Research Progress Of Carbon Dioxide Capture Materials

As global warming intensifies, reducing greenhouse gas emissions such as carbon dioxide and protecting the environment have become one of the core global issues to be solved. To achieve carbon dioxide emission reduction, it is necessary to go through the processes of capture, transportation, storage, application, and conversion. The capture cost in these processes accounts for about 75%, or even higher. At present, the CO2 concentration of most large emission sources is less than 15%, while a small part (less than 2%) of industrial sources based on fossil fuels has a CO2 emission concentration of more than 95%. High-concentration sources are potential targets for early implementation of carbon dioxide capture and storage technology (CCS). CO2 is one of the main gases that cause the greenhouse effect and is also a potential carbon resource. CO2 has a wide range of uses in various sectors of the national economy as a chemical raw material, refrigerant, oil field production enhancer, inert medium, solvent and pressure source. Therefore, all countries are committed to reducing carbon dioxide emissions from the burning of fossil fuels. At present, carbon dioxide capture technology is widely used in chemical, power plant, automobile manufacturing and other industries. However, the disadvantages of carbon dioxide capture materials at this stage are poor recycling performance, toxicity, low capture efficiency, scarce raw materials, and high energy consumption. Therefore, the development of new carbon capture materials has become a research focus.

Keywords: carbon dioxide; capture materials; research; progress

Solution adsorption mainly uses solutions containing amine functional groups to capture CO2 through chemical absorption. The commonly used adsorbents are alcohol amine solutions, including primary alcohol amines (such as ethanolamine), secondary alcohol amines (such as diethanolamine and diisopropanolamine) and tertiary alcohol amines (such as methyldiethanolamine and triethanolamine). This chemical absorption method uses the absorbent to react with CO2 to achieve the purpose of recovering CO, and uses its reverse reaction to regenerate the absorbent. This method has a high CO2 removal rate and is one of the most effective methods for recovering CO2. It is also suitable for treating mixed gases with low partial pressure of CO. However, there are still many disadvantages that limit the use of this method: amines are prone to oxidative degradation, which reduces the absorption performance and also increases the viscosity of the solution, which is not conducive to gas transmission; amines and their degradation products are easy to volatilize during the regeneration of the absorbent, which reduces its absorption capacity: the strong alkalinity of the amine solution is particularly corrosive to instruments and equipment; the operation is relatively cumbersome; the regeneration energy consumption is high.

Since CO2 is an acidic gas, it is easily adsorbed on the surface of slightly alkaline materials. There are three main alkaline adsorbents currently under research and development: one is alkaline metal oxides, such as Na2O2, K2O, CaO, MgO and AI2O3. Metal oxides have good adsorption capacity at high temperatures, especially alumina. When alkali metals (such as Li2O, K2O, Na2O) are added, its adsorption capacity at high temperatures can be greatly improved compared with physical adsorbents; the second is alkaline metal salts, such as calcium carbonate, silicates, lithium silicate, and lithium zirconate; the third is a hydrotalcite mixture. Hydrotalcite contains alkaline metal compounds and has a microporous structure. It is a natural composite material. The adsorption of carbon dioxide by hydrotalcite has aroused people's research interest.

Carbon materials mainly include activated carbon and carbon fiber.

(1) Activated carbon is the most common black porous adsorbent with a large specific surface area. Its main components are amorphous carbon, as well as a small amount of hydrogen, oxygen, nitrogen, sulfur and ash. The physical and chemical properties and surface chemical properties of the activated carbon produced will vary greatly depending on the raw materials, preparation process and activation method. The main factors that determine the adsorption capacity of activated carbon are the specific surface area, pore structure characteristics, surface properties and adsorption characteristics of the adsorbate. The adsorption performance of several activated carbon adsorbents on CO2 at high temperature was studied. For different types of adsorbents, the adsorption amount of CO2 is proportional to the specific surface area and total pore volume of the activated carbon; while for the same adsorbent, the adsorption amount is proportional to the pressure and inversely proportional to the temperature.

(2) Activated carbon fiber is obtained by carbonizing and activating organic fibers. It is the third generation of carbon material after activated carbon powder and activated carbon particles. Activated carbon fiber has a more developed specific surface area than granular activated carbon, a smaller micropore diameter (about 1nm), and the micropore volume accounts for more than 90% of the total pore volume. At the same time, it is directly opened on the fiber surface, so it has the advantages of large adsorption capacity, high adsorption efficiency, and fast adsorption and desorption speed. Due to the particularity of its structure and performance, the use of activated carbon fiber to adsorb air pollutants has become a research hotspot for scientific researchers and has shown great application prospects.

Zeolite molecular sieve is a natural or synthetic crystalline aluminosilicate containing alkali metal and alkaline earth metal oxides. It has a strict structure and pores. The pore size varies slightly due to structural differences, and can separate substances of different molecular weights. Zeolite molecular sieve adsorbents are often used for gas separation and purification, such as nitrogen production from air, separation and purification of CO2, etc. Its adsorption capacity also decreases with increasing temperature. Lila et al. used ASRTSA molecular sieve to adsorb and remove CO2 from the space capsule. Experiments show that when the temperature rises to 175 ℃, the adsorption amount is only 24% of that at 25 ℃. Under the same conditions, the adsorption amount of zeolite molecular sieve, which is also a physical adsorption, is higher than that of activated carbon.

Scientists at the French National Center for Scientific Research have developed a new material called MIL-101, which can absorb a large amount of carbon dioxide gas. This material is expected to enhance the ability to resist global warming. This material is synthesized from chromium and terephthalic acid. It is a porous composite nanomaterial with a surface covered with small holes with a diameter of 3.5nm. Therefore, the adsorption capacity is very strong. MIL-101 with a volume of 1m³ can store 400m³ of carbon dioxide at 25℃. The storage capacity of the current general adsorption material under the same conditions is only 200m³. This new material can be placed on the car to filter the carbon dioxide it emits, thereby achieving the purpose of reducing greenhouse gas emissions.

The CO adsorption performance of two silica gel adsorbents was compared, the adsorption isotherms of N2 and CO2 on silica gel and activated carbon adsorbents were determined, and the dynamic adsorption penetration curves of CO2 in different systems were investigated. The results show that the adsorption amount of CO2 by silica gel adsorbent is comparable to that of activated carbon, and the adsorption selectivity is better than that of activated carbon; the larger specific surface area and high pore content are beneficial to the adsorption of CO2, and the appropriate pore distribution is conducive to reducing the internal diffusion resistance of silica gel adsorbent.

The study used mesoporous molecular sieve powder as a carrier and loaded with different organic amines to prepare CO2 adsorption materials. Solid amine CO2 adsorbents can selectively adsorb acidic gas CO2 through chemical reactions and are less affected by water vapor. The solid amine CO2 adsorbent prepared by using mesoporous materials with high specific surface area and pore volume as carriers showed the characteristics of high adsorption capacity. In particular, the template micelles contained in the original powder of the mesoporous material are retained, and "meshes" of different scales are formed in the mesoporous space to intercept and adsorb CO2 in the airflow, with high adsorption efficiency. The carbon dioxide adsorption of zeolites loaded with amine compounds was studied, and the results showed that the CO2 adsorption capacity of zeolites increased by 20%~30% after loading amines. This is because both physical adsorption and chemical adsorption occur in the adsorption process of composite materials, and the dual effects have a synergistic effect.

Unlike traditional organic solvents, ionic liquids do not produce volatile organic compounds during the decarbonization process due to their low vapor pressure and are easy to use. At the same time, ionic liquids can be used repeatedly. With the joint funding of the U.S. Department of Energy's Office of Fossil Energy and the U.S. National Energy Technology Laboratory, Jennifer L, Wang Zhongni, and others conducted a variety of ionic liquids. Physical properties and CO2 absorption mechanism studies. The results show that among given ionic liquids, ionic liquids have better selectivity for CO2; at the same time, it is found that ionic liquids have high CO, absorption load and lower regeneration heat requirements.

Studied the adsorption of CO2 by strong alkaline ion exchange fiber. They simulated the gas adsorption and desorption process and found that strong alkaline ion exchange fiber can adsorb CO2 gas well. The study of various factors affecting the adsorption of CO2 gas by strong alkaline anion exchange fiber showed that: the change of water content has the greatest impact on adsorption, and high water content is conducive to the adsorption of gas by fiber; slow gas flow rate is conducive to the adsorption of gas by fiber, and fast flow rate can also adsorb gas well. As long as the concentration of gas does not exceed a certain limit, the adsorption of fiber will be less affected; the shape of the exchange column also affects the adsorption performance of the fiber, and slender exchange columns are better than short and thick ones.

Membrane-based absorption is a new membrane separation technology that combines membrane technology with gas absorption technology. Membrane-based absorption is a new membrane separation technology that couples membrane separation and liquid absorption. The membrane material suitable for CO2 capture is polypropylene hollow fiber, and the membrane absorption liquid is an activated polyamine aqueous solution. The CO2 component in the mixed gas preferentially passes through the membrane and is absorbed by the polyamine aqueous solution. Then the waste liquid is regenerated by membrane distillation, and its regeneration rate can exceed 98%. Not only does it occupy a small area and have friendly operating conditions, but also the hollow fiber membrane area is large, the CO2 pass rate is high, and the solution regeneration rate is high, making this method the development trend of CO2 capture technology in the future.

As people's environmental awareness gradually increases, various countries have increased their efforts to protect the environment, which will inevitably play a positive role in promoting the development of carbon dioxide capture materials. In recent years, the research work on carbon dioxide capture materials has made great progress. Carbon dioxide capture technology is developing in the direction of low price, simple operation process, low operating cost, and long-term recycling. This requires that carbon dioxide capture materials must have the characteristics of easy availability of production raw materials and low price, simple and environmentally friendly production process, good regeneration ability, and recyclable use, and it is required to be able to simultaneously treat multiple pollutants such as carbon dioxide, hydrogen sulfide, and nitrogen oxides. This makes intelligent carbon dioxide capture materials a future development trend. New materials can appropriately adjust their own surface properties and enhance adsorption in different atmospheres according to environmental changes.