A process for CO2 capture and synergistic in-situ conversion of calcium-iron bifunctional materials with spatially distributed reaction timing.

Abstract

This invention relates to a process for CO2 capture and synergistic in-situ conversion of calcium-iron bifunctional materials with spatially distributed reaction timing, belonging to the field of resources and environment. This invention constructs spatially precisely arranged calcium-iron-shell and iron-calcium-shell bifunctional materials using a sol-gel method. It utilizes the sequential exposure to a CH4 gas atmosphere to directionally control the reaction timing of dry reforming and partial oxidative reforming, achieving a three-in-one process of CO2 carbon capture, synergistic in-situ conversion, and material regeneration within a single fixed-bed reactor, encompassing CO2 carbonation, CH4 dual reforming, and air / CO2 oxidation. The 3Ca@1Fe material achieves a maximum CO2 adsorption capacity of 10.29 mmol. CO2 / g CaO， The fastest adsorption-desorption rate is attributed to the outer Fe2O3 layer providing more oxygen vacancies to promote CO2 diffusion. The 4Fe@3Ca material achieves a maximum total syngas yield of 21 mmol / g, with a stable H2 / CO ratio close to 2 and a low carbon deposition of 1.13 mmol / g. This is attributed to the outer CaO shell inhibiting Fe2O3 aggregation and promoting oxygen ion migration. By adjusting the calcium-iron molar ratio, tunable syngas with an H2 / CO ratio of 2.04–3.25 can be obtained to meet the needs of different downstream processes. This invention achieves precise control of reaction timing through material spatial structure design, significantly improving the synergistic efficiency of CO2 capture and in-situ conversion. The process is simple, recycles stably, and is low-cost, providing a novel industrial technology route for integrated CO2 capture and resource utilization.

Description

Technical Field

[0001] This invention belongs to the field of resources and environment and is related to carbon capture, utilization and storage (CCUS) technology. Specifically, it relates to a spatially distributed reaction-sequence-guided process for the synergistic in-situ conversion of CO2 by calcium-iron bifunctional materials. Technical Background

[0002] As global industrialization continues, the large-scale consumption of fossil fuels has led to a continuous rise in atmospheric CO2 concentrations, resulting in increasingly severe greenhouse effects, climate anomalies, and ecological and environmental problems. Against this backdrop, carbon capture, utilization, and storage (CCUS) technology is widely recognized as a core supporting technology for promoting the green and low-carbon transformation of the economy and society. In the CCUS technology system, captured CO2 is directly and directionally converted into high-value-added chemicals such as syngas and hydrocarbon fuels, achieving an integrated "emission reduction-resource utilization-high-value" process with significant environmental and economic benefits. This has become a key research direction in the fields of resources and environment both domestically and internationally.

[0003] Traditional CO2 emission reduction technologies typically employ a stepwise "capture-then-conversion" strategy. This involves first capturing and concentrating CO2 using amine liquid absorption or solid adsorbents (such as CaO), then transporting the high-purity CO2 to another reaction unit for hydrogenation conversion using a specific catalyst (such as Ni, Ru, or Fe-based catalysts). This process suffers from inherent drawbacks such as complex equipment, high energy consumption, and high costs. In particular, the compression, transportation, and repeated heating and cooling processes of the reactor result in significant energy losses. To overcome these shortcomings, Integrated CO2 and In-situ Conversion (ICCC) technology has emerged. This technology couples the capture and conversion processes within the same reactor, allowing for cyclical operation by switching the reactant gases (CO2 / H2), greatly simplifying the process and reducing energy consumption and equipment investment. The core of ICCC technology lies in developing highly efficient dual functional materials (DFMs) that simultaneously possess both CO2 capture and catalytic conversion capabilities.

[0004] Among numerous DFM designs, the combination of calcium (Ca) and iron (Fe) is considered a promising technological approach. Calcium-based materials (CaO) are excellent high-temperature CO2 traps, widely available (e.g., limestone), inexpensive, and possessing a high theoretical absorption capacity (0.78 g). CO2 / g CaOFurthermore, it possesses suitable thermodynamic and kinetic properties within the temperature range of 600~700℃. Iron-based materials (Fe2O3) are highly efficient catalytic components, exhibiting high activity and selectivity for reverse water-gas shift reaction (RWGS) and partial oxidation of methane to syngas (POM). Moreover, they are abundant, inexpensive, and environmentally friendly, possessing the potential for large-scale industrial applications. Therefore, constructing calcium-iron bifunctional materials is expected to achieve low-cost and high-efficiency ICCC processes. The literature (Industrial & Engineering Chemistry Research, 2020, 59(3): 1298-1307.) proposes and verifies a chemical chain reforming process based on CaO and Fe2O3. By coupling the calcium cycle (CO2 capture) with the redox cycle of the iron-based oxygen carrier, in-situ integrated CO2 capture and conversion are achieved in a single process. The results show that the synergistic effect of CaO and Fe2O3 significantly improves the syngas yield, and the H2 / CO ratio is close to 2, while effectively inhibiting carbon deposition. For example, the literature (Chemical Engineering Science, 2025, 309: 121486.) uses impregnation and co-precipitation methods to synthesize Ca-Fe-Ni bifunctional materials for CO2 capture and conversion. These materials achieve a CO2 retention rate of 88.18% after 10 CO2 adsorption cycles, and the highest syngas space-time yield reaches [missing value]. and However, existing technologies mostly employ physical mixing or simple loading methods to construct calcium-iron bifunctional materials. Although these methods can achieve certain synergistic effects of capture and conversion, they generally suffer from technical bottlenecks such as high reaction temperatures, insufficient catalytic activity, weak oxygen storage and release capabilities, poor high-temperature cycling stability, large carbon deposition, and uncontrollable syngas ratios. Core-shell structures can achieve spatial separation, interface confinement, and sequential effects of functional components, significantly improving the material's activity, stability, and sintering resistance. This is an effective solution to the above problems. For example, the literature (International Journal of Hydrogen Energy, 2019, 44(13): 6491-6504.) prepared Fe2O3@CeO using the sol-gel method. Fe2O3@CeO exhibited superior catalytic activity, stability, and sintering resistance compared to Fe2O3 / CeO. Despite the significant advantages of the core-shell structure, current research on calcium-iron based materials still focuses on structural and basic performance optimization, without combining the spatial arrangement of the core and shell with the directional control of reaction timing. This makes it difficult to fully leverage the synergistic effect of the calcium-iron bifunctional phase and fails to meet the industrial demands for process stability, syngas tunability, and efficient carbon resource utilization.

[0005] To this end, this invention proposes a spatially distributed, reaction-sequential-guided process for CO2 capture and synergistic in-situ conversion using calcium-iron bifunctional materials, achieving efficient CO2 capture, in-situ reforming, and resource utilization. This invention uses CaO as the CO2 carbonation capture carrier and Fe2O3 as the CH4 reforming oxygen carrier, employing a sol-gel method to prepare two core-shell structured materials: CaO@Fe2O3 and Fe2O3@CaO. By comparing the reaction behavior of single CaO, single Fe2O3, and physically mixed CaO / Fe2O3 systems, the interfacial interaction and reaction mechanism between the capture carrier and oxygen carrier under the core-shell structure are systematically studied. The process reaction performance and operating conditions are optimized, providing a new technical route for the industrial application of CO2 capture and synergistic conversion. Summary of the Invention

[0006] This invention aims to provide a process for the synergistic in-situ conversion of CO2 by calcium-iron bifunctional materials with spatially distributed reaction timing. By constructing a core-shell spatial structure, the precise spatial arrangement of CaO and Fe2O3 is achieved, the reaction timing and pathway are directionally controlled, the synergistic efficiency of CO2 capture and in-situ conversion is improved, the formation of carbon deposits is reduced, the syngas ratio is controlled, and it is adaptable to different downstream processes.

[0007] This invention also provides a preparation scheme for a calcium-iron core-shell material suitable for the three-in-one carbon capture synergistic in-situ conversion and material regeneration of CO2 carbonation-CH4 dual-air / CO2 oxidation, comprising the following steps: adding citric acid to a calcium nitrate solution and stirring at 50 °C for 0.5 h to form a chelate solution; dispersing Fe2O3 particles in 60% ethanol and sonicating for 1 h, allowing to stand overnight to remove the supernatant, and obtaining a particle suspension; adding the suspension to the chelate solution and stirring vigorously at 50 °C for 0.5 h, adding ethylene glycol, and stirring at 80 °C to form a gel; drying the gel in a vacuum drying oven at 100 °C for 24 h, calcining and grinding in a muffle furnace to obtain Fe2O3@CaO material; CaO@Fe2O3 material is prepared by the same method, the difference being that CaO particles are used as the core phase and the iron chelate solution formed by iron nitrate and citric acid is used as the shell precursor for coating, and after calcination and grinding, a reverse core-shell structure with CaO as the core and Fe2O3 as the shell is formed. In this invention, the molar ratio of citric acid to metal ions was 2, the molar ratio of ethylene glycol to citric acid was 2, the gel stirring time was 8 h, the muffle furnace calcination temperature was 900 ℃, the calcination time was 6 h, and the calcination rate was 5 ℃ / min, to ensure the integrity of the core-shell structure and uniform coating. This invention precisely synthesized bifunctional materials with spatially ordered iron-core calcium-shell and calcium-core iron-shell structures via the sol-gel method, achieving controllable preparation of the core-shell structure and providing crucial support for directional regulation of reaction timing, CO2 capture, and in-situ synergistic conversion.

[0008] The CO2 adsorption performance of the calcium-iron core-shell material was tested using a thermogravimetric analyzer: approximately 20 mg of the material was weighed into an Al2O3 crucible, purged with N2 at room temperature for 10 min, then CO2 was switched, and the temperature was increased to 600, 700, 800, and 900 ℃ at a rate of 10 ℃ / min and held at each temperature for 1 h to test the CO2 adsorption capacity.

[0009] The process for CO2 capture and synergistic in-situ conversion of a spatially distributed, reaction-sequential calcium-iron bifunctional material is as follows: The prepared material is placed in a quartz tube reaction zone with an inner diameter of 12 mm. Quartz wool is laid in the groove at the bottom of the reaction zone to prevent powder loss. The quartz tube is heated by an electric furnace equipped with an N-type thermocouple with controlled temperature control, and the flow rate is 300 mL / min throughout the process. CO2 capture stage: adsorption temperature is 700 ℃, heating rate is 10 ℃ / min, adsorption time is 0.5 h, and CO2 concentration is 10 vol% CO2 / N2; CH4 reforming stage: reforming temperature is 900 ℃, CH4 concentration is 4.5 vol% CH4 / N2, and reaction time is 0.5 h; oxidation stage: oxidation temperature is 700 ℃, O2 concentration is 21 vol% O2 / N2, and oxidation time is 0.5 h. The changes in gas concentrations (CO2, CO, H2, CH4, and O2) in the three processes were detected by an online infrared gas analyzer.

[0010] Compared with the prior art, the advantages of the present invention are as follows:

[0011] (1) To address the unclear mechanism and complexity of calcium-iron materials in the methane dual reforming process using physical mixing methods, the core-shell structure is applied to the synthesis of calcium-iron bifunctional materials. This innovatively separates the properties of calcium and iron while organically combining them into one. Compared with conventional core-shell materials with single properties, this constructs a functional material that simultaneously performs dual functions, achieving carbon dioxide capture and resource conversion while exploring the structure-property relationship of the material, thereby improving the reforming reaction performance.

[0012] (2) Since the construction of calcium-iron materials by physical mixing or simple loading methods cannot achieve orderly synergy between bifunctional materials and directional control of reaction time, it is difficult to give full play to the capture and conversion advantages of calcium-iron materials. By preparing two types of calcium-iron bifunctional materials with core-shell structure, the core-shell spatial positions of CaO and Fe2O3 are precisely controlled, and the directional control of reaction time and path is achieved innovating. This strengthens the synergistic effect of calcium-iron components at the interface and significantly improves the efficiency of CO2 capture and in-situ synergistic conversion. Attached Figure Description

[0013] Figure 1 A schematic diagram of the process flow for preparing calcium-iron core-shell materials.

[0014] Figure 2The figure shows the effect of different core-shell structures and different calcium-iron molar ratios on the reforming performance of calcium-iron bifunctional materials.

[0015] Figure 3 The figure shows the effect of different calcium-iron molar ratios on the CO2 adsorption performance of calcium-iron bifunctional materials.

[0016] Figure 4 The graph shows the gas concentration changes of the 4Fe@3Ca material over five cycles.

[0017] Figure 5 A comparison chart showing the reforming reaction performance of single CaO, single Fe2O3, and physically mixed CaO / Fe2O3 systems. Detailed Implementation

[0018] To make the technical solution and implementation details of the present invention clearer and more complete, the present invention will be further described in detail below with reference to preferred embodiments.

[0019] Example 1

[0020] In this embodiment, a CaO@Fe2O3 bifunctional material with a calcium-to-iron molar ratio of 3:4 was prepared using the sol-gel method. 16.4068 g of ferric nitrate nonahydrate was dissolved in 200 mL of deionized water and stirred at 30 °C for 0.5 h. Then, 15.4471 g of citric acid was added, and the mixture was stirred at 50 °C for 0.5 h to form a chelate solution. 1.7167 g of CaO was dispersed in 100 mL of 60% ethanol solution, sonicated for 1 h, and allowed to stand overnight. The supernatant was removed. The suspension was added to the chelate solution and stirred vigorously at 50 °C for 0.5 h. Ethylene glycol with a molar ratio of 2:1 to citric acid was added, and the mixture was stirred at 80 °C for 8 h. The gel was dried at 100 °C for 24 h, calcined at 900 °C for 6 h, and ground to obtain the core-shell material, which was named 3Ca@4Fe. The preparation process is as follows: Figure 1 As shown. 0.5 g of the material prepared above was placed in a fixed-bed reactor for testing of a three-in-one process: CO2 capture-CH4 dual rectification-air / CO2 oxidation regeneration. (See figure) Figure 2 As shown, the results indicate that the total synthesis gas (H2 and CO) yield of this material is 9.86 mmol / g, and the carbon deposition is 0.49 mmol / g.

[0021] Example 2

[0022] In this embodiment, a Fe2O3@CaO bifunctional material with a calcium-to-iron molar ratio of 3:4 was prepared using the sol-gel method. 7.156 g of calcium nitrate tetrahydrate was dissolved in 200 mL of deionized water and stirred at 30 °C for 0.5 h. Then, 11.5851 g of citric acid was added, and the mixture was stirred at 50 °C for 0.5 h to form a chelate solution. 3.226 g of Fe2O3 was dispersed in 100 mL of 60% ethanol solution, sonicated for 1 h, and allowed to stand overnight. The supernatant was removed. The suspension was added to the chelate solution and stirred vigorously at 50 °C for 0.5 h. Then, ethylene glycol with a molar ratio of 2:1 to citric acid was added, and the mixture was stirred at 80 °C for 8 h. The gel was dried at 100 °C for 24 h, calcined at 900 °C for 6 h, and ground to obtain the core-shell material, which was named 4Fe@3Ca. The preparation process is as follows. Figure 1 As shown. 0.5 g of the material prepared above was placed in a fixed-bed reactor for testing of a three-in-one process: CO2 capture-CH4 dual rectification-air / CO2 oxidation regeneration. (See figure) Figure 2 As shown, the results indicate that the total syngas yield of this material reaches a maximum of 21 mmol / g, which is 112.98% higher than that of the 3Ca@4Fe material, and the H2 / CO ratio is close to 2.3, making it suitable for Fischer-Tropsch synthesis. This suggests that the spatial distribution of the CaO shell in the 4Fe@3Ca structure can effectively inhibit the aggregation of the Fe2O3 core, promote the migration of oxygen ions from the core to the shell, and improve the conversion efficiency. In contrast, the Fe2O3 shell in the 3Ca@4Fe structure is prone to aggregation, leading to a reduction in active sites.

[0023] Example 3

[0024] While keeping the raw materials, preparation steps, and process conditions unchanged as described in Example 2, only the calcium-iron molar ratio was changed to 3:1, 3:2, and 3:3 to prepare 1Fe@3Ca, 2Fe@3Ca, and 3Fe@3Ca materials. 0.5 g of each material was placed in a fixed-bed reactor for testing of the integrated CO2 capture-CH4 dual rectification-air / CO2 oxidation regeneration process. Figure 2 As shown, H can be obtained. 2 / Syngas with CO concentrations of 2.04–3.25% can meet the needs of various downstream processes.

[0025] Example 4

[0026] CO2 adsorption performance was tested using a thermogravimetric analyzer. Approximately 20 mg of the prepared calcium-iron core-shell material was weighed into an Al2O3 crucible, purged with N2 at room temperature for 10 min, then CO2 was switched, and the temperature was increased to 600, 700, 800, and 900 °C at a rate of 10 °C / min and held at each temperature for 1 h. Figure 3 As shown, the CO2 adsorption capacity of the 3Ca@1Fe material reached a maximum of 10.29 mmol. CO2 / g CaOThe adsorption and desorption rates are the fastest, which is related to the fact that the outer Fe2O3 provides more oxygen vacancies to promote the diffusion of CO2.

[0027] Example 5

[0028] Five cycles of testing were performed on the 4Fe@3Ca material. Figure 4 As shown, in the first cycle, a large amount of CO2 was initially produced. This was because no gas was introduced during the heating process to 900 °C after CO2 adsorption, and the CO2 from the decomposition of CaCO3 was purged out of the pipeline by CH4. The H2 concentration showed an increasing trend, while the CO concentration showed an initial increasing and then decreasing trend. This is related to the insufficient oxygen source in the later stages of the reaction, which led to a decrease in the reforming reaction performance, and the reduction of Fe2O3 to produce elemental Fe, which promoted the cracking of CH4 to produce carbon deposits and H2. In cycles 2-5, the gas concentrations were stable, and the H2 and CO concentrations showed consistent trends, indicating good overall cycle stability of the material.

[0029] Comparative Example 1

[0030] The reforming reaction performance of single CaO, single Fe2O3, and physically mixed CaO / Fe2O3 systems was compared using a fixed-bed reactor. Figure 5 As shown, the results indicate that physically mixed CaO / Fe2O3 materials have the following advantages: (i) under the same conditions, the amount of carbon deposited by mixed reforming is lower; (ii) although the H2 yield is slightly lower than that of single CaO materials (the reason for the high H2 yield of single CaO materials is the presence of a large amount of CH4 cracking, which produces a large amount of carbon deposits), it is still beneficial for the H2 / CO ratio to be close to 2 in mixed reforming, making it suitable for Fischer-Tropsch synthesis. These obvious advantages are attributed to two points: (1) there is an interaction between the Fe-based oxygen carrier and the CO2 carbon carrier; (2) the interaction between the gases produced by the two reforming reactions affects the composition of the atmosphere inside the reaction tube.

Claims

1. A process for the synergistic in-situ conversion of CO2 by calcium-iron bifunctional materials with spatially distributed reaction timing, characterized in that: Using a calcium-iron core-shell bifunctional material as the core medium, the timing of the CH4 reforming reaction is guided and controlled by the spatial distribution difference between the core and shell. This allows for a three-in-one process of CO2 carbonation, CH4 dual reforming, air / CO2 oxidation, carbon capture, synergistic in-situ conversion, and material regeneration within a single fixed-bed reactor. This achieves synergistic effects in CO2 capture and in-situ conversion while reducing process costs. The calcium-iron core-shell bifunctional material uses CaO as the carbon carrier for CO2 carbonation and Fe2O3 as the oxygen carrier for CH4 reforming. It is synthesized using a sol-gel method, forming two spatial distribution structures: Fe2O3@CaO (Fe2O3 as the core, CaO as the shell) and CaO@Fe2O3 (CaO as the core, Fe2O3 as the shell). The ratio of the two carriers is optimized to obtain syngas products with adjustable H2 / CO ratios, making it suitable for a wider range of downstream applications.

2. The process according to claim 1, characterized in that: The process uses single CaO, single Fe2O3, and physically mixed CaO / Fe2O3 as control systems, and performs a three-in-one process of CO2 capture, CH4 reforming, and air oxidation regeneration under the same conditions.

3. The process according to claim 1, characterized in that: The preparation parameters of the calcium-iron core-shell bifunctional material are as follows: the molar ratio of citric acid to metal ions is 1~3, the gel stirring time is 6~8 h, the vacuum drying time is 12~48 h, the muffle furnace calcination temperature is 400~900 ℃, the calcination time is 3~6 h, the calcination rate is 2~10 ℃ / min, and the calcium-iron molar ratio is 3:1~3:

4.

4. The process according to claim 1, characterized in that: With the synergistic effect of Fe-based materials, the process conditions for CO2 capture are as follows: the adsorption temperature of calcium-based materials is extended from below 720 ℃ to 600~900 ℃, the heating rate is 2~10 ℃ / min, the adsorption time is 0.5~2 h, and the adsorption pressure is atmospheric pressure.

5. The process according to claim 1, characterized in that: Under the influence of the spatial distribution difference of calcium and iron elements to guide the timing of CH4 reforming reaction, the process conditions of the CH4 dual-in-situ reforming stage are as follows: the reforming temperature is extended from the single high temperature requirement above 900 ℃ to 700~900 ℃, the CH4 concentration is 1.5~4.5 vol%, the reaction time is 0.5~1 h, and the reaction pressure is atmospheric pressure.

6. The process according to claim 1, characterized in that: To achieve material recycling and carbon removal in the process, the process conditions for the air oxidation regeneration stage are: oxidation temperature of 600~900 ℃ and oxidation time of 0.5~1 h.

7. The process according to claim 1, characterized in that: The CaO@Fe2O3 structure allows the outer Fe2O3 layer to provide more oxygen vacancies, promoting CO2 diffusion and increasing CO2 capture rate and adsorption capacity. The Fe2O3@CaO structure allows the CaO shell to delay direct contact between CH4 and the Fe2O3 core layer through steric hindrance, achieving directional control of reaction timing. At the same time, it inhibits Fe2O3 core layer aggregation, promotes oxygen ion migration, and improves conversion efficiency.

8. The process according to claim 1, characterized in that: Carbon removal and carbon capture are achieved by utilizing the two-step oxidation process of O2 in the oxidation regeneration stage and CO2 in the capture stage, and the carbon removal and oxygen carrier oxidation regeneration are completed. O2 first reacts with the active components of the shell layer of the calcium-iron core-shell material to oxidize the reduced iron-based material into the initial oxygen carrier state, and then diffuses into the core layer. Combining the carbonic and oxidizing properties of CO2, the carbon deposits generated during the reforming process are converted into CO while material regeneration and CO2 capture are achieved.

Images