Synthesis and textural characteristics of organic aerogels, transition-metal-containing organic aerogels and their carbonized derivatives

Carbon 37 (1999) 1199–1205

Synthesis and textural characteristics of organic aerogels, transition-metal-containing organic aerogels and their carbonized derivatives ´ ´ J. Rivera-Utrilla, C. Moreno-Castilla* F.J. Maldonado-Hodar, M.A. Ferro-Garcıa, ´ en Carbones, Departamento de Quımica ´ ´ Inorganica , Facultad de Ciencias, Universidad de Granada, 18071 Grupo de Investigacion Granada, Spain Received 28 July 1998; accepted 3 November 1998

Abstract Four organic aerogels were prepared by the sol-gel method from polymerization of resorcinol with formaldehyde catalyzed by Na 2 CO 3 . These aerogels were further pyrolyzed in N 2 in order to obtain their corresponding carbon aerogels, and after that activated with either steam or CO 2 to obtain activated carbon aerogels. Another series of organic aerogels and their pyrolyzed and activated derivatives were prepared without a catalyst or in the presence of Ag, Pd or Pt. All these samples were texturally characterized, which has shown the porous modifications undergone by the aerogels during the pyrolysis and activation processes. The textural characteristics of the transition-metal-containing activated carbon aerogels depend on the nature of the metal. Thus, whereas the sample containing Pt, in a very small amount, had the largest meso and macropore volume (0.822 and 2.982 cm 3 g 21 ) found, those containing either Pd or Ag were essentially microporous.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon aerogel; B. Activation; Carbonization; D. Surface properties; Textures

1. Introduction Materials prepared by the sol-gel approach have rapidly become a fascinating new field of research in material science. Aerogels are prepared from gels by supercritical drying methods. The skeletal structure of wet aerogels is maintained through supercritical drying, obtaining solids with high porosity and specific surface area [1–4]. The sol-gel process is a chemical synthesis method initially used for the preparation of inorganic materials such as glasses and ceramics. Thus, the most common aerogels are inorganic, usually derived from the sol-gel polymerization of metal alkoxides (e.g. tetramethoxysilane, tetraisopropoxy titanate), followed by supercritical drying [1,5–7]. Pekala et al. [8–12] have found that certain organic monomers can also be used to prepare aerogels. Thus, polycondensation of resorcinol with formaldehyde in aqueous solutions leads to gels that can be supercritically dried *Corresponding author. Fax: 134-958-248-526. E-mail address: [email protected] (C. Moreno-Castilla)

with carbon dioxide to form organic aerogels. These resorcinol–formaldehyde (RF) aerogels can be pyrolyzed in an inert atmosphere to form carbon aerogels. Because of the chemical and textural characteristics of these materials, they are expected to be used as thermal and phonic insulators, electric double layer capacitors, chromatographic packings, adsorbents, and catalyst supports [13,14]. The structure and properties of RF and carbon aerogels are largely determined by polymerization conditions, the dominant factor that controls this process being the resorcinol / catalyst (R / C) ratio [15]. The catalyst used by Pekala [8–12] and other authors [15,16] to carry out this polymerization was a basic catalyst such as Na 2 CO 3 . The main objective of the present paper is to study the possibility of preparing transition-metal-containing RF aerogels and their carbonized derivatives. The transitionmetals studied were Pt, Pd and Ag. For this purpose, several RF aerogels following the Pekala method were obtained and in some cases, the basic catalyst (Na 2 CO 3 ) was substituted by different salts of the transition metals chosen. After that all aerogels were carbonized and activated both in CO 2 or steam. The influence of the

0008-6223 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 98 )00314-5

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presence of either the transition metal or Na 2 CO 3 on the textural characteristics of the solids obtained was studied with all the samples prepared. It is expected that some of these samples find a proper use as adsorbents and catalysts of different reactions. Therefore, the analyses of the pore texture of these aerogels are very important from the view point of the above applications.

2. Experimental Four RF aerogels were synthesized following the method described by Pekala [8–12]. Briefly, different amounts of resorcinol (R) and formaldehyde (F), both from Aldrich, were mixed with the appropriate amounts of distilled water and Na 2 CO 3 , as polymerization catalyst. The mixtures were cast into glass molds (25 cm length30.5 cm internal diameter) and cured. After that, the gel rods were cut in 5 mm pellets and introduced in acetone to remove the water inside the pores. The gels were then supercritically dried with carbon dioxide to obtain the corresponding aerogels. Three transition-metal-containing RF aerogels were also prepared following the recipe of sample D, but changing the catalyst, Na 2 CO 3 , by [Pt(NH 3 ) 4 ]Cl 2 , PdCl 2 or AgOOC–CH 3 . These aerogel samples will be referred to in the text as N–Pt, N–Pd and N–Ag, respectively. In addition, an aerogel sample was obtained in the same experimental conditions as sample D, but without a polymerization catalyst. This aerogel will be referred to as N. The recipes of the different aerogels prepared are given in Table 1. Pyrolysis of all aerogels to obtain the corresponding carbon aerogels was carried out in N 2 flow (100 cm 3 min 21 ) by heating up to 1273 K with a heating rate of 1.5 K min 21 and a soak time of 5 h. Some of these carbon aerogels were activated at 1173 K either in steam, for 25 min, or in CO 2 , for 2 h. Carbon aerogels will be referred to in the text adding the letter C to the name of the corresponding aerogel and the activated carbon aerogels will be referred to in the text adding the activation agent, S Table 1 Recipes of the aerogels; R, resorcinol; F, formaldehyde; W, water; C, catalyst Samples

R

F

W

C

Amounts in moles

Catalyst precursor

A C D E

0.112 0.112 0.112 0.112

0.224 0.112 0.224 0.336

0.850 0.850 0.850 0.850

5.6310 24 1.4310 24 1.4310 24 1.4310 24

Na 2 CO 3 Na 2 CO 3 Na 2 CO 3 Na 2 CO 3

N N–Pt N–Pd N–Ag

0.112 0.112 0.112 0.112

0.224 0.224 0.224 0.224

0.850 0.850 0.850 0.850

– 1.4310 24 1.4310 24 1.4310 24

– [Pt(NH 3 ) 4 ]Cl 2 PdCl 2 Ag(CH 3 COO)

(H 2 O) or CO 2 , to the denomination of the corresponding carbon aerogel. Textural characterization of all samples was carried out by adsorption of N 2 and CO 2 at 77 K and 273 K, respectively, and mercury porosimetry up to 4200 kg cm 22 (Quantachrome Autoscan 60 porosimeter). The BET equation was applied to the N 2 adsorption isotherms to obtain the nitrogen surface area, SN 2 . Adsorption isotherms of CO 2 at 273 K were analyzed using the Dubinin-Radushkevich Eq. (1): W 5 Wo exp[2B(T /b )2 ln 2 (Po /P)

(1)

where W is the volume filled at temperature T and relative pressure P/Po , Wo is the micropore volume and b is the affinity coefficient [17]. In our experimental conditions b50.46 [18]. From the corresponding plots of Eq. (1), the values of Wo (intercept) and B (slope) were obtained. Parameter B is called the structural constant, and is related to the characteristic adsorption energy, Eo , by Eq. (2): Eo (kJmol 21 ) 5 8.3144 3 10 23 /B 1 / 2

(2)

Eo is a function of the micropore size distribution of the adsorbent. It has been shown [19] that for the slit-shaped and cylindrical model micropores, the adsorption potential is inversely related to the width of these pores [20] according to Eq. (3): Lo (nm) 5 4.699exp(20.0666Eo )

(3)

where Lo is the mean pore width accessible to the adsorbate. From the mercury porosimetry experiments the following textural parameters were obtained: pore size distribution of pores with a diameter greater than 3.7 nm, surface area of these pores, which is known as external surface area, S ext , pore volume corresponding to pores with a diameter between 3.7 and 50 nm, V2 , pore volume of pores with a diameter higher than 50 nm, V3 and particle density, r p.

3. Results and discussion

3.1. Organic aerogels and their carbonized derivatives Firstly the results obtained with the aerogels prepared by using Na 2 CO 3 as polymerization catalyst will be discussed, as well as the corresponding carbon aerogels and activated carbon aerogels obtained from them. The aqueous solutions containing the reactants and the catalyst were initially transparent and colorless, but they turned progressively to yellow, orange and dark red color as the polymerization process progressed during the cure period. All the aerogel pellets were externally of dark red

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color, and that with the highest catalyst concentration, sample A, had internally the same color and was hard, rigid and fragile. Nevertheless, the aerogel pellets with the lower catalyst concentration, C, D and E samples, but specially C, were externally of dark red color and hard, whereas internally they were of orange color and soft, which indicates that the polymerization was not complete. The surface characteristics of these aerogels are compiled in Table 2 and the pore size distributions for pores with a diameter greater than 3.7 nm are depicted in Fig. 1. All these aerogels are mainly mesoporous materials. The pore size distributions show that the maxima are located around 50 nm in diameter, the limit of mesopores and macropores. Sample A, with a complete polymerization, shows the highest particle density and, therefore, lower V2 and V3 pore volumes. Samples C, D and E have lower particle density and, evidently, the above porosity more developed. These three last samples have different values of the R / F molar ratio (Table 1) which essentially affects the macropore volume, V3 . Thus, there is a significant increase in this volume when the R / F ratio decreases from 1 / 1 to 1 / 2. The external surface area markedly increases with the R / C ratio. The nitrogen surface area, SN 2 , and the micropore volume, Wo , were not practically affected by the amount of catalyst used, when the other ingredients in the recipe remained unchanged (samples A and D). When the amount of formaldehyde increased or the R / F ratio decreased, maintaining constant the amount of catalyst, samples C, D and E, there was an increase in the micropore volume although the micropore width practically did not change. Similar trends in the textural characteristics of aerogels to those mentioned above were found by different researchers [13,15,21], which were explained according to the gelation process. In the sol-gel polycondensation, formaldehyde is consumed to form highly cross-linked particles. Before gelation, the aggregated particles are not interconnected but consist mostly of linear and branched chains of particles. After gelation, the particles become interconnected, showing a structure composed of rings of particles with 20–50 particles per ring [22]. According to Ruben et al. [23], the particle size ranges from 3 to 20 nm, and depends on the R / C ratio. It has been also found [11] that aquagels shrink during supercritical drying, and this effect is also dependent on the R / C value. Thus, this effect

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Fig. 1. Pore size distributions of the organic aerogels.

is enhanced for lower R / C values. Therefore, the textural characteristics of these aerogels are determined by the balance of the particle size of aquagels and the drying shrinkage. The aerogels were pyrolyzed as indicated in Section 2. The TG–DTG curves of sample D are depicted in Fig. 2 as an example. The shapes of these curves are similar to that found for carbon xerogels by Lin et al. [16]. Roughly, two different regions can be distinguished, one up to 523 K with around a 15% weight loss which would be associated with the desorption of water and residual organic precursor. The other region between 523 and 1023 K and with

Fig. 2. TG–DTG curves of sample D.

Table 2 Textural characteristics of aerogels obtained by using Na 2 CO 3 as polymerization catalyst Sample

rp (cm 3 g 21 )

V2

V3

S ext

(cm 3 g 21 ) A C D E

1.14 0.74 0.60 0.72

0.111 0.557 0.509 0.531

0.060 0.053 0.366 0.226

SN 2

Wo (cm 3 g 21 )

Eo (kJ mol 21 )

Lo nm

373 116 361 391

0.082 0.037 0.070 0.102

21.45 22.87 23.84 23.54

1.12 1.02 0.96 0.98

(m 2 g 21 ) 21 112 146 149

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around 35% weight loss presents two peaks centered at around 623 K and another peak at around 823 K. These peaks would be related with the carbonization reaction of the aerogel. According to Lin et al. [16] this reaction would involve breaking of C–O bonds at the lower temperature and breaking of C–H bonds at higher temperature. Textural characteristics of carbon aerogels, obtained by pyrolyzing the aerogel samples, are shown in Table 3, and their pore size distributions for pores with diameter greater than 3.7 nm are depicted in Fig. 3. Comparing data in Table 3 with those in Table 2, it is evident that during pyrolysis some porous transformations took place. Thus, in all carbon aerogels the macropore volume, V3 , completely disappeared, and the pore volume V2 increased except for sample AC. Therefore, the pore size distribution is shifted to pores with smaller diameter (Fig. 3) in comparison to that obtained in the case of the corresponding aerogels (Fig. 1). However, SN 2 and the micropore volume, Wo , are higher in the carbon aerogels than in the aerogels. According to these results, the release of volatile matter during pyrolysis increased the micropore volume, without affecting practically its mean width, Lo , which was accompanied by the loss of macroporosity, probably due to the shrinkage of the aerogel structure during this process. Recently, Kaneko et al. [24] showed that activated carbon aerogels with very large meso and micropore volumes could be obtained by CO 2 activation of a carbon aerogel prepared by the Pekala method. Thus we have tried to obtain also activated carbon aerogels from our carbon aerogels by their activation both in CO 2 and steam flow. The results obtained in the textural characterization of these samples are summarized in Table 4 and Fig. 4. The degree of activation, % burn-off (BO), obtained with CO 2 was always lower than that obtained with steam. Thus, the steam activated carbon aerogels have a higher porosity development, which mainly takes place in the range of pores V2 . It is noteworthy that samples DC-S and EC-S have a narrow pore size distribution with their maximum centered at a pore diameter around 15 and 13 nm, respectively. The activation process largely increased the micropore volume (Wo ), which was accompanied by an increase in its mean width. It is noteworthy that some of these activated carbon aerogels have a high SN 2 value, as in the case of sample CC-S, whose value is around 1600

Fig. 3. Pore size distribution of the carbon aerogels.

m 2 g 21 , likely because this sample underwent the highest degree of activation in steam (42%). This is probably due to its higher resorcinol / formaldehyde ratio (see Table 1), because when this ratio decreased, in samples D and E, the percentages of burn-off during the activation steps, samples DC-S and EC-S, decreased.

3.2. Pt, Pd and Ag-containing organic aerogels and their carbonized derivatives As mentioned in Section 2, the aerogels prepared with Pt, Pd and Ag were obtained following the recipe of sample D (Table 1) but without adding Na 2 CO 3 . The pellets of all these samples had the same aspect both external and internally. In addition, an aerogel and its corresponding carbonized and activated samples were prepared using the same experimental conditions as sample D but without adding any metal, sample N. The values of the textural parameters of some of these aerogels, carbon aerogels and activated carbon aerogels are given in Table 5. The aerogels N, N–Pt, N–Ag and N–Pd have no pores in the V2 range, although the aerogels prepared with Na 2 CO 3 as polymerization catalyst (samples A to E) have in this range the greatest pore volume (Table 2). Samples N–Ag and N–Pd have lower macropore volume than sample N, whereas N–Pt has a V3 value fairly close to that of sample N.

Table 3 Textural characteristics of carbon aerogels Sample

Weight loss (%)

rp (cm 3 g 21 )

V2

V3

S ext

(cm 3 g 21 ) AC CC DC EC

51 54 53 50

1.18 0.94 0.76 0.78

0.024 0.384 0.613 0.743

0.000 0.000 0.000 0.000

SN 2

Wo (cm 3 g 21 )

Eo (kJ mol 21 )

Lo nm

483 585 586 550

0.204 0.251 0.210 0.224

22.17 23.36 23.39 23.40

1.07 0.99 0.99 0.99

(m 2 g 21 ) 24 102 185 249

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Table 4 Textural characteristics of activated carbon aerogels Sample

Burn-off (%)

rp (cm 3 g 21 )

V2

V3

Sext

SN 2

(cm 3 g 21 ) AC-S CC-S DC-S EC-S DC-CO 2 EC-CO 2

33 42 29 27 13 16

0.83 0.73 0.55 0.55 0.70 0.70

0.327 0.382 1.113 0.898 0.846 0.628

Wo (cm 3 g 21 )

Eo (kJ mol 21 )

Lo nm

0.290 0.399 0.363 0.337 0.302 –

20.41 18.66 19.81 20.19 22.29 –

1.21 1.35 1.25 1.22 1.06 –

(m 2 g 21 )

0.000 0.000 0.018 0.027 0.000 0.000

Fig. 4. Pore size distribution of the activated carbon aerogels.

An interesting behaviour with regard to the development of porosity is found with samples containing Pt. Thus, when N–Pt is carbonized to obtain (N–Pt)C there is an increase in the micropore and macropore volumes, with the

246 113 322 282 241 218

1183 1598 1260 1173 925 –

micropore width, Lo , remaining practically unchanged. This increase in macropore volume, V3 , was not observed in the case of carbonization of aerogel N to obtain NC. The activation of (N–Pt)C sample in steam to obtain sample (N–Pt)C-S produced a large increase in all surface characteristics: pore volumes (V2 , V3 and Wo ), external and nitrogen surface area. It is noteworthy that this sample has a very large pore volume V2 (0.822 cm 3 g 21 ) and the largest macropore volume (2.982 cm 3 g 21 ) of all samples studied, and that all these changes in textural characteristics were produced by the presence of a very small amount of Pt present in the aerogel matrix. Thus, the Pt content of sample (N–Pt)C-S was around 0.5%, which was obtained by weighing the residue left after burning completely a portion of sample in air. This sample could find a potential use in many technological processes where solids with a very open porosity are required (macromolecule adsorption, catalyst support, etc). All these aspects are being studied in our laboratory. This large increase, specially in V2 and V3 values, is in part due to the high degree of activation (BO) of sample (N–Pt)C-S, because Pt acted as

Table 5 Textural characteristics of series N and carbon-metal aerogel series Sample

WL or BO (%)a

rp (cm 3 g 21 )

V2

V3

Sext

(cm 3 g 21 )

SN 2

Wo (cm 3 g 21 )

Eo (kJ mol 21 )

Lo nm

(m 2 g 21 )

N NC NC-S NC-CO 2

– 54 25 7

0.69 0.72 0.55 0.64

0.000 0.000 0.035 0.008

0.695 0.628 0.984 0.763

4 18 35 22

– – 1085 –

– – 0.377 0.314

– – 20.18 22.40

– – 1.29 1.12

N–Pt (N–Pt)C (N–Pt)C-S (N–Pt)C-CO 2

– 54 60 31

0.82 0.76 0.23 0.60

0.000 0.000 0.822 0.041

0.611 0.749 2.982 0.819

5 14 230 26

83 490 699 847

0.128 0.282 0.347 0.316

22.96 22.65 20.60 21.89

1.07 1.10 1.26 1.15

N–Ag (N–Ag)C-S

– 24

1.25 0.73

0.000 0.000

0.168 0.455

,1 1

– 1140

– 0.410

– 19.31

– 1.37

N–Pd (N–Pd)C-S

– 31

1.16 1.08

0.000 0.000

0.036 0.031

,1 ,1

– 1302

– 0.417

– 19.45

– 1.36

a

Weight loss for the pyrolyzed samples and burn-off for the activated ones.

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Fig. 5. Pore size distributions of the Pt-containing aerogel and its carbonized derivatives.

a gasification catalyst during the activation step. Thus, sample NC-S, without any metal in its composition, underwent a lower degree of activation and therefore its meso and macropore volumes were less developed. The degree of activation of sample (N–Pt)C-CO 2 is lower than the same sample activated in steam, and so, its textural characteristics values were smaller. However, Pt also catalyzed the gasification in CO 2 because the BO obtained is higher than in the case of sample NC-CO 2 . Meso and macropore size distributions of samples containing Pt are shown in Fig. 5. It is interesting that the pyrolysis of the aerogel N–Pt to obtain the sample (N– Pt)C decreased the mean width of the pores up to around 200 nm in diameter, and its steam activation largely increased the pore volume with this width. Moreover, a smaller maximum also appeared in the region of the mesopores, around 16 nm in diameter. Sample (N–Pd)C-S is microporous with high Wo and SN 2 values. The microporosity of this sample is similar to that of (N–Ag)C-S, in value and mean width (Wo and Lo ), although this last sample has a more developed macroporosity, V3 . In order to observe the surface morphology of these materials, samples NC-S and (N–Pt)C-S were analyzed by SEM. The corresponding micrographs are shown in Fig. 6. The surface of these samples is composed by interconnected spherical particles, and the Pt-containing activated carbon aerogel shows smaller particle diameters than sample NC-S due to its higher degree of activation.

4. Conclusions Organic aerogels are mainly mesoporous materials, and when they were carbonized, the macroporosity disappeared and in general the mesoporosity increased as well as the microporosity and the nitrogen surface area. The activation process in steam yielded higher degrees of activation than

Fig. 6. SEM micrographs of samples: (A) NC-S and (B) (N–Pt)CS.

in CO 2 . Steam activation increased the microporosity and the mesoporosity essentially in a narrow range of pore sizes. A nitrogen surface area of up to 1600 m 2 g 21 was obtained. In contrast, transition-metal-containing aerogels are macroporous. It is noteworthy for the case of the Ptcontaining activated carbon aerogel that with a very low Pt content the sample can develop the greatest meso and macropore volume of all samples studied, 0.822 and 2.982 cm 3 g 21 , respectively. In contrast to this sample, the Pdand Ag-containing activated carbon aerogels are essentially microporous materials with surface areas of 1300 and 1100 m 2 g 21 , respectively. Finally, the results presented in this work show that transition-metal-carbon aerogels can be prepared with

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different pore texture and surface characteristics, depending on the nature of the metal dissolved in the original aerogel and the heat treatments given to them.

Acknowledgements The authors wish to acknowledge the financial support of DGCYT, Project no. PB94-0754.

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