Membrane-based separations are becoming increasingly relevant for a number of applications due to their low energy requirements, potentially low fabrication cost, and steady-state operation. Polymeric membranes are amenable to large-scale fabrication processes, for example in the form of hollow fibers. Polymeric hollow-fiber membrane modules typically have attractive, large surface area/volume ratios (>1000 m2/m3). However, polymeric membranes also have an intrinsic “upper bound” on their performance, reflecting a trade-off between their permeability and selectivity. Over the last decade, inorganic membranes have been shown to possess high permeability, tunable selectivity, and high thermal and chemical resistance. Their applications are yet limited by the difficulty of fabricating inorganic membranes on a technologically scalable, low-cost platform. For example, polymeric hollow fiber substrates cannot withstand the high temperatures often needed for post-synthesis processing of molecular sieving zeolite membranes, and usually cannot withstand the hydrothermal membrane synthesis conditions. In exceptional cases that only require mild synthesis conditions, the fabrication of zeolite membranes on tubular polymer supports has been demonstrated.
Ordered mesoporous silica materials, prepared using surfactant templates, have uniform pore channels with a diameter range of 2-10 nm. The mesopores are advantageous for rapid diffusion of target molecules, can modified in a variety of ways, and have high chemical and thermal stability, allowing for specific separation applications for a range of molecular sizes (e.g., from small gas molecules to larger pharmaceutical or biological molecules). Mesoporous silica membranes have been proposed as a potential candidates for separation applications, and have been synthesized by hydrothermal methods on ceramic substrates. In this talk, we describe the technologically scalable fabrication of an inorganic membrane platform based upon mesoporous silica membranes on polymeric hollow fibers. These asymmetric mesoporous silica membranes are continuous over large areas, are defect-free, and have high gas flux. Furthermore, we also modify the mesopores via the use of amine-containing polysilsesquioxane (POSS) molecules. The resulting modified membranes exhibit high permeability and selective behavior in separation of CO2 from various gas streams.
MCM-48, a mesoporous silica with three-dimensional channels of 2.7 nm diameter pores, was selected as the membrane material. The process comprises the fabrication of a macroporous polymeric hollow fiber, followed by the formation of a thin mesoporous silica layer on the hollow fiber at room temperature by static immersion in an acidic silica/surfactant precursor solution. The initial mesoporous framework formed in this process is then fully condensed by supplying additional silica species via a TEOS (tetraethylorthosilicate) vapor treatment at 100 °C, and the mesopores are then activated by a room-temperature surfactant extraction step. The membranes are subsequently ready for surface modifications, such as impregnation with POSS molecules considered in this work. The resulting modified membranes display selective permeation behavior for CO2/N2 and CO2/CH4 separations, and are promising for applications as gas separation membranes.