# Role of Ion-Exchange Resins in Hydrogen Reactions
## 1 Introduction  
To date, the role of ion-exchange resins (IERs) as catalysts for a number of industrial applications is a subject that has already been covered in great extension (e.g., [1; 2; 3; 4; 5; 6; 7]). Likewise, the role of IERs as support materials for metal-doped catalysts for hydrogenation reactions is not a new field either, since very complete reviews dating back more than twenty years ago can be found in the available literature, such as those by [PERSON] and [PERSON] (2000, 2001) [8; 9]. However, scarce relevant works can be found in more recent times comprehensively dealing with the use of metal-doped IERs specifically as catalysts for hydrogenation reactions. Noteworthy, a very complete review dealing with the use of polymeric materials as supports for a variety of reagents can be found in [PERSON] et al. (2006) [10]. In particular, Section 3.1 of that work offers an overview of a number of relevant aspects regarding the properties of the different types of polymeric backbones that can be used in this field. Regarding IER, a remarkable whole section (that is, Section 10 of that work) is devoted to the use of Nafion(tm) as support for metal catalysts, but little reference is made to metal-doped IERs of other types. Scarce mentions to the role of IERs as catalysts for several one-pot synthesis processes can be found in [PERSON] et al. (2011, 2014) [11; 12]. Furthermore, two reviews authored by [PERSON] and [PERSON] were published recently [13; 14], where some works can be found cited therein regarding the use of IERs in different hydrogations reactions, together with references related to the use of polymeric frameworks different from IERs as supports for metal catalysts (e.g., [15; 16; 17; 18]). Table 1 lists some review paperscovering different aspects related to this field. At this point, it is worth mentioning that previous remarkable works (e.g., [19; 20; 21; 22; 23]) have not been explicitly included in the present review since, for the sake of brevity, it is limited to references dating back to the early 2000s (albeit a few exceptions have been made in Table 2). Thus, the present work focuses on the use of IERs as catalysts, more precisely, as catalyst supports for hydrogenation reactions. A brief introduction is conducted in Section 1, which includes some generalities regarding IERs types and applications. Section 2 is focused on relevant morphological implications stemming from the introduction of metal nanoparticles into polymeric frameworks. Section 3, which is devoted to particular hydrogenation types applications, has been divided into several subsections, containing specific processes where IERs have been used as support for metal catalysts. Finally, overall concluding remarks and future scope are presented in Section 4. Ion-Exchange Resins: Main Types and ApplicationsIERs are insoluble, organic materials consisting of a polymeric, amorphous backbone with a hydrophobic character with evenly distributed hydrophilic functional groups. Depending on their application, IER can present a wide variety of functional groups: acid (or cationic) resins usually contain sulfonic groups (-SO\({}_{3}\)H), carboxylic groups (-COOH), or phenolic groups (C\({}_{6}\)H\({}_{4}\)OH) that can exchange cations in solution with H\({}^{+}\) ions, whereas basic (or anionic) resins can contain various amino groups (-NH\({}_{3}\), =NH\({}_{2}\), =NH) or quaternary substituted ammonium, being able to promote anions exchange with OH\({}^{-}\) ions [31]. IERs polymeric backbone, which is a crosslinked copolymer, consists of an irregular three-dimensional matrix of hydrocarbon chains. Typically, polymeric backbones can be based on perfluorosulfonic acid (i.e., Nafion\({}^{\text{TM}}\) resins), on polymethacrylate, or on polystyrene, among other monomers [3; 32; 33].