Table 1 summarizes the main characteristics of the different technologies analyzed in the present perspective paper for the conversion of biomass into liquid hydrocarbon fuels. Various routes have been compared in terms of important parameters (e. g., pretreatment required, external chemicals and hydrogen requirements, reaction conditions, cleaning/separation steps, number of reactors, use of precious metal catalysts) involved in the processing from the initial lignocellulosic biomass to the final liquid hy­drocarbon fuel. Additionally, technologies are compared in terms of the overall yield of LHF for each route, calculated according to recent data available in literature. The ability to use the entire organic matter in lig — nocellulose represents the main advantage of thermal routes (i. e., BTL and pyrolysis + upgrading) versus aqueous-phase approaches which, as indicated in Section 4.3, only can process the sugar fraction of lignocel — lulose (typically 60-80% depending on the source [13]). This constraint in aqueous-phase routes has important consequences for the overall process: (i) it limits the final yield to LHF and (ii) it negatively affects the econom­ics since additional reactors for pretreatment/hydrolysis steps are required to solubilize the sugar feedstocks (Table 1). On the other hand, aqueous — phase processes are carried out at milder conditions compared to thermal routes which allows for better control of the chemistry and, with it, higher selectivities to targeted hydrocarbon fuels. This control over the chemistry in aqueous phase technologies has important implications on the cleaning/ separation steps as well. Thus, unlike BTL and pyrolysis which produce intermediate fractions with high degrees of impurities that require deep cleaning and conditioning before the upgrading process, aqueous-phase routes achieve high-purity organic streams that spontaneously separate from water, with no further cleaning/ conditioning steps required. The higher number of reactors employed in the upgrading process for aque­ous-phase routes versus thermal routes is another important difference be­tween both approaches. In some cases (for example reforming/reduction of sugars, HMF and GVL platforms), the use of additional upgrading reac­tors is justified by the production of a final liquid hydrocarbon fuel with well-defined characteristics of molecular weight and structure that could be directly used for gasoline, jet fuel and diesel applications (see Fig. 7-9). This control of the final hydrocarbon fuel is more difficult to achieve by BTL and pyrolysis, which produce a mixture of hydrocarbons with broad molecular weight distributions and low control of the final chemical struc­ture. Consequently, thermal routes might need additional refining reactors to produce fuel-grade compounds.

As indicated in previous sections, two parameters are important to as­sess the economic feasibility of aqueous-phase catalytic routes: the num­ber of reactors and the use of external hydrogen. Thus, glycerol reforming, with only 2 reactors (reforming and F-T) and no hydrogen requirements (Table 1), would represent an interesting route. However, the final hydro­carbon yield (0.011 g of LHF per g of dry biomass), negatively affected by the low content of oils in the biomass source (20% in soybeans137), is low compared with other aqueous routes. With respect to these two pa­rameters, reforming/reduction of sugars over Pt-Re is a promising route since no external hydrogen is required (hydrogen required for reduction of sugars and hydrogenation of the ketone formed by aldol-condensation or ketonization is internally supplied in sufficient amounts by aqueous-phase reforming of a fraction of the sugar, Fig. 8), and only 4 reactors (hydroly­sis, reforming, C-C coupling and dehydration/hydrogenation) are needed to produce gasoline, diesel and jet fuel components from lignocellulosic biomass with an overall yield comparable to that of BTL (0.21 g of LHF per g of dry biomass). The main drawback of this approach is the high cost of the Pt-Re (10 wt%) reforming catalysts. The recently developed GVL platform to produce butene oligomers offers an attractive alternative with minimum external hydrogen utilization (required only during the final al — kene hydrogenation step) and no precious metal catalysts, which should give this process a promising economic assessment. The HMF platform route can achieve good yields to LHF (with a maximum yield close to 0.3) at the expense of needing external chemicals such as acetone and organic solvents (typically produced from fossil fuels like petroleum) and moderate amounts of hydrogen to carry out APD/H. The GVL C9 route offers versatility to produce gasoline and C9-C27 diesel components with acceptable yields, but it would require multiple reactors to transform bio­mass into the final hydrocarbon fuel depending on the upgrading process used. Finally, we note that taking into consideration all the parameters as a whole, pyrolysis coupled with upgrading processes appears to be a promising route to convert lignocellulose into LHF with high yields and low complexity. While advances have been made recently in the pyroly­sis step, challenges for this route are currently focused on the upgrading process, with particular emphasis on two crucial aspects: (i) designing strategies for the reduction of hydrogen consumption during HDO and (ii) development of hydrothermally stable catalysts (preferably without pre­cious metals) with high resistance to sulfur and alkaline impurities typi­cally present in bio-oils.

4.5 CONCLUSIONS

The production of liquid transportation fuels from renewable sources such as biomass is a promising route that can help to reduce our depen­dence on fossil fuels and to mitigate global warming effects. Ethanol, the most abundantly produced biofuel at the present time, suffers from low energy-density and compatibility issues with the existing transportation infrastructure, which is based on petroleum-derived liquid hydrocarbons. The current limitations of ethanol as a fuel can be overcome by develop­ing cost-effective technologies that allow conversion of non-edible lig — nocellulosic biomass into liquid hydrocarbon fuels chemically identical to those currently used in the transportation sector. In this respect, sev­eral promising routes are currently being developed worldwide, includ­ing gasification of biomass to syngas coupled with F-T synthesis (BTL), pyrolysis integrated with bio-oils upgrading processes, and aqueous-phase catalytic processing of biomass-derived sugars and derivatives. BTL and pyrolysis-upgrading are thermochemical routes that allow utilization of all the organic matter in lignocellulose, and aqueousphase processing is a catalytic route designed to operate with water-soluble sugars (and plat­form chemicals derived from them). While aqueous-phase routes require that the lignocellulosic biomass be subjected to pretreatment/hydrolysis steps, these routes offer the opportunity to selectively carry out a variety of reactions to achieve the deep chemical transformations and C-C cou­pling reactions required when converting sugars into liquid hydrocarbons. To be economically viable, these aqueous-phase routes should be carried out with a small number of reactors and with minimum utilization of ex­ternal fossil fuel-based hydrogen sources, as illustrated in the examples presented in the present paper.