Solution-State Nuclear Magnetic Resonance Spectroscopy of Crystalline Cellulosic Materials using a Direct Dissolution Ionic Liquid Electrolyte

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Solution-State Nuclear Magnetic Resonance Spectroscopy of Crystalline Cellulosic Materials using a Direct Dissolution Ionic Liquid Electrolyte
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While you are probably not consciously aware of it, you are most likely in contact with cellulosic materials on a daily basis. Either through your clothing made of cotton, your morning newspaper, the cardboard in different packaging applications and the paper tissues in your pocket all made of cellulosic pulp. Indeed, cellulose has extensively been utilized, as a material, long before the advent of petrochemical-based polymers. However, if you have stumbled across this blog post by accident, you may ask yourself: “Why is there a need for a protocol to measure solution-state NMR of cellulosic materials?” There is now the possibility to perform sophisticated 2D correlation NMR experiments on materials as unsophisticated as toilet paper. But really, why is this significant?

 Obviously, there is an increasing accumulation of fossil-fuel derived and poorly degradable materials in the biosphere, even in the most remote places, far from human proliferation.1 Their persistency in the environment, in the form of microplastics, has led to some urgency in how to transition to a more sustainable way of living. Thus, we find ourselves increasingly looking back to cellulose as the feedstock of choice to replace those established materials. Cellulose chemistry has been studied for more than hundred years – so what can we now offer to bring to widen the scope of materials beyond those developed during the golden years of chemistry?

 Cellulose has outstanding mechanical properties, which might partly explain why nature chose it as the main structural element in plants. Equally important are our demonstrated capacities to harvest this sustainable resource from the environment, at a reasonable cost. However, as cellulose has a highly rigid polymer backbone, the stability of native crystalline phases prevents further low-cost melt processing, into a range of shaped objects. Therefore, it is obvious that native cellulose cannot be used to substitute each and every ‘plastic’ right away.

To solve this problem, sustainable chemical modifications must be developed – beyond the current commercial chemistries, which already resulted in a multitude of materials utilized in consumer products. For example, different rheology thickeners are produced from different etherification reactions of cellulose, and acetylation leads to a thermoplastic best known for its use in cigarette filters. Most of these industrially relevant cellulose derived products rely on a bulk modification, meaning that a high input of other chemicals is needed for their preparation. Consequently, also their bio-combability can be impaired. This is probably best exemplified by the presence of slowly rotting cigarette butts on our streets, as another cellulosic material encountered on an almost daily basis.

 Arguably, just because a material is bio-derived it does not necessarily mean that it represents a more sustainable or bio-degradable alternative. This is a conundrum that researchers in the cellulose community are more and more concerned about. For example, modifications of cellulose that are restricted to fibril surfaces inherently require less chemical input but can still strongly alter its surface interactions, while preserving the favorable mechanical properties, porosity, and biocompatibility. In the continuing hot topic of cellulosic nanomaterials, this is especially relevant.2-5 Overall, the aims and modification strategies in the field over the last years have become more and more sophisticated. However, the available analytical protocols have not developed in line with the researchers’ aspirations. Techniques like infrared spectroscopy, elemental analysis, certain titrations, or different destructive protocols are still standard for reaction control in the community. In classic organic chemistry they have been effectively replaced by higher resolution techniques, such as high-performance chromatography, mass spectrometry, single-crystal X-ray crystallography or solution-state NMR spectroscopy. These methods of course work well with low molecular weight species but are unfortunately not easily adaptable to cellulose research.

 

Solution state NMR spectroscopy is arguably one of the most powerful analytical techniques available in organic chemistry and thus emerged as its go-to method. It can deliver both quick and dirty insights on occurred reactions with 1D experiments and allows to solve more complex problems via more sophisticated multidimensional experiments. However, due to the insolubility of cellulose in all commercial (per)deuterated solvents, NMR investigations were practically restricted to lower resolution 13C solid state NMR spectroscopy (Figure1). Over the last decades several direct dissolution systems for cellulose were developed, significantly accelerated by the discovery that certain ionic liquids (ILs) can dissolve technical celluloses.6 Nonetheless, most of these solutions are not ideal for NMR applications, due to instabilities causing artifact formation and peak overlap with the polysaccharide resonances.7

 

Figure 1. Comparison of a solution state 13C NMR obtained in the [P4444][OAc] electrolyte (Top) and a solid state 13C NMR spectrum, as is standard in cellulose research (Bottom), of the same microcrystalline cellulose model compound. With the featured protocol the inherently higher chemical resolution of solution state NMR can now also be exploited in cellulose research.

 In their investigations focusing on tetra-n-alkyl phosphonium ILs for biomass processing a team around Alistair King at the University of Helsinki found that they have several peculiar properties that qualify them as excellent NMR solvents for cellulosic materials. “Foremost their ability to dissolve cellulose as an electrolyte in the co-solvent DMSO-d6, the absence of overlapping solvent peaks and their inertness under dissolution and analysis conditions, render them outstanding” says Alistair King, now a Principal Scientist at VTT Finland. “Especially, the high degree of solvation has allowed for well resolved spectra. We conducted extensive internal screenings and identified [P4444][OAc] mixed to 80 wt% with DMSO-d6 as the most promising direct dissolution electrolyte. As the system only requires comparably low temperatures of 65 °C it is compatible with most spectrometers already present in research institutions without specialized adaption of the NMR setup.”

 After reporting the first spectra in these solvent systems in 20148 further articles expanded the protocol and signals of several model compounds were charted to give a spectral catalogue for comparison.9,10 While the focus so far was laid on cellulose nanocrystals (CNCs), tentative investigations showed that the solvent system can be applied for a broad spectrum of biopolymers, and some fossil fuel-based plastics.9 Tracking different spatioselective modifications of reducing end groups in CNCs – known as a distinctive analytical challenge in the field – lately emerged as one of the most powerful applications of the protocol.11,12

We observed a clear interest in the method from certain parts of the cellulose community, measurable in an increased number of invitations for collaborations and packages with different samples sent to us for analysis.” Alistair remarks. “However, there was also a clear reluctancy to implement the method in the respective research institutions based on the less user-friendly previous publications. This is admittedly a problem and has prompted us to develop this protocol – for the average user.

 Besides providing simplified syntheses for the needed high purity [P4444][OAc] another major issue that was addressed in the Protocol were the translational barriers faced when transferring the NMR know-how from Helsinki University to Aalto University. “I was familiar with the basics of NMR from my studies, but when I read the papers or Alistair tried to explain polymer NMR related phenomena to me, I was initially not able to follow at all.” Lukas, a doctoral candidate at Aalto University, confesses. “Honestly, the theory behind NMR is complicated. But to be frank, to benefit from NMR in everyday lab work I really don’t need to understand all of it. If there is a new peak in a certain spectral area, compared to the starting material, that is in many cases enough information. For more complex spectral interpretations there is a plethora of information on the internet and most institutions have employed NMR experts or someone with a background in organic chemistry to consult.”

We have put a lot of work in the preparation of this Protocol, and we just hope that it will prove to be useful to the cellulose community.” both conclude. “To be honest, given the impact of solution-state NMR on other research areas, we are quite optimistic that this Protocol will find wide utility.”

 If investigations in 20 years will still rely on the same dissolving electrolyte as proposed is however not certain to the authors. Alistair says: “Definitely there will be a huge improvement of the procedure once a (per)deuterated dissolution medium is available. Thus, we hope that once the practicality of solution state NMR is more broadly accepted, this will also lead to further investigations in this regard.”

Let’s see in a few years how the topic develops,” Lukas argues “maybe the protocol will help to unravel some cool phenomena leading to a cellulose super material. Maybe we just saved some poor future PhD students from one additional analytical headache. A good thing in either case!

References:

 [1]        MacLeod, M., Arp, H. P. H., Tekman, M. B. & Jahnke, A. The global threat from plastic pollution. Science 373, 61–65 (2021).

[2]        Dufresne, A. Nanocellulose: a new ageless bionanomaterial. Mater. Today 16, 220–227 (2013).

[3]        Li, T. et al. Developing fibrillated cellulose as a sustainable technological material. Nature 590, 47–56 (2021).

[4].       Eyley, S. & Thielemans, W. Surface modification of cellulose nanocrystals. Nanoscale 6, 7764–7779 (2014).

[5]        Heise, K. et al. Chemical Modification of Reducing End-Groups in Cellulose Nanocrystals. Angw. Chem. Int. Edit. 60, 66–87 (2021).

[6]        Swatloski, R. P., Spear, S. K., Holbrey, J. D. & Rogers, R. D. Dissolution of cellulose with ionic liquids. J. Am. Chem. Soc. 124, 4974–4975 (2002).

[7]        Ebner, G., Schiehser, S., Potthast, A. & Rosenau, T. Side reaction of cellulose with common 1-alkyl-3-methylimidazolium-based ionic liquids. Tetrahedron Lett. 49, 7322–7324 (2008).

[8]        Holding, A. J., Heikkilä, M., Kilpeläinen, I. & King, A. W. T. Amphiphilic and phase-separable ionic liquids for biomass processing. ChemSusChem 7, 1422–1434 (2014).

[9]        King, A. W. T. et al. Liquid-State NMR Analysis of Nanocelluloses. Biomacromolecules 19, 2708–2720 (2018).

[10]      Koso, T. et al. 2D Assignment and quantitative analysis of cellulose and oxidized celluloses using solution-state NMR spectroscopy. Cellulose 27, 7929–7953 (2020).

[11]      Heise, K. et al. Knoevenagel Condensation for Modifying the Reducing End Groups of Cellulose Nanocrystals. ACS Macro Lett. 8, 1642–1647 (2019).

[12]      Delepierre, G. et al. Challenges in Synthesis and Analysis of Asymmetrically Grafted Cellulose Nanocrystals via Atom Transfer Radical Polymerization. Biomacromolecules 22, 2702–2717 (2021).

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Solution-state NMR
Physical Sciences > Chemistry > Physical Chemistry > Spectroscopy > Nuclear Magnetic Resonance Spectroscopy > Solution-state NMR
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