VO Supported on Functionalized CNTs for Oxidative Conversion of Furfural to Maleic Anhydride

Abstract

Commercial non-functionalized (CNTs) and functionalized carbon nanotubes (CNT-COOH and CNT-NH2) were used as supports to synthesize vanadium-supported catalysts to be used in the gas phase partial oxidation of furfural towards maleic anhydride (MA). The CNTs and the VO2-V2O5/CNTs, so-called VO/CNT catalysts, were characterized by AAS, TGA, XRD, N2 adsorption isotherms at −196 °C, Raman, NH3-TPD and XPS. The surface area values, TGA and XRD results indicate that the larger thermal stability and larger dispersion of vanadium species is reached for the VO/CNT-NH2 catalyst. XPS indicates presence of surface VO2 and V2O5 species for the non-functionalized (CNT) and functionalized (CNT-COOH and CNT-NH2) catalysts, with a large interaction of the functional group with the surface vanadium species only for the VO/CNT-NH2 catalyst. The catalytic activity, evaluated in the range 305 °C to 350 °C, indicates that CO, CO2 and MA yield (%) and MA productivity are associated to the redox properties of the vanadium species, the oxygen exchange ability of the support and the vanadium–support interaction. For the reaction temperatures between 320 °C and 335 °C, the maximum MA yield (%) is found in the functionalized VO/CNT-COOH and VO/CNT-NH2 catalysts. This behavior is attributed to a decreased oxidation capability of the CNT with the functionalization. In addition, VO/CNT-NH2 is the more active and selective catalyst for MA productivity at 305 °C and 320 °C, which is related to the greater interaction of the surface vanadium species with the -NH2 group, which enhances the redox properties and stabilization of the VO2 and V2O5 surface active sites. Recycling at 350 °C resulted in 100% furfural conversion for all catalysts and a similar MA yield (%) compared to the fresh catalyst, indicating no loss of surface active sites.