(297c) Dispersed Ta(V): A Site-Isolated Epoxidation Catalyst and Unexpected Reactivity In Selective Hydrogenation / Hydrodenitrogenation

Highly dispersed Ta^V species are known, albeit
relatively infrequently studied, epoxidation catalysts with H₂O₂.
We recently reported on replacing traditional TaCl₅ precursors with calixarene?Ta(V)
complexes on SiO₂, which resulted in epoxidation catalysts of higher
H₂O₂ utilization and selectivity for the direct
epoxidation of cyclohexene (without formation of cyclohexenol) as compared to other
highly dispersed Ta-SiO₂ catalysts and as compared to the benchmark
Ti-SiO₂ catalysts. Calixarene-derived catalysts had initial
cyclohexene direct epoxidation turnover rates of 3.9 ± 0.1 × 10^-2 s^-1
unaffected by surface density, demonstrating single-site character.

In parallel, soluble Ta^III organometallic
complexes have been studied as HDN models, showing direct C-N insertion of
pyridine without ring hydrogenation, and highly dispersed, supported Ta are
known to be active in a number of unusual reactions including alkane
metathesis, and direct N₂ activation. As such, we investigated our
highly dispersed Ta catalysts for HDN of quinoline. Catalysts were used after
calcination at 550°C for 6 h in air to and activation of the calcined catalyst under
H₂ at up to 350°C, 4 h. HDN reactions were run up to 12h at 40 bar
and 275-350°C, followed by GC/FID with a TR-WaxMS column. Some catalysts were
activated with small amounts of Pd deposited by incipient wetness.

Table
1 compares dispersed Ta vs Pd catalysts. Pd is used here as a typical
hydrogenation catalyst and, as expected, shows much higher HDN conversion (to
hydrocarbons), forms essentially only propylcyclohexane (PCH). Although quinoline
HDN was slower over Ta catalysts, it proceeded via a less H₂-consuming
route with measurable formation of propylaniline (PA), propylbenzene (PB), and
propylcyclohexene (PCHE). Combining Pd and Ta increases both HDN activity and
yields of propylbenzene. The direct cleavage of aniline to benzene was observed
over Pd-Ta/Al₂O₃ catalysts by in situ diffuse reflectance
UV-visible spectroscopy-mass spectrometry and gave evidence for a key Ta imido
intermediate.