Membrane preparation
Zeolite activation is of great importance which affects gas permeation properties. Accordingly, zeolite powder was dried in vacuum oven at 180 °C for 24 h in order to remove water from its cavities. Weight comparison of the zeolite before and after drying in vacuum oven shows that the dried powder was almost 50% after activation. Activated particles were put in desiccator to be away from humidity and other organic sorption.
In order to have defect-free membranes some parameters should be considered. Polymer concentration in the solution is the most important factor that can lead to good dispersion of zeolite and eliminate sedimentation in the prepared membranes. But increased polymer content in the solution, leads to higher solution viscosity and weaker homogenous dispersion of particles, resulting in thicker membrane with lower permeance. In this case the amount of casting solution should be reduced. In this research, 15 wt% polymeric solution was prepared by dissolving Pebax1074 in 1-buthanol at 90–100 °C under reflux for 24 h. For preparation of MMMs, in the first step zeolite was added to solvent and the mixture was stirred for 2 h. Then 10–15 wt% of polymer was added to mixture in order to prime the solution. This priming causes coating of fillers with polymer in dilute solution and helps dispersion and adhesion between polymer and filler [25]. After 24 h, a homogeneous solution was achieved and the remaining amount of the polymer was added to the solution to have a 15 wt% polymer solution and zeolite content ranging from 10 wt% to 35 wt% of polymer. MMMs with 35 wt% zeolite loading were fabricated but they were too brittle for performance and characterization tests, thereby they were not used. Subsequently, the Pebax-zeolite suspension was casted on Teflon coated petri dish, warmed at 60 °C. Petri dish was held in oven at 60 °C, overnight. After solvent evaporation MMM was easily peeled off from petri dish and membrane was put in a vacuum oven at 50 °C for another 24 h to remove residual solvent. Finally, the fabricated membrane was allowed to cool to room temperature, followed by storing in desiccator for further experiments. Thickness of the films was determined using a digital micrometer and was about 68 μm for the neat one, 80 μm for 10%MMM, 93 μm for 20%MMM and 105 μm for 30%MMM. Fig. 1 shows the step-by-step methodology of MMM preparation.