**4. Conclusions**

The carboxylation of epoxides is a sustainable pathway for the fixation of CO2 into valuable chemicals, considering the industrial utilization of cyclic and polymeric carbonates. The effect of homogeneous organocatalysts published in recent literature is presented. We hope that this review affords insights into the recent research and development of efficient metal-free homogeneous catalysts.

Hopefully, the next development of homogeneous catalysts, including organocatalysts (e.g., organic salt, ILs and DESs), will facilitate the expanding of the spectrum of available metal-free organocatalysts applicable for the reaction of CO2 with terminal epoxides even at CO2 pressures of 1 bar and reaction temperatures of less than around 50 ◦C. Except high catalytic activity, the simple catalyst separability should be profitable because of the necessity of high catalytic loading for the effective course of cycloaddition reaction. The bulky tetrabutylammonium or tetrabutylphosphonium cations in Bu4NX or Bu4PX enable the high nucleophilic activity of the appropriate naked anions of X−, such as bromide or iodide, in most cases. Onium salts are widely applied as part of multicomponent catalytic systems in the research and development of epoxides' carbonation processes. Most of all, deep eutectic solvents constitute an important group of multicomponent lowcost homogeneous organocatalysts. In particular, DESs containing choline chloride and urea exhibit high catalytic activity [84,85]. Besides the above-mentioned onium halides, some ion pairs produced by the mixing of DBU with amidine-based alcohols are highly active [95]. The most effective halide free IL-based homogeneous catalyst was recognized to be the Bu4N salt of histidine [103]. Several ion pairs based on *N* -carboxymethylated MIM bromides neutralized with tetraalkylguanidines enable CO2 cycloaddition at ambient pressure [100–102]. Searching for simple and cheap catalytic systems that are active at mild reaction conditions is attractive not only due to the environmental point of view (lower energy consumption) but even due to the thermodynamic reasons. As the formation the cyclic carbonate is exothermic, the lower reaction temperature affects the shifting of the reaction equilibrium in the products.

As we illustrate in this review, many simple molecules are known to act as effective mediators and/or catalysts, including Lewis and Bronsted acids such as water, ascorbic acid, cellulose, etc. Based on generally accepted mechanisms of carboxylation of epoxides, research focused on utilization of other HBDs such as bidentate nucleophiles could be profitable. The promising groups of simply available catalytically active compounds such

as polyalkyl guanidines [100,101,104], enaminones [105], *N*-hydroxylamines [106] and amidoximes [107] should, in our opinion, be investigated in more detail.

The utilization of tandem reactions such as the one-pot production of cyclic carbonates starting directly from biobased unsaturated fatty acids esters [8], the one-pot production of ethylene carbonate from ethylene produced by low-energy-demanding methods [2,108] or the production of HMEC from chlorinated bio-based glycerol [109] seems to be very promising for effective CO2 fixation.

It is evident from the recently published results that both the possible utilization of CO2 from flue gas and the carbonation of internal bio-based epoxides such as epoxidized fatty acid esters are the main developing areas of research focused on CO2 capture and utilization. However, the mild reaction conditions and lower catalytic loading are still challenging for both the carboxylation of internal epoxide substrates such as epoxidized fatty acid esters as well as for the direct utilization of waste CO2 from power plant flue gas.

**Author Contributions:** T.W. conceived, designed and wrote the paper. B.K. provided technical support. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Faculty of Chemical Technology, University of Pardubice, with the support of excellent research.

**Conflicts of Interest:** The authors declare no conflict of interest.
