Uniform Chemical Functionality of Technical Lignin Using Ethylene Carbonate for Hydroxyethylation and Subsequent Greener Esterification

Abstract

Full utilization of plant-based feedstocks for sustainable societies must include the valorization of lignin, an abundant aromatic component of the cell wall of plant stems, in processes that follow green chemistry principles. However, one of the major issues limiting lignin utilization is the chemical nonuniformity of the isolated polymer, along with the thermal sensitivity of the free phenolics which hinders processing at elevated temperatures. To address these issues, free phenolics and carboxylic acids of industrial lignins were hydroxyalkylated through the use of ethylene carbonate, which served as both the reagent and the solvent. This facile and safe reaction resulted in near complete conversion of phenolics and carboxylic acids into aliphatic hydroxyls resulting in uniform chemical functionality and enhanced thermal stability. However, the increased reaction temperature decreased the total hydroxyl content and increased the molecular weight of the lignin; the work identified a narrow processing window that achieved derivitazation without extensive structural modification, which plagued earlier work with carbonate modification of lignin. 13C NMR spectroscopy of hydroxyalkylated lignin showed a low degree of condensation and limited to no copolymerization of the ethylene carbonate when reacted under modest conditions. The hydroxyethylated derivative had enhanced solubility in propionic acid, which was used as a solvent and reagent in order to directly esterify the lignin. The reaction achieved over 95% substitution of the lignin hydroxyls creating ethylpropionate derivatives where excess propionic acid could be recycled under vacuum. The hydroxyalkylation followed by direct esterification provided a route toward the development of greener lignin esters by avoiding added solvents, carcinogens such as ethylene oxide, and halogens like acid chlorides for lignin-based polymeric materials synthesized utilizing green chemistry principles.

Aqueous Dispersions of Esterified Lignin Particles for Hydrophobic Coatings

“…After purging nitrogen gas to remove air, the reaction proceeded in an oil bath under 120°C for 2.5 h. At the end of the reaction, the solution was poured slowly into 400 ml pH = 2 aqueous sulfuric acid with continuous stirring to precipitate modified lignin. The mixture was then filtered by 0.45 μm PVTF membrane, followed by washing with another 400 ml distilled water until neutral pH and dried by freeze dryer (Liu et al, 2018).…”

“…The direct esterification (catalyst-free esterification) was adopted to obtain esterified lignin following our previous procedure (Liu et al, 2018, 2019). Specifically, 3 g hydroxyethyl lignin was mixed with 30 ml oleic acid in a round bottom flask, which was heated under 180°C (oil bath) for 24 h. After cooling down, the samples were mixed with filter aids (Celite 545, Sigma Aldrich) and washed by 300 ml aqueous ethanol (75% v/v).…”

“…Working from this approach, softwood kraft lignin (extracted from LignoForce system) (Kouisni et al, 2012) was modified utilizing a two-step direct esterification to obtain an ethyl oleate lignin. Specifically, the lignin was modified by ethylene carbonate to obtain lignin only containing aliphatic hydroxyl groups (Liu et al, 2018). Then oleic acid was utilized as both solvent and reagent to realize the direct esterification of hydroxyethyl lignin derivative.…”

Sorption behavior of malachite green onto pristine lignin to evaluate the possibility as a dye adsorbent by lignin

“…However, lignin is less useful due to this complicated structure, and only about 2% is used as a dispersant, an adhesive, or a surfactant, and most of the lignin produced as a by-product is discarded or incinerated [6]. Most recently, Yin et al [7] reported that lignin has a number of functional groups including hydroxyl, methoxyl and carbonyl groups, and thus can be utilized as a raw material for chemical manufacture through chemical modification techniques such as oxypropylation and epoxidation [8]. Some researchers have found that materials derived from the chemical modification of lignin can be utilized as raw materials for making plastics such as polyurethanes and polyesters in the manufacture of plastics [9,10], as well as raw materials for phenolic resins, epoxy resins and carbon fiber products.…”

Isolation of phenolic monomers from kraft lignin using a magnetically recyclable TEMPO nanocatalyst

“…ASKL is reported to have a higher phenolic content and a highly modified structure that may have led to the increase in yield and vanillin selectivity. 11,42 Further, AIKL that has a relatively greater amount of β-O-4 linkages showed limited production of vanillin relative to ASKL. Along similar lines, milled wood lignin from pine, subjected to significantly less processing than kraft lignin, was also depolymerized under the same conditions with Fe@MagTEMPO.…”