(395ar) Chemical Aspects of CO2 Adsorption On Mesoporous Silica Modified With n-Propylamines

Today, pilot plants for CO₂ capture use chemical absorption processes with various amines. These amines selectively absorb CO₂ from flue gases. The major drawbacks with such amines are the high energy demand for regeneration, corrosion of equipments, and the evaporation and leaching of the amine. Tethered amines on porous solid supports could possibly reduce the energy required for regeneration, limit the corrosion, and the evaporation of the amines.

We have studied the detailed chemistry that is involved when CO₂ reacts with tethered amines on model supports. Chromatographic silica and mesostructured silicas with cubic structures were modified with propylamine groups and studied by in situ FTIR spectroscopy and volumetric carbon dioxide uptake experiments. A range of equilibrium and transient spectra were recorded after and during uptake of CO₂ at different partial pressures. In addition, we will discuss how a trace component of carbonyl sulfide (COS) in the flue gas reacts and interacts with such propylamine groups. COS is known to interfere with the liquid phase chemistry of some amines used for CO₂ capture.

Numerous studies have suggested that certain ion pairs (ammonium carbamates) form with dry carbon dioxide, and that bicarbonates form under moist conditions. Complexities relating to analyzing and assigning the infrared spectra have rendered conflicting interpretations. We try to resolve such outstanding disagreement about the species that form during chemisorption of carbon dioxide on propylamines tethered on silica. We show that propylammonium-propylcarbamate ion pairs form for both dry and moist carbon dioxide. Significant amounts of propylcarbamic acid and its stabilized form of silylpropylcarbamate ester are formed by slow condensation on silanol groups under dry conditions. More ammonium carbamates are formed with moist carbon dioxide than with dry carbon dioxide, but no bicarbonates or carbonates were observed.

Infrared spectroscopy was also applied to directly quantify physisorbed and chemisorbed CO₂ on the propylamine-modified silicas. Irrespectively of the amine density, only a minor part of the CO₂ physisorbed at low (10 kPa) partial pressure of CO₂. At 101 kPa of CO₂ the relative contribution of physisorption of CO₂ to the total uptake of CO₂ increased to ~35 % at 101 kPa of CO₂ when the propylamine surface density was low or medium (0.87-1.67 NH₂/nm²). When the amine density was >2.0 NH₂/ nm², chemisorption dominated (up to 90 %) also at 101 kPa of CO₂. At low or medium amine surface densities, the concentration of ammonium carbamate scaled linearly with the density of amine, but the concentration of such ion pairs increased very significantly when the amine density was high. The significant increase in the formation of ammonium carbamate ion pairs is consistent with that sufficiently close propylamine groups are needed to form such ion pairs. This thesis was supported by the observation of a significant fraction of other chemisorbed moieties (propylcarbamic acid and silylpropylcarbamate) for sorbents with low amine surface densities, where significant amount of ion pairs could not form.