(583l) First-Principles Assessment of Carbon Dioxide (CO2) Capture Mechanisms in Aqueous Piperazine (PZ) Solution

Energy-efficient and low-cost methods for carbon dioxide (CO₂) capture from flue gases have been sought to curb greenhouse gas emissions while fossil fuels will likely remain a dominant energy source in the next few decades.^1,2 A wet-scrubbing approach using amines has been considered the most promising short-term solution.³Â Its widespread implementation is limited by the high cost associated with the high parasitic energy consumption during solvent regeneration, along with amine degradation and corrosion problems.^4–6 Aqueous piperazine (PZ), a cyclic diamine with two secondary amine groups, has been proposed as an effective solvent for CO₂ capture due to its many advantages such as low regeneration energy, low rates of thermal and oxidative degradation, and low corrosivity relative to other widely used amines including MEA.^7–9 In addition, PZ exhibits a relatively high absorption rate.

Here, we attempt to elucidate molecular mechanisms underlying the reaction of CO₂ with aqueous PZ using combined ab initio and classical force field calculations, including the roles of PZ carbamate (PZCOO^-) and protonated PZ (PZH⁺).¹⁰ From AIMD simulations combined with RDF analysis, we confirmed that PZ and PZCOO^- may be protonated, while protonation of PZH⁺ hardly occurs. As the available basic sites are well-linked via the hydrogen-bonded water network, the relative concentrations of protonated/deprotonated amine species can be predicted from their relative basicities. We also find the probability of protonated PZ carbamate (H⁺PZCOO^-) formation to increase with CO₂ loading. This is not only because of the greater availability of PZ relative to PZCOO^-, but also because of the thermodynamic favorability of proton transfer from PZH⁺ to PZCOO^- (i.e., PZH⁺ + PZCOO^- → PZ + H⁺PZCOO^-).

Regarding the tendency of PZH⁺ and PZCOO^- to react with CO₂, we found that while PZCOO^- can easily react with CO₂ forming dicarbamate (COO^-PZCOO^-), the direct CO₂ reaction with PZH⁺ hardly occurs. Rather, our AIMD simulations consistently demonstrate that the proton is first released from PZH⁺, followed by the PZ + CO₂ reaction forming PZCOO^-, which can then easily abstract a proton to form H⁺PZCOO^-. This behavior has largely been attributed to the low pKa of PZH⁺. However, our spatial distribution analysis using classical MD simulations suggests that H₂O tends to more densely pack around PZH⁺, which may hinder CO₂ accessibility relative to PZ/PZCOO^-, further suppressing the PZH⁺ + CO₂ reaction.¹⁰

While CO₂ capture by PZCOO^- appears to be kinetically possible, our free energy calculations predict COO^-PZCOO^- formation to be highly unlikely at thermodynamic equilibrium relative to the monocarbamates.¹⁰ However, the formation of COO^-PZCOO^- at high CO₂ loadings has been reported from nuclear magnetic resonance (NMR) measurements, albeit in small concentrations.^11,12 Analysis of the distributions of PZ/PZCOO^-/COO^-PZCOO^- near the gas-liquid surface shows that although PZ accumulates at the gas-solvent interface, PZCOO^- also remains near the surface.¹⁰ This hints that while PZ may predominantly capture CO₂, the PZCOO^- + CO₂ reaction to form COO^-PZCOO^- may be more likely to occur with increasing CO₂ loading as PZ is depleted. Based on our results, we speculate that COO^-PZCOO^- is mostly produced from the PZCOO^- + CO₂ reaction near the gas-solvent interface, rather than interconversion with the monocarbamates (PZCOO^-/H⁺PZCOO^-) in bulk solution.

The results of this study provide important insights into fundamental CO₂ absorption mechanisms in aqueous PZ solution. The improved mechanistic understanding may aid in the development of better kinetic and thermodynamic models to predict and optimize the performance of PZ in concentrated solution and as a rate-promoter in PZ-blends. Furthermore, it may provide valuable guidance on how to mitigate the disadvantages of PZ as well as in the design of new PZ-containing mixed solvents.

References

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