Aluminum oxide precipitation catalyst

Li JL, Inui T. Characterization of precursors of methanol synthesis catalysts, copper zinc aluminum oxides, precipitated at different pHs and temperatures. Appl Catal A Gen. 1996 137(1) 105 17. 

The more or less simultaneously developed Phillips process likewise uses a xylene solution, but at lower pressures (30-50 bar) and temperatures (150°C) and with a partially reduced chromic oxide on aluminum oxide or aluminum silicate as supporting agent. The catalyst, which is used in relatively large quantities (10% of ethylene content), is activated by heating and filtered off after an almost 100% yield of poly(ethylene) has been obtained. The poly(ethylene) precipitates from the cooled solution the solvent, before distillation, still contains low-molecular-weight poly-... 

Inorganic precursors are much cheaper and easier to handle than metal alkoxides. Therefore the industrial production of oxide powders for ceramics and catalysts is mainly based on the precipitation or coprecipitation of inorganic salts from aqueous solutions. Gibbsite, Al(OH)3, (see Aluminum Inorganic Chemistry) is precipitated from aluminate solutions. Ti02 powders are made via the controlled hydrolysis of titanium salts. Stabilized zirconia is coprecipitated from aqueous solutions of zirconium oxychloride, ZrOC, and yttrium nitrate, YlKOsjs. 

By altering the mode of preparation of alumina catalysts in such a way that the molecular porosity was affected, Adkins 40 has been able to activate preferentially the catalyst for either decarboxylation or dehydration. This was experimentally accomplished by forming aluminum hydroxide and hydrated alumina by precipitation from salts or esters in water or xylene solution and by preparation by the action of water vapor on aluminum alk-oxides, pure and supported on pumice. Variation in ethylene efficiency" from 34 to 150 was obtained in the different catalysts. On the basis of the results, Adkins advanced the hypothesis that molecular porosity of the catalysts determined the directional activity and that large pores favored decarboxylation and small pores dehydration reactions. 

There are many other methods of preparing active synthetic silica-alumina catalysts. A fair catalyst can be made by impregnating dried silica gel with an aluminum compound which is easily converted to the oxide by calcination, e.g., A1(NC>3)3. A preferred impregnation technique is to soak a sodium-free silica hydrogel in a solution of an aluminum salt and to follow this with an aqueous ammonia treatment to precipitate the hydrous alumina on the silica (Thomas, 16 Ryland and Tamele, 17). It should be noted that silica hydrogel can easily be freed of sodium ions by water washing, since it is not a zeolite. Exceptionally pure silica-alumina composites can also be prepared by the hydrolysis of mixtures of ethyl orthosilicate and aluminum alkoxides (Thomas, 18). 

AIPO4 (AP) catalysts were obtained by precipitation, firom aluminum chloride and H3PO4 aqueous solutions, with aqueous ammonia (A), ethylene oxide (E) or propylene oxide (P). 

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