Microwave synthesis of MFI-type zeolite membranes by seeded secondary growth without the use of organic structure directing agents

This paper reports the microwave synthesis of MFI-type zeolite membranes by secondary growth of nanocrystalline silicalite seed layers in pure inorganic precursors. The inorganic precursor solutions contained SiO₂, Al₂(SO₄)₃, NaOH, and H₂O. It was found that the membrane formation and quality were sensitive to the Si/Al ratio of the precursor solution. Good quality MFI membranes were obtained from precursors with Si/Al ratios in range 111-130. The membranes had H₂/SF₆ permselectivity of >1700 and H₂ permeance >3 × 10^-7 mol m^-2 s^-1 Pa^-1 at 23 °C when helium was used as sweeping gas. The membrane synthesis duration can be greatly shortened by microwave heating as compared to the conventional heating method. Furthermore, membranes obtained from template-free inorganic precursors exhibited higher permselectivties of H₂ over N₂, and SF₆ than the membrane synthesized with TPAOH template. The studies on small gas permeation mechanisms and BET characterization of zeolite particles suggested that the membranes derived from inorganic solutions contained minimized intercrystalline pores by avoiding the high temperature template removal process and possessed slightly smaller effective pore size likely due to trapping of SiO₂ molecular species in the zeolite microporosity.