## 3 HCL Polystyrene Based Nanocomposites
### Nanocomposites Containing Metal Nanoparticles  
Metallic nanoparticles supported onto inorganic or carbon-based materials are widely used as heterogeneous catalysts in many industrial sectors, including chemical manufacturing and energy-related applications [85]. Nevertheless, catalysts that are immobilized on these supports often show poor stability as their catalytic activity is reduced by aggregation and leaching phenomena. The use of high SSA materials, such as hyper-crosslinked polymers, can represent a valid alternative to inorganic and carbon-based materials as supports for various metal nanoparticles, allowing for the realization of microheterogeneous catalysts that combine the advantages of both homo- and heterogeneous systems and are suitable for various catalytic applications [86]. The first studies on the possible use of nanocavities typical of hyper-crosslinked polystyrenes as nanoreactors are due to [PERSON] et al. [87], who grew the Co nanoparticle by the impregnation of ST/DVB resins with Co organometallic solutions, followed by drying, hydrolysis, and thermolysis. By this method, a wide range of Co contents (2-8 wt %) and nanoparticle sizes of 1-3 nm, which are comparable to the average pore size of the polymeric matrix, were obtained. With a similar strategy, Pt nanoparticles were grown within the pores of the same microporous resin by impregnation with platinic acid solutions, followed by reduction of the obtained Pt(II) complexes. These materials were highly selective (up to 98% at 100% conversion) for the catalytic oxidation of l-sorbose to 2-keto-l-gulonic acid [88]. Further studies on Pt nanoparticles that are grown on commercial micro/macroporous polystyrene resins (Macronet MN 270/386, Purolite Int., UK), demonstrated that the presence of macropores can increase the catalytic activity of these systems by a factor 4.6, as large pores improve the accessibility of Pt nanoparticles to the reaction species [89]. A similar system with Pt nanoparticles in different forms (unmodified, reduced with various reagents, and impregnated with cinchonidine) was tested for the hydrogenation of ethylpyruvate, demonstrating its potential for the enantioselective catalysis of activated ketones hydrogenation [90]. A commercial PS resin containing from about 0.1 wt % up to about 5 wt % (evaluated by X-ray fluorescence analysis [91]) of mixed Pt-containing nanoparticles, was also proposed for water purification applications, and was tested for phenol catalytic wet air oxidation. Nanoparticles size that was obtained within the pores of the HCL resin measured about 2.1-2.3 nm, and the obtained Pt species included Pt(II), Pt(IV) and relatively lower amounts of Pt(0). The highest conversion, activity, and selectivity, markedly exceeding those of the conventional Al\({}_{2}\)O\({}_{3}\)-supported Pt catalysts, were obtained for the HCL resin containing 0.95 wt % Pt, while the catalyst containing the highest Pt amount was found the less active and selective. This result was explained when considering that nanoparticle agglomeration phenomena and pore blockage both contribute to the reduction of the catalytic activity of nanoparticles within micro/macroporous resins when the nanoparticle loading exceeds an optimal amount [92]. A subsequent work of the same research group [93] was focused on the growth of Pd complexes within the pores of HCL polystyrenes. They demonstrated that by varying the organic components of the Pd complex, different morphologies could be obtained within the pores of the resins based on the compatibility between the resin and the complex. Large nanoparticles were obtained for the bis-acetonitrile/Pd complex (about 5 nm diameter), whereas no nanoparticles were obtained in the case of the highly compatible styrene/Pd complex (Figure 7).