Controllable preparation, structural characterization and application of rare earth element - containing layered double hydroxides
Layered double hydroxides (LDHs) and rare earth elements (REEs) have both attracted extensive attention because of their structures and properties. The layered structure of LDHs makes it possible for many functional ions to be incorporated and the uniform dispersion of metal - organic anions in interlayer galleries or metal cations in layers extends the applications of layered materials to a wide variety of fields, whilst the electronic structure of REEs offers many advantages in the areas of optical, magnetic and catalytic materials. Catalytic wet oxidation (CWO) is a promising technique for destruction of organic pollutants, of which phenol is a simple representative, in water under mild conditions. Common heterogeneous catalysts for CWO can be divided into three series: noble metals, transition metals (especially copper) and rare earths, each of which has their own advantages and shortcomings. Co-existence of two types of catalyst components may show a higher catalytic efficiency. It is, therefore, of interest to prepare hybrid materials based on ZnAl- or CuZnAl-LDHs incorporating rare earth elements such as Z. Chang : Controllable preparation, structural characterization and application of rare earth element - containing layered double hydroxides II Ce or Eu in order to investigate the potential synergistic effect between the two components and the resultant influence on catalytic properties. A continuous co-precipitation method under steady-state conditions was first developed for the preparation of nanometer-size LDH particles using Zn2Al(OH)6(CO3)0.5*2H2O as prototype. The effects of varying the operating conditions on the structural and textural properties of LDHs were studied, including total cation concentration, solvent, residence time, pH and intercalation anion. Increasing either the cation concentration or the fraction of ethylene glycol (EG) in EG/H2O mixtures affects salt solubility and supersaturation, which results in smaller crystallites, larger surface areas and more amorphous compounds. The new method employs a short residence time of less than 15 min, allows large-scale production and maintains a constant supersaturation level in the reactor, and was shown to be a promising alternative to the conventional batch method. The second part of this research was to prepare a series of novel REE-containing LDH materials by anion-exchange or co-precipitation methods, and to investigate the effects of varying experimental parameters in order to optimize the product properties. Using the anion-exchange method, pyridine-2,6-dicarboxylic acid (H2dipic) and diethylenetriaminepentaacetic acid (DTPA) were employed as the ligands in REE-containing complex anions. [Ce(dipic)3]3- complexes were firstly introduced into ZnAl-NO3-LDH and other LDH precursors with varying layer metal ions like CuZnAl-NO3-LDH, and then different REE-containing complexes such as [Ce(DTPA)]2- and [Eu(dipic)3]3- were incorporated into LDHs. The products were Z. Chang : Controllable preparation, structural characterization and application of rare earth element - containing layered double hydroxides III characterized by many physicochemical techniques, including XRD, IR, UV, ICP, BET, TEM, TG/Mass, in situ HT-XRD and XPS. Ce-containing ZnAl-LDHs prepared by an anion-exchange method with [Ce(dipic)3]3- were found to be always mixed with a CO3 2--containing LDH. The intercalation of [Ce(dipic)3]3- in the layered host was confirmed by an increase in interlayer spacing to 1.24 nm. Geometrical considerations suggest the complex has a tilted orientation between the layers. However, a fraction of the [Ce(dipic)3]3- anions decomposed into dipic2- and Ce3+ (or Ce4+) ions during the exchange process, resulting in formation of a dipic2--containing LDH. Different experimental conditions were optimized, including the host layer composition, charge density and other synthesis parameters such as temperature, pressure, reagent concentration, and reaction time. The most promising product was obtained with a ratio of layer cations M2+/M3+ = 2, with a [Ce(dipic)3]3- concentration around 5 mmol/l and an exchange period around 10 h at room temperature. For some other host matrix compositions, the decomposition of [Ce(dipic)3]3- and co-intercalation of CO3 2- could both be reduced. The interlayer spacing values varied for different layer metals. Another Ce-containing ZnAl-LDH product was synthesized by a ion-exchange process with [Ce(DTPA)]2-. The product showed characteristics of a well-crystallized LDH and the interlayer spacing was enlarged to 1.46 nm. Eu-containing LDHs with [Eu(dipic)3]3- anions in the interlayer galleries were prepared under the same conditions as for [Ce(dipic)3]3--intercalated LDHs and the structure and properties of the products were similar. The immobilized luminescent Z. Chang : Controllable preparation, structural characterization and application of rare earth element - containing layered double hydroxides IV materials had excellent stability and luminescence properties. Eu3+ was used as a structural probe to study the interaction between the layered host and the complex guest by monitoring its luminescence properties. The structural information obtained can reasonably be transposed to [Ce(dipic)3]3--intercalated LDHs. On the basis of studies with REE-containing ZnAl-LDH materials, CuZnAl-Ce(dipic)-LDHs with cerium ions located in the interlayer galleries were synthesized by the ion-exchange method. The decomposition of [Ce(dipic)3]3- and co-intercalation of CO3 2- were relatively insignificant with this host matrix. Another type of CeX-LDHs (where X represents Ce content) was prepared by a co-precipitation method. It was demonstrated that the products were a mixture of CuZnAl-LDH and CeO2, with cerium uniformly dispersed on the surface of small LDH particles. Thermal behavior of Ce-containing CuZnAl-LDHs was found to be influenced by the presence of cerium oxides and the temperature of formation of spinel phases was significantly increased. The effects of varying some experimental conditions such as preparation method, layer composition, calcination temperature/time and Ce/Al molar ratio on the specific surface area were investigated. When the product CuZnAl-Ce(dipic)-CLDH with a stoichiometric Ce/Al ratio was calcined at 500 °C for 6 h, a composite Cu-Ce-O solid solution consisting of mixed metal oxides and having a large specific surface area was obtained. The final part of research was to prepare a series of new catalysts by calcination of Ce-containing CuZnAl-LDHs, to study the catalytic abilities of these materials in the phenol oxidation reaction and investigate the interaction between Cu and Ce centers in Z. Chang : Controllable preparation, structural characterization and application of rare earth element - containing layered double hydroxides V the catalysts. It was shown that the difference in catalytic performances of the two types of Ce-containing CuZnAl-CLDHs is related to the structure and composition of the catalysts. For CuZnAl-Ce(dipic)-CLDHs obtained by calcination of [Ce(dipic)3]3--intercalated CuZnAl-LDHs, the presence of cerium significantly improved the catalytic activity and control over the product distribution in phenol oxidation. The uniform dispersion of Ce-complexes in the interlayer galleries of the LDH precursors results in the presence of a Cu-Ce synergistic effect in the Cu-Ce-O solid solution. The strong interaction between Cu and Ce enhances the catalyst performance. For CeX-CLDHs obtained by calcination of the mixture of CuZnAl-LDH and CeO2, the presence of cerium enhanced deep oxidation of phenol and reduced the extent of leaching of metal elements, resulting in improved catalyst selectivity and stability. The interaction between Cu and Ce in the two different phases was weak, not enough to give significant increase in catalytic activity, but sufficient to stabilize the active component, Cu, against leaching.