# Role of Ion-Exchange Resins in Hydrogen Reactions
## 2 Ion-Exchange Resins as Metal Catalysts Supports
A similar idea is briefly mentioned by [PERSON] and [PERSON] (2019), who simply stated that, depending on the pre-surface area and the bulk structure of the IER, the metal could migrate from the polymer to the pores after the reduction of the catalyst [13]. In any case, the fact that metal nanoparticles are heterogeneously distributed within macroreticular IERs compared to the homogeneous distributions observed in gel-type resins is a revealing finding to highlight. The same commercial macroreticular resin (i.e., UCP 118, in different forms: without any metal, in both acidic and Na\({}^{+}\) forms, and with Pd in Na\({}^{+}\) form) was analyzed by electron spin resonance (ESR) and pulse field gradient spin echo-nuclear magnetic resonance (PSSE-NMR) techniques to assess their molecular accessibility to 2,2,6,6-tetramethyl-4-oxo-1-oxyppiperidine (TEMPONE) dissolved in water in [PERSON] et al. (2000) [43]. The authors observed that both water and TEMPONE can penetrate inside the swollen resin, including the macro- and microporous domains, regardless of the Pd presence. In a different study, [PERSON] and coworkers used both gel-type and macroreticular IERs as supports for either Au or Pd nanoclusters and tested those catalysts in several reactions [48; 51; 52]. For instance, Pd\({}^{\rm II}\) and Au\({}^{\rm II}\) were linked onto an acrylic, moderately crosslinked, gel-type resin, obtaining well-sized-controlled Au\({}^{0}\) (2.2 nm) and Pd\({}^{0}\) (2.3 nm) nanoclusters, as expected according to their TCS approach (Figure 1). The Au\({}^{0}\)/IER catalyst was tested in the oxidation of pentanal to pentanoic acid by molecular oxygen under mild conditions, revealing itself as twice as active as classic Au\({}^{0}\)/C catalysts [51]. Later on, other acrylic resins were used following the same approach with success, and the catalytic tests revealed that the synthesized Au\({}^{0}\) catalyst supported on a very hydrophilic resin--i.e., poly-(4-vinylpyridine-acrylic acid-\(N\),\(N^{\prime}\)-methylenebisacrylamide)--was particularly promising for the selective oxidation of \(n\)-butanol to \(n\)-butanol [48]. The direct synthesis of hydrogen peroxide from molecular hydrogen and oxygen was tested with two acidic Pd\({}^{0}\)/IER(H\({}^{+}\)) and one non-acidic Pd\({}^{0}\)-Au\({}^{0}\)/IER catalysts, based on a commercial, macroreticular cation exchange resin (i.e., Lewati\({}^{\rm\tiny\textregistered}\) K2621/H\({}^{+}\)) and two gel-type resins with -SO\({}_{3}\)H and -SMe functional groups [52]. Interestingly, the Pd-Au nanoalloy supported on a hydrophilic resin prepared in this work was an effective catalyst for hydrogen peroxide synthesis. In this line, [PERSON] and coworkers prepared bimetallic Pd-Au (Pd: 1.0 wt.%; Au: 0.25-1.0 wt.%) and Pd-Pt (Pd: 1.0 wt.%; Pt: 0.1-1.0 wt.%) catalysts over commercial Lewati\({}^{\rm\tiny\textregistered}\) K2621 by simple ion-exchange in water and reduction with aqueous formaldehyde, which they tested in the direct synthesis of H\({}_{2}\)O\({}_{2}\)[58]. Reported nanoparticle distributions varied depending on the catalyst composition, with average sizes of ca. 7 nm and 11 nm. Later on,  
Figure 1: Schematic representation of the Template Controlled Synthesis (TCS) approach for a gel-type (top) and a macroreticular (bottom) IER. Inspired in [27; 41; 42]. through X-ray absorption fine structure spectroscopy (EXAFS) under reaction conditions for the same synthesis, [PERSON] et al. (2015) observed that their Pd/K2621 catalyst was practically neither affected by the reaction medium (methanol) nor by the reaction mixture (CO\({}_{2}\)/H\({}_{2}\)/O\({}_{2}\)), but that some of the PdO contained in the resin underwent reduction to metal Pd and some leaching in the presence of bromide ions [59]. The effect of adding Au to Pt/IER catalysts was studied by [PERSON] and associates, who used Diaion(tm) WA30, a macroporous weak anion exchange resin with tertiary amine functional groups in an ST-DVB polymer matrix, as the support [60].