Drying of resorcinol–formaldehyde gels with CO2 medium

Microporous and Mesoporous Materials 148 (2012) 34–42

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Drying of resorcinol–formaldehyde gels with CO2 medium Orsolya Czakkel a,b,⇑, Edit Székely c, Béla Koczka d, Erik Geissler e, Krisztina László a a

Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, H-1521 Budapest, Hungary European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France c Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, H-1521 Budapest, Hungary d Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gellért tér 4, H-1111 Budapest, Hungary e Laboratoire Interdisciplinaire de Physique, CNRS UMR 5588, Université J. Fourier de Grenoble, BP 87, 38402 St Martin d’Hères Cedex, France b

a r t i c l e

i n f o

Article history: Received 17 February 2011 Received in revised form 1 July 2011 Accepted 21 July 2011 Available online 28 July 2011 Keywords: Supercritical drying RF gels Porosity Structure

a b s t r a c t The effect of using real supercritical conditions in the CO2 drying process on the structure and texture of resorcinol–formaldehyde networks is investigated by low temperature nitrogen adsorption, scanning electron microscopy and by small and wide angle X-ray scattering. If supercritical conditions are maintained throughout the whole extraction process the resulting networks exhibit much more developed porosity. The surface area of the supercritically dried gel, in excess of 500 m2/g, is more than twice that of the sample dried with liquid CO2. Pore volumes are also significantly higher in all pore classes. In the supercritical region the applied pressure strongly affects the porosity, while the effect of temperature is limited. Drying time also influences the total pore volume of the samples, but not the mesopore and micropore volumes. The volume filling character of the molecular adsorption process in this system is illustrated by the difference in surface areas measured by small angle X-ray scattering and that by nitrogen adsorption. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Mesoporous carbons with high specific surface area have attracted much attention in recent years. The chemical and textural characteristics of these materials predispose them for use as thermal and phonic insulators, electric double layer capacitors, chromatography packing, adsorbents, catalyst supports, etc. Carbon gel precursors are most often prepared from resorcinol and formaldehyde under controlled conditions [1]. Since the polymerization reaction between resorcinol and formaldehyde is sensitive to the initial conditions [1], the structure of the resorcinol–formaldehyde (RF) hydrogels can be tailored to specific needs. The reaction consists of two steps, addition followed by condensation. The rate of both steps depends strongly on the synthesis conditions such as the nature of the catalyst, the pH of the initial solution, the temperature of synthesis, the resorcinol/formaldehyde and resorcinol/catalyst molar ratios, and the overall concentration. Heteroatoms present during the polymerization also give rise to significantly different structures [2]. The structure of the polymer gel and the corresponding carbon gel is strongly affected by the drying procedure employed [3]. Prior to drying, the water is often substituted with a more volatile ⇑ Corresponding author at: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France. Tel.: +33 4 38 88 19 33; fax: +33 4 76 88 27 08. E-mail address: [email protected] (O. Czakkel). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.07.008

solvent. Network deformation during drying is reduced by using substitute solvents with low surface tension. Three techniques are used for the subsequent drying: heat treatment in an inert atmosphere, freeze drying and extraction, most often with CO2 [4]. The use of CO2 extraction in preparing different kinds of aerogels is well established [4,5]. It is also widely used for preparing RF aerogels, but in spite of the large literature on these systems, to our knowledge no systematic study of the conditions of this process has been reported. The most widely used method is to exchange the CO2-compatible solvent (usually acetone) in the pores of the RF gels with liquid CO2 (liqCO2). This operation is often called supercritical extraction although during the extraction the CO2 is liquid. It is transformed to the supercritical state only immediately prior to the final, depressurizing step. The procedure can be performed in a batch-type process [1,6–8] or with a continuous flow of liquid CO2 [9–11]. To reduce structural damage during the depressurizing step, the temperature of the system is raised above the critical point of CO2, and the pressure is then slowly reduced. Besides scCO2, acetone [12,13] and ethanol [14] have also been reported as supercritical drying media for RF gels. The effect of liquid CO2 is of particular interest in RF gels. Liquid CO2 being a good solvent of the polymer, acts as a plasticizer [15], allowing the segments in the network chains to rearrange under the local elastic constraint. The balance between the pressuredependence of the solvent quality and the surface tension, which vanishes at the critical point, can therefore be expected to give rise to pore structures that depend in a complex way on the kinetics of

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42

drying. A comparison of the influence of liquid and true supercritical CO2 extraction on the morphology and texture of polymer RF aerogels is therefore of considerable practical relevance. In this paper we report the results of a systematic study of how these properties are affected by the various parameters of the supercritical procedure. 2. Experimental 2.1. Materials 2.1.1. Sample preparation The preparation of the monolithic RF hydrogels was based on the method of Lin and Ritter using sodium carbonate as the catalyst [16]. Samples were prepared in cylindrical tubes of diameter 5 mm, the reaction being left to proceed for 1 week at 358 K. Prior to drying the gel rods were placed in three times their own volume of acetone for 3 days, which was renewed with fresh solvent every day. A detailed description is given elsewhere [3]. 2.1.2. Supercritical drying Fig. 1 shows the experimental arrangement of the instrument constructed in our laboratory for the scCO2 drying. After the water to acetone solvent exchange the gel rods were transferred into the pressure cell along with excess acetone. The 15 cm3 pressure cell was thermostated throughout the whole process. The mass flow rate of CO2 was maintained in the range of 0.98–1.15 g/min in all experiments. The pressure was varied between 100 and 300 bar. The applied temperature range was 306–326 K, and the overall drying time was in the range 50– 250 min. The drying process consists of three steps: (i) pressurization, (ii) extraction at constant pressure with a fixed flow rate of CO2, (iii) slow decompression. A crucial point in the process is the third step. To avoid damage to the rods, the decompression rate was kept below 5 bar/min. 2.1.3. Drying with liquid CO2 For comparison, samples were also dried with liquid CO2 [3]. The reactor was filled with liquid CO2 at 80 bar and ambient temperature. Batch-type acetone to CO2 solvent exchange was performed by replacing the solvent in the extractor three times.

35

Prior to the final slow decompression step the temperature of the system was raised to 318 K (critical point of CO2: 304 K, 73.8 bar). 2.2. Methods 2.2.1. Scanning electron microscopy (SEM) Scanning electron micrographs were made with a JEOL 5500 (JEOL, Japan) electron microscope in high vacuum mode with a secondary electron detector. The accelerating voltage was 20 kV and the working distance varied between 5 and 27 mm. Samples were fastened to a copper sample holder by adhesive carbon tape. The samples were coated with Au/Pd to prevent charging. 2.2.2. Nitrogen adsorption Nitrogen adsorption/desorption isotherms were measured at 77 K with an Autosorb-1 (Quantachrome, USA) computer controlled volumetric gas adsorption apparatus. Pre-treatment of the samples was performed at 293 K and p < 3  104 mbar, for 24 h. The apparent surface area was calculated using the Brunauer– Emmett–Teller (BET) model (SBET). The total pore volume (VTOT) was derived from the amount of vapour adsorbed at relative pressure p/p0 ? 1, assuming that the pores are then filled with liquid adsorbate. The micropore volume (W0) was derived from the Dubinin–Radushkevich (DR) plot. The pore size distribution in the micropore region was computed from non-linear density function theory (NLDFT), using a NLDFT equilibrium model and assuming cylindrical/slit-type pore geometry. The size distribution of the mesopores was calculated from the desorption branch by the method of Barrett et al. [17]. Transformation of the primary adsorption data and the (micro)pore analysis were performed by the Quantachrome software. 2.2.3. Small and wide angle X-ray scattering (SAXS/WAXS) SAXS/WAXS measurements of the dry samples were made on the BM2 small angle camera in the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The transfer momentum range explored was 0.008 < q < 5.7 Å1, where q = 4p sin(h/2)/k, and k is the wavelength of the incident radiation. An indirect illumination CCD detector (Princeton Instruments) with effective pixel size 50 lm was used. Intensity curves, I(q), obtained by azimuthal averaging, were corrected for grid distortion, dark current, sample transmission and also for background scattering. The powdered samples were placed in glass capillary tubes of diameter 1.5 mm and heated to 110 °C for 24 h, which were then flame sealed [18]. Intensities were normalised with respect to a standard sample (Lupolen), assuming an effective sample thickness of 1 mm to account approximately for the filling factor of the powder. The resulting error in the stated absolute values of the intensity is expected to be no greater than 25%. All samples exhibited an extended region at 0.1 Å1 < q < 0.5 Å1 in which the SAXS intensity I(q) varies according to

IðqÞ ¼ Kq4 þ b;

ð1Þ

where K is the final slope of the scattering curves [19] and b is a qindependent scattering intensity due to local disorder of the molecular subunits. The internal X-ray surface area SX was estimated using the relationship

SX ¼

Fig. 1. Experimental setup for scCO2 drying. 1 – CO2 cylinder, 2 – high pressure pump with chilled pump head, 3 – pressurization valve, 4 – extractor, 5 – filters, 6 – back pressure control valve, 7 – water bath, 8 – separator, 9 – decompression valve (to atmospheric pressure), 10 – solvent absorption in water, P – manometer, T – thermocouple.

pK

V TOT ; Q ð1 þ V TOT dHe Þ

ð2Þ

where Q is the second moment of the SAXS response [20] and dHe is the helium density of the samples. To quantify the differences in scattering response, the elementary cluster units composing the networks were approximated as independent uniform solid spheres of external radius R [21]. The

36

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42

scattering intensity is thus defined by the structure factor of a single sphere,

h i2 Ssphere ðqÞ / 3ðsin qR  qR cos qRÞ=ðqRÞ3 :

ð3Þ

At large q, this function exhibits the q4 power law behaviour characteristic of smooth surfaces. The plot of Ssphere(q)  q4 vs. q is a characteristic oscillatory function with its first maximum at qR = 2.74 and first minimum at 4.49. In this representation, for a measured scattering intensity I(q), the intensity of the maxima is proportional to (4pR3/3)2NSsphere(q)  q4, where N is the number of spheres per unit volume. Since Ssphere(q) is a function of qR only, for otherwise identical samples having the same overall density (4pR3/3)N, but composed of spheres of different radii R, the intensity of the maxima is inversely proportional to R, i.e.,

IðqÞ  q4 / 1=R:

ð4Þ

3. Phase behaviour of the acetone–CO2 mixture To avoid collapse of the gel structure, the density of the fluid phase must be homogenous throughout the drying process. As shown in Fig. 2, the critical pressure of acetone–CO2 mixtures is much higher than that of CO2. Fig. 3 shows a detailed phase diagram of acetone and CO2, indicating the experimental points and the saturation line of pure CO2. Since liquid CO2 and liquid acetone form a completely miscible mixture, at the time of pressurization and solvent exchange of acetone to CO2 a homogenous phase prevails both in the extractor and within the pores, independently of the composition of the liquid phase. However, this is not always the case at a higher temperature. Thus, the degree of replacement of the acetone by CO2 as well as the selected pressure and temperature are all key parameters in the drying process. It is also important to note that at 313 K, 80 bar the phase is homogeneous if CO2 alone is present, i.e., solvent exchange is complete in all the pores. However, if small amounts of acetone remain in the pore structure, the mixture can quickly enter the two-phase region during the decompression step. According to literature [26–32], as well as modelling by Global Phase Equilibrium Calculations (GPEC), with increasing temperature, the maxi-

300

P3

P5

Vapor pressure line of CO2 Vapor pressure line of acetone Critical line C2 C1 Samples

250

p,bar

200

P2,P6

150

100

P7-14 P1 P4

50

0 270

320

370

420

470

520

T, K Fig. 2. Critical line of acetone–CO2 mixtures. Dashed and dotted curves are the gas– liquid phase boundaries of CO2 and acetone, respectively, ending at their critical points, C1 and C2. Solid circles denote the experimental conditions explored by the various samples. Green solid curve is the line of critical points of the mixtures. The diagram was obtained by the GPEC software [18], using Redlich–Kwong–PengRobinson Equation of State with van der Waals combination rules [19–21]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Drying steps with liqCO2 in the phase diagram of the CO2–acetone mixtures ( Δ ). Constructed from Refs. [22–28]. The arrow 1 ? 2 represents the pressurization step, arrow 2 ? 3 the solvent exchange process with CO2 followed by temperature increase (3 ? 4) above the critical point of the CO2. The final step is the decompression (4 ? 5). None of the lines may cross any phase boundary. The black dotted line is the gas/liquid phase boundary of CO2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

mum allowable acetone molar fraction x of the pores that maintains single phase behaviour during decompression also increases (at 306 K, x = 0.003, at 316 K, x = 0.01, at 326 K, x = 0.021). 4. Results and discussion 4.1. Supercritical experiments In the experimental arrangement of Fig. 1, the parameters most likely to be of key importance in influencing the structure of the dry gel are pressure, temperature and time. The range of these parameters studied, all in the supercritical region, was selected using a Box–Behnken 22 scheme [33]. The values of the experimental parameters explored are listed in Table 1. All the set p values were well above the critical pressure of the mixture at the selected temperatures (experimental setup conditions are also shown in Fig. 2). However, since temperature and pressure both affect the density and diffusivity of the mixtures, they may have an influence on the drying of the gels. Since both the mass flow rate of the CO2 and the amount of acetone were kept constant during the experiments, the CO2/acetone ratio is proportional to the extraction time, i.e., to the mass of CO2 used. However, in a different experimental setup this ratio can be used directly to calculate the requisite mass of drying CO2. 4.1.1. Effect of pressure and temperature The three different pressures and temperatures selected are listed in Table 1. The samples discussed in this section are designated as P1–P6. After drying, samples with short drying time still smelled of acetone, indicating that solvent exchange was incomplete. Shrinkage during the drying process was negligible. The low temperature N2 adsorption isotherms (Fig. 4) are of Type II, exhibiting a steep, narrow hysteresis loop. Owing to the high final slope of the isotherms, the calculated total pore volumes Vtot at p/p0 ? 1 are only estimations. The pore volume was also determined at p/p0 = 0.94, corresponding to a pore diameter of

37

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42 Table 1 Supercritical drying parameters. Sample

p (bar)

T (K)

CO2/ acetone ratio (g/ml)

Extraction time (min)

Decompression time (min)

Total drying timea (min)

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14

100 200 300 100 300 200 100 100 100 100 100 100 100 100

306 316 306 326 326 316 316 316 316 316 316 316 316 316

6.4 9.9 7.2 9.0 10.8 7.9 3.3 13.0 6.5 6.5 6.5 6.3 3.5 12.5

95 134 Ii94 164 103 116 50 200 100 100 100 100 50 200

20 20 20 20 20 20 20 30 40 20 40 40 40 40

150 245 250 225 235 230 98 258 184 142 167 157 184 256

a Total drying time includes pressurizing, extraction and the decompression steps.

4000

Adsorbedvolume,cm3/g(STP)

3500

100

3000

2500

2000

0.00

0.05

0.10

0.15

Adsorbedvolume,cm3/g(STP)

200

0 0.20

all the distributions span the range 10–20 Å with two or three maxima around 13 Å, 16 and 19 Å. The PSD curves in the mesopore region (Fig. 5b) overlap, except the slightly higher mesoporosity of the P2 and P4 samples in the lower mesopore region (d 6 50 Å). The steep final section of the isotherms indicates macroporosity. The N2 adsorption method by itself is not suited for macropore analysis, but the macroporosity (Vmacro, Table 2) was estimated from the difference between the pore volumes VTOT and V0.94. Table 2 shows that all the samples are highly macroporous. This is the pore size range that is most influenced by the pressure in supercritical conditions. Evaluation by ANOVA methods, based on the nitrogen adsorption results, reveal that the influence of the temperature on the porosity was insignificant. Furthermore, no coupling between the effects of T and p was detected either in the porosity or the surface area of the samples. The effect of pressure was investigated further by SAXS and WAXS observations on samples prepared at 100, 200 and 300 bar. To narrow the temperature range P11, P2 and P3 were measured. The overall structure of these samples is very similar (Fig. 6a). The slope of the steep region around q = 0.1 Å1, namely 3.7 ± 0.1, is sometimes interpreted in terms of scattering from a rough surface. In the resolution range 0.1 P q P 0.5 Å1, however, this scattering can be described satisfactorily by Eq. (1). The apparent roughness may therefore be attributed to the molecular disorder, represented by the constant term b. The surface scattering feature of the SAXS response is separated by a knee at q  0.05 Å1 from another power law region at lower q, where the slope changes to about 1.2. A plot of the same data in the representation Iq4 vs. q (see Section 2) is used in Fig. 6b to determine the size of the scattering units. The radius of the elementary beads depends on the pressure applied. In the sample dried at 100 bar the characteristic radius is 78 Å, while it is almost 92 Å at the higher pressures. The increase in size of the building blocks may be due to swelling of the polymer matrix in the CO2 diluent, in analogy with the well known effect of CO2 on conventional

p/p0

1500

Table 2 Porosity data of aerogels from N2 adsorption (Fig. 4) and from SAXS.a

1000

500

0 0.0

0.2

0.4

0.6

0.8

1.0

p/p0 Fig. 4. Low temperature N2 adsorption/desorption isotherms of the RF aerogels listed in Table 1 (P1: h, P2: , P3: , P4: , P5: , P6: ). Insert shows the region p/p0 < 0.2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

340 Å. In this article, pores within the 20–340 Å range were considered as mesopores. Below p/p0 = 0.3, the isotherms practically overlap, while in the higher relative pressure region the divergence gradually increases. The drying parameters hardly affect the BET surface area of the samples. Their average is 550 ± 27 m2/g, i.e., the standard deviation is less than 5%. The respective microporosity and mesoporosity of all the samples are closely similar (Table 2). However, small differences in the pore size distribution (PSD) are noticeable both in the micro- and mesopore domains (Fig. 5). In the micropore region

Sample SBET (m2/g)

SX VTOT (m2/g) (cm3/g)

V0.94 (cm3/g)

W0 Vmeso (cm3/g) (cm3/g)

Vmacro (cm3/g)

P1 P2 P3 P4 P5 P6

– 280 – – 323 –

0.72 0.72 0.77 0.71 0.75 0.66

0.22 0.20 0.21 0.21 0.21 0.20

2.11 2.90 4.93 1.01 3.67 0.73

578 533 577 541 562 511

2.83 3.62 5.70 1.72 4.42 1.39

0.50 0.52 0.56 0.50 0.54 0.46

a SBET: apparent surface area; SX: surface area from SAXS; VTOT: total pore volume at p/p0 ? 1; V0.94: pore volume at p/p0 = 0.94; W0: micropore volume from DR analysis; Vmeso = V0.94  W0; Vmacro = VTOT  V0.94.

Table 3 Porosity data of aerogels from SAXS and N2 adsorption isotherms of Fig. 7.a Sample SBET (m2/g)

SX VTOT (m2/g) (cm3/g)

V0.94 (cm3/g)

W0 Vmeso (cm3/g) (cm3/g)

Vmacro (cm3/g)

P7 P8 P9 P10 P11 P12 P13 P14

273 – – 300 280 – – –

0.93 0.84 0.85 0.87 0.87 0.78 0.75 0.90

0.20 0.18 0.19 0.21 0.19 0.18 0.29 0.24

0.66 2.15 0.80 1.30 1.10 0.91 1.13 1.02

663 580 591 660 607 568 779 638

1.59 2.99 1.65 2.17 1.97 1.69 1.87 1.92

0.73 0.66 0.66 0.66 0.68 0.60 0.46 0.66

a SBET: apparent surface area; SX: surface area from SAXS; VTOT: total pore volume at p/p0 ? 1; V0.94: pore volume at p/p0 = 0.94; W0: micropore volume from DR analysis; Vmeso = V0.94  W0; Vmacro = VTOT  V0.94.

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42

a

0.07

b

0.02

Dv(d),cm3/Å/g

38

0.01

dV(w),cc/Å/g

0.06 0.05 0.04 0.03 0.02 0.01 0.00

0.00

5

10

15

20

25

100

dBJH,Å

dDFT,Å Fig. 5. Pore size distributions of the RF aerogels listed in Table 1 by (a) NLDFT and (b) BJH methods of RF aerogels (P1: h, P2: of the references to colour in this figure legend, the reader is referred to the web version of this article.)

a

10

4

b

10

-3

10

-4

, P3:

, P4:

, P5:

, P6:

). (For interpretation

1000 slope-1.2

Iq4 (cm-1Å-4)

I(q) (cm-1)

100

10

1 slope-3.7±0.1

78Å

92Å

0.1

0.01

0.001

10 0.01

0.1

q

1

-5

0

0.02

(Å-1)

q

Fig. 6. (a) SAXS/WAXS response curves, and (b) Iq4  q representation of RF aerogels dried at 100 ( , P11) 200 ( references to colour in this figure legend, the reader is referred to the web version of this article.)

polymers [34–36]. The oscillations in Fig. 6b, together with the slope 1.2 at low q suggest that the beads are arranged in roughly linear sequences in the network. Absence of Bragg peaks the WAXS region (q P 1 Å1, partly shown here) indicates that in all cases the gel matrix is amorphous. We conclude that while the temperature has a limited effect, the pressure applied during the extraction step strongly modifies the total porosity, influencing particularly the macroporosity of the samples. Higher pressures favour greater macroporosity.

4.1.2. Effect of drying time Since the applied flow rate is constant, the drying time is proportional to the mass of CO2 used. In samples P7–P14 the effect of drying time was investigated at 316 K, 100 bar, with a constant rate of pressure change, 5 bar/min. Parameters are listed in Table 1. P7 and P10, for which the total drying times were the shortest, emitted a slight acetone odour. Shrinkage was again negligible.

0.04

0.06

0.08

0.1

(Å-1)

, P2) and 300 (

, P5) bar. (For interpretation of the

The N2 adsorption/desorption results (Fig. 7) are all practically identical, i.e., independent of the drying time. Significant differences appear only at p/p0 > 0.9. The porosity data are summarized in Table 3. The PSD curves in the micropore region (Fig. 8a) practically overlap, except for sample P7, which had the shortest drying time. In the mesopore region the distribution curves exhibit a peak around 30 Å, which is most pronounced in sample P7 (Fig. 8b). The drying time (i.e., the integrated flow of scCO2) affects the total pore volume, but not the mesopore and micropore volumes. The change in total pore volume arises from the macropore region, like the effect of pressure described above. Possible interactions between the extraction and decompression were also investigated: such coupling is insignificant for VTOT, Vmacro and W0, and weak for Vmeso and the apparent surface area SBET. The SAXS responses of the measured samples in Table 3 (not shown here) are similar to those in Fig. 6. A steep region at q  0.1 Å1 with slope 3.7 ± 0.1 once again mimics scattering from rough surfaces. In the lowest q region the slope (1.2) is iden-

39

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42

0.003

200

Adsorbed volume, cm3/g (STP)

P2

100

1000

0.00

0.05

0.10

0.15

-1 - 4

0.002

4

1500

Iq (cm Å )

Adsorbed volume, cm3/g (STP)

2000

0.001

0 0.20

p/p0

0

500

0

0.02

0.04

0.06

4

0.08

0.1

-4

q (Å ) Fig. 9. Surface scattering response from sample P2, in the representation I(q)q4 vs. q4.

0 0.0

0.2

0.4

0.6

0.8

1.0

p/p0 Fig. 7. Low temperature N2 adsorption/desorption isotherms of the RF aerogels listed in Table 1 (P7: j, P8: , P9: , P10: , P11: , P12: , P13: , P14: ). Insert shows the region p/p0 < 0.2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

tical to that observed in the case of the pressure effect. The size of the elementary beads in all these samples is 78 Å, i.e., independent of the drying time, which confirms their structural similarity on the micropore length scale. As described in Section 2, the surface area SX is calculated from the region of the SAXS spectra where the intensity I(q) is described by the relationship, drawn from Eq. (1) 4

IðqÞq4 ¼ K þ bq ;

ð5Þ

where K is the Porod final slope and b is a constant due to molecular disorder in the system. This linear behaviour, which validates Eq.

a

(1) and yields a well defined intercept K, is illustrated in Fig. 9 for sample P2. All the samples investigated displayed similar behaviour in the same q region. It is important to note that between 0.5 and 1 Å1 additional scattering is observed, such that Eqs. (1) and (5) no longer hold. This implies that, on the distance scale 2p/q  6–12 Å, the surfaces are not flat, a result that is predictable from the size of the bulky repeat units of the RF polymer. The values of the SAXS surface area SX (Tables 2 and 3) are all substantially smaller than (less than one half) those derived from nitrogen adsorption. This result is surprising, since for microporous systems such as activated carbons the reverse is generally true, i.e., SX > SBET, because SAXS tends to detect surfaces that are inaccessible to gas adsorption. In the present case, the discrepancy can be due to several contributory factors. Firstly, in a system where the characteristic size of the surface roughness due to the constituent molecular segments is situated in the range 6–12 Å, it is evident that more nitrogen molecules, of size 3.5 Å, can be accommodated than if the surface were flat. This point was already noted for RF gels dried with liquid CO2 [3], where the anomaly was attributed

b 0.02

0.07 0.06

Dv(d),ccÅ/g

Dv(d),ccÅ/g

0.05 0.04 0.03 0.02

0.01

0.01 0.00 5

10

15

dDFT,Å

20

25

0.00 20

100

dBJH,Å

Fig. 8. Pore size distributions of the aerogels listed in Table 1 by (a) NLDFT and (b) BJH methods of RF aerogels (P7: j, P8: , P9: (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

, P10:

, P11:

, P12:

, P13:

, P14:

).

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42

to anfractuosities in the pore walls that, at low q, are perceived as smooth in the X-ray response. A second factor is that the micropores are wide enough to accommodate more than two layers of nitrogen molecules. Such volume filling leads to an overestimate of SBET [18]. Lastly, the solid phase is not chemically homogeneous. During the gel synthesis it cannot be excluded that polar groups on the polymer surface are preferentially directed towards the water phase. As the electron density of polar groups is lower than that of the bulk polymer, the X-ray contrast at the polymer surface is diminished, with the consequence that the estimated value of SX is lower than if the electronic density of the material were uniform [37]. It can be concluded that the drying time also affects the structure of the aerogels, although its effect on the pore volume is smaller than that of pressure. Differences in surface area among the samples were greater than those found among samples P1–P6. 4.2. Liquid vs. supercritical CO2 extraction The properties of RF aerogels dried in liqCO2 (room temperature, 80 bar) are compared to the scCO2 dried sample P10. The diameter of the gel rods dried with liqCO2 shrank from 5 to 3 mm, while, as noted earlier, practically no shrinkage is observed with scCO2 extraction. The spherical polymer domains developing during the polycondensation reaction are conserved in both drying protocols (Fig. 10), but liqCO2 treatment results in a more compact large scale structure. The adsorption measurements (Fig. 11), and the derived numerical data (Table 4) show that in the supercritical case the looser structure results in a notably higher surface area, comparable to that of the freeze-dried gel [3]. The higher pore volumes in all categories also reflect the looser structure. For both samples the pore size distributions (PSDs) in the micropore range (Fig. 12a) are complex. Microporosity is much more developed in the scCO2 gel and appears in the range 12– 18 Å. The mesoporosity of this gel is also significantly higher, and the PSD in this range exhibits a maximum at 30 Å, which is absent in the other sample (Fig. 12b). In the SAXS response, the low q region of the curves reveals differences in the larger scale structure of the samples (Fig. 13). While the response curve of the supercritically extracted sample indicates linear arrays of interconnected particles, in the case of the liqCO2 extraction the elementary units are clustered together, forming large objects. At q  0.1 Å1 (Fig. 13a) the spectra display a power law of slope  3.87, i.e., the apparent surface roughness is very similar. As before, however, in the range 0.1 P q P 0.5 Å1, these samples also comply with the linear relationship equation (5). The size of the elementary beads calculated from Fig. 13b is more than three times larger in the liqCO2 sample than in scCO2. This increase in size may be the result of clustering.

1400

1200

Adsorbedvolume,cm3/g(STP)

40

1000

800

600

400

200

0.0

0.2

0.4

0.6

0.8

1.0

p/p0 Fig. 11. Low temperature N2 adsorption/desorption isotherms of polymer aerogels dried with liquid ( ) and supercritical CO2 ( ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

These results show that removing the acetone in true supercritical conditions yields polymer aerogels with significantly larger surface area and higher porosity. At the critical point the surface tension of CO2 disappears. In supercritical conditions, however, the drying temperature affects not only the phase equilibrium of the acetone–CO2 system, but also influences its density and diffusivity. The density of the mixture at 298 K is 0.81–0.85 g/cm3 [38]. From measurements performed in the ‘expanded solvent’ state of the acetone (acetone diluted with CO2) the diffusion coefficient was found to be 2–5 times higher in the supercritical regime than in the liquid state [39,40]. The latter was estimated by Eckert et al. to be 7  109 m2/s, at 293 K and 78 bar. This accelerated diffusion should also operate during the drying process of the RF gels, requiring in principle 2–5 times less time to complete the solvent exchange in the supercritical state than with liqCO2. As the gel rods have an open pore structure, however, the diffusion length within

Fig. 10. SEM images of the polymer aerogels dried with (a) liquid and (b) supercritical CO2 extraction (scale bar = 1 lm).

41

O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42 Table 4 Porosity data of aerogels from N2 adsorption and SAXS measurements.a Extraction medium

SBET (m2/g)

SX (m2/g)

VTOT (cm3/g)

V0.94 (cm3/g)

W0 (cm3/g)

Vmeso (cm3/g)

Vmacro (cm3/g)

liqCO2 scCO2

270 607

71 280

1.00 1.97

0.47 0.87

0.01 0.19

0.46 0.68

0.53 1.10

a SBET: apparent surface area; SX: surface area from SAXS; VTOT: total pore volume at p/p0 ? 1; V0.94: pore volume at p/p0 = 0.94; W0: micropore volume from DR analysis; Vmeso = V0.94  W0; Vmacro = VTOT  V0.94.

a

0.07

b

0.02

0.06

dV(d),cm3/Å/g

dV(w),cm3/Å/g

0.05 0.04 0.03

0.01

0.02 0.01 0.00

0.00 5

10

15

20

20

25

100

dBJH,Å

dDFT,Å

Fig. 12. Pore size distributions by (a) NLDFT and (b) BJH methods of aerogels dried with liquid ( ) and supercritical ( ) CO2 extraction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

a

10 10

5

b

10

-3

4

1000

4 -1 -4 Iq (cm Å )

78 Å

I(q (cm-1)

100 10 1 slope -3.87

10

-4

0.1 0.01 0.001 0.001

278 Å 10 0.01

0.1

1

10

-5 0

q(Å-1)

0.02

0.04

0.06

0.08

0.1

q(Å-1)

Fig. 13. SAXS response curves (a) and Iq4  q representation of the initial part of the SAXS spectrum (b) of aerogels dried with liquid ( ) and supercritical ( ) CO2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the gel is approximately equal to the radius of the rod, 2.5 mm. In supercritical mixtures of acetone and CO2 the temperature is higher and the density slightly lower, but these effects are more than compensated by the increase in the diffusion coefficient of the acetone, leading to a reduction by a factor between 2 and 5 of the drying time compared with liquid solvent exchange. Our results show that continuous supercritical drying is faster and more efficient than the liquid CO2 procedure. When supercritical drying is undertaken, the choice of pressure and temperature should be based on the phase equilibrium of the solvent mixture, which may, however, be affected by impurities dissolved from the sample.

5. Summary The comparison of the performance of liquid and supercritical CO2 extraction shows that scCO2 is more effective in conserving the porosity over the whole size range. However, even if supercritical conditions are maintained throughout the whole extraction process the duration of the extraction and the applied pressure affect the pore structure of the RF aerogels. The effect of the temperature is found to be insignificant. Above the critical point of CO2, the surface area and the micro- and mesopore content of the aerogels are independent of the drying parameters. By contrast, the total pore volume and macropore content are strongly affected.

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O. Czakkel et al. / Microporous and Mesoporous Materials 148 (2012) 34–42

Higher pressures favour higher porosity. Changing the extraction conditions modifies the total pore volume of the aerogels in the range 1.39–5.70 cm3/g, while the macropore volume varies between 1.01 and 4.93 cm3/g. The effect of the duration of the scCO2 extraction is less pronounced than that of pressure. In this case the total pore volume of the samples lies between 1.59 and 2.99 cm3/g, while the macropore content is in the range 0.66–2.15 cm3/g. The amount of micro- and mesopores is constant. The surface area SX of the networks measured by small angle X-ray scattering is substantially smaller than that measured by low temperature nitrogen adsorption. This is believed to be principally due to the surface roughness associated with the bulky repeat units of the RF polymer. Macroscopically, the gels dried with scCO2 exhibited no shrinkage. Microscopically, however, the structure of the two systems differs significantly. The size of the elementary building units was 3.5 times smaller in the case of the scCO2, while the porosity and the measured surface area was much higher, becoming comparable with the freeze-dried gels. Owing to the higher diffusion constant of the acetone in the acetone–scCO2 mixture, the time required to complete solvent exchange is estimated to be 2–5 times shorter in the supercritical regime than in the case of liqCO2. As an overall conclusion it can be stated that to increase the efficiency of ‘conventional’ CO2 drying of resorcinol–formaldehyde gels the parameters can be chosen freely (and can be used to fine-tune the desired structure of the gels), as long as they are above the critical point. Acknowledgements We are grateful to the European Synchrotron Radiation Facility for access to the French CRG beam line BM2 and to Sándor Kemény, Cyrille Rochas, György Bosznai and Bence Nagy for their invaluable assistance. O. Czakkel expresses gratitude for an Erasmus scholarship. This research was supported by the EU Grant No. 218138 and the EU–Hungarian Government joint fund GVOP-3.2.2-2004-070006/3.0. References [1] R.W. Pekala, J. Mater. Sci. 24 (1989) 3221–3227. [2] O. Czakkel, E. Geissler, A. Moussaïd, B. Koczka, K. László, Microporous Mesoporous Mater. 126 (2009) 213–221.

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