# Role of Ion-Exchange Resins in Hydrogen Reactions
## 2 Ion-Exchange Resins as Metal Catalysts Supports  
Regarding the use of IERs as supports for metal catalysts, several works published in the early 2000s by [PERSON], [PERSON], and coworkers need to be highlighted, since they offer a detailed overview assessing the morphological implications of introducing metal nanoparticles into the resins backbone [8; 9; 25; 27; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56]. Mostly, acrylic gel-type resins were used in those works, but macroreticular ST-DVB resins are also considered. For instance, [PERSON] et al. (2000a) and [PERSON] et al. (2004) discussed the generation of size-controlled palladium nanoclusters inside gel-type resins [41; 47], whereas the use of macroreticular resins is studied in [PERSON] et al. (2000) and [PERSON] et al. (2000b) [42; 43]. Notably, the issue of metal nanoclusters size is comprehensibly discussed in [PERSON] et al. (2008) [57]. [PERSON] and coworkers fixed Pd\({}^{\text{II}}\) to acrylic, mildly crosslinked, gel-type resins by contacting them with palladium acetate in a tetrahydrofuranwater solution, reaching metalation yields of about 90% [41]. Reduction of the metal was accomplished by exposure to sodium borohydride (NaBH\({}_{4}\)) in ethanol. Through this procedure, they obtained 2-4 nm Pd crystallites that were homogeneously distributed in the resins. They concluded that the average size and size distribution of the generated Pd nanoclusters was independent of the metal content and of the crosslinking degree of the resins. The explanation given to their results was linked to the relatively low polymer chain density of the employed resins at which the nucleation and growth of the metal nanoclusters preferentially occur, giving rise to nanoparticles matching the dimensions of the nanometer-sized cavities inside the swollen polymer domains upon swelling [41]. From this, it follows that the resin-swollen polymer morphology can in fact act as a template to generate size-controlled metal nanoclusters suitable for the synthesis of resin-supported metal catalysts, since metal clusters would be blocked inside the largest available polymer cavities and, consequently, metal aggregates would not be able to continue growing. Such an approach was named Template Controlled Synthesis (TCS) approach (sketched in Figure 1), and it was confirmed by [PERSON] et al. (2004) [47] using three independent morphological analyses: transmission electron microscopy (TEM), X-ray diffraction (XRD) and Inverse Steric Exclusion Chromatography (ISEC). With respect to macroreticular resins, [PERSON] et al. (2000b), used commercial Lewati\({}^{\text{\textregistered}}\) SPC 118 and UCP 118 to support Pd [42]. Textural features of the resins were determined by ISEC, revealing that the polymer mass was largely distributed in dense fractions. TEM of the obtained Pd/resins showed a heterogeneous distribution of Pd crystallites, appearing to be preferentially located either at the macropores surface or in the gel-type surface layer covering the polymer. Finally, CO chemisorption indicated that, in the dry state, Pd sites were practically inaccessible. From those results, the authors concluded that metalationoccurs both at the macroproces surface and inside the highly crosslinked polymer mass, but while NaBH\({}_{4}\) reduction in the macroproces surface is fast, it would be comparably slower inside the polymer denser fractions. Accordingly, some differences in Pd\({}^{0}\) concentration could exist within the resin which, ultimately, would act as a gradient to promote the migration of Pd\({}^{\rm II}\) from the internal parts of the polymer towards the macroproces surface by coalescence. The final consequence would be that the crystallites are located in well-defined gel-type domains at the macroproce surface, which become accessible only in the swollen state [42].