Understanding interactions between lignin and ionic liquids with experimental and theoretical studies during catalytic depolymerisation

• Published in: Catalysis today 2018-07, Vol.309, p.98-108
• Main Authors: Singh, Sandip Kumar, Banerjee, Subhrashis, Vanka, Kumar, Dhepe, Paresh Laxmikant
• Format: Article
• Language: English
• Subjects: Brønsted acidic ionic liquids; Depolymerization; Experimental and DFT studies; Interaction; Lignin
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2017.09.050

Extensive weak and strong hydrogen bonding between acidic ionic liquid and lignin/vanillin (experimental and computational studies), helps achieve higher depolymerization activity. [Display omitted] •Brønsted acidic ionic liquids performed better than H2SO4 in lignin depolymerization.•NMR (1D, 2D-HMBC) & RAMAN proves transfer of electron density from substrate to IL.•DFT calculations suggest weak and strong H-bonding between IL-substrate adduct.•Substrate-IL Interaction improves low mol. wt. products yield in depolymerization. Compared to H2SO4, Brønsted acidic ionic liquid (BAIL, [C3SO3HMIM][HSO4]) catalyzed depolymerization of lignin yielded higher concentration of low molecular weight products under mild conditions (120°C) as proven by GC and GC–MS. To comprehend this disparity in catalytic activity among H2SO4 and BAIL (at similar H+ concentration), experimental techniques [1D(1H) NMR, 2D(15N/1H HMBC) NMR and RAMAN] have been employed. Based on these studies, it has been proven that the transfer of electron density from substrate to the electron deficient imidazolium ring via formation of hydrogen bond between OH/OCH3 (substrate) and N1C2H2N3− (cation) is possible, while the anion plays an important role. Further, density functional theory (DFT) calculations also corroborated this fact by showing a change in the bond angle and decrease in bond length (C2H2 in imidazole), due to the presence of weak and strong hydrogen bonding between the substrate and IL.