Glycerol (1,2,3-propanetriol) is a water-soluble biomass-derived polyol with versatile chemical reactivity. [94,96,97] Approximately 100 kg of glycerol per ton of biodiesel in the form of concentrated aqueous solutions (80 wt%) are annually produced worldwide in the biodiesel industry as a byproduct of the transesterification of vegetable oils and animal fats with methanol. [98] The high number of applications of glycerol in different fields such as cosmetics, pharmaceuticals, foods, and drinks cannot ab­sorb the surplus of this compound created by the biodiesel industry. Extra glycerol could be consumed as a fuel in internal combustion engines but, unlike ethanol, glycerol cannot be used blended with gasoline because of its low solubility in hydrocarbons and high viscosity. Furthermore, the purification of crude aqueous glycerol for chemical purposes is costly and energy-consuming. Consequently, new technologies, with potential to up­grade aqueous solutions of glycerol, would be valuable. In this respect, a promising route for glycerol conversion involves the production of syn­gas through aqueous-phase reforming (APR) processes. [99] By means of this route, concentrated aqueous solutions of glycerol can be converted to gaseous H2, CO and CO2 mixtures over supported metal catalysts at mod­erate temperatures (498-548 K). Platinum is the preferred metal for this conversion because it favors C-C cleavage reactions (leading to CO, H2 and CO2) versus C-O cleavage reactions (leading to light hydrocarbons). [73,100] To selectively produce syngas versus CO2-enriched mixtures, re­action conditions and catalytic materials must be selected to control WGS reactions. Thus, by operating with inert materials like carbon as a support (instead of using inorganic oxide supports that can activate water), low total pressures (to avoid high partial pressures of water), and high glycerol concentrations (allowing water to become the limiting reagent in WGS), syngas streams with adequate H2/CO ratios for Fischer-Tropsch synthesis can be produced from aqueous glycerol over Pt/C catalysts. [99]

To allow for coupling of endothermic glycerol reforming with exo­thermic Fischer-Tropsch synthesis in a single reactor, it is crucial that the former process is carried out efficiently at low temperatures. However, under these conditions, the metallic surface of Pt is likely to be saturated with adsorbed CO, [101] thus decreasing the overall catalytic rate. One effective strategy to overcome this limitation involves the utilization of alloys such as Pt-Re and Pt-Ru, on which the strength of adsorption of CO is lower compared to Pt. [102,103] This new route involving low — temperature gasification of aqueous glycerol to syngas integrated with Fischer-Tropsch synthesis represents an interesting alternative to complex BTL approaches (Section 4.1). Thus, unlike biomass gasification, glycerol reforming can be carried out at temperatures within the range employed for Fischer-Tropsch synthesis, thereby allowing effective integration of both processes in a single reactor [104] with improved thermal efficiency (since heat required for endothermic reforming is provided by exother­mic F-T process). Furthermore, concentrated aqueous solutions of glyc­erol (as produced in biodiesel facilities) can be converted to undiluted and impurity-free syngas, thereby eliminating the need for large gasifiers with oxygen-production plants and expensive gas-cleaning units. Thus, unlike BTL, this aqueous-phase route allows for costcompetitive operations at small scale, which is advantageous for the processing of distributed bio­mass resources.