Amine-dependent CO2 sorption on amine-impregnated Mg2(dobpdc) MOF under humid conditions

Abstract

Amine-functionalized Mg₂(dobpdc) sorbents are prepared and studied systematically using three amines of different sizes: N,N′-dimethylethylenediamine (m-2-m), tetraethylenepentamine (TEPA), and poly(ethyleneimine) (PEI), in order of increasing size. A prototypical amine-appended Mg-based metal–organic framework, m-2-m-Mg₂(dobpdc), is tested under dry direct air capture (DAC) conditions at cold temperatures (<25 °C) with the objective of increasing the CO₂ capture fraction by shifting the step pressure in the isotherm to lower pressures. We observe that the theoretical amine efficiency (one CO₂ to one diamine) could not be achieved due to the failure of the established amine insertion mechanism. In contrast, TEPA-impregnated Mg₂(dobpdc) shows a significant increase in its CO₂ adsorption capacity under humid conditions (3.9 mmol g⁻¹ and 0.33 amine efficiency at 25 °C) compared to dry conditions (0.54 mmol g⁻¹), aided by hydration of amines by water at elevated relative humidities (≥50% RH), which frees some amine chains and alleviates diffusion resistances along the MOF pore. On the other hand, both branched and linear PEI-impregnated Mg₂(dobpdc) samples undergo morphological degradation after humid adsorption/desorption cycles, likely due to the ineffective protection of the open metal sites in the MOF from water by the higher molecular weight amines. While degradation of PEI-impregnated Mg₂(dobpdc) raises a concern about the overall stability of poly(amine)-impregnated Mg₂(dobpdc) materials, the TEPA-impregnated sample shows stable performance over 20 humid sorption/desorption cycles with N₂ purge for desorption and over 8 humid cycles with vacuum desorption.

Keywords: Direct air capture; Humidity; Metal organic frameworks; DAC; Hydration; Degradation.