POLYOXOMETALATE CATALYSIS OF THE AEROBIC OXIDATION OF HYDROGEN-SULFIDE TO SULFUR

Polyoxometalate-based catalytic systems for the effective and selective aerobic oxidation of H2S to S(0) (H2S + 1/2O(2) --> H2O + 1/8S(8), eq 1) are reported. K-5[ZnPW11O39], alpha-K-8[SiW11O39], alpha-K-6[ZnSiW11O39], and K-14- [NaP5W30O110] (1) are effective catalysts for eq 1 exhibiting 40, 29, 21, and 7.4 turnovers, respectively at 60 degrees C with 1.1 atm of O-2 for 24 h in a sealed, constant pressure system. Under these same conditions, K-7[PW11O39] produces 50 turnovers and partially decomposes while sulfide quantitatively demetalates many substituted Keggin complexes: H2S + [(TM)X(n+)W(11)O(39)]((10-n))(-) --> (TM)S + [X(n+)W(11)O(39)]((12-n)-) + 2H(+), TM = Fe-II, Co-II, Ni-II or Cu-II and X(n+) = P-V or Si-IV. An evaluation of the redox potentials of water compatible and redox active polyoxometalates and all sulfur species pertinent to eq 1 at pH 0 and pH 8.5, the practically optimal pH for eq 1, indicates that many polyoxometalates should selectively facilitate eq 1, including 1. The rates for the reduction of 1 by HS-, the likely sulfur redox intermediates, polysulfide (S-n(2-)), S2O62-, S2O32-, and the product, S-8, (pH 8.5, borax buffer, [1] = 2.0 mM, [substrate] = 2.6 mM, 298K) are V-0 = 0.96, 22.2, 159, 0, and 0, mu M s(-1) respectively. The experimental rate law for oxidation of HS- by 1 is first order in HS- and second order in 1, with a complex pH dependence. A comparison of eq 1 catalyzed by 1 versus the chelated Fe compounds used industrially indicates that, for 1, the reaction is slower initially, lasts far longer since the catalyst is not degraded, and produces higher purity S-8 (>99.5% for 1 versus 89.8% for Fe-III(EDTA)(-)). The success limiting, factor for eq 1 catalyzed by 1 derives from appreciable undesirable uncatalyzed background oxidations, particularly the production of sulfate (HS- + 2O(2) --> HSO4-).