Fabrication of nanoporous silicon carbide membranes for gas separation applications

Silicon carbide (SiC) microporous membranes were prepared by the pyrolysis of thin allyl-hydridopolycarbosilane (AHPCS) films coated, using a combination of slip-casting and dip-coating techniques, on tubular SiC macroporous supports. Combining slip-casting with dip-coating significantly improved the reproducibility in preparing high quality membranes. The membranes prepared exhibited an ideal H2/CO2 selectivity in the range of (42-96), and a H2/CH4 ideal selectivity in the range of (29-78). Separation factors measured with the same membranes, using equimolar binary mixtures of H2 in CO2 and H2 in CH 4, were similar to the ideal selectivity values. Steam stability experiments with the membranes lasting 21 days, using an equimolar flowing mixture of He/H2O at 200 °C, indicated some initial decline in the permeance of He, after which the permeance became stable at these conditions.;Further, a novel method was presented based on periodic and alternate coatings of polystyrene sacrificial interlayers and SiC pre-ceramic layers on the top of slip-casted tubular SiC supports. Membranes prepared by this technique exhibit single gas ideal separation factors of helium and hydrogen over argon in the ranges (176-465) and (101-258), respectively, with permeances that are typically two to three times higher than those of SiC membranes prepared previously by the more conventional techniques. Mixed-gas experiments with the same membranes indicate separation factors as high as 117 for an equimolar H2/CH4 mixture.;In the next phase of this research, SiC fibers were fabricated by immersion of anodized aluminum oxide templates in AHPCS solutions and subsequent pyrolysis at 750 °C. The effect of polystyrene (the pore former) on the formation of the nanofibers in the confined channels of the templates was studied. Fibers prepared by adding polystyrene exhibit higher surface area than the ones prepared without adding polystyrene.;Finally, the effect of porous (SiC nanofibers) and nonporous (SiC powders) fillers on the performance of SiC membranes was studied. We found membranes prepared with SiC powders in their structure exhibited a higher performance. This was attributed to better packing of SiC powders compared to SiC fibers, as well as, the loss of porosity of the fibers after being implemented into the membrane structure as revealed by BET analysis.