A method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology

Abstract

This invention discloses a method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology. In this invention, under low-temperature conditions of 400–600℃, a carbon precursor is loaded into a pressure-resistant reactor. Liquid CO2 is first injected and the pressure is adjusted to 8–15 MPa to achieve a supercritical state. Simultaneously, ozone is directly introduced, with an ozone mass concentration of 10%–30%. During the activation process, ozone partially decomposes at high temperature to generate highly reactive free radicals. These free radicals play a role in directional etching and pore control of the carbon precursor. At the same time, oxygen and supercritical CO2 themselves synergistically promote the activation of the carbon material through oxidation and physical activation effects. The resulting porous carbon material has a specific surface area greater than or equal to 1000 m² / g to 2500 m² / g, with a pore volume distribution concentrated in the range of 0.6–1.2 cm³. 3 The material contains oxygen-doped porous carbon with a density between 1 / g and 1 / 2g. This invention significantly reduces energy consumption and equipment load, while also reducing operating costs and maintenance frequency.

Description

Technical Field

[0001] This invention relates to carbon material preparation technology, and in particular to a method for rapidly preparing porous carbon materials with hierarchical pore structure, controllable pore size distribution and abundant surface functional groups at low temperature (400~600 ℃) using supercritical CO2 and ozone mixed fluid activation technology. It is applicable to fields such as supercapacitors, CO2 adsorption, electrocatalysis and flexible energy storage devices. Background Technology

[0002] Traditional carbon material preparation technologies mainly fall into two categories: physical activation and chemical activation. Physical activation typically uses steam or CO2 as activating agents, conducting the activation reaction at high temperatures of 800–950°C. While this method can generate a large number of micropores, the high temperature can easily damage some surface functional groups, and it also results in high energy consumption, severe equipment wear, and difficulty in controlling the pore size distribution. On the other hand, chemical activation methods usually involve reacting strong bases such as KOH and NaOH with carbon precursors. Although this can achieve higher specific surface areas and richer pore structures at lower temperatures, this method often suffers from problems such as impurity introduction, environmental pollution, and complex post-processing steps. Furthermore, the types and densities of functional groups introduced during chemical activation are difficult to control precisely, thus limiting the application of materials in high-end energy storage, catalysis, and adsorption fields.

[0003] In recent years, with the widespread application of novel carbon materials in energy storage, catalysis, and environmental remediation, higher demands have been placed on preparation methods. These methods not only require the preparation of porous carbon materials with high specific surface areas and ideal pore size distributions, but also demand controllable functional groups on the material surface to meet specific functionalization requirements. Furthermore, more precise requirements have been placed on the pore size distribution of porous carbon materials. In supercapacitors, micropores are needed to provide high specific surface areas (energy storage) while mesopores ensure rapid ion transport (high power). In gas adsorption, the size and distribution of micropores directly affect the selective adsorption capacity for gases such as CO2 and CH4. In catalytic support applications, hierarchical pore structures (micropore-mesopore-macropore synergy) can simultaneously achieve high dispersion loading and rapid mass transfer. These demands highlight the shortcomings of traditional activation methods in precisely controlling pore size.

[0004] Supercritical CO2 activation technology, possessing both gaseous and liquid properties, offers advantages such as high diffusivity and high density, enabling rapid penetration into the interior of carbon precursors. Ozone, as a strong oxidant, decomposes at relatively low temperatures to generate highly reactive free radicals. These free radicals can not only react directly with carbon precursors, promoting pore formation and expansion, but also introduce abundant oxygen-containing functional groups. Combining supercritical CO2 and ozone leverages their respective advantages: the excellent permeability and relatively low-temperature physical activation effect of supercritical CO2, combined with the activation effect of ozone's partial decomposition at high temperatures to generate highly reactive free radicals, forms a synergistic, temperature-controlled, and energy-efficient new activation process. It should be noted that in this process, the free radicals generated by ozone decomposition are only one of several mechanisms promoting the activation reaction. Oxygen and supercritical CO2 themselves can also activate carbon precursors through oxidation and physical processes, thereby achieving precise control over hierarchical pore structure and surface functional groups. Summary of the Invention

[0005] This invention provides a method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology. The core of this method lies in giving full play to the advantages of supercritical CO2 and ozone to achieve low-temperature and high-efficiency activation. At the same time, oxygen and the activation effect of CO2 itself are introduced into the reaction process, thereby precisely controlling the pore structure and surface functional groups of the carbon material at a lower temperature.

[0006] The technical solution adopted by this invention to solve its technical problem includes the following steps:

[0007] (1) The carbon precursor is loaded into a rotatable reactor, with a loading amount of 10-40% of the reactor volume;

[0008] (2) Use CO2 to replace the air in the reactor. After the replacement is completed, the reactor is heated to 400℃~600℃. Then, liquid CO2 is injected into the reactor. The pressure in the reactor is adjusted to make the CO2 in a supercritical state. The reactor is rotated to make the supercritical CO2 and carbon precursor mix evenly.

[0009] (3) A mixture of ozone and oxygen is introduced into the reactor. The volume of the introduced mixture at normal pressure (1 standard atmosphere) is 1% to 10% of the effective volume of the reactor.

[0010] (4) Activate at 400-600℃ for 0.5-2 hours. After the reaction is completed, reduce the pressure to atmospheric pressure to obtain porous carbon material.

[0011] The core technical means of this invention are described below:

[0012] First, this invention utilizes the high diffusivity and high density of supercritical CO2, making the activation process easier and the pores more likely to develop inward.

[0013] Secondly, the present invention directly introduces an ozone / oxygen mixture (ozone concentration of 10% to 30%, volume of 100 to 500 mL), and at a reaction temperature of 400 to 600°C, the ozone partially decomposes to generate highly active free radicals. CO2, oxygen and these free radicals have an etching effect on the carbon precursor, promoting the formation and expansion of pores.

[0014] In addition, oxygen and supercritical CO2 themselves produce a synergistic activation effect on carbon precursors through oxidation and physical processes, jointly achieving precise control of pore structure and surface functional groups.

[0015] The beneficial effects of this invention are as follows:

[0016] Traditional steam activation typically requires temperatures of 800–950°C, resulting in extremely high energy consumption and severe equipment wear. This invention, however, completes activation at a low temperature of 400–600°C, significantly reducing energy consumption and equipment load, while also decreasing operating costs and maintenance frequency.

[0017] Compared to traditional activation processes, this method significantly shortens the reaction time (typically only 1-3 hours), thanks to the synergistic effect of the high mass transfer efficiency of supercritical CO2 and the high reactivity of ozone radicals. Traditional physical activation often requires 5-8 hours of high-temperature maintenance, while chemical activation, although shorter in time, is limited by the time-consuming post-processing steps and environmental factors. This technology, through in-situ activation and the rapid penetration characteristics of supercritical fluids, greatly improves activation efficiency under low-temperature conditions, avoiding the consumption of equipment and energy due to prolonged high-temperature processes.

[0018] This invention achieves activation through the synergistic effect of supercritical CO2 and ozone. Supercritical CO2, with its high diffusivity, can rapidly penetrate into the interior of the carbon precursor, allowing the activation reaction to proceed uniformly from the inside out. Simultaneously, the active free radicals generated by the partial decomposition of ozone at high temperatures promote directional etching, which not only accelerates the formation of pores but also introduces abundant oxygen-containing functional groups onto the material surface, thereby optimizing the pore structure and surface chemical properties. Attached Figure Description

[0019] Figure 1 This is the FTIR spectrum of the product in Example 1.

[0020] Figure 2 This is the nitrogen adsorption-desorption curve of the product in Example 1.

[0021] Figure 3 This is a pore size distribution diagram of the product in Example 1.

[0022] Figure 4 The images are SEM images (a) and EDS elemental (O element) mapping images (b) of the porous carbon product from Example 1. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] like Figures 1-4 As shown, this invention involves loading a carbon precursor into a pressure-resistant reactor at a low temperature of 400–600 °C. Liquid CO2 is first injected and the pressure is adjusted to 8–15 MPa to achieve a supercritical state, while ozone (10%–30% mass concentration) is directly introduced. During activation, ozone partially decomposes at high temperature to generate highly reactive free radicals. These free radicals directionally etch and regulate the porosity of the carbon precursor. Simultaneously, oxygen and supercritical CO2 also synergistically promote the activation of the carbon material through oxidation and physical activation effects. The resulting porous carbon material has a specific surface area greater than 1000 m² / g, with a pore volume distribution concentrated between 0.6 and 1.2 cm³. 3 It is between / g and rich in oxygen-doped porous carbon materials.

[0025] Example 1

[0026] Step 1: Coconut shells were selected as raw material, crushed, and sieved to a particle size of 200±20 μm. The coconut shell particles were soaked in a 5% HCl solution and stirred at 80 °C for 6 hours to remove ash and metallic impurities. They were then repeatedly washed with deionized water until neutral, and subsequently dried at 105 °C for 12 hours. The dried coconut shell particles were placed in a tube furnace and heated to 800 °C at a heating rate of 5 °C / min under nitrogen protection for 2 hours to obtain a coconut shell carbon precursor (specific surface area 300±30 m² / g, pore volume 0.3±0.02 cm³ / g, average pore size 1.2±0.1 nm).

[0027] Step 2: Evenly load the carbonized coconut shell char precursor into a 1 L rotary reactor, filling it to 30% of the reactor volume (approximately 130 g). Close the reactor, connect the CO2 gas source, and slowly introduce high-purity carbon dioxide gas to displace the air inside the reactor, ensuring no oxygen remains in the system. After air replacement, raise the reactor temperature to the activation temperature of 500℃. Then continue to introduce liquid CO2, using a high-pressure plunger pump to increase the pressure inside the reactor to 10 MPa, bringing the CO2 to a supercritical state. Maintain this pressure at the supercritical state for 10 minutes to confirm no leaks in the system. Simultaneously, start the rotary reactor (10 rpm) and maintain this position for 1 hour to ensure thorough and uniform gas distribution.

[0028] Step 3: Using an external ozone generator, ozone gas is generated from oxygen. A predetermined amount of ozone / oxygen mixture (120 mL at atmospheric pressure, ozone concentration of 20 wt%) is introduced into the rotary reactor before sealing. Subsequently, the inlet of the rotary reactor is closed to ensure that the internal gas is not disturbed by the outside world. At the same time, the rotary reactor is kept rotating continuously (10 rpm) and maintained at this temperature (activation temperature 500 ℃) for 1.5 hours.

[0029] Step 4: After the reaction is complete, the reactor pressure is reduced to atmospheric pressure at a rate of 2 MPa / min to avoid rapid pressure reduction that could cause the pore structure to collapse. The obtained porous carbon material is then dried in a vacuum drying oven at 60 °C for 12 hours to obtain the final product. Figure 1 The image shown is the FTIR spectrum of the product from Example 1. Figure 2 The figure shows the nitrogen adsorption-desorption curve of the product in Example 1. Figure 3 The image shown is a pore size distribution diagram of the product in Example 1. Figure 4 The image shown is a SEM image and an EDS elemental mapping image of the porous carbon product of Example 1.

[0030] Example 2 (Temperature Gradient Optimization)

[0031] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the activation temperature is set to 400℃ (other parameters are the same as in Example 1).

[0032] Example 3 (Temperature Gradient Optimization)

[0033] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the activation temperature is set to 600℃ (other parameters are the same as in Example 1).

[0034] Example 4 (Ozone Concentration Control)

[0035] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the ozone mass concentration is 10% (the volume is fixed at 120 mL, and other parameters are the same as in Example 1).

[0036] Example 5 (Ozone Concentration Control)

[0037] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the ozone mass concentration is 30% (the volume is fixed at 120 mL, and other parameters are the same as in Example 1).

[0038] Example 6 (Supercritical CO2 Pressure Change)

[0039] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the pressure is set to 8 MPa (activation temperature 500 °C).

[0040] Example 7 (Supercritical CO2 Pressure Change)

[0041] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the pressure is set to 15 MPa (activation temperature 500 °C).

[0042] Example 8 (Ozone / Oxygen Mixture Concentration Control)

[0043] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the volume of the mixed gas is 60 ml (other parameters are the same as in Example 1).

[0044] Example 9 (Ozone / Oxygen Mixture Concentration Control)

[0045] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the volume of the mixed gas is 300 ml (other parameters are the same as in Example 1).

[0046] Example 10 (Phenolic Resin Carbonization Material)

[0047] Step 1: Preparation of carbon precursor

[0048] Phenolic resin was selected as the raw material and pulverized and sieved to a particle size of 250±20 μm. The particles were placed in a tube furnace and carbonized at 600℃ for 3 hours under nitrogen protection at a rate of 5 ℃ / min (the resin pyrolysis temperature is relatively low) to obtain a phenolic resin carbon precursor (specific surface area 280±20 m² / g, pore volume 0.25±0.03 cm³ / g, average pore size 1.0±0.2 nm).

[0049] Steps 2-4: The process and conditions are the same as in Example 1.

[0050] Example 11 (Petroleum coke carbide)

[0051] Step 1: Preparation of carbon precursor

[0052] Petroleum coke was selected as raw material, crushed and sieved to 160±20 μm particles, soaked in 5% HCl (80℃ / 8h) to remove ash, washed and dried, and then carbonized at 900℃ for 4 hours under nitrogen protection at a rate of 10℃ / min to obtain a petroleum coke precursor (specific surface area 180±15 m² / g, pore volume 0.18±0.02 cm³ / g, average pore size 0.9±0.1 nm). Steps 2-4: The process and conditions are the same as in Example 1.

[0053] Comparative Example 1 (Low Temperature)

[0054] The process and conditions are the same as in Example 1, except that the parameters are adjusted as follows: the activation temperature is set to 300℃ (other parameters are the same as in Example 1).

[0055] Comparative Example 2 (Activation with pure supercritical CO2)

[0056] The process and conditions are the same as in Example 1, except that the process parameters are as follows: ozone removal step (i.e., step 3: close the air inlet of the rotary reactor to ensure that the internal gas is not disturbed by the outside, while the rotary reactor continues to rotate (10 rpm) and is maintained at this temperature (activation temperature 500 ℃) for 1.5 hours), using only 12 MPa supercritical CO2 (500 ℃).

[0057] Comparative Example 3 (Low concentration CO2 + ozone activation)

[0058] The process and conditions are the same as in Example 1, except that the process parameters are adjusted as follows: the pressure is set to 5 MPa (keeping it in a gaseous state, not reaching the supercritical state), the ozone mass concentration is 20% (addition volume 200 mL), and the activation temperature and time are maintained at 500 °C and 1.5 hours.

[0059] Other steps, including carbonization of the carbon precursor, ozone injection, and gradient depressurization, are the same as in Example 1.

[0060] Experimental data analysis:

[0061] Oxygen content was determined using an Elementar vario EL cube oxygen, nitrogen, and hydrogen elemental analyzer; specific surface area and pore size distribution were obtained using a Micromeritics ASAP 2460 physical adsorption analyzer, calculated using the N2 adsorption / desorption isotherm (77 K) combined with BET theory and the NLDFT model; microstructure was observed using a Zeiss Sigma 500 field emission scanning electron microscope (SEM). Based on experimental data and comparative analysis, this study reveals the key mechanism of supercritical CO2 / ozone mixed fluid activation technology in the preparation of porous carbon materials. By comparing the data of the examples and the control group, the high diffusivity of supercritical CO2 and the strong oxidizing power of ozone showed a significant synergistic effect: under the conditions of 500 ℃ and 10 MPa (Example 1), the material had a specific surface area of ​​2019 m² / g, an oxygen content of 8.1%, and a total pore volume of 1.02 cm³ / g, which is 3.7 times better than the single activator system (such as the specific surface area of ​​only 432 m² / g in Comparative Example 2), confirming that CO2 etches the pore framework through supercritical permeation. Furthermore, combined with oxygen, nitrogen, and hydrogen elemental analysis and infrared spectroscopy, it was confirmed that ozone selectively oxidized the pore walls and introduced abundant oxygen-containing functional groups. Temperature control showed that low temperature (400 °C) suppressed activation efficiency (specific surface area 1321 m² / g), while high temperature (600 °C), although increasing the specific surface area to 2231 m² / g, accelerated the pyrolysis of functional groups (oxygen content only 8.5%), indicating that 500 °C is the optimal equilibrium point. Ozone concentration and mixed gas volume exhibited a non-linear effect: a 20% ozone concentration (120 mL) could balance pore expansion (pore volume 1.02 cm³ / g) and surface modification (oxygen content 8.1%), while increasing the concentration to 30% or the volume to 300 mL (Example 9), although further increasing the oxygen content to 10.8%, resulted in a widening of the pore size distribution due to excessive oxidation (pore volume 1.30 cm³ / g, but the risk of micropore collapse increased). Supercritical CO2 pressure regulation verifies its physical-chemical synergistic effect. At 10 MPa pressure (Example 1), the micropore / mesopore ratio is balanced, while at high pressure (15 MPa), the specific surface area increases to 2114 m² / g.

[0062] Meanwhile, by controlling the activation temperature, ozone concentration (and mixed gas volume), and supercritical CO2 pressure, this study achieved precise control over the pore size distribution of porous carbon materials: at low temperatures (approximately 400 °C), the reaction is mild, the ozone decomposition rate is low, and the microporous structure is mainly preserved; at high temperatures (approximately 600 °C), ozone decomposition and free radical generation are intensified, resulting in a significant increase in mesopores and macropores, but at the same time, functional group pyrolysis may occur; while at 500 °C (as in Example 1), a relatively balanced distribution of micropores, mesopores, and macropores is observed, while obtaining a high specific surface area (approximately 2019 m² / g), a suitable pore volume (approximately 1.02 cm³ / g), and abundant surface oxygen-containing functional groups (approximately 8.1%). In addition, low concentrations of ozone and smaller mixed gas volumes mainly promote micropore formation, while high concentrations of ozone or larger volumes enhance the oxidative etching effect, resulting in a decrease in the proportion of micropores (as low as 56.5%) and an increase in the proportion of mesopores and macropores, and an increase in total pore volume (up to 1.30 cm³ / g). However, excessive oxidation poses a risk of micropore collapse. Meanwhile, a supercritical CO2 pressure of 10 MPa helps with uniform penetration and etching, while higher pressures (15 MPa) will further increase the specific surface area but may increase the number of mesopores.

[0063] Table 1. Specific surface area, pore volume, and oxygen content of porous carbon

[0064]

[0065] In summary, this invention, through comprehensive parameter control, can achieve continuous control from micropore dominance to a gradual increase in mesopores and macropores according to different application requirements, as shown in Table 1.

Claims

1. A method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology, characterized in that, The method includes the following steps: (1) The carbon precursor is loaded into a rotatable reactor, with a loading amount of 10-40% of the reactor volume; (2) Use CO2 to replace the air in the reactor. After the replacement is completed, the reactor is heated to 400℃~600℃. Then, liquid CO2 is injected into the reactor. The pressure in the reactor is adjusted to make the CO2 in a supercritical state. The reactor is rotated to make the supercritical CO2 and carbon precursor mix evenly. (3) Introduce a mixture of ozone and oxygen into the reactor. The volume of the mixture under normal pressure is 1% to 10% of the effective volume of the reactor. (4) Activate at 400-600℃ for 0.5-2 hours. After the reaction is completed, reduce the pressure to atmospheric pressure to obtain porous carbon material.

2. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 1, characterized in that, The carbon precursor has the following specifications: its specific surface area is 250–350 m². 2 The pore volume is between 0.2 and 0.4 cm³ / g. 3 The average pore size is between 0.8 and 1.4 nm.

3. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 1, characterized in that: In step (3), a mixture of supercritical CO2 and ozone / oxygen is used as an activator, wherein the pressure of supercritical CO2 is 8MPa-15MPa; the ozone mass concentration in the mixture of ozone and oxygen is 10%-30%, and this concentration has a regulating effect on the pore size distribution.

4. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 3, characterized in that, By adjusting the ozone concentration and volume of the mixed gas, and jointly controlling the activation temperature and activation time, the proportion of micropores, mesopores, and macropores in porous carbon materials can be continuously controlled.

5. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 4, characterized in that, The continuous control of the ratio of micropores, mesopores, and macropores in porous carbon materials is achieved as follows: (a) When the activation temperature is 400-500℃, the activation time is 1-2h, the ozone mass concentration is 10%-20%, and the volume of the mixed gas accounts for 1%-3% of the effective volume of the reactor, the micropore volume accounts for 70%-85%; the remainder are mesopores and macropores. Or (b) when the activation temperature is 500-600℃ (excluding 500℃), the activation time is 2-3h, the ozone mass concentration is increased by 20%-30% (excluding 20%), and the mixed gas volume is increased by 3%-10% (excluding 3%), the free radical activity generated by the high-temperature decomposition of ozone is enhanced, the proportion of mesopores and macropores is increased, and the proportion of micropores is reduced to 55%-70%.

6. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 4, characterized in that, The continuous control of the ratio of micropores, mesopores, and macropores in porous carbon materials is achieved as follows: (a) When the activation temperature is 430–480℃, the activation time is 1–2h, the ozone mass concentration is 15%–18%, and the volume of the mixed gas accounts for 1%–2% of the effective volume of the reactor, the micropore volume accounts for 70%–85%; the remainder are mesopores and macropores. Or (b) when the activation temperature is 530-580℃ (excluding 530℃), the activation time is 2-3h, the ozone mass concentration is increased to 25%-28% (excluding 25%), and the mixed gas volume is increased to 5%-8% (excluding 5%), the free radical activity generated by the high-temperature decomposition of ozone is enhanced, the proportion of mesopores and macropores is increased, and the proportion of micropores is reduced to 55%-70%.

7. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 5 or 6, characterized in that, The carbon precursor mentioned above is biomass with a particle size of 120-300 μm. Alternatively, petroleum coke particles can be obtained by soaking in a 4-6% HCl solution at 20-90℃ for 4-48 hours, washing with water, drying, and carbonizing at 700-900℃ for 1-12 hours. Alternatively, it can be obtained by carbonizing phenolic resin particles with a particle size of 160-300μm at 600-800℃ for 1-12 hours.

8. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 5 or 6, characterized in that, The rotary reactor is a reactor that can rotate along a rotation axis; the rotation speed range of the rotary reactor is 5-20 rpm.

9. The method for preparing porous carbon materials based on supercritical CO2 / ozone mixed fluid activation technology according to claim 5 or 6, characterized in that, The ozone / oxygen mixture introduced into the reactor is generated by an ozone generator that uses oxygen as a raw material.

10. A porous carbon material, characterized in that, The porous carbon material is prepared by any one of the methods described in claims 1 to 6, and has a specific surface area of ​​1000 m². 2 / g~2500m 2 / g, pore volume distribution between 0.6 and 1.2 cm³ 3 It has a range of / g and a hierarchical pore structure and controllable pore size distribution, while being rich in oxygen-doped porous carbon materials.

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