Probing the Adsorption/Desorption of CO₂ on Amine Sorbents by Transient Infrared Studies of Adsorbed CO₂ and C₆H₆

CO₂ diffusion limitations and readsorption of desorbed CO₂ during removal from immobilized amine sorbents could significantly reduce the effectiveness of CO₂ capture processes. To decouple CO₂ diffusion from desorption/readsorption on silica and tetraethylenepentamine (TEPA)/silica sorbents, a new transient diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method was carried out by using benzene as a surrogate probe molecule. Comparison of the infrared intensity profiles of adsorbed CO₂ and Si–OH (which adsorbs benzene) revealed that slow rates of CO₂ uptake and desorption are a result of (i) CO₂ diffusion through an interconnected network produced from CO₂ adsorbed inside of the amine/silica sorbent pores and (ii) readsorption of CO₂ on the amine sites inside of the pores and at the external surface of the sorbents. High rates of CO₂ adsorption/desorption onto/from the immobilized amine sorbents could be achieved by sorbents with low amine density at the external surfaces and pore mouths

In Situ ATR and DRIFTS Studies of the Nature of Adsorbed CO₂ on Tetraethylenepentamine Films

CO₂ adsorption/desorption onto/from tetraethylenepentamine (TEPA) films of 4, 10, and 20 μm thicknesses were studied by in situ attenuated total reflectance (ATR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques under transient conditions. Molar absorption coefficients for adsorbed CO₂ were used to determine the CO₂ capture capacities and amine efficiencies (CO₂/N) of the films in the DRIFTS system. Adsorption of CO₂ onto surface and bulk NH₂ groups of the 4 μm film produced weakly adsorbed CO₂, which can be desorbed at 50 °C by reducing the CO₂ partial pressure. These weakly adsorbed CO₂ exhibit low ammonium ion intensities and could be in the form of ammonium-carbamate ion pairs and zwitterions. Increasing the film thickness enhanced the surface amine–amine interactions, resulting in strongly adsorbed ion pairs and zwitterions associated with NH and NH₂ groups of neighboring amines. These adsorbed species may form an interconnected surface network, which slowed CO₂ gas diffusion into and diminished access of the bulk amine groups (or amine efficiency) of the 20 μm film by a minimum of 65%. Desorption of strongly adsorbed CO₂ comprising the surface network could occur via dissociation of NH₃⁺/NH₂⁺···NH₂/NH ionic hydrogen bonds beginning from 60 to 80 °C, followed by decomposition of NHCOO^–/NCOO^– at 100 °C. These results suggest that faster CO₂ diffusion and adsorption/desorption kinetics could be achieved by thinner layers of liquid or immobilized amines

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 <i>N</i>-(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

Recovering Rare Earth Elements from Aqueous Solution with Porous Amine–Epoxy Networks

Recovering aqueous rare earth elements (REEs) from domestic water sources is one key strategy to diminish the U.S.’s foreign reliance of these precious commodities. Herein, we synthesized an array of porous, amine–epoxy monolith and particle REE recovery sorbents from different polyamine, namely tetraethylenepentamine, and diepoxide (E2), triepoxide (E3), and tetra-epoxide (E4) monomer combinations via a polymer-induced phase separation (PIPS) method. The polyamines provided −NH₂ (primary amine) plus −NH (secondary amine) REE adsorption sites, which were partially reacted with C–O–C (epoxide) groups at different amine/epoxide ratios to precipitate porous materials that exhibited a wide range of apparent porosities and REE recoveries/affinities. Specifically, polymer particles (ground monoliths) were tested for their recovery of La³⁺, Nd³⁺, Eu³⁺, Dy³⁺, and Yb³⁺ (Ln³⁺) species from ppm-level, model REE solutions (pH ≈ 2.4, 5.5, and 6.4) and a ppb-level, simulated acid mine drainage (AMD) solution (pH ≈ 2.6). Screening the sorbents revealed that E3/TEPA-88 (88% theoretical reaction of −NH₂ plus −NH) recovered, overall, the highest percentage of Ln³⁺ species of all particles from model 100 ppm- and 500 ppm-concentrated REE solutions. Water swelling (monoliths) and ex situ, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (ground monoliths/particles) data revealed the high REE uptake by the optimized particles was facilitated by effective distribution of amine and hydroxyl groups within a porous, phase-separated polymer network. In situ DRIFTS results clarified that phase separation, in part, resulted from polymerization of the TEPA-E3 (<i>N</i>-<i>N</i>-diglycidyl-4-glycidyloxyaniline) species in the porogen via C–N bond formation, especially at higher temperatures. Most importantly, the E3/TEPA-88 material cyclically recovered >93% of ppb-level Ln³⁺ species from AMD solution in a recovery–strip–recovery scheme, highlighting the efficacy of these materials for practical applications