Surface Organometallic Chemistry of Titanium on Silica−Alumina and Catalytic Hydrogenolysis of Waxes at Low Temperature

Ti(CH₂tBu)₄ (1) reacts selectively with the surface silanols of a silica−alumina partially dehydroxylated at 500 °C to provide the monosiloxy species [(SiO)Ti(CH₂tBu)₃]_SA (2a) and the bisiloxy species [(SiO)₂Ti(CH₂tBu)₂]_SA (2b) in a ca. 40:60 ratio, with concomitant evolution of 1.6 ± 0.2 equiv of neopentane per Ti. These surface complexes were characterized via the combined use of several techniques such as IR spectroscopy, ¹H MAS, ¹³C-CP/MAS, 2D ¹H−¹³C HETCOR, and J-resolved solid-state NMR, as well as mass balance analysis. By treatment under hydrogen at 150 °C the neopentyl ligands in complexes 2a,b undergo hydrogenolysis and a mixture of supported titanium species is obtained. IR, ESR, ¹H MAS, and DQ solid-state NMR spectroscopies show the presence of ca. 3% [(SiO)(M_sO)TiH₂]_SA (3a; M_s = Si, Al), 5% [(SiO)(M_sO)Ti(Me)−H]_SA (3b), 75−80% [(SiO)(M_sO)₂Ti−H]_SA (3c), and 14% [(SiO)(M_sO)₂Ti^III]_SA (3d), along with (SiH_x) and (AlH_x) fragments whose formation arise from the opening of adjacent Si−O−M bridges (M = Si, Al). Species 3a−d are efficient catalysts for the hydrogenolysis of waxes with a diesel selectivity higher than 60%. Comparison with the silica-based system shows a beneficial role of the silica−alumina support on the activity of the Ti centers, attributed to a direct interaction of the surface with the active site, which possibly facilitates the β-alkyl transfer, the key C−C bond cleavage step in the proposed mechanism.