Demethylation of Alkali Lignin with Halogen Acids and Its Application to Phenolic Resins

Abstract

Lignin, a byproduct from the chemical processing of lignocellulosic biomass, is a polyphenolic compound that has potential as a partial phenol substitute in phenolic adhesive formulations. In this study, HBr and HI were used as reagents to demethylate an alkali lignin (AL) to increase its hydroxyl content and thereby enhance its reactivity for the preparation of phenolic resins. Analyses by FT-IR, 1H-NMR and 2D-NMR(HSQC) demonstrated both a decrease in methoxyl groups and an increase in hydroxyl groups for each demethylated lignin (DL). In addition, the molar amounts of phenolic hydroxyls, determined by 1H-NMR, increased to 0.67 mmol/g for the HI-DL, and 0.64 mmol/g for the HBr-DL, from 0.52 mmol/g for the AL. These results showed that HI, a stronger nucleophilic reagent than HBr, provided a higher degree of AL demethylation. Lignin-containing resins, prepared by copolymerization, met the bonding strength standard for exterior plywood with DL used to replace as much as 50 wt.% of phenol. The increased hydroxyl contents resulting from the lignin demethylations also imparted faster cure times for the lignin-containing resins and lower formaldehyde emissions. Altogether, the stronger nucleophilicity of HI, compared to HBr, impacted the degree of lignin demethylation, and carried through to measurable differences the thermal properties and performance of the lignin-containing PF resins.

Recent Research Progress on Lignin-Derived Resins for Natural Fiber Composite Applications

“…In recent years, lignin has been used with various polymers to prepare composites and epoxy resins for different applications, i.e., surfactants, coatings, lubricants, plasticizers, carbon fiber, wastewater treatment, stabilizing agents, and fire retardant [ 1 , 8 , 45 , 46 , 47 ]. Several studies have been described the phenolization/demethylation of lignin by using various types of catalysts such as H 2 SO 4 [ 45 ], hydrogen bromide (HBr), hydrogen iodide (HI) [ 48 ], 1-dodecanethiol (DSH) [ 44 ], and Na 2 SO 3 [ 49 ] to improve the reactivity and also to reduce the molecular weight of lignin for thermoset applications ( Figure 2 a) [ 45 , 50 , 51 ]. Since HI has strong nucleophile behavior, it is a more effective catalyst for lignin demethylation than H 2 SO 4 , DSH, HBr, and Na 2 SO 3 .…”

Cleavage of ethers and demethylation of lignin in acidic concentrated lithium bromide (ACLB) solution

“…On the other hand, demethylation (converting methoxyl to a phenolic hydroxyl group) can improve lignin reactivity toward crosslinking, degradation, and other modifications. For example, demethylated lignin had a higher reactivity in lignin-based phenolic resins, resulting in a stronger binding strength and a lower formaldehyde emission [16][17][18][19][20] . Demethylated lignin showed a higher adsorption capacity of heavy metals 21 .…”

Strong, Reusable, and Self‐Healing Lignin‐Containing Polyurea Adhesives

“…The results showed that the introduction of lignin significantly enhanced the mechanical performance of polyurethane adhesives. Wang et al [12] demethylated alkali lignin with HBr and HI and then applied it in the synthesis of phenolic resin adhesives. The demethylation of lignin shortened the curing time and lowered the formaldehyde emissions of lignin-containing resins.…”