, and nitric acid.

[0012] A preferred method for preparing Lewis acid-rich γ-Al2O3 nanosheets is as follows: 2-10 g of aluminum isopropoxide is dispersed in 20-80 mL of isopropanol, and deionized water is added dropwise and stirred until homogeneous to obtain a dispersion. The dispersion is placed in a stainless steel autoclave lined with polytetrafluoroethylene and hydrothermally heated for 2 hours. After cooling, it is centrifuged and dried to obtain a precursor. The precursor is then calcined in a muffle furnace at 200-700 °C to obtain Lewis acid-rich γ-Al2O3 nanosheets.

[0013] Beneficial effects of this invention:

[0014] 1. This invention employs a thermocatalytic hydrolysis method in H... + The combined action of the γ-Al2O3 nanosheet catalyst with Lewis acid sites accelerates the hydrolysis of lithium hexafluorophosphate, which can completely hydrolyze hexafluorophosphate into fluoride ions and phosphate ions.

[0015] 2. This invention decomposes lithium hexafluorophosphate in water using thermocatalytic hydrolysis technology. Compared with existing technologies, the process is simple, easy to operate, and stable.

[0016] 3. The materials used in this invention are inexpensive and readily available, the synthesis method is simple, and the synthesis yield and purity are high, making it suitable for large-scale production. Attached Figure Description

[0017] The accompanying drawings, which are provided to further illustrate the invention and constitute a part of this invention, are illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention.

[0018] Figure 1 These are X-ray powder diffraction comparison images of the γ-Al2O3 catalyst before and after the reaction in this invention.

[0019] Figure 2 This is a transmission electron microscope image of the γ-Al2O3 catalyst of the present invention;

[0020] Figure 3 The accompanying diagram shows the temperature-programmed ammonia removal process of the γ-Al₂O₃ catalyst of this invention.

[0021] Figure 4 The infrared spectrum of the γ-Al₂O₃ catalyst pyridine of the present invention;

[0022] Figure 5 This is a graph showing the change in the decomposition rate of lithium hexafluorophosphate in simulated wastewater with different acidities over fifteen days, as presented in this invention.

[0023] Figure 6 This is a graph showing the thermal decomposition rate of lithium hexafluorophosphate in unacidified simulated wastewater at different temperatures according to the present invention.

[0024] Figure 7 The graph shows the hydrolysis rate of lithium hexafluorophosphate in simulated wastewater catalytically acidified by the γ-Al2O3 catalyst of the present invention under the condition of 100 degrees Celsius for 5 hours.

[0025] Figure 8 This is a comparison chart of the hydrolysis effect under the condition of 100 degrees Celsius for 5 hours according to the present invention. Detailed Implementation

[0026] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0027] Reference Figures 1-8 A preferred embodiment of the present invention provides a method for the pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid materials, wherein the pyrolysis method is based on constructing PF6 using γ-Al2O3 nanosheets with Lewis acid sites. - The complete hydrolysis thermocatalytic system reduces the thermal stability of lithium hexafluorophosphate, activates the PF bond, and adds an inorganic acid as an additional polarizing cation, namely H. + The source of lithium hexafluorophosphate is obtained through catalytic pyrolysis, and the specific steps are as follows:

[0028] Step 1, Acidification treatment: Add lithium hexafluorophosphate simulated wastewater to the magnetically stirred reactor, and then add inorganic acid to adjust the pH of the lithium hexafluorophosphate simulated wastewater to 2-3.

[0029] Step 2, Catalytic pyrolysis: Measure 40-60 ml of acidified lithium hexafluorophosphate simulated wastewater and add it to a magnetically stirred reactor. Then add 0.5-1 wt% of Lewis acid-rich γ-Al2O3 nanosheet catalyst based on the mass of the lithium hexafluorophosphate simulated wastewater. Turn on the magnetically stirred reactor and heat it up. After heating, keep the temperature and carry out the thermal catalytic hydrolysis reaction.

[0030] Specifically, PF6 was constructed based on Lewis acid sites γ-Al2O3 nanosheets. - In a complete hydrolysis thermocatalytic system, γ-Al₂O₃ nanosheets rich in Lewis acid sites are used as catalysts, with inorganic acids providing cation donors. The unsaturated coordinated Al₂O₃ nanosheets... III The site is the catalytic active center, Al III Strong Lewis acidity at the site for PF6 - It has a strong binding effect and helps to fix PF6. - And prompt PF6 - It is more susceptible to attack by nucleophiles such as water, and at the same time, polarized cations in water can further weaken PF6. - Structural stability and reduced reaction conditions enable γ-Al₂O₃ to effectively promote PF₆ under a thermocatalytic medium. - Complete hydrolysis.

[0031] Furthermore, in step 2, the temperature range for heating inside the magnetically stirred reactor is 80–120°C, and the holding time is 4–6 hours.

[0032] Furthermore, in step 2, the γ-Al₂O₃ nanosheets rich in Lewis acid sites provide catalytic active sites, while the inorganic acid provides polarized cations, namely H₂O. + .

[0033] Furthermore, the inorganic acid added in step 1 is one or more of hydrochloric acid, sulfuric acid, and nitric acid.

[0034] Furthermore, the preparation method of γ-Al2O3 nanosheets rich in Lewis acid sites is as follows: 2-10 g of aluminum isopropoxide is dispersed in 20-80 mL of isopropanol, and deionized water is added dropwise and stirred evenly to obtain a dispersion. The dispersion is placed in a stainless steel autoclave lined with polytetrafluoroethylene and hydrothermally heated for 2 hours. After cooling, it is centrifuged and dried to obtain a precursor. The precursor is placed in a muffle furnace and calcined at 200-700 °C to obtain γ-Al2O3 nanosheets rich in Lewis acid sites.

[0035] The technical principle of this invention is that high temperature conditions accelerate the autocatalytic decomposition of LiPF6 to form LiF and PF5 (as shown in Formula 1). The strong Lewis acid PF5 generated by the reaction readily undergoes continuous hydrolysis with water to generate H3PO4 and HF (as shown in Formulas 4 and 6). However, PF6 in wastewater... - Since it exists stably and its content is much higher than that of PF5, it is necessary to use high temperature as a medium and a catalyst to break the PF bond, so as to generate PF5 and thus accelerate the hydrolysis process.

[0036] The hydrolysis reaction formula for lithium hexafluorophosphate is as follows:

[0037] LiPF6→PF5+LiF (1)

[0038]

[0039] PF5 + H2O → POF3 + 2HF (4)

[0040] POF3 + H2O → HPO2F2 + HF (5)

[0041] H₂PO₂F₂ + H₂O → H₂PO₃F + HF (6)

[0042] H2PO3F + H2O → H3PO4 + HF (7)

[0043] Specifically, through Figure 1 Mid-X-ray powder diffraction analysis showed that the γ-Al2O3 nanosheet catalyst was successfully prepared and that no change in crystal form occurred after the reaction, indicating that the γ-Al2O3 catalyst rich in Lewis acid sites has high stability after the hydrolysis reaction of lithium hexafluorophosphate in simulated wastewater.

[0044] pass Figure 2Transmission electron microscopy confirmed that the γ-Al2O3 nanosheet catalyst exposed the (110) crystal plane. This crystal plane is rich in unsaturated coordinated Al. III Site, Al III Sites are the main source of Lewis acidity.

[0045] pass Figure 3 , Figure 4 Ammonia temperature-programmed desorption and pyridine infrared spectroscopy confirmed that the synthesized γ-Al₂O₃ nanosheet catalyst is rich in acid sites, which are Lewis acid sites, PF₆. - As a Lewis base and γ-Al2O3(110) crystal surface, there are a large number of Lewis acid sites (Al III It has a strong binding effect, and the H provided by the inorganic acid + The polarization effect distorts the Al III PF6 binding site - The originally stable octahedral structure reduces its thermal stability, Al III The presence of the site fixed PF6 - It is more conducive to H + For PF6 - The attack, at the same time, Al III Hydroxyl groups bound to the site surface act as nucleophiles, attacking H-positive sites. + Twisted PF6 - The central P atom activates the PF bond, accelerates the breaking of the PF bond, and thus promotes the complete hydrolysis of LiPF6 into fluoride ions and phosphate ions.

[0046] According to the present invention, after acidification of lithium hexafluorophosphate water-simulated wastewater, it is added together with Lewis acid site-rich γ-Al2O3 nanosheet catalyst into a magnetically stirred reactor, and lithium hexafluorophosphate can be completely hydrolyzed into phosphoric acid and hydrogen fluoride under relatively low temperature conditions.

[0047] Example 1

[0048] This invention provides an application method for a new thermocatalytic hydrolysis technology. In a magnetically stirred reactor, 50% nitric acid is added to adjust the pH of simulated lithium hexafluorophosphate wastewater to 1, 2, and 3, respectively. The adjusted 1 mmol / L simulated lithium hexafluorophosphate wastewater and the unadjusted original solution (pH 4-5) are stored for 15 days, and the change in hexafluorophosphate concentration is continuously monitored. Figure 5 As shown in the graph, the decomposition rate of lithium hexafluorophosphate in simulated wastewater with different acidities changes over 15 days. It can be seen that lithium hexafluorophosphate in acidic simulated wastewater has high stability and did not undergo hydrolysis within 15 days.

[0049] Example 2

[0050] In a magnetically stirred reactor, 1 mM / L of unacidified lithium hexafluorophosphate simulated wastewater was added to the reactor body. Stirring was started at 150 rpm, and the reactor was heated to initiate a thermocatalytic hydrolysis reaction. The reaction was carried out at 40, 60, 80, 100, 120, and 140°C for 2, 4, 6, 8, 10, and 12 hours, respectively. Figure 6 As shown, the thermal decomposition rate of unacidified lithium hexafluorophosphate solution varies considerably at different temperatures. The initial thermal dissociation temperature of hexafluorophosphate is around 120℃, and complete hydrolysis of hexafluorophosphate occurs when the temperature rises to 140℃. This process requires a high temperature and a long time.

[0051] Example 3

[0052] This invention also provides an application method for a new thermocatalytic hydrolysis technology. In a magnetically stirred reactor, 50% nitric acid is added to adjust the pH of a 1 mmol / L lithium hexafluorophosphate simulated wastewater to 2. 50 mL of this solution is then measured and added to the reactor. Next, a Lewis acid-rich γ-Al₂O₃ nanosheet catalyst is added, with the catalyst amount being 0.5 wt% of the mass of the lithium hexafluorophosphate simulated wastewater. Stirring is then started and the temperature is increased to carry out the thermocatalytic hydrolysis reaction. Figure 7 As shown, when the reactor temperature is heated to 100℃ and the thermal catalytic reaction is carried out for 5 hours, the decomposition rate of hexafluorophosphate in the control group without catalyst is only 38%, while the dissociation rate of hexafluorophosphate in the γ-Al2O3 catalyst experimental group is over 99%.

[0053] Without causing conflict, those skilled in the art can freely combine and use the above-mentioned additional technical features.

[0054] The above description is only a preferred embodiment of the present invention. Any technical solution that achieves the purpose of the present invention by essentially the same means is within the protection scope of the present invention.

Claims

1. A method for pyrolyzing lithium hexafluorophosphate catalyzed by Lewis acid materials, characterized in that: The pyrolysis method is based on the construction of PF6 using Lewis acid site γ-Al2O3 nanosheets. - The complete hydrolysis thermocatalytic system reduces the thermal stability of lithium hexafluorophosphate, activates the PF bond, and adds an inorganic acid as an additional polarizing cation, namely H. + The source of lithium hexafluorophosphate is obtained through catalytic pyrolysis, and the specific steps are as follows: Step 1, Acidification treatment: Add lithium hexafluorophosphate simulated wastewater to the magnetically stirred reactor, and then add inorganic acid to adjust the pH of the lithium hexafluorophosphate simulated wastewater to 2-3. Step 2, Catalytic pyrolysis: Measure 40-60 ml of acidified lithium hexafluorophosphate simulated wastewater and add it to a magnetically stirred reactor. Then add 0.5-1 wt% of Lewis acid-rich γ-Al2O3 nanosheet catalyst based on the mass of the lithium hexafluorophosphate simulated wastewater. Turn on the magnetically stirred reactor and heat it up. After heating, keep the temperature and carry out the thermal catalytic hydrolysis reaction.

2. The method for pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid materials according to claim 1, characterized in that: In step 2, the temperature range for heating inside the magnetically stirred reactor is 80–120°C, and the holding time is 4–6 hours.

3. The method for pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid materials according to claim 1, characterized in that: In step 2, the γ-Al₂O₃ nanosheets rich in Lewis acid sites provide catalytic active sites, while the inorganic acid provides polarized cations, namely H₂O. + .

4. The method for pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid materials according to claim 1, characterized in that: The inorganic acid added in step 1 is one or more of hydrochloric acid, sulfuric acid, and nitric acid.

5. The method for pyrolysis of lithium hexafluorophosphate catalyzed by Lewis acid materials according to claim 1, characterized in that: The preparation method of γ-Al2O3 nanosheets rich in Lewis acid sites is as follows: 2-10 g of aluminum isopropoxide is dispersed in 20-80 mL of isopropanol, and deionized water is added dropwise and stirred evenly to obtain a dispersion. The dispersion is placed in a stainless steel autoclave with a polytetrafluoroethylene liner and hydrothermally heated for 2 hours. After cooling, it is centrifuged and dried to obtain a precursor. The precursor is calcined in a muffle furnace at 200-700 °C to obtain γ-Al2O3 nanosheets rich in Lewis acid sites.

Images