Characterization Of Synthesised Layered Double Hydroxides Materials
Nadia Boukhalfa1, Nassima Djebri1, 2, Mokhtar Boutahala1
1Laboratory of Chemical Process Engineering Setif-1 University, Setif, Algeria
2Laboratory of Materials and Electronic Systems B.B.Arreridj University, B.B.Arreridj, Algeria
e-mail: nadouchette2011@hotmail.fr
Abstract—In this study, layered double hydroxides are synthesised by co-precipitation and reconstruction method.
Layered double hydroxides were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, surface measurment by Brunauer, Emmett and Teller method, and determination of point zero charge pH. Calcined layered double hydroxide are obtained by thermal treatment. The removal of water and carbonate during calcination lead to the formation of channels and pores which increased its specific surface area. The The X-ray pattern of calcined LDH show that the structure was destroyed and converted to an amorphous material after calcination and can be reconstructed by simple contact with anionic solution. The point zero charge pH value of these solids confirm that these materials are a good candidate for adsorption and catalysis applications.
Keywords—co-precipitation; layered double hydroxides;
calcination; characterization
I. INTRODUCTION
Layered double hydroxides or hydrotalcite-like compounds are a family of natural or synthetic materials with a formulation of [M12XMX3(OH) ][2 A zH Ox nn/ 2 ] where M2+ and M3+ are, respectively divalent and trivalent metals, and An- is the intercalated hydrated counter-anion. Layered double hydroxides (LHD), also called anionic clays, display unique physical and chemical properties surprisingly close to the properties of clay minerals. The general terms hydrotalcite- type (HT) compounds is also widely used, probably due to the fact that most of its characterizations have been carried out on hydrotalcite (a Mg-Al hydroxycarbonate), and that it can be easily and inexpensively synthesized [1]. The name LDH is derived from the early works of Feithnecht, who called these compounds “Doppelschichtstrukturen” (double sheet structures), hypothesizing a structure with intercalated hydroxide layers. This hypothesis was refuted many years later on the basis of single crystal XRD analysis, which showed that all the cations are placed in the same layer, with the anions and water molecules located in the interlayer region [2] as shown in Fig. 1.
Fig. 1. Schematic representation of the structure of LDH
These materials form successive positively charged layers, compensated by intercalation of hydrated negatively charged species. LDH has a structure similar to that of brucite (the common name for magnesium hydroxide, with a basal distance of 4.8 Å) and their interlayer distance depends on the size of the intercalated hydrated anion [1].
The most interesting properties of these layered double hydroxides that they present a high specific surface area and an important thermally stability [3]. In addition, synergetic effects between the elements, due to the intimate dispersion, which favors, for example, the development of unusual basic or hydrogenating properties [2]. It is worth noting that basic properties depend significantly on the composition and the calcination temperature.
They present “Memory effect”, which allows reconstruction under mild conditions (after calcination until 773 kelvin) of the original structure by contact with solutions containing various anions and they have a good anion exchange capacities [1]. Synthetic layered double hydroxides after thermal decomposition, find many industrial applications: they are used in catalyst as a support, in medicine as an antiacid and stabilizer and in water treatment as an adsorbent.
This paper summarizes the studies on preparation and characterization of ZnAl hydrotalcite-type material containing CO32- as the intercalated hydrated counter anion, we have also tried to reconstructed the hydrotalcite after calcined at 773k by imerging it in an anionic solution contain a pharmaceutical
compound. The samples have been prepared by anion exchange and direct coprécipitation procedure and characterized by different physicochemical techniques.
II. EXPERIMENTAL A. Materials
ZnCl26H2O is obtained from Biochem, AlCl36H2O is procured from PRS Panreac, and Na2CO3 is obtained from Flucka. NaOH is obtained from Rectapur, without further purification.
B. Preparation of LDH
The pristine CO3-LDH were synthesized by a standard co- precipitation method. A base solution was prepared by dissolving an amount of Na2CO3 in 800mL of deionized water. This solution was then added dropwise to 800mL of a mixed salt solution prepared by dissolving Zn(CO3)26H2O and Al (CO3)39H2O under vigorous stirring. The solution pH was adjusted to 10 by dropwise addition of 1 M NaOH solution into the mixed solution. A suspension was formed and aged at 353k for 24 hours. The products were filtered, washed with deionized water to remove any precursor impurities and then dried at 333k for 24 hours.
C. Preparation of reconstructed LDH
The intercalation products were prepared by reconstruction method. First, the oxide precursors were prepared by the thermal decomposition of the CO3-LDH at 773k for 4 hours in the absence of air and oxygen. Then, 1g of oxide precursor was added to an aqueous solution (100mL) containing an sodium diclofenac. We have also tried to reconstruct calcined LDH by adding 1 g of solid into water only. The solution was stirred vigorously at room temperature for 24 hours, then filtered, washed and dried as described above.
D. Characterization of LDH materials
Fourier transform infrared spectroscopy (FTIR) analysis of CO3-LDH, calcined LDH ( C-LDH), reconstructed LDH with water (H-LDH) and with diclofenac solution (D-LDH) samples was carried out in KBr pel-lets in the range of 4000- 400 cm−1 using FTIR 8400S Shimadzu spectrometer model.
Powder X-ray diffraction (XRD) data were collected on a Bruker Advanced X-ray diffractometer using CuKα radiation (λ =1.54Å) at a scan range between 6° and 90° in 2 theta (2θ).
The specific surface area of the CO3-LDH and C-LDH samples was analyzed using Brunauer-Emmett-Teller (BET) equation from N2 adsorption-desorption isotherm at 77k (liquid nitrogen temperature) by surface analyzer (Micromeritics Gemini VII). The samples were degassed at 200◦C for 2 hours before BET analysis.
The point of zero charge (pHPZC) of CO3-LDH and C-LDH was determined according to the method described by Benhouria et al. [4]. In brief, the initial pH (pHi) of 0.01 M NaCl solutions (50 mL) were adjusted to a pH range of 2-12 using 0.1 M HCl or NaOH. Then, 0.2 g of solid was added to
each sample. The dispersions were stirred for 24 hours at 295k, and the final pH of the solutions (pHf) was determined.
The point of zero charge was obtained from a plot of (pHf-pHi) versus pHi.
III. RESULTS AND DISCUSSION
The XRD diagrams for the host solids are shown in Fig. 2.
The XRD patterns exhibit the characteristic reflections of LDHs with a series of peaks, which are sharp and symmetric at low 2θ angle, but broad and asymmetric at higher 2θ angle [5]. The XRD patterns of the CO3-LDH showed three equally spaced peaks at 2θ values of 11.5°, 23° and 34.5° consistent with hydrotalcites with a hexagonal crystal structure [6].
In all the samples, a good crystallinity was observed. At 2θ of 11.5° and 11.6°, the basal plane d003 spacing was 7.83Å for CO3-LDH and H-LDH, and 7.67Å for D-LDH, respectively. The interlayer distance, value of d003, representing the summation of thickness of brucite-like layer and the galleriy heigh, which is a function of the number, the size and the orientation of intercalated anions. The X-ray pattern of C-LDH shows that LDH structure was destroyed and converted to an amorphous material with formation of metal oxides corresponding to d200 and d220 reflections. The X-ray pattern of H-LDH and D-LDH show the possibility of reconstruction of C-LDH by simple addition in anion solutions or simply in water.
Fig. 2. XRD patterns of precursors: CO3-LDH, C-LDH, H-LDH and D- LDH
Fig. 3 shows the reference FT-IR spectra for CO3-LDH, C- LDH, H-LDH, diclofenac sodium and D-LDH. The diclofenac sodium have characteristic absorption bands in the carbonyl region at 1507 and 1453 cm-1, which appear at 1508 and 1450
cm-1 in D-LDH spectrum, which indicate the intercalation of diclofenac in the surface of C-LDH. The reconstruction of C- LDH is confirmed by the spectrum of H-LDH wich is similar to CO3-LDH spectrum. The bands at 3500–3600 cm-1 and 1600–1650 cm-1 are indicative of OH group in the brucite-like layer [7] and the interlayer water. The temperature at which the LDH is calcined should be sufficiently high to eliminate most of the carbonate anions and any other anions in the interlayer region and low enough that it does not preclude the structural reconstruction. After calcination, these bands become less intense whish is shown in the spectrum of C- LDH. IR Bands Observed in all simples are summarized in Fig. 3 and TABLE I.
TABLE I. SUMMARY OF IR BANDS OBSERVED IN PREPARED SAMPLES
Assignment Band Position (cm-1)
O―H stretch (hydrogen bonded) 3500–3600 O―H stretch (interlayer water) 1600–1650
CO32- (v3) 1350–1380
CO32- (v2) 770–800
Aromatic skeletal vibration + C=O stretching 1450-1508
Fig.3. FT-IR spectra of CO3-LDH, C-LDH, H-LDH, diclofenac and D- LDH
As this materials are synthesis for adsorption application, the specific surface area of a solid is one of the first thing must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Brunauer,
Emmett and Teller showed how to extent Langmuir’s approach to multilayer adsorption and their equation has come to be known as the BET equation [8].
Brunauer, Emmett and Teller (BET) specific surface area of the two samples CO3-LDH and C-LDH are found to be 35 and 77 m2/g, respectively. The thermal treatment lead to the elimination of the interlayer water and to the liberation of the pores. [9].
For the point zero charge measurement, the CO3-LDH and C-LDH materials are added to NaCl solution, which lead to the reconstruction of C-LDH and to the increase in CO3
content. A higher CO3 content would increase the charge density of the LDH and consequently causing an increase in the magnitude of the zeta potential from 7.8 to 10.5. Fig. 4 shows the results. pHPZC is an important physical property of synthesis solids designed for adsorption applications. There are three domain of pH:
At pH< pHPZC, the surface is positively charged, adsorption of anions is advantageous.
At pH= pHPZC, the positive and the negative charges are equal, which indicates the electrical neutrality of the adsorbent.
At pH> pHPZC, the surface is negatively charged, adsorption of cations is advantageous.
Fig.4. pHPZC of CO3-LDH and C-LDH
IV. CONCLUSIONS
Four layered double hydroxide materials have been synthesized by co-precipation and reconstruction method and characterized by powder XRD, FTIR, BET surface mesurment and pHPZC determination. The XRD patterns of the CO3-LDH indicate the hexagonal crystal structure. The thermal treatment of LDH lead to the formation of metal oxides with the possibility to be reconstructed by contact with anionic solution, which is confirmed by FTIR results. The thermal treatment increases the specific surface area of CO3-LDH from 35 to 77 m2/g which lead to an important capacity of adsorption of this material. The value of pH of point zero charge of C-LDH indicates the large interval of adsorption pH
feasibility. These materials could be a high effective materials for adsorption and catalyst applications.
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