Indoor direct air capture using amorphous MOF pellets from blast furnace slag: Waste to porous functional materials
Baljeet Singh, Marianna Kemell, Mikko Heikkilä, Timo Repo
Abstract
• A simple and cost-effective method was developed to recycle BFS into valuable components, including Si, Ca, Mg, and Al. • Al-MOFs were synthesized using Al-extracted from BFS and pelletized into different shapes, such as spherical, cylindrical and hollow cylindrical. • MOF pellets were tested under ultra diluted CO 2 conditions (500 and 1000 ppm), demonstrating an average adsorption capacity of 1 mmol/g. • The pellets exhibited stable performance over 15 adsorption–desorption cycles, maintaining an average adsorption capacity of ∼ 1 mmol/g. • This study showcases a sustainable approach to repurpose BFS into valuable materials, effectively reducing waste while offering cost-effective CO 2 capture solutions for industrial, indoor, and direct air capture (DAC) applications. Steel slag is a waste, could be a valuable source of metals, and these extracted metals from slag can serve as precursors for producing porous materials, such as metal–organic frameworks (MOFs). By refluxing blast furnace slag (BFS) in a 1 M HCl solution facilitates the dissolution of metals like Al, Mg, and Ca, while careful pH control enables effective separation of these metals. Therefore, the Al extracted from BFS was used to synthesize Al-1,3,5-benzene tricarboxylic acid (Al-BTC) and Al-fumarate (Al-FUM) MOFs. The MOFs powder was then formed into pellets and PEI-modified pellets were explored for CO 2 adsorption, using 1000 and 500 ppm CO 2 in N 2 , which simulates indoor CO 2 concentrations. Pellets were examined to assess the impact of binders, and pellet shapes on adsorption capacity and kinetics. Pellets exhibited an average adsorption capacity of 0.6 to 1.2 mmol/g for 500 ppm CO 2 , depending on the shape and binder. The most effective samples were subjected to 15 adsorption–desorption cycles from 500 ppm CO 2 . MOF pellets demonstrated cyclic stability with an average adsorption capacity of approximately 1 mmol/g. This research presents a sustainable, circular method to utilize solid waste to synthesize MOFs, offering a significant reduction in production cost for CO 2 adsorption solid sorbents applications.