Chemical filter reduces the carbon output of power plants

A team from the National Institute of Standards and Technology (NIST) and other institutions have developed a material that acts as a chemical filter to combat the carbon dioxide emitted from smokestacks in power plants. The material, which is easy to synthesize and is simple, cost-effective and potentially reusable, helps to block the greenhouse gas from reaching the atmosphere.

Although removing CO₂ from flue gas before it reaches the atmosphere in the first place is a logical approach, it has not been easy to identify an effective scrubber. As the different gases that flow from the smokestacks of coal-fired power plants are usually hot, humid and corrosive, it is difficult to find a material that can do this effectively.

Although CO₂ capture is a critical strategy for controlling greenhouse gas emissions, there is a critical gap in bridging typical synthesis scales in the lab for potential applications that might require tonnes of the material. As reported in Science Advances [Evans et al. Sci. Adv. (2022) DOI: 10.1126/sciadv.ade.1473], this inspired the team to investigate affordable adsorbents that are easy to synthesize and retain useful CO₂ separation performance, here focusing on aluminum hydroxide and formic acid (ALF), one of the metal–organic frameworks (MOFs) of substances.

MOFs, offering high surface areas, tunable pore sizes, surface functionality and structural diversity, have already demonstrated useful potential for filtering and separating organic materials. ALF resembles a 3D wire cage with numerous tiny holes just big enough to allow CO₂ molecules to enter and then be trapped but small enough to exclude the bigger nitrogen molecules that make up the majority of flue gas.

ALF compares well with other high-performing CO₂ adsorbents, showing outstanding CO₂ uptake and CO₂/N₂selectivity, highlighting the effect of optimal pore size and pore environment for molecular gas separation, as well as that complex networks are not always required for good performance. In addition, ALF offers useful mechanical properties that allow pelletizing and ball-milling without compromising its CO₂ uptake properties.

As co-lead author Dan Zhao told Materials Today, “The excellent CO₂ adsorption properties, ready availability, low cost and excellent mechanical properties make ALF a competitive adsorbent for practical and affordable CO₂ capture processes.” However, it still requires a procedure for large scales, as coal-fired plants would also need a compatible process to reduce the humidity of the flue gas before scrubbing, but this should not be cost-prohibitive.

The ALF structure is simple and flexible enough to be further tuned by using different metal nodes, or even smaller molecular ligands. The team are now assessing the design and development of feasible post-combustion processes based on ALF and related materials, as well as exploring a range of analogs of ALF material for gas separations.

“The excellent CO2 adsorption properties, ready availability, low cost and excellent mechanical properties make ALF a competitive adsorbent for practical and affordable CO2 capture processes”Dan Zhao