(465a) Amine-Silica Nanobubbles for CO2 Sorption

Recent efforts in the development of post-combustion CO₂ capture materials have focused on combining the advantages of solid capture materials with those of liquid amines by both grafting and embedding amine groups onto solids.  Here, we report on the application of this sorbent design principle to the encapsulation of polyethylenimine and tetraethylenepentamine (PEI and TEPA, two liquid amines widely investigated for CO₂ capture) in silica “nanobubbles”, as nanoencapsulation may result in unique properties for CO₂ sorption.

Porous silica shells were prepared in a reverse microemulsion using a straightforward one-pot synthesis method developed in our laboratory.  The resulting “nanobubbles” consist of a microporous silica shell with wall thickness which can be tailored between ~4-30nm encapsulating a pronounced cavity of ~15 nm diameter. Liquid amines were introduced into these porous silica nanobubbles by a simple wet impregnation approach, resulting in controllable liquid amine loadings of ~25 – 75wt%. The CO₂ sorption kinetics were then measured via isothermal thermo­gravimetric analysis (TGA) over a temperature range of 60 – 105^oC during alternating exposure to a high purity CO₂ stream (P_CO2 = 1 bar), followed by desorption into a pure inert gas stream (He) at the same temperature.

Results show distinct contributions from kinetic and thermodynamic effects with increasing temperature, and, more importantly, a drastic acceleration of the sorption kinetics for the nanoencapsulated PEI in comparison to the free liquid PEI by about three orders of magnitude.  Furthermore, the specific CO₂ sorption capacity (per mole of PEI) is twice as high for the nanoencapsulated PEI than for the unconfined liquid PEI (~125 mg^.CO₂/g^.PEI vs ~250 mg^.CO₂/g^.PEI, respectively).  The strongly accelerated kinetics can be explained by the drastic reduction of the diffusion length for the nanoconfined material, and hence a complete removal of mass transfer limitations during CO₂ capture.  The reason for the enhanced sorption capacity is still under investigation, but is likely due to configurational changes of PEI in the nanoconfinement.  

The nanoencapsulated TEPA performed best at a 1:1 ratio for greater amounts of TEPA and silica (142 mg CO₂/g sorbent), whereas the PEI embedded material performed best at a 1:1 ratio of PEI to silica.  This suggests that the MeOH solvent plays a different role in entraining the material into the silica nanobubbles.  The TEPA material is less stable than PEI under impregnation conditions, and it is likely that some TEPA volatilizes during the embedding procedure.  This effect appears to increase with higher concentrations of MeOH.  Furthermore, TEPA-nanobubble material is not stable under TGA operating conditions, as the weight of the material decreases past the initial weight even after the first cycle.  This loss in mass is likely due to the high vapor pressure of TEPA at T > 60 °C.  In contrast, PEI@SiO₂ showed stable operation over multiple cycles at temperatures as high as 75 °C, suggesting that the PEI-nanobubble sorbent might constitute a feasible sorbent material with extremely fast sorption kinetics.  Further investigations into the mechanism of PEI sorption into the nanobubbles are on-going.  Overall, the nanoencapsulated PEI shows promise as a hybrid liquid-solid CO₂ sorbent material, and results to-date suggest that further improvement of the performance will be possible based on appropriate nanostructuring of the “nanobubbles”.