Readily regenerable amine-free CO2 sorbent based on a solid-supported carboxylate ionic liquid

Abstract

Accumulation of anthropogenic CO₂ is undoubtedly the major cause of global warming. In addition to reducing emissions, minimising the threatening effects of climate change in the near future might also require the capture of enormous amounts of CO₂ from point sources or from the atmosphere. In this regard, the development of novel affordable and energetically attainable capture technologies is greatly needed. In this work, we report rapid and greatly facilitated CO₂ desorption for amine-free carboxylate ionic liquid hydrates as compared to a benchmark amine-based sorbent. Complete regeneration was achieved at moderate temperature (60 °C) over short capture-release cycles using model flue gas on a silica-supported tetrabutylphosphonium acetate ionic liquid hydrate (IL/SiO₂), whereas the polyethyleneimine counterpart (PEI/SiO₂) only recovered half its capacity after the first cycle in a rather sluggish release process under the same conditions. The IL/SiO₂ sorbent achieved a slightly superior working CO₂ capacity than PEI/SiO₂. The easier regeneration of carboxylate ionic liquid hydrates, which behave as chemical CO₂ sorbents leading to bicarbonate in a 1:1 stoichiometry, is due to their relatively low sorption enthalpies (≈40 kJ mol^-1). The faster and more efficient desorption from IL/SiO₂ fits a first-order kinetic model (k = 0.73 min^-1), whereas a more complex process was observed for PEI/SiO₂ (pseudo-first order initially, k = 0.11 min^-1, pseudo-zero order at later stages). The remarkably low regeneration temperature, the absence of amines and the non-volatility of the IL sorbent are favourable assets to minimise gaseous stream contamination. Importantly, regeneration heats -a crucial parameter for practical application- are advantageous for IL/SiO₂ (4.3 kJ g (CO₂)^-1) vs. PEI/SiO₂, and fall within the range of typical amine sorbents indicating a remarkable performance at this proof-of-concept stage. Further structural design will enhance the viability of amine-free ionic liquid hydrates for carbon capture technologies.

Keywords: Absorption; Carbon dioxide capture; Climate change; Ionic liquids; Kinetics; Regeneration energy.