Zinc chloride/acetamide deep eutectic solvent‐mediated fractionation of lignin produces high‐ and low‐molecular‐weight fillers for phenol‐formaldehyde resins

Abstract

Lignin readily dissolves in a deep eutectic solvent (DES) composed of ZnCl 2 /acetamide 1:3 M ratio in concentrations up to 16.7 wt %, and upon addition of water, two fractions were obtained; one composed of lignin with low-molecular weight that remained dissolved in the DES/water mixture, whereas the other fraction of lignin with higher molecular weight (regenerated lignin, RL) was regenerated by precipitation. Both the RL fraction and the whole solution of lignin in DES were used to replace part of phenol (20 wt %) in the condensation of phenol and formaldehyde to introduce biomass in the resulting resins. The lignin treatment that produced the higher yield of RL consisted of lignin/DES ratio of 1:10 (w/w) stirred for 4 h at 100 C. By using 13 C NMR spectroscopy, a preferential cleavage of the S unit of lignin during DES treatment was identified. The modified phenol-formaldehyde resin (RLPF) containing the RL exhibited higher bonding strength (1.28 AE 0.16 MPa) and shorter Sunshine gel time (557 s) than PF (587 s), showing that the RL helps to improve the polycondensation process. The resin modified with the whole solution of lignin in DES exhibited the shortest Sunshine gel time due to the effect of zinc(II) accelerating the curing process.

Comparison of Lignin Fractions Isolated from Wheat Straw Using Alkaline and Acidic Deep Eutectic Solvents

“…DESs are often considered as a sub-class of ionic liquids; however, compared to traditional ionic liquids, DESs are of advantage due to its simple preparation, high abundance of raw materials, low price, environmental friendliness, and biodegradability. Previous DESs have also been used in the removal of lignin and fractionation of other components from biomasses [19][20][21][22][23] . It has been shown that DESs can be used for efficient lignin removal, in order to obtain a high-quality lignin [24][25][26][27] .…”

“…DESs are often considered as a subclass of ionic liquids; however, compared to traditional ionic liquids, DESs are of advantage due to its simple preparation, high abundance of raw materials, low price, environmental friendliness, and biodegradability. Previous DESs have also been used in the removal of lignin and fractionation of other components from biomasses. − It has been shown that DESs can be used for efficient lignin removal in order to obtain a high-quality lignin. − We previously used six different deep eutectic solvent (DES) treatmentsfive acidic (natural organic acid-choline chloride) and one alkaline (K 2 CO 3 -glycerol)to compare the delignification and nanofibrillation of agricultural byproducts . However, the properties of lignin fractions are known to be strongly associated with the chemical characteristics of used solvent systems; furthermore, the characteristics and differences of lignins separated using different DESs remain to be poorly understood.…”

Processing of Functional Composite Resins Using Deep Eutectic Solvent

“…DESs impacted the interaction between the intermolecular and intramolecular in the lignin network due to the action of the ionic environment, and the chemical structure of lignin was found to be influenced by Zn by comparing the FTIR spectra of lignin and relignin, where a similar conclusion was drawn by Sun et al [98]. Hong et al [54] compared the phenolic resins modified by lignin (LPF), DES/lignin (DLPF) and DES-modified lignin (MLPF) and reported that the curing time was in a trend of DLPF > MLPF >LPF. MLPF exhibited higher bond strength and shorter curing time than the phenolic resin.…”

Greener, Faster, Stronger: The Benefits of Deep Eutectic Solvents in Polymer and Materials Science

“…Polycondensation of formaldehyde using this eutectic in the presence of iron acetate resulted in the creation of high surface area monoliths with graphitic character and very high electrical conductivity (up to 31 S cm −1 [ 89 ]). DES systems have also been prepared as catalysts for the polycondensation of furfuryl alcohol to ultimately produce porous carbon electrodes [ 87 ], as well for the effective solubilization of lignin to effectively incorporate biomass fillers into phenol-formaldehyde resins [ 92 ].…”