Tech Briefs

Oxygen-Permeable, Hydrophobic Membranes of Silanized α-Al2O3

These membranes perform better than do organic polymer oxygen-diffusion membranes.

Membranes made of silanized alumina have been prepared and tested as prototypes of derivatized ceramic membranes that are both highly permeable to oxygen and hydrophobic. Improved oxygen-permeable, hydrophobic membranes would be attractive for use in several technological disciplines, including supporting high-temperature aqueous-phase oxidation in industrial production of chemicals, oxygenation of aqueous streams for bioreactors, and oxygenation of blood during open-heart surgery and in cases of extreme pulmonary dysfunction. In comparison with organic polymeric oxygen-permeable membranes now commercially available, the derivatized ceramic membranes are more chemically robust, are capable of withstanding higher temperatures, and exhibit higher oxygen-diffusion coefficients.

ImageMembranes made from alumina as well as such other ceramics as titania and zirconia are permeable to oxygen and capable of withstanding higher temperatures. However, without modification, these ceramics are also hydrophilic. Hence, it is necessary to modify the surface properties of these ceramics to render them hydrophobic. For a series of experiments, the prototype membranes were made from α-Al2O3 with pore sizes from 5 to 200 nm. Hydrophobic molecular groups were attached to each α-Al2O3 membrane through silanization, using a suitable trimethoxy- or triethoxysilane (see figure).

In the experiments, both the silanized α-Al2O3 membranes and an organic polymer membrane based on polydimethylsiloxane (PDMS) were used as media for the transport of oxygen from a constant-pressure gas phase into a recirculating aqueous stream. Coefficients of diffusion of O2 and H2O across the membranes were measured. At room temperature, the silanized α-Al2O3 membranes exhibited oxygen-diffusion coefficients ranging from 1.24 to 5.75 times that of the PDMS membrane, the value in each case depending on the pore size and on which hydrophobic functional groups were present. Water-loss rates of the silanized α-Al2O3 membranes were found to be as much as two orders of magnitude below that of the PDMS membrane. In one test at a temperature of 90 °C, one of the silanized α-Al2O3 membranes exhibited an oxygen-diffusion coefficient 23.9 times that of the PDMS membrane at 23 °C.

This work was done by James E. Atwater and James R. Akse of Umpqua Research Co. for Johnson Space Center. For further information, contact the Johnson Innovative Partnerships Office at (281) 483-3809. MSC-23384