Stability as catalysts

Zeolites are microporous crystalline aluminosilicates built from corner-connected TO4 (T = Si or Al) tetrahedra. Their micropores with uniform dimensions give rise to molecular sieve properties. Tetracoordinated aluminum atoms lead to ion-exchange properties and make internal acid sites possible. They are economically important as detergent builders, as adsorbents, and, because zeolites have high thermal stability, as catalysts in the petrochemical industry. They are also considered to have a high potential as catalysts in the synthesis of fine chemicals. 

Amorphous aluminas have moderate thermal stabdity in dry atmospheres, that is, up to temperatures of less than 700 °C. However, the thermal stabihty of amorphous alumina is reported to significantly increase with decreasing particle size (141). Upon hydrothermal treatment, they tend to convert easily into bayerite, Y-AI2O3, or TI-AI2O3 (110,131,142). Amorphous aluminas appear to have only moderate activity as acid—base catalysts (110,130). Surprisingly, they have been reported to have redox properties (143). In summary, amorphous aluminas have so far not shown high activity for any catalytic reaction, and because of their Hmited structural stability, they are also expected to have poor stability as catalysts. However, they can serve as reference materials in the catalysis field. 

Since cyclometalation reactions proceed extremely easily, many reactions have been tried with many substrates, and the number of reaction substrates employed has increased rapidly as a result. Many representative substrates are shown in Fig. 5.11 [58]. These substrates are derived from bidentate compounds or tridentate compounds. The tridentate substrates are usually called pincer compounds. Tridentate pincer compounds are used as catalysts, with an phasis on their stability as catalysts. 

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