Investigaton Of Active Sites And Oxidative Carbonylation Mechanism Over Cu-zeolite Catalysts

Cu-exchanged zeolites are widely used in various classes of reactions, such assynthesis of organic carbonates by oxidative carbonylation, selective catalyticreduction by ammonia (NH3-SCR) or hydrocarbons (HC-SCR), adsorption of CO andN2, and decomposition of NO, etc. Characterization of active sites over Cu-zeolitecatalysts is essential for understanding the oxidation, coordination and active ceters,while mechanism investigation of oxidative carbonylation will help to understand thereaction in the microcosm. This dissertation focuses on the characterization of Cuactive sites over Cu-SAPO-34catalyst and mechanism investigation of methanoloxidative carbonylation on CuY catalyst by using diffuse reflectance infrared Fouriertransform spectroscopy (DRIFTS).CO was applied as a probe molecule to study the state of Cu in Cu-SAPO-34catalyst prepared by calcination-activation method. IR spectra of CO adsorptiondemonstrated that Cu+can occupy two distinct cationic positions in Cu-SAPO-34catalyst: the band at2154cm-1was assigned to Cu+located at site I (center ofsix-membered rings connected to the large cages of the chabazite structure) while theband at2136cm1characterized Cu+at site II (center of four-membered ringsconnected to the large cages of the chabazite structure). Moreover, the distribution ofCu+was greatly affected by Cu loading: Cu+preferred to locate at site II with a lowCu loading, while an increasing number of Cu+located at site I with the increasementof Cu loading. Furthermore, migration of Cu+at site II to site I occurred with theincrease of temperature due to the instability of Cu+positioned at site II. Besides, theeffect of three different synthetic methods was also investigated: aqueous ionexchange, solid state ion exchange and calcination-activation method on the state ofCu species. The results indicated that Cu+can not be effectively introduced to theframework of H-SAPO-34zeolite by aqueous ion exchange, whilecalcination-activation method was the most effective approach.The interaction of the CuY catalyst with different reactants and reactant mixtures(CH3OH, CH318OH, HCHO, O2, CH3OH/HCHO and CH318OH/CO/O2) was studied indetail. It was found that CH3OH was oxidized to methoxide species with the O-Hscission in the presence of CuOxor O2. Reaction of methoxide species with O2resulted in the formation of formaldehyde (HCHO) by a breaking of the C-O bond inOCH3group, and formate species was formed by consecutive reaction of HCHO with O2or CuOx. Oxidation of methoxide species to HCHO was found to be therate-controlling step in oxidation of CH3OH to formate species, and HCHO wasidentified as the reaction intermediate in the synthesis of dimethoxy methane (DMM)and methyl formate (MF), the two major by products in dimethyl carbonate (DMC)synthesis.