C ONTROLLING P ORE D IMENSIONS

The distance between the silica layers as well as the distance between the pillars, the interlayer and interpillar spacings characterize the pore structure of pillared clay, respectively. Changing either interlayer or interpillar spacing can vary the pore dimensions. With different pillaring species, the interlayer spacing can be altered due to the different structures of the intercalating species. The interpillar distance can be

controlled by the amount of pillars introduced between the layers. The charge density and distribution on the silicate layers of the clay are the two factors that determine the pillar density. The clay with a higher CEC can ion-exchange more pillaring oligomers, and subsequently form a pillared clay with a smaller interpillar spacing, which can serve as another adjustable parameter for achieving desired pore dimensions.

Table 3.1.2. Nitrogen adsorption data of the Hungarian parent clays

Table 3.1.3. Nitrogen adsorption data of the Hungarian pillared layered clays

The specific surface area is typically obtained from the adsorption branch by applying the BET equation. Approximately 0.21 - 0.26 g sample was degassed at 250ºC for 48 - 72 hours under high vacuum (10^-5 mbar) in the outgassing section of the apparatus. The isotherms were then obtained at liquid nitrogen temperature, at 77K.

Name BET surface area Vmicro

[m²/g] [cm³/g]

Na-Mád-10 204 0.17

Na-Mád-15 197 0.17

Na-Koldu-10 151 0.1

Na-Koldu-15 176 0.11

Name BET surface area V_micro

[m²/g] [cm³/g]

Mád 91 0.17

Na-Mád 97 0.15

Koldu 40 0.08

Na-Koldu 26 0.05

Istenmezeje 33 0.07

Na-Istenmezeje 84 0.14

Figure 3.1.14. Data of Mád montmorillonite from nitrogen adsorption-desorption

Figure 3.1.15. Data of Mad-15 clay from adsorption-desorption measurements

BET specific surface areas and micropore volume data of Hungarian clays (Table 3.1.2.

and Table 3.1.3.) were calculated using the N2 adsorption data in the relative pressure (P/P0) range between 0.01 and 0.1 where the BET plots are linear, or the correlation higher than the average surface area value (50 m²/g) of overseas bentonites. Probably, this property is related to the relatively high CEC of Hungarian clays. Alumina pillared Hungarian clays (Na-Mád and Na-Koldu) have specific surface area around 200 m²/g (Table 3.1.3.) that is nicely meets the accepted requirements. The isotherm of alumina-pillared clay is Type I isotherm. A significant increase in N2 adsorption at a lower relative pressure interval is observed for all these samples, indicating the presence of micropores. The mean micropore size of PILCs could be different from the interlayer free spacing obtained by XRD. The d001 peak reflects the regulatory in the arrangement of the clay sheets, while there is certainly an inevitable degree of irregularity in the arrangement. This results in various pores of different sizes and accordingly, PILCs have a pore size distribution (PSD). The mean micropore size is the average of all micropores so that it should be an intermediate value between the pore sizes of the main pore groups. The mean pore size has a higher value, 19 Å for alumina pillared Koldu clays and 25 Å for alumina pillared Mád clays, compared to the gallery height measured by XRD. The pore structure of these samples is much complicated than that was anticipated previously [122]. The sheets deviate from the geometry of a flat plane.

Part of the resultant pores could be larger and part of them smaller, compared to the parent samples. The pore structure of the products are even more irregular and their pore size distributions are more broadened. Pillared clays usually exhibit a broader pore size distribution than zeolites. Dimov and coworkers [143] proposed an excellent structural model of Al13-pillared montmorillonite. The model is based on literature data and on a comparative investigation using TEM. According this model the structure of the pillared montmorillonite may be represent by unit cell of an enlarged interlayer montmorillonite with OVO (Occupied-Vacant-Occupied), OVVO, or OVVVO alternating

pillar layers in the interlayer. The calculated pillar-site occupancy of the OVO alternation is 25%, OVVO modification is 11.1%, and OVVVO variation is 6,25%.

However, an appropriate model can represent the structure of a pillared montmorillonite with different pillar-site occupancies by choosing the correct O-V alternation. The modeled pillar layer consists of lower, middle, and upper atomic planes containing Al atoms with equal z coordinates, respectively.

Table 3.1.4. Si / Al ratio of original and pillared clays on the basis of elemental composition

Thirty one atomic positions may be statistically occupied by 52 Al atoms of the four pillar types, namely such as deformed (P), rotated (R), mirror (M), and centrally inverted pillar (C). They confirmed that the atomic structure of the modeled pillar montmorillonite retains the C2/m symmetry of the modeled enlarged-interlayer montmorillonite and respectively the C2/m symmetry of the 2:1 montmorillonite layer.

The distribution of the pillars of Hungarian clays is required additional investigation on the basis of elemental composition. As expected the Si / Al ratio is the highest for the starting Koldu and Mád montmorillonite, 2.14 and 2.83 respectively (Table 3.1.4.), as well as in the pillared clays, this value decreases down showing that a high amount of aluminum has been incorporated into the network. The exact structure of the pillars is required additional investigation and careful calculation taking high attention for error range and variation in Si /Al ratio due to presence of more pillars.

Sample Si / Al

(atomic ratio)

Na-Mád 2.83

Na-Mád-10 1.49

Na-Mád-15 2.01

Na-Koldu 2.14

Na-Koldu-10 1.27

Na-Koldu-15 1.69

Concluding Remarks

Clays may be modified introducing alumina pillars between the layers to obtain microporous solids. In general, the pillaring process is carried out with diluted systems:

a diluted clay suspension (less than 1wt%) is brought in contact with a diluted pillaring solution (0.5M for aluminum). From an industrial viewpoint, in which large amounts of clay have to be prepared, this process may result uneconomic, since enormous equipments as well as the manipulation of huge volumes of water are required.

Schoonheydt et al. [43] have reported the preparation of alumina-pillared saponite using 6-10wt% clay suspensions. Molina et al. [144] have used a dialysis technique for intercalation and washing to prepare alumina-pillared montmorillonite from a 40wt%

clay slurry. They observed that the intercalation occurs mainly during the washing step, where, indeed, the system is highly diluted. Fetter et al. [145] have investigated alumina pillaring with 10, 15, 20, 40, 50, and 70wt% clay slurry concentrations using microwave irradiation. When 70wt% suspension was used the pillaring was not successful, only a small fraction of the clay turned out to be pillared; in the other cases the intercalation was favourable. The advantage of the microwave irradiation during clay pillaring is not only time shortening but a resulting higher surface area than in the conventionally prepared clays. Using Hungarian bentonites, namely such as Istenmezeje, Koldu and Mád, the alumina-pillaring was succesfuly carried out in the case of highly concentrated clay suspensions (10 and 15wt%) and highly concentrated aluminum solutions with simple stirring engineering at room temperature. This method provides an economical way to synthesize alumina-pillared clays in a high scale production.

Results and Discussion 2