Preparation superacidic metal oxides

Preparation of Superacidic Metal Oxides and Their Catalytic Action... 

Superacidic metal oxides prepared by calcination at a high temperature can be used at elevated temperatures and, thus, provide new trends for developing environmentally benign processes. Superacidity is generated on the oxides of Fe, Ti, Zr, Hf, Sn, Si, and A1 by treatment with sulfate, tungstate, and molybdate. Sulfated and tungstated zirconias have attracted much attention as potential catalysts the latter are thermally stable superacids and can be calcined at temperatures above 1000°C. 

Although the sulfate superacids are stable enough because of preparatory heat treatment at elevated temperatures, elimination of the sulfate is sometimes observed during reaction as a result of catalyst deactivation, especially in a solid-liquid system. It is hoped to synthesize superacids with the system of metal oxides. We have succeeded in preparing another type of superacid, not containing any sulfate ion but consisting of metal oxides, which can be used at temperatures over 800°C (188-192). 

Superacidity is generally created by adsorbing sulfate ions onto amorphous metal oxides followed by calcination in air to convert to the crystalline forms. However in the case of AI2O3, a superacid is prepared from the crystallized oxide. 

A double metal oxide sulfate solid superacid (alumina-zirconia/ persulfate, SA-SZ) can be prepared by treatment of a mixture of aluminum hydroxide and zirconium(IV) hydroxide with an aqueous solution of ammonium persulfate, followed by calcination at 650°C. This catalyst can be efficiently utilized in the benzoylation of arenes with benzoyl and parfl-nitrobenzoyl chloride (Table 4.22), giving BPs in interesting yields. Even if 1 g of catalyst is needed for 40 mmol of chloride, the process seems to be quite useful because the catalyst can be readily regenerated by heating after washing with acetone and diethyl ether and reused four times. 

Certain metal oxides treated with SbFs exhibit superacidic character. The SbFs-treated metal oxides can catalyze skeletal isomerization of saturated hydrocarbons at room temperatures. The catalysts were prepared by repeated exposure of the heat-treated metal oxides to SbFs vapor followed by outgassing to remove excess SbFs. 

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