Hydrodenitrogenation by bifunctional catalysts combining Pd and highly-dispersed Ta sites

Ta organometallic complexes have been studied as HDN models and dispersed, supported Ta is active in reductions and isomerizations, but related HDN catalysts are not known. Here we use highly-dispersed, supported Ta derived from calixarene-Ta(V) complexes in quinoline HDN. Supported Ta is minimally reactive, but has high selectivity to propylaniline and propylbenzene, consistent with (unusual) direct C-N bond hydrogenolysis. Propylbenzene appears neither over catalysts derived from TaCl ₅ at any loading, nor from partially reduced catalysts. XAS and FTIR show Ta(V) is stable during reaction. Adding Pd increases propylbenzene yields, especially for Pd-Ta/Al ₂O ₃. Large enhancements suggest spillover-mediated cooperativity between initial hydrogenation over Pd, followed by hydrogenolysis of subsequent intermediates coordinated to Lewis acidic Ta(V). In model reactions, adsorbed aniline is reduced to cyclohexane over Pd-Al ₂O ₃, but benzene over Pd-Ta/Al ₂O ₃, and DRUV-vis spectra are consistent with the formation of Ta imido species, indicating direct C-N bond hydrogenolysis.