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

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.

A graphical approach for mixed ratio optimisation in the binary mixed amine solution

Chemical absorption method plays an important role in the process of CO2 separation. One major problem for chemical absorption is huge energy consumption, which is affected by the performance of absorbents. Developing a type of absorbent with high absorption capacity and low regenerative energy consumption is a research topic that attracts attention. The combination of two or more amines is one way to develop new solvents. However, the change of amine liquid ratio can cause a series of complex nonlinear changes in absorption capacity, absorption heat, the heat of vaporisation and sensible heat. It is of interest to visualise the amine solution mixing ratio optimisation to help reduce the energy consumption and increase the absorption capacity. Derivative analysis of standardised vs variables diagram (DSVD), a kind of graphical method based on maximum benefit and minimum consumption, is proposed to determine the optimal mixing ratio of binary amine solution. This novel approach helps to visualise what kind of amines are not suitable for compounding, what kind of amines have the best compounding ratio, and how to determine the optimal compounding ratio. The optimal mixing ratio of the Methyldiethanolamine (MDEA) - Piperazine (PZ) system and MDEA - Monoethanolamine (MEA) were optimised by this method. The optimal ratio of MDEA - PZ and MDEA - MEA are 0.6 (PZ: MDEA = 0.6:0.4, wt.%) and 0.8 (MEA: MDEA = 0.8:0.2, wt.%).

Experimental studies on CO2 absorption and solvent recovery in aqueous blends of monoethanolamine and tetrabutylammonium hydroxide

Solvent-based post-combustion CO₂ capture process is recently carried out using chemical absorption with aqueous blends of Monoethanolamine (MEA) and Ionic Liquids (IL) as promising solvents. In the present work, the blends of MEA and TetraButylAmmonium Hydroxide [TBA][OH] have been used for CO₂ absorption and desorption process. The solubility of CO₂ is investigated with aqueous mixtures for various carbon loading time by varying compositions of MEA and [TBA][OH] as 30 wt%, 28 wt%, 25 wt%, 20 wt% and 0 wt%, 2 wt%, 5 wt%, 10 wt% respectively. It increases with increasing IL concentration for all the aqueous mixtures. The solvent regeneration was also studied at different temperatures in order to recover and reuse the solvent for cyclic absorption. The slight decrease in CO₂ solubility was noted for 20 wt% MEA +10 wt% [TBA][OH] mixture. However, this mixture exhibits higher absorption/desorption rate and regeneration efficiency than other mixtures. The regeneration energy of this mixture was also calculated as 28.6 kJ/mol of CO₂, which is 32% less than that of baseline 30 wt% MEA. Furthermore, the physicochemical properties such as density, viscosity and surface tension for all the solvent blends were studied experimentally.