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Applied Catalysis B: Environmental 101 (2011) 451–460

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

journa l homepage: www.e lsev ier .com/ locate /apcatb

atalytic wet hydrogen peroxide oxidation of para-chlorophenol over Al/Feillared clays (AlFePILCs) prepared from different host clays

ezar Catrinescu ∗, Daniela Arsene, Carmen TeodosiuGheorghe Asachi” Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Environmental Engineering and Management,3 A, D. Mangeron Blvd, 700050 Iasi, Romania

r t i c l e i n f o

rticle history:eceived 21 July 2010eceived in revised form 11 October 2010ccepted 14 October 2010vailable online 21 October 2010

eywords:-Chlorophenolydrogen peroxidexidatione-containing pillared clay

a b s t r a c t

In this work, three AlFePILCs prepared from different host clays were synthesized, characterized andtested in the catalytic wet hydrogen peroxide oxidation of 4-chlorophenol. Two reference clays, withwidely different cation exchange capacities (1.2 meq/g for SAz-1 and 0.87 meq/g for SWy-2), and a Roma-nian montmorillonite (Mt) were used for the preparation of the catalysts, their structural and texturalproperties being determined by X-ray diffraction and nitrogen adsorption-desorption isotherms. The cat-alyst based on SAz-1 has a more ordered structure and a higher surface area than the other two catalysts,prepared from SWy-2 and Mt, and this was attributed to its higher layer charge. The 4-chlorophenol oxi-dation proceeds with the formation of 4-chlorocatechol (main reaction intermediate) and hydroquinone.Other chlorinated benzenediols and triols as well as dimerization products have been also identified byderivatization and GC–MS analysis. All the catalysts allowed the total elimination of 4-chlorophenoland significant removal efficiencies for the total organic carbon, of 60, 52 and 45%, for Mt, SWy-2 and

SAz-1 based catalysts, respectively. The iron leaching was very low, but the most active catalyst pro-duced the higher amount of dissolved iron (1 ppm), as compared with AlFePILCs based on SWy-2 andSAz-1 (0.5 ppm). To explain the differences in the catalytic properties, Mössbauer and diffuse-reflectanceUV–vis spectroscopies were used to investigate the nature of the active sites. Both methods suggested thepresence of two iron species: low-nuclearity ferric oxides and well-ordered hematite-like nanoparticles.The low-nuclearity ferric oxides seem to be responsible for the iron leaching and for the differences in the catalytic activity.

. Introduction

The sustainability approach in water resources managementnvolves increased demands on wastewater treatment technolo-ies in order to minimize the negative impacts on the naturalater bodies and to facilitate recycling and reuse of wastewater.dvanced wastewater treatment technologies efficiently eliminateriority pollutants such as non-biodegradable organics (includingersistent organic pollutants, POPs), or inorganic toxic compoundshrough their destruction or separation by: advanced oxidationrocesses (AOPs) (catalytic and photocatalytic oxidation, catalyticet air oxidation, ultrasound, plasma processes, etc.), chemical pre-

ipitation, adsorption on different media, membrane processes, etc.

1].

The various AOPs present high removal efficiencies of prior-ty/refractory organic pollutants, based on a chemical feature, i.e.he generation of reactive radicals, such as hydroxyl radicals, which

∗ Corresponding author. Tel.: +40 232 237594; fax: +40 232 237594.E-mail address: ccatrine@ch.tuiasi.ro (C. Catrinescu).

926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcatb.2010.10.015

© 2010 Elsevier B.V. All rights reserved.

oxidize the organic pollutants up to mineralization or generationof easily biodegradable small molecules, at near-ambient temper-atures and pressures [1,2]. These processes use different reagents,such as: ozone, hydrogen peroxide, oxygen, air or their combina-tion, as well as the influence of UV radiation or different catalysts,so as to increase pollutant removal and mineralization potential.Among the multitude of AOPs, Fenton’s reagent (Fe2+/Fe3+/H2O2)has been one of the most common homogeneous systems for thetreatment of priority pollutants, including highly toxic and rela-tively water-soluble chlorophenols [3]. Although the homogeneousprocesses are efficient and cost-effective, there are two major draw-backs that limit the industrial application of this technology: (i) thetight range of pH in which the reaction proceeds and (ii) the need forrecovering and further treating the precipitated catalyst after thetreatment. The resulting sludge may contain organic substances aswell as heavy metals and has to be further treated, thus increasing

the overall costs.

To circumvent these drawbacks a diversity of solids has beenused as catalysts for the oxidation of various organic pollutantsfrom wastewaters [4]. Among these catalysts, pillared interlayeredclays (PILCs) containing mixed aluminium oxide/iron oxide pillars

452 C. Catrinescu et al. / Applied Catalysis B: Environmental 101 (2011) 451–460

Table 1Chemical composition, BET surface area and CEC of the host clays used for the preparation of PILCs.

Clay Chemical composition (% oxide) ABET (m2/g) CEC (mEq./100 g)

Na Mg Al Si K Ca Ti Fe LOCa

SAz-1 0.06 6.73 19.98 59.65 0.19 3.15 0.25 1.77 8.17 60 1208

(sPwmadfps

m4a

(nasci

oomt(icTta

acdy

eSnosiSracd

cficwa

SWy-2 1.47 2.94 22.05 61.46 0.2 1.1Mt 1.31 2.41 19.17 69.61 0.18 0.9

a Loss on calcination.

AlFePILCs) seem to be promising catalysts, combining a good (rea-onable) catalytic activity with a high stability against leaching.henol and its derivates represent common organic pollutants inater discharged by several industries, and have particularities thatake them valuable as a model pollutants, such as: toxicity even

t low concentrations, formation of substituted compounds duringisinfection and oxidation processes (such as those used currentlyor the treatment of natural surface water for drinking purposes),hytotoxicity and ability to bioaccumulate in organisms, that haveimilarities with persistent organic pollutants (POPs) [5,6].

Previous studies conducted on these catalysts, using simpleodel pollutants such as: phenol [2,7–22], p-coumaric acid [23,24],

-chlorophenol [5,25,26], and toluene [27] have pointed out thedvantages of these solid catalysts.

The heterogeneous catalytic wet hydrogen peroxide oxidationCWHPO) has several major advantages over the classical homoge-eous Fenton-like process, such as: enhancement of the catalyticctivity, the lack of secondary pollution due to the iron containingludge [7,14], the widening of the pH range in which the CWHPOould be performed [7,16] and the possibility of reusing the catalystn successive cycles.

Most of the CWHPO studies addressed primarily the impactf the reaction parameters on conversion rate and kinetics, whilether important aspects, such as the influence of the preparationethod or the nature of the active sites were less studied. Never-

heless, Guelou et al. [12] deduced from an electron spin resonanceESR) study that, besides the isolated iron from the clay structure,ron is also present as oxide clusters and isolated iron species asso-iated with the pillars. Concerning the preparation of AlFePILCs,imofeeva et al. [16] found that the speciation of iron can be con-rolled by different factors, such as the Fe/Al ratio, OH/(Fe + Al) ratio,geing time and calcination temperatures.

Unfortunately, in these studies, the AlFePILCs are prepared fromvariety of host clays, most of them of limited availability, so

omparison of data reported by different research groups is ratherifficult, thus making very difficult a technical and economical anal-sis of different CWHPO alternatives.

This article presents the results of studies that used two ref-rence “source” clays (SAz-1 and SWy-2), from The Clay Mineralociety Source Clay repository and a commercially available Roma-ian montmorillonite (Mt), as host structures for the preparationf AlFePILCs. SAz-1 and SWy-2 have different octahedral compo-ition: SAz-1 is richer in magnesium, but contains less octahedralron, than SWy-2. Thus, the main difference between SAz-1 andWy-2 is the density of the layer charge in the octahedral layer,esulting in different cation exchange capacities (CEC). These claysre considered as model ‘low’ (SWy-2) and ‘high’ (SAz-1) surface-harge smectites and were chosen to reflect the effect of chargeensity on the properties of the corresponding pillared clays.

During this study, the assessment of the influence of hostlay on the structural, textural and catalytic properties was per-

ormed. In particular, we attempted to identify the structure ofron-containing species in the AlFePILCs prepared from differentlays, in order to analyze the catalytic properties in correlationith the nature of these active phases. We used 4-chlorophenol

s a model pollutant due to several reasons: (a) its characteristics

0.09 4.37 5.76 24 87– 1.27 5.15 21 82.5

and similarities with other types of refractory pollutants (includingPOPs), (b) lack of consistent studies on AlFePILCs catalysts (exceptsame preliminary studies [5,25,26]), (c) part of a complex study thatanalyses several advanced treatment processes and their integra-tion with conventional treatment.

2. Experimental

2.1. Materials

For the preparation of PILCs catalysts three starting clays havebeen used: two reference “source” clays (SAz-1 and SWy-2) and acommercially available Romanian montmorillonite (Mt). The highsurface-charge density montmorillonite type “Cheto” SAz-1 andthe low surface-charge Wyoming SWy-2 montmorillonite werereceived from The Clay Mineral Society Source Clay repository(Purdue University). The raw bentonite Mt was obtained from theValea Chioarului deposit and was purified according to a referencemethod [28]. The clay dispersion was improved by ultrasonicationfor 20 min, before being separated by centrifugation. The chemicalcomposition and the CEC for the host clays are listed in Table 1.

2.2. Synthesis of pillared clays

The Al–Fe PILCs catalysts were prepared by pillaring the hostclay using the powder method. The pillaring solution was pre-pared by titration of an Al3+/Fe3+ cationic solution (90 cm3 0.1 mol/LAl(NO3)3 and 5 cm3 0.2 mol/L Fe(NO3)3 with 0.2 mol/L NaOH. TheAl3+/Fe3+ molar ratio was 9:1. The NaOH solution was slowly addedto the cationic solution at 60 ◦C until the OH/cation mole ratio wasequal to 2. The resulting solution was aged at room temperature for24 h. The dry clay powder (1 g) was directly dispersed in the pillar-ing solution, stirred for 24 h, then separated by filtration, washed bycentrifugation with demineralized water, dried overnight at 60 ◦Cand finally calcinated at 500 ◦C for 4 h. The catalysts obtained fromSAz-1, SWy-2 and Mt were named as AlFeSAzPILC, AlFeSWyPILCand AlFeMtPILC, respectively.

2.3. Instrumental measurements

The XRD patterns were recorded using a Shimadzu LabX XRD-6000 diffractometer with Cu K� radiation (� = 1.54184 A). The BETsurface areas were determined from nitrogen physisorption dataat 77 K by using a Quantachrome Autosorb 1MP analyzer, afterdegassing about 250 mg of calcinated samples, at 350 ◦C for 3 h,under vacuum.

The DR UV–vis spectra were recorded on a UV-2501 PC Shi-madzu in the 190–900 nm range, with a resolution of 2 nm.BaSO4 was used as standard. The reflectance was recalculated toKubelka–Munk absorbance and the positions of the absorptionmaxima were estimated from the minima of the 2nd derivative

of the 20 point FFT smoothed spectra.

Mössbauer spectra were recorded at the Institute de Ciencia deMateriales de Barcelona, by using powdered solid samples. Thespectra were collected at 300 K and 80 K (liquid nitrogen) usinga constant acceleration Mössbauer spectrometer with a 57Co/Rh

C. Catrinescu et al. / Applied Catalysis B: Environmental 101 (2011) 451–460 453

Table 2The iron content, the main textural properties and the basal spacings (d0 0 1) for AlFePILCs.

Catalyst Fe2O3 (wt.%) ABET (m2/g) S(� (m2/g) Vpore (cm3/g) V(� (cm3/g) d0 0 1, (nm)

2

.6

s�witr

2

sarsiua6tttp[dwvftamtesanMbtc(mcu

3

3

Aie

didic

The steep low-pressure region of the adsorption isotherm indi-cates that the PILCs contain narrow micropores, more prominent inAlFeSAzPILC which have a higher proportion of micropores (highermicroporosity) than the other two PILCs. The hysteresis loop is

400

500

600

700

800

900

Inte

nsity

, a.u

.

AlFeSAzPILC

AlFeSWyPILC

AlFeMtPILC

AlFeSAzPILC 5.9 319 20AlFeSWyPILC 7.7 192 92AlFeMtPILC 8.2 159 75

ource. The source was moved via triangular velocity wave and the-counts were collected in a 512 multi-channel analyser. The dataere folded, plotted and fitted by a computer procedure. Veloc-

ty calibration was done using a 25 �m thick metallic iron foil andhe Mössbauer spectral parameters (presented in Table 5) are givenelative to this standard at room temperature.

.4. Catalytic tests

In order to evaluate 4-chlorophenol (4-CP) removal by CWHPO,ynthetic wastewater samples were prepared and analyzed usingn experimental set-up as described below. The oxidation was car-ied out in a reaction system of 250 mL equipped with a magnetictirrer and a pH electrode. The solid catalyst (1 g/L) was introducednto 100 mL of an aqueous 4-chlorophenol solution (125 mg/L),nder continuous stirring. After the stabilization of the temperaturet 50 ◦C and the correction of pH at 3.5, the solution was stirred for0 min and then analyzed to confirm the absence of adsorption byhe pillared clay. Then, a solution of 3% H2O2 was added to achievehe selected H2O2/4-CP ratio, these being the basic conditions forhe experimental tests. The concentration of the phenolic com-ounds was determined by 4-aminoantypirine standard method29]. Leaching of the catalyst at the end of the oxidation process wasetermined with an Atomic Absorption Spectroscopy (GBC Avantaith system 3000 Graphite Furnace). Total organic carbon (TOC)

alues were analyzed using a Total carbon analyzer (TOC-VCSNrom Shimadzu Model TNM1). In order to identify the main reac-ion intermediates, a separate catalytic run was performed, usingconcentrated aqueous solution of 4-CP (1 g/L) and a substoichio-etric amount of H2O2 (H2O2:4-CP = 2.7:1 molar ratio). 10 mL of

he resulting solution was extracted twice with 2.5 mL portions ofthylic ether. The combined extracts were dried over anhydrousodium sulfate, evaporated under vacuum and concentrated undernitrogen stream. To derivatize the hydroxyl groups of the phe-olic compounds, N-Methyl-N (trimethylsilyl) trifluoroacetamideSTFA (0.1 mL) was added to the concentrated solution, followed

y heating at 70 ◦C for 60 min. An Agilent 6890N/MSD GC–MS sys-em, equipped with electron impact (EI) ionization source and aapillary column (30 m HP 5 ms), working with He as carrier gas1 mL/min) was used. The injection was performed in the splitless

ode and the data acquisition was carried out in full scan mode. Theolumn was held at 80 ◦C for 3 min and then heated with 15 ◦C/minp to 300 ◦C and maintained at this value for 7 min.

. Results and discussions

.1. Characterization of AlFePILCs

The amount of iron in the studied AlFePILCs, as determined byAS, is listed in Table 2. The total iron content of the catalysts

s higher than that of the corresponding host clay, indicating theffective introduction of iron in the AlFePILCs.

The XRD patterns of PILCs, prepared from different clays, are

isplayed in Fig. 1. The values of the basal spacings (d0 0 1) are listed

n Table 2. AlFeSAzPILC gives an intense and well-defined (0 0 1)iffraction peak while AlFeSWyPILC and AlFeMtPILC produce less

ntense, but perceptible peaks. Thus, the XRD data show that theatalyst prepared from SAz-1 has a more ordered structure than

0.26 0.088 1.720.36 0.04 1.680.45 0.035 1.68

the other two catalysts, prepared from SWy-2 and Mt. This couldbe related to the higher cation exchange capacity (CEC) of SAz-1,leading to a more homogeneous distribution of the pillar precur-sor in the clay gallery and to a higher extent of ordered pillars inthe calcinated form. The poor XRD patterns, with broad reflection,could also be an indication of a presence of a fraction of unmodifiedclay, or could show that the pillaring species were intercalated inan irregular manner. The interlayer space (0.74 nm for AlFeSAzPILCand of 0.70 nm for AlFeSWyPILC and AlFeMtPILC), calculated tak-ing into account a thickness of 0.98 nm for the 2:1 silicate layers,compares well to the data reported in the literature [30,31].

The nitrogen adsorption/desorption isotherms of the pillaredclays are depicted in Fig. 2, and main textural characteristics ofAl–Fe PILCs are presented in Table 2. The adsorption isothermscorrespond to a combination of Type I and Type IIb, reflecting therelative extents of the interlamellar and external surface areas [32].

4 5 6 7 8 92q

300

Fig. 1. X-ray diffraction patterns for Al–Fe PILCs prepared from SAz-1, SWy-2 andMt.

454 C. Catrinescu et al. / Applied Catalysis B: E

0

50

100

150

200

250

300

Vol

ume,

cc/

g S

TP AlFeSAzPILC

AlFeSWyPILCAlFeMtPILC

otaT(ApmlTpp

3

3C

f

rinated) diols and triols through oxidative cleavages, occurring

0 0.2 0.4 0.6 0.8 1

p/p0

Fig. 2. Nitrogen adsorption/desorption isotherms for AlFePILCs.

f Type H3 in the IUPAC classification, and this is consistent withhe model proposed for PILCs (a material having slit-shaped poresnd plate-like particles with spaces between the parallel plates).he BET surface area (ABET) of the AlFeSAzPILC is much higher319 m2/g) than the surface areas of AlFeSWyPILC (192 m2/g) and oflFeMtPILC (159 m2/g), confirming the trend observed in the XRDatterns. The values of the surface area for these studied PILCs areuch larger than the typical values for non-expanded montmoril-

onites (Table 1) and are in agreement with the literature data [30].hus, the adsorption/desorption isotherms provide additional sup-ort for envisaging these samples as clay structures with layersropped apart by pillars, as indicated by the XRD patterns.

.2. CWHPO of 4-chlorophenol over AlFePILCs

.2.1. Identification of the reaction intermediates by Gashromatography–Mass Spectrometry (GC/MS)

Fig. 3 shows a typical total ion chromatogram (TIC) obtainedrom the oxidation of 1 g/L 4-CP, followed by extraction and TMS

5 10

Retenti

0

2x106

4x106

6x106

8x106

Abu

ndan

ce

1

2

3

4

5

6

7

8

0

100000

200000

300000

400000

500000

600000

Abu

ndan

ce

Fig. 3. Typical total ion chromatogram and TMS derivatization. Conditions:

nvironmental 101 (2011) 451–460

derivatization. Some compounds, such as 4-CP, 2,4-dichlorophenol,4-chlorocatechol and hydroquinone were identified by compar-ing retention times and mass spectra with those of authenticstandards. Table 3 presents the retention times and the charac-teristic ions of the major degradation products detected in thisstudy.

Based on the reaction intermediates presented above, twomajor pathways may be proposed (Scheme 1). In the first one(A), it can be supposed that the reaction starts with the hydroxylradical (HO•) attack, directed by the position of the hydroxylgroup, which is a stronger ortho–para director than chlorine. Inthe case of 4-chlorophenol, the main reaction intermediate 4-Chlorocatechol (4-CC), is the result of HO• attack at ortho positionto the hydroxyl group. The hydroxylation of 4-CC leads to 5-chloro-1,2,4-benzenetriol.

The formation of non-chlorinated products (hydroquinone)indicates that the presence of a chlorine group did not prevent theHO• attack at that particular position on the ring (second path-way, B). Hydroxyl radical attack did however take place morereadily at ortho position, not occupied by chlorine group, becausedechlorination occurs via a different reaction mechanism, whichis probably not as efficient as the reactions without dechlorina-tion. Chlorohydroquinone could be formed by further chlorinationat the ortho-position of hydroquinone. Benzoquinone and 1,2,4-benzentriol, which might be formed from hydroquinone, were notobserved in these experiments.

Similar product distributions, with chlorinated benzenediolsand triols as the main reaction intermediates were reported byPoerschmann et al. in the Fenton oxidation of 2-chlorophenol[33] and by Li et al. [34] in the photocatalytic oxidation of 4-chlorophenol and 4-chlorocatechol.

The ring opening products might then be formed from the (chlo-

mainly between ortho hydroxy-substituted carbons to give diacids,but also between one hydroxy and one unsubstituted carbon to giveacid-aldehydes, as proposed by Li et al. [34] for the photocatalyticdegradation of 4-chlorocatechol.

15 20 25

on time, min

14.5 15 15.5 16 16.5 17 17.5

Retention time, min

9 10

11

1213

14 15

16

17

1 g/L 4-CP, H2O2:4-CP = 2.7:1 molar ratio, 1 g/L catalyst, 50 ◦C, pH 3.5.

C. Catrinescu et al. / Applied Catalysis B: Environmental 101 (2011) 451–460 455

Table 3Retention times and mass spectral characteristic ions of the major products obtained by CWHPO of 4-chlorophenol over AlFeMtPILC.

Ret. time (min) Peak nr. MW Compound Characteristic ion, m/z

7.6 1 200 4-Chlorophenol-OTMS 185,187, 2008.9 2 234 2,4-Dichlorophenol-OTMS 219, 221, 939.4 3 254 Hydroquinone-(OTMS)2 239, 254, 73

10.2 4 288 Chlorocatechol-(OTMS)2 288, 290, 27310.5 5 288 Chlorohydroquinone-(OTMS)2 73, 288, 29011.3 6 322 Dichlorocatechol-(OTMS)2 73, 322, 32411.8 7 376 Chlorinated benzenetriol-(OTMS)3 376, 7312.2 8 376 Chlorinated benzenetriol-(OTMS)3 376, 73

Cl

OH

Cl

Cl

OH

OH

OH

Cl

OH

OH

OH

Cl

OH

Cl

OH Cl

Cl

OH

OH

OH

4-chlorocatechol (4)Hydroquinone (3)

4-Chlorophenol (1)

2,4-dichlorophenol (2)Chloro-Hydroquinone (5) dichlorocatechol (6) 5-chloro-1,2,4- benzenetriol (7 or 8)

AB

Other intermediates

many steps

OH

r the d

bctc

3

dlp

PP

TR

CO2H2O

Scheme 1. Reaction pathway fo

High molecular-weight compounds (dimers, oligomers), proba-ly formed by oxidative coupling reaction, were also present in thehromatogram but only in low amounts. Based on the mass spec-ra reported by Poerschmann et al. [33] and Hong et al. [35] theondensation products listed in Table 4 were tentatively identified.

.2.2. Comparison of the catalytic performance of AlFePILCsPreliminary tests, with no H2O2 added to the reaction mixture,

emonstrated that 4-CP removal, caused by adsorption, was neg-

igible. In addition, no iron leaching was observed after 60 min, atH 3.5 and 50 ◦C, before the addition of H2O2.

Fig. 4 shows the catalytic performance of the Al–FeILCs, which indicates a sequence in which AlFeMt-ILC > AlFeSWyPILC > AlFeSAzPILC. For the 4-CP removal curves,

able 4etention times and mass spectral characteristic ions of the minor (high molecular weigh

Retention time (min) Peak nr. MW (TMS) Te

14.57 9 398 Di15.079 10 380 Ch15.28 11 432 Di15.137 and 15.81 12,13 414 Di15.9 and 17.27 14,15 468 Tr16.2 16 486 Di16.43 17 394 Ch

Cl

egradation of 4-CP by CWHPO.

these differences are more evident at short reaction times, e.g.30 min, all the catalysts allowing a complete elimination of thephenols at extended reaction times. Simultaneously, the TOCremoval efficiency increases with time until it reaches a plateauafter 120 min of reaction. The removal efficiencies expressed asTOC are smaller than these obtained for 4-CP, confirming that4-CP oxidation occurs in multiple steps and results in severalby-products, which are more resistant to oxidation than the initialpollutant. The data shown in Fig. 4 demonstrate that AlFeMtPILC

was not only the most active but also the most efficient catalystfor TOC removal. In addition, as the concentration of H2O2 at theend of the process is zero, the catalyst efficiency in the use of H2O2follows the same order as the catalytic activity, more precisely:AlFeMtPILC > AlFeSWyPILC > AlFeSAzPILC.

t) products obtained by CWHPO of 4-chlorophenol over AlFeMtPILC.

ntative identification Characteristic ion, m/z

chlorodihydroxybiphenyl-(OTMS)2 398, 400, 73lorodihydroxydiphenyl ether-(OTMS)2 380, 382, 73hydroxytrichlorobiphenyl-(OTMS)2 339, 73, 75, 129chlorodihydroxydiphenil ethers-(OTMS)2 414, 416, 73ihydroxychlorodiphenylether-(OTMS)3 468, 470, 73chlorotrihydroxybiphenyl-(OTMS)3 486, 73lorodihydroxydibenzo-p-dioxin-(OTMS)2 394, 396, 73

456 C. Catrinescu et al. / Applied Catalysis B: Environmental 101 (2011) 451–460

F ovalT SAzPILH

stpitiita

3

ootiA

cp1P

FR(

ig. 4. 4-Chlorophenol and TOC removal efficiencies vs. time for AlFePILCs. 4-CP remOC removal efficiency (dashed lines) for AlFeMtPILC (1′), AlFeSWyPILC (2′) and AlFe2O2 (H2O2:4-CP = 13.5:1 molar ratio).

Returning to the characterization data one can assume that thetructural and textural properties of the catalysts could not be usedo explain the differences in the catalytic properties. AlFeSAzPILC,ossessing the highest surface area, gave the lowest catalytic activ-

ty/removal efficiency. On the other hand, the enhanced activity ofhe SWy-2 and Mt derived catalysts may result from their higherron content. Although these results support the fact that the totalron content is an important feature for this reaction, the nature ofhe iron species in the catalysts should also be considered to arrivet a more detailed understanding of the process.

.2.3. Iron leachingThe performance of a solid catalyst cannot be assessed based

nly on the analysis of the organic pollutants removal. The stabilityf the catalyst against leaching is another important aspect that haso be taken into account. The concentration of the total iron ionsn solution, during the late phase of the oxidation, as measured bytomic Absorption Spectroscopy (AAS), is depicted in Fig. 5.

3+

The Fe concentration in solution is very low (<1 mg/L), indi-ating a good stability against leaching for all the catalysts. Theercentages of iron leached in solution from the catalysts were:.0% for AlFeSAzPILC and AlFeSWyPILC and 1.9% for AlFeMt-ILC. The most active catalyst (AlFeMtPILC) produced the higher

ig. 5. Iron leaching at the end of the reaction (240 min) for different Al–Fe PILCs.eaction conditions: 125 mg/L 4-CP, 1 g/L catalyst, 50 ◦C, pH 3.5, 13.09 mmol/L H2O2

H2O2:4-CP = 13.5:1 molar ratio).

efficiency (solid lines) for AlFeMtPILC (1), AlFeSWyPILC (2) and AlFeSAzPILC (3) andC (3′) Reaction conditions: 125 mg/L 4-CP, 1 g/L catalyst, 50 ◦C, pH 3.5, 13.09 mmol/L

amount of Fe3+ in solution (1.028 mg/L), followed by AlFeSWyPILC(0.508 mg/L) and AlFeSAzPILC (0.447 mg/L). Consequently, besidesthe catalytic properties, the quantities of iron leached in solu-tion appear to be related to the total iron content of the catalyst.However, the data presented in Figs. 4 and 5 provide considerablesupport for the lack of a direct correlation between the iron concen-tration in solution and the TOC removal efficiency. In this context,itis important to point out that the typical concentrations of iron ionsin solution, used in a classical Fenton process, range from 5.6 to840 mg/L [36], at least one order of magnitude higher than the con-centration observed in our experiments. However, the oxidation of4-CP by Fenton’s reagent takes place even at Fe2+ concentrationsbelow 1 mg/L. Kwon et al. [37] showed that the decomposition of4-CP proceeded with a very small quantity of Fe2+ (0.01 mM) andstopped only when H2O2 was completely depleted. Thus, the con-tribution of the homogeneous catalytic reaction (leached iron ions)to the global process could not be ruled out. Moreover, it could bea partial rationalization of the differences observed in the catalyticactivity of the PILCs. In order to explain both catalytic and leachingdata, a more complex phenomenon should be envisaged, by con-sidering the nature of the iron species present on the surface of thecatalysts.

3.3. Iron speciation in AlFePILCs by Mössbauer spectroscopy

The 300 K Mössbauer spectrum for AlFeMtPILC is displayedin Fig. 6. The Mössbauer hyperfine parameters of AlFeMtPILCs,extracted from the spectra recorded at 300 and 77 K are listed inTable 5. Both spectra show two paramagnetic doublets, suggestingthe existence of two distinct environments for the ferric ion, bothin distorted octahedral symmetry.

The major form (around 70%), with a IS = 0.36 mm/s and a highvalue of the QS = 0.96 mm/s (at RT), could be attributed to a severelydistorted octahedral symmetry, due either to the substitution of Alfor Fe or/and to the small size of the oxide particles. In syntheticmicrocrystalline hematite decreasing particle size results in latticeexpansion which causes the quadrupole splitting of this mineral toincrease whereas the isomer shift remains essentially unchanged

[38]. A similar doublet (IS = 34 mm/s and QS = 0.92 mm/s) was foundby Bakas et al. [39] in a detailed Mössbauer study of the iron speciesin AlFePILCs and was attributed to small particles of Al-substitutedmaghemite. The second doublet, with a lower quadrupole split-ting (IS = 0.39 mm/s and QS = 0.55 mm/s in 300 K spectrum), was

C. Catrinescu et al. / Applied Catalysis B: Environmental 101 (2011) 451–460 457

record

nsltpip

stsaowsu

3

tFnsdAimap

TM7

is well-established [43] that the spectra of ferric oxides exhibitcharge-transfer absorption (LMCT) bands centered around 260 nm,followed by medium intense d–d electron pair transition (EPT)bands around 450–560 nm and weak d–d bands above 600 nm. By

297 nm

Fig. 6. Mössbauer spectrum

ot observed by these authors. Guélou et al. [12] observed twoignals of equal intensity in the Mössbauer spectrum of FAZA cata-ysts (IS = 0.35 mm/s and QS = 0.9 mm/s), but no attempt was madeo elucidate the spectrum. Carriazzo et al. [50] found that iron isresent as hematite, while Aouad et al. [51] found that heating the

ntercalated clay at 300 ◦C resulted in a mixture of akaganeite andoorly ordered hematite.

Refined analysis showed that the room temperature Mössbauerpectra of small particles (<70 A) of hematite can be fitted withwo partly overlapping doublets that have different quadrupoleplittings of 0.52 and 0.90 mm/s [40,41]. These components weressigned to well-ordered inner and poorly ordered surface regionsf the particles, respectively. These observations correlate wellith the observed spectrum of AlFeMtPILC, which has quadrupole

plittings that are almost identical with those given by Kraan forltrafine hematite [40].

.4. Iron speciation in AlFePILCs by DR UV–vis spectroscopy

The DR UV–vis spectra of Al–Fe pillared clays, prepared fromhree different clays (SAz-1, SWy-2 and Mt), are presented inig. 7. The spectra exhibit a strong absorption band in theear-UV, with one side extending in the visible region of thepectra. Looking at the total area under the curves, it can beeduced that the total iron content varies in the following order:lFeMtPILCs > AlFeSWyPILCs > AlFeSAzPILC. Three absorption max-

ma could be observed for each spectrum. The first and the thirdaxima are common for all the catalysts and are positioned at 256

nd 512 nm, respectively. Important differences are noted in theosition of the second absorption maximum, situated at 297 nm

able 5össbauer hyperfine parameters of AlFeMtPILC from spectra recorded at 300 K and

7 K.

IS (mm s−1) QS (mm s−1) S (%)

Mt 300 K0.35 0.51 90.750.39 1.37 9.25

AlFe(Mt)PILC 300 K0.39 0.55 260.36 0.96 74

AlFe(Mt)PILCs 77 K0.52 0.6 320.46 1.0 68

ed at 300 K for AlFeMtPILC.

for AlFeMtPILC, at 333 nm for AlFeSWyPILC and at 354 nm for AlFe-SAzPILC.

The noise removal, based on the application of Fourier filter, fol-lowed by the second derivative (SD) of the spectra was proposedas the best way for the analysis of the spectra for iron oxides mixedwith other Fe-containing minerals [42]. The Fourier filter was cho-sen because it does not alter the shape of the spectrum, includingthe weak bands (shoulders). Typical SD curves are shown in Fig. 8.

The positions of the minima extracted from the SD curves for allof the Al–Fe pillared clays are presented in Table 6.

The SD curves of our PILCs display an intense minimum ataround 260 nm (LMCT1) followed by a second region (288–450 nm)composed of three convoluted bands (LMCT2, LMCT3 and LMCT4),a third region with a medium-intense minimum at 512 nm (EPT1)and a shoulder at about 560 nm (EPT2), and finally several bandsabove 580 nm (d–d bands).

The first step in our attempt to identify the iron specia-tion was to look for the absorption bands which are typicallypresent in “classical”, three-dimensional (3-d) ferric oxides. It

200 300 400 500 600 700 800 900

Wavelength, nm

2 x10-4

4 x10-4

6 x10-4

8 x10-4

1x10-3

F(R

)

AlFeMtPILC

AlFeSWyPILC

AlFeSAzPILC

333 nm

354 nm

256 nm

512 nm

Fig. 7. UV–vis diffuse reflectance spectra of catalysts.

458 C. Catrinescu et al. / Applied Catalysis B: E

500

-1.5x10-7

-1x10-7

-5x10-8

0.0

5x10-8

2nd

deriv

ativ

e of

inte

nsity

LMCT1

LMCT2 LMCT3LMCT4

EPT1

EPT2

d-d transitions

AlFeMtPILC

300 400 600 700 800

Fa

altoth

ottpo(5pcaonEbtpodct

fbMt3

TSs

Wavelength, nm

ig. 8. Second derivative UV–vis diffuse reflectance spectra of AlFeMtPILC withssigned LMCT, EPT and d-d transition bands.

nalogy, the band at 260 nm from our spectra could be attributed toigand (oxygen)-to-metal charge transfer (LMCT) excitations fromhe O(2p) non-bonding valence bands to the Fe(3d) ligand filedrbitals. This band is not phase-specific and is normally encoun-ered as an intense band in the spectra of iron oxides and oxide-ydroxides.

The minima at 512 nm and 560 nm (shoulder) in the SD curves ofur PILCs could be both assigned to EPT. In contrast to LMCT bands,he EPT bands (450–580 nm) are most appropriate for the iden-ification of iron oxides and oxide-hydroxides. For example, theosition of EPT band above 500 nm is characteristic to face-sharingctahedral in hematite (bands at 521–565 nm) and maghemiteband at 515 nm). The other iron oxides posses EPT bands below00 nm, e.g. between 478 and 492.6 nm in goethite. In our case, theositions of these minima indicate the occurrence of 3-d, highlyondensed Fe2O3 iron oxides, precipitated on the PILCs surface. Inddition, the presence of two minima could suggest the existencef two Fe2O3 phases. In our opinion, no further speculations on theature of these phases could be made from the positions of thePT bands, because the corresponding absorptions are influencedy several factors which are not controlled in this study. Amonghese factors the most important are (i) the size of the iron oxidearticles and (ii) the isomorphic substitution of Al for Fe in the ironxide structure [43]. The minima above 580 nm, corresponding to-d transitions in iron oxides, can also be distinguished in the SDurves and confirm the existence of 3-d iron oxide-like particles onhe catalyst surface.

After the identification of the iron oxides, the next step was toocus on the 300–450 nm range, where several medium-intense

ands are observed. The attribution of these bands is rather difficult.ost of the iron oxides posses weak (spin and parity forbidden) d-d

ransition bands in this range, e.g. at around 315 nm (6A1 → 4T1) and70 nm (6A1 → 4E) for both hematite and maghemite and around

able 6ummary of band positions identified as minima in the second derivative smoothedpectra of PILCs.

Transition/origin Band position, nm

AlFeSAz AlFeSWy AlFeMt

LMCT1 260 260 260LMCT2 307 307 301LMCT3 334 329 338LMCT4 384 380 378EPT1 2(6A1) → 2(4T1) 509 509 508EPT2 2(6A1) → 2(4T1) 561s 568s 568sd–d bands 672, 691 674, 692 670, 690

nvironmental 101 (2011) 451–460

359 nm in lepidocrocite (6A1 → 4E d–d transition). In our opinion,the bands in the 300–400 nm range of our spectra could not beassociated with the d–d transitions, being rather intense and well-defined.

On the other hand, UV absorption bands at around 357 nm werefound in Fe-ZSM-5, particularly in samples prepared by chemicalvapor deposition, and were assigned to ferric oxide nanoclus-ters [44,45] or to bidimentional (2-d) FexOy species, forming thin“rafts/patches” on the surface of the solid support [46]. Similarassignments were proposed for iron oxides supported on SiO2[47,48]. Furthermore, these bands originate, according to Smo-lakova et al., [45] from ligand-to-metal charge transfers. By analogy,we tentatively ascribe the absorption bands in the 300–400 nmrange from our samples to charge transfers in low-nuclearity fer-ric oxides (FexOy). However, this assignment should be taken withcaution because a band centered between 345 and 357 nm wasalso observed in ferrihydrite, calcined ferrihydrite (near amorphousFe2O3) and calcined Fe-exchanged smectite [45].

Another important aspect is related to the complexity of the300–400 nm regions, which contains at least three well-definedminima in the SD curves. The convolution of these bands giverise to Kubelka–Munk absorption maxima at 370 nm, 355 and312 nm for AlFeSAzPILCs, AlFeSWyPILCs and AlFeMtPILCs, respec-tively. According to the current knowledge [44], the increase inthe number of Fe–O–Fe bonds with increasing cluster size of FexOy

leads to a shift of the LMCT bands towards higher wavelengths.This interpretation was based on the fact that oligomerization of[Fe (O, OH, H2O)x] polyhedral shifts the LMCT bands towards near-UV and even VIS region [49]. Consequently, we can deduce that theaverage size of the oligonuclear clusters from PILCs increases in thefollowing order: AlFeMtPILCs < AlFeSWyPILCs < AlFeSAzPILCs. Thiscorrelates well with the values of the surface areas and with XRDdata, suggesting that a more ordered and opened structure is asso-ciated with oligonuclear species of increasing size. On the otherhand, assuming that the stability of the FexOy in the reaction con-ditions increases with their size, the differences in iron leaching andcatalytic properties, obtained for our PILCs, could be thus justified.

In summary, based on DR UV–vis and Mössbauer data wecan envisage that iron can be found in two forms: (i) as poorly-ordered/low-nuclearity FexOy phases (major form), characterizedby UV absorption bands in the 300–400 nm range and by ahigh quadrupole splitting in the Mössbauer spectra and (ii) aswell-ordered hematite-like nanoparticles (minor form), with char-acteristic EPT bands above 500 nm and a smaller quadrupolesplitting. The low nuclearity ferric oxides are mainly responsiblefor the catalytic properties of Al–Fe PILCs.

By adding a known volume of 30% H2O2 or H2O on the cata-lyst tablet surface, a change in color and a subsequent increase ofthe intensity of the Kubelka–Munk absorption (Fig. 9) are clearlyobserved. However, the spectrum of the H2O wet sample main-tains the original shape while the addition of H2O2 strongly altersthe absorption in the UV region. The first question is whether thisstrong absorption below 285 nm is due only to liquid H2O2 or to itsinteraction with the iron species. To exclude the first hypothesis,we added the same amount of H2O2 to a simple BaSO4 (withoutcatalyst) tablet and only a slight increase of the absorption in theUV range, as compared with the catalyst tablet, was noticed. More-over, by adding increasing amounts of H2O2 to this catalyst tabletthe spectra remain almost unchanged (curves not shown). Conse-quently, the intense band centered at 197 nm, observed only whenboth AlFePILC and H2O2 are present, could be attributed to a LMCT

associated to an interaction between H2O2 and Fe (III). Assumingthat this interaction will lead to the formation of reactive species,e.g. HO., we can expect a correlation between the intensity of thisband and the catalytic activity. For comparison the spectra of BaSO4,after the contact with 0.5 mL 30% H2O2 is also shown.

C. Catrinescu et al. / Applied Catalysis B: Environmental 101 (2011) 451–460 459

Fig. 9. UV–vis diffuse reflectance spectra of the catalyst before (AlFeSAzPILC) and after the

0

0.005

0.01

0.015

0.02

0.025

0.03

F(R

)

AlFeMtPILCAlFeSWyPILC

AlFeSAzPILC

F

tPwtf

4

oatcp

ctpOst

200 300 400 500 600 700 800 900

Wavelength, nm

ig. 10. UV–vis diffuse reflectance spectra of the catalysts after the addition of H2O2.

The DR UV–vis spectra, recorded after the addition of H2O2 to thehree catalysts, are depicted in Fig. 10. AlFeSWyPILC and AlFeMt-ILC display similar intensities of the absorption bands at 197 nmhile for AlFeSAzPILC this band is less intense. This ordering seems

o correlate well with the catalytic activity, which proceed in theollowing order: AlFeMtPILC > AlFeSWyPILC » AlFeSAzPILC.

. Conclusions

This study presents the assessment of the influence of host clayn the structural, textural and catalytic properties. In particular, wettempted to identify the structure of iron-containing species inhe AlFePILCs prepared from different clays, in order to analyze theatalytic properties in correlation with the nature of these activehases.

Following our initial investigation on the CWHPO of 4-hlorophenol we have shown that, using AlFePILCs as catalysts,

wo reactions compete as the first transformation of the pollutant,roducing 4-chlorocatechol (major pathway) and hydroquinone.ther reaction intermediates were also identified. We have also

hown that the nature of the host clay influences the structural,extural and catalytic properties of the pillared clay catalysts in

[[

[

[

contact with 0.5 mL of H2O (AlFeSAzPILC + H2O) and 30% H2O2 (AlFeSAzPILC + H2O2).

the catalytic wet hydrogen peroxide oxidation of 4-chlorophenol.Significant differences were observed not only in the catalytic activ-ity/pollutant removal efficiency, but also in the stability againstleaching. Structural and textural properties of the catalysts couldnot be used to explain these differences in the catalytic propertieswhile diffuse reflectance UV–vis spectroscopy gives valuable infor-mation on the solid state speciation of ferric oxides from AlFePILCs.The description of the structure-activity relationship in the cat-alytic wet hydrogen peroxide oxidation of 4-chlorophenol shownthat the low-nuclearity ferric oxides appear to be responsible forthe differences observed in the catalytic properties of the catalysts.Further studies are conducted towards the understanding of theiron dissolution process and of the stability of different solid ironspecies in the reaction conditions.

Acknowledgements

This work was supported by the CNCSIS–UEFISCSU, projectnumber PNII–IDEI code 368/2008, financed by Romanian Gov-ernment, and by the project BRAIN “Doctoral scholarships as aninvestment in intelligence”, financed by the European Social Foundand Romanian Government.

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