Despite the large number of catalytic studies involving Ag/Al2O3 catalysts, especially in deNOx reactions, the phenomenon of ageing observed during catalysts storage after preparation by impregnation followed by a calcination treatment, has not been reported earlier and rises questions. The present paper highlights firstly the colors of the calcined Ag/Al2O3 catalysts (white to yellowish) and the associated UV-visible spectra as a function of the Ag loading (0.5 to 4 wt% Ag), and secondly the gradual changes of color to grey/black and the associated UVvisible spectra when the samples are stored in ambient air or in vacuum in a desiccator in dark. These changes were found to be reversible when the samples are re-calcined. The physico-chemical changes in the silver species during ageing were explained thanks to a comprehensive characterization study of the Ag/Al2O3 catalysts after calcination and after ageing. Several techniques such as UV-Visible spectroscopy, XRD, electron microscopy, XAS and photoluminescence were used. After calcination, in addition to highly dispersed Ag + species on alumina, the presence of Agn clusters of size smaller than 1 nm was found to be responsible for the yellowish color of the Ag/Al2O3 catalysts with Ag loadings higher than 2 wt% Ag and for the associated plasmon band at 350 nm. The ageing process was explained based on characterizations, coupled to a close examination of the literature data on the mechanisms of reduction of Ag + species, and on physico-chemical phenomena that can influence the color of various silver-containing materials, such as Ag 0 particle size, aggregation/redispersion and surface alteration. Consequently, the ageing process of the calcined Ag/Al2O3 catalysts, associated to sample darkening and to Ag plasmon band shift and broadening in the entire visible range, was proposed to result from the modification of the nature of the supported Ag phase: the reduction of Ag + species (by auto-reduction and/or photoreduction), followed by the growth of Ag 0 particles, It is proposed that the Agn clusters may act as nucleation sites for the formation of larger Ag 0 particles and that the formation of aggregates is favored by an easy migration of Ag on alumina.

In this work, the combined use of p-sulfonic acid-calix[n]arene and microwave energy to hydrolyze the 1,3-dioxolane ketal of isatin was evaluated with excellent results. This is the first time that p-sulfonic acid-calix[n]arene has been used as the catalyst in a ketal hydrolysis reaction and the deprotection of the ketone carbonyl of isatin is reported. The presence of 2.5 mol% of p-sulfonic acid-calix[4,6]arene at 160 ºC resulted in over 96% conversion of this ketal in 10 min, with the additional advantage of using water as a solvent. This catalytic system (aqueous phase containing p-sulfonic acid-calix[4]arene) was reused for five consecutive cycles, with a conversion above 96% maintained. This reusable system is not practicable using p-toluenesulfonic acid and p-hydroxybenzenesulfonic acid as catalysts since both are extracted to the organic phase with the reaction product. The hydrolysis of 1,3-dioxolane ketal of isatins with different substituents (CH3, I, Br, Cl, F, NO2) in the aromatic ring was also evaluated. The protecting group of 5-methyl-isatin was removed with 73% conversion using 2.5 mol% of p-sulfonic acid-calix[4]arene at 160 ºC for 5 min. In contrast, the ketal of 5-nitro-isatin reached 80% conversion using the same conditions after 40 min.