Zirconia superacidic metal oxides

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. 

Out of the metal oxides, sulfated titania and tin oxide performed slightly better than the sulfated zirconia (SZ) catalyst and niobic acid (Nb205). However, SZ is cheaper and readily available on an industrial scale. Moreover, it is already applied in several industrial processes (7,8). Zirconia can be modified with sulfate ions to form a superacidic catalyst, depending on the treatment conditions (11-16). In our experiments, SZ showed high activity and selectivity for the esterification of fatty acids with a variety of alcohols, from 2-ethylhexanol to methanol. Increasing... 

This review summarizes the recent works on syntheses of solid superacids and their catalytic action, including Lewis acids and liquid superacids in the solid state, as discussed in Sections Il-IV. Sections VI and VII describe new types of solid superacids we have studied in this decade sulfate-supported metal oxides and tungsten or molybdenum oxide supported on zirconia. Perfluorinated sulfonic acid, based on the acid form of DuPont s Nafion brand ion membrane resin, is also gaining interest as a solid superacid catalyst Nafion-H-catalyzed reactions are reviewed in Section V. 

Solid catalysts can be used at elevated temperatures, though their acidities are much weaker than those of liquid ones. From this point of view, solid superacids based on Lewis acids and liquid superacids discussed in Sections II—1V are not sufficiently stable Nafion-H is also unsatisfactory, its maximum operating temperature being below 200°C. A new type of the sulfate-supported metal oxides is more stable because of preparatory heat treatment at high temperatures, but elimination of the sulfate is sometimes observed during reaction, thus it is hoped to synthesize superacids with the system of metal oxides. Another type of superacid, tungsten or molybdenum oxide supported on zirconia, has been prepared by a new preparation method, and its stability is satisfactory so far. It is hoped that the preparation method will be extensively applied to other metal oxides for new solid superacids. 

A range of metal oxides have been compared as methane combustion catalysts. The effect of modification to generate superacidic behaviour on Zr02 and Fe203 systems has been studied. It has been shown that whilst sulfation lowers the activity of Fe203, sulfation and, particularly, molybdation enhance the performance of Zr02. Despite enhancing the activity of the unmodified base oxides, the addition of low levels of platinum has been demonstrated to poison the activity of superacidic zirconias. Potential reasons for these observations are discussed. 

The isomerization of light paraffin using superacid solid catalysts is a clean way to increase the octane number of hydrocarbons. On this basis, sulfated metal oxides have attracted the attention of many research groups owing to their high activity in acid catalyzed reactions [1]. Sulfated zirconia was found to be a promising catalyst in this field and at the industrial level [2],... 

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. 

Although the super acidity of sulphated zirconium oxide is widely accepted, Farca iu indicated that traditional acidity measurements, i.e. Hammett type, can not be applied to solid acids. He showed that the exceptional catalytic activity of sulphated zirconia and other sulphated metal oxides (SMO) is not justified by their acid strength and that they are in fact not superacidic at all. He argues that sulphated zirconium exhibits a bifunctional character in which oxidation in the initiation step is combined with acid catalysed reaction of the intermediates formed. Oxidation is normally achieved through the reduction of the sulphate or added redox promotors, which increases the catalytic activity but not the acid strength. 

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