Characterization of sol–gel derived membranes and zeolite membranes by nanopermporometry

Abstract

Abstract Silica–zirconia membranes having pore sizes in the range 0.5–2 nm, determined using water as a condensable gas (vapor) by nanopermporometry, were prepared by the sol–gel process, and used in gas permeation experiments. The permeability ratio of He/N2 approached the Knudsen value (=2.6) for pore sizes larger than 2 nm. It then decreased with decreasing pore size, probably because of an enhanced contribution of surface diffusion in the small pore, and showed a minimum at an approximate pore size of 1 nm. It then increased to approximately 50 for a membrane having pore sizes of 0.3 nm. For the case of He/SF6, the curve appears to shift to a larger Kelvin diameter, probably because of the larger molecular size of SF6 as well as adsorption. The effects of non-condensable gases (He and N2) were examined using silica–zirconia membranes of 2 and 0.8 nm in pore size. The pore size distribution (PSD) curves measured by He and N2 were in good agreement with each other for membranes having a pore size as large as 2 nm. On the other hand, for the case of porous membranes having small pore sizes, PSD curves measured using He were shifted to a smaller pore size, compared with those measured by N2. This suggests the existence of micropores, which allowed the permeation of only He. Moreover, nanopermporometry was applied to MFI zeolite membranes to characterize selective (intracrystalline) and non-selective pores (intercrystalline) using hexane, and the data were in reasonable agreement with the observed separation performances.