Microwave synthesized quaternized celluloses for cyclic carbonate synthesis from carbon dioxide and epoxides

Abstract

The efficacy of microwave irradiation for the quaternization of ethylenediamine-functionalized celluloses (en-Cell), as well as the catalytic activity of the resultant microwave quaternized celluloses (mQCs) in the cycloaddition of carbon dioxide with epoxide substrates under solvent-free conditions has been studied. A series of mQCs with various alkyl chains and anions were synthesized and characterized by solid-state 13C NMR, FT-IR, XPS, and EA. The mQCs employed were found to yield cyclic carbonates in an atom-efficient manner with excellent selectivity (99%) under a low catalytic loading (0.4mol%) and elevated pressures of CO2 (1.2MPa), thereby realizing a turnover number (TON) of 242. A quite remarkable BET surface area ranging from 50 to 71m2g−1 for the microwave-synthesized quaternized celluloses was obtained, compared to the very low surface area of 2m2g−1 of the starting cellulosic material. The indispensable role played by the vicinal hydroxyl groups of cellulose during catalysis was investigated using hydroxyl-protected celluloses. By rationalizing the obtained results, a plausible mechanism, in which all the functional moieties of the mQCs operate synergistically, is proposed. The mQC catalysts were found to exhibit excellent thermal and moisture stability, and they were reusable more than six times without any significant reduction in activity.