Zeolite-based mixed matrix membranes for hazardous gas removal
Well known as adhesion promoters between polymers and inorganics, silane coupling agents are usually introduced directly in the mixture of polymer/solvent/fillers as holistic mixing before forming a membrane, on the filler surface, or in the polymer solution prior to mixing the two components. Typical silane coupling agents include (3-aminopropyl)-triethoxymethyl silane (APTES), (3-aminopropyl)-diethoxymethyl silane (APDEMS), and aminopropyldimethylethoxy silane (APDMES). The addition of silane coupling agents can increase the interaction between polymer and filler by forming a chain linker between inorganic fillers and polymers. Typically, the silane group first reacts with hydroxyl groups (–OH) on the zeolite surface. After that, the free organic tails from coupling agents can form entangled contact with polymer phase, hydrogen bonds, or covalent bonds with the polymer chain if the coupling agents have active functional groups like an amino group (–NH). An example of APDEMS as the coupling agent to link zeolite with Matridmid is illustrated in Fig. 6.9.
Modified from Ebadi Amooghin, A., Omidkhah, M., Kargari, A., 2015. The effects of aminosilane grafting on NaY zeolite–Matrimid®5218 mixed matrix membranes for CO2/CH4 separation. J. Membr. Sci. 490, 364–379, with permission from Elsevier.
Ismail et al. (2008a) modified the zeolite surface with 3-amino triethoxy methyl silane (ATMS) prior to mixing with PES. Interfacial-void free MMM was obtained. The addition of the coupling agent was believed to enhance the interaction between filler and polymer via a hydrogen bond. The CO2 permeance of the membrane declined from 6.67 to 1.62 GPU while its selectivity increased almost two times. Chen et al. (2012) studied the effects of ethoxy (ET) groups in silane coupling agents on the performance of MMMs. Three silane coupling agents with different numbers of ET groups, namely APTES, APMDES, and APDMES for 1, 2, and 3 ET groups, respectively, were grafted on the surface of intergrowth faujasites and EMC-2 zeolites (FAU/EMT). All investigated coupling agents resulted in an improved zeolite dispersion and could prevent particle sedimentation. However, different morphologies were observed in each membrane composition, contributed from the change in rheological properties of the dope solutions. In addition, the MMMs showed zeolite clusters (alveoli) dispersed in the polymer continuous phase. Different coupling agents containing different numbers of ET groups resulted in different sizes and shape of alveoli. More ET groups in coupling agents resulted in larger and more regular dispersion of alveoli and consequently obtained a better performance (Chen et al., 2012).
When using the bridging route with silane coupling agents, though the interfacial gap-free could be ensured, one should not overexercise the crosslinking. Too high degree of linkage typically results in a huge loss in permeability due to rigidified polymer and partial pore blockage.