Silica-alumina phase transformations

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

Synthetic silica-alumina catalysts containing 25% alumina are converted to gamma-alumina and mullite when thermally treated at 700-1260°. The phase transformation is accompanied by loss of catalytic activity and by collapse of the porous structure of the catalyst. The gamma-alumina phase is formed apparently by crystallization of the amorphous alumina in the catalyst, while the mullite formation apparently results from a combined silica-alumina amorphous phase. At sufficiently high temperature all alumina is converted to mullite. Silica-alumina catalysts made from more stable silicas have a greater tendency to form gamma-alumina. Such catalysts have lower initial catalytic activity and maintain relatively high catalytic activity after steam deactivation. 

Fig. 1. Phase transformations of silica alumina catalysts during sintering.

A series of papers was published by Vansant and coworkers dealing with the gas-phase deposition and thermal transformation of Cr(acac)3 to chromia on the surface of alumina and silica supports. Cr(acac)3 binds to the hydroxyl-terminated alumina surface by hydrogen bonding and/or a donor-acceptor interaction with coordinatively unsaturated Al + ions as outlined in Figure 23 . ... 

The use of metal phthalocyanine compounds has also been described (66,67). The catalysts can either be supported on an inert carrier or used in a liquid-liquid two-phase system. Various functionalized phthalocyanine ruthenium complexes have been mentioned for the reaction of interest. For instance, ruthenium phthalocyanine disulfonate transformed hept-3-ene into 3- -propylpentanol (80°C, 18 hours, 120 bar) in a two-phase system. Further details (67) on the preparation of metal complexes supported on silica- and alumina-type supports have appeared. Generally, a mixture of metal phthalocyanine sulfonate and hydrated alumina pro-... 

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