Inter- and Intra-Primary-Particle Structure of Monolithic Carbon

Feb 8, 2008 - To carry out this study, various techniques were used, including high-resolution transmission and scanning electron microscopy, mercury ...
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Inter- and Intra-Primary-Particle Structure of Monolithic Carbon Aerogels Obtained with Varying Solvents D. Faire´n-Jime´nez, F. Carrasco-Marı´n, and C. Moreno-Castilla* Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias, UniVersidad de Granada, 18071 Granada, Spain ReceiVed October 30, 2007. In Final Form: December 18, 2007 Different carbon aerogels were obtained by carbonization of organic aerogels prepared from the polymerization of resorcinol and formaldehyde using potassium carbonate as catalyst. Various solvents were added to the initial mixture to study their effects on the inter- and intra-primary-particle structure of the carbon aerogels. To carry out this study, various techniques were used, including high-resolution transmission and scanning electron microscopy, mercury porosimetry, mechanical tests, N2 and CO2 adsorption at -196 and 0 °C, respectively, and immersion calorimetry into benzene. Variation of the solvent used produced changes in the gelation time of the organic aerogels, which gave rise to variations in the inter- and intra-primary-particle structure of the carbon aerogels obtained. The monolith density of the carbon aerogels ranged from 0.37 to 0.87 g/cm3. Samples with a density higher than 0.61 g/cm3 had micropores and mesopores but no macropores. Macro- and mesoporosity had a monomodal pore size distribution. The elastic modulus showed a scaling relationship with density. In all samples studied, which had a mean micropore width of 0.62-1.06 nm, the surface area obtained by enthalpy of immersion into benzene yielded a realistic value of their total surface area.

Introduction Monolithic carbon aerogels (MCAs) are porous carbon materials obtained by carbonization of organic aerogels prepared by the sol-gel process from certain organic monomers such as resorcinol and formaldehyde following Pekala’s method.1,2 Carbon gels have a network structure of interconnected nanosized primary particles. With regard to their pore texture, micropores are developed in the intra-primary-particle structure, whereas meso- and macropores are produced in the inter-primary-particle structure initially occupied by the solvent. Because the structure and texture of carbon aerogels can be designed and controlled on the nanometer scale, they were recently classified as nanostructured carbons.3 It is therefore possible to control the concentration of micropores and mesopores independently, which is one of the advantages of carbon gels as porous carbons, and makes them promising materials in adsorption,4-6 catalysis,7 and the field of energy storage.8,9 The pore texture of a carbon gel depends on the nature of the original ingredients (reactants, solvent, and catalyst used to prepare the organic aerogel), the curing and drying methods, and the carbonization conditions.10 We previously investigated11 the effect of different alkaline carbonates and acid catalysts, e.g., oxalic acid and p-toluene* To whom correspondence should be addressed. Fax: +34-958248526. E-mail: [email protected]. (1) Pekala, R. W. U.S. Patent 4,873,218, 1989. (2) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221-27. (3) Inagaki, M.; Kaneko, K.; Nishizawa, T. Carbon 2004, 42, 1401-17. (4) Goel, J.; Kadirvelu, K.; Rajagopal, C.; Garg, V. K. Carbon 2005, 43, 197-200. (5) Sa´nchez-Polo, M.; Rivera-Utrilla, J.; Salhi, E.; von Gunten, U. J. Colloid Interface Sci. 2006, 300, 437-41. (6) Faire´n-Jime´nez, D.; Carrasco-Marı´n, F.; Moreno-Castilla, C. Langmuir 2007, 23, 10095-101. (7) Moreno-Castlla, C.; Maldonado-Ho´dar, F. J. Carbon 2005, 43, 455-65. (8) Li, J.; Wang, X.; Huang, Q.; Gamboa, S.; Sebastian, P. J. Power Sources 2006, 158, 784-88. (9) Liu, N.; Zhang, S.; Fu, R.; Dresselhaus, M. S.; Dresselhaus, G. Carbon 2006, 44, 2430-36. (10) Al-Muhtaseb, S. A.; Ritter, J. A. AdV. Mater. 2003, 15, 101-14. (11) Faire´n-Jime´nez, D.; Carrasco-Marı´n, F.; Moreno-Castilla, C. Carbon 2006, 44, 2301-07.

sulfonic acid, on the surface area and pore texture of the carbon aerogels obtained. The present study describes the effect of the addition of different solvents (methanol, ethanol, acetone, and tetrahydrofurane) to the initial resorcinol-formaldehyde mixture on the intra- and inter-primary-particle structure. The effect of using acetone instead of water as solvent was previously studied.12,13 Results obtained indicated that the structure of the organic aerogels was modified by variations in the gelation process. Likewise, the carbon aerogels obtained from them showed important differences in a broad range of lengths, from a few to hundreds of nanometers. The techniques used in this study included scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), N2 and CO2 adsorption at -196 and 0 °C, respectively, immersion calorimetry into benzene, mercury porosimetry, and mechanical property measurement. Experimental Section Monolithic organic aerogels were prepared by sol-gel polymerization reaction of resorcinol and formaldehyde in water or water containing an organic solvent such as methanol, ethanol, tetrahydrofuran, or acetone. The polymerization catalyst was K2CO3. Molar ratios were resorcinol/formaldehyde ) 0.5 and resorcinol/catalyst ) 800. Mixtures were stirred to obtain a homogeneous solution that was cast into glass molds (45 cm length × 0.5 cm i.d.). The glass molds were sealed, and the mixture was cured.11 Recipes of organic aerogels are given in Table 1, which also shows the gelation time of the mixtures. This gelation time was defined as the time interval between the beginning of the curing cycle and the point when the mixture changes to an opaque solid. After the curing cycle, gel rods were cut into 5 mm pellets and placed in acetone to exchange water and were then supercritically dried with carbon dioxide to form the corresponding monolithic organic aerogels. Pyrolysis was carried out in N2 flow at 100 cm3/ (12) Berthon, S.; Barbieri, O.; Ehrburger-Dolle, F.; Geissler, E.; Achard, P.; Bley, F.; Hecht, A. M.; Livet, F.; Pajonj, G. M.; Pinto, N.; Rigacci, A.; Rochas, C. J. Non-Cryst. Solids 2001, 285, 154-61. (13) Berthon-Fabry, S.; Langohr, D.; Achard, P.; Charrier, D.; Djurado, D.; Ehrburger-Dolle, F. J. Non-Cryst. Solids 2004, 350, 136-44.

10.1021/la703386q CCC: $40.75 © 2008 American Chemical Society Published on Web 02/08/2008

Structure of MCAs Obtained with Varying SolVents

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Table 1. Water and Other Solvent Contents of Organic Aerogel Prepared with Resorcinol (0.224 mol), Formaldehyde (0.112 mol), and K2CO3 (1.4 × 10-4 mol) as Catalyst and the Gelation Time of the Mixtures sample

water vol, mL

B C D F K L M

15.3 24.5 24.5 11.4 13.1 26.7 24.5

other solvents (vol, mL)

gelation time, h

EtOH (2.3) THF (2.3) EtOH (12.9) + MeOH (2.3) MeOH (2.3)

50 nm, designated macropore volume, V3; monolith density, F, determined at atmospheric pressure. Adsorption isotherms were measured using conventional volumetric equipment made of Pyrex glass and free of mercury and grease, which reached a dynamic vacuum of 10-6 mbar at the sample location. The equilibrium pressure was measured with a Baratron transducer from MKS. Before adsorption measurements, samples were outgassed at 110 °C overnight under high vacuum. N2 adsorption isotherms were analyzed by the BET equation in the relative pressure range between 10-3 and 10-6, where there was a linear plot. In addition, the Dubinin-Radushkevich (DR) equation (1) was applied to N2 and CO2 adsorption isotherms at -196 and W ) W0 exp -

Once the E0 value was known, the mean micropore width, L0, was obtained by applying the Stoeckli equation (2).16 In addition to SBET,

(3)

those obtained with CO2, because the former yield the total volume of micropores and their mean width, respectively, if there are no micropore constrictions at their entrance.15 The enthalpy of immersion into benzene, ∆iH(C6H6), was determined using a model C80-D Setaram calorimeter. The samples were previously outgassed overnight under a dynamic vacuum of 10-6 mbar at 120 °C. Measurements were carried out at least twice for each sample. The enthalpy of immersion into benzene can be converted into a surface area value, following the method proposed by Denoyel et al.18 and used by other authors.19-21 The enthalpy of immersion is assumed to be proportional to the surface area accessible to the wetting liquid, since the benzene molecule has no specific interactions with surface groups. A surface area value was obtained by taking into account the enthalpy of immersion into benzene of a nonporous graphitized carbon black,18 -0.114 J/m2. The mechanical properties of the samples were measured under uniaxial compression using model AGS-J 10kN Shimazdu equipment. Tests were performed at a strain rate of 1 mm/min. All measurements were made at room temperature (around 20 °C) and 40% relative humidity. Specimens were cylinders carefully machined to have perfectly parallel bases and a length/diameter ratio of 1.5. No precautions were taken to prevent moisture adsorption by the carbon aerogels. The compression stress-strain curves obtained were used to determine the elastic modulus, E, and the compressive strength, RF. The elastic modulus corresponds to the slope of the linear region of the curve at low strains. The compressive strength is the maximum stress supported by the specimen during the test, i.e., the stress at which macroscopic failure occurs.

Results and Discussion Inter-Primary-Particle Structure. Polymerization of resorcinol and formaldehyde was dependent on the solvent used. Thus, the gelation time of the samples prepared with water as solvent was less than 24 h, whereas for samples prepared with a mixture of water and an organic solvent the gelation time was around 48 h (Table 1). Therefore, the nature of the solvent affects the polymerization kinetics. Figure 1 depicts SEM microphotographs of samples of different densities, showing them to be composed of rounded primary particles which are interconnected to each other. This leaves well-opened accessible macro- and/or mesopores, as shown in the HRTEM microphotographs in Figure 2. When the interconnectivity among the primary particles increases, the larger pores dissapear and the monolith density increases. The macroand mesopore network constitutes the inter-primary-particle structure and was studied by mercury porosimetry, analyzing intrusion curves by means of the Washburn equation.14 Precaution (16) Stoeckli, F. In Porosity in carbons-characterization and applications; Patrick, J., Ed.; Arnold: London, 1995; pp 67-97. (17) Stoeckli, F.; Centeno, T. A. Carbon 1997, 35, 1097-00. (18) Denoyel, R.; Ferna´ndez-Colinas, J.; Grillet, Y.; Rouquerol, J. Langmuir 1993, 9, 515-8. (19) Gonza´lez, M. T.; Sepu´lveda-Escribano, A.; Molina-Sabio, M.; Rodrı´guezReinoso, F. Langmuir 1995, 11, 2151-5. (20) Silvestre-Albero, J.; Go´mez de Salazar, C.; Sepu´lveda-Escribano, A.; Rodrı´guez-Reinoso, F. Colloids Surf., A 2001, 187, 151-65. (21) Villar-Rodil, S.; Denoyel, R.; Rouquerol, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Carbon 2002, 40, 1376-80.

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Figure 2. HRTEM microphotographs of samples F, M, and C.

Figure 1. SEM microphotographs of samples with the lowest, medium, and highest density, samples F, M, and C, respectively.

must be taken when this technique is applied to highly porous materials, because some of them can be densified under isostatic pressure without any mercury intrusion into their pores.22,23 This has been observed in many inorganic xerogels and some organic aerogels under isostatic pressure, with densification produced by shrinkage of the material due to hierarchical pore collapse. In this case, the Washburn equation cannot be applied to determine the pore size distribution of the solid.23 (22) Brown, S. E.; Lard, E. W. Powder Technol. 1974, 9, 187-90. (23) Job, N.; Pirard, R.; Pirard, J. P.; Alie´, C. Part. Part. Syst. Charact. 2006, 23, 72-81.

Mercury porosimetry curves obtained for the MCAs with the lowest, medium, and highest density (samples F, M, and C, respectively) are shown in Figure 3 as an example. These curves show that there was no volume variation during depressurization. The shape of the intrusion curves may indicate shrinkage of these materials under isostatic pressure. To test this possibility, it is recommended23 to weigh the samples before and after the mercury porosimetry run to determine the volume of mercury remaining within the pore network after depressurization. This volume agreed with the total pore volume, VP, obtained from the intrusion curves (see Table 2). Consequently, the mercury porosimetry data obtained correspond to irreversible intrusion of mercury into the meso- and macropores of the material and not to compression of the material under isostatic pressure. Mercury porosimetry results are compiled in Table 2, which shows the order of samples from lowest to highest density.

Structure of MCAs Obtained with Varying SolVents

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Highly porous materials, e.g., aerogels, show a scaling relationship of the elastic modulus with density25-33 according to eq 4, where A is a pre-exponential factor and n is the scaling

E ) AFn

Figure 3. Intrusion-extrusion curves from mercury porosimetry for samples F (∆), M (0), and C (O). The symbols on the curves are only to indentify the sample: intrusion, closed symbols; extrusion, open symbols.

Samples with low density, F and L, had a large macropore volume. However, samples with densities >0.61 g/cm3 only had mesopores, ranging from 0.67 to 0.96 cm3/g, with no macropores detected. Pore size distributions (PSDs) determined by mercury porosimetry are depicted in Figures 4 and 5. Monomodal PSDs were obtained, which is characteristic of carbon aerogels.24 The mean pore size in the meso- to macropore range varied from 666 nm for sample L to 9 nm for samples D and C. CO2 activation of sample K produced virtually no change in the PSD, only increasing the mesopore volume. Reduction in the amount of water in the original recipe (sample B vs sample L) produced an increase in the monolith density and a decrease in the mean pore size above 3.7 nm width (from 666 nm in sample L to 34 nm in sample B). The replacement of a small amount of water in the original recipe by an organic solvent had the same effect on the density and mean pore width (samples C, D, and M vs sample L and sample K vs sample B). The highest increase in density was obtained with ethanol (sample C) and the lowest with acetone (sample M). With regard to the mean pore size, samples C, D, and M were mesoporous, whereas L was macroporous. Likewise, sample K was mesoporous and sample B macroporous. A lower amount of water in the original recipe is expected to produce an MCA with a higher monolith density and narrower pores, because meso- and macropores are formed in the space occupied by the solvent in the gel. However, the effect of small amounts of an organic solvent on the density is more difficult to explain. The organic solvent may affect the gelation process by increasing the interconnectivity or cross-linking of the polymer clusters that give rise to the primary particles. CO2 activation of sample K produced a decrease in the density due to an increase in the mesopore volume and had practically no effect on the mean pore width. This is because new mesopores are created as a consequence of the micropore widening. Mechanical properties of carbon aerogels were obtained from the compression stress-strain curves. The shape of these curves is typical of brittle materials, and the different parameters obtained from them are compiled in Table 2. Mechanical properties are as important as the pore texture when MCAs are used as adsorbents or catalyst supports in fixed bed reactors, since they must resist the weight of the material packed in the bed and the stress produced by vibration or movements. (24) Maldonado-Ho´dar, F. J.; Ferro-Garcı´a, M. A.; Rivera-Utrilla, J.; MorenoCastilla, C. Carbon 1999, 37, 1199-05.

(4)

exponent. The scaling exponent value found was 1.7 ( 0.2 (see the Supporting Information), lower than the value of 2.7 ( 0.2 reported by Pekala et al.27 for their carbon aerogels. This can be due to the fact that the resorcinol/catalyst ratio used in this work, 800, was higher than that used by Pekala, 50-300, which produced MCAs with higher monolith densities, 0.37-0.87 cm3/ g, in comparison with their finding of 0.05-0.5 cm3/g. The density of sample K was decreased by CO2 activation, thereby decreasing its mechanical properties, because carbon gasification produces an increase in porosity and a decrease in the thickness of the carbon network. Hence, carbon aerogels are stiffer and have a higher compressive strength compared with activated carbon aerogels with similar density. Thus, the density values of K11 and K18 were close to those of M and B, respectively, but their E and RF values were lower. Intra-Primary-Particle Structure. The intra-primary-particle structure is determined by the microporosity of the MCAs. This microporosity is formed during carbonization of the organic aerogels. Thus, the Simm value of the organic aerogels markedly increased after their carbonization, as shown by the Simm(CA)/ Simm(OA) ratio in Table 3. In general, with exception of sample C, the Simm of the organic aerogels, which is a measure of their external surface because they have no micropores, was greater than the Sext of the carbon aerogels (Table 2), which can be attributed to the shrinkage of the organic aerogel during the carbonization step. Values of the micropore volume and mean micropore width obtained with N2 and CO2 at -196 and 0 °C, respectively, are compiled in Table 4, showing the order of samples from narrowest to widest L0(N2) values. The micropore volume obtained from CO2 adsorption at 0 °C yields the volume of narrow micropores (below about 0.7 nm width), while the micropore volume from N2 adsorption at -196 °C yields the total micropore volume if there are no micropore constrictions.15 In the case of samples B, D, and L both micropore volumes are similar, which indicates a homogeneous micropore size distribution with a mean size of 0.6-0.7 nm. Remaining samples show a higher W0(N2) than W0(CO2) value, indicative of a more heterogeneous micropore size distribution. The difference between the two values increased with CO2 activation of sample K due to the widening of the narrower micropores, L0(CO2). The surface area of the samples was obtained by using different approaches, as indicated in the Experimental Section. However, some observations have to be taken into account before its (25) Fricke, J. J. Non-Cryst. Solids 1988, 101, 169-73. (26) Pekala, R. W.; Alviso, C. T.; LeMay, J. D. J. Non-Cryst. Solids 1990, 125, 67-75. (27) Gross, J.; Scherer, G. W.; Alviso, C. T.; Pekala, R. W. J. Non-Cryst. Solids 1997, 211, 132-42. (28) Saliger, R.; Bock, V.; Petricevic, R.; Tillotson, T.; Geis, S.; Fricke, J. J. Non-Cryst. Solids 1997, 211, 144-50. (29) Woignier, T.; Reynes, J.; Hafidi, A. A.; Beurroies, I.; Phalippou, J. J. Non-Cryst. Solids 1998, 241, 45-52. (30) Ma, H. S.; Roberts, A. P.; Prevost, J. H.; Jullien, R.; Scherer, G. W. J. Non-Cryst. Solids 2000, 277, 127-41. (31) Ma, H. S.; Prevost, J. H.; Jullien, R.; Scherer, G. W. J. Non-Cryst. Solids 2001, 285, 216-21. (32) Fischer, F.; Rigacci, A.; Pirard, R.; Berthon-Fabry, S.; Achard, P. Polymer 2006, 47, 7636-45. (33) Gibson, L. J.; Ashby, M. F. Cellular solids, structure and properties; Pergamon: New York, 1988.

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Table 2. Particle Density, Meso- and Macropore Volume, Mean Pore Width and External Surface Area from Mercury Porosimetry, and Mechanical Properties of Monolithic Carbon and Activated Carbon Aerogels sample

F, g/cm3

Vp, cm3/g

V2, cm3/g

V3, cm3/g

d, nm

Sext, m2/g

E, MPa

RF, MPa

F L K18 B K11 M D K C

0.37 0.38 0.61 0.62 0.65 0.68 0.70 0.72 0.87

1.88 (1.81)a 1.67 0.96 0.79 0.93 0.89 (0.87) 0.87 0.81 0.67 (0.64)

0.28 0.00 0.96 0.79 0.93 0.89 0.87 0.81 0.67

1.60 1.67 0.00 0.00 0.00 0.00 0.00 0.00 0.00

60 666 14 34 12 14 9 12 9

129 13 337 116 327 260 80 275 308

213 166 127 569 278 617 812 489 624

5 5 3 18 9 14 17 13 10

a Values in parentheses are the mercury volumes introduced into the pellets and determined by weighing them before and after the mercury porosimetry run.

Table 3. Enthalpies of Immersion into Benzene, Surface Areas of Organic Aerogels, and Carbon (CA) to Organic (OA) Aerogel Surface Area Ratios

Figure 4. Pore size distribution from mercury porosimetry.

Figure 5. Pore size distribution from mercury porosimetry for K-series samples.

discussion. Thus, although the BET model34 is widely used to obtain an apparent surface area, SBET, it should be kept in mind35 that it is not the most appropriate model to describe N2 adsorption at -196 °C on microporous carbons because it describes multilayer adsorption on an open, nonporous surface. On the other hand, with regard to Simm, Stoeckli et al.36 recently reported that the immersion calorimatry method is inadequate to determine surface areas in activated carbons where L0 > 1 nm. This is because the enthalpy of immersion into benzene appears to depend on the Smic and Sext values and the volume of liquid found between the surface layers of the micropores. (34) Gregg, S. J.; Sing, K. S. W. Adsorption surface, area and porosity; Academic Press: New York, 1982. (35) Rodrı´guez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21, p 11. (36) Stoeckli, F.; Centeno, T. A. Carbon 2005, 43, 1184-90.

sample

-∆ιH(C6H6), J/g

Simm, m2/g

Simm(CA)/Simm(OA)

D-OA B-OA L-OA F-OA M-OA C-OA K-OA

-21.3 ( 0.2 -20.9 ( 0.2 -13.9 ( 0.1 -39.7 ( 0.3 -43.5 ( 0.3 -23.2 ( 0.2 -64.9 ( 0.4

187 183 122 348 382 203 569

5.1 5.1 8.3 3.6 2.3 5.0 1.7

Surface area values of MCAs displayed in Table 4 show two different trends. Samples with L0(N2) values between 0.62 and 0.69 (D, B, and L) had similar Smic and Simm values, and even the Smic + Sext value was quite close to these values due to the low influence of Sext on the total surface area of these samples. In this case, SBET was lower than the above surface areas. The minimal dimensions21 of N2 (0.36 nm) and benzene (0.37 nm) are almost the same; therefore, both molecules should have access to a similar slit-shaped micropore range. However, the surface area determined by the BET method is underestimated with regard to Simm or Smic, because the micropore width does not allow one molecular layer of N2 to be accommodated on each micropore wall. In contrast, benzene molecules interact with both micropore walls in the immersion method, leading to a higher surface area value similar to the Smic value. This was previously reported in studies of activated carbons19 and aramid base activated carbon fibers.21 Hence, differences between SBET and Smic or Simm can be expected to increase when the micropore width approaches the minimal dimensions of N2 or benzene.19,21 In these cases, the BET model will yield an unrealistic value of the surface area for reasons related to the method itself. The second group of samples contained those with L0(N2) values of 0.96-1.06 nm. In this case, Smic + Sext was similar to Simm, and in most cases, SBET was close to these surface area values. This was found by Stoeckli et al.36 in activated carbons with a micropore width of 0.8-1 nm and by Gonza´lez et al.19 in activated carbons with a micropore width of 0.7-1.05 nm. Therefore, in all carbon aerogels investigated in the present study, with micropore widths ranging from 0.62 to 1.06 nm, the enthalpy of immersion into benzene yielded a realistic total surface area value. This result is likely due to the small influence on the Simm value of the volume of liquid benzene between the surface layers of the micropores.

Conclusions Changes in the solvent used in the preparation of the organic aerogels produced changes in their gelation time. This gave rise

Structure of MCAs Obtained with Varying SolVents

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Table 4. Microporosity Characteristics, Heat of Immersion into Benzene, and Surface Areas of Monolithic Carbon and Activated Carbon Aerogels sample D B L F M C K K11 K18

L0(N2), nm

W0(N2), cm3/g

L0(CO2), nm

W0(CO2), cm3/g

-∆ιH(C6H6), J/g

Smic, m2/g

Smic + Sext, m2/g

SBET, m2/g

Simm, m2/g

0.62 0.68 0.69 0.96 0.96 1.01 1.04 1.06 1.06

0.30 0.32 0.34 0.47 0.35 0.37 0.36 0.42 0.47

0.59 0.60 0.60 0.67 0.57 0.59 0.54 0.63 0.69

0.30 0.33 0.33 0.42 0.29 0.27 0.27 0.31 0.33

109.0 ( 0.5 106.8 ( 0.7 115.2 ( 0.6 143.7 ( 1.6 101.5 ( 0.7 116.4 ( 0.8 113.5 ( 0.7 137.8 ( 1.2 151.0 ( 0.8

968 941 985 979 729 732 692 792 887

1048 1057 998 1108 989 1040 967 1112 1232

753 813 873 1164 887 853 914 1073 1199

956 937 1010 1260 891 1021 995 1209 1325

to variations in the inter- and intra-primary-particle structure of the MCAs obtained after carbonization of the organic aerogels. The monolith density of the carbon aerogels ranged from 0.37 to 0.87 g/cm3. Samples with a density >0.61 g/cm3 had mesopores and micropores but no macropores. The mesopore volume ranged from 0.67 to 0.96 cm3/g. The meso- and macroporosity showed a monomodal pore size distribution. There was a scaling relationship between the elastic modulus and density, with a scaling exponent close to 2. CO2 activation of sample K produced a decrease in the density due to an increase in the mesopore volume but had practically no effect on the mean pore width. This was due to the creation of new mesopores as a consequence of micropore widening. The surface area of the organic aerogels determined from the enthalpy of immersion into benzene was much lower than that of the carbon aerogels due to the development of microporosity during carbonization. The surface area of MCAs was determined from N2 adsorption by using the BET and the Stoeckli equations and from the enthalpy of immersion into benzene. In all studied samples, which had mean micropore widths ranging from 0.62 to 1.06 nm, the enthalpy of immersion into benzene yielded a realistic total surface area value. This result was likely due to the small influence on the Simm value of the volume of liquid

benzene between the surface layers of the micropores. In samples with a micropore width below 0.67 nm the SBET value was underestimated with regard to Simm. This was due to the fact that the micropore width does not allow one molecular layer of N2 to be accommodated on each micropore wall. In contrast, benzene molecules interact with both micropore walls in the immersion method, leading to a higher surface area value. CO2 activation of sample K produced an increase in the volume and width of the micropores, producing larger surface areas at higher degrees of activation. Acknowledgment. This research was supported by MEC and FEDER Project CTQ2007-61324 and Junta de Andalucı´a Project RNM 547. D.F.-J. acknowledges a predoctoral fellowship from MEC. Supporting Information Available: DR plots for N2 and CO2 adsorption and the relationship between the elastic modulus and density. This material is available free of charge via the Internet at http:// pubs.acs.org. LA703386Q