Preparation method of biomass carbon gel material and application thereof

Abstract

The application belongs to the technical field of adsorption materials, and particularly relates to a preparation method of biomass carbon gel material and application thereof. The biomass carbon gel material has excellent methanol adsorption performance, and can realize efficient adsorption of methanol and good water molecule adsorption capacity on the basis of unique pore structure characteristics and surface functional group distribution. With the dual adsorption characteristics, the biomass carbon gel material is not only suitable for a methanol adsorption refrigeration system, but also can be applied to water adsorption refrigeration, air dehumidification, water vapor recovery and other fields, greatly improving the multifunctionality and application scenarios of the adsorption material, and providing a new technical path for development and industrial application of multifunctional adsorption materials.

Description

Technical Field

[0001] This invention relates to the field of adsorption materials technology, specifically to a method for preparing biomass carbon gel material and its application. Background Technology

[0002] With the urgent global demand for sustainable development, the development of efficient and environmentally friendly refrigeration technologies has become a major issue in the energy and environment fields. Adsorption refrigeration technology can utilize low-grade heat energy such as solar energy and industrial waste heat, and uses natural working fluids such as water, methanol, and ammonia. Therefore, it is regarded as a key path to replace traditional high-energy-consuming and high-global-warming-potential vapor compression refrigeration systems.

[0003] However, existing adsorption refrigeration technologies generally suffer from low cooling capacity and energy efficiency in practical applications. One fundamental reason for this is the limited adsorption capacity of adsorbent materials for refrigerants, which fails to meet the demands of efficient adsorption refrigeration cycles. Methanol, due to its low boiling point, high saturated vapor pressure, and small molecular size, is considered an ideal working fluid for medium- and low-temperature heat source-driven applications. However, the equilibrium adsorption capacity of commonly used adsorbent materials is generally only at the 0.1-1.4 g / g level, resulting in limited cyclic adsorption capacity that cannot support high cooling capacity output requirements. Even with improvements in specific surface area or optimization of pore structure, existing materials have not achieved a breakthrough in methanol adsorption capacity on a significant scale.

[0004] In theory, increasing the adsorption capacity of adsorbents for methanol, especially significantly improving the methanol storage capacity per unit mass of adsorbent, can substantially enhance the cooling capacity and energy efficiency of adsorption refrigeration systems without increasing system complexity. However, existing adsorbent materials still have significant shortcomings in pore structure design, adsorption site distribution, and capacity to accommodate methanol molecules, making it difficult to achieve high-density adsorption and efficient desorption of methanol. This restricts the development of adsorption refrigeration systems towards higher cooling capacity and higher energy efficiency.

[0005] From the perspective of refrigeration energy efficiency, the coefficient of performance (COP) of an adsorption refrigeration system is positively correlated with the effective adsorption capacity of the adsorbent for methanol. Because existing materials have a relatively low methanol adsorption capacity, the cooling capacity generated per unit refrigeration cycle is limited. To obtain the same cooling capacity, more driving heat is required, leading to a decrease in the overall refrigeration energy efficiency of the system.

[0006] In summary, in the methanol working fluid system, existing adsorption refrigeration systems still urgently need a new type of adsorbent material that can significantly improve the methanol adsorption capacity and break through the traditional limitations of adsorption capacity per unit mass, so as to greatly improve the cooling capacity and energy efficiency of the system without significantly increasing the system volume and energy consumption. Summary of the Invention

[0007] The main objective of this invention is to develop a biomass carbon gel material for methanol adsorption refrigeration systems. This material achieves complete dissolution of biomass using molten salt hydrate as a solvent, obtains a homogeneous solution, and then undergoes cooling gelation, controlled displacement, freeze-drying, and carbonization. This invention solves the problems of low methanol adsorption performance and low coefficient of performance (COP) of the adsorbent-methanol working fluid in existing adsorption refrigeration systems, achieving a breakthrough improvement in adsorption performance and a significant increase in COP. Furthermore, based on the porous structure and surface chemical properties of this adsorbent, the adsorbent provided by this invention can also effectively adsorb water, thereby broadening its application range and enhancing its multifunctionality.

[0008] On the one hand, this application provides a method for preparing biomass carbon gel material, which includes the following steps: (1) After mixing biomass with molten salt hydrate, heat and stir until the biomass is completely dissolved, pour into a hot mold, and cool to obtain biomass hydrogel; (2) The biomass hydrogel obtained in step (1) is placed in deionized water to displace part of the solvent, and then freeze-dried to obtain biomass aerogel; the part of the solvent refers to a solvent retention rate of 1%-3%; (3) The biomass aerogel obtained in step (2) is pretreated under a nitrogen atmosphere and then carbonized to obtain biomass carbon gel material.

[0009] Furthermore, the amount of biomass added in step (1) is 1wt%-7wt% of the total mass of the material.

[0010] Furthermore, in step (3), the aerogel carbonization temperature is 200-500 ℃ and the time is 30 min.

[0011] Furthermore, the pretreatment temperature in step (3) is 160 °C and the time is 30 min.

[0012] Furthermore, in step (1), the mold is preheated in an oven at 110 ℃ for 20 min.

[0013] Furthermore, the solvent replacement time in step (2) is 2-10 h.

[0014] Furthermore, the molten salt hydrate in step (1) is one or more of lithium bromide, lithium chloride, ferric chloride, zinc chloride, calcium chloride, and aluminum chloride.

[0015] Furthermore, the biomass includes cellulose, a hemicellulose-containing analog system, corn stalks, corn cobs, rice straw, bamboo leaves, and wood branches.

[0016] On the other hand, this application also provides a biomass carbon gel material, which is prepared by the preparation method according to any one of claims 1-8.

[0017] On the other hand, this application also provides an application of the above-mentioned biomass carbon gel material in methanol adsorption refrigeration.

[0018] Beneficial effects

[0019] (1) This invention provides a method for preparing biomass carbon gel. This method uses only two materials—biomass (solute) and molten salt hydrate (solvent)—as components, and prepares the carbon gel through a process of dissolution, cooling gelation, controlled displacement, freeze-drying, and carbonization. The preparation process of the biomass carbon gel of this invention is short and simple. The biomass carbon gel has a three-dimensional ordered hierarchical porous structure with a porosity of 83.76%; simultaneously, the material exhibits excellent hydrophilicity, with a hydrophilic angle of 15°.

[0020] (2) The biomass carbon gel obtained by the preparation method of the present invention can be subjected to oxidation at 25 °C. p / p 0 The methanol adsorption capacity under the condition of p / p0=0.95 was 10.35 g / g, which was significantly better than that of conventional carbon materials (p / p0=0.95, m 核桃壳活性炭 =0.65 g / g, m 烟杆活性炭 =0.61 g / g, m 核咖啡壳活性炭 =0.57 g / g, m 活性碳纤维 =0.49 g / g), at evaporation temperature T ev =5 °C, condensation temperature T con =25 °C, regeneration temperature T des At 65 °C, the COP is 0.92, demonstrating highly efficient cooling performance.

[0021] (3) The methanol adsorption capacity of the biomass carbon gel prepared by the method of the present invention is significantly improved, which is more than 7 times that of the methanol adsorption capacity of the currently reported high-performance adsorbents.

[0022] (4) The molten salt hydrate selected in this invention can be recovered by rotary evaporation after controlled replacement and can be used again for the preparation of biomass carbon gel. The biomass selected is agricultural and forestry waste or biomass extract cellulose, which makes the preparation method green, environmentally friendly, widely available and low cost. It is expected to achieve green large-scale preparation and has the potential for practical application in industry.

[0023] (5) The present invention provides a method for preparing biomass carbon gel, which can obtain biomass carbon gel with high methanol adsorption performance and COP. The biomass carbon gel also has the advantages of simple composition, green environmental protection and low cost, and can replace traditional adsorption and refrigeration materials. It is expected to be practically applied in the future.

[0024] (6) The adsorbent material prepared by the method of preparing biomass carbon gel provided by the present invention, based on the pore structure characteristics and surface group distribution of the adsorbent, can also adsorb water molecules while achieving efficient adsorption of methanol. This water adsorption function enables the adsorbent of the present invention to be applied not only to methanol adsorption refrigeration systems, but also to related technical fields such as water adsorption refrigeration, air dehumidification, and water vapor recovery, significantly improving the multifunctionality of the adsorbent.

[0025] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0026] The above and other objects, features and advantages of this application will become more apparent from the more detailed description of exemplary embodiments thereof in conjunction with the accompanying drawings.

[0027] Figure 1 This is a schematic diagram of the structure of a method for preparing biomass carbon gel material for methanol adsorption refrigeration system provided by the present invention.

[0028] Figure 2 The adsorption isotherms of the biomass carbon gel materials in Examples 1-3 at 25 °C are shown.

[0029] Figure 3 The adsorption isotherms at 25 °C are for different activated carbon materials in Comparative Examples 1-3. Detailed Implementation

[0030] Embodiments of this application will now be described in more detail with reference to the accompanying drawings. While embodiments of this application are shown in the drawings, it should be understood that this application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make this application more thorough and complete, and to fully convey the scope of this application to those skilled in the art.

[0031] The method for preparing biochar gel material provided in this application involves the following steps: the cations of the molten salt hydrate coordinate with the oxygen atoms of the hydroxyl groups in the biomass, and the anions interact with the hydrogen atoms of the hydroxyl groups, thereby weakening the strong intermolecular hydrogen bonding between cellulose chains; water molecules in the molten salt hydrate form a hydration layer around the ions and cellulose chains, promoting the dispersion of cellulose molecular chains; the cellulose chains reassemble through hydrogen bonding and molecular chain entanglement to form a three-dimensional network structure; during freeze-drying, water is removed by sublimation, preserving the three-dimensional network structure of the gel and forming a porous aerogel; the subsequent carbonization process transforms the cellulose skeleton into carbon material while essentially maintaining the hierarchical pore structure formed by the hydrogel template, thus obtaining a carbon aerogel.

[0032] This aerogel material achieves effective adsorption of methanol through its specific pore structure and surface chemical properties. The adsorption principle is as follows: First, methanol molecules undergo physical adsorption in the carbon gel adsorbent via van der Waals forces; this process is dominant under low concentration conditions. Second, methanol has strong polarity, which allows it to generate ion-dipole interactions with metal ions in the molten salt hydrate and form hydrogen bonds with the carrier surface, thereby enhancing adsorption capacity. Furthermore, the molten salt hydrate further absorbs methanol vapor to form a liquid phase, in which methanol dissolves, exhibiting an "adsorption-absorption" coupled process. Finally, weak coordination interactions, where oxygen atoms in methanol form coordinate bonds with metal ions, may also promote adsorption performance.

[0033] Example 1 This embodiment provides a method for preparing biomass carbon gel material for methanol adsorption refrigeration systems, the steps of which are as follows: 1) Mix 50g of molten salt hydrate with 3wt% of biomass of the total material mass until homogeneous. Place the resulting material in an oil bath at 105℃ and stir magnetically until the cellulose is completely dissolved. The molten salt hydrate is one or more of lithium bromide, lithium chloride, ferric chloride, zinc chloride, calcium chloride, and aluminum chloride; this embodiment uses lithium bromide (hydration value of 3) as an example. The biomass includes cellulose, a simulated system containing hemicellulose (from Aladdin Chemical Reagent Network, Shanghai Aladdin Biochemical Technology Co., Ltd.), and corn stalks, corn cobs, rice straw, bamboo leaves, and wood branches (from agricultural and forestry waste in Yunnan Province); this embodiment uses cellulose (mass fraction of 3wt%) as an example.

[0034] 2) After dissolving, the mixed solution is poured into a hot mold, cooled and gelled, and then placed in 1 L of deionized water for 3 h for controlled displacement. The solvent retention rate is 2%. The solution is then directionally frozen using liquid nitrogen and dried at -80°C. The hot mold is preheated in an oven at 110°C for 20 min.

[0035] 3) The dried cellulose aerogel was pretreated for 30 min under nitrogen atmosphere protection at a temperature of 160 ℃, and then carbonized at 320 ℃ for 30 min to obtain cellulose carbon gel material.

[0036] Example 2 This embodiment provides a method for preparing biomass carbon gel material for methanol adsorption refrigeration systems. Except for the following steps, the remaining steps are the same as in Embodiment 1.

[0037] 2) Pour the dissolved mixture into a hot mold, cool and gel, and then place it in 2 L of deionized water for solvent replacement. Change the water every 2 hours until no lithium ions are detected in the solution, and then freeze-dry it.

[0038] Example 3 This embodiment provides a method for preparing biomass carbon gel material for methanol adsorption refrigeration systems. Unlike embodiment 1, step 3 is omitted.

[0039] Comparative Example 1 Coffee shells (derived from agricultural waste) were carbonized in a tube furnace at 650 °C for 40 min, then cooled to room temperature to obtain carbonized coffee shell material. The carbonized coffee shell material was mixed with potassium hydroxide (from Aladdin Chemical Reagents website) at a mass ratio of 1:3 and activated in a high-temperature atmosphere furnace at 800 °C for 10 min; both carbonization and activation were carried out under a nitrogen atmosphere. The activated product was washed 4-5 times with hot distilled water (approximately 90 °C), then thoroughly mixed with a 1 mol / L HCl solution and ultrasonically impregnated at 30 °C for 40 min (ultrasonic frequency 45 kHz, power 300 W) to remove acid-soluble substances and excess activator. The product was then washed with hot distilled water until the pH value was between 6.5 and 7. After washing, the final product was dried at 110 °C for 12 h to obtain activated coffee shell carbon.

[0040] Comparative Example 2 First, tobacco stalks (derived from agricultural waste) were mixed with a zinc chloride solution (from Aladdin Chemical Reagent Network) using an ultrasonic-assisted impregnation process and treated at 70 °C for 7 h. Next, the impregnated tobacco stalks were dried in a drying oven at 110 °C to remove residual moisture. Then, the impregnated and dried samples were activated using a horizontal quartz reactor placed in a tube furnace. During activation, the horizontal quartz reactor was purged with nitrogen at a flow rate of 1000 mL / min. Finally, the activated samples were thoroughly washed with 0.1 mol / L hydrochloric acid, followed by sequential rinsing with hot water until the pH of the washing solution was between 6.5 and 7.0. These wet samples were then dried overnight at 110 °C to obtain activated carbon from the tobacco stalks.

[0041] Comparative Example 3 1. Place the dried walnut shells (derived from agricultural waste) in the center of a tube furnace. Raise the furnace temperature from room temperature to 700 °C at a rate of 10 °C / min, maintain the temperature for 10 min, and then cool it to room temperature to obtain carbonized material.

[0042] 2. Mix approximately 120 g of KOH activator (from Aladdin Chemical Reagent Network) with the carbonized material (40 g) obtained in step 1 in a porcelain mortar, then add 10 g of water and stir thoroughly.

[0043] 3. The mixture obtained in step 2 was dehydrated in an atmosphere furnace at 380 °C for 30 min, then activated at 700 °C for 60 min, and finally cooled to room temperature. These activated samples were thoroughly washed with 0.1 mol / mL hydrochloric acid, followed by sequential washing with boiling water until the pH of the washing solution was between 6.5 and 7.0. Finally, these wet samples were dried at 110 °C for 12 h to obtain walnut shell activated carbon.

[0044] Comparative Example 4 Commercial activated carbon fiber (Nantong Senyou Carbon Fiber Co., Ltd., item number: Y-1500) was cleaned in an ultrasonic cleaner for 30 minutes, then rinsed with deionized water, and then dried in a 100 °C oven for 12 hours.

[0045] Example 4 1. Testing the methanol adsorption performance of different adsorption materials The methanol adsorption performance of the gel materials prepared in Examples 1-3 and the activated carbon materials prepared in Comparative Examples 1-4 was tested using a fully automated adsorption analyzer (Autosorb IQ, Anton Paar). The results are as follows: Figure 2 and Figure 3 As shown. By Figure 2It can be seen that Example 1 (cellulose carbon gel) exhibits excellent methanol adsorption performance, achieving a methanol adsorption capacity of 1.3 g / g at a relative pressure of 0.2. Furthermore, within the medium relative pressure range (0.4-0.6), the methanol adsorption capacity changes gradually. At a relative pressure of 0.8, the adsorption capacity increases sharply, reaching as high as 10.35 g / g at a relative pressure of 0.95. The methanol adsorption isotherm of Example 3 (cellulose gel) shows the same trend as that of Example 1, with a final adsorption capacity of 9.53 g / g. Compared to Examples 1 and 3, the methanol adsorption capacity of Example 2 (cellulose carbon gel - complete displacement) is almost zero, only 0.17 g / g. Therefore, the methanol adsorption performance of the material largely depends on the available retention of hygroscopic salts. Comparative Examples 1 (coffee shell activated carbon), 3 (walnut shell activated carbon), and 4 (activated carbon fiber) showed a rapid increase in methanol adsorption capacity with increasing relative pressure when the relative pressure was below 0.3. When the relative pressure exceeded 0.3, adsorption tended to reach equilibrium, with final methanol adsorption capacities of 0.57 g / g, 0.65 g / g, and 0.49 g / g, respectively. In contrast, Comparative Example 2 (tobacco stem activated carbon) showed a slow increase in methanol adsorption capacity with increasing relative pressure when the relative pressure was below 0.6. When the relative pressure exceeded 0.6, it tended to reach equilibrium, with a final methanol adsorption capacity of 0.61 g / g.

[0046] In addition, the methanol adsorption performance was compared with that of other reported adsorption-type refrigeration adsorbents, as shown in Table 1. As shown in the table, the methanol adsorption performance of the cellulose carbon gel of the present invention is far superior to the methanol adsorption capacity of currently reported high-performance adsorbents (Gordeeva LG, Freni A., Krieger TA, et al. Composites“lithium halides in silica gel pores”: Methanol sorption equilibrium [J]. Microporous and Mesoporous Materials, 2008, 112(1): 254-261.https: / / doi.org / 10.1016 / j.micromeso.2007.09.040; Jeremias F., Fröhlich D., Janiak C., et al. Water and methanol adsorption on MOFs for cycling heat transformation processes [J]. New Journal of Chemistry, 2014, 38(5): 1846-1852.10.1039 / c3nj01556d; El-Sharkawy II, Hassan M., Saha BB, et al. Study onadsorption of methanol onto carbon based adsorbents [J]. InternationalJournal of Refrigeration, 2009, 32(7): 1579-1586. 10.1016 / j.ijrefrig.2009.06.011; Kumita M., Yamawaki N., Shinohara K., et al. Methanoladsorption behaviors of compression-molded activated carbon fiber with PTFE[J]. International Journal of Refrigeration, 2018, 94: 127-135. 10.1016 / j.ijrefrig.2018.07.036; Grekova A., Strelova S., Gordeeva L., et al.“LiCl / vermiculite - Methanol” as working pair for adsorption heat storage: Adsorption equilibrium and dynamics [J]. Energy, 2019, 186: 115775. 10.1016 / j.energy.2019.07.105;De Lange MF, Van Velzen BL, Ottevanger CP, et al. Metal–organic frameworks in adsorption-driven heat pumps: the potential of alcohols as working fluids [J]. Langmuir, 2015, 31(46): 12783-12796.10.1021 / acs.langmuir.5b03272.), which is more than 7 times the methanol adsorption capacity of currently reported high-performance adsorbents.

[0047] Table 1. Comparison of methanol adsorption performance of the biomass carbon gel material prepared in Example 1 with that of previously reported adsorbents.

[0048] 2. Methanol adsorption and refrigeration performance of different adsorption materials The methanol adsorption and refrigeration performance of the gel materials and activated carbon materials prepared in Examples 1-3 and Comparative Examples 1-4 were calculated, and the results are shown in Table 2. Under the same operating conditions, the COP of the biomass gel materials prepared in Examples 1 and 3 was higher than that of the adsorption and refrigeration materials prepared in Examples 2 and Comparative Examples 1-4. At the same time, the COP of Example 1 and the reported adsorbents under different conditions were calculated and compared, and the results are shown in Table 3. As can be seen from the figure, the COP of the cellulose carbon gel of the present invention for methanol adsorption is as high as 0.91, which is much higher than that of other adsorbents under the same operating conditions (Tobie J.Matemb Ma Ntep, Journal of Materials Chemistry A, 2019, 7(43): 24973-24981;Martijn F. de Lange, Langmuir, 2015, 31(46): 12783-12796.).

[0049] Table 2 Examples 1-3 and Comparative Examples 1-4 at T eva =5 °C, T ads=25 °C, T des COP at 65 °C

[0050] Table 3. Comparison of COP of the biomass carbon gel material prepared in Example 1 with reported adsorbents.

[0051] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing a biomass carbon gel material, characterized in that, The method includes the following steps: (1) After mixing biomass with molten salt hydrate, heat and stir until the biomass is completely dissolved, pour into a hot mold, and cool to obtain biomass hydrogel; (2) The biomass hydrogel obtained in step (1) is placed in deionized water to displace part of the solvent, and then freeze-dried to obtain biomass aerogel; the part of the solvent refers to a solvent retention rate of 1%-3%; (3) The biomass aerogel obtained in step (2) is pretreated under a nitrogen atmosphere and then carbonized to obtain biomass carbon gel material.

2. The preparation method according to claim 1, characterized in that, In step (1), the amount of biomass added is 1wt%-7wt% of the total mass of the material.

3. The method for preparing biomass carbon gel material according to claim 1, characterized in that, In step (3), the aerogel carbonization temperature is 200-500 ℃ and the time is 30 min.

4. The preparation method according to claim 1, characterized in that, The pretreatment temperature in step (3) is 160℃ and the time is 30 min.

5. The preparation method according to claim 1, characterized in that, In step (1), the mold is preheated in an oven at 110 ℃ for 20 min.

6. The preparation method according to claim 1, characterized in that, The solvent replacement time in step (2) is 2-10 hours.

7. The preparation method according to claim 1, characterized in that, The molten salt hydrate in step (1) is one or more of lithium bromide, lithium chloride, ferric chloride, zinc chloride, calcium chloride, and aluminum chloride.

8. The preparation method according to claim 1, characterized in that, The biomass includes cellulose, a hemicellulose-containing analog system, corn stalks, corn cobs, rice straw, bamboo leaves, and wood branches.

9. A biomass carbon gel material, characterized in that, The biomass carbon gel material is prepared by the preparation method according to any one of claims 1-8.

10. The application of the biomass carbon gel material according to claim 9 in methanol adsorption refrigeration.

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