Partially reduced heteropolyanions for the oxidative dehydrogenation of ethane to ethylene and acetic acid at atmospheric pressure

Niobium- and pyridine-exchanged salts of phosphomolybdic (NbPMo12Pyr) and phosphovanadomolybdic acids, NbPMo12Pyr and NbPMo11VPyr, respectively, were investigated as precursors to materials that catalyze the oxidative dehydrogenation of ethane to ethylene and acetic acid at atmospheric pressure. The effects of feed composition, steam flow, temperature, and precursor composition on catalytic activity and selectivity are presented for both ethane and ethylene oxidation. The productivity of ethylene and acetic acid from ethane using the catalytic materials formed from these precursors exceeds those reported in the literature for Mo-V-Nb-Ox systems under atmospheric or elevated pressure. Also, the production of acetic acid from ethylene is greater than that observed from Mo-V-Nb-Ox systems. Water is found to aid in desorption of acetic acid, thereby limiting deep oxidation to carbon oxides. Addition of vanadium reduces catalytic activity and selectivity to both ethylene and acetic acid, whereas niobium is essential for the formation of acetic acid from ethane. Other metals, such as titanium, tantalum, zirconium, antimony, iron, and gallium, do not provide the same beneficial effect as niobium. A balance of niobium and protonated pyridine in the catalyst precursor is required to produce an active catalyst by thermal treatment in nonoxidizing environments. Molybdenum and niobium are also necessary for the creation of the active site for ethane activation, and niobium is directly involved in the transformation of ethylene to acetic acid. A reaction scheme is proposed for the production of acetic acid from ethane over the catalytic materials.