Method for catalyzing pyrolysis of lithium hexafluorophosphate by using Lewis acid material

Abstract

The invention discloses a method for catalyzing pyrolysis of lithium hexafluorophosphate by using a Lewis acid material, a PF6 <-> complete hydrolysis thermocatalytic system is constructed based on a Lewis acid site gamma-Al2O3 nanosheet, in the thermocatalytic system, the Lewis acid site-rich gamma-Al2O3 nanosheet is used as a catalyst, inorganic acid is used for providing a cation donor, an unsaturated coordinated AlIII site is used as a catalytic activity center, and the PF6 <-> complete hydrolysis thermocatalytic system is obtained. The strong Lewis acidity of the AlIII site has a strong binding effect on PF6-, PF6-can be fixed and promoted to be attacked by nucleophilic substances such as water more easily, meanwhile, polarized cations in water can further weaken the structural stability of PF6-, the reaction conditions are reduced, and gamma-Al2O3 can effectively promote complete hydrolysis of PF6-under a thermocatalytic medium; according to the method, technical support is provided for solving the problem that lithium hexafluorophosphate in lithium battery production wastewater is difficult to dispose by constructing Lewis material mediated thermocatalysis.

Description

Technical Field

[0001] The invention relates to the technical field of sewage treatment, and in particular to a method for thermally decomposing lithium hexafluorophosphate using Lewis acid materials as catalysts. Background Art

[0002] In recent years, with the popularization of the new energy industry, the lithium battery industry required for new energy has developed rapidly. As one of the four major materials of lithium batteries, lithium battery electrolyte is a carrier of ion transmission in lithium batteries, and plays a role in conducting lithium ions between the positive and negative electrodes. Lithium hexafluorophosphate (LiPF6) electrolyte has the advantages of good ionic conductivity and good electrochemical stability, and is currently the most widely used lithium ion electrolyte. Whether it is the production process of LiPF6 or the scrapping process of waste lithium batteries, high-fluorine and high-phosphorus wastewater with LiPF6 as the main pollution source will be generated. If not properly disposed of, the highly stable LiPF6 will persistently release fluoride ions and phosphate ions in the water body, causing serious harm to surface water, and even causing environmental problems such as eutrophication of water bodies and red tides.

[0003] Due to the unique electronic structure of LiPF6, it exhibits strong Lewis alkalinity and has high stability and biotoxicity. Therefore, traditional biochemical processes are difficult to effectively dispose of due to their high biotoxicity. Although traditional physicochemical methods can achieve partial decomposition of LiPF6 to a certain extent by adding alkaline substances such as calcium hydroxide, they cannot completely hydrolyze it, and there are problems such as large reagent usage and high solid waste output. Under the high temperature and high pressure conditions of thermal catalysis, the hydrolysis energy barrier of LiPF6 can theoretically be reduced, further promoting the hydrolysis of LiPF6. γ-Al2O3 is widely used in the field of thermal catalysis due to its good thermodynamic and chemical stability and abundant Lewis acid sites. In theory, γ-Al2O3 rich in Lewis acid sites can efficiently bind to Lewis base PF6 - , thereby achieving complete hydrolysis of LiPF6.

[0004] Based on this, the present invention constructs a new technology for the thermal decomposition of lithium hexafluorophosphate catalyzed by Lewis acid-rich materials, aiming to fundamentally solve environmental problems such as excessive fluorine and phosphorus caused by the difficulty in decomposing LiPF6, and provide new ideas 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] In view of the problem that lithium hexafluorophosphate is difficult to treat in wastewater in the lithium battery industry and it is difficult to achieve deep hydrolysis of lithium hexafluorophosphate, the present invention provides a new technology for thermal decomposition of lithium hexafluorophosphate catalyzed by Lewis acid materials. By constructing a thermal catalytic system mediated by materials rich in Lewis acid sites, its thermal stability is reduced, PF bonds are activated, and the thermal catalytic hydrolysis efficiency of lithium hexafluorophosphate is improved, so as to achieve deep treatment of LiPF6 and effectively avoid the problem of excessive fluorine and phosphorus content in water bodies caused by incomplete hydrolysis of LiPF6.

[0006] To achieve the above object, the technical solution provided by the present invention is: a method for pyrolyzing lithium hexafluorophosphate catalyzed by Lewis acid materials, wherein the pyrolysis method is based on Lewis acid site γ-Al2O3 nanosheets to construct PF6 - Thermocatalytic system for complete hydrolysis reduces the thermal stability of lithium hexafluorophosphate, activates the PF bond, and adds inorganic acid as an additional polarizing cation, namely H + The specific steps are as follows:

[0007] Step 1, acidification treatment: adding lithium hexafluorophosphate simulated wastewater into a magnetic stirring reactor, and then adding inorganic acid into the magnetic stirring reactor to adjust the pH of the lithium hexafluorophosphate simulated wastewater to 2-3;

[0008] Step 2, catalytic pyrolysis: 40 to 60 ml of acidified lithium hexafluorophosphate simulated wastewater is measured and added into a magnetic stirring reactor, and then 0.5 to 1 wt% of the mass of the lithium hexafluorophosphate simulated wastewater is added with a Lewis acid site-rich γ-Al2O3 nanosheet catalyst, the magnetic stirring reactor is started to stir and heat, and the temperature is kept after heating to carry out a thermal catalytic hydrolysis reaction.

[0009] Preferably, the temperature range of heating in the magnetic stirring reactor in step 2 is 80 to 120° C., and the insulation time is 4 to 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, i.e., H + .

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

[0012] Preferably, the preparation method of Lewis acid site-rich γ-Al2O3 nanosheets is as follows: 2 to 10 grams of aluminum isopropoxide is dispersed in 20 to 80 milliliters of isopropanol, and deionized water is added dropwise and stirred to obtain a dispersion, the dispersion is placed in a stainless steel autoclave lined with polytetrafluoroethylene, hydrothermal for 2 hours, and centrifugally dried after cooling to obtain a precursor, and the precursor is placed in a muffle furnace and calcined at 200 to 700°C to obtain Lewis acid site-rich γ-Al2O3 nanosheets.

[0013] Beneficial effects of the present invention:

[0014] 1. The present invention adopts thermal catalytic hydrolysis method to + The hydrolysis of lithium hexafluorophosphate is accelerated under the joint action of the Lewis acid site-rich γ-Al2O3 nanosheet catalyst, and the hexafluorophosphate can be completely hydrolyzed into fluoride ions and phosphate.

[0015] 2. The present invention decomposes lithium hexafluorophosphate in water by thermal catalytic hydrolysis technology. Compared with the prior art, the present invention has simple process, simple operation and stable operation.

[0016] 3. The material of the present invention is cheap and readily available, the synthesis method is simple, the yield and purity of the synthesis are high, and it is suitable for the requirements of scale-up production. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings described herein are used to provide further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0018] Figure 1 This is a comparison diagram of X-ray powder diffraction of the γ-Al2O3 catalyst before and after the reaction of the present invention;

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

[0020] Figure 3 This is a diagram of the ammonia-programmed temperature desorption of the γ-Al2O3 catalyst of the present invention;

[0021] Figure 4 This is the infrared image of the pyridine of the γ-Al2O3 catalyst 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 within 15 days of the present invention;

[0023] Figure 6 It is a graph showing the thermal decomposition rate of lithium hexafluorophosphate in the un-acidified simulated wastewater at different temperatures of the present invention;

[0024] Figure 7 It is a graph showing the hydrolysis rate of lithium hexafluorophosphate in simulated wastewater catalyzed by the γ-Al2O3 catalyst of the present invention at 100 degrees Celsius for 5 hours;

[0025] Figure 8 This is a comparison chart of the hydrolysis effects of the present invention under the conditions of 100 degrees Celsius for 5 hours. DETAILED DESCRIPTION

[0026] This section will describe in detail the specific embodiments of the present invention. The preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the accompanying drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but it cannot be understood as a limitation on the scope of protection of the present invention.

[0027] Reference Figure 1-Figure 8 The preferred embodiment of the present invention is a method for thermally decomposing lithium hexafluorophosphate catalyzed by Lewis acid materials, wherein the thermal decomposition method is based on the construction of PF6 - Thermocatalytic system for complete hydrolysis reduces the thermal stability of lithium hexafluorophosphate, activates the PF bond, and adds inorganic acid as an additional polarizing cation, namely H + The specific steps are as follows:

[0028] Step 1, acidification treatment: adding lithium hexafluorophosphate simulated wastewater into a magnetic stirring reactor, and then adding inorganic acid into the magnetic stirring reactor to adjust the pH of the lithium hexafluorophosphate simulated wastewater to 2-3;

[0029] Step 2, catalytic pyrolysis: 40 to 60 ml of acidified lithium hexafluorophosphate simulated wastewater is measured and added into a magnetic stirring reactor, and then 0.5 to 1 wt% of the mass of the lithium hexafluorophosphate simulated wastewater is added with a Lewis acid site-rich γ-Al2O3 nanosheet catalyst, the magnetic stirring reactor is started to stir and heat, and the temperature is kept after heating to carry out a thermal catalytic hydrolysis reaction.

[0030] Specifically, PF6 was constructed based on Lewis acid site γ-Al2O3 nanosheets. - In the complete hydrolysis thermal catalytic system, γ-Al2O3 nanosheets rich in Lewis acid sites are used as catalysts, and inorganic acids are used as cation donors. III The site is the catalytic active center, Al III The strong Lewis acidity of the site has an important effect on PF6 - Has a strong binding effect, helps to fix PF6 - And prompted PF6 - It is more susceptible to attack by nucleophilic substances such as water. At the same time, polarized cations in water can further weaken PF6 - Structural stability reduces the reaction conditions, making γ-Al2O3 effectively promote PF6 under thermal catalytic media - Completely hydrolyzed.

[0031] Furthermore, in step 2, the temperature range of heating in the magnetic stirring reactor is 80 to 120° C., and the insulation time is 4 to 6 hours.

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

[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 Lewis acid site-rich γ-Al2O3 nanosheets is as follows: 2 to 10 grams of aluminum isopropoxide is dispersed in 20 to 80 milliliters of isopropanol, and deionized water is added dropwise and stirred to obtain a dispersion, the dispersion is placed in a polytetrafluoroethylene-lined stainless steel autoclave, hydrothermal for 2 hours, and centrifugally dried after cooling to obtain a precursor, and the precursor is placed in a muffle furnace and calcined at 200 to 700°C to obtain Lewis acid site-rich γ-Al2O3 nanosheets.

[0035] The technical principle of the present invention is that under high temperature conditions, the self-catalytic decomposition of LiPF6 will be accelerated to form LiF and PF5 (as shown in Formula 1). The strong Lewis acid PF5 generated by the reaction is easy to undergo continuous hydrolysis reaction with water to generate H3PO4 and HF (as shown in Formula 4 and Formula 6). However, PF6 in wastewater - It exists stably and its content is much higher than PF5, so high temperature is needed to use a catalyst to break the PF bond and produce PF5, thereby accelerating the hydrolysis process.

[0036] The hydrolysis reaction of 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 The X-ray powder diffraction test showed that the γ-Al2O3 nanosheet catalyst was successfully prepared and no crystal change occurred after the reaction, indicating that the γ-Al2O3 catalyst rich in Lewis acid sites had high stability after catalyzing the hydrolysis of lithium hexafluorophosphate in simulated wastewater.

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

[0045] pass Figure 3 , Figure 4 :The ammonia temperature-programmed desorption and pyridine infrared test showed that the synthesized γ-Al2O3 nanosheet catalyst is rich in acid sites, and the acid sites are Lewis acid sites. - As a Lewis base, it interacts with the large number of Lewis acid sites (Al III ) has a strong binding effect, and the H provided by the inorganic acid + The Al III Site-bound PF6 - The originally stable octahedral structure reduces its thermal stability, Al III The presence of the site fixes PF6 - , which is more conducive to H + PF6 - At the same time, Al III The hydroxyl group bound to the surface of the site acts as a nucleophile and attacks the H + Twisted PF6 - The central P atom activates the PF bond and accelerates the PF bond breaking, thereby promoting the complete hydrolysis of LiPF6 into fluoride ions and phosphate.

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

[0047] Example 1

[0048] The present invention provides an application method of a new thermal catalytic hydrolysis technology. In a magnetic stirring reactor, 50% nitric acid is added to adjust the pH of lithium hexafluorophosphate simulated wastewater to 1, pH 2, and pH 3 respectively. The 1 mmol per liter lithium hexafluorophosphate simulated wastewater after acid adjustment and the unadjusted stock solution (pH = 4-5) are stored for 15 days, and the change of hexafluorophosphate concentration is continuously monitored. Figure 5 As shown in the graph of the change in the decomposition rate of lithium hexafluorophosphate in simulated wastewater with different acidity within fifteen days, it can be found that the lithium hexafluorophosphate in the acidic simulated wastewater is very stable and no hydrolysis occurs within 15 days.

[0049] Example 2

[0050] In a magnetic stirring reactor, 1 mmol / L of unacidified lithium hexafluorophosphate simulated wastewater was added into the reactor body, stirring was started, the speed was 150 rpm, the temperature was increased, and the thermal catalytic hydrolysis 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 in the figure, the thermal decomposition rate of unacidified lithium hexafluorophosphate solution at different temperatures varies greatly. The starting temperature of thermal dissociation of hexafluorophosphate is between 120°C. When the temperature rises to 140°C, hexafluorophosphate can be completely hydrolyzed. This process requires a higher temperature and a longer time.

[0051] Example 3

[0052] The present invention also provides an application method of the new thermal catalytic hydrolysis technology. In a magnetic stirring reactor, 50% nitric acid is added, and the pH of 1 mmol per liter of lithium hexafluorophosphate simulated wastewater is adjusted to 2, and 50 ml is measured and added to the body of the magnetic stirring reactor, and then a Lewis acid site-rich γ-Al2O3 nanosheet catalyst is added, and the amount of the catalyst added is 0.5wt% of the mass of the lithium hexafluorophosphate simulated wastewater. Stirring is started and the temperature is increased to perform a thermal catalytic hydrolysis reaction. Figure 7 As shown, when the reactor temperature was heated to 100°C for 5 hours of thermal catalytic reaction, the decomposition rate of hexafluorophosphate in the control group without adding catalyst was only 38%, while the dissociation of hexafluorophosphate in the γ-Al2O3 catalyst experimental group reached more than 99%.

[0053] Under the premise that no conflict occurs, those skilled in the art may freely combine and superimpose the above-mentioned additional technical features.

[0054] The above description is only a preferred implementation mode of the present invention. Any technical solution that achieves the purpose of the present invention by substantially the same means belongs to the protection scope of the present invention.

Claims

1. A method for thermally decomposing lithium hexafluorophosphate catalyzed by Lewis acid material, characterized in that: The pyrolysis method is based on Lewis acid site γ-Al2O3 nanosheets to construct PF6 - Thermocatalytic system for complete hydrolysis reduces the thermal stability of lithium hexafluorophosphate, activates the PF bond, and adds inorganic acid as an additional polarizing cation, namely H + The specific steps are as follows: Step 1, acidification treatment: adding lithium hexafluorophosphate simulated wastewater into a magnetic stirring reactor, and then adding inorganic acid into the magnetic stirring reactor to adjust the pH of the lithium hexafluorophosphate simulated wastewater to 2-3; Step 2, catalytic pyrolysis: 40 to 60 ml of acidified lithium hexafluorophosphate simulated wastewater is measured and added into a magnetic stirring reactor, and then 0.5 to 1 wt% of the mass of the lithium hexafluorophosphate simulated wastewater is added with a Lewis acid site-rich γ-Al2O3 nanosheet catalyst, the magnetic stirring reactor is started to stir and heat, and the temperature is kept after heating to carry out a thermal catalytic hydrolysis reaction.

2. The method for thermally decomposing lithium hexafluorophosphate catalyzed by a Lewis acid material according to claim 1, characterized in that: In step 2, the temperature range of heating in the magnetic stirring reactor is 80 to 120° C., and the insulation time is 4 to 6 hours.

3. The method for thermally decomposing lithium hexafluorophosphate catalyzed by a Lewis acid material according to claim 1, characterized in that: In step 2, the Lewis acid site-rich γ-Al2O3 nanosheets provide catalytic active sites, and the inorganic acid provides polarized cations, namely H + .

4. The method for thermally decomposing lithium hexafluorophosphate catalyzed by a Lewis acid material 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 thermally decomposing lithium hexafluorophosphate catalyzed by a Lewis acid material according to claim 1, characterized in that: The preparation method of Lewis acid site-rich γ-Al2O3 nanosheets is as follows: 2 to 10 grams of aluminum isopropoxide is dispersed in 20 to 80 milliliters of isopropanol, and deionized water is added dropwise and stirred to obtain a dispersion, the dispersion is placed in a stainless steel autoclave lined with polytetrafluoroethylene, hydrothermal for 2 hours, and centrifugally dried after cooling to obtain a precursor, and the precursor is placed in a muffle furnace and calcined at 200 to 700° C. to obtain Lewis acid site-rich γ-Al2O3 nanosheets.

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