Silver-exchanged zeolite LTA molecular sieving membranes with enhanced hydrogen selectivity

•A seeding-free synthesis strategy is developed for synthesis of Na-LTA membranes.•Both morphology and structure of zeolite LTA keeps unchanged after silver exchange.•Zeolite Ag-LTA membrane displays higher separation selectivity than Na-LTA membrane.

In the present work, we report a new road to enhance the gas separation performances of the zeolite Na-LTA membrane by tuning the pore size of zeolite LTA through silver cation exchange. Through the functionalization of the alumina support by using 3-aminopropyltriethoxysilane (APTES), a thin, phase-pure and well intergrown zeolite Na-LTA membrane with a thickness of about 5.0 µm can be facilely prepared on the APTES-modified macroporous α-Al2O3 tube. After a following silver exchange treatment of the as-synthesized zeolite Na-LTA membranes, the sodium ions in zeolite Na-LTA framework are replaced by silver ions, thus forming zeolite Ag-LTA membranes with a narrower pore diameter. The zeolite Ag-LTA membranes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It is found that both the morphology and structure of the zeolite Na-LTA membrane keep unchanged after silver-exchange, and no cracks, pinholes or other defects are observed in the membrane layer. The zeolite Ag-LTA membrane shows high hydrogen selectivity due to the reduction of pore size of the zeolite Ag-LTA. For the separation of the binary mixture at 50 °C and 2.0 bar, the separation factor of H2/C3H8 is α≈120.8, which by far exceeds α≈19.4 of the starting Na-LTA membrane, and are also much higher than the separation factors previously reported for the H2/C3H8 mixture on zeolite membranes.

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