Determination of Phenolic Hydroxyl Groups in Technical Lignins by Ionization Difference Ultraviolet Spectrophotometry (∆ε-IDUS method)

Werner Marcelo Goldmann,Anu M Kantola,Sanna Komulainen,Otto Mankinen,Juha Ahola,Juha Tanskanen,Ville-Veikko Telkki

doi:10.3311/ppch.9269

Abstract

The amount of hydroxyl groups, particularly phenolic, is one of the most important parameters in lignins, as it is an indicator of lignin reactivity. Ultraviolet (UV) Spectrophotometry is a simple and inexpensive method for determining phenolic hydroxyls in lignin. Ionization Difference Ultraviolet Spectrophotometry (Δe-method) relies on the analysis of solubilized lignin at neutral and alkaline conditions with a UV spectrophotometer. We added a slope analysis to the ∆e-method and dubbed the resulting method ∆e-IDUS (Ionization Difference UV Spectrophotometry). We assessed the reliability of ∆e-IDUS by studying the well-known Indulin AT lignin. Additionally, ∆e-IDUS was applied to a previously uncharacterized milox lignin. When compared to 13 C-NMR, ∆e-IDUS underestimated the amount of phenolic hydroxyls for Indulin AT, possibly due to neglecting second phenolic hydroxyls in some lignin units, which resist ionization because of steric hindrance. Nevertheless, the results agreed with previously reported values and confirm that ∆e-IDUS is useful to screen lignins based on their phenolic hydroxyl group content.

Highlights

Despite being one of the most abundant biomaterials available, lignin has its potential mostly untapped, as to this day there are no widespread applications
Even with the limited information available in the literature about on the condensed and uncondensed amounts of OHph in Indulin AT, the available data (Table 1) compared to the results shows that the Δε-IDUS method determines the amount of uncondensed OHph reliably and within range of the reported values
4 Conclusions The Δε-method was modified to include a slope analysis to reduce the experimental variation in calculation of the mass extinction coefficient Δamax, and the resulting method was named Δε-IDUS

Summary

Introduction

Despite being one of the most abundant biomaterials available, lignin has its potential mostly untapped, as to this day there are no widespread applications. It can be used as a low-grade fuel for the recovery boiler in the pulp and paper industry; its potential lies in its phenolic structure. The R substituents in positions 3 and 5 depend on the alcohol of origin They can be hydrogens in both positions (p-coumaryl), one methoxy group and one hydrogen (coniferyl), or two methoxy groups (sinapyl) (Fig. 1a). The aromatic portions or units (without the propanoid side-chain) are named p-hydroxyphenyl (H, additative of p-coumaryl), guaiacyl (G, additative of coniferyl), and syringyl (S, additative of sinapyl) and they form the aromatic backbone of lignin fragments (Fig. 1b). Lignins that contain a large proportion of one of these aromatic units can be named such as guaiacyl lignins from softwood [3]

Methods

Results

Conclusion