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Hierarchical materials originated frommesoporous MCF material and Betazeolite nanoparticles – synthesis andcatalytic activity in N2O decompositionMałgorzta Rutkowska a , Lucjan Chmielarz a , Daniel Macina a ,

Barbara Dudek a , Cynthia Van Oers b & Pegie Cool ba Faculty of Chemistry, Jagiellonian University, Kraków, Polandb Department of Chemistry, University of Antwerpen, Wilrijk,BelgiumVersion of record first published: 05 Apr 2013.

To cite this article: Małgorzta Rutkowska , Lucjan Chmielarz , Daniel Macina , Barbara Dudek ,Cynthia Van Oers & Pegie Cool (2013): Hierarchical materials originated from mesoporous MCFmaterial and Beta zeolite nanoparticles – synthesis and catalytic activity in N2O decomposition,Journal of the Chinese Advanced Materials Society, 1:1, 48-55

To link to this article: http://dx.doi.org/10.1080/22243682.2013.772318

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Hierarchical materials originated from mesoporous MCF material

and Beta zeolite nanoparticles – synthesis and catalytic activity

in N2O decomposition

Ma»gorzta Rutkowskaa�, Lucjan Chmielarza, Daniel Macinaa, Barbara Dudeka,

Cynthia Van Oersb and Pegie Coolb

aFaculty of Chemistry, Jagiellonian University, Ingardena Krak�ow, Poland; bDepartment ofChemistry, University of Antwerpen, Universiteitsplein, Wilrijk, Belgium

(Received 2 January 2013; revised 22 January 2013; accepted 30 January 2013)

Hierarchical micro-mesoporous materials with the properties of Beta zeolite and meso-porous cellular foam (MCF) material were prepared by impregnation of MCF with asuspension of zeolite nanoparticles. Four different methods were used for MCFmodification: (1) wet impregnation, (2) wet impregnation with acidification of Betananoseeds suspension, (3) incipient wetness impregnation, (4) incipient wetness impreg-nation with acidification of Beta nanoseeds suspension. The obtained materials werecharacterized using nitrogen sorption measurements, thermogravimetric analysis(TGA), X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform(DRIFT) techniques. The use of different impregnation methods resulted in introductionof various forms of iron species to the samples. The iron modified samples (iron wasintroduced using ion-exchange method) as well as mesoporous MCF silica and Beta zeo-lite (as reference samples) were tested as catalysts in the N2O decomposition reaction.

Keywords: nitrous oxide; catalytic decomposition; MCF; Beta zeolite; zeolitenanoparticles

1. Introduction

One of the new trends in designing of advanced materials for catalysis and adsorption is

related to the synthesis of materials with hierarchical porous structure. An example could

be a combination of microporous, crystalline zeolite characterized by acidic and ion-ex-

change properties and high surface area mesoporous silica. It is expected that such hybrid

composite materials, which show properties of both mesoporous silica materials and zeo-

lites will be active, selective and stable catalysts of many reactions [1–3].

The origin of mesoporosity in zeolites can be fundamentally different, what greatly

extends the areas of the synthesis methods. Cejka et al. [4] and Egebald et al. [5] pro-

posed the following routes to obtain materials with hierarchical pore sizes distribution:

(1) recrystallization of amorphous mesoporous materials, (2) supramolecular templating,

(3) mesoporous framework build from the zeolite nanoseeds, (4) deposition of zeolite

seeds onto mesoporous materials, (5) desilication and dealumination, (6) solid templating,

(7) nontemplating method and (8) delamination of 2D zeolites. These methods can be

matched and mixed together in many different combinations opening new possibilities in

this field of science.

*Corresponding author. Email: ma.rutkows@chemia.uj.edu.pl

� 2013 Chinese Advanced Materials Society

Journal of the Chinese Advanced Materials Society, 2013

Vol. 1, No. 1, 48–55, http://dx.doi.org/10.1080/22243682.2013.772318

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Presented studies were focused on preparation of hierarchical micro-mesoporous materi-

als, which integrate the properties of Beta zeolite and mesopous MCF silica. These mate-

rials were obtained by impregnation of Beta zeolite nanoseeds on the MCF surface using

four different impregnation methods. The obtained materials were modified with iron by

wet ion-exchange method and tested as catalysts in N2O decomposition reaction.

2. Experimental methods

2.1. Catalysts preparation

The Beta zeolite nanoparticles solution, prepared according to Van Oers et al. [6], was

hydrothermal treated at 423 K for 24 h. The resulting milk-like suspension was impreg-

nated on MCF silica (synthesis recipe presented in [7]) using different impregnation

methods: (1) wet impregnation (bMCF-w), (2) wet impregnation with acidification of

Beta nanoseeds suspension (bMCF-wa), (3) incipient wetness impregnation (bMCF-i),

(4) incipient wetness impregnation with acidification of Beta nanoseeds suspension

(bMCF-ia). In case of wet impregnation 15 mL of the nanoparticles suspension per 1 g

of MCF was added. The acidification of the Beta nanoparticles slurry (5 mL of concen-

trated HCl per 18 mL of nanoseeds suspension) resulted in limitation of the zeolite crys-

tals growth. The impregnated samples were dried in ambient conditions and calcined at

823 K for 6 h. Conventional b zeolite used as the reference sample was obtained by hy-

drothermal treatment of the Beta nanoparticles solution at 423 K for 7 days.

Iron was introduced to the samples by ion-exchange method (6 h at 358 K in

anaerobic atmosphere), using 0.06 M solution of FeSO4�7H2O (Sigma-Aldrich). The Fe-

modified samples were dried in ambient conditions and calcined at 823 K for 6 h.

2.2. Catalysts characterization

The X-ray diffraction (XRD) patterns of the samples were recorded using Bruker D2

Phaser instrument, in the 2 theta ranges of 5–60� with steps of 0.02�.The infrared spectroscopy (IR) measurements were performed using Nicolet 6700 FT-

IR spectrometer (Thermo Scientific) equipped with DRIFT (diffuse reflectance infrared

Fourier transform) accessory. The measurements were carried out in the wavenumber range

of 400–4000 cm�1 with a resolution of 2 cm�1, for the 4 wt.% of the samples grounded in

dried KBr.

The low-temperature nitrogen adsorption–desorption measurements were done using

an ASAP 2010 sorptomat, while UV-vis-DR (diffuse reflectance ultraviolet–visible spec-

troscopy) spectra of the samples were recorded using an Evolution 600 (Thermo) spectro-

photometer. Details of these measurements were presented in our previous work [8].

Catalytic studies of N2O decomposition were performed in a fixed-bed quartz micro-

reactor. Composition of outlet gases was analyzed by a gas chromatograph (SRI 8610C)

equipped with thermal conductivity detector (TCD). The experiments were performed

using 0.1 g of the catalyst (particles sizes in the range of 0.160–0.315 mm) in the tempera-

ture range from 473 to 823 K in the intervals of 25 K. The reaction mixture with the follow-

ing composition: 1000 ppm of N2O, 40,000 ppm of O2 and 200 ppm of NO (optionally)

diluted in helium was used. Total flow rate of the reaction mixture was 50 ml/min.

3. Results and discussion

The nitrogen adsorption–desorption isotherms of mesoporous MCF and b zeolite are pre-

sented in Figure 1. The isotherm obtained for MCF is type IV (according to the IUPAC

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classification) and is characteristic of mesoporous materials (Figure 1a). The hysteresis

loop at P/P0 relative pressure of about 0.8–0.9 proves the presence of large and uniform

mesopores. The type I isotherm, which was obtained for b zeolite, confirms its micropo-

rous structure (Figure 1b).

The textural parameters of MCF and b zeolite are presented in Table 1. The MCF

mesoporous material is characterized by larger surface area as well as meso and macro-

pore volume (634 m2/g and 2.069 cm3/g, respectively) in comparison to Beta zeolite,

which possesses significantly higher micropore volume (0.235 cm3/g).

Figure 2a shows the X-ray powder diffraction patterns of b zeolite and the micro-

mesoporous composites (bMCF-w, bMCF-wa, bMCF-i, bMCF-ia). The XRD patterns

Table 1. Textural properties of the MCF and b samples determined from N2-sorption measure-ments at 77 K.

Sample SBET [m2/g] VMIC [cm3/g] VMESþMAC [cm3/g]

MCF 634 0.071 2.069b 513 0.235 0.095

Figure 1. Nitrogen adsorption–desorption isotherms recorded at 77 K, for (a) MCF and (b) Betazeolite.

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of the all-examined samples correspond to the pattern of Beta family topology, although

in case of the impregnated samples a decrease in the samples crystallinity (decrease in the

content of the crystalline phase) was observed. The reflections for the impregnated sam-

ples are shifted toward higher angles (decrease in d spacings) what could be explained by

the shorter crystallization time and acid treatment. It is possible that acid treatment

resulted in incomplete build-in of aluminum atoms into the zeolite framework or leaching

of aluminum atoms form the zeolite structure (the average Si�O bond length is shorter

than that of Al–O) [9,10].

The intensity of the reflections in diffractograms of the samples obtained by incipient

wetness impregnation (bMCF-i, bMCF-ia) is lower than that observed for the samples

obtained by wet impregnation (bMCF-w, bMCF-wa). It could be explained by different

amounts of deposited b zeolite nanoseeds.

In case of the acidified samples (bMCF-wa, bMCF-ia), three new reflections

appeared. Probably they are connected with the structural changes in the zeolite seeds

structure (stopping of the crystals growth, aluminum leaching) and also the formation of

new phases (such as NaCl, Al(OH)3) during acid treatment of the samples cannot be ex-

cluded. This interesting phenomenon needs additional, more detailed studies.

The DRIFT spectra of b zeolite and micro-mesoporous composites (bMCF-w,

bMCF-wa, bMCF-i, bMCF-ia) in the region of OH stretching vibrations (Figure 2b)

show three bands at about 3745, 3637 and 3000–3500 cm�1 [9–12]. The small and sharp

band at 3745 cm�1, which corresponds to terminal silanols (Si–OH), increased for the

acidified samples. It can be clearly visible by comparison of the spectra recorded for the

bMCF-i and bMCF-ia samples. It proves that the acidification stopped the crystals

growth because the increase in intensity of this band is connected with a decrease of the

zeolite crystal sizes. The intensity of the band at 3637 cm�1, assigned to Si–OH–Al

groups in the zeolite framework, decreased for the impregnated samples in comparison to

b zeolite. This difference is a result of smaller content of the zeolite phase (shorter crys-

tallization time of Beta nanoseeds) in the impregnated MCF samples. There is also a dif-

ference in intensity of this band in the spectra of the acidified and nonacidified samples.

Because Si–OH–Al groups are considered to be responsible for Brønsted acidity, there-

fore it could be expected that in the bMCF-wa and bMCF-ia samples is lower content of

this type acidity than in the bMCF-w and bMCF-i samples. The broad band at 3000–

3500 cm�1 present in the all spectra is attributed to internal H-bonded silanols.

The diffusive-reflectance UV-vis spectra of iron-exchanged and calcined Fe-b zeolite,

micro-mesoporous composites (Fe-bMCF-w, Fe-bMCF-wa, Fe-bMCF-i, Fe-bMCF-ia)

Figure 2. (a) XRD patterns and (b) DRIFT spectra of b zeolite and micro-mesoporous composites.

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and Fe-MCF sample are shown in Figure 3. The spectrum of iron species absorbance

depending on the nature and distribution of Fe3þ species can be divided into three

regions: (1) l < 300 nm, for isolated iron cations, (2) 300 < l < 400 nm, for oligonu-

clear FexOy clusters and (3) l > 400 nm, for Fe2O3 nanoparticles. In case of isolated

Fe3þ, two oxygen to metal CT bands are expected (t1!t2 and t1!e transitions) in the

ranges of 190–250 nm and 250–300 nm [13,14].

The Fe-b zeolite shows three absorption bands, which can be attributed to octahedral-

ly coordinated Fe3þ ions (220 and 270 nm) and to oligonuclear FexOy species (350 nm).

The same absorption bands were observed for the impregnated Fe-bMCF-w sample con-

taining the highest amount of b nanoseeds among the impregnated samples. The intensi-

ties of absorption bands for this sample were significantly higher than for the Fe-bsample, what means that larger amount of iron was introduced to Fe-bMCF-w. It was

probably caused by the smaller sizes of the zeolite crystals, what enhanced the accessibili-

ty of ion-exchange positions.

In the spectra of Fe-bMCF-wa, Fe-bMCF-i and Fe-bMCF-ia the new band character-

istic of Fe2O3 clusters appeared. It could be explained by smaller content of the zeolitic

phase in these samples, what resulted in accommodation of smaller amount of Fe3þ ions

in the ion-exchange positions of zeolite and therefore, deposition of iron species also out-

side of the zeolite nanoseeds. Possibly these species were aggregated during calcinations

with the formation of hematite particles. For the Fe-MCF sample only small absorption

band, attributed to isolated octahedrally coordinated Fe3þ ions, was observed.

Figure 3. DR UV-vis spectra of iron exchanged b zeolite, micro-mesoporous composites and MCFmaterial.

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The temperature dependencies of N2O conversion over Fe-b zeolite, micro-mesoporous

composites (Fe-bMCF-w, Fe-bMCF-wa, Fe-bMCF-i, Fe-bMCF-ia) and Fe-MCF sample

are shown in Figure 4a. All the hierarchical samples were found to be active catalysts for

N2O decomposition. The used impregnation methods only slightly differentiated activity

of the samples. For all the studied catalysts 95% of N2O conversion was obtained at about

823 K. The samples prepared using wet impregnation method (Fe-bMCF-w, Fe-bMCF-

wa) presented slightly higher activity comparing to Fe-bMCF-i and Fe-bMCF-ia. It could

be explained by higher content of zeolitic phase in the Fe-bMCF-w, Fe-bMCF-wa sam-

ples. The Fe-MCF sample was practically inactive in the studied temperature range, what

shows a very important role of the zeolite phase in the process of N2O decomposition.

In the presence of the most active Fe-b zeolite sample, 100% of N2O conversion was

reached at about 798 K, but it is also worth to mention that the activity of the micro-mes-

oporous composites is shifted only by about 50 K into higher temperatures in comparison

to conventional Beta zeolite.

Figure 4b shows the influence of O2 and NO (other components, beside N2O, present

in flue gases from the nitric acid plants) on the N2O conversion over the chosen hierarchi-

cal sample – Fe-bMCF-wa. The presence of oxygen did not affect significantly the cata-

lyst activity, while the addition of NO to the feed greatly enhances the rate of N2O

decomposition. The strong promotion of N2O decomposition by NO is a known feature

of Fe-zeolites [15]. NO removes oxygen atom deposited on the catalysts surface with the

formation of NO2 (2). The surface oxygen is a product of N2O decomposition (1) and its

desorption is considered as a rate determining step in this process. The NO2 molecule

reacts with another oxygen atom, liberating O2 and NO (3) [16],

N2Oþ� !O� þ N2 ð1ÞNO� þ O�!NO2

�þ� ð2ÞNO2

� þ O�!2� þ NOþ O2; ð3Þ

where: � denotes surface active sites.

Figure 4. Temperature dependence of N2O conversion for (a) b zeolite, micro-mesoporous com-posites and MCF material; 1000 ppm N2O in He, total flow 50 ml/min, weight of catalyst 0.1 g. (b)Fe-bMCF-wa sample at different conditions; 1000 ppm N2O, 40,000 ppm O2, 200 ppm NO in He;total flow 50 ml/min; weight of catalyst 0.1 g.

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Despite only small amount of the zeolitic phase impregnated on the Fe-bMCF-wa

sample, the small addition of NO to the feed gases caused the characteristic for Fe-zeo-

lites increase in catalytic activity.

Despite only small amount of the zeolitic phase impregnated on the Fe-bMCF-wa

sample, the small addition of NO to the feed gases caused the increase in catalytic activi-

ty, which is a typical feature observed for Fe-modified zeolites.

It should be noted that an opposite effect of NO addition is usually observed for other

catalytic systems [15].

4. Conclusions

Four different impregnation methods were used for synthesis of micro-mesoporous mate-

rials. All of them resulted in successful introduction of the zeolitic phase into MCF, which

was confirmed by XRD method. Depending on the used impregnation method different

amounts and characteristics of b zeolite phase deposited on the MCF surface were re-

ceived. These differences affected the form of iron introduced to the samples by ion-ex-

change method. The iron-exchanged hierarchical materials as well as Fe-MCF and Fe-b(as reference samples) were tested as catalysts in the N2O decomposition reaction. The

deposition of Beta nanoparticles on the surface of MCF significantly increased its catalyt-

ic activity. The highest activity presented conventional Beta zeolite (100% of N2O con-

version at about 798 K), but the activity of hierarchical samples, despite the significantly

lower content of the zeolitic phase, was shifted to higher temperatures only by about

50 K. It should be also noted that the production of the impregnated samples is preferable

from the economical point of view (smaller amount the zeolitic phase and shorter crystal-

lization time).

Acknowledgements

M.R. acknowledges the financial support from the International PhD-studies programme at theFaculty of Chemistry Jagiellonian University within the Foundation for Polish Science MPDProgramme co-financed by the EU European Regional Development Fund. The research wascarried out with the equipment purchased thanks to the financial support of the European RegionalDevelopment Fund in the framework of the Polish Innovation Economy Operational Program(contract no. POIG.02.01.00-12-023/08).

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