Synergic catalysis of W and Ni originating from substitution of trivacant phosphotungstate for the selective oxidation of aniline to azoxybenzene

Partial substitution of polyoxometalate (POM) is an efficient route to modulate the catalytic property of maternal POM. In this work, a new Keggin type POM involving {Ni6} cluster, {[Ni(H2O)2(Dach)2][Ni(Dach)2]2}{[Ni6Cl(μ-OH)3(H2O)(Dach)3(WO4)(PW9O34)][Ni6(μ-OH)3(H2O)2(Dach)3(WO4)(PW9O34)]}Cl·27H2O, (1, Dach = 1,2-diaminocyclohexane) was synthesized. Compounds 1 shows excellent catalytic performance in the selective oxidation of aniline to azoxybenzene (AOB) in water. The apparently different results from that with the matrix {PW9O34} ({PW9}) suggest the successful regulation of the catalytic property of {PW9} by the introduction of the {Ni6} cluster into the skeleton. The experimental results indicate that the highlighted performance of 1 is contributed by the synergy of W and Ni sites, which are respectively responsible for the oxidation and condensation steps in the production of AOB. The good selectivity to AOB is essentially attributed to the effective modulation of the reaction rates of oxidation and condensation steps by W and Ni sites, respectively.