Review articleFurfural and 5-(hydroxymethyl)furfural: Two pivotal intermediates for bio-based chemistry
Graphical abstract
Introduction
A new chemical tree from C5–C6 carbohydrates via furfural and 5-(hydroxymethyl)furfural (HMF) is now established. These two furanic aldehydes offer rare opportunities for designing novel bio-based products exhibiting promising applications as commodities or specialty chemicals including monomers, solvents, fuels, and fine chemicals [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14].
Although renewability is an important point, its beneficial contribution (renewable versus fossil) to the overall product’s carbon footprint should not be overstated. All other contributions (reagents, catalysts, solvents, heat, separation, utilization of the product and its end of life) account indeed, to the major part of the cumulative ‘carbon cost’. This is why, in keeping with the modern vision of green chemistry and engineering [15], the topic must be considered globally and address all issues, including cheaper and more sustainable raw materials, safer and cleaner processes, and diversification in molecular design of the derivatives.
The difficulty lies on the overlap between several key fundamental issues: adapting the catalyst to different starting resources, finding appropriate ‘catalyst/solvent’ couples with higher ‘dehydration–isolation’ efficiency, or looking for more selective and efficient subsequent transformations. In all of these directions, innovation results from a subtle combination of catalyst design, solvent and process optimization, and mechanistic considerations. Overall, several facets of green chemistry subtly overlap, such as catalyst design, use of non-edible biomass, clean solvent and processes, molecular design, selectivity, making this topic a fascinating school case (Figure 1).
This short opinion account aims at highlighting first the variety of approaches by picking a selection of very recent papers exemplifying the ‘resource–catalyst–solvent’ triple challenge of furfural and HMF synthesis. Next, among applications and uses, a focus is made on furfural-derived bio-based solvents and on HMF oxidations and reductions toward monomers. A short section also covers the bridge between C5–C6 chemistries. This account gives the viewpoint of synthetic organic chemists, stressing the benefits of the progress in the design of catalysts and processes and encouraging further synthetic exploration for diversifying the realm of possible products and applications.
Section snippets
Synthesis
Xylose is the substrate of choice for the synthesis of furfural. However, there are still some limitations to the efficacy of its dehydration to furfural, with selectivity issues due to the formation of humins necessitating to start from low xylose concentrations. Recent investigations have focused on the exploration of different catalysts, various media, and possible activation by additives.
Chatterjee et al. [16] reported that activated coal fly ash at 170 °C allowed achieving 85% conversion
Synthesis
HMF is the acid-catalyzed triply dehydrated product of C6 carbohydrates. In this reaction, the key issues are the selectivity, because of the limited stability of HMF in acidic conditions, and the possible use of hexose-containing starting materials cheaper than the very reactive fructose. Many studies reported recently (Table 1) have addressed both issues, by proposing specifically designed catalysts and solvents.
Reactivity of HMF
The recent literature on HMF-derived products is extremely rich and diversified
Bridging the C5 and C6 branches of the furfural/HMF chemical tree
Adding a carbon atom (+C1) to furfural or removing a carbon atom from HMF (−C1) is also an interesting strategy to switch from C5 to C6 sugars or vice versa. This may be related to price and availability issues, as furfural is still significantly cheaper and more abundant than HMF. It may also be appealing in terms of reactivity, when one useful C5 target can be obtained more easily or only from HMF. In the following passages, two examples illustrate the ±C1 options.
Carbonylation of furfural to
Conclusions
The vitality of the chemistry of C5 and C6 sugars is so high that furfural and HMF will very likely play a central role in tomorrow's chemistry, benefiting from efforts of chemists in all disciplines, from theoretical to engineering via synthesis and catalysis. Although the ‘furfural tree’ is rather mature already, the HMF one is still incomplete. Apart from the need to widen the range of resources to raw biomass that should replace refined sugars to improve the economic viability of the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank the Ministère de l’Enseignement Supérieur, de la Recherche et de l’Innovation (MESRI) and CNRS for their financial support, as well as the Chinese Scholarship Council (Université de Lyon-CSC call) for the fellowship to GS.
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