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 {dollar}rm MoSsb2,{dollar} 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 {dollar}rm MoSsb2{dollar} were synthesized. Different Co species and Co loadings were introduced into the {dollar}rm MoSsb2{dollar} host by flocculation of aqueous suspensions of single-layer {dollar}rm MoSsb2.{dollar} The structures of the intercalation compounds were determined by EXAFS, electron diffraction, and powder X-ray diffraction. The {dollar}rm MoSsb2{dollar} 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-{dollar}rm MoSsb2{dollar} and {dollar}rm Cosb9Ssb8{dollar} 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 {dollar}rm Cpsb2MOsb2COsb2Ssb3(CO)sb4{dollar} (7b, {dollar}rm Cp = etasp5{dollar}-{dollar}rm Csb5Hsb5){dollar} 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 {dollar}rm 110spcirc C{dollar} by cluster 7b to form the cubane cluster, {dollar}rm CPsb2MOsb2Cosb2Ssb4(CO)sb2,{dollar} 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 {dollar}rm (ksb{lcub}abs{rcub} = 3.7times 10sp6 Msp{lcub}-1{rcub}ssp{lcub}-1{rcub}).{dollar} 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.