The term polymetalate acid or simply poly acid may be defined as the condensed or polymerized formof the weak acids of amphoteric metals like vanadium, niobium, tantalum (VB group metals) or chromium,molybdenum, and tungsten (VIB group metals) in the +5 and +6 oxidation states. The anions of these polyacids contain several molecules of the acid anhydride and the corresponding salts are called as polysalts.
Furthermore, if these polymerized acids contain only one type of acid anhydride, they are called asisopoly acids. However, these anhydrides can also condense with some other acids like phosphoric or silicicacid to form heteropoly acids. In other words, isopoly acids contain only one metal along with hydrogen andoxygen while heteropoly acids contain two elements other than hydrogen and oxygen. The corresponding saltsof isopoly and heteropoly acids are called as isopoly and heteropoly salts, respectively.
Iso Poly And Heteropoly Acids And Salts Pdf Download
The applications of heteropolyacids, HPAs, in the field of catalysis are growing continuously. These compounds possess unique properties such as Brönsted acidity, possibility to modify their acid-base and redox properties by changing their chemical composition (substituted HPAs), ability to accept and release electrons, high proton mobility, easy work-up procedures, easy filtration, and minimization of cost and waste generation due to reuse and recycling of these catalysts [3,4,5,6,7]. Because of their stronger acidity, they generally exhibit higher catalytic activity than conventional catalysts such as mineral acids, ion exchange resins, mixed oxides, zeolites, etc. [8]. In the context of Green Chemistry, the substitution of harmful liquid acids by solid reusable HPAs as catalysts in organic synthesis is the most promising application of these acids [9,10].
In connection with our program of using heteropolyacids in organic reactions [20] we wish to report the results of a study on the use of three type of HPAs, including Preyssler, H14[NaP5W30O110], Keggin, H4[PMo11VO40] and Wells-Dawson types H6[P2W18O62] in the synthesis of 3,1,5-benzoxa-diazepines and the effects of reaction parameters such as the type of HPA, temperature and reaction times on the reaction yields.
With respect to the catalytic performance of these catalysts and the overall effects of all isomers, we cannot control the reaction conditions for the synthesis of positional vanadium-substituted isomers separately, indicating that the relationship between the H3+xPMo12-xVxO40 (x = 1) structures and hence study of their catalytic activity is difficult. However, because the metal substitution may modify the energy and composition of the LUMO and redox properties, for mentioned heteropolyacids with different charges, the energy and composition of the LUMOs have significant effects on the catalytic activity. Substitution of vanadium ions into the molybdenum framework stabilize the LUMOs because these orbitals derive, in part, from vanadium d-orbitals which have been assumed to be more stable than those of molybdenum and tungsten [22]. The abundance of different isomers may also play an important role in catalytic performance. In addition, different positional Mo atom(s) substituted by the V atom(s) in [PMo12O40]3- may create different vanadium chemical environments, thus causing these catalysts to exhibit varying catalytic performances. Considering the above explanations we suggest that the rigidity, steric hindrance and lower number of protons in H4[PMo11VO40] are tentatively assumed to be responsible for its observed lower activity. The larger number of protons may lower the activation barrier to the reaction. 2ff7e9595c
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