Carbon dioxide selective mixed-matrix membranes formulation and characterization using rubbery substituted polyphosphazene

Substituted polyphosphazene (PPZ) used in the formulation of mixed-matrix membranes conforms to make good interface contact with the molecular sieve particles due to its rubbery nature. The methoxy ethoxy ethanol (MEE) substituted polyphosphazene is CO2 selective with CO2/N2 and CO2/CH4 ideal separation factors of 43 and 15, respectively, at 22 °C. This study shows the improvement of the CO2 separation factor over light gases by formulating mixed-matrix membranes using silicoaluminophosphate (SAPO-34) and modified 2D layered aluminophosphate (ALPO). The synthesis and characterization of these molecular sieve materials is described. The CO2/CH4 and CO2/N2 ideal separation factors increase to 17 and 54 for the membrane formed by PPZ and SAPO-34. A more significant increase in separation factors is observed with decreasing membrane temperature. The pore blocking effect by CO2 in the PPZ–SAPO mixed-matrix membranes was not observed even though this behavior was reported in literature for SAPO-34 molecular sieve membranes. This is potentially due to lower adsorption of CO2 in the pores of SAPO-34 when dispersed in the continuous polymer phase as compared to SAPO-34 particles. The layered aluminophosphate zeolite was exfoliated to create particles with high aspect ratio that were dispersed in the PPZ polymer to form composite membranes. The CO2/H2 separation factor observed in these composite membranes was 12.0 as compared to a value of 8.5 for PPZ membrane at 22 °C. The structure of these membranes is expected to be CO2 selective flakes with high aspect ratio dispersed in the polymer which retard the transport of less interactive gases resulting in the improvement of separation factors. A comparison of gas transport data between the two types of mixed-matrix membranes also provides information regarding the structure of these membranes.