Macroporous support coatings for molecular separation membranes having a minimum defect density

Bubble number porometry and Hg porosimetry were used to investigate the density and size of percolating channels in gross macroporous support tubes, support tubes coated with one or two macroporous α-alumina layers and the latter coated with a thin mesoporous γ-alumina layer. The key factor in obtaining macroporous thin layers with bulk properties is found in the application of multiple layers rather than the use of a single somewhat thicker layer. The breakthrough pressure of the layered macroporous substrate system drastically increases only when two coatings are applied with sufficient total thickness to shield larger voids. The bubble percolation behaviour of a mesoporous γ-alumina coating is determined by the width of the Hg intrusion curve of the underlying macroporous substrate coating(s). The average size of the macropore distribution is sufficiently low for building up a sol–gel layer, but the large pores cause percolating channels in the mesoporous γ-alumina layer of the same size. This leads to a bubble point size/mean pore size ratio of the γ-alumina coating ≫1. The present results show an increase in the reproducibility of the pore properties up to the γ-alumina layer.