Boehmite membranes

Figure X7. Idealized model of the boehmite membrane structure, d is the distance between the 2 boehmite crystals A and B, r is the thickness of the boehmite plates (Leenaars and Burggraaf 1985).

A last complication results from the measurement of the thickness of supported layers. This can easily be done with SEM for calcined layers. Wet lyogel, or even dry xerogel, layers cannot be measured in this way. Reproducible thickness measurements on wet lyogel films could not be obtained with other easy-to-perform methods. Consequently layer thicknesses were measured after calcination. Estimates of the shrinkage in the thickness direction were made for supported alumina (boehmite) membranes dried at 40°C and 60% RH made with a standard precursor solution of 1 mol AlOOH/1 stabilised at pH = 4 by... 

Fig. 8.18. Reversible stress diagram of boehmite membranes dried at 40°C with RH values changing cyclic between 60 and 90% RH. From Voncken et al. [26].

It is well documented that the addition of organic polymeric additives to precursor sols promotes the formation of defect-free membranes [4,12,33]. To investigate this effect, stress measurements were performed on boehmite membranes obtained from standard sols (1 mole AlOOH/1) mixed with different amounts of PVA solutions (containing 35 ml PVA/1, molecular weight 72000). 

Fig. 8.19. Stress in the constant stress region of a drying boehmite membrane as a function of the amount of PVA added to the precursor solution. Drying conditions 40°C and 60% RH. From Kumar [13].

The development of stress during calcination is shown in Fig. 8.20 for boehmite membranes calcined at 600°C (thickness after calcination is 5 pm). Curve c in Fig. 8.20 represents the curve which is corrected for support effects (see the preceding section on this subject). Three heating and cooling cycles are shown. During the first heating the Al-hydroxide particles of the gel are transformed to boehmite and subsequently to (hydrated) y-aluminium oxide particles and the shape of the first peak of curve c differs from the subsequent peaks. The maximum tensile stress calculated from the deflection amounts about 30 MPa. 

Fig. 8.20. Deflection (stress) versus time diagram during the cyclic heat treatment (calcination) of boehmite membranes converted to y-alumina at 600°C. Heating and cooling rates were 60°C/h. From Kumar [13]. Curve a (dotted) blank run, support only curve b actual run with supported membrane curve c deflection of b corrected for support effects.

FIGURE 10.1.3 Pore structure of y-alumina (boehmite) membrane proposed by Leenaars and Burggraaf [19]. 

The mode of synthesis of alumina membranes through the colloidal suspension route is given in Figure 2.6. The first step involves the preparation of a slip consisting of boehmite particles. These arc plate-shaped in the form of pennies with a diameter of 25-50 nm and a thickness of 3.5-5.5nm (Leenaars ct al. 1984,1985). The synthesis chemistry of the colloidal boehmite (y-AlOOH) solution is described in detail by Leenaars and Yoldas (1975) and to some extent by Anderson, Gieselman and Xu (1988) and by Larbot et al. (1987). 

There exist a maximum allowable thickness of the supported gel layers above which it is not possible to obtain crack-free membranes after calcination. For Y-alumina membranes this thickness depends on a number of (partly unknown) parameters and has a value between 5 and 10 /im. One of the important parameters is certainly the roughness and porosity of the support system, because unsupported membranes (cast on teflon) are obtained crack-free up to 100 )xm. The xerogel obtained after drying was calcined over a wide range of temperatures. At 390°C the transition of boehmite to y-AljOj takes place in accordance with the overall reaction... 

This transition produces an isomorphous phase and the resulting y-alumina has the same morphology and texture as its boehmite precursor. With increasing temperature and time the mean pore diameter increases gradually and other phases appear (S-, 6-alumina). Due to the broad XRD lines, the distinction between y- and S-alumina cannot be made 6-alumina occurs at about 900°C while the conversion to the chemically very stable a-alumina phase takes place at T> 1000°C. Some typical results for alumina membranes synthesized without binders are given in Table 2.4. When PVA was used as a binder, thermogravimetric analysis showed that, provided the appropriate binder type was used, the binder could be effectively removed at T > 400°C. The ash residue is of the order of 0.01 wt.%. Mean pore size and... 

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,... 

The starting material is a state-of-the-art flat y-alumina membrane prepared by dipcoating of a boehmite solution on a macroporous a-alumina support and subsequent firing at 600°C as described in [4], The a-alumina support is prepared from AKP-30 powder by making a colloidal suspension of this powder in diluted nitric acid and subsequent filtration. After filtration the wet cake is dried overnight and sintered for 1 hour at 1100°C. The resulting flat a-alumina supports have a mean pore diameter of 80 nm. A detailed description of the support synthesis is provided in chapter 4. 

The magnitude of the stress has been studied for a boehmite gel layer on a porous a-alumina support Voncken et al [1992] have found that among various stress measurement methods the cantilever principle is most suitable for studying porous thin films like gels. Using a laser displacement meter to detect the deflection of gel and support layers, they found that the tensile stress exerted on the drying membrane (gel)... 

Liquid phase modifications. Alternatively a porous membrane can be reduced in pore size by a liquid deposition prcx ess where the membrane is dipped into a solution or sol to form deposits inside the membrane pores. For example, a silicon nitride tube with a mean pore diameter of 0.35 pm is first immersed in a solution of aluminum alcoholate (aluminum isopropylate or 2aluminum tris(ethyl acetoacetate) or ethyl acetoacetate aluminum diisopropylate) in an organic solvent (hexane, cyclohexane, benzene, isopropanol, etc.). It is then treated with saturated water vapor to hydrolyze the alcoholate or chelate to form bochmite inside the pores, thus changing the pore diameter to as small as 20 nm [Mitsubishi Heavy Ind., 1984a and 1934b]. Upon calcining at 800X, boehmite transforms into transition-alumina. 

Contrary to SEM, TEM does not provide the striking three-dimensional images. Shown in Figure 4.5 are some well dispersed crystallites of boehmite as the precursor to a thin, unsupported partially calcined alumina membrane the TEM image of which is given in Figure 4.6. It is estimated from the TEM that many of the crystallites appear to be smaller than about 50 nm. The ordered suiicture of the very thin, partially calcined alumina membrane is evident in Figure 4.6. 

Figuic 4.5 Transmission electron microscopy image of well dispersed boehmite particles as a precursor to fine-pore alumina membranes... 

Fig. 6.46. The effect of the PVA content of boehmite sols on the maximum stress developed during drying for alumina membranes made and dried under identical conditions. (Redrawn from...

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... 

In sol-gel processing, particulate sols of hydrous metal oxides can be formed using a peptization reaction to prevent aggregation of the primary particles. This is the case for y-alumina obtained from boehmite. Usually mesoporous membranes with a pore diameter down to 5 nm are easily elaborated from commercial boehmite using nitric acid as the peptization agent. Larbot et al. [53]... 

The initial layer formation step has hardly been investigated. Some qualitative guidelines can be abstracted from theoretical considerations and experimental observations. The first intensive and systematic study of the kinetics and mechanism of membrane formation was performed by Leenaars et al. [2-4] on boehmite and Y-AI2O3 membranes with pore diameters in the range of 3-5 nm. 

It should be stressed that this is a very simplified model. For example, the magnitude and size of the electrical charges on the pore wall and particle surface will be important in cases where pore and particle size are not too different. No data are available on initial membrane formation. A similar situation exists to explain the positive effect of some additions (e.g. PVA to boehmite solutions). The trends in experimental observations and the qualitative model discussed above holds also for the formation of other types of mesoporous membranes (titania, zirconia, silica) [6] (Chapter 7). 

The membranes were synthesised from AlOOH (boehmite) sols with a typical (standard) concentration of 1 mol AlOOH/1, stabilised at pH = 4 with HNO3. Ti02 sols had a typical concentration of 0.3 mol Ti02/1, composite membranes of AlOOH and Ti02 were obtained by mixing the sols. 

That bonds are formed between particles is inferred by the fact that the gel layers are able to bear considerable stresses. These bonds are sensitive to the presence of stresses and allow stress relaxation to occur. The relation between stress relaxation and cracking on one hand and particle shape on the other hand is not known. The relative ease of preparing y-alumina membranes might be due to the relative ease of rearrangement of the particles and easy stress relaxation in plate-shaped boehmite particles and the isomorphous transitions to plate-shaped y-alumina at about 300°C, the transition also being accompanied by a relatively small volume change [2-4]. With spherical particles (titania, zirconia) stress relaxation might be more difficult. The easier formation of defect poor composites of alumina and titania (with spherical particles) supports the beneficial effect of plate-shaped particles. 

In comparison with pure "j alumina, mixing the precursor boehmite sol with 3% LaNOs or impregnation of calcined y-alumina with LaNOs solution stabilised the unsupported membrane. After 120 h at 800°C the La doped system had a pore diameter of about 5 nm compared with about 9 nm for the pure y-alumina. 

In practice it is simpler to treat the tortuosity x as an empirical factor and to determine it experimentally (see below). The same holds for other geometrical parameters like 0 and (3, discussed in connection with Eqs. (9.6 and (9.2). In porous pellets of packed particles a correlation of the type e/x = constant is frequently found [3]. The validity of this expression is not shown however for low values of the porosity (e < 0.30) and very small pore sizes. Experimental tortuosity values generally fall in the region 2 < x < 5, but in special cases much larger values have been reported. Leenaars et al. [17] reported values of x = 6-7 for membranes consisting of a packing of plate-shaped (boehmite, gamma alumina) particles. 

X.L. Pan et al., Mesoporous spinel MgA1204 prepared by in situ modification of boehmite sol particle surface I Synthesis and characterization of the unsupported membranes. Colloids Surf. A-Physicochem. Eng. Asp. 179(2-3), 163-169 (2001). 

The methods used to make these membranes are diverse and highly proprietary. However, one method for preparing gamma-alumina membrane films on porous supports (which can be made from alpha-alumina) has been reported by Dutch researchers.13 They use a sol/gel technique with Boehmite (7-AIO-OH) as the precursor because it can be easily dispersed with acids. 

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