# Role of Ion-Exchange Resins in Hydrogen Reactions
## 4 Concluding Remarks and Future Outlook  
As can be deducted throughout the work, numerous IER applications exist as catalyst support for metal-catalyzed hydrogenation reactions, making it a wide, vibrant field, with both a solid trajectory and a promising future prospective. Extensive work has already been conducted regarding metal dispersion over IERs, with multiple data being published regarding metal precursor salts, solvents, and conditions for the ion exchange between the resin functional groups and the metal ions. Likewise, and strongly dependent on the final application of the metal/IER catalyst, different procedures to activate the metal, that is, to reduce the metal ion to its zero-valence species, can be retrieved from the covered literature, with some works reporting that the reduction protocol would affect the metal distribution within the resin beads. Both gel-type and macroreticular, basic or acid (yet, with acidic resins being the most widely used ones), IERs have been used for many metal-catalyzed hydrogenation processes, either in the gas and the liquid phase, including hydrogenation of alkenes, alkynes, carbonyls, arenes, nitroaromatics, and nitrates, among others. Of particular interest are the one-pot multistep synthesis processes including hydrogenation, together with hydration, dehydration, condensation, or isomerization reactions, covered in Section 3.7, since they constitute a particular type of application of IERs in the field of hydrogenation, not only as metal supports but also as acid sites-containing catalysts. Such technology allows for the integration and optimization of processes and is encompassed with improved sustainability. In this regard, current trends aiming at the transformation of biomass derivatives into high-value-added chemicals match the potential use of metal-doped IERs as bifunctional catalysts to obtain industrially acceptable target product yields under mild conditions. Even more so when, according to reported data, metal species leaching after extended use of metal-doped IER catalysts does not appear to be a cause for concern. Another aspect worth mentioning that can be inferred from data provided in Tables 3-7 is the fact that mild operating conditions (that is, temperatures in the range of room temperature to 150 \({}^{\circ}\)C and H\({}_{2}\) pressures typically below 40 bar) are needed to obtain high yields towards desired products, which contrasts with the more demanding operating conditions usually associated to inorganic supports. This adds to the fact that the metal loading in IERs is usually achieved by simple ion exchange at room temperature, without any thermal-intensive processes such as calcination. Therefore, lower energy-demanding operations are involved in both the manufacturing and the utilization of metal-doped IERs as catalysts when compared to catalysis over inorganic supports. In addition, another potential benefit of using IERs would be related to their effect on critical parameters for the catalytic behavior of these materials, such as metal nanoparticles size as well as nature and number of defects, since IERs offer distinct microenvironments surrounding the nanoparticles that can boost their effectiveness for a given reaction. As a matter of fact, the advantages and limitations of using IERs in this field could be summarized as follows: on the one hand, IERs combine hydrophilic and hydrophobic regions, which may provide unique microenvironments to metal nanoparticles and/or promote specific interactions of interest between reactants and the polymer material, ultimately affecting reaction selectivity for a given application. In addition, and depending on the IER composition (e.g., DVB content) and morphology, a molecular sieve action can be in effect when using IERs which also would impact reaction selectivity when steric effects are in place.