A titanium-doped hollow flower-shaped nano-alumina adsorbent and its preparation method and application

Abstract

The present invention provides a titanium-doped hollow flower-spherical nano-alumina adsorbent, a preparation method and application thereof, and belongs to the technical field of adsorbent preparation. The present invention utilizes titanium sulfate as a titanium source, aluminum nitrate nonahydrate as an aluminum source, and urea as a precipitant, and obtains the titanium-doped hollow flower-spherical nano-alumina adsorbent through a hydrothermal co-precipitation method and high-temperature calcination. The interaction between the titanium-doped hollow flower-spherical nano-alumina adsorbent and PCl3 can efficiently remove trace amounts of PCl3, and the adsorbent can be recycled and reused multiple times after regeneration, thereby having good practical value.

Description

Technical Field

[0001] The invention belongs to the technical field of adsorbent preparation, and particularly relates to a titanium-doped hollow flower-spherical nano-alumina adsorbent, a preparation method and application thereof. Background Art

[0002] Solar energy is considered one of the most promising clean energy sources. Currently, over 95% of solar energy development and utilization is silicon-based, with polysilicon being the most important and fundamental material in the photovoltaic cell, semiconductor, and electronic information industries. Polysilicon used in these industries typically requires extremely high purity. For example, phosphorus impurity levels must be controlled to within the parts per billion (ppb) level, as otherwise, performance will be severely impacted. Therefore, in polysilicon production, phosphorus impurity levels must be reduced to the required level.

[0003] PCl3, the most important phosphorus-containing impurity in the polysilicon production process, is currently removed primarily through distillation. However, this method is complex, requires high equipment investment and energy consumption, and can pose problems such as equipment clogging and corrosion. In recent years, adsorption has been considered one of the most promising technologies for removing PCl3 due to its high efficiency, ease of operation, and low energy consumption. However, various adsorbents such as activated carbon, silicates, alumina, and ion exchange resins can only desorb PCl3 to the ppm level; removal to the ppb level is difficult. Therefore, there is a need to develop an adsorbent capable of removing PCl3 to the ppb level. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the present invention provides a titanium-doped hollow flower-spherical nano-alumina adsorbent and its preparation method and application; the present invention uses titanium sulfate as a titanium source, aluminum nitrate nonahydrate as an aluminum source, and urea as a precipitant, and obtains the titanium-doped hollow flower-spherical nano-alumina adsorbent through a hydrothermal co-precipitation method and high-temperature calcination; the interaction between the titanium-doped hollow flower-spherical nano-alumina adsorbent and PCl3 can efficiently remove trace amounts of PCl3, and the adsorbent can be recycled and reused multiple times after regeneration, and has good practical value.

[0005] The present invention achieves the above technical objectives through the following technical means.

[0006] The present invention first provides a titanium-doped hollow flower-ball-shaped nano-alumina adsorbent, in which blade-shaped alumina is uniformly embedded on the surface of titanium dioxide balls and stacked to form a hollow flower-ball-shaped structure; the adsorbent has macropores and mesopores.

[0007] Preferably, the specific surface area of ​​the titanium-doped hollow flower-shaped nano-alumina adsorbent is 116.74 m 2 / g, pore volume is 0.41cm3 / g, pore size is 10~100nm.

[0008] The present invention also provides a method for preparing the titanium-doped hollow flower-shaped nano-alumina adsorbent, which specifically comprises the following steps:

[0009] Titanium sulfate, aluminum nitrate nonahydrate and urea are dissolved in ultrapure water, mixed evenly and then subjected to hydrothermal reaction. After the reaction is completed, the mixture is filtered, washed and dried to obtain a precursor.

[0010] The precursor is calcined at 500-900° C. and cooled to room temperature after the calcination to obtain the titanium-doped hollow flower-shaped nano-alumina adsorbent.

[0011] Preferably, the molar ratio of titanium sulfate to aluminum nitrate nonahydrate is 1:1-3.

[0012] Preferably, the amount of urea used is 0.5 to 1.2 times the total mass of titanium sulfate and aluminum nitrate nonahydrate.

[0013] Preferably, the ratio of the total mass of the titanium sulfate, aluminum nitrate nonahydrate and urea to the mass of the ultrapure water is 1:30-50.

[0014] Preferably, the hydrothermal reaction is carried out at 140-220° C. for 1-24 hours.

[0015] Preferably, the calcination time is 2 hours.

[0016] The present invention also provides the use of the titanium-doped hollow flower-shaped nano-alumina adsorbent in the removal of trace PCl3.

[0017] Preferably, the application is the adsorption of PCl3 at 25-45°C.

[0018] Preferably, after the titanium-doped hollow flower-shaped nano-alumina adsorbent adsorbs PCl3, it is calcined at 120°C for 1 hour under nitrogen protection to regenerate the adsorbent.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention utilizes a hydrothermal coprecipitation method to produce a titanium-doped hollow flower-shaped nano-alumina adsorbent. By utilizing its interaction with PCl₃, it not only efficiently removes trace amounts of PCl₃, but also allows for multiple recycling and reuse after desorption, reducing adsorbent consumption and saving costs. This invention provides a new method for the removal of trace amounts of PCl₃.

[0021] The titanium-doped hollow flower-spherical nano-alumina adsorbent described in the present invention is an electrophilic adsorbent, which can be combined with electron-rich PCl3, thereby achieving the removal of trace PCl3. The present invention adds titanium sulfate in the process of preparing the titanium-doped hollow flower-spherical nano-alumina adsorbent. Both sulfate and titanium ions exhibit strong electron-withdrawing properties, and adding them to alumina can effectively increase its electrophilic point. At the same time, titanium is known as the king of metals and has the characteristics of light weight, non-toxicity, strong stability and strong corrosion resistance. Titanium sulfate is used as a titanium source to modify alumina from the source. The addition of this reagent causes a significant change in the growth direction of the crystal structure of alumina, from a branch-like shape to a flower-like shape. Among them, the blade-shaped alumina is evenly embedded in the surface of the titanium dioxide ball, stacked to form a hollow flower-spherical structure, and there are macropores and mesoporous structures in the adsorbent. The prepared titanium-doped hollow flower-spherical nano-alumina adsorbent has a specific surface area of ​​116.74m 2 / g, pore volume is 0.41cm 3 / g, and the pore size is mainly distributed between 10-100 nanometers.

[0022] The present invention explores the preparation conditions of the titanium-doped hollow flower-spherical nano-alumina adsorbent and optimizes it. The study found that with the increase of titanium content, a large number of accumulated smooth balls appear in the adsorbent, affecting the growth of flower-spherical alumina and reducing the performance of removing PCl3; if the temperature is too low, the degree of adsorbent accumulation is particularly large, and if the temperature is too high, titanium oxide will be converted into titanium pentoxide, which will affect the performance of removing PCl3; in addition, if the hydrothermal reaction time is too short, the adsorbent crystal growth is incomplete, and if the hydrothermal reaction time is too long, the adsorbent morphology will grow in other directions, resulting in damage to the flower-spherical alumina. In addition, high-temperature calcination will cause titanium dioxide to transform into titanium pentoxide, and as the temperature rises, its degree of accumulation becomes larger and larger. The present invention prepares a nanocrystalline structure with a special morphology and excellent performance by controlling the reaction conditions, thereby achieving the removal of trace amounts of PCl3.

[0023] The adsorbent of the present invention is doped with titanium to increase electrophilic sites. In addition to simple physical adsorption, Cl in PCl3 can interact with Al and Ti in the adsorbent through charge transfer, thereby achieving the purpose of efficiently removing PCl3. The adsorbent can remove PCl3 to below 20 ppb, which is superior to the commercial resins in the prior art. The adsorbent of the present invention circumvents the problems of poor stability, high replacement frequency and difficulty in regenerating the adsorbent in the prior art. It can be regenerated by a heating regeneration method, which is simple and efficient. In addition, the adsorbent of the present invention is prepared using inorganic materials, which greatly reduces the introduction of impurities and does not introduce new trace impurities during its application. The preparation method of the adsorbent is simple, easy to operate, and suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the SEM image of the titanium-doped hollow flower-ball-shaped nano-alumina adsorbent removing trace PCl3. In the figure, a is the hollow structure of the adsorbent at this magnified scale, and b is the flower-ball-shaped morphology of the adsorbent.

[0025] Figure 2 This is the EDS image of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent, a is the EDS electron image, b is the Al element distribution map, and c is the Ti element distribution map.

[0026] Figure 3 These are the infrared spectra of titanium-doped hollow flower-shaped nano-alumina adsorbent before adsorption, after adsorption, and after five cycles of adsorption.

[0027] Figure 4 This is a comparison chart of the activity data of titanium-doped hollow spherical nano-alumina adsorbent and other commonly used adsorbents in removing trace amounts of PCl3.

[0028] Figure 5 This is the data of five regenerations of titanium-doped hollow flower-shaped nano-alumina adsorbent for removing trace PCl3. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0030] Example 1:

[0031] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, and placed in 50mL of ultrapure water with stirring. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O.

[0032] Figure 1 This is an SEM image of titanium-doped hollow flower-spherical nano-alumina adsorbent removing trace amounts of PCl3. As can be seen from the figure, at a scale of 500 nanometers, the titanium-doped hollow flower-spherical nano-alumina adsorbent is displayed in the shape of a hollow flower-spherical morphology. It not only has a very beautiful morphology, but also increases its specific surface area, allowing its active sites to be fully displayed, which is beneficial to the adsorption of PCl3. As can be seen from the figure, blade-shaped alumina is evenly embedded in the surface of the titanium dioxide spheres, stacked to form a hollow flower-spherical structure; there are macropores and mesoporous structures in the adsorbent. According to measurements, the specific surface area of ​​the titanium-doped hollow flower-spherical nano-alumina adsorbent is 116.74m 2 / g, pore volume is 0.41cm3 / g, pore size is 10~100nm.

[0033] Figure 2 This is the EDS image of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent. It can be seen from the figure that Ti and Al are evenly distributed in the adsorbent, indicating that the titanium-doped hollow flower-shaped nano-alumina adsorbent is successfully prepared.

[0034] In this example, the removal efficiency of the titanium-doped hollow flower-shaped nano-alumina adsorbent prepared above for trace PCl3 was also investigated. The specific investigation steps were as follows:

[0035] In sealed polypropylene conical flasks, 20 mL of chloroform solution, 10-200 ppb PCl₃, and 0.2 g of the aforementioned titanium-doped hollow flower-shaped nanoalumina adsorbent were added. Adsorption was carried out at 25°C until saturation. After adsorption, the remaining PCl₃ impurity content in the solution was measured by ICP-MS, and the removal rate of trace PCl₃ impurities was calculated. When the titanium-doped hollow flower-shaped nanoalumina adsorbent reached saturation with PCl₃, it was regenerated by calcining it at 120°C for 1 hour under nitrogen protection.

[0036] Among them, the removal rate formula of trace PCl3 impurities is:

[0037]

[0038] Where, R is the removal rate (%); C0 is the concentration of PCl3 in the initial solution (μg / L); C e is the concentration of PCl3 remaining in the solution after adsorption reaches saturation (μg / L).

[0039] After testing and analysis, the removal rate of PCl3 by Ti1Al3O prepared in this embodiment is 90.4%.

[0040] When the Ti1Al3O3 adsorbent reaches saturation with PCl3, it is regenerated by calcining it at 120°C for 1 hour under nitrogen protection. In this example, the regenerated adsorbent is then used to adsorb PCl3 again. Testing and analysis show that after this second recycling, the adsorbent achieves a final PCl3 removal rate of 88.7%.

[0041] The Ti1Al3O adsorption-regeneration experiment was carried out for 5 times. After testing and analysis, the removal rates of PCl3 by the adsorbent for 5 times were 90.4%, 88.7%, 87.6%, 87.3%, 87.2% respectively. Figure 5 ). As can be seen from the figure, the adsorption performance is relatively stable and the regeneration performance is good.

[0042] Figure 3The infrared spectra of the titanium-doped hollow flower-shaped nanoalumina adsorbent before, after, and after five cycles of adsorption are shown. As can be seen from the figure, the infrared spectrum did not change significantly after five consecutive adsorption-regeneration experiments, indicating the adsorbent's good stability.

[0043] Example 2:

[0044] 0.48g of titanium sulfate and 0.48g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al1O.

[0045] In this example, the method described in Example 1 was used to investigate the removal efficiency of the titanium-doped hollow flower-shaped nano-alumina adsorbent for trace amounts of PCl3. After testing and analysis, the removal rate of PCl3 by the Ti1Al1O prepared in this example was 81.4%.

[0046] Example 3:

[0047] 0.72g of titanium sulfate and 0.24g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti3Al1O.

[0048] In this example, the method described in Example 1 was used to investigate the removal efficiency of the titanium-doped hollow flower-shaped nano-alumina adsorbent for trace amounts of PCl3. Detection and analysis showed that the removal rate of the Ti3Al1O prepared in this example for PCl3 was 76.8%.

[0049] Example 4:

[0050] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 1 hour. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O-1.

[0051] In this example, the method described in Example 1 was also used to investigate the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent on trace amounts of PCl3. After detection and analysis, the removal rate of Ti1Al3O-1 prepared in this example on PCl3 was 62.2%.

[0052] Example 5:

[0053] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 24 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O-24.

[0054] In this example, the method described in Example 1 was also used to investigate the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent on trace amounts of PCl3. After detection and analysis, the removal rate of Ti1Al3O-24 prepared in this example on PCl3 was 67.0%.

[0055] Example 6:

[0056] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 140°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O-140.

[0057] In this example, the method described in Example 1 was also used to investigate the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent on trace amounts of PCl3. After detection and analysis, the removal rate of Ti1Al3O-140 prepared in this example on PCl3 was 87.1%.

[0058] Example 7:

[0059] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 220°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O-220.

[0060] In this example, the method described in Example 1 was also used to investigate the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent on trace amounts of PCl3. After detection and analysis, the removal rate of Ti1Al3O-220 prepared in this example on PCl3 was 84.5%.

[0061] Example 8:

[0062] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 700°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O-700.

[0063] In this example, the method described in Example 1 was also used to investigate the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent on trace amounts of PCl3. After detection and analysis, the removal rate of Ti1Al3O-700 prepared in this example on PCl3 was 79.3%.

[0064] Example 9:

[0065] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, placed in 50mL of ultrapure water, and stirred until uniformly mixed. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 900°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O-900.

[0066] In this example, the method described in Example 1 was also used to investigate the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent on trace amounts of PCl3. After detection and analysis, the removal rate of Ti1Al3O-900 prepared in this example on PCl3 was 81.2%.

[0067] Example 10:

[0068] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, and placed in 50mL of ultrapure water with stirring. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O.

[0069] In this example, the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent for trace PCl3 was also investigated using the method described in Example 1. However, the adsorption temperature was adjusted to 35°C. After detection and analysis, the removal rate of PCl3 by the Ti1Al3O prepared in this example at 35°C was 87.1%.

[0070] Example 11:

[0071] 0.24g of titanium sulfate and 0.72g of aluminum nitrate nonahydrate were weighed, added to 0.5g of urea, and placed in 50mL of ultrapure water with stirring. The mixture was then transferred to a 100mL high-pressure hydrothermal autoclave and reacted at 180°C for 6 hours. After cooling naturally, the synthesized product was washed with ultrapure water and dried at 100°C for 6 hours. It was then calcined in a muffle furnace at 500°C for 2 hours to obtain a titanium-doped hollow flower-shaped nanoalumina adsorbent, designated Ti1Al3O.

[0072] In this example, the removal efficiency of the prepared titanium-doped hollow flower-shaped nano-alumina adsorbent for trace PCl3 was also investigated using the method described in Example 1. However, the adsorption temperature was adjusted to 45°C. After detection and analysis, the removal rate of PCl3 by the Ti1Al3O prepared in this example at 45°C was 85.7%.

[0073] Comparative Example 1:

[0074] 0.2 g of commercial activated carbon, 13X molecular sieve, silica, γ-Al2O3 and commercial resin were added to 20 ml of 200 ppb PCl3 solution respectively, and adsorbed at 25°C until saturation, and their removal efficiency for trace PCl3 was analyzed.

[0075] After testing and analysis, the removal efficiency of trace PCl3 by commercial activated carbon, 13X molecular sieve, silica, γ-Al2O3 and commercial resin were 43.6%, 59.1%, 66.0%, 67.4% and 69.4% respectively.

[0076] The results of the investigation are as follows Figure 4 As shown in the figure, you can see that common adsorbents all have a certain removal effect on PCl3, but their removal efficiency is significantly lower than that of the titanium-doped hollow flower-shaped nano-alumina adsorbent prepared by the present invention. This shows that the adsorbent prepared by this method has the performance of highly efficient removal of trace amounts of PCl3 and has good development and utilization prospects.

[0077] In summary, the interaction between the titanium-doped hollow flower-shaped nano-alumina adsorbent and PCl3 of the present invention can efficiently remove trace amounts of PCl3, and the adsorbent can be recycled and reused multiple times after regeneration, which has great practical value.

[0078] The embodiments described are preferred implementations of the present invention, but the present invention is not limited to the above implementations. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essence of the present invention are within the scope of protection of the present invention.

Claims

1. Application of titanium-doped hollow flower-shaped nano-alumina adsorbent in the removal of trace PCl3, characterized in that: The preparation method of the titanium-doped hollow flower-shaped nano-alumina adsorbent comprises: Titanium sulfate, aluminum nitrate nonahydrate and urea are dissolved in ultrapure water, mixed evenly and then subjected to hydrothermal reaction. After the reaction is completed, the mixture is filtered, washed and dried to obtain a precursor. The precursor is calcined at 500-900° C. and cooled to room temperature after calcination to obtain the titanium-doped hollow flower-shaped nano-alumina adsorbent; The molar ratio of titanium sulfate to aluminum nitrate nonahydrate is 1:1 to 3; The amount of urea used is 0.5 to 1.2 times the total mass of titanium sulfate and aluminum nitrate nonahydrate; The hydrothermal reaction conditions are 140-220° C. for 1-24 hours.

2. The use according to claim 1, characterized in that The ratio of the total mass of the titanium sulfate, aluminum nitrate nonahydrate and urea to the mass of the ultrapure water is 1:30-50.

3. The use according to claim 1, characterized in that The calcination time is 2 hours.

4. The use according to claim 1, characterized in that The application is the adsorption of PCl3 at 25 ~ 45 °C.

5. The use according to claim 1, characterized in that After the titanium-doped hollow flower-shaped nano-alumina adsorbent adsorbed PCl3, it was calcined at 120℃ for 1 h under nitrogen protection to regenerate the adsorbent.

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