(311b) Effect of Residual Solvent On the Gas Separation Properties of Mixed Matrix Membranes

Mixed matrix membranes (MMMs), typically composed of inorganic particles, such as zeolites, distributed in a polymeric matrix, have been studied extensively over the last two decades. Many factors including the physical properties of the polymer forming the matrix, and the zeolite particles (such as particle size and zeolite pore size), as well as the interaction between the zeolite and the polymer (resulting in the formation of a zeolite/polymer interphase with distinct properties) have a tremendous effect on the MMM performance, which is hard to predict prior to experimentation. It is a well-known phenomenon that smaller particles provide more zeolite/polymer contact area, thus possibly favor a more pronounced effect of the interphase on the final membrane performance [1]. Additionally, the solvent type used in MMM fabrication and thermal treatment conditions are among the parameters which may affect the gas permeation properties. Therefore, a suitable annealing process must be applied to the membranes, in order to control the effect of residual solvent on the overall properties of MMMs. In this study, we investigated the effect of thermal treatment conditions and hence the residual solvent on the single and binary gas transport properties of MMMs.

MMMs are prepared by embedding micron- and submicron-sized zeolites (SAPO-34) into Ultem® (T_g, 215°C) polymeric matrix. NMP (n-methyl pyrrolidone) (b.p. 205°C) was used as solvent.  SAPO-34 is a silicaaluminophosphate with a pore size of 3.8 Å, favoring the passage of CO₂ over CH₄ and N₂, and that of O₂ over N₂. MMMs were prepared with 20% zeolite content through optimisation and application of a priming procedure to enhance interaction between the zeolite and the polymer phases. The MMMs prepared were annealed at 150°C (below T_g and b.p. of NMP), 200°C (below T_g,close to b.p. of NMP), and 220°C (above T_g and b.p. of NMP) for 48 hours.

SEM analysis showed good adhesion, TGA analysis showed decreased amounts of solvent, and DSC measurements showed increased T_g values, all increasing with higher annealing temperature. Permeabilities (measured at 35°C and 4 bar upstream pressure) of all gases in MMMs generally decreased whereas selectivities increased, which became more pronounced with decreasing zeolite particle size. CO₂ permeability decreased for membranes annealed below T_g, whereas a slight increase was observed for membranes annealed above T_g. CO₂/CH₄ selectivity significantly increased with increasing annealing temperature reaching to an ideal selectivity of 120 for the membrane containing submicron-sized particles and annealed at 220°C. This was attributed to the plasticization effect of residual solvent favoring the smaller penetrant, as well as the preferential sorption of CO₂ in NMP compared to CH₄, decreasing with the decreasing amount of residual solvent. Despite the competitive sorption phenomena occurring in permeation of gas mixtures, mixed gas measurements (at 35°C and 4 bar) of MMM with submicron-sized particles and annealed above T_g showed a CO₂/CH₄ selectivity of 52 at 50/50 molar composition which is higher than that of pure polymer (39 at 50/50) but lower than ideal selectivity for the same membrane. These results lead us to the conclusion that residual solvent affects the gas transport properties in MMMs significantly; the residual solvent trapped in the zeolite/polymer interphase is relatively more difficult to remove than that in the bulk of the polymer and therefore smaller size zeolite particles enhance the influence of residual solvent on the MMM transport properties.

[1] Tantekin-Ersolmaz, S.B., Atalay-Oral, C., Tatlıer, M., Erdem-Senatalar, A., Schoeman, B., Sterte, J., “Effect of Zeolite Particle Size on the Performance of Polymer-Zeolite Mixed- Matrix Membranes”, Journal of Membrane Science, 175(2), 285-288 (2000).