Kings and Prof. Kilpeläinens work (Sustainable Chemsitry). Indeed, the main feature we sought for new ionic liquids was the ability to dissolve cellulose, as the main component in woody biomass. Thus, structures developed have found application in, e.g., dry-jet wet-spinning of cellulose (IONCELL process), and other applications.
However, maintaining the crystalline structure and morphology of cellulose substrates has considerable value also. By applying an expedient and low-cost surface modification of cellulose (e.g. chemical pulp, nanocellulose, fibres, films or aerogels) the materials may be valorised with relatively low development costs, compared to the application of highly novel solvent systems. There are many methods in the literature demonstrating these modification methods and performance of the materials in application. However, few give detailed structural characterisation after modification, to assess regioselectivity, leading to solid mechanistic information.
Therefore, we have invested significant time into development of solution-state NMR methods for analysis of crystalline celluloses, i.e. native celluloses or those which have undergone a surface modification. We apply these methods to analyse novel chemical processing schemes, designed to be regioselective surface chemistries. The reaction products are characterised and we work with collaborators (particularly in Aalto University) to assess their potential in application.
It was determined that key cellulose-dissolving phase-separable ionic liquids, were highly effective solvents for cellulose, as their DMSO electrolytes (DOI: 10.1002/cssc.201301261; DOI: 10.1039/c7ra01662j). Tetrabutylphosphonium acetate, [P4444][OAc], was even found to dissolve 5 wt% cellulose at a weight ratio of 1:4, with DMSO-d6. This yielded highly resolved 1H and 13C spectra. The initial results were published in two recent papers: The first with [P8881][OAc] (DOI: 10.1002/cssc.201501511) and the second with [P4444][OAc], in collaboration with the Cranston Lab in UBC (DOI: 10.1021/acs.biomac.8b00295). More recently we have given highly accurate assignments for cellulose and oxidised celluloses and tested the quantitivity of applying the Q-CAHSQC (quantitative HSQC sequence) for quantifying key species (DOI: 10.1007/s10570-020-03317-0).
Now that we have a tool that is able to give us such resolution of chemical species, performing selective chemistries can yield characterisation of materials to such a standard to begin to satisfy the analytical standards of the organic chemist. A recent example of this was the demonstration of a 'Knoevenagel Condensation for Modifying the Reducing End Groups of Cellulose Nanocrystals'. In this publication it was demonstrate that a 1,3-diketone could be reacted at the reducing end groups of cellulose nanocrystals. The combination of, model compound assignments with diffusion-editing (edit out ionic liquid signals) and band-selective TOCSY, yieded confirmation of reaction at reducing end groups.
Figure. Confirmation of the 'Knoevenagel Condensation for Modifying the Reducing End Groups of Cellulose Nanocrystals'.
Further chemistries are being explored. Of particular interest are modification of regenerated cellulose fibres for novel textile applications. Addition of higher functionalities are also of interest, such as those that may have biomedical application. Please contact us if you have ideas you wish to try.
Despite its low cost, compared to other petrochemical polymers, the major limitation to the wider use of cellulose in materials applications is its native crystalline form, which does not melt (it decomposes before melting). This limits the opportunity for low-cost melt-processing, as is typically performed on petrochemical thermoplastic materials. However, it also means that cellulose is quite non-reactive, except under harsh conditions (in strong acid or base). There are established 'heterogeneous' modification reactions that have lead to long-standing and specified materials. However, no new industrially competetive chemistry is appearing that can be performed at low-cost with economy of scale.
Therefore much of our reserch efforts concern the reduction or modulation of the crystallinity of celluloses, combined with specific chemical modifications, to yield low-cost or novel products. One recent article 'Crystallinity reduction and enhancement in the chemical reactivity of cellulose by non-dissolving pre-treatment with tetrabutylphosphonium acetate' shows that cellulose can be pre-treated with ionic liquid, leading to increased chemical reactivity under aqueous and organic conditions.
With woody biomass, despite our ability to dissolve cellulose, the situation becomes more complicated, due to the complex nature of the material. This is commonly termed as 'biomass recalcitrance'. However, it is possible to dissolve wood in certain novel solvents with extended thermal (DOI: 10.1021/jf071692e), mechanical (DOI: 10.1039/c3gc41273c; DOI: 10.1021/jf901095w) or chemical treatment (DOI: 10.1039/c6gc00183a). As such, there is clear interest in the investigation of low-cost pre-treatments, due to the significantly lower cost of wood chips, compared to chemical pulp or other more processed materials.