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

Abstract

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 CO 2 selective with CO 2/N 2 and CO 2/CH 4 ideal separation factors of 43 and 15, respectively, at 22 °C. This study shows the improvement of the CO 2 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 CO 2/CH 4 and CO 2/N 2 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 CO 2 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 CO 2 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 CO 2/H 2 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 CO 2 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.