Potentialities of microporous membranes for H2/CO2 separation in future fossil fuel power plants: Evaluation of SiO2, ZrO2, Y2O3-ZrO2 and TiO2-ZrO2 sol-gel membranes

In this work, an experimental study is made on the preparation, the morphological characterization and the gas permeation of graded ceramic multilayer membranes with silica and non-silica toplayers. The membranes were prepared on porous α-Al2O3 or 8Y2O3-ZrO2 supports by dip-coating methods, where sols with different particle sizes were used as coating liquids. In a first step, mesoporous alumina or graded zirconia sublayers with a pore size of 7-3 nm were deposited, starting from sols with a particle size in the range 60-30 nm. The active toplayer of the membrane is a SiO2, ZrO2, 8Y2O3-ZrO2 or 50TiO2-50ZrO2 thin film with a thickness in the range 50-200 nm. Nano-particles of these materials were prepared by a precipitate-free hydrolysis-condensation synthesis method, starting from metal-organic precursors. An important consideration is that the properties of the novel zirconia based sublayers and toplayers, which are developed for typical higher steam pressure areas, are comparable to the commonly used γ-Al2O3 and silica layers. Gas permeation tests showed a decrease of permeation in the order He > H2 > CO2 > N2 and suggested that the membranes with silica toplayers are microporous. Moreover, optimising all conditions in the membrane manufacturing procedure in our lab, such as the properties of the support and the sols and the use of cleanroom coating, resulted in a 100% H2/CO2 selectivity for a few samples. The formation of crack-free non-silica toplayers was initially experienced as very difficult, but after optimization of the sol synthesis and coating methodology and by restricting the layer thickness below 100 nm, comparable ultra-thin toplayers are obtained. Further, extensive gas permeation testing confirmed that each of these toplayers posses a very low number of defects and a comparable low or zero CO2 permeation is obtained. On the other hand, extremely low He and H2 permeation - especially for the samples fired at 400 and 500 °C - suggested the formation of thin dense toplayers. This behaviour is found for the crystalline ZrO2 and 8Y2O3-ZrO2 layers as well as the amorphous 50TiO2-50ZrO2 layers and indicates the presence of a totally different structure in the non-silica material. The amorphous silica toplayer is probably formed of 5-, 6-, 7-, 8- and also larger Si-O bonded rings which enable H2 permeation, while our results suggest that the non-silica toplayers are characterized by a more dense atomic packing hindering H2 permeation. For future H2/CO2 separation in a power plant, membranes will however need to out-perform the discussed materials. Amorphous silica toplayers hold the potential to combine an excellent selectivity with a relatively high gas permeation, but lack the required stability to operate over a wide range of conditions, and the discussed non-silica toplayers which are considered as the best until now in the literature - based on their zero CO2 permeation - are to dense to allow the passage of H2.