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Liquid chromatography method for quantification of surface connected mesoporosity in ultrastable Y zeolites
Microporous and Mesoporous Materials 100 (2007) 6–11 www.elsevier.com/locate/micromeso
Liquid chromatography method for quantification of surface connected mesoporosity in ultrastable Y zeolites L. Teyssier a, M. Thomas a, C. Bouchy a, J.A. Martens b, E. Guillon b
a,*
a Institut Franc¸ais du Pe´trole, B.P. no. 3, 69390 Vernaison, France Center for Surface Chemistry and Catalysis, K.U.Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
Received 9 May 2006; received in revised form 22 September 2006; accepted 28 September 2006 Available online 15 November 2006
Abstract Liquid phase breakthrough experiments were performed in order to characterize the accessible mesoporosity of a dealuminated Y zeolite. The methodology presented herein shows that the use of a bulky molecular probe such as 1,3,5-triisopropylcyclohexane allows discrimination between the occluded mesoporosity accessible via the micropores only, and mesopores connected to the external surface of the zeolite crystals. The results, compared to nitrogen physisorption and mercury intrusion data, show that in commercial ultrastable Y CBV780 zeolite mesoporosity is mainly connected to the surface of the zeolite crystals. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Chromatographic breakthrough curve; Dealuminated ultrastable Y zeolite; Mesoporosity; Nitrogen adsorption; Mercury intrusion
1. Introduction Acidic microporous materials, and in particular Y type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and durability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary pore network inside individual zeolite crystals composed of mesopores measuring from 2 to 50 nm. In view of catalytic applications, it is essential to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3]. Common techniques for characterization of mesoporosity such as N2 physisorption [4], Hg porosimetry [5] and catalytic test reactions [6] give little information on
*
Corresponding author. Tel.: +33 4 78 02 28 75; fax: +33 4 78 02 20 66. E-mail address: [email protected] (E. Guillon).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.09.044
the accessibility of the mesopores. The connectivity of mesopores in individual crystals can be estimated with 3D transmission electron microscopy [2]. In this paper we show that breakthrough experiments using probe molecules can be used to quantify the accessible mesopore volume in ultrastable Y zeolites.
2. Experimental 2.1. Materials Two Y zeolite samples (Y CBV300 and Y CBV780, with a Si/Al global molar ratio of 2.5 and 40, respectively) were provided by Zeolyst International. Zeolite Y CBV300 (labelled NaY in this paper) is a Na–Y zeolite which has been ammonium-exchanged. This sample underwent three additional NaNO3 exchanges, so that it can be considered as a pure Na form. Zeolite Y CBV780 (labelled HUSY) is prepared by two steamings of zeolite Y CBV300, followed by a mineral acid leaching [7]. This sample was used in its H form. For the two samples, the zeolite powder was pelletised without binder by compaction under a pressure of ca.
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
17 MPa and then crushed. About 10 g of 315–500 lm particle fraction was used for the liquid phase experiments and the nitrogen adsorption and mercury intrusion measurements. 2.2. Characterization Nitrogen physisorption isotherms were recorded at 77 K on a Micromeritics ASAP 2405 apparatus. Prior to measurements, samples were outgassed under 105 bars at 773 K for 12 h. Mercury intrusion was performed on a Micromeritics Autopore IV 9500 apparatus. Prior to measurements, samples were dried at least 2 h at 523 K. For the electron microscopy experiments a tilt series of about 100 images was taken from +67° to 67° carried out on a Tecnai 20G microscope. Gold beads were used as reference markers for the alignment of the data set during the 3D reconstruction.
2.4. Liquid phase breakthrough experiments The experimental setup is shown in Fig. 1. The dimensions of the stainless-steel column used are 20 cm in length and 1 cm of internal diameter. The column was filled with the zeolite sample, which was then dried overnight at 723 K under a nitrogen flow in a tubular furnace. The chromatographic experiments were performed in an HP 5890 chromatograph at 423 K and 1.2 MPa pressure. Solvent and probe were injected by two independent pumps each providing a flow rate of 0.5 cm3/min. Switching between solvent and probe was done with a 4-way valve. The effluents were collected every minute (collect time: 1 min) in 80 sampling vials. The vials were analysed on an Agilent 6890N-GC equipped with a FFAP column. Experimental uncertainty was determined by carrying out similar breakthrough experiments twice or thrice. Good reproducibility was obtained: uncertainty was below 3%.
2.3. Dimensions of the molecular probes
3. Results and discussion
The dimensions of the different molecular probes used in this study are listed in Table 1. For measuring these dimensions, the molecule geometry was optimized by universal force field (UFF) via the Materials Studio interface marketed by Accelrys. The molecular dimensions were obtained from the shadow of the three-dimensional molecule projected onto a plane according to the method of Rohrbaugh and Jurs [8]. A molecular probe is considered not to penetrate into a cylindrical pore if two of its dimensions are greater than the pore diameter [9].
3.1. Characterization of Y zeolites
Table 1 ˚ ) of the molecular probes Molecular dimensions (A Molecule
x
y
z
n-Heptane n-Octane Toluene p-Xylene Norbornane Adamantane 1,3,5-Triisopropylcyclohexane
4.2 4.2 4.2 4.2 5.9 7.3 6.5
4.6 4.8 6.7 6.7 6.7 7.4 11.1
11.8 13.0 8.3 9.2 6.7 7.1 10.8
The nitrogen isotherms of NaY and HUSY samples prepared as described in Section 2.1 are shown in Fig. 2. Sample NaY has a type I isotherm, characteristic of purely microporous materials. The irreversibility of the isotherm at very high relative pressure (P/P0 > 0.9) could be assigned to the adsorption of nitrogen in the interparticular void generated by the agglomeration of the zeolite crystals. Sample HUSY shows a clearly visible hysteresis loop (type IV isotherm), indicating the presence of mesopores in this material. The BET area SBET, the micropore volume Vl (pore size below 2 nm), the mesopore volume Vm (pore size between 2 and 50 nm), the macropore volume VM (pore size between 50 and 1000 nm) and the interparticular volume VI (defined as the void between the sample particles within the column) are reported in Table 2. The interparticular volume VI was calculated according to the following expression: eI ¼
VI VCVP VP q ¼ ¼1 ¼1 B VC VC VC qP
PI pump
4 way valve
feed
column
BPR
PI pump BPR
solvent
7
oven
vials thermostated sampling bath
waste
Fig. 1. Experimental setup. BPR: backpressure regulator; PI: pressure indicator.
8
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
0.55
NaY
Adsorbed volume (cm3/g)
0.50
HUSY
0.45 0.40 0.35 0.30 0.25 0.20 0
0.2
0.4
0.6
0.8
1
P/P0 Fig. 2. Nitrogen physisorption isotherms at 77 K of NaY and HUSY samples. Table 2 BET specific surface area, micropore, mesopore, macropore and interparticular volumes determined by nitrogen physisorption and mercury intrusion of samples NaY and HUSY Sample
BET area (m2/g)
Microporous volume Vl (cm3/g)a
Mesoporous volume Vm (cm3/g)b
Macroporous volume VM (cm3/g)c
Interparticular volume VI (cm3/g)d
NaY HUSY
783 772
0.33 0.29
0.02e 0.19
0.51 0.68
0.98 0.95
a b c d e
t-Plot method. N2 adsorbed volume at P/P0: 0.96 minus the micropore volume. Determined by Hg intrusion. Determined by low pressure Hg intrusion. Interparticular mesoporosity.
with eI the interparticular porosity (void fraction of the bed), VI the interparticular volume (cm3), VC the column volume (cm3), VP the particle volume (cm3), qP the particle density (g/cm3) obtained by low pressure Hg intrusion, and qB the bulk density (g/cm3). qP was corrected of the volume corresponding to the rearrangement of the powder in the penetrometer at low intrusion pressures. The large BET specific surface areas (783 and 772 m2/g for NaY and HUSY samples, respectively) and large micropore volumes (0.33 and 0.29 cm3/g for NaY and HUSY samples, respectively) (Table 2) reveal that the investigated zeolites are highly crystalline. As seen previously, the mesoporous volume found with sample NaY (0.02 cm3/g) stems from the adsorption of nitrogen in the interparticular void formed by the zeolite crystals. The high mesoporous volume found with sample HUSY (0.19 cm3/ g) stems from the severe dealumination treatment carried out on this material [7]. Mesopore size distribution determined by mercury intrusion (Fig. 3) confirms that sample NaY has no mesopore, as expected by the nitrogen physisorption data. For HUSY sample, mesopore diameters range from 10 to 30 nm. Compaction of the zeolite powder into pellets leads to close macroporous volumes: 0.51 and 0.68 cm3/g for samples NaY and HUSY samples, respectively. However, whereas the macroporous volume in
NaY sample exclusively stems from the compaction process, part of the macroporous volume in HUSY sample could stem from the dealumination treatment carried out on this zeolite, leading to a crystal agglomeration and thus explaining the somewhat higher macroporous volume of HUSY sample compared to NaY sample. The interparticular volumes are very close (0.98 and 0.95 cm3/g for NaY and HUSY samples, respectively) since the same particle size fraction was introduced in the column. A 3D-TEM slice of a zeolite crystal is shown in Fig. 4. Mesopores are visualized as bright spots. The mesopores estimated width is in good agreement with the pore size distribution determined by mercury intrusion (10–30 nm, see Fig. 3). One cylindrical mesopore connected to the external surface of the crystal can be observed as a brighter line at the right side of the particle. For the other bright spots visualizing mesopores the TEM picture does not provide evidence for connection to the exterior. 3.2. Liquid phase breakthrough experiments The methodology for determining the accessible mesoporosity presented in this paper is based on the exploitation of chromatographic breakthrough curves of probe molecules [10,11]. The first moment l of the chromatographic
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
9
0.5
NaY HUSY
dV/dLogV (cm3/g)
0.4
0.3
0.2
0.1
0.0 1
10 Mean pore size (nm)
100
Fig. 3. Pore size distribution of NaY and HUSY samples determined by Hg intrusion.
Fig. 4. 3D-TEM slice of a crystal of HUSY sample.
signal depends on the accessible porosity. For a step response of a breakthrough experiment, the first moment l is calculated as Z 1 C l¼ 1 dt C0 0 where C0 is the inlet concentration and C the outlet concentration of the probe molecule. Knowing the flow rate and the first moment, the eluted volume can be calculated. Using the chromatographic method we attempted determination of the total porous volume (VI + VM + Vm + Vl) of the samples. For this purpose, the molecular probe has to fulfil the following conditions: (i) its size should allow to enter the micropores (no steric exclusion) and (ii) it must be able to easily replace the molecules of the solvent on the adsorption sites. For this latter condition, the molecular probe must have, at least, the same adsorption properties of the solvent. Table 3 gathers results obtained with different potential solvent/feed couples fulfilling these conditions, that is n-alkane/aromatics or aromatics/aromatics couples for instance. For sample HUSY, it can be seen that
the experimental eluted volumes are (i) very close and (ii) smaller than the total porous volume (2.11 cm3/g) calculated with nitrogen and mercury data of Table 2. The fact that the eluted volumes are close the ones compared to the others, whereas the molecules couples are quite different in nature, confirms the validity of the method. The eluted volumes are smaller than the calculated one because alkanes and aromatics molecules do not pack within the Y zeolite supercages in the efficient way nitrogen does (this will be referred as ‘‘packing effect’’ in the following. Note that for the sake of a more simple discussion, the packing effect will be assumed to be the same for all the probe molecules employed in the following, which is certainly not the case for instance for two molecules such as p-xylene and 1,3,5triisopropylcyclohexan). Since the toluene/p-xylene couple gives the eluted volume the closest to the calculated one for HUSY sample, this couple was used to determine the total porous volume of NaY sample (Table 3). For the same reason given above, the eluted volume (1.57 cm3/g) found for NaY sample is smaller than the volume calculated with nitrogen adsorption and mercury intrusion data (1.84 cm3/g). Note that the contribution of the packing effect leads for the two samples to an eluted volume equals to about 90% of the volume determined by nitrogen and mercury data. Our goal was to characterize the externally accessible mesopore volume Vem (Vm = Vem + Vim, with Vim the ‘‘internal mesoporous’’ volume, that is the volume located in the mesoporous cavities only accessible via the micropores) of sample HUSY through determination of the sum (VI + VMVem). For this determination the molecular probe must not penetrate into the micropores, either by steric restrictions or because of a weaker adsorption strength compared to the solvent molecules. Therefore, we first carried out experiments on the NaY sample to know if the chosen molecular probes, namely norbornane, adamantane and 1,3,5-triisopropylcyclohexane (Fig. 5), could penetrate into the micropores of Y zeolite.
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L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
Table 3 Determination of total porous volume of NaY and HUSY samples Sample
Solvent/feed
(VI + VM + Vm + Vl)a (cm3/g)
Experimental eluted volume (cm3/g)
NaY
Toluene/pxylene
1.84
1.57 ± 0.03
HUSY
n-Heptane/ toluene n-Heptane/ n-octane Toluene/pxylene p-Xylene/ toluene
2.11
1.88 ± 0.01 1.85 ± 0.01 1.91 ± 0.01 1.88 ± 0.01
a As calculated with nitrogen adsorption and mercury intrusion data of Table 2.
Fig. 5. Representation of norbornane (1), adamantane (2) and 1,3,5triisopropylcyclohexane (3).
For sample NaY, the (VI + VM + Vem) volume to be determined is expected to be situated between 1.49 cm3/g (VI + VM) and 1.51 cm3/g (VI + VM + Vm) according to values of Table 2. The eluted volumes found with the three hindered probe molecules norbornane, adamantane and 1,3,5-triisopropylcyclohexane are still smaller than the volume determined with nitrogen adsorption and mercury intrusion data (see Table 4), due to the packing effect reported above. The eluted volumes found with adamantane (1.32 cm3/g) and 1,3,5-triisopropylcyclohexane (1.35 cm3/g) are very close, whereas the eluted volume found with norbornane is larger (1.43 cm3/g). Provided the molecular dimensions reported in Table 1, this clearly shows that norbornane penetrates into the Y zeolite ˚ ), whereas adamantane micropores (pore diameter of 7.4 A
and 1,3,5-triisopropylcyclohexane do not in the chosen experimental conditions. Again, the eluted volume found with adamantane and 1,3,5-triisopropylcyclohexane represents about 90% of the volume determined with nitrogen and mercury data, resulting from the packing effect. For sample HUSY, the (VI + VM + Vem) volume to be determined is expected to be situated between 1.63 cm3/g (VI + VM) and 1.82 cm3/g (VI + VM + Vm) according to values of Table 2. Table 4 shows that the eluted volume decreases as the molecular dimensions of the molecular probe increase (see Table 1): 1.80, 1.77 and 1.63 cm3/g for norbornane, adamantane and 1,3,5-triisopropylcyclohexane, respectively. Note that the uncertainty found with 1,3,5-triisopropylcyclohexane is higher than the one found with the other probe molecules because the former was used more diluted than the latters, resulting in well less defined breakthrough curves. Taking heed of the packing effect, the eluted volumes found with norbornane and adamantane are out of the volume range calculated with nitrogen and mercury data, that is larger than 1.82 cm3/g. For norbornane, this result is coherent with the results obtained on NaY sample (Table 4), since norbornane has access to micropores. For adamantane, experiments carried out on the NaY sample (Table 4) show that this probe molecule can not enter in the micropores of this zeolite. Thus results obtained on the HUSY suggest that adamantane penetrates into the micropores of this dealuminated zeolite. This could be explained by a partial destruction of the zeolite ˚ diameter) during the dealumination treatsupercages (13 A ment, facilitating the diffusion of adamantane in the HUSY micropores, and thus increasing the eluted volume. Results obtained with 1,3,5-triisopropylcyclohexane on the HUSY sample are very interesting, since if one corrects the eluted volume with the packing effect and if one takes heed of the experimental uncertainty, an experimental volume range between 1.77 and 1.86 cm3/g is found. Comparing this result with the volume range determined with nitrogen and mercury data, this leads to the conclusion that more than ca. 74% ((1.77–1.63)/0.19, 0.19 being the mesoporous volume determined by nitrogen adsorption) of the mesoporous volume is located in the cylindrical mesopores connected to the external surface of the HUSY crystals. This high percentage is in good agreement with the observations of individual ultrastable Y zeolite crystals with 3D TEM in
Table 4 Determination of externally accessible mesoporous volume of NaY and HUSY samples Sample
Solvent/feeda
(VI + VM + Vem)b (cm3/g)
Experimental eluted volume (cm3/g)
NaY
Toluene/10% norbornane Toluene/7.5% adamantane Toluene/5% triisopropylcyclohexane
1.49 < V < 1.51
1.43 ± 0.01 1.32 ± 0.01 1.35 ± 0.03
HUSY
Toluene/10% norbornane Toluene/7.5% adamantane Toluene/5% triisopropylcyclohexane
1.63 < V < 1.82
1.80 ± 0.01 1.77 ± 0.02 1.63 ± 0.04
a b
The percentage represents the dilution level (wt.%) of the feed in the solvent. As calculated with nitrogen adsorption and mercury intrusion data of Table 2.
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
the work by Janssen et al. [2], who found that ca. 71% of the mesoporous volume is located in the cylindrical mesopores of the Y CBV780 zeolite. However, it is not excluded that the partial destruction of the supercages phenomenon that was pointed out in the case of adamantane also occurs in the case of 1,3,5-triisopropylcyclohexane, which would lead to a slight overestimation of the calculated percentage of cylindrical mesopores. 4. Conclusions Chromatographic breakthrough experiments in liquid phase were used to characterize porosity of Y zeolite samples. Determination of total porous volume of samples was achieved with the use of the probe molecule p-xylene, since this molecule can penetrate into the Y zeolite micropores. When the dimensions of the probe molecule increased, the eluted volume decreased, reflecting steric exclusion phenomena. Discrimination between accessible and occluded mesoporosity in ultrastable Y zeolite could be achieved via breakthrough experiments with the probe 1,3,5-triisopropylcyclohexane. Using this technique it was found that more than ca. 74% of mesoporous volume of commercial ultrastable Y zeolite CBV780 is externally accessible. The method may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores.
11
Acknowledgments The authors are grateful to Dr. Fanny Tihay (IFPLyon), the Institut de Ge´ne´tique et de Biologie Mole´culaire et Cellulaire (IGBMC) and the Institut de Physique et de Chimie des Mate´riaux de Strasbourg (IPCMS) for the 3D-MET image, and to Dr. Mathieu Digne (IFP-Lyon) for the calculation of the molecular dimensions. JAM acknowledges the Flemish Government for a concerted research action (GOA). References [1] A. Corma, V. Forne´s, F. Rey, Appl. Catal. 59 (1990) 267. [2] A.H. Janssen, A.J. Koster, K.P. de Jong, J. Phys. Chem. B 106 (2002) 11905. [3] A. Corma, Stud. Surf. Sci. Catal. 49 A (1989) 49. [4] S.W. Sing, R.T. Williams, Part. Part. Syst. Charact. 21 (2004) 71. [5] C.A. Leo´n y Leo´n, Adv. Colloid Interface Sci. 76-77 (1998) 341. [6] J. Martens, P. Jacobs, J. Mol. Catal. 78 (1993) L47. [7] M.J. Remy, D. Stanica, G. Poncelet, E.J.P. Feijen, P.J. Grobet, J.A. Martens, P.A. Jacobs, J. Phys. Chem. 100 (1996) 12440. [8] R.H. Rohrbaugh, P.C. Jurs, Anal. Chim. Acta 199 (1987) 99. [9] C.E. Webster, R.S. Drago, M.C. Zerner, J. Am. Chem. Soc. 120 (1998) 5509. [10] Y.H. Ma, Y.S. Lin, AIChE Symp. Ser. 259 (83) (1987) 1. [11] D.M. Ruthven, J. Ka¨rger, Diffusion in Zeolites and Other Microporous Solids, John Wiley & Sons, 1992.
Liquid chromatography method for quantification of surface connected mesoporosity in ultrastable Y zeolites L. Teyssier a, M. Thomas a, C. Bouchy a, J.A. Martens b, E. Guillon b
a,*
a Institut Franc¸ais du Pe´trole, B.P. no. 3, 69390 Vernaison, France Center for Surface Chemistry and Catalysis, K.U.Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
Received 9 May 2006; received in revised form 22 September 2006; accepted 28 September 2006 Available online 15 November 2006
Abstract Liquid phase breakthrough experiments were performed in order to characterize the accessible mesoporosity of a dealuminated Y zeolite. The methodology presented herein shows that the use of a bulky molecular probe such as 1,3,5-triisopropylcyclohexane allows discrimination between the occluded mesoporosity accessible via the micropores only, and mesopores connected to the external surface of the zeolite crystals. The results, compared to nitrogen physisorption and mercury intrusion data, show that in commercial ultrastable Y CBV780 zeolite mesoporosity is mainly connected to the surface of the zeolite crystals. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Chromatographic breakthrough curve; Dealuminated ultrastable Y zeolite; Mesoporosity; Nitrogen adsorption; Mercury intrusion
1. Introduction Acidic microporous materials, and in particular Y type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and durability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary pore network inside individual zeolite crystals composed of mesopores measuring from 2 to 50 nm. In view of catalytic applications, it is essential to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3]. Common techniques for characterization of mesoporosity such as N2 physisorption [4], Hg porosimetry [5] and catalytic test reactions [6] give little information on
*
Corresponding author. Tel.: +33 4 78 02 28 75; fax: +33 4 78 02 20 66. E-mail address: [email protected] (E. Guillon).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.09.044
the accessibility of the mesopores. The connectivity of mesopores in individual crystals can be estimated with 3D transmission electron microscopy [2]. In this paper we show that breakthrough experiments using probe molecules can be used to quantify the accessible mesopore volume in ultrastable Y zeolites.
2. Experimental 2.1. Materials Two Y zeolite samples (Y CBV300 and Y CBV780, with a Si/Al global molar ratio of 2.5 and 40, respectively) were provided by Zeolyst International. Zeolite Y CBV300 (labelled NaY in this paper) is a Na–Y zeolite which has been ammonium-exchanged. This sample underwent three additional NaNO3 exchanges, so that it can be considered as a pure Na form. Zeolite Y CBV780 (labelled HUSY) is prepared by two steamings of zeolite Y CBV300, followed by a mineral acid leaching [7]. This sample was used in its H form. For the two samples, the zeolite powder was pelletised without binder by compaction under a pressure of ca.
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
17 MPa and then crushed. About 10 g of 315–500 lm particle fraction was used for the liquid phase experiments and the nitrogen adsorption and mercury intrusion measurements. 2.2. Characterization Nitrogen physisorption isotherms were recorded at 77 K on a Micromeritics ASAP 2405 apparatus. Prior to measurements, samples were outgassed under 105 bars at 773 K for 12 h. Mercury intrusion was performed on a Micromeritics Autopore IV 9500 apparatus. Prior to measurements, samples were dried at least 2 h at 523 K. For the electron microscopy experiments a tilt series of about 100 images was taken from +67° to 67° carried out on a Tecnai 20G microscope. Gold beads were used as reference markers for the alignment of the data set during the 3D reconstruction.
2.4. Liquid phase breakthrough experiments The experimental setup is shown in Fig. 1. The dimensions of the stainless-steel column used are 20 cm in length and 1 cm of internal diameter. The column was filled with the zeolite sample, which was then dried overnight at 723 K under a nitrogen flow in a tubular furnace. The chromatographic experiments were performed in an HP 5890 chromatograph at 423 K and 1.2 MPa pressure. Solvent and probe were injected by two independent pumps each providing a flow rate of 0.5 cm3/min. Switching between solvent and probe was done with a 4-way valve. The effluents were collected every minute (collect time: 1 min) in 80 sampling vials. The vials were analysed on an Agilent 6890N-GC equipped with a FFAP column. Experimental uncertainty was determined by carrying out similar breakthrough experiments twice or thrice. Good reproducibility was obtained: uncertainty was below 3%.
2.3. Dimensions of the molecular probes
3. Results and discussion
The dimensions of the different molecular probes used in this study are listed in Table 1. For measuring these dimensions, the molecule geometry was optimized by universal force field (UFF) via the Materials Studio interface marketed by Accelrys. The molecular dimensions were obtained from the shadow of the three-dimensional molecule projected onto a plane according to the method of Rohrbaugh and Jurs [8]. A molecular probe is considered not to penetrate into a cylindrical pore if two of its dimensions are greater than the pore diameter [9].
3.1. Characterization of Y zeolites
Table 1 ˚ ) of the molecular probes Molecular dimensions (A Molecule
x
y
z
n-Heptane n-Octane Toluene p-Xylene Norbornane Adamantane 1,3,5-Triisopropylcyclohexane
4.2 4.2 4.2 4.2 5.9 7.3 6.5
4.6 4.8 6.7 6.7 6.7 7.4 11.1
11.8 13.0 8.3 9.2 6.7 7.1 10.8
The nitrogen isotherms of NaY and HUSY samples prepared as described in Section 2.1 are shown in Fig. 2. Sample NaY has a type I isotherm, characteristic of purely microporous materials. The irreversibility of the isotherm at very high relative pressure (P/P0 > 0.9) could be assigned to the adsorption of nitrogen in the interparticular void generated by the agglomeration of the zeolite crystals. Sample HUSY shows a clearly visible hysteresis loop (type IV isotherm), indicating the presence of mesopores in this material. The BET area SBET, the micropore volume Vl (pore size below 2 nm), the mesopore volume Vm (pore size between 2 and 50 nm), the macropore volume VM (pore size between 50 and 1000 nm) and the interparticular volume VI (defined as the void between the sample particles within the column) are reported in Table 2. The interparticular volume VI was calculated according to the following expression: eI ¼
VI VCVP VP q ¼ ¼1 ¼1 B VC VC VC qP
PI pump
4 way valve
feed
column
BPR
PI pump BPR
solvent
7
oven
vials thermostated sampling bath
waste
Fig. 1. Experimental setup. BPR: backpressure regulator; PI: pressure indicator.
8
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
0.55
NaY
Adsorbed volume (cm3/g)
0.50
HUSY
0.45 0.40 0.35 0.30 0.25 0.20 0
0.2
0.4
0.6
0.8
1
P/P0 Fig. 2. Nitrogen physisorption isotherms at 77 K of NaY and HUSY samples. Table 2 BET specific surface area, micropore, mesopore, macropore and interparticular volumes determined by nitrogen physisorption and mercury intrusion of samples NaY and HUSY Sample
BET area (m2/g)
Microporous volume Vl (cm3/g)a
Mesoporous volume Vm (cm3/g)b
Macroporous volume VM (cm3/g)c
Interparticular volume VI (cm3/g)d
NaY HUSY
783 772
0.33 0.29
0.02e 0.19
0.51 0.68
0.98 0.95
a b c d e
t-Plot method. N2 adsorbed volume at P/P0: 0.96 minus the micropore volume. Determined by Hg intrusion. Determined by low pressure Hg intrusion. Interparticular mesoporosity.
with eI the interparticular porosity (void fraction of the bed), VI the interparticular volume (cm3), VC the column volume (cm3), VP the particle volume (cm3), qP the particle density (g/cm3) obtained by low pressure Hg intrusion, and qB the bulk density (g/cm3). qP was corrected of the volume corresponding to the rearrangement of the powder in the penetrometer at low intrusion pressures. The large BET specific surface areas (783 and 772 m2/g for NaY and HUSY samples, respectively) and large micropore volumes (0.33 and 0.29 cm3/g for NaY and HUSY samples, respectively) (Table 2) reveal that the investigated zeolites are highly crystalline. As seen previously, the mesoporous volume found with sample NaY (0.02 cm3/g) stems from the adsorption of nitrogen in the interparticular void formed by the zeolite crystals. The high mesoporous volume found with sample HUSY (0.19 cm3/ g) stems from the severe dealumination treatment carried out on this material [7]. Mesopore size distribution determined by mercury intrusion (Fig. 3) confirms that sample NaY has no mesopore, as expected by the nitrogen physisorption data. For HUSY sample, mesopore diameters range from 10 to 30 nm. Compaction of the zeolite powder into pellets leads to close macroporous volumes: 0.51 and 0.68 cm3/g for samples NaY and HUSY samples, respectively. However, whereas the macroporous volume in
NaY sample exclusively stems from the compaction process, part of the macroporous volume in HUSY sample could stem from the dealumination treatment carried out on this zeolite, leading to a crystal agglomeration and thus explaining the somewhat higher macroporous volume of HUSY sample compared to NaY sample. The interparticular volumes are very close (0.98 and 0.95 cm3/g for NaY and HUSY samples, respectively) since the same particle size fraction was introduced in the column. A 3D-TEM slice of a zeolite crystal is shown in Fig. 4. Mesopores are visualized as bright spots. The mesopores estimated width is in good agreement with the pore size distribution determined by mercury intrusion (10–30 nm, see Fig. 3). One cylindrical mesopore connected to the external surface of the crystal can be observed as a brighter line at the right side of the particle. For the other bright spots visualizing mesopores the TEM picture does not provide evidence for connection to the exterior. 3.2. Liquid phase breakthrough experiments The methodology for determining the accessible mesoporosity presented in this paper is based on the exploitation of chromatographic breakthrough curves of probe molecules [10,11]. The first moment l of the chromatographic
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
9
0.5
NaY HUSY
dV/dLogV (cm3/g)
0.4
0.3
0.2
0.1
0.0 1
10 Mean pore size (nm)
100
Fig. 3. Pore size distribution of NaY and HUSY samples determined by Hg intrusion.
Fig. 4. 3D-TEM slice of a crystal of HUSY sample.
signal depends on the accessible porosity. For a step response of a breakthrough experiment, the first moment l is calculated as Z 1 C l¼ 1 dt C0 0 where C0 is the inlet concentration and C the outlet concentration of the probe molecule. Knowing the flow rate and the first moment, the eluted volume can be calculated. Using the chromatographic method we attempted determination of the total porous volume (VI + VM + Vm + Vl) of the samples. For this purpose, the molecular probe has to fulfil the following conditions: (i) its size should allow to enter the micropores (no steric exclusion) and (ii) it must be able to easily replace the molecules of the solvent on the adsorption sites. For this latter condition, the molecular probe must have, at least, the same adsorption properties of the solvent. Table 3 gathers results obtained with different potential solvent/feed couples fulfilling these conditions, that is n-alkane/aromatics or aromatics/aromatics couples for instance. For sample HUSY, it can be seen that
the experimental eluted volumes are (i) very close and (ii) smaller than the total porous volume (2.11 cm3/g) calculated with nitrogen and mercury data of Table 2. The fact that the eluted volumes are close the ones compared to the others, whereas the molecules couples are quite different in nature, confirms the validity of the method. The eluted volumes are smaller than the calculated one because alkanes and aromatics molecules do not pack within the Y zeolite supercages in the efficient way nitrogen does (this will be referred as ‘‘packing effect’’ in the following. Note that for the sake of a more simple discussion, the packing effect will be assumed to be the same for all the probe molecules employed in the following, which is certainly not the case for instance for two molecules such as p-xylene and 1,3,5triisopropylcyclohexan). Since the toluene/p-xylene couple gives the eluted volume the closest to the calculated one for HUSY sample, this couple was used to determine the total porous volume of NaY sample (Table 3). For the same reason given above, the eluted volume (1.57 cm3/g) found for NaY sample is smaller than the volume calculated with nitrogen adsorption and mercury intrusion data (1.84 cm3/g). Note that the contribution of the packing effect leads for the two samples to an eluted volume equals to about 90% of the volume determined by nitrogen and mercury data. Our goal was to characterize the externally accessible mesopore volume Vem (Vm = Vem + Vim, with Vim the ‘‘internal mesoporous’’ volume, that is the volume located in the mesoporous cavities only accessible via the micropores) of sample HUSY through determination of the sum (VI + VMVem). For this determination the molecular probe must not penetrate into the micropores, either by steric restrictions or because of a weaker adsorption strength compared to the solvent molecules. Therefore, we first carried out experiments on the NaY sample to know if the chosen molecular probes, namely norbornane, adamantane and 1,3,5-triisopropylcyclohexane (Fig. 5), could penetrate into the micropores of Y zeolite.
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L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
Table 3 Determination of total porous volume of NaY and HUSY samples Sample
Solvent/feed
(VI + VM + Vm + Vl)a (cm3/g)
Experimental eluted volume (cm3/g)
NaY
Toluene/pxylene
1.84
1.57 ± 0.03
HUSY
n-Heptane/ toluene n-Heptane/ n-octane Toluene/pxylene p-Xylene/ toluene
2.11
1.88 ± 0.01 1.85 ± 0.01 1.91 ± 0.01 1.88 ± 0.01
a As calculated with nitrogen adsorption and mercury intrusion data of Table 2.
Fig. 5. Representation of norbornane (1), adamantane (2) and 1,3,5triisopropylcyclohexane (3).
For sample NaY, the (VI + VM + Vem) volume to be determined is expected to be situated between 1.49 cm3/g (VI + VM) and 1.51 cm3/g (VI + VM + Vm) according to values of Table 2. The eluted volumes found with the three hindered probe molecules norbornane, adamantane and 1,3,5-triisopropylcyclohexane are still smaller than the volume determined with nitrogen adsorption and mercury intrusion data (see Table 4), due to the packing effect reported above. The eluted volumes found with adamantane (1.32 cm3/g) and 1,3,5-triisopropylcyclohexane (1.35 cm3/g) are very close, whereas the eluted volume found with norbornane is larger (1.43 cm3/g). Provided the molecular dimensions reported in Table 1, this clearly shows that norbornane penetrates into the Y zeolite ˚ ), whereas adamantane micropores (pore diameter of 7.4 A
and 1,3,5-triisopropylcyclohexane do not in the chosen experimental conditions. Again, the eluted volume found with adamantane and 1,3,5-triisopropylcyclohexane represents about 90% of the volume determined with nitrogen and mercury data, resulting from the packing effect. For sample HUSY, the (VI + VM + Vem) volume to be determined is expected to be situated between 1.63 cm3/g (VI + VM) and 1.82 cm3/g (VI + VM + Vm) according to values of Table 2. Table 4 shows that the eluted volume decreases as the molecular dimensions of the molecular probe increase (see Table 1): 1.80, 1.77 and 1.63 cm3/g for norbornane, adamantane and 1,3,5-triisopropylcyclohexane, respectively. Note that the uncertainty found with 1,3,5-triisopropylcyclohexane is higher than the one found with the other probe molecules because the former was used more diluted than the latters, resulting in well less defined breakthrough curves. Taking heed of the packing effect, the eluted volumes found with norbornane and adamantane are out of the volume range calculated with nitrogen and mercury data, that is larger than 1.82 cm3/g. For norbornane, this result is coherent with the results obtained on NaY sample (Table 4), since norbornane has access to micropores. For adamantane, experiments carried out on the NaY sample (Table 4) show that this probe molecule can not enter in the micropores of this zeolite. Thus results obtained on the HUSY suggest that adamantane penetrates into the micropores of this dealuminated zeolite. This could be explained by a partial destruction of the zeolite ˚ diameter) during the dealumination treatsupercages (13 A ment, facilitating the diffusion of adamantane in the HUSY micropores, and thus increasing the eluted volume. Results obtained with 1,3,5-triisopropylcyclohexane on the HUSY sample are very interesting, since if one corrects the eluted volume with the packing effect and if one takes heed of the experimental uncertainty, an experimental volume range between 1.77 and 1.86 cm3/g is found. Comparing this result with the volume range determined with nitrogen and mercury data, this leads to the conclusion that more than ca. 74% ((1.77–1.63)/0.19, 0.19 being the mesoporous volume determined by nitrogen adsorption) of the mesoporous volume is located in the cylindrical mesopores connected to the external surface of the HUSY crystals. This high percentage is in good agreement with the observations of individual ultrastable Y zeolite crystals with 3D TEM in
Table 4 Determination of externally accessible mesoporous volume of NaY and HUSY samples Sample
Solvent/feeda
(VI + VM + Vem)b (cm3/g)
Experimental eluted volume (cm3/g)
NaY
Toluene/10% norbornane Toluene/7.5% adamantane Toluene/5% triisopropylcyclohexane
1.49 < V < 1.51
1.43 ± 0.01 1.32 ± 0.01 1.35 ± 0.03
HUSY
Toluene/10% norbornane Toluene/7.5% adamantane Toluene/5% triisopropylcyclohexane
1.63 < V < 1.82
1.80 ± 0.01 1.77 ± 0.02 1.63 ± 0.04
a b
The percentage represents the dilution level (wt.%) of the feed in the solvent. As calculated with nitrogen adsorption and mercury intrusion data of Table 2.
L. Teyssier et al. / Microporous and Mesoporous Materials 100 (2007) 6–11
the work by Janssen et al. [2], who found that ca. 71% of the mesoporous volume is located in the cylindrical mesopores of the Y CBV780 zeolite. However, it is not excluded that the partial destruction of the supercages phenomenon that was pointed out in the case of adamantane also occurs in the case of 1,3,5-triisopropylcyclohexane, which would lead to a slight overestimation of the calculated percentage of cylindrical mesopores. 4. Conclusions Chromatographic breakthrough experiments in liquid phase were used to characterize porosity of Y zeolite samples. Determination of total porous volume of samples was achieved with the use of the probe molecule p-xylene, since this molecule can penetrate into the Y zeolite micropores. When the dimensions of the probe molecule increased, the eluted volume decreased, reflecting steric exclusion phenomena. Discrimination between accessible and occluded mesoporosity in ultrastable Y zeolite could be achieved via breakthrough experiments with the probe 1,3,5-triisopropylcyclohexane. Using this technique it was found that more than ca. 74% of mesoporous volume of commercial ultrastable Y zeolite CBV780 is externally accessible. The method may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores.
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Acknowledgments The authors are grateful to Dr. Fanny Tihay (IFPLyon), the Institut de Ge´ne´tique et de Biologie Mole´culaire et Cellulaire (IGBMC) and the Institut de Physique et de Chimie des Mate´riaux de Strasbourg (IPCMS) for the 3D-MET image, and to Dr. Mathieu Digne (IFP-Lyon) for the calculation of the molecular dimensions. JAM acknowledges the Flemish Government for a concerted research action (GOA). References [1] A. Corma, V. Forne´s, F. Rey, Appl. Catal. 59 (1990) 267. [2] A.H. Janssen, A.J. Koster, K.P. de Jong, J. Phys. Chem. B 106 (2002) 11905. [3] A. Corma, Stud. Surf. Sci. Catal. 49 A (1989) 49. [4] S.W. Sing, R.T. Williams, Part. Part. Syst. Charact. 21 (2004) 71. [5] C.A. Leo´n y Leo´n, Adv. Colloid Interface Sci. 76-77 (1998) 341. [6] J. Martens, P. Jacobs, J. Mol. Catal. 78 (1993) L47. [7] M.J. Remy, D. Stanica, G. Poncelet, E.J.P. Feijen, P.J. Grobet, J.A. Martens, P.A. Jacobs, J. Phys. Chem. 100 (1996) 12440. [8] R.H. Rohrbaugh, P.C. Jurs, Anal. Chim. Acta 199 (1987) 99. [9] C.E. Webster, R.S. Drago, M.C. Zerner, J. Am. Chem. Soc. 120 (1998) 5509. [10] Y.H. Ma, Y.S. Lin, AIChE Symp. Ser. 259 (83) (1987) 1. [11] D.M. Ruthven, J. Ka¨rger, Diffusion in Zeolites and Other Microporous Solids, John Wiley & Sons, 1992.