Ageing process Alumina

The alumina supports of the fresh commercial catalysts were originally y-AI2O3 which became more crystallized during the ageing process (8 and 0)-... 

Fu G., Nazar L.F., Bain A.D. Aging processes of alumina sol-gel characterization of new aluminum polyoxycations by Al NMR spectroscopy. Chem. Mater. 1991 3 602-610 Furusaki T., Kodaira K. Preparation and properties of IniOs thin films by the sol-gel method. In High Performance Ceramic Film and Coatings. Elsevier Sdaice Pub, 1991, pp. 241-247 Furusaki T., Takahashi J., Takaha H., Kodaira K. Formation process ofSn02 thin films by sol-gel method. J. Ceram. Soc. Jpn. 1993 101 451 55 Furusaki T., Takahashi J., Kodaira K. Preparation ofTTO thin films by sol-gel method. J. CCTam. Soc. Jpn. 1994 102 200-205... 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

In view of these considerations, a large amount of effort is reported in the scientific press on the development of a process to produce benzene from n-hexane by combined cyclization and dehydrogenation. w-Hexane has a low Research octane number of only 24.8 and can be separated in fair purities from virgin naphthas by simple distillation. Recently, an announcement was made of a process in the laboratory stage for aromatiza-tion of n-hexane (16). The process utilizes a chromia-alumina catalyst at 900° F., atmospheric pressure, and a liquid space velocity of about one volume of liquid per volume of catalyst per hour. The liquid product contains about 36% benzene with 64% of hexane plus olefin. The catalyst was shown to be regenerable with a mixture of air and nitrogen. The tests were made on a unit of the fixed-bed type, but it was indicated that the fluid technique probably could be used. If commercial application of this or similar processes can be achieved economically, it could be of immense help in relieving the benzene short-age. 

With amorphous silica-alumina catalysts [5,6], the primary mode of aging involves steam-induced loss of surface area by the growth of the ultimate gel particles, resulting also In loss of porosity. While amorphous catalysts deactivate thermally as well as hydrothermally, thermal deactivation is a significantly slower process. 

Microsphere Formation. Because the microspheres were fabricated using a batch process, we monitored the viscosity and pH of the catalyst slurry as it aged. Figure 2 shows that the viscosity of the slurry was dependent on both the age of the slurry and the additive type. The reference formula was stable for 3 h, but the CP-alumina and pseudoboehmite formulations thickened or gelled in the same time period. A typical batch starting at pH 3.0 increased to about pH 3.3 before the onset of thickening (about 100 cP). For CP formulations, the onset of thickening may be related to the median particle size of the powder. 

It can be concluded that weakly to moderated branched specimen (as indicated by D vcilues) with particle sizes given by Rg can be obtained by careful control of processing parameters and of ageing time. Note that the particle size can be brought into the range of the pore diameter of the y-alumina support (4 nm). Similar results are reported by Drinker et al. [42] using somewhat different synthesis parameter values. 

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