[en] In the context of fossil resources dependence and climate change due to green-house gas emissions, increasing efforts are devoted to the synthesis of plastic materials from renewable resources using less energy consuming processes. A possible path to produce plastic resins precursors is the transformation of plant saccharides to furan derivatives like 5-hydroxymethylfurfural (5-HMF) and 2-furfural (2-F).
Furan derivatives are obtained through acid-catalyzed dehydration of saccharides and can be converted to new and more performant plastic materials. Their selective synthesis at moderate temperature remains however an important challenge and is the focus of this work.
The first part of the research consists in an extensive review of monosaccharides dehydration mechanisms. Monosaccharides can be separated into two categories: ketoses (e.g. fructose) possessing a ketose moiety under acyclic form and aldoses (e.g. glucose) possessing an aldehyde moiety under acyclic form. The dehydration of ketoses through cyclic intermediates and their different reactivities are clarified. The reaction medium components required for their selective conversion to furan derivatives are methodically examined. Aldoses are more abundant in plants than ketoses and also more difficult to convert to furan derivatives. The study reveals why aldoses structure limits their dehydration and how catalysts and reaction media can be combined to enable a selective transformation.
Given the promising results achieved with imidazolium chloride ionic liquids according to the literature, reaction media based on melted mixtures of choline chloride and organic acids were investigated as a possibly cheaper alternative. The dehydration of a ketose, fructose, was attempted at 90 °C in different mixtures with the aim to understand the role of each reaction medium components. Through reduction of the amount of organic acid and by increasing the acid strength, an 80% 5-HMF yield could be achieved in only 1h at 90 °C. The addition of small amount of an organic solvent like isopropanol enabled the reaction at 60 °C with a similar selectivity.
The potential of choline chloride/organic acids melted mixtures for aldoses dehydration (glucose, mannose, galactose, xylose, arabinose) was then explored. To enable the formation of furan derivatives, boric acid was required for the prior isomerization of aldoses to ketoses. Aldoses transformation remained limited (e.g. 5% 5-HMF yield and 23% glucose conversion after one hour at 90 °C with maleic acid). Combined with α-hydroxyacids however, the reaction was faster and more selective (e.g. 19% 5-HMF yield and 61% glucose conversion after one hour at 90 °C). The synergy between α-hydroxyacids and boric acid was explained by formation of tetrahydroxyborate esters (THBE). Compared to boric acid, THBE reaction with glucose is energetically favored. THBE formation is associated with H3O+ release in the medium increasing its acidity. Dark polymers called humins were the main observed side-product of the reaction and are a major hurdle to the development of selective 5-HMF and 2-F synthesis processes.
To better understand and inhibit their formation, humins were produced from different monosaccharides (hexoses, pentoses, 2-deoxyglucose) and analyzed by infrared spectroscopy and nuclear magnetic resonance. Humins formation during ketoses dehydration can be limited using organic solvents but humins production from aldoses persists even in organic reaction media. Those polymers are suspected to form from acyclic monosaccharides after one or several dehydration steps leading to a conjugated structure. The resulting conjugated product is sensitive to aldol addition/condensation with monosaccharides and furan derivatives. The experiments suggest that isomerization catalysts required for furan derivatives synthesis also exacerbate humins formation in two ways: they stabilize monosaccharides acyclic form and therefore expose their carbonyl moiety; humins possess functional groups which interact evenly or more strongly with the catalyst than reaction medium components or monosaccharides.
Based on humins structure and a better understanding of their formation, advices are provided regarding the design of new catalysts including transition metals or boronic acids. We suggest that the catalyst should be strongly bonded to an organic structure which must prevent simultaneous interactions of several molecules with the catalytic site. The bulkiness of the organic structure must be precisely controlled to prevent humins of large size to inhibit the catalyst but still enable interaction with monosaccharides. The provided strategy should be valid for choline chloride-based medium as well as other solvents.
This research supports the possibility to perform selective dehydration of abundant aldoses (e.g. glucose, xylose) at moderate temperature in commercially available and relatively cheap reaction media based on choline chloride. If polysaccharides depolymerization can be achieved in such media, they will certainly represent a promising path to generate bio-based monomers from renewable resources.
