Mesoporous alumina membranes

Structural studies of mesoporous alumina membranes by small angle X-ray scattering... 

Mesoporous alumina membranes ( anodic aluminium oxide , or AAO) are prepared by anodic oxidation of aluminium metal [1,2]. The cylindrical pores, perpendicular to the membrane surface, form hexagonal arrays of straight non-intersecting channels with pore densities up to lO Vcm. Their diameters are controllable within the range 5 - 100 nm as a linear function of anodisation voltage. These membranes are used as molecular sieves, and have also found application as templates for metallic nanowires [3,4,5,6], metal elusters and colloids [7,8], and carbon nanotubes [9,10]. 

Figl Structural features of the mesoporous alumina membranes Schematic cross section (before removal of A1 backing) and schematic top view, together with an atomic force microscope AFM (Voltage) image for a membrane produced by anodisation at 40 V. 

Fig 3. Experimental SAXS intensity profQe (log-log plot) for mesoporous alumina membranes grown at 20 V, 40 V and 60 V. The membranes were face-on to the X-ray beam. 

Binkerd CR, Ma YH, Moser WR, and Dixon AG. An experimental study of the oxidative coupling of methane in porous ceramic radial-flow catalytic membrane reactors. Proceedings of ICIM4 (Inorganic Membranes), Gatlinburg, TN D.E. Fain (ed.), 1996 441-450. Yeung AKL, Sebastian JM, and Varma A. Mesoporous alumina membranes synthesis, characterization, thermal stability and nonuniform distribution of catalyst. J. Membr. Sci. 1997 131 9-28. 

SAS can detect the presence of non-accessible to vapor pores, which cannot be detected by the intrusive methods, such as sorption. If no such pores exist, a good agreement is expected between the intrusive sorption and the non-intrusive SANS, as illustrated in Fig. I where, an experimental water adsorption isotherm on a mesoporous alumina membrane, made by compressing non-porous spherical particles, is compared to the corresponding one reconstructed from the SANS data. 

Mesoporous alumina membranes have been studied extensively mostly for... 

S. Haag, M. Burgard and B. Ernst, Pure Nickel Coating on a Mesoporous Alumina Membrane Preparation by Electroless Plating and Characterization, Surf. Coat. Technol., 201, 2166-73 (2006). 

Yeung AKL, Sebastian JM, and Varma A. Mesoporous alumina membranes Synthesis, characterization, thermal stability and nonuniform distribution of catalyst. J. Membr. Sci. 1997 131 9-28. 

At the start of the project microporous sol-gel silica membranes were under investigation in various research groups. An extensive literature review is provided in the introduction of the thesis of De Vos [43], Common supports for sol-gel silica membranes are mesoporous y-alumina membranes [44-47], These mesoporous membranes are either home made [43-45] or obtained from commercial sources [46,47],... 

A standard membrane as prepared by de Lange [45] consisted of a die-pressed a-alumina support, fired at 1360°C with a pore diameter of 160 nm on which a y-alumina membrane was coated with a home-prepared boehmite sol. The coated y-alumina layer was calcined at 600°C, had a thickness of 7 pm with a pore diameter of 5 nm. On top of this mesoporous membrane,... 

Finally, in chapter 9, conclusions are drawn and suggestions made for further research on (steam-stable) molecular sieving silica membranes or mesoporous y-alumina membranes. Though not all of the project objectives were obtained, progress was made in the synthesis of micro- and mesoporous membranes. Especially the development of steam stable membranes may be a large step forward in the development of ceramic membranes. 

Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely y-alumina membranes on OC-AI2O3 supports. These mesoporous y-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the y-alumina membrane from the OC-AI2O3 supports in hot steam was found to be a major compli-... 

Figure la SEM micrograph of a delaminated conventional mesoporous y-alumina membrane after SASRA... 

A common and well-known method to prepare silica membranes with molecular sieving properties is sol-gel coating [3-5], With this technique, microporous silica layers with a pore-size of about 0.5 nm are dip-coated on top of supported y-alumina membranes. The supports are porous a-alumina disks with pore diameters in the range from 100-200 nm. On top of these macroporous supports a 3 pm thick mesoporous y-alumina layer is coated, with a pore size of 3 nm. 

To obtain thinner layers with a reasonable hydrogen permeance, Lin et al. [9] used a-alumina supported mesoporous y-alumina membranes with a pore size of 4 nm. However, they did not succeed in depositing highly selective layers. Most likely this is caused by defects in the y-alumina layer. 

As can be seen from Table 1, the starting membrane exhibits almost pure Knudsen separation characteristics, as can be expected from a state-of-the-art mesoporous y-alumina membrane. The ideal Knudsen permselectivities are H2 N2=3.74, He N2=2.65, H2 CC>2=4.69, which are in agreement with the measured permselectivities. After 43 minutes of CVI in experiment num-ber 1, the hydrogen permeance had decreased to 4.0 10 mol/m sPa, but the permselectivity towards nitrogen had increased to 43, which is far above the Knudsen value. 

A few publications have reported the permeation of capillary condensate in mesoporous materials. Caiman and Raal [14], measured permeability of CFiCb in Linde silica porous plugs at 240 and 251.5 K. Lee and Hwang [15], measured freon and water vapor permeabilities on vycor membranes. These permeabilities were found to exhibit maxima at relative pressures around 0.6-0.8, with values 20-50 times the Knudsen permeability. Ulhom et al. [16], reported a similar behavior for propylene at 263K in y-alumina membranes. Sperry et al. [17] demonstrated the ability of mesoporous y-alumina membranes in methanol separation at 473 K, provided the applied pressure... 

Gamma-alumina membranes were the first and most investigated mesopor-ous membranes to follow the colloidal preparation method. Based on a sol-gel process developed by Yoldas [13], a boehmite sol can be prepared by hydrolysis... 

Extensive stress measurements has been reported in mesoporous alumina [13,26], titania and alumina-titania composite membranes [13,27] supported by... 

Xiang and Ma [86] also recognised the problems of treating mesoporous y-alumina membranes with highly alkaline zeolite precursor solutions. They... 

As discussed in the introduction, hydrogen transports through mesoporous y-alumina membranes by Knudsen diffusion, and its selectivity is, therefore, not very high for light alkane dehydrogenation, y-alumina, furthermore, is a metastable material, which at high... 

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