DIFFERENT KIND OF IRON-PILLARED MONTMORILLONITES

The iron oxide pillared clays formed by the reaction of sodium montmorillonite with hydrolyzed Fe³⁺ solutions depended critically on the hydrolysis conditions. The aqueous chemistry of Fe³⁺ is known to yield polymeric cations of substantial size.

Polymerization of iron typically begins at low pH (<1.5) and propagates by deprotonation of coordinated water molecules (olation) and hydroxy groups (oxolation).

The hydrolysis reactions of Fe³⁺ can lead to discrete spherical polycations as large as 30 Å in diameter. Aggregation of the spheres produces rods and eventually rafts of rods. The polymerization process is dependent on base to metal ratio, temperature, nature of the counterion, pH, and other factors. In this study, we have examined the intercalated products formed by the reaction of montmorillonite with iron-chloride and iron-benzoate ([Fe3benzoato6(OH)2]benzoate) solutions under different hydrolysis conditions.

3.2.1. Pillaring with iron-chloride

The speciation of hydrolyzed ferric chloride complexes is certainly less known than for aluminium salts. Before the flocculation threshold (n = (base) / (Fe) ~ 2.7), large colloids are formed during the hydrolysis. The size of polymers generally determined from electron microscopy varied with time from 2-4 nm in diameter to 7 nm. X-ray diffraction studies carried out on fully hydrated precipitates showed that a structural continuity exists. The evolution of the local range order progressively tends to that of Goethite when the hydrolysis ratio (n= (base) / (Fe)) is increased. Fe is always octahedrally coordinated and the condensed phases are formed by Fe sharing at both edges (olation) and corners (oxoolation). Table 3.2.1. shows the relationship between the base to metal ratio (B/M) used for the hydrolysis of ferric chloride solution, along with the basal spacing of the products formed upon reaction with sodium montmorillonite. In each case the time allowed for aging of the pillaring reagent was held constant at 24 hours.

Table 3.2.1. Iron pillared clays formed from hydrolyzed ferric-chloride solution

Incremental increases in B / M ratio over the range 1.0 to 2.5 brought increases in basal spacing in the range 24 to 25 . The observed gallery heights are consistent with the previously estimated size for polycations formed by iron hydrolysis in this B/M range.

All of the products described in Table 3.2.1. washed many times with distilled water prior to being air-dried. The washing process is extremely important for producing crystalline pillared products. At least ten wash-cycles were normally needed before the flocculation of the product was observed. The relationship between the product crystallinity, flocculation, and degree of washing suggested that hydrolysis of the clay-bound polycations continued during the wash cycle.

Table 3.2.2. Nitrogen adsorption data of iron-pillared samples B/M ratio

The aging time of the hydrolyzed pillaring solutions also was important in determining the nature of the pillared products. The relatively small increase in basal spacing (24.1 Å, 24.2 Å, 24.8 Å, and 25.2 Å) despite the appreciable drop in pH and extent of hydrolysis was suggestive of uniaxial polymer growth.

BET surface area measurements by nitrogen adsorption are shown in Table 3.2.2. For B / M = 1.0, the specific surface area of the sample (182 m²/g) is mainly non-microporous as the non-non-microporous surface area accounts for 171 m²/g. For B / M = 1.5, the sample has the same non microporous surface area (171 m²/g) but has more micropores which explains the increase of the total surface area to 218 m²/g. For B / M = 2.0, the same tendency is more pronounced with the same non-microporous surface area (164 m²/g), and much more micropores. By looking at the corresponding X-ray diffraction patterns, it seems that pillaring progresses with increasing hydrolysis ratio (B / M). These BET surface area measurements values are clearly demonstrated the existence of iron oxide pillared layered montmorillonite samples. All samples were sintered at 450ºC for 2 hours, and they showed an relatively poor thermal stability, therefore the pillared structure was collapsed. This is probably due to the iron species decomposing during the pillaring procedure. Bradley et al. [93] proposed the being of a tridecamer (Fe13) species similar to Al13 Keggin ion as a pillaring agent but its structure has not been unambiguously characterized. The hydrolysis of ferric chloride solutions afforded polycations of iron that were suitable for the pillaring of montmorillonite. The products obtained by the reaction of the polycations with sodium-montmorillonite crystallographically ordered along the 001 layer stacking direction.

3.2.2. Iron-benzoate as a pillaring agent

The [Fe3benzoato6(OH)2]benzoate, shortly iron-benzoate (or Fe3benz6) was chosen as an other iron pillaring precursor. This complex is very well-characterized [146-147], but not soluble in water; consequently this agent was dispersed in aceton, and then added to the clay suspension. To obtain the best iron intercalation procedure, the following preparation technologies were applied: (1) reflux, (2) shaking, and (3) stirring. The preparation of iron intercalated clays by reflux was not really appropriate synthesis way,

since the higher temperature is not favourable for the hydrolysis of iron, and consequently the reached d-spacings were mainly around 13-14 Å.

Table 3.2.3. Representative iron intercalated clay samples were prepared with shaking

Mark Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 Fe7 Fe8

Clay

[mg] 200 200 200 200 200 200 200 200

H2O

[ml] 18 18 18 18 18 18 18 15

Fe3benz6

[CEC] 1 1.5 2 3 2 2 2 3

Acetone

[ml] 2 2 2 2 2 2 2 5

Additive Na2SO4 NaBr Et4NCl

D001

[Å] 20.6 14.0 19.0 12.8 14.2 14.0 18.6 24.1

Table 3.2.4. Representative iron intercalated clay samples were prepared with stirring

Mark A1 A2 A3 A4 A5 A6

Clay [mg] 1000 200 150 150 200 200

H2O [ml] 30 20 10 10 20 20

Fe3benz6 [mg] 1500 1000 150 150 150 150

Acetone [ml] 70 20 8 8 20 20

few drops 10 drops few drops 50mg 5 ml

Additive ccHNO3 HClO4 ccHNO3 NaNO3 HClO4

50mg NaNO3

24.35 28.48 26.25 16.95 16.81 16.13

d-spacing [A] 12.28 15.51 13.03 12.53

15.04 13.52 13.8

13.53 12.65

The iron oxide intercalated clays, were prepared by shaking at room temperature, showed higher basal reflection values then samples formed by reflux as it can be seen in Table 3.2.3. The d-spacing values of obtained products are from 14 Å to 24 Å. The bulk of the intercalated structures were collapsed after calcination at 450°C, even in the case of high d-spacing values. The pillaring with stirring (see Table 3.2.4.) looks like is the best available procedure to make iron-intercalated materials at room temperature, because in this way were reached d-spacing around 24 - 28 Å, and the lowest value is around 16 Å. Calcination at 450°C causes the partial break down of pillared structures.

At room temperature, the benzoate was a suitable pillaring agent for preparing iron-pillared clays, and the exchange process was really critical to form porous structures because of this complex is not water-soluble.

Conclusion

Iron-intercalated clays were prepared using two kinds of iron-pillaring agents, namely iron-chloride and iron-benzoate. Iron-intercalated structures were depended critically on the hydrolysis conditions of iron. The pH of the solutions were kept under 3.5, because at this value the iron is formed iron(II)oxide precipitate on the surface of the silicate layers. By increasing the base to metal ratio it was detected higher surface area and bigger basal spacing values. The structures of the intercalated clays were collapsed under calcination process at 450 °C with only a few exceptions.

Results and Discussion 3