A characterization and catalytic properties of copper (II) exchanged zirconium phosphate
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1982
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Abstract
In order to characterize a copper (II) exchanged (alpha)-zirconium phosphate, ZrCu(PO(,4))(,2), as a catalyst, the reduction of this material by hydrogen was studied. The reduction proceeded through two states: Cu(II) to Cu(I) and Cu(0) with activation energies of 7.0 and 5.7 Kcal/mole, respectively. The presence of Cu(I) was evidenced by the change of the catalyst's color, from ESCA spectra, from the disappearance of characteristic absorption band in the UV-visible reflectance spectrum and from consideration of E.s.r. signals. X-ray diffraction did not show any existence of copper metal or copper oxide. The rates of both reactions were found to conform to an Elovich-type equation and to be strongly pressure dependent. These results were interpreted in terms of sorption and diffusion of hydrogen to the metal as the rate-determining step. The concept of active-site generation was introduced to account for the observed induction period. The oxidative dehydrogenation of cyclohexene was investigated at different mole ratios of oxygen to cyclohexene and at different temperatures in a conventional flow reactor. By either keeping the pressure of cyclohexene or that of oxygen constant, the rates of formation of benzene and carbon dioxide were found to be first order in the pressures of cyclohexene and oxygen. The activation energies were determined to be 11.7 and 6.45 Kcal/mole for the formation of benzene and carbon dioxide. During the reaction, the catalyst remained unchanged. Catalytic selectivity of the reaction was found to be dependent on pretreatment temperatures, cupric exchange loadings and surface areas. This was suggested to be the result of phase changes due to dehydration, the presence of acidic sites and Cu(II) sites as two reaction centers for the process.
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Chemistry