Direct hydrogenation of CO2 to ethylene

This project sought initially to evaluate a novel solid base catalyst system for the production of valuable synthetic precursors from resources available on long-duration space missions – namely, CO2 and H2 gas. Preliminary results of experiments carried out at elevated temperature and pressure in a batch reactor configuration had suggested that a variety of products, including ethylene gas, could be formed by CO2 hydrogenation in the presence of an alkali carbonate (M2CO3) salt. We aimed to 1) characterize M2CO3 catalysts to better understand their properties relating to solid base activity in the presence of gas-phase substrates, and 2) transition our preliminary batch results to a continuous reaction paradigm.

In the process of transitioning to a flow reactor system, we investigated preparation of M2CO3 catalysts by dispersing the M2CO3 salt onto a mesoporous catalyst support, thereby increasing its accessibility to gas-phase substrates. Characterization of these dispersed carbonates by solid state nuclear magnetic resonance (NMR) spectroscopy revealed structural changes that occur in these dispersed carbonates relative to their bulk analogues, and provided unprecedented insight into the structure and dynamics of these active solid base materials. To evaluate the CO2 hydrogenation activity of supported M2CO3 catalysts, we designed and constructed a custom pressurized fixed bed reactor. Using this reactor, it was found that the system’s selectivity shifted notably from the product distribution observed in prior batch reactions. While ethylene was found to be inaccessible in this regime, M2CO3 displayed notable activity as a catalyst for the Reverse Water-Gas Shift (RWGS) reaction, i.e. the production of CO gas and H2O from CO2 and H2. Specifically, M2CO3 was found to catalyze RWGS with high selectivity and near-equilibrium conversion at intermediate temperature (400–500 °C) and high gas hourly space velocity (GHSV 4800 h–1).

CO gas is a synthetic precursor that can be used in the preparation of a variety of more complex molecules, making its preparation from CO2 a potentially valuable first-stage transformation for ISRU purposes. Thermochemical methods for CO2 conversion to CO are compelling alternatives to existing electrochemical methods (e.g. solid oxide CO2 electrolysis) in situations where high single-pass conversion is not critical. For example, performing single-pass RWGS at equilibrium conversion can be used to upgrade a gas stream containing CO2 and H2 into a CO/CO2/H2 stream with a composition suitable as a feedstock for acetogenic fermentation to higher alcohols. Relative to metal catalysts typically employed for RWGS, carbonate catalysts can be expected to offer advantages in terms of stability and impurity intolerance due to the inherently reversible nature of binding at an acid-base active site. Hence, this NSTRF project has unveiled a new class of solid base catalysts for the RWGS reaction, thereby creating new opportunities for CO2 utilization.