Supports boehmite

Gelatinous boehmite, called alumina gel in commeicial use, is used in the piepaiation of adsorbents, desiccants (qv), catalysts, and catalyst supports (see Catalysts, SUPPORTBd). A significant amount is used in pharmaceutical preparations. 

Liquid film state under reactive distillation conditions for the dehydrogenation of decalin on platinum supported on active carbon and boehmite... 

However the employment of AI2O3 and Al(OH)0 as a support afforded the dissimilar results as shown in Figs. 1 (b) and (c), respectively. Active carbon does not repel decalin while boehmite and alumina repel extensively decalin, indicating that the affinity between the liquid substrate and the support should be considered in the preparation of the catalysts. 

The gel layer thickness increases linearly with the square root of dipping time indicating that indeed a slip-casting process is operative. The rate constant depends on gel structure and pore size of the support. If the modal pore size of the support is increased from 0.12 /im (type 1 support) to 0.34 /xm (type 2 support) the casting rate is decreased in accordance with theory. Typical casting rates for type 1 and type 2 supports are 4.4 /xm/s and 2.8 /xm/s, respectively for HNO3-stabilized sols with a concentration of 1.22 mol boehmite/L. 

Ageing of the sol profoundly affects the casting behavior. After ageing (e.g. one week) gel layers could be formed on type 2 supports while before ageing this was not found to be the case. This points to an increase of the agglomerate size of the boehmite particles in accordance... 

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... 

Support used The type of support, e.g., gamma-, eta-, boehmite-alumina, could greatly affect the disposition and/or degree of interaction with Mo and Co. Also, impurities in the support could change the acidic character of the support and consequent interactions. Some commercial aluminas contain sodium, chloride, sulfate, or silica impurities. 

Such an effect might be expected when boehmite supported cobalt is being calcined, viz. during the phase transition AIO(OH) - y-Al203. Figure 7 shows spectra of pyridine, adsorbed on the sample CoMo-124 B, which has been prepared in this way. Spectra for MoCo-122, -123 and -124, containing 2, 3 and 4 wt% CoO resp. are shown for comparison. All these catalysts have had a final calcination of 650°C. Comparison of the spectra of CoMo-124 B and MoCo-124 indicates that the intensity of the 1612 cm l band, which is introduced by the interaction of the cobalt ions and the molybdate layer, is lower for CoMo-124 B than for MoCo-124. The spectrum for CoMo-124 B resembles that of CoMo-123, indicating that a part of the cobalt ions does not participate in this interaction. 

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)... 

The most common catalyst supports are the transitional aluminas, particularly y alumina, which is best prepared by heating v)/ boehmite. This process gives a y alumina having a surface area between 150-300 m /g, a pore volume between 0.5-1 cm /g and a large number of pores in the 3-12 nm range. The Y aluminas prepared from other sources have significantly lower surface areas and pore volumes. In contrast, a alumina, the most dehydrated form of alumina, is essentially non-porous with surface areas between 0.1 and 5 m /g. ... 

The hydroxy oxides, especially y boehmite, have excellent coalescing properties so they are particularly good for forming large support pellets and granules. They are also used as wash coats to provide porous surfaces for catalyst adhesion to non-porous materials such as metals and ceramics. 

Given a certain support diameter, the immediate formation of a gel film could be promoted by (i) increasing the boehmite concentration, (ii) ageing the colloidal solution and, (iii) in some cases the dipping time [3]. In later studies [12,14] additions such as PVA were added which enhance the viscosity of the colloidal solution and promote the formation of a lyogel film. 

The influence of the type of support (pore size) and the peptising acid on the formation of gel layers during dip-coating, hr all cases the sol contains 1.2 mol A1 (boehmite) per litre. 

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