Stability

Abstract

A large number of publications have reported studies of the thermal stability of heteropoly oxometalates as part of investigations of other aspects of these materials. Although these reports may have been discussed elsewhere in this monograph where relevant, they have also been included here. The present discussion of thermal stability is, for convenience, restricted to unsupported heteropoly oxometalates with that pertaining to the supported materials included in the section on the properties of the latter. A cautionary note should be added here. The results from studies of stability are dependent not only on the method employed for such evaluations, but in addition are frequently dependent on the conditions under which the experiment was performed.One of the earliest studies of the stability of the heteropoly acids employed differential thermal analysis and X-ray diffraction. (1) With each of the four acids examined, an endotherm and an exotherm are observed at 300 C or less and 340 C or more, respectively (Table 5.1). The low-temperature endotherm is attributed to the removal of water whereas the high-temperature exotherm signals the decomposition of the anion. The authors note that metatungstic acid H 8 [W 12 O 40 ]ÁxH 2 O decomposes at 50 C. When the void in the center of the [W 12 O 40 ] 8À anion is occupied by Si 4 or P 5 , the decomposition temperature increases by approximately 400 or 500 C, respectively. The presence of a positively charged species reduces the effect of thermal vibrations and thus of decomposition.Thermogravimetric analyses of H 3 XW 12 O 40 with X being zinc, iron, cobalt, boron, silicon, or phosphorus, their potassium salts, and their ether-addition compounds have been reported. (2) With the ®rst three acids, three peaks were observed whereas only two were obtained with the latter three acids (Table 5.2). The third peak was attributed to decomposition of the anion. 42CHAPTER 5 STABILITY 43