Comparative study of the coprecipitation methods for the preparation of Layered Double Hydroxides

[摘要] Layered double hydroxides (LDHs) have received considerable attention due to their properties and potential applications as catalysts1-4, catalyst precursors or catalyst support5-6, adsorbents7, and anion exchangers8-10. LDHs consist of stacks of positively charged metal hydroxide layers which require the presence of interlayer anions to maintain overall charge neutrality. The most important class of LDHs can be represented by the general formula [M2+1-x M3+x (OH)2]x+ Am-x/m.nH2 O, where M2+ is a bivalent cation, M3+ is a trivalent cation and A an interlamellar anion with charge m-. The layers have a brucite-like structure [Mg(OH)2] in which the isomorphic substitution of some divalent cations by trivalent ones gives rise to the positive charge. In order to keep the electroneutrality, the interlamellar domain must be occupied by an adequate number of anions which are generally hydrated. A great variety of this kind of compounds can be obtained by varying the di- and trivalent cations, their proportions and the interlamellar anions.The direct synthesis of such materials can be performed by various methods such as coprecipitation (or salt-base method)11,12, salt-oxide method10,13 and hydrothermal synthesis11. Other methods of direct synthesis are found in the literature, such as electrochemical synthesis14,15 and sol-gel synthesis16, but these methods have been barely studied up to now. Among the direct methods, the most utilized one is coprecipitation, which can be carried out under constant or variable pH, depending on the applied conditions. Even though many reports are available in the literature on LDH preparations using this method, a comparative systematic study on the effect of the different conditions involved in this method has not been carried out.In the present study LDHs were prepared by the coprecipitation method using constant and variable pH values. The cation pairs used in the preparations were Mg(II)-Al(III) and Zn(II)-Cr(III), some of the most frequently used combinations. In order to prepare representative materials, two kinds of anions were used, a carboxylate (terephthalate, represented by TA) and a sulfated surfactant (dodecylsulfate, represented by DS). The aim of this investigation was to study and compare properties such as crystallinity, phase purity, thermal stability, particle size distribution, average particle diameter, and textural properties of the prepared materials using the two variations of the coprecipitation methods.  ExperimentalAll cations salts used, Mg(NO3)2.6H2O, Al(NO3)3 .9H2O, Zn(NO3)2.6H2O and Cr(NO3)3.9H2O were of analytical grade and were purchased from Merck. Sodium dodecylsulfate was also of the highest available purity (Merck), and was further purified by the Soxhlet extraction with hexane in order to extract residual 1-dodecanol. The elimination of 1-dodecanol was confirmed by measurements of the CMC of the surfactant, using a conductiometric method17. Sodium terephthalate was prepared by addition of a NaOH (Merck >98%) solution to a suspension of terephthalic acid (99.9%) which was obtained from Rhodiaco Indústrias Químicas Ltda.The synthesis conducted at constant pH was carried out by the addition of the cation solution (M2+=2.5´10-2 mol and M3+= 8.3 ´ 10-3 mol both dissolved in 17 cm3 of water) to a solution of the anion to be intercalated (SDS = 1.66 ´ 10-2 mol and terephthalate = 8.3 ´ 10-3 mol each one dissolved in 70 cm3 of water). A 2 mol dm-3 NaOH solution was concomitantly added in order to keep the pH constant. Different pH values were used to prepare the LDHs, i.e., 7.0 ± 0.2 for the Zn-Cr-LDHs and 8.5 ± 0.2 for the Mg-Al-LDHs.In the preparation at variable pH values, the cation solution with an MII:MIII ratio of 3:1 was added (identical to the one used in the constant pH method) to an alkaline solution (0.133 mol of NaOH) of the anion in 70 cm3 of water. The amounts of cations and anions were identical to those used in the method of coprecipitation at constant pH.In both cases, the materials obtained were water washed by centrifugation and dried under vacuum. Part of the materials obtained was separated before drying and submitted to hydrothermal treatment (100ºC for 18 h at atmospheric pressure), after which it was centrifuged again and vacuum dried.The materials were characterized using powder X-ray diffraction (PXRD), thermogravimetric and differential thermal analysis (TG/DTA), light scattering, N2-BET and scanning electron microscopy (SEM).A Siemens D5005 X-ray diffractometer with a Cu Ka radiation (l = 1.5406 Ã…) and a graphite crystal as monochromator, was used to obtain the PXRD patterns ranging from 2 to 70º 2q, using a step of 0.020º s-1. TG/DTA analyses were performed using an SDT 2960 Simultaneous DTA-TGA coupled to a Thermal Analyst 2100, both from TA Instruments, in dry synthetic air at a heating rate of 10ºC min-1.Size distribution and average diameter of the particles in suspension were determined by light scattering. A Zetasizer 4 coupled to a SX/16 microcomputer from Malvern was used to obtain the measurements. Specific surface area and average pore diameter were determined by the N2-BET isotherm technique using a Micromeritics ASAP 2010 apparatus. Before the beginning of each measurement, the samples were outgassed at 40°C (higher temperatures were avoided because of the thermal decomposition, see Figure 2) for 1 h.      The SEM images were taken with a Zeiss DSM 960 Digital Scanning Microscope. The powder solids were deposited on the sample-holder by dip-coating the supports in the suspension of each material and submitted to gold sputtering with a Sputter Coater Balzers SCD 050.  Results and DiscussionPrepared materialsBy varying the cation combination, interlamellar anion and pH control during coprecipitation, with or without submission to hydrothermal treatment a total amount of 16 samples was obtained. The prepared materials are organized in Table 1 according to the denomination adopted.     Powder X-ray diffraction patternsFigure 1 shows the PXRD patterns for the materials prepared under different conditions of pH during precipitation and hydrothermal treatment. The basal spacing of ~14 and ~27 Ã… was found for the TA- and DS-containing LDHs, respectively, depending on the system and on the submission to hydrothermal treatment. The Miller indexes indicated were based on literature data18, and the values of basal spacing were then calculated by the Bragg equation, using the average 1/3(d003 + d006 + d009). The basal spacings obtained agree with those reported in the literature for these anions19.A general analysis of these patterns shows that the materials prepared with the Zn-Cr cation combination presented higher crystallinity than those containing the Mg-Al cation combination with the same interlamellar anion. Such difference in crystallinity is attributed to the higher similarity of ionic radii for the ion pair Cr(III)/Zn(II) (0.61/0.75 Ã…) than that for Al(III)/Mg(II) (0.53/0.72 Ã…).It can be seen that in all of the four systems the materials obtained presented higher crystallinity when the preparation was conducted at constant pH than at variable pH values. Moreover, this crystallinity was little affected by hydrothermal treatment.Figure 1a shows that the MgAlTA system presented a phase impurity in all cases. The vMgAlTA shows broad reflections, characteristic of LDHs intercalated with hydroxyl anions11. This behavior could be expected, since coprecipitation at variable pH facilitates the intercalation of this anion. When the material was submitted to a hydrothermal treatment (vMgAlTA-ht), this phase disappeared, and two new phases appeared. These two phases were also present in both cMgAlTA and cMgAlTA-ht, and could be attributed to: (i) the TA-containing LDH phase and (ii) a carbonate-containing LDH phase. The presence of this second phase was due to CO2 dissolution, since the hydrothermal treatment was conducted in an open system, and the medium was alkaline (pH 8.5). It is also possible to observe that coprecipitation at constant pH led to the desired material even without treatment, and after hydrothermal treatment the material was more crystalline in relation to the TA-containing LDH, when compared to that obtained under the same treatment but at variable pH.Finally, the materials belonging to the MgAlDS system showed a broad reflection of about 20º (2-theta) under all synthesis conditions. This broad reflection was probably due to the Mg(DS)2 salt, which may be formed during the synthesis and present many x-ray diffraction peaks in that 2-theta region.Thermogravimetric analysis and differential thermal analysis Figure 2 presents the thermograms obtained for each prepared material, showing that each set of materials presents almost the same decomposition steps. Furthermore, the samples prepared at constant pH and those submitted to hydrothermal treatment presented more defined decomposition ranges. This behavior may be correlated with the higher crystallinity and phase purity of the cited materials. DTA curves (not shown) presented only endo- and isothermic processes, including those involving DS decomposition. The first decomposition step was a common feature of all of the materials prepared. This decomposition is related to water loss3, which occurs from room temperature to about 150ºC, and represents a mass loss of 5 to 10%.The TA-containing LDHs (Figures 2a and 2c) present three decomposition steps, with the first one related to water loss, as described above. The second decomposition was related to the first step of dehydroxylation, and represented a mass loss of almost 10%3. This decomposition occurred between 200 and 500 ºC for the materials prepared with the MgAl cation combination, and between 150 to 350ºC with the ZnCr cation combination20. The last decomposition, around 20%, was due to the second step of dehydroxylation and to TA decomposition19, occurring at different temperatures depending on the cation combination used, about 550 and 350ºC for the MgAl- and ZnCr-containing LDHs, respectively. This difference may be attributed to the different TA salts formed during the heating process in each case19. An overall view of these Figures shows that the materials obtained at variable pH, without hydrothermal treatment, i.e. vMgAlTA and vZnCrTA, present a distinct curve shape, which was related to the amorphous material present under these conditions. However, the materials obtained at constant pH without further treatment showed almost the shape of those treated, indicating again that they were obtained with a considerable crystallinity without treatment.The DS-containing materials (Figures 2b and 2d) presented four decomposition steps. The first one has been already explained. The second decomposition represents a mass loss of more than 20% and is related to the first dehydroxylation step and to the DS decomposition. This decomposition step occurs at about 150-200ºC; however, TG/MS measurements (data not shown) indicated that the DS decomposition proceeded until about 600ºC. Besides the decomposition of the residual DS, the second step of dehydroxylation occurred in the same temperature range, i.e., 250-600 ºC. Finally, the last decomposition can be observed at about 950 and 750ºC for the MgAl and ZnCr cation combination respectively, and represents a 10% mass loss. This step is due to the decomposition of the residual sulfate salt originated by the DS hydrophobic chain decomposition (as observed by TG/MS measurements). The difference in the decomposition temperature is related to the distinct salts formed in each

[学科分类] 化学（综合）

[关键词] layered double hydroxides;coprecipitation;intercalation;hydrotalcite.