Halloysite nanotubes with a large specific surface area, preparation method thereof and application

Abstract

The present invention relates to the technical field of the preparation of halloysite nanotubes, and discloses a halloysite nanotube with a large specific surface area, a preparation method thereof and an application; the method comprises the following steps: mixing halloysite nanotubes with potassium acetate and grinding, then adding water, carrying out ultrasonic treatment and standing; then adding ethanol, stirring and washing, separating solid and liquid, and drying the solid to obtain a potassium acetate-halloysite nanotube composite; calcining the potassium acetate-halloysite nanotube composite to obtain a calcined potassium acetate-halloysite nanotube composite; treating the calcined potassium acetate-halloysite nanotube composite with a hydrochloric acid solution, and then washing and drying to obtain a halloysite nanotube with a large specific surface area. The reaction conditions of the present invention are mild and easy to control, and the halloysite nanotubes can maintain their tubular structures. In addition, potassium acetate is a kind of salt, which is environmentally friendly, pollution-free, and can be prepared in large quantities, facilitating production. The specific surface area can be increased to 5-27 times of the original one.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of halloysite nanotubes, and discloses a halloysite nanotube with a large specific surface area, a preparation method thereof, and an application thereof. Background Art

[0002] Halloysite nanotubes (Al2(OH)4Si2O5·2H2O) are a kind of aluminosilicate mineral material with a hollow tubular structure. The tube wall is composed of a layered curly structure formed by alternating silicon-oxygen tetrahedrons and aluminum-oxygen octahedrons. The two ends of the halloysite nanotubes are open. The tube length is 0.2 - 1.0 μm, the inner diameter is 10 - 20 nm, the outer diameter is 20 - 50 nm, and the number of layers is 15 - 25 layers. The layers on the tube wall are separated by single-layer water molecules. Therefore, the layer spacing of the hydrated halloysite nanotubes is Due to the weak retention force of the interlayer water, halloysite nanotubes are prone to irreversible dehydration in the environment to obtain the corresponding halloysite nanotubes. The inner surface of the halloysite nanotubes is positively charged Al-OH groups, and the outer surface is negatively charged Si-OH groups. Compared with other one-dimensional nanomaterials such as carbon nanotubes, halloysite nanotubes are cheap, easy to obtain, environmentally friendly, biocompatible, rich in reserves, and do not need to be synthesized through complex reaction routes.

[0003] In view of the unique tubular structure and surface characteristics of halloysite nanotubes, the applications of this material in adsorption, separation, catalysis, drug slow release, flame retardancy, energy storage, etc. have been widely studied in recent years. However, due to the relatively low specific surface area of halloysite nanotubes, the number of their active sites is limited, which is not conducive to the adsorption of pollutants and the loading of nanoparticles, etc., and to a certain extent limits their practical applications. Therefore, it is of great significance to increase the specific surface area of halloysite nanotubes by a certain method for their practical applications.

[0004] Currently, the main method to increase the specific surface area of halloysite nanotubes is to use strong acid treatment. For example, Abdullayev et al. used sulfuric acid to selectively etch the internal cavity of halloysite nanotubes, increasing their specific surface area to 250 m 2 g -1 , which is 6 times the specific surface area of the original halloysite nanotubes (ACS Nano, 2012, 6, 7216). However, when more aluminum is etched, the halloysite nanotubes become SiO2 nanospheres. Zhang et al. found that when using sulfuric acid to treat halloysite nanotubes, with the extension of time, their specific surface area first increases to a maximum of 267.1 m 2 g -1, and then shows a decreasing trend. The reason for the decrease is the collapse of its tubular structure caused by the decomposition of the silica layer (Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2012, 396, 182 - 188). These results indicate that although acid etching can increase the specific surface area of halloysite nanotubes, it causes severe corrosion and damage to their inner cavities or tubular structures, and the highest specific surface area is about 6 times that of the initial halloysite nanotubes, with a low possibility of further improvement, which is not conducive to their subsequent applications. At the same time, the strong acid corrosion and high pollution seriously restrict their large-scale applications. Inserting some organic small molecules and surfactants between the layers can also increase the layer spacing and specific surface area of halloysite nanotubes. Previous studies have shown that the strong hydrogen bonds and polarity between the layers of halloysite nanotubes are not conducive to the entry of molecules, and only a few small molecules (such as urea, dimethyl sulfoxide, formamide, potassium acetate, etc.) can directly enter the interlayer of halloysite nanotubes, increasing the layer spacing from 0.7 nm to at most 1.0 nm. It is worth noting that Salaa et al. used a two-step intercalation method of dimethyl sulfoxide and cetyltrimethylammonium bromide to increase the layer spacing of halloysite nanotubes from 0.7 nm to 2.6 nm, which is the largest increase in the layer spacing of halloysite nanotubes reported so far, but some layers are peeled off (Chemical Engineering Journal, 2020, 396, 125226), and the increase in the specific surface area of halloysite nanotubes is limited. In summary, it is urgent to find a scalable and environmentally friendly method that can increase the specific surface area of halloysite nanotubes without damaging their tubular structures, which is conducive to their practical applications. Summary of the Invention

[0005] The object of the present invention is to overcome the above deficiencies and provide a preparation method of halloysite nanotubes with a large specific surface area to improve their adsorption performance and the hydrogen production performance of ammonia borane pyrolysis.

[0006] To achieve the above object, the present invention is implemented according to the following technical scheme:

[0007] A preparation method of halloysite nanotubes with a large specific surface area, comprising the following steps:

[0008] S1, preparing a potassium acetate - halloysite nanotube composite

[0009] Mix halloysite nanotubes and potassium acetate, grind them, add water, perform ultrasonic treatment and then let it stand; then add ethanol, stir and wash, separate the solid and liquid, and dry the solid to obtain the potassium acetate - halloysite nanotube composite;

[0010] S2, calcining the potassium acetate - halloysite nanotube composite

[0011] The potassium acetate-halloysite nanotube composite is calcined to obtain the calcined potassium acetate-halloysite nanotube composite;

[0012] S3. The calcined potassium acetate-halloysite nanotube composite obtained in step S2 is treated with a hydrochloric acid solution, and then washed and dried to obtain halloysite nanotubes with a large specific surface area, denoted as M-HNTs.

[0013] Preferably, in step S1, the weight ratio of halloysite nanotubes to potassium acetate is 1:0.6 - 20.

[0014] Preferably, in step S1, the standing time is 2 - 7 days.

[0015] Preferably, in step S2, the calcination temperature is 320 - 425 °C.

[0016] Preferably, in step S2, the heating rate of calcination is 5 °C / min. -1

[0017] Preferably, in step S3, the process of treating with a hydrochloric acid solution is to mix the calcined potassium acetate-halloysite nanotube composite with the hydrochloric acid solution and then place it in a constant temperature water bath.

[0018] Preferably, in step S3, the drying process uses freeze-drying.

[0019] Specifically, a method for preparing halloysite nanotubes with a large specific surface area includes the following steps:

[0020] S1. Prepare the potassium acetate-halloysite nanotube composite

[0021] The weight ratio of halloysite nanotubes to potassium acetate is 1:0.6 - 20. The two are mixed and ground in a mortar for 20 min, then a small amount of water is added, and ultrasonic treatment is carried out for 30 min. After taking it out and standing for 2 - 7 days, potassium acetate enters the interlayer of halloysite nanotubes. Then ethanol is added and stirred for 4 h, and the excess potassium acetate is removed by washing with excessive ethanol. It is dried in a drying oven at 70 °C for 8 h to obtain the potassium acetate-halloysite nanotube composite.

[0022] S2. Calcinate the potassium acetate-halloysite nanotube composite

[0023] Place 1 g of the potassium acetate-halloysite nanotube composite in a tubular furnace and calcine it for 3 h in an air atmosphere to obtain the calcined potassium acetate-halloysite nanotube composite; the specific calcination conditions are: the calcination temperature is 320 - 425 °C, and the heating rate is 5 °C / min. -1

[0024] ​​S3. Use hydrochloric acid solution to wash and remove impurities from the calcined potassium acetate - halloysite nanotube composite to obtain the modified halloysite nanotubes, which are the halloysite nanotubes with a large specific surface area of the present invention, denoted as M - HNTs;

[0025] Specifically, use 4M (4 mol L -1 ) hydrochloric acid solution to treat the calcined potassium acetate - halloysite nanotube composite, let it stand in a constant temperature water bath at 70 °C for 3 h, wash it to neutral after the treatment, and then freeze - dry it for 8 h to obtain the finished product M - HNTs.

[0026] A kind of halloysite nanotubes with a large specific surface area is prepared by the above - mentioned preparation method.

[0027] The present invention also includes the application of the halloysite nanotubes with a large specific surface area in the adsorption of methylene blue.

[0028] Specifically, directly putting M - HNTs into the solution containing methylene blue can complete the adsorption of methylene blue.

[0029] The present invention also includes the application of the halloysite nanotubes with a large specific surface area in the encapsulation of ammonia borane and hydrogen production by thermal decomposition.

[0030] Specifically, the process of ammonia borane encapsulation and hydrogen production by thermal decomposition is as follows: Weigh 50 mg of M - HNTs and add it to 5 mL of tetrahydrofuran (THF), ultrasonically disperse it evenly for 10 min, denoted as A. Then add 25 mg of ammonia borane (referred to as AB) to 5 mL of THF, ultrasonically disperse it into a 0.162 M solution, denoted as B. Add A to B and mix and ultrasonically disperse for 30 min, then stir at room temperature for 4 - 6 h, denoted as C. Finally, place C in a vacuum drying oven at 30 °C to evaporate the THF solvent to obtain a dry sample AB@M - HNTs (halloysite nanotubes encapsulated with ammonia borane).

[0031] Use a thermogravimetric - mass spectrometry (PerkinElmer) to test the hydrogen production process by thermal decomposition of AB@M - HNTs. The mass of the sample used is 10 mg, the atmosphere used is helium, and the gas flow rate is 10 °C min -1 , and the heating temperature is 20 - 260 °C.

[0032] The present invention introduces potassium acetate into the interlayer of halloysite nanotubes. By calcining the halloysite nanotube - potassium acetate composite, using the characteristics that the high - temperature melting of potassium acetate leads to an increase in the degree of molecular polymerization or the high - temperature decomposition into acetic acid, the specific surface area is increased, and its tubular morphology is not damaged, improving the adsorption performance, encapsulation performance, and ammonia borane hydrogen production performance by thermal decomposition of halloysite nanotubes, etc. The specific surface area of the obtained M - HNTs in the present invention is increased to 5 - 27 times of the original, which can effectively improve the adsorption performance of methylene blue, increase the encapsulation amount of ammonia borane, and reduce the ammonia borane hydrogen production temperature by thermal decomposition.

[0033] Principle of action of the present invention:

[0034] The halloysite nanotubes with a large specific surface area (abbreviated as M-HNTs) obtained by the preparation of the present invention are introduced into the interlayer of halloysite nanotubes with potassium acetate as the modification material (this process can also be called intercalation), and the interlayer space and specific surface area of halloysite nanotubes are increased by calcination at a high temperature (320 - 425 °C). In the present invention, the solid-phase grinding method is first used to grind halloysite nanotubes and potassium acetate to make them evenly mixed. After adding a small amount of water and standing for a sufficient time, potassium acetate enters the interlayer of halloysite nanotubes, and then the excess potassium acetate that has not entered the tube wall and interlayer on the halloysite nanotubes is washed away with ethanol. Potassium acetate molecules form hydrogen bonds with the hydroxyl groups on the inner surface of halloysite nanotubes, serving as molecular supports in the interlayer of halloysite nanotubes and interacting with the tetrahedral structure of the next adjacent layer, enhancing the stability of halloysite nanotubes in aqueous solution.

[0035] The principle of the calcination process is as follows:

[0036] (1) When the calcination temperature is lower than the decomposition temperature of potassium acetate (320 - 407 °C) but higher than the melting point of potassium acetate (292 °C), the potassium acetate entering the interlayer of halloysite nanotubes melts into a liquid state. The potassium acetate molecules are conducive to their aggregation in the liquid state, resulting in further expansion of the interlayer of halloysite nanotubes and the formation of pores on the layer wall. The more potassium acetate enters the interlayer of halloysite nanotubes, the more the liquid molecules aggregate during high-temperature melting, the larger the interlayer distance, and thus the larger the specific surface area. In addition, potassium acetate is a salt of a strong base and a weak acid, and can etch the tube wall of halloysite nanotubes thermally in the molten state, so it can also increase the specific surface area of halloysite nanotubes.

[0037] (2) When the calcination temperature is higher than the decomposition temperature of potassium acetate (407 - 425 °C), the potassium acetate entering the interlayer of halloysite nanotubes decomposes into acetic acid, which reacts with Al-OH on the tube wall of halloysite nanotubes, etching away part of the aluminum, further increasing the layer domain space and specific surface area of halloysite nanotubes. The higher the temperature, the higher the etching degree of aluminum by the generated acetic acid, and the more the specific surface area increases. The increase in specific surface area and pore volume can provide more adsorption sites and encapsulation spaces for pollutants and ammonia borane, and can change the pyrolysis environment of ammonia borane, reducing its hydrogen production temperature by pyrolysis.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] 1. The present invention uses calcined potassium acetate to intercalate halloysite nanotubes to increase their specific surface area. The reaction conditions are mild and easy to control, and the halloysite nanotubes can maintain their tubular structure. In addition, potassium acetate is a salt, which is environmentally friendly, pollution-free, and can be prepared in large quantities, facilitating production.

[0040] 2. The existing methods for increasing the specific surface area mentioned in the current literature can increase the specific surface area by up to 6 times. However, the method of the present invention uses calcined halloysite nanotubes intercalated with potassium acetate, and the specific surface area can be increased to 5 - 27 times the original.

[0041] 3. Compared with the original halloysite nanotubes, the removal rate of methylene blue by the intercalated and calcined halloysite nanotubes is increased by 75.45% compared with that of the original halloysite nanotubes, indicating that the adsorption performance of the intercalated and calcined halloysite nanotubes for methylene blue is significantly enhanced.

[0042] 4. Compared with the original halloysite nanotubes, the encapsulation efficiency of ammonia borane by the intercalated and calcined halloysite nanotubes is increased by 6% - 12%, and the temperature for thermal decomposition of ammonia borane to produce hydrogen is reduced from 106 °C to 50 °C, effectively reducing the energy consumption. Description of the Drawings

[0043] Figure 1 N2 adsorption - desorption isotherm and BJH pore size distribution diagram of HNTs in Comparative Example 1 and M - HNTs(1:4)-425 in Example 16;

[0044] Figure 2 Infrared absorption spectra of HNTs in Comparative Example 1, M - HNTs(1:1)-320 in Example 4, M - HNTs(1:1)-350 in Example 5, M - HNTs(1:1)-400 in Example 6, and M - HNTs(1:1)-425 in Example 7;

[0045] Figure 3 TEM detection diagrams of HNTs in Comparative Example 1, M - HNTs(1:1)-425 in Example 7, and HCl - HNTs in Comparative Example 2;

[0046] Figure 4 Contact angle test result diagrams of HNTs in Comparative Example 1 and M - HNTs(1:0.6)-400 in Example 3;

[0047] Figure 5 XRD diagrams of HNTs in Comparative Example 1, M - HNTs(1:1)-320 in Example 4, M - HNTs(1:1)-350 in Example 5, M - HNTs(1:1)-400 in Example 6, and M - HNTs(1:1)-425 in Example 7;

[0048] Figure 6 Isoelectric point (PZC) curve diagrams of HNTs in Comparative Example 1 and M - HNTs(1:1)-400 in Example 6;

[0049] Figure 7Relevant results of HNTs of Comparative Example 1 and M-HNTs(1:1)-400 of Example 6 for methylene blue treatment; wherein Figure a is the ultraviolet-visible spectral diagram of methylene blue after adsorption of MB (methylene blue); Figure b is the bar chart of the removal rate of methylene blue.

[0050] Figure 8 TG curve and DTG curve of ammonia borane encapsulated by HNTs of Comparative Example 1 and M-HNTs(1:1.5)-320 of Example 8, wherein Figure a is the TG curve and Figure b is the DTG curve.

[0051] Figure 9 Thermogravimetry-mass spectrometry (TG-MS) curve of hydrogen production from the pyrolysis of ammonia borane encapsulated by HNTs of Comparative Example 1 and M-HNTs(1:1.5)-320 of Example 8. Detailed implementation mode

[0052] A preparation method of halloysite nanotubes with a large specific surface area, comprising the following steps:

[0053] Mix 1 g of halloysite nanotubes with 0.6 - 20 g of potassium acetate in a mortar, gently grind for 20 min by solid grinding method, add a small amount of water and ultrasonicate for 30 min, let stand for 2 - 7 days, take out, add ethanol, stir and wash for 4 h, then centrifuge and wash twice, and dry the sample in a drying oven at 70°C. After the dried sample is ground into powder, put it into a tubular furnace at 320 - 425°C and calcine for 3 h in an air atmosphere. Finally, further purify the calcined potassium acetate-halloysite nanotube composite with 4M hydrochloric acid, keep it in a water bath at 70°C for 3 h, and obtain the sample after freeze-drying.

[0054] The following describes the present invention with specific examples. The illustrative embodiments and explanations of this invention are used to explain the present invention, but do not limit the present invention.

[0055] Example 1

[0056] A preparation method of halloysite nanotubes with a large specific surface area, comprising the following steps:

[0057] S1, Prepare potassium acetate-halloysite nanotube composite

[0058] Mix 1 g of halloysite nanotubes with 0.6 g of potassium acetate in a mortar, gently grind for 20 min by solid grinding method, add a small amount of water and ultrasonicate for 30 min, let stand for 2 - 7 days, take out, add ethanol, stir and wash for 4 h, then centrifuge and wash twice, and dry in a drying oven at 70°C to obtain potassium acetate-halloysite nanotube composite.

[0059] S2, Calcine potassium acetate-halloysite nanotube composite

[0060] After grinding the potassium acetate-halloysite nanotube composite into powder, it was placed in a tube furnace at 320 °C and calcined in an air atmosphere for 3 h to obtain the calcined potassium acetate-halloysite nanotube composite.

[0061] S3. The calcined potassium acetate-halloysite nanotube composite was further purified with 4M hydrochloric acid, kept in a water bath at 70 °C for 3 h under constant temperature, and freeze-dried to obtain halloysite nanotubes with a large specific surface area, labeled as M-HNTs(1:0.6)-320.

[0062] Example 2

[0063] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:0.6, and the calcination temperature was 350 °C; the remaining steps were the same as those in Example 1.

[0064] Halloysite nanotubes with a large specific surface area were obtained, labeled as M-HNTs(1:0.6)-350.

[0065] Example 3

[0066] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:0.6, and the calcination temperature was 400 °C; the remaining steps were the same as those in Example 1.

[0067] Halloysite nanotubes with a large specific surface area were obtained, labeled as M-HNTs(1:0.6)-400.

[0068] Example 4

[0069] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1, and the calcination temperature was 320 °C; the remaining steps were the same as those in Example 1.

[0070] Halloysite nanotubes with a large specific surface area were obtained, labeled as M-HNTs(1:1)-320.

[0071] Example 5

[0072] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1, and the calcination temperature was 350 °C; the remaining steps were the same as those in Example 1.

[0073] Halloysite nanotubes with a large specific surface area were obtained, labeled as M-HNTs(1:1)-350.

[0074] Example 6

[0075] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1, and the calcination temperature was 400 °C; the remaining steps were the same as those in Example 1.

[0076] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:1)-400.

[0077] Example 7

[0078] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0079] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:1)-425.

[0080] Example 8

[0081] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1.5, and the calcination temperature was 320 °C; the remaining steps were the same as those in Example 1.

[0082] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:1.5)-320.

[0083] Example 9

[0084] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1.5, and the calcination temperature was 350 °C; the remaining steps were the same as those in Example 1.

[0085] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:1.5)-350.

[0086] Example 10

[0087] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1.5, and the calcination temperature was 400 °C; the remaining steps were the same as those in Example 1.

[0088] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:1.5)-400.

[0089] Example 11

[0090] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:1.5, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0091] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:1.5)-425.

[0092] Example 12

[0093] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:2, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0094] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:2)-425.

[0095] Example 13

[0096] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:3, and the calcination temperature was 400 °C; the remaining steps were the same as those in Example 1.

[0097] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:3)-400.

[0098] Example 14

[0099] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:3, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0100] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:3)-425.

[0101] Example 15

[0102] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:4, and the calcination temperature was 400 °C; the remaining steps were the same as those in Example 1.

[0103] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:4)-400.

[0104] Example 16

[0105] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:4, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0106] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:4)-425.

[0107] Example 17

[0108] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:6, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0109] Halloysite nanotubes with a large specific surface area were obtained and labeled as M-HNTs(1:6)-425.

[0110] Example 18

[0111] In this example, the mass ratio of halloysite nanotubes to potassium acetate was 1:10, and the calcination temperature was 425 °C; the remaining steps were the same as those in Example 1.

[0112] Halloysite nanotubes with large specific surface area were obtained and labeled as M-HNTs(1:10)-425.

[0113] Example 19

[0114] In this embodiment, the mass ratio of halloysite nanotubes to potassium acetate is 1:20, and the calcination temperature is 425° C.; the remaining steps are the same as those in Example 1.

[0115] Halloysite nanotubes with large specific surface area were obtained and labeled as M-HNTs(1:20)-425.

[0116] Comparative Example 1

[0117] In this comparative example, the halloysite nanotubes used in step S1 of Example 1 were directly used, and were labeled as HNTs.

[0118] Comparative Example 2

[0119] This comparative example uses halloysite nanotubes treated with hydrochloric acid, and the treatment process is as follows:

[0120] 1 g of the halloysite nanotubes used in step S1 of Example 1 was placed in a tube furnace at 320° C. and calcined for 3 h in air atmosphere, then purified with 4 M hydrochloric acid, kept in a constant temperature water bath at 70° C. for 3 h, and freeze-dried to obtain a sample.

[0121] The obtained products were labeled as HCl-HNTs.

[0122] The halloysite nanotubes obtained in Examples 1 to 19, Comparative Example 1, and Comparative Example 2 were subjected to BET tests. The results are shown in Table 1.

[0123] Table 1 BET analysis of halloysite nanotubes of Examples 1 to 19, Comparative Example 1, and Comparative Example 2

[0124]

[0125]

[0126] It can be seen from Table 1 that, first, with the increase of temperature and the increase of potassium acetate dosage, the specific surface area and pore volume (total pore volume) of halloysite nanotubes gradually increase. When the ratio of halloysite nanotubes to acetic acid increases to 1:10, the specific surface area increases to the maximum value. The maximum specific surface area and pore volume are 1003.466 m 2 g -1 and 1.2546m 3 g -1, which are 27 times and 5.4 times that of the original halloysite nanotubes (HNTs in Comparative Example 1), respectively. Continuing to increase the dosage of potassium acetate, the intercalation space within the halloysite nanotubes reaches saturation, and the specific surface area remains unchanged. This indicates that calcining potassium acetate-intercalated halloysite nanotubes can significantly increase the specific surface area and pore volume of halloysite nanotubes.

[0127] As Figure 1 shown, the N2 adsorption-desorption isotherm and BJH pore size distribution diagram of HNTs in Comparative Example 1 and M-HNTs(1:4)-425 in Example 16 are presented.

[0128] From Figure 1 it can be seen that by characterizing the specific surface area and pore size changes of HNTs and M-HNTs(1:4)-425 through the BET gas adsorption method, Figure 1 are the N2 adsorption / desorption curves of HNTs and M-HNTs and the corresponding pore size distributions. As can be seen from a and b in the figure, a hysteresis loop appears at a relative pressure of 0.6 - 1.0, which is a typical type IV isotherm. Therefore, the halloysite nanotubes have a mesoporous structure. According to the BET test results, the specific surface area of HNTs is 37.270 m 2 g -1 , while the specific surface area of the modified halloysite nanotubes M-HNTs(1:4)-425 is 749.022 m 2 g -1 , so the specific surface area increases significantly. Using the BJH calculation method to analyze the pore size distribution, the pore volume of HNTs is 0.2323 cm 3 g -1 , and the pore diameter is 3.817 nm; while the pore volume of M-HNTs(1:4)-425 is 1.0760 cm 3 g -1 , and the pore diameter (hole diameter) is 3.814 nm. The increase in pore volume is beneficial for the adsorption and encapsulation of ammonia borane.

[0129] As Figure 2 shown, the infrared absorption spectra of HNTs in Comparative Example 1, M-HNTs(1:1)-320 in Example 4, M-HNTs(1:1)-350 in Example 5, M-HNTs(1:1)-400 in Example 6, and M-HNTs(1:1)-425 in Example 7 are presented.

[0130] From Figure 2 it can be seen that for the infrared spectrum of HNTs in Comparative Example 1, the absorption peaks at 467 cm -1 and 910 cm -1 can be attributed to the stretching vibration and bending vibration of Al-OH. At 532 cm -1 and 753 cm -1The absorption peak at [location] is attributed to the bending vibration of Si-O and the vertical stretching of Si-O-Al, 753 cm -1 The intensity of the absorption peak at [location] weakens. It is possible that part of Al is etched away by potassium acetate (KAc) during the heating process. The absorption peaks at 3625 cm -1 and 3699 cm -1 are the characteristic peaks of the outer and inner hydroxyl groups on the wall of halloysite nanotubes respectively. After modification, these two characteristic peaks of halloysite nanotubes weaken, indicating that molten potassium acetate reacts with the wall of halloysite nanotubes under high-temperature conditions.

[0131] As Figure 3 shown, they are the TEM detection images of HNTs in Comparative Example 1, M-HNTs(1:1)-425 in Example 7, and HCl-HNTs in Comparative Example 2.

[0132] Figure 3 a, Figure 3 b are untreated halloysite nanotubes (HNTs in Comparative Example 1). It can be seen that the structure of the original halloysite nanotubes is a hollow tubular shape, the tube wall is relatively smooth, the tube length is about 500 nm, the tube diameter is about 20 nm, and the dispersion degree of halloysite nanotubes is not very ideal. Figure 3 c, Figure 3 In the TEM images of M-HNTs (M-HNTs(1:1)-425 in Example 7) in d, the halloysite nanotubes show a single-tube dispersion state, the tube length decreases, there are broken tubes, most of them are 100 - 300 nm, the interlayer structure of the M-HNTs tube wall is more obvious, the tube wall surface becomes rough, and it can be seen that there are etched defects on the tube wall. Figure 3 e, Figure 3 f are halloysite nanotubes treated only with hydrochloric acid (HCl-HNTs in Comparative Example 2). It can be seen that the dispersibility of HCl-HNTs is still not very ideal, and the change in the tube wall is not as obvious as that of the halloysite nanotubes modified by this method (M-HNTs), which can also prove the success of modifying halloysite nanotubes by this method.

[0133] As Figure 4 shown, they are the contact angle test result images of HNTs in Comparative Example 1 and M-HNTs(1:0.6)-400 in Example 3.

[0134] To test the original halloysite nanotubes (HNTs in Comparative Example 1, Figure 4 a diagram of [specific content]) and the modified halloysite nanotubes (M-HNTs(1:0.6)-400 in Example 3, Figure 4Figure b) Hydrophilicity, and its contact angle was measured. It can be seen that the contact angle of M-HNTs(1:0.6)-400 in Example 3 became 0 within 33 ms, while that of HNTs took a longer time of 333 ms to become 0. Therefore, it can be seen that M-HNTs has stronger hydrophilicity.

[0135] As Figure 5 shown, the XRD patterns of HNTs in Comparative Example 1, M-HNTs(1:1)-320 in Example 4, M-HNTs(1:1)-350 in Example 5, M-HNTs(1:1)-400 in Example 6, and M-HNTs(1:1)-425 in Example 7 are presented.

[0136] It can be Figure 5 seen that the peaks of the original halloysite nanotubes (HNTs in Comparative Example 1) correspond to those of the standard card PDF#09-0453, and the peak positions are 11.79°, 20.07°, 24.5°, 35.02°, 62.68°, and 73.79° respectively. The M-HNTs (M-HNTs(1:1)-320 in Example 4, M-HNTs(1:1)-350 in Example 5, M-HNTs(1:1)-400 in Example 6, M-HNTs(1:1)-425 in Example 7) also correspond to the peak positions on the standard card. However, the peak of M-HNTs at 20.07° shows a leftward shift, indicating that intercalation calcination can increase the layer spacing of halloysite nanotubes.

[0137] As Figure 6 shown, the isoelectric point (PZC) curve graphs of HNTs in Comparative Example 1 and M-HNTs(1:1)-400 in Example 6 are presented.

[0138] It can be Figure 6 seen that by using the isoelectric point (PZC) test method, the pH pzc of HNTs (HNTs in Comparative Example 1) can be obtained as 2.67, and the isoelectric point pH pzc of M-HNTs (M-HNTs(1:1)-400 in Example 6) is 5.1. The pH pzc value of M-HNTs is greater than that of HNTs, indicating that the molecular structure of M-HNTs is more stable, with better hydrophilicity and greater stability in aqueous solution.

[0139] Methylene blue adsorption

[0140] Weigh 10 mg of the original halloysite nanotubes (HNTs in Comparative Example 1) and M-HNTs(1:1)-400 obtained in Example 6 respectively, and place them in a prepared methylene blue solution (20 mL) with a concentration of 50 mg L -1 and shake overnight.

[0141] After oscillation and standing for a period of time, the supernatant was taken, and then the absorbance was measured by ultraviolet-visible spectroscopy to compare the removal rates of methylene blue dye by HNTs and M-HNTs(1:1)-400.

[0142] As Figure 7 shown, the relevant results of the treatment of methylene blue by HNTs of Comparative Example 1 and M-HNTs(1:1)-400 of Example 6 are shown. For the comparison of the adsorption performance of methylene blue dye, the results are shown in Figure 7 a; the removal rates of methylene blue are shown in Figure 7 b.

[0143] From Figure 7 the ultraviolet absorption curve of a, it can be seen that the maximum absorption wavelength of methylene blue with a concentration of 50 mg L -1 is at 664 nm. The adsorption performance of halloysite nanotubes intercalated with potassium acetate and calcined (M-HNTs(1:1)-400 of Example 6) for methylene blue is greatly improved. The concentration of methylene blue adsorbed by the original halloysite nanotubes (HNTs of Comparative Example 1) was calculated to be 35.80 mg L -1 through the standard curve of methylene blue, while the concentration of methylene blue adsorbed by M-HNTs was 0.775 mg L -1 . According to the removal rate (R, %) formula: R = 100(C0 - C e ), where C0 is the concentration before adsorption and C e is the concentration after adsorption equilibrium, the removal rate of HNTs for methylene blue was calculated to be 23%, and the removal rate of M-HNTs for methylene blue was 98.45%. The results are shown in Figure 7 b. It can be concluded that the removal rate of methylene blue by the modified halloysite nanotubes is greatly improved.

[0144] Ammonia borane encapsulation and hydrogen production performance

[0145] ① The samples used were HNTs obtained from Comparative Example 1, M-HNTs(1:1)-320 obtained from Example 4, M-HNTs(1:1)-350 obtained from Example 5, M-HNTs(1:1)-400 obtained from Example 6, and M-HNTs(1:1.5)-320 obtained from Example 8.

[0146] ②Ammonia borane encapsulation: Weigh 50 mg of M-HNTs(1:1)-400 and add it to 5 mL of tetrahydrofuran (THF). Ultrasonicate for 10 min to disperse evenly, denoted as A. Then add 25 mg of AB to 5 mL of THF and ultrasonically disperse it into a 0.162 M solution, denoted as B. Add A to B, mix and ultrasonicate for 30 min, then stir at room temperature for 4 - 6 h, denoted as C. Finally, place C in a vacuum drying oven at 30 °C to evaporate to dryness, and the THF solvent gives the dry sample AB@M-HNTs(1:1)-400.

[0147] ③Similarly, use HNTs, M-HNTs(1:1)-320, M-HNTs(1:1)-350, M-HNTs(1:1.5)-320 to perform ammonia borane encapsulation according to the above steps, and successively obtain AB@HNTs, AB@M-HNTs(1:1)-320, AB@M-HNTs(1:1)-350, AB@M-HNTs(1:1.5)-320.

[0148] ④Thermogravimetric analysis was used to measure the TG curves and DTG curves of AB, AB@HNTs, and AB@M-HNTs(1:1.5)-320, and compare the mass losses of the samples during the temperature ranges of 30 - 350 °C and 30 - 800 °C, so as to obtain the initial hydrogen release temperature and the encapsulation rate of ammonia borane for the samples respectively.

[0149] As Figure 8 shown, they are the TG curve and DTG curve of the ammonia borane encapsulated by HNTs of Comparative Example 1 and M-HNTs(1:1.5)-320 of Example 8; the TG curve is shown in Figure 8 a, and the DTG curve is shown in Figure 8 b.

[0150] When the sample is heated in the temperature range of 30 °C to 350 °C, it will decompose to produce gas, so the mass will decrease, that is, the weight loss of the sample is the amount of gas produced. However, the weight loss of pure ammonia borane (AB) is much greater than the theoretical hydrogen production amount. Therefore, other impurity gases (ammonia, diborane, borazine) escape along with the hydrogen release. At about 150 °C, AB will produce toxic gases such as ammonia. As can be seen from the Figure 8 TG curve in a, the initial decomposition temperature of the composite material encapsulating AB decreases, and the modified halloysite nanotubes (AB@M-HNTs(1:1.5)-320) effectively inhibit the thermal decomposition of AB at about 150 °C, indicating that encapsulating AB in M-HNTs can enhance the practical application effect of hydrogen production by AB thermal decomposition. Differentiate the curve of pure AB decomposition once to obtain the DTG curve, as shown in Figure 8As shown in Fig. b, it can be obtained that the first-step decomposition of AB starts at about 106 °C and then the second-step decomposition occurs at 115 °C. After AB is encapsulated in HNTs, the initial decomposition temperature of AB@HNTs is about 87 °C, which is significantly lower than that of pure AB. Moreover, Figure 8 The thermogravimetric curve in Fig. a shows that the weight loss of AB@HNTs (21.55%) is less than that of pure AB (52.9%). From Figure 8 the DTG curve in Fig. b, it can be seen that by encapsulating AB with halloysite nanotubes intercalated and calcined with potassium acetate, the initial decomposition temperature of AB@M-HNTs(1:1.5)-320 is about 50 °C, which greatly reduces the initial decomposition temperature of AB, and the weight loss of AB@M-HNTs(1:1.5)-320 (12.76%) is much lower than that of AB. This indicates that M-HNTs can effectively reduce the initial decomposition temperature of AB.

[0151] As Figure 9 shown, it is the thermogravimetry-mass spectrometry (TG-MS) curve of the hydrogen generation from the thermal decomposition of the encapsulated ammonia borane of HNTs in Comparative Example 1 and M-HNTs(1:1.5)-320 in Example 8.

[0152] From Figure 9 the TG-MS curve of hydrogen in it, it can be seen that the first-step decomposition of pure AB to generate hydrogen starts at about 100 °C, while the initial decomposition temperature of hydrogen generation of halloysite nanotubes intercalated and calcined with potassium acetate (AB@M-HNTs(1:1.5)-320) is about 50 °C, which is consistent with the Figure 8 results in it, indicating that M-HNTs can effectively reduce the hydrogen release temperature, and the defects and pores formed by the halloysite nanotubes intercalated and calcined with potassium acetate can increase the encapsulation amount of AB.

[0153] The encapsulation effects of halloysite nanotubes HNTs and modified halloysite nanotubes M-HNTs on AB are shown in Table 2.

[0154] Table 2 Encapsulation rate of modified halloysite nanotubes on ammonia borane

[0155]

[0156] It can be seen from Table 2 that the encapsulation amount of ammonia borane by halloysite nanotubes intercalated and calcined with potassium acetate (M-HNTs) is significantly improved.

[0157] From the above data, it can be known that: the halloysite nanotubes intercalated and calcined with potassium acetate (M-HNTs) can still maintain the tubular structure and morphology. The surface of the tube wall is rough, and a porous structure formed by thermal calcination is formed between layers, and the pore volume increases to 0.4965 - 1.0760 m 3 g -1, the specific surface area increases to 171.808 - 749.022 m 2 g -1 . In addition, with the increase of the dosage of potassium acetate and temperature, the specific surface area gradually increases. Comparing the original halloysite nanotubes (HNTs) and the halloysite nanotubes treated with hydrochloric acid under the same conditions only (HCl-HNTs), the surface and the interlayer are smooth, and the pore volume and specific surface area are small, which are 0.232 / 0.3919 m 3 g -1 and 37.270 / 98.737 m 2 g -1 . M-HNTs can significantly improve its adsorption of methylene blue, improve its encapsulation of ammonia borane and reduce the temperature of hydrogen production by thermal decomposition of ammonia borane.

[0158] The technical solution of the present invention is not limited to the limitations of the above specific embodiments. Any technical deformation made according to the technical solution of the present invention falls within the protection scope of the present invention.

Claims

1. A preparation method of halloysite nanotubes with a large specific surface area, characterized in that: It includes the following steps: S1. Prepare potassium acetate-halloysite nanotube composite Mix and grind halloysite nanotubes with potassium acetate, add water, perform ultrasonic treatment and then let it stand; subsequently, add ethanol, stir and wash, separate the solid from the liquid, and dry the solid to obtain potassium acetate-halloysite nanotube composite; S2. Calcinate potassium acetate-halloysite nanotube composite Calcinate potassium acetate-halloysite nanotube composite to obtain the calcined potassium acetate-halloysite nanotube composite; S3. Treat the calcined potassium acetate-halloysite nanotube composite obtained in step S2 with hydrochloric acid solution, and then wash and dry to obtain halloysite nanotubes with a large specific surface area; In step S2, the calcination temperature is 320 - 425 °C.

2. The preparation method of halloysite nanotubes with a large specific surface area according to claim 1, characterized in that: In step S1, the weight ratio of halloysite nanotubes to potassium acetate is 1:0.6 - 20.

3. The preparation method of halloysite nanotubes with a large specific surface area according to claim 1, characterized in that: In step S1, the standing time is 2 - 7 days.

4. The preparation method of halloysite nanotubes with a large specific surface area according to claim 1, characterized in that: In the step S2, the heating rate of the calcination is 5 °C / min -1 .

5. The preparation method of halloysite nanotubes with a large specific surface area according to claim 1, characterized in that: In step S3, the process of treating with hydrochloric acid solution is to mix the calcined potassium acetate-halloysite nanotube composite with hydrochloric acid solution and then place it in a constant temperature water bath.

6. The preparation method of halloysite nanotubes with a large specific surface area according to claim 1, characterized in that: In step S3, the drying process adopts freeze-drying.

7. Halloysite nanotubes with a large specific surface area, characterized in that: Prepared by the preparation method according to any one of claims 1 - 6.

8. Application of the halloysite nanotubes with a large specific surface area prepared by the preparation method according to any one of claims 1 - 6 in methylene blue adsorption.

9. Application of the halloysite nanotubes with a large specific surface area prepared by the preparation method according to any one of claims 1 - 6 in ammonia borane encapsulation and pyrolysis hydrogen production.

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