Stephanie A. Didas

Dramatic Enhancement of CO2 Uptake by Poly(ethyleneimine) Using Zirconosilicate Supports

The CO(2) adsorption characteristics of prototypical poly(ethyleneimine)/silica composite adsorbents can be drastically enhanced by altering the acid/base properties of the oxide support via incorporation of Zr into the silica support. Introduction of an optimal amount of Zr resulted in a significant improvement in the CO(2) capacity and amine efficiency under dilute (simulated flue gas) and ultradilute (simulated ambient air) conditions. Adsorption experiments combined with detailed characterization by thermogravimetric analysis, temperature-programmed desorption, and in situ FT-IR spectroscopy clearly demonstrate a stabilizing effect of amphoteric Zr sites that enhances the adsorbent capacity, regenerability, and stability over continued recycling. It is suggested that the important role of the surface properties of the oxide support in these polymer/oxide composite adsorbents has been largely overlooked and that the properties may be even further enhanced in the future by tuning the acid/base properties of the support.

Tuning Cooperativity by Controlling the Linker Length of Silica-Supported Amines in Catalysis and CO2 Capture

Cooperative interactions between aminoalkylsilanes and silanols on a silica surface can be controlled by varying the length of the alkyl linker attaching the amine to the silica surface from C1 (methyl) to C5 (pentyl). The linker length strongly affects the catalytic cooperativity of amines and silanols in aldol condensations as well as the adsorptive cooperativity for CO(2) capture. The catalytic cooperativity increases with the linker length up to propyl (C3), with longer, more flexible linkers (up to C5) providing no additional benefit or hindrance. Short linkers (C1 and C2) limit the beneficial amine-silanol cooperativity in aldol condensations, resulting in lower catalytic rates than with the C3+ linkers. For the same materials, the adsorptive cooperativity exhibits similar trends for CO(2) capture efficiency.

Role of amine structure on carbon dioxide adsorption from ultradilute gas streams such as ambient air.

A fundamental study on the adsorption properties of primary, secondary, and tertiary amine materials is used to evaluate what amine type(s) are best suited for ultradilute CO(2) capture applications. A series of comparable materials comprised of primary, secondary, or tertiary amines ligated to a mesoporous silica support via a propyl linker are used to systematically assess the role of amine type. Both CO(2) and water adsorption isotherms are presented for these materials in the range relevant to CO(2) capture from ambient air and it is demonstrated that primary amines are the best candidates for CO(2) capture from air. Primary amines possess both the highest amine efficiency for CO(2) adsorption as well as enhanced water affinity compared to other amine types or the bare silica support. The results suggest that the rational design of amine adsorbents for the extraction of CO(2) from ambient air should focus on adsorbents rich in primary amines.

Structural changes of silica mesocellular foam supported amine-functionalized CO2 adsorbents upon exposure to steam

Three classes of amine-functionalized mesocellular foam (MCF) materials are prepared and evaluated as CO(2) adsorbents. The stability of the adsorbents under steam/air and steam/nitrogen conditions is investigated using a Parr autoclave reactor to simulate, in an accelerated manner, the exposure that such adsorbents will see under steam stripping regeneration conditions at various temperatures. The CO(2) capacity and organic content of all adsorbents decrease after steam treatment under both steam/air and steam/nitrogen conditions, primarily due to structural collapse of the MCF framework, but with additional contributions likely associated with amine degradation during treatment under harsh conditions. Treatment with steam/air is found to have stronger effect on the CO(2) capacity of the adsorbents compared to steam/nitrogen.

Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species

Three primary amine materials functionalized onto mesoporous silica with low, medium, and high surface amine coverages are prepared and evaluated for binary CO2/H2O adsorption under dilute conditions. Enhancement of amine efficiency due to humid adsorption is most pronounced for low surface amine coverage materials. In situ FT-IR spectra of adsorbed CO2 on these materials suggest this enhancement may be associated with the formation of bicarbonate species during adsorption on materials with low surface amine coverage, though such species are not observed on high surface coverage materials. On the materials with the lowest amine loading, bicarbonate is observed on longer time scales of adsorption, but only after spectral contributions from rapidly forming alkylammonium carbamate species are removed. This is the first time that direct evidence for bicarbonate formation, which is known to occur in liquid aqueous amine solutions, has been presented for CO2 adsorption on solid amine adsorbents.

Spectroscopic Characterization of Adsorbed 13CO2 on 3-Aminopropylsilyl-Modified SBA15 Mesoporous Silica

Multiple chemisorption products are found from the interaction of CO2 with the solid-amine sorbent, 3-aminopropyl silane (APS), bound to mesoporous silica (SBA15) using solid-state NMR and FTIR spectroscopy. We employed a combination of both 15N{13C} rotational-echo double-resonance (REDOR) NMR and 13C{15N} REDOR to determine the chemical identity of these products. 15N{13C} REDOR measurements are consistent with a single 13C-15N pair and distance of 1.45 Å. In contrast, both 13C{15N} REDOR and 13C CPMAS are consistent with multiple 13C products. 13C CPMAS shows two neighboring resonances, whose chemical shifts are consistent with carbamate (at 165 ppm) and carbamic acid. The 13C{15N} REDOR experiments resonant at 165 ppm show an incomplete buildup of the REDOR data to ∼90% of the expected maximum. We conclude this 10% missing intensity corresponds to a 13C NMR species that resonates at the identical chemical shift but that is not in dipolar contact with 15N. These data are consistent with the presence of bicarbonate, HCO3-, since it is commonly observed at ∼165 ppm and lacks 15N for dipolar coupling.

15N Solid State NMR Spectroscopic Study of Surface Amine Groups for Carbon Capture: 3-Aminopropylsilyl Grafted to SBA-15 Mesoporous Silica

Materials composed of high-porosity solid supports, such as SBA-15, containing amine-bearing moieties inside the pores, such as 3-aminopropylsilane (APS), are envisioned for carbon dioxide capture; solid-state 15N NMR can be highly informative for studying chemisorption reactions. Two 15N-enriched samples with different APS loadings were studied to probe the identity of the pendant molecules and structure of the chemisorbed CO2 species. 15N cross-polarization magic-angle spinning NMR provides unique information about the amines, whether they are rigid or dynamic, by measuring contact time curves and rotating frame, T1ρ(15N), relaxation. Both carbamate and carbamic acid are formed; carbamic acid is shown to be less stable than carbamate. After desorption, a steady state for the chemisorbed reaction product is reached, leaving behind carbamate. 15N NMR monitors the evolution of the species over time. During desorption, APS is regenerated, but the ammonium propylsilane intensity does not change, leading us to conclude that carbamic acid desorbs, while carbamate (to which ammonium propylsilane is ion paired) persists. A secondary ditehtered amine present does not react with CO2, and we posit this may be due to its rigidity. These findings demonstrate the versatility of solid-state NMR to provide information about these complex CO2 reactions with solid amine sorbents.