Direct Air Capture of CO₂ via Reactive Crystallization

Atmospheric CO₂ removal using engineered chemical processes, aka direct air capture (DAC), has become an essential component of our available portfolio for mitigating climate change. Here we describe a promising approach to DAC based on reactive crystallization of atmospheric CO₂ (RC-DAC) with aqueous guanidine and amino acid. Compared to the previously studied phase-changing DAC processes involving initial CO₂ absorption by an aqueous alkaline solvent followed by carbonate crystallization in a second step, RC-DAC combines the CO₂ absorption and carbonate crystallization into a single step. Thus, as the insoluble carbonate salts are removed from solution by crystallization, more CO₂ is pulled from the air into solution, further driving the DAC process. The RC-DAC was performed in a household humidifier as the air-liquid contactor, which can handle solid-liquid slurries effectively. The crystallization was monitored in situ by pH measurements, real-time imaging with a microscope probe, and by Raman spectroscopy, and ex situ by NMR spectroscopy, powder X-ray diffraction, and total inorganic carbonate analysis. The investigation provided a detailed mechanistic picture of the RC-DAC process, involving formation of carbamate and carbonate anions in solution, followed by sequential crystallization of different guanidinium carbonate phases.