Spectroscopic Investigation of the Mechanisms Responsible for the Superior Stability of Hybrid Class 1/Class 2 CO₂ Sorbents: A New Class 4 Category

Hybrid Class 1/Class 2 supported amine CO₂ sorbents demonstrate superior performance under practical steam conditions, yet their amine immobilization and stabilization mechanisms are unclear. Uncovering the interactions responsible for the sorbents robust features is critical for further improvements and can facilitate practical applications. We employ solid state ²⁹Si CP-MAS and 2-D FSLG ¹H-¹³C CP HETCOR NMR spectroscopies to probe the overall molecular interactions of aminosilane/silica, polyamine [poly(ethylenimine), PEI]/silica, and hybrid aminosilane/PEI/silica sorbents. A unique, sequential impregnation sorbent preparation method is executed in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) setup to decouple amine binding mechanisms at the amine-silica interface from those within bulk amine layers. These mechanisms are correlated with each sorbents resistance to accelerated liquid H₂O and TGA steam treatments (H₂O stability) and to oxidative degradation (thermal stability). High percentages of CO₂ capture retained (PCR) and organic content retained (OCR) values after H₂O testing of N-(3-(trimethoxysilyl)propyl)ethylenediamine (TMPED)/PEI and (3-aminopropyl)trimethoxysilane (APTMS)/PEI hybrid sorbents are associated with a synergistic stabilizing effect of the amine species observed during oxidative degradation (thermal gravimetric analysis-differential scanning calorimetry, TGA-DSC). Solid state NMR spectroscopy reveals that the synergistic effect of the TMPED/PEI mixture is manifested by the formation of hydrogen-bonded PEI-NH₂···NH₂-TMPED and PEI-NH₂···HO-Si/O-Si-O (TMPED, T²) linkages within the sorbent. DRIFTS further determines that PEI enhances the grafting of TMPED to silica and that PEI is dispersed among a stable network of polymerized TMPED in the bulk, utilizing H-bonded linkages. These findings provide the scientific basis for establishing a Class 4 category for aminosilane/polyamine/silica hybrid sorbents.