Characterization of ceramic membranes II. Modified commercial membranes with pore size under 40 Å

Abstract

A methodology was developed for characterizing a series of membranes with pore size ⩽40 Å. These membranes were modified from commercial microporous γ-Al 2O 3 membranes (40 Å) by pore size reduction. SEM/EDAX analysis showed a modified layer with a thickness of ≈1.5 μm deposited within the existing porous structure. A flow-weighted pore size distribution analyzer was used to determine pore size versus flow contribution for the pore size ranging from 40 Å down to ≈15 Å. Size exclusion with selected gaseous molecules was employed to determine pore size <6 Å. Pore size not accounted for by the above two methods was assigned to 15−6 Å. Gas permeation of helium and nitrogen through the pore size ranging from > 40 to ⩽5 Å at 25°C was investigated in detail. Selectivity decreased slightly along with the pore size reduction in the Knudsen regime (i.e., 40 Å down to ≈20 Å), then increased dramatically in the molecular sieving regime. The unexpected decrease of the selectivity in the Knudsen regime was likely to have resulted from the contribution of surface diffusion of nitrogen.