Lignins – functional group analysis

One of the key parameters of lignins and technical lignins is the content of methoxy groups, which depends on the one hand of the monomer ratio and on the other hand on chemical changes during processing (pulping, biomass pretreatment, etc.). For certain organosolv variants also ethoxy/alkoxy moieties are of interest (Lin, 1992). A reliable quantification of those groups provides not only structural information, but also indicates structural changes during processing or modification; in many cases the methoxyl group content serves as a stable internal reference in the analysis of other lignin functional groups.

We provide a robust method based on headspace-isotope dilution (HS-ID) GC-MS for the quantitative analysis of methoxy, ethoxy and other alkoxy groups in any kind of lignin, such as kraft, organosolv, lignosulfonates and other biorefinery lignins (Sumerskii et al., 2017). Due to the implementation of isotopically labeled internal standards the HS-ID GC-MS method is robust with high-throughput and requires just a few milligrams of sample and has exceptional precision and accuracy.  

The analysis is performed on an Agilent 6890N gas chromatograph equipped with an Agilent 7697A headspace autosampler and coupled with an Agilent 5975B inert XL mass selective detector.

Sample amount required: 20 mg.

References

Lin, S. Y. (1992).
Commercial Spent Pulping Liquors.
In S. Y. Lin & C. W. Dence (Eds.), Methods in Lignin Chemistry (pp. 75-80). Berlin, Heidelberg: Springer Berlin Heidelberg.

Sumerskii, I., Zweckmair, T., Hettegger, H., Zinovyev, G., Bacher, M., Rosenau, T., & Potthast, A. (2017).
A fast track for the accurate determination of methoxyl and ethoxyl groups in lignin.
Rsc Advances, 7(37), 22974-22982. doi: 10.1039/c7ra00690j

The phenolic and aliphatic hydroxyl groups in lignin are fundamental functionalities affecting the physical and chemical properties of lignins, such as reactivity in condensation reactions or antioxidative properties. Quantitative determination of hydroxyl moieties provides crucial information on the structure, the degree of lignin degradation vs. condensation, antioxidant activity, thermal and oxidative behavior, compatibility with other solvents or polymer matrices, as well as the reactivity of lignin in various reactions involved in the development of lignin-based applications.

Only a rather limited number of methods is able to precisely monitor the hydroxyl group content in lignin, especially in lignins from various biorefinery streams. We are offering several methods for hydroxy group quantification.

Hydroxyl groups by ³¹P NMR

Phosphitylation followed by quantitative ³¹P NMR spectroscopy of lignin enables the quantification of different types of hydroxyl groups, such as aliphatic, different phenolic and carboxyl groups (Balakshin & Capanema, 2015). Due to the high sensitivity of the ³¹P NMR the method is able to cope with very small sample amounts. Usually phosphorous derivatives possess limited stability which, in practice, represented a bottleneck when it came to high-throughput measurements. However, with appropriately selected derivatization reagents and internal standards as well as properly optimized derivatization and NMR conditions, the approach provides hitherto unparalleled precision, accuracy, and robustness (Kortner et al., 2015). The analysis is performed on a Bruker Avance II 400 NMR spectrometer, operating at 162 MHz (³¹P frequency).

Sample amount: 30 mg

References

Balakshin, M., & Capanema, E. (2015).
On the Quantification of Lignin Hydroxyl Groups with P-31 and C-13 Nmr Spectroscopy.
Journal of Wood Chemistry and Technology, 35(3), 220-237. doi: 10.1080/02773813.2014.928328

Korntner, P., Sumerskii, I., Bacher, M., Rosenau, T., & Potthast, A. (2015).
Characterization of technical lignins by NMR spectroscopy: optimization of functional group analysis by P-31 NMR spectroscopy.
olzforschung, 69(6), 807-814. doi: 10.1515/hf-2014-0281

Hydroxyl groups by ¹H NMR after peracetylation

If only information on the total amount of aliphatic and aromatic hydroxy groups is required, the ¹H NMR of peracetylated lignins provides reproducible results combined with good long-term sample stability (Lundquist, 1992). In this case, a relatively long sample preparation (exhaustive acetylation) which may take up to 10 days, has to be taken into account. Due to the overall simplicity and fast NMR acquisition, the method offers the potential for analysis of large sample numbers. The analysis is performed on a Bruker Avance II 400 NMR spectrometer, operating at 400.13 MHz (¹H frequency).

Sample amount required: 30 mg.

References

Lundquist, K. (1992).
Proton (1H) NMR Spectroscopy.
In S. Y. Lin & C. W. Dence (Eds.), Methods in Lignin Chemistry (pp. 242-249). Berlin, Heidelberg: Springer Berlin Heidelberg.

Hydroxyl groups by methylation headspace isotope-dilution GC-MS

A brand-new method for the fast and precise determination of total hydroxyl groups in lignins involves a quantitative methylation, followed by accurate and reproducible robotic lignin isolation and purification with subsequent lignin demethylation and headspace-isotope dilution GC-MS analysis. The methoxy groups present in the original sample serve as the internal standard, which renders the method predestined to be applied in tandem with the lignin methoxy group determination. The high robustness of the method was achieved by the implementation of the isotope dilution approach, which allows compensating for any error or inaccuracy. The method provides the overall hydroxyl group content, i.e. it cannot differentiate between different hydroxyl groups present in the lignin structure. It requires just a few milligrams of the sample and possesses high robustness and sufficient high-throughput features which positively distinguishes it from alternative methods.

The analysis is performed on an Agilent 6890N gas chromatograph equipped with an Agilent 7697A headspace autosampler and coupled with an Agilent 5975B inert XL mass selective detector.

Sample amount required: 20 mg.

Functional groups containing sulfur, most importantly sulfonic acid groups, are usually introduced upon one of the most prominent, commercially relevant pulping processes, the sulfite processes (Sixta et at., 2006), the products usually being denoted as lignosulfonates. The sulfonic acid groups are introduced mostly in the benzylic positions of the lignin macromolecule; their presence is one of the driving factors for the dissolution of the lignin upon sulfite pulping. Alternatively, in some cases also sulfonation of biorefinery of Kraft lignins is performed in order to increase their hydrophilicity and water solubility in developments towards water-soluble value-added products.  

Although the presence of sulfur may not be desirable for some applications, such as work with catalysts that might be poisoned, the sulfonic acid groups in lignins provide unique physicochemical properties of a polyelectrolyte and amphiphile, allowing the lignosulfonates to be utilized in many applications, such as oil drilling additives, dispersants, emulsion stabilizers and plasticizers in concrete (Xu & Ferdosian, 2017).

Quantitative determination of sulfonic acid groups may not just provide information regarding the degree to which the lignin has been modified, but also allows predicting the behavior of lignin in certain applications. There are just few available techniques to address sulfonic acid groups (Korntner et al., 2018). Our core facility provides the two most efficient approaches, either by means of titration technique or by elemental analysis.

Conductometric titration is performed on a Metrohm Titrando device, equipped with an 856-conductivity module and an electrode with a 5-ring conductivity cell combined with a PT1000 thermosensor. The commissioned elemental analysis is performed as C/H/N/S (oxygen can be determined indirectly) on an EA 1108 CHNS-O (Carlo Erba Instruments, CE Elantech, Inc.) elemental analyzer.

Sample amount required: 300 mg.

References

Korntner, P., Schedl, A., Sumerskii, I., Zweckmair, T., Mahler, A. K., Rosenau, T. & Potthast, A. (2018)
Sulfonic Acid Group Determination in Lignosulfonates by Headspace Gas Chromatography.
ACS Sustainable Chemistry & Engineering, 6, 6240-6246. doi: 10.1021/acssuschemeng.8b00011

Sixta, H., Potthast, A. & Krotscheck, A. (2006)
Chemical pulping processes.
In: Handbook of pulp, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Chapter 4, pp. 109-229. doi: 10.1002/9783527619887.ch4b

Xu, C., & Ferdosian, F. (2017).
Utilization of Lignosulfonate as Dispersants or Surfactants Conversion of Lignin into Bio-Based Chemicals and Materials (pp. 81-90).
Berlin, Heidelberg: Springer Berlin Heidelberg.