Development of cesium-incorporated carbon membranes for CO2 separation under humid conditions

Cesium-incorporated carbon membranes were developed to overcome the problem of separation performance decline under humid conditions. A cardo-type polyimide was chosen as the precursor for the carbon membranes. Cesium carbonate (Cs2CO3) was chosen as a Cs source and blended with precursor solution. Carbonization was conducted at 600 °C for 3 h under a nitrogen atmosphere. Analyses by atomic absorption spectrometry, inductively coupled plasma mass spectrometry and electron probe microanalysis revealed that Cs was incorporated into the carbon membranes uniformly and that the concentration of the incorporated Cs could be controlled by the concentration of Cs2CO3 in the precursor. The separation performance was evaluated using a CO2/N2 gas mixture with controlled relative humidity at 40 °C at atmospheric pressure. The original carbon membrane without Cs had a lower CO2 permeance and CO2/N2 separation factor under humid conditions than it did under dry conditions. On the other hand, Cs-incorporated carbon membranes had a higher CO2 permeance and separation factor under humid conditions. The CO2 permeance image and separation factor image using a CO2/N2 gas mixture (CO2/N2 = 5/95%) at 100% relative humidity (RH%) for Cs-incorporated carbon membranes (Cs2CO3 concentration in precursor: 1.4 wt%) were ca. 2.2 × 10−10 m3 (STP) m−2 s−1 Pa−1 and 44, respectively, and were greater than values for the original carbon membrane image. It was revealed by a water vapor sorption experiment and nanopermporometry, that carbon pores became more hydrophilic and that the pore size distribution was shifted to larger pore size by the incorporation of Cs into the carbon membranes. It was suggested that the change in pore size distribution and the pore surface properties played an important role in improving CO2 separation performance under humid conditions.