# Role of Ion-Exchange Resins in Hydrogen Reactions
## 3 Ion-Exchange Resins Application to Hydrogenation Reactions
### Hydrogenation of Alkenes  
Four Pd-doped (0.25-0.45 wt.%), sulfonated, synthetic IERs were tested in the liquid-phase hydrogenation of cyclohexene to cyclohexane and cyclohexen-1-one to cyclohexanone by [PERSON] and coworkers [44]. All resins were exchanged with palladium acetate, and Pd\({}^{\text{II}}\) was reduced to Pd\({}^{0}\) with NaBH\({}_{4}\) in ethanol. Depending on their composition, the solvent affinities of the resins spanned from moderately hydrophobic to clearly hydrophilic--the lower the ST amount in the resin, the higher its hydrophilicity. No effect of the hydrophobic/hydrophilic character of the resin was observed on the reported catalytic activity. Interestingly, resins with more nitrogen atoms in their structures (i.e., those based on \(N\),\(N^{\prime}\)-methylene(bis)acrylamide and \(N\),\(N\)-dimethylacrylamide) were found to be the most active ones, with rates increasing linearly with the nitrogen/Pd molar ratio, and no metal leaching was observed in any case. The role of the gel-type IERs properties in the hydrogenation of the C=C bonds of maleic and fumaric acids, viz. the _cis_ and _trans_ isomers of butenedioic acid, was investigatedin [PERSON] et al. (2008) [78]. They synthesized Pd-doped (0.25-2.0 wt.%, 2-3 nm) acrylic resins with crosslinking degrees of 3 and 10%. The observed swelling of the polymer mass during the hydrogenation was linked to the catalytic activity: the same hydrogenation rates were observed for both maleic and fumaric acids in the absence of swelling, while the hydrogenation rate of maleic acid was much higher than that of fumaric acid in the swollen state. Interestingly, a \"bell-like\" dependence between initial rates and metal content was found, with maximum activity at 0.5-1.0 wt.% Pd content. Among the reactions tested in [PERSON] et al. (2012), hydrogenation of the C=C bond in methyl 2-acetamidoacrylate and in _trans_-4-phenyl-3-buten-2-one was studied over Pd-doped, gel-type, cationic and anionic resins [67]. Reported conversions were 91.7% and 98.2%, respectively, with selectivities of 100% and 83.7%, over lithiated Pd/DowexTM 50W \(\times\) 2 resin without any separated pre-reduction step of the ionic PdII to PdO. Remarkably, neither activity loss upon recycling of the catalyst nor Pd leaching was observed and selectivity was unaffected after 6 cycles using methanol as the reaction solvent. [PERSON] and associates hydrogenated fatty acid methyl esters (i.e., methyl undecenoate, methyl oleate, methyl ricinoleate, and methyl linoleate) and vegetable oil-based compounds (i.e., glycerol trioleate, castor oil, jojoba oil, olive oil, and very high-oleic sunflower oil) over Ru/DowexTM 50W \(\times\) 2 under mild conditions (30 \({}^{\circ}\)C and 10 bar H\({}_{2}\)) [79]. Regarding the metal impregnation procedure (consisting of ion exchange with an aqueous metal salt solution followed by NaBH\({}_{4}\) reduction), the authors indicate that the resin must be in the Na+ form to improve metalation yield and to avoid Ru leaching due to the presence of acidic species. According to their results, the catalytic activity of the catalysts was higher for shorter hydrocarbon chain length: hydrogenation of methyl undecenoate (C\({}_{12}\)) was faster than methyl linoleate (C\({}_{19}\)), showing 100% conversion in 60 min in front of about 50% conversion in 210 min, respectively. Moreover, the double bond in the larger unsaturated esters was located in an internal position, which hindered the reaction rate [79]. In a different approach, a lithiated DowexTM 50W \(\times\) 2 resin was used to immobilize phosphine-rho complexes to catalyze the asymmetric hydrogenation of prochiral olefins, such as methyl 2-acetamidoacrylate, under mild conditions (room temperature, under 1-5 bar H\({}_{2}\)) [80; 81; 82]. Reported yields mount up to 99.9%, even in the second recycle, with a detected Rh leaching of 2.0% [80]. Hydrogenation of cyclohexene in supercritical CO\({}_{2}\) over metal-doped AmberlystTM 15 was studied by [PERSON] and associates [83].