Experimental-theoretical study of the epoxide structures effect on the CO2 conversion to cyclic carbonates catalyzed by hybrid titanate nanostructures

Hybrid nanostructures were produced from several ionic liquids (ILs) anchored on titanate nanotubes (TNT). Ionic liquids ILs composed by the 1-methyl-3-(3-trimethoxysilylpropyl)imidazolium cation associated with the [Cl-], [BF4-], [PF6-] or [Tf2N-] anions were studied. The highest anchored amounts of IL were obtained with the [Cl-] and [Tf2N-] anions, respectively, of 31.5 and 43.5 % w/w. These hybrid nanostructures were used in the direct conversion of CO2 to cyclic carbonates through the coupling reaction with various epoxides (epichlorohydrin, styrene oxide, glycidyl isopropyl ether, and propylene oxide). With styrene oxide, the best results were obtained using the [Cl-] and [BF4-] IL, showing TON values of 35.6 and 18.4, and TOF values of 17.2 and 9.2 h(-1), respectively. The TNT-[IL][Cl] was also used with different epoxides (i.e. epichlorohydrin, styrene oxide, glycidyl isopropyl ether and propylene oxide) being more reactive in the reactions with epichlorohydrin (TON = 86.3, TOF =21.6 h(-1)) and styrene oxide (TON = 47.0 TOF =11.7 h(-1)). Theoretical studies showed that the high reactivity of styrene oxide can be associated with their lower HOMO-LUMO energy gap, while epichlorohydrin presented high electron withdrawing effect by chlorine, generating the higher electronegativity. Besides that, by NBO analysis, both molecules presented lower donor - acceptor interact energy which may favor the opening of the ring.