PNNL

Abstract

Hydrosulfurization (HDS) is carried out by exposure of a substrate fuel source to a regime of hydrogen pressure and high temperatures in the presence of Co/MoS or Ni/MoS catalysts on ?-Al2O3 supports [Topsoe et al., 1996]. At temperatures of HDS, and in the presence of organosulfur structures, thermal free radical chemistry will accompany or even dominate many pathways of organic structure transformation. Hydrogenation and dehydrogenation of organic structure will be particularly sensitive to the nature of organosulfur intermediates. A major pathway of hydrogenolysis and hydrogenation is initiated by the transfer of hydrogen atom from a closed shell molecule to an acceptor [Rüchardt et al. 1997]. This reaction is the starting point for hydrogenation of unsaturated organic structure. The reaction is the reverse of the disproportionation reaction of two free radicals, and depends on the enthalpy change of the forward reaction. To predict the rate of reaction in eq 1, key information required is the bond strength of ?Hº = ?H* R-H + CH2=CH2 ? R• + CH3=CH2• (1) the participating organic or catalyst function (R-H, eq 1). In this paper, we present results of a density functional theory investigation of bond strengths of a series of thiols, and of the HDS prototype model, CpMo[(µ-S)2(µ-SH)2]MoCp, 1, Cp = ?5-C5H5 [Rakowski DuBois, 1998]. This system is representative of a class of MoS cluster systems that are models of exfoliated MoS2 catalysts [Curtiss, et al. 1996]. The level of theory for prediction of accurate (ca. 1-2 kcal/mol error) S-H bonds is explored. The reactivity of the prototype MoSH functionality in initiating homolytic hydrogenation steps is provided by the computational predictions.