Growing interest in the hydrogen economy is motivating research on inorganic, hydrogen-permselective membranes, which can be used in processes (related to H₂ production) that take place at high temperatures and pressures. A promising candidate material for a variety of inorganic membrane applications is silicon carbide due to its many unique properties, such as high thermal conductivity, thermal shock resistance, biocompatibility, resistance in acidic and alkali environments, chemical inertness, and high mechanical strength [1,2]. Previously, using a combination of slip-cating and dip-coating [1] we prepared SiC membranes typically showing an ideal H₂/CO₂ selectivity in the range of (42-96), and a H₂/CH₄ ideal selectivity in the range of (29-78). One of the challenges for applying membrane technology in reaction environment is to produce membranes with high steam stability. Steam stability experiments with our SiC membranes lasting 21 days, using an equimolar flowing mixture of He/H₂O at 200^oC, indicated some initial decline in the permeance of He, after which the permeance became stable at these conditions. However, for a better perforamce higher permselectivity for hydrogen is needed. The next phase of our project was to increase the permeance of the membrane while keeping the separation factor at high levels. To do so, a novel method is presented for the preparation of microporous silicon carbide membranes. The strategy comprises periodic coating of sacrificial interlayers and SiC pre-ceramic layers. In order to keep in place the interlayers; a solvent has been selected for the pre-ceramic polymer which does not dissolve the sacrificial interlayer during the dip-coating process or afterwards. The new preparation method involves first dip-coating the slip-casted supports in polystyrene (PS) solution, and then drying at 100^oC. The supports were subsequently dip-coated in allylhydridopolycarbosilane (AHPCS) solution, and then pyrolyzed at 750^oC resulting in complete decomposition of the polystyrene, and the forming of a SiC membrane layer on the top. The procedure of coating, drying and pyrolyzing of PS and AHPCS layers was carried out for three additional times. Membranes prepared, so far, show single gas ideal separation factors of helium and hydrogen over argon in the range (176-420) and (100-200), respectively, with an increase of about two to three times in permeance compared to the our previously prepared membranes [1]. To get some insight of how these sacrificial interlayes promote membrane performance, SEM pictures were taken after the last layer coating of PS. Within our SEM resolution, no continuous distinguishable PS film was seen on the top. We speculate that the sacrificial interlayers fill the pores and prevent their blockage by the pre-ceramic polymer. Since the interlayer barrier can be decomposed in inert or oxidizing environments, the new method has promising potential for application to a variety of other inorganic membrane systems.

[1] Bahman Elyassi, Muhammad Sahimi, Theodore T. Tsotsis, Silicon carbide membranes for gas separation applications, Journal of Membrane Science 288 (2007) 290–297.

[2] Richard J. Ciora, Babak Fayyaz, Paul K.T. Liu, Varaporn Suwanmethanond, Reyes Mallada, Muhammad Sahimi, Theodore T. Tsotsis, Preparation and reactive applications of nanoporous silicon carbide membranes, Chemical Engineering Science 59 (2004) 4957-4965.