(751b) Direct Measurement of Adsorption Thermodynamics of CO2 on Type I Adsorbents

The combination of an automated volumetric adsorption apparatus (Micromeritics ASAP 2020) and a commercial differential scanning calorimetry (DSC) is versatile apparatus to study adsorption energies of CO2 on solid aminosilica adsorbents.  Since volumetric adsorption instrument controls the gas dosing precisely, high energy sites can be investigate at low coverages while interfaced with the DSC, which delivers high resolution data on the heat of adsorption.  Another capability of this combination is to measure differential and integral heat of adsorption separately for each equilibrated pressure point while the adsorption isotherm is collecting via the volumetric adsorption apparatus.  Therefore, one can investigate the energetic heterogeneity of adsorption sites and lateral interaction of adsorbed gas at various gas coverage. This combination has been used to measure the complete thermodynamic profile such as heat of adsorption, Gibbs free energy, and entropy at various temperatures on polyethyleneamine (PEI), which was impregnated on silica material for CO₂ adsorption.

A unique behavior has been observed for the adsorption of CO₂ over 40wt% PEI/SiO₂ at low CO₂ partial pressures (<0.4 Torr).  In general, it is expected that the highest heat of adsorption will be measured at the lowest coverage, but we have determined that the heat of adsorption actually increases with inceasing coverage of CO2, which suggest a minimum requirement of CO₂ partial pressures to chemically adsorb the CO₂ gas on the amine groups.