Tetragonal system germanium dioxide and hydrothermal synthesis method thereof

Abstract

The invention discloses tetragonal system germanium dioxide and a hydrothermal synthesis method thereof. According to the method, hexagonal crystal system germanium dioxide is taken as a raw material, a hydrothermal reaction is carried out in an acidic aqueous solution, and efficient conversion from a hexagonal crystal system to a tetragonal crystal system is realized by controlling the reaction temperature to be 120-240 DEG C, the reaction time to be 24-48 hours and the pH value to be 1-3. According to the preferable scheme, the raw materials are subjected to calcination pretreatment at the temperature of 500-600 DEG C, the yield of the tetragonal crystal phase can be remarkably increased, and the synthesis temperature can be reduced to 120 DEG C. According to the invention, stable synthesis of tetragonal system germanium dioxide is realized for the first time under the mild condition of 240 DEG C and below, and the technical problem that high-temperature transformation at 1000 DEG C or above is needed in the traditional method is solved. The tetragonal crystal system germanium dioxide prepared by the method is of a regular polyhedral structure, has the particle size of 400-500nm, and can be used as a high-efficiency catalyst for polyester synthesis. The process is energy-saving and environment-friendly, the total utilization rate of the raw materials is increased by recycling the waste liquid, and the process has remarkable industrial application value.

Description

Technical Field

[0001] This invention relates to a tetragonal germanium dioxide and its hydrothermal synthesis method, belonging to the interdisciplinary field of inorganic functional material synthesis technology and catalytic chemistry. Background Technology

[0002] Polyethylene terephthalate (PET), as an important polymer material, is highly dependent on the performance of its catalyst during synthesis. An ideal catalyst should possess high catalytic activity, excellent stability, few side reactions, and the ability to produce high-molecular-weight, high-quality polyester. Among numerous catalysts, germanium dioxide (GeO2) is widely recognized as an excellent catalyst because it effectively meets the above requirements, resulting in PET products with good color and high quality. However, germanium dioxide, as a precious metal oxide, is expensive, significantly increasing the production cost of PET. Therefore, developing more efficient and economical germanium dioxide catalysts to improve its cost-effectiveness and utilization rate has become an important and clear research direction in this field.

[0003] Germanium dioxide exists primarily in two crystal structures: hexagonal (α-Quartz type) and tetragonal (rutile type). These two crystal forms differ significantly in their physicochemical properties due to their different atomic arrangements, which may in turn affect their catalytic performance. Currently, most commercially available germanium dioxide catalysts are hexagonal. However, literature and theoretical studies suggest that tetragonal germanium dioxide may exhibit superior catalytic activity, offering the potential to replace or surpass traditional hexagonal catalysts in PET synthesis.

[0004] However, the preparation of tetragonal germanium dioxide faces significant challenges. Traditional synthesis methods typically require extremely high temperatures (above 1000℃) through a solid-state phase transformation of hexagonal germanium dioxide. This method suffers from inherent drawbacks such as high energy consumption, demanding equipment requirements, and high costs, severely restricting the commercial production and application of tetragonal germanium dioxide. Therefore, exploring a mild, low-energy synthesis route to lower the preparation threshold of tetragonal germanium dioxide has become a pressing technical bottleneck in this field. Summary of the Invention

[0005] To address the problems of existing methods for synthesizing tetragonal germanium dioxide, such as extremely high reaction temperatures (>1000℃), high energy consumption, high cost, low yield, uncontrollable morphology, and insufficient raw material utilization in existing hydrothermal methods, this invention provides a tetragonal germanium dioxide and its hydrothermal synthesis method.

[0006] Specifically, the present invention can solve the following technical problems: how to achieve stable synthesis of tetragonal germanium dioxide without relying on ultra-high temperature solid-state transformation; how to significantly improve the yield of tetragonal germanium dioxide and control its formation of regular micromorphology through process optimization; and how to achieve recycling of raw materials in the reaction system to reduce production costs and waste emissions.

[0007] The method for preparing tetragonal germanium dioxide provided by this invention includes the following steps: S1. Disperse hexagonal germanium dioxide raw material in an acidic aqueous solution to form a reaction mixture; S2. The reaction mixture is placed in a hydrothermal reactor to carry out a hydrothermal reaction to obtain the final product; The conditions for the hydrothermal reaction are as follows: The reaction temperature is 120℃-240℃, the reaction time is 24-48h, and the pH value is 1-3.

[0008] Preferably, before dispersing the hexagonal germanium dioxide raw material in the acidic aqueous solution, the method further includes a step of calcining the hexagonal germanium dioxide raw material for pretreatment. The calcination pretreatment conditions are calcination at 500-600℃ for 4-8 hours, preferably calcination at 550℃ for 6 hours; The calcination pretreatment step can effectively change the crystal defects and reactivity of the raw materials: on the one hand, it can significantly improve the yield of tetragonal products (e.g., from 42.35% to 61.63%); on the other hand, it can significantly reduce the temperature required for hydrothermal reaction, and tetragonal germanium dioxide can be successfully synthesized even at lower temperatures of 120℃-180℃, although the yield decreases with decreasing temperature.

[0009] Preferably, the conditions for the hydrothermal reaction are as follows: The reaction temperature was 240℃, the reaction time was 36h, and the pH value was 3. By precisely controlling the reaction time and solution pH, the morphology and size of the product can be controlled. A reaction time that is too short (e.g., 12 hours) will not form a tetragonal crystal phase. Extending the time to 36 hours will yield a regular polyhedral structure, while further extending it to 48 hours will result in uneven particle size. pH has a significant impact on morphology; at pH=1, long prismatic rod-like structures are easily formed, while at pH=2 and pH=3, regular dodecahedral particles are more easily formed, and the particles obtained at pH=3 are smaller and more uniform in size.

[0010] Preferably, the acidic aqueous solution is an aqueous solution of sulfuric acid.

[0011] Preferably, after the hydrothermal reaction is completed, the process further includes a waste liquid reuse step to improve the overall utilization rate of the raw materials: New hexagonal germanium dioxide raw material is added to the collected reaction waste liquid, and the hydrothermal reaction is carried out again.

[0012] The tetragonal germanium dioxide prepared by this invention has a regular polyhedral structure with a particle size of 400-500 nm.

[0013] The tetragonal germanium dioxide prepared by this invention can be used as a catalyst to catalyze the synthesis of polyester. The synthesized PET polyester has advantages such as high molecular weight, good transparency, good color, and strong thermal stability.

[0014] Compared with the prior art, the present invention has the following significant advantages: Mild reaction conditions and energy saving: The synthesis temperature of tetragonal germanium dioxide has been successfully reduced from more than 1,000°C in traditional methods to 240°C or even lower (120°C after pretreatment), which greatly reduces energy consumption and equipment requirements, laying the foundation for large-scale production.

[0015] High yield and excellent product quality: By optimizing process parameters and introducing raw material pretreatment, the yield of tetragonal germanium dioxide was increased to over 60%. The prepared product is a regular polyhedral structure with good crystallinity and high purity, with a particle size mainly distributed in the range of 400-500 nm. When used as a catalyst, it has a large specific surface area and abundant active sites.

[0016] The process is green and economical with high resource utilization: it realizes the recycling of reaction waste liquid, and after multiple cycles, it can significantly improve the total utilization rate of raw materials (the total yield reaches 76.73% after five cycles), reduce raw material waste and waste liquid discharge, and is in line with the development direction of green chemistry.

[0017] Broad application prospects: The high-quality tetragonal germanium dioxide prepared by this invention shows the potential to replace traditional catalysts in the synthesis of polyester (such as PET), and is expected to improve the quality of polyester products. Attached Figure Description

[0018] Figure 1 The effect of different temperatures (240℃, 180℃, 150℃, 120℃) on the morphology of GeO2 samples under the condition that the raw material is not calcined.

[0019] Figure 2 These are XRD characterization diagrams of GeO2 samples at different hydrothermal temperatures under the condition that the raw material is not calcined.

[0020] Figure 3 This study investigates the effects of different temperatures (240℃, 180℃, 150℃, 120℃) on the morphology of GeO2 samples after calcination of the raw materials.

[0021] Figure 4These are XRD patterns of GeO2 formed at different reaction temperatures after calcination.

[0022] Figure 5 This is a graph showing the yield of tetragonal GeO2 at different temperatures.

[0023] Figure 6 The effect of reaction time (24h, 36h, 48h) on the morphology of GeO2 samples.

[0024] Figure 7 These are XRD patterns of GeO2 formed at different reaction times.

[0025] Figure 8 This reflects the effect of pH (pH=1, pH=2, pH=3) on the morphology of GeO2 samples.

[0026] Figure 9 These are XRD patterns of GeO2 formation at different reaction pH values.

[0027] Figure 10 This is the XRD pattern of GeO2 formed by the reuse of reaction waste liquid. Figure 11 This is a graph showing the yield of tetragonal GeO2 under different reaction cycles.

[0028] Figure 12 These are FT-IR images of PET synthesized using GeO2 catalysts with different crystal forms.

[0029] Figure 13 This is a transmittance diagram of PET synthesized using GeO2 catalysts with different crystal forms.

[0030] Figure 14 This is a chromaticity diagram of PET synthesized using GeO2 catalysts with different crystal forms.

[0031] Figure 15 These are TGA spectra of PET synthesized using GeO2 catalysts with different crystal forms.

[0032] Figure 16 These are DTG spectra of PET synthesized using GeO2 catalysts with different crystal forms.

[0033] Figure 17 These are DSC spectra of PET synthesized by GeO2 catalysts with different crystal forms. Among them, (a) is the heating curve of PET1, (b) is the heating curve of PET2, (c) is the cooling curve of PET1, and (d) is the cooling curve of PET2.

[0034] Figure 18 These are XRD patterns of different PET samples. Detailed Implementation

[0035] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0036] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0037] This invention provides a mild hydrothermal method for preparing tetragonal germanium dioxide, aiming to overcome the harsh conditions and high costs of traditional high-temperature phase transformation methods for preparing tetragonal GeO2. Its features are: using hexagonal germanium dioxide as raw material, in an acidic hydrothermal system, by precisely controlling key parameters such as reaction temperature, time, and pH value, the hexagonal crystal system is transformed into the tetragonal crystal system with better catalytic activity, achieving efficient and low-temperature conversion.

[0038] A key improvement of this invention lies in the introduction of a raw material calcination pretreatment step, which significantly improves the yield of the target product and substantially reduces the critical reaction temperature required to achieve the crystal transformation. Furthermore, this invention enables the recycling of reaction waste liquid; by reusing the synthesized mother liquor in subsequent reactions, the overall utilization rate of raw materials is significantly improved, while costs and waste emissions are reduced.

[0039] The raw materials used in the following examples are as follows: hexagonal germanium dioxide was purchased from Yunnan Chihong International Germanium Co., Ltd., sulfuric acid was purchased from Sinopharm Chemical Reagent Co., Ltd., anhydrous ethanol was purchased from Beijing Chemical Plant, pH standard buffer solution (pH=4.01) was purchased from Shanghai Aladdin Chemical Reagent Co., Ltd., and pH standard buffer solution (pH=9.18) was purchased from Shanghai Aladdin Chemical Reagent Co., Ltd.

[0040] The detection methods in the following embodiments are as follows: X-ray diffraction (XRD) is primarily used to analyze the phase composition, structure, crystallinity, and unit cell parameters of a sample. Working principle: After the sample is irradiated with X-rays, the scattered waves interfere with each other, forming diffraction patterns, thus obtaining the sample's diffraction pattern. The current is 40 mA, the accelerating voltage is 40 kV, and the scanning speed is 5 ° / min, with a scanning range of 10–80 °.

[0041] Scanning electron microscope (SEM): Primarily used for observing and analyzing the morphology, size, and structure of materials. Working principle: It uses an electron beam to scan the sample surface and forms an image by detecting secondary electrons and backscattered electron signals. Sample preparation method: Take the sample and disperse it in ethanol using ultrasound. Use tweezers to select a silicon wafer and attach it to the stage. Use a dropper to pick up the ultrasound-treated sample and drop it onto the silicon wafer. After drying, spray gold onto it for analysis.

[0042] X-ray photoelectron spectroscopy (XPS): Primarily used to analyze the surface chemical composition and elemental chemical state of samples. Working principle: X-rays are used to irradiate the sample, exciting the inner-shell electrons or valence electrons of atoms or molecules, which are then emitted. Their energy is measured to obtain a photoelectron spectrum.

[0043] Using hexagonal germanium dioxide as raw material, tetragonal germanium dioxide was prepared by a hydrothermal method. The specific process is as follows: First, prepare a sulfuric acid aqueous solution with a specific pH. Weigh 0.4 g of hexagonal germanium dioxide and disperse it in 60 mL of the sulfuric acid aqueous solution, then place the solution in a hydrothermal reactor. Place the hydrothermal reactor in an electrically heated constant-temperature drying oven at the appropriate temperature for the corresponding reaction time. After the reaction is complete, remove the solution, take out the white precipitate, and place the precipitate back into deionized water. Disperse and wash the precipitate using an ultrasonic water bath. Centrifuge three times at 5000 r / min to remove any residual sulfuric acid. Then, dry the centrifuged powder in a 60 ℃ oven for 24 h to obtain the tetragonal germanium dioxide product.

[0044] Yield calculation formula:

[0045] In the formula: Y is the yield, %; m is the mass of the dried product, g; M is the mass of the added hexagonal GeO2 raw material, g.

[0046] The specific process for preparing tetragonal GeO2 by reusing waste liquid is as follows: After the tetragonal germanium dioxide preparation experiment, the reaction solution was collected, and hexagonal germanium dioxide raw material of the same mass as the product from the previous round was added back into it. After dispersion, the solution volume and pH were adjusted, and the solution was transferred into a 100 mL hydrothermal reactor. The hydrothermal reactor was placed in a 240 ℃ electric thermostatic drying oven and reacted for 36 h. The solution and precipitate were separated, and the solution was used for the next round of synthesis, while the precipitate was washed and dried.

[0047] Formula for calculating the yield of waste liquid recycling:

[0048] In the formula: Y' is the yield of this round, %; m1 is the mass of hexagonal GeO2 added in this round, g; m2 is the mass of the product in this round, g; Y1 is the yield of the first round product, %; M is the mass of hexagonal GeO2 raw material fixed in the solution, g.

[0049] Example 1: Effects of reaction temperature and raw material calcination on the synthesis of tetragonal GeO2 This embodiment studies the effects of hydrothermal reaction temperature and raw material calcination treatment on the morphology, crystal form, and yield of GeO2 synthesis.

[0050] The hydrothermal reaction was carried out for 36 hours at a pH of 3, and at controlled temperatures of 240 ℃, 180 ℃, 150 ℃, and 120 ℃. The results showed that after the hydrothermal reaction at 240 ℃, a precipitate was formed in the reactor lining, while no precipitate was formed in the reactor lining under the conditions of 180 ℃, 150 ℃, and 120 ℃. The solid sample was obtained after evaporating the solution to dryness.

[0051] To investigate the effect of reaction temperature on the morphology of synthesized GeO2, the morphology of samples under four temperature conditions was characterized by SEM, and the results are as follows: Figure 1 As shown.

[0052] from Figure 1 As shown in Figure a, at a reaction temperature of 240 ℃, the sample exhibits a polyhedral shape with a particle size of approximately 600 nm. This morphology is similar to the bulk structure of tetragonal germanium dioxide synthesized in the literature (Zou Xu. Preparation and High-Pressure Structural Phase Transition of Germanium Oxide Nanostructures [D]. Jilin: Jilin University, 2011.), thus preliminarily inferring that this sample is tetragonal germanium dioxide. At a reaction temperature of 180 ℃ ( Figure 1 (Figure b) The synthesized sample is a dried GeO2 sample with a particle size of about 1 μm, exhibiting a starfruit-like shape. The particle surface has many small protrusions and is not smooth. This morphology is similar to that of hexagonal germanium dioxide prepared by reverse micelle method in the literature (Chiu Y W, Huang M H. Formation of hexabranched GeO2 nanoparticles via a reversemicelle system[J]. The Journal of Physical Chemistry C, 2009, 113(15): 6056-6060.). Therefore, it is preliminarily speculated that the solid obtained after evaporating the reaction solution is hexagonal germanium dioxide. However, the particle size prepared in this invention is larger, which may be due to the different synthesis method and pH used. It is believed that the appearance of this morphology is due to the aggregation of many irregular particles through rapid nucleation and multi-site growth mechanism. The many small protrusions on the surface of the starfruit-like particles also corroborate this view. Figure 1Figure c shows the sample obtained by evaporating the solution at 150 ℃. This sample is spherical with severe agglomeration and a particle size of about 100 nm. This morphology and size are similar to the hexagonal GeO2 sample prepared by the reverse microemulsion method in the literature (Wu HP, Liu JF, Ge MY, et al. Preparation of monodisperse GeO2 nanoocubes in a reverse micelle system[J]. Chemistry of Materials, 2006, 18(7): 1817-1820.). However, the dispersibility of the sample prepared in this invention is poor, which may be due to the different synthesis methods. When the reaction temperature is 120 ℃, the sample is also spherical with poor dispersibility and a particle size of about 80 nm. Figure 1 As shown in Figure d, the samples prepared at 150 ℃ and 120 ℃ are presumably hexagonal germanium dioxide. This demonstrates that the temperature conditions of the hydrothermal reaction have a significant impact on the morphology of germanium dioxide, with different morphologies observed in germanium dioxide prepared under different hydrothermal temperatures.

[0053] The morphology of the samples was determined and the crystal type was preliminarily identified by SEM characterization. To further clarify the crystal forms of the four prepared samples, XRD characterization will be used for verification. The results are as follows: Figure 2 As shown.

[0054] from Figure 2 As can be seen from the graph, the precipitated sample at 240 ℃ exhibits sharp peaks, indicating good crystallinity. Three distinct strong diffraction peaks appear at 2θ = 28.674°, 37.471°, and 56.720°, while weaker diffraction peaks appear at 2θ = 40.990°, 42.907°, 59.373°, 72.345°, and 72.634°. This spectrum matches well with the PDF (ICDD 71-0651) card of tetragonal GeO2, and the absence of other impurity phases indicates good purity. The peak shapes of the samples obtained by evaporating the solution at 180 ℃, 150 ℃, and 120 ℃ were similar. Three obvious strong diffraction peaks appeared at approximately 2θ=20.559°, 25.969°, and 38.049°, while weaker diffraction peaks appeared at approximately 2θ=36.001°, 39.467°, 41.804°, 48.684°, and 58.847°. The spectrum matched well with the PDF (ICDD 85-0437) card of hexagonal GeO2 and there were no other impurity phases, indicating good purity.

[0055] Combined with scanning electron microscopy Figure 1It was found that although the samples obtained at 180 ℃, 150 ℃, and 120 ℃ were all hexagonal germanium dioxide, the morphology of the sample at 180 ℃ was different from that at 150 ℃ and 120 ℃. This indicates that the hydrothermal method can also prepare hexagonal germanium dioxide with different morphologies. In summary, tetragonal germanium dioxide can only be prepared at a reaction temperature of 240 ℃, indicating that temperature is also an important factor affecting the hydrothermal synthesis of tetragonal germanium dioxide. This is consistent with the mechanism that the crystallization process is thermodynamically controlled. The formation of tetragonal germanium dioxide requires overcoming a higher energy barrier and requires certain temperature conditions to complete the crystallization process.

[0056] This invention involves pretreating hexagonal germanium dioxide raw material by calcination at 550 °C for 6 hours, followed by a hydrothermal reaction at a controlled time of 36 hours, a pH of 3, and hydrothermal reaction temperatures of 240 °C, 180 °C, 150 °C, and 120 °C. After the reaction, precipitated samples were formed on the inner lining of the hydrothermal reactor.

[0057] To investigate the effect of reaction temperature on the morphology of synthesized GeO2 after calcination of the raw materials, the precipitated samples under four temperature conditions were characterized by scanning electron microscopy. The results are as follows: Figure 3 As shown. From Figure 3 Figure a shows the SEM image of the hydrothermal reaction at 240 ℃ after calcination. The sample is polyhedral in shape with a smooth, flat surface and a particle size of approximately 600 nm. The SEM image of the sample reacting at 180 ℃ after calcination is shown below. Figure 3 As shown in Figure b, the particles are irregularly shaped, severely aggregated, and approximately 600 nm in size. From... Figure 3 As shown in Figure c, the sample calcined at 150 °C exhibits poor dispersibility, forming lumps with a particle size of approximately 650 nm. The sample calcined at 120 °C also shows irregular lumps with agglomeration, varying particle sizes, and some incomplete lumps, such as... Figure 3 As shown in Figure d. In summary, the samples under the four calcination conditions were all blocky and had a similar morphology to the uncalcined sample at 240 °C. Therefore, it is preliminarily inferred that all four samples are tetragonal germanium dioxide.

[0058] To further verify the crystal form of the samples under four temperature conditions after calcination of the raw material, XRD characterization was performed. Hexagonal GeO2 raw material was calcined at 550 ℃ for 6 h, followed by a hydrothermal reaction at 240 ℃, 180 ℃, 150 ℃, and 120 ℃, with a hydrothermal reaction time of 36 h and a pH of 3. The XRD results of the products are shown below. Figure 4 As shown. By Figure 4It can be seen that the XRD curves of the four groups of precipitated product samples are almost identical, and the diffraction characteristic peaks are strong and sharp, indicating good crystallinity. The diffraction peak intensity of the product at 120 ℃ after calcination is weaker, which may be due to the lower reaction temperature and the lower degree of crystallinity compared to other temperature conditions. All four groups of precipitated product samples are consistent with the tetragonal germanium dioxide PDF (ICDD 71-0651) standard card at 2θ at 28.685°, 37.457°, 41.015°, 56.746°, 59.397°, 67.272°, 72.268°, and 72.663°, with no other extra peaks and high purity, indicating that the products are all tetragonal germanium dioxide. It can be seen that the operation step of calcining the raw materials can effectively reduce the temperature conditions for the hydrothermal synthesis of tetragonal germanium dioxide.

[0059] Figure 5 The graph shows the yield of tetragonal germanium dioxide produced from calcined and uncalcined raw materials at different temperatures. Figure 5 It was found that when the reaction temperature was 240 ℃, the raw material was not calcined, the reaction time was 36 h, and the reaction pH was 3, the yield of tetragonal germanium dioxide was 42.35%. Under the uncalcined conditions, no tetragonal germanium dioxide was produced at temperatures of 180 ℃, 150 ℃, and 120 ℃, so the yield was 0. Therefore, the hydrothermal reaction temperature was selected as 240 ℃. After calcining the raw material at 550 ℃ for 6 h, and controlling other hydrothermal conditions the same, the yield of tetragonal germanium dioxide synthesized at 240 ℃ was 61.63%, which was nearly 20% higher than that of the uncalcined raw material. Tetragonal germanium dioxide appeared after calcination at 180 ℃, with a yield of 48.32%. When the reaction temperature after calcination was 150 ℃, the product yield was 37.58%; when the reaction temperature after calcination was 120 ℃, the product yield was only 7.58%. The results indicate that calcination effectively increases the yield of germanium dioxide and lowers the temperature requirements for the hydrothermal reaction. However, the yield of tetragonal germanium dioxide decreases with decreasing hydrothermal temperature after calcination. It is speculated that the crystal absorbs heat and undergoes slight thermal expansion at 550 °C. Furthermore, the inherent defects in hexagonal germanium dioxide lead to varying degrees of thermal expansion, generating localized stress that damages the crystal and makes it more susceptible to phase transitions in the subsequent hydrothermal reaction. Therefore, the optimal temperature condition, 240 °C, is selected as the hydrothermal condition with the highest yield.

[0060] Example 2: Effect of reaction time on the synthesis of tetragonal GeO2 To investigate the effect of reaction time on the synthesis of tetragonal germanium dioxide, the hydrothermal reaction temperature was controlled at 240 ℃ and the hydrothermal reaction pH=2. Comparative experiments were conducted with hydrothermal reaction time gradients of 12 h, 24 h, 36 h, and 48 h. No precipitation appeared on the inner substrate at 12 h, so it was preliminarily determined that no tetragonal germanium dioxide was produced at this time. White precipitates appeared on the inner substrate of the reactor at 24 h, 36 h, and 48 h, which were preliminarily identified as tetragonal germanium dioxide. To explore the effect of hydrothermal reaction time on the morphology and structure of the formed tetragonal GeO2, the product samples were characterized using scanning electron microscopy. The results are as follows: Figure 6 As shown.

[0061] Depend on Figure 6 As shown in Figure a, when the reaction time is 24 h, the sample dispersion is poor, and agglomeration occurs. Some samples appear as complete polyhedral blocks, while others appear as polyhedral structures with missing parts. When the reaction time is 36 h ( Figure 6 (Figure b) The sample also exhibits agglomeration. The synthesized product presents a regular polyhedral structure with relatively uniform size, an average particle size of approximately 1 μm, and a smooth, flat surface. At a reaction time of 48 h, the synthesized sample also exhibits a regular polyhedral structure with a smooth, flat surface, but the particle size is not uniform, with large particles appearing, such as... Figure 6 As shown in the diagram, the hydrothermal reaction time also has a significant impact on the morphology of GeO2. The longer the hydrothermal time, the more complete and larger the tetragonal germanium dioxide particles become.

[0062] To verify the crystal form of the prepared samples, GeO2 obtained at different reaction time gradients was characterized by XRD. The results are as follows: Figure 7 As shown. From Figure 7 As can be seen, no precipitation occurred when the reaction time was 12 h. XRD analysis of the sample obtained after evaporating the solution showed results consistent with hexagonal GeO2, exhibiting three strong peaks at 2θ = 20.548°, 25.942°, and 38.020°. No other impurity phases were observed, and the peaks were sharp, indicating good crystallinity. This suggests that a hydrothermal reaction of 12 h was insufficient to synthesize tetragonal GeO2. Precipitation occurred at the inner substrate when the reaction temperature was 24 h, 36 h, and 48 h. The XRD curves of these three samples were similar and consistent with the standard spectrum of tetragonal GeO2, exhibiting three strong peaks at 2θ = 28.685°, 37.457°, and 56.746°. The peaks were sharp, and no other impurity phases were observed. This indicates that the hydrothermal reaction time plays a crucial role in the formation of tetragonal GeO2, and only a certain hydrothermal reaction time can synthesize tetragonal GeO2.

[0063] Generally, for catalytic reactions, the smaller the particle size of the catalyst, the larger the specific surface area, the more active sites it contains, and the better its catalytic performance. The product sample with a reaction time of 36 h has a more complete crystal structure than the product sample with a reaction time of 24 h. Compared with the sample with a reaction time of 48 h, the particle size is more uniform and the average particle size is smaller, which may result in better catalytic efficiency. Therefore, the optimal hydrothermal reaction time condition of 36 h was selected.

[0064] Example 3: Effect of reaction pH on the synthesis of tetragonal GeO2 To investigate the effect of pH, the hydrothermal reaction temperature was controlled at 240 ℃ and the reaction time at 36 h. A pH sequence of pH=1, pH=2, and pH=3 was designed for experimental study. Settling was observed at the bottom of the inner lining of all three experimental reactors, indicating that the sediment was tetragonal germanium dioxide.

[0065] The product samples were characterized by scanning electron microscopy. To further investigate the effect of different hydrothermal reaction pH on the morphology and structure of the formed tetragonal GeO2, the results are as follows: Figure 8 .like Figure 8 As shown in Figure a, at reaction pH=1, the sample is in the shape of a long prismatic rod, approximately 1 μm wide. At reaction pH=2, the sample exhibits poor monodispersity and obvious aggregation. The synthesized product displays a regular dodecahedral structure with relatively uniform size, an average particle size greater than 1 μm, and a relatively smooth particle surface. Figure 8 As shown in Figure b. From Figure 8 As shown in the figure, at pH 3, the synthesized sample also exhibits a regular dodecahedral structure with relatively uniform particle size, an average particle diameter of less than 1 μm, and a smooth surface. This morphological change is similar to the growth mechanism of tetragonal rutile TiO2 synthesis. The mineralizing agent used is sulfuric acid, and at pH 1, the chloride ion concentration is also higher. Literature reports on the synthesis of tetragonal rutile TiO2 have shown that chloride ions tend to adsorb onto specific crystal faces, reducing their surface energy and thus decreasing their growth rate. Therefore, in a high chloride ion environment, crystals tend to grow along other crystal faces, forming rod-like shapes. At pH 2 and pH 3, the chloride ion concentration is lower, and the effect is less pronounced, making granular formation more likely.

[0066] Figure 9 XRD patterns of GeO2 synthesized under three different pH conditions are shown below. Figure 9 The XRD diffraction peaks of the samples synthesized under three different pH conditions were observed to be sharp, with high diffraction intensity and good crystallinity. Furthermore, the diffraction peak positions of the three product samples fit well with the PDF card of tetragonal GeO2, and there were no other impurity peaks, indicating that tetragonal GeO2 was prepared under all three pH conditions.

[0067] In summary, the pH of the hydrothermal reaction also plays a significant role in the morphology of tetragonal GeO2. When the reaction pH is 3, the synthesized tetragonal GeO2 is smaller and has a larger specific surface area than the rod-shaped sample synthesized at pH 1 and the granular sample synthesized at pH 2, potentially exhibiting higher catalytic efficiency. Therefore, the optimal hydrothermal reaction pH is selected as pH 3.

[0068] Example 4: The effect of reusing reaction waste liquid on GeO2 synthesis As shown in section 10, the highest yield of tetragonal germanium dioxide (61.63%) was observed in the experiment, with nearly 40% of germanium dioxide remaining in the solution. XRD analysis of the sample obtained after evaporating the solution to dryness revealed it to be hexagonal germanium dioxide. Furthermore, since the reactant was also hexagonal germanium dioxide, to improve its utilization rate, the waste liquid was repeatedly subjected to hydrothermal reactions to investigate the effect of waste liquid reuse on the yield of tetragonal germanium dioxide. Therefore, five rounds of hydrothermal reaction experiments were conducted in this section. Figure 10 The image shows the XRD patterns of the precipitates from the five hydrothermal reactions. Figure 10 As shown, the XRD peaks of the five-stage product are relatively clear and sharp, and match well with the tetragonal germanium dioxide PDF card. There are no obvious impurity peaks, indicating that the product is relatively pure. However, with the increase of the number of reactions, the diffraction intensity of the peaks gradually decreases. This may be due to the increased number of reactions and the deterioration of the quality of the reactants, leading to an increase in internal defects in the synthesized product crystals and a decrease in crystallinity.

[0069] Figure 11 The graph shows the yield of tetragonal GeO2 at different reaction cycles. The yields of tetragonal germanium dioxide from the waste liquid of the second hydrothermal reaction in the five hydrothermal cycles were 58.70%, 33.33%, 27.93%, 17.41%, and 19.75%, respectively. With increasing reaction cycles, the overall yield of tetragonal germanium dioxide showed a decreasing trend. The yield drop was significant in the first reuse of the waste liquid (i.e., the second reaction cycle), and below 20% after three reuses (i.e., the fourth reaction cycle). This low yield indicates that the waste liquid is reusable, but the yield of tetragonal germanium dioxide from the waste liquid decreased significantly. After five reaction cycles, the total yield of tetragonal germanium dioxide was 76.73%, an increase of approximately 18% compared to the first reaction cycle, improving raw material utilization, reducing losses, and minimizing waste.

[0070] Example 5: Preparation of PET Polyester (1) Esterification reaction: Weigh 46.5 g (0.75 mol) ethylene glycol (EG) and add it to a 250 mL three-necked flask, then add 83 g (0.5 mol) terephthalic acid (PTA). Set up the esterification reaction apparatus, introduce nitrogen (N2) into the apparatus to purge the air from the system, set the rotation speed to 150 r / min, and heat under normal pressure. Observe the amount of water flowing out of the graduated cylinder. The first drop of water is taken as the starting point of the esterification reaction. When the reactants show a clear spot, the esterification reaction is considered to be over, and the esterified product BHET is discharged. During the reaction, control the temperature at the top of the fractionation column to not exceed 120 ℃.

[0071] (2) Polycondensation reaction: After weighing the product BHET from the esterification reaction, place it in a quartz liner. Weigh 0.0131 g (0.25‰) of GeO2 catalyst (hexagonal germanium dioxide raw material was pretreated by calcination at 550 °C for 6 hours, and then the hydrothermal reaction time was controlled at 36 h, the hydrothermal reaction pH=3, and the hydrothermal reaction temperature was 240 °C), and disperse it in BHET. Place the quartz liner in the reactor, seal it tightly, and build the polycondensation device. Open the inlet valve and outlet valve, introduce N2 into the device, and purge the air in the system. Close the inlet valve and outlet valve, start heating the system, and after the temperature rises to 230 °C, turn on the stirrer at a speed of 300 r / min, and turn on the vacuum oil pump and outlet valve for pre-polycondensation for 0.5 h. The vacuum degree is continuously increased, and the temperature gradually rises to 280 °C. The polycondensation reaction is carried out at this temperature for 3.5 h, then the stirring is stopped, the outlet valve is closed, and the vacuum is stopped. Open the nitrogen cylinder, open the inlet valve, and slowly introduce nitrogen gas. Discharge the material to prepare PET chips.

[0072] FT-IR analysis of PET polyester: The structures of two samples were analyzed using infrared spectroscopy. One sample was prepared using tetragonal germanium dioxide as a catalyst at a final polycondensation temperature of 280 °C and a catalyst concentration of 0.20‰. The other sample was prepared using hexagonal germanium dioxide as a catalyst at the same conditions. Their FT-IR spectra are shown below. Figure 12 As shown.

[0073] exist Figure 12 Middle, 2920 cm -1 2850 cm -1 1408 cm -1 The absorption peaks at 1715 cm⁻¹ are caused by the asymmetric stretching vibration, symmetric stretching vibration, and bending vibration of the methylene group (-CH₂-), respectively. -1 The absorption peak at 1650-1430 cm⁻¹ is a characteristic peak of the stretching vibration of the carbonyl group (C=O); -1 The absorption peaks that appear intermittently are attributed to the skeletal vibrations of substituted benzene compounds; 1236 cm⁻¹ -11090 cm -1 1016 cm -1 The absorption peaks at 725 cm⁻¹ are all caused by the stretching vibrations of the -COC- structure. -1 The peak at that point is caused by the bending vibration of the CH bond.

[0074] Depend on Figure 12 As can be seen, both samples exhibit typical infrared spectral characteristics of aromatic polyesters, and the groups present are also present in PET polyesters. Comparison of the spectra with the standard infrared spectra of PET polyester shows consistent results. Therefore, based on the FTIR spectra, it can be determined that both synthesized samples are PET polyesters.

[0075] Example 6: Evaluation of the catalytic activity of germanium dioxide with different crystal forms The experimental synthesis conditions and intrinsic viscosity measurement results of PET polyester are shown in Table 1.

[0076] The optimal final polycondensation reaction temperature of 280 °C and catalyst concentration of 0.2‰ were obtained through screening experiments for polyester synthesis. Hexagonal germanium dioxide and the tetragonal germanium dioxide prepared in this invention were selected as catalysts. Generally, for polyester catalysts, under the same conditions (i.e., the same polycondensation temperature, the same catalyst concentration, and the same polycondensation time), the higher the intrinsic viscosity of the prepared PET polyester, the higher the catalytic activity of the catalyst. Table 1 shows that, under the same reaction conditions, the intrinsic viscosity of PET1 polyester prepared using the tetragonal germanium dioxide catalyst is 0.429 dL / g, which is higher than the intrinsic viscosity of PET2 polyester prepared using hexagonal germanium dioxide as the catalyst (0.402 dL / g), and the molecular weight is also larger. This further indicates that the tetragonal germanium dioxide catalyst has higher activity than the hexagonal germanium dioxide catalyst, which is consistent with the analysis showing that tetragonal germanium dioxide has more oxygen defects than hexagonal germanium dioxide.

[0077] Table 1. PET Synthesis Conditions and Intrinsic Viscosities

[0078] Example 7: Optical property analysis of PET polyester The polyester PET1 and PET2 obtained after the final polycondensation reaction at a final polycondensation reaction temperature of 280 °C, a catalyst concentration of 0.2‰, and a polycondensation time of 3.5 h were coated onto a flat surface using a coater to form a 0.50 mm thick polyester film, with an actual thickness of 0.48 mm.

[0079] The transmittance of PET1 and PET2 was measured using a UV spectrophotometer in transmittance mode. The results are as follows: Figure 12As shown, the transmittance of PET1 is 82.3%, which is higher than that of PET2 (78.2%). This indicates that the PET polyester prepared by the tetragonal germanium dioxide catalyst has better transparency. This is presumably due to the fact that the PET polyester prepared by the tetragonal germanium dioxide catalyst is more regular, has fewer impurities, and fewer crystalline regions.

[0080] Figure 13 The chromaticity diagrams for the two PET polyesters are shown. In terms of whiteness, PET1 has an L value of 92.2, slightly better than PET2. However, in terms of yellowness, PET1 has a lower b value of 4.45, a decrease of 16.67% compared to PET2, which is significantly better. This indicates that the tetragonal germanium dioxide catalyst produces fewer side reactions, resulting in a lower content of byproducts containing yellowing groups such as aldehydes. In conclusion, PET1 prepared using tetragonal germanium dioxide as a catalyst is superior to PET2 prepared using hexagonal germanium dioxide as a catalyst in both intrinsic viscosity and chromaticity.

[0081] Example 8: Thermal Performance Analysis of PET Polyester TGA spectra can determine the mass change of PET polyester when heated. Figure 15 These are TGA spectra of PET polyester samples prepared using germanium dioxide with different crystal forms as catalysts at 30-550 °C. Figure 15 It can be seen that the thermal degradation of PET1 and PET2 polyesters is almost the same, with the degradation temperature range being basically between 380-470 ℃. The 5% weight loss temperatures of PET1 and PET2 are 396.8 ℃ and 392.9 ℃, respectively, and the maximum weight loss temperature is around 460 ℃.

[0082] Figure 16 DTG spectra of PET polyester samples prepared with germanium dioxide catalysts of different crystal forms at 30-550 °C. Figure 16 It can be seen that although the maximum weight loss rate of PET1 is slightly higher than that of PET2, the temperature at which the maximum weight loss rate of PET1 occurs is 430.8 ℃, which is higher than that of PET2 (424.4 ℃). This is because hexagonal germanium dioxide has slightly stronger degradation reactivity than tetragonal germanium dioxide. Under the influence of hexagonal germanium dioxide, PET2 underwent degradation earlier, resulting in the difference in the curves. Furthermore, the thermal weight loss of PET polyester is due to the volatilization of small molecules remaining in the polyester itself and the thermal breakage of ester bonds, generating terminal carboxyl and hydroxyl groups, which in turn lead to the volatilization of small molecule products. The higher temperature at which the maximum weight loss rate of PET1 occurs, and the combination... Figure 15 It can be seen that the 5% weight loss temperature of PET1 is higher than that of PET2, and PET1 has a greater final residual mass, both indicating that PET1 polyester has a more stable structure and better stability. At the same time, it has fewer small molecule impurities remaining, indicating that PET1 polyester has fewer impurities, which is consistent with the light transmittance test results.

[0083] The DSC curve can intuitively reflect the crystallization properties, melting behavior, and melt crystallization behavior of PET samples. The glass transition temperature (T0) of the PET sample can be obtained from the DSC curve. g ), cold crystallization temperature (T) c ), melting point (T) m ), melt cooling crystallization temperature (T) mc ), enthalpy of cold crystallization (Δ H c ), enthalpy of fusion (Δ H m ), thermal crystallization enthalpy (Δ H mc ) and other characteristics.

[0084] Figure 17 The DSC spectra of PET1 prepared with tetragonal germanium dioxide as a catalyst and PET2 prepared with hexagonal germanium dioxide as a catalyst are shown at heating rates of 10 °C and cooling rates of 10 °C, respectively. Figure 17 Figures a and b in the middle are the heating curves of PET1 and PET2, respectively. Figure 17 As shown in Figures a and b, there is a peak around 70 °C, with step-like height variations on both sides of the peak. This is due to the heat absorption caused by PET1 and PET2 transitioning from a harder glassy state to a softer, more elastic state. The temperature corresponding to the midpoint of the step height variation is the glass transition temperature. Therefore, we can determine the Tg of PET1. g The T value of PET2 is 68.5 °C. g The temperature is 64.0 ℃. The T value of PET1 g T higher than PET2 g This is presumably due to the difference in intrinsic viscosity between the two types of PET polyesters. PET1 has a higher intrinsic viscosity, resulting in longer molecular chains, stronger interactions between the chains, and potentially more chain entanglement. Therefore, the chain segments have to overcome greater resistance to movement, requiring a higher temperature.

[0085] from Figure 17 As shown in Figures a and b, the DSC curve of PET1 exhibits an exothermic peak at 127.0 ℃, while that of PET2 appears at 120.9 ℃. The presence of this peak indicates the presence of amorphous PET within the original PET polyester. The temperature corresponding to this peak value is the cold crystallization temperature; under these conditions, PET releases heat from its amorphous state and rearranges to transform into a partially crystalline state. The Tc of PET2... cThe temperature is lower because PET2 has shorter molecular chains, less interaction between molecular chains, and less chain entanglement, making it easier for chain segments to move, resulting in a faster crystallization rate and thus a lower crystallization temperature. The integral area of ​​the peak corresponding to this temperature is the enthalpy of cold crystallization, so the Δcrystallization of PET1 is lower. H c The Δ value of PET2 is 38.9 J / g. H c It is 41.3 J / g.

[0086] Depend on Figure 17 As shown in Figure a, a large endothermic peak appears at 247.7 ℃. Figure 17 A large endothermic peak also appears at 240.4 ℃ in Figure b. This peak is the melting peak of the crystal, and the temperature corresponding to this peak is the melting point of polyester. PET1 has a higher melting point and a narrower peak, which to some extent indicates that PET1 has fewer impurities and is more stable. The integral area value of this peak is the enthalpy of fusion, and the Δfusion of PET1 is... H m The Δ value of PET2 is 54.5 J / g. H m It is 57.5 J / g.

[0087] Figure 17 Figures c and d show the cooling curves of PET1 and PET2, respectively. Figure 17 As can be seen from graphs c and d, there is only one peak, the temperature of which is the melt cooling crystallization temperature of PET1. mc The T value of PET2 is 194.5 °C. mc The temperature was 190.2℃. When the temperature of PET1 decreased... mc A higher T value indicates better regularity and flexibility of the PET1 molecular chain, and fewer impurities. At this temperature, PET polyester gradually arranges itself from the molten state to form regular crystals. The T value of PET1... mc A higher T value indicates a faster crystallization rate, allowing for earlier crystallization in the preparation of highly crystalline PET, shortening cooling time, and improving efficiency. Furthermore, the T value of PET1... mc The higher temperature and ability to crystallize at higher temperatures also indicate that its structure is relatively more stable than PET2 at high temperatures. The integral area of ​​this peak is the enthalpy of thermal crystallization. The ΔE of PET1... H mc The Δ value of PET2 is 48.8 J / g. H mc The thermal enthalpy of PET2 is 49.1 J / g, which is slightly higher than that of PET2, indicating that PET2 may form crystals to a greater extent.

[0088] The various characteristic parameters obtained from the DSC curves of PET1 and PET2 are summarized in Table 2. The crystallinity was calculated using Formula 4 based on the enthalpy of cold crystallization and the enthalpy of melting. The results are shown in Table 2. The crystallinity of PET1 is 11.14%, slightly lower than that of PET2 (11.57%). Generally, the lower the crystallinity of PET, the greater its transparency. The crystallinity results are also consistent with the previously measured transparency results.

[0089] Table 2 DSC analysis results of PET samples

[0090] Example 9: Analysis of the Crystallization Properties of PET Polyester Crystallization properties play an important role in subsequent processing and predicting material properties. Preliminary calculations using DSC testing showed that PET1 and PET2 had low crystallinity. However, this calculation was affected by the DSC test results and the theoretical enthalpy of melting. Therefore, XRD testing was conducted to verify the crystallization properties of the prepared PET1 and PET2. Figure 18 The XRD patterns are shown for two groups of PET polyesters and two groups of heat-treated PET polyesters.

[0091] The samples exhibited low crystallinity, predominantly consisting of amorphous polyester, consistent with DSC results. After heating both groups of polyesters at 140℃ for 0.5 h followed by slow cooling, X-ray diffraction was performed, revealing diffraction peaks. Comparison with standard PET cards showed that 16.4° and 17.5° corresponded to the 010 crystal plane, 21.6° and 22.7° to the 100 crystal plane, 25.8° to the 110 crystal plane, and 32.6° to the 021 crystal plane, indicating that amorphous polyesters can be processed into crystalline PET polyesters under certain conditions.

Claims

1. A method for preparing tetragonal germanium dioxide, comprising the following steps: S1. Disperse hexagonal germanium dioxide raw material in an acidic aqueous solution to form a reaction mixture; S2. The reaction mixture is placed in a hydrothermal reactor to carry out a hydrothermal reaction to obtain the final product; The conditions for the hydrothermal reaction are as follows: The reaction temperature is 120℃-240℃, the reaction time is 24-48h, and the pH value is 1-3.

2. The preparation method according to claim 1, characterized in that: Before dispersing the hexagonal germanium dioxide raw material in the acidic aqueous solution, the method further includes a calcination pretreatment step of the hexagonal germanium dioxide raw material; the calcination pretreatment conditions are calcination at 500-600℃ for 4-8 hours.

3. The preparation method according to claim 2, characterized in that: The hydrothermal reaction temperature is 120℃-80℃.

4. The preparation method according to any one of claims 1-3, characterized in that: The conditions for the hydrothermal reaction are as follows: The reaction temperature was 240℃, the reaction time was 36h, and the pH value was 3.

5. The preparation method according to any one of claims 1-4, characterized in that: The acidic aqueous solution is an aqueous solution of sulfuric acid.

6. The preparation method according to any one of claims 1-5, characterized in that: After the hydrothermal reaction is completed, the following waste liquid reuse step is also included: New hexagonal germanium dioxide raw material is added to the collected reaction waste liquid, and the hydrothermal reaction is carried out again.

7. Tetragonal germanium dioxide prepared by the method according to any one of claims 1-6; The tetragonal germanium dioxide has a regular polyhedral structure.

8. The tetragonal germanium dioxide according to claim 7, characterized in that: The tetragonal germanium dioxide has a particle size of 400-500 nm.

9. The use of the tetragonal germanium dioxide of claim 7 or 8 as a catalyst in the synthesis of polyesters.

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