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Pore size control in activated carbons obtained by pyrolysis under different conditions of chemically impregnated cork
J. Anal. Appl. Pyrolysis 75 (2006) 120–127 www.elsevier.com/locate/jaap
Pore size control in activated carbons obtained by pyrolysis under different conditions of chemically impregnated cork P.J.M. Carrott *, M.M.L. Ribeiro Carrott, P.A.M. Moura˜o Centro de Quı´mica de E´vora and Departamento de Quı´mica, Universidade de E´vora, Cole´gio Luı´s Anto´nio Verney, 7000-671 E´vora, Portugal Received 21 December 2004; accepted 27 April 2005 Available online 13 June 2005
Abstract Activated carbons were prepared by the pyrolysis of cork impregnated with potassium and sodium hydroxides and carbonates as well as phosphoric acid and the effect of five experimental parameters, namely method of impregnation, impregnant concentration, mass ratio, precursor particle size and pyrolysis temperature, were studied. It is shown that cork is a versatile precursor and allows us to prepare a wide variety of materials with quite different pore structural characteristics by precise control of the impregnation and pyrolysis conditions. Even under relatively mild conditions, it was possible to produce cork based carbons with high pore volumes, in the range 0.5–0.7 cm3 g1, and to simultaneously control the mean pore width over a three-fold range from a value as low as 0.7 nm up to a value as high as 2.2 nm. The best materials produced present pore structural characteristics which are significantly different to the vast majority of commercial activated carbons. In particular, the possibility of obtaining such high pore volumes in essentially microporous materials, containing virtually no mesoporosity in most cases, is noteworthy. Furthermore, the fact that it was possible with some samples to combine high pore volume and very narrow micropore size is a particularly notable achievement. # 2005 Elsevier B.V. All rights reserved. Keywords: Activated carbon; Cork; Chemical activation; Microporosity
1. Introduction High micropore volume is one of the major advantages of activated carbons over other types of adsorbent material. However, of equal importance in many applications is the pore size distribution. In very broad terms, small pores are often required for gas phase applications and larger pores, including some mesoporosity, for liquid phase applications. Hence, with a view to developing a new precursor for activated carbon manufacture, it is important to establish the extent to which the pore size can be controlled by judicious control of the pyrolysis conditions used during the carbonisation and activation treatment. Previous work had indicated that it would be possible to obtain high pore volume materials with variable pore widths using cork as a * Corresponding author. Tel.: +351 266 745320; fax: +351 266 744971. E-mail address: [email protected] (P.J.M. Carrott). URL: http://www.cqe.uevora.pt 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2005.04.013
precursor [1–3]. A more detailed study of the pyrolysis of chemically impregnated cork under a wider variety of conditions with the specific objective of controlling the pore size has now been carried out and the results presented here. As a precursor material, cork is significantly different to wood and other predominantly lignocellulosic materials with regard to both its chemical constitution [4] and its microstructure [5]. The latter is formed from an ordered packing of hollow polyhedral prismatic cells. When viewed from the radial direction (relative to the tree trunk), the cork has a (very imperfect) honeycomb structure with each cell having, on average, about six sides. The diameter of the cells is approximately 15 mm and the thickness of the cell walls is about 1–2 mm. When viewed from transversal directions, on the other hand, the cork has a brick wall structure with the size of the cells being about 15 mm in width and between about 10 and 40 mm in length. These walls also have a thickness of about 1–2 mm, but tend to be somewhat corrugated. With regard to chemical composition, a typical
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cork sample contains 45% suberin, 27% lignin, 12% polysaccharides (cellulose and hemicellulose), 6% waxes and 6% tannins. The major component, suberin, appears to be a polyester formed from glycerol and mainly C18 a,vdicarboxylic acids with a high percentage of epoxide or vicinal diol groups as well as some C–C double bonds on carbons 9 and 10 [4]. A small percentage of the aliphatic chains are also terminated by phenolic groups. A considerable amount of work has been published on the preparation of activated carbon materials by pyrolysis of natural products and industrial residues, including in this journal [6–13]. It is now well known that, for a given precursor, the properties of the activated carbon product depend on a number of factors, which include the impregnant, the pyrolysis temperature and flow-rate and the degree of impregnation, as well as the precise experimental procedure used to carry out the impregnation. However, results obtained using different precursor materials and different impregnants under a variety of conditions indicate that pore volumes in the range 0.5–0.7 cm3 g1, corresponding to BET apparent surface areas of about 1000– 1500 m2 g1, are very reasonable. Hence, one of the goals of the present work was to identify the conditions necessary for achieving this level of porosity in materials with different mean pore sizes. Furthermore, with a view to possible industrial production of the materials, it was also our aim to achieve suitable materials under moderate conditions of temperature, flow-rate and impregnant to precursor ratio. The influence of impregnant, conditions and method of impregnation and pyrolysis temperature were all studied. In addition, as previous work concerned with the physical activation of cork had indicated a particle size effect, this was also taken into consideration [2].
2. Experimental 2.1. Materials The adult cork used in this study was taken from a single board of cork oak (Quercus Suber L.) from the Alentejo region of Portugal and cut into pieces of diameter d = 5 or 1 mm. The age of the cork was 9 years and the age of the tree from which it was stripped was estimated to be about 70 years. In all cases, the cork was pre-washed for 24 h with a 20% solution of sulfuric acid, in order to remove weakly bound impurities, followed by washing with distilled water until complete removal of the acid and then oven dried at 100 8C. Two types of impregnation, referred to as wet and dry, were carried out. In the first case, 2.5 g of cork was impregnated for 3 days with solutions of phosphoric acid, potassium hydroxide or sodium hydroxide of different absolute concentrations, C, of the impregnant and mass ratios, R = impregnant/precursor, equal to 1 or 2. At the end of the impregnation, any remaining liquid was removed by
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heating to dryness. In the case of dry impregnation, the cork was mixed with the impregnant, potassium hydroxide, sodium hydroxide, sodium carbonate or potassium carbonate in the mass ratio of 2. Pyrolysis was carried out by heating the sample in a horizontal tubular furnace in a flux of dry nitrogen with flowrate 85 cm3 min1. The heating rate was 8 8C min1, the maximum temperature, T, was 450 or 700 8C (or 600 8C for one sample only) and the dwell time 120 min. The pyrolysed samples were washed until complete removal of the impregnant. Finally, the samples were dried at 100 8C for 3 days. The samples are designated by AAdC/R/T where AA is the initials of the impregnant (PH, potassium hydroxide; SH, sodium hydroxide; SC, sodium carbonate; PC, potassium carbonate; and PA, phosphoric acid), d the particle size (A = 5 mm and C = 1 mm), C the concentration of the impregnant in mol L1, R the mass ratio of impregnant to cork and T is the pyrolysis temperature in 8C. For instance, PAA6.9/1/700 refers to the phosphoric acid impregnated sample which has d = 5 mm, C = 6.9 mol L1, R = 1 and T = 700 8C. 2.2. Methods Elemental analyses of combustible C, H, N, O and S were carried out using a EuroVector CHNOS analyser and each analysis was the result of three replicate determinations. Combustible C, H, N and S were determined simultaneously, while combustible O was determined directly, but in separate experiments. For a number of samples, the ash content was estimated from the mass of the residue left after combustion of the samples in air. SEM measurements were determined using an RJ Lee Personal SEM. N2 adsorption measurements at 77 K were determined after outgassing the samples at 250 8C using a CE Instruments Sorptomatic 1900 equipped with an Edwards turbomolecular vacuum pump and high precision low and high pressure MKS Baratron gauges. Dry N2 of purity 99.999% was used. The N2 isotherms were analysed by means of the as method [14,15] using published standard data [16] in order to obtain the external surface area, As, and total pore volume vs. They were also analysed by means of the DR equation [14,15] in order to obtain the DR pore volume, vo, and the mean pore width, Lo, from the expression [17,18]: Lo ¼
10:8 ðEo 11:4Þ
(1)
where Eo is the characteristic energy estimated from the slope of the DR plot. In much published work, no estimate of mean pore size is given and a variety of different methods are used to estimate the pore volume. Hence, in order to facilitate comparison between different authors, the BET equation was also used to calculate apparent surface areas.
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3. Results 3.1. Yields and elemental and ash analysis The pyrolysis yields and the results of elemental and ash analysis are given in Table 1. Sulfur was not detected in any of the samples and is therefore not included in the table. In those cases where the ash content was determined, it was found that the percentages of ash and combustible C, N, H and O added up to 100.0 1.3%. For the phosphoric acid impregnated samples, the pyrolysis yields are about 30–40% using the larger precursor particle size and 60% when using the smaller particle size. Taking into account the high pore volume (discussed below) in particular, this last value is excellent. For the carbonate impregnated samples, the yields are about 20–30% and for the hydroxide impregnated samples, they are 10–20%. The values obtained, even for the hydroxide impregnated samples, are higher than would normally be found during physical activation to a similar level of pore volume [2]. The Table 1 Results of elemental (combustible) and ash analysis of samples Yield (%)
N (%)
H (%)
C (%)
O (%)
Ash (%)
KOH PHA5.2/1/450 PHA7.1/1/450 PHA10.1/1/450 PHA7.1/1/700 PHA7.1/2/450 PHC4.0/2/450 PHCdry/2/450 PHCdry/2/700
16 17 18 17 13 12 14 14
0.5 0.4 0.6 0.0 0.5 0.4 0.7 1.1
2.0 2.2 1.9 0.3 1.8 1.9 1.6 0.4
78.1 77.8 78.7 87.0 71.1 70.0 64.5 65.2
14.9 15.8 19.5
4.5 3.8 0.0
25.3 18.6
1.3 9.1
NaOH SHA7.0/1/450 SHA7.0/1/700 SHA7.0/2/450 SHC4.0/2/450 SHCdry/2/450 SHCdry/2/700
16 14 6 19 13 13
0.6 0.2 0.4 0.6 0.2 2.5
2.1 0.3 0.1 2.0 2.2 0.8
82.2 88.5 65.6 69.3 57.1 71.7
13.8
0.0
29.3
0.0
Na2CO3 SCC4.0/2/450 SCCdry/2/450 SCCdry/2/700
21 27 21
0.8 0.6 1.9
5.2 1.9 0.5
78.8 75.2 75.9
K2CO3 PCCdry/2/450 PCCdry/2/700
26 23
0.4 0.7
1.7 0.6
75.0 66.7
H3PO4 PAA5.0/1/450 PAA6.9/1/450 PAA14.8/1/450 PAA6.9/1/700 PAA6.9/2/450 PAC4.0/2/450 PAC4.0/2/600
33 37 42 43 35 59 61
0.3 0.3 0.3 0.5 0.2 0.2 0.8
2.8 2.5 2.8 2.6 2.3 2.5 1.0
59.3 61.6 60.3 75.0 71.7 64.5 70.8
24.6 24.0 25.2
13.0 11.6 11.8
17.4 22.6
8.4 10.2
reasons for the difference in yields obtained with the different impregnants and particle sizes are discussed more fully in Section 4. The elemental analysis shows that the phosphoric acid impregnated samples have high oxygen and ash contents. The values are, in fact, higher than previously found with physically activated samples. Furthermore, the C/O ratios vary from 2.4 to 4.1, with an average value of 2.9, which is quite low. These results suggest that in spite of the extremely prolonged washing period used, a certain amount of phosphate remained chemically bound inside the pore structure. Commercial phosphoric acid activated carbons are also known to contain about 2–3% phosphorous. Using the basic impregnating agents, the ash contents are lower than previously found [2] indicating that some of the ash present in the precursor was leached out either during the prewashing with sulfuric acid or during the final post-activation washing of the product. In some cases, all of the ash appeared to have been leached giving an ash-free activated carbon product. The C/O ratios are mostly 4 which are also quite low. For the samples prepared at 450 8C, the H contents are higher than previously found after physical activation [2], which was shown to produce mainly non-hydrogen containing carbonyl type groups. On the other hand, after preparation at 700 8C, the H contents were reduced to very low values. 3.2. Porosity development during pyrolysis
Samples are designated AAdC/R/T where AA is the initials of the impregnant, d the particle size (A = 5 mm and C = 1 mm), C the concentration of the impregnant in mol L1, R the mass ratio of impregnant to cork and T is the pyrolysis temperature in 8C.
Illustrative N2 isotherms determined on the samples that gave the highest pore volumes for each impregnant are shown in Fig. 1 (wet impregnation) and Fig. 2 (dry impregnation). All of the isotherms were essentially Type I of the IUPAC classification. However, there were significant variations in the exact shapes of the isotherms, depending on the nature of the impregnant and the precise conditions of preparation. In general, the isotherms determined on the samples prepared with basic impregnants gave fairly
Fig. 1. N2/77K adsorption isotherms determined on selected samples after wet impregnation. From top to bottom at P0 0.9: PAC4.0/2/450, PHA7.1/1/ 700, SHA7.0/2/450. Open points are the adsorption branch and closed points the desorption branch of the isotherms.
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Fig. 2. N2/77K adsorption isotherms determined on selected samples after dry impregnation. From top to bottom at P0 0.9: SHCdry/2/700, PHCdry/2/ 700, PCCdry/2/700, SCC/dry/2/700. Open points are the adsorption branch and closed points the desorption branch of the isotherms.
rectangular isotherms, while those determined on the phosphoric acid impregnated samples exhibited a much slower approach to the plateau, indicating the presence of a broader pore size distribution. This was confirmed by the presence of a small Type H4 hysteresis loop on many of the
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isotherms determined on the phosphoric acid impregnated samples. With the other samples, low-pressure hysteresis was sometimes observed on the isotherms, indicating the presence in some samples of constricted micropore entrances. The textural parameters obtained by application of the as, DR and BET methods are given in Table 2. In those cases where low-pressure hysteresis was found, the DR equation cannot be applied as the adsorption isotherm will not be in equilibrium at very low pressures and for this reason, the values of vo and Lo are not given in the table. The external surface areas, As, of all the samples were found to be <50 m2 g1. In the present case, As can be identified as the non-microporous area, that is, the area of the macropores and the inter- and intra-particle mesopores. Hence, the low values obtained indicate, in agreement with the qualitative interpretation of the isotherms, that very little significant mesoporosity was developed under most conditions. The main exception to this was the phosphoric acid impregnated sample PAC4.0/2/450 which was the one that gave the highest pore volume of the wet impregnated samples and the highest external surface area of all the
Table 2 Textural characteristics obtained by application of the as, DR and BET methods to N2/77K adsorption isotherms determined on activated carbon samples vs (cm3 g1)
vo (cm3 g1)
Lo (nm)
As (m2 g1)
ABET (m2 g1)
KOH PHA5.2/1/450 PHA7.1/1/450 PHA10.1/1/450 PHA7.1/1/700 PHA7.1/2/450 PHC4.0/2/450 PHCdry/2/450 PHCdry/2/700
0.22 0.21 0.21 0.53 0.20 0.25 0.22 0.64
0.22 0.20 0.21 0.52 0.21 0.25 0.22 0.61
1.00 0.95 1.03 0.69 1.32 0.72 0.83 0.89
18 14 13 19 29 19 13 17
554 511 523 1370 517 646 552 1616
NaOH SHA7.0/1/450 SHA7.0/1/700 SHA7.0/2/450 SHC4.0/2/450 SHCdry/2/450 SHCdry/2/700
0.11 0.13 0.35 0.19 0.55 0.73
lph lph 0.33 0.20 0.47 0.55
lph lph 1.12 0.82 1.34 1.55
25 40 25 43 10 19
215 350 875 510 1287 1615
Na2CO3 SCC4.0/2/450 SCCdry/2/450 SCCdry/2/700
0.07 0.07 0.16
lph lph 0.15
lph lph 1.12
14 14 16
170 161 386
K2CO3 PCCdry/2/450 PCCdry/2/700
0.27 0.46
lph 0.45
lph 0.80
6 11
625 1175
H3PO4 PAA5.0/1/450 PAA6.9/1/450 PAA14.8/1/450 PAA6.9/1/700 PAA6.9/2/450 PAC4.0/2/450 PAC4.0/2/600
0.21 0.19 0.19 0.21 0.21 0.56 0.48
0.17 0.16 0.15 0.19 0.20 0.38 0.36
1.58 1.59 1.73 1.28 1.85 2.21 1.41
29 33 32 21 33 51 13
463 428 418 488 566 1066 1026
vs, Pore volume from as plot; vo, pore volume from DR plot; Lo, mean pore width; As, external surface area; ABET, apparent surface area; and lph, low-pressure hysteresis.
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samples and for which the mean pore width was slightly greater than the micropore limit of 2 nm. For many of the samples, the pore volumes were quite low. However, it can be seen from Table 2 that by suitable control of the conditions, it was possible to obtain samples with total pore volumes (vs) as high as 0.56 cm3 g1 by wet impregnation (PAC4.0/2/450) or 0.73 cm3 g1 by dry impregnation (SHCdry/2/700). It is also extremely interesting to notice from Table 2 that for those samples with high pore volumes, within the desired range of 0.5– 0.7 cm3 g1, there was a very large three-fold variation in mean pore width from 0.7 to 2.2 nm. That is, the primary goals of the work, referred to in Section 1, were achieved and the results confirm that by judicious choice of impregnant and conditions, it is possible to fine-tune the pore size of cork based activated carbons, while always managing to maintain a reasonably high pore volume. It is important to stress that the samples are essentially nonmesoporous. With the exception of the so-called superactivated carbons, currently produced by the Kansai Coke Company of Japan and marketed under the brand name MAXSORB, and certain activated carbon fibres, it has proven to be extremely difficult to produce exclusively microporous activated carbons with such high pore volumes [19]. 3.3. Potassium hydroxide impregnation The results in Table 2 indicate that when using potassium hydroxide as impregnant and a pyrolysis temperature of 450 8C, the total pore volume is essentially independent of impregnant concentration (including dry impregnation), mass ratio or particle size. However, the pore structures of the materials obtained are quite clearly different, as evidenced by the large variations in the values of Lo for these samples. On one hand, increasing the mass ratio provokes a small increase in Lo from 1.0 to 1.3 nm. On the other hand, the Lo values are much lower when the precursor has the smaller particle size. High pore volumes are only obtained at the higher temperature 700 8C and under these conditions, it is also found that the mean pore widths are particularly low. In fact, within the required range of high pore volumes of 0.5–0.7 cm3 g1, the lowest mean pore width, 0.69 nm, was achieved using potassium hydroxide as impregnant (PHA7.1/1/700). In a recent paper, potassium hydroxide impregnation followed by pyrolysis at 800–900 8C of a commercial phosphoric acid activated carbon was used in an attempt to obtain activated carbons with high pore volumes but narrow pore widths [19]. The pore volumes (vo) achieved varied between 0.4 and 0.5 cm3 g1 and the corresponding mean pore widths were 1.0–1.1 nm. It can be seen from Table 2 that higher pore volume and even narrower pore width (samples PHA7.1/1/700 and PHCdry/2/700) can be obtained by single step potassium hydroxide impregnation at a significantly lower pyrolysis temperature when using cork as the precursor.
3.4. Sodium hydroxide impregnation The samples prepared with the sodium impregnants were those most prone to activated entry effects as shown by the presence of low-pressure hysteresis on many of the corresponding isotherms. Where these effects are not present, it appears that both the total pore volume and the mean pore width are somewhat higher when using sodium hydroxide, in comparison to potassium hydroxide. In fact, the highest pore volume which appears in the tables, 0.73 cm3 g1 was achieved using sodium hydroxide as impregnant (SHCdry/2/700). The variation of pore volume and mean pore width as a function of impregnant concentration, mass ratio, particle size and pyrolysis temperature may be similar to that found with potassium hydroxide. However, the presence of lowpressure hysteresis complicates the analysis. For instance, the results in Table 2 indicate that at d = 5 mm and T = 450 8C, increasing the mass ratio, R = 1–2, has no effect on vs(KOH) (samples PHA7.1/1/450 and PHA7.1/2/450) but that vs(NaOH) increases significantly (samples SHA7.0/1/ 450 and SHA7.0/2/450). We believe that this is due to the simultaneous increase in pore widths, which occurs with both impregnants and which in the case of sodium hydroxide also results in the removal of pore entrance constrictions, thereby freeing microporosity which was not accessible at the lower mass ratio. Another example are the pore volumes at d = 5 mm, R = 1 and T = 700 8C. vs(KOH) is high (PHA7.1/1/700), but vs(NaOH) is low (SHA7.0/1/700). We believe this difference is due to the particularly lowmean pore widths which are found at the higher pyrolysis temperature and which, in the case of sodium hydroxide, are accompanied by pore entrance constrictions. 3.5. Sodium carbonate and potassium carbonate impregnation These two impregnants were only studied using the smaller precursor particle size as this was generally found to be more favourable with the other impregnants. However, in no case was the minimum required value of pore volume, 0.5 cm3 g1, achieved. Sodium carbonate was found to be particularly ineffective. On the other hand, one of the carbonate impregnated samples, PCCdry/2/700, came very close to the value of 0.5 cm3 g1 and it should be noted that once again, as found with the high temperature potassium hydroxide samples, the mean pore width was low. Other workers have also found that carbonate impregnants develop some porosity at low temperatures, but that it is only possible to achieve high pore volumes at temperatures above 800 8C [20,21]. It has been suggested that this difference is due to a change in the mechanism of activation at the higher temperature [21]. On the other hand, an additional factor which may contribute to the different behaviour of the carbonates in comparison with the hydroxides could be the stronger hydrolysing activity of
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the latter. It is well known that treating lignocellulosic materials with sodium hydroxide, for example, is effective in breaking the bonds between the polysaccharides and the lignin and also results in some depolymerisation of both [22,23]. Alkali treatment is also one of the techniques used to separate suberin from the lignin and polysaccharides present in cork, prior to its chemical analysis [4]. The extent of debonding and depolymerisation, which occurs during the impregnation of the precursor, should be less with the weaker carbonate bases and the mean molar mass of the molecules will therefore be higher. It would therefore also be expected that the carbonate impregnated samples would exhibit better thermal stability resulting in higher yield and lower porosity development, at least during the initial lower temperature stages of pyrolysis. 3.6. Phosphoric acid impregnation In agreement with other published studies, phosphoric acid was found to be different from basic impregnating agents in two major respects [24,25]. Firstly, as already mentioned, there was a greater degree of pore widening. Secondly, increasing the pyrolysis temperature did not
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improve the pore volume but, with the smaller precursor particle size, decreased it. On the other hand, there are also a number of similarities between phosphoric acid and the basic impregnants. With the larger precursor particle size, the pore volume was found to be independent of impregnant concentration and mass ratio, while the mean pore width increased with increasing impregnant concentration or mass ratio. Although increasing the pyrolysis temperature did not improve the pore volume, the same decrease in mean pore width found with the basic impregnants was also observed with phosphoric acid. 3.7. SEM One surprising feature of the results is that the impregnant concentration and mass ratio had very little effect on the pore volume. Hence, in order to verify that our samples were effectively and uniformly impregnated, SEM images of selected samples were determined and illustrative examples of three samples prepared under very similar conditions but with different impregnants are shown in Fig. 3. For comparison, an SEM image of a physically activated sample is also shown in Fig. 3 and it can be seen that the
Fig. 3. Representative SEM images (all on the same scale, indicated in part (a)): (a) CO2 activation [16], (b) PAA6.9/1/450, (c) PHA7.1/1/450 and (d) SHA7.0/ 1/450.
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characteristic cork membrane structure is present on this image. The other SEM images show that during pyrolysis of chemically impregnated samples, the cork structure was extensively modified. In some cases, such as PAA6.9/1/450 shown in Fig. 3(b), remnants of the membrane structure are still visible, while in other cases, such as PHA7.1/1/450 and SHA7.0/1/450 shown in Fig. 3(c and d), the cork appears to have been almost completely digested by the impregnant. A more detailed study aimed at relating the adsorption and SEM data is currently under way.
4. Discussion Pyrolysis of natural materials impregnated with phosphoric acid or with basic impregnants involves different chemical mechanisms and our results show that this is reflected in quite different pore structures. One particularly important difference in chemical mechanism is that phosphoric acid intervenes in the initial stages of carbonisation of the precursor, which begin at lower temperatures [24–26], whereas the basic agents only react after the precursor has been at least partially carbonised at higher temperatures [20,21,24]. This is one of the reasons why the development of porosity in phosphoric acid activation was found to be better at low temperature, while in basic activation, it was better at high temperature. In addition, it is also consistent with the higher yields found with phosphoric acid. In this case, the agent promotes char formation and inhibits tar formation at low temperatures and therefore reduces volatilisation and loss of carbon. With basic agents, on the other hand, significant volatilisation of the organic compounds formed as a result of low-temperature thermal decomposition of the precursor can occur before the subsequent chemical reactions involving the activating agent become thermodynamically feasible at higher temperatures [21]. Another important difference is that with bases, it is believed that there is a stoichiometric reaction between the impregnant and the carbon, which leads to the formation of volatile or soluble products (removed during the final washing of the product), such as potassium carbonate [27]. As a result of these reactions, C atoms are quantitatively separated from the polymeric carbon matrix. This may be one of the reasons for the higher yields found here with the carbonate impregnants, in comparison with the hydroxides. Although with all four basic impregnants, the yield was low, due to the low-temperature thermal decomposition reactions previously referred to, the yield was somewhat higher with the carbonates. One reason for this difference, already mentioned in Section 3.5, may be the weaker hydrolysing activity of the carbonates during impregnation. However, we believe the fact that the porosity development was lower, indicates that the additional carbon loss at high temperatures, caused by reaction of the impregnating agent with the carbon, was also less significant. With phosphoric acid, on the other hand, the agent, whether by a catalytic or
mechanistic pathway, promotes bond cleavage in thermally less stable structures and subsequent formation of new bonds in thermally more stable polymeric aromatic structures. Hence, there is not a quantitative loss of C atoms at any temperature and the yield is much higher. There may be various reasons for the higher pore widths obtained with phosphoric acid. Firstly, the bond cleavage and formation reactions can lead to cross-linking and eventually to the formation of buckled or folded graphene layers. Furthermore, at the lower temperatures where these reactions begin the precursor molecules are disordered and there may exist relatively large gaps between the molecules. The combination of these two factors will lead to a relatively open and poorly ordered packing of the graphene layers which, in turn, will make the high temperature structural transformations which could lead to densification more difficult. An additional factor is grafting of phosphate species to the graphene layers. The presence of large phosphate or polyphosphate species will also prevent the graphene layers from adopting an oriented dense structure [26]. In contrast, when basic impregnants are used, the chemical reactions involved may not lead to such a significant level of cross-linking but, on the contrary, may remove carbon atoms which are impeding reorientation of the growing graphene layers. Furthermore, these reactions only begin after the initial disordered structure of the precursor has been partially reorganised during carbonisation. As in our previous work on physical activation, a particle size effect was also found here [2]. However, as the SEM showed that chemical impregnation leads to the destruction of the cell walls, the cause of this effect must be different. It was found that decreasing the particle size results in an increase in pore volume when using either phosphoric acid or potassium hydroxide. On the other hand, the mean pore width increased when using phosphoric acid, but decreased when using potassium hydroxide. Other workers have shown that increasing the flow-rate improves the porosity development due to the faster removal of gaseous products which inhibit the pyrolysis reactions [21]. Hence, it is possible that the particle size effect that we observed is due to a more efficient removal of gaseous products when the particle size is low. As a result, the pyrolysis rates will be higher and higher pore volumes are obtained. With phosphoric acid, cross-linking is enhanced and the pore size increases. With potassium hydroxide, reorientation of the graphene layers is enhanced and the mean pore width goes down.
5. Conclusions The results presented here show that by appropriate control of the conditions of impregnation and pyrolysis, it is possible to obtain cork based activated carbons with high pore volumes, in the range 0.5–0.7 cm3 g1, and with mean pore widths of 0.7–1.3 nm using basic impregnants (1.6 nm with sodium hydroxide under dry conditions and at higher
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activation temperature) and 1.3–2.2 nm using phosphoric acid. The best materials produced present pore structural characteristics which are significantly different to the vast majority of commercial activated carbons. In particular, the possibility of using an agricultural waste product as a precursor for obtaining materials with such high pore volumes, which are essentially microporous and contain virtually no mesoporosity in most cases, is noteworthy. Furthermore, the fact that it was possible with some samples to combine high pore volume and very narrow micropore size is a particularly notable achievement of the results. The effect of five experimental parameters, namely method of impregnation, impregnant concentration, mass ratio, precursor particle size and pyrolysis temperature, were studied. Over the range of conditions used, it was found that increasing the impregnant concentration or mass ratio had little effect on pore volume but increased the mean pore width. A decrease in the precursor particle size resulted, in general, in higher pore volumes. Increasing the pyrolysis temperature resulted in an increase in pore volume when using basic impregnants but a decrease when using phosphoric acid. However, in all cases, higher pyrolysis temperature resulted in particularly low-mean pore widths. On the basis of the results presented, it is possible to identify the preparation conditions that ought to be most suited for different types of application. For applications, such as gas separation, where a narrow distribution of small micropore widths is essential, wet potassium hydroxide impregnation followed by pyrolysis at high temperature would be the best choice and allows a mean micropore width of 0.7 nm with micropore volume of 0.5 cm3 g1, to be obtained. For liquid phase applications where a broad micropore size distribution is required, wet phosphoric acid impregnation followed by pyrolysis at a lower temperature would be one suitable choice. This allows us to obtain a pore volume of 0.6 cm3 g1 with a pore size distribution partially extending into the region of small mesopores and has the particular advantage of a very high pyrolysis yield. On the other hand, sodium hydroxide, under conditions of dry impregnation, also allows very high pore volume and reasonably broad pore size distributions to be obtained and has the additional advantage of producing low ash or in certain cases ash-free products.
Acknowledgements This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (Project N8 POCTI/CTM/38255/2001) with national and European community funds.
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References [1] P.J.M. Carrott, M.M.L. Ribeiro Carrott, R.P. Lima, Carbon 37 (1999) 515–517. [2] P.J.M. Carrott, M.M.L. Ribeiro Carrott, P.A.M. Moura˜o, R.P. Lima, Adsorpt. Sci. Technol. 21 (2003) 669–681. [3] A.P. Carvalho, B. Cardoso, J. Pires, M. Brotas de Carvalho, Carbon 41 (2003) 2873–2876. [4] J.A.R. Grac¸a, H.M.N. Pereira, in: J.S. Seixas de Melo, M.J. Moreno, H.D. Burroughs, M.H. Gil (Eds.), Polymer Chemistry, Coimbra University Press, 2004, pp. 619–640. [5] H. Pereira, M.E. Rosa, M.A. Fortes, IAWA Bull. 8 (3) (1987) 213– 218. [6] P.T. Williams, A.R. Reed, J. Anal. Appl. Pyrolysis 71 (2) (2004) 971– 986. [7] A.H. El-Sheikh, A.P. Newman, H.K. Al-Daffaee, S. Phull, N. Cresswell, J. Anal. Appl. Pyrolysis 71 (1) (2004) 151–164. [8] P.T. Williams, A.R. Reed, J. Anal. Appl. Pyrolysis 70 (2) (2003) 563– 577. [9] M.C. Baquero, L. Giraldo, J.C. Moreno, F. Sua´rez-Garcı´a, A. Martı´nez-Alonso, J.M.D. Tasco´n, J. Anal. Appl. Pyrolysis 70 (2) (2003) 779–784. [10] F. Sua´rez-Garcı´a, A. Martı´nez-Alonso, J.M.D. Tasco´n, J. Anal. Appl. Pyrolysis 63 (2) (2002) 283–301. [11] E. Iniesta, F. Sa´nchez, A.N. Garcı´a, A. Marcilla, J. Anal. Appl. Pyrolysis 58–59 (1) (2001) 967–981. [12] J. Pastor-Villegas, C.J. Dura´n-Valle, J. Anal. Appl. Pyrolysis 57 (1) (2001) 1–13. [13] J. Pastor-Villegas, V. Go´mez-Serrano, C.J. Dura´n-Valle, F.J. HigesRolando, J. Anal. Appl. Pyrolysis 50 (1) (1999) 1–16. [14] P.J.M. Carrott, J.M.V. Nabais, M.M.L. Ribeiro Carrott, J.A. Pajares, Carbon 39 (2001) 1543–1555. [15] P.J.M. Carrott, M.M.L. Ribeiro Carrott, R.A. Roberts, Colloids Surf. 58 (1991) 385–400. [16] P.J.M. Carrott, R.A. Roberts, K.S.W. Sing, Carbon 25 (1987) 769– 770. [17] P.J.M. Carrott, M.M.L. Ribeiro Carrott, Carbon 27 (1999) 647– 656. [18] H.F. Stoeckli, P. Rebstein, L. Ballerini, Carbon 28 (1990) 907– 909. [19] I. Martin-Gullon, J.P. Marco-Lozar, D. Cazorla-Amoro´s, A. LinaresSolano, Carbon 42 (2004) 1339–1343. [20] K. Okada, N. Yamamoto, Y. Kameshima, A. Yasumori, J. Colloid Interface Sci. 262 (2003) 179–193. [21] M.A. Lillo-Ro´denas, D. Cazorla-Amoro´s, A. Linares-Solano, Carbon 41 (2003) 267–275. [22] C.P. Neto, D. Evtuguin, in: J.S. Seixas de Melo, M.J. Moreno, H.D. Burroughs, M.H. Gil (Eds.), Polymer Chemistry, Coimbra University Press, 2004, pp. 545–569. [23] D.N.S. Hon, Chemical Modification of Lignocellulosic Materials, Marcel Dekker, New York, 1996. [24] M. Jagtoyen, F. Derbyshire, R. Rathbone, Extended Abstracts CARBON ’92, Germany, 1992, pp. 250–252. [25] C. Toles, S. Rimmer, J.C. Hower, Carbon 34 (1996) 1419–1426. [26] M. Molina-Sabio, F. Rodrigues-Reinoso, F. Caturla, M.J. Selle´s, Carbon 33 (1995) 1105–1113. [27] M.A. Lillo-Ro´denas, J. Juan-Juan, D. Cazorla-Amoro´s, A. LinaresSolano, Carbon 42 (2004) 1371–1375.
Pore size control in activated carbons obtained by pyrolysis under different conditions of chemically impregnated cork P.J.M. Carrott *, M.M.L. Ribeiro Carrott, P.A.M. Moura˜o Centro de Quı´mica de E´vora and Departamento de Quı´mica, Universidade de E´vora, Cole´gio Luı´s Anto´nio Verney, 7000-671 E´vora, Portugal Received 21 December 2004; accepted 27 April 2005 Available online 13 June 2005
Abstract Activated carbons were prepared by the pyrolysis of cork impregnated with potassium and sodium hydroxides and carbonates as well as phosphoric acid and the effect of five experimental parameters, namely method of impregnation, impregnant concentration, mass ratio, precursor particle size and pyrolysis temperature, were studied. It is shown that cork is a versatile precursor and allows us to prepare a wide variety of materials with quite different pore structural characteristics by precise control of the impregnation and pyrolysis conditions. Even under relatively mild conditions, it was possible to produce cork based carbons with high pore volumes, in the range 0.5–0.7 cm3 g1, and to simultaneously control the mean pore width over a three-fold range from a value as low as 0.7 nm up to a value as high as 2.2 nm. The best materials produced present pore structural characteristics which are significantly different to the vast majority of commercial activated carbons. In particular, the possibility of obtaining such high pore volumes in essentially microporous materials, containing virtually no mesoporosity in most cases, is noteworthy. Furthermore, the fact that it was possible with some samples to combine high pore volume and very narrow micropore size is a particularly notable achievement. # 2005 Elsevier B.V. All rights reserved. Keywords: Activated carbon; Cork; Chemical activation; Microporosity
1. Introduction High micropore volume is one of the major advantages of activated carbons over other types of adsorbent material. However, of equal importance in many applications is the pore size distribution. In very broad terms, small pores are often required for gas phase applications and larger pores, including some mesoporosity, for liquid phase applications. Hence, with a view to developing a new precursor for activated carbon manufacture, it is important to establish the extent to which the pore size can be controlled by judicious control of the pyrolysis conditions used during the carbonisation and activation treatment. Previous work had indicated that it would be possible to obtain high pore volume materials with variable pore widths using cork as a * Corresponding author. Tel.: +351 266 745320; fax: +351 266 744971. E-mail address: [email protected] (P.J.M. Carrott). URL: http://www.cqe.uevora.pt 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2005.04.013
precursor [1–3]. A more detailed study of the pyrolysis of chemically impregnated cork under a wider variety of conditions with the specific objective of controlling the pore size has now been carried out and the results presented here. As a precursor material, cork is significantly different to wood and other predominantly lignocellulosic materials with regard to both its chemical constitution [4] and its microstructure [5]. The latter is formed from an ordered packing of hollow polyhedral prismatic cells. When viewed from the radial direction (relative to the tree trunk), the cork has a (very imperfect) honeycomb structure with each cell having, on average, about six sides. The diameter of the cells is approximately 15 mm and the thickness of the cell walls is about 1–2 mm. When viewed from transversal directions, on the other hand, the cork has a brick wall structure with the size of the cells being about 15 mm in width and between about 10 and 40 mm in length. These walls also have a thickness of about 1–2 mm, but tend to be somewhat corrugated. With regard to chemical composition, a typical
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cork sample contains 45% suberin, 27% lignin, 12% polysaccharides (cellulose and hemicellulose), 6% waxes and 6% tannins. The major component, suberin, appears to be a polyester formed from glycerol and mainly C18 a,vdicarboxylic acids with a high percentage of epoxide or vicinal diol groups as well as some C–C double bonds on carbons 9 and 10 [4]. A small percentage of the aliphatic chains are also terminated by phenolic groups. A considerable amount of work has been published on the preparation of activated carbon materials by pyrolysis of natural products and industrial residues, including in this journal [6–13]. It is now well known that, for a given precursor, the properties of the activated carbon product depend on a number of factors, which include the impregnant, the pyrolysis temperature and flow-rate and the degree of impregnation, as well as the precise experimental procedure used to carry out the impregnation. However, results obtained using different precursor materials and different impregnants under a variety of conditions indicate that pore volumes in the range 0.5–0.7 cm3 g1, corresponding to BET apparent surface areas of about 1000– 1500 m2 g1, are very reasonable. Hence, one of the goals of the present work was to identify the conditions necessary for achieving this level of porosity in materials with different mean pore sizes. Furthermore, with a view to possible industrial production of the materials, it was also our aim to achieve suitable materials under moderate conditions of temperature, flow-rate and impregnant to precursor ratio. The influence of impregnant, conditions and method of impregnation and pyrolysis temperature were all studied. In addition, as previous work concerned with the physical activation of cork had indicated a particle size effect, this was also taken into consideration [2].
2. Experimental 2.1. Materials The adult cork used in this study was taken from a single board of cork oak (Quercus Suber L.) from the Alentejo region of Portugal and cut into pieces of diameter d = 5 or 1 mm. The age of the cork was 9 years and the age of the tree from which it was stripped was estimated to be about 70 years. In all cases, the cork was pre-washed for 24 h with a 20% solution of sulfuric acid, in order to remove weakly bound impurities, followed by washing with distilled water until complete removal of the acid and then oven dried at 100 8C. Two types of impregnation, referred to as wet and dry, were carried out. In the first case, 2.5 g of cork was impregnated for 3 days with solutions of phosphoric acid, potassium hydroxide or sodium hydroxide of different absolute concentrations, C, of the impregnant and mass ratios, R = impregnant/precursor, equal to 1 or 2. At the end of the impregnation, any remaining liquid was removed by
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heating to dryness. In the case of dry impregnation, the cork was mixed with the impregnant, potassium hydroxide, sodium hydroxide, sodium carbonate or potassium carbonate in the mass ratio of 2. Pyrolysis was carried out by heating the sample in a horizontal tubular furnace in a flux of dry nitrogen with flowrate 85 cm3 min1. The heating rate was 8 8C min1, the maximum temperature, T, was 450 or 700 8C (or 600 8C for one sample only) and the dwell time 120 min. The pyrolysed samples were washed until complete removal of the impregnant. Finally, the samples were dried at 100 8C for 3 days. The samples are designated by AAdC/R/T where AA is the initials of the impregnant (PH, potassium hydroxide; SH, sodium hydroxide; SC, sodium carbonate; PC, potassium carbonate; and PA, phosphoric acid), d the particle size (A = 5 mm and C = 1 mm), C the concentration of the impregnant in mol L1, R the mass ratio of impregnant to cork and T is the pyrolysis temperature in 8C. For instance, PAA6.9/1/700 refers to the phosphoric acid impregnated sample which has d = 5 mm, C = 6.9 mol L1, R = 1 and T = 700 8C. 2.2. Methods Elemental analyses of combustible C, H, N, O and S were carried out using a EuroVector CHNOS analyser and each analysis was the result of three replicate determinations. Combustible C, H, N and S were determined simultaneously, while combustible O was determined directly, but in separate experiments. For a number of samples, the ash content was estimated from the mass of the residue left after combustion of the samples in air. SEM measurements were determined using an RJ Lee Personal SEM. N2 adsorption measurements at 77 K were determined after outgassing the samples at 250 8C using a CE Instruments Sorptomatic 1900 equipped with an Edwards turbomolecular vacuum pump and high precision low and high pressure MKS Baratron gauges. Dry N2 of purity 99.999% was used. The N2 isotherms were analysed by means of the as method [14,15] using published standard data [16] in order to obtain the external surface area, As, and total pore volume vs. They were also analysed by means of the DR equation [14,15] in order to obtain the DR pore volume, vo, and the mean pore width, Lo, from the expression [17,18]: Lo ¼
10:8 ðEo 11:4Þ
(1)
where Eo is the characteristic energy estimated from the slope of the DR plot. In much published work, no estimate of mean pore size is given and a variety of different methods are used to estimate the pore volume. Hence, in order to facilitate comparison between different authors, the BET equation was also used to calculate apparent surface areas.
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3. Results 3.1. Yields and elemental and ash analysis The pyrolysis yields and the results of elemental and ash analysis are given in Table 1. Sulfur was not detected in any of the samples and is therefore not included in the table. In those cases where the ash content was determined, it was found that the percentages of ash and combustible C, N, H and O added up to 100.0 1.3%. For the phosphoric acid impregnated samples, the pyrolysis yields are about 30–40% using the larger precursor particle size and 60% when using the smaller particle size. Taking into account the high pore volume (discussed below) in particular, this last value is excellent. For the carbonate impregnated samples, the yields are about 20–30% and for the hydroxide impregnated samples, they are 10–20%. The values obtained, even for the hydroxide impregnated samples, are higher than would normally be found during physical activation to a similar level of pore volume [2]. The Table 1 Results of elemental (combustible) and ash analysis of samples Yield (%)
N (%)
H (%)
C (%)
O (%)
Ash (%)
KOH PHA5.2/1/450 PHA7.1/1/450 PHA10.1/1/450 PHA7.1/1/700 PHA7.1/2/450 PHC4.0/2/450 PHCdry/2/450 PHCdry/2/700
16 17 18 17 13 12 14 14
0.5 0.4 0.6 0.0 0.5 0.4 0.7 1.1
2.0 2.2 1.9 0.3 1.8 1.9 1.6 0.4
78.1 77.8 78.7 87.0 71.1 70.0 64.5 65.2
14.9 15.8 19.5
4.5 3.8 0.0
25.3 18.6
1.3 9.1
NaOH SHA7.0/1/450 SHA7.0/1/700 SHA7.0/2/450 SHC4.0/2/450 SHCdry/2/450 SHCdry/2/700
16 14 6 19 13 13
0.6 0.2 0.4 0.6 0.2 2.5
2.1 0.3 0.1 2.0 2.2 0.8
82.2 88.5 65.6 69.3 57.1 71.7
13.8
0.0
29.3
0.0
Na2CO3 SCC4.0/2/450 SCCdry/2/450 SCCdry/2/700
21 27 21
0.8 0.6 1.9
5.2 1.9 0.5
78.8 75.2 75.9
K2CO3 PCCdry/2/450 PCCdry/2/700
26 23
0.4 0.7
1.7 0.6
75.0 66.7
H3PO4 PAA5.0/1/450 PAA6.9/1/450 PAA14.8/1/450 PAA6.9/1/700 PAA6.9/2/450 PAC4.0/2/450 PAC4.0/2/600
33 37 42 43 35 59 61
0.3 0.3 0.3 0.5 0.2 0.2 0.8
2.8 2.5 2.8 2.6 2.3 2.5 1.0
59.3 61.6 60.3 75.0 71.7 64.5 70.8
24.6 24.0 25.2
13.0 11.6 11.8
17.4 22.6
8.4 10.2
reasons for the difference in yields obtained with the different impregnants and particle sizes are discussed more fully in Section 4. The elemental analysis shows that the phosphoric acid impregnated samples have high oxygen and ash contents. The values are, in fact, higher than previously found with physically activated samples. Furthermore, the C/O ratios vary from 2.4 to 4.1, with an average value of 2.9, which is quite low. These results suggest that in spite of the extremely prolonged washing period used, a certain amount of phosphate remained chemically bound inside the pore structure. Commercial phosphoric acid activated carbons are also known to contain about 2–3% phosphorous. Using the basic impregnating agents, the ash contents are lower than previously found [2] indicating that some of the ash present in the precursor was leached out either during the prewashing with sulfuric acid or during the final post-activation washing of the product. In some cases, all of the ash appeared to have been leached giving an ash-free activated carbon product. The C/O ratios are mostly 4 which are also quite low. For the samples prepared at 450 8C, the H contents are higher than previously found after physical activation [2], which was shown to produce mainly non-hydrogen containing carbonyl type groups. On the other hand, after preparation at 700 8C, the H contents were reduced to very low values. 3.2. Porosity development during pyrolysis
Samples are designated AAdC/R/T where AA is the initials of the impregnant, d the particle size (A = 5 mm and C = 1 mm), C the concentration of the impregnant in mol L1, R the mass ratio of impregnant to cork and T is the pyrolysis temperature in 8C.
Illustrative N2 isotherms determined on the samples that gave the highest pore volumes for each impregnant are shown in Fig. 1 (wet impregnation) and Fig. 2 (dry impregnation). All of the isotherms were essentially Type I of the IUPAC classification. However, there were significant variations in the exact shapes of the isotherms, depending on the nature of the impregnant and the precise conditions of preparation. In general, the isotherms determined on the samples prepared with basic impregnants gave fairly
Fig. 1. N2/77K adsorption isotherms determined on selected samples after wet impregnation. From top to bottom at P0 0.9: PAC4.0/2/450, PHA7.1/1/ 700, SHA7.0/2/450. Open points are the adsorption branch and closed points the desorption branch of the isotherms.
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Fig. 2. N2/77K adsorption isotherms determined on selected samples after dry impregnation. From top to bottom at P0 0.9: SHCdry/2/700, PHCdry/2/ 700, PCCdry/2/700, SCC/dry/2/700. Open points are the adsorption branch and closed points the desorption branch of the isotherms.
rectangular isotherms, while those determined on the phosphoric acid impregnated samples exhibited a much slower approach to the plateau, indicating the presence of a broader pore size distribution. This was confirmed by the presence of a small Type H4 hysteresis loop on many of the
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isotherms determined on the phosphoric acid impregnated samples. With the other samples, low-pressure hysteresis was sometimes observed on the isotherms, indicating the presence in some samples of constricted micropore entrances. The textural parameters obtained by application of the as, DR and BET methods are given in Table 2. In those cases where low-pressure hysteresis was found, the DR equation cannot be applied as the adsorption isotherm will not be in equilibrium at very low pressures and for this reason, the values of vo and Lo are not given in the table. The external surface areas, As, of all the samples were found to be <50 m2 g1. In the present case, As can be identified as the non-microporous area, that is, the area of the macropores and the inter- and intra-particle mesopores. Hence, the low values obtained indicate, in agreement with the qualitative interpretation of the isotherms, that very little significant mesoporosity was developed under most conditions. The main exception to this was the phosphoric acid impregnated sample PAC4.0/2/450 which was the one that gave the highest pore volume of the wet impregnated samples and the highest external surface area of all the
Table 2 Textural characteristics obtained by application of the as, DR and BET methods to N2/77K adsorption isotherms determined on activated carbon samples vs (cm3 g1)
vo (cm3 g1)
Lo (nm)
As (m2 g1)
ABET (m2 g1)
KOH PHA5.2/1/450 PHA7.1/1/450 PHA10.1/1/450 PHA7.1/1/700 PHA7.1/2/450 PHC4.0/2/450 PHCdry/2/450 PHCdry/2/700
0.22 0.21 0.21 0.53 0.20 0.25 0.22 0.64
0.22 0.20 0.21 0.52 0.21 0.25 0.22 0.61
1.00 0.95 1.03 0.69 1.32 0.72 0.83 0.89
18 14 13 19 29 19 13 17
554 511 523 1370 517 646 552 1616
NaOH SHA7.0/1/450 SHA7.0/1/700 SHA7.0/2/450 SHC4.0/2/450 SHCdry/2/450 SHCdry/2/700
0.11 0.13 0.35 0.19 0.55 0.73
lph lph 0.33 0.20 0.47 0.55
lph lph 1.12 0.82 1.34 1.55
25 40 25 43 10 19
215 350 875 510 1287 1615
Na2CO3 SCC4.0/2/450 SCCdry/2/450 SCCdry/2/700
0.07 0.07 0.16
lph lph 0.15
lph lph 1.12
14 14 16
170 161 386
K2CO3 PCCdry/2/450 PCCdry/2/700
0.27 0.46
lph 0.45
lph 0.80
6 11
625 1175
H3PO4 PAA5.0/1/450 PAA6.9/1/450 PAA14.8/1/450 PAA6.9/1/700 PAA6.9/2/450 PAC4.0/2/450 PAC4.0/2/600
0.21 0.19 0.19 0.21 0.21 0.56 0.48
0.17 0.16 0.15 0.19 0.20 0.38 0.36
1.58 1.59 1.73 1.28 1.85 2.21 1.41
29 33 32 21 33 51 13
463 428 418 488 566 1066 1026
vs, Pore volume from as plot; vo, pore volume from DR plot; Lo, mean pore width; As, external surface area; ABET, apparent surface area; and lph, low-pressure hysteresis.
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samples and for which the mean pore width was slightly greater than the micropore limit of 2 nm. For many of the samples, the pore volumes were quite low. However, it can be seen from Table 2 that by suitable control of the conditions, it was possible to obtain samples with total pore volumes (vs) as high as 0.56 cm3 g1 by wet impregnation (PAC4.0/2/450) or 0.73 cm3 g1 by dry impregnation (SHCdry/2/700). It is also extremely interesting to notice from Table 2 that for those samples with high pore volumes, within the desired range of 0.5– 0.7 cm3 g1, there was a very large three-fold variation in mean pore width from 0.7 to 2.2 nm. That is, the primary goals of the work, referred to in Section 1, were achieved and the results confirm that by judicious choice of impregnant and conditions, it is possible to fine-tune the pore size of cork based activated carbons, while always managing to maintain a reasonably high pore volume. It is important to stress that the samples are essentially nonmesoporous. With the exception of the so-called superactivated carbons, currently produced by the Kansai Coke Company of Japan and marketed under the brand name MAXSORB, and certain activated carbon fibres, it has proven to be extremely difficult to produce exclusively microporous activated carbons with such high pore volumes [19]. 3.3. Potassium hydroxide impregnation The results in Table 2 indicate that when using potassium hydroxide as impregnant and a pyrolysis temperature of 450 8C, the total pore volume is essentially independent of impregnant concentration (including dry impregnation), mass ratio or particle size. However, the pore structures of the materials obtained are quite clearly different, as evidenced by the large variations in the values of Lo for these samples. On one hand, increasing the mass ratio provokes a small increase in Lo from 1.0 to 1.3 nm. On the other hand, the Lo values are much lower when the precursor has the smaller particle size. High pore volumes are only obtained at the higher temperature 700 8C and under these conditions, it is also found that the mean pore widths are particularly low. In fact, within the required range of high pore volumes of 0.5–0.7 cm3 g1, the lowest mean pore width, 0.69 nm, was achieved using potassium hydroxide as impregnant (PHA7.1/1/700). In a recent paper, potassium hydroxide impregnation followed by pyrolysis at 800–900 8C of a commercial phosphoric acid activated carbon was used in an attempt to obtain activated carbons with high pore volumes but narrow pore widths [19]. The pore volumes (vo) achieved varied between 0.4 and 0.5 cm3 g1 and the corresponding mean pore widths were 1.0–1.1 nm. It can be seen from Table 2 that higher pore volume and even narrower pore width (samples PHA7.1/1/700 and PHCdry/2/700) can be obtained by single step potassium hydroxide impregnation at a significantly lower pyrolysis temperature when using cork as the precursor.
3.4. Sodium hydroxide impregnation The samples prepared with the sodium impregnants were those most prone to activated entry effects as shown by the presence of low-pressure hysteresis on many of the corresponding isotherms. Where these effects are not present, it appears that both the total pore volume and the mean pore width are somewhat higher when using sodium hydroxide, in comparison to potassium hydroxide. In fact, the highest pore volume which appears in the tables, 0.73 cm3 g1 was achieved using sodium hydroxide as impregnant (SHCdry/2/700). The variation of pore volume and mean pore width as a function of impregnant concentration, mass ratio, particle size and pyrolysis temperature may be similar to that found with potassium hydroxide. However, the presence of lowpressure hysteresis complicates the analysis. For instance, the results in Table 2 indicate that at d = 5 mm and T = 450 8C, increasing the mass ratio, R = 1–2, has no effect on vs(KOH) (samples PHA7.1/1/450 and PHA7.1/2/450) but that vs(NaOH) increases significantly (samples SHA7.0/1/ 450 and SHA7.0/2/450). We believe that this is due to the simultaneous increase in pore widths, which occurs with both impregnants and which in the case of sodium hydroxide also results in the removal of pore entrance constrictions, thereby freeing microporosity which was not accessible at the lower mass ratio. Another example are the pore volumes at d = 5 mm, R = 1 and T = 700 8C. vs(KOH) is high (PHA7.1/1/700), but vs(NaOH) is low (SHA7.0/1/700). We believe this difference is due to the particularly lowmean pore widths which are found at the higher pyrolysis temperature and which, in the case of sodium hydroxide, are accompanied by pore entrance constrictions. 3.5. Sodium carbonate and potassium carbonate impregnation These two impregnants were only studied using the smaller precursor particle size as this was generally found to be more favourable with the other impregnants. However, in no case was the minimum required value of pore volume, 0.5 cm3 g1, achieved. Sodium carbonate was found to be particularly ineffective. On the other hand, one of the carbonate impregnated samples, PCCdry/2/700, came very close to the value of 0.5 cm3 g1 and it should be noted that once again, as found with the high temperature potassium hydroxide samples, the mean pore width was low. Other workers have also found that carbonate impregnants develop some porosity at low temperatures, but that it is only possible to achieve high pore volumes at temperatures above 800 8C [20,21]. It has been suggested that this difference is due to a change in the mechanism of activation at the higher temperature [21]. On the other hand, an additional factor which may contribute to the different behaviour of the carbonates in comparison with the hydroxides could be the stronger hydrolysing activity of
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the latter. It is well known that treating lignocellulosic materials with sodium hydroxide, for example, is effective in breaking the bonds between the polysaccharides and the lignin and also results in some depolymerisation of both [22,23]. Alkali treatment is also one of the techniques used to separate suberin from the lignin and polysaccharides present in cork, prior to its chemical analysis [4]. The extent of debonding and depolymerisation, which occurs during the impregnation of the precursor, should be less with the weaker carbonate bases and the mean molar mass of the molecules will therefore be higher. It would therefore also be expected that the carbonate impregnated samples would exhibit better thermal stability resulting in higher yield and lower porosity development, at least during the initial lower temperature stages of pyrolysis. 3.6. Phosphoric acid impregnation In agreement with other published studies, phosphoric acid was found to be different from basic impregnating agents in two major respects [24,25]. Firstly, as already mentioned, there was a greater degree of pore widening. Secondly, increasing the pyrolysis temperature did not
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improve the pore volume but, with the smaller precursor particle size, decreased it. On the other hand, there are also a number of similarities between phosphoric acid and the basic impregnants. With the larger precursor particle size, the pore volume was found to be independent of impregnant concentration and mass ratio, while the mean pore width increased with increasing impregnant concentration or mass ratio. Although increasing the pyrolysis temperature did not improve the pore volume, the same decrease in mean pore width found with the basic impregnants was also observed with phosphoric acid. 3.7. SEM One surprising feature of the results is that the impregnant concentration and mass ratio had very little effect on the pore volume. Hence, in order to verify that our samples were effectively and uniformly impregnated, SEM images of selected samples were determined and illustrative examples of three samples prepared under very similar conditions but with different impregnants are shown in Fig. 3. For comparison, an SEM image of a physically activated sample is also shown in Fig. 3 and it can be seen that the
Fig. 3. Representative SEM images (all on the same scale, indicated in part (a)): (a) CO2 activation [16], (b) PAA6.9/1/450, (c) PHA7.1/1/450 and (d) SHA7.0/ 1/450.
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characteristic cork membrane structure is present on this image. The other SEM images show that during pyrolysis of chemically impregnated samples, the cork structure was extensively modified. In some cases, such as PAA6.9/1/450 shown in Fig. 3(b), remnants of the membrane structure are still visible, while in other cases, such as PHA7.1/1/450 and SHA7.0/1/450 shown in Fig. 3(c and d), the cork appears to have been almost completely digested by the impregnant. A more detailed study aimed at relating the adsorption and SEM data is currently under way.
4. Discussion Pyrolysis of natural materials impregnated with phosphoric acid or with basic impregnants involves different chemical mechanisms and our results show that this is reflected in quite different pore structures. One particularly important difference in chemical mechanism is that phosphoric acid intervenes in the initial stages of carbonisation of the precursor, which begin at lower temperatures [24–26], whereas the basic agents only react after the precursor has been at least partially carbonised at higher temperatures [20,21,24]. This is one of the reasons why the development of porosity in phosphoric acid activation was found to be better at low temperature, while in basic activation, it was better at high temperature. In addition, it is also consistent with the higher yields found with phosphoric acid. In this case, the agent promotes char formation and inhibits tar formation at low temperatures and therefore reduces volatilisation and loss of carbon. With basic agents, on the other hand, significant volatilisation of the organic compounds formed as a result of low-temperature thermal decomposition of the precursor can occur before the subsequent chemical reactions involving the activating agent become thermodynamically feasible at higher temperatures [21]. Another important difference is that with bases, it is believed that there is a stoichiometric reaction between the impregnant and the carbon, which leads to the formation of volatile or soluble products (removed during the final washing of the product), such as potassium carbonate [27]. As a result of these reactions, C atoms are quantitatively separated from the polymeric carbon matrix. This may be one of the reasons for the higher yields found here with the carbonate impregnants, in comparison with the hydroxides. Although with all four basic impregnants, the yield was low, due to the low-temperature thermal decomposition reactions previously referred to, the yield was somewhat higher with the carbonates. One reason for this difference, already mentioned in Section 3.5, may be the weaker hydrolysing activity of the carbonates during impregnation. However, we believe the fact that the porosity development was lower, indicates that the additional carbon loss at high temperatures, caused by reaction of the impregnating agent with the carbon, was also less significant. With phosphoric acid, on the other hand, the agent, whether by a catalytic or
mechanistic pathway, promotes bond cleavage in thermally less stable structures and subsequent formation of new bonds in thermally more stable polymeric aromatic structures. Hence, there is not a quantitative loss of C atoms at any temperature and the yield is much higher. There may be various reasons for the higher pore widths obtained with phosphoric acid. Firstly, the bond cleavage and formation reactions can lead to cross-linking and eventually to the formation of buckled or folded graphene layers. Furthermore, at the lower temperatures where these reactions begin the precursor molecules are disordered and there may exist relatively large gaps between the molecules. The combination of these two factors will lead to a relatively open and poorly ordered packing of the graphene layers which, in turn, will make the high temperature structural transformations which could lead to densification more difficult. An additional factor is grafting of phosphate species to the graphene layers. The presence of large phosphate or polyphosphate species will also prevent the graphene layers from adopting an oriented dense structure [26]. In contrast, when basic impregnants are used, the chemical reactions involved may not lead to such a significant level of cross-linking but, on the contrary, may remove carbon atoms which are impeding reorientation of the growing graphene layers. Furthermore, these reactions only begin after the initial disordered structure of the precursor has been partially reorganised during carbonisation. As in our previous work on physical activation, a particle size effect was also found here [2]. However, as the SEM showed that chemical impregnation leads to the destruction of the cell walls, the cause of this effect must be different. It was found that decreasing the particle size results in an increase in pore volume when using either phosphoric acid or potassium hydroxide. On the other hand, the mean pore width increased when using phosphoric acid, but decreased when using potassium hydroxide. Other workers have shown that increasing the flow-rate improves the porosity development due to the faster removal of gaseous products which inhibit the pyrolysis reactions [21]. Hence, it is possible that the particle size effect that we observed is due to a more efficient removal of gaseous products when the particle size is low. As a result, the pyrolysis rates will be higher and higher pore volumes are obtained. With phosphoric acid, cross-linking is enhanced and the pore size increases. With potassium hydroxide, reorientation of the graphene layers is enhanced and the mean pore width goes down.
5. Conclusions The results presented here show that by appropriate control of the conditions of impregnation and pyrolysis, it is possible to obtain cork based activated carbons with high pore volumes, in the range 0.5–0.7 cm3 g1, and with mean pore widths of 0.7–1.3 nm using basic impregnants (1.6 nm with sodium hydroxide under dry conditions and at higher
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activation temperature) and 1.3–2.2 nm using phosphoric acid. The best materials produced present pore structural characteristics which are significantly different to the vast majority of commercial activated carbons. In particular, the possibility of using an agricultural waste product as a precursor for obtaining materials with such high pore volumes, which are essentially microporous and contain virtually no mesoporosity in most cases, is noteworthy. Furthermore, the fact that it was possible with some samples to combine high pore volume and very narrow micropore size is a particularly notable achievement of the results. The effect of five experimental parameters, namely method of impregnation, impregnant concentration, mass ratio, precursor particle size and pyrolysis temperature, were studied. Over the range of conditions used, it was found that increasing the impregnant concentration or mass ratio had little effect on pore volume but increased the mean pore width. A decrease in the precursor particle size resulted, in general, in higher pore volumes. Increasing the pyrolysis temperature resulted in an increase in pore volume when using basic impregnants but a decrease when using phosphoric acid. However, in all cases, higher pyrolysis temperature resulted in particularly low-mean pore widths. On the basis of the results presented, it is possible to identify the preparation conditions that ought to be most suited for different types of application. For applications, such as gas separation, where a narrow distribution of small micropore widths is essential, wet potassium hydroxide impregnation followed by pyrolysis at high temperature would be the best choice and allows a mean micropore width of 0.7 nm with micropore volume of 0.5 cm3 g1, to be obtained. For liquid phase applications where a broad micropore size distribution is required, wet phosphoric acid impregnation followed by pyrolysis at a lower temperature would be one suitable choice. This allows us to obtain a pore volume of 0.6 cm3 g1 with a pore size distribution partially extending into the region of small mesopores and has the particular advantage of a very high pyrolysis yield. On the other hand, sodium hydroxide, under conditions of dry impregnation, also allows very high pore volume and reasonably broad pore size distributions to be obtained and has the additional advantage of producing low ash or in certain cases ash-free products.
Acknowledgements This work was supported by the Fundac¸a˜o para a Cieˆncia e a Tecnologia (Project N8 POCTI/CTM/38255/2001) with national and European community funds.
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