- Email: [email protected]
Resorcinol–formaldehyde aerogels prepared by supercritical acetone drying
Journal of Non-Crystalline Solids 271 (2000) 167±170
www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Resorcinol±formaldehyde aerogels prepared by supercritical acetone drying Changhai Liang *, Guangyan Sha, Shucai Guo Open Laboratory of Comprehensive Utilization for Carbon Resources, Institute of Coal Chemical Engineering, Dalian University of Technology, Dalian 116012, People's Republic of China Received 26 July 1999; received in revised form 21 September 1999
Abstract Preparation of resorcinol±formaldehyde (RF) aerogels by the aqueous polycondensation of resorcinol with formaldehyde and supercritical acetone drying is reported. A comparison of the basic physical properties between an RF aerogel from supercritical acetone drying and an RF aerogel from supercritical CO2 drying was carried out. It is found that the shrinkage and density of the RF aerogel from supercritical acetone drying are larger than those of the RF aerogel with supercritical CO2 drying. The experimental use of an initial nitrogen pressure shows that the gel texture can be preserved using supercritical acetone drying. Examination of the materials obtained by SEM, TEM and FT-infrared spectroscopy shows similar data to those of RF aerogel prepared by supercritical CO2 drying. Ó 2000 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Aerogels are unusual porous materials because of their unique microstructure consisting of pores and particles with nanometer size. Aerogels are typically made using sol±gel chemistry to form solvent-containing gels which are dried in a way that does not shrink or collapse the weak structure of the solid matrix. Supercritical drying is the most common because there are no liquid/gas interfaces during drying.
* Corresponding author. Present Address: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, People's Republic of China. Tel.: +86-411 4671991 726; fax: +86-411 4694447. E-mail address: [email protected] (C. Liang).
Since the ®rst aerogel was prepared by Kistler [1] in 1931, inorganic aerogels have been extensively studied in some ®elds, for optical, thermal, acoustic, electronic and catalytic applications [2± 6]. But organic aerogels based on the aqueous polycondensation of resorcinol with formaldehyde were ®rst reported by Pekala et al. [7,8] in 1987. Because these aerogels consist of cross-linked polymer and are thermosetting, they can be carbonized in an inert atmosphere to form carbon aerogels [9]. Organic and carbon aerogels have exhibited excellent properties in the ®eld of thermal, electronics, chemistry, etc. [7±12]. Therefore, it is interesting to exploit a low-cost and timesaving preparation method. Although the properties of organic aerogels have been widely investigated, the preparation of organic aerogels is limited to sol±gel processing of organic monomers with supercritical CO2 drying
0022-3093/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 1 0 8 - 3
168
C. Liang et al. / Journal of Non-Crystalline Solids 271 (2000) 167±170
[7±12] which is energy-saving, safe and environmentally benign. However, one of the disadvantages with supercritical CO2 drying is that it needs a long time to exchange between CO2 and solvent. (The preparation period of a resorcinol±formaldehyde (RF) aerogel takes at least two weeks; for CO2 exchange with solvent and drying, it requires 3±4 days [7].) Pajonk et al. [13] have reported a new method of preparing an RF aerogel through sol±gel processing in acetone with acid catalyst and then drying with supercritical CO2 . This method is less time-consuming than PekalaÕs method because of avoiding the acetone exchange with water, but the aerogel obtained is dierent from PekalaÕs aerogel in structure [8,13]. However, few papers are found on preparing organic aerogels and carbon aerogels by organic solvent supercritical drying. By comparison with supercritical CO2 drying, the high-temperature process is time-saving and also friendly to the environment if the process is reasonably designed.
2. Experimental RF gel preparation has been described in previous references [7±9]. Brie¯y, resorcinol and formaldehyde were mixed in deionized and distilled water at a molar ratio of 1:2. Sodium carbonate was then added as the basic catalyst. RF solutions were poured into glass vials sealed and cured for seven days at 85°C. Upon removal from their container, the gels were placed in a 5% acetic acid solution to increase the cross-link density through further condensation of hydroxymethyl groups. The aquagels were solvent-exchanged with acetone for four days to remove residual water
before supercritical drying. The solvent-®lled gels were placed in an autoclave with about 25 cm3 volume and added 15 cm3 acetone. The autoclave was taken above the critical point (235°C, 4.7 MPa). The resultant RF aerogels were dark red in color but still transparent. In addition, the acetone-®lled gel was exchanged for the lique®ed CO2 in the autoclave. After partial exchange of the acetone with CO2 , part of the acetone was drained, then the autoclave was re®lled with CO2 . The CO2 ®lled gel was dried at 45°C and 7.5 MPa (supercritical CO2 ). All mass and dimensional measurements were made as soon as the aerogels were taken out of the autoclave. BET surface area measurement was made using nitrogen adsorption (ST-03) with an error of below 10%. The morphology of the RF aerogel sample was examined with scanning electron microscopy and transmission electron microscopy. Pieces of the RF aerogel about 0.3 mm thick were analyzed by FT-IR spectroscopy to determine chemical composition.
3. Results and discussion Table 1 shows some representative data taken on materials from dierent preparation conditions. It is found that the drying process is a key step in the preparation of sol±gel derived materials. RF xerogel (sample 1) with bulk density of about 1.214 g cmÿ3 shrinks 74% in the radial direction during the conventional drying. RF aerogel (samples 2 and 3) with bulk density of about 0.058 and 0.146 g cmÿ3 only shrink 8% and 34% in the radial direction during the drying process and their surface area also increases from 176 to 718 and 716 m2 gÿ1 ,
Table 1 The preparation condition and corresponding density, speci®c surface area and shrinkage of materials Sample
Solids (%)
Drying condition
Theoretical density (g cmÿ3 )
Actual density (g cmÿ3 )
Surface area (m2 gÿ1 )
Radial shrinkage (%)
1 2 3 4
5 5 5 5
0.050 0.050 0.050 0.050
1.214 0.058 0.146 0.085
171 718 716 1148
74 8 34 14
5
4
Direct evaporation CO2 (7.5 MPa, 45°C) Acetone (5 MPa, 240°C) N2 (5 MPa) and acetone (22 MPa, 240°C) Acetone (6.5 MPa, 250°C)
0.040
0.073
819
15
C. Liang et al. / Journal of Non-Crystalline Solids 271 (2000) 167±170
respectively. At the same time, the dierent drying media result in dierent densities and degrees of shrinkage. It is clear that acetone supercritical drying increases shrinkage and density of the aerogel. The color of the aerogel from supercritical acetone drying is darker than that of the aerogel from supercritical CO2 drying. Therefore, this may be attributed to partial decomposition and condensation of the gel and solvation of solvent at high temperatures. An improvement of the aerogel drying method has been claimed by Mulder and van Lierop [14] who applied a prepressure of 80 bar of nitrogen prior to the heating step in the autoclave in order to avoid shrinkage. In the present work, we try to use this method to prepare an organic aerogel. It should be noted that the initial pressure of nitrogen has an eect on the properties of RF aerogels. Its actual density, speci®c surface area and shrinkage have to a great extent been improved by applying an initial nitrogen prepressure of 5 MPa and their values attain 0.085 g cmÿ3 , 1148 m2 gÿ1 and 14%, respectively. Thus, with further improvement it should be possible to produce organic gels without shrinkage. In addition, we have obtained the 4% solidcontaining gel. When the drying temperature and pressure were increased to 6.5 MPa and 250°C, the aerogel obtained showed a lower density of 0.073 g cmÿ3 , a higher speci®c surface area of 819 m2 gÿ1 and a shrinkage of 15% in the radial direction. The shrinkage for supercritical drying may be due to inecient solvent exchange and partial condensation or decomposition of RF gels during drying. But it should be complete for solvent exchange between acetone and water according to Refs. [7± 9]. We have observed that acetone obtained from the supercritical drying process is color-containing. The color in the acetone could result from solvation by acetone of some compounds of the RF gel. As shown in Fig. 1, the IR spectrum for sample 4 is similar to that for RF aerogel prepared by CO2 supercritical drying. IR absorption bands at 2942, 2874 and 1479 cmÿ1 were associated with CH2 stretching and scissor vibrations. The absorption band at 1614 cmÿ1 was assigned to aromatic ring stretching vibrations. The IR bands at 1222 and
169
1092 cmÿ1 were associated with C±O±C stretching vibrations of methylene ether bridges between resorcinol molecules. The broad band at above 3200 cmÿ1 was characteristic of OH stretching vibrations [9]. In Fig. 2, an SEM examination of sample 4 shows that the morphology of an aerogel prepared by supercritical acetone drying has an open-celled structure with continuous porosity and cell sizes less than 100 nm and the structure is homogenous and interconnected. TEM of several samples from
Fig. 1. IR spectrum of RF aerogel prepared from supercritical acetone drying.
Fig. 2. SEM of RF aerogel prepared from supercritical acetone drying.
170
C. Liang et al. / Journal of Non-Crystalline Solids 271 (2000) 167±170
acetone supercritical drying further show the solid phase is composed of particles with diameters about 20 nm and a typical pore size less than 100 nm [15,16]. This structure is analogous to that from supercritical CO2 drying, but the pore/cell size is larger than that prepared from supercritical CO2 drying, which was reported by Pekala [10]. In addition, Pajonk et al. [13] have reported that the RF aerogel is dierent from PekalaÕs aerogel in structure. The former exhibits a particular porous texture showing the presence of only micro- and macropores which is retained by the pyrolysed carbon material, while the pore size of PekalaÕs aerogel was non-macroporous (<50 nm). The reason results probably from dierent catalysts because the ®nal resin from acid catalysis should be a thermoplastic material in general, and that from base catalysis is thermosetting. In this paper, the RF aerogel is synthesized with base catalyst, so that its properties are dierent from Ref. [13] because of change in the nature of the catalyst. The pore size of the RF aerogel mainly distributes in the mesopore range, typical value of pore size is about 40 nm, while that of the aerogel of Pajonk et al. [13] only showed micro- and macropores.
are similar to those of the RF aerogel prepared by supercritical CO2 drying, but the pore/cell size is larger than that prepared from supercritical CO2 drying. Further work consists of studies on the in¯uence of supercritical drying conditions in detail and preparation of carbon aerogels derived from RF aerogels prepared by high-temperature supercritical drying. Acknowledgements This work was performed under the auspices of the Natural Sciences Foundation of China. The authors gratefully acknowledge Professor Jieshan Qiu for some bene®cial suggestions. References [1] [2] [3] [4] [5] [6] [7]
4. Conclusion The above data indicate that RF aerogels can be produced by supercritical acetone drying. A prepressure of initial nitrogen greatly improves performance by preserving the texture of the organic gel during the drying process. The suitable temperature and pressure of supercritical acetone drying can avoid the collapse of the organic gel structure during the drying process. The chemical composition and the morphology of the organic aerogel prepared by supercritical acetone drying
[8] [9] [10] [11] [12] [13] [14] [15] [16]
S.S. Kistler, Nature 127 (1931) 741. J. Fricke, J. Non-Cryst. Solids 147&148 (1992) 356. G.M. Pajonk, Catal. Today 35 (1997) 319. H.D. Gesser, P.C. Goswami, Chem. Rev. 89 (1989) 765. L.W. Hrubesh, J.F. Poco, J. Non-Cryst. Solids 188 (1995) 46. N. Husing, U. Schubert, Angew. Chem. Int. Ed. 37 (1998) 22. L.M. Hair, R.W. Pekala, R.E. Stone, C. Chen, S.L. Buckley, J. Vac. Sci. Technol. A 6 (1988) 2559. R.W. Pekala, US Patent 4873218, 1989. R.W. Pekala, R.M. Kong, Polym. Prepr. 30 (1989) 221. R.W. Pekala, D.W. Schaefer, Macromolecules 26 (1993) 5487. R.W. Pekala, J. Mater. Sci. 24 (1989) 3221. R.W. Pekala, C.T. Alviso, F.G. Kong, S.S. Hulsey, J. NonCryst. Solids 145 (1992) 90. G.M. Pajonk, A.V. Rao, N. Pinto, F.E. Dolle, M.B. Gil, Stud. Surf. Sci. Catal. 118 (1998) 167. C.A.M. Mulder, van Lierop, in: J. Fricker (Ed.), Aerogels, Springer, Berlin, 1986, p. 68. G. Qin, Doctoral Thesis, Dalian University of Technology, 1999. G. Qin, S. Guo, Carbon 37 (1999) 1168.
www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Resorcinol±formaldehyde aerogels prepared by supercritical acetone drying Changhai Liang *, Guangyan Sha, Shucai Guo Open Laboratory of Comprehensive Utilization for Carbon Resources, Institute of Coal Chemical Engineering, Dalian University of Technology, Dalian 116012, People's Republic of China Received 26 July 1999; received in revised form 21 September 1999
Abstract Preparation of resorcinol±formaldehyde (RF) aerogels by the aqueous polycondensation of resorcinol with formaldehyde and supercritical acetone drying is reported. A comparison of the basic physical properties between an RF aerogel from supercritical acetone drying and an RF aerogel from supercritical CO2 drying was carried out. It is found that the shrinkage and density of the RF aerogel from supercritical acetone drying are larger than those of the RF aerogel with supercritical CO2 drying. The experimental use of an initial nitrogen pressure shows that the gel texture can be preserved using supercritical acetone drying. Examination of the materials obtained by SEM, TEM and FT-infrared spectroscopy shows similar data to those of RF aerogel prepared by supercritical CO2 drying. Ó 2000 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Aerogels are unusual porous materials because of their unique microstructure consisting of pores and particles with nanometer size. Aerogels are typically made using sol±gel chemistry to form solvent-containing gels which are dried in a way that does not shrink or collapse the weak structure of the solid matrix. Supercritical drying is the most common because there are no liquid/gas interfaces during drying.
* Corresponding author. Present Address: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, People's Republic of China. Tel.: +86-411 4671991 726; fax: +86-411 4694447. E-mail address: [email protected] (C. Liang).
Since the ®rst aerogel was prepared by Kistler [1] in 1931, inorganic aerogels have been extensively studied in some ®elds, for optical, thermal, acoustic, electronic and catalytic applications [2± 6]. But organic aerogels based on the aqueous polycondensation of resorcinol with formaldehyde were ®rst reported by Pekala et al. [7,8] in 1987. Because these aerogels consist of cross-linked polymer and are thermosetting, they can be carbonized in an inert atmosphere to form carbon aerogels [9]. Organic and carbon aerogels have exhibited excellent properties in the ®eld of thermal, electronics, chemistry, etc. [7±12]. Therefore, it is interesting to exploit a low-cost and timesaving preparation method. Although the properties of organic aerogels have been widely investigated, the preparation of organic aerogels is limited to sol±gel processing of organic monomers with supercritical CO2 drying
0022-3093/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 1 0 8 - 3
168
C. Liang et al. / Journal of Non-Crystalline Solids 271 (2000) 167±170
[7±12] which is energy-saving, safe and environmentally benign. However, one of the disadvantages with supercritical CO2 drying is that it needs a long time to exchange between CO2 and solvent. (The preparation period of a resorcinol±formaldehyde (RF) aerogel takes at least two weeks; for CO2 exchange with solvent and drying, it requires 3±4 days [7].) Pajonk et al. [13] have reported a new method of preparing an RF aerogel through sol±gel processing in acetone with acid catalyst and then drying with supercritical CO2 . This method is less time-consuming than PekalaÕs method because of avoiding the acetone exchange with water, but the aerogel obtained is dierent from PekalaÕs aerogel in structure [8,13]. However, few papers are found on preparing organic aerogels and carbon aerogels by organic solvent supercritical drying. By comparison with supercritical CO2 drying, the high-temperature process is time-saving and also friendly to the environment if the process is reasonably designed.
2. Experimental RF gel preparation has been described in previous references [7±9]. Brie¯y, resorcinol and formaldehyde were mixed in deionized and distilled water at a molar ratio of 1:2. Sodium carbonate was then added as the basic catalyst. RF solutions were poured into glass vials sealed and cured for seven days at 85°C. Upon removal from their container, the gels were placed in a 5% acetic acid solution to increase the cross-link density through further condensation of hydroxymethyl groups. The aquagels were solvent-exchanged with acetone for four days to remove residual water
before supercritical drying. The solvent-®lled gels were placed in an autoclave with about 25 cm3 volume and added 15 cm3 acetone. The autoclave was taken above the critical point (235°C, 4.7 MPa). The resultant RF aerogels were dark red in color but still transparent. In addition, the acetone-®lled gel was exchanged for the lique®ed CO2 in the autoclave. After partial exchange of the acetone with CO2 , part of the acetone was drained, then the autoclave was re®lled with CO2 . The CO2 ®lled gel was dried at 45°C and 7.5 MPa (supercritical CO2 ). All mass and dimensional measurements were made as soon as the aerogels were taken out of the autoclave. BET surface area measurement was made using nitrogen adsorption (ST-03) with an error of below 10%. The morphology of the RF aerogel sample was examined with scanning electron microscopy and transmission electron microscopy. Pieces of the RF aerogel about 0.3 mm thick were analyzed by FT-IR spectroscopy to determine chemical composition.
3. Results and discussion Table 1 shows some representative data taken on materials from dierent preparation conditions. It is found that the drying process is a key step in the preparation of sol±gel derived materials. RF xerogel (sample 1) with bulk density of about 1.214 g cmÿ3 shrinks 74% in the radial direction during the conventional drying. RF aerogel (samples 2 and 3) with bulk density of about 0.058 and 0.146 g cmÿ3 only shrink 8% and 34% in the radial direction during the drying process and their surface area also increases from 176 to 718 and 716 m2 gÿ1 ,
Table 1 The preparation condition and corresponding density, speci®c surface area and shrinkage of materials Sample
Solids (%)
Drying condition
Theoretical density (g cmÿ3 )
Actual density (g cmÿ3 )
Surface area (m2 gÿ1 )
Radial shrinkage (%)
1 2 3 4
5 5 5 5
0.050 0.050 0.050 0.050
1.214 0.058 0.146 0.085
171 718 716 1148
74 8 34 14
5
4
Direct evaporation CO2 (7.5 MPa, 45°C) Acetone (5 MPa, 240°C) N2 (5 MPa) and acetone (22 MPa, 240°C) Acetone (6.5 MPa, 250°C)
0.040
0.073
819
15
C. Liang et al. / Journal of Non-Crystalline Solids 271 (2000) 167±170
respectively. At the same time, the dierent drying media result in dierent densities and degrees of shrinkage. It is clear that acetone supercritical drying increases shrinkage and density of the aerogel. The color of the aerogel from supercritical acetone drying is darker than that of the aerogel from supercritical CO2 drying. Therefore, this may be attributed to partial decomposition and condensation of the gel and solvation of solvent at high temperatures. An improvement of the aerogel drying method has been claimed by Mulder and van Lierop [14] who applied a prepressure of 80 bar of nitrogen prior to the heating step in the autoclave in order to avoid shrinkage. In the present work, we try to use this method to prepare an organic aerogel. It should be noted that the initial pressure of nitrogen has an eect on the properties of RF aerogels. Its actual density, speci®c surface area and shrinkage have to a great extent been improved by applying an initial nitrogen prepressure of 5 MPa and their values attain 0.085 g cmÿ3 , 1148 m2 gÿ1 and 14%, respectively. Thus, with further improvement it should be possible to produce organic gels without shrinkage. In addition, we have obtained the 4% solidcontaining gel. When the drying temperature and pressure were increased to 6.5 MPa and 250°C, the aerogel obtained showed a lower density of 0.073 g cmÿ3 , a higher speci®c surface area of 819 m2 gÿ1 and a shrinkage of 15% in the radial direction. The shrinkage for supercritical drying may be due to inecient solvent exchange and partial condensation or decomposition of RF gels during drying. But it should be complete for solvent exchange between acetone and water according to Refs. [7± 9]. We have observed that acetone obtained from the supercritical drying process is color-containing. The color in the acetone could result from solvation by acetone of some compounds of the RF gel. As shown in Fig. 1, the IR spectrum for sample 4 is similar to that for RF aerogel prepared by CO2 supercritical drying. IR absorption bands at 2942, 2874 and 1479 cmÿ1 were associated with CH2 stretching and scissor vibrations. The absorption band at 1614 cmÿ1 was assigned to aromatic ring stretching vibrations. The IR bands at 1222 and
169
1092 cmÿ1 were associated with C±O±C stretching vibrations of methylene ether bridges between resorcinol molecules. The broad band at above 3200 cmÿ1 was characteristic of OH stretching vibrations [9]. In Fig. 2, an SEM examination of sample 4 shows that the morphology of an aerogel prepared by supercritical acetone drying has an open-celled structure with continuous porosity and cell sizes less than 100 nm and the structure is homogenous and interconnected. TEM of several samples from
Fig. 1. IR spectrum of RF aerogel prepared from supercritical acetone drying.
Fig. 2. SEM of RF aerogel prepared from supercritical acetone drying.
170
C. Liang et al. / Journal of Non-Crystalline Solids 271 (2000) 167±170
acetone supercritical drying further show the solid phase is composed of particles with diameters about 20 nm and a typical pore size less than 100 nm [15,16]. This structure is analogous to that from supercritical CO2 drying, but the pore/cell size is larger than that prepared from supercritical CO2 drying, which was reported by Pekala [10]. In addition, Pajonk et al. [13] have reported that the RF aerogel is dierent from PekalaÕs aerogel in structure. The former exhibits a particular porous texture showing the presence of only micro- and macropores which is retained by the pyrolysed carbon material, while the pore size of PekalaÕs aerogel was non-macroporous (<50 nm). The reason results probably from dierent catalysts because the ®nal resin from acid catalysis should be a thermoplastic material in general, and that from base catalysis is thermosetting. In this paper, the RF aerogel is synthesized with base catalyst, so that its properties are dierent from Ref. [13] because of change in the nature of the catalyst. The pore size of the RF aerogel mainly distributes in the mesopore range, typical value of pore size is about 40 nm, while that of the aerogel of Pajonk et al. [13] only showed micro- and macropores.
are similar to those of the RF aerogel prepared by supercritical CO2 drying, but the pore/cell size is larger than that prepared from supercritical CO2 drying. Further work consists of studies on the in¯uence of supercritical drying conditions in detail and preparation of carbon aerogels derived from RF aerogels prepared by high-temperature supercritical drying. Acknowledgements This work was performed under the auspices of the Natural Sciences Foundation of China. The authors gratefully acknowledge Professor Jieshan Qiu for some bene®cial suggestions. References [1] [2] [3] [4] [5] [6] [7]
4. Conclusion The above data indicate that RF aerogels can be produced by supercritical acetone drying. A prepressure of initial nitrogen greatly improves performance by preserving the texture of the organic gel during the drying process. The suitable temperature and pressure of supercritical acetone drying can avoid the collapse of the organic gel structure during the drying process. The chemical composition and the morphology of the organic aerogel prepared by supercritical acetone drying
[8] [9] [10] [11] [12] [13] [14] [15] [16]
S.S. Kistler, Nature 127 (1931) 741. J. Fricke, J. Non-Cryst. Solids 147&148 (1992) 356. G.M. Pajonk, Catal. Today 35 (1997) 319. H.D. Gesser, P.C. Goswami, Chem. Rev. 89 (1989) 765. L.W. Hrubesh, J.F. Poco, J. Non-Cryst. Solids 188 (1995) 46. N. Husing, U. Schubert, Angew. Chem. Int. Ed. 37 (1998) 22. L.M. Hair, R.W. Pekala, R.E. Stone, C. Chen, S.L. Buckley, J. Vac. Sci. Technol. A 6 (1988) 2559. R.W. Pekala, US Patent 4873218, 1989. R.W. Pekala, R.M. Kong, Polym. Prepr. 30 (1989) 221. R.W. Pekala, D.W. Schaefer, Macromolecules 26 (1993) 5487. R.W. Pekala, J. Mater. Sci. 24 (1989) 3221. R.W. Pekala, C.T. Alviso, F.G. Kong, S.S. Hulsey, J. NonCryst. Solids 145 (1992) 90. G.M. Pajonk, A.V. Rao, N. Pinto, F.E. Dolle, M.B. Gil, Stud. Surf. Sci. Catal. 118 (1998) 167. C.A.M. Mulder, van Lierop, in: J. Fricker (Ed.), Aerogels, Springer, Berlin, 1986, p. 68. G. Qin, Doctoral Thesis, Dalian University of Technology, 1999. G. Qin, S. Guo, Carbon 37 (1999) 1168.