(298b) Guanidinium-Based Adsorbents for CO2 Capture

Over the past 200 years—correlated with the beginning of the industrial revolution—atmospheric carbon dioxide (CO₂) concentration has increased as a direct result of human activity. Current CO₂ concentrations are around 420 parts-per-million (ppm). The accumulation of CO₂ and other greenhouse gases in the atmosphere have caused an increase in global average temperatures that has led to increased severity and frequency of extreme weather and climate events. To meet the Paris Agreement objective of keeping the average global temperature under 2 °C above pre-industrial temperature, removal of 10 billion metric tons per year of atmospheric CO₂ must occur before 2050 with another 20 billion metric tons per year after 2050. One of several negative emissions technologies proposed to achieve this aim is direct air capture (DAC) of CO₂. One approach to DAC that aims to lower the energy demand of the DAC process is the use of solid adsorbents.

Of the numerous solid sorbent classes proposed for CO₂ capture, such as amine-modified porous materials, zeolites, metal-organic frameworks, covalent-organic frameworks, and polymers, amine-modified materials are interesting because of their high selectivity and adsorption capacity even at low CO₂ concentrations as they have the ability to perform physisorption within the pores and chemisorption due to the acid-base interactions between amine groups and CO₂.

Here we present a series of activated carbon (AC) adsorbents functionalized with guanidine carbonate (G₂CO₃) via wet impregnation at varying loading levels with the objective of improving the CO₂ adsorption capacity of activated carbon in low CO₂ concentrations and in the presence of humidity. We find that the BET surface area of the adsorbent shows a decreasing trend with increasing guanidine loading, going from 1014 m²/g in the AC control to 329 m²/g at 20% nominal guanidine weight loading and 90 m²/g at 60% nominal guanidine weight loading. In single-component CO₂ adsorption, non-functionalized activated carbon performs better than 10%, 15% and 20% nominal G₂CO₃ at CO₂ pressures above 60 kPa, however, all functionalized adsorbents surpass it for CO₂ pressures under 8 kPa. In both dry and humid DAC conditions, the addition of G₂CO₃ improves CO₂ adsorption from 1% nominal loading. The adsorbent with 20% nominal G₂CO₃ shows the highest adsorption capacity overall, adsorbing approximately 3.5 times more CO₂ than the AC control in dry DAC conditions and 12 times more in humid (70% RH) DAC conditions. These adsorbents could present a viable alternative to other classes of adsorbents that are adversely affected by the presence of humidity.

Some directions of future work are replacing the anion in G₂CO₃ with longer groups to improve amine accessibility and testing analogous AC adsorbents loaded with Na₂CO₃ to evaluate the effect of the carbonate ion on CO₂ adsorption.