Laminaria digitata and Palmaria palmata Seaweeds as Natural Source of Catalysts for the Cycloaddition of CO2 to Epoxides

Abstract

Seaweed powder has been found to act as an effective catalyst for the fixation of CO2 into epoxides to generate cyclic carbonates under solvent free conditions. Model background reactions were performed using metal halides and amino acids typically found in common seaweeds which showed potassium iodide (KI) to be the most active. The efficacy of the seaweed catalysts kelp (Laminaria digitata) and dulse (Palmaria palmata) was probed based on particle size, showing that kelp possessed greater catalytic ability, achieving a maximum conversion and selectivity of 63.7% to styrene carbonate using a kelp loading of 80% by weight with respect to epoxide, 40 bar of CO2, 120 °C for 3 h. Maximizing selectivity was difficult due to the generation of diol side product from residual H2O found in kelp, along with a chlorinated by-product thought to form due to a high quantity of chloride salts in the seaweeds. Data showed there was loss of organic matter upon use of the kelp catalyst, likely due to the breakdown of organic compounds and their subsequent removal during product extraction. This was highlighted as the likely cause of loss of catalytic activity upon reuse of the Kelp catalyst.

Solvent-free synthesis of cyclic carbonates from CO2 and epoxides catalyzed by reusable alumina-supported zinc dichloride

“…The coordination of Zn on the surface of Al2O3 to the epoxide is also supported by the appearance in the FT-IR spectrum of the broad intense bands (at 1132 cm -1 and 1157 cm -1 ), probably attributed to the stretching modes of Zn-O-C group along with the characteristic C-O-C band of epoxide ring at 1259 cm -1 (See ESI for spectra). Iodide ion attack on the terminal or internal oxirane carbon of the alkyl or aryl-substituted epoxides [99][100][101], respectively, would furnish the alkoxide species ready to add to CO2. Intramolecular 5-exo-tet cyclization of the acyclic mixed carbonate would release the cyclic carbonate and iodide ion, with the latter entering the catalytic cycle again.…”