Preparation And Catalytic Properties Of Copper/Iron-containing Layered Double Hydroxides And Their Derivatives

Layered double hydroxides (LDHs) and their derivatives, mixed oxides by calcining LDHs, are one of important potential materials in the catalysis and adsorption fields.In the thesis, Cu-Ni-Al LDHs, Cu-Mg-Al LDHs, Fe-Ni-Al LDHs and Fe-Mg-Al LDHs were prepared by coprecipitation, from the corresponding nitrate solutions and mixed precipitants (NaOH and Na₂CO₃). Mixed oxides were obtained by calcining LDHs at different temperatures. The catalytic behaviours of the aforementioned LDHs and their derivatives for the catalytic wet hydrogen peroxide oxidation(CWPO) of phenol aqueous solutions were tested. Under the same experimental conditions, it was observed that iron catalysts showed bad activities (< 10% of phenol conversion), and the copper catalysts, especially Cu-Ni-Al mixed oxide, showed higher activities. It was said that a few of iron doping into the copper catalyst could improve the dispersal of the active element (Cu). So the quaternary mixed oxide was prepared, containing copper, nickel, iron and aluminum. According to the conversion of phenol, the ratios of four elements in the catalyst were ascertained:ΣM²⁺/ΣM³⁺=3, Fe³⁺/ΣM³⁺ =0.25, Cu²⁺/ΣM²⁺=0.25.We performed thermogravimetric analysis, X-ray diffraction and FT-IR. spectroscopy to characterize the Cu-Ni-Fe-Al LDHs and the chemical changes which take place during the calcining process. By a series of experiments, the optimum parameters in the CWPO process, using Cu-Ni-Fe-Al mixed oxide as catalyst, were ascertained. When initial phenol concentration was 400mg·L^-1, optimum conditions required for 97.5% phenol conversion were as follows: catalyst concentration was 400 mg·L^-1, M_H2O2/M_phenol (molar ratio) was equal to 12, reaction time was 60 min, reaction temperature was 60℃and the initial pH value was not adjusted (=6.5). The catalyst also showed good activities after reusing 4 times, and the conversion of phenol was 93.5%.We also performed high performance liquid chromatography (HPLC) to analyze the products of phenol degradation at different reaction time in the paper. The analytical results indicated that hydroquinone, catechol, maleic acid and other compounds were existed. It was concluded that the hydroxylation of phenol to hydroquinone and catechol as a first step with a further oxidation of the dihyoxylbenzenes to benzoquinones. Then hydroxyl radicals sequentially attacked the aromatic rings of these compounds to cause ring opening. At this stage, maleic acid and fumaric acid were the major products. Maleic acid and fumaric acid could be oxidized to a number of other organic acids, including oxalic acid and formic acid, which can be oxidized to CO₂ and H₂O.