Study on the structure and solid-state CO2 absorption behavior of N,N′-di-tert-butylethylenediamine hydrates and their ionic complexes with poly（acrylic acid）

Abstract

In this thesis, we present a crystallization-driven CO₂ capture system based on the crystalline monohydrate of N,N′-di-tert-butylethylenediamine （DBEDA）. DBEDA is a sterically hindered secondary diamine in which bulky tert-butyl substituents on both nitrogen centers suppress conventional carbamate formation and promote water-mediated CO₂ capture. At ambient temperature, DBEDA co-crystallizes with an equimolar amount of water to form a monoclinic monohydrate （Tm = 47.5 °C）. Upon CO₂ exposure, the hydrate undergoes solid-state transformation into a CO₂-loaded crystalline phase with an approximate 1:1:1 stoichiometry of DBEDA, H₂O, and CO₂, accompanied by a 34% expansion in unit cell volume. FT-IR and Raman spectroscopy confirm the formation of protonated amine （R₂NH₂⁺） and bicarbonate （HCO₃⁻） species within the crystal lattice. These results are consistent with a water-mediated bicarbonate capture pathway. DSC analysis gives a CO₂ absorption enthalpy of ~32 kJ/mol, substantially lower than that of conventional amine absorbents. The CO₂-loaded crystal releases CO₂ between 65–90 °C through melting-induced structural destabilization, and the resulting DBEDA/H₂O liquid recrystallizes upon cooling to regenerate the original hydrate. However, the neat hydrate system shows a critical morphological limitation upon repeated cycling. During thermal regeneration, the molten DBEDA/H₂O phase coalesces and recrystallizes into dense crystalline aggregates rather than returning to a finely divided powder. This uncontrolled recrystallization severely restricts CO₂ diffusion in subsequent cycles and reduces the practical reusability of the neat material without mechanical regrinding. The following study introduces poly（acrylic acid） （PAA） as a functional polymer matrix to address this limitation. PAA interacts with DBEDA through acid–base protonation between carboxylic acid groups and secondary amine sites, partially immobilizing DBEDA within the polymer network and suppressing macroscopic melt coalescence during regeneration. In addition, the hygroscopic nature of PAA retains water around the amine sites, sustaining the bicarbonate-forming pathway across repeated cycles. Incorporation of DBEDA into the PAA matrix shifts the morphology from a highly crystalline hydrate to an ionically confined, amorphous complex structure, shortening CO₂ diffusion pathways to reactive amine sites. At the optimized PAA/DBEDA molar ratio of 4, the complex achieves a CO₂ uptake capacity of 2.05 mol CO₂ per mol DBEDA with near saturation within approximately 5 minutes under pure CO₂. In cyclic testing over ten consecutive absorption–desorption cycles at 80 °C, the complex retains 90–95% of its initial CO₂ capacity with regeneration efficiency exceeding 99.9%, without mechanical reprocessing between cycles. These overall findings demonstrate that reversible phase transitions of organic amine hydrates can serve as a viable design principle for low-energy solid CO₂ sorbents, and that a functional polymer matrix can resolve the morphological limitations of crystalline phase-change sorbents while enhancing CO₂ uptake capacity, kinetics, and cycling stability.