Amorphous alumina gels, formation

Preparation of X-ray amorphous ZSM-5 crystallites according to procedure BT It is important that the gel formation takes place as homogeneously as possible. Because of the particular sensitivity of various silica and alumina species to the pH (63,64), the pH range between 4.5 and 8.5 was avoided. Nucleation was performed at pH 3.5-4, where a low viscous gel containing essentially monomeric silica species is rapidly formed (65).The, pH is theii raised to about 9, in order to form tetrahedral A1(0H) entities and to favour the further A1 incorporation within the zeolitic framework. NaCl was used to increase the (super)saturation of the gel, which will flocculate into a macromolecular colloid (V) and initiate the nucleation. This procedure yields 100 % crystalline zeolite after... 

Finally, intermediate gels are often used which derive from a mixture of a silica hydrosol with an aqueous solution of an aluminum salt. Although the silicon and aluminum bearing species are actually mixed at small scales in all these sol-gel precursors, the formation at relatively low temperatures of homogeneous and dense mullite bodies from amorphous alumina-silica... 

The use of polystyrene-based materials has also been investigated for porous carbon formation with the hard template method using amorphous silica gel, mesoporous alumina, and microporous zeolites as inorganic templates [90]. 

Our synthetic routes to ZK4 were modifications of those described by Kerr (21). An amorphous, basic aluminosilicate gel containing tetramethylammonlum (TMA) Ion was heated at 100°C to promote formation of ZK4 crystals. Preparation of the gel Involved the vigorous mixing at room temperature of one component acting as a source of sodium and alumina, with another component acting as a source of TMA and silica. The alumina used was sodium... 

In the present study, amorphous silica-alumina nanomaterials with controlled mesoporous distribution have been synthesized by two templateless approaches (1) vacuum-sol process, and (2) ultrasonic-sol process. It is found that the preparation method affects the precursor sol properties and the specific surface area and pore volume of the final materials. Ultrasonic-sol method favors the formation of monodispersed sol particles with narrow size distribution. Because of several base-exchange cycles and absence of drying process prior to heat treatment, the gel derived fiom ultrasonic-sol method may have enough stiffiiess to protect the network fiom pore collapse by capillary force, thus, leading to produce the materials with... 

For intermediate mixing scales, crystallization of diphasic gels involves an intermediate exothermic step at 980°C. This step has been assigned to the formation of a poorly crystallized Al-Si spinel (alone or mixed with tetragonal alumina-rich mullite) that further reacts exothermically with the amorphous phase at 1270° C to give stable orthorhombic mullite [15] [40-41]. In this case, sintering can take place before mullitization, and is enhanced by the low crystallization state of the material (Figure 4). 

Moreover, solutes may correspond to different solid phases with different solubility. Surface dissolution can, therefore, be accompanied by phase transition on the particle surface. For instance, surfaces of fused quartz swell under water to form gel layers of amorphous silica (Her 1979, pp. 34—40 Yaminsky et al. 1998). Similarly, it could be shown that the dissolution of y-alumina (Y-AI2O3) is coupled with the gradual formation of a hydroxide phase on the particle surface (e.g. Roelofs and Vogelsberger 2006 Carrier et al. 2007 BoneUi et al. 2010). 

The continuous mullite fibers were first made by heat-treating the gel fibers spun from the sol prepared by hydrolyzing the stochiometric mixture of TEOS and acac-modified AIP (Tucker, 1990). The amorphous gel fibers were crystallized to mullite fibers above 1000°C. Thereafter, several cost-effective sol-gel methods of the mullite fiber formation have been developed. In one method, the mixture of silica sol and alumina sol prepared from aluminum salts were spun with the aid of polyethylene oxide (PEO) (Bhattacharya, 1996a). In others the stoichiometric mixture of inexpensive alumina sources (nitrate, acetates. 

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