Adsorption and Gas Chromatographic Properties of Fluorinated Carbon

Transcription

1 585 Adsorption and Gas Chromatographic Properties of Fluorinated Carbon Tatiana M. Roshchina, Natalia K. Shonia and Svetlana V. Glazkova* Laboratory of Adsorption & Chromatography, Chemistry Department, M.V. Lomonosov State University, Leninskie Gory, Moscow, Russia. (Received 9 July 2004; revised form accepted 11 May 2005) ABSTRACT: Carbon fibre (C) and adsorbent prepared from fluorinated carbon fibre (FC) have been studied by adsorption and gas chromatography (GC). An extreme growth in the specific surface area of C was observed as a result of fluorination, although the homogeneity of FC was increased in comparison to that of C. Heats and standard adsorption entropies for a number of compounds have been measured by GC. It was shown that the FC sample was characterized by a highly hydrophobic and chemically homogeneous surface of low polarity. INTRODUCTION Scientific and technical routes for the synthesis and practical application of fluorocarbons have been actively developed recently. Carbon fluorides are mainly synthesized by the direct combination of different allotropic forms of carbon with elemental fluorine. Natural graphites, cokes, blacks, fibres and textiles, nanotubes and fullerenes can be used as carbon matrixes (Touhara and Okino 2000). Fluorination results in the appreciable modification of the physical and physicochemical properties and structure of carbon matrices. It has been shown that the formation of a completely fluorinated carbon (CF) n leads to a change in the hybridization of the carbon atoms from sp 2 to sp 3. The structure of 1:1 fluorinated graphite is presumed to be composed of perfluorinated polycyclohexane units (Nanse et al. 1997). However, the average interlayer distances, d, of fluoro- carbons are considerably greater than those of graphite and because of this the interactions between the carbon sheets are weaker. This is the reason for the ready formation of intercalated compounds (Sato et al. 2003). A vast variety of physicochemical methods has been applied for the characterization of the chemistry and structure of fluorocarbon surfaces. Of these, adsorption methods are quite informative (Sato et al. 2003; Li et al. 1995; Kuznetsov and Moreva 1996). Most works highlight the low surface energy of fluorine-containing materials as fluorocarbons adsorb substances, including nitrogen, methanol and ethanol, to a lesser extent than the original carbon irrespective of the nature of the carbon matrix (Li et al. 1995). The unique property of fluorocarbons is their extraordinarily high hydrophobicity (Li et al. 1995; Watanabe 1995). Aside from classical adsorption methods, gas chromatography (GC) can also be used for the investigation of the surface properties of fluorinated carbons. GC has the double advantage of being relatively simple and available. It is also applicable to the study of a large number of test compounds over a wide range of temperatures at near-zero surface coverage, i.e. in the region where static adsorption methods are of little use (Donnet and Park 1991; Gurevich et al. 2001; Kiselev 1986). *Author to whom all correspondence should be addressed. Rosh@phys.chem.msu.ru.

2 586 T.M. Roshchina et al./adsorption Science & Technology Vol. 23 No Analysis of the literature data reveals a lack of information concerning the adsorption properties of fluorocarbons. At the same time the relationship between the adsorption characteristics and the structure of an adsorbate is of great interest both from the viewpoint of an assessment of the surface properties and for the practical application of fluorocarbon materials as adsorbents, supports and catalysts. In the present study, static adsorption and gas chromatographic measurements have been applied to the characterization of the surfaces of fluorocarbons and the original carbon. EXPERIMENTAL The pristine carbon fibre (C) and the fluorocarbon adsorbent (FC) were prepared at the Institute of Electrocoal Articles, Russia. According to the manufacturer, the fluorinated carbon fibre possessed a C/F ratio of 1:1. In order to prepare the granular fluorocarbon adsorbent, polytetrafluoroethylene (PTFE) was added as a binding agent (Polyakova et al. 1995). The PTFE employed was prepared at the Kirovo-Chepetsk Chemical Combine, Russia. Fraction separation was performed using appropriate sieves with adsorbent particles in the range mm being employed. All the adsorbates used were of analytical grade as purchased from Reakhim, Russia. Static adsorption isotherms of benzene, hexane and water on C and FC were studied at 298 K using a vacuum McBain Bakr balance (Nikitin and Petrova 1990). Nitrogen adsorption isotherms were measured using a Micromeritics 2100D analyzer at 77 K. Prior to such measurements, all adsorbents were degassed at 423 K and 1.3 mpa for 20 h. The measurements made were accurate to ± 0.02 mmol/g. Nitrogen adsorption data in the range 0 < P/P s < 0.2 (where P/P s is the relative pressure, P is the equilibrium pressure and P s is the saturated vapour pressure) was used to estimate the BET specific surface area. The cross-sectional area of the nitrogen molecule, ω, was assumed to be nm 2. The average value of the apparent pore size was obtained using the benzene adsorption isotherm in conjunction with the Dubinin Radushkevich (DR) equation (Gregg and Sing 1982). Gas chromatographic measurements were performed on Chrom-5 (Czech Republic) and Tsvet-100 (Russia) instruments equipped with flame ionization detectors employing high-purity helium and nitrogen as carrier gases (flow rate F = ml/min). The columns employed were of glass ( mm 2; 2.5 mm i.d.). All adsorbents were preliminarily heated at 453 K for h in a helium flow. Test compounds ( ml volume) were injected into the column in the form of diluted vapours. Retention volumes, V a (ml/m 2 ) and V g (ml/g), were calculated from the relationships V a =V N /gs and V g =V N / g, where V N, g and S are the net retention volume, adsorbent mass and the specific surface, respectively, as described earlier (Nikitin and Petrova 1990; Kiselev and Yashin 1969). The thermodynamic adsorption characteristics were obtained from chromatographic parameters corresponding to zero surface coverage by the adsorbate, i.e. from the Henry region of the adsorption isotherms (Nikitin and Petrova 1990; Kiselev and Yashin 1969). The heat, q, and the standard adsorption entropy change, S 0, were obtained from the temperature dependence of V a, i.e. ( ) ln Va = q/ RT + S 0 + R / R assuming that q and S 0 are independent of the temperature. (1)

3 Adsorption and Gas Chromatographic Properties of Fluorinated Carbon 587 The Kovátz retention indices, I, were calculated from the relationship (Nikitin and Petrova 1990): I = log V ( X) log V ( n) g log V ( n + 1) log V ( n) g g g n (2) where V g (n) and V g (n + 1) are the retention volumes of n-alkanes with n and n + 1 carbon atoms, respectively, provided that V g (n + 1) V g (X) V g (n). RESULTS AND DISCUSSION Adsorption measurements Nitrogen adsorption isotherms on C and FC are presented in Figure 1. The adsorption isotherm on FC demonstrates an essential increase in the adsorption ability of the carbon material after fluorination. This resulted mainly from the changes in the porous structure induced by fluorination. Estimations of the specific surface area (S) derived from the nitrogen adsorption isotherms confirm this conclusion, since these values were 1.7 m 2 /g and 190 m 2 /g for the original carbon and the fluorinated carbon, respectively. The amounts of benzene and hexane adsorbed onto C were small and close to the level of accuracy in the measurements (Figure 2). It will be noted from this figure that the benzene adsorption/desorption isotherm on FC exhibited a horizontal hysteresis loop which is usually indicative of the existence of slit-like pores (Gregg and Sing 1982). Their presence is in agreement with known conceptions of the flaky structure of fluorocarbons (Park et al. 2003). In addition, Amount adsorbed, A (ml/g) P/P s Figure 1. Nitrogen adsorption isotherms at 77 K on the original carbon fibre (C, ) and on the fluorocarbon (FC, ). In the figure, the amount of N 2 adsorbed onto the initial carbon has been multiplied by 100.

4 588 T.M. Roshchina et al./adsorption Science & Technology Vol. 23 No Amount adsorbed, A (mmol/g) P/P s Figure 2. Adsorption/desorption isotherms of benzene at 298 K on the original carbon fibre (C,, ) and on the fluorocarbon (FC,, ). Open symbols correspond to adsorption while solid symbols correspond to desorption. the continuation of the hysteresis loop to the lowest pressures should be noted as a peculiarity of this particular adsorption isotherm. There are several explanations for the irreversibility of the benzene isotherm but the majority of them are associated with the retention of adsorbate molecules in the micropores of the adsorbent (Gregg and Sing 1982). However, benzene chemisorption can be excluded in this case since benzene was adsorbed to a lesser extent than hexane in the low-pressure region (Figure 3). A comparison of the benzene and hexane molecules allows the extent of surface polarity to be evaluated as both molecules contribute similar dispersive interaction energies to the adsorption process (Gregg and Sing 1982). Isirikyan and Kiselev (1961) showed that the same results were obtained from an adsorption analysis of the behaviour of these compounds on the homogeneous and non-polar surface of graphitized thermal carbon black (GTCB). As the π-electron systems of hydrocarbons interact electrostatically with the active polar centres on the surface, this provides convincing proof that the fluorocarbon surface is non-polar towards aromatic hydrocarbons. Thus, it is most likely that the reason for the irreversibility of the benzene adsorption isotherm is the residual microporosity in the fluorocarbon structure. According to the results of the application of the DR equation to the benzene adsorption isotherm (Gregg and Sing 1982), small mesopores with an apparent pore size x = 1.4 nm existed in the FC structure. The most obvious property of all fluorinated surfaces is their high hydrophobicity, which was also characteristic for the FC sample. This may be established from the corresponding water adsorption isotherms which demonstrate that the amounts of water adsorbed onto the sample were virtually zero over a wide range of P/P s values. Thus, at P/P s 1, an insignificant increase of the amount of adsorbed water up to a value of 0.05 mmol/g (= 0.25 µmol/m 2 ) was observed.

5 Adsorption and Gas Chromatographic Properties of Fluorinated Carbon Amount adsorbed, A (µmol/m 2 ) Pressure, P (kpa) Figure 3. Adsorption isotherms for benzene ( ) and hexane ( ) on the fluorocarbon at 298 K. According to Carrott (1992), the relative capacity of a compact monolayer of water on a carbon surface is 15 µmol/m 2, which is 10-times larger than the maximum value observed for FC. In addition, the amount of water adsorbed at P/P s 1 was much less than that adsorbed on the original carbon material (6 µmol/m 2 ). From these results it may be inferred that the FC sample was characterized by a low-polarity, highly hydrophobic and chemically homogeneous surface. Gas chromatography The GC method is more sensitive than static adsorption and therefore more convenient for the investigation of materials with low surface areas. The retention volumes, V g (ml/g), which are proportional to the adsorption values, A, referred to 1 g adsorbent at low and equal concentration (or pressure) of a substance in the gaseous phase (Kiselev 1986), are presented in Table 1. It will be seen from the data listed that the magnitudes of the V g values on FC for all the compounds investigated were much larger than on the original carbon matrix. This result is in agreement with the static adsorption data. Since the amount adsorbed depends on the specific surface area, it is possible to use retention volumes relative to the surface area, i.e. values of V a (ml/m 2 ), for the characterization of the surface properties of C and FC, as is customary in the literature (Kiselev 1986). Let us first compare the V a values for the interaction of n-alkanes. Such interaction usually occurs via a dispersive mechanism with various surfaces. As shown in Figure 4, the retention volumes, V a, for n-alkanes on FC were lower than those on C. This decrease correlates with literature data arising from a combined investigation of the adsorption properties of carbons and fluorocarbons (Li et al. 1995; Setoyama et al. 1997). However, in order to investigate the materials in detail, the adsorption data for molecules exhibiting similar non-specific interactions

6 590 T.M. Roshchina et al./adsorption Science & Technology Vol. 23 No TABLE 1. Kovátz Retention Indices, I, and Retention Volumes,V g (ml/g), at 373 K of Various Adsorbates on the Initial Carbon Fibre (C) and the Fluorinated Carbon (FC) Adsorbate C FC V g I V g I Pentane Hexane Benzene Propan-1-ol Propan-2-ol Butan-1-ol Diethyl ether Acetone Acetonitrile ln V a n 7 8 Figure 4. Dependence of the retention volumes, V a, on the original carbon fibre (C, ) and on the fluorocarbon (FC, ) for n-alkanes on the number of carbon atoms, n, in their chains, as measured at 373 K. were also compared. Two pairs which are traditional for gas chromatography were used in this case, i.e. pentane/diethyl ether and hexane/benzene (Kiselev 1986; Gregg and Sing 1982). Figure 5 shows that the retention of benzene on C was greater than that of hexane while that of diethyl ether was stronger than that of pentane. This indicates that the surface contained oxygen complexes, i.e. the carbon fibre possessed a chemically non-homogeneous surface. The fluorinated material

7 Adsorption and Gas Chromatographic Properties of Fluorinated Carbon ln V a /T (1/K) Figure 5. Dependence of the retention volumes, V a, on the inverse temperature. Open symbols relate to FC while solid symbols relate to C. The data points correspond to the following adsorbates:, hexane;, benzene;, diethyl ether;, pentane. behaved differently. In this case, the retention volumes of hexane and benzene were almost equal. However, the retention of diethyl ether was much less than that of pentane. This is typical for nonpolar adsorbents. With GC techniques it is traditional to compare the retention Kovátz indices, I, for substances capable of interacting via electrostatic and donor acceptor mechanisms or forming hydrogen bonds. The Kovátz indices for all investigated compounds were smaller on FC relative to C by ca units (see data listed in Table 1). This demonstrates the substantial decrease in the role of specific interactions during adsorption onto the fluorocarbon. It is also necessary to emphasize that, in contrast to C, the chromatographic peaks of all the investigated compounds (including alcohols) on FC were symmetric. This provides evidence that FC possessed a chemically homogeneous surface. Thus, fluorination led not only to a variation in the structural parameters, but also to a substantial diminution in the role of specific interactions during the adsorption process. Because dispersive interactions contribute substantially to adsorption onto graphitized thermal carbon black (GTCB) (Avgul et al. 1975), the latter has been employed as a model adsorbent in the present studies. Table 2 presents the retention volumes, V a, and heats of adsorption, q, determined for FC and GTCB (Avgul et al. 1975; Davydov et al. 1996). For the vast majority of adsorbates studied, the retention volumes on FC appear to be less than those on GTCB, which is in good agreement with a widespread concept of the low surface energy of fluorinated materials. In contrast to the retention volumes, the heats of adsorption on FC were higher and the entropies were smaller than for GTCB. The reason for this behaviour could be the residual microporosity of FC. In other words, the presence of micropores resulted in a corresponding increase in interaction with FC and a decrease in adsorption entropy due to the lowering of molecule mobility in small pores.

8 592 T.M. Roshchina et al./adsorption Science & Technology Vol. 23 No TABLE 2. Retention Volumes, V a (ml/m 2 ), at 373 K, Heats of Adsorption, q (kj/mol), and the Standard Entropies of Adsorption, S 0 [J/(mol K)], on Fluorinated Carbon (FC) and Graphitized Thermal Carbon Black (GTCB) a Adsorbate FC GTCB V a q S 0 V a q S 0 Butane Pentane Hexane Benzene Diethyl ether Acetone Propan-2-ol Butan-1-ol Pentan-1-ol Nitromethane Acetonitrile a Taken from the data of Avgul et al. (1975) and Davydov et al. (1996). TABLE 3. Kovátz Indices, I, at 403 K on Graphitized Thermal Carbon Black (GTCB) a, Fluorinated Carbon (FC) and Polytetrafluoroethylene (PTFE) Adsorbate GTCB FC PTFE Benzene Propan-2-ol Butan-1-ol Diethyl ether Acetone Acetonitrile a Taken from the data of Avgul et al. (1975). It is worthy of note that the heat of adsorption for benzene as determined from GC data (46 kj/mol) was virtually identical with that obtained by calorimetry (44 kj/mol) (Kuznetsov and Moreva 1996). This fact suggests the possible application of GC in the investigation of materials such as fluorinated carbons. In contrast to GTCB, FC exhibited a smaller difference in its retention capabilities towards hexane and benzene as well as towards pentane and diethyl ether. Butan-1-ol was retained even more strongly than pentane while pentan-1-ol was retained more strongly than hexane (see data in Table 2). At first sight, these data suggest some retained surface heterogeneity, e.g. the existence of oxygen complexes on the surface of the adsorbent. However, before making such a conclusion, the Kovátz indices on GTCB and FC should be compared with those on PTFE (see Table 3). PTFE possesses a highly hydrophobic, chemically inert and homogeneous surface (Leibnitz and

9 Adsorption and Gas Chromatographic Properties of Fluorinated Carbon 593 Struppe 1984). In moving from GTCB to FC, the Kovátz indices appear to increase and a further increase may be noted in moving to PTFE, i.e. an increase in the fluorine content of the sample leads to a relative increase in its polarity. Let us now compare the retention of acetone and propan-2-ol, two molecules with close structures, molecular weights and polarizabilities. These compounds were retained almost equally by GTCB. FC, as well as PTFE, retained acetone more strongly than alcohol (Tables 2 and 3). Such a retention order for propan-2-ol and acetone suggests that the surface is non-polar (Leibnitz and Struppe 1984). Thus, oxygen complexes, which are capable of forming hydrogen bonds, are unlikely to exist on the surface of FC. This assumption is confirmed by both literature data and our experimental water adsorption data. It is also possible to suggest that adsorption centres exist on the FC surface which are capable of supporting donor acceptor interactions. However, acetonitrile whose molecule has the greatest dipole moment was retained by FC to the smallest extent of all the adsorbates investigated. Nitromethane was also retained to a smaller extent than most compounds. This suggests that most probably neither donor acceptor nor electrostatic interactions occur, an observation which accords with those of Kuznetsov and Moreva (1996). CONCLUSIONS The data obtained indicate that the fluorocarbon examined in the present study possessed a homogeneous and hydrophobic surface which exhibited low polarity. Such data could be useful in the utilization of fluorocarbons as catalysts, supports and adsorbents. REFERENCES Avgul, N., Kiselev, A. and Poshkus, D. (1975) Adsorption of Gases and Vapors on Homogeneous Surfaces, Khimia, Moscow. Carrott, P.J.M. (1992) Carbon 30, 201. Davydov, V.Ya., Roshchina, T.M., Filatova, G.N. and Khrustaleva, N.M. (1996) Zh. Fiz. Khim. 70, Donnet, J. and Park, S. (1991) Carbon 29, 955. Gregg, S.J. and Sing, K.S.W. (1982) Adsorption, Surface Area and Porosity, Academic Press, London. Gurevich, K., Roshchina, T., Shonia, N., Kustov, L. and Ivanov, A. (2001) Adsorp. Sci. Technol. 19, 291. Isirikyan, A. and Kiselev, A. (1961) J. Am. Chem. Soc. 65, 601. Kiselev, A. (1986) Intermolecular Interactions in Adsorption and Chromatography, Vysshaya Shkola, Moscow. Kiselev, A. and Yashin, Y. (1969) Gas-Adsorption Chromatography, Plenum, New York. Kuznetsov, B. and Moreva, A. (1996) Zh. Fiz. Khim. 70, Leibnitz, E. and Struppe, H. (1984) Handbuch der Gaschromatographie, Academische Verlagsgesellschaft Geest & Portig K.-G., Leipzig, East Germany. Li, G., Kaneko, K., Ozeki, S., Okino, F., Ishikawa, R., Kanda, M. and Touhara, H. (1995) Langmuir 11, 716. Nanse, G., Papirer, E., Fioux, P., Moguet, F. and Tressaud, A. (1997) Carbon 35, 175. Nikitin, Yu. and Petrova, R. (1990) Experimental Methods in Adsorption and Molecular Chromatography, MSU, Moscow. Park, S.-J., Seo, M.-K. and Lee, Y.-S. (2003) Carbon 41, 723. Polyakova, N.V., Koldyshev, A.E., Pimenova, L.M., Orlova, H.I., Garbuzov, V.G., Kirgotenko, V.M., Kuznetsov, Yu.N. and Wulf, V.A. (1995) Russian Pat (27 June).

10 594 T.M. Roshchina et al./adsorption Science & Technology Vol. 23 No Sato, Y., Shiraishi, S., Mazej, Z., Hagiwara, R. and Ito, Y. (2003) Carbon 41, Setoyama, N., Li, G., Kaneko, K., Okino, F., Ishikawa, R., Kanda, M. and Touhara, H. (1997) Adsorption 2, 293. Touhara, H. and Okino, F. (2000) Carbon 38, 241. Watanabe, N. (1995) J. Fluorine Chem. 71, 173.