(591d) Chemical Looping for the Production of Value-Added Oxygenates from CO2 and Natural Gas

Natural gas is an abundant and relatively low-carbon energy source, but it still contributes to emissions of greenhouse gases. However, when used to make value-added oxygenates such as acetic acid, ethylene glycol, and propionic acid, natural gas’s chemical composition is conducive to the beneficial utilization of CO₂. For example, the net reaction of methane with CO₂ to produce acetic acid is mildly exothermic and consumes one mol of CO₂ per mole of methane. Unfortunately, conventional routes to producing acetic acid from natural gas include several parasitic energy losses that prevent the system from achieving net CO₂ consumption. In these routes, natural gas is first converted to syngas in an energy-intensive reforming step, followed by extensive water gas shift and/or cryogenic separation to achieve proper ratios of hydrogen and CO for the needed chemicals. The high temperature of reforming needed for high yields is not conducive to sustainable energy sources such as renewable or waste energy, and the parasitic losses generally require additional energy imports into the system and create a significant net carbon footprint for the process.

To address these issues, we propose to utilize a chemical looping reforming approach. In the first step, methane is partially oxidized with a mixed metal oxide to form a 2:1 H₂ to CO syngas mixture. In a subsequent step, the metal-oxide is regenerated. Although regenerating the metal oxide with air makes the system thermally sufficient, it can also be regenerated with CO₂ to form carbon monoxide. The operating temperature of the process allows the integration of renewable and/or industrial waste heat sources, allowing for the net reaction endotherm to be made up without contributing to the carbon footprint of the process. The inherently separated syngas and CO streams are ideal for efficient chemical synthesis. The syngas can be converted directly to methanol, which subsequently reacts with CO to form acetic acid through carbonylation.

We present experimental and modeling data showing that the chemical looping approach can facilitate the production of acetic acid from natural gas with a negative net CO₂ emission. We also briefly describe chemical looping routes to other value-added chemicals with net utilization of CO₂.