## 4 Cu-Exchanged Clays and Other Inorganic Materials  
Although purely inorganic materials fall outside the main focus of the present review, for the sake of completeness recent advances on the topic are only briefly summarized in this section. ZMS-5 zeolite exchanged with Cu(II) and Co(II) was used by [PERSON] and [PERSON] in the three-component CuAAC reaction of benzyl bromides and propargyl alcohol, carried out in water under thermal or US-promoted conditions [199]. The most interesting findings of the study were the observation that the mixed Cu/Co system was more effective than an analogous ZMS-5 zeolite exchanged with Cu(II) alone and the fact that, under sonication, the reaction time could be shortened from several hours to 5 min. The possibility was also mentioned of reusing the catalyst eighth times without significant loss of activity. However, no specific data was provided, neither in this respect nor for what it concerns potential leaching issues. A comparative evaluation of different catalysts for the CuAAC reaction between dipropylated alloxan and various benzyl azides was undertaken by [PERSON] et al. [200]. Apart for the nature of the reactants, the study strictly mirrored in methods and conclusions those of [PERSON] et al. for three-components CuAAC reactions involving aromatic boronic acids (Scheme 2d) ([PERSON], 2016). Among the systems included in the screening, a Cu(II)-exchanged clay proved superior to other Cu(I/II) catalysts examined in the work. Recovery and recycling of the catalyst was possible, but some decrease of the product yield was observed in the course of five reaction cycles with a fixed reaction time of 3 h (96% \(\rightarrow\) 81%). In their continuing interest in flow-chemistry, [PERSON] et al. examined a Cu(II)-Fe(III) layered double hydroxide (LDH) as an heterogeneous catalyst for _in continous_ two-components CuAAC reactions ([PERSON], 2016). When the material was used in a stainless-steel reactor provided with a back-pressure regulator set to 100 bar, optimal results (77%-98% yield) were obtained in CH\({}_{2}\)Cl\({}_{2}\) at 100 \({}^{\circ}\)C with contact time as short as 100 s. With this experimental set-up, the system could be kept working for 10 h without any evident decrease in activity and provided the expected triazole product with a final TON around 5 and a residual copper content of 45.3 ppm. Although the resting state of the catalyst was found to be Cu(II), its activity in CuAAC was explained by reasoning that under the vigorous reaction conditions cupric ions might undergo in situ reduction (e.g., by dehydrogenase homocoupling of the alkyne substrate), to generate Cu(I) active sites as defects in the LDH lattice. [PERSON] et al. carried out a comparison of the catalytic properties in CuAAC of a Cu-Al LDH, with those of the Cu(Al)O mixed oxide obtained by high temperature calcination of the former ([PERSON], 2016). The mixed oxide was found to be more active and better fit for recycling than the LDH, especially when calcination was used before each new run in order to get rid of any organic material deposited on its surface. Both materials displayed some tendency to leach copper in the solution, albeit to an extent significantly larger for LDH than for the mixed oxide (0.4% and 4%, respectively). The phenomenon, attributed to the capability of sodium ascorbate to chelate metal ions, translated into an appreciable catalytic activity in the filtrate. This let the Authors to conclude that, at variance with most of the supported CuAAC catalysts reported in related studies, homogeneous and heterogeneous type of catalysis coexisted in their system. Another instance of a copper-exchanged clay material, in this case natural montmorillonite, was described recently by [PERSON] et al. (2014). Rather surprisingly in view of the very low solubility of CuI in water (approximate 4.2 \(\times\) 10\({}^{-4}\) M at 18-20 \({}^{\circ}\)C) ([PERSON], 2016), the material was reportedly prepared by vigorous stirring of the clay in a saturated aqueous solution of the copper salt.