2. PREPARATION OF PILLARED CLAYS
3.1. ALUMINA PILLARED MONTMORILLONITES FROM HUNGARY
3.1.1. X- RAY ANALYSIS OF ALUMINA INTERCALATED AND PILLARED SAMPLES
The quickest way to determine whether pillar intercalation was successful is to record an X-ray diffraction pattern of an oriented film of the product. Such films are formed by allowing a suspension of the product to evaporate on a glass slide. Oriented film samples favor 001 Bragg reflections. The width at half maximum of the x-ray diffraction peaks can be an indication of the crystallinity of the pillared clay. Bigger line broadening will be appeared when the product’s crystallinity is lower. However, there are different sources of line broadening:
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.
Smectite clays exhibit a turbostratic stacking, which means that the layers stack flat (face – face) on each other, but without alignment of the ab planes. This defect can
contribute to variations in basal spacing and increase the line-broadening of the 001
It is also known that the Al13 reaction products of clays with very small particle sizes can exhibit an amorphous X-ray diffraction pattern. 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.
A basal spacing around 18.5-19.5 Å indicates right away that alumina pillaring occurred since the layer thickness of a smectite is 9.5 Å and the size of the Al137+
Keggin-like ion is about 9 Å. The Keggin-ion is rather symmetrical and contains all symmetry elements of a tetrahedra. Along its diagonal axis, the Al13 oligomer is 8.8 Å from the upper to the lower oxygen. If OH groups are present, then 0.6 Å is added for each O – H bond length and an approximate value of 10 Å is obtained. The largest direction is found along C2 axis 10.3 Å or 11.5 Å if OH groups are assumed. The tetrahedral structure is revealed in the macrostructure of these Keggin ion crystals. Table 3.1.1.
and Figure 3.1.8., 3.1.10., 3.1.12. show the basal-spacings of all samples of Hungarian montmorillonites, intercalated and alumina-pillared clays. X-ray diffraction patterns of the alumina - intercalated clays (black) and calcined alumina - PILCs at 350°C (red) and at 500°C (green) for all Mád samples and Na-Koldu samples are shown in series of Figure 3.1.1. – Figure 3.1.6. In Table 3.1.1. it is apparent that the peak positions of the d001 reflection of intercalated clay samples are between 17.85 Å and 19.45 Å;
indicating that all intercalated samples were indeed well intercalated with aluminum-polyoxycations. The lowest value was coming from Istenmezeje intercalated clay, and showed that intercalation was poorer and more unstable in this case. Basically, Istenmezeje clay is different than the other two since the change in basal spacing is the highest value, precisely 37% both for original and Na-form clays as it can be seen in Figure 3.1.7.
Table 3.1.1. The basal-spacings of the Hungarian montmorillonites, intercalated clays and alumina-pillared clays
Room
After the calcination
After the calcination Name of Montmorillonite Temperature on 350°C for 3h on 500°C for 2h
d001[Å] d001[Å] d001[Å]
Mád 15.37 10.05 10.03
Na-Mád 15.11 9.95 9.86
Mád-10 18.71 18.63 18.17
Mád-15 18.67 18.27 18.88
Na-Mád-10 19.30 18.81 18.03
Na-Mád-15 19.01 19.05 18.36
Istenmezeje 15.83 10.02 10.01
Na-Istenmezeje 15.58 9.76 9.75
Istenmezeje-10 19.39 14.74 14.02
Istenmezeje-15 19.43 15.02 15.19
Na-Istenmezeje-10 19.34 18.67 17.56
Na-Istenmezeje-15 17.85 17.89 17.50
Koldu 15.01 10.05 9.98
Na-Koldu 15.61 10.04 9.92
Koldu-10 19.03 14.85 13.55
Koldu-15 19.45 14.56 14.01
Na-Koldu-10 19.45 18.38 18.04
Na-Koldu-15 19.30 18.32 18.21
0 10 20 30 40 50 60 0
5000 10000 15000 20000 25000 30000
2h 500°C 3h 350°C Na-Mád 15
2Θ angle
Figure 3.1.1. The X-ray diffractograms of alumina pillared Na-Mád-15 montmorillonite
Figure 3.1.2. The X-ray diffractograms of alumina pillared Na-Mád-10 montmorillonite
0 10 20 30 40 50 60
0 5000 10000 15000 20000 25000 30000
2h 500°C 3h 350°C Na-Mád 10
2Θ angle
0 10 20 30 40 50 60 0
5000 10000 15000 20000 25000 30000 35000 40000
2h 500°C 3h 350°C Mád 15
2Θ angle
Figure 3.1.3. The X-ray diffractograms of alumina pillared Mád-15 montmorillonite
0 10 20 30 40 50 60
0 5000 10000 15000 20000 25000 30000 35000
40000 2h 500°C
3h 350°C Mád 10
2Θ angle
Figure 3.1.4. The X-ray diffractograms of alumina pillared Mád-10 montmorillonite
0 10 20 30 40 50 60 0
5000 10000 15000 20000 25000 30000 35000
2h 500°C 3h 350°C Na-Koldu 10
2Θ angle
Figure 3.1.5. The X-ray diffractograms of alumina pillared Na-Koldu-10
0 10 20 30 40 50 60
0 5000 10000 15000 20000 25000 30000
35000 2h 500°C
3h 350°C Na-Koldu 15
2Θ angle
Figure 3.1.6. The X-ray diffractograms of alumina pillared Na-Koldu-15 montmorillonite
Koldu and Mád clays have very similar behavior according to change in basal spacing (between 33%-36%) by calcination at 350ºC and also 500ºC as it can be seen in Figure 3.1.7. These per cent values are correspond with morphological data, namely such as the appearance of Koldu and Mád clay is very similar to each other, and Istenmezeje clay has a bit different features; the most jagged, frilled surface of the adhered bunches with the smallest grain size. The X-ray patterns of Istenmezeje clay have showed low intensity, well-defined basal reflections after exchange process, and then the calcined samples had line broadening and basal peaks were returned almost to original position.
More or less the sodium-form samples have kept the interlayer distance with little line broadening. It looks like these samples have no long range face-to-face order what could be observed by X-ray diffraction, probably the materials are delaminated.
In the case of original and sodium-form of Mád and Istenmezeje intercalated clay samples (Figure 3.1.8.-Figure 3.1.10.) the per cent change in basal spacings exhibits
Figure 3.1.7. Change in basal spacing by calcination of (A) Mád, (B) Na-Mád, (C) Koldu, (D) Na-Koldu, (E) Istenmezeje and (F) Na-Istenmezeje clay samples
C h a n g e in b a s a l s p a c in g b y c a lc in a tio n
Figure 3.1.8. Basal spacing of all Istenmezeje clay samples at room temperature, at 350ºC and at 500ºC
Figure 3.1.9. Change in basal spacing by calcination of pillared samples of (A) Istenmezeje-10, (B) Istenmezeje-15, (C) 10, and (D) Na-Istenmezeje-15 clay samples
Bas al s pacing of Is tenm ezeje clay s am ples
0
Figure 3.1.10. Basal spacing of all Koldu clay samples at room temperature, at 350ºC and at 500ºC
Figure 3.1.11. Change in basal spacing by calcination of pillared samples of (A) Koldu-10, (B) Koldu-15, (C) Na-Koldu-Koldu-10, and (D) Na-Koldu-15 clay samples
Basal spacing of Koldu clay samples
Figure 3.1.12. Basal spacing of all Mád clay samples at room temperature, at 350ºC and at 500ºC
Figure 3.1.13. Change in basal spacing by calcination of pillared samples of (A) Mád-10, (B) Mád-15, (C) Na-Mád-Mád-10, and (D) Na-Mád-15 clay samples
Basal spacing of Mád clay samples
three-steps columns for both 350ºC and 500ºC sintering temperatures. However, Mád clay samples (Figure 3.1.12.) show two-steps columns because both original and sodium-form samples keep about the same basal spacings values over 18Å after calcination.
Sintering at
500ºC:
Taking into consideration the change of basal spacing in the case of intercalated structures (Figure 3.1.9.-Figure 3.1.13.) at 500ºC sintering temperature; Koldu clay samples have the biggest decrease (29%, 28% and 5%, 6%for sodium forms) that it means these pillared structures are least of all stationary.
Besides Istenmezeje clay samples have very similar layered structure behavior to Koldu with high loss values (28%, 22% and 0%, 2% for sodium forms). The d(001) values of Istenmezeje intercalated clays were the smallest of all, and then the calcined samples had line broadening, as well as the basal peaks of clays were returned partly.
The sodium-form Istenmezeje clay samples have kept the interlayer distance, but also had little line broadening. Consequently, it means these layered clays have no long range face-to-face order can be observed by X-ray diffraction, probably the material is delaminated. Somehow, according to our long time experiences Istenmezeje clay could not forget its original layered structure and therefore has a high tendency to rebuild that instead of sodium form or aluminum-oxide pillared layered formation. It is similarly correlated to its swelling ability (see Figure 2.21.), which was the smallest among the Hungarian clays, and besides Istenmezeje clay has a doublet d001 peak. Istenmezeje pillared layered montmorillonite samples have lower thermal stability than other clays since the basal spacing of sintered pillared products is under 18Å. Mád clay samples have the smallest decrease in d-spacing (3%, 0% and 7%, 3% for sodium forms) that indicates the best pillared form samples both for original and sodium form, such as lower and higher concentration of clay suspension.
Pillared samples prepared by 15g/L concentration of clay suspension disclose smaller d-spacing loss for all clay than 10g/L at 500ºC sintering temperature.
Sintering at
350ºC:
Mád clay samples have again the smallest decrease in d-spacing (0%, 2% and 3%, 0% for sodium forms) at the lower sintering temperature that shows the best pillared layered structures in all cases. Both the Istenmezeje and the Koldu clay samples have valuable decrease (Figure 3.1.9.-Figure 3.1.11.) in basalspacing ( less for sodium forms) but Istenmezeje clay has the biggest drop in d(001) presenting the most unstable pillared formations.
Pillared samples prepared by 15g/L concentration of clay suspension disclose smaller d-spacing loss for sodium-clay samples, and larger loss for original forms than in the case of 10g/L suspension at 350ºC sintering temperature.
Furthermore, the data mentioned about Hungarian pillared clays also demonstrate the well-known fact for overseas (non-Hungarian) clays that the sodium forms have higher tendency or they are more suitable for good pillaring by producing controlled structures than the original bentonite structures. As we know, the minimum criteria for a material to be called pillared [142] are chemical and thermal stability and molecular distribution of pillars, but no order of the pillars is required in the interlayered region. The lamellae of the layered compound must be ordered so as to give an XRD pattern which allows the determination of the d(001) distance. The XRD pattern must show clearly the d(001) reflection, but a rational series of d(001) lines is not required. Applying the same pillaring procedure for all chosen Hungarian clay after sintering at lower and higher temperature it can be detected the expected d(001) spacing value about 19Å for Mád and Koldu clay but not for Istenmezeje clay samples. The basal spacings of Istenmezeje samples are under 18Å that demonstrate the not too high thermal stability of these pillared clays. It is a little bit surprising since Istenmezeje clay has the biggest (90 meq/100g for original and 95 meq/100g for sodium form) cation exchange capacity (CEC) compared other Hungarian clays (Table 2.4.). Mád and Koldu pillared layered clay samples absolutely fulfill the minimum criteria of pillared material after X-ray and thermal analysis. Since the final criterion is accessibility of the interlayer region by molecules at least as large as nitrogen, consequently porosity and surface area investigation of these two clays was required.