Microporous and Mesoporous Materials 116 (2008) 723–726

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Microporous and Mesoporous Materials

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Correspondence

Fabrication of molecular-sieve-type carbons from Salix viminalis

Jerzy P. Łukaszewicz *, Radosław P. WesołowskiFaculty of Chemistry, Nicholas Copernicus University, 87-100 Torun, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 December 2007Received in revised form 13 April 2008Accepted 22 April 2008Available online 25 May 2008

Keywords:PorosityCarbon molecular sievesCarbonizationSalix viminalis

1387-1811/$ - see front matter � 2008 Elsevier Inc. Adoi:10.1016/j.micromeso.2008.04.034

* Corresponding author. Tel.: +48 60 5314300.E-mail address: lukaszju@chem.uni.torun.pl (J.P. Ł

The discovery of a novel raw material for the fabrication of strictly nanoporous carbons is described. Theinvention deals with the application of widely accessible wood from Salix viminalis as a precursor for car-bonization. High temperature carbonization intermediately yields carbons of very narrowed pore sizedistribution (PSD) in the range below 1 nm. Regarding narrowed PSD the carbons may be consideredin future as an effective molecular sieves for gas separation. The determined effective pore dimensionis similar to the inner diameter of opened carbon nanotubes which size is essential in other applicationslike gas (hydrogen) storage. Estimated production cost of the production of nanoporous carbons is severalorders lower than for carbon nanotubes.

� 2008 Elsevier Inc. All rights reserved.

1. Introduction

Molecular sieves (MS) are often defined as carbonaceous mate-rials possessing narrowed pore sizes distribution [1]. The men-tioned existence of ‘‘tinny” and uniform pores in a solid is themain factor distinguishing MS from other porous solids. Accordingto some researchers, the term MS (carbon molecular sieves CMS,too) should be applied to adsorbents exhibiting molecular sievingeffect in practice [2]. Because of the effect, CMS are widely appliedfor gas separation [3,4], mixture purification or catalytic processes[5]. The phenomenon of selective adsorption depends not only onthe size of pores, but also on other properties of CMS, such as shapeof pores and/or electron properties of CMS.

Unlike the other known molecular sieving materials of mineralcharacter [6], CMSs posses adequate chemical (pH) and thermal (ininert atmosphere) stability and high hydrophobicity (if not chem-ically modified). Number of procedures and precursors for prepar-ing CMS have been proposed and developed since the discovery ofmolecular sieving effect of Saran char in the late 1940s [7]. Morerecently, highly-ordered mesoporous materials, like zeolites orMCM-48 silicas, have been found convenient to use as templatesfor the preparation of mesoporous CMS. In such a matrix, car-bon-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 CMSsremains complicated as consisted of several steps and requiringhazardous reagents. Also the yield of the fabrication method seems

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ukaszewicz).

to be rather low and therefore the whole method is hardly trans-ferable to industrial scale.

One may also obtain specific carbon structures of relatively uni-form pore size distribution by the deposition of a carbon precursorinto the existing pore system. Some methods reported in literature,exploited chemical vapor deposition of coal tar pitch [11], poly-furfuryl alcohol [12], or benzene [13], into the pores of activatedcarbon. Also in this case, the fabrication is rather complicatedand limited to laboratory scale.

Nearly the same shortcomings deal with fabrication of carbonnanotubes (CNTs). In general, after opening and fragmentation,CNTs might be regarded as a carbon-type adsorbent of narrowedPSD due to the frequently reported, uniform inner diameter ofthe opened tubes [14]. Additionally, in some cases the uniformspaces between aligned CNTs can contribute to the total pore vol-ume of such samples [15]. Narrowed PSD can be observed also forsome active carbons obtained by the usual carbonization/activa-tion procedure of some sorts of natural and/or synthetic precur-sors. Such a narrowed PSDs were mentioned for active carbonsfor which preparation starting materials like wood shells, as ofwalnut [16], or palm fruits [17], were used. The announcementsclaim the such fabricated active carbons to posses pore structuretypical for molecular sieves but in many cases the determinedPSD is not narrowed and therefore being far from the ideal oneand one may find that pores of differentiated size contribute con-siderably to the total pore volume [18].

Regarding the mentioned applications of solids of narrowed PSD(including CMSs) and the shortcomings of some already discoveredCMS fabrication routes, one has to state that there is an obviousneed for an inexpensive carbon-type adsorbent in which poresare 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 ofunconventional and inexpensive raw material.

2. Experimental

The need for inexpensive CMSs of narrowed PSD can be solvedeither by finding more and more complex fabrication proceduresor by searching for unconventional precursors for carbonization.We came to the conclusion that such a raw material should be acarbon-rich matter possessing specific genuine structure thatcould quite automatically transform into carbon of molecular-sieve-type pore structure upon a heat-treatment. After several at-tempts 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 iswidely known due to its contribution to ‘‘green” energy produc-tion. The use of S. viminalis for other purposes, beside energeticuse, or phytoremediation of waters and soil, is a rare case.

In this work we describe the application of S. viminalis wood asa precursor for the fabrication of carbons exhibiting narrowed PSD(almost perfect CMS structure). The fabrication is potentiallyexpandable to the industrial scale. It was found that the carbonadsorbents obtained by carbonization of willow wood, withoutany extraordinary treatment, exhibit very narrowed PSD in therange of micropores (typically effective pore size is less than1 nm in diameter). This phenomenon is an original and uniqueproperty of S. viminalis wood discovered exclusively in the currentstudy. Aiming at the better expression of the originality of suchcarbon molecular sieves we propose to name the so fabricated car-bons as CarboSal� [20].

Manufacturing procedure of CarboSal� is typical for the prepa-ration of active carbons [21]. Harvested S. viminalis rods are driedand ground into shavings of circa 1 cm long, then pyrolysed. Car-bonization is carried out in two stages:

– The preliminary stage: 1 h at 673 K (±1 K) for expelling somevolatile species.

– The secondary stage: 1 h at arbitrarily chosen temperature (973,1073 or 1173 K, ±1 K) for expelling residual volatile fractionsand the formation of micropore-rich polycrystalline carbonmatrix.

The collected volatile fraction is itself interesting because it is asource of valuable organic compounds including salicylic acid andits derivatives which usefulness will be studied soon. The heat-treatment was run in a quartz-glassy tube in oxygen-free condi-tions (argon atmosphere).

In contrast to some instrumental methods being complex ininterpretation, low temperature nitrogen adsorption is a rapidmethod for the evaluation of pore size distribution (PSD) in meso-pore range [22]. However, some important limitations occur in thecase of PSD determination for micropores from N2 adsorption data(at 77 K). The obstacles mainly result from the limitations existingtheoretical models of adsorption, [23] which proper selection influ-ences the finally calculated PSD [24].

Micromeritics micropore analyzer ASAP 2010 was applied tothe collection of nitrogen adsorption data (N2 adsorption vs. rela-tive pressure of nitrogen).

Fig. 1. Nitrogen adsorption isotherms on carbons obtained from Salix viminaliswood and corresponding pore size distribution functions (a – carbonization in700 �C with no activation; b – activation at 800 �C in CO2 stream).

3. Results

The all determined nitrogen adsorption isotherms are of the typeI of IUPAC classification [25], with the plateau reached for very lowvalues of relative pressures (p/ps) of the adsorbate (Fig. 1). Such ashape of isotherms allows to assume that the adsorbent is poten-

tially a strictly microporous solid. In order to confirm the presenceof real nanopores in CarboSal� (‘‘micropores” according to IUPACclassification), the recorded nitrogen adsorption data needed tobe regressed according to a certain adsorption theory.

It is known that some adsorption models do not consider thephenomenon of unlocalized (‘‘scattered”) adsorption being only fo-cused only on the condensation of adsorbate in pores of particularsize at particular partial pressure [26]. This may lead to PSD beingdifferent from those obtained from molecular probing (advised byIUPAC) [27]. Therefore, two considerably different regressionmethods were applied for PSD calculation from the collected lowtemperature N2 adsorption data: Horvath–Kawazoe (H–K) method[28], and Nguyen–Do (N–D) approach [29]. Each of the methods isbased on radically different assumptions on the mechanism of gas

Fig. 2. SEM images of carbons obtained from Salix viminalis (carbonization in800 �C, no activation).

J.P. Łukaszewicz, R.P. Wesołowski / Microporous and Mesoporous Materials 116 (2008) 723–726 725

(nitrogen) adsorption. However, in both cases slit-like shape ofpores in carbon matrix is assumed what was widely accepted foractive carbons [30]. The H–K model attracted our attention sinceit was originally devoted to calculating the effective micropore sizedistribution of slit-shaped pores in carbon molecular sieves [31]. Itconsiders a phase transition (at a specified relative pressure) in thelayer of adsorbed gas leading to the complete filling of pores hav-ing a strict specific size. On the contrary, the N–D model takes intoaccount simultaneous formation of adsorbate layer on the walls ofpores of differentiated size. Thus, the latter model considers thephenomenon of scattered adsorption (not only in pores of definitesize) which lack in the H–K model is regarded as a serious limita-tion. The scattered adsorption ought to play and important role incarbons having pore structure more complex than in typicalmolecular sieves. Such a complex pore structure was not excludedfor CarboSal carbons (prior to nitrogen adsorption investigations)and therefore a model accounting scattered adsorption was neededbeside the H–K method. Moreover, it was proven in other studiesthat careful interpretation of N2 adsorption (at 77 K) data by meansof N–D model may lead to identical PSD function for microporouscarbons (different pore structure and origin) as in the case of ap-proaches relying on density functional theory [32].

Both regression approaches yielded practically identical resultspointing out very narrowed PSD what was the main target of theperformed research: inexpensive fabrication of molecular-sieve-looking carbons. The effective diameter of pores is definitely below1 nm, what makes CarboSal� carbons a unique and promisingadsorbent for separation applications, especially for gas separation.Beside gas separation, gas accumulation may be regarded as an-other possible field of application for the carbons. In the case ofsimple carbonization of S. viminalis wood, specific surface area ofthe raw CarboSal� carbons (calculated by the application of multi-point BET method) reaches the value of 300–400 m2/g (examplecase 342 m2/g). It is not very impressive if compared to some puri-fied CNT samples, [33] or some active carbons [34].

However, just obtained raw pyrolytic carbons and/or organicprecursors for fabrication of them, can be subjected to several pro-cedures leading to the development of the total micropore volumeand specific surface area. One may quote gas phase activationmethods of carbon adsorbents like carbon dioxide and argon/watervapor treatment. The first method, beside other activation proce-dures, was applied to fabricate CarboSal� of well developed surfacearea and preserving very narrowed PSD. In some other experi-ments, the wood from S. viminalis prior to carbonization was trea-ted with different chemical activators like inorganic acids, alkalimetal hydroxides and selected salts yielding carbons of well devel-oped surface area, too. These activation measures are under anextensive investigation but the so far obtained results let us to satethat most of the mentioned activation measures lift the value ofspecific surface area (example case 1108 m2/g). Regarding theachieved specific surface area values and calculated PSD (from N2

adsorption data), CarboSal� adsorbents become competitive toopened and fragmented carbon nanotubes.

This is obvious that some significant differences must exist be-tween CarboSal� and opened/purified CNTs. The main three are asfollows:

– Pore shape (cylindrical in CNTs and slit-like in CarboSal�).– 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 Carbo-

Sal� due to the plant origin of the precursor for carbonization).

The last feature is typical for carbons obtained from naturalproducts due to the enrichment of plant tissues in metal ions col-lected by roots from soil and transported over the whole organism,

but the salts are removable by typical deashing procedure. Never-theless, it has to be stated that in particular applications the sizeand volume of pores plays the most important role.

Scanning electron microscopy (SEM) images (Fig. 2a) indicatethat carbons of CarboSal� family obtained in the way presentedabove, retain some features of their natural wooden precursor de-spite the mentioned sever heat-treatment (carbonization). Thus,we assume that the unique pore structure of CarboSal� adsorbentsis likely a derivative of the natural structure of S. viminalis wood.Other types of wood subjected to similar carbonization yield car-bon adsorbents of different pore structure [35]. Thus, particularpore structure of such obtained carbons is also predestinated bydifferent properties of other wooden precursors.

We have noticed the influence of the proposed fabrication path-ways on the pore structure of CarboSal� is somehow limited. Theperformed CO2-activation is a good example. Figs. 1a and b doubt-lessly indicate that CO2 – treatment changed the total nitrogen up-take at p/ps above 0.95 but the shape of nitrogen adsorptionisotherm and PSD remained unchanged. The same effect i.e. un-changed PSD have been found as a result of fabrication proceduresperformed at different carbonization temperatures.

The texture of CarboSal� resembling original wood structure isvisible in Fig. 2a, while better magnification gives some informa-tion on the nanostructure of raw CalboSal� (Fig. 2b). It is visiblethat the structure of carbon matrix is a granular one. The matrixconsists of carbon granules which dimensions are of tens of nano-

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meters. The intergranular spaces are often considered as slit-like inshape and are described as slit-like pores in literature.

4. Summary

The study reports an unique way of microporous (in fact nano-porous) carbon fabrication with a pore structure resembling car-bon molecular sieves. The fabrication procedure is based onunconventional raw material i.e. S. viminalis wood. The plant isinexpensive, easy to grow, harvest and typically used as a ‘‘greenfuel”. Specific properties of S. viminalis wood lead directly to theformation of carbons of very narrowed PSD (typical property ofmolecular sieves) during oxygen-free pyrolysis of the raw material.Additional activation procedures are useful for the development oftotal micropore volume and specific surface area while PSD func-tion remains very narrowed and unchanged. The characteristicpore structure of the obtained microporous (nanoporous) carbonslet consider them as a possible CMS (for gas separation) and a com-petitor to CNTs in some applications like gas storage. It opens theway to wide application of novel microporous (nanoporous) car-bons in practice.

Acknowledgment

We express deep appreciation to Mr. Adam Presz from HighPressure Research Centre UNIPRESS (Warsaw, Poland) for the helpat the recording of SEM images.

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