Effective pretreatment of lignin-rich coconut wastes using a low-cost ionic liquid

Abstract

Coconut husks and shells are underutilised agricultural feedstocks in the bio-based industry. These biomass wastes have a higher lignin content than other woody biomass and have excellent potential as raw materials for the production of lignin-based materials. This work demonstrates the performance of a low-cost protic ionic liquid, N,N,N-dimethylbutylammonium hydrogen sulfate ([DMBA][HSO4]), for ionoSolv pretreatment of coconut husk and shell at 150 °C for 45–90 min and 170 °C for 15–60 min. Optimum pretreatment conditions were observed at 170 °C and 45 min for both feedstocks. At these conditions, [DMBA][HSO4] was able to remove almost 77 wt% of the lignin from the husk; leaving a cellulosic rich pulp behind, which released 82 % of the theoretical maximum glucose after enzymatic saccharification. The pretreated shell, by comparison, achieved 82 wt% lignin removal and 89 % glucose yield and these higher values could be attributed to the highly porous structure of coconut shell cell walls. The cleavage of the β-O-4 aryl ether linkages of lignin followed by extensive C–C condensation in the lignin at longer pretreatment times was shown by HSQC NMR analysis. This extensive condensation was evidenced by molecular weights > 10,000 g/mol exhibited by lignin precipitated after pretreatment at high temperature and long times. The high degree of lignin removal and high glucose release from both feedstocks demonstrate that [DMBA][HSO4] is an excellent ionic liquid for fractionation of very lignin-rich biomass.

Effects of the Ionic Liquid Structure on Porosity of Lignin-Derived Carbon Materials

“…The coconut shell lignin used for this study resembles hardwood biomass, because it contains a larger proportion of syringyl (3,5-dimethoxy-4-hydroxyphenyl, S) than guaiacyl (4-hydroxy-3-methoxyphenyl, G) subunits after characterization, which results in a S/G ratio of 1.5 . Nonetheless, these subunits are linked together various types of aryl ether (C–O, e.g., β-O-4) and condensed (C–C) linkages. , There is no general consensus on the lignin pyrolysis mechanism; it is widely proposed that lignin pyrolysis involves a two-step mechanism: primary and secondary reactions. , An alternative suggestion of an intermediate lignin pyrolysis reaction has also been proposed. ,, …”

History repeats itself again: Will the mistakes of the past for ILs be repeated for DESs? From being considered ionic liquids to becoming their alternative: the unbalanced turn of deep eutectic solvents

“…Indeed, by modifying the operating conditions, it proved to be efficient with biomasses such as hardwood, 141 softwood, 142 forest residues and switchgrass, 143 agricultural wastes such as rice straw, rice husk, wheat straw, sugarcane bagasse and coconuts waste. [144][145][146] This process has been scaled-up to pilot plants of 200 L capacity and plans for a unit able to produce ca. 200 t per year of pulp have been drafted.…”

Apparent Molar Properties and Viscosity Studies of Ternary Systems Composed of Model Lignin and Xylose in DBU Based Ionic Liquid–DMSO Solutions

“…ILs can process a wide range of feedstocks including the fruit wastes, vegetable wastes, , hardwood, , and softwood, etc. Cellulose, , lignin, − and hemicellulose , are some of the biomolecules (biopolymers) present in the biomass in which lignin is the hardest and is the protective layer over the other components. , Lignin is widely used in cosmetics, hydrogel formation, electrochemical applications, nanoparticle synthesis, and packaging, etc. The poor solubility of lignin in different solvents restricts the uses of lignin though.…”