Catalytic activity and stability of carbon supported v oxides and carbides synthesized via pyrolysis of MIL-47 (V)

Substantial leaching of active V species often limits the reusability of V-based soild catalysts in liquid-phase oxidation reactions and therefore requires the development of more stable, novel materials. This paper first reports the synthesis of active vanadium (V) oxide and carbide species dispersed on a carbon support via the pyrolysis of MIL-47 (V), a V-based metal-organic framework (MOF) template. The phase transition of V species present in this MOF template was achieved by varying the pyrolysis temperatures ranging from 600 to 1100°C to synthesize a series of carbon catalysts with different surface and bulk phases of V. Notably, the pyrolysis of MIL-47 (V) provided carbon supports with high surface areas (∼ 350 m² g^-1), high mesoporosities (V_MESO/V_PORE ∼0.88), high V quantities (35-70 wt.%), and small (∼ 18 nm) V crystallites dispersed on the surface. These desired properties were not observed when V was supported on activated carbon (V/AC) via conventional impregnation. The V/AC catalyst showed lower mesoporosity (∼0.63), lower V quantity (∼25 wt.%), and larger V crystallites (∼ 27 nm) compared to the catalyst produced from MIL-47 (V) pyrolysis under identical conditions. Of additional note, the pyrolysis of MIL-47 (V) could yield an isolated bulk phase of V carbide at low pyrolysis temperatures (i.e., >900°C). This phase was not attainable when V/AC was synthesized via pyrolysis even at higher temperatures (i.e., 1100°C). The catalytic performance of the resulting V on carbon catalysts was evaluated in a liquid-phase oxidation reaction of dibenzothiophene. The V carbide catalysts exhibited good activities and enhanced stabilities, as evidenced by lower amounts of V species leached (<20%) during recycle runs compared to the conventional V/AC catalyst (V leaching ∼56%). This study marks a signifcant improvement in the synthesis of supported V catalysts with reduced leaching in liquid-phase oxidation reactions compared to materials synthesized via conventional impregnation techniques.