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.

 

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