Polymer Supported Heterogenous Catalysts for Direct Synthesis of Hydrogen Peroxide in Absence of Selectivity Enhancers

Autor: Sterchele, Stefano
Jazyk: angličtina
Rok vydání: 2013
Předmět:
Popis: The research program developed during the Ph.D. School is focused on the study of metal catalysts supported on cross-linked functional polymers (CFPs) for the direct synthesis of hydrogen peroxide. In the last twenty years this compound has become a commodity with a constant increasing demand because of its strong oxidant properties and the formation of water as the reduction byproduct. In particular, H2O2 is widely employed as environmentally-friendly bleaching and cleaning agent. The best alternative to the current process, in particular for the small-scale production, is certainly the synthesis of H2O2 from the elements (direct synthesis). This is generally carried out with a heterogeneous catalyst under triphase condition. For safety reasons, the hydrogen-oxygen mixtures, according to the wide explosion range, are properly diluted with an inert gas, usually nitrogen or carbon dioxide. The catalyst is generally composed by one or more nanostructured noble metals, supported on an inorganic solid, carbon or organic materials. It is well known in literature that the presence in solution of additives, like halides (bromide and chloride) and mineral acids, dramatically improves the catalytic performances, in particular the selectivity towards H2O2. However, the use of these additives presents some process drawbacks, such as corrosion, leaching of catalyst, etc, which do not allow the straightforward use of the H2O2 solutions obtained from the direct synthesis. It is therefore mandatory a further step of purification to remove the additives. As a consequence, in order to evaluate the effective performance of the catalysts,the research activity during this PhD thesis aim at the investigation of catalytic systems free of selectivity enhancers. In particular, their presence has been avoided not only in the reaction mixture, but also during the preparation of the catalysts. In the frame of this PhD Thesis, a few sets of mono- and bimetallic catalysts, supported on the commercially available macroreticular resin, Laxness Lewatit K2621, have been studied in detail. This work has been performed in a research group with a long standing experience in the investigation of polymer-based metal catalysts for industrially relevant reactions and, for a few months, in the Laboratory of Industrial Chemistry and Reaction Engineering of Akademi Prof. Tapio Salmi (Department of Chemical Engineering, Process Chemistry Centre, Åbo Akademi University, Turku, Finland) for the detailed study of the catalysts performances. The research program is based on the synthesis, the characterisation and the investigation of the catalytic behaviour of the catalysts, obtained by carefully controlling a few essential parameters during the synthesis, such as the nature of the precursor, the reducing agent and the experimental conditions. These three key-points remarkably affect the features of metal nanoparticles (size distribution, difective structure, etc...) and, hence, the behaviour of the catalysts. In particular, the use of tetraaminepalladium (II) sulfate as the metal precursor and the reductive treatment with hydrogen under mild condition lead to a catalyst with noteworthy catalytic performances, specially a remarkable selectivity (70%). The investigation has also included four libraries of bimetallic materials, Au/Pd and Pt/Pd catalysts based on K2621, obtained as followed: - by keeping constant the content of palladium (1 wt.%) and changing the one of the second metal (0.1, 0.25, 0.5, 1 wt.% of Pt or Au); - by treating the material with two different reduction protocols (formaldehyde under reflux temperature and hydrogen (5 bar) under 60°C ). The catalytic results clearly show that the addition of platinum and gold to palladium improves the catalytic performances, although apparently with different mechanisms. The best catalysts are consistent with the empiric trends so far reported in literature. Finally a new class of mesoporous cross-linked polymers, featured by high surface area at the dry state, has been studied. This non-commercial polymer, quite promising as catalytic support, has been investigated in details and used for the preparation of palladium nanoparticles. This material, in view of its peculiar morphology, shows unique catalytic properties, exhibiting simultaneously a modest activity and a remarkable (70 – 80) and constant H2O2 selectivity: this unique features makes this catalyst a good candidate for a mechanistic study of the direct synthesis of hydrogen peroxide.
Databáze: OpenAIRE
Externí odkaz: