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Porous texture of activated carbons modified with carbohydrates
Carbon Vol. 35, No. 4, pp. 447-453, 1997 Copyright 0 1997 Elsevier Science Ltd
Pergamon
Printed in Great Britain. All rights reserved 0008-6223/97$17.00+ 0.00 PIk 80008-6223(96)00170-4
POROUS TEXTURE OF ACTIVATED CARBONS MODIFIED WITH CARBOHYDRATES R. TORREGROSA-MAC&’
J. M. MART~N-MART~NEZ~‘*
and
M. C. MITTELMEIJER-HAZELEGER~ “Departamento de Quimica InorgAnica, Universidad de Alicante, 03080, Alicante, Spain bDepartment of Chemical Engineering, University of Amsterdam, 1018WV Amsterdam, The Netherlands (Received 23 May 1996; accepted in revisedform
11 September
1996)
Abstract-A modification of the porous texture of activated carbons to develop meso- and macroporosity is described. Three gas-activated carbons of different origin, have been modified by impregnation with two carbon compounds (n-glucose and o-glucosamine) from aqueous solution followed by carbonization at 1173 K and activation with CO,. The most important results of this procedure are (1) the development of meso- and macroporosity depends on the mesopore size distribution of the original activated carbon; (2) the gasification rates are different for each impregnated carbon in the activation step, and (3) there was no or little development of the original microporosity. 0 1997 Elsevier Science Ltd Key Words-A.
Activated carbon, B. impregnation, activation, C. adsorption, D. microporosity, porosity.
1. INTRODUCTION
D-glucose and D-glucosamine have been shown to be effective in the selective catalytic reduction of NO, with NH, [7,8]. In this study, two carbohydrates (D-glucose and D-glucosamine) were used. Thus, the activated carbons (RX1 Extra, D55/2 and Carbon Y) were impregnated with the two carbon compounds from aqueous solution and then submitted to carbonization (N,/1173 K - 2 hours) and physical activation (CO,/1 123 K - 2 hours). One of the most important features to take into account for an impregnation treatment is the characteristics of the raw material (i.e. the pore system of the activated carbon to be treated, and the relative affinity between the carbon and the solvent); depending on these characteristics the solute will be carried to different pore ranges. In fact, the development of the meso- and macroporosity will be more affected, the more accessible the mesopores are to the impregnating carbon compound, because the impregnating agent will produce a carbon deposit, during pyrolysis, on the surface of these pores.
Activated carbons can be very useful in some industrial applications where well-developed meso- and macropore systems and a high purity of the adsorbents is highly desirable, i.e. for adsorption from solution or for catalyst supports [l-13]. Usually, the most appropriate relatively wide porous system for a given industrial application is better obtained by chemical activation, which, however, leaves contaminating agents in the activated carbon, than by physical or gas activation, where the development of an important volume of meso- and macropores requires high burn-offs, resulting in poor mechanical properties of the activated carbon. To avoid such drawbacks, it is proposed in this paper to modify the porous system in activated carbons, produced by “clean” preparation procedures (i.e. by physical activation), by impregnating the carbons with carbon compounds such as carbohydrates or their nitrogencontaining derivatives. Subsequent carbonization and physical activation will result in a modified activated carbon whose porous system, being properly developed for those applications (i.e. wide porosity can be developed), will have no contaminants. To analyze the factors affecting the effectiveness of this kind of treatment in the development of mesothree activated carbons of and macroporosity, different origin and produced by physical activation with different activating agents have been used in this study. According to the literature [1416], the most appropriate organics to modify the porous texture of the activated carbons are hydrocarbons, polymeric derivatives or carbohydrates. Furthermore, the impregnation of activated carbons with
2. EXPERIMENTAL The activated carbons selected for this study were: (1) RX1 Extra (NORIT, Amersfort, Netherlands), a steam-activated carbon prepared from peat with narrow pores; (2) D55/2 (Bergbau Forschung, Essen, Germany), a steam-activated carbon prepared from a subbituminous coal with a well-developed porous texture (i.e. pores in all ranges are present); and (3) Carbon Y, an activated carbon prepared in the laboratory from plum stones having a well-developed porous texture. These carbons were selected considering that different precursor materials (i.e. peat, subbituminous coal), activated by the same method,
*Corresponding author. 441
R. TORREGROSA-MACI~et al.
448
result in different porous systems, and different raw materials (i.e. peat, plum stone), activated by different agents (i.e. COZ, steam), exhibit a similar porous texture. Figure 1 (a) shows schematically the experimental procedures followed in the modification of the porous texture of the selected carbons by impregnating the original activated carbons with glucose (G) or glucosamine (GM). For comparison, the original activated carbons were also re-carbonized, without impregnation, under similar experimental conditions and re-activated during different periods of time, to obtain the desired degree of burn-off, leading to the corresponding reactivated carbons (R) (Fig. 1 (b)). The activated carbons, dried in air at 383 K for 12 hours, were impregnated with 35 wt% aqueous solutions of D-glUCOSt? and D-glucosamine, respectively, a carbon:solution ratio of 1:lO in volume being used. The mixture of carbon and solution were dried at 383 K (12 hours) and carbonized in a fixed-bed oven in Nz (120 mL min-‘) at different temperatures and soaking periods, progressively increasing from 623 K/30 minutes, 773 K/30 minutes and, finally, at 1173 K/2 hours. Except for carbons obtained from D55/2, the activation of the other impregnated carbons was performed in situ at the end of the carbonization period, by changing the N, stream for CO,
Ia)
(120 mL min-‘), during 24 hours, to obtain the corresponding modified activated carbon. The impregnation plus carbonization process was followed with D55/2 taking samples of both carbonized impregnated carbons, G(c) with glucose and GM(c) with glucosamine, in order to evaluate the porous texture parameters obtained before activation. In each case, the porous texture of the original and the corresponding modified activated carbons was determined from adsorption isotherms of N, at 77 K and CO, at 273 K, measured by use of a Sorptomatic 1800 Carlo Erba instrument and by mercury porosimetry with a Carlo Erba Mercury Porosimeter 4000. Furthermore, SEM photographs of the external surface of the carbon particles, goldcoated, were taken in a Jeol JSM 840 scanning electron microscope.
3. RESULTS AND DISCUSSION
The porous texture parameters for all carbons, obtained from the experimental data by applying the Dubinin-Radushkevich equation [ 171, cc,-method [ 181 by using the N, adsorption standard isotherm published elsewhere [19], and from mercury poroare summarized in Table 1 which also simetry,
0) drying impregnatton Mth addithn do%!
1
383K/ Ith
36vA%inH2C 1 383Kl17.h 1
f
I
623K/ 30m 77310 3om
carbonization in N 2
1 1173KI
2h
I
1
activation
62310 3Omin
carbonization inN 2
Pctivation in CO2
in CO2
1173Kl variable
time
1
773Kl3Omin 1
I
1173W 24h
Fig. 1. Modification of the porous texture of activated Preparative
carbons. (a) Preparative scheme for the re-activated
scheme of the modified carbons.
activated
carbons.
(b)
Porous texture of activated carbons modified with carbohydrates
449
Table 1. Textural properties of activated carbons. Volumes (V) in cm3 g-r and external surfaces (S,) in mz g-’ N, at 77 K a, method
Mercury porosimetry
Weight loss (%)
V, DR
V0
&
CO, at 273 K V, DR
_ -5 14 -3 25 25
0.30 0.20 0.30 0.21 0.40 0.31
0.28 0.20 0.28 0.20 0.39 0.29
22 15 17 14 34 40
0.26 0.21 0.25 0.22 0.30 0.35
0.03 _ 0.05 _ 0.04 0.00
0.36 _ 0.38 _ 0.47 0.21
RX1 Extra 0 G GM R
54 62 35
0.66 0.71 0.76 0.73
0.63 0.86 0.86 0.79
137 252 372 148
0.51 0.37 0.36 0.36
0.14 0.45 0.52 0.26
0.53 1.27 1.55 0.60
Carbon Y 0 G GM R
80 77 38
0.65 0.62 0.83 0.64
0.64 0.80 1.11 1.03
62 180 101 97
0.38 0.59 0.50 0.48
0.20 0.45 0.49 0.43
0.27 0.53 0.35 0.39
Carbon D55/2 0 G(c) G GM(c) GM R
includes the global weight loss percentage with respect to the original activated carbon. At first sight, the differences in weight loss percentage between glucose- and glucosamine-impregnated activated carbons from RX1 Extra and D55/2 could be supposed to be normal (i.e. a significant weight loss) and the behaviour of Carbon Y to be the exception (i.e. a small weight loss). However, according to the volumes of meso- and macropores in the original activated carbons, it can be suspected that the meso- and macroporosity of these might be important in determining the reactivity towards COZ of the carbonized-impregnated carbons, since the major differences between the original and the modified porous system do not occur in the micropore but in the meso- and macropore ranges. For this reason, the modification treatment will be analyzed separately for every activated carbon. 3. I Carbon 05512 The Nz adsorption isotherms at 77 K of the original activated carbon (0) and the various treated carbons are shown in Fig. 2(a). All isotherms correspond to the type I of the IUPAC classification [20] exhibiting a knee at low pressures, typical for relatively narrow micropores. The impregnation plus carbonization process (samples G, and GM,) results in a partial blocking in the entrance of the micropores for the adsorption of N2 at 77 K, but the subsequent activation in CO, develops both the micropore and mesopore systems, in the GM sample to a larger extent than in the G-modified carbon, where only a slight increase in the slope of the plateau is observed with respect to the original activated carbon isotherm. Comparing the results of the modification procedure proposed in this study with the reactivated carbon
0.0
0.2
0.4
Vmcso
0.6
Vln~cro
0.8
1.0
P/P0 15
s
:
I
I GM
10
CR
E”
E. E
0
5 A(GM-0)
_++----------
A(R-0)
0 0.0
0.5
1.0
1.5
2.0
2.5
Fig. 2. (a) Adsorption isotherms of NJ77 K on activated carbons obtained from D55/2. (b) c(, plots for the original and modified carbons, and difference between the plots of each modified carbon and the original.
R. TORREGROSA-MACI.~ et al.
450
(R), this differs only from GM but no major differences from G are observed. The c[, plots, in Fig. 2(b), indicate that the differences between the original and the modified carbons are found in the amount adsorbed in the range of PIP o z 0.04-0.24 and, to a lesser extent, in the slopes of the straight line at higher P/P'.The difference between the amount adsorbed by the modified carbons and the original was larger for the GM modified carbon than for the other samples in the range of showing the pore range where the P/PO% 0.04-0.24, activation of carbon deposit from glucosamine modifies the pore structure of the original carbon. From the micropore volumes calculated from CO, adsorption at 273 K (Table 1) it can be deduced that both carbonized and impregnated carbons have the micropore system blocked in a similar way, and the volume of micropores measured by CO, is practically the same as obtained from N, adsorption at 77 K, showing that the microporosity accessible to both adsorbates is in a range of narrow sizes. Furthermore, the residual external surface area (from a!s plots) after carbonization is practically the same for G- and GM-impregnated carbonized carbons. The differences in slope in the c(, plots show that the meso- and macroporosity were changed during the treatment, as is confirmed by mercury porosimetry (Table 1 ), the major changes occurring in the macroporosity; thus, in order to analyze this pore range, the mercury porosimetry data are presented in Fig. 3. The range of pore diameters covered by this technique (from 10 000 up to 4 nm), shows that the mesoporosity is nil not only for the original carbon but also for the modified activated carbons obtained from it. In this kind of porous system, with a poorly developed mesoporosity, the impregnating solute must be deposited mainly on the surface of the macropores in the carbon particles. Therefore, the subsequent carbonization process would result in a blocking of the entrance to the pore system resulting 1lower adsorption capacity than that of the
original activated carbon. When the carbon deposit, originating from the pyrolysis of the impregnating G or GM, reacts with the activating agent, the micropore system becomes again accessible to the adsorptive, and the adsorption capacity of the modified activated carbon will depend on the reactivity of the carbon deposit - more than on that of the original carbon ~ as a function of the kind and number of active surface groups created by the carbonization of the impregnating agent. In Fig. 4 representative SEM micrographs of original (0), G- and GM-modified, and activated D55/2 carbons are compared. These micrographs show the erosion of the smooth parts of D55/2 produced by the G and GM treatment which is more marked for GM-modified carbon. Therefore, it appears that the glucosamine carbon deposit has on its surface such species of nitrogen-containing groups (depending on the nature of the oxygen-containing groups initially present on the surface [21]) that make this carbon
dp (nm) Fig. 3. Cumulative pore volume versus pore diameter for modified activated carbons from D55/2 (mercury porosimetry).
Fig. 4. SEM micrographs
of D55/2 and modified and GM.
carbons
G
Porous
texture of activated
carbons
more reactive towards CO2 than the glucose carbon deposit. In the case of G- and GM-modified D55/2, with 14% and 25 wt% weight loss, respectively (Table l), CO, activation results in an increase of the macroporosity of both G and GM, the microporosity being only moderately increased in the GM modified carbon. Contrarily, in the reactivated carbon (R), the mesoporosity is zero and the macroporosity is reduced to 58% of that in the original activated carbon, showing that the external burn-off predominates in this case, i.e. the ablation of the particle surface by a pore widening mechanism. In the case of the activated carbon D55/2, the impregnation seems to result in favouring the pore creation by gasification wherever the impregnating agent was deposited, and, in consequence, the more reactive the carbon deposit, the greater the difference in porosity with respect to the original activated carbon. This is confirmed (Table 1) for the GM-impregnated activated carbon by the values, in cm3 gg’, of V,( DR) for both adsorbates, CO, (0.30) and N, (0.40) and the V,,,,, (0.47) in contrast to the G-impregnated activated carbon for which the V,(DR) of both adsorbates, CO* (0.25) and N, (0.30), and the V,,,,, (0.38) are lower. The differences in reactivity between the two carbon deposits of G and GM are in agreement with the results obtained by Mang et al. [ 151 who found the reactivity in CO, of carbonized mixtures of glucose and glucosamine, at temperatures higher than 843 K, exceeded that of carbonized glucose alone. This effect is probably due to the different nature of the surface groups originating in the pyrolysis of both carbon compounds and, furthermore, the greater the surface covered by the impregnating agent, the higher a reactivity should be expected. 3.2 RX1 Extra In this carbon, with a well-developed pore system in all pore ranges (Table 1 ), the impregnation results in a highly developed micro-, meso- and macroporosity, although the volume of the narrow micropores (measured with CO, adsorption at 273 K) is not decreased. A comparison with the development of the porosity in the reactivated carbon (R) shows that the impregnated and activated carbons have higher macroposity but similar micro- and mesopore volumes (Table 1). Figure 5 shows the GI,plots of N, at 77 K for the corresponding original and modified carbons. In this case, a higher meso and macropore volume than in D55/2 (Table 1) allows the glucose and glucosamine to be deposited in the interior of the particles rather than on its surface. As a consequence, the meso- and macroporosity in the original carbon is protected by the carbon deposit against widening by the CO2 reaction; more micropores are created by gasification in the G and GM carbons. As a consequence of this, the activating agent cannot penetrate efficiently to the inner, narrower micropores, so that the volume
modified
with carbohydrates
451
40 s :
30
ki & c
20 10 0 0.0
0.5
1.0
1.5
2.0
as Fig. 5. CI, plots for the original and modified carbons from RX1 Extra, and difference between the plots of each modified carbon and the original.
of this pore range is maintained approximately the same than in the reactivated carbon (R) in both cases - the values for the GM carbon are larger because of the higher weight loss. These differences can also be seen using SEM (micrographs are not given) which show the erosion produced by the two impregnating agents on the external surface of the carbon particles. In Fig. 6, the evolution of meso- and macropore volumes determined by mercury porosimetry shows a great development of both pore ranges in the two impregnated activated carbons with respect to the reactivated carbon (R), leading to the assumption that the high reactivity originated by the GM- and G-carbon deposits (62% and 52% respectively, in Table 1) is not caused by widening of the original pores but by creation of new pores by gasification on the bulk surface where the impregnating agent was deposited. In this way, the faster the pore deepening, the greater the enlargement of the pores by conecting all the adjacent pores.
3.0 6
2.0
-0 -___. _-, r2
-
g
-__. GM
-*\
P 2
10”
10’
lo*
10'
IO4
dp Mm) Fig. 6. Cumulative modified activated
pore volume versus pore diameter for carbons from RX1 Extra (mercury porosimetry).
452
R. TORREGROSA-MACI.=~ et al.
3.3 Carbon Y The pore size distribution of this carbon differs from that of the two preceding carbons, the main difference being in the mesopore range (Table 1). According to Table 1 and the N, adsorption isotherms at 77 K (not given here) for this carbon and the modified carbons obtained from it, the micropore size distribution is wider than in the original sample for every additional treatment (G, GM and R). This effect could be explained by more internal deposition of the glucose and glucosamine allowed by the mesoporosity of the original carbon; the reactivity of both carbon deposits is higher than with the preceding activated carbons, reflecting a larger surface covered by the carbon deposits (80% burn-off for G and 77% burn-off for GM, in Table 1). Comparing the differences in micropore volume developed by G and GM with respect to the original activated carbon, in Table 1, and the cumulative pore volumes for both in Fig. 7, it is seen that the differences in reactivity between them result in different pore ranges being increased in each case. The reaction with COZ with the glucose carbon deposit generates a volume increase in the macropore range more than in the meso- or micropore range, as compared to the reactivated carbon (R). In contrast, the reaction of the glucosamine carbon deposit develops mainly the micropore range, the meso- and macropore ranges being developed in the same way as in the reactivated carbon (R). Taking into account that the two meso- and macropore volumes are almost equal (0.20 and 0.27 cm3 g-i, respectively) in Carbon Y, this can show the different extent of the permeation of both impregnating agents whilst in the other activated carbons studied this was not the case because the macropore volumes were ostensibly greater than those of the mesopores. Presumably, the mesopores are more accessible to glucose. If this was the case, the elimination of the carbon gasification products that retard the gasification (i.e. CO [22]) by diffusion into the 1.5 -0 1.2
.
0.9
.
___-. R --. G -.. GM
dp Mm) Fig. 7. Cumulative modified activated
pore volume versus carbons from Carbon simetry).
pore diameter for Y (mercury poro-
CO, stream will be the less effective the narrower the pores are where the reaction takes place. This could be the case with the GM carbon, with a lower burnoff than that of the G carbon, where the development of porosity by gasification of organic deposits seems to be slower than in the case of the G carbon. 4. CONCLUSIONS
(1) The addition
of carbohydrates produces an increase in the macropore volume of activated carbons with no appreciable reduction in other ranges of pore size. (2) The degree of development of meso- and macroporosity depends on the meso- and macropore characteristics of the original activated carbon, the higher the surface area of the larger pores, the more important is its development by impregnation with carbohydrates. containing nitrogen functional (3) The additives groups produced a higher enhancement of porosity together with the highest total burn-off percentage. REFERENCES 1. Smisek, M. and Cerny, S., Active Carbons. Elsevier, Amsterdam, 1970. 2. Bansal, D. C., Donnet, J. B. and Stoeckli, H. F., Active Carbon. Marcel Dekker, New York, 1988. J. M., Rodriguez-Reinoso, F. and 3. Martin-Martinez, Vannice, M. A., Appl. Catal., 1989, 51, 93. J. M. and Vannice, M. A., Ind. Eng. 4. Martin-Martinez, Chem. Res., 1991, 30, 2263. J. M.. Fuel. 5. Sell&-Perez. M. J. and Martin-Martinez. 1991, 70, 877. M. C. and Martin-Martinez, 6. Mittelmeijer-Hazeleger, J. M., Carbon, 1992, 30, 695. L., Kapteijn, F., Moulijn, J. A., Martin7. Singoredjo, Martinez, J. M. and Boehm, H. P., Carbon, 1993, 31, 213. J. M., Singoredjo, L., Mittelmeijer8. Martin-Martinez, Hazeleger, M. C., Kapteijn, F. and Moulijn, J. A., Carbon, 1994, 32, 897. M., Martin-Martinez, J. M. and 9. Perez-Candela, Torregrosa-Macia, R., Water Research, 1995, 29, 2174. F., Martin-Martinez, J. M., 10. Rodriguez-Reinoso, Molina-Sabio, M., Perez-Lledo, I. and Prado-Burguete, C., Carbon, 1985, 23, 19. 11. Jtintgen, H., Fuel, 1986, 65, 1436. 12. Richter, E., Catalvsis Today, 1990, 7. 93 H. and Lesclaux, R., Chem. Phys. Lefrers, 13. Kurasawa, 1979, 66, 602. 14. Miura, K. and Hayashi, J., Carbon, 1991, 29, 653. 15. Mang, D., Boehm, H. P., Stanczyk, K. and Marsh, H., Carbon, 1993, 30, 391. J. and Hashimoto, K., Carbon, 16. Miura, K., Hayashi, 1992, 30, 946. M. M., Progress in Surface and Membrane 17. Dubinin, Science, Vol. 9, ed. E. Matijevich. Academic Press, London, 1975, p. 1. 18. Carrott, P. J. M. and Sing, K. S. W., Characterization of Porous Solids, ed. K. K. Unger, J. Rouquerol, K. S. W. Sing and H. Kral. Elsevier, Amsterdam, 1988, p. 77. J. M., .I. Chem. 19. Sell&s-Perez, M. J. and Martin-Martinez, Sot., Faraday Trans. I, 1991, 87, 1237.
Porous texture of activated carbons modified with carbohydrates 20. Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pieritti, R. A., Rouquerol, J. and Siemieniewska, T., Reporting physisorption data for gas/solid systems, Pure and Appl. Chem., 1985, 57, 603.
21. Puri, R. B., Chemistry and Physics of Carbon, Vol. 6,
453
ed. P. L. Walker Jr. Marcel Dekker, New York, 1970, p. 257. 22. Ergun, S. and Mentser, M., Chemistry and Physics of Carbon, Vol. 1, ed. P. L. Walker Jr. Marcel Dekker, New York, 1965, p. 257.
Pergamon
Printed in Great Britain. All rights reserved 0008-6223/97$17.00+ 0.00 PIk 80008-6223(96)00170-4
POROUS TEXTURE OF ACTIVATED CARBONS MODIFIED WITH CARBOHYDRATES R. TORREGROSA-MAC&’
J. M. MART~N-MART~NEZ~‘*
and
M. C. MITTELMEIJER-HAZELEGER~ “Departamento de Quimica InorgAnica, Universidad de Alicante, 03080, Alicante, Spain bDepartment of Chemical Engineering, University of Amsterdam, 1018WV Amsterdam, The Netherlands (Received 23 May 1996; accepted in revisedform
11 September
1996)
Abstract-A modification of the porous texture of activated carbons to develop meso- and macroporosity is described. Three gas-activated carbons of different origin, have been modified by impregnation with two carbon compounds (n-glucose and o-glucosamine) from aqueous solution followed by carbonization at 1173 K and activation with CO,. The most important results of this procedure are (1) the development of meso- and macroporosity depends on the mesopore size distribution of the original activated carbon; (2) the gasification rates are different for each impregnated carbon in the activation step, and (3) there was no or little development of the original microporosity. 0 1997 Elsevier Science Ltd Key Words-A.
Activated carbon, B. impregnation, activation, C. adsorption, D. microporosity, porosity.
1. INTRODUCTION
D-glucose and D-glucosamine have been shown to be effective in the selective catalytic reduction of NO, with NH, [7,8]. In this study, two carbohydrates (D-glucose and D-glucosamine) were used. Thus, the activated carbons (RX1 Extra, D55/2 and Carbon Y) were impregnated with the two carbon compounds from aqueous solution and then submitted to carbonization (N,/1173 K - 2 hours) and physical activation (CO,/1 123 K - 2 hours). One of the most important features to take into account for an impregnation treatment is the characteristics of the raw material (i.e. the pore system of the activated carbon to be treated, and the relative affinity between the carbon and the solvent); depending on these characteristics the solute will be carried to different pore ranges. In fact, the development of the meso- and macroporosity will be more affected, the more accessible the mesopores are to the impregnating carbon compound, because the impregnating agent will produce a carbon deposit, during pyrolysis, on the surface of these pores.
Activated carbons can be very useful in some industrial applications where well-developed meso- and macropore systems and a high purity of the adsorbents is highly desirable, i.e. for adsorption from solution or for catalyst supports [l-13]. Usually, the most appropriate relatively wide porous system for a given industrial application is better obtained by chemical activation, which, however, leaves contaminating agents in the activated carbon, than by physical or gas activation, where the development of an important volume of meso- and macropores requires high burn-offs, resulting in poor mechanical properties of the activated carbon. To avoid such drawbacks, it is proposed in this paper to modify the porous system in activated carbons, produced by “clean” preparation procedures (i.e. by physical activation), by impregnating the carbons with carbon compounds such as carbohydrates or their nitrogencontaining derivatives. Subsequent carbonization and physical activation will result in a modified activated carbon whose porous system, being properly developed for those applications (i.e. wide porosity can be developed), will have no contaminants. To analyze the factors affecting the effectiveness of this kind of treatment in the development of mesothree activated carbons of and macroporosity, different origin and produced by physical activation with different activating agents have been used in this study. According to the literature [1416], the most appropriate organics to modify the porous texture of the activated carbons are hydrocarbons, polymeric derivatives or carbohydrates. Furthermore, the impregnation of activated carbons with
2. EXPERIMENTAL The activated carbons selected for this study were: (1) RX1 Extra (NORIT, Amersfort, Netherlands), a steam-activated carbon prepared from peat with narrow pores; (2) D55/2 (Bergbau Forschung, Essen, Germany), a steam-activated carbon prepared from a subbituminous coal with a well-developed porous texture (i.e. pores in all ranges are present); and (3) Carbon Y, an activated carbon prepared in the laboratory from plum stones having a well-developed porous texture. These carbons were selected considering that different precursor materials (i.e. peat, subbituminous coal), activated by the same method,
*Corresponding author. 441
R. TORREGROSA-MACI~et al.
448
result in different porous systems, and different raw materials (i.e. peat, plum stone), activated by different agents (i.e. COZ, steam), exhibit a similar porous texture. Figure 1 (a) shows schematically the experimental procedures followed in the modification of the porous texture of the selected carbons by impregnating the original activated carbons with glucose (G) or glucosamine (GM). For comparison, the original activated carbons were also re-carbonized, without impregnation, under similar experimental conditions and re-activated during different periods of time, to obtain the desired degree of burn-off, leading to the corresponding reactivated carbons (R) (Fig. 1 (b)). The activated carbons, dried in air at 383 K for 12 hours, were impregnated with 35 wt% aqueous solutions of D-glUCOSt? and D-glucosamine, respectively, a carbon:solution ratio of 1:lO in volume being used. The mixture of carbon and solution were dried at 383 K (12 hours) and carbonized in a fixed-bed oven in Nz (120 mL min-‘) at different temperatures and soaking periods, progressively increasing from 623 K/30 minutes, 773 K/30 minutes and, finally, at 1173 K/2 hours. Except for carbons obtained from D55/2, the activation of the other impregnated carbons was performed in situ at the end of the carbonization period, by changing the N, stream for CO,
Ia)
(120 mL min-‘), during 24 hours, to obtain the corresponding modified activated carbon. The impregnation plus carbonization process was followed with D55/2 taking samples of both carbonized impregnated carbons, G(c) with glucose and GM(c) with glucosamine, in order to evaluate the porous texture parameters obtained before activation. In each case, the porous texture of the original and the corresponding modified activated carbons was determined from adsorption isotherms of N, at 77 K and CO, at 273 K, measured by use of a Sorptomatic 1800 Carlo Erba instrument and by mercury porosimetry with a Carlo Erba Mercury Porosimeter 4000. Furthermore, SEM photographs of the external surface of the carbon particles, goldcoated, were taken in a Jeol JSM 840 scanning electron microscope.
3. RESULTS AND DISCUSSION
The porous texture parameters for all carbons, obtained from the experimental data by applying the Dubinin-Radushkevich equation [ 171, cc,-method [ 181 by using the N, adsorption standard isotherm published elsewhere [19], and from mercury poroare summarized in Table 1 which also simetry,
0) drying impregnatton Mth addithn do%!
1
383K/ Ith
36vA%inH2C 1 383Kl17.h 1
f
I
623K/ 30m 77310 3om
carbonization in N 2
1 1173KI
2h
I
1
activation
62310 3Omin
carbonization inN 2
Pctivation in CO2
in CO2
1173Kl variable
time
1
773Kl3Omin 1
I
1173W 24h
Fig. 1. Modification of the porous texture of activated Preparative
carbons. (a) Preparative scheme for the re-activated
scheme of the modified carbons.
activated
carbons.
(b)
Porous texture of activated carbons modified with carbohydrates
449
Table 1. Textural properties of activated carbons. Volumes (V) in cm3 g-r and external surfaces (S,) in mz g-’ N, at 77 K a, method
Mercury porosimetry
Weight loss (%)
V, DR
V0
&
CO, at 273 K V, DR
_ -5 14 -3 25 25
0.30 0.20 0.30 0.21 0.40 0.31
0.28 0.20 0.28 0.20 0.39 0.29
22 15 17 14 34 40
0.26 0.21 0.25 0.22 0.30 0.35
0.03 _ 0.05 _ 0.04 0.00
0.36 _ 0.38 _ 0.47 0.21
RX1 Extra 0 G GM R
54 62 35
0.66 0.71 0.76 0.73
0.63 0.86 0.86 0.79
137 252 372 148
0.51 0.37 0.36 0.36
0.14 0.45 0.52 0.26
0.53 1.27 1.55 0.60
Carbon Y 0 G GM R
80 77 38
0.65 0.62 0.83 0.64
0.64 0.80 1.11 1.03
62 180 101 97
0.38 0.59 0.50 0.48
0.20 0.45 0.49 0.43
0.27 0.53 0.35 0.39
Carbon D55/2 0 G(c) G GM(c) GM R
includes the global weight loss percentage with respect to the original activated carbon. At first sight, the differences in weight loss percentage between glucose- and glucosamine-impregnated activated carbons from RX1 Extra and D55/2 could be supposed to be normal (i.e. a significant weight loss) and the behaviour of Carbon Y to be the exception (i.e. a small weight loss). However, according to the volumes of meso- and macropores in the original activated carbons, it can be suspected that the meso- and macroporosity of these might be important in determining the reactivity towards COZ of the carbonized-impregnated carbons, since the major differences between the original and the modified porous system do not occur in the micropore but in the meso- and macropore ranges. For this reason, the modification treatment will be analyzed separately for every activated carbon. 3. I Carbon 05512 The Nz adsorption isotherms at 77 K of the original activated carbon (0) and the various treated carbons are shown in Fig. 2(a). All isotherms correspond to the type I of the IUPAC classification [20] exhibiting a knee at low pressures, typical for relatively narrow micropores. The impregnation plus carbonization process (samples G, and GM,) results in a partial blocking in the entrance of the micropores for the adsorption of N2 at 77 K, but the subsequent activation in CO, develops both the micropore and mesopore systems, in the GM sample to a larger extent than in the G-modified carbon, where only a slight increase in the slope of the plateau is observed with respect to the original activated carbon isotherm. Comparing the results of the modification procedure proposed in this study with the reactivated carbon
0.0
0.2
0.4
Vmcso
0.6
Vln~cro
0.8
1.0
P/P0 15
s
:
I
I GM
10
CR
E”
E. E
0
5 A(GM-0)
_++----------
A(R-0)
0 0.0
0.5
1.0
1.5
2.0
2.5
Fig. 2. (a) Adsorption isotherms of NJ77 K on activated carbons obtained from D55/2. (b) c(, plots for the original and modified carbons, and difference between the plots of each modified carbon and the original.
R. TORREGROSA-MACI.~ et al.
450
(R), this differs only from GM but no major differences from G are observed. The c[, plots, in Fig. 2(b), indicate that the differences between the original and the modified carbons are found in the amount adsorbed in the range of PIP o z 0.04-0.24 and, to a lesser extent, in the slopes of the straight line at higher P/P'.The difference between the amount adsorbed by the modified carbons and the original was larger for the GM modified carbon than for the other samples in the range of showing the pore range where the P/PO% 0.04-0.24, activation of carbon deposit from glucosamine modifies the pore structure of the original carbon. From the micropore volumes calculated from CO, adsorption at 273 K (Table 1) it can be deduced that both carbonized and impregnated carbons have the micropore system blocked in a similar way, and the volume of micropores measured by CO, is practically the same as obtained from N, adsorption at 77 K, showing that the microporosity accessible to both adsorbates is in a range of narrow sizes. Furthermore, the residual external surface area (from a!s plots) after carbonization is practically the same for G- and GM-impregnated carbonized carbons. The differences in slope in the c(, plots show that the meso- and macroporosity were changed during the treatment, as is confirmed by mercury porosimetry (Table 1 ), the major changes occurring in the macroporosity; thus, in order to analyze this pore range, the mercury porosimetry data are presented in Fig. 3. The range of pore diameters covered by this technique (from 10 000 up to 4 nm), shows that the mesoporosity is nil not only for the original carbon but also for the modified activated carbons obtained from it. In this kind of porous system, with a poorly developed mesoporosity, the impregnating solute must be deposited mainly on the surface of the macropores in the carbon particles. Therefore, the subsequent carbonization process would result in a blocking of the entrance to the pore system resulting 1lower adsorption capacity than that of the
original activated carbon. When the carbon deposit, originating from the pyrolysis of the impregnating G or GM, reacts with the activating agent, the micropore system becomes again accessible to the adsorptive, and the adsorption capacity of the modified activated carbon will depend on the reactivity of the carbon deposit - more than on that of the original carbon ~ as a function of the kind and number of active surface groups created by the carbonization of the impregnating agent. In Fig. 4 representative SEM micrographs of original (0), G- and GM-modified, and activated D55/2 carbons are compared. These micrographs show the erosion of the smooth parts of D55/2 produced by the G and GM treatment which is more marked for GM-modified carbon. Therefore, it appears that the glucosamine carbon deposit has on its surface such species of nitrogen-containing groups (depending on the nature of the oxygen-containing groups initially present on the surface [21]) that make this carbon
dp (nm) Fig. 3. Cumulative pore volume versus pore diameter for modified activated carbons from D55/2 (mercury porosimetry).
Fig. 4. SEM micrographs
of D55/2 and modified and GM.
carbons
G
Porous
texture of activated
carbons
more reactive towards CO2 than the glucose carbon deposit. In the case of G- and GM-modified D55/2, with 14% and 25 wt% weight loss, respectively (Table l), CO, activation results in an increase of the macroporosity of both G and GM, the microporosity being only moderately increased in the GM modified carbon. Contrarily, in the reactivated carbon (R), the mesoporosity is zero and the macroporosity is reduced to 58% of that in the original activated carbon, showing that the external burn-off predominates in this case, i.e. the ablation of the particle surface by a pore widening mechanism. In the case of the activated carbon D55/2, the impregnation seems to result in favouring the pore creation by gasification wherever the impregnating agent was deposited, and, in consequence, the more reactive the carbon deposit, the greater the difference in porosity with respect to the original activated carbon. This is confirmed (Table 1) for the GM-impregnated activated carbon by the values, in cm3 gg’, of V,( DR) for both adsorbates, CO, (0.30) and N, (0.40) and the V,,,,, (0.47) in contrast to the G-impregnated activated carbon for which the V,(DR) of both adsorbates, CO* (0.25) and N, (0.30), and the V,,,,, (0.38) are lower. The differences in reactivity between the two carbon deposits of G and GM are in agreement with the results obtained by Mang et al. [ 151 who found the reactivity in CO, of carbonized mixtures of glucose and glucosamine, at temperatures higher than 843 K, exceeded that of carbonized glucose alone. This effect is probably due to the different nature of the surface groups originating in the pyrolysis of both carbon compounds and, furthermore, the greater the surface covered by the impregnating agent, the higher a reactivity should be expected. 3.2 RX1 Extra In this carbon, with a well-developed pore system in all pore ranges (Table 1 ), the impregnation results in a highly developed micro-, meso- and macroporosity, although the volume of the narrow micropores (measured with CO, adsorption at 273 K) is not decreased. A comparison with the development of the porosity in the reactivated carbon (R) shows that the impregnated and activated carbons have higher macroposity but similar micro- and mesopore volumes (Table 1). Figure 5 shows the GI,plots of N, at 77 K for the corresponding original and modified carbons. In this case, a higher meso and macropore volume than in D55/2 (Table 1) allows the glucose and glucosamine to be deposited in the interior of the particles rather than on its surface. As a consequence, the meso- and macroporosity in the original carbon is protected by the carbon deposit against widening by the CO2 reaction; more micropores are created by gasification in the G and GM carbons. As a consequence of this, the activating agent cannot penetrate efficiently to the inner, narrower micropores, so that the volume
modified
with carbohydrates
451
40 s :
30
ki & c
20 10 0 0.0
0.5
1.0
1.5
2.0
as Fig. 5. CI, plots for the original and modified carbons from RX1 Extra, and difference between the plots of each modified carbon and the original.
of this pore range is maintained approximately the same than in the reactivated carbon (R) in both cases - the values for the GM carbon are larger because of the higher weight loss. These differences can also be seen using SEM (micrographs are not given) which show the erosion produced by the two impregnating agents on the external surface of the carbon particles. In Fig. 6, the evolution of meso- and macropore volumes determined by mercury porosimetry shows a great development of both pore ranges in the two impregnated activated carbons with respect to the reactivated carbon (R), leading to the assumption that the high reactivity originated by the GM- and G-carbon deposits (62% and 52% respectively, in Table 1) is not caused by widening of the original pores but by creation of new pores by gasification on the bulk surface where the impregnating agent was deposited. In this way, the faster the pore deepening, the greater the enlargement of the pores by conecting all the adjacent pores.
3.0 6
2.0
-0 -___. _-, r2
-
g
-__. GM
-*\
P 2
10”
10’
lo*
10'
IO4
dp Mm) Fig. 6. Cumulative modified activated
pore volume versus pore diameter for carbons from RX1 Extra (mercury porosimetry).
452
R. TORREGROSA-MACI.=~ et al.
3.3 Carbon Y The pore size distribution of this carbon differs from that of the two preceding carbons, the main difference being in the mesopore range (Table 1). According to Table 1 and the N, adsorption isotherms at 77 K (not given here) for this carbon and the modified carbons obtained from it, the micropore size distribution is wider than in the original sample for every additional treatment (G, GM and R). This effect could be explained by more internal deposition of the glucose and glucosamine allowed by the mesoporosity of the original carbon; the reactivity of both carbon deposits is higher than with the preceding activated carbons, reflecting a larger surface covered by the carbon deposits (80% burn-off for G and 77% burn-off for GM, in Table 1). Comparing the differences in micropore volume developed by G and GM with respect to the original activated carbon, in Table 1, and the cumulative pore volumes for both in Fig. 7, it is seen that the differences in reactivity between them result in different pore ranges being increased in each case. The reaction with COZ with the glucose carbon deposit generates a volume increase in the macropore range more than in the meso- or micropore range, as compared to the reactivated carbon (R). In contrast, the reaction of the glucosamine carbon deposit develops mainly the micropore range, the meso- and macropore ranges being developed in the same way as in the reactivated carbon (R). Taking into account that the two meso- and macropore volumes are almost equal (0.20 and 0.27 cm3 g-i, respectively) in Carbon Y, this can show the different extent of the permeation of both impregnating agents whilst in the other activated carbons studied this was not the case because the macropore volumes were ostensibly greater than those of the mesopores. Presumably, the mesopores are more accessible to glucose. If this was the case, the elimination of the carbon gasification products that retard the gasification (i.e. CO [22]) by diffusion into the 1.5 -0 1.2
.
0.9
.
___-. R --. G -.. GM
dp Mm) Fig. 7. Cumulative modified activated
pore volume versus carbons from Carbon simetry).
pore diameter for Y (mercury poro-
CO, stream will be the less effective the narrower the pores are where the reaction takes place. This could be the case with the GM carbon, with a lower burnoff than that of the G carbon, where the development of porosity by gasification of organic deposits seems to be slower than in the case of the G carbon. 4. CONCLUSIONS
(1) The addition
of carbohydrates produces an increase in the macropore volume of activated carbons with no appreciable reduction in other ranges of pore size. (2) The degree of development of meso- and macroporosity depends on the meso- and macropore characteristics of the original activated carbon, the higher the surface area of the larger pores, the more important is its development by impregnation with carbohydrates. containing nitrogen functional (3) The additives groups produced a higher enhancement of porosity together with the highest total burn-off percentage. REFERENCES 1. Smisek, M. and Cerny, S., Active Carbons. Elsevier, Amsterdam, 1970. 2. Bansal, D. C., Donnet, J. B. and Stoeckli, H. F., Active Carbon. Marcel Dekker, New York, 1988. J. M., Rodriguez-Reinoso, F. and 3. Martin-Martinez, Vannice, M. A., Appl. Catal., 1989, 51, 93. J. M. and Vannice, M. A., Ind. Eng. 4. Martin-Martinez, Chem. Res., 1991, 30, 2263. J. M.. Fuel. 5. Sell&-Perez. M. J. and Martin-Martinez. 1991, 70, 877. M. C. and Martin-Martinez, 6. Mittelmeijer-Hazeleger, J. M., Carbon, 1992, 30, 695. L., Kapteijn, F., Moulijn, J. A., Martin7. Singoredjo, Martinez, J. M. and Boehm, H. P., Carbon, 1993, 31, 213. J. M., Singoredjo, L., Mittelmeijer8. Martin-Martinez, Hazeleger, M. C., Kapteijn, F. and Moulijn, J. A., Carbon, 1994, 32, 897. M., Martin-Martinez, J. M. and 9. Perez-Candela, Torregrosa-Macia, R., Water Research, 1995, 29, 2174. F., Martin-Martinez, J. M., 10. Rodriguez-Reinoso, Molina-Sabio, M., Perez-Lledo, I. and Prado-Burguete, C., Carbon, 1985, 23, 19. 11. Jtintgen, H., Fuel, 1986, 65, 1436. 12. Richter, E., Catalvsis Today, 1990, 7. 93 H. and Lesclaux, R., Chem. Phys. Lefrers, 13. Kurasawa, 1979, 66, 602. 14. Miura, K. and Hayashi, J., Carbon, 1991, 29, 653. 15. Mang, D., Boehm, H. P., Stanczyk, K. and Marsh, H., Carbon, 1993, 30, 391. J. and Hashimoto, K., Carbon, 16. Miura, K., Hayashi, 1992, 30, 946. M. M., Progress in Surface and Membrane 17. Dubinin, Science, Vol. 9, ed. E. Matijevich. Academic Press, London, 1975, p. 1. 18. Carrott, P. J. M. and Sing, K. S. W., Characterization of Porous Solids, ed. K. K. Unger, J. Rouquerol, K. S. W. Sing and H. Kral. Elsevier, Amsterdam, 1988, p. 77. J. M., .I. Chem. 19. Sell&s-Perez, M. J. and Martin-Martinez, Sot., Faraday Trans. I, 1991, 87, 1237.
Porous texture of activated carbons modified with carbohydrates 20. Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pieritti, R. A., Rouquerol, J. and Siemieniewska, T., Reporting physisorption data for gas/solid systems, Pure and Appl. Chem., 1985, 57, 603.
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ed. P. L. Walker Jr. Marcel Dekker, New York, 1970, p. 257. 22. Ergun, S. and Mentser, M., Chemistry and Physics of Carbon, Vol. 1, ed. P. L. Walker Jr. Marcel Dekker, New York, 1965, p. 257.