Furan compounds such as furfural (2-furaldehyde) and HMFare obtained by dehydration of sugars over mineral acid catalysts such as HCl or H2SO4 at moderate temperatures (e. g., 423 K). These furan compounds find ap­plications as chemical intermediates in the production of industrial sol­vents, polymers and fuels additives. Furfural is obtained by dehydration of C5 sugars like xylose in a well-developed industrial process. [105] On the other hand, the large-scale production of HMF from C6 sugars is more complicated, and several aspects still remain a challenge, one of which is the utilization of glucose as a sugar feedstock. Thus, HMF is typically produced from glucose with low yields, and current technologies include an additional isomerization step to fructose, since dehydration of fructose to HMF takes place with better selectivity and higher rates. [106,107] Fur­thermore, the control over the unwanted side reactions involving the reac­tant, intermediates and the final HMF product is critical. It is particularly important to prevent HMF from overreacting in the aqueous phase, and utilization of biphasic reactors where HMF is continuously extracted into an organic solvent has shown promising results. [108]

Furfural and HMF can be used as building blocks for the production of linear hydrocarbons (in the molecular weight appropriate for diesel and jet fuel) by means of a cascade process involving dehydration, hydrogenation and aldol-condensation reactions, [109,110] as shown in Fig. 7 for HMF. The process starts with acid-catalyzed deconstruction of polysaccharides (e. g., starch, cellulose or hemicellulose) to yield C5 and C6 sugar mono­mers, which are subsequently dehydrated (under the same acid environ­ment) to form carbonyl-containing furan compounds such as furfural and HMF. In a further step, the carbonyl group in the furan compounds serves as a reactive center for C-C coupling through aldol condensation reactions with carbonyl-containing molecules such as acetone, which can be also obtained from biomass-derived sources. [111,112] These condensations are base-catalyzed (e. g., NaOH, Mg-Al oxides) and are typically carried out in polar solvents like water. As a result of the aldol-condensation, a larger compound containing unsaturated C==C and C==O bonds (i. e., al — dol-adduct) is formed and, owing to its hydrophobic character, this adduct precipitates out of the aqueous solution. Recently, improvements have been made in the aldol-condensation process by utilizing biphasic reactors where furan compounds (dissolved in organic THF) are contacted with aqueous NaOH, thus allowing continuous extraction of aldol-adducts into the organic phase. [110] As represented in Fig. 7, the molecular weight of final alkanes can be increased by allowing adducts to undergo a second aldolcondensation process with the initial furanic feedstock. The unsatu­rated C==C and C==O bonds in aldol adducts are subsequently hydroge­nated over metal catalysts such as Pd to yield large water-soluble polyol compounds. The complexity of the process can be reduced by using a bifunctional (metal and basic sites) water-stable Pd/MgO-ZrO2 catalyst. [113] Thus, both aldolcondensation and adduct hydrogenation can be car­ried out simultaneously in a single reactor. The last step of the process involves complete oxygen removal from the hydrogenated aldoladducts to produce liquid alkanes through aqueous-phase dehydration/hydrogena — tion (APD/H) reactions. [114] Oxygen is progressively removed from the water-soluble adducts over a bifunctional metal-acid catalyst by cycles of dehydration and hydrogenation reactions. APD/H can be achieved over Pt-SiO2- Al2O3 in a four-phase reactor involving aqueous solution of ad­ducts, a hydrogen gas inlet stream, a hexadecane sweep stream, and the solid catalyst. [109] The hexadecane stream is important in that it prevents intermediate organic species from overreacting to coke over acid sites. Recently, the utilization of a bifunctional Pt/NbPO4 catalyst, with superior dehydration activity under water environments, [115] has allowed elimi-

nation of the hexadecane sweep stream step and, consequently, production of a pure organic stream of liquid hydrocarbon fuels with targeted molecu­lar weights (C9-C15 for HMF and C8-C13 for furfural) that spontaneously separates from water and retains 60% of the carbon of the initial sugar feedstock. [110]