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Carbon spheres prepared from zeolite Beta beads
Carbon 43 (2005) 2474–2480 www.elsevier.com/locate/carbon
Carbon spheres prepared from zeolite Beta beads Lubomira Tosheva a, Julien Parmentier a,*, Valentin Valtchev a, Cathie Vix-Guterl b, Joe¨l Patarin a a
Laboratoire de Mate´riaux a` Porosite´ Controˆle´e, UMR-7016 CNRS, ENSCMu, Universite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France b Institut de Chimie des Surfaces et Interfaces, UPR CNRS 9069, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France Received 8 February 2005; accepted 29 April 2005 Available online 27 June 2005
Abstract Porous carbon beads were prepared from macroporous anion-exchange resin beads preliminary converted into resin–zeolite Beta composite or pure zeolite Beta spheres. Two synthesis procedures were used depending on the initial template employed. In a series of experiments, the resin from the resin–zeolite Beta composite was directly carbonized into carbon. In another series of experiments, the resin was removed by oxidation at 600 °C leaving behind self-bonded zeolite Beta beads, which were filled with carbon by chemical vapor deposition (CVD) of propylene. As a final step for both procedures, the zeolite was dissolved in hydrofluoric acid. All the carbons prepared inherited the macroscopic spherical shape of the template spheres as well as the morphology of the primary particles building up the beads. The synthesis procedure and the carbonization temperature or the temperature for CVD of carbon employed influenced the ordering and the pore structure of the produced carbons. The carbons prepared by direct carbonization showed relatively low surface areas, less than 1000 m2 g1, and no zeolite structural regularity. The samples obtained via CVD main˚ and had surface areas of over 2000 m2 g1. tained the zeolite ordering with a periodicity of 11.7 A Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Porous carbon; Carbon beads; Chemical vapor deposition; X-ray diffraction; Adsorption
1. Introduction The carbon replication of ordered silicate materials and zeolites in particular is an interesting subject from scientific and application point of view. The microporosity of such carbon replicas arises from the unique zeolite channel system, which can bring different properties into the carbon replicas compared to microporous carbons prepared by other methods. The application of porous carbons of well-defined microporosity includes areas such as storage media for methane gas, electrodes for
*
Corresponding author. Tel.: +33 3 89 33 68 87; fax: +33 3 89 33 68
85. E-mail address: [email protected] (J. Parmentier). 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.04.030
electric double-layer capacitors, chemical sensing and separation processes. Most of the works on carbon replicas of zeolites have been performed by the group of Kyotani [1–5]. In a number of papers, they have described and optimized the replication of zeolite Y. They succeeded in preparing high surface area carbons having the structural regularity of zeolite Y with almost no mesoporosity and strong morphological resemblance to the zeolite template [1–3]. A zeolite Y carbon replica with a BET surface area as large as 4000 m2 g1 and details of its microporous structure has recently been reported by this group [4]. Further, the authors have attempted to replicate other zeolite structure types such as Beta, L, mordenite and ZSM-5 [5]. They have found that the optimum carbon filling method for zeolite Y is not an optimum one for the other zeolites and the zeolite type is of paramount
L. Tosheva et al. / Carbon 43 (2005) 2474–2480
importance for the success of the replication. The regularity of the carbons prepared from zeolite Beta was superior compared to the one for the other zeolites replicas but poor in comparison with the ordering of zeolite Y replicas. Other groups have also prepared carbon materials from zeolites [6–9]. Zeolites Y, Beta and L have been infiltrated with polymers followed by pyrolysis of the zeolite–resin composites and HF etching [6]. Zeolite Y has been replicated by chemical vapor infiltration of the zeolite [7] or by impregnation [8]. Zeolite Beta has been used as a template in another work to prepare carbon with a structural regularity [9]. All the above examples concern the preparation of carbons in the form of powders. For a number of applications, e.g., in catalysis and chromatography, the use of macroscopic beads is beneficial. Porous carbon beads have been prepared by different methods, e.g., by esterification of silica gel beads with alcohol or phenol, pyrolysis and silica dissolution [10], pyrolysis of polymer beads [11] and carbonization of bead-shaped polymers in the presence of inorganic salts [12]. However, the carbon replication of zeolite macroscopic beads has not yet been realized. Recently, we reported the preparation of porous carbon spheres by the direct carbonization of the resin from silicate-exchanged resin beads [13]. The resin was acting as a shape-directing template, whereas the silicate anions exchanged preserved the macroscopic form of the particles from collapsing during the carbonization. In the present paper, the use of resin–zeolite Beta beads for the preparation of porous carbons is presented. Two preparation procedures were employed, namely direct carbonization of the resin from the resin–zeolite composites or removal of the resin by oxidation followed by chemical vapor deposition of propylene.
2. Experimental Solid zeolite Beta spheres were prepared following the procedure described in Ref. [14]. An Amberlite IRA-900 macroporous strongly basic anion-exchange resin (chloride form, Avocado) was employed in the experiments as received. In brief, a mixture of the resin and the zeolite precursor solution was hydrothermally treated at 170 °C for 24 h. As a result of the zeolite crystallization within the resin pore structure, resin–zeolite Beta composite beads were obtained. In a series of experiments, the resin from the as made resin–zeolite composites was directly carbonized. In another series of experiments, the resin template was removed from the resin–zeolite composite beads by oxidation at 600 °C in air. For the direct carbonization (CD), the as synthesized resin–zeolite Beta composites were heated at 5 °C min1 up to 700 °C or 900 °C for 2 h under an Ar flow. The
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zeolite was removed from the composite beads by a two-step dissolution in an excess of hydrofluoric acid (40 wt.%) followed by a treatment in concentrated hydrochloric acid (37 wt.%), where the duration of each step was 24 h. The resultant carbon samples were repeatedly rinsed with distilled water, filtrated and dried at 80 °C. The two carbon samples obtained were designated as CD700 and CD900. The calcined zeolite Beta spheres were used for the preparation of carbon beads by chemical vapor deposition of propylene (2.5% in Ar). This procedure was previously found to be the optimum procedure for zeolite Beta replication and the infiltration of mesoporous silica [5,15,16]. Two carbon samples were prepared using two different temperatures for the carbon infiltration, 700 °C and 800 °C, for 4 h and 2 h, respectively. A heating rate of 5 °C was employed. The carbon samples were recovered using the same procedure as described for the CD samples. The mixed zeolite–carbon samples were designated as BC700 and BC800 and the pure carbon ones—C700 and C800. The carbon content of the mixed zeolite–carbon samples was determined by thermogravimetry using a Setaram TG-ATD LABSYS thermal analyzer at a heating rate of 5 °C min1 in air. The crystallinity of the samples was evaluated with a STOE STADI-P X-ray diffractometer (XRD) using Ge monochromated Cu Ka1 radiation. Before the XRD analysis the beads were ground into powder in an agate mortar. Sample morphology was studied by scanning electron microscopy (SEM) using a Philips XL 30 scanning electron microscope (SEM) equipped with a LaB6 emission source. Nitrogen adsorption–desorption measurements were performed using a Micromeritics ASAP 2010 surface area analyzer after outgassing the samples overnight at 300 °C prior to analysis. Specific surface areas were calculated using the BET equation in the range of relative pressures 0.01–0.05 [4]. Pore-size distributions were determined from the desorption branch of the isotherms by the BJH method. The micropore volumes were calculated from the Dubinin–Radushkevich equation. The total pore volumes were obtained by converting the amount adsorbed at a relative pressure of 0.99 to the volume of liquid adsorbate. DFT pore-size distributions were determined from the isotherms using the DFT Plus data software available in the Micromeritics ASAP 2010.
3. Results and discussion SEM micrographs of the carbon samples prepared by direct carbonization are shown in Fig. 1. A certain degree of shrinkage was observed for the CD spheres and although no measurements were made it seemed that the shrinkage was more pronounced for the CD700 sample (Fig. 1a and b). The surface and the
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interior of the two samples were similar in appearance to each other. Larger particles were observed on the bead surfaces replicating the larger zeolite Beta crystals formed on the surface of the resin–Beta composite particles (Fig. 1a 0 and b 0 ). The growth of those larger zeolite crystals on the sphere surface is due to the fact that the sphere surface is exposed to the synthesis solution and open for crystal growth [14]. The crystal growth within the spheres is space hindered by the presence of the resin polymer chains and limited to the size of the pores of macroporous resins, 100 nm on an average. Thus, the interior zeolite particles were well replicated in the carbon spheres as well (Fig. 1a00 and b00 ). The majority of the carbon beads prepared in a previous work by direct carbonization of the resin from resin–silicate composite spheres using the same initial macroporous resin collapsed upon silica dissolution [13]. It is not likely that the slightly different experimental conditions used in this work are the sole reason for the differences observed. Most probably, the zeolite structure and the texture of the zeolite primary particles building up the spheres enhanced the binding between the particles in the resultant carbon replicas thus preserving the spherical macroscopic shape throughout the entire preparation process. The CD beads were further analyzed by XRD. No diffraction peaks at 2h below 10°, related to the zeolite regularity could be observed in the corresponding XRD patterns (Fig. 2). This result is not surprising considering the synthesis procedure used. The direct carbonization of the resin probably resulted in a carbon deposition onto the surface of the zeolite crystals and not within the zeolite channels. The structure-directing template inside the zeolite channels (tetraethylammonium cation, TEA+) did not provide enough carbon to replicate the zeolite structure either. Indeed, thermogravimetric analysis performed on the zeolite Beta/carbon composites obtained by direct carbonization gave
Intensity / a.u.
Fig. 1. SEM micrographs of the carbon beads prepared by direct carbonization of the resin–zeolite Beta composite spheres: sample CD700 (a) and sample CD900 (b). Micrographs (a 0 and b 0 ) and (a00 and b00 ) correspond to images of the bead surface and interior, respectively.
CD900 CD700
10
20
30
40
50
2θ/ ˚
Fig. 2. XRD patterns of the carbon samples prepared by direct carbonization of the resin–zeolite Beta composite spheres.
a relatively low carbon content close to 13 wt.%. A broad peak was observed around 24° (2h) in both patterns, which can be ascribed to graphite-like carbon layers of poor structural organization deposited on the external area of the zeolite particles [3]. The d value of the C(002) XRD peak is used to estimate the carbon crystallinity; an increased value of d002 indicates growing disorder in the materials. The d002 values for the CD700 ˚ and CD900 samples were in the range 3.7–3.8 A thus much larger than that of ideal graphite (d002 = ˚ ), indicating low crystallinity of these carbons. 3.354 A The nitrogen adsorption isotherms for the CD samples reported in Fig. 3a revealed the presence of microand mesopores. Indeed, these isotherms were type IV with a small hysteresis loop at high relative pressures indicative of mesopores and a steep increase at low relative pressures related to the presence of micropores. The BJH pore-size distributions showed that a substantial part of the pore volume was found in mesopores ˚ (Fig. 3b). Similar mesopore with a size of ca. 500 A
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Table 1 BET surface area (SA), micropore (Vmicro), mesopore (Vmeso) and total (Vtotal) pore volumes for the calcined zeolite Beta spheres and the different carbon beads
700
CD700 CD900
Volume absorbed /cm3g-1
600 500 400 300 200 100 0 0.0
dV/dlog(D)/cm3g-1
0.4
0.6
0.8
1.0
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 10
b
0.2
Relative pressure P/P0
a
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100
1000
Pore diameter / Å
Fig. 3. Nitrogen adsorption–desorption isotherms for the carbon beads prepared by direct carbonization (a) and the corresponding BJH desorption pore-size distributions (b). Solid symbols, adsorption branch; open symbols, desorption branch.
Sample
SA (m2 g1)
Vmicro (cm3 g1)
Vmeso (cm3 g1)
Vtotal (cm3 g1)
Beta CD700 CD900 BC700 BC800 C700 C800
661 740 980 170 108 2011 2391
0.25 0.28 0.37 0.06 0.04 0.79 0.96
0.39 0.36 0.58 0.28 0.25 0.93 0.90
0.64 0.64 0.95 0.34 0.29 1.72 1.86
structures were observed for zeolite spheres prepared by resin templating, e.g. the zeolite Beta spheres used in this work, and the formation of these mesopores was related to the removal of the resin polymer chains [14]. The ˚ is an artefact related to the presence peak at about 35 A of an unstable meniscus in small mesopores upon desorption [17]. The BET surface area, micro-, mesoand total pore volumes were larger for the CD900 sample (Table 1). This result indicated that under the experimental conditions employed, the use of higher temperature favors the formation of a high surface area carbon material [3,8]. The second series of experiments was performed using calcined zeolite Beta spheres as templates and
Fig. 4. SEM micrographs of the calcined zeolite Beta spheres (a), the mixed zeolite–carbon beads prepared by CVD of propylene (sample BC800) (b) and the corresponding carbon beads after silica dissolution (c). Micrographs (a 0 –c 0 ) and (a00 –c00 ) correspond to images of the bead surface and interior of fractured spheres, respectively.
L. Tosheva et al. / Carbon 43 (2005) 2474–2480
BC800
Intensity / a.u.
CVD of propylene. SEM images of the initial Beta spheres, the mixed carbon–zeolite samples and the porous carbons are shown in Fig. 4. The zeolite spheres and the mixed spheres were identical in size and shape, whereas the carbon beads were reduced in size (Fig. 4a–c). The C700 sample was fragile and the beads were easy to break during the laboratory manipulations but the C800 sample was hard and relatively stable. Cracked particles were observed in the SEM images of C700 (not shown). A possible explanation for the different stability of the carbon beads could be the different carbon content of the two samples. This content was determined by TG analysis as 17.7 wt.% and 24.8 wt.% for the BC700 and BC800 samples, respectively. The higher temperature used to prepare the BC800 sample probably results in a higher external carbon content, which is binding the primary particles together thus strengthening the bead macrostructure [3]. Further, the primary particles building up the bead surface and bead interior seemed similar for all samples, the initial Beta, the mixed and the pure carbon ones (Fig. 4a 0 –c 0 and a00 –c00 ). Similar to the CD samples, larger particles were observed on the bead surface and fine particles with a size of less than 100 nm within the bead interiors. XRD analysis was performed in order to investigate the crystallinity of the samples (Fig. 5). The mixed carbon–zeolite samples showed XRD patterns typical of a highly crystalline Beta indicating that the high temperature treatment did not affect the zeolite crystallinity (Fig. 5a). The XRD patterns of the pure carbon samples obtained upon removal of the zeolite are shown in Fig. 5b. In the pattern of the C800 sample, a well-pronounced peak at about 7.5° 2h can be seen. The presence of this peak indicates that the structural regularity of zeolite Beta was maintained in the carbon replicas with a peri˚ . Only a shoulder was observed at that odicity of 11.7 A position in the XRD pattern of the C700 sample. From the latter it may be concluded that a higher temperature of carbon infiltration yields carbons of a higher ordering. In addition, the XRD peak around 23° (2h) attributed to C(002) was less pronounced in the XRD patterns of the C700 and C800 samples compared to the CD samples indicating a lower amount of poorly organized graphite-like carbon. This could be related to the lower amount of external carbon in case of the former samples. The more effective filling of the zeolite Beta sphere pores was further confirmed by the nitrogen adsorption analysis. The mixed carbon–zeolite samples showed BET surface areas of 170 and 108 m2 g1 for the BC700 and BC800 samples, respectively, due to the presence of the remaining mesopores (Table 1). This result indicated that the micropores were either filled with carbon or not accessible to the nitrogen molecule. On the other hand, the slight decrease of the mesopore volume (0.25 instead of 0.28) of the BC800 sample was in agree-
BC700
Beta
a
10
20
30
40
50
2θ /˚
Intensity / a.u.
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C800 C700
b
10
20
30
40
50
2θ /˚
Fig. 5. XRD patterns of the calcined zeolite Beta spheres and the mixed zeolite–carbon samples (a) and the carbon beads after silica dissolution (b).
ment with its higher carbon content and revealed better filling of the interparticular porosity [3]. From the nitrogen adsorption isotherms presented in Fig. 6a, a substantial increase in the microporosity of the carbon samples compared to the initial calcined Beta spheres was observed. The BET surface areas of the carbon beads increased to over 2000 m2 g1 with a higher value for the C800 sample (Table 1). The meso- and total pore volumes of the porous carbons were also much higher in comparison with the pure Beta sample. From the volumes adsorbed at low relative pressures, the volume of micropores in the C800 sample was higher than the C700 sample (Fig. 6a). Judging from the BJH pore-size ˚ , the mesopore volume of distributions around 500 A C800 was lower (Fig. 6b), although the latter was not evidenced from the data presented in Table 1. The increased microporosity of the C800 sample may be related to the higher quality of the carbon filling leading to an improved carbon replica as confirmed by the XRD results. On the other hand, the slight decrease in the mesoporosity of this sample can be attributed to its higher carbon content associated with a higher amount of external carbon. A comparison with the results obtained for the samples prepared by direct carbonization showed that the use of calcined zeolite Beta
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4. Conclusions
1200
Beta C700 C800
Volumeadsorbed/cm3g-1
1000 800 600 400 200 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressureP/P0
a 3.0
dV/dlog(D)/cm3g-1
2.5 2.0 1.5 1.0 0.5 0.0 10
b
100
1000
Pore diameter /Å
Fig. 6. Nitrogen adsorption–desorption isotherms for of the zeolite Beta spheres and the carbon beads prepared by CVD of propylene (a) and the corresponding BJH desorption pore-size distributions (b). Solid symbols, adsorption branch; open symbols, desorption branch.
spheres as templates and CVD yielded porous carbons of much higher surface areas and pore volumes (Table 1). Further information about the micropore-size distribution in the four carbon samples prepared was obtained from the DFT pore-size distributions (Fig. 7). ˚ The size distributions in the pore width range 10–20 A were similar for all the samples with the microporosity increasing in the order CD700 < CD900 < C700 < C800.
600
CD700 CD900 C700 C800
Incremental area /m2g-1
500 400 300 200 100 0 0
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10
20
30
Pore width / Å
Fig. 7. DFT pore-size distributions for the carbon beads prepared.
Porous carbon beads of controlled macroscopic shape and pore structure were obtained using ion-exchange resins as templates. Zeolite Beta was firstly crystallized within the resin followed by either direct carbonization of the resin or by resin oxidation and subsequent carbon infiltration by chemical vapor deposition of propylene. Finally, the zeolite was dissolved in hydrofluoric acid leaving behind porous carbon spheres. The size of the carbon spheres was reduced compared to the original resin beads and the morphology of the zeolite sphere surface and interior was replicated in the carbon samples with a high fidelity. The carbons prepared by direct carbonization of the resin did not preserve the structural regularity of the zeolite and showed low carbon crystallinity. Their surface areas were in the range 740– 980 m2 g1 depending on the carbonization temperature. ˚ was observed in the XRD patA periodicity of 11.7 A terns of the samples obtained via CVD, the ordering being higher for the sample prepared at 800 °C. BET surface areas of over 2000 m2 g1 were determined for those samples. The high surface areas and pores in both the micro- and meso-range together with the spherical macroscopic shape of the porous carbons prepared make them potentially interesting for a number of applications such as catalysis, separation and chromatography. Acknowledgement Financial supports from the CNRS-DFG bilateral program, Procope, the French Ministry of Research (ACI Nanosciences et Nanotechnologies, project NN060 and ACI Energie Pr1-2) are gratefully acknowledged. References [1] Kyotani T, Nagai T, Inoue S, Tomita A. Formation of new type of porous carbon by carbonization in zeolite nanochannels. Chem Mater 1997;9:609–15. [2] Ma Z, Kyotani T, Liu Z, Terasaki O, Tomita A. Very high surface area microporous carbon with a three-dimensional nano-array structure: Synthesis and its molecular structure. Chem Mater 2001;13:4413–5. [3] Ma Z, Kyotani T, Tomita A. Synthesis methods of preparing microporous carbons with a structural regularity of zeolite Y. Carbon 2002;40(13):2367–74. [4] Matsuoka K, Yamagishi Y, Yamazaki T, Setoyama N, Tomita A, Kyotani T. Extremely high microporosity and sharp pore size distribution of a large surface area carbon prepared in the nanochannels of zeolite Y. Carbon 2005;43(4):876–9. [5] Kyotani T, Ma Z, Tomita A. Template synthesis of novel porous carbons using various types of zeolites. Carbon 2003;41(7):1451–9. [6] Johnson SA, Brigham ES, Ollivier PJ, Mallouk TE. Effect of micropore topology in the structure and properties of zeolite polymer replicas. Chem Mater 1997;9:2448–58.
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[7] Rodriguez-Mirasol J, Cordero T, Radovic LR, Rodriguez JJ. Structural and textural properties of pyrolytic carbon formed within a microporous zeolite template. Chem Mater 1998;10:550–8. [8] Su F, Zhao XS, Lv L, Zhou Z. Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y. Carbon 2004;42(14):2821–31. [9] Barata-Rodrigues PM, Mays TJ, Moggridge GD. Structured carbon adsorbents from clay, zeolite and mesoporous aluminosilicate templates. Carbon 2003;41(12):2231–46. [10] Kamegawa K, Yoshida H. Preparation and characterization of swelling porous carbon beads. Carbon 1997;35(5):631–9. [11] Li F, Huang J, Zou J, Pan P, Yuan G. Preparation and characterization of porous carbon beads and their application in dispersing small metal crystallites. Carbon 2002;40(15): 2871–7. [12] Kocˇ´ırˇ´ık M, Brych J, Hradil J. Carbonization of bead-shaped polymers for application in adsorption and in composite membranes. Carbon 2001;39(12):1919–28.
[13] Tosheva L, Parmentier J, Saadallah S, Vix-Guterl C, Valtchev V, Patarin J. Carbon and SiC macroscopic beads from ion-exchange resin templates. J Am Chem Soc 2004;126:13624–5. [14] Tosheva L, Mihailova B, Valtchev V, Sterte J. Zeolite beta spheres. Micropor Mesopor Mater 2001;48:31–7. [15] Vix-Guterl C, Boulard S, Parmentier J, Werckmann J, Patarin J. Formation of ordered mesoporous carbon materials from a silica template by a one-step chemical vapour infiltration process. Chem Lett 2002;10:1062–3. [16] Parmentier J, Saadhallah S, Reda M, Gibot P, Roux M, Vidal L, et al. New carbons with controlled nanoporosity obtained by nanocasting using a SBA-15 mesoporous silica host-matrix and different preparation routes. J Phys Chem Solids 2004;65:139–46. [17] Groen JC, Peffer LAA, Pe´rez-Ramı´rez J. Comments on ‘‘Vanadium- and chromium-containing mesoporous MCM-41 molecular sieves with hierarchical structure [Micropor. Mesopor. Mater. 43 (2001) 227–236]’’. Micropor Mesopor Mater 2002;51:75–8.
Carbon spheres prepared from zeolite Beta beads Lubomira Tosheva a, Julien Parmentier a,*, Valentin Valtchev a, Cathie Vix-Guterl b, Joe¨l Patarin a a
Laboratoire de Mate´riaux a` Porosite´ Controˆle´e, UMR-7016 CNRS, ENSCMu, Universite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France b Institut de Chimie des Surfaces et Interfaces, UPR CNRS 9069, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France Received 8 February 2005; accepted 29 April 2005 Available online 27 June 2005
Abstract Porous carbon beads were prepared from macroporous anion-exchange resin beads preliminary converted into resin–zeolite Beta composite or pure zeolite Beta spheres. Two synthesis procedures were used depending on the initial template employed. In a series of experiments, the resin from the resin–zeolite Beta composite was directly carbonized into carbon. In another series of experiments, the resin was removed by oxidation at 600 °C leaving behind self-bonded zeolite Beta beads, which were filled with carbon by chemical vapor deposition (CVD) of propylene. As a final step for both procedures, the zeolite was dissolved in hydrofluoric acid. All the carbons prepared inherited the macroscopic spherical shape of the template spheres as well as the morphology of the primary particles building up the beads. The synthesis procedure and the carbonization temperature or the temperature for CVD of carbon employed influenced the ordering and the pore structure of the produced carbons. The carbons prepared by direct carbonization showed relatively low surface areas, less than 1000 m2 g1, and no zeolite structural regularity. The samples obtained via CVD main˚ and had surface areas of over 2000 m2 g1. tained the zeolite ordering with a periodicity of 11.7 A Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Porous carbon; Carbon beads; Chemical vapor deposition; X-ray diffraction; Adsorption
1. Introduction The carbon replication of ordered silicate materials and zeolites in particular is an interesting subject from scientific and application point of view. The microporosity of such carbon replicas arises from the unique zeolite channel system, which can bring different properties into the carbon replicas compared to microporous carbons prepared by other methods. The application of porous carbons of well-defined microporosity includes areas such as storage media for methane gas, electrodes for
*
Corresponding author. Tel.: +33 3 89 33 68 87; fax: +33 3 89 33 68
85. E-mail address: [email protected] (J. Parmentier). 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.04.030
electric double-layer capacitors, chemical sensing and separation processes. Most of the works on carbon replicas of zeolites have been performed by the group of Kyotani [1–5]. In a number of papers, they have described and optimized the replication of zeolite Y. They succeeded in preparing high surface area carbons having the structural regularity of zeolite Y with almost no mesoporosity and strong morphological resemblance to the zeolite template [1–3]. A zeolite Y carbon replica with a BET surface area as large as 4000 m2 g1 and details of its microporous structure has recently been reported by this group [4]. Further, the authors have attempted to replicate other zeolite structure types such as Beta, L, mordenite and ZSM-5 [5]. They have found that the optimum carbon filling method for zeolite Y is not an optimum one for the other zeolites and the zeolite type is of paramount
L. Tosheva et al. / Carbon 43 (2005) 2474–2480
importance for the success of the replication. The regularity of the carbons prepared from zeolite Beta was superior compared to the one for the other zeolites replicas but poor in comparison with the ordering of zeolite Y replicas. Other groups have also prepared carbon materials from zeolites [6–9]. Zeolites Y, Beta and L have been infiltrated with polymers followed by pyrolysis of the zeolite–resin composites and HF etching [6]. Zeolite Y has been replicated by chemical vapor infiltration of the zeolite [7] or by impregnation [8]. Zeolite Beta has been used as a template in another work to prepare carbon with a structural regularity [9]. All the above examples concern the preparation of carbons in the form of powders. For a number of applications, e.g., in catalysis and chromatography, the use of macroscopic beads is beneficial. Porous carbon beads have been prepared by different methods, e.g., by esterification of silica gel beads with alcohol or phenol, pyrolysis and silica dissolution [10], pyrolysis of polymer beads [11] and carbonization of bead-shaped polymers in the presence of inorganic salts [12]. However, the carbon replication of zeolite macroscopic beads has not yet been realized. Recently, we reported the preparation of porous carbon spheres by the direct carbonization of the resin from silicate-exchanged resin beads [13]. The resin was acting as a shape-directing template, whereas the silicate anions exchanged preserved the macroscopic form of the particles from collapsing during the carbonization. In the present paper, the use of resin–zeolite Beta beads for the preparation of porous carbons is presented. Two preparation procedures were employed, namely direct carbonization of the resin from the resin–zeolite composites or removal of the resin by oxidation followed by chemical vapor deposition of propylene.
2. Experimental Solid zeolite Beta spheres were prepared following the procedure described in Ref. [14]. An Amberlite IRA-900 macroporous strongly basic anion-exchange resin (chloride form, Avocado) was employed in the experiments as received. In brief, a mixture of the resin and the zeolite precursor solution was hydrothermally treated at 170 °C for 24 h. As a result of the zeolite crystallization within the resin pore structure, resin–zeolite Beta composite beads were obtained. In a series of experiments, the resin from the as made resin–zeolite composites was directly carbonized. In another series of experiments, the resin template was removed from the resin–zeolite composite beads by oxidation at 600 °C in air. For the direct carbonization (CD), the as synthesized resin–zeolite Beta composites were heated at 5 °C min1 up to 700 °C or 900 °C for 2 h under an Ar flow. The
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zeolite was removed from the composite beads by a two-step dissolution in an excess of hydrofluoric acid (40 wt.%) followed by a treatment in concentrated hydrochloric acid (37 wt.%), where the duration of each step was 24 h. The resultant carbon samples were repeatedly rinsed with distilled water, filtrated and dried at 80 °C. The two carbon samples obtained were designated as CD700 and CD900. The calcined zeolite Beta spheres were used for the preparation of carbon beads by chemical vapor deposition of propylene (2.5% in Ar). This procedure was previously found to be the optimum procedure for zeolite Beta replication and the infiltration of mesoporous silica [5,15,16]. Two carbon samples were prepared using two different temperatures for the carbon infiltration, 700 °C and 800 °C, for 4 h and 2 h, respectively. A heating rate of 5 °C was employed. The carbon samples were recovered using the same procedure as described for the CD samples. The mixed zeolite–carbon samples were designated as BC700 and BC800 and the pure carbon ones—C700 and C800. The carbon content of the mixed zeolite–carbon samples was determined by thermogravimetry using a Setaram TG-ATD LABSYS thermal analyzer at a heating rate of 5 °C min1 in air. The crystallinity of the samples was evaluated with a STOE STADI-P X-ray diffractometer (XRD) using Ge monochromated Cu Ka1 radiation. Before the XRD analysis the beads were ground into powder in an agate mortar. Sample morphology was studied by scanning electron microscopy (SEM) using a Philips XL 30 scanning electron microscope (SEM) equipped with a LaB6 emission source. Nitrogen adsorption–desorption measurements were performed using a Micromeritics ASAP 2010 surface area analyzer after outgassing the samples overnight at 300 °C prior to analysis. Specific surface areas were calculated using the BET equation in the range of relative pressures 0.01–0.05 [4]. Pore-size distributions were determined from the desorption branch of the isotherms by the BJH method. The micropore volumes were calculated from the Dubinin–Radushkevich equation. The total pore volumes were obtained by converting the amount adsorbed at a relative pressure of 0.99 to the volume of liquid adsorbate. DFT pore-size distributions were determined from the isotherms using the DFT Plus data software available in the Micromeritics ASAP 2010.
3. Results and discussion SEM micrographs of the carbon samples prepared by direct carbonization are shown in Fig. 1. A certain degree of shrinkage was observed for the CD spheres and although no measurements were made it seemed that the shrinkage was more pronounced for the CD700 sample (Fig. 1a and b). The surface and the
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interior of the two samples were similar in appearance to each other. Larger particles were observed on the bead surfaces replicating the larger zeolite Beta crystals formed on the surface of the resin–Beta composite particles (Fig. 1a 0 and b 0 ). The growth of those larger zeolite crystals on the sphere surface is due to the fact that the sphere surface is exposed to the synthesis solution and open for crystal growth [14]. The crystal growth within the spheres is space hindered by the presence of the resin polymer chains and limited to the size of the pores of macroporous resins, 100 nm on an average. Thus, the interior zeolite particles were well replicated in the carbon spheres as well (Fig. 1a00 and b00 ). The majority of the carbon beads prepared in a previous work by direct carbonization of the resin from resin–silicate composite spheres using the same initial macroporous resin collapsed upon silica dissolution [13]. It is not likely that the slightly different experimental conditions used in this work are the sole reason for the differences observed. Most probably, the zeolite structure and the texture of the zeolite primary particles building up the spheres enhanced the binding between the particles in the resultant carbon replicas thus preserving the spherical macroscopic shape throughout the entire preparation process. The CD beads were further analyzed by XRD. No diffraction peaks at 2h below 10°, related to the zeolite regularity could be observed in the corresponding XRD patterns (Fig. 2). This result is not surprising considering the synthesis procedure used. The direct carbonization of the resin probably resulted in a carbon deposition onto the surface of the zeolite crystals and not within the zeolite channels. The structure-directing template inside the zeolite channels (tetraethylammonium cation, TEA+) did not provide enough carbon to replicate the zeolite structure either. Indeed, thermogravimetric analysis performed on the zeolite Beta/carbon composites obtained by direct carbonization gave
Intensity / a.u.
Fig. 1. SEM micrographs of the carbon beads prepared by direct carbonization of the resin–zeolite Beta composite spheres: sample CD700 (a) and sample CD900 (b). Micrographs (a 0 and b 0 ) and (a00 and b00 ) correspond to images of the bead surface and interior, respectively.
CD900 CD700
10
20
30
40
50
2θ/ ˚
Fig. 2. XRD patterns of the carbon samples prepared by direct carbonization of the resin–zeolite Beta composite spheres.
a relatively low carbon content close to 13 wt.%. A broad peak was observed around 24° (2h) in both patterns, which can be ascribed to graphite-like carbon layers of poor structural organization deposited on the external area of the zeolite particles [3]. The d value of the C(002) XRD peak is used to estimate the carbon crystallinity; an increased value of d002 indicates growing disorder in the materials. The d002 values for the CD700 ˚ and CD900 samples were in the range 3.7–3.8 A thus much larger than that of ideal graphite (d002 = ˚ ), indicating low crystallinity of these carbons. 3.354 A The nitrogen adsorption isotherms for the CD samples reported in Fig. 3a revealed the presence of microand mesopores. Indeed, these isotherms were type IV with a small hysteresis loop at high relative pressures indicative of mesopores and a steep increase at low relative pressures related to the presence of micropores. The BJH pore-size distributions showed that a substantial part of the pore volume was found in mesopores ˚ (Fig. 3b). Similar mesopore with a size of ca. 500 A
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Table 1 BET surface area (SA), micropore (Vmicro), mesopore (Vmeso) and total (Vtotal) pore volumes for the calcined zeolite Beta spheres and the different carbon beads
700
CD700 CD900
Volume absorbed /cm3g-1
600 500 400 300 200 100 0 0.0
dV/dlog(D)/cm3g-1
0.4
0.6
0.8
1.0
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 10
b
0.2
Relative pressure P/P0
a
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100
1000
Pore diameter / Å
Fig. 3. Nitrogen adsorption–desorption isotherms for the carbon beads prepared by direct carbonization (a) and the corresponding BJH desorption pore-size distributions (b). Solid symbols, adsorption branch; open symbols, desorption branch.
Sample
SA (m2 g1)
Vmicro (cm3 g1)
Vmeso (cm3 g1)
Vtotal (cm3 g1)
Beta CD700 CD900 BC700 BC800 C700 C800
661 740 980 170 108 2011 2391
0.25 0.28 0.37 0.06 0.04 0.79 0.96
0.39 0.36 0.58 0.28 0.25 0.93 0.90
0.64 0.64 0.95 0.34 0.29 1.72 1.86
structures were observed for zeolite spheres prepared by resin templating, e.g. the zeolite Beta spheres used in this work, and the formation of these mesopores was related to the removal of the resin polymer chains [14]. The ˚ is an artefact related to the presence peak at about 35 A of an unstable meniscus in small mesopores upon desorption [17]. The BET surface area, micro-, mesoand total pore volumes were larger for the CD900 sample (Table 1). This result indicated that under the experimental conditions employed, the use of higher temperature favors the formation of a high surface area carbon material [3,8]. The second series of experiments was performed using calcined zeolite Beta spheres as templates and
Fig. 4. SEM micrographs of the calcined zeolite Beta spheres (a), the mixed zeolite–carbon beads prepared by CVD of propylene (sample BC800) (b) and the corresponding carbon beads after silica dissolution (c). Micrographs (a 0 –c 0 ) and (a00 –c00 ) correspond to images of the bead surface and interior of fractured spheres, respectively.
L. Tosheva et al. / Carbon 43 (2005) 2474–2480
BC800
Intensity / a.u.
CVD of propylene. SEM images of the initial Beta spheres, the mixed carbon–zeolite samples and the porous carbons are shown in Fig. 4. The zeolite spheres and the mixed spheres were identical in size and shape, whereas the carbon beads were reduced in size (Fig. 4a–c). The C700 sample was fragile and the beads were easy to break during the laboratory manipulations but the C800 sample was hard and relatively stable. Cracked particles were observed in the SEM images of C700 (not shown). A possible explanation for the different stability of the carbon beads could be the different carbon content of the two samples. This content was determined by TG analysis as 17.7 wt.% and 24.8 wt.% for the BC700 and BC800 samples, respectively. The higher temperature used to prepare the BC800 sample probably results in a higher external carbon content, which is binding the primary particles together thus strengthening the bead macrostructure [3]. Further, the primary particles building up the bead surface and bead interior seemed similar for all samples, the initial Beta, the mixed and the pure carbon ones (Fig. 4a 0 –c 0 and a00 –c00 ). Similar to the CD samples, larger particles were observed on the bead surface and fine particles with a size of less than 100 nm within the bead interiors. XRD analysis was performed in order to investigate the crystallinity of the samples (Fig. 5). The mixed carbon–zeolite samples showed XRD patterns typical of a highly crystalline Beta indicating that the high temperature treatment did not affect the zeolite crystallinity (Fig. 5a). The XRD patterns of the pure carbon samples obtained upon removal of the zeolite are shown in Fig. 5b. In the pattern of the C800 sample, a well-pronounced peak at about 7.5° 2h can be seen. The presence of this peak indicates that the structural regularity of zeolite Beta was maintained in the carbon replicas with a peri˚ . Only a shoulder was observed at that odicity of 11.7 A position in the XRD pattern of the C700 sample. From the latter it may be concluded that a higher temperature of carbon infiltration yields carbons of a higher ordering. In addition, the XRD peak around 23° (2h) attributed to C(002) was less pronounced in the XRD patterns of the C700 and C800 samples compared to the CD samples indicating a lower amount of poorly organized graphite-like carbon. This could be related to the lower amount of external carbon in case of the former samples. The more effective filling of the zeolite Beta sphere pores was further confirmed by the nitrogen adsorption analysis. The mixed carbon–zeolite samples showed BET surface areas of 170 and 108 m2 g1 for the BC700 and BC800 samples, respectively, due to the presence of the remaining mesopores (Table 1). This result indicated that the micropores were either filled with carbon or not accessible to the nitrogen molecule. On the other hand, the slight decrease of the mesopore volume (0.25 instead of 0.28) of the BC800 sample was in agree-
BC700
Beta
a
10
20
30
40
50
2θ /˚
Intensity / a.u.
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C800 C700
b
10
20
30
40
50
2θ /˚
Fig. 5. XRD patterns of the calcined zeolite Beta spheres and the mixed zeolite–carbon samples (a) and the carbon beads after silica dissolution (b).
ment with its higher carbon content and revealed better filling of the interparticular porosity [3]. From the nitrogen adsorption isotherms presented in Fig. 6a, a substantial increase in the microporosity of the carbon samples compared to the initial calcined Beta spheres was observed. The BET surface areas of the carbon beads increased to over 2000 m2 g1 with a higher value for the C800 sample (Table 1). The meso- and total pore volumes of the porous carbons were also much higher in comparison with the pure Beta sample. From the volumes adsorbed at low relative pressures, the volume of micropores in the C800 sample was higher than the C700 sample (Fig. 6a). Judging from the BJH pore-size ˚ , the mesopore volume of distributions around 500 A C800 was lower (Fig. 6b), although the latter was not evidenced from the data presented in Table 1. The increased microporosity of the C800 sample may be related to the higher quality of the carbon filling leading to an improved carbon replica as confirmed by the XRD results. On the other hand, the slight decrease in the mesoporosity of this sample can be attributed to its higher carbon content associated with a higher amount of external carbon. A comparison with the results obtained for the samples prepared by direct carbonization showed that the use of calcined zeolite Beta
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4. Conclusions
1200
Beta C700 C800
Volumeadsorbed/cm3g-1
1000 800 600 400 200 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressureP/P0
a 3.0
dV/dlog(D)/cm3g-1
2.5 2.0 1.5 1.0 0.5 0.0 10
b
100
1000
Pore diameter /Å
Fig. 6. Nitrogen adsorption–desorption isotherms for of the zeolite Beta spheres and the carbon beads prepared by CVD of propylene (a) and the corresponding BJH desorption pore-size distributions (b). Solid symbols, adsorption branch; open symbols, desorption branch.
spheres as templates and CVD yielded porous carbons of much higher surface areas and pore volumes (Table 1). Further information about the micropore-size distribution in the four carbon samples prepared was obtained from the DFT pore-size distributions (Fig. 7). ˚ The size distributions in the pore width range 10–20 A were similar for all the samples with the microporosity increasing in the order CD700 < CD900 < C700 < C800.
600
CD700 CD900 C700 C800
Incremental area /m2g-1
500 400 300 200 100 0 0
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10
20
30
Pore width / Å
Fig. 7. DFT pore-size distributions for the carbon beads prepared.
Porous carbon beads of controlled macroscopic shape and pore structure were obtained using ion-exchange resins as templates. Zeolite Beta was firstly crystallized within the resin followed by either direct carbonization of the resin or by resin oxidation and subsequent carbon infiltration by chemical vapor deposition of propylene. Finally, the zeolite was dissolved in hydrofluoric acid leaving behind porous carbon spheres. The size of the carbon spheres was reduced compared to the original resin beads and the morphology of the zeolite sphere surface and interior was replicated in the carbon samples with a high fidelity. The carbons prepared by direct carbonization of the resin did not preserve the structural regularity of the zeolite and showed low carbon crystallinity. Their surface areas were in the range 740– 980 m2 g1 depending on the carbonization temperature. ˚ was observed in the XRD patA periodicity of 11.7 A terns of the samples obtained via CVD, the ordering being higher for the sample prepared at 800 °C. BET surface areas of over 2000 m2 g1 were determined for those samples. The high surface areas and pores in both the micro- and meso-range together with the spherical macroscopic shape of the porous carbons prepared make them potentially interesting for a number of applications such as catalysis, separation and chromatography. Acknowledgement Financial supports from the CNRS-DFG bilateral program, Procope, the French Ministry of Research (ACI Nanosciences et Nanotechnologies, project NN060 and ACI Energie Pr1-2) are gratefully acknowledged. References [1] Kyotani T, Nagai T, Inoue S, Tomita A. Formation of new type of porous carbon by carbonization in zeolite nanochannels. Chem Mater 1997;9:609–15. [2] Ma Z, Kyotani T, Liu Z, Terasaki O, Tomita A. Very high surface area microporous carbon with a three-dimensional nano-array structure: Synthesis and its molecular structure. Chem Mater 2001;13:4413–5. [3] Ma Z, Kyotani T, Tomita A. Synthesis methods of preparing microporous carbons with a structural regularity of zeolite Y. Carbon 2002;40(13):2367–74. [4] Matsuoka K, Yamagishi Y, Yamazaki T, Setoyama N, Tomita A, Kyotani T. Extremely high microporosity and sharp pore size distribution of a large surface area carbon prepared in the nanochannels of zeolite Y. Carbon 2005;43(4):876–9. [5] Kyotani T, Ma Z, Tomita A. Template synthesis of novel porous carbons using various types of zeolites. Carbon 2003;41(7):1451–9. [6] Johnson SA, Brigham ES, Ollivier PJ, Mallouk TE. Effect of micropore topology in the structure and properties of zeolite polymer replicas. Chem Mater 1997;9:2448–58.
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