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

Ti(CH 2 t Bu) 4 ( 1 ) reacts selectively with the surface silanols of a silica−alumina partially dehydroxylated at 500 °C to provide the monosiloxy species [(≡SiO)Ti(CH 2 t Bu) 3 ] SA ( 2a ) and the bisiloxy species [(≡SiO) 2 Ti(CH 2 t Bu) 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, 1 H MAS, 13 C-CP/MAS, 2D 1 H− 13 C 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, 1 H MAS, and DQ solid-state NMR spectroscopies show the presence of ca. 3% [(≡SiO)(M s O)TiH 2 ] SA ( 3a; M s = Si, Al), 5% [(≡SiO)(M s O)Ti(Me)−H] SA ( 3b ), 75−80% [(≡SiO)(M s O) 2 Ti−H] SA ( 3c ), and 14% [(≡SiO)(M s O) 2 Ti 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.