Description of gas transport in perfluoropolymer/SAPO-34 mixed matrix membranes using four-resistance model

Mixed matrix membranes (MMMs) are considered a viable solution for coupling the outstanding gas separation performances of inorganic porous materials with the ease of fabrication of polymeric membranes. It is known that the heterogeneous system may include a polymer shell of modified permeability surrounding the filler particles. In addition to that, modifications of the filler were evidenced from the comparison of the permeability of selective perfluoropolymer/SAPO-34 MMMs with existing extended Maxwell-like model calculations. This is in agreement with experimental and theoretical evidences of the presence of barriers to gas transport in the filler. A 4-phase approach for the transport of gas through MMMs is proposed here to quantitatively describe the modified layers, both within the Maxwell model and by finite element analysis. The four phases are: bulk and modified polymer and bulk and modified filler. The latter explicitly accounts for the barriers to mass transport. The changes in the modified layers are quantitatively determined by numerical optimization procedures based on Maxwell’s effective medium analytical formula and a finite element solution of the transport differential equations. The new approach is applied to the minimization of the residuals between each model and an extensive set of experimental permeability data (He, H2, CO2, N2) through Hyflon AD60X/SAPO-34 MMMs, including different filler particle sizes (0.2, 1.5 and 2 micron), aspect ratios (2, 3 and 9) and loadings (20, 30, 35 and 44 vol %). A MMM subset containing impermeable fillers gives information on the transport properties of the sole matrix. Both the effective medium and the microscopic 4-phase approaches yield physically sound descriptions of the permeability trends, but FEM simulations also provide spatially resolved gas flows and concentrations throughout the systems. The focus on the real physical phenomena makes the four-phase approach a powerful tool for understanding the MMM transport and improving membrane design