(679f) Catalytic Ethane Conversion through Ammoxidation

The US shale gas revolution over the past decade resulted in an unprecedented increase in the natural gas (light alkanes) production, which benefits the US/global chemical sector by shifting the raw materials from naphtha to cheap and abundant natural gas/natural gas liquids. Although ethane makes up only 5-15% of the shale gas, it has so far had the biggest impact on the chemical industry since it can be easily converted into ethylene via steam cracking. While steam cracking is well established in the industry, it is very expensive to build the facilities (up to $10 billion) and such a non-catalytic process requires a reaction temperature above 900^oC. Hence, the catalytically-driven processes for ethane conversion, such as aromatization and dehydrogenation, have recently been extensively studied, however, such non-oxidative catalytic process suffered from significant catalyst deactivation due to coking.

Ammoxidation is one of the most important industrial processes in the production of acrylonitrile from propylene. Ammoxidation was also employed for ethane conversion, which produces ethylene and acetonitrile, simultaneously. However, the traditional propylene ammoxidation catalyst, mixed metal oxide, was found ineffective in ethane ammoxidation probably due to the different reaction mechanism. Nonetheless, former studies by Li and Armor from Air Products and Chemical, Inc. observed that cobalt exchanged zeolites are effective for ethane ammoxidation, and they suggested that the exchanged Co²⁺ cations to be responsible for the formation of acetonitrile. Differently, our recent study suggested that Co/HZSM-5 catalyst prepared through impregnation shows higher activity and selectivity than the Co exchanged HZSM-5, and the catalytic performance dependent highly upon the Co loading. Over the optimal 2 wt% Co/HZSM-5 catalyst at 475^oC, the space-time yields (STY) of ethylene, acetonitrile, and CO₂ are 91.5, 219.6, and 136.7 μmol/g/min, respectively, which corresponds to a sum selectivity of acetonitrile and ethylene more than 80%. Beside the Co/HZSM-5 catalyst, our recent study identified that Sn/HZSM-5 catalyst was also pretty effective in the ethane ammoxidation. Although the overall activity of the Sn/HZSM-5 is lower than the Co/HZSM-5, the former shows higher sum selectivity of ethylene and acetonitrile (with respect to ethane conversion) and higher acetonitrile selectivity with respect to the NH₃ conversion. Here we will present our recent results on the ethane ammoxidation over both catalysts, and compare their catalytic performance with respect to STY and selectivity. Additionally, extensive characterization results of the catalysts by NH₃-TPD, n-propylamine-TPDec, and TEM will also be presented, and the possible structure/performance relationships will be discussed.