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

Authors: 
Notestein, J. M., Northwestern University
Morlanes-Sanchez, N., Northwestern University
Bachrach, M., Northwestern University
Marks, T. J., Northwestern University


Highly dispersed TaV 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 TaIII 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.

 

catalyst

Yields

THQ or DHQ

PA

PCHE

PCH

PB

Pd/Al2O3

32

0

0

66

0

Pd/SiO2

24

0

1

65

0

Ta/Al2O3

42

2

4

0

0

Ta/SiO2

47

1

0

0

1

Ta/SiO2a

56

2

1

0

0

Ta/SiO2b

67

1

0

0

0

0.5%Pd-Ta/SiO2

30

3

7

47

5

0.5%Pd-Ta/Al2O3

24

5

21

29

19

Ta/Al2O3+Pd/Al2O3

84

0

5

3

0

 

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