Gel precipitation process

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

Proteins that remain in whey after removing casein from milk are recovered as whey protein concentrates by precipitation with added polyphosphate or other polyvalent anionic compounds, ultrafiltration, ion exchange adsorption, gel filtration, or a combined acid and heat precipitation process. Whey protein concentrates are also manufactured by a combined process involving electrodialysis, concentration, lactose crystallization, and drying (Richert 1975 Morr 1979 Marshall 1982 Anon. 1982 Muller 1982B). 

On the basis of pilot-scale experimental results, the AAH precipitation process can be scaled-up to field systems. In such systems AAH can be obtained by pouring AI2(804)3 solution into NaOH solution in a dosage time not exceeding 40 sec. The final concentration of Al2(S04)3 solution, after mixing, should be within 60-100 kg Al2(S04)3 18 H2 0/m the final pH should be within the range pH 7.0-8.5. The reactor should be equipped with a mixer for converting the AAH precipitates from a gel into a sol form. For excess salts removal, AAH suspension may be rinsed with water or dosed to the treatment processes directly. 

Active aluminas (various oxides and hydrated oxides) with high specific areas, good absorption characteristics, catalytic properties and high chemical reactivity are either produced by precipitation processes from aluminum salt solutions e.g. via thermal post-treatment of aluminum hydroxide gels, or by the calcination of a-aluminum hydroxide under specific conditions (low temperatures, very rapid heating). 

The wet chemical manufacturing proces.ses can be divided into precipitation processes, gel processes and hydrothermal proces,ses, of which the precipitation processes are the most important. 

The sol-gel process was first used for the preparation of silicates used in phase equilibrium studies. Often the sol-gel process makes use of a concentrated hydrous sol, a colloidal dispersion of (hydrated) oxide particles produced by controlled precipitation. It is also a precipitation process that makes use of immobilization of ions in a gel or glassy structure. 

The precipitation process, the reverse of solid dissolution, is dictated by solution thermodynamics. The solution reaches saturation state when the dissolution rate equals the precipitation rate. Nucleation starts when the solution concentration exceeds the saturation concentration. The solution concentration affects the crystallite size of the precipitated catalyst. For example, diluted solution is beneficial to crystal growth due to slow nucleation and the presence of a few nuclei. In contrast, submicrometer- or even nano-sized amorphous gel or sol can be formed starting with a more concentrated solution. 

This suggests that in spite of substantial distinctions in composition of co-precipitated gels, the process of their structure formation is accompanied by building-up of particles... 

There are various nanoparticle production methods reported. Most common approaches include solid-state methods (grinding and milling), vapor methods (physical vapor deposition and chemical vapor deposition), chemical synthesis/ solution methods (sol-gel approach and colloidal chemistry), and gas-phase synthesis methods [1]. Chemical approaches are the most popular methods for the production of nanoparticles. Other novel production methods include microwave techniques, a supercritical fluid precipitation process, and biological techniques. 

When a sol flocculates it becomes a gel. This process is reminiscent of the precipitation of a salt and leads to the appearance of colloidal turbidity. The reverse phenomenon, called dispersion, is similar to the dissolving of a salt. Coagulation and syneresis correspond to the formation and disappearance of jellies. 

Legrand and co-workers [8] and the important NMR work by Brinker and co-workers on the formation of silica in the sol-gel process [9]. We assume that much of this is covered elsewhere in this volume. In addition, the huge subject of template-synthesized mesoporous silicas, for which there is a very large recent NMR literature, is omitted from this paper, which focuses on what is commonly referred to as amorphous silica, e.g., silica gels, precipitated silicas and fumed silicas. 

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