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BETA zeolite nanowire synthesis under non-hydrothermal conditions using carbon nanotubes as template
Carbon 42 (2004) 1941–1946 www.elsevier.com/locate/carbon
BETA zeolite nanowire synthesis under non-hydrothermal conditions using carbon nanotubes as template Cuong Pham-Huu a,*, Gauthier Wine a, Jean-Philippe Tessonnier a, Marc-Jacques Ledoux a, Severine Rigolet b, Claire Marichal b a
b
Laboratoire des Materiaux, Surfaces et Procedes pour la Catalyse, UMR 7515 CNRS, ECPM, Universite Louis Pasteur, 25, rue Becquerel, F-67087 Strasbourg cedex 02, France Laboratoire des Materiaux Mineraux, UMR 7016 CNRS, ENSCMu, 3, rue Alfred Werner, F-68093 Mulhouse cedex, France Received 25 July 2003; accepted 16 March 2004 Available online 4 May 2004
Abstract The properties of carbon nanotubes, used as a nanometric template and as a reactor for the synthesis of BETA zeolite, have been investigated. The confinement effect of the carbon nanotubes, induced by the high aspect ratio of the tubes could be effectively used for the synthesis of one-dimensional nanowire zeolitic materials under non-hydrothermal macroscopic conditions. Zeolite material is easily recovered by combustion of the nanotubes. The average sizes of the zeolite particles are about 20 nm. The BETA zeolite was successfully used as a catalyst for benzoylation of anisol. The zeolite catalyst exhibits a high activity compared to a commercial BETA, essentially due to its high external surface area. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Catalyst; C. Scanning electron microscopy; C. Transmission electron microscopy; D. Catalytic properties
1. Introduction Since their discovery among the products of arc discharge machine in 1991 by Iijima [1], carbon nanotubes have been the subject of an increasing research effort due to their unique mechanical and physical properties as summarised in the series of reviews devoted to this field [2–4]. The combination of the one-dimensional morphology and the presence of tubules with high aspect ratios leads to a peculiar behavior which has never been observed within traditional solids. Dravid [5] have reported that nickel particles located inside the tubule can withstand several months in aqua regia without damage. Gogotsi and co-authors [6] have reported the elastic behavior of water trapped inside the tubule with a large elongation amplitude. Apart from the peculiar physical properties cited above, the hollow tubule could also act as a nanometric template for solid synthesis under milder conditions which cannot occur in bulk material *
Corresponding author. Tel.: +33-3-90-24-26-75; fax: +33-3-90-2426-76/74. E-mail address: [email protected] (C. Pham-Huu). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.03.027
according to the recent reports [7–10]. It is expected that the combination of the exceptional mechanical strength of the carbon nanotube and its peculiar tubular morphology could give rise to a new kind of nanoscale engineering and the tailoring of nanomaterials which were not possible before, using conventional materials. Jacobsen et al. [11] have reported the use of confined space synthesis for the preparation of nanosized zeolites, with controlled zeolite crystal size, by classical hydrothermal synthesis using an activated charcoal template. The zeolite was recovered by controlling the combustion of the carbon template. As far as the literature results are concerned no article dealing with the synthesis of zeolite with a one-dimensional morphology and under non-hydrothermal conditions has been reported up to now. The aim of the present article is to report the first synthesis of zeolite nanowire under non-hydrothermal conditions using multi-walled carbon nanotubes (MWNTs) (one-dimensional material with extremely high aspect ratio) as a confinement medium. A new concept of nanoreactor, based on the confinement effect of this material is put forward to explain the results. The BETA zeolite which
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belongs to an interesting class of inorganic heterogeneous catalysts for petrochemical reactions, was chosen to illustrate our purpose. Using the confinement effect, pure BETA zeolite nanowires, i.e. one-dimensional zeolite, were synthesized at a temperature as low as 140 °C and under atmospheric pressure instead under hydrothermal conditions which is more complex to carry out and more costly in the scaling-up process. In addition, the final product was recovered without any need for the ultracentrifugation which is usually required with the nanosized BETA zeolite. The BETA zeolite synthesized under nonhydrothermal conditions was then tested as catalyst for the Friedel-Crafts benzoylation which is one of the most important reactions for the preparation of fine chemical intermediates [12].
2. Experimental 2.1. Multi-walled carbon nanotubes Multi-walled carbon nanotubes (MWNTs) with an average inner diameter between 50 and 80 nm and lengths up to several micrometers (average aspect ratio of approximately 30) were supplied by Applied Sciences Ltd. (Ohio, USA) and were used as received. The material had a relatively low specific surface area of 10 m2 g1 , measured by N2 absorption at liquid nitrogen temperature with a mainly mesoporous system; no trace of microporosity was observed. A representative transmission electron microscopy (TEM) image of the
MWNTs is displayed in Fig. 1. A large amount of these tubes are hollow from tip to tail even if some bamboolike or closed tubes are also observed. The tubes are relatively straight with few defects according to TEM images. It is worth noting the presence of an amorphous carbon layer on the outer surface of the tubes (inset of Fig. 1) which could be attributed to carbon formed by pyrolysis of the hydrocarbon during the tube synthesis. In some areas, the remains of the iron catalyst which was used to grow the carbon nanotubes are observed. However, these iron particles are completely encapsulated inside graphite layers and are not accessible for further catalytic activity. 2.2. BETA zeolite synthesis The BETA zeolite synthesis solution was prepared by dissolving a silica source (Aerosil 200) in an aqueous solution of tetraethylammonium hydroxide (TEAOH) at 50 °C under vigorous stirring. At the same time, an aluminium source was dissolved in a portion of TEAOH at room temperature. The aluminium solution was then added into the silica source under vigorous stirring. For the synthesis, a known amount of CNTs (5 g) was slowly added to the zeolite precursor gel (30 ml) under vigorous stirring at atmospheric and ambient conditions followed by a sonication treatment for 30 min. The resulting paste-like solid was then heated in air at 140 °C for different durations. The solid obtained was washed several times with distilled water in order to remove the unreacted soluble fraction and then heated to 550 °C in flowing argon for 2 h in order to decompose the structure directing agent (TEAOH). The carbon template was then removed by submitting the sample to a calcination in air at 650 °C for 14 h. Finally, the acidic form of the BETA zeolite was obtained by submitting the sample to a cationic exchange using a NH4 Cl solution and calcinating the recovered solid at 550 °C in air for 10 h. 2.3. Characterization techniques
Fig. 1. TEM image of the multi-walled carbon nanotubes used as nanosized mould for the synthesis of one-dimensional zeolite. (Inset: high-resolution TEM image showing the presence of an amorphous layer of carbon on the outer surface of the nanotube.)
Structural characterization of the samples was done by powder XRD. XRD measurements were carried out with a Siemens Diffractometer Model D-5000, using CuKa radiation and operated at 40 kV and 20 mA with a h=2h mode. The nature of the different phases present in the sample was checked using the database of the Joint Committee on Powder Diffraction Standards (JCPDS). Scanning electron microscopy (SEM) was carried out on a field emission scanning microscope model Jeol JSM-6700F working under low accelerating voltage, 3– 15 kV, and allowing microstructural investigations down to a few nanometers. TEM and energy dispersive spectroscopy (EDS) microanalysis were used to provide
C. Pham-Huu et al. / Carbon 42 (2004) 1941–1946
information on the location and dispersion of the active phase, together with its particle size, morphology and composition. TEM and EDS were carried out in a Topcon Model EM002B operating at 200 kV, equipped with beryllium window detectors, with a point-to-point resolution of 0.17 nm. BET surface areas were measured by means of a commercial BET unit (Coulter SA 3100, Coultronics SA) using N2 adsorption at 77 K. Before the N2 adsorption, samples were heated at 325 °C for 14 h under dynamic vacuum in order to desorb the impurities on the surface. SBET is the surface area of the sample calculated from the nitrogen isotherm using the BET method. The micropore surface area and pore volume were calculated using the t-plot method developed by de Boer [13]. 27 Al (I ¼ 5=2) magic angle spinning nuclear magnetic resonance (MAS-NMR) was carried out with a Bruker DSX 400 spectrometer operating at B0 ¼ 9:4 T (Larmor frequency m0 ¼ 104:2 MHz). A single pulse of 0.8 ls with a recycle delay of 1 s was used for all experiments. The spinning frequency was 8 kHz. Measurements were carried out at room temperature with [Al(H2 O)6 ]3þ as external standard reference.
3. Results and discussion
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present work, due to the absence of dissolution and recrystallization processes at macroscopic level, the average size of the zeolite particles is modified. The formation of zeolite was confirmed by 27 Al and 29 Si MAS-NMR techniques. The presence of zeolite was confirmed by 27 Al MAS-NMR technique (Fig. 3A). The aluminium NMR signal was visible at 50 ± 2 ppm and could be unambiguously attributed to the tetrahedral coordinated aluminium atoms of the zeolite framework in agreement with the earlier 27 Al MAS-NMR studies of zeolite samples in the literature [15]. The absence of 27 Al peak at around 0 ppm indicates that all the aluminium atoms were incorporated into the framework of the zeolite and no extra-framework aluminium was formed. The full width at half maximum (FWHM) of the 27 Al band was around 13 ppm. This value reflects a relatively low primary crystal size of the zeolite in agreement with the XRD results of the present work and those reported by Jacobsen et al. [11]. Jacobsen et al. [11] have reported a FWHM of 7.1 ppm for large ZSM-5 crystals (8 lm) whereas for small size ZSM-5 crystals (20–30 nm) the FWHM significantly increased to between 10.6 and 11.1 ppm. 29 Si MAS-NMR spectrum (Fig. 3B) presents a broad resonance in the range from )120 to )90 ppm that accounts for three components which are assigned to Si (OSi)4 , AlOSi (OSi)3 and HOSi (OSi)3 sites in the framework of the BEA structure [16].
3.1. Physical characterizations The XDR pattern only showed a very broad diffraction line located at around 23 in 2h angle (Fig. 2). The BETA zeolite particles were probably too small to be detected by means of the XRD technique. Similar XRD pattern was already reported by Jacobs et al. [14] for ZSM-5 with small crystal size (<8 nm), and by Camblor et al. [15] for nanocrystals of BETA (<10 nm). Here, the small particle size was attributed to the synthesis mode, i.e. non-hydrothermal instead of hydrothermal synthesis. The same starting gel under hydrothermal conditions gives a well crystallized BETA zeolite. In the
Al Td FWHM = 13 ppm
150
100
(A)
50
0
-50
Chemical Shift (ppm) Si*(OSi)4
AlOSi*(OSi)3
HOSi*(OSi)3
-70
5
10
15
20
25
30
35
40
Two-Theta angle (deg.) Fig. 2. XRD pattern of the as-synthesized BETA zeolite inside the MWNTs under a non-hydrothermal conditions.
(B)
-80
-90
-100
-110
-120
-130
Chemical shift (ppm)
Fig. 3. (A) 27 Al MAS-NMR and (B) 29 Si MAS-NMR spectra of the BETA zeolite encapsulated inside the MWNTs showing a resonance peak of Al in tetrahedral coordination.
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The location of the zeolite inside the MWNTs was investigated by microscopy techniques and the representative images are displayed in Fig. 4A and B. Zeolite was observed as relatively long nanowires along the MWNTs axis (Fig. 4A). The zeolite nanowires mostly filled half of the inner tubule from the tube tip to deeper inside. The high magnification TEM image (Fig. 4B) clearly shows the low contact angle between the zeolite and the tube’s inner wall which resulted from a high wetting of the inner tube wall by the zeolite gel precursor. After removal of the MWNTs template a relatively long zeolite nanowire was clearly observed as shown by the SEM image in Fig. 5A. A high magnification SEM image (Fig. 5B) shows that the zeolite nanowires formed were made up of an assembly of several small zeolite particles with an average diameter of around 10 nm (indicated by arrow) which is in good agreement with the results deduced from the NMR technique above. Statistical SEM observation has also revealed the formation of a small amount of zeolite outside the carbon nanotubes. However, the amount of zeolite formed out of the carbon nanotubes was relatively small, i.e. few
Fig. 5. (A) SEM image of the zeolite nanowires obtained after thermal treatment to remove the MWNTs template. (B) High magnification SEM image showing the individual zeolite particles which constitute the whole zeolite nanowire with an average particle size of approximately 10 nm.
Fig. 4. (A) TEM images of the zeolite encapsulated inside the MWNTs tubule after synthesis. (B) TEM image showing the low contact angle between the zeolite and the inner tube wall.
percents, which indicates the high filling of the synthesis method. The observed results clearly show the fast and easy filling of the MWNTs by the zeolite precursor aqueous solution similar to that observed with water alone according to the literature reports [7–10]. The relatively large inner diameter of the MWNTs used in the present work also probably facilitated such filling. Ugarte et al. [17] have reported that the capillarity of narrow tubes is reduced with respect to large cavity tubes. It is expected that the wetting and filling process only occurs when a perfect fit is achieved between the filling solution and the filled tube diameter. In a previous investigation, we have shown that fluids trapped inside the MWNTs exhibit unusual behavior and the liquid-to-solid transformation is significantly altered due to the existence of a confinement effect [10]. During the synthesis at 140 °C, the water contained in the zeolite gel precursor was slowly vaporized leading to the occurrence of a local hydrothermal condition inside the nanosized reactors, i.e. individual carbon nanotubes playing the role of confinement medium, which provides nanohydrothermal conditions necessary to the formation of the zeolite.
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3.2. Catalytic test The catalytic test is also a final proof to confirm the formation of BETA zeolite under our condition. The BETA nanowires were used as catalyst in the benzoylation of anisole by benzoyle chloride in a slurry mode (Fig. 6). The desired product is the p-methoxybenzophenone. The reaction activity of the BETA zeolite nanowires is depicted in Fig. 7. The results obtained, i.e. conversion and selectivity to ketone, have shown that R3
O
R2 Ph
1-2
O
OMe
R1 - MeOH
+
Cl - HCl O
O
100 H-BEA (NH) 80
Conversion (%)
The formation of zeolite nanowire inside the MWNTs tubule was accompanied by a significant increase in the specific surface area of the composite. The overall surface area was increased from 10 m2 g1 for the pristine MWNTs to about 700 m2 g1 for the sample obtained after 24 h of synthesis. There was also an increase in the micropore surface area from 0 to about 200 m2 g1 which was developed by the channel network inside the primary zeolite particles. A large part of the surface area was composed of mesopores, i.e. pores located on the outer surface area of the zeolite particles, which indicated the relatively low particle size of the synthesized zeolite. Zeolite materials are characterized by the presence of channels and cavities of molecular dimensions (<1 nm) which lead to a high microporous surface area. The relatively low microporous surface contribution was attributed to the three-dimensional network inside the zeolite particle being less ordered which was probably linked with the non-hydrothermal synthesis conditions used. These results have already been reported by Camblor et al. [15] for BETA zeolite nanocrystallites. The authors have observed a sharp decrease in the micropore volume when the zeolite crystal size was below 100 nm. The low crystallinity of the zeolite synthesized allows to explain the absence of any diffraction lines in the XRD pattern.
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40 H-BEA (H) 20
0 0
5
10
15
20
25
Time on stream (h) Fig. 7. Friedel-Crafts activity and selectivity obtained in a liquid phase over the BETA nanowires zeolite synthesized under non-hydrothermal conditions and the commercial BETA zeolite (Zeolyst International). Reaction conditions: anisole:benzoylchloride, 8:1 mol:mol, temperature: 120 °C, catalyst weight: 0.2 g.
the zeolite synthesized under non-hydrothermal conditions exhibits a relatively high conversion with an extremely high selectivity towards aromatic p-ketone. The other products, i.e. o-methoxybenzophenone and phenyl benzoate, amounted to less than 5%. It has been reported that the reduction of the crystal size of zeolite Y resulted in an increase in activity and selectivity in fluid catalytic cracking (FCC) due to the significant improvement in the diffusion of both the reactants and the products through the catalyst pores. Typically, the traditional synthesis of BETA zeolite gives nanoscopic size of crystal which forms macroscopic aggregates. The use of carbon nanotubes as matrix prevents the formation of such aggregates, and allowed as a maximum size of the zeolite, the inner diameter of the nanotube. The nanometric size combined with a relatively high aspect ratio of the nanowire-like zeolite also provided a high external surface area contact between the catalyst and the reactants compared to ordinary zeolite crystals synthesized under hydrothermal conditions. Work is ongoing to check the stability of this catalyst.
E E
4. Conclusions
1 = p – methoxybenzophenone (R1 = OMe, R2 = R3 = H) 2 = o – methoxybenzophenone (R1 = R2 = H, R3 = OMe) E = Phenyl benzoate
Fig. 6. Reaction pathway of the benzoylation of anisole by benzoyle chloride.
In summary, BETA zeolite of nanometric size was successfully synthesized by means of the confinement effect inside the MWNT tubules under non-hydrothermal synthesis conditions despite the relatively low aspect ratio of the used MWNTs, i.e. 30. The avoidance of the hydrothermal process during the synthesis is of interest for industrial scale-up processes due to the significant
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cost reduction. The high volume-to-weight ratio of the MWNTs allows the synthesis of a large amount of zeolite with a small amount of template. Using MWNTs as template also allows the easy recovery, washing and post-treatment of the zeolite when compared to the classical hydrothermal synthesis method where highspeed centrifugation is necessary to recover the zeolite material due to its colloidal aspect. The confinement effect synthesis can also be extended to the synthesis of zeolites with a non-conventional gel precursor composition and allows tailoring of the crystal size which is not an easy task using the conventional synthesis method. Acknowledgements The authors gratefully acknowledge Mrs. A.C. Faust (LMM, UMR 7016 CNRS, Mulhouse, France) and G. Ehret (IPCMS, UMR 7504 CNRS, Strasbourg, France) for SEM and TEM experiments. References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes. New York: Academic Press; 1996. [3] Ebbessen TW. Carbon nanotubes, preparation and properties. New York: CRC Press; 1996.
[4] Bonard JM, Forro L, Ugarte D, de Herr WA, Ch^atelain A. Physics and chemistry of carbon nanostructures. ECC Res 1998;1:9–16. [5] Dravid VP. Controlled-size nanocapsules. Nature 1995;374:602. [6] (a) Libera J, Gogotsi Y. Hydrothermal synthesis of graphite tubes using Ni catalyst. Carbon 2001;39:1307–18; (b) Gogotsi Y, Naguib N, Libera JA. In situ chemical experiments in carbon nanotubes. Chem Phys Lett 2002;365:354–60. [7] Koga K, Gao GT, Tanaka H, Zeng XC. Formation of ordered ice nanotubes inside carbon nanotubes. Nature 2001;412:802–5. [8] Sansom MSP, Biggin PC. Biophysics: water at the nanoscale. Nature 2001;414:156–9. [9] Gordillo MC, Marti J. Hydrogen bonding in supercritical water confined in carbon nanotubes. Chem Phys Lett 2001;341:250–4. [10] Pham-Huu C, Keller N, Estournes C, Ehret G, Ledoux MJ. Synthesis of CoFe2 O4 nanowire in carbon nanotubes. A new use of the confinement effect. Chem Commun 2002;1:1882–3. [11] Jacobsen CJH, Madsen C, Janssens TVW, Jakobsen HJ, Skibsted J. Zeolites by confined space synthesis––characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and 27Al/ 29Si MAS-NMR spectroscopy. Micropor Mesopor Mater 2000;39:393–401. [12] Derouane EG. Catalysis in the 21st century: lessons from the past, challenges for the future. CaTTech 2001;5:214–25. [13] de Boer JH. In: Everett DH, Stone FS, editors. The structure and properties of porous materials. London: Butterworth; 1958. [14] Jacobs PA, Derouane EG, Weitkamp J. Evidence for X-rayamorphous zeolites. J Chem Soc Chem Commun 1981:591–2. [15] Camblor MA, Corma A, Valencia S. Characterization of nanocrystalline zeolite Beta. Micropor Mesopor Mater 1998;25:59–74. [16] Engelhardt G, Michel D. High-resolution solid-state NMR of silicates and zeolites. Chichester: Wiley; 1987. [17] Ugarte D, Ch^atelain A, de Herr WA. Nanocapillarity and chemistry in carbon nanotubes. Science 1996;274:1897–9.
BETA zeolite nanowire synthesis under non-hydrothermal conditions using carbon nanotubes as template Cuong Pham-Huu a,*, Gauthier Wine a, Jean-Philippe Tessonnier a, Marc-Jacques Ledoux a, Severine Rigolet b, Claire Marichal b a
b
Laboratoire des Materiaux, Surfaces et Procedes pour la Catalyse, UMR 7515 CNRS, ECPM, Universite Louis Pasteur, 25, rue Becquerel, F-67087 Strasbourg cedex 02, France Laboratoire des Materiaux Mineraux, UMR 7016 CNRS, ENSCMu, 3, rue Alfred Werner, F-68093 Mulhouse cedex, France Received 25 July 2003; accepted 16 March 2004 Available online 4 May 2004
Abstract The properties of carbon nanotubes, used as a nanometric template and as a reactor for the synthesis of BETA zeolite, have been investigated. The confinement effect of the carbon nanotubes, induced by the high aspect ratio of the tubes could be effectively used for the synthesis of one-dimensional nanowire zeolitic materials under non-hydrothermal macroscopic conditions. Zeolite material is easily recovered by combustion of the nanotubes. The average sizes of the zeolite particles are about 20 nm. The BETA zeolite was successfully used as a catalyst for benzoylation of anisol. The zeolite catalyst exhibits a high activity compared to a commercial BETA, essentially due to its high external surface area. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Catalyst; C. Scanning electron microscopy; C. Transmission electron microscopy; D. Catalytic properties
1. Introduction Since their discovery among the products of arc discharge machine in 1991 by Iijima [1], carbon nanotubes have been the subject of an increasing research effort due to their unique mechanical and physical properties as summarised in the series of reviews devoted to this field [2–4]. The combination of the one-dimensional morphology and the presence of tubules with high aspect ratios leads to a peculiar behavior which has never been observed within traditional solids. Dravid [5] have reported that nickel particles located inside the tubule can withstand several months in aqua regia without damage. Gogotsi and co-authors [6] have reported the elastic behavior of water trapped inside the tubule with a large elongation amplitude. Apart from the peculiar physical properties cited above, the hollow tubule could also act as a nanometric template for solid synthesis under milder conditions which cannot occur in bulk material *
Corresponding author. Tel.: +33-3-90-24-26-75; fax: +33-3-90-2426-76/74. E-mail address: [email protected] (C. Pham-Huu). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.03.027
according to the recent reports [7–10]. It is expected that the combination of the exceptional mechanical strength of the carbon nanotube and its peculiar tubular morphology could give rise to a new kind of nanoscale engineering and the tailoring of nanomaterials which were not possible before, using conventional materials. Jacobsen et al. [11] have reported the use of confined space synthesis for the preparation of nanosized zeolites, with controlled zeolite crystal size, by classical hydrothermal synthesis using an activated charcoal template. The zeolite was recovered by controlling the combustion of the carbon template. As far as the literature results are concerned no article dealing with the synthesis of zeolite with a one-dimensional morphology and under non-hydrothermal conditions has been reported up to now. The aim of the present article is to report the first synthesis of zeolite nanowire under non-hydrothermal conditions using multi-walled carbon nanotubes (MWNTs) (one-dimensional material with extremely high aspect ratio) as a confinement medium. A new concept of nanoreactor, based on the confinement effect of this material is put forward to explain the results. The BETA zeolite which
1942
C. Pham-Huu et al. / Carbon 42 (2004) 1941–1946
belongs to an interesting class of inorganic heterogeneous catalysts for petrochemical reactions, was chosen to illustrate our purpose. Using the confinement effect, pure BETA zeolite nanowires, i.e. one-dimensional zeolite, were synthesized at a temperature as low as 140 °C and under atmospheric pressure instead under hydrothermal conditions which is more complex to carry out and more costly in the scaling-up process. In addition, the final product was recovered without any need for the ultracentrifugation which is usually required with the nanosized BETA zeolite. The BETA zeolite synthesized under nonhydrothermal conditions was then tested as catalyst for the Friedel-Crafts benzoylation which is one of the most important reactions for the preparation of fine chemical intermediates [12].
2. Experimental 2.1. Multi-walled carbon nanotubes Multi-walled carbon nanotubes (MWNTs) with an average inner diameter between 50 and 80 nm and lengths up to several micrometers (average aspect ratio of approximately 30) were supplied by Applied Sciences Ltd. (Ohio, USA) and were used as received. The material had a relatively low specific surface area of 10 m2 g1 , measured by N2 absorption at liquid nitrogen temperature with a mainly mesoporous system; no trace of microporosity was observed. A representative transmission electron microscopy (TEM) image of the
MWNTs is displayed in Fig. 1. A large amount of these tubes are hollow from tip to tail even if some bamboolike or closed tubes are also observed. The tubes are relatively straight with few defects according to TEM images. It is worth noting the presence of an amorphous carbon layer on the outer surface of the tubes (inset of Fig. 1) which could be attributed to carbon formed by pyrolysis of the hydrocarbon during the tube synthesis. In some areas, the remains of the iron catalyst which was used to grow the carbon nanotubes are observed. However, these iron particles are completely encapsulated inside graphite layers and are not accessible for further catalytic activity. 2.2. BETA zeolite synthesis The BETA zeolite synthesis solution was prepared by dissolving a silica source (Aerosil 200) in an aqueous solution of tetraethylammonium hydroxide (TEAOH) at 50 °C under vigorous stirring. At the same time, an aluminium source was dissolved in a portion of TEAOH at room temperature. The aluminium solution was then added into the silica source under vigorous stirring. For the synthesis, a known amount of CNTs (5 g) was slowly added to the zeolite precursor gel (30 ml) under vigorous stirring at atmospheric and ambient conditions followed by a sonication treatment for 30 min. The resulting paste-like solid was then heated in air at 140 °C for different durations. The solid obtained was washed several times with distilled water in order to remove the unreacted soluble fraction and then heated to 550 °C in flowing argon for 2 h in order to decompose the structure directing agent (TEAOH). The carbon template was then removed by submitting the sample to a calcination in air at 650 °C for 14 h. Finally, the acidic form of the BETA zeolite was obtained by submitting the sample to a cationic exchange using a NH4 Cl solution and calcinating the recovered solid at 550 °C in air for 10 h. 2.3. Characterization techniques
Fig. 1. TEM image of the multi-walled carbon nanotubes used as nanosized mould for the synthesis of one-dimensional zeolite. (Inset: high-resolution TEM image showing the presence of an amorphous layer of carbon on the outer surface of the nanotube.)
Structural characterization of the samples was done by powder XRD. XRD measurements were carried out with a Siemens Diffractometer Model D-5000, using CuKa radiation and operated at 40 kV and 20 mA with a h=2h mode. The nature of the different phases present in the sample was checked using the database of the Joint Committee on Powder Diffraction Standards (JCPDS). Scanning electron microscopy (SEM) was carried out on a field emission scanning microscope model Jeol JSM-6700F working under low accelerating voltage, 3– 15 kV, and allowing microstructural investigations down to a few nanometers. TEM and energy dispersive spectroscopy (EDS) microanalysis were used to provide
C. Pham-Huu et al. / Carbon 42 (2004) 1941–1946
information on the location and dispersion of the active phase, together with its particle size, morphology and composition. TEM and EDS were carried out in a Topcon Model EM002B operating at 200 kV, equipped with beryllium window detectors, with a point-to-point resolution of 0.17 nm. BET surface areas were measured by means of a commercial BET unit (Coulter SA 3100, Coultronics SA) using N2 adsorption at 77 K. Before the N2 adsorption, samples were heated at 325 °C for 14 h under dynamic vacuum in order to desorb the impurities on the surface. SBET is the surface area of the sample calculated from the nitrogen isotherm using the BET method. The micropore surface area and pore volume were calculated using the t-plot method developed by de Boer [13]. 27 Al (I ¼ 5=2) magic angle spinning nuclear magnetic resonance (MAS-NMR) was carried out with a Bruker DSX 400 spectrometer operating at B0 ¼ 9:4 T (Larmor frequency m0 ¼ 104:2 MHz). A single pulse of 0.8 ls with a recycle delay of 1 s was used for all experiments. The spinning frequency was 8 kHz. Measurements were carried out at room temperature with [Al(H2 O)6 ]3þ as external standard reference.
3. Results and discussion
1943
present work, due to the absence of dissolution and recrystallization processes at macroscopic level, the average size of the zeolite particles is modified. The formation of zeolite was confirmed by 27 Al and 29 Si MAS-NMR techniques. The presence of zeolite was confirmed by 27 Al MAS-NMR technique (Fig. 3A). The aluminium NMR signal was visible at 50 ± 2 ppm and could be unambiguously attributed to the tetrahedral coordinated aluminium atoms of the zeolite framework in agreement with the earlier 27 Al MAS-NMR studies of zeolite samples in the literature [15]. The absence of 27 Al peak at around 0 ppm indicates that all the aluminium atoms were incorporated into the framework of the zeolite and no extra-framework aluminium was formed. The full width at half maximum (FWHM) of the 27 Al band was around 13 ppm. This value reflects a relatively low primary crystal size of the zeolite in agreement with the XRD results of the present work and those reported by Jacobsen et al. [11]. Jacobsen et al. [11] have reported a FWHM of 7.1 ppm for large ZSM-5 crystals (8 lm) whereas for small size ZSM-5 crystals (20–30 nm) the FWHM significantly increased to between 10.6 and 11.1 ppm. 29 Si MAS-NMR spectrum (Fig. 3B) presents a broad resonance in the range from )120 to )90 ppm that accounts for three components which are assigned to Si (OSi)4 , AlOSi (OSi)3 and HOSi (OSi)3 sites in the framework of the BEA structure [16].
3.1. Physical characterizations The XDR pattern only showed a very broad diffraction line located at around 23 in 2h angle (Fig. 2). The BETA zeolite particles were probably too small to be detected by means of the XRD technique. Similar XRD pattern was already reported by Jacobs et al. [14] for ZSM-5 with small crystal size (<8 nm), and by Camblor et al. [15] for nanocrystals of BETA (<10 nm). Here, the small particle size was attributed to the synthesis mode, i.e. non-hydrothermal instead of hydrothermal synthesis. The same starting gel under hydrothermal conditions gives a well crystallized BETA zeolite. In the
Al Td FWHM = 13 ppm
150
100
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50
0
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Chemical Shift (ppm) Si*(OSi)4
AlOSi*(OSi)3
HOSi*(OSi)3
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5
10
15
20
25
30
35
40
Two-Theta angle (deg.) Fig. 2. XRD pattern of the as-synthesized BETA zeolite inside the MWNTs under a non-hydrothermal conditions.
(B)
-80
-90
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Chemical shift (ppm)
Fig. 3. (A) 27 Al MAS-NMR and (B) 29 Si MAS-NMR spectra of the BETA zeolite encapsulated inside the MWNTs showing a resonance peak of Al in tetrahedral coordination.
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The location of the zeolite inside the MWNTs was investigated by microscopy techniques and the representative images are displayed in Fig. 4A and B. Zeolite was observed as relatively long nanowires along the MWNTs axis (Fig. 4A). The zeolite nanowires mostly filled half of the inner tubule from the tube tip to deeper inside. The high magnification TEM image (Fig. 4B) clearly shows the low contact angle between the zeolite and the tube’s inner wall which resulted from a high wetting of the inner tube wall by the zeolite gel precursor. After removal of the MWNTs template a relatively long zeolite nanowire was clearly observed as shown by the SEM image in Fig. 5A. A high magnification SEM image (Fig. 5B) shows that the zeolite nanowires formed were made up of an assembly of several small zeolite particles with an average diameter of around 10 nm (indicated by arrow) which is in good agreement with the results deduced from the NMR technique above. Statistical SEM observation has also revealed the formation of a small amount of zeolite outside the carbon nanotubes. However, the amount of zeolite formed out of the carbon nanotubes was relatively small, i.e. few
Fig. 5. (A) SEM image of the zeolite nanowires obtained after thermal treatment to remove the MWNTs template. (B) High magnification SEM image showing the individual zeolite particles which constitute the whole zeolite nanowire with an average particle size of approximately 10 nm.
Fig. 4. (A) TEM images of the zeolite encapsulated inside the MWNTs tubule after synthesis. (B) TEM image showing the low contact angle between the zeolite and the inner tube wall.
percents, which indicates the high filling of the synthesis method. The observed results clearly show the fast and easy filling of the MWNTs by the zeolite precursor aqueous solution similar to that observed with water alone according to the literature reports [7–10]. The relatively large inner diameter of the MWNTs used in the present work also probably facilitated such filling. Ugarte et al. [17] have reported that the capillarity of narrow tubes is reduced with respect to large cavity tubes. It is expected that the wetting and filling process only occurs when a perfect fit is achieved between the filling solution and the filled tube diameter. In a previous investigation, we have shown that fluids trapped inside the MWNTs exhibit unusual behavior and the liquid-to-solid transformation is significantly altered due to the existence of a confinement effect [10]. During the synthesis at 140 °C, the water contained in the zeolite gel precursor was slowly vaporized leading to the occurrence of a local hydrothermal condition inside the nanosized reactors, i.e. individual carbon nanotubes playing the role of confinement medium, which provides nanohydrothermal conditions necessary to the formation of the zeolite.
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3.2. Catalytic test The catalytic test is also a final proof to confirm the formation of BETA zeolite under our condition. The BETA nanowires were used as catalyst in the benzoylation of anisole by benzoyle chloride in a slurry mode (Fig. 6). The desired product is the p-methoxybenzophenone. The reaction activity of the BETA zeolite nanowires is depicted in Fig. 7. The results obtained, i.e. conversion and selectivity to ketone, have shown that R3
O
R2 Ph
1-2
O
OMe
R1 - MeOH
+
Cl - HCl O
O
100 H-BEA (NH) 80
Conversion (%)
The formation of zeolite nanowire inside the MWNTs tubule was accompanied by a significant increase in the specific surface area of the composite. The overall surface area was increased from 10 m2 g1 for the pristine MWNTs to about 700 m2 g1 for the sample obtained after 24 h of synthesis. There was also an increase in the micropore surface area from 0 to about 200 m2 g1 which was developed by the channel network inside the primary zeolite particles. A large part of the surface area was composed of mesopores, i.e. pores located on the outer surface area of the zeolite particles, which indicated the relatively low particle size of the synthesized zeolite. Zeolite materials are characterized by the presence of channels and cavities of molecular dimensions (<1 nm) which lead to a high microporous surface area. The relatively low microporous surface contribution was attributed to the three-dimensional network inside the zeolite particle being less ordered which was probably linked with the non-hydrothermal synthesis conditions used. These results have already been reported by Camblor et al. [15] for BETA zeolite nanocrystallites. The authors have observed a sharp decrease in the micropore volume when the zeolite crystal size was below 100 nm. The low crystallinity of the zeolite synthesized allows to explain the absence of any diffraction lines in the XRD pattern.
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40 H-BEA (H) 20
0 0
5
10
15
20
25
Time on stream (h) Fig. 7. Friedel-Crafts activity and selectivity obtained in a liquid phase over the BETA nanowires zeolite synthesized under non-hydrothermal conditions and the commercial BETA zeolite (Zeolyst International). Reaction conditions: anisole:benzoylchloride, 8:1 mol:mol, temperature: 120 °C, catalyst weight: 0.2 g.
the zeolite synthesized under non-hydrothermal conditions exhibits a relatively high conversion with an extremely high selectivity towards aromatic p-ketone. The other products, i.e. o-methoxybenzophenone and phenyl benzoate, amounted to less than 5%. It has been reported that the reduction of the crystal size of zeolite Y resulted in an increase in activity and selectivity in fluid catalytic cracking (FCC) due to the significant improvement in the diffusion of both the reactants and the products through the catalyst pores. Typically, the traditional synthesis of BETA zeolite gives nanoscopic size of crystal which forms macroscopic aggregates. The use of carbon nanotubes as matrix prevents the formation of such aggregates, and allowed as a maximum size of the zeolite, the inner diameter of the nanotube. The nanometric size combined with a relatively high aspect ratio of the nanowire-like zeolite also provided a high external surface area contact between the catalyst and the reactants compared to ordinary zeolite crystals synthesized under hydrothermal conditions. Work is ongoing to check the stability of this catalyst.
E E
4. Conclusions
1 = p – methoxybenzophenone (R1 = OMe, R2 = R3 = H) 2 = o – methoxybenzophenone (R1 = R2 = H, R3 = OMe) E = Phenyl benzoate
Fig. 6. Reaction pathway of the benzoylation of anisole by benzoyle chloride.
In summary, BETA zeolite of nanometric size was successfully synthesized by means of the confinement effect inside the MWNT tubules under non-hydrothermal synthesis conditions despite the relatively low aspect ratio of the used MWNTs, i.e. 30. The avoidance of the hydrothermal process during the synthesis is of interest for industrial scale-up processes due to the significant
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cost reduction. The high volume-to-weight ratio of the MWNTs allows the synthesis of a large amount of zeolite with a small amount of template. Using MWNTs as template also allows the easy recovery, washing and post-treatment of the zeolite when compared to the classical hydrothermal synthesis method where highspeed centrifugation is necessary to recover the zeolite material due to its colloidal aspect. The confinement effect synthesis can also be extended to the synthesis of zeolites with a non-conventional gel precursor composition and allows tailoring of the crystal size which is not an easy task using the conventional synthesis method. Acknowledgements The authors gratefully acknowledge Mrs. A.C. Faust (LMM, UMR 7016 CNRS, Mulhouse, France) and G. Ehret (IPCMS, UMR 7504 CNRS, Strasbourg, France) for SEM and TEM experiments. References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes. New York: Academic Press; 1996. [3] Ebbessen TW. Carbon nanotubes, preparation and properties. New York: CRC Press; 1996.
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