The Effect of Sample Preparation Techniques on Lignin Fourier Transform Infrared Spectroscopy

Abstract

The characterization and quantification of functional groups in technical lignins are among the chief obstacles of the utilization of this highly abundant biopolymer. Although several techniques were developed for this purpose, there is still a need for quick, cost-efficient, and reliable quantification methods for lignin. In this paper, three sampling techniques for fourier transform infrared (FTIR) spectroscopy were assessed both qualitatively and quantitatively, delineating how these affected the resultant spectra. The attenuated total reflectance (ATR) of neat powders and DMSO-d6 solutions, as well as transmission FTIR using the KBr pelleting method (0.5 wt%), were investigated and compared for eight lignin samples. The ATR of neat lignins provided a quick and easy method, but the signal-to-noise ratios in the afforded spectra were limited. The ATR of the DMSO-d6 solutions was highly concentration dependent, but at a 30 wt%, acceptable signal-to-noise ratios were obtained, allowing for the lignins to be studied in the dissolved state. The KBr pelleting method gave a significant improvement in the smoothness and resolution of the resultant spectra compared to the ATR techniques. Subsequently, the content of phenolic OH groups was calculated from each FTIR mode, and the best correlation was seen between the transmission mode using KBr pellets and the ATR of the neat samples (R2 = 0.9995). Using the titration measurements, the total OH and the phenolic OH group content of the lignin samples were determined as well. These results were then compared to the FTIR results, which revealed an under-estimation of the phenolic OH groups from the non-aqueous potentiometric titration, which was likely due to the differences in the pKa between the lignin and the calibration standard 4-hydroxybenzoic acid. Further, a clear correlation was found between the lower Mn and the increased phenolic OH group content via SEC analyses. The work outlined in this paper give complementary views on the characterization and quantification of technical lignin samples via FTIR.

The Influence of TiO2–Lignin Hybrid Fillers in Low-Density Polyethylene Composites on Photocatalytic Performance and UV-Barrier Properties

“…The next analyzed infrared spectrum was that obtained for kraft lignin. Typical bands of functional groups present in the structure of this biopolymer were observed in earlier works [ 37 , 39 , 40 , 41 , 42 ]. Namely, the band at the wavenumber 3423 cm −1 originates from the stretching vibrations of hydroxyl groups.…”

Interfacial Adsorption of Oil-Soluble Kraft Lignin and Stabilization of Water-in-Oil Emulsions

“…The removal of low-molecular-weight compounds also decreased the polydispersity of the lignin, which is marked by a narrower main-peak. A decrease in polydispersity also makes sense, as the lignin macromolecules with a lower molecular weight tend to exhibit more reactive sites . More alkyl groups will therefore be added per lignin macromolecules, as compared to the lignin with a higher molecular weight.…”

“…A decrease in polydispersity also makes sense, as the lignin macromolecules with a lower molecular weight tend to exhibit more reactive sites. 51 More alkyl groups will therefore be added per lignin macromolecules, as compared to the lignin with a higher molecular weight. Interestingly, the butyrated Kraft lignin exhibited a longer tail at the high-end of the molecular weight.…”

Lignin-Based Wax Inhibitors

“…53 The abundance of hydroxyl groups, which is of importance for subsequent modification reactions, has been published in our previous article. 54 An overview of the waxes used in the study is presented in Table 1. The waxes Sasolwax 6805 (Wax A) and Sasolwax 5405 (Wax B) with congealing points of 66−70 °C and 53− 55 °C, respectively, were supplied by Sasol GmbH, Germany.…”

“…The organosolv lignin (OL) was produced from Norwegian spruce as previously described by Ruwoldt and Tanase Opedal . The abundance of hydroxyl groups, which is of importance for subsequent modification reactions, has been published in our previous article . An overview of the waxes used in the study is presented in Table .…”