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Applied Catalysis A: General 476 (2014) 175–185

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom ep age: www.elsev ier .com/ locate /apcata

eria and lanthana as blocking modifiers for the external surface ofFI zeolite

oi-Gu Janga, Kwang Haa, Jong-Ho Kima, Yoshihiro Sugib,c, Gon Seoa,∗

School of Applied Chemical Engineering and Research Institute for Catalysis, Chonnam National University, Gwangju 500-757, Republic of KoreaAustralian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld 4072, AustraliaDepartment of Materials Science and Technology, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan

r t i c l e i n f o

rticle history:eceived 25 October 2013eceived in revised form 27 January 2014ccepted 19 February 2014vailable online 28 February 2014

eywords:FI zeolite

eria and lanthana impregnationTO reaction

a b s t r a c t

The ceria- and lanthana-modified MFI zeolites were prepared and extensively investigated to verify thefunction of lanthanide oxides as blocking modifiers. Their catalytic behavior in the methanol-to-olefin(MTO) conversion was explained by the characterization results regarding crystallinity, agglomeration,porosity, surface composition, and the uptakes of o-xylene and methanol. The ceria impregnated on theMFI formed nano-sized small particles with two oxidation states of +3 and +4 (Ce2O3 and CeO2) on thesurface and was located predominantly on the external surface. So, the ceria impregnation maintainedthe porosity and activity of the MFI, and induced little change in the conversion and product compositionof MTO. However, the lanthana impregnated on the MFI dispersed as a single lanthana (La2O3) phase inits micropores, as well as on the external surface. The blocking of the micropores and reduction of acidity

atalytic performance by lanthana impregnation lowered the activity of the lanthana-modified MFI in MTO. The ceria locatedon the external surface of MFI did not have any negative effects on the catalytic performance, whilethe lanthana dispersed in the micropores lowered the conversion in MTO. The locations and dispersedstates of ceria and lanthana were systematically discussed in the relation to their catalytic performanceas blocking modifiers.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Zeolites have been widely used as catalysts in refineries andhe petrochemical industry due to their specific framework, largeurface area, high acidity, and excellent thermal and mechanicaltabilities [1]. Zeolites are very efficient in acid-catalyzed cracking,lkylation, and isomerization processes in terms of high selectiv-ty to desired products [2–4]. Uniform pores of zeolites with similarizes of reactant molecules increase the feasibility of the material asatalysts with high selectivity. Furthermore, various modificationethods enhance the catalytic performance of zeolites. For exam-

le, the ion exchange of zeolite cations with various other cationsaries their acidity, maximizing their catalytic activity and selectiv-ty. Also the impregnation of phosphorous or potassium hydroxideartially neutralizes strong acid sites, significantly reducing coke

eposit, whereas the impregnation of lanthanide oxides suppresseshe reaction occurred on the external surface, resulting in betterelectivity [5,6].

∗ Corresponding author. Tel.: +82 62 530 1876; fax: +82 62 530 1899.E-mail address: gseo@chonnam.ac.kr (G. Seo).

ttp://dx.doi.org/10.1016/j.apcata.2014.02.028926-860X/© 2014 Elsevier B.V. All rights reserved.

The particular pore structure of zeolites causes a specific cat-alytic property called the shape-selectivity, which is defined by achange at the rate or the product composition of catalytic reac-tions originating from the shape and size of catalyst pores [2,6].Three kinds of shape-selective catalysis by reactants, products, andtransition states enhance the usefulness of zeolites as catalysts[7]. The high selectivity to p-xylene in the alkylation of toluenewith methanol is a typical example, which minimizes the separa-tion cost of p-xylene from xylene mixtures [8]. The combinationof a reaction with a separation step due to the specific pore sizeand shape improves the selectivity to a given desired product,and achieves higher yield than that expected from thermodynamicequilibrium.

In order to enhance the selectivity to a high-selective catalysisover a zeolite, large and highly crystalline zeolites are preferredto suppress the side reactions that occur on the external surface,because the regulation exerted by the shape and size of pores isonly effective in zeolite pores [9]. Since thermodynamically equili-

brated products are produced on the external surface, the blockingof the external surface increases the selectivity to the desired reac-tions that occur in the pores [10]. Suggested as feasible methodsinclude the selective poisoning of the acid sites with large basic

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aterials which cannot enter the pores, the silination by the reac-ion between surface acid sites and silane molecules, and thempregnation of lanthanide metal oxides on the external surface9–11]. The impregnation of ceria deactivates only the acid sitesn the external surface, while the impregnation of lanthana con-iderably reduces strong acid sites because it can intrude into theores.

Among these methods, the impregnation of ceria on morden-te has been studied to improve the selectivity to linear productsn the alkylation of various aromatic materials [12]. Ceria- andanthana-modified MFI zeolites show enhanced selectivity to p-iethylbenzene among diethylbenzene isomers without significant

oss of catalytic activity. The ceria and lanthana impregnated onhe external surface successively prevent further isomerization of-diethylbenzene. The high improvement of para-selectivity on theanthana-modified MFI zeolite compared to that modified by cerias due to the adjustment of pore entrances as well as the deacti-ation of external acid sites, suggesting a certain difference in theocation between ceria and lanthana.

Although the ceria is selectively impregnated on the externalurface, the oxides of the lanthanide metals such as lanthanum,amarium, dysprosium, and yttrium do not show such specificity inheir impregnation position [13]. The lanthanide oxides except foreria concomitantly disperse in the pores as well as on the exter-al surface, significantly lowering the acidity and narrowing theore entrances of zeolite catalysts. The extraordinary preference oferia in its impregnation on the external surface is explained by theormation of agglomerates comprising small particles rather than

lump that can extend toward pore entrances. However, the rea-on why the ceria impregnated does not form a continuous solidhase has not been investigated. Furthermore, the isopropylationn ceria-modified mordenites is carried out below 300 ◦C, so pre-ious studies have not provided any information on the state oferia at elevated temperatures [9,10,13]. The difference in the statef lanthanide oxides impregnated on MOR zeolites is clear, but itsause has not been systematically investigated.

Methanol-to-olefin (MTO) conversion zeolite catalysts produc-ng lower olefins from methanol requires high shape-selectivityy suppressing the formation of large and long hydrocarbons tochieve a high yield of lower olefins and to extend catalyst life with-ut regeneration [14]. The sinusoidal channels of MFI zeolite do notllow for the formation of longer aliphatic hydrocarbons than C12nd polyaromatic hydrocarbon in the MTO conversion, resultingn an extraordinary long catalyst life compared to other zeolites,ncluding SAPO-34 [15,16]. Furthermore, the partial neutralizationf strong acid sites of MFI zeolites by phosphorous modificationncreases the selectivity to propylene, and lengthens their catalystifetime [17,18].

We have prepared ceria- and lanthana-impregnated MFI zeo-ites to investigate their impregnated states and to discuss the effectf the impregnation on their catalytic performance in the MTO con-ersion. The impregnated amount of ceria and lanthana increasedo 40% to achieve the complete masking of the external surfacend to observe their catalytic contribution to the MTO conversion.he behavior of ceria and lanthana as blocking modifiers of MFIeolite was systematically deduced from their agglomeration, sur-ace composition, crystallinity, porosity, uptakes of o-xylene and

ethanol, acidic properties obtained using transmission electronicroscopy (TEM), scanning electron microscope (SEM), X-ray pho-

oelectron spectroscopy (XPS), X-ray diffraction (XRD), nitrogendsorption, IR spectroscopy of adsorbed pyridine and collidine, andemperature-programmed desorption (TPD) of ammonia. The vari-

tions of the activity and selectivity to lower olefins with reactionemperature over MFI zeolites modified by ceria and lanthana inhe MTO conversion clearly revealed differences in their dispersedtates.

: General 476 (2014) 175–185

2. Experimental

2.1. Catalyst preparation

A commercial MFI zeolite with Si/Al molar ratio of 25 waspurchased from Zeolyst and was used as a starting material. Themolar ratio was written in parentheses, like with MFI(25). Ceriumnitrate hexahydrate (Yakuri, 98%) and lanthanum nitrate hexahy-drate (Wako, 97%) were impregnated on MFI(25) by an incipientwetness method with varying their impregnated amounts withinthe range of 10–40 wt%. After impregnating the aqueous solutionsof lanthanide oxides on MFI(25), the zeolites were dried at 100 ◦Cfor 6 h. The lanthanide oxide-impregnated MFI(25) catalysts wereobtained by calcination of the dried zeolites at 550 ◦C for 4 h anddenoted by M(x)-MFI, with M representing the lanthanide metaland x standing for the impregnated amount of ceria or lanthanaas wt%. For comparison, Ce(40)/Al2O3 and La(40)/Al2O3 were pre-pared by impregnating the precursors of ceria and lanthana on�-Al2O3 (Sasol).

2.2. Characterization

Powder XRD patterns were recorded on a Rigaku Ultima IIIX-ray diffractometer with Cu K� radiation. In situ XRD patternswere recorded using a PANalytical X’pert Pro with Cu K� radia-tion. The samples were purged with oxygen and hydrogen duringheating from 25 to 600 ◦C. The shape and size of zeolites wereobserved using a Hitachi S-4700 SEM. N2 adsorption isothermswere recorded on a Mirae SI nanoPorosity-XG analyzer. Surfaceareas were calculated from the adsorption isotherms using theBrunauer–Emmett–Teller (BET) equation, and the pore size dis-tribution was obtained using the Barrett–Joyner–Halenda (BJH)method. TPD of ammonia was carried out using a laboratory-made apparatus following a procedure described in the literature[19]. The samples were previously saturated with ammonia (AirKorea, 1000 ppmv/He balance) and purged with helium (Sinil,99.999%) at 150 ◦C before being heated to 800 ◦C at 10 ◦C min−1.The desorbed ammonia was monitored by a Balzers QMS200 massspectrometer.

The dispersion of ceria and lanthana was observed by TEM (JEM2000 FXII, JEOL). Catalysts dispersed in acetone were sonicated andsampled using a carbon grid. The surface compositions of ceriaand lanthana on the prepared catalysts were investigated by XPS(VG MultiLab 2000) using a monochromated Mg K� X-ray source(300 W). The binding energies of surface elements were calibratedusing the C1s (285 eV) peak as a reference.

Uptakes of o-xylene (Yakuri, 99%) and methanol (Aldrich, 99.8%)on the M(x)-MFI catalysts were measured using a gravimetricadsorption system equipped with a quartz spring [19]. The sam-ples were evacuated at 300 ◦C for 1 h prior to exposure to theseadsorbates. The mass gain due to the uptake of o-xylene was mea-sured under 7 Torr of o-xylene vapor at 90 ◦C for 90 min, and thatof methanol was measured under 37 Torr of methanol vapor at30 ◦C for 60 min. IR spectra of pyridine (Aldrich, 99.8%) and colli-dine (Aldrich, 99%) adsorbed on the zeolite catalysts were recordedon a BIO-RAD 175C FT-IR spectrophotometer equipped with aGraseby Specac in situ cell. A self-supported catalyst wafer (about10 mg) was activated in a nitrogen (Sinil, 99.9%) of 50 ml min−1 at500 ◦C for 1 h. 1 �l pyridine or collidine was injected at 50 ◦C, andmaintained for 30 min. Differential IR spectra of adsorbed pyridine

were recorded with increasing temperature to 400 ◦C across therange 4000–700 cm−1 with a resolution of 4 cm−1. The spectra ofadsorbed collidine were obtained after purging at 50 ◦C because ofits easy desorption.

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.3. The MTO conversion

The MTO conversion was carried out at atmospheric pressuren a continuous-flow micro reactor, as described elsewhere [19]. Inypical runs, 0.1 g of catalyst charged in the center of a quartz tubeO.D. 1/2′′) was activated at 550 ◦C for 1 h and used at 350 ◦C and.5, 5, and 15 h−1 WHSV. The products were analyzed by an on-lineonam DS 6200 gas chromatograph equipped with a CP-Volamineapillary column (60 m × 0.32 mm) and a flame ionization detector.onversion was defined as the percentage of methanol consumeduring the MTO conversion, and dimethyl ether was not considereds a product. The yield of each product was calculated to representts percentage in products on carbon basis.

ethanol conversion (%) = [Amount of methanol reacted

Amount of methanol fed] × 100

he yield of a given product (%)

=[

Amount of methanol consumed for the production of the product

Amount of methanol fed

]× 100

GC/MS analysis was performed by a modified procedure in theiterature to determine the organics occluded in the used cata-ysts [20]. 0.1 g of the used catalyst was completely dissolved in0 ml of 20% hydrofluoride solution for 30 min and neutralized withotassium carbonate (Daejung, 99.5%). Dichloromethane (Aldrich,9.9%) and hexachloroethane (Aldrich, 99%) were employed asxtracting materials for the organic species and as the internaltandard. The GC/MS total ion chromatograms of extracted organichases were obtained by a Shimadzu GC-2010 gas chromatographquipped with a Shimadzu GCMS-QP2010 mass selective detectorelectron impact ionization: 70 eV). The column used was a DB-5MS60 m × 0.25 mm) with flowing He (1.00 ml min−1).

. Results

.1. Physico-chemical states of ceria and lanthana impregnatedn MFI zeolite

Since the amounts of ceria and lanthana on MFI(25) are highp to 40 wt%, the impregnation of these lanthanide oxides onFI zeolite decreases the content of zeolite and thereby causes

he weakening of its characteristic diffraction peaks. Fig. 1 showsRD patterns of M(x)-MFI. The characteristic diffraction peaksttributed to MFI zeolite significantly decreased with the impreg-ation, regardless of their species. The extents of the decreases iniffraction peaks by the impregnation of ceria and lanthana wereigher than those expected from the dilution of a zeolite frameworky them. The strong affinity between the lanthanide oxides andeolite framework considerably weakened the diffraction peaks.

Ce(40)-MFI exhibited high diffraction peaks of ceria at 2� = 32.7◦

nd 47.4◦, indicating the presence of ceria as a crystalline phase22]. On the other hand, La(40)-MFI did not show any diffractioneaks attributed to lanthana, even though the amount of lanthana

mpregnated on MFI was 40 wt%, like Ce(40)-MFI. The absence ofharacteristic peaks suggested that lanthana was dispersed on MFIeolite as an amorphous phase, thin film, or very small particlesuch that it did not induce diffraction peaks. Although the amountsf the lanthanide metal oxides impregnated on Ce(40)-MFI anda(40)-MFI were the same, the shape and crystallinity of ceria andanthana deduced from the XRD patterns were definitely differ-nt: ceria made crystalline particles inducing diffraction peaks,

hile lanthana did not form any crystallites that were detectable

y powder XRD. The small diffraction peaks of ceria observed one(40)-MFI were insufficient to estimate its particle size using thecherrer equation.

: General 476 (2014) 175–185 177

Fig. 2 shows SEM photos and mapping images of Ce(40)-MFIand La(40)-MFI catalysts. The similar mapping images indicatedthat ceria and lanthana were dispersed throughout zeolite particles,and did not form large particles, even on Ce(40)-MFI. The differ-ence observed in the XRD patterns of Ce(40)-MFI and La(40)-MFIwas probably caused by the different states of ceria and lanthanaimpregnated on MFI zeolite. Fig. 3 shows TEM images of Ce(20)-, Ce(40)-, and La(40)-MFI. Many small particles of ceria less than5 nm were dispersed on the surface of Ce(20)-MFI, and the highcontent of ceria impregnated caused the agglomeration on Ce(40)-MFI. In contrast, no agglomerates of lanthana were observed, evenon La(40)-MFI. Ceria impregnated on MFI(25) formed many nano-sized small particles and produced agglomerates when the amountof ceria was high, but the external surface of La(40)-MFI wassmooth, regardless of the same amount of lanthana impregnated.Ceria and lanthana exhibited a definite different behavior in termsof particle formation.

The adsorption isotherms of nitrogen on the M(x)-MFI catalystsare shown in Fig. S1. MFI(25) showed a Langmuir type isothermwithout any hysteresis, but the increase in the amount of ceriaimpregnated on Ce(x)-MFI caused the appearance of hysteresisat high P/P0, implying the formation of mesopores. In contrast,the impregnation of lanthana did not induce hysteresis even onLa(40)-MFI. The appearance of hysteresis on Ce(40)-MFI confirmedthe agglomeration of ceria particles with the formation of meso-pores, as shown in the TEM images, while the lack of hysteresison La(40)-MFI indicated no formation of lanthana particles to beagglomerated.

Fig. 4 shows the pore size distribution of Ce(x)-MFI andLa(x)-MFI catalysts calculated for the adsorption isotherms. Theimpregnation of ceria on MFI(25) produced mesopores with thediameter of about 10 nm, and the amount of mesopores gradu-ally increased with the amount of ceria impregnated. Althoughthe formation of mesopores confirmed the agglomeration of nano-sized small ceria particles on Ce(x)-MFI, the pore size distributionsof La(x)-MFI did not confirm the formation of mesopores bythe impregnation of lanthana. A small peak at 3 nm on La(30)-MFI might be an exception. The lanthana impregnated on MFI(25) did not make particles with sufficient size to form mesoporesamong them.

The TPD profiles of ammonia from the Ce(x)-MFI and La(x)-MFIcatalysts are shown in Fig. S2, and the deconvoluted amounts of acidsites by their strength are listed in Table S1. Most profiles could benicely deconvoluted with three desorption peaks: the desorptionpeak with the temperature at maximum (Tm) less than 270 ◦C cor-responded to the ammonia adsorbed on weak acid sites, and thatwith Tm at 360–400 ◦C was attributed to the ammonia adsorbedon strong acid sites [17]. Since the third peak with Tm around500–550 ◦C was due to the desorption of ammonia at higher tem-peratures, it might be reasonable to include them into strong acidsites. The third peaks were very weak, and their contribution toacidity was not significant, although high accordance between theexperimental and simulated profiles was required.

La(40)-MFI showed another desorption peak with Tm above640–680 ◦C, while Ce(40)-MFI did not show any desorption peaksin this temperature range. In order to verify the source of ammo-nia desorbed at higher temperatures, TPD profiles of ammoniafrom Al2O3, Ce(40)/Al2O3, and La(40)/Al2O3 were investigated (Fig.S3). The lanthana catalyst supported on alumina La(40)/Al2O3 alsoshowed the desorption peak with Tm above 600 ◦C. Since the des-orption peak appeared only on the catalysts with impregnatedlanthana regardless of support, it was obvious that the peak might

have originated from the strong interaction between lanthana andammonia, and not the desorption of ammonia from the strong acidsites. A study on the formation of new compounds between lan-thana and ammonia is under way.

178 H.-G. Jang et al. / Applied Catalysis A: General 476 (2014) 175–185

10 20 30 40 50

Ce(10)-M FI

Ce(20)- MFI

2 th eta

Inte

nsity

MFI( 25)

Ce(3 0)-M FI

Ce(40)-MFI

1000 cps

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La(1 0)-M FI

La(20)-MFI

La(3 0)-M FI

2 theta

MFI(25)

La(40)-MFI100 0 cp s

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cttltitd

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The desorption peaks of ammonia relative to strong acid sitesommonly decreased with increasing amounts of ceria and lan-hana impregnated on MFI(25). The masking of strong acid sites onhe external surface and the lowering of the relative content of zeo-ite per mass of catalysts by ceria and lanthana caused a decrease of

he strong acid sites. In contrast, the desorption peak correspond-ng to weak acid sites increased with their impregnation, indicatinghat the weak acid sites were formed on the ceria and lanthana, andid not originate from the zeolite framework.

Fig. 2. SEM mapping images of (A) Ce(40

FI and (B) La(x)-MFI catalysts.

The impregnation of ceria and lanthana on MFI zeolite also influ-ences the species of acid sites remaining. Fig. 5 shows IR spectra ofthe MFI(25), Ce(40)-MFI, and La(40)-MFI catalysts recorded afteractivation at 500 ◦C (Fig. 5A), after pyridine adsorption at 50 ◦C fol-lowed by purging at 400 ◦C (Fig. 5B), and after collidine adsorption

at 50 ◦C followed by purging (Fig. 5C). MFI(25) activated at 500 ◦Cshowed two clear IR absorption bands at 3736 and 3590 cm−1.The former was attributed to isolated hydroxyl groups dispersedon the external surface, and the latter was attributed to Brønsted

)-MFI and (B) La(40)-MFI catalysts.

H.-G. Jang et al. / Applied Catalysis A: General 476 (2014) 175–185 179

Fig. 3. TEM images of (A) Ce(20)-MFI, (B) Ce(40)-MFI, and (B) La(40)-MFI catalysts.

1 10 10 0Pore diame ter ( nm)

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Ce(30)-MFI

Ce(20 )-MFI

Ce(10)-MFI

MFI(25 )

1 10 10 0Pore diame ter (nm )

0.05La(40)-MF I

La(30)-MFI

La(20)-MFI

La(10)-MF I

MFI(25)

Fig. 4. Pore size distributions of (A) Ce(x)-MFI and (B) La(x)-MFI catalysts.

4000 380 0 3600 3400 32 00 300 0

La(40)-MFI

Ce(40)-MFI

MFI(25)

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Wave number (cm-1)

Abs

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Temp.: 50 oC

La(40)-MFI

Ce(40)-M FI

MFI(25)1574

1618

1638

Fig. 5. IR spectra of MFI, Ce(40)-MFI, and La(40)-MFI catalysts: (A) activated at 500 ◦C, (B) exposed to pyridine followed by purging with increasing temperature up to 400 ◦C,and (C) exposed to collidine followed by purging at 50 ◦C.

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80 H.-G. Jang et al. / Applied Catal

cidic hydroxyl groups [15]. The impregnation of ceria inducedhe appearance of the band at 3726 cm−1 accompanying the dis-ppearance of the band at 3736 cm−1, but the band at 3590 cm−1

as still retained on Ce(40)-MFI. The disappearance of the bandt 3736 cm−1 indicated the whole masking of the external surface,nd the observation of the new 3726 cm−1 band was relative tohe isolated hydroxyl groups on ceria, not the hydroxyl groups of

FI zeolite [15]. The retention of the band at 3590 cm−1 on Ce(40)-FI indicated the preservation of Brønsted acid sites, even after the

eria impregnation. In contrast, La(40)-MFI did not show any bandsttributed to isolated hydroxyl groups, indicating their disappear-nce by the impregnation of lanthana. In addition, the small bandst 3590 cm−1 on La(40)-MFI reflected the partial diminishment ofhe Brønsted acid sites in pores.

The IR spectra of Ce(x)- and La(x)-MFI after activation at 500 ◦Cre shown in Fig. S4. The maintenance of the band at 3590 cm−1 one(x)-MFI was clear, regardless of the amount of ceria impregnated,

mplying the preservation of Brønsted acid sites. On the other hand,he band at 3590 cm−1 gradually reduced with increasing amountf lanthana impregnated.

The absorption of pyridine on Brønsted and Lewis acid sitesrovides valuable information to the effect of ceria and lan-hana impregnation on the species of the acid sites dispersed in

FI zeolites. Fig. 5(B) shows the IR spectra of pyridine adsorbedn the strong acid sites of MFI(25), Ce(40)-MFI, and La(40)-FI catalysts, since the zeolites were purged with nitrogen at

00 ◦C. All catalysts exhibited absorption bands at 1537, 1480, and450 cm−1, but their intensity considerably varied with the impreg-ation of ceria and lanthana. The absorption band at 1537 cm−1

s attributed to pyridinium ions formed on Brønsted acid sites,nd that at 1450 cm−1 is attributed to pyridine coordinated toewis acid sites [17]. The strong bands observed at 1537 cm−1

n MFI(25) and Ce(40)-MFI after purging at 400 ◦C indicated thereservation of strong Brønsted acid sites on Ce(40)-MFI. How-ver, the band at 1537 cm−1 was weak on La(40)-MFI, revealinghe reduction of strong acid sites by the impregnation of lan-hana. The weak bands at 1450 cm−1 on all catalysts representedhe small number of strong Lewis acid sites after activation at00 ◦C.

In addition, Fig. S5 shows the IR spectra of pyridine adsorbedn Ce(x)-MFI and La(x)-MFI at 50 ◦C. The steady observation of theand at 1547 cm−1 on Ce(x)-MFI revealed the retention of Brønstedcid sites even after the high loading of ceria. On the contrary,he gradual reduction of the band at 1547 cm−1 on La(x)-MFI withncreasing the amount of lanthana impregnated revealed a partial

asking of Brønsted acid sites. The appearance of the bands at 1600nd 1445 cm−1 attributed to pyridine adsorbed physically on Ce(x)-FI and La(x)-MFI indicated the formation of weak acid sites on the

eria and lanthana impregnated on MFI [23,24].Since the molecular size of collidine is larger than the pore

ntrance of MFI [21], the IR spectra of collidine adsorbed on MFIeolites reflect the acid sites located on their external surface.ig. 5(C) shows IR spectra of the collidine that remained on MFI(25),e(40)-MFI, and La(40)-MFI after purging at 50 ◦C. The bands at638 and 1574 cm−1 are attributed to Brønsted and Lewis acid sitesn the external surface of MFI(25), respectively [21]. The 1618 cm−1

and corresponded to the physically adsorbed and chemisorbedollidine. The small bands at 1638 and 1574 cm−1 on MFI(25)ndicated small amounts of Brønsted and Lewis acid sites on thexternal surface. The slight reduction of the band at 1638 cm−1

eflected the masking of Brønsted acid sites by ceria impregnation.owever, the high band at 1574 cm−1 on Ce(40)-MFI illustrated the

resence of a large amount of Lewis acid sites on impregnated ceria.a(40)-MFI exhibited very weak bands attributed to Brønsted andewis acid sites, indicating the negligible amounts of acid sites onts external surface.

: General 476 (2014) 175–185

Since o-xylene has a similar molecular size to the pore entranceof MFI zeolite, the changes in the entrance and volume of zeolitepores by the impregnation of ceria and lanthana may be reflectedin the uptake curve of o-xylene in terms of rate and amount.Fig. 6 shows the uptake curves of o-xylene on M(x)-MFI at 90 ◦C.The uptake rates of o-xylene on Ce(x)-MFI were almost the same,regardless of the amount of ceria impregnated, while the uptakeamount of o-xylene per gram of catalyst gradually decreased withincreasing the amount of ceria impregnated. The uptake amountof o-xylene on Ce(x)-MFI per unit gram of MFI zeolite calculatedby compensating for the increase of mass by the impregnationof ceria exceeded that on MFI(25), indicating that the pores ofMFI(25) still remained after the impregnation of ceria, and a partof o-xylene should be adsorbed on the surface of ceria as well asin the pores of MFI zeolite. In other words, the impregnation ofceria did not block the pores of MFI zeolite, and the ceria impreg-nated on the external surface produced mesopores that could beoccupied by o-xylene. In contrast, the impregnation of lanthanasignificantly reduced both the uptake rate and amount of o-xylene.The uptake amount of o-xylene on La(40)-MFI per gram of MFI(25)calculated by the compensation of lanthana mass was less than ahalf of that on MFI(25) because a considerable amount of pores wasblocked.

Table 1 lists the summary of physical properties obtained inthe characterization of M(x)-MFI. The uncompensated BET surfaceareas of Ce(x)-MFI decreased with the impregnation of ceria, whilethe compensated ones increased with the impregnation. The for-mation of other pores in addition to the micropores of MFI(25)resulted in an increase of the compensated BET surface area, imply-ing that the ceria impregnated did not block the micropores andwas located on the external surface. The impregnation of lanthanacaused considerably different behavior. Even the compensated SBETalso decreased with the impregnation of lanthana, indicating theimpregnated lanthana partially blocked the pores of MFI(25).

Fig. 7 shows the uptake curves of methanol on the M(40)-MFI catalysts. Since a methanol molecule is very small and highlyhydrophilic, methanol accomplished the adsorption equilibriumin a short time of less than 5 min, while the uptake amount ofmethanol varied with the species of impregnated lanthanide metaloxides. The uptake amount of methanol on Ce(40)-MFI was lessthan that on MFI(25), but the compensated uptake amount ofmethanol on the former exceeded that on MFI(25). This meantthat the ceria impregnated did not block the pores of MFI(25) andformed new pores for methanol adsorption. The uptake amount ofmethanol on La(40)-MFI was less than that on MFI(25), but the sim-ilar values of the compensated amount of methanol on La(40)-MFIand MFI(25) illustrated that the pores of La(40)-MFI were suffi-ciently large for the adsorption of small molecules, like methanol.

The agglomeration of ceria particles can produce mesopores andincrease the uptake amounts of o-xylene and methanol. The widespreading of lanthana without forming agglomerates on MFI(25)reduced the uptake amount of o-xylene by narrowing the poreentrances or blocking mass transfer in pores, but small methanolmolecules without suffering such restrictions could maintain theuptake amount.

Fig. 8 shows the XPS spectra of Ce(40)-MFI and La(40)-MFI. Ce(40)-MFI exhibited two very complicated XPS peaks at910–880 eV, while La(40)-MFI showed two doublets of XPS peaks at860–830 eV. The complicated Ce 3d peaks on Ce(40)-MFI could bedeconvoluted to Ce3+ and Ce4+, but the La 3d peaks on La(40)-MFIwere in good accord with those of La3+ species [22,25]. There-fore, the ceria impregnated on MFI was a mixture of Ce2O3 and

CeO2, and these ceria particles were segregated and did not form asingle phase. In contrast, the lanthana impregnated on MFI wasa single phase of La2O3. Although both the catalysts were pre-pared with the same method, the ceria impregnated on MFI(25) was

H.-G. Jang et al. / Applied Catalysis A: General 476 (2014) 175–185 181

0 30 60 900

10

20

30

40

50

60

Time (min)

MFI(25) La(1 0)-M FI La(20)-MFI La(30)-MFI La(4 0)-M FI

0 30 60 900

10

20

30

40

50

60

Upt

ake

ofo-

xyle

ne (m

g g-1

)

Time (min)

MFI( 25) Ce(1 0)-M FI Ce(20)-MFI Ce(30)-MFI Ce(4 0)-M FI

Fig. 6. Uptake curves of o-xylene at 90 ◦C over (A) Ce(x)-MFI, (B) La(x)-MFI catalysts. The pressure of o-xylene was adjusted to 7 Torr throughout the uptake measurement.

Table 1Characterization data for M(x)-MFI catalysts.

Catalysts SBET (m2 g−1) Pore volume (cm3g−1) Adsorption amount (mgg−1)

Aa Bb Vmicro Vmeso o-Xylene Methanol

MFI(25) 365 365 0.15 0.29 55 106Ce(10)-MFI 330 367 0.14 0.30 53 –Ce(20)-MFI 295 368 0.12 0.20 48 –Ce(30)-MFI 278 397 0.11 0.22 46 –Ce(40)-MFI 243 405 0.10 0.31 42 81La(10)-MFI 225 250 0.10 0.17 31 –La(20)-MFI 200 250 0.08 0.21 16 –La(30)-MFI 126 180 0.05 0.17 14 –

03

ct

3c

eiosldoshanaMiatiCt

In contrast, the impregnation of lanthana on MFI(25) graduallyreduces the conversion, as shown in Fig. 10. The conversion onLa(40)-MFI at a TOS of 150 min became less than 10%. The prod-uct composition on La(x)-MFI also largely varied with the amount

0 10 20 30 40 50 600

20

40

60

80

100

120

Upt

ake

of m

etha

nol (

mg

g-1)

MFI(25) Ce(40)-MFI La(40)-MFI

La(40)-MFI 70 175 0.

a Without compensation by the increase of mass by the impregnation.b With compensation by the increase of mass by the impregnation.

omposed of ceria particles with different oxidation states, whilehe impregnated lanthana was presented in a single phase.

.2. MTO conversion over ceria- and lanthana-impregnated MFIatalysts

The impregnation of lanthanide oxides on zeolites causes sev-ral changes in their catalytic behavior. The lanthanide metal oxidempregnated on the external surface masks the acid sites locatedn it, whereas that dispersed near the pore entrances reduces poreize, and that impregnated in micropores inactivates the acid sitesocated there. Nevertheless, the impregnation of ceria on MFI(25)id not induce any significant changes in the catalytic performancef MFI zeolite in terms of conversion and product composition, ashown in Fig. 9. The conversion of methanol was maintained veryigh above 95% over all Ce(x)-MFI at 350 ◦C, even though the actualmount of MFI(25) in Ce(40)-MFI was reduced to 60%. The impreg-ation of ceria did not cause any appreciable deactivation in thectivity of MFI(25). The product compositions of MTO over all Ce(x)-FI were very similar. The only appreciable changes due to the

mpregnation of ceria were a slight decrease of aromatic yield and slight increase of propene yield. These small changes might be due

o the loss of strong acid sites on the external surface by the ceriampregnation. The strong acid sites located in the micropores ofe(40)-MFI zeolite still worked as active sites of MTO, maintaininghe high conversion and the same product composition.

0.14 11 59

Time (min)

Fig. 7. Uptake curves of methanol at 30 ◦C over MFI, Ce(40)-MFI, and La(40)-MFIcatalysts. The pressure of methanol was adjusted to 37 Torr throughout the uptakemeasurement.

182 H.-G. Jang et al. / Applied Catalysis A: General 476 (2014) 175–185

920 910 900 890 880

Ce4+

Ce3+

Inte

nsity

(a.u

.)

Binding energy (eV)

5000

870 860 850 840 830

La3+

50000

Inte

nsity

(a.u

.)

Bin ding energy (eV)

Fig. 8. XPS spectra of (A) Ce(40)-MFI and (B) La(40)-MFI catalysts.

0 50 10 0 150 200 25 00

20

40

60

80

100

Con

vers

ion

(%)

Time on stream (min)

MFI( 25) Ce( 10)- MFI Ce( 20)- MFI Ce( 30)- MFI Ce( 40)- MFI

0 20 40 60 80 100

Time on stream: 70 min MeOH+DME Other s Aroma tics C=

5~C=6

C=4

C=3

C=2

Ce(30)-MFI

Ce(20)- MFI

Ce(1 0)-M FI

Ce( 40)-MFI

MFI(25)

Compo sition (%)

Fig. 9. MTO conversions over Ce(x)-MFI: (A) the conversion of methanol with respect to time on stream and (B) product composition at 70 min reaction time at 350 ◦C and5.0 h−1 WHSV.

0 50 10 0 150 20 0 2500

20

40

60

80

100

Con

vers

ion

(%)

Time on stream (min )

MFI (25 ) La(10 )-MF I La(20 )-MF I La(30 )-MF I La(40 )-MF I

0 20 40 60 80 10 0

Time on strea m: 70 min MeOH+D ME Others Aroma tics C=

5~C=6

C=4

C=3

C=2

La(30)- MFI

La(20)-MFI

La(10)- MFI

La(40)-MFI

MFI(25)

Composition (%)

Fig. 10. MTO conversions over La(x)-MFI: (A) the conversion of methanol with respect to time on stream and (B) product composition at 70 min reaction time at 350 ◦C and5.0 h−1 WHSV.

H.-G. Jang et al. / Applied Catalysis A: General 476 (2014) 175–185 183

0 20 40 60 80 100

~90%

~80%

Conve rsion : ~55%La(30)-MFI_5 h-1

Ce(30 )-MFI_15 h-1

La(20)- MFI_1.5 h-1

MeO H+DM E Oth ers Aromatics C=

5~C=6

C=4

C=3

C=2

MFI(25)_15 h-1

Composition (%)

La(40)- MFI_1.5 h-1

Ce(30)-MFI_15 h-1

MFI(25)_15 h-1

0 100 200 15 00 2000 2500 30000

20

40

60

80

100

La(20)-MFI_1.5 h-1

La(40)- MFI_1.5 h-1

La(30)- MFI_5 h-1

MFI(25)_15 h-1

Ce(30)-MF I_15 h-1

Con

vers

ion

(%)

Time on str eam (min)

F MFI, C

oanaSconoawe

tmrobv

dTtMcctylalctonst

cpcaho

ig. 11. Comparison of (A) methanol conversions and (B) product compositions over

f lanthana impregnated. The yields of higher olefins (C=5 − C=

6 ) andromatics decreased by increasing the amount of lanthana impreg-ated. The yields of lower olefins (C=

2 and C=3 ) increased on La(10)-

nd La(20)-MFI, but they also decreased largely on La(40)-MFI.ince the dimerization of lower olefins followed by the dehydrocy-lization over strong acid sites produced aromatics, the reductionf aromatics with the impregnation of lanthana suggested a sig-ificant loss of strong acid sites. A part of lanthana impregnatedn MFI(25) moved into its micropores and masked some strongcid sites. In addition, the significant decrease of aromatic yieldith higher amounts of lanthana impregnation implied that the

ntrances of the pores were also narrowed by lanthana.Fig. S6 shows the conversion profiles and product composi-

ions over La(x)-MFI at WHSV = 1.5 h−1. The low concentration ofethanol in feed retarded the deactivation of La(x)-MFI, but the

eduction of conversion with time on stream was inevitable evenver La(10)-MFI. The increases in the yields of ethane and propeney the impregnation of lanthana accompanied the decrease of con-ersion due to the loss of strong acid sites.

The product compositions over MFI, Ce(x)-MFI, and La(x)-MFI atifferent conversions were compared in Fig. 11 by varying WHSV.he MFI(25) and Ce(x)-MFI exhibited similar product composi-ions at higher conversions such as ∼80% and ∼90%, because the

TO conversion mainly occurs in the pores of MFI zeolites. Theeria on Ce(30)-MFI dispersed only on the external surface did notause a considerable change in the product composition. However,he impregnation of lanthana caused considerable increases in theields of ethane and propene at high conversion of ∼90%. A part ofanthana on La(20)-MFI induced into pores and masked acid sites,nd thereby the impregnation of lanthana increased the yield ofower olefins by suppressing their further reactions. The productompositions over Ce(30)-MFI and La(30)-MFI were very similar athe conversion of ∼55%, because the low concentration of lowerlefins lowered the rates of their further reactions. The impreg-ation of ceria and lanthana on MFI(25) reduced the number oftrong acid sites in pores and resulted in appreciable differences inhe product composition.

The blocking of micropores on La(x)-MFI by lanthana can beonfirmed by the analysis of occluded organic species in micro-ores of the catalysts used. Fig. S7 shows the GC–MS total ion

hromatograms of the extracts from the used MFI(25), Ce(20)-MFI,nd La(20)-MFI. Polymethylbenzenes including tetra-, penta-, andexamethylbenzenes were formed and occluded in the microporesf the used MFI(25) catalyst. The amount of polymethylbenzenes

e(x)-MFI, and La(x)-MFI at different conversion levels by adjusting WHSV at 350 ◦C.

decreased on used Ce(20)-MFI, while no organic materials wereobserved from La(20)-MFI. Although the detection sensitivity forthe polymethylbenzenes formed in the micropores of MFI zeolitesin the MTO conversion was not high, the absence of occluded poly-methylbenzenes reflected a significant reduction of the catalyticactivity of MFI zeolite by the impregnation of lanthana.

Fig. 12 compares the conversion and product compositionover cerium- and lanthanum-containing catalysts impregnated onMFI(25) and alumina. The catalysts prepared by the impregnationof ceria and lanthana on Al2O3 and Ce(40)/Al2O3 exhibited negligi-ble conversion. The low conversion less than 20% over La(40)/Al2O3was due to the formation of methane, not lower olefins. Theseresults indicated low catalytic activities of ceria and lanthana them-selves. However, Ce(40)-MFI and La(40)-MFI prepared by the sameimpregnation method exhibited the definite differences in theircatalytic performance in MTO indicated that the ceria and lanthanadid not have comparable activity to MFI. The desorption peaks ofammonia around 250 ◦C from ceria and lanthana impregnated onalumina and MFI(25) confirmed the formation of weak acid siteson ceria and lanthana, but the negligible formation of lower olefinsover Al2O3, Ce(40)/Al2O3, and La(40)/Al2O3 indicated that weakacid sites did not contribute to the MTO conversion (Fig. 12). Thestrong acid sites that could produce polymethylbenzenium radicalcations were responsible for the formation of lower olefins.

Nevertheless, the XRD patterns of Ce(40)- and La(40)-MFIrecorded with increasing temperature from 25 to 600 ◦C, anddecreasing from 600 to 25 ◦C did not show any significant changesin their diffraction peaks in terms of position and size, as shownin Fig. S8. The XRD peak of the ceria impregnated on Ce(40)-MFIdid not alter appreciably with the increasing temperature. It mightbe possible not to observe small changes in the surface of ceriacrystallites, because the amount of ceria impregnated was large at40%.

4. Discussion

The XRD patterns of Ce(x)- and La(x)-MFI, their TEM images, theuptake curves of o-xylene and methanol on them, the amounts ofstrong acid sites on them measured by NH3-TPD profiles, and theircatalytic performance in MTO coincidently suggested a definite

difference in the location and dispersed state of ceria and lan-thana when they were impregnated on MFI(25). The maintenanceof the characteristic peaks attributed to MFI zeolite on Ce(x)-MFIindicated negligible feasibility for the intrusion of ceria into the

184 H.-G. Jang et al. / Applied Catalysis A: General 476 (2014) 175–185

0 50 100 150 200 2500

20

40

60

80

100

Al2O3

Ce(40)/Al2O3

La(40)/Al2O3

MFI(25) Ce(40)-MFI La(40)-MFIC

onve

rsio

n (%

)

Time on stre am (min )

0 20 40 60 80 100

Time on stream: 70 min

MFI(25)

Al2O3

Ce(40 )-MFI

La(40)-MFI

La(40)/Al2O3

Ce(40)/Al2O3

MeOH+DME Others Aromatics C=

5~C=6

C=4

C=3

C=2

Composition (%)

F -MFI,

s

mpiioamtsMptlrvrlsdo

niCostb

lXonoMswLoe

t

ig. 12. MTO conversions over Al2O3, Ce(40)/Al2O3, La(40)/Al2O3, MFI(25), Ce(40)tream and (B) product composition at 350 ◦C and 5.0 h−1 WHSV.

icropores of MFI zeolite, while the considerable reduction of theeaks on La(x)-MFI indicated a partial filling of micropores by the

mpregnated lanthana. The TEM images of Ce(20)- and Ce(40)-MFIndicated the presence of ceria particles on the external surfacef MFI(25), while there were no agglomerates on La(40)-MFI. Thedsorption isotherms of nitrogen on Ce(x)-MFI confirmed the for-ation of mesopores among the nano-sized small ceria particles by

he appearance of hysteresis at high P/P0. The agglomeration of themall particles made mesopores, but the smooth surface of La(40)-FI indicated the absence of particles on its external surface. The

resence of ceria on the external surface of Ce(x)-MFI did not reducehe uptake rate and amount of o-xylene, while the impregnation ofanthana on MFI(25) resulted in the retardation of the rate and theeduction of the uptake amount of o-xylene. The actual microporeolume of MFI(25) measured by the uptake of methanol was noteduced by the impregnation of ceria and lanthana. Therefore, theanthana dispersed in the micropores as well as on the externalurface of MFI zeolite worked as obstacles in the micropores to theiffusion of larger molecules, reducing the uptake rate and amountf o-xylene.

The negligible changes in the acidity of MFI(25) with the impreg-ation of ceria also supported the predominant location of ceria on

ts external surface. The retention of strong Brønsted acid sites one(40)-MFI clearly showed that the most of ceria should be locatedn the external surface of MFI(25). The gradual reduction of thetrong acid sites of La(x)-MFI with an increasing amount of lan-hana impregnated exhibited a partial blocking of strong acid sitesy the impregnated lanthana.

The differences in the location and dispersed state of ceria andanthana on MFI(25) were induced from their oxidation states. ThePS spectra of Ce(40)-MFI confirmed the presence of ceria with twoxidation states of +3 and +4 on its outer surface, so the ceria couldot make a single homogeneous phase. The agglomerated particlesf ceria were observed on the TEM photos of Ce(20)- and Ce(40)-FI. However, the lanthana impregnated on MFI(25) comprised a

ingle phase with a single oxidation state of +3, and no particlesere observed on La(40)-MFI. The easy expansion of lanthana on

a(x)-MFI on the external surface might be due to its homogeneous

xidation state. The partial intrusion of lanthana into pores wasxpected.

The different location and dispersed state of ceria and lan-hana on MFI(25) also cause the different catalytic performance of

and La(40)-MFI catalysts: (A) the conversion of methanol as a function of time on

Ce(x)- and La(x)-MFI in the MTO conversion. The predominant dis-persion of ceria on the external surface of MFI(25) resulted inlittle change in the conversion and product composition of Ce(x)-MFI. However, a partial blocking of strong acid sites located in themicropores by lanthana gradually lowered the conversion and theformation of aromatics. The pore narrowing by the impregnation oflanthana was also inevitable. The conversion and product compo-sition of MTO on Ce(x)- and La(x)-MFI at 350 ◦C could be explainedby the difference in the location of the dispersed state of ceria andlanthana.

5. Conclusions

Ceria and lanthana impregnated on MFI zeolite work as block-ing modifiers for the external surface, but their detailed functionswere considerably different. Ceria was predominantly dispersedon the external surface with forming agglomerates and did notinduce the reduction of both the entrance and of pores and theamount of strong acid sites of MFI zeolite. However, lanthana wasdispersed in the micropores as well as on the external surface, sothe decrease of strong acid sites was inevitable. These differencesbetween ceria and lanthana in their location and state of dispersioninduced different catalytic performance in the MTO conversion. Nodefinite deactivation of MFI zeolite by the impregnation of ceria wasobserved in MTO at 350 ◦C, but the conversion of MTO graduallydecreased when increasing the amount of impregnated lanthana.The ceria with two oxidation states of Ce3+ and Ce4+ did not makea single homogenous phase, but segregated into many particles.Therefore, the ceria particles were mainly located on the externalsurface of MFI zeolite, blocking it’s the external surface withoutintruding into the micropores. On the other hand, the formation ofa single lanthana phase with a +3 oxidation state made it possiblefor some lanthana to intrude into the micropores of MFI zeolites,and for the definite reduction of its activity by the impregnation oflanthana.

Acknowledgments

This research was supported by the Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF),funded by the Ministry of Education (NRF-2009-0094055). SEMand TEM photos of the Caria and lanthana impregnated MFI

ysis A

cGNaa

A

f2

R

[

[[

[

[

[

[

[[

[[[

[

[

H.-G. Jang et al. / Applied Catal

atalysts were obtained from the Korea Basic Science Institute,wangju Branch. Heesung Catalyst Corporation (especially Mr. S.C.a) helped with obtaining in situ X-ray diffraction patterns of Ce-nd La-MFI with increasing temperature in oxidation and reductiontmospheres, and we are deeply grateful for their help.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the on-line version, at http://dx.doi.org/10.1016/j.apcata.014.02.028.

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