Lignocellulosic residues as catalysts for CO 2 fixation: complementary experimental and computational approaches

Lignocellulosic wastes obtained from vegetal residues in combination with nucleophiles were used as catalysts for carbon dioxide fixation in epoxides to form cyclic carbonates. An adequate combination of the residue and the nucleophile was essential to obtain active catalytic systems. The best binary systems were formed by olive bones, grape waste, date pit and corn leaves husk residues with tetrabutylammonium bromide (TBAB). High conversions in the cycloaddition of CO 2 to propylene oxide (76–84%) and 1,2-epoxyhexane (68–79%) were achieved at very low nucleophile loading (0.47 mol % respect to the substrate) under mild conditions (95 °C and 10 atm of CO 2 ). The vegetal wastes were stable under catalytic conditions and could be recycled after adequately supporting the nucleophile TBAB in silica gel. The mechanistic computational study carried out with Density Functional Theory calculations on model catalysts describes the contribution of individual lignin and cellulosic components to the experimental substrate conversion into cyclic carbonate. The energy barriers obtained and the experimental data suggest that the contribution of lignin to the total catalytic activity (barrier energy of 9.9 kcal/mol) may be more important than the contribution of cellulose (energy barrier 11.9 kcal/mol). Graphic abstract