Comparative Study of Molecular Basket Sorbents Consisting of Polyallylamine and Polyethylenimine Functionalized SBA-15 for CO2 Capture from Flue Gas.

Polyallylamine (PAA)-based molecular basket sorbents (MBS) have been studied for CO2 capture in comparison with polyethylenimine (PEI)-based MBS. The characterizations including N2 physisorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermogravimetric analysis (TGA) showed that PAA (Mn =15 000) is more rigid and has more steric hindrance inside SBA-15 pores than PEI owing mainly to its different polymer structure. The effects of temperature and PAA loading on the CO2 sorption capacity of PAA-based MBS have been examined by TGA by using 100 % CO2 gas stream and compared with PEI/SBA-15. It was found that the capacity of the PAA/SBA-15 sorbent increased with increasing temperature. The optimum capacity of 88 mgCO2  gsorb-1 was obtained at 140 °C for PAA(50)/SBA-15 whereas the optimum sorption temperature was 75 and 90 °C for PEI-I(50)/SBA-15 (PEI-I, Mn =423) and PEI-II(50)/SBA-15 (PEI-II, Mn =25 000), respectively. The capacity initially increased with the increase of PAA loading and then dropped at high amine contents, owing to the increased diffusion barrier. The highest CO2 capacity of 109 mgCO2  gsorb-1 was obtained at a PAA loading of 65 wt %, whereas the PAA(50)/SBA-15 sorbent gave the best amine efficiency of 0.23 molCO2  molN-1 . The effect of moisture was examined in a fixed-bed flow system with simulated flue gas containing 15 % CO2 and 4.5 % O2 in N2 . It was found that the presence of moisture significantly enhanced CO2 sorption over PAA(50)/SBA-15 and greatly improved its cyclic stability and regenerability. Compared with PEI/SBA-15, PAA/SBA-15 possesses a better thermal stability and higher resistance to oxidative degradation. However, the CO2 sorption rate over the PAA(50)/SBA-15 sorbent was much slower.