Advancing Molten Alkali Borate Sorbents for High-Temperature Carbon Capture by Structural Elucidation of the Ionic-Oxide Reaction Mechanism

Abstract

The structural complexity of alkali borates, evident in the wide range of distinct structures that typically comprise these compounds, is responsible for the significant differences observed in the physicochemical properties of their corresponding melts. In this work, the structural transformations arising from carbon capture using molten lithium-sodium orthoborate ((Li_0.5Na_0.5)₃BO₃), a promising new alkali borate sorbent for carbon capture, are investigated to better understand the evolution of various physicochemical properties of the melt by employing in situ high-temperature Fourier transform infrared spectroscopy in conjunction with density functional theory. The carbon capture mechanism is shown to proceed via polymerization of small orthoborate segments (BO₃^{3 −}) into larger structural units, ultimately reaching the metaborate composition (${\mathrm{BO}}_2^{-}$) in the form of Li₃NaB₄O₈ and Na₃B₃O₆ upon complete CO₂ saturation. The introduction of carbonate ions in the non-polymerized orthoborate melt via CO₂-capture has a substantial diluting effect on the density and viscosity of the resulting polymerized CO₂-saturated melt. Investigations into the melting points associated with the various compounds involved in the capture mechanism and the discovery of an apparent second-order phase transition of lithium-sodium pyroborate (Li₂Na₂B₂O₅) past 520 °C further provide new and valuable information as to potential energetically favorable operating conditions for this absorber/regenerator-based carbon capture technology.