Study On The Preparation, Performances And Application Of Hydrotalcite-Like-Compounds

Hydrotalcite, a kind of natural mineral, has an ideal formula Mg₆Al₂(OH)₁₆CO₃·4H₂O with a sheet structure, in which Mg²⁺ ions are arranged in sheets and each of them is octahedrally surrounded by six hydroxide groups while each hydroxide spans three magnesium ions. The sheet structure shows a positive charge of the layer when the divalent cations(Mg²⁺ ) are partially substituted with some comparable size trivalent ones such as Al³⁺, balanced by the anions CO₃^2- between the hydroxylated layers. Under certain condition, Mg²⁺ and Al³⁺ can be substituted by other divalent and trivalent cations with similar radius, respectively; and CO₃^2- can also be replaced with NO₃^-, Ac^-, SO₄^2- and etc.. Thus a large number of hydrotalcite-like compounds (HTLcs) can be synthesized in the same structure but with different compositions, whose general formula can be therefore shown like [M (II)_1-xM (III)_x (OH)₂]^x+(A^n-)_x/2·mH₂O, where M (II) and M (III) represent the divalent and trivalent cations, respectively; A^n- represents anions in the octahedral positions; 'm' is the number of the water molecules and 'x' is the ratio of trivalent to all the cations.With the characteristics of exchangeable anions in the interlayer, adjustable cations on the sheets and variable pore sizes, HTLcs have different acidic-basic capacity and oxidation-reduction property if given different cation species and ratio of n(M²⁺) to n(M³⁺) and thereout have been widely used to catalyze the reactions of hydrogenation , degradation , catalytic hydrogenation , esterification and Fischer-Tropsch low-carbon alcohol. Moreover, HTLcs have also potential to be utilized as new type materials and carriers of catalysts.However, the application of the natural HTLcs have been limited because of fewer species, lower crystallization, higher impurity, and unstable constituent. Therefore, it is urgent to build a system of producing a series of structurally refined HTLcs with different compositions on the sheets, different isomorphous replacement degree or different charge density on the plates and different particle sizes under certain conditions.In this current project, we started with measuring the NaOH titration curves of nitrate salt by co-precipitation method to study the effect of pH, raw material, procedure, temperature and time of crystallization on the synthesis of HTLcs; then identifying the phases of complex with different pH values by XRD; eventually selecting an optimal condition of preparing HTLcs through characterization of physicochemical performances of the complex by using the modern instrumentals of FT-IR, ICP, BET and TG-DTA. More importantly, we proposed a reasonable mechanism and a general principle of synthesizing HTLcs that will provide a theoretical basis for further study in this field.The result showed that 1) through the optimal procedure, CO₃² was not detectable in the product and NO₃^- was the unique ion in the interlayer of complex when MgAl-HT, NiAl-HTLc and ZnAl-HTLc solution were prepared by the freshly boiled distilled water without protection of N₂. 2) We found that the hydrothermal treatment with shorter time could replace the conventional reflux of over 20h at 80℃completely , and such procedure would simplify the synthesis process and shorten the preparation time significantly. 3 ) The pH value is a crucial factor in the preparation of HTLcs. The pH value in the synthesis of bi-HTLcs should be higher than in the preparation pH of Al(OH)₃ and lower than in that of M(OH)₂. 4) The preparation mechanism of HTLcs by co-precipitation was describe as: by adding NaOH aqueous into the mixed salt solutions, the pH value in the system increased generally, Al(OH)₃ was got and then subsequently, with the M replacing Al³⁺ partially, HTLcs with layered structure formed.HTLcs are facile to be decomposed when heated and the obtained complex oxides will have fine application prospects in some acid-base catalysis or redox catalysis reactions. Because of their typical characteristics such as big specific surface area, even distribution of catalytic active sites and mild reduction condition of metallic cations. However, the thermal decomposition procedure is complicated and the properties of complex oxides are mainly determined by the composition of the precursor and calcination temperatures of thermal decomposition process. In this paper, the thermal decomposition process of HTLcs was investigated and the decomposition mechanism was also proposed in order to obtain such complex oxides possessing perfect performances. TG-DTA technology was adopted to study the thermal decomposition processes of MgAl-HT, NiAl-HTLcs and ZnAl-HTLcs. Kinetic parameters of the process were calculated and analyzed by Ozawa method and Kissinger method, and then the reaction mechanism was deduced afterwards. The results showed that the thermal stability of the three kinds of HTLcs was different and the stability order was: MgAl-HT>NiAl-HTLcs>ZnAl-HTLcs. The temperatures of stability were 400℃, 300℃and 200℃, respectively. In the thermal decomposition process of MgAl-HT, when Mg/Al=1～2, the TG curves showed three mass loss stages: the weight loss of the deformation of the interlayer water molecules under the first stage, the removal of the Al-OH on the sheets of the second and the loss of Mg-OH on the sheets and nitrate ions of the third; while when the Mg/Al=3～6, the curves showed two stages: the loss of interlayer water molecules and dehydroxylation of the sheets as well as the loss of nitrate ions. For the thermal behavior of NiAl-HTLcs, all the samples showed two weight loss stages: one stage to the loss of interlayer water molecules and the other to dehydroxylation of the sheets as well as the loss of nitrate ions in the interlayer. As to the ZnAl-HTLcs, the thermal decomposition was completed under one stage; i.e. the loss of interlayer water molecules and dehydroxylation of the sheets as well as the loss of nitrate ions in the interlayer. The active energy values calculated by Ozawa method were higher than that by Kissinger method under the same decomposition stage of the same samples. Active energy values of the corresponded stages presented the order of E_MgAl-HT> E _NiAl-HTLcs >E_ZnAl-HTLcs. The calculated results by Ozawa method showed that the active energy values of thermal decompositions kept changing dynamically.HTLcs are new type functional materials and their application fields are mainly dependent on their compositions, structures and particle sizes. Particularly in many applications, the particle sizes were specified to make full use of their advantages. In this paper, NiAl-HTLcs with different particle size distribution were prepared by methods of equilibrium crystallization, variation of the degree of super-saturation and ultrosonic crystallization. And also the effect of many factors on particle size was systematically studied. The results showed that the big particle sizes of HTLcs could be obtained when increasing temperature and extending time of the hydrothermal treatment during equilibrium crystallization, in which the average particle size of samples would increase from 1.01μm to 4.39μm. However, an effective means to acquire HTLcs with small particle sizes and narrow particle sizes distribution was by the method of ultrosonic crystallization, from which the average particle size of samples was 0.38μm and took 83.04% of (0.26-0.51)μm.In order to study the catalytic activity of HTLcs, NiAl-HTLcs was introduced to the oxidation reaction of phenyl aldehyde into benzoic acid where acetic acid was solvent and oxygen in the air as the oxidant. Effects of different factors on the reaction were investigated in detail, and then the reaction mechanism was deduced by the calculation of kinetics parameters including the reaction activity energy and reaction grades. The result showed that the NiAl-HTLcs could be used as an excellent catalyst in the synthesis of benzoic acid with acetic acid and phenyl aldehyde. In this reaction, oxygen could be dissolved and diffused much more easily because NiAl-HTLcs activated the oxygen molecules in the air and at the same time acetic acid as solvent was associated with oxygen in the presence of hydrogen bond. Under optimal condition, both the conversional rate of phenyl aldehyde and the selectivity of benzoic acid were up to 100%. The kinetic study showed this reaction was a typical controlled diffuse reaction or could be also called zero progression reaction, in which the active energy was got to be 15.277kJ·mol^-1 following the free radical reaction. This experiment pioneered a completely new path of reaction where phenyl aldehyde was oxidized into benzoic acid using protonic solvent as medium .NiAl-HTLcs was also introduced into another synthetic reaction of benzoin ethyl ether with ethanol and phenyl aldehyde, in which the conversion of phenyl aldehyde was up to 54.86% and the selectivity of benzoin ethyl ether was nearly 100% at the optimal condition. This reaction was kind of one progression reaction and the reaction activity energy was 42.189kJ·mol^-1 where NiAl-HTLcs and ethanol were as the catalyst and the solvent, respectively. This technology completely innovated the traditional design of the synthesis of benzoin ethyl ether, i.e. NiAl-HTLcs replaced cyanide as the catalyst in the reaction. By this new method, not only the cyanide poisoning was avoided but also the synthesis of benzoin ethyl ether could be completed in one step instead of the traditional two steps with both condensation and etherification.As the John-Tellor effect of Cu²⁺, when Cu²⁺ was used to prepare HTLcs with trivalent cations by coprecipitation method, Cu would form deforming octahedral complex rather than HTLcs. Even if it could be introduced into the preparation of HTLcs, the preparation conditions would be harsh and the stability of the resultant would be poor. However, catalysts loading copper have wide applications in alkali and redox catalysis. Moreover, the complex oxides of HTLcs with copper as the precursor possess potential application capacity because of whose typical characteristics such as big specific surface area, high dispersion degree of Cu and even particle sizes. Here, CuMAl-HTLcs (M=Mg²⁺ or Zn²⁺) were prepared by coprecipitation method. Effects of different factors on the preparation were studied elaborately. Based on the above research, the most optimal preparation conditions were acquired and the obtained HTLcs was also used in the reaction of phenol hydroxylation to study its catalytic performances. The results showed that the feasible pH value range for preparation of CuMAl-HTLcs (M=Mg²⁺ or Zn²⁺) fell in the range of that of Al(OH)₃, M(OH)₂ and Zn(OH)₂. By increasing Cu content in CuZnAl-HTLcs, it catalytic activity in the reaction of phenol hydroxylation would be elevated gradually and the highest conversion of phenol could reach to 56.1%.Rare metals with special structures have shown champion effectiveness in many reactions. Here, by introducing rare metals with larger ionic radius into the structure of HTLcs by controlling the synthesis factors. The complex oxides obtained from calcinating the above HTLcs were expected to have advantages to be an environment benign catalyst such as big specific surface area, even distribution of rare metal elements and fine coordinating action among the metal cations. Starting by the titration curve of mixed salt solution of several metal ions (Ce³⁺, Al³⁺, Zn²⁺ and Ni²⁺) with NaOH solution as the precipitator, a feasible way was modified to introduce Ce³⁺ with larger ionic radius into the structure of HTLcs and the optimal conditions were nvestigated elaborately for preparing NiAlCe-HTLcs and ZnAlCe-HTLcs. Subsequently, the posibility of the complex oxides with HTLcs containing rare metal as the precursor in elimination reaction of NO was preliminarily researched. The experiments results showed that the feasible pH value range for preparation of MAlCe-HTLcs (M=Ni²⁺ or Zn²⁺) was among the ranges of that of Al(OH)₃, Zn(OH)₂ and Ce(OH)₃. The complex oxides obtained from calcination of ZnAlCe-HTLcs at 750℃was used in catalytic reduction reaction of NO; when the reaction temperature was 680℃, the conversion of NO was up to 100%. NiAlCe-HTLcs showed high activity at lower temperatures in the catalytic reduction reaction of NO; when the reaction temperature was 400℃, the conversion of NO could reach 95%.