Robust, cost-effective and energy efficient methods to capture carbon dioxide from the atmosphere are made possible with novel materials like porous organic frameworks

 

Reducing greenhouse gas emissions, particularly carbon dioxide, is paramount in combating climate change. One way to do this is to capture the carbon dioxide (CO₂) emissions directly from the source before they enter the atmosphere.

 

Dr. Georgios Karanikolos, Associate Professor of Chemical Engineering, Dr. Vengatesan Rangaraj, Research Scientist, and Dr. K. Suresh Kumar Reddy, Research Scientist, designed and developed a new material for use in carbon capture. The properties and efficacy of their phosphazene-core Covalent Triazine Framework were examined and tested at various conditions, with their results published in Chemical Engineering Journal.

 

“Carbon dioxide is the primary cause of global warming, which has had adverse effects on climate change in the last few decades and with even more negative consequences predicted for the near future,” Dr. Karanikolos said. “Combusting fossil fuels increases atmospheric CO₂ levels, and since fossil fuels are currently the predominant energy source for industry and the transportation sector, it is essential that we explore robust, and cost- and energy-efficient methods to capture the CO₂ emitted from combustion.”

 

Carbon capture, utilization and storage (CCUS) is the most widely accepted and promising strategy currently in use, and can be further developed to improve efficiency, energy consumption, and cost.

 

Successful carbon capture needs a sorbent material that will selectively grab CO₂ in a stream of gas and then readily release it when desired so that the material can be reused, while the released CO₂ can be utilized or sent for long-term storage.

 

In adsorption, CO₂ reversibly collects in the pores in the material that serve as active capture sites. When, for instance, temperature is lowered, CO₂ adheres to the surface, and when temperature is raised, CO₂ is released. Changes in pressure can also bring about these capture and release cycles.

 

Currently, aqueous amine solutions, which are solutions containing water and organic compounds called amines that contain nitrogen atoms attached to hydrogen and carbon atoms, are used to capture CO₂ in industrial applications. Amine solutions are excellent at trapping the CO₂, making them the most popular and developed carbon capture technology. However, their disadvantage  is that in order to recover the trapped CO₂ from the amine solution, the solution has to be heated, requiring large amounts of thermal energy and resulting in some amines being lost to the environment in this high-energy process.

 

To overcome the shortcomings of amine solutions, solid sorbent materials are a viable alternative. Solid sorbents can selectively adsorb CO₂, however some solid sorbent materials perform better than others. The KU research team focuses on investigating a variety of solid sorbents including zeolites, porous carbon nanostructures, metal-organic frameworks (MOFs), and porous organic frameworks (POFs).

 

“Over the last few years, MOFs and POFs have been studied extensively for various applications due to their superior textural properties, high structural flexibility and the various functional groups they can contain,” Dr. Karanikolos said. “However, MOFs typically possess low thermal and chemical stability, restricting their use especially in harsh environments. On the other hand, POFs are made of organic building blocks closely connected through covalent bonds that enhance chemical and thermal stability. This means they can be used in environments where MOFs are not suitable.”

 

Covalent Triazine Frameworks (CTFs) are a class of porous organic frameworks with properties that can be tuned through careful design for a wide range of applications. One such application is carbon dioxide adsorption. CTFs are easily manufactured and can be designed to include functionalities that are CO₂-philic, meaning they can selectively attract the CO₂ from the atmosphere to adsorb into the CTF for removal. CTFs can also include elements such as sulfur, phosphorous, boron, and oxygen to improve the chemical properties of the framework, which can be highly advantageous for CO₂ adsorption.

 

The KU research team designed and manufactured phosphonitrilic core CTFs (Pz-CTFs) and tested CO₂ adsorption, selectivity, and regeneration at various temperatures and pressures. These CTFs used a phosphorus-based core with a nitrile group to increase crosslinking, which created a material with a high porosity and surface area (one gram of the material has about 1,000 square meters of surface area), providing a large space for CO₂ adsorption.

 

High surface area, low density, excellent thermal and chemical stability and a large number of nitrogen functional groups make Pz-CTFs excellent potential candidates for CO₂ capture. The team’s Pz-CTFs can work in temperatures up to 500 °C, meaning they can be used in various industries that require high temperatures. Even at these high temperatures, the team’s material exhibited excellent CO₂ uptake. Furthermore, the material does exhibit significant hydrophobic character, meaning it is less impacted by the presence of water in a CO2-containing mixture, it is selective in the presence of other gas species, and it can be reused. Hence, it is a high-capacity and reversible adsorbent for selective carbon capture in extreme environments.

 

Jade Sterling
Science Writer
29 December 2021