# Role of Ion-Exchange Resins in Hydrogen Reactions
## 2 Ion-Exchange Resins as Metal Catalysts Supports
In previous work, they synthesized Pt/WA30 catalysts for the oxidation of glycerol to glyceric acid, showing high activity, selectivity, and stability [61]. Different preparation procedures were applied for the bimetallic catalysts, involving that either both Au and Pt were simultaneously exchanged with the resin (noted as PtAu samples), or that the exchange took place sequentially (and, in this case, Au-Pt catalyst denotes that Au was incorporated and reduced prior to incorporation of Pt; whereas, for Pt-Au catalysts, Pt was the first metal to be exchanged with the IER). In all cases, bimetallic catalysts formed an alloy, according to XPS results reported by the authors. Pt-Au catalysts showed similar behavior to monometallic Pt/WA30, but both PtAu and Au-Pt were more active. Regarding these two catalysts, Au-Pt samples were more active than PtAu samples with the same amount of metal, which was attributed to smaller metal nanoparticles (3.6 nm for Au-Pt in front of 5.4 nm) [60]. Pd (0.6-2.5 wt.%) and Pt (0.01-1.2 wt.%) were introduced in Nafion(tm)-silica composites and Amberlyst(tm) 15, aiming at the direct hydroxylation of benzene with O\({}_{2}\)/H\({}_{2}\) gas mixtures to produce phenol [62; 63]. Amberlyst(tm) 15 is a commercial, strongly-acidic macroreticular IER with an ST-DVB matrix, functionalized with sulfonic groups. Nafion(tm) counts with an inert polymeric backbone and highly acidic sulfonic groups, with hydrophobic (-CF\({}_{2}\)CF\({}_{2}\)-) and hydrophilic (-SO\({}_{3}\)H) regions. The influence of the solvent and the reduction protocol on the catalytic performance over the best catalyst (i.e., a Nafion(tm)-based one) was evaluated, and the best results were obtained for a 3:1 water:methanol solvent and a reduced sample at 150 \({}^{\circ}\)C, yet little influence of the reduction conditions was observed [63]. The catalyst could be recycled after methanol washing followed by reduction, but catalytic activity could not be fully restored, which was attributed to the formation of heavy compounds [62]. In [PERSON] et al. (2012), two hyper-crosslinked resins with permanent, accessible nanoporosity were prepared from a gel-type poly-chloromethylated styrene-divinylbenzene resin which, according to the authors, are promising supporting materials for metal catalysts [64]. The authors highlighted the importance of the solvent choice to be used in a catalyzed reaction, since the resin-solvent environment would strongly influence the mobility of the solute and, consequently, the effective reactivity. [PERSON] et al. (2013) investigated the same catalysts by elemental analysis, thermogravimetric analysis (TGA), SEM, X-ray microanalysis (XRMA), TEM, high-resolution TEM, selected area electron diffraction (SAED), nitrogen adsorption/desorption (BET analysis), ISEC, and specific absorption volume (SAV) measurements [65]. They evidenced that hyper-crosslinked resins present a permanent pore system comprising both microproes (<2.0 nm in diameter) and mesopores (2.2 nm), and that the metal nanoparticles had different sizes distribution all in the 1-6 nm range. Interestingly, they found that when formaldehyde was used as a reducing agent, more than 90% of the nanoparticles were less than 3 nm and that their radial distribution throughout the polymer beads was quite homogeneous [65]. More recently, an investigation on the effects of the support morphology for Pd/ST-DVB resins with different crosslinking degrees (that is, conventional macroreticular sulfonated ST-DVB resin and hyper-crosslinked sulfonated ST-DVB) can be found [66]. In their work, [PERSON] and coworkers hypothesize that the very open pore structure of hyper-crosslinked resins would allow at the same time a fast inwards diffusion of the reagents and a fast outwards diffusion of the product in the direct synthesis of H\({}_{2}\)O\({}_{2}\), making it a more suitable support than macroreticular resins, with its sulfonic groups located in the gel phase and mainly accessible upon swelling.