(728g) Direct Evaluation of CO2 Adsorption Thermodynamics and Origins of Lower Amine Efficiency on Molecular Type I Adsorbents

The combination of a breakthrough reactor (BTR) and a commercial differential scanning calorimetry (DSC) is a versatile apparatus to study adsorption thermodynamic and kinetics of CO₂ adsorption on solid aminosilica materials under humid and dry conditions.  The breakthrough reactor is utilized to measure CO₂ adsorption capacity and efficiency while the heat of adsorption and desorption are assessed in the DSC simultaneously.

In this study, a variety of linear molecular amines including diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA) on silica material have been utilized to synthesize impregnated Type I sorbents with various surface densities of amines on silica material have been used as models for polyethyleneimine (PEI).  These amines contain two primary amines with varying primary:secondary amine ratios which increase in the order of DETA, TETA, TEPA, and PEHA.  The experiments were carried out in a 10%CO₂/He gas flow at four different temperatures.  The CO₂ adsorption capacity, heat of adsorption, and nitrogen efficiency of aminosilica materials for all loadings have been studied as a function of secondary amine increment.

The results indicate that at low surface densities, CO₂ adsorption decreases as temperature increases, while at high weight loadings, it increases with temperature.  In addition, it was found that at low surface densities, lower secondary to primary amine ratios are in the favor of CO₂ adsorption while it behaves conversely at high weight loadings. Adsorption efficiency increases as surface density rises up to a moderate value (around 30 wt. % amine/SiO₂), and then decreases with further increases in the amine loading.  It was also found that at low surface densities, the efficiency is directly proportional to the primary to secondary amine ratio while inversely proportional to temperature.  At low surface densities, increasing amine weight loading leads to higher heat of adsorption even when normalized per mole of CO₂ adsorbed suggesting that a number of primary amines remain inaccessible to CO2 at lower temperatures.  At high weight loadings, heat of adsorption passed through a maximum as temperature increases.  Heats of adsorption are higher for lower secondary to primary amine ratios, which indicates that primary amines always have higher heats of adsorption.  

These results demonstrate the adsorption thermodynamics in a real scenario (rather in a combined static adsorption equilibrium apparatus combined with calorimetry) and have recently been used to describe the spatiotemporal dynamics of an isothermal aminosilica adsorption bed.