Metal-organic gel material mog-808-pasa-5:5 for proton exchange membranes and method for its preparation

Abstract

This invention belongs to the field of energy materials and fuel cell technology, specifically relating to a metal-organic gel material MOG-808-pASA-5:5 for proton exchange membranes and its preparation method. Using Zr-based metal salts, tricarboxylic acid organic ligands, and p-aminobenzenesulfonic acid as raw materials, the metal-organic gel MOG-808 structure is induced by a solvothermal reaction in a mixed solvent system containing DMF, formic acid, and a small amount of water by controlling the feed ratio of the raw materials. Subsequently, residual reactants and weakly coordinating solvent molecules are removed through high-temperature solvent replacement and solvent exchange treatment. Finally, after vacuum drying and grinding, the p-aminobenzenesulfonic acid-functionalized metal-organic gel material MOG-808-pASA-5:5 is obtained. This invention achieves a synergistic distribution of acidic and basic sites in a flexible network, constructing a continuous and stable proton transport channel, improving the proton conduction performance of the material under low humidity and wide temperature conditions, while reducing the complexity and cost of using functionalized ligands, thus promoting the practical application of proton exchange membrane fuel cells.

Description

Technical Field

[0001] This invention belongs to the field of energy materials and fuel cell technology, specifically relating to a metal-organic gel (MOG) material and its preparation method, as well as the application of this material in the proton exchange membrane (PEM) of a hydrogen-oxygen fuel cell. Background Technology

[0002] Driven by the "dual-carbon" strategic goal, efficient and clean energy conversion and storage technologies have received widespread attention. Fuel cells, as devices that directly convert chemical energy into electrical energy, possess high energy conversion efficiency and environmental friendliness, making them valuable for applications in new energy vehicles and distributed energy systems. Among these, proton exchange membrane fuel cells (PEMFCs) are currently the most technologically mature type of fuel cell, with their performance primarily dependent on the proton conductivity and structural stability of the proton exchange membrane. Currently, the most widely used commercial proton exchange membranes are perfluorosulfonic acid polymer materials (such as Nafion membranes). These materials can achieve high proton conductivity under fully hydrated conditions, but their conduction process is highly dependent on the hydrogen bond network formed by water molecules. Under low humidity or high temperature conditions, moisture loss or rearrangement easily occurs, leading to interruption of the proton transport channel and a significant decrease in conductivity, thus limiting their practical application under a wide range of operating conditions.

[0003] In recent years, porous materials have received continuous research attention in the field of proton conduction. Among them, metal-organic frameworks (MOFs) are considered important candidate materials for constructing novel proton conduction systems due to their high specific surface area, ordered pore structure, and tunable structure. They can improve the proton migration environment through framework functionalization or pore regulation. In the development of MOF systems, metal-organic gels (MOGs), as their amorphous or low-crystallinity derivatives, have gradually attracted attention. Compared with crystalline MOFs, MOGs have a three-dimensional cross-linked flexible network structure, stronger pore continuity, and better solvent and moisture retention capabilities, which is conducive to constructing continuous hydrogen bond networks, thus providing more favorable channels for proton transport.

[0004] Currently, the main strategies for improving the performance of MOF or MOG matrix proton-conducting materials fall into two categories: one is to introduce exogenous proton-conducting components (such as inorganic acids, small organic molecules, or ionic liquids) to increase the proton carrier concentration; the other is to introduce acidic groups through framework functionalization to enhance the intrinsic proton conductivity of the material. The former is susceptible to component migration or loss, leading to insufficient stability; while the latter, although improving structural stability, often focuses on modifying a single acidic group, making it difficult to simultaneously address proton supply and the synergistic construction of the hydrogen bond network. Furthermore, the complex synthesis and high cost of some functionalized ligands limit their large-scale application. Therefore, developing a MOG matrix proton-conducting material system that combines structural stability, continuous proton transport capability, and controllable cost is of significant research importance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the technical problem to be solved by this invention is to provide a proton-conducting material based on a metal-organic gel structure and its preparation method. By introducing organic ligands containing sulfonic acid groups and amino groups during the gelation process, acidic and basic sites are synergistically distributed in a flexible network, thereby constructing a continuous and stable proton transport channel, improving the proton conduction performance of the material under low humidity and wide temperature conditions, while reducing the complexity and cost of using functionalized ligands, and promoting the practical application of proton exchange membrane fuel cells.

[0006] The technical solution adopted by the present invention to achieve the above objectives is as follows: A method for preparing a metal-organic gel proton-conducting material MOG-808-pASA-5:5 is disclosed. Using Zr-based metal salts, tricarboxylic acid organic ligands, and p-aminobenzenesulfonic acid as raw materials, the method involves controlling the feed ratio of the raw materials and inducing the formation of a metal-organic gel MOG-808 structure through a solvothermal reaction in a mixed solvent system containing DMF, formic acid, and a small amount of water. Subsequently, residual reactants and weakly coordinating solvent molecules are removed through high-temperature solvent replacement and solvent exchange treatments. Finally, the material is vacuum dried and ground to obtain the p-aminobenzenesulfonic acid-functionalized metal-organic gel material MOG-808-pASA-5:5.

[0007] The proton conduction mechanism of MOG-808-pASA-5:5 is as follows: On the one hand, the sulfonic acid groups in the material have strong acidity and can act as proton donors, promoting proton dissociation and providing the main transport carrier under certain humidity conditions; on the other hand, multiple hydrogen bonds can be formed between amino groups, sulfonic acid groups, and water molecules, which is conducive to building a continuous hydrogen bond network structure, thereby providing channels for the rapid migration of protons in the gel framework. Simultaneously, the flexible three-dimensional cross-linked structure of MOG has strong solvent and moisture retention capabilities, maintaining a stable local aqueous environment, reducing the energy barrier during proton migration, and achieving efficient proton conduction.

[0008] This invention also includes a method for preparing MOG-808-pASA-5:5, the preparation steps of which include: (1) Weigh ZrOCl2·8H2O, pyromellitic acid and p-aminobenzenesulfonic acid precisely according to the set molar ratio and add them to a mixed solvent composed of DMF, formic acid and distilled water. Sonicate the mixture to disperse it fully and form a homogeneous solution system. (2) The resulting mixed solution was transferred to a polytetrafluoroethylene-lined high-pressure reactor and heated at 100 °C for 2 h to induce the formation of a metal-organic gel structure; (3) The obtained gel material was added to DMF for high-temperature solvent replacement treatment, kept at 120 °C and centrifuged and washed, and repeated several times to remove unreacted precursors and residual solvents. (4) Use ethanol for secondary solvent exchange treatment, and repeat the replacement and centrifugation process at 60 °C to further improve the purity and structural stability of the pores; (5) The final product was dried under vacuum at 70 °C to obtain MOG-808-pASA-5:5 powder material, which was then ground for use.

[0009] Furthermore, by adjusting the feeding ratio of p-aminobenzenesulfonic acid to pyromellitic acid in the metal-organic gel framework precursor, the distribution density and interaction degree of acidic and basic groups in the material can be controlled, thereby achieving the control of the continuity of the proton conduction path and the conduction efficiency, so as to obtain MOG matrix proton conductive materials with different proton conductivity properties.

[0010] The beneficial effects of this invention are: This invention introduces organic ligands containing sulfonic acid and amino groups during the formation of the MOG structure, achieving the synergistic construction of acidic and basic sites in a flexible gel network, which is beneficial for forming continuous and stable hydrogen bond transport channels. At the same time, the metal-organic gel structure has a strong water retention capacity, which can maintain a stable local proton conduction environment. In addition, the synthesis steps of this method are relatively simple, which helps to reduce the dependence on functionalized ligands and the cost of material preparation, thereby enhancing the application potential of the material in proton exchange membrane fuel cells. Attached Figure Description

[0011] Figure 1 The X-ray powder diffraction pattern of the synthesized product in the embodiments of the present invention; Figure 2-5 The impedance energy quister plot of the synthesized product MOG-808-pASA-5:5 in Example 2 of this invention at 30-100% RH; Figure 6This is a humidity dependence plot of the synthesized product MOG-808-pASA-5:5 in Example 2 of the present invention; Figure 7 The impedance energy quister plot of the synthesized product MOG-808-pASA-4:6 in Example 3 of this invention at 100% RH; Figure 8 The impedance energy quister plot of the synthesized product MOG-808-pASA-5:5-anhydrous in Example 3 of this invention at 100% RH; Figure 9-12 This is a line graph comparing the conductivity of the synthesized products in the embodiments of the present invention; Figure 13 This is a stability test diagram of the synthesized product MOG-808-pASA-5:5 in Example 2 of the present invention; Detailed Implementation The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0012] Example 1 A method for preparing MOG-808-pASA-5:5 proton-conducting material, which is prepared by a one-step metal-organic gel formation and post-treatment solvent replacement method.

[0013] The specific implementation steps are as follows: At room temperature, 0.0033 mol ZrOCl2·8H2O, 0.0024 mol p-aminobenzenesulfonic acid (pASA), and 0.0024 mol trimellitic acid were accurately weighed and placed in a beaker (total molar volume: 4.8 mmol, molar ratio: 5:5). A mixed solvent system consisting of 10 mL N,N-dimethylformamide (DMF), 10 mL formic acid, and 0.3 mL distilled water was added, and the mixture was sonicated for 30 min to ensure thorough dispersion and form a homogeneous solution. The resulting mixed solution was transferred to a 100 mL polytetrafluoroethylene-lined high-pressure reactor and heated at 100 °C for 2 h to induce the formation of a metal-organic gel structure, MOG-808. After the reaction, twice the gel volume of DMF was added to the resulting gel material, and the mixture was subjected to high-temperature solvent displacement at 120 °C for 5 h, followed by centrifugation. This process was repeated three times to remove residual reactants and weakly coordinating solvent molecules. Subsequently, ethanol was used as the replacement solvent, and the solvent exchange step was repeated three times at 60 °C to further improve the pore purity and structural stability. Finally, the obtained product was collected by centrifugation and dried under vacuum at 70 °C to obtain the MOG-808-pASA-5:5 sample, which was then ground for later use.

[0014] Example 2 A method for preparing MOG-808-pASA-4:6 proton-conductive material is disclosed, which controls the distribution of acidic and basic groups in the material structure by adjusting the feeding ratio of p-aminobenzenesulfonic acid and pyromellitic acid.

[0015] The specific preparation steps are as follows: Accurately weigh 0.0033 mol ZrOCl2·8H2O, 0.00192 mol p-aminobenzenesulfonic acid (pASA), and 0.00288 mol trimellitic acid (total molar amount 4.8 mmol, molar ratio 4:6) and place them in a beaker. Add a mixed solvent system consisting of 10 mL N,N-dimethylformamide (DMF), 10 mL formic acid, and 0.3 mL distilled water. Sonicate for 30 min to ensure complete dissolution and form a homogeneous system. Then, transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined high-pressure reactor and heat at 100 °C for 2 h to induce the formation of a MOG-808 gel network structure. After the reaction, add twice the volume of DMF to the resulting gel material and perform solvent replacement at 120 °C for 5 h, followed by centrifugation. Repeat this step three times. Subsequently, perform a second solvent replacement treatment with ethanol at 60 °C three times to further purify the pore structure. The product was finally dried under vacuum at 70 °C to obtain the MOG-808-pASA-4:6 sample, which was then ground for later use.

[0016] Example 3 A method for preparing anhydrous MOG-808-pASA-5:5-proton-conductive material is disclosed, which achieves the control of the distribution of acidic and basic groups in the material structure by adjusting the feeding ratio of p-aminobenzenesulfonic acid, pyromellitic acid and deionized water.

[0017] The specific preparation steps are as follows: Accurately weigh 0.0033 mol ZrOCl2·8H2O, 0.0024 mol p-aminobenzenesulfonic acid (pASA), and 0.0024 mol trimellitic acid (total molar amount 4.8 mmol, molar ratio 5:5) and place them in a beaker. Add a mixed solvent system consisting of 10 mL N,N-dimethylformamide (DMF) and 10 mL formic acid, and sonicate for 30 min to ensure complete dissolution and form a homogeneous system. Then, transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined high-pressure reactor and heat at 100 °C for 2 h to induce the formation of a MOG-808 gel network structure. After the reaction, add twice the volume of DMF to the resulting gel material and perform solvent replacement at 120 °C for 5 h, followed by centrifugation. Repeat this step three times. Then, perform a second solvent replacement treatment with ethanol at 60 °C three times to further purify the pore structure. Finally, the product was dried under vacuum at 70 °C to obtain an anhydrous MOG-808-pASA-5:5- sample, which was then ground for later use.

[0018] Table 1: Raw material ratios for each synthesized product in Examples 1-3 Table 1 shows the feed ratios of the raw materials in the three examples. The performance of the final materials was compared by changing the ratio of pyromellitic acid and p-aminobenzenesulfonic acid in the initial synthesis, as well as whether or not water was introduced.

[0019] Figure 1 The X-ray powder diffraction (XPD) patterns of the synthesized products in Examples 1-3 are shown. A comparison of the XPD patterns of the MOG-808-pASA materials synthesized in this invention with simulated data reveals that each sample retains characteristic diffraction peaks similar to those of the simulated MOF-808 in the low-angle region (6°-10°), indicating that the synthesized MOG-808-pASA series materials generally inherit the basic topological structure of MOF-808, and the framework has not collapsed. Compared with the sharp and high-intensity characteristic peaks in the simulated spectra, the diffraction peaks of the actual synthesized samples are significantly broadened and their intensity reduced, especially exhibiting more obvious diffuse peaks in the 10°~30° range, indicating a decrease in the crystallinity of the material. This phenomenon is mainly attributed to the change in reaction conditions, which alters the crystallization rate and concentration of the seed crystals. Simultaneously, the introduction of p-aminobenzenesulfonic acid (pASA) also interferes with the ordered growth of the crystals to some extent, causing the system to tend towards a partially amorphous or low-crystallinity state. This reduction in crystallinity helps to reduce the grain boundary effect and improve the stability and conductivity of the material.

[0020] Figure 2-5 Table 2 shows the impedance energy quist plot of the synthesized product MOG-808-pASA-5:5, and its proton conductivity.

[0021] Table 2. Proton conductivity of MOF-808-pASA-5:5 The MOG-808-pASA-5:5 material exhibits relatively stable proton conductivity within the range of 303–353 K and 30%–100% RH, generally showing a pattern of "temperature and humidity synergistically promoting conductivity." Combined with... Figure 6 As can be seen from the line graph and the specific data in Table 2, under low humidity conditions (30% RH), the overall conductivity of the material remains at 10. -2 The conductivity is on the order of S / cm, indicating that the material itself possesses relatively stable proton transport capabilities. With increasing humidity, especially under 75% RH and 100% RH conditions, the conductivity significantly increases, indicating that water molecules adsorbed within the pores can form a continuous hydrogen bond network with sulfonic acid groups, thereby promoting proton migration. Specifically, at 353 K and 100% RH, the conductivity reaches 1.01 × 10⁻⁶. -1 The S / cm value is the highest for this material, indicating that high temperature and high humidity environments are more conducive to proton hopping conduction. At the same time, the conductivity changes consistently at all temperatures, without any significant abnormal attenuation, indicating that the material framework still has good structural stability and environmental adaptability under rising temperature and low humidity conditions, and can maintain a continuous and effective proton conduction channel, demonstrating good thermal and humidity stability.

[0022] Figure 7 Table 3 shows the impedance energy quister plot of the synthesized product MOG-808-pASA-4:6 at 100% RH, and its proton conductivity.

[0023] Table 3. Proton conductivity of MOF-808-pASA-4:6 from Figure 9-12 The comparison of the line graph and the data in Table 3 shows that MOG-808-pASA-4:6 exhibits a significant decrease in both overall conductivity and environmental stability compared to MOG-808-pASA-5:5. Under 30% RH conditions, the conductivity of MOG-808-pASA-5:5 consistently remains at 10%. -2 S·cm -1 Order of magnitude, reaching a maximum of 3.69 × 10 -2 S·cm -1 MOG-808-pASA-4:6 is only at level 10. -5 -10 -4 S·cm -1The conductivity range decreased by approximately 2-3 orders of magnitude; under 50% RH conditions, the conductivity of MOG-808-pASA-5:5 remained stable at 1.33 × 10⁻⁶. -2 -1.67×10 -2 S·cm -1 While the conductivity of MOG-808-pASA-4:6 increases with increasing temperature, it decreases significantly at 343 K, indicating poor stability of its proton migration channels. Under 75% RH and 100% RH conditions, the conductivity of MOG-808-pASA-4:6 increases rapidly in the high-temperature region, reaching 8.74 × 10⁻⁶ at 353 K and 75% RH. -2 S·cm -1 However, the overall conductivity fluctuates significantly, and the conductivity is low in the low-temperature region, indicating that its conductivity process is highly dependent on temperature and humidity. In contrast, MOG-808-pASA-5:5 maintains a high and relatively stable conductivity over a wide temperature and humidity range, reaching 1.01 × 10⁻⁶ at 100% RH and 353 K. -1 S·cm -1 This demonstrates superior proton transport capabilities. The proton conductivity of MOGs can be controlled by altering the ratio of pyromellitic acid to p-aminobenzenesulfonic acid in the MOG structure. The highest conductivity is achieved when the ratio of pyromellitic acid to p-aminobenzenesulfonic acid is 1:1 (i.e., MOG-808-pASA-5:5). Therefore, the proton conductivity of materials can be more precisely controlled by changing the ratio of pyromellitic acid to p-aminobenzenesulfonic acid to meet specific proton conductivity requirements, enabling the preparation of proton-conducting materials of different specifications and promoting the application and widespread use of proton exchange membrane fuel cells.

[0024] Figure 8 The impedance energy quister plot of the synthesized product MOG-808-pASA-5:5-anhydrous at 100% RH is shown in Table 4, which is a table of its proton conductivity.

[0025] Table 4. Proton conductivity of MOF-808-pASA-5:5-anhydrous Combination Figure 9-12 The contrasting line graphs show that the introduction of water molecules during the synthesis process plays a crucial role in improving the proton conductivity of the material. The conductivity of the anhydrous MOG-808-pASA-5:5 sample is significantly lower than that of MOG-808-pASA-5:5 under various humidity conditions; for example, its conductivity is only 10 ohms under 30% RH conditions. -6 -10 -3 S·cm -1 The order of magnitude, while the corresponding MOG-808-pASA-5:5 remains consistently at 10.-2 S·cm -1 Order of magnitude; under 75% RH conditions, the highest conductivity of the anhydrous sample was only 2.08 × 10⁻⁶. -3 S·cm -1 The MOG-808-pASA-5:5 can reach 6.37×10⁻⁵. -2 S·cm -1 Even under 100% RH conditions, the conductivity of the anhydrous sample was only 1.27 × 10⁻⁶. -2 S·cm -1 It is still significantly lower than the 1.01×10 of MOG-808-pASA-5:5. -1 S·cm -1 Meanwhile, the conductivity improvement of the anhydrous sample under different temperatures and humidity levels was relatively limited, indicating that the introduction of a small amount of water during the initial synthesis had a significant impact on the formation of the material's internal microstructure. In contrast, MOG-808-pASA-5:5 maintained high conductivity and good stability under various humidity conditions, suggesting that an appropriate amount of water can promote the formation of continuous proton transport channels between acidic sites, thereby improving the material's proton migration efficiency. Therefore, MOG-808-pASA-5:5 exhibits superior proton conductivity and hydrothermal stability compared to the anhydrous sample, making it more valuable for applications in proton exchange membranes and related electrochemical fields.

[0026] Table 5. Proton conductivity of MOF-808 Table 6 Proton Conductivity of MOG-808 The proton conductivity of the MOG-808-pASA-5:5 synthesized in this invention is significantly improved compared to the matrix materials MOF-808 and MOG-808 under low temperature and low humidity conditions. The conductivity of MOF-808 and MOG-808 under low temperature and low humidity conditions (303 K, 30% RH) is 1.52 × 10⁻⁶. -7 S·cm -1 and 1.57×10 -6 S·cm -1 Under the same conditions, MOG-808-pASA-5:5 achieved a proton conductivity of 3.69 × 10⁻⁶. -2 S·cm -1 This represents an improvement of 4-5 orders of magnitude. Under high temperature and humidity conditions (353 K, 100% RH), the proton conductivity of MOF-808 and MOG-808 materials is 7.38 × 10⁻⁶. -3 S·cm -1 and 1.69×10-2 S·cm -1 The proton conductivity of MOG-808-pASA-5:5 reached 1.01 × 10⁻⁶ under the same conditions. -1 S·cm -2 These figures are 13.69 times and 5.89 times higher than those of MOF-808 and MOG-808 materials, respectively.

[0027] like Figure 13 As shown, the proton conductivity stability of MOG-808-pASA-5:5 material was tested continuously for 7 days under conditions of 353 K and 100% RH. The results show that the initial proton conductivity of this material is approximately 1.01 × 10⁻⁶. -1 S·cm -1 The decrease was more pronounced in the first two days (down to approximately 8.0 × 10⁻⁶). -2 S·cm -1 (), and then rapidly decreased to about 3.5 × 10 on the 3rd day. -2 S·cm -1 After the third day, the decreasing trend of conductivity slowed significantly, and remained basically at 2~3×10 from the third to the seventh day. -2 S·cm -1 Within the specified range, the variation is relatively small, exhibiting good later-stage stability and superior final stability. This indicates that after undergoing initial structural or conductive channel rearrangement under high temperature and humidity conditions, the material gradually reaches a relatively stable conductive state.

[0028] The above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made based on the essence of the content of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a metal-organic gel proton-conducting material MOG-808-pASA-5:5, characterized in that, Using Zr-based metal salts, tricarboxylic acid organic ligands, and p-aminobenzenesulfonic acid as raw materials, the metal-organic gel MOG-808 structure was induced by a solvothermal reaction in a mixed solvent system containing DMF, formic acid, and a small amount of water by adjusting the feed ratio of the raw materials. Subsequently, residual reactants and weakly coordinating solvent molecules were removed by high-temperature solvent replacement and solvent exchange treatment. Finally, the p-aminobenzenesulfonic acid-functionalized metal-organic gel material MOG-808-pASA-5:5 was obtained by vacuum drying and grinding.

2. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 1, characterized in that, Includes the following steps: (1) Weigh ZrOCl2·8H2O, pyromellitic acid and p-aminobenzenesulfonic acid precisely according to the set molar ratio and add them to a mixed solvent composed of DMF, formic acid and distilled water. Sonicate the mixture to disperse it fully and form a homogeneous solution system. (2) The resulting mixed solution was transferred to a polytetrafluoroethylene-lined high-pressure reactor and heated to induce the formation of a metal-organic gel structure; (3) The obtained gel material was added to DMF for high-temperature solvent replacement treatment, kept warm and centrifuged and washed, and repeated several times to remove unreacted precursors and residual solvents. (4) Use ethanol for secondary solvent exchange treatment, repeat the replacement and centrifugation process to further improve the purity and structural stability of the pores; (5) The final product was dried under vacuum to obtain MOG-808-pASA-5:5 powder material, which was then ground for later use.

3. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 2, characterized in that, The molar ratio of ZrOCl2·8H2O, pyromellitic acid, and p-aminobenzenesulfonic acid is 11:6-8:8-10.

4. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 2, characterized in that, The volume ratio of DMF, formic acid, and distilled water is 1:1:0.025-0.

035.

5. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 2, characterized in that, The reaction conditions in step (2) are: heating the reaction at 100 °C for 2 h.

6. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 2, characterized in that, The insulation temperature in step (3) is 120 ℃.

7. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 2, characterized in that, The temperature of the displacement in step (4) is 60 ℃.

8. The preparation method of the metal-organic gel proton-conducting material MOG-808-pASA-5:5 according to claim 2, characterized in that, The vacuum drying temperature in step (5) is 70 °C.

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