(179k) Carbonaceous Adsorbents for Carbon Dioxide Capture

Removing carbon dioxide from the atmosphere has become necessary to meet the climate goals and limit the global warming. Since their 4^th Assessment Report in 2007, the IPCC indeed includes Carbon capture and Storage alongside with the other necessary technological options (reduce fossil fuels consumption, increase energy efficiency, transition to carbon intensive fuels and renewable energy, enhance biological absorption) to reduce atmospheric CO₂ levels. In contrast to environmental engineering solutions such as ocean alkalinization or fertilization that could have unpredictable effects, industrial carbon capture provides a controlled way of removing CO2 from the atmosphere.

The development of carbon capture processes has thus gained much interest in the two last decades. Membrane, cryogenic or sorption-based CO2 separations emerged as technologically feasible solutions, for instance. To date, sorption processes stand out for both economical and technical process maturity. In particular, adsorption (fixation of CO2 molecules on solid adsorbents) benefits from a good hourly productivity and from a lower energy demand for sorbent regeneration, at a comparatively low opex. Temperature/Pressure swing adsorption plants are thus relevant to separate CO2 from large volumes of gas.

Despite the first industrial projects, the current CO2 removal capacity shall be increased of several orders of magnitude to balance the annual emissions and existing technology needs to be improved. Key material for the sorption based CO2 separation is the sorbent. Improvements as well as scalability and economics are strongly linked to its performance. This triggers strong need for further sorbent development.

This contribution provides a review of the current sorbent candidates for carbon capture such as porous zeolites, metal-organic frameworks and carbons and a perspective for future research needs. Their relevance in CO2 adsorption is compared in light of both nano-scale chemistry and process considerations. With a holistic view on the whole DAC process the review addresses not only the sorption capacity, but the sum of important material properties like heat and mass transport properties, volume-based characteristics, desorption temperature, selectivity to water, stability, etc...

In a condensed way, costly metal-organic frameworks are water vapor sensitive and face stability issues. Silica and zeolites are more affordable and reliable sorbents with traditional high adsorption capacity. However, they require more energy for regeneration and face selectivity issues, since they readily adsorb moisture. Moreover, the functionalization strategies employed to increase CO₂/N₂ selectivity lead to an increase of hydrophilic character and a higher desorption heat.

Carbon in turn provides a very high surface area at a low cost, associated with a good stability over regeneration cycles and towards moisture. Furthermore, carbon-based sorbents are versatile materials that can be tuned for structure, texture and surface functionalization, have low density, chemical durability and very high resistance to rapid changes in temperature and pressure.

In this contribution, a focus will be given to these carbon materials, which are also appreciated for their availability and production procedure. Materials included in this review vary from biomass derived activated carbons via graphene sheets to synthetic heteroatom-doped carbons, where a flourishing literature emerged in the last decade to separate CO2 from the air and mitigate global warming.

The review will comprehensively summarize strategies employed in the synthesis and development of such porous sorbents and nanomaterials. The advances made in adsorption science and surface engineering are relevant for neighbouring fields, as many of those materials are found in heterogeneous catalysis for instance. Finally, the contribution will address the current challenges and perspectives in carbon sorbent engineering.