influence of iron removal of the synthesis of pillared clays - a surface study by nitrogen...

8/10/2019 Influence of Iron Removal of the Synthesis of Pillared Clays - A Surface Study by Nitrogen Adsorption

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Research paper

Inuence of iron removal on the synthesis of pillared clays: A surface study bynitrogen adsorption, XRD and EPR

J.G. Carriazo Chemistry Department, Science Faculty, Universidad Nacional de Colombia, Carrera 30 # 45-03, Bogot, Colombia

a b s t r a c ta r t i c l e i n f o

Article history:

Received 1 March 2011

Received in revised form 9 June 2012Accepted 16 July 2012Available online 5 September 2012

Keywords:

Pillared claysCharacterisation of pillared claysIron-oxide removalSurface study of pillared clays

The present paper reports a surface study on the synthesis of pillared clays before and after removal of ironoxides (with sodium dithionite) from the clay used as starting material. Four pillared clays were synthesisedwith Al- or Al-Fe-polyhydroxocationic solutions: two solids were prepared from the iron-removed clay andthe others from the non-pre-treated clay. All the solids were characterised by X-ray diffraction (XRD), elec-tron paramagnetic resonance (EPR), differential adsorption potential distributions (DAPDs) and fractaldimension from nitrogen adsorption. The natural clay was also analysed by transmission electron microscopy(TEM) to verify iron oxide clusters. XRD and EPR analyses revealed that iron-reduction with sodiumdithionite, under the studied conditions, did not affect the clay mineral structure and the clay became suc-cessfully pillared in all cases; however, nitrogen adsorption showed a decrease of external surface areasand an increase of micropore areas and volumes as a consequence of chemical pre-treatment. DAPDs indicat-ed that iron-oxide removal reduced the surface heterogeneity of the pillared clays but enhanced the micro-pore fraction in the solids.

2012 Elsevier B.V. All rights reserved.

1. Introduction

Modifying clay minerals via pillaring currently constitutes an im-portant and broad eld of materials and surface science research.Pillared clays are complex microporous systems, having enormouspotential for application in adsorption and catalytic processes(Carriazo et al., 2008, 2010). Many researchers thus show interest inclays which have been modied through pillaring and their applica-tion as catalysts in a wide range of reactions ( Carriazo et al., 2005,2007a, 2007b; Centi and Perathoner, 2008; De Stefanis andTomlinson, 2006; Ding et al., 2001; Gil et al., 2000, 2008; Serwickaand Bahranowski, 2004). Introducing inorganic pillars, in addition toimproving clay mineral strength and stability, increases microporosi-ty and provides greater surface area on the solid, thereby facilitatingreagentsaccess to potentially active sites for the catalysis of some re-actions (Barrera-Vargas et al., 2007). The synthesis of pillared clayshas been widely described in the literature (Aouad et al., 2005;Carriazo et al., 2009; Gil et al., 2000; Vicente and Lambert, 2003)and characterising their porous structure has been recently reviewed(Gil et al., 2008).

However, the synthesis of pillared clays requires several optimisa-tions to enable such a procedure to be applied at industrial level andmarketing them in the future. One such improvement is to reduce

previous purication treatments (minimal rening) of natural clays

(Storaro et al., 1996; Vaughan, 1998), such as eliminating organicmatter traces, soluble salts and iron oxides. Surface iron oxides arecommonly removed from some natural clays before pillaring becauseit is considered that they affect the synthesis and textural propertiesof the nal product (pillared clay), but a systematic study have notbeen reported in the literature. Previous works use puried clays forboth lab synthesis and preliminary studies of the properties of the re-sultant solids; however, using raw clays is preferred for scaling-upthe process. Excessive content of iron oxides in natural clays may in-terfere on both the preparation procedures and the performanceof the nal solids synthesised from these clays, because high iron-oxide content may modify the colloidal and rheological behaviourof clay suspensions as well as the cation exchange capacity of theclay mineral present. For more advanced processes, such as catalystmanufacturing, some good properties of shaped macro-structures(extruded solids, pellets, agglomerates, monoliths, etc) are desired,but the high iron content may decrease the mechanical and thermalstability of the nal products. Furthermore, both the acidic andredox properties ofnal pillared clays could be modied because ofthe high iron content in the starting natural clay.

Iron oxides are usually eliminated from clay minerals by reducingFe3+ to Fe2+ by sodium dithionite (Na2S2O4)(Bertolino et al., 2010;Drits and Manceau, 2000; Kunze, 1965), but this process can modifynatural clays'structural and surfaceproperties.In fact, redox reactionsmodify the chemical and physical properties of iron-containing smec-tites, such as cation exchange capacity, specic surface area, swellingbehaviour and ability to x interlayer cations (Drits and Manceau,

Applied Clay Science 6768 (2012) 99105

Tel.: +57 1 3165000x14403; fax: +57 1 3165220.E-mail address:[email protected].

0169-1317/$ see front matter 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.clay.2012.07.010

Contents lists available at SciVerse ScienceDirect

Applied Clay Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c l a y
http://dx.doi.org/10.1016/j.clay.2012.07.010http://dx.doi.org/10.1016/j.clay.2012.07.010http://dx.doi.org/10.1016/j.clay.2012.07.010http://dx.doi.org/10.1016/j.clay.2012.07.010http://dx.doi.org/10.1016/j.clay.2012.07.010mailto:[email protected]://dx.doi.org/10.1016/j.clay.2012.07.010http://www.sciencedirect.com/science/journal/01691317http://www.sciencedirect.com/science/journal/01691317http://dx.doi.org/10.1016/j.clay.2012.07.010mailto:[email protected]://dx.doi.org/10.1016/j.clay.2012.07.010


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2000; Komadel et al., 2006; Stucki et al., 2002). Consequently the tex-ture of iron-removed clays and that of pillared clays synthesised fromthese minerals may change. However, most of the studies on the re-ducing effect of sodium dithionite on clay minerals have been carriedout under inert conditions (using N2atmosphere) (Anastcio et al.,2008; Fialips et al., 2002; Komadel et al., 1999; Neumann et al.,2011; Stucki et al., 1984), and the extent of iron reduction is notcompletely understood. So, the effect of sodium dithionite on the

structure and texture of clay minerals under atmospheric air condi-tions is less known. On the other hand, some models about the reduc-tion mechanism of iron (III) to iron(II) under inert atmosphere havebeen proposed (Drits and Manceau, 2000; Manceau et al., 2000).Most of the rst models assumed that Fe(II) in reduced dioctahedralsmectites was ve-coordinated, butManceau et al. (2000) revealedthat iron is much more likely to maintain six-fold coordination aftercomplete reduction, with possible migration of some of the ironatoms from cis- to trans-sites duringthe reduction reaction, and creat-ing defects (vacant sites)in the octahedral sheets(Drits and Manceau,2000; Komadel et al., 2006; Manceau et al., 2000; Stucki et al., 2002).Recently Neumann et al. (2011) showed that reactions of Fe(II)/Fe(III) in clay minerals depend on a variety of mineralogical and envi-ronmental factors, and that iron content, the overall cationic composi-tion and the location of the negative excess charge determine whichstructural Fe(II) arrangement forms during Fe reduction using sodiumdithionite. Furthermore, they suggest that these observations can beused to rene the current model for structural Fe reduction proposedby Drits andManceau (2000). Thus, a model for reduction of iron in aniron-bearing smectite under air atmosphere is not discussed in the lit-erature, which makes difcult to predict theextent to which thestruc-ture of this mineral can be altered through the pre-treatment withsodium dithionite to remove iron oxides before a pillaring process,since the synthesis of pillared clays, as in the present work, is devel-oped under atmospheric pressure.

Al- and mixed Al\Fe pillared clays showing an excellent catalyticactivity in environmental oxidation reactions have recently beensynthesised from an iron oxide-rich smectite-type clay (Carriazo etal., 2005, 2007a); however, for optimising the synthesis, it should

be determined whether removing the natural iron oxides containedon the surface of this raw clay can enhance or reduce the nal pillaredclaystextural properties. The aim of this paper has thus been to pres-ent a study of the effect of removing iron-oxides from a clay (underatmospheric air conditions) on the synthesised pillared-clayssurfaceand textural properties.

2. Experimental

2.1. Pillared clay synthesis

Two pillared clays were synthesised from a bentonite from Valledel Cauca-Colombia and polyhydroxocationic aqueous solutions ofAl or Al\Fe(10%), as described elsewhere (Carriazo et al., 2005,

2008): Al-Pilc and Al\Fe(10%)-Pilc, labelled here as B-AlNR andB-AlFeNR respectively. Additionally, the same solids were synthesisedby an identical method but after chemical pre-treatment with sodiumcitrate, sodium bicarbonate and sodium dithionite (Na2S2O4) to re-move the iron oxides from the clay surface, as described in the litera-ture (Kunze, 1965). These pillared clays are named B-AlR and B-AlFeR(characters NR and R respectively indicate non-removed iron-oxidesor removed iron-oxides).

To remove the iron oxides from the clay surface, 10 g of clay wasadded to a mixture of 400 mL of 0.3 M sodium citrate solution with50 mL of 1 N sodium bicarbonate solution (Kunze, 1965). The resul-tant suspension was heated to 80 C, and then 10 g of sodiumdithionite was added, maintaining this temperature and a continuousstirring during 15 min. After this digestion period a volume of 100 mL

of a NaCl saturated solution was added to occulate the suspension

(Kunze, 1965). The resultant solid was centrifuged at 2200 rpm andthe supernatant extracted, and then washed several times up to theconductivity of supernatant was closed to that of distilled water. Thisprocedure of removing iron oxides was repeated on the solid.

2.2. Characterising solids

Transmission electron microscopy (TEM) analysis of the natural-

clay samples was performed using a Philips CM 120 (at 120 kV)transmission microscope with EDX analyser. The clay samples weredispersed in ethanol and then placed on a small copper grid coveredwith a carbon lm. The EPR spectra were obtained on an X-BandESP 300 BRUKER spectrometer at 77 K, 100 kHz eld modulationand 9.44 GHz frequency; 10 mW microwave power, 8000 sweptwidth, 10.25 G modulation amplitude, 40.96 ms time constant and2103 gain were used.

Powder X-ray diffraction (XRD) analysis was carried out in anX-Pert Pro MPD PANalytical equipment using 2 geometry and aBragg-Brentano conguration at room temperature and using a 0.01step size and 5 s step time. Nitrogen adsorption isotherms weretaken at 77 K using a Micromeritic TriStar 3000 adsorption analyserin the 104 to 0.99 P/P0 range. The samples were outgassed at90 C for 1 h and then at 350 C for 8 h. All calculations were devel-oped assuming both a 16.2 2 area for covering a nitrogen molecule(N2) and =0.81 g/cm

3 as density of nitrogen condensed in thepores.-Curves and t-plots were used to verify the micropore forma-tion: -curves were made using a non-porous silica as reference(Gregg and Sing, 1982), =Vads/V(0.4), and t-values (thickness ofmultimolecular layer) were calculated from Halsey equation:

t 3:54 5

LnP

P0

0BB@

1CCA

1=3

. Micropore areas and volumes were obtained

from t-plots. Adsorption potential distribution functions X(A) and thecharacteristic adsorption curves v(A) were used for evaluating thesolids energetic heterogeneity (Carriazo et al., 2008; Jaroniec et al.,1991, 1996). Total energetic heterogeneity results fromboth the contri-

bution of functional groups on the surface, impurities and defects, andthe contribution of microporosity (Carriazo et al., 2008; Jaroniec et al.,1991, 1996). X(A) was expressed as a differential potential distributionfunction (Carriazo et al., 2008):

X A dv A

dA

Fractal dimension (D) was determined from adsorption data (P/P0between 0.08 and 0.2 (Carriazo et al., 2008; Gil et al., 2004), using theAvnir and Jaroniec equation (Avnir and Jaroniec, 1989; Jaroniec,1995):

Ln x K 3D Ln A

where x is the adsorbed amount, K is a constant and A is the adsorp-tion potential.

A G RTLn P0

P

R is the universal gas constant, T is the absolute temperature andP0 and P are, respectively, the saturation and equilibrium pressuresduring gas adsorption.

3. Results and discussion

Microscopic observation (TEM images) showed the existence of

iron-oxide particles on the surface of the natural bentonite-clay

100 J.G. Carriazo / Applied Clay Science 6768 (2012) 99105


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(Fig. 1). These iron oxides or oxyhydroxides might be hematite, goe-thite or magnetite (Bertolino et al., 2010) or mixed aluminium-ironoxide nanoparticles (Schwertmann et al., 2000) supported on the

clay surface. In the present case iron oxide (or mixed aluminium-iron oxide) clusters were considered.

Typical values of g =2.0, g =4.3 and g =9.3 for iron paramagneticspecies were observed from EPR analysis (Fig. 2). Therst value (g =2.0) was attributed to iron oxides in cluster form, while g=4.3 andg=9.3 were assigned to Fe3+ in octahedral or tetrahedral chemicalenvironments (Bertolino et al., 2010; Carriazo et al., 2005). The lasttwo values could be interpreted as being iron ions in the octahedralsheets (Fe3+ replacing Al3+) of the clay mineral (Bertolino et al.,2010; Carriazo et al., 2005) plus iron ions contained in mixedaluminium-iron oxides deposited on the clay surface. An importantEPR result was the signicant decrease of signal intensity (decreaseof area under curve) for the B-AlR and B-AlFeR solids due to clay min-eral treatment with sodium dithionite. This conrmed an effective re-

moval of iron oxides before synthesis of pillared clay. However, thesmaller reduction of signal intensity for the B-AlFeR sample resultedfrom iron introduction during the course of this solid's synthesis.Moreover, the EPR spectra indicated that clay mineral structural fea-tures were maintained and, therefore, the chemical treatment foriron removal, under these conditions, did not have an apparent effecton the clay mineral structure of the smectite.

XRD analysis revealed the successful synthesis of pillared clay inall cases. The shift of the 001 diffraction maximum (d001basal spac-ing) from 14.7 (natural clay) to 17.3 for all the modied solids(Fig. 3a) clearly indicated that the smectite was pillared with thepolyoxocationic species (Carriazo et al., 2005, 2007a, 2009). The com-plete XRD traces (Fig. 3b) did not show other modications in thesesolids structure, revealing similar XRD patterns for the pillared

clays synthesised with or without iron removal. Only slightly higher

Fe

Fe

Si

Al

O

100 nmBentonite

200 nmBentonite

Iron oxide particles

Energy (keV)

Intensity

1.0 2.0 5.0 6.0 7.0

Fig. 1.TEM micrographs of iron oxide particles observed on the natural clay (bentonite), and EDX spectrum of the sample.

1000 3000 5000 7000

150

100

50

0

-50

-150

-100

B-AlNR

B-AlR

g = 2.0

g = 4.3

g = 9.3

Magnetic field (Gauss)

Intensity

1000 3000 5000 7000

150

100

50

0

-50

-150

-100

B-AlFeNR

B-AlFeR

g = 2.0

g = 4.3

g = 9.3

Magnetic field (Gauss)

Intensity

Fig. 2.EPR spectra for the pillared clays.

101J.G. Carriazo / Applied Clay Science 6768 (2012) 99105


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intensity of peaks at ~2=21, 2=26 and 2=28 for the iron-removed samples was observed, which correspond to quartz and

feldspar impurities in the natural clay. The intensity of the diffractionmaxima of these minerals increases because their surfaces arecleaned by iron-oxide elimination. This result conrmed that chemi-cal treatment with sodium dithionite did not affect the smectitestructure. The iron oxide species in the materials were not detectedby XRD because they were present in small quantities and had poorcrystallinity.

The nitrogen adsorption isotherms for the natural and pillaredclays are shown inFig. 4. The isotherms display H3 type hysteresis(IUPAC classication), characteristic of slit shape pores, indicatingthat the conguration of parallel plates of clay minerals wasmaintained (Carriazo et al., 2008). The adsorption capacity of pillaredclays was higher than that of natural clay, due to the formation of mi-cropores by the effective pillaring. Moreover, a slight difference in

adsorbed volumes was observed between pillared clays synthesisedwith or without iron-oxide removal; the volume adsorbed by B-AlNR was higher than that adsorbed by B-AlR and that of B-AlFeNRwas higher than B-AlFeR. The -curves (taking a nonporous silica asreference) and t-plots (Fig. 5) conrmed the increase of microporos-ity of the synthesised solids: when micropores were introduced intothe solids, adsorption in the low-pressure region was enhanced andthe (- or t-) plots were bent, in accordance with previous reports(Gregg and Sing, 1982). Pillared clays synthesised after iron-oxideelimination thus showed lower nitrogen uptake than their naturalcounterparts; B-AlFeNR was the material having higher apparentcomplexity (Fig. 5a).

Table 1shows the textural properties of the solids. Specic surfaceareas and pore volumes for all the pillared clays increased compared

to the initial natural clay values; BET and external surface areas of

0 2 4 6 8 10 12 142 theta, CuK

2 theta, CuK

Inte

nsity

(a.u.

)

Natural clay

B-AlNR

B-AlR

B-AlFeNR

B-AlFeR

d=17.3

d=14.7

(a)

0 10 20 30 40 50 60 70 80

Intensit

y

(a.u.

)

B-AlNR - (1)

B-AlR - (2)

B-AlFeNR - (3)

B-AlFeR - ( 4)

(1)

(2)

(3)

(4)

(b)

Fig. 3. XRD patterns for the pillared clays. a) Amplifying the d001peak. b) Completediffractograms.

0

5

10

15

20

25

30

35

40

45

50

55

Relative pressure (P/P0)

Volume

adsorbed(cm

3/g),STP B-AlNR

B-AlR

Natural clay

0.10 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 10

5

10

15

2025

30

35

40

45

50

55

60

65

Relative pressure (P/P0)

Volumeadsorbed(cm

3/g),STP B-AlFeNR

B-AlFeR

Natural clay

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fig. 4.Nitrogen adsorption isotherms for the natural clay and pillared solids.

0

10

20

30

40

50

60

0 1 2 3Volumeadsorbed(cm

3/g;STP)

(Vads/V(0.4))

Natural clayB-AlNR

B-AlR

B-AlFeNR

B-AlFeR

nonporous silica(reference)

0.5 1.5 2.5 3.5

00 2 4 6 8 10 12 14 16 18 20V

olumeadsorbed(liquid)(cm

3g-1)

t value ()

Natural clayB-AlNRB-AlRB-AlFeNRB-AlFeR

0.010.020.030.040.050.060.070.080.090.1

(a)

(b)

Fig. 5. -curves (a), and t-plots (b) for the pillared solids and the natural clay.



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B-AlR and B-AlFeR were lower than those of B-AlNR and B-AlFeNR asa consequence of previous treatment for iron removal, which indi-cates that treatment with sodium dithionite, under this conditions,allowed reduction of the free iron-oxides to decrease the number ofadsorption sites on outer smectite surface. Moreover, increase of mi-cropore areas and volumes were observed for B-AlR and B-AlFeRregarding B-AlNR and B-AlFeNR, although this result was more evi-dent between B-AlFeR and B-AlFeNR. Removing iron oxides fromclay surface allowed access of N2molecules to micropores generatedby the pillaring. Difference between B-AlFeR and B-AlFeNR microporeareas indicates a higher effect of pore blocking because introducingiron oxides in the synthesis of pillared clays. The adverse inuenceof natural iron-oxide particles on the pillaring with iron species prob-ably is related to the nucleation of larger iron polyoxocations yieldinglarger nanoparticles than those of aluminium oxide, since hydrolysisof iron cations forms colloidal species with much higher degree ofcondensation (Jolivet et al., 2000). On the other hand, the total specif-ic surface area of the pillared clays is given as the sum of microporearea plus external surface area (Stotal=Smp+Sext); therefore thetotal specic surface area is higher than BET area for all the pillaredclays.

Important differences were observed in the characteristic adsorp-tion curves for the solids (Fig. 6), indicating a possible surface

energetic variation. Pillared clays showed higher gas adsorption vol-umes than the natural clay throughout the adsorption potentialvalues as a consequence of their microporous structure, whilst solidsobtained after iron removal showed lower gas adsorption volumesthan clays pillared without such a chemical treatment probably be-cause the external specic surface area of former materials wasreduced.

Differential adsorption potential distributions for the pillaredsolids (Fig. 7) clearly revealed a shift of the maximum of the curvesto higher adsorption potential values compared to the natural clay(from 3.3 to 4.05.0 kJ/mol), indicating effective formation of micro-pore structures in the solids. However, the highest derivative valuesfor B-AlNR and B-AlR in the low pressure region (high adsorption po-tential values: 4.05.0 kJ/mol) suggest the formation of a higher frac-tion of micropore in these solids than for Al\Fe pillared clays.Pillaring with iron or aluminiumiron aqueous solutions leads tothe formation of iron oxide nanoclusters on the pillared clays(Carriazo et al., 2005) which are able to partly block microporosity.Furthermore, an additional complexity in the high adsorption poten-tial region was observed for non-pre-treated solid curves (B-AlNRand B-AlFeNR) (Fig. 7b) showing multimodal functions due to higher

Table 1

Textural properties of natural and pillared clays.

Solid BET surface area(m2g1)

Micropore area, Smp(m2g1), from t-plot

External surface area,Sext(m

2g1)Micropore volume, Vmp(cm3g1), from t-plot

Total pore volume (cm3g1)from Gurvitsch's method

Natural clay 42.8 3.4 35 0.0012 0.0470B-AlNR 72.1 74.0 18 0.0262 0.0829B-AlR 66.6 74.6 13 0.0264 0.0704B-AlFeNR 76.8 38.2 44 0.0135 0.1032B-AlFeR 69.3 60.5 25 0.0214 0.0866

0 1 2 3 4 5 6Volumeadsorbed(liquid)(cm

3g-1)

Adsorption potential (A) (kJ mol-1)

Natural Clay

B-AlNRB-AlR

0.01

0.00

0.02

0.03

0.04

0.05

0 1 2 3 4 5 6Volumeadsorbed(liquid)(cm

3g-1)


Natural clayB-AlFeNR

B-AlFeR

0.01

0.00

0.02

0.03

0.04

0.05

Fig. 6.Characteristic adsorption curves for the solids (natural and pillared clays).

0 1 2 3 4 5 6

-dV/dA(molcm

3g-1k

J-1)


Natural clayB-AlNRB-AlR

B-AlFeNRB-AlFeR

0.002

0.004

0.006

0.008

0.010

0.012

0.014(a)

2 3 4 5

-dV/dA(molcm

3g-1kJ-1)


B-AlNR

B-AlFeNR

0.003

0.004

0.005

0.006

0.007

0.008

0.009(b)

Fig. 7.Differential adsorption potential distributions dv A dA

h ifor the natural and pillared

clays. a) Complete curves and b) Expansion of an interval for non-pre-treated synthesised

solids.

103J.G. Carriazo / Applied Clay Science 6768 (2012) 99105


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heterogeneity on their surface. In this way, lack of removal of iron ox-ides contributed to higher surface heterogeneity of the pillared clays,but it reduced the micropore fraction in the as-synthesised solids.This effect was associated with a variation in the external andmicroporo surface areas (Table 1). On the other hand, the rise in ad-sorption potential distribution functions at low adsorption potentialvalues (high relative pressures) revealed the multilayer adsorptionand capillary condensation (Carriazo et al., 2008) which was moreimportant on the natural and Al\Fe pillared clays due to their higherexternal surface areas (Table 1).

The fractal dimension (D) determined for the solids (Fig. 8andTable 2) showed an increase in surface roughness as a result of thesuccessful pillaring of the natural clay to produce microporous struc-tures (D from 2.6941 to 2.8583). In general, D value may vary from 2to 3, with the lowest value (2) corresponding to a perfectly smoothsurface, and the upper limit (3) corresponding to a totally irregularor rough surface (Avnir and Jaroniec, 1989; Jaroniec, 1995). D valuesfor B-AlNR and B-AlR were higher than those for Al-Fe pillared clays,conrming the higher microporosity of the former solids. Althoughthese differences in D parameter were important, the fractal dimen-sion could not verify any variation induced by the iron-oxide removalpre-treatments, probably because all the synthesised solids have highmicroporosity and therefore elevated roughness.

4. Conclusions

In this work, XRD and nitrogen adsorption analyses conrmed thesuccessful pillaring-intercalation of a Colombian bentonite, whichyielded similar microporous structures with or without previouschemical treatment with sodium dithionite to remove iron oxides.The pre-treatment with sodium dithionite did not affect the structureof smectites in the bentonite. Removal of iron oxides from the clay re-duced the external surface area and increased the micropore area andvolumes for Al- and Al-Fe-pillared clays (this effect being higher inthe Al-Fe-pillared clay) compared to the solids prepared withoutsuch a treatment. Differential adsorption potential distributions andthe characteristic adsorption curves revealed some variations in ener-

getic terms, indicating that eliminating iron oxides in this bentonitecontributed to a lower surface heterogeneity of pillared clays and in-creased the fraction of micropores in the solids. These results contrib-ute in understanding some chemical pre-treatments and optimisationprocedures required for the pillaring of this Colombian bentonite.

Acknowledgments

The author gratefully wishes to acknowledge the UniversidadNacional de Colombia (Bogot) for supplying the resources for thisscientic investigation.

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Langmuir 7, 2719

2722.

Ln (A)

Ln(V)

Natural clay

B-AlNR

B-AlR

6.7 6.9 7.37.1 7.52.5

3.3

3.1

3.5

2.7

2.9

Ln (A)

Ln(V)

Natural clay

B-AlFeNR

B-AlFeR

6.7 6.9 7.37.1 7.52.5

3.3

3.1

3.5

2.7

2.9

Fig. 8.Linear relationship (AvnirJaroniec equation) for determining the fractal dimensio n of pillared clays by nitrogen adsorption.

Table 2

Fractal dimension (AvnirJaroniec equation) of solids studied here.

Solid Equation Fractal dimension (D) r

Natural c lay y=0.3059x+4.9799 2.6941 0.9998B-AlNR y=0.1417x+4.1129 2.8583 0.9988B-AlR y=0.1464x+4.0682 2.8536 0.9987B-AlFeNR y =0.2261x+4.7293 2.7739 0.9997B-AlFeR y=0.2086x+4.5119 2.7914 0.9995

Correlation coefcient.



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influence of iron removal of the synthesis of pillared clays - a surface study by nitrogen...

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