Metal-Organic Frameworks for CO2 Capture

By: Paweesuda Natewong

Developing an effective carbon dioxide capture system is essential to reducing greenhouse gas emissions and moving toward a cleaner energy future. As an emerging new class of porous solids, metal-organic frameworks (MOFs) adsorbents are particularly promising as CO2 capture materials because they have high internal surface areas, low heat capacity, and adjustable pore functionality enabling the selective adsorption of large quantities of CO2.

MOFs are formed by assembling metal-containing nodes (metal ions or metal-based clusters) that function as structural building units and organic linkers. One gram of the MOFs has an internal surface area equal to 1.5 football (soccer) pitches or two American football fields. The large surface area offers more space for chemical reactions and adsorption of molecules. Consequently, if appropriately designed, a small amount of MOFs can remove enormous CO2 from the exhaust gas produced by fossil fuel combustion.

The capacity to rationally select the metal-organic framework components is expected to allow the affinity of the internal pore surface toward CO2 to be exactly controlled, facilitating materials properties that are optimized for the specific type of CO2 capture to be performed (post-combustion, pre-combustion, or oxy-fuel combustion) and potentially even for the specific power plant in which the capture system is to be installed. For this reason, a crucial effort has been made in recent years to improve the gas separation performance of metal-organic frameworks, and some studies evaluating the prospects of deploying these materials in real-world CO2 capture systems have begun to emerge.

MOFs could be accomplished using much smaller temperature changes than required for other technologies, giving them a significant advantage over conventional ways to capture CO2. (The adsorbed CO2 can then be utilized in other products.) This strategy eliminates the need to divert high-value, high-temperature steam away from power production, avoiding a large increase in the cost of electricity. In the course of these efforts, we also showed that variants of the MOFs could be efficient for the removal of CO2 from other gas mixtures, including biogas, natural gas, and even directly from the air.