Preparation and characterization of active carbons from olive stones

Carbon Vol. I& pp.413-418 @ Pergamon PressLtd., 1980. Printed in GreatBritain

PREPARATION AND CHARACTERIZATION OF ACTIVE CARBONS FROM OLIVE STONES J. de D. LOPEZ-GONZALEZ,F.MARTINEZ-VILCHEZ and F. RODRIGUEZ-REINOSO Department of Inorganic Chemistry, Universtiy of Granada, Spain (Received

4 Nouember

1978)

Abstract-Olive stones have been carbonized under a flow of nitogen in the temperature range from 700 to 900°C and activated in a CO2 flow in the range from 675 to 875°C.ZnCI, was used in some of the activation processes. The adsoptive characteristics of the carbonized and activated samples have been determined by adsorption of nitrogen (77 and 90 K), carbon dioxide (195 and 273K), n-butane (273K) and methylene blue (aqueous solution at 298 K).Meso and macroporosity have been followed by mercury porosimetry. The resulting activated carbons have very large surface areas as well as a highly developed microporosity. The most adequate experimental conditions for the preparation of active carbons, highly microporous but with a well developed meso and macroDorosity, are discussed. All active carbons prepared have a very low ash content and complete absence of sulphir, both-very attractive characteristics.

LINTRODUCTION

Some basic information about the adsorptive properties of activated carbons prepared from olive stones was presented in a previous paper[l]. The results obtained made clear that olive stones, a very abundant agricultural by-product in Mediterranean countries, could be a very adequate raw material to obtain active carbons. The preparation of these activated carbons is economical and they have, besides some special properties, good adsorptive properties and hardness, which could be of interest in some specific industrial applications as well as in future environmental amelioration programs. In view of the possible application of this type of activated carbon, an extensive study of the experimental conditions of preparation and influence on their adsorptive properties has been carried out. This paper presents the information on such study. 2.EXPERIMENTAL Olive stones were freed either from their fruit or from the bagasse obtained as a by-product in the olive oil industry; subsequently, they were crushed and washed in a 10% solution of sulphuric acid and refluxed in distilled water to zero acid removal. The stones were sized to about 3-4 mm diameter and carbonized under a flow of nitrogen at 700, 750, 800, 850 and 900°C using a heating rate of 5°C min-‘; once the carbonization temperature was reached, it was kept for 1 hr before the furnace was allowed to cool down to room temperature. Since the olive bagasse contained the crushed stones, in this case the particle size used was as received l-2 mm diameter. Yield of the carbonization process is low in any case, as it could be expected from the botanical structure of the olive stones [I], and it decreases with increasing temperature although not in a very marked manner, ranging from 2% (700°C) to 23% (90°C). The particle size of all carbonized olive stones was reduced to the range 500-850 pm diameter. Subsequently, they were activated in a continous flow of carbon dioxide (0.2 cm* set-‘) at different temperatures

(conditioned by the carbonization temperature) and for various periods of time (from 2 to 16 hr); in all cases the heating rate was that of the own furnace, without using a proportional temperature controller. However, in order to find the influence of the heating rate on the characteristics of the final products, some activations were carried out using a much slower and controlled heating rate of 5°C min-‘; under these later conditions yield is consistently lower, the difference becoming larger with increasing activation time. In this way a total of 36 activated carbons have been prepared by activation with carbon dioxide, to cover a wide range of burn-offs (referred to the carbonized products) from 4 to 65%. Another type of activation was carried out using ZnClz. The crushed olive stones were impregnated with solutions of different concentrations of ZnCl, given by the weight ratio stone/ZnCl, (I : 0.5; I : 1 and I : 2) and for various periods of time before being carbonized at 600 or 700°C following the same procedure described above. A total of eight samples have been prepared using this activation procedure. Adsorption isotherms were determined using conventional McBain silica spring balances. Nitrogen was adsorbed at 77 K (in some of the samples also at 90 K), carbon dioxide at 195 and 273 K and n-butane at 273 K. The time to reach equilibrium was short except in the cases of the carbonized products. Adsorption data were complemented by mercury porosimetry measurements (an Aminco prososimeter was used). The adsorption at 298 K of methylene blue in aqueous solution has been followed spectrophotometrically using a Beckman Acta III spectrophotometer. Chemical analysis of the stones, and of two carbonized products and two activated carbons are included in Table 1. It is important to note in this table the total absence of sulphur in all samples and the low ash content of the activated carbons. Electron probe analysis detected traces of calcium, sodium, potasium and chlorine within the original olive stones. The large content of oxygen 413

J. de D. L6PEZ-GONZALEZ

414

Table 1. Chemical analysis of some samples Sample

C%

H%

N%

S%

Ash%

Stones C-4 C-5 A-25 ACZ-4t

47.31 89.26 88.40

5.88 0.91 0.79 0.63 1.45

0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00

0.08 0.11 0.14 0.14 -

ISeveral runs were made in samples activated with ZnC12and in all cases the ash content was found to be nil. (deduced by difference) in the activated carbons may be another interesting feature (Table 1). 3. RESULTS

A great number of adsorption isotherms has been obtained, but only an example for each adsorbate and adsorption temperature used on the sample prepared by carbonization at 850°C and activation at 82X, with 5°C min-’ heating rate, has been included in Fig. 1. All the isotherms are of type I and consequently their

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analysis may present some problems [2,3]. To calculate surface areas, the BET equation has been used since it is customary to express adsorptive capacity of porous materials in this way; Dubinin-Radushkevich (D-R) equation of adsorption in the conventional form[4] log V = log V,,- D log*(Al?,) has also been used to determine V,,, the volume of micropores, from which an effective surface area has been deduced[l]. Molecular areas for the adsorbates used are: 16.2A’ for nitrogen (77 K); 17.0A’ for nitrogen (90 K); 17.0A2 for carbon dioxide (195 K); 18.7A’ for carbon dioxide (273 K) and 32.3 A’ for n-butane (273 K) (see Refs. [I, 63). The surface areas of the five carbonized products have been listed in Table 2; the nomenclature of the samples is made by the letter C followed by the carbonization temperature. Since a table with the surface areas of all activated products prepared would be too large, the surface areas of only two series of active carbons (those carbonized at 850°C and activated at 825°C with-samples A-23-A-27-and without-samples A-3%A42+ontrolled heating rate of 5°C min-‘) have been included in Table 3. Finally, surface areas of the samples obtained by activation with ZnC12 (samples ACZ-I-ACZ-8) have been listed in Table 4. The variation of surface area with percentage burn-off of the activation process (which is directly related to the activation time) is shown in Fig. 2; the surface area values plotted have been those deducted from adsorption of nitrogen at 77 K using the Dubinin-Radushkevich equation of adsorption. The variation of the gradient “II” in Dubinin-Radushkevich equation corresponding to adsorption of carbon dioxide at 273 K is shown in Fig. 3. The variations of pore volume with pore radius, obtained using the mercury porosimeter in some of the samples, including the original stones, the carbonized products and the activated carbons, are shown in Fig. 4. The surface area values obtained using adsorption of methylene blue in aqueous solution are listed in the last column of Tables 2-4; the molecular area for methylene blue has been taken as 130A’[S]. 4. DISCUSSION

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Fig. 1. Typical adsorption isotherms (sample c&onized at 850°C and activated at 825°Cwith 5°C min-’ heating rate).

The carbonization of olive stones under a flow of nitrogen and their subsequent activation with either carbon dioxide or ZnCl, yields activated carbons having very high surface areas. However, carbonized products have a very low surface area when determined by adsorption of nitrogen at 77 K, but the values of the corresponding surface areas obtained from adsorption of carbon dioxide at 195 or 273 K are much higher (Table 2). These results show that the entry of the nitrogen molecule at 77 K is partially restricted in micropores of dimensions only slightly larger than the nitrogen molecule, whereas the somewhat larger molecule of carbon dioxide can readily penetrate those micropores at 195 or 273 K. This must mean that there is an activated adsorption process and that the role of the temperature is

415

Preparation and characterization of active carbons from olive stones Table 2. Surface areas (m2g-‘) of carbonized samples

Sample c-700 c-750 C-800 C-850 C-900

W77 K) BET D-R 29 33 35 38 74

34 37 42 4s 98

C&(195 K) BET D-R

COJ273 K) BET D-R

398 411 434 456 484

338 357 3% 397 417

482 489 SO8 530 583

Methylene blue (298K)

t&H,&273 K) BET DR

539 54s 565 551 571

38 -

5 8 9 -

48

Table 3. Surface areas (m’g-‘) of some samples activated with CO? Methylene blue (298K)

N2(77K) BET DR

WJO K) BET ILR

CO2(19SK) BET t&R

CO?,273K) BET D-R

n-&H,&273 K) BET D-R

A-23 A-24 A-25 A-26 A-27

545 525 668 758 872

699 793 868 999 1121

520 549 663 762 953

646 726 861 982 1094

457 518 582 701 766

640 725 828 1010 1097

352 488 S29 643 636

635 695 765 856 981

341 424 474 536 624

426 519 575 675 776

93 50 98 333 441

A-38 A-39 A-40 A-41 A-42

693 792 934 1071 1307

870 1051 122s 1404 1724

683 762 1016 1089 1238

871 1012 1083 1394 1626

592 703 864 %O 1192

832 973 1092 1368 1747

530 662 681 787 969

714 829 983 II47 1338

467 512 657 782 897

614 657 847 1012 1230

193 202 35s 410 745

Sample

Table 4. Surface areas (m2g-‘) of samples activated with ZnClz

Sample

BET

D-R

CO,( 195K) BET D-R

ACZ-I ACZ-2 ACZJ ACZ-4 ACZ-5; ACZd ACZ-7 ACZ-8

2084 14% 1917 1761 1489 1548 994 IISO

1656 1612 1784 1751 1564 1711 1426 1257

1412 1429 1464 1583 1137 915

N2(77 K)

1074 1014 1170 1274 951 1159

since the nitrogen is being adsorbed at 77 K whereas the carbon dioxide is adsorbed at a much higher temperature, 195 or 273 K. Consequently, the carbonized materials must be esentially microporous, with surface areas of about 500 rn’g-’ basically in microporosity only slightly larger than the carbon dioxide molecule diameter. Moreover, when the surface areas of these carbonised samples deduced from adsorption of carbon dioxide (D-R equation) at 195 and 273 K are compared, it can be found (see Table 2) that the values obtained at 273 K are in general larger than those obtained at 195K. This further confirms the diffusional restrictive process which must be present in the adsorption on these ultramicroporous carbons. On the other hand, the surface area values for sample C-850 obtained from adsorption of n-butane at 273 K are almost identical to those calculated from adsorption of nitrogen at 77 K and much lower than those obtained from adsorption of carbon dioxide. This is a clear confirmation of the microporosity mentioned above since the n-butane molecule, even at a temperature as high as 273 K, can not penetrate in the important,

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co2(273K) BET D-R

&H,,,(273 K) BET D-R

-

-

547 585 599 552

481

1281

137s

529 710 640

1166

1358 -

-

-

Methylene blue (298K) 663 70s 655 638 622 542 488

ultramicropores because of its larger dimensions to nitrogen and carbon dioxide molecules.

respect

The surface area values obtained for the activated products show that these restrictions can be removed upon activation of the carbonized samples. Thus, even for samples with a relatively low percentage burn-off (< 10%) the increase in surface area as deduced from adsorption of nitrogen at 77K is very large, in most cases above tenfold (Fig. 2). This effect, often observed in activation processes[7], is attributed to removal of carbon atoms which restricted entry of the adsorbate molecules into the microporosity. Moreover, in these samples with low burn-off (< 10%) there is still some degree of restriction and activated adsorption since the surface areas obtained from adsorption of nitrogen at 77 K are lower than those obtained from carbon dioxide at 195K and these even lower than the ones from carbon dioxide at 273 K. For samples with larger degree of burn-off ( > 10%)the restrictions seems to be completely removed since, in general and for a given equation of adsorption of the two

416

J. de D. MPEZ-GONZALEZ

et al.

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Fig. 4. Variation of pore volume with pore radius, obtained using a mercury porosimeter.

Preparation and characterization of active carbons from olive stones

used in this work, the surface area values obtained from adsorption of nitrogen at 77 K (Fig. 2) are very similar to those calculated from adsorption of nitrogen at 90 K and larger than those from carbon dioxide at 195 and 273 K. On the other hand, surface areas of carbons increase with increasing percentages of burn-off. It is also important to note that for these activated carbons the surface areas obtained by adsorption of nitrogen (77 K) and carbon dioxide (195 and 273 K) are in a reasonable agreement and this is indicative of a microporosity which is almost equally accessible to both adsorbates at the different adsorption temperatures. However, the surface areas deduced from adsorption of n-butane at 273 K are consistently lower than those obtained from adsorption of carbon dioxide at the same temperature, specially in the samples activated without using a proportional heating rate. In these later samples, about 25% of the micropores are not accessible to n-butane at 273 K and this is indicative of a certain molecular sieve character of these carbons, which decreases with increasing degree of activation. In this sense, it is also interesting to compare the surface area values of the series of samples activated using a heating rate of 5°C min-’ (samples A-38-A-42 in Table 3) with those prepared under the same conditions but without a proportional heating rate (samples A-23-A27). In the former samples the surface area is always larger than in the later, the difference being larger the higher is the time of activation. On the other hand, the surface area values for the samples activated with a heating rate of 5°C mini’, obtained by adsorption of carbon dioxide and n-butane at 273 K (IXR equation) are now rather similar. All these data indicate that the heating rate of 5°C min-’ produces a larger development of the porosity of the carbons, the micropores becoming enlarged in such a way as to make them almost equally accessible to both gases. However, the two series of surface area values mentioned are consistently lower than those obtained from adsorption of nitrogen at 77 K or carbon dioxide at I95 K which are rather similar to each other. These differences must be related to the different relative pressures of the adsorbates at the temperatures of the adsorption measurements. This different adsorptive behaviour of the two series of activated carbons prepared by carbonization at 850°C and activation at 825°C is a clear indication of the role played by the heating rate used during the activation process. The results show that the use of a heating rate of 5°C min-’ develops a more uniform porosity than the faster heating rate of the furnace directly connected to the power. Surface area values for carbons activated using ZnCl* (Table 4) are extremely high, specially some of those deduced from the BET equation applied to adsorption of N2 at 77 K. Values obtained from D-R equation probably represent a maximum in possible internal microporosity developed by the activation process. Finally, surface areas of activated carbons prepared using the olive bagasse are almost identical to those obtained for activated carbons prepared directly from the olive stones. This result is of industrial significance,

417

since it means that the bagasse, which is the more abundant by-product from olives in mediterranean countries, can be used directly for the preparation of activated carbons. To obtain a general information about the influence of the carbonization and activation conditions on the adsorptive properties of the final activated carbons, the surface area values have been plotted in Fig. 2 as a function of the percentage burn-off of the activation process with carbon dioxide. The surface area values used are those deduced from adsorption of Nz at 77 K (D-R equation). The general trend is an increase in surface area with increasing burn-off for any activation temperature; however, there are some interesting features in the plots of Fig. 2. First, the influence of the heating rate used in the activation process is very well shown for the two series activated at 825°C since the surface area increases more drastically when a heating rate of 5°C min-’ is used. Second, the rate of surface area development is much larger at low percentages burn-off but after about IO-20% burn-off, this rate falls down noticeably. Mercury porosimetry measurements show the development of meso- and macroporosity by activation. Figure 4 shows that the pore volume of the original olive stones and the carbonized products have very small proportion of meso and macroporosity. When the degree of activation with carbon dioxide is low (for example, sample A-4) there is a small development of such porosity but on further activation this porosity is considerably developed. It is interesting to compare plots of samples A-23 and A-24 (no proportional heating rate) with samples A-38, A-39 and A-41 (5°C min-‘, heating rate). The development of meso and macroporosity is much larger in the later set of samples, a phenomenon which corresponds with that already discussed about the microporosity. Thus, sample A-41 has about 0.4 cm3 g-’ of pore volume contained within macropores in addition to about 0.5 cm3 g-’ in microporosity (assuming the density of liquid carbon dioxide is 1.0g cm-‘[S]). Again, the pore volume measured with the mercury porosimeter for samples activated with ZnCIZ is much higher than that of the carbons activated with carbon dioxide, being in the case of sample ACZ-2 above 0.6cm3g-‘. The existence of this marked meso- and macroporosity in many of the activated carbons obtained may be of advantage in many industrial applications since these pores can be considered as arteries of transportation of the adsorbate to the interior of the carbon particles. The gradient "D"of the Dubinin-Radushkevich equation can be taken as a function of the mean porosity diameter of the active carbons and has a semiquantitative significance[l]. Figure 3 shows the variation of the gradient “D” (calculated from adsorption isotherms of carbon dioxide at 273 K) with the extent of activation of the olive stones with carbon dioxide. There is a general trend of increasing “D" value as burn-off increases and, again, the activation at 825°C using a heating rate of SUmin-’ produces larger increase in the gradient "D"per percentage burn-off unit. Surface area values of carbons deduced from methy-

418

J. de D. MPEZ-GONZALEZ et al.

lene blue adsorption (Tables 2-4) show that in the cases of carbonized samples and activated carbons with low degree of activation the adsorption of this adsorbate is very small, as it could be expected for such ultramicroporous adsorbents. Among the samples activated with carbon dioxide, only those activated at 825°C and specially those using a 5°C min-’ heating rate exhibit a surface area accessible to methylene blue relatively significant although always this surface area is less than 50% of that determined by adsorption of N2 at 77 K. Samples activated with ZnCl, retain much more methylene blue from aqueous solution than most of the samples activated with carbon dioxide. The activated carbons which exhibit larger retention capacity for methylene blue are also those which have a larger pore volume as determined by mercury porosimetry, as it could be expected given the large molecular size of the dye. On the other hand, the results obtained by adsorption of methylene blue in aqueous solution constitute a confirmation of the microporous nature of the activated carbons prepared from olive stones, and that the porosity is more uniformly developed when a heating rate of 5°C min-’ is used. As it can be deduced from the results given above, olive stones are an excellent raw material for the preparation of activated carbons with very high surface areas, of possible application in gas-phase adsorption. The experimental conditions of the carbonization and activation processes considerably affect the adsorptive

properties of the activated carbons as a consequence of the effect they have on the development of micro- and mesoporosity. In this way it has been deduced that carbonization in nitrogen at 850°C and activation with carbon dioxide at 825°C produce active carbon with high surface areas in which micro- and mesoporosity is well developed, specially if a heating rate of 5°C min-’ is used during both processes. It has also been shown that activation with ZnC12yields activated carbons with better adsorptive properties either in gas or liquid phase, this latter aspect being worthy of further research.

REFERJCNCFS

M. Iley, H. Marsh and F. Rodriguez-Reinoso, Carbon II, 633 (1973). S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity. Academic Press, London (1%7). K. S. W. Sing, Colloid Science, Vol. 1, p. 1(1973). M. M. Dubinin, Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 2, p. 51. Edward Arnold, London

we.

5. J. de D. L6pez-Gonzalez and F. Rodriguez-Reinoso, Anales de Ouimica 68.977 WZ’2).

6. J. de D. Lopez-Gonzalez, F. Rodriguez-Reinoso, M. A. Baiiares and C. Moreno-Castilla, Anales de Quimica 72, 759 (1976). E. M. Freeman and H. Marsh, Carbon 8, 19 (1970). s7.M. hf. Dubinin, B. P. Bering, V. V. Serpinsky and B. N. Vasil’ev, The properties of substances in the adsorbed state. In Surface Phenomena in Chemistry and Biology (Edited by J. F. Danielli), p. 172.Pergamon Press, London (1958).