Fabrication of Si-CHA/SSZ-13 bilayer membrane for CO2/CH4 separation in wet conditions

One of the most challenging issues in the SSZ-13 membranes is the negative impact of moisture on gas permeance and separation properties. Increasing the surface hydrophobicity of the membranes layer with a layer which has a high Si/Al ratio and can reduce the moisture effect on gas permeation. Therefore, in this paper, Si-CHA/SSZ-13 bilayer membrane was synthesized and evaluated for CO2/CH4 separation in wet conditions. Also, the existence of ion exchange property of the bilayer membrane was evaluated due to the cations in the SSZ-13 structure. Characterizations including FESEM, XRD, and EDX revealed that the Si-CHA layer has been successfully grown on the SSZ-13 layer, while there was no damage in crystal structure. The results showed that in the wet condition, CO2 permeance reduced about 89% in the SSZ-13 membrane, while this amount was 70% in the bilayer membrane at 100 kPa pressure difference and 30 °C, indicating that the bilayer membrane is more resistant in wet conditions. Also, the ideal selectivity of CO2/CH4 raised from 43 to 54 in wet gas conditions after adding the Si-CHA layer on the SSZ-13 membrane. Moreover, ion exchanging of the membranes demonstrated that CO2/CH4 selectivity increased almost five times in bilayer membranes, while that of the Si-CHA membrane slightly changed. Si-CHA layer made a hydrophobic surface on SSZ-13 layer and reduced the negative effect of the humidity on gases separation with maintaining the ion-exchanging capability of the membrane, simultaneously. These results confirm the advantages of ion-exchange property of the bilayer membrane over the Si-CHA membrane.