Nanoparticles of aromatic biopolymers catalyze CO2 cycloaddition to epoxides under atmospheric conditions

Lignin and melanin are aromatic biopolymers that are contained in large amounts in plants and animals. Biopolymers containing hydrogen bond donor (HBD) moieties such phenols, or some amino acids, are sustainable catalytic components for the synthesis of useful cyclic carbonates by cycloaddition reaction of CO2 to epoxides. However, their application is typically restricted to the use of bulk materials requiring harsh reaction conditions for efficient conversion. In the search for more efficient and sustainable bio-based heterogeneous HBDs, we extract and produce microparticles of melanin from various sources as well as kraft lignin nanoparticles and apply them for the cycloaddition of carbon dioxide to epoxides. Importantly, nanoparticles from lignin and melanin can efficiently catalyze the reaction under atmospheric conditions at moderate temperatures (60 °C) using low loadings of external nucleophiles. Moreover, lignin nanoparticles are functionalized with catechol groups, leading to more efficient catalysts for the conversion of a series of epoxides to the corresponding cyclic carbonates. The phenolated lignin nanoparticles, acting as heterogeneous hydrogen bond donors, can be recovered and used at least 10 times with similar performance. Additionally, the origin of the catalytic activity of the biopolymers is investigated in relation to their morphology and composition. Nanoparticles of aromatic biopolymers are promising and inexpensive renewable platforms for efficient CO2 fixation to produce useful chemicals.