Carbon fixing materials are a new class of self-healing, self-reinforcing materials that utilize ambient CO2 to chemically add to an ever-extending carbon backbone. This class of materials can utilize biological or non-biological photocatalysts and support a wide range of potential backbone chemistries. However, there is no analysis to date that describes their fundamental limits regarding chemical kinetics and mass transfer.<br/><br/>Here, we employ a reaction engineering and materials science analysis to answer basic questions about the maximum growth rate, photocatalytic requirements, and limits of applicable materials. Our proposed mathematical framework envelops three main functions required for carbon fixing materials: (1) adsorption of CO2 from the air, (2) photocatalytic reduction of CO2 into selected monomers, and (3) polymerization of CO2-derived monomers. First, by performing a Damköhler number analysis, we derive criteria for the cross-over from kinetic control to mass transfer limited growth, setting upper limits on the performance of a potential photocatalyst. Next, we analyze the photocatalytic reduction of CO2 to single carbon products, using known catalytic pathways and kinetic data. We identify formaldehyde as a C1 intermediate having unique potential for incorporation into the material backbone of carbon fixing materials. Finally, as an applied case study, we consider and analyze the example of a carbon fixing poly(oxymethylene) system with embedded catalyst promoting the photocatalytic reduction of CO2 to formaldehyde, conversion of formaldehyde to trioxane, and its subsequent polymerization.<br/><br/>This reaction engineering analysis introduces benchmarks for carbon-fixing materials with respect to achievable rates of photocatalysis, adsorption, and polymerization. We determine ranges of catalytic activity and CO2 adsorption capacity that will allow for maximum polymer growth from atmospheric CO2 and present some mechanistic insights on the photocatalytic reduction of CO2 to C1 intermediates. Modeling also indicates that it may be possible to continuously regenerate polymers at their natural degradation rates, hence mimicking a key aspect of living tissues and biomaterials. These results should prove valuable for designing, evaluating, and benchmarking this emergent and new class of environmentally sustainable materials.

Materials exhibiting biomimetic carbon fixation and self-repair: A mathematical analysis

ACS Spring 2022

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