One interesting option to in situ generate the required hydrogen for hydrogenolysis reac­tions is through aqueous phase reforming (APR). APR is a quite well known process in which a polyol is converted to hydrogen and CO2 in the presence of water. The hydrogen generated can be further used in the hydrogenolysis reaction. The specific case for combined glycerol APR and hydrogenolysis to 1,2-PDO is shown in Figure 14. If the process is perfect­ly balanced, glycerol is fully converted into 1,2-PDO, being CO2 and H2O the only byprod­uct. Tailored metal-acid bifunctional catalysts or combination of catalysts are required to obtain high yields to 1,2-PDO. Indeed, there must be a proper balance between the C-C bond cleavage reactions that lead to the production of hydrogen, and the C-O bond cleavage reactions that lead to the formation of PDOs [76]. While Pt is known to be active in C-C bond cleavage, its combination with other metals active in C-O bond hydrogenolysis, like Ni, Sn or Ru, over acidic supports appears as promising formulations to obtain high yields to 1,2-PDO [77]. However, glycerol APR itself runs at elevated pressure and therefore the advantage over conventional hydrogenolysis at high hydrogen pressure is marginal with re­gard to equipment and safety costs.

The same benefits that have been previously addressed for the use of in situ generated hy­drogen in glycerol hydrogenolysis can be applied to the conversion of other higher polyols, like sorbitol or xylitol. However, the considerable research effort that has been directed to the conversion of glycerol yet has not been paid to other biomass based polyols. Therefore, the amount of works related to high polyol hydrogenolysis with in situ generation of the re­quired hydrogen is quite scarce. As a consequence of this, it is a really interesting and open research field.

Huber et al. [78] studied the production of renewable alkanes (C1-C6) from the aqueous phase reforming of sorbitol using a Pt/SiO2-Al2O3catalyst. They suggested a multistep bifunc­tional reaction pathway. The first step involves the formation of CO2 and H2 on the Pt sites, and the dehydration of sorbitol on the acid sites of the silica-alumina support. These initial steps are followed by hydrogenation of the dehydrated reaction intermediates on the metal catalyst (Scheme 9). 64 % alkane selectivity at 92% sorbitol conversion were recorded at 498 K and 39.6 bar. When hydrogen was co-fed, alkane selectivity significantly increased up to 91%. Glucose showed to be less active than sorbitol over a Pt/Al2O3 catalyst at 538 K and 52.4 bar of N2pressure, achieving moderate alkane selectivities (49.5%) [79]. Therefore, it seems that initial hydrogenation of glucose to sorbitol and subsequent aqueous phase reforming of the sugar is more effective than direct aqueous phase reforming of glucose.