Organocatalytic Keratin Sponges for the Chemical Fixation of CO₂ into Cyclic Carbonates

Abstract

The fabrication of a novel macroporous organic material from keratin wool fiber waste is herein described together with its utilization as a heterogeneous matrix for the immobilization of organocatalysts, namely, methyl imidazolium iodide, TBD, and a combination thereof. Application of the freeze-drying method and utilization of the cross-linking agent hexamethylene diisocyanate (HDI) allow for the preparation of keratin sponges suitably shaped in the form of cylindrical pellets with an interconnected threedimensional porous structure (main pore size, 72 μm; porosity, 68%). Additional features of the cross-linked keratin sponges include their insolubility in water and common organic solvents, as well as their exposure to the surface of isocyanate groups for organocatalyst immobilization. The anchoring step is effectively performed in anhydrous acetone-DMSO with either 1-hydroxyethyl-3-methyl imidazolium iodide (HO-EtMImI; 50 °C) or TBD (room temperature) to produce the corresponding heterogeneous catalysts with a satisfactory loading of active units (0.47 and 1.80 mmol g −1 , respectively). The unprecedented monofunctionalization of HDI with HO-EtMImI affords a novel monoisocyanate imidazolium iodide derivative, which, in turn, is grafted on the immobilized TBD catalyst to form a one-component multifunctional keratin sponge bearing hydrogen bond donor, nucleophilic, and CO 2 activating sites. This heterogeneous catalyst can effectively function in the cycloaddition of CO 2 to epoxides free of solvent and cocatalysts, producing five-membered cyclic carbonates in high yields (79− 94%) at moderate temperature (35 °C) and pressure (10 atm), with satisfactory recyclability (five runs) and the possibility of complete biodegradation in 2 days by enzymatic treatment (protease XIV).