Thermal multi-electron transfer catalysis by polyoxometalates. Application to the practical problem of sustained, selective oxidation of hydrogen sulfide to sulfur

Novel polyoxometalate-based catalytic systems for the effective and selective aerobic oxidation of H2S to S(O) are reported. The base stable catalysts TMA(9)[Nb3P2W15O62], 1, Li-33[H7P8W48O184], 2, K-10[Zn4P2W18O68], 3, and K-4[Nb2W4O19] produce 17, 135, 5.7 and 8.8 turnovers of S-8, respectively (23 degrees C; 1.0 atm O-2; 4 h; sealed vessel). The stability of these catalysts during sulfide oxidation are evaluated. The redox potentials of these polyoxometalates and the primary fragmentation products pertinent to sulfide oxidation are given. Early time kinetics of the reduction of 1 by sulfide proceeds according to the rate law +d[Nb3P2W15O6210-]/dt=k[1][HS-], with a complex pH dependence. The buffering agent is shown to be non-innocent and at parity of reaction conditions, the reduction of K-14[NaP5W30O110], 4, by sulfide in media buffered with PO43-, CO32-,B4O102-, and OH- (a blank) results in 40, 9.0, 49, and 10 turnovers of S-8, respectively.