(716g) Fabrication of Hydrogen-Selective Silicon-Based Ceramic Membranes Via Pyrolysis of Vapor Deposited Polymer Films

The efficient separation of H₂ under steam reforming conditions is important for the development of the clean energy industry and has helped drive inorganic membrane research for several decades. Although several types of nanoporous membranes have been synthesized, the fabrication of high-temperature and steam-stable inorganic membranes with high fluxes and large separation factors still remains a challenge. Silicon-based ceramic membranes are particularly promising due to their high temperature resistance and excellent chemical and mechanical stability. In this work, we propose a new synthetic route towards manufacturing nanoporous asymmetric membranes by the pyrolysis of pre-ceramic siloxane-based polymer films deposited by initiated chemical vapor deposition (iCVD) on macroporous silicon carbide (SiC) supports. The iCVD technique is a one-step, solventless, substrate-independent process that can be used to create a wide variety of polymer coatings. It can be used to deposit conformal films on a variety of geometries, giving it an advantage over conventional solution-based methods typically used for polymer precursor films. In this research, we systematically investigated the change in the chemical structure of silicon-based polymer films under different pyrolysis conditions. Diffuse reflectance infrared Fourier transform spectroscopy was used to examine in situ the change in the IR spectra of the films as a function of temperature. Fourier transform IR and X-ray photoelectron spectroscopy were utilized to confirm the chemical bonding and composition at the different pyrolysis conditions ex situ. We also analyzed the permeation properties of the resulting ceramic films and the effect of the support composition on the membrane transport characteristics. Single-gas permeation tests were performed to determine the permeance and binary separation factors of H₂ and He over Ar for each film. In our preliminary efforts to date, poly(2,4,6,8-tetramethyl cyclotetrasiloxane) was deposited via iCVD on highly permeable, mechanically strong, flat-disk SiC supports. We found that the complete oxidation of the polymer film to form a pure silica membrane does not occur before 1100 °C. Three different support types were tested for membrane preparation: SiC supports composed of 50% of particles with an average diameter less than 80 nm and 50% of particles with average diameter of 0.6 μm, supports made of pure 0.6 μm SiC powders, and supports made with pure <80 nm particles. It was found that for all supports, membranes formed with this process were microporous with seperation factors several times above the Knudsen factor. We found, furthermore, that the membranes formed with the 50% 0.6 μm/50% 80 nm SiC powder mixture had the highest separation factors.