Temperature 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. 

Solid superacids may be made by treating ordinary solid add catalysts with strong Br0nsted or Lewis acids. For example, if freshly precipitated titanium hydroxide or zirconium hydroxide is treated with sulfuric acid and calcined in air at 500 °C. a very active solid acid catalyst results. The solids consist mainly of the metal dioxides with sulfate ions coordinated to the metal ions on the surface. Likewise, a superacid solid catalyst can be made by treating these metal oxides with antimony penlafluonde. Both catalysts contain both Br nsted and Lewis acid sites, and they arc sufficiently active to catalyze the isomerization of n-butane at room temperature.26... 

Benzylation of toluene with benzyl chloride, which is a typical example of Friedel-Crafts alkylation, is known to be catalyzed by Lewis-type superacids such as A1C13 and BF3. This type of catalyst has been mostly used for the Friedel-Crafts reaction, which is one of the most studied of organic reactions. This reaction was performed over several metal oxides and sulfates, and iron sulfates showed an unexpected effectiveness for the reaction (102-104). The catalytic activities of FeS04 and Fe2(S04)3 for the reaction were examined in detail the activities were remarkably dependent on calcination temperature, the maximum activity being observed with calcination at 700°C (105-107). Catalytic actions analogous to the above case were also observed with other Friedel-Crafts reactions, the benzoyl-ation of toluene with benzoyl chloride (108), the isopropylation of toluene with isopropyl halides (109), and the polycondensation of benzyl chloride UIO). 

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). 

Sulfated metal oxides were claimed to be superacidic on the basis of the following observations. Their acid strength mc-asured by the Hammett indicators method goes up to Ho <-16.4. Adsorbed benzene, which is a very weak base, desorbs at very high temperatures in TPD experiments. These materials are able to isomerizc n-butanc at temperatures below lOO C. 

The predominant application of metal oxide catalysts is due to their oxidation and acid-base behavior. In the following, these areas are discussed separately, although it is clear that in many materials, for example, heteropolyacids, which combine both strong acidity and oxidation efficacy (37,38), and the sulfated metal oxides, where controversy exists as to whether the observed low temperature isomerization pathways are catalyzed by superacid or redox mechanisms (39-41), the distinction between acid-base and oxidation properties is somewhat arbitrary. To illustrate their principles, a number of different reaction types are discussed. Dehydrogenation reactions, ammoxidation, and the WGS reaction have been included imder oxidation catalysts since they constitute major industrial applications of metal oxide-based catalysts. In the case of acid-base catalysis, some of the recent activity in the area of biodiesel is described as an illustration of the complementarity of both acid catalysis and base catalysis. There are a number of additional applications of oxides as catalysts, such as in photocatalysis (42), which have not been reviewed here because of limitations of space. Oxidation Activity. 

The use of model reactions as probes of acid-base characteristics has been adopted in a number of studies. This type of test is frequently applied in a qualitative manner. In principle, careful determination of kinetics may yield parameters that relate to site strength and site density. However, this approach is not without limitations, the most signiflcant of which is that the pathway must be unique to the process to which it is being applied. For this point, in the case of sulfated metal oxides, there has been discussion as to the pathways for low temperature isomerization of ra-butane, and a number of which are nonreliant upon the superacidity previously assumed have been identified. Nevertheless in appropriate circumstances, probe reactions can be of use and some examples are detailed in the following sentences. COS hydrolysis is sensitive to basicity and has been used to probe basic surfaces (293), but the most frequently used reaction is the... 

Metal halides complexed with certain compounds exhibit superacidic character as evidenced by conversion of saturated hydrocarbons at room temperature or below. Among these materials are AICI3 complexed with other halides or sulfates, and SbFs mounted on metal oxides. 

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. 

These superacidic solutions of d- and /-transition element cations can provide a controllable low-temperature route to compounds containing transition metals in unusually low oxidation states. Also they are perceived as the precursors to transition metal carbonyl cations, as indicated in Sec. 11.3.4.3 immediately below. 

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