Alumina gel preparation

Fig. 3.35 Mercury porosimetry intrusion-extrusion plots of alumina gels prepared from solutions of aluminium monohydrate in A, propan-2-ol (2-5w/v%) B, propan-2-ol (4-9w/v%) C, 2-methylpropan-2-ol (4-9 w/v%) D, 2-methylpropan-2-ol (9-5 w/v%) E,butan-2-ol (9-5 w/v%). -------, ascending, intrusion curve -----, descending, extrusion curve.

Pyridinium / -toluenesulfonate, 5°, 1 h." Similar selectivity can be achieved using a silica-alumina gel prepared by the solgel method. ... 

Ether cleavage. Silica gel on which oxalic acid is deposited hydrolyzes enol ethers. This reagent is useful for converting 3-methoxy-2,5(10)-diene steroids (Birch reduction products of A-aromatic steroids) into the corresponding 5(10)-en-3-ones. For the cleavage of other ethers in methanol, the silica-alumina gel (prepared by the sol-gel method) is very valuable. The ease of deprotection follows the order TMS > 1 -methyl-1 -methoxyethyl 1 -ethoxyethyl > THP methoxymethyl. 

Ikoma S., Kawakita K., Yokoi H. Characterization ofpolyamine copper(ll) complex-doped alumina gels prepared by the sol-gel technique. J. Non-Cryst Solids 1990 122 183-192 Ilharco L.M., Santos A.M., Silva M. J., Martinho J.M.G. Intramolecular pyrene excimer in probing the sol-gel process. Langmuir 1995 11 2419-2422 Innocenzi P., Kozuka H., Yoko T. Dimer-to-monomer transformation of rhodamine 6G in sol-gel silica films. J. Non-Cryst. Solids 1996 201 26-36... 

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. 

Suzuki, F., K. Onozato and Y. Kurokawa. 1987. Gas permeability of a porous alumina membrane prepared by the sol-gel process. J. Non-Cryst. Solids 94 160-62. 

When cracking catalysts are prepared with a silica or silica-alumina gel base, the pore volume and average pore... 

Laboratory investigation revealed that sodium, which was present in the support to the extent of several tenths of 1%, had a profound effect on stability and activity of the moiybdena-alumina catalyst. Over a period of time it was possible to alter the procedure for preparing the support on successive occasions until the catalyst contained much less than 0.1% sodium oxide. The reduction in sodium content of the support was immediately reflected in improved catalyst life. Ultimately the life was extended to 9 to 12 months before replacement. Various forms of alumina have been used as a support, including alumina gel and a stabilized alumina gel. Moiybdena-alumina catalyst has been employed exclusively in the eight commercial plants previously referred to. Today the majority of refiners who operate hydroformers are using molybdena on alumina gel as a catalyst. The molybdic oxide content of the catalyst is somewhat below 10%. Although similar to the original catalyst as far as chemical composition is concerned, it possesses superior activity and life. 

Fig. 8.13. Diagram of a set up for continuous sol-gel preparation of alumina spheres. (1) sol preparaton (2) dropping of sol into layer of petrol ether (3) gelation reactor filled with a NH4OH solution (4) circulation pump (5) supply vessel for the gelation fluid (6) particle separation ...

Gel Preparation NiO/A C gels are prepared using the method suggested by Teichner and co-workers (4). Aluminum sec-Butylate is dissolved in sec-Butanol while Nickel Acetate is dissolved in Methanol separately. Water is added to the second mixture in near stoichiometric amounts necessary for the hydrolysis reactions. The two solutions are mixed and a precipitate of alumina is immediately formed. Gels are dried immediately after preparation, in order to eliminate the effect of aging (2). 

The name activated alumina is generally applied to an adsorbent alumina (usually an industrial product) prepared by the heat treatment of some form of hydrated alumina (i.e. a crystalline hydroxide, oxide-hydroxide or hydrous alumina gel). It has been known for many years that certain forms of activated alumina can be used as powerful desiccants or for the recovery of various vapours. It was apparent at an early stage that the adsorbent activity was dependent on the conditions of heat treatment. For example, in 1934 Bayley reported that the adsorption of H2S by a commercial sample of activated alumina was affected by prior heating of the adsorbent at different temperatures, the maximum uptake being obtained after heat treatment at SS0°C. During an investigation of the catalytic dehydration of alcohols, Alekseevskii (1930) found that a calcination temperature of c. 400°C was required to optimize the adsorption of the alcohol reactants, whereas calcination at 600°C was preferable for the adsorption of the olefine products. 

Other catalyst preparation methods include cementation of iron oxide powders with compounds such as Al3(N03)j potasaum waterglass or sodium borate. The imprecation of high surface-area supports such as silica or alumina gels with metal salts have also been investigated. These methods usually lead to catalysts of low activity. 

We will next consider the case of a lew silica content co-gel. A 5% silica-content silica-alumina was prepared by precipitation of aluminum isdsutoxide and tetraethoxv-silane as described for the silica-free gel. After gelation water was added just sufficient to fill the pore voids of the gel. The added water led to formation of a boehmite-rich hase during recrystallization. After drying at 120 and calcination at 500 0 for 16 hours, a transitional alumina hase is formed with a surface area of 410 m /g and a pore volume of 1.9 oc/g. This silica-alumina had an average pore diameter of 18 nm, similar to the silica-free material discussed previously. Steam treatment of this 18 nm pore diameter silica-alumina at 870°C (1600 ) in 90% H20-10% N2 for 16 hours resulted in a material with surface area of 196 m /g. This surface area is much hi er than expected for an amori ous gel and is consistent with silica enrichment of the outer surface during the recrystallization step vhere water was added to the pores of the amoridious gel. Silica stabilization of bodunite alumina by formation of a surface Aiase complex has been reported in recent work (9). ESCA analysis also indicates silica surface enrichment vhen compared to the amori ous gel. 

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