Selective Modification of Aliphatic Hydroxy Groups in Lignin Using Ionic Liquid

Abstract

A facile, sustainable method for the selective modification of aliphatic hydroxy (R–OH) groups in Kraft lignin was developed using an ionic liquid, 1-ethyl-3-methylimidazolium acetate (EmimOAc), as a solvent and catalyst. Selective R–OH modification was achieved by a one-pot, two-step homogeneous reaction: (i) acetylation of R–OH and aromatic OH (Ar–OH) groups with isopropenyl acetate (IPAc) as an acyl donor and (ii) subsequent selective deacetylation of the generated aromatic acetyl (Ar–OAc) groups. In step (i), IPAc reacts rapidly with Ar–OH but slowly with R–OH. The generated Ar–OAc is gradually deacetylated by heating in EmimOAc, whereas the aliphatic acetyl (R–OAc) groups are chemically stable. In step (ii), all R–OH is acetylated by IPAc and Ar–OAc which is a better acyl donor than IPAc, contributing to the rapid acetylation of the remaining R–OH, and selective deacetylation of the residual Ar–OAc is completed by adding a tiny amount of water as a proton source. This two-step reaction resulted in selective R–OH modification (>99%) in Kraft lignin with the remaining being almost all Ar–OH groups (93%). Selectively modified Kraft lignin was obtained with an acceptably high isolated yield (85%) and repeatability (N = 3). Furthermore, despite the lower substitution degree, it exhibited solubility in common solvents, heat-meltability, and thermal stability comparable to completely acetylated Kraft lignin.

Green Conversion of Total Lignocellulosic Components of Sugarcane Bagasse to Thermoplastics Through Transesterification Using Ionic Liquid

“…The content (mmol g –1 ) of each ester group in the esterified bagasse was calculated from the integral ratio between the internal standard and the protons corresponding to the terminal methyl (CH 3 −) group in the ester substituents. A quantitative 31 P NMR ( 31 P qNMR) analysis was conducted to quantify the unreacted OH contents for the per-phosphitylated sample using a predetermined concentration of internal standards in accordance with previous studies. ,− Finally, the DS of each ester group was calculated from the concentration ratio of long-/short-chain acyl groups and unreacted OH groups.…”

Isopropenyl Esters (iPEs) in Green Organic Synthesis

“…[Emim][OAc] was active also in the catalysis of the acetylation of lignin with iPAc [226] . In this case, a one‐pot, two‐step process was observed during which iPAc first rapidly reacted with aromatic −OH groups (Ar−OH) but slowly with aliphatic ones (R−OH); then, the generated Ar−OAc species were subjected to a gradual deacetylation by heating in the presence of [Emim][OAc], while the aliphatic acetyl esters (R−OAc) were thermodynamically more stable and persisted as such [227] . In another paper, Takahashi's group proposed a holistic approach for the treatment of sugarcane bagasse.…”

“…[226] In this case, a one-pot, two-step process was observed during which iPAc first rapidly reacted with aromatic À OH groups (ArÀ OH) but slowly with aliphatic ones (RÀ OH); then, the generated ArÀ OAc species were subjected to a gradual deacetylation by heating in the presence of [Emim][OAc], while the aliphatic acetyl esters (RÀ OAc) were thermodynamically more stable and persisted as such. [227] In another paper, Takahashi's group proposed a holistic approach for the treatment of sugarcane bagasse. A homogeneous transesterification of the bagasse was carried out at 50 °C using iPAc as the acetyl donor and [Emim][OAc] as a catalyst/solvent.…”

Alginate Modification and Lectin-Conjugation Approach to Synthesize the Mucoadhesive Matrix

“…This functionalization could perform to both available functional groups, either carbonyl or hydroxyl. An additional catalyst is needed for esterification of the hydroxyl groups for a more selective reaction [41].…”