- Email: [email protected]
Fabrication of molecular-sieve-type carbons from Salix viminalis
Microporous and Mesoporous Materials 116 (2008) 723–726
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Correspondence
Fabrication of molecular-sieve-type carbons from Salix viminalis Jerzy P. Łukaszewicz *, Radosław P. Wesołowski Faculty of Chemistry, Nicholas Copernicus University, 87-100 Torun, Poland
a r t i c l e
i n f o
Article history: Received 9 December 2007 Received in revised form 13 April 2008 Accepted 22 April 2008 Available online 25 May 2008 Keywords: Porosity Carbon molecular sieves Carbonization Salix viminalis
a b s t r a c t The discovery of a novel raw material for the fabrication of strictly nanoporous carbons is described. The invention deals with the application of widely accessible wood from Salix viminalis as a precursor for carbonization. High temperature carbonization intermediately yields carbons of very narrowed pore size distribution (PSD) in the range below 1 nm. Regarding narrowed PSD the carbons may be considered in future as an effective molecular sieves for gas separation. The determined effective pore dimension is similar to the inner diameter of opened carbon nanotubes which size is essential in other applications like gas (hydrogen) storage. Estimated production cost of the production of nanoporous carbons is several orders lower than for carbon nanotubes. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Molecular sieves (MS) are often defined as carbonaceous materials possessing narrowed pore sizes distribution [1]. The mentioned existence of ‘‘tinny” and uniform pores in a solid is the main factor distinguishing MS from other porous solids. According to some researchers, the term MS (carbon molecular sieves CMS, too) should be applied to adsorbents exhibiting molecular sieving effect in practice [2]. Because of the effect, CMS are widely applied for gas separation [3,4], mixture purification or catalytic processes [5]. The phenomenon of selective adsorption depends not only on the size of pores, but also on other properties of CMS, such as shape of pores and/or electron properties of CMS. Unlike the other known molecular sieving materials of mineral character [6], CMSs posses adequate chemical (pH) and thermal (in inert atmosphere) stability and high hydrophobicity (if not chemically modified). Number of procedures and precursors for preparing CMS have been proposed and developed since the discovery of molecular sieving effect of Saran char in the late 1940s [7]. More recently, highly-ordered mesoporous materials, like zeolites or MCM-48 silicas, have been found convenient to use as templates for the preparation of mesoporous CMS. In such a matrix, carbon-containing precursor is placed inside and then carbonized, for instance sucrose in MCM-48 [8], divinylbenzene in MCM-48 [9], or furfuryl alcohol in bentonite and taeniolite [10]. However, the application of proposed methods in mass production of CMSs remains complicated as consisted of several steps and requiring hazardous reagents. Also the yield of the fabrication method seems * Corresponding author. Tel.: +48 60 5314300. E-mail address: [email protected] (J.P. Łukaszewicz). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.04.034
to be rather low and therefore the whole method is hardly transferable to industrial scale. One may also obtain specific carbon structures of relatively uniform pore size distribution by the deposition of a carbon precursor into the existing pore system. Some methods reported in literature, exploited chemical vapor deposition of coal tar pitch [11], polyfurfuryl alcohol [12], or benzene [13], into the pores of activated carbon. Also in this case, the fabrication is rather complicated and limited to laboratory scale. Nearly the same shortcomings deal with fabrication of carbon nanotubes (CNTs). In general, after opening and fragmentation, CNTs might be regarded as a carbon-type adsorbent of narrowed PSD due to the frequently reported, uniform inner diameter of the opened tubes [14]. Additionally, in some cases the uniform spaces between aligned CNTs can contribute to the total pore volume of such samples [15]. Narrowed PSD can be observed also for some active carbons obtained by the usual carbonization/activation procedure of some sorts of natural and/or synthetic precursors. Such a narrowed PSDs were mentioned for active carbons for which preparation starting materials like wood shells, as of walnut [16], or palm fruits [17], were used. The announcements claim the such fabricated active carbons to posses pore structure typical for molecular sieves but in many cases the determined PSD is not narrowed and therefore being far from the ideal one and one may find that pores of differentiated size contribute considerably to the total pore volume [18]. Regarding the mentioned applications of solids of narrowed PSD (including CMSs) and the shortcomings of some already discovered CMS fabrication routes, one has to state that there is an obvious need for an inexpensive carbon-type adsorbent in which pores are really uniform and their size is around 1 nm. This paper reports
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first results of a research attempt aiming at the application of unconventional and inexpensive raw material. 2. Experimental The need for inexpensive CMSs of narrowed PSD can be solved either by finding more and more complex fabrication procedures or by searching for unconventional precursors for carbonization. We came to the conclusion that such a raw material should be a carbon-rich matter possessing specific genuine structure that could quite automatically transform into carbon of molecularsieve-type pore structure upon a heat-treatment. After several attempts our attention was focused on Salix viminalis. S. viminalis is an example of so-called short-rotation coppice, planted as an easily renewable source of energy [19]. The plant is widely known due to its contribution to ‘‘green” energy production. The use of S. viminalis for other purposes, beside energetic use, or phytoremediation of waters and soil, is a rare case. In this work we describe the application of S. viminalis wood as a precursor for the fabrication of carbons exhibiting narrowed PSD (almost perfect CMS structure). The fabrication is potentially expandable to the industrial scale. It was found that the carbon adsorbents obtained by carbonization of willow wood, without any extraordinary treatment, exhibit very narrowed PSD in the range of micropores (typically effective pore size is less than 1 nm in diameter). This phenomenon is an original and unique property of S. viminalis wood discovered exclusively in the current study. Aiming at the better expression of the originality of such carbon molecular sieves we propose to name the so fabricated carbons as CarboSalÒ [20]. Manufacturing procedure of CarboSalÒ is typical for the preparation of active carbons [21]. Harvested S. viminalis rods are dried and ground into shavings of circa 1 cm long, then pyrolysed. Carbonization is carried out in two stages:
tially a strictly microporous solid. In order to confirm the presence of real nanopores in CarboSalÒ (‘‘micropores” according to IUPAC classification), the recorded nitrogen adsorption data needed to be regressed according to a certain adsorption theory. It is known that some adsorption models do not consider the phenomenon of unlocalized (‘‘scattered”) adsorption being only focused only on the condensation of adsorbate in pores of particular size at particular partial pressure [26]. This may lead to PSD being different from those obtained from molecular probing (advised by IUPAC) [27]. Therefore, two considerably different regression methods were applied for PSD calculation from the collected low temperature N2 adsorption data: Horvath–Kawazoe (H–K) method [28], and Nguyen–Do (N–D) approach [29]. Each of the methods is based on radically different assumptions on the mechanism of gas
– The preliminary stage: 1 h at 673 K (±1 K) for expelling some volatile species. – The secondary stage: 1 h at arbitrarily chosen temperature (973, 1073 or 1173 K, ±1 K) for expelling residual volatile fractions and the formation of micropore-rich polycrystalline carbon matrix. The collected volatile fraction is itself interesting because it is a source of valuable organic compounds including salicylic acid and its derivatives which usefulness will be studied soon. The heattreatment was run in a quartz-glassy tube in oxygen-free conditions (argon atmosphere). In contrast to some instrumental methods being complex in interpretation, low temperature nitrogen adsorption is a rapid method for the evaluation of pore size distribution (PSD) in mesopore range [22]. However, some important limitations occur in the case of PSD determination for micropores from N2 adsorption data (at 77 K). The obstacles mainly result from the limitations existing theoretical models of adsorption, [23] which proper selection influences the finally calculated PSD [24]. Micromeritics micropore analyzer ASAP 2010 was applied to the collection of nitrogen adsorption data (N2 adsorption vs. relative pressure of nitrogen). 3. Results The all determined nitrogen adsorption isotherms are of the type I of IUPAC classification [25], with the plateau reached for very low values of relative pressures (p/ps) of the adsorbate (Fig. 1). Such a shape of isotherms allows to assume that the adsorbent is poten-
Fig. 1. Nitrogen adsorption isotherms on carbons obtained from Salix viminalis wood and corresponding pore size distribution functions (a – carbonization in 700 °C with no activation; b – activation at 800 °C in CO2 stream).
J.P. Łukaszewicz, R.P. Wesołowski / Microporous and Mesoporous Materials 116 (2008) 723–726
(nitrogen) adsorption. However, in both cases slit-like shape of pores in carbon matrix is assumed what was widely accepted for active carbons [30]. The H–K model attracted our attention since it was originally devoted to calculating the effective micropore size distribution of slit-shaped pores in carbon molecular sieves [31]. It considers a phase transition (at a specified relative pressure) in the layer of adsorbed gas leading to the complete filling of pores having a strict specific size. On the contrary, the N–D model takes into account simultaneous formation of adsorbate layer on the walls of pores of differentiated size. Thus, the latter model considers the phenomenon of scattered adsorption (not only in pores of definite size) which lack in the H–K model is regarded as a serious limitation. The scattered adsorption ought to play and important role in carbons having pore structure more complex than in typical molecular sieves. Such a complex pore structure was not excluded for CarboSal carbons (prior to nitrogen adsorption investigations) and therefore a model accounting scattered adsorption was needed beside the H–K method. Moreover, it was proven in other studies that careful interpretation of N2 adsorption (at 77 K) data by means of N–D model may lead to identical PSD function for microporous carbons (different pore structure and origin) as in the case of approaches relying on density functional theory [32]. Both regression approaches yielded practically identical results pointing out very narrowed PSD what was the main target of the performed research: inexpensive fabrication of molecular-sievelooking carbons. The effective diameter of pores is definitely below 1 nm, what makes CarboSalÒ carbons a unique and promising adsorbent for separation applications, especially for gas separation. Beside gas separation, gas accumulation may be regarded as another possible field of application for the carbons. In the case of simple carbonization of S. viminalis wood, specific surface area of the raw CarboSalÒ carbons (calculated by the application of multipoint BET method) reaches the value of 300–400 m2/g (example case 342 m2/g). It is not very impressive if compared to some purified CNT samples, [33] or some active carbons [34]. However, just obtained raw pyrolytic carbons and/or organic precursors for fabrication of them, can be subjected to several procedures leading to the development of the total micropore volume and specific surface area. One may quote gas phase activation methods of carbon adsorbents like carbon dioxide and argon/water vapor treatment. The first method, beside other activation procedures, was applied to fabricate CarboSalÒ of well developed surface area and preserving very narrowed PSD. In some other experiments, the wood from S. viminalis prior to carbonization was treated with different chemical activators like inorganic acids, alkali metal hydroxides and selected salts yielding carbons of well developed surface area, too. These activation measures are under an extensive investigation but the so far obtained results let us to sate that most of the mentioned activation measures lift the value of specific surface area (example case 1108 m2/g). Regarding the achieved specific surface area values and calculated PSD (from N2 adsorption data), CarboSalÒ adsorbents become competitive to opened and fragmented carbon nanotubes. This is obvious that some significant differences must exist between CarboSalÒ and opened/purified CNTs. The main three are as follows:
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but the salts are removable by typical deashing procedure. Nevertheless, it has to be stated that in particular applications the size and volume of pores plays the most important role. Scanning electron microscopy (SEM) images (Fig. 2a) indicate that carbons of CarboSalÒ family obtained in the way presented above, retain some features of their natural wooden precursor despite the mentioned sever heat-treatment (carbonization). Thus, we assume that the unique pore structure of CarboSalÒ adsorbents is likely a derivative of the natural structure of S. viminalis wood. Other types of wood subjected to similar carbonization yield carbon adsorbents of different pore structure [35]. Thus, particular pore structure of such obtained carbons is also predestinated by different properties of other wooden precursors. We have noticed the influence of the proposed fabrication pathways on the pore structure of CarboSalÒ is somehow limited. The performed CO2-activation is a good example. Figs. 1a and b doubtlessly indicate that CO2 – treatment changed the total nitrogen uptake at p/ps above 0.95 but the shape of nitrogen adsorption isotherm and PSD remained unchanged. The same effect i.e. unchanged PSD have been found as a result of fabrication procedures performed at different carbonization temperatures. The texture of CarboSalÒ resembling original wood structure is visible in Fig. 2a, while better magnification gives some information on the nanostructure of raw CalboSalÒ (Fig. 2b). It is visible that the structure of carbon matrix is a granular one. The matrix consists of carbon granules which dimensions are of tens of nano-
– Pore shape (cylindrical in CNTs and slit-like in CarboSalÒ). – Chemical reactivity of carbon atoms in the matrix (low in CNTs due to the saturation of carbon atom valences). – The presence of foreign atoms (low in CNTs but higher in CarboSalÒ due to the plant origin of the precursor for carbonization). The last feature is typical for carbons obtained from natural products due to the enrichment of plant tissues in metal ions collected by roots from soil and transported over the whole organism,
Fig. 2. SEM images of carbons obtained from Salix viminalis (carbonization in 800 °C, no activation).
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meters. The intergranular spaces are often considered as slit-like in shape and are described as slit-like pores in literature. 4. Summary The study reports an unique way of microporous (in fact nanoporous) carbon fabrication with a pore structure resembling carbon molecular sieves. The fabrication procedure is based on unconventional raw material i.e. S. viminalis wood. The plant is inexpensive, easy to grow, harvest and typically used as a ‘‘green fuel”. Specific properties of S. viminalis wood lead directly to the formation of carbons of very narrowed PSD (typical property of molecular sieves) during oxygen-free pyrolysis of the raw material. Additional activation procedures are useful for the development of total micropore volume and specific surface area while PSD function remains very narrowed and unchanged. The characteristic pore structure of the obtained microporous (nanoporous) carbons let consider them as a possible CMS (for gas separation) and a competitor to CNTs in some applications like gas storage. It opens the way to wide application of novel microporous (nanoporous) carbons in practice. Acknowledgment We express deep appreciation to Mr. Adam Presz from High Pressure Research Centre UNIPRESS (Warsaw, Poland) for the help at the recording of SEM images. References [1] J. Arvelius, S. Roslin, H. Nilsson, S. Kirkwood, in: Proceedings of the XX Quadrennial Ozone Symposium, 2004, p. 515. [2] K.S.W. Sing, R.T. Williams, Part. Part. Syst. Charact. 21 (2004) 71. [3] A.I. Shirley, N.O. Lemcoff, Adsorption 8 (2002) 147. [4] I. Mochida, S. Yatsunami, Y. Kawabuchi, Y. Nkayama, Carbon 33 (1995) 1611.
[5] I.I. Ivanova, A.S. Kuznetsov, V.V. Yuschenko, E.E. Knyazeva, Pure Appl. Chem. 76 (2004) 1647. [6] A. Corma, Chem. Rev. 97 (1997) 2373. [7] P.H. Emmet, Chem. Rev. 43 (1948) 69. [8] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [9] S.K. Yoon, J.Y. Kim, J.S. Yu, Chem. Commun. 6 (2001) 559. [10] T.J. Bandosz, J. Jagiełło, K. Putyera, J.A. Schwarz, Langmuir 11 (1995) 3964. [11] D. Lozano-Castello, J. Alcaniz-Monge, D. Cazorla-Amoroz, A. Linarez-Solano, W. Zhu, F. Kapteijn, J.A. Moulijn, Carbon 43 (2005) 1643. [12] R.F.P.M. Moreira, H.J. Jose, A.E. Rodrigues, Carbon 39 (2001) 2269. [13] C. Gomez de Salazar, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, Adsorption 11 (1) (2005) 663. [14] M. Endo, H. Muramatsu, et al., Nature 433 (7025) (2005) 476. [15] Ch. Bossuot, G. Bister, A. Fonseca, J. B. Nagy, J.-P. Pirard,in: H. Kuzmany, J. Fink, M. Mehring, S. Roth (Eds.), Electronic Properties of Molecular Nanostructures: Proceedings of the XVth International Winterschool/Euroconference, AIP Conference Proceedings, Melville-New York, vol. 591, 2001, p. 393. [16] Z. Hu, E.F. Vansan, Carbon 33 (1995) 561. [17] J.S. Tan, F.N. Ani, Sep. Purif. Technol. 35 (2004) 47. [18] S. Manocha, Sadhana 28 (2003) 335. [19] M. Labrecque, T.I. Teodorescu, S. Daigle, Biomass Bioenerg. 12 (1997) 409. [20] J.P. Lukaszewicz, R.P. Wesołowski, Trade Mark Registration Pending, Patent Office of Poland, 2007. [21] J.P. Lukaszewicz, A. Arcimowicz, B. Klemp-Dyczek, Patent Pending, 2006, Patent Office of Poland. [22] IUPAC Recommendation for the characterization of porous solids, Pure Appl. Chem. 66 (1994) 1739. [23] K.S.W. Sing, Colloid. Surf. A 241 (2004) 3. [24] C. Lastoskie, K.E. Gubbins, N. Quirke, Stud. Surf. Sci. Catal. 87 (1994) 51. [25] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Everett, J.H. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, IUPAC recommendation for the characterization of porous solids, Pure Appl. Chem. 66 (1994) 1739. [26] R.J. Dombrowski, C.M. Lastoskie, D.R. Hyduke, Colloid. Surf. A 187/188 (2001) 23. [27] Z. Hu, N. Maes, E.F. Vansant, J. Porous Mater. 2 (1995) 19. [28] G. Hórvath, K. Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470. [29] C. Nguyen, D.D. Do, Langmuir 15 (1999) 3608. [30] R.J. Dombrowski, D.R. Hyduke, C.M. Lastoskie, Langmuir 16 (2000) 5041. [31] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corp., Norcross, USA, 1997. [32] P. Gauden, A. Terzyk, P. Kowalczyk, J. ColIoid. Interf. Sci. 300 (2006) 453. [33] S. Chakraborty, J. Chattopadhyay, H. Peng, Z. Chen, A. Mukherjee, R.S. Arvidson, R.H. Hauge, W.E. Billups, J. Phys. Chem. B 110 (2006) 24812. [34] X. Deng, Y. Yue, Z. Gao, J. Colloid. Interf. Sci. 192 (1997) 475. [35] M. Lopez, M. Labady, J. Laine, Carbon 34 (1996) 825.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Correspondence
Fabrication of molecular-sieve-type carbons from Salix viminalis Jerzy P. Łukaszewicz *, Radosław P. Wesołowski Faculty of Chemistry, Nicholas Copernicus University, 87-100 Torun, Poland
a r t i c l e
i n f o
Article history: Received 9 December 2007 Received in revised form 13 April 2008 Accepted 22 April 2008 Available online 25 May 2008 Keywords: Porosity Carbon molecular sieves Carbonization Salix viminalis
a b s t r a c t The discovery of a novel raw material for the fabrication of strictly nanoporous carbons is described. The invention deals with the application of widely accessible wood from Salix viminalis as a precursor for carbonization. High temperature carbonization intermediately yields carbons of very narrowed pore size distribution (PSD) in the range below 1 nm. Regarding narrowed PSD the carbons may be considered in future as an effective molecular sieves for gas separation. The determined effective pore dimension is similar to the inner diameter of opened carbon nanotubes which size is essential in other applications like gas (hydrogen) storage. Estimated production cost of the production of nanoporous carbons is several orders lower than for carbon nanotubes. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Molecular sieves (MS) are often defined as carbonaceous materials possessing narrowed pore sizes distribution [1]. The mentioned existence of ‘‘tinny” and uniform pores in a solid is the main factor distinguishing MS from other porous solids. According to some researchers, the term MS (carbon molecular sieves CMS, too) should be applied to adsorbents exhibiting molecular sieving effect in practice [2]. Because of the effect, CMS are widely applied for gas separation [3,4], mixture purification or catalytic processes [5]. The phenomenon of selective adsorption depends not only on the size of pores, but also on other properties of CMS, such as shape of pores and/or electron properties of CMS. Unlike the other known molecular sieving materials of mineral character [6], CMSs posses adequate chemical (pH) and thermal (in inert atmosphere) stability and high hydrophobicity (if not chemically modified). Number of procedures and precursors for preparing CMS have been proposed and developed since the discovery of molecular sieving effect of Saran char in the late 1940s [7]. More recently, highly-ordered mesoporous materials, like zeolites or MCM-48 silicas, have been found convenient to use as templates for the preparation of mesoporous CMS. In such a matrix, carbon-containing precursor is placed inside and then carbonized, for instance sucrose in MCM-48 [8], divinylbenzene in MCM-48 [9], or furfuryl alcohol in bentonite and taeniolite [10]. However, the application of proposed methods in mass production of CMSs remains complicated as consisted of several steps and requiring hazardous reagents. Also the yield of the fabrication method seems * Corresponding author. Tel.: +48 60 5314300. E-mail address: [email protected] (J.P. Łukaszewicz). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.04.034
to be rather low and therefore the whole method is hardly transferable to industrial scale. One may also obtain specific carbon structures of relatively uniform pore size distribution by the deposition of a carbon precursor into the existing pore system. Some methods reported in literature, exploited chemical vapor deposition of coal tar pitch [11], polyfurfuryl alcohol [12], or benzene [13], into the pores of activated carbon. Also in this case, the fabrication is rather complicated and limited to laboratory scale. Nearly the same shortcomings deal with fabrication of carbon nanotubes (CNTs). In general, after opening and fragmentation, CNTs might be regarded as a carbon-type adsorbent of narrowed PSD due to the frequently reported, uniform inner diameter of the opened tubes [14]. Additionally, in some cases the uniform spaces between aligned CNTs can contribute to the total pore volume of such samples [15]. Narrowed PSD can be observed also for some active carbons obtained by the usual carbonization/activation procedure of some sorts of natural and/or synthetic precursors. Such a narrowed PSDs were mentioned for active carbons for which preparation starting materials like wood shells, as of walnut [16], or palm fruits [17], were used. The announcements claim the such fabricated active carbons to posses pore structure typical for molecular sieves but in many cases the determined PSD is not narrowed and therefore being far from the ideal one and one may find that pores of differentiated size contribute considerably to the total pore volume [18]. Regarding the mentioned applications of solids of narrowed PSD (including CMSs) and the shortcomings of some already discovered CMS fabrication routes, one has to state that there is an obvious need for an inexpensive carbon-type adsorbent in which pores are really uniform and their size is around 1 nm. This paper reports
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first results of a research attempt aiming at the application of unconventional and inexpensive raw material. 2. Experimental The need for inexpensive CMSs of narrowed PSD can be solved either by finding more and more complex fabrication procedures or by searching for unconventional precursors for carbonization. We came to the conclusion that such a raw material should be a carbon-rich matter possessing specific genuine structure that could quite automatically transform into carbon of molecularsieve-type pore structure upon a heat-treatment. After several attempts our attention was focused on Salix viminalis. S. viminalis is an example of so-called short-rotation coppice, planted as an easily renewable source of energy [19]. The plant is widely known due to its contribution to ‘‘green” energy production. The use of S. viminalis for other purposes, beside energetic use, or phytoremediation of waters and soil, is a rare case. In this work we describe the application of S. viminalis wood as a precursor for the fabrication of carbons exhibiting narrowed PSD (almost perfect CMS structure). The fabrication is potentially expandable to the industrial scale. It was found that the carbon adsorbents obtained by carbonization of willow wood, without any extraordinary treatment, exhibit very narrowed PSD in the range of micropores (typically effective pore size is less than 1 nm in diameter). This phenomenon is an original and unique property of S. viminalis wood discovered exclusively in the current study. Aiming at the better expression of the originality of such carbon molecular sieves we propose to name the so fabricated carbons as CarboSalÒ [20]. Manufacturing procedure of CarboSalÒ is typical for the preparation of active carbons [21]. Harvested S. viminalis rods are dried and ground into shavings of circa 1 cm long, then pyrolysed. Carbonization is carried out in two stages:
tially a strictly microporous solid. In order to confirm the presence of real nanopores in CarboSalÒ (‘‘micropores” according to IUPAC classification), the recorded nitrogen adsorption data needed to be regressed according to a certain adsorption theory. It is known that some adsorption models do not consider the phenomenon of unlocalized (‘‘scattered”) adsorption being only focused only on the condensation of adsorbate in pores of particular size at particular partial pressure [26]. This may lead to PSD being different from those obtained from molecular probing (advised by IUPAC) [27]. Therefore, two considerably different regression methods were applied for PSD calculation from the collected low temperature N2 adsorption data: Horvath–Kawazoe (H–K) method [28], and Nguyen–Do (N–D) approach [29]. Each of the methods is based on radically different assumptions on the mechanism of gas
– The preliminary stage: 1 h at 673 K (±1 K) for expelling some volatile species. – The secondary stage: 1 h at arbitrarily chosen temperature (973, 1073 or 1173 K, ±1 K) for expelling residual volatile fractions and the formation of micropore-rich polycrystalline carbon matrix. The collected volatile fraction is itself interesting because it is a source of valuable organic compounds including salicylic acid and its derivatives which usefulness will be studied soon. The heattreatment was run in a quartz-glassy tube in oxygen-free conditions (argon atmosphere). In contrast to some instrumental methods being complex in interpretation, low temperature nitrogen adsorption is a rapid method for the evaluation of pore size distribution (PSD) in mesopore range [22]. However, some important limitations occur in the case of PSD determination for micropores from N2 adsorption data (at 77 K). The obstacles mainly result from the limitations existing theoretical models of adsorption, [23] which proper selection influences the finally calculated PSD [24]. Micromeritics micropore analyzer ASAP 2010 was applied to the collection of nitrogen adsorption data (N2 adsorption vs. relative pressure of nitrogen). 3. Results The all determined nitrogen adsorption isotherms are of the type I of IUPAC classification [25], with the plateau reached for very low values of relative pressures (p/ps) of the adsorbate (Fig. 1). Such a shape of isotherms allows to assume that the adsorbent is poten-
Fig. 1. Nitrogen adsorption isotherms on carbons obtained from Salix viminalis wood and corresponding pore size distribution functions (a – carbonization in 700 °C with no activation; b – activation at 800 °C in CO2 stream).
J.P. Łukaszewicz, R.P. Wesołowski / Microporous and Mesoporous Materials 116 (2008) 723–726
(nitrogen) adsorption. However, in both cases slit-like shape of pores in carbon matrix is assumed what was widely accepted for active carbons [30]. The H–K model attracted our attention since it was originally devoted to calculating the effective micropore size distribution of slit-shaped pores in carbon molecular sieves [31]. It considers a phase transition (at a specified relative pressure) in the layer of adsorbed gas leading to the complete filling of pores having a strict specific size. On the contrary, the N–D model takes into account simultaneous formation of adsorbate layer on the walls of pores of differentiated size. Thus, the latter model considers the phenomenon of scattered adsorption (not only in pores of definite size) which lack in the H–K model is regarded as a serious limitation. The scattered adsorption ought to play and important role in carbons having pore structure more complex than in typical molecular sieves. Such a complex pore structure was not excluded for CarboSal carbons (prior to nitrogen adsorption investigations) and therefore a model accounting scattered adsorption was needed beside the H–K method. Moreover, it was proven in other studies that careful interpretation of N2 adsorption (at 77 K) data by means of N–D model may lead to identical PSD function for microporous carbons (different pore structure and origin) as in the case of approaches relying on density functional theory [32]. Both regression approaches yielded practically identical results pointing out very narrowed PSD what was the main target of the performed research: inexpensive fabrication of molecular-sievelooking carbons. The effective diameter of pores is definitely below 1 nm, what makes CarboSalÒ carbons a unique and promising adsorbent for separation applications, especially for gas separation. Beside gas separation, gas accumulation may be regarded as another possible field of application for the carbons. In the case of simple carbonization of S. viminalis wood, specific surface area of the raw CarboSalÒ carbons (calculated by the application of multipoint BET method) reaches the value of 300–400 m2/g (example case 342 m2/g). It is not very impressive if compared to some purified CNT samples, [33] or some active carbons [34]. However, just obtained raw pyrolytic carbons and/or organic precursors for fabrication of them, can be subjected to several procedures leading to the development of the total micropore volume and specific surface area. One may quote gas phase activation methods of carbon adsorbents like carbon dioxide and argon/water vapor treatment. The first method, beside other activation procedures, was applied to fabricate CarboSalÒ of well developed surface area and preserving very narrowed PSD. In some other experiments, the wood from S. viminalis prior to carbonization was treated with different chemical activators like inorganic acids, alkali metal hydroxides and selected salts yielding carbons of well developed surface area, too. These activation measures are under an extensive investigation but the so far obtained results let us to sate that most of the mentioned activation measures lift the value of specific surface area (example case 1108 m2/g). Regarding the achieved specific surface area values and calculated PSD (from N2 adsorption data), CarboSalÒ adsorbents become competitive to opened and fragmented carbon nanotubes. This is obvious that some significant differences must exist between CarboSalÒ and opened/purified CNTs. The main three are as follows:
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but the salts are removable by typical deashing procedure. Nevertheless, it has to be stated that in particular applications the size and volume of pores plays the most important role. Scanning electron microscopy (SEM) images (Fig. 2a) indicate that carbons of CarboSalÒ family obtained in the way presented above, retain some features of their natural wooden precursor despite the mentioned sever heat-treatment (carbonization). Thus, we assume that the unique pore structure of CarboSalÒ adsorbents is likely a derivative of the natural structure of S. viminalis wood. Other types of wood subjected to similar carbonization yield carbon adsorbents of different pore structure [35]. Thus, particular pore structure of such obtained carbons is also predestinated by different properties of other wooden precursors. We have noticed the influence of the proposed fabrication pathways on the pore structure of CarboSalÒ is somehow limited. The performed CO2-activation is a good example. Figs. 1a and b doubtlessly indicate that CO2 – treatment changed the total nitrogen uptake at p/ps above 0.95 but the shape of nitrogen adsorption isotherm and PSD remained unchanged. The same effect i.e. unchanged PSD have been found as a result of fabrication procedures performed at different carbonization temperatures. The texture of CarboSalÒ resembling original wood structure is visible in Fig. 2a, while better magnification gives some information on the nanostructure of raw CalboSalÒ (Fig. 2b). It is visible that the structure of carbon matrix is a granular one. The matrix consists of carbon granules which dimensions are of tens of nano-
– Pore shape (cylindrical in CNTs and slit-like in CarboSalÒ). – Chemical reactivity of carbon atoms in the matrix (low in CNTs due to the saturation of carbon atom valences). – The presence of foreign atoms (low in CNTs but higher in CarboSalÒ due to the plant origin of the precursor for carbonization). The last feature is typical for carbons obtained from natural products due to the enrichment of plant tissues in metal ions collected by roots from soil and transported over the whole organism,
Fig. 2. SEM images of carbons obtained from Salix viminalis (carbonization in 800 °C, no activation).
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meters. The intergranular spaces are often considered as slit-like in shape and are described as slit-like pores in literature. 4. Summary The study reports an unique way of microporous (in fact nanoporous) carbon fabrication with a pore structure resembling carbon molecular sieves. The fabrication procedure is based on unconventional raw material i.e. S. viminalis wood. The plant is inexpensive, easy to grow, harvest and typically used as a ‘‘green fuel”. Specific properties of S. viminalis wood lead directly to the formation of carbons of very narrowed PSD (typical property of molecular sieves) during oxygen-free pyrolysis of the raw material. Additional activation procedures are useful for the development of total micropore volume and specific surface area while PSD function remains very narrowed and unchanged. The characteristic pore structure of the obtained microporous (nanoporous) carbons let consider them as a possible CMS (for gas separation) and a competitor to CNTs in some applications like gas storage. It opens the way to wide application of novel microporous (nanoporous) carbons in practice. Acknowledgment We express deep appreciation to Mr. Adam Presz from High Pressure Research Centre UNIPRESS (Warsaw, Poland) for the help at the recording of SEM images. References [1] J. Arvelius, S. Roslin, H. Nilsson, S. Kirkwood, in: Proceedings of the XX Quadrennial Ozone Symposium, 2004, p. 515. [2] K.S.W. Sing, R.T. Williams, Part. Part. Syst. Charact. 21 (2004) 71. [3] A.I. Shirley, N.O. Lemcoff, Adsorption 8 (2002) 147. [4] I. Mochida, S. Yatsunami, Y. Kawabuchi, Y. Nkayama, Carbon 33 (1995) 1611.
[5] I.I. Ivanova, A.S. Kuznetsov, V.V. Yuschenko, E.E. Knyazeva, Pure Appl. Chem. 76 (2004) 1647. [6] A. Corma, Chem. Rev. 97 (1997) 2373. [7] P.H. Emmet, Chem. Rev. 43 (1948) 69. [8] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [9] S.K. Yoon, J.Y. Kim, J.S. Yu, Chem. Commun. 6 (2001) 559. [10] T.J. Bandosz, J. Jagiełło, K. Putyera, J.A. Schwarz, Langmuir 11 (1995) 3964. [11] D. Lozano-Castello, J. Alcaniz-Monge, D. Cazorla-Amoroz, A. Linarez-Solano, W. Zhu, F. Kapteijn, J.A. Moulijn, Carbon 43 (2005) 1643. [12] R.F.P.M. Moreira, H.J. Jose, A.E. Rodrigues, Carbon 39 (2001) 2269. [13] C. Gomez de Salazar, A. Sepulveda-Escribano, F. Rodriguez-Reinoso, Adsorption 11 (1) (2005) 663. [14] M. Endo, H. Muramatsu, et al., Nature 433 (7025) (2005) 476. [15] Ch. Bossuot, G. Bister, A. Fonseca, J. B. Nagy, J.-P. Pirard,in: H. Kuzmany, J. Fink, M. Mehring, S. Roth (Eds.), Electronic Properties of Molecular Nanostructures: Proceedings of the XVth International Winterschool/Euroconference, AIP Conference Proceedings, Melville-New York, vol. 591, 2001, p. 393. [16] Z. Hu, E.F. Vansan, Carbon 33 (1995) 561. [17] J.S. Tan, F.N. Ani, Sep. Purif. Technol. 35 (2004) 47. [18] S. Manocha, Sadhana 28 (2003) 335. [19] M. Labrecque, T.I. Teodorescu, S. Daigle, Biomass Bioenerg. 12 (1997) 409. [20] J.P. Lukaszewicz, R.P. Wesołowski, Trade Mark Registration Pending, Patent Office of Poland, 2007. [21] J.P. Lukaszewicz, A. Arcimowicz, B. Klemp-Dyczek, Patent Pending, 2006, Patent Office of Poland. [22] IUPAC Recommendation for the characterization of porous solids, Pure Appl. Chem. 66 (1994) 1739. [23] K.S.W. Sing, Colloid. Surf. A 241 (2004) 3. [24] C. Lastoskie, K.E. Gubbins, N. Quirke, Stud. Surf. Sci. Catal. 87 (1994) 51. [25] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Everett, J.H. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, IUPAC recommendation for the characterization of porous solids, Pure Appl. Chem. 66 (1994) 1739. [26] R.J. Dombrowski, C.M. Lastoskie, D.R. Hyduke, Colloid. Surf. A 187/188 (2001) 23. [27] Z. Hu, N. Maes, E.F. Vansant, J. Porous Mater. 2 (1995) 19. [28] G. Hórvath, K. Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470. [29] C. Nguyen, D.D. Do, Langmuir 15 (1999) 3608. [30] R.J. Dombrowski, D.R. Hyduke, C.M. Lastoskie, Langmuir 16 (2000) 5041. [31] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corp., Norcross, USA, 1997. [32] P. Gauden, A. Terzyk, P. Kowalczyk, J. ColIoid. Interf. Sci. 300 (2006) 453. [33] S. Chakraborty, J. Chattopadhyay, H. Peng, Z. Chen, A. Mukherjee, R.S. Arvidson, R.H. Hauge, W.E. Billups, J. Phys. Chem. B 110 (2006) 24812. [34] X. Deng, Y. Yue, Z. Gao, J. Colloid. Interf. Sci. 192 (1997) 475. [35] M. Lopez, M. Labady, J. Laine, Carbon 34 (1996) 825.