# Role of Ion-Exchange Resins in Hydrogen Reactions
## 3 Ion-Exchange Resins Application to Hydrogenation Reactions
### Hydrogenation of Carbonyl Compounds  
Pd-doped, highly lipophilic gel-type IERs, based on styrene or dodecyl methacrylate, were used as catalysts for the hydrogenation of 2-ethylanthraquinone to 2-ethylanthrahydroquinone, which involves the consecutive hydrogations of C=O and C=C bonds, in both [PERSON] et al. (2002) and [PERSON] et al. (2003). Reported selectivity for the tested ST-DVB supported catalyst was 50 and 65%, depending on the reducing step procedure (H\({}_{2}\) or NaBH\({}_{4}\), respectively), and catalytic activity was clearly lower than a benchmark industrial catalyst (Pd/silicoaluminate). On the other hand, while activity was also low, the resins based on dodecyl methacrylate reached selectivity values as high as 98%, which were slightly above the commercial alternative at the time [45; 90]. Hydrogation of citral (3,7-dimethyl-2,6-ocatdeniol) to geraniol (_trans_-3,7-dimethyl-2,6-ocatdeniol) and nerol (_cis_-3,7-dimethyl-2,6-ocatdeniol) was accomplished by [PERSON] and coworkers over Pt (0.43-2.5 wt.%) catalysts supported on two sets of functional gel-type resins synthesized by the authors, as well as a commercial 4-vinylpyridine (VP) polymer crosslinked with 2 mol% of DVB [50]. The first set of resins contained \(N\),_N_-dimethyl-2-aminoethylmethacrylate (DMAEMA), cyanoethyl-acrylate (CEA) or methacrylic acid (MAA) as the functional monomer, and DVB as the crosslinker, while the second one contained either CEA or MAA, as the functional monomer, together with \(N\),_N_-dimethylacrylamide (DMAA) and DVB. In addition, two different approaches to incorporate Pt into the resins were adopted: one based on the impregnation of the resin with mesitylene solutions of colloidal platinum, and another one involving immobilization of Pt precursors in pre-swollen resins, followed by chemical reduction. In addition, some of the tested resins were further modified by incorporation of a second metal, acting as Lewis acids (i.e., the introduction of 6.2-15.2 wt.% FeII, CoII, or ZnII in Pt-containing resins with -COOH groups). The authors state that improved selectivity can be achieved thanks to the presence of a more electropositive metal than the noble one because it would release electronic density to the noble active metal, which ultimately hinders C=C hydrogenation and favors that of C=O bonds. Reported conversions span from relatively low values (20-30%) to \(\geq\)80-90%, depending on the tested catalyst, with selectivity values reaching values above 90%, particularly with the bimetallic catalysts. The C=O bond in methyl benzoyformate (methyl 2-oxo-2-phenylacetate) and 2,2,2-trifluoracetophenone (2,2,2-trifluoro-1-phenylethan-1-one) was hydrogenated by [PERSON] and coworkers over a lithiated Pd/DowexTM 50W \(\times\) 2 IER, yielding selectivity values over 99.5% at acceptably high conversion levels (_ca._ 90% and 62%) when no pre-reduction of the PdII species supported on the IER was conducted [67]. Another noteworthy application of IERs in the field of carbonyl hydrogenation is that of [PERSON] and associates, using DowexTM 50W \(\times\) 2 resin as support for immobilizing an Ir complex. Their catalyst was used for the hydrogenation reactions of a number of substrates, including several mines, the \(\alpha\)-keto ester dihydro-4,4-dimethyl-2,3-furandione, and terpenes like (\(R\))-carvone, with similar results to homogeneous catalysts [91]. On the other hand, Pt-doped hyper-crosslinked polystyrene modified with cinchonidine was used as the catalyst for enantioselective hydrogenation of the C=O bond in ethylpyruvate into (\(R\))-ethyllactate and (\(S\))-ethyllactate in [PERSON] et al. (2009) [92]. The authors found that enantioselectivity of activated ketone hydrogenation with platinated hyper-crosslinked polystyrene depends on the solvent, reducing agent, temperature, substrate, catalyst, and the modifier concentrations.