5-Hydroxymethylfurfural (HMF) was discussed in chapter 3 as a toxic product of acidic pretreatment techniques for biomass. The boiling point of HMF is too high (291°C) to be considered as a liquid fuel, but if HMF is subject to chemical hydroge — nolysis of two of its C-O bonds, a more volatile product, 2,5-dimethylfuran (DMF) is formed (figure 7.3).12 DMF has a boiling point of 93°C, 20°C higher than ethanol, and has a Research Octane Number of 119 — the by-product 2-methylfuran has an even higher RON (131) but is more water-soluble than DMF.

HMF is most readily formed by the dehydration of fructose, a naturally occur­ring sugar and a straightforward isomerization product of glucose; mineral acids such as hydrochloric acid (HCl) can be used to catalyze the reaction, 88% conversion being achieved at 180°C.12 A solvent such as n-butanol (chapter 6, section 6.3.3) can then be employed to extract the HMF before hydrogenolysis over a mixed Cu-Ru catalyst at 220°C.

Such production routes, beginning with enzymic conversion of glucose to fruc­tose and proceeding via entirely thermochemical processes, have been described as “hybrid.”13 They have the advantage of avoiding reliance on large fermentation vessels for the production step(s), therefore being potentially much more rapid. Their economics could be similar to, or an improvement on, those for Fischer-Tropsch liquid fuels (chapter 6, section 6.2). The conversion of glucose to fructose, catalyzed by the enzyme glucose isomerase, has been a major industrial application of enzy- mology since the 1960s, the product (high-fructose corn syrup) being introduced as a substitute for Cuban sugar in the U. S. reduced-calorie sweetener market.14 The enzyme technology has been continuously improved, evolving to immobilized forms of the enzyme; the potential of enzymes from hyperthermophilic microbes has now been explored, with a stability at 80oC rivaling that of conventional enzyme pro­cesses operated at 55-650C.15 Rapid and efficient processing of glucose solutions to high concentrations of fructose is feasible if the desirable biocatalytic and thermo­stability properties of suitable enzymes can be realized.