- Email: [email protected]
Effect of hydrothermal treatments on adsorption properties and structural characteristics of carbon-silica adsorbents (carbosils)
MATERIALS CHEM;~T$V?isND ELSEVIER
Materials
Chemistry
and Physics
39 (1994) 136-141
Effect of hydrothermal treatments on adsorption properties and structural characteristics of carbon-silica adsorbents (carbosils) R. Leboda a, V.V. Sidorchuk b, J. Skubiszewska-Ziqba
a
aFaculty of Chemishy, Maria Curie-Skiodowska b Institute of Surface Chemistry,
University 20031 Lublitq Poland Ukrainian Academy of Science, 252028 Kiev, Ukraine
Received 16 February 1994; accepted 20 July 1994
Abstract The paper presents the effects of hydrothermal treatment of complex carbon-silica adsorbents (carbosils) on their adsorption properties and porous structures. The treatment was conducted under various conditions: temperatures ranged from 100 to 300 “C and the duration from 5 to 6 h. Water and 5% peroxide solution were used as active media in the modification process. The carbosils contained 5-36 wt.% carbon. In the process of hydrothermal treatment of the carbosils, new materials were obtained, characterized by increased activity in relation to the primary unmodified carbosils in the process of physical and chemical adsorption. Hydrogen peroxide was a more active medium for the modification of the chemical and geometrical structure of the carbosils. Keywords: Carbosils; Hydrothermal
treatments; Adsorption
1. Introduction
2. Experimental
Data concerning the synthesis, physico-chemical properties and thermal modification methods of a new type of inorganic carbon-mineral adsorbent have been presented previously [1,2], while some properties of highly dispersed nonporous powders of such adsorbents prepared from pyrogenic silica-aerosil have also been discussed [3]. As already known [4,5], hydrothermal treatment (HTT) is one of the most effective methods for regulating the surface properties of various silicas. In the previous papers the possibility of improving the surface properties of carbon+ilica adsorbents has been shown. This involves taking advantage of HTT processes either at high temperature (1073 K) under superatmospheric pressure [6], or at moderate temperature (250 “C) under a high reagents pressure in the autoclave [7]. The purpose of this paper is to show a wider range of possibilities for regulating the physical properties of complex adsorbents (their porous structure) as well as the chemical character and structure of their adsorption centers. These processes open up many possibilities for using complex adsorbents in technological applications and in scientific research [1,2,8-111.
One of the carbosils was the adsorbent prepared from silica gel, Kieselgel 60, by pyrolysis of n-octanol on its surface. The reaction proceeded in a steel autoclave (1 1) at 500 “C for over 6 h. For 60 g of silica gel 54.5 ml of alcohol were used. Tarry products not firmly bound to the surface of complex adsorbent were removed by extraction withiV,N-dimethylformamide and acetone in a Soxhlet apparatus. Then the prepared carbosil was treated hydrothermally for 6 h at 250 “C. Both the initial and the hydrothermally modified carbosils were silanized, either with toluene hexamethylor with trimethylchlorsilane disilazane (HMDS) (TMCS) solution. The reaction proceeded in a Teflona autoclave for over 12 h at 200 “C. Under these conditions the adsorbents were silanized first with HMDS and then with TMCS. In all samples prepared the heat and adsorption isotherms of n-hexane and chloroform were examined by the standard chromatographic method [12]. The isotherms were measured with a Mera Elwro (Poland) N-504 gas chromatograph at 160 “C. The test mixture of ketones was separated by gas chromatography on selected modified carbosils. Carbosils containing 5-36 wt.% pyrogenic carbon were prepared according to the procedure presented in Ref.
0254-0584/94/$07.00 0 1994 Elsevier Science S.A. All rights reserved SSDI 0254-0584(94)01421-C
R. Leboda
et al. I Materials
Chemistry
[3] by pyrolysis of methylene chloride at 500 “C in a nitrogen stream in a dynamic reactor [13]. All of these carbosils were subjected to hydrothermal treatment with pure water or 5% hydrogen peroxide at temperatures in the range loo-350 “C for 5 h and then modified with n-butanol in an autoclave at 250 “C for 5 h. All hydrothermal treatment processes were performed in an autoclave under saturated water vapour conditions corresponding to given temperatures [14]. The minimum amounts of water needed for formation of saturated vapour were calculated as described earlier [15]. Adsorption isotherms of ammonia and water vapour were measured gravimetrically at 20 “C for all samples. The specific surface area, SBET, and pore volume V, in all carbosils were calculated from low-temperature nitrogen adsorption isotherms measured on a Carlo Erba Sorptomat 1800 apparatus. The average pore radius was calculated from the formula R=2V,/S,,,, assuming a cylindrical shape for the pores. The pyrocarbon content in the samples and the content of bonded organic groups were determined on a Hewlett-Packard 185 CHN apparatus, as well as by differential thermoanalysis on a Paulik (Hungary) Q-1500 D derivatograph. The surface concentration of bonded organic groups was calculated from Berendsen and Galen’s formula [16]:
(Y(mol m-‘)=
106P, 12oon, -P&4
137
and Physics 39 (1994) 136-141
penetration even for small HF molecules. At high temperatures such deposits undergo activation owing to the action of water vapours [6], in a way similar to that of typical active carbons. From the above considerations the carbosils prepared by pyrolysis of alcohols should undergo either hydrothermal or chemical treatment more easily. The hydrothermal modification processes described here were performed under moderate conditions in order not to cause sudden crystallographic changes in the amorphous structure of the silica skeleton and to avoid total loss of carbon deposit. In hydrothermal treatment of carbosils and silica (water steam), a decrease in the specific surface area with increasing temperature is generally observed (Figs. 1 and 2). Regarding carboaerosils, the higher their pyrocarbon content, the smaller the S,,, area decrease at a given temperature (Fig. 1). Thus, in carbosil containing 36 wt.% pyrocarbon, the specific surface area practically does not change at 350 “C. A comparison of the course of respective curves in Figs. 1 and 2 shows the effect of a stronger oxidizer (H,O,) on the process of hydrothermal modification of carbosils con-
1 - 1) ( s,,,
)
(1)
where PC denotes the percentage carbon content, nc the number of carbon atoms in a bound organic molecule, S,,, the adsorbent specific surface area, and M the molecular weight of the organic molecule. I
3. Results
CH,Cl, - ‘cmp’ C + 2HCl
200
(2)
it results that such an adsorbent may consist mainly of carbon forming agglomerates of different shapes, including globules [ 19,201. Such deposits adhere strongly to the surface of the modified adsorbent, so that in the case of full coverage this surface is resistant to
Fig. 1. Dependence of the specific surface the temperature of hydrothermal treatment (2) carboaerosil 5 wt.% C; (3) carboacrosil aerosil 36 wt.% C.
I
100
200
400
300
, in
temperature
and discussion
Complex adsorbents were prepared by pyrolysis of two different substances, i.e., n-octanol and methylene chloride, on the silica surface. From earlier investigations it was known that carbon deposits obtained by pyrolysis of these substances differ in either their chemical or physical structure [13,17-201. The pyrolysis products of n-octanol have a ‘polymeric’ carbon nature [17,18], characterized by a high degree of hydrogenation, and they form the uniform layers on the silica surface [20], whereas the carbon deposit formed from CH,Cl, is compact and hard [l&19]. From the stoichiometry of the reaction
I
1
100
“C
area of carboaerosils on with water: (1) aerosil; 19.6 wt.% C: (4) carbo-
300
temperature,
400
fn“C
Fig. 2. Dependence of the specific surface area of carboaerosils the temperature of hydrothermal treatment with a 5% solution hydrogen peroxide: (1) carboaerosil 5 wt.% C; (2) carboaerosil wt.% C; (3) carboaerosil 36 wt.% C.
on of 19
R. Leboda
et al. I Materials
Chemistty
and Physics 39 (1994) 134-141
taining different pyrocarbon amounts. In hydrothermal treatment in vapours of a 5% water solution of hydrogen peroxide (Fig. 2), for samples containing 5 and 19.6 wt.% carbon; the rate of S,,, decrease is higher than in the case of the analogous sample treated with pure water. This difference may result from the fact that hydrothermal treatment in oxidizing atmosphere always leads to liberation of a large surface area from carbon and, as a consequence, to intensification of silica transport during depol~erization and recondensation. As already known [4,5], reactions of this type are the basis of the hydrothermal modification of silica gels. In the sample with a high carbon content (36 wt.%), the specific surface area is observed to increase above 150 “C (Fig. 2). This may be due to microporosi~ developed in a relatively thick layer of carbon deposited on the surface of aerosil particles (according to data [3], the carbon layer is about 40 A thick). Similar effects were observed earlier [6], but in that study a high temperature (1075 K) was used in the hydrothermal process. As indicated by the data in Tables 1 and 2, the carbon content in the samples distinctly decreases with increasing temperature, and, as expected, it decreases even more strongly when the sample is treated with hydrogen peroxide. Such gasification of carbon under the influence of oxidizers (water or hydrogen peroxide) finally results not only in geometrical modification and morphological changes in the carbon layer deposited on the silica surface, but also, as shown, in an increased concentration of active adsorption centers on the surface, which is an essential change in its chemical structure. The consequence of these changes is increased activity of hydrothe~ally treated carbosils in physical and chemical adsorption. Thus, for example, for the carbosil prepared from silica gel after its hydrothermal modification (with water), the concentration of chemically bonded trimethylsilyl groups increases from 3.46 to 7.25 pmol m-2 (Table 1). These values refer to the total number of such groups coming from HMDS and TMCS subsequently bonded with the surface of the adsorbents compared, i.e., (3.13+0.33) and (5.40 + 1.85) pmol mB2, respectively. For carboaerosils modified with water and a 5% water solution of hydrogen peroxide, the concentration of surface-ended butoxy groups reaches 8.45 Fmol mm2 at a 16 wt.% carbon content in the sample (Table 2). At smaller and bigger carbon contents in the aerosilogel the concentration of bonded groups decreases; however, it may also exceed that of free silanol groups on the surface of maximally hydro~lated silicas: 4.6-4.8 pmol mm2 [4,12,22]. As the content of oxygen-containing functional groups on the carbon surface (also oxidized) does not exceed this value [23,24], it is of course the surface structure of carbosils, particularly of those treated hydrothermally, that causes the occurrence of such abnormally high concentrations of bonded organic groups. It is possible
R. Leboda Table 2 Effect of hydrothermal No.
treatment
Adsorbent,
on surface
conditions
et al. / Materials
properties
Chemistry and Physics 39 (1994) 136-141
139
of carboaerosils
of modification
C (wt.%)
&lET (m’ g-‘)
I II III IV V VI VII VIII IX X XI XII XIII XIV xv XVI XVII XVIII XIX XX XXI XXII
Aerosil (A) Adsorbent I modified with C4HpOH/250 “C Adsorbent I modified with HZ0/250 “C (HIT) Adsorbent III modified with C,H,OH/250 “C Carbosil 5 wt.% C (C-5%) Adsorbent V modified with C4H90H/250 “C Adsorbent V modified with 5% H20J100 “C (H’IT) Adsorbent VII modified with C4H90H/250 “C Adsorbent V modified with H,0/250 “C (HIT) Adsorbent IX modified with C4H90H/250 “C Adsorbent V modified with H,0/350 “C (HIT) Adsorbent XI modified with C4H90H/250 “C Carbosil 19.6 wt.% C (C-19.6%) Adsorbent XIII modified with C4H90H/250 “C Adsorbent XIII modified with Ha0/350 “C (HTT) Adsorbent XV modified with C4H90H/250 “C Carbosil 36 wt.% C (C-36%) Adsorbent XVII modified with C4H90H/250 “C Adsorbent XVII modified with 5% Hz02/250 “C (HIT) Adsorbent XIX modified with C4H90H/250 “C Adsorbent XVII modified with H,0/350 “C (HIT) Adsorbent XXI modified with C4H,0H/250 “C
Note: In the parentheses are given the concentrations the reaction with hexamethyldisilazane [4].
295 203 161 101 236 196 202 178 151 91 71 60 188 180 111 93 176 172 210 190 177 161
of free OH groups
that for such complex adsorbents synergism of their properties is revealed. However, this problem requires further investigation. Figs. 3 and 4 present absolute adsorption isotherms of n-hexane and chloroform on pure silica gel, on nonmodified carbosil prepared on its basis, and on this carbosil hydrothermally modified at 250 “C. The adsorption heats of the adsorbates mentioned on these
(3.43) 3.88 (4.58) 4.88 _
5.0 4.9 3.2 3.8 4.0 3.9 3.1 3.0 19.6 19.4 16.3 16.1 36.0 35.8 22.3 22.0 30.7 30.4
on the silica surface,
determined
2.10 6.40 5.27 7.30 0.15 8.45
_ 3.70 1.52
on the basis of the results
t
Fig. 4. Absolute adsorption the same as in Fig. 3.
atm
Fig. 3. Absolute adsorption isotherms of n-hexane on (1) silica gel, (2) carbosil prepared on the basis of this silica gel, and (3) the same carbosils having undergone hydrothermal treatment with water at 250 “C.
of
3
pressure,
pressure,
Tprnol m-‘)
isotherms
atm
of chloroform.
Symbols
are
adsorbents are given in Table 1. The data indicate that a sharp increase in the physical adsorption (and an appropriate increase in the adsorption heats) of both pairs of substances occurs on hydrothermally treated carbosils. The same effects for carboaerosil gels treated hydrothermally with water and 5% H,O, water solution can be seen in Figs. 5 and 6. The adsorbates here are ammonium and water vapours. This also confirms the formation of functional groups with an acidic character on their surfaces.
140
R. Leboda
et al. i Materials
Chemistry
and Physics 39 (1994) 136-141
heterogenei~ (of polar surface energy centers) - on hydrothermally treated carbosils in comparison with analyses made on silica gels and nonmodified carbosils.
21 4. Conclusions
I
03
0.2
I
I
0.3
0,4
retotive
0.5
0.6
0.1
0.8
pressure, p/ps
Fig. 5. Absolute adsorption isotherms of water vapour on (1) aerosil modified hydrothermally (HTT) with water at 250 “C, and on the following carboaerosils: (2) carboaerosil 5 wt.% C; (3) carboaerosil 5 wt.% C, I-ITT, H,Oz, 100 “C; (4) carboaerosil 19.6 wt.% C, HIT, HzOz, 350 “C.
Hydrothermal treatment of carbon-silica adsorbents (carbosils) with water or a hydrogen peroxide water solution leads to physical ~geometrical~ and chemical modification of the adsorbents. In this way new materials with properties quite different from those of the initial adsorbents are obtained. As a result of the changes occurring during modification, a sharp increase in the activity of treated carbosils in processes of physical and chemical adsorption takes place. Also, an improvement in the chromatographic properties of the modified adsorbents is observed. HIT processes do not require high temperatures, which is significant for mineral materials (constituting the base of carbon deposits), which undergo sintering at high temperatures. Using hydrogen peroxide as the modification medium, rather than normal water, makes it possible to obtain greater effects of physical and chemical modification, as with the former lower temperatures for the modification process are generally required.
Financial support from KBN, Project No. 205119101, is gratefully acknowledged.
6
References
4
113R. 121R.
2
0
20
40
60
80
100
pressure
120
140
160
180
200
, mmtlg
Fig. 6. Absolute adsorption isotherms of ammonium on (1) aerosii modified hydrothermally with water at 250 “C, and on the following carboaerosils: (2) carboaerosil 5 wt.% C, (3) carboaerosil 5 wt.% C, HIT, HzO, 250 “C, (4) carboaerosit 19.6 wt.% C, HIT, H,O, 350 “C.
Hence it is very likely that in hydrothermal treatment of carbosils, homogenization (increased homogeneity) of the surface occurs owing to more complete hy droxylation. Moreover, the kinetics of mass transport in adsorption/desorption processes in pores is improved, owing to their size increase. This observation has allowed us to obtain better chromatographic separations, e.g., a mixture of ketones - substances sensitive to energetic
Leboda, Mater. Chem. Phys, 31 (1992) 243, Leboda, Mater. Chem. Phys., 34 (1993) 123. V. Sidorczuk, V. Tertykh [31 R. Leboda, J. Skubiszewska-Zisba, and V. Zarko, J. Non-Ctyst. Solids, submitted for publication. [41 R. Iler, The Chemistty of Silica, Wiley, New York, 1979. PI R. Leboda and E. Mendyk, Mater. Chem. Phys., 27 (1991) 189. (61 A. Gierak, F. Czechowski and R. Leboda, Mater. Chem. Phys., 36 (1994) 264. 171 R. Leboda, A. Gierak, B. Charmas and A. todyga, React. Kinet. Catal. Lett., 50 (1993) 63. PI Yu.1. Tarasevich, Khimoa Tiechnd Wody, 9 (1989) 699. PI N. Sonobe, T. Kyotani and A. Tomita, Car&err, 29 (1991) 61. WI CA. Leon y Leon, A.W. Scaroni and L.R. Radovic, 1. Colloid Interfhce Sci., 148 (1992) 1. and H. Yoshida, J. Colloid Interface Sci., I59 IllI K. Kamegawa (1993) 324. WI A.V. Kiselev and Ya.1. Yashin, Gas Adsorption Chromatography, PWN, Warsaw, 1969. P31 R. Leboda, Chromatographia, 14 (1981) 524. P41 Physico-chemical Handbook (in Polish), WNT, Warsaw, 1974, p. 132. WI E. Mendyk, R. Leboda and A. Gierak, Mater. Chem. Phys., 31 (1992) 355.
R. Leboda [16] [17] [18] [19] [20]
et al. / Materials
Chemistry and Physics 39 (1994) 136-141
G.E. Berendsen and J. Galen, J. LI$. Chromatogr., 1 (1978) 561. R. Lehada, Chromatographia, I1 (1980) 703. R. Leboda, PO/. J. Chem., II (1980) 2305. A. Gierak and R. Leboda, Mater. Chem. Phys., I9 (1988) 503. A. Gierak, R. Leboda and E. Tracz, J. Anal. Appl. Pyrol., 13 (1988) 89.
[21] (221
[23] [24]
141
R. Leboda and A. Eodyga, J. Anal. Appl. Pyrol., I4 (1988) 203. C.J. Brinker and G.W. Scherer, Sol-Gel Science: ;Tlre Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990. I.A. Tarkovskaya, Okulennyj ugol’, Naukovaya dumka, Kiev, 1981, p. 197. HF. Boehm, High. Temp. - High Pressures, 22 (1090) 275.
Materials
Chemistry
and Physics
39 (1994) 136-141
Effect of hydrothermal treatments on adsorption properties and structural characteristics of carbon-silica adsorbents (carbosils) R. Leboda a, V.V. Sidorchuk b, J. Skubiszewska-Ziqba
a
aFaculty of Chemishy, Maria Curie-Skiodowska b Institute of Surface Chemistry,
University 20031 Lublitq Poland Ukrainian Academy of Science, 252028 Kiev, Ukraine
Received 16 February 1994; accepted 20 July 1994
Abstract The paper presents the effects of hydrothermal treatment of complex carbon-silica adsorbents (carbosils) on their adsorption properties and porous structures. The treatment was conducted under various conditions: temperatures ranged from 100 to 300 “C and the duration from 5 to 6 h. Water and 5% peroxide solution were used as active media in the modification process. The carbosils contained 5-36 wt.% carbon. In the process of hydrothermal treatment of the carbosils, new materials were obtained, characterized by increased activity in relation to the primary unmodified carbosils in the process of physical and chemical adsorption. Hydrogen peroxide was a more active medium for the modification of the chemical and geometrical structure of the carbosils. Keywords: Carbosils; Hydrothermal
treatments; Adsorption
1. Introduction
2. Experimental
Data concerning the synthesis, physico-chemical properties and thermal modification methods of a new type of inorganic carbon-mineral adsorbent have been presented previously [1,2], while some properties of highly dispersed nonporous powders of such adsorbents prepared from pyrogenic silica-aerosil have also been discussed [3]. As already known [4,5], hydrothermal treatment (HTT) is one of the most effective methods for regulating the surface properties of various silicas. In the previous papers the possibility of improving the surface properties of carbon+ilica adsorbents has been shown. This involves taking advantage of HTT processes either at high temperature (1073 K) under superatmospheric pressure [6], or at moderate temperature (250 “C) under a high reagents pressure in the autoclave [7]. The purpose of this paper is to show a wider range of possibilities for regulating the physical properties of complex adsorbents (their porous structure) as well as the chemical character and structure of their adsorption centers. These processes open up many possibilities for using complex adsorbents in technological applications and in scientific research [1,2,8-111.
One of the carbosils was the adsorbent prepared from silica gel, Kieselgel 60, by pyrolysis of n-octanol on its surface. The reaction proceeded in a steel autoclave (1 1) at 500 “C for over 6 h. For 60 g of silica gel 54.5 ml of alcohol were used. Tarry products not firmly bound to the surface of complex adsorbent were removed by extraction withiV,N-dimethylformamide and acetone in a Soxhlet apparatus. Then the prepared carbosil was treated hydrothermally for 6 h at 250 “C. Both the initial and the hydrothermally modified carbosils were silanized, either with toluene hexamethylor with trimethylchlorsilane disilazane (HMDS) (TMCS) solution. The reaction proceeded in a Teflona autoclave for over 12 h at 200 “C. Under these conditions the adsorbents were silanized first with HMDS and then with TMCS. In all samples prepared the heat and adsorption isotherms of n-hexane and chloroform were examined by the standard chromatographic method [12]. The isotherms were measured with a Mera Elwro (Poland) N-504 gas chromatograph at 160 “C. The test mixture of ketones was separated by gas chromatography on selected modified carbosils. Carbosils containing 5-36 wt.% pyrogenic carbon were prepared according to the procedure presented in Ref.
0254-0584/94/$07.00 0 1994 Elsevier Science S.A. All rights reserved SSDI 0254-0584(94)01421-C
R. Leboda
et al. I Materials
Chemistry
[3] by pyrolysis of methylene chloride at 500 “C in a nitrogen stream in a dynamic reactor [13]. All of these carbosils were subjected to hydrothermal treatment with pure water or 5% hydrogen peroxide at temperatures in the range loo-350 “C for 5 h and then modified with n-butanol in an autoclave at 250 “C for 5 h. All hydrothermal treatment processes were performed in an autoclave under saturated water vapour conditions corresponding to given temperatures [14]. The minimum amounts of water needed for formation of saturated vapour were calculated as described earlier [15]. Adsorption isotherms of ammonia and water vapour were measured gravimetrically at 20 “C for all samples. The specific surface area, SBET, and pore volume V, in all carbosils were calculated from low-temperature nitrogen adsorption isotherms measured on a Carlo Erba Sorptomat 1800 apparatus. The average pore radius was calculated from the formula R=2V,/S,,,, assuming a cylindrical shape for the pores. The pyrocarbon content in the samples and the content of bonded organic groups were determined on a Hewlett-Packard 185 CHN apparatus, as well as by differential thermoanalysis on a Paulik (Hungary) Q-1500 D derivatograph. The surface concentration of bonded organic groups was calculated from Berendsen and Galen’s formula [16]:
(Y(mol m-‘)=
106P, 12oon, -P&4
137
and Physics 39 (1994) 136-141
penetration even for small HF molecules. At high temperatures such deposits undergo activation owing to the action of water vapours [6], in a way similar to that of typical active carbons. From the above considerations the carbosils prepared by pyrolysis of alcohols should undergo either hydrothermal or chemical treatment more easily. The hydrothermal modification processes described here were performed under moderate conditions in order not to cause sudden crystallographic changes in the amorphous structure of the silica skeleton and to avoid total loss of carbon deposit. In hydrothermal treatment of carbosils and silica (water steam), a decrease in the specific surface area with increasing temperature is generally observed (Figs. 1 and 2). Regarding carboaerosils, the higher their pyrocarbon content, the smaller the S,,, area decrease at a given temperature (Fig. 1). Thus, in carbosil containing 36 wt.% pyrocarbon, the specific surface area practically does not change at 350 “C. A comparison of the course of respective curves in Figs. 1 and 2 shows the effect of a stronger oxidizer (H,O,) on the process of hydrothermal modification of carbosils con-
1 - 1) ( s,,,
)
(1)
where PC denotes the percentage carbon content, nc the number of carbon atoms in a bound organic molecule, S,,, the adsorbent specific surface area, and M the molecular weight of the organic molecule. I
3. Results
CH,Cl, - ‘cmp’ C + 2HCl
200
(2)
it results that such an adsorbent may consist mainly of carbon forming agglomerates of different shapes, including globules [ 19,201. Such deposits adhere strongly to the surface of the modified adsorbent, so that in the case of full coverage this surface is resistant to
Fig. 1. Dependence of the specific surface the temperature of hydrothermal treatment (2) carboaerosil 5 wt.% C; (3) carboacrosil aerosil 36 wt.% C.
I
100
200
400
300
, in
temperature
and discussion
Complex adsorbents were prepared by pyrolysis of two different substances, i.e., n-octanol and methylene chloride, on the silica surface. From earlier investigations it was known that carbon deposits obtained by pyrolysis of these substances differ in either their chemical or physical structure [13,17-201. The pyrolysis products of n-octanol have a ‘polymeric’ carbon nature [17,18], characterized by a high degree of hydrogenation, and they form the uniform layers on the silica surface [20], whereas the carbon deposit formed from CH,Cl, is compact and hard [l&19]. From the stoichiometry of the reaction
I
1
100
“C
area of carboaerosils on with water: (1) aerosil; 19.6 wt.% C: (4) carbo-
300
temperature,
400
fn“C
Fig. 2. Dependence of the specific surface area of carboaerosils the temperature of hydrothermal treatment with a 5% solution hydrogen peroxide: (1) carboaerosil 5 wt.% C; (2) carboaerosil wt.% C; (3) carboaerosil 36 wt.% C.
on of 19
R. Leboda
et al. I Materials
Chemistty
and Physics 39 (1994) 134-141
taining different pyrocarbon amounts. In hydrothermal treatment in vapours of a 5% water solution of hydrogen peroxide (Fig. 2), for samples containing 5 and 19.6 wt.% carbon; the rate of S,,, decrease is higher than in the case of the analogous sample treated with pure water. This difference may result from the fact that hydrothermal treatment in oxidizing atmosphere always leads to liberation of a large surface area from carbon and, as a consequence, to intensification of silica transport during depol~erization and recondensation. As already known [4,5], reactions of this type are the basis of the hydrothermal modification of silica gels. In the sample with a high carbon content (36 wt.%), the specific surface area is observed to increase above 150 “C (Fig. 2). This may be due to microporosi~ developed in a relatively thick layer of carbon deposited on the surface of aerosil particles (according to data [3], the carbon layer is about 40 A thick). Similar effects were observed earlier [6], but in that study a high temperature (1075 K) was used in the hydrothermal process. As indicated by the data in Tables 1 and 2, the carbon content in the samples distinctly decreases with increasing temperature, and, as expected, it decreases even more strongly when the sample is treated with hydrogen peroxide. Such gasification of carbon under the influence of oxidizers (water or hydrogen peroxide) finally results not only in geometrical modification and morphological changes in the carbon layer deposited on the silica surface, but also, as shown, in an increased concentration of active adsorption centers on the surface, which is an essential change in its chemical structure. The consequence of these changes is increased activity of hydrothe~ally treated carbosils in physical and chemical adsorption. Thus, for example, for the carbosil prepared from silica gel after its hydrothermal modification (with water), the concentration of chemically bonded trimethylsilyl groups increases from 3.46 to 7.25 pmol m-2 (Table 1). These values refer to the total number of such groups coming from HMDS and TMCS subsequently bonded with the surface of the adsorbents compared, i.e., (3.13+0.33) and (5.40 + 1.85) pmol mB2, respectively. For carboaerosils modified with water and a 5% water solution of hydrogen peroxide, the concentration of surface-ended butoxy groups reaches 8.45 Fmol mm2 at a 16 wt.% carbon content in the sample (Table 2). At smaller and bigger carbon contents in the aerosilogel the concentration of bonded groups decreases; however, it may also exceed that of free silanol groups on the surface of maximally hydro~lated silicas: 4.6-4.8 pmol mm2 [4,12,22]. As the content of oxygen-containing functional groups on the carbon surface (also oxidized) does not exceed this value [23,24], it is of course the surface structure of carbosils, particularly of those treated hydrothermally, that causes the occurrence of such abnormally high concentrations of bonded organic groups. It is possible
R. Leboda Table 2 Effect of hydrothermal No.
treatment
Adsorbent,
on surface
conditions
et al. / Materials
properties
Chemistry and Physics 39 (1994) 136-141
139
of carboaerosils
of modification
C (wt.%)
&lET (m’ g-‘)
I II III IV V VI VII VIII IX X XI XII XIII XIV xv XVI XVII XVIII XIX XX XXI XXII
Aerosil (A) Adsorbent I modified with C4HpOH/250 “C Adsorbent I modified with HZ0/250 “C (HIT) Adsorbent III modified with C,H,OH/250 “C Carbosil 5 wt.% C (C-5%) Adsorbent V modified with C4H90H/250 “C Adsorbent V modified with 5% H20J100 “C (H’IT) Adsorbent VII modified with C4H90H/250 “C Adsorbent V modified with H,0/250 “C (HIT) Adsorbent IX modified with C4H90H/250 “C Adsorbent V modified with H,0/350 “C (HIT) Adsorbent XI modified with C4H90H/250 “C Carbosil 19.6 wt.% C (C-19.6%) Adsorbent XIII modified with C4H90H/250 “C Adsorbent XIII modified with Ha0/350 “C (HTT) Adsorbent XV modified with C4H90H/250 “C Carbosil 36 wt.% C (C-36%) Adsorbent XVII modified with C4H90H/250 “C Adsorbent XVII modified with 5% Hz02/250 “C (HIT) Adsorbent XIX modified with C4H90H/250 “C Adsorbent XVII modified with H,0/350 “C (HIT) Adsorbent XXI modified with C4H,0H/250 “C
Note: In the parentheses are given the concentrations the reaction with hexamethyldisilazane [4].
295 203 161 101 236 196 202 178 151 91 71 60 188 180 111 93 176 172 210 190 177 161
of free OH groups
that for such complex adsorbents synergism of their properties is revealed. However, this problem requires further investigation. Figs. 3 and 4 present absolute adsorption isotherms of n-hexane and chloroform on pure silica gel, on nonmodified carbosil prepared on its basis, and on this carbosil hydrothermally modified at 250 “C. The adsorption heats of the adsorbates mentioned on these
(3.43) 3.88 (4.58) 4.88 _
5.0 4.9 3.2 3.8 4.0 3.9 3.1 3.0 19.6 19.4 16.3 16.1 36.0 35.8 22.3 22.0 30.7 30.4
on the silica surface,
determined
2.10 6.40 5.27 7.30 0.15 8.45
_ 3.70 1.52
on the basis of the results
t
Fig. 4. Absolute adsorption the same as in Fig. 3.
atm
Fig. 3. Absolute adsorption isotherms of n-hexane on (1) silica gel, (2) carbosil prepared on the basis of this silica gel, and (3) the same carbosils having undergone hydrothermal treatment with water at 250 “C.
of
3
pressure,
pressure,
Tprnol m-‘)
isotherms
atm
of chloroform.
Symbols
are
adsorbents are given in Table 1. The data indicate that a sharp increase in the physical adsorption (and an appropriate increase in the adsorption heats) of both pairs of substances occurs on hydrothermally treated carbosils. The same effects for carboaerosil gels treated hydrothermally with water and 5% H,O, water solution can be seen in Figs. 5 and 6. The adsorbates here are ammonium and water vapours. This also confirms the formation of functional groups with an acidic character on their surfaces.
140
R. Leboda
et al. i Materials
Chemistry
and Physics 39 (1994) 136-141
heterogenei~ (of polar surface energy centers) - on hydrothermally treated carbosils in comparison with analyses made on silica gels and nonmodified carbosils.
21 4. Conclusions
I
03
0.2
I
I
0.3
0,4
retotive
0.5
0.6
0.1
0.8
pressure, p/ps
Fig. 5. Absolute adsorption isotherms of water vapour on (1) aerosil modified hydrothermally (HTT) with water at 250 “C, and on the following carboaerosils: (2) carboaerosil 5 wt.% C; (3) carboaerosil 5 wt.% C, I-ITT, H,Oz, 100 “C; (4) carboaerosil 19.6 wt.% C, HIT, HzOz, 350 “C.
Hydrothermal treatment of carbon-silica adsorbents (carbosils) with water or a hydrogen peroxide water solution leads to physical ~geometrical~ and chemical modification of the adsorbents. In this way new materials with properties quite different from those of the initial adsorbents are obtained. As a result of the changes occurring during modification, a sharp increase in the activity of treated carbosils in processes of physical and chemical adsorption takes place. Also, an improvement in the chromatographic properties of the modified adsorbents is observed. HIT processes do not require high temperatures, which is significant for mineral materials (constituting the base of carbon deposits), which undergo sintering at high temperatures. Using hydrogen peroxide as the modification medium, rather than normal water, makes it possible to obtain greater effects of physical and chemical modification, as with the former lower temperatures for the modification process are generally required.
Financial support from KBN, Project No. 205119101, is gratefully acknowledged.
6
References
4
113R. 121R.
2
0
20
40
60
80
100
pressure
120
140
160
180
200
, mmtlg
Fig. 6. Absolute adsorption isotherms of ammonium on (1) aerosii modified hydrothermally with water at 250 “C, and on the following carboaerosils: (2) carboaerosil 5 wt.% C, (3) carboaerosil 5 wt.% C, HIT, HzO, 250 “C, (4) carboaerosit 19.6 wt.% C, HIT, H,O, 350 “C.
Hence it is very likely that in hydrothermal treatment of carbosils, homogenization (increased homogeneity) of the surface occurs owing to more complete hy droxylation. Moreover, the kinetics of mass transport in adsorption/desorption processes in pores is improved, owing to their size increase. This observation has allowed us to obtain better chromatographic separations, e.g., a mixture of ketones - substances sensitive to energetic
Leboda, Mater. Chem. Phys, 31 (1992) 243, Leboda, Mater. Chem. Phys., 34 (1993) 123. V. Sidorczuk, V. Tertykh [31 R. Leboda, J. Skubiszewska-Zisba, and V. Zarko, J. Non-Ctyst. Solids, submitted for publication. [41 R. Iler, The Chemistty of Silica, Wiley, New York, 1979. PI R. Leboda and E. Mendyk, Mater. Chem. Phys., 27 (1991) 189. (61 A. Gierak, F. Czechowski and R. Leboda, Mater. Chem. Phys., 36 (1994) 264. 171 R. Leboda, A. Gierak, B. Charmas and A. todyga, React. Kinet. Catal. Lett., 50 (1993) 63. PI Yu.1. Tarasevich, Khimoa Tiechnd Wody, 9 (1989) 699. PI N. Sonobe, T. Kyotani and A. Tomita, Car&err, 29 (1991) 61. WI CA. Leon y Leon, A.W. Scaroni and L.R. Radovic, 1. Colloid Interfhce Sci., 148 (1992) 1. and H. Yoshida, J. Colloid Interface Sci., I59 IllI K. Kamegawa (1993) 324. WI A.V. Kiselev and Ya.1. Yashin, Gas Adsorption Chromatography, PWN, Warsaw, 1969. P31 R. Leboda, Chromatographia, 14 (1981) 524. P41 Physico-chemical Handbook (in Polish), WNT, Warsaw, 1974, p. 132. WI E. Mendyk, R. Leboda and A. Gierak, Mater. Chem. Phys., 31 (1992) 355.
R. Leboda [16] [17] [18] [19] [20]
et al. / Materials
Chemistry and Physics 39 (1994) 136-141
G.E. Berendsen and J. Galen, J. LI$. Chromatogr., 1 (1978) 561. R. Lehada, Chromatographia, I1 (1980) 703. R. Leboda, PO/. J. Chem., II (1980) 2305. A. Gierak and R. Leboda, Mater. Chem. Phys., I9 (1988) 503. A. Gierak, R. Leboda and E. Tracz, J. Anal. Appl. Pyrol., 13 (1988) 89.
[21] (221
[23] [24]
141
R. Leboda and A. Eodyga, J. Anal. Appl. Pyrol., I4 (1988) 203. C.J. Brinker and G.W. Scherer, Sol-Gel Science: ;Tlre Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990. I.A. Tarkovskaya, Okulennyj ugol’, Naukovaya dumka, Kiev, 1981, p. 197. HF. Boehm, High. Temp. - High Pressures, 22 (1090) 275.