Synthesis and Properties of Phloroglucinol−Phenol−Formaldehyde

Langmuir 2009, 25, 2461-2466

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Synthesis and Properties of Phloroglucinol-Phenol-Formaldehyde Carbon Aerogels and Xerogels Hana Jirglova´,† Agustı´n F. Pe´rez-Cadenas, and Francisco J. Maldonado-Ho´dar* Department of Inorganic Chemistry, Faculty of Sciences, UniVersity of Granada, AVda. FuentenueVa s/n, 18071 Granada, Spain ReceiVed September 29, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008 Carbon aerogels and xerogels were successfully prepared from phloroglucinol-phenol mixtures and characterized by different techniques to determine their potential. We examined the influence of the phloroglucinol/phenol ratio, reactant concentration, cure conditions, and drying method on the morphology and porosity of the samples. The gelation time was found to be independent of the phloroglucinol/phenol ratio in spite of the different reactivities of both monomers. In general, carbon aerogels have a high volume of mesopores and of micropores without diffusion restrictions. Carbon xerogels are denser materials without mesopores but with a well-developed microporosity that shows a strong molecular sieve effect. Therefore, while micro-/mesoporous carbon aerogels can be used as catalyst supports or VOC adsorbents, the microporous carbon xerogel could offer high selectivity in the separation of small molecules from gaseous mixtures.

1. Introduction Carbon aerogels and xerogels are one of the most promising new carbon forms. These materials were first synthesized by Pekala et al.1,2 from the sol-gel polycondensation of resorcinol (R) and formaldehyde (F) in aqueous solutions. Their characteristics depend on careful control of the experimental conditions of the four main steps of the synthesis process: sol-gel, solvent exchange, drying, and carbonization/activation.3 In recent years, significant efforts have been made to improve the characteristics of these materials, reduce the costs, and enhance possible applications by analyzing the influence of reactant ratios,1-3 catalyst or solvent type,4-6 metal doping,7-9 drying method,10-13 surfactants,6,14 and so forth. Although resorcinol is the product normally used, other carbon precursors have been progressively gaining interest as a means of reducing the cost of the materials involved, while maintaining * Corresponding author. E-mail: [email protected]. Tel.: +34 958240444. Fax: +34 958248526. † Permanent address: J. Heyrovsky´ Institute of Physical Chemistry v. v. i., Academy of Sciences of the Czech Republic, Dolejskova 3, 18223, Prague, Czech Republic. (1) Pekala, R. W.; Alviso, C. T.; LeMay, J. D. J. Non-Cryst. Solids 1990, 125(1-2), 67–75. (2) Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. J. Non-Cryst. Solids 1992, 145, 90–98. (3) Al-Muhtaseb, S. A.; Ritter, J. A. AdV. Mater. 2003, 15, 101–14. (4) Pekala, R. W.; Schaefer, D. W. Macromolecules 1993, 26, 5487–5493. (5) Tamon, H.; Ishizaka, H. J. Colloid Interface Sci. 2000, 223(2), 305–307. (6) Fujikawa, D.; Uota, M.; Sakai, G.; Kijima, T. Carbon 2007, 45, 1289– 1295. (7) Maldonado-Ho´dar, F. J.; Ferro-Garcia, M. A.; Rivera-Utrilla, J.; MorenoCastilla, C. Carbon 1999, 37, 1199–1205. (8) Maldonado-Hodar, F. J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; FerroGarcı´a, M. A. Stud. Surf. Sci. Catal. 2000, 130, 1007–1012. (9) Cotet, L. C.; Baia, M.; Baia, L.; Popescu, I. C.; Cosoveanu, V.; Indrea, E.; Popp, J.; Danciu, V. J. Alloys Compd. 2007, 434-435, 854–857. (10) Czakkel, O.; Marthi, K.; Geissler, E.; Laszlo, K. Microporous Mesoporous Mater. 2005, 86, 124–133. (11) Tonanon, N.; Wareenin, Y.; Siyasukh, A.; Tanthapanichakoon, W.; Nishihara, H.; Mukai, S. R.; Tamon, H. J. Non-Cryst. Solids 2006, 352, 5683– 5686. (12) Leonard, A.; Blacher, D.; Crine, M.; Jomaa, W. J. Non-Cryst. Solids 2008, 354, 831–838. (13) Jung, H.; Hwang, S. W.; Hyun, S. H.; Lee, S. H.; Kimwere, G. T. Desalination 2007, 216, 377–385. (14) Matos, I.; Fernandes, S.; Guerreiro, L.; Barata, S.; Ramos, A. R.; Vital, J.; Fonseca, I. M. Microporous Mesoporous Mater. 2006, 92, 38–46.

their potential. Phenol (Ph) is the most abundant and inexpensive precursor, but it is poorly reactive. Nevertheless, phenolformaldehyde materials were prepared successfully by using low phenol concentration and high (Na2CO3) catalyst concentration15 or, alternatively, by using stronger bases such as NaOH.16 Phenolic-furfural,17 melamine-formaldehyde,18 or cresolformaldehyde19,20 mixtures were also used to prepare raw organic gels. The use of phloroglucinol (Phl) for this purpose21 has so far been the subject of relatively little research, even when the higher reactivity of this monomer could reduce the cost in terms of carbon materials by decreasing the preparation time. In our previous works we prepared different series of R-F carbon xerogels or aerogels, in the form of monoliths, powder, or coatings, and applied them as catalyst supports or adsorbents.7,8,22-25 The aim of this work is to obtain carbon materials from mixtures of phloroglucinol and phenol so as to obtain the synergies offered by the specific advantages presented by these two monomers, namely, the high polymerization rate of phloroglucinol and the cheapness of phenol. This could provide an interesting alternative to the use of resorcinol. Different samples were prepared in which phloroglucinol was progressively substituted by phenol, and then characterized using different techniques. The evolution of the porous characteristics of the samples is related to their phenol content, drying method, morphology, and thermal behavior. (15) Mukai, S. R.; Tamitsuji, C.; Nishihara, H.; Tamon, H. Carbon 2005, 43, 2628–2630. (16) Wu, D.; Fu, R.; Sun, Z.; Yu, Z. J. Non-Cryst. Solids 2005, 351(10-11), 915–921. (17) Pekala, R. W.; Alviso, C. T.; Lu, X.; Gross, J.; Fricke, J. J. Non-Cryst. Solids 1995, 188, 34–40. (18) Ruben, G. C.; Pekala, R. W. J. Non-Cryst. Solids 1998, 225, 64–68. (19) Li, W. C.; Lu, A. H.; Guo, S. C. Carbon 2001, 39, 1989–1994. (20) Pe´rez-Caballero, F.; Peikolainen, A. L.; Uibu, M.; Kuusik, R.; Volobujeva, O.; Koel, M. Microporous Mesoporous Mater. 2008, 108(1-3), 230–236. (21) Barral, K. J. Non-Cryst. Solids 1998, 225, 46–50. (22) Maldonado-Ho´dar, F. J.; Morales-Torres, S.; Ribeiro, F.; Ribeiro-Silva, E.; Pe´rez-Cadenas, A. F.; Carrasco-Marı´n, F.; Costa-Oliveira, F. Langmuir 2008, 24(7), 3267–3273. (23) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J. Phys. Chem. Chem. Phys. 2000, 2(20), 4818–4822. (24) Maldonado-Ho´dar, F. J.; Moreno-Castilla, C.; Carrasco-Marı´n, F.; Pe´rezCadenas, A. F. J. Hazard. Mater. 2007, 148, 548–552. (25) Padilla-Serrano, M. N.; Maldonado-Ho´dar, F. J.; Moreno-Castilla, C. Appl. Catal. B 2005, 61, 253–258.

10.1021/la803200b CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009

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Table 1. Phloroglucinol/Phenol Molar Ratio and Shrinkage after the Samples Were Dried sample

Phl/Ph (mol)

drying method

drying shrinkage (%)

PA AA BA CA AA-900*a

100/0 60/40 40/60 20/80 60/40

supercritical CO2 supercritical CO2 supercritical CO2 supercritical CO2 supercritical CO2

29 25 34 56 31

PX AX BX CX

100/0 60/40 40/60 20/80

oven, 120 °C oven, 120 °C oven, 120 °C oven, 120 °C

49 43 46 55

a

Aerogel obtained after 24 h of curing.

2. Experimental Section 2.1. Synthesis Conditions. The organic precursors were prepared by sol-gel polymerization reactions. Due to the poor solubility of phloroglucinol in water, we decided to use ethanol (E) as the solvent. Different mixtures of phloroglucinol (Phl), phenol (Ph), and formaldehyde (F) were dissolved in the appropriate amount of ethanol under magnetic stirring to obtain homogeneous solutions. Sodium carbonate, Na2CO3, was used as a polymerization catalyst (C). The molar ratios (Phl + Ph)/F, (Phl + Ph)/C, and (Phl + Ph)/E were maintained constant at 1:2, 300:1, and 1:8, respectively. More diluted mixtures, using a (Phl + Ph)/E molar ratio of 1:16, were also prepared, but the gels obtained were weak. A pure Phl-F gel (sample P) was obtained as a reference, but in these experimental conditions, no gelation was achieved in the case of pure Ph-F solutions. Phloroglucinol was progressively replaced by phenol in the preparation recipe (Table 1), while all other synthesis conditions were maintained constant. The pH values of the starting solutions ranged between 6.5 and 7.0. The solutions were homogenized by stirring, then poured into glass molds (25 cm length × 0.5 cm internal diameter), sealed to avoid evaporation, and cured at 80 °C for 1 or 5 days. After curing, the gel rods were cut into 5 mm long pellets and divided into two portions. The first portion was exchanged in acetone for 24 h before being dried in supercritical CO2, so obtaining organic aerogels. The second portion was dried in an oven at 120 °C in static air, and the corresponding xerogels were obtained. In this paper we have distinguished between these samples by adding the letter A (aerogel) or X (xerogel) to the name of the sample. Both raw materials were finally carbonized at 900 °C using a 100 cm3 min-1 N2 flow, a heating rate of 1.5 °C min-1, and a soak time of 5 h. The carbonization temperature was also added to the sample name to differentiate the carbonized samples. 2.2. Characterization of Organic and Carbon Aerogels and Xerogels. The chemical structure of the organic aerogels or xerogels was analyzed by TG and FTIR. TG-DTG experiments were performed in a Shimadzu TGA-50H thermobalance using a heating rate of 10 °C min-1 up to 900 °C in N2 flow. IR spectra were recorded on a NICOLET 20SXB FTIR spectrophotometer. Pellets of KBr containing 0.1 wt % of sample were used. The morphology of both organic and carbonized samples was observed by SEM using a LEO scanning electron microscope (Carl Zeiss) GEMINI-153. Textural characterization was carried out using mercury porosimetry up to a pressure of 4200 kg cm-2 (Quantachrome Autoscan60). With this technique we obtained the pore size distribution (PSD) of pores with a diameter greater than 3.7 nm and their external surface area Sext. The bulk density of the samples, F, was also determined. The pore volume of pores with diameters of between 3.7 and 50 nm is referred to as mesopore volume, Vmeso (note that the mesopore volume range26 is defined as 2-50 nm). The volume of the pores with a diameter of more than 50 nm is referred to as macropore volume, Vmacro. Adsorption isotherms of N2 (77 K) and CO2 (273 K) were measured using a volumetric device (Autosorbe-1 Quantachrome). The apparent (26) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. ActiVe Carbon; Marcel Dekker: New York, 1988.

BET surface areas were calculated from the nitrogen adsorption isotherms. The micropore volume, W0, and the characteristic adsorption energy, E0, were obtained by the application of the Dubininin-Raduskevich (DR) equation to both CO2 and N2 adsorption isotherms.26 Then the mean micropore width, L0, was obtained by applying the Stoeckli equation27

L0(nm) ) 10.8/(E0 - 11.4 kJ mol-1)

(1)

and the micropore surface area was obtained by the equation28

SCO2 ) 2000W0(cm3 g-1)/L0(nm)

(2)

The enthalpies of the immersion in benzene of the previously outgassed samples (383 K for 12 h) were measured at 303 K in a Setaram C-80, Tian-Calvet type isothermal calorimeter. Corrections were made to take into account the bulb breaking and the liquid vaporization energy. Once corrected, the immersion enthalpy was converted into surface area values (Sbenz) according to the Denoyel method.29

3. Results and Discussion 3.1. Organic Gels. In view of the different reactivities of both monomers (phloroglucinol and phenol), we observed the gelation time of each mixture. Gelation time was considered to be the time at which the solution no longer flows in a vial tilted at an angle of 45°.16 We found that this parameter was independent of the Phl/Ph ratio, and in all cases it was around 50 min. It should be noted that under our experimental conditions the pure Ph-F solutions never provided the gel, even after long curing periods. All the solutions that contained Phl produced homogeneous, opaque, red-brown gels with no apparent volume changes compared to the original liquid solution. Precipitation was not observed in any cases. The process of gelation is therefore controlled by the presence of phloroglucinol. The Phl concentration in all the starting solutions is large enough to polymerize forming a homogeneous gel, which prevents mobility inside the container and homogenizes the gelation time as defined. Nevertheless, polymerization is not complete at the gelation time; polymerization progresses along the curing period, as will be described below. Although gelation occurs without any change in volume, a large shrinkage occurs during the drying and carbonization processes. The shrinkage percentage, defined as the reduction of the pellet diameter, is shown in Table 1. The degree of shrinkage is always greater for xerogels when compared to that of the corresponding aerogels. Short polymerization time and high phenol content also favored shrinkage. In contrast, the presence of Ph in small concentration seems to prevent shrinkage with respect to sample P. The gelation process of both Phl and Ph monomers develops by addition and condensation reactions, as happens with resorcinol.3,16,21 After the formaldehyde addition to the aromatic ring, hydroxymethyl groups are formed, which allow the development of a three-dimensional network by formation of methylene (-CH2-) and methylene ether (-CH2OCH2-) bridges through condensation reactions. These cross-linked structures of the organic gels were studied by FTIR (Figure 1). The main bands observed were located at 1620, 1460, and 1115 cm-1. When the phenol ratio in the samples was increased, new bands were observed at 1010 and 1520 cm-1 that did not appear in the (27) Stoeckli, F. In Porosity in Carbons. Characterization and Applications; Patrick, J. W., Ed. Edward Arnold: London, 1995; pp 67-97. (28) Stoeckli, F.; Centeno, T. A. Carbon 1997, 35, 1097–2000. (29) Denoyel, R.; Ferna´ndez-Colinas, J.; Grillet, Y.; Rouquerol, J. Langmuir 1993, 9, 515–518.

Phl-Ph-F Carbon Aerogels and Xerogels

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FTIR spectrum of the P sample, while bands located at 1260 and 1350 cm-1 also became more intense. The bands at 1460 and 1260 cm-1 are associated respectively in the literature16,19 with the formation of methylene and methylene ether bridges between the aromatic rings. These bands correspond respectively to the C-H bending and C-O stretching vibrations in C-O-C structures. The main band in the region of 1600 cm-1 is due to the CdC stretching in the aromatic rings, while the bands at around 1115 and 1350 cm-1 can also be attributed to C-OH stretching and deformation in phenolic groups. Brandt et al.30 associated the band at 1520 cm-1 to aromatic rings in resorcinol (R)-formaldehyde (F) carbon aerogels synthesized by an acetic acid catalyzed process, while Bouchelta et al.31 related it to the CdC skeletal stretch in the condensed aromatic system. The increasing phenol content seems therefore to induce the formation of such condensed aromatic species. The FTIR spectra corresponding to xerogel samples are similar, but show weak bands. Similarly, after carbonization the bands at 1460, 1350, and 1260 cm-1 disappeared, as a consequence of the structure breaking and gases being released. Only the band corresponding to the aromatic rings is clearly observed, although it has shifted to higher frequencies (at about 1635 cm-1). There is also a small band at 1115 cm-1. The morphology of the samples was studied using SEM (Figure 2). The sample structure consists of a three-dimensional network of interconnected nanosized primary particles, similar to that described by different authors3,7,8 for resorcinol-formaldehyde (R-F) aerogels. The particle size decreases with the phenol content and is also always smaller for xerogels, in agreement with the evolution of the shrinkage. The carbonization process was simulated by TG experiments. Figure 3 compares the DTG curves obtained for A and C aerogel samples. The shapes of these curves were also similar to those previously found for R-F xerogels or aerogels.7 Two different regions can be distinguished, one up to 200 °C with around a 15% w/w loss, which was associated with the elimination of solvents and residual organic precursors, and a second between 200 and 800 °C in which the samples underwent a weight loss of about 45% w/w as a result of the carbonization reaction of

the organic gels. This reaction takes place in two steps: the breaking of C-O bonds between 200 and 400 °C and of C-H bonds at higher temperatures with the subsequent reorganization into C-C bonds. The main weight loss is associated with the breaking of C-O bonds, eliminating COx and water, as pointed out by the stronger DTG peak. It is also worth noting that as the phenol content of the sample is increased, the bonds begin to break at lower temperatures, in such a way that the DTG curves are shifted downward by between 40 and 50 °C. 3.2. Carbon Aerogel and Xerogels. After carbonization, the cumulative shrinkage of PA-900 and AA-900 carbon aerogels reaches values of about 50%. The other samples lose their pellet shape in the carbonization process, becoming rounded, which means that it was impossible to estimate shrinkage by the method used. This effect was more pronounced as the phenol ratio was increased, particularly in the case of xerogel samples. Although Mukai et al.15 reported that Ph-F carbon gels maintained their monolithic shape, Wei et al.32 observed that the Ph-F pellets became deformed, something which they associated with the fact that this type of material can melt during the pyrolysis processes. In our case, the deformation of the pellets was related to the weaker chemical structure of these samples. In previous works,3,33 the carbonized R-F samples maintained the structure of their organic precursors and carbonization produced only a certain degree of shrinkage. In this case, the morphology of both the carbonized samples and the organic samples was strongly influenced by both the phenol ratio and the drying method (Figure 4). In the absence of phenol, sample PA900 presented the typical coral-like structure described above. With a Phl/Ph ratio of 60/40, sample AA-900 retained a corallike structure similar to its parent aerogel, where all the primary particles were of a similar size. Nevertheless, in the corresponding xerogel (AX-900) this homogeneity disappeared and two types of particles with different sizes could be clearly observed. The particle size of the carbon matrix tended to decrease, while another larger particle appeared homogeneously dispersed. This effect is enhanced as the phenol content is increased. Thus, this latter larger particle was also visible in the carbon aerogel CA-900 (Phl/Ph ) 20/80) but they were larger and polyhedral in shape in the case of the carbon xerogel, sample CX-900, while the carbon matrix became a smooth surface on which it was impossible to make out the primary particles. It is therefore clear that, during carbonization, the mobility of the chemical species increases and a severe reorganization of the structure occurs. This effect is stronger in the case of xerogels and as phenol content is increased. The analysis of the external surface of the pellets (i.e., the surface in contact with the glass mold) confirmed that the sample partially melts during carbonization (Figure 4), and the liquid phase seems to diffuse across the primary particles filling the pores. These transformations strongly influence the porosity of the samples. The results obtained by mercury porosimetry are summarized in Table 2 and in the PSD in Figure 5. It is important to note the absence of macropores in both the carbon aerogel and xerogel series. Nevertheless, the different drying method strongly influences the mesoporosity of the samples. Carbon aerogel (Figure 5a) obtained from pure phloroglucinol (PA-900) is a mesoporous material with a monomodal PSD around pores with a radius of 5.5 nm and high Sext. The mesopore volume and the PSD are quite similar to those obtained for R-F carbon aerogels.7 The replacement of phloroglucinol by phenol in a

(30) Brandt, R.; Petricevic, R.; Probstle, H.; Fricke, J. J. Non-Porous Mater. 2003, 10, 171–178. (31) Bouchelta, Ch.; Salah-Medjram, M.; Bertrand, O.; Bellat, J. P. J. Anal. Appl. Pyrol. 2008, 82, 70–77.

(32) Wei, W.; Qin, G.; Huc, H.; You, L.; Chen, G. J. Membr. Sci. 2007, 303, 80–85. (33) Tamon, H.; Ishizaka, H.; Araki, T.; Okazaki, M. Carbon 1998, 36(9), 1257–1262.

Figure 1. FTIR spectra of organic aerogels with different phloroglucinol/ phenol ratios.

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Figure 2. Influence of the phenol content and drying method on the primary particle size of the samples.

Figure 3. DTG profiles of organic aerogels with different phloroglucinol-phenol content.

40/60 molar ratio leads to an increase in the mesopore volume (Table 2) without modifying the PSD (Figure 5a), which is associated with a smaller degree of shrinkage (Table 1). The smaller mesopore volume and narrow pores of the sample AA900* (Figure 5b) may be related to an incomplete polymerization after 24 h of curing. Polymerization was considered complete after 5 days at 80 °C because no textural transformations were observed for longer curing periods. However, sample CA-900, with a higher phenol content, does not present any mesoporosity. This fact is related to the melting and pore filling processes observed in this sample. Similarly, carbon xerogels do not present any macro- or mesoporosity, and therefore have higher density values. In this case, two factors contribute to this behavior. These samples undergo the largest drying shrinkage and present the smallest particle size. This means that during carbonization their mesoporosity (interparticle voids) can be easily blocked by the mobile phase. The results obtained from N2 and CO2 adsorption isotherms are presented in Table 3. The micropore volume W0 obtained

from CO2 adsorption at 273 K yields the volume of narrower micropores (below 0.7 nm in width). In the absence of diffusion restriction, the total micropore volume is obtained by N2 adsorption.34,35 When the apparent surface area from N2 adsorption (SBET) is compared with that obtained by benzene immersion calorimetry (Sbenz), the best agreement between the two techniques occurs in the case of wider micropores. The narrowest microporosity leads to the underestimation of surface areas using the N2 adsorption technique, if the pore dimension is less than twice the molecular dimension of nitrogen because it is impossible for a nitrogen monolayer to form on each wall of the pore.36 The narrowest microporosity W0 (CO2), and consequently the SCO2 surface area values, are barely influenced by the drying method and the sample composition. These parameters present similar values in all the carbonized samples. Nevertheless, the mean micropore width (L0) is greater for xerogels than for their corresponding aerogels (Table 3). Raw organic aerogels and xerogels present low microporosity, which develops inside the primary particles during carbonization due to the release of gases.3,24 When these results are compared with those obtained by N2 adsorption and benzene microcalorimetry, the different behaviors of carbon aerogels and xerogels are observed at a glance: carbon aerogels present high surface areas accessible to both N2 and benzene, while in the case of carbon xerogels the access to both compounds is completely restricted. Moreover, carbon aerogels present a similar surface area accessible to each adsorbate, except in the cases of CA-900 and AA-900* which show behavior patterns that fall between those for carbon aerogels and xerogels. (34) Cazorla-Amoros, D.; Alcaniz-Monge, J.; De la Casa-Lillo, M. A.; LinaresSolano, A. Langmuir 1998, 14, 4589–4596. (35) Cazorla-Amoros, D.; Alcaniz-Monge, J.; Linares-Solano, A. Langmuir 1996, 12, 2820–2824. (36) Molina-Sabio, M.; Nakagawa, Y.; Rodrı´guez-Reinoso, F. Carbon 2008, 46329-334.

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Figure 4. SEM images of the cross section and external surface* of pellet samples. Table 2. Characterization of Carbon Aerogels and Xerogels by Mercury Porosimetry sample

pyrolysis (%) F (g cm-3) Vmeso (cm3 g-1) Sext (m2 g-1)

PA-900 AA-900 BA-900 CA-900 AA-900*

59 56 55 53 55

0.77 0.74 0.78 1.54 0.90

0.45 0.58 0.59 0.00 0.32

167 218 224 0.00 160

PX-900 AX-900 BX-900 CX-900

51 46 51 53

1.74 1.78 1.81 1.87

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

The N2 adsorption isotherms for carbon aerogels are shown in Figure 6a. After adsorption in micropores at low relative pressure P/P0, N2 was absorbed in larger pores by increasing P/P0. The isotherms show an important slope in the plateau and an important hysteresis loop at high relative pressure that confirms the presence of mesopores in these samples. The PSDs obtained by the BJH method applied to desorption branches are shown in Figure 6b. A monomodal PSD was obtained with a maximum radius for pores of about 5 nm, which is in good agreement with the mercury porosimetry results (see Figure 5). The different behavior of sample CA-900 could also be clearly observed. In

this case, N2 was mainly adsorbed at a low relative pressure, although the micropore volume W0 (N2) was significantly smaller than those for the other carbon aerogels. The adsorption at high relative pressure was also small, and desorption took place in a large pressure range, indicating a small volume of narrow mesopores, mainly below the detection limit of the porosimeter. Given that the dimensions of each molecule are similar, 0.33 nm (CO2), 0.36 nm (N2), and 0.37 nm (liquid benzene) respectively,32,34,36 in the absence of diffusion restriction, the accessibility to the micropores should also be similar. This behavior was previously described for carbon aerogels with low phenol content. However, AA-900* and CA-900 samples present a significant molecular sieve effect in the micropore range that is stronger in the case of carbon xerogels. Thus, xerogels provide high micropore volume which is accessible for CO2, but completely inaccessible for both N2 and benzene. Because greater micropore width L0 is observed in all xerogel samples, constrictions at the micropore entrance must have been generated by shrinkage and/or by the pore-filling process after melting. The absence of mesopores could also make access to these micropores more difficult. Therefore, while micro-/mesoporous carbon aerogels can be used as catalyst supports or VOC adsorbents,8,24,28 the mi-

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Figure 5. Influence of the phloroglucinol/phenol ratio (a) and polymerization time (b) on the PSD of carbon aerogels obtained by mercury porosimetry. Table 3. Microporosity and Surface Area Characteristics of Carbon Samples

sample

W0 L0 W0 (CO2) (CO2) SBET (N2) SCO2 Sbenz (m2 g-1) (cm3 g-1) (nm) (m2 g-1) (cm3 g-1) (m2 g-1)

PA-900 AA-900 BA-900 CA-900 AA-900*

723 843 776 705 806

0.20 0.24 0.22 0.22 0.22

0.54 0.57 0.56 0.63 0.55

636 659 672 335 510

0.26 0.28 0.28 0.17 0.24

694 839 726 395 455

PX-900 AX-900 BX-900 CX-900

750 773 830 896

0.27 0.32 0.26 0.28

0.72 0.81 0.62 0.63

26 50 39 50

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

croporous carbon xerogel could present a high selectivity in the separation of small molecules. The separation of mixtures such as N2/H2, CH4/H2, CO/H2, or O2/N2 is very important in applications such as fuel cells or oxygen enrichment of air for combustion applications.37,38

4. Conclusions Carbon aerogels and xerogels were successfully synthesized from phloroglucinol-phenol mixtures. The gelation process occurs only in the presence of phloroglucinol and under our experimental conditions the pure phenol-formaldehyde gel could not be obtained. The characteristics of the samples depend highly on the drying method and the composition. The characteristics of xerogels are (37) Li, Y.; Chung, T.-S. Microporous Mesoporous Mater. 2008, in press, doi 10.1016/j.micromeso.2007.11.038. (38) Moon, J. H.; Bae, J. H.; Bae, Y. S.; Chung, J. T.; Lee, C. C. J. Membr. Sci. 2008, 318(1-2), 45–55.

Figure 6. Nitrogen adsorption isotherms and PSDs of carbon aerogels obtained by the BJH method.

determined by their greater drying shrinkage. As for the composition, the characteristics of the samples change abruptly when phenol content goes beyond 60% (Phl/Ph ) 40/60). The structure of the organic samples consists of a threedimensional network of nearly spherical particles. The particle size decreased at high phenol content and was also smaller for xerogels due to a higher degree of shrinkage. The chemical structure of organic samples becomes thermally weaker at high phenol content, which favors the partial melting of these samples during carbonization. The formation of a mobile phase sharply modifies the morphology and porosity of the carbonized samples because it tends to fill the adjacent porosity. In this case, a distribution of big particles on a smooth surface is generated from the coral-like structure of the organic samples. At low phenol content, carbon aerogels are micromesoporous materials, while carbon xerogels do not present any mesoporosity. Mesoporosity is also blocked in phenol-rich carbon aerogels. The micropore volume accessible to CO2 is similar for all the carbonized samples. Nevertheless, in the case of phenol-rich carbon aerogel and in all the carbon xerogels, the N2 and benzene accessibility to this microporosity was completely restricted by the appearance of constrictions at the micropore entrance, showing a strong molecular sieve effect. Acknowledgment. H. Jirglova´ and A. F. Pe´rez-Cadenas acknowledge the Spanish AECI and Ministry of Education and Science for an MAE fellowship and for a Ramo´n y Cajal research contract, respectively. These investigations were supported by the Junta de Andalucı´a, and the MEC-FEDER, projects RNM547, CTQ2006-04702, and CTQ2007-61324. LA803200B