Lactic acid (2-hydroxypropanoic acid) is the most widely occurring car­boxylic acid in nature. Annual production reaches 120,000 tons per year, 90% of which is produced by bacterial fermentation of biomass sugars [25], including pentoses [26]. The bacterial route affords lactic acid in high yields (e. g. 90%) although it possesses important drawbacks namely low reaction rates and troublesome separation/purification from the re­action broth of the lactic acid product. Lactic acid is obtained in form of calcium salt, and subsequent neutralization generates large amounts of residual CaSO4 (1 kg per kg of lactic acid) which raises production costs and produces a waste disposal problem. Recently, a non-biological route for the conversion of aqueous sugars into lactic acid based on eas­ily separable and recyclable solid zeolites has been developed [27]. This promising technology opens the possibility of producing lactic acid from biomass sugars at more competitive prices in near future thereby consider­ably increasing the platform potential of lactic acid. The classical market for lactic acid involves food and food-related applications; however, the development and commercialization of new applications in the field of polymers and chemicals has caused steady expansion of the lactic acid market since the early 1990s [28].

Lactic acid possesses a rich chemistry based on its two functional groups (e. g. single bondOH and single bondCOOH). Thus, a variety of transformations to useful compounds such as acetaldehyde [29] (via de — carbonylation/decarboxylation), acrylic acid [30] (via dehydration), pro­panoic acid [31] (via reduction), 2,3-pentanedione [32] (via condensation) and polylactic acid (PLA) [33] (via self-esterification to dilactide and subsequent polymerization) has been described. All these transformations convert lactic acid in an attractive feedstock for the renewable chemicals industry [34].

Lactic acid, with its two adjacent functional groups concentrated in a small molecule of three carbon atoms, can be considered as a proto­type of an over-functionalized biomass-derived molecule. This chemical structure determines its high reactivity as well as its natural tendency to decompose with temperature [35]. As indicated in the Introduction, an ef­fective approach for the conversion of biomass derivatives into advanced biofuels involves a requisite oxygen removal step that helps to reduce re­activity leaving molecule more amenable for subsequent Csingle bondC coupling upgrading processes. Following this approach, lactic acid can be converted into hydrophobic C4-C7 alcohols suitable as high energy density gasoline-compatible liquid fuels for the transportation sector (Fig. 2) [31]. In this scheme, lactic acid is first deoxygenated to generate two reactive intermediates, namely propanoic acid and acetaldehyde, by means of dehydration-hydrogenation and decarbonylation/decarboxyl — ation processes, respectively. These intermediates were detected at low lactic acid conversions indicating that they are primary products in the synthesis [36]. Importantly, these intermediates are less reactive than lactic acid but still preserve oxygen functionality for subsequent Csingle bondC coupling upgrading. Thus, acetaldehyde can undergo self-coupling by aldol-condensation to generate butanal (after hydrogenation of the cor­responding C4 unsaturated aldol-adduct), whereas propanoic acid is self­coupled into 3-pentanone via ketonization. As shown in Fig. 2, successive aldol condensations between acetaldehyde (which is present in the reactor in high amounts) and butanal and 3-pentanone products generate C6 and C7 ketones. There are several important aspects of this process: (i) lactic acid is processed solved in water which the classical medium in which this molecule is obtained after microbial fermentation of sugars; (ii) the num­ber of reactions leading from lactic acid to C4-C7 carbonyl compounds can be carried out in a single reactor by employing a multifunctional and water-stable Pt/Nb2O5 catalyst in which niobic support plays a crucial role catalyzing dehydration, decarboxylation/decarbonylation and Csingle bondC coupling reactions; and (iii) C4-C7 carbonyl compounds (precur­sors of the corresponding alcohols by simple hydrogenation) are stored in a spontaneously separating from water organic layer accounting for 50% of the carbon in the lactic acid feed.