Behind the Science: Improved Structural Characterization of Lignins

Behind the Science: Improved Structural Characterization of Lignins

Author: David Smith, Pieter Bruijnincx

Dr. David J. Smith, Editor of ChemSusChem, talked to Professor Pieter Bruijnincx, Utrecht University, The Netherlands, about his work on faster, simpler lignin characterization that was recently published in ChemSusChem.

Lignin is an attractive material for the production of renewable chemicals, materials, and energy. However, its use is hampered by its highly complex and variable chemical structure, which usually requires an extensive suite of analytical instruments to characterize.

Could you explain the key findings of your study?

We have shown that important information on the chemical structure of a host of different lignins can be obtained simply from their attenuated total reflection-infrared (ATR-IR) spectra, combined with a chemometrics approach. Lignin is one of the main components of lignocellulosic biomass. As a highly aromatic biopolymer, it is heavily investigated as a source of renewable aromatics.

The challenge with lignin, however, is that it has a rather complex structure. The structure varies from plant species to species and readily changes when it is extracted from the biomass. Multitechnique characterization is required for each lignin feedstock that is to be studied. This requires access to high-end equipment and takes a considerable amount of time. Lignin analytics rapidly becomes a bottleneck.

We wanted to address this by using data from a simple and readily-available technique to see if all key structural information could be extracted. It was previously shown that properties such as the lignin content in the biomass, the syringyl/guaiacyl (S/G) ratio, and the hydroxyl group content could be determined by IR spectroscopy, but we have shown that more detailed chemical structure information can also be obtained.

Using multivariate analysis and building on a gel‐permeation chromatography (GPC) and NMR data set for calibration, we were able to accurately predict both the molecular weight and inter-unit linkage abundance of various lignins from different botanical origins and isolated by different pretreatment methods. This means that, once these models are in place, lignin sample analysis requires only a single IR measurement rather than the use of an entire suite of traditional analysis methods, thus potentially removing lignin analysis as a bottleneck.

Research into lignin valorization has become a lot more common recently. Why do you think this is?

The economic viability of competitive biorefinery operations requires that the value of the lignin fraction of the lignocellulosic feed is maximized. This provides an industrial or commercial driver for lignin valorization. In addition, academics enjoy getting their teeth into a difficult challenge, and lignin certainly provides that.



What are the major sources of technical lignins?

About 50–70 million tons of lignins are extracted annually in the pulp and paper industry. Only about 1–2 % of the lignin extracted ends up in value-added products, with the remainder mainly being used for energy generation. Most often used for materials applications are the lignosulfonates, followed by kraft lignins (produced in the conversion of wood to wood pulp).

Recent industrial innovations have made kraft lignin more readily available commercially on a large scale. This is a promising development for this lignin to serve as a feedstock for further valorization. Other sources of lignin are associated with biorefinery efforts currently being operated at the pilot or demo scale, e.g., steam explosion lignins, organosolv lignins, and acid hydrolysis lignins. These are lignins produced in different pretreatment/fractionation processes.

You have explored the structure of technical lignins using a combined spectroscopic and modeling method. How does this improve the prospects for lignin valorization?

It may take away some analytical hurdles that people can run into when developing lignin valorization technology. We often heard from our industrial partners that they would like to have more analytical protocols available that are simple to operate, do not require very specialized equipment, and allow the high-throughput screening of samples, e.g., for quality or feed control at a particular site.

As you state in your paper, a vast array of spectroscopic techniques has previously been used for the characterization of lignin. Could you explain a little about how the combination of IR spectroscopy and multivariate analysis provides unique insights?

It is not so much that the insights obtained from ATR-IR/multivariate analysis are unique, but rather that a number of important characteristics of the lignin structure (linkage abundance, molecular weight distribution, hydroxyl content) can be obtained from one single measurement of an IR spectrum in a matter of minutes and without any significant sample preparation.

Traditionally, one would need three separate experiments, a GPC analysis, a 2D heteronuclear single quantum coherence (HSQC) NMR spectrum, and a 31P NMR spectrum. This requires considerable efforts for sample preparation and derivatization, access to (high-end) equipment, and at least a couple of hours of experimentation time. Extracting all this information from a simple IR spectrum, provided that a good calibration data set is available, thus saves time and money.

What do you see as the main obstacles to the use of lignin as a major industrial feedstock?

There are a couple of chicken-and-egg type of issues with the large-scale application of lignin-derived products. Increases in its availability as a feedstock will follow when a few tangible, new applications hit the market. The viability of close-to-market applications is, in turn, somewhat limited by the availability of the feed.

When it comes to developing actual applications, be it as a macromolecule for materials use or as a supply of (aromatic) chemical building blocks, we have to keep in mind that our lignin feeds are highly heterogeneous when it comes to molecular weight, chemical structure, and composition. Just like a barrel of crude oil is not used as such, but rather fractionated to serve different outlets, we might want to do the same with the different lignins we work with.

Currently, the majority of industrially obtained lignin is burned as fuel. How far away is lignin from widespread adoption as a renewable source of chemicals?

Having lignin-derived pure chemicals such as BTX (benzene, toluene, and xylenes), phenol, or others, is further off than a tailored macromolecular application of lignin, for which there are now many examples at a relatively developed stage, or the application of lignin-derived mixtures. Having the latter two succeed commercially will also further pave the way for the development of lignin-to-chemicals technology, if it is supported with significant fundamental research efforts regarding technology development.

How will you follow up on this study?

We intend to see if we can still get more information out of our dataset, both in terms of types of data and in terms of the accuracy of the predicted results.

The article they talked about


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