Use of CO2 as a synthetic feedstock is considered a potentially promising strategy to mitigate emissions and decrease the carbon footprint of the chemical industry. However, to feasibly incentivize CO2 capture, scalable CO2 utilization processes must be developed using non-energy-intensive reagents. We have demonstrated that alkali carbonate salts (M2CO3) can act as surprisingly effective base promoters for CO2 carboxylation reactions when employed under solvent-free reaction conditions. Carbonates are abundant, thermally regenerable, and unlike many traditional solid base catalysts, retain their activity in the presence of CO2 gas. The use of solvent-free thermochemical reaction conditions maintains locally high ionic concentrations capable of stabilizing charged carboxylation reaction intermediates. The key to enabling carbonate reactivity under these conditions is to introduce disorder to the ordinarily crystalline M2CO3 salt. For implementation in solid-gas processes, disorder can be engendered to M2CO3 by dispersing it onto a mesoporous catalyst support. Here, the use of supported alkali carbonates as promoters or catalysts for gas-phase CO2 insertion reactions is discussed. Characterization of supported carbonate materials demonstrates that dispersion leads to the formation of a chemically-distinct, surface-bound carbonate moiety that is stable up to high temperatures and displays increased molecular mobility relative to the bulk crystalline species. The presence of this surface-dispersed species is correlated to higher activity in thermochemical CO2 carboxylation reactions. Reactions with a variety of gas-phase substrates, including aromatic hydrocarbons and H2 gas, are demonstrated. Effects of the support will be discussed, as well as the importance of process and reactor design. Finally, strategies for the implementation of closed-loop or continuous catalytic processes are considered, highlighting challenges and opportunities for further process optimization.

Alkali carbonate solid bases for solvent-free CO2 utilization

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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