(595f) A Generalized High-Viscosity Gel Conversion Method for Highly Selective Small-Pore and Medium-Pore Zeolitic Hollow Fiber Membranes

High-silica zeolite membranes made of small-pore CHA and medium-pore MFI zeolites have previously demonstrated excellent performance for important separations involving smaller molecules like CO₂ and hydrocarbons such as butanes and xylenes. To accelerate the scale-up and industrial application of these zeolite membranes, more versatile and scalable fabrication techniques are highly desired. Gel-based conversion methods for zeolite membrane formation have the advantages of being amenable to coating-based membrane deposition approaches and not requiring bulk hydrothermal solutions. However, gel conversion methods have not been studied and controlled in detail to obtain high-quality membranes. Here we discuss a versatile, high-viscosity gel conversion method for more generalized use in zeolite membrane fabrication, and demonstrate its application to both CHA and MFI-type zeolite membrane fabrication on ceramic hollow fiber substrates. The highly concentrated precursor gels - with viscosities high enough to overcome gravitational effects on uniformity of coatings - were coated on the hollow fibers and transformed into membranes in a water vapor atmosphere. Important parameters, such as H₂O/SiO₂ ratio, seeding and synthesis duration, were investigated in detail for better understanding of the membrane formation and optimization of synthesis conditions. The excellent separation properties of the membranes are discussed in detail. For example, CHA hollow fiber membranes with CO₂/propane separation factors > 10,000 and CO₂ permeance over 5000 GPU, and MFI hollow fiber membranes with n-butane permeances > 400 GPU and n-/i-butane separation factors ~40, are obtained. We discuss a path forward for larger-scale fabrication of these membranes.