Experimental and Theoretical Study of <i>n</i>‑Butanal Self-Condensation over Ti Species Supported on Silica
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Abstract
The effects of the coordination environment
and connectivity of
Ti on the rate of <i>n</i>-butanal self-condensation over
Ti-silica catalysts were investigated. Ti was introduced in two ways,
either during the synthesis of mesoporous SBA-15 or via grafting onto
amorphous silica with a disordered pore structure. The connectivity
of Ti was then characterized by XANES, UV–vis, and Raman spectroscopy.
For the lowest Ti loadings, the Ti is found to be predominantly in
isolated monomeric species, irrespective of the manner of sample preparation,
and as the Ti loading is increased, a progressively larger fraction
of Ti is present in oligomeric species and anatase nanoparticles.
The turnover frequency for butanal condensation decreased monotonically
with increasing Ti loading, and the apparent activation energy increased
from 60 kJ mol<sup>–1</sup> for monomeric species to 120 kJ
mol<sup>–1</sup> for oligomeric species. A kinetic H/D isotope
effect was observed over isolated titanol and Ti dimer catalysts suggesting
that α-H abstraction is the rate-determining step. This conclusion
is supported by theoretical analysis of the reaction mechanism. In
agreement with experimental results, the calculated activation barrier
for alkanal condensation over a Ti dimer is roughly two times greater
than that over Ti-OH sites. The cause for this difference was explained
by energy decomposition analysis of the enolate formation step which
showed that there is a large energetic penalty for the substrate to
distort over the Ti–O–Ti dimer than the Ti-OH monomer