M ETHODS AND EQUIPMENTS FOR STRUCTURE ELUCIDATION OF PILLARED CLAYS

The methods to be overlooked in the following were employed in our scheme of the structure characterization of the starting and the produced materials in this research work.

2.1.1. X-ray diffraction measurements

The X-ray powder diffraction method is unique in that it is the only analytical method that is capable of providing qualitative and quantitative information about the compounds present in a solid sample. The X-ray diffraction measurements provide much useful information about the structure of materials. The identification of a species from its powder diffraction pattern is based upon the position of the liQHVLQWHUPVRI RU DQG WKHLU UHODWLYH LQWHQVLWLHV 7KH GLIIUDFWLRQ DQJOH LV GHWHUPLQHG E\ WKH

spacing between a particular set of planes; with the aid of the Bragg equation, this distance d is readily calculated from the known wavelength of the source and the measured angle. Line intensities depend upon the number and kind of atomic

reflection centers that exist in each set of planes. Identification of crystals is empirical on the basis of database system of International Centre for Diffraction Data. If the sample contains two or more crystalline compounds, identification becomes more complex. By measuring the intensity of the diffraction lines and comparing with standards, it is also possible to make a quantitative analysis of crystalline mixtures.

The quickest way to determine whether pillar intercalation was successful is to record the X-ray diffraction pattern of an oriented film of the product, and compare the d-spacing values of the host, intercalated and pillared clays. Such films, clay mineral aggregates prepared by suspending a sample in deionized water and pipetting the suspension onto an glass slide/plate. Note that the resulting mounts are not infinitely thick, and the intensities of higher angle peaks are therefore weaker than otherwise for an infinitely thick sample. Oriented film samples favor 001 Bragg reflections; the expansion of the inter-lamellar spacing, corresponding to the c lattice parameter, results

LQ D PDUNHG VKLIW RI WKH UHIOHFWLRQ WR D ORZHU YDOXH 7KH VPHctite basal (001)-diffraction pattern varies as a function of numerous factors, including the interlayer-cation type and relative humidity. The width at half maximum of the X-ray diffraction peaks can be an indication of the crystallinity of the pillared clay. The meaning of measured data is ambiguous since there are different sources of line broadening.

(1) Particle-size broadening arises as a consequence of the small size of clay crystallites. The diffraction peak width can be used to estimate quantitatively the particle size, or more precisely, the size of the scattering domain, by the Scherrer equation: / . FRV , where L is the mean crystalline dimension in Ångstroms along the c-axis, K is a constant very near unity and is the width of a 2 UHIOHFWLRQDWKDOI-height expressed in radians.

(2) Smectite clays exhibit a turbostatic stacking, which means that the layers stack flat (face – face) on each other, but without alignment of the a b planes. This defect can contribute to variations in basal spacing and increase the line-broadening of the 001 reflection.

(3) Also, since hydrated pillars prop the layers apart, differences in pillar density or hydration can lead to a distribution of basal-spacings. This distribution may be correlated to the density of the pillars and thus to the cationic exchange capacity

(CEC) of the clay. Charge localization on the layer can also be an important factor in determining pillar distribution. For example, an uncalcined pillared form of a fluorohectorite with a relatively large and localized charge density of 140 meq / 100 g exhibits much sharper XRD peaks than an analogous Wyoming montmorillonite with a low and delocalized layer charge of 75 meq / 100 g.

Pillared clays with very small particle sizes can exhibit an amorphous X-ray diffraction pattern. As previously discussed, the absence of X-ray diffraction peaks shows that no long range face-face layer aggregation is present and that the material is an edge-face delaminated clay.

X-ray powder diffraction (XRD) data were collected on a D500 Siemens, and also a Philips PW 1730/10 (type of goniometer PW 1050/70) diffractometer using CuKD (40 KV, 35 mA) radiation and secondary beam graphite monochromator. Data were collected from 2 to 30º2 XVLQJDVWHSsize of 0.03º2 DQGDFRXQWWLPHRIVHFSHU

step.

2.1.2. Elemental analysis

Elemental analysis enables one to determine the amount of metal per unit cell that has been incorporated in the clay by pillaring. The first step in this determination is to obtain a satisfactory analysis of the starting clay. Several techniques can be used to solubilize the sample for chemical analysis for instance fusion with lithium metaborate.

The composition is then analyzed by X-ray flourescence spectrometry, ICP emission or atomic absorption.

The explanation how to obtain a unit cell formula from the elemental analysis will be explain in chapter of alumina pillared results. Difficulties in determining the unit cell composition can arise from the presence of mineral impurities such as quartz, kaolin, and other fine grain materials, and from inaccurate subtraction of such impurities from the bulk analysis.

It is not necessary to obtain a complete unit cell formula to obtain the amount of aluminum incorporated for one O20(OH)4 unit. This amount can be obtained from the

difference in the aluminum content before and after the pillaring reaction. It is given by the following simple formula:

NAl = NSi ((AlA / SiA) – (AlB / SiB))

where NAl and NSi are , respectively, the number of incorporated aluminum atoms and the number of layer silicon atoms for one O20(OH)4 unit of the host clay and AlA, AlB,

SiA, SiB are, respectively, the relative molar amounts of aluminum and silicon after and before the pillaring reaction. The amount of metal incorporated often is correlated with the charge density of the clay and thus with the CEC. This correlation, however, requires that the same synthetic procedures are used to prepare each sample.

X-ray fluorescence spectra were taken on a Philips 1480 spectrometer (50 kV, 50 mA) using Cr anode, PE crystal and flow counter. Quantitative evaluation was possible for some components using analytical standards for calibration.

Atomic absorption measurements were carried out on a Perkin-Elmer photometer.

2.1.3. Electron Microscopy

Morphological characteristic of parent clays can be seen using Scanning Electron Microscopy (SEM). SEM measurements were carried out with JEOL JSM 50A microscopy at 25 kV accelerates voltage. Furthermore, it was applied EDAX analysis.

The structure of pillared clays can be confirmed by lattice imaging using high resolution Transmission Electron Microscopy (TEM), but this is not a routine technique.

2.1.4. Pore structure by adsorption-desorption techniques.

Adsorption-desorption isotherms of probe molecules can provide quantitative information on the pore structure of pillared clays. The main information obtained from N2 adsorption is the specific surface area of the sample. Usually, a non-pillared clay exhibits a surface area of less than 50 m²/g whereas a pillared clay has a surface area in the 200-400 m²/g range. The surface area is typically obtained from the adsorption branch by applying the BET equation. However, for microporous solids like pillared clays, the BET equation does not apply over the usual partial pressure range between

0.05 and 0.25, and the Langmuir treatment has then been used by some authors. The Langmuir equation is derived from a very simple model of monolayer adsorption which does not take into account lateral interactions between the adsorbate molecules. This equation usually fits surprisingly well to the adsorption isotherm of microporous solids.

However, this does not mean that the Langmuir model actually describes the adsorption process in the micropores. Since the Langmuir model is very different from the BET model of multilayer adsorption, there is no utility in comparing surface area values derived from these two models. As many workers typically report BET surface areas of solids, it is customary to report BET surface areas for microporous solids like pillared clays. However, one should ensure applying the BET equation in the pressure range where it is valid, ie. wHere the correlation coefficient is near one. For pillared clays, the range of validity of the BET equation is usually between P/P0 = 0.01 and 0.1.

The validity of the Langmuir equation is an indication of the presence of micropores.

However, it is much more useful to use the t - plot or the Ds - plot methods, which not only indicate the presence of micropores, but also allow one to quantify them [137].

The main idea of the t - plot and the αs - plot methods is to compare the adsorption isotherm of a porous solid. The t – plot is a plot of the adsorbed volume on a sample versus the statistical thickness of the adsorbed layer on the non – porous reference.

The values of the standard adsorbate layer statistical thickness, t, are obtained with the help of the BET theory. In the αs - plot method, the BET theory is not used. The αs

values are the ratios of the number of adsorbed molecules per unit area at P/P0 to the number of adsorbed molecules per unit area at P/P0 = 0.4. For a non microporous solid, a plot of the adsorbed volume versus t or Ds will be a straight line passing through the origin. The slope of this resulting straight line is proportional to the surface area. For microporous solids like pillared clays, the t – plot exhibits two regions. In the first region of the t – plot, the first few data points are fitted to a straight line passing through the origin. The slope of this line yields an equivalent surface area. A second domain of data points below t = 6 are fitted to another straight line. The intercept of this line gives the microporous STP volume (VSTP) which can be converted to a microporous liquid volume (Vliq = 0.00154VSTP) and to an equivalent microporous surface area (Smic

= KVSTP). The slope of this second line (bt1) provides the mesopore plus macropore surface area.

Pillared clays usually exhibit a broder pore size distribution than zeolites. With reliable data for argon or nitrogen adsorption in the low pressure range, one also can estimate the micropore size distribution from the isotherm [138]. Most workers favor the desorption branch of an adsorption/desorption isotherm to evaluate the mesopore size distribution. With pillared clays, a slit-shaped or parallel pore model is appropriate. The mesopore plus external surface areas obtained from this method should be in agreement with the t – plot results.

The nitrogen adsorption experiments were carried out employing a Quantachrome AUTOSORB-1 automatic analyzer, and furthermore MICROMETRITICS ASAP-2000 equipment, and using the volumetric method to calculate the adsorbed amount of nitrogen. The samples were outgassed at 523K for 48-72 h under high vacuum (10^-5 mbar) in the outgassing section of the apparatus. Then they were placed in the sample cell station and the experiment was carried out at 77 K in a constant level liquid nitrogen bath. Using the software of the analyzer one can either perform an experiment choosing the appropriate conditions (equilibration time, P/P0 tolerance etc.) or analyze the data calculating the surface area and the pore volume with a variety of methods (BET, Langmuir, t-method etc.).

2.1.5. FTIR spectroscopy.

Infrared spectra in the region 400-4000 cm^-1 were measured with a Nicolet 550 infrared spectrometer equipped with a DTGS detector. Each spectrum was the average of 200 scans collected at 2 cm^-1 resolution, by means a SPECAC variable-angle attachment. Samples were in the form of KBr pellets containing ca. 2% wt sample.

2.1.6. Thermal analysis

Thermal analysis involves a dynamic phenomenological approach to the study of materials by observing the response of these materials to a change in temperature.

The differential thermal analysis (DTA) curves show the effect of energy changes

(endothermic or exothermic reactions) in a sample [139]. For clays, endothermic reactions involve desorption of surface water and dehydration (e.g. interlayer water) at low temperatures (<100ºC), dehydration and dehydroxylation at more elevated temperatures, and eventually, melting. Exothermic reactions are related to recrystallization at high temperatures that may be nearly concurrent with or after dehydroxylation and melting. The thermal gravimetric (TG) curves ideally show only weight changes during heating. The derivative of the TG curve, the DTG curve, shows changes in the TG slope that may not be obvious from the TG curve. Thus, the DTG curve and the DTA curve may show strong similarities for those reactions that involve weight and enthalpy changes, such as desorption, dehydration and dehydroxylation reactions. A Hungarian made simultaneous TG – DTG - DTA instrument, Derivatograph-C was utilized at a heating rate ( RIGHJPLQWRGHWHUPLQHWKH

thermal gravimetric (TG), derivative thermal gravimetric (DTG), and differential thermal analysis (DTA) curves of the original clays in temperature range 25 - 1000°C. Each of the powdered clay material was placed on a platinum plate sample holder and investigated in air.

2.1.7. Cyclic Voltammetry.

Voltammograms were recorded on a PAR Model 174A polarographic analyzer operated in conjunction with a Model 175 universal programmer. A conventional H-cell was used with pyrolytic graphite as working electrode (0.25cm²), a saturated calomel reference electrode and a platinum gauze as the counterelectrode. APTEOS-clay films were cast onto the freshly cleaved pyrolytic graphite electrodes by allowing 10 µm of a aqueous 1% organosilane-clay suspension to evaporate at room temperature on the electrode surface. The APTEOS-clay suspension was prepared by ultra-sonication for fifteen minutes followed by settling time of 10 minutes.

Concluding Remarks

A wide reange of host clays and pillaring reagents can be used for pillaring synthesis.

Two methods are essential for the characterization of pillared clays, namely, X-ray diffraction and surface area measurements. The former method reveals whether an intercalation reaction has ocurred between the clay layers; the latter demonstrates that the intercalated pillars are sufficiently laterally to generate an internal microporous volume. Taken with additional refinements, these methods can also provide more information on the crystallinity and on the pore structure of the pillared clay.