# Role of Ion-Exchange Resins in Hydrogen Reactions
## 3 Ion-Exchange Resins Application to Hydrogenation Reactions  
Hydrogenation can be defined as the incorporation of hydrogen atoms, either from gaseous H\({}_{2}\) or from other sources, to unsaturated compounds, that is compounds with double (C=C) or triple (C\(\equiv\)C) bonds, including those with carbonyl (C=O) or nitrile (C\(\equiv\)N) groups.  
The application of metal-doped IERs in hydrogenation reactions has been the focus of continuous attention for the research community with many relevant works published in the field. For instance, only in [PERSON] and [PERSON] (2000), the following hydrogenation reactions are mentioned: methyl isobutyl ketone (MIBK) production; hydrogenation of diolefins, acetylenes and carbonyl compounds from C4 hydrocarbon streams; ethrification-hydrogenation of mixtures of unsaturated hydrocarbons to give blends of alkanes and branched ethers for the manufacture of unleaded petrol (i.e., BP Etherol process [73; 74]); O\({}_{2}\) removal from industrial waters; hydrogenation of cyclohexene; hydrogenation of nitroaroomatics; hydrogenation of 2-ethylanthraquinone; and hydrogenation of nitrate and nitrite ions to ammonia [8]. In their work, [PERSON] and [PERSON] considered applications at temperatures ranging from room temperature to 120 \({}^{\circ}\)C, macroreticular and gel-type resins, and metals such as Pd, Ni, Cu, or Pt. They also provided details regarding both the dispersion of the metal in the IERs (i.e., metalation step) and the activation protocol (i.e., reduction step) of the metal-doped IERs, which have already been included to some extent in Section 2 of the present work.  
Further work can be found with respect to some of the quoted applications, such as cyclohexene hydrogenation into cyclohexane (e.g., [44]), hydrogenation of alkynes such as 2-butyne-1,4-diol and phenylacetylene (e.g., [75]), or hydrogenation of 2-ethylanthraquinone to 2-ethylanthraydroquinone (e.g., [45]). Other studies deal specifically with multistep reaction processes, such as the mentioned MIBK or, for instance, \(\gamma\)-valerolactone (GVL) production (e.g., [12; 29; 76; 77]), involving catalytic activity not only for hydrogenation because of the metal sites, but also for at least another reaction typically requiring acid/basic sites (e.g., hydrolysis, lactonization, esterification, cyclization, or condensation). Therefore, metal-doped IERs for such applications must present a bifunctional nature, incorporating active sites of different characteristics on the same support. Regarding reaction conditions, pressures ranging from 1-50 bar and temperatures as high as 150 \({}^{\circ}\)C can be found, depending on the application and IER used.  
A general overview suggests that the range of potential applications of metal-doped IERs in hydrogenation reactions is notoriously wide, as is the range of experimental conditions specifically required for a particular process. Aiming to scrutinize such scenarios and for the sake of clarity, the next sections have been divided into the following: (1) alkenes hydrogenation; (2) alkynes hydrogenation; (3) carbonyl hydrogenation; (4) substituted arenes hydrogenation; (5) hydrogenation of nitroaroomatic compounds to their corresponding amines; (6) reduction of nitrites and nitrates; (7) one-pot, multistep syntheses involving hydrogenation, together with other reactions (e.g., esterification, cyclization, condensation); and (8) other related reactions, including the direct synthesis of hydrogen peroxide from molecular oxygen.