^(51)VNMR研究磷钼钒系杂多酸的热稳定性

^(51)V NMR STUDY ON THERMAL STABILITY OF MOLYBDOVANADOPHOSPHORIC ACIDS

用NMR及IR、TG-DTA等手段研究具有Keggin结构的磷钼钒系杂多酸在空气中的热稳定性。实验结果表明:合成的杂多酸在各个Keggin单元中具有不同的钒含量。样品的热稳定性与其钒含量有关.随着钒含量的增加,keggin结构的热稳定性逐渐降低,但H_4PMo_(11)V_1O_(40)具有比H_3PMo_(12)O_(40)还要高的热稳定性。由于钒的取代,钼钒磷酸的热分解过程不同于钼磷酸。失去结构水使Keggin结构遭到破坏,但在室温下与水重新作用可使钼钒磷酸的Keggin结构恢复。

The thermal stability of H3+nPMo12-nO40 1,2,3) in air was studied by 51V NMR as well as IR, TG-DTA and XRD. It was found that the synthesized heteropolyacids (HPA) were mixtures with different vanadium contents in different Keggin units. Three NMR peaks were observed, and the isomer shifts were -530 ppm, -534 ppm and -542 ppm(relative to VOCl3) respectively. The narrow peak (δ= -530ppm) could be attributed to H4PMo11VO40. As the vanadium contents in Keggin units increased, the thermal stability decreased. It is noteworth that the thermal stability of H4PMo11VO40 is higher than that of H3PMo12O40 It was also found that the decomposition process of molybdovanadophosphoric acid was different from that of H3PMo12O40. The HPA containing vanadium became insoluble in water after being treated at 200-400℃, and no IR peak could be observed. The HPA was converted into MoO3and other substances at higher temperature.Howerer,the thermal decomposition of H3PMo12O40 to form MoO3 seem much easier than that of other samples. The Keggin units containing vanadium could be reconstructed by reacting with water at room temperature.