- Email: [email protected]
NMR investigation on the occurrence of Na species in porous carbons prepared by NaOH activation
Carbon 45 (2007) 1097–1104 www.elsevier.com/locate/carbon
NMR investigation on the occurrence of Na species in porous carbons prepared by NaOH activation Jair C.C. Freitas a,*, Miguel A. Schettino Jr. a, Alfredo G. Cunha a, Francisco G. Emmerich a, Antonio C. Bloise b, Eduardo R. de Azevedo b, Tito J. Bonagamba b b
a Departamento de Fı´sica, Universidade Federal do Espı´rito Santo, 29060-900 Vito´ria, ES, Brazil Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, P.O. Box 369, 13560-970 Sa˜o Carlos, SP, Brazil
Received 15 September 2006; accepted 4 December 2006 Available online 7 February 2007
Abstract A detailed investigation was conducted about the process of alkali activation of charred rice hulls using NaOH. A carbon-rich precursor was initially prepared from the pyrolysis of rice hulls under N2 atmosphere, part of it being leached with HF to remove silica. The precursor was then mixed with NaOH, heat-treated at activation temperatures from 600 to 800 C, and part of the product was finally washed with distilled water. Thermogravimetric curves under O2 flux showed a strong reduction in the ash content of the activated samples, indicating the consumption of silica during the activation process. From X-ray diffractometry, 29Si, and 23Na NMR spectroscopy, it was possible to identify the formation of sodium carbonate and silicates in the non-washed samples. After washing, all these compounds were removed and specific surface area measurements indicated a substantial porosity development, with larger surface area values obtained for the samples prepared from the HF-leached precursor. The use of 23Na NMR spectroscopy indicated the retention of sodium in the washed samples, in a chemical environment distinct from carbonates and silicates. The shapes and positions of the observed resonance lines pointed to a disordered environment, associated with oxygenated surface groups within the porous structure of the activated carbons. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The use of alkali hydroxides (specially NaOH and KOH) as activating agents for the production of microporous active carbons has attracted great interest due to the valuable properties of the materials produced by this process [1–6]. The method of chemical activation presents several advantages over the physical activation, leading to materials with large specific surface area and elevated microporosity with relatively narrow pore-size distribution. This subject has been the focus of many detailed investigations in recent years due the necessity of novel methods for natural gas storage aiming automotive applications and the *
Corresponding author. Tel.: +55 27 3335 2483; fax: +55 27 3335 2823. E-mail address: [email protected] (J.C.C. Freitas).
0008-6223/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.12.006
potentially promising use of porous carbons for that purpose [5,7]. Nevertheless, a complete understanding of the chemical activation process is still lacking. A huge effort has been devoted to clarify the interaction between the alkali compounds and the carbon matrix and to correlate this to the external parameters involved in the activation process, such as temperature, time, atmosphere, and type of precursors [3–5]. The most frequently reported results indicate the formation of alkali carbonate, hydrogen, and the alkaline metal, according to the overall reaction: 6MeOH þ 2C ! 2Me þ 3H2 þ 2Me2 CO3 ;
ð1Þ
where Me = Na or K [8,9]. The detection of alkaline metal traces in the activation equipment has been in fact reported and this corroborates the proposed reaction [5,8,10]. The
1098
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
hydrogen evolution has been detected starting from temperatures in the range 200–600 C, depending on the reactivity of the carbon precursor [9]. The evolution of CO and CO2 has also been reported, but this can be associated both with the decomposition of alkali carbonates at high temperatures (above 800 C) and with the devolatilization reactions of the carbon precursor itself, especially in the case of activation of lignocellulosic precursors and high-volatile content coals [2,8,10]. More recently, the participation of nitrogen gas (which is usually present in the dynamic atmosphere of heat-treatment) in the activation reactions has been proposed, with detection of cyanide groups in the structure of the activated materials [11]. The formation of surface complexes involving the participation of the alkali in oxygenated groups of the type C–O–Me has long been invoked in many reports. Marsh et al., in a study on the KOH activation of petroleum cokes, suggested that the addition of KOH to a green coke could lead to the formation of alkalides in the form –OK, with the subsequent separation of aromatic lamellae by oxidation of cross-linking carbon atoms and formation of functional groups on the edges of these lamellae [1]. The K atoms liberated at high-temperature reactions might as well intercalate and cause an increase in the separation of the lamellae. The removal of potassium salts by water leaching would then allow the development of microporosity. Ehrburger et al. also mentioned the existence of surface complex salts, acting as active gasification sites [2]. In a detailed study on the influence of NaOH on the carbonization of a phenol-formaldehyde resin, Yamashita and Ouchi showed that Na atoms gave rise to –ONa groups attached to aromatic rings, being later decomposed to form carbonates and oxides [12]. An important distinction between the behavior of K and Na with respect to the activation mechanism was addressed in recent works dealing with materials with different degrees of crystallinity, such as graphite, carbon fibers, mesocarbon microbeads, and nanotubes [10,13–15]. While there are strong evidences of K intercalation between graphene layers, especially in the case of well ordered materials, the trend for Na is distinct: activation with NaOH is mostly effective for disordered carbon materials, with no indication of formation of intercalation compounds. Otherwise, in this case the mechanism involves the occurrence of surface reactions with high defective sites. This work describes the NaOH activation of charred rice hulls and the investigation on the nature of the products generated in the process, mainly by using X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) spectroscopy. Rice hulls are known as a rich and abundant source for the preparation of silicon-based products of technological interest, such as silicon carbide, silicon nitride, and high purity silica, among many others [16– 18]. There is a particular interest in the research on the profitable utilization of rice hulls as they represent a waste product largely produced in many countries. In recent
studies, several authors reported the preparation of active carbons with high surface area (in the range 2500– 3000 m2/g) from chemical activation of rice husks [19–21]. The presence of silicon in rice hulls and husks has been long associated with the occurrence of amorphous hydrated silica, but some results have indicated that there is a substantial part of the silicon species that are chemically bonded to organic groups [22,23]. The influence of these silicon compounds on the reactions occurring during the NaOH activation of charred rice hulls is investigated here by 29Si NMR, while 23Na NMR is used to detect the formation of Na species and particularly the presence of surface Na groups, which are retained in the carbon structure even after washing the activated materials. 2. Experimental methods 2.1. Samples preparation First, a carbon-rich precursor was prepared from the pyrolysis of raw rice hulls under N2 atmosphere at 700 C for 4 h. This material contained around 53 wt.% of fixed carbon, with the remaining part being mineral matter, mostly silica. Part of this material was leached with HF in order to obtain a nearly silica-free precursor. Next, the precursor was mixed with NaOH by two methods: by direct powder maceration or by impregnation within an aqueous solution. The results to be presented later refer to the samples prepared by physical mixture, but no considerable difference was observed between the general characteristics of the samples prepared by either method (physical mixture of NaOH and char or aqueous impregnation) while keeping unaltered the other conditions. The NaOH/ carbon weight ratio was kept equal to 3:1, by taking into account the fixed carbon content of the precursor. The activation heat-treatments were conducted under N2 flow at temperatures ranging from 600 to 800 C. For each heat-treated sample, part of the material was washed with distilled water and another part was separated for comparison (these are respectively called washed and non-washed samples). The precursor previously leached with HF was submitted to similar activation methods, at the temperature of 800 C. Further details about samples preparation and basic characterization of the activated samples can be found elsewhere [24].
2.2. Measurements X-ray diffraction (XRD) measurements were conducted on powdered samples, in a Rigaku diffractometer using Cu Ka radiation ˚ ), with 2h ranging from 10 to 80 in steps of 0.05. Thermo(k = 1.5418 A gravimetric (TG) curves were recorded in a Shimadzu thermobalance up to 1000 C under O2 flow, using powdered samples with initial mass around 15–20 mg. Specific surface area measurements were performed in an Autosorb equipment, using the BET method to analyze the nitrogen adsorption at 77 K. Solid-state NMR spectra were recorded at room temperature in a Varian INOVA 400 spectrometer, operating at 79.4 MHz for 29Si and 105.7 MHz for 23Na nuclei (magnetic field of 9.4 T). 29Si chemical shifts – given in parts per million (ppm) – were externally referred to tetramethylsilane (TMS) by using kaolin as a secondary reference. 23Na shift values were referred to an aqueous solution of NaCl (0.1 M). All spectra were acquired with single pulse excitation (pulse widths of 5.1 and 3.5 ls for 29Si and 23Na, respectively) and magic angle spinning (MAS) with a rotor speed of 6.0 kHz. Recycle delays varied in the range 1–10 s, being adjusted to avoid saturation problems. Nearly 100–500 transients were recorded for 29Si and 23Na NMR measurements on non-washed samples, whereas typically 4000 transients were accumulated for 23Na NMR measurements on washed samples (which gave no 29Si NMR signal, as detailed below).
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
3. Results 3.1. Structure and textural properties Fig. 1 shows the TG curves recorded under oxidizing conditions for the precursors and some activated and washed samples. A strong reduction in the ash content of the activated samples compared to the silicon-rich precursor can be observed from the drop in the residue fraction left after complete combustion, which indicates the consumption of silica during the activation process. This is expected based on the possible chemical reactions involving NaOH and SiO2 at room temperature and above [19], which generate mainly water-soluble sodium silicates. These products have been clearly identified in the XRD patterns of the non-washed activated samples, which are shown in Fig. 2. Through the whole temperature range here investigated, sodium silicates and sodium carbonate
Fig. 1. TG curves recorded under oxidizing atmosphere for the precursor, the HF-leached precursor, and the washed activated samples prepared from these precursors at the indicated activation temperatures.
Fig. 2. XRD patterns of the non-washed activated samples, prepared at the indicated temperatures. The HF labeled sample was prepared from the HF-leached precursor.
1099
(including hydrated forms) constituted the main phases formed. No XRD peaks associated with crystalline SiO2 were detected, neither in the precursor nor in the activated samples, which is in accordance with previous results showing the amorphous character of silica in raw rice hulls and in chars produced at such low temperatures [23]. In the case of the HF-leached precursor, Na2CO3 was the only detected product. The abundant presence of Na2CO3 in the structure of the activated samples is an important indication of the success of the activation process, as indicated by the overall chemical reaction (1), and this fact has been corroborated by many related works [2–5]. The XRD patterns of the activated and washed samples (not shown) exhibited the profile typical of the turbostratic structure. From the carbon 002 diffraction peak in these patterns, ˚ ) and enlarged interone obtained reduced Lc values (7–9 A ˚ planar spacing (4.0–4.5 A) for the activated samples, which is in accordance with other published results on activated carbons [6]. A further point to be noted in the TG curves in Fig. 1 is the increase in ash content of the 800 C activated sample prepared from the HF-leached precursor (curve indicated as 800 C – HF) when compared to the ash content of this precursor (curve indicated as Precursor – HF). This constitutes an indication of the production of an inorganic material as result of chemical reactions occurring during activation and its retention in the activated carbon structure even after water washing. As this sample is essentially silica-free, this product is expected to be related to some sodium-containing compounds. Elemental Na contents of the samples activated at 800 C, determined from spectrophotometer analysis of the ashes, confirm this result. The sample prepared from the silicon-rich precursor presented a Na content of 42 wt.%, whereas the value corresponding to the sample prepared from the HF-leached precursor was 31 wt.% (both values referred to the ash weight in each sample). This shows that samples prepared from the silicon-rich precursor had higher ability to retain Na compounds after water washing, which was consistent with the trend shown in the TG curves (Fig. 1) of a larger residue fraction left after combustion for the activated samples prepared from the silicon-rich precursor as compared to the sample prepared from the HF-leached precursor. The TG curves exhibited in Fig. 1 for the washed activated samples clearly showed a decrease in the oxidation temperature when compared to the precursors. This increased reactivity with oxygen is related to the development of the porous structure of the activated carbon, with enlargement of the surface area accessible to oxygen molecules, as shown in previously works [25]. The measurements of BET surface area confirm this result. All nonwashed samples exhibited negligible BET surface areas. After water washing, the removal of the inorganic products allowed the porosity development. The BET surface area then reached values around 450 m2/g for activated samples prepared from the silicon-rich precursor. In the case of the samples prepared from the HF-leached precursor, the
1100
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
surface area reached a much higher value (1400 m2/g). The pore size distribution determined for these materials indicated that all activated samples were essentially micro˚ porous, with average pore diameter around 16–18 A [24]. These values of specific surface area are just moderate when compared to activated carbons obtained from other natural precursors by similar activation methods [3,4,19– 21]. The increase in the surface area of the activated samples when compared to the precursors (around 60 and 530 m2/g for the silicon-rich and the HF-leached precursors, respectively) is, nevertheless, evident, confirming the success of the activation process. The relatively low values of specific surface areas for the samples prepared from the silicon-rich precursor can be understood considering that a substantial part of the NaOH initially mixed with the precursor was consumed in reactions with the silicon species. This caused a significant reduction in the effective NaOH/carbon ratio, which was therefore deviated from the value of 3:1. This ratio is usually quoted as the optimum one for attaining the best porosity development [4,19], but this depends obviously on the type of precursor used. In our case, for the silicon-rich precursor a higher NaOH/carbon ratio would likely lead to a larger porosity development, since more hydroxide would be available after reaction with silica. In fact, when a precursor with low silicon content was used, more NaOH was available for the reaction with carbon, which lead to a larger surface area for the sample prepared from the HF-leached precursor. Furthermore, the HF leaching process itself also contributes to a porosity development, due the removal of silicon species which were intimately bonded to the carbon matrix [21]. Both effects lead to the attainment of an activated material with large surface area (1400 m2/g), although still below some values previously quoted in the literature [19,20]. Nevertheless, in spite of the only partially efficient activation method here employed, the produced samples showed reasonable development of microporosity and were thus appropriate for the investigation on the local chemical environment of the Na and Si species in the structure of the activated materials, as detailed below. 3.2. NMR measurements Fig. 3 shows the 29Si MAS NMR spectra of the precursor and the non-washed activated samples. It is worth noting that the washed activated samples did not give any 29Si NMR signal, even with changes in recycle delays and increase in the number of transients. This is in accordance with the previously discussed TG and XRD results, which revealed drop in ash content and absence of crystalline silicon compounds in the washed activated samples, respectively. Therefore, we can conclude that the water washing process was effective in the removal of essentially all silicon compounds formed by reaction with NaOH. These watersoluble compounds can be recovered and recycled from the solution resulting of the washing step.
Fig. 3. 29Si MAS NMR spectra of the precursor, the non-washed activated samples prepared at the indicated activation temperatures, and a sample prepared from rice hulls ashes under similar conditions used for the activated samples.
The spectrum shown in Fig. 3 for the precursor exhibited a broad resonance centered at ca. 115 ppm, which is characteristic of an amorphous form of silica [23]. In the activated samples, no signal at similar chemical shifts was detected, indicating the complete consumption of silica by the chemical reactions occurring during the activation process with NaOH. On the other hand, in all spectra there was a narrow line at 76.8 ppm, associated with crystalline Na2SiO3 [26], which is consistent with the previous XRD results. An ill-defined resonance appeared near 68 ppm in the spectra of the samples activated from 600 C upward, being more easily detectable for samples activated at 700 and 800 C. The broad shape and the chemical shift range of this resonance suggest the occurrence of Si groups connected through oxygen bridges to Na-containing groups in a structurally disordered network, as it is found in sodium-silicate glasses [27,28]. Besides these resonances, a narrow peak was observed at 62.0 ppm for the sample with activation temperature of 800 C. This last peak was also detected in the 29Si NMR spectrum of a sample prepared by mixing NaOH powder with rice hulls ashes (obtained after combustion in air) and heat-treating this mixture at 700 C (see the top spectrum in Fig. 3), which indicates it is associated with a reaction occurring between silica and NaOH and not involving the carbonaceous matrix. The reported chemical shift value points to a monomeric Q0 unit, i.e., a silica tetrahedron not connected to other similar tetrahedra [27,29], which, also by comparison with the XRD patterns of these samples, indicates that this peak can be attributed to the compound Na4SiO4. The 23Na MAS NMR spectra recorded for the nonwashed samples activated at temperatures ranging from 600 to 800 C are displayed in Fig. 4, along with the corresponding spectrum of synthetic Na2CO3. As can be clearly seen, the spectrum of all non-washed samples exhibited the same features as the Na2CO3 one, with two broad bands extending nearly from 10 to 38 ppm. The broad character
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
1101
Fig. 4. 23Na MAS NMR spectra of the non-washed activated samples, prepared at the indicated activation temperatures, compared to the spectrum corresponding to sodium carbonate. The HF labeled sample was prepared from the HF-leached precursor.
Fig. 5. 23Na MAS NMR spectra of the washed activated samples, prepared at the indicated activation temperatures, compared to the spectrum corresponding to sodium carbonate. The HF labeled sample was prepared from the HF-leached precursor.
of these spectra is related to the quadrupolar nature of the 23 Na nuclei, which are therefore sensitive to electric field gradients besides the local magnetic fields created by the surroundings [29–31]. Also, the presence of small but perceptible sharp features around 3.8, 1.8, 8.0, and 21.6 ppm indicates the occurrence of second-order quadrupolar broadening effects not removed by MAS. In the spectra of the samples prepared from the silicon-rich precursor, a resonance pattern was also observed around 16 ppm, associated with crystalline Na2SiO3 [30], in agreement with the previous XRD and 29Si NMR data. Also, with increasing the activation temperature, it was observed a small increase in the relative intensity in the region near 10 ppm, concomitantly with the decrease in the contribution due to crystalline Na2SiO3. This suggests the consumption of Na2SiO3 by the reaction with Na2CO3, leading to the formation of new (Na,Si)-based products probably in a disordered network, in accordance with the 29Si NMR results presented before and also with previous work on glass-forming reactions between Na2CO3 and SiO2 [32]. These resonances associated with Na species related to Si were obviously missing in the spectrum corresponding to the activated sample prepared from the HF-leached precursor. The 23Na MAS NMR spectra of the washed activated samples presented in Fig. 5 are of a completely different nature. Just for comparison, the spectrum corresponding to Na2CO3 is shown again. As it can be observed, no signal associated with sodium carbonate or silicates was observed in the NMR spectra of the activated and washed samples, which agrees with the TG and XRD results. On the other hand, a broad and featureless maximum extending from 7 to 30 ppm and centered around 5 to 10 ppm was readily detected, in spite of the low ash content of the washed samples. Similar NMR lines have been reported in sodium-containing glassy materials, where the distribution of isotropic chemical shifts and structural parameters in a disordered network explains the broad shape [29,31].
In the present case, this resonance is associated with the presence of oxygenated surface Na groups at the edges of the aromatic lamellae, as detailed below. 4. Discussion Quadrupolar nuclei (i.e., those with spin >1/2) such as Na usually give rise to severely broadened NMR spectra in solid-state materials. The interaction between the electric quadrupole moment of the nucleus and the electric field gradient (EFG) at the nuclear site leads to an orientation-dependent frequency shift for each crystallite. Thus, in a static polycrystalline sample, a powder spectrum is produced, with overall linewidth falling typically into the kHz to MHz range for ordinary magnetic field strengths [29,30,33]. For half-integer spin nuclei, as it is the case of 23 Na (nuclear spin 3/2), the central transition (+1/2 M 1/2) is the only one usually observed, because it is not affected by the quadrupolar interaction to first-order and is thus much narrower than the satellite transitions [29]. Under fast MAS, the second-order residual broadening still affects the central transition frequency, which gives rise to spectra with a typical lineshape containing singularities, whose center of gravity and linewidth depend on the magnitude of the applied magnetic field and on the MAS speed employed [33]. From this lineshape it is possible, at least in principle, to extract the quadrupole parameters and the isotropic chemical shift of the resonance line [30,31]. For materials with no long-range structural order, the distribution of interatomic distances and bond angles gives rise to a featureless broad resonance pattern with a characteristic tail to low frequency, whose broadening is usually associated with both isotropic chemical shift and quadrupole parameters distribution. The direct determination of these distributions is not possible from just one such spectrum, although relevant information can be acquired from the changes in the spectral parameters with the magnetic field strength [29,33]. 23
1102
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
Taking into account this background information, one can recognize the patterns shown in Fig. 4 for the nonwashed samples as being typical of the crystalline arrangements involving the 23Na nuclei in the compounds Na2SiO3 [30] and Na2CO3, as also shown by XRD data. For the latter, the existence of two distinct crystalline sites is evident, as previously reported elsewhere [31,32]. On the other hand, the 23Na NMR spectra detected in the case of the washed samples (Fig. 5) constitutes an evidence for the presence of Na sites in a structurally disordered environment within the activated carbon network. These lines are broad and in some cases slightly asymmetric, pointing to a distribution of quadrupole parameters superimposed to a dispersion of isotropic chemical shifts [29]. A more detailed analysis of these lineshapes using several magnetic field strengths and multiple-quantum (MQ) methods is worthwhile and this is in our plans for the near future in order to clarify the chemical nature of these Nacontaining groups. However, some qualitative conclusions can be drawn from the spectra shown: First, it is clear that the mere detection of such 23Na NMR spectra in these samples is an evidence of the retention of Na groups in the structure of the washed activated materials. The washing with water leads to the release of sodium silicates and carbonates, but some sodium is retained, possibly bound to the carbon network. It is worth mentioning that, although the results here presented refer only to water washed samples prepared from rice hulls char, some preliminary analyses were also conducted with HNO3-treated samples derived from other lignocelullosic precursors and similar results were found. It seems therefore that the sodium retention in activated carbons prepared from chemical activation with NaOH is a general issue and not a particular characteristic of rice hulls derived materials. As for the values of the resonance shifts corresponding to the spectra shown in Fig. 5, some caution is needed when one tries to correlate these with chemical environments. The position of the centre of gravity of a quadrupolarbroadened NMR resonance line is in general different from the isotropic chemical shift associated with that peak, due to the isotropic shifts caused by the second-order quadrupolar effects. Furthermore, the magnitude of such effects is dependent on instrumental factors, such as the magnetic field strength and even the spinning speed employed in MAS [33]. Therefore, it is not immediate to assign the peak positions based on previously established isotropic chemical shifts of known chemical groups or structural units, as it is usual in NMR of spin-1/2 nuclei. Nevertheless, a qualitative comparison of the present results with other NMR data found in the literature for materials analyzed under similar experimental conditions can help in the identification of the chemical environments of the 23Na nuclei in the materials here investigated. Broad and featureless resonance lines with shapes and shifts similar to the ones here presented for the washed samples (Fig. 5) have been reported in many Na-containing glasses [29,31,32,34,35].
Usually these shifts have been associated with Na+ ions in a disordered local environment involving coordination to oxygen ions and/or water molecules. More interesting to the present work is the comparison of the shifts here found with the ones reported for Na groups occurring in coal samples. Previous works by Howarth et al. [36,37], using the same magnetic field strength here employed, have reported the detection of narrow 23Na MAS NMR resonance lines in several coal samples of different origins, with peak positions ranging from +2.3 to 1.7 ppm for raw coals. A noticeable high-field shift to the range from 0.9 to 15.4 ppm and a severe broadening of the resonance lines were reported for dried samples. For one particular coal sample the reverse effect was observed: a broad resonance line was found at 5.4 ppm, which was narrowed and shifted to 0.5 ppm upon controlled wetting. A consistent trend was found between the shifts and the coal ranks, which was interpreted as pointing to a close association between Na and organic matter in coal. The values of the shifts were interpreted as indicating the presence of hydrated Na+ ions bound to organic functional groups, specially carboxylates and phenoxides [29,30], at the surface of the coal pores. Also, the deliberate addition of sodium (from an NaCl aqueous solution) to some low-sodium raw coal samples lead to the observation of 23Na NMR spectra very similar to the ones detected in other coal samples naturally containing sodium, with no peak associated with NaCl. This indicated that coal acts as an ion-exchangeable medium, with Na+ ions being incorporated as similar surface groups whatever the Na source. More recently, 23Na MAS NMR spectra were also reported in both raw and demineralized coal samples presenting very low (a few ppm) Na contents, with broad asymmetric lines extending from 0 down to values beyond 100 ppm, which were associated with ionexchangeable Na species interacting with oxygen functional groups in the organic matter of the coal samples [38]. Taking into account these findings, we can associate the NMR lines shown in Fig. 5 for the washed samples with the presence of oxygenated surface Na groups at the edges of the aromatic lamellae in the porous carbon network. These groups would have been formed during the activation process by the reaction between alkali and carbon, remaining in the structure of the activated material even after water washing. As mentioned in Section 1, there have appeared many reports suggesting the formation of such surface Na groups in the literature on alkali reactions with carbons. But the retention of these groups even after extensive washing is a relevant feature clearly demonstrated by the NMR results reported here. The previous findings of similar Na surface groups in coals corroborate this interpretation and show that Na species can be easily incorporated into the activated carbon matrix, forming chemical functionalities that are, to some extent, resistant to water washing (or even to acid treatment). The existence of these surface Na groups can be of great relevance for catalytic applications of porous carbons, as it can bring some simi-
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
larity to what occurs for example in reactions involving ion-exchange in zeolites. This stimulates the pursue of more detailed investigations both on the chemical nature of these Na-containing groups and on the factors that influence their occurrence, such as the type of precursor used for the preparation of the activated carbon, the experimental conditions used in the activation, and the washing step employed to the attainment of the porous material. Finally, some interesting aspects related to the possible effect of Si on the occurrence of the Na surface groups deserve to be commented here. It was mentioned above that the activated sample prepared from the HF-leached precursor contains lesser amount of Na remaining after washing than the samples prepared from the silicon-rich precursor. Thus, it seems that the presence of silicon aids in the retention of Na surface groups in the porous structure of the carbon material. At this point it is not clear if this is a consequence of some chemical interaction involving Si, Na, and the organic functionalities at the surface of the pores or it is just a result of the more efficient water washing for the sample with increased surface area derived from the HF-precursor. This also constitutes a subject to be clarified by comparing samples prepared from different sources (including perhaps synthetic precursors with no silicon content) submitted to the same activation treatments. 5. Conclusion Chemical activation of charred rice hulls has been achieved by reaction with NaOH. After water washing, the activated samples showed reasonable development of porosity, specially when prepared from a precursor with reduced silicon content. XRD and 29Si and 23Na NMR allowed the identification of sodium carbonate and silicates as the main compounds produced in the activation process, but these products were readily removed by water washing. As for the retention of sodium in the structure of the activated samples, the following issues can be inferred from the present results: • The presence of residual Na in a disordered environment in the structure of the activated and washed samples was verified by the detection of broad lines in 23Na MAS NMR spectra. • These resonance lines, presenting large broadening due to the superposition of second-order quadrupolar effects and distribution of isotropic chemical shifts, were associated with the formation of complex Na-containing surface groups at the edges of the aromatic lamellae inside the porous structure of the activated samples. • The presence of silicon species in the precursor (rice hulls char) exerts great influence on the final product obtained by activation with NaOH, as they lead to the partial consumption of the activating agent (with formation of sodium silicates) and also because they appear to favor the retention of the Na groups even after water washing.
1103
Acknowledgements The partial support from Brazilian agencies CNPq, CAPES, FINEP, and FAPESP is gratefully acknowledged. The authors are also grateful to Dr. L.C. Machado, Dr. H.C. de Jesus, Dr. P.R.N. Silva e MSc., A.B. Soares for helping with many analyses. References [1] Marsh H, Yan DS, O’Grady TM, Wennerberg A. Formation of active carbons from cokes using potassium hydroxide. Carbon 1984;22:603–11. [2] Ehrburger P, Addoun A, Addoun F, Donnet JB. Carbonization of coals in the presence of alkaline hydroxides and carbonates: formation of activated carbons. Fuel 1986;65:1447–9. [3] Lozano-Castello D, Lillo-Ro´denas MA, Cazorla-Amoro´s D, LinaresSolano A. Preparation of activated carbons from Spanish anthracite. I. Activation by KOH. Carbon 2001;39:741–9. [4] Lillo-Ro´denas MA, Lozano-Castello D, Cazorla-Amoro´s D, LinaresSolano A. Preparation of activated carbons from Spanish anthracite. II. Activation by NaOH. Carbon 2001;39:751–9. [5] Perrin A, Celzard A, Albiniak A, Kaczmarczyk J, Mareˆche´ JF, Furdin G. NaOH activation of anthracites: effect of temperature on pore textures and methane storage ability. Carbon 2004;42: 2855–66. [6] Yoshizawa N, Maruyama K, Yamada Y, Ishikawa E, Kobayashi M, Toda Y, Shiraishi M. XRD evaluation of KOH activation process and influence of coal rank. Fuel 2002;81:1717–22. [7] Lozano-Castello D, Alcan˜iz-Monge J, de la Casa-Lillo MA, CazorlaAmoro´s D, Linares-Solano A. Advances in the study of methane storage in porous carbonaceous materials. Fuel 2002;81(14): 1777–803. [8] Lillo-Ro´denas MA, Cazorla-Amoro´s D, Linares-Solano A. Understanding chemical reactions between carbons and NaOH and KOH. An insight into the chemical activation mechanism. Carbon 2003;41:267–75. [9] Lillo-Ro´denas MA, Juan-Juan J, Cazorla-Amoro´s D, Linares-Solano A. About reactions occurring during chemical activation with hydroxides. Carbon 2004;42:1371–5. [10] Diaz-Te´ran J, Nevskaia DM, Fierro JLG, Lo´pez-Peinado AJ, Jerez A. Study of chemical activation process of a lignocellulosic material with KOH by XPS and XRD. Micropor Mesopor Mater 2003;60: 173–81. [11] Robau-Sa´nchez A, Cordero-de La Rosa F, Aguilar-Pliego J, AguilarElgue´zabal A. On the reaction mechanism of the chemical activation of Quercus Agrifolia char by alkaline hydroxides. J Porous Mater 2006;13:123–32. [12] Yamashita Y, Ouchi K. Influence of alkali on the carbonization process – I. Carbonization of 3,5-dimethylphenol-formaldehyde resin with NaOH. Carbon 1982;20:41–5. [13] Raymundo-Pin˜ero E, Azaı¨s P, Cacciaguerra T, Cazorla-Amoro´s D, Linares-Solano A, Be´guin F. KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organisation. Carbon 2005;43:786–95. [14] Xue R, Shen Z. Formation of graphite-potassium intercalation compounds during activation of MCMB with KOH. Carbon 2003;41: 1851–64. [15] Macia´-Agullo´ JA, Moore BC, Cazorla-Amoro´s D, Linares-Solano A. Chemical activation by KOH and NaOH of carbon materials with different crystallinity. Extended Abstracts, Carbon 2003, Oviedo (Spain); 2003. [16] Lee J-G, Cutler IB. Formation of silicon carbide from rice hulls. Am Ceram Soc Bull 1975;54:195–8. [17] Sun L, Gong K. Silicon-based materials from rice husks and their applications. Ind Eng Chem Res 2001;40:5861–77.
1104
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
[18] Freitas JCC, Moreira JS, Emmerich FG, Bonagamba TJ. Development of Si/C/N/O ceramics from pyrolyzed and heat-treated rice hulls. J Non-Cryst Solids 2004;341:77–85. [19] Guo Y, Yang S, Yu K, Zhao J, Wang Z, Xu H. The preparation and mechanism studies of rice husk based porous carbon. Mater Chem Phys 2002;74:320–3. [20] Oh GH, Park CR. Preparation and characteristics of rice-straw-based porous carbons with high adsorption capacity. Fuel 2002;81:327–36. [21] Yun CH, Park YH, Oh GH, Park CR. Contribution of inorganic components in precursors to porosity evolution in biomass-based porous carbons. Carbon 2003;41:2009–25. [22] Patel M, Karera A, Prasanna P. Effect of thermal and chemical treatments on carbon and silica contents in rice husk. J Mater Sci 1987;22:2457–64. [23] Freitas JCC, Emmerich FG, Bonagamba TJ. High resolution solidstate NMR study of the occurrence and thermal transformations of silicon-containing species in biomass materials. Chem Mater 2000;12:711–8. [24] Schetinno Jr MA, Freitas JCC, Cunha AG, Emmerich FG, Soares AB, Silva PRN. Preparation and characterization of chemically activated carbon from rice hulls. Quim Nova, in press. [25] Jiang W, Nadeau G, Zaghib K, Kinoshita K. Thermal analysis of the oxidation of natural graphite – effect of particle size. Thermochim Acta 2000;351:85–93. [26] Ma¨gi M, Lipmaa E, Samoson A, Engelhardt G, Grimmer AR. Solidstate high-resolution 29Si chemical shifts in silicates. J Phys Chem 1954;88:1518–22. [27] Engelhardt G, Michel D. High-resolution solid-state NMR of silicates and zeolites. Chichester: John Wiley and Sons; 1987. [28] Schneider J, Mastelaro VR, Papenucci H, Zanotto ED. 29Si MASNMR studies of Qn structural units in metasilicate glasses and their nucleating ability. J Non-Cryst Solids 2000;273:8–18.
[29] Mackenzie KJD, Smith ME. Multinuclear solid-state NMR of inorganic materials. Amsterdam: Pergamon; 2002. [30] Koller H, Engelhardt G, Kentgens APM, Sauer J. 23Na NMR spectroscopy of solids: interpretation of quadrupole interaction parameters and chemical shifts. J Phys Chem 1994;98:1544–51. [31] Egan JM, Mueller KT. Detection and identification of corrosion products of sodium aluminoborosilicate glasses by 23Na MQMAS and 1H ! 23Na CPMAS NMR. J Phys Chem B 2000;104:9580–6. [32] Jones AR, Winter R, Greaves GN, Smith IH. 23Na, 29Si, and 13C MAS NMR investigation of glass-forming reactions between Na2CO3 and SiO2. J Phys Chem B 2005;109:23154–61. [33] Smith ME, van Eck ERH. Recent advances in experimental solid state NMR methodology for half-integer spin quadrupolar nuclei. Prog Nucl Magn Reson Spec 1999;34:159–201. [34] Pandya N, Muenow DW, Sharma SK, Sherriff BL. The speciation of water in hydrated alkali silicate glasses. J Non-Cryst Solids 1994;176:140–6. [35] Belke´bir A, Rocha J, Esculcas AP, Berthet P, Poisson S, Gilbert B, Gabelica Z, Llabres G, Wijzen F, Rulmont A. Structural characterization of glassy phases in the system Na2O–Ga2O3–P2O5 by MASNMR, EXAFS and vibrational spectroscopy. I. Cations coordination. Spectrochim Acta Part A 2000;56:423–34. [36] Howarth OW, Ratcliffe GS, Burchill P. Solid state nuclear magnetic resonance studies of sodium and aluminum in coal. Fuel 1987;66:34–9. [37] Burchill P, Howarth OW, Sword BJ. MAS NMR studies of inorganic elements in coals and combustion residues. Fuel 1991;70:361–6. [38] Sakanishi K, Akashi E, Nakazato T, Tao H, Kawashima H, Saito I, Takarada T. Characterization of eluted metal components from coal during pretreatment and solvent extraction. Fuel 2004;83: 739–43.
NMR investigation on the occurrence of Na species in porous carbons prepared by NaOH activation Jair C.C. Freitas a,*, Miguel A. Schettino Jr. a, Alfredo G. Cunha a, Francisco G. Emmerich a, Antonio C. Bloise b, Eduardo R. de Azevedo b, Tito J. Bonagamba b b
a Departamento de Fı´sica, Universidade Federal do Espı´rito Santo, 29060-900 Vito´ria, ES, Brazil Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, P.O. Box 369, 13560-970 Sa˜o Carlos, SP, Brazil
Received 15 September 2006; accepted 4 December 2006 Available online 7 February 2007
Abstract A detailed investigation was conducted about the process of alkali activation of charred rice hulls using NaOH. A carbon-rich precursor was initially prepared from the pyrolysis of rice hulls under N2 atmosphere, part of it being leached with HF to remove silica. The precursor was then mixed with NaOH, heat-treated at activation temperatures from 600 to 800 C, and part of the product was finally washed with distilled water. Thermogravimetric curves under O2 flux showed a strong reduction in the ash content of the activated samples, indicating the consumption of silica during the activation process. From X-ray diffractometry, 29Si, and 23Na NMR spectroscopy, it was possible to identify the formation of sodium carbonate and silicates in the non-washed samples. After washing, all these compounds were removed and specific surface area measurements indicated a substantial porosity development, with larger surface area values obtained for the samples prepared from the HF-leached precursor. The use of 23Na NMR spectroscopy indicated the retention of sodium in the washed samples, in a chemical environment distinct from carbonates and silicates. The shapes and positions of the observed resonance lines pointed to a disordered environment, associated with oxygenated surface groups within the porous structure of the activated carbons. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The use of alkali hydroxides (specially NaOH and KOH) as activating agents for the production of microporous active carbons has attracted great interest due to the valuable properties of the materials produced by this process [1–6]. The method of chemical activation presents several advantages over the physical activation, leading to materials with large specific surface area and elevated microporosity with relatively narrow pore-size distribution. This subject has been the focus of many detailed investigations in recent years due the necessity of novel methods for natural gas storage aiming automotive applications and the *
Corresponding author. Tel.: +55 27 3335 2483; fax: +55 27 3335 2823. E-mail address: [email protected] (J.C.C. Freitas).
0008-6223/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.12.006
potentially promising use of porous carbons for that purpose [5,7]. Nevertheless, a complete understanding of the chemical activation process is still lacking. A huge effort has been devoted to clarify the interaction between the alkali compounds and the carbon matrix and to correlate this to the external parameters involved in the activation process, such as temperature, time, atmosphere, and type of precursors [3–5]. The most frequently reported results indicate the formation of alkali carbonate, hydrogen, and the alkaline metal, according to the overall reaction: 6MeOH þ 2C ! 2Me þ 3H2 þ 2Me2 CO3 ;
ð1Þ
where Me = Na or K [8,9]. The detection of alkaline metal traces in the activation equipment has been in fact reported and this corroborates the proposed reaction [5,8,10]. The
1098
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
hydrogen evolution has been detected starting from temperatures in the range 200–600 C, depending on the reactivity of the carbon precursor [9]. The evolution of CO and CO2 has also been reported, but this can be associated both with the decomposition of alkali carbonates at high temperatures (above 800 C) and with the devolatilization reactions of the carbon precursor itself, especially in the case of activation of lignocellulosic precursors and high-volatile content coals [2,8,10]. More recently, the participation of nitrogen gas (which is usually present in the dynamic atmosphere of heat-treatment) in the activation reactions has been proposed, with detection of cyanide groups in the structure of the activated materials [11]. The formation of surface complexes involving the participation of the alkali in oxygenated groups of the type C–O–Me has long been invoked in many reports. Marsh et al., in a study on the KOH activation of petroleum cokes, suggested that the addition of KOH to a green coke could lead to the formation of alkalides in the form –OK, with the subsequent separation of aromatic lamellae by oxidation of cross-linking carbon atoms and formation of functional groups on the edges of these lamellae [1]. The K atoms liberated at high-temperature reactions might as well intercalate and cause an increase in the separation of the lamellae. The removal of potassium salts by water leaching would then allow the development of microporosity. Ehrburger et al. also mentioned the existence of surface complex salts, acting as active gasification sites [2]. In a detailed study on the influence of NaOH on the carbonization of a phenol-formaldehyde resin, Yamashita and Ouchi showed that Na atoms gave rise to –ONa groups attached to aromatic rings, being later decomposed to form carbonates and oxides [12]. An important distinction between the behavior of K and Na with respect to the activation mechanism was addressed in recent works dealing with materials with different degrees of crystallinity, such as graphite, carbon fibers, mesocarbon microbeads, and nanotubes [10,13–15]. While there are strong evidences of K intercalation between graphene layers, especially in the case of well ordered materials, the trend for Na is distinct: activation with NaOH is mostly effective for disordered carbon materials, with no indication of formation of intercalation compounds. Otherwise, in this case the mechanism involves the occurrence of surface reactions with high defective sites. This work describes the NaOH activation of charred rice hulls and the investigation on the nature of the products generated in the process, mainly by using X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) spectroscopy. Rice hulls are known as a rich and abundant source for the preparation of silicon-based products of technological interest, such as silicon carbide, silicon nitride, and high purity silica, among many others [16– 18]. There is a particular interest in the research on the profitable utilization of rice hulls as they represent a waste product largely produced in many countries. In recent
studies, several authors reported the preparation of active carbons with high surface area (in the range 2500– 3000 m2/g) from chemical activation of rice husks [19–21]. The presence of silicon in rice hulls and husks has been long associated with the occurrence of amorphous hydrated silica, but some results have indicated that there is a substantial part of the silicon species that are chemically bonded to organic groups [22,23]. The influence of these silicon compounds on the reactions occurring during the NaOH activation of charred rice hulls is investigated here by 29Si NMR, while 23Na NMR is used to detect the formation of Na species and particularly the presence of surface Na groups, which are retained in the carbon structure even after washing the activated materials. 2. Experimental methods 2.1. Samples preparation First, a carbon-rich precursor was prepared from the pyrolysis of raw rice hulls under N2 atmosphere at 700 C for 4 h. This material contained around 53 wt.% of fixed carbon, with the remaining part being mineral matter, mostly silica. Part of this material was leached with HF in order to obtain a nearly silica-free precursor. Next, the precursor was mixed with NaOH by two methods: by direct powder maceration or by impregnation within an aqueous solution. The results to be presented later refer to the samples prepared by physical mixture, but no considerable difference was observed between the general characteristics of the samples prepared by either method (physical mixture of NaOH and char or aqueous impregnation) while keeping unaltered the other conditions. The NaOH/ carbon weight ratio was kept equal to 3:1, by taking into account the fixed carbon content of the precursor. The activation heat-treatments were conducted under N2 flow at temperatures ranging from 600 to 800 C. For each heat-treated sample, part of the material was washed with distilled water and another part was separated for comparison (these are respectively called washed and non-washed samples). The precursor previously leached with HF was submitted to similar activation methods, at the temperature of 800 C. Further details about samples preparation and basic characterization of the activated samples can be found elsewhere [24].
2.2. Measurements X-ray diffraction (XRD) measurements were conducted on powdered samples, in a Rigaku diffractometer using Cu Ka radiation ˚ ), with 2h ranging from 10 to 80 in steps of 0.05. Thermo(k = 1.5418 A gravimetric (TG) curves were recorded in a Shimadzu thermobalance up to 1000 C under O2 flow, using powdered samples with initial mass around 15–20 mg. Specific surface area measurements were performed in an Autosorb equipment, using the BET method to analyze the nitrogen adsorption at 77 K. Solid-state NMR spectra were recorded at room temperature in a Varian INOVA 400 spectrometer, operating at 79.4 MHz for 29Si and 105.7 MHz for 23Na nuclei (magnetic field of 9.4 T). 29Si chemical shifts – given in parts per million (ppm) – were externally referred to tetramethylsilane (TMS) by using kaolin as a secondary reference. 23Na shift values were referred to an aqueous solution of NaCl (0.1 M). All spectra were acquired with single pulse excitation (pulse widths of 5.1 and 3.5 ls for 29Si and 23Na, respectively) and magic angle spinning (MAS) with a rotor speed of 6.0 kHz. Recycle delays varied in the range 1–10 s, being adjusted to avoid saturation problems. Nearly 100–500 transients were recorded for 29Si and 23Na NMR measurements on non-washed samples, whereas typically 4000 transients were accumulated for 23Na NMR measurements on washed samples (which gave no 29Si NMR signal, as detailed below).
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
3. Results 3.1. Structure and textural properties Fig. 1 shows the TG curves recorded under oxidizing conditions for the precursors and some activated and washed samples. A strong reduction in the ash content of the activated samples compared to the silicon-rich precursor can be observed from the drop in the residue fraction left after complete combustion, which indicates the consumption of silica during the activation process. This is expected based on the possible chemical reactions involving NaOH and SiO2 at room temperature and above [19], which generate mainly water-soluble sodium silicates. These products have been clearly identified in the XRD patterns of the non-washed activated samples, which are shown in Fig. 2. Through the whole temperature range here investigated, sodium silicates and sodium carbonate
Fig. 1. TG curves recorded under oxidizing atmosphere for the precursor, the HF-leached precursor, and the washed activated samples prepared from these precursors at the indicated activation temperatures.
Fig. 2. XRD patterns of the non-washed activated samples, prepared at the indicated temperatures. The HF labeled sample was prepared from the HF-leached precursor.
1099
(including hydrated forms) constituted the main phases formed. No XRD peaks associated with crystalline SiO2 were detected, neither in the precursor nor in the activated samples, which is in accordance with previous results showing the amorphous character of silica in raw rice hulls and in chars produced at such low temperatures [23]. In the case of the HF-leached precursor, Na2CO3 was the only detected product. The abundant presence of Na2CO3 in the structure of the activated samples is an important indication of the success of the activation process, as indicated by the overall chemical reaction (1), and this fact has been corroborated by many related works [2–5]. The XRD patterns of the activated and washed samples (not shown) exhibited the profile typical of the turbostratic structure. From the carbon 002 diffraction peak in these patterns, ˚ ) and enlarged interone obtained reduced Lc values (7–9 A ˚ planar spacing (4.0–4.5 A) for the activated samples, which is in accordance with other published results on activated carbons [6]. A further point to be noted in the TG curves in Fig. 1 is the increase in ash content of the 800 C activated sample prepared from the HF-leached precursor (curve indicated as 800 C – HF) when compared to the ash content of this precursor (curve indicated as Precursor – HF). This constitutes an indication of the production of an inorganic material as result of chemical reactions occurring during activation and its retention in the activated carbon structure even after water washing. As this sample is essentially silica-free, this product is expected to be related to some sodium-containing compounds. Elemental Na contents of the samples activated at 800 C, determined from spectrophotometer analysis of the ashes, confirm this result. The sample prepared from the silicon-rich precursor presented a Na content of 42 wt.%, whereas the value corresponding to the sample prepared from the HF-leached precursor was 31 wt.% (both values referred to the ash weight in each sample). This shows that samples prepared from the silicon-rich precursor had higher ability to retain Na compounds after water washing, which was consistent with the trend shown in the TG curves (Fig. 1) of a larger residue fraction left after combustion for the activated samples prepared from the silicon-rich precursor as compared to the sample prepared from the HF-leached precursor. The TG curves exhibited in Fig. 1 for the washed activated samples clearly showed a decrease in the oxidation temperature when compared to the precursors. This increased reactivity with oxygen is related to the development of the porous structure of the activated carbon, with enlargement of the surface area accessible to oxygen molecules, as shown in previously works [25]. The measurements of BET surface area confirm this result. All nonwashed samples exhibited negligible BET surface areas. After water washing, the removal of the inorganic products allowed the porosity development. The BET surface area then reached values around 450 m2/g for activated samples prepared from the silicon-rich precursor. In the case of the samples prepared from the HF-leached precursor, the
1100
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
surface area reached a much higher value (1400 m2/g). The pore size distribution determined for these materials indicated that all activated samples were essentially micro˚ porous, with average pore diameter around 16–18 A [24]. These values of specific surface area are just moderate when compared to activated carbons obtained from other natural precursors by similar activation methods [3,4,19– 21]. The increase in the surface area of the activated samples when compared to the precursors (around 60 and 530 m2/g for the silicon-rich and the HF-leached precursors, respectively) is, nevertheless, evident, confirming the success of the activation process. The relatively low values of specific surface areas for the samples prepared from the silicon-rich precursor can be understood considering that a substantial part of the NaOH initially mixed with the precursor was consumed in reactions with the silicon species. This caused a significant reduction in the effective NaOH/carbon ratio, which was therefore deviated from the value of 3:1. This ratio is usually quoted as the optimum one for attaining the best porosity development [4,19], but this depends obviously on the type of precursor used. In our case, for the silicon-rich precursor a higher NaOH/carbon ratio would likely lead to a larger porosity development, since more hydroxide would be available after reaction with silica. In fact, when a precursor with low silicon content was used, more NaOH was available for the reaction with carbon, which lead to a larger surface area for the sample prepared from the HF-leached precursor. Furthermore, the HF leaching process itself also contributes to a porosity development, due the removal of silicon species which were intimately bonded to the carbon matrix [21]. Both effects lead to the attainment of an activated material with large surface area (1400 m2/g), although still below some values previously quoted in the literature [19,20]. Nevertheless, in spite of the only partially efficient activation method here employed, the produced samples showed reasonable development of microporosity and were thus appropriate for the investigation on the local chemical environment of the Na and Si species in the structure of the activated materials, as detailed below. 3.2. NMR measurements Fig. 3 shows the 29Si MAS NMR spectra of the precursor and the non-washed activated samples. It is worth noting that the washed activated samples did not give any 29Si NMR signal, even with changes in recycle delays and increase in the number of transients. This is in accordance with the previously discussed TG and XRD results, which revealed drop in ash content and absence of crystalline silicon compounds in the washed activated samples, respectively. Therefore, we can conclude that the water washing process was effective in the removal of essentially all silicon compounds formed by reaction with NaOH. These watersoluble compounds can be recovered and recycled from the solution resulting of the washing step.
Fig. 3. 29Si MAS NMR spectra of the precursor, the non-washed activated samples prepared at the indicated activation temperatures, and a sample prepared from rice hulls ashes under similar conditions used for the activated samples.
The spectrum shown in Fig. 3 for the precursor exhibited a broad resonance centered at ca. 115 ppm, which is characteristic of an amorphous form of silica [23]. In the activated samples, no signal at similar chemical shifts was detected, indicating the complete consumption of silica by the chemical reactions occurring during the activation process with NaOH. On the other hand, in all spectra there was a narrow line at 76.8 ppm, associated with crystalline Na2SiO3 [26], which is consistent with the previous XRD results. An ill-defined resonance appeared near 68 ppm in the spectra of the samples activated from 600 C upward, being more easily detectable for samples activated at 700 and 800 C. The broad shape and the chemical shift range of this resonance suggest the occurrence of Si groups connected through oxygen bridges to Na-containing groups in a structurally disordered network, as it is found in sodium-silicate glasses [27,28]. Besides these resonances, a narrow peak was observed at 62.0 ppm for the sample with activation temperature of 800 C. This last peak was also detected in the 29Si NMR spectrum of a sample prepared by mixing NaOH powder with rice hulls ashes (obtained after combustion in air) and heat-treating this mixture at 700 C (see the top spectrum in Fig. 3), which indicates it is associated with a reaction occurring between silica and NaOH and not involving the carbonaceous matrix. The reported chemical shift value points to a monomeric Q0 unit, i.e., a silica tetrahedron not connected to other similar tetrahedra [27,29], which, also by comparison with the XRD patterns of these samples, indicates that this peak can be attributed to the compound Na4SiO4. The 23Na MAS NMR spectra recorded for the nonwashed samples activated at temperatures ranging from 600 to 800 C are displayed in Fig. 4, along with the corresponding spectrum of synthetic Na2CO3. As can be clearly seen, the spectrum of all non-washed samples exhibited the same features as the Na2CO3 one, with two broad bands extending nearly from 10 to 38 ppm. The broad character
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
1101
Fig. 4. 23Na MAS NMR spectra of the non-washed activated samples, prepared at the indicated activation temperatures, compared to the spectrum corresponding to sodium carbonate. The HF labeled sample was prepared from the HF-leached precursor.
Fig. 5. 23Na MAS NMR spectra of the washed activated samples, prepared at the indicated activation temperatures, compared to the spectrum corresponding to sodium carbonate. The HF labeled sample was prepared from the HF-leached precursor.
of these spectra is related to the quadrupolar nature of the 23 Na nuclei, which are therefore sensitive to electric field gradients besides the local magnetic fields created by the surroundings [29–31]. Also, the presence of small but perceptible sharp features around 3.8, 1.8, 8.0, and 21.6 ppm indicates the occurrence of second-order quadrupolar broadening effects not removed by MAS. In the spectra of the samples prepared from the silicon-rich precursor, a resonance pattern was also observed around 16 ppm, associated with crystalline Na2SiO3 [30], in agreement with the previous XRD and 29Si NMR data. Also, with increasing the activation temperature, it was observed a small increase in the relative intensity in the region near 10 ppm, concomitantly with the decrease in the contribution due to crystalline Na2SiO3. This suggests the consumption of Na2SiO3 by the reaction with Na2CO3, leading to the formation of new (Na,Si)-based products probably in a disordered network, in accordance with the 29Si NMR results presented before and also with previous work on glass-forming reactions between Na2CO3 and SiO2 [32]. These resonances associated with Na species related to Si were obviously missing in the spectrum corresponding to the activated sample prepared from the HF-leached precursor. The 23Na MAS NMR spectra of the washed activated samples presented in Fig. 5 are of a completely different nature. Just for comparison, the spectrum corresponding to Na2CO3 is shown again. As it can be observed, no signal associated with sodium carbonate or silicates was observed in the NMR spectra of the activated and washed samples, which agrees with the TG and XRD results. On the other hand, a broad and featureless maximum extending from 7 to 30 ppm and centered around 5 to 10 ppm was readily detected, in spite of the low ash content of the washed samples. Similar NMR lines have been reported in sodium-containing glassy materials, where the distribution of isotropic chemical shifts and structural parameters in a disordered network explains the broad shape [29,31].
In the present case, this resonance is associated with the presence of oxygenated surface Na groups at the edges of the aromatic lamellae, as detailed below. 4. Discussion Quadrupolar nuclei (i.e., those with spin >1/2) such as Na usually give rise to severely broadened NMR spectra in solid-state materials. The interaction between the electric quadrupole moment of the nucleus and the electric field gradient (EFG) at the nuclear site leads to an orientation-dependent frequency shift for each crystallite. Thus, in a static polycrystalline sample, a powder spectrum is produced, with overall linewidth falling typically into the kHz to MHz range for ordinary magnetic field strengths [29,30,33]. For half-integer spin nuclei, as it is the case of 23 Na (nuclear spin 3/2), the central transition (+1/2 M 1/2) is the only one usually observed, because it is not affected by the quadrupolar interaction to first-order and is thus much narrower than the satellite transitions [29]. Under fast MAS, the second-order residual broadening still affects the central transition frequency, which gives rise to spectra with a typical lineshape containing singularities, whose center of gravity and linewidth depend on the magnitude of the applied magnetic field and on the MAS speed employed [33]. From this lineshape it is possible, at least in principle, to extract the quadrupole parameters and the isotropic chemical shift of the resonance line [30,31]. For materials with no long-range structural order, the distribution of interatomic distances and bond angles gives rise to a featureless broad resonance pattern with a characteristic tail to low frequency, whose broadening is usually associated with both isotropic chemical shift and quadrupole parameters distribution. The direct determination of these distributions is not possible from just one such spectrum, although relevant information can be acquired from the changes in the spectral parameters with the magnetic field strength [29,33]. 23
1102
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
Taking into account this background information, one can recognize the patterns shown in Fig. 4 for the nonwashed samples as being typical of the crystalline arrangements involving the 23Na nuclei in the compounds Na2SiO3 [30] and Na2CO3, as also shown by XRD data. For the latter, the existence of two distinct crystalline sites is evident, as previously reported elsewhere [31,32]. On the other hand, the 23Na NMR spectra detected in the case of the washed samples (Fig. 5) constitutes an evidence for the presence of Na sites in a structurally disordered environment within the activated carbon network. These lines are broad and in some cases slightly asymmetric, pointing to a distribution of quadrupole parameters superimposed to a dispersion of isotropic chemical shifts [29]. A more detailed analysis of these lineshapes using several magnetic field strengths and multiple-quantum (MQ) methods is worthwhile and this is in our plans for the near future in order to clarify the chemical nature of these Nacontaining groups. However, some qualitative conclusions can be drawn from the spectra shown: First, it is clear that the mere detection of such 23Na NMR spectra in these samples is an evidence of the retention of Na groups in the structure of the washed activated materials. The washing with water leads to the release of sodium silicates and carbonates, but some sodium is retained, possibly bound to the carbon network. It is worth mentioning that, although the results here presented refer only to water washed samples prepared from rice hulls char, some preliminary analyses were also conducted with HNO3-treated samples derived from other lignocelullosic precursors and similar results were found. It seems therefore that the sodium retention in activated carbons prepared from chemical activation with NaOH is a general issue and not a particular characteristic of rice hulls derived materials. As for the values of the resonance shifts corresponding to the spectra shown in Fig. 5, some caution is needed when one tries to correlate these with chemical environments. The position of the centre of gravity of a quadrupolarbroadened NMR resonance line is in general different from the isotropic chemical shift associated with that peak, due to the isotropic shifts caused by the second-order quadrupolar effects. Furthermore, the magnitude of such effects is dependent on instrumental factors, such as the magnetic field strength and even the spinning speed employed in MAS [33]. Therefore, it is not immediate to assign the peak positions based on previously established isotropic chemical shifts of known chemical groups or structural units, as it is usual in NMR of spin-1/2 nuclei. Nevertheless, a qualitative comparison of the present results with other NMR data found in the literature for materials analyzed under similar experimental conditions can help in the identification of the chemical environments of the 23Na nuclei in the materials here investigated. Broad and featureless resonance lines with shapes and shifts similar to the ones here presented for the washed samples (Fig. 5) have been reported in many Na-containing glasses [29,31,32,34,35].
Usually these shifts have been associated with Na+ ions in a disordered local environment involving coordination to oxygen ions and/or water molecules. More interesting to the present work is the comparison of the shifts here found with the ones reported for Na groups occurring in coal samples. Previous works by Howarth et al. [36,37], using the same magnetic field strength here employed, have reported the detection of narrow 23Na MAS NMR resonance lines in several coal samples of different origins, with peak positions ranging from +2.3 to 1.7 ppm for raw coals. A noticeable high-field shift to the range from 0.9 to 15.4 ppm and a severe broadening of the resonance lines were reported for dried samples. For one particular coal sample the reverse effect was observed: a broad resonance line was found at 5.4 ppm, which was narrowed and shifted to 0.5 ppm upon controlled wetting. A consistent trend was found between the shifts and the coal ranks, which was interpreted as pointing to a close association between Na and organic matter in coal. The values of the shifts were interpreted as indicating the presence of hydrated Na+ ions bound to organic functional groups, specially carboxylates and phenoxides [29,30], at the surface of the coal pores. Also, the deliberate addition of sodium (from an NaCl aqueous solution) to some low-sodium raw coal samples lead to the observation of 23Na NMR spectra very similar to the ones detected in other coal samples naturally containing sodium, with no peak associated with NaCl. This indicated that coal acts as an ion-exchangeable medium, with Na+ ions being incorporated as similar surface groups whatever the Na source. More recently, 23Na MAS NMR spectra were also reported in both raw and demineralized coal samples presenting very low (a few ppm) Na contents, with broad asymmetric lines extending from 0 down to values beyond 100 ppm, which were associated with ionexchangeable Na species interacting with oxygen functional groups in the organic matter of the coal samples [38]. Taking into account these findings, we can associate the NMR lines shown in Fig. 5 for the washed samples with the presence of oxygenated surface Na groups at the edges of the aromatic lamellae in the porous carbon network. These groups would have been formed during the activation process by the reaction between alkali and carbon, remaining in the structure of the activated material even after water washing. As mentioned in Section 1, there have appeared many reports suggesting the formation of such surface Na groups in the literature on alkali reactions with carbons. But the retention of these groups even after extensive washing is a relevant feature clearly demonstrated by the NMR results reported here. The previous findings of similar Na surface groups in coals corroborate this interpretation and show that Na species can be easily incorporated into the activated carbon matrix, forming chemical functionalities that are, to some extent, resistant to water washing (or even to acid treatment). The existence of these surface Na groups can be of great relevance for catalytic applications of porous carbons, as it can bring some simi-
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
larity to what occurs for example in reactions involving ion-exchange in zeolites. This stimulates the pursue of more detailed investigations both on the chemical nature of these Na-containing groups and on the factors that influence their occurrence, such as the type of precursor used for the preparation of the activated carbon, the experimental conditions used in the activation, and the washing step employed to the attainment of the porous material. Finally, some interesting aspects related to the possible effect of Si on the occurrence of the Na surface groups deserve to be commented here. It was mentioned above that the activated sample prepared from the HF-leached precursor contains lesser amount of Na remaining after washing than the samples prepared from the silicon-rich precursor. Thus, it seems that the presence of silicon aids in the retention of Na surface groups in the porous structure of the carbon material. At this point it is not clear if this is a consequence of some chemical interaction involving Si, Na, and the organic functionalities at the surface of the pores or it is just a result of the more efficient water washing for the sample with increased surface area derived from the HF-precursor. This also constitutes a subject to be clarified by comparing samples prepared from different sources (including perhaps synthetic precursors with no silicon content) submitted to the same activation treatments. 5. Conclusion Chemical activation of charred rice hulls has been achieved by reaction with NaOH. After water washing, the activated samples showed reasonable development of porosity, specially when prepared from a precursor with reduced silicon content. XRD and 29Si and 23Na NMR allowed the identification of sodium carbonate and silicates as the main compounds produced in the activation process, but these products were readily removed by water washing. As for the retention of sodium in the structure of the activated samples, the following issues can be inferred from the present results: • The presence of residual Na in a disordered environment in the structure of the activated and washed samples was verified by the detection of broad lines in 23Na MAS NMR spectra. • These resonance lines, presenting large broadening due to the superposition of second-order quadrupolar effects and distribution of isotropic chemical shifts, were associated with the formation of complex Na-containing surface groups at the edges of the aromatic lamellae inside the porous structure of the activated samples. • The presence of silicon species in the precursor (rice hulls char) exerts great influence on the final product obtained by activation with NaOH, as they lead to the partial consumption of the activating agent (with formation of sodium silicates) and also because they appear to favor the retention of the Na groups even after water washing.
1103
Acknowledgements The partial support from Brazilian agencies CNPq, CAPES, FINEP, and FAPESP is gratefully acknowledged. The authors are also grateful to Dr. L.C. Machado, Dr. H.C. de Jesus, Dr. P.R.N. Silva e MSc., A.B. Soares for helping with many analyses. References [1] Marsh H, Yan DS, O’Grady TM, Wennerberg A. Formation of active carbons from cokes using potassium hydroxide. Carbon 1984;22:603–11. [2] Ehrburger P, Addoun A, Addoun F, Donnet JB. Carbonization of coals in the presence of alkaline hydroxides and carbonates: formation of activated carbons. Fuel 1986;65:1447–9. [3] Lozano-Castello D, Lillo-Ro´denas MA, Cazorla-Amoro´s D, LinaresSolano A. Preparation of activated carbons from Spanish anthracite. I. Activation by KOH. Carbon 2001;39:741–9. [4] Lillo-Ro´denas MA, Lozano-Castello D, Cazorla-Amoro´s D, LinaresSolano A. Preparation of activated carbons from Spanish anthracite. II. Activation by NaOH. Carbon 2001;39:751–9. [5] Perrin A, Celzard A, Albiniak A, Kaczmarczyk J, Mareˆche´ JF, Furdin G. NaOH activation of anthracites: effect of temperature on pore textures and methane storage ability. Carbon 2004;42: 2855–66. [6] Yoshizawa N, Maruyama K, Yamada Y, Ishikawa E, Kobayashi M, Toda Y, Shiraishi M. XRD evaluation of KOH activation process and influence of coal rank. Fuel 2002;81:1717–22. [7] Lozano-Castello D, Alcan˜iz-Monge J, de la Casa-Lillo MA, CazorlaAmoro´s D, Linares-Solano A. Advances in the study of methane storage in porous carbonaceous materials. Fuel 2002;81(14): 1777–803. [8] Lillo-Ro´denas MA, Cazorla-Amoro´s D, Linares-Solano A. Understanding chemical reactions between carbons and NaOH and KOH. An insight into the chemical activation mechanism. Carbon 2003;41:267–75. [9] Lillo-Ro´denas MA, Juan-Juan J, Cazorla-Amoro´s D, Linares-Solano A. About reactions occurring during chemical activation with hydroxides. Carbon 2004;42:1371–5. [10] Diaz-Te´ran J, Nevskaia DM, Fierro JLG, Lo´pez-Peinado AJ, Jerez A. Study of chemical activation process of a lignocellulosic material with KOH by XPS and XRD. Micropor Mesopor Mater 2003;60: 173–81. [11] Robau-Sa´nchez A, Cordero-de La Rosa F, Aguilar-Pliego J, AguilarElgue´zabal A. On the reaction mechanism of the chemical activation of Quercus Agrifolia char by alkaline hydroxides. J Porous Mater 2006;13:123–32. [12] Yamashita Y, Ouchi K. Influence of alkali on the carbonization process – I. Carbonization of 3,5-dimethylphenol-formaldehyde resin with NaOH. Carbon 1982;20:41–5. [13] Raymundo-Pin˜ero E, Azaı¨s P, Cacciaguerra T, Cazorla-Amoro´s D, Linares-Solano A, Be´guin F. KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organisation. Carbon 2005;43:786–95. [14] Xue R, Shen Z. Formation of graphite-potassium intercalation compounds during activation of MCMB with KOH. Carbon 2003;41: 1851–64. [15] Macia´-Agullo´ JA, Moore BC, Cazorla-Amoro´s D, Linares-Solano A. Chemical activation by KOH and NaOH of carbon materials with different crystallinity. Extended Abstracts, Carbon 2003, Oviedo (Spain); 2003. [16] Lee J-G, Cutler IB. Formation of silicon carbide from rice hulls. Am Ceram Soc Bull 1975;54:195–8. [17] Sun L, Gong K. Silicon-based materials from rice husks and their applications. Ind Eng Chem Res 2001;40:5861–77.
1104
J.C.C. Freitas et al. / Carbon 45 (2007) 1097–1104
[18] Freitas JCC, Moreira JS, Emmerich FG, Bonagamba TJ. Development of Si/C/N/O ceramics from pyrolyzed and heat-treated rice hulls. J Non-Cryst Solids 2004;341:77–85. [19] Guo Y, Yang S, Yu K, Zhao J, Wang Z, Xu H. The preparation and mechanism studies of rice husk based porous carbon. Mater Chem Phys 2002;74:320–3. [20] Oh GH, Park CR. Preparation and characteristics of rice-straw-based porous carbons with high adsorption capacity. Fuel 2002;81:327–36. [21] Yun CH, Park YH, Oh GH, Park CR. Contribution of inorganic components in precursors to porosity evolution in biomass-based porous carbons. Carbon 2003;41:2009–25. [22] Patel M, Karera A, Prasanna P. Effect of thermal and chemical treatments on carbon and silica contents in rice husk. J Mater Sci 1987;22:2457–64. [23] Freitas JCC, Emmerich FG, Bonagamba TJ. High resolution solidstate NMR study of the occurrence and thermal transformations of silicon-containing species in biomass materials. Chem Mater 2000;12:711–8. [24] Schetinno Jr MA, Freitas JCC, Cunha AG, Emmerich FG, Soares AB, Silva PRN. Preparation and characterization of chemically activated carbon from rice hulls. Quim Nova, in press. [25] Jiang W, Nadeau G, Zaghib K, Kinoshita K. Thermal analysis of the oxidation of natural graphite – effect of particle size. Thermochim Acta 2000;351:85–93. [26] Ma¨gi M, Lipmaa E, Samoson A, Engelhardt G, Grimmer AR. Solidstate high-resolution 29Si chemical shifts in silicates. J Phys Chem 1954;88:1518–22. [27] Engelhardt G, Michel D. High-resolution solid-state NMR of silicates and zeolites. Chichester: John Wiley and Sons; 1987. [28] Schneider J, Mastelaro VR, Papenucci H, Zanotto ED. 29Si MASNMR studies of Qn structural units in metasilicate glasses and their nucleating ability. J Non-Cryst Solids 2000;273:8–18.
[29] Mackenzie KJD, Smith ME. Multinuclear solid-state NMR of inorganic materials. Amsterdam: Pergamon; 2002. [30] Koller H, Engelhardt G, Kentgens APM, Sauer J. 23Na NMR spectroscopy of solids: interpretation of quadrupole interaction parameters and chemical shifts. J Phys Chem 1994;98:1544–51. [31] Egan JM, Mueller KT. Detection and identification of corrosion products of sodium aluminoborosilicate glasses by 23Na MQMAS and 1H ! 23Na CPMAS NMR. J Phys Chem B 2000;104:9580–6. [32] Jones AR, Winter R, Greaves GN, Smith IH. 23Na, 29Si, and 13C MAS NMR investigation of glass-forming reactions between Na2CO3 and SiO2. J Phys Chem B 2005;109:23154–61. [33] Smith ME, van Eck ERH. Recent advances in experimental solid state NMR methodology for half-integer spin quadrupolar nuclei. Prog Nucl Magn Reson Spec 1999;34:159–201. [34] Pandya N, Muenow DW, Sharma SK, Sherriff BL. The speciation of water in hydrated alkali silicate glasses. J Non-Cryst Solids 1994;176:140–6. [35] Belke´bir A, Rocha J, Esculcas AP, Berthet P, Poisson S, Gilbert B, Gabelica Z, Llabres G, Wijzen F, Rulmont A. Structural characterization of glassy phases in the system Na2O–Ga2O3–P2O5 by MASNMR, EXAFS and vibrational spectroscopy. I. Cations coordination. Spectrochim Acta Part A 2000;56:423–34. [36] Howarth OW, Ratcliffe GS, Burchill P. Solid state nuclear magnetic resonance studies of sodium and aluminum in coal. Fuel 1987;66:34–9. [37] Burchill P, Howarth OW, Sword BJ. MAS NMR studies of inorganic elements in coals and combustion residues. Fuel 1991;70:361–6. [38] Sakanishi K, Akashi E, Nakazato T, Tao H, Kawashima H, Saito I, Takarada T. Characterization of eluted metal components from coal during pretreatment and solvent extraction. Fuel 2004;83: 739–43.