(703c) Direct Air Capture (DAC) of CO2 Using a Tailored Chelating Exchanger at Ambient Temperature

Climate change is considered to be the worst existential threat confronting humanity today and, as always is the case, the suffering of marginalized populations is disproportionately high. An appropriate CO₂ capture technology that engages the public at the grassroots level can be a transformative driver and an equalizer. We, at Lehigh University, have developed a process to capture CO₂ directly from the atmosphere at any location and store as pure CO₂. The proposed technology: (i) is amenable to deployment in local communities including schools, churches, NGOs, and wastewater plants; and (ii) operates at ambient temperature without requiring steam or waste heat. Besides solar driven electricity for electrolysis, no other form of energy is required.

The fundamental scientific tenet of the proposed Direct Air Capture (DAC) of CO₂ stems from the unique weak acid-weak base (WAWB) properties of CO₂ and its interaction with a new class of tailored chelating ion exchangers. When a transition metal cation, namely Cu(II), is covalently attached to a chelating polymer with nitrogen donor atoms (polyamine) through Lewis Acid-Base (LAB) interaction, its two positive charges are not neutralized. The resulting material, referred to as PolyLAB-Cu²⁺, is essentially an anion exchanger with a very high affinity for carbonate or CO₃^2- through both electrostatic and concurrent Lewis acid-base (LAB) interaction. Sorption and desorption processes occur as follows:

CO₂ Sorption and Desorption: Use of OH^- as the counter-ion for PolyLAB-Cu²⁺ leads to irreversible uptake of CO₂ from the atmosphere in the following sequence: i) dissolution of CO₂ in the moisture/humidity at the sorbent interface; ii) transport of non-ionized H₂CO₃ inside the ion exchanger; and iii) rapid neutralization followed by selective binding of CO₃² onto Cu(II). Note that it is a three phase (air-water-exchanger) process where the presence of humidity or moisture in between air and the exchanger facilitates dissolution of CO₂ followed by selective uptake.

Upon exhaustion, passage of 3% Na₂SO₄ at near-neutral pH causes desorption of CO₃^2- leading to an exit stream with high alkalinity which again is passed through a weak-acid cation (WAC) exchanger to recover pure CO₂. PolyLAB-Cu²⁺ is subsequently converted into hydroxyl or OH^- form through passage NaOH. Also, the WAC is regenerated with dilute 2-3% H₂SO₄.

Both NaOH and H₂SO₄ are generated through electrolysis of Na₂SO₄ which again is reproduced in the process, thus avoiding external addition of chemicals. Thus, it is essentially a cyclic two-step self-sustaining process where CO₂ is first captured in the first step from the ultra-dilute ambient air and then recovered in the second step as pure CO₂. The entire sorption-desorption cycle operates at ambient temperature and besides electricity for the electrolysis cell, no other chemicals or energy is needed.

Experimental Results: We evaluated various polymers with nitrogen groups that are commercially available and then identified one chelating polymer that is durable, chemically stable and possesses high affinity toward Cu(II). During the last two years, we have carried out an extensive number of laboratory experiments and three key significant findings which make the process quite distinctive are presented below:

5. Under ambient conditions, PolyLAB-Cu²⁺ offered a sorption capacity of 5.1 M CO₂/g dry resin and that is 2.5-3.0 times greater than the capacity of other materials currently in use. Also, these flow-through high capacities are obtained for an empty bed contact time (EBCT) of only 1.0 second.
6. One major downside of direct air capture is the ultra-low concentration of CO₂ in the ambient air (approximately 0.04% or 400 ppm by volume). Understandably, for increased uptake, point-source flue gas streams with higher CO₂ concentrations have been desirable targets. Although counter-intuitive, PolyLAB-Cu²⁺ offers identical CO₂ capture capacity at ambient conditions, with 10% CO₂ and 50% CO₂, the rest being nitrogen. The scientific explanation is being avoided here for the sake of brevity but the observation is very significant from an application viewpoint.

• For flow-through systems, five consecutive runs were carried out with regenerations in between. Capacities did not change or deteriorate. Dilute NaOH and H₂SO₄ produced from the electrolysis cell were used for regeneration and Na₂SO₄ is reproduced at the end of the process and reused.

Path Forward: The proposed process operates at ambient temperature and does not need any thermal energy or waste heat. Consequently, the process is more amenable to implementation around the world including developing countries and marginalized communities with appropriate carbon credit. The work is under progress to put the electrolysis cell in tandem with solar electric panels to make the process a true Negative Emission Technology (NET). Also, the scale-up of the process is relatively straightforward and a skid-mounted prototype is currently under constructio