As indicated in Fig. 4, aqueous solutions of sugars, derived from the car­bohydrate fraction of lignocellulose (i. e., cellulose and hemicellulose), can be used to produce second generation ethanol fuel through fermenta­tion routes. Alternatively, these sugars can be transformed into a variety of useful derivatives by means of chemical and biological processes. [90,91] As will be addressed in this section, sugars and some of their derivatives can be catalytically processed in the aqueous phase to produce liquid fuels chemically identical to those currently used in the transportation sector. The key advantage of this route, in comparison with BTL and pyrolysis­upgrading approaches, is derived from the mild reaction conditions used, allowing for better control of conversion selectivity. However, costly pre­treatment and hydrolysis steps are required to hydrolyze solid lignocel — lulose to soluble sugar feeds, and the lignin fraction, once isolated, is typi­cally combusted to provide heat and power.

The production of liquid hydrocarbon transportation fuels from bio­mass derivatives involves deep chemical transformations. In this respect, sugars (and chemicals derived from them) are molecules with high degrees of functionality (e. g., — OH, — C==O and — COOH groups) and a maximum number of carbon atoms limited to six (derived from glucose monomers). On the other hand, hydrocarbon fuels are larger (up to C20 for diesel ap­plications) and completely unfunctionalized compounds. Consequently, a number of reactions involving oxygen removal (e. g., dehydration, hy­drogenation, and hydrogenolysis), combined with C-C coupling (e. g., al — dol condensation, ketonization, and oligomerization), will be required to convert sugars into hydrocarbon transportation fuels, and aqueous-phase catalytic processing offers the opportunity to selectively carry out those transformations. Importantly, two aspects are crucial to ensure econom­ic feasibility of the aqueous-phase route: (i) reduction of the number of processing steps by means of catalytic coupling approaches [92] and (ii) deoxygenation of biomass feedstocks with minimal consumption of hy­drogen from an external source. [93]

The main aqueous-phase routes to upgrade sugars and derivatives into liquid hydrocarbon transportation fuels are schematically shown in Fig.

6. The biomass derivatives have been selected in view of their potential to produce liquid hydrocarbon fuels. First, we will describe the catalytic route designed to convert glycerol into liquid hydrocarbon fuels. This route involves the integration of two processes: aqueous-phase reforming (APR) of glycerol to syngas and F-T synthesis. This approach is particu­larly interesting because glycerol is produced in large amounts as a waste

stream of the growing biodiesel industry. [94] Furthermore, glycerol can be co-produced, along with ethanol, by bacterial fermentation of sugars [95] (Fig. 4). Secondly, we will address furfural and hydroxymethylfur — fural (HMF) as important compounds obtained by chemical dehydration of biomass-derived sugars. Furfural and HMF can be used as platform chemicals for green diesel and jet fuel production through dehydration, hydrogenation and aldol-condensation reactions. More recently, our group has developed a two-step (involving sugar reforming/reduction and C-C coupling processes) cascade catalytic approach to convert aqueous solutions of sugars and polyols into the full range of liquid hydrocarbon fuels, and this process will be described in Section 4.3.3. Finally, we will

Dehydration
HMF
Dehydration
GVL
x Liquid Hydrocarbon. Fuels /
Reforming Reduction	C-C coupling
Lignocellulose
Aqueous
Fermentation
sugars
-X-
F-T Synthesis
Transesterification
APR
FIGURE 6: Main catalytic routes for the aqueous-phase conversion of sugars and derivatives into liquid hydrocarbon transportation fuels.

analyze the potential of two important biomass derivatives, levulinic acid (LA, obtained from sugars or HMF through dehydration processes) and g-valerolactone (GVL, obtained by hydrogenation of LA), to produce liq­uid hydrocarbon fuels.