# Role of Ion-Exchange Resins in Hydrogen Reactions
## 2 Ion-Exchange Resins as Metal Catalysts Supports
The most favored hyper-crosslinkedPd/ST-DVB resin in that work was more active than the tested Pd/C catalyst, despite its larger metal nanoparticles [66]. With regards to the effect that the resin functionality can exert on the preparation of metal-doped IERs, [PERSON] and collaborators focused on the synthesis of Pd catalysts over two commercial gel-type resins (i.e., DowexTM 50W \(\times\) 2 and DowexTM 1 \(\times\) 2, that is a cationic resin with sulfonic groups and an anionic resin with trimethylbenzyl ammonium groups, respectively, both being ST-DVB resins with 2% crosslinking) [67]. By simply contacting the resins with aqueous solutions of palladium (II) nitrate (for the cationic resin) and potassium tetrachloropalladate (II) (for the anionic one) they obtained 60% metalation yields, reaching final Pd content of 1.1-5.1 wt.%. Three different approaches were tested regarding the activation of the catalysts: (1) reduction with aqueous NaBH\({}_{4}\), (2) reduction with H\({}_{2}\) gas, or (3) no pre-reduction of the catalyst at all, but, simply, allowing PdII to undergo reduction to PdO under reaction conditions. Observed Pd nanoparticles ranged 2.2-6.0 nm according to TEM, XRD, and small-angle X-ray scattering (SAXS), and reaction conversions as high as about 90% were obtained. Other factors investigated in that work were the solvent effect, IER bead size, and H\({}_{2}\) pressure. Regarding the solvent effect, it was seen that irrespectively of the substrate or the catalyst preparation method, the presence of water in the reaction solution invariably caused a decrease in catalyst activity, a loss of productivity upon reuse, and significant amounts of palladium leached, compared to pure methanol. Smaller bead sizes invariably yielded higher productivity, which entails resistance to internal mass transfer. With respect to H\({}_{2}\) pressure, conversion was higher at higher pressures (in the range of 1-8 bar), reaching a plateau, and selectivity towards fully hydrogenated species was promoted [67]. Concerning the catalytic activity for C-C coupling reactions, [PERSON] and colleagues studied Pd-doped IERs as catalysts for Suzuki cross-coupling reactions [68]. They prepared Pd-supported resins from the following commercial macrorecular resins: Ambersep(r) GT74 (ST-DVB with thiol (-SH) groups), Lewatit(r) K2629 (ST-DVB with sulfonic groups), and Lewatit(r) MP500 OH (ST-DVB with quaternary amine (-CH\({}_{2}\)N(CH\({}_{3}\))\({}_{3}\)OH) groups). Results were compared to AmbersipTM CH28, which is a commercial, macrorecular Pd-doped ST-DVB resin with sulfonic groups. They reported nanoparticles with average sizes of 1.34 nm (Ambersep(r) GT74, with a Pd load of 0.11 wt.%), 2.42 nm (Lewatit(r) K2629, Pd 0.08 wt.%), and 2.59 nm (Lewatit(r) MP500 OH, Pd 0.11 wt.%), which contrast with the commercial AmbersipTM CH28 (Pd \(\geq\)0.70 wt.%), with particles of ca. 7 nm, which the authors attribute to its higher metal loading. It was claimed that the metal size differences between the prepared catalysts are related to the stabilization strength of their functionalities, being stronger for smaller nanoparticles. Pd leaching, which was low for all catalysts, also seems to be related to the resin functionality, since the thiolic resin (Ambersep(r) GT74) yielded no leaching, Lewatit(r) MP500 OH produced a 1.1% leaching, and Lewatit(r) K2629 showed a 4.8% leaching [68]. At this point, it is worth mentioning the work by [PERSON] and coworkers [69]. Yet aiming at a different goal compared to other references contained in the present review (that is, they pursued the preparation of Pd/C, Pt/C, and Ni/C catalysts, after carbonization of metal-doped IER), high metal loadings (>10 wt.%) with small metal particle sizes (2.7-3.6 nm) were reported using commercial weakly basic and acidic IERs (i.e., DiaionTM WA30 and DiaionTM WK11, respectively).