Preparation and characterization of Cr and Fe-pillared bentonites by using CrCl 3, FeCl 3, Cr(acac) 3 and Fe(acac) 3 as precursors

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Microporous and Mesoporous Materials 111 (2008) 211–218

Preparation and characterization of Cr- and Fe-pillared bentonitesby using CrCl3, FeCl3, Cr(acac)3 and Fe(acac)3 as precursors

Muruvvet Yurdakoc a, Mehmet Akcay b, Yalcın Tonbul b, Fatih Ok b, Kadir Yurdakoc a,*

a Dokuz Eylul University, Faculty of Arts and Sciences, Department of Chemistry, Kaynaklar Kampusu, 35160 Buca- _Izmir, Turkeyb Dicle University, Faculty of Arts and Sciences, Department of Chemistry, 21280 Diyarbakır, Turkey

Received 12 October 2005; received in revised form 13 July 2007; accepted 18 July 2007Available online 28 July 2007

Abstract

Four pillared bentonites (Cr-PILB, Cr(acac)3-PILB, Fe-PILB and Fe(acac)3-PILB were prepared and characterized by X-ray diffrac-tion (XRD), nitrogen sorption measurements and thermogravimetric (TG) analysis. The surface acidities of the samples and their struc-tures were also investigated in the gas phase adsorption data of pyridine by the aid of FT-IR spectroscopic techniques. The XRD dataand FT-IR spectra of the samples reflected mainly the structure of bentonite. The nitrogen adsorption–desorption isotherms of the sam-ples were Type II shaped and showed in general mesoporous structures with pore openings of 4 nm. Two steps mass losses were observedin the TGA thermograms of B, Cr-PILB and Fe-PILB, while three steps mass losses were detected in the case of Cr(acac)3-PILB andFe(acac)3-PILB. IR study by the adsorption of pyridine on the samples showed both Lewis and Bronsted acid sites on their surfaces.� 2007 Elsevier Inc. All rights reserved.

Keywords: Bentonite; Pillared bentonite; Cr-pillared bentonite; Fe-pillared bentonite; Surface acidity; Cr(acac)3; Fe(acac)3

1. Introduction

It has been well known that pillared clays are synthe-sized by cation exchange of the Na+ or Ca2+ ions in a claywith polymer oxy-hydroxy cations of Al, Fe, Cr, Zr, Ti, etc.the resultant materials, after calcinations, contain oxide pil-lars which open the clay sheets and thus expose the internalsurfaces of the clay layers. On this concept, a special issuewas published in Catalysis Today [1] and a review wasgiven by Figueras [2]. The main uses of pillared clays (PIL-Cs) are seen to be in acid catalysis. Cr- and Fe-pillaredclays, in particular, have attracted the researchers’ forbeing used as catalysts in a wide range of reactions, likecatalytic conversion of hydrocarbons, hydrocracking ofheavy liquid fuels [3–6], catalytic acylation of alcohols [7],for phenol hydroxylation [8], for ethylbenzene dehydroge-nation [9]. The chromium and iron species inserted are

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.07.032

* Corresponding author. Tel.: +90 232 4128695; fax: +90 232 4534188.E-mail address: [email protected] (K. Yurdakoc).

mostly polynuclear hydroxo-species formed by hydrolysisof the Cr(III) and Fe(III) monomer. Several methods havebeen developed to produce the oligomeric species that areto be incorporated into the interlamellar space of smectite[10–12]. Specific types of Cr-, Fe- and Fe sulphide pillaredclays were also prepared, characterized and used as acidcatalysts [3–21]. In the literature, we can see the usage ofvarious pillar precursors. Pillaring smectites with ironand chromium polyhydroxy polymers is more difficult asfar as hydrolysis conditions are concerned. Usages of metalhalides as pillaring reagents are very common. On the otherhand, usages of metal organic complexes, especially metalacetyl acetonates are still not known yet. It has beenplanned also to use these catalysts in the Friedel–Craftsalkylation reactions.

Therefore, the aim of study is to prepare and character-ize Cr- and Fe-PILBs by using Turkish bentonite as a clayresource and to show the possible usage of metal acetylacetonates as pillaring precursors for the preparation ofthe pillared clay catalysts. In this study, CrCl3, FeCl3,Cr(acac)3 and Fe(acac)3 were used as pillaring precursors.

212 M. Yurdakoc et al. / Microporous and Mesoporous Materials 111 (2008) 211–218

The purpose of this study was also to compare the proper-ties of these catalysts which were prepared by using differ-ent precursors. The morphology and the surface acidities ofthe catalysts can be expected to be different each otherdepending on the pillaring agent.

2. Experimental

2.1. Parent clay

Bentonite used for a raw material for the preparation ofthe pillared samples came from a bed located in Res�adiye/Tokat, Turkey. This clay was first suspended with distilledwater. After sedimentation, middle fraction was collected,centrifuged and dried at 110 �C. This raw material wasground, sieved to 100 meshes, labeled as B and then usedin the preparation of the pillared samples. Chemical com-position of the sample can be found elsewhere [22]. The cat-ion exchange capacity (CEC) of bentonite was determinedusing ammonium acetate saturation method [23] and resultwas expressed as 92 meq per 100 g of clay.

2.2. Synthesis of pillared clays

Four samples of pillared clays from B were prepared inthe following ways

2.2.1. Preparation of chromium pillared bentonite by using

CrCl3 as pillaring precursor

CrCl3 Æ 6 H2O (13.323 g) was dissolved in 500 mL of dis-tilled water. Na2CO3 (0.053 g) was carefully added to thesolution of CrCl3 at 25 �C. At this stage, the pH of thesolution was measured as 3.10. This solution was then agedat 95 �C under reflux for 40 h. After aging, the pH of theresultant pillaring agent was measured as 1.46. At the sametime, 10 g of bentonite was suspended with 500 mL of dis-tilled water. Afterwards, the bentonite suspension and thehydrolyzed chromium solution were mixed while the for-mer solution was still warm and then the mixture was stir-red for 24 h. The Cr-pillared bentonite suspension was thenfiltered by suction, washed with distilled water until chlo-ride free, and dried at 60 �C and calcinated at 200 �C for2 h. The resulting material was called Cr-PILB.

2.2.2. Preparation of iron pillared bentonite by using FeCl3as pillaring precursor

NaOH (400 mL, 0.1 M) solution was added with a rateof 2–3 mL min�1 to a rapidly stirred 200 mL solution of0.1 M FeCl3. The amount of base added was 2 meq mol�1

of Fe3+. The pH of the solution at this stage was measuredas 2.5. Distilled water (400 mL) was then added to thissolution to obtain the final volume of 1 L of solution. Thissolution was then aged at 25 �C for 24 h. pH of the solutionwas recorded as 2.32. Bentonite (10 g) was then added tothe 500 mL of above pillaring solution to obtain 10 mmolesof Fe3+ per grams of bentonite. Afterwards, 500 mL of dis-tilled water was also added to this suspension. The reaction

mixture was stirred for six additional hours for 25 �C. Theproduct was then filtered by suction and excessively washedwith distilled water until chlorine free. After separation,this product was dried at 60 �C and calcinated at 200 �Cfor 2 h. The resulting material was called Fe-PILB.

2.2.3. Preparation of chromium pillared bentonite by usingCr(acac)3 as pillaring precursor

In order to obtain 0.01 M Cr(acac)3 solution, 1.7467 g ofCr(acac)3 was dissolved in 500 mL of distilled water. Thissolution was aged at 80 �C for 5 h under reflux. Bentonite(10 g) and 500 mL of distilled water were added to thishydrolyzed chromium solution while the former solutionwas still warm and then the suspension was stirred at25 �C for 24 h. At the end of this stage, a colloidal suspen-sion was obtained with a pH value of 9.27. This colloidalsuspension was flocculated by the drop wise addition of1 mL of conc. acetic acid to maintain the pH value of5.18. This mixture was then gently heated and stirred forthe completion of the flocculation. This end product wasfiltered, dried at 40 �C and calcinated at 200 �C for 2 h.This material was then labeled as Cr(acac)3-PILB.

2.2.4. Preparation of iron pillared bentonite by using

Fe(acac)3 as pillaring precursor

Fe(acac)3 (1.7659 g) was dissolved in 500 mL of distilledwater. This solution was aged at 75 �C for 5 h under reflux.Initial pH of the solution was measured as 4.17. Afteraging, the pH of the solution was raised to 8.14. Bentonite(10 g) and 500 mL of distillated water were slowly added tothe above warm solution. This mixture was then stirredadditionally at 25 �C for 24 h. To continue, the same pro-cedure was followed as in the preparation of the Cr(a-cac)3-PILB. In this case, the product was labeled asFe(acac)3-PILB.

2.3. Characterization

N2 adsorption–desorption isotherms at 77 K were mea-sured on SORPTOMATIC 1990 after a degassing undervacuum for 2 h at 130 �C. BET specific surface area (SBET)specific pore volume (Vp) total adsorbed volume (VT)monolayer coverage (Vm) were determined by usingMILES-200 Advanced Data Processing Sorption SoftwareVersion 3.00. For the porosity analysis, the Dollimore–Heal method [24] was used.

X-ray powder diffraction (XRD) patterns were obtainedin a Seifert-XRD 3000 under 40 kV, 40 mA current inten-sity in the 2h 3–10� range and using Ni-filtered Cu Ka radi-ation. For the range of 2h 8–63�, a HZG4 XRD powderdiffractometer was used under the same conditions. Theinterlayer free spacing was obtained by lowering the valueof the d(001) basal spacing with the thickness of a claylayer, 9.6 A.

Thermogravimetric analysis (TGA) of the samples werecarried out in N2 atmosphere with a flow rate of10 mL min�1 by using Shimadzu TGA 50 thermal analyzer

Table 1Textural characteristics of the samples (after BET(3)-parameters fit)

Sample Vm

(cm3 g�1)C SBET

(m2 g�1)Vp

(cm3 g�1)Vads

(cm3 g�1)

B 13.0 154 57 0.130 125Fe-PILB 20.4 137 89 0.133 270Fe(acac)3-PILB 14.8 91 64 0.084 62Cr-PILB 21.1 170 92 0.099 240Cr(acac)3-PILB 4.5 112 20 0.038 56

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in the temperature range of 10–850 �C at a heating rate of10� min�1.

FT-IR measurements for the surface acidity determina-tion were performed using self-supporting pressed discs(0.1–0.2 g) contained in a cell with NaCl windows, whichallowed discs to be heated in vacuo or in the presence ofgases. IR spectra have been recorded in situ at room or ele-vated temperatures by a Mattson 1000 FT-IR instrument,equipped with a conventional evacuation-gas manipulationramp (10�3 Pa). The FT-IR instrument was typically oper-ated at a scan speed of 0.1 cm�1 s�1 and a resolution of2 cm�1, collecting 50 scans per spectrum. All the sampleswere subjected to a standard pretreatment involving heattreatment at 673 K in vacuo for 8 h. All the adsorptionstudies were carried out at 298 K and with the partial pres-sure of pyridine at 298 K in vacuo. After adsorption stud-ies, desorption experiments were also done at elevatedtemperatures (373–673 K) for 10 min for the comparisonof the strength of Bronsted and Lewis acid sites. The detailsof the experiment can be found elsewhere [25].

3. Results and discussion

Fig. 1 shows the N2 adsorption–desorption isotherms at77 K for B, Cr-PILB, Fe-PILB, Cr(acac)3-PILB andFe(acac)3-PILB with a linear relative pressure axis. As

Fig. 1. The N2 adsorption–desorption isotherms at 77 K for B, Cr-PILB,Fe-PILB, Cr(acac)3-PILB and Fe(acac)3-PILB.

can be seen from Fig. 1 that the isotherms of all samplesexhibit Type II behaviors according to the IUPAC classifi-cation or BDDT classification [26], characteristic of nonpo-rous or mesoporous adsorbents. A hysteresis loop closingat a relative pressure of 0.4 is also recorded and can beascribed to type H4 [27] which are often associated withnarrow slit-like pores. The textural characteristics of thesamples obtained from the analysis of these isotherms aregiven in Table 1.

As can be seen from Table 1 that usage of metal halidesas pillaring precursor is more effective compared with theacetyl acetonato complexes. Specific surface area valuesof Cr-PILB and Fe-PILB were greater than the surfacearea of B, while Cr(acac)3-PILB had a smallest specific sur-face area value. As an example, pore size distribution,cumulative- and differential pore volume of Cr-PILB andbentonite is given in Fig. 2a and b. Pore size distributionanalysis data which was obtained from N2 adsorption–desorption experiments on the samples and evaluatedaccording to the Dollimore–Heal method [24] showed thatall the samples had a pore width of 4 nm; however, theircumulative and differential pore volumes were very differ-ent from each other. Pore volumes and specific surfaceareas of Cr(acac)3-PILB and Fe(acac)3-PILB were smallerthan Cr-PILB and Fe-PILB. In some cases, some of theprecursor may not decompose properly and placed in thepores structure of end product. During the calcination innitrogen atmosphere, coke formation might be possible inwhich fills the pores or closed the pore openings. In thiscase, decrease naturally in BET specific surface area ofthe samples and also pore volume can be observed in somedegree. This may the main result of the importance of theprecursor (metal halides or metal acetylacetonates) in thepreparation pillared clays.

X-ray diffraction patterns of the samples are shown inFig. 3a. The XRD pattern of the parent clay (i.e., benton-ite, B) exhibits seven main X-ray peaks around 7�, 20�, 27�,29�, 35�, 55� and 62�. The first peak is commonly assignedto the basal (001) reflection, while the others are attributedto the two-dimensional (h k) planes. It has been observedthat all the characteristic peaks of bentonite were con-served in the PILB samples. The peak around 30� graduallydecreases during the pillaring procedure. In particular, the(001) peak was found to shift towards the lower 2h region,which is a clear indicative of the enlargement of the basalspacing of B. However, this peak is not clearly seen in thisfigure due to experimental difficulties. This may also

Fig. 2. Pore size distribution, cumulative- and differential pore volume of (a) Cr-PILB and (b) bentonite.

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collapse of the layered structure during the pillaring pro-cess and calcination condition after preparation, revealingpoor microporosity and low surface area values, especiallyin the case of Cr(acac)3-PILB and Fe(acac)3-PILB samples.

It has been reported that full glycolation of the clays waspossible after heating to 473 K (d(001) spacing 17.4 A)which was consisted with findings for the Cr(III) oligomerintercalated clays [28]. After calcinations at 200 �C for 2 hin N2 atmosphere, XRD peaks especially in 2h = 0–10�region became broader, with it being difficult to assign amaximum. In order to maximize the (001) reflection inten-sities, ethylene glycol oriented specimens were prepared byspreading them on an inert slide for 2 h in desiccators.

After this procedure, XRD diffractograms were recordedimmediately at room temperature (Fig. 3b). The d spacingsof the samples were 17.3, 17.5, 17.8, 17.6 and 17.4 A for B,Cr-PILB, Fe-PILB, Cr(acac)3-PILB and Fe(acac)3-PILB,respectively. From the XRD data, it did not clearlybelieved that the influence of usage of metal acetyl aceto-nates as compared with metal halides were valuable. Thisis due to the matrix influence of B. XRD data was reflectedthe main structure of B.

The TGA/DTG curves for B, Cr-PILB, Fe-PILB,Cr(acac)3-PILB and Fe(acac)3-PILB are given in Fig. 4. B,Cr-PILB and Fe-PILB give two stages of mass losses:15–300 and 300–850 �C. While the first step can be assigned

Fig. 3a. X-ray diffraction patterns of the samples.

Fig. 3b. X-ray diffraction patterns of the ethylene glycol oriented samples.

Fig. 4. The TGA/DTG curves for B, Cr-PILB, Fe-PILB, Cr(acac)3-PILBand Fe(acac)3-PILB.

M. Yurdakoc et al. / Microporous and Mesoporous Materials 111 (2008) 211–218 215

to desorption of water, the second step can be related to thedehydroxylation of OH groups on the internal and/orexternal surface of the samples. The mass losses for B inthe temperature range of 15–300 and 300–850 �C are3.06% and 6.88%, respectively. The total mass losses forCr-PILB and Fe-PILB at the temperature range of 15–850 �C are 11.13% and 10.62%, respectively. In the caseof Cr(acac)3-PILB, three steps mass losses was observedat the range of 310–410 �C with a maximum temperatureof 373 �C in the DTG thermogram. This may be due tothe decomposition of acetyl acetonato complex. On theother hand, in the TGA/DTG data (not shown here)Cr(acac)3 decomposes at the temperature range of 200–260 �C with a sharp maximum at a temperature of254 �C in its DTG. This suggests that the pillaring processmay lead to a shift of this decomposition temperature to ahigher temperature region which can be related to theimprovement of the thermal stability. The maxima of thetemperatures in DTG profiles of Cr-PILB and Cr(acac)3-PILB were for former 80, 640 �C and for later 62, 373and 644 �C. The mass losses for Cr(acac)3-PILB in the tem-perature range of 15–300 �C and 300–850 �C are 9.63% and

9.78%, respectively. Three steps mass losses were observedin the TG thermograms of Fe(acac)3-PILB. The tempera-ture maxima of the DTG profile of Fe(acac)3-PILB are80, 332 and 666 �C. The mass losses are due to the dehydra-tion, decomposition of acac complex and the dehydroxyla-tion of the sample. The total mass loss at the temperaturerange of 20–850 �C is 15.6%.

The acidity is one of the important properties requiredfrom pillared clays. Therefore, an IR study has been

Fig. 6. The IR spectra of Cr(acac)3-PILB in the adsorption–desorption ofpyridine. (a) Reference, (b) initial ads., (c) 30 min ads., (d) 1 h ads., (e)desorp. at room temp., (f) desorp. at 100 �C, (g) desorp. at 200 �C and (h)desorp. at 250 �C.

216 M. Yurdakoc et al. / Microporous and Mesoporous Materials 111 (2008) 211–218

performed by the gas phase adsorption of pyridine on theclay surface to identify the surface acidity, i.e., Bronstedand Lewis acid sites. Figs. 5, 6, 7, and 8 show the IR spec-tra of Cr-PILB, Cr(acac)3-PILB, Fe-PILB, and Fe(acac)3-PILB in the adsorption–desorption of pyridine in theregion of IR band from 1200 to 1800 cm�1, respectively.As shown in Fig. 5b, after adsorption of pyridine on toCr-PILB at room temperature exhibits IR bands at 1440–1446, 1490, 1600, 1623 and 1635–1640 cm�1. The bandsat 1440–1446, 1600 and 1623 cm�1 are commonly assignedto a Lewis acid site [25,29]. The band at 1490 cm�1 ismainly attributed to both Lewis and Bronsted acid sitesof the Cr-PILB. A small band at 1560 cm�1 and a shoulderafter desorption at 100 �C (Fig. 5f) at 1635–1640 cm�1 areobserved due to Bronsted acid sites on the surface of Cr-PILB. The intensities of the bands assigned to pyridinecoordinated onto Lewis and Bronsted acid sites arereduced with respect to the evacuated temperature of thesample (Fig. 5f–h). Especially, the bands at 1446 cm�1

and 1490 cm�1 reduced and the band at 1600 cm�1 disap-peared as a result of evacuation at a temperature of100 �C. However, the intensities of the bands at 1490,1560 and 1638 cm�1 assigned to Bronsted acid sites areslightly reduced, and can be still seen after evacuation at240 �C. Fig. 6 shows the IR spectra of pyridine adsorbedonto Cr(acac)3-PILB. However, the spectra are more com-plex as compared with Fig. 5 and there are lots of intensebands which can not be interpreted easily. This may bedue to high resolution and decomposition the acac complexin vacuum during the pretreatment before the adsorptionof pyridine. After evacuation at 250 �C, the bands at1612, 1600, 1568, 1517, 1456 and 1419 cm�1 are stillobserved. On the other hand the band, 1635–1640 cm�1

related with Bronsted acid center was not observed clearlyas compared with the bands for characteristics of Lewiscenters. In the case of IR spectra of pyridine after initial

Fig. 5. The IR spectra of Cr-PILB in the adsorption–desorption ofpyridine. (a) Reference, (b) initial ads., (c) 30 min ads., (d) 1 h ads., (e)desorp. at room temp., (f) desorp. at 100 �C, (g) desorp. at 200 �C and (h)Desorp. at 250 �C.

adsorption onto Fe-PILB (Fig. 7b) the bands 1446,1490 cm�1 and a shoulder at 1600 cm�1 are observed andassigned to Lewis acid sites as in the case as in Fig. 5.The band at 1635 cm�1 is strongly reduced after evacuationat 100 �C and nearly disappeared at 300 �C. The intensitiesof the bands for the characteristics of Lewis acid sites areminimized after evacuation at 100 �C. Similar bands arealso observed in the case of Fe(acac)3-PILB. The bandsat 1445, 1490, 1597–1600 and 1634 cm�1 are observed inFig. 8. The band at 1445 cm�1 is shifted to 1450 cm�1 afterevacuation at 300 �C. The intensities of the bands werestrongly reduced after degassing in vacuum at 200 �C.When the spectra of g and h of Figs. 7 and 8 are compared,the intensity of the band, 1635–1640 cm�1 was decreasedafter desorption at 200 �C and 300 �C. However, the bandsfor Lewis centers can be still observed. It was also observedin general that the Lewis-type acidity of the samples is notmuch affected by degassing at higher temperature whileBronsted acid centers practically disappeared.

Fig. 7. The IR spectra of Fe-PILB in the adsorption–desorption of pyridine. (a) Reference, (b) initial ads., (c) 30 min ads., (d) 1 h ads., (e) desorp. at roomtemp., (f) desorp. at 100 �C, (g) desorp. at 200 �C and (h) desorp. at 300 �C.

Fig. 8. The IR spectra of Fe(acac)3-PILB in the adsorption–desorption ofpyridine. (a) Reference, (b) initial ads., c) 30 min ads. (d) 1 h ads. (e)desorp. at room temp., (f) desorp. at 100 �C, (g) desorp. at 200 �C and (h)desorp. at 300 �C.

M. Yurdakoc et al. / Microporous and Mesoporous Materials 111 (2008) 211–218 217

4. Conclusions

The adsorption isotherms of the samples are of Type IIin BDDT classification. The hysteresis loop (all samples H4type in the IUPAC classification) indicates the presence ofmesoporosity. All samples presented a similar maximum inthe pore volume distribution at ca. 4 nm. Usage of acetyl-acetonato salts reduced the specific surface area. XRD pat-terns of the samples reflected the main structure ofbentonite. TGA/DTG thermograms of the samples showedsimilar thermal behavior. The samples presented also bothBronsted and Lewis acid sites. Desorption of the samples invacuum at elevated temperatures after pyridine adsorptionshowed that the Lewis acid centers are stronger. Analysisof the pyridine adsorption data indicated an increase inthe acidity of the pillared samples with respect to the start-ing bentonite. Usage of metal chloride precursor in thepreparation of the pillaring may be more effective thanthe acetylacetonato salts. However, metal acetylacetonatocomplexes may be used as pillaring agents in the prepara-tion of the pillared clays. On the other hand, for the usageof acetylacetonato salts as precursors, it should be neces-sary to know hydrolysis conditions of these metal saltswhich are still in question.

Acknowledgments

We express our thanks to Prof. Dr. Ing. Dieter Honickefor the N2 adsorption and XRD experiments. The scholar-ship support for K. Yurdakoc by DAAD/Germany isgratefully appreciated. This work has been supported

218 M. Yurdakoc et al. / Microporous and Mesoporous Materials 111 (2008) 211–218

partly by the Research Foundation of Dokuz Eylul Univer-sity, Turkey, through the Project 0903.01.01.01.

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