Journal of Hazardous Materials 300 (2015) 590597

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Journal of Hazardous Materials

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

Intrins ontoward

Mhamed Assebban, Achraf El Kasmi, Sanae Harti, Tarik Chak

Laboratory LGCVR-UAE/L01FST, Faculty of Sciences and Techniques, University Abdelmalek Essaadi, B.P. 416 Tangier, Morocco

h i g h l

Clay monotoward airevaluated.

17 air poland variou

All testedtively comand CO2.

Intrinsic cawas tentatposed acid

Dissimilariinduced diactivity.

a r t i c l

Article history:Received 21 JuReceived in reAccepted 28 JuAvailable onlin

Keywords:Catalytic oxidaCarbon monoxVOCsNatural clayHoneycomb m

1. Introdu

Volatile nized as m

CorresponE-mail add

http://dx.doi.o0304-3894/ i g h t s

lith catalytic performance pollution abatement was

lutants including CO, CH4s VOCs were tested.

compounds were effec-pletely oxidized to H2O

talytic activity of the clayively attributed to predis-

centers.ties between reactantsfferences in clays catalytic

g r a p h i c a l a b s t r a c t

e i n f o

ne 2015vised form 25 July 2015ly 2015e 29 July 2015

tionide

onolith

a b s t r a c t

The present work highlights the intrinsic catalytic properties of extruded clay honeycomb monolithtoward complete oxidation of various air pollutants namely CO, methane, propane, acetylene, propene,n-butene, methanol, ethanol, n-propanol, n-butanol, acetone, dimethyl ether, benzene, toluene, o-xylene,monochlorobenzene and 1,2-dichlorobenzene. Total catalytic conversion was achieved for all testedcompounds with different behaviors depending on pollutants structural and chemical nature. The com-parison of T50 values obtained from light-off curves allowed the establishment of the following reactivitysequence: ketone > alcohol > ether > CO > alkyne > aromatic > alkene > chlorinated aromatic > alkane. Theintrinsic catalytic performances of the natural clay was ascribed to the implication of a quite complexmixture constituted by OH groups (Brnsted acids) and coordinately-unsaturated cations, such as Al3+,Fe3+ and Fe2+ (Lewis acids). Hence, the combination of the clays intrinsic catalytic performances andeasier extrudability suggests a promissory potential for application in air pollution control.

2015 Published by Elsevier B.V.

ction

organic compounds (VOCs) have been widely recog-ajor source of air pollution due to their high potential

ding author. Fax: +212 539 39 39 53.resses: t.chak@fstt.ac.ma, tchak@yahoo.com (T. Chak).

toxicity, as well as their role in the formation of photochemi-cal smog and destruction of ozone layer [1,2]. Catalytic oxidationhas been proved to be less costly and environmentally advanta-geous when compared to the conventional thermal incinerationfor simultaneous destruction of CO and VOCs encountered in var-ious kinds of industrial gaseous efuents [3,4]. The key to thisprocess is a highly efcient catalyst that could prompt the com-plete oxidation of VOCs at rather low temperatures. Generally, the

rg/10.1016/j.jhazmat.2015.07.0672015 Published by Elsevier B.V.ic catalytic properties of extruded clay h complete oxidation of air pollutantseycomb monolith

M. Assebban et al. / Journal of Hazardous Materials 300 (2015) 590597 591

catalysts that are used fall into two categories: supported noblemetal-based catalysts, typically platinum, palladium or gold andoxides of the transition metals, mainly oxides of cobalt, copper,nickel and lanthanum [58]. Among the noble metal-based cata-lysts, Pt andof their higand vulneraous drawbaalthough thoxides are asoning as wcontaining in VOCs oxias functioncost and opoxidation, cowing to thture [21]. Oacid sites isalytic perfo[22]. So farmainly derLewis acidit(Al3+) and ilayer and aLewis acid sinant and/ocatalytic pethree VOCsthe need ofintercalatioful binderlewithin the rbility [28]. Tterms of lobed, especia[2931]. It iout with cotive three wbest of our kmonolith toscarce.

The presof extrudedterms of cogeted compn-butene, dimethyl ezene and 1as represenurated hydchlorinatedpollutants. in reactivitystructural dproperties

2. Experim

The extrused in thiswas comprtural and tebrief, chemorescence w

Table 1Main characteristics of the used M Clay honeycomb monolith.

Specic surf 2

Monolith gensitytrengsectioels sh

the chickneraction

: stan

.29%.02%cal cte an

M C inve

in aatm

d betich wller (lly intemC. Tht + 1ond

ith) e an

of mhe re

con syrilar aand tpresserefotely on ral setC to geactixturbecaconvment

ext wooel reservlacerodu

remesults not shown). Reactants and products formed duringn were monitored on-line using FTIR spectroscopy (Nicoletasco 410) equipped with homemade gaseous cell. Accord-reactant conversion values were calculated by subtractingctant outlet and inlet concentrations and then divided by thencentration. Therefore, the collected data permit obtainingff curves indicating conversion proles versus temperatureow extraction of T10, T50 and T90 values dened as temper-

at which, respectively, 10%, 50% and 90% conversion wased. Au supported catalysts are the preferred ones becauseh specic activity [911]. However, their high costbility to chlorine and sulfur poisoning constitute seri-cks for their application [12,13]. On the other hand,ey are less active than noble metals, transition metal

suitable alternative because of their resistance to poi-ell as their lower price [14,15]. In particular, cobalt

mixed oxides have been claimed for their effectivenessdation [1619]. Much attention has been paid to claysal materials in the eld of catalysis, due to their lowerational simplicity [20]. Particularly, vis--vis VOCslays have recently caught interest as efcient catalystseir unique features in composition, structure and tex-f interest, the presence of suitable Brnsted and Lewis

known as a prevailing parameter governing the cat-rmance of clays toward the deep oxidation of VOCs, it is believed that Brnsted acidity (proton donor) isiving from the clay structural hydroxyl groups, whiley (electron pair acceptor) is attributed to the aluminumron (Fe2+ and Fe3+) species located in the octahedralt broken edges [23,24]. Generally, both Brnsted andites are common, whereas one of them might be dom-r catalytically benecial [25]. Recently, we introducedrformances of a natural clay toward the oxidation of, namely, n-butanol, acetylene and propene without

any chemical treatment or modication (e.g. pillaring,n) [26,27]. Furthermore, noteworthy is the clay success-ss extrusion into a honeycomb-like monolithic shapeange of required mechanical strength and thermal sta-his conguration offers additional major advantage in

w pressure drop as compared to conventional packedlly when large volumetric ow rates have to be handleds known that monolithic structures are typically maderdierite and are commonly associated with automo-ay catalyst and industrial applications [3234]. To thenowledge, the application of honeycomb extruded clay

the abatement of common air pollutants is relatively

ent work aims to investigate the catalytic performances clay with honeycomb monolithic shape (M Clay) inmplete oxidation of carbon monoxide (CO) and16 tar-ounds, namely, methane, propane, acetylene, propene,methanol, ethanol, n-propanol, n-butanol, acetone,ther (DME), benzene, toluene, o-xylene, chloroben-,2-dichlorobenzene. These compounds were selectedtatives of different types of VOCs (i.e., alkanes, unsat-rocarbons, oxygenated hydrocarbons, aromatics and

aromatics) all of which are considered as hazardousThis issue will be addressed considering the differences

of the tested compounds induced by their chemical andissimilarities with respect to the clay intrinsic catalytic

ental

usion process of the clay honeycomb monolith (M Clay) work has been described elsewhere [26]. The M Clayehensively characterized in terms of chemical, struc-xtural properties in our previous publication [27]. Inical composition of the clay as revealed by X-ray u-as; SiO2 (58.50 wt.%), Al2O3 (23.90%), Fe2O3 (11.11%),

Cell deAxial sCross-ChannSize ofWall tVoid f

a CPSI

K2O (2ZrO2 (0eralogikaolinistudied

Theductedunder stacketor whcontrocoaxiaof the of 2 reactancorrespmonolpropanmeanswhile tlowingusing aParticulimits vapor tor. Thaccurainjectiimentaat 70

liquid rtant moutlet of the experiine thequartzless stewas obtakes pthe repmeasulith (rereactio5700/Jingly, the reainlet colight-oand allaturesachievace area (SBET) 41.7 m /g

ometry Cylindrical250CPSIa

th 2.5 MPanal diameter 14.7 mmape Squarehannel openings 1.2 mmss 0.2 mm

55.8%

ds for Cells Per Square Inch.

), MgO (1.55%), Na2O (1.46%), TiO2 (0.63%), CaO (0.22%),), and MnO2 (0.02%), loss on ignition (0.30%). The min-omposition was found to be a mixture of quartz, illite,d vermiculite. Briey, the main characteristics of thelay are depicted in Table 1.stigations of M Clay catalytic performances were con-

continuous ow stainless steel reactor (i.d. 1.47 cm)ospheric pressure. Typically, 1 g of the M Clay wasween two quartz wool plugs and loaded into the reac-as heated stepwise at a rate of 5 C min1 by a heat

Horst). A K-type thin thermocouple was positioned the middle of the monolith for accurate measurementperature prole during the reaction with a precisione total ow rate of the reactant feed mixture (1 vol.%0 vol.% O2 diluted in Ar or N2) was set at 15 ml min1,ing to a GHSV (considering the compact volume of theof 2300 h1. CO, DME, methane, acetylene, propene,d n-butene were introduced into the gaseous stream byass ow controllers (MKS and Bronkhorst instruments),maining VOCs were generated through evaporation fol-tinuous injection into O2/N2 mixture gaseous streamnge pump and Hamilton micro-syringe (250 l) (Fig. 1).ttention has been paid to avoid encountering ammableo hold the VOC partial pressure always lower than theure value so as to prevent its condensation in the reac-re, the reactant desired inlet concentration is obtainedby adjusting the syringe pump to the correspondingte. Moreover, all the tubes and connections of the exper-up were wrapped with a heating tape and maintaineduarantee an instantaneous evaporation of the injectedant. Note that the catalytic test was launched once reac-e reached steady state (i.e., the concentration at theme stable) in order to avoid eventual overestimationersion due to adsorption on the clay material. Blanks were conducted without the clay monolith to exam-ent of homogenous reaction and adsorption over thel as well as the inertness of the inner walls of the stain-actor. In this work, neither conversion nor adsorptioned in the temperature range where the catalytic reaction

with respect to each tested compound. Furthermore,cibility of the results was checked by repeating thents at least twice using either fresh or used clay mono-

592 M. Assebban et al. / Journal of Hazardous Materials 300 (2015) 590597

et-up used for the catalytic tests.

Fig. 2. Light-o

3. Results

The stuand the reindicated bseparate rated hydrand aromafour gurewith respecurves presmost reacttive one; (ivketone > alcaromatic > a

Table 2 sthe differenby-productall vanishedof the VOCschlorinated

Among athe highestFig. 1. Schematic representation of the experimental sff curves of CO, methane and propane oxidation reactions over M Clay.

and discussion

died compounds were arranged within four groupssults of their catalytic conversion on M Clay, asy light-off curves, were superposed and plotted ingure as following: CO and alkanes (Fig. 2), unsatu-ocarbons (Fig. 3), oxygenated hydrocarbons (Fig. 4)tic hydrocarbons (Fig. 5). The comparison of theses shows that (i) complete conversion was achievedct to each compound; (ii) all the obtained light-offented a typical sigmoidal shape; (iii) acetone is theive compound, whereas methane is the least reac-) the overall reactivity decreases following the order;ohol > ether > CO > alkyne > aromatic > alkene > chlorinatedlkane.ummarizes the values of T10, T50 and T90 resulting fromt testing as well as the presence or not of any detected(s) during the oxidation processes that were, by the way,

at total conversion. Note that the complete oxidation yields to CO2 and H2O in addition to HCl in the case of

VOCs.ll tested hydrocarbons, alkanes were found to present

refractoriness against oxidation. As displayed in Fig. 2,

Fig. 3. Light-off curves of unsaturated hydrocvarbons (acetylene, propene and n-butene) complete catalytic oxidation over M Clay.

Fig. 4. Light-off curves of oxygenated hydrocarbons catalytic oxidation over M Clay.

M. Assebban et al. / Journal of Hazardous Materials 300 (2015) 590597 593

Table 2T10, T50, T90 values, apparent activation energies (Eappa) and by-products detected during the oxidation processes.

Compounds T10 (C) T50 (C) T90 (C) Eappa (kJ mol1) By-products

CO 270 295 316 137 n.d.a

Methane Propane Acetylene Propene n-ButeneMethanol Ethanol n-Propanol n-Butanol Acetone DME Benzene Toluene o-Xylene Chlorobenze1,2-Dichloro

a n.d.: none

Fig

methane un445 C andaround 660verted withrespectivelythat of an aNieuwenhuwith regardcase of CO, aing temperabetween itscarbons undfound to presion prolecompoundslene oxidat50% and 90ing propeneoverlappedsion 50%)over a broaing rise to Regarding ocatalytic peest T90 at 2

of 28e oxiese

ay inimila

a wre garopa90. A445 512 585 359 420 473 255 299 337 277 355 429 280 351 385 223 261 301 233 272 298 226 271 324 242 278 313 179 228 259 265 286 307 306 345 387 266 329 369 234 321 386

ne 353 395 444 benzene 371 414 449

detected.

value acetonclay, thural clquite swithinperatufor n-ptheir T. 5. Catalytic oxidation proles of aromatics over M Clay.

dergoes 10% and 50% conversion at temperatures of 512 C, respectively, and it is completely oxidizedC. Comparatively, propane is rather easy to be con-

T10, T50 and T90 values at 359, 420 and 473 C,. These results make the performance of M Clay surpass

lumina-supported gold catalyst reported by Gluhoi andys [35], which featured a T50 value of 592 and 447 C

to methane and propane oxidation, respectively. In sharp increase of conversion was observed by increas-ture as indicated by the small difference of only 46 C

T10 (270 C) and T90 (316 C). The unsaturated hydro-er scrutiny (i. e. acetylene, propene and n-butene) weresent intermediate reactivity as revealed by the conver-s depicted in Fig. 3. Although the oxidation of the three

starts at approximately the same temperature, acety-ion proceeds afterwards to a higher extent undergoing% conversion at 299 and 337 C, respectively. Concern-

and n-butene, the obtained light-off curves are nearly in the low and medium conversion regime (i. e. conver-. Nonetheless, the prole of propene becomes spreadd range of temperature beyond 50% conversion, giv-a difference of 44 C at T90 between the two curves.xygenated hydrocarbons, Fig. 4 illustrates interestingrformances particularly for acetone showing the low-59 C. Taking into account the comparatively high T50

alcohols is with CO. Nobutanal wen-propanoloxygenatedconverted aever, its cothe temperathe values oboth acetonout formatireactivity ohydrocarboamong the were foundwas achievand o-xylenwhich is 17306 C, andtemperaturdation prodoxidation wmethyl-subchlorobenzslight diffeture increaless pronouother than It is of inteactivity in tature workset al. [37] azene at 600Nogueira emontmorill90% toluen165 CO133 CO117 CO78 CO63 CO92 CO; Formaldehyde97 CO; Acetaldehyde96 CO; Propanal99 CO; Butanal77 n.d.a

221 n.d.a

109 n.d.a

78 CO71 CO

133 CO173 CO

1 C obtained in the work of Gil et al. [36] throughdation over platinum catalyst supported on Al-pillaredresults show the good catalytic potential of the nat-

the present work. As far as alcohols are concerned,r oxidation proles were obtained and almost overlapide range of temperature. Hence, no signicant tem-p was noticed between their T10, T50 and T90, exceptnol and ethanol showing a difference of 26 C betweens illustrated in Table 2, the oxidation of the testedaccompanied by the formation of aldehydes togetherte that formaldehyde, acetaldehyde, n-propanal and n-re produced during the oxidation of methanol, ethanol,

and n-butanol, respectively. As compared to the other hydrocarbons, DME seems to be more difcult to bes indicated by its higher T10 at around 265 C. How-

nversion increases sharply to reach 90% by increasingture by 42 C only (T90 = 307 C), which is comparable tobtained with alcohols. It should be pointed out that fore and DME, the oxidation yields to CO2 and H2O with-on of any detectable by-products. As shown in Fig. 5, thef aromatics is denitely lower than that of oxygenatedns but still higher than that of alkanes. Furthermore,investigated aromatics, methyl-substituted molecules

to be the most reactive. 10% conversion of o-xyleneed at 32 C lower than that of toluene. T50 of toluenee are quite similar. However, T90 of toluene is 369 C,C lower than that of o-xylene. Benzene shows T10 at

the conversion increases steeply in a narrow range ofe to attain 90% without giving rise to any partial oxi-ucts. In the case of chlorinated aromatics, the catalytic

as found to proceed with a lesser extent than non- andstituted aromatics. Moreover, the conversion prole ofene is adjacent to that of 1,2-dichlorobenzene with arence of 18 C between their T10 values. As tempera-ses, the curves get closer and the difference becomesnced at 90% conversions (i.e. 5 C). Besides, by-productsCO were not detected during the catalytic oxidation.rest to note that the M Clay presents a competitive

he oxidation of the aromatics as compared to some liter- dealing with clay based catalysts. For instance, Oliveirachieved complete conversion of xylene and chloroben-C, over Al-PILC bentonite impregnated with 5 wt.% Pd.t al. [38] investigated the oxidation of toluene over aonite-rich clay impregnated with iron oxide, obtainede conversion at 378 C. Whilst Zuo and Zhou [39] oxi-

594 M. Assebban et al. / Journal of Hazardous Materials 300 (2015) 590597

dized 90% benzene over Pd catalyst supported on Na-exchangedmontmorillonite (0.2 wt.% Pd contenant) at a temperature of 384 C.

The aforementioned light-off curves have been used to esti-mate apparent activation energy (Eappa) by using Arrhenius plotof the exp[17] and ththat amongEappa (77 kues were f137 kJ mol

ences wereto 99 kJ mohols has nopresence odifference ba much higacetylene. propane is 133 and 16obtained Eativity ordedichlorobenof activatiodecreasing tion with reis to be undby high conthe one givevation ener(i.e. the easmight be nEa is alwaythe identifer limitatioboth reactiocase, the m(Hads) fortion and rerelated to buration, naionization pspecic suraccessibilitymatter of faan essentiaOf particulaproperties, groups and vailing parathe elemenclay were lpresumablyters (Brnst[47].

On the odation of saactivation otion proces[48,49]. Thuvation procsuch as thepropane ofhigher activvalues obta(C C) is illuand propen

proceeds to a higher extent than propane, giving rise to a drasticdecrease in the Eappa value. This is attributed to the implication ofelectrons of the C C double bond in unsaturated adsorption onLewis acid sites, which makes them more prone to subsequent reac-

254in, eactia valuppa vred t

an e [56rptioed tic acimumsorptic beME. l gro2+/Feen. Te of

lonehichningclay sized

Al2Ovolvs Lewor alrableting ductbetwLewiundsay ols tof theith

oceetion

boned alen oxls OHationhe clling

weas we[68,6ion e or

Indece, Evarrieberg

n V2Oopos

Cl sion ond ver Perimental data where conversion is lower than 15%e results are listed Table 2. As expected, it was found

oxygenated compounds, acetone exhibited the lowestJ mol1). As for DME and CO, calculated Eappa val-ound to be exceedingly high amounting to 221 and1, respectively. Regarding alcohols, no major differ-

observed between Eappa values (i. e. ranging from 92l1), indicating that the chain length of primary alco-

signicant effect on their reactivity. Nevertheless, thef one extra methyl group yields to about 15 kJ mol1

etween Eappa values of propene and n-butene, whileher Eappa amounting to 117 kJ mol1 is obtained forIn the case of saturated hydrocarbons, the Eappa forpredictably lower than that of methane with values of5 kJ mol1, respectively. Concerning the aromatics, theppa values range from 71 to 173 kJ mol1 following reac-r; o-xylene > toluene > benzene > chlorobenzene > 1,2-zene. Accordingly, it can be obviously seen that the easen of the studied aromatic compounds decreases withdegree of alkylation and increasing degree of chlorina-spect to non-substituted benzene. On the other hand, iterlined that the overall order of reactivity as indicatedversion at lowest T10 appears to be partly different fromn by obtained Eappa values. In general, the intrinsic acti-gy (Ea) follows the same order as the ease of oxidationier the molecule is oxidized, the lower the Ea) [40]. Thiso longer valid for apparent activation energy, becauses associated with overall elementary steps or at leasted rate-determining step in the absence of mass trans-n [41]. In fact, the catalytic reaction is controlled byn kinetics and reaction equilibrium [42]. Thus, in our

easured Eappa combines both Ea and heat of adsorption all the intermediates involved in the overall adsorp-action processes. In this sense, multiple factor effectsoth characteristics of the VOC (e.g. degree of unsat-

ture of functional groups, molecular polarization andotential) and intrinsic properties of the catalyst (e.g.face area, acidity, presence of promoters, nature and

of active sites) need to be taken into account [43]. As act, it is well established that acidity of the catalyst playsl role in the adsorption and oxidation of VOCs [22,44].r interest, beside chemical composition and texturalthe clays acidic characteristics stemming from hydroxylcoordinately-unsaturated cations were found to be pre-meters for their catalytic performances [45,46]. So far,tal and mineralogical characterization of the studiedikely correlated to the intrinsic catalytic performance

associated with total acidity due to terminal OH cen-ed acids) and Al3+, Fe3+ and Fe2+ cations (Lewis acids)

ther hand, the crucial role of the acidic sites in the oxi-turated hydrocarbons, as well as the importance of ther rupture of the weakest C H bond during the adsorp-s, have been recently highlighted by several authorss, lower activation energy generally implies easier acti-ess associated with lower C H bonding energy [50,51],

case of C-H bond dissociation energy for methane and 439 kJ mol1 and 410 kJ mol1, respectively, yield toation energy for methane in agreement with the Eappained in this work. Moreover, the effect of double bondstrated by comparing the oxidation behavior of propanee. As more already mentioned, the oxidation of propene

tion [5the chamore rin Eapplene Eacompalene assurfaceof adsoso-callcatalytan optthe adcatalytwith Dmethyand Feof oxygthe cascarbonsites, wConcerto the emphaby thetions inacted a

As fcompasuggesby-proaction and a compotion malcohoview omade whols printeracof O Hadsorbbetwealcohodrogen

In tcontroof thebond ibonds activatlow th(C C).instantions cLichtenzene oand prweak CconcluC Cl bzene o]. Furthermore, when an extra methyl group is added toelectrons in the C C bond of n-butene become slightlyve than in propene [55], which explains the differencees found between the two molecules. However, acety-

alue was found to be curiously nearly two folds higher aso propene, apparently due to the high capacity of acety-lectron donor which makes it strongly adsorbed on the]. In fact, the qualitative relationship between strengthn and catalytic activity is suggested to be attributed to

he volcano plot reported by Bond [57]. Accordingly, thetivity increases with adsorption strength until reaching

(crest of the volcano), then the activity decreases asion gets stronger. This postulate might also explain thehavior as indicated by the higher Eappa value obtainedThe latter possess an oxygen atom, attached to twoups, that seems to have a strong afnity toward Al3+3+ centers via interaction with the electron lone pairhis could also explain the high Eappa value obtained in

CO oxidation due to the strong interaction between the pair of electrons and the clay predisposed Lewis acid

gives rise to stable adsorbed species hardly reactive. acetone, it seems that catalytic reaction is attributedLewis acid sites, according to Zaki et al. [58]. The latter

that surface reactions of acetone is critically controlled3 surface acidic properties through adsorptive interac-ing coordination of the ketones carbonyl group to Al3+

is acid sites.l investigated alcohols, similar conversion proles and

Eappa values were obtained regardless the chain length,a common reaction pathway yielding to aldehyde as

along with CO. Recently, it was reported that inter-een the lone pair electrons on the oxygen in alcohols

s acid center is the primary mode of sorption of these [5961]. While it was also suggested that the sorp-ccur through hydrogen bonding from the hydroxyl inward oxygen atoms in Brnsted acid centers [62]. Inse propositions, two reasonable interpretations can berespect to the clay under study. (i) The oxidation of alco-ds through adsorption on Al3+, Fe3+ and Fe2+ centers via

with oxygen lone pair which results in the weakeningd that will undergo abstraction, thus leaving a stronglycoxyl at the surface [63]. (ii) A hydrogen bond is formedygen of the octahedral hydroxyl groups and hydrogen of. Subsequently, the adsorbed alcohol undergoes dehy-

giving rise to a weakly adsorbed aldehyde [64].ase of aromatics, it is strongly accepted that the rate-

step is the dissociative adsorption involving the breakkest bond [6567]. Thus, since C Cl (390 kJ mol1)aker than C H (470 kJ mol1) and CC (432 kJ mol1)9], and so more likely to undergo abstraction duringprocess, the reactivity sequence is expected to fol-der: chlorobenzene (C Cl) > benzene (C H) > tolueneed, such sequence was observed by many authors, foreraert et al. reported similar order for oxidation reac-d out with mixed oxide V2O5WO3/TiO catalyst [70].er and Amiridis stated that the activation of chloroben-5/TiO2 catalyst is easier than the activation of benzene

ed a mechanism involving a nucleophilic attack in theposition rather than in the C H position [71]. Similarwas drawn by van den Brink et al. [72] implicating thescission as a rst step in the oxidation of chloroben-t/-Al2O3. A point to note however, is that the above

M. Assebban et al. / Journal of Hazardous Materials 300 (2015) 590597 595

noted interpretations were obtained with metal and metal oxide-based catalysts, and that the reactivity trend (and Eappa likewise)obtained in our work is completely the reverse of the forego-ing; that is to say: toluene > benzene > chlorobenzene. Therefore, itappears placlay predisption of thesadsorption interruptionpared to bento the electthe aromatbenzene rinnicant incintensied rendering tby the slighparison witdifcult to beffect of chlby reducingrupture is ond chlorinassociated wnation degr[7779], thas obtaineddichloroben[71]. Convechlorobenz1,2,4,5-tetrincreasing cchlorinationchlorobenz

Althoughposed acid toward therequire deechemical cotent in thesuch as, hywhich mighover, it is wof surface sites offerinFurther clasuggested tcenters forreactivity aand introdu[8486].

4. Conclus

In this wextruded hcomplete obons such hydrocarbooxidized afollows: ketchlorinatedsion and apcurves, an sic catalyti

predisposed active sites. It is assumed that the clay active centersfor catalytic oxidations involve quite complex mixture constitutedby OH groups (acting as Brnsted acids) and coordinately-unsaturated cations such as, Al3+, Fe3+ and Fe2+ (acting as Lewis

Finles,

ound

wled

s wond F/Moncere Naco). Asch efeldhse-ld wUnivion. Td Drions

nces

Spivey. 26 (1. Cen

atile oal. A Gjala, Stejovaimk11) 12Mallcture38.aillet

pene,micalahouseryki

oble mSpiveyay 10hen, Seira, Jkel ox13) 79apaefylacet92.T. Basueiredtemp

(201.C. Carestryaporteal. Todrishna-dichl00) 26. AgarwhloroBaldi, rocar51.hen, Seira, Jium-carcia,alyst fracte

Yorkusible to suggest that the activation process involvingosed acid sites proceeds differently. Hence, the activa-e aromatic compounds is most likely initiated by thevia -bonding of the aromatic ring followed by the

of aromaticity at catalyst surface [7375]. As com-zene, toluene showed higher reactivity apparently dueron-donating effect of the methyl substituent towardic ring system. The methyl substituent destabilizes theg by increasing its electron density resulting in a sig-rease in reactivity. This destabilization is even morewhen the degree of methyl substitution increases, thushe destruction of the aromatic ring easier as indicatedtly lower Eappa value obtained for o-xylene in com-h toluene. On the other hand, chlorobenzene is moree oxidized than benzene due to electron-withdrawingorine which protects the aromatic ring from oxidation

its electronic density [76]. Furthermore, aromaticitysubstantially more difcult in the presence of a sec-e substituent as indicated by the higher Eappa valuesith increased chlorination degree. The effect of chlori-

ee has been formerly examined by several investigatorsough different conclusions were drawn. Similar trend

in the present study with chlorobenzene and 1,2-zene was also asserted by Lichtenberger and Amiridisrsely, Weber [80] disclosed that the rate of oxidation ofenes (i.e., 1,2-dichlorobenzene, 1,2,3-trichlorobenzene,achlorobenzene, hexachlorobenzene) decreases withhlorination degree. However, Furrer et al. reported that

degree has no signicant effect on the conversions ofenes [81].

the proposed interpretations based on clay predis-sites explains ttingly the observed catalytic behavior

oxidation of the studied organic molecules, it mightper investigation considering the complexity of claymposition. For example, given the important iron con-

investigated clay (11.11 wt.%), other forms of irondroxides and oxides may possibly be present [82,83],t suggest the implication of redox interactions. More-ell established that clay minerals present a variety

oxygens [25], which might act as additional activeg probably an extra route for the catalytic process.y components such as, K, Na, Ca and Mg are alsoo act as promoters providing another type of active

oxygen adsorption, which may in turn increase thend reduce activation energy of the overall processcing thus efcient contribution into reaction network

ions

ork, intrinsic catalytic performances of natural clayoneycomb monolith were tested with respect toxidation of CO and different kinds of hydrocar-as, saturated, unsaturated, oxygenated and aromaticns. All the tested compounds were completelynd the reactivity sequence was found to be asone > alcohol > ether > CO > alkyne > aromatic > alkene >

aromatic > alkane. In light of the gathered conver-parent activation energy data extracted from light-offattempt is made to explain the promising intrin-

c behavior of the clay monolith considering several

acids).molecuwere f

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to have signicant inuence on their reactivity.

gements

rk was supported by Bundes Ministerium fr Bil-orschung within the project (BMBF 3681/Project androcco/Germany). M. Assebban and A. El Kasmi expresse thanks to the research fellowships provided by CNRSTtional de Recherche Scientique et Technique, Rabat-. El Kasmi is grateful to the Deutscher Akademischer

Dienst (DAAD) for research fellowship during his stay, Germany. The authors acknowledge Prof. Dr. Katha-Hinghaus for the access to her laboratory facilities inhere part of this work was performed. The team FQM-ersity of Cadiz, Spain is acknowledged for clay monolithhe authors express their gratitude to Prof. Dr. Zhen-Yu. Naoufal Bahlawane for their helpful suggestions and.

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Intrinsic catalytic properties of extruded clay honeycomb monolith toward complete oxidation of air pollutants1 Introduction2 Experimental3 Results and discussion4 ConclusionsAcknowledgementsReferences