Properties superacidic metal oxides

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 natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

Earlier reference has been made to sulphonic acid resins and supported mineral acids as heterogeneous catalysts. The chlorination of alumina provides a strongly acidic surface, while the performance of a number of solid superacid catalysts (— Ho values up to 16) has recently been reported. However a number of mixed oxides of metals and metalloids also show useful acidic properties. 

PFSA membranes have excellent chemical inertness and mechanical integrity in a corrosive and oxidative environment, and their superior properties allowed for broad application in electrochemical devices and other fields such as superacid catalysis, gas drying or humidification, sensors, and metal-ion recovery. Here, we refer their important applications in electrochemical devices for energy storage and conversion including PEMFC, chlor-alkali production, water electrolysis, vanadium redox flow batteries, lithium-ion batteries (LIBs), and solar cells. 

---