Catalyst-free fixation of carbon dioxide into value-added chemicals: a review

In the context of global warming and the future circular economy, chemical fixation of carbon dioxide (CO₂) is a fast-developing research area because gaseous CO₂ could be trapped and converted into value-added products, industrial chemicals and pharmaceuticals. The latest advancements focus on homogeneous and heterogeneous catalysts, yet drawbacks of high price, complicated design and difficult recovery need to be circumvented. Although the catalyst-free concept has been extensively used in synthetic chemistry, this paradigm remains underexplored in the domain of CO₂ conversion. Therefore, here we review catalyst-free chemical and photochemical fixation of CO₂ for the manufacture of products such as ureas, carbamates, oxazolidinones, quinazoline-2,4(1H,3H)-diones, benzimidazoles, 1,6-dioxospiro/1,6-dioxaspiro derivatives, polymers, N-formamides, N-methylamines and carboxylic acids. Noteworthy, the catalyst-free conversion of CO₂ can be accomplished in water or in solvent-less conditions. We also discuss the reaction mechanisms leading to the formation of CO₂-derived products in the absence of any catalyst or additives.