Role of the direct reaction H2S + SO2 in the homogeneous Claus reaction.

Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H(2)S, SO(2), and S(2)O such as might occur in the front-end furnace of the Claus process. The direct reaction between H(2)S and SO(2) occurs via a 5-centered transition state with an initial barrier of approximately 135 kJ mol(-1) and an overall barrier of approximately 153 kJ mol(-1) to produce S(2)O and H(2)O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by approximately 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H(2)S and S(2)O can occur via two possible pathways; the analogous reaction to H(2)S + SO(2) has an initial barrier of approximately 117 kJ mol(-1) and an overall barrier of approximately 126 kJ mol(-1) producing S(3) and H(2)O, and a pathway with a 6-centred transition state has a barrier of approximately 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.