Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane

Abstract

Several niobium phosphate phases have been prepared, fully characterized and tested as catalysts for the selective oxidation of ethane to ethylene. Three distinct niobium phosphate catalysts were prepared, and each was comprised predominantly of a different bulk phase, namely Nb₂P₄O₁₅, NbOPO₄ and Nb_1.91P_2.82O₁₂. All of the niobium phosphate catalysts showed high selectivity towards ethylene, but the best catalyst was Nb_1.91P_2.82O₁₂, which was produced from the reduction of niobium oxide phosphate (NbOPO₄) by hydrogen. It was particularly selective for ethylene, giving ca. 95% selectivity at 5% conversion, decreasing to ca. 90% at 15% conversion, and only produced low levels of carbon oxides. It was also determined that the only primary product from ethane oxidation over this catalyst was ethylene. Catalyst activity also increased with time-on-line, and this behaviour was ascribed to an increase of the concentration of the Nb_1.91P_2.82O₁₂ phase, as partially transformed NbOPO₄, formed during preparation, was converted to Nb_1.91P_2.82O₁₂ during use. Catalysts with predominant phases of Nb₂P₄O₁₅ and NbOPO₄ also showed appreciable activity and selectivities to ethylene with values around 75% and 85% respectively at 5% ethane conversion. The presence of phosphorous is required to achieve high ethylene selectivity, as orthorhombic and monoclinic Nb₂O₅ catalysts showed similar activity, but displayed selectivities to ethylene that were <20% under the same reaction conditions. To the best of our knowledge, this is the first time that niobium phosphates have been shown to be highly selective catalysts for the oxidation of ethane to ethylene, and demonstrates that they are worthy candidates for further study.