Low-Temperature Nonoxidative Dehydrogenation of Ethane to Ethylene over Copper-Containing Zeolites via Chemical Looping

Abstract

In this work, we report a low-temperature chemical looping strategy for the selective nonoxidative dehydrogenation (NODH) of ethane (C2H6) into ethylene (C2H4) using copper-containing zeolites. The combination of in situ Fourier transform infrared (FTIR) spectroscopy, in situ X-ray absorption spectroscopy (XAS), and reactor tests shows that MFI and MOR frameworks with isolated Cu(I) sites are capable of ethane NODH to ethylene at temperatures below 615 K with up to 100% ethylene selectivity: the conditions much milder than that of steam cracking or catalytic dehydrogenation. Copper-containing FAU and CHA show very limited ethylene yield. The significant activity of copper-containing zeolites is achieved through C2H4···Cu(I) π-complexation after dehydrogenation, as revealed by in situ FTIR spectroscopy and XAS, which shifts the thermodynamic equilibrium toward the products by separating ethylene from the gas-phase hydrogen and protecting the formed olefin from the side reactions. The complex can be decomposed upon contact with diluted steam, allowing regeneration of active sites without losing any products. Reactor tests show that MFI is the best performing zeolite among all the frameworks with similar Si/Al ratio, while in situ FTIR spectroscopy indicates that the presence of dual-Cu(I) active sites in the MFI framework leads to high low-temperature C2H6 NODH activity. The contribution of distinct Cu(I) sites hosted by the MFI framework in NODH can be precisely controlled upon fine-tuning of the copper weight loading, Si/Al ratio, and dehydrogenation temperature. This work demonstrates that chemical looping dehydrogenation is compatible with a range of copper-containing zeolites and shows promising potential for further optimization of selective and mild dehydrogenation processes since low operating temperatures would potentially simplify reactor design, remove the necessity for a transfer line exchanger, and enable easier process electrification.