Experimental and theoretical studies about the adsorption of toluene on ZSM5 and mordenite zeolites modified with Cs

Microporous and Mesoporous Materials 147 (2012) 17–29

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Experimental and theoretical studies about the adsorption of toluene on ZSM5 and mordenite zeolites modified with Cs Ramiro M. Serra a, Eduardo E. Miró a, Pablo Bolcatto b, Alicia V. Boix a,⇑ a b

Instituto de Investigaciones en Catálisis y Petroquímica – INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2829, 3000 Santa Fe, Argentina Departamento de Física, Facultad de Ingeniería Química y Facultad de Humanidades y Ciencias, Universidad Nacional del Litoral, Santiago del Estero 2829, 3000 Santa Fe, Argentina

a r t i c l e

i n f o

Article history: Received 12 November 2010 Received in revised form 28 April 2011 Accepted 18 May 2011 Available online 27 May 2011 Keywords: Toluene adsorption TPD DFT Hydrocarbon traps

a b s t r a c t In this work, we studied the adsorption capacity and thermal stability of toluene on ZSM5 and mordenite zeolites with different exchange cations (H, Na and Cs). The interaction of the hydrocarbon molecules with the adsorption sites was also addressed with both experimental and theoretical techniques. Flowadsorption measurements were carried out in order to obtain breakthrough curves as well as TPD experiments. Molecular simulations were performed using the Density Function Theory (DFT). The results show that, in general, mordenite samples have a better adsorption capacity than ZSM5 samples and that the thermal stability of the adsorbed toluene is more influenced by the exchanged cations than by the zeolite structure. The overall basicity of the samples depends on the exchanged cations, following the Cs > Na > H order. The main interaction of toluene takes place between the p electrons of the aromatic ring and the Lewis sites generated, and another interaction takes place between C–H groups and the zeolite network oxygen. The increase of the Cs loading decreases the surface area and pore volume of the zeolites, thus decreasing the available sites for the overall toluene adsorption at 100 °C. Through TPD experiments both the U parameter, which represents the fraction of toluene retained at temperatures higher than 100 °C, and the temperature at which the toluene desorbs were measured. It was observed that in the Cs exchanged samples, U is higher but the thermal stability (temperature of the maximum desorption rate on TPD) follows the order NaMOR = NaZSM5 > CsNaZSM5 > CsNaMOR > CsHMOR = CsHZSM5 > HMOR > HZSM5. The molecular calculations for the NaMOR structure determined that the interaction energies were higher for NaMOR when compared with the HMOR structure and increased even more when Cs was exchanged. These facts are in line with the experimental results. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Combustion processes both in industry and transportation constitute one of the main sources of pollution. Alcohols, alkenes and aromatics are among the compounds released which greatly contribute to environmental contamination [1]. The simultaneous removal of NOx, CO and hydrocarbons emitted by Otto cycle engines is currently accomplished using the socalled three-way catalysts (TWC). However, the ultra-low emission vehicle (ULEV) standard is particularly demanding with respect to lowering hydrocarbon emissions. It is well known that the key to reduce hydrocarbon emissions is to achieve a fast increase of the catalytic converter temperature after the cold start. However, standard vehicles require at least 1–2 min to reach the minimum temperature at which the catalytic converter is effective (175 °C). During this period, the largest emission of pollutants takes place, ⇑ Corresponding author. Tel.: +54 03424536861. E-mail address: aboix@fiq.unl.edu.ar (A.V. Boix). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.05.016

which frees up to 80% of unburned hydrocarbons [2–4]. Several potential solutions to the cold-start problem have been proposed: one of these involves trapping hydrocarbons in a suitable porous material during the cold start and release them after the TWC has reached operating temperature. Molecular sieves such as zeolites are used in many catalytic processes and as adsorbents due to their orderly micropore structure and hydrothermal stability [5]. The availability of zeolites with different structures, composition, and degree of hydrophobicity, makes them interesting materials in developing hydrocarbon traps. The adsorption of a variety of hydrocarbons in zeolites has been widely studied and the sorption properties depend on the number, strength, distribution and accessibility of adsorption sites and the framework topology. The replacement of an atom (Si4+) in a zeolite framework by another with a lower valency (Al3+) creates a negative charge on the framework which has to be neutralized by a proton or a metal cation. Consequently, the conjugated acid–base pairs are then formed. The cation acts as a Lewis acid center, while

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the framework oxygen bearing partial negative charge behaves as a Lewis base center. It has been proposed that the cations in zeolites are adsorption sites for molecules with p electrons, and many molecules of this type have been studied, particularly benzene. The two main modes of benzene adsorption involve the interaction through its p electron cloud with the counterions and the interaction of the CH group with framework oxygens bearing the negative charge of the lattice [6]. Wesson and Snurr [7] studied the controlled release of propane in the presence of toluene using several zeolite samples with different pore sizes and different pore network connectivity. In the same way, Elangovan et al. [8] reported results using SSZ-33, MCM-48 molecular sieves for their use as hydrocarbon traps employing temperature-programmed desorption (TPD) of toluene as a model system. Kaliaguine and coworkers [3] synthesized a series of one-dimensional channel molecular sieves with MTW structure, which were modified by isomorphous metal substitutions and cation exchange. The samples were tested as hydrocarbon-trap adsorbents and the results demonstrated that a silver exchanged MTW zeolite exhibited high and stable trapping capacities for toluene and ethylene under different mixture conditions. Recently, we studied the adsorption capacity and the dynamics of the adsorption–diffusion of toluene in NaMOR modified with cesium. These properties are influenced by the basicity of the solid, which in turn is a function of the charge density of network oxygen and the steric effects [9]. On the other hand, quantum chemistry is a powerful tool to study aspects of heterogeneous catalysis that cannot be directly addressed with experimental techniques. The geometry and properties of the active sites, the structure and nature of the intermediates and transition states of catalyzed reactions, as well as the energies involved in each of the stages or steps that form the basic mechanics of a catalytic reaction are some of the aspects that can be studied using quantum chemistry. In this field, many studies about the adsorption of organic molecules in different zeolites have been reported [10–13]. In this work, several zeolite samples having different pore sizes and pore network connectivity were examined. ZSM5 (MFI) and mordenite (MOR) zeolites, with different Si/Al ratios and modified with different loadings of exchangeable cations, Na+, H+ or Cs+ were evaluated as hydrocarbon traps with controlled release. The microporous structure of MFI zeolites consists of a 10-membered straight and sinusoidal channel running along [0 1 0] and [1 0 0], respectively, which constitutes a three-dimensional (3-D) pore network. The micropore system of the mordenite is a one-dimensional structure (1-D) which consists of a main 12-MR channel running parallel to the c-axis, and another 8-MR channel termed the side pocket which runs parallel to the b-axis [5]. The toluene molecule was used as representative of the aromatic hydrocarbons released in different effluent streams. The adsorption and desorption temperatures were investigated. Furthermore, a theoretical simulation using Density Functional Theory (DFT) was also performed to support the experimental results. In addition, all samples were characterized by X-ray diffraction (XRD) and N2 adsorption at 196 °C. A thermogravimetric analysis of samples that were subjected to successive cycles of adsorption– desorption was also performed in order to assess the possibility of carbon deposition.

2. Experimental 2.1. Materials preparation The parent zeolites used in the preparation of adsorbent materials were:

NaZSM-5 (Linde, Si/Al = 10, Na9[Al9Si87O192](H2O)16); NH4ZSM-5 (Zeolyst-CBV-3024E, Si/Al = 15, (NH4)6[Al6Si90O192] (H2O)16); NH4ZSM-5 (Zeolyst-CBV-28,014, Si/Al = 140, (NH4)0.68[Al0.68Si95.32O192](H2O)16); NaMOR (Zeolyst-CBV-10A, Si/Al = 6.5, Na6.4[Al6.4Si41.6O96] (H2O)24); NH4MOR (Zeolyst-CBV-21A, Si/Al = 10, (NH4)4.4[Al4.4Si43.6O96] ((H2O)24). The method used to incorporate cesium into the zeolite was ionic exchange Cesium solutions were prepared with cesium acetate (CsAc) 99.5 wt.%, Sigma–Aldrich. In each case, 10 g of support were mixed with 1 L of cesium acetate solution of appropriate concentration. The mixture was stirred during 24 h at 60 °C and afterwards it was filtered several times, washed and dried at 100 °C for 8 h. Then the materials were calcined at 500 °C for 8 h on a stream of 25 cm3 min1 of air. The exchange degree achieved was determined by atomic absorption spectroscopy (AAS). 2.2. Characterization techniques 2.2.1. Determination of pore volume and surface area Nitrogen adsorption–desorption isotherms were obtained at 196 °C on a Quantachrome Autosorb instrument. Previously, the samples were outgassed at 250 °C for 6 h (0.013 Pa). The Brunauer–Emmett–Teller (BET) equation was used for calculating the specific surface area of the materials from nitrogen adsorption isotherms. The external surface area and the micropore volume were determined by applying the t-plot method proposed by Lippens et al. [14]. 2.2.2. X-ray diffraction The X-ray diffraction measurements were taken using an XD-D1 model Shimadzu diffractometer operated with Cu Ka radiation at 30 kV and 40 mA, using a scanning rate of 1°C min1. The database employed was the one provided by the manufacturer. The crystallinity was estimated from the ratio of the sums of the heights of the prominent peaks for the cesium-exchanged samples and the unexchanged supports [15]. 2.3. Toluene adsorption and temperature-programmed desorption The breakthrough curves were obtained using a continuous flow system. Toluene was used as probe molecule. The stream of toluene diluted in N2 was obtained by passing pure N2 through two connected saturators that contained toluene (99.9%, Sigma Aldrich), whose temperature was kept at 0 °C with an ice bath achieving approximately 8000 ppm of toluene. The sample (ca. 100 mg) was placed in a tubular quartz reactor of ca. 5 mm i.d., which was placed inside an electrical furnace equipped with a temperature controller. Subsequently, the sample was dehydrated in a N2 flow at 500 °C for about 6 h. After cooling the solid sample to 100 °C, it was exposed to toluene/N2 flow during 1 h before TPD. All TPD experiments were carried out in N2 flow at 50 cm3 min1. Desorption was performed by a temperature-programmed method that included heating from 100 to 550 °C with a heating rate of 5 °C min1 and maintaining it at the final temperature for about 10 min. Toluene concentration (m/e = 91) was continuously monitored by on-line mass spectrometry (Pfeiffer/Balzers Quadstar, QMI422, QME125). Before the TPD experiments, a sweep with N2 flow at 100 °C was carried out. The amount of toluene adsorbed during the breakthrough experiments (adsorption capacity) was calculated as the difference between the area under the curve in a blank experiment and the area under the breakthrough curve.

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We also measured the area under the curve during the TPD experiments, from which we could calculate the amount of toluene desorbed between 100 and 550 °C. This amount corresponds to the toluene retained in the sample after purging with N2 at 100 °C. A U coefficient, a comparison parameter, was defined as the amount of toluene retained during TPD divided by the toluene adsorbed at 100 °C. On the other hand, the toluene concentration in the feed was changed between 4000 to 10,000 ppm and the amount of adsorbed toluene for each one was calculated. From these values the adsorption isotherms for Na- and H-MOR were shaped at 100, 150 and 200 °C. In addition, the adsorption heat of toluene was also calculated from the adsorption constant at different temperatures.

ure of the terminal cation (Na+, H+ and/or Cs+ extra-framework) used to compensate the negative charge introduced by replacing Si (4+) by Al (3+) was also studied. The geometries of the clusters were optimized using the criterion of the minimum of the total energy. The optimization was finished when the energy difference between two interaction cycles was less than 0.001 atomic units (a.u. = hartree). The adsorption energy or interaction energy of toluene on the zeolitic cluster was also determined using the following equation:

2.4. Coke formation studied by thermal analysis (TGA)

where Etot(cluster + toluene) describes the total energy of the cluster with the adsorbed toluene molecule, Etot(cluster) is the total energy of the clean cluster and Etot(toluene) is the total energy of the ground state of toluene. Thus, the energy of each of the proposed clusters and the interaction energy of these clusters with one molecule of toluene were determined.

It is known that zeolites, especially the protonic ones, catalyze hydrocarbon cracking. Consequently, the coke formed during the experiments of adsorption/desorption of toluene were analyzed by the TG method using a Mettler Toledo Star, TGA/SDTA 851 instrument. An amount of 15 mg of adsorbent material was loaded within an alumina crucible of 70 lL. The temperature was increased from 25 to 900 °C at a rate of 10 °C min1 under a stream of 30 cm3 min1 of air. The mass loss was recorded. 2.5. Molecular simulations All geometrical optimizations and energies of interaction results were performed applying the Density Function Theory (DFT) using the Gaussian 03 package [16], which uses Gaussiantype basis functions to describe molecular orbitals and charge density. The gradient correlation, which considers Becke’s threeparameter hybrid functional B3LYP [17] and the correlation of Lee et al. [18], were used in all cases. The core potentials and basis sets used on all atoms were 6-31G, but when the clusters contained cesium, the basis used was LanL2DZ. The structures of the solids studied in this work were analyzed with the cluster model. The model employed to represent the main channel of the mordenite framework included (SiO4)(12  Y)(AlO4)Y, where Y = 0, 1, 2. These centers were formed from a simple structure consisting of a silicon tetrahedron [Si(OH)4], in which the number of atoms was later increased to reach a ring of 12 tetrahedrons (12 MR). Afterwards, two Si atoms were exchanged by two Al atoms at T1 and T2 positions of the ring [19]. The effect of the nat-

Eads ðtolueneÞ ¼ Etot ðcluster þ tolueneÞ  ½Etot ðclusterÞ þ Etot ðtolueneÞ

3. Results and discussion 3.1. Properties of the adsorbent materials Table 1 shows the chemical composition, the total area calculated by the BET method and pore volume using the ‘‘t-plot’’ method. Zeolites have a high specific surface due to their typical structure of micropores. The surface area and pore volume of all ZSM5 samples are lower than those of HMOR and NaMOR (Table 1), and they are in agreement with those reported by the supplier. In the mordenite structure, both the surface area and the pore volume increase when increasing the Si/Al ratio from 6.5 to 10. Similarly, in ZSM5, the BET area increases when increasing the Si/Al ratio, although the pore volume slightly decreases. The framework of ZSM5 exhibits a 10-MR two-channel system with openings of 5.3  5.6 Å and 5.1  5.5 Å, respectively. On the other hand, the micropore system of mordenite consists of two types of channels: a main 12-MR (membered ring, 6.5  7.0 Å) channel running parallel to the c-axis, and another 8-MR (2.6  5.7 Å) channel, termed the side pocket, running parallel to the b-axis [5]. To modify the adsorption capacity of the original zeolite supports, Cs+ ions were exchanged with Na+ or H+ as appropriate. In

Table 1 Textural properties of zeolites exchanged with cesium.

a b c d e

Sample

% Cs+a

Cs/u.c.b

Cs/Alb

SBET (m2 g1)c

Pore volume (cm3 g1)d

Sext (m2 g1)d

% Cryst.e

NaZSM5 Si/Al = 10 Cs7NaZSM5 Si/Al = 10 HZSM5 Si/Al = 15 Cs7HZSM5 Si/Al = 15 HZSM5 Si/Al = 140 Cs7HZSM5 Si/Al = 140 NaMOR Si/Al = 6.5 Cs2NaMOR Cs7NaMOR Cs19NaMOR HMOR Si/Al = 10 Cs2HMOR Cs7HMOR Cs14HMOR

0 6.6 0 6.7 0 6.9 0 1.9 7 19 – 1.8 6.8 14

– 3.5 – 3.4 – 3.4 – 0.6 2.2 6.1 – 0.5 2.5 4.0

– 0.39 – 0.57 – 5.0 – 0.08 0.30 0.91 – 0.12 0.43 0.93

366 300 378 299 410 360 409 408 344 128 479 457 429 212

0.159 0.145 0.138 0.134 0.131 0.120 0.165 0.158 0.138 0.055 0.189 0.182 0.175 0.096

13.05 25.30 98.23 31.18 68.49 103.2 1.130 1.960 4.135 24.27 1.21 1.15 1.07 29.17

100 70.4 100 60.8 100 48.0 100 97.0 70.0 56.1 100 99.9 91.0 62.6

Measured by atomic absorption spectrometry. Cesium atoms per unit cell. Calculated from N2 isotherms at (196 °C). External surface area (Sext) and micropore volume (Vl) calculated by t-plot method. 100% crystallinity corresponds to the support.

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the ZSM5 samples, about 7 wt.% of cesium were added, which corresponded to different levels of ion exchange according to the Si/Al ratio present in the solid. The BET area decreased by about 20% after the exchange with cesium both in sodium and protonated ZSM5 samples with Si/Al = 10 and 15, respectively. Meanwhile, in the CsHZSM5 sample with Si/Al = 140 ratio the BET area decreased by about 10%. In the same way, the micropore volume decreased due to the addition of cesium in the three materials under study. It is known that mordenite and ZSM5 zeolites have three types of cationic sites (denoted as a, b and c) [20]. While the a site is placed on the walls of the main channel of the framework, b and c sites are inside channels of difficult access. The bare cesium ions have an atomic radius of 3.34 Å [21] and when they occupy the sites inside the channels, a decrease in the surface area and pore volume is provoked. The external surface area was also determined by the t-plot method and it increased after the addition of cesium both in the NaZSM5 (Si/Al = 10) and in the HZSM5 (Si/Al = 140) samples. Similarly Hu et al. [22] showed that the addition of magnesium on

A a b

HZSM5 markedly increased the external surface area due to the presence of magnesium oxide species. In the case of mordenites, three ion-exchange sites are also present which are similar to those mentioned above for the ZSM5 solid. Both the NaMOR and HMOR samples showed a similar behavior when 7 wt.% of cesium were added by ionic exchange. Other samples were also prepared with low (about 10%) and high (about 90%) exchange levels (Table 1). In HMOR and NaMOR with 2 wt.% of cesium, both the BET surface area and pore volume were similar to those of the unexchanged supports. When the Cs load was increased from 2% to 19% in NaMOR; and from 2% to 14% in HMOR, it was observed that the BET area and pore volume markedly decreased. Then, in samples with high Cs loadings (Cs14HMOR and Cs19NaMOR), bulky particles such as oxides and/or hydroxides could occupy the zeolite channels thus decreasing the amount of N2 adsorbed. Concepcion-Heydorn et al. [23] also observed the same effect. Fig. 1 shows diffraction patterns of adsorbents prepared from ZSM5, with different Si/Al ratios, and HMOR, in both cases cesium-modified. When 7 wt.% of Cs were added to the ZSM5 samples, a decrease in the intensity peaks was observed compared with the parent zeolite (Fig. 1A). The last column in Table 1 shows the crystallinity values calculated from the intensity of the main peaks (see Section 2). For the same amount of cesium, crystallinity decreases when increasing the Si/Al ratio. The greater crystallinity decrease (close to 50%) is observed for Cs7HZSM5 (Si/Al = 140). This sample has a few exchange sites due to a high Si/Al ratio; therefore, during the calcination treatment the cesium ions form oxide and

A

100

c d Crystallinity

0.15

e f

80

0.10

Crystallinity 5

10

15

20

25

60

30

0.05

Pore Volume (cm3g-1)

Intensity (a.u.)

0.20

Pore Volume

2θ 0

10

5

B

0.00 20

15

Cesium percentage (wt%)

B

100

0.20

0.15

Crystallinity

a b c d

80

0.10

15

20

Pore Volume

25

2θ Fig. 1. X-ray diffraction patterns of adsorbents; (A) (a) NaZSM5 (Si/Al = 10), (b) Cs7NaZSM5 (Si/Al = 10), (c) HZSM5 (Si/Al = 15), (d) Cs7HZSM5 (Si/Al = 15), (e) HZSM5 (Si/Al = 140) and (f) CsHZSM5 (Si/Al = 140) and (B) HMOR (Si/Al = 10) modified with: (a) 0, (b) 2, (c) 7 and (d) 14 wt.% of cesium.

0.05

Crystallinity

60

Pore Volume (cm3g-1)

Intensity (a.u.)

Cs2O

0

5

10

15

0.00 20

Cesium percentage (wt%) Fig. 2. Modification of crystallinity and pore volume of (A) HMOR and (B) NaMOR adsorbents with different amounts of exchanged cesium.

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A

1.0

c

b

a

0.8

0.8

0.6

0.6

C/C0

C/C0

C

1.0

c

0.4

0.4

0.2

0.2

0.0 1.0

0.0

b

a b c

B

D

1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

a

0.4

b

100 °C 150 °C 200 °C

c

b

a

0.4

a

a b c

0.2

0.2

100 °C 150 °C 200 °C

0.0

0.0 0

10

20

30

40

0

50

10

20

Time (min.)

30

40

50

Time (min.)

Fig. 3. Breakthrough curves of toluene adsorption at 100 °C on: (A) ZSM5 with different ions (a) NaZSM5 (Si/Al = 10), (b) HZSM5 (Si/Al = 15) and (c) HZSM5 (Si/Al = 140); (B) MOR with different ions (a) NaMOR (Si/Al = 6.5) and (b) HMOR (Si/Al = 10). Breakthrough curves of toluene adsorption at (a) 100, (b) 150 and (c) 200 °C on: (C) NaMOR and (D) HMOR. Conditions: 100 mg. of sample, 20 cm3 min1 of toluene (8000 ppm)/N2.

hydroxide clusters outside the zeolite network, in agreement with an increased external surface area. Fig. 1B shows XRD results for CsHMOR with 2, 7 and 14 wt.% of cesium. A signal at 18.2° can be observed in diffractograms c and d, which can be associated with the Cs2O phase (PDF No. 09-0104) [24]. In the same way, the diffraction patterns of the CsxNaMOR samples show this peak for samples with 7 and 19 wt.% of cesium. These oxide particles could be responsible for blocking the zeolite channels and decreasing the pore volume determined by N2 adsorption measurement (Table 1). Fig. 2 shows the changes in crystallinity observed for HMOR (Fig. 2A) and NaMOR (Fig. 2B) against the exchange level: as the latter increases, the crystallinity degree decreases. The picture also shows the change in pore volume due to the addition of cesium. The loss of crystallinity is higher in Cs14HMOR and Cs19NaMOR.

kinetic diameter of toluene which is about 5.8 Å [26], close to the cross-section of the MFI channels [5]. This causes a steric hindrance for bulky hydrocarbon adsorption inside the zeolite channels. The same behavior was reported by Malherbe and Wendelbo [27] when they studied the interaction of benzene, toluene and ethylbenzene with the HZSM5 structure. The breakthrough curves of the ZSM5 supports showed slight differences by changing the cation, Na+ by H+ or by increasing the Si/Al ratio. The shape of the curves for HZSM5 with different Table 2 Adsorption and retention capacity of toluene. Basicity of the materials.

3.2. Adsorption and retention capacity of toluene 3.2.1. Determination of breakthrough curves The evolution of the toluene concentration, measured at the reactor outlet during the isothermal adsorption process allowed the monitoring of adsorbent loading as a function of time. Fig. 3 shows the toluene adsorption breakthrough curves of selected adsorbents. The shape of the toluene adsorption curves in all the adsorbent materials is typical of microporous materials [25]. Fig. 3A shows breakthrough curves of adsorbed toluene at 100 °C on: (a) NaZSM5 (Si/Al = 10), (b) HZSM5 (Si/Al = 15) and (c) HZSM5 (Si/Al = 140). During the adsorption process, there is no signal at the reactor outlet (C/C0 = 0) for a period between 10 and 15 min in all materials. After that, the outlet toluene concentration increases gradually to reach a plateau indicating the adsorbent saturation. The NaZSM5 sample showed the higher adsorption capacity. Moreover, diffusive limitations can be found due to the

a b c d e

Adsorbent

QA (lmole mg1)a

QD (lmole mg1)b

Uc

Tend (°C)d

(doxygen)e

NaZSM-5 Cs7NaZSM-5 HZSM-5 (Si/Al = 15) Cs7HZSM-5 (Si/Al = 15) HZSM-5 (Si/Al = 140) Cs7HZSM-5 (Si/Al = 140) NaMOR Cs2NaMOR Cs7NaMOR Cs19NaMOR HMOR Cs2HMOR Cs7HMOR Cs14HMOR

0.92 0.88 0.69

0.32 0.34 0.41

34.9 38.2 59.1

530 525 291

0.2717 0.2865 0.3151

0.62

0.28

45.1

394

0.3359

0.50

0.08

16.0

200

0.2255

1.08

0.32

29.4

380

0.2306

1.34 1.40 0.89 0.55 1.07 1.06 1.07 0.68

0.88 1.01 0.67 0.50 0.51 0.59 0.61 0.42

65.5 72.0 75.0 90.3 47.8 55.7 60.2 63.3

540 520 450 450 325 354 370 376

0.2841 0.2867 0.2935 0.3124 0.2197 0.2273 0.2541 0.2782

Toluene amount adsorbed at 100 °C. Toluene amount desorbed at temperatures above 100 °C. Toluene retention capacity above 100 °C; U = QD/QA. Desorption end temperature. Zeolitic oxygen charge [Ref. [6]].

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1.0

A

0.8

0.8

0.6

0.6

C/C0

C/C0

1.0

D

0.4

0.4

b

0.2

0.2

b

a 0.0

0.0 0

10

20

30

40

0

50

10

B

1.0

0.8

0.6

0.6

C/C0

0.8

b

30

40

50

40

50

E

1.0

0.4

20

Time (min.)

Time (min.)

C/C0

a

0.4

a

a b

0.2

0.2 0.0

0.0 0

10

20

30

40

50

Time (min.)

0

10

20

30

Time (min.)

C

1.0

C/C0

0.8 0.6 0.4

a

b

0.2 0.0 0

10

20

30

40

50

Time (min.) Fig. 4. Breakthrough curves of toluene adsorption at 100 °C on: (A) NaZSM5 (Si/Al = 10), (B) HZSM5 (Si/Al = 15), (C) HZSM5 (Si/Al = 140), (D) NaMOR (Si/Al = 6.5) and (E) HMOR (Si/Al = 10). (a) Support without cesium and (b) support exchanged with 7 wt.% of cesium. Conditions: 100 mg of sample, 20 cm3 min1 of toluene (8000 ppm)/N2.

Si/Al ratios (15 and 140) showed that the time to reach saturation with toluene is slightly shorter in the solid with higher Si/Al ratio. Table 2 shows the results obtained from studies of adsorption and desorption of toluene. In column 2, the amount of toluene adsorbed at 100 °C (QA) is presented, which was calculated from breakthrough curves. The values are 0.92, 0.69 and 0.5 lmole mg1 for samples NaZSM5 (Si/Al = 10), HZSM5 (Si/Al = 15) and HZSM5 (Si/Al = 140), respectively. This sequence can be explained by comparing micropore volumes, type of cations exchanged and their concentration. NaZSM5 (Si/Al = 10) has a micropore volume higher

than that of HZSM5, and the aromatic ring interacts more strongly with Na+ cations than with Brönsted acid sites [28,29]. This is in agreement with Choudhary et al. [29] who found that the adsorption heat of different aromatic molecules (benzene, toluene and xylene) increases with Na+ concentration. In agreement with these observations, the NaZSM5 sample adsorbs 25% more of toluene than HZSM5. On the other hand, by comparing the protonic samples, HZSM5 (Si/Al = 15) and HZSM5 (Si/Al = 140), it can be observed that they have approximately the same micropore volume. However, the sample with lower Si/Al

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In the CsHZSM5 material with Si/Al = 140 (Fig. 4C), the amount of hydrocarbon adsorbed increased significantly; however, the BET area and micropore volume only barely changed. This result can be explained with the increase on the external surface. The oxide or hydroxide clusters located on the external surface formed by cesium excess increases the oxygen charge (doxygen) and could improve the interaction strength between CH groups with basic sites. Fig. 4D and E shows the breakthrough curves for the NaMOR and HMOR materials, respectively. In the NaMOR sample, the effect of the addition of 7 wt.% of cesium decreases the adsorption capacity, reducing it by about 35% compared to the material without cesium (see Table 2). In this case, as in the ZSM5 materials, the lower adsorption capacity is the result of a decrease of pore volume of the structure. Instead, in the HMOR sample, the addition of 7 wt.% of cesium does not modify the adsorption capacity of toluene. Finally, we analyzed the adsorption of toluene on the NaMOR and HMOR materials with opposite levels of exchanged cesium, close to 10% and 90% (see Table 1). On the NaMOR samples with 2 and 19 wt.% of cesium, it was found that a low cesium loading (2%) slightly enhanced the adsorption capacity of the original support. However, when cesium was 19 wt.%, there was a decrease in

Intensity (a.u.)

A

a

b

c

%2 100

200

300

400

500

Temperature (ºC)

B a b

Intensity (a.u.)

ratio is able to adsorb more toluene. As a matter of fact, Table 2 shows that HZSM5 (Si/Al = 15) adsorbs 27% more than HZSM5 (Si/Al = 140). Analyzing the adsorption of toluene at 100 °C on NaMOR and HMOR (Fig. 3B), we can see a marked difference in the breakthrough curves due to the presence of either Na+ or H+ ions in the structure (Table 2). The curve corresponding to NaMOR (Fig. 3B, curve a) shows that more time is required to reach the saturation value comparing to the curve corresponding to HMOR (Fig. 3B, curve b). The adsorption capacity values were 1.34 and 1.07 lmole mg1, respectively. In a previous work [30], we studied the toluene adsorption on NaMOR and HMOR by FTIR spectroscopy. In both cases, bands at 1493 and 1450 cm1, assigned to the C@C stretching vibration corresponding to the aromatic ring are slightly shifted to a lower frequency (15 cm1) if compared with the values of the gas toluene molecule. Similar shifts are observed in the higher wavenumber region for the 2924 cm1 and 3034 cm1 bands corresponding to –C–H stretching of the methyl group and @C–H stretching of the aromatic ring, respectively. These signals indicate that the toluene molecule interacts through both p electrons of the aromatic ring and the hydrogen atoms of the CH groups. In the former case, the interaction is with exchanged cations and in the latter case, it occurs with negatively charged framework oxygen [6]. In both structures, the adsorption capacity in the NaZSM5 and NaMOR samples was higher than in their proton forms. These results are also consistent with other data from the literature reporting that different hydrocarbons (toluene, benzene, etc.) have higher interaction strength with sodium zeolite than with the proton form [31]. Comparing the two types of structures, the amount of toluene adsorbed on mordenite structure was higher than that for ZSM5. This is because the 12 MR channel in mordenite is larger than that of 10 MR in ZSM5 and does not cause steric hindrance. Another difference between frameworks is the way the tetrahedra are interconnected, i.e. the framework topology which directs the network of the bonds beyond the first layer of four tetrahedra around a central atom. Barthomeuf [32] reported that the topology of the framework influences both the basic and the acid character of the Si–Al zeolites whether they are in a cationic or protonic form. In addition, we studied the effect of adsorption temperature on adsorption capacity in the NaMOR and HMOR samples (Fig. 3C and D, respectively). Breakthrough curves were measured at different temperatures, namely 100, 150 and 200 °C, showing that the adsorbed amount was lower at higher temperature. This was consistent with the thermodynamics of adsorption since it is an exothermic process. The adsorption heats (DHa) were calculated from adsorption isotherms at 100, 150 and 200 °C. The obtained values were 22.52 and 15.55 kJ mol1 for NaMOR and HMOR, respectively. The effect of the addition of cesium in the adsorption capacity of toluene was examined through the study of the breakthrough curves obtained for all adsorbents. Fig. 4 compares the curves corresponding to the above described zeolites with and without 7 wt.% of cesium. In all cases, the shape of the breakthrough curve for materials with cesium was similar to that of the unexchanged supports. The addition of cesium to the NaZSM5 (Si/Al = 10) and HZSM5 (Si/Al = 15) samples, Fig. 4A and B, respectively, slightly decreases the adsorption capacity of toluene (see Table 2). The adsorption capacity decreases because the Cs+ ions inside the zeolitic channels occupy a larger volume than the Na+ cations and could hinder the toluene adsorption. This effect is due to the fact that the atomic radius of Cs+ is 3.34 Å [22], close to the channel diameter in the ZSM5 structure (5.3  5.6 Å) [33,34].

100

200

300

400

500

Temperature (ºC) Fig. 5. Profiles of temperature-programmed desorption (TPD) of adsorbed toluene at 100 °C with (8000 ppm) toluene/N2 on: (A) ZSM5 adsorbent: (a) HZSM5 (Si/ Al = 140), (b) HZSM5 (Si/Al = 15), (c) NaZSM5 (Si/Al = 10); and (B) MOR adsorbents: (a) HMOR (Si/Al = 10) and (b) NaMOR (Si/Al = 6.5). Desorption in N2 flow; heating rate 10 °C min1.

24

R.M. Serra et al. / Microporous and Mesoporous Materials 147 (2012) 17–29

the amount adsorbed [9]. In the same way, it was found that the addition of 14 wt.% of cesium to HMOR decreased the adsorbed toluene from 1.07 to 0.68 lmole mg1 (see Table 2). At this point, it is clear that Na+ and Cs+ cations exchanged in both the ZSM5 and mordenite frameworks behave better than exchanged protons. On the other hand, the ion exchange of Na+ ions by Cs+ ions changes the chemical composition of the adsorbent and modifies the adsorption properties of hydrocarbon. In the alkalies cation-exchanged zeolites, the cations behave as Lewis acid sites while the framework oxygen bearing partial negative charge behaves as a Lewis base center, i.e. the lower the compensating cation M+ acidity the higher the oxygen basicity. In turn, cation M+ acidity diminishes when its volume is increased, the positive charge being diluted in a higher volume (less electronegativity). By the Sanderson principle equalization it is possible to calculate the intermedi-

A

ate electronegativity of zeolites, which gives access to the mean charge on the oxygen atoms in the framework (doxygen). The basicity of oxygen increases when the absolute value of doxygen also increases. It was shown that in the alkaline series, the basicity follows the order of cationic zeolite form, Li < Na < K < Rb < Cs, and increases with the Al content. It should be noted that the approach takes into account only the chemical composition of the zeolite and no effect of structure is considered [35]. Our experimental results show that the adsorption capacity of toluene increases with the increase of the Lewis acidity of the exchanged-cation. Accordingly, a higher Cs/Na ratio in the zeolites decreases the adsorbed toluene. In addition, the highest cesium loading inside the zeolites hinders the toluene adsorption. The results shown by samples with 2 and 7 wt.% Cs suggest that the adsorption capacity is more strongly influenced by the cation

D

NaZSM5 (Si/Al=10)

412

200

b

300

400

100

500

b

a

Intensity (a.u.)

Intensity (a.u.) 100

415

180

262

a

200

300

Temperature (°C)

B

E

HZSM5 (Si/Al=140)

500

HMOR (Si/Al=10)

225 293

Intensity (a.u.)

Intensity (a.u.)

400

Temperature (°C)

144 250

a

100

350

NaMOR (Si/Al=6.5)

b

200

300

400

500

Temperature (°C)

C

b

a

100

200

300

400

500

Temperature (°C)

HZSM5 (Si/Al=15)

Intensity (a.u.)

177 282

a

100

200

b

300

400

500

Temperature (°C) Fig. 6. Temperature-programmed desorption (TPD) of adsorbed toluene at 100 °C with (8000 ppm) toluene/N2 on different zeolites modified with cesium; profiles (a) Support unchanged cesium and (b) support with 7 wt.% of cesium exchanged. Desorption in N2 flow; heating rate 10 °C min1.

25

R.M. Serra et al. / Microporous and Mesoporous Materials 147 (2012) 17–29

interaction with the p electron cloud than by the interaction of CH groups with the negative charge on network oxygen.

a

Intensity (a.u.)

b c d

100

150

200

250

300

350

400

450

Temperature (°C) Fig. 7. Profiles of temperature-programmed desorption (TPD) of adsorbed toluene on: (a) HMOR, (b) Cs2HMOR, (c) Cs7HMOR and (d) Cs14HMOR after adsorption at 100 °C with (8000 ppm) toluene/N2, desorption in N2 flow; heating rate 10 °C min1.

A

Relative mass

1.00

a b c

NaMOR Cs2NaMOR Cs7NaMOR

0.96

0.92

c a b

0.88 0

200

400

600

800

Temperature (°C)

1.00

Relative mass

3.2.2. Temperature-programmed desorption Fig. 5 shows the desorption profiles of toluene chemisorbed on ZSM5 (Fig. 5A) and MOR samples (Fig. 5B). Fig. 5A highlights two factors that affect the retention of toluene in the adsorbent. First, the effect of the Al content on the HZSM5 samples. Profile (b) corresponds to a sample with lower Si/Al ratio (=15) which retains toluene up to 280 °C, whereas in the sample with Si/Al = 140 (profile a), the toluene disappears before reaching 190 °C. This behavior is due to the fact that the higher number of tetrahedral aluminum increases the interactions between toluene and the zeolite structure. The presence of Na+ ions instead of proton ions in NaZSM5 (Si/ Al = 10) significantly improves the retention temperature; the toluene desorption begins at 220 °C and finishes at 500 °C (Fig. 5A, profile c). The peak width of NaZSM5 suggests overlapping adsorption sites with different interaction strengths. On the other hand, the HZSM5 samples show a lower desorption temperature, which proves the weak interaction with the protons. Chourdhary and Mantri [35] assigned this behavior to the preference of the electrons of aromatic ring with Na+ cations rather than with the protons in the surface on mesoporous materials. Fig. 5B shows the desorption profiles of toluene of samples HMOR (profile a) and NaMOR (profile b), after adsorption at 100 °C. The HMOR sample shows a single desorption peak with a maximum at around 200 °C; instead, in NaMOR the toluene desorption starts at 150 °C and shows two peaks, one with a maximum at around 180 °C, corresponding to weakly adsorbed toluene, and another at 418 °C assigned to toluene with a higher interaction energy with the structure. Yoshimoto et al. [36] studied the TPD profiles of toluene adsorbed on HNa-MFI samples and showed a large peak at relatively high temperature (ca. 287 °C), which was weakened with decreasing Na+ and increasing H+ ions. On the other hand, they reported that the heat of toluene adsorption on Na+ was in the order MOR > MFI > BEA > FAU. The retention coefficient u (Table 2) relates the amount of chemisorbed hydrocarbon that was desorbed at temperatures above 100 °C (temperature of toluene adsorption and purge of toluene physisorbed). It is noted that the u value calculated for HZSM5 (Si/Al = 15) was higher than that of NaZSM5 (Si/Al = 10) but the maximum desorption temperature was much lower. This behavior confirms that the interaction strength of toluene is higher with NaZSM5 than with H-ZSM5. The order of u values shown in Table 2 was: NaMOR > HZSM5 (Si/Al = 15) > HMOR > NaZSM5 (Si/ Al = 10) > HZSM5 (Si/Al = 140); however, the sodium form retains the hydrocarbon adsorbed at higher temperature. Fig. 6 shows the profiles of toluene desorption after adsorption with 8000 ppm of toluene/N2 at 100 °C, on the ZSM5 and MOR samples modified with 7 wt.% of cesium. Fig. 6A compares the profiles of toluene desorption of the NaZSM5 sample with and without cesium. It can be seen that the toluene is desorbed in two well-defined zones, one at a low temperature, close to 250 °C and another at a higher temperature, close to 410 °C. It can be noticed that the presence of Cs in NaZSM5 increases the amount of retained toluene at higher temperature, close to 410 °C. Fig. 6B and C compare the profiles of toluene desorption of the HZSM5 samples (Si/Al = 140 and Si/Al = 15, respectively) with and without the addition of cesium. In both cases the addition of cesium increases the maximum desorption temperature above 100 °C. In sample HZSM-5 (Si/Al = 15), the addition of cesium increases the maximum desorption temperature of toluene from 177 to 282 °C (Fig. 6C); however, the retention coefficient decreases. This negative effect could be due to the blocking of the zeolite channels by agglomerated cesium species. A similar

B

a b c

HMOR Cs14HMOR Cs14HMOR fresh

0.95

0.90

0.85 0

200

400

600

800

Temperature (°C) Fig. 8. Thermal gravimetric analysis of samples fresh and used in toluene adsorption–desorption cycles: (A) NaMOR modified with cesium and (B) HMOR modified with cesium. Conditions: 30 cm3 of air stream, temperature from 25 to 900 °C at 10 °C min1.

26

R.M. Serra et al. / Microporous and Mesoporous Materials 147 (2012) 17–29

temperature increase was observed comparing the samples NaMOR and HMOR exchange with cesium cations Fig. 6D and E, respectively. Table 2 shows the effect of the addition of different cesium loadings, 2, 7 and 19 wt.% on NaMOR samples. In a previous work, we observed that the increase in Cs content led to an increase in the retention capacity of toluene (u), which was consistent with the increased interaction of toluene when increasing the basicity of the material [9]. Fig. 7 shows the profiles of toluene desorption (adsorbed at 100 °C) on the HMOR samples modified with 2, 7 and 14 wt.% of cesium. The presence of Cs ions increases the stability of toluene adsorbed, increasing the desorption temperature with increasing cesium content. The maximum desorption temperature shifts from 223 to 251, 294 and 300 °C when the HMOR is modified with 2, 7 and 14 wt.% of cesium, respectively. On the other hand, it can be noticed that the toluene is desorbed from sample Cs14HMOR in two temperature zones, one close to 180 °C and another at 300 °C. Two desorption peaks correspond to two sites of different interaction energy: one site corresponds to toluene adsorbed on protonic sites and the other to the interaction with Cs cation sites. 3.3. Thermogravimetric analysis Fig. 8 shows the thermogravimetric profiles obtained on air flow for the CsNaMOR (Fig. 8A) and CsHMOR (Fig. 8B) samples, which were used in 10 successive cycles of toluene adsorption–desorption. All samples show a mass loss between 100 and 300 °C corresponding to desorption of water physisorbed from the zeolite framework. The Cs14HMOR and HMOR adsorbents used in the adsorption cycles (Fig. 8B) present an additional mass loss between 480 and 650 °C, which is attributed to the combustion of coke remaining in the adsorbent materials because the ignition temperature of the carbon particles in air flow is above 550 °C [37]. By XPS spectroscopy the C1s region was measured (not shown). A distinct peak observed at 280.5 eV was assigned to deposited carbon species of the graphite type which appeared after the adsorbents were used in adsorption cycles. During the temperature-programmed desorption of toluene, different reactions may occur on the surface of the protonic zeolite as in the case of the dehydrogenation of hydrocarbons, which leads to the formation of coke. Burke et al. [38] observed the formation of oligomers of carbon resulting from the cracking of aromatic hydrocarbons such as benzene and toluene in zeolite BEA. They argued

that the formation of oligomers is due to the presence of terminal hydroxyl groups on the surface of the zeolite. On the other hand, Chen Gui-mei et al. [39] demonstrated that toluene is prone to form benzylic carbocation when combined with protons supplied by acid solids, leading to a series of reactions in which coke is being formed. From Fig. 8B the amount of carbon burned with air was determined: about 0.039 mg in HMOR and 0.028 mg in Cs14HMOR, considering 1 mg of zeolite. The addition of cesium decreases the amount of carbon formed, because it decreases the number of acid sites in the material. 3.4. Molecular simulations 3.4.1. Optimization of the structure To simulate the mordenite structure [5], a cluster model was considered. For optimizing the ring in the main channel of the mordenite, the number of tetrahedra was increased to reach 12 members (12-MR). In order to evaluate the adsorption sites, the Si4+ atom was replaced by Al3+ and a hydrogen atom was added to have a neutral charge in the ring, thus obtaining an HMOR cluster. Sodium and cesium atoms were also used to maintain a neutral charge ring; NaMOR, CsNaMOR, and CsHMOR clusters were obtained, respectively.

A d= 0.97 Å

B

d= 1.99 Å

Table 3 Energy and geometrical characteristics of zeolitic clusters. Cluster

Terminal atom

Bond distance Si–O/Al–O (Å)

Bond angle Si–O–Al (degree)

Energy (kJ mol1)

Si(OH)4

H Na Cs H Na CS H Na Cs 2H 2Na 2Cs

1.680 1.640 1.680 1.665/1.738 1.658/1.736 1.669/1.735 1.667/1.842 1.653/1.827 1.650/1.780 1.660/1.810 1.667/1.817 1.654/1.820

– – – 111 113 116 128 130 134 126 141 149

– – – 8.20E6a 8.61E6a 3.88E6b 1.61E7a 1.65E7a 7.28E6b 1.60E7a 1.68E7a 7.36E6b

5Si(OH)4Al(OH)4

11Si(OH)4Al(OH)4

10Si(OH)42Al(OH)4

a b

Energy of system calculated using basis 6-31G. Energy of system calculated using basis LanL2DZ.

C

d= 3.01 Å

Fig. 9. Optimized geometry of the silanol group with different terminal cations: (A) H, (B) Na and (C) Cs.

R.M. Serra et al. / Microporous and Mesoporous Materials 147 (2012) 17–29

Table 3 shows the structural properties calculated for each of the clusters under study, such as the bond distance and the SiO–Al bond angle, in the spatial coordinates corresponding to positions of lower energy. It can be observed that an increase in the number of atoms in the cluster slightly increased the angle of Si–O–Al, although the bond distances Si–O and Al–O were not significantly changed. Fig. 9A–C shows a tetrahedron made up of Si (OH)4 with a H, Na or Cs atom, respectively. The increase in the ionic radius of the cation (M) attached to the terminal oxygen increases the bond distance between the oxygen and the cation (dOM). The values of the bond distance dOM were: 0.967, 1.992 and 3.012 Å for M = H, Na and Cs; respectively.

27

3.4.2. Determination of the interaction energy between toluene and zeolitic clusters The value of the interaction energy or energy of adsorption is a measure of the stability of the adsorbate (toluene) on the substrate surface and it was calculated using the following equation:

Eads ¼ Esubsþads  ðEsubs þ Eads Þ where Esubs+ads, Esubs, and Eads correspond to the energy of the whole system (substrate + adsorbate), energies of the substrate and adsorbate energy, respectively. In this way, a negative value of Eads means that the adsorbate is adsorbed on the surface. Fig. 10A and B shows a cluster of 12 members, formed by tetrahedral T–O–T (T = 10Si, 2Al) and 2 Na atoms (NaMOR cluster) that

A

B

Fig. 10. Interactions between toluene and a zeolite cluster consisting of 10 Si(OH)4, 2 Al(OH)4 and 2 Na atoms.

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R.M. Serra et al. / Microporous and Mesoporous Materials 147 (2012) 17–29

Table 4 Toluene adsorption energy on differents zeolitic clusters.

a

Clusters

Base

Energy (a.u.)a

Adsorption energy (kJ mol1)a

NaMOR toluene NaMOR/Tol CsNa-MOR toluene CsNaMOR/Tol CsMOR Toluene CsMOR/Tol H-MOR (Si/Al = 5) Toluene HMOR/Tol CsHMOR Toluene CsHMOR/Tol H-MOR (Si/Al = 11) Toluene H-MOR/Tol

6-31G

6427.67 271.51 -6699.55 2787.56 271.52 3059.14 2806.98 -271.52 -3078.62 6104.34 271.51 6375.83 2787.68 271.52 3059.24 6150,71 271,52 6422,24

971.43

Landl2DZ

Landl2DZ 6-31G

Landl2DZ

6-31G

a.u. (Energy) = Hartree; 1 Hartree = 2625.5 kJ mol

157.53

315.06

52.51

105.02

26.25

1

.

capacity, the defined parameter (U) is higher for the Cs exchanged samples. In order to further analyze the toluene-zeolite interactions, we performed molecular simulations. The interaction energies are higher for NaMOR when compared with the HMOR structure and further increase when Cs is exchanged, which is in line with the experimental results. Acknowledgements The authors wish to acknowledge the financial support received from UNL (CAI+D 2009: PI 55-265 and PI 68-344), CONICET (PIP 2010-190) and ANPCyT. Thanks are given to Elsa Grimaldi for the English language editing. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2011.05.016. References

interact with a molecule of toluene. Table 4 shows the values of toluene adsorption energy calculated for each cluster analyzed using DFT. Similarly, we examined the system consisting of toluene interacting with a 12-MR cluster containing cesium atoms as cation. The evaluated system showed that by increasing the cesium atom number from 1 to 2, the energy of adsorption increased from 157.53 to 315.06 kJ mol1 (Table 4). This is possible due to the increase of the interaction of the C–H groups of toluene with the oxygen atoms of the zeolite network, which takes place due to the electro affinity of Cs atoms. When we studied the HMOR clusters, it was observed that the adsorption energy values were worse compared to the NaMOR clusters, (+52.51 kJ mol1) and (971.43 kJ mol1), respectively. Similar results were obtained when an atom of Cs was introduced in both samples, CsHMOR (105.02 kJ mol1) and CsNaMOR (157.53 kJ mol1), in agreement with the experimental results. 4. Conclusions Both adsorption capacity and thermal stability for the toluene adsorption in zeolites are strongly determined by the zeolite structure and the exchanged cation. The one-dimensional structure of mordenite appears to be more convenient than the threedimensional ZSM5 structure because, in general, mordenite samples have better adsorption properties than ZSM5 ones. As a matter of fact, the parent samples show the following adsorption capacity order: NaMOR > HMOR > NaZSM-5 > HZSM-5. However, the thermal stability of the adsorbed toluene, is more influenced by the exchanged cation than by the zeolite structure. This effect is evidenced by the desorption peaks in the TPD experiments, which developed at higher temperatures in the case of the Na samples. The overall basicity of the samples is determined by the exchanged cation, which in turn develops Lewis acidity. The Cs exchange increases the zeolitic network oxygen charge (doxygen) which determines the overall basicity. Besides, the Cs loading decreases the surface area and pore volume of the zeolites, thus decreasing the available sites for toluene adsorption. For a low Cs loading (2 wt.%), the adsorption capacity remains constant, but for higher loadings it decreases. In order to separate the effects of the nature of the exchanged cation and zeolite channels block, we calculated the amount of toluene retained at temperatures higher that 100 °C, which is a measure of the toluene adsorption capacity at these temperatures. Despite the lower adsorption

[1] Ching-Hieu Wang, Chemosphere 55 (2004) 11–17. [2] J.M. López, M.V. Navarro, T. García, R. Murillo, A.M. Mastral, F.J. Varela-Gandía, D. Lozano-Castelló, A. Bueno-López, D. Cazola-Amorós, Microporous and Mesoporous Materials 130 (2010) 239–247. [3] Z. Sarshar, M.H. Zahedi-Niaki, Q. Huang, M. Eic´, S. Kaliaguine, Applied Catalysis B: Environmental 87 (2009) 37–45. [4] A. Iliyas, M.H. Zahedi-Niaki, M. Ei’c, S. Kaliaguine, Microporous and Mesoporous Materials 102 (2007) 171–177. [5] D.W. Breck, Zeolite Molecular Sieves: Structure Chemistry and Use, John Wiley &Sons, New York, 1974. [6] D. Barthomeuf, Catalysis Reviews 38 (4) (1996) 521–612. [7] P.J. Wesson, R.Q. Snur, Microporous and Mesoporous Materials 125 (2009) 35– 38. [8] S.P. Elangovan, M. Ogura, S. Ernst, M. Hartamann, S. Tontisirin, M.E. Davis, T. Okubo, Microporous and Mesoporous Materials 96 (2006) 210–215. [9] R.M. Serra, E.E. Miró, M.K. Sapag, A.V. Boix, Microporous and Mesoporous Materials 138 (2011) 102–109. [10] B. Mentzen, G. Bergeret, H. Emerich, H.P. Webe, Journal of Physical Chemistry B 110 (2006) 97–106. [11] L.A. García-Serrano, C.A. Flores-Sandoval, I.P. Zaragoza, Journal of Molecular Catalysis A: Chemistry 200 (2003) 205–212. [12] X. Rozanska, R.A. van Santen, F. Hutschka, J. Hafner, Journal of Catalysis 215 (2003) 20–29. [13] E. Kukulska-Zajac, P. Kozyra, J. Datka, Applied Catalysis A: General 307 (2006) 46–50. [14] B. Lippens, B. Linsen, J. de Boer, Journal of Catalysis 3 (1964) 32–37. [15] M.L. Mignoni, D.I. Petkowicz, N.R.C. Fernandez Machado, S.B.C Pergher, Applied Clay Science 41 (2008) 99–104. [16] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.Ochterski, P.Y. Ayala, K. Morokuma, G.A.Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT, 2004. [17] A.D. Becke, Journal of Chemical Physical 98 (1993) 5648–5652. [18] C. Lee, W. Yang, R.G. Parr, Journal of Physical Review B 37 (1988) 785–789. [19] W.J. Mortier, Journal of Physical Chemistry 81 (1977) 1334–1338. [20] D. Kaucky´, A. Vondrová, J. Dedecek, B. Wichterlová, Journal of Catalysis 194 (2000) 318–329. [21] E.R. Nightingale Jr., Journal of Physical Chemical 63 (1959) 1381–1387. [22] Z. Hu, L. Wei, J. Dong, Y. Wang, S. Chen, S. Peng, Microporous and Mesoporous Materials 28 (1999) 49–55. [23] P. Concepción-Heydorn, C. Jia, D. Herein, N. Pfänder, H.G. Karge, F.C. Jentoft, Journalof Molecular Catalysis A: Chemical 162 (2000) 227–246. [24] A. Band, A. Albu-Yaron, T. Livnch, H. Cohen, Y. Feldman, L. Shimon, R. PopovitzBiro, V. Lyahovitskaya, R. Tenne, Journal of Physical Chemistry B 108 (2004) 12360–12367.

R.M. Serra et al. / Microporous and Mesoporous Materials 147 (2012) 17–29 [25] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catalysis Today 41 (1998) 207–219. [26] B.F. Mentzen, P. Gélin, Material Research Bulletin 33 (1998) 109–116. [27] R.R. Malherbe, R. Wendelbo, Thermochimica Acta 400 (2003) 165–173. [28] V. Choudhary, K. Srinivasan, Journal of Catalysis 102 (1986) 328–337. [29] V. Choudhary, K. Srinivasan, A. Singh, Zeolites 10 (1990) 16–20. [30] R.M. Serra, E.E. Miró, A.V. Boix, Microporous and Mesoporous Materials 127 (2010) 182–189. [31] B.L. Su, V. Norberg, Colloids and Surfaces A: Physicochemistry Engineering Aspects 187–188 (2001) 311–318. [32] D. Barthomeuf, Microporous and Mesoporous Materials 66 (2003) 1–14. [33] U.D. Joshi, P.N. Joshi, S.S. Tamhankar, V.V. Joshi, C.V. Rode, V.P. Shiralkar, Applied Catalysis A: General 239 (2003) 209–220.

29

[34] B. Su, V. Norberg, J.A. Marten, Microporous and Mesoporous Materials 25 (1998) 151–167. [35] V.R. Choudhary, K. Mantri, Microporous and Mesoporous Materials 46 (2001) 47–55. [36] R. Yoshimoto, K. Hara, K. Okumura, N. Katada, M. Niwa, Journal of Physical Chemistry C 111 (2007) 1474–1479. [37] V.G. Milt, M.A. Ulla, E.E. Miró, in: A. Gil, S.A. Korili (Eds.), Recent Research Developments in Environmental Technology, Trans world Research Network, 2008, p. 57. [38] N.R. Burke, D.L. Trimma, R.F. Howe, Applied Catalysis B: Environmental 46 (2003) 97–104. [39] C. Gui-mei, Z. Xiang-wen, M. Zhen-tao, Journal of Fuel Chemistry and Technology 35 (2007) 211–216.