Synthesis of Niobium Carbonitride by Thermal Decomposition of Guanidine Oxaloniobate and Its Application to the Hydrodesulfurization of Dibenzothiophene

A production method that uses lower temperatures than the temperatures of commonly utilized methods (950–1,100 °C) is proposed for the synthesis of niobium carbonitride and niobium carbide. This methodology consists of three stages: (i) the synthesis of guanidine oxaloniobate via a solid-state reaction at 150 °C between monohydrated ammonium oxotris(oxalate)niobate (niobium ammoniacal complex) and guanidine carbonate; (ii) the thermal decomposition of the guanidine oxaloniobate at 400 °C under an inert atmosphere; and (iii) the subsequent decomposition under an inert atmosphere at temperatures varied between 450 and 900 °C. The structure of the guanidine oxaloniobate was also calculated by density functional theory. The characterization results suggest that at a decomposition temperature of 700 °C, niobium carbonitride (NbC x N1−x , x < 1) was formed, which was transformed into NbC at higher temperatures (≥850 °C). From the X-ray refinement results, which were performed using the Rietveld method, the formation of NbC was proposed to occur in stages: oxaloniobate → oxynitride → carbonitride → carbide. The materials synthesized at 600, 650 and 700 °C exhibited steady state activities for more than 24 h in the hydrodesulfurization of dibenzothiophene, which predominantly occurred via the direct desulfurization route.