(646g) Commodifying Flue Gas: Molten-Carbonate-Assisted Olefin Synthesis

The idea of using CO₂ as a soft oxidant for thermochemical ethane dehydrogenation (CO₂-ODH) has generated extensive interest in recent years, as the process could utilize waste CO₂ to offset the substantial emissions associated with conventional ethylene production. Unfortunately, CO₂-ODH has a high activation energy and is equilibrium-limited, with both factors contributing to low single-pass yields. To circumvent these constraints, we propose to oxidatively dehydrogenate ethane via a molten-salt-mediated chemical looping scheme (MM-ODH). MM-ODH valorizes ethane-rich shale gas and concurrently upgrades CO₂ emissions by using a molten carbonate medium to segregate ethane and CO₂ into gas and melt phases, respectively. In the first step, ethane cracks to form ethylene and hydrogen as it bubbles through a carbonate melt. The generated hydrogen then interacts with carbonate anions in the reaction medium to form hydroxides and CO. In the second step, CO₂ from flue gas re-carbonates the melt to enable cyclic operation, generating steam in the process. By separating the reduction and oxidation steps into two chemical phases, equilibrium limits on conversion are alleviated. Lab-scale experiments have demonstrated ethane conversions as high as 80%, with CO₂ conversions and ethylene selectivities surpassing 90% under several salt compositions. Super-equilibrium CO yields (greater than ~45%) have also been realized. The resulting ~1:1 ratio of ethylene to CO produced is particularly well suited for the production of propionic acid. Although C₃H₆O₂ constitutes a relatively small market, it is representative of a large number of specialty oxygenates that can be produced from ethylene and CO. With waste heat integration and carbon capture and utilization, the MM-ODH system can reduce the net carbon footprint of propionic acid production from 0.5 ton CO₂ emitted per ton of product to a net consumption of 0.4 ton CO₂ per ton of product.