For my first post on the Biobased Chemicals blog, it is my pleasure to highlight an excellent new review on 5-hydroxymethylfurfural (HMF),one of the most tantalizing renewable chemicals sought by chemists. This six-carbon furan with several versatile handles for manipulation is a starting point for multiple products such as 2,5-furandicarboxylic acid (polymers), 5-alkoxymethylfurfurals and 2,5-dimethylfuran (fuel components), pharmaceuticals, and flavors, and it can be made in high yield from fructose - the sugar which is already mass-produced in high fructose corn syrup. Despite all of these attractive qualities, HMF has lagged behind while other bio-based chemicals like lactic acid, 1,3-propanediol, and even its cousin furfural have been scaled up, commercialized, and transformed into myriad consumer products. Practical and economical production of HMF and its derivatives remains a challenge which is being attacked by many researchers and companies.
Heeres, de Jong, de Vries and the other authors provide a wide-ranging look at HMF, covering topics such as toxicological aspects, dehydration mechanisms, process chemistry for HMF production, reactor systems, and attempts to scale up HMF production. They also take a look at HMF's position as a platform chemical and its derivatives such as FDCA, adipic acid, fuel components, and fine chemicals. Throughout, the authors take a critical look at the topic while providing some humor along the way to keep the nearly 100 pages of dense chemistry interesting. For instance, they note that "ever since mankind started heating its food, furan compounds have been part of the human diet, as HMF is formed during the thermal decomposition of carbohydrates."
One highlight of the publication is the discussion of the potential mechanisms by which HMF is formed from sugars, in which the authors pull together insights and proposals from more than 50 years of research on this topic. Despite many studies on this topic, it is difficult to draw many conclusions. One is that HMF is much more readily formed from fructose than from other sugars, and another is that a cyclic mechanism from fructose is more likely than an acyclic mechanism. Finally, "the literature strongly indicates that the first dehydration of fructose is the rate determining step in the formation of HMF, with all subsequent steps proceeding much faster. This makes it almost impossible to determine the reactive intermediates by analysis. A second complicating factor is the inherent reactivity of sugars, facilitating a wide array of side-reactions of the sugar."
These side reactions have plagued efforts toward practical production of HMF, and the review discusses the many variations on process chemistry used to avoid side reactions and increase yields. "Until the 1980s, research in this field almost exclusively focused on the use of homogeneous acids as the catalyst in water, a common solvent for traditional sugar chemistry. The past three decades have seen a shift from water-based chemistry to alternative solvent systems to improve the overall yield of HMF." The three major alternative solvent systems have been organic solvents such as DMSO, room temperature ionic liquids, and mixed solvents such as biphasic water/organic systems. In some solvents, derivatives of HMF such as ethers and esters, are formed, and Avantium itself has focused on the production of HMF ethers in alcohol solvents such as methanol.
A further complication to HMF production is its isolation from the reaction mixture, and the review provides an overview of the more promising methods (extraction and adsorption) and the less promising methods (distillation: "the distillation of crude HMF is recognized as troublesome due to thermal degradation of HMF associated with the formation of tarry carbonaceous materials.")
The review closes with a perspective on the challenge and promise of HMF:
It has always been a challenge to obtain HMF in an efficient way. Although a lot of improvements have been made in understanding the mechanism and kinetics of the dehydration process, significant challenges still remain in transferring it to an industrial scale. The vast majority of the research summarized in this review has been performed on lab-scale in batch reactions without a real focus on finding an efficient and economically viable process. Improvements have been made in recent years by applying different solvent types and extraction methods, and by applying bifunctional catalyst systems. The use of biphasic systems appears to be straightforward; good yields of HMF have been obtained, and such processes would appear to be scalable although they require relatively large amounts of extracting solvents. There has been little focus on the purification of HMF and the efficient recycling of reaction- and extraction solvents or catalysts . . . .
The biggest challenge still to overcome is the use of glucose and glucose-based polymers, like starch and cellulose, as the carbohydrate feedstock because of the unstable nature of HMF and the severe reaction conditions required. Key to the success of HMF will be an economical production process. So far two pilot plant efforts have not been successful in the direct production of HMF. Recently, Avantium Chemicals started operating a pilot plant for production of furan-based chemicals and plastics in which HMF is in situ converted to its ether to help improve yield and purification. A similar strategy is followed by Mascal in a process that produces 5-chloromethyl-furfural, another more stable derivative of HMF to obtain high yields in combination with efficient separation . . . .I believe that this review will guide and encourage further progress toward that goal.
This review underlines the progress that has been made toward an economic production of furan derivatives from biomass; a development which will open the way to a wealth of interesting materials all the way from fuels and bulk chemicals to fine chemicals and pharmaceuticals applications.
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