Encyclopedia
Please note this is a comparison between Version 2 by Conner Chen and Version 1 by Supriyo Ray.
Carbon is the most important element of organic matter, forming the major part of its dry weight. Elemental carbon exists in the atmosphere as carbon dioxide (CO2) and is dissolved in water, rocks, and soils as carbonic acid (H2CO3), bicarbonates, and carbonates [1]. In fossil fuels, it exists as carbon rings or chains and in biomolecules such as nucleic acids, carbohydrates, proteins, and lipids. Atmospheric CO2 is the major cause for global warming through the greenhouse effect; hence, carbon dioxide emissions must be strictly regulated to minimize the greenhouse effect.
- Metal-Based Nanocatalyst
- Metal-Organic Frameworks
- Carbon Nanotube
- flame spray pyrolysis
- Capturing Post-Combustion Carbon Dioxide
- Nanotechnology Based Carbon Dioxide Fixation
- Fixing Carbon Dioxide into Plastics
- non-photosynthetic methods for carbon capture
1. Introduction
Carbon is the most important element of organic matter, forming the major part of its dry weight. Elemental carbon exists in the atmosphere as carbon dioxide (CO2) and is dissolved in water, rocks, and soils as carbonic acid (H2CO3), bicarbonates, and carbonates [1]. In fossil fuels, it exists as carbon rings or chains and in biomolecules such as nucleic acids, carbohydrates, proteins, and lipids. Atmospheric CO2 is the major cause for global warming through the greenhouse effect; hence, carbon dioxide emissions must be strictly regulated to minimize the greenhouse effect [2,3,4][2][3][4]. The Intergovernmental Panel on Climate Change (IPCC) report from 2018 forewarns us that catastrophe in the form of floods, poverty, drought, etc., will be unleashed by a mere change in global temperature of a degree. In the Paris agreement in February 2021, 186 countries that are responsible for 90% of global emissions pledged carbon emission reduction through 2025 or 2030. India (the third-largest carbon emitter) has committed to generating 50% of its energy needs by renewable means by 2030 and reaching net-zero carbon emissions by 2070. China (the largest carbon emitter) has not made any such commitments, since they are the manufacturing hub and are wary of disrupting the supply chain worldwide. Nevertheless, irrespective of the verbal commitment, China leads the world in electricity production through renewable energy resources and also in the investment in renewable energy innovation and capacity building according to the UN Environmental Program Report.
Though the top three major carbon-emitting countries, i.e., China, the United States, and India, are aggressively working on cutting down their emissions by reducing their dependency on fossil fuels, it will still take a few more decades to slow down before we see any observable effects.
2. Chemical Alternatives for Fixing Carbon Dioxide
2.1. Carbon Nanotube (CNT) Synthesis
Synthesis of carbon nanotubes from carbon dioxide is one of the most promising non-photosynthetic methods for carbon capture, utilization, and storage [103][5]. The carbon material developed from gaseous carbon dioxide can have multiple utilities. Compared to amorphous carbon, carbon nanotubes are superior due to their superior electronic/ionic properties and mechanical, chemical, and thermal stability [104,105,106][6][7][8]. Hence, they are used to make supercapacitors, lithium-ion batteries, and fuel cells with high performance. This method, though promising, did not seem feasible, as it was hard to scale up due to extreme experimental conditions. For instance, it required supercritical carbon dioxide with magnesium and lithium as reducing agents [104,106][6][8]. This made it more reactive compared to regular carbon dioxide. To conduct such an experiment, one needs to maintain a high temperature around 1000 °C and pressure of 10 kbar, which made it highly improbable to scale up [107,108,109,110][9][10][11][12]. Recent advances in this production technique have not only made it more feasible but scalable as well. CNTS can now be synthesized from gaseous carbon dioxide by one-step chemical vapor deposition in the presence of sodium borohydride as a reducing agent and nickel catalyst at 500 °C and 1 atm pressure [105][7]. Another recent advancement was made using another greenhouse gas; i.e., reforming of methane was done by reacting it with carbon dioxide in the presence of a catalyst to produce syngas using a series combination of two reactors (carbon generator or CARGEN), which converts the gasses into multi-walled carbon nanotubes. Carbon nanotubes are in use in electronics, lithium-ion batteries, aerospace technology, and carbon fiber, to name a few applications [111][13].
2.2. Fixing Carbon Dioxide into Plastics
Recent research has shown that using a technique known as flame spray pyrolysis (FSP) generates zinc oxide nanoparticles that can convert carbon dioxide into syngas [112][14]. Syngas when mixed with hydrogen and carbon monoxide at various ratios in turn can generate various chemicals such as diesel, alcohol, and plastics [111,112][13][14]. With this method, carbon dioxide can be converted into precursors, which in turn can be used to generate various chemicals including plastics [111][13].
2.3. Metal-Organic Frameworks (MOFs)
MOFs hold a lot of promise because they are porous and have a large surface area [113][15]. Though they are being used as a heterogeneous catalyst, more improvements are required to lower the cost of production and energy requirement. Recently, a MOF was created that has acrylamide groups that are accessible and has a copper group exposed, thereby increasing the efficiency of carbon dioxide adsorption [114][16]. In these MOFs, they extended the tricarboxylate ligand backbone using click chemistry that helped create larger pores. In another study, Zn-MOF-184 was made that had strong Lewis acidity due to zinc cations, and it showed the highest catalytic activity upon cycloaddition of carbon dioxide under mild conditions, i.e., 80 °C for 6 h under mild solvent conditions and balloon pressure [115][17]. This is an evolving field and holds a lot of promise for further improvement and research for sequestering carbon dioxide. Presently, there is enough information to develop intuitive models to optimize acid-base properties to improve and expedite the search for MOFs with remarkably improved catalytic properties [116][18].
2.4. Nanotechnology Based Carbon Dioxide Fixation
2.4.1. Metal-Based Nanocatalyst
Similar to MOFs, metal-based magnetic nanocatalysts hold a lot of promise. In one of the recent studies, a copper-based magnetic nanocatalyst was found to be a simple yet efficient way of carbon dioxide fixation. This nanocatalyst adds epoxide and carbon dioxide to form cyclic carbonates through cycloaddition [117][19].
Nickel-based nanoparticles hold a lot of promise, as they can catalyze the hydration of carbon dioxide. This is important, as during carbon dioxide absorption, the hydration reaction is the rate-limiting step. These nanoparticles showed optimal activity around 20–30 °C with pH less than 8. When these nanoparticles were mixed with potassium carbonate solution, the adsorption capacity was enhanced by 77%. At a commercial level, heating at 150 °C regenerated the potassium-carbonate-saturated nanofluid. Interestingly, the nanoparticles did not show any surface oxidation post-regeneration [118][20].
2.4.2. Nanoparticles for Capturing Post-Combustion Carbon Dioxide
Fossil fuel derivatives such as coal, natural gas, and crude oil are utilized to generate approximately 67% of the world power supply. Hence, one of the largest sources of carbon dioxide emitters is constituted by the power plants that consume fossil fuels such as coal and natural gas to generate electricity [119][21]. Water-based nanofluids have been tested for the elimination of carbon dioxide. It was found that aluminum oxide and calcium carbonate were the best-performing nanoparticles. The assessment for efficiency was made based on the surface area, temperature, and pressure to adsorb carbon dioxide.
