Gheller, Stefano (2014) Ossidazione dell'alcol benzilico mediata da catalizzatori metallici supportati. [Magistrali biennali]
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The selective oxidation of benzyl alcohol to benzaldehyde is of great interest to the great utility of this carbonilic compound as an intermediate in organic synthesis but also because benzaldehyde is much used as such in the food, pharmaceutical and in the dyes industry. The processes currently used provide for the use of stoichiometric oxidants or otherwise toxic to humans and the environment. The purpose of this thesis work is the preparation of supported metal catalysts based on palladium nanoparticles and their use in the reaction in question, using molecular oxygen as an oxidant and water as a solvent and with the objective of obtaining only water as by-product. Have been used as supports various functional organic polymers or simply resins having different chemical composition and morphology. On the basis of the composition are distinguished those of type PS-DVB, obtained by copolymerization of styrene (monomer structural) and divinylbenzene (crosslinking monomer) and those acrylamidic (DAE), obtained by copolymerization of N,N-dimetilacrilammide (monomer structural), 2-acrylamido-2-methyl sulfonic acid (functional monomer) and ethylene dimethacrylate (crosslinking monomer). Some resins also contain sulfonic groups, which are useful to make the material hydrophilic but also as ion exchangers in the metallation step; the sulfonation is usually carried out by treatment with concentrated sulfuric acid. The morphology strongly depends on the amount of crosslinking agent contained in the polymer and, based on this parameter, we distinguish resins of gel type, macroreticular or hyper-crosslinked. The content of crosslinking agent in gel type resins is about 2-8%; these resins have a nanometric porosity only in the swollen state. The macroreticular resins, however, contain a quantity of crosslinking equal to 8-40%; they are characterized, in the dry state, by the presence of macropores and also by nanopores on the swollen state. The hyper-cross-linked resins can be obtained from both gel type and macroreticular resins, creating new bonds between the polymer chains when the material is in the swollen state. The activity of the catalysts prepared was evaluated by catalytic tests carried out in different conditions of temperature, pressure and duration of the reaction. During the tests, and/or to their end, the reaction mixture has been sampled, from the sample it was extracted the organic phase, which was then analyzed by gas-chromatography using anisole as an internal standard. The gas-chromatographic analysis have allowed to determine the conversion, selectivity and yield of each reaction. The most active catalysts result to be Pd/DAE 26-4 and Pd/DAE 26-40 (82-85% conversion) in the conditions of 80 ° C, 4 bar and 2 hours. The increased activity is probably due to better affinity for the solvent. The more selective is Pd/DAE 26-40 (36%), in which the resin is macroreticular; the best selectivity with respect to the gel-type (DAE 26-4) is presumably attributable to the presence of the macropores and the thinner layer through which reactants and products can diffuse; these two aspects imply that the benzaldehyde, once formed, is able to exit from the catalyst before being further oxidized to benzoic acid.
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