## 1 Introduction
Quite recently, [PERSON], [PERSON] and co-workers have described an efficient heterogeneous catalyst, consisting of inexpensive ferrite-nickel magnetic nanoparticles (Fe\({}_{3}\)O\({}_{4}\)-Ni MNPs), for the transfer hydrogenation of carbonyl compounds in glycerol (Scheme 5) [52]. This system (8.85 mol% of Ni), associated with KOH (2 equivalents), was able to convert selectively benzaldehyde, different acetophenone derivatives and cyclohexanone to the corresponding alcohols in good to excellent yields after heating the reaction mixtures at 80 \({}^{\circ}\)C for 2-2.5 h. Remarkably, unlike the examples discussed above, no differences in reactivity between aldehyde and ketone functionalities were observed in this case. Figure 2: Structure NHC-based iridium catalysts **2–5**. **Scheme 5. ** TH of carbonyl compounds in glycerol using heterogeneous Fe\({}_{3}\)O\({}_{4}\)-Ni magnetic nanoparticles (MNPs). \( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig1}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig2}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig3}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig4}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig5}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig5}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig6}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig5}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig6}\end{array} \)\( \begin{array}{c}\includegraphics[width=142.26378 pt]{Fig5}\end{array} \)\(\begin{array}{c}\includegraphics[width=142. KOH). As in the preceding cases, conversion of glycerol to dihydroxyacetone was proposed without experimental confirmation. **Scheme 7. ** Ir-catalyzed transfer hydrogenation of \(\alpha\),\(\beta\)-unsaturated carbonyl compounds in glycerol. Reduction of the C=C bond of allylic alcohols in both pharmaceutical (99.5% purity) and technical (87% purity) grade glycerol has been described employing a family of arene-ruthenium(II) complexes [RuCl\({}_{2}\)(\(\eta^{6}\)-arene)L] (arene = C\({}_{6}\)H\({}_{6}\), \(p\)-cymene, mesitylene, C\({}_{6}\)Me\({}_{6}\); L = hydrophilic phosphine ligand) as catalysts [54]. Among them, the best results were obtained with [RuCl\({}_{2}\)(\(\eta^{6}\)-C\({}_{6}\)H\({}_{6}\))(DAPTA)] (**6**; DAPTA = 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane), which associated with KOH was able to generate the desired saturated alcohols in high yields (up to 90% after 6-15 h of heating at 100 \({}^{\circ}\)C) regardless of the purity of the solvent employed (Scheme 8). We must note that this reduction process does not involve a direct transfer of hydrogen from glycerol to the C=C bond of the substrate, but the initial redox-isomerization of the allylic alcohol through C=C bond migration and subsequent transfer hydrogenation of the resulting carbonyl compound. Remarkably, contrary to previous studies carried out in water and 2-propanol [55,56], the use of glycerol as the reaction medium enabled the recycling of the catalyst (up to four consecutive runs). **Scheme 8. ** Ruthenium-catalyzed reduction of allylic alcohols in glycerol. In addition to these examples, reduction of linoleic to oleic acid was found to occur during the hydrolysis of soybean oil and tallow using nickel catalysts supported on alumina, effect that was best manifested under 3 h of reaction at 250-270 \({}^{\circ}\)C with NiO/Al\({}_{2}\)O\({}_{3}\) as catalyst (Scheme 9) [57]. The glycerol formed during the hydrolysis of the triglycerides behaves as the hydrogen donor, allowing the reduction of the _in-situ_ produced di-unsaturated fatty acid into the mono-unsaturated one. **Scheme 9. ** Transformation of linoleic acid to oleic acid by catalytic TH from glycerol.