Hybrid Catalysts for Artificial Photosynthesis: Merging Approaches from Molecular, Materials, and Biological Catalysis

Abstract

Conspectus Increasing demand for sustainable energy sources continues to motivate the development of new catalytic processes that store intermittent energy in the form of chemical bonds. In this context, photosynthetic organisms harvest light to drive dark reactions reducing carbon dioxide, an abundant and accessible carbon source, to store solar energy in the form of glucose and other biomass feedstocks. Inspired by this biological process, the field of artificial photosynthesis aims to store renewable energy in chemical bonds spanning fuels, foods, medicines, and materials using light, water, and CO2 as the primary chemical feedstocks, with the added benefit of mitigating the accumulation of CO2 as a greenhouse gas in the atmosphere. As such, devising new catalyst platforms for transforming CO2 into value-added chemical products is of importance. Historically, catalyst design for artificial photosynthesis has been approached from the three traditional fields of catalysis: molecular, materials, and biological. In this Account, we show progress from our laboratory in constructing new hybrid catalysts for artificial photosynthesis that draw upon design concepts from all three of these traditional fields of catalysis and blur the boundaries between them. Starting with molecular catalysis, we incorporated biological design elements that are prevalent in enzymes into synthetic systems. Specifically, we demonstrated that proper positioning of intramolecular hydrogen bond donors or addition of intermolecular multipoint hydrogen bond donors with classic iron porphyrin and nickel cyclam platforms can substantially increase rates of CO2 reduction and break electronic scaling relationships. In parallel, we incorporated a key materials design element, namely, high surface area and porosity for maximizing active site exposure, into molecular systems. A supramolecular porous organic cage molecule was synthesized with iron porphyrin building blocks, and the porosity was observed to facilitate substrate and charge transport through the catalyst film. In turn, molecular design elements can be incorporated into materials catalysts for CO2 reduction. First, we utilized molecular synthons in a bottom-up reticular approach to drive polymerization/assembly into a bulk framework material. Second, we established an organometallic approach in which molecular ligands, including chelating ones, are adsorbed onto a bulk inorganic solid to create and tune new active sites on surfaces. Finally, we describe two examples in which molecular, materials, and biological design elements are all integrated to catalyze the reduction of CO2 into CH4 using a hybrid biological-materials interface with sustainably generated H2 as the reductant or to reduce CO into value-added C2 products acetate and ethanol using a hybrid molecular-materials interface to construct a biomimetic, bimetallic active site. Taken together, our program in catalysis for energy and sustainability has revealed that combining more conventional design strategies in synergistic ways ...

Achieving selectivity in porphyrin bromination through a DoE-driven optimization under continuous flow conditions

“…In order to better describe the yield variance, a (Fig. 2) was constructed by adding six additional experiments as axial points (Table 1 entries [12][13][14][15][16][17] to the previous design. The so-obtained data were fitted through a multiple quadratic regression.…”

“…Covalent linking of porphyrins to a wide variety of systems, such as carbon nanostructures [10] or biopolymers, [11][12][13] is commonly exploited to obtain functional materials. Peripheral functionalization may also be used to create light harvesting systems [14]. For instance, zinc porphyrins substituted at the β-pyrrolic position with groups bearing an extended π-system are known as a highly performing class of photosensitizers employed for Grätzel solar cells.…”

Recent Developments in Metalloporphyrin Electrocatalysts for Reduction of Small Molecules: Strategies for Managing Electron and Proton Transfer Reactions

“…However, it remains a challenge to reproduce some of those features in heterogeneous systems. If molecular catalysts are to be incorporated into materials for large‐scale application, the field must continue to bridge conceptual gaps between molecules and materials [5,6] . Herein, we highlight advances in both homogeneous and heterogenous catalysts and present some forward‐looking views on challenges and opportunities in metalloporphyrin‐catalyzed activation of small molecules.…”

An NADH-Inspired Redox Mediator Strategy to Promote Second-Sphere Electron and Proton Transfer for Cooperative Electrochemical CO2 Reduction Catalyzed by Iron Porphyrin

“…Catalyzing the CO 2 reduction reaction by electrochemical means offers an approach for converting this greenhouse gas into value-added chemical products with sustainable energy input. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Efforts in molecular electrocatalysts for CO 2 reduction have predominantly focused on the 2electron/2-proton reduction of CO 2 into CO, which can then be fed into numerous existing industrial processes for chemical synthesis. Iron porphyrins, in particular, have been the subject of extensive research owing to their high selectivity for CO 2 versus proton reduction, structural tunability, and compatibility with various electrolyte media.…”