Highly stable VSi-beta zeolite synthesized by a post-synthesis strategy for efficiently catalyzing styrene epoxidation
To enhance the doping of vanadium in zeolites and achieve high stability, the selection of vanadium precursors containing specific vanadium species can effectively overcome this crucial problem. Two series of V x Si-Beta(A) and V x Si-Beta(N) zeolites have been prepared by a two-step post-synthesis method at pH = 1 and pH = 7. The properties and environments of vanadium in samples are investigated by UV–vis, XPS, EPR, H 2 -TPR and 51 V MAS NMR. The bonding between vanadium and framework in VSi-Beta(A) is stronger than that of VSi-Beta(N). After the two are calcined or hydrated, the vanadium species in the former are relatively stable, while the pseudo-tetrahedral V(V) species and pseudo-octahedral V(V) species in the latter are interconverted. The changes of vanadium species in the supernatant during the preparation have been investigated by 51 V static NMR. At low concentration and pH = 1, there is a unique mononuclear cation (VO 2 + ) in the aqueous NH 4 VO 3 solution, which is favorable for the interaction with the negatively charged hydroxyl nests generated after the nitric acid dealumination , and enables the V atoms to be tightly inserted into the framework. In styrene epoxidation, the type of V(V) species in the catalyst could significantly affect its catalytic performance, and the catalytic activity and stability of V x Si-Beta(A) are superior than that of V x Si-Beta(N).
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