Modeling hydrodesulfurization catalysis (HDS) by solid-state synthesis, homogeneous desulfurization and heterogeneous probe reactions.

Three approaches towards modeling the HDS catalyst are discussed: structural investigations and catalytic measurements of intercalation compounds of $\rm MoS\sb2,$ reactivity studies of soluble bimetallic clusters, and HDS of probe molecules over heterogeneous catalysts. In order to test the viability of the pseudo-intercalation model for the structure of the HDS active site, intercalation compounds of $\rm MoS\sb2$ were synthesized. Different Co species and Co loadings were introduced into the $\rm MoS\sb2$ host by flocculation of aqueous suspensions of single-layer $\rm MoS\sb2.$ The structures of the intercalation compounds were determined by EXAFS, electron diffraction, and powder X-ray diffraction. The $\rm MoS\sb2$ layers in these materials are distorted 1T-type structures consisting of linked trimolybdenum clusters. After using these materials as catalysts for the HDS of thiophene, the initial structures of the intercalates were found to be unstable: the sulfide phases 2H-$\rm MoS\sb2$ and $\rm Co\sb9S\sb8$ were the only phases detected by EXAFS. These results strongly suggest that the pseudo-intercalation model would not be thermodynamically nor kinetically stable under the conditions of HDS catalysis. The reactivity of the butterfly cluster $\rm Cp\sb2MO\sb2CO\sb2S\sb3(CO)\sb4$ (7b, $\rm Cp = \eta\sp5$-$\rm C\sb5H\sb5)$ for the desulfurization of thiols was investigated as a homogeneous model for the HDS mechanism. Both cyclopropylmethyl thiol (CPMT) and 5-hexenethiol (HET) were stoichiometrically desulfurized at $\rm 110\sp\circ C$ by cluster 7b to form the cubane cluster, $\rm CP\sb2MO\sb2Co\sb2S\sb4(CO)\sb2,$ and rearranged organic products expected of radical clocks (1-butene and methylcyclopentane, respectively). In neat HET, the reaction with cluster 7a resulted in a mixture of rearranged and unrearranged (1-hexene) desulfurization products which allowed the calculation of the rate of hydrogen abstraction $\rm (k\sb{abs} = 3.7\times 10\sp6\ M\sp{-1}s\sp{-1}).$ Thus the reagent thiol was identified as the hydrogen atom donor. The free radical mechanistic model was tested by reacting these same thiols over a conventionally prepared heterogeneous HDS catalyst. The initial product from the HDS of CPMT was 1-butene, while the HDS of HET formed a mixture of hexenes. In this case, the rearranged product (methylcyclopentane) was not formed. A free radical mechanism is proposed with the rate of hydrogen abstraction faster in the heterogeneous system than in the homogeneous one.PhDApplied SciencesChemical engineeringInorganic chemistryMaterials sciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130945/2/9825207.pd