This contribution reports a detailed kinetic and mechanistic study of heterogeneous olefin hydrogenation by Cp′2Th(CH3)2 (Cp′ = η5-(CH3)5C5) adsorbed on dehydroxylated alumina (DA). For both propylene and isobutylene, the kinetics can be accommodated by a two-step sequence: (i) olefin insertion into Th-H (rate constant ν2); (ii) hydrogenolysis of the resulting Th-alkyl bond (rate constant ν3). Kinetic data can be fit to the rate law Nt (turnover frequency) = ν2ν3PH2/(ν2 + ν3PH2), where the reaction is zero-order in olefin and where ν2/ν3 = 0.55 bar-1 (-63°C) and 0.47 bar-1 (0°C) for propylene and isobutylene, respectively. Kinetic isotope measurements indicate Nt(H2)/Nt(D2) = 1.5(1) for propylene hydrogenation at -63°C and 2.0(2) for isobutylene hydrogenation at 0°C. In reaction with D2, propylene yields exclusively 1,2-propane-d2. Arrhenius activation energies for catalytic hydrogenation are 3.6(2) kcal mol-1 (propylene, -63 to -23°C) and 5.3(2) kcal mol-1 (isobutylene, -23 to +64°C); for isobutylene, it could be determined that the activation enthalpies for olefin insertion and alkyl hydrogenolysis are comparable (6.2(10) and 4.2(10) kcal mol-1, respectively). Protonolytic poisoning experiments indicate that ≤4 ± 1% and ≤2 ± 1% of the thorium surface sites are responsible for the bulk of the propylene and isobutylene hydrogenation, respectively. As a function of olefin, the relative rates of Cp′2Th(CH3)2/DA-catalyzed hydrogenation are cis-2-butene > trans-2-butene > propylene > isobutylene; 1,3-butadiene yields only 1-butene and cis-2-butene as initial hydrogenation products. For several organoactinides supported on DA, the relative ordering of propylene hydrogenation activity is Cp′2Th(CH3)2 ≥ Me2SiCp″2Th(n-Bu)2 (Cp″ = n5-(CH3)4C5) ≥ [Me2SiCp″2Th(μ-H)2]2 ≥ Cp′2Th(CH3)(O3SCF3) ≥ [Me2SiCp″2U(μ-H)2]2, ≫ U(η8-n-BuC8H7)2. As a function of support, the relative rates of Cp′2Th(CH3)2catalyzed propylene hydrogenation are DA > dehydroxylated SiO2-Al2O3 > dehydroxylated MgCl2 ≫ dehydroxylated SiO2-MgO. Protonolytic poisoning experiments indicate that 35 ± 10% of the Cp′2Th(CH3)2/MgCl2 sites are catalytically significant. Informative parallels can be drawn between these observations and homogeneous solution phenomenology for similar types of compounds.
|Number of pages||13|
|Publication status||Published - 1990|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Colloid and Surface Chemistry