Surface organometallic chemistry of titanium on silica-alumina and catalytic hydrogenolysis of waxes at low temperature

Abstract

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 1R 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)(MsO)TiH 2]SA (3a; MS, = Si, Al), 5% [≡=SiO)(MSO)Ti(Me)-H]SA (3b), 75-80% [(=SiO)(M sO)2Ti-H]SA (3c), and 14% [(SSiO)(M SO)2Ti-H]SA (3d), along with (SiHx) and (AlHx) 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. © 2009 American Chemical. Society.