Carbon Dioxide Capture and Separation

Introduction

Human society has developed into a stage where economic prosperity is predominantly dependent on the exploitation of fossil fuels, which are the main energy sources. However, combustion of fossil fuels for power has inevitably brought the emission of large amounts of carbon dioxide into the atmosphere and led to global warming. Several serious issues associated with the rise of temperature, the increase of sea level, and the species extinction may occur according to the reports of Intergovernmental Panel on Climate Change (IPCC). The atmospheric carbon dioxide concentration has continued to increase to reach 415 ppm at the present, and the pH value in sea surface water would drop from 8.2 to 7.8 by 2095 year due to the accumulation effect of the dissolved carbon dioxide. Developing effective techniques to reduce the carbon emission and minimize greenhouse effect are thus becoming more and more urgent than ever.

Current carbon dioxide scrubbers are mostly aqueous alkanolamine solutions as adsorbent materials to capture carbon dioxide, which exist from serious issues such as equipment corrosion, solvent loss, and toxicity that retard their further applications. And, one of the most disadvantages was the high energy input for amine regeneration. Significant work is still needed to identify the promising adsorbent materials so as to overcome these problems. Different from other gases, carbon dioxide has no dipole moment but exhibits a high quadrupole moment. Using this basic feature of carbon dioxide in designing a porous structure to enhance interactions with carbon dioxide is necessary to increase the uptake capacity and selectivity. Owing to the combined features of the precise structural design and chemical stability, MOFs and COFs offer a new platform for designing porous structures to capture and separate carbon dioxide.

Carbon Dioxide Capture and Separation Using MOFs

Since metal/metal oxide shows high adsorption capacities, open/unsaturated metal sites (OMSs) are widely introduced in MOFs materials, which are primarily utilized as sorbents to achieve carbon dioxide capture and separation. This is based on differences in interaction between gas species and frameworks and the consequent selective adsorption of target gas. In recent years, a growing number of explorations have utilized MOFs for carbon dioxide capture and separation. For instance, in 2017, Liang et al. constructed a new Cu-based MOF, FJI-H14, which is anticipated to have a high carbon dioxide uptake. The results showed that FJI-H14 displays an extraordinarily high volumetric carbon dioxide capacity of 171 cm³/cm³ at 298 K and 1 atm with CO₂/N₂ selectivity as high as 51 for a 15/85 CO₂/N₂ mixture^[1].

Fig.1 The schematic diagram of carbon dioxide capture and separation using MOFs

In addition, the researcher studied the strengths and weaknesses between MOFs and other adsorbent materials in carbon dioxide capture and separation, the results are shown in below. It can be concluded that MOFs have the highest working capacity among all other adsorbent materials^[2].

Table1.Strengths and weaknesses comparison of selected adsorbent materials

Carbon Dioxide Capture and Separation Using COFs

COFs with structural, thermal, and hydrothermal stabilities are superior to most porous polymers in carbon dioxide capture and are compatible with the best performing MOFs, such as Mg₂(dodbc). For example, an ionic-liquid-immobilized [Et₄NBr]_50%-Py-COF exhibits an impressive carbon dioxide uptake capacity of 164.6 mg/g at 273 K and 1 bar^[3]. The [HO₂C]_100%-H₂P-COF reaches a capacity of 174 mg/g at 273 K and 1 bar^[4]. Moreover, these COFs exhibit excellent cycle stability. Generally, total control over the surface area, pore size, and pore volume of COFs can enhance the capacity and selectivity of carbon dioxide. More importantly, incorporating functional groups that enhance interactions with carbon dioxide to the pore walls of COFs is an efficient way to facilitate carbon dioxide capture.

Approaches for Capturing Carbon Dioxide Using MOFs and COFs

The key approaches for capturing carbon dioxide using MOFs and COFs for capture carbon dioxide are generally four types: pre-combustion capture, post-combustion capture, oxy-fuel combustion capture and direct capture from air.

• Pre-combustion capture: Pre-combustion capture technology involves capturing the carbon by MOFs and COFs from the fuel before completing the combustion process.

Fig. 2. Block flow diagram of a pre-combustion capture system

• Post-combustion capture: In the post-combustion capture technology, the carbon-based fuel first undergoes combustion before the separation of the carbon dioxide by MOFs and COFs from the flue gas is carried out.

Fig. 3. Block flow diagram of a post-combustion capture system

• Oxy-fuel combustion capture: Oxy-fuel combustion is the process at which hydrocarbon fuel is combusted in a nearly pure oxygen environment, the flue gas generated is thus almost completely carbon dioxide after removal of water, rendering a more facile capture step of carbon dioxide by MOFs and COFs.

Fig. 4. Block flow diagram of an oxy-fuel combustion capture system

• Direct capture from air: Directly capturing the carbon dioxide which from air.

What Can Alfa Chemistry Do

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