Preparation Of Copper-Silicon Membrane And Its Performance On Separation Of H₂,N₂ And CO₂

Membrane separation for H2/CO2 in the hydrogen production process from fossil fuel is considered the most cost-effective and potential technology. The US department of energy calculated that if membrane separation process replaced pressure swing adsorption process, the unit cost could save 0.31＄/kg. Silicalite-1 zeolite membrane and microporous SiO2 membrane are two kinds of conventional inorganic membranes for gas separation. The zeolite membrane has good hydrothermal stability and poor gases selectivity because the pore size is much bigger than molecular dynamics diameter of small molecular gases; The latter has dense Si-O-Si network structure, therefore, the selectivity of small molecular gases is good, but the hydrothermal stability is poor due to the surface hydroxyl.Transition metal modification is a common method for inorganic membrane, which can be incorporated into the Si-O-Si network structure to decorate aperture and change pore canal surface properties. In this work, Cu was used to modify Silicalite-1 membrane and microporous SiO2 membrane.α-Al2O3 porous ceramics as supporter, Cu-Silicalite-1 and microporous Cu-SiO2 membranes were prepared by direct hydrothermal synthesis and dip-coating process with tetraethylorthosilicate(TEOS) and copper nitrate as precursors. The status of copper in the membranes was detected by FT-IR and XRD. The permeation of H2, N2 and CO2 was measured by differential-pressure method.The results show that, H2 permeate flux of the membrane tubes is between (11.95～14.93)×10-7 mol·s-1Pa-1m-2 at 250℃. Silicalite-1 membranes conform to a combination of Knudsen diffusion mechanism and CO2 surface diffusion mechanism, with a permselectivity of 2.64 for H2/N2 and 1.74 for H2/CO2. Using H3PO4 as complexing agent, the permselectivity of H2/N2 and H2/CO2 on Cu-Silicalite-1 membrane is 3.55 and 3.08, increasing by 38.5% and 82.4% than Silicalite-1 membrane because copper entered the pore and reduced the pore size. Copper incorporated into Si-O-Si network structure of Cu-SiO2 microporous membrane resulted in permselectivity of 4.31 for H2/N2 and 4.63 for H2/CO2, which increased by 65.8% and 172.4% than Silicalite-1 membrane, and showed a good permeability and selectivity. However, H2 reduction under high temperature can make Cu in pore canal migrate to the surface of the membrane, leading to the increase in gases permeability and the decrease in selectivity.