The catalytic transformation of sugars to liquid hydrocarbon fuels is a complicated process that ideally should combine deep oxygen removal and adjustment of the molecular weight using a small number of reactors and with minimal utilization of fossil fuel-based external hydrogen. This goal can be achieved by (i) using multifunctional catalysts able to carry out different reactions in the same reactor [92] and (ii) utilizing a fraction of the sugar feedstock as a source of in situ hydrogen through aqueous — phase reforming reactions. [73]

Both approaches are combined by Kunkes et al. in a recent process that transforms aqueous solutions of sugars and sugaralcohols into liquid hydrocarbon fuels in a two-step cascade process [116] (Fig. 8). Firstly, aqueous sugars and polyols (typically glucose and sorbitol) are converted into a mixture of monofunctional compounds (e. g., acids, alcohols, ke­tones and heterocycles) in the C4-C6 range, which are stored in an organic phase that spontaneously separates from water. This step is carried out at temperatures near 500 K over a Pt-Re/C catalyst, which achieves deep de­oxygenation (up to 80% of the oxygen in the initial feedstock is removed) by means of C-O hydrogenolysis reactions. Importantly, the hydrogen required to accomplish the C-O cleavage step is internally supplied by aqueous-phase reforming (involving C-C cleavage and WGS reactions) of a fraction of the feed (Fig. 8). The Pt-Re/C material allows production of hydrogen and removal of oxygen in a single reactor. Unlike bio-oils produced by pyrolysis (Section 4.2), the organic stream of monofunctional compounds produced by sugar processing over Pt-Re/C is completely free of water and has a well-defined composition that is controlled by the feed­stock type (e. g., sugars or polyols) and the reaction conditions. [117]

The retention of functionality in the organic intermediates is key to control reactivity and to allow subsequent C-C coupling upgrading strate­gies. This approach has been demonstrated to be conceptually adequate to process sugars into fuels, [3] and important biomass derivatives such as lactic acid (3-hydroxypropanoic acid) [89,118] and levulinic acid [119] have been upgraded following this strategy. Each group of compounds (e. g., alcohols, ketones, acids) in the monofunctional stream can be up­graded to targeted hydrocarbons through different C-C coupling reactions (e. g., oligomerization, aldol-condensation and ketonization). For example, the organic stream enriched in alcohols by hydrogenation of ketones can be processed over an acidic H-ZSM5 zeolite at atmospheric pressure to yield 40% of C6+ aromatic gasoline components. Ketones can be upgraded to larger hydrocarbon compounds (C8-C12) with low extents of branching by means of aldol-condensation reactions over bifunctional Cu/ Mg10Al7Ox catalysts. However, carboxylic acids present in the organic stream caused deactivation of the basic sites responsible for aldol-condensation, and ap­proaches based on upstream removal of acids by ketonization (similar to those proposed herein for bio-oils upgrading, Section 4.2) and subsequent aldolcondensation have been successfully developed. [120-122] Ketoni — zation acquires special relevance when the organic stream is rich in car­boxylic acids, as is the case when the feed is glucose.